Premixed MILD Combustion of Propane in a Cylindrical Furnace with

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Combustion

Premixed MILD Combustion of Propane in a Cylindrical Furnace with a Single Jet Burner: Combustion and Emission Characteristics Kin-Pang Cheong, Guochang Wang, Jianchun Mi, Bo Wang, Rong Zhu, and Wei Ren Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01587 • Publication Date (Web): 03 Jul 2018 Downloaded from http://pubs.acs.org on July 4, 2018

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Energy & Fuels

Premixed MILD Combustion of Propane in a Cylindrical Furnace with a Single Jet Burner: Combustion and Emission Characteristics

Kin-Pang Cheong†,§, Guochang Wang†, Jianchun Mi*,†, Bo Wang†, Rong Zhu‡, Wei Ren§ †

‡

College of Engineering, Peking University, Beijing 100871, China

School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China

§

Department of Mechanical and Automation Engineering, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong SAR, China *

Corresponding author. Email: [email protected]; Tel: +86-010-62767074

Abstract This paper reports the combustion and emission characteristics of the premixed MILD combustion of propane established by a single jet burner in a laboratory-scale cylindrical furnace. Measurements are made of spatial distributions of the furnace temperature and species concentrations (O2, CO2, CO, NO) and also exhaust emissions of CO and NO. Experiments are conducted for different values of thermal input, injection diameter and global equivalence ratio (Φ). Results are analyzed with the aids of computational fluid dynamics (CFD) simulations and chemical kinetic calculations, which use a simplified perfectly stirred reactor (PSR) system with exhaust gas recirculation (EGR). It is observed that the premixed MILD combustion of propane in the present furnace can be established once the injection momentum rate is sufficiently high to enable the flue gas recirculation rate Kv > 2.5 (critical value) for Φ = 1.0. The critical Kv increses as Φ falls. Inlet

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conditions of the transition regime should be avoided to prevent the occurrence of instability and flashback for this regime. The present premixed MILD combustion of propane generates low CO and NO emissions. At a sufficient residence time, an increased speed injection reduces NO emission mainly by growing Kv and thus reducing the local peak temperature. In the present premixed MILD combustion, the prompt and reburning routes of NO formation are important. The effects of temperature, equivalence ratio, recirculation rate and residence time should be systematically considered when optimizing the combustion system for ultralow NO emission.

Keywords: MILD combustion; flameless combustion; premixed combustion; NOx emission; residence time


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1 Introduction The MILD combustion1 is a highly practical technology to reduce NOx emissions from fossil-fuel combustion.2 Over the past three decades, extensive investigations have been focused on the fundamentals of MILD combustion and its applications. However, most previous investigations were based on non-premixed combustion3–7 because: (1) MILD combustion was discovered and developed from high-temperature exhaust gas recirculation (EGR) combustion2 that requires nonpremixed mode; (2) The delayed mixing of initially separate fuel and oxidant streams may facilitate the establishment of MILD combustion. It was later discovered that MILD combustion can be established by controlling the inlet momentum rate (≡ mass flow rate × inlet velocity) and internal recirculation rate with premixing and/or without preheating.8–11 Therefore, for better understanding the MILD combustion, more attention should be paid to its premixed mode. Premixed MILD combustion in a perfectly-stirred reactor (PSR) assists the classification of combustion regimes under different initial conditions.12–15 de Joannon et al.12 investigated the dynamic behavior of the premixed MILD combustion in a PSR and obtained the stability map. They observed temperature oscillations when the inlet temperature is around 1000 ~ 1200 K, except for the near stoichiometric condition. Temperature oscillations are mainly caused by thermal conductivity and CH3 recombination path of the methane oxidation. Sabia et al.13 reported the auto-ignition delay times for atmospheric MILD combustion and the detailed combustion regime for methane. Wang et al.14 characterized the oxidation routes of CH4/O2/N2 mixture in a PSR over a wide range of temperature, dilution level and equivalent ratio with the classification of combustion regime. These combustion regime maps are useful for analytically identifying the combustion behaviors of different inlet conditions and fuels. For instance, Zhang et al.15 found that MILD combustion is more likely to occur with CO2


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dilution than N2 by comparing the corresponding regime maps, in good agreement with the experimental observations. The feasibility of premixed MILD combustion in a furnace was demonstrated by Özdemir and Peter16 using a rectangular furnace with heat recycling of the flue gas. By investigating the aerodynamics and chemical features of premixed and non-premixed MILD combustion, these authors found the importance of flue gas entrainment and strong shear flow condition for the initiation of MILD combustion. They also revealed that the chemical and flow time scales are comparable in the reaction zone. Cavigiolo et al.17 introduced a laboratory-scale furnace with preheated and premixed reactants to simulate the MILD combustion under EGR. They focused on the effect of exhaust gas recirculation, which can be quantitatively indicated by the recirculation rate Kv = Me / (Mf + Mair) proposed in Wünning and Wünning2, where Me, Mf and Mair are the mass flow rates of internally entrained flue gas, inlet fuel and air respectively. Cavigiolo et al. obtained the flue gas recirculation rate Kv and furnace temperature required for establishing MILD combustion, such as Kv > 4 for methane and Kv > 3.5 for ethane. Mi and Li et al.10,18–20 conducted a series of experimental and numerical investigations on the influence of the initial conditions on the premixed MILD combustion, emphasizing the establishment of MILD combustion using aerodynamic approach without initial preheat and dilution. They observed the critical momentum rate for reactant injection, above which stable MILD combustion occurs with low exhaust emissions.10 The critical momentum rate was then further developed into a critical Reynolds number, so that the criterion can be extended to different furnace systems.18 The findings of Mi and Li et al.10,18 provide new ideas of realizing the MILD combustion. Especially for the premixed condition, the aerodynamic approach instead of EGR preheating alleviates the risk of flashback so that the premixed MILD combustion can be applied for practical applications. Khalil et al.21 proposed a dual-injection arrangement, in which one


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maintained fuel-lean and the other was fuel-rich, so that both inlet equivalence ratios were out of the flammable limit. While this strategy obviously avoids the potential flashback of premixed MILD combustion, it also gains extremely low NO and CO emissions. Additionally, it has been confirmed that the premixed MILD combustion is fuel flexible 22–27

for gaseous hydrocarbon fuels, hydrogen-containing fuels, liquid fuels and biogas.

