The impact of zeolites during co-combustion of municipal sewage sludge with alkali and chlorine rich fuels

Abstract

Municipal sewage sludge has proven to eliminate alkali metals and chlorine related problems during combustion of straw and refuse derived fuels (RDF). However, the mechanisms involved have not been clarified. The aim of this work was to gain more knowledge about the behaviour of sewage sludge and detergent zeolites in combustion and about their effects on alkali metal chemistry. Co-combustion tests with combinations of municipal sewage sludge, wood and straw were carried out in a 12 MW fluidised bed (FB) boiler. In addition, a detergent zeolite, Doucil A24, was used as additive during co-combustion of wood and straw. The chemical characteristics of fuels and fly ashes were studied using several methods, such as chemical fractionation and scanning electron microscopy with element analysis by energy dispersive fluorescence detection (SEM-EDX) and X-ray diffraction (XRD). In the co-combustion tests involving sewage sludge no KCl was found in the flue gas prior to the convection pass. The zeolite addition was less effective but the KCl concentration was reduced to some degree in favour of HCl compared to the reference case. Both SEM-EDX and XRD confirmed the presence of potassium-aluminium-silicates in the fly ash fraction in all cases. In addition, the laboratory study showed that Doucil A24 had the ability to capture potassium and KCl at temperatures in the range of 700-900°C.