Biomass gasification in a downdraft gasifier with a two-stage air supply: Effect of operating conditions on gas quality

• Gasification systems with a two-stage air supply have the advantage to produce a gas with low tar content.
• The addition of a second air supply allows to increase the temperature in the pyrolysis zone.
• The tar yield decreases with the increase on the total air flow and AR.
• The cracking of the tar is favored by a higher combustion temperature.

The gasification of the biomass is an attractive technology for the production of electricity, heat, chemicals and liquid fuels. This paper presents an experimental evaluation of the quality of the producer gas in a two-stage, air supply downdraft gasifier, referred to its tar and particle content for different operating conditions. The gas composition and its lower heating value were also determined. Experimental tests were performed varying the operating conditions of the gasifier: the air flow between 18 Nm3/h and 22 Nm3/h (the proximate equivalence ratio from 3.03 to 0.279) and the air flow ratio in the two stages (AR) between 0% and 80%, evaluating the effects of these parameters over the quality of the gas. The results show that a fuel gas, with tar and particulate matter content of 54.25 ± 0.66 mg/Nm3 and 102.4 ± 1.09 mg/Nm3, respectively, was obtained, for a total air flow rate of 20 ± 0.45 Nm3/h and an air ratio, between the two stages, of 80%. For these conditions, the lower heating value of the gas was 4.74 ± 0.5 MJ/Nm3. The two stage air supply in the gasification allowed to reduce the tar content in the producer gas up-to 87% with even a slight increase in the gasifier efficiency. This results can be explained by an increase of the temperatures in the pyrolysis and combustion regions.