Especially, the hydrogen addition increases the stability of MILD combustion and reduces the formations of soot and polycyclic aromatic hydrocarbons (PAHs).28 In addition, premixed MILD combustion appears to achieve even better thermal efficiency and pollutant reduction than non-premixed MILD combustion as the premixed flame was demonstrated to burn more sufficiently and to produce less NOx and soot.10,18,20 All those studies noted above have pointed to the importance of premixed MILD combustion in both fundamental research and applications. Hence, it deserves more attention. Besides, premixed MILD combustion provides a more ideal/simpler environment than the non-premixed counterpart does for the research of MILD combustion. On one hand, the fuel has already mixed with the oxidizer before entering the combustion chamber, which results in nearly-constant local equivalence ratio within the chamber and eliminates the influence of oxidizer-fuel mixing on combustion. On the other hand, the inlet of premixed MILD combustion can be treated as simply as a single injection10 so that the complexity and uncertainty of the flow field can be substantially reduced. Moreover, the unique stability and emission characteristics of premixed MILD combustion needs further investigations for its optimal applications. In this paper, we investigate the premixed MILD combustion systematically in a laboratory-scale furnace with a maximum thermal capacity of 40 kW at Peking University. We mainly report the experimental results, including the stability and detailed in-furnace measurements and emissions, of the premixed MILD combustion of C3H8/air. Computational


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fluid dynamics (CFD) simulations are performed to explain the measured results. The formation mechanisms of NO from the present premixed MILD combustion are comprehensively analyzed to provide insight into the reduced NO emission. Our results can be used for further understanding the premixed MILD combustion and CFD simulations in the future.

2 Experimental Details 2.1 Experimental setup Pressure Relief Valve

Pressure Gauge Gas Analyzer Cooling& Desiccation

Heat Exchanger Fuel

Thermocouple

Mass Flow Controller

Exhaust

Window Data Convertor f2 Roots Blower

f1

Camera

Computer

Variable Frequency Driver

(a) System diagram

(b) On-site view

Figure 1 (a) Schematic and (b) on-site views of the experimental platform.

Figure 1 depicts the experimental platform that consists of a gas supply system, a furnace with a maximum thermal capacity of 40 kW and a data acquisition and processing system. High-pressure liquefied propane (purity 98%) is used for fuel supply. After evaporation and depressurization in a heat exchanger, the gaseous propane is mixed with air and piped into the furnace. The flow rate of the fuel is adjusted by a Sevenstar mass flow controller. A SIEMENS variable frequency driver MicroMaster 440 is used to adjust the


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rotation rate of the Roots-type blower for manipulating the flow rate of air. The uncertainties in the flow rates of fuel and air are ±0.2％ and ±2%, respectively. Measuring Port

Flame Mode Air C3H8

R=150mm

Air

L=800mm

Φ=80mm Outlet

MILD Mode Air

D = 9, 12, 15, 18 mm

C3H8

r

1 x

2

3

4

5

Observation Window (a) Geometry of the furnace

Air Dair = 20 mm Df = 6 mm

side view Lp = 50 mm

(b) Burner Design

Figure 2 Combustion system: (a) furnace geometry and (b) flow patterns under flame and MILD modes.

The combustion chamber of the cylindrical furnace is 800 mm in length (x) and 150 mm in radius (r), shown in Figure 2(a). The chamber is well insulated by the 150 mm thick mullite ceramic wall with an operating temperature limit of 1873 K. Five observation windows with fused silica glass are equally spaced (x = 80, 240, 400, 560 and 720 mm) on a sidewall of the furnace. These observation windows can be sealed with the mullite ceramic bricks to minimize the impact on the flow dynamics when not in use. On the opposite side of the windows, there are five measuring ports for either temperature or species measurements. The furnace outlet has a diameter of 80 mm. The furnace is slightly pressurized to prevent ambient air leakage when in operation. Figure 2(b) presents the two operation modes of the burner: diffusion flame mode with a bluff body and premixed MILD mode using a single jet. The fuel and oxidant streams are separately transported initially. When the furnace needs to be heated up, the fuel flows in the central tube (Df = 6 mm) and the oxidant flows in the outer region of the annular air tube (Dair = 20 mm). The step-like bluff body in the front part of the burner induces a reversed flow and


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stabilizes the diffusion flame. Once the wall temperature of the furnace is above the autoignition temperature (≈ 750 K for the stoichiometric mixture of C3H8/air), the air flow can be switched to the inner annular tube. For the present experiments, Twall > 1273 K is chosen as a standard for the switching from flame mode to MILD mode. The fuel and air then meet and mix before they enter the chamber in a premixing tube with a constant length of Lp = 50 mm and varying nozzle diameters of D = 9, 12, 15, 18 mm. Consequently, the MILD combustion within the furnace is established by the single turbulent jet of premixed reactants. The corresponding observations during the establishing process of premixed MILD combustion are provided in the next section. Table 1 summarizes the inlet conditions of the single jet nozzle for the tested cases. With the thermal input of 15 kW and global equivalence ratio of 0.9, the total flow rate of fuel and air is 16.04 m3/h. Note that the fuel and air are supplied at 300 K. Table 1 Summary of the inlet conditions for all the experiments. Thermal input

Qfuel

Qair

Pin (kW)

3

(m /h)

3

(m /h)

Φ

1

10, 15

0.386, 0.579

9.19~22.98

2

10, 15, 20

0.386~0.772

3

10, 15

0.386, 0.579

4

15

0.579

Case

D

U

Re=UD/v

(mm)

(m/s)

(×104)

1.0~0.6

9

41.8~102.8

2.37~5.82

9.19~30.63

1.0~0.6

12

23.5~77.1

1.77~5.82

9.19~22.98

1.0~0.6

15

15.1~37.0

1.43~3.49

13.79~22.98 1.0~0.6

18

15.7~25.7

1.78~2.91

The images of the combustion states in the furnace are taken by a Cannon 700D with a prime lens of 50 mm focal length in front of the observation windows. The settings of the camera are kept the same (f 2.8, 1/4000 s, ISO100) during the observations of MILD combustion. Since no quantitative criteria for the MILD combustion in a practical furnace have been available yet, the occurrence of MILD combustion is qualitatively determined based on the visual observation of no flame front existing in the furnace, and checked by the


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T-Kv map proposed by Wunning and Wunning2 with CFD simulations; see Section 4.1 for more details. Typical images are presented and discussed in Sections 3.1. Under MILD combustion, the time-averaged temperature (T) in combustion chamber is measured radially at the five measuring ports by traversing the B-type thermocouples (Pt30%Rh vs. Pt-6%Rh). The thermocouple wires are 0.5 mm in diameter and 500 mm in length, protected by corundum tube with a diameter of 6 mm. All the thermocouples are connected to an 8-channel data recording and converting instrument that synchronizes the data to the computer every second for instant monitoring. In order to characterize the temperature distribution in the furnace, 15 uniformly distributed measuring points are made at each measuring port for the radial temperature profiles (see Fig. 5 for more details). Note that a stabilization period of 5 minutes is required after moving the thermocouple to a new measuring point. Then the mean temperatures are determined by averaging the data acquired over another 5 minutes. The measured temperature data (Tm) are corrected for the radiation effect following Szegö et al.29, which is based on the balance of net heat flux caused by convection and radiation heat transfer, i.e.,

aε ∑ i =1 σ Gi (Tm4 − Ti 4 ) N

T = Tm +

.

