Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 65 (2015) 352 – 361
Conference and Exhibition Indonesia - New, Renewable Energy and Energy Conservation (The 3rd Indo-EBTKE ConEx 2014)
Computational Fluid Dynamics Simulation: Hydrogen Production from Biogas Autothermal in Monolithic Catalyst Reactor Thoharudina*, Arif Setyo Nugrohoa, Edy Suryonoa a
Akademi Teknologi Warga Surakarta, Jl. Raya Solo-Baki km. 2, Sukoharjo 57552, Indonesia
Abstract This research aims to find out the characteristic of autothermal biogas on catalyst monolithic reactor especially the influence of inlet gas temperature and H2O/C ratio. Monolithic catalyst has dimension 45 mm × 45 mm × 225 mm with cell density 600 cpsi. The research with Computational Fluid Dynamics (CFD) simulation involved four reactions: oxidation, steam reforming, water gas shift, and decomposition. This result elucidated that the higher inlet gas temperature, the higher methane conversion, hydrogen concentration, and carbon monoxide including the lowering H2/CO ratio. In addition, this research resulted that the higher H2O/C ratio, the higher is the hydrogen concentration with the lowering carbon monoxide concentration involving the increase of H2/CO ratio. © Authors. Published by Elsevier Ltd. ThisPublished is an openby access articleLtd. under the CC BY-NC-ND license © 2015 2015The Thoharudin, A.S. Nugroho, E. Suryono. Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the Scientific Committee of EBTKE ConEx 2014. Peer-review under responsibility of the Scientific Committee of EBTKE ConEx 2014 Keywords: Biogas; computational fluid dynamics; hydrogen; monolithic catalyst
Nomenclature ATR CFD
autothermal reforming computational fluid dynamics
K iC
Kej
adsorbtion constant of species i (i=CH4, O2) equilibrium constant of reaction j (j=2-4)
* Corresponding author. Tel.: +62 856 4244 6638; fax: +62 271-621 176. E-mail address:
[email protected]
1876-6102 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the Scientific Committee of EBTKE ConEx 2014 doi:10.1016/j.egypro.2015.01.064
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MSR methane steam reforming Koj constant of reaction j (j=1-4) POX partial oxidation Pi partial pressure of species i (i=CH4, O2, CO, CO2, H2, H2O) Ej activation energy (kJ/kmol) R universal gas constant (kJ/kmol K) H enthalpy (kJ/mol) T temperature (K) kj kinetic rate constant of reactions j (j=1-4) ηj effectiveness factor of reaction j (j=1-4) Ki adsorbtion constant of species i (i=CO, CO2, H2, H2O) cpsi cells per square inch 1. Introduction Energy is essential for development both in industry and economy. However, the greater development of the industry results the greater demand for energy which the currently most of the demand is supplied from fossil energy. Beside the availability of fossil energy in the earth is limited and not able to be renewed, the combustion of fossil fuels also gives some problems on the increase temperature resulted from increasement of gas emissions from greenhouse in the atmosphere. Nowadays, the various methods in making fuel are developed to replace the need for fossil fuels with renewable fuels derived from vegetable. One of the renewable energy is biogas. Biogas is produced by the decomposition of waste or organic materials in a room without air commonly called digester with the help of methane bacteria. Biogas has the major gas components namely CH 4 and CO2, besides there are also other gases such as N2, H2, H2S, Ar, or CO with a small percentage [1]. Hydrogen is an interesting fuel to develop because it has a high calorific value. One kilogram of hydrogen gas has a calorific value of 143 MJ which is equivalent to 5 L of petrol. Hydrogen has the main characteristic which is categorized into clean energy when the process of combustion is happening, there is the release of H-H bond chemical energy which then bonds with oxygen to form the compounds of water (H 2O) [2]. Water is a compound that is friendly to the environment because it has the number of Global Warming Potential (GWP) and Ozone Depletion Potential (ODP) which is equal to zero [3]. Hydrogen is generally produced from the synthesis process of Methane Steam Reforming (MSR) by reacting methane with steam to produce hydrogen and carbon monoxide gases. Methane Steam Reforming is an endothermic reaction that requires heat to react as shown in Equation 1 [4]. CH4 H 2O l CO 3H 2
