COMBUSTION BEHAVIOURS OF LIGNIN-LEAN BLACK LIQUOR AND LIGNIN, 2014 PEERS Conference

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For kraft pulp mills where recovery boilers are thermally-limited, lignin removal from black liquor has become an attractive process option for increasing pulp production by allowing more black liquor to be processed through the boiler. This study systematically examined the combustion behaviours of lignin-lean black liquor and precipitated lignin from three kraft mills using a thermogravimetric (TG) combustor. The results show 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 swelling, however, was insignificant for mixed liquors that contained less than 20% lignin-lean black liquor on a dry basis. This implies that for a given mill, up to 20% of black liquor could be treated to extract lignin without altering the burning behaviour of black liquor. As with black liquor, the combustion of lignin was found to occur through 3 main stages: drying, volatile burning and char burning. During the volatile burning stage, hardwood (HW) lignin swelled significantly, softwood (SW) lignin did not swell much, while mixed HW/SW lignin was somewhere in between. Lignin that swelled more burned more quickly than lignin that swelled less. 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.