Thermodynamic analysis of hydrogen production from glycerol autothermal reforming

In this work, thermodynamics was applied to investigate the glycerol autothermal reforming to generate hydrogen for fuel cell application. Equilibrium calculations employing the Gibbs free energy minimization were performed in a wide range of temperature (700–1000 K), steam to glycerol ratio (1–12) and oxygen to glycerol ratio (0.0–3.0). Results show that the most favorable conditions for hydrogen production are achieved with the temperatures, steam to glycerol ratios and oxygen to glycerol ratios of 900–1000 K, 9–12 and 0.0–0.4, respectively. Further, it is demonstrated that thermoneutral conditions (steam to glycerol ratio 9–12) can be obtained at oxygen to glycerol ratios of around 0.36 (at 900 K) and 0.38–0.39 (at 1000 K). Under these thermoneutral conditions, the maximum number of moles of hydrogen produced are 5.62 (900 K) and 5.43 (1000 K) with a steam to glycerol ratio of 12. Also, it should be noted that methane and carbon formation can be effectively eliminated.     

Thermodynamic study of the supercritical water reforming of glycerol

Hydrogen can be produced by steam reforming, partial oxidation, autothermal, or aqueous-phase reforming processes using various noble metal based catalysts, but also by supercritical water (SCW) reforming. Using AspenPlus™, a systematic thermodynamic analysis of glycerol reforming using supercritical water has been carried out by the total Gibbs free energy minimization method, which computes the equilibrium composition of synthesis gas (syngas). The predictive Soave-Redlich-Kwong equation of state (EOS) has been used as thermodynamic method in the simulation of the supercritical region, after evaluating it against other EOS methods. A sensitivity analysis has been conducted on supercritical water reforming of pure and pretreated crude glycerol, as obtained from biodiesel production. The effect of the main operating parameters (temperature, concentration of glycerol feed, glycerol purity in the feed of crude glycerol, and pressure) aimed to the hydrogen production has been investigated in the reforming process, by obtaining the mole fraction and molar flow-rate of components in syngas, as well as the hydrogen yield. Selectivity to the different compounds has been also calculated. By this way, the thermodynamic favorable operating conditions at which glycerol may be converted into hydrogen by SCW reforming have been identified. The simulation results agree well with some few experimental data from the literature. This study is the first of a series addressed to glycerol reforming using SCW.     

Thermodynamic study of hydrogen production from crude glycerol autothermal reforming for fuel cell applications

This study presents a thermodynamic analysis of hydrogen production from an autothermal reforming of crude glycerol derived from a biodiesel production process. As a composition of crude glycerol depends on feedstock and processes used in biodiesel production, a mixture of glycerol and methanol, major components in crude glycerol, at different ratios was used to investigate its effect on the autothermal reforming process. Equilibrium compositions of reforming gas obtained were determined as a function of temperature, steam to crude glycerol ratio, and oxygen to crude glycerol ratio. The results showed that at isothermal condition, raising operating temperature increases hydrogen yield, whereas increasing steam to crude glycerol and oxygen to crude glycerol ratios causes a reduction of hydrogen concentration. However, high temperature operation also promotes CO formation which would hinder the performance of low-temperature fuel cells. The steam to crude glycerol ratio is a key factor to reduce the extent of CO but a dilution effect of steam should be considered if reforming gas is fed to fuel cells. An increase in the ratio of glycerol to methanol in crude glycerol can increase the amount of hydrogen produced. In addition, an optimal operating condition of glycerol autothermal reforming at a thermoneutral condition that no external heat to sustain the reformer operation is required, was investigated.     

Thermodynamic analysis of hydrogen production by autothermal reforming of ethanol

Ethanol steam reforming (ESR) is a strong endothermic reaction and ideally it only produces hydrogen and carbon dioxide.     

Dry autothermal reforming of glycerol with in situ hydrogen separation via thermodynamic evaluation

On the basis of the Gibbs free energy minimization principle, the dry autothermal reforming performance of crude glycerol in situ hydrogen separation is investigated via thermodynamic analysis. The impact of hydrogen separation fraction on gas composition in product, carbon formation and reaction heat is studied. It can be found that the hydrogen separation promotes the hydrogen production and hinders methane formation. The hydrogen removal is selective to the reduction of carbon deposition, which improves the carbon formation at a low feed CO₂ to glycerol molar ratio and the impact is reverse for high feed CO₂ to glycerol molar ratio. When the reaction temperature varies from 850 K to 900 K, the required oxygen to glycerol molar ratio of thermal neutral condition is obviously increased from 0.15 to 0.4 with hydrogen removal. Meanwhile, the glycerol impurities evaluation indicates that the syngas yield is significantly reduced with the increase of the glycerol impurities. At a high temperature, the hydrogen removal is in favor of the achievement of autothermal process.     

