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MILD Combustion under Different Premixing Patterns and Characteristics of the Reaction Regime P. Li,† F. Wang,† J. Mi,*,† B. B. Dally,‡ and Z. Mei† †

State Key Laboratory of Turbulence and Complex Systems, Department of Energy & Resources Engineering, College of Engineering, Peking University, Beijing 100871, China ‡ Center of Energy Technology and School of Mechanical Engineering, The University of Adelaide, Adelaide, South Australia 5005, Australia ABSTRACT: Through experiment and numerical modeling, this study investigated the establishment of moderate or intense low-oxygen dilution (MILD) combustion in a laboratory-scale furnace when fuel and air are fully premixed (FP), partially premixed (PP), or non-premixed (NP). Experiments were carried out at firing rates from 7.5 to 15 kW and equivalence ratios (Φ) ranging from 0.5 to 1. The furnace thermal fields and exhaust NOx emissions for the three mixing patterns were compared. Validated computational fluid dynamics was used to aid in better understanding the flow and compositional structures in the furnace. Natural gas was used as the fuel. The eddy dissipation concept (EDC) model and the GRI-Mech 3.0 mechanism were used. Additional chemical kinetics calculations were also performed to examine reaction pathways under the MILD combustion regime. Moreover, the characteristics of the reaction regime of MILD combustion were examined and are discussed in detail. Estimation of the initial jet momentum rate (J) showed that JFP > JNP > JPP, and consistently the recirculating rate of internal flue gas (Kv) was found to be in the order Kv,FP > Kv,NP > Kv,PP. Correspondingly, the highest values of both furnace temperature and NOx emission were experimentally measured in the PP case, while the lowest values were found in the FP case. The measured NOx emission was negligibly low for the FP case. Numerical results revealed that in all the three cases of firing natural gas (FP, PP, NP), more than 80% of the total NO formation results from the N2O intermediate route while other NO mechanisms are unimportant. As Φ is increased from 0.5 to 1.0, both the measured and simulated NO emissions in the three cases initially increase and then decrease. Moreover, for Φ > 0.9, the NO-reburning reaction becomes significant and the resulting reduction of NO is notable. The rates of both turbulent mixing and chemical reaction were found to play a significant role in the structure and establishment of MILD combustion, with estimated Damköhler numbers in the range Da = 0.01−5.35. experimentally studied by research groups at Adelaide18−20 and Delft1,21,22 and was further numerically investigated by research groups in Iran23−26 and Michigan.27,28 On the application side, Weber and co-workers 8,13 investigated the MILD combustion of gaseous, liquid, and solid fuels with high-temperature air and large quantities of the recirculated flue gas. The fuel input was 0.58 MW, and the combustion air was preheated to approximately 1600 K. These authors conducted comprehensive in-furnace measurements of velocities, temperature, and gas composition (O2, CO2, CO, and NOx). They found that high and uniform heat fluxes as well as dramatic reductions in NOx, CO, and CO2 were obtained in the MILD combustion process. Derudi et al.11 found that with respect to the well-established MILD combustion of methane, the establishment of MILD combustion of hydrogen-containing fuels requires a higher jet velocity. Derudi and Rota14 also investigated the sustainability of MILD combustion for liquid hydrocarbons (i.e., n-octane, n-octane/isooctane, and n-octane/ isooctane/n-decane). They found that MILD combustion of liquid hydrocarbon fuels can operate at a wide range of furnace temperatures and internal recirculation rates in comparison with gaseous fuels, and very low amounts of NOx and CO as

1. INTRODUCTION There is no doubt that the moderate or intense low-oxygen dilution (MILD) combustion has attracted increasing attention from the international combustion community because of its concurrently high-efficiency and low-NOx characteristics.1−4 The achievement of MILD combustion requires the local temperature to exceed the autoignition temperature of the reactant mixture and a sufficiently low local oxygen or fuel concentration. The MILD combustion regime is obtained by strong recirculation of the exhaust gas, where reactants are intensely diluted and thus reactions occur volumetrically without a visible flame front. Relative to the conventional counterpart, MILD combustion increases the in-furnace thermal uniformity and efficiency and simultaneously suppresses NOx emission.5−7 Considerable work on MILD combustion has been published2,8−51 and reviewed by Cavaliere and de Joannon and5 Tsuji et al.6 and more recently by Li et al.7 Numerous fundamental and practical studies have found that when a proper burner and furnace system is used, MILD combustion can be established (and extremely low NOx emissions are obtained) no matter what fuel (i.e., gaseous,8−12 liquid,2,13−15 or solid fuel13,15−17) and oxidant (e.g., air or an O2/CO2 mixture9,10,15,16) are used. Moreover, to gain fundamental understanding of MILD combustion, the flame of a jet-in-hotcoflow (JHC) burner, which emulates the MILD condition, was © 2014 American Chemical Society

Received: November 29, 2013 Revised: March 3, 2014 Published: March 3, 2014 2211

dx.doi.org/10.1021/ef402357t | Energy Fuels 2014, 28, 2211−2226

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Table 1. Summary of Experimental Investigations of MILD Combustion fuel

oxidant

capacity (kW)

oxidant temperature (K)

CH4 CH4 CH4 CH4 CH4 CH4 CH4 CH4, C2H4, LPG CH4, LPG natural gas natural gas natural gas natural gas natural gas, biogas CH4/H2 mixture CH4/H2 mixture CH4/H2 mixture CH4/H2 mixture C3H8 LPG, producer gas biogas kerosene (C12H23) organic waste liquids dried sludge coal coal

air air air air air air air air, CO2/O2, CO2/N2/O2 air air air N2/O2 air air air air air air O2 air air air air air air, Ar/O2, CO2/O2 air, CO2/O2

3.91−6.25 5.4 6.25 7−13 8−11 10 25 13 15−20 7.5, 10 10 20 300 20 0.2−0.3 0.2−0.3 13 20 200 3−150 8 20 665 6 5, 8, 40 22−41

873 N/A 300 973 N/A 673 300 288 298−748 288 288 873 1123 953 1573 1573 N/A 288, 873 288 288 950 288 1173 288 288, 573 288

premixing pattern NP, NP, NP, NP NP NP NP, NP NP FP NP, NP NP NP NP PP NP NP NP NP NP NP NP NP NP NP

