Research of Methane Reforming and Combustion Characteristics in

Jan 16, 2014 - In this study, the methane steam reforming test bed is designed and built under the parameters of a chemically recuperated gas turbine ...
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Research of Methane Reforming and Combustion Characteristics in Chemically Recuperated Gas Turbine Xiao Liu,* Hongtao Zheng, and Qian Liu College of Power and Energy Engineering, Harbin Engineering University, Harbin 150001, People’s Republic of China ABSTRACT: In this study, the methane steam reforming test bed is designed and built under the parameters of a chemically recuperated gas turbine (CRGT); relevant experimental research about fuel reforming is also conducted. Three kinds of reforming are used, including catalyst reforming, plasma reforming, and apposition synergy reforming. Methane conversion and fuel calorific value increase rate are discussed, and the performance of synergy is best. Synergy reforming is an effective measure to improve disadvantages of the plasma and catalyst reaction process. In addition, combustion characteristics are compared and analyzed using reformed gas in a diffusion flame. The flame is shortened and becomes narrow due to the inflammability of H2, and the maximum temperature of flame drops almost 500 K because of the water steam in reformed gas. Ultralow NOx emission is calculated in a jet flame, which is the key advantage of CRGT. pathways under nonthermal plasma. De Bie et al.18 developed a one-dimensional fluid model for a dielectric barrier discharge in methane to study underlying reaction pathways of higher hydrocarbons. Combined plasma and catalyst has been explored to exploit possible synergetic effects by Nair et al.19 and Nozaki et al.;20 the chemical kinetics model was obtained. Furthermore, methane reforming under different discharge forms (DBD, BEC) combined with catalyst was researched by theoretical calculations and experimental studies, and the main influencing factors of methane conversion were discussed.21−25 A numerical simulation method with its high efficiency and economic advantages has been more and more recognized in recent years:26 the combustion characteristics such as temperature, component distribution, pollutant emissions, etc. In this paper, the methane steam reforming test bed and the generator were designed and built matching the parameters of CRGT. The methane conversion rate and fuel calorific value increase rate are discussed under three reforming forms, which are catalyst reforming, plasma reforming, and synergy reforming. Then, according to experiments data of reformed gas compositions, combustion characteristics were calculated and compared with numerical simulation in coflowing turbulent jet flame.

1. INTRODUCTION Chemical recuperation is one of several innovative concepts applicable to natural gas-fired gas turbine-based power generation cycles. The key advantages of a chemically recuperated gas turbine (CRGT) is potential for high cycle efficiency and ultralow NO emissions.1,2 Figure 1 shows the CRGT concept. The exhaust thermal energy is recovered in the steam reformer (SR) replacing the superheater section and in the heat recovery steam generator (HRSG). The steam produced by the HRSG is mixed with the fuel and fed to the SR. In the reformer, a mixture of natural gas and steam is heated by the combustion turbine exhaust, and an endothermic reaction occurs between the fuel and the steam in the presence of a nickel-based catalyst. The reformed gas, which is composed chiefly of CO, CO2, H2, H2O, and unconverted CH4, is then fed to the combustion chamber.3 In the past decade, many scholars have done a lot of experimental and numerical studies on CRGTs.4−8 As the kernel component of CRGT, the SR, and methane catalytic reforming have been studied, a thorough study of the reaction mechanism, influencial factors, and type of catalyst was carried out.9−13 However, most of the research is used for preparation of hydrogen industrially,14 and many disadvantages of SR are found such as the recuperator size being too large, the fuel−steam reaction depth being low, the reaction not being controllable, and problems related to catalyst poisoning using conventional catalyst reforming. In addition, the optimal range of temperatures for operating the nickel catalyst reforming reaction is around 800 °C or higher.15 However, the export of the simple gas turbine cycle (or partial-load condition) exhaust temperature is typically around 400−500 °C,16 so the performance of the catalyst is relatively poor. Fuel steam reforming reaction depth and efficiency of chemically recuperated cycle are restricted. Therefore, nonthermal plasma and the idea of a plasma catalysis effect has been investigated as a possible means to enhance or replace traditional thermal or catalytic reforming systems. Sugasawa et al.17 investigated the effects of initial water content on hydrocarbon conversion and proposed reaction © 2014 American Chemical Society

