CO 2 Capture and Sequestration from Power Generation; Studies by the IEA Greenhouse Gas R&D Programme

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1 Capture and Sequestration from Power Generation; Studies by the IEA Greenhouse Gas R&D Programme Kelly Thambimuthu, Chairman, IEA Greenhouse Gas R&D Programme c/o CANMET Energy Technology Centre, Natural Resources Canada, Ottawa, Ontario K1A 1M1, Canada. and Paul Freund, Project Director, IEA Greenhouse Gas R&D Programme, CRE Group Ltd, Cheltenham GL52 4RZ, United Kingdom. Summary Current understanding of the world s climate indicates that human-induced changes are occurring and may be sufficient in magnitude to require preventative action, such as limiting atmospheric concentrations of greenhouse gases. The main anthropogenic greenhouse gas is carbon dioxide and its largest source is combustion of fossil fuels for power generation. There are many strategies that can be adopted to tackle greenhouse gas levels in the atmosphere. Many different technologies can be used for reducing emissions, as well as increasing the removal of from the atmosphere through enhancement of natural sinks, such as by forestry. Some of these options are available today and could be implemented at relatively little overall cost. For example, improving energy efficiency and switching from high carbon fuels to low carbon fuels, if suitable supplies are available. These can achieve significant reductions in emissions. Introduction of renewable sources of energy or nuclear power to displace fossil fuels would achieve deep reductions in emissions if applied widely. However, to avoid disruptive changes, it will also be necessary to find ways of continuing to use fossil fuels but with much less emissions. Capture and storage of is a technology that could deliver deep reductions in emissions from fossil fuels. In this paper, methods of removing from the flue gas streams of coal and gas-fired power plants are examined, considering both currently available technology as well as possible future variants. Methods of storage are also discussed. The results on capture and storage of are put into perspective by comparison with studies of the large-scale application of forestry for sequestering atmospheric, and also large-scale use of renewable energy sources, in this case growth and harvesting of woody biomass for power generation. Each of these options has different characteristics, providing a range of choices of ways of tackling climate change

2 Introduction Climate change is a world-wide issue and everyone, not least the power generation industry, will be involved in finding solutions. The Intergovernmental Panel on Climate Change (IPCC) has reported [1] that the balance of evidence suggests a discernible human influence on global climate. As a result, the third Conference of the Parties to the UN Framework Convention on Climate Change met in Kyoto in December 1997 to decide on targets for reducing emissions and methods of achieving those targets. Emissions of greenhouse gases from anthropogenic sources are the main contributors to the anticipated change in the climate - principally carbon dioxide but methane and other gases also making significant contributions (Figure 1). Human-induced emissions of carbon dioxide arise mainly from combustion of fossil fuels (about 6GtC/y; 1GtC is equivalent to 3.6 Gt of ) with about 2 GtC/y from deforestation (1). The main uses of fossil fuels are transport, heating and power generation. On a global basis, power generation accounts for over 30% of carbon dioxide emissions, making it the single largest source. A further characteristic of power generation is that the flue gas streams are large and relatively few in number, in contrast to heating and transport, where there are many, relatively small, dispersed sources. This suggests that technical changes to reduce emissions may be applied more easily in the power generation sector than elsewhere. CARBON DIOXIDE (63.5%) CFC'S N 2O CH 4 CFC'S (11.5%) METHANE (20.5%) NITROUS OXIDE (4.5%) Figure 1. Contributions to climate change from anthropogenic sources [1] At the Kyoto conference, the so-called Annex 1 countries agreed to an average reduction of 5.2% in greenhouse gas emissions by the period Targets for later periods have yet to be set. The extent to which these reductions will effectively combat climate change is unclear. Some indication of the extent of the change that may be needed is provided by the IPCC s scenarios (1). Table 1 shows the predicted change in atmospheric loading for two scenarios - IS92a is often described as business as usual ; this is compared with stabilisation of the level of at 450 ppmv (today it is at 360ppmv). The IS92a scenario is expected to lead to levels of 750 ppmv by the year 2100, something which could produce warming of 1.5 to 3.5 o C (1) for the world as a whole (and much greater change in some places)

