Optimal Equivalence Ratio to Minimize NO Emission during Moderate

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Optimal equivalence ratio to minimize NO emission during MILD combustion fan hu, Pengfei Li, Junjun Guo, Feifei Wang, Kai Wang, Xudong Jiang, Zhaohui Liu, and Chuguang Zheng Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03162 • Publication Date (Web): 01 Feb 2018 Downloaded from http://pubs.acs.org on February 6, 2018

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Optimal equivalence ratio to minimize NO emission during MILD combustion

Fan Hu1, Pengfei Li1, *, Junjun Guo1, Feifei Wang2, Kai Wang1, Xudong Jiang1, Zhaohui Liu1, *, and Chuguang Zheng1 1

U.S.-China Clean Energy Research Center, State Key Laboratory of Coal Combustion, School of

Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, China 2

School of Environmental Science and Engineering, Huazhong University of Science and

Technology, Wuhan 430074, China

*

E-mail address: [email protected] (P. Li), [email protected] (Z. Liu).

ACS Paragon Plus Environment

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Abstract: The influence of injection conditions on NO emission during moderate or intense low-oxygen dilution (MILD) combustion was investigated through experiments and numerical simulations. The detailed GRI-Mech 2.11 mechanism and finite-rate chemistry effects were considered in simulations. The model was systematically validated using detailed in-furnace and exhaust emission measurements. The initial conditions investigated included the equivalence ratio (Φ), oxidant preheating temperature (TO) and thermal input (P). Notable modeling results were found: under MILD combustion, with Φ increasing from 0.5 to 1.2, NO production first decreases and then increases, with the minimum NO emission obtained at Φ ≈ 0.8. Moreover, this trend is obtained at various P and TO, although NO emission increases monotonically with increasing P or TO. Furthermore, we found that CO emission is generally extremely low and can be ignored when Φ ≤ 0.85 but is much higher when Φ > 0.9. Consequently, for the furnace considered here, an optimal equivalence ratio (Φ) exists to minimize the NO and CO emissions of MILD combustion; this equivalence ratio is approximately 0.8 regardless of the P and TO. These findings were confirmed by our experiments; in fact, nearly zero NO emission was achieved at this optimal Φ in the present experiment, which is significantly lower than that of previous studies on MILD combustion with typical NO emissions of 15-40 mg/m3 (@ 15% O2). Therefore, the present study improves MILD combustion technology significantly to relatively low or even nearly zero NO emission when using the optimal equivalence ratio. Our detailed analysis of NO mechanisms further reveals that all NO formation routes (particularly the N2O-intermediate route and the prompt route) are strongly suppressed at this optimal Φ, thus minimizing NO emission.

Keywords: MILD combustion, Equivalence ratio, CFD modeling, NO emission


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1. Introduction Moderate or intense low-oxygen dilution (MILD) combustion[1-3] can increase the thermal efficiency and reduce emissions. This technology has received significant attention owing to its semi-uniform temperature distribution, high thermal performance, low nitrogen oxide (NOx) formation, and favorable combustion stabilization[4]. In the MILD regime, reactions occur volumetrically[5], and the flame is invisible. Therefore, this technology is also called flameless oxidation (FLOX)[6], homogeneous combustion or colorless distribution combustion (CDC)[7]. The MILD combustion has been successfully used in industries for heating combustors[8] and gas turbines[9] and has great potential for applications in other fields[10].

The influence of initial operating conditions on MILD combustion is of vital importance; selected relevant studies are summarized in Table 1. Szegö et al.[11-12] experimentally investigated the influence of certain operating conditions (i.e., air preheating, heat extraction, the equivalence ratio and fuel dilution) on reaction temperatures and emissions in a 20 kW furnace. They discovered that the fuel jet momentum rate dominated the combustion stability. Mi et al.[13] numerically and experimentally found that for a specific combustor, MILD combustion can be established only when the initial jet momentum rate exceeds a critical value. Mi et al.[14] also found that the stability limit of the MILD combustion varies with different initial conditions and combustor configurations. Veríssimo et al.[8, 15-17] experimentally examined the influence of the inlet air velocity[15], thermal input[16], excess air coefficient[8] and preheating temperature[17] on MILD combustion performance. They concluded that when the excess air coefficient is maintained at 1.3, as the inlet jet momentum is increased, the main reaction region does not vary significantly due to the similarity of the internal flow field. However, in their experiments, when a surplus of air was supplied further, MILD combustion could not be achieved regardless of the initial velocity of the air jet. They also found that as the input capacity, preheating temperature or excess air coefficient was increased, the central


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jet momentum also increased, leading to stronger internal recirculation. Consequently, the main reaction region tended to approach the nozzle exit. Castela et al.[18] also found experimentally that the initial velocity of the air jet significantly influenced the NOx emissions at a given air initial preheating temperature. Li et al.[19] found that, there is a dimensionless parameter, namely, the initial Reynolds number (Re) of the air-fuel fully premixed jet, dominating the stability and characteristics of the MILD combustion under the fully premixed fuel-air condition, irrespective of changes in Φ, A, or f. Li et al.[20] also found that the initial jet momentum controls the reaction regime and NOx formation of MILD combustion. For their burner and furnace, the initial jet momentum of the fully premixed jet was the highest. The high jet momentum led to significant internal recirculation. The temperature peak was avoided, and thus the NOx emission was the lowest for the fully premixed case. Conversely, the initial jet momentum of the reactant partially premixed pattern was the lowest. The NOx emission for this case was the highest, although the MILD combustion could still be established using a partially premixed fuel-air pattern. Ayoub et al.[21] experimentally examined influences of hydrogen addition and air preheating on MILD combustion. Their experiments showed that as the hydrogen level is increased, NOx emissions decrease due mainly to the inhibition of the prompt NO production route. They also found that as the air temperature is decreased, more air must be provided to suppress CO emission. Sorrentino et al.[22] studied effects of initial conditions, including the preheating temperature and mixture composition, on the MILD combustion. As the inlet preheating temperature increases, the flame gradually disappears and that flameless MILD combustion can ultimately be established. Tu et al.[23] studied influences of the furnace configuration on the characteristics of MILD combustion via numerical modeling. They found that as the angle between the furnace sidewall and the roof was increased, the internal recirculation became stronger and thus a lower combustion peak temperature and NOx emission rate were obtained.


