Author: Marcello De Falco – Associate Professor, University UCBM – Rome (Italy)

 

1. Theme description

Elastomers are polymers made of long chains of atoms (mainly, carbon, hydrogen and oxygen), composed of repeated units of a single monomer or of two or more different monomers.

Elastomers are characterized by a high level of both viscosity and elasticity and they are probably the most versatile material, exhibiting the particular combination of elasticity and resilience, having the capacity to substantially deform and then return to the original shape as the forces causing the deformation are removed.

They can be divided in two main categories:

• Thermoset elastomers, as acrylic, butadiene, butyl, chlorinated polyethylene, ethylene propylene, fluorocarbon, isoprene, nitrile, polysulphide, polyurethane, silicone, etc.
• Thermoplastic elastomers, as thermoplastic urethane elastomers, styrenic block copolymers, copolyether ester elastomers, polyester amide elastomers.

In the Oil & Gas sector, they are used in a wide range of applications, which can be divided in [1]:

• sealing – the high elasticity characteristics of elastomers makes them a proper choice for sealing applications. The elastomers are used as seals in shafts, flanges, pump pistons, rods, etc.
• not sealing –  applications include elastomeric belts for drive systems and power transmission, flexible hoses, personal protection products (face masks, nasal units, neck seals, etc.), expansion joints.

Elastomers are widely used also in the civil engineering sector (mounting structures, bridge bearings, expansion joints, pipe couplings, as barrier to water, roofing membranes, rubberised asphalt, concrete texturing) and in the automotive sector (tyres, suspension systems).

In the following pages, the main high performance elastomers production processes and the main innovations for Oil & Gas sector applications are described.

2. Production process

Basically, the elastomers production process is composed by two steps [2]:

• Compounding step – elastomer is mixed with additives as pigments, re-enforced agents, antioxidants, etc. The compounding is performed in a dispersion kneader at high pressure and temperature. Then, the compound mixture is further mixed in a two-roll mill, where curing agents are added. The final compound is then extracted as sheets with specific thickness.

Elastomer dispersion kneader (left) and two-roll mill (right) [3]

 

• Moulding/Extrusion – the final elastomeric product is shaped by an extrusion or a moulding process. The extrusion consists of three units in series: an extruder (a rotating screw inside a heated barrel), a shaping die (a metal disk with a machined opening in the desired shape) and a vulcanizator, able to give to the product the physical properties and to maintain the shape. In the moulding process, the elastomer sheets are cut and fed into heated moulds of the desired shape. Then, the elastomer is baked to vulcanize.

Elastomer Extruder [4] (left) and compression moduling press [5] (right)

 

3. Innovations in the Oil &Gas

Elastomers are widely used in Oil & Gas sector for many applications and a strong research effort is devoted to improve the materials properties. Specifically, the innovation actions are focused on:

• improving the chemical resistance to new products;
• increasing the operating temperature and pressure;
• improving the abrasion resistance.

In the following, some applications and product innovations in the Oil & Gas sector are described.

3.1 Offshore Drilling

Drilling sector makes an extensive use of elastomeric seals. When the drilling has to be performed in deep water, the seals have to operate at critical conditions and, consequently, their properties have to be improved. For these applications, the best elastomers are NBR (acrylonitrilebutadiene rubber), HNBR (hydrogenated acrylonitrile-butadiene rubber) and fluoroelastomers, since they are characterized by high performance in terms of operating temperature / pressure and chemical stability.

The main problems occur for the subsea equipment, which works at cold temperatures and high pressures. The elastomeric seals are used in the rotary control device, in the riser gas handling and in the flex joint and have to guarantee optimal performance in terms of stability and abrasion resistance when applied in rotational devices.

Elastomeric seals are also applied in blowout preventers (BOP), used to control the wellbore pressure. When the BOP bore is closed, the elastomer has to be able to assure large elongation properties (+ 400%) and, when the bore is opened again, the elastomer has to retract completely. Moreover, the elastomers have to resist at pressures within the range 15000 – 25000 psi.

Annular Blowout Preventers [6]

 

3.2 O-Rings rapid gas decompression resistance in EOR

Enhanced Oil Recovery (EOR) is a technology that uses supercritical carbon dioxide (s-CO₂) to increase well pressure and reduce the viscosity of the crude oil to be extracted, thus strongly improving the well yield. But, the high content of CO₂ in the crude oil can damage the elastomeric seals for two main reasons:

• the plastification of the elastomer, leading to a softening of the seal [7], [8];
• the rapid gas decompression damage, particularly serious with s-CO₂ [9], [10].

Generally, two materials are used for sealing in EOR: fluoroelastomers (FKM) and Hydrogenated Nitrile Butadiene Rubber (HNBR).

The FKM suffers to the swollen (plastification) due to CO₂ more than HNBR since its CO₂ solubility is higher, so that usually HNBR is preferred and demonstrates good performance in applications at high level of CO₂. However, many studies are in progress concerning with the improvement of elastomeric seals under s-CO₂ environment.

HNBR sample after 5 days immersion in CO2 [11]

 

3.3 Materials for Sealing Solutions in HPHT conditions

Saint-Gobain Seals Group is going to develop innovative seals elastomers for High Pressure/High Temperature (HPHT) applications, based on specific thermoplastic elastomer formulations [12].

The tests performed under critical conditions (25 kpsi at 200°C for 4 hours, 30 kpsi at room temperature and at 250°C) reveal that the innovative materials show very good thermal and chemical properties and a strong resistance to rapid gas decompression. The main application thought for the innovative thermoplastic elastomers is the fulfillment of seals for Subsea Production system in the deep sea.

 

 3D drawing of a Saint-Gobain Seals Group innovative thermoplastic elastomer seal [12]

 

3.4 Bio-elastomers

Extender oils are additives widely used in the tyre industry as plasticising substances for improving the elastomer workability and decreasing the rigidity after vulcanization. Usually, they derive from crude oil refining and contain high contents of toxic substance as polycyclic-aromatic hydrocarbons (PAH).

The European Union Directive 2005/69/EC limits the use of oils containing PAHs in the tyre industry, thus boosting the research efforts toward the development of innovative and sustainable extender oils production processes.

Among the developed studies on this topic, the one fulfilled by Matrica, a joint venture between Versalis and Novamont, is particularly interesting since it identifies a number of vegetable oils derived from biomass to be used in rubbers for tyre industry [13]. The company is developing a modified styrene-butadiene rubber (SBR) with properties similar to the PAH high content oils but derived from biomass and called bio-elastomers, with a reduced environmental impact and paving the way to a new elastomeric materials market for environmentally friendly tyres production.

3.5 Shell

SO.F.TER. Group develops high-performance thermoplastic elastomer (TPE) compounds for automotive sector using the high-quality white oil produced by the Gas-to-Liquids (GTL) technology developed by SHELL, called Shell Risella X 430. The low volatility, high viscosity and thermal color stability, makes the Shell Risella an optimal ingredient for the production of TPE for automotive dashboard applications.

The first tests performed on innovative TPE samples have demonstrated that the material is able to assure fogging characteristics 60% better than the TPE which uses conventional paraffinic oils, confirming the high potentialities of these materials [14].

 

____________

[1] http://www.jameswalker.biz/en/pdf_docs/148-elastomer-engineering-guide

[2] http://www.siltonrubber.com/manufacturing_process.html

[3] http://www.rcmc.com/equip/display.php?id=13

[4] http://www.davis-standard.com/de/extruders

[5] http://naskahku.tk/compression/compression-molding-press-compression-molding-presses-.html

[6] http://aea-al.org/wp-content/uploads/2014/11/Polymers.pdf

[7] Danny Hertz III – Elastomers in CO2 -2012 Smithers High Performance Elastomers & Polymers for Oil & Gas Applications conference

[8] Elastomeric Materials based on Hydrogenated Nitrile Rubber for Seals in Carbon Dioxide (R 744) High Pressure Service. Improving the Resistance against Explosive Decompression. Dr Hans Maag, Achim Welle, Dr Matthias Soddemann, Dr Kevin Kulbaba. Merl Oilfield Engineering with Polymers 2010

[9] Z Major, K Lederer, M Moitzi, T Schwarz and RW Lang – Development of a Test and Failure Analysis for Elastomeric Seals Exposed to Explosive Decompression. Oilfield Engineering with polymers, 2006.

[10] AF George, S Sully and OM Davies – Carbon Dioxide Saturated Elastomers: The loss of Tensile Properties and the Effects of Temperature Rise and pressure Cycling. Fluid sealing – BHR Group, 1997.

[11] P. Warren, S. Winter, A. Douglas. Rapid Gas Decompression resistance of elastomeric o-rings to supercritical CO2.  High Performance Polymers for Oil & Gas 2014 (15-16 April 2014 – Edinburgh, Scotland), Paper 5.

[12] http://www.seals.saint-gobain.com/news_detail.aspx?id=278038

[13] http://www.eni.com/en_IT/innovation-technology/technological-focus/bio-elastomers/bio-elastomers.shtml?home_2010_en_tab=editorial

[14] http://www.shell.com/content/dam/shell-new/local/corporate/lubricants/downloads/pdf/softer-d8.pdf

Author: Andrea Milioni, Chemical Engineer – On Contract Cooperator – University UCBM – Rome (Italy)

 

1. Theme description

Scientific progress in the last two centuries has allowed a great development of industrial production activities, modifying the relationship between mankind and the environment. In particular, the exploitation of natural resources has led to changes in the environment, often irreversibly. This raises the need to develop, in parallel to the new technologies, research aimed on the one hand at preventing potential ecological disasters, the other at remedying in case of contamination. In this frame, great are the efforts by developed countries to stimulate applied research for the rehabilitation of polluted sites.

One of the industrial sectors with the largest impact on the environment relates to the Oil and Gas industry. Pollution is due to several activities, including drilling, stimulation and the separation and dehydration operations. Two couples of categories can be individuated in the first place: pollution from punctual of diffuse sources and from chronic or accidental release of pollutants. From the pollutant point of view, this can be either organic or inorganic with, consequently, different environmental fates and repercussion on air, soil and groundwater [1].

The main categories of chemical standards for developing soil remediation guidelines are represented by: inorganic parameters, metals, hydrocarbons, halogenated aliphatics, pesticides, other organics and radionuclides. Both before and after the remediation is necessary to perform some important procedures. The first concerns monitoring and planning methods of remediation, where chemical, hydrogeological and microbiological analysis are made to characterize the soil and the pollution typology. The individuation of standards depends both on human and ecological features (as the use of land and water) as well as the kind of the contaminated matrix (physic-chemical characteristics)[2]. The second concerns the methods effectiveness control: the treatment is complete when the remedial target levels have been achieved for the specified use of the soils and the risk to the ecosystem is minimized. The number of samples to collect should be adequate to provide a statistically reliable result and strictly depends on the use of the soils and the possible contaminants migration. During the decontamination procedure, any possible impact on the related matrixes (such as air in case of VOC emission) should be constantly monitored [3]. The Soil Screening Guidance [4] is an EPA tool for the standardization of the step-by-step evaluation and clean-up of contaminated soils destined to possible residential use of land. In the following, main techniques for soil remediation will be presented focusing on the cleaning up after hydrocarbon contamination.

 

2. Remediation methods

The goal of remediation is to remove, or to make harmless, substances contaminating the soil or groundwater. The remediation processes can be applied directly to the site of contamination, in situ, or after removing the contaminated soil, ex situ. Among the latter, there are the treatments on site, when working on the excavation site or off site, when you need to transport in plants located elsewhere. It is possible a classification of the different processes in accordance with the mechanism for cleaning up: non-organic (chemical, physical or thermal) or biological. The choice of different types of treatment is linked to several factors related to the nature of the pollutant, the polluted site, the type of technology (basically to its efficiency and cost). Briefly, the main techniques of both non-biological and biological remediation will be described below.

3. Non-biological Remediation

Soil Washing

Description: this technique, generally carried out on site, consists in the soil decontamination by washing with water and possible addition of other substances (chelating agents, surfactants, acids or bases), according to the needs, in order to improve the ‘solid-liquid extraction in contact with the ground. The latter undergoes an initial pre-treatment to remove coarse material and then switch to the washing stage where there is the process of extraction-solubilisation of the pollutant, which passes into the aqueous solution. Subsequently it has a solid-liquid separation that provides the clean solid and starts the water to the treatment thanks to which the pollutant is separated, concentrated, and recirculates the process water for the next wash. The capacity of reclamation is quite high (about 25 t / h). The duration of treatment Soil Washing is usually short, from one to three months and increases as the percentage of clay and silt content in the soil.

