Experimental investigation on the effect of iron-rich coal ash on biomass-volatile combustion characteristics in the fluidized bed

The adoption of alternative bed material may effectively address issues in the biomass-fired circulating fluidized bed (CFB) boiler, such as uneven fuel-oxygen mixing and low combustion efficiency. This study presents a novel additive, iron-rich coal ash, which is expected to enhance the combustion performance of biomass-fired CFB boilers while may affect the transformation mechanism of volatiles and nitrogen oxides. In this paper, the Indonesian lignite ash was applied as the active bed material to test its effect on the combustion characteristics of volatiles yield from biomass in a lab-scale bubbling fluidized bed. Experimental results indicate that the addition of iron-rich lignite ash (54.4 % Fe2O3) can evidently improve the oxidation of volatiles, which is comparable to steel slag and ilmenite. By contrast, the other two bituminous coal ashes show weaker reactivity. Through composition analysis, the effect of alternative bed material is positively correlated with the iron content. In addition, increasing the additive substitution ratio can promote volatile combustion, but tends to saturate after reaching 50 %. Although reducing the particle size exhibits poorer mass transfer in bed, smaller ash particles can provide more reactive surface areas, thus the conversion rate of volatiles increases. A substitution ratio of 50 % and a bed material size of 150–200 μm are suitable options. Besides, with increasing temperature and excess oxygen coefficient, the combustion of volatile becomes more complete. However, for all cases, as combustion efficiency improves, there is an increase in NOx emission. That means the addition of iron-rich coal ash may bring negative effect in the NOx emission control, which should be noticed in the industrial application. In conclusion, this work offers important engineering recommendations for the large-scale utilization of solid waste coal ash and the enhancement of conversion efficiency in biomass-fired CFB boilers.