Development and integration of acid precipitation based lignin biorefineries in Kraft pulping mills

摘要

The use of fossil fuel resources for manufacture of goods and services has dire consequences on the environment. To reduce the impact, the use of renewable raw materials and energy has been suggested, leading to a sustainable bioeconomy. The pulp and paper industry in North America is currently facing stiff economic conditions due to declining demand for traditional paper commodities, international competition and rising energy prices. The biorefinery technologies, which convert wood components to value added bio-based chemicals and products, can be a means for the pulp and paper industry to increase its revenue, diversify the product portfolio and become more sustainable in the long run. The industry is uniquely positioned to accommodate this task due to access to biomass, existing wood handling and transportation infrastructures and expertise in traditional wood products.;The extraction and conversion of lignin to value added products has been identified as one of the most profitable biorefining pathways in Kraft pulping mills. Extraction of lignin, a crucial step of the integrated lignin biorefinery, has a high degree of interaction with the receptor Kraft pulping mill. The importance of the interaction is further enhanced by the possibility of incremental pulp production with lignin extraction. For successful implementation of the lignin biorefinery in Kraft pulping mills, we should understand and address the interactions between lignin extraction process and receptor Kraft pulping mill. Furthermore, it is necessary to enhance the profitability and environmental performance of the lignin extraction processes by improving its operations and meeting the internal energy and chemical demands. In this thesis, the development and integration of acid precipitation process in Kraft pulping mills to improve its economic and environmental performance have been investigated.;In the first part of this research, the energy impacts of implementing the acid precipitation process on the Kraft pulping mill was evaluated. A representative Canadian softwood Kraft pulping mill have been used as the reference. Although, the acid precipitation process did not have considerable energy demands, the lignin removal reduced the recovery boiler energy production and increased the evaporator steam demand. The electricity production was also decreased due to reduced recovery boiler steam production. A total of 15% lignin extraction reduced the recovery boiler energy production by 13.5% and allowed an increase of pulp production capacity by 15% through recovery boiler debottlenecking. The energy deficits in the Kraft process was addressed by measures combining energy savings projects identified using Pinch analysis, biomass boiler energy production increase and biomass gasification combined cycle. It was shown that addressing the energy deficits internally was feasible, but requires high degree of energy integration.;In the second part of the work, the impact of integrating a lignin biorefinery on the chemical balance of Kraft pulping mill have been investigated. A modeling approach based on electrolyte equilibrium, which was proposed to estimate pH and chemical composition variations of the black liquor, have been used to simulate the acid precipitation process. A calculation sequence based on Na/S balances was utilized to calculate the chemical balances of the liquor cycle of the Kraft pulping mill. Balancing Na and S have been performed according to several strategies based on mill constraints and wash filtrate recycle. Complete disposal or excessive recycle of wash filtrates have been found to increase the make-up caustic requirement. The caustic demand could be minimized by recycling a part of most concentrated wash filtrates so that disruption of the Na/S balance was minimum. The efficiency of the washing setup was also found to affect the make-up caustic demand. The Na and S balancing strategy also affected the loads to the recovery cycle operations including evaporator train, recausticizing and lime kiln. Therefore, the bottlenecks in the other recovery operations have to be taken into account in selecting a strategy so that chemical demands are minimized and maximum pulp production increase is obtained.;In the third and fourth parts of this research, possibilities of reducing the costs of acid precipitation process have been investigated. As filtration and washing equipment represent a significant portion of capital costs in a lignin precipitation plant, the feasibility of improving lignin filtration by hydrodynamic optimization has been evaluated. The hydrodynamic conditions during lignin coagulation and flocculation were varied and the precipitated lignin particles were characterized in terms of particle and filtration properties. The turbulent shear rate was found to be the determining hydrodynamic parameter that controlled the size and shape of the precipitated lignin particles. More compact and larger particles, which could be obtained by hydrodynamic control, provided superior filtration and washing properties.;The systematic parameter optimization was used as means of improving the yield and filtration performance of the acid precipitation process. The experiments were performed using the Taguchi statistical design of experiments approach and the results were analyzed using signal-to-noise ratio and analysis of variance statistical methods. The black liquor solid content have been identified as the most critical parameter, which had the largest influence on both lignin yield and filtration resistance. The optimal parameters set for filtration performance, which produced lignin with lowest ash content of 0.09%, did not provide the highest lignin yield. Therefore, considering trade-offs between lignin purity and process economics would be required, when developing lignin precipitation processes for specific applications.;Lastly, the economic and technical feasibility of using lime kiln flue gases for black liquor acidification was investigated. The experimental results showed that it is feasible to obtain filterable lignin, however, the kinetics of the acidification was considerably slower. A simulation and economic analysis revealed that despite the elimination of CO2 cost, the lime kiln flue gas supply was less attractive economically than external CO2 purchase due to the gas cleaning requirements and increased size of acidification vessels. The sensitivity analysis showed that CO2 price and CO 2 demand of the process has to be considerably higher that current values for the use of flue gases to be economically attractive.