(54b) Process Synthesis and Economical Evaluation of Torrefied Wood Pellet Production Processes

Torrefied wood pellet is a solid biofuel derived from forestry, agriculture, or industrial woody residues. The torrefied wood pellets, with properties close to coal [1], can be potentially used to replace coal for power generation. The design and optimization of the wood pellet production process can help save energy consumption, reduce CO₂ emissions, and increase economic benefits.

A torrefied wood pellet production process involves several unit operations including drying, grinding, torrefaction, pelletizing and cooling, with major heat consumption in drying and torrefaction, and power mainly consumed for grinding and pelletization. During torrefaction combustible organic volatiles are also generated from biomass decomposition, which can be used to provide heat for the drying and torrefaction to make the overall process auto-thermal, thus improve process energy and material efficiency [2]. The heating value of organic volatiles and their mass fraction can be adjusted by controlling torrefaction reactor conditions, such as reactor temperature and mean solid residence time. There are several potential configurations of the above biomass torrefaction and densification processes based on desired pellet product quality and energy efficiency, such as 1) drying " torrefaction " grinding " pelletization; 2) drying " grinding " torrefaction " pelletization, 3) drying " grinding " pelletization " torrefaction. Previous studies of the torrefied wood pellet production process have focused on either single energy balances or single reactor models. Our study combined both considering detail reactor equipment design with particle mechanics, thermal dynamics, and kinetics etc., as well as different heat integration strategies.

In this study, we conducted a systematic investigation of the above three configurations for the production of torrefied pellets using biomass feedstocks of different physical properties. Different biomass feedstocks of different properties require the use of different equipment for drying and torrefaction. For wood chips, rotary dryers and torrefiers are considered with combined direct and indirect heat transfer from combustion flue gases. For sawdust powders, fluidized bed dryer and torrefier with or without build-in heat exchangers are applied. The equipment types and heat transfer mode are selected based on the constraints of operating conditions e.g. inlet gas temperature, gas velocity and availability of gas blowers for high temperature flue gases etc., as well as the overall flowsheet heat integration strategies. Sensitivity analysis was also performed to search for the optimum operating conditions. After the flowsheet synthesis, we carried out economical evaluation to identify the most promising configurations for making desired products. Different strategies were proposed and discussed for different purpose e.g. high energy qualified product, low energy demand, or low production costs. All the synthesized processes were modeled and simulated using Aspen Plus under steady state.The developed flowsheet simulation packages could be applied to assist the development and scale-up of commercial biomass torrefaction and densification process.

References

[1] WH Chen, J Peng, XT Bi., 2015. A state-of-the-art review of biomass torrefaction, densification, and applications. Renewable and Sustainable Energy Reviews, 44, 847-866.

[2] Prins, M.J., 2005. Thermodynamic analysis of biomass gasification and torrefaction. Ph.D. Thesis. Eindhoven University of Technology.