Bioenergy systems for the future 6 thermodynamic analysis of ethanol reforming for hydrogen production

Bioenergy systems for the future 6 thermodynamic analysis of ethanol reforming for hydrogen production Bioenergy systems for the future 6 thermodynamic analysis of ethanol reforming for hydrogen production Bioenergy systems for the future 6 thermodynamic analysis of ethanol reforming for hydrogen production Bioenergy systems for the future 6 thermodynamic analysis of ethanol reforming for hydrogen production Bioenergy systems for the future 6 thermodynamic analysis of ethanol reforming for hydrogen production Bioenergy systems for the future 6 thermodynamic analysis of ethanol reforming for hydrogen production

Thermodynamic analysis of ethanol reforming for hydrogen production G Tamasi, C Bonechi, A Magnani, G Leone, A Donati, S Pepi, C Rossi University of Siena, Siena, Italy Abbreviations ESR WGSR ΔH ΔS ΔG Keq 6.1 ethanol steam reforming water-gas shift reaction enthalpy entropy Gibbs free energy equilibrium constant Introduction In the recent years, environmental problems, like climate change, air pollution, and natural resources depletion, have attracted attention due to their direct and indirect impacts on human health and ecosystem services It is widely recognized that the modern lifestyles require high consumption of energy, the generation of which still relies heavily on the use of fossil fuels derivatives, a nonrenewable source A new approach to meet the global energy request is mandatory, and the need for renewable alternatives is urgent Among the renewable resources for energy production, the solar, wind, and biomass are the most promising However, they are usually site-specific and seasonally intermittent Hydrogen has been identified as a good “energy carrier” to support sustainable energy development (Goltsov et al., 2006; Ni et al., 2006) and can be used in fuel cells to generate electricity with high efficiency The use of hydrogen is very clean as the only final by-product is water However, in order to support sustainable hydrogen economy, it is mandatory to produce hydrogen in a clean and renewable way At present, almost 90% of the hydrogen is commercially produced in an economically competitive method, by gasification, partial oxidation reactions of fossil fuels (Das and Veziroglu, 2001; Haryanto et al., 2005), via steam reforming reactions of hydrocarbons, for example, coal, natural gas, liquefied petroleum gas, propane, methane (CH4), gasoline, and light diesel The current worldwide production is around  1011 N m3/year (Vaidya and Rodrigues, 2006) The hydrogen is mostly used as a feedstock in the chemical industry and in the manufacture of ammonia and methanol, in refinery reprocessing Bioenergy Systems for the Future http://dx.doi.org/10.1016/B978-0-08-101031-0.00006-5 © 2017 Elsevier Ltd All rights reserved 188 Bioenergy Systems for the Future and conversion processes (Vaidya and Rodrigues, 2006; Sun et al., 2004), but it is also used as feedstock in the production of ethylene, acetaldehyde, acetone, etc Fossil-fuel-based (e.g., natural gas) production of hydrogen fails to provide a solution to deal with the huge amount of carbon dioxide emissions during the steam reforming at high-temperature processes As a result, studies on possible effective alternatives to produce renewable hydrogen in a clean and safe way have attracted considerable attention Particularly interesting is the production of hydrogen through pyrolysis, gasification, and steam reforming processes of lignocellulosic biomass, a renewable resource (Wang et al., 1997; Garcia et al., 2000; Stiegel and Maxwell, 2001) or through intermediate liquid biofuels (Fatsikostas et al., 2002) Among the liquid biofuels, ethanol (C2H5OH) is a good candidate for several reasons: (i) Ethanol is renewable and is becoming increasingly available; (ii) it is easy to transport, biodegradable, low in toxicity, and low volatile; and (iii) it could be easily decomposed in the presence of water to generate a hydrogen-rich mixture This latter process, steam reforming, is conducted at 200–800°C in the presence of catalysts The main advantage of liquid biofuels in general and ethanol in particular is their high energy density, ease of handling, and “on-demand” production to feed fuel cells, with applications in mobile and stationary grid-independent power systems In addition, ethanol can be produced renewably from several biomass sources, including energy plants, lignocellulosics, waste materials from agricultural and agro-industrial processes, forestry residue materials, and organic fraction of municipal solid waste The ethanol produced in this way is called bioethanol, which is a mixture of ethanol and water with a molar ratio of 1:13 (about 18 wt% ethanol; Roh et al., 2006; Benito et al., 2005; D€ om€ ok et al., 2007) Furthermore, in contrast to other proposed fossil-fuel-based systems (like methanol and gasoline), bioethanol has the significant advantage of being a nearly CO2 neutral process, since the carbon dioxide produced was created by biomass growth, thus offering a nearly closed carbon loop, with high efficiency The proposed overall process for the hydrogen (and electricity) production from biomass is schematically shown in Fig 6.1 (Fatsikostas et al., 2002) The biomass from plant cultivation (1) and/or residues of agricultural and agro-industrial processes (2) are used for the production of bioethanol by saccharification/fermentation reactions (3) The aqueous mixture is then distilled to 45%–55% ethanol (4), meanwhile the fermented solid can feed an