Technology for gasifying pulverised bio-fuels including agricultural residues

Abstract

This paper presents both the fundamental and technological aspects of gasification of pulverisable agro-residues, more particularly rice husk. The fundamental aspects are related to the times required for flaming and char conversion processes. The technological aspects described here are related to the gasification process using a cyclone reactor. The aspects of reactor design, cooling and cleaning systems are addressed. Limiting the tar fraction in the gas needs good mixing between the gas and the red hot char arranged in a circulating fluid bed for the ash handling system. Cleaning the gas (of particulates and some tar) is achieved by a centrifugal separation process aided by the injection of a water jet. The experience gained from the developmental programme on the gasifier operation leading to power generation via the dual-fuel mode on an existing diesel engine is also described. The gasifier operates on an air-to-fuel ratio of 0.95 to 1.0 and the consumption rate of husk when the gas is fed to an internal combustion engine in dual-fuel mode with 80% diesel replacement is 1.5-1.55 kg/kWh of electrical energy, confirmed from several long duration tests. This is lower than the value of to 2 to 2.2 kg/kWh reported for Chinese gasifiers. The conversion-fixed carbon in the rice husk char achieved in the reactor is about 50%, the cold gas efficiency being about 70%. The fuel is pulverised to a mean size of 0.75 mm with about 35% by weight less than 0.4 mm. The reduction in the size of the agricultural residues is to enable the same gasifier to be used for all the pulverisable fuels; accordingly, the same gasifier is able to successfully gasify sawdust and sugarcane trash. Some issues related to handling sugarcane trash are also discussed. The treatment of the ash/residue is in the dry mode and therefore, the requirement of water is comparable to that in wood gasifiers and is much less than in Chinese fixed bed gasifiers. Another important aspect of the study is the measurement of the NO_x in the engine exhaust which has shown that the NO_x level for the dual-fuel mode is one-eighth of that in diesel-alone mode over the entire power range up to full load.