Kinetic and CFD Modeling of Exhaust Gas Reforming of Natural Gas in a Catalytic Fixed-Bed Reactor for Spark Ignition Engines

Fuel reforming is an attractive method for performance enhancement of internal combustion engines fueled by natural gas, since the syngas can be generated inline from the reforming process. In this study, 1D and 2D steady-state modeling of exhaust gas reforming of natural gas in a catalytic fixed-bed reactor were conducted under different conditions. With increasing engine speed, methane conversion and hydrogen production increased. Similarly, increasing the fraction of recirculated exhaust gas resulted in higher consumption of methane and generation of H₂ and CO. Steam addition enhanced methane conversion. However, when the amount of steam exceeded that of methane, less hydrogen was produced. Increasing the wall temperature increased the methane conversion and reduced the H₂/CO ratio.     

Analysis of hydrogen production from methane autothermal reformer with a dual catalyst-bed configuration

This paper presents a performance analysis of a dual-bed autothermal reformer for hydrogen production from methane using a non-isothermal, one dimensional reactor model. The first section of Pt/Al₂O₃ catalyst is designed for oxidation reaction, whereas the second one based on Ni/MgAl₂O₄ catalyst involves steam reforming reaction. The simulation results show that the dual-bed autothermal reactor provides higher reactor temperature and methane conversion compared with a conventional fixed-bed reformer. The H₂O/CH₄ and O₂/CH₄ feed ratios affect the methane conversion and the H₂/CO product ratio. The addition of steam at lower temperatures to the steam reforming section of the dual-bed reactor can produce the synthesis gas with a higher H₂/CO product ratio.     

Thermodynamic Equilibrium Calculations for the Reforming of Coke Oven Gas with Gasification Gas

Thermodynamic analyses of the reforming of coke oven gas with gasification gas for syngas were investigated as a function of coke oven gas‐to‐gasification gas ratio (1–3), oxygen‐to‐methane ratio (0–1.56), pressure (25–35 bar) and temperature (700–1100 °C). Thermodynamic equilibrium results indicate that the operating temperature should be approximately 1100 °C and the oxygen‐to‐methane ratio should be approximately 0.39, where about 80 % CH₄ and CO₂ can be converted at 30 bar. Increasing the operating pressure shifts the equilibrium toward the reactants (CH₄ and CO₂); increasing the pressure from 25 to 35 bar decreases the conversion of CO₂ from 73.7 % to 67.8 %. The conversion ratio of CO₂ is less than that in the absence of O₂. For a constant feed gas composition (7 % O₂, 31 % gasification gas, and 62 % coke oven gas), a H₂/CO ratio of about 2 occurs at temperatures of 950 °C and above. Pressure effects on the H₂/CO ratio are negligible for temperatures greater than 750 °C. The steam produced has an effect on the hydrogen selectivity, but its mole fraction decreases with temperature; trace amounts of other secondary products are observed.     

Chemical‐looping combustion process: Kinetics and mathematical modeling

Chemical Looping Combustion technology involves circulating a metal oxide between a fuel zone where methane reacts under anaerobic conditions to produce a concentrated stream of CO₂ and water and an oxygen rich environment where the metal is reoxidized. Although the needs for electrical power generation drive the process to high temperatures, lower temperatures (600–800°C) are sufficient for industrial processes such as refineries. In this paper, we investigate the transient kinetics of NiO carriers in the temperature range of 600 to 900°C in both a fixed bed microreactor (WHSV = 2‐4 g CH₄/h/g oxygen carrier) and a fluid bed reactor (WHSV = 0.014‐0.14 g CH₄/h per g oxygen carrier). Complete methane conversion is achieved in the fluid bed for several minutes. In the microreactor, the methane conversion reaches a maximum after an initial induction period of less than 10 s. Both CO₂ and H₂O yields are highest during this induction period. As the oxygen is consumed, methane conversion drops and both CO and H₂ yields increase, whereas the CO₂ and H₂O concentrations decrease. The kinetics parameter of the gas–solids reactions (reduction of NiO with CH₄, H₂, and CO) together with catalytic reactions (methane reforming, methanation, shift, and gasification) were estimated using experimental data obtained on the fixed bed microreactor. Then, the kinetic expressions were combined with a detailed hydrodynamic model to successfully simulate the comportment of the fluidized bed reactor. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

