Abstract
The prospect of ethanol dry reforming process to utilize CO2 for conversion to hydrogen, syngas, and carbon nanofilaments using abundantly available biofuel—ethanol, and widely available environmental pollutant CO2 is very enthusiastic. A thermodynamic analysis of ethanol CO2 reforming process is done using Gibbs free energy minimization methodology within the temperature range 300–900°C, 1–10 bar pressure, and CO2 to carbon (in ethanol) ratio (CCER) 1–5. The effect of individual as well as combined effect of process parameters such as temperature, pressure, and CCER was determined on the product distribution. Optimum process conditions for maximising desired products and minimizing undesired products for applications such as gas to liquids (GTL) via fischer tropsch synthesis, syngas generation for Solid oxide fuel cells, and carbon nanofilament manufacture were found in this study.
1. Introduction
CO2 reforming (also known as dry reforming) is a useful way to utilize CO2 to transform it into valuable species such as hydrogen, syngas, and carbon (nanofilaments). CO2 reforming is analogous to steam reforming which has been widely used to produce hydrogen for different applications. Although dry reforming (DR) is a known process in the chemical literature, catalyst deactivation due to carbon formation was its major drawback. However, with increase in CO2 pollution awareness, researchers have started fresh studies in dry reforming to utilize (and thus sequester) CO2. This move has come with a bonus: carbon nanofilament formation was reported in some experimental studies of dry reforming. Dry reforming of natural gas was a major research area. Many research studies on dry reforming of methane have been reported [1–6]. Dry reforming of butanol [7], glycerol [8], and coke oven gas [9] has also been studied by some researchers. The popularity of biofuels and use of biomass has brought an alternative to natural gas. Ethanol is cheaply available in many countries. It is easily manufactured by biomass fermentation can be stored and transported safely. Hence, ethanol is a potential fuel that can be easily used in dry reforming processes. Dry reforming of ethanol has been studied by some researchers: De Oliveira-Vigier et al. [10] have experimentally studied the dry reforming of ethanol using a recyclable and long-lasting SS 316 catalyst and have obtained a hydrogen yield that is 98% of the theoretical value. Blanchard et al. [11] have experimentally studied the ethanol dry reforming using a carbon steel catalyst to produce syngas and nanocarbons. Bellido et al. [12] have experimentally studied the dry reforming of ethanol using Ni/Y2O3-ZrO2 catalysts and achieved a maximum CO2 conversion of 61% at 800°C.
Ethanol dry reforming primarily results in the formation of species such as H2, CO, CH4, H2O, and C. Formation of any other by-products in significant quantities has not been reported in the literature. These major value-added products have different applications. Hydrogen is used as reactant in many reducing reactions, hydrogenation reactions, and refinery processes such as hydrocracking and platforming. and also as fuel in fuel cells. CO is also a powerful reducing agent and a fuel, but it is usually used with hydrogen as syngas (H2 + CO). Syngas is the basic building block of petrochemical industry. Many speciality chemicals are manufactured using fischer tropsch (FT) synthesis from syngas (of ratio 1–3). Carbon (as nanofilament) formed in dry reforming is nowadays a highly precious commodity.
Thermodynamic studies are vital steps for process development. Many thermodynamic studies have been reported for various processes, for example, steam reforming of ethanol [13–16], sorption enhanced steam reforming of butanol [17] and propane [18], oxidative steam reforming of propane [19], glycerol steam reforming with in-situ hydrogen separation [20], steam reforming of dimethyl ether [21], dry autothermal reforming of glycerol [22], and so forth. Some thermodynamic studies of ethanol dry reforming have also been reported: Jankhah et al. [23] have presented a thermodynamic equilibrium analysis and experimental data on thermal and catalytic ethanol cracking and dry reforming reactions at various CO2/ethanol ratios using carbon steel catalyst and reported that highest hydrogen and carbon (nanofilament) yields were obtained at 550°C. W. Wang and Y. Wang [24] have studied the thermodynamics of ethanol reforming with carbon dioxide for hydrogen production and have reported that optimum conditions gave over 94% yield of syngas with complete conversion of ethanol without carbon deposition.
