Ash transformation by co-firing of coal with high ratios of woody biomass and effect on slagging propensity

Abstract The co-firing of coal with biomass is a promising method for reducing net CO 2 emissions from existing coal-fired power plants, as well as for the utilization of forest resources. This present study examined the effect of the co-firing of coal with woody biomass on the produced combustion ash and slagging propensity under different conditions representative of the wall furnace region of pulverized-coal boilers. The slagging tests were conducted in a drop-tube furnace by inserting a water–air-cooled deposition probe to the point where the inner furnace temperature was 1300 °C. Bituminous coal was mixed with up to 70% (energy basis) of four different Japanese woody biomasses, namely; sakura wood ( Prunus spp.), sugi ( Cryptomeria japonica ), nara (Japanese oak), and bark of sugi respectively. For comparison purposes, pure coal firing was also performed. The collected combustion ashes and ash deposits were characterized by computer-controlled scanning electron microscopy analysis while the slagging propensity was evaluated by determining the ratio of the deposited ash to the fuel ash, the so-called ash deposition ratio. The results showed no increase in the ash deposition ratio or significant change in the properties of the ash for co-firing using up to 50% of the low-ash (0.4 wt%) biomasses ( sugi and sakura ). In contrast, there was significant change in the properties of the ash and an increase in the ash deposition ratio with increasing biomass ratio for co-firing using the high-ash (>1.0 wt%) biomasses ( nara and bark of sugi ). The significant transformation of ash with regard to its morphology and chemical composition, together with the formation of eutectic calcium mineral mixtures in the deposit layer enhanced the slagging propensity during the co-firing process, particularly when using the high-ash biomasses. The use of a low-ash biomass was found to be suitable for preventing severe slagging during the co-firing process.