An energy and exergy analysis of the supercritical water reforming of glycerol for power production

As a continuation of a previous work, a conceptual design is proposed for reforming glycerol using supercritical water to produce maximum electrical power in an energy self-sufficient system. The scheme of the process is simulated after discussing some routes to achieve the aim. The selected way takes advantage of the huge pressure energy of reformate products just at the outlet of the reforming process. The expanded product gas is used as a fuel gas to provide the thermal energy required by the reforming process. The evaluation of the thermodynamic performance of the process is carried out by an energy and exergy analysis. As relevant outputs measurements of the process performance, the net work and exergetic efficiencies as well as the mole fraction and molar flow-rates of hydrogen obtained. Glycerol feed concentration in aqueous solution at which no external heat source is needed was obtained, both for pure and pretreated crude glycerol, at 800 °C and 240 atm. The effect of the main operating parameters has been investigated by sensitivity analysis to identify optimal conditions that maximize power production. In the exergy analysis, the thermodynamic efficiencies used for the overall process and for its individual units are suitably discussed. The computation has been made with the aid of AspenPlus™, using the predictive Soave-Redlich-Kwong equation of state as thermodynamic method in the simulation of the supercritical region. The next study in this series of glycerol reforming using SCW will aim to maximize hydrogen production, including the syngas purification, to generate electricity via fuel cells.     

Catalytic reforming of glycerol in supercritical water with nickel-based catalysts

The catalytic performance of nickel catalysts supported on La₂O₃, α-Al₂O₃, γ-Al₂O₃, ZrO₂, and YSZ for supercritical water reforming of glycerol was investigated. Experiments were conducted in a tubular reactor made of Inconel-625 with the temperature range of 723–848 K under a pressure of 25 MPa. Carbon formation causing operation failure was observed for α-Al₂O₃, γ-Al₂O₃ and ZrO₂ at temperatures higher than 748, 798 and 823 K, respectively. Ni/La₂O₃ exhibited the highest H₂ yield where almost complete conversion was obtained at 798 K. Moderate space velocities (WHSV = 6.45 h⁻¹) and glycerol feed concentration (5wt.%) favor high hydrogen selectivity and yield. Methanation is favored at a low WHSV or high glycerol feed concentration, resulting in a lower H₂ yield. Increasing Ni loading on the Ni/La₂O₃ catalyst strongly promoted the reforming, water–gas shift, and methanation reactions, which contributed significantly to the product species distribution.     

Hydrogen production from catalytic supercritical water reforming of glycerol with cobalt-based catalysts

Glycerol reforming under catalytic supercritical water at temperatures in the range of 723–848 K using Co catalyst deposited on various supports including ZrO₂, yttria-stabilized zirconia (YSZ), La₂O₃, γ-Al₂O₃, and α-Al₂O₃ was investigated. An increase in operating temperature promoted the continued increase in glycerol conversion; however, carbon formation causing system operation failure was observed for γ-Al₂O₃ and α-Al₂O₃ at high operating temperatures (i.e. 748–798 K). Co supported on YSZ provided the most efficient performance for hydrogen production. 10 wt.% Co loading on YSZ support was an optimum amount to enhance the reaction. The increase in glycerol conversion and reduction of the amount of liquid products were observed for lower weight hourly space velocity (WHSV), higher operating temperature or higher cobalt loading. On Co/YSZ catalyst, glycerol conversion of 0.94 and hydrogen yield of 3.72 was obtained with WHSV of 6.45 h⁻¹at 773 K.     