anaerobic digestion unit (5) where biogas is produced (mixture of CH4 and CO2) In addition, the anaerobic digester could also be fed by the organic fraction of municipal solid waste (6) Finally, a gas mixture rich in H2 is produced by reformation of bioethanol (7) and biogas (8) A water-gas shift reactor (WGSR) is then used for the transformation of CO into H2 and CO2 (9) The final mixture would be further purified by selective oxidation of residual CO (10) for feeding a fuel cell (11) and produce electricity In addition, a postcombustion reactor (12) may be used to clean up the effluent of the fuel cell (Fatsikostas et al., 2002) Thermodynamic analysis of ethanol reforming for hydrogen production 189 Solar energy CO2 Fertilizer CO2 Plant cultivation Biomass (1) Solid residue Saccharification/ Anaerobic fermentation digestion (3) (5) Aqueous broth 8%–10% ethanol Distillation (4) Residues of agroindustries and cultivations (2) Heat 45%–55% ethanol CH4, CO2 Biogas reformation (8) CO, H2 Reformation of ethanol (7) Municipal solid waste (organic fraction) (6) Heat CO, H2 Shift reactor (9) Heat CO2, CO, H2 Selective oxidation of CO (10) Heat Heat CO2, H2 Heat Fuel cell (11) CO2 Post combustion (12) Electricity Fig 6.1 Overall process for the renewable production of hydrogen and electricity from biomass-derived ethanol with high efficiency and zero emissions Reproduced with permission from Fatsikostas, A.N., Kondarides, D.I., Verykios, X.E., 2002 Production of hydrogen for fuel cells by reformation of biomass-derived ethanol Catal Today 75, 145–155 6.1.1 Bioethanol production In this overview, it is interesting to underline that among the biomass materials that are available from ethanol production, sugar cane, switch grass, potatoes, corns, and other starch-rich materials can be effectively converted into ethanol by yeast-assisted fermentation, but the cost of this process is high because of the expensive feedstock plantation 190 Bioenergy Systems for the Future yeast assisted fermentation C6 H12 O6 ƒƒƒƒƒƒƒƒƒƒƒ! 2CH3 CH2 OH + 2CO2 In contrast, about 50% of biomass in the world is represented by lignocellulosic biomasses that could be used for the same purposes (Galbe and Zacchi, 2002; Dien et al., 2003), even if the total process is more difficult due to its more complex molecular structures (Sun and Cheng, 2002) Lignocellulosics mainly consist of three components: cellulose, hemicellulose, and lignin; the first two being composed of sugar molecule chains (Fig 6.2) Possible processes for producing bioethanol are represented in Fig 6.3 (Galbe and Zacchi, 2002; Ni et al., 2007) They mainly consist of (i) hydrolysis of cellulose and hemicellulose to monomeric sugars, and (ii) fermentation The processes are different in the hydrolysis steps, in which concentrated acid, diluted acid, or enzymes can be used The enzymatic path is more convenient with respect to the acid hydrolyses that are characterized by lower glucose yield and equipment corrosion Moreover, the simultaneous saccharification and fermentation process results more advantageous due to simpler reactor configuration and higher glucose conversion (Ni et al., 2007) HO OH O HO CH3 OH HO O O O O HO OH O O HO (B) O HO OH OH HO O O O O O O O HO O (C) OH Fig 6.2 Cellulose (A), hemicelluloses (B), and lignin (C) structures CH3 O O O OH HO HO O O HO (A) O O O O OH OH OH O HO Thermodynamic analysis of ethanol reforming for hydrogen production 191 Cellulose/hemicellulose Concentrated acid hydrolysis Diluited acid hydrolysis Pretreatment Enzyme production Acid recovery Sugars Simultaneous saccharification and fermentation Enzymatic hydrolysis Fermentation Bioethanol Fig 6.3 Possible processes for producing bioethanol from lignocellulosic biomass 6.1.2 Ethanol steam reforming The target reaction for the ethanol steam reforming (ESR) is the stoichiometric “ideal” reaction of ethanol (C2H5OH) that produces hydrogen (H2) that can be subsequently used for feeding fuel cells The ESR reaction is C2 H5 OH + 3H2 O ! 2CO2 + 6H2 (6.1) However, it is very well known that the ESR “real” process is more complex and consists of several subreactions that lead to the formation of several intermediates and by-products in the final mixture, depending on the temperature, pressure, ethanol/ water ratio, nature of the catalyst, specific plant characteristics, etc It is, therefore, fundamental to take into account the possible by-products that could inactivate the catalyst used in the reforming process and/or the fuel cell themselves Given this challenge, it is fundamental to explore the subreactions without discarding the many reactions on secondary species that could form Some of those reactions (Fig 6.4; Haryanto et al., 2005) are summarized as C2 H5 OH + 3H2 O ! 2CO2 + 6H2 + BY À PRODUCTS (6.2) 192 Bioenergy Systems for the Future H H O C C OH H H O H C C C H Metal catalyst Dissociative adsorption H H H C Decomposition H2O Coking Metal catalyst Metal catalyst H H2O H H2O Steam reforming C H H OH Water-gas-shift H2, CO Metal catalyst Metal catalyst Dehydrogenation Decomposition Metal catalyst H2O Metal catalyst C Steam reforming H Metal catalyst H2, CO2 Methanation Metal catalyst H2 Decomposition Metal catalyst Dehydration Metal catalyst H2O H2 CH4, C2H4, H2, H2O, CO2 H2O C Polymerization Metal catalyst Water-gas-shift CO H H O C C Metal catalyst Decomposition H Metal catalyst Steam reforming Metal catalyst H H2O Fig 6.4 Possible reactions that can occur in the ethanol steam reforming (ESR) process Reproduced with permission from Haryanto, A., Fernando, S., Murali, N., Adhikari, S., 2005 Current status of