FP FP FP

FP

FP, PP

ref 32 33 34 35 12 36 37 10 38, 39 40 41 42 29 43 44 11 45 46 9 47 48 2 49 50 16 51

instance, the application of MILD combustion to gas turbine combustors was introduced in ref 53. Therefore, to further extend the applications, more investigation of MILD combustion in the FP and PP modes is needed. Ö zdemir and Peters conducted perhaps the first study of FP MILD combustion.33 They found that when MILD combustion occurs for both the FP and NP cases, the main reaction zone is closer to the burner exit in the FP case than in the NP case. Arghode and co-workers found that FP MILD combustion produces ultralow NOx emissions (e.g., only 1 ppm at Φ = 0.5).32,54 When investigating the effects of hydrogen (H2) addition on methane−air FP MILD combustion, Arghode and Gupta55 found that for FP MILD combustion the addition of H2 results in a slight increase in NO emission at same equivalence ratio, possibly because of a higher flame temperature. Li and co-workers40,56,57 investigated the impacts of initial conditions on the characteristics of FP MILD combustion by both experiment and numerical simulation. They systemically examined different initial conditions, including the area of the nozzle (A), the equivalence ratio (Φ), the thermal input (P), and the initial dilution of the reactants. They found very low emissions of NOx, CO, and H2 under MILD conditions when the furnace was operated under the FP pattern. Their numerical results showed that FP MILD combustion can occur only for the injection Reynolds numbers greater than a critical value (i.e., Re > Rec). MILD combustion, if established, can be stably sustained irrespective of the variation of A, Φ, or P. The previous study41 also investigated the influence of the initial jet momentum and air−fuel premixing on MILD combustion. It was found that MILD combustion for all of the cases (FP, NP, and PP) can occur only when the total initial jet momentum rate is sufficiently high. Although there have been some previous investigations of

well as negligible PAH and soot precursors were recorded. Stadler et al.16 further investigated the application of MILD combustion to coal combustors. Their experiments showed an overall NOx reduction capability of about 20−50% depending on the coal type (i.e., lignite or bituminous coals), oxidant type (i.e., air, Ar/O2, or CO2/O2 atmosphere), and stoichiometry. Kumar et al.31 found that MILD combustion can be achieved by using air at ambient temperature at a high recirculation rate. Krishnamurthy et al.9 discovered that MILD combustion can be established even when pure oxygen is used as the oxidant by means of a slight asymmetric injection of oxygen at near sonic velocities. Li et al.10 investigated the characteristics of MILD oxycombustion using natural gas (NG), liquefied petroleum gas (LPG), and ethylene (C2H4). They found that MILD combustion can be established for all three fuels even when pure oxygen is used as the oxidant, albeit with sufficiently high fuel jet momentum. Recently, Li et al.15 investigated by experiment the global characteristics of both MILD oxycombustion and air combustion of firing light oil and pulverized coal in a pilot-scale furnace. The MILD combustion was found to reduce the NO emission more effectively in the oxycombustion case than in the air combustion case. Useful information concerning the experimental work is summarized in Table 1. Obviously, although a good volume of work on MILD combustion has been carried out, most of the previous investigations have focused on the fuel−air nonpremixed (NP) combustion pattern. Premixed MILD combustion [i.e., fully premixed (FP) and partially premixed (PP)] is not yet well-understood. Turbulent premixed combustion has various important applications such as spark-ignition engines, gas-turbine engines, and industrial premixed burners.52,53 If those devices can achieve MILD combustion, the thermal field should be uniform and the NOx emission should be low. For 2212


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high-temperature ceramic fiber boards, allowing only about 20% of the total heat input to be conducted through the walls. This assists with the establishment and stability of the MILD regime and results in a warm-up time of about 1.5 h from a cold state to steady-state operation. The furnace has five openings that are equally spaced vertically down three sides of the furnace, as shown in Figure 1. These openings can accommodate interchangeable insulating window plugs or UV-grade fused silica windows. Two U-shaped cooling tubes with variable heat exchange areas are used to control the heat load. The heat exchangers can be inserted through any of the window openings, but for this investigation they were positioned in windows A3 and C3 and their exposed surface areas were 0.03 m2 each. These heat exchangers remove 4.01, 4.46, and 4.93 kW of heat on average for firing rates (P) of 7.5, 10, and 15 kW, respectively. The time-averaged furnace reference temperature was measured with a bare, fine-wire, type R (Pt−Pt−13% Rh) thermocouple of 254 μm diameter wire with a bead diameter of 1.2 mm under steady-state conditions at the location denoted as Tf in Figure 1 (x = 0, y = 0, z = 542.5 mm). This position was chosen because it is located in the postcombustion zone39 and was expected to have only products of the combustion and hence offer a reference point for all cases. The exhaust temperature was measured with a stainless steel sheath type K (Ni− Cr) thermocouple. Global emission levels of CO, CO2, NO, NO2, and O2 were measured using a TESTO 350 XL portable gas analyzer. While the uncertainties in the mean temperatures measured by thermocouples were about ±4 K, the analyzer measurement accuracies were estimated to be the following: [O2], ±0.8% of the measured value; [CO], ±10 ppm or 5% of the measured value (whichever is smaller); [NO], ±5 ppm; [NO2], ±5 ppm; [CO2], ±0.3% of the measured value. The analyzer was checked with a calibration gas to yield total NOx emission accuracies better than ±5 ppm. Total NOx emission (NO + NO2) concentrations are reported by volume on a dry basis corrected to 3% O2 concentration. Correcting to a specific O2 level allows true comparisons of emissions levels to be made because the effect of various degrees of dilution is removed while still retaining a familiar mole-fraction-like variable. The furnace was operated with thermal inputs of 7.5, 10, and 15 kW using natural gas as fuel. The properties and composition of natural gas used in the present experiment are shown in Table 2. The equivalence ratio was varied from Φ = 0.5 to Φ = 1. The fuel and air were introduced into the furnace at room temperature (288 K). Three different burner arrangements were considered in this study (Figure 2). The burner design consists of a single central tube (Do = 7.2 mm) located on the axis of the furnace, an annular channel around the tube, and four exhaust ports arranged symmetrically in a ring pattern on the same wall, as shown in Figures 1 and 2. The variation of the burner arrangements involves the retraction of the central fuel jet, which is surrounded by a bluff body inside a larger tube, thus forming an annulus air jet (Figure 2). The bluff body has a diameter of 18 mm, whereas the outer tube has a diameter of 26.6 mm. For the NP burner, the bluff body was at the same height as the furnace bottom. For the PP burner, the bluff body was lowered 90 mm from the furnace bottom to allow the fuel and the surrounding air to premix before emerging into the furnace. As the jet is retracted, the air and fuel are mixed to a varied extent but do not reach the fully mixed level. For the FP burner, the fuel and air were fully premixed externally and introduced through a central nozzle into the furnace. Table 3 summarizes all of the test conditions of the present experiments. The initial fuel−air jet momentum rate (J) was calculated from the expression J = ∫ ρU2 dA, where ρ is the mixture density, U is the velocity, and A is the nozzle exit area. At a fixed firing rate and equivalence ratio, the mass flow rate of the air−fuel mixture (ṁ = ∫ ρU dA) is constant, and thus, different values of the exit velocity U for the three premixing patterns produce different values of J. Because the exit area of the FP burner is smallest, the FP burner produces the highest U and thus the highest J. By comparison, for the PP burner, the exit area is largest, so the momentum rate is smallest. In the present study, MILD combustion is defined as flameless combustion where no visible flame front occurs at all. Cavaliere and de Joannon5 defined MILD combustion quantitatively as “a combustion