Thermodynamic analysis of hydrogen production via autothermal steam reforming of selected components of aqueous bio-oil fraction

From a technical and economic point of view, autothermal steam reforming offers many advantages, as it minimizes heat load demand in the reformer. Bio-oil, the liquid product of biomass pyrolysis, can be effectively converted to a hydrogen-rich stream. Autothermal steam reforming of selected compounds of bio-oil was investigated using thermodynamic analysis. Equilibrium calculations employing Gibbs free energy minimization were performed for acetic acid, acetone and ethylene glycol in a broad range of temperature (400–1300 K), steam to fuel ratio (1–9) and pressure (1–20 atm) values. The optimal O₂/fuel ratio to achieve thermoneutral conditions was calculated under all operating conditions. Hydrogen-rich gas is produced at temperatures higher than 700 K with the maximum yield attained at 900 K. The ratio of steam to fuel and the pressure determine to a great extent the equilibrium hydrogen concentration. The heat demand of the reformer, as expressed by the required amount of oxygen, varies with temperature, steam to fuel ratio and pressure, as well as the type of oxygenate compound used. When the required oxygen enters the system at the reforming temperature, autothermal steam reforming results in hydrogen yield around 20% lower than the yield by steam reforming because part of the organic feed is consumed in the combustion reaction. Autothermicity was also calculated for the whole cycle, including preheating of the organic feed to the reactor temperature and the reforming reaction itself. The oxygen demand in such a case is much higher, while the amount of hydrogen produced is drastically reduced.     

Exergy analysis of glycerol steam reforming in a heat recovery reactor

A detailed exergy analysis was performed for the steam reforming process of glycerol by means of a series of experiments in a bench scale apparatus. The reforming was conducted in a fixed bed reactor, which operated in heat recovery mode by extracting the demanded energy from hot exhaust gases provided by a diesel engine. In order to determine the role of the main operational parameters into the exergy efficiency of the studied process, the experiments were carried out with glycerol feed concentrations in water ranging from 10% up to 90% weight, whereas the outlet reactor temperature was varied from 600 °C up to 800 °C. Detailed exergy balances revealed a compromise between the exergy destruction within the reforming reactor and liquid separator versus the exergy losses associated to the tar and char outputs. This trade-off was favourable to the 50% and 70% glycerol feed concentration regimes and plateaus of about 74% exergy efficiency and 24 MJ/kg dry syngas exergy content were verified from 650 to 800 °C reactor temperatures.     

Thermodynamic evaluation of hydrazine assisted glycerol reforming for syngas production and coke inhibition

Thermodynamic investigation of glycerol reforming has been performed to study hydrogen production, carbon dioxide evolution and coke formation. Gibb's free energy direct minimization procedure was used to calculate the concentration of different components at equilibrium. The analysis was performed at temperatures from 300K to 1000K under unit atmospheric pressure. A comparative study on steam reforming of glycerol (SRG) and glycerol reforming reaction with hydrazine has been conducted in the presence of hydrazine that act as a suitable reducing agent. Incorporation of hydrazine into glycerol reforming system helped in minimizing coke formation, maximizing hydrogen and syn-gas production. A complete conversion of glycerol with coke free products, along with reduced level of carbon dioxide and maximum hydrogen generation was obtained when glycerol steam reforming process (S/G = 1) was combined with higher moles of hydrazine. Reformation at higher temperatures could enhance the hydrogen production and decrease carbon generation due to methanation reaction and hence optimum results were accomplished at 1000K and atmospheric pressure.     

Hydrogen production via steam and autothermal reforming of beef tallow: A thermodynamic investigation

A thermodynamic analysis of hydrogen production via steam and autothermal reforming of beef tallow has been carried out via the Gibbs free energy minimization method. Equilibrium calculations are performed at atmospheric pressure with a wide range of temperatures (400–1200 °C), steam-to-beef tallow ratios (1–15) and oxygen-to-beef tallow ratios (0.0–2.0).     

Hydrogen production from supercritical water reforming of glycerol in an empty Inconel 625 reactor

Glycerol reforming was investigated under supercritical water conditions (450–575 °C, 250 bar). A feed containing 5 wt.% of glycerol was continuously fed to an empty Inconel 625 reactor. The products of the reaction were separated into gas and liquid phases in a condenser. At a feed rate of 2.15 g/min, the glycerol conversion significantly increased from 0.05 to 0.97 when increasing operating temperature from 450 to 575 °C. Although lowering the feed rate (i.e. increasing the residence time) could considerably improve the conversion, carbon formation became a problem especially at high operating temperatures (550–575 °C). The major gaseous products were hydrogen (approximately 60 mol%), carbon monoxide, carbon dioxide and methane with some traces of ethane, ethylene, propane, and propylene. Various liquid products were detected including acetaldehyde, acetol, methanol, acetic acid, propionaldehyde, allyl alcohol, acetone, acrolein, ethanol, ethylene glycol, and acrylic acid but the major liquid components were acetaldehyde and acetol. With a feed glycerol concentration of 2.5 wt.% and operating temperature of 525 °C, glycerol conversion of 0.91 and H₂ yield of 2.86 can be obtained without carbon formation. Finally, it was demonstrated that higher H₂ yield with much lower carbon formation was observed in supercritical water reforming (250 bar) compared to conventional steam reforming at 1 bar under similar temperatures.     

