Torrefied biomass fuels as a renewable alternative to coal in co-firing for power generation

摘要

This study aims to assess the torrefaction of biomass as an alternative renewable energy fuel to coal during co-firing. It was evaluated that torrefaction improves biomass grind ability to such an extent that it can be used in coal mills with coal in co-firing without capital intensive modification. Torrefaction of beech wood was performed on a batch-scale reactor at three different temperatures (200°C, 250°C, and 300°C) with 30 min of residence time. The chemical structural changes in torrefied biomass were investigated with binding energies and FTIR (Fourier transform infrared) analysis. Monocombustion and co-combustion tests were performed to examine the combustion behaviour regarding flue gas emissions (CO, NO_x, and SO_2) at 0.5, 1.5, and 2.5 m distance from the burner opening along with fly ash analysis. The FTIR and binding energies showed that lignin hardly affected during light torrefaction, while the hemicellulosic material was significantly depleted. The Hardgrove grindability index (HGI) was calculated with three methods (DIN51742, IFK, and ISO). The medium temperature torrefied biomass (MTTB) yielded HGI value in the range of 32-37, which was comparable with HGI of El Cerrejon coal (36-41). A slight change in temperature enabled the torrefied beech wood to be co-milled with coal without capital intensive modification and improved grindability. Regarding the combustion behaviour of single fuels, low temperature torrefied biomass (LTTB) produces a less amount of NO_x (426 mg/m~3), CO (0.002 mg/m~3), and SO_2 (2 mg/m~3) as compared MTTB and raw beech wood. In the case of co-combustion, it was found that blending of coal with raw biomass does not show a stable behaviour. However, premixing of 50% of coal with 50% of torrefied biomasses (MTTB and LTTB) gives most stable behaviour and reduces NO_x almost 30% and SO_x up to almost 50% compared to coal. The fly ash content analysis proved that K_2O content decreased much during co-firing of coal and torrefied fuels, which could cause ash-related issues during combustion of raw biomass.