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Production of Activated Carbon within the Dual Fluidized Bed Gasification Process

Tchoffor, Placid A., Davidsson, Kent O., Thunman, Henrik
Industrial & Engineering Chemistry Research 2015 v.54 no.15 pp. 3761-3766
activated carbon, alkalis, biomass, clean energy, cleaning, cost effectiveness, engineering, fluidized beds, gasification, hydrochloric acid, hydrogen sulfide, particulates, steam, wood products
A key step in the production of clean energy from biomass gasification is the cleaning of the product gas to remove impurities, such as tars, H₂S, HCl, particulates, and alkali compounds. Some gas cleaning systems employ activated carbon (AC), with Brunauer–Emmett–Teller (BET) surface areas in the range of 800–1300 m²/g. Because the operational conditions for the production of AC are similar to those applied in the gasification chamber of a dual fluidized bed gasifier, the possibility to produce AC within the dual fluidized bed gasification (DFBG) process was investigated in the present work. Wood pellets were gasified with steam in a fluidized bed reactor at 800 and 840 °C. Part of the unconverted char from this process was further activated with steam in a fixed bed reactor at 850 °C. The results obtained indicate that if ≥44% of the char that results from the devolatilization of the wood pellets is gasified (the extent of char conversion), the unconverted char acquires a BET surface area >900 m²/g, which makes it suitable for use in gas cleaning processes. Achieving this extent of char conversion in the gasification chamber of industrial dual fluidized gasification plants would remove the need and cost for a second reactor for further activation of the unconverted char. The cost of the produced activated carbon (AC) has been estimated as being 15-fold lower than that of commercially available AC. This suggests that the production of AC as part of the dual fluidized bed gasification process would be cost-effective.