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Table 1. Selected investigations on influences of initial conditions on the stability of MILD combustion Fuel

Factor(s) studied

NG, LPG

Fuel type, diluent type, fuel jet momentum Air preheating, heat extraction, equivalence ratio, fuel dilution Air-fuel premixing pattern, initial jet momentum Excess air coefficient Equivalence ratio, inlet area, fuel capacity, dilution of the reactants Separation distance between the fuel and air nozzles, inlet area, air preheating temperature Composition of methane/hydrogen mixture, air preheating Air inlet preheating temperature, equivalence ratio Air inlet velocity Thermal input Premixed patterns Furnace chamber shape Air preheating temperature Preheating temperatures, mixture composition

NG, LPG NG Methane NG Methane Methane/hydrogen mixtures NG Methane Methane NG NG Methane Propane a

Methodology Exp.a Sim.b √ -

2008 [11]

√

-

2009 [12]

√

CFD

2009 [13]

√

-

2011 [8]

√

CFD

2011 [19]

-

CFD

2011 [14]

√

-

2012 [21]

√

-

2012 [18]

√ √ √ √

CFD CFD -

2013 [15] 2013 [16] 2014 [20] 2015 [23] 2015 [17]

√

CFD

2016 [22]

Reference

Experiment, bSimulation, LPG (liquefied petroleum gas), NG (natural gas), CFD (computational

fluid dynamics).

The studies mentioned above focused mainly on impacts of initial conditions on the establishment, stability, reaction and global NOx characteristics of MILD combustion. However, information on the influence of operating conditions on the detailed NO production mechanisms during MILD combustion is lacking. Previous studies on the influence of the initial conditions on NO formation


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mechanisms/routes were conducted mainly through zero-dimensional (0-D)[24] or one-dimensional (1-D) chemical kinetic analysis[25] or two-dimensional (2-D) jet into a hot coflow (JHC) open flame simulation[26-27], as listed in Table 2. Li et al.[24] studied NO-relevant mechanisms under MILD combustion of methane and hydrogen fuel blends through well-stirred reactor (WSR) simulations. They found that the N2O-intermediate route controls the NO formation in MILD combustion of CH4/H2 fuel blends. With increasing reaction temperature, the NO formation from the thermal NO pathway is enhanced, while that of the N2O-intermediate reduces. Moreover, when the oxygen level rises, the N2O-intermediate pathway becomes stronger, whereas the prompt and NNH pathways weaken.

Gao et al.[26] examined influences of hydrogen addition and air temperature on NO formation in H2-CH4 JHC flames. Their modeling results demonstrated that with decreasing hydrogen level or air temperature, NO emission is suppressed. Wang et al.[27] also investigated detailed NO-relevant mechanisms in CH4/H2 jet flames through CFD simulations. They found that additional hydrogen increases the importance of the NNH NO mechanism because more H atoms are available.

Table 2. Selected investigations on the influence of initial conditions on NO-relevant mechanisms during MILD combustion Fuel

Factor(s) studied

CH4/H2

Reactor temperature

CH4

Oxidizer temperature

CH4/H2 CH4/H2

Methodology Chemical kinetics analysis Chemical kinetics analysis

Coflow temperature, hydrogen CFD addition Oxygen concentration, coflow CFD temperature, hydrogen addition

Model

Reference

0-D WSR

2014 [24]

1-D counterflow

2017 [25]

2-D JHC

2013 [26]

2-D JHC

2015 [27]

Generally, 0-D or 1-D chemical kinetic analysis and JHC open flames are significantly different from in-furnace combustion. Moreover, previous studies typically characterized exhaust NO


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emissions using MILD combustion rates of 15-40 mg/m3 (@ 15% O2). Potential remains to further reduce the exhaust NO emission from furnace MILD combustion. Therefore, more systematic investigation is needed to further examine effects of injection conditions on NO-relevant mechanisms during MILD combustion in furnaces to lower NO emissions further. This study is designated to address this gap. The purpose of the present work is (1) to experimentally and numerically optimize operational conditions (i.e., the equivalence ratio, thermal input and oxidant preheating temperature) to achieve lower NO emission from methane MILD combustion and (2) to perform detailed experiments under the preferred initial conditions to minimize NO emission. The description of the present experiments is first reviewed in Section 2. The numerical simulation details are given in Section 3. Results and analyses are presented in Section 4. Experimental results under the optimal conditions and a detailed analysis of the NO suppression mechanism are further discussed in Section 5, and the conclusions are summarized finally.

2. Experimental details 2.1 Experimental facility system The experiments are conducted in a 20 kW laboratory-scale facility system. This facility can support typical MILD and traditional flame combustion for gaseous and solid fuels; the associated schematic diagram is presented in Figure 1. The complete experimental system consists of six subsystems, namely, the combustion chamber, flue gas cleaning system, fuel feeder system, oxidizer supply system, dust removal system and distributed control system. A gas electrical heater is used to preheat the air. Methane with a purity of 99.9% is used as the fuel. The fuel is kept at ambient temperature, and a maximum thermal input of 20 kW can be supplied.


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(a) Experimental system

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(b) Flow chart

Figure 1. Schematic diagram of the 20 kW MILD combustion laboratory-scale facility system.

Reactants are injected through a burner at the top wall, while the flue gas is exhausted from the bottom (as shown in Figure 2(a)). The system includes a cubic furnace with base dimensions of 400 × 400 mm2 and a height of 800 mm. Due to the 50 mm thick corundum and 200 mm thick ceramic fiber board housing, stable MILD combustion can occur in the combustion chamber after suitable preheating. Observation windows are located at furnace sidewalls, with five observation windows per side. Only one window is used for observation during the experiments; the other windows are blocked to prevent heat and mass loss. There are 5 × 6 sampling holes at one side of the furnace to measure in-furnace temperatures and species compositions (Figures 1(a) and 2(a)). Moreover, wall temperatures are also recorded using thermocouples. A water cooling heat exchanger is used to adjust the furnace temperature.


