FBC in boilers can be particularly useful for high ash coals, and/or those with variable characteristics although PFBC has also been used on a commercial scale in Sweden and Japan with traded coals of higher quality. It is used with a combined-cycle system incorporating both steam and gas turbines. Considerable effort has been devoted to the development of PFBC during the 1990s, and the other demonstration units were in Germany, Spain and the USA.
FBC in pressurized boilers can be undertaken in compact units, and can be potentially useful for low grade coals and those with variable characteristics. As with atmospheric FBC, two formats are possible, one with bubbling beds, the other with a circulating configuration. Currently commercial-scale operating units all use bubbling beds, and hence the acronym PFBC is normally used in the literature to refer to pressurized bubbling bed units. A pressurized circulating fluidized bed combustion (PCFBC) demonstration unit was considered, but no gas turbine was available for the combined cycle configuration.
In PFBC, the combustor and hot gas cyclones are all enclosed in a pressure vessel. Both coal and sorbent have to be fed across the pressure boundary, and similar provision for ash removal is necessary. For hard coal applications, the coal and limestone can be crushed together, and then fed as a paste, with 25% water. As with atmospheric FBC (CFBC or BFBC), the combustion temperature between 800-900°C has the advantage that NOx formation is less than in PCC, but N2O is higher. SO2 emissions can be reduced by the injection of a sorbent, and its subsequent removal with the ash.
Units operate at pressures of 1-1.5 MPa with combustion temperatures of 800-900°C. The pressurized coal combustion system heats steam, in conventional heat transfer tubing, and produces a hot gas supplied to a gas turbine. Gas cleaning is a vital aspect of the system, as is the ability of the turbine to cope with some residual solids. The need to pressurize the feed coal, limestone and combustion air, and to depressurize the flue gases and the ash removal system introduces some significant operating complications. The combustion air is pressurized in the compressor section of the gas turbine.
The proportion of power coming from the steam:gas turbines is approximately 80:20%.
PFBC and generation by the combined cycle route involves unique control considerations, as the combustor and gas turbine have to be properly matched through the whole operating range. The gas turbines are rather special, in that the maximum gas temperature available from the FBC is limited by ash fusion characteristics. As no ash softening should take place and alkali metals should not be vaporised (otherwise they will recondense later in the system), the maximum gas temperature is around 900°C. As a result a high pressure ratio gas turbine with compression intercooling is used. This is to offset the effects of the relatively low temperature at the turbine inlet.
Heat release per unit bed area is much greater in pressurized systems, and bed depths of 3-4 m are required in order to accommodate the heat exchange area necessary for the control of bed temperature. At reduced load, bed material is extracted, so that part of the heat exchange surface is exposed.
The current PFBC demonstration units are all of about 80 MWe capacity, but two larger units have started up in Japan at Karita and Osaki. These are of 360 and 250 MWe capacity respectively, and the Karita unit uses supercritical steam. (Their size is tied to the capacity of the gas turbine).
PFBC units are intended to give an efficiency value of over 40%, and low emissions, and developments of the system using more advanced cycles are intended to achieve efficiencies of over 45%.
Flue gas cleaning/emissions
Combustion takes place at temperatures from 800-900°C resulting in reduced NOx formation compared with PCC. N2O formation is, however, increased. SO2 emissions can be reduced by the injection of sorbent into the bed, and the subsequent removal of ash together with reacted sorbent. Limestone or dolomite are commonly used for this purpose.
The residues consist of the original mineral matter, most of which does not melt at the combustion temperatures used. Where sorbent is added for SO2 removal, there will be additional CaO/MgO, CaSO4 and CaCO3 present. There may be a high free lime content and leachates will be strongly alkaline. Carbon-in-ash levels are higher in FBC residues that in those from PCC.