Effects of Contact Conditions between Particles and Volatiles during Co-Pyrolysis of Brown Coal and Wheat Straw in a Thermogravimetric Analyzer and Fixed-Bed Reactor

Abstract

Biomass is a clean and renewable energy source. In order to partially replace fossil fuels and break the limitations of the usage of biomass alone, co-pyrolysis of coal and biomass has been increasingly focused on by researchers, but few articles have investigated the effects of contact conditions between volatiles produced by thermal decomposition and particles from each other during co-pyrolysis. In the present work, co-pyrolysis behavior of wheat straw (WS) and brown coal (HKN) was investigated in a thermogravimetric analyzer (TGA) up to 1100 °C and a fixed bed reactor up to 800 °C, with different contact conditions of particles (biomass was placed above, below and well-mixed with coal). The results showed that the most obvious interactions occurred for mixed sample when 10 wt.% biomass was placed below the coal and mixed sample when 50 wt.% biomass was placed above the coal, both in TGA and fixed bed reactor, with different mechanisms. The synergy effect related to interactions that occurred during co-pyrolysis lead to different behaviors compared to simply addition of coal and biomass: In TGA this was caused by longer reaction time between particles and volatile products produced in primary pyrolysis process. However, for the fixed bed reactor, much more volatiles and catalytic compounds were produced to promote the particles decomposition of WS and HKN. Therefore, opposite to TGA, obvious synergy effects occurred for the blend with less contact time and were caused by volatiles containing more H2 and catalytic materials, which reacted with particles of the other fuel species along the gas flow direction. The kinetic parameters obtained by the Coats–Redfern method agreed with experimental behaviors and synergy effects.