The nd Joint International Conference on Sustainable Energy and Environment (SEE 6) A- (O) - November 6, Bangkok, Thailand Thermodynamic Analysis of Hydrogen Production from Ethanol in Three Different Technologies Nawadee Srisiriwat, Apichai Therdthianwong and Supaporn Therdthianwong,* Department of Chemical Engineering, King Mongkut s University of Technology Thonburi, Bangkok, Thailand Constructionism-Chemical Engineering Practice School (C-ChEPS), King Mongkut s University of Technology Thonburi, Bangkok, Thailand Abstract: A thermodynamic analysis of hydrogen production from ethanol was performed in AspenPlus TM program where both material and energy balances are solved simultaneously. The effect of temperature, steam to carbon molar ratio and air to carbon molar ratio, on thermodynamic equilibrium of product distribution was investigated in three different fuel processing systems, namely steam reforming (SR), partial oxidation (POX), and autothermal reforming (ATR). The optimum operating conditions giving maximum hydrogen yield for each case were obtained. At those conditions, the SR system required the highest total energy whereas the POX system had the lowest value. However, the SR system gave the maximum hydrogen yield following ATR system and POX system. Keywords: Hydrogen Production, Steam Reforming, Partial Oxidation, Autothermal Reforming, Ethanol. INTRODUCTION Hydrogen is an attractive alternative energy source in the near future because of its cleanliness and environmental reasons. Hydrogen can be used as a fuel in a fuel cell system, generally more efficient than combustion engines and essentially zero emission, so the increasing demand of hydrogen for fuel cell application is expected []. Three technologies of hydrogen production from ethanol are steam reforming (SR), partial oxidation (POX), and autothermal reforming (ATR) which is a combination of POX and SR in a single unit. The oldest SR technology has been widely used, but disadvantage of this process is a slow start-up []. The ATR is practical for moderate size fuel processor because it reduces the size and heat transfer limitation of the steam reformer; in addition, it has advantage of high hydrogen concentrations with faster start-up and less coking []. Both POX and ATR require low energy and have high gas space velocity []. Several primary fuels used in hydrogen production have been considered [, -6], compared and investigated for the choice of primary fuels. The use of many fuels such as methane, methanol, ethanol, and gasoline is possible that if green fuel from renewable resources is considered, both light alcohols, methanol and ethanol, are competitive. Ethanol is a renewable resource that is playing an increasingly important role in decreasing emissions, improving air quality, and increasing energy security. Garcia and Laborde [7] carried out the first thermodynamic analysis of hydrogen production by ethanol steam reforming. Vasudeva et al. [8] improved the model by considering carbon formation and compared with the results of Garcia and Larbode. Later, there are many studies dealing with the thermodynamic analysis of ethanol steam reforming [, 9-5] while few thermodynamic studies of ethanol partial oxidation [-] and ethanol autothermal reforming [6]. However, no report dealing with the comparison three different fuel processings for hydrogen production from ethanol. The objectives of this research were to analyze thermodynamically operating conditions of hydrogen production from ethanol in the SR, POX, and ATR processes and to investigate the optimum operating condition for each case. AspenPlus TM was used to simulate the reforming process. The effect of operating condition on product compositions was characterized thermodynamically. In addition, the total thermal energy required in the reforming system was evaluated for each case.. METHODOLOGY In this study, AspenPlus TM, a commercially available software, was used in order to analyze the thermodynamic behavior of hydrogen production. The minimization of the Gibbs free energy was used. In the simulation, three different technologies, SR, POX and ART were considered that the system consists of a steam generator, a vaporizer of ethanol, a preheater, and a reforming reactor. The equilibrium compositions were calculated when operating conditions were given in the simulation. Moreover, the optimum operating conditions for each system in the equilibrium simulation were investigated and at these conditions, the energy utilization in the three systems of ethanol was compared to obtain economical process. The energy consumptions were calculated as total energy. In this study, the ethanol equivalent refers to the amount of ethanol combusted to produce the required heat.. RESULTS AND DISCUSSION. Steam reforming The stoichiometry of the reaction for maximum hydrogen production by ethanol SR reaction