Experimental study on biomass gasification in a double air stage downdraft reactor

This work presents an experimental study of the gasification of a wood biomass in a moving bed downdraft reactor with two-air supply stages. This configuration is considered as primary method to improve the quality of the producer gas, regarding its tar reduction. By varying the air flow fed to the gasifier and the distribution of gasification air between stages (AR), being the controllable and measurable variables for this type of gasifiers, measuring the CO, CH4 and H2 gas concentrations and through a mass and energy balance, the gas yield and its power, the cold efficiency of the process and the equivalence ratio (ER), as well as other performance variables were calculated. The gasifier produces a combustible gas with a CO, CH4 and H2 concentrations of 19.04, 0.89 and 16.78% v respectively, at a total flow of air of 20 Nm3 h−1 and an AR of 80%. For these conditions, the low heating value of the gas was 4539 kJ Nm−3. Results from the calculation model show a useful gas power and cold efficiency around 40 kW and 68%, respectively. The resulting ER under the referred operation condition is around 0.40. The results suggested a considerable effect of the secondary stage over the reduction of the CH4 concentration which is associated with the decreases of the tar content in the produced gas. Under these conditions the biomass devolatilization in the pyrolysis zone gives much lighter compounds which are more easily cracked when the gas stream passes through the combustion zone.

► Obtained results an important for a better phenomenological understanding of processes occurring in two stage gasification reactors.
► The air flow is the fundamental parameter in the operation of downdraft gasifiers.
► CH4 reduction is associated with a decreases in the tar content.
► An enhancement in the thermal cracking of tar is carried out in the two-air downdraft gasifier.