(707a) Capture and in-House Utilizations of Carbon Dioxide in Pulp and Paper Manufacturing

In pulp and paper manufacturing, the majority of carbon dioxide (CO₂) released is of biogenic origin. In a Kraft pulp mill, the recovery boiler, the bark boiler and the limekiln are the largest sources of carbon dioxide. The recovery of quick lime [CaO] from lime mud [Ca(OH)₂] takes place in a rotary limekiln where the lime mud is introduced uphill, feed end and slowly makes way to the downhill end. A burner on the downhill side burns the fuel. Carbon dioxide is released from the limekiln because of the calcination of calcium carbonate to CO₂ and the combustion-related reactions. As a result, limekiln has the highest concentration of CO₂ as compared to other flue gases from the recovery boiler and the bark boiler. Carbon dioxide, depending on the mill-specific details such as the chosen practices and the raw material, also has many applications in the pulp and paper manufacturing. The use of CO₂ for precipitated calcium carbonate (PCC) as a filler material for white paper mill is well established. Apart from PCC, CO₂ can be used in the tall oil manufacturing, in the lignin manufacturing, for pH control in the brown stock washing and effluent treatment, the stock preparation and near-neutral bleaching.

This research work focuses on the capture of CO₂ from the limekiln flue gas using the conventional Monoethanol Amine (MEA) solvent absorption process and the in-mill application of CO₂. A process simulator, ASPEN Plus, is used to model the carbon dioxide capture process. The CO₂ capture rate and the purity used in the simulation is adjusted to meet the CO₂ amount required for in-mill applications. We performed a preliminary techno-economic analysis (TEA) for the CO₂ capture process. Our results match the results presented in the literature: The equipment costs in the absorption setup are shadowed by the operating costs, which include the reboiler steam, cooling water and electricity. We further coupled the CO₂ capture process simulation with a derivative-free optimization (DFO) tool to identify the overall optimal configuration by minimization of the total capture cost. The absorber stages, stripper stages, economizer temperature, solvent lean loading and the MEA weight concentration of the lean solvent were the decision variables. The process configurations suggested by the DFO revealed that the carbon dioxide capture cost is minimum for a process with three absorber stages, eleven stripper stages, and which is run at an economizer temperature of 98 °C, solvent lean loading of 0.215 with a 46.92 wt% MEA concentration. The corresponding CO₂ capture cost for this process is $38.85 per ton of CO₂ captured.