Ind. Eng. Chem. Res. 2002, 41, 1425-1435
1425
Plasma Pyrolysis of Methane to Hydrogen and Carbon Black James R. Fincke,* Raymond P. Anderson, Timothy A. Hyde, and Brent A. Detering Idaho National Engineering and Environmental Laboratory, P.O. Box 1625, Idaho Falls, Idaho 83415-2211
The plasma-driven gas-phase thermal decomposition of methane yielding hydrogen and solidphase carbon has been suggested as an environmentally friendly alternative to conventional methods of producing hydrogen from natural gas. The advantage of the process is that hydrogen is obtained directly from methane without producing CO2 as a byproduct. The process was experimentally examined using a modified version of a dc plasma reactor originally developed for the conversion of methane to acetylene. Carbon yields of 30%, a factor of 6 increase, with a corresponding decrease in acetylene yield were obtained by simply increasing the residence or reaction time. A detailed kinetic model that includes the reaction mechanisms resulting in the formation of acetylene and heavier hydrocarbons through benzene is described. A model for solid carbon nucleation and growth is included. The model is compared to experimental results and is used to examine process optimization. 1. Introduction The most common conventional method of obtaining hydrogen from natural gas is by steam reforming followed by the water gas shift reaction,1,2 resulting in the overall reaction CH4 + 2H2O f CO2 + 4H2. In this process, half of the hydrogen produced originates from the hydrocarbon and the other half comes from water. Steam reforming is applicable to hydrocarbons other than methane; however, feedstock heavier than naphtha cannot be used.1 Hydrogen is also routinely produced from natural gas by partial oxidation (CH4 + 1/2O2 f CO + 2H2), with the advantage that heavier hydrocarbons such as residual oil can be processed. However, the overall efficiency of the process (50%) is less than that of steam-methane reforming (65-75%), and a source of pure oxygen is required.1,2 The technologies for hydrogen production from natural gas are mature and widely practiced on a large scale for the production of methanol, ammonia, and hydrogen for petrochemical plants.1,2 Both the steam reforming and partial oxidation processes have the drawback that large quantities of CO2 are produced. If the goal is to minimize the emission of greenhouse gases, the CO2 byproduct must be captured and sequestered, increasing the cost of the hydrogen produced. The direct thermal decomposition, or cracking, of methane to produce hydrogen and solidphase carbon is potentially an environmentally friendly alternative. Because the cracking process is highly endothermic, the energy source must also be efficient with low CO2 emissions or renewable with essentially zero net emission to maximize the net greenhouse gas reduction. It is possible to realize a significant net reduction in CO2 emission using conventional, fossil generated, electrical power.3 The thermal cracking process can, in principle, coproduce two valuable products, hydrogen and carbon black, improving process economics.3 Commercially, carbon black is often obtained by the partial oxidation or thermal decomposition of acetylene, although other hydrocarbon feedstocks * Corresponding author. Fax: (208) 526-2031. E-mail: JF1@ inel.gov.
Figure 1. Simplified equilibrium diagram for 1 mol of methane.
such as ethylene are also used. The thermal cracking process can use less costly methane directly. The equilibrium dissociation of methane begins at around 500 °C and is complete by 1000 °C. The equilibrium products between about 1000 and 2500 °C are solid carbon and molecular hydrogen (Figure 1). Assuming that the product stream is at 1000 °C and that the process is 100% thermally efficient, the specific energy requirement (SER) corresponding to Figure 1 is 0.933 (kW‚h)/(Nm3‚H2). Thermal inefficiencies increase this number, as does an increase in the temperature of the product stream and the use of additional gases; process heat recovery decreases it. Recovery of enough product stream energy to preheat the incoming feedstock to 400 °C lowers the SER to 0.83 (kW‚h)/(Nm3‚ H2). If decomposition occurs on a solid (perhaps catalytic) surface, lower value coke is formed rather than high-quality, generally higher value, carbon black. To produce high-quality carbon black, the decomposition and soot formation process preferably occurs in the gas phase. In practice, the rate of formation of unsaturated hydrocarbons, primarily acetylene, ethylene, and benzene, is much faster than the complete decomposition to hydrogen and the subsequent formation of solid carbon soot, and thermodynamics alone cannot adequately describe soot formation. It is inherently a kinetically limited process composed of a nucleation step followed by mass growth. It is known that the formation of polycyclic aromatic hydrocarbons (PAHs) is a major
10.1021/ie010722e CCC: $22.00 © 2002 American Chemical Society Published on Web 02/16/2002
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Ind. Eng. Chem. Res., Vol. 41, No. 6, 2002
Figure 2. Simplified equilibrium diagram for 1 mol of methane where the solid phase of carbon is not included as a species.