Fluidised bed membrane reactor for ultrapure hydrogen production via methane steam reforming: Experimental demonstration and model validation

Hydrogen is emerging as a future alternative for mobile and stationary energy carriers in addition to its use in chemical and petrochemical applications. A novel multifunctional reactor concept has been developed for the production of ultrapure hydrogen from light hydrocarbons such as methane for online use in downstream polymer electrolyte membrane fuel cells. A high degree of process intensification can be achieved by integrating perm-selective hydrogen membranes for selective hydrogen removal to shift the methane steam reforming and water-gas-shift equilibriums in the favourable direction and perm-selective oxygen membranes for selective oxygen addition to supply the required reaction energy via partial oxidation of part of the methane feed and enable pure CO₂ capture without costly post-treatment. This can be achieved in a proposed novel multifunctional bi-membrane bi-section fluidised bed reactor [Patil, C.S., van Sint Annaland, M., Kuipers, J.A.M., 2005. Design of a novel autothermal membrane assisted fluidized bed reactor for the production of ultrapure hydrogen from methane. Industrial and Engineering Chemistry Research 44, 9502-9512]. In this paper, an experimental proof of principle for the steam reforming/water-gas-shift section of the proposed novel fluidised bed membrane reactor is presented. A fluidised bed membrane reactor for steam reforming of methane/water-gas-shift on a commercial noble metal-based catalyst has been designed and constructed using 10 H₂ perm-selective Pd membranes for a fuel cell power output in the range of 50-100 W. It has been experimentally demonstrated that by the insertion of the membranes in the fluidised bed, the thermodynamic equilibrium constraints can indeed be overcome, i.e., increased CH₄ conversion, decreased CO selectivity and higher product yield (H₂ produced/CH₄ reacted). Experiments at different superficial gas velocities and also at different temperatures and pressures (carried out in the regime without kinetic limitations) revealed enhanced reactor performance at higher temperatures and pressures (3-4 bar). With a phenomenological two-phase reactor model for the fluidised bed membrane reactor, incorporating a separately developed lumped flux expression for the H₂ permeation rate through the used Pd-based membranes, the measured data from the fluidised bed membrane reactor could be well described, provided that axial gas back-mixing in the membrane-assisted fluidised bed reactor is negligible. This indicates that the membrane reactor behaviour approached that of an ideal isothermal plug flow reactor with maximum H₂ permeation.     

Hydrogen production via chemical looping dry reforming of methane: Process modeling and systems analysis

The present study presents and analyses a family of “chemical looping dry reforming” (CLDR) processes that produce inherently separated syngas (H₂ and CO) streams via a combination of methane cracking in a “cracker reactor” and the Boudouard reaction (i.e., conversion of the formed carbon with CO₂ to CO) in a “CO₂ reactor,” and then further maximize the H₂ yield via conversion of the produced CO via water-gas-shift. Remaining CO₂ emissions are minimized via CO₂ capture and sequestration. Four different configurations are evaluated which differ in how the heat required for the highly endothermic dry reforming reaction is supplied: (i) combustion of additional CH₄ feed (CLDR-CH₄); (ii) combustion of some of the CO produced in the CO₂ reactor (CLDR-CO); and combustion of some of the carbon produced in the cracker reactor with (iii) pure oxygen (CLDR-C-oxy); or (iv) with air (CLDR-C-air). Process models are developed to comparatively analyze the mass and energy balances of these configurations, and benchmark them against H₂-production via conventional dry reforming and steam reforming of methane. Our results show that CLDR-C-oxy is the most promising H₂-production pathway among the chemical looping and conventional technologies both in terms of chemical energy efficiency and in terms CO₂ emissions. Thus, the unique flexibility offered by the production of inherently separated syngas streams in CLDR enables overcoming the disadvantage of the strongly endothermic dry reforming reaction by combusting carbon internally in the reactor and thus achieving highly effective heat integration. Overall, the results support the technical viability and demonstrate the promise for strong process intensification of CLDR compared to conventional dry reforming and even steam reforming, the most widely used H₂-production pathway to-date.     