The reaction for DR of ethanol is given as
However, minor amounts of by-products such as CH4 (g), H2O (g), and C (solid) are formed in the process. The product formation is governed by a combination of process parameters such as temperature, pressure, and feed CO2 to carbon (in ethanol) ratio (CCER). A change in any one or more than one process parameter results in a change in the quantity of product formation. Thermodynamic studies reported so far on dry reforming of ethanol have generally considered single parameter variation, for example, study of hydrogen generation at constant pressure and constant ethanol to CO2 ratio with variation in temperature. Similarly, these studies have been limited to hydrogen or syngas generation only without any comprehensive study on optimum thermodynamic conditions to maximize the desired combined products and minimize the undesired products. Hence, this comprehensive theoretical thermodynamic study was initiated to study the product distribution with combined effect of process parameters and find the optimum conditions to maximise the desired products for certain important industrial applications. Such studies are very important to start experimental programs for catalyst and process development.
2. Methodology
HSC Chemistry version 5.1 [25] has been used for this thermodynamic equilibrium study. It uses the Gibbs free minimization algorithm to find the equilibrium compositions using species and not chemical reaction equations. This software is extremely user-friendly. Equilibrium calculations can be made using Gibbs energy minimization method, simultaneous solution of nonlinear reaction equations using MATLAB programs, and equilibrium reactor modules of commercial software like Design II, HYSYS, Aspen Plus, or fluent. Sometimes, simultaneous solution of nonlinear reaction equations based on equilibrium constants might become nonsolvable. An alternate procedure that takes account of chemical species only (not chemical reactions) is based on minimization of the total Gibbs energy Gt shown by the expression
The Gibbs program finds the most stable phase combination and seeks the phase composition where the Gibbs energy of the system reaches its minimum at a fixed mass balance, constant pressure and temperature. It shows that all irreversible processes occurring at constant (temperature) and (pressure) proceed in a direction to the equilibrium state which has the lowest total Gibbs energy attainable. The method is based on the set of species and is better than that of independent reactions among the species as the number and nature of equations are not always known perfectly. The software gives the individual product moles, along with overall reaction enthalpy at the temperature and pressure condition.
The input species fed to the software were ethanol (both gaseous and liquid state) and CO2 (g). The output species fed to the software are H2, CO, CO2, CH4 (all in gaseous state), H2O (both gas and liquid state), and C (solid), which are common reaction products in dry reforming processes. Some side products such as formaldehyde and methanol. were also considered in the preliminary analysis but were later ignored as their concentrations in the product gas were found to be negligible. The software results gave the individual product moles at the desired input condition. One mole of ethanol was used at all conditions for this study. The thermodynamic analysis was carried out in the temperature range 300–900°C at 1, 5, and 10 bar reaction pressure with CCER ranging from 1 to 5. These conditions were carefully chosen to represent a realistic view of ethanol dry reforming process. The individual product distribution data thus generated was analyzed and discussed in the proceeding section.
3. Results and Discussion
3.1. H2 Yield
Hydrogen is one of the most desired products of reforming processes. Figure 1 shows the variation in hydrogen yield with change in pressure, CCER, and temperature conditions. It was observed that the hydrogen yield initially increased with increase in temperature from 300°C, reached a maximum value, and then slightly decreased at higher temperatures at constant CCER and pressure; this can be explained by the fact that Kp of (1) increases with increase in temperature which is already studied [24]. For example, the hydrogen yield increased from 0.09 to 2.54 moles up to 850°C and then slightly decreased to 2.51 moles at 900°C (CCER = 1) at 1 bar pressure. It was also observed that the hydrogen yield decreased with increase in pressure at constant CCER and temperature; for example, the hydrogen yield decreased from 2.24 to 1.12 moles (CCER = 1) at 700°C for increase in pressure from 1 to 10 bar. It was also observed that the hydrogen yield decreased with increase in CCER at constant pressure and temperature; for example, the hydrogen yield obtained at constant pressure and 700°C decreased from 2.24 to 1.46 moles (1 bar) for increase in CCER from 1 to 5. It was also seen that the