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.
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 planned, and there have been a number of pilot-scale investigations. It has possible advantages, in terms of reductions in unit size, but the work is currently at pilot scale. The demonstration plant was cancelled as no suitable gas turbine was available.
As with PFBC using bubbling beds, the combustor and cyclones are held inside a pressure vessel. The pressure used is 1-1.5 MPa with combustion temperatures of 800-900°C. 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. Hot gas filtration may be necessary before the flue gases are fed to the turbine, and this is an area of technology under development.
Approximately 15% of the power produced will come from the gas turbine and 85% from the steam turbine. 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.
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.
As indicated above, hot gas filtration may be needed before the flue gases are passed through the turbine.
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.