Development and applicability: currently, it is developed on a large scale and found more applications in Europe than in America especially in the removal of heavy metals. It can be applied, however, to a wide range of pollutants, including hydrocarbons and pesticides.

Critical issues: May be limited by silty and clayey soils that make it more difficult to solid-liquid separation. Physical separation is generally not effective for treating the chemically adsorbed metals [5].

 

Figure 1 – Soil Washing [6]

Solvent Extraction

Description: technique operating on site using a solvent to improve the efficiency of extraction, in a process very similar to that previously described. Since traces of solvent may remain in the ground at the end of the treatment, a criterion for its choice concerns the degree of toxicity.

Development and applicability: the solvent extraction has proven effective in removing a wide range of organic pollutants, from hydrocarbons to organochlorine pesticides, VOCs and petroleum wastes. The plants in full scale come to treat 20 t / h of soil.

Critical issues: It is not applicable for the removal of inorganic pollutants and some processes are limited by the solid matrix moisture content and fine particles. The presence of detergents and emulsifiers can unfavourably influence the extraction performance [7].

 

Soil Vapor Extraction (SVE)

Description: this technique, also called Soil Vacuum Extraction, is applied in situ and used in the reclamation of the unsaturated zone of the soil, the area in which the pores of the soil contain air or water at a pressure lower than the atmospheric one (by capillarity). Using a system of wells, vacuum is applied so as to induce a controlled flow of air from outside which brings with it the volatile compounds and some semivolatiles. This system comprises a gas treatment extracts made from activated carbon filters, systems of incineration or cold traps; the treated gas is released into the atmosphere or re-injected into the ground.

Development and applicability: This method finds application mainly in soils at medium depth and permeability to avoid the short-circuiting of the steam flow or a difficulty in its circulation. The pollutants to be removed must have a vapor pressure greater than 1 mm Hg at 20°C. Both SVE and air sparging are used to clean up several acres of contaminated soil and groundwater at the Vienna PCE Superfund site in West Virginia [8].

Critical issues: Soils with moisture content above 50%, tend to adsorb the pollutant compounds, compromising the effectiveness of the technology.

 

 Figure 2 – Soil Vapor Extraction (SVE) System for Vadose Zone Remediation Air sparging

 

Air Sparging

Description: technology operating in situ in which air is bubbled through a contaminated aquifer. The air bubbles cause stripping of volatile organic compounds present in the saturated zone, the part of the subsurface in which the pores of the soil are filled with water at pressure equal to or greater than the atmospheric one. In general, the exit gas from the underground are conveyed by means of a suction system, which often coincides with a SVE inserted into the unsaturated zone.

Development and applicability: it finds ideal application in homogeneous soils with high permeability and unconfined aquifers, polluted by volatile compounds, halogenated  and hydrocarbons.

Critical issues: if it is not used in conjunction with SVE, an unwanted migration of pollutants outside the contaminated area may occur. Special attention should be given in the event of large doses of pollutant supernatants (eg. Hydrocarbons in suspension), to prevent the push and the bubbling cause aerosols in the surrounding areas.

 

 Figure 3 – Combined Air Sparging and Soil Vapor Extraction system

 

Dual Phase Extraction

Description: this technology allows the in situ simultaneous removal of the contaminants present in the unsaturated zone and the saturated zone of the soil (In case the contamination concerns both stages) by the means of a vacuum pump. In this way extends the applicability of the SVE to the saturated zone of the soil. Downstream of the vacuum pump it is necessary to separate the liquid from the vapour phase and proceed to the train of treatments for the different phases.

Development and applicability: suitable for this technology are the low permeability soils, usually clay, in which the cone of depression extends in depth, going to increase the thickness of the unsaturated zone. An important factor to consider concerns the hydrogeology of the site, crucial to understand the degree of applicability and effectiveness of this treatment.

Critical issues: The technique is not recommended in case of deep contamination of the aquifer and in the case where the contamination is very extensive.

 

Figure 4 – Typical Dual Phase Extraction Scheme [11]

 

 

Solidification / Stabilization

Description: in the solidification processes, the pollutants are physically linked, or trapped in a solid matrix, while in the stabilization, chemical reactions transforming the pollutant in a less mobile species, are favoured. An example is given by the cement that immobilizes many metal contaminants by forming insoluble hydroxides, carbonates and silicates (stabilization) as well as providing an encapsulating matrix for the leaching attenuation (solidification) [12].

Development and applicability: this technique is used mainly for the treatment of inorganic pollutants, including radionuclides, while the presence of organic material may constitute an obstacle for the success of the neutralizing process.

Critical issues: increase in volume of the final product (up to twice the volume to be treated) and long-term stability of the material inertization.

 

Thermal Desorption

Description: This ex-situ treatment consists in the desorption of volatile pollutants through the supply of heat from outside. The material polluted, is sent to a rotary kiln or to a heated auger system, where, by increasing the temperature the formation of gases and vapours of polluting compounds is guaranteed. The contaminants destruction is realised using a secondary treatment units [13].

Development and applicability: there are two processes in response to operating temperature, low (90-320°C) and high (320-560°C). In the first case, generally suitable for non-halogenated hydrocarbons, no thermal oxidation occurs and the physical characteristics of the soil remain unchanged. In the second case, instead, semivolatile organic compounds, volatile metals and polycyclic aromatic compounds are removed by operating, often in combination with the incineration processes, solidification/stabilization and dechlorination [14].

Critical issues: the economy of the process is affected by the moisture content of the material to be reclaimed.

 

Incineration

Description: technology that works off site used for the final disposal of contaminated materials resulting from the treatment of soil washing, solvent extraction and thermal desorption. The contaminated material is fed into a burner where takes place the volatilization and oxidation of organic compounds at temperatures between 870°C and 1200°C, in the presence of oxygen. Often it is necessary to supply the burner with an auxiliary fuel, both to trigger and to maintain the combustion.

Development and applicability: used especially if contamination concerns explosives, chlorinated hydrocarbons, polychlorinated biphenyls and dioxins. In the presence of heavy metals, it is necessary to inert ashes.

Critical issues: at the exit of the burner, gas treatment, particulate abatement and neutralization of acids (HCl, NO_x and SO_x) systems are needed.

 

4. Biologic Remediation

Bioventing

Description: technology operating in situ, for the soils treatment in the unsaturated zone. It stimulates the degrading action of microorganisms already present in the soil (native microbial flora), providing oxygen and, where necessary, mineral nutrients into the ground by percolation or by direct input with specific spargers. Oxygen is normally provided through direct input or air suction through spears stuck in the ground.

Development and applicability: it is useful in the remediation of hydrocarbons contaminated soils and is adaptable to soils with high permeability. Process often coupled with SVE: first making an SVE with the removal of the more volatile hydrocarbons, then performs a bioventing, simply by reducing the air flow and injecting nutrients for degrading residual non-volatile hydrocarbon components.

Critical issues: avoid the application when soils are: too heterogeneous, so contaminated by pollutants to create a saturated zone and in areas  close to aquifers.

Figure 5 – Typical Bioventing system

 

Biosparging

Description: technology similar to bioventing, which operates in situ for the treatment of saturated soils and groundwater. The acceleration of the native microbial flora degradation is done through direct air and appropriate nutrients entering the contaminated area.

Development and applicability: it is normally used to degrade the contaminants that are dissolved in the groundwater, adsorbed on soil particles below the groundwater level or in the capillary fringe area. Effective is the application in petroleum products reduction, usually realised for underground storage tank sites [16].

Critical issues: the presence of an hydrocarbons concentration such as to decrease the permeability or to be toxic, can make ineffective the action of biosparging.

 

Natural attenuation

Description: in this way is performed an intrinsic bioremediation, exploiting the nature ability to restore a polluted environment. Set up a site for a natural attenuation essentially means: run a targeted monitoring to know the precise boundary between the contaminated area and the clean zone, a campaign of analysis for the measurement of some basic parameters (temperature, pH, redox potential, concentration nitrate, nitrite and ammonia, phosphates) and enumeration of bacterial populations specific for the different types of biodegradation.

Critical issues: the presence of non-biodegradable pollutants, existence of contamination phenomena able to convey hazardous substances towards targets of environmental interest and need to complete the remediation in a short time.

 

Landfarming

Description: technique operating on site which consists of arranging the contaminated material on a non-permeable surface in a layer normally less than one meter, ensuring, during the decontamination period, the maintenance of the best conditions for the microbial populations development. It is essential to ensure, from the beginning of the treatment, a correct balance of the main nutritional components of the system: carbon, nitrogen and phosphorus, in relations respectively 100: 5: 1 in addition to the content of water content (60-70% of the saturation value), and the soil pH, which must be neutral. Furthermore it is necessary to facilitate the air entry for the correct oxygen supply to the bacterial populations, generally by mixing the soil to be treated or by entering bulking agents (wood chips, expanded silicon, etc.). This process requires an extended time frame, possibly up to 24 months, depending on a number of factors, including the nature of the contamination, the concentrations of contaminants, types of soil and volume of soil to be remediated. Landfarming to remove volatile constituents from soils through evaporation, without biological degradation, is not acceptable, unless are realised the volatile constituents capture and treatments [17].

Critical issues: the presence of volatile pollutants that threaten the operators health, unavailability of adequate land area.

Figure 6 – Typical Landfarming operation

 

Biopiles

Description: technique very similar to landfarming, with the main difference residing in the method of oxygen transfer. In the preparation of the biopile soil layers are superimposed with interspersed perforated tubes, used to distribute, air and solutions containing the necessary nutrients [19]. In the presence of volatile pollutants, the biopile can be covered with waterproof sheets with appropriate openings to let out the steam to be sent to treatment. The presence of the sheets also facilitates the monitoring of the parameters indicated in the landfarming section.

Development and applicability: The application of biopiles has been guaranteed by numerous studies that have shown good removal efficiencies especially for total petroleum hydrocarbon (THP) [20].

 

Figure 7 – Typical Biopiles Scheme

 

Bioslurry

Description: technique consisting in the remediation of soil within fermentation reactors. Inside the reactors can be controlled effectively operating parameters or use non-native bacterial populations (bioaugmentation). Theoretically it would be possible to use genetically modified bacteria, practice today banned in open field [22].

Development and applicability: to date, this method has been applied only to remove substances not readily degradable.

Critical issues: high costs and reduced volumes of treated soils limit the operational capabilities of bioslurry.

 

Figure 8 – Bioslurry system

 

Microbiological Barriers

Description: A system for the in situ treatment of groundwater. Inside the aquifer is placed transversally a barrier consisting of soil or suitable solid support colonized by microorganisms. Therefore the barrier is “passive”, and biodegradation occurs by contact between the water that runs through it and microorganisms adhering to it. It is also necessary to construct a series of wells for the air and nutrients intake for the present microorganisms.

Development: the microbiological barriers, have had a good spread in Italy with some application examples [24].

 

Emerging Technologies

In recent years, research in the field of bioremediation is evolving to try to increase the contamination cases resolved by biological remediation. In this regard two research lines are being developed to exploit synergistic actions respectively between microorganisms and plants (bioremediation/phytoremediation) and between bacteria and fungi [25]. Not to be overlooked are the research of genetics to modify microorganisms and make them capable of degrading substance considered recalcitrant and the research to make possible actions in situ even in situations where it is very difficult to convey sufficient amount of oxygen. Have particular importance the cases of hydrogen or magnesium peroxides use [26].

In the non-bio field, permeable reactive barriers (continuous and non), characterized by high conductive reactive media capturing the contaminants by deviating the natural flows, are producing invigorated research results [27]. Another interesting field of research can be individuated in the use of hydraulic fracture technology for soil and groundwater remediation. FRx promotes the Hydraulic fractures as an optimal in situ treatment for the removal of materials from extraction wells. They utilizes US EPA practices and individuates four key factors distinguishing the uses of hydraulic fractures for soil and groundwater remediation from those ideated for the oil and gas production: volume, depth pressure and chemical additives [28].

