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Experimental study of syngas production from methane dry reforming with heat recovery strategy
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 2 ( 2 0 1 7 ) 2 5 2 1 3 e2 5 2 2 4
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Experimental study of syngas production from methane dry reforming with heat recovery strategy Cheng-Hsun Liao, Rong-Fang Horng* Department of Mechanical Engineering, Intelligent Vehicle Research Center, Kun Shan University, No.195, Kunda Rd., Yungkang Dist, Tainan City 710, Taiwan, ROC
article info
abstract
Article history:
This study employed the concept of heat recovery to design a set of reformer to facilitate
Received 16 August 2016
the methane dry reforming (MDR), through which syngas (H2þCO) could be generated. The
Received in revised form
MDR involves an endothermic reaction and thus additional energy is required to sustain it.
22 November 2016
According to the concept of industrial heat recovery, the energy required to facilitate the
Accepted 6 January 2017
MDR was recovered from waste heat. In addition, after the reforming reaction, the waste
Available online 11 September 2017
heat inside the reformer was used for internal heat recovery to preheat the reactants
Keywords:
the parameter settings in the experiments, the CH4 feed flow rate was set at 1e2.5 NL/min
(CO2þCH4) to reduce the amount of energy required for the reforming reaction. Regarding Methane dry reforming
and the mole ratio for CO2/CH4 was set at 0.43e1.22. Subsequently, an oven was used to
Heat recovery
simulate a heat recovery environment to facilitate the dry reforming experiment. The
Unconventional natural gas
experimental results indicated that an increase in oven temperature could increase the
Reactants preheating
reforming reaction temperature and elevate the energy for the reformer. H2 and CO production could increase when the reformer gained more energy. The high-temperature gas generated from the reforming reaction was applied to facilitate internal heat recovery of reformer and preheat the reactants; thus, the efficiency of reforming and CO2 conversion were evidently elevated. The theoretical equilibrium analysis indicated that the thermal efficiency of reforming increased with the increase of CO2/CH4 molar ratio. While, the thermal efficiency of reforming by experiments decreased with the increase of the CH4 feed rate, but increased with the increase of CO2/CH4. In summary, the experimental results revealed that the overall H2 was 0.017e0.019 mol/min. In addition, the reforming efficiency was 8.76%e78.08%, the CO2 conversion was 53.92%e96.43%, and the maximum thermal efficiency of reforming was 102.3%. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction This study employed the concept of industrial heat recovery to facilitate the methane dry reforming (MDR) to generate syngas (H2 þ CO). In recent years, intelligent technology has
been successfully applied to daily life in contexts such as housing, transportation, and entertainment. However, the demand for energy is increasing because of the steadily rising global population [1]. Energy is constantly used to facilitate livelihoods and industries. Fossil fuels are currently the main type of energy used worldwide. However, the reduction
* Corresponding author. E-mail address: [email protected] (R.-F. Horng). http://dx.doi.org/10.1016/j.ijhydene.2017.01.238 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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in petroleum output in recent years has caused numerous countries to actively seek alternative and emerging energy sources. Recently, many countries, such as the United States and Australia, have made efforts to the development and exploitation of unconventional natural gas, mainly including shale gas, tight-sands gas, biogas, landfill, methane hydrate (iced methane), coal base methane, and coal mine methane [2]. Because of international breakthroughs in mining technology, the amount of shale gas exploitation is projected to increase yearly during 2010e2035, and the use patterns of conventional and unconventional natural gas will change accordingly. This type of unconventional natural gas exists in shale formations; the main components of the gas are methane (CH4), ethane, butane, pentane, carbon dioxide (CO2), and carbon monoxide (CO) [3]. Hydrocarbon fuels produce a large amount of end gas after they are combusted. This end gas contains CO2, CO, NOx, and unburned hydrocarbons. However, the end gas was emitted into the atmosphere to cause air pollution and the greenhouse effect. In addition, fossil energy cannot be regenerated, causing problems such as dwindling energy supplies and rising energy costs. Substantial amounts of fossil energy are consumed during industrial processes, such as in the petroleum refining industry, power plants, and solid waste combustion and disposal plants. After metal is melted in the metal manufacturing industry, the temperature of the slag reaches as high as 1650 C. Currently, the main development for slag energy is in gas heat recovery, which is transformed into chemical energy and used as fuels and for thermoelectric generation. Slag energy has received little attention so far in the metal manufacturing industry [4]. According to the regulations on waste combustion and disposal stipulated by the European Union, the temperature for operating municipal solid waste cannot be lower than 850 C. The general operating temperature must be maintained above 900 Ce950 C, and thus the temperature of the emitted gases is approximately 1000 C-1200 C [5e8]. A huge amount of energy is also consumed during the ceramic production process, especially in the firing stage, during which more than 50% of the input energy is lost through the combustion of end gas (flue gas) and cooling gas exhaust stacks [9]. The temperature of molten steel in steelmaking is approximately 2700 C; however, only approximately 20% of the input energy is applied to steel making, with the rest expelled through heat loss, which is also placed for cooling or discharged by high-temperature end gases. Large amounts of chemical energy are contained in high-temperature gases, such as coke oven gas, blast furnace gas, and basic oxygen furnace exhaust [10,11]. In addition to actively researching alternative energy sources, countries worldwide have also implemented heat recovery procedures after fuel combustion; the recovered heat is then applied to numerous systems to elevate energy efficiency and reduce energy consumption. Morgan et al. [12] applied new intracycle waste heat recovery technology to an internal combustion engine, a design that enables the recovery of waste heat generated from compression and combustion chambers, through which the intake air flow can be preheated. A theoretical analysis showed that the indicated thermal efficiency of the combined cycle is approximately
44.2%, which is 3.8% higher than that of the diesel cycle. However, the systematic thermal efficiency of the split cycle in intracycle waste heat recovery was found to be 52.2%, indicating that the efficiency is higher than that of the combined cycle. Peris et al. [13] applied the organic Rankine cycle to the ceramic industry to conduct an experiment on low-level waste heat recovery. The experimental results indicated that the heat input power was 128.19e179.87 kW, and the output net electric power was 21.79 kW and 18.51 kW. When the system was at maximum cycle efficiency, the value for the total electric power was 12.47% and that for the net electric power is 10.94%. Among the various types of alternative energy used for combustion, hydrogen energy is associated with numerous advantages, namely: relatively more favorable lean combustion capacity, low ignition energy (0.02 mJ), fast flame propagation speed (290 cm/s), and friendly to environment after combustion [14,15]. In addition, the combustion effect can be improved and pollutant emission can be reduced when H2 serves as an auxiliary fuel. The dry reforming reaction is the most appropriate reforming method for producing syngas from CO2 and CH4; however, carbon deposits are easily generated during the reforming reaction process. Catalysts will be deactivated and catalyst life will be shortened if the catalyst surface is covered with carbon. Khani et al. [16] executed the MDR, the steam reforming of methane, and the combined dry-steam reforming of methane to test four catalysts. During the MDR and steam reforming of methane, the catalyst with 3% of Ru/ZnLaAlO4 exhibited the optimal anticarbon deposit and methane conversion efficiency and yield. Sumrunronnasak et al. [17] introduced a catalyst with 5% of Ni/Ce0.6Zr0.4O2 into a Pd76Ag19Cu5 alloy membrane reformer, which produced H2 from CH4 and CO2 through dry reforming. They set the experimental parameters CH4/CO2 ratio at 1, the syngas feed rate at 20 ml/min, and the temperature of the reactor at 550 C. In comparison with conditions in which the alloy membrane reformer was not incorporated, the overall H2 yield increased from 10% to 35% and H2 selectivity increased from 47% to 53% when the alloy membrane reformer was incorporated. In addition, parameter collocation plays an important role in dry reforming. Lai et al. [18] conducted the research on the parameter operating range regarding dry autothermal reforming and determined that the conversion efficiency of CO2 increased with the increase of the reforming reaction temperature, and the conversion efficiency was more favorable when CO2/CH4 ratios were larger than 1. Currently, H2 can be generated from various fossil fuel reforming methods, which can be mainly divided into exothermic, endothermic, and autothermal reactions. However, when the endothermic reforming reaction is collocated with waste heat recovery, no additional fuel is required to produce heat for reforming. Chen et al. [19] used rhodium catalyst to perform partial oxidation of CH4, in which CO2 was also added and excess enthalpy recovery was collocated to facilitate oxidation methane reforming. This study suggested that the catalytic partial oxidation reforming of CH4 would be facilitated when O2/CH4 ¼ 0.6 because the following values were relatively higher under these conditions: CH4 conversion efficiency, syngas output, CO2 conversion efficiency, and the energy efficiency of the system. The proportion of the conversion of CO2 to CO was 18.2%e77.0%, and the CH4 conversion
