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

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Abstract
In this study, biomass of rice straw (RS) and wood (WD) and their torrefied biomass (RST and WDT) were used as solid bio-fuel (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 air flow rate. The effects of key system parameters on the gaseous and particulate pollutions and ash were examined. These include SBF type and blending ratio (RBL) 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 RBL = 5-10%), WD (2-15%), RST (2-10%), and WDT (2-20%), respectively with coal decreases the emissions of NOx and SO2, but increases that of CO (except RST). The emission of HCl is little. The addition of biomass also increases the emission of fine particulate matters (PM) especially PM2.5 in the flue gases, raising PM2.5/PM100 from 34.87 to 78.35 wt.% (Case 50%WDT). 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, RST, and WDT for co-firing with coal.

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