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Figure 2. Schematic diagram of the burner and the experimental furnace: (a) furnace, (b) the MILD burner, (c) top view of the furnace, (d) visible flame combustion and (e) flameless MILD combustion.

For the detailed burner system, Figures 2(b) & 2(c) show that a central annular nozzle and two symmetric nozzles are mounted on the furnace top wall. As shown in the annular tube, a 4 mm


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diameter pole of the bluff-body is set in the center, and the radius of the bluff-body is 6 mm; this component is used to maintain a stable flame and preheat the furnace. The inner diameters of the fuel tube and swirl jet tube are 14 mm and 28 mm, respectively. Two symmetrical 8 mm diameter air nozzles are separated by 200 mm. As a result, both swirl jet flame combustion and MILD combustion can be established using this burner by switching the air supply. For swirl jet flame combustion, the flame is established by the central annular fuel jet at ambient temperature and the swirl air with a preheating temperature of approximately 500 K. Figure 2(d) displays a typical visible flame picture. For MILD combustion, the fuel at ambient temperature is injected in the same way as flame combustion. The low preheated air (≈500 K) is introduced into the furnace through the two parallel 8 mm diameter nozzles with high momentum, resulting in a stable flameless MILD condition as shown in Figure 2(e).

2.2 Measurement details and experimental conditions The distribution of the two symmetry planes and 30 sampling points is displayed in Figure 3(a). The furnace temperature and major species of O2, CO2, and CO are measured at plane A and plane B through the array measuring holes. S-type thermocouples are used for temperature measurements with uncertainties less than ± 0.5% of the measured value. A customized gas sampling probe is applied to sample the species inside the furnace. The in-furnace major species concentrations (O2, CO2 and CO) are determined using an MRU VARIO PLUS gas analyzer. The exhaust species concentrations of O2, NO, CO2, CO are recorded with a HORIBA PG-350 gas analyzer. The analyzers can achieve accuracies of (O2: ± 0.2%, 0 - 25%), (CO2: ± 2%, 0 - 20%), and (CO: ± 5%, 0 - 4000 ppm) for the MRU analyzer and (O2: ± 0.1%, 0 - 25%), (CO2: ± 1%, 0 - 20%), (CO: ± 20 ppm, 0 - 1000 ppm), and (NO: ± 0.3%, 0 - 25 ppm) for the HORIBA analyzer. The CO and NO measurements (both in-furnace and exhaust) are calibrated to 15% O2 concentration (dry basis) to eliminate the dilution effects and ensure direct comparisons. For NO emissions, to compare with


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emission standards, the unit of the exhaust NO is converted from ppm to mg/m3 through Yi [mg/m3] = (M / 22.4) × Xi [ppm] × (273 / T) × (p / 101,325), where Yi is the species concentration in mg/m3, M is the gas molecular weight (g/mol), Xi is the species concentration (ppm), and p is the pressure (Pa). In addition, note that in China, different calibrated O2 concentrations are used for different emission standards for boilers of gaseous fuels. For example, the calibrated O2 concentration is 3.5% for an industrial boiler of gaseous fuels, 3% for a gas-fired boiler for power generation, and 15% for a gas turbine. To compare the NO emissions among various cases, the exhaust oxygen level is calibrated to 15% in our study. The correction of the O2 concentration can be obtained as

Y2 = Y1 ×

21 − C2 (O2 ) , where Y2 and Y1 are the calibrated and measured oxygen levels (mg/m3), 21 − C1 (O2 )

respectively, and C2(O2) and C1(O2) are the calibrated and measured oxygen levels (%), respectively.

0 50 150 Plane A 300

(a) 450

600

750 -150 -75 0


75 150

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(b)

Figure 3. Schematic diagram of the measurement system: (a) distributions of the measurement planes and sampling points and (b) customized sampling probe.

The customized sampling probe (as shown in Figure 3(b)) for in-furnace gas sampling contains three concentric stainless-steel pipes. The internal tube is used for gas sampling, the circular tube between the internal and middle tubes transports inlet cooling water, and the circular tube between the middle and outer wall transports outlet cooling water. The probe is introduced into the furnace through sampling holes from plane A (Y-Z plane) and plane B (X-Z plane). As shown in Figure 3(a), each plane contains 30 measurement points. The sampling probe can be used at various locations; thus, a total of 56 points inside the furnace can be measured. Before the experiment, the gas analyzers were checked, and the airtightness of the measurement system was evaluated. The temperature measurement was also corrected for radiation after experiments according to Ref. [11] and [28]. We found that the temperature correction results in a maximum correction of 2.2%. The typical correction between the measured and real gas temperatures is less than 1.1%. Consequently, the uncertainty is less than 1.2%.

3. Numerical simulation details 3.1 Brief description of CFD modeling The Reynolds-averaged Navier–Stokes (RANS) CFD simulation is used to comprehensively examine influences of injection conditions on NO formation during MILD combustion. The


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realizable k–ε model[29] with the standard wall function are adopted to describe flow fields. Considering finite-rate slow reactions of methane MILD combustion, the eddy dissipation concept (EDC) model[30] is used to predict the turbulence-chemistry interaction. The detailed GRI-Mech 2.11 mechanism[31] is used to model the reaction.

