What are the costs and environmental impacts of IGCC (integrated gasification combined cycle) relative to PC (pulverised coal)? A recent report prepared for the Environmental Protection Agency (EPA) offers some answers, in the US context.
The report, by EPA subcontractor Nexant, Environmental footprints and costs of coal-based IGCC and PC technologies (EPA-430/R-06/006), attempts to give, it says, “a snapshot of conditions in the changing industry as of February 2006,” noting that, as of June 2006, there were some 24 IGCC plants proposed in the USA (according to http://www.netl.doe.gov/coal/refshelf/ncp.pdf).
The fuels and feedstocks for each type of plant studied in the report include bituminous, subbituminous, and lignite coals. The PC plant configurations include subcritical, supercritical, and ultrasupercritical boiler designs. A coal–water slurry feed type of gasifier (typified by the Texaco, now GE Energy technology) is assumed for the bituminous and subbituminous feedstocks. A solid feed gasifier (such as the Shell technology) is assumed with lignite. The technology options included in the IGCC and PC plant designs are restricted to those that are projected by the report’s authors to be commercially applied by 2010.
The authors observe that “development and implementation of the IGCC technology is relatively immature compared with the PC technology that has hundreds or thousands of units in operation globally.” While there are a number of gasification units installed at petroleum and chemical plants, there are only a very few installations using coal to generate power as the primary product. “Most of these IGCC installations were installed with government subsidies and have experienced technical and commercial problems common to the startup of new technologies,” say the authors. “While many of the problems with operability and maintainability have been mitigated,” they acknowledge, “successful application of the IGCC technology at additional commercial installations is needed to address any remaining concerns.”
Relatively little research or commercial work has been done to investigate gasification of low rank coals, including subbituminous and lignite, for electric generation purposes, the report says. The existing IGCC plants use bituminous coals as feedstocks. Almost four million tons of subbituminous coal was gasified at the Louisiana Gasification Technology Inc facility located at Dow’s Plaquemine, Louisiana, chemical plant under a Synfuels Corporation contract from 1987 to 1995. However, without additional research or commercial experience with the gasification of low rank coals, it is difficult to compare the gasification technology development with low rank coals to that of bituminous coal, the authors say.
The ultrasupercritical PC technology considered in the study exists in several installations in Japan and Europe and the thermal performance of plants using this technology may match or exceed IGCC performance, the report notes. However, as yet, there is no commercial experience with the technology in the USA, “therefore, for application in this country, the technology is considered unproven with potential technical and economic risks,” the report says.
Advanced technologies are also being developed to improve IGCC performance: new technologies for air separation and oxygen production, higher temperature gas cleaning methods, advanced gas turbines, and fuel cells. These technologies are being developed with the goal of raising thermal efficiency (HHV) to 50-60%. However, these advances are not likely to be accomplished in the 2010 timeframe of the study, the authors say.
Table 1 summarises the results of the performance estimates for IGCC and PC plants. The IGCC plant performance in particular can vary depending on design and site specific factors, and the estimates for IGCC plants using subbituminous and lignite coals are based on process models which were developed with limited test or other actual data. The ultrasupercritical plant performance is also estimated from modeling calculations and values found in the literature.
According to Table 1, the IGCC has significantly better thermal performance than the subcritical and supercritical PC plants in commercial operation in the USA. The estimates developed from limited data on ultrasupercritical technology show its thermal performance to exceed that of the IGCC for bituminous and sub-bituminous coal cases.
With the exception of controls for CO2, the control systems included in the report for reducing emissions of air pollutants from IGCC plants have been demonstrated at the two existing coal and petroleum-coke based US plants, and very similar systems are broadly used within the petroleum and chemical industries. The one remaining uncertainty appears to be the long-term continuous operational proof for the generation industry that the emission control processes/equipment will work in the IGCC power generation context. Such proof would involve the use of coal, which has physical and chemical properties that tend to be much more heterogeneous than refinery feedstocks, and the individual plant’s capability to generate baseload power without significant planned or unplanned interruptions. Partly this uncertainty is related to “the more general lack of information about IGCC system upsets, reliability, and a well-engineered definition of redundancy requirements,” the authors say.
Compared with PC plants, the IGCC more closely resembles a chemical plant than one for power generation. However, the power industry has incorporated and learned to use chemical processes for flue gas desulphurisation, ammonia-based selective catalytic NOx reduction processes, and a variety of water treatment and cleanup operations, “so operation of an IGCC plant by the power industry is possible.”
Based on the investigations conducted for the study, IGCC technology can offer environmental advantages over PC technologies for most emissions. In addition to the reduced air emissions from IGCC technology, the plants typically consume significantly less water and generate less solid waste in comparison with PC plants, depending on coal properties and whether or not the solid waste streams are sold as industrial byproducts.
Table 2 presents environmental impact estimates for the specific control technologies and coal types. The estimates are based on literature review, including recent air permits and related documents, contacts with certain potential suppliers of the control technologies, and power generation modelling software. In general, the estimates represent typical control technology capabilities, which, in many cases, reflect the levels determined through best available control technology reviews conducted during the processing of air permits for recent power plants. In some cases, such as subbituminous coal and lignite based IGCC plants, relevant air permit or operating data were not available. For these plants, information from other study sources, including vendor contacts, were used to develop the emission estimates.
The emissions and (in parallel) the removal capabilities are similar across the technologies and coals, with the clearest distinction being that IGCC emissions are less than for PC plants for all pollutants. The IGCC cases studied do not include SCR for the syngas turbines. MDEA amine type acid gas cleaning is used along with a system for sulphur recovery. The PC plants have wet limestone flue gas desulphurisation (WL-FGD) for the bituminous and lignite coals, lime spray dryer absorber (SDA) desulphurisation for the low-sulphur subbituminous coal, and all the PC plants have selective catalytic reduction (SCR) post-combustion NOx controls.
