RELEASE OF CARBON AND NITROGEN DURING PYROLYSIS OF REDUCED LIGNIN BLACK LIQUORS – EXPERIMENTAL RESULTS

Removing lignin from black liquor is of increasing industrial interest. It has the potential to create a new product stream for pulp mills as well as being a method for reducing the load of over-loaded recovery boilers. Currently, there is very little information on the combustion properties of reduced lignin black liquors. Of particular importance is the split of carbon and nitrogen. The carbon split is especially important because it plays an important role in how much carbon reaches the char bed. This in turn affects the lower furnace temperature, which plays an important role in reduction efficiency; sulfur and alkali release; and NO and cyanate formation. The nitrogen split is of interest because a part of the black liquor nitrogen precipitates with the lignin, potentially changing the behavior of black liquor N. The purpose of this work is to investigate the effect of lignin removal on carbon and nitrogen distribution between volatiles and char during pyrolysis.

In this work, black liquors (one SW and one eucalyptus) with different levels of lignin removal were pyrolysed in a single particle furnace for 2 seconds at temperatures from 900°C to 1100°. Additional softwood and hardwood liquors were also pyrolyzed in order to study liquor to liquor variability. The carbon and nitrogen splits were obtained by analyzing the chars from the pyrolysis experiments using a CHNS elemental analyzer. Results show a similar carbon split for liquors with up to 25% lignin removal. The nitrogen split was also found to be fairly consistent with approximately 45% of the nitrogen staying with the char. This information has partly been used in another study in the CFD modeling of the combustion of black liquor with different levels of lignin removal.