hydrogen yield decreased with simultaneous increase in CCER and pressure, at constant temperature; that is, the hydrogen yield decreased from 2.54 (CCER = 1, = 1 bar) to 1.21 moles (CCER = 5, = 10 bar) at 800°C. However, it was observed that the hydrogen yield increased at lower temperatures (up to ~700°C) and decreased at higher temperatures for simultaneous increase in CCER and decrease in pressure at constant temperature; that is, the hydrogen yield increased from 1.12 to 1.46 moles at 700°C but decreased from 2.01 to 1.14 moles at 850°C for increase in CCER from 1 to 5 with decrease in pressure from 10 to 1 bar. Vice versa, it was also seen that the hydrogen yield initially decreased and then increased at high temperature, when process CCER was decreased and pressure was increased simultaneously at constant temperature, that is, the H2 yield decreased from 1.46 to 1.12 moles at 700°C but increased from 1.23 to 1.72 moles at 800°C for decrease in CCER from 1 to 5 and increase in pressure from 1 to 10 bar. It was seen that, with simultaneous increase in temperature and CCER at constant pressure, the hydrogen yield increased initially at lower temperatures but decreased at higher temperatures above ~700°C. It was also seen that the H2 yield increased at constant pressure with simultaneous increase in temperature and decrease in CCER, while it decreased with simultaneous increase in CCER, and decrease in temperature at constant pressure. It was also seen that the hydrogen yield showed a mixed trend with simultaneous increase in temperature and pressure at constant CCER. It was observed that the hydrogen yield increased with simultaneous increase in temperature and decrease in pressure at constant CCER and vice versa. It was also seen that the hydrogen yield decreased with simultaneous increase in pressure, CCER, and temperature. The maximum hydrogen yield for every CCER was obtained at 1 bar pressure and higher temperatures. Considering all the data points, the maximum H2 yield was found to be 2.54 moles (CCER = 1, = 1 bar) at 800°C and 850°C, while the minimum H2 yield was found to be 0.02 moles for CCER = 3,4,5 at pressure 10 bar at 300°C. Thus, maximum hydrogen yield was obtained at lower CCER, lower pressure, and higher temperatures.
3.2. CO Yield
Carbon monoxide is a desired component of syngas for GTL (gas-to-liquids) manufacture as well as for use in solid oxide and molten carbonate fuel cells. Figure 2 shows the variation in CO yield with change in process pressure, CCER, and temperature. It was seen that the CO yield increased with increase in temperature from 300 to 900°C, at constant CCER and pressure, as the boudouard reaction plays more important role in CO production than WGS because H2O is present in reaction at smaller quantity than CO2; for example, the CO yield increased from 0.0 to 3.48 moles (CCER = 1) at 1 bar with increase in temperature from 300–900°C. It was also observed that the CO yield decreased slowly with increase in pressure at constant CCER and temperature; for example, the CO yield decreased from 3.39 to 2.10 moles at CCER = 1 at 800°C for increase in pressure from 1 to 10 bar. It was also seen that the CO yield increased with increase in CCER from 1 to 5 at constant pressure and temperature; for example, the CO yield increased from 3.39 to 4.77 moles (1 bar) at 800°C for increase in CCER from 1 to 5. The CO yield showed an increase with a simultaneous increase in CCER and pressure at constant temperature; that is, the CO yield increased from 3.39 to 4.73 moles at 800°C for simultaneous increase in CCER from 1 to 5 and pressure from 1 to 10 bar. The CO yield also increased with simultaneous increase in CCER and decrease in pressure at constant temperature; that is, it increased from 2.10 to 4.77 moles at 800°C for increase in CCER from 1 to 5 and decrease in pressure from 10 to 1 bar. An exact reverse trend was observed for vice versa parameter variations. The CO yield increased with simultaneous increase in temperature and CCER, at constant pressure, while it also increased with simultaneous increase in temperature and decrease in CCER at constant pressure. But the CO yield decreased at lower temperatures and increased at high temperatures with simultaneous increase in CCER and decrease in temperature at constant pressure. The CO yield showed a mixed trend with simultaneous increase in temperature and pressure at constant CCER. Similarly, the CO yield increased with simultaneous increase in temperature and decrease in pressure at constant CCER and vice versa. The maximum CO yield for every CCER was obtained at 1 bar pressure and higher temperatures, while the minimum CO yield was obtained at 10 bar pressure and lower temperatures. The maximum CO yield obtained was 4.95 moles at CCER = 5 for pressure 1, 5, 10 bar at 900°C, while the minimum CO yield was found to be 0.00 for all CCER and pressures at 300°C. The maximum CO yield was obtainable at higher CCER, lower pressure, and higher temperatures.
