Article pubs.acs.org/EF
Experimental Research on Low-Temperature Methane Steam Reforming Technology in a Chemically Recuperated Gas Turbine Qian Liu and Hongtao Zheng* College of Power and Energy Engineering, Harbin Engineering University, Harbin 150001, Heilongjiang Province, China ABSTRACT: Under the operating parameters of a chemically recuperated gas turbine (CRGT), the low-temperature methane steam reforming test bench is designed and built; systematic experimental studies about fuel steam reforming are conducted. Four different reforming forms are employed, including catalyst-only reforming, plasma-only reforming, piecewise synergistic reforming, and parallel synergistic reforming. A better reforming form for CRGT was determined by analyzing the effect of methane space velocity, steam-to-carbon ratio of fuel reforming (S/C), wall temperature, and plasma input power. All of the above factors had an effect on the reforming performance, but a direct and significant effect separately on effective carbon recovery rate, total enthalpy increasing rate, methane conversion, and fuel heating value increasing rate. The effective carbon recovery rate was taken as the chief indicator for choosing the right operating conditions, determining a best methane space velocity of 680 mL/(gcat·h). With the increase of S/C, total enthalpy increasing rate had a significant improvement; wall temperature had a positive effect on reforming, especially in synergistic catalysis. The effect of input power was linked with wall temperature in parallel synergistic reforming, with a greater effect at a higher temperature. Combining the experimental results with the theory analysis, parallel synergistic reforming is best, with a methane conversion of 51.23% and total enthalpy increasing rate of 25.73% at a wall temperature of 500 °C, an input power of 84.53 W, and an S/C value of 2.
1. INTRODUCTION Chemically recuperated gas turbine (CRGT), as a promising advanced cycle gas turbine, can greatly improve gas turbine thermal efficiency and reduce NOx emission.1,2 In the past decades, many scholars have done a lot of experimental and theoretical work on CRGTs.3−7 The CRGT concept is shown in Figure 1. Turbine exhaust is cascaded utilized by steam reformer (SR) and steam generator (SG), respectively. The steam produced by SG is mixed with the fuel and fed into SR. In the reformer, the expected endothermic reforming catalytic conversion of CH4 and H2O(g) to H2 and CO or CO2 occurs, as shown in eqs 1 and 2. CH4 + H 2O = 3H 2 + CO CH4 + 2H 2O = 4H 2 + CO2
Δr H = 206 kJ/mol
Open literature relating to the techniques of reforming methane and producing hydrogen from a plasma reformer and their applications has been gradually increasing over the years.14−16 Bromberg et al.17 first proposed the combination of a thermal plasma reformer operating in partial oxidation mode with a catalyst bed for hydrogen produced in small-tomedium-scale plants, and determined the conditions for system optimization and cost minimization. However, the thermal plasma devices as plasmatrons would generate very high temperatures (>2000 °C),18 which is not consistent with the concept of heat recovery. In this study, we first propose and systemically assess the applicability of hybrid dielectric barrier discharge (DBD)− catalytic methane steam reforming for heat recovery in CRGT. The methane steam reforming test bench is designed and built under the operating parameters of a CRGT; systematic experimental studies about fuel steam reforming are also conducted at low temperature (350−500 °C). Four types of reforming are used, including catalyst reforming, plasma reforming, piecewise synergistic reforming, and parallel synergistic reforming. The effect of methane space velocity, reactor wall temperature, steam-to-carbon ratio of fuel reforming (S/C), and plasma input power on methane conversion and total enthalpy increasing rate is studied to determine a better reforming form.
