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 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.     

Numerical investigation of coal gasification in supercritical water with the ReaxFF molecular dynamics method

Studies on the coal gasification process in supercritical water (SCW) were carried out with the ReaxFF molecular dynamics (MD) method, in which the Wiser model of the coal molecule was adopted. The results show that hydrogen production increases with increase of temperature and water–coal mass ratio. It is also found that the coal molecule breaks into small fragments before it reacts with water molecules. The detailed chemical reactions and pathways of hydrogen generation during the gasification process are disclosed. H ions are found to be the main source of hydrogen generation, and C–H–O compounds or radicals are the most essential reactants throughout the reactions producing H₂ and H ions. OH ions can significantly accelerate the oxidization of organic fragments to produce C–H–O compounds and radicals, which explains how catalysts of alkali salts such as NaOH and KOH improve hydrogen production.     

Hydrogen production from glycerol by supercritical water gasification in a continuous flow tubular reactor

In this work, glycerol was used for hydrogen production by supercritical water gasification. Experiments were conducted in a continuous flow tubular reactor at 445∼600 °C, 25 MPa, with a short residence time of 3.9∼9.0 s. The effects of reaction temperature, residence time, glycerol concentration and alkali catalysts on gasification were systematically studied. The results showed that the gasification efficiency increased sharply with increasing temperature above 487 °C. A short residence time of 7.0 s was enough for 10 wt% glycerol gasification at 567 °C. With the increase of glycerol concentration from 10 to 50 wt%, the gasification efficiency decreased from 88% to 71% at 567 °C. The alkali catalysts greatly enhanced water-gas shift reaction and the hydrogen yield in relation to catalysts was in the following order: NaOH > Na₂CO₃>KOH > K₂CO₃. The hydrogen yield of 4.93 mol/mol was achieved at 526 °C with 0.1 wt% NaOH. No char or tar was observed in all experiments. The apparent activation energy and apparent pre-exponential factor for glycerol carbon gasification were obtained by assuming pseudo first-order kinetics.     

Catalytic supercritical water gasification mechanism of coal

Catalytic supercritical water gasification (SCWG) for H₂ production is a hopeful way of coal conversion to replace the traditional coal utilization mode. At present, the detailed catalytic mechanism in the process remains unknown. Herein, a comprehensive catalytic SCWG mechanism of coal is proposed by establishing a novel catalytic kinetic model. It shows that catalysts (K₂CO₃) break up the coal matrix by a cyclic redox reaction to produce plenty of mesopores, accelerating steam reforming of fixed carbon and coal pyrolysis. Water-gas shift reaction is facilitated by K₂CO₃ via formation of formate, which then promotes steam reforming of CH₄ at high temperature (≥700 °C) due to the decreasing CO. The proposed mechanism provides important insights in catalytic SCWG process of coal.     

Molecular dynamics investigation on the gasification of a coal particle in supercritical water

Coal gasification technology in supercritical water provides a clean and efficient way to convert coal to H₂. In the present paper, the whole supercritical water（SWC）gasification process of a coal particle is studied with the reactive force field (ReaxFF) molecular dynamics (MD) method for the first time. First, the detailed reaction mechanism which can't be clearly illustrated in experiments, such as the evolution of the carbon structure during the gasification process and the detailed reaction mechanism of the main products, is obtained. According to the generation mechanism of H₂, it is found that the supercritical water gasification process of a coal particle can be divided into two stages with different reaction mechanisms, namely the rapid reaction stage and the stable reaction stage. Then, the effects of temperature and coal concentration in the reaction system on the yield of H₂ are studied. Finally, the transition of N in the coal particle is revealed, in which the precursors of NH₃ such as CN, CHN, and CHON are the basic molecular structures for nitrogen atoms during the gasification process at high temperature.     

Hydrogen generation by gasification of phenol and alcohols in supercritical water

The conversion of phenol, cyclohexanol (a hydrogenated analog of phenol for comparison with phenol), and ethanol into gas products in supercritical water (SCW) was studied with the goal to compare the reactivity of their aqueous solutions with the structural features obtained by the method of classical molecular dynamics. Transformation of phenol and alcohols occurs in different ways. In the case of alcohols, the conversion of 75–100% is achieved at 600 °C with noticeable gasification. At the same time, the conversion of phenol is only 47% and no gas products are formed at all. The complete conversion of phenol is achieved at a temperature of 750 °C, while the degree of gasification does not exceed 30%. It is shown that an increase in the phenol gasification degree is possible by pre-catalytic hydrogenation of phenol into cyclohexanol.     