MILD combustion, the different MILD combustion characteristics for the NP, PP, and FP cases are not yet well-understood. This has stimulated the present study, which aims to narrow the deficit in knowledge. Two specific objectives were designated for the present study: (1) to systematically investigate both experimentally and numerically the effect of the fuel−air premixing (i.e., NP, PP, or FP) on the performance of MILD combustion and (2) to examine the characteristics of the reaction regime of MILD combustion. For the first objective, experiments were carried out at thermal input rates of 7.5 to 15 kW and equivalence ratios ranging from 0.5 to 1. The flow field, temperature, and intermediate species (e.g., the radicals CH2O and OH) were also investigated using computational fluid dynamics (CFD) with the eddy dissipation concept (EDC) model and the detailed chemical reactions of GRI-Mech 3.0. Effects of the reactant mixing pattern on the flow field, temperature, intermediate species (e.g., CH2O and OH), and emissions of NOx and CO from MILD combustion were also examined. The NOx formation mechanisms of the combustions with different mixing modes were also analyzed. For the second objective, the Damköhler number (Da) for FP MILD combustion was calculated and the reaction regime of the FP MILD combustion was examined in detail. Finally, the reaction pathway under the MILD combustion regime was analyzed.

2. EXPERIMENTAL DETAILS The present study used a laboratory-scale MILD combustion furnace, shown in Figure 1. Details of the furnace have been given elsewhere,38,39,58 and only a brief description is provided here. The combustion chamber is well-insulated with four layers of 38 mm thick

Figure 1. Schematic figure of the furnace (distances in mm). 2213


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Table 2. Properties and Composition of Natural Gas Used in the Present Experiments fuel natural gas a

a

LHVb

CH4

C2H6

CO2

N2

C3H8

C4H10

C5H12

C6H14

51.154

91.36

4.364

2.084

1.278

0.62

0.20

0.055

0.04

Gas analysis provided by Origin Energy Australia. bLower heating value (MJ/kg). with a detailed chemical kinetic mechanism (GRI-Mech 3.064) was used to model the chemical reactions of natural gas, approximated as 91.4% CH4 and 8.6% C2H6 by volume in the model. The EDC model has been widely used in the modeling of MILD combustion and performs reasonably well.23−26,30,44,45,62,65,66 Especially, Christo and Dally66 used the ξ/PDF model, flamelet models, the eddy dissipation model (EDM), the eddy dissipation/finite-rate model (EDM/FR), and the EDC model to simulate MILD conditions for a JHC burner. They concluded that the EDC model produced better results than the other models. Christo and Dally also evaluated the transported probability density function (PDF) model in the simulation of MILD conditions for the JHC burner.67 They found that the transported PDF predictions are of comparable quality as the EDC model predictions, but the former is sensitive to the level of velocity fluctuations, which cannot be validated independently. Moreover, De et al.62 performed a detailed analysis of the quality of the EDC prediction of the MILD conditions of their JHC burner. They found that the EDC model globally captures the trend of the temperature profile, although it predicts too-early ignition because of the low turbulent Reynolds number effect. The prediction of too-early ignition was avoided by using modified EDC model constants.62 On the fundamental side, the EDC model is an extension of the EDM to include detailed chemical mechanisms in turbulent flows, and it can capture finite-rate chemistry effects with a relatively low computational cost, for example, in comparison with more advanced models such as the transported PDF method. The EDC model assumes that reactions occur in small turbulent structures or at fine scales. The combustion at fine scales is assumed to occur as a constantpressure reactor. The characteristic length fraction of fine scales (ξ) and the chemical residence time scale (τ) of fluid in the fine structures are expressed as

Figure 2. Schematic illustrations of the burner arrangements for the (a) fully premixed (FP), (b) non-premixed (NP), and (c) partially premixed (PP) experiments.

Table 3. Summary of All Experimental Conditions premixing pattern

capacity (kW)

Φ

102·J (kg·m/s2)a

mode

FP

7.5 10 7.5 10 15 7.5 10 15

0.58−0.99 0.52−0.99 0.71−0.99 0.52−0.99 0.61−0.99 0.66−0.99 0.51−0.99 0.62−0.98

26.0−6.5 58.3−15.0 5.6−2.2 17.4−4.4 28.9−10.5 1.9−0.8 5.8−1.6 9.7−3.7

MILD MILD MILD MILD MILD MILD MILD MILD

NP

PP

a

J denotes the initial air−fuel jet momentum rate.

process when the inlet temperature of the reactant mixture is higher than mixture self-ignition temperature whereas the maximum allowable temperature increase with respect to inlet temperature during combustion is lower than mixture self-ignition temperature.” It is well-known that the MILD combustion regime is obtained by strong exhaust gas recirculation. Wünning and Wünning59 mentioned that either external or internal exhaust gas recirculation is effective to lower the peak flame temperature and achieve MILD combustion. Traditionally, MILD combustion accompanies heat recovery, which can highly preheat air to a temperature well above the mixture selfignition temperature (which is termed as high-temperature air combustion in industry).6 In the present experiment, the roomtemperature reactants were injected into the furnace and then were preheated to beyond the self-ignition temperature of their mixture inside the furnace and simultaneously highly diluted before the main reaction occurred by means of strong internal exhaust gas recirculation. That is, MILD combustion was established without preheating the combustion air. Actually, since this non-preheating MILD combustion was achieved by Kumar et al.,31 considerable work has been carried out using oxidant at room temperature.2,9,10,16,37,39−41

⎛ νε ⎞1/4 ξ = Cξ⎜ 2 ⎟ , ⎝k ⎠

⎛ ν ⎞1/2 τ = Cτ ⎜ ⎟ ⎝ε⎠

(1)

where Cξ is the volume fraction constant (=2.1377) and Cτ is the time scale constant (=0.4082). The evolutions of species concentrations are then computed by integrating the chemistry within those fine scales. In the EDC model, the species conservation equation takes the following general form:

∂(ρYi ) + ∇· (ρ vYi ) = −∇Ji + R i ∂t

(2)

where Yi is the local mass fraction of each species i, Ji is the diffusion flux, and Ri is the net rate of production by chemical reaction, given by Ri =

ρξ 2 (Y i* − Yi ) τ(1 − ξ 3)

(3)

where Yi* is the fine-scale species mass fraction after reaction for time τ. The evolution of Yi* depends also on the chemical kinetic mechanism. Note that the eq 3 is valid only when the turbulence Reynolds number (Ret) is greater than 65.62 When Ret < 65, the early ignition problem may occur in the model. This problem can be avoided by modifying the EDC model constants.62 For the present simulation, since Ret > 65 (see section 5.2), the default values of the EDC model constants (Cξ = 2.1377 and Cτ = 0.4082) were adopted, as in the literature.60 To reduce the computational cost of time integration, the in situ adaptive tabulation (ISAT) model of Pope68 was used. The accuracy was checked by lowering the ISAT error tolerance and ensuring that results were unchanged. The ISAT error tolerance was set to 10−5 finally. The discrete ordinate (DO) radiation model69,70 was used with a weighted sum of gray gas model (WSGGM) to model the radiation in

