In order to reduce CO, emission, ELSAM (the electric utility company of the western part of Denmark) is looking into the possibilities for using biomass - mainly straw - for combustion in high-efficiency power plants. In this connection ELSAM has investigated 3 ultra supercritical boiler concepts for combustion of straw only or together with coal: 1) pulverized fuel boilers (PF-boilers); 2) circulating fluidized bed boilers (CFB-boilers); and 3) vibrating grate boilers with 100% straw. These investigations have mainly been full-scale tests with straw fed into existing boilers. Corrosion tests have been performed in these boilers using temperature regulated probes and in-plant test tubes in existing superheaters. The corrosion has been determined by detailed measurements of wall thickness reduction and light optical microscopic measurements of the material degradation due to high temperature corrosion. Corrosion mechanisms have been evaluated using SEM/EDX together with thermodynamic considerations based on measurements of the chemical environment of the flue gas. Great differences were found in the corrosion mechanisms for superheaters in PF-boilers and the CFB-boilers fired with almost the same share of straw and at the same metal temperatures.
Keywords: Biomass, straw, combustion, co-combustion, corrosion, PF-boiler, CFB-boiler, vibrting grate boiler, ultra supercritical power plants