Supercritical Water Reforming Of Glycerol Research Articles

CFD modelling of supercritical water reforming of glycerol for hydrogen production

Abstract Glycerol, as a main by-product of biodiesel synthesis, can be used in a large variety of applications including food, personal care, pharmaceutical and chemical industries However, due to the large production of biodiesel, the glycerol market was depressed. The conversion of glycerol into an energy carrier (syngas or hydrogen) could be a very interesting route to providing value as a renewable energy source. The reforming of glycerol leads to an almost complete conversion and very high carbon-to-gas efficiency with short residence time. In this work, the performances of packed bed reactor for hydrogen production from glycerol in supercritical conditions, by using a Ni-based catalyst supported on Al2O3 and SiO2, through CFD modelling in three-dimensions were studied. The parameters of kinetic model were determined by using an optimization method to fit the experimental data. The developed model was been validated based on experimental results published in literature for three different feed concentration of glycerol of 5, 10 and 20 wt% (R2 = 0.969). Varying the reaction temperature, between 500 and 800 °C, and residence time, between 1.5 and 10 s, the concentration of hydrogen increased with increasing the temperature and decreasing the residence time. At high temperature, the hydrogen can achieve a concentration of 65% and the present of methane is less than 5% and carbon monoxide maintain lower concentration. The simulation results show that high hydrogen yield can be obtained in short residence time with conversion of glycerol almost completed.

Hydrogen and power generation from supercritical water reforming of glycerol and pressurized SOFC integrated system: Use of different CO2 adsorption process

The performance analysis of an integrated system of glycerol supercritical water reforming and pressurized SOFC was presented. The use of different CO2 adsorption processes that include in situ and ex situ processes was compared to determine the suitable process for hydrogen and power generations. The influence of operating condition, e.g., temperature and pressure of reformer, supercritical water to glycerol (S/G) molar ratio, and calcium oxide to glycerol (CaO/G) molar ratio was examined. Then, the electrical performance of each integrated process was considered with respect to the SOFC conditions comprising temperature, pressure, and current density. The simulation results revealed that both processes have same favourable conditions for temperature and pressure operated at 800 °C and 240 atm, respectively. The suitable S/G and CaO/G molar ratios for in situ process are 10 and 2 whereas those for ex situ process are 20 and 1. Under these conditions, maximum hydrogen can be achieved as 87% and 75% for in situ and ex situ processes, respectively. When both integrated processes are operated at the optimal SOFC conditions as 900 °C, 4 atm, and current density of 10,000 A/m2, the SOFC efficiency of 71.56% and 62.12% can provide for in situ and ex situ processes, respectively.

Optimization of hydrogen production from three reforming approaches of glycerol via using supercritical water with in situ CO2 separation

A pathway for hydrogen production from supercritical water reforming of glycerol integrated with in situ CO2 removal was proposed and analyzed. The thermodynamic analysis carried out by the minimizing Gibbs free energy method of three glycerol reforming processes for hydrogen production was investigated in terms of equilibrium compositions and energy consumption using AspenPlus™ simulator. The effect of operating condition, i.e., temperature, pressure, steam to glycerol (S/G) ratio, calcium oxide to glycerol (CaO/G) ratio, air to glycerol (A/G) ratio, and nickel oxide to glycerol (NiO/G) ratio on the hydrogen production was investigated. The optimum operating conditions under maximum H2 production were predicted at 450 °C (only steam reforming), 400 °C (for autothermal reforming and chemical looping reforming), 240 atm, S/G ratio of 40, CaO/G ratio of 2.5, A/G ratio of 1 (for autothermal reforming), and NiO/G ratio of 1 (for chemical looping reforming). Compared to three reforming processes, the steam reforming obtained the highest hydrogen purity and yield. Moreover, it was found that only autothermal reforming and chemical looping reforming were possible to operate under the thermal self-sufficient condition, which the hydrogen purity of chemical looping reforming (92.14%) was higher than that of autothermal reforming (52.98%). Under both the maximum H2 production and thermal self-sufficient conditions, the amount of CO was found below 50 ppm for all reforming processes.

Turnover rates for the supercritical water reforming of glycerol on supported Ni and Ru catalysts

Supercritical water reforming (SCWR) of glycerol conversion data to hydrogen are reported as reaction rates, normalized by the number of ostensible catalytic sites (i.e., as turnover rates). The process was studied in a tubular fixed-bed reactor using commercial Ni/Al2O3–SiO2 and Ru/Al2O3 catalysts. The kinetic results were captured as rate equations, and the Arrhenius parameters (pre-exponential factor and apparent activation energy) were obtained. In this way, a quantitative measure of catalytic activity of these catalysts was achieved and can be used as a reference in other studies.

