Moderate or Intense Low-Oxygen Dilution Oxy-combustion

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Moderate or Intense Low-Oxygen Dilution Oxy-combustion Characteristics of Light Oil and Pulverized Coal in a Pilot-Scale Furnace P. Li,† F. Wang,† Y. Tu,‡ Z. Mei,† J. Zhang,† Y. Zheng,‡ H. Liu,‡ Z. Liu,‡ J. Mi,*,† and C. Zheng‡ †

State Key Laboratory of Turbulence and Complex Systems, Department of Energy and Resources Engineering, College of Engineering, Peking University, Beijing 100871, People’s Republic of China ‡ State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan, Hubei 430074, People’s Republic of China ABSTRACT: This study investigates by experiment the global characteristics of both moderate or intense low-oxygen dilution (MILD) oxy-combustion and air combustion of firing light oil and pulverized coal in a pilot-scale furnace. There are three burner configurations used, i.e., (I) central straight (primary) jet + swirl (secondary) jet, (II) central straight (primary) jet + two side symmetrical (secondary) jets, and (III) central straight (primary) jet + side asymmetrical jet. The furnace centerline temperature, species concentrations, and exhaust emissions are measured and compared for the MILD and conventional combustion cases. For light oil and pulverized coal, the MILD air combustion or oxy-combustion occurs with burner II or III, while the conventional combustion takes place when using burner I. For the light oil, the MILD oxy-combustion can be reached even using pure oxygen. As the MILD combustion is reached, a fairly uniform temperature distribution and low emissions of NO and CO are obtained. Note that burner III produces the largest internal recirculation of the flue gas, lowest peak temperature, and most uniform temperature, whereas the opposite occurs for burner I. Importantly, the MILD combustion is found to reduce the NO emission much more effectively in the oxy-combustion case than in the air combustion case. Moreover, the appearance of the MILD combustion of light oil and pulverized coal differs from the invisible MILD combustion of gaseous fuels. Dark sparks from burning oil droplets or char particles are present in the MILD combustion of light oil or pulverized coal. It is also revealed that the char burnout under the MILD combustion is weaker than that under the conventional combustion.

1. INTRODUCTION It is becoming well-known that moderate or intense low-oxygen dilution (MILD) combustion can achieve both high combustion efficiency and low pollutant emissions. In this combustion, reactants are strongly diluted because of the large-scale recirculation of flue gas and chemical reactions occur slowly; therefore, heat releases mildly, over a large volume. As a result, the peak temperature is reduced and the NOx production is, hence, suppressed. This technology was developed nearly simultaneously in Germany, 1,2 the International Flame Research Foundation (IFRF),3−5 and Nippon Furnace Kogyo Kaisha, Ltd. (NFK, Japan)6,7 in the 1990s; the international combustion community has well-accepted the term “MILD combustion” introduced by Cavaliere and de Joannon.8 In the last 2 decades or so, Dally’s group,9−12 Weber’s group,13−16 Gupta’s group,17−19 Rota’s group,20−25 Cavaliere’s group,26,27 Costa’s group,28,29 Kumar’s group,30−32 Blasiak’s group,33,34 and Mi’s group35−41 have extensively investigated the MILD combustion characteristics, both experimentally and numerically. Useful information of the experimental work is summarized in Table 1. Most of the previous work on MILD combustion focused on gaseous fuels and was carried out in laboratory-scale furnaces using highly preheated air, including several studies associated with oxy-combustion. To extend this, the oxy-combustion of liquid (e.g., fuel oil) or solid (e.g., pulverized coal) fuels in pilot-scale furnaces, without the requirement of highly oxidant preheating, calls for more investigations. © 2014 American Chemical Society