hydrogen production techniques by steam reforming of ethanol: a review Energy Fuels 19, 2098–2106 A list of a selected group of the possible by-reaction is also reported in Table 6.1 (without considering reaction that can bring about NOx by-products, coming from the N2 present in the air) Several experimental and theoretical studies have been performed in the last couple of decades to shed light on this complex process (Sun et al., 2004; Freni et al., 1996; Fishtik et al., 2000; Ioanides, 2001; Comas et al., 2004; Goula et al., 2004; Fatsikostas and Verykios, 2004; Llorca et al., 2001; Diagne et al., 2002; Marin˜o et al., 2003; Batista et al., 2003; Sheng et al., 2004; Segal et al., 2003; Rasko et al., 2004) and the relation with the catalyst used (Sun et al., 2004; Fatsikostas and Verykios, 2004; Llorca et al., 2001, 2009; Diagne et al., 2002; Marin˜o et al., 2003; Table 6.1 ESR reaction and selected subreactions taken into account Reaction C2H5OH + 3H2O ! 2CO2 + 6H2 C2H5OH + 2H2O + ½O2 ! 2CO2 + 5H2 C2H5OH + ½O2 ! 2CO + 3H2 ID “Ideal” C2H5OH steam reforming to H2 Autothermal C2H5OH steam reforming to H2 Autothermal C2H5OH steam reforming to H2 Eq (6.9) Eq (6.27) Eq (6.28) Subreaction group A: other possible steam reforming reactions for C2H5OH C2H5OH + H2O ! CH4 + CO2 + 2H2 C2H5OH + H2O ! 2CO + 4H2 2C2H5OH + H2O ! CH3COCH3 + CO2 + 4H2 C2H5OH + H2O ! CH3COOH + 2H2 2C2H5OH + H2O ! CH3COH + 2CO +5H2 C2H5OH + 3H2O ! 2CO2 + 6H2 + C C2H5OH steam C2H5OH steam syngas C2H5OH steam CH3COCH3 C2H5OH steam CH3COOH C2H5OH steam CH3COH reforming to CH4 reforming to Eq (6.10) Eq (6.11) reforming to Eq (6.12) reforming to Eq (6.13) reforming to Eq (6.14) Eq (6.15) Subreaction group B: reaction for CH4 CH4 + H2O ! CO + 3H2 CH4 + 2H2O ! CO2 + 4H2 CH4 + CO2 ! 2CO + 2H2 CH4 steam reforming to CO CH4 steam reforming to CO2 Eq (6.16) Eq (6.17) Eq (6.18) Subreaction group C: reaction for CO CO + H2O ! CO2 + H2 2CO ! CO2 + C Water-gas shift reaction (WGSR) Boundouard reaction Eq (6.19) Eq (6.20) Subreaction group D: reaction for CH3COCH3, CH3COH, CH3COOH CH3COCH3 + 5H2O ! 3CO2 + 8H2 CH3COCH3 + 2H2O ! 3CO + 5H2 CH3COH + 3H2O ! 2CO2 + 5H2 CH3COH + H2O ! 2CO + 3H2 CH3COOH ! CO2 + CH4 CH3COH ! CO + CH4 CH3COCH3 steam reforming to CO2 CH3COCH3 steam reforming to CO CH3COH steam reforming to CO2 CH3COH steam reforming to CO CH3COOH decarboxylation CH3COH methanation Eq (6.21) Eq (6.22) a a a a Sub-reaction group E: other reactions for C2H5OH C2H5OH ! CH4 + CO + H2 2C2H5OH ! CH3COCH3 + CO + 3H2 C2H5OH ! C2H4 + H2O C2H5OH + 2H2 ! 2CH4 + H2O C2H5OH ! CH3COH + H2 C2H4 ! 2H2 + 2C a C2H5OH methanation C2H5OH oxydation to CH3COCH3 C2H5OH dehydration to C2H4 C2H5OH hydrogenation to CH4 C2H5OH dehydrogenation to CH3COH C2H4 dehydrogenation Eq (6.23) Eq (6.24) Eq (6.25) Eq (6.26) a a These reactions have not been studied because REFPROP software is not implemented for CH3COH, CH3COOH and C (Coke) species (see also Table 6.2) 194 Bioenergy Systems for the Future Batista et al., 2003; Iwasa et al., 1999; Kaddouri and Mazzocchia, 2004; Liguras et al., 2003; Frusteri et al., 2004) Several experimental studies showed effect of the temperature on the conversion and selectivity toward the main products and by-products Studies at different temperatures have allowed optimizing the experimental conditions to maximize hydrogen yield, to limit by-product formation, and to propose reaction schemes (Sun et al., 2004; Fishtik et al., 2000; Ioanides, 2001; Comas et al., 2004; Goula et al., 2004; Fatsikostas and Verykios, 2004; Llorca et al., 2001; Diagne et al., 2002; Batista et al., 2003) The behavior of the catalysts versus time has also been considered (Sun et al., 2004; Goula et al., 2004) 6.1.3 Brief overview on the catalyst for the ESR process The main types of reforming could be classified as (i) steam reforming (reaction 6.1, Table 6.1), (ii) autothermal reforming (see below, reaction 6.27), and (iii) partial oxidation (see below, reaction 6.28) All these processes have common aspects of a primary reforming reaction that brings about a mixture of gases rich in H2, starting from the reactants, and processes of purification/separation of the stream mixture Such catalytic reforming processes of ethanol can occur through various catalysts with a metallic phase that acts as the catalytic activator and of a supporting phase on which the metal microparticles are loaded and spread out at a variable weight concentration From analyses by laboratory research studies and experimental data from plants, the initial step of the reforming process consists of the adsorption of ethanol molecules on the surface of the catalyst where the breakage of the bonds CdC, CdO, CdH, and OdH takes place and intermediate species form (ethylene, acetone, and acetaldehyde) The concentrations of these latter species depend on the catalyst (nature of metal and the support, the concentration of the metal, and other experimental parameters) Commonly, the catalysts can be noble metals (Pt, Pd, Rh, and Ru) or other “block-d” metals (Ir, Cu, Co, and Ni), loaded on supporting phase by metal oxides (Al2O3, CeO2, SiO2, ZrO2, TiO2, MgO, La2O, and Y2O3) or mixed metal oxides As examples, on using Rh/CeO2 catalyst, not only the yields are excellent, as high as 95% for H2 (Deluga et al., 2004), but also catalysts based on iridium and cobalt have been reported to bring about quite good yields (Wang et al., 2009; Iulianelli and Basile, 2010) Another important step of the overall process is the purification of the gas from the reforming reactions for the production of H2 ESR can be carried out in traditional catalytic reactors