Combustion behaviors of lignin-lean black liquor and lignin

For kraft pulp mills that have thermally limited recovery boilers, lignin removal from black liquor has become an attractive option for increasing pulp production by allowing more black liquor to be processed through the boiler. This study systematically examined the combustion characteristics of lignin-lean black liquor and precipitated lignin from three kraft mills using a thermogravimetric combustor. The results confirm that adding lignin-lean black liquor to its original black liquor decreased the heating value and the degree of swelling of the mixed liquor. The effect on liquor swelling, however, was insignificant for mixed liquors that contained less than 20 wt% of lignin-lean liquor. As with other biofuels, the combustion of precipitated lignin was found to occur through three main stages: drying, volatile burning, and char burning. During the volatile burning stage, hardwood lignin swelled significantly, softwood lignin did not swell much, and mixed hardwood and softwood lignin was somewhere in between. Although the char content in lignin was about half of the volatile content, it took 10 times longer for the char to burn compared to the volatiles.