Hydrogen production from bio-oil: A thermodynamic analysis of sorption-enhanced chemical looping steam reforming

The steam reforming of pyrolysis bio-oil is one proposed route to low carbon hydrogen production, which may be enhanced by combination with advanced steam reforming techniques. The advanced reforming of bio-oil is investigated via a thermodynamic analysis based on the minimisation of Gibbs Energy. Conventional steam reforming (C-SR) is assessed alongside sorption-enhanced steam reforming (SE-SR), chemical looping steam reforming (CLSR) and sorption-enhanced chemical looping steam reforming (SE-CLSR). The selected CO₂ sorbent is CaO(s) and oxygen transfer material (OTM) is Ni/NiO. PEFB bio-oil is modelled as a surrogate mixture and two common model compounds, acetic acid and furfural, are also considered. A process comparison highlights the advantages of sorption-enhancement and chemical looping, including improved purity and yield, and reductions in carbon deposition and process net energy balance.The operating regime of SE-CLSR is evaluated in order to assess the impact of S/C ratio, NiO/C ratio, CaO/C ratio and temperature. Autothermal operation can be achieved for S/C ratios between 1 and 3. In autothermal operation at 30 bar, S/C ratio of 2 gives a yield of 11.8 wt%, and hydrogen purity of 96.9 mol%. Alternatively, if autothermal operation is not a priority, the yield can be improved by reducing the quantity of OTM. The thermodynamic analysis highlights the role of advanced reforming techniques in enhancing the potential of bio-oil as a source of hydrogen.     

Hydrogen production via steam reforming of coke oven gas enhanced by steel slag-derived CaO

Steel slag, a waste from steelmaking plant, has been proven to be good candidate resources for low-cost calcium-based CO₂ sorbent derivation. In this work, a cheap and sintering-resistance CaO-based sorbent (CaO (SS)) was prepared from low cost waste steel slag and was applied to enhance catalytic steam reforming of coke oven gas for production of high-purity hydrogen. This steel slag-derived CaO possessed a high and stable CO₂ capture capacity of about 0.48 g CO₂/g sorbent after 35 adsorption/desorption cycles, which was mainly ascribed to the mesoporous structure and the presence of MgO and Fe₂O₃. Product gas containing 95.8 vol% H₂ and 1.4 vol% CO, with a CH₄ conversion of 91.3% was achieved at 600 °C by steam reforming of COG enhanced by CaO (SS). Although high temperature was beneficial for methane conversion, CH₄ conversion was remarkably increased at lower operation temperatures with the promotion effects from CaO (SS), and CO selectivity has been also greatly decreased. Reducing WHSV could increase methane conversion and reduce CO selectivity due to longer reactants residence time. Reducing C/A could increase methane conversion and hydrogen recovery factor, and also decrease CO selectivity. When being mixed with catalyst during SE-SRCOG, CaO (SS) with a uniform size distribution favored methane conversion due to the high utilization efficiency of catalyst. Promising stability of CaO (SS) in cyclic reforming/calcination tests was evidenced with a hydrogen recovery factor >2.1 and CH₄ conversion of 82.5% at 600 °C after 10 cycles using CaO (SS) as sorbent.     

Hydrogen-rich syngas production by chemical looping steam reforming of acetic acid as bio-oil model compound over Fe-doped LaNiO3 oxygen carriers

Ni-based perovskites are promising oxygen carriers for chemical looping steam reforming to produce H₂-rich gas from organics. In this study, a series of Fe-doped LaNiO₃ perovskites with various Ni/Fe ratios (LaNi_xFe_1-xO₃ (0 ≤ x ≤ 1)) were investigated for chemical looping steam reforming of acetic acid as a model compounds of bio-oil. Results illustrated that although LaNiO₃ showed higher activity for gas production, the Ni–Fe bimetallic perovskites were more stable during the steam reforming reactions. It was found that Fe doping can promote the content of lattice oxygen in the perovskite which could be released during the steam reforming reaction, thus coking resistant of the perovskite was effectively improved. Among the LaNi_xFe_1-xO₃ (0 ≤ x ≤ 1) perovskites, LaNi_0.8Fe_0.2O₃ exhibited the best synergistic effect between Ni and Fe to achieve the highest H₂/CO for H₂-rich gas production. Operational variables of the steam reforming reactions catalyzed by LaNi_0.8Fe_0.2O₃ for H₂ production were further optimized.     

