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Study on hydrogen-rich syngas production by dry autothermal reforming from biomass derived gas
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Study on hydrogen-rich syngas production by dry autothermal reforming from biomass derived gas Wei-Hsiang Lai a,b, Ming-Pin Lai a, Rong-Fang Horng c,* a
Department of Aeronautics and Astronautics, National Cheng Kung University, No.1, University Road, Tainan City 701, Taiwan Research Center for Energy Technology and Strategy, National Cheng Kung University, No.1, University Road, Tainan City 701, Taiwan c Department of Mechanical Engineering, Clean Energy Center, Kun Shan University, No. 949, Da-Wan Road, Yung-Kang District, Tainan City 710, Taiwan b
article info
abstract
Article history:
In this study, the H2-rich syngas (H2 þ CO) production from biomass derived gas (BDG) by
Received 17 December 2011
dry autothermal reforming (DATR) is investigated. Methane and carbon dioxide is the
Received in revised form
major composition of biomass derived gas. DATR reaction combined benefits of partial
5 March 2012
oxidation (POX) and dry reforming (DR) reaction was carried out in this study. The
Accepted 16 March 2012
reforming parameters on the conversion of methane and syngas selectivity were explored.
Available online 14 April 2012
The reforming parameters included the fuel feeding rate, CO2/CH4 and O2/CH4 molar ratios. The experimental results demonstrated that it not only supplied the energy required for
Keywords:
self-sustained reaction, but also avoided the coke formation by dry autothermal reforming.
Carbon dioxide
It has a wide operation region to maintain the moderate production of the syngas. During
Dry autothermal reforming
the reforming process, the reformate gas temperature was between 650 and 900 C, and
Syngas production
energy loss percentage in reforming process was between 15 and 30%. Further, high CO2
Reverse wateregas shifting
concentration in the reactant had a considerable influence on the heat release of oxidation, and thereby decreased the reformate gas temperature. It caused the reduction of synthesis gas concentration and assisting/impeding combustion composition (A/I ) ratio. However, it was favorable to CO selectivity because of the reverse water-gas shifting reaction. The H2/ CO molar ratio between 1 and 2 was achieved by varying CO2/CH4 molar ratio. However, the syngas concentrations were affected by CO2/CH4 and O2/CH4 molar ratio. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Increasing energy usage efficiency and reducing exhaust emissions have recently become the worldwide focus because of environmental protection demands and energy shortage. To confront these problems, research in alternative fuels and low carbon technology may be the focal topics in the future. Biomass derived gas (BDG), including biogas, landfill, and digester, is a valuable source of alternative
clean energy. Recently published papers related to energy policy and sustainable energy indicated that several countries are promoting low carbon energy from biomass derived gas [1,2]. Applications of biomass derived gas were gradually expanded, such as direct burning, chemical synthesis, and electricity generation [3]. If managed properly, BDG may be a considerable source of alternative and renewable energy and could be helpful to achieve greenhouse gas reduction.
* Corresponding author. Fax: þ886 6 2050509. E-mail address: [email protected] (R.-F. Horng). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2012.03.076
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BDG was made from an anaerobic fermentation process using anaerobic decomposition with microorganisms. Sources of BDG were abundant, such as livestock waste, agricultural residues, domestic sewage, and landfill. BDG is an alternative energy, and it also reduces the cost of sewage treatment and waste management problems. The composition of BDG is complicated, including CH4, CO2, CO, H2, H2O, H2S, HCs, NH3, and N2 [4], which were determined by environmental conditions and waste sources. A typical composition of BDG is shown in Table 1 [1,4e7]. In addition to CO2 and N2, most of the composition in BDG would corrode the components of the combustion chamber, such as H2S, HCs, and NH3. Thus, if BDG is directly applied to a combustor or solid oxide fuel cell (SOFC), it may damage the equipment and increase maintenance costs. Shiratori et al. [8] indicated that the electrochemical reaction with 1 ppm H2S caused an approximately 40% decrease in reaction rate of dry reforming and approximately 9% voltage drop in SOFC. The deactivation of catalyst on anode may be caused by coke formation with unsuitable CO2/CH4 ratio [9]. The coke formation is resulted from the deactivation of catalyst by the chemisorption of impurities. Coke formation would adhere on the catalyst surface and cover the active site [10,11]. BDG combustion will damage the combustor components because of corrosion. In addition, both the reactant heating value and flame propagation speed will decrease with a high concentration of CO2. Liu et al. [12] stated that methane mixed with carbon dioxide will decrease the laminar flame speed and adiabatic flame temperature, by calculating with GRI-Mech3.0. In addition, the adiabatic flame temperature was raised with high CO concentration, and laminar flame speed was increased considerably with high H2 concentration in the syngas. The heating value of fuel and flame propagation speed may be reduced with the dilution gas, such as CO2, in BDG. Therefore, literature data indicated that the reaction rate and the lean flammability limit were improved by adding hydrogen to BDG as the auxiliary fuel of engines. The addition of hydrogen-rich gas in engine not only increased brake thermal efficiency, but also improved the cycle-to-cycle variation and the exhaust emissions under fuel lean conditions [13]. The peak pressure and the heat release rate of the engine may be increased with the addition of hydrogen, and thus, the ignition timing must be retarded to prevent the knock. The addition of hydrogen up to 10% in BDG, it may improve the performance and reduce exhaust emissions of the engine [14]. BDG is produced mainly from agricultural waste and landfill. It contains a large amount of greenhouse gases, such
as CH4 and CO2; therefore, dry reforming is the suitable approach for fuel processing to produce hydrogen. If BDG is used properly and converted into syngas, not only would it increase the overall energy usage, but also decrease greenhouse gases substantially. The produced H2/CO ratio is close to 1.0 in reformate gas by dry reforming with methane and CO2. This is the advantage of this fuel processing approach and is usually used for the synthesis of high-value chemicals and fuels [15,16]. Furthermore, syngas, which contains the main composition of H2 and CO, has a wider flammable range. It may extend the lean limit and maintain flame stability if it fuels the internal combustion engine and burner; hence, the fuel conversion and exhaust emissions would be improved [17,18]. Fisher and Tropsch initiated the investigation on dry reforming by methane with CO2 [19]. However, it is a high temperature and carbon-rich reaction. Deactivation of the catalyst caused by sintering and coking inhibited commercialization development. Recently, the need in greenhouse gas reduction, dry reforming has become a renewed interest for CO2 reforming, and a considerable number of studies focused on finding suitable catalyst, including cost, activation, and selectivity of the catalyst for the reforming [20], further improving the coke resistance ability of catalyst and preventing the sintering [21,22]. Dry reforming is a strong endothermic and carbon-rich reaction. It exhibits the reactions of methane decomposition and CO disproportionation, and causes carbon deposit on catalyst and deactivation of catalyst at low temperature [23,24]. BDG has a wide range of CO2/CH4 molar ratio; thus, the surface reaction of the catalyst may be terminated by the carbon deposition if the operating parameters are not well set. Most studies on dry reforming investigated the problems of carbon formation by water addition, and the simulated gas feeding ratio of CH4/CO2 was set as 1.5. Effendi et al. [25] studied biogas reformation with a Ni/Al2O3 catalyst. Their results indicated that considerable carbon deposition occurred under steam/biogas molar ratio less than 0.4. Although steam addition may prevent carbon formation, it reduced CO2 conversion. Kolbitsch et al. [26] investigated biogas reformation with a NiO/CaOeAl2O3 catalyst. They demonstrated that optimal H2 yield might be achieved under the H2O/CH4 ratio of 2.2 and a reactor temperature of 750 C. The negative CO2 conversion occurred when the H2O/CH4 was higher than 3. Araki et al. [27] studied reforming by using model biogas and discovered that the maximal H2 concentration was obtained with H2O/CH4 2 and O2/CH4 ¼ 0.5.
