The valorization of volatiles released during torrefaction of various kinds of agricultural biomass
J.J. de Koning (TU Delft - Applied Sciences)
AB De Haan – Graduation committee member (TU Delft - ChemE/Transport Phenomena)
Wiebren De Jong – Mentor (TU Delft - Large Scale Energy Storage)
L. Cutz – Coach (TU Delft - Large Scale Energy Storage)
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Abstract
Pretreatment and densification processes such as torrefaction coupled with pelletization have proven to be valuable techniques to convert biomass to solid biofuels. Presently, waste streams from the torrefaction process in the form of volatiles are usually combusted to generate heat required for the torrefaction process. Nevertheless, the chemical characteristics of the torrefaction gas combined with recent technologies have opened new alternatives that could be used for further valorization of volatiles in other ways than combustion. The aim of this Thesis was to find alternative pathways other than straight up combustion of volatiles to valorize the volatiles released during torrefaction of biomass. Two types of biomass were investigated: corn stalk and sugarcane bagasse. Four different configurations for valorization were developed for their prospects of valorization: (1) valorization via straight up combustion, (2) chemical looping combustion, (3) chemical looping steam reforming and (4) a combination of chemical looping combustion and straight up combustion. A model based on experimental data was build in Aspen Plus to simulate the drying, torrefaction and valorization for a torrefaction plant with a capacity of 100 kton/year. The results were analysed in terms of process efficiency and economics. Results indicate that chemical looping combustion of all volatiles is the most attractive option of valorization, both in terms of efficiency and economics. The gross LHV efficiency for this option is 92 % for corn stalk and 84 % for sugarcane bagasse. The production costs in these scenarios are €208/ton for torrefied corn stalk pellets and €180/ton for torrefied sugarcane bagasse pellets. The capital investment required for the torrefaction plant for this configuration is €527/kW installed for corn stalk and €650/kW installed for sugarcane bagasse.