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efficiency was 83.5%e89.9%. Lai et al. [20] applied biomassderived gas to a porous medium to facilitate the dry autothermal reforming of excess enthalpy. Porous media can store thermal energy through heat transfer and facilitate internal heat recovery to preheat the reactants, thereby increasing the effect of the reforming reaction and the production of H2-rich syngas. When the experimental parameters CO2/CH4 and O2/ CH4 were set at 1 and 0.75, respectively, methane conversion efficiency was increased by 18% and energy loss rate was at 20.7%. Chen et al. [21] used catalytic partial oxidation of methane (CPOM) and water gas shift reaction (WGSR) to investigate the behavior of rhodium catalysts in hydrogen production and observed that the parameters yielded maximum hydrogen production. Therefore, numerical results indicated that increase of O/C ratio enhances CH4 conversion for given S/C ratio, but will reduce H2 and CO selectivity. While, it also found that recycling heat can preheat reactants and trigger HTSR or LTSR. Bueno et al. [22] employed diesel engine heat recovery, in which the recovered heat was used in the gasification of glycerin to produce H2 and CH4. When the weight ratio of glycerin in water was 50%e70%, the temperature of the backend exit of the catalyst was above 700 C, during which syngas with a lower heating value of more than 22 MJ/kg could be attained and the conversion efficiency of glycerin was approximately 85%. The lower heating value of the produced syngas accounted for 140% of the input heating value of the reformed glycerin. Chen et al. [23] applied heat recovery technology to spiral tube to preheat reactants. The experimental results indicate that the employing spiral preheating the reactants, the methanol conversion and hydrogen yield are higher than 95 and 90%, respectively. Liao et al. [24] used the exhaust waste heat recovery for spark ignition engine, which served as the energy for facilitating methanol steam reforming. The results indicated that the maximum methanol conversion efficiency was as high as 93% and the molar flow rate of H2 generated was approximately 1.34 mol/min when the S/C ratio was set at 1.2, the methanol feed was set at 15.8 g/min, and the reforming reaction temperature was controlled at 300 C. In addition, the output of the hydrogen mole generated from each
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unit of the recovered heat was approximately 1.6 mol/MJ, and the reforming thermal efficiency was 104.2%. Thanga et al. [25] reported that consuming large amount of fossil fuels causes high levels of pollution and affects the global environment. H2 contains relatively more favorable combustion characteristics and can elevate the combustion efficie ncy of hydrocarbon fuels and reduce exhaust emissions when H2 is used to spark ignition engines and compression ignition engines. It also indicated that using H2-rich gas in internal combustion engines can reduce emissions of CO and HC by up to 7%e20% and 5%e9.5%, respectively. Using H2-rich gas can also reduce approximately 19.3% of particulate matter and 50% of NOx. On the basis of the aforementioned literature review, CO2 and CH4 are established as the main greenhouse gases. This study adopted the concept of heat recovery to facilitate reforming, in which syngas can be generated through the conversion of CH4 and CO2. H2-rich syngas can be used in engines, the steel industry, incinerators, and the ceramic industry, and can be inputted into fuel cells after purification. Therefore, H2-rich syngas can elevate energy usage efficiency and reduce energy consumption, environmental pollution, and global warming.
Experiments and methods Experimental setup The equipment for this experiment consisted of four main parts: the reforming system, fuel supply system, condensing system (gas/liquid phase separation), and the analysis system for data acquisition and measurement. Fig. 1 shows the schematic diagram of the equipment. The experiment was conducted using self-designed reforming equipment for heat recovery, as shown in Fig. 2. The reforming reaction chamber measured 427.2 mm 60 mm, the catalyst was PteRu based, and the substrates were spherical ceramic particles 1e3 mm in diameter (see Table 1). A preheating pipe for the reactants was embedded inside the reformer. The pipe and the reformer
Fig. 1 e Schematic of the experimental setup.
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Fig. 2 e Schematic of (a) internal structure and (b) internal heat recovery structure of reformer unit.
Table 1 e Reformer specifications. Reformer Catalyst
Inner diameter Length Material Geometry Volume
427.2 mm 60 mm PteRu/Al2O3 41e3 mm 20 mm3
were configured as concentric cylinders. The waste heat generated from the reforming reaction in the catalyst downstream was mainly applied to internal heat recovery to preheat reactant gases, thereby increasing the sensible enthalpy of the reactants and elevating the efficiency of the reforming reaction. To supply the gases (CH4 and CO2) for the reforming reaction, the mass flow controllers were adopted to control the reactant flow rates to achieve the appropriate proportion for experiment. After the reforming reaction was completed, the produced gas was inputted into the condensation and drying separation section to separate the gas and liquid phase products; subsequently, a dry basis analysis was performed on the produced gas. The analysis system was adopted to analyze the composition and concentration of the gas phase products. The gas chromatograph Agilent 6850 GC was employed to perform a quantitative analysis, and the gas analyzer HORIBA MEXA-554JA was used to monitor the realtime changes in product concentration to determine the extent of stability of the reforming reaction. Moreover, the Ktype thermocouples were installed in the inlet and outlet of the reactants preheating pipe, catalyst upstream and downstream, and reformer outlet. The measured temperature of oven was used to controlled oven power.
Experimental methods
used to simulate the heat derived from external heat recovery. In addition, to reduce energy consumption, the heat recovery was facilitated in the catalyst downstream inside the reformer, and the reactants inside the preheating pipe were preheated; after the temperature of the reactants was elevated, the reactants were added to the fluidized-bed catalyst to initiate the reforming reaction. Subsequently, cooling coils were used to cool the outputted high-temperature gas to room temperature, and a condensing device was employed to separate the gas and liquid phase products. Finally, desiccant adsorption was performed on the remaining trace moisture to ensure that the liquid phase product did not flow into the analyzer to damage it or cause measurement errors. For the measurement analysis, instant and quantitative analyses were performed on the yielded gas phase through a gas chromatograph and instant gas analyzer. In this study, the CH4 and CO2 were used to simulate the unconventional natural gas for the reforming reaction. The parameters for the reforming reaction were set as the CH4 feed flow rate at 1e2.5 NL/min, with every 0.5 NL/min serving as an interval. The compositions of the unconventional natural gas differed according to the sources of gas; the calculation of CO2/CH4 molar ratio is according to Equation (1). In this equation, mt and Vt represent the mass flow rate and volume flow rate, respectively. The CO2/CH4 molar ratio was set at 0.43e1.22. For the temperature control, the oven temperature was set at 800 C, 900 C, and 1000 C, and different feed flow rates were assigned to perform a series of experiments involving dry reforming. rVt mt= CO CO2 M CO 2 M ¼ ¼ rVt 2 CH4 mt=M CH4 M CH4
(1)
Chemical equilibrium and experimental calculations CH4 and CO2 were employed to facilitate the dry reforming, which demands a huge amount of energy compared with other types of reforming reactions. Therefore, an oven was
The composition and concentration of the gas generated after the dry reforming reaction changed according to the
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proportion of inputted reactants and the set temperature. Accordingly, chemical equilibrium Equations (2)e(4) were adopted to theoretically calculate the product and assess the reforming effects of different proportions of CO2/CH4. Subsequently, chemical equilibrium Equation (5) was applied to the products generated after the dry reforming experiment to calculate the coefficients m, n, p, q, r, s, and k, through which the mole coefficients derived from the actual reaction could be derived. Where m, n, p, q, r, s, and k are the equilibrium constants in the actual reaction, and x is the proportionality constant, respectively. While, p, q, r, and s are the product concentrations analyzed using a gas chromatograph by dry base analysis. Therefore, the mole coefficients of experiment could be employed to calculate the mole flow rates of H2 and CO, conversion efficiency of CH4 and CO2, heat produced in reactions, energy loss percentage, reforming efficiency, and thermal efficiency of reforming. Through a series of data measurements and calculations implemented in this experiment, the reaction effects and syngas production characteristics were determined when different parameter setups were applied in the MDR. aCO2 þbCH4 /dCO þ eH2 þ fC
(2)
aCO2 þbCH4 /dCO þ eH2
(3)
aCO2 þbCH4 /dCO þ eH2 þ gCO2
(4)
mCO2 þ nCH4 /xðpCO þ qH2 þ rCO2 þ sCH4 Þ þ kC
(5)
a < 1:0
a¼1:0
a > 1:0
Results and discussion Reformate products Because the MDR is an endothermic reaction, it requires the input of additional energy for reforming. A large amount of waste heat is produced after industrial energy is consumed. Applying the heat recovered from these processes to reforming reactions can reduce energy consumption, and reusing the recovered heat can elevate energy use. In this study, an oven was used to simulate a heat recovery environment and facilitate the MDR. Fig. 3 shows the H2 and CO mole flow rate at different parameters. Fig. 3(a)e(c) show that the oven temperatures were set at 800 C, 900 C, and 1000 C, respectively. Fig. 3(a) shows that H2 mole flow rate decreased slightly with the increase of CO2/CH4, and that an increased amount of CH4 feed rate did not evidently influence H2 production, which was approximately 0.019e0.010 mol/min. The influences of the CO2/CH4 ratio and CH4 feed rate on CO mole flow rate were not substantial; the overall output was 0.017e0.022 mol/min. The trend of H2 mole flow rate displayed in Fig. 3(b) was similar to that shown in Fig. 3(a); however, H2 was influenced by the increased CH4 feed rate. This influence was especially evident when CO2/CH4 was higher than 1.0, during which H2 produced was 0.028e0.032 mol/min. It was observed that CO increased with the increase of CO2/CH4, in which CO produced was 0.026e0.042 mol/min. The overall trends of H2 and CO as shown
Fig. 3 e The H2 and CO molar flow rate under various conditions. (CO2/CH4 ratio: 0.43~1.22, CH4: 1~2.5 L/min).
in Fig. 3(c) were similar to those shown in Fig. 3(b); however, clear changes were observed with the increase in the CH4 feed rate. The influences on the mole flow rate were the most evident when CO2/CH4 was 0.43, during which H2 and CO produced were respectively 0.030e0.058 mol/min and 0.027e0.068 mol/min.