nucleation mechanism for soot4 and that the formation of benzene is a precursor to the formation of higher aromatics. The PAHs increase in molecular weight by acetylene addition and become hydrogen-deficient through hydrogen abstraction by atomic hydrogen, leading to the formation of primary soot particles. Primary soot particles continue to grow via the decomposition of acetylene on their surfaces. If the product stream is rapidly cooled to temperatures where the products are stable, before soot has time to nucleate and grow, the composition is frozen and the equilibrium condition of Figure 1 is never attained. A second, modified equilibrium diagram excluding solid carbon as a product is shown in Figure 2. The result is the formation of acetylene with a maximum theoretical yield of 98.5% at a temperature of around 1875 °C. This process has been exploited for the production of acetylene from methane (see, for example, refs 5 and 6 and references therein) with the source of energy being an electrical arc discharge or plasma. Electrically generated plasmas are the preferred method of introducing large amounts of energy into a hydrocarbon gas stream at high temperature while avoiding coke formation on surfaces and the limitations inherent in heat exchanger materials. Several authors have reported the plasma-driven thermal decomposition of methane and other light hydrocarbons. In general, the goal was to produce carbon black utilizing methane as a source of carbon directly rather than more expensive acetylene or ethylene with hydrogen as a secondary product. Bolouri and Amouroux7 describe the production of carbon (acetylene) black by directly processing methane in an RF thermal plasma. They demonstrated a methane conversion efficiency approaching 100% and carbon yields as high as 45%. Most of the remainder of the carbon appeared as acetylene. In a series of papers, Fulcheri and coworkers8-11 describe a novel three-phase alternating
current plasma process for the production of carbon black from methane. They reported carbon yields of 50% for methane10 and 60% for ethylene11 feeds. Kaverner Engineering has also reported the coproduction of hydrogen and carbon black from methane with high yield, reported to be in the 90-100% range with a specific energy requirement of ≈1.1 (kW‚h)/(Nm3‚H2), although detailed process information in the available literature are lacking and a complete analysis of the product stream is apparently unpublished.3,12,13 Other information suggests that the process operates at a maximum temperature of 1600 °C14 and at a pressure between 2 and 5 atm.15 It is unclear if the high carbon yield reported is for “single pass” operation, if unconverted products or products other than hydrogen and solid carbon are recirculated and reprocessed, or if process heat recovery is used. Our goal is to better understand the details of the chemistry of the plasma thermal conversion of methane to hydrogen and solid-phase carbon. Furthermore, we wish to examine the influence of various parameters (temperature, pressure, residence time, etc.) on process performance. The challenge is to maximize the yield of solid carbon and hydrogen while minimizing energy consumption and gaseous species other that hydrogen. Our approach is the development of a reasonably detailed kinetic model that is validated by comparison to experimental results. The kinetic model includes the reaction mechanisms resulting in the formation of acetylene and heavier hydrocarbons through benzene and solid carbon nucleation and growth. The model is benchmarked against experimental results and is used to examine reaction timescales and optimization of carbon and hydrogen yields. 2. Experimental Results The test apparatus used was originally designed to investigate the plasma thermal conversion of methane to acetylene.5 The original configuration includes an efficient four-port injector and a carbon-lined reactor section that provides residence time for reactions to take place. The original geometry is given in Figure 3. The apparatus was later modified to increase residence time by replacing the 12.7 cm reactor section with a 61 cm reactor section of the same diameter. The carbon liner was retained in an attempt to minimize thermal losses to the water-cooled reactor shell. The measured thermal efficiency of the plasma torch was between 60% and 80%, depending on the gas mixture and flow rates. The dc plasma torch that is used will not operate for extended periods of time on pure hydrogen without severe anode erosion; hence, all test data is acquired using at least some Ar plasma gas. The processing of
Figure 3. Schematic of torch, injector, and reactor; flow is left to right.
