Process Integration Approaches to Improve the Techno-Economic Feasibility of Torrefaction Process

Over the past few years, the torrefaction process has evolved into a promising pre-treatment process to improve the properties of biomass to a level at which it is competitive with coal. However, in order to make torrefied biomass pellets an economically viable alternative to coal and wood pellets, the techno-economic feasibility of the torrefaction process needs to be improved. Thus, new process configurations are required to produce torrefied biomass pellets and other high value products from the torrefaction process. This thesis presents new process configurations, which have been evaluated with laboratory experiments, process simulations and mathematical modeling.Two different biomass samples i.e. eucalyptus clone and pinewood were used in torrefaction experiments. Initially, the effect that torrefaction pretreatment has on the kinetics, reaction mechanisms and heat flow during biomass pyrolysis was studied using TGA and DSC analysis. The results showed that the pyrolysis reaction mechanism varied significantly with torrefaction treatment. The heat flow data from DSC showed that torrefied biomass pyrolysis requires more energy than dried biomass in order to initiate the pyrolysis reactions.In the second stage, the anaerobic digestion of torrefaction condensate for the efficient utilization of torrefaction volatiles was studied through batch anaerobic digestion assays. Torrefaction condensate produced at 225, 275 and 300 °C was used at various substrate to inoculum ratio i.e. 0.1, 0.2 and 0.5. The methane yield was in the range of 430 - 492 mL/g volatile solids (VS) and 430 - 460 mL/g VS under mesophilic and thermophilic conditions, respectively. With the higher loading, i.e. > 0.2 VSsubstrate:VSinoculum, the production of methane was inhibited because of the inhibitory compounds in the torrefaction condensate, such as furfural and guaiacol.Large quantities of binders are required to make the pelletization process effective and to improve the quality of the pellets. An innovative process configuration is hereby proposed for detoxifying the torrefaction condensate and to reduce the binders’ requirement. The removal of a major inhibitory compound, i.e. furfural, through adsorption using torrefied biomass as an adsorbent was also studied. The adsorption of furfural from the torrefaction condensate at 250 g/L dosage was around 54%. Finally, the influence of the detoxification of the torrefaction condensate on the AD process was studied through batch assays.Finally, the experimental results were used to simulate industrial scale operations to evaluate the feasibility of integrating the torrefaction process with anaerobic digestion. In addition, different process integration approaches were studied to identify possible heat energy recovery options in the torrefaction process, on its own, and also when integrated with AD. The standalone torrefaction process was compared with three different process configurations, which varied according to the intended application for the produced biogas. The mass balance showed that biomethane can be produced at 369 m3/h, at 10 t/h of torrefied biomass pellets production capacity. A sensitivity analysis showed that the cost of the feedstock has a significant effect on the economics of the overall process. The economic analysis showed that the price of torrefied biomass pellets could be significantly reduced if the torrefaction process is integrated with AD.