Lignin can be precipitated from kraft black liquor (BL) through the addition of an acidifying agent such as carbon dioxide or sulfuric acid. In most of the existing lignin precipitation processes that are using acid addition, sufficient acid is added to drop the pH of the black liquor from about 13−14 to about 9−10, followed by lignin particle coagulation, lignin slurry filtration, and lignin cake washing with sulfuric acid and water. At pH values of less than 11, the potential exists for the generation of significant quantities of totally reduced sulfur (TRS) compounds and other volatile sulfur species. Such compounds which include hydrogen sulfide, methyl mercaptan, dimethyl sulfide, and dimethyl disulfide are strongly odorous compounds with well-known negative effects on human health and other forms of life. To address this problem, as well as other problems associated with existing lignin recovery processes, FPInnovations and Noram recently developed a new process called the LignoForce System. This process employs a black liquor oxidation step to convert TRS compounds present in kraft black liquor to nonvolatile species. This paper discusses the applicability of the LignoForce System to several feedstock black liquors (e.g., softwood, hardwood, and eucalyptus) as well as the sulfur compound outgassing potential from various stages of this process compared to a reference case in which the black liquor was not oxidized. In addition, the emission of volatile sulfur and organic compounds from the two lignin products at different temperatures is discussed and compared.

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.