2. EXPERIMENTAL SYSTEM DESCRIPTION The experimental apparatus and process are illustrated in Figure 2. The feed gas was composed of methane and steam, where steam was generated by a steam boiler. The flow rate of methane and water was separately controlled with a mass flow controller (Brooks 5850E) and tranquil flow pump (Beijing Xingda Co., 2PB00C). The feed gas flew into the reactor after homogeneous mixing. The reactor was heated by a ceramic heating pipe and maintained a constant wall temperature Received: Revised: Accepted: Published: 1940

September 12, 2013 December 20, 2013 January 16, 2014 January 16, 2014 dx.doi.org/10.1021/ie403014c | Ind. Eng. Chem. Res. 2014, 53, 1940−1946

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Figure 1. Chemically recuperated gas turbine concept.

Figure 2. Schematic diagram of experimental setup.

Figure 3. Structures of reactor and plasma in a planar configuration.

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through the temperature controller. Qualitative and quantitative analysis of plasma products were carried out online with a gas chromatograph. Before flowing into the gas chromatograph, the products were cooled by a cold trap and dried by allochroic silica gel. The concentrations of hydrocarbons and synthesis gas were determined by GC-FID (2 m DB-1+ 25 m HP-Al/S Column) and GC-TCD (Haysep Q Column + Molecular Sieve 5A Column), respectively. Experiments were carried out at a constant steam/carbon ratio (S/C). The inlet temperature was fixed at 673 K, and the operating pressure was atmospheric in all cases. During the experiment, to ensure that the temperature in the catalytic bed maintained the set value, the reactor was heated by the ceramic heater for 2 h before starting the experiment. Temperature transmitters and controllers were used for temperature measurement. Due to thermal inertia, measured values were deviated from the set point of about 5 °C, seen from the controller display. The structures of the reactor and nonthermal dielectric barrier discharge plasma are shown in Figure 3.

Figure 4. Variation of methane conversion and fuel calorific value increase rate.

the conversion rate of the plasma form is 3% higher than catalyst. This is because, only for plasma reforming, the export component contains C2H6 and C3H8. This shows that, under the effect of plasma, the conversion reaction of methane is a multipath: steam reforming and a coupling reaction. Various types of free radicals from ionization of the plasma generate high-carbon hydrocarbons. The coupling reaction is an exothermic reaction at low temperature; therefore, although the conversion rate of methane is increased, the calorific value of reformed gas changes little or even reduces. When the catalyst is added in the discharge region, synergy reforming is an effective measure to improve the disadvantages of the plasma chemical reaction process with high energy consumption and poor selectivity; a radical reaction and conventional steam methane reforming reaction occur. Therefore, the methane conversion and fuel calorific value increase rates are highest (51.8% and 22.3%) in all reforming forms. The reseach27 indicates shows that the stretching vibrations of chemically excited methane molecules are up to 1600 times more reactive on a clean Ni(100) surface than are molecules in the ground vibrational state; thus, the effect will enhance the rate of adsorption and desorption on the catalyst at a low temperature (400 °C) and accelerate the reforming reaction. 3.2. Combustion Characteristics Analysis. 3.2.1. Flame and Model Description. The flame configuration is an axisymmetric, coflowing turbulent jet flame, so one-half of the cross section is given in Figure 5. The nozzle radius of the

3. RESULTS AND DISCUSSION 3.1. Methane Reforming analysis. Table 1 shows the mole percentage of the reformed gas component. The nickel Table 1. Mole Percentage of the Reformed Gas Component gas composition in volume (%) reforming form

CH4

CO2

CO

H2

C2H6

C3H8

H2O

methane catalyst plasma synergy

100.00 26.76 29.97 12.18

0.00 2.69 0.00 6.78

0.00 0.00 0.14 5.67

0.00 12.19 2.32 44.54

0.00 0.00 0.74 0.00

0.00 0.00 0.17 0.00

0.00 58.36 66.65 30.83

catalyst was used, well-known as a good catalyst for methane steam reforming. The steam/carbon ratio was kept at 2, and the discharge power and frequency were 70 W and 42 kHz, respectively. The inlet temperature was fixed at 673 K, and the operating pressure was atmospheric in all cases. The methane conversion rate YCH4 is defined as mCH4,in − mCH4,out YCH4 = × 100% mCH4,in (1) where mCH4,in is the mass flow of the inlet methane, g/min. The calorific value increase rate R is defined as R= =

Q out Q in

× 100% Figure 5. Flame configuration.

