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Title: Effect of in-situ CO2 sorption and chemical looping on steam reforming of unconventional gaseous feedstocks
Author: Sarkin Gobir Adiya, Zainab Ibrahim
Awarding Body: University of Leeds
Current Institution: University of Leeds
Date of Award: 2017
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Detailed chemical equilibrium analysis based on minimisation of Gibbs Energy is conducted to illustrate the benefits of integrating sorption enhancement (SE) and chemical looping (CL) together with the conventional catalytic steam reforming (C-SR) process for hydrogen production from a typical shale gas feedstock. CaO(S) was chosen as the CO2 sorbent and Ni/NiO is the oxygen transfer material (OTM) doubling as steam reforming catalyst. Results are presented and compared for the separate processes of C-SR, SE-SR, CL-SR and finally the coupled SE-CLSR. Up to 49 % and 52 % rise in H2 yield and purity respectively were achieved with SE-CLSR with a lower enthalpy change compared to C-SR at S:C 3 and 800 K. A minimum energy of 159 kJ was required to produce 1 mol of H2 at S:C 3 and 800 K in C-SR process, this significantly dropped to 34 kJ/mol of produced H2 in the CaO(S)/NiO system at same operating condition without regeneration of the sorbent. When the energy of regenerating the sorbent at 1170 K was included, the enthalpy rose to 92 kJ/mol H2, i.e., significantly lower than the Ca-free system. The presence of inert bed materials in the reactor bed such as catalyst support or degraded CO2 sorbent introduced a very substantial heating burden to bring these materials from reforming temperature to sorbent regeneration temperature or to Ni oxidation temperature. The choice of S:C ratio in conditions of excess steam represents a compromise between the higher H2 yield and purity and lower risk of coking, balanced by the increased enthalpy cost of raising excess steam. Furthermore, the effect of gas composition, inert N2 gas and CO2, on the SE steam reforming processe was also investigated. It was found that H2 yield and purity from sorption enhanced steam reforming (SE-SR) are determined by temperature S:C ratio in use, and feed gas composition in hydrocarbons N2 and CO2. Gases with high hydrocarbons composition had the highest H2 yield and purity. The magnitude of SE effects compared to C-SR, i.e. increases in H2 yield and purity, drop in CH4 yield were remarkably insensitive to alkane (C1-C3) and CO2 content (0.1-10 vol%), with only N2 content (0.4-70 vol%) having a minor effect. Although the presence of inert (N2) decreases the partial pressure of the reactants which is beneficial in steam reforming, high inert contents increase the energetic cost of operating the reforming plants. A high operating temperature and low pressure favours the C-SR and CL-SR processes while conversely a low/medium operating temperature favours the SE processes. The aim of the thermodynamic equilibrium study is to demonstrate the effect of coupling SE and CL in C-SR process as well as identify the optimum operating conditions of the studied processes. Experimental studies to demonstrate the theoretical benefits identified in the thermodynamic equilibrium study were perfomed in a bench scale fixed bed reactor. The experimental study evaluates the performance of NiO based oxygen carriers on Al2O3 and CaO/Al2O3 support as C-SR and SE-SR catalyst as well as OTM for CL-SR and SE-CLSR processes when using a model shale gas as the feedstock. Ca based CaO sorbent was used as adsorbent for the SE processes. High operating temperatures were found to be in favour of the strong endothermic steam reforming reaction but to the detriment of the water gas shift reaction. The effect of Ni loading and catalyst support on C-SR process comparing 18 wt. % NiO on Al2O3 and 15 wt. % NiO on CaO/Al2O3 was not evident in low/medium temperature range (600-650 ℃). However, at higher temperature (700 and 750 ℃), the NiO on CaO/Al2O3 support catalyst showed better performance than the Ni on Al2O3 support owing to the alkalinity of CaO suppressing solid carbon formation. The effect of SE using Ca based CaO sorbent and CL using NiO based oxygen carriers as OTM/catalyst on C-SR process was separately investigated and discussed in detail prior to coupling both processes together in a single process termed SE-CLSR process. The process (SE-CLSR) was investigated at 1 bar, GHSV 0.498, S:C 3 and 650 ℃ for 20 redox-oxidation-calcination cycles using CaO and 18 wt. % NiO on Al2O3 as sorbent and OTM/catalyst respectively. The feedstock (shale gas) showed good reduction/reforming properties in the presence of the CaO sorbent and Ni based OTM/catalyst, with high H2 yield and purity for example H2 yield of 31 wt. % and purity of 92 % was obtained in the 4th cycle during the pre-breakthrough period. This was equivalent to 80 and 43 % enhancement compared to the C-SR process respectively. The experimental results were found to be away from equilibrium results which could be mainly attributed to reaction kinetics and mass transfer limitation. Comparison of the SE-CLSR process post breakthrough period with the C-SR process, shows that the SE-CLSR did not degenerate back fully to the C-SR process due to OTM/catalyst bed dilution with the sorbent material. The CaO sorbent demonstrated significant decrease in CO2 adsorption capacity after the first 9 cycles due to sintering and agglomeration of particles, before stabilizing at a certain point limit to the end of the 20 cycles. An increase in the oxidation/regeneration temperature (10-25℃ roughly) during the air feed was observed due to the exothermic nature of the oxidation reaction. This was accompanied with burning off of the solid carbon (coke) that had deposited on surface of the OTM/catalyst, coincidental with evolution of CO2 and CO.
Supervisor: Dupont, Valerie ; Mahmud, Tariq Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available