Chemical Looping Steam Methane Reforming via Ni-ferrite Supported on Cerium and Zirconium Oxides

Pure hydrogen was produced by means of chemical looping steam methane reforming with novel oxygen carriers. Ni-ferrite, Ni-ferrite-ZrO₂, and Ni-ferrite-CeO₂ were synthesized as oxygen carriers and characterized by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) method, scanning electron microscopy (SEM), dynamic light scattering (DLS), and hydrogen temperature-programmed reduction (H₂-TPR). The performances of the as-synthesized oxygen carriers in the cyclic oxidation-reduction were investigated in the packed-bed microreactor at a defined temperature range and under atmospheric pressure. Ni_0.39Fe_2.61O₄-ZrO₂ exhibited the best performance and maximum methane conversion among the other oxygen carriers. In addition, high selectivities for H₂ and CO were reached.     

Partial Oxidation of Methane to Syngas in a Packed Bed Catalyst Membrane Reactor

The planar membrane reactor configuration was explored for partial oxidation of methane (POM) to syngas. A supported membrane composed of yttria‐stabilized zirconia and La_0.8Sr_0.2Cr_0.5Fe_0.5O_3‐δ was sealed to a stainless holder, and a Ni/Al₂O₃ catalyst bed was placed under the membrane plane with a small slit between them. This reactor configuration would facilitate the POM reaction via oxidation‐reforming mechanism: the oxidation reaction occurring at the membrane surface and the reforming reaction taking place in the catalyst bed. At 800°C and a methane feed rate of 32 mL min^?1, the reactor attained methane throughput conversion over 90%, CO and H₂ selectivity both over 95%, and an equivalent oxygen permeation rate 1.4 mL cm^?2 min^?1. The membrane and catalyst remained intact after the POM testing. The planar membrane reactor configuration explored in this study may lead to the development of a compact reactor for syngas production. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2170–2176, 2016     

Hydrogen production through chemical looping using NiO/NiAl2O4 as oxygen carrier

A new autothermal route to produce hydrogen from natural gas via chemical looping technology was investigated. Tests were conducted in a micro-fixed bed reactor loaded with 200 mg of NiO/NiAl₂O₄ as oxygen carrier. Methane reacts with a nickel oxide in the absence of molecular oxygen at 800 °C for a period of time as high as 10 min. The NiO is subsequently contacted with a synthetic air stream (21% O₂ in argon) to reconstitute the surface and combust carbon deposited on the surface. Methane conversion nears completion but to minimize combustion of the hydrogen produced, the oxidation state of the carrier was maintained below 30% (where 100% represents a fully oxidized surface). Co-feeding water together with methane resulted in stable hydrogen production. Although the carbon deposition increased with time during the reduction cycle, the production rate of hydrogen remained virtually constant. A new concept is also presented where hydrogen is obtained from methane with inherent CO₂ capture in an energy neutral 3-reactors CFB process. This process combines a methane combustion step where oxygen is provided via an oxygen carrier, a steam methane reforming step catalyzed by the reduced oxygen carrier and an oxidizing step where the O-carrier is reconstituted to its original state.     