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On the basis of the results displayed in Fig. 3(a)e(c), H2 and CO productions have been elevated when the CH4 feed rate increased at high oven temperature. This phenomenon was most evident when the oven temperature was 1000 C and CO2/CH4 was 0.43. However, CO production increased with the increase of CO2/CH4 when the oven temperature was above 900 C.
Gas hourly space velocity and reforming temperature The gas hourly space velocity (GHSV) is a critical factor determining the effects of the reforming reaction, and refers to the ratio of reactant gas flow rate to catalyst volume entering the reformer. Equation (6) presents the calculation of GHSV. GHSV ¼
ðVt ÞCH4 þ ðVt ÞCO2 Vcatalyst
fed
(6)
Fig. 5 e The effect of gas hourly space velocity on preheating temperature of reactants under various reaction parameters.
Fig. 4 shows that the GHSV increased when the CH4 feed rate and CO2/CH4 increased, during which the overall GHSV was 4200e16800 h1. The catalyst volume of the reformer served as the constant value; thus, the GHSV was determined according to the CO2/CH4 ratio and the reactant feed rate. Fig. 5 shows the influence of the GHSV on the preheating temperature of reactants (CH4 þ CO2), and shows that this temperature declined when GHSV increased and it increased with the increase of oven temperature. The overall preheating temperature was 724 C-378 C. When the oven temperature was 1000 C, the changes in the preheating temperature of reactants slowed down with the increase in GHSV. However, Fig. 6 shows the influence of GHSV on H2 yield, which decreased with the increase in GHSV but increased with the increase in oven temperature. The H2 yield was 4.50%e33.52%, and the maximum yield was 51.63%. From Fig. 6, it shows that GHSV increased when the CO2/CH4 ratio or feed flow rate of reactants increased, thus accelerating the flow speed of the reactants inside the reformer. Consequently, the preheating time for the reactants shortened and the preheating temperature could not be elevated.
Furthermore, a large GHSV indicated that the residence time of CH4 inside the catalyst shortened; therefore, CH4 could not be effectively applied to facilitate the reforming reaction and H2 yield declined. The reforming temperature refers to the temperature of the catalyst of the reformer. Higher levels of energy received by the reformer were correlated with higher temperature values. Figs. 7 and 8 demonstrate the influence of the reforming temperature. Specifically, Fig. 7 shows the reforming efficiency ððhÞRe Þ and the relationships between the preheating temperature of reactants and the reforming temperature. Equation (7) presents the calculation of reforming efficiency. The reforming efficiency and preheating temperature of reactants increased correspondingly with the reforming temperature; the reforming efficiency was 8.76%e78.08% and the preheating temperature of reactants was 378 C-724 C. Fig. 8 shows the relationships between the reforming temperature and CO2 conversion efficiency
Fig. 4 e The gas hourly space velocity under various feeding parameters. (CO2/CH4 ratio: 0.43~1.22, CH4: 1~2.5 L/min).
Fig. 6 e Effect of gas hourly space velocity on hydrogen yield under various oven temperatures.
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greater amount of energy was applied, evidently elevating the reforming efficiency and CO2 conversion. In addition, the preheating source of the reactants was derived from the heat flow rate of the products generated after the reforming reaction; therefore, with a high reforming temperature, a relatively larger amount of energy in the catalyst downstream could be used to preheat the reactants and elevate their temperatures. The obtained results in this study are compared with other studies in Table 2. As shown in the table, on MDR of methane and carbon dioxide, it can be found that the CO2 and CH4 conversions of this study are in the reasonable range. They are even better than that of others in some cases.
Fig. 7 e The relationship between reforming efficiency and temperatures (reforming temperature and preheating temperature of reactants).
ðhÞRe ¼
ðmt LHVÞH2 þ ðmt LHVÞCO 100% ðmt LHVÞCH4 fed
ðhconv: ÞCO2 ¼
(7)
moles of CO2 consumed 100% moles of CO2 fed
(8)
Heat produced in reactions and preheated energy to reactants The heat produced in reactions referred to the produced enthalpy from the methane dry reforming reaction, which includes lower heating value and sensible heat of syngas (H2þCO) due to temperature increase. In addition, the preheated energy of the reactants referred to the enthalpy added to raise the temperature of reactants for the MDR. Equation (9) presents the calculations for the heat produced in reactions and the preheated energy of reactants. ZT ðDhi ÞT ¼
Cp i dT
(9)
298:15
i ¼ CO2 ; CH4 ; H2 ; CO
Fig. 8 e Relationship between reforming temperature and carbon dioxide conversion.
ððhconv: ÞCO2 Þ. Equation (8) presents the calculation of CO2 conversion efficiency, which increased correspondingly with the reforming temperature and had a range of approximately 53.92%e96.43%. From these two figures, it was found that relatively higher reforming temperatures could be attained when the oven temperature was increased from 800 C to 1000 C; that is, the reforming reaction was improved when a
Fig. 9 shows the relationships among the heat produced in reactions, the preheated energy of reactants, and the oven temperature. Fig. 9(a)(d) show that the CH4 feed rate was set at 1.0, 1.5, 2.0, and 2.5 L/min, respectively. From Fig. 9(a), it showed that the changes in heat produced in reactions were less evident when the oven temperature increased; however, the increase in heat was relatively more apparent (ranging between 196 and 462 kJ/kg) when CO2/CH4 increased. However, the lightly colored section at the bottom of the figure refers to the preheated energy of reactants, displaying a trend similar to that of the heat produced in reactions. Relatively greater changes were observed in the preheated energy of reactants when CO2/CH4 increased, with the energy ranging
Table 2 e Comparison for operating conditions and the conversion of methane and carbon dioxide on MDR. Catalyst
Ru/ZnLaAlO4 Pt/HAP IWI La3.5Ru4.0O3 Ru/Mg3(Al)O PteRu/Al2O3
CO2/CH4 ratio
Temperature (oC)
GHSV (h1)
e 1 1 1 0.43e1.22
600e800 700 800 800 800e1000
10500 e e e 4200e20100
Conversion (%) CH4
CO2
z38e89.2 50 60 87 52e76
z50e90 67 70 90 54e97
Reference
[16] [26] [27] [28] This study
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Fig. 9 e The relationship between heat produced in reactions, preheated energy of reactants and oven temperatures.
between 22 and 40 kJ/kg. Subsequently, the overall trend as shown in Fig. 9(b)e(d) was similar to that shown in Fig. 9(a). It also illustrated that when the CH4 feed rate increased, the produced heat in reactions and the preheated energy of reactants increased substantially; the highest values for these two parameters were attained when CH4 was set at 2.5 L/min. The heat produced in reactions and the preheated energy of reactants were respectively 505e1141 kJ/kg and 46e100 kJ/kg when CO2/CH4 increased. Fig. 10 shows the effects of the different heat produced in reactions on the preheated energy of reactants and the heat recovery ratio ðQRecycled Þ, which refers to the ratio of the preheated energy of reactants to the heat produced in reaction as demonstrated in Equation (10). QRecycled ¼
Preheated energy of reactants 100% Reactions produced heat
(10)
Fig. 10 shows that the preheated energy of reactants clearly increased with the increase in heat produced in reactions, that the heat recovery ratio decreased slightly with the increase of heat produced in reactions, and that the overall heat recovery ratio was 11.73%e8.73%. However, when the oven temperature was 800 C and 900 C and the heat produced in reactions was approximately 800 J/sec, the preheated energy of reactants slowed down, causing the heat recovery ratio to decline. When the oven temperature was 1000 C, the preheated energy of reactants increased and the heat recovery ratio decreased with the increase of heat produced in
Fig. 10 e Effects of heat produced in reactions on preheated energy and heat recovery ratio.
reactions. However, this phenomenon was slow compared with that when the oven temperature was at 800 C and 900 C. An increase in the heat produced in reactions represented that the amount of heat generated by the reforming reaction increased. In contrast, an oven temperature of 1000 C provided sufficient energy to facilitate the reforming and could effectively preheat the reactants.
