Hydrogen enrichment of biogas through reforming & its utilization in compression ignition engine under dual fuel mode

Abstract

Today’s worldwide energy sectors are mainly focused on power generation by using unsustainable fossil fuels. However, from long-term perspective, power must be produced from renewable resources because of unpredictability and shortage of worldwide fossil fuel reserves. Gaseous fuel induction in compression ignition engine has picked up much attention over the most recent few years, particularly when it is produced from renewable resources. Catalytic reforming has been considered as an effective technique to produce H2-enriched biogas (HEB) from the various feedstocks. The reforming techniques which proven their potential for HEB production have been comprised of steam reforming (SR), dry reforming (DR) and partial oxidation (PO). Among these, only SR has been commercialized for the H2 production, and another (DR and PO) have their own drawbacks which limit their implementation in industries. The combined DR and PO which is so-called dry oxidative reforming (DOR), has an approach to overcome issues occurred in DR technique. In the present study, HEB production has been accomplished with commercially available Ni and synthesized catalysts under DR and DOR processes at temperature ranges from 650 to 900°C, with a CH4/CO2 ratio of 1.5:1. Moreover, attention has been focused to elucidate the influence of catalyst performance on the reactant (CH4 and CO2) conversion and product (H2 and CO) yield, as well as, on the H2/CO ratio of the DR and DOR processes. Box-Behnken design (BBD) has been used to optimize the DOR process parameters for HEB production by integrating response surface methodology (RSM) in the presence of Ni nanoparticle. The effect of the CH4/CO2 (1-2) and O2/CH4 (0.1-0.3) ratios on the catalytic performance of DOR has been assessed in the temperature range of 800 to 900°C. The empirical regression models have been developed to identify the influential and most significant parameters. Various Ni based novel catalysts have been synthesized at laboratory scale with wet impregnation method using metal oxides viz., CeO2, Al2O3, TiO2, and ZnO. The textural and structural properties of synthesized catalysts have been assessed by XRD (X-ray diffraction), BET (Brunauer–Emmett–Teller), FESEM (Field emission scanning electron microscopy) and TPR (Temperature programmed reduction). The carbon deposition in wt.% over the spent catalysts has been determined using CHNS Elemental Analyzer. Influence of reaction temperature was strong on H2/CO ratio in both reforming processes, whereas, weight hour space velocity (WHSV) showed variation in products yields. In DR, Ni showed better performance at high temperature and low WHSV. Highest CH4 conversion and H2 selectivity of 77.1 and 36.7%, respectively, were observed at 900°C temperature and 20,000 NmL g-1 h-1 WHSV, whereas, increased WHSV to 40,000 NmL g-1 h-1, 21.6 and 26.3% decrement in CH4 conversion and H2 selectivity was observed. DOR employed at 0.17 O2/CH4 ratio with high reaction temperature (≥850°C) showed improved performance in terms of reactant conversion and H2 yield. At 900°C, CH4 conversion and H2 selectivity of 80.8 and 35.9 %, respectively, were obtained at 0.17 O2/CH4 ratios in DOR. Carbon deposition of 0.40 wt % was examined under dry reforming at 900°C, whereas, negligible carbon deposition (0.003 wt.%) was observed in case of DOR after 2h of continuous reaction stream. More than 95% value of determination coefficients by analysis of variance proved that the developed regression models were highly satisfactory. Experimentally, maximum H2 enrichment of 38.7% with 82.9 and 90.8% of CH4 and CO2 conversions, respectively, were achieved at optimal reaction conditions of 900°C, 1.5 of CH4/CO2 ratio and 0.10 ratio of O2/CH4. The H2-TPR results revealed that 11 wt. % Ni impregnation on TiO2 support makes the catalyst with strong metal-support interaction which moderates the metal sintering. Also, the addition of CeO2 effectively improved the CH4 and CO2 conversions as well as H2 enrichment. At 850°C, 11 wt. % Ni/TiO2 catalyst leads to 70.5% CH4 conversion with 32.0% H2 enrichment, whereas, Ni0.11/Ce0.20 (Al2O3-TiO2) yielded high CH4 conversion (84.9%) with 40.6% of H2 enrichment. No significant change in the activity of the catalyst was observed with 22.6 wt. % of carbon deposited on the Ni0.11/Ce0.20 (Al2O3-TiO2) catalyst, after 18h of continuous reforming. Moreover, under DOR biogas, the stoichiometric H2/CO ratio (1.2) was observed at 0.47 O2/CH4 ratios with negligible carbon deposition. Thus, Ni0.11/Ce0.20 (Al2O3-TiO2) catalyst exhibited better activity and selectivity with high catalyst stability at 850°C. Further, in order to reduce carbon deposition on the catalyst bed, dry oxidative reforming was carried out at 650°C with varying proportions of O2/CH4 ratio which resulted in significantly higher CH4 conversion with low catalyst deactivation. The effect of Zn loading (10 and 20 wt. %) on the catalytic activity was assessed in terms of reactant (CH4 and CO2) conversion, product (H2 and CO) yield and H2/CO ratio. At 650°C, CH4 conversion and H2 selectivity achieved were 18.1 and 4.3%, respectively, whereas, at 900°C, enhanced CH4 conversion (78.5%) and H2 (29.8%) selectivity were achieved with Ni0.1/CeO2 catalyst. It was also observed that Ni supported on mixed support exhibited higher reactant conversions when compared to Ni supported with ceria. At 900°C, Ni0.10/ (Zn0.1-Ce0.9) catalyst showed higher CH4 and CO2 conversion of 83.1 and 97.0%, respectively, with 40.3% of H2 enrichment. HEB (gaseous fuel) has also been utilized in a CI engine under dual fuel mode with diesel and B20 as pilot fuels. Palm oil was used for biodiesel production, and B20 biodiesel blend was used to achieve diesel substitution. Experimentation was carried out on the 3.5 kW CI engine test rig by varying brake mean effective pressure (bmep) between 0 and 3.5 bar, as well as, HEB (0.1 to 0.5 kg/h). HEB induction effects on various engine characteristics (combustion, performance, and emission) were studied at rated engine speed (1500 rpm). Results revealed that the ignition delay period and peak cylinder pressure increased with increasing HEB proportion (0.1 to 0.5 kg/h) in comparison to diesel and B20 mode. The heating value of HEB (57.0 MJ/Kg) is higher when compared to diesel (42.0 MJ/Kg) and (41.2 MJ/kg), which led to improved brake thermal efficiency in dual fuel modes. The emission results showed that with an increase of HEB rates, the NOx emission mildly decreases, but smoke opacity and hydrocarbon emissions majorly reduce. The influence of compression ratio (16:1, 17:1 and 18:1) on various engine characteristics by fuelling diesel and B20 along with HEB under dual fuel mode in compression ignition engine was also investigated. Experimental observations revealed that ignition delay (ID) period decreased persistently, whereas the peak cylinder pressures and brake thermal efficiency increased with increasing CR (16:1 to 18:1) under HEB-Diesel and HEB-B20 dual fuel modes. The hydrocarbon, carbon monoxide and smoke emissions decrease continuously, while, the increase of oxide of nitrogen with increasing CR (16:1 to 18:1) was observed. Thus, the HEB induction may be a feasible technological solution to overcome the issue of low BTE and high hydrocarbon emissions in biogas operated CI engines.