Improvement of syngas quality from two-stage downdraft wood gasification through steam and oxygen injection

Abstract

The transition from fossil fuels to clean energy sources requires the supply of gaseous fuels from renewable origin for a number of applications. Such applications include in particular high-temperature heat for industries, and Combined Heat and Power (CHP) for flexible, decentralized power production. Gasification of lignocellulosic biomass is a good candidate to this end, converting solid biomass such as wood residues into a more versatile and CO2-neutral gaseous fuel. Today, the use of syngas remains limited due to its relatively low energy density and challenges related to its residual tar content.To improve the quality of syngas on these two aspects, we have studied the injection of steam and oxygen in a pilot two-stage downdraft gasifier. Replacing secondary air by oxygen enhances the Lower Heating Value (LHV) by 65%, while steam injection boosts the H2/CO ratio and damps the temperature rise due to oxygen. The gasification efficiency is enhanced by both oxygen and steam. As a drawback, oxy-steam conditions yield more Class III and IV tars in the syngas. The combined use of oxygen and steam can enhance the volumetric power of syngas-based CHP engines, and provide a sustainable fuel for high-temperature industrial applications.In future research, longer runs should be done to confirm the effects of steam and oxygen on conversion efficiency. The injection of steam and oxygen at the primary stage should also be studied. The presented results will help choosing whether to use oxygen and steam in downdraft gasification applications, and support the validation of numerical models.