Ind. Eng. Chem. Res., Vol. 41, No. 6, 2002 1427 Table 1. Experimental Parameters Test I
Table 2. Chemical Species Considered Test II
Test Parameters pressure (kPa) 85 Ar (slm) 140 H2 (slm) 100 net power (kW) 60 avg plasma temperature (K) 6200 reactor length (cm) 12.7 CH4 (slm) 120.8
77.1 140 30 60 11000 61 90
Measured Energy Loss (W/m2) injector ring 5.95 × 106 reactor 1.73 × 105 downstream contraction 6.64 × 105 downstream piping 2.0 × 105
not present 2.56 × 105 4.52 × 105 5.83 × 104
Measured Process Performance conversion efficiency (%) 99.1 carbon yield (%) 4.67 C2H2 yield (%) 85.5 C6H6 yield (%) 8.1 C2H4 yield (%) 1.7 other hydrocarbons yield (%) 1.0
99.4 30.5 47.5 13.9 6.7 1.4
Inlet Boundary Condition temperature (K) 3450 velocity (m/s) 531.25 [Ar] (mole fraction) 0.3613 [H2] (mole fraction) 0.1888 [H] (mole fraction) 0.1382 [CH4] (mole fraction) 0.3117
3900 44.63 0.5070 0.0510 0.1160 0.3260
methane directly in the discharge is precluded by the severe erosion of the tungsten cathode via the formation of volatile tungsten carbides. The power to the plasma torch was adjusted to give a constant 60 kW deposited in the plasma gas. Because the torch voltage is determined by the argon-to-hydrogen ratio, the desired power was obtained by adjusting the discharge current. The measurements have been previously described in detail,5 so only a brief description will appear here. The composition of the product stream is analyzed by a Hewlett-Packard series 6890 model G1540A gas chromatograph. Other instrumentation include gas flow rates, cooling water flow rates, and various temperatures and pressures. All instrumentation except for the gas chromatograph are directly interfaced to a data acquisition system for continuous recording of system parameters during a test run. Once the specified process power levels, pressures, and gas flow rates are established, a residual gas analyzer continuously samples the gas stream to verify that major species concentrations stabilize. This ensures that steady state is achieved, that the system temperatures equilibrate, and that thermal masses such as the graphite reactor liner do not influence the results. All cooling water flow rates and inlet and outlet temperatures are monitored and recorded, allowing a complete system energy balance to be calculated, in many cases on individual components such as the injector ring, reactor, and so forth. The presence of inert Ar has the advantage that it provides a built in reference for validating the overall process mass balance. In general, the measured system mass balance was within (5%. This accuracy is not sufficient to resolve small amounts of soot production but does provide a valuable check on an overall measurement system performance. In the test cases described, the solid carbon soot was physically collected and weighed to provide an accurate measure of the amount of solid carbon produced. We have chosen two data sets, summarized in Table 1, to use in benchmarking the model. The first of these
species no.