h

(1)

Here a and ε are the surface area and the emissivity30 of the thermocouple bead, respectively; N is the total number of furnace walls, σ is the Stefan-Boltzmann constant, Gi is the fraction of radiation absorbed by the ith wall emitting from the thermocouple30 and h is the convection coefficient estimated from a sphere31. The typical correction of gas temperature is within 1.2%. The in-furnace and exhaust gas species concentrations are measured by a TESTO 350 portable flue gas analyzer with a self-designed sampling probe. The sampling probe is a corundum tube with an outer diameter of 6 mm and an inner diameter of 3 mm with no


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water-cooling. In this way, the local extinction caused by probing can be avoided. After being quickly extracted from the furnace, the sample gas was cooled down to less than 300 K and dried by the porous spheres of calcium chloride anhydrous, as indicated in Figure 1(a). Then the cooled and dried sample gas enters the analyzer for component analysis. The gas analyzer is calibrated with standard gases before each measurement to ensure the reliability. Table 2 lists the nominal accuracies of the thermocouples and gas sensors, together with the uncertainties in the present measurements after considering the thermocouple correction and the fluctuations in gas supplies. All the species data obtained are on dry basis. To compare the NO emissions from the different cases, the NO readings of the flue gas are normalized to the 3% O2 level using the following relation: NO @3% O2 = NO Reading × (20.9% – 3%) / (20.9% – O2 Reading),

(2)

whereas the NO readings within the furnace are presented directly on dry basis. Table 2 Nominal accuracies and uncertainties of temperature and gas composition Data

T (K)

O2 (Vol%)

CO2 (Vol%)

Accuracy

±0.25%

±0.2

± (1%+0.3)

Uncertainty

±4%

± (2%+0.2Vol%)

± (3%+0.3)

CO (ppmv)

NO (ppmv)

±5% or ±10 ppmv ±5% or ±2 ppmv ±(5%+10ppmv)

±(5%+2ppmv)

CFD simulations and chemical kinetic calculations are performed for a better interpretation of the experimental results. Brief descriptions and validations of these two methods can be found in Section 4 and Supporting Information.

2.2 Establishment of MILD combustion Figure 3 shows the typical process of establishing MILD combustion in the present furnace using the burner illustrated in Fig. 2(b). Initially, the furnace temperature is below the auto-ignition point and a warm up by the diffusion flame mode was necessary. During the warm-up stage, visible flame fronts are clearly observed in Windows 1 and 3 shown in Fig.


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3(a), and the luminance of the wall increases as the higher furnace temperature raises. To accelerate this stage, a thermal input of Pin = 20 kW is applied with a global equivalence ratio of Φ = 0.9 to reduce the warm-up time < 1.0 hour. The wall temperature recorded by the thermocouples at the positions of x = 80, 400, 720 mm are plotted in Fig. 3(b). At 20 kW flame mode, the furnace can be heated up to 1200 K in 30 minutes. When the wall temperature is sufficiently high to ensure the autoignition of the reactants, the burner is switched to the single jet mode with the flow rates of the fuel and air adjusted as required. Then the MILD combustion is established in a few seconds as all the glaring flame fronts disappeared, leaving only a uniform luminance shown in Fig. 3(a). All the measurements are conducted after the MILD combustion lasted for about 2 hours (Fig. 3(b)) before the furnace becomes thermally steady. (b) Wall temperature time-histories

(a) Observations

1600

Window 1 Ignitor

Thermocouple

Window 3

Switch

Temperature (K)

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1200 start of measurement

800

15kW 20kW Flame mode MILD mode

400 00:00

Temp1 Temp3 Temp5

switch to MILD 00:30

Time

01:00

01:30

02:00

02:30

Elapsed Time (HH:MM)

Figure 3 Establishment of MILD combustion in the cylindrical furnce with D = 12 mm. (a) Observations at different times from Windows 1 and 3. (b) Wall temperature time-histories measured at x = 80 mm (Temp1), 400 mm (Temp2) and 720 mm (Temp3).


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3 Experimental Results 3.1 Observations After switching the burner to the MILD mode, the occurrence of MILD combustion in the furnace are examined by visual observations. Figure 4 presents typical images of the premixed MILD combustion for Pin = 10, 15, 20 kW, Φ = 1.0 ~ 0.6 and D = 9, 12, 15, 18 mm. It is observed that the premixed MILD combustion can be established easily for D = 9, 12, 15 mm at the thermal input of Pin = 10 ~ 20 kW and the global equivalence ratio Φ = 1.0 ~ 0.6. Once the MILD combustion is established, no flame front can be observed and the luminous intensity is uniform in the furnace. As Pin or Φ is increased, the mean furnace temperature also increases, as seen from the luminosity of the furnace wall. It is interesting to note that the luminosity observed for D = 9 mm is higher than those for D = 12 and 15 mm. Hence, a higher furnace temperature of the front part can be achieved for D = 9 mm. The in-furnace temperature distributions under different conditions are presented in the next section. Meanwhile, for D = 18 mm, combustion cannot sustain even at the auto-ignition condition (where the furnace temperature > 1400 K) due to the repetitive ignition and extinction. The pressure waves induced by the instable combustion causes the furnace oscillations and periodic noise. The instability becomes even more intense when Φ is varied from 1.0 to 0.6, and measurements for these cases are terminated for safety reason. Further discussion on the stability and combustion regime will be given in Section 4.1.


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Figure 4 Typical photographs from Window 1 taken by camera (f2.8, 1/4000 s, ISO100) under different inlet conditions.


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3.2 In-furnace Measurements

Figure 5 Measured temperature field in the furnace for (a) different thermal inputs (D = 12 mm, Φ = 0.9), (b) different inlet diameters (Pin = 15 kW, Φ = 0.9), and (c) detailed plots for the contours in (b). The circles in (a) represent the measuring positions.