(1)
Δr H = 165 kJ/mol (2)
As the key unit of CRGT, the SR, and methane steam reforming processes have been widely studied in terms of influencing factors, reaction mechanism and reaction kinetics, the type of catalyst, and reactor design.8−12 However, most of the research is carried out for the production of hydrogen industrially, with many disadvantages such as a large recuperator size, slow response, the uncontrollable reaction process, and a series of problems related to catalyst deactivation. Moreover, the requisite reaction conditions of high pressure (2−3 MPa) and high temperature (700−900 °C) are not amenable to the CRGT. Turbine exhaust has a temperature range of 350−500 °C, so the cracking depth by conventional catalytic reforming is too low to use this exhaust heat efficiently.13 To solve these problems, plasma and the idea of a plasma catalysis has been put forward as a possible means to enhance or replace conventional catalytic reforming systems. © 2014 American Chemical Society
2. EXPERIMENTAL SETUP The experimental process and apparatus are depicted in Figures 2a and 2b, respectively. A small-scale test bench of methane steam reforming was employed to study the working process of heat recovery units in Received: August 1, 2014 Revised: September 21, 2014 Published: September 24, 2014 6596
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Figure 1. Chemically recuperated gas turbine (CRGT) concept.
Figure 2. (a) Schematic diagram of experimental setup. (b) Photograph of experimental equipment. CRGT, where SR and SG could be substituted with a ceramic-heating reactor and steam boiler, respectively. The reactants were steam and methane; an amount of 10 vol % nitrogen was added in the reactants as a reference for quantitative analysis of reformed gases. The volume flow rate of nitrogen and methane were controlled with a flow controller (Brooks, Model 5850E) and a tranquil flow pump was employed to inject water (Beijing Xingda Co., Model 2PB00C). After mixing, they flowed into the reactor. Four different types of reactors were designed: a packed-bed reactor (i.e., catalyst-only reactor), a dielectric barrier discharge (DBD) reactor (i.e., plasma-only reactor), a piecewise synergistic reactor, and a parallel synergistic reactor. These reactors were denoted as No.1, No.2, No.3, and No.4 (see Table 1), for the sake of analysis. Figures 3a and 3b shows the structures of synergistic reactors. The parallel synergistic
Table 1. Identification of Each Reactor reactor identification
type
No.1 No.2 No.3 No.4
catalyst-only reactor plasma-only reactor piecewise synergistic reactor parallel synergistic reactor
reactor (reactor No.4) was fabricated from double quartz tubes with a discharge gap of 3.5 mm. Catalysts were packed in the annular discharge area that was enclosed by the double quartz tubes. When the plasma source was turned off, the reactor served as a catalyst-only reactor (reactor No.1). Piecewise synergistic reactor (reactor No.3) 6597
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methane conversion: nCH4,inlet − nCH4,outlet χ= nCH4,inlet
(3)
The effective carbon recovery rate is an indicator for the form of carbon existence in reformed gases; it is defined as the ratio of the total number of moles of carbon (except high hydrocarbon) in reformed gases to the number of moles of carbon in methane from inlet. This indicator was proposed by the reason that the nonequilibrium characteristic of DBD made it easy to produce C2, C3, C4, and C4+ hydrocarbons using a form of adiabatic or even exothermic process, which is bad for recycling turbine exhaust heat. In addition, high hydrocarbons have some disadvantages on combustion efficiency or the leanblowout performance of the combustor. Thus, this indicator will be used as the primary consideration in the experimental analysis. effective carbon recovery rate:
Figure 3. Schematics of the reactor structures: (a) parallel synergistic reactor and (b) piecewise synergistic reactor.