species
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Ar H2 H C C(s) C2 CH4 CH3 CH2 CH C 2H C2H2 C2H3 C2H4 C2H5 C2H6 C3H2 C3H3 a-C3H4 p-C3H4
21 22 23 24 25 26 27 28 29
C3H5 C4H4 n-C4H5 i-C4H5 1,3-C4H6 C4H8 C6H5 C6H6 Csoot(s)
name argon diatomic hydrogen atomic hydrogen carbon (gas) carbon (graphite) diatomic carbon methane methyl methylene methylidyne ethynyl acetylene vinyl radical ethylene ethyl radical ethane C3H2 radical 2-propynl allene methylacetylene (propyne) prop-1-enyl vinyl acetylene trans-1,3-butadiene t-2-butene phenyl benzene carbon (graphite)
CHEMKIN symbol AR H2 H C C(S) C2 CH4 CH3 CH2 CH C2H C2H2 C2H3 C2H4 C2H5 C2H6 C3H2 H2CCCH C3H4 C3H4P CH2CHCH2 CH2CHCCH CH2CHCHCH CH2CHCCH2 CH2CHCHCH2 C4H8 C6H5 C6H6 C(S)
data sets utilizes the original (short) reactor section used in our previous acetylene work (Test I), and the second utilizes the 61 cm reactor section (Test II). In Test II, the methane injector ring was removed, and the injection ports were relocated in the modified anode exit to decrease wall heat losses. In Table 1, the conversion efficiency (CE) is defined as CE ) [1 - Q˙ CH4P/Q˙ CH4in] × 100, where Q˙ CH4 is the volumetric flow rate of methane at standard temperature and pressure and the superscripts “P” and “in” denote the product stream and process feedstock inflow, respectively. The carbon basis yield (y) is defined as y ) (mass carbon contained in a particular product)/(mass carbon input in feedstock inflow) × 100. Methane conversion is essentially complete in both tests. The measured carbon yields are 4.7% for the short reactor and 30% for the 61 cm reactor. The majority of the remaining carbon appears as acetylene, benzene, and ethylene. The average plasma temperature is obtained from the calculated average enthalpy (power deposited in the plasma/mass flow rate of plasma gas) using the tables in ref 16. The inlet boundary condition assumes that the injected methane is instantaneously mixed without dissociation. The resulting mixture enthalpy is used to determine the inlet boundary condition temperature. The plasma composition is assumed to be in equilibrium at this temperature. Ionization is neglected; at the average plasma temperature the argon is less than 5% ionized in Test II, and ionization is negligible in Test I. 3. Kinetic Modeling A detailed kinetic model was developed to aid in understanding the pyrolysis process. The model is a onedimensional representation of the fluid dynamics and chemistry of the process implemented in the plug flow reactor application in CHEMKIN, version 3.6.17 The chemical species included and their CHEMKIN thermodynamic database symbols are given in Table 2. The
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inlet boundary conditions were given in Table 1. Downstream of the inlet, the temperature and pressure are allowed to vary, as required by conservation of mass, momentum, and energy. Wall heat transfer, based on actual energy loss measurements, is included. The kinetic model includes the reaction mechanisms resulting in the formation of acetylene and heavier hydrocarbons through benzene. Also included in the model is a solid carbon nucleation and growth mechanism, as are reactions resulting in gas-phase carbon as C and C2. The reverse rates were obtained using equilibrium constants calculated from thermodynamic data. Argon is included as a third-body collision partner, and its effect on the equilibrium state is included. The thermodynamic data is taken from the CHEMKIN17 database. A simplified soot nucleation and growth model has been combined with the gas-phase chemistry model. The soot nucleation and growth reactions are linked to the gas-phase chemistry by the simplifying assumptions that the local concentration of benzene determines the rate of nucleation and that soot mass growth is controlled by the local acetylene concentration. The model that we will employ is based on the semiempirical model of Lindstedt.18,19 The model does not have the generality of the very detailed PAH formation and growth models of Frenklach and co-workers;4,20,21 however, with appropriate benchmarking, the simplified model should be adequate to examine timescales and process sensitivity over a limited range. The nucleation step used in the present approach is written as C6H6 T 6Csoot(s) + 3H2, where Csoot(s) has the thermodynamic and physical properties of solid carbon. The reaction rate suggested by Lindstedt19 is first order in C6H6. Note that incipient soot particulates do not contain hydrogen in this representation. This is an oversimplification and one of the limitations inherent in simplified approaches. The issue is that newly formed soot particles with significant hydrogen content tend to display a higher reactivity than older particles (as