Moderate or Intense Low-Oxygen Dilution Combustion of Methane

Article pubs.acs.org/EF

Moderate or Intense Low-Oxygen Dilution Combustion of Methane Diluted by CO2 and N2 Jianpeng Zhang,† Jianchun Mi,*,† Pengfei Li,† Feifei Wang,‡ and Bassam B. Dally§ †

State Key Laboratory of Turbulence and Complex Systems, College of Engineering, Peking University, Beijing 100871, People’s Republic of China ‡ Department of Building Environment and Equipment Engineering, School of Environmental Science and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, People’s Republic of China § Centre of Energy Technology and School of Mechanical Engineering, The University of Adelaide, Adelaide, South Australia 5005, Australia ABSTRACT: Moderate or intense low-oxygen dilution (MILD) combustion is regarded as one of the most effective technologies to achieve extremely low NOx emissions of combustion. MILD combustion diluted by CO2 and by N2 are herein termed “MILD oxy-combustion” and “MILD air combustion”, respectively. The present study is to investigate the difference of the two by experimental observation in a furnace of 20 kW and chemical kinetics calculation of a well-stirred reactor (WSR). Also, to identify their difference in mechanism, reaction paths of combustion diluted with N2 and CO2 are examined. It is revealed that the region of MILD oxy-combustion is notably larger than that of MILD air combustion for gaseous fuels, which suggests that the requirement for establishing MILD combustion is less stringent with dilution by CO2 than by N2. The key reason is that the CO2 dilution substantially lowers the temperature rise because of combustion, delays the ignition, and slows the overall reaction rate, thus facilitating the occurrence of MILD combustion. Detailed analyses show that the temperature reduction derives from the physical effect of CO2 dilution, while the ignition delay results mainly from the chemical effect. Moreover, the investigation of reaction paths suggests that the CO2 dilution increases the local CO production mainly through H + CO2 → OH + CO and CO2 + CH2(s) → CO + CH2O. effectively achieve the goal of “near-zero emission” for fossil fuel combustion. Although much progress has been made in the research and development of both oxy-combustion and MILD combustion in parallel, only limited investigations7−11 have been reported by considering their combination, even broadly in terms of any gaseous, liquid, and solid fuels. Blasiak et al.7 examined the main characteristics of MILD oxy-combustion at an industrialscale furnace. Those investigators found that higher combustion efficiency and lower emissions of exhaust NOx, CO, soot, and particulate matter can be achieved in the MILD oxycombustion than the traditional case. Especially, NOx emission becomes insensitive to air ingress in the MILD oxy-combustion. In their study, they developed an overall two-regime heattransfer model for the combustion and found that heat flux to the wall increases when flame volume becomes larger. It is thus implied that MILD oxy-combustion can enhance the heat transfer. Krishnamurthy et al.8 established MILD oxycombustion in a 200 kW furnace by a slightly asymmetric, high-speed oxygen injection. By experiment and numerical computation, those investigators obtained and compared the combustion characteristics of the conventional flame and MILD modes. They found that the flame of MILD oxy-combustion is bluish and almost invisible. Lower NOx emission and more uniform total heat flux were observed under MILD oxy-

1. INTRODUCTION The oxy-combustion has been extensively investigated for reduced emissions of CO2 and pollutants, such as NOx, SOx, and soot, from burning fossil fuels.1−4,7−13 To realize this combustion, the mixture of O2 and CO2 is used as the oxidizer, which creates a natural advantage in combining with carbon capture and storage (CCS)1,2 because of the composition of its exhaust gas being predominantly CO2 and some H2O. The oxycombustion is also proposed to eliminate the NOx production in an ideal situation. However, there are a number of challenges that have limited the oxy-combustion in practical applications. For instance, the air ingress to a high-temperature environment in practical applications of oxy-combustion of gaseous fuels often brings about high NOx emission.3 Also, the related ignition is delayed by the dilution of CO2, so that the flame stability needs to be enhanced.2,4 Meanwhile, over the last 20 years or so, moderate or intense low-oxygen dilution (MILD) combustion or flameless oxidation (FLOX) has developed and been regarded as the most potential technology for substantial reduction of NOx emission during combustion.5 As long as the MILD mode is established, combustion becomes very stable over an expanded reaction zone across which the burning temperature is quite uniform.5,6 Thus, the MILD combustion is expected to be an ideal solution to the above problems of the oxy-combustion. It follows that a combination of these two technologies, herein termed as “MILD oxy-combustion”, has been investigated;7−11 note that the MILD oxy-combustion is the MILD mode of oxy-combustion. In such a way, the combined advantages of the two technologies would help to © 2015 American Chemical Society

Received: March 10, 2015 Revised: May 27, 2015 Published: May 27, 2015 4576

DOI: 10.1021/acs.energyfuels.5b00511 Energy Fuels 2015, 29, 4576−4585

Article

Energy & Fuels

Figure 1. Schematic of the MILD combustion furnace.

combustion. Stadler et al.9 investigated NOx emissions under different diluent atmospheres for coal combustion and confirmed the advantage of low NOx emissions in MILD conditions. The thermal NO was drastically reduced, while the fuel NO was slightly increased because of the lower temperature under MILD mode. Also, they found that the alteration from conventional to MILD oxy-combustion allows for a lower oxygen concentration in the oxidizer to establish stable combustion. In their recent experimental study of oxycoal combustion, Stadler et al.10 analyzed the formation and reduction mechanisms of NOx in the conventional and MILD oxy-combustion processes and found that the flue gas recirculation reduces both the total NOx yield and the conversion of fuel N to NO. Particularly, their investigation indicates that wet recirculation is more preferable in terms of NOx emission. Our recent study by Li et al.11 investigated the MILD oxy-combustion of firing both light oil and pulverized coal by experiment at a pilot-scale furnace of 300 kW and examined the effects of burner design and dilution level on the combustion appearance of liquid and solid fuels and emissions of CO and NOx. It was found that NOx reduction is more efficient under MILD oxy-combustion than the air case, while the burnout rate needs to be improved. Different dilution methods should result in non-identical requirements for establishing MILD air combustion and MILD oxy-combustion. Indeed, our numerical investigation of jet in hot co-flow (JHC) by Mei et al.12 has demonstrated that the reaction zone is larger, whereas the temperature rise because of combustion is lower, with co-flow diluted by CO2 than by N2. In other words, the MILD combustion should be achieved more easily with dilution of CO2. In a different study,13 we have investigated the MILD oxy-combustion of natural gas, liquefied petroleum gas, and ethylene at a laboratory-scale furnace of 13 kW. It was revealed that the MILD combustion could be wellestablished for the three fuels with diluted oxidant or even pure oxygen. Under MILD combustion, both the N2O intermediate formation and NO-reburning destruction mechanisms are important for the NOx emission. Particularly, Li et al.13 found by analyzing the stability limits of MILD combustion diluted by CO2 and N2 that the MILD oxy-combustion may be established with a lower equivalence ratio and a lower dilution rate