Table 1 e Composition of biomass derived gas. Reference
Hao Ryckebosch Speight Persson Deublein Lai
Gas source
Landfill gas Biogas gas Biogas gas Biogas gas Biogas gas Simulated gas
Compositions (Vol. %)
Source term
CH4
CO2
N2
CO
H2
H2S
O2
NH3
50 40e75 50e75 53e70 45e75 50e75
45 15e60 25e50 30e47 25e55 25e50
5 0e2 0e10 0e1 0e5 e
e <0.6 e e 0e0.2 e
e e 0e1 0 0.5 e
21 ppm v 0e2 0e3 0e1 0e1 e
0e1 0e1 0e2 0 0e2 e
e <1 e e e e
[1] [4] [5] [6] [7] This study
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Avraam et al. [28] studied the steam reforming of biogas with a 5% Ru/Al2O3 catalyst by experimental method and theoretical calculation. Optimal results were obtained with the parameters of the CH4/CO2 molar ratio between 1.0 and 1.5, the H2O/CH4 molar ratio between 3.0 and 5.0, and a reaction temperature approximately around 700e800 C. Moreover, Muradov et al. [29] investigated BDG reforming by various noble metals (Ru, Ir, Pt, Rh, Pd), and compared the results with those by Ni catalyst under a feeding ratio of CH4/CO2 set at 1.3. The Ru (0.5 wt.%)/Al2O3 and Ir (1.0 wt.%)/Al2O3 demonstrated high activity and selectivity, and no carbon deposition was detected. In addition, Pd and Rh exhibited high activity on methane decomposition reaction, and carbon deposition was observed in the reforming. The analysis of XRD and SEM indicated that the deactivation of Ni catalyst was caused by a large number of graphitic-type carbons on the catalyst surface, and NiO was reduced to metallic Ni. Several advantages were revealed in BDG reforming for H2rich synthesis gas production. A feasible concept was proposed in this study, as shown in Fig. 1. BDG of identified composition was fed to a fuel reformer, and a pre-reforming reaction was applied to produce H2-rich syngas. The syngas was subsequently fed back to the original fuel supply system for co-fire with BDG. The syngas could be used as the fuel for engine, fuel cell or combustor to extend the flammability limit and maintain the combustion stability for reducing the incomplete combustion and decreasing the carbon deposition. The aim of energy recirculation was to use waste heat and exhaust gas, namely CO2-rich gas, which may improve overall energy utilizing efficiency. However, dry reforming is a carbon-rich and strong endothermic reaction. The main concern of this study was to prevent carbon deposition and to provide the heat required for self-sustaining reaction. Although water addition may efficiently reduce carbon deposition on the catalyst surface, it requires a large amount of latent heat of evaporation; that is, water addition increases energy consumption and decreases the conversion of carbon dioxide [25,26]. The molecule of carbon dioxide is a doublebond structure; therefore, bond breaking is difficult. It must provide an external heat to supply the required energy for reaction. Moreover, the active sites of the catalyst were sintered and aggregated easily while the temperature distribution was inhomogeneous. Further reviewing some literature for DATR, it was found that most of the studies were carried out with an external heat source; and some researchers
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conducted the study by theoretical equilibrium calculations [30,31]. The method adopted in this study was different from those performed by other researchers. This study proposed a concept of syngas production by dry autothermal reforming from BDG. BDG has a wide range of composition, and is a carbon-rich fuel, which provokes carbon deposition on the catalyst in a dry reforming process. The dry autothermal reforming may control the reaction temperature with CO2/CH4 and O2/CH4 ratios, and which provide the oxidant to prevent carbon from covering the active site of the catalyst. Consequently, methane conversion efficiency may be controlled by oxygen addition, namely O2/CH4 molar ratio. The heat produced in the exothermic reaction can preheat the reactant to raise intake enthalpy. Dry autothermal reforming can not only provide the thermal energy required for a self-sustaining reaction, but also avoid coke formation. Therefore, the energy usage efficiency could be improved by this design. This study focused on the parametric investigation for dry autothermal reforming under various CO2/CH4 and O2/CH4 ratios.
2.
Relative calculations
Eq. (1) is the theoretical reaction while Eq. (2) is the actual reaction equation. a is the stoichiometric factor in the theoretical reaction; a0, b, a, b, d, e, f, g, h and j are stoichiometric factors in the actual reaction, and A is the proportionality constant, respectively. CH4 þ aCO2 þ
1a ðO2 þ 3:76N2 Þ/2H2 þ ð1 þ aÞCO 2
þ 1:88ð1 aÞN2
(1)
a ¼ 0ðPartial oxidationÞ a ¼ 0w1ðDry autothermal reformingÞ a ¼ 1ðDry reformingÞ CH4 þ a0 CO2 þ bðO2 þ 3:76N2 Þ/AðaCO2 þ bCH4 þ dCO þ eH2 þ f N2 þ gO2 Þ þ hH2 O þ jC
(2)
The energy loss percentage was calculated by Eq. (3). The CO2 in oxidation (U) was calculated by Eq. (4). Selectivity of CO was calculated by Eq. (5), while selectivity of H2 was calculated by Eq. (6).
Fig. 1 e The feasible concept of syngas production via reforming for enhancing energy efficiency.
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Energy loss percentage ¼ U¼
absðDHÞ LHVSimulated fuel
100%
V_ CO2 Volume flow rate of CO2 in the oxidizer ¼ _ Total volume flow rate of oxidizer VO2 þ V_ CO2
(3)
(4)
SCO ¼
Moles of CO formed 100% Moles of CH4 consumed
(5)
SH2 ¼
2 Moles of H2 formed 100% 4 Moles of CH4 consumed
(6)
In Eq. (3), LHVSimulated fuel is the lower heating value of simulated BDG; and DH represents the enthalpy of reaction. In Eq. (4), V_ CO2 is the volume flow rate of CO2 and V_ O2 þ V_ CO2 represents the total volume flow rate of oxidizer. The theoretical equilibrium composition was calculated by using the commercialized HSC Chemistry software (ªChemSW Software, Inc.). The results of the experiments and the theoretical calculations were compared. For this research, a new parameter, namely A/I molar ratio, was defined in Eq. (7). The A/I molar ratio of gas phase indicates the molar ratio of combustion assisting gases to combustion impeding gases. Ag =Ig ¼
n_ H2 þ n_ CO þ n_ CH4 n_ CO2 þ n_ N2
3.
Experimental set-up and method
3.1.
Experimental set-up
(7)
The experimental apparatus included a reforming unit, reactant supply system, data acquisition system, gas sampling/analyzing system. The schematic of experimental arrangement is shown in Fig. 2. The reforming unit was
composed of the nozzle of reactants, mixing chamber, catalyst reaction bed, arc generation device and products collection zone. In order to enhance the mixing of reactants and reduce the uniformity, the gas phase reactants were induced by the tangential flow into the reaction chamber to form the swirling flow. Furthermore, the intake swirling flow with the sudden-expansion design in mixing chamber was favorable to the mixing of reactants, and it could avoid the formation of carbon-rich and hot spot zones in the reforming zone. Some researchers indicated noble metals had high activation and decoking ability, and facilitated the CO2 reduction reaction [21,24,29]. Therefore, in this study, the catalyst reaction bed was made by PteRh based noble metals, coated over the washcoat (CeO2eAl2O3) on metallic monolith (FeeCreAl). The detailed specifications of catalyst and surface morphology are shown in Table 2. The substrate of the catalyst was made of metallic monolith with low pressure drop, high strength and high thermal conductivity. It was favorable to the transient response and cold starting. Ignition device includes spark plug (NGK CR8EGP) and DC power supply (Glassman EQ series). The power supply was used to provide the power of igniter to trigger the reforming during cold start, and it was shut down while the reforming launched. Gas collection zone was used to collect the products for analysis. In order to separate the phases of gas and liquid, a cooling unit with a fin-and-tube heat exchanger was used to cool the stream down to room temperature. After cooling process, the residual moisture was absorbed by drying-agent (SiO2) to prevent the liquid phase species to pollute the analyzer. A mass flowmeter (Brooks Instrument Model 5850S, made in USA) was used to determine the feeding rate of the reactants. A gas chromatograph (Agilent GC-6850) was used to determine the gas species; an emission analyzer (Horiba MEXA-584L) was used for the real time monitoring of the gas emission, such as
Fig. 2 e Schematic of experimental arrangement.
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Table 2 e Specifications of catalyst. Items
Surface morphology
Catalyst Washcoat Monolith material Monolith geometry mm2 Cell per square inch (CPSI) BET surface area (m2/g) BJH desorption pore size (nm)
Pt-Rh based CeO2eAl2O3 FeeCreAl j48.6*L50 100 139.53
18.58
CO, CO2, O2.etc. A data acquisition system was used to record the catalyst temperature, reactant feeding rate and real time concentration of products. The temperatures in the upstream, middle-stream and downstream inside the monolithic catalysts were measured by K-type thermocouples. The mounting locations of thermocouples are shown in Fig. 2.
3.2.
Experimental method
This study focused on reforming performance of dry autothermal reforming, using the main composition of simulated biomass derived gas, including CH4 and CO2. In the reforming process, it provided the thermal energy required for carbon dioxide reduction reaction from the heat release of oxidation, and the selectivity of H2-rich syngas was improved with the surface reaction of catalysis. The commercialized HSC Chemistry software (ChemSW Software, Inc.) was used to calculate the equilibrium production. Fig. 3 shows the relationship of CO2/CH4 molar ratio and theoretical equilibrium molar yield of products under various reaction temperature for DR reaction, which could provide the information for the parameter setting of DATR reaction. The temperature was set at 600, 800 and 1000 C, respectively. Because the low methane conversion efficiency was obtained at 600 C, hence H2 and CO yields were insignificant. From theoretical calculation, the carbon free region and high syngas production could be achieved when CO2/CH4 ratio is 1.0 at high temperature conditions (T 1000 C). However, the dry reforming is a strong endothermic reaction, and it needs to provide the heat for reaction. When CO2/CH4 ratio was set between 1.0 and 2.0, the methane and the carbon dioxide were not effectively reformed. Nevertheless, the CO2 could be converted into CO by the reverse water-gas shifting reaction. Moreover, coke formation was found under CO2/CH4 molar ratio between 1.0 and 2.0. So, it must supply the suitable oxygen to raise the carbon monoxide yield, and thereby increase the yield of synthesis gas. Fig. 4 shows enthalpy of reaction of methane reforming under different O2/CH4 ratios. Hollow square (A) represents thermal decomposition of methane, being an endothermic reaction. During the decomposition reaction, it could precipitate a large number of carbon atoms because of lacking of oxidant and reducer in the reaction. Hollow squares (B and C) represent partial oxidation of methane. Heat of oxidation was
Fig. 3 e CO2/CH4 molar ratio on the theoretical equilibrium molar yield of products under various reaction temperatures.
