Strengths and chemistry of filter cakes from pressurized fluidized bed combustion

Excessively hard filter cakes have occurred during filtration of fine parti cles from hot gases in pressurized fluidized bed combustion (PFBC) power pl ants. Hence, shear strengths were measured (at 22 degreesC) for cakes forme d from powders taken from the hot-gas filtration vessel of a PFBC demonstra tion power plant. The measurements were made both before and after heating at 750 degreesC (1 h) or at 950 degreesC (1or 3 h). Sample strengths were s ignificantly increased by the heat treatment at 950 degreesC, but not by he ating to 750 degreesC. In filter vessels operating at 750 degreesC, formati on of Mg2Ca(SO4)(3) from MgO, SOx, and CaSO4 is the main cause of the devel opment of excessive cake strengths. However, these reactions and the concom itant strength increases cannot occur in the laboratory, if plant material is heated in the absence of one or more of the reactants (MgO and/or SOx). At 950 degreesC and in the absence of coal ash, the formation of Mg2Ca(SO4) (3) occurs even more rapidly. However, at 950 degreesC, Fe2O3 and other com ponents of coal ash promote the decomposition of Mg2Ca(SO4)3. Hence, depend ing on the temperature, sorbent, amount of coal ash, and its composition, f ormation of Mg2Ca(SO4)(3) and/or sintering of CaSO4 may be the important ca use(s) of cake hardening.