Figure 5 shows the contours and plots of temperature distributions in the furnace under different Pin and D values at Φ = 0.9, together with the measuring positions marked by small


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circles. As shown in Fig. 5(a), for D = 12 mm and Φ = 0.9 , uniformly distributed thermal fields are obtained when Pin is increased from 10 kW to 20 kW and the maxima of the measured temperatures are 1410 K, 1520 K and 1607 K, respectively. The temperature differences between the main reaction zone and the wall are only ~150 K. Fig. 5(b) displays that as D varies from 9 mm to 15 mm, the high temperature region becomes more centralized around the axis of the furnace, which degrades the volumetric distribution of MILD combustion. The temperature uniformity may be quantified approximately by the relative temperature variance T* = |T - Tavg| / Tavg proposed by Veríssimo et al.7, where T is the local temperature and Tavg is the overall averaged temperature. The averaged T* of the temperature fields for D = 9, 12, 15 mm are estimated to be 3.06%, 3.31% and 4.46%, respectively. Note that the growth of D means the drop of the jet momentum rate J [= 4(Mair+Mf)2/(πρD2)]. So, a higher inlet momentum rate enhances the uniformity of the reaction zone. A better uniformity for D = 9 mm results in higher temperatures for the front part of the furnace, resulting in the higher luminosity observed in window 1 for D = 9 mm than those for D = 12 and 15 mm, see Fig. 4. Figure 6 compares the distributions of species volume fractions (O2, CO2, CO and NO) for the cases of D = 9 mm and 12 mm at Pin = 15 kW and Φ = 0.9. All the species distribute uniformly in the furnace, indicating the homogeneous oxidation of propane for the present MILD combustion. High CO concentrations only appear in the small region adjacent to the burner where x < 300 mm. By switching D = 12 mm to D = 9 mm, the species distributions become more uniform and the CO and NO formations reduce. The averaged NO concentration for D = 9 mm is less than 25 ppmv while that for D = 12 mm is over 35 ppmv. Veríssimo et al.11 also found that a high injection velocity suppresses the NO emission under diffusion MILD combustion. They attributed the NO reduction to the faster and stronger


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dilution of the reactants induced by the increased injection velocity. However, a higher velocity does not always help reduce NO emission, which will be discussed in Section 3.4.

Figure 6 Measured species concentrations of O2, CO2, CO and NO (from top to bottom) for Pin = 15 kW and Φ = 0.9: (a) D = 9 mm; (b) D = 12 mm. Those circles in the top (a) represent the measuring positions.

3.3 Exhaust Emissions The emissions of the present premixed MILD combustion of propane are obtained by analyzing the composition of the exhaust gas. Figure 7 presents the volume fractions of emitted CO, O2 and CO2 at different global equivalence ratios for Pin = 15 kW. The measured emissions of O2 and CO2 (symbols) are very close to the theoretical values (lines), indicating good burnouts. The complete combustion of the fuel is also well indicated in Fig. 7 by extremely low CO emission for all the cases except from Φ = 1.0. For the cases of Φ < 1.0, the exhaust CO is all below 5 ppmv in volume fraction and almost undetectable by the


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current gas analysor. For the cases of Φ ≈ 1.0, however, the CO emissions rise drastically. This might be caused by either the formations of intermediate radicals and minor species like OH and NO that consume O2 and prevent the further oxidation of CO at Φ = 1.0, or the

CO2 O2 CO

5400 5300 5200 5100

Pin = 15 kW

400

60 50 40

D = 9 mm D = 12 mm D = 15 mm

300 200

30

100

CO

0

20

CO (dry ppmv)

uncertainties in the flow rates of fuel and air that lead to a slightly fuel-rich condition.

O2or CO2(dry Vol%)

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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CO2

10

Complete combustion

0

O2 0.6

0.7

0.8

0.9

1.0

Φ

Figure 7 Volume fractions of O2, CO2 and CO in the exhaust gas measured at different values of Φ and D for Pin = 15 kW. Symbols, measurements; lines, theoretical calculations.

Figure 8 presents the exhaust NO emissions and furnace temperatures as a function of Φ for the premixed MILD combustion of propane for the three values of D with Pin = 10 kW and 15 kW. Generally, the NO emission and furnace temperature rise as Φ increases from 0.6 to 1.0. Due to the suppression of thermal NO under MILD combustion, the overall NO emissions from all the test cases are < 50 ppmv, which are below the emission limits of China32 for gas boilers and turbines. Also, the volumetric thermal intensity in the present experiments can reach up to 354 kW/m3, within the thermal-intensity range of industrial furnaces and boilers.33 This suggests a good potential of the premixed MILD combustion for industry application. Generally, a high volumetric thermal intensity specification is preferred


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since it leads to compact and low-cost design for the combustion equipment.34 Figure 8(a) demonstrates that increasing the injection speed by decreasing the nozzle diameter generally facilitates the reduction of NO emission. This also applies for Φ = 1.0 ~ 0.8 under Pin = 15 kW, as seen from Fig. 8(b). However, for Φ = 0.7 ~ 0.6, the NO emission from D = 15 mm is lower than those from D = 9 mm and 12 mm, even though the mean exhuast temperature is higher. On one hand, this may due to the measurement and operation uncertainties during the experiments; on the other hand, it also suggests that other non-temperature factors may significantly affect the NO emission, which will be examined by the chemical calculations in Section 4.2. 100

100

(a) Pin = 10 kW

(b) Pin = 15 kW 1600

D = 9 mm D = 12 mm D = 15 mm

80

1600 80

60

1400

40

60

1400

40 1200

20

0

Temperature (K)

NO (dry ppmv @3% 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

Page 18 of 35

1200 20

0.6

0.8

1.0

1000

0

0.6

Φ

0.8

1.0

1000

Φ

Figure 8 Measured exhaust NO emissions and temperatures in MILD combustion as a function of equivalence ratio (Φ) for (a) Pin = 10 kW and (b) Pin = 15 kW. All the temperatures were measured at x = 720mm, r = 0 mm.


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4 Discussion 4.1 Stability of the premixed MILD combustion of C3H8 The stability of MILD combustion in a practical furnace has been widely investigated for the non-premixed mode2, e.g., see Refs.. Wünning and Wünning2 proposed a combustion regime map based on the exhaust gas recirculation rate Kv and furnace temperature Tfurn. They mapped such four regions: (a) stable flame for low Kv and high Tfurn; (b) transition zone; (c) stable MILD combustion for high Kv and high Tfurn and (d) no reaction zone. Due to the characteristics of non-premixed flame, the transition zone mainly consists of unstable and lift-off flames, combustion still sustains once Tfurn is above the self-ignition temperature Tsi. However, the transition zone for the premixed MILD combustion seems rather different, which may cause furnace trembling and even flashback when D is too large, as we have mentioned in Section 3.1 and Fig. 4. This phenomenon is worth further investigation below. The experimental cases have been simulated by RANS modeling, see Appendix A for more details of the numerical setup and meshing of the computational domain. The present CFD simulation well captures the temperature and major species distributions within the furnace, except in the region near the nozzle, as shown in Fig. 9. The overestimated ignition delay results in a longer distance for ignition, which is seen in the distributions of temperature and O2 and CO2 concentrations (Figs. 9(b) and 9(c)) in the vicinity of the nozzle. These discrepancies may be caused by the errors induced by the reduced mechanism of propane and models for combustion and turbulence. Improvement of these models are still needed in the future, as indicated by Parente et al.35 and Evens et al.36 Mostly within the furnace, the predicted results agrees well with the measurements, confirming that the present simulations are sufficient for investigating the major combustion and flow characteristics in the present furnace.