The EDC model is extensively applied in the modeling of MILD combustion and obtains satisfactory performance[13-14, 19-20, 23, 32-34]. Theoretically, this model can consider detailed chemistry and capture the finite-rate reaction influences. This combustion model considers timescales of both turbulence and kinetics and assumes that reactions occur at fine scales. For fine scales, the vε 1 4 ) , where Cξ is the volume fraction k2 v constant. The chemical residence time scale (τ) is obtained as τ = Cτ ( )1 2 , where Cτ is the time

characteristic length fraction (ξ) is obtained through ξ = Cξ (

ε

scale constant (= 0.4082). Integrating the chemistry within those fine scales can determine the evolution of species concentrations. For the EDC model, the conservation equation for a species is v uv ∂ ( ρYi ) + ∇ ⋅ ( ρ vYi ) = −∇ J i + Ri . ∂t

(1)

uv In equation (1), Ri is the net rate of formation through chemical reaction, J i is the diffusion flux and Yi is the mass fraction of each species i. The chemistry source term (Ri) is obtained according to

Ri =

ρξ 2 (Y * − Yi ) τ (1 − ξ 3 ) i

(2)

where Yi* is the mass fraction of the fine-scale species after reacting over time (τ). This equation is valid when Ret > 65 (where Ret is the turbulence Reynolds number)[35]. When Ret < 65, the default values of the EDC parameters can lead to early ignition of the reaction. This issue can be resolved by revising the parameters of this model[35]. For this study, Ret > 65, and thus the default EDC model parameter values[30] of Cξ = 2.1377 and Cτ = 0.4082 are used.


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For the detailed chemical kinetic mechanism, according to our previous work and the work of other investigators[24, 36-39], both GRI-Mech 2.11 and GRI-Mech 3.0 can accurately predict the oxidation of methane MILD combustion. However, the prediction of NO emission by GRI-Mech 2.11 is found to be better than that of the other one[36-39] because GRI-Mech 3.0 over-estimates the prompt NO production[36-39]. Consequently, GRI-Mech 2.11 is used to model the NO production. The in situ adaptive tabulation (ISAT) model[40] is used to reduce the computational integration cost. The discrete ordinate radiation model[41] is adopted for radiation. For the absorption coefficient, the weighted sum of the gray gas model (WSGGM)[42] is adopted. In this modeling, a structured hexahedral grid with approximately 420,000 cells is adopted, and a mesh-independent analysis shows that the number of cells is sufficient.

3.2 NO mechanisms For methane MILD combustion, NO is formed through the following four production paths: the thermal route[43], prompt route[43], N2O-intermediate route[43], and NNH route[44]. The NO formation routes are described in detail as follows: Thermal route[43]: The N2 triple bond is broken by the O atom at high temperatures, and the N atoms are oxidized by O2 and OH radicals. The thermal route has a strong dependence on temperature and becomes significant for high-temperature flames. Prompt route[43]: N2 reacts with CHx radicals (e.g., CH, CH2, and CH3) through the path of N2 → NH/NCN/H/HCN/HNCN → NCO/N → NO. For this route, the rate-limiting step is the reaction of CH + N2. N2O-intermediate route[43]: for this route, NO is produced from the reaction of N2O with O atoms. This route may be important at moderate temperatures because the activation energy for this route is low. As more O atoms are provided, the reaction of N2O with O is enhanced, and thus the N2O-intermediate route becomes stronger at fuel-lean conditions.


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NNH route[44]: The main reaction pathway for this route is N2 → NNH → NO. This mechanism has a minor effect on the combustion of hydrocarbon fuels, but its contribution may significantly increase with hydrogen addition. With hydrogen addition, more H atoms can enhance the production of NNH radicals, thus producing more NO.

The NO destruction routes involve the NO-reburning[45] and NO-NO2 reactions[43]. More detailed information is given below: NO-reburning route[45]: NO can be reduced through reactions with CHx radicals (e.g., CH, CH2, and CH3). The importance of this route is significant under fuel-rich conditions. NO-NO2 reactions[43]: in which NO is oxidized to NO2 with OH radicals and O atoms.

For exhaust NO emissions, both the NO emission index (EINO) and NO concentration are discussed. EINO is adopted to quantify the exhaust NO emissions for various conditions and is characterized as the ratio between the NO formation rate and fuel consumption rate without being affected by the dilution of the products[43]. EINO can be expressed as EINO =

∫ MW − ∫ MW

ω NO dxdydz

NO

CH 4

ωCH dxdydz

( kg − NO / kg − fuel )

(3)

4

where MW is the molecular weight, and ωi is the production/consumption rate of the subscript species.

3.3 Summary of all calculation cases Table 3 summarizes all the CFD modeling cases to investigate the dependence of NOx emission on the equivalence ratio (Φ), thermal input (P) and oxidant preheating temperatures (TO). Case EXP I is the present detailed measurement case for validation. Case EXP I is also discussed in detail in the reaction path and EINO contribution analysis. Case CFD I is used to examine various equivalence


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ratios (Φ = 0.5, 0.7, 0.8, 0.9, 1.0 and 1.2). Case CFD II is used to investigate the influence of various thermal inputs (P = 5, 10 and 20 kW) at various equivalence ratios. Case CFD III employs various oxidant preheating temperatures (TO = 300, 500, 700, 900 and 1100 K) at various equivalence ratios.

Table 3. Summary of the injection conditions investigated Cases EXP I CFD I CFD II CFD III

Fuel (methane) Tf (K) Vf (m/s) 300 3.291 300 3.247 300 2.165-4.330 300 3.247

Oxidant (air) To (K) Vo (m/s) 93.449 500 144.968-60.403 500 138.065-48.323 500 227.807-43.490 300-1100

Ф

P (kW)

0.79 0.5-1.2 0.7-1.0 0.7-1.0

15.2 15 10-20 15

4. Results and analysis 4.1 Modeling validation with the present experiments Comparisons of the predicted temperatures and O2, CO2 and CO concentrations with experiments (Case EXP I) on the two central planes (i.e., the Y-Z and X-Z planes) are shown in panels (a) - (d) of Figure 4 for validation. There are 6 different Z locations and a total of 56 measurement points in two planes of the detailed in-furnace measurements. Figure 4(a) shows that the measured temperature distributions are relatively uniform, which is the main feature of MILD combustion[2]. Moreover, the O2 concentration also has satisfactory uniformity, and most of the values are in the typical MILD combustion range of 2 - 5% by volume[46]. The distribution of the Y-Z plane is less uniform than the distribution of the X-Z plane because the inlet streams are injected into the Y-Z plane through the burner at the top of the furnace (Figure 3). As shown in Figure 3(a) and Figures 4 (a-d), there are no measurement points at the jet downstream region; thus, the jet core zone was not characterized in the experiments. There are 30 sampling holes and a total of 56 measurement points in this furnace, and it is not realistic to measure all locations inside the real furnace. Overall, Figures 4 (a-d) show that the predictions of temperature and major species concentrations are quantitatively consistent


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with the available experimental data. Moreover, Table 4 lists the exhaust emissions from the CFD modeling and experiments. The simulation reproduces the experiments well. We conclude that both the in-flame measurements and exhaust emissions can be predicted accurately through this CFD modeling approach. Therefore, the current modeling has sufficient accuracy to simulate both oxidation and NO formation during MILD combustion. Numerical simulations with this well-validated model were carried out to investigate influences of Φ, P and TO on NO emission during MILD combustion.

