Experimental Study on Slow Pyrolysis of Biomass by Direct and Indirect Contact Heat Transfer in a Continuous Reactor

Abstract

Biochar from pyrolysis is emerging as a new option for biomass utilization for various purposes including soil amelioration and carbon sequestration. This study investigated the slow pyrolysis characteristics of wood pellet in a continuous reactor varying the heat transfer type (direct and indirect contact) at various temperatures (400~700°C) with different carrier gas compositions (inert, O2 2%, and O2 4%). Direct contact heat transfer by preheated carrier gas accelerated the heat up of biomass, promoting the pyrolysis process as well as the temperature increase of indirect contact cases. The presence of O2 in the carrier gas resulted in limited oxidation reactions with pyrolytic vapor, increasing the reactor temperature to 529.4°C and 591.7°C at 2% and 4% O2, respectively, compared to the inert case having a final temperature of 492.4°C. This can be exploited to save the external heat supply but a careful temperature control is required to yield uniform biochar properties. The pyrolysis products from different test conditions were characterized for mass yield and detailed properties of biochar, bio-oil, and non-condensable gases.