Effect of nickel loading on hydrogen production and chemical oxygen demand (COD) destruction from glucose oxidation and gasification in supercritical water

Gasification and partial oxidation of 0.25 molar glucose solution was conducted over different metallic nickel (Ni) loadings (7.5, 11, and 18 wt%) on different catalyst supports (θ-Al₂O₃ and γ-Al₂O₃) in supercritical water. Experiments were carried out at three different temperatures (T) of 400, 450, and 500 °C at constant pressure of 28 MPa and a 30 min reaction time (t). For comparison, some experiments were conducted using high loading commercial catalyst (65 wt% Ni on Silica–alumina). Hydrogen peroxide (H₂O₂) was used as a source of oxygen in the partial oxidation experiments. Oxygen to carbon molar ratios (MR) of 0.5–0.9 were examined to increase the hydrogen production via carbon monoxide (CO) production. Results showed that in the absence of the catalyst, the optimum molar ratio was 0.8 i.e. 80% of the amount of oxygen required for complete oxidation of glucose. At a molar ratio of 0.8, the hydrogen yield was 0.3 mol/mol, as compared to 0.2 mol/mol glucose at molar ratio of 0.5 and 0.9. This optimized oxygen dose was adopted as a base line for catalysts evaluation. The main gaseous products were carbon dioxide (CO₂), carbon monoxide (CO), hydrogen (H₂), and methane (CH₄). Results also showed that the presence of Ni increased the total gas yield increased in the 7.5–18 wt Ni/Al₂O₃ catalyst. An increase in MR from 0.55 to 0.8 increased the of carbon dioxide and hydrogen yields from 1.8 to 3.8 mol/mol glucose and from 0.9 to 1.1 mol/mol. The carbon monoxide and methane yields remain constant at 2 and 0.5 mol/mol glucose, respectively. The introduction of hydrogen peroxide (H₂O₂) prior to the feed injection inhibited the catalyst activity and did not increase the hydrogen yield whereas the introduction of H₂O₂ after 15 min of reaction time increased the hydrogen yield from 0.62 mol/mol to 1.5 mol/mol. This study showed that approximately the same hydrogen yield can be obtained from the synthesized low nickel alumina loading (18 wt%) catalyst as with the 65 wt% nickel on silica–alumina loading commercial catalyst. The highest H₂ yield of 1.5 mol/mol glucose was obtained with commercial Ni/silica–alumina with a BET surface area of 190 m²/g compared to 1.2 mol/mol with the synthesized Ni/θ alumina with a BET surface area of 46 m²/g.     

Hydrogen production by catalytic gasification of coal in supercritical water with alkaline catalysts: Explore the way to complete gasification of coal

Supercritical water gasification (SCWG) of coal is a promising technology for clean coal utilization. In this paper, hydrogen production by catalytic gasification of coal in supercritical water (SCW) was carried out in a micro batch reactor with various alkaline catalysts: Na₂CO₃, K₂CO₃, Ca(OH)₂, NaOH and KOH. H₂ yield in relation to the alkaline catalyst was in the following order: K₂CO₃ ≈ KOH ≈ NaOH > Na₂CO₃ > Ca(OH)₂. Then, hydrogen production by catalytic gasification of coal with K₂CO₃ was systematically investigated in supercritical water. The influences of the main operating parameters including feed concentration, catalyst loading and reaction temperature on the gasification characteristics of coal were investigated. The experimental results showed that carbon gasification efficiency (CE, mass of carbon in gaseous product/mass of carbon in coal × 100%) and H₂ yield increased with increasing catalyst loading, increasing temperature, and decreasing coal concentration. In particular, coal was completely gasified at 700 °C when the weight ratio of K₂CO₃ to coal was 1, and it was encouraging that raw coal was converted into white residual. At last, a reaction mechanism based on oxygen transfer and intermediate hybrid mechanism was proposed to understand coal gasification in supercritical water.     