∆H= + 206 kJ/mol
(1)
It is not only with Methane Steam Reforming; hydrogen gas can also be produced by Partial Oxidation (POX) and Autothermal Reforming (ATR). Partial Oxidation has faster reaction rate than Methane Steam Reforming but the yields H2 of each carbon of fuel which is produced lower because on the Partial Oxidation, there is combustion on a half of fuels so that the gases resulted from the Partial Oxidation has a high concentration of carbon dioxide. While the method of hydrogen production with Autothermal Reforming method is a combination of Partial Oxidation and Methane Steam Reforming therefore beside most of the fuel reacts with oxygen through the combustion process, the fuel also reacts with water vapor to form hydrogen and carbon monoxide [5]. The current researches are focused on improving the efficiency of the reactor. The use of fixed bed reactor is recommended because it has higher heat transfer and contact frequencies when it is compared with the fluidized bed reactor. One of fixed bed reactor types which are interesting to study is the monolithic catalyst which has several strengths which are: a low pressure drop and large surface area geometry [6]. Modeling and simulation methods are used to provide an overview of virtual prototype and design of the reactor hence the efficiency and optimization reactor can be predicted. Hydrogen production from natural gas has been examined by using process simulation of the Partial Oxidation in a fixed bed reactor with diameter of 1 800 mm and height 11 580 mm [7]. The Methane Steam Reforming was used in hydrogen production process simulation on the monolith reactor with surface basis and volume basis reaction model [8]. Those studies indicate that surface basis reaction is closer to the results of an experimental study on a volume basis reaction model. CFD simulation of
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hydrogen production is also conducted in a tubular membrane reactor [9] and a catalytic micro reactor using Methane Steam Reforming which the process involves three kinds of reactions which are Steam Reforming, Water Gas Shift Reaction, and Methane Decomposition [10]. Based on some references which have been done, this paper will discuss the autothermal biogas on monolithic catalyst reactor. The research was conducted in simulation which aimed to determine the effect of temperature and ratio of H2O/C on the concentration of hydrogen and carbon monoxide, methane conversion and H 2/CO ratio. 2. Method 2.1. Autothermal reformer Autothermal biogas was done on the monolithic catalyst reactor which has dimension 45 mm × 45 mm × 225 mm, 600 cpsi. Biogas with the composition of 48.9 % of CH4 and 51.1 % of CO2 was flown through pressure of atmosphere in reactor of 11 L · min-1. Biogas, water vapor, and oxygen were heated up so it reaches certain temperature with H2O/C rate in 2.6 and O2/C in 0.49. Then, water to carbon ratio was varied from (0 to 5) H2O/C ratio. The conversion of CH4 was estimated by using the calculation of Equation 2 [11].
X CH 4 (%)
CH 4conversion x100 % CH 4inlet
(2)
Reformer was modeled 2D with assumption compressible flow, steady state, turbulent flow with k-ɛ model. The boundary conditions at the inlet using a mass flow inlet while the outlet using a pressure outlet. Wall reactor was assumed to be smooth so that there were no slip and adiabatic condition. A chemical reaction only occured on the surface of the reactor wall. The mixture of gas species consists of six gases such as CH4, CO2, H2O, O2, CO, H2, and CO. Gas concentration in the reaction products were taken on a dry basis. 2.2. Reaction of kinetic The chemical reaction of autothermal process includes eight chemical reactions, which are the main reaction is an oxidation reaction, followed by the reaction of steam reforming, water gas shift, and CH 4-CO2 reforming reaction. In addition, there are also other reactions including the cracking of methane and carbon monoxide to carbon deposition, carbon gasification by steam and oxygen, and methane decomposition. However, from eight reactions only four reactions which have a major influence on the autothermal process, which are oxidation, steam reforming, and water gas shift, and methane decomposition as shown in Equation 3, Equation 4, Equation 5, and Equation 6 [12]. R1 : CH 4 2O2 o CO2 2 H 2 O