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600

x = 240mm

300 1800 1500 1200 900 600

x = 400mm

300 1800 1500 1200 900 600

x = 560mm

300 1800 1500 1200 900 600

x = 720mm

300

8 4

x = 240mm

0 16 12 8 4

x = 400mm 0 16 12 8 4

x = 560mm 0 16 12 8 4

x = 720mm 0

0

30

60

90

120

150

x = 80mm

4 0 20 16 12 8

x = 240mm

4 0 20 16 12 8

x = 400mm

4 0 20 16 12 8

x = 560mm

4 0 20 16 12 8

x = 720mm

4 0

0

30

R (mm)

60

90

120

150

CO (dry-vol %)

8

CO (dry-vol %)

900

12

12

CO (dry-vol %)

1200

x = 80mm

0 16

16

CO (dry-vol %)

1500

4

(d) CO

20

CO (dry-vol %)

x = 80mm

300 1800

8

O2 (dry-vol %)

600

O2 (dry-vol %)

900

O2 (dry-vol %)

1200

12

O2 (dry-vol %)

1500

(c) O2

16

O2 (dry-vol %)

(b) CO2

1800

CO2 (dry-vol %) CO2 (dry-vol %) CO2 (dry-vol %) CO2 (dry-vol %) CO2 (dry-vol %)

(a) Temperature Temperature (K) Temperature (K) Temperature (K) Temperature (K) Temperature (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

Page 20 of 35

1.5

Exp CFD

1.2 0.9 0.6 0.3

x = 80mm

0.0 1.5 1.2 0.9 0.6 0.3

x = 240mm

0.0 1.5 1.2 0.9 0.6 0.3

x = 400mm

0.0 1.5 1.2 0.9 0.6 0.3

x = 560mm

0.0 1.5 1.2 0.9 0.6 0.3

x = 720mm

0.0 0

30

R (mm)

60

90

R (mm)

120

150

0

30

60

90

120

150

R (mm)

Figure 9 Validations for the present CFD simulations.

The simulations suggest that the in-furnace flow structures and reaction zones for all the test cases are geometrically similar to the schematic given in Fig. 10(a). While the reactants are preheated and diluted by the recycled flue gas in the front furnace, major recirculation and reactions both occur in the mid part, then followed by combustion product or flue gas near the furnace exit, some of which exits out as the exhaust gas and the rest recycles to the front part. The cross-sectional Kv along the axis of the furnace and the corresponding maximum values as a function of Φ are shown in Fig. 10(b). Clearly, reductions of the injection diameter and equivalence ratio both result in higher Kv. The minimal value of Kv, max is about 2.5 and 3.0, respectively, at Φ = 1.0 and 0.6 for the stable premixed MILD combustion in the present furnace.


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Figure 10 Internal flue gas recirculation in the furnace: (a) illustration of combustion zones and flow patterns, (b) internal recirculation rates for different nozzle diameters and equivalence ratios.

Through the classification of combustoin regimes, Wang et al.14 found that the key factors for the establishment of MILD combustion are the preheating temperature and the dilution ratio. As D increases, the entrained hot flue gas reduces37, thus lowering preheating temperatures and reactant dilution. Therefore, a steady MILD combustion cannot be successfully established presently for D = 18 mm. The preheated temperature cannot reach the self-ignition point and the reactants may be ignited by the hot furnace wall instead. Then fast reactions occur, producing flame fronts and pressure waves travelling quickly upstream that significantly influences the flow and mixing. Consequently, local extinction takes place. As the new premixed reactants issue downstream and ignite, a flashback occurs. This explains the repetitive ignition and extinction for D = 18 mm mentioned early in Section 3.1.


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

The non-premixed combustion can avoide the flashback and sustain even with unstable lifted flame2. The flashback and pressure waves that might cause furnace oscillations, likely damaging the combustion system. In addition, de Joannon et al.12,38 found that dynamic behaviors of highly diluted premixed reactants occur when temperature ranges from 1000 K to 1150 K. Hence, the inlet conditions related to the transition regime and the temperature range of dynamic behaviors should be avoided for the premixed MILD combustion. Base on the above discussion, the regime map of Tfurn-Kv is obtained by the RANS modeling for the present combustion system, as shown in Fig. 11. The results (dots) for the four cases of D = 9 ~ 18 mm are also plotted in the figure. It is clear that as D increases, the combustion in the furnace changes from stable MILD (D = 9 mm and 12 mm) to the transitional combustion (D = 18 mm), suggesting that smaller values of D are preferred for establishing the MILD combustion. From Fig. 10, the injection nozzle of D ≤ 12 mm and the global equivalence ratio Φ ≥ 0.5 are recommanded for stable, efficient and safe realization of the premixed MILD combustion in the present furnace.


Page 22 of 35

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Figure 11 Tfurn–Kv map for the occurance of stable MILD combustion in the present furnace for different nozzle diameters and equivalence ratios. Each symbol represents the combustion mode of a case based on the CFD simulated Tfurn and Kv.

4.2 Influence factors of NO emission in premixed MILD combustion Inlet

Mixer

PSR Exhaust

C3H8/air

m& 1

m& 1 +m& EGR

constant T

m& EGR

Recirculation K v = m& EGR / m& 1

Figure 12 Schematic of the perfectly stirred reactor with exhaust gas recirculation for the present furnace.

In Section 3.3, Fig. 8(b) suggests that the furnace temperature is not the sole factor significantly influencing the NO emission from the premixed MILD combustion. And no previous work has appeared to clarify the influential factors of NO emission from premixed MILD combustion with EGR. To address this deficit, here we examine such factors using chemical calculations in a variable-controlling manner. The present combustion occurs with a rather simple reaction zone in the furnace and may be emulated as a simplified PSR system with EGR, as illustrated in Fig. 12. The PSR-EGR system is solved using CHEMKIN39. With such setup, the calculated exhaust gas will be splitted and recycled to the mixer for preheating and diluting the reactant of C3H8/air until the difference of the species concentration between two sequential iterations is less than 10-9. Then we can obtain the final solution with steady internal recirculation similar to those in our experiments and find out the significant factors for NO formation in premixed MILD combustion. To limit the discussion in this section, we only focus on the practical MILD combustion region and choose the