3. COMPUTATIONAL DETAILS The present modeling work consisted of two parts. The first was CFD modeling of the flow inside the furnace, and the second was a chemical analysis of the firing of natural gas in a well-stirred reactor (WSR). The FLUENT code60 was used for the first part, and the CHEMKIN code was adopted for the second part. Some descriptions of the CFD modeling are provided below. The renormalization group (RNG) k−ε model with the standard wall function was implemented to model the turbulent flows. The RNG k−ε model is expected to address the well-documented shortfalls of the standard k−ε model, such as predicting round jets, swirling flows, and flows with a recirculation region.61,62 The EDC model63 2214


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the furnace. For the DO mode, each octant of the angular space 4π was discretized into 3 × 3 solid angles, and a total of 72 radiative transfer equations were solved in the three-dimensional space. Christo and Dally66 found that differential diffusion effects have a strong influence on the prediction of a JHC burner firing a CH4−H2 mixture. Mardani et al.23 further showed that for a JHC burner firing a CH4−H2 mixture, the influence of molecular diffusion increases with increasing fraction of H2 in the fuel mixture. However, for MILD combustion firing natural gas, the effect of differential diffusion was found to be small.12,62 Parente et al.45 also reported that the effect of molecular diffusion on the temperature field was negligible for their MILD combustion system. Therefore, it appears that the role of molecular diffusion is system-dependent. For the present study, differential diffusion was considered by representing the molecular diffusion coefficient for each species as a fourth-order polynomial function of temperature. It was found that there was no difference in the results of the simulations of firing natural gas with and without differential diffusion terms. However, in the present simulation the molecular diffusion was still taken into account. A primary orthogonal structured mesh with about 400 000 cells was used. The adequacy of this mesh (i.e., the grid independence) was verified by comparing the results with those obtained using a finer grid with 800 000 cells. The comparison between these two cases showed a high consistency in the results. The detailed grid is shown in ref 57 and is not displayed here. The inlet reactant velocity was set as the inlet boundary condition, and a pressure of 1.0 atm was set as the furnaceoutlet boundary condition; the measured wall temperatures were used as the wall boundary conditions. The calculated heat loss was validated by the measurements. The finite-volume method and the implicit method were used to discretize and solve the model equations. The SIMPLE algorithm method was utilized to solve pressure−velocity coupling. The secondorder upwind scheme was employed to discretize the equations in order to improve the simulation accuracy. The energy and species equations were subsequently solved, and their convergence was checked. Convergence was obtained when the residuals were less than 10−6 for the energy and 10−5 for all other variables. The outlet temperature and velocity were monitored, and their variations were allowed to be within 1 K and 0.1 m/s, respectively, to achieve convergence of their solutions. In order to better understand the mechanisms responsible for NO emission in the MILD combustion regime, the thermal-NO, promptNO, N2O-intermediate, NNH, and NO-reburning mechanisms were considered in the present investigation. The NO mechanism was solved as a postprocessing operation in FLUENT60 because the NO production typically appears in low concentrations and the NO chemistry has a negligible influence on the predicted flow field, temperatures, and major combustion product concentrations. Once the flow and thermal fields were obtained from CFD and validated with the experiment, NO route paths were calculated on the basis of the calculated thermal and species fields. 60 The O radical concentration in modeling of NO formation from the thermal-NO and N2O-intermediate routes as well as the OH radical concentration in the thermal-NO mechanism were predicted using the EDC model with the GRI-Mech 3.0 mechanism (this method is termed the Instantaneous Approach in FLUENT60). The prompt NO formation was modeled following De Soete.71 The N2O-intermediate mechanism was assumed at the quasi-steady-state.60 In order to take into account the effect of turbulent fluctuations on the mean reaction rates, the calculations for the thermal-NO, prompt-NO, and N2O-intermediate routes were based on kinetic mechanisms with Arrhenius equations integrated over a PDF of assumed β shape for temperature. For the NNH route, because no commercial code is available, we used the EDC model with the NNH mechanism72 to simulate its production (see Table 4 for details).

Table 4. Chemical Reactions in the NHH Mechanism for NO Formation72 rate coefficient parametersa A

reaction NNH ⇄ N2 + H NNH + O ⇄ NH + NO NH + O ⇄ NO + H NH + O2 ⇄ NO + OH

1.0 5.2 9.2 1.3

× × × ×

109 1011 1013 106

n

E

0.000 0.388 0.000 1.500

0 −409 0 100

a

The reaction rate coefficient was assumed to have the form k = ATn exp(−E/RT), where R is the universal gas constant and T is the absolute temperature. Units are cm, mol, s, and cal.

previous measurements on NP MILD combustion by Szegö and co-workers39,58 were made. Axial mean velocity and temperature distributions were measured using laser doppler anemometry (LDA) and thermocouples, respectively. Only the axial velocity data were recorded and presented here because the lateral velocity components (vx and vy) were too small to be measured reliably by LDA.58 The same furnace was used for the present measurements, and the burner configuration was similar to the NP MILD burner in Figure 2. It is worth noting that the burner of Szegö et al.39 was arranged as four side fuel jets (Df = 2 mm) and a central air jet (Da = 26.6 mm) (see ref 39 for details). For the comparison case, the air was preheated to 723 K with Φ = 0.8. Figure 3 compares lateral profiles of the axial mean velocity (vz) at z = 60.5 and 176.5 mm with x = 0 and −10 mm. The red solid lines represent the present numerical results obtained using the RNG k−ε model, while the black dashed lines denote the previous results obtained using the standard k−ε model.73 The profiles of the jets in the downstream planes (z = 60.5 and 176.5 mm) obviously reflect the upward central air jet and side fuel jets. The slightly negative axial velocities reveal the downward exhaust streams. The evolution of the velocity profiles shown in Figure 3 is asymmetric because the measured inlet velocity profile of the central air jet is not symmetrical.58 Clearly, the mean axial velocity (vz) distributions for the present numerical simulations (red lines) and the LDA measurements are reasonably consistent. Moreover, the present numerical predictions obtained from the RNG k−ε model (red solid lines) are better than the previous results of the standard k−ε model73 (black dashed lines). The RNG k−ε turbulent model offers accuracy in predicting the jet spreading, decay, and mixing as well as the recirculated downstream flow. The internal flow field is qualitatively and even quantitatively predicted by the present CFD simulation. Figure 4 shows comparisons of the experimental and numerical mean temperature profiles along the y axis at different axial locations (z = 142.5 and 442.5 mm) and yz planes. The prediction captures the asymmetric distribution of the in-furnace temperature caused by the asymmetric central inlet jet. In the lower part of the furnace (z = 142.5 mm), the CFD result well predicts the low temperature of the initial air streams. In the upper part of the furnace (z = 442.5 mm), the simulation results reproduce the measured uniform thermal field of the MILD combustion. Therefore, according to the present comparisons, it is appropriate to investigate the thermal field of MILD combustion using CFD calculations with the GRI-Mech 3.0 mechanism. 4.2. Effects of Reactant Mixing Pattern on the Flow Field and Internal Recirculation. Figure 5 displays contours of the z component of the mean velocity (vz) in the central xz