∆H= - 802 kJ/mol
(3)
R2 : CH 4 H 2 O l CO 3H 2
∆H= + 206 kJ/mol
(4)
∆H= - 41 kJ/mol
(5)
∆H= + 165 kJ/mol
(6)
R3 : CO H 2 O l CO2 H 2 R4 : CH 4 2 H 2 O l CO2 4 H 2
The rate of chemical reactions associated the Equation 7 to Equation 10 refers to research that has been conducted by Xuan et al. [13]. R1 K 1
k 1 PCH4 PO02.5 C ( 1 K CH P K OC2 PO02.5 ) 2 4 CH 4
(7)
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R2 K 2
PH3 PCO ¸· 1 k 2 §¨ P P 2 2.5 ¨ CH4 H 2 O K e 2 ¸ Q22 PH 2 © ¹
R3 K 3
k3 PH 2
R4 K 4
PH4 2 PCO2 k 4 §¨ 2 P P CH4 H 2 O K e4 PH3.25 ¨©
P P § ¨ PCO PH O H 2 CO2 2 ¨ K e3 ©
(8)
· 1 ¸ ¸ Q2 ¹ 2
(9)
and · 1 ¸ ¸ Q2 ¹ 2
(10)
In which Qr
1 K CO PCO K H 2 PH 2 K CH4 PCH4
kj
K oj exp( E j / RT )
K iC
K H 2O PH 2O
(11)
PH 2
(12)
K oiC exp( 'H Cj / RT ) ,
(13)
and Ki
K oi exp( 'H j / RT )
(14)
The kinetic parameters of the reactions R1-R4 can be seen in Table 1 [13]. The constant equilibrium of reaction R2-R4, Kej, it can be seen in Table 2 [13]. The constant adsorbtion of gas species i (i=CH4, O2) in the reaction R1 and the constant adsorbtion of gas species i (i=CO, H2, CH4, H2O) in the reaction of R2-R4 can be seen in Table 3 [13]. To calculate the mass transfer limitations intra particle, effectiveness factor included in the equation R1-R4 as shown in Table 4 [13]. Table 1.Parameter of reaction kinetic R1-R4. Reaction
Koj (kmol/kgcat h) 17
-1.5
-Ej (kJ/kmol)
R1
5.852 x 10 bar
R2
4.225 x 1015 bar 0.5
-240 000
R3
1.955 x 106 bar -1
-67 130
R4
15
1.020 x 10 bar
0.5
Table 2.Equilibrium constant. Reaction
Equilibrium Constant Kej.
R2
5.75 x 1012exp (-11,476/T) (bar2)
R3
1.26 x 10-2exp (4,639/T)
R4
7.24 x 1010exp (-21,646/T) (bar2)
-204 000
-243 900
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Thoharudin et al. / Energy Procedia 65 (2015) 352 – 361 Table 3.Adsorption constant. Species
Koi (bar -1)
-∆Hj (kJ/kmol)
CH4
4.02 x 105
-103 500
O2
5.08 x 104 bar0.5
-66 200
CH4
6.65 x 10
-4
38 280
CO
8.23 x 10-5
70 650
H2
6.12 x 10-9
82 900
H2O
5
1.77 x 10 bar
-88 680
Table 4.Effectiveness factor. Reaction
Effectiveness
R1
0.05
R2
0.07
R3
0.7
R4
0.06
3. Results and discussion 3.1. Effect of inlet temperature In general, as seen in Fig. 1, it can be shown that the higher temperature of the inlet gas, stimulate high concentration of hydrogen gas and carbon monoxide, while concentrations of methane, oxygen, and carbon dioxide are lower. It is because the higher the temperature of the gas, the greater the energy contained in the gas so that chemical reactions can occur with faster reaction rates. It is especially in the reaction of steam reforming and methane decomposition in which the methane is decomposed by steam turning into hydrogen, carbon monoxide, and carbon dioxide. Steam reforming reaction and the decomposition of methane is endothermic reaction that requires heat or energy to react. Thus the higher temperature contributes to the high concentration of hydrogen and carbon monoxide also in high level. Fig. 1 also shows that the hydrogen concentration rises from 500 °C to 700 °C, but after the 700 °C, the hydrogen concentration falls accompanied by the increase of the significant carbon monoxide concentration. This is because the effects of the water gas shift reaction in which the higher the temperature, the reaction goes to the reverse direction where there is a reaction between hydrogen and carbon dioxide to form carbon monoxide and water vapor so that the concentration of hydrogen and carbon dioxide falls followed by the increase of carbon monoxide concentration. In Fig. 1 also shows that between 500 °C to 650 °C, the oxygen concentration falls gradually and at temperatures above 650 °C, oxygen concentration approaches zero. This is because oxygen acts as an oxidant in the combustion process and one of the factors in combustion is the enough energy or heat as energy activation on the combustion process. Biogas has a burning point/auto ignition, namely around 650 °C [14] hence on that temperature, biogas combustion process is just started happening.
