Optimizing the gasification operating conditions of forest residues by coupling a two-stage equilibrium model with a response surface methodology

Abstract

A response surface methodology (RSM) was combined with a thermodynamical dual stage model to optimize the hydrogen syngas yield and the cold gas efficiency from the gasification of forest residues. The studied factors were the temperature, the steam to biomass ratio (SBR) and the oxygen content. The selected factors and their corresponding ranges were based on pre-screening results obtained from the dual stage model. The hydrogen yield was the first studied response. The computed results were also compared with semi-industrial experimental data gathered from a bubbling fluidized bed gasifier. Equilibrium results differ from semi-industrial data but in both cases the hydrogen molar fraction shows similar responses regarding the effect of different operating conditions. It was concluded that the hydrogen molar composition increases with the temperature and SBR but decreases with the oxygen content. On the other hand, it was found that very near from the optimal conditions there are several sets of temperature/SBR/O2 content that could produce similar hydrogen outputs but at less energy demanding conditions and a new optimization making use of desirability functions was carried out considering also the cold gas efficiency. The optimization procedure indicated that changing the operating conditions considerable economical savings are possible without reducing the hydrogen yield and cold gas efficiency outputs.