 

__________________________

[1] www.saveballona.org/gasoilfields/Oil&gaspollution.pdf

[2] http://esrd.alberta.ca/lands-forests/land-industrial/inspections-and-compliance/documents/AlbertaTier1Guidelines-May23-2014.pdf

[3] G.J. Graening 2007, Remediation Journal, 17(4), Wiley

[4] http://www.epa.gov/superfund/health/conmedia/soil/#fact

[5] G. Dermont et al. 2008, Journal Of Hazardous Materials, 152, 1-31, Elsevier

[6] “A Citizen’s guide to soil washing” infohouse.p2ric.org

[7] http://www.frtr.gov/matrix2/section4/4-15.html

[8] https://clu-in.org/download/Citizens/a_citizens_guide_to_soil_vapor_extraction_and_air_sparging.pdf

[9] “Conceptual Diagram of Basic Soil Vapor Extraction (SVE) System for Vadose Zone Remediation” by Gwremed

[10] Hinchee (1994), Airsparging for Site Remediation, Lewis Publishers

[11] http://www.responsiblebusiness.eu/display/rebwp8/Dual+Phase+Extraction

[12] http://clu-in.org/techfocus/default.focus/sec/Solidification/cat/Overview/

[13] http://www.midwest soil.com/thermal-desorption/

[14] http://www.frtr.gov

[15] http://pixshark.com/bioventing.htm

[16] http://www.epa.gov/oust/cat/biosparg.htm

[17] SA EPA 2005

[18] US EPA 1994

[19] US EPA 2011

[20] Kao et al. 2009, World Environmental ad Water Resources Congress, pp. 1-10.

[21] http://www.tankonyvtar.hu/hu/tartalom/tamop425/0032_kornyezettechnologia_en/ch02s02.html

[22] UK EA 2002

[23] http://www.alken-murray.com/bioslurry.html

[24] Buzzelli et al. (1997), In Situ and On Site Bioremediation, Vol.1, pp. 423-428, Battelle Press

[25] http://ergobalance.blogspot.it/2013/11/applications-of-bioremediation-and.html

[26] http://www.h2o2.com/remediation/in-situ-soil-and-groundwater-treatment.aspx?pid=91&name=In-situ-Soil-and- Groundwater-Treatment, A.Goi et al. 2011, Chemosphere 82, pp. 1196-1201, Elsevier

[27] http://www.aidic.it/acos/13/11/007.pdf

[28] http://frx-inc.com/envfrac/

Author: Mauro Capocelli,  Researcher, University  UCBM – Rome (Italy)

 

1. Theme description

The growth of economy and consumes, combined with the modern models of production, have resulted in a constant increasing generation of waste plastics. The global production of municipal solid wastes (MSW) overcame the value of 3·10⁶ ton/day and should almost double in 2025 [1]. The plastic waste (PSW) represents more than the 10% in the industrialized as well in the less developed countries (see Figure 1). The plastic production has overcame the 200 millions of tonnes in 2007 and is increasing at a rate by 5% per year [2]. The highest percentage is given by containers, packaging, soft drinks, commonly synthetized using non-renewable resources. In this frame, the recycle of plastic represents a challenge as well as a great opportunity, with the aim of save land (used for disposal) and reduce the production impact, i.e. raw-material utilization and emissions of greenhouse gases. At the present, an increasing effort has been devoted to the development of recycling techniques and integral strategies. In 2012, North Americans generated about 251 million tons of trash and recycled (and/or composted) almost the 35% of it [3]. The existent recycling pathways are represented in Figure 1; each of them provides unique advantages that make it suitable for specific applications, locations and requirements. The mechanical recycling (primary through re-extrusion of selected materials and secondary from mixed feedstock) involves physical treatment aimed at reinserting the materials into the production cycle with low energy consumption and almost zero pollutant emission. The thermochemical conversion techniques involve the complete or partial oxidation of the material producing heat, power and/or gaseous fuels, oils and chars. These kind of treatments generates by-products that must be treated and/or disposed. The conversion methods of waste plastics into fuel (tertiary) depend on the types of plastics to be targeted and is commonly realized through processes schemes involving gasification and pyrolysis. In general, the conversion of waste plastic into fuel requires non-hazardous and combustible feedstock. In this field, the incineration technique with energy recovery (quaternary) is the major competitor obtaining the best efficiencies with, on the higher hand, a severe impact on the environment due to release of harmful gases like dioxins, hydrogen chloride, particulate matters and carbon dioxide. In the thermochemical conversion process it is also possible to combine different feedstocks in hybrid schemes exploiting the value of low quality fuels.

Figure 1 – Composition of Municipal Solid Waste in 2012 by regions [4]

 

Figure 2 –  Plastic Solid Waste recycling scheme (adapted from Panda et al., 2010) [5]

 

The available consolidated methods for the conversion of plastics to liquid fuel are primarily based on the pyrolysis and/or gasification processes producing three different phases: a solid phase (char, 5–25 wt%), a liquid phase (tar, 10–45 wt%) and a gas phase [6]. The primarily produced C20–C50  hydrocarbon are cracked in the gas phase to obtain lighter hydrocarbons, as ethane and propene, which are unstable at high temperatures and react to form aromatic compounds as benzene or toluene. Higher temperatures (above 500°C) and longer residence time disadvantage the tar formation and increase the production of coke, methane and hydrogen. The chloride content in the feedstock and the fluid dynamics are the main parameters to control. The oils produced from plastics have high calorific value, comparable to that of gas oil derived from petroleum (see Table 1, extracted from the related Bulletin of the Royal Society of Chemistry [7]).

Table 1 – Fuel properties of oils derived from the pyrolysis of various wastes

 

2. Applications

The application of thermo-chemical conversion can be represented through the scheme propose by Mastellone et al., 1999 [8]. Pyrolysis consists of the thermal decomposition under an inert gas like nitrogen;  the typical scheme is reported in Figure 3. The plastic materials are introduced into a reactor where they decompose at 400°C-600°C; the major product of the process is an oily mixture of liquid hydrocarbons obtained through the condensation of the decomposed vapours. The evaporated oil may also be further cracked with a catalyst. The boiling point of the produced oil is controlled by the operation conditions of the reactor, the type of reactor, and presence of catalyst. Commonly, the hydrocarbons with high boiling points such as diesel, kerosene and gasoline are then fractionated through fractional distillation. The two main problems associated with thermal cracking are the conversion limits and large molecular weight distribution in the pyrolysis product, resulting in limited market value. In this frame, the use of catalysts enable to decrease the cracking temperature and/or to increase the reaction rate as well as to increase the selectivity and consequently the product quality. The main advantages of the catalytic process are reported in Table 2, while the main drawback is the loss in catalyst activity due to coke formation.

Figure 2 – Scheme of thermolysis process of Plastic Solid Waste [9]

 

Table 2 – Comparison of thermal and catalytic cracking [10]

 

One of the first widespread technology for pyrolysis appeared in 1978 with the name of PYROPLEQ. It is based on pyrolysis at circa 500 °C in an externally heated rotary drum and gas combustion at 1200 °C. The system has been coupled with a co-generative rankine cycle (2.2 MW) and alimented with  PSW.  The BP process consists of a fluidized bed heated at 500 °C in the absence of air; the decomposition generates hydrocarbon vapors with a high content of monomers as and a relative low methane percentage and  pollutants as HCl which has to be neutralized by lime adsorption. The yield of liquid fuel production is 85% while the solids produced are typically up to 0.2 kg/kg of total solids feed. The system has been established in Scotland with a capacity of 25,000 tonnes/year after a series of pilot trails. The BASF process started in 1994 with a pilot plant capacity of 15,000 ton/year in Germany. It consists of a pre-treatment stage (grinding and separation) and a multi-stage melting and reduction process. The HCl is processed in the hydrochloric acid production plant. The NKT process realizes the thermal conversion of waste mixtures in a reactor at a low pressure (2–3 bars) and a moderate temperature (375 °C) and has proven to be succsfull for PSW treatment, especiallin for PVC cables [11].

The gasification is achieved by reaction at high temperature (600-800°C) with a controlled amount of oxygen and/or steam (the gasifying agent). The air factor is generally 20% – 40% of the amount of air needed for the combustion of the PSW.[12] The process oxidizes the hydrocarbon feedstock to generate the endothermic depolymerisation heat and produce (primary) a gaseous mixture of carbon monoxide and hydrogen, with minor percentages of gaseous hydrocarbons. The produced syngas can be used as a source of energy for combustion processes or as a source of chemical building blocks from which chemicals may be manufactured. The Texaco process, sketched in Figure 4, represents one of the major industrial example of PSW gasification. It was firstly tested at the pilot scale in the USA (10 ton/day) in 1997. In the liquefaction step, the plastic waste is cracked (depolymerisation in mild conditions) into a synthetic heavy oil and a gas fractions. In the second step oil and condensed gas are injected to the entrained gasifier with oxygen and steam at 1200-1500 ºC. The gasification pressure can be adjusted to the pressure of the served downstream process. The syngas contains small amounts of CH₄, CO₂ and inert besides the main components CO and H₂ and is treated to remove HCl and HF and. In recent years, the gasification of plastics has been intensively conducted with useful results. Many research studies are focusing on fluidized bed, co-gasification [13] and two stage gasification in order to overcome the related technological issues: handling of feedstocks, variability of the feedstock physical characteristics, low heating values and tar formation.

Figure 3 – Apparatus for pyrolysis of waste polymers

(1. Transportation, 2. Selective collection, 3. shredding, 4. Washing, 5. Drying, 6. Waste storage, 7. Catalyst storage, 8. Reactor, 9. Heating gas storage, 10. Separation unit, 11. Catalyst filter) [14].

 

 

 

Figure 4 – Texaco gasification process schematic diagram [15]

 

The chemical conversion technique can also count on the hydrogenation process. It represents a potential alternative for breaking down the long polymer chains generating highly saturated products and avoiding the presence of olefins in the liquid fractions. Moreover, compared to the other treatments, the hydrogen promotes the removal of Cl, N and S. Main drawbacks are related to the supplying of hydrogen and the need to use a catalyst and/or operate at higher pressure. The main technology applied in PSW recycling via hydrogenation technology is based on the Veba process, born in the field of the coal liquefaction technologies. The Veba Combi-Cracking [16] (VCC™) is a slurry phase hydrogenation process with integrated hydrocracking for converting petroleum residues at very high conversion rates (>95 wt%, 524 ºC). The process, as described in the Veba report [17], converts the solid hydrocarbons into light distillates by hydrogen addition through two stage reactors detached by a hot separator that recovers the unconverted high boiling material and the additive; the bottom product is fed into a vacuum distillation for recovery of dissolved distillates from the residue. The recovered products are fed to the distillation step together with the top products of the hot separator and are therefore sent to the second hydrotreating/hydrocracking stage in a a catalytic fixed bed reactor (see Figure 5).

Worth of mention is the project “Converting Waste Plastic into Fuel” by the United Nation Environmental Programme (UNEP) [18]. The UNEP also compiled in 2009 a compendium of the industrial plants appeared in this field, with a detailed list of technology providers. A list of the technologies, together with the related main features has been extracted from the UNEP compendium [19] and reported in the Table 3.

