Expanding the application range of conventional biological gas treatment techniquesdevelopment of a biotechnological reactor for energy-rich gases desulfurization

Abstract

This thesis is focused on developing a biological system for gaseous effluents desulfurization. The initial steps were aimed at proving the biological and technical feasibility of biologically treating H2S concentrations of up to 10,000 ppmv of H2S. For this purpose, a preliminary reactor setup was built using a packing material that had been previously reported to successfully perform for biotrickling filter reactors. After reaching elimination capacities of 280 g H2S m-3 h-1 at a gas contact time of 180 s and identifying some of the most important aspects for the system operation, an improved lab-scale setup was built and used to study other operational parameters. A different packing material with a more open and solid structure was tested and proved to reduce the unwanted elemental sulfur accumulation, although when operating at very low O2/H2S supply ratios also elemental sulfur formation and accumulation was observed. Once the new setup maximum elimination capacity had been tested and the effect of various system perturbations had been preliminary studied (loading peaks and operational stops), a few modifications were introduced to obtain the final version of the lab-scale design. It included a separate oxygenation compartment for a better use of the supplied oxygen and a pH and liquid level control. At a controlled pH and stable sulfide supply operation, a very fast startup phase was obtained using activated sludge from a municipal wastewater treatment plant. Also, an intensive study on some of the most significant operational parameters was carried out, including assessment of the effect of reduced gas contact times, substrate supply shutdowns, wide pH changes, increased trickling liquid velocities and operation at different O2/H2S supply ratios. Throughout the development process, it was concluded that if not sufficient oxygen was guaranteed, sulfur accumulation could become a problem upon treatment of very high H2S loads. Therefore, elemental sulfur biological oxidation was investigated as a possible remediation action for reactor unclogging. Experimental results proved that it could be considered as a remediation alternative since considerably high oxidation rates were measured upon sulfide supply shutdown and accumulated sulfur oxidation. Since fuel-gases do also contain volatile organic sulfur compounds (VOSC), co-treatment of VOSC and H2S would improve the fuel-gas sweetening process, and hence, VOSC biological oxidation was investigated. Experimental bioreactor runs were carried out in a different biological system which consisted of an air-lift bioreactor for sulfide removal at natron-alkaliphilic conditions. Hence, also a different type of sulfide oxidizing biomass was used. Results indicated that VOSC and H2S co-treatment was feasible although it introduced some important changes in the air-lift reactor normal operating conditions. Preliminary biodegradability experiments at neutrophilic conditions indicated that VOSC biological degradation did also occur and, therefore, reactor runs with the biotrickling filter reactor could be performed next. Also, a preliminary assessment of respirometry and titrimetry application to sulfur oxidizing biomass (SOB) activity monitoring was carried out. When sulfide was used as substrate, mainly experimental limitations were identified and different hypothesis formulated to explain the obtained results. However, results on dissolved-elemental sulfur or thiosulfate did show the potential of these two techniques for SOB activity monitoring if an improved methodology is obtained. Finally, the design of a full-scale prototype of the desulfurization system applied to biogas desulfurization is presented, as well as the results from an operation and economical viability study which both encourage the full-scale application of the developed technology.