Spatial comprehensive comparative analysis of updraft and downdraft fixed bed gasification reactors by computational fluid dynamics approach with industrial data validation

Energy and chemical production through small to medium scale biomass thermochemical conversion plants has propel fixed bed gasification process to the forefront of research and development. The updraft and downdraft fixed-bed gasification reactors have similar operational characteristics but are distinct in performance output regarding syngas quality, tar production, and efficiencies. Emissions, solid residues, and conversion efficiency also vary, making their comparisons essential for evaluating sustainability and environmental impact depending on the gasifying agent used. Additionally, comparing reactor designs helps understand which configuration suits which industrial conditions and the scaling needs of specific gasification processes under different operational conditions. A comprehensive biomass conversion computational fluid dynamics model with different gasifying medium - air, steam (H2O(g)), and carbon dioxide (CO2) is studied to comparatively assess their effect on gas composition, lower heating value (LHV), tar yield, carbon conversion efficiency (CCE), and cold gas efficiency (CGE) in industrial updraft and downdraft gasifiers. The validity of the developed models was established by contrasting with two sets of industrial data obtained from air gasification, with a root mean square error of 1.025 and 1.321 for updraft and downdraft, respectively. Increasing the equivalence ratio (0.18–0.34), steam-fuel ratio (0.4–1.2), and CO2 to fuel ratio (0.18–0.9) enhanced gas yield and tar cracking but reduced the gas quality in both configurations. The updraft recorded the highest LHV of 6.81, 8.23, 7.48 MJ/Nm3, and the lowest tar contents of 1.81, 1.03, and 1.61 g/Nm3 were predicted by downdraft for air, steam, and CO2, respectively. The highest CCE of 97.3 % was recorded by updraft during the steam gasification process, while downdraft recorded the highest CCE of 92.4 and 86.24 % in air and CO2 gasification processes, respectively. Finally, the updraft recorded the highest CGE of 83.5 and 72.5 % for steam and CO2 gasification, while the downdraft recorded the highest CGE of 71.1 % for air gasification. The study will distinctively aid the understanding of these reactor configurations under different operational conditions and which design suits which industrial conditions and scaling needs of specific gasification processes.