 

 

Figure 5 – VCC™ Process Flow Diagram [20]

 

Table 3 – Fuel production plants

 

_______________________________

[1] http://siteresources.worldbank.org/INTURBANDEVELOPMENT/Resources/336387-1334852610766/Chap3.pdf

[2] Patni et al., 2013. ISRN Renewable Energy http://dx.doi.org/10.1155/2013/902053

[3] http://www.epa.gov/epawaste/nonhaz/municipal/

[4] http://www.proparco.fr/jahia/webdav/site/proparco/shared/PORTAILS/Secteur_prive_developpement/PDF/SPD15/SPD15_key_data_uk.pdf

[5] A.K. Panda et al. / Renewable and Sustainable Energy Reviews 14 (2010) 233–248

[6] S.M. Al-Salem et al., 2009_Waste Management 29. 2625–2643

[7] Royal Society of Chemistry, Environmental Chemistry Group, 2012. ISSN 1758-6224 (Print) 2040-1469 (Online)

[8] Mastellone, M.L., 1999. Thermal treatments of plastic wastes by means of fluidized bed reactors. Ph.D. Thesis, Department of Chemical Engineering, Second University of Naples, Italy

[9] S.M. Al-Salem et al., 2009_Waste Management 29. 2625–2643

[10] Butlet et al., 2011_Waste Biomass Valor

[11] S.M. Al-Salem et al., 2009_Waste Management 29. 2625–2643

[12] A. Brems et al. / Natural Science 5 (2013) 695-704

[13] Brachi et al., 2014. Fuel 128. 88–98

[14] N. Miskolczi et al., 2009. Fuel Processing Technology 90. 1032–1040

[15] A. Brems et al. / Natural Science 5 (2013) 695-704

[16] http://www.kbr.com/Technologies/Refining/Veba-Combi-Cracking/

[17] http://www.kbr.com/Newsroom/Publications/Brochures/Veba-Combi-Cracking-Technology.pdf

[18] http://www.unep.org/ietc/OurWork/WasteManagement/Projects/wastePlasticsProject/tabid/79203/Default.aspx

[19] United Nations Environmental Programme. Division of Technology, Industry and Economics. International Environmental Technology Centre, Osaka/Shiga, Japan Converting Waste Plastics into a Resource. Compendium of Technologies

[20] http://www.kbr.com/Technologies/Refining/Veba-Combi-Cracking/

Author: Vincenzo Piemonte, Associate Professor, University  UCBM – Rome (Italy)

 

1. Theme description

Around 50% of the produced crude petroleum in the world is refined into transportation fuels which represent the fastest growing component of the energy sector.

This massive use of a non-renewable feedstock leads to a significant footprint on the environment especially in terms of greenhouse gas emissions. In this framework, biomass can be view as the only reliable green energy source that can be converted into liquid transportation fuels in competition with fossil-fuels (see Figure 1) [1].

Figure 1 – Overview of biofuels production from biomass

 

Actually, the most common process to produce biofuels from biomass is the biodiesel production process. As valid and interesting alternative it is possible to produce biofuels for transportation purpose by blending or co-processing of green feedstock within traditional petroleum refineries by properly revamped processes. To this end, ENI has developed a new biofuel production process, called Ecofining, to produce Green Diesel in alternative to fatty acid methyl ester (FAME) also known as biodiesel.

If from one hand, biodiesel has many desirable qualities such as high cetane number, from other hand there are some disadvantages associated with its use, such as poor stability and high solvency, leading to filter plugging problems.

In order to overcome these problems, ENI has developed its new process based on a revamping of a conventional hydroprocessing technology that is widely deployed in refineries and uses the existing refinery infrastructure and fuels distribution system. As already said, this new technology process use widely available vegetable-oil feedstocks to produce a high-cetane, low-gravity, low-aromatics and sulphur-free diesel fuel, known as Green Diesel. Table 1 reports a comparison between physical properties of biodiesel and green diesel [2].

Table 1 – Comparison of Biodiesel and Green Diesel properties

 

2. Green Diesel Production Processes

The biodiesel is conventionally produced via transesterification of triglycerides in presence of methanol. The reaction is  catalyzed by caustic soda to produce FAME and glycerol as a byproduct (see Figure 2) [3].

The Green Diesel production process, implementing the hydroprocessing route, uses hydrogen to remove oxygen from the triglyceride molecules (see Figure 2). Then, the oxygen is easily removed via two competing reactions: decarboxylation and hydrodeoxygenation. The extent for each reaction depends on the catalyst and process conditions.

Figure 2 – Biodiesel vs. Green Diesel production processes

 

The revamping of the hydroprocessing technology to produce green diesel is achievable mainly by two different approaches (see Figure 3)[4]:

• Co-processing in an existing distillate hydroprocessing unit;
• Building a standalone unit.

 

Figure 3 – Hydroprocessing scheme for Green Diesel production

 

The co-processing route shows good potentially due to the possibility of reuse existing equipments, resulting in a lower implementation cost of the whole plant. Anyway co-processing is affected by some problems related to the processing of vegetable oils which contain trace of metal contaminants such as phosphorous, sodium, potassium and calcium. In fact, in this case a pre-treating reactor may be needed to remove the aforementioned contaminants by catalytic reactions. Generally existing reactors in the refinery plants not have sufficient catalyst volume to carry out the required reactions. Furthermore, processing vegetable oil reactions are fairly exothermic and may require quench facilities which may not be available. By-products (H₂O, CO, CO₂) coming from the deoxygenation reaction have to be removed in a revamped recycle-gas system, or by using a big amount of purge gas. In addition, it was found that the deoxygenation reactions had a tendency to compete with the primary desulfurization reactions taking place within the hydrotreating unit. This was seen to present too much risk in a fuel environment where ultra-low-sulfur diesel (ULSD) is required and rerun of off-spec products is expensive.

In view of the several difficulties related to co-processing route, building a dedicated unit optimized for vegetable-oil processing, as shown in Figure 4, seems more cost effective.

Figure 4 – Green Diesel production flow scheme

 

Figure 4 reports a simplified flow scheme of the green diesel production process in a standalone unit. Vegetable oil is combined with hydrogen and brought to reaction temperature. Then the mixture is sent in the reaction unit where the vegetable oil is converted in Green Diesel. The last one is separated from the recycle gas in the separator unit while the liquid product is sent to a fractionation section. The design of the fractionation section can vary from a one-column system to produce diesel and unstabilized naphtha to a three-column system to produce propane, naphtha and diesel products. The recycle gas is treated in an amine system to remove CO₂.

 

3. The ENI’s Project

The project Green Refinery is the first example in the world of conversion of an oil refinery in conventional biorefinery. ENI has identified an opportunity to reuse the catalytic hydrodesulfurization section present in Venice refinery (see Figure 5) [5] by reconfiguring it in a biorefinery. The new biorefinery is based on the proprietary technology EcofiningTM previously developed by ENI in the laboratories of San Donato Milanese in collaboration with Honeywell-UOP.

After conversion, the biorefinery will be able to produce high quality biofuels especially green diesel, but also green naphtha, LPG and potentially jet fuel from raw materials of biological origin. It will be possible this way to meet the requirements of the EU directive on renewable energy.

At moment, have been done extensive process performance tests to determine the optimum process conditions, the catalyst stability and the obtained products properties. A range of vegetable-oil feedstocks has been processed in the pilot plants ranging from soybean to rapeseed and palm oil. An extensive program is underway to evaluate other potential feedstocks including tallow and greases derived from animals. Pilot-plant tests have shown that there is no measurable deactivation after over 2,000 hours on stream.

Figure 5 – Venice refinery reconfiguring it in biorefinery

 

_____________________________ 

[1] J. Holmgren et al., Refining Biofeedstock innovations, PTQ Q3, 2007, 119-125

[2] T. N. Kalnes et al., Green Diesel production by hydrorefining renewable feedstocks, Biofuels Technology, 7-11

[3] J. Holgren et al., Opportunities for biorenewables, Hydrocarbon Engineering, June 2007.

[4] J. Holgren et al., New developments in renewable fuels offer more choices, Hydrocarbon processing, September 2007

[5] www.Eni.com

Author: Vincenzo Piemonte, Associate Professor, University UCBM – Rome (Italy)

 

1. Theme description

 

Produced water is typically generated in large quantity for the lifespan of a well. It is made up of natural formation water as well as the uphole return of water injected into the formation as part of an enhanced recovery operation.

Technologies and strategies applied to produced water comprise (Figure 1):

• (1) Minimization
• (2) Recycle / Re-use
• (3) Disposal

Onshore produced water can be re-injected to provide additional oil and gas recovery, treated and used for civil usage or disposed on the reservoir according to regulations and permits.

Offshore produced water can be re-injected into reservoir, disposed as discharge to the sea after suitable treatment or transported onshore for suitable treatments and subsequent disposal.

Figure 1 – Produced water treatment overview

 

Nowadays, it has become essential to recover and use this water again due to the more and more stringent environmental limits (the components of produced water must exhibit persistent, toxic or bioaccumulative properties) and the always increasing water demand.

Produced waters are characterized by a high content of salt and oil that render necessary to device a specific treatment train in order to decontaminate them, as for example with respect to a municipal wastewater. Typically, produced water contains high concentrations of aromatic hydrocarbons e.g BTEX (benzene, toluene, ethylbenzene, xylene), NPD (naphthalene, phenanthrene e dibenzotiophene) and PAH (polycyclic aromatic compounds), minerals, radioactive substances, dissolved gases, scale products, waxes, microorganisms and dissolved oxygen [1].

To remove hydrocarbon components from produced water, biological, physical and chemical methods are available. In offshore extraction facilities due to space constraints, compact physical and chemical treatment technologies are preferred such as photo-electrocatalyticprocesses, hydrocyclones, coagulation and flocculation [2]. Most of these techniques are only suitable for pretreatment of wastewater for in situ reuse, e.g. reinjection for enhancing oil recovery [3]. On the contrary, membrane technology may be successfully used to remove hydrocarbons from oil-contaminated wastewater, also in the presence of a high salinity. Membrane processes offer several advantages over conventional treatments such as compact module, lower energy consumption, environmental friendliness and high quality product independently on fluctuations in feed quality. Because of the presence of dissolved and suspended oil in untreated produced water, the membrane equipment may become fouled, thus increasing operation costs [4]. At this purpose, the vibrating membrane technology process VSEP® (Vibratory Shear Enhances Process) is a technology that limits membrane fouling [5].

 

 Figure 2 – Produced water treatment steps

 

Actually, the most important treatments applied to produced water are classified as primary, secondary and tertiary processes. In the following it is possible to find a short description of these processes.

 

2. Processes

2.1 Primary Treatments

Primary Treatments are mainly devoted to remove suspended hydrocarbons components and solids from produced water. Figure 1 reports the block scheme of the primary processes[6]. The produced water, leaving the three-phase separator which separates oil, gas and water coming from the wellhead, is sent to the gravity separator (API separator), which removes, from water surface, oils and other light fractions with a lower density than water. These fractions, removed by an oil skimmer, are then sent to the oil recovery stage. The settled particles are conveyed on the bottom of the separator and transferred to the oil sludge processing.

The water separated from oil reaches the free surface on the side opposite to the power supply and it is sent to a mixing tank, where appropriate amounts of coagulants and flocculants are added to facilitate the particles coalescence by sedimentation.

 

                                                        

   Figure 3 – Primary Treatment Scheme

 

Then, water enters into the dissolved gas flotation (DGF) stage, where nitrogen or natural gas (to avoid explosions upon contact with hydrocarbons) is blown to separate oil, suspended solids and other macromolecules.

The produced water coming from the DGF stage is further deoiled and then sent in another mixing tank, where chemical reagents are added (polyelectrolytes, caustic soda and aluminum chloride); then, the stream is treated in a sedimentation tank to remove metals. Before the secondary treatment stages, produced water passes through sand filters to ensure a further reduction of pollutants.

 

2.2 Secondary Treatments

The main “secondary” treatments are known as Best Available Techniques for produced water treatment [7]. These techniques (based on adsorption, biodegradation, stripping, membrane separation) not only allow the removal of organic compounds, suspended solids and oil, but are also able to eliminate the dissolved aromatic hydrocarbons such as BTEX and NPD. It was possible to establish that the treatment with the vibrating membrane system VSEP (Vibratory Shear Enhances Process) turns out to be the best process to be applied to the produced water, managing to get with a single operation an effluent with an extremely high degree of purity without the addiction of any other chemical and without the production of a large amount of waste.

In the VSEP membrane system, patented by New Logic Research, the feed slurry remains nearly stationary, moving in a leisurely, meandering flow between parallel membrane leaf elements. Shear cleaning action is created by vigorously vibrating the leaf elements in a direction tangent to the faces of the membranes: the propagation of shear waves from the sinusoidal membrane surface favors the suspension of particles on it, facilitating the flow, thus reducing the membrane fouling.

 

Figure 4 – VSEP Membrane scheme

 

2.3 Tertiary Treatments

Tertiary treatment are focused on the salts removal from treated produced water coming from secondary processes. By using the reverse osmosis as tertiary treatment [8], mainly reducing the levels of nitrates and phosphates, it is possibleto ensure the necessary attributes required by the law for the reuse of water for industrial and agricultural purposes. Figure 3 shows an example of Reverse Osmosis Process applied to produced water treatment

To prevent precipitation of low soluble salts on the membrane surface, antiscalant and chemicals are added to water in the upstream of reverse osmosis stage. The permeate is then pressurized and sent to the reverse osmosis stage.The process scheme includes also an energy recovery device (R) and a booster pump.

