Modeling and Optimal Design of Biomass Torrefaction Process

Abstract

Although the potential of biomass is immense as fuel, raw biomass has poor properties such as low energy density, hydrophilicity and high grinding energy requirement. Torrefaction is thermal pretreatment technology to enhance the biomass property. It is called as a mild pyrolysis occurring in the temperature range of 200–300°C and at atmospheric pressure in the absence of oxygen. Biomass torrefaction has been known as a feasible method to convert the biomass feedstock into attractive solid fuel which is utilized for combustion and gasification. Moreover, the torrefied biomass has high energy density, hydrophobic property, high grindability and a lower oxygen to carbon ratio. However, the economical infeasibility of torrefaction process is major difficulty in commercial production of torrefied biomass. In order to overcome the difficulty in economics, structural improvement of process design and optimization of design variables are required. Torrefaction reactor model is needed for the development of reactor and process designs for biomass torrefaction. In the thesis, a one-dimensional reactor model was developed based on the kinetic model describing volatiles components and solid evolution and the thermochemical model proposed by Bates and Ghoniem [1, 2] considering heat and mass balance. The developed reactor model used the temperature and flow rate of the recycled gas, which can be used as the practical manipulated variables instead of the torrefaction temperature, under operating conditions based on the process scheme proposed by Bergman et al. [3]. The temperature profiles of the gas and solid phase were generated, depending on the practical thermal conditions, using developed model. Moreover, the effects of each selected operating variables on the parameters of the torrefaction process and effect of whole operating variables with particular energy yield were analyzed. Through the results of sensitivity analysis, it is represented that the residence time insignificantly influenced to energy yield when the flow rate of recycled gas is low. Moreover, higher temperature of recycled gas with lower flow rate of recycled gas and residence time produces the attractive properties, including HHV and grindability, of torrefied biomass when the energy yield is specified. Using the developed model, the optimization of operating variables in the basic process design was carried out. For the formulation of optimization problem, the assessment method to evaluate the torrefaction process was analyzed and chosen. In order to develop the objective function of optimization problem, the economic evaluation model was made based on reasonable assumptions. It includes the capital cost of main facilities and operating cost of natural gas and electricity. To enhance the basic process design, the drawbacks in the base case reactor was analyzed and found several opportunities to improve the process efficiency. Based on the opportunities to improve the process, three process alternatives was proposed. The operating variables of process alternatives were optimized and compared to propose the optimal process design of biomass torrefaction.