A combined thermodynamic and experimental study on chemical‐looping ethanol reforming with carbon dioxide capture for hydrogen generation

Chemical‐looping ethanol reforming with carbon dioxide capture is proposed. It combines chemical‐looping reforming and carbon dioxide capture for pure hydrogen generation from ethanol with inherent separation of carbon dioxide. A thermal analysis of the process using NiO oxygen carrier is performed by simulating reactions using the Gibbs energy minimization method. The promising systems are investigated further with respect to temperature, NiO/C₂H₅OH molar ratio, CaO/C₂H₅OH molar ratio and pressure changes as well as possible carbon formation in the reformer. Favorable operation conditions in the presence of CaO are: pressures around 3 atm, reactor temperatures around 850 K, NiO/C₂H₅OH molar ratio = 3 and CaO/C₂H₅OH = 3. The H₂ yield and thermal efficiency with CaO addition are higher than that without CaO addition, showing that the addition of a CO₂ sorbent in the process increases the H₂ production. Copyright © 2012 John Wiley & Sons, Ltd.     

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.     

Co catalysts modified by rare earths (La,Ce or Pr) for hydrogen production from ethanol

Co/MgAl₂O₄ catalysts modified with La, Pr or Ce were prepared, characterized by different techniques and tested in ethanol steam reforming reaction to produce hydrogen. The catalytic behavior at 650 °C depended on the nature of rare earth. The amount of carbon on promoted catalysts was significantly lower than that on unpromoted one. The Pr and La containing catalysts produced a high acetaldehyde selectivity which decreased the hydrogen production. The superior performance of the catalyst promoted with 7.8% Ce could be partially explained by a higher dispersion and a high reduction of Co species.     

Continuous sorption-enhanced steam reforming of glycerol to high-purity hydrogen production

In this study, the continuous sorption-enhanced steam reforming of glycerol to high-purity hydrogen production by a simultaneous flow concept of catalyst and sorbent for reaction and regeneration using two moving-bed reactors has been evaluated experimentally. A Ni-based catalyst (NiO/NiAl₂O₄) and a lime sorbent (CaO) were used for glycerol steam reforming with and without in-situ CO₂ removal at 500 °C and 600 °C. The simultaneous regeneration of catalyst and sorbent was carried out with the mixture gas of N₂ and steam at 900 °C. The product gases were measured by a GC gas analyzer. It is obvious that the amounts of CO₂, CO and CH₄ were reduced in the sorption-enhanced steam reforming of glycerol, and the H₂ concentration is greatly increased in the pre-CO₂ breakthrough periods within 10 min both 500 °C and 600 °C. The extended time of operation for high-purity hydrogen production and CO₂ capture was obtained by the continuous sorption-enhanced steam reforming of glycerol. High-purity H₂ products of 93.9% and 96.1% were produced at 500 °C and 600 °C and very small amounts of CO₂, CH₄ and CO were formed. The decay in activity during the continuous reaction-regeneration of catalyst and sorbent was not observed.     

Hydrogen production via chemical looping steam reforming of ethanol by Ni-based oxygen carriers supported on CeO2 and La2O3 promoted Al2O3

This work studies the effects of Ce⁴⁺ and/or La³⁺ on NiO/Al₂O₃ oxygen carrier (OC) on chemical looping steam reforming of ethanol for hydrogen production - alternating between fuel feed step (FFS) and air feed step (AFS). Suitable amount of Ce- and La-doping increases OC carbon tolerance. The solubility limit is found at 50 mol% La in solid solution. At higher La-doping, La₂O₃ disperses on the surface and adsorbs CO₂ forming La₂O₂CO₃ during FFS. From the 1st cycle, 12.5 wt%Ni/7 wt%La₂O₃-3wt%CeO₂–Al₂O₃ (N/7LCA) displays the highest averaged H₂ yield (3.2 mol/mol ethanol) with 87% ethanol conversion. However, after the 5th cycle, 12.5 wt%Ni/3 wt%La₂O₃-7wt%CeO₂–Al₂O₃ (N/3LCA) exhibits more stability and presents the highest ethanol conversion (88%) and H₂ yield (2.7 mol/mol ethanol). Amorphous coke on the OCs decreases with increasing La³⁺ content and can be removed at 500 °C during AFS; nevertheless, fibrous coke and La₂O₂CO₃ cannot be eliminated. Therefore, after multiple redox cycles, highly La-doped OCs exhibits rather low stability.     

