STRAW COMBUSTION IN CIRCULATING FLUIDIZED BED AT LOW-TEMPERATURE: TRANSFORMATION AND DISTRIBUTION OF POTASSIUM

摘要

Potassium in straw is relatively high and exists mainly as readily available potassium (K(r)). The release and transformation of the K(r) during biomass combustion mean a decrease or loss of potash fertiliser value. The purpose of this work is to quantify the transformation and the distribution of the K(r) under conditions that can obviously reduce the loss of it, and then to try to offer some suggestions for new biomass projects. A pilot-scale circulating fluidized bed (CFB) combustor (0.5 MW(th)) has been applied to combust rice straw. The combustion temperature was kept at a low level (lower than 700 degrees C) to reduce the transformation of potassium. The K(r) and the total potassium (K(t)) in the rice straw, the bottom ash, and the flyash were determined after chemical fractionation and wet acid digestion, respectively. The mass balance of the biomass ash, the K(r) and the K(t) in stable operation were quantified. The experimental results showed that the flyash accounted for more than 95% of the total straw ash. The flyash contained around 92% of the K(t) and 86% of the K(r) of the straw. In the flyash about 3% of the K(r) of straw was transformed to unavailable potassium (K(u)) during combustion. On the basis of the obtained results, four suggestions that reduce the loss of the K(r) are given for the better development of biomass-fired power plants.