Wood pyrolysis is overall endothermic and is a complex process influenced by several parameters which directly affects the yields and characteristics of the products obtained. One of the lesser known aspects of wood pyrolysis concerns the actual energy change happening during the process within the biomass matrix. To fully understand the reaction sequences occurring, thermal analysis methods must be coupled with evolved gas, char and tar analysis.
The aim of this paper is to determine the endothermicity and exothermicity change throughout the pyrolysis process by making use of an innovative intra-particle gas sampling technique developed at CCNY and of thermodynamic calculations.
The experimental tests have been performed in a tubular reactor on biomass spheres, at operating temperatures of 500°C under non-oxidant controlled conditions and in the presence of an internal standard (IS) represented by a noble gas (Kr). In order to close the mass balance and to develop an accurate calculation based on an enthalpy balance, the thermal decomposition solid and liquid by-products were analyzed in terms of CHNS, calorimetry and GC/MS.
The intra-particle gas evolution only gives us more insight on the decomposition mechanism of the biomass than any other thermal analysis technique alone. CO and CO2 are being released almost coincident reaching a maximum at 325°C, with a maximum concentration of 49.5% for CO2. This is expected given the highly oxygenated structure of biomass, seen also in the proximate analysis of the char collected at different temperatures, which shows a decrease in the oxygen content from 45.3% to 12.1%, at 200°C to 500°C, respectively. The reactions happening might form CO2 from the decomposition of bound water combining with a CO release from the biomass matrix. The H2 profile reveals some possible hydrogenation reactions while the concentration of hydrogen decreased from its maximum of 0.72% at 310°C to concentrations near 0.40% at higher temperatures and forming of light hydrocarbons (CH4, C2H4, C2H6). By characterizing the three gas, char and tar by-products with the techniques mentioned before, a Carbon mass balance for the entire system was closed within ±10%.