Figure 5 – Reverse Osmosis application to produced water treatment

 

3. Projects and Innovation

The treatment of produced water is an hard challenge for all the major oil companies. Both Eni and Shell for the Blacktip Field (north Australia) and Auk platform, respectively, have adopted an adsorption based technology licensed from CETCO Oilfield Services which appears to be extremely effective. This type of treatment package utilizes water polishing adsorption filters which are specifically designed to remove oil, grease and soluble organics from water based streams. The adsorption media is based on resin, polymer, and clay technology.

A completely different approach has been followed in a Montana oilfield (U.S.A.) using a mobile station to design a plant to cost efficiently treat the produced water for agricultural irrigation. In this case it has been used a combination of physical and chemical treatment in order to comply with reuse and discharge limits. This mobile station consists of three stages: pretreatments, membrane filtration and post treatment. Two spiral-wound membrane units were employed and the rejections of various constituents were examined. Cost analysis showed that the treatment cost of produced water is less expensive than to dispose of it by injection. This solution may be of great value in water-poor regions [9].

 

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[1] E.T. Igunnu, G.Z. Chen, Produced water treatment technologies, Int. J. Low-Carbon Tech. (2012) 1-21

[2] F.R. Ahmadun, A. Pendashte, L.C. Abdullah, D.R.A. Biaka, S.S. Madaeni, Z.Z Abidin,  Review of technologies for oil and gas produced water treatment” J. Hazardous Mat. 170 (2009) 530–551.

[3] A.R. Pendashteh, A. Fakhru’l-Razi, T.G. Chuah, A.B DayangRadiah, S.S. Madaeni, Z.A. Zurina, Biological treatment of produced water in a sequencing batch reactor by a consortium of isolated halophilic microorganisms, Environ. Tech. 31 (2010) 1229–1239

[4] J. Tian, M. Ernst, F. Cui, M. Jekel,Effect of particle size and concentration on the synergistic UF membrane fouling by particles and NOM fractions, J. Mem. Science 446 (2013) 1–9.

[5] W. Shi, M.M. Benjamin, Membrane interactions with NOM and an adsorbent in a vibratory shear enhanced filtration process (VSEP) system, J. Mem. Science 312 (2008) 23–33.

[6] http://www.all-llc.com/publicdownloads/ALLConsulting-WaterTreatmentOptionsReport.pdf

[7] http://www.ogp.org.uk/pubs/324.pdf; http://www.usbr.gov/research/AWT/reportpdfs/report157.pdf

[8] http://static1.squarespace.com/static/52fe73f6e4b05f81d6be283d/t/549f2649e4b00eb7a7a6b2ca/1419716169831/Is+Reverse+Osmosis+Effective+for+Produced+Water+Purification%2C+Viability+and+Econ+Analysis.pdf

[9] http://www.environmental-expert.com/articles

Author: Marcello De Falco, Associate Professor, University UCBM – Rome (Italy)

 

1. Theme description

Human technology has always looked for solutions to exploit the wide and always available energy provided by the earth and by the sun. The beginning of the industrial era, totally focused on the use of fossil fuels to supply the huge amount of energy needed,  slowed down the research on renewable energy technology but, from 1970s, when the communities understood the environmental, societal and political negative effects of an extensive use of fossil fuels, the renewable energies gained a new thrust forward.

Today, renewables supply an important share of the total energy, with an average increase of 6% per year. The European Commission imposes the target of producing 20% of the total energy from renewable by 2020 [1] and devotes many financial resources to achieve it [2].

A first classification of the enormous number of technologies developed in the past and already applied or in progress can be made dividing them on the derivation of the energy:

• from the Sun;
• from the Wind;
• from the Water;
• from the Organic Matter.

In the following, an overview of the main technologies and applications is reported and some references are given to examine in depth some of them.

2. Solar Energy

The production technologies which exploit solar energy can be divided in three classes:

• Photovoltaic;
• Solar thermal energy;
• Solar Power concentration.

Photovoltaic technology exploits the photovoltaic effect, by which a voltage is created in particular materials (mainly semiconductors) upon exposure to sun light.

A wide number of photovoltaic plant with Si-based modules is installed worldwide, thanks to the high incentives made available in the last 10 years. Such modules assure a low conversion efficiency (8% – 13%), calculated as the ratio between the electricity produced to the solar energy received.

 

Fig. 1 – SunEdison’s 70 megawatt (MW) photovoltaic (PV) power plant in Northeast Italy [3]

 

The research is mainly focused on the development of new materials for the improvement of the efficiency [4]. As an example, ENI is developing a polymer solar cell, printed on small-sized flexible substrate [5]. The production process has the targets of producing flexible modules with an efficiency in line with the commercial modules at competitive costs.

Moreover, researches are devoted to find always new applications for the photovoltaic technology, as the solar-driven pumps [6].

Solar thermal technologies are able to exploit solar energy to produce streams of hot water, steam at low pressure or to supply low-temperature heat for various processes. Many solar thermal plants have been installed in the last years for domestic applications (domestic hot water). Concerning with solar thermal energy supplied for processes, surely one of the most interesting application is the solar powered desalination [7].

Solar Power concentration systems are able to heat up a thermal fluid or a system to high temperature (400-1000°C) exploiting the sunrays concentration in focal points or lines. The high temperatures allow the production of clean electricity or the supplying of high quality energy to industrial processes. Four technologies are developed or under development:

• Parabolic Trough Power [8], [9]
• Solar Thermal Tower Power [10]
• Dish-Stirling Systems [11]
• Solar Chimney Power [12]

 

Fig. 2 – Solar Power concentration technologies

3. Wind energy

Wind-powered plant can convert wind power to electricity by exploiting the rotating movement of a rotor.

Many wind farms have been installed worldwide in the last years, also thanks to the high incentives that promoted the technology diffusion.

But, the traditional horizontal-axis turbines has a series of problems that limits the applications, as the low efficiency, mainly due to the fact that the turbine can work only within a range of wind velocity (not too low and not too high), an expensive maintenance and the noise which annoy local residents.

 

Fig.3 – Horizontal-axis wind turbine

 

Many innovations are proposed to solve these problems. The most interesting is the vertical-axis turbine. The shaft is mounted on a vertical axis, perpendicular to the ground, thus allowing the alignment with the wind without the necessity of adjustment when the wind direction changes. At the present, the only one commercial product is the Darrieus turbine [13], but many research and industrial efforts are devoted to find innovative and competitive solutions.

 

Fig. 4 – Darrieus turbine

 

Many other wind turbine shapes are proposed by the technological and scientific communities, all aimed to reduce the noise, enlarge the wind velocity range and increase the conversion efficiency [14].

4. Hydro and tidal energy

Hydroelectric power exploits the energy from the movement of water streams to generate electricity by rotating a rotor of a turbine connected to a generator.

Among the number of developed technologies, the most common is a system composed by a dam where water is stored until it is needed to produce electricity.

 

Fig. 5 – Hydroelectric dam layout [15]

 

Another technology is the run-of-the-river type, located on swift flowing streams or rivers and able to extract the energy from the water as it passes through the station.

 

Fig. 6 – Run-of-the-river hydroelectric technology [15]

 

Moreover, different types of turbines are installed to convert the water flow movement in mechanical energy to be exploited for electricity generation, among which:

• Francis turbine;
• Propeller turbine;
• Pelton turbine.

Many research efforts are devoted to reduce the power size of hydroelectric plants and to reduce the impact on the environment, with the scope to increase the application potentialities [16].

Another form of hydropower is the tidal energy, which converts the energy of the tides into electricity or other useful forms of power thanks to submerged turbines [17]. The tide is created by the gravitational effect of the sun and the moon on the earth which provoke cyclical seas movement. The main benefit of the tidal energy is that tides are predictable, unlike sun or wind.

The main innovations proposed concern the turbine technology and, in general, the system to convert wave and tidal energy in electricity [18], [19], [20].

5. Biomass

The biomass is an organic matter that stores energy through the photosynthesis process and can release it by a combustion or a by conversion process. Since the biomass contains carbon, its combustion emits carbon dioxide. On the other hand, the same amount of carbon dioxide has been adsorbed during the growth, therefore biomass is considered a clean renewable energy since the CO₂ balance is equal to zero.

Biomass exists in the form of wood products, dried vegetation, crop residues, aquatic plants or animal/human wastes. There are four main processes to generate energy from biomass:

• The Direct Combustion [21] – it is the most frequently used process to extract energy from biomass. The biomass energy is released by burning the solid biomass and energy is recovered in the form of heat, which can be used for thermal requirements or converted in electricity by means of a steam / gas turbine or organic Rankine cycles (ORC).
• The Pyrolysis [22] – it is a thermal process, performed in absence of oxidizing agents and at relatively low temperature (500-800°C). Generally, different fractions are produced, depending on the operating temperature and the biomass composition: a gas fraction, composed by methane, hydrogen, carbon monoxide and carbon dioxide, a biochar and a bio-oil, which is the most interesting pyrolysis product.
• The Gasification [23] – is a thermo-chemical process which converts the solid biomass in a high heating value gas mixture composed by H_2, CH₄, CO, H₂ In the gasification process, the biomass reacts at high temperature (850-1000°C) with a fumigator (air, oxygen, steam or CO₂), which provide oxygen for the process. Gasification by-products are char coal, ash with varying carbon contents and condensable low molecular hydrocarbons. Many pre-industrial plants have been developed, among which the thermal gasificator FICFB (Fast Internally Circulating Fluidized Bed) can be cited [24].

 

 

 

Fig. 7 – Pyrolysis process layout [25]

 

• Biofuels – Biofuels are fuels derived by production processes whose feedstock is composed by biomasses. Biofuel can be derived from a wide number of different biomasses, from the marine algae to solid wood-based wastes. Among the various technologies developed or under study, the ENI developing processes for the production of lipids from lignocellulosic biomass and the subsequent conversion of lipids into a biofuel to be fed in traditional engines can be cited [26].

 

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[1] http://www.eea.europa.eu/highlights/renewable-energy-production-must-grow

[2] http://ec.europa.eu/programmes/horizon2020/en/h2020-section/secure-clean-and-efficient-energy

[3] http://www.pv-magazine.com/news/details/beitrag/70-mw-italian-pv-power-plant-grid-connected_100001644/#axzz3RMIS7ZqB

[4] http://oilprice.com/Alternative-Energy/Solar-Energy/New-Efficient-Materials-Promise-a-Photovoltaic-Revolution.html

[5] http://www.eni.com/en_IT/innovation-technology/technological-focus/celle-solari/organic-solar-cells.shtml

[6] http://www.sunelco.com/planning_pumping.html

[7] http://www.hitachi.com/environment/showcase/solution/industrial/desalination_plant.html

[8] http://www.schott.com/csp/english/parabolic-through-technology.html

[9] http://www.enea.it/it/enea_informa/events/techitaly2012/DiMario_3ott2012.pdf

[10] http://www.brightsourceenergy.com/technology#.VNo1A-aG8gE

[11] http://www.unitedsunsystems.com/

[12] http://news.nationalgeographic.com/news/energy/2014/04/140416-solar-updraft-towers-convert-hot-air-to-energy/

[13] http://www.reuk.co.uk/Darrieus-Wind-Turbines.htm

[14] http://www.treehugger.com/wind-technology/future-wind-power-9-cool-innovations.html

[15] http://re.emsd.gov.hk/english/other/hydroelectric/hyd_tech.html

[16] http://www.british-hydro.org/uploads/11202007120238PM.pdf

[17] http://www.tidalenergyltd.com/?page_id=1370

[18] http://www.renewableenergyworld.com/rea/tech/ocean-energy

[19] http://cdn.intechopen.com/pdfs-wm/9342.pdf

[20] http://uk.whales.org/sites/default/files/wdc-marine-renewable-energy-report.pdf

[21] http://www.wisions.net/technologyradar/technology/direct-biomass-combustion

[22] http://www.fao.org/docrep/t4470e/t4470e0a.htm

[23] http://www.bios-bioenergy.at/en/electricity-from-biomass/biomass-gasification.html

[24] http://www.guessingrenewable.com/htcms/en/wer-was-wie-wo-wann/wie/thermische-vergasungficfb-reaktor.html

[25] http://www.altenergymag.com/emagazine.php?issue_number=09.02.01&article=pyrolysis

[26] https://www.eni.com/en_IT/attachments/innovazione-tecnologia/technological-answers/scheda-pt-biodiesel-da-alghe-rev-dic10-eng.pdf

Author: Marcello De Falco – Associate Professor,  University UCBM – Rome (Italy)

 

1. Theme description

In the last years, in parallel with the development of Carbon Capture and Storage (CCS) technologies, a new vision about the CO₂ is rising, focused on the development of technologies able to reused CO₂ instead of storing or emitting it to the atmosphere [1], [2].

