Air-steam gasification of biomass based on a multi-composition multi-step kinetic model: A clean strategy for hydrogen-enriched syngas production

Abstract

Biomass, as a renewable energy source, has high potential for supplying the energy needs of modern societies. Gasification is a thermochemical route for converting biomass into combustible gas at high temperatures. The main purpose of the present study was to develop an Aspen Plus model of air-steam gasification of biomass (sawdust) to predict the gasification characteristics and performances. The prediction capability of the model was evaluated by comparison with experimental data obtained in a fluidized bed biomass gasifier. First, the influence of gasification temperature on gas composition, product yields and gasifier performances was investigated. The biomass feeding rate and air flow rate were set at~0.445 kg/h and 0.5 Nm3/h, respectively, while the gasifier temperature was varied between 700 °C to 800 °C. With the increase of temperature, the gas yield (DGY) increased steadily from 1.72 to 2.0 Nm3/kg, while the HHV of the produced syngas (HHVgas) increased initially from 5.38 to 5.73 MJ/Nm3 and then decreased to 5.69 MJ/Nm3. After determining optimal temperature (800 °C), the influence of equivalence ratio (ER) and steam/biomass ratio (S/B) on gasification characteristics, dry gas yield (DGY) and tar yield (TRY) was studied. As ER increased from 0.19 to 0.23, TRY decreased from 9.13 g/Nm3 to 8.45 g/Nm3. In contrast, DGY initially increased from 2.02 Nm3/kg to 2.43 Nm3/kg as ER increased from 0.19 to 0.21 and then dropped to 2.24 Nm3/kg at ER of 0.23. An increase in S/B from 0.61 to 2.7 also resulted in a slight increase in HHVgas; however, TRY showed a decreasing trend (from 9.65 g/Nm3 to 8.95 g/Nm3). The results showed that the model developed in this paper is a promising tool for simulating the biomass gasification at various operating conditions.