Development of a Fe/Mg-bearing metallurgical waste stabilized-CaO/NiO hybrid sorbent-catalyst for high purity H2 production through sorption-enhanced glycerol steam reforming

In this work, a Fe/Mg-bearing metallurgical waste (upgraded slag oxide, UGSO) was, for the first time, investigated as a stabilizer for increasing the cyclic stability of CaO-based sorbents. The sorbents were prepared through the wet mixing of the ball-milled UGSO particles with the limestone-derived calcium citrate under sonication. The sorption capacity of samples containing different waste loadings (5, 10, 15, and 25 wt%) was studied for 18 carbonation/regeneration cycles under conditions similar to the sorption-enhanced glycerol steam reforming process. A significant improvement of the cyclic stability was observed for all doped sorbents; however, the sample with 10 wt% UGSO showed the highest sorption capacity among all tested samples. This optimum sorbent was further used to synthesize a UGSO stabilized CaO–NiO hybrid sorbent-catalyst material (20 wt% NiO loading), whose performance was tested in sorption-enhanced steam reforming of glycerol. A H₂ purity of around 95% was obtained in the pre-breakthrough period that lasted for about 30 min. In summary, the results showed a better stability of UGSO stabilized sorbents compared to pure CaO and a good performance of the CaO-UGSO10/NiO sorbent-catalyst hybrid material in the sorption-enhanced reforming process.     

Performance comparison among different multifunctional reactors operated under energy self-sufficiency for sustainable hydrogen production from ethanol

Four ethanol-derived hydrogen production processes including conventional ethanol steam reforming (ESR), sorption enhanced steam reforming (SESR), chemical looping reforming (CLR) and sorption enhanced chemical looping reforming (SECLR) were simulated on the basis of energy self-sufficiency, i.e. process energy requirement supplied by burning some of the produced hydrogen. The process performances in terms of hydrogen productivity, hydrogen purity, ethanol conversion, CO₂ capture ability and thermal efficiency were compared at their maximized net hydrogen. The simulation results showed that the sorption enhanced processes yield better performances than the conventional ESR and CLR because their in situ CO₂ sorption increases hydrogen production and provides heat from the sorption reaction. SECLR is the most promising process as it offers the highest net hydrogen with high-purity hydrogen at low energy requirement. Only 12.5% of the produced hydrogen was diverted into combustion to fulfill the process's energy requirement. The thermal efficiency of SECLR was evaluated at 86% at its optimal condition.     

Towards H2-rich gas production from unmixed steam reforming of methane: Thermodynamic modeling

In this work, the Gibbs energy minimization method is applied to investigate the unmixed steam reforming (USR) of methane to generate hydrogen for fuel cell application. The USR process is an advanced reforming technology that relies on the use of separate air and fuel/steam feeds to create a cyclic process. Under air flow (first half of the cycle), a bed of Ni-based material is oxidized, providing the heat necessary for the steam reforming that occurs subsequently during fuel/steam feed stage (second half of the cycle). In the presence of CaO sorbent, high purity hydrogen can be produced in a single reactor. In the first part of this work, it is demonstrated that thermodynamic predictions are consistent with experimental results from USR isothermal tests under fuel/steam feed. From this, it is also verified that the reacted NiO to CH₄ (NiO_reacted/CH₄) molar ratio is a very important parameter that affects the product gas composition and decreases with time. At the end of fuel/steam flow, the reforming reaction is the most important chemical mechanism, with H₂ production reaching ∼75 mol%. On the other hand, at the beginning of fuel/steam feed stage, NiO reduction reactions dominate the equilibrium system, resulting in high CO₂ selectivity, negative steam conversion and low concentrations of H₂. In the second part of this paper, the effect of NiO_reacted/CH₄ molar ratio on the product gas composition and enthalpy change during fuel flow is investigated at different temperatures for inlet H₂O/CH₄ molar ratios in the range of 1.2-4, considering the USR process operated with and without CaO sorbent. During fuel/steam feed stage, the energy demand increases as time passes, because endothermic reforming reaction becomes increasingly important as this stage nears its end. Thus, the duration of the second half of the cycle is limited by the conditions under which auto-thermal operation can be achieved. In absence of CaO, H₂ at concentrations of approximately 73 mol% can be produced under thermo-neutral conditions (H₂O/CH₄ molar ratio of 4, with NiO_reacted/CH₄ molar ratio at the end of fuel flow of ∼0.8, in temperature range of 873-1073 K). In the presence of CaO sorbent, using an inlet H₂O/CH₄ molar ratio of 4 at 873 K, H₂ at concentrations over 98 mol% can be obtained all through fuel/steam feed stage. At 873 K, carbonation reaction provides all the heat necessary for H₂ production when NiO_reacted/CH₄ molar ratio reached at the end of fuel/steam feed is greater or equal to1. In this way, the heat released during air flow due to Ni oxidation can be entirely used to decompose CaCO₃ into CaO. In this case, a calcite-to-nickel molar ratio of 1.4 (maximum possible value) can be used during air flow. For longer durations of fuel/steam feed, corresponding to lower NiO_reacted/CH₄ molar ratios, some heat is necessary for steam reforming, and a calcite-to-nickel molar ratio of about 0.7 is more suitable. With the USR technology, CaO can be regenerated under air feeds, and an economically feasible process can be achieved.     

