(70) Therefore, the risk of sulphuric acid dew point attack ... - DTI Home

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of biomass fuels and the land area and fuel supply infrastructure required to support even a relatively small plant. There are a variety of approaches to biomass gasification including fixed bed, rotary kiln, pressurised circulating fluidised bed (CFB), atmospheric CFB and the Batelle process [72] . The air blown CFB, fixed bed and rotary kiln approaches produce a low calorific value (typically 5-6MJNm -3 ) ‘syngas’ while the Batelle process by excluding air and using steam as the gasifying medium produces a medium calorific value gas (~15 MJNm -3 ). The ‘syngas’ from all of these processes can be burnt in a GT or a reciprocating engine with exhaust heat recovery. If syngas is to be burnt in an engine, it must be cleaned first to remove particulates and in some cases ‘tars’ as well. Current gas clean up technology requires that the gas be cooled from the gasification temperature (typically 850 - 950°C) before filtration. There are therefore two HRSGs in the system, one to recover heat from the hot process gas prior to filtering and the second to recover heat from the engine exhaust. The ‘syngas’ cooler HRSG will be exposed to carry over of bed material and ash. Biomass fuels may be rich in alkali metals which increase the potential for tube fouling. This needs to be taken into account in the design. Very few BIG-CC projects have yet been successfully developed. If BIG-CC technology can be made to operate reliably and economically it will open up a new market for industrial scale HRSGs. 4.4.3 Microturbines In general, microturbines are unlikely to be coupled with HRSGs. The relatively low exhaust temperature (if recuperated as most are) and flow are not normally sufficient for economic steam generation at useful steam conditions. However, microturbine suppliers are working on scaling up their units. Bowman Power Systems expect to release a 200kWe unit soon and believe that microturbines up to 500kWe are feasible. At the larger sizes they are more likely to be coupled to HRSGs to provide steam flows for smaller consumers in some specialist applications. It is possible that they could be used for air pre-heating or provide heat for an LP circuit as part of a larger boiler system. 4.5 Conclusions • Future increases in HRSG operating conditions will largely be dictated by increases in GT exhaust temperature. • One area of significant interest is once through design. The main benefit of this technology at present is its suitability for flexible operation. In the long-term future, it should pave the way for supercritical cycles with even higher thermal efficiencies. • Another area of significant interest is the use of HRSG steam for GT blade cooling in the latest class of GTs. This presents significant challenges for HRSG design in attaining the high steam qualities required. (96)
• The use of HRSGs within IGCC plant is now approaching the status of commercial operation, although the costs still remain relatively high. • Other development areas include modular design to reduce build costs, improving reliability, and improving access. The latter two items address specific problems experienced by plant operators. Improvements in these areas are perceived to provide product differentiation in an extremely competitive market place. • Industrial scale HRSG technology is relatively mature. Most development comes from the integration of HRSGs within new processes, and the trickling down of technology from utility scale HRSGs. One exception is the use of once through technology which is already standard practice for one HRSG supplier. (97)
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Page 45 and 46: Detailed studies of CCTs within the
Page 47 and 48: knowledge, expertise and experience
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