Z=50

T [K]

1500 1500 1000 1000

500 0

0

Z=150

T [K]

1500 1500 1000 1000

500

Z=450

1500

1000 1000

1000

500

500

1500 0 1500

0

T [K]

Z=300

T [K]

0 1500 1500

1000

1000 500

1500 0

1500

Z=600

T [K]

1500

1000

1000

1000

500

500

1500 0

0 1500

1500

Z=750

T [K]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1000

1000

1000

500

500

00 -0.2

-0.1

0.0 Y [mm]

0.1

40 0.2 -0.2

(a) T [K]


-0.1

0.0 X [mm]

0.1

0.2

O2 [%]

O2 [%]

Z=300

Z=450

O2 [%]

Z=150

Z=600

O2 [%]

Z=50

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8 6 4 2 0 8 6 4 2 0 8 6 4 2 0 8 6 4 2 0 8 6 4 2 0 8

Z=750

O2 [%]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

O2 [%]

Energy & Fuels

6 4 2 00 -0.2

-0.1

0.0 Y [mm]

0.1

4 0.2 -0.2

(b) Dry O2 concentration [%]


-0.1

0.0 X [mm]

0.1

0.2

Page 19 of 40

Z=50

CO2 [%]

10 8 6 4

Z=150

CO2 [%]

10 8 6 4

Z=300

CO2 [%]

10 8 6

Z=450

CO2 [%]

4 10 8 6

Z=600

CO2 [%]

4 10

8 6

4 10

Z=750

CO2 [%]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

8 6 4 0 -0.2

-0.1

0.0 Y [mm]

0.1

4 0.2 -0.2

-0.1

(c) Dry CO2 concentration [%]


0.0 X [mm]

0.1

0.2

Energy & Fuels

Z=50

CO

10

-1

10-3 10-5

Z=150

CO

10-1 10

-3

10-5

Z=300

CO

10-1 10-3

10-5

Z=450

CO

10-1 10-3

Z=600

CO

10-5 10-1 10-3

10-5 10-1

Z=750

CO

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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10-3 10-50 -0.2

-0.1

0.0 Y [mm]

0.1

4 0.2 -0.2

-0.1

0.0 X [mm]

0.1

0.2

(d) Dry CO concentration Figure 4. In-furnace results from the CFD modeling and experiments: (a) temperature, and mole fractions of (b) O2, (c) CO2, and (d) CO. Table 4. Exhaust emissions from the CFD modeling and experiments on a dry basis Experiment 4.509 ± 0.005 8.88 ± 0.04 0, where ρ is the gas density. Therefore, Kv can be calculated from the modeling results. Moreover, the initial air jet momentum rate (Ga) is significantly larger (approximately 200 times or more) than that of the fuel jet (Gf). Therefore, for the present furnace, the injection momentum rate is approximately the same as that of the air jet[14].

Figure 5 displays predicted temperature contours and calculated recirculation ratios (Kv) at various Z planes for various equivalence ratios (Φ). As Φ is increased, Kv is reduced, and the temperature inside the furnace increases slightly because the air supply decreases; thus, the inlet air jet momentum rate (Ga) decreases, which weakens the amount of flue gases recirculated. Consequently, the reaction temperature increases due to the reduction in dilution[20, 47].


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5. Predicted temperature distributions (K) at plane A (Y-Z plane) and recirculation rates (Kv) at various Z locations for various equivalence ratios (Φ) for Case CFD I.

The uniformity of the in-furnace thermal field can be described using the temperature variance ratio (T*)[8]. This ratio is defined as: T * = T − T / T , where T is the local value, and T is obtained by averaging all values in the statistical domain. Figure 6 displays contours of T* at plane A (i.e., the Y-Z plane) with the dependence of equivalence ratio. Note that symmetrical air jets are distributed at plane A, for which the variables are strongly influenced by the inlet turbulent jet streams[48]. As shown in Figure 6, only slight temperature variations appear with MILD combustion for the present conditions; namely, the thermal field is extremely uniform. The temperature distribution becomes non-uniform downstream of the reactant jets only. The burner can thus be protected because the main reaction zone of MILD combustion is not near the burner exit. Moreover, as Φ is increased, T* first decreases and then increases gradually. The lowest T* with the greatest thermal uniformity is expected to be obtained between Φ = 0.8 and 0.9 in the furnace considered here.


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Page 23 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure 6. Calculated temperature variance ratio distributions (T*) at plane A (Y-Z plane) at various equivalence ratios (Φ) for Case CFD I.

The maximum temperature (Tmax), average temperature (Tavg), maximum temperature increment △T (△T = Tmax – TO), average recirculation ratio Kv-avg, average temperature variance ratio T*-avg and EINO and NO concentration are displayed in Figure 7 as a function of Φ. The data of Tmax, Tavg, △T, Kv-avg and T*-avg in Figure 7(a) are calculated from the entire furnace. Figure 7(a) reveals that as Φ is increased, Tavg increases, while both Tmax and △T first increase and then decrease, consistent with the thermal distributions shown in Figures 5 and 6. The trends of Kv-avg and T*-avg are also consistent with the above discussion. The value of T*-avg of Figure 7(a) is different from the value of T*-avg in Figure 6 due to the difference between the corresponding statistical regions.