3.3. CH4 Formation
Methane is an inevitable by-product of reforming processes. Figure 3 shows the effect of change in pressure, CCER, and temperatures on CH4 formation in the dry reforming of ethanol. It was seen that, at constant CCER and pressure with increase in temperature from 300 to 900°C, the CH4 formation slightly increased at lower temperature and then decreased to zero at higher temperature as the rate of H2O and CO2 reforming of methane becomes significant at higher temperature; that is, the CH4 yield increased from 0.38 to 0.40 moles up to 400°C and then decreased to 0.00 at 900°C for CCER = 1 at 1 bar. It was also observed that the CH4 yield decreased with increase in CCER from 1 to 5 at constant pressure and temperature; that is, the methane yield decreased from 0.19 to 0.00 moles at 10 bar pressure and 850°C. It was also seen that the CH4 yield increased with increased in pressure from 1 to 10 bar, at constant CCER and temperature; that is, it increased from 0.02 to 0.13 moles at CCER = 5 and temperature 650°C. Similarly, the methane yield decreased at constant temperature with simultaneous increase in CCER and pressure, and similar trend was observed when CCER was increased and pressure was decreased at constant temperature and vice versa. The methane yield decreased with simultaneous increase in temperature and CCER at constant pressure, but it increased at constant pressure with simultaneous increase in temperature and decrease in CCER and vice versa. The methane yield also increased with simultaneous increase in temperature and pressure at constant CCER, but it decreased with simultaneous increase in temperature and decrease in pressure at constant CCER and vice versa. The maximum methane yield for every CCER was obtained at 10 bar pressure and lower temperatures, while the minimum methane yield was obtained at 1 bar pressure and higher temperatures. The maximum methane yield was found to be 0.46 moles (CCER = 1, = 10 bar) at 450°C and 500°C, while the minimum CH4 yield was observed to be 0.00 moles for all CCER and pressures at higher temperatures in almost all cases. Thus, it can be seen that the undesirable methane formation can be minimized by operating the process at lower pressure, higher CCER, and higher temperatures.
3.4. Water Formation
Water formation is generally undesirable as it reduces the hydrogen output of the process. But water formation takes place in almost all reforming processes. Although water is also a by-product similar to methane, its formation was much higher compared to methane formation in this process. Figure 4 depicts the variation in H2O formation at different pressure, CCER, and temperature conditions. As seen from the figure, it was observed that the moles of H2O produced decreased up to certain temperature and then increased at higher temperatures with increase in temperature from 300°C to 900°C, at constant CCER and pressure; that is, the moles of water formed decreased from 2.65 to 1.22 moles till 750°C and then increased to 1.51 moles (900°C) for CCER = 3 at 5 bar pressure. The water formation also increased with increase in CCER at constant pressure and temperature; that is, the water formation increased from 0.81 to 1.65 moles for 5 bar pressure at 750°C with increase in CCER from 1 to 5. It was also seen that the water formation generally increased for all CCERs with increase in pressure from 1 to 10 bar at constant CCER and temperature except at 750°C, where the moles of water increased from 0.39 to 1.02 moles at CCER = 1 and also increased from 1.23 to 1.30 moles at CCER = 3 but decreased slightly from 1.66 to 1.63 moles at CCER = 5. It was also observed that the H2O yield increased with simultaneous increase in both CCER and pressure at constant temperature while it also increased with simultaneous increase in CCER and decrease in pressure at constant temperature and vice versa. But the H2O yield showed mixed trend (sometimes it increased/decreased) with simultaneous increase in temperature and CCER at constant pressure. The water formation decreased at constant pressure with simultaneous increase in temperature and decrease in CCER and vice versa. It was also seen that the H2O yield showed a mixed trend with simultaneous increase in temperature and pressure at constant CCER and the same trend was observed with simultaneous increase in temperature and decrease in pressure at constant CCER. The H2O yield showed a mixed trend with simultaneous increase in pressure and decrease in temperature at constant CCER. It was also seen that the H2O yield increased with simultaneous increase in pressure, CCER, and temperature. The data analysis confirmed that higher H2O yield was obtained at 10 bar pressure and lower temperatures, while lower H2O yield was obtained at 1 bar pressure and temperature range 700–750°C for all CCERs considered in this study. The maximum H2O yield obtained was found to be 2.78 moles at 300°C, CCER = 5, and 10 bar pressure, while the minimum H2O yield was found to be 0.39 moles at 750°C, CCER = 1, and 1 bar pressure. Since H2O formation is undesirable, minimum H2O formation can be obtained by operating the process at lower pressure and lower CCER between 700 and 750°C.