The information on the MILD combustion characteristics of firing liquid or solid fuels is limited. Weber et al.15 investigated the MILD combustion of natural gas, heavy and light oils, and pulverized coal in highly preheated air in a 0.58 MW furnace. They found that the in-furnace temperature and the chemistry fields are fairly uniform because of the slow combustion process of MILD combustion. CO was not found in the exhaust emission for any of the fuels. Reddy et al.32 investigated the MILD combustion of kerosene in a 20 kW two-stage combustor. They used high swirl flow to increase both the internal recirculation and dilution of fresh reactants, hence resulting in MILD combustion. They found that, as the combustion mode is switched from conventional to MILD, the CO and NOx emissions may be reduced by an order of magnitude. Derudi et al.25 investigated the sustainability of MILD combustion of liquid hydrocarbons (namely, n-octane, n-octane/isooctane, and n-octane/isooctane/n-decane) using a dual-nozzle laboratory-scale burner. They found that the MILD combustion can occur and very low amounts of NOx, CO, and polycyclic aromatic hydrocarbon (PAH) and soot precursors are produced. Stadler et al.46 investigated the MILD combustion of pulverized coal in air, Ar/O2, and CO2/O2. They found that the intensive mixing in MILD combustion allows for a reduction of the oxidizer oxygen concentration Received: November 14, 2013 Revised: December 28, 2013 Published: January 6, 2014 1524

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Table 1. Summary of Experimental Investigations of MILD Combustion fuel

oxidant

capacity

oxidant temperature

reference

CH4 CH4 CH4 CH4 CH4 CH4, C2H6 CH4, C2H6 CH4, C2H4, C3H8 CH4, LPG CH4, C2H4, LPG CH4 and H2 mixture CH4 and H2 mixture CH4 and H2 mixture CH4 and H2 mixture natural gas natural gas, biogas C3H8 LPG, producer gas natural gas, light oil, heavy oil, coal sawdust coal coal coal

air air air air air air air air air air, CO2/O2, CO2/N2/O2 air air air air air air pure oxygen air air air air, Ar/O2, and CO2/O2 air and oxy-fuel air

5.4 kW 10 kW 10 kW 7−13 kW 25 kW 0.13 kW 0.13 kW 5−8 kW 15−20 kW 13 kW 0.2−0.3 kW 0.2−0.3 kW 6.25 kW 20 kW 0.3−1.5 MW 0.1−0.3 kW 200 kW 3 kW, 150 kW 0.58 MW 7.5−10 kW 5, 8, and 40 kW 22−41 kW 12 MW

N/A 773 K 673 K 673 K 300 K 1573 K for CH4, 1173 K for C2H6 1573 K 1352−1453 K 298−748 K 288 K 1573 K 1573 K 300 K 298 and 858 K 963−1300 K 1573 K 288 K 288 K 1573 K 288 K 298−573 K N/A 453 K

42 2 28 29 17 22 20 11 9, 10, 35, and 36 37 23 24 19 43 44 21 45 30 and 31 15 12 46 47 48

large asymmetric recirculation pattern, resulting in no visible flame with little soot and NOx formation, which is the occurrence of MILD combustion. Stadler et al.47 investigated the coal MILD combustion in three oxidizer atmospheres, namely, air, CO2/O2, and O2-enriched flue gas recirculated. They found that, as the oxygen ratio is increased, NOx emissions increase for air and oxy-fuel in both the conventional and MILD combustion. They also found that the reduction of NOx emissions by the flue gas recirculation is attributed to both the reduction of recirculated NOx and the reduced conversion of fuel N to NO. Recently, Li et al.37 investigated the characteristics of MILD oxy-combustion using natural gas, liquefied petroleum gas (LPG), and ethylene at a firing rate of 13 kW. They found that, when diluting oxidant by CO2 at a fixed rate, the MILD oxy-combustion can be established as long as the equivalence ratio is sufficiently high. They also found that the N2O intermediate route plays a crucial role in forming NO and that the NO-reburning reaction should not be ignored in the MILD combustion. The objective of the present study is to investigate the global characteristics of MILD combustion in a pilot-scale furnace (300 kW) firing light oil and pulverized coal as fuel with air or O2-enriched recycled flue gas as the oxidant. The oxidant is only slightly preheated (35%) in their coal MILD experiments. To examine the applicability of the MILD combustion with the low volatile coals, one type of bituminous coal with high ash and low volatile, obtained from the Qingshan thermal power plant of China, is used in the present experiment. The ultimate and proximate analyses of the bituminous coal are given in Table 5. It is also hard to fire the pulverized coal with a large particle size. In the IFRF experiment of coal MILD combustion,15 the coal was milled to a particle size distribution of 80% < 90 μm. Stadler et al.47 even