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

Page 24 of 35

ranges of the key variables for the PSR-EGR system accordingly, as listed in Table 3. Other factors such as turbulent mixing and radiation will not be discussed as turbulence has neglegible effect on the overall NO emission40 and the effect of radiation generally lowers the reaction temperature of PSR. Table 4 provides the typical inlet conditions for the chemical calculations. Appendix B of Supporting Information shows the proper selections of detailed oxidation and NO formation mechanisms for the premixed MILD combustion of C3H8/air. It turns out that the USCMech II with the Konnov NOx sub-mechanism provides reliable predictions for the NO formation. The value of Kv = 5 is chosen as the typical EGR ratio in the following calculations since it ensures the dilution ratio to be over 90% and the preheating temperature exceeds the self-ignition temperature of propane (≈ 750 K) for the premixed reactants. From the definition of MILD combustion proposed by Cavaliere and de Joannon1, Wang et al.14 and Zhang et al.15 found that the combustion at Kv = 5 is well within the MILD regime. Table 3 Summary of the cases using chemical kinetic calculations T (K)

Φ

Kv

τ (s)

(1) NO vs. T

1100 ~ 1600

0.8

5

1

(2) NO vs. Φ

1350

0.5 ~ 0.9

5

1

(3) NO vs. Kv

1350

0.8

3 ~ 15

1

(4) NO vs. τ

1350

0.8

5

0.001 ~ 10

Table 4 Typical inlet conditions for current chemical calculations of MILD combustion Φ

0.8

C 3 H8

Air

Qf = 0.2976 g/s Qair = 5.6670 g/s T = 300 K

T = 300 K

Premixed Reactant XC3H8 = 0.03251 XN2 = 0.76432 XO2 = 0.20317


EGR (Kv = 5) QEGR = 29.8229 g/s (initial guess value)

Page 25 of 35

In the following discussion on the formation routes of NO, only the 10 most important nitrogen-containing reactions are listed, which are quantitatively identified by sensitivity analysis. Here the sensitivity coefficient is defined as the normalized species concentration sensitivity to the pre-exponential factor CS,ik = (Ai∂Yk) / (Yk∂Ai), where Ai is the Arrhenius preexponential factor of the ith reaction and Yk is the concentration of the kth species. The magnitude of CS,ik indicates the importance of the ith reaction to the formation of the kth species, while the sign of CS,ik represents whether the contribution of the reaction is positive (+) or negative (-) to the NO formation. The formation routes are classified based on the recent review on the nitrogen chemistry by Glarborg et al.41, which mainly includes thermal, prompt, N2O, NNH and reburning routes42.

4.2.1 Effect of reaction temperature T C2O+N2↔NCN+CO C2O+N2→NCO+CN HCCO+NO↔ HCCO+NO↔HCN+OH NCO+O NCO+O↔ ↔NO+CO N2O+O O+O↔ ↔NO+NO NCO+NO↔ NCO+NO↔N2+CO2 NCO+NO↔ NCO+NO↔N2O+CO HCCO+NO HCCO+NO↔ ↔HCNO+CO HCNO+O HCNO+O↔ ↔HCO+NO CH2+N2 ↔ CN+N

35 30

NO (dry ppmv@3% 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

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(a)

25 20 Experiment range 15 10 5 0

1100 1200 1300 1400 1500 1600

C2O+N2↔NCN+CO C2O+N2→NCO+CN NCO+NO NCO+NO↔ ↔N2+CO2 NCO+O↔ NCO+O↔NO+CO HCCO+NO HCCO+NO↔ ↔HCNO+CO NCO+NO NCO+NO↔ ↔N2O+CO HCCO+NO HCCO+NO↔ ↔HCN+CO2 N2+O↔ +O↔NO+N HCNO+O HCNO+O↔ ↔HCO+NO HCNO HCNO↔ ↔HCN+O

Temperature (K)

R789 Prompt R1238 Reburning R1145 R1201 Prompt R818 N2O R1207 R1206 R1144 Reburning R1152 R1120 Prompt (b1) 1350 K -0.10 -0.05 0.00 0.05 0.10 Normalized R789 Prompt sensitivity coefficient R1238 R1207 R1201 Reburning R1144 R1206 R1145 R812 Thermal R1152 Reburning R1147 (b2) 1600 K -0.10 -0.05 0.00 0.05 0.10

Normalized sensitivity coefficient

Figure 13 Effect of reaction temperature on (a) NO emission and (b) NO formation routes of premixed MILD combustion of C3H8/air at Φ = 0.8, Kv = 5 and τ = 1.0 s.

It is well-known that the NO formation during combustion is highly correlated to the flame temperature. Under MILD combustion, the thermal field and reaction zone are nearly


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uniform so that the temperatures over 1600 K do not occur, resulting in a large suppression of the thermal NO relative to the conventional case which have high temperature flame fronts2. Even so, the NO emission from the premixed MILD combustion is still dependent strongly on the reaction temperature. For instance, Fig. 13(a) shows that the NO emission from the combustion at 1600 K is almost 6 times as that from the case at 1350 K. The increase of NO emission results from all the NO formation routes which all become more active with increasing temperature. Among them, the thermal route contributes the most to the increament and dominates the NO formation when temperature is over 1600 K43. This is also demonstrated by the sensitivity analysis for NO formation shown in Fig. 13(b1) and 13(b2). At 1350K, NO mainly forms through the prompt and N2O routes while the thermal NO shows its importance in NO formation when the temperature increases to 1600 K.

4.2.2 Effect of equivalence ratio Φ 0.001

25 OH O H CH3 CH2

20

15

1E-4 1E-5 1E-6 1E-7

CH

1E-12

10 NO

1E-16

5 N2O dominated

0 0.4

0.6

Prompt dominated

0.8

1.0

1E-20

Mole Fraction

(a) NO (dry ppmv@3% 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

Page 26 of 35

N 2O Prompt

O+O↔NO+NO N2O+O↔ C2O+N2→NCO+CN C2O+N2↔NCN+CO N2O+OH O+OH↔ ↔N2+HO2 CH2+N2 ↔ CN+N N2O+H↔ O+H↔N2+OH O+H↔NH+NO N2O+H↔ HCCO+NO HCCO+NO↔ ↔HCN+CO2 NCO+O↔ NCO+O↔NO+CO NCO+NO↔ NCO+NO↔N2+CO

R818 R1238 R789 R894 R1120 R891 R890 R1145 R1201 R1207

HCCO+NO↔ HCCO+NO↔HCN+CO2 C2O+N2↔NCN+CO C2O+N2→NCO+CN NCO+NO NCO+NO↔ ↔N2+CO NCO+O NCO+O↔ ↔NO+CO NCO+NO NCO+NO↔ ↔N2O+CO HCCO+NO↔ HCCO+NO↔HCNO+CO HCNO+O HCNO+O↔ ↔HCO+NO HCNO HCNO↔ ↔HCN+O HCNO+H↔ HCNO+H↔NH2+CO

Normalized sensitivity coefficient R1145 R789 Reburning R1238 Prompt R1207 R1201 R1206 R1144 R1152 Reburning R1147 R1189 Fuel-N (b2) Φ = 0.9 -0.1 0.0 0.1

Φ

N 2O Prompt N 2O Reburning Prompt (b1) Φ = 0.5 Reburning -0.1 0.0 0.1


Figure 14 Effect of equivalence ratio on (a) NO emission and (b) NO formation routes of premixed MILD combustion of C3H8/air at T = 1350 K, Kv = 5 and τ = 1.0 s.