γ=
was fabricated from a single quartz tube with a discharge gap of 1 mm and a length of 130 mm and catalysts were packed downstream of the discharge region. When no catalyst was packed downstream, the reactor served as a plasma-only reactor (reactor No.2). Except for reactor No.2, the amount of catalyst packed in each reactor was 35.36 g, with a packed void fraction of 50%. The catalysts employed in the experiments were supplied by Qilu Keli Chemical Institute Co., Ltd., which supplied the catalysts in reduced state, with the composition of 40 wt % Ni/SiO2 and a column size of ⌀2.5 mm × 2.5 mm. The catalysts then were activated and we could use them directly. The primary distinctions between reactor No.3 and reactor No.4 were the differences of catalysts filling position and discharge gap that it presents. However, these differences can be taken as intrinsic property of the reactors and the effects on reforming will be ascribed to the effect of reactor types in the next study. In order to maintain a constant temperature in the reaction zone, the reactors were heated by ceramic radiant heaters. Because of thermal inertia, the actual measurements deviated from the set point by ∼5 °C. The reactor was energized by a maximum ac high voltage of 6 kV at a frequency of 41.93 kHz (Nanjing Suman Co., CTP-2000K). The qualitative and quantitative analysis of the reformed gas were conducted by online gas chromatography (Agilent Model GC 7890A). Before flowing into the gas chromatograph, the reformed gas were cooled by a cold-trap and dried by allochroic silica gel. GC-FID (2 m DB-1 column + 25 m HP-Al/S column), GC-TCD B (Haysep Q column + molecular sieve 5 Å column), and GC-TCD C (Haysep Q column + molecular sieve 5 Å column) were employed to measure the volume fraction of hydrocarbons, CO/CO2/N2, and H2, respectively. In order to ensure the accuracy of measured results, the data were recorded after stabilizing the reforming system in any setting for 20 min.
nCO,outlet + nCO2,outlet + nCH4,outlet nCH4,inlet
(4)
Here, ni,inlet and ni,outlet are the component mole flows in the inlet and outlet, respectively (where i = CH4, CO, and CO2). The rate of increase of the fuel heating value can be taken as a comprehensive reflection of methane conversion and the product molar fraction. fuel heating value, rate of increase: ηLHV = =
Q LHV,reformed gas − Q LHV,CH Q LHV,CH ΔQ LHV Q LHV,CH
4
× 100
4
× 100 (5)
4
Here, QLHV,CH4 is the fuel low heating value of 1 mol of methane under standard conditions; QLHV,reformed gas is the fuel low heating value of the reformed gas generated from 1 mol of methane by steam reforming, where the effective components of reformed gas are residual CH4, H2, CO, and a small amount of C2H6 and C3H8. The definition of the total enthalpy increasing rate indicates the total contributions on energy utilization of the chemical and physical recuperation: total enthalpy increasing rate: ηtotal =
3. RESULTS AND DISCUSSION 3.1. Indicators of Steam Reforming Characteristics. There are some possible definitions to characterize methane reforming and hydrogen production in reactors, mainly including methane conversion and molar fraction of outlet components. Considering the nonequilibrium characteristic of DBD, we took the concept of effective carbon recovery rate as an comprehensive evaluation indicator for heat recovery and combustion. To be clear, we also analyzed the effect of different types of reactors on fuel heating value increasing rate and total enthalpy increasing rate under different working conditions. It shows the contributions of physical and chemical recuperation to the reactor performance, which also a reflection of energy utilization.
