The emissions from co-firing of biomass and torrefied biomass with coal in a chain-grate steam boiler

In this study, biomass of rice straw (RS) and wood (WD) and their torrefied biomass (RS_T and WD_T) were used as solid biofuel (SBF) for co-firing individually with coal in a commercial continuous chain-grate steam boiler system, which was conducted at fixed input rate of heating value of mixture of SBF and coal and at fixed airflow rate. The effects of key system parameters on the gaseous and particulate pollutions and ash were examined. These include SBF type and blending ratio (R_BL) of biomass (i.e., SBF) in the mixture of coal and biomass based on heating values for co-firing.The results indicated that wood, which possesses high heating value while less amount of ash, is more suitable for co-firing with coal than rice straw. Torrefaction can increase the heating value of biomass and homogenize its property, being beneficial to co-firing. Also, torrefaction can decompose the hydroxyl group of biomass, which makes biomass tending to possess hydrophobicity. This, in turn, helps the storage and transportation of biomass. Generally, adding the RS (with R_BL = 5-10%), WD (2-15%), RS_T (2-10%) and WD_T (2-20%), respectively, with coal decreases the emissions of NO_x and SO₂, but increases that of CO (except RS_T). The emission of HCl is little. The addition of biomass also increases the emission of fine particulate matters (PM) especially PM_2.5 in the flue gases, raising PM_2.5/PM₁₀₀ from 34.87 to 78.35 wt.% (Case 50%WD_T). These emissions for the Cases tested satisfy with Taiwanese emission standards of stationary sources which set limitations of NOx, SO₂, CO and HCl < 350, 300, 2000 and 80 ppmv, while PM < 50 mg/m³, respectively. The results support the use of RS, WD, RS_T and WD_T for co-firing with coal.Implications: This study examined the suitability of using solid bio-fuels to co-fire with coal in an industrial chain-grate steam boiler system with a capacity of 100 kW, in order to achieve carbon-free emissions. Both biomass and torrefied biomass of solid bio-fuel were tested. The findings would be useful for proper design and rational operation of solid bio-fuel/coal co-firing combustion matching the appeal of sustainable material management and circular economy of biomass, and of adaptation of global warming induced by greenhouse gases. It also provides information for policy-makers to promote the co-firing application of biomass and related bio-waste materials.