Q cv,out + Q sn,out − (Q cv,in + Q sn, i n) Q cv,in

× 100%

burner is 2 mm with an annulus of 0.16 mm width for the premixed pilot flame; the length and radius of the burner are 1000 mm and 77.5 mm, respectively. The mean inlet velocity of the fuel jet is 20.3 m/s, and the jet Reynolds number of the flame is 5000. Experimental data28 of the axial and radial profiles of the mean temperature are available in the case of CH4 (100%) and used for model validation. The structured mesh used for numerical simulation is shown in Figure 6. The two-dimensional mesh was generated with the axial symmetry hypothesis to save the calculation expend, and the optimal grid size was 23 482 cells after grid-independent

(2)

where Qcv,in is the equivalent low heating value (LHV) of inlet methane, kJ, Qcv,out is the equivalent low heating value (LHV) of reformed gas, kJ, and Qsn,in and Qsn,out are the sensible enthalpy of each component, kJ. The methane conversion and fuel calorific value increase rate are the main parameters for the degree of methane reforming, and Figure 4 shows the variation for three reforming forms. The calorific value increase rate of the reformed gas is about 16.4% for catalyst reforming and 11.2% for plasma, although 1942

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Figure 6. Structured mesh used for numerical simulation.

Figure 7. Profile of predicted temperature against measurements at different locations.

verification. The SIMPLE method was used for velocity− pressure coupling, and all governing equations were solved using a second-order upwind discretization scheme. The convergence criterion demanded that the sum of the residuals of the discretized equations over the domain dropped below 10−4, except for the energy, where a tolerance of 10−5 was used. Many combustion models have been developed to simulate turbulent nonpremixed flames, such as the eddy dissipation (ED) model,29 eddy dissipation concept (EDC) model,30 steady flamelet model (SFM),31 and composition PDF transport model.32 The ED model is based on a fast chemical reaction; it can take into account the overall chemical reaction rate of one or two steps and is sometimes used for modeling combustion equipment having a turbulent nonpremixed flame. The EDC, laminar flamelet, and composition PDF transport models take into consideration the detailed chemical mechanisms to predict the mass fractions of chemical species, including intermediate species such as H, O, and OH. The DRM19 chemical mechanism33 was used in this work to describe the combustion of natural gas which is a subset of the GRI-Mech 1.2 full mechanism, with 19 species and 84 reactions, developed to obtain the smallest set of reactions

needed to closely reproduce the main combustion characteristics predicted by the full mechanism. As the simplest “complete model” to predict the turbulent combustion reaction, the k − ε model has been widely used in the turbulent diffusion flame in the past few years;34−37 it has been extensively validated for a wide range of flows. The discrete ordinate radiation model was used in this work, as it is applicable across a wide range of optical thicknesses. In addition, thermal and prompt NOx are considered in hightemperature combustion; the NOx model is carried out in the postprocessing stage. 3.2.2. Model Validation. In order to validate the numerical model, predicted axial and radial (x = 150, 200, 250 mm) temperature gradients were compared to flame temperature measurements, and the distribution is shown in Figure 7a−d. The predicted axial temperature gradient shows satisfactory agreement with the measurements in terms of both trend and value with SFM. The temperature predicted by the ED and PDF models is about 200 K higher than experimental data. The performance of the EDC model is the worst; it cannot capture the features in the fuel jet region well, although a detailed chemical mechanism is used. The radial temperature gradients 1943