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Thermal efficiency of reforming and energy loss percentage Fig. 11 demonstrates the influences of CO2/CH4 on the thermal efficiency of reforming, namely the product to reactant energy ratio. Where, the product energy is the lower heating value (LHV) of H2 and CO, and the reactant energy is the lower heating value of consumed CH4. Equation (11) presents the calculation of thermal efficiency of reforming ðhth ÞRe . In addition, the reforming efficiency is the heating value ratio of syngas (H2þCO) to methane fed to the reformer. While, thermal efficiency of reforming is the heating value ratio of (H2þCO þ CH4) to methane fed. ðhth ÞRe ¼
ðmt LHVÞH2 þ ðmt LHVÞCO 100% ðmt LHVÞCH4 consumed
(11)
Fig. 11(a)e(c) shows that the oven temperature was set at 800 C, 900 C, and 1000 C, respectively. The curves in the figures represent the thermal efficiency of the theoretical chemical equilibrium analysis. Theoretical chemical equilibrium analysis was calculated by using the commercialized software package (HSC Chemistry), and the equilibrium results of the reforming reactions were obtained from the calculations (See Table 3). Therefore, the thermal efficiency of the theoretical chemical equilibrium analysis was calculated according to Eq. (11). However, the products generated from the reforming reaction (and therefore the thermal efficiency) differed according to the changes in CO2/CH4. Fig. 11(a) shows that thermal efficiency of reforming increased with CO2/CH4 and ranged from 88.4% to 125.3% according to the theoretical analysis. The thermal efficiency of reforming ranged from 56.5% to 66.5% by experiment. Changes in the thermal efficiency of reforming by experiment resulting from an increase of CO2/CH4 were not evident; however, the changes in efficiency resulting from an increase of the CH4 feed rate were relatively more evident. Fig. 11(b) shows the thermal efficiency of reforming for 900 C, in which the overall efficiency increased with the increase of CO2/CH4 and ranged from 64.6% to 74.2%. Changes in the thermal efficiency of reforming were the most evident when the CH4 feed rate was 1.0 L/min, and the value of 86.8% could be attained when CO2/CH4 was 1.2. The overall trend of the thermal efficiency of reforming by experiment as shown in Fig. 11(C), was similar to that shown in Fig. 11(a) and (b), in which thermal efficiency of reforming was approximately 73.3%e75%. However, the thermal efficiency increased when the CH4 feed rate was 1 L/min. The highest value of the thermal efficiency of reforming was 102.3% when the CH4 feed rate was 1.0 L/min and CO2/CH4 was 1.2. On the basis of the aforementioned three figures, this study determined that GHSV increased when the CH4 feed rate and CO2/CH4 increased, thereby shortening the contact time of CH4 and CO2 with the catalyst and reducing the reforming effect. In addition, an over high feed rate would reduce the reforming reaction and take heat away from the reformer, thus reducing the temperature of the fluidized-bed catalyst. Consequently, the energy in the reforming reaction would be insufficient, preventing CH4 from being effectively conversed and reducing the output results. It also can be found in Fig. 11, the theoretical thermal
Fig. 11 e The effect of CO2/CH4 ratio on thermal efficiency of reforming under various CH4 flow rates.
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Table 3 e Syngas production by theoretical calculations. volume flow rate (L/min) Reactants
CO2 CH4 CO H2
Products operational parameters temperature pressure
0.43 1 0.86 1.95
0.66 1 1.31 1.96
0.82 1 1.63 1.96
1 1 1.98 1.97
1.22 1 2.12 1.87
1000 C 1 atm
efficiency of reforming is about 90% at CO2/CH4 of 0.4, and it is over 100% when CO2/CH4 is larger than 0.6. However, the experimental results are lower than the results obtained by theoretical calculation obviously. The deviation between the experimental and theoretical results is thought that the actual reaction cannot reach chemical equilibrium due to low reaction temperature or other different operation parameters. Figs. 12 and 13 show the discussions regarding the energy loss percentage (ELP), which refers to the proportion of the loss of the inputted energy of CH4 after the dry reforming reaction. Equation (12) presents the calculation of ELP. ELP ¼
ðmt LHVÞCH4
ðmt LHVÞH2 ðmt LHVÞCO 100% ðmt LHVÞCH4 fed
fed
(12) First, Fig. 12 shows the influences of the heat produced in reactions on the ELP. When the CH4 feed rate was set at 1.0 L/min, the ELP declined rapidly with the increase in the heat produced in reactions, during which the ELP value was approximately from 2.3% to 33.5%. However, when the CH4 feed rate increased to above 1.5 L/min, the ELP gradually slowed according to the changes in the heat produced in reactions, during which the overall ELP was 26.7%e39.1%. The lowest ELP value was 2.3%, which represented that the heating value of the produced syngas was higher than that of the reactant (CH4). This phenomenon occurred when the oven temperature was set at 1000 C, the CH4 feed rate was
Fig. 13 e Relationship between efficiencies (thermal efficiency of reforming and reforming efficiency) and energy loss percentage under various reaction parameters.
set at 1.0 L/min, and the CO2/CH4 was set at 1.22. Subsequently, Fig. 13 shows that an increase in the ELP would lower the reforming efficiency and thermal efficiency of reforming. A low ELP indicated that relatively more CH4 participated in the reforming reaction and thus more syngas was produced. Syngas could be effectively generated from the reforming reaction through the energy supplied by the high-temperature reformate gases; thus, the reforming efficiency and thermal efficiency of reforming could be elevated. According to the aforementioned two figures, the heat produced in reactions continued to increase but the ELP declined slightly when the CH4 feed rate was relatively high. The main cause was that temperature elevation heightened the sensible heat, and not because of the increased heat produced in reactions. Therefore, the ELP increased and the reforming efficiency declined when the energy used to facilitate the reforming reaction was relatively insufficient, and the thermal efficiency of reforming reaction decreased substantially.
Conclusions
Fig. 12 e Effect of heat produced in reactions on energy loss percentage under various reaction parameters.
In this study, an oven was used to simulate the structure of an external heat recovery system; subsequently, the CH4 feed rate and CO2/CH4 were applied to a series of experiments to explore the output characteristics of the reforming reaction. The results are presented as follows:
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1. An increase in the oven temperature could elevate the reforming temperature, and an increase in the energy for reforming benefited the H2 and CO production. When the oven temperature was above 900 C, CO production increased with the increase of CO2/CH4. The overall H2 yield was 0.017e0.058 mol/min. 2. The preheating source of the reactants was the heat recovered from the high-temperature products generated by the reforming reaction. When the reforming temperature was high, a relatively higher amount of energy was contained in the catalyst downstream gases to preheat the reactants. Elevating the preheating temperature of the reactants increased the reforming efficiency and CO2 conversion, in which their highest values were 78.08% and 96.43%, respectively. 3. An increase in the feed flow rate elevated the GHSV; thus, the preheating time for the reactants was shortened and the preheating temperature diverged from the expected value. In addition, a large GHSV indicated that the residence time of CH4 in the catalyst had shortened, thus preventing CH4 from effectively facilitating the reforming reactions and in turn reducing H2 yield. The maximum H2 yield was 51.63% when the GHSV was relatively lower with the oven temperature of 1000 C. 4. The preheated energy of reactants increased when the heat produced in reactions increased. Because the MDR is an endothermic reaction, a relatively smaller energy can be recovered after its completion. Consequently, the reactants could not be effectively preheated, and the overall heat recovery ratio was 8.73%e11.73%. 5. The thermal efficiency of reforming by the theoretical chemical equilibrium analysis increased with the increase of CO2/CH4, and the distribution was 88.4%e125.3%. The thermal efficiency of reforming by experiment decreased when the CH4 feed rate increased, but it increased when CO2/CH4 increased. When the CH4 feed rate was 1.0 L/min and CO2/CH4 was 1.2, the highest thermal efficiency of the reforming was 102.3%. 6. The thermal efficiency of reforming and reforming efficiency both decreased with the increase in the ELP, in which a low ELP indicated that relatively more CH4 participated in the reaction during the dry reforming process. The minimum ELP value was 2.3% when the parameters were as follows: the oven temperature was set at 1000 C, the CH4 feed flow rate was set at 1.0 L/min, and CO2/CH4 was set at 1.22.
Acknowledgments This work was supported by Ministry of Science and Technology, Taiwan. (MOST 104-2221-E-168-016)
Nomenclature Dh cp M mt
change in enthalpy (kJ) specific heat capacity (kJ kg1 K1) molecular weight mass flow rate (kg sec1)
LHV Q V Vt ELP
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lower heating value (kJ kg1) heat flow rate (kW) volume (m3) volume flow rate (m3 sec1) energy loss percentage (%)
Greek letters h efficiency (%) r density (kg m3) thermal efficiency of reforming (%) hth reforming efficiency (%) ðhÞRe ðhconv: Þ conversion efficiency (%) Subscripts Conv conversion i species Recycled heat recovery Re Reforming Th Thermal T temperature
references
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[14] Jamal Y, Wyszynski ML. On-board generation of hydrogenrich gaseous fuels-a review. Int J Hydrogen Energy 1994;19:557e72. [15] Verhelst S, Wallner T. Hydrogen-fueled internal combustion engines. Prog Energy Combust Sci 2009;35:490e527. [16] Khani Y, Shariatinia Z, Bahadoran F. High catalytic activity and stability of ZnLaAlO4 supported Ni, Pt and Ru nanocatalysts applied in the dry, steam and combined drysteam reforming of methane. Chem Eng J 2016;299:353e66. [17] Sumrunronnasak S, Tantayanon S, Kiatgamolchai S, Sukonket T. Improved hydrogen production from dry reforming reaction using a catalytic packed-bed membrane reactor with Ni-based catalyst and dense PdAgCu alloy membrane. Int J Hydrogen Energy 2016;41:2621e30. [18] Lai MP, Horng RF, Lai WH, Lee CH. Determination of the operating range of CO2 conversion and syngas production in dry auto-thermal reforming. Int J Hydrogen Energy 2013;38:5705e12. [19] Chen WH, Lin SC. Characterization of catalytic partial oxidation of methane with carbon dioxide utilization and excess enthalpy recovery. Appl Energy 2016;162:1141e52. [20] Lai MP, Lai WH, Horng RF. Dry autothermal reforming from biomass derived gas under excess enthalpy with porous medium. J Power Sources 2012;217:407e16. [21] Chen WH, Chiu TW, Hung CI. Hydrogen production from methane under the interaction of catalytic partial oxidation, water gas shift reaction and heat recovery. Int J Hydrogen Energy 2010;35:12808e20.