Hydrogen from steam methane reforming by catalytic nonthermal plasma using a dielectric barrier discharge reactor

A scaled-up dielectric barrier discharge (DBD) reactor has been developed and demonstrated for the production of hydrogen from steam methane reforming (SMR) by catalytic nonthermal plasma (CNTP) technology. Compared to SMR, CNTP offers conversion at ambient pressure (101.325 kPa), low temperature with better efficiency, making it suitable for distributed hydrogen production with small footprint. There have been several lab-scale DBD reactors reported in the literature. Dimension of the scaled-up DBD reactor is about six times the lab-scale version and can produce 0.9 kg H₂/day. The scale-up is, however, nonlinear; several technical innovations were required including spray nozzle for homogeneous introduction of steam, perforated tube central electrodes for generation of homogeneous plasma. Conversion efficiency of the scaled-up DBD reactor is 70–80% at 550°C and 500 W. A continuous run of 8 hr was demonstrated with typical product gas composition of 69% H₂, 6% CO₂, 15% CO, 10% CH₄.     

Autothermal Reforming of Methane with Integrated CO2 Capture in a Novel Fluidized Bed Membrane Reactor. Part 1: Experimental Demonstration

Two fluidized bed membrane reactor concepts for hydrogen production via autothermal reforming of methane with integrated CO₂ capture are proposed. Ultra-pure hydrogen is obtained via hydrogen perm-selective Pd-based membranes, while the required reaction energy is supplied by oxidizing part of the CH₄ in situ in the methane combustion configuration or by combusting part of the permeated H₂ in the hydrogen combustion configuration (oxidative sweeping). In this first part, the technical feasibility of the two concepts has been studied experimentally, investigating the reactor performance (CH₄ conversion, CO selectivity, H₂ production and H₂ yield) at different operating conditions. A more detailed comparison of the performance of the two proposed reactor concepts is carried out with a simulation study and is presented in the second part of this work.     

Adsorption of Off‐Gases from Steam Methane Reforming (H2, CO2, CH4, CO and N2) on Activated Carbon

Abstract

Hydrogen is the energy carrier of the future and could be employed in stationary sources for energy production. Commercial sources of hydrogen are actually operating employing the steam reforming of hydrocarbons, normally methane. Separation of hydrogen from other gases is performed by Pressure Swing Adsorption (PSA) units where recovery of high‐purity hydrogen does not exceed 80%.

In this work we report adsorption equilibrium and kinetics of five pure gases present in off‐gases from steam reforming of methane for hydrogen production (H₂, CO₂, CH₄, CO and N₂). Adsorption equilibrium data were collected in activated carbon at 303, 323, and 343 K between 0‐22 bar and was fitted to a Virial isotherm model. Carbon dioxide is the most adsorbed gas followed by methane, carbon monoxide, nitrogen, and hydrogen. This adsorbent is suitable for selective removal of CO₂ and CH₄. Diffusion of all the gases studied was controlled by micropore resistances. Binary (H₂‐CO₂) and ternary (H₂‐CO₂‐CH₄) breakthrough curves are also reported to describe the behavior of the mixtures in a fixed‐bed column. With the data reported it is possible to completely design a PSA unit for hydrogen purification from steam reforming natural gas in a wide range of pressures.     

Simulating and Optimizing Auto-Thermal Reforming of Methane to Synthesis Gas Using a Non-Dominated Sorting Genetic Algorithm II Method

Conventional synthesis gas production plants consist of a natural gas steam reforming to CO + 3H₂ on Ni catalysts in a furnace. An alternative method for highly endothermic steam reforming is auto-thermal reforming. In this work, synthesis gas production by auto-thermal reforming was simulated based on a heterogeneous and one-dimensional model in two cases. The first case was the auto-thermal reformer of Dias and Assaf's study. The present work was validated by the reported experimental results. The second case was the fixed-bed catalytic auto-thermal reactor operated at high pressure, which was suitable for methanol production and Fischer–Tropsch reactions (baseline case). Then, the effect of operating variables on the system behavior was studied. Finally, Pareto-optimal solutions were determined by non-dominated sorting genetic algorithm II. The objectives included obtaining a H₂/CO ratio of 2 in the produced synthesis gas and the maximum methane conversion. The adjustable parameters were the feed temperature, mass flux, and O₂/CH₄ and H₂O/CH₄ ratios in the feed.     

