The effect of pressurized induced flow on downdraft gasifire performance with low emission burner

Abstract

Malaysian agriculture industries produce large amount of biomass residue every year. Oil palm solid waste, rice husk, coconut shell and sawdust from wood are being disposed in an uncontrolled manner presently. Alternatively, this biomass can be used to produce energy by controlled combustion system or gasification process. This process produces producer gas that could generate electrical or mechanical energy. Pressurized induced flow was utilized in a downdraft gasifier with low emission burner. In this research, downdraft gasifier was coupled together with an air ejector, orifice cylinder and gas burner. Palm oil (Elaeis guineensis) shell and coal were used as feed. The solid feeds were combusted at a controlled condition inside the gasifier. Primary compressed air was supplied to assist the process. The process produced producer gas, which was consisted of methane, carbon monoxide, carbon dioxide and hydrogen. Producer gas flowed out from the gasifier to the orifice cylinder and gas burner to re-burn again. Secondary compressed air was supplied to the gas burner through an air ejector to assist the burning process. This ejector also acted as an induced flow. High pressure and high velocity air streaming through the ejector caused pressure inside of the gas burner to drop. This situation allowed producer gas inside of gasifier, which was at a higher pressure to flow into the gas burner. The orifice cylinder’s function was to enhance the air-fuel ratio mixture before it was being burnt. Air ejector configuration was carried out using computer simulation software, FLUENT 5.3. Five orifice cylinders with the diameters of 60, 70, 80, 100 and 120 mm were simulated to investigate the profile of gas flow mixing. From the simulation, orifice cylinders with 60 and 70 mm diameter produced choked gas flow. Thus, only orifice cylinder with 80, 100 and 120 mm diameters were tested in the experiment. Secondary air pressure supplied was varied from 0.5 to 1.0, 1.5, 2.0, 2.5 and 3.0 bar to obtain the emission level. From the experiment by using palm oil shell as feed based, and at 2.5 bar secondary air pressure supplied, it was found that orifice cylinder with 80 mm diameter produced 53% NOx less than the gas burner without orifice cylinder. The combustion efficiency was also increased by 13%. Furthermore, gas burner with 80 mm diameter orifice cylinder and 40o vane decreased NOx by 15% besides increasing combustion efficiency by 3%. Gasification with coal feed based produced less than 20% NOx compared to palm oil shell feed based, at 92.1% combustion efficiency.