IN-SITU GASIFICATION OF METAL IMPREGNATED BIOMASS IN SUB/- AND SUPERCRITICAL WATER

Abstract

Depletion of fossils fuels and rise in global warming due to its extensive use drives attention towards renewable energy sources like biomass. Biomass being rich in cellulosic components is complex in nature and requires energy to degrade into gaseous fuels. Thermochemical conversion routes are often limited for biomass due to high moisture content that demands extra energy for drying. Supercritical water gasification of biomass has been found to be effective to handle variety of biomass to generate H2 rich product gaseous mixture. Catalysts are usually implemented to selectivity promote H2 yields with a control on the operating parameters. However, these catalysts are prone to deactivation with time. Henceforth, nanocatalysts and their integration into the biomass gained importance over the last few years. This research gives insights on impregnation of nickel (Ni), ruthenium (Ru) and iron (Fe) salts in banana pseudo-stem (Musa acuminate), sugarcane bagasse (Saccharum officinarum) and mosambi peels (Citrus limetta) with the variation of pH at 30 ⁰C for 48 h. The highest metals loading into banana pseudo-stem was attained at pH 6.4, 5.8 and 5.3 for Ni (0.98 mol/kg biomass), Ru (0.79 mol/kg biomass) and Fe (0.83 mol/kg biomass), respectively. Ni loading (1.07 mol/kg biomass) into sugarcane bagasse was maximum at pH 6.5, whereas Ru loading (0.41 mol/kg biomass) into mosambi peels was least at pH 4.6 of all the synthesized metal impregnated samples. As compared to raw biomass, various physico-chemical and morphological changes were observed for metal impregnated biomass samples, that were characterized by proximate and ultimate analysis, FTIR, TGA/DTG, XRD, XPS, TEM and FE-SEM. During the metal impregnation, formation of metal nanoparticles (majorly in the form of oxide or hydroxides) were observed into biomass. The transition of M(+n) to metallic nanoparticles (M(0)) during in-situ hydrothermal treatment was confirmed by XRD and XPS spectral analysis. FESEM-EDX analysis shows the rough and porous structure of metal impregnated samples that infers the uptake of metals by the biomass. From TEM analysis, the size of nanoparticles imbedded into biomass matrix were found to be less than 50 nm. Thermal degradation behavior of raw and metal impregnated biomass was performed at different heating rates to evaluate the kinetic parameters. Three isoconversional models such as Flynn-Wall-Ozawa (FWO), Kissinguer-Akahira-Sinose (KAS), and Kissinger’s methods have been employed to calculate the activation energy and pre-exponential factor from TGA/DTG profiles. The ii activation energy of raw sugarcane bagasse (113.61 kJmol-1) has found to be minimum followed by banana pseudo-stem (116.22 kJmol-1) and mosambi peels (138.54 kJmol-1) supporting the component analysis of the three biomass samples. Lower activation energy and reduced weight loss were observed for metal impregnated biomass over the raw biomass. Among the metal impregnated biomass, Ni impregnated bagasse resulted in lower activation energy (56.95 kJmol-1) because of its high loading and activity towards degradation reforming reactions. The impregnated metal biomass samples were subjected to in-situ hydrothermal gasification over the temperature range of 300-600 ⁰C, biomass-to water ratio (feed concentration) of 1:4 to 1:10 for the residence time of 30-60 min. The carbothermal reduction of metal oxides to nanometals during sub/- and supercritical water gasification accelerate the degradation of organic intermediates and thereby contributes to gas yields. Temperature being the most crucial parameter for the degradation of organic biomass decides the quality of the product gaseous mixture. Cracking and reforming reactions that are endothermic demands high temperature for the production of fuel gases. Maximum gas yields are attained at the operated supercritical temperature of 600 ⁰C for all the raw and metal impregnated biomass samples. Low biomass concentration compliments the water as reactant and free radical generator and thereby significantly enhance the product gas yields of reforming reactions (H2 and CO2). Longer residence time provides the ample time for the degradation of organic intermediates formed during the supercritical gasification of biomass. The experimental results confirmed the maximum total gas yields (TGY) and carbon gasification efficiency (CGE) at biomass to water ratio of 1:10 and 60 min residence time for both the raw and metal impregnated biomass samples. Ni that have been loaded to maximum extent in all the three different biomass showed highest activity in terms of TGY, CGE and H2 gas yields. The high catalytic activity of Ni towards cracking and reforming reactions results in enhancement of H2 and CH4 yields. Ru have promoted both H2 and CH4 yields whereas Fe has been found to increase H2 yields to a greater extent over the raw samples. Of all the metal impregnated biomass, Ni impregnated sugarcane bagasse resulted in maximum H2 yield (14.93 mmol/kg biomass), total gas yield (30.23 mmol/kg biomass) and carbon gasification efficiency (81.78%) at the operated supercritical temperature of 600 ⁰C and 1:10 biomass to water ratio for the residence time of 60 min. On the basis of performance parameters, the order of the activity of three metals in three biomass followed as: Ni > Ru > Fe.