Hydrogen production by glycerol steam reforming with in situ hydrogen separation: A thermodynamic investigation

Thermodynamic features of hydrogen production by glycerol steam reforming with in situ hydrogen extraction have been studied with the method of Gibbs free energy minimization. The effects of pressure (1–5 atm), temperature (600–1000 K), water to glycerol ratio (WGR, 3–12) and fraction of H₂ removal (f, 0–1) on the reforming reactions and carbon formation were investigated. The results suggest separation of hydrogen in situ can substantially enhance hydrogen production from glycerol steam reforming, as 7 mol (stoichiometric value) of hydrogen can be obtained even at 600 K due to the hydrogen extraction. It is demonstrated that atmospheric pressure and a WGR of 9 are suitable for hydrogen production and the optimum temperature for glycerol steam reforming with in situ hydrogen removal is between 825 and 875 K, 100 K lower than that achieved typically without hydrogen separation. Furthermore, the detrimental influence of increasing pressure in terms of hydrogen production becomes marginal above 800 K with a high fraction of H₂ removal (i.e., f = 0.99). High temperature and WGR are favorable to inhibit carbon production.     

Thermodynamic analysis of hydrogen production from methane via autothermal reforming and partial oxidation followed by water gas shift reaction

Reaction characteristics of hydrogen production from a one-stage reaction and a two-stage reaction are studied and compared with each other in the present study, by means of thermodynamic analyses. In the one-stage reaction, the autothermal reforming (ATR) of methane is considered. In the two-stage reaction, it is featured by the partial oxidation of methane (POM) followed by a water gas shift reaction (WGSR) where the temperatures of POM and WGSR are individually controlled. The results indicate that the reaction temperature of ATR plays an important role in determining H₂ yield. Meanwhile, the conditions of higher steam/methane (S/C) ratio and lower oxygen/methane (O/C) ratio in association with a higher reaction temperature have a trend to increase H₂ yield. When O/C ≤ 0.125, the coking behavior may be exhibited. In regard to the two-stage reaction, it is found that the methane conversion is always high in POM, regardless of what the reaction temperature is. When the O/C ratio is smaller than 0.5, H₂ is generated from the partial oxidation and thermal decomposition of methane, causing solid carbon deposition. Following the performance of WGSR, it suggests that the H₂ yield of the two-stage reaction is significantly affected by the reaction temperature of WGSR. This reflects that the temperature of WGSR is the key factor in producing H₂. When methane, oxygen and steam are in the stoichiometric ratio (i.e. 1:0.5:1), the maximum H₂ yield from ATR is 2.25 which occurs at 800 °C. In contrast, the maximum H₂ yield of the two-stage reaction is 2.89 with the WGSR temperature of 200 °C. Accordingly, it reveals that the two-stage reaction is a recommended fuel processing method for hydrogen production because of its higher H₂ yield and flexible operation.     

Thermodynamic analysis of supercritical water gasification of methanol,ethanol, glycerol,glucose and cellulose

In the present work the Gibbs free energy minimization, using a non-linear programming formulation and an approximation in the gas fugacities, was used to calculate the equilibrium composition for supercritical water gasification of methanol, ethanol, glycerol, glucose and cellulose. The proposed formulation mathematically ensures finding the global optimal solution with no need of an initial estimate and the numerical results are close to the ones calculated using non-ideal gas formulation. Therefore, the proposed approach is reliable and easy to use, without numerical difficulties, such as an undesirable local minimum. The model predictions show a good agreement with the experimental studies in all cases studied in this work.     

Effect of mode of operation on hydrogen production from glycerol at thermal neutral conditions: Thermodynamic analysis