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In light of the above, the operating parameters of experiments were set as methane feeding rate of 10 NL/min, the CO2/CH4 molar ratio between 0.0 and 1.0 and the O2/CH4 molar ratio between 0.5 and 1.0. The reforming of BDG was carried out under the condition of atmospheric pressure.
Fig. 4 e Enthalpy of reaction of carbon dioxide reforming with methane under different O2/CH4 ratios.
released in the reaction. Solid sphere (D) is dry reforming with methane, which is a strong endothermic reaction. Although dry reforming reaction could increase syngas production, it requires external heat source provided. Solid balls (E and F) represent the dry autothermal reforming, which has the merits of partial oxidation and dry reforming reaction. The heat required for CO2 reduction was supplied by the heat of methane oxidation, and no external heat source was needed. However, the reaction gives a low heat of reaction because of the high enthalpy of formation of carbon dioxide. Consequently, O2/CH4 ratio must be set above 0.5 to reach the exothermic reaction. In this study, the parameters were set in the range of BeCeFeE. From the theoretical equilibrium calculation, as shown in Fig. 3, the dry reforming has the problem of carbon formation under CO2/CH4 between 0.0 and 1.0. Therefore, oxygen was required to supply to avoid coking problem. Furthermore, the reactions of the dry reforming are very complex, such as reverse water-gas shifting, CO2 methanation, CO2 decomposition, Boudouard reaction.etc. Relative chemical reaction and enthalpy of reaction are shown in Table 3. Table 1 shows the main composition and concentration of the simulated biomass derived gas. The CO2 concentration was between 25 and 50%, and methane in balance. The CO2/CH4 molar ratio was set between 0.33 and 1.0.
4.
Results and discussion
4.1.
Effect of reaction gas temperature
The enthalpy of reaction indicates that raising the O2/CH4 ratio may release corresponding energy through oxidation reaction to provide the necessary energy for the reforming process. As the catalysis of a catalyst is dominated by surface reactions, this study needs to find a method for providing the necessary energy for reforming on the catalyst surface. The effect of reformate gas temperature on energy loss and methane conversion will be discussed. The relationships between the equilibrium adiabatic temperature, the measured reformate gas temperature, and energy loss percentage are shown in Fig. 5(A). The experimental parameters used a methane feeding rate of 10 NL/min, an O2/CH4 ratio between 0.5 and 1.0, and a CO2/CH4 ratio between 0.0 and 1.0. In the figure, each set of symbols consists of six data points and represents from left to right of the X-axis, the O2/CH4 molar ratio of 0.5, 0.6, 0.7, 0.8, 0.9 and 1.0, respectively. The average gas temperature of the whole catalyst section is defined as the reformate gas temperature. The range of reformate gas temperature in the experiments is from 650 to 900 C. The result shows the equilibrium adiabatic temperature is a highly linear relationship with reformate gas temperature, with more pronounced differences at higher temperatures. This is mainly due to the heat loss through the wall of the reactor as the temperature gradient between the catalytic reaction zone and the surrounding environment increases at higher temperatures. According to the regression of measured data, it indicated that the reformate gas temperature had a highly linear relationship with energy loss percentage. The photo in Fig. 5(B) illustrates the phenomenon of heat dissipation through the wall in the reforming. Because of the reactor was not insulated, the higher catalyst bed temperature caused higher heat loss percentage. Total energy loss percentage lies between 15 and 30% under this set of parameters. Heat loss can be divided into the heat lost through the wall surface, and the sensible heat carried away by the gaseous products. Energy loss percentage was defined in Eq. (3).
Table 3 e Chemical equations of the reforming reactions and their associated enthalpy of reaction. Reaction CO2 reforming of CH4 Reverse water-gas shifting Boudouard reaction CO2 decomposition CO2 methanation Partial oxidation Complete oxidation
Chemical equation
Enthalpy of reaction (kJ/mol-NTP)
CO2(g) þ CH4(g) / 2CO(g) þ 2H2(g) CO2(g) þ H2(g) / CO(g) þ H2O(g) CO2(g) þ C(s) / 2CO(g) CO2(g) / CO(g) þ 0.5O2(g) CO2(g) þ 4H2(g) / CH4(g) þ 2H2O(g) CH4(g) þ 0.5O2(g) / CO(g) þ 2H2(g) CH4(g) þ 2O2(g) / CO2(g) þ 2H2O(g)
247.3 41.2 172.5 282.9 165.0 35.6 802.2
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Fig. 5 e (A) Relationship of the equilibrium adiabatic temperature with reformate gas temperature and energy loss percentage under varying reforming parameters. (B) Observation at outer surface of reactor wall.
Fig. 6 illustrates the reformate gas temperature on the methane conversion efficiency under the well regulated feeding ratio. In figure, each set of symbols consists of six data points and represents from left to right of the X-axis, the oxygen to carbon ratio of 0.5, 0.6, 0.7, 0.8, 0.9 and 1.0, respectively. Theoretically, better methane conversion would be achieved at higher reaction temperature. From a physical implication, the energy required for DATR reaction was provided by heat release of oxidation in the reforming process. The higher O2/CH4 molar ratio provokes the corresponding higher reformate gas temperature, and thus it is helpful to improve the methane conversion efficiency. If the higher CO2/ CH4 ratio is adopted, the endothermic reaction may result in
the lower temperature in the reforming. It is inauspicious to methane conversion efficiency. Fig. 7 shows that the equilibrium constant (kp) of methane oxidation is far higher than the reductive reaction between methane and CO2, or the reformation reaction between methane and steam. The partial oxidation reaction of methane could remain its self sustainable reaction under the experimented parameters, and next followed the reformation of methane and steam. The reformation of methane with CO2 occurs only at a high temperature range. Thus, O2/CH4 ratio is the dominant factor in the dry autothermal reforming. A high O2/CH4 ratio implies a high temperature in the reforming process. From the experimental results, it shows that the methane conversion efficiency is influenced by reformate gas temperature [32,33]. That is, the
120
100
O
110
80
100
60
90
40
80
20
70
0
0
0.33 0.5 1.0 0.5 0.6 0.7 0.8 0.9 1.0
60 50 40 30 600
lnk
Methane conversion efficiency (%)
130
650
700
750
800
850
900
950
1000
0
Reformate gas temperature ( C)
CH4+0.5O2 = CO+2H2 CH4+H2O = CO+3H2 CH4+CO2 = 2CO+2H2
Partial Oxidation
-20
Steam reforming
-40 -60
CO2 reforming
-80 -100 5
10
15
20
25 4
Fig. 6 e Effect of reformate gas temperature on methane conversion efficiency under various O2/CH4 and CO2/CH4 ratio.
30
35
40
-1
1/Tx10 /K
Fig. 7 e Relationship of side reaction and chemical equilibrium constant under varying reaction temperature.
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Effect of CO2 in oxidizer
Oxygen addition promotes the oxidation reaction and heat release, which provides the heat required in dry autothermal reforming. However, it also generates CO2. The oxidizer is comprised of both oxygen and CO2 in this study. Calculation of CO2 in oxidizer is shown in Eq. (4). The effect of CO2 concentration in oxidizer on the reforming performance will be discussed. Fig. 8 describes the effect of CO2 concentration on product molar fractions. The product consists of hydrogen, carbon monoxide, carbon dioxide, methane, water vapor, and nitrogen. The reaction without CO2 addition in the reactants represents the methane partial oxidation reforming, which gives good syngas yield. As CO2 concentration increases, the molar fraction of CO in the products increases initially, then it dips down and the syngas production decreases due to the dilution effect caused by excessive CO2. Fig. 9 shows the CO2 concentration on the syngas selectivity. As the CO2 concentration increases, a more effective reverse water-gas shifting reaction linearly increases CO selectivity. Considering both selectivity and yield, a 30% CO2 concentration in the oxidizers shows an effective reverse water-gas shifting reaction. If CO2 concentration in oxidizer is higher than 50% and O2/CH4 molar ratio between 0.7 and 0.8, 100% CO selectivity could be achieved. However, if the CO2 concentration becomes excessive, it will only dilute the syngas and reduce products usability. Calculations of selectivity for H2 and CO are shown in Eqs (5) and (6). Fig. 10 demonstrates the relationship between fuel input low heating value (LHV) power and syngas output LHV power. The input LHV power comes from the methane consumed, and the syngas output LHV power is derived from H2 and CO. The energy of reaction in the reforming process can be divided into chemical energy and thermal energy. Chemical energy includes the heating value from H2 and CO. Thermal energy includes the sensible heat in the stream and heat loss through
110
Methane feeding rate = 10 L/min 105 Operation pressure = Atmospheric
105
100
100
95
95
90
90
85
0.5 0.6 0.7
80
70
70 65
POX
60
60
55
55 -10
0
10
20
30
40
50
60
70
CO2 in oxidizer (vol. %) Fig. 9 e Relationship of CO2 in oxidizer and products selectivity under varying O2/CH4 and CO2/CH4 ratio.
