Biohydrogen production from Imperata cylindrica bio-oil using non-stoichiometric and thermodynamic model

This paper presents a non-stoichiometric and thermodynamic model for steam reforming of Imperata cylindrica bio-oil for biohydrogen production. Thermodynamic analyses of major bio-oil components such as formic acid, propanoic acid, oleic acid, hexadecanoic acid and octanol produced from fast pyrolysis of I. cylindrica was examined. Sensitivity analyses of the operating conditions; temperature (100–1000 °C), pressure (1–10 atm) and steam to fuel ratio (1–10) were determined. The results showed an increase in biohydrogen yield with increasing temperature although the effect of pressure was negligible. Furthermore, increase in steam to fuel ratio favoured biohydrogen production. Maximum yield of 60 ± 10% at 500–810 °C temperature range and steam to fuel ratio 5–9 was obtained for formic acid, propanoic acid and octanol. The heavier components hexadecanoic and oleic acid maximum hydrogen yield are 40% (740 °C and S/F = 9) and 43% (810 °C and S/F = 8) respectively. However, the effect of pressure on biohydrogen yield at the selected reforming temperatures was negligible. Overall, the results of the study demonstrate that the non-stoichiometry and thermodynamic model can successfully predict biohydrogen yield as well as the composition of gas mixtures from the gasification and steam reforming of bio-oil from biomass resources. This will serve as a useful guide for further experimental works and process development.     

Carbon deposition behavior in steam reforming of bio-oil model compound for hydrogen production

Catalyst deactivation caused by coke formation is a bottleneck in steam reforming of bio-oil for hydrogen production. The investigation of carbon deposition behavior can make a contribution to the improvement of catalyst and the knowledge of reaction mechanism. In this paper, m-cresol (C₇H₈O, one of the organic compounds present in bio-oil) was chosen as model compound. The experiment was carried out on a commercial Ni/MgO catalyst. As a comparative test, m-cresol decomposition showed carbon deposition can be formed more easily under higher temperature. In steam reforming process, for the competition of carbon deposition and carbon elimination, a peak value of coking formation rate was obtained in a broad range of temperature (575–900 °C). The increase of steam to carbon ratio can favor the carbon elimination. Final coking formation rate curve was determined under optimal reaction conditions and the results showed the severity of carbon deposition maintained a very low level during the entire reaction time. Based on the distribution of reforming products, high temperature and sufficient water feeding can favor the carbon conversion from solid and liquid phase to gaseous phase. Unreacted m-cresol is the main organic compound detected in liquid condensate. Some secondary reactions can be deduced through the other compounds detected. The carbon deposition state on catalyst surface can be in the form of nanofiber, but their concrete shapes can be different due to different reaction conditions.     

A thermodynamic equilibrium analysis of hydrogen and synthesis gas production from steam reforming of acetic acid and acetone blends as bio-oil model compounds

Thermodynamic analysis of steam reforming of blends of two model oxygenates, acetic acid and acetone, representing carboxylic acids and ketones in bio-oil is performed to investigate the effects of their potential interactions on hydrogen yield, synthesis gas composition and progress of reaction network. The results show that both acetic acid and acetone reach complete conversion at all operating conditions. Higher S/C molar ratio results in higher H₂ and CO₂ yields for both acetic acid and acetone. With the increase in pressure, H₂ and CO yields are diminished whereas CH₄ and CO₂ yields are enhanced. H₂ and CO₂ yields increase with the decrease in acetone concentration in the feed blend. CO and CH₄ production are affected adversely for acetic acid rich blends. The maximum H₂ yield values are 75.54%, 78.34%, 80.09%, 81.78% and 84.17% at 700 °C for acetic acid/acetone blends of 0.0/1.0, 0.3/0.7, 0.5/0.5, 0.7/0.3 and 1.0/0.0, respectively.     

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

This work presents thermodynamics analysis of hydrogen production via steam reforming of bio-oil components. The model compounds, acetic acid, ethylene glycol and acetone, representatives of the major classes of components present in the aqueous fraction of bio-oil were used for the study. The equilibrium product compositions were investigated in a broad range of conditions like temperature (400–1300 K), steam to fuel ratio (1–9) and pressure (1–20 atm). Any of the three model compounds can be fully reformed even at low temperatures producing hydrogen with maximum yield ranging from 80% to 90% at 900 K. Steam to fuel ratio positively affect the hydrogen content over the entire range of temperature studied. Conversely, higher pressure decreases the hydrogen yield. The formation of solid carbon (graphite) does not constitute a problem provided that reforming temperatures higher than 600 K and steam to fuel ratios higher than 4 for acetic acid and ethylene glycol and 6 for acetone are to be used. Thermal decomposition of the bio-oil components is thermodynamically feasible, forming a mixture containing _C(s)${\mathrm{C}}_{(\mathrm{s})}$, CH₄, H₂, CO, CO₂, and H₂O at various proportions depending on the specific nature of the compound and the temperature. Material and energy balances of complete reforming system demonstrated that the production of 1 kmol/s hydrogen from bio-oil steam reforming requires almost the same amount of energy as with natural gas reforming.     