Kinetic simulation of a compact reactor system for hydrogen production by steam reforming of higher hydrocarbons

A bubbling fluidized bed membrane reactor for steam reforming of higher hydrocarbons is modelled, using n‐heptane as a model component to represent steam reforming of naphtha. The reformer is modelled as a bubbling fluidized bed reactor, consisting of two pseudo phases, a dense phase and a bubble phase, both in plug flow. In situ H₂ permselective membranes remove H₂ continuously as a pure product, greatly enhancing the H₂ yield per mole of heptane fed. A fluidized bed membrane reformer for higher hydrocarbons could give a very compact reactor system combining all the units from the pre‐reformer to the hydrogen purification system in a traditional steam reforming plant into a single unit.     

Carbon dioxide reforming of methane with a free energy minimization approach

Carbon dioxide reforming of methane to syngas is one of the primary technologies of the new poly-generation energy system on the basis of gasification gas and coke oven gas. A free energy minimization is applied to study the influence of operating parameters (temperature, pressure and methane-to-carbon dioxide ratio) on methane conversion, products distribution, and energy coupling between methane oxidation and carbon dioxide reforming methane. The results show that the methane conversion increases with temperature and decreases with pressure. When the methane-to-carbon dioxide ratio increases, the methane conversion drops but the H₂/CO ratio increases. By the introduction of oxygen, an energy balance in the process of the carbon dioxide reforming methane and oxidation can be realized, and the CO/H₂ ratio can be adjusted as well without water-gas shift reaction for Fischer-Tropsch or methanol synthesis. This work was presented at the 6 ^th Korea-China Workshop on Clean Energy Technology held at Busan, Korea, July 4–7, 2006.     

A new approach for hydrogen production in Claus sulfur recovery process

In this study, the decomposition of hydrogen sulfide and reforming of hydrocarbons were employed for producing hydrogen. Thermolysis of H₂S in the platinum-based catalytic reactor led to the production of hydrogen and sulfur. It is observed that the presence of catalyst increased the conversion of H₂S decomposition up to 99.6%. Also, the hydrocarbon content of acid gas stream (CH₄) was converted to syngas, especially hydrogen in a catalytic reforming process. The produced hydrogen was separated using a Pd/Ag membrane. The simulation results showed that hydrogen production in a sulfur recovery unit provided a green source of energy by incinerating gasses. By hydrogen production during the process, the molar flow rate of incinerating gasses reduced from 2555 to 621?Kmol/h. Moreover, the hazardous sulfur compounds of the stack were removed, while hydrogen was produced by 256?Kmol/hr.     

CO2 reforming of CH4 in coke oven gas to syngas over coal char catalyst

The CO₂ reforming of methane (in coke oven gas) on the coal char catalyst was performed in a fixed bed reactor at temperatures between 800 and 1200 °C under normal pressure. The effects of the coal char catalyst pretreatment and the ratio of CO₂/CH₄ were studied. Experimental results showed that the coal char was an effective catalyst for production of syngas, and addition of CO₂ did not enhance the CH₄ reforming to H₂. It was also found that the product gas ratio of H₂/CO is strongly influenced by the feed ratio of CO₂/CH₄. The modified coal char catalyst was more active during the CO₂–CH₄ reforming than the coal char catalyst based on the catalyst volume, furthermore the modified catalyst exhibited high activity in CO₂–CH₄ reforming to syngas. The conversion of methane can be divided into two stages. In the first stage, the conversion of CH₄ gradually decreased. In the second stage, the conversion of methane maintained nearly constant. The conversion of CO₂ decreased slightly during the overall reactions in CO₂–CH₄ reforming. The coal char catalyst is a highly promising catalyst for the CO₂ reforming of methane to syngas.     