the wall surface from the oxidation reaction. Under the experimented conditions, roughly 70% of total input energy consists of chemical energy; the rest is mostly heat loss in the oxidation reaction. This graph corresponds to the heat loss percentage shown in Fig. 5(A). Greater input energy leads to more heat from produced gas flow; and the greater temperature gradient between the outer wall of the reaction zone and the surrounding environment leads to a relatively significant loss via convection. Viewed together, the ratio of H2 and CO in chemical energy can be controlled by reforming parameters. A higher CO2/CH4 ratio lowers the LHV power fraction of hydrogen in the produced gases. A higher hydrogen LHV fraction is obtained for the condition without CO2 addition, namely CO2/CH4 ¼ 0. Increasing the CO2/CH4 ratio lowers hydrogen yield. This is because the CO2 is converted into CO and H2O with H2 via the reverse water-gas shifting reaction. That is, controlling the CO2/CH4 ratio would modulate the
Output LHV power of syngas (kW)
Molar fraction of products (mole/mole)
Methane feeding rate = 10 L/min Operation pressure = Atmospheric O2/CH4=0.5~1.0, CO2/CH4=0, 0.33, 0.5, 1.0
POX 0.25
Hydrogen
0.20
0.15
Carbon monoxide
0.10
0.05
Carbon dioxide
0.00
Methane feeding rate = 10 L/min 5.0 O /CH = 0.5~1.0, CO /CH = 0, 0.33, 0.5, 1.0 Catalyst = Pt/Rh based (Monolith) Energy 4.5 Operation pressure = Atmospheric loss rate 4.0
0
0.5 0.6 0.7 0.8 0.9 1.0
3.5 3.0 2.5 2.0
0
10
20
30
40
50
60
CO2 in oxidizer (vol. %) Fig. 8 e Effect of CO2 in oxidizer on the molar fraction of products under varying reforming parameters.
70
5.5 5.0 4.5
region
0.33 0.5 1.0
4.0 3.5 3.0 CO region
2.5
LHV(syngas)
2.0
R =0.9645
1.5
1.5 H region
1.0
1.0
0.5 -10
80 75
5.5 0.30
85
0.8 0.9 1.0
75
65
Hydrogen selectivity (%)
4.2.
110
Output LHV power of H2 (kW)
reaction temperature was affected by both of O2/CH4 and CO2/ CH4 ratios.
Carbon momoxide selectivity (%)
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0.5 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
Input LHV power of fuels (kW) Fig. 10 e Relationship between input LHV power of fuels and output LHV power of syngas.
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H2/CO ratio in the products, and does not affect the total power output significantly.
Gaseous products in a dry autothermal reforming reaction mainly consist of hydrogen, carbon monoxide, carbon dioxide, nitrogen, and unconverted methane. Among these, H2 and CO contain good heating value. Hydrogen also has a high flame speed and can assist combustion under the conditions of highly diluted gas concentration. Even though the CO2, H2O, and N2 in the reformed product do not provide heating value, they have the effect like an exhaust gas recirculation (EGR) for engine. These gases effectively reduce the high temperature in combustion and suppress the generation of thermal NOx under high temperature condition. Hydrogen also benefits in its short quenching distance, which enhances combustion efficiency and lowers the ratio of unburned gas from the combustion chamber. With regard to these advantages, here the effect of feeding ratio on H2 þ CO syngas concentration, and the relationship between A/I ratio and H2/ CO ratio in reforming process will be investigated. Fig. 11(A) indicates the influence of O2/CH4 ratio on H2 þ CO concentration under different CO2/CH4 ratios. A higher O2/CH4 ratio raises the overall reforming gas temperature and promotes fuel conversion into syngas. An excessively high CO2/CH4 ratio dilutes syngas concentration, as additional CO2 can not join reduction reaction effectively. Thus, syngas concentration is dominated by the O2/CH4 and CO2/CH4 ratios. Overall syngas concentration range lies within 20%e40% under the experimented conditions. Fig. 11(B) shows the effect of O2/CH4 ratio on A/I ratio under different CO2/CH4 ratios. The assisting/impeding (A/I ) combustion composition ratio is calculated by Eq. (7). In this equation, the combustible gas from fuel reforming are the numerator, such as H2, CO, CH4, and the gases which suppress the high temperature of combustion to reduce NOx generation are the denominator, such as CO2, N2. Generally, the A/I ratio decreases with the increase of both the O2/CH4 and CO2/CH4 ratios. Overall A/I ratio falls within a range of 0.43e0.84. Different CO2/CH4 ratios produce different corresponding ranges of A/I ratios. Lower O2/CH4 molar ratios give larger operation ranges. This result gives the parameter setting of reforming for matching the different operating conditions of engine. In the future applications, the combustion efficiency and exhaust emissions of engines can be improved with the compromised A/I ratio, and the operating parameters obtained in this study. Fig. 11(C) shows the influence of O2/CH4 ratio on the produced H2/CO ratio under different CO2/CH4 ratios. The applications of H2/CO ratio can be classified as either combustion or the production of chemical material by synthesis. If it is used in combustion, both hydrogen and CO increase the flammability limit and stabilize the combustion. Among them, hydrogen can be used to raise the overall flame propagation speed, while CO can increase the adiabatic flame temperature [12]. However, the applications in chemical product synthesis depend upon the suitable H2/CO ratio needed in further reactions. Generally, H2/CO ratios controlled to within 1.0e2.0 are more useful [34]. From the experimental results, one can find
A 50 H +CO concentration (Vol. %)
Effect of O2/CH4 on performance index
Methane feeding rate: 10 L/min Pressure = Atmospheric 40 CO /CH =0, 0.33, 0.5, 1
CO CO CO CO
/CH /CH /CH /CH
=0 = 0.33 = 0.5 =1
30
20 10 0 0.5
0.6
0.7 0.8 0.9 O /CH molar ratio
1.0
H2+CO concentration B Assisting / Impeding molar ratio
4.3.
1.00 Methane feeding rate: 10 L/min
0.75
0.50
0.25
0.00 0.5
0.6
0.7
0.8
0.9
1.0
0.9
1.0
A/I ratio C
3.5 Methane feeding rate: 10 L/min
3.0 2.5 2.0 1.5 1.0 0.5 0.0 0.5
0.6
0.7
0.8
H2/CO ratio Fig. 11 e Effect of O2/CH4 ratio on H2 D CO concentration, A/ I ratio and H2/CO ratio under varying CO2/CH4 ratio.
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that high H2/CO ratio is obtained by partial oxidation reforming, but it is not conducive to chemical synthesis. Adding proper CO2 in the reactants to aid the reforming reaction, with the CO2/CH4 ratio ranging between 0.0 and 1.0, has been proven to effectively control the H2/CO ratio between 1.0 and 2.0. This ratio is favorable for the production of chemical product by synthesis. The ratio of H2/CO is easily controlled by adjusting CO2/CH4 ratio; the influence of O2/CH4 ratio is insignificant.
5.
Conclusions
The parametric design and hydrogen-rich syngas production by dry autothermal reforming with BDG were investigated in this study. The reforming parameters included fuel feeding rate, and CO2/CH4 and O2/CH4 molar ratios. A series of experiments were conducted for hydrogen-rich gas production. The conclusions are as follows: 1. The theoretical calculation indicated that a reaction temperature higher than 1000 C may avoid coke formation for dry reforming (CO2/CH4 molar ratio ¼ 1). Therefore, CO2/ CH4 ratio should be higher than the coking boundary, or oxygen should be introduced to prevent the coking phenomenon. 2. The results also indicated that good correlations occurred among the reformate gas temperature, energy loss percentage, and equilibrium adiabatic temperature. The energy loss percentage increases approximately linearly with the gradient of the reformate gas and environment temperatures. Under the reforming parameter setting, the reformate gas temperature was between 650 and 900 C, and energy loss percentage was between 15 and 30%. 3. The important findings including: it could provide the energy required for a self-sustaining reaction in DATR; and carbon deposition could be avoided with the appropriate addition of oxidants. In addition, the O2/CH4 ratio was used to control the syngas yield and thereby regulated the heating value of the syngas. The lower percentage of hydrogen was obtained with a higher CO2/CH4 ratio because of the reverse water-gas shifting reaction, which was favorable to CO selectivity. 4. For dry autothermal reforming, methane conversion was dominated by reaction temperature. High CO2 concentration affected the overall heat release of oxidation, thereby reducing the reformate gas temperature. Although a high CO2/CH4 ratio decreased the A/I ratio and syngas concentration, it was helpful to increase CO selectivity. In addition, the H2/CO molar ratio in the reformate gas between 1 and 2 was achieved by adjusting the CO2/CH4 molar ratio. However, syngas concentrations were affected by both the CO2/CH4 and O2/CH4 molar ratios.