ΔQ total Q LHV,CH
=
ΔQ SH + ΔQ LHV Q LHV,CH
4
(6)
4
Here, ΔQSH is the change in fuel low heating value (as defined by eq 7) and ΔQLHV is the change in sensible enthalpy (as defined by eq 8). ΔQ SH =
∑ ΔHj = ∑ noutxj(HT ,j − H25 °C,j) (7)
j
ΔQ LHV =
∑ noutxjQ LHV,j − Q LHV,CH j
4
(8)
Here, nout is the total molar number after reforming 1 mol of methane, HT is the enthalpy of each product component at the exit temperature T, and xj is the molar fraction of each 6598
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component (the subscript j is the product component, which includes CH4, CO2, CO, H2, C2H6, C3H8, and H2O). 3.2. Effect of Methane Space Velocity on Reforming Characteristics. We define methane space velocity to be the volume flow rate of methane per unit mass of catalyst, as shown in eq 9. It is used as an indicator of the processing capacity of reactors. vCH4,SP =
VCH4 mc
× 60
Table 3. Effective Carbon Recovery Rate, Relative to the Methane Space Velocity, in Different Reactors Effective Carbon Recovery Rate vCH4,sp [mL/(gcat·h)]
No.1
No.2
No.3
No.4
171 340 510 680
94.04 96.25 97.29 97.36
74.79 86.39 90.73 99.02
80.13 90.96 100.00 100.00
82.82 94.48 100.00 100.00
(9)
velocity in different reactors. Since a higher effective carbon recovery rate was the primary condition for a higher ηLHV or ηtotal, a higher methane space velocity was chosen in the next experiments, although methane conversion decreased, as shown in Figure 4. At last, the methane space velocity of 680 mL/(gcat· h) was employed. In Table 2, it was notable that the effect of methane space velocity on reactor No.3 or No.4 was less than that in reactor No.2. This was due to the packed catalysts in reactors No.3 and No.4. The selectivity of Ni catalysts will promote the conversion of hydrocarbon (CH4, C2H6, C3H8, or C4+) and H2O to CO, CO2, and H2. In reactor No.3, Ni catalyst packed downstream made C2H6 or higher hydrocarbons generated in the discharge region be further converted into CH4, CO, CO2, and H2.19 In reactor No.4, the Ni catalyst packed in the discharge region inhibited the coupling of CH3 and directly reduced the yield of higher hydrocarbons.20 3.3. Effect of S/C on Reforming Characteristics. Figure 5 shows the effect of S/C on methane conversion in each
Here, VCH4 is the inlet volume flow rate of methane and mc is the mass of the catalyst. Since the inlet volume flow rate in reactor No.2 is the same as that in others, for the sake of comparison, methane space velocity is still employed to study the effect of inlet volume flow rate on reforming characteristics, despite the absence of catalyst in reactor No.2. Figure 4 shows the effect of methane space
Figure 4. Effect of methane space velocity on methane conversion.
velocity on methane conversion in different reactors. The operating conditions included a wall temperature of 400 °C, an input power of 70.13 W, a frequency of 41.93 kHz, and S/C = 2. When methane space velocity was at a minimum of 171 mL/ (gcat·h), the methane conversion in each reactor was at a maximum. Besides, methane conversion in reactors No.2, No.3, and No.4 were higher than that in reactor No.1. However, both ηLHV and ηtotal were lower than that in reactor No.1, even to be negative, which was not expected, as shown in Table 2. The direct reason was that reactors No.2, No.3, and No.4 had a lower effective carbon recovery rate than 83%, which meant more than 17% of methane was converted into higher hydrocarbons. This was due to the effect of nonequilibrium characteristic of DBD. CH3 generated by electron collision was coupled with each other to form C2H6 and C3H8, which would be further dehydrogenated to couple into C4 or higher (C4+) hydrocarbons as the residence time increased. Table 3 shows effective carbon recovery rate increased with methane space
Figure 5. Effect of S/C on methane conversion.