or in inorganic membrane reactors with an inner tubular-shaped inorganic membrane that is selective for the permeation of H2 These membranes often consist of metals like Pd that are extremely selective toward H2 permeation (but unfortunately very costly), and/or its alloys (like Pd/-Ag; Basile et al., 2015) As an example, on a laboratory scale, an autosupported tubular membrane reactor consisting of Pd-Ag allows to get a conversion proximal to 100%, for ethanol reforming and a recovery of H2 of c.90%, when operating at 400–500°C and relatively low reaction pressure by 1–3 bar (Iulianelli and Basile, 2010; Iulianelli et al., 2009, 2010a,b; Basile et al., 2008a,b) Thermodynamic analysis of ethanol reforming for hydrogen production 195 In conclusion, the chemical nature of the species produced needs to be determined, and the reaction paths identified Subsequently, a thermodynamic analysis can be carried out to evaluate the maximum yield reachable in terms of the characteristic of the reactor The literature offers several thermodynamic models for the ESR process (Wang and Wang, 2008, 2009; Graschinsky et al., 2012; Mas et al., 2006), and a recent study based on an exergetic approach was published (Casas-Ledo´n et al., 2012) On the basis of this reasoning, this chapter reports on the state of the art of ESR processes and a theoretical thermodynamic analysis of process paths with the goal to determine the optimal working conditions for high yield production of H2 from bioethanol and, from it, to produce electric power, that is, through fuel cells 6.2 Calculation method The analysis utilized classical thermodynamic properties (ΔH, ΔS, ΔG, and Keq) for a selected number of subreactions (Table 6.1) to compare values calculated for the main ESR, reaction (6.1), and for hypothesizing the optimal working conditions as regards temperature (T, K) and pressure (P, atm) The autothermal ESR reactions (6.27) and (6.28) were also considered for comparative purposes The calculation was performed applying the basic rules of the thermodynamics: Enthalpy of formation, as a function of T and P for each species H T , Pị ẳ H + ẵCp ΔT Š ¼ ΔH° + h  i CpðT , PÞ À Cp° ðT À T°Þ (6.3) where Cp° is the specific heat capacity at standard conditions (expressed in J/K) Enthalpy of reaction, as a function of T and P Hreaction ẳ products H T , Pị reagents HT , PÞ (6.4) Entropy of formation, as a function of T and P, for each species ST , Pị ẳ S° + CpðT , PÞ ln ðT=T°Þ (6.5) where S° is the standard molar entropy expressed as J/mol/K Entropy of reaction, as a function of T and P ΔSreaction ¼ Σproducts ΔSðT , PÞ À Σreagents ΔSðT , PÞ (6.6) Gibbs free energy, as a function of T and P ΔGreaction ¼ ΔHreaction À TΔSreaction (6.7) Equilibrium constant as a function of T and P Keq ẳ eG=RTị (6.8) 196 Bioenergy Systems for the Future The ΔH, ΔS, ΔG, and Keq parameters for selected reactions (Table 6.1) were studied by varying the temperature T in the range 274–973 K (steps by K) and pressure P in the range 0.5–10 atm (steps by 0.25 atm) Particular attention was devoted to focus on values obtained for parameters P ¼ atm and T > 450 K, comparable with typical experimental working conditions for ESR plants The data for standard condition parameters (T° ¼ 298 K and P° ¼ atm) for ΔH°, ΔS°, and Cp° were obtained from the CRC Handbook of Chemistry and Physics (CRC, 2005–2006), whereas the theoretical data as a function of T and P were calculated by using the REFerence fluid PROPerties software (Lemmon et al., 2013) and the PRODE software (PRODE software, 2014) for comparison REFPROP is a program developed by the National Institute of Standards and Technology (NIST) that calculates the thermodynamic properties of selected fluids and mixtures It is not a database and does not contain any experimental information, aside from the critical and triple points of the pure fluids REFPROP is based on the most accurate pure fluid and mixture models currently available: equations of state explicit in Helmholtz energy, the modified Benedict-Webb-Rubin equation of state, and an extended corresponding states (ECS) model High accuracy was obtained by using many coefficients in the equations that are generally valid over the entire vapor and liquid regions of the fluid, including supercritical states; the upper temperature limit was usually near the point of decomposition of the fluid, and the upper pressure limit was defined by the melting line of the substance Table 6.2 reports the references relevant to the equation of state used for each single fluid, and selected parameters relevant to them, like critical point parameters (temperature, Tc, K; pressure, Pc, MPa), triple-point temperature (TT, K), liquid-gas transition temperature (TLG, K), decomposition temperature (TD, K), and maximum temperature as limit of the model (Tmax, K) Selected details about uncertainties in the equation of state are also reported Furthermore, it is important to note that not all the fluids are implemented in REFPROP software; thus, some of the by-reactions reported in Table 6.1 were not considered in this work (i.e., reactions in which the formation of carbon powder appear) N2 (from air) reactions producing NOx were not considered at that stage Each reaction was singularly studied as without taking into account any synergic and direct competitive effect among them, as in real experimental systems ESR plants are usually catalytically assisted, and the present study did not consider any preferential path on the basis of possible kinetic effects 6.3 Analysis of thermodynamic properties for the single reactions 6.3.1 Reaction (6.9): Ideal ESR C2 H5 OH + 3H2 O ! 