compared to the case where MILD air combustion occurs. Thus, a deduction is made here that the occurrence of MILD oxy-combustion is more attainable than that of MILD air combustion. However, because the turbulence−chemistry interaction is complicated and differs from furnace to furnace, it is uncertain whether or not the deduction works in general. Also, the previous results noted above are qualitative and have not identified the dissimilarity between MILD oxy-combustion and MILD air combustion. For example, for the difference in establishing the two combustion processes or, more specifically, when the diluent is changed from N2 to CO2, the differences between the physical and chemical effects on the characteristics of MILD combustion require further investigation. The present study is designated to address the above issues by looking at the MILD combustion when it is diluted by N2 and by CO2. The specific aim is 2-fold: (1) to distinguish the difference in establishing MILD mode under N2 and CO2 dilutions and (2) to compare the characteristics of MILD oxycombustion and MILD air combustion, so that the physical and chemical effects on making these discrepancies can be identified. The rest of this paper is organized as follows: The experimental and numerical methods are introduced in section 2. In section 3, observational experiments and detailed chemical kinetic calculations using CHEMKIN14 are conducted to testify whether MILD oxy-combustion is more realizable than the MILD air combustion. Then, the temperature rise and ignition delay time under MILD oxy-combustion and MILD air combustion are compared under different equivalence ratios and reactant temperatures through kinetic calculations in section 4. Particularly, fictituous diluents are introduced to quantitatively separate the physical and chemical effects of diluents. Chemical characteristics, including species formation, reaction paths, and reaction sensitivity, are also analyzed, which may advance our understanding to the chemical process of MILD oxy-combustion. Section 5 provides the main conclusions drawn from the present study.

2. INVESTIGATION METHODS 2.1. Experiment. To compare the establishments of MILD combustion under CO2 and N2 dilutions, observational experiments were conducted of firing natural gas (NG) and liquefied petroleum gas 4577


Article

Energy & Fuels (LPG). Here, the MILD combustion is identified as being flameless, i.e., invisible at all. The configuration of the MILD combustion furnace is shown in Figure 1. There are 10 pluggable observation windows (A1−A5 and C1−C5) and an adjustable heat exchanger installed on the cuboid furnace. The images for the present observational experiment were taken through the C5 window. The burner consists of a single fuel nozzle on the center axis of the furnace, and four exhaust and four air ports arrange symmetrically in a ring pattern on the same wall. These four air nozzles and the central fuel nozzle have inner diameters of Da = 4.0 mm and Df = 7.2 mm, respectively. The exhaust ports are 25 mm in diameter. The thermal input of the furnace in the present study is approximately 13 kW by firing NG (XCH4 = 91.36%; XC2H6 = 4.36%; XC3H8 = 0.62%; XCO2 = 2.08%; and XN2 = 1.28%) and LPG (XC3H8 = 96.32%; XC2H6 = 1.12%; XC4H10 = 1.78%; XCO2 = 0.05%; and XN2 = 0.73%). Reactants at room temperature (288 K) were injected into the furnace. The oxygen concentration of the oxidant streams was kept at 21 or 20% for consistency with the practical operative conditions of MILD combustion. Note that the present experiments were designed from the following considerations: first, experiments for NG combustion diluted with CO2 and N2 were conducted to distinguish the requirements for the establishments of MILD oxy-combustion and MILD air combustion, so that the deduction of Li et al.13 can be examined; second, a check of the fuel dependence of the deduction was performed by repeating the above experiments using LPG; and third, partially replacing N2 with CO2 to further test whether the CO2 replacement is beneficial for establishing MILD combustion. 2.2. Numerical Calculation. The combustion of CH4 in a wellstirred reactor (WSR) is considered to investigate the establishment of MILD conditions with different temperatures, equivalence ratios, and dilution levels based on the definition of MILD combustion proposed by Cavaliere and de Joannon.5 Accordingly, as listed in Table 1, the

with inlet injection conditions, reactant−jet mixing, and flue gas recirculation.

