Influence of chlorine and potassium on operation and design of chemical recovery equipment

Abstract

The main objective of this study is to evaluate the effect of chlorine (Cl) and potassium (K) on the operation and design of equipment in the chemical recovery cycle. Due to stricter environmental regulations, mill closure tends to increase, causing the accumulation of undesirable elements such as Cl and K in the kraft liquor cycle. Total inorganic solids in pulping liquors increase in the presence of Cl and K, which affects the operation of chemical recovery equipment. The objective here is to show the magnitude of these impacts and estimate operational cost differences by using different Cl and K contents in liquors. Pulp mill material and energy balances for each case are used for this purpose, and a modern Brazilian pulp mill served as a base case model. The results show that for one specific range, the solids content in black liquor can be 6.6% higher by increasing the mass percentage of Cl and K in black liquor. This difference reduces the black liquor higher heating value by 6.2% and increases the amount of dry solids to burn in the recovery boiler, also by 6.6%. The evaporation load increases along with steam consumption. This lowers total electricity output by up to 1.6 MW due to reduced flow to the condensing stage of the steam turbine. The balances also demonstrate that some pumping costs can be 12% higher when operating from a low to high concentration of Cl and K in black liquor.