much as an order of magnitude). To include this behavior in the soot surface growth step requires that the rate constant display a temporal dependence on the age of the particle during the initial growth stage and the adoption of additional “aging” equations. Lindstedt’s approach avoids this issue by combining the nucleation and initial mass growth process by assuming that incipient soot particles contain a certain number of carbon atoms, nc min. Hence, the number density of primary soot particles is given by [Csoot(s)]NA/nc min, where NA is Avogadro’s number, 6.02 × 1023 g mol-1. Lindstedt further assumed that nc min is equal to a C-60 shell, which is the most abundant of the smaller PAH ions. This assumption gives an initial particle size of about 1 nm. The rate constant has the Arrhenius form and an activation energy of about 174 600 J mol-1 was found to yield good results.22 Lindstedt determined the pre-exponential factor by comparison to an ethylene flame. The second reaction step, which is primarily responsible for the increase in soot mass, is assumed to be surface growth due to the adsorption of acetylene on the surface of soot particles and abstraction of the hydrogen.4,20 On the basis of measurements, this process was found to be approximately first-order in acetylene concentration, and the activation energy was determined to have a value of around 100 600 J mol-1 in diffusion flames and premixed flames.18,22 The depen-
dence of the soot mass growth reaction step on particle surface area is less clear, and many approaches have been suggested. These idealized approaches can generally be categorized as follows. (1) The reactivity is proportional to the available surface area, and the effects of surface chemistry are accounted for by the use of a steady-state approximation for the acetylene addition/hydrogen abstraction sequence. (2) The reaction rate is simply proportional to the local surface area and acetylene concentration. (3) The reaction rate is proportional to the number of particles but independent of surface area. (4) The reaction rate is proportional to the number of particles, independent of surface area, and the number of particles is constant. Lindstedt examined all four models and obtained the best agreement in diffusion flames using approach 3s the mass growth step is independent of surface area but dependent on the number of particles.18,22 The basis for approach 3 is the work of Brockhorn et al.,23 who showed that soot growth was not related only to surface area and could, in fact, be independent of area.23 In essence, this assumption accounts for, in an approximate way, the reduced activity of older soot particles. As particles age, there is a depletion of active surface acetylene adsorption sites, resulting in a depressed growth rate as the surface area continues to increase. To maintain consistency with the original Lindstedt model and the ease of incorporation into the kinetics model, this is the approach that we will use. The form of the mass growth reaction is given by C2H2 T 2C(s) + H2, where C(s) has the thermodynamic and physical properties of solid carbon. In the model, both [Csoot(s)] and [C(s)] are calculated giving both an estimate of the unagglomerated number of primary soot particles, [Csoot(s)]NA/nc min, and the total amount of soot produced, [C(s)]. The nucleation reaction C6H6 T 6Csoot(s) + 3H2 has a rate given by
rC soot(s) ) kC soot(s)[C6H6] ) AC soot(S)e-174 600/RT[C6H6] where AC soot(s) and [C6H6] are in mol/cm3 and RT is in J mol-1. The rate of mass growth of the primary particles is first order in [C2H2] and is proportional to the number of primary soot particles. The rate expression is given by
[
]
NA [C H ] rC(s) ) AC(s)e-100 600/RT [Csoot(s)] nc min 2 2 The pre-exponential factors for both the nucleation and growth reactions will be further discussed when we compare calculated to experimental results. The complete set of gas-phase reactions and their forward rates are summarized in Table 3. 4. Comparison of Kinetic Model to Experiment The calculated axial position-dependent temperature and velocity corresponding to Test I is shown in Figure 4. The high axial velocity present in the injector section drops when the larger diameter reactor section is entered, then increases again in the downstream contraction. The temperature decreases monotonically, with the initial sharp drop corresponding to the rapid dissociation of CH4 and formation of C2H2. The subsequent temperature decrease is due to wall heat transfer. The destruction of methane and formation of C1 and C2 product species is shown in Figures 5 and 6, and the
Ind. Eng. Chem. Res., Vol. 41, No. 6, 2002 1429 Table 3. Reaction Mechanism and Forward Rate Coefficientsa C1 and C2 Mechanism Kf ) R1TR2 exp(-E/RT) no.
