INVESTIGATING VIABLE CO 2 CAPTURE AND SEPARATION OPTIONS

Size: px

Start display at page:

Download "INVESTIGATING VIABLE CO 2 CAPTURE AND SEPARATION OPTIONS"

Oscar Stafford
6 years ago
Views:

1 INVESTIGATING VIABLE CO 2 CAPTURE AND SEPARATION OPTIONS Sandra Kentish, Research Project Leader, CRC for Greenhouse Gas Technologies, Department of Chemical and Biomolecular Engineering, University of Melbourne, Victoria 3010 INTRODUCTION CO 2 capture refers to the separation of carbon dioxide from other gases, and is a necessary precursor to geological sequestration. The cost of capture can be 70-90% of the total cost of sequestration and hence is the critical component in any economic analysis of this greenhouse gas technology. Fossil fuelled power generation represents the largest potential source of carbon dioxide. Carbon dioxide can be captured from this source by a number of mechanisms: by separation of oxygen from air prior to combustion. This leads to a flue gas rich in carbon dioxide that will require little or no treatment prior to separation. by gasification of the fuel source, reaction with water and subsequent separation of the carbon dioxide from product hydrogen. This water-shifted syngas is typically at relatively high pressure (20 atm) and in a reducing condition. CO 2 concentrations are typically 35-40% 1. by post-combustion capture of CO 2 from the combustion flue gases. These flue gases are usually unpressurised, and in an oxidising condition. CO 2 concentrations range from 3-5% v/v in gas plants to 13-15% v/v in coal plants 2. The focus of this paper will be on post-combustion capture, a technology that can be retrofitted to existing power plants. Air separation and gasification technologies generally require the development of new infrastructure. Thus while they potentially offer greater potential for step change reductions in the cost of capture, this potential will only gradually be realised. Several industrial processes also produce more concentrated streams of CO 2 as a byproduct. While these streams are limited in quantity, they have great potential as a CO 2 capture target, due to this high concentration. Examples of such industrial processes include wellhead natural gas which may contain 3-30% v/v carbon dioxide in a reducing atmosphere, mineral processing operations such as iron ore smelting (20-50% CO 2 ) fermentation processes (>90% CO 2 ) cement manufacture (20-40% CO 2 )

2 These processes are also amenable to retrofitted CO 2 capture options as discussed below. COMPARING CAPTURE TECHNOLOGY OPTIONS Solvent Technology Solvent absorption technology is the only proven method of post-combustion flue gas CO 2 capture. Currently such separation is practiced in at least a dozen facilities worldwide 2. All of these plants service a commercial CO 2 market, thus making them economically viable. Most utilise the Fluor Econamine solvent. In a solvent absorption process, flue gas is contacted with a solvent in an absorption column and the CO 2 is selectively absorbed. The loaded solvent is then heated and passes to a stripping column, where steam is used to remove the CO 2. For post-combustion CO 2 capture, chemical solvents, principally based around monoethanolamine (MEA) are preferred. These solvents chemically react with the carbon dioxide and hence are useful at low pressure. At the higher pressures available in syngas purification, physical solvents such as Selexol are preferred and have been commercially proven. While solvent technology has been practised for many years, there are a number of issues that make it exceedingly expensive for flue gas recovery. As pressures are low and processing volumes are high, equipment sizes soon become unrealistically large. The presence of oxygen increases corrosion rates, often necessitating the use of expensive stainless steel materials of construction. The presence of impurities such as SOx and NOx leads to the formation of heat stable salts that consume solvent. As solvent regeneration is achieved by stripping with steam, there are also significant energy penalties. There is a huge amount of research currently being conducted worldwide to overcome these issues. As an example, Mitsubishi Heavy Industries and Kansai Electric Company have developed a range of innovative solvents referred to as KS-1 and KS-2. These hindered amine solvents have been successfully used for capture of 160 tonne/day of CO 2 from the steam reformer of a Malaysian ammonia plant since In a recent paper 4, they forecast the economics of utilising their KS-1 solvent for CO 2 capture from natural gas fired boilers and gas turbines in the Middle East area. These results suggest that a CO 2 capture cost of 20US$/tonne is feasible using this technology. A number of other research groups are similarly developing new absorption liquids 5,6. Kvaerner Oil and Gas have developed a membrane contactor, which replaces the classical packed tower with a system of microporous membranes. These semi-permeable membranes keep the gas and liquid phases separate but allow the passage of CO 2 between the two. They find Gore-tex eptfe to be the most suitable for this purpose 7. However, other workers are experimenting with polypropylene hollow fibres that are cheaper, but require the use of alternative solvent formulations 6. This technology reduces the size of the absorber and stripper units by 65% and lowers the reboiler duty and solvent loss. By

