Greenhouse gas mitigation technologies, an overview of the CO2 capture, storage and future activities of the IEA Greenhouse Gas R&D programme

Pergamon

Energy Convers.MgmtVol.37, Nos 6-8, pp. 665-670, 1996 Copyright© 1996Elsevier ScienceLtd 0196-8904(95)00237-5 Printed in Great Britain. All rights reserved 0196-8904/96 $15.00 + 0.00

Greenhouse Gas Mitigation Technologies, an Overview of the C02 Capture, Storage and Future Activities of the IEA Greenhouse Gas R&D Programme Dr Pierce Riemer The lEA Greenhouse Gas R&D Programme, CRE Group Ltd, Stoke Orchard, Cheltenham, Gloucestershire, GL52 4RZ. UK, Telephone: +44 (0) 1242 680753. Fax: +44 (0)1242 680758. e-mail: pierce@ieagreen, demon, co. uk

Abstract The IEA Greenhouse gas R&D programme is an international collaboration supported by 16 countries and several industrial organisations. During the first three years (phase 1) the programme has evaluated technologies for reducing emissions of greenhouse gases from power stations. The main types of fossil fuel power plant were investigated and the costs and emissions associated with power generation were calculated. Technologies for capturing CO2, were then evaluated with reference to the power generation technologies. Once captured and compressed, storage, transportation and utilisation of carbon dioxide was studied. It was found that the most appropriate technology for capture of CO2 depends upon the type of power plant and in most cases proven technology is available to carry this out. Capture of CO2 adds substantially to the cost of power generation and reduces plant efficiency. In contrast, storage of CO2, in deep aquifers, the oceans or in exhausted oil and gas fields is unproven but would be relatively inexpensive. There are major uncertainties about disposal, in particular in terms of environmental impact and long term security of storage. Utilisation of CO2, in the manufacture of chemicals, has only a limited potential capacity for carbon sequestration. The second phase of the programme has now commenced, developed in the light of key issues identified in the first phase. Such key issues are; can the cost of capture be substantially reduced? How can the uncertainties of disposal be reduced? What are the full fuel cycle costs of mitigation technologies? How effective are means of mitigating emissions of other greenhouse gases? Major elements of the phase 2 programme include: •

Methane emissions from the: Coal industry Oil and gas industry Landfills Other anthropogenic sources • power generation/radical approaches to C02 capture • advanced separation techniques • ocean storage • full fuel cycle studies

Introduction Concern about possible climate changes has led many nations to co-operate in developing scientific understanding of global warming. After the Rio summit in 1992, they adopted strategies to limit emissions, initially through improved energy efficiency and changes in fuels. If further action is necessary, one option would be to sequester the CO2 produced by burning fossil fuels. This would allow continued use of the fossil fuel infrastructure, built up over many decades, and avoid the disruption of changing to alternative sources of energy. However, better information is required about the feasibility of mitigation options. To meet this need, the International Energy Agency formed an R&D Programme on Greenhouse Gas Technologies, which completed its first 3 years of work in late 1994, having carried out 24 major studies. The key achievements are reported here. Initial attention, through phase I, was given to power generation since it is a concentrated source of greenhouse gases (relative to diffuse sources such as transport). A common basis for 665

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assessment of the options was developed to ensure that differences, e.g. in location or in plant specification, do not obscure conclusions about their relative merits. As a result, a balanced view has been produced, without bias to any particular type of fuel or technology. International collaboration ensures that the programme benefits from a wide range of views. Organisations in different countries are able to contribute to the study activities in accordance with their expertise in particular technologies. As the whole world is facing a common problem in global warming, co-operative investigation of the options is the best way to use available resources.

Power generation It was necessary to establish the cost, performance and emissions of the principal types of power generation plant. Cost, efficiency and emissions data were derived for a representative range of power stations, covering both current and future technology. The likelihood of technical developments in each of the main types of plant was examined. A key aspect of each study was the identification of likely requirements for capture of CO: from the plant. Four power generation schemes were studied, all producing 500MW net electrical output: Pulverised coal (PF) with flue gas desuiphurisation, representing the most commonly used type of plant; this provided a marker against which to assess other technologies. Sub-critical steam conditions were assumed and the impact of super-critical steam cycles was also considered. Natural gas-fired combined-cycle (NGCC) is another widely available technology. In this case, natural gas is burnt in a gas turbine operated in conjunction with a steam turbine. This is the most efficient option studied, emits the least amount of CO:/kWh and, with low cost supplies of gas, is the cheapest means of generating electricity. Integrated gasification combined-cycle (IGCC), representative of emerging technology, appropriate for a future when CO: mitigation is practised. The base case involved a coal-slurry fed gasifier; many variations were evaluated, including type of gasifier and shift conversion of synthesis gas CO to CO2. Combustion of coal in an atmosphere of oxTgen and recycled CO2, a potential option for the longer-term. Schemes of this type have been suggested because they raise the concentration of CO2 in the exhaust gas, thereby making capture easier. Evaluation of the different power generation systems on a consistent basis allows clear understanding of the merits of each. Improved efficiency in a power plant can produce significant reductions in CO: emissions - a 1% point gain in efficiency is roughly equivalent to reducing CO2 emissions by 2%. An NGCC system provides low generating costs and relatively low emissions of CO2. For coal-fired plant, an IGCC is better suited to the capture of CO2 than a pulverised-coal plant.