3.5. Carbon Formation
Generally, carbon (coke) is an undesired component of reforming processes as it deactivates the catalyst and increases pressure drop in reactors. However, carbon (in the form of carbon nanofilaments) is a highly valuable product obtained by some researchers in ethanol dry reforming experiments. Carbon formation may occur due to the boudouard reaction, methane decomposition, and reduction of carbon oxides. Figure 5 shows the trend of carbon formation at various pressures, CCER, and temperatures. It was seen from the figure that the carbon formation gradually decreased to 0.00 moles with increase in process temperature from 300 to 900°C at constant CCER and pressure; that is, the moles of carbon formed decreased from 2.79 to 0.00 moles at CCER = 5 and 10 bar pressure. The carbon formation also decreased with increase in CCER from 1 to 5 at higher temperatures at constant pressure and temperature; that is, the carbon formation decreased from 1.57 to 1.36 moles at 10 bar and 650°C. A reverse trend was observed at lower temperatures with increase in CCER from 1 to 5 at constant pressure. However, the carbon formation increased with increase in pressure from 1 to 10 bar at constant CCER and temperature; that is, the carbon formation increased from 0.05 to 1.59 moles at CCER = 3 and at 650°C with increase in pressure from 1 to 10 bar. It was also observed that the carbon formation increased with simultaneous increase in CCER and pressure at constant temperature, but it showed a mixed trend when CCER was increased with a simultaneous decrease in pressure at constant temperature and also with simultaneous decrease in CCER and increase in pressure at constant temperature. It was also observed that the carbon formation increased up to CCER = 3 and then slightly decreased at CCER = 5 (at lower temperatures), but it decreased to zero at higher temperatures with simultaneous increase in temperature and CCER at constant pressure. The carbon formation also decreased with simultaneous increase in temperature and decrease in CCER at constant pressure and vice versa. The carbon formation showed a mixed trend for simultaneous increase in temperature and pressure at constant CCER while the carbon formation decreased with simultaneous increase in temperature and decrease in pressure at constant CCER and vice versa. Higher value of carbon formation was observed at 10 bar pressure and lower temperatures, while low carbon formation was observed at all pressures above ~700°C for almost all cases of CCERs considered in this study. The maximum carbon formation was found to be 2.79 moles at CCER = 5 and 5 and 10 bar pressures at 300°C, while zero carbon formation was seen for all CCERs and pressures at higher temperatures. Thus, carbon was significantly formed at higher CCER and lower temperatures at all pressures, while low carbon formation was seen at higher temperatures.
3.5.1. Carbon Nanofilaments
The conditions to obtain maximum carbon from the process are already discussed in the earlier section. The exact nature of carbon (nanofilaments, etc.) can be ascertained only after analysis of carbon obtained in experimental studies and has been confirmed by some researchers. Considering all the data points, it was observed that the maximum carbon formation occurred at lower temperatures (~300°C) at high pressures and CCER conditions considered in this study. It was also observed that the carbon formation increased with increase in process CCER from 1 to 5 at constant pressure; that is, the carbon formation increased from 2.20 to 2.77 moles at 1 bar pressure, while it increased from 2.20 to 2.79 moles for 5 and 10 bar pressures. It was also seen that the carbon formation remained almost constant at constant CCER with increase in process pressure from 1 to 10 bar; that is, the carbon formation was 2.2 moles for CCER = 1 at all pressures, while at CCER = 5, the carbon formation was found to be 2.77 moles at 1 bar and 2.79 moles at 5 and 10 bar. Thus, the carbon formation was more dependent on process CCER than pressure at lower process temperatures. The maximum carbon formation was noted to be 2.79 moles for CCER = 5, pressure 5 and 10 bar at 300°C. Hence, it was desirable to operate the process at lower temperatures to obtain maximum carbon formation.