2CO2 + 6H2 (6.9) The values for ΔH, ΔS, ΔG, and the equilibrium constants (Keq and pKeq) were calculated for the stoichiometric and “ideal” C2H5OH steam reforming to H2 (ESR), 202 Bioenergy Systems for the Future Selected calculated parameters (ΔHreaction, kJ/mol; ΔSreaction, kJ/mol K; ΔGreaction, kJ/mol; and Keq) at selected temperature and pressure values (T 298, 673, 723, and 973 K; P 1, 5, and 10 atm) for the ideal ESR, reaction (6.9) Table 6.3 C2H5OH + 3H2O ! 2CO2 + 6H2 Eq (6.9) P atm ΔH298,1 ¼ 173.4530 ΔS298,1 ¼ 0.3615 ΔG298,1 ¼ 65.7260 Keq298,1 ¼ 3.012  10À12 ΔH673,1 ¼ 126.8319 ΔS673,1 ¼ 0.3992 ΔG673,1 ¼ À 141.4295 Keq673,1 ¼ 1.020  1011 ΔH ΔS ΔG ¼ À 169.5250 Keq723,1 ¼ 1.771  1012 ΔS973,1 ¼ 0.3911 ΔG973,1 ¼ À 305.3434 Keq973,1 ¼ 2.470  1016 ΔH298,5 ¼ 173.4530 ΔS298,5 ¼ 0.3615 ΔG298,5 ¼ 65.7260 Keq298,5 ¼ 3.012  10À12 ΔH673,5 ¼ 126.0849 ΔS673,5 ¼ 0.3976 ΔG673,5 ¼ À 141.4843 Keq673,5 ¼ 9.586  1010 ΔH ΔS ΔG ¼ À 169.2175 Keq723,5 ¼ 1.682  1012 ΔS973,5 ¼ 0.3906 ΔG973,5 ¼ À 305.1571 Keq973,5 ¼ 2.414  1016 ΔH298,10 ¼ 173.4530 ΔS298,10 ¼ 0.3615 ΔG298,10 ¼ 65.7260 Keq298,10 ¼ 3.012  10À12 ΔH673,10 ¼ 125.1155 ΔS673,10 ¼ 0.3955 ΔG673,10 ¼ À141.0365 Keq673,10 ¼ 8.849  1010 ΔH ΔS ΔG ¼ À168.8237 Keq723,10 ¼ 1.576  1012 ΔG973,10 ¼ À304.9232 Keq973,10 ¼ 2.345  1016 723,1 ¼ 118.8860 ΔH973,1 ¼ 75.1636 723,1 ¼ 0.3989 723,1 P 5 atm 723,5 ¼ 118.2804 ΔH973,5 ¼ 74.8996 723,5 ¼ 0.3977 723,5 P 10 atm 723,10 ¼ 117.5047 ΔH973,10 ¼ 74.5682 723,10 ¼ 0.3960 ΔS973,10 ¼ 0.3900 723,10 Selected calculated parameters (ΔHreaction, kJ/mol; ΔSreaction, kJ/mol K; ΔGreaction, kJ/mol; and Keq) at selected temperature and pressure values (T 673, 723, and 973 K; P 5 atm) for the subreactions (6.10)–(6.12) Table 6.4 C2H5OH + H2O ! CH4 + CO2 + 2H2 Eq (6.10) ΔH673,5 ¼ À 10.7751 ΔH723,5 ¼ À 13.8320 ΔH937,5 ¼ À 27.7861 ΔG673,5 ¼ À 143.6210 ΔG723,5 ¼ À 156.3210 ΔG937,5 ¼ À 210.0010 ΔS673,5 ¼ 0.1974 ΔS723,5 ¼ 0.1971 ΔS937,5 ¼ 0.1945 C2H5OH + H2O ! 2CO + 4H2 Eq (6.11) ΔH ¼ 225.9884 723,5 ΔH ¼ 220.3029 ΔH750,5 ¼ 217.1147 ΔG673,5 ¼ À 87.5388 ΔG723,5 ¼ À 115.2170 ΔG750,5 ¼ À 130.0740 673,5 ΔS ¼ 0.4659 723,5 ΔS ¼ 0.4641 ΔS750,5 ¼ 0.4629 673,5 2C2H5OH + H2O ! CH3COCH3 + CO2 + 4H2 Eq (6.12) ΔH673,5 ¼ 79.5984 ΔH723,5 ¼ 75.1455 ΔH825,5 ¼ 65.2997 ΔG673,5 ¼ À 121.4810 ΔG723,5 ¼ À 140.4470 ΔG825,5 ¼ À 178.6520 ΔS673,5 ¼ 0.2988 ΔS723,5 ¼ 0.2982 ΔS825,5 ¼ 0.2957 Keq673,5 ¼ 1.405  1011 Keq723,5 ¼ 1.969  1011 Keq937,5 ¼ 5.096  1011 Keq673,5 ¼ 6.231  106 Keq723,5 ¼ 2.111  108 Keq750,5 ¼ 1.147  109 Keq673,5 ¼ 2.686  109 Keq723,5 ¼ 1.404  1010 Keq825,5 ¼ 2.050  1011 Thermodynamic analysis of ethanol reforming for hydrogen production 203 The subreaction (6.10) considers the ESR producing methane, hydrogen, and carbon dioxide: C2 H5 OH + H2 O ! CH4 + CO2 + 2H2 (6.10) The ΔHreaction calculated parameter was lower and slightly exothermic (ΔH673,5 ¼ À 10.7751, ΔH723,5 ¼ À 13.8320, and ΔH937,5 ¼ À 27.7861 kJ/mol) when compared with value computed for reaction (6.9) (i.e., ΔH673,5 ¼ 126.0849 kJ/mol, Table 6.3) Furthermore, the ΔGreaction, and the Keq values, revealed a spontaneous reaction, shifted toward the products at relatively low temperature (ΔG673,5 ¼ À 143.6210 kJ/mol, and Keq673,5 ¼ 1.405  1011) The reaction (6.10) was a good competitor for the ideal ESR (reaction 6.9) A second possible ethanol steam reaction can be described stoichiometrically as reaction (6.11) and can produce syngas, a mixture of hydrogen and carbon monoxide: C2 H5 OH + H2 O ! 2CO + 4H2 (6.11) The carbon monoxide represents one of the most undesired species, poisoning fuel cells even at relatively low concentrations (10 ppm; Vaidya and Rodrigues, 2006) Reaction (6.11) was a very endothermic process (ΔH673,5 ¼ 225.9884 kJ/mol), revealing values much higher with respect to reaction (6.9) Although, it was spontaneous at common working temperature (ΔG673,5 ¼ À 87.5388 kJ/mol), the computed Gibbs free energy was lower when compared with the values at the same condition for reaction (6.9) (ΔG673,5 ¼ À 141.4843 kJ/mol) On this basis, it is reasonable to state that the reaction of syngas formation is thermodynamically unfavorable Computed ΔHreaction value was in good agreement with experimental data at standard condition: ΔH298,1(calcd) ¼ 247 kJ/mol, and ΔH298,1(exp) ¼ 256 kJ/mol (Vaidya and Rodrigues, 2006) A third possible ESR reaction can produce acetone as final product, a reaction (6.12) starting stoichiometrically by two moles of ethanol: 2C2 H5 OH + H2 O ! CH3 COCH3 + CO2 + 4H2 (6.12) The calculation revealed that the reaction is endothermic but had ΔHreaction values (ΔH673,5 ¼ 79.5984 kJ/mol) lower with respect to reaction (6.9) (Table 6.3) The reaction has negative values of free Gibbs energy, revealing a spontaneous process at the studied conditions (ΔG673,5 ¼ À 121.4810 kJ/mol, and Keq673,5 ¼ 2.686  109) Therefore, reaction (6.12) would be competitive with the ideal reaction (6.9), particularly at the usual working conditions of temperature and pressure (673 K and atm) Finally, the subreactions of other ESR by-processes are as follows: C2 H5 OH + H2 O ! CH3 COOH + 2H2 (6.13) 2C2 H5 OH + H2 O ! CH3 COH + 2CO + 5H2 (6.14) C2 H5 OH + 3H2 O ! 