Design and fabrication of a downdraft gasifier coupled with a small-scale diesel engine

Abstract

Gasification technology plays a significant role to increase the potential exploitation of agro-waste products. This technology can convert solid dried biomass into a combustible gas, widely called syngas or producer gas. Producer gas can be used to run a diesel engine in a dual fuel mode to partially reduce diesel fuel consumption. However, determination of the dimensions of a gasifier and gas cleaning elements for a pre-specified capacity of a pilot-scale diesel engine remains unexplored. This study, therefore, intends to provide a heuristic concept to avoid an oversize, cumbersome design of a gasifier and a gas cleaning unit based on basic theory. A 5.7 kW diesel engine and a Jatropha seed feedstock were selected for our study. A type of closed top, throatless, downdraft gasifier with an annular space was chosen. A cyclone filter, a shell-tube heat exchanger, and a dried-bed filter were selected as the components of the gas cleaning unit. The cyclone filter’s dimensions were determined using the ideal gas principle. Next, basic heat transfer theory and fluid dynamics were applied to design a shell-tube heat exchanger. The results highlighted that the inner diameter of the designed gasifier should be 150 mm, and gasifier produced up to 27 kg/h of gas. At a biomass consumption rate of 5 kg/h, the gas production rate was 20 kg/h that substituted the diesel fuel consumption rate by up to 49% at 70% of the fuel engine load.  It is noteworthy that the gasification efficiency of Jatropha seed was 77%. The proposed concept is expected to be informative for the design of other gasifier types suitable for different biomass types. This technical article is written in a simple way, which is very useful for practitioners to understand and apply it for the fabrication of a small-scale gasifier-engine system.