Modeling of biomass gasification implies the representation of chemical and physical phenomena constituting all the four zones of the gasifier (pyrolysis, combustion, reduction, and drying) in the mathematical form. The models of downdraft biomass gasification can be categorized into two groups: (1) Equilibrium models and (2) Combined transport and kinetic models. Kinetics-free equilibrium models can predict the exit gas composition, given the solid composition and the equilibrium temperature, but they cannot be used for reactor design (Di Blasi, 2000). An equilibrium model can not predict the concentration or temperature profiles across the axis of gasifier and hence results generated using an equilibrium model would give the same final composition for different lengths of reduction zone of biomass gasifier. Hence, there is a need to develop a combined transport and kinetic model which takes into account of the kinetics of homogeneous and heterogeneous chemical reactions, transport of volatiles produced, heat and mass transfer between solid and gaseous phase and pyrolysis reactions. In a survey of gasifier manufacturers, it is reported that 75% of gasifiers offered commercially were downdraft, 20% were fluid beds (including circulation fluid beds), 2.5% were updraft, and 2.5% were of other types (Bridgwater, 2002). Taking into account of the importance of downdraft biomass gasifier and its commercial applications, it is essential to have a complete model for such a configuration.
In the present study, a transient one-dimensional model is developed for the throated close-top downdraft biomass gasifier. The model takes into account of the pyrolysis, secondary tar reactions, homogeneous gas reactions and heterogeneous combustion/gasification reactions. Eight gaseous species namely O2, N2, CO2, CO, H2O, H2, CH4 and tar are considered in the gas phase. In the pyrolysis and combustion zone, the solid phase is a biomass, whereas in the reduction zone it is charcoal. The developed model is divided into three parts according to three prevailing zones in the gasifier: (1) pyrolysis, (2) oxidation, and (3) reduction. Pyrolysis is a process by which a biomass feedstock is thermally degraded in the absence of oxygen/air to produce solid (charcoal), liquid (tar and other organics) and gaseous (H2, CO2, CO, etc.) products. Released volatiles from each biomass particle flow downward in packed pyrolysis bed. Rate of release of volatiles depends on the particle size and temperature within a single particle. The drying zone is indirectly incorporated in the developed model. The composition of volatiles is found using the experimental data of Boroson et al. (1989), which predicts the release of mainly water vapor from the pyrolyzing particle below 120 °C. Pyrolysis bed is modeled as a stack of particles in one-dimension. To consider the temperature gradient, the entire bed is divided into two subsystems, i.e., gas phase inside the bed and the individual particles. To describe the chemical process of pyrolysis in a single solid particle, an unsteady state one-dimensional variable property model of transport phenomena is required. It should include heat (conductive, convective and radiative modes), mass (diffusive and convective modes) and momentum transport of the products formed within the solid (volatiles and gases). The model developed and modified by Babu and Chaurasia (2004 a-d) is used in the present study to model the single particle in the pyrolysis zone. The single particle modeling equations are clubbed with the conservation equation of the gaseous species flowing inside the bed of pyrolysis zone. The volatile products generated in the pyrolysis zone flow downwards and enter into the oxidation zone where a part of the volatiles gets oxidized. In complete combustion, carbon present in biomass is completely converted to carbon dioxide while hydrogen is converted to water vapor. It is an exothermic reaction and yields temperatures in the range of 1000 °C to 1500 °C. In the present model, complete combustion of biomass is assumed which is ensured by supplying excess air (usually around 20% more than the stoichiometric requirement). It is assumed that the tar present in the pyrolysed gas mixture completely gets decomposed due to very high temperature present in the oxidation zone. The main components of the gaseous mixture leaving the combustion zone are carbon dioxide, water vapor, inert nitrogen, carbon monoxide, hydrogen and some amount of low molecular weight hydrocarbons such as methane, ethane, ethylene etc. In the reduction zone, the gaseous mixture passes through the hot porous charcoal bed resting above the grate. The endothermic reactions are carried out where the degree of temperature drop depends upon the extents of reactions. Giltrap et al. (2003) developed a model of reduction zone of downdraft biomass gasifier to predict the composition of producer gas under steady state operation. In our earlier simulation study (Babu and Sheth, 2006), Giltrap's model (2003) was modified by incorporating the variation of CRF along the reduction zone of downdraft biomass gasifier. It is assumed that CRF is exponentially increasing along the bed length of the reduction zone. Solid carbon in the form of char is assumed to be present throughout the reduction zone.
The experimental data obtained in our earlier study (Sheth and Babu, 2009) are used to validate the simulation results of the complete combined transport and kinetic model. The fraction of initial moisture content, air flow rate, temperature of the pyrolysis zone, and chemical composition of the biomass are required as input data for the model to predict the composition of producer gas. The variation of molar fraction of producer gas components with time is predicted and compared with the experimental data. It is found that the model predicted molar fraction of nitrogen decreases first during the few initial minutes (5-10 min) of gasification. After that it increases and attains a steady value (after 10-15 min). The molar amount of nitrogen is constant for a particular flow rate of air as nitrogen acts as an inert but its composition varies due to the changes in molar amount of other components of gaseous mixture. It is found that the simulated molar fractions of carbon monoxide and hydrogen increase first with time and subsequently decrease and finally attain steady values. It is observed that the model predicted molar fraction of methane is very less and almost remains constant; while the model predicted molar fraction of carbon dioxide decreases initially and attains a steady value.
The simulation results of the molar composition of various components of the producer gas match well with the experimental data of 10 minutes or higher from the start of an experimental run. For the experimental data of 5 min and 10 min, the simulation results differ more. This is because of the assumption taken in the model that all the gas generated in pyrolysis or reduction zone travel downwards in the gasifier. However, it is observed while carrying out the experiments that the gas produced in the pyrolysis zone first travels upwards and occupies the empty space above the biomass. After 5 - 10 minutes from the start of the run, the accumulated gas builds up the pressure and the producer gas starts flowing downwards. Because of this, the model predicts higher concentration of hydrogen and carbon monoxide and lower concentration of nitrogen in comparison with the experimental data for initial 5 -10 minutes from the start of a particular experimental run. It is concluded from the present study that the developed model can predict the performance of the biomass gasifier, a priori. The results of this study are also useful in the design of a downdraft biomass gasifier.
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