reaction
R1
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
H + H + M T H2 + M CH4 + H T CH3 + H2 CH4 + CH T C2H4 + H CH4 + CH2 T CH3 + CH3 CH3 + H T CH4 CH3 + CH3 T C2H6 CH3 + CH3 T C2H5 + H CH3 + CH3 T C2H4 + H2 CH3 + M T CH2 + H + M CH3 + M T CH + H2 + M CH2 + H T CH + H2 CH2 + CH2 T C2H2 + H2 CH2 + CH2 T C2H2 + H + H CH2 + CH3 T C2H4 + H CH2 +C2H T C2H2 + CH CH2 +C2H3 T C2H2 + CH3 C2H + H T C2H2 C2H + CH4 T C2H2 + CH3
1.80 × 1.30 × 104 3.00 × 1013 1.30 × 1013 1.93 × 1036 1.69 × 1053 3.01 × 1013 1.00 × 1016 1.00 × 1016 6.90 × 1014 6.00 × 1012 1.20 × 1013 1.10 × 1014 4.20 × 1013 1.81 × 1013 1.81 × 1013 1.81 × 1014 1.81 × 1012
R2 1018
-1.0 3.0 0.0 0.0 -7.0 -12 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
E 0 33.6 -1.7 39.9 38.00 81.24 56.54 134 379 345 -7.5 3.4 3.4 0 0 0 0 2.08
Kf ) R1TR2 exp(-E/RT) ref 24 25 25 25 25 25 26 25 25 27 25 25 25 25 28 28 28 28
no.
reaction
R1
19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
C2H + C2H3 T C2H2 + C2H2 C2H + C2H5 T C2H2 + C2H4 C2H2 + H T C2H + H2 C2H3 T C2H2 + H C2H3 + H T C2H2 + H2 C2H3 + CH3 T C2H2 + CH4 C2H3 + C2H3 T C2H2 + C2H4 C2H4 + CH3 T C2H3 + CH4 C2H4 + H T C2H3 + H2 C2H4 + M T C2H2 + H2 + M C2H4 + M T C2H3 + H + M C2H5 T C2H4 + H C2H5 + C2H2 T C2H6 + C2H C2H5 +CH3 T C2H4 + CH4 C2H5 + C2H5 T C2H4 + C2H6 C2H6 + H T C2H5 + H2 C2H6 + CH2 T C2H5 + CH3 C2H6 + CH3 T C2H5 + CH4
9.64 × 1.81 × 1012 6.02 × 1013 4.73 × 1040 1.2 × 1013 3.92 × 1011 9.64 × 1011 4.16 × 1012 1.70 × 1015 2.5 × 1017 1.7 × 1018 1.02 × 1043 2.7 × 1011 1.15 × 1012 1.39 × 1012 1.44 × 109 2.2 × 1013 1.50 × 10-7 1011
R2
E
ref
0.0 0.0 0.0 -8.8 0.0 0.0 0.0 0.0 0.0 0.0 0.0 -9.1 0.0 0.0 0.0 1.5 0.0 6.0
0 0 116.4 194.5 0 0 0 45.56 62.9 320 404 224.15 98.11 0 0 31.1 36.3 25.4
28 28 26 25 25 28 28 26 25 25 25 25 28 26 29 26 30 26
Benzene Mechanism Kf ) R1TR2 exp(-E/RT) no.
reaction
37 C2H2 + CH T C3H2 + H 38 39 40 41
C2H2 + CH2 T C3H3 + H CH3 + C2H T C3H3 + H a-C3H4 + H T C3H3 + H2 a-C3H4 + C2H T C2H2 + C3H3
R1
R2
E
Kf ) R1TR2 exp(-E/RT) ref no.