2CO2 + 6H2 + C (6.15) 204 Bioenergy Systems for the Future Bring about acetic acid (Eq 6.13), acetaldehyde (Eq 6.14), and carbon dioxide and carbon powder (Eq 6.15) These were not computed as the data relevant to CH3COOH and CH3COH fluids (as well as solid carbon) could not be implemented in the software 6.3.3 Subreaction group B: Methane reactions Table 6.5 reports selected calculated thermodynamic parameters for the subreaction of the group B that involve methane (CH4) as reactant The reaction (6.16) was a steam reforming process for CH4 for CO formation: CH4 + H2 O ! CO + 3H2 (6.16) It was an endothermic reaction, showing values of enthalpy much higher than those for reaction (6.9) (Eq 6.16: ΔH673,5 ¼ 186.8118; 1: ΔH673,5 ¼ 126.0849 kJ/mol) In addition to that, the reaction was not spontaneous up to the highest temperature allowed by the software, being 938 K for CH4, but just 750 K for CO (Table 6.2): ΔG673,5 ¼ 29.1098 and ΔG750,5 ¼ 6.0543 kJ/mol Thus, the reaction (6.16) was less probable than the ESR ideal reaction (6.9), at least at the experimental working conditions This is also important from a technical point of view to reduce carbon monoxide production The literature reports experimental data for reaction (6.16) that are in good agreement with the data computed in this work, although the experimental ones were obtained in systems working at standard conditions and catalytically assisted by noble Selected calculated parameters (ΔHreaction, kJ/mol; ΔSreaction, kJ/mol K; ΔGreaction, kJ/mol; and Keq) at selected temperature and pressure values (T 673, 723, 750, and 973 K; P 5 atm) for the subreactions (6.16)–(6.18) Table 6.5 CH4 + H2O ! CO + 3H2 Eq (6.16) ΔH673,5 ¼ 186.8118 ΔH723,5 ¼ 183.1236 ΔH750,5 ¼ 181.0405 ΔG673,5 ¼ 29.1098 ΔG723,5 ¼ 14.1041 ΔG750,5 ¼ 6.0543 ΔS673,5 ¼ 0.2343 ΔS723,5 ¼ 0.2338 ΔS750,5 ¼ 0.2333 CH4 + 2H2O ! CO2 + 4H2 Eq (6.17) ΔH673,5 ¼ 136.8600 ΔH723,5 ¼ 132.1123 ΔH937,5 ¼ 109.2858 ΔG673,5 ¼ 2.1371 ΔG723,5 ¼ À 12.8961 ΔG937,5 ¼ À 75.8834 ΔS673,5 ¼ 0.2002 ΔS723,5 ¼ 0.2006 ΔS937,5 ¼ 0.1976 CH4 + CO2 ! 2CO + 2H2 ΔH ¼ 236.7635 ΔH723,5 ¼ 234.1348 ΔH750,5 ¼ 232.6362 673,5 Keq673,5 ¼ 5.503  10À3 Keq723,5 ¼ 9.572  10À2 Keq750,5 ¼ 3.787  10À1 Keq673,5 ¼ 6.825  10À1 Keq723,5 ¼ 8.546  100 Keq937,5 ¼ 1.700  104 Eq (6.18) ΔS ¼ 0.2685 ΔS723,5 ¼ 0.2670 ΔS750,5 ¼ 0.2661 673,5 ΔG673,5 ¼ 56.0826 ΔG723,5 ¼ 41.1043 ΔG750,5 ¼ 33.0812 Keq673,5 ¼ 4.436  10À5 Keq723,5 ¼ 1.072  10À3 Keq750,5 ¼ 4.965  10À3 Thermodynamic analysis of ethanol reforming for hydrogen production 205 metal catalysts (ΔH298,1(exp) ¼ 205 kJ/mol (Vaidya and Rodrigues, 2006); ΔH298,1(exp) ¼ 206.1 kJ/mol (Basile et al., 2008a,b)) Similarly to the reaction (6.16), the by-reaction (6.17) concerned a possible methane steam reforming that produces CO2 (instead of CO) CH4 + 2H2 O ! CO2 + 4H2 (6.17) The computed parameters showed that the reaction had ΔHreaction values comparable with the ideal ESR (Eq 6.17: ΔH673,5 ¼ 136.8600 and ΔH723,5 ¼ 132.1123 kJ/mol) Looking at ΔGreaction computed parameters, the reaction was not spontaneous at 673 K (Eq 6.17: ΔG673,5 ¼ 2.1371 kJ/mol), but become slightly spontaneous at just higher temperature, ΔG723,5 ¼ À 12.8961 kJ/mol On the basis of these data, the reaction could be a competitor for the ESR ideal reaction While reaction (6.16) was undesired in the final process due to the formation of CO, reaction (6.17) was less dangerous by-reaction that enriches the final gas mixture of H2 allowing the consumption of CH4 that can come from other by-reactions, like Eq (6.10) It can be also added that in the presence of oxygen (air) the formation of CO2 is favored with respect to CO Finally, the Eq (6.18) chemical equation showed a possible reaction between two of the most important by-products of the ideal ESR process, methane and carbon dioxide: CH4 + CO2 ! 2CO + 2H2 (6.18) From an enthalpic point of view, the reaction was endothermic (ΔH750,5 ¼ 232.6362 kJ/mol, higher than reaction 6.9) Furthermore, it was also disadvantaged on the basis of the ΔGreaction parameter (ΔG750,5 ¼ 33.0812 kJ/mol) Similar considerations can be obtained analyzing the Keq values computed for other reactions from the group B The equilibrium constant values tend to increase by increasing the temperature, although the order of magnitude was less with respect to reaction (6.9) As an example, reactions (6.16) and (6.17), that produce CO, showed low Keq673,5 values (by c.10À3–10À5) when compared with reaction (6.17), had Keq673,5 ¼ 6.825  10À1 and produced the less dangerous CO2 6.3.4 Subreaction group C: Carbon monoxide reactions Among the possible by-reactions of the ESR process, the water-gas shift reaction (WGSR, Eq 6.19) was particularly important, as it involved the “steam reforming” of CO to produce hydrogen CO + H2 O ! CO2 + H2 (6.19) ΔHreaction, ΔSreaction, ΔGreaction, and Keq were computed (Table 6.6), and values showed that the WGSR was favored from a thermodynamic point of view It was exothermic and spontaneous with ΔH673,5, ΔG673,5, and Keq673,5 of À49.9518, À26.9728 kJ/mol, and 1.240  102, respectively Furthermore, this reaction was 206 Bioenergy Systems for the Future Selected calculated parameters (Hreaction, kJ/mol; ΔSreaction, kJ/mol K; ΔGreaction, kJ/mol; and Keq) at selected temperature and pressure values (T 673, 723, and 750 K; P 5 atm) for the sub-reaction (6.19) Table 6.6 CO + H2O ! CO2 + H2 ΔH ΔG673,5 ¼ À26.9728 Keq673,5 ¼ 1.240  102 ΔH723,5 ¼ À 51.0113 ΔS723,5 ¼ À0.0332 ΔG723,5 ¼ À27.0002 Keq723,5 ¼ 8.928  101 ΔH ΔS ΔG Keq750,5 ¼ 7.628  101 750,5 ¼ À 49.9518 Eq (6.19) ¼ À0.0341 673,5 ¼ À 51.5957 ΔS 673,5 750,5 ¼ 0.0328 750,5 ¼ À27.0268 particularly favored when working in excess water (Basile et al., 2008a,b), and this was required from a technological point of view to reduce CO as an intermediate product, thus reducing the subsequent formation of coke, through the Boudouard reaction (Eq 6.20), and enhancing the production of hydrogen: 2CO ! CO2 + C (6.20) Also, in this work, it was possible to compare the computed data for the WGSR (Eq 6.19) and the