4. RESULTS AND DISCUSSION 4.1. Validation of the Model. To validate the present simulations, comparisons between the present predictions and 2215


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Figure 3. Comparisons of the experimental and numerical axial velocities in the two yz planes located at x = 0 and −10 mm: (a) x = 0 mm and z = 60.5 mm; (b) x = 0 mm and z = 176.5 mm; (c) x = −10 mm and z = 60.5 mm; (d) x = −10 mm and z = 176.5 mm. The red solid lines represent the present numerical results obtained using the RNG k−ε model, while the black dashed lines denote the previous results obtained using the standard k−ε model.73

Figure 4. Comparisons of the experimental and numerical mean temperature profiles in the three yz planes located at x = −100 (left), 0 (center), and 100 (right) for z = 142.5 and 442.5 mm.

plane (top panels) and the xy plane at z = 200 mm (bottom panels) for the modeling cases. The color scale is from −5 m/s (blue) to 20 m/s (red), with the blue and red colors applying for vz ≤ −5 m/s and vz ≥ 20 m/s, respectively. For the three modeling cases, the thermal input and equivalence ratio were constant at P = 10 kW and Φ = 0.8. The black lines are the contours of vz = 0. The upward and downward velocities can be identified by means of the colored contours and the black lines. Evidently, although the premixing patterns of the three cases are different, the flow fields of these three cases are generally similar. They are all characterized by a central upward (initially fuel−air mixture) flow (vz > 0) and side downward flue gas flows mainly at the four corners (vz < 0). The most dynamical

and strongest recirculation clearly occurs in the FP case, whose initial jet momentum rate is the highest. As the premixing pattern varies from FP to NP and finally to PP mode, J decreases from 0.4643 to 0.0341 kg·m/s2, and thus, both the cross-sectional area of the upward flow and its velocity at z = 200 mm decrease, so that the jet entrainment of the flue gas (ṁ e) also decreases. Wünning and Wünning59 defined the relative recirculation rate as Kv = ṁ e/(ṁ a + ṁ f), where ṁ e, ṁ a, and ṁ f are the recirculation exhaust gas mass flux and the initial air and fuel mass fluxes, respectively. For the present backwardflowing furnace, the total recirculated exhaust gas flux can be estimated as ṁ e = ṁ up − (ṁ a + ṁ f), where the upward-flow mass flux ṁ up is calculated from the expression ṁ up = ∫ ∫ ρvz(x, 2216


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occurs in the FP MILD case, whose J and Kv are higher than in the other two cases. The reaction appears to take place homogeneously in the upper furnace, and the in-furnace temperature difference is less than 50 K (Figure 6a). However, for the PP MILD case, whose J and Kv are the lowest, the temperature peak (∼1700 K) is the highest and obviously exists. The temperature peak arises because the J and Kv values are lowest, resulting in the weakest dilution effect of the recirculated exhaust gas on the reactants and thus the highest reaction rate. The reaction rate can be reflected by the OH radical. For conventional flames, the OH radical gives a good indication of the location of the flame front and is usually used as a flame marker in flame diagnostics. For the present MILD combustion, the location of the peak OH radical concentration is treated as the main reaction zone. Evidently, for the PP case, the OH radical is mainly concentrated in the downstream boundary of the mixture jet and the peak OH radical mass fraction is the highest. For the FP and NP cases, the OH radical is more distributed and evenly spread in most areas of the upper furnace. The peak OH radical mass fraction in the FP case is the lowest because its J and Kv are the highest, and thus, the reactants are intensely diluted by the recirculated exhaust gas, resulting in the lowest reaction rate. Figure 7 shows the effects of the premixing pattern on the CO and H2CO mass fraction distributions in the central xz

Figure 5. Contours of the z component of mean velocity (vz) in the xz plane at y = 0 (top) and the xy plane at z = 200 mm (lower) for (a) FP, (b) NP, and (c) PP obtained by CFD. The black contours on the plots are those of vz = 0. The values of Kv obtained at z = 400 mm are also shown in the plots. The unit of initial jet momentum rate (J) is kg· m/s2.

y) dxdy for vz > 0. It follows that the ratio ṁ e/(ṁ a + ṁ f) is equal to [ṁ up/(ṁ a + ṁ f)] − 1, and thus, Kv can be estimated according to the modeling results. The magnitudes of Kv at z = 400 mm are shown in the upper plots in Figure 5. Clearly, as the premixing pattern changes from FP to NP and finally to PP mode, J decreases rapidly from 0.4643 to 0.0341 kg·m/s2 and Kv decreases fast from 8.11 to 1.18. Therefore, variation of the premixing pattern leads to changes in J and thus influences Kv, resulting dilution of the reactants by different amounts of flue gas, thereby influencing the reaction. 4.3. Effects of Reactant Mixing Pattern on Temperatures and Species. Figure 6 displays the effects of the premixing pattern on the temperature and OH mass fraction distributions in the central xz plane (y = 0) obtained by CFD. It is evident from Figure 6 that for all three cases operating in MILD mode, the temperature distribution is uniform and generally less than 1600 K. The most uniform thermal field

Figure 7. Effects of the premixing pattern on the CO and H2CO mass fraction distributions in the central xz plane (y = 0) obtained by CFD: (a) FP; (b) NP; (c) PP.

plane (y = 0) obtained by CFD. As a reducing agent, CO mainly exists in the reducing atmosphere of the reaction zone. In Figure 7 for the three cases, CO is concentrated in the main reaction zone and, more importantly, its amount is not low. The existence of CO may result in reduction reactions. For instance, NO may be reduced to N2 by CO and other hydrocarbon radicals (CHi) in the fuel-rich zone, which is termed as the “NO-reburning” mechanism.74,75 Therefore, the peak CO region is the fuel-rich region where reduction reactions (e.g., the NO-reburning mechanism) may occur and thus may reduce the total NO emission. For the NP mode, the fuel and air are not premixed before their injection into the furnace, and thus, the peak equivalence ratio in the fuel-rich zone should be higher than in other two cases (i.e., Φ ≫ 1). It is hence expected that the CO distribution should be the highest for the NP case, which is indeed reflected by Figure 7. The fact that the CO level is highest for the NP case implies that its NO-reburning reaction may be the strongest. In

Figure 6. Effects of the premixing pattern on the temperature and OH mass fraction distributions in the central xz plane (y = 0) obtained by CFD: (a) FP; (b) NP; (c) PP. 2217


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Figure 8. Effect of the premixing pattern on furnace reference temperatures (Tf) obtained from experiments with firing rates of 7.5, 10, and 15 kW with equivalence ratios (Φ) changing from 0.5 to 1: (a) FP; (b) NP; (c) PP. Tad is the adiabatic flame temperature calculated by CHEMKIN.