3 Table 1. Accumulated emissions in the period Scenario Accumulated emissions(gtc) IS 92a 1500 Stabilise at 450 ppmv 650 Difference 850 The striking aspect of these figures is that, to reduce atmospheric levels of sufficiently to achieve stabilisation would require a cut in emissions of more than 50% over the whole of the next century. If these simulations are correct, deep reduction in emissions would be needed to avoid such changes in climate. Measures for reducing emissions Reducing emissions of carbon dioxide could be achieved, for example, by reducing the demand for energy, by altering the way that energy is used, or by changing the method of producing/delivering energy. Demand for energy can be influenced by fiscal measures and changes in behaviour but, from a technology perspective, there are 4 distinct types of option for reducing emissions: _ improving energy efficiency _ switching to low carbon fuels _ switching to no-carbon fuels _ capture and storage The first two options are cost-effective in many places today and will deliver useful reductions; whether this will be sufficient is not yet known. Deep reductions could be provided by third option, such as switching to renewable energy or nuclear power, but the fact remains that the world is heavily dependent on fossil fuels. For this reason, it is important that there should also be technology options that would allow continued use of fossil fuels without substantial emissions of. This can be done with capture and storage of from flue gases, which is the main topic of this paper. In the early stages of development of this technology, a number of countries recognised a common need to learn more about it. The obvious way to do this was to collaborate. A collaboration was formed under the aegis of the International Energy Agency (IEA) and is called the Greenhouse Gas R&D Programme. This programme is an international collaboration for learning about technologies to reduce greenhouse gas emissions from use of fossil fuels. It is supported by 15 member countries, the European Commission and 6 industrial sponsors. It delivers new information to its members at low cost, as well as providing opportunities for practical research. Much of the work has been directed at carbon dioxide from its main source, power generation, but work has also been done on other greenhouse gases, especially methane, and other sources. Capture and storage of The main steps in the process of capture and storage of from post combustion operations are shown in figure 2. After the fossil fuel has been burnt to produce power, the is separated from - 3 -

4 the flue gas stream. Then the would be stored, for a long time, if it cannot be put to some useful purpose. Other approaches to the removal of can be with its pre-combustion separation, for example, from synthesis gas streams as produced from the steam reforming of methane or the gasification of coal. In these operations, the CO present in the synthesis gas is also converted to via a water gas shift reaction prior to its separation. Utilisation of CO2 Fossil Fuel Power Generation Capture of CO2 Storage of CO2 Figure 2. Capture and storage of carbon dioxide from power generation The technology of capture is in use today to supply to the food industry. It makes use of the technique of chemical or physical solvent absorption or of adsorption onto a solid surface (i.e. pressure or thermal swing adsorption). Systems could also be developed based on membranes or cryogenics, which would be particularly suitable for flue gas streams with high concentrations of (2). Studies of these separation processes have been conducted for the common types of power plant: natural gas combined-cycle and pulverized coal plant, as well as possible future plant including oxygen-blown combustion, gasification combined cycles and fuel cells; 3 representative fuels have been considered: natural gas, coal and, to a lesser extent, Orimulsion. Some results from the assessment of power generation with capture are described below. Power plant incorporating capture Standardised assessment conditions have been used, to ensure comparability between the various studies - for example, each plant is rated at 500 MW e (sent out);, once captured, is dried and pressurised for transport to the storage site; fuel costs are $3/GJ for gas and $2/GJ for coal and Orimulsion, plus a range of sensitivities; levelised costs are calculated at 10% discount rate; results are presented in terms of the amount of emissions avoided (rather than the amount removed) by comparison with a similar type of plant but without capture; this is expressed in terms of tonnes of avoided

5 Natural gas-fired plant The base-case plant is a combined cycle (NGCC). Cost of power from the base-case plant is 3.5 US /kwh, with emissions of 406 g /kwh so. Capture of is by solvent absorption (using monoethanolamine, MEA) with a scrubber in the flue gas stream. The concentration in the flue gas is low (3.4%) making removal of quite expensive. Compression & D rying Natural Gas Scrubber Stack Gas Combustion Chamber Heat Recovery Steam Generation Steam Turbine S Compressor Gas Turbine S Air Figure 3. Schematic diagram of NGCC with capture of The NGCC plant with capture is illustrated in Figure 3; some data on cost and performance are shown in Table 2. For comparison, a future technology, the molten carbonate fuel cell (MCFC), has also been examined with removal from the exhaust gas using a MEA scrubber. The data used for the MCFC are intended to describe such plant when produced in large quantities, rather than the prototypes being tested today. The MCFC power plant also incorporates a bottoming steam cycle for optimal heat recovery. Table 2. Performance of natural gas plant with and without capture Efficiency %(LHV) Power cost US /kwh in flue or captured gas (vol % dry) Cost of avoided emissions ($/t ) NGCC no capture N/A NGCC with MEA solvent absorption MCFC with internal reforming and MEA capture

