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

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.
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