the oil gun at 7 bar pressure (Figure 2). The product code of the Danfoss oil nozzle is 030B1129,52 and the spray cone angle of the fuel oil injector is 60°. For case 1, the air is injected through the annular swirl jet (Figure 3d). For cases 2−4, the oxidant is introduced via the asymmetrical jet (Figures 2c and 3f) and the small oxygen nozzles are not used. For the oxy-fuel combustion of case 4 (Table 5), the oxygen is mixed with the hot recirculated exhaust gas as the oxidant injected as the asymmetrical jet. For the combustion of the pulverized coal, the coal is carried by the primary stream and introduced into the furnace 1527

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Table 5. Ultimate and Proximate Analyses of the Bituminous Coal (Air-Dried Basis) ultimate analysis (wt %)

proximate analysis (wt %)

C

H

O

N

S

moisture

volatiles

fixed carbon

ash

LCV (kJ/kg)

84.60

4.30

8.58

1.45

1.07

1.15

18.74

47.53

32.58

21861

milled the coal to a particle size distribution of 90% < 48 μm. To test the pulverized coal being prepared originally for the power plant, the coal in the present experiment is obtained directly from the Qingshan thermal power plant of China without further milling. The present particle size distribution is shown in Figure 4, with the mean size of approximately 110 μm, which is larger than that used in refs 15 and 47.

conditions of the coal air combustion (cases 5−7). Only the primary and secondary air streams are modeled to obtain the flow field. The commercial software FLUENT code53 is used for the steady-state, implicit, finite-volume-based simulation. The pressure velocity coupling is the SIMPLE algorithm. A full hexahedral grid (about 2 000 000 hexahedral cells) is adopted to minimize the grid size, and an appropriate refinement of the grid is performed in the regions with higher gradients. A realizable k−ε model with standard wall functions is used to model turbulence. A realizable k−ε model has been shown to provide a more accurate prediction of flow evolution and flow spreading of round jets.54 Boundary conditions are velocity inlet, pressure outlet, and temperature wall boundary. To improve the accuracy of the simulations, the second-order scheme is employed for pressure and the second-order upwind scheme is employed for momentum, turbulent kinetic energy, turbulent dissipation, species transport, and energy. Solution convergence is obtained when (a) the residuals are less than 10−6 for the energy and 10−5 for all of the other variables and (b) the variations of the outlet temperature and velocity are allowed to be within 1.0 K and 0.1 m/s, respectively.

4. RESULTS AND DISCUSSION 4.1. Flow Patterns of Three Burner Configurations from Numerical Simulation. Figure 5 shows contour distributions of the in-furnace non-reacting velocity magnitude and flow pathlines, numerically simulated, for burners I, II, and III. In the cases of burners II and III, there is a large-scale recirculation zone present downstream from the burner. In addition, the recirculation zone for burner III is significantly

Figure 4. Particle probability distribution of the pulverized coal.

3. BRIEF DESCRIPTION OF NUMERICAL SIMULATION To investigate the flow pattern and internal gas recirculation caused by the three burner configurations (i.e., burners I, II, and III), numerical simulations are carried out for non-reacting

Figure 5. Contour distributions of the non-reacting velocity magnitude and flow pathlines, numerically simulated, for configurations of (a) the swirl jet, burner I, (b) the symmetrical jet, burner II, and (c) the asymmetrical jet, burner III. 1528