3.6. CO2 Conversion
CO2 utilization by conversion to value-added product such as syngas and carbon (nanofilaments) is an important feature of dry reforming processes. Hence, CO2 conversion is a vital component of this study. Figure 6 shows the change in CO2 conversion at different pressures, CCER, and temperatures. It was observed that, with increase in temperature from 300 to 900°C at constant CCER and pressure, the CO2 conversion slightly decreased and then increased at higher temperatures; it can be explained by the fact that the CO2 reforming of methane is more at higher temperature and minimum CO2 conversion was obtained at 550°C; that is, the CO2 conversion decreased from 8.71 to 7.72% (450°C) and then increased to 29.49% at 900°C (CCER = 5) at 1 bar pressure. It was seen that the CO2 conversion increased at lower temperatures but decreased at slightly higher temperatures with increase in pressure at constant CCER and temperature; that is, the CO2 conversion decreased from 51.22 to 26.73% (CCER = 1) at 700°C with increase in pressure from 1 to 10 bar. The CO2 conversion also decreased with increase in CCER at constant pressure and temperature, that is, the CO2 conversion decreased from 31.23 to 24.28% (5 bar) for increase in CCER from 1 to 5 at 700°C at constant pressure. It was also seen that with simultaneous increase in CCER and pressure, the CO2 conversion decreased at constant temperature; that is, the CO2 conversion decreased from 51.22% (CCER = 1, = 1 bar) to 20.18% (CCER = 5, = 10 bar) at 700°C. However, the CO2 conversion decreased with simultaneous increase in CCER from 1 to 5 and decrease in pressure from 10 to 1 bar, at constant temperature, and vice versa. The CO2 conversion showed a mixed trend with simultaneous increase in temperature and CCER at constant pressure, but it increased with simultaneous increase in temperature and decrease in CCER at constant pressure and vice versa. Also, the CO2 conversion showed a mixed trend with simultaneous increase in temperature and pressure at constant CCER. It was also observed that the CO2 conversion decreased at lower temperatures but increased at higher temperatures above 600°C with simultaneous increase in temperature and decrease in pressure at constant CCER and showed a reverse trend for simultaneous decrease in temperature and increase in pressure at constant CCER. High CO2 conversion was seen at CCER = 1 for all pressures and higher temperatures, while low CO2 conversion was obtained at CCER = 5 for all pressures and lower temperatures. The maximum CO2 conversion was found to be 74.17% at CCER = 1, 1 bar pressure and 900°C, while the minimum CO2 conversion was observed to be 7.72% at CCER = 5, 1 bar pressure and 450°C. Thus, higher CO2 conversion in dry reforming of ethanol can be obtained by operating the process at lower CCER and higher temperatures for all pressures considered in this study.
