Chlorine in Deposits During Co-Firing of Biomass, Peat, and Coal in a Full-Scale CFBC Boiler

Co-combustion of coal with biomass or firing biomass alone is increasingly used as a first step to meet the Finnish commitments under the Kyoto agreement. Fluidized bed combustors are commonly used when co-firing, however, even if FBC’s have a wide tolerance for different fuel qualities, co-combustion of biomass or firing biomass alone may lead to unwanted ash-related problems. A deposit measurement campaign was done, at the 550 MW_th biofuelled CFB in Jakobstad, Finland. During the campaign a total of 16 different fuel blends were burned. The deposits were sampled with air-cooled probes with detachable rings. The deposits were sampled at two different locations, one where the flue gas temperature was about 730°C (probe surface temp. 540°C) and the second where the flue gas temperature was about 530°C (probe surface temp. 350°C). From every deposit sample three elemental analyses were done — one from the wind side, one from the lee side, and one from an angle of about 50° from the wind side. The analyses were done with a SEM/EDX analyzer. The fuels used during the measurement campaigns were sampled and analyzed. In addition to proximate and ultimate fuel analysis so called fuel fractionation was applied. The fractionation method is based on selective leaching by water, ammonium acetate, and hydrochloric acid, consecutively. After each leaching step the solutions are analyzed for the most important elements. The method can be used to determine how the elements are bound in the fuel and how they may behave during combustion. The analysis results from the measurement campaign and from the advanced fuel analysis were combined and are reported in this paper, with emphasis on the fate of chlorine.