Entrained flow pyrolysis and gasification of selected biomass – an experimental and modeling study

This PhD thesis presents the work carried out by kinetic modeling incorporated with particle simulation on selected plant based biomass during pyrolysis and gasification followed by an experimental investigation of those processes under entrained flow to satisfy the engineering requirement. Renewable energy sources are becoming a significant part of the primary energy share for mitigating the CO₂ emission along with addressing the issue of fossil fuel depletion. According to the lifecycle of biomass, it is CO₂ neutral and can be a potential replacement for fossil fuels. Being a solid fuel, it can be consumed using the existing technology for solid fossil fuels, however, with modification. To modify any available technology, significant research effort is needed in both fundamental and engineering level to find out optimum reaction conditions. One appropriate technology for solid fuel conversion is entrained flow gasification which uses a high heating rate and low residence time to produce high energy gas. Non-conventional biomass (e.g. algae) along with woody biomass might be consumed by this technology. At the same time, fossil fuel (coal) can be potentially replaced by blending biomass with it. To model the inherent kinetics involved in the pyrolysis process, a new algorithm was proposed for higher order distributed activation energy model. The new algorithm was found to be versatile in estimating the intrinsic pyrolysis kinetics for different types of biomass (algae, sawdust, and coconut shell) along with predicting the pyrolysis behavior of the blends of one those biomass samples and coal. To link this fundamental development to the engineering application, entrained flow pyrolysis experiments on biomass were performed on biomass by varying different operating conditions. After that, a particle model was developed for this process to predict the conversion profile of the solid biomass particle using apparent kinetics which showed good agreement with the experimental data. A guideline was also generated on the basis of parametric study (particle size, temperature, gas velocity, residence time etc.) to design a laminar entrained flow reactor for pyrolysis. Further development of the particle model was achieved by incorporating the intrinsic kinetic parameters obtained by the newly developed algorithm. It was proposed that inclusion of pyrolysis heat of reaction would improve the prediction of the model if intrinsic kinetic parameters were to be used. At the same time, effect of operating parameters (temperature, particle size) and species variation on entrained flow pyrolysis was evaluated. The study was limited to the particle size ranges under 600 µm (suitable particle size for entrained flow gasification) and up to a temperature of 1000 °C. Among all the parameters, particle size was found to be the most critical because of its influence on both residence time and heating rate. Also, temperature was found be very important for achieving full conversion in case of larger particle size. At 1000 °C, pyrolysis of all types of biomass under consideration reached completion. In case of a lower temperature (800 °C), some unconverted particles were observed for larger size (500-600 µm). Tar production was minimized at 1000 °C for the smaller particle size (150-250 µm). At a higher temperature, the gas yield was also increased considerably due to the increase in conversion efficiency. Morphological study on the char particles showed that sawdust experienced a molten phase during its pyrolysis and due to the gas release from inside, the smaller particles were converted into cenospheres where no morphology of the parent particle was visible. This resulted in highly reactive char with an extremely porous structure. However, this observation could not be generalized as algae and coconut shell char showed different morphological development. As the char obtained from the entrained flow pyrolysis process were different from fixed bed chars, they were studied for their reactivity and kinetics under CO₂. Generally, gasification kinetics of most of the chars was predicted well by random pore model. Only the algal char obtained from rapid pyrolysis was different because of its low amount of gasifiable mass attached to the surface which did not show any porous structure, therefore, followed the volumetric reaction model. Along with the fixed bed chars, only coconut shell char from entrained flow reactor showed very low reactivity. This difference in the reactivity was attributed to the lack of mesopores along with the variation of indigenous alkali in the ash among the biomass species to a lesser extent. This low reactivity of coconut shell char resulted in the entrained flow gasification experiments which were performed by varying the temperature, particle size and also concentration of gasifying agent (CO₂). The char from coconut shell did not show any significant increase in conversion due to the decrease in particle size whereas a steady linear increase was observed for temperature. In contrast, the sawdust char was highly reactive and reached its highest conversion (50%) at 1000 °C under 20% CO₂ for a reactor length of 1.885 m. Remarkable increase in the conversion was observed with decrease in the particle size and increase of temperature. The increase in reactor length also showed positive effect on char conversion and gas production. These findings have important implications on the gasifier design and sample preparation meaning there will be no benefit of reduced particle size on conversion if the sample itself is less reactive in the first place. Also it was revealed that if raw biomass was gasified, these effects would have been indistinguishable because of the dominance of pyrolysis. No tar was observed during char gasification process at 1000 °C as most of it removed during the pyrolysis process. Along with the above studies, a new analytical technique (Synchrotron based Infrared spectrum) was used to study the pyrolysis process of biomass. The study delineated the evolution of functional groups from the surface of biomass along with the effect of heating rate during the process. This was a preliminary study which opened up new possibilities in energy research considering in situ gasification of biomass.