3.7. Syngas (H2 + CO) Yield
Syngas is the most important product of this process. Figure 7 shows the effect of pressure, CCER, and temperature on syngas yield in the process. It was observed that the syngas yield generally increased with increase in process temperature from 300 to 900°C and reached its maximum value at constant CCER and pressure; that is, the syngas increased from 0.04 to 6.00 (CCER = 5) at 5 bar pressure with increase in temperature from 300 to 900°C. But the syngas yield decreased with increase in pressure at constant CCER and temperature; that is, the syngas yield decreased from 5.99 to 4.88 moles (CCER = 3) at 750°C with increase in pressure from 1 to 10 bar. It was also seen that the syngas yield increased with increase in CCER at constant pressure and temperature; that is, the syngas yield increased from 2.36 to 4.68 moles (1 bar) with increase in CCER from 1 to 5 at 600°C. Similarly, with simultaneous increase in CCER and pressure at constant temperature, the syngas yield decreased slightly at lower temperature till 700°C, but above 800°C—it first decreased and then increased slightly before reaching a saturation value. But the syngas yield increased with simultaneous increase in CCER and decrease in pressure at constant temperature and vice versa. The syngas yield also increased with simultaneous increase in both temperature and CCER at constant pressure. It also increased at constant pressure with simultaneous increase in temperature and decrease in CCER and vice versa. This shows that the syngas yield was controlled by process temperature at constant pressure but not much by CCER. It was also seen that the syngas yield first decreased and then increased with simultaneous increase in both temperature and pressure at constant CCER. The syngas yield increased with increase in temperature and decrease in pressure at constant CCER and vice versa. The higher syngas yield was obtained at 1 bar and higher temperatures, while the lower syngas yield was obtained at 10 bar pressure and lower temperatures for all CCERs considered in this study. The maximum syngas yield obtained in this thermodynamic analysis using 1 mole ethanol was found to be ~6 moles at CCER = 2, 3, 4, and 5, 1 bar pressure between 800 and 900°C, while the minimum syngas yield of 0.03 moles was obtained at all CCERs at 10 bar and 300°C. Thus, it can be concluded that higher syngas yield can be obtained for all CCERs at lower pressure and higher temperatures.
Syngas for Fuel Cells. The syngas produced in the dry reforming process can be used for use as fuel in fuel cells. The solid oxide fuel cells can easily operate with CO contaminated hydrogen gas. The product gas from the process can be passed through suitable gas-solid separators and directly fed to the SOFC. It is assumed by some researchers that the CO undergoes in-situ water gas shift reaction (reaction (3)) with water present in the product gas to produce hydrogen, which is used in the SOFC:
As seen from reaction (3), the product gas containing equimolar or higher H2O/CO ratio can be easily used as feed to the SOFC. However, low temperature PEM fuel cells generally cannot tolerate CO more than 1% in the product gas. Hence, the syngas needs to be processed through separate water gas shift reactors to reduce the CO content by converting it to hydrogen (reaction (3)) and further CO reduction is achieved by preferential oxidation (PrOx) reactors. But the WGS catalysts require H2O/CO ratio >4.5 to convert the CO to hydrogen. Figure 8 shows the regions of suitable H2O/CO ratio of the product syngas that can be used in SOFC and PEMFC. As seen from the figure, the syngas obtained in the temperature range 300–750°C at all pressures and CCERs and having H2O/CO ratio above 1 can be used in fuel cells. The syngas having H2O/CO ratio >4.5 can be used for both SOFC and PEMFC.
3.8. Syngas Ratio
Syngas ratio (H2/CO) is an important criterion for petrochemical manufacture by FT synthesis. Hence, a detailed analysis of syngas ratio of the product gas obtained in dry reforming of ethanol was also done in this thermodynamic study. Figure 9 shows the individual as well as combined effect of pressure, CCER, and temperature on the syngas ratio of the product gas obtained in the process. It was observed that the syngas ratio generally decreased with increase in temperature from 300 to 900°C at constant CCER and pressure; that is, the syngas ratio decreased from 68.75 to 0.68 (CCER = 1) at 10 bar with increase in temperature from 300 to 900°C. But it was also seen that the syngas ratio increased slightly with increase in pressure at constant CCER and temperature; that is, the syngas ratio increased from 5.32 to 5.73 (CCER = 3) at 400°C with increase in pressure from 1 to 10 bar. It was observed that the syngas ratio decreased with increase in process CCER at constant pressure and temperature; that is, the syngas ratio decreased from 15.66 to 3.42 (5 bar) with increase in CCER from 1 to 5 at constant pressure and 400°C. It was also observed that, with simultaneous increase in both CCER and pressure, the syngas ratio of the product gas decreased at constant temperature. It also showed a decrease with simultaneous increase in CCER and decrease in pressure at constant temperature and vice versa and also showed a decrease with simultaneous increase in temperature and CCER at constant pressure, while it showed a mixed trend with simultaneous decrease in temperature and increase in CCER at constant pressure. The syngas ratio decreased with simultaneous increase in temperature and pressure at constant CCER and also with simultaneous increase in temperature and decrease in pressure at constant CCER and vice versa. Thus, the syngas ratio was more influenced by process temperature than pressure at constant CCER. The product gas of higher syngas ratio (higher hydrogen content) was obtained for CCER = 1 at all pressures and lower temperatures, while the product gas of lower syngas ratio was obtained at all conditions at higher temperatures. The maximum syngas ratio of 68.75 was seen at CCER = 1 for 10 bar pressure at 300°C, while the minimum syngas ratio of 0.21 was observed for CCER = 5 at 1, 5, and 10 bar pressures at 900°C. It was seen that the product gas of lower syngas ratio (1–3) was obtainable at all conditions at high temperatures in the process.
