TOWARDS CLOSED-CYCLE KRAFT; ECF versus TCF Case Studies

摘要

Case studies are developed for closed-cycle mills following a design approach which initially considers effluent volume reduction and heat balance issues, then the chemical balance and recovery cycle issues, and finally the dynamic balance, control behaviour and economic optimisation. Water, energy and chemical balances are presented for closed-cycle cases in which alkaline bleach effluent is used in brownstock washing and acidic effluent is separately concentrated and added to the black liquor. Evaporator condensates are treated for reuse as process water supply. Both the chemical balances, and the water and energy balances are compared to a base case with no bleach effluent recycle, represented by a recent-design mill. The recent-design mill has fresh water consumption of 67.7 m~3/ADt, and an effluent flow of 61.9 m~3/ADt, with evaporative losses of 7.9 m~3/ADt. The closed-cycle case has a fresh water consumption of 9.1 m~3/ADt, zero liquid discharge, and evaporative losses of 11.2 m~3/ADt. Total energy release from the recent-design mill (excluding losses in dry flue-gases) is 34.8 GJ/ADt, electricity export is 3.8 GJ/ADt, evaporative cooling tower heat rejection is 14.1 GJ/ADt and heat loss with the effluent is 4.5 GJ/ADt. For the closed-cycle mill, total energy release is 36.4 GJ/ADt, electricity export is 3.6 GJ/ADt, and evaporative cooling tower heat rejection is 10.2 GJ/ADt. Chemical balances over the recovery cycle for the soluble elements sodium, sulphur, potassium and chloride are calculated for cases using elemental chlorine free (ECF) and totally chlorine free (TCF) bleach sequences on a softwood kraft pulp of kappa 14 entering the bleach plant. The effects of a variety of chemical purging techniques were modelled. Chloride concentrations in white liquor ranges from 10 and 20 g/L NaCl in the ECF cases and 0.2-5.0 g/L in the TCF cases. The potassium mole fraction in the liquor ranges from 0.9 mol% K/(Na+K) in the ECF case to 10 mol% in some TCF cases. The recovery boiler sticky ash temperature range in the base case was 715-760℃, in the ECF cases 515-720℃ and TCF cases 515-820℃. The effect on liquor compositions and recovery boiler ash chemistry were used to draw conclusions important in recovery boiler design.