Process Intensification of Biomass Slow Pyrolysis Through Autothermal Operation of Auger Reactor

The main aim of the present study is to assess the feasibility of a self-sustained process within a continuous auger-type pyrolyzer (SPYRO) and to increase plant throughput through autothermal operation. To the author’s knowledge, autothermal operation has been successfully achieved and validated on different types of reactors, including fluidized bed reactor, fixed bed reactor, and spouted bed reactor However, it has not yet been investigated in auger reactors. Auger reactors present a simple design, which is less sensitive to feedstock characteristics compared to fixed bed or fluidized bed reactors. However, the heat transfer in auger reactors is a limiting factor for scaling up, especially when the energy needed for the pyrolysis process is implemented indirectly. Therefore, this work deals with experimental investigation of oxidative slow pyrolysis within a continuous auger reactor (SPYRO), which was designed by RE-CORD to work both under inert and oxidative atmospheres. As a second objective, the study aimed at performing a detailed characterization of the obtained products and the overall product yields and energy recoveries. Pyrolysis products from inert and oxidative pyrolysis trials were comprehensively characterized in RE-CORD analytical laboratory. Finally, the techno-economic feasibility of autothermal slow pyrolysis for biochar and bioenergy production was evaluated to promote the further development of pyrolysis technologies. The first chapter introduces the heat demand and the heat supply mechanism in biomass pyrolysis. In particular, progress study on oxidative and autothermal pyrolysis and their implications will be discussed. The second chapter describes the material and methods used in this study, including a thorough description of the feedstock, experimental apparatus, and methodology. In particular, the experimental setup consisted of continuous auger-type pyrolyzer (SPYRO), a condensation unit for pyrogas cooling and liquid recovery and a Micro GC for permanent gas sampling. The third chapter is related to the commissioning of the experimental apparatus. This involved calibrating the feeding system and auger, as well as assessing the thermal loss of the reactor. In the fourth chapter oxidative slow pyrolysis of poplar wood chips and the feasibility of a self-sustained process were investigated in SPYRO pilot unit. Pyrolysis trials were carried out at a feed flow rate of 1.5 kg h-1, reaction temperature of 500 °C and solid residence time of 30 min, at distinct amount of air injected into the pyrolysis reactor. In this chapter are reported the effect of air on plant power consumption, as well as on products’ yield and composition. The fifth chapter focuses on the intensification of slow pyrolysis process in SPYRO pilot unit. Four pyrolysis experiments were carried out at 500°C by increasing biomass flow rate both in allothermal and autothermal modes. The main aim was to experimentally assess that plant throughput can be increased under autothermal operation while keeping constant electricity consumption and the quality and yield of output products. In this chapter are presented and compared the results obtained under allothermal operation and autothermal operation with process intensification. The last chapter describes the techno-economic assessment of allothermal and autothermal slow pyrolysis. The results of the experimental activity reported in chapters 4 and 5 were used to assess the mass and energy balances for four different slow pyrolysis plants. The modelling results were then used as input for the final economic analysis of the considered cases. The main aim is to investigate the techno-economic feasibility of autothermal slow pyrolysis compared to allothermal slow pyrolysis to promote the further development of pyrolysis technologies.