follows: C H 5 OH + H O = CO + 6H. () In fact, the stoichiometric coefficient of H can not reach 6 because in the SR conditions other ethanol reactions can take place such as decomposition, dehydrogenation, etc. As the results, at reactor temperature of 7 o C and steam-to-carbon ratio of 6, the yields of H, CO, CO, CH, and H O are.8,.66,.5,.9, and., respectively. In the simulation, the steam-to-carbon molar ratio was varied between.5 and and the temperature of SR reactor was varied from to 9 o C. First, the SR reactor was simulated for various steam-to-carbon molar ratios and temperatures. The main products are found to be H, CO, CO, CH, and H O whereas the others are negligible and very tiny amount of carbon formation Corresponding author: supaporn.the@kmutt.ac.th
The nd Joint International Conference on Sustainable Energy and Environment (SEE 6) A- (O) - November 6, Bangkok, Thailand was found. The effects of steam-to-carbon molar ratio and reactor temperature on H yield is illustrated in Fig.. Although an increase of both temperature and steam-to-ethanol ratio also has the effect on increasing amount of H yiled, the thermal energy consumption in system increases because of the increase of steam generation required. Therefore, the total thermal energy of system increases. H yield from SR reactor increases as the temperature and steam-to-ethanol ratio increase when the reactor temperature increases up to 8 o C and the steam-to-ethanol ratio increases above as shown in Fig.. However, the maximum reactor temperature is restrained because of the thermal durability of the catalysts and the maximum amount of steam-to-ethanol ratio is restricted according to the energy cost of the reactor system. However, increasing H and decreasing of CO are advantageous to the increase of water in system. The optimum operating conditions in the ethanol SR reactor at the proper reactor temperature of 7 o C and suitable steam-tocarbon ratio of were selected. In the simulation, the pressure was kept constant at bar and ethanol conversion was more than.99. At these conditions, the yields of H, CO, CO, CH, and H O are 5.5,.,.88,., and.9, respectively and the result shows no coke formation. The thermodynamic conclusions obtained in this study were also compared to the results in terms of response reactions (RERs) by Fishtik [] who solved that at pressure of bar the maximum amount of hydrogen is located at temperature of 98 K ( 77 o C). 6 5 H yield S:C =.5 S:C = S:C =.5 S:C = S:C = S:C = 5 6 7 8 9 Reactor temperature ( o C) Fig. Effect of steam-to-carbon molar ratio and reactor temperature on H yield in SR reactor Fig. shows the effect of temperature on product distribution of H, CO, CO, CH and H O. The order of gas yield is H O > CH > CO > H > CO at low temperature whereas it is H > H O > CO > CO > CH at high temperature (7 o C). Both H and CO yields increase when increasing temperature, whereas the CH yield shows the opposite trend. The temperature affects the amount of CH which is slowly decreased by increasing temperature up to 8 o C. This is the point where the CH yield is nearly zero. However, an increase of temperature increases the energy to be supplied to the system..7.6.5.... 5 6 7 8 9 Temperature ( o C) Fig. Effect of reactor temperature on H, CO, CO, CH and H O mole fraction in SR reactor at steam-to-carbon ratio =.5. Partial oxidation The POX is a reaction that oxygen is added, making the exothermic reaction at which an oxygen-to-ethanol molar ratio of / corresponds to the reactions producing H and CO as follows: C H 5 OH + / O = CO + H ()
The nd Joint International Conference on Sustainable Energy and Environment (SEE 6) A- (O) - November 6, Bangkok, Thailand and an oxygen-to-ethanol molar ratio of / corresponds to the following ideal reaction: C H 5 OH + / O = CO + H. () The POX reactor is less efficient than SR reactor, but the exothermic nature of the POX reaction makes it more responsive than SR. The effect of air-to-carbon molar ratio was studied between and 5 at an adiabatic temperature and a constant pressure of bar to avoid the heat management due to exothermic reaction. Both POX and ATR reactors were simulated at temperature operating under autothermal condition, namely adiabatic temperature, which means that there are no heat generated and used in the reactor as shown in Fig.. at steam-to-carbon ratio of zero. H yield increases as air-to-carbon ratio increases from to and then H yield gradually decreases because the excess air reacts with H to form H O. To determine the optimum air-to-carbon ratio for POX reactor, the H yield was considered as a function of air-to-carbon ratio as shown in Fig. The air-to-carbon ratio of according to oxygen-tocarbon ratio of. gives the maximum H yield at approximately adiabatic temperature of 8 o C. At the optimum POX reaction, the yields of H, CO, CO, CH, and H O are.59,.9,.7,., and., respectively. In