(144c) Biomass Gasification in a Fixed Bed Downdraft Gasifier: Experimental Study & Comprehensive Reactor and Particle Modelling

Authors: 
Yao, Z., National University of Singapore
You, S., National University of Singapore
Yan, W. C., National University of Singapore
Dai, Y., Shanghai Jiaotong University
Tong, Y. W., National University of Singapore
Wang, C. H., National University of Singapore
Gasification of biomass waste is considered as a green and sustainable solution to reduce waste volumes and recover energy by producing syngas. In this study, both experiment and simulation have been conducted to understand the various physical and chemical effects in a fixed bed downdraft gasifier and to provide researchers with a tool to overcome the need to conduct cost intensive experiments for predicting and optimizing energy efficiencies.

A combined transient single particle and fuel-bed model is developed to describe the entire packed bed in the flow direction of primary air and intraparticle phenomena. The reactor is discretized and in each cell one representative particle is chosen to be modelled as shrinking sphere. The reactor was divided into three regions due to the difference of air velocity in each region: natural convection region, mixed convection region and forced convection region. Sensitivity analysis has been conducted to determine the boundary location of each region. The presented model considers drying, pyrolysis, homogeneous gas reactions, and heterogeneous combustion/gasification reactions. In the gas phase eight species (O2, N2, CO, CO2, H2, H2O, CH4 and tar) are considered. The solid phase is biomass waste, in which all the components obtained from approximate analysis (moisture, volatiles, fixed carbon and ash) and ultimate analysis (C, H, O, N) are treated as dependent variables of time and space. This model can predict production rate and quality of both synthetic gas and produced char from gasifier reactor. The optimized biomass properties, equivalence ratio and air inlet location for maximum energy efficiency can also be obtained from this model.

Experiments have been conducted to investigate the effects of different parameters on gasification performance, such as biomass moisture content, particle size and shape, equivalence ratio, etc. The experimental results are also used to validate against simulation results and the deviation between the numerical and experimental results obtained in this study is lower than 10%.