Thermodynamic analysis of hydrogen production from glycerol under thermal neutral conditions is studied in this work. Heat requirement from the process can be achieved from the exothermic reaction of glycerol with oxygen in air fed to the system. Two modes of operation for air feeding are considered including (i) Single-feed mode in which air is fed in combination with water and glycerol to the reformer, and (ii) Split-feed mode in which air and part of glycerol is fed to a combustor in order to generate heat. The thermal neutral conditions are considered for two levels including Reformer and System levels. It was found that the H₂ yield from both modes is not significantly different at the Reformer level. In contrast, the difference becomes more pronounced at the System level. Single-feed and Split-feed modes offer high H₂ yield in low (600–900 K) and high (900–1200 K) temperature ranges, respectively. The maximum H₂ yields are 5.67 (water to glycerol ratio, WGR = 12, oxygen to glycerol ratio, OGR = 0.37, T = 900 K, Split-feed mode), and 3.28 (WGR = 3, OGR = 1.40, T = 900 K, Single-feed mode), for the Reformer and System levels, respectively. The difference between H₂ yields in both levels mainly arises from the huge heat demand for preheating feeds in the System level, and therefore, a higher amount of air is needed to achieve the thermal neutral condition. Split-feed mode is a favorable choice in term of H₂ purity because the gas product is not diluted with N₂ from the air. The use of pure O₂ and afterburner products (ABP) stream were also considered at the System level. The maximum H₂ yield becomes 3.75 (WGR = 5.21, OGR = 1.28, T = 900 K, Split-feed mode) at thermal neutral condition when utilizing heat from the ABP stream. Finally comparisons between the different modes and levels are addressed in terms of yield of by-products, and carbon formation.     

Comparison of steam and autothermal reforming of methanol using a packed-bed low-cost copper catalyst

Small-scale reformers for hydrogen production via steam and autothermal reforming of hydrocarbon feedstocks can be a solution to the lack of hydrogen distribution infrastructure. A packed-bed reactor is one possible design for such purpose. However, the two reforming processes of steam and autothermal methods have different characteristics, thus they have different and often opposite design requirements. In implementing control strategy for small-scale reformers, understanding the overall chemical reactions and the reactor physical properties becomes essential. This paper presents some inherent features of a packed-bed reactor that can both improve and/or degrade the performance of a packed-bed reactor with both reforming modes.The high thermal resistance of the packed bed is disadvantageous to steam reforming (SR), but it is beneficial to the autothermal reforming (ATR) mode with appropriate reactor geometry. The low catalyst utilization in steam reforming can help to prevent the unconverted fuel leaving the reactor during transient by allowing briefly for higher reactant fuel flow rates. In this study, experiments were performed using three reactor geometries to illustrate these properties and a discussion is presented on how to take advantages of these properties in reactor design.     

Catalytic gasification of glycine and glycerol in supercritical water

Glycine and glycerol were used as the model compounds of protein and fattiness, respectively. A continuous tubular-flow reactor was used for the gasification experiments operated at 380–500 °C and 25 MPa with or without Na₂CO₃ catalyst. Compared with a negative effect on glycerol gasification, Na₂CO₃ could increase hydrogen yield and Chemical Oxygen Demand (COD) destruction efficiency, and the catalytic performance of 0.1 wt% Na₂CO₃ was better than that of 0.2 wt% for glycine gasification. When 1 wt% glycine solution with 0.1 wt% Na₂CO₃, or 1 wt% glycerol solution without Na₂CO₃ was gasified at 500 °C with the residence time of 0.98 min, their corresponding gasification efficiencies were up to 95.8% and 98%, and hydrogen yields could reach 4.14 and 5.08 mol/mol, respectively. Hydrogen molar fraction in gaseous product was about 60% and liquid effluents could be reutilized. Correspondingly, the ideal overall reaction equations for glycine and glycerol gasification were proposed.     

Thermodynamic study on the integrated supercritical water gasification with reforming process for hydrogen production: Effects of operating parameters

In this paper, a conceptual process design of the integrated supercritical water gasification (SCWG) and reforming process for enhancing H₂ production has been developed. The influence of several operating parameters including SCWG temperature, SCWG pressure, reforming temperature, reforming pressure and feed concentration on the syngas composition and process efficiency was investigated. In addition, the thermodynamic equilibrium calculations have been carried out based on Gibbs free energy minimization by using Aspen Plus. The results showed that the higher H₂ production could be obtained at higher SCWG temperature, the H₂ concentration increased from 5.40% at 400 °C to 38.95% at 600 °C. The lower feed concentration was found to be favorable for achieving hydrogen-rich gas. However, pressure of SCWG had insignificant effect on the syngas composition. The addition of reformer to the SCWG system enhanced H₂ yield by converting high methane content in the syngas into H₂. The modified SCWG enhanced the productivity of syngas to 151.12 kg/100kg_feed compared to 120.61 kg/100kg_feed of the conventional SCWG system. Furthermore, H₂ yield and system efficiency increased significantly from 1.81 kg/100kg_feed and 9.18% to 8.91 kg/100kg_feed, and 45.09%, respectively, after the modification.