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Available online at www.sciencedirect.com
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Experimental study of syngas production from methane dry reforming with heat recovery strategy Cheng-Hsun Liao, Rong-Fang Horng* Department of Mechanical Engineering, Intelligent Vehicle Research Center, Kun Shan University, No.195, Kunda Rd., Yungkang Dist, Tainan City 710, Taiwan, ROC
article info
abstract
Article history:
This study employed the concept of heat recovery to design a set of reformer to facilitate
Received 16 August 2016
the methane dry reforming (MDR), through which syngas (H2þCO) could be generated. The
Received in revised form
MDR involves an endothermic reaction and thus additional energy is required to sustain it.
22 November 2016
According to the concept of industrial heat recovery, the energy required to facilitate the
Accepted 6 January 2017
MDR was recovered from waste heat. In addition, after the reforming reaction, the waste
Available online 11 September 2017
heat inside the reformer was used for internal heat recovery to preheat the reactants
Keywords:
the parameter settings in the experiments, the CH4 feed flow rate was set at 1e2.5 NL/min
(CO2þCH4) to reduce the amount of energy required for the reforming reaction. Regarding Methane dry reforming
and the mole ratio for CO2/CH4 was set at 0.43e1.22. Subsequently, an oven was used to
Heat recovery
simulate a heat recovery environment to facilitate the dry reforming experiment. The
Unconventional natural gas
experimental results indicated that an increase in oven temperature could increase the
Reactants preheating
reforming reaction temperature and elevate the energy for the reformer. H2 and CO production could increase when the reformer gained more energy. The high-temperature gas generated from the reforming reaction was applied to facilitate internal heat recovery of reformer and preheat the reactants; thus, the efficiency of reforming and CO2 conversion were evidently elevated. The theoretical equilibrium analysis indicated that the thermal efficiency of reforming increased with the increase of CO2/CH4 molar ratio. While, the thermal efficiency of reforming by experiments decreased with the increase of the CH4 feed rate, but increased with the increase of CO2/CH4. In summary, the experimental results revealed that the overall H2 was 0.017e0.019 mol/min. In addition, the reforming efficiency was 8.76%e78.08%, the CO2 conversion was 53.92%e96.43%, and the maximum thermal efficiency of reforming was 102.3%. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction This study employed the concept of industrial heat recovery to facilitate the methane dry reforming (MDR) to generate syngas (H2 þ CO). In recent years, intelligent technology has
been successfully applied to daily life in contexts such as housing, transportation, and entertainment. However, the demand for energy is increasing because of the steadily rising global population [1]. Energy is constantly used to facilitate livelihoods and industries. Fossil fuels are currently the main type of energy used worldwide. However, the reduction
* Corresponding author. E-mail address: [email protected] (R.-F. Horng). http://dx.doi.org/10.1016/j.ijhydene.2017.01.238 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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in petroleum output in recent years has caused numerous countries to actively seek alternative and emerging energy sources. Recently, many countries, such as the United States and Australia, have made efforts to the development and exploitation of unconventional natural gas, mainly including shale gas, tight-sands gas, biogas, landfill, methane hydrate (iced methane), coal base methane, and coal mine methane [2]. Because of international breakthroughs in mining technology, the amount of shale gas exploitation is projected to increase yearly during 2010e2035, and the use patterns of conventional and unconventional natural gas will change accordingly. This type of unconventional natural gas exists in shale formations; the main components of the gas are methane (CH4), ethane, butane, pentane, carbon dioxide (CO2), and carbon monoxide (CO) [3]. Hydrocarbon fuels produce a large amount of end gas after they are combusted. This end gas contains CO2, CO, NOx, and unburned hydrocarbons. However, the end gas was emitted into the atmosphere to cause air pollution and the greenhouse effect. In addition, fossil energy cannot be regenerated, causing problems such as dwindling energy supplies and rising energy costs. Substantial amounts of fossil energy are consumed during industrial processes, such as in the petroleum refining industry, power plants, and solid waste combustion and disposal plants. After metal is melted in the metal manufacturing industry, the temperature of the slag reaches as high as 1650 C. Currently, the main development for slag energy is in gas heat recovery, which is transformed into chemical energy and used as fuels and for thermoelectric generation. Slag energy has received little attention so far in the metal manufacturing industry [4]. According to the regulations on waste combustion and disposal stipulated by the European Union, the temperature for operating municipal solid waste cannot be lower than 850 C. The general operating temperature must be maintained above 900 Ce950 C, and thus the temperature of the emitted gases is approximately 1000 C-1200 C [5e8]. A huge amount of energy is also consumed during the ceramic production process, especially in the firing stage, during which more than 50% of the input energy is lost through the combustion of end gas (flue gas) and cooling gas exhaust stacks [9]. The temperature of molten steel in steelmaking is approximately 2700 C; however, only approximately 20% of the input energy is applied to steel making, with the rest expelled through heat loss, which is also placed for cooling or discharged by high-temperature end gases. Large amounts of chemical energy are contained in high-temperature gases, such as coke oven gas, blast furnace gas, and basic oxygen furnace exhaust [10,11]. In addition to actively researching alternative energy sources, countries worldwide have also implemented heat recovery procedures after fuel combustion; the recovered heat is then applied to numerous systems to elevate energy efficiency and reduce energy consumption. Morgan et al. [12] applied new intracycle waste heat recovery technology to an internal combustion engine, a design that enables the recovery of waste heat generated from compression and combustion chambers, through which the intake air flow can be preheated. A theoretical analysis showed that the indicated thermal efficiency of the combined cycle is approximately
44.2%, which is 3.8% higher than that of the diesel cycle. However, the systematic thermal efficiency of the split cycle in intracycle waste heat recovery was found to be 52.2%, indicating that the efficiency is higher than that of the combined cycle. Peris et al. [13] applied the organic Rankine cycle to the ceramic industry to conduct an experiment on low-level waste heat recovery. The experimental results indicated that the heat input power was 128.19e179.87 kW, and the output net electric power was 21.79 kW and 18.51 kW. When the system was at maximum cycle efficiency, the value for the total electric power was 12.47% and that for the net electric power is 10.94%. Among the various types of alternative energy used for combustion, hydrogen energy is associated with numerous advantages, namely: relatively more favorable lean combustion capacity, low ignition energy (0.02 mJ), fast flame propagation speed (290 cm/s), and friendly to environment after combustion [14,15]. In addition, the combustion effect can be improved and pollutant emission can be reduced when H2 serves as an auxiliary fuel. The dry reforming reaction is the most appropriate reforming method for producing syngas from CO2 and CH4; however, carbon deposits are easily generated during the reforming reaction process. Catalysts will be deactivated and catalyst life will be shortened if the catalyst surface is covered with carbon. Khani et al. [16] executed the MDR, the steam reforming of methane, and the combined dry-steam reforming of methane to test four catalysts. During the MDR and steam reforming of methane, the catalyst with 3% of Ru/ZnLaAlO4 exhibited the optimal anticarbon deposit and methane conversion efficiency and yield. Sumrunronnasak et al. [17] introduced a catalyst with 5% of Ni/Ce0.6Zr0.4O2 into a Pd76Ag19Cu5 alloy membrane reformer, which produced H2 from CH4 and CO2 through dry reforming. They set the experimental parameters CH4/CO2 ratio at 1, the syngas feed rate at 20 ml/min, and the temperature of the reactor at 550 C. In comparison with conditions in which the alloy membrane reformer was not incorporated, the overall H2 yield increased from 10% to 35% and H2 selectivity increased from 47% to 53% when the alloy membrane reformer was incorporated. In addition, parameter collocation plays an important role in dry reforming. Lai et al. [18] conducted the research on the parameter operating range regarding dry autothermal reforming and determined that the conversion efficiency of CO2 increased with the increase of the reforming reaction temperature, and the conversion efficiency was more favorable when CO2/CH4 ratios were larger than 1. Currently, H2 can be generated from various fossil fuel reforming methods, which can be mainly divided into exothermic, endothermic, and autothermal reactions. However, when the endothermic reforming reaction is collocated with waste heat recovery, no additional fuel is required to produce heat for reforming. Chen et al. [19] used rhodium catalyst to perform partial oxidation of CH4, in which CO2 was also added and excess enthalpy recovery was collocated to facilitate oxidation methane reforming. This study suggested that the catalytic partial oxidation reforming of CH4 would be facilitated when O2/CH4 ¼ 0.6 because the following values were relatively higher under these conditions: CH4 conversion efficiency, syngas output, CO2 conversion efficiency, and the energy efficiency of the system. The proportion of the conversion of CO2 to CO was 18.2%e77.0%, and the CH4 conversion
