A gasification-based single step process is reported for synthesizing moderately activated char (surface area in the range of 100–600 m2/g) from ligno-cellulosic biomass. The novelty of the process lies in the use of mixtures of O2-CO2 and O2-steam as gasification agents to obtain significantly higher activation in a single-step as compared to gasification with air (<100 m2/g) in a self-sustained process. Experiments were conducted in a counter-current packed bed reactor with three biomass (agro-residue pellets, wood based pellets and coconut shells). Yield and Brunauer-Emmett-Teller (BET) surface area are reported for char obtained from experiments covering an overall equivalence ratio range of 3.9–1.2. The volatiles equivalence ratio (ϕv), identified as the unifying parameter for analysis from the earlier studies of the authors, emerges as the relevant parameter to characterize the relationship between yield and activation as well. With mixtures of O2-CO2, as ϕv decreases from 2.7 to 1.1 (fuel-rich to stoichiometric) the yield decreases from 100 to 10% (of fixed carbon content) with a corresponding increase in BET surface area from 10 to 570 m2/g. With mixtures of O2-steam, as ϕv decreases from 2.1 to 1.04 the yield decreases from 92 to 5% with a corresponding increase in BET surface area from 106 to 561 m2/g. The zone of transition of activation from reaction rate limited to diffusion limited conditions is identified from the experimental results. It occurs at around ϕv ~ 1.7 (T ~ 1450 K) for O2-CO2 mixtures and around ϕv ~ 2.1 (T ~ 1200 K) for O2-steam mixtures. The activation obtained with the current single step process is an order of magnitude higher compared to that from other single step processes like pyrolysis and air-gasification. Char obtained from the current process can be used as activated biochar and for other applications requiring only moderate activation (100<SBET<600 m2/g).
|Journal||Data powered by TypesetFuel Processing Technology|
|Publisher||Data powered by TypesetElsevier BV|