1989

Mojtahedi, Wahab; Backman, Rainer

A combined cycle power generation system incorporating pressurised fluidised bed combustion (PFBC) or gasification (PFBG) is considered a promising approach for electricity generation using solid fuels such as peat. In these systems, the high pressure hot flue gas is expanded in a gas turbine. Peat contains sodium and potassium which are released by combustion and gasification.These are corrosive elements that can cause severe damage to the turbine blades if not suppressed.Multicomponent, multiphase equilibrium calculations were carried out for atmospheric and pressurised fluidised bed operating conditions to determine the distribution of sodium and potassium in the gas and condensed phases.Dependence of the alkali volatilisation on the operating temperature and pressure, and on the sulphur content, total alkali content and chlorine content of the feedstock was studied.The results show that the alkali release in the vapour phase could be much higher than acceptable to a gas turbine, particularly under gasification conditions.Hence, the necessity to remove the volatilised alkali metal compounds is more acute in gasification than in combustion.The results are also compared with the measured alkali content of pressurised fluidised bed combustor and gasifier effluent streams.Both sodium and potassium are present as chlorides and to a lesser extent as hydroxides in the gas phase in both modes of operation (i.e. combustion and gasification).However, whereas under combustion conditions both metals seem to condense as sulphates (Na2SO4 and K2S04), in gasification, chlorides and carbonates dominate in the condensed phase.The alkali metals volatilisation shows strong dependence on the operating pressure of the system as well as on the chlorine content of the feedstock.It decreases markedly with the former but increases sharply with the latter.