Dry-oxidative Reforming for Synthesis of Hydrogen Augmented Biogas & its Utilization in Internal Combustion Engine

Abstract

The over-reliance on conventional fossil fuels has created multifaceted problems for society. Moreover, the fluctuating prices and limited availability are also pushing for non-conventional fuels. The exploration of domestically available clean and renewable fuel is essential for attaining sustainable development goals. Biogas is an attractive alternative gaseous fuel as it can be produced from various locally available feedstocks. However, its lower calorific value and CO2 content retard the utilization in energy applications. Hydrogen-augmented biogas has recently gained interest due to its higher calorific value and suitability for energy applications. Dry reforming is a catalyst-based technique that could generate hydrogen-augmented biogas from raw biogas. It is a reaction between methane and carbon dioxide for producing a mixture of hydrogen and carbon monoxide; however, it suffers from high energy requirements and frequent catalyst deactivation. Dry oxidative reforming is the coupling of both dry and partial oxidative reforming. It could help lower the energy requirements and increase the catalytic activity and stability. In the present study, various nickel-based supported catalysts were synthesized through the wet-impregnation method and utilized for reforming application. This study involved the assessment of the effects of various support systems, promoters, and bimetallic catalysts. The catalysts were characterized using BET analysis, X-ray diffraction (XRD), Field-emission Scanning Electron Microscope (FESEM), and H2- temperature-programmed reduction (H2-TPR) techniques. The study on effects of support system was performed using various metal oxides (CeO2, Sm2O3, TiO2, and Y2O3). The reforming of synthetic biogas was performed in the temperature range of 650-800 °C under atmospheric pressure conditions with varying O2/CH4 from 0 to 0.5. The conversion of both reactants (CH4 and CO2) and yield of both products (H2 and CO) were increased with the increasing temperature for all the catalysts due to the endothermic nature of dry reforming. The Ni/TiO2 catalyst has shown the highest activity due to the strong metal-support interaction, superior metal dispersion, and presence of suitable rutile phases of TiO2. The maximum CH4 and CO2 conversion of 82.1 and 88.2%, respectively with maximum H2 and CO yield of 36.1 and 41.2%, respectively and H2/CO ratio of 0.88 were obtained with Ni/TiO2 catalyst at 800 °C. The addition of oxygen was proved to be beneficial for CH4 conversion, H2 yield, and H2/CO ratio. The maximum CH4 conversion of 91.9%, H2 yield of 49.05%, and H2/CO ratio of 1.11 were observed with O2/CH4 ratio of 0.5. The CHNS analysis of spent Ni/TiO2 catalyst also shows that only 0.074% carbon was deposited on the catalytic surface during 24 h of continuous reforming due to the presence of O2 in the inlet stream resulting in simultaneous removal of carbon through the gasification reaction. The promotion effect of alkaline earth metals (Ca and Mg) on the catalytic activity of Ni/TiO2 catalyst was also assessed. The promoted catalysts were synthesized through the two-step impregnation method. The XRD spectrum of all lab-synthesized catalysts (un-promoted and promoted) showed that the alkaline earth metals have a good interaction with the support material of the catalyst. For the promoted catalysts, the EDS analysis confirmed the presence of Ca and Mg metals on the surface of corresponding promoted catalysts. The higher reduction temperature of calcium promoted catalysts evident from the H2-TPR analysis demonstrated the strong metal-support interaction for the catalyst. The higher activity of Ca-Ni/TiO2 catalyst could be attributed to the higher basicity of the CaO, strong metal-support interaction, and higher dispersion of active metal sites. The Ca-Ni/TiO2 catalyst showed 81.13% CH4 conversion and 94.65% CO2 conversion at 800 °C with H2 and CO yield of 37.5 and 41.52%, respectively. The oxygen introduction has shown the positive effects for CH4 conversion, H2 yield, and H2/CO ratio. The CH4 conversion, H2 yield, and H2/CO ratio attained the maximum value of 96%, 57.6%, and 1.25 with an O2/CH4 ratio of 0.5 at 800 °C. The Ca-Ni/TiO2 catalyst holds excellent stability for reforming conditions up to 20 hours of continuous reaction. The nickel and cobalt bimetallic catalysts (with 11 wt% metals content) supported on Titanium oxide were synthesized through a wet-impregnation approach and investigated for biogas reforming. The H2-TPR analysis revealed that bimetallic catalysts have sturdy metal-support synergy, restraining the metallic sintering. The catalytic activity for biogas reforming was explored under ambient pressure conditions with temperatures varying from 650 to 900 °C and GHSV range of 24,000 to 72,000 ml/g/h with CH4/CO2 ratio of 1.5. The experimental results revealed that the catalytic performance is strongly dependent on the appropriate Ni/Co ratio for bimetallic catalysts. The bimetallic catalyst (7 wt.%) Ni-(4 wt.%) Co/TiO2 showed better catalytic activity and stability due to the synergistic effects of Ni and Co. The Ni/Co ratio could be fine-tuned to enhance pore textural properties, which assisted the metal particle distribution and led to reduction in metallic particle size, increment in activity, and retardation to coke deposition. At 900 °C, (7 wt.