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represent the breadth of the reaction zone of the flame. The predicted radial gradients of temperature imply a slightly broader reaction zone, when compared to their corresponding measurements by SFM, whose performance is better than the ED, PDF, and EDC models. The basic concept of flamelet models views the turbulent flames as an ensemble of laminar flamelets. Each laminar flamelet is subjected to the local flow field convecting and stretching in terms of the instantaneous scalar dissipation rate at the stoichiometric condition. A steady-state assumption for the laminar flamelet is often made, so that the local flame structure can be described only by the local mixture fraction and local scalar dissipation rate at a stoichiometric mixture. This is the so-called steady flamelet model (SFM). It is computationally tractable for the SFM since the laminar flamelet structure can be precalculated and tabulated into a flamelet library with the mixture fraction and stoichiometric scalar dissipation rate as independent variables, which makes the SFM very popular in turbulent combustion studies. These results demonstrate that the realizable k − ε model within the flamelet approach predicts correctly the flame shape and the distribution of the flame temperature. 3.2.3. Combustion Characteristics. The temperature distribution of methane and reformed gas by synergy reforming is shown in Figure 8. Due to the change of inlet temperature

Figure 9. Temperature profile of axis and radial distance at X = 200.

Figure 8. Temperature distribution of methane and reformed gas.

(673 K) and the inflammable component (H2), the shape is totally different: the high-temperature region moves forward and the length of the flame is shortened by almost one-half. The maximum temperature of the flame drops almost 500 K because of the water steam existence in reformed gas. The flame shape of the three forms is almost the same, but the temperature of synergy reforming is highest because of the high methane conversion and fuel calorific value increase rate, obviously. The simulated temperature along the axis and radial direction is plotted in Figure 9a and 9b. It is easily seen that the reformed gas reacts completely before x = 500 mm and the flame becomes narrower. The reduction in flame thickness can be related to enhanced mixing of fuel and oxidant. Turbulent mixing is of great importance in turbulent nonpremixed flames as chemical reactions occur only if fuel and oxidant are mixed at the molecular level. The radial profiles (x = 200) of the mixture fraction and its variance are plotted in Figure 10a and 10b. The mixture fraction and its variance are considered as an indicator of

Figure 10. Mixture fraction and variance of radial distance at X = 200.

mixing quality.38 From these predictions, it can be observed that the radial spreading rate of the mixture fraction and its corresponding fluctuations are reduced; the reducing proportion is in agreement with the content of H2 in reformed gas. This can be taken as a sign of mixing enhancement. The radial profile of the mole fraction of H2 and CH4 is presented in Figure 11a and 11b. The initial mole fraction of CH4 is different due to the methane reforming reaction, but the variation tendency is unanimous: reduced with an increase of the axial distance. The tendency of the H2 mole fraction is generally similar for the three reformed gases, but for methane combustion, the mole fraction of H2 rises at first and then decreases to 0. This is because methane is converted into 1944

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(2) The flame shape is totally different: the flame is shortened and becomes narrow due to the inflammability of H2. The maximum temperature of the flame drops almost 500 K because of the water steam existence in reformed gas. (3) NO production is in agreement with the region of high temperature, and the maximum NO emission decreases sharply from 140 to almost 0 ppm using reformed gas.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Fundamental Research Funds for the Central Universities of China (HEUCF120303).

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Figure 11. Species (CH4 and H2) profile of radial distance at X = 200.

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hydrogen by intramolecular recombination under thermal reforming according to the mechanism and then burns out. The contour of NO (ppm) is presented in Figure 12. The rich region of NO is consistent with the high-temperature

Figure 12. NO distribution of methane and reformed gas.

distribution; this is because the thermal NO formation mechanism in the combustion chamber is mainly controlled by the temperature, so the rate is largely dependent on the temperature rather than the type of fuel. The maximum content of NO decreases sharply from 140 to almost 0 ppm, and the potential for ultralow NO emission is one of the key advantages using reformed gas in CRGT.

4. CONCLUSION Methane steam reforming was analyzed under three kinds of reforming including catalyst, plasma, and synergy reforming. In addition, a turbulent nonpremixed flame was calculated and compared using methane and reformed gas. The main conclusions include the following. (1) The methane conversion rate of catalyst and plasma and synergy reforming is 17.5%, 20.8%, and 51.8%, respectively, and the fuel calorific value increase rate is 16.4%, 11.2%, and 22.3%. Although the conversion rate of plasma reforming is higher than catalyst, the calorific value increase rate is lower because of the coupling reaction of free radicals. The performance of synergy reforming is best due to the promotion and complement of catalyst and plasma. 1945

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