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oil combustion, case 1) to no flame but dark flamelet existing (see Figure 7). Moreover, as the burner switches from burner I to burner III, the furnace temperature and the NO emissions rapidly decrease (this will be discussed in detail in section 4.3). The oxy-oil MILD combustion (case 3) can be established when pure oxygen is supplied as the oxidant, which has never been reported previously. When the exhaust gas is externally recirculated, mixed with the oxygen, and discharged into the furnace as the oxidant mixture, the MILD oxy-fuel combustion of light oil can be reached (see Figure 7c). For the present experiment of burning light oil, the appearance of the MILD oxy-combustion is similar to that of the MILD air combustion (panels b and c of Figure 7) and there is no big flame existing; however, the flamelet can be observed. For burning coal, the furnace has been fully warmed and burner I is used for conventional air and oxy-combustion. The coal MILD air or oxy-combustion can be established when the burner is switched from burner I to burner II or III, because of the stronger recirculation caused by burners II and III (Figure 5). Panels d−f of Figure 7 show the photographs of the coal combustion operated at the conventional and MILD conditions. Obviously, in the conventional condition, the visible yellow flame vividly exists. However, under the coal MILD combustion, the large yellow flame disappears, although “ghost” sparks sweep rapidly inside the furnace, regardless of air or oxygen with recirculated flue gas as the oxidant. However, for the MILD combustion of light oil and coal in the present experiment, the true “flameless” combustion has never been observed and there are always flamelets or sparks existing. Speculatively, this is because dark flamelets are associated with burning droplets and sparks are related to firing char particles. In this case, the big yellow flame appearing in the conventional case becomes invisible. Therefore, the volatile combustible gas is burning invisibly, while the droplets or char particles are just firing under the volatile MILD condition and, thus, remain visible. Likewise, Smart et al.55 and Weber et al.15 found that sparks exist under the coal MILD combustion. Dally et al.12 also observed that the glow red particle is firing under the sawdust MILD combustion. These observations differ significantly from the MILD combustion of gaseous fuel. Panels g−i of Figure 7 display the photographs of the conventional and MILD combustion of natural gas, obtained from our previous work.37 There is no flame visible under the MILD combustion of natural gas. The appearance difference leads to an interesting question: what are the qualitative criteria for the MILD combustion of firing liquid or solid fuels? For gaseous fuels, the MILD combustion may be qualitatively defined as the combustion where no visible flame front occurs at all.1,7,8 Therefore, for the MILD combustion of liquid or solid fuels, the burning of volatiles does not yield any yellow flame or is invisible, while visible dark flamelets or sparks may exist. On the fundamental side, the appearance difference of the MILD combustion of gaseous, liquid, and solid fuels is caused by their different ignition delay and burnout times. For instance, both the ignition delay and burnout times of natural gas are approximately several milliseconds.56 However, the combustion of pulverized coal is relatively complicated and mainly proceeds in four stages. First, the pulverized coal is preheated. Second, its volatile matter is emitted and ignited. Then, the char burns after reaching its firing temperature. Finally, the char burns out. The heating and devolatilization of the volatiles are mainly completed in the first and second

larger than that from burner II. However, for burner I, the central primary jet is surrounded by the secondary swirl jet and the internal recirculation is relatively small. The above observations from the computational fluid dynamics (CFD) work suggest that the straightly discharging side jets for the secondary oxidant from burners II and III have a higher injection axial momentum, thus leading to a stronger recirculation than the central swirl jet configuration (burner I). Therefore, it is deduced that, for the present furnace, the side direct injection burner configuration should be a good choice to realize the MILD oxy-combustion of burning liquid and solid fuels. 4.2. Establishment of the MILD Combustion for Burning Light Oil and Pulverized Coal and Their Appearances. The light oil is first tested in the experiment. To warm the furnace, burner I is adopted with air as the oxidant and the furnace operates at the conventional mode (case 1). The temperature variations measured at several centerline locations along the furnace in the warming process are shown in Figure 6. It takes approximately 2 h to warm the furnace.

Figure 6. Temporal variations of the measured temperatures at the furnace warming process.

There are two stages in the warming process: the fast preheat process (stage 1) and the slow preheat process (stage 2), although the thermal input of the firing oil is the same. For stage 1, the temperature at T3 (z = 840 mm) is rapidly heated from approximately 600 to 1140 K in 20 min. However, as the preheat process continues, the in-furnace temperature growth rate slows. For stage 2, it takes about 100 min to heat the temperature of T3 from 1140 to 1360 K. For the present experiment, the MILD combustion of light oil cannot be established at stage 1. The reason is that, to achieve MILD combustion, the whole furnace must be well warmed. For gaseous fuels, the MILD combustion can be reached when the temperature of the entire furnace exceeds the ignition of the mixture. However, for the present experiment using light oil, the MILD combustion cannot be established when the whole furnace temperature exceeds its ignition in air (approximately 650 K), because the light oil is hard to volatilize and burn. When the preheat process goes into stage 2 and the entire furnace temperature exceeds 800 K (T7 in Figure 6), Burner I is switched to burner III, and then the MILD combustion of light oil is achieved (case 2) because of larger recirculation caused by burner III (Figure 5). The appearance of the combustion changes from the big yellow flame (conventional 1529

