Subcritical water gasification of lignocellulosic wastes for hydrogen production with Co modified Ni/Al2O3 catalysts

A Review of the Design and Performance of Catalysts for Hydrothermal Gasification of Biomass to Produce Hydrogen-Rich Gas Fuel

Supercritical water gasification has emerged as a promising technology to sustainably convert waste residues into clean gaseous fuels rich in combustible gases such as hydrogen and methane. The composition and yield of gases from hydrothermal gasification depend on process conditions such as temperature, pressure, reaction time, feedstock concentration, and reactor geometry. However, catalysts also play a vital role in enhancing the gasification reactions and selectively altering the composition of gas products. Catalysts can also enhance hydrothermal reforming and cracking of biomass to achieve desired gas yields at moderate temperatures, thereby reducing the energy input of the hydrothermal gasification process. However, due to the complex hydrodynamics of supercritical water, the literature is limited regarding the synthesis, application, and performance of catalysts used in hydrothermal gasification. Hence, this review provides a detailed discussion of different heterogeneous catalysts (e.g., metal oxides and transition metals), homogeneous catalysts (e.g., hydroxides and carbonates), and novel carbonaceous catalysts deployed in hydrothermal gasification. The article also summarizes the advantages, disadvantages, and performance of these catalysts in accelerating specific reactions during hydrothermal gasification of biomass, such as water-gas shift, methanation, hydrogenation, reforming, hydrolysis, cracking, bond cleavage, and depolymerization. Different reaction mechanisms involving a variety of catalysts during the hydrothermal gasification of biomass are outlined. The article also highlights recent advancements with recommendations for catalytic supercritical water gasification of biomass and its model compounds, and it evaluates process viability and feasibility for commercialization.

Review on Catalytic Biomass Gasification for Hydrogen Production as a Sustainable Energy Form and Social, Technological, Economic, Environmental, and Political Analysis of Catalysts

Sustainable energy production is a worldwide concern due to the adverse effects and limited availability of fossil fuels, requiring the development of suitable environmentally friendly alternatives. Hydrogen is considered a sustainable future energy source owing to its unique properties as a clean and nontoxic fuel with high energy yield and abundance. Hydrogen can be produced through renewable and nonrenewable sources where the production method and feedstock used are indicators of whether they are carbon-neutral or not. Biomass is one of the renewable hydrogen sources that is also available in large quantities and can be used in different conversion methods to produce fuel, heat, chemicals, etc. Biomass gasification is a promising technology to generate carbon-neutral hydrogen. However, tar production during this process is the biggest obstacle limiting hydrogen production and commercialization of biomass gasification technology. This review focuses on hydrogen production through catalytic biomass gasification. The effect of different catalysts to enhance hydrogen production is reviewed, and social, technological, economic, environmental, and political (STEEP) analysis of catalysts is carried out to demonstrate challenges in the field and the development of catalysts.

Catalytic pyrolysis of film waste over Co/Ni pillared montmorillonites towards H2 production

The transformation of plastic waste into valuable fuel products via catalytic pyrolysis is a promising and eco-friendly strategy. Herein, a series of Co/Ni pillared montmorillonites were developed as low-cost and effective catalysts for the pyrolysis of post-consumer film waste, which is one of the representative plastic wastes in nature. The best-performing catalyst produced 80.2% of liquid product, with a high selectivity of 43.5% of hydrocarbons at C10-C13 range, and 42.0 vol% of H₂ which is nearly increased by 40-fold as compared to that in non-catalytic run. The improved results were ascribed to the pillared structure, the oxidation state of Co/Ni, and the distribution of acid sites. Particularly, the Lewis acidity (which governs the cyclization and alkanisation) coupled with high surface area and uniform dispersion of transition metallic sites, were found to promote the selectivity of condensable product. The pyrolytic mechanism towards H₂ production was explored by theoretical calculations. The lattice oxygen bonded to both Ni and Co in an octahedral environment was found to promote the adsorption of the fragment of polymer in dehydrogenation. Additionally, the solid residues are potentially applied for the production of valuable carbonaceous materials since they displayed high heating value. This work is expected to provide a direction for the development of pyrolysis technology for fuel production with sustainability and economic viability.

