Corn stover steam gasification

Abstract

Biomass corn stover is a promising renewable energy resource. One of the most widely used technologies of utilizing biomass is fluidized bed steam gasification due to its flexibility of handling fuels and the high-energy-content gas produced. In this study a complex unsteady-state two-phase kinetic model including fluid dynamics and reaction kinetics is developed. An un-reacted core shrinking model is employed to describe chemical reactions, and particle entrainment is considered. In addition, a pyrolysis model including the effect of particle size and temperature is developed and incorporated in the gasification model. This model is able to reflect the effect of particle size, temperature, pressure, the steam/biomass ratio, mass flow rate, and superficial gas velocity on gasifier performance. In addition, this model can provide detailed information on the evolution of gas, char, and particle size in the bed, and percentages of particles consumed during reactions, turned to fines by friction, or entrained out of the bed. There are many models with difference levels of complexity and modeling concepts for a fluidized bed but there are few studies available on comparing these models. Therefore, six other widely used models are also developed and compared to study the importance of modeling complexity on model selection. These models are the zero-dimensional non-stoichiometric equilibrium model, zero-dimensional stoichiometric equilibrium model, zero-dimensional kinetic model, one-dimensional one-phase kinetic model, one-dimensional two-phase kinetic model-all char in bed with particle size, and one-dimensional two-phase kinetic model-all char in bed without particle size. Gasification results show that the one-dimension two-phase kinetic model and the one-dimensional one-phase kinetic model are equivalent, and both predict the same gasification results in terms of the gas volumetric fraction, yields of char and dry tar-free gas, the history of the evolution of particles, and higher heating values (HHVs). Therefore, it can be concluded that the number of phases in the fluidized bed does not affect the simulation results. Since it takes less time to finish a run for the one-phase kinetic model than for the two-phase kinetic model, the one-dimensional one-phase kinetic model is better than the two-phase kinetic model. In addition, the one-dimensional two-phase kinetic models-all char in the bed predict the same gas volumetric fractions and the yields of char and dry tar-free gas at steady state, but predict different amounts of time for the bed to reach steady state and different amounts of char in the bed at steady state. The main reason is because different methods are used to calculate the reactive surface area for reaction rates. Through model comparison, it is found that models with similar modeling concepts tend to have similar results. Gasification models developed in this study are incorporated into a biomass integrated gasification combined cycle (BIGCC) system to provide heat and power for a corn ethanol plant. The effect of different gasification models on the overall BIGCC system performance is evaluated. Results show that BIGCC systems using different gasification models have similar but not identical overall system performances.