Numerical and experimental investigation of downdraft gasification of wood chips

Biomass is widely perceived as a potential renewable energy source. Thermo-chemical conversion technologies including gasification, co-firing, and pyrolysis are of primary interest due to their higher conversion efficiency and throughput when compared with the low temperature digestion and fermentation for lignocellulose and wood-based feedstock. In this paper, a small scale, air blown, downdraft gasification system is operated using wood to investigate its conversion efficiency. Wood chips of 0.5 cm thickness, 1–2 cm width, and 2–2.5 cm length constitute the feedstock to the downdraft gasifier that is assembled and instrumented at Masdar Institute’s Waste-to-Energy laboratory. The experimental investigation of the temperature field inside the gasifier is followed by high fidelity numerical simulation using CFD to model the Lagrangian particle coupled evolution. The numerical simulation is conducted on a high resolution mesh accounting for the solid and gaseous phases, k–ε turbulence, and reacting CFD model. The temperature distribution and the evolution of species are computed and compared with the experimental results and with the ideal equilibrium, zero dimensional case.

► We examined the efficiency of gasifying wood chips in a downdraft reactor using numerical and experimental methods.
► Feedstock density and heating value is important as it controls the flame stability.
► Improvements modeled gasifier include decreasing the heat losses, controlling the system leaks and enhancing the mixing.
► Max. temp. measured at the combustion zone as expected with a cold gas efficiency of 70% (CFD) vs. 89% (equilibrium model).