reactor. Operating conditions included a wall temperature of 450 °C, an input power of 70.13 W, and a frequency of 41.93 kHz. In order to keep the inlet volume flow rate of reactants constant in each reactor, the inlet flow rate of methane was reduced correspondingly as S/C increased, with methane space velocities of 680, 507, 406, and 336 mL/(gcat·h) for S/C values of 2, 3, 4, and 5, respectively. For any of the S/C values,
Table 2. Comparative Analysis of Reforming Characteristics in Different Reactors at a Methane Space Velocity of 171 mL/(gcat·h) Molar Fraction of Outlet Components [%] reactor
χ [%]
γ [%]
ηLHV[%]
ηtotal[%]
CH4
CO2
CO
H2
C2H6
C3H8
H2O
No.1 No.2 No.3 No.4
15.56 31.66 26.96 26.19
94.04 74.79 80.13 82.82
−4.48 −22.42 −19.42 −16.64
10.95 −6.58 −4.11 −1.42
26.87 23.97 25.24 25.05
3.05 0.00 2.30 2.68
0.00 0.53 0.00 0.18
12.56 4.78 8.62 10.56
0.00 0.69 0.00 0.00
0.00 0.19 0.00 0.00
57.52 69.84 63.84 61.52
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trend of methane conversion increased first and then leveled off. There were two reasons: one was related to the setting of experimental conditions, where methane content decreased with S/C; the other was that the concentration of OH and H generated by energetic electron-impact H2O increased as S/C increased, which accelerated the chain reactions between methane molecules and free radicals. With the increase of S/C, methane conversion in reactors No.3 and No.4 increased gradually. The growth trend was slightly higher than that in reactor No.1, which was attributed to the synergistic catalysis between plasma and catalyst. As shown in Table 5, ηtotal at an S/C value of 5 reached or exceeded 30%, because of more steam and its higher molar specific heat capacity. However, the effective carbon recovery rate varied in the range of 81%−88% in each reactor, because of the low methane space velocity of 336 mL/(gcat·h). Although the effective carbon recovery rate in reactor No.4 was slightly lower than that in reactor No.1, the higher methane conversion resulted in the increased hydrogen concentration in reformed gas. 3.4. Effect of Wall Temperature on Reforming Characteristics. Figure 7 shows the effect of wall temperature
methane conversion in each reactor decreased in the following order: No.4 > No.3 > No.2 > No.1 For reactor No.1, methane conversion linearly increased as the S/C value increased, namely, more steam contents led to a greater methane conversion, which was consistent with Le Chatelier’s Principle. However, plasma reforming has an intrinsic difference from catalyst reforming. In order to reveal plasma-catalysis mechanism on steam reforming, we calculated Boltzmann equation with BOLSIG+ solver21 based on electron collision cross-section data22 and obtained the energy deposition on the main electron-impact reactions in CH4/H2O gas mixture, as seen in Figure 6. Because of the dissociation threshold of H2O
Figure 6. Effect of S/C on energy deposition (E/N = 188 Td, T = 450 °C) (note that 1 Td = 10−17 V cm2).
Table 4. Electron-Impact Threshold for Methane and Steam reaction R1 R2 R3 R4 R5 R6 R7
reaction equation E* E* E* E* E* E* E*
+ + + + + + +
threshold [eV]
CH4 = CH4(v1) + E CH4 = CH4(v2) + E CH4 = CH3 + H + E H2O = H2O (v1) + E H2O = H2O (v2) + E H2O = H2O (v3) + E H2O = OH + H + E
Figure 7. Effect of wall temperature on methane conversion.
0.162 0.361 9.00 0.206 0.459 1.058 7.00
on methane conversion in each reactor. The operating conditions included a methane space velocity of 680 mL/ (gcat·h), an S/C value of 2, an input power of 70.13 W, and a frequency of 41.93 kHz, respectively. Figure 7 shows that methane conversion in each reactor decreased in the following order under any wall temperature: No.4 > No.3 > No.1 > No.2
lower than that of CH4 (as seen in Table 4), with an increase of S/C, energy deposition on reaction R7 increased and correspondingly decreased on reaction R3. This result suggested that the high-energy electrons were partitioned between methane and steam in proportion to their contents. In other words, higher S/C values led to a lower methane conversion by plasma catalysis, which was consistent with the literature.23 Actually, for reactor No.2, it was notable that the
For reactor No.2, methane conversion was only slightly affected by wall temperature, with a constant value of 4%. For reactors No.1, No.3, and No.4, methane conversion increased notably as the wall temperature increased, thanks to the catalytic activity of Ni catalysts. Moreover, attributing to the synergistic catalysis between plasma and catalysts, methane conversion in reactor No.3 or reactor No.4 was higher than that in reactor No.1.
