Ultra-rich carbonization through flash devolatilization for synthesis of biochar from biomass

This paper presents a scalable ultra-rich carbonization process in counter-current packed bed (CCPB) for synthesizing biochar/charcoal with a yield of more than 33% and fuel consumption rate of more than 200 g/m2s (which is at least 2 times more than the maximum achievable with conventional gasification) from coconut shells (CS). The novelty of the process lies in the use of reactant pre-heating to slightly below the devolatilization temperature of CS as a strategy to achieve steady and controlled conversion of CS to biochar/charcoal. The basis for this idea is that the ignition time (tig) of biomass particles decreases significantly with pre-heating while the time for devolatilization (tv) remains more or less the same. Single particle experiments with CS as fuel and atmospheric air as oxidizer show that tv/tig increases from 1.3 to 7.7 with a corresponding increase in pre-heating temperature Ti from 30 to 170 ∘C. The bed operation, in terms of flame propagation, equivalence ratio, and charcoal yield, with pre-heating to slightly below devolatilization temperature in CCPB systems, shows several interesting behaviors not observed in traditional gasifier systems. Two prominent examples of such behaviors are steady flame propagation under extremely rich conditions (volatile equivalence ratio, ϕv > 10) and reduction in peak bed temperature (Tpb) well below 600 ∘C. The biochar yield of the developed process is at least ∼ 25% more than traditional CCPB gasification system. The feedstock flexibility, self-sustainability, and scalability of the developed process are also brought out.