Time-dependent variations of activation energy during rapid devolatilization of biomass

• Char deactivation is correlated well with effective fuel residence time.
• Gas composition of rapid solid fuel devolatilization was quantified as a function of time and temperature.
• Previous devolatilization models are compared to empirical findings and well correlated.
• A definition of ‘apparent activation energy’ is proposed.

Industrial gasifiers and combustors are assumed to reach particle heating rates of 105–106 °C/s and understanding how particles behave in these extreme conditions can improve the utilization of solid fuels in these reactors and in downstream applications. By studying intermediate devolatilization processes during solid fuel pyrolysis, detailed models for solid fuel conversion can be formulated. Key objectives of this study included (1) investigate possible mechanisms that promote the formation of synthesis gas components and char, (2) compare the devolatilization behavior of pyrolysis by varying particle size, hold time, and temperature and (3) correlate char deactivation with hold time. The objectives of the study were accomplished using a wire-mesh reactor with a uniform heating rate of 500 °C/s in nitrogen under atmospheric pressure. A design of experiments approach was used to quantify the effects that hold time, temperature, and particle size had on char yield, evolved gas composition, and apparent activation energy of pine stem wood and wheat straw. Key results indicate that with increased temperature and hold time more volatiles evolve from the fuels and favor carbon monoxide and methane production at higher temperatures. Apparent activation energy of the volatile matter decreases with hold time. An abbreviated model for apparent activation energy correlates well with experimental data and assumes that along a devolatilization pathway, that not all volatiles are driven from the fuel.