Figure 9. Effect of the premixing pattern on exhaust temperatures (Tex) obtained from experiments with firing rates of 7.5, 10, and 15 kW and Φ varying from 0.5 to 1: (a) FP; (b) NP; (c) PP.

comparison, the NO-reburning reaction of the FP case may be the lowest because when the fuel and air are fully premixed the peak equivalence ratio in the fuel-rich zone should be the lowest among the three patterns. The reduction of the NOreburning reaction was verified by the CFD simulation, as shown in section 5.1 which specially investigates the NOx formation mechanism. The formaldehyde (H2CO) intermediate species is an important first-step flame intermediate formed in the lowtemperature regions of combustion of hydrocarbons76 and thus is predominant at low temperatures. For the three cases, H2CO mainly exists in the jet-mixing or prereaction region, while the OH radical is located downstream. It is also shown in Figure 7 that the H2CO distributions in the FP and NP cases are wider than in the PP case. Therefore, for the PP case, both the main reaction zone and the jet-mixing (or prereaction) zone are the smallest among the three cases. To further investigate the effect of the premixing pattern, experiments were carried out at firing rates (P) of 7.5, 10, and 15 kW with Φ varying from 0.5 to 1. The data for the FP mode at P = 15 kW are lacking because the air flow rate could not be

further increased as a result of the quite large pressure loss caused by the central tube with an extremely small diameter (for the FP burner, D = 7.2 mm). Figure 8 shows the experimental results for the effect of the premixing pattern on the furnace reference temperature (Tf). There is no significant difference (0.8 (i.e., more than 80% of the total NO formation is from the N2O-intermediate route). The thermal-NO, prompt-NO, and NNH mechanisms are unimportant, together contributing less than 20% (thus, their contributions are not shown). Moreover, the modeling NO emission results (red square symbols) for the three premixing modes have the same trend: for Φ < 0.8, the NO emissions generally increase with increasing Φ, and for Φ > 0.8, the emissions decrease as Φ is further increased. Comparison of the results in Figure 13 with those in Figure 11 indicates that the modeling results for P = 10 kW agree qualitatively with the measurements for P = 10 kW. The N2Ointermediate route is influenced by both the reaction temperature and the O radical concentration.77 For Φ < 0.8, there is enough O radical, and the N2O-intermediate mechanism is mainly influenced by reaction temperature. Therefore, as Φ increases from 0.5 to approximately 0.8, both the reaction temperature and the residence time increase, and

thus, the NO produced from the N2O-intermediate mechanism increases. However, when Φ further increases from 0.8 to 1, the O radical concentration decreases, and hence, the NO formation from the N2O-intermediate mechanism decreases. Galletti et al.44 and Mardani and Tabejamaat25 found that in the presence of hydrogen the NNH and N2O-intermediate routes are the dominant formation pathways under MILD combustion conditions. For the present investigation of firing natural gas without hydrogen addition, more than 80% of the total NO formation results from the N2O-intermediate route, while little comes from the NNH route. Therefore, it appears that the NNH route is important only when hydrogen is added to the fuel. This can be explained by considering NO production by the NNH route, particularly the following two reactions: N2 + H (+ M) ⇄ NNH (+ M)

(4)

NNH + O ⇄ NO + NH

(5)

The addition of hydrogen will increase the concentration of H radical and thus increase the importance of the NNH route. Moreover, Figure 13 shows that as Φ is increased, the NO reduction increases and therefore the NO-reburning reaction rate increases. For Φ > 0.9, the reburning reaction becomes sufficiently strong that the total NO emission is less than the NO produced from the N2O-intermediate route. The NOreburning mechanism78 occurs in the fuel-rich zone (Figure 7), where NO is reduced by reductive species such as CO and small radicals such as CH2, CH, and C; as noted above, CO mainly exists in the fuel-rich region. Figure 7 shows that the highest CO concentration occurs in the NP case and the lowest in the FP case. This is consistent with the NO-reburning results. For Φ > 0.95, the ratio of NO reduction by reburning to the total NO formation for the NP pattern (∼0.287) is the highest among the three patterns, demonstrating that the corresponding NO-reburning reaction is strongest. In contrast, the NO-reburning reaction is weakest in the FP case, with a ratio of ∼0.135. 2220


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= δL/SL, where S 0 is the integral scale, v′rms is the root-meansquare fluctuating velocity, δL is the laminar flame thickness, and SL is the laminar flame speed. It should be noted that δL characterizes the thickness of a reaction zone controlled by molecular (not turbulent) transport of heat and mass. For MILD combustion, although no turbulent flame front is present, the molecular-controlled reaction zone still exists. Turns52 suggests that S 0 and vrms ′ should be estimated as 1/10 of the characteristic length and the mean velocity, respectively. Also, δL = 2α/SL, where α denotes the thermal diffusivity, and α and SL can both be obtained from the thermal properties.52 Consequently, Da can be estimated using the above approach and the present experimental data. Figure 15 presents the

4.6. Effect of the Reactant Mixing Pattern on CO Emissions. Figure 14 displays the exhaust emissions of O2,

Figure 14. Exhaust emissions of CO, H2, O2, and CO2 for all of the FP, NP, and PP MILD combustion cases as obtained by experiments.

CO2, CO, and H2 for all of the MILD combustion cases in the present experiments. The emissions of CO and H2 are reported on logarithmic scales to better represent their low values. For Φ < 0.97, both the emission of CO ( 1. Generally, Da is in the range of 0.01−5.35 for the MILD combustion of the present investigation. Therefore, the reaction regime of MILD combustion is controlled by both the characteristic flow mixing time τflow and chemical reaction time τchem, neither of which can be ignored. 5.2. Reaction Regime of the Premixed MILD Combustion. Figure 16 shows the dependence of Da on the turbulence Reynolds number Ret (=vrms ′ S 0/ν) for the FP MILD combustion. The two data sets for P = 7.5 and 10 kW are presented. For the present FP MILD combustion, as Ret increases from 345 to 971, Da decreases rapidly from 5.35 to 0.01. This suggests that the increase in the large-scale turbulent mixing strongly promotes flue gas dilution of the reaction and thus decreases the reaction rate. Especially, in Figure 16 there is a series of red and black dots overlapping with each other, but one isolated red dot is located at much lower Da as a result of both high characteristic velocity and low laminar flame speed (SL). For this isolated case, Φ = 0.52 and P = 10 kW. Therefore, a large amount of injection air leads to both high in-furnace characteristic velocity and root-mean-square fluctuating velocity (v′rms). The low Φ also corresponds to low SL. Consequently, τflow (=S 0/v′rms) is rather low and τchem (=δL/SL) is quite high. As a result, for this particular case Da is extremely low relative to other cases.