6 The original evaluation of the base-case NGCC was conducted some years ago; since then, the efficiency of NGCC plant has increased and costs have been reduced further, so the figures in Table 2 probably understate the advantage that the NGCC has over the MCFC. Overall, the results in Table 2 show that for capture, the cost of capture and electricity production are broadly similar from the NGCC and MCFC power plants. Coal-fired plant Options for burning coal are more varied - base-case plant are conventional pulverised coal (PC) and (oxygen blown) integrated gasification combined cycle (IGCC). These are also assessed with capture (using MEA or Selexol solvent absorption), as shown in Table 3. Table 3. Performance of various types of coal-fired plant with and without removal Efficiency %(LHV) Power cost US /kwh in flue or captured gas Cost of avoided emissions ($/t ) (vol % dry) PC-FGD no capture N/A PC-FGD (MEA capture) IGCC without capture N/A IGCC with shift; capture using Selexol ABGCC without N/A capture ABGCC with shift; capture with amine Compression Coal Gasifier Gas Cleanup Shift Reactor Heat Recovery Scrubber Stack Gas Water Oxygen Air Separation Unit Nitrogen H 2 Combustion Chamber Heat Recovery Steam Generation Steam Turbine S Compressor Gas Turbine S Air Figure 4. Schematic diagram of IGCC with shift and removal - 6 -

7 A variant of the IGCC incorporating a shift reaction stage, to convert CO to and hydrogen, would make easier the subsequent capture of ; this is illustrated in Figure 4. Air-blown gasification has also been considered. Emissions from the base-case plant are about 800 g /kwh, with power costs around 5 US /kwh. Although the IGCC type of plant is only now at commercial demonstration stage, this would appear to offer advantages over other coal-fired plant in terms of minimising the cost of removing, providing this is done by incorporation of the shift reaction stage to optimise the conditions for removal. However, it should be noted that turbines which burn a mainly hydrogen (>70%) fuel gas in the combined cycle part of the plant are currently not available and may require development. Orimulsion-fired plant This fuel has been considered as representative of liquid fuels for central station power plant. An IGCC is assumed to be used (although none are in commercial use at present with this fuel); base case efficiency is 41%, with emissions of 700g /kwh at a cost of 4.6 US /kwh. For the removal case, a shift reaction stage is introduced, as for the coal-fired IGCC (see Table 4). The combined cycle power generation scheme also relies on the combustion of hydrogen fuel gas in the gas turbine. The relatively high efficiency is thought to be due to regeneration of the solvent by use of the large amount of waste heat generated as a result of the water in the fuel. Table 4. Performance of an Orimulsion -fired IGCC incorporating removal Efficiency % (LHV) Power cost US /kwh in flue or captured gas (vol % dry) Cost of avoided emissions ($/t ) IGCC without Capture N/A IGCC with shift stage and 36.5% Selexol removal of Oxygen/ recycle combustion schemes In addition to the power plant options and capture technologies examined in the previous sections, schemes incorporating a modification which relies on the combustion of fossil fuels in an oxygen and recycled flue gas (mainly ) medium were also evaluated. Figure 5 illustrates the concept as it is applied to a steam boiler or a gas turbine combustor. By using pure oxygen from an air separation unit for combustion, a flue gas stream with a high concentration of is produced at the combustor outlet. Part of the gas stream from the combustor outlet is recycled as a diluent to moderate furnace temperatures, while the balance of a mainly exhaust gas is dried and further purified before its removal for transport and storage