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efficiency was 83.5%e89.9%. Lai et al. [20] applied biomassderived gas to a porous medium to facilitate the dry autothermal reforming of excess enthalpy. Porous media can store thermal energy through heat transfer and facilitate internal heat recovery to preheat the reactants, thereby increasing the effect of the reforming reaction and the production of H2-rich syngas. When the experimental parameters CO2/CH4 and O2/ CH4 were set at 1 and 0.75, respectively, methane conversion efficiency was increased by 18% and energy loss rate was at 20.7%. Chen et al. [21] used catalytic partial oxidation of methane (CPOM) and water gas shift reaction (WGSR) to investigate the behavior of rhodium catalysts in hydrogen production and observed that the parameters yielded maximum hydrogen production. Therefore, numerical results indicated that increase of O/C ratio enhances CH4 conversion for given S/C ratio, but will reduce H2 and CO selectivity. While, it also found that recycling heat can preheat reactants and trigger HTSR or LTSR. Bueno et al. [22] employed diesel engine heat recovery, in which the recovered heat was used in the gasification of glycerin to produce H2 and CH4. When the weight ratio of glycerin in water was 50%e70%, the temperature of the backend exit of the catalyst was above 700 C, during which syngas with a lower heating value of more than 22 MJ/kg could be attained and the conversion efficiency of glycerin was approximately 85%. The lower heating value of the produced syngas accounted for 140% of the input heating value of the reformed glycerin. Chen et al. [23] applied heat recovery technology to spiral tube to preheat reactants. The experimental results indicate that the employing spiral preheating the reactants, the methanol conversion and hydrogen yield are higher than 95 and 90%, respectively. Liao et al. [24] used the exhaust waste heat recovery for spark ignition engine, which served as the energy for facilitating methanol steam reforming. The results indicated that the maximum methanol conversion efficiency was as high as 93% and the molar flow rate of H2 generated was approximately 1.34 mol/min when the S/C ratio was set at 1.2, the methanol feed was set at 15.8 g/min, and the reforming reaction temperature was controlled at 300 C. In addition, the output of the hydrogen mole generated from each
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unit of the recovered heat was approximately 1.6 mol/MJ, and the reforming thermal efficiency was 104.2%. Thanga et al. [25] reported that consuming large amount of fossil fuels causes high levels of pollution and affects the global environment. H2 contains relatively more favorable combustion characteristics and can elevate the combustion efficie ncy of hydrocarbon fuels and reduce exhaust emissions when H2 is used to spark ignition engines and compression ignition engines. It also indicated that using H2-rich gas in internal combustion engines can reduce emissions of CO and HC by up to 7%e20% and 5%e9.5%, respectively. Using H2-rich gas can also reduce approximately 19.3% of particulate matter and 50% of NOx. On the basis of the aforementioned literature review, CO2 and CH4 are established as the main greenhouse gases. This study adopted the concept of heat recovery to facilitate reforming, in which syngas can be generated through the conversion of CH4 and CO2. H2-rich syngas can be used in engines, the steel industry, incinerators, and the ceramic industry, and can be inputted into fuel cells after purification. Therefore, H2-rich syngas can elevate energy usage efficiency and reduce energy consumption, environmental pollution, and global warming.
Experiments and methods Experimental setup The equipment for this experiment consisted of four main parts: the reforming system, fuel supply system, condensing system (gas/liquid phase separation), and the analysis system for data acquisition and measurement. Fig. 1 shows the schematic diagram of the equipment. The experiment was conducted using self-designed reforming equipment for heat recovery, as shown in Fig. 2. The reforming reaction chamber measured 427.2 mm 60 mm, the catalyst was PteRu based, and the substrates were spherical ceramic particles 1e3 mm in diameter (see Table 1). A preheating pipe for the reactants was embedded inside the reformer. The pipe and the reformer
Fig. 1 e Schematic of the experimental setup.
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Fig. 2 e Schematic of (a) internal structure and (b) internal heat recovery structure of reformer unit.
Table 1 e Reformer specifications. Reformer Catalyst
Inner diameter Length Material Geometry Volume
427.2 mm 60 mm PteRu/Al2O3 41e3 mm 20 mm3
were configured as concentric cylinders. The waste heat generated from the reforming reaction in the catalyst downstream was mainly applied to internal heat recovery to preheat reactant gases, thereby increasing the sensible enthalpy of the reactants and elevating the efficiency of the reforming reaction. To supply the gases (CH4 and CO2) for the reforming reaction, the mass flow controllers were adopted to control the reactant flow rates to achieve the appropriate proportion for experiment. After the reforming reaction was completed, the produced gas was inputted into the condensation and drying separation section to separate the gas and liquid phase products; subsequently, a dry basis analysis was performed on the produced gas. The analysis system was adopted to analyze the composition and concentration of the gas phase products. The gas chromatograph Agilent 6850 GC was employed to perform a quantitative analysis, and the gas analyzer HORIBA MEXA-554JA was used to monitor the realtime changes in product concentration to determine the extent of stability of the reforming reaction. Moreover, the Ktype thermocouples were installed in the inlet and outlet of the reactants preheating pipe, catalyst upstream and downstream, and reformer outlet. The measured temperature of oven was used to controlled oven power.
Experimental methods
used to simulate the heat derived from external heat recovery. In addition, to reduce energy consumption, the heat recovery was facilitated in the catalyst downstream inside the reformer, and the reactants inside the preheating pipe were preheated; after the temperature of the reactants was elevated, the reactants were added to the fluidized-bed catalyst to initiate the reforming reaction. Subsequently, cooling coils were used to cool the outputted high-temperature gas to room temperature, and a condensing device was employed to separate the gas and liquid phase products. Finally, desiccant adsorption was performed on the remaining trace moisture to ensure that the liquid phase product did not flow into the analyzer to damage it or cause measurement errors. For the measurement analysis, instant and quantitative analyses were performed on the yielded gas phase through a gas chromatograph and instant gas analyzer. In this study, the CH4 and CO2 were used to simulate the unconventional natural gas for the reforming reaction. The parameters for the reforming reaction were set as the CH4 feed flow rate at 1e2.5 NL/min, with every 0.5 NL/min serving as an interval. The compositions of the unconventional natural gas differed according to the sources of gas; the calculation of CO2/CH4 molar ratio is according to Equation (1). In this equation, mt and Vt represent the mass flow rate and volume flow rate, respectively. The CO2/CH4 molar ratio was set at 0.43e1.22. For the temperature control, the oven temperature was set at 800 C, 900 C, and 1000 C, and different feed flow rates were assigned to perform a series of experiments involving dry reforming. rVt mt= CO CO2 M CO 2 M ¼ ¼ rVt 2 CH4 mt=M CH4 M CH4
(1)
Chemical equilibrium and experimental calculations CH4 and CO2 were employed to facilitate the dry reforming, which demands a huge amount of energy compared with other types of reforming reactions. Therefore, an oven was
The composition and concentration of the gas generated after the dry reforming reaction changed according to the
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proportion of inputted reactants and the set temperature. Accordingly, chemical equilibrium Equations (2)e(4) were adopted to theoretically calculate the product and assess the reforming effects of different proportions of CO2/CH4. Subsequently, chemical equilibrium Equation (5) was applied to the products generated after the dry reforming experiment to calculate the coefficients m, n, p, q, r, s, and k, through which the mole coefficients derived from the actual reaction could be derived. Where m, n, p, q, r, s, and k are the equilibrium constants in the actual reaction, and x is the proportionality constant, respectively. While, p, q, r, and s are the product concentrations analyzed using a gas chromatograph by dry base analysis. Therefore, the mole coefficients of experiment could be employed to calculate the mole flow rates of H2 and CO, conversion efficiency of CH4 and CO2, heat produced in reactions, energy loss percentage, reforming efficiency, and thermal efficiency of reforming. Through a series of data measurements and calculations implemented in this experiment, the reaction effects and syngas production characteristics were determined when different parameter setups were applied in the MDR. aCO2 þbCH4 /dCO þ eH2 þ fC
(2)
aCO2 þbCH4 /dCO þ eH2
(3)
aCO2 þbCH4 /dCO þ eH2 þ gCO2
(4)
mCO2 þ nCH4 /xðpCO þ qH2 þ rCO2 þ sCH4 Þ þ kC
(5)
a < 1:0
a¼1:0
a > 1:0
Results and discussion Reformate products Because the MDR is an endothermic reaction, it requires the input of additional energy for reforming. A large amount of waste heat is produced after industrial energy is consumed. Applying the heat recovered from these processes to reforming reactions can reduce energy consumption, and reusing the recovered heat can elevate energy use. In this study, an oven was used to simulate a heat recovery environment and facilitate the MDR. Fig. 3 shows the H2 and CO mole flow rate at different parameters. Fig. 3(a)e(c) show that the oven temperatures were set at 800 C, 900 C, and 1000 C, respectively. Fig. 3(a) shows that H2 mole flow rate decreased slightly with the increase of CO2/CH4, and that an increased amount of CH4 feed rate did not evidently influence H2 production, which was approximately 0.019e0.010 mol/min. The influences of the CO2/CH4 ratio and CH4 feed rate on CO mole flow rate were not substantial; the overall output was 0.017e0.022 mol/min. The trend of H2 mole flow rate displayed in Fig. 3(b) was similar to that shown in Fig. 3(a); however, H2 was influenced by the increased CH4 feed rate. This influence was especially evident when CO2/CH4 was higher than 1.0, during which H2 produced was 0.028e0.032 mol/min. It was observed that CO increased with the increase of CO2/CH4, in which CO produced was 0.026e0.042 mol/min. The overall trends of H2 and CO as shown
Fig. 3 e The H2 and CO molar flow rate under various conditions. (CO2/CH4 ratio: 0.43~1.22, CH4: 1~2.5 L/min).
in Fig. 3(c) were similar to those shown in Fig. 3(b); however, clear changes were observed with the increase in the CH4 feed rate. The influences on the mole flow rate were the most evident when CO2/CH4 was 0.43, during which H2 and CO produced were respectively 0.030e0.058 mol/min and 0.027e0.068 mol/min.