Figure 14(a) presents the effect of fuel-air equivalence ratio Φ on the NO emission from premixed MILD combustion with EGR. Evidently, the NO emission increases with Φ, even


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though the reaction temperature is kept constant at 1350 K (Table 3). This is because the volume fraction of fuel increases with Φ, resulting in more H, O, and hydrocarbon radicals that are beneficial for the NO formation. Meanwhile, the main routes for NO formation vary with Φ for lean MILD combustion. At Φ = 0.5, when air is abundent, the sensitivity coefficient of R818 is significantly larger than those of other reactions, indicating that the N2O route predominates the NO formation, see Fig. 14(b1). This agrees well with Li et al.44 who investigated the relative importance of NO mechanisms for MILD combustion of methane with hydrogen addition. As Φ approaches 1.0, the NO forms and destructs mainly through the prompt and reburnning routes as more hydrocarbon radicals such as CH, CH2 and CH3 are available, enhancing the initiation reactions of these two routes, as indicated by Fig. 14(a) and 14(b2). For the present PSR system, the swith of main NO formation from the N2O route to the prompt route occurs when Φ is around 0.8.

4.2.3 Effect of recirculation rate Kv Figure 15 shows the effect of Kv on the NO emission and formation routes for the premixed MILD combustion of propane. It turns out that large Kv not only facilitates the occurance of MILD combustion, but also reduces the NO emission. About 35% of NO emission is reduced as Kv increases from 3 to 15 shown in Fig. 15(a). The recirculated exhaust gas serves as a diluent to reduce the activities of fuel oxidation and NO formation, thus lowering NO emission as Kv rises. The comparsion of sensitivity coefficients in Figs. 15(b1) and 15(b2) also supports this conclusion. The sensitivity coefficients are notably smaller for Kv = 15 than those for Kv = 3. Moreover, the main formation routes, i.e., prompt and reburning, change their relative contributions Kv increases. Therefore, increasing Kv will strengthen the NO reburning.


Energy & Fuels

8

NO (dry ppmv@3% O2)

(a) 6

4

C2O+N2↔NCN+CO C2O+N2→NCO+CN HCCO+NO↔ HCCO+NO↔HCN+CO2 NCO+O↔ NCO+O↔NO+CO N2O+O↔ O+O↔NO+NO NCO+O↔ NCO+O↔NO+CO NCO+NO↔ NCO+NO↔N2O+CO CH2+N2 ↔ CN+N HCCO+NO↔ HCCO+NO↔HCNO+CO HCNO+O↔ HCNO+O↔HCO+NO

R789 R1238 R1145 R1201 R818 R1207 R1206 R1120 R1144 R1152

Prompt Reburning Prompt N2O Reburning Prompt Reburning -0.1

HCCO+NO↔ HCCO+NO↔HCN+CO2 C2O+N2↔NCN+CO C2O+N2→NCO+CN NCO+NO↔ NCO+NO↔N2+CO NCO+O↔ NCO+O↔NO+CO NCO+NO↔ NCO+NO↔N2O+CO HCCO+NO↔ HCCO+NO↔HCNO+CO HCNO+O↔ HCNO+O↔HCO+NO HCNO↔ HCNO↔HCN+O HCNO+H↔ HCNO+H↔NH2+CO 16

Experiment range 2

0

4

8

12

R1145 R789 R1238 R1201 R1207 R1144 R818 R1206 R1152 R1147

0.0

(b1) Kv = 3 0.1

0.0

(b2) Kv = 15 0.1

Reburning Prompt Reburning N2O Reburning -0.1

Kv


Figure 15 Effect of recirculation rate on (a) NO emission and (b) NO formation routes of premixed MILD combustion of C3H8/air at T = 1350 K, Φ = 0.8 and τ = 1.0 s.

4.2.4 Effect of residance time τ

20

NO (dry ppmv@3% 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

Page 28 of 35

12

(a)

8 4

15

0

4

τ (s)

8

10

5 One-way residence time in 15 kW cases

0 0.001

0.01

0.1

1

C2O+N2↔NCN+CO C2O+N2→NCO+CN HCCO+NO↔ HCCO+NO↔HCN+CO2 HCN+O↔ HCN+O↔NCO+H CH2+N2 ↔ CN+N NCO+O NCO+O↔ ↔NO+CO HCN+O HCN+O↔ ↔NH+CO NO2+H↔ +H↔NO+OH O+H↔ ↔NH+NO N2O+H HCCO+NO↔ HCCO+NO↔HCNO+CO

C2O+N2↔NCN+CO C2O+N2↔NCO+CN HCCO+NO HCCO+NO↔ ↔HCN+CO2 NCO+NO NCO+NO↔ ↔N2+CO NCO+O NCO+O↔ ↔NO+CO NCO+NO NCO+NO↔ ↔N2O+CO HCCO+NO HCCO+NO↔ ↔HCNO+CO HCNO+O HCNO+O↔ ↔HCO+NO HCNO HCNO↔ ↔HCN+O 10 HCNO+H HCNO+H↔ ↔NH2+CO

τ (s)

R1238 R789 R1145 R1175 R1120 R1201 R1176 R908 R890 R1144

Prompt Reburning

Prompt

Reburning NO2 N2O (b1) 0.01 s Reburning -0.1 0.0 0.1 Normalized R789 Prompt sensitivity coefficient R1238 Reburning R1145 R1201 Prompt R818 N2O R1207 R1206 R1144 Reburning R1152 R1120 Prompt (b2) 1.0 s -0.1 0.0 0.1


Figure 16 Effect of residence time on (a) NO emission and (b) NO formation routes of premixed MILD combustion of C3H8/air at T = 1350 K, Φ = 0.8 and Kv = 5.