Acknowledgments The authors are grateful to the support of the National Science Council of Taiwan under grant number NSC 100-2221-E-168038-MY2.
references
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Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Study on hydrogen-rich syngas production by dry autothermal reforming from biomass derived gas Wei-Hsiang Lai a,b, Ming-Pin Lai a, Rong-Fang Horng c,* a
Department of Aeronautics and Astronautics, National Cheng Kung University, No.1, University Road, Tainan City 701, Taiwan Research Center for Energy Technology and Strategy, National Cheng Kung University, No.1, University Road, Tainan City 701, Taiwan c Department of Mechanical Engineering, Clean Energy Center, Kun Shan University, No. 949, Da-Wan Road, Yung-Kang District, Tainan City 710, Taiwan b
article info
abstract
Article history:
In this study, the H2-rich syngas (H2 þ CO) production from biomass derived gas (BDG) by
Received 17 December 2011
dry autothermal reforming (DATR) is investigated. Methane and carbon dioxide is the
Received in revised form
major composition of biomass derived gas. DATR reaction combined benefits of partial
5 March 2012
oxidation (POX) and dry reforming (DR) reaction was carried out in this study. The
Accepted 16 March 2012
reforming parameters on the conversion of methane and syngas selectivity were explored.
Available online 14 April 2012
The reforming parameters included the fuel feeding rate, CO2/CH4 and O2/CH4 molar ratios. The experimental results demonstrated that it not only supplied the energy required for
Keywords:
self-sustained reaction, but also avoided the coke formation by dry autothermal reforming.
Carbon dioxide
It has a wide operation region to maintain the moderate production of the syngas. During
Dry autothermal reforming
the reforming process, the reformate gas temperature was between 650 and 900 C, and
Syngas production
energy loss percentage in reforming process was between 15 and 30%. Further, high CO2
Reverse wateregas shifting
concentration in the reactant had a considerable influence on the heat release of oxidation, and thereby decreased the reformate gas temperature. It caused the reduction of synthesis gas concentration and assisting/impeding combustion composition (A/I ) ratio. However, it was favorable to CO selectivity because of the reverse water-gas shifting reaction. The H2/ CO molar ratio between 1 and 2 was achieved by varying CO2/CH4 molar ratio. However, the syngas concentrations were affected by CO2/CH4 and O2/CH4 molar ratio. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Increasing energy usage efficiency and reducing exhaust emissions have recently become the worldwide focus because of environmental protection demands and energy shortage. To confront these problems, research in alternative fuels and low carbon technology may be the focal topics in the future. Biomass derived gas (BDG), including biogas, landfill, and digester, is a valuable source of alternative
clean energy. Recently published papers related to energy policy and sustainable energy indicated that several countries are promoting low carbon energy from biomass derived gas [1,2]. Applications of biomass derived gas were gradually expanded, such as direct burning, chemical synthesis, and electricity generation [3]. If managed properly, BDG may be a considerable source of alternative and renewable energy and could be helpful to achieve greenhouse gas reduction.
* Corresponding author. Fax: þ886 6 2050509. E-mail address: [email protected] (R.-F. Horng). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2012.03.076
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BDG was made from an anaerobic fermentation process using anaerobic decomposition with microorganisms. Sources of BDG were abundant, such as livestock waste, agricultural residues, domestic sewage, and landfill. BDG is an alternative energy, and it also reduces the cost of sewage treatment and waste management problems. The composition of BDG is complicated, including CH4, CO2, CO, H2, H2O, H2S, HCs, NH3, and N2 [4], which were determined by environmental conditions and waste sources. A typical composition of BDG is shown in Table 1 [1,4e7]. In addition to CO2 and N2, most of the composition in BDG would corrode the components of the combustion chamber, such as H2S, HCs, and NH3. Thus, if BDG is directly applied to a combustor or solid oxide fuel cell (SOFC), it may damage the equipment and increase maintenance costs. Shiratori et al. [8] indicated that the electrochemical reaction with 1 ppm H2S caused an approximately 40% decrease in reaction rate of dry reforming and approximately 9% voltage drop in SOFC. The deactivation of catalyst on anode may be caused by coke formation with unsuitable CO2/CH4 ratio [9]. The coke formation is resulted from the deactivation of catalyst by the chemisorption of impurities. Coke formation would adhere on the catalyst surface and cover the active site [10,11]. BDG combustion will damage the combustor components because of corrosion. In addition, both the reactant heating value and flame propagation speed will decrease with a high concentration of CO2. Liu et al. [12] stated that methane mixed with carbon dioxide will decrease the laminar flame speed and adiabatic flame temperature, by calculating with GRI-Mech3.0. In addition, the adiabatic flame temperature was raised with high CO concentration, and laminar flame speed was increased considerably with high H2 concentration in the syngas. The heating value of fuel and flame propagation speed may be reduced with the dilution gas, such as CO2, in BDG. Therefore, literature data indicated that the reaction rate and the lean flammability limit were improved by adding hydrogen to BDG as the auxiliary fuel of engines. The addition of hydrogen-rich gas in engine not only increased brake thermal efficiency, but also improved the cycle-to-cycle variation and the exhaust emissions under fuel lean conditions [13]. The peak pressure and the heat release rate of the engine may be increased with the addition of hydrogen, and thus, the ignition timing must be retarded to prevent the knock. The addition of hydrogen up to 10% in BDG, it may improve the performance and reduce exhaust emissions of the engine [14]. BDG is produced mainly from agricultural waste and landfill. It contains a large amount of greenhouse gases, such
as CH4 and CO2; therefore, dry reforming is the suitable approach for fuel processing to produce hydrogen. If BDG is used properly and converted into syngas, not only would it increase the overall energy usage, but also decrease greenhouse gases substantially. The produced H2/CO ratio is close to 1.0 in reformate gas by dry reforming with methane and CO2. This is the advantage of this fuel processing approach and is usually used for the synthesis of high-value chemicals and fuels [15,16]. Furthermore, syngas, which contains the main composition of H2 and CO, has a wider flammable range. It may extend the lean limit and maintain flame stability if it fuels the internal combustion engine and burner; hence, the fuel conversion and exhaust emissions would be improved [17,18]. Fisher and Tropsch initiated the investigation on dry reforming by methane with CO2 [19]. However, it is a high temperature and carbon-rich reaction. Deactivation of the catalyst caused by sintering and coking inhibited commercialization development. Recently, the need in greenhouse gas reduction, dry reforming has become a renewed interest for CO2 reforming, and a considerable number of studies focused on finding suitable catalyst, including cost, activation, and selectivity of the catalyst for the reforming [20], further improving the coke resistance ability of catalyst and preventing the sintering [21,22]. Dry reforming is a strong endothermic and carbon-rich reaction. It exhibits the reactions of methane decomposition and CO disproportionation, and causes carbon deposit on catalyst and deactivation of catalyst at low temperature [23,24]. BDG has a wide range of CO2/CH4 molar ratio; thus, the surface reaction of the catalyst may be terminated by the carbon deposition if the operating parameters are not well set. Most studies on dry reforming investigated the problems of carbon formation by water addition, and the simulated gas feeding ratio of CH4/CO2 was set as 1.5. Effendi et al. [25] studied biogas reformation with a Ni/Al2O3 catalyst. Their results indicated that considerable carbon deposition occurred under steam/biogas molar ratio less than 0.4. Although steam addition may prevent carbon formation, it reduced CO2 conversion. Kolbitsch et al. [26] investigated biogas reformation with a NiO/CaOeAl2O3 catalyst. They demonstrated that optimal H2 yield might be achieved under the H2O/CH4 ratio of 2.2 and a reactor temperature of 750 C. The negative CO2 conversion occurred when the H2O/CH4 was higher than 3. Araki et al. [27] studied reforming by using model biogas and discovered that the maximal H2 concentration was obtained with H2O/CH4 2 and O2/CH4 ¼ 0.5.