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Thoharudin et al. / Energy Procedia 65 (2015) 352 – 361 60% CH4
CO
CO2
H2
O2
Concentration
50%
40% 30% 20% 10% 0%
500
550
600
650
700
750
800
Inlet temperature (oC)
Fig. 1. Effect of gas inlet temperature on the concentration of the gases.
Fig. 2 shows the relationship between the temperature on the methane conversion and H 2/CO ratio. In Fig. 2, it describes that the higher the temperature, the greater the value of methane conversion. This is because by the increase of temperature, the reaction rate both oxidation reactions, steam reforming, and also methane decomposition are greater so the concentration of methane is lower resulting the increase of methane conversion values. The Fig. 2 also presents that the optimum value of methane conversion at 650 °C temperature, then the value of the conversion of methane after 650 oC, there is lowering although it is not significant. The decreasement is resulted by the steam reforming reaction and methane decomposition in which if the concentration of CO, CO 2, and H2 is higher, it leads to the partial pressure also high so the reaction will go to the reverse direction. Thus, there is a reverse reaction between CO, CO2, and H2 to form CH4 and H2O. Fig. 2 also proves that the higher the temperature, the lower H2/CO ratio. This is because the higher the temperature, the faster the water gas shift reaction in which the reaction goes to the reverse direction where there is a reaction between hydrogen and carbon monoxide to form carbon dioxide and water vapor. Thus, although the hydrogen formed has a high concentration, but on the reaction which runs simultaneously, the hydrogen formed is reacted with carbon monoxide turning into carbon dioxide hence the concentrations of carbon monoxide increases. 100%
CH4 Conversion
90%
8
H2/CO
7 6
70%
60%
5
50% 4
40% 30%
3
20%
2
10%
0%
1 500
550
600
650
700
750
800
Inlet temperature (oC) Fig. 2. The effect of inlet gas toward methane conversion and H2/CO ratio.
H2/CO
CH4 Conversion
80%
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3.2. Effect of reactor length Concentrations profile of gas along the reactor is shown in Fig. 3 while the profile methane conversion and H2/CO ratio is shown in Fig. 4. Fig. 3 shows that with the increase length of the reactor, the greater the concentration of hydrogen and carbon monoxide and the lower carbon dioxide, methane, and the oxygen concentration. The longer the length of the reactor, the longer the reaction time and it results the greater methane conversion which is shown in Fig. 4. The greater the conversion of methane, the lower the concentration of methane gas with the increase concentrations of the reaction products namely hydrogen and carbon monoxide. In Fig. 4 also shows that the H2/CO ratio along the reactor is getting down. This is due to an increase in the concentration of H2 from methane steam reforming and water gas shift reaction. Due to the greater concentration of H2, the lower the rate of reaction of water gas shift to form hydrogen and carbon dioxide from carbon monoxide and water vapor. The lower the rate of reaction of water gas shift produce higher carbon monoxide concentration along the reactor and results the decrease of H2/CO ratio. 60% 50% Concentration
H2 40%
CO2
30% 20%
CO
10%
CH4
O2
0% 0
0.2
0.4 0.6 Relative length
0.8
1
Fig. 3. The profile of gas concentration of inlet temperature 700 oC.