Table I

Electricity generation costs for the four base cases Efficiency (%)

CO2 conc. (% dry)

Generation Cost (mills/kWh)

PF+FGD

39.9

14

49

NGCC

52.0

4

35

IGCC

41.7

7

52

CO: recycle

32.8

91

78

The power generation studies provided a foundation for later work and also served to identify gaps in knowledge, including the performance of novel options such as natural gas combustion in turbines using recycled CO:, the competitiveness of hybrid gasifier/combustor schemes using super-critical steam cycles, and investigation of alternative ox~ygenproduction techniques.

Carbon dioxide capture Systems are available for capturing CO2 but there is a limited choice, so the technologies evaluated included methods under development. Each capture technology, described below, was considered as applied to each of the four types of power plant described above.

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667

Adsorption of the gas using molecular sieves - a key aspect is release of the gas into a closed system after it has been captured; in all the cases studied, varying the pressure to release the gas is preferable to varying the temperature, because the adsorber can be put back into service faster. Capture of CO2 by an adsorbent is most effective when the concentration in the gas is between 400ppm and 15000ppm, lower than is normally the case with power stations. Coupled with limited capacity and poor selectivity, this makes adsorption unattractive for CO2 capture from conventional generation processes. Physical and chemical absorption - several solvents were evaluated for each type of power plant. For low partial pressures of CO2 in the flue gases, a chemical solvent such as monoethanolamine is preferred; where the CO2 partial pressure is high, a physical solvent is favoured; in either case, additional processing is required if there is much SO2 in the flue gas (as with a coal-fired piano to avoid excessive loss of solvent. Use of cryogenic processes - is only worth considering where there is a high concentration of CO2 in the flue gas, as could be achieved in future IGCC designs. Cryogenic processes have the advantage of producing liquid CO2 ready for transportation by pipeline. Membranes - although used commercially (e.g. in hydrogen separation) development is required before they could be used on a significant scale for the capture of CO2. The extent to which their present high cost could be reduced is not clear. One attraction of membranes is that they require less energy for operation than other methods of capture. One system which showed promise was the combination of membranes with chemical solvents. There are a wide range of potential CO2 capture technologies; the cost and performance of the main options have been assessed and the most appropriate capture technologies for each power plant identified. An important aspect of CO2 capture is the extra amount of energy used, this reduces the overall energy efficiency of power generation (typically by 10 percentage points) which is a large penalty; allowance must be made for this in estimating the amount of CO2 emissions avoided and hence the economics of capture. Figure 1 illustrates the concept for one of the PF cases studied.

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Figure 1: Emissions nith and Hithout capture shox~ng the total C02 generated

The main conclusions on capture options are, that the cost of capturing CO2 varies with plant type, being greater for gas-fired plant (because the flue gases are more dilute in CO2) than for a coal-fired plant. For each tonne of CO2 avoided through capture, a cost of about 40 US $ would be borne by the producer, increasing the cost of electricity generation by at least 2c/kWh or 40% above current levels. This is broadly similar to other proposed mitigation options such as a carbon tax. Radical approaches to capture or to combustion of fossil fuels a r e required in order to reduce these costs substantially.

668

RIEMER: GREENHOUSE GAS MITIGATION TECHNOLOGIES

Carbon dioxide storage If the capture of CO2 is to help mitigate greenhouse gas emissions, methods of storage will have to be developed. Storage options are largely independent of the type of power plant and capture technology employed; they are far more dependent on geographical location. Four primary disposal options have been studied: . • • •

ocean disposal disposal in aquifers disposal in depleted oil/gas reservoirs storage as a solid in a thermally insulated repository

Table 2 gives the estimated cost of disposal for each of the above options and also includes the cost of pipeline transport derived in each of the studies; the costs are based on schemes designed to dispose of 150kg COz/s (the CO2 product from a 500MW(e) PF+FGD power plant fitted with an MEA-based CO2 capture system). The studies have shown that the cost of disposal, with one exception, is inexpensive compared to capture costs; with modest transport distances disposal is estimated to add only 0.2 cents/kWh to electricity costs; even with distances of 1000km, the cost addition for disposal is estimated to be <1 cent/kWh (Figure 2). The natural gas combined cycle case would again benefit from having less CO2 to transport but there are significant advantages of scale so this benefit would not be pro-rata. The benefits of scale in reducing the cost of CO2 pipeline transport has also been estimated by other workers and the costs reported are, if anything, lower than those reported in Table 2. The storage of CO2 as a solid was proposed as a theoretical concept; the concept is valid but the study has shown that translating this concept into a practical scheme results in prohibitively high capital cost and there is also a significant power consumption penalty for the production of solid CO2.