Petrochemical Manufacture. The dry reforming process product gas does not contain any diluents like nitrogen. Hence, the syngas has a high concentration in the product gas. The syngas ratio in the range of 1–3 (desirable for use in petrochemical manufacture) is easily obtained in this process. This product gas can be processed through gas-solid separators to remove any catalyst/coke and can be compressed to a suitable pressure (if required) for use in petrochemical manufacture by FT synthesis. Figure 10 shows the process parameters, temperatures, pressures, and CCER, to obtain a product gas of exact syngas ratio 1, 2, and 3. It was observed that the product gas of exact syngas ratio 1 can be obtained between the temperature ranges 510°C to 680°C, 530°C to 735°C, and 535°C to 750°C for increase in CCER from 1 to 5 at 1, 5, and 10 bar pressure, respectively. Similarly, it was seen that the product gas of exact syngas ratio 2 can be obtained between the temperature ranges 445°C to 590°C, 450°C to 620°C, 455°C to 635°C at 1, 5, and 10 bar pressure, respectively, while the product gas of exact syngas ratio 3 was obtainable within the temperature range 405°C to 545°C, 415°C to 570°C, 410°C to 580°C for 1, 5, 10 bar pressure, respectively, for increase in CCER from 1 to 5. It was also observed that the temperature for exact syngas ratio = 1, 2, and 3 decreased with increase in CCER from 1 to 5; that is, the temperature for exact H2/CO = 1 decreased from 750°C to 535°C at 10 bar with increase in CCER from 1 to 5. It was also seen that the process temperatures to obtain syngas of exact syngas ratios (1, 2, and 3) increased with increase in pressure from 1 to 10 bar at constant CCER; that is, the temperature increased from 680°C to 750°C for syngas ratio = 1 at CCER = 1 with increase in pressure from 1 to 10 bar. The maximum temperature for all cases of CCER to obtain the desired syngas ratio (1, 2 or 3) was observed to be 750°C at 10 bar pressure for syngas ratio 1, while minimum temperature was found to be 405°C at 1 bar for exact syngas ratio 3.
3.9. Optimum Process Conditions
Considering all the data generated in this thermodynamic study, it was observed that H2, CO, carbon, and CO2 conversion are desired points for the process, while methane formation is undesirable for the process. Carbon, in the form of nanofilaments, is a highly desired product, while other forms of carbon are undesired products of the process. However, the form of carbon cannot be predicted by the results of this thermodynamic study and can only be ascertained only after analysis of experimentally obtained carbon and has already been reported by some researchers. The results were analyzed to find the optimum conditions to maximize the desired products and minimize the undesired products for the process and are shown in Table 1.
4. Conclusion
Thermodynamic analysis for dry reforming of ethanol via Gibbs free energy minimization method to evaluate the effect of reaction temperature, CO2/C in ethanol molar ratio and pressure has been studied for single parameter as well as multiparameter variation. The effect of these process parameters on the individual product distribution was studied and analysed in detail for both desired and undesired products obtained in the process. Some results obtained in this study were found to be similar to the earlier publications on this topic. Optimum thermodynamic conditions to maximise certain products (combined) were determined. Complete conversion of ethanol, that is, 100%, was observed for all cases considered in this study. A maximum of 6 mole syngas/mole ethanol can be obtained at some optimum conditions in the process. Lower pressure operation favoured higher hydrogen production, lower methane, and water formation, while higher pressure operation favoured higher CO2 conversion and sometimes higher carbon formation. Conditions for obtaining desired products for specific applications such as syngas of ratio 1–3, maximising carbon (nanofilament) formation, and syngas for use in fuel cells were identified. The results obtained in this comprehensive study can be used for experimental programs.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.