Thermodynamic analysis of aqueous phase reforming of three model compounds in bio-oil for hydrogen production

Thermodynamic analysis with Gibbs free energy minimization was performed for aqueous phase reforming of methanol, acetic acid, and ethylene glycol as model compounds for hydrogen production from bio-oil. The effects of the temperature (340-660 K) and pressure ratio Psys/PH₂O (0.1-2.0) on the selectivity of H₂ and CH₄, formation of solid carbon, and conversion of model compounds were analyzed. The influences of CaO and O₂ addition on the formation of H₂, CH₄, and CO₂ in the gas phase and solid phase carbon, CaCO₃, and Ca(OH)₂ were also investigated. With methanation and carbon formation, the conversion of the model compounds was >99.99% with no carbon formation, and methanation was thermodynamically favored over hydrogen production. H₂ selectivity was greatly improved when methanation was suppressed, but most of the inlet model compounds formed solid carbon. After suppressing both methanation and carbon formation, aqueous phase reforming of methanol, acetic acid and ethylene glycol at 500 K and with Psys/PH₂O = 1.1 gave H₂ selectivity of 74.98%, 66.64% and 71.38%, respectively. These were similar to the maximum stoichiometric hydrogen selectivity of 75.00% (methanol), 66.67% (acetic acid), and 71.43% (ethylene glycol). At 500 K and 2.90 MPa, as the molar ratio of CaO/BMCs increased, the normalized variation in H₂ increased and that for CH₄ decreased. Formation of solid carbon was effectively suppressed by addition of O₂, but this was at the expense of H₂ formation. With the O₂/BMCs molar ratio regulated at 1.0, oxidation and CO₂ capture increased the normalized variation in H₂ to 33.33% (methanol), 50.00% (acetic acid), and 60.00% (ethylene glycol), and the formation of solid carbon decreased to zero.     

Sustainable hydrogen production from steam reforming of bio-oil model compound based on carbon deposition/elimination

Steam reforming of crude bio-oil or some heavy component present in bio-oil is a great challenge for sustainable hydrogen production due to the extensive coke formation and catalyst deactivation. Catalyst regeneration will be an unavoidable operation in this process. In this paper, m-cresol (a model compound derived from bio-oil) was steam reformed on commercial Ni-based catalyst. Two conventional carbon elimination methods for coked catalyst were applied and the results showed that sustainable hydrogen production can be obtained based on carbon deposition/elimination. The carbon deposition can be gasified easily under certain temperature. The activity of regenerated catalyst samples can be nearly recovered as the fresh ones. Under the reaction conditions of 850 °C and steam to carbon ratio 5:1, >66% hydrogen mole fraction, >81% hydrogen yield, and >97% carbon conversion can be achieved based on regenerated catalyst. Catalyst characterization indicated that the loss of active metal can be considered as the main reason for tiny activity drop. Ni redispersion and Fe contamination may be another two factors that influence catalyst activity.     

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.     

Mechanistic study of bio-oil catalytic steam reforming for hydrogen production: Acetic acid decomposition

To clarify the understanding of the mechanism of bio-oil catalytic steam reforming, we selected acetic acid as a typical bio-oil model compound to study its detailed behavior in decomposition over an active stepped Ni surface by density functional theory calculations. The adsorption geometries and energies of various intermediates were reported. Linear correlations between the adsorption energy and the number of hydrogen atoms removed for CH_xCOOH, CH_xCOO, and CH_x species (x = 1–3) were found, with increments of ?1.56, ?0.81, and ?1.80 eV, respectively. Thirty-seven possible elementary reactions of acetic acid decomposition were proposed, and their activation energies, reaction energies, rate constants, and equilibrium constants were calculated. Acetic acid dissociation likely started via α-carbon dehydrogenation, OH dehydrogenation, and dehydroxylation. Combined with microkinetic modeling, the most preferable decomposition pathway was suggested as CH₃COOH → CH₃CO → CO + CH₃. The rate-determining step was CH₃COOH dehydroxylation to CH₃CO with an activation energy of 0.68 eV and a rate constant of 3.82 × 10⁸ s^?1. The formation of CH₃COO was dominant at high temperatures, whereas its decomposition occurred with difficulty.     

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.     

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%.     

