Prediction of the Combustion Behavior of Black Liquor by Infrared Spectroscopy

Combustion behavior (swelling during combustion and combustion time) of the kraft black liquors varies significantly between pulp mills and also time-dependent variations within a mill occur. In addition, due to the different raw materials and pulping conditions, some further variations in these combustion properties are typically observed. Therefore, by means of the proper process control and optimization, it would be beneficial if these characteristic changes could be rapidly detected from black liquor prior to its combustion in the recovery boiler. In this paper, the possibility of applying infrared spectroscopy (utilizing mid-infrared range, MIR) to predict the combustion behavior of softwood black liquor was studied. INTRODUCTION FTIR spectroscopy has been shown to be a versatile method for indicating different chemical bonds and thus different functional groups in wood and pulp samples [1-4]. For this reason, with respect to the topics related to chemical pulping, there are several FTIR-based techniques for measuring, for example, the kappa number of pulp and, on the other hand, the contents of lignin and carbohydrates in wood and pulp. Since there exist correlations between the chemical composition and the combustion properties of black liquor [5-8], it could be further concluded that correlations between these properties and, for example, easily detectable FTIR data offer an interesting possibility of developing a proper method for characterizing also the combustion behavior of black liquor. The purpose of this study was to find out one suitable way to predict the combustion behavior of black liquor with different chemical compositions. In this study, FTIR spectroscopy, as a rapid analysis technique, was selected to obtain composition data on industrial softwood black liquors. EXPERIMENTAL The black liquors studied comprised of industrial softwood black liquor from kraft pulping treated with different conditions [9]. The single black liquor droplet combustion technique developed by Hupa and coworkers [10] was applied in combustion experiments. In this study, the overall duration of the combustion stages of black liquor is expressed for each liquor as a term “combustion time”, which is Corr. Coeff.: 0.896 RMSEC: 0.0184 0.85 0.9 0.95 1 1.05 0.85 0.9 0.95 1 1.05 Measured combustion time, s/mg Pr ed ic te d co m bu st io n tim e, s/ m g Corr. Coeff.: 0.937 RMSEC: 2.06