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Figure 7. Images showing the in-furnace appearance of the combustion of the light oil, coal, and natural gas.

specific heat of CO2 is higher than that of N2, the furnace temperature of the O2/CO2 MILD combustion (case 4) is lower than that of the MILD air combustion (case 2). Correspondingly, because there is no inert gas being supplied as the diluent and the combustion heat is used to warm the exhaust gas, the furnace temperature of the oxy-oil MILD combustion (case 3) is higher than that of the other two MILD combustion cases (cases 2 and 4). At z > 1500, furnace temperatures of the four cases gradually decrease. Interestingly, at the lower part of the furnace (z > 1500), the temperature in case 1 is lower than the other three MILD cases. This is because the MILD combustion still occurs at the lower part of the furnace, while for the conventional combustion (case 1), the fuel basically burns out at the upper furnace because of the relatively high reaction rate. Figure 9 shows O2 distributions along the furnace centerline in different cases of firing light oil. In case 1, because the

stages, and this takes about several milliseconds. Nevertheless, it takes about several seconds to completely burn out the char particle.56 The local residence time of the coal particle inside the furnace is typically larger than several milliseconds but less than several seconds. Therefore, for a limited residence time, the relatively long firing time of char particles leads to visible char particles, while the combustion of gaseous volatiles still occurs in flameless mode without any big yellow flame. 4.3. MILD Combustion of Light Oil. Figure 8 shows the temperature distribution along the furnace centerline under

Figure 8. Temperature distributions along the furnace centerline under different combustion conditions of light oil (cases 1−4).

different conditions of oil combustion. Clearly, the maximal temperature of the conventional flame condition (case 1) is the highest, and it is approximately 1610 K at z = 840 mm. For the other three MILD cases (cases 2−4), the temperature distributions are relatively uniform at z < 1500 mm. Note that the initial oxygen concentration of the oxy-fuel combustion (case 4) is 21 vol % and the hot exhaust gas is externally recycled without removing H2O (i.e., wet recycle). Because the

Figure 9. Oxygen distributions along the furnace centerline under different combustion conditions of light oil (cases 1−4). 1530

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oxidant (air) issues through the central swirl nozzle, the centerline oxygen concentration at the first measurement point is much higher than those for the other cases, where the injection location of the oxidant is far off the centerline. On the other hand, in case 1, the atomized oil is ejected at a location not far from the air nozzle and, thus, soon mixes with highconcentration oxygen that results in chemical reactions occurring at much higher rates than those for the other cases. Consequently, the conventional flame occurs in case 1. For the other cases, the oxygen concentrations at measurement locations 1−3 are relatively much lower and, hence, reactions occur slowly, so that the MILD combustion is established. Moreover, the oxygen concentration of MILD O2/CO 2 combustion (case 4) is slightly higher than that of the MILD air combustion (case 2). This can be explained here. When O2/ CO2 is used as the oxidant, the reactant mixture is less flammable than the mixture of fuel and O2/N2.49 Plus, the high CO2 concentration of the O2/CO2 atmosphere enhances the backward reaction of O2 + CO ⇔ CO2. In addition, Figure 9 indicates that the centerline oxygen concentration is lowest in case 3, where pure oxygen is injected far away from the oil nozzle. This is likely due to the fact that the furnace temperature is overall higher (see Figure 8), so that the oxidation rate should be higher when using pure oxygen, because there is no inert gas initially injecting into the furnace and, thus, less heat is needed to warm the oxidant. Figure 10 displays the NO distribution along the furnace centerline. The highest NO production is obtained for the

Figure 11. Exhaust emissions under different conditions of firing light oil.

are all below the threshold of the gas analyzer, and thus, no CO emissions are considered. 4.4. MILD Combustion of Pulverized Coal. Figure 12 shows the temperature distributions along the furnace center-