Table 1 e Composition of biomass derived gas. Reference
Hao Ryckebosch Speight Persson Deublein Lai
Gas source
Landfill gas Biogas gas Biogas gas Biogas gas Biogas gas Simulated gas
Compositions (Vol. %)
Source term
CH4
CO2
N2
CO
H2
H2S
O2
NH3
50 40e75 50e75 53e70 45e75 50e75
45 15e60 25e50 30e47 25e55 25e50
5 0e2 0e10 0e1 0e5 e
e <0.6 e e 0e0.2 e
e e 0e1 0 0.5 e
21 ppm v 0e2 0e3 0e1 0e1 e
0e1 0e1 0e2 0 0e2 e
e <1 e e e e
[1] [4] [5] [6] [7] This study
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Avraam et al. [28] studied the steam reforming of biogas with a 5% Ru/Al2O3 catalyst by experimental method and theoretical calculation. Optimal results were obtained with the parameters of the CH4/CO2 molar ratio between 1.0 and 1.5, the H2O/CH4 molar ratio between 3.0 and 5.0, and a reaction temperature approximately around 700e800 C. Moreover, Muradov et al. [29] investigated BDG reforming by various noble metals (Ru, Ir, Pt, Rh, Pd), and compared the results with those by Ni catalyst under a feeding ratio of CH4/CO2 set at 1.3. The Ru (0.5 wt.%)/Al2O3 and Ir (1.0 wt.%)/Al2O3 demonstrated high activity and selectivity, and no carbon deposition was detected. In addition, Pd and Rh exhibited high activity on methane decomposition reaction, and carbon deposition was observed in the reforming. The analysis of XRD and SEM indicated that the deactivation of Ni catalyst was caused by a large number of graphitic-type carbons on the catalyst surface, and NiO was reduced to metallic Ni. Several advantages were revealed in BDG reforming for H2rich synthesis gas production. A feasible concept was proposed in this study, as shown in Fig. 1. BDG of identified composition was fed to a fuel reformer, and a pre-reforming reaction was applied to produce H2-rich syngas. The syngas was subsequently fed back to the original fuel supply system for co-fire with BDG. The syngas could be used as the fuel for engine, fuel cell or combustor to extend the flammability limit and maintain the combustion stability for reducing the incomplete combustion and decreasing the carbon deposition. The aim of energy recirculation was to use waste heat and exhaust gas, namely CO2-rich gas, which may improve overall energy utilizing efficiency. However, dry reforming is a carbon-rich and strong endothermic reaction. The main concern of this study was to prevent carbon deposition and to provide the heat required for self-sustaining reaction. Although water addition may efficiently reduce carbon deposition on the catalyst surface, it requires a large amount of latent heat of evaporation; that is, water addition increases energy consumption and decreases the conversion of carbon dioxide [25,26]. The molecule of carbon dioxide is a doublebond structure; therefore, bond breaking is difficult. It must provide an external heat to supply the required energy for reaction. Moreover, the active sites of the catalyst were sintered and aggregated easily while the temperature distribution was inhomogeneous. Further reviewing some literature for DATR, it was found that most of the studies were carried out with an external heat source; and some researchers
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conducted the study by theoretical equilibrium calculations [30,31]. The method adopted in this study was different from those performed by other researchers. This study proposed a concept of syngas production by dry autothermal reforming from BDG. BDG has a wide range of composition, and is a carbon-rich fuel, which provokes carbon deposition on the catalyst in a dry reforming process. The dry autothermal reforming may control the reaction temperature with CO2/CH4 and O2/CH4 ratios, and which provide the oxidant to prevent carbon from covering the active site of the catalyst. Consequently, methane conversion efficiency may be controlled by oxygen addition, namely O2/CH4 molar ratio. The heat produced in the exothermic reaction can preheat the reactant to raise intake enthalpy. Dry autothermal reforming can not only provide the thermal energy required for a self-sustaining reaction, but also avoid coke formation. Therefore, the energy usage efficiency could be improved by this design. This study focused on the parametric investigation for dry autothermal reforming under various CO2/CH4 and O2/CH4 ratios.
2.
Relative calculations
Eq. (1) is the theoretical reaction while Eq. (2) is the actual reaction equation. a is the stoichiometric factor in the theoretical reaction; a0, b, a, b, d, e, f, g, h and j are stoichiometric factors in the actual reaction, and A is the proportionality constant, respectively. CH4 þ aCO2 þ
1a ðO2 þ 3:76N2 Þ/2H2 þ ð1 þ aÞCO 2
þ 1:88ð1 aÞN2
(1)
a ¼ 0ðPartial oxidationÞ a ¼ 0w1ðDry autothermal reformingÞ a ¼ 1ðDry reformingÞ CH4 þ a0 CO2 þ bðO2 þ 3:76N2 Þ/AðaCO2 þ bCH4 þ dCO þ eH2 þ f N2 þ gO2 Þ þ hH2 O þ jC
(2)
The energy loss percentage was calculated by Eq. (3). The CO2 in oxidation (U) was calculated by Eq. (4). Selectivity of CO was calculated by Eq. (5), while selectivity of H2 was calculated by Eq. (6).
Fig. 1 e The feasible concept of syngas production via reforming for enhancing energy efficiency.
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Energy loss percentage ¼ U¼
absðDHÞ LHVSimulated fuel
100%
V_ CO2 Volume flow rate of CO2 in the oxidizer ¼ _ Total volume flow rate of oxidizer VO2 þ V_ CO2
(3)
(4)
SCO ¼
Moles of CO formed 100% Moles of CH4 consumed
(5)
SH2 ¼
2 Moles of H2 formed 100% 4 Moles of CH4 consumed
(6)
In Eq. (3), LHVSimulated fuel is the lower heating value of simulated BDG; and DH represents the enthalpy of reaction. In Eq. (4), V_ CO2 is the volume flow rate of CO2 and V_ O2 þ V_ CO2 represents the total volume flow rate of oxidizer. The theoretical equilibrium composition was calculated by using the commercialized HSC Chemistry software (ªChemSW Software, Inc.). The results of the experiments and the theoretical calculations were compared. For this research, a new parameter, namely A/I molar ratio, was defined in Eq. (7). The A/I molar ratio of gas phase indicates the molar ratio of combustion assisting gases to combustion impeding gases. Ag =Ig ¼
n_ H2 þ n_ CO þ n_ CH4 n_ CO2 þ n_ N2
3.
Experimental set-up and method
3.1.
Experimental set-up
(7)
The experimental apparatus included a reforming unit, reactant supply system, data acquisition system, gas sampling/analyzing system. The schematic of experimental arrangement is shown in Fig. 2. The reforming unit was
composed of the nozzle of reactants, mixing chamber, catalyst reaction bed, arc generation device and products collection zone. In order to enhance the mixing of reactants and reduce the uniformity, the gas phase reactants were induced by the tangential flow into the reaction chamber to form the swirling flow. Furthermore, the intake swirling flow with the sudden-expansion design in mixing chamber was favorable to the mixing of reactants, and it could avoid the formation of carbon-rich and hot spot zones in the reforming zone. Some researchers indicated noble metals had high activation and decoking ability, and facilitated the CO2 reduction reaction [21,24,29]. Therefore, in this study, the catalyst reaction bed was made by PteRh based noble metals, coated over the washcoat (CeO2eAl2O3) on metallic monolith (FeeCreAl). The detailed specifications of catalyst and surface morphology are shown in Table 2. The substrate of the catalyst was made of metallic monolith with low pressure drop, high strength and high thermal conductivity. It was favorable to the transient response and cold starting. Ignition device includes spark plug (NGK CR8EGP) and DC power supply (Glassman EQ series). The power supply was used to provide the power of igniter to trigger the reforming during cold start, and it was shut down while the reforming launched. Gas collection zone was used to collect the products for analysis. In order to separate the phases of gas and liquid, a cooling unit with a fin-and-tube heat exchanger was used to cool the stream down to room temperature. After cooling process, the residual moisture was absorbed by drying-agent (SiO2) to prevent the liquid phase species to pollute the analyzer. A mass flowmeter (Brooks Instrument Model 5850S, made in USA) was used to determine the feeding rate of the reactants. A gas chromatograph (Agilent GC-6850) was used to determine the gas species; an emission analyzer (Horiba MEXA-584L) was used for the real time monitoring of the gas emission, such as
Fig. 2 e Schematic of experimental arrangement.
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Table 2 e Specifications of catalyst. Items
Surface morphology
Catalyst Washcoat Monolith material Monolith geometry mm2 Cell per square inch (CPSI) BET surface area (m2/g) BJH desorption pore size (nm)
Pt-Rh based CeO2eAl2O3 FeeCreAl j48.6*L50 100 139.53
18.58
CO, CO2, O2.etc. A data acquisition system was used to record the catalyst temperature, reactant feeding rate and real time concentration of products. The temperatures in the upstream, middle-stream and downstream inside the monolithic catalysts were measured by K-type thermocouples. The mounting locations of thermocouples are shown in Fig. 2.
3.2.
Experimental method
This study focused on reforming performance of dry autothermal reforming, using the main composition of simulated biomass derived gas, including CH4 and CO2. In the reforming process, it provided the thermal energy required for carbon dioxide reduction reaction from the heat release of oxidation, and the selectivity of H2-rich syngas was improved with the surface reaction of catalysis. The commercialized HSC Chemistry software (ChemSW Software, Inc.) was used to calculate the equilibrium production. Fig. 3 shows the relationship of CO2/CH4 molar ratio and theoretical equilibrium molar yield of products under various reaction temperature for DR reaction, which could provide the information for the parameter setting of DATR reaction. The temperature was set at 600, 800 and 1000 C, respectively. Because the low methane conversion efficiency was obtained at 600 C, hence H2 and CO yields were insignificant. From theoretical calculation, the carbon free region and high syngas production could be achieved when CO2/CH4 ratio is 1.0 at high temperature conditions (T 1000 C). However, the dry reforming is a strong endothermic reaction, and it needs to provide the heat for reaction. When CO2/CH4 ratio was set between 1.0 and 2.0, the methane and the carbon dioxide were not effectively reformed. Nevertheless, the CO2 could be converted into CO by the reverse water-gas shifting reaction. Moreover, coke formation was found under CO2/CH4 molar ratio between 1.0 and 2.0. So, it must supply the suitable oxygen to raise the carbon monoxide yield, and thereby increase the yield of synthesis gas. Fig. 4 shows enthalpy of reaction of methane reforming under different O2/CH4 ratios. Hollow square (A) represents thermal decomposition of methane, being an endothermic reaction. During the decomposition reaction, it could precipitate a large number of carbon atoms because of lacking of oxidant and reducer in the reaction. Hollow squares (B and C) represent partial oxidation of methane. Heat of oxidation was
Fig. 3 e CO2/CH4 molar ratio on the theoretical equilibrium molar yield of products under various reaction temperatures.