By this way, CO₂ is not more considered a problem or a waste to be treated with a significant economic impact, but a key valuable element to be used for the sustainable future of chemical industry.

Carbon dioxide has already its market, since it is used as blanketing agent, fire extinguisher, drying ice, refrigerating fluid, aerosol propellant, shielding gas in welding, for carbonation of beverages, etc. But, new processes are going to be developed for the massive use of CO₂ as reactant or feedstock in crucial industrial sectors. Among the different application of CO₂ for chemical processes, the following classification can be made:

• CO₂ for the production of raw materials (methane, light olefin);
• CO₂ for the synthesis of advanced materials (CO₂-based polymers);
• CO₂ for the production of fuels (methanol, biodiesel).
• CO₂ as a solvent (supercritical extraction)

In the following, some of the most interesting technologies developed are reported and described.

 

2. Technologies

2.1 Dry reforming of methane for the production of synthetic fuels

The dry reforming is a reaction through which the carbon dioxide reacts with a methane stream to produce the syngas, a mixture of carbon monoxide and hydrogen:

The syngas mixture can be used for the production of synthetic fuels by the Fischer–Tropsch synthesis or for the production of compounds as the methanol or the dimethyl ether (DME).

Many studies are reported on this syngas production process [3], mainly about the development of proper catalysts. The most interesting application is with the biogas derived from biological processes as anaerobic digestion or gasification, which is typical composed by methane and carbon dioxide and can be fed directly to the dry reforming reactor. But, methane dry reforming can be also applied by adding to a methane flow the CO₂ stream captured from thermoelectrical energy production processes.

2.2 Production of methane from CO₂

The reduction of CO₂ with hydrogen for the production of methane is a process that attracted interest since the beginning of the last century. The first scientist who studied the reaction was Sabatier, which discovered that one mole of methane can be obtained by the reaction of one mole of carbon dioxide and four moles of hydrogen, according to:

CO2 + 4H2 = CH4 + 2H2O

 

CO₂ can be generated from the combustion of coal or biomass. The CO₂ based methanation interest is increasing mainly in countries with large coal resources, as U.S., China and India. The synthetic methane produced can be distributed in natural gas pipeline and can feed the natural gas internal combustion engine for the automotive sector or can be used in boilers for domestic heating.

The only industrial plant already operating worldwide is located in South Dakota (U.S.) [4]: the Great Plains Synfuels Plant (GPSP) in Beulah is a coal-to-natural gas plant able to produce 1.53 billion Nm³/year of synthetic methane.

 

Great plains synfuels plant (GPSP) in Beulah

 

Other pilot plants are under development [5], mainly in the U.S. (Taylorville, Kentucky, Lake Charles) and China (Shenhua, Huayin, Hexigten Qi).

 2.3 CO₂ to methanol

Methanol is a compound used as feedstock for hundreds of chemical products. Moreover, in the last years, methanol has been considered as a potential energy vector to be applied as a fuel in the transportation sector or as a feedstock for electricity production through DMFC [6] (Direct Methanol Fuel Cell).

Traditionally, methanol derives from the syngas, produced from fossil fuels as coal or natural gas. But interesting processes able to synthesize methanol directly from CO₂ are under development.

The basis of these processes is the reverse Water Gas Shift reaction [7], by which carbon monoxide (CO) can be produced by converting CO₂:

CO2 + H2 = CO + H2O

Then, carbon monoxide can react with hydrogen to produce methanol:

CO + 2H2 = CH3OH

If the hydrogen is produced by means of an electrolysis unit, the global process can convert electricity in a liquid fuel as methanol, easily to be stored, distributed and used.

 

 

 

2.4 CO₂ for biodiesel production

Biodiesel production from microalgae is one of the most promising solutions for the development of innovative and clean fuels.

Carbon dioxide can be fed to greenhouses for microalgae cultivation photosynthetic processes. Such microalgae are cultivated in open ponds or photo-bio-reactors where carbon dioxide can be bubbled from the down, thus increasing the photosynthetic efficiencies and the biomass productivity in comparison with other cultivated biomass [8],  [9].

The most interesting application of CO₂ use for microalgae cultivation has been realized by Seambiotic (Israel) [10], which realized a pilot at the power plant of the Israeli Electric Corporation in Ashkelon of about 1.000 meter square of open ponds.

 

Microalgae open ponds developed by the Israeli Company Seambiotic

 

2.5 CO₂ supercritical extraction

The Supercritical Fluid Extraction (SFE) process is a pre-industrial technology, already applied in some high added-value productions. The technology exploits the particular characteristics of supercritical fluids to extract components from solid matrices [11], [12]. Supercritical state is achieved when the temperature and the pressure are over the critical values, giving to the substance characteristics typical of both gases and liquids. Compared with the traditional extraction process with liquid solvents, supercritical fluids (SFs) have several benefits, as the capability to adjust the solvent density and, therefore, the extraction properties, the lower viscosity and higher diffusion, thus increasing the extraction rates, the easiness in separation of the supercritical fluid from the extracted compounds.

Among the supercritical fluids that can be applied, carbon dioxide is the most interesting for the low cost, the high availability and the low critical temperature (31°C at 73 bar).

Many applications of the CO₂-SFE have been realized in the food processing [13].

 

 Supercritical CO₂ extraction system developed by Neo Farms [14]

 

__________

[1] http://pubs.acs.org/doi/abs/10.1021/cr4002758?journalCode=chreay

[2] http://www.springer.com/energy/policy,+economics,+management+%26+transport/book/978-1-4471-5118-0

[3] https://web.anl.gov/PCS/acsfuel/preprint%20archive/Files/45_4_WASHINGTON%20DC_08-00_0690.pdf

[4] http://www.dakotagas.com/About_Us/

[5] Z. Kowalczyk, K. Stolecki, W. Rarng-Pilecka, E. Miskiewicz, E. Wilczkowska, Z. Karpiniski (2008). Appl. Catal. A 342, 35

[6] https://www.wpi.edu/Pubs/ETD/Available/etd-051205-151955/unrestricted/A.Hacquard.pdf

[7] http://pubs.acs.org/doi/abs/10.1021/ie5002185

[8] M. Tredici, in Symposium ‘‘I Biocarburanti di seconda e terza generazione’’ Roma, April 14 363 2011

[9] http://www.academia.edu/1777643/An_overview_of_CO2_mitigation_using_Algal_Cultivation_Technology

[10] http://www.seambiotic.com/research/microalgae-speices/

[11] Chemat S., Lagha A., AitAmar H., Bartels P. V. and Chemat, F. (2004) Comparison of conventional and ultrasound-assisted extraction of carvone and limonene from caraway seeds. Flavour and Fragrance Journal, 19, 188–195.

[12] Cherchi G., Deidd, D., De Gioannis B.,Marongiu, B., Pompei, R., and Porcedda, S. (2001) Extraction of Santolina insularis essential oil by supercritical carbon dioxide: Influence of some process parameters and biological activity. Flavour and Fragrance Journal, 16, 35–43.

[13] https://www.uic.edu/labs/trl/1.OnlineMaterials/SCEinFoodTechnology.pdf

[14] http://neo-farms.com/portfolio_item/5000-5l-modular-supercritical-co2-system/

Author: Marcello De Falco, Associate Professor, University  UCBM – Rome (Italy)

 

1. Theme description

Carbon Capture and Storage (CCS) is the solution to “close the balance” between the carbon dioxide (CO₂) emissions due to the combustion of fossil fuels (coal, oil and natural gas) for electricity generation and industrial processes and the CO₂ natural capture, with the scope to mitigate the GreenHouse effect which would be a consequence of a CO₂ content increase in the atmosphere and would have catastrophic effects on the ecosystems.

The interest on CCS technologies is increasing in the last years, mainly thanks to the targets imposed by the European Commissions to cut 20% of GreenHouse Gases emissions in EU by 2020: it will be impossible to reach the goal without implementing proper CCS solutions in the production plants.

In this framework, the EU policy has promoted the Emission Trading System (ETS) [1] scheme, known as the Carbon Tax, to drive the CO₂ emissions reduction by introducing a tax on the emissions over an allowed level. The EU ETS system operates in the 28 EU countries plus Iceland, Liechtenstein and Norway and is focused on industries as the power and heat generation and the energy-intensive activities including oil refineries, steel works and production of iron, aluminum, metals, cement, lime, glass, ceramics, pulp, paper, cardboard, acids and bulk organic chemicals [2]. According to the EU ETS system, the companies receive allowances to emit tons of CO₂ beyond the cap. If a company overcomes the cap, it has to buy other emissions allowances; on the other hand, companies that have implemented efficient technologies to reduce CO₂ emissions can stay under the cap and sell the allowances to other less efficient companies, thus creating the ETS market. This mechanism creates an incentive to invest in technologies that cut CO₂ emissions, as the CCS technology, and the ETS market is becoming huge, considering that in 2012 7.9 billions of allowance have been traded (total value = €56 billions).

2. Technologies

The CCS technology consists of three parts:

• the CO₂ capture directly in the production plant;
• the CO₂ transport to the storage site;
• the CO₂ storage underground in depleted oil and gas fields or in deep saline aquifer formations.

The CO₂ capture is surely the most interesting step from a technological point of view; the transport represents a no negligible cost (€5/tons of CO₂ by onshore pipelines, decreasing at €3.7/tons if pipelines length is increased to 500 km; much more if the CO₂ is transported by road tankers or ships); the storage is usually made in  selected geological rock formation, typically located several kilometers below the earth’s surface.

2.1 CO₂ capture technologies

Typically, CO₂ is captured from the exhaust gas of a combustion process after the fuel combustor: this method is called post-combustion capture.

The most used process is the absorption process which uses MDEA (Methyl DiEthanol Amine) as the solvent. The exhaust gas containing carbon dioxide is fed to a packed column where a MDEA solvent is sent in counter-current configuration: CO₂ is thus absorbed by the solvent and separated from the gas. Then, the solvent is stripped, releasing the CO₂ stream, which has to be compressed and sent to the transportation step or directly to the storage, while the solvent is re-circulated to the absorption column (Figure 1).

 Fig. 1 – Absorption process for CO2 capture 

 

Other post-combustion capture methods for separating CO₂ are:

• high pressure membrane filtration [3] – a selective membrane allows the separation of CO₂ from the other components of the exhaust gas by exploiting a pressure gradient driving force.
• Adsorption/desorption processes [4] – a sorption solid material, typically a mineral zeolite, is packed in a column where the exhaust gas is fed. CO₂ is adsorbed in the solid pellets (operation stage) and separated from the clean gas, which is emitted to the atmosphere. Then, the solid is purified (regeneration stage) increasing the column temperature and thus desorbing the carbon dioxide. Such a cyclic operation uses two columns in parallel at least, one in the operation stage and the other in the regeneration stage.
• Cryogenic distillation [5] – the exhaust gas is cooled down to the CO₂ sublimation temperature (-100 to -135°C), thus separating the solid CO₂ from the light gases.

An alternative to the post-combustion capture processes is the pre-combustion CO₂ capture technology.

By this process, the carbon dioxide is removed before the combustion of the fuel. Basically, the coal or the natural gas fuels are not directly burned but are converted to hydrogen and CO₂ by steam reforming process (for natural gas) or by a gasification (for coal). Then, CO₂ is separated in a similar way as in the post combustion process (typically by an absorption unit) and the hydrogen-rich gas is finally burned (Figure 2).