3. ESTABLISHMENTS OF MILD COMBUSTION DILUTED BY CO2 AND N2 3.1. Experimental Observation. During the observational experiments, a great number of instantaneous images of the flames were taken in different combustion states and under different dilution ratios for comparison. Figure 2 presents some typical images of establishing MILD air combustion and MILD oxy-combustion in the furnace when firing (a) NG and (b) LPG with an oxygen concentration of XO2 = 21% in the oxidant streams and (c) NG diluted by the mixture of N2 and CO2 with XO2 = 20% and an equivalence ratio Φ = 0.83. Note that Φ was varied by changing the flow rate of the oxidant stream, while the fuel supply was kept constant. When burning NG with air (21% O2 + 79% N2) (see Figure 2a), the state of combustion changed from MILD to flame mode as Φ was decreased from 1.0 to 0.78. It means that, with N2 dilution, MILD combustion can be established only when Φ is above 0.78. The underlying reason is that, as Φ decreases, the oxygen concentration within the furnace will increase, which is opposite the requirement of the low oxygen level environment for establishing MILD combustion. Thus, decreasing Φ eventually leads to the occurrence of the flame mode. The critical equivalence ratio for establishing MILD combustion is denoted by Φ*, below which flame is visible. With O2 + CO2 instead of air, Φ* was expanded to 0.71. Similarly, when firing LPG, the replacement of N2 by CO2 expands Φ* from 0.9 to 0.85 (see Figure 2b). The reduced range of Φ for LPG MILD combustion results likely from its much higher heat value compared to that of NG. On the basis of the above observations, the establishment of MILD oxy-combustion is less stringent than that for MILD air combustion, because the critical Φ* is expanded by CO2 dilution when burning either NG or LPG. To further investigate the importance of CO2 dilution, oxidant streams consisting of various volume fractions of N2 and CO2 were injected into the furnace when firing NG. Figure 2c shows that, as the volume fraction of CO2 in the oxidant streams was increased from XCO2 = 0 to 20%, sporadic flame fronts disappeared and MILD conditions were established therewith. It is thus demonstrated that either fully or partially replacing N2 with CO2 facilitates the establishment of MILD combustion. To see better the variation of Φ* for the three cases, a chart of the combustion state against Φ and XCO2 is shown in Figure 2d. In summary, the present experimental investigation confirms that MILD combustion is more likely to occur with dilution of CO2 than N2 for different gaseous fuels. 3.2. Numerical Investigation. Figure 3a shows the relationship between the WSR temperature (TWSR) and the inlet temperature (Tin), i.e., the TWSR−Tin curves, for the stoichiometric CH4/O2/N2 mixture at Φ = 1.0, p = 1.0 atm, and tR = 1.0. The line annotated “no combustion” represents TWSR = Tin, as a baseline for the temperature rise ΔT during combustion. By preheating the mixture until Tin is above a certain threshold, combustion reactions take place. This threshold is defined as the self-ignition temperature Tsi. It is seen that Tsi = 1010 K for XO2 = 5%. As XO2 is increased, Tsi drops. Under the WSR combustion, for each XO2, as Tin is increased, TWSR grows along the upper curve. However, when Tin is gradually decreased to a value of Tex, the extinction of

Table 1. Classification of Different Regimes of the WSR Combustion combustion regimes

conditions

(a) feedback combustion (FC) (b) high-temperature combustion (HTC) (c) MILD combustion (MC)

ΔT > Tsi, and Tin < Tsi ΔT > Tsi, and Tin > Tsi ΔT < Tsi, and Tin > Tsi

WSR combustion can be classified into three regimes under different conditions. In the classification, ΔT = TWSR − Tin is the temperature rise during combustion, TWSR is the reacting temperature at a residence time tR, Tin is the inlet temperature of reactants, and Tsi is the selfignition temperature of the flammable reactant mixture. This definition is chosen because (1) it can provide some physical insights into MILD combustion,15−18 (2) it is mathematically stated and simple to use for the WSR combustion, and (3) by drawing up the three regimes, the difference between MILD oxy-combustion and MILD air combustion can be distinguished. In addition, the numerical WSR was applied to study the temperature rise and the chemical routes of the CH4 combustion, while a closed homogeneous reactor was chosen to calculate the ignition delay time. The setup of chemical kinetic calculation is briefly described as follows. CHEMKIN 4.114 incorporated with GRI-Mech 3.019 is applied for present calculations under constant pressure (p = 1 atm) over a residence time of tR = 1.0 s. The GRI-Mech 3.0 is selected for the present chemical calculation because it is an optimized detailed mechanism desgined for natural gas combustion under the conditions of the temperature varying from 1000 to 2500 K, pressure from 10 Torr to 10 atm, and equivalence ratio from 0.1 to 5.0. The present study uses the residence time of tR = 1.0 s to ensure the equilibrium of various reactions, which is vital for appropriate predictions of the temperature and reaction paths.20 Similar settings of tR were used in the previous studies.5,20,21 It is worth noting that the WSR combustion is for the premixed case, and thus, it is considered approximately to emulate the well-mixed combustion in the furnace but not correlated 4578


Article

Energy & Fuels

Figure 2. Typical images of establishing MILD combustion with N2 and CO2 dilutions for different fuels: (a) NG, (b) LPG, and (c) NG with N2/ CO2 blended dilution at Φ = 0.83.

WSR combustion will occur; this inlet temperature denoted by Tex is called the “extinction temperature”. The dashed line connecting the points for Tsi and Tex represents the unstable combustion states, which cannot be obtained experimentally. Figure 3b presents the variations of Tsi and Tex against XO2 for the WSR combustion of methane. Evidently, Tsi,N2 and Tsi,CO2 for the cases diluted with N2 and CO2, respectively, are apparently identical at XO2 > 5% and exhibit a minor difference at XO2 ≤ 5%, so that it is appropriate to treat Tsi,N2 ≈ Tsi,CO2 in the following discussion. On the other hand, there is a significant difference between Tex,N2 and Tex,CO2 for any XO2; actually, Tex,N2 is substantially smaller than Tex,CO2. In other words, the extinction of WSR combustion diluted by N2 occurs at a lower temperature than that diluted by CO2. Note that the extinction occurs only when XO2 < 7.5 and 11% for dilutions of

Figure 3. (a) WSR temperature TWSR versus inlet temperature Tin for the stoichiometric CH4/O2/N2 mixture at Φ = 1.0, p = 1.0 atm, and tR = 1.0 s. (b) Self-ignition temperature Tsi and extinction temperature Tex of the mixture versus XO2 with different dilutions.

