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UBC Theses and Dissertations

Kraft lignin as a fuel for the rotary lime kiln Richardson, Brian


For many kraft pulp mills the chemical recovery boiler, in which concentrated black liquor is burned, represents the principal obstacle to increased production. However, studies have shown that the removal of even a small portion of the lignin from the black liquor would permit an incremental increase in furnace capacity. Fortuitously, precipitation of lignin from black liquor followed by filtration, washing and drying yields a solid fuel which could be burned in the lime kiln. In order to test the suitability of the precipitated lignin as an alternate fuel, a 0.4 m ID by 5.5 m pilot rotary kiln was modified with a computer controlled screw feed system and a water-cooled lance in order to burn dry powdered lignin, with or without natural gas. In a series of trials, crushed limestone was calcined in experiments using three different kraft lignins at various levels of replacement for natural gas. Lignins precipitated from black liquor by both the mineral acid and the carbon dioxide process were burned successfully as the sole fuel, or in conjunction with natural gas, to yield a stable orange luminous flame. On a basis of constant total energy input to the kiln, comparisons are made of axial profiles of gas temperature, solid bed temperature and percent calcination as a function of the percentage of natural gas replaced by lignin. Gas and solids bed temperatures and percent calcination were found to be slightly higher at a given axial position in the kiln when burning lignin as compared to natural gas. All three lignins tested were found to produce similar results. Impurities such as sodium and sulphur, which were present in the powdered lignin, did not significantly affect the quality of the lime. All three lignins were able to produce fully calcined limestone with no detrimental effect on the slaking properties of the lime. No significant difference in reactivity and slaking times could be observed between the lime produced with natural gas and with 100% lignin firing. The lignin fuel was found to be more efficient at supplying heat to the solids bed when compared to natural gas and this was reflected in the axial calcination profiles. It was concluded that dry lignin could be an acceptable fuel in either partial or complete replacement of natural gas, without penalty in lime quality or kiln productivity. However, full scale trials will be necessary to confirm these results and to identify any potential long-term effects on white liquor or lime quality.

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