Towards practical application of gasification: a critical review from syngas and biochar perspectives

Syngas and biochar production are mainly influenced by temperature, feedstock properties, gasifying agent, pressure, and the mass ratio between gasifying agent and feedstock with temperature being the most significant factor. Increasing temperature generally promotes syngas production while suppressing biochar production. The selection of gasifiers (fixed bed, fluidized bed, and entrained flow) is highly dependent on scale requirement (e.g., volume of feedstock and energy demand), feedstock characteristics (e.g., moisture and ash content), and the quality of syngas and biochar. Updraft fixed bed gasifiers are suitable for the feedstocks with a moisture content up to 50 wt.%. High ash feedstocks such as Indian coal, dried sewage sludge, and municipal solid waste that are not suitable for fixed bed gasifiers, have been successfully gasified in bubbling fluidized bed reactors. Woody biomass is not suitable for entrained flow gasifiers unless specialized feeding methods are employed such as wood torrefaction and grinding followed by the existing feeding methods for pulverized coals, biomass-oil biochar slurry preparation followed by pumping, wood or torrefied wood slurry preparation followed by pumping, etc. Syngas and biochar can potentially be contaminated by NH₃, H₂S, and tar, which can be removed using catalysts (e.g., Ni-based), metal oxides-based sorbents, and thermal and catalytic cracking methods. Existing syngas and biochar upgrading methods suffered from various problems such as economic infeasibility, limited productivity, and fouling, and future syngas and biochar upgrading methods should be aimed to have the features of reliability, security, affordability, and sustainability, towards the practical, large-scale production of syngas- and biochar-based products. One potential solution is to develop integrated systems by combining biochar upgrading and application with syngas upgrading, which warrants an integrated perspective based on both life cycle assessment and economic analysis.