5. FURTHER DISCUSSION OF CHARACTERISTICS OF THE REACTION REGIME OF MILD COMBUSTION 5.1. Damkö hler Number of the Premixed MILD Combustion. The Damköhler number (Da) is defined as the ratio of a characteristic flow time τflow to a characteristic chemical time τchem (i.e., Da = τflow/τchem). Da is an important dimensionless parameter for many combustion problems and is vital in understanding turbulent flames.52 For Da ≫ 1, the chemical reaction rate is much higher than the fluid mixing rate, and a fast-chemistry regime is defined. Conversely, for Da ≪ 1, the mixing rate is much higher than the reaction rate. For Da ≈ 1, the chemical reaction rate is close to the mixing rate. According to Turns,52 particularly useful characteristic times are the lifetime of large eddies in the flow, defined as τflow = S 0/ vrms ′ , and a laminar-flame-based chemical time, defined as τchem 2221


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B, typified by 1 < Da < 10 000. Although small portions of those regions lie in regime B, their Da is generally higher than that of the present FP MILD combustion. The MILD reactions occur at lower rates and thus are influenced more significantly by chemistry. Therefore, to make these devices operate in the MILD mode, their Da has to be decreased (e.g., by increasing the reaction dilution using a high-momentum jet). 5.3. Reaction Pathways under the MILD Combustion Regime. Although pathways of the conventional oxidation of natural gas have been investigated and are understood reasonably well, those for MILD combustion of natural gas have yet to be examined. Here we investigated the influence of premixing patterns on the reaction pathways for low (1200 K) and moderate (1600 K) temperatures and the global equivalence ratios Φ = 0.5, 1.0, and 1.5. It should be noted that, as shown in section 4.3, the typical furnace average temperature is 1200 K while the maximum peak temperature is 1600 K for the MILD combustion. Figure 17 shows the effect of the MILD reaction temperature (TWSR) on the reaction pathways of natural gas in a well-stirred reactor (WSR) at Φ = 1 for the residence time τ = 0.1 s. Panels (a) and (b) display the reaction pathways for TWSR = 1200 and 1600 K, respectively. Each arrow represents an elementary

Figure 16. Reaction regimes of the premixed turbulent combustion. Three types of reaction regimes are shown: (A) reaction sheets, (B) flamelets in eddies, and (C) distributed reactions. They are classified according to the two bold lines (S K = δL and S 0 = δL), where S K, S 0, and δL represent the Kolmogorov scale, the integral scale, and the laminar flame thickness, respectively. The black and red solid symbols correspond to the present experimental premixed MILD combustion with P = 7.5 and 10 kW, respectively.

Also shown on the plot of Figure 16 are the reaction regimes of premixed turbulent combustion from Turns 52 and Williams.79 The two bold lines define three separate regions, A, B, and C. Above the bold line of S K/δL = 1 (where S K is the Kolmogorov microscale), reactions can occur in thin sheets in the wrinkled laminar-flame region (region A); below the bold line of S 0/δL = 1 reactions, take place over a distributed region (region C). The region between the two bold lines is the flamelets-in-eddies regime (region B). In summary, the three regions are the following: regime A (wrinkled laminar-flames), where δL ≤ S K; regime B (flamelets-in-eddies), where S 0 > δL > S K; and regime C (distributed flames), where δL > S 0. Obviously, the present FP MILD combustion is located in Regime B, which is typified by moderate Da and high turbulence intensities. It is obtained in Section 5.1 and also from Figure 16 that the MILD reaction thickness δL is smaller than the integral scale S 0. Therefore, the whole reaction zone can be moved from one position to another by the large-scale motion, and simultaneously, the reaction can be sustained even if the large-scale velocity is high. Consequently, the entire furnace is filled with numerous separate distributed reaction zones as a result of the strong large-scale recirculation. On the other hand, δL is larger than the Kolmogorov microscale S K, and thus, there is not an obvious visible flame front. It is of interest to note that the present FP MILD combustion is not located in the distributed regime C, although it occurs volumetrically. This result is consistent with the direct numerical simulation results for premixed turbulent MILD combustion carried out by Minamoto and co-workers.80,81 These authors also found that the MILD combustion is not an idealized homogeneous reactive mixture and has thin reaction regions while the reaction zone in the MILD combustion is non-flamelet-like.80 Williams79 also presented the estimated reaction regimes of spark-ignition engine combustion (the green boxed region in Figure 16), diesel engines and offshore flares (the blue round domain), and supersonic combustion (the pink trapezoidal region). Those practical devices operate in both regimes A and

Figure 17. WSR reaction pathways of natural gas in the MILD regime for (a) TWSR = 1200 K and (b) TWSR = 1600 K at Φ = 1 for τ = 0.1 s. Numbers in parentheses denote the reaction rates (e.g., 2.4−7 means 2.4 × 10−7 gmol·cm−3·s−1) for the WSR model in CHEMKIN. 2222


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reaction or set of reactions, with the primary reactant species at the tail and the primary product species at the head. Additional reactant species are shown along the length of the arrow, and the corresponding reaction number is indicated. The width of the arrow gives a visual indication of the relative importance of a particular reaction path, while the parenthetical numerical values quantify the destruction rate of the reactant (i.e., the reaction rates are shown in parentheses, e.g., 2.6−7 means 2.6 × 10−7 gmol·cm−3·s−1). Those pathways with reaction rates less than 1 × 10−7 gmol·cm−3·s−1 are not shown in the diagram. At the low temperature (1200 K; Figure 17a), the prominent reaction pathways are CH4 → CH3 → (CH3O) → CH2O → HCO → CO → CO2 and CH4 → CH3 → CH2(s) → CH2 → CO → CO2. The red arrows in the diagram for the moderate temperature (1600 K; Figure 17b) show the pathways that become more dominant and the blue arrows the pathways that are weaker relative to the pathways at the lower temperature (1200 K). At the moderate temperature (1600 K), CH3 is more likely to be converted to CH2(s) and CH2 and then to form CO. The prominent reaction pathway is CH4 → CH3 → CH2(s) → CH2 → CO → CO2. Meanwhile, the reaction pathways CH3 → C2Hx → CO → CO2 and CH3 → CH3O → CH2O → HCO → CO → CO2 weaken. Obviously, there are significant differences between the leading reaction loops for TWSR = 1200 and 1600 K. Mardani et al.24 also found that under MILD conditions chemical pathways including conversion of CH3 to higher hydrocarbons (i.e., C2H6) are activated and some portions of CH4 are oxidized to CO through conversion to heavier C2H6 molecules. Namely, the methane lowtemperature reaction pathways are activated under MILD conditions. Simplified mechanisms such as the Westbrook and Dryer mechanism82 and the Jones and Lindstedt mechanism,83 are not appropriate for accurate prediction of MILD combustion. A detailed reaction mechanism should be adopted. Figure 18 shows the effects of the equivalence ratio on the reaction pathways of natural gas in a well-stirred reactor at low temperature (1200 K) for the residence time τ = 0.1 s. As Φ increases, the reaction pathways CH3 → (CH2(s) → CH2) → CO weaken (blue arrows), while the reaction routes CH4 → CH3 → CH3O → CH2O and CH4 → CH3 → C2Hx → CO are boosted (red arrows). The increase in the importance of the C2Hi species on reaction pathways with increasing Φ suggests that the C2Hi reaction mechanism should be considered in those combustion mechanisms simplified from the detailed one. In addition, the products of the reaction of C2 species include soot, so more soot is produced as Φ is increased. We also investigated the effect of residence time (τ) on the reaction pathways at low temperature (1200 K). The reaction calculations were performed with τ varying from 0.01 to 1.0 s. We discovered that τ has a minor influence on the elementary reaction pathways of the natural gas−air mixture at Φ = 1. The reason is that the ignition time of the natural gas−air mixture is extremely short (e.g., for methane, 84 τ = 2.5 × 10 −15 exp(26700/T) Y[CH4]0.32 Y[O2]−1.02; when Y[CH4] = 0.03, Y[O2] = 0.22, and T = 1200 K, τ ≈ 1.76 × 10−5 s). The actual reaction time is generally longer, so the reaction path is near the chemical equilibrium state. It is worth noting that Mardani et al.24 used a residence time of 0.1 s for their WSR simulation.