Supercritical water reforming of glycerol: Performance of Ru and Ni catalysts on Al2O3 support

Supercritical water reforming of glycerol was studied in a tubular fixed-bed reactor using a Ru/Al2O3 catalyst, and was compared with our previous study using a Ni-based catalyst, with the aim of enhancing the performance of a global process designed under energy self-sufficient conditions. Relatively high glycerol concentrations of up to 25 wt.% and temperatures from 500 to 800 °C were tested. Glycerol conversion was very high (>99%) at temperatures of 600 °C and above, but low at 500 and 550 °C (<50%) using the Ru/Al2O3 catalyst. The gas product (dry basis) was mainly CH4 and CO2, while H2 production was quite low, against expectations. Under the same operating conditions, the behavior of the catalysts is quite different as the Ni catalyst promotes H2 production much more than the Ru catalyst. A detailed discussion is provided on our results and those of previous studies using the Ru/Al2O3 catalyst, thus acquiring more insight into the catalyst behavior. The Ru catalyst showed a large increase in its crystalline phase after testing, although the oxidation state of ruthenium did not change.

Hydrogen production from supercritical water reforming of glycerol over Ni/Al2O3–SiO2 catalyst

Hydrogen production from the supercritical water reforming of glycerol was studied in a tubular fixed-bed reactor by using a Ni-based catalyst supported on Al2O3 and SiO2. Tests were carried out at a pressure of 240 bar, temperatures of 500–800 °C, glycerol feed concentrations of 5–30 wt.%, and weight hourly space velocity from 1.25 to 22.5 gGly h−1 gCat−1 (residence time from 1.6 to 4.8 s through the bed). The dry gas is mainly composed of H2, CO2, CO, CH4. The results showed that the glycerol conversion was almost complete, except at the highest glycerol feed concentration and lowest temperature. Hydrogen yields were very close to those values predicted by equilibrium at a short residence time. Nickel on catalyst was completely reduced, and structured carbon nanotubes were encountered at glycerol concentrations higher than 20 wt.%. This study illustrates that the reforming of glycerol using supercritical water over a Ni catalyst makes it possible to reduce the reforming temperature needed when no catalyst is used (from 800 °C to 600 °C), achieving a high-yield hydrogen production, very close to equilibrium, and requiring less energy.

Life cycle assessment of hydrogen and power production by supercritical water reforming of glycerol

The environmental performance of hydrogen and electricity production by supercritical water reforming (SCWR) of glycerol was evaluated following a Life Cycle Assessment (LCA) approach. The heat-integrated process was designed to be energy self-sufficient. Mass and energy balances needed for the study were performed using Aspen Plus 8.4, and the environmental assessment was carried out through SimaPro 8.0. CML 2000 was selected as the life cycle impact assessment method, considering as impact categories the global warming, ozone layer depletion, abiotic depletion, photochemical oxidant formation, eutrophication, acidification, and cumulative energy demand. A distinction between biogenic and fossil CO2 emissions was done to quantify a more realistic GHG inventory of 3.77kg CO2-eq per kg H2 produced. Additionally, the environmental profile of SCWR process was compared to other H2 production technologies such as steam methane reforming, carbon gasification, water electrolysis and dark fermentation among others. This way, it is shown that SCWR of glycerol allows reducing greenhouse gas emissions and obtaining a favorable positive life cycle energy balance, achieving a good environmental performance of H2 and power production by SCWR of glycerol.

Techno-economic assessment of hydrogen and power production from supercritical water reforming of glycerol

Hydrogen and power production from supercritical water reforming of glycerol was techno-economically assessed, considering future states of technology because there is no demonstration plant using this technology. Two different configurations were proposed: supercritical water reforming (SCWR) and autothermal supercritical water reforming (ASCWR). A plant size of 1000kg/h of glycerol was considered on a process flow-sheet simulated by Aspen Plus with the criterion of being energy self-sufficient. The results reveal that, although ASCWR presents better performance than SCWR in terms of energy efficiency, the investment capital and operational difficulties of ASCWR process leads to higher hydrogen production costs. The levelized production cost of hydrogen was evaluated using a discounted cash flow analysis with a discount rate of 10% and 100% equity financing. Thus, the minimum hydrogen selling price (achieved when net present value is zero) is 5.36$/kg for SCWR and 5.75$/kg for ASCWR. These values are somewhat higher than in a few conventional technologies, such as steam methane reforming, although lower than other renewable processes, such as wood gasification. In a future scenario, possible improvements in SCW reforming performance may lead to a decrease in the estimation of renewable hydrogen price.

