Techno-economic analysis of integrated biomass gasification for green chemical production

Sammanfattning: Production of renewable motor fuels and green chemicals is important in the development towards a more sustainable society where fossil fuels are replaced. The global annual production of methanol and ammonia from fossil fuels is very large. Alternative production pathways are therefore needed to reduce emission of anthropogenic greenhouse gases and to reduce the fossil fuel dependency. Gasification of lignocellulosic biomass is one promising renewable alternative for that purpose. However, to be able to compete with fossil feedstocks, a highly efficient production of biomass-based products is required to maximize overall process economics and to minimize negative environmental impact. In order to reach reasonable production costs, large production plants will likely be required to obtain favourable economy-of-scale effects.Integrating large scale biofuel or green chemical production processes in existing pulp mills or in other large forest industries may provide large logistical and feedstock handling advantages due to the already existing biomass handling infrastructure. In addition, there are large possibilities to make use of different by-products. In chemical pulp mills, black liquor, a residue from pulp making, provides a good feedstock for the production of chemicals. It has previously been shown that investment in a black liquor gasification plant is advantageous regarding efficiency and economic performance compared to investment in a new recovery boiler. The potential production volume of green chemicals from black liquor is however limited since the availability of black liquor is strongly connected to pulp production. Increased chemical production volumes and thereby potential positive scale effects can be obtained either by adding other types of raw material to the gasification process or by increasing the syngas production by other gasification units operating in parallel. Several publications can be found regarding biomass gasification using one single feedstock and/or gasifier, but only a few consider cogasification of different fuels and dual gasification units. The overall aim of this thesis has therefore been to investigate technoeconomically the integration of biomass gasification systems in existing pulp and paper mills for green chemical production with the focus on creating economy-of-scale effects. The following system configurations were selected: i) a solid biomass gasifier that replaces the bark boiler in a pulp mill for methanol or ammonia production, ii) a solid biomass gasifier operated in parallel with a black liquor gasifier for methanol production, and iii) methanol production from gasification of black liquor blended with biomass-based pyrolysis oil. The main objectives were to find possible and measurable technically and economically added values for different integrated system solutions.The gasifier, the gas conditioning and synthesis were modelled in the commercial software Aspen Plus for material and energy balance calculations. A thermodynamic model developed for gasification of black liquor was used to simulate co-gasification of black liquor blended with pyrolysis oil. The outputs served as inputs for the process integration studies, where models based on Mixed Integer Linear Programming (MILP) were used. An iterative modelling approach between the two models was adopted to ensure that all constraints of the pulp and paper mill as well as for the gasification plant were met. The resulting material and energy balances were used to analyze the different system configurations in terms of overall energy efficiency and process economics. The results show that replacing the recovery or bark boiler with a biomass gasifier for green chemical production improves the overall energy system efficiency and the economic performance compared to the original operation mode of the mill and a non-integrated standalone gasification plant. Significant economy-of-scale effects were obtained when co-gasifying black liquor and pyrolysis oil. This adds extra revenue per produced unit of methanol compared to gasification of pure black liquor, even for pyrolysis oil prices that are considerably higher than projected future commercial scale production costs. Ingeneral, methanol sold to replace fossil gasoline showed good investment opportunities if exempted from taxes. Ammonia produced via gasification of lignocellulosic biomass is per unit of produced chemical significantly more capital intensive than methanol. The investment opportunity of the ammonia configuration is therefore diminished in comparison to methanol production.The main conclusion is that production of green chemicals via biomass gasification integrated in a pulp and paper mill is advantageous compared to stand-alone alternatives. Highest efficiencies and economic benefits are obtained for the systems where co-utilization of upstream (air separation unit) as well as downstream process equipment (gas conditioning units and synthesis loop) is possible.