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

Abstract

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, H2, 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 m3), cold gas efficiency (29.66% to 31.00%), and carbon conversion (79.59% to 80.87%) are observed.

Hardwood Branch Char Gasification with CO2: Characterization of Reactivity and Proposal of a Semiempirical Kinetic Model

“…Siripaiboon et al 30 compared experimental results in a downdraft gasifier with results from two-dimensional numerical simulations using computational fluid dynamics. Good agreement was found between the two considering three gasification parameters: fuel consumption, temperature profile, and syngas composition.…”

CFDs Modeling and Simulation of Wheat Straw Pellet Combustion in a 10 kW Fixed-Bed Downdraft Reactor

“…Reactors are tools used to convert solid biomass material into gaseous fuel [41]. Two widely employed configurations for fixed-bed reactors are the downdraft (co-current flow) and updraft (cross-current flow) approaches, distinguished by the relative movement of the feedstock and combustion agent [42]. In the downdraft reactor, biomass enters from the top, air flows from the side, and a multi-stage combustion process (primary, intermediate, and dilution zones) efficiently converts the biomass into gas [43].…”

“…Ensuring reliable simulations necessitated careful consideration of operating and boundary conditions, beginning with selecting the reaction phase. The boundary and operating conditions for the combustion of WSPs in downdraft reactors were derived from a combination of experimental operations and data from the literature [42]. A summary of these operating conditions is provided in Table 4.…”

CFD Simulation and Experimental Validation of Wheat Straw Pellet Gasification in a 10-kW Fixed Bed Gasifier: Enhanced Syngas Production

“…Two widely employed configurations for fixed bed gasifiers are the downdraft (co-current flow) and updraft (cross-current flow) approaches, distinguished by the relative movement of the feedstock and gasifying agent [29]. In the downdraft gasifier, three main actions occur: (i) biomass flows on top of the reactor, (ii) air flows from the side into the reactor, and (iii) combustion/gasification takes place inside the reactor [30].…”