Combustion properties of black liquors from alkaline pulping of wheat straw and reed canary grass.

Application: Understanding the changes in combustion properties and FTIR spectrum of nonwood black liquors during heating leads to better control methods for black liquor recovery.

Recovery boiler combustion of black liquor after evaporation to recover energy and cooking chemicals is becoming a necessary technique for nonwood pulping. However, due to the high viscosity of nonwood-based liquors, their heat treatment prior to combustion to reduce viscosity is necessary. This study evaluated the combustion properties of black liquors from soda-anthraquinone (AQ) pulping of wheat straw and soda-AQ and kraft pulping of reed canary grass as such, and after heating at 190[degrees]C for 30 min, in a laboratory furnace at 800[degrees]C by a single droplet technique. For each liquor, we also determined swelling properties and the duration of different burning stages, i.e., devolatilization and char burning. Finally, to develop rapid methods for predicting the combustion behavior of black liquor, we looked for adequate correlations between the FTIR (Fourier transform infrared) spectral data (indicating chemical composition) and the combustion properties of the studied black liquors.

Wheat straw and reed canary grass black liquors obtained from laboratory soda-AQ and kraft pulping showed similar burning behavior, which is also typical for the corresponding birch black liquors. For kraft pulping, the sulfidity used in delignification had a significant influence on the swelling properties of the formed black liquors. In the case of soda-AQ liquors the heat treatment, which is necessary for nonwood black liquors to reduce their viscosity prior to evaporation, resulted in a slight decrease in burning time, but had practically no influence on swelling. In addition, the results indicated that the primary combustion properties could be predicted by means of FTIR spectral data. Further work is needed, especially the detailed chemical analysis of these types of black liquors, to realize the potential of this method for characterizing the burning behavior of different nonwood black liquors.

Louhelainen and Alen are with the University of Jyvaskyla, Department of Chemistry, Laboratory of Applied Chemistry, P.O. Box 35, FIN-40014 University of Jyvaskyla, Finland; Zhinan Feng is with the Department of Chemical Engineering, McMaster University, 1280 Main Street West, Hamilton, Ontario L8S 4L7, Canada. Email Louhelainen at [e-mail], Alen at [e-mail].