Coal gasification, by which coal is converted into a fuel gas rich in hydrogen and carbon monoxide, has been undertaken on an industrial basis for over two hundred years. The oil crises of the early 1970s prompted a renewed interest in advanced coal utilisation technologies as an alternative to increasingly expensive oil supplies. One technology, integrated (coal) gasification in a combined cycle (IGCC), offered the promise of generating power from coal at high efficiency with low emissions and this sparked research and development activity aimed at demonstrating and commercialising these plants.
IGCC has today reached a status where experience is available from first and second generation plants, built in the 1970s/1980s and in the 1990s respectively, as commercial-scale demonstration plants for coal-based applications. These plants feature variations on gasification technology and subsequent environmental controls and in operating them a number of technical and commercial lessons have been learned that will help to improve the next generation of IGCC projects.
This report reviews and summarises the state-of-the-art and operating experience of several commercial IGCC plants worldwide, setting out the experience gained and the plans for future development, embracing such issues as the changes or modifications to plant made to overcome the operational problems and to improve the reliability and availability of the plant. Since IGCC is considered a ‘capture ready’ technology for CO2 abatement, the current status with regard to the incorporation of carbon capture and storage systems (CCS) has been reviewed. The report also outlines the issues associated with assessing the risks associated with commercialising IGCC plant.
The IGCC plants described in the study have accumulated many thousands of hours operation on a range of coal and, in some cases, co-fuel feedstocks. The plants feature important differences in technology, especially in the selection of gasifiers, but also commonalities. The one development that runs across all IGCC operations is the development of high performance gas turbines for the utilisation of syngas. Unlike natural gas which mainly consists of CH4, the combustible components of syngas from coal are mostly H2 and CO. The combustion properties of H2 and CO are quite different from those of CH4 and the high flame speed, high flame temperature and wide flammability range of H2, along with low ignition energy and low density, may cause blowout and flashback.
Future IGCC plants will require CO2 reduction and a typical decarbonised syngas with 90% CO2 removal contains over 90% H2. More diluent is required for NOx reduction, depending on several factors such as the type of gasifier, heat recovery and air separation unit. This affects the pressure ratio of the gas turbine with restrictions on compressor performance. Decarbonised syngas may require derating of the turbine firing temperature. The thermal barrier coatings may be adversely affected. Zero air integration, which reduces operating complexity, may be advisable for a plant, which is carbon capture ready. Various configurations for CO2 reduction in IGCC for hard coal result in a wide range of relative losses in the net plant efficiency. The effect of converting to H2 firing on performance of an IGCC which had been initially optimised for syngas is currently uncertain.
IGCC plant is inherently a high efficiency technology from its use of a closely-coupled combined cycle generation, but the complexity necessary for this configuration can also introduce problems with plant availability. IGCC availability is therefore, perhaps, the most important technical issue governing the success or failure of this plant. All the plant studied show a similar trend in ‘getting to grips’ with plant operation and maintenance issues.
Clean coal and pulverised coal
Many of the improvements described arise from a careful study of the performance of the plant and the reasons for failure. The improvement of plant has primarily been undertaken by the plant operators, but the existence of a considerable body of published information and regular systems of information of exchange such as the Freiberg and Gasification Technologies Council Conferences means that developments can benefit from cross-fertilisation. The Nakoso IGCC has demonstrated an impressive accumulation of operating experience in a very short time and this may be due to learning lessons from earlier IGCC plant. However, despite the impressive technical gains that have been made, IGCC still appears to be less attractive to investors than conventional pulverised coal combustion plant.
Specifically there are few incentives for ultra-clean technology, such as IGCC, since new pulverised coal fired plants have reduced emissions and in most locations can meet the current environmental standards. Importantly, pulverised coal is a very well-known and accepted technology in the power industry and most of the new coal plants that are being developed worldwide are pulverised coal fired power plants, with high efficiency cycles and emissions control technologies. A large number of IGCC projects have been announced, offered for sale, and then failed to proceed: this alone has created some uncertainty in the minds of potential IGCC customers. Specifically in the US, there is currently an excess of generating capacity in many US regions brought about by the building of natural gas fired combined cycle plants in 1999-2003 and a general expectation of continuing low natural gas prices from the widespread adoption of shale gas fracking technology. This has been exacerbated by the extended economic downturn with little growth, and a limited need for new power plants.
Elsewhere, countries with large and strategically important coal deposits are continuing to develop IGCC plants. In April 2012, China’s first coal-gasification power plant opened in Tianjin. The facility, known as GreenGen, is the world’s largest integrated gasification combined cycle generator and is also the first plant built explicitly as a test bed for capturing carbon. The future prospects for IGCC will depend heavily on the performance of plant such as this, and the way in which alternative energy sources such as shale gas develop.
The full report ‘Recent operating experience and improvement of commercial IGCC, CCC/222, by Ian Barnes, is available from the IEA CCC website www.iea-coal.org.
Read the article online at: https://www.worldcoal.com/coal/23092013/global_development_of_igcc_use_67/