Figure 4. Combustion classifications of the CH4/O2/N2 and CH4/O2/CO2 mixtures in a WSR at (a) Φ = 0.5, (b) Φ = 1.0, and (c) Φ = 2.0. On the map, TC = traditional combustion, HTC = high-temperature combustion, NC = no combustion, and MILD = moderate or intense low-oxygen dilution. 4579


Article

Energy & Fuels N2 and CO2, respectively. Moreover, Figure 3b demonstrates that both Tsi and Tex decrease as XO2 increases and significantly that the decrease rate in Tex is much greater than that in Tsi. The latter suggests that the extinction is more sensitive to the change in the oxygen concentration than the self-ignition. Because ΔT cannot be provided a priori, one cannot determine the combustion state from the Tin−ΔT map based on the common operative conditions, such as reactant temperature Tin, concentration Xi and equivalence ratio Φ. Here, we transform the Tin−ΔT map into the Tin−Xdil* map, where Xdil* is the dilution ratio with the subscript “dil” denoting diluent, as Figure 4 shows. It is straightforward to see that the three combustion regions (MILD, TC, and HTC) are quite different for the two cases of N2 and CO2 dilutions. For different equivalence ratios from fuel-lean to fuel-rich, the MILD region with CO2 dilution is significantly larger than that with N2 dilution. Namely, the limit for establishing MILD combustion with CO2 dilution is extended to lower values of Xdil* and Tin. The extension of the MILD combustion regime by altering the diluent becomes most significant under the stoichiometric circumstance. Therefore, it is numerically demonstrated that less stringent requirements of preheating and dilution are expected to establish MILD oxy-combustion than MILD air combustion. Also, it is interesting to compare the extinction regime for the N2 and CO2 dilutions in Figure 4. When changing N2 to CO2, the “no combustion” region is enlarged. Although the difference of the NC region is quite significant below 900 K, the local temperature within a furnace is usually above 1000 K, at which the difference of the NC region for N2 and CO2 dilutions is negligible. Hence, no obvious difference between the stabilities of MILD oxycombustion and MILD air combustion is expected. It can be summarized that, on the basis of the MILD combustion definition,5 MILD conditions are more likely to occur with CO2 than N2 dilution under the same conditions (XO2 and Φ) as long as the reactants are well-mixed. The mechanism behind this phenomenon is investigated in the following section.

Figure 5. Physical properties of gaseous CO2 and N2 against their temperature. (a) Molar specific heat capacity, (b) thermal conductivity, (c) density, and (d) kinematic viscosity. Symbols, ref 23; curves, (a) polynomial from CHEMKIN, (b) power law, (c) ideal gas law, and (d) Sutherland’s formula.

monotonic with respect to temperature. Specifically, the heat capacity of CO2 (Cp,CO2) is much greater than that of N2 (Cp,N2) at any temperature; for example, Cp,CO2 ≈ 1.7Cp,N2 at the typical working temperature of MILD combustion. It is thus deduced that the temperature rise because of combustion diluted by CO2 is considerably lower than that by N2 because of the enthalpy change Δh = ∫ TT21Cp dT. Figure 5b shows that Cp,CO2 is slightly larger than Cp,N2 at the working temperature of MILD combustion. Accordingly, CO2 dilution should be significantly more effective than N2 dilution in lowering the furnace temperature or making it more uniform to establish the MILD combustion. Also, CO2 is denser and less viscous than N2, (see panels c and d of Figure 5), so that a better turbulent mixing in the furnace is expected to occur with CO2 dilution. To conclude, CO2 dilution can physically enhance the turbulent mixing and heat transfer, thus facilitating the occurrence of MILD conditions. Next, to quantify the effects of Cp and chemical factor of CO2 dilution on the adiabatic temperature rise ΔTab, calculations of CH4 combustion in a WSR with various dilutions are performed. Figure 6 shows the results against different equivalence ratios and reactant temperatures. Because none of FN2 and FCO2 is involved in chemical reactions, their difference in Cp results in the ΔTab discrepancy between FN2 and FCO2, which is the physical effect, and the chemical effect can be quantified by comparing it to that between N2 and CO2. From Figure 6a, one can see that the Cp effect rather than the chemical effect accounts largely for the great temperature reduction under varying Φ when changing the diluent from N2 to CO2. Relatively, the chemical effect of CO2 is minor in decreasing the temperature rise because of combustion and reaches its maximum when Φ is around 1.0. Figure 6b strengthens the dominating effect of Cp over the chemical effect when varying the inlet temperature. Quantitatively, the Cp effect contributes over 80% in reducing the temperature. It is interesting to note that, as Tin is varied, the discrepancy between ΔTab,N2 and ΔTab,CO2 is nearly constant, which is about 400 K. Note also that the chemical effect of CO2 becomes more important as Tin increases. That is, the chemical effect of CO2

4. PHYSICAL AND CHEMICAL EFFECTS OF CO2 VERSUS N2 DILUTION To distinguish physical and chemical effects of N2 and CO2 dilutions, two additional fictitious species FN2 and FCO2 are considered. The physical properties of FN2 and FCO2 are set exactly the same as those of N2 and CO2, but neither of them is involved in chemical reactions. It follows that the combustion characteristics differing because of real and fictitious diluents should result from the chemical effect of the dilution. Because the working temperature of MILD combustion typically ranges from 1100 to 1600 K (e.g., see refs 13 and 22), the value of (1100 K + 1600 K)/2 = 1350 K is chosen as the typical inlet temperature in the following calculations. 4.1. Temperature Rise. Under the well-mixed conditions, the furnace temperature distribution is determined by chemical reaction and heat transfer during combustion. The furnace temperature in turn affects the preheating and ignition of the injected mixture of fuel and oxidant, which is strongly related to the occurrence of MILD combustion. Here, we attempt to investigate how different properties of N2 and CO2 affect the final temperature of adiabatic combustion to reveal physical and chemical effects of the diluents on MILD combustion. Figure 5 presents those properties of gaseous N2 and CO2 varying with the temperature. It is obvious that overall those properties are 4580


Article

Energy & Fuels

Figure 6. Effects of Cp and chemistry on ΔTab by replacing N2 with CO2 for burning CH4/O2 at XO2 = 5%, p = 1 atm, and tR = 1.0 s. (a) Tin = 1350 K, various Φ; (b) Φ = 1.0, various Tin.