Hydrogen production from the gasification of lignin with nickel catalysts in supercritical water

Hydrogen production from the gasification of lignin with Ni/MgO catalysts in supercritical water was conducted using stainless steel tube bomb reactor. Ni/MgO catalysts were prepared by impregnation method and were calcined at 773–1173 K in air for 8 h. The results of characterization for reduced Ni/MgO catalysts showed that Ni metal and NiO–MgO phase are formed after the reduction of calcined catalyst by _H2${\mathrm{H}}_2$. Furthermore, Ni metal surface area, which was calculated by CO chemical adsorption technique, decreased with increase in calcination temperatures. It was found that the carbon yield of gas products was increased with increase in Ni metal surface area except 10 wt% Ni/MgO (773 K) catalyst. Thus, it can be supposed that there is an optimal Ni particle size for the gasification of lignin in supercritical water. It should be noted that 10 wt% Ni/MgO (873 K) catalyst showed the best catalytic performance (carbon yield 30%) under reaction condition tested. It was concluded that Ni/MgO catalyst is a promising system for the gasification of lignin in supercritical water.     

Hydrogen production by non-catalytic partial oxidation of coal in supercritical water: Explore the way to complete gasification of lignite and bituminous coal

Supercritical water gasification (SCWG) of coal is a promising technology for clean coal utilization. In this paper, hydrogen production by non-catalytic partial oxidation of coal was systematically investigated in supercritical water (SCW) with quartz batch reactors for the first time. The influences of the main operating parameters including residence time, temperature, oxidant equivalent ratio (ER) and feed concentration on the gasification characteristics of coal were investigated. The experimental results showed that H₂ yield and carbon gasification efficiency (CE) increased with increasing temperature and decreasing feed concentration. CE increased with increasing ER, and H₂ yield peaked when ER equaled 0.1. CE increased quickly within 1 min and then tended to be stable between 2 and 3 min. In particular, complete gasification of lignite was obtained at 950 °C when ER equaled 0.1, as for bituminous coal, at a higher temperature of 980 °C when ER equaled 0.2.     

Hydrogen production from supercritical water gasification of alkaline wheat straw pulping black liquor in continuous flow system

Supercritical water gasification of alkaline black liquor was investigated in a continuous flow system. The experiments were carried out at 400–600 °C, 25 MPa, with residence times ranging from 4.94 to 13.71 s. The results showed that the increase of temperature and residence time and the decrease of feeding concentration enhanced the gasification. The gaseous product contained high level of hydrogen (40.26–61.02%). Maximum COD removal efficiency (88.69%) was obtained at 600 °C. The alkalis in black liquor were found to be precipitated in the reactor during the gasification, which decreased the pH of the effluent to the neutral region (6.4–8.0). The precipitated alkalis were dissolved in the water when the fluid temperature in the reactor was cooled to about 360 °C which increased the pH of the effluent to 11.0. A simplified kinetic study for COD removal efficiency was done by the pseudo-first order reaction assumption. The apparent activated energy was 74.38 kJ/mol and the apparent pre-exponential factor was 10^4.05 s⁻¹.     

Oleic acid gasification over supported metal catalysts in supercritical water: Hydrogen production and product distribution

Oleic acid was examined as a model compound for lipids, which was gasified in supercritical water (SCW) using a batch reactor from 400 to 500 °C at 28 MPa. The influence of operating temperature and several commercial catalysts on the gasification efficiency, hydrogen gas yield, and residual liquid product quality was examined and discussed. The main gaseous components measured were carbon dioxide (CO₂), hydrogen (H₂), methane (CH₄), and traces of carbon monoxide (CO). The residual liquid after reaction was characterized by analyzing the chemical oxygen demand (COD), total organic carbon (TOC), volatile fatty acids (VFAs), and the long chain fatty acids (LCFAs), namely, palmitic, myristic, stearic, linoleic, and oleic acids. The results showed that an increase of temperature coupled with the use of catalyst enhanced the gas yield dramatically. The H₂ yield was 15 mol/mol oleic acid converted using both the pelletized Ru/Al₂O₃ and powder Ni/Silica-alumina catalysts which gave 4 times higher than the equilibrium yield. The COD reduction efficiency ranged from 31% at 400 °C without catalyst to 96 % at 500 °C in the presence of Ni/Silica-alumina catalyst. The composition of residual liquid products was studied using gas chromatography/mass spectrometry (GC-MS), with a generalized reaction pathway for oleic acid decomposition in SCW reported.     