experimental one An excellent agreement at standard conditions (ΔH298,1(calcd) ¼ À 41.1640 kJ/mol; ΔH298,1(exp) ¼ À 41.2 kJ/mol (Vaidya and Rodrigues, 2006)) was found The Boudouard reaction (Eq 6.20) is one of the most undesired reactions, because of the production of carbon solid particles, which can also be formed in other ways, that is, reaction (6.15) In the present study, it was not possible to compute the thermodynamic parameters for Eq (6.20), but the literature reports that it is an exothermic reaction, ΔH298,1(exp) ¼ À 171.5 kJ/mol (Vaidya and Rodrigues, 2006; Aupretre et al., 2005) 6.3.5 Subreaction group D: Acetone reactions The values for ΔHreaction, ΔSreaction, ΔGreaction, and Keq as computed for selected subreaction from group D and are reported in Table 6.7 The reaction (6.21) is relevant to the steam reforming process on acetone (CH3COCH3) to form CO2 and H2: CH3 COCH3 + 5H2 O ! 3CO2 + 8H2 (6.21) This was an endothermic process and had ΔHreaction values slightly larger (i.e., ΔH673,5 ¼ 172.5714 kJ/mol) than those obtained for ESR (reaction 6.9) The reaction was highly spontaneous even at T > 673 K (ΔG673,5 ¼ À 161.4875 kJ/mol), therefore the reaction was in competition with ESR (Eq 6.9) This latter conclusion was in agreement with an ideal ESR full process requiring the consumption of the acetone, formed from the reaction (6.12), that on reacting with H2O, significantly increased the final amount of H2 Thermodynamic analysis of ethanol reforming for hydrogen production 207 Selected calculated parameters (ΔHreaction, kJ/mol; ΔSreaction, kJ/mol K; ΔGreaction, kJ/mol; and Keq) at selected temperature and pressure values (T 673, 723, and 750 K; P 5 atm) for the subreactions (6.21) and (6.22) Table 6.7 CH3COCH3 + 5H2O ! 3CO2 + 8H2 Eq (6.21) ΔH ¼ 0.4964 ΔG673,5 ¼ À 161.4875 Keq673,5 ¼ 3.422  1012 ΔH723,5 ¼ 161.4152 ΔS723,5 ¼ 0.4971 ΔG723,5 ¼ À 197.9878 Keq723,5 ¼ 2.016  1014 ΔH ΔS ΔG Keq825,5 ¼ 1.637  1017 673,5 825,5 ¼ 172.5714 ¼ 137.3812 ΔS 673,5 825,5 ¼ 0.4961 825,5 ¼ À 271.8720 CH3COCH3 + 2H2O ! 3CO + 5H2 Eq (6.22) ΔH 673,5 ¼ 332.4216 ΔH 723,5 ¼ 314.4490 ΔH750,5 ¼ 309.9998 ΔS 673,5 ¼ 0.5988 ΔG673,5 ¼ À 80.5692 Keq673,5 ¼ 1.793  106 ΔS 723,5 ¼ 0.5967 ΔG ¼ À 116.9871 Keq723,5 ¼ 2.833  108 ΔG750,5 ¼ À 136.5477 Keq750,5 ¼ 3.239  109 ΔS750,5 ¼ 0.5954 723,5 Another possible steam reforming reaction on acetone (Eq 6.22) brings about the formation of CO, instead of CO2 CH3 COCH3 + 2H2 O ! 3CO + 5H2 (6.22) The reaction was very endothermic, and it had ΔHreaction values much higher than those for ESR (reaction 6.9) and for Eq (6.21) (Eq 6.22, ΔH673,5 ¼ 332.4216 kJ/mol) However, the reaction was spontaneous at 673 K (ΔG673,5 ¼ À 80.5692 kJ/mol), but at a lesser extent than ESR (Table 6.3) and Eq (6.21) (see just above) Acetone consumption toward the formation of H2 was favored, but that the reaction that brings about CO2 was much more favored with respect to that produces carbon monoxide 6.3.6 Subreaction group E: Other reactions on C2H5OH The values for ΔH, ΔS, ΔG, and Keq are reported in Table 6.8 Reaction (6.23) represents the decomposition of ethanol to produce methane and was an endothermic process, whose ΔHreaction values (ΔH673,5 ¼ 39.1766 and ΔH723,5 ¼ 37.2205 kJ/mol) were smaller than those computed for ESR (reaction 6.9): C2 H5 OH ! CH4 + CO + H2 (6.23) Furthermore, the reaction was spontaneous at 673 K (ΔG673,5 ¼ À 116.6486 kJ/mol), and therefore, it could compete with reaction (6.9) That reaction can be considered as a subprocess that allowed the formation of H2 through the consumption of ethanol and brings about two undesired species, methane and carbon monoxide 208 Bioenergy Systems for the Future Selected calculated parameters (ΔHreaction, kJ/mol; ΔSreaction, kJ/mol K; ΔGreaction, kJ/mol; and Keq) at selected temperature and pressure values (T 673, 723, and 750 K; P 5 atm) for the subreactions (6.23)–(6.26) Table 6.8 C2H5OH ! CH4 + CO + H2 ΔH ΔG673,5 ¼ À 116.6486 Keq673,5 ¼ 1.132  109 ΔH723,5 ¼ 37.2205 ΔS723,5 ¼ 0.2303 ΔG723,5 ¼ À 129.3211 Keq723,5 ¼ 2.205  109 ΔH ΔS ΔG Keq750,5 ¼ 3.028  109 750,5 ¼ 39.1766 ¼ 36.0741 ΔS Eq (6.23) ¼ 0.2315 673,5 673,5 750,5 ¼ À 0.2296 750,5 ¼ À 136.1283 2C2H5OH ! CH3COCH3 + CO + 3H2 Eq (6.24) ΔH 673,5 ¼ 129.5501 ΔS ¼ 0.3329 ΔG673,5 ¼ À 94.5084 Keq673,5 ¼ 2.165  107 ΔH 723,5 ¼ 126.2264 ΔS ¼ 0.3314 ΔG ¼ À 113.4470 Keq723,5 ¼ 1.572  108 ΔG750,5 ¼ À 123.6003 Keq750,5 ¼ 4.061  108 ΔH750,5 ¼ 124.2295 673,5 723,5 ΔS750,5 ¼ 0.3304 C2H5OH ! C2H4 + H2O ΔH 673,5 ¼ 51.4130 723,5 Eq (6.25) ΔS 673,5 ¼ 0.1223 ΔG673,5 ¼ À 30.8720 C2H5OH + 2H2 ! 2CH4 + H2O Eq (6.26) Keq673,5 ¼ 2.490  102 ΔH 673,5 ¼ À 147.6351 ΔS ¼ À 0.0028 ΔG673,5 ¼ À 145.7580 Keq673,5 ¼ 2.058  1011 ΔH 723,5 ¼ À 145.9443 ΔS ¼ À 0.0035 ΔG Keq723,5 ¼ 2.304  1010 ΔH750,5 ¼ À 137.0719 673,5 723,5 ΔS750,5 ¼ À 0.0037 723,5 ¼ À 143.4250 ΔG750,5 ¼ À 142.1830 Keq750,5 ¼ 7.995  109 Comparative thermodynamic analyses of the steam reforming and of the decomposition of ethanol Eq (6.23) showed that at 500 K, the steam reforming of ethanol hardly occurs because of ΔG° > 0, whereas the decomposition of ethanol should occur at the same temperature, because ΔG° was sufficiently negative (Haga et al., 1997) Reaction (6.24) consisted of the oxidation of ethanol and production of acetone, carbon monoxide, and hydrogen: 2C2 H5 OH ! CH3 COCH3 + CO + 3H2 (6.24) The process was endothermic, and ΔHreaction values were comparable with those for ESR (Eq 6.9) (Eq 6.24, ΔH673,5 ¼ 129.5501 kJ/mol) The reaction was spontaneous at 673 K (ΔG673,5 ¼ À 94.5084 kJ/mol) and therefore was competitive to reaction (6.9) It produces mol H2 from mol of ethanol Reaction (6.25) consisted of the dehydration of ethanol and production of ethane: C2 H5 OH ! C2 H4 + H2 O (6.25) Thermodynamic analysis of ethanol reforming for hydrogen production 209 It was endothermic by ΔHreaction values lower than those for reaction (6.9) and it was spontaneous at 673 K and thus in competition with ESR (Eq 6.25, ΔH673,5 ¼ 51.4130 kJ/mol and ΔG673,5 ¼ À 30.8720 kJ/mol) The reaction could be important because all processes that bring about ethene finally produce coke powder This was dangerous for the overall ESR process because deposition of coke on catalysts causes the inactivation Finally, reaction (6.26) is the hydrogenation of ethanol to methane: C2 H5 OH + 2H2 ! 