Steam reforming of shale gas in a packed bed reactor with and without chemical looping using nickel based oxygen carrier

The catalytic steam reforming of shale gas was examined over NiO on Al₂O₃ and NiO on CaO/Al₂O₃ in the double role of catalysts and oxygen carrier (OC) when operating in chemical looping in a packed bed reactor at 1 bar pressure and S:C 3. The effects of gas hourly space velocity GHSV (h^?1), reforming temperatures (600–750 °C) and catalyst type on conventional steam reforming (C-SR) was first evaluated. The feasibility of chemical looping steam reforming (CL-SR) of shale gas at 750 °C with NiO on CaO/Al₂O₃ was then assessed and demonstrated a significant deterioration after about 9 successive reduction-oxidation cycles. But, fuel conversion was high over 80% approximately prior to deterioration of the catalyst/OC, that can be strongly attributed to the high operating temperature in favour of the steam reforming process.     

Chemical looping gasification for hydrogen production: A comparison of two unique processes simulated using ASPEN Plus

The research compares the simulations of two chemical looping gasification (CLG) types using the ASPEN Plus simulation software for the production of H₂. The simulated biomass type was poultry litter (PL). The first CLG type used in situ CO₂ capture utilizing a CaO sorbent, coupled with steam utilization for tar reforming, allowing for the production of a CO₂-rich stream for sequestration. Near-total sorbent recovery and recycle was achieved via the CO₂ desorption process. The second type utilized iron-based oxygen carriers in reduction–oxidation cycles to achieve 99.8% Fe₃O₄ carrier recovery and higher syngas yields. Temperature and pressure sensitivity analyses were conducted on the main reactors to determine optimal operating conditions. The optimal temperatures ranged from 500 to 1250 °C depending on the simulation and reactor type. Atmospheric pressure proved optimal in all cases except for the reducer and oxidizer in the iron-based CLG type, which operated at high pressure. This CLG simulation generated the most syngas in absolute terms (2.54 versus 0.79 kmol/kmol PL), while the CO₂ capture simulation generated much more H₂-rich syngas (92.45 mol-% compared to 62.94 mol-% H₂).     

Experimental investigation of improved calcium-based CO2 sorbent and Co3O4/SiO2 oxygen carrier for clean production of hydrogen in sorption-enhanced chemical looping reforming

In this study, highly pure hydrogen is produced in sorption enhanced chemical looping steam methane reforming (SE-CLSMR) using cobalt-based oxygen carrier (OC) and cerium promoted CaO-based sorbent. In addition, the CO₂ removal from a gas stream at high temperatures is investigated via calcium looping process prior to SE-CLSMR process. The prepared samples are characterized by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) and energy dispersive X-ray spectroscopy (EDX) techniques. The effect of Ca/Ce molar ratio (100/0.00–0.91/0.09), sorption temperature (550–650 °C) and sorbent lifetime are studied to find the optimal sorbent. The characterization results show the uniform and orderly CeO₂ dispersed sorbent nanoparticles that notably improved the sorbent morphology compared with blank CaO. The sorption results revealed the negative effect of temperature on CO₂ uptake of all the samples. In addition, the CO₂ sorption evaluations indicate that the molar ratio of cerium to calcium plays a significant role in the stability of sorbent and improved the CO₂ sorption capacity significantly. The high CO₂ removal efficiency in the cerium modified sorbents could be due to decrease in diffusion resistance of CO₂ through the sorbent structure during the carbonation reaction. Furthermore, results show that the addition of cerium to the sorbent structure, effectively improves the thermal resistance of synthesis sorbents. The SE-CLSMR results showed that the H₂ purity could be increased up to about 95% considering Co₃O₄/SiO₂ oxygen carrier and cerium promoted calcium-based sorbent at relatively low temperature of 550 °C, which is comparable with 84% in CLR process.     