Figure 18. Effects of the equivalence ratio on reaction pathways of natural gas in a well-stirred reactor in the low-temperature (1200 K) MILD regime for the residence time τ = 0.1 s: (a) Φ = 0.5; (b) Φ = 1; (c) Φ = 1.5.

partially premixed (PP), and fully premixed (FP) patterns of the fuel and air streams in a laboratory-scale furnace. The characteristics of the reaction regime of MILD combustion have also been investigated. Specifically, the influences of the equivalence ratio (Φ) and firing rate (P) have been examined.

6. CONCLUDING REMARKS The present study has investigated the global characteristics of MILD combustion of natural gas using the non-premixed (NP), 2223


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Experiments have demonstrated that MILD combustion in the present furnace can be achieved in the three air−fuel premixing patterns. When MILD combustion is established, low emissions of both NOx and CO are generally obtained regardless of the air−fuel pattern and firing rate. On the basis of the results shown in Sections 4 and 5, the main concluding remarks are made below: (1) Variation of the premixing pattern leads to a change in the initial jet momentum and thus influences the internal flue gas recirculation. This results in dilution of the reactants by different mass fluxes of the recirculated flue gas and thus influences the reaction rate. For the present investigation, the FP burner produces the highest jet momentum rate and thus the strongest mixing, resulting in the lowest peak temperatures and hence the lowest exhaust NOx emissions. In comparison, the PP burner yields the lowest jet momentum and thus the highest peak temperature and exhaust NOx emissions. (2) For the present burner and furnace configurations at Φ < 0.97, MILD combustion can be established with nearly zero exhaust CO emission, regardless of the premixing pattern and firing rate. (3) For the three premixing patterns, more than 80% of the total NO emission derives from the N2O-intermediate route, with the rest from the thermal-NO, prompt-NO, and NNH mechanisms. As Φ is increased from 0.5 to 1.0, the NOx emission first increases and then decreases. For the FP MILD case, the reaction temperature is relatively very low, and hence, the NOx emissions from all of the NO routes are negligible. Moreover, for Φ > 0.9, the NO-reburning reaction becomes strong (with the NP and FP patterns being strongest and weakest, respectively) and cannot be ignored. (4) As Φ is increased, the characteristic flow time (τflow) increases while the characteristic chemical time (τchem) decreases, and hence, the Damköhler number (Da) increases rapidly. For Φ ≈ 0.73, τflow ≈ τchem and Da ≈ 1. Generally, Da for the MILD combustion is in the range of 0.01−5.35. Thus, the MILD combustion is controlled by both τflow and τchem. The FP MILD reactions are located in the flamelet-in-eddy regime, which is characterized by moderate Da and high turbulence intensities. (5) The premixing pattern affects the chemical reaction pathways under MILD conditions through its influences on the reaction temperature and equivalence ratio locally in the reaction zone. At low reactor temperature (1200 K), the prominent reaction pathways are CH4 → CH3 → (CH3O) → CH2O → HCO → CO → CO2 and CH4 → CH3 → CH2(s) → CH2 → CO → CO2. However, at the moderate reactor temperature (1600 K), CH3 more likely to be converted to CH2(s) and CH2 and then to form CO. Meanwhile, the reaction pathways CH3 → C2Hx → CO → CO2 and CH3 → CH3O → CH2O → HCO → CO → CO2 weaken. In addition, at 1200 K, as Φ is increased, the reaction pathways CH3 → (CH2(s) → CH2) → CO weaken, but the reaction routes CH4 → CH3 → CH3O → CH2O and CH4 → CH3 → C2Hx → CO are boosted. The significantly different routes of the chemical reactions for various temperatures and equivalence ratios suggest that simplified mechanisms such as the Westbrook and Dryer mechanism82 and the Jones and Lindstedt mechanism83 are not appropriate for prediction of MILD combustion. A detailed reaction mechanism should be adopted.

Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the Specific Research Fund for the Doctoral Program of Higher Education of China (Grant 20110001130014), the National Natural Science Foundation of China (51276002), and the Centre for Global New Energy Strategy Studies of Peking University.

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NOMENCLATURE

Symbols

A = area of the nozzle exit (m2) Cξ = volume fraction constant (=2.1377) Cτ = time scale constant (=0.4082) Da = diameter of the central air jet (m) Df = diameter of the side fuel jet (m) Do = diameter of the single central tube (m) Etotal‑NO = total NO emission considering all the NO routes (including NO reburning) FN2O‑route = NO formation from the N2O route Ftotal‑NO = total NO formation considering only NO formation routes (without NO reburning) J = initial jet momentum rate (kg·m/s2) Kv = relative recirculation rate ṁ = mass flow rate of the fuel−air mixture (kg/s) P = thermal input power rate (kW) RNO‑reburn = the amount of NO reduction by the NO-reburn route SL = laminar flame speed (m/s) T = temperature (K) Tad = adiabatic flame temperature (K) Tex = exhaust temperature (K) Tf = furnace reference temperature (K) Tmax = maximum temperature (K) U = velocity of the initial reactant (m/s) v = velocity (m/s) v′rms = root-mean-square fluctuating velocity (m/s) Yi = mass fraction of species i Yi* = mass fraction of fine-scale species i after reaction YOH = mass fraction of OH radical Greek Letters

■

α = thermal diffusivity (m2/s) δL = laminar flame thickness (m) ρ = density of the mixture (kg/m3) τ = residence chemical time scale (s) τchem = characteristic chemical time (s) τflow = characteristic flow time (s) Φ = equivalence ratio

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