Figure 16 shows the effect of residence time on the NO emission and formation under MILD combustion. As the residence time grows from 0.001 s to 10 s, the NO emission first


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increases and then decreases, reaching its maximum arount τ = 0.01 s. For τ < 0.01 s, combustion becomes incomplete and NO is reduced by the reductants such as CO and CHi radicals. Hence, we should ensure a long residence time for the products to reduce NO emissions. Figs. 16(b1) and 16(b2) demonstrate that the NO emission mainly results from the balance of the prompt and reburning routes for τ = 0.01 ~ 1.0 s. At τ = 0.01 s, the prompt NO prevails since the sensitivity coefficients of R789 and R1238, which relate to the prompt route, are much larger than those of the reburning and N2O routes. While at τ = 1.0 s, the reburning route is enhanced by the reactions R1145, R1207, R1206 and R1144. Therefore, the NO emission for τ = 0.01 ~ 1 s decreases drastically, see the inset graph with linear axis in Fig. 16(a). At τ = 1.0 s, the N2O route also makes an important contribution to the total NO emission. Table 5 Estimated one-way residence time τ1 (in ms) for the cases at Pin = 15 kW Φ

D = 9 mm D = 12 mm D = 15 mm

0.6

12.8

22.7

35.4

0.7

10.9

19.4

30.3

0.8

9.6

17.0

26.5

0.9

8.5

15.1

23.6

1.0

7.7

13.6

21.2

In the above, the individual effects of the temperature T, equivalence ratio Φ, recirculation rate Kv and residence time τ have been clarified on the NO emissions from premixed MILD combustion with EGR. Among these factors, the reaction temperature T is the predominent one since a 20% increase of T results in about 600% increase in NO emission. Now let us look at Fig. 8 again for the NO emissions from the furnace combustion under different conditions. As demonstrated above, apart from the furnace temperature, the NO emission also depends on Φ, Kv and τ. When decreasing Φ, more air is injected into the furnace, which reduces the furnace temperature and increases the recirculation rate in the


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

furnace (Fig. 10(b)). All these changes in T, Φ and Kv facilitate the reduction of NO emission. However, decreasing Φ also shortens the residence time, which increases the NO emission. The competition between the τ effect and the integrated effect of T, Φ and Kv results in the final variations of NO emission against Φ and D. For Pin = 15 kW, the one-way residence time τ1 (≡ Lfurn / U) is calculated and listed in Table 5. It is suggested that the residence times for complete furnace combustion are distinct for different cases. For Pin = 15 kW and D = 9 mm, the resitance time is short, under which the NO emission might reach its maximum (Fig. 16(a)). The effect of τ might balance out part of the integrated effects of T, Φ and Kv that causes NO emissions to vary slightly with Φ. However, for D = 15 mm, the residence time is much longer, and so the effect of τ on NO emission is weaker, resulting in lower NO emission than those cases of D = 9 mm for Φ = 0.7 and 0.6. Therefore, the effects of T, Φ, Kv and τ on NO emission are very important to comprehensively understand NO emissions from premixed MILD combustion. It is worth noting that the discussion in Section 4.2 is limited in a steady and homogeneous PSR, the uniformity of temperature and species in the furnace still deviates from the ideal PSR, even under premixed MILD combustion. The local characteristics such as local residence time, heat release or temperature fluctuations may play important roles on NO emission. The importance of these local characteristics will be further investigated in our future work.

5 Concluding Remarks This study has reported the in-furnace measurements and emission characteristics of the premixed MILD combustion of propane established by a single jet burner in a laboratoryscale cylindircal furnace. Visual observations and detailed measurements in furnace of the mean temperature and concentrations of O2, CO2, CO, NO are provided. Moreover, the


Page 30 of 35

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measured emissions of exhaust CO and NO at two thermal inputs, three injection diameters and five global equivalence ratios are presented and analyzed. (These datasets should be very significant and useful for future’s numerical studies of the present combustion system that require experimental validation.) Particularly, we have proposed a simplified PSR system with EGR to examine the individual effects of temperature, equivalence ratio, recirculation rate and residence time on the NO formation of premixed MILD combustion. The main conclusions from the present work are summerized below: (1) The premixed MILD combustion of propane can be established over a wide range of equivalence ratios at least between 0.6 ~ 1.0, as long as the maximal internal recirculation rate Kv reaches the critical value. The critical Kv for Φ = 1.0 is about 2.5 and increses as Φ reduces. Once the premixed MILD combustion is established, the temperature and major species distribute nearly uniformly, as confirmed by visual observations and in-furnace measurements. Additionally, decreasing the nozzle diameter will improve the uniformity over the reaction region. (2) Inlet conditions for the transition between traditional combustion and MILD regime should be avoided, since this premixed combustion may become unstable and flashback. Injection nozzle of D ≤ 12 mm is recommanded for stable realization of premixed MILD combustion in the present furnace. (3) For the present premixed MILD combsution of propane, the CO emissions for the fuel-lean cases are almost undetectable, and the NO emissions for all the tested cases are less than 50 ppmv (dry at 3%O2). (4) For a sufficiently long residence time, a high speed injection reduces the NO emission mainly by decreasing the local peak temperatures and increasing Kv for the premixed MILD combustion, which decrease the reactivity of NO formation and


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enhance the NO reburning. The effects of T, Φ, Kv and τ on NO emission should be considered comprehensively to achieve the optimal NO emission. (5) In the present premixed MILD combustion, the prompt and reburning routes are important. The N2O route prevails at lean conditions and the thermal route becomes important as the reaction temperature beyond 1600 K.

Acknowledgements The authors gratefully acknowledge the support of Natural Science Foundation of China under Grants 51776003 & 51276002. They also appreciate the support of Beijing Peking University Pioneer Technology Co., Ltd. Moreover, the first author’s gratitude goes to National Supercomputing Center (Shenzhen) for providing CHEMKIN-Pro (15131) software and computational facilities.

Nomenclature Symbols Ai a CS,ik Dair Df D Gi h J Kv Lp Lfurn Mair Me Mf Pin

Arrhenius pre-exponential factor of the ith reaction (mol-cm-s-K) surface area of the thermocouple bead (m2) normalized ith species concentration sensitivity to the kth reaction diameter of the air tube (mm) diameter of the fuel tube (mm) diameter of the nozzle exit (mm) fraction of radiation absorbed by the ith furnace wall convection coefficient (W/m2K) momentum rate of the nozzle exit (kg/s2) internal recirculation rate premixing length (mm) length of the furnace (mm) mass flow rate of air (kg/s) mass flow rate of the entrained flue gas (kg/s) mass flow rate of fuel (kg/s) thermal input of the furnace (kW)


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Energy & Fuels

Qair volumetric flow rate of air (m3/h) Qfuel volumetric flow rate of fuel (m3/h) Re Reynolds number r radial position (mm) T temperature (K) * T relative temperature variance Tavg Overall averaged temperature (K) Twall wall temperature of the furnace (K) U bulk velocity of the nozzle exit (m/s) x axial location (mm) Yk mass fraction of the kth species Greek letters ε ρ τ τ1 ν Ф

emissivity density (kg/m3) residence time (s) one-way residence time (ms) kinematic viscosity (m2/s) equivalence ratio

Declarations of interest None.

Supporting Information. Details of CFD simulation and the selections of chemistry mechanism for the chemical calculation in the PSR-EGR system.

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Premixed MILD Combustion of Propane in a Cylindrical Furnace with

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