Table 2 Estimated costs and potential storage capacities for C02 CO2 storage option Cost ($/tC) Global capacity (GtC) Ocean disposal 4.1 > 1000 Saline aquifers 4.7 > 100 Depleted gas reservoirs 8.2 > 140 Depleted oil reservoirs 8.2 >40 Improved forestry & reforestation 5 - 20 50 - 100 The oceans are the ultimate natural 'sink' and have the greatest long-term potential; not all countries have access to a deep ocean. Deep, saline aquifers and exhausted oil and gas reservoirs also have large storage capacities. Oil and gas reservoirs have the advantage of known geology to provide a seal to contain the CO2 in the store; they are an immediately available option. Afforestation is, in principle, a feasible approach, but the land areas required are huge (for example, about 2000km2 would be required to absorb the CO2 produced during the life of a 5O0MW coal-fired power station). Uncertainties about land availability, elasticity of price and security of CO2 storage raise questions.

BASE CASE ~PF ms NC_.CC

PLUS CAPTURE

]

PLUS COMPRESSION PLUS STORAGE 0

2

4

centCkW

6

S

10

Figure 2 Additional cost of capture, compression and storage More effective management of forests could sequester substantial amounts of carbon at relatively low cost. However, the long-term fate of forests planted specifically for this purpose needs further examination, as do the political aspects of establishing such forests in countries different from those emitting the CO2. The

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669

environmental impact of storing large quantities of CO2 is far from certain - how long the CO2 would stay in the deep ocean, the reaction between CO., and host rock in an underground reservoir and many other questions must be answered. Storage on a large-scale could be achieved without major developments in technology and would be less expensive than the CO2 capture step. As illustrated in Figure 2 the costs of storage are very small compared to the other costs involved. However, all storage schemes are site specific and involve environmental uncertainties. Utilisation of carbon dioxide Captured CO2 could be used commercially, as a feedstock from which to make chemicals. This offers the twin benefits of sequestering the gas as well as replacing other, manufactured feedstocks. CO2 is already used for a wide range of purposes in the food and oil industries although, in most cases, the gas is not permanently stored in the products but is quickly lost to the atmosphere. Ways of putting it to use include: • • •

as a feedstock for manufacture of chemical products for enhancement of the production of crude oil in growth of plants or algae (for use as a bio-fuei)

Income from selling the products would help offset the cost of capturing CO2. Significant costs are incurred in producing a chemical product and the amount of energy consumed is also significant; consequently, the net benefit of utilising CO2 is much less than the amount nominally contained in the product. Utilisation of captured CO2 as a feedstock for production of chemicals is an attractive concept as long as the additional energy required is small; this is a tough target to meet but this approach could be implemented quickly if the economics are right. Enhanced oil recovery (EOR) has the largest potential for utilising CO2 (Table 3) and is employed commercially in a number of oil fields. However, there is little economic incentive to use captured CO2 for this purpose, as most of the carbon dioxide used commercially is derived from natural sources or from a process where it is already being removed as a by-product. Direct use to grow algae in order to make bio-fuels might be viable but only in certain locations, and a similar conclusion has been reached about growth of plants to produce liquid fuels, currently an option of popular discussion. Indirect use such as short-rotation cropping of trees, to produce wood chip fuel, is attractive in some countries. Artificial biomass schemes based on production of algae are uncompetitive at present but potentially offer high rates of take-up of carbon. Overall, utilisation may help to remove CO2 from the environment and can make a contribution, but it is unlikely to solve the problem.

Table 3

Estimated potential storage capacities for C02

COz Utilisation options Enhanced Oil Recovery Biofixation - indirect Biofixation - direct Chemicals

Global capacity (GtC) 65 1.2 0.15

0.09

Conclusions

The assessment studies undertaken by the lEA Greenhouse Gas R&D Programme have shown that the concept of applying CO2 capture and disposal technologies to large stationary sources of CO2 such as power plant stacks is a viable method of reducing CO2 emissions from the combustion of fossil fuel. Applying the technology to power plant would increase present day electricity costs by at least 50% but the overall power generation costs still remain favourable compared to many renewable energy options. Most of the cost increase is associated with the capture technology and in particular with the parasitic energy demands of the process. Further R&D offers the potential of reducing these costs. Storage/disposal of CO: in the deep ocean, in aquifers and in depleted oil or gas reservoirs can be achieved at relatively low cost but further R&D effort is required to provide CO2-spocific data to establish the environmental credibility of each of the disposal options. The specific knowledge available on oil and gas wells and the experience of using CO2 for enhanced oil recovery, suggests that depleted gas reservoirs would be the first disposal option for a practical demonstration.