Hydrogen production through biomass gasification in supercritical water: A review from exergy aspect

Hydrogen production through supercritical water gasification (SWG) of biomass has been widely studied. This study reviews the main factors from exergy aspect, and these include feedstock characteristics, biomass concentration, gasification temperature, residence time, reaction catalyst, and reactor pressure. The results show that the exergy efficiencies of hydrogen production are mainly in the range of 0.04–42.05%. Biomass feedstock may affect hydrogen production by changing the H₂ yield and the heating value of biomass. Increases in biomass concentrations decrease the exergy efficiencies, increases in gasification temperatures generally increase the exergy efficiencies, and increases in residence times may initially increase and finally decrease the exergy efficiencies. Reaction catalysts also have positive effects on the exergy efficiencies, and the reviewed results show that the effects are followed KOH > K₂CO₃ > NaOH > Na₂CO₃. Reactor pressure may have positive, negative or negligible effects on the exergy efficiencies.     

Experimental basic factors in the production of H2 via supercritical water gasification

This study aims to discuss some of the factors that influence the production of hydrogen via the gasification of organic matter in supercritical water. These factors have been investigated based on the reactions of organic matter with relatively simple chemical structures, such as ethanol, glycerol, and glucose. Investigations of these relatively simple organic materials demonstrate the characteristics and trends in the gasification in supercritical water. The results reported in the literature for these organic compounds can also be extrapolated to the reactions of biomass containing ethanol, glucose, (sugar cane industry) and glycerol (biodiesel industry) in supercritical water. Many organic compounds with different levels of molecular complexity can be used to produce hydrogen, which represents an interesting form of energy storage. Supercritical water (T_c ≥ 374 °C, P_c ≥ 22.1 MPa) has unique physical and chemical properties that minimize mass transport limitations, making it an excellent medium for the decomposition of organic compounds. Thus, understanding the key factors that influence organic compound gasification in supercritical water is extremely important. In this study, we summarize some of the key factors involved in these reactions. The main experimental factors were confirmed to be the temperature, concentration of organic matter in the feed, space time/feed rate, catalysts, oxidants, material and design of the reactor, and pressure. In addition, operational challenges, namely, catalyst deactivation and corrosion are mentioned in the text. Furthermore, the operational challenges were discussed, and the state of the art regarding the gasification of ethanol-, glycerol-, and glucose-containing biomass is also presented.     

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

In this work, gasification of sewage sludge in supercritical water was investigated in a fluidized bed reactor. Effect of operating parameters such as temperature, concentration of the feedstock, alkali catalysts and catalyst loading on gaseous products and carbon distribution were systematically studied. The results showed that the increase of temperature and the decrease of feedstock concentration were both favorable for gasification, and the addition of catalyst enhanced the formation of hydrogen better. The K₂CO₃ catalyst could better enhance gasification efficiency and the catalytic activity of different catalysts for hydrogen production was in the following order: KOH > K₂CO₃ > NaOH > Na₂CO_3. The maximum molar fraction and yield of hydrogen reached to 55.96% and 15.49 mol/kg respectively with KOH at 540 °C. Most carbon in feedstock existed in gaseous and liquid products, and alkali catalysts mainly promoted the water-gas shift reaction rather than steam reforming.     

Experimental investigation on organic functional groups evolution in hydrogen production process by coal gasification in supercritical water

Supercritical water gasification (SCWG) of coal has great application prospect for converting coal into hydrogen-rich gas efficiently and cleanly. However, the previous study on the reaction mechanism for SCWG of coal is relatively macroscopic rather than reflects the reaction essence deeply. The evolution of organic functional groups in Zhundong lignite (ZD), Hongliulin bitumite (HLL) and Ningxia anthracite (NX) during SCWG, as well as the correlation with gaseous products were analyzed quantitatively in this paper. It was found that the lower rank coal contained more free radicals and produced more H₂ with SCW. H₂ yield of the three types of coal exceeded 2 times the hydrogen content in coal at 800 °C. The organic functional groups evolve in 2–4 stages during SCWG process. The decomposition and gasification of organic functional groups mainly took place in low or medium temperature range. About 95% of C=O groups and 90% of aromatic C=C groups cracked and were gasified. Aromatic ether (C_ar-O) groups were formed in high temperature range. The reasonable functional relationship between the parameters of gaseous products and organic functional groups was established, providing a new approach to predict organic functional groups through gaseous products. This research may lay the foundation for further optimization design of reactor.     

