Analysis and Removal of Product Gas Contaminants Present in Gasification Processes

Energy demand and economic development are intrinsically linked. Earlier economic development came at the price of serious environmental pollution, posing a risk to life on earth. Thus, a transition from fossil to renewable energy is needed. Among the renewable energy technologies, gasification of biomass promises high process efficiency and flexibility concerning the feedstock and products. In such a process, gas contaminants have to be removed to protect downstream equipment and the environment. Finding an optimal configuration of process steps in gas cleanup chains, offering reliability, availability and high efficiency at low operating and capital costs is still a challenge and one of the major hurdles for commercial deployment of biomass gasification processes. This thesis aimed to investigate the gas cleanup chain as part of gasification processes. Two aspects, the analysis of gas contaminants and their elimination, were examined. At first, three analytical methods, aimed at the detection of sulphur containing hydrocarbons, tars and trace elements, were developed. In the next step, these analytical tools were employed to investigate the performance of gas cleanup units, intended for the removal of the sulphur containing hydrocarbon thiophene, tars and the trace element Se. The removal of thiophene by activated carbons (AC) was explored experimentally and theoretically. Experiments were performed in a packed bed column between 100-200°C. The adsorption process was described by a 1D approach, including mass transfer phenomena and axial dispersion. Experimental validation showed good agreement between measured and predicted breakthrough times and capacities. A global sensitivity analysis (GSA) was performed to rank the importance of model input factors and gain further process insight. The work showed that ACs are a promising option for thiophene removal and provided a generic approach for the application of GSA to adsorption models. In this context, the analysis demonstrated that the selection of common model assumptions deserves as much attention as the model parameters. Further work chapter aimed at understanding the effect of H2O, H2 and H2S on the decomposition of thiophene over CaO. Experiments were performed in a fixed bed reactor at 810°C. The factors were varied as part of a full factorial design at two levels. Results were fitted to a linear model with interactions. Analysis showed that H2O and H2S had a major effect on thiophene conversion, while that of H2 was less pronounced. Interactions between varied factors were found to be negligible. A UV-Vis method was developed to obtain online information on tar compound concentrations in real process gases in the low ppmv region. Spectra were analysed for their tar composition by two chemometric approaches. Applied to two case studies, the developed tool proved to be a rapid, sensitive tool, applicable to qualitative process monitoring with the added benefit of quantification in gases with a limited number of tar compounds. An ICP-MS was employed as a tool for the online detection of trace elements. The low detection limits (< 2 ppbv for H2Se, PH3; < 0.1 ppbv for AsH3; < 0.01 ppbv for Hg) and the high temporal resolution render this method a quick and robust tool for the performance evaluation of sorbent materials. A case study investigated the capture of H2Se by a Zn/Al2O3 sorbent between 150-350°C.


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