Figure 7. (a) Present definition of ignition delay and (b) predicted ignition delay time validated by the experiment of Zhang et al.26 (mixture: 0.998% CH4 + 3.99% O2 + 95.102% Ar) and Frenklach and Bornside27 (mixture: 9.5% CH4 + 19% O2 + 71.5% Ar).

in reducing the peak temperature should not be neglected under a high inlet temperature and stoichiometric conditions. Additionally, as shown in Figures 4 and 5b, increasing the inlet temperature will lower the temperature rise because of the larger heat capacity of either diluent. It is indicated that a higher preheating/furnace temperature is more preferable for the occurrence of MILD combustion, regardless of diluents. To summarize, with respect to N2 dilution, CO2 dilution has physical and chemical effects on lowering the adiabatic temperature rise that are beneficial to the establishment of MILD combustion according to the definition.5 Particularly, the Cp effect far outweighs the chemical effect. 4.2. Ignition Delay Time. Ignition significantly influences flame propagation and stability24 and, therefore, is an important process for combustion. Delayed ignition may increase the flame instability or cause extinction for conventional combustion, but it is contrarily beneficial for achieving MILD combustion because (1) a delayed ignition ensures better mixing between reactants and flue gases and allows for more turbulence interaction with the chemical process25 and (2) a delayed ignition also retards the heat release process of chemical reactions, thus reducing the local temperature rise to achieve a more uniform temperature in a combustion furnace. Although the MILD combustion is established on the basis of fuel self-ignition, which depends upon the reactant temperature rather than heat release,5 ignition is still critical for MILD combustion and deserves more attention. The physical and chemical effects of CO2 on ignition delay are thus quantified below. Several methods have been explored to determine ignition delay time τ in both experiments and numerical simulations, e.g., the interval of reflected shock passage,28 the onset of rapid pressure rise,29 the maximum radical (e.g., OH) production,28−32 the maximum temperature rise rate,33 or certain temperature rise.30 In the present study, the ignition delay time τ is defined as the period from the beginning of calculation to the occurrence of the maximum OH production, as indicated in Figure 7a for XO2 = 5 and 10% with Φ = 1.0 and T = 1350 K. As the oxygen fraction is increased, the ignition delay time is shortened, which is consistent with van Oijen’s result.39 The present ignition delay times are validated with the measurements by Zhang et al.,26 and Frenklach and Bornside27 in Figure 7b. Good agreement between the predictions and the measurements is obtained, which confirms the accuracy of the present predictions for ignition of pure CH4 under different

pressures, oxygen levels, and equivalence ratios. Therefore, the present model and chemical mechanism are proper for prediction of the CH4 ignition delay time under MILD conditions. Figure 8 shows the predicted ignition delay time (τ) of CH4 combustion, diluted by N2 and CO2, against equivalence ratio

Figure 8. Cp and chemical effects on ignition delay times for CH4 combustion with different diluents under p = 1 atm, XO2 = 5%, and (a) Tin = 1350 K, various Φ; (b) Φ = 1.0, various T.

(Φ) and temperature (T) under p = 1 atm and XO2 = 5%. Apparently, the ignition delay time increases with increasing Φ (Figure 7a) and decreases with increasing T (Figure 7b). These observations agree qualitatively with the relationship obtained by Spadaccini and Colket34 between the ignition delay time and CH4/O2 concentrations τ = A exp(E/T )[O2 ]−1.05 [CH4]0.66 = A* exp(E /T )[O2 ]−0.39 Φ0.66

where A, A*, and E are experimental constants. However, the expression of τ cannot describe the chemical effect of dilution gas. Thus, to quantify the effect, the calculations after introducing fictitous species, FN2 and FCO2, have been presently made and shown in Figure 8. Note that FN2 and FCO2 will not involve any chemical reactions. It is evident that N2 has little impact on the ignition delay time regardless of whether or not it is involved in chemical reactions. This is expected because N2 is not involved in the main paths of CH4 oxidation but mostly the reactions of NOx formation. 4581


Article

Energy & Fuels

considered. In the diagram of CO2 dilution, paths colored by red represent that the net reaction rates are enhanced, while the blue color means being weakened, for an order of magnitude, when changing the diluent from N2 to CO2. Figure 10b clearly demostrates that the CO → CO2 conversion and the paths started from CH2(s) are significantly influenced by the chemical effect of CO2 when using CO2 as the diluent. However, the CO2 impact on the main reaction path CH4 → CH3 → CH2O → HCO → CO and the C2 paths located on the rightside of the diagram is small. The forward, reverse, and net rates of the reactions related to the influenced paths are summarized in Table 2, with red and blue colors representing the accelaration and deccelaration of the reactions by CO2. From Figure 10b, two main reactions CO + OH ⇌ CO2 + H (R99) and CH2(s) + CO2 ⇌ CO + CH2O (R153) are identified to increase the CO concentration under CO2 dilution, as shown in Figure 9b. These two reactions also indicate that the decomposition of CO2 under MILD oxycombustion depletes H and CH2(s) radicals substantially, which explains the suppression of H and CH2(s) formations observed in Figure 9a. The depletion of H radicals by CO2 through R99 is found to reduce the burning rate of the fuel and the overall reaction rate of the combustion.38 Thus, under CO2 dilution, combustion reactions of firing methane take place at lower rates, resulting in the chemical time scale being close to the turbulence mixing time scale, which is the typical characteristic of MILD combustion. This is another supportive evidence for the MILD combustion to occur more likely under CO2 dilution from a chemical view. On the other hand, the consumption of CH2(s) through R153 significanlty weaken the paths of CH2(s) → CH2 → CH → CH2O, CH2(s) → CH2 → CH2O, and CH2(s) → CO, as presented by the blue arrows in Figure 10b. Note that, although the concentration of CH2O is expected to be lowered because of the reduction of CH2(s), the decomposition of CO2 through R153 increases CH2O formation. Thus, these two impact balance and the CH2O concentration remains nearly unchanged when N2 is replaced by CO2 (see Figure 9a). Figure 11 shows the first-order sensitivity analysis for CO under N2 and CO2 dilution with T = 1350 K, XO2 = 5%, p = 1 atm, and tR = 1.0 s. The linear sensitivity coefficients (Ai/ Xj)(δXj/δAi) are performed, where Ai is the pre-exponential factor for reaction i, and Xj is the molar fraction of the jth species. It can be interpreted that a negetive/positive sensitivity coefficient of a reaction for certain species implies the importance of the reaction in inhibiting/promoting the species formation. As demonstrated in Figure 11, the CO production of the MILD combustion diluted by N2 is most sensitive to H + O2 → O + OH (R38) and CO + OH → H + CO2 (R99). R38 is important because, during combustion, especially under MILD conditions (low temperature and highly diluted reactants), it is the main chain-branching reaction to consume/produce the chain carriers H, O, and OH, which will initiate the combustion reaction by attacking CH439 and provide OH for CO + OH → H + CO2, as indicated in Figure 10. Under CO2 dilution, the importance of R38 is promoted, while that of R99 is weakened remarkably. It is deduced that, with CO2 dilution, R99 is so active in producing CO and OH that the slight variation of its reaction rate affects little on the final CO formation. Simultaneously, the CO2 dilution intensifies the competition of H radicals between R38 and R99 and shifts the OH balance; eventually the CO output