Hydrogen production by non-catalytic partial oxidation of coal in supercritical water: The study on reaction kinetics

Supercritical water gasification (SCWG) has attracted great attention for efficient and clean coal conversion recently. A novel kinetic model of non-catalytic partial oxidation of coal in supercritical water (SCW) that describes formation and consumption of gas products (H₂, CO, CH₄ and CO₂) is reported in this paper. The model comprises 7 reactions, and the reaction rate constants are obtained by fitting the experimental data. Activation energy analysis indicates that steam reforming of fixed carbon (FC) is the rate-determining step for the complete gasification of coal. Once CH₄ is produced by pyrolysis of coal, steam reforming of CH₄ will be the rate-determining step for directional hydrogen production.     

Study on gasification mechanism of biomass waste in supercritical water based on product distribution

Supercritical water gasification technology is widely applied to convert organic waste into valuable substances as a clean and efficient method. Biomass gasification in SCW is a complex process and complicated chemical reactions like decomposition and poly-condensation take place, thus, reaction mechanism of real biomass needs to be further investigated. In this paper, experimental study on cornstalk gasification in SCW was conducted at the temperature of 500–800 °C, reaction time of 1–15min and feedstock concentration of 1–9%. The effects of various operating parameters on evolution of gas, liquid and solid products were conducted. It was discovered that pore structure and carbon microspheres appeared successively on the surface of solid residue. Mechanism study showed that the biomass was first depolymerized into monomer and its derivatives, then cracked and poly-condensed into a nuclear to generate carbon microspheres as its concentration reached the critical concentration. As the reaction proceeds, reduction reaction, coke combustion and secondary reaction occurred, thus carbon microspheres decreased. The results indicated that higher reaction temperature, longer reaction time and lower feed concentration were conducive to improving reaction performance of biomass. Finally, it was discovered that carbon gasification efficiency reached 99% at the temperature of 700 °C, reaction time of 15 min and biomass concentration of 3%.     

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 from catalytic gasification of switchgrass biocrude in supercritical water

Biomass can be liquefied to produce biocrude for ease of transportation and processing. Biocrude contains oxygenated hydrocarbons of varying molecular structure and molecular weights, including lignin derived products, sugars and their decomposition products. In this work several catalysts were screened for hydrogen production by gasification of switchgrass biocrude in supercritical water at 600 °C and 250 bar. Nickel, cobalt, and ruthenium catalysts were prepared and tested on titania, zirconia, and magnesium aluminum spinel supports. Magnesium aluminum spinel was seen to be an inappropriate support as reactors quickly plugged. Ni/ZrO₂ gave 0.98 mol H₂/mol C, the highest hydrogen yield of all tested catalysts; however, over time, increase in pressure drop lead to reactor plugging with all zirconia supported catalysts. Titania supported catalysts gave lower conversions, however they did not plug during the course of the study. Charring of all catalysts was seen to occur at the entrance of the reactor as the biocrude was heated. All support materials suffered significant surface area loss due to sintering.     

Hydrogen production via supercritical water gasification of glycerol enhanced by simple structured catalysts

The supercritical water gasification (ScWG) technology is a promising alternative for H₂-rich gas production from renewable sources, such as residual glycerol from biodiesel manufacture. Combined with heterogeneous catalysts, the ScWG process can achieve improved selectivity towards the desired products and high conversion efficiency in short reaction times. In this work, the efficiency of a synthesized Ni-based catalyst supported in cordierite (CRD) honeycomb structure on the ScWG of glycerol was evaluated and compared with two commercial automotive catalysts. Initially, to determine the best experimental conditions, the ScWG experiments were conducted in the absence of catalysts at constant conditions pressure (25 Mpa) and volumetric flow rate (10 mL min⁻¹). The temperature range of 400–700 °C and glycerol feed composition between 10 and 34 wt% were evaluated. The catalysts evaluated were characterized by SEM-EDS, XRD, N₂ adsorption/desorption, XRF, WDS and TGA. The liquid and gaseous products were analyzed by TOC and gas chromatography, respectively. Results indicated that Ni/CRD catalyst showed the highest H₂ yield (5.38 mol H₂ per mol of glycerol fed) and long-term stability. Additionally, a comparison between the experimental results on the ScWG of glycerol and simulated thermodynamic equilibrium data was also reported. Thus, results demonstrated the great potential of the prepared catalyst to improve H₂-rich gas production from glycerol gasification.