Figure 7(b) reveals that both the EINO and NO concentrations first decrease and then increase as Φ is increased and that the minimum values occur between Φ = 0.8 and 0.9. The NO emission trend is independent of the temperature variation. More importantly, the NO emissions are far below the typical emission standard (i.e., 50 mg/m3 @ 15% O2 according to the emission standards of air


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

pollution of China[49], the EU[50] and the USA[51]). Furthermore, a stricter NO emission standard has been implemented in Beijing (the capital of China) since 2017; the new NO emission concentration limit is 30 mg/m3. Figure 7(b) shows the NO emission concentrations of all equivalence ratios are also significantly lower than the new emission standard of 30 mg/m3, indicating that the MILD combustion technology is extremely promising to accommodate the new emission standard.

On the fundamental side, NO is produced from the thermal, N2O-intermediate, prompt and NNH pathways, while NO destruction occurs mainly through the NO-reburning route. The rate-limiting steps of the five routes are R178 N + NO ⇌ N2 + O for thermal[43], R240 CH + N2 ⇌ N + HCN for prompt[43], R182 N2O + O ⇌ 2NO for N2O-intermediate[43] and R205 NNH + M ⇌ H + M + N2 for NNH[44], and R212 NO + H + M ⇌ HNO + M for NO-reburning[27]. Figure 8 displays the average reaction rates from the entire furnace of the rate-limiting steps of the five routes at various equivalence ratios (Φ). We can qualitatively investigate the NO formation by analyzing the rate-limiting steps. For MILD combustion, reactions occur volumetrically. The average reaction rates can be used to compare various cases. Generally, all the reaction rates of the four formation routes first decrease and then increase, and the reactions rate of the NO-reburning pathway increases, leading to the lowest EINO and NO concentrations occurring between Φ = 0.8 and 0.9.

We find that the CO emission is extremely low (and can be ignored) when Φ ≤ 0.85 and that the CO emission increases dramatically when Φ > 0.9. Consequently, considering both the NO and CO emissions, the optimal equivalence ratio of MILD combustion for the minimum NO and CO emissions is approximately 0.8.


Page 24 of 40

15

1000 800

10

600 400 200 0

Tmax. Tavg.

5

Average recirculation ratio

1200 0.03

0.02

0.01

∆Τ Kv-avg. T*-avg.

0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2

0

60

0.01

0.04

Emission standard [mg/m3]

0.00

1E-3

50

EINO NO concentration

40

30

1E-4

High CO

20

1E-5

NO [mg/m3] @ 15% O2

20

EINO [kg-NO / kg-fuel]

1400

Temperature [K]

10

1E-6

Equivalence ratio

0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 Equivalence ratio

(a)

0

(b)

Figure 7. Dependence of (a) Tmax, Tavg, △T, Kv-avg and T*-avg, and (b) EINO and NO concentration on the equivalence ratio (Φ) for Case CFD I.

10-5 Reaction rate [kmol/(m3-s)]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Average temperature variance ratio

Page 25 of 40

10-7

Φ = 0.5 Φ = 0.7 Φ = 0.8 Φ = 0.9 Φ = 1.0 Φ = 1.2

10-9

10-11

10-13

R178

R182

R205

R240

R212

R178-thermal, R182-N2O, R205-NNH, R240-prompt, R212-reburning

Figure 8. Reaction rates of the rate-limiting steps of the five routes at various equivalence ratios (Φ) for Case CFD I.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

4.3 Influence of the thermal input (P) The influence of selected thermal inputs (P = 5, 10 and 20 kW) at various equivalence ratios are numerically investigated in this section. Figure 9 plots Tmax, Tavg, △T, Kv-avg, T*-avg and EINO and NO concentration at various thermal inputs (P) and equivalence ratios (Φ). Figure 9(a) shows that as P is increased at constant Φ, Tavg is sharply elevated, while the increase of Tmax and △T is relatively low. Moreover, Kv-avg and T*-avg also increase with increasing P. For the present study, as the input capacity is increased from 10 kW to 20 kW, the reactant mass flow rate rises from 0.004124 kg/s to 0.008247 kg/s. Then the in-furnace upward-flow (mup) increases from 0.03686 kg/s to 0.07511 kg/s. Consequently, Kv increases from 7.938 to 8.108. Furthermore, at constant P, as Φ is increased, the trends of Tmax, Tavg, △T, and Kv-avg, T*-avg are similar.

Figure 9(b) indicates that the EINO and NO concentrations increase as P is increased at constant Φ. Moreover, the optimal equivalence ratio for minimum NO emission found in Section 4.2 is also obtained between 0.8 and 0.9 regardless of the thermal input. Similarly, the CO emission rises sharply when Φ > 0.9.

Figure 10 shows the averaged reaction rates of the entire furnace of the rate-limiting steps of the five routes at various thermal inputs (P) and equivalence ratios (Φ): (1) as the thermal input is increased at a constant equivalence ratio, each NO route is accelerated, and thus, the total NO emissions are increased; (2) at a constant thermal input, the trends in the reaction rates of the four formation routes are consistent with the trends in the reaction rates of Figure 8. The minimum NO emission is obtained between Φ = 0.8 and 0.9.


Page 26 of 40

Page 27 of 40

(a)

60

0.01

(b)

Emission standard [mg/m3]

1E-3

50

40 20 kW

1E-4

30

15 kW 10 kW

20 1E-5

NO [mg/m3] @ 15% O2


EINO [ kg - NO / kg - fuel ]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

High CO

20 kW

10 15 kW

1E-6

0.7

0.8

0.9

10 kW 0 1.0

Equivalence ratio

Figure 9. Dependence of (a) Tmax, Tavg, △T, Kv-avg and T*-avg and (b) the EINO and NO concentration on the thermal input (P) and equivalence ratio (Φ) for Case CFD II.


Energy & Fuels

10-5 Reaction rate [kmol/(m3- s)]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

10-7

Φ = 0.7 Φ = 0.8 Φ = 0.9 Φ = 1.0

Page 28 of 40

P = 20 kW P = 15 kW

P = 10 kW

10-9

10-11

10-13

R178 R182 R205 R240 R212

R178 R182 R205 R240 R212

R178 R182 R205 R240 R212

R178-thermal, R182-N2O, R205-NNH, R240-prompt, R212-reburning

Figure 10. Reaction rates of the rate-limiting steps of the five routes at various thermal inputs (P) and equivalence ratios (Φ) for Case CFD II.