CO2 conversion through combined steam and CO2 reforming of methane reactions over Ni and Co catalysts

Ni‐Co bimetallic and Ni or Co monometallic catalysts prepared for CO₂ reforming of methane were tested with the stimulated biogas containing steam, CO₂, CH₄, H₂, and CO. A mix of the prepared CO₂ reforming catalyst and a commercial steam reforming catalyst was used in hopes of maximizing the CO₂ conversion. Both CO₂ reforming and steam reforming of CH₄ occurred over the prepared Ni‐Co bimetallic and Ni or Co monometallic catalysts when the feed contained steam. However, CO₂ reforming did not occur on the commercial steam reforming catalyst. There was a critical steam content limit above which the catalyst facilitated no more CO₂ conversion but net CO₂ production for steam reforming and water‐gas shift became the dominant reactions in the system. The Ni‐Co bimetallic catalyst can convert more than 70% of CO₂ in a biogas feed that contains ~33 mol% of CH₄, 21.5 mol% of CO₂, 12 mol% of H₂O, 3.5 mol% of H₂, and 30 mol% of N₂. The H₂/CO ratio of the produced syngas was in the range of 1.8‐2. X‐ray absorption spectroscopy of the spent catalysts revealed that the metallic sites of Ni‐Co bimetallic, Ni and Co monometallic catalysts after the steam reforming of methane reaction with equimolar feed (CH₄:H₂O:N₂ = 1:1:1) experienced severe oxidation, which led to the catalytic deactivation.     

Hydrogen production from steam methane reforming coupled with in situ CO2 capture: Conceptual parametric study

The work deals with fundamental analysis of the sorption-enhanced steam methane reforming (SE-SMR) process in which the simultaneous removal of carbon dioxide by hydrotalcite-based chemisorbent is coupled. A two-section reactor model is developed to describe the SE-SMR reactor, decoupling the complexity in process analysis. The model defines two subsequent sections in the reactor: an equilibrium conversion section (upstream) and an adsorption reforming section (downstream). The material balance relationship in the equilibrium conversion section is directly determined by thermodynamic equilibrium calculation, providing an equilibrated atmosphere to the next section. The adsorption reforming section is described using an isothermal multi-component dynamic model into which the SMR reactions and the high-temperature CO₂ adsorption are embedded. The multiple requirements (including H₂ purity, H₂ productivity, CH₄ conversion enhancement, and carbon oxides concentrations) are taken into account simultaneously so as to analyze and define feasible operation window for producing high-purity hydrogen with ppm-level CO impurity. The performances of the reactors with different dimensions (laboratory-scale and pilot-scale) are explored, highlighting the importance of operation parameter control to the process feasibility.     

Hydrogen production from bioethanol reforming in supercritical water

Hydrogen production by reforming and oxidative reforming of ethanol in supercritical water (SCW) at the intermediate temperature range of 500-600 °C and pressure of 25 MPa were investigated at different ethanol concentrations or water to ethanol ratios (3, 20 and 30), with the absence and the presence of oxygen (oxygen to ethanol ratio between 0 and 0.156). Hydrogen was the main product accompanied with relatively low amounts of carbon dioxide, methane and carbon monoxide. Some liquid products, such as acetaldehyde and, occasionally, methanol were present. The ethanol conversion and hydrogen yield and selectivity increased substantially as the water to ethanol ratio and the reaction temperature increased. Ethanol was almost completely reformed and mainly converted to hydrogen giving a H₂/CO ratio of 2.6 at 550 °C and water to ethanol ratio of 30 without carbon formation. Coke deposition was favored at low water to ethanol ratio, especially at high temperatures (≥550 °C). The hydrogen yield improved as the ethanol was partially oxidized by the oxygen added into the feed at oxygen to ethanol ratios <0.071. It was evidenced that the metal components in Inconel 625 reactor wall reduced by a hydrogen stream acted as a catalyst promoting hydrocarbon reforming as well as water-gas-shift reactions while dehydrogenation of ethanol forming acetaldehyde can proceed homogeneously under the SCW condition. However, at high oxygen to ethanol ratio, the reactor wall was gradually deactivated after being exposed to the oxidant in the feed. The loss of the catalytic activity of the reactor surface was mainly due to the metal oxide formation resulting in reduction of catalytic activity of the reactor wall and reforming of carbon species was no longer promoted.