2CH4 + H2 O (6.26) It was strongly exothermic and spontaneous even at 673 K (ΔH673,5 ¼ À 147.6351 kJ/mol and ΔG673,5 ¼ À 145.7580 kJ/mol) and favored with respect to reaction (6.9) This reaction brought about the consumption of H2 and therefore would need to be inhibited Furthermore, all subreactions from group E were characterized by high Keq values (Table 6.8); the yield for their subproducts was high and undesired for the ideal ESR process 6.3.7 Ethanol autothermal steam reforming At this stage, the reactions of autothermal steam reforming of ethanol in the presence of oxygen, reaction (6.27), and of ethanol oxidation through oxygen, reaction (6.28), were evaluated for the possibility of an autosustainable reforming process: C2 H5 OH + 2H2 O + ½O2 ! 2CO2 + 5H2 (6.27) C2 H5 OH + ½O2 ! 2CO + 3H2 (6.28) The values for ΔHreaction, ΔGreaction, Keq, and pKeq as a function of T and P for reaction (6.27) were reported in Figs 6.8–6.10, respectively The computed data for ΔH, ΔS e, ΔG, and Keq for selected values of T and P were reported in Table 6.9 The reaction was strongly exothermic and spontaneous at 673 K, bringing about H2, as reaction (6.9) Being spontaneous (ΔG673,5 ¼ À 337.0630 kJ/mol) and largely exothermic (ΔH673,5 ¼ À 103.7842 kJ/mol), it was possible, by properly modulating the reaction conditions and the initial mixture of fluids, to obtain a process that was thermally autosustainable On the contrary, the reaction of ethanol oxidation through O2 (Eq 6.28) was slightly exothermic, even though spontaneous at 673 K (ΔH673,5 ¼ À 3.8807 kJ/mol and ΔG673,5 ¼ À 283.1180 kJ/mol) Reaction (6.28) occurs in the absence of water, otherwise reaction (6.27) takes place The theoretically computed value for reaction (6.27) regarding ΔH298,1(calcd) ¼ À 68.3630 kJ/mol, compared well with the experimental value ΔH298,1 (exp) ¼ À 50.16 kJ/mol (Ni et al., 2007) –80 –90 –100 –120 –130 DH (kJ/mol) –110 –140 600 700 T (K) 800 atm P( 500 ) 10 900 1000 Fig 6.8 ΔHreaction (kJ/mol) as a function of temperature (K) and pressure (atm) conditions for the autothermal ESR, reaction (6.27) –200 –250 –350 –400 P (atm ) –450 DG (kJ/mol) –300 500 600 700 T (K) 10 800 900 1000 Fig 6.9 ΔGreaction (kJ/mol) as a function of temperature (K) and pressure (atm) conditions for the autothermal ESR, reaction (6.27) Thermodynamic analysis of ethanol reforming for hydrogen production 211 Keq 1,00E+027 8,00E+026 6,00E+026 4,00E+026 2,00E+026 0,00E+000 500 600 700 T (K) 800 900 (A) pKeq P (atm) 10 –25,5 –26,0 –26,5 –27,0 (B) P (atm) 700 10 600 800 900 T (K) 500 Fig 6.10 Keq (A) and pKeq (B) as a function of temperature (K) and pressure (atm) conditions for the autothermal ESR, reaction (6.27) 212 Bioenergy Systems for the Future Selected calculated parameters (ΔHreaction, kJ/mol; ΔSreaction, kJ/mol K; ΔGreaction, kJ/mol; and Keq) at selected temperature and pressure values (T 673, 723, and 973 K; P 5 atm) for the autothermal ethanol steam reforming reactions (6.27) and (6.28) Table 6.9 C2H5OH + 2H2O + ½O2 ! 2CO2 + 5H2 Eq (6.27) ΔH ¼ À 103.7842 ΔH723,5 ¼ À 109.9229 ΔH973,5 ¼ À 144.3383 ΔG673,5 ¼ À337.0630 ΔG723,5 ¼ À360.2760 ΔG973,5 ¼ À473.7670 673,5 ΔS ¼ 0.3466 ΔS723,5 ¼ 0.3463 ΔS973,5 ¼ 0.3386 673,5 C2H5OH + ẵO2 ! 2CO + 3H2 Eq (6.28) H ẳ À 3.8807 ΔH723,5 ¼ À 7.9004 ΔH750,5 ¼ À 10.1704 ΔG673,5 ¼ À283.1180 ΔG723,5 ¼ À306.2760 ΔG750,5 ¼ À318.6930 673,5 6.4 ΔS ¼ 0.4149 ΔS723,5 ¼ 0.4127 ΔS750,5 ¼ 0.4114 673,5 Keq673,5 ¼ 1.452  1026 Keq723,5 ¼ 1.071  1026 Keq973,5 ¼ 2.721  1025 Keq673,5 ¼ 9.438  1021 Keq723,5 ¼ 1.344  1022 Keq750,5 ¼ 1.572  1022 Conclusion In conclusion, the thermodynamic investigation showed a series of reactions and subreactions that occur during the ESR process The theoretical estimation of ΔH, ΔS, ΔG, and Keq, as a function of T and P allowed us to evaluate the favorite and competitive reactions with respect to the ideal ESR reaction (6.9) The full analysis allowed for the identification of the optimal temperature and pressure to reduce the formation of undesired species, such as CO and C2H4 Finally, the possibility of an autothermal process conducted in the monitored presence of oxygen, reaction (6.27), and aimed at improving the sustainability of the ESR overall process, was taken into account This latter investigation, even though at a preliminary stage, showed that the strategy is promising both from the enthalpic standpoint 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C2 H4 + H2 O (6. 25) Thermodynamic analysis of ethanol reforming for hydrogen production. .. (Table 6. 8); the yield for their subproducts was high and undesired for the ideal ESR process 6. 3.7 Ethanol autothermal steam reforming At this stage, the reactions of autothermal steam reforming of. .. function of temperature (K) and pressure (atm) conditions for the autothermal ESR, reaction (6. 27) Thermodynamic analysis of ethanol reforming for hydrogen production 211 Keq 1,00E+027 8,00E+0 26 6,00E+026