Steam reforming of acetic acid for hydrogen production over Ni/CaxFeyO catalysts

Three Ni/Ca_xFe_yO (x/y = 2:1, 1:1, 1:2) catalysts are prepared by impregnation method and applied in steam reforming of acetic acid as the model compound of bio-oil for hydrogen production. The effects of reaction temperature, steam to carbon ratio (S/C), liquid hourly space velocity (LHSV) on gas contents and H₂ yield are carefully investigated and optimized. The fresh and used catalysts are characterized by BET, XRD, H₂-TPR, CO₂-TPD, SEM and TG methods. The experimental and characterization results show that the Ni/CaFe₂O₄ catalyst displays the best activity and stability among the three catalysts, providing 92.1% of H₂ yield under S/C = 5, LHSV = 3.4 h⁻¹ and at 600 °C. The strong interaction between Ni and CaFe₂O₄ support result in the formation of Ni–Fe alloy and Ca₂Fe₂O₅, which shows the synergistic effects on the resistant to carbon deposition and metal sintering, thereby improving the activity and stability of the Ni/CaFe₂O₄ catalyst.     

Hydrogen production of bio-oil steam reforming combining heat recovery of blast furnace slag: Thermodynamic analysis

Hydrogen production via steam reforming of bio-oil combining heat recovery of blast furnace slag was investigated via thermodynamic analysis in this paper. The addition of blast furnace slag just had a slight enhancement for hydrogen production from the steam reforming process of bio-oil at low temperature, and had almost no thermodynamic effect (either promotion or restraint) for the steam reforming reaction equilibrium at high temperature where higher H₂ yield were obtained, no matter how much blast furnace slag was added. However, different masses of blast furnace slag as heat carrier supply different amounts of heat, so the optimal blast furnace slag addition was performed via energy balance. If the sensible heats of the reformed gas and the slag after steam reforming reactions were unrecycled, the required mass of blast furnace slag was over 30 times of bio-oil mass, while the required slag mass was just 11.5 times of bio-oil mass if the sensible heats after the steam reforming reactions were recycled. For the latter, about 0.144 Nm³ H₂ per kg blast furnace slag was obtained at the reforming temperature of 700–750 °C and the steam/carbon mole ratio of 6.     

Renewable hydrogen production via steam reforming of simulated bio-oil over Ni-based catalysts

The production of hydrogen via steam reforming (SR) of simulated bio-oil (glycerol, syringol, n-butanol, m-xylene, m-cresol, and furfural) was investigated over Ni/CeO₂-Al₂O₃ and Me-Ni/CeO₂-Al₂O₃ (Me = Rh, Ru) catalysts. Monometallic (Ni) and bimetallic (Rh-Ni and Ru-Ni) catalysts were prepared by the wetness impregnation technique of the CeO₂-Al₂O₃ support previously synthesized by the surfactant-assisted co-precipitation method. The as-prepared powders were systematically characterized by N₂-physisorption, XRD, H₂-TPR, and TEM measurements to analyze their structure, morphology, and reducibility properties. Experiments were performed in a continuous fixed-bed reactor at atmospheric pressure, temperature of 800 °C, steam to carbon (S/C) ratio of 5, and WHSV of 21.15 h⁻¹. Then, the temperature was decreased to 700 °C and increased afterwards to 800 °C. After the experiments TPO and TEM analysis were performed on the spent catalysts to check any evidence of catalyst deactivation. The results showed that the incorporation of noble metal (Ru or Rh) promoter positively affected the activity of the Ni/CeO₂-Al₂O₃ catalysts by enhancing the reducibility of Ni²⁺ species. Ni-based catalyst deactivated under the studied conditions, whereas Ru- and mainly Rh-promoted systems showed increased resistance to carbon deposition by favouring the gasification of adsorbed carbon species. Between all tested catalysts, the Rh-Ni/CeO₂-Al₂O₃ provided the highest H₂ yield and coking-resistance in SR of simulated bio-oil.     

Thermodynamic analysis of propane dry and steam reforming for synthesis gas or hydrogen production

Thermodynamics was applied to investigate propane dry reforming (DR) and steam reforming (SR). Equilibrium calculations employing the Gibbs free energy minimization were performed upon a wide range of pressure (1–5 atm), temperature (700–1100 K), carbon dioxide to propane ratio (CPR, 1–12) and water to propane ratio (WPR, 1–18). From a thermodynamic perspective, it is demonstrated that DR is promising for production of synthesis gas with low hydrogen content, as opposite to SR which favours generation of synthesis gas with high hydrogen content. Complete conversion of propane was obtained for the range of pressure, temperature, CPR and WPR considered in this study. Atmospheric pressure is shown to be preferable for both DR and SR. Approximately 10 mol of synthesis gas can be produced per mole of propane at a temperature greater than 1000 K from DR when CPR is higher than 6. The optimum conditions for synthesis gas production from DR are found to be 975 K (CPR = 3) for a H₂/CO ratio of 1 and 1100 K (CPR = 1) for a H₂/CO ratio of 2. The greatest CO₂ conversion (95%) can be obtained also at 1100 K and CPR = 1. Preferential conditions for hydrogen production from SR are achieved with the temperatures between 925 and 975 K and WPRs of 12–18. The maximum number of moles of hydrogen produced is 9.1 (925 K and WPR = 18). Under conditions that favour hydrogen production, methane and carbon formation can be eliminated to negligible level.     