3 making the process equipment more tolerant to the solvent, the technology enables additional solvent optimization, which should further reduce energy requirements and associated costs. Membrane Technology Gas separation membranes are semi-permeable materials that permit the direct passage of carbon dioxide but retain other molecules. While these systems have not been proven in flue gas service, they have had commercial success in the separation of CO 2 from natural gas at the well head. Initially cellulose acetate spiral wound membranes were used in this service, but the trend more recently has been towards hollow fibre polyimide membranes. Gas separation membranes are small in volume and occupy a small footprint. They will thus fill niche markets for carbon capture such as in offshore and remote locations. They will continue to capture market share for wellhead gas separation, where feed pressures are higher. The major technology barrier to more widespread use of membrane technology is the currently short useful life span. After several years of operation in carbon dioxide service, the membranes fail in a process known variously as compaction, aging or plasticisation. This failure occurs by the chemical interaction of the carbon dioxide, or heavier hydrocarbons with the membrane material itself. To address this issue, RITE pursued the development of a novel cardo-type polyimide with UBE Industries for many years. However, this research program has now been substantially curtailed. Our work in this area attempts to strengthen the membrane structure through the introduction of an epoxy content 8. Membranes are also showing promise for high temperature pre-combustion capture 9. It is well known that the water gas shift reaction is equilibrium limited. One approach for overcoming this limit is to carry out the reaction in a reactor with walls that are CO 2 permeable (a membrane reactor). This requires a ceramic or other inorganic membrane that is viable at temperatures in excess of 600C. Again, there is a large amount of research being conducted in this area worldwide and the development of a cost-effective high temperature membrane is likely in the near future. Pressure Swing Adsorption Pressure swing adsorption processes have been commercially proven for the separation of oxygen from nitrogen. They are also used to separate CO 2 and methane from landfill gas. While they have not been proven for flue gas capture, this represents another potentially attractive technology. Tohoku Electric Power Company has tested a PSA process with a zeolite adsorbent at a coal-fired power plant at a scale of 1,700 Nm 3 /hour 10. In PSA processes, a column packed with solid adsorbent is exposed to pressurised flue gases and the carbon dioxide is selectively adsorbed. Dropping the pressure to a lower level regenerates the adsorbent and releases the CO 2. Adsorbents currently available for carbon dioxide separation (zeolites) selectively adsorb

