Modified stoichiometric equilibrium model for sewage sludge gasification and its validation based on experiments in a downdraft gasifier

Appropriate selection of reactor configuration, proper sizing and optimization of operating conditions is crucial for the efficient biomass to energy conversion in a gasifier. Simulation of gasification processes by mathematical modelling helps to optimise the gasifier operating conditions and parameters. A suitable mathematical model can furnish valuable information about the impact of operating parameters on the gasification process. This paper deals with developing a modified stoichiometric thermodynamic equilibrium modelling with sewage sludge as the feedstock for gasification. The developed model investigates the effects of operating variables by solving the equations using Newton-Raphson method in the MATLAB (R) platform. It incorporates the presence of sulphur, which is usually present in the sewage sludge in a substantial amount. The char conversion and the four types of tar components produced during the gasification of sewage sludge were also included. Effect of temperature and equivalence ratio (ER) on the mole fraction of product gas components (H-2, CO, CO2, CH4, H2S and tar components) was predicted using the model. Experimental investigations were conducted in a downdraft gasifier with SS as the feedstock and the assessment of the predictions by the developed model was done by evaluating the average absolute error, percentage average absolute error, sum of squares error and root mean square error. All the error values estimated showed an acceptable deviation (< 10%) from the experimental results. A high coefficient of regression (> 0.9) and low p values (<0.05) proved that there exists a statistically significant relationship between the model prediction and experimental results.