100%
8
90%
CH4 Conversion
70%
6
60%
5
50% 40%
4
30%
3
20%
2
10%
1
0% 0
0.2
0.4 0.6 Relative Length
0.8
1
Fig. 4. The profile of methane conversion and H2/CO ratio in inlet temperature 700 oC.
H2/CO
7
80%
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3.3. Effect of H2O/C ratio Fig. 5 shows the relationship between the moles of steam to carbon toward gas concentration. It can be seen that the higher the ratio of H2O/C, the higher the concentration of hydrogen which is formed followed by the lowering of carbon monoxide concentration. It happens due to the higher concentration of steam, the faster the steam reforming reaction, water gas shift and methane decomposition. In Equation 8 to Equation 10 shows that the reaction rate of steam reforming, water gas shift, and methane decomposition in which the reaction rate is equal to the partial pressure of H2O. So the greater the concentration, the greater H2O partial pressure resulting the bigger the growing reaction of hydrogen formation. Fig. 5, it shows that a decline in the concentration of hydrogen, carbon monoxide, and carbon dioxide in the ratio of H2O/C above 4 followed by increasing of small methane. This is due to a decrease of methane partial pressure and the competition between methane and steam on active sites on the catalyst surface. It could be concluded that too large of H2O/C ratio is not always favor to the hydrogen production. From the view of energy, the excess steam would consume plenty of heat in the process of reaction and the separation of product gas from steam condensation and dryness. 60%
H2
CO
CH4
O2
CO2
Concentration
50% 40% 30% 20% 10%
0% 0
1
2
3
4
5
H2O/C Fig. 5. The effect of H2O/C ratio toward gas concentration in inlet temperature 700 oC.
Fig. 6 shows the relationship between the effect of comparisons H 2O/C to CH4 conversion and H2/CO ratio. It can be seen that the greater the ratio of H2/CO occurs, the raising of CH4 conversion value is not significant, but in the H2/CO ratio, there is a significant increase. It is shown in Fig. 6 that the increase in hydrogen to carbon monoxide ratio of (1.47 to 3.58) H2/CO in the ratio of water to carbon from (0 to 5) H2O/C. Such increase is the result of the water gas shift reaction in which the higher concentration of H 2O, the greater the partial pressure of H2O. Due to the increase of partial pressure of H2O, the greater the rate of the water gas shift reaction with the reaction to the right which is the reaction between carbon monoxide and water vapor to form hydrogen and carbon dioxide. Thus, there is an increase in the concentration of hydrogen followed by a decrease in the concentration of carbon monoxide hence the H2/CO ratio increases.
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100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0%
8 7
H2/CO
6 5
4
H2/CO
CH4 Conversion
CH4 Conversion
3 2 1 0
1
2
3
4
5
H2O/C Fig. 6. The effect of H2O/C ratio toward CH4 conversion and H2/CO ratio in inlet temperature 700 oC.
The hydrogen production from biogas ATR in this study is 38.9 % to 52.6 %. Base on studies reported in the literature to compare with POX and MSR, it mentioned that POX of biogas produce hydrogen concentration around 2 % to 30 % depended on the CH4/O2 ratio [15] while MSR of biogas produce higher concentration of hydrogen than ATR, namely around 60 % to 65 % depended on H2O/C ratio [16]. 4. Conclution Autothermal biogas was influenced by inlet gas temperature. The greater the temperature, the greater the increase of the methane conversion due to the increase of hydrogen and carbon monoxide concentrations. The concentration of hydrogen was optimally produced at a temperature of 700 °C. In addition, The high ratio of H2/CO could be obtained from biogas reforming in the low temperature. Conversion of methane and steam into hydrogen and carbon monoxide occured along the monolithic catalyst reactor. The longer the relative lengths of the reactor, the greater the conversion of methane and hydrogen, and carbon monoxide concentrations through the decline of H 2/CO ratio. Autothermal biogas was also influenced by the ratio of H2O/C where the higher the ratio of it, the higher the concentration of hydrogen by the decrease of carbon monoxide. Optimal concentration of hydrogen was in the water to carbon ratio of 4 H2O/C. In addition, due to the increase in the ratio of H2O/C, the higher is H2/CO ratio. References [1] [2] [3] [4] [5] [6] [7] [8]
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