| dc.description.abstract | Conversion of synthesis gas from biomass gasification to fuels or other valuable chemicals is regarded as a sustainable and renewable pathway to solve the energy crisis. However, the sulfur impurities, normally in the form of H2S, in the raw products are highly corrosive and poisoning to the equipment and downstream catalysts, hence, it requests to be thoroughly eliminated. Conventionally, The sulfur compounds can be removed chemically or physically by low-temperature cleaning methods, which normally involve liquid solvent and require to be operated at temperatures lower than 100 °C, even down to -60 °C. Considering the high temperature of raw gas products from the biomass gasifier and the reaction temperature for subsequent syngas applications, such as over 200 °C for Fischer-Tropsch synthesis, the cooling and reheating steps are expected, which means high investment and low energy efficiency. Hence, high-temperature desulfurization with metal-based solid sorbent is of interest since this route can potentially avoid energy loss and reduce the investment.
The thesis focuses on developing Mn-based solid sorbent and the corresponding operational procedures related to the sorbent. The Mo-addition effects on the Mn-based sorbent were firstly studied. A series of Mn-based desulfurization sorbents with different amounts of Mo addition were prepared, characterized, and tested for H2S sorption and regeneration. The sorbent with the largest Mo-addition, 15Mn8Mo, exhibited improved and promising performance regarding sorption capacity and stability. Formation of the mixed oxide, MnMoO4, is proposed to play a role in promoting the sorbent. Comparing to the unpromoted sorbent 15Mn, the promoted sorbent 15Mn8Mo showed a 38% increase of the initial capacity and significantly less deactivation, with only 9.7% capacity loss after 10 sorption/ regeneration cycles. This shows that the material is well suited as a regenerable sorbent in a chemical looping sorption process. The impacts of regeneration conditions on the desulfurization performance of the sorbent, 15Mn8Mo, were then investigated. A sufficiently severe combination of temperature and oxygen concentration is necessary to achieve the complete removal of sulfur from the sorbent. SO2 formation was observed during sorption with O2 regenerated sorbent, which was approved to originate from sulfate decomposition and H2S conversion by oxide species with higher oxidation states. The SO2 formation is a transient phenomenon that can be avoided by the pre-reduction of the sorbent following the oxidative regeneration. The repeated desulfurization test as well as residual level tests demonstrate that the pre-reduction step can help to maintain good stability, in terms of either capacity and sulfur residue. In addition, a new evaluation standard for the sorbent performance, sulfur residue in the gas phase (both H2S and SO2), was introduced and studied. The options of active metals for the sorbents, the operational parameters, including temperature, space velocity, gas composition, and the steam content, all pose impacts on the effluent H2S and SO2 levels. By properly combining and adjustment of all parameters, the H2S residue could be down to lower than 1ppm and almost no SO2 was present with the sorbent 15Mn8Mo. Furthermore, a kinetic study on the sulfidation of the sorbent, 15Mn8Mo, for high-temperature desulfurization is described. A modified grain model, taking structural changes of the grains sizes into account was applied, and the model shows a good fit to the experimental data.
To summarize, the Mo-promoted Mn-based solid sorbent shows promising high-temperature desulphurization ability for biomass-derived syngas. By applying proper operation conditions, the sorbent can achieve high capacity, good stability, and low sulfur residue in the gas phase. | en_US |