Fig. 2 – Pre-combustion CO2 capture thecnology [6]

 

Another process widely applied for CO₂ capture is the oxy-fuel combustion. Before the fuel combustion, the oxygen required as combustive agent is separated from the N₂. Then, the oxy-combustion produces a exhaust gas containing mainly H₂O and CO₂ (without N₂) and the higher concentration of CO₂ makes easier the post-combustion capture processes (Figure 3).

Fig. 3 – Oxy-fuel combustion process layout [7]

2.2 CO₂ storage

Once the CO₂ has been transported by pipelines, ships or road tankers, it is stored in geological formations located several kilometers under the earth’s surface. The injection is performed at temperatures and pressures such as the CO₂ is in liquid or supercritical phase.

Suitable storage sites include:

• former gas and oil fields;
• deep saline formations, which represent a high potential carbon dioxide storage capacity, even if they are not well understood yet;
• depleting oil fields, where the injected carbon dioxide may increases the amount of oil recovered.

Once injected, the carbon dioxide moves up through the storage site until it reaches an impermeable layer of rock (known as the cap rock). The layer traps the carbon dioxide in the storage formation and such a mechanism is called structural storage.

Moreover, when the CO₂ is moving up towards the cap rock, an amount of it is stored in the pores of the rocks: this mechanism is called residual storage.

Then, the stored carbon dioxide starts to be dissolved in the surrounding salty water, by a process known as dissolution storage, and, after long time, the CO₂ binds chemically to the surrounding rock by a mechanism called mineral storage.

 

3. Innovations and Applications

All the principal companies operating in the power and heat generation and in the energy-intensive activities are devoting many efforts for finding competitive solutions to apply CCS technologies in their production plants.

ENI is studying geochemical models with the aim to assess the compatibility of its wells as CO₂ storage site. Moreover, ENI is studying the CO₂ pipelining for the transport and, in cooperation with ENEL, it performs  a feasibility study on large-capacity integrated projects in Brindisi where carbon dioxide is produced and separated by MDEA unit in ENEL owned power stations, then piped to the storage site and injected into hydrocarbon fields or saline aquifers [8].

SHELL is conducting the Quest Carbon Capture and Storage Project [9], constructing a complete CCS plant at its Scotford oil sands Upgrader near Edmonton (Canada) on behalf of a joint venture and with the support of the Canadian and Alberta governments. The CCS unit will be able to store more than 1 million tons of CO₂ per year, reducing the emissions of Scotford Upgrader by up to 35%. The total cost of the project is estimated at $1.35 billion, the  CO₂ will be transported by dedicated pipelines and stored in the Cambrian Basal Sands at a depth of 2.000 – 2.500 meters.

TOTAL participates in a number of international projects for the development of the different aspects of CCS, such as capture technologies, the behavior of CO₂ during injection and storage, the long-term integrity of storage reservoirs and the risk analysis. Among them, the European programs DECARBit on the pre-combustion capture technology development and ReMove on monitoring and safety of reservoirs to be used for geological storage have to be cited [10].

Interesting studies are focused on Enhanced Hydrocarbon Recovery (EHR), including Enhanced Oil Recovery (EOR), Enhanced Gas Recovery (EGR) and Enhanced Coalbed Methane Recovery (ECBM): by injecting high-pressure CO₂ in Oil&Gas fields it is possible to recover oil or gas that would be not extracted, leading to an economic revenue. A number of studies stated that the combining the the CCS and the EHR technologies would allow boosting the carbon capture applications [11]. A report of the Durham University [12] calculates that the CO₂ captured through CCS and used for EOR in oil fields of North Sea could lead to a surplus of oil production of  $200 billion.

Other interesting projects on storage technologies are:

• Sleipner CO₂ Storage Project [13], by Statoil, ExxonMobil E&P Norway and Total E&P Norge, operative from 1996, with a pre-combustion capture technology based on absorption chemical solvent process (Amine), without CO₂ transportation since the storage site is close to the production plant and able to store 0.9 million tons/year of CO₂ in a sandstone at a depth approximately 800-1,100 metres.

• In Salah CO₂ Storage Project [14], a joint venture with BP, Sonatrach and Statoil, started from 2004 and able to store CO₂ in a carboniferous sandstone at a depth of approximately 1.900 metres.

 
Fig. 4 – In Salah onshore industrial-scale CO2 storage site

 

• Weyburn-Midale Project in Canada, by which the CO₂ is piped via an onshore pipeline for EOR in 2 carbonate fields.

 

__________

[1] Directive on emissions trading adopted by the European Parliament and Council (comprising member countries) in 2003/87/CE; substantially revised in 2009 to strengthen the EU ETS from 2013.

[2] The EU Emission Trading System (EU ETS). European Union, 2013

[3] http://www.utwente.nl/tnw/mtg/publications/mstpublications/pdf/2010_Simons.pdf

[4] http://www.co2crc.com.au/aboutccs/cap_adsorption.html

[5] http://www.sustainablees.com/documents/Clearwater.pdf

[6] http://bellona.org/ccs/technology/capture/pre-combustion.html

[7] http://www.nrcan.gc.ca/energy/coal/carbon-capture-storage/4307

[8] http://sequestration.mit.edu/tools/projects/enel_1.html

[9] http://www.shell.ca/en/aboutshell/our-business-tpkg/upstream/oil-sands/quest/ccs.html

[10] http://www.total.com/en/society-environment/environment/climate-and-carbon/carbon-capture-and-storage/our-capabilities-and-know-how/rd-programs

[11] https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/47992/1006-optimization-of-co2-storage-in-co2-enhanced-oil-re.pdf

[12] http://www.sciencedaily.com/releases/2010/10/101013193533.htm

[13] http://www.globalccsinstitute.com/project/sleipner%C2%A0co2-injection

[14] http://www.globalccsinstitute.com/project/salah-co2-storage

Author: Mauro Capocelli, Researcher, University UCBM – Rome (Italy)

 

1.Theme description

In the light of the recent trend in regulations about greenhouse emissions and environmental protection and towards more cost-effective production systems, there is an increasing demand to develop combustion systems to reduce pollutant emissions and fuel consumption. In this frame, a breakthrough technology, called flameless combustion has appeared about thirty years ago. Firstly though for energy-efficient combustion in steel furnaces, is now a consolidated part of several research projects in matter of advanced combustion. During experiments with a self-recuperative burner in the fields of in 1989¹, a surprising phenomenon was observed: at furnace temperatures of 1000°C and about 650°C air preheated temperature, no flame could be seen, but the fuel was completely burnt. Furthermore, the CO and NO_x emissions from the furnace were considerably low[1] and, thanks to the air preheating, a great energy efficiency was reached. In order to define the conditions of flameless combustion, it can be said that, the reactants must exceed self-ignition temperature and must have entrained enough inert combustion products to reduce the final reaction temperature well below adiabatic flame temperature, so much that a flame front cannot be stabilized[2]. Figure 1 shows the conventional and flameless firing of heavy oil with preheated, vitiated air. The visible differences result in diverging reaction steps that follow chemistry paths different resulting in quite different pollutant formation and heat flux distribution of hot combustion products. The mixing of 2-4 recirculating volumes (low Damköhler number), has the double aim to preheat and to reduce the outlet temperature peaks. Other features of the technologies is that the fuel is oxidized in a low oxygen environment with a substantial amount of inert (flue) gases by spontaneous ignition with no visible or audible signs of the flames usually associated with burning; the chemical reaction zone is quite diffuse, and this leads to almost uniform heat release and a smooth temperature profile. All these factors result in an extremely efficient process as well as reduced emissions; furthermore the flame supervision may be dispensed with according to safety rules, as there is no danger of the reaction extinguishing and hence no risk of explosion.

Figure 1 – Flame and flameless firing of heavy fuel oil (left: flame mode – right: flameless)

A steady flame front is reached in conventional combustion systems and the local temperature approaches the adiabatic one. The front is characterized by a sharp gradient of temperature and composition due to the radical reactions and quenching by convection. Hot peak temperatures stabilize the flame, but at the same time cause the the formation of thermal NO. On the contrary, in a flameless burner, the flame front is avoided and combustion reactions occur at the mixing of fuel, air and recirculated combustion products; the mixing is also controlling mechanism for the heat transfer and consequently for the temperature profile. This last, consequently, cannot deviate too much from the temperature of the recirculated combustion products. A characterization of temperature peak regime in flameless combustion is given by Oberlack et al., 2011[3] and shown in Figure 2 reported by Cavaliere and de Johannon in 2004.

The consequence of reduced temperature peaks in lower emission of thermal NO is shown in figure 3, where data from natural gas burners were reported for the purpose of direct comparison[4] (right side).

Figure 2 – Typical region of Mild Combustion in relation to temperature differences, according to Oberlak et al., 2011.

Figure 3 – Typical flameless and conventional temperature profile [5](left) and NOx concentration in conventional and flameless burners (right)[6].

 

The regime of flameless combustion can be individuated through some well-consolidated maps. Different combustion zones against rate of dilution and oxygen content are depicted in the Figure 4: in the typical flameless regime, the oxidation of fuel occurs with a very limited oxygen supply at a very high temperature.

Figure 4 – Combustion regimes in relation to the dilution and reactants’ temperature

The geometry of burners varies according to the fuel characteristics and the need of assuring flameless regime conditions. The concept of high swirl flows has been widely adopted to achieve internal recirculation rates for increasing the dilution of the fresh reactants. Two examples of properly designed geometries for liquid fuel combustion are depicted in Figure 5. The trapped vortex combustor (TVC) (a) is based on mixing hot combustion products and reactants at a high rate by a cavity stabilization concept[7] and has been implemented mainly for aircraft combustors. The name “trapped” came from the cavity that contains the injected reactants for realizing typical flameless regime and providing significant reduction of the pressure drop. A two stage combustor design (b) was proposed for an experimental campaign conduced with liquid fuels (diesel, Kerosene, gasoline) for different thermal heat inputs of 20-60 kW and heat release density of 5–15 MW/m³, showing very low pollutant and sound emission[8].

Figure 5 – Flameless burner geometries

2.Applications

The most noticeable advantages of flameless conditions are the contemporary use of hot preheated air for avoiding uncontrolled NOx emission and thermal stress to materials (ENEA). Since the early experiments of 90’, such benefits justified further research under different “names” in relation to the specific applications or to the chronological and geographical “birth”. In the last decade several research papers have appeared in the field of modelling and simulating this kind of combustion applied in several fields[9],[10]^,[11]. A remarkable scientific review by Cavaliere & De Joannon gave a comprehensive analysis of the development and all the related fundamental phenomenological aspects.

The technology is often named as FLOX (a registered trademark belonging to the company WS) in Germany or Low NOx Injection (LNI) in the USA. Similarly, the High Temperature Air Combustion (HiTAC), born in Japan, a leading country in during the early stage of developing, refers to increase the air temperature by preheating systems such as regenerators and was originally named Excess Enthalpy Combustion. The technology is also known as Moderate and Intensive Low oxygen Dilution (MILD) or Colorless Distributed Combustion (CDC) combustion in Italy where radiant tube as well as flameless burner application in the steel industry have represented a pioneering application of flameless combustion. An example is the Regemat^®, a single regenerative burner which can replace self recuperative burner, being applied at the steel plant of AST-Terni (Figure 6) and Acc. Pietra di Brescia.

Figure 6 – Regenerative burners in an annealing and pickling line installed at Terni (11)

The applied research also concerned the field of power generating equipment, from gas turbine combustors to small reformers for decentralized H₂ production. In particular, in the utilization of gas mixtures H₂-rich, some problem may arise because of the hydrogen combustion peculiarities (high laminar flame speed, high adiabatic flame temperature and heating value, large flammability range, high reactivity and short delay time) which make the performances of conventional burners unsatisfactory. Moreover, the intrinsic flameless temperature found an ideal application in steam reformers for hydrogen production thanks to the proven temperature uniformity, the easy control and the minimized thermic stress outside of the tubes thanks to the low front temperature gradients.