addition, Comas [6] summarized that the presence of oxygen in the feed can reduce carbon formation and oxygen-to-carbon ratio of above. is an optimum value. In the simulation, however, at high reactor temperature above 9 o C, H and CO yields are approximately.9 and.9, respectively, which are almost equal to the stoichiometric coefficient of H and CO.. Autothermal reforming The effects of both steam-to-carbon molar ratio and air-to-carbon molar ratio were investigated between -5 and -5, respectively. The results at the air-to-carbon molar ratio of zero refer to that of POX reaction. Heat transfer between the high temperature partial oxidation and steam reforming reaction allows for better heat integration and lower operating temperature of the primary reformer, but requires a catalyst to perform both reactions. The air-to-carbon molar ratio and steam-to-carbon molar ratio affect the adiabatic temperature in ATR reactor and H yield. The adiabatic temperature increases by increasing air-to-carbon molar ratio because of the predominant exothermic reaction. At air-tocarbon molar ratio of above.75, the adiabatic temperature rapidly increases. Increasing the steam-to-carbon molar ratio causes the adiabatic temperature of the reactor to decrease because of the dominant endothermic reaction. The H yield as a function of air-tocarbon molar ratio and steam-to-carbon molar ratio under adiabatic temperature are illustrated in Fig.. H yield increases as the airto-carbon molar ratio increases. On the other hand, if the air-to-carbon molar ratio increases above.75, H yield rapidly decreases. Hence, H yield peaks at air-to-carbon molar ratio of.75. At this condition, H yield increases as steam-to-carbon molar ratio increases up to. As a result, the optimum operating condition for the ATR reactor is obtained at the air-to-carbon molar ratio of.75 and steam-to-carbon molar ratio of at an adiabatic temperature of 6 o C. For the favorable operating condition of ethanol ATR reaction, the yields of H, CO, CO, CH, and H O are.6,.,.58,.9, and., respectively. In addition, Liguras [7] proposed that at least.6 mol of oxygen per mole of ethanol is required to achieve thermal neutrality as shown in Eq. (). C H 5 OH +.78H O +.6O CO +.78H () H yield.5.5.5.5.5 POX S:C = S:C = S:C = S:C = S:C = 5.5.5.5.5 Fig. Effect of steam-to-carbon molar ratio and air-to-carbon molar ratio on H yield at adiabatic temperature. Fig. shows the effect of air-to-carbon ratio on product distribution of H, CO, CO, CH and H O of POX reaction. The order of gas yield is H > CO > H O ~ CO >CH at optimal conditions whereas it is H O > CO > H > CO > CH at high air-to-carbon ratio. Both H and CO yields increase as air-to-carbon ratio increases up to.75, whereas the CH yield rapidly decreases.
The nd Joint International Conference on Sustainable Energy and Environment (SEE 6) A- (O) - November 6, Bangkok, Thailand.7.6.5.....5.5.5.5.5 5 Fig. Effect of air-to-carbon ratio on H, CO, CO, CH and H O mole fraction in POX reactor at adiabatic temperature. Fig. 5 shows the effect of air-to-carbon ratio on product distribution of H, CO, CO, CH and H O of ATR reaction at steam-tocarbon ratio to be.5. The order of gas yield is H O > CH > H > CO > CO at low air-to-carbon ratio and H > H O > CO >CO > CH at optimal conditions whereas it is H O > CO > H > CO > CH at high air-to-carbon ratio. Both H and CO yields increase as air-to-carbon ratio increases up to.75, whereas the CH yield rapidly decreases which is the same as the POX reaction results..5.5..5..5..5..5 S:C =.5.5.5.5.5.5 5 Fig. 5 Effect of air-to-carbon ratio on H, CO, CO, CH and H O mole fraction in ATR reactor at steam-to-carbon ratio =.5.. CONCLUSION The optimum operating conditions giving the maximum hydrogen yield for each case were obtained. H yield from SR reactor increases with increasing temperature and steam to carbon ratio. H yield decreases continuously when steam-to-carbon ratio is more than at 7 o C. However, the increasing H and decreasing of CO are the advantages for increasing water in system. Because POX is exothermic reaction, the POX reactor at adiabatic temperature was considered to avoid the heat management. Both of steam-tocarbon ratio and air-to-carbon ratio strongly affect the adiabatic temperature and H yield. The adiabatic temperature increases by increasing air-to-carbon ratio because of the predominant exothermic reaction and decreases by increasing steam-to-carbon ratio because of the endothermic reaction. Table the optimum conditions in three different technologies. Fuel processor S:C ratio Air:C ratio Temperature ( o C) Total energy (MW) SR - 7 76.67 5.6 POX - 8 (Adia.) 76.976.587 ATR.75 6 (Adia.) 7.85.6 At conditions studied, the SR system requires the highest total energy whereas the POX system has the lowest value. However, Y H
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