Effects of Reduced Bed Temperature in Laboratory- and Full-Scale Fluidized-Bed Boilers: Particle, Deposit, and Ash Chemistry

• Published in: Energy & fuels 2013-08, Vol.27 (8), p.4999-5007
• Main Authors: Jones, Frida, Niklasson, Fredrik, Lindberg, Daniel, Hupa, Mikko
• Format: Article
• Language: English
• Subjects: Ashes; Boilers; Deposition; Emission analysis; Flue gases; Fluidized beds; Sand; Superheaters
• ISSN: 0887-0624; 1520-5029
• DOI: 10.1021/ef400836e

This study focuses on how the temperature of the bed in a waste-fired fluidized-bed boiler affects the chemical composition of ash and deposits. The aim was to reduce the concentration of corrosive elements in the convection pass, which can lead to both less frequent soot blowing intervals and extended superheater lifetimes. Complementary laboratory-scale tests were carried out in a single-pellet reactor to study online alkali and Zn emissions during temperature changes, using an ICP-MS instrument. The full-scale study was based on full-scale experiments at a plant consisting of two 20 MWth fluidized-bed boilers firing a mixture of municipal solid waste and industrial waste. The boilers are normally operated at bed temperatures of about 870 °C. This normal operation was compared in this work to a test case in which the bed temperature was reduced below 720 °C by altering the air staging and flue gas recycling. The experimental work included collecting samples of fuel, ash, and particles under the two different sets of operating conditions. Furthermore, deposits on temperature-controlled probes were sampled upstream of the superheaters. By reducing the bed temperature, the sand consumption of the plant could be reduced by roughly 25%. The measurements showed that the amount of submicrometer particles decreased and the fouling rate on deposit probes was reduced by about 20%. The measured concentration of HCl in the flue gas increased as the bed temperature was reduced. This might be a consequence of the reduced formation of alkali chlorides. In addition, results from the laboratory-scale tests indicated a trend of reduced alkali emissions from the fluidized bed with reduced temperature, and thermodynamic equilibrium calculations confirmed the trends.