Design and Development of a Catalytic Fixed-Bed Reactor for Gasification of Banana Biomass in Hydrogen Production

Hydrogen produced from biomass is an alternative energy source to fossil fuels. In this study, hydrogen production by gasification of the banana plant is proposed. A fixed-bed catalytic reactor was designed considering fluidization conditions and a height/diameter ratio of 3/1. Experimentation was carried out under the following conditions: 368 °C, atmospheric pressure, 11.75 g of residual mass of the banana (pseudo-stem), an average particle diameter of 1.84 mm, and superheated water vapor as a gasifying agent. Gasification reactions were performed using a catalyzed and uncatalyzed medium to compare the effectiveness of each case. The catalyst was Ni/Al2O3, synthesized by coprecipitation. The gas mixture produced from the reaction was continuously condensed to form a two-phase liquid–gas system. The synthesis gas was passed through a silica gel filter and analyzed online by gas chromatography. To conclude, the results of this study show production of 178 mg of synthesis gas for every 1 g of biomass and the selectivity of hydrogen to be 51.8 mol% when a Ni 2.5% w/w catalyst was used. The amount of CO2 was halved, and CO was reduced from 3.87% to 0% in molar percentage. Lastly, a simulation of the distribution of temperatures inside the furnace was developed; the modeled behavior is in agreement with experimental observations.

Hydrothermal flames for subaquatic, terrestrial and extraterrestrial applications

Hydrothermal flames are formed in supercritical water in the presence of a fuel and an oxidant (usually air or oxygen). Integrating hydrothermal flames as the heat source for supercritical water oxidation helps to minimize the reaction time (to milliseconds), improve the reaction kinetics and reduce the chances of corrosion and reactor plugging. This review outlines state-of-the-art research on hydrothermal flames including the impacts of process parameters on flame ignition. The ignition and sustainability of hydrothermal flames are dependent on several factors such as the type of fuel and its concentration, type of oxidant (air and oxygen) as well as the temperatures and flow rate of the feed and oxidant. The article describes some novel applications of hydrothermal flames for clean energy production, geothermal energy recovery, deep well spallation, wastewater treatment, degradation of recalcitrant nitrogen-containing compounds and heavy oil upgrading. Finally, the challenges and future perspectives of hydrothermal flame applications are discussed. This review also highlights some technical considerations relating to hydrothermal flames such as the choice of organic solvent and its characteristics, preheating, ignition mechanism, flame stability and propagation, advanced reactor configurations, mixing with subcritical and supercritical components, recirculation zones, cooling mechanisms, corrosion and salt precipitation.

Sub- and Supercritical Water Gasification of Rice Husk: Parametric Optimization Using the I-Optimality Criterion

In this study, rice husk biomass was gasified under sub- and supercritical water conditions in an autoclave reactor. The effect of temperature (350-500 °C), residence time (30-120 min), and feed concentration (3-10 wt %) was experimentally studied using the response surface methodology in relation to the yield of gasification products. The quadratic models have been suggested for both responses. Based on the models, the quantitative relationship between various operational conditions and the responses will reliably forecast the experimental outcomes. The findings revealed that higher temperatures, longer residence times, and lower feed concentrations favored high gas yields. The lowest tar yield obtained was 2.98 wt %, while the highest gasification efficiency and gas volume attained were 64.27% and 423 mL/g, respectively. The ANOVA test showed that the order of the effects of the factors on all responses except gravimetric tar yield follows temperature > feed concentration > residence time. The gravimetric tar yield followed a different trend: temperature > residence time > feed concentration. The results revealed that SCW gasification could provide an effective mechanism for transforming the energy content of RH into a substantial fuel product.