The coal characteristics and types of control technologies used for the study plants influence the estimates in Table 2. Changes in design assumptions can result in different estimates and new developments continue to take place for both the PC and IGCC technologies, the authors emphasise, with estimates subject to change in the future. The data in Table 2 also show the IGCC plants generating less solid waste than the PC plants. This comparison assumes that no waste is sold for industrial use, except for the relatively small amount of sulphur produced from IGCC. IGCC plants can also produce sulphuric acid as an alternative to sulphur, should the market conditions require this change.
All solid waste products from both PC and IGCC plants have varying degrees of potential for industrial use. Therefore, if it is assumed that these plants can sell some or all of their solid wastes, the differences between the amounts of solid waste generated as shown in Table 2 would either reduce or be eliminated.
The study investigations showed that while about 24% of the PC plants were able to sell the gypsum produced from the wet FGD systems in 2004, only 5% were able to do so for the SO2 wastes from the SDA systems. So, even though the industrial use of PC solid wastes is projected to increase in the future, it appears that a large number of such plants may not be able to sell their wastes. If an IGCC plant cannot sell its sulphur byproduct, it would have to be disposed of as a waste.
The study investigations included a comparison of major non-criteria and hazardous air pollutant emissions for the PC and IGCC technologies. In most cases, these emissions are heavily influenced by the concentration of impurities in the coal being used. Therefore, emissions of certain pollutants can vary over a wide range, depending on the coal characteristics.
Industry and government organisations have recently begun considering the application of SCR technology to reduce NOx from syngas-fired turbines at IGCC plants, the authors note, but suggest that industry is reluctant to install SCR units because of impacts on the overall operation, performance uncertainties and marginal cost. The study estimates a cost of $7290 to $13 120 per ton of NOx removed based on the difference between 15 parts per million by volume, dry basis (ppmvd) emissions with syngas dilution combustion controls, and three ppmvd after the SCR is added. The wide range of cost estimates results from uncertainty about the degree of sulphur control installation required to operate the catalytic NOx control technology.
The use of SCR with the coal-based IGCC synthesis gas-fired turbine combined cycle system has no commercial operating experience and is still evolving, which makes the evaluation difficult and necessarily limited to the present level of understanding and criteria defined for the study, the authors note. SCR performance and the quality of the synthesis gas going to the turbine are issues that are being continually examined to determine the limits of contaminants in the synthesis gas, especially sulphur, which causes fouling in the downstream heat recovery steam generator, the authors say.
The technology to remove sulphur from the synthesis gas and the removal requirement strongly impact costs and introduces the major uncertainty about cost estimates, the authors argue, while a second major economic uncertainty is the SCR catalyst life and replacement costs over time.
Also, SCR uses ammonia to reduce NOx emissions, and depending on how the SCR is operated some ammonia will be released to the atmosphere (“ammonia slip”) and is a pollutant. The methods for balancing NOx reduction and ammonia slip in the presence of sulphur in the flue gas and thus minimising total emission impacts are not yet well defined for IGCC plants. Despite the present uncertainties, and perhaps as an indicator of how future installations may develop, it is noted that the “reference” IGCC plant being engineered by GE Energy and Bechtel Corporation includes SCR. In addition, certain recently filed or amended IGCC permit applications propose use of SCR technology. These applications are not covered in the report, however, the authors say, since the information on the applications became available after the study investigations were completed.
Cost and availability
Cost and availability are areas of uncertainty for IGCC technology, the report’s authors say. Even given higher thermal efficiency and lower emissions, the cost and availability differences between IGCC and PC plants continue to be a major hurdle to commercial IGCC deployment, they observe. While the differences in cost estimates for PC and IGCC reported by several sources are not that great, less than $100 per kW in some cases, the actual cost disparities when it comes to IGCC demonstration facilities have been much greater. The IGCC estimates presented are for plants that assume commercial performance, and unfortunately the cost for the first generation of plants is bound to be more than for the “Nth plant,” the report notes. Similarly, the authors say, the availability of the currently operating IGCC plants has been around 80% (higher availability levels were achieved only by operating the combined cycle portion of the plant on natural gas or oil). But these plants were designed with single gasifier trains and “it is expected that the future commercial facilities, designed with a spare gasifier train, would achieve availability levels of 85 percent and higher. In comparison, the subcritical and supercritical PC can generally achieve greater than 90 percent availability levels.”
Capital and annual operating costs estimated for the plants are shown in Table 3. While the capital costs for IGCC plants are higher than the costs for all three PC plant configurations, there are only small differences between the operating costs for all plants. The risk and uncertainty issues surrounding performance estimates apply equally to cost estimates.
Only limited information is available from operating plants showing the impact of coal quality on IGCC and PC plants, the report notes, with even conceptual engineering studies much less available for IGCC plants using low rank coals than for plants using bituminous coal.
The costs reported in the study are derived from recent literature and experience with similar PC and IGCC studies conducted by Nexant. If the cost uncertainties are to be reduced, a more detailed engineering and design project would be required with site and technology specific criteria, the authors caution.
Carbon dioxide capture and sequestration
IGCC technology is enjoying renewed attention in the light of greenhouse gas issues and carbon management and the report concludes that “The currently available carbon management technologies for IGCC are much more cost effective than similar technologies for removing CO2 from PC plant flue gases.”
The major performance and economic impacts of applying these technologies to IGCC and supercritical PC plants for achieving 90% CO2 capture (or thereabouts) are reported in the table below.
This, says the report, “highlights the potential advantage for IGCC to capture and sequester CO2 at significantly lower costs than PC technologies.”
Tampa Electric’s Polk IGCC power station, one of only four coal based IGCC power plants in the world