8 Recycle and/or H 2 O Air AIR SEPARATION O 2 Combustor or boiler rich product gas Nitrogen Coal, oil or natural gas Remove H 2 O Figure 5. Oxygen/CO2 recycle scheme applied to a gas turbine combustor or boiler Table 5 presents data for the efficiency, gas purity and the power and capture costs for a NGCC plant, a coal-fired IGCC plant and a PC fired plant operating in this mode of combustion. In the case of the NGCC and coal-fired IGCC plants, oxygen-flue gas ( ) recycle combustion was carried out around the gas turbine combustor, with expansion of a mainly and moisture rich gas through the gas turbine. It should be noted that gas turbines able to operate with a mainly expansion gas do not exist and require development for the implementation of these power plant schemes. In the case of the PC-fired plant, modifications to a steam boiler to accommodate the change in composition and heat transfer characteristics of the combustion medium is not generally considered to be a major impediment for the implementation of this power generation scheme. The data in Table 5 shows that for the coal-fired PC and IGCC options, the oxygen/ recycle combustion scheme produces among the lowest efficiency penalties and cost of capture, albeit with a lower outlet flue gas purity in the vol% range. Table 5. Performance of Oxygen/ recycle combustion schemes with removal NGCC with 50% flue gas recycle, cryogenic O 2 & MEA purification Efficiency % (LHV) Power cost US /kwh in flue or captured gas (vol % dry) Cost of avoided emissions ($/t ) IGCC with flue gas recycle and cryogenic O 2 PC with flue gas recycle & cryogenic O 2-8 -

9 Storage of Having captured the, it would have to be put to use or stored somewhere, separate from the atmosphere. One option for utilisation of is enhanced oil recovery (EOR); this is done now with naturally occurring ; captured is somewhat more expensive but applications for it in EOR are now being developed(3). Other methods of utilising captured have been examined but none have been found which would offer substantial net savings in emissions. To store the would require very large reservoirs - for example, disused oil or gas fields, deep saline-water reservoirs or the deep ocean (see Table 6). Disused oil and gas fields have the attraction that the geology has been well studied and, in principle, a geological seal is known to exist, which will ensure long term storage. However, as yet, no storage has been undertaken in such fields. Instead, the first commercial-scale storage of uses a deep saline-water reservoir under the North Sea. This project began operation in It is storing 1 million tonnes of /year extracted from the natural gas stream produced from the Sleipner West gas field. The field s operator, Statoil, have installed an MEA plant to separate from the produced gas stream. It is then pressurised and injected into the Utsira formation, 800 m below the sea-bed. Another option considered for storage of is the deep ocean. Eventually most atmospheric will be deposited in the deep ocean, so this could be thought of as an accnowneration of the natural process. The technique involves pumping to a depth of 1000m or more (where it might be dispersed or induced to form a sinking plume) or injecting it as a liquid at 3000 m depth, where it would be deposited on the sea-bed. There are many ideas for this type of scheme but there are substantial obstacles too. In the longer term, this storage option might be needed but, in the meantime, much research is required to clarify likely performance and environmental impact, not to mention the legal basis for such actions. Another option, that of building artificial stores for on land (4), has also been examined but the cost is prohibitive. Table 6. Options for storage of in natural reservoirs and injection costs Global capacity Gt C Storage (Injection) Cost ($/t ) Disused oil fields Disused gas fields Deep saline reservoirs Ocean Estimates of the global storage capacity of these options are shown in Table 6, which indicates there would be many years of capacity available. In all of the cases shown, the cost of transporting the and storing it should be only a few $/t, substantially less than the cost of capturing it. Some costs for the injection of (excluding transportation) in the various storage sites are shown in Table

10 Enhancement of natural sinks for An alternative to end-of-pipe removal of is to remove it from the atmosphere by enhancing the take-up by natural sinks - typically in growth of trees but options for enhancing take-up by the oceans are now also being discussed. Such methods have received much interest in recent years and the agreement reached in Kyoto in 1997 specifically allows countries to take advantage of enhancement of natural sinks on land to offset emissions of greenhouse gases. The terrestrial options are: _ reduction in deforestation _ afforestation of land which has not supported forests previously _ reforestation of land which has been under forestry at sometime in the (recent) past It has been estimated that afforestation and reforestation could sequester as much as 0.8GtC/y worldwide; the main limitation on this being the amount of land required million ha (5). Increases in carbon stored in the soil could usefully contribute as well. For the first such schemes, covering areas of typically ha, the costs are thought to be very low ($0.80-4/t sequestered) in developing countries (6). In developed countries and at larger scale of operation, the costs of sequestration rise, not least because of the pressure on available land. Thus for large-scale schemes in tropical countries, costs of sequestration are estimated to be in excess of $7/t ; comparable schemes in a mid-latitude developed country would be over $19/t (7). Having started to sequester carbon in trees, the forest must be maintained for long periods of time in order to ensure the carbon is not re-released to the atmosphere. Thus when the trees mature (and the rate of take-up of carbon slows down), they serve less as a sink than a store (providing they are suitably managed). Otherwise, the timber may be harvested and the area replanted in a regular fashion, which maintains a dynamic store of carbon. Short rotation cropping to provide a -neutral fuel In some places, rather than growing trees to sequester carbon, they could be grown to provide a renewable source of fuel. Such biomass provides fuel for power generation with little net emission and many such schemes are underway. In order to understand the implications of the large-scale use of biomass, a study has been conducted of short-rotation cropping of a woody biomass, to provide fuel sufficient to produce 1GW e electricity. The specific location selected for this study was Spain but the results are probably relevant to other developed countries too. The biomass grown was mainly eucalyptus and acacia; currently available technology was assumed for most of the power plant, based on fluid bed combustors rated at MW e each. The effect of using (future) gasification combined-cycle plant was also examined. Locations for the plants were selected on the basis of: _ Availability of cooling water _ Proximity to small towns (for staff accommodation) but not close to large towns _ Within 1km of roads and having access to the grid