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In light of the above, the operating parameters of experiments were set as methane feeding rate of 10 NL/min, the CO2/CH4 molar ratio between 0.0 and 1.0 and the O2/CH4 molar ratio between 0.5 and 1.0. The reforming of BDG was carried out under the condition of atmospheric pressure.
Fig. 4 e Enthalpy of reaction of carbon dioxide reforming with methane under different O2/CH4 ratios.
released in the reaction. Solid sphere (D) is dry reforming with methane, which is a strong endothermic reaction. Although dry reforming reaction could increase syngas production, it requires external heat source provided. Solid balls (E and F) represent the dry autothermal reforming, which has the merits of partial oxidation and dry reforming reaction. The heat required for CO2 reduction was supplied by the heat of methane oxidation, and no external heat source was needed. However, the reaction gives a low heat of reaction because of the high enthalpy of formation of carbon dioxide. Consequently, O2/CH4 ratio must be set above 0.5 to reach the exothermic reaction. In this study, the parameters were set in the range of BeCeFeE. From the theoretical equilibrium calculation, as shown in Fig. 3, the dry reforming has the problem of carbon formation under CO2/CH4 between 0.0 and 1.0. Therefore, oxygen was required to supply to avoid coking problem. Furthermore, the reactions of the dry reforming are very complex, such as reverse water-gas shifting, CO2 methanation, CO2 decomposition, Boudouard reaction.etc. Relative chemical reaction and enthalpy of reaction are shown in Table 3. Table 1 shows the main composition and concentration of the simulated biomass derived gas. The CO2 concentration was between 25 and 50%, and methane in balance. The CO2/CH4 molar ratio was set between 0.33 and 1.0.
4.
Results and discussion
4.1.
Effect of reaction gas temperature
The enthalpy of reaction indicates that raising the O2/CH4 ratio may release corresponding energy through oxidation reaction to provide the necessary energy for the reforming process. As the catalysis of a catalyst is dominated by surface reactions, this study needs to find a method for providing the necessary energy for reforming on the catalyst surface. The effect of reformate gas temperature on energy loss and methane conversion will be discussed. The relationships between the equilibrium adiabatic temperature, the measured reformate gas temperature, and energy loss percentage are shown in Fig. 5(A). The experimental parameters used a methane feeding rate of 10 NL/min, an O2/CH4 ratio between 0.5 and 1.0, and a CO2/CH4 ratio between 0.0 and 1.0. In the figure, each set of symbols consists of six data points and represents from left to right of the X-axis, the O2/CH4 molar ratio of 0.5, 0.6, 0.7, 0.8, 0.9 and 1.0, respectively. The average gas temperature of the whole catalyst section is defined as the reformate gas temperature. The range of reformate gas temperature in the experiments is from 650 to 900 C. The result shows the equilibrium adiabatic temperature is a highly linear relationship with reformate gas temperature, with more pronounced differences at higher temperatures. This is mainly due to the heat loss through the wall of the reactor as the temperature gradient between the catalytic reaction zone and the surrounding environment increases at higher temperatures. According to the regression of measured data, it indicated that the reformate gas temperature had a highly linear relationship with energy loss percentage. The photo in Fig. 5(B) illustrates the phenomenon of heat dissipation through the wall in the reforming. Because of the reactor was not insulated, the higher catalyst bed temperature caused higher heat loss percentage. Total energy loss percentage lies between 15 and 30% under this set of parameters. Heat loss can be divided into the heat lost through the wall surface, and the sensible heat carried away by the gaseous products. Energy loss percentage was defined in Eq. (3).
Table 3 e Chemical equations of the reforming reactions and their associated enthalpy of reaction. Reaction CO2 reforming of CH4 Reverse water-gas shifting Boudouard reaction CO2 decomposition CO2 methanation Partial oxidation Complete oxidation
Chemical equation
Enthalpy of reaction (kJ/mol-NTP)
CO2(g) þ CH4(g) / 2CO(g) þ 2H2(g) CO2(g) þ H2(g) / CO(g) þ H2O(g) CO2(g) þ C(s) / 2CO(g) CO2(g) / CO(g) þ 0.5O2(g) CO2(g) þ 4H2(g) / CH4(g) þ 2H2O(g) CH4(g) þ 0.5O2(g) / CO(g) þ 2H2(g) CH4(g) þ 2O2(g) / CO2(g) þ 2H2O(g)
247.3 41.2 172.5 282.9 165.0 35.6 802.2
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Fig. 5 e (A) Relationship of the equilibrium adiabatic temperature with reformate gas temperature and energy loss percentage under varying reforming parameters. (B) Observation at outer surface of reactor wall.
Fig. 6 illustrates the reformate gas temperature on the methane conversion efficiency under the well regulated feeding ratio. In figure, each set of symbols consists of six data points and represents from left to right of the X-axis, the oxygen to carbon ratio of 0.5, 0.6, 0.7, 0.8, 0.9 and 1.0, respectively. Theoretically, better methane conversion would be achieved at higher reaction temperature. From a physical implication, the energy required for DATR reaction was provided by heat release of oxidation in the reforming process. The higher O2/CH4 molar ratio provokes the corresponding higher reformate gas temperature, and thus it is helpful to improve the methane conversion efficiency. If the higher CO2/ CH4 ratio is adopted, the endothermic reaction may result in
the lower temperature in the reforming. It is inauspicious to methane conversion efficiency. Fig. 7 shows that the equilibrium constant (kp) of methane oxidation is far higher than the reductive reaction between methane and CO2, or the reformation reaction between methane and steam. The partial oxidation reaction of methane could remain its self sustainable reaction under the experimented parameters, and next followed the reformation of methane and steam. The reformation of methane with CO2 occurs only at a high temperature range. Thus, O2/CH4 ratio is the dominant factor in the dry autothermal reforming. A high O2/CH4 ratio implies a high temperature in the reforming process. From the experimental results, it shows that the methane conversion efficiency is influenced by reformate gas temperature [32,33]. That is, the
120
100
O
110
80
100
60
90
40
80
20
70
0
0
0.33 0.5 1.0 0.5 0.6 0.7 0.8 0.9 1.0
60 50 40 30 600
lnk
Methane conversion efficiency (%)
130
650
700
750
800
850
900
950
1000
0
Reformate gas temperature ( C)
CH4+0.5O2 = CO+2H2 CH4+H2O = CO+3H2 CH4+CO2 = 2CO+2H2
Partial Oxidation
-20
Steam reforming
-40 -60
CO2 reforming
-80 -100 5
10
15
20
25 4
Fig. 6 e Effect of reformate gas temperature on methane conversion efficiency under various O2/CH4 and CO2/CH4 ratio.
30
35
40
-1
1/Tx10 /K
Fig. 7 e Relationship of side reaction and chemical equilibrium constant under varying reaction temperature.
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Effect of CO2 in oxidizer
Oxygen addition promotes the oxidation reaction and heat release, which provides the heat required in dry autothermal reforming. However, it also generates CO2. The oxidizer is comprised of both oxygen and CO2 in this study. Calculation of CO2 in oxidizer is shown in Eq. (4). The effect of CO2 concentration in oxidizer on the reforming performance will be discussed. Fig. 8 describes the effect of CO2 concentration on product molar fractions. The product consists of hydrogen, carbon monoxide, carbon dioxide, methane, water vapor, and nitrogen. The reaction without CO2 addition in the reactants represents the methane partial oxidation reforming, which gives good syngas yield. As CO2 concentration increases, the molar fraction of CO in the products increases initially, then it dips down and the syngas production decreases due to the dilution effect caused by excessive CO2. Fig. 9 shows the CO2 concentration on the syngas selectivity. As the CO2 concentration increases, a more effective reverse water-gas shifting reaction linearly increases CO selectivity. Considering both selectivity and yield, a 30% CO2 concentration in the oxidizers shows an effective reverse water-gas shifting reaction. If CO2 concentration in oxidizer is higher than 50% and O2/CH4 molar ratio between 0.7 and 0.8, 100% CO selectivity could be achieved. However, if the CO2 concentration becomes excessive, it will only dilute the syngas and reduce products usability. Calculations of selectivity for H2 and CO are shown in Eqs (5) and (6). Fig. 10 demonstrates the relationship between fuel input low heating value (LHV) power and syngas output LHV power. The input LHV power comes from the methane consumed, and the syngas output LHV power is derived from H2 and CO. The energy of reaction in the reforming process can be divided into chemical energy and thermal energy. Chemical energy includes the heating value from H2 and CO. Thermal energy includes the sensible heat in the stream and heat loss through
110
Methane feeding rate = 10 L/min 105 Operation pressure = Atmospheric
105
100
100
95
95
90
90
85
0.5 0.6 0.7
80
70
70 65
POX
60
60
55
55 -10
0
10
20
30
40
50
60
70
CO2 in oxidizer (vol. %) Fig. 9 e Relationship of CO2 in oxidizer and products selectivity under varying O2/CH4 and CO2/CH4 ratio.