Interestingly, the ignition is accelerated slightly by the chemical effect of N2, which is exhibited in the discrepancy between τFN2 and τN2. Guo and Chen35 found the similar result and considered that NO addition enhances the OH production and, thus, the oxidation process. In contrast, CO2 dilution remarkably slows the ignition and, therefore, affects reaction equilibrium because of its role as a major production of methane oxidation. Figure 8 reveals that, relative to the N2 case, the CO2 dilution delays, both physically and chemically, the ignition of CH4, under varying Φ and T. Importantly, the chemical effect of CO2 dilution is much more prominent than the physical effect. It is evident from Figure 8a that replacing N2 with CO2 for dilution decelerates the ignition of CH4 for about 50% under typical MILD conditions. This deceleration provides more time for reactants to mix flue gases and, hence, dilute themselves, eventually promoting the occurrence of MILD combustion. 4.3. Chemical Mechanism. To examine the diluent effect on the chemical process under MILD conditions, the WSR combustion is calculted under different temperatures with XO2 = 5%, p = 1.0 atm, tR = 1.0 s, and Φ = 1.0. Figure 9 shows

Figure 9. Mole fractions varying with the temperature of (a) CH2O, H, OH, and CH2(s) and (b) H2O, CO, and O2 under MILD WSR combustion with N2 and CO2 dilutions.

temperature-dependent mole fractions of CH2O, H, OH, CH2(s), H2O, CO, and O2 under MILD WSR combustion diluted by N2 and CO2. It is observed that the mole fractions of H and CH2(s) are considerably lower, while those of CO and O2 increase drastically, under CO2 dilution. The second observation is consistent with those of Heil et al.36 and Glarborg and Bentzen.37 However, a sharp rise of the CO concentration because of CO2 dilution is not expected to cause a high CO emission in practical applications because the reaction between CO and O2 will be completed, thus converting to CO2 under typical cooling rates of a power plant furnace.37 In addition, our simulations also indicate a negligible influence on productions of H2O and CH2O when changing the diluent from N2 to CO2. To understand the above observations more fundamentally, the reaction paths of MILD combustion of CH4 under N2 and CO2 dilutions are calculated on the basis of GRI 3.0. The results are diplayed by diagrams shown in Figure 10. In each diagram, primary reactants and products of each elementary reaction are shown on the arrow tails and heads, respectively. Secondary reactants and net reaction rates (e.g., 1.7−7 means 1.7 × 10−7 g mol cm−3 s−1) calculated by CHEMKIN are provided beside the arrows. Less important reactions, whose rates are below 1 × 10−12 g mol cm−3 s−1, have not been 4582


Article

Energy & Fuels

Figure 10. Reaction paths of CH4 with (a) N2 dilution and (b) CO2 dilution under T = 1350 K, XO2 = 5%, p = 1 atm, and tR = 1.0 s. In the diagram, the color red means enhanced, while the color blue means weakened by the chemical effect of CO2.

FCO2 are introduced to quantitatively separate the contributions from physical and chemical factors to the difference between the two combustions. It has been found that using CO2 instead of N2 lowers the temperature rise, delays the ignition, and slows the combustion reaction, thus facilitating the establishment of MILD combustion. More specifically, several conclusions can be made as follows: (1) The MILD oxy-combustion is established with less stringent conditions of preheating, dilution, and equivalence ratio than the MILD air combustion, regardless of any gaseous fuel. These are proven quantitatively by detailed chemical kinetic calculations and qualitatively by experimental observa-

becomes much more sensitive to the rate of R38. The importance of H + O2 → O + OH (R38) implicates that resulting CO for MILD oxy-combustion can be lowered by operating at oxygen-enriched and hydrogen-added conditions. Therefore, it will be interesting to investigate the MILD oxycombustion of hydrogen-blended fuels.

5. CONCLUSION The present study has investigated both the MILD combustion diluted with N2 (air combustion) and that with CO2 (oxycombustion), by experiment (furnace; methane and LPG) and kinetic calculation (WSR; methane). Fictitious species FN2 and 4583


Article

Energy & Fuels Table 2. Reaction Paths Prominently Influenced by the Chemical Effect of CO2 Dilution path

reaction

diluent N2 CO2 N2 CO2 N2 CO2 N2 CO2 N2 CO2 N2 CO2 N2 CO2 N2 CO2

CO ↔ CO2

R99

CO + OH ⇌ CO2 + H

CO2 → CO

R153

CH2(s) + CO2 ⇌ CO + CH2O

CH2(s) → CH2 CH2(s) → CO

R142 R152 R144

CH2(s) + N2 ⇌ CH2 + N2 CH2(s) + CO2 ⇌ CH2 + CO2 CH2(s) + O2 ⇌ H + OH + CO

CH2 → CH2O

R92

OH + CH2 ⇌ H + CH2O

CH2 → CH

R93

OH + CH2 ⇌ CH + H2O

R126

CH + H2 ⇌ H + CH2

R127

CH + H2O ⇌ H + CH2O

CH → CH2O

forward rate (g mol cm−3 s−1) 1.16 7.62 2.67 1.27 8.69 6.36 1.30 6.63 6.15 6.34 2.07 2.13 1.49 4.50 3.13 3.94

× × × × × × × × × × × × × × × ×

reverse rate (g mol cm−3 s−1)

net rate (g mol cm−3 s−1)

9.2 × 10−7 7.33 × 10−6 2.86 × 10−17 4.76 × 10−16 5.43 × 10−8 5.76 × 10−8 8.45 × 10−26 1.14 × 10−25 5.59 × 10−18 2.14 × 10−18 8.34 × 10−11 1.05 × 10−12 3.63 × 10−8 8.85 × 10−10 7.06 × 10−16 2.71 × 10−16

2.4 × 10−7 2.9 × 10−7 2.67 × 10−8 1.27 × 10−7 8.15 × 10−7 6.06 × 10−9 1.30 × 10−9 6.63 × 10−10 6.15 × 10−9 6.34 × 10−10 1.99 × 10−9 2.12 × 10−10 3.48 × 10−8 8.81 × 10−10 3.13 × 10−8 3.94 × 10−10

10−6 10−6 10−8 10−7 10−7 10−8 10−9 10−10 10−9 10−10 10−9 10−10 10−9 10−12 10−8 10−10

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors gratefully acknowledge the support of the Special Research Fund for the Doctoral Program of Higher Education of China (Grant 20110001130014), the China Postdoctoral Science Foundation (2014M550011), and the National Natural Science Foundation of China (Grants 51276002 and 51406001).