Optimization of two bio-oil steam reforming processes for hydrogen production based on thermodynamic analysis

Thermodynamics of hydrogen production from conventional steam reforming (C-SR) and sorption-enhanced steam reforming (SE-SR) of bio-oil was performed under different conditions including reforming temperature, S/C ratio (the mole ratio of steam to carbon in the bio-oil), operating pressure and CaO/C ratio (the mole ratio of CaO to carbon in the bio-oil). Increasing temperature and S/C ratio, and decreasing the operating pressure were favorable to improve the hydrogen yield. Compared to C-SR, SE-SR had the significant advantage of higher hydrogen yield at lower desirable temperature, and showed a significant suppression for carbon formation. However excess CaO (CaO/C > 1) almost had no additional contribution to hydrogen production. Aimed to achieve the maximum utilization of bio-oil with as little energy consumption as possible, the influences of temperature and S/C ratio on the reforming performance (energy requirements and bio-oil consumption per unit volume of hydrogen produced, Q_D/H2 (kJ/Nm³) and Y_Bio-oil/H2 (kg/Nm³)) were comprehensively evaluated using matrix analysis while ensuring the highest hydrogen yield as possible. The optimal operating parameters were confirmed at 650 °C, S/C = 2 for C-SR; and 550 °C, S/C = 2 for SE-SR. Under their respective optimal conditions, the Y_Bio-oil/H2 of SE-SR is significant decreased, by 18.50% compared to that of C-SR, although the Q_D/H2 was slightly increased, just by 7.55%.     

H2 production from oxidative steam reforming of 1-propanol and propylene glycol over yttria-stabilized supported bimetallic Ni–M (M = Pt,Ru, Ir) catalysts

This paper reports hydrogen production from oxidative steam reforming of 1-propanol and propylene glycol over Ni–M/Y₂O₃–ZrO₂ (10% wt/wt Y₂O₃; M = Ir, Pt, Ru) bimetallic catalysts promoted with K. The results are compared with those obtained over the corresponding monometallic catalyst. The catalytic performance of the calcined catalysts was analyzed in the temperature range 723–773 K, adjusting the total composition of the reactants to O/C = 4 and S/C = 3.2–3.1 (molar ratios). The bimetallic catalysts showed higher hydrogen selectivity and lower selectivity of byproducts than the monometallic catalyst, especially at 723 K. Ni–Ir performed best in the oxidative steam reforming of both 1-propanol and propylene glycol. The presence of the noble metal favours the reduction of the NiO and the partial reduction of the support. The NiO crystalline phase present in the calcined catalysts was transformed to Ni° during oxidative steam reforming. The adsorption and subsequent reactivity of both 1-propanol and propylene glycol over Ni–Ir and Ni catalysts were followed by FTIR; C–C bond cleavage was found to occur at a lower temperature in propylene glycol than in 1-propanol.     

Study of the reactivity by pulse of CH4 over NiO/Fe-doped MgAl2O4 oxygen carriers for hydrogen production

The reactivity of Ni-based oxygen carrier (OC) was studied by CH₄ pulse test. The MgAl₂O₄ spinel was synthesized by microwave assisted combustion method and Ni and Fe were added by wet impregnation method. The results of CH₄ pulse test revealed that the OCs were more reactive for partial oxidation reaction. The XRD analysis of OCs after the test confirms the presence of NiO and MgAl₂O₄ without the secondary phases like NiAl₂O₄ and FeAl₂O₄. Among the OCs, Ni15Fe2MA was the most reactive producing the highest amounts of H₂ and exhibiting good re-oxidation capacity, illustrating its potential for use in Chemical Looping Reforming (CLR). The high reactivity is associated to a change on the NiO-support interaction.     