11 Areas for growing the biomass were chosen on the basis of: _ non-irrigated agricultural land _ rainfall more than 400mm/y _ availablity of ha within 40km of each plant Use of irrigated agricultural land was considered unacceptable because this would mean displacing high value crops. A total of 28 sites were identified, sufficient to supply 1GW e. The competition for land is reflected in the cost of power (Figure 6) which rises significantly even in the first 1GWe tranche. New (gasification) technology would provide significant cost reduction but, even then, electricity costs of more than 8c/kWh were anticipated. It was estimated that another 1GW e could be produced from biomass in Spain in a similar way but to expand beyond that would need use of less suitable land, and costs would rise. The cost of carbonavoided was estimated at $47/t for FBC plant, based on the existing mix of plant on the grid. Electricity cost (c/kwh) FBC Gasifier Cumulative installed capacity (MWe) Figure 6. Cost of power from biomass plants, located in Spain, using fluid bed combustion (FBC) and future gasification combined cycle plant. Capture and storage of but with substantially reduced emissions. The cost of this option is comparable with the cost of other mitigation options such as renewable (biomass) fuel and large-scale enhancement of sinks. Each of

12 these options has different characteristics, providing a range of choices of ways of tackling climate change. Bearing in mind the time required to develop and deploy new energy supply technologies, it is only sensible to adopt a precautionary stance by investigating a range of options. Through international collaboration, many countries and organisations are able to understand the potential of technologies not yet in widespread use and participate in the development of the more promising ones. References 1) Climate Change. The science of climate change. Houghton, J. T., Meira Filho, L. G., Callander,B. A., Harris, N., Kattenberg, A., Maskell, K., (Eds), Cambridge University Press ) Capture of carbon dioxide from power stations. IEA Greenhouse Gas R&D Programme, Cheltenham, UK ISBN ) Tontiwachwuthikul, P., Chan, C.W., Kritpiphat, W., Skoropad, D., Gelowitz, D., Aroonwilas, A., Jordan, C., Mourtis, F., Wilson, M., Ward, L., New feasibility study of carbon dioxide production from coal-fired power plants for enhanced oil recovery: a Canadian perspective Energy Convers. Mgmt. 37 (6-8), pp , ) Seifritz, W. The terrestrial storage of -ice as a means to mitigate the greenhouse effect in Hydrogen Energy Progress IX pp 59-68, International Association for Hydrogen Energy, ) Utilisation of carbon dioxide IEA Greenhouse Gas R&D Programme, Cheltenham, UK. 1995, ISBN ) Verweij, J.A. Re/afforestation and the market for Joint Implentation in Greenhouse Gas Mitigation - Technologies for Activities Implement Jointly pp , published by Pergamon, Oxford, ) Global warming damage and the benefits of mitigation IEA Greenhouse Gas R&D Programme, Cheltenham, UK. 1996, ISBN Speaker : Kelly Thambimuthu Company : Chairman, IEA Greenhouse Gas R&D Programme Country : Canada Kelly Thambimuthu is chairman of the IEA Greenhouse Gas R&D Programme based at Cheltenham in England. Dr. Thambimuthu represents Canada on the executive committee that supervises the work program of the IEA-GHG Programme. In his other duties, Kelly is a senior research scientist at the CANMET Energy Technology Centre in Ottawa Canada. At CANMET, Kelly is responsible for the development of O 2 / recycle combustion cycles and is an adviser on greenhouse gas management and fossil energy utilisation issues for the government of Canada He holds degrees at the batchelor s, master s and doctorate levels in chemical engineering from the University of Birmingham, England, McGill University, Canada and Cambridge University, England