Table 5. Comparative Analysis of Reforming Characteristics in Different Reactors at S/C = 5 Molar Fraction of Outlet Components [%] reactor
χ [%]
γ [%]
ηLHV[%]
ηtotal[%]
CH4
CO2
CO
H2
C2H6
C3H8
H2O
No.1 No.2 No.3 No.4
26.49 18.87 32.91 43.85
87.72 83.33 81.12 83.85
−7.00 −15.49 −9.13 −3.60
30.39 23.09 26.81 32.76
11.94 13.88 13.02 8.58
2.29 0.00 3.04 4.10
0.00 0.12 0.00 0.13
10.43 1.04 11.25 20.27
0.00 0.15 0.00 0.00
0.00 0.00 0.00 0.00
75.34 84.81 72.68 66.92
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Table 6. Comparative Analysis of Reforming Characteristics in Different Reactors at a Wall Temperature of 500 °C Molar Fraction of Outlet Components [%] reactor
χ [%]
γ [/%]
ηLHV[%]
ηtotal[%]
CH4
CO2
CO
H2
C2H6
C3H8
H2O
No.1 No.2 No.3 No.4
20.14 3.97 25.06 46.51
97.07 95.13 100.00 100.00
1.36 −1.82 5.34 10.15
18.30 16.26 21.93 25.28
23.83 30.68 22.13 13.63
4.16 0.00 4.84 6.78
0.57 0.14 0.15 4.92
21.32 1.65 24.17 42.18
0.00 0.42 0.00 0.00
0.00 0.08 0.00 0.00
50.12 67.02 48.71 32.50
There were two main influencing factors: excited species and heat generated by plasma. On one hand, high-energy electron densities increased with an increase of input power, which generated more vibrationally excited components and radicals and thus to enhance synergistic effect. On the other hand, electric field intensity increased as the input power increased, resulting in an increase of polarization losses and ohmic losses inside the dielectric, which were finally converted into heat. However, the latter one was not expected and should be forbidden as much as possible. Table 7 shows that ηLHV had nearly doubled to 10.76% at an input power of 84.53 W from 5.92% at an input power of 19.58 W, which was consistent with the change of methane conversion, since γ was uniform. While ηtotal only increased by 16.6%, from 22.06% to 25.73%, because the physical recuperation gradually reduced as the steam concentration decreased. 3.6. Theory Analysis of Parallel Synergistic Catalytic Reforming Technology Application. In a synergistic reaction, vibrationally excited species and radicals generated by electron impact with a threshold energy of 0.162−0.361 eV/ CH 4 and 9−12 eV/CH 4 , respectively, could enhance chemisorption and chemical desorption. The dissociation energy of methane in C−H bond was 4.26 eV/CH4. This indicated that, when radicals were responsible for methane reforming, more than half of the input power would be wasted as excess input energy. However, if vibrationally excited species were taken as the main intermediate products, they would not consume excess input energy and could still greatly enhance chemical conversion at a low temperature. Therefore, in order to reduce extra energy consumption and improve methane conversion, the principal thing is to seek a best-reduced electric-field intensity to improve the concentration of vibrationally excited species. Figure 9a shows the effect of reduced field intensity on energy deposition of the electronimpact methane reactions in a CH4/H2O(g) mixture under different temperatures. It was calculated by BOLSIG+ code with the electron-impact cross sections of CH4 and H2O(g). The reaction temperatures had no effect on electron-impact reactions, and all of the lines at different temperatures for a certain reaction were coincident. The energy deposition first increased and then decreased for excitation, while the ernegy deposition first increased and then leveled off for dissociation and a growth trend was maintained for ionization. Since the excitation is mainly used for the production of vibrationally excited species, when the S/C value was 2, there existed an optimum value of 60 Td (1 Td = 10−17 V cm2). In addition, Figure 9b shows that there always existed an optimum value of more or less 60 Td, whatever the S/C value. Therefore, to reduce energy loss and make full use of parallel synergistic reforming technology, the ideal plasma source should be characterized by moderate electron density (1013− 1015 cm−3) and low reduced field (
