Chemical kinetics of biomass and sorbent blends for gasification purposes

Abstract

The main aim of this research was to determine the chemical kinetics of biomass and biomass/sorbent mixtures, which is crucial in assessing parameters including the feasibility, design, and scaling of industrial biomass conversion applications such as pyrolysis and gasification. Many studies have been conducted for co-gasification of coal and biomass, however little has been done for the biomass with sorbent mixtures. Biomass is one of the main renewable energy sources and coupled with carbon dioxide absorbent material such as calcium oxide (CaO) and/or magnesium oxide (MgO) sorbent it increases the biomass conversion efficiency during gasification. The thermogravimetric analyzer (TGA) was conducted in order to establish the material thermal behavior under temperatures for which gasification takes place with specific reference to the pyrolysis stage of gasification. Kinetics of thermal decomposition of biomass with sorbent mixtures of pine wood (wood), CaO and MgO, were investigated using the TGA technique. The different ratios of these materials, which ultimately determines the gasification characteristics of the blends, were investigated. The measurements were carried out in a nitrogen atmosphere at different heating rates of 10, 15 and 20 ºC/min. It was found that the material fully degraded in the devolatilazation zone, which is in the temperature range of 200-800 ºC. At higher temperature some samples were more stable compared to others. The significant mean difference in the rate of degradation between the samples was compared using one way analysis of variance (ANOVA). The kinetic parameters (activation energy, pre-exponential factor and the reaction order) were obtained from both model free (Kissinger and Flynn-Wall-Ozawa (FWO)) and the regression models. A computer simulation was performed to establish the ratio that resulted in higher conversion efficiency during gasification. A mixture of 80% pine wood and 12,5% CaO and 12.5%MgO to make 25%CaO.MgO has resulted in the highest thermal stability compared to other samples. In other words, at higher temperatures these samples degrade at a slower rate compared to others. The activation energy of this sample, 139.63 kJ/mol and the average value of 143.74 kJ/mol was obtained from the Kissinger and FWO method, respectively. The values obtained from the regression models were higher compared to model free methods. Both the model free and model fitting methods were effective in determining the kinetic parameters. However, for accurate results, it is crucial to develop your own model which can fit the data perfectly. Compared to other blends the 25%CaO.MgO sample was found to be suitable to perform better during gasification.