Review of heterogeneous catalysts for sub- and supercritical water gasification of biomass and wastes

Nowadays, substantial efforts are devoted to decrease our dependence on fossil fuels. This change will heavily rely on development of new and improved catalytic processes. Over the past two decades, catalytic hydrogen production from wet biomass and organic compounds in sub- and supercritical water (SCW) has gained significant attention. In this process, catalysts are employed to enhance the gas formation rate at moderate temperatures. Catalysts can be also utilized to shift the product distribution toward a more desirable compound (e.g. hydrogen). The effectiveness of various types of heterogeneous catalysts, mainly containing nickel and ruthenium, have been demonstrated for hydrothermal gasification of organic compounds. Catalyst formulation along with operating conditions such as temperature and feed concentration can significantly affect the conversion and selectivity of the process. This paper reviews the major findings of hydrothermal gasification over the past two decades with the aid of heterogeneous catalysts in terms of activity, hydrogen selectivity and stability. Commercially available and laboratory-prepared catalysts including supported and skeletal metal catalysts, activated carbon, oxides, metal wires and other innovative catalysts are considered. Results of supercritical water gasification (SCWG) of various feedstocks reported in the literature are compared and possible mechanisms and rates of deactivation of heterogeneous catalysts are discussed.     

Activity of Ni/CeO2 catalyst for gasification of phenol in supercritical water

An effective Ni/CeO₂ catalyst prepared by the polyol reduction method for degrading phenol into CH₄, H₂ and CO₂ in supercritical water (SCW) was developed. About 80% carbon gasification efficiency can be achieved at 525 °C and 60 min with 5 wt% phenol, 0.098 kg/m³ water density and 0.5 g Ni/CeO₂/g phenol catalyst, forming CH₄ and H₂ as the main gaseous products. Comparison study indicated that the efficiency of present Ni/CeO₂ catalyst was about 20% higher than that of a commercial catalyst, i.e., Ni/SiO₂Al₂O₃ from Sigma-Aldrich with 65 wt%Ni, at a reaction conditions of 500 °C and 30 min. The characterization analyses of BET, TPR, XRD, XPS and TEM indicated that there was a NiCe alloy formed in Ni/CeO₂, which could be important to enhance the activities of the carbon gasification efficiencies and gas yields. A kinetic modelings were conducted and the results showed that the lnA and the activation energy (Ea) of gasification were 7.1 ± 0.5 and 58.1 ± 3.2 kJ/mol for the gaseous product, and were 2.6 ± 0.9 and Ea is 36.6 ± 5.6 kJ/mol for the char formation, respectively. The present Ni-based-metal Ni/CeO₂ catalyst is cheaper and has a potential application for the gasification to convert phenol into gases fuels in SCW process.     

Reactions of different food classes during subcritical water gasification for hydrogen gas production

The reactions of different food classes during alkaline subcritical water gasification have been investigated with a view on hydrogen gas production. Experiments were conducted with sub-stoichiometric amounts of H₂O₂ for partial oxidation. NaOH was added to aid sample decomposition, reduce char/tar formation and to promote water–gas shift reaction. In general, hydrogen gas production depended on the class of food wastes including their chemical structure. Carbohydrate-rich food waste (glucose, molasses, tropical fruit mixture, whey powder) produced higher H₂ gas yields than others (proteins and lipids). Lipid-rich samples were the most difficult to decompose into gasifiable intermediates and therefore produced the lowest H₂ yield. Generally, the addition of NaOH led to higher H₂ generation from all sample types. However, two separate side reactions namely, neutralization and saponification involving NaOH with protein- and lipid-rich samples, respectively were significant. Hydrogen production from carbohydrate-rich samples was most suited for the reaction conditions applied.     

