Fate of Alkali Metals during Co-Combustion of Biodiesel Residues with Coal in a Semi-Industrial CFB Boiler

Paper in proceeding, 2008

The use of biodiesel, especially of Rapeseed Methyl Ester (RME), has recently increased in the European countries. This gives rise to an increasing amount of after production residues. Rapeseed cake is the main residue of the RME production process, and may become available in large amounts for energy recovery. Since rapeseed cake contains high levels of alkali metals, it may cause bed agglomeration, deposit formation and corrosion upon combustion in fluidized beds. This paper presents the fate of alkali metals formed during co-combustion of coal and up to 25% of rapeseed cake pellets in a 12 MW Circulating Fluidized Bed Boiler at Chalmers University of Technology (Sweden). Standard fuel analyses combined with sequential leaching and SEM/EDX, showed the alkali entering the boiler. Gaseous emissions, bed samples, fly ash and deposits were analyzed in order to monitor the alkali metals distribution. Gaseous alkali chlorides were measured upstream from the convective pass at a flue gas temperature of 800 oC using an In-situ Alkali Chloride Monitor (IACM). In the same location, deposits were collected on a deposit probe fitted with rings for SEM/EDX analysis. Re-circulated bed material was analyzed as well. Two-day tests showed successful co-firing of up to 25% rapeseed cake on energy basis in the semiindustrial fluidized-bed boiler. Alkali metals were enriched in fly ash. No significant difference could be noticed in the distribution of alkali metals when increasing the share of rapeseed cake from 15% to 25% on energy basis. In the near future, further studies will be carried out to determine the influence of limestone on the performed tests.