Two-dimensional CFD simulation and pilot-scale experimental verification of a downdraft gasifier: effect of reactor aspect ratios on temperature and syngas composition during gasification

This paper focuses on a two-dimensional CFD simulation of a downdraft gasifier and a pilot-scale experiment for verification using wood pellet fuel. The simulation work was carried out via the ANSYS-Fluent CFD software package with in-house coding via User Defined Function. Three gasification parameters were taken into account in the simulation and validation to achieve highly accurate results; namely, fuel consumption, temperature profile, and syngas composition. After verification of the developed model, the effects of aspect ratios on temperature and syngas composition were investigated. Results from simulation and experimental work indicated that the fuel consumption rate during the steady state gasification experiment was 1.750 +/- 0.048 g/s. The average steady state temperature of the experiment was 1240.32 +/- 14.20 K. In sum, the fuel consumption and temperature profile during gasification from modeling and experimentation show an error lower than 1.3%. Concentrations of CO, CO2, H-2, and CH4 were 20.42 vol%, 15.09 vol%, 8.02 vol%, and 2.6 vol%, respectively, which are comparable to those of the experiment: 20.00 vol%, 15.48 vol%, 8.00 vol%, and 2.65 vol%. A high concentration of syngas is observed in the outer radial part of the reactor because of the resistive flow of the air inlet and the synthesis gas produced. The average temperatures during the steady state of the gasifier with aspect ratios (H/D) of 1.00, 1.38 (experiment), and 1.82 were 978.77 +/- 11.60, 1256.46 +/- 9.90, and 1368.94 +/- 9.20 K, respectively. The 1.82 aspect ratio reactor has the smallest diameter, therefore the radiative heat transferred from the reactor wall affects the temperature in the reactor. Syngas compositions are comparable. Inverse relationships between the aspect ratios and the syngas LHV, (4.29-4.49 MJ/N m(3)), cold gas efficiency (29.66% to 31.00%), and carbon conversion (79.59% to 80.87%) are observed.