Co-gasification of catechol and starch in supercritical water for hydrogen production

Hydrogen production from waste feedstocks using supercritical water gasification (SCWG) is a promising approach towards cleaner fuel production and a solution for hard to treat wastes. In this study, the catalytic co-gasification of starch and catechol as models of carbohydrates and phenol compounds was investigated in a batch reactor at 28 MPa, 400–500 °C, from 10 to 30 min. The effects of reaction conditions, and the addition of calcium oxide (CaO) as a carbon dioxide (CO₂) sorbent and TiO₂ as catalyst on the gas yields and product distribution were investigated. Employing TiO₂ as a catalyst alone had no significant effect on the H₂ yield but when combined with CaO increased the hydrogen yield by 35% and promoted higher total organic carbon (TOC) reduction efficiencies. The process liquid effluent was characterized using GC–MS, with the results showing that the major non-polar components were phenol, substituted phenols, and cresols. An overall reaction scheme is provided.     

Hydrogen production by coal gasification in supercritical water with a fluidized bed reactor

The technology of supercritical water gasification of coal can converse coal to hydrogen-rich gaseous products effectively and cleanly. However, the slugging problem in the tubular reactor is the bottleneck of the development of continuous large-scale hydrogen production from coal. The reaction of coal gasification in supercritical water was analyzed from the point of view of thermodynamics. A chemical equilibrium model based on Gibbs free energy minimization was adopted to predict the yield of gaseous products and their fractions. The gasification reaction was calculated to be complete. A supercritical water gasification system with a fluidized bed reactor was applied to investigate the gasification of coal in supercritical water. 24 wt% coal-water-slurry was continuously transported and stably gasified without plugging problems; a hydrogen yield of 32.26  mol/kg was obtained and the hydrogen fraction was 69.78%. The effects of operational parameters upon the gasification characteristics were investigated. The recycle of the liquid residual from the gasification system was also studied.     

Hydrogen production from coal gasification in supercritical water with a continuous flowing system

The technology of supercritical water gasification can convert coal to hydrogen-rich gaseous product efficiently and cleanly. A novel continuous-flow system for coal gasification in supercritical water was developed successfully in State Key Laboratory of Multiphase Flow in Power Engineering (SKLMF). The experimental device was designed for the temperature up to 800 °C and the pressure up to 30 MPa. The gasification characteristics of coal were investigated within the experimental condition range of temperature at 650–800 °C, pressure at 23–27 MPa and flow rate from 3 kg h⁻¹ to 7 kg h⁻¹. K₂CO₃ and Raney-Ni were used as catalyst and H₂O₂ as oxidant. The effects of main operation parameters (temperature, pressure, flow rate, catalyst, oxidant, concentration of coal slurry) upon gasification were carried out. The slurry of 16 wt% coal + 1.5 wt% CMC was successfully transported into the reactor and continuously gasified in supercritical water in the system. The hydrogen fraction reached up to 72.85%. The experimental results demonstrate the bright future of efficient and clean conversion of coal.     

High performance noble-metal-free NiCo/AC bimetal for gasification in supercritical water

We report high performance Noble-metal-free NiCo/AC bimetal for gasification in supercritical water by the polyol reduction method with chloroplatinic acid as nucleation seed. At 475 °C and 20 min, the carbon efficiency (CE) for gasification of phenol with NiCo/AC has reached about 65%, which represents a more than 20-fold enhancement of carbon efficiency compared to the Co/AC and 6-fold compared to the Ni/AC. The CE of commercial noble Ru/C (5 wt%) catalyst is only about 15% higher than that of NiCo/AC catalyst. The catalyst was characterized by BET, XRD, XPS and TEM. The results indicate smaller and higher disperse of NiCo alloy has been achieved. By using NiCo/AC catalyst, the CE can reach about 90% at 525 °C and 60 min with a 5 wt% phenol, 0.098 kg/m³ density and catalyst loading of 0.5 g/g (the mass of NiCo/AC catalyst/the mass of phenol). The results of GC–MS indicate NiCo/AC can suppress tar successfully. A kinetic modeling was also proposed to describe gaseous product and tar formation, which gives the activation energy (Ea) 123.56 ± 31 kJ/mol and the frequency factor 16.93 ± 0.48 for gaseous products. While for tar formation, the Ea is 112.95 ± 24 kJ/mol and the frequency factor is 13.99 ± 0.37. The successful use of noble-metal-free NiCo/AC shows the potential to replace noble Ru-catalysts in SCWG process for gases fuels by bimetal.