Thermodynamic analysis of steam reforming and oxidative steam reforming of propane and butane for hydrogen production

Thermodynamic analyses of cracking, partial oxidation (POX), steam reforming (SR) and oxidative steam reforming (OSR) of butane and propane (for comparison) were performed using the Gibbs free energy minimization method under the reaction conditions of T = 250–1000 °C, steam-to-carbon ratio (S/C) of 0.5–5 and O₂/HC (hydrocarbon) ratio of 0–2.4. The simulations for the cracking and POX processes showed that olefins and acetylene can be easily generated through the cracking reactions and can be removed by adding an appropriate amount of oxygen. For SR and OSR of propane and butane, predicted carbon formation only occurred at low S/C ratios (<2) with the maximum level of carbon formation at 550–650 °C. For the thermal-neutral conditions, the TN temperatures decrease with the increase of the S/C ratio (except for O/C = 0.6) and the decrease of the O/C ratio. The simulated results for SR or OSR of propane and butane are very close under the investigated conditions.     

Thermodynamic analysis of steam reforming of ethanol and glycerine for hydrogen production

In the past few years there has been a growing interest in environmentally clean renewable sources for hydrogen production. In this context new technologies have been developed for ethanol and glycerine reforming. Hydrogen production varies significantly according to the operating conditions such as pressure, temperature and feed reactants ratio. The thermodynamic analysis provides important knowledge about the effects of those variables on the process of ethanol and glycerine reforming. The present work was aimed at analyzing the thermodynamic steam reforming of ethanol and glycerine, using Gibbs free energy minimization using actual temperature and pressure data found in the literature. The nonlinear programming model was implemented in GAMS^® and the CONOPT2 solver was used to solve the equations. The ideality in gaseous phase and the formation of solid carbon was considered. The methodology used reproduced the most relevant papers involving experimental studies and thermodynamic analysis.     

Catalytic steam reforming of bio-oil

Hydrogen and synthesis gas can be produced in an environmentally friendly and sustainable way through steam reforming (SR) of bio-oil and this review presents the state-of-the-art of SR of bio-oil and model compounds hereof. The possible reactions, which can occur in the SR process and the influence of operating conditions will be presented along with the catalysts and processes investigated in the literature.     

Hydrogen production from catalytic steam reforming of biomass pyrolysis oil or bio-oil derivatives: A review

Steam reforming of biomass pyrolysis oil or bio-oil derivatives is one of the attractive approaches for hydrogen production. The current research focused on the development of promising catalysts with favorable catalytic activity and high coke resistance. Noble metal such as Rh has been proven to achieve promising reforming reaction efficiencies. However, Ni has attracted considerable attention owing to its stability, cost effectiveness, and good activity in breaking C–C and C–H bonds. Nevertheless, Ni-based catalysts have serious carbon deposition problems arising from chemical poisoning, metal sintering, and poor metal dispersion. This paper attempted to review the current trends in catalyst development considering the aspects of supports, metals, and promoters as an effort to find possible solutions for the limitations of Ni-based catalysts. The present review also covered the current understanding on the reaction mechanisms as well as the future prospects in the field of steam reforming catalysts.     

Hydrogen production from a model bio-oil/bio-glycerol mixture through steam reforming using Zeolite L supported catalysts

Zeolite L featuring different size and shape (nanocrystals and discs), with and without alkaline metal exchange (Cs or Na), was used as catalyst support in bio-oil/bio-glycerol mixture Steam Reforming (SR). Zeolites were modified with CeO₂ to improve support properties before the impregnation of nickel. Then, prepared catalysts were tested in SR of a multi-component synthetic bio-oil/bio-glycerol mixture at 1073 and 973 K, under atmospheric pressure and using a Steam to Carbon (S/C) ratio of 5.0. Activity tests showed that catalysts deactivated during the experiments at 973 K. In addition, the sodium exchange produced the sintering of the Zeolite L crystals. Thus, Na containing catalysts produced low conversions and hydrogen yields lower than 30%. On the other hand, Cs containing catalysts resulted in slightly lower hydrogen yields than the supports without this metallic cation. Regarding the morphology of the zeolites, the ones with disc shape were the most active for bio-oil SR purposes producing hydrogen yields close to 80% in the first reaction stage at 1073 K and hydrogen yields close to the 50% at the last reaction stage at 1073 K.     

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.