4.4 Influence of the oxidant preheating temperature (TO) Figure 11 displays Tmax, Tavg, △T, Kv-avg, T*-avg and the EINO and NO concentrations at various oxidant preheating temperatures (TO) and equivalence ratios (Φ) obtained through CFD simulation. Figure 11(a) shows that at constant Φ, as TO is increased, Tmax, Tavg and Kv-avg increase, while △T and T*-avg decrease. For cases of varying TO, as the initial air temperature is increased, warmer air is provided, the reaction trends occur earlier, and the main reaction zone tends to shift closer to the burner exit. Moreover, as the air preheating temperature rises, the air initial jet momentum rate is also increased. The higher jet momentum rate results in stronger internal recirculation and leads to a more uniform thermal field. Moreover, the dependence of Tmax, Tavg, △T, Kv-avg and T*-avg on the equivalence ratio (Φ) is similar at various oxidant preheating temperatures.

Figure 11(b) displays the increase in EINO and NO concentrations as TO is increased at constant Φ. Again, the abovementioned trend in EINO and NO concentrations and CO emission with Φ is found


Page 29 of 40

at various oxidant preheating temperatures. The optimal equivalence ratio to minimize NO and CO emissions remains approximately 0.8, irrespective of the oxidant preheating temperature, for reasons similar to that associated with the reaction rates of the rate-limiting steps of the five NO routes shown in Figure 12.

(a1)

(a4)

(a2)

(a3)

(a5)

(b) 60 Emission standard [mg/m3]

1E-3

50

1100K

40

700K 300K

30 1E-4 20

High CO

1100K

NO [mg/m3] @ 15% O2


EINO [ kg - NO / kg - fuel ]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

10 700K

1E-5

300K

0.7

0.8 0.9 Equivalence ratio

1.0

0

Figure 11. Dependence of (a) Tmax, Tavg, △T, Kv-avg and T*-avg and (b) the EINO and NO concentration on the oxidant preheating temperature (TO) and equivalence ratio (Φ) for Case CFD III.


Energy & Fuels

10-5 Reaction rate [kmol/(m3- s)]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Φ = 0.7 Φ = 0.8 Φ = 0.9 Φ = 1.0

10-7

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TO = 900 K

TO = 700 K

TO = 1100 K

TO = 500 K

TO = 300 K

10-9

10-11

10-13

1 2 3 4 5

1 2 3 4 5

1 2 3 4 5

1 2 3 4 5

1 2 3 4 5

1-R178-thermal, 2-R182-N2O, 3-R205-NNH, 4-R240-prompt, 5-R212-reburning

Figure 12. Reaction rates of the rate-limiting steps of the five routes at various oxidant preheating temperatures (TO) and equivalence ratios (Φ) for Case CFD III.

5. Further discussion According to the above discussion on influences of injection conditions, when MILD combustion occurs, the equivalence ratio significantly influences NO emissions, and the optimal equivalence ratio for minimum NO and CO emissions is approximately 0.8, irrespective of the thermal input and oxidant preheating temperature. The thermal input and oxidant preheating temperature can therefore be used to adjust the capacity and furnace temperature.

A series of experiments were carried out in our MILD combustion furnace. Table 5 presents experimental injection conditions. During experiments, the swirl jet flame combustion was used to preheat the furnace. When in-furnace temperatures exceeded approximately 900 K, air was switched from the swirl jet to the two symmetrical 8 mm diameter air jets (Figure 2b); stable MILD


Page 31 of 40

combustion occurred with an invisible flame, and extremely low exhaust emissions were recorded. Figure 13 displays the experimental results of influences of Φ on NO and CO emissions (Case EXP II). As Φ is increased from 0.70 to 0.87, the exhaust NO reduces. Moreover, all NO emissions are far lower than the emission standard limit value. However, the CO emission increases sharply when the equivalence ratio is higher than 0.87. An extremely high CO emission when Φ is close to 1 was also found by Ref. [4, 11-12]. The present experimental NO and CO results are also consistent with the numerical simulations. The present experiments suggest that the optimal equivalence ratios for minimum NO and CO emissions lie in the range of 0.78 to 0.85 (i.e., approximately 0.8).

Table 5. Summary of the present experimental injection conditions Case

Fuel (methane)

Oxidizer (air)

Φ

Flow rate [L/min]

Tf [K]

Flow rate [L/min]

TO [K]

EXP I

28

300

563

500

0.79

EXP II

28

300

635-494

500

0.7-0.9

100

60 50 40

NO emission standard [mg/m3]

80 NO CO

60 30 40 20

High CO

20

10

CO [mg/m3] @ 15% O2

NO [mg/m3] @ 15% O2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

0 0 0.70 0.75 0.80 0.85 0.90 0.95 Equivalence ratio (Φ) Figure 13. Experimental results of influences of Φ on NO and CO emissions (CASE EXP II).

We choose an experimental case (Case EXP I in Tables 3 and 5) to conduct comprehensive in-flame


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measurements and detailed numerical simulation. This is the same case as that used for CFD validation in Section 4.1 and is further discussed in detail in this section. Nearly zero NO emission (< 1 mg/m3) is realized in the present experiment, which is significantly lower than the emission standards of China, the USA and the EU. Figure 14 shows the contour map of modeling velocity and simplified aerodynamics and the simulated and experimental temperatures and O2 concentrations at plane A (i.e., the Y-Z plane) for the present experimental case. Figure 14(a) shows that a jet core zone exists below the air nozzles. The recirculation zone is close to the furnace wall. The flow field is almost symmetrical, although there is a cooling loop in the lower left of the plane. In Figures 14(b-c), the distributions of temperature and O2 are extremely uniform except for the downstream zone of the fuel and air jets. Overall, the predicted results agree well with that of the experiments except for the region downstream of the jets; the reason for this discrepancy was explained in Section 4.1.

