Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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In Situ Sub- and Supercritical Water Gasification of Nano-Nickel (Ni2+) Impregnated Biomass for H2 Production Ashutosh Kumar and Sivamohan N. Reddy* Department of Chemical Engineering, Indian Institute of Technology Roorkee, Roorkee, Uttarakhand 247 667 India
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ABSTRACT: Nanocatalysts and their integration into biomass have gained importance over the past few years. This research gave insights on impregnation of Ni salts in bagasse and mosambi peels with the variation of pH at 30 °C for 48 h. The highest loading was attained at pH 6.5 for bagasse and 5.2 for mosambi peels. The impregnated metal biomass samples were subjected to in situ hydrothermal gasification over the temperature range 300−500 °C. The transition of Ni2+ to metal nickel nanoparticles during in situ hydrothermal treatment was confirmed by X-ray diffraction and X-ray photoelectron spectroscopy. Maximum yields of H2, 13.82 mmol/g of bagasse and 9.52 mmol/g of mosambi peel, were attained at the operated temperature of 500 °C and 1:8 biomass to water ratio with carbon gasification efficiency approaching 73.7 and 60.6% for bagasse and mosambi peels, respectively. The performance of hydrothermal gasification of impregnated biomass was compared with Raney Ni and washed samples at the above operated supercritical temperature and biomass to water ratio. The results conveyed that in situ gasification of Ni2+ impregnated biomass enhanced not only H2 yields but also improved overall gas yields contributing to the carbon gasification efficiency of both feedstocks.
1. INTRODUCTION The substantial increase in energy demand across the globe and the depletion of fossil fuels enable exploration of alternate renewable energy sources. Biomass, being rich in carbon content, gains more attention over the years due to net zero CO2 emissions.1 The bioavailability and high energy content directs the consideration of biomass as a potential renewable energy source. Tropical countries are known for their rich agricultural production, with sugarcane being the most cultivated crop. The processing of sugarcane for the production of sugar and other valuable products result in one-third of the feed as residue which has the potential to generate power.2 Annual production of nearly 280 million metric tons (MMT) of sugarcane bagasse paves a path to generate fuel gas by gasification process.3 Annually, residues from the citrus fruit family contribute nearly 15 Mt of total food waste generated, with India being in fourth place in the production of citrus fruits.4 The agricultural and fruit residues are renewable and abundant with high cellulose/hemicellulose content that could be converted into fuels.5 Biomass is vulnerable to generate fuel gases via conventional thermo© XXXX American Chemical Society
chemical routes such as pyrolysis and gasification but is constrained due to high moisture content.6−8 The application of hydrothermal gasification can rule out the extra energy demand for drying in the thermochemical routes. The thermophysical (low density, high diffusivity, low dielectric constant) properties of water beyond its critical point (Tc = 374 °C; Pc = 22.1 MPa) makes it a promising medium for degradation of organic matter into fuel gases.9 Further, at the operated supercritical conditions water behaves as a catalyst and reactant simultaneously.10 The complete dissolution of organics at supercritical conditions rules out the mass transfer barriers and restricts the unwanted polymerization reactions retarding the formation of tar.11 Temperature is the key parameter for the product gas yields during the hydrothermal gasification.12 Depending on the operating temperature, hydrothermal gasification can be noted as low Received: Revised: Accepted: Published: A
January 23, 2019 February 25, 2019 March 4, 2019 March 4, 2019 DOI: 10.1021/acs.iecr.9b00425 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research temperature (500 °C) gasification.13 Methane is the main gas product yield in low temperature gasification, while H2 can be found majorly in the product gas mixture at high temperatures.9,13 The implication of catalysts can be an alternative to reduce harsh operating conditions of supercritical water gasification with a motive to selectively promote hydrogen.14 The catalysts implemented should have the potential to catalyze the breakage of C−C, C−O, C−H, and O−H bonds to yield H2 rich gas mixture.10,15 Researchers have tested a variety of both homogeneous and heterogeneous catalysts to selectively enhance H2 gas yields.16−20 Alkali and alkaline earth metals have been found to be active and selective for H2 by catalyzing the water gas shift (WGS) reaction. However, at high concentrations, hydroxides and carbonates of these alkali and alkaline earth metals precipitate out in supercritical water, limiting their industrial application.21 In addition, transition metals along with the other metal oxides were also tested for the upgrading of fuel gas yields. Both supported and unsupported Ni and Ru are choices of implementation for the biomass degradation and selectively promote H2-rich product gas yields. Recently, supercritical water gasification of model compounds (glucose, cellulose, and lignin) and real biomass (wheat straw, timothy grass, and canola meal) were performed in the presence of Ni and Ru supported on TiO2, activated carbon, ZnO, MgO, and Al2O3 along with promoters (Ce, Cu, Co, and Ru) in batch and continuous flow reactors.22−24 Nanocatalysts of Ni, Co, Zn, and Cu supported on TiO2 which were synthesized by a hydrothermal route were tested for furfural gasification, with Zn exhibiting better performance than the others.25 Sugarcane bagasse was effectively catalyzed to selectively enhance H2 with Cu and potassium promoted Ni and Ru supported catalysts on Al2O3.26−29 Catalytic supercritical water gasification of oil frond biomass with NiO, CuO, and ZnO supported on MgO was conducted, and ZnO displayed high selectivity and conversion.30 The above-mentioned catalysts are prone to deactivation, poisoning, sintering, and phase transformation leading to stability issues.13 Further, the synthesis of catalysts requires a secondary setup for impregnation followed by calcination at high temperatures. Moreover, heavy metals present in soil were known to accumulate in plant roots, stems, and leaves during their growth through phytoextraction of contaminated soils.31 These biomasses when subjected to gasification yields give different yields depending on the composition of various metals. Researchers made an attempt for direct impregnation of metals onto the feed material to understand their influence during the generation of gaseous fuels.32,33 Wood chips were impregnated with Ni to increase the yields of H2 during the pyrolysis in the temperature range 400−500 °C.32 Sequentially, Ni was integrated into a matrix of beech wood chips to reduce tar during pyrolysis below 500 °C with H2 yield increasing beyond 90%.33 Recently, Nanda et al.34 performed sub- and supercritical water gasification of biomass (pinewood and wheat straw) and reported a maximum H2 yield of 5.8 mmol/g of biomass with a carbon gasification efficiency of nearly 32.6%. Only a few studies are available in the literature on in situ pyrolysis and gasification studies of metal impregnated biomass to raise the productivity and quality of gas products. Moreover, in situ gasification of metal impregnated biomass for energy applications needs special
attention due to its enhanced product gas yields along with nanometal synthesis. The current research work aims for the integration of metal into the potential renewable energy source (biomass) to produce high yields of clean fuel gas (H2) and to understand their influence on the gasification process. Sugarcane bagasse and peels of Citrus limetta (mosambi) were selected as biomass that can adsorb/integrate nickel ions from the solution into the lignocellulosic matrix. The loading of metal species depend mainly on the pH of the solution. The impregnated biomass was subjected to in situ gasification at sub- and supercritical conditions. The key operating parameters (temperature and concentration) for hydrothermal gasification of Ni-impregnated biomass samples were varied, and the gas yields were noted along with Ni transformations. In addition, the performance of metal impregnated biomass was compared with that of Raney Ni and washed biomass at the supercritical water gasification operating parameters.
2. MATERIALS AND METHODS 2.1. Biomass and Chemicals. Sugarcane bagasse was obtained from a nearby juice industrial source, and citrus fruit peels were collected from a local juice center located in Roorkee, India. The selected biomass was crushed and reduced to a size of