■

NOMENCLATURE

Symbols

Da = diameter of the air nozzle exit (mm) Df = diameter of the fuel nozzle exit (mm) P = pressure (atm) tR = residence time (s) T = temperature (K) ΔT = maximum temperature rise (K) Tin = inlet mixture temperature (K) TWSR = temperature of the well-stirred reactor (K) Tsi = self-ignition temperature of the fuel (K) Tex = extinction temperature of the fuel (K) Cp = molar heat capacity (kJ mol−1 K−1) Δh = molar enthalpy change of gas (kJ/mol) XO2 = molar fraction of O2 (%)

Figure 11. First-order sensitivity analysis for CO with N2 and CO2 dilution under T = 1350 K, XO2 = 5%, p = 1 atm, and tR = 1.0 s.

tions of NG and LPG combustion in a furnace. (2) The heat capacity of CO2 > the heat capacity of N2 is the main physical factor that enables the CO2 dilution to be more efficient in lowering the adiabatic temperature rise, thus promoting the occurrence of the MILD state. By comparison, the chemical factor of CO2 is minor. (3) The ignition delay time of methane by CO2 dilution is significantly longer than that by N2 dilution, which facilitates the occurrence of MILD combustion. For this difference, the chemical factor plays a more important role than the physical factor. (4) The local CO production increases from using N2 dilution to CO2 dilution mainly because of the reactions: CO2 + H → OH + CO and CO2 + CH2(s) → CO + CH2O. The depletion of H radicals by CO2 dilution through CO2 + H → OH + CO inhibits the main chain-branching reaction H + O2 → OH + O, reducing the overall reaction rate and obstruction of the first oxidation step CH4 → CH3 under MILD oxy-combustion. This further enhances the conclusion that MILD combustion is more attainable with CO2 dilution than N2 dilution from the chemical view.

■

Greek Letters

■

ε = emissivity λ = thermal conductivity (W m−1 K−1) ρ = density (kg/m3) τ = ignition delay time (ms) ν = kinematic viscosity (m2/s) Φ = equivalence ratio

REFERENCES

(1) Ghoniem, A. F. Needs, resources and climate change: Clean and efficient conversion technologies. Prog. Energy Combust. Sci. 2011, 37 (1), 15−51. (2) Chen, L.; Yong, S. Z.; Ghoniem, A. F. Oxy-fuel combustion of pulverized coal: Characterization, fundamentals, stabilization and CFD modeling. Prog. Energy Combust. Sci. 2012, 38 (2), 156−214.

AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-010-62767074. E-mail: [email protected]. 4584


