Investigating tar formation at low pressures in wood gasification systems, applying a novel thermo-chemical simulation model

Abstract

Even-though wood gasification remains a promising technology regarding de-centralized sustainable energy supply, its main limitations, namely the issues of unsteady operation, excessive tar-formation and consequential high maintenance requirements, have never been fully overcome. In order to tackle these deficiencies and to increase the understanding of thermo-chemical wood-gas phase reaction dynamics, a numerical model has been created. After validating the simulator against comparable software, it has been applied to predict and thus understand tar-formation phenomena within a small experimental co-current gasification system. This work particularly focuses on the investigation and minimization of tar-formation phenomena within low-pressure zones (e.g. downstream of valves) at temperatures T≤500K. Model-based analysis has led to a range of recommended measures, which reduce the occurrence of tars in low-pressure zones. Said recommendations are: i) Decrease gas residence time and ii) increase temperatures in low-pressure zones; iii) Increase hydrogen to carbon ratio as well as iv) oxygen to carbon ratio in the wood gas. While measures i) and ii) require modifications to the plant and/or process itself (e.g. by installing modified pipes or by recirculating thermal energy via heat-exchangers), measures iii) and iv) can be implemented either by removing coal from the reaction zone or by adding either water or process air to the process.