(a)

(b)

(c)

Figure 14. Contour maps of (a) predicted and simplified aerodynamics, (b) predicted and experimental temperatures, and (c) predicted and experimental O2 concentrations at plane A (Y-Z


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plane) for the present experimental case (Case EXP I).

Figure 15 displays the reaction path from N2 to NO for the experimental and nearly zero NO emission case. The number inside the parentheses shows the relevant reaction step from GRI-Mech 2.11. The value beside the arrow is the reaction rate, which was obtained by integrating the net rate of each reaction over the entire computational domain. The unit of reaction rate is kmol/(m3·s).

Table 6. Main reactions of various NO formation routes in GRI Mech 2.11. Route Thermal[43] Prompt[43]

N2O-intermediate [43]

NNH[44]

NO-reburning[45]

Reactions R178 NO + N ⇌ O + N2 R180 N + OH ⇌ NO + H R222 NCO + O ⇌ NO + CO R231 O + HCN ⇌ H + NCO R240 CH + N2 ⇌ HCN + N R181N2O + O ⇌ N2 + O2 R184 OH + N2O ⇌ HO2 + N2 R199 NH + NO ⇌ N2O + H R226 NCO + O2 ⇌ NO + CO2. R190 O + NH ⇌ NO + H R193 OH + NH ⇌ N + H2O R196 N + NH ⇌ N2 + H R199 NO + NH ⇌ N2O + H R206 O2 + NNH ⇌ HO2 + N2 R209 H + NNH ⇌ H2 + N2 R212 NO + H + M ⇌ HNO + M R215 OH + HNO ⇌ H2O + NO R183 H + N2O ⇌ OH + N2 R199 NH + NO ⇌ N2O + H R220 CN + O2 ⇌ NCO + O R231 O + HCN ⇌ H + NCO R234 HCN + OH ⇌ HOCN + H R244 C + NO ⇌ CN + O R247 CH + NO ⇌ H + NCO R250 NO + CH2 ⇌ HCN + OH R253 NO + CH2(S) ⇌ HCN + OH R256 NO + CH3 ⇌ H2CN + OH

R179 N + O2 ⇌ NO + O R191 NH + H ⇌ N + H2 R223 NCO + H ⇌ NH + CO R233 HCN ⇌ CN + H R242 CH2 + N2 ⇌ HCN + NH R182 N2O + O ⇌ 2NO R185 N2O (+M) ⇌ N2 + O(+M) R222 NCO + O ⇌ NO + CO

R180 N + OH ⇌ NO + H. R220 CN + O2 ⇌ NCO + O R230 HCN + M ⇌ CN + H + M R239 C + N2 ⇌ CN + N R243 CH2(S) + N2 ⇌ NH + HCN. R183 N2O + H ⇌ N2 + OH R190 O + NH ⇌ NO + H R224 NCO + OH ⇌ NO + H + CO

R191 NH + H ⇌ N + H2 R194 NH + O2 ⇌ HNO + O R197 NH + H2O ⇌ HNO + H2 R204 NNH ⇌ N2 + H R207 NNH + O ⇌ OH + N2 R210 NNH + OH ⇌ H2O + N R213 HNO + O ⇌ NO + OH R216 HNO + O2 ⇌ HO2 + NO. R196 NH + N ⇌ N2 + H R212 H + NO + M ⇌ HNO + M R223 NCO + H ⇌ NH + CO R232 HCN + O ⇌ NH + CO R235 HCN + OH ⇌ HNCO + H R245 C + NO ⇌ CO + N R248 CH + NO ⇌ N + HCO R251 CH2 + NO ⇌ H + HCNO R254 CH2(S) + NO ⇌ H + HCNO R274 HCCO + NO ⇌ HCNO + CO.

R192 NH + OH ⇌ HNO + H R195 NH + O2 ⇌ NO + OH R198 NH + NO ⇌ N2 + OH R205 NNH + M ⇌ N2 + H + M R208 NNH + O ⇌ NH + NO R211 NNH + CH3 ⇌ CH4 + N2 R214 HNO + H ⇌ H2 + NO R198 NH + NO ⇌ N2 + OH R218 CN + OH ⇌ NCO + H R229 NCO + NO ⇌ N2 + CO2 R233 HCN + O ⇌ CN + OH R236 HCN + OH ⇌ NH2 + CO R246 CH + NO ⇌ HCN + O R249 CH2 + NO ⇌ H + HNCO R252 CH2(S) + NO ⇌ H + HNCO R255 CH3 + NO ⇌ HCN + H2O

Figure 15 reveals that N2 is transmuted to NO via four paths, i.e., the pathways of N2 → N → NO (the thermal route), N2 → HCN → CN/NCO/N/NH/HNO → NO (prompt), N2 → NNH → NO (NNH), and N2 → N2O → NO (N2O-intermediate). In general, relative to the similar NO formation path described in Ref. [27], the present reaction rates are lower. More specifically, the production of


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NNH is not significant. The reaction rate of the N2 → NNH/NH → NO pathway is also small. The N2O-intermediate route N2 → N2O/NH → NO is found to be suppressed due to the low reaction rate. Moreover, the reaction rates of the reaction paths of N2 → N → NO and N2 → HCN → NO are relatively small; thus, the thermal and prompt routes are inhibited. Furthermore, the NO-reburning route is found to have an important influence on reducing NO emission under the conditions presented here. Overall, the present detailed reaction path analysis shows that (1) the reaction rates of four NO formation routes are extremely low, and thus, all NO production routes (i.e., thermal, N2O-intermediate, prompt and NNH) are strongly suppressed; and (2) NO is further reduced through NO-reburning reactions. Therefore, nearly zero NO emissions can be obtained experimentally under the conditions studied here.


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Figure 15. Reaction paths of NO formation for the present experimental case (Case EXP I). Unit: kmol/(m3·s).

Figure 16 quantitatively shows the EINO and the contribution of each route (as described in Table 6) to the total EINO for the nearly zero NO emission case (Case EXP I). Figure 16 shows that the EINO values produced from all routes are extremely low and thus that nearly zero NO (

Optimal Equivalence Ratio to Minimize NO Emission during Moderate

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