Article

Energy & Fuels (3) Normann, F.; Andersson, K.; Leckner, B.; Johnsson, F. Emission control of nitrogen oxides in the oxy-fuel process. Prog. Energy Combust. Sci. 2009, 35 (5), 385−397. (4) Suda, T.; Masuko, K.; Sato, J.i.; Yamamoto, A.; Okazaki, K. Effect of carbon dioxide on flame propagation of pulverized coal clouds in CO2/O2 combustion. Fuel 2007, 86 (12), 2008−2015. (5) Cavaliere, A.; de Joannon, M. Mild combustion. Prog. Energy Combust. Sci. 2004, 30 (4), 329−366. (6) Wünning, J. A.; Wünning, J. G. Flameless oxidation to reduce thermal NO-formation. Prog. Energy Combust. Sci. 1997, 23 (1), 81− 94. (7) Blasiak, W.; Yang, W. H.; Narayanan, K.; Von Schéele, J. Flameless oxyfuel combustion for fuel consumption and nitrogen oxides emissions reductions and productivity increase. J. Energy Inst. 2007, 80 (1), 3−11. (8) Krishnamurthy, N.; Paul, P. J.; Blasiak, W. Studies on lowintensity oxy-fuel burner. Proc. Combust. Inst. 2009, 32 (2), 3139− 3146. (9) Stadler, H.; Ristic, D.; Förster, M.; Schuster, A.; Kneer, R.; Scheffknecht, G. NOx-emissions from flameless coal combustion in air, Ar/O2 and CO2/O2. Proc. Combust. Inst. 2009, 32 (2), 3131−3138. (10) Stadler, H.; Christ, D.; Habermehl, M.; Heil, P.; Kellermann, A.; Ohliger, A.; Toporov, D.; Kneer, R. Experimental investigation of NOx emissions in oxycoal combustion. Fuel 2011, 90 (4), 1604−1611. (11) Li, P.; Wang, F.; Tu, Y.; Mei, Z.; Zhang, J.; Zheng, Y.; Liu, H.; Liu, Z.; Mi, J.; Zheng, C. Moderate or intense low-oxygen dilution oxycombustion characteristics of light oil and pulverized coal in a pilotscale furnace. Energy Fuels 2014, 28 (2), 1524−1535. (12) Mei, Z.; Mi, J.; Wang, F.; Zheng, C. Dimensions of CH4-jet flame in hot O2/CO2 coflow. Energy Fuels 2012, 26 (6), 3257−3266. (13) Li, P.; Dally, B. B.; Mi, J.; Wang, F. MILD oxy-combustion of gaseous fuels in a laboratory-scale furnace. Combust. Flame 2013, 160 (5), 933−946. (14) Kee, R. J.; Rupley, F. M.; Miller, J. A.; Coltrin, M. E.; Grcar, J. F.; Meeks, E.; Moffat, H. K.; Lutz, A. E.; Dixon-Lewis, G.; Smooke, M. D. CHEMKIN Release 4.1; Reaction Design: San Diego, CA, 2007. (15) Li, P.; Wang, F.; Mi, J.; Dally, B. B.; Mei, Z.; Zhang, J.; Parente, A. Mechanisms of NO formation in MILD combustion of CH4/H2 fuel blends. Int. J. Hydrogen Energy 2014, 39 (33), 19187−19203. (16) Chen, S.; Mi, J.; Liu, H.; Zheng, C. First and second thermodynamic-law analyses of hydrogen−air counter-flow diffusion combustion in various combustion modes. Int. J. Hydrogen Energy 2012, 37 (6), 5234−5245. (17) de Joannon, M.; Sorrentino, G.; Cavaliere, A. MILD combustion in diffusion-controlled regimes of hot diluted fuel. Combust. Flame 2012, 159 (5), 1832−1839. (18) de Joannon, M.; Sabia, P.; Sorrentino, G.; Cavaliere, A. Numerical study of mild combustion in hot diluted diffusion ignition (HDDI) regime. Proc. Combust. Inst. 2009, 32 (2), 3147−3154. (19) Smith, G. P.; Golden, D. M.; Frenklach, M.; Moriarty, N. W.; Eiteneer, B.; Goldenberg, M.; Bowman, C. T.; Hanson, R. K.; Song, S.; Gardiner, W. C., Jr. GRI-Mech 3.0; www.me.berkeley.edu/gri-mech/. (20) Wang, F.; Li, P.; Mei, Z.; Zhang, J.; Mi, J. Combustion of CH4/ O2/N2 in a well stirred reactor. Energy 2014, 72 (1), 242−253. (21) de Joannon, M.; Saponaro, A.; Cavaliere, A. Zero-dimensional analysis of diluted oxidation of methane in rich conditions. Proc. Combust. Inst. 2000, 28 (2), 1639−1646. (22) Szegö, G. G.; Dally, B. B.; Nathan, G. J. Operational characteristics of a parallel jet MILD combustion burner system. Combust. Flame 2009, 156 (2), 429−438. (23) Lide, D. R. CRC Handbook of Chemistry and Physics, 90th ed.; CRC Press (Taylor and Francis Group): Boca Raton, FL, 2010. (24) Turns, S. R. An Introduction to Combustion: Concepts and Applications; McGraw-Hill: New York, 1996. (25) van Oijen, J. A. Direct numerical simulation of autoigniting mixing layers in MILD combustion. Proc. Combust. Inst. 2013, 34 (1), 1163−1171. (26) Zhang, Y.; Huang, Z.; Wei, L.; Zhang, J.; Law, C. K. Experimental and modeling study on ignition delays of lean mixtures

of methane, hydrogen, oxygen, and argon at elevated pressures. Combust. Flame 2012, 159 (3), 918−931. (27) Frenklach, M.; Bornside, D. E. Shock-initiated ignition in methane−propane mixtures. Combust. Flame 1984, 56 (1), 1−27. (28) Curran, H.; Simmie, J. M.; Dagaut, P.; Voisin, D.; Cathonnet, M. The ignition and oxidation of allene and propyne: Experiments and kinetic modeling. Symp. (Int.) Combust., [Proc.] 1996, 26 (1), 613− 620. (29) Donohoe, N.; Heufer, A.; Metcalfe, W. K.; Curran, H. J.; Davis, M. L.; Mathieu, O.; Plichta, D.; Morones, A.; Petersen, E. L.; Güthe, F. Ignition delay times, laminar flame speeds, and mechanism validation for natural gas/hydrogen blends at elevated pressures. Combust. Flame 2014, 161 (6), 1432−1443. (30) de Joannon, M.; Cavaliere, A.; Donnarumma, R.; Ragucci, R. Dependence of autoignition delay on oxygen concentration in mild combustion of high molecular weight paraffin. Proc. Combust. Inst. 2002, 29 (1), 1139−1146. (31) Medwell, P. R.; Kalt, P. A. M.; Dally, B. B. Simultaneous imaging of OH, formaldehyde, and temperature of turbulent nonpremixed jet flames in a heated and diluted coflow. Combust. Flame 2007, 148 (1− 2), 48−61. (32) Parente, A.; Galletti, C.; Tognotti, L. Effect of the combustion model and kinetic mechanism on the MILD combustion in an industrial burner fed with hydrogen enriched fuels. Int. J. Hydrogen Energy 2008, 33 (24), 7553−7564. (33) Petrova, M. V.; Williams, F. A. A small detailed chemical-kinetic mechanism for hydrocarbon combustion. Combust. Flame 2006, 144 (3), 526−544. (34) Spadaccini, L. J.; Colket Iii, M. B. Ignition delay characteristics of methane fuels. Prog. Energy Combust. Sci. 1994, 20 (5), 431−460. (35) Guo, P.; Chen, Z. Ignition enhancement of ethylene/air by NOx addition. Chin. J. Aeronaut. 2013, 26 (4), 876−883. (36) Heil, P.; Toporov, D.; Förster, M.; Kneer, R. Experimental investigation on the effect of O2 and CO2 on burning rates during oxyfuel combustion of methane. Proc. Combust. Inst. 2011, 33 (2), 3407−3413. (37) Glarborg, P.; Bentzen, L. L. B. Chemical effects of a high CO2 concentration in oxy-fuel combustion of methane. Energy Fuels 2007, 22 (1), 291−296. (38) Liu, F.; Guo, H.; Smallwood, G. J. The chemical effect of CO2 replacement of N2 in air on the burning velocity of CH4 and H2 premixed flames. Combust. Flame 2003, 133 (4), 495−497. (39) Mardani, A.; Tabejamaat, S.; Hassanpour, S. Numerical study of CO and CO2 formation in CH4/H2 blended flame under MILD condition. Combust. Flame 2013, 160 (9), 1636−1649.

4585


Moderate or Intense Low-Oxygen Dilution Combustion of Methane

Recommend Documents