Thermochemical Analysis of Biomass Gasification by Gibbs Free Energy Minimization Model—Part: I (Optimization of Pressure and Temperature)

Abstract

The choice of gasifying medium significantly affects the fuel gas composition and its heating value in biomass gasification. Generally, steam gasification is superior to air or steam-air or pure oxygen gasification in terms of hydrogen concentration and heating value of product gas. In this context, a thermodynamic equilibrium model for steam gasification of biomass based on minimization of Gibbs free energy is presented. Gas phases are modeled as real gases obeying the Redlich–Kwong equation of state. Minimization of Gibbs free energy of the gasification system is carried out with simulated annealing (SA) algorithm. The parametric variation is studied by varying temperature and pressure for computing the composition and lower heating value of the product gas and energy efficiency of the gasification process for arriving at the optimum values of these variables. Simulations are performed with apricot stones as biomass material. Sensitivity analysis is done for the model and corresponding sensitivity coefficients are computed. The maximum mole fraction of hydrogen is found to be 63.5% at 870 K and the maximum LHV is 14.3 MJ/Nm3 at 1050 K at a pressure of 1 bar. Similarly, the highest mole fraction percentage of 62% and LHV of 15.5 MJ/Nm3 are obtained at pressures of 8 bar and 4 bar, respectively, for the gasification temperature of 1000 K. Energy efficiency is consistently above 60% with a peak of 65.7% at 973 K and 1 bar. From the simulation results, the global optimum conditions for operation of gasifier are arrived as 973 K and 1 bar.