Syngas methanation from the supercritical water reforming of glycerol

An overall heat-integrated process of SCW (supercritical water) reforming of glycerol for methanation of the syngas obtained and power generation is proposed and analyzed. Methanation is the methane synthesis from the hydrogenation of CO and CO2. The SCW reforming is performed at 240 bars. Reforming temperatures from 700 °C to 900 °C and glycerol feed concentrations between 25 wt.% and 50 wt.%, needed to reach an energy self-sufficient process, are studied. For methanation, three adiabatic, fixed-bed reactors are connected in series with intermediate gas cooling, operating at 30 bars. The exit temperatures of these reactors range from 600 °C to 300 °C, respectively. The feed for the methanation section is previously conditioned by a Pressure Swing Adsorption unit to achieve a stoichiometric number of 3. The recommended operating conditions are a reforming temperature of 800 °C and a glycerol concentration of 33 wt.% to obtain 0.166 kg CH4/kg glycerol, 0.433 kWe/kg glycerol and an overall energy efficiency of 61.6%, which may increase up to 76.1% if the hot water leaving the process at 90 °C is considered (cogeneration water). The results of this process were compared to those of the methanol synthesis, previously published, resulting in a better performance, because the carbon proportion converted into methane is higher than into methanol from SCW reforming of glycerol, and the higher specific overall value for the methane production, which considers the price of the product and the electricity jointly.

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 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.

Experimental study of the supercritical water reforming of glycerol without the addition of a catalyst

Hydrogen production from the supercritical water reforming of glycerol was studied in a tubular reactor without adding a catalyst. Experiments were carried out at a pressure of 240bar, temperatures of 750–850°C, and glycerol feed concentrations of 5–30wt.%. Likewise, the residence time was changed from 12 to 160s, by handling the feed flow-rate. The dry gas is mainly consisted of H2, CO2, CO, CH4. In addition, small concentrations of glycerol were measured in the liquid phase analysis, but barely traces of others like glycolaldehyde, glyceraldehyde, dihydroxyacetone and lactic acid were detected. Thus, two probable reaction pathways are discussed, which makes it possible to explain the experimental results by using a method applicable to other similar processes. The results showed that the glycerol conversion was almost complete, except at the highest glycerol feed concentration, in which the conversion was of 88%. Hydrogen yields from 2 to 4molH2/molglycerol were obtained at high and low glycerol feed concentrations, respectively, when operating at high temperature and residence time. Besides, it was verified the catalytic effect of the reactor material (Inconel 625) from the trend of the gas product yields with time and the structured carbon nanotubes encountered. The catalytic activity of the reactor material was decreasing to reach a steady state after a few tens of operating hours. This study illustrates that the reforming of glycerol using supercritical water without added catalyst is feasible to achieve a high-yield hydrogen production, and it encourages to continue the research line, to obtain a process economically interesting.

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.

Methanol synthesis from syngas obtained by supercritical water reforming of glycerol

A process for producing methanol from the synthesis gas obtained by reforming of glycerol using supercritical water is studied. The process also produces power from the huge pressure energy of product gas just at the outlet of the reformer by a turbine. The expanded product gas is conditioned in a PSA system, which has three sections so as to produce a H2-rich gas stream, a CO-rich gas stream and CO2 for sequestration. Thus, it can be achieved the feed required for the methanol synthesis. The surplus hydrogen is sent to a fuel cell to generate power, and the PSA off-gas, purge from the methanol loop and gases separated from the crude methanol are burnt in a furnace to achieve an energy self-sufficient process. By changing the reforming temperature, the water-to-glycerol mass ratio and the purge from the methanol loop, the conditions for optimizing the overall process relative to methanol and power productions were achieved. Thus, by reforming at 1000°C and 240atm, and performing the methanol synthesis at 250°C and 85atm, the optimal conditions were a water-to-glycerol mass ratio of 1.68 with a purge ratio of 0.2. Under these conditions 0.270kg MeOH/kg glycerol and overall energy efficiency of 38.0% were obtained. The separated CO2 for sequestration is 0.38kg/kg of glycerol.

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.

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.