4 water. Consequently moisture must be removed in an expensive pretreatment step. A major focus of current research is therefore to develop novel adsorbents that are insensitive to moisture 11. There is also a need to develop and demonstrate large-scale vacuum pumps and valves for this type of service. Hydrate Formation A further option for flue gas CO 2 capture is simply to cool and compress the gas stream until the carbon dioxide condenses. This cryogenic approach is unlikely to ever be economically viable. However, in the presence of small quantities of water, the carbon dioxide can be made to condense at a more reasonable temperature and pressure (0C and 20 atm 12, in the form of a hydrate, a solid ice-like structure. If conditions are carefully controlled, these carbon dioxide hydrates can be made to form selectively, leaving other gases behind. Once these gaseous components have been separated, the carbon dioxide can be regenerated from the gas phase, or indeed sequestered in the hydrate form. Recent research indicates that the use of hydrate formation for high pressure shifted syngas has the potential to provide separation costs as low as US$8/ton CO CALCULATING THE COST OF CAPTURE Current estimates for the cost of capture vary from US$20 to US$60/tonne CO 2 (or US /kwh). Variation within this range is to be expected and can be explained by the following factors. 1. The basis of the estimate Some cost estimates are quoted as 'per tonne CO 2 captured' or as a CO 2 'delivery cost'. This term differs substantially from the more correct 'per tonne of CO 2 avoided'. Because of the high energy costs involved, capturing a tonne of CO 2 may in itself release say an extra 200 kg of CO 2 as flue gas. Thus while 1 tonne of CO 2 is captured, the CO 2 avoided is only 0.8 tonne. The electricity output of the power plant is also significantly affected in this process. Rubin and co-workers formally define two parameters i.e. 14 : ( TCR)( FCF) + FOM Cost of Electricity, COE ($/kwh) = + VOM + ( HR)( FC) ( CF)(8760)( kw ) Where: TCR = total capital requirement ($) FCF = Fixed Charge Factor (fraction/year) FOM = fixed operating costs ($/year) CF = capacity factor (fraction) KW = net plant power (kw) VOM = variable operating costs ($/kwh) HR = net plant heat rate (kj/kwh) FC = fuel cost ($/kj) Cost of CO 2 Avoided = ( CO ( COE) capture 2 / kwh) ref ( COE) ( CO 2 ref / kwh) capture Where 'Ref' refers to a reference plant of the same type and size as the plant with CO 2 capture. Many cost estimates are also not full 'engineering' estimates and thus do not include the

5 full cost of capital, contingency costs etc. This is particularly true for the 'unproven' technologies such as membranes and PSA as there is usually no allowance for the teething problems that inevitably occur with new technologies. Often, costs estimates also assume best case scenarios of plant efficiency, fuel costs and fuel quality. 2. Flue Gas Condition High temperatures, low pressures and low CO 2 concentrations all increase the cost of capture. The presence of impurities such as oxygen, SOx and NOx will also increase costs. In particular, Australian environmental regulations of SOx/NOx emissions are less stringent than in many overseas locations. Thus most of our power plants do not incorporate SOx/NOx removal facilities, which are common elsewhere. The presence of these contaminants results in high levels of solvent degradation in classical amine absorption capture. Dave and co-workers from CSIRO Energy Technology have shown that operating costs for CO 2 capture for typical Australian black coal-fired power plants go through a minimum when 60% of these contaminants are removed through low NOx burners and the injection of sorbent into the furnace Economies of Scale It is much cheaper to capture flue gases from a 1200 MW facility than from a 400 MW facility Technology Enhancements Solvent technology is currently the cheapest of the capture methods (for onshore use). However, as this represents a mature technology, costs will fall more slowly in this area than with other options such as PSA or membrane technology. These latter options are immature technologies with respect to CO 2 capture and costs should fall rapidly with R&D input. CONCLUSION Carbon dioxide capture is currently prohibitively expensive. However, worldwide there is an enormous amount of research dollars being directed at this problem. Consequently it is inevitable that the cost of capture will fall. Technology gains will initially be centred around solvent technology where newer, more stable solvents will reduce solvent losses and better column internals, including the use of membrane contactors will reduce pressure drop and equipment size. In the longer term, emerging technologies such as gas separation membranes and PSA technology will provide the opportunity for deeper cuts into capture costs.

