Biomass Gasification for Power and Fuel Applications using a Bench-Scale Bubbling Fluidized Bed Gasifier

Abstract

Today fossil fuels are a major source of energy. Coal is used extensively around the world for power applications, while petroleum products are used to fulfill liquid fuel needs. This extensive use of fossil fuels has led to environmental pollution, including release of greenhouse gases and hazardous impact on human health. These factors combined with the possibility of the extinction of fossil fuels have led to the development of alternate and renewable sources of energy. Renewable resources like wind and solar provide an excellent option for alternative modes of electricity production. However, biomass is the only source which has a potential to fulfill not only our electric power needs but also liquid fuel demands. Lignocellulosic biomass such as agricultural and forest waste are found in abundance in the southeastern U.S. It can be processed into fuel via either a biochemical or thermochemical route. A popular thermochemical process, gasification, yields a gaseous fuel known as synthesis gas or “syngas”, composed primarily of a mixture of carbon monoxide and hydrogen, which can be further converted into liquid fuel via Fischer-Tropsch synthesis or fermentation. The syngas composition depends upon several factors such as type of biomass used, type of gasifier selected, type of bed material in the gasifier, gasifying media used and the operating condition (temperature, pressure, and equivalence ratio). These factors affect the primary syngas composition and the amount of tar and contaminants released during the gasification process, which need to be controlled for downstream use of the syngas produced. This dissertation is presented as a series of three chapters, each of which represents a manuscript that has been submitted for archival publication. The content of these chapters is preceded by an overarching introductory commentary that includes an exhaustive survey of the literature. The dissertation ends with a segment that indicates an overall summary of key conclusions and a series of recommendations on possible extensions of this effort. Among different types of lignocellulosic biomass, energy crops such as switchgrass are attractive because they can be grown on marginal lands with minimal maintenance and in a short period. In this dissertation, gasification of switchgrass has been studied in Chapter 3. The objective of this study is to understand the effect of temperature (790, 935 and 1000˚C), equivalence ratio (0.21, 0.24, 0.27), biomass ash content (2.71, 3.19, 4.59%), and feed rate (3.63, 6.38, 9.34 g/min) on switchgrass gasification in a bench-scale bubbling fluidized bed gasifier with sand as bed material. In this study, nitrogen was used as a fluidizing agent and oxygen was used as a gasifying agent. The effect of process variables were evaluated in terms of (i) product yield, (ii) syngas components, (iii) carbon balance, (iv) syngas energy content, and (v) contaminants. It is shown that carbon monoxide (CO) and hydrogen (H2) yields increased with an increase in temperature resulting in an increase in energy content of the syngas. The CO and H2 yield increased with an increase in ER. This was due to the decomposition of acetylene in the presence of excess oxygen. The gas yield was not affected by an increase in the ER in the range under study (0.21 to 0.27). The ash content in the range of 2.71 to 4.59% did not affect syngas composition and syngas yield, These results obtained from the ash content study were unique, it can be said that a bubbling fluidized bed gasifier can efficiently handle a biomass ash content (up to 4.6 wt%) material, without significantly affecting the performance or the syngas composition. The increase in biomass feed rate (g/min) helped improve the volumetric concentrations of primary components in the syngas, thereby improving the syngas heating value. However, the overall yield of components per kg of biomass, and the carbon balance were not affected; suggesting that the conversion of biomass was independent of feed rate. The focus of Chapter 4 is on gasification of switchgrass with a bench-scale bubbling fluidized bed gasifier using naturally occurring olivine as a bed material (or in-situ catalyst). Olivine primarily contains iron and magnesium silicate and is found in abundance on earth. As in the sand study presented in Chapter 3, the study was carried out to understand the effect of temperature (790, 935 and 1000˚C) and equivalence ratio, ER, (0.20, 0.25, 0.30 ERs) with olivine as bed material on i) product yield, ii) syngas components, iii) syngas energy, iv) the carbon balance and v) the energy efficiencies. The data obtained were then compared with that of sand as a bed material. It was found that with an increase in temperature, the CO and H2 yield increased, while the CO2 and methane yield decreased. With an increase in ER, the CO2 and water yield increased due to oxidation of CO and hydrogen in presence of increased oxygen. When compared with use of sand as bed material, the olivine use resulted in higher concentration of CO, H2 and CH4 at 790°C. However, with increase in temperature the sand performed better, resulting in gas with higher energy content and higher concentration of CO, H2 and CH4. Olivine did not reduce the tar yield from switchgrass. The elutriation and thermal decomposition of olivine particles accompanied by coking of the olivine in the reactor could be the possible reason for the unexpectedly ordinary performance of olivine. Lastly in Chapter 5, gasification of thermally pretreated biomass has been studied. Torrefied biomass has higher C:O ratio, resulting in improved heating value and reduced hygroscopic nature of the biomass, thus enabling longer storage times. In the southeastern United States, pine is abundant and has been identified as a potential feedstock for energy production. Thus, torrefaction of pine has been proposed to improve the properties of pine. The objective of this component of the study was to understand the performance of the torrefied biomass as a gasification fuel in a bench-scale bubbling fluidized bed gasifier. As expected the CO and H2 concentrations increased and CH4 concentration decreased significantly with an increase in temperature; while with an increase in ER, only the CO2 concentrations increased in the syngas. The performance of torrefied pine was comparable with pine and switchgrass under similar experimental conditions. Torrefied pine gasification led to much higher char yield (more than twice) than pine i.e. lower carbon conversion; however, it produced less than half as much tar and ammonia.