reaction
1.9 0.0 0.0
31.52 0 8.37
4 4 4
3.00 × 1013 2.72 × 1018
0.0 -1.97
0 84.56
4 4
2.00 × 1013 5.00 × 1012 3.00 × 1013
0.0 0.0 0.0
5.70 × 1036 4.90 × 1051 1.60 × 1046 1.20 × 1022 3.10 × 1026 1.50 × 1067
-6.27 470.5 -11.92 74.1 -10.98 77.86 -2.44 57.35 -3.35 72.83 -16.89 247.4
4 4 4 4 4 4
1,3-C4H6 Forming Reactions 61.62 4 70 C2H3 + C2H3 T 1,3-C4H6
1.50 × 1042
-8.84
4
2-C4H8 Forming Reactions 334.72 42 72 2-C4H8 T 1,3-C4H6 + H2
1.00 × 1013
0.0
49 C2H2 + CH3 T C3H5
6.03 × 1011 0.0
C3H5 Forming Reactions 32.23 28 C4H4 Forming Reactions 20.92 31 54 C3H3 + CH2 T C4H4 + H 194.14 32 55 C3H2 + CH3 T C4H4 + H 0 4 56 i-C4H5 + H T C4H4 + H2 0 4
50 51 52 53
C2H3 + C2H2 T C4H4 + H C2H2 + C2H2 T C4H4 C2H4 + C2H T C4H4 + H n-C4H5 + H T C4H4 + H2
2.00 × 1012 2.45 × 1014 1.20 × 1013 1.50 × 1013
0.0 0.0 0.0 0.0
57 58 59 60 61 62
C4H4 + H T n-C4H5 C2H3 + C2H2 T n-C4H5 C2H3 + C2H3 T n-C4H5 + H 1,3-C4H6 + H T n-C4H5 + H2 1,3-C4H6 T n-C4H5 + H 1,3-C4H6 + H T i-C4H5 + H2
1.30 × 1051 2.51 × 105 2.40 × 1020 1.33 × 106 5.30 × 1044 6.65 × 105
C4H5 Forming Reactions -11.92 69.5 4 63 1,3-C4H6 T i-C4H5 + H 1.9 8.8 4 64 C4H4 + H T i-C4H5 -2.04 64.46 9 65 C2H2 + C2H3 T i-C4H5 2.53 51.2 4 66 C2H3 + C2H3 T i-C4H5 + H -8.62 517.4 4 67 n-C4H5 + H T i-C4H5 + H 2.53 38.7 4 68 n-C4H5 T i-C4H5
2.80 × 1021 -2.44 1.00 × 1013 0.0
73 C6H5 + H + M T C6H6
9.30 × 1014 0.0
74 C2H2 + C4H4 T C6H6 75 n-C4H5 + C2H2 T C6H6 + H
4.47 × 1011 0.0 1.60 × 1016 -1.33
a
CH2 + CH2 T CH3 + CH CH2 + M T CH + H C + H2 T CH + H CH + M T C + H + M CH2 + M T C + H2 + M
ref
C3H3 Forming Reactions 26.48 4 42 p-C3H4 + H T C3H3 + H2 1.15 × 108 0 4 43 p-C3H4 + C2H T C2H2 + C3H3 1.00 × 1013 31.52 4 44 i-C4H5 + H T C3H3 + CH3 2.00 × 1013 0 4
0.0 0.0 1.9 0.0
C3H4 Forming Reactions 0 4 47 C3H3 + H + M T p-C3H4 + M 131.86 4 48 C2H2 + CH3 T p-C3H4 + H
78 79 80 81 82
E
1.20 × 1013 2.41 × 1013 1.15 × 108 1.00 × 1013
3.00 × 1013 0.0 5.72 × 1020 -2.36
71 2-C4H8 T CH3 + C3H5
R2
3.00 × 1013 0.0
45 C3H3 + H + M T a-C3H4 + M 46 C2H2 + CH3 T a-C3H4 + H
69 C2H4 + C2H3 T 1,3-C4H6 + H
R1
C3H2 Forming Reactions 0 4
2.40 × 1014 4.00 × 1015 4.00 × 1014 1.90 × 1014 1.30 × 1014
0.0 0.0 0.0 0.0 0.0
0 0 0
52.3
9 4 4
271.96 43
C6H6 Destruction Reactions 53.35 4 C6H6 Forming Reactions 125.90 33 76 C2H3 + n-C4H5 T C6H6 + H2 22.6 4 77 C3H3 + C3H3 T C6H6 Gas-Phase Carbon Reactions 41.57 37 83 C2 + M T C + C + M 347.54 38 84 C2H + C2H T C2H2 + C2 97.28 39 85 C2H2 + M T C2 + H2 + M 280.19 38 86 C2H + M T C2 + H + M 246.93 38 87 C2H + H T C2 + H2
1.84 × 10-13 7.07 3.00 × 1011 0.0
-15.11 35 0 36
3.72 × 1014 1.81 × 1012 4.57 × 107 3.61 × 1015 3.61 × 1013
580.53 0 17.20 598.17 118.25
0.0 0.0 0.0 0.0 0.0
40 28 41 40 40
Units are cm, mol, s, and E in kJ mol-1.
C3-C6 species along with solid carbon soot appear in Figures 7 and 8. The concentration of diatomic gaseous carbon, C2, is
