The ash chemistry in fluidised bed gasification of biomass fuels. Part I: predicting the chemistry of melting ashes and ash–bed material interaction

Abstract

This paper is part I in a series of two describing the modelling of the ash-chemistry of seven biomass fuels under reducing, pressurised conditions in fluidised bed gasification by means of thermodynamic multi-phase multi-component equilibrium (TPCE) calculations. The fuels considered were Salix, a Scandinavian forest residue, Miscanthus, Reed Canary Grass, Eucalyptus, Arundo Donax and Lucerne. The composition and amount of phases have been calculated for the gasification of the fuel as such and in presence of an excess amount of calcite, dolomite, magnesium olivine sand and sand by using TPCE calculations in a temperature interval of 600–900°C and a pressure of 10 bar. It was found that interaction of inorganic compounds released from the fuels with bed material is a prerequisite for the formation of bed agglomerates. The presence of an excess of dolomite decreased the amount of alkali components in the bed, thereby increasing the amount of alkali components volatilised. A silica bed, however, binds most alkali released from the fuel, retaining it in the bed as low melting alkali silicates. The chances of experiencing operating problems due to bed agglomeration may increase hereby significantly. Calculations at atmospheric pressure show that the amounts of melt present will be smaller when compared to pressurised conditions, thereby decreasing the chances of bed agglomeration. In a pressurised gasifier using calcite or dolomite as bed material a small amount of an alkali carbonate rich melt can be expected at temperatures above 620°C with each of the seven biomass fuels fired. In silica-rich cases such as when firing Miscanthus, Reed Canary Grass, Arundo Donax or using a Si-rich bed material a melt can be expected at temperatures above 770°C. The amount of melt is rather high, i.e. 12–100% of the original ash formed. In the case of a magnesium olivine sand bed an alkali melt can be expected at 620°C. At temperatures above 800°C, a silicate melt can form as well. The amount of melt was high, i.e. 60–300% of the original ash formed, showing a significant contribution of the bed material.