Globally, the flameless oxidation burners can be designed for every application, where the flame front stabilization is a critical issue, for overcoming the problem of fluctuations or “humming” that affects premix-based combustors; other applicative fields worth of mention are the ceramic and glass industry as well as the chemical industry[11]. In particular, this last has representative examples in petrochemical and reformer processes where Oxy-Flameless is a valid alternative in order to further reduce CO₂ emissions. Furthermore, fewer studies have been carried out on the use of solid fuels, including biomass under flameless combustion conditions. The combustion characteristics and emissions under high temperature air combustion as well as innovative boiler design, are described in several papers for convention fuels (as in the case of the oxycoal technology) [12]^,[13] and biomass[14]. The growing interest in biofuel is in fact pushing the FLOX technology forward in order to overcome the issue related to their inhomogeneous nature: several EU research projects born with this aim[15], as the BOFcom^®, aimed to develop low-carbon option in cola fired plant by integrating oxyfuel and flameless combustion to coal-biomass co-firing[16].

---

[1] M. Khosravy el Hossaini 2014. Review of the New Combustion Technologies in Modern Gas Turbines

[2] Cavaliere A., De Joannon 2004. Mild Combustion. Prog En Comb Sci 30. 329-366.

[3] Oberlack M, Arlitt R, Peters N. On stocastic Damko¨hler number variations in a homogeneous flow reactor. Combust Theory Modell 2000;4:495–509.

[4] Giammartini et al. ENEA. La combustione “senza fiamma”

[5] el Hossaini M.K., 2014  Review of the New Combustion Technologies in Modern Gas Turbines. InTech

[6] Flamme et al., 2001. Low NOx Combustion technologies for high temperature applications. Energy Conversion and Management 42 (2001) 1919-1935.

[7] M. Khosravy and el_Hossaini. Review of the New Combustion Technologies in Modern Gas Turbines. http://dx.doi.org/10.5772/54403

[8] V. Mahendra Reddy and Sudarshan Kumar. Development of high intensity low emission combustor for achieving flameless combustion of liquid fuels. Propulsion and Power Research 2013;2(2):139–147

[9] Galletti et al., 2009. Numerical and experimental investigation of a mild combustion burner. Combustion and Flame 151 (2007) 649-664.

[10] Weber et al., 2008. On the (MILD) combustion of gaseous, liquid, and solid fuels in high temperature preheated air. Proceedings of the Combustion Institute 30 (2005) 2623-2629.

[11] Wunning 2005 Flameless Combustion and its application

[12] Suda et al., 2002. A study of combustion behavior of pulverized coal in high-temperature air. Proc Combust Inst 2002; 29: 503-9.

[13] Zhang et al., 2007. Development of high temperature air combustion technology in pulverized fossil fuel fired boilers. Proceedings of the Combustion Institute; 31:2779-85.

[14] Abuelnuor et al., 2014. Characteristics of biomass in flameless combustion: A review. Renewable and Sustainable Energy Reviews 33: 363-370.

[15] Wunning 2005 Flameless Combustion and its application

[16] http://bookshop.europa.eu/it/application-of-the-biomass-oxyfuel-and-flameless-combustion-for-the-utilisation-of-pulverised-coals-for-electricity-generation-bofcom–pbKINA25128/

Author: Marcello De Falco, Associate Professor, University UCBM – Rome (Italy)

 

1. Theme description

 

The negative environmental effects due to the extensive use of fossil fuel are boosting the research of innovative solutions to exploit renewable energies for the production of clean electricity without emitting pollutants or GreenHouse gases (GHGs).

Among the renewable energies, solar energy is the most interesting. Sun is widely available and radiates on Earth’s atmosphere 1367 W/m². But, most techniques for generating electricity from heat need high temperatures to achieve reasonable efficiencies and the output temperatures of non-concentrating solar collectors are limited to temperatures lower than 200°C. Therefore, concentrating systems have to be used to reach operating temperatures suitable for electricity production.

Many interesting solar concentration technologies have been developed and proposed during the last years and the solar electrical energy production feasibility has been demonstrated, even if more efforts to reduce cost are needed to boost a wide commercialization.

The main elements composing a solar power concentration plant are:

• The reflector, which concentrates the sunlight to a focal line or focal point and has a various shapes (parabolic, flat, hemispheric). The reflector has to be tracked and a distinction can be made between one-axis and two-axis tracking: one-axis tracking systems concentrate the sunlight onto an absorber tube in the focal line, while two-axis tracking systems do so onto a relatively small absorber surface near the focal point.
• The absorber, through which a thermal fluid (liquid or gas) flows, collecting the solar concentrated heat and increasing its temperature. The absorber has to be able to absorb the main share of the solar radiation but, at the same time, has to avoid the energy leakage towards the external.
• The storage systems, able to store the hot thermal fluid thus avoiding a discontinuous behavior of the power generation system.
• The power unit, by which the high temperature thermal fluid energy is converted in electricity.

In the following, some of the main solar concentration power technologies are described.

 

2. Technologies

2.1 Parabolic Trough Power Plants

Parabolic trough power technology is the widest applied solar concentration technology, being the total electric power installed worldwide 1.095 MW approx.

The technology consists of large curved mirrors, which concentrate the sunlight by a factor of 80 or more to a focal line. Parallel collectors build up a 300–1200 m long collector row, and a multitude of parallel rows form the solar collector field and the tracking system is one-axis type.

After the thermal fluid flowing through the absorber has been heated up, it is stored in a hot storage system and then pumped to the power unit, where its sensible heat is converted in electricity by a water steam cycle (Rankine cycle). Then, the cold stream is stored in the cold storage system and pumped again to the absorber.

 

Parabolic trough power plants are classified in function of the thermal fluid flowing through the absorber:

• Mineral oil, able to reach a temperature of 400°C;
• Molten Salt, developed by ENEA Research Centre [1] and able to reach temperature of 550°C;
• Water/superheated steam.

The most common thermal fluid used is the thermal oil, but the molten salt technology is becoming more and more interesting and a first pre-industrial application has been installed in Italy (Priolo) in 2014 [2].

ENI has identified an original mixture of salts able to replicate the outlet temperature obtained by the ENEA technology (up to 550°C). Moreover, ENI has developed in 2012 a selective coating in metal-ceramic (CERMET) alloy able to increase the absorbance of solar radiation by reducing emissivity, thus also reducing the heat losses of the receiver unit [3].

The efficiency of the solar thermal power plant is the product of the collector efficiency, field efficiency and steam-cycle efficiency [4].

 

2.2 Solar Thermal Tower Power Plants

Central receiver systems such as solar thermal tower plants can reach higher temperatures and therefore achieve higher efficiencies than Parabolic Trough.

Hundreds or even thousands of large two-axis tracked mirrors are installed around a tower. The slightly curved mirrors are also called heliostats; a computer calculates the ideal position for each of these, and a motor moves them into the sun.

The absorber is installed on the top of the tower and it is heated up to temperatures of 1000°C or more. Hot air or molten salt then transports the heat from the absorber to a steam generator, where superheated water steam is produced, driving a turbine and electrical generator.

There are two main types of solar tower typology:

• Open Volumetric Air Receiver. A blower transports ambient air through the receiver, which consists of wire mesh or ceramic or metallic materials in a honeycomb structure. The air stream is drawn through this and heated up to temperatures between 650°C and 850°C. On the front side, incoming cold air cools down the receiver surface. Therefore, the volumetric structure produces the highest temperatures inside the receiver material, reducing the heat radiation losses on the receiver surface. Then, the air reaches the heat boiler, where steam is produced.

 Open Volumetric Air Reciver

 

• Pressurized Air Receiver, where a compressor pressurizes air to about 15 bar. A transparent glass dome covers the receiver and separates the absorber from the environment. Inside the pressurized receiver, the air is heated to temperatures of up to 1100°C, and the hot air drives a gas turbine. This turbine feds both the air compressor and the generator that produces electricity. The waste heat of the gas turbine goes to a heat boiler and in addition to this drives a steam-cycle process. The combined gas and steam turbine process can reach efficiencies of 50%, whereas the efficiency of a simple steam turbine cycle is only 35%.

 Pressurized Volumetric Air Reciver

No commercial tower power plant exists at present. However, prototype systems – in Almería, Spain [5], in Barstow, California, US [6], and in Rehovot, Israel [7] – have proven the functionality of various tower power plant concepts.

 

2.3 Dish-Stirling Systems

Dish–Stirling systems can be used to generate electricity for small and medium applications (some kWs). A parabolic concave two-axis tracked mirror (the dish) concentrates sunlight in the focus, where a receiver is placed and heated up to 650°C. The absorbed heat drives a Stirling motor, which converts the heat into motive energy and mechanical drives a generator to produce electricity.

If sufficient sunlight is not available, combustion heat from either fossil fuels or biofuels can also drive the Stirling engine and generate electricity. The system efficiency of Dish–Stirling systems can reach 20% or more.

Some Dish–Stirling system prototypes have been successfully tested in a number of countries. However, the electricity generation costs of these systems are much higher than those for trough or tower power plants, and only series production can achieve further significant cost reductions for Dish–Stirling systems.

Two solar parks using the Stirling Energy Systems are currently being developed in US with a capacity of  1.4GW.

 

2.4  Solar Chimney Power Plants

The technologies described above use only the direct irradiance. The Chimney Power plant is able to collect the global irradiance and converts it into electricity.

A solar chimney power plant has a high chimney (tower), with a height of up to 1000 metres, and this is surrounded by a large collector roof, up to 130 metres in diameter, that consists of glass or resistive plastic supported on a framework. Towards its centre, the roof curves upwards to join the chimney, creating a funnel.

The sun heats up the ground and the air underneath the collector roof, and the heated air follows the upward incline of the roof until it reaches the chimney. There, it flows at high speed through the chimney and drives wind generators at its bottom.

The efficiency of the solar chimney power plant is below 2%, and depends mainly on the height of the tower, and so these power plants can only be constructed on land, typically in desert regions, which is very cheap or free.

However, the whole power plant is not without other uses, as the outer area under the collector roof can also be utilized as a greenhouse for agricultural purposes. As with trough and tower plants, the minimum economical size of solar chimney power plants is also in the multi-megawatt range.

3D Rendering of 5 MW solar chimney power plant [8]

 

3. Applications

The main application of solar concentration power plants is the clean electricity production.

Of course, the plants have to be installed in regions with a minimum of around 2000 full-load hours, i.e. in regions with a direct normal irradiance of more than 2000 kWh/m² or a global irradiance of more than 1800 kWh/m². However, a proper thermal storage assembling allows significantly increasing the number of full-load hours also in region with lower normal irradiance.

The specific system costs are between 2 €/W and 5 €/W depending on the system size, technology and storage size. Hence, a 10 MWe solar thermal power plant will cost €20–100 million. At very good sites, today’s solar thermal power plants can generate high revenues, with a payback time of 5-10 years.

The potential for solar thermal power plants is enormous: for instance, about 1% of the area of the Sahara desert covered with solar thermal power plants would theoretically be sufficient to meet the entire global electricity demand. Therefore, solar thermal power systems will hopefully play an important role in the world’s future electricity supply.

Solar concentrating power plant can also be installed in a cogeneration configuration (CHP – Combined Heat and Power), thus producing both electricity and thermal energy flow (hot water, low-medium steam, etc.).

Moreover, an increasing interest is focused on the coupling of solar concentrating power plant with chemical processes, for which a high heat duty is required. Some studies are reported in the literature about the possibility to exploit the high-temperature thermal flows generated from the solar plant for supporting industrial production processes [9].

 

 

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[1] http://www.enea.it/it/enea_informa/events/techitaly2012/DiMario_3ott2012.pdf

[2] http://www.nrel.gov/csp/solarpaces/project_detail.cfm/projectID=19

[3] Eni Technology Report 2011-2012

[4] http://www.volker-quaschning.de/articles/fundamentals2/index_e.php

[5] http://www.dlr.de/media/en/desktopdefault.aspx/tabid-4987/8424_page-3//8424_read-7297

[6] http://www.solaripedia.com/13/31/solar_one_and_two_%28now_defunct%29.html

[7] http://www.rotemi.co.il/p-753/

[8] SCHLAICH BERGERMANN SOLAR (SBS) GMBH, STUTTGART (www.sbp.de)

[9] De Falco M, Giaconia A, Marrelli L, Tarquini P, Grena R, Caputo G (2009). Enriched methane production using solar energy: an assessment of plant performance. International Journal of Hydrogen Energy 34: 98-109.