Hydrogen‐rich syngas production from chemical looping steam reforming of bio‐oil model compound: Effect of bimetal on LaNi0.8M0.2O3 (M = Fe,Co, Cu,and Mn)

Chemical looping steam reforming (CLSR) of acetic acid as bio‐oil model compound is a suitable way to produce hydrogen‐rich syngas. The LaNiO₃ and LaNi_0.8M_0.2O₃ (M = Fe, Co, Mn, and Cu) perovskites were prepared via the sol‐gel method. The perovskites were characterized by X‐ray diffraction (XRD), thermogravimetry (TG), and test activity of hydrogen‐rich syngas in a fixed bed. XRD and TG results showed that the coke generation on LaNi_0.8Fe_0.2O₃ needs a lower decomposition temperature, and a stable structure was appeared after reaction. The activity order of hydrogen‐rich syngas on five types of perovskite is LaNi_0.8Fe_0.2O₃ > LaNi_0.8Co_0.2O₃ > LaNiO₃ > LaNi_0.8Mn_0.2O₃ > LaNi_0.8Cu_0.2O₃ at 650°C, mole ratio of steam/carbon (2:1), and sample mixture flow (GHSV = 34 736 g of feed/(g catalyst h)). The CLSR of acetic acid with LaNi_0.8Fe_0.2O₃ at GHSV = 43 992 g of feed/(g catalyst h) showed good stability. Correlated to experimental results, the adsorption energy of steam and acetic acid on five types of perovskite were calculated using density functional theory (DFT). The adsorption energy of steam (?0.61 eV) and acetic acid (?0.93 eV) of LaNi_0.8Fe_0.2O₃ is maximum. This value of DFT calculation is well explained in the experimental results.     

Autothermal reforming of ethanol on noble metal catalysts

Autothermal reforming of ethanol on zirconia-supported Rh and Pt mono- and bimetallic catalysts (0.5 wt-% total metal loading) was studied as a source of H₂-rich gas for fuel cells. The results were compared with those obtained on a commercial steam reforming catalyst (15 wt-% NiO/Al₂O₃). The Rh-containing catalysts exhibited the highest selectivity for H₂ production and were stable in 24 h experiments. The formation of carbonaceous deposits was lower on the noble metal catalysts than on the commercial NiO/Al₂O₃ catalyst. Thus, the Rh-containing catalysts are more suitable than the commercial NiO/Al₂O₃ catalysts for the ATR of ethanol.     

Transient reaction phenomena of sorption-enhanced steam methane reforming in a fixed-bed reactor

The transient chemical reaction phenomena of the sorption-enhanced steam methane reforming (SE-SMR) by using Ni/Al₂O₃ catalyst and CaO sorbent in a tubular fixed-bed reactor were numerically investigated by an experimentally verified unsteady 2D model. Four chemical reactions are involved in SE-SMR including steam reforming (SR), water gas shift (WGS), global steam reforming (GSR), and CO₂ sorption. The reaction process in time is divided into period 1, transient period, and period 2. The high-purity H₂ is produced in period 1 which is defined as the outlet molar fractions of H₂ ≥ 90% and CO ≤ 1% (dry basis) in this work. In the first half of period 1, the endothermic reaction rates of SR and GSR are dominant in the entrance region of catalyst/sorbent bed. The WGS and CO₂ sorption reactions are triggered by SR and GSR reactions. The heat transfer from the wall plays an important role. Higher CaO conversion, temperature, and reaction rates appear first near the wall region, then they gradually expand to the central region.In the second half of period 1, a sharp wave-shaped curve of strong CO₂ sorption reaction occurred in downstream becomes dominant and it moves to downstream almost at a constant speed, as time progresses. The peak value of the CO₂ sorption reaction is more than twice larger than that of SR or WGS. The SR and WGS reaction rates are significantly enhanced by CO₂ sorption reaction. The great sorption, WGS, and SR reactions result in a high-purity H₂ production with the outlet molar fractions of 95.8% H₂, 0.998% CO, and 0.73% CO₂ at the end of period 1, based on the parameters used in this work such as reactor temperature of 600 °C. The maximum CaO conversion is about 76% in end of period 1 and the average CaO conversion in the reactor is 51%. The 2D distributions of CaO conversion, temperature, and reaction rates are also presented and discussed.