Towards Environmental Resilience in Pulp and Paper Manufacturing: Water Consumption and Carbon Dioxide Emission Reductions

Abstract

The objective of this research is to reduce freshwater consumption and carbon dioxide emission from the pulp and paper industry. Specifically, we investigated the feasibility of reducing freshwater consumption in the bleach section, one of the largest freshwater consumers in pulp and paper manufacturing by recycling white water from the paper machine. Then, we carried out a techno-economic analysis of solvent-based carbon dioxide capture from limekiln flue gas, the primary source of fossil fuel-based emissions from the pulp and paper industry. Windows General Energy and Material Balance Systems (WinGEMS) was used to develop a process simulation model of an integrated softwood line of a Kraft pulp mill for the bleaching section and the paper machine section capturing the material, water, energy, and the Non-Process Elements (NPEs). Steady-state equations for the Elemental Chlorine Free (ECF) bleaching sequence D0EOPD1EPD2 are included in the simulation capturing the effects of Brown-stock washers (BSW) and the extraction stage washers on the chemical consumption and the pulp brightness. The simulation model prepared can predict changes in the kappa number, brightness, and COD values with changes in the operating parameters. Industry and literature data were used to validate the ECF bleaching simulation model, and the difference was in the range of 0.2 to 5.5% for final stage brightness and the post-first extraction pulp kappa number, respectively. Then we simulated the effect of stepwise replacement of the freshwater used in the bleach washer showers with the white water on scaling tendency in the bleach section. The partition of calcium ions, the primary NPE in white water, between the fiber walls and the surrounding liquor was studied using the Donnan equilibrium model. The dynamic data exchange (DDE) feature in WinGEMS was used to link the Donnan equilibrium model with the bleaching section simulation. After simulating white water addition, the free calcium ions were calculated using the value of the distribution coefficient ‘λ’ and taking the inputs of the metal ion concentration from the ECF bleaching section of the WinGEMS model. The free calcium ions predicted by the Donnan equilibrium model were used to calculate the saturation index (SI) in the bleaching section. The maximum amount of white water recycled in the bleaching section without the scale formation is determined by the SI value sensitivity analysis. Of the three major sources of CO2 in pulp and paper manufacturing, the Lime Kiln is the only source of fossil fuel-derived CO2 and has the highest concentration of CO2. In this work, we, for the first time, performed a techno-economic analysis (TEA) of a monoethanolamine (MEA) solvent-based CO2 capture from the pulp and paper mill’s Lime Kiln section using the CAPCOST modular program. The flue gas specifications were obtained from published limekiln data of a theoretical pulp and paper mill. The process was simulated in Aspen Plus and linked to CAPCOST using a python script for the cost calculations. The CO2 capture cost estimates were compared to the only CO2 capture costs data available in the literature for limekiln flue gas. Comparing the cost breakdown between the published data and this study, the capital cost difference was found to be highest for the stripper and the compression and dehydration sections. We further examined the TEA and process flowsheet optimization by processing flue-gas using actual mill data from two different mills. A derivative-free optimization (DFO) solver was used to optimize the flowsheet and minimize the CO2 capture costs. Additionally, we analyzed possible steam integration from within the mills and explore potential in-mill CO2 applications and their impact on the total CO2 capture costs. After taking into account the steam savings and CO2 utilization, the total capture costs were -$2.5 per tonne of CO2 for Mill A and $2.6 per tonne of CO2 for Mill B.