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On the basis of the results displayed in Fig. 3(a)e(c), H2 and CO productions have been elevated when the CH4 feed rate increased at high oven temperature. This phenomenon was most evident when the oven temperature was 1000 C and CO2/CH4 was 0.43. However, CO production increased with the increase of CO2/CH4 when the oven temperature was above 900 C.
Gas hourly space velocity and reforming temperature The gas hourly space velocity (GHSV) is a critical factor determining the effects of the reforming reaction, and refers to the ratio of reactant gas flow rate to catalyst volume entering the reformer. Equation (6) presents the calculation of GHSV. GHSV ¼
ðVt ÞCH4 þ ðVt ÞCO2 Vcatalyst
fed
(6)
Fig. 5 e The effect of gas hourly space velocity on preheating temperature of reactants under various reaction parameters.
Fig. 4 shows that the GHSV increased when the CH4 feed rate and CO2/CH4 increased, during which the overall GHSV was 4200e16800 h1. The catalyst volume of the reformer served as the constant value; thus, the GHSV was determined according to the CO2/CH4 ratio and the reactant feed rate. Fig. 5 shows the influence of the GHSV on the preheating temperature of reactants (CH4 þ CO2), and shows that this temperature declined when GHSV increased and it increased with the increase of oven temperature. The overall preheating temperature was 724 C-378 C. When the oven temperature was 1000 C, the changes in the preheating temperature of reactants slowed down with the increase in GHSV. However, Fig. 6 shows the influence of GHSV on H2 yield, which decreased with the increase in GHSV but increased with the increase in oven temperature. The H2 yield was 4.50%e33.52%, and the maximum yield was 51.63%. From Fig. 6, it shows that GHSV increased when the CO2/CH4 ratio or feed flow rate of reactants increased, thus accelerating the flow speed of the reactants inside the reformer. Consequently, the preheating time for the reactants shortened and the preheating temperature could not be elevated.
Furthermore, a large GHSV indicated that the residence time of CH4 inside the catalyst shortened; therefore, CH4 could not be effectively applied to facilitate the reforming reaction and H2 yield declined. The reforming temperature refers to the temperature of the catalyst of the reformer. Higher levels of energy received by the reformer were correlated with higher temperature values. Figs. 7 and 8 demonstrate the influence of the reforming temperature. Specifically, Fig. 7 shows the reforming efficiency ððhÞRe Þ and the relationships between the preheating temperature of reactants and the reforming temperature. Equation (7) presents the calculation of reforming efficiency. The reforming efficiency and preheating temperature of reactants increased correspondingly with the reforming temperature; the reforming efficiency was 8.76%e78.08% and the preheating temperature of reactants was 378 C-724 C. Fig. 8 shows the relationships between the reforming temperature and CO2 conversion efficiency
Fig. 4 e The gas hourly space velocity under various feeding parameters. (CO2/CH4 ratio: 0.43~1.22, CH4: 1~2.5 L/min).
Fig. 6 e Effect of gas hourly space velocity on hydrogen yield under various oven temperatures.
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greater amount of energy was applied, evidently elevating the reforming efficiency and CO2 conversion. In addition, the preheating source of the reactants was derived from the heat flow rate of the products generated after the reforming reaction; therefore, with a high reforming temperature, a relatively larger amount of energy in the catalyst downstream could be used to preheat the reactants and elevate their temperatures. The obtained results in this study are compared with other studies in Table 2. As shown in the table, on MDR of methane and carbon dioxide, it can be found that the CO2 and CH4 conversions of this study are in the reasonable range. They are even better than that of others in some cases.
Fig. 7 e The relationship between reforming efficiency and temperatures (reforming temperature and preheating temperature of reactants).
ðhÞRe ¼
ðmt LHVÞH2 þ ðmt LHVÞCO 100% ðmt LHVÞCH4 fed
ðhconv: ÞCO2 ¼
(7)
moles of CO2 consumed 100% moles of CO2 fed
(8)
Heat produced in reactions and preheated energy to reactants The heat produced in reactions referred to the produced enthalpy from the methane dry reforming reaction, which includes lower heating value and sensible heat of syngas (H2þCO) due to temperature increase. In addition, the preheated energy of the reactants referred to the enthalpy added to raise the temperature of reactants for the MDR. Equation (9) presents the calculations for the heat produced in reactions and the preheated energy of reactants. ZT ðDhi ÞT ¼
Cp i dT
(9)
298:15
i ¼ CO2 ; CH4 ; H2 ; CO
Fig. 8 e Relationship between reforming temperature and carbon dioxide conversion.
ððhconv: ÞCO2 Þ. Equation (8) presents the calculation of CO2 conversion efficiency, which increased correspondingly with the reforming temperature and had a range of approximately 53.92%e96.43%. From these two figures, it was found that relatively higher reforming temperatures could be attained when the oven temperature was increased from 800 C to 1000 C; that is, the reforming reaction was improved when a
Fig. 9 shows the relationships among the heat produced in reactions, the preheated energy of reactants, and the oven temperature. Fig. 9(a)(d) show that the CH4 feed rate was set at 1.0, 1.5, 2.0, and 2.5 L/min, respectively. From Fig. 9(a), it showed that the changes in heat produced in reactions were less evident when the oven temperature increased; however, the increase in heat was relatively more apparent (ranging between 196 and 462 kJ/kg) when CO2/CH4 increased. However, the lightly colored section at the bottom of the figure refers to the preheated energy of reactants, displaying a trend similar to that of the heat produced in reactions. Relatively greater changes were observed in the preheated energy of reactants when CO2/CH4 increased, with the energy ranging
Table 2 e Comparison for operating conditions and the conversion of methane and carbon dioxide on MDR. Catalyst
Ru/ZnLaAlO4 Pt/HAP IWI La3.5Ru4.0O3 Ru/Mg3(Al)O PteRu/Al2O3
CO2/CH4 ratio
Temperature (oC)
GHSV (h1)
e 1 1 1 0.43e1.22
600e800 700 800 800 800e1000
10500 e e e 4200e20100
Conversion (%) CH4
CO2
z38e89.2 50 60 87 52e76
z50e90 67 70 90 54e97
Reference
[16] [26] [27] [28] This study
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Fig. 9 e The relationship between heat produced in reactions, preheated energy of reactants and oven temperatures.
between 22 and 40 kJ/kg. Subsequently, the overall trend as shown in Fig. 9(b)e(d) was similar to that shown in Fig. 9(a). It also illustrated that when the CH4 feed rate increased, the produced heat in reactions and the preheated energy of reactants increased substantially; the highest values for these two parameters were attained when CH4 was set at 2.5 L/min. The heat produced in reactions and the preheated energy of reactants were respectively 505e1141 kJ/kg and 46e100 kJ/kg when CO2/CH4 increased. Fig. 10 shows the effects of the different heat produced in reactions on the preheated energy of reactants and the heat recovery ratio ðQRecycled Þ, which refers to the ratio of the preheated energy of reactants to the heat produced in reaction as demonstrated in Equation (10). QRecycled ¼
Preheated energy of reactants 100% Reactions produced heat
(10)
Fig. 10 shows that the preheated energy of reactants clearly increased with the increase in heat produced in reactions, that the heat recovery ratio decreased slightly with the increase of heat produced in reactions, and that the overall heat recovery ratio was 11.73%e8.73%. However, when the oven temperature was 800 C and 900 C and the heat produced in reactions was approximately 800 J/sec, the preheated energy of reactants slowed down, causing the heat recovery ratio to decline. When the oven temperature was 1000 C, the preheated energy of reactants increased and the heat recovery ratio decreased with the increase of heat produced in
Fig. 10 e Effects of heat produced in reactions on preheated energy and heat recovery ratio.
reactions. However, this phenomenon was slow compared with that when the oven temperature was at 800 C and 900 C. An increase in the heat produced in reactions represented that the amount of heat generated by the reforming reaction increased. In contrast, an oven temperature of 1000 C provided sufficient energy to facilitate the reforming and could effectively preheat the reactants.