the wall surface from the oxidation reaction. Under the experimented conditions, roughly 70% of total input energy consists of chemical energy; the rest is mostly heat loss in the oxidation reaction. This graph corresponds to the heat loss percentage shown in Fig. 5(A). Greater input energy leads to more heat from produced gas flow; and the greater temperature gradient between the outer wall of the reaction zone and the surrounding environment leads to a relatively significant loss via convection. Viewed together, the ratio of H2 and CO in chemical energy can be controlled by reforming parameters. A higher CO2/CH4 ratio lowers the LHV power fraction of hydrogen in the produced gases. A higher hydrogen LHV fraction is obtained for the condition without CO2 addition, namely CO2/CH4 ¼ 0. Increasing the CO2/CH4 ratio lowers hydrogen yield. This is because the CO2 is converted into CO and H2O with H2 via the reverse water-gas shifting reaction. That is, controlling the CO2/CH4 ratio would modulate the
Output LHV power of syngas (kW)
Molar fraction of products (mole/mole)
Methane feeding rate = 10 L/min Operation pressure = Atmospheric O2/CH4=0.5~1.0, CO2/CH4=0, 0.33, 0.5, 1.0
POX 0.25
Hydrogen
0.20
0.15
Carbon monoxide
0.10
0.05
Carbon dioxide
0.00
Methane feeding rate = 10 L/min 5.0 O /CH = 0.5~1.0, CO /CH = 0, 0.33, 0.5, 1.0 Catalyst = Pt/Rh based (Monolith) Energy 4.5 Operation pressure = Atmospheric loss rate 4.0
0
0.5 0.6 0.7 0.8 0.9 1.0
3.5 3.0 2.5 2.0
0
10
20
30
40
50
60
CO2 in oxidizer (vol. %) Fig. 8 e Effect of CO2 in oxidizer on the molar fraction of products under varying reforming parameters.
70
5.5 5.0 4.5
region
0.33 0.5 1.0
4.0 3.5 3.0 CO region
2.5
LHV(syngas)
2.0
R =0.9645
1.5
1.5 H region
1.0
1.0
0.5 -10
80 75
5.5 0.30
85
0.8 0.9 1.0
75
65
Hydrogen selectivity (%)
4.2.
110
Output LHV power of H2 (kW)
reaction temperature was affected by both of O2/CH4 and CO2/ CH4 ratios.
Carbon momoxide selectivity (%)
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0.5 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
Input LHV power of fuels (kW) Fig. 10 e Relationship between input LHV power of fuels and output LHV power of syngas.
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H2/CO ratio in the products, and does not affect the total power output significantly.
Gaseous products in a dry autothermal reforming reaction mainly consist of hydrogen, carbon monoxide, carbon dioxide, nitrogen, and unconverted methane. Among these, H2 and CO contain good heating value. Hydrogen also has a high flame speed and can assist combustion under the conditions of highly diluted gas concentration. Even though the CO2, H2O, and N2 in the reformed product do not provide heating value, they have the effect like an exhaust gas recirculation (EGR) for engine. These gases effectively reduce the high temperature in combustion and suppress the generation of thermal NOx under high temperature condition. Hydrogen also benefits in its short quenching distance, which enhances combustion efficiency and lowers the ratio of unburned gas from the combustion chamber. With regard to these advantages, here the effect of feeding ratio on H2 þ CO syngas concentration, and the relationship between A/I ratio and H2/ CO ratio in reforming process will be investigated. Fig. 11(A) indicates the influence of O2/CH4 ratio on H2 þ CO concentration under different CO2/CH4 ratios. A higher O2/CH4 ratio raises the overall reforming gas temperature and promotes fuel conversion into syngas. An excessively high CO2/CH4 ratio dilutes syngas concentration, as additional CO2 can not join reduction reaction effectively. Thus, syngas concentration is dominated by the O2/CH4 and CO2/CH4 ratios. Overall syngas concentration range lies within 20%e40% under the experimented conditions. Fig. 11(B) shows the effect of O2/CH4 ratio on A/I ratio under different CO2/CH4 ratios. The assisting/impeding (A/I ) combustion composition ratio is calculated by Eq. (7). In this equation, the combustible gas from fuel reforming are the numerator, such as H2, CO, CH4, and the gases which suppress the high temperature of combustion to reduce NOx generation are the denominator, such as CO2, N2. Generally, the A/I ratio decreases with the increase of both the O2/CH4 and CO2/CH4 ratios. Overall A/I ratio falls within a range of 0.43e0.84. Different CO2/CH4 ratios produce different corresponding ranges of A/I ratios. Lower O2/CH4 molar ratios give larger operation ranges. This result gives the parameter setting of reforming for matching the different operating conditions of engine. In the future applications, the combustion efficiency and exhaust emissions of engines can be improved with the compromised A/I ratio, and the operating parameters obtained in this study. Fig. 11(C) shows the influence of O2/CH4 ratio on the produced H2/CO ratio under different CO2/CH4 ratios. The applications of H2/CO ratio can be classified as either combustion or the production of chemical material by synthesis. If it is used in combustion, both hydrogen and CO increase the flammability limit and stabilize the combustion. Among them, hydrogen can be used to raise the overall flame propagation speed, while CO can increase the adiabatic flame temperature [12]. However, the applications in chemical product synthesis depend upon the suitable H2/CO ratio needed in further reactions. Generally, H2/CO ratios controlled to within 1.0e2.0 are more useful [34]. From the experimental results, one can find
A 50 H +CO concentration (Vol. %)
Effect of O2/CH4 on performance index
Methane feeding rate: 10 L/min Pressure = Atmospheric 40 CO /CH =0, 0.33, 0.5, 1
CO CO CO CO
/CH /CH /CH /CH
=0 = 0.33 = 0.5 =1
30
20 10 0 0.5
0.6
0.7 0.8 0.9 O /CH molar ratio
1.0
H2+CO concentration B Assisting / Impeding molar ratio
4.3.
1.00 Methane feeding rate: 10 L/min
0.75
0.50
0.25
0.00 0.5
0.6
0.7
0.8
0.9
1.0
0.9
1.0
A/I ratio C
3.5 Methane feeding rate: 10 L/min
3.0 2.5 2.0 1.5 1.0 0.5 0.0 0.5
0.6
0.7
0.8
H2/CO ratio Fig. 11 e Effect of O2/CH4 ratio on H2 D CO concentration, A/ I ratio and H2/CO ratio under varying CO2/CH4 ratio.
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that high H2/CO ratio is obtained by partial oxidation reforming, but it is not conducive to chemical synthesis. Adding proper CO2 in the reactants to aid the reforming reaction, with the CO2/CH4 ratio ranging between 0.0 and 1.0, has been proven to effectively control the H2/CO ratio between 1.0 and 2.0. This ratio is favorable for the production of chemical product by synthesis. The ratio of H2/CO is easily controlled by adjusting CO2/CH4 ratio; the influence of O2/CH4 ratio is insignificant.
5.
Conclusions
The parametric design and hydrogen-rich syngas production by dry autothermal reforming with BDG were investigated in this study. The reforming parameters included fuel feeding rate, and CO2/CH4 and O2/CH4 molar ratios. A series of experiments were conducted for hydrogen-rich gas production. The conclusions are as follows: 1. The theoretical calculation indicated that a reaction temperature higher than 1000 C may avoid coke formation for dry reforming (CO2/CH4 molar ratio ¼ 1). Therefore, CO2/ CH4 ratio should be higher than the coking boundary, or oxygen should be introduced to prevent the coking phenomenon. 2. The results also indicated that good correlations occurred among the reformate gas temperature, energy loss percentage, and equilibrium adiabatic temperature. The energy loss percentage increases approximately linearly with the gradient of the reformate gas and environment temperatures. Under the reforming parameter setting, the reformate gas temperature was between 650 and 900 C, and energy loss percentage was between 15 and 30%. 3. The important findings including: it could provide the energy required for a self-sustaining reaction in DATR; and carbon deposition could be avoided with the appropriate addition of oxidants. In addition, the O2/CH4 ratio was used to control the syngas yield and thereby regulated the heating value of the syngas. The lower percentage of hydrogen was obtained with a higher CO2/CH4 ratio because of the reverse water-gas shifting reaction, which was favorable to CO selectivity. 4. For dry autothermal reforming, methane conversion was dominated by reaction temperature. High CO2 concentration affected the overall heat release of oxidation, thereby reducing the reformate gas temperature. Although a high CO2/CH4 ratio decreased the A/I ratio and syngas concentration, it was helpful to increase CO selectivity. In addition, the H2/CO molar ratio in the reformate gas between 1 and 2 was achieved by adjusting the CO2/CH4 molar ratio. However, syngas concentrations were affected by both the CO2/CH4 and O2/CH4 molar ratios.
Acknowledgments The authors are grateful to the support of the National Science Council of Taiwan under grant number NSC 100-2221-E-168038-MY2.
references
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