Kinetic Modelling of Biomass Gasification in an Indirectly Heated Bubbling Fluidized Bed Steam Reformer

The effort to reduce global anthropogenic carbon dioxide emissions has caused a growing interest in the use of sustainable energy sources. One of the potentially sustainable energy sources considered as an alternative for the currently used fossil sources is biomass. Biomass is organic matter that originates from plants or animals which can be used as chemical energy carrier for fuels or for the production of chemical feed stock[Basu, 2010]. Gasification is a thermochemical process that converts materials like biomass into useful convenient gaseous fuels or chemical feedstock[Basu, 2010]. Gasification takes place at high temperature in the presence of a gaseous agent. Gasification is aimed at increasing the overall energy density of biomass by producing gaseous energy carriers with a higher energy density than the original biomass feed. Conventional gasifiers combust a fraction of the biomass or char in order to provide heat to the reduction reactions. This has two disadvantages. First, the flue gasses are mixed with the product. Secondly, the air that is used for partial biomass combustion mainly consists out of nitrogen. The nitrogen dilutes the gasification product significantly. The Indirectly Heated Bubbling Fluidized Bed Steam Reformer [IHBFBSR] set-up at the TU Delft is a novel indirectly heated gasifier or allothermal gasifier. The combustion of natural gas takes place in a separate combustion chamber inside the reactor chamber. Steam gasification can therefore be performed in the absolute absence of oxygen. A radiant tube facilitates heat transfer from the combustion to the reaction chamber. The radiant tubes are installed at the top and bottom of the IHBFBSR and work according to the heat from inside to outside principle. The aim of this study is to assess the performance of the IHBFBSR set-up at the TU Delft and compare its performance with other existing allothermal gasifiers. The main research question of this study is stated as: "How does the Indirectly Heated Fluidized Bed Reactor perform in terms of product yield, product quality and energy efficiency and how does its performance compare to other indirectly heated gasifiers?". In order to answer this question, both an equilibrium model and a kinetic model have been designed. Sub-optimal conditions for the IHBFBSR take place at a reactor temperature of 850 degrees C, which is the maximal controlable temperature of the reactor. At this temperature, the carbon limit is reached for an equivalent ratio of 0.23. The CGE at the carbon limit equals 69.7%. Optimal process conditions are where the particle size of the bed material is maximized without losing the fluidization behaviour of the bed. The maximum particle size equals 600 micron. The overall efficiency of the system can be increased by heat transfer between the inlet and outlet gasses of the burners and by the separation and combustion of tars and char to add heat to the process. This study shows that the pyrolysis step in the IHBFBSR is rate limited by internal heat transfer. In addition, the char oxidation reaction is limited by mass transfer of oxygen to the char particle.Electrical Engineering | Sustainable Energy Technolog