6 REFERENCES 1 D.Wallace 'Captuer and Storage of CO 2 : What Needs to be Done' in COP6 (Sixth Conference of the parties to the UN Framework Convention on Climate Change) 2000 The Hague 2 Herzog, H., "What Future for Carbon Capture and Sequestration?" Environmental Science and Technology, 35:7, pp 148 A 153 A, April 1, T. Mimura, K. Matsumoto, M. Iijima, S. Mitsuoka, Development and Applications of Flue Gas Carbon Dioxide Recovery Technology 5 th International Conference on Greenhouse Gas Control Technologies, Cairns, Australia, August (2000) 4 Iijima M., Kamijo T., "Flue Gas CO 2 Recovery and Compression Cost Study for CO 2 Enhanced Oil Recovery" 6 th International Conference on Greenhouse Gas Control technologies, Kyoto Japan, Oct. 1-4 (2002) 5 A. Chakma P. Tontiwachwuththikul, Economics and Cost Studies of Designer Solvents for Energy Efficient CO 2 Separation From Flue Gas Streams 5 th International Conference on Greenhouse Gas Control Technologies, Cairns, Australia, August (2000) 6 P.S. Kumar, J.A. Hogenfoorn, P.H.M. feron, G.F. Versteeg, "New Absorption Liquids for the removal of CO 2 from Dilute Gas Streams Using Membrane Contactors", Chemical Engineering Science 57 (2002) O. Falk Pederson, H. Dannstrom, M. Gronvold, D. Stuksrud, O. Ronning, Gas Treatment Using Membrane Gas/Liquid Contactors 5 th International Conference on Greenhouse Gas Control Technologies, Cairns, Australia, August (2000) 8 Nagai, K., Booker, N., Mau, A., Hodgkin, J., Kentish, S., Stevens, G., Geertsema, A., Gas Permeation Properties of Polyimide/Epoxy Composite Materials, 222nd ACS Fall National Meeting, Chicago, August Klara S.M., Srivastava R.D., "US DOE Integrated Collaborative Technology Development Program for CO2 Separation and Capture" Environmental Progress 21(4) Yokoyama T., "Japanese R&D on CO 2 Capture" 6 th International Conference on Greenhouse Gas Control technologies, Kyoto Japan, Oct. 1-4 (2002) 11 SW Delaney, GP Knowles, AL Chaffee, Hybrid Mesoporous Materials for Carbon Dioixde Separation, American Chemical Society, Division of Fuel Chemistry (2002) 47(1) Kiyono F., Yamazaki A., Ogasawara K., "Cost Estimation of CO 2 Recovery Technology Using Hydrates" Preprints of Symposia - American Chemical Society, Division of Fuel Chemistry (2002), 47(1), Spencer, D. F.; Tam, S. S.; Deppe, Gordon; Currier, R. F.; Young, J. S.; Anderson, G. K Carbon dioxide separation from a high pressure shifted synthesis gas. Proceedings - Annual International Pittsburgh Coal Conference (2002), 19th Rubin E.S., Rao A.B., 'Uncertainties in CO 2 Capture and Sequestration Costs" " 6 th International Conference on Greenhouse Gas Control technologies, Kyoto Japan, Oct. 1-4 (2002) 15 N.C. Dave, G.J. Duffy and J. H. Edwards, Economic Evaluation of Capture and Sequestration of CO 2 From Australian Black Coal-Fired Power Stations 5 th International Conference on Greenhouse Gas Control Technologies, Cairns, Australia, August (2000) 16 H. Undrum, O. Bolland and Aarebrot, Economical Assessment of Natural Gas Fired Combined Cycle Power Plant with CO 2 Capture ands Sequestration 5 th International Conference on Greenhouse Gas Control Technologies, Cairns, Australia, August (2000)

Fluor's Econamine FG Plus SM Technology

Fluor's Econamine FG Plus SM Technology An Enhanced Amine-Based CO 2 Capture Process Satish Reddy Jeff Scherffius Stefano Freguia Fluor Enterprises, Inc. Aliso Viejo, CA Christopher Roberts Fluor Ltd.,