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Outstanding Issues and Future Activities

Tackling emissions of greenhouse gases by capture and sequestration of CO2 produced by power stations is feasible with a range of possible options. The cost can be high and major uncertainties remain about the consequences of adopting such technologies. Costs and performance can now be compared with other means of CO2 mitigation. Uncertainties about the technology have been reduced but key questions remain, such as: can the costs of CO2 capture be substantially reduced? What is the environmental impact and long-term security of CO2 disposal? Can chemicals be manufactured which would be significant net users of CO2? How do the full-fuel cycle costs compare with those of other mitigation options? To answer these questions requires practical investigations into sequestration, further examination of possible uses of CO2, and stimulation of novel ideas about combustion and capture. Through international collaboration, the feasibility of mitigating the effects of CO2 emissions has been established. Good understanding has been produced of the costs and implications of using such technology. Some major issues are outstanding and phase 2 of the IEA Greenhouse Gas P-,&D Programme, which has just begun, has been designed to answer these questions. An increased number of countries and organisations are now supporting this work. Future topics will include expansion of the studies assessing global impact, examination of the mitigation of other greenhouse gases, as well as further studies on capture, utilisation and disposal of CO2. Work is in progress on: methane emissions from anthrophogenic activities and has been separated into studies covering the coal, oil and gas industries, landfill, and other anthropenegic options. These studies are concentrating on technical approaches to methane mitigation options. In addition there is a study assessing the use of fuel cell applications, with fossil fuels, in a carbon dioxide free environment. Pre-combnstion decarbonisation and hydrogen production, chemicals utilisation, gasification and forestry are all subjects being covered within phase 2. References

1. Jack A R, Audus H, Riemer P W F. Energy Convers. Mgmt, 1992, 33,813-818 2. Webster I C, Audus H, Ormerod W G, Riemer P W F. "International Cooperation in Greenhouse Gas Mitigation" in the Second World Coal Institute Conference, London, March 1993 3. Skovoit O. Energy Convers. Mgmt. 1993, 34, 1095-1103 4. Seifritz W. "The terrestrial storage of CO2-ice as a means to mitigate the greenhouse effect." in: Hydrogen Energy Progress 1X (Pottier C D J, Veziroglu T N eds.), 1992, pp 59-68. 5. Ormerod W G, Webster I C, Audus H, Riemer P W F. Energy Convers. Mgmt. 1993, 34, 833-840 6. Wong C S, Matear R. Energy Convers. Mgmt. 1993, 34, 873-880 7. Koide H, Tazaki Y, Nochuchi Y, Nakayama S, Iijima M, Ito K and Shindo Y Energy Convers. Mgmt,1992, 33, 619-626. 8. Hendriks C A, Blok K. Energy Convers. Mgmt. 1993, 34, 949-957 9. Holt T, Lindeberg E. Energy Convers. Mgmt, 1992, 33, 595-602 10. Stegen G R, Cole K H. Energy Convers. Mgmt. 1993, 34, 857-864 11. Nakashiki N, Ohsumi T, Katano N. "Technical view on CO2 transportation onto the deep ocean floor and dispersion at intermediate depths", in The Second International Workshop on Interaction between C02 and Ocean, Tsukuba, Japan, 1993 12. Marchetti C. Climate Change, 1977, 1, 59-68 13. Drange H, Haugan P M. Nature, 1992, 357, 318-320. 14. Adams E E, Goiomb D, Zhang X Y, Herzog H J. "Confined Release of CO2 inti Shallow Seawater" in The Second International Workshop on Interaction between CO: and Ocean, Tsukuha, Japan, 1993 15. Gunter W D, Perkins E H. Energy Convers. Mgmt. 1993, 34, 941-948 16. Riemer, P. *International perspectives and the results of carben dioxide capture and disposal studies" The Second International Conference on Carbon Dioxide Removal, Kyoto, October 1994. Energy Conversion and Management, 36, p813-818, 1995. The conclusions reached and opinions expressed do not necessarily reflect those of the lEA Greenhouse Gas R&D Programme, its supporting organisations, its Operating Agent or the International Energy Agency, each of whom disclaims liability from the contents of this paper