%) Ni-(4 wt.%) Co/TiO2 catalyst showed 87.13% CH4 and 92.6% CO2 conversion with 41.1% H2 production. The bimetallic catalysts also withstand the catalytic performance during 15 h of continuous reforming as an insignificant decrease was observed in the activity. For optimizing the dry oxidative reforming, the response surface methodology using a three-level, three-factor experimental design was employed over the nickel-cobalt bimetallic catalyst. The Box-Behnken design of experimentation was employed to assess the interaction and discrete effect of reforming temperature and ratios of CH4/CO2 and O2/CH4. The effects of CH4/CO2 ratio (1–2) and O2/CH4 ratio (0.3–0.5) on catalytic activity were assessed in the temperature range of 700–800 °C. The conversion of both reactants (CH4 and CO2), yield of products (H2 and CO), and ratio of products (H2/CO ratio) were selected as responses for statistical study. The analysis of variance demonstrated that reforming temperature and O2/CH4 ratio have a statistically considerable impact on the H2 enrichment of biogas by virtue of the higher endothermic nature of the reaction. Experimentally, the maximum H2 enrichment of 44.04% was obtained at 800 °C with 1.5 and 0.5 CH4/CO2 and O2/CH4 ratios, respectively. However, from the statistical model, the optimum H2 enrichment of 36.9% was obtained at 725.68 °C with CH4/CO2 and O2/CH4 ratios of 1.32 and 0.42, respectively. The close agreement between predicted and experimental data shows that the combination of response surface methodology and dry oxidative reforming could be an efficient approach for optimizing the H2 enrichment of biogas and the generation of environment-friendly fuel. In the engine study, the hydrogen-augmented biogas was used as a fuel in a common rail diesel engine with diesel as pilot fuel. The experimentations were performed in the load range from 6 to 24 N•m with two different flow rates of gaseous fuel (0.5 and 1.5 kg/h) at a constant speed of 1800 RPM. The effects on engine performance parameters (brake thermal efficiency, brake specific energy consumption, and brake specific diesel consumption), combustion parameters (rate of pressure rise and maximum heat release rate), and emission parameters (Unburnt hydrocarbons, nitrogen oxides, carbon monoxide, and carbon dioxide) were assessed. The induction of gaseous fuel led to an increase in brake thermal efficiency by 10.5%, reduction in brake specific energy consumption by 13.6%, and a reduction of 26.4% in brake specific diesel consumption with a flow rate of 0.5 kg/h when compared to diesel-only mode at 24 N•m load. The HC, NOX, and CO2 emissions were reduced by 18.2, 7.4 and 1.4% with a flow rate of 0.5 kg/h when compared to diesel-only mode at 24 N•m load due to lower availability of carbon content in the combustible mixture. For the efficient utilization of gaseous fuel, the input variables of the engine, i.e., engine load, injection timing, and injection pressure were optimized using response surface methodology with a three-factor, three-level approach. The optimization study involved five responses, namely: brake thermal efficiency (BTE), brake specific energy consumption (BSEC), NOX emissions, HC emissions, and CO emissions. The statistical analysis showed that engine load more statistical impact on BTE and BSEC when compared to injection timing and injection pressure. The engine emission characteristics were statistically dependent on engine load, injection timing, and injection pressure. The response surface optimization showed that optimum engine operating conditions are 69.02% engine load, 20.66° bTDC injection timing, and 30 MPa injection pressure. The corresponding responses at optimized conditions were predicted as 28.98% BTE, 11.07 MJ/kW.h BSEC, 1817 ppm NOX emission, 13.53 ppm HC emission, and 0.0744 vol.% CO emissions. The prepared statistical model has a high desirability value of 77%, which indicates the high accuracy of the given model for predicting the responses. The model's predicted values were also found to be in close agreement with experimental values. This research work comprises both production and utilization of the hydrogen augmented biogas. The hydrogen-augmented biogas was produced through the dry-oxidative reforming of biogas using Ni-based catalysts. The catalytic study showed that Ni/TiO2 has the maximum efficiency when compared to other catalysts due to the favourable metal-support interaction and uniform metal particle distribution. Furthermore, the promotion of Ni/TiO2 catalyst with calcium improved the activity as well as the stability of the catalyst. The use of bimetallic catalyst also led to improved catalytic activity and stability for reforming reaction due to the synergic effects of Ni and Co. The utilization of the hydrogen-augmented biogas in the CI engine under dual fuel mode showed improvement in the performance and emission characteristics of the engine. It led to the improved BTE and reduced diesel consumption with 0.5 kg/h flow rate of gaseous fuel. Moreover, the HC, NOX and CO2 emissions were significantly reduced with introduction of hydrogen-augmented biogas. Lastly, the optimization of engine operating parameters, i.e. engine load, injection timing, and injection pressure also improved the utilization of gaseous fuel in the CI engine under dual fuel mode.