Biomass torrefaction: Modeling of reaction thermochemistry

Abstract

ABSTRACT:Based on the evolution of volatile and solid products predicted by a previous model for torrefaction (Bates and Ghoniem, 2012) a model has been developed which describes their thermal, chemical, and physical properties as well as the rates of heat release. The first stage of torrefaction, associated with hemicellulose decomposition, is exothermic releasing between 40-280 kJ/kg initial . The second stage is associated with the decomposition of the remaining lignocellulosic components, completes over a longer period, and is predicted to be either endothermic or exothermic depending on the temperature. Increasing torrefaction severity, as quantified by the mass loss, is predicted to cause greater heat release. The rate of mass loss and rate of heat release increase with higher temperatures. The higher heating value of volatiles produced during torrefaction was estimated to be between 4.4 and 16 MJ kg -1 increasing with the level of mass loss. Highlights: Model developed to describe the energy balance during willow torrefaction  First stage is exothermic releasing 40-280 kJ/kg  Second stage is either exothermic or endothermic  Higher torrefaction temperatures result in increased reaction rate and heat release rate

Properties of Torrefied U.S. Waste Blends

“…It was a challenge to find values for h r as the torrefied material was not well defined, it comprises fibers (mostly cellulose) and a large variety of plastic materials. Cellulose torrefaction in the 25-300 • C temperature range starts as an endothermic reaction and continues as an exothermic reaction (Bates and Ghoniem, 2013). Enthalpies of reaction for plastic in the same temperature range were always positive and vary in the range (12.55-147.86 J/kg) (Zhao et al, 2017), which is smaller than the value of c p (T-T o ) (∼400 kJ/kg) in Equation (4).…”

A Comprehensive Review on Biomass Torrefaction

“…Longer residence time releases more volatiles gases and leads to the evolution of a secondary volatiles gases, which has relatively a higher energy value than the first stage volatiles. For instance, Bates and Ghoniem (2013) found that the first stage volatile in the two-step torrefaction kinetics has heating the value of only 4.4 MJ/kg compared to 16 MJ/kg in the second stage volatiles. The carbon usually contributes 60-85% of the total mass of the coal composition, whereas the oxygen content ranges from 5-20% (Prins et al, 2007).…”

Modeling kinetics-transport interactions during biomass torrefaction: The effects of temperature, particle size, and moisture content

“…The advantage of this kinetic mechanism and set of parameters is that all pseudo-components (A,B,C,V1,V2) have welldefined chemical compositions thus enabling estimation of their thermodynamic properties, namely their heat of formation and specific heat capacity [58]. Thus, the elemental balance enables the estimation of the solid product composition and energy yield [59] not considered in previous single particle models.…”

“…Thermodynamic properties of the solid phases (A,B,C,S) and V1 and V2 are summarized in Table 2. For further information on the thermodynamic property estimation of the solid and volatile products see [58]. The effective conductivity of the solid matrix described in Table 2 includes a contribution from radiation heat transfer through the pore spaces [12] as wells as a contribution from the moisture [43,48].…”

“…First, both the solid mass loss kinetics mechanism and thermochemical submodel were developed using temperature data below 300 ○ C, specifically for willow feedstock [6,58]. This commonly used temperature limit was originally chosen to avoid excessive decomposition of the relatively carbon-rich lignocellulose components-namely cellulose and lignin) [6,57].…”