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Thermal efficiency of reforming and energy loss percentage Fig. 11 demonstrates the influences of CO2/CH4 on the thermal efficiency of reforming, namely the product to reactant energy ratio. Where, the product energy is the lower heating value (LHV) of H2 and CO, and the reactant energy is the lower heating value of consumed CH4. Equation (11) presents the calculation of thermal efficiency of reforming ðhth ÞRe . In addition, the reforming efficiency is the heating value ratio of syngas (H2þCO) to methane fed to the reformer. While, thermal efficiency of reforming is the heating value ratio of (H2þCO þ CH4) to methane fed. ðhth ÞRe ¼
ðmt LHVÞH2 þ ðmt LHVÞCO 100% ðmt LHVÞCH4 consumed
(11)
Fig. 11(a)e(c) shows that the oven temperature was set at 800 C, 900 C, and 1000 C, respectively. The curves in the figures represent the thermal efficiency of the theoretical chemical equilibrium analysis. Theoretical chemical equilibrium analysis was calculated by using the commercialized software package (HSC Chemistry), and the equilibrium results of the reforming reactions were obtained from the calculations (See Table 3). Therefore, the thermal efficiency of the theoretical chemical equilibrium analysis was calculated according to Eq. (11). However, the products generated from the reforming reaction (and therefore the thermal efficiency) differed according to the changes in CO2/CH4. Fig. 11(a) shows that thermal efficiency of reforming increased with CO2/CH4 and ranged from 88.4% to 125.3% according to the theoretical analysis. The thermal efficiency of reforming ranged from 56.5% to 66.5% by experiment. Changes in the thermal efficiency of reforming by experiment resulting from an increase of CO2/CH4 were not evident; however, the changes in efficiency resulting from an increase of the CH4 feed rate were relatively more evident. Fig. 11(b) shows the thermal efficiency of reforming for 900 C, in which the overall efficiency increased with the increase of CO2/CH4 and ranged from 64.6% to 74.2%. Changes in the thermal efficiency of reforming were the most evident when the CH4 feed rate was 1.0 L/min, and the value of 86.8% could be attained when CO2/CH4 was 1.2. The overall trend of the thermal efficiency of reforming by experiment as shown in Fig. 11(C), was similar to that shown in Fig. 11(a) and (b), in which thermal efficiency of reforming was approximately 73.3%e75%. However, the thermal efficiency increased when the CH4 feed rate was 1 L/min. The highest value of the thermal efficiency of reforming was 102.3% when the CH4 feed rate was 1.0 L/min and CO2/CH4 was 1.2. On the basis of the aforementioned three figures, this study determined that GHSV increased when the CH4 feed rate and CO2/CH4 increased, thereby shortening the contact time of CH4 and CO2 with the catalyst and reducing the reforming effect. In addition, an over high feed rate would reduce the reforming reaction and take heat away from the reformer, thus reducing the temperature of the fluidized-bed catalyst. Consequently, the energy in the reforming reaction would be insufficient, preventing CH4 from being effectively conversed and reducing the output results. It also can be found in Fig. 11, the theoretical thermal
Fig. 11 e The effect of CO2/CH4 ratio on thermal efficiency of reforming under various CH4 flow rates.
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Table 3 e Syngas production by theoretical calculations. volume flow rate (L/min) Reactants
CO2 CH4 CO H2
Products operational parameters temperature pressure
0.43 1 0.86 1.95
0.66 1 1.31 1.96
0.82 1 1.63 1.96
1 1 1.98 1.97
1.22 1 2.12 1.87
1000 C 1 atm
efficiency of reforming is about 90% at CO2/CH4 of 0.4, and it is over 100% when CO2/CH4 is larger than 0.6. However, the experimental results are lower than the results obtained by theoretical calculation obviously. The deviation between the experimental and theoretical results is thought that the actual reaction cannot reach chemical equilibrium due to low reaction temperature or other different operation parameters. Figs. 12 and 13 show the discussions regarding the energy loss percentage (ELP), which refers to the proportion of the loss of the inputted energy of CH4 after the dry reforming reaction. Equation (12) presents the calculation of ELP. ELP ¼
ðmt LHVÞCH4
ðmt LHVÞH2 ðmt LHVÞCO 100% ðmt LHVÞCH4 fed
fed
(12) First, Fig. 12 shows the influences of the heat produced in reactions on the ELP. When the CH4 feed rate was set at 1.0 L/min, the ELP declined rapidly with the increase in the heat produced in reactions, during which the ELP value was approximately from 2.3% to 33.5%. However, when the CH4 feed rate increased to above 1.5 L/min, the ELP gradually slowed according to the changes in the heat produced in reactions, during which the overall ELP was 26.7%e39.1%. The lowest ELP value was 2.3%, which represented that the heating value of the produced syngas was higher than that of the reactant (CH4). This phenomenon occurred when the oven temperature was set at 1000 C, the CH4 feed rate was
Fig. 13 e Relationship between efficiencies (thermal efficiency of reforming and reforming efficiency) and energy loss percentage under various reaction parameters.
set at 1.0 L/min, and the CO2/CH4 was set at 1.22. Subsequently, Fig. 13 shows that an increase in the ELP would lower the reforming efficiency and thermal efficiency of reforming. A low ELP indicated that relatively more CH4 participated in the reforming reaction and thus more syngas was produced. Syngas could be effectively generated from the reforming reaction through the energy supplied by the high-temperature reformate gases; thus, the reforming efficiency and thermal efficiency of reforming could be elevated. According to the aforementioned two figures, the heat produced in reactions continued to increase but the ELP declined slightly when the CH4 feed rate was relatively high. The main cause was that temperature elevation heightened the sensible heat, and not because of the increased heat produced in reactions. Therefore, the ELP increased and the reforming efficiency declined when the energy used to facilitate the reforming reaction was relatively insufficient, and the thermal efficiency of reforming reaction decreased substantially.
Conclusions
Fig. 12 e Effect of heat produced in reactions on energy loss percentage under various reaction parameters.
In this study, an oven was used to simulate the structure of an external heat recovery system; subsequently, the CH4 feed rate and CO2/CH4 were applied to a series of experiments to explore the output characteristics of the reforming reaction. The results are presented as follows:
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 2 ( 2 0 1 7 ) 2 5 2 1 3 e2 5 2 2 4
1. An increase in the oven temperature could elevate the reforming temperature, and an increase in the energy for reforming benefited the H2 and CO production. When the oven temperature was above 900 C, CO production increased with the increase of CO2/CH4. The overall H2 yield was 0.017e0.058 mol/min. 2. The preheating source of the reactants was the heat recovered from the high-temperature products generated by the reforming reaction. When the reforming temperature was high, a relatively higher amount of energy was contained in the catalyst downstream gases to preheat the reactants. Elevating the preheating temperature of the reactants increased the reforming efficiency and CO2 conversion, in which their highest values were 78.08% and 96.43%, respectively. 3. An increase in the feed flow rate elevated the GHSV; thus, the preheating time for the reactants was shortened and the preheating temperature diverged from the expected value. In addition, a large GHSV indicated that the residence time of CH4 in the catalyst had shortened, thus preventing CH4 from effectively facilitating the reforming reactions and in turn reducing H2 yield. The maximum H2 yield was 51.63% when the GHSV was relatively lower with the oven temperature of 1000 C. 4. The preheated energy of reactants increased when the heat produced in reactions increased. Because the MDR is an endothermic reaction, a relatively smaller energy can be recovered after its completion. Consequently, the reactants could not be effectively preheated, and the overall heat recovery ratio was 8.73%e11.73%. 5. The thermal efficiency of reforming by the theoretical chemical equilibrium analysis increased with the increase of CO2/CH4, and the distribution was 88.4%e125.3%. The thermal efficiency of reforming by experiment decreased when the CH4 feed rate increased, but it increased when CO2/CH4 increased. When the CH4 feed rate was 1.0 L/min and CO2/CH4 was 1.2, the highest thermal efficiency of the reforming was 102.3%. 6. The thermal efficiency of reforming and reforming efficiency both decreased with the increase in the ELP, in which a low ELP indicated that relatively more CH4 participated in the reaction during the dry reforming process. The minimum ELP value was 2.3% when the parameters were as follows: the oven temperature was set at 1000 C, the CH4 feed flow rate was set at 1.0 L/min, and CO2/CH4 was set at 1.22.
Acknowledgments This work was supported by Ministry of Science and Technology, Taiwan. (MOST 104-2221-E-168-016)
Nomenclature Dh cp M mt
change in enthalpy (kJ) specific heat capacity (kJ kg1 K1) molecular weight mass flow rate (kg sec1)
LHV Q V Vt ELP
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lower heating value (kJ kg1) heat flow rate (kW) volume (m3) volume flow rate (m3 sec1) energy loss percentage (%)
Greek letters h efficiency (%) r density (kg m3) thermal efficiency of reforming (%) hth reforming efficiency (%) ðhÞRe ðhconv: Þ conversion efficiency (%) Subscripts Conv conversion i species Recycled heat recovery Re Reforming Th Thermal T temperature
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