Figure 12. Temperature distribution along the furnace centerline under different conditions of coal combustion (cases 5−10).

line for cases 5−10. Note that the initial oxygen concentration of the coal oxy-combustion (cases 8−10) is about 40 vol % and the hot flue gas is externally recycled without removing H2O (i.e., wet recycle). It is demonstrated that, overall, for all of the cases, as the downstream distance (z) from the burner increases, initially the centerline temperature increases before reaching its maximum and then decreases. However, a clear distinction exists between those results measured in the conventional and MILD conditions. For instance, the maximal temperature of the conventional combustion (cases 5 and 8) is higher than that of the MILD combustion taken under both the air fuel and oxy-fuel conditions. Under the air fuel conditions (cases 5−7), the maximal temperature for the conventional case is approximately 1580 K, while those for the two MILD cases are close to 1400 K; differently, the corresponding maximal temperatures are higher (≈1721 versus 1550 K) under the oxyfuel conditions (cases 8−10). As a result, the temperature drop at z = 3500 mm is quite large (>550−675 K) for the conventional cases and relatively small ( 1500 mm, the temperature for the MILD cases exceeds that for the conventional cases. It is thus deduced that chemical reactions occur more uniformly inside

Figure 10. NO distributions along the furnace centerline under different combustion conditions of light oil (cases 1−4).

conventional flame (case 1). For this case, the maximal temperature is the greatest, therefore, the highest thermal NO is formed. Thermal NO is suppressed under the MILD combustion, and thus, NO formations of cases 2−4 are considerably lower than the conventional conditions (there is nearly no fuel NO formation for this type of oil; see Table 4). For the three MILD cases, the maximal temperature of O2/CO2 MILD combustion (case 4) is the lowest, and hence, its NO distribution is lower than the other two MILD cases. The exhaust emissions of NO and O2 are presented in Figure 11. Generally, according to the present experiment, the NO emission is strongly dependent upon the maximal temperature: The higher the maximal temperature, the higher the NO emission. The exhaust O2 emissions for each case are similar (approximately 5.5%) because of the same air excess ratio. In addition, the concentrations of exhaust CO for the four cases 1531

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the MILD condition occur more slowly, and thus, the infurnace temperature is lower. Figure 14 illustrates the NO distribution along the furnace centerline in different conditions of coal combustion (cases 5−

the furnace in the MILD combustion than in the conventional combustion. Furthermore, as seen in Figure 12, the maximum temperatures for the MILD combustion from burner III are lower than those from burner II, because burner III can produce a higher internal recirculation (Figure 5). Of note, in the present experiment using the identical burner but different oxidant mixtures, the centerline maximum temperature obtained for the oxy-combustion is approximately 150 K greater than that for the air combustion (Figure 12). The reason is that the initial oxygen concentration of the oxycombustion (40 vol %) is much higher than that of the air combustion (21 vol %). Of note, to obtain a nearly identical furnace temperature under both oxy-fuel and air fuel conditions, the initial oxygen has to be higher in the former case than that in the latter because the specific heat of CO2 is higher than N2. This has been verified by the previous work.57,58 Andersson et al.57 and Hjartstam et al.58 found that, when the initial oxygen is 29 vol %, the temperature of lignite coal oxy-combustion is 100 K higher than that of air combustion. Likewise, Croiset et al.59 used the initial oxygen of 35 vol % for the oxy-combustion of Canadian western subbituminous coal and obtained the temperature to be 80 K higher than that for the air combustion case. Moreover, if H2O participates in the recycle, different conditions will result. That is, the furnace temperatures of wet recycle oxy-combustion and dry recycle oxy-combustion are different. Wall et al.60 carried out equilibrium calculations and found that, to match the adiabatic flame temperature of the coal air combustion, the theoretical initial oxygen level (vol %) should be about 28 or 35%, for wet and dry recycle coal oxy-combustion, respectively. Overall, the initial oxygen level, coal type, oxidant mixture content (O2/N2 or O2/CO2), and combustion condition (conventional or MILD) all affect the furnace temperature. Those aspects should be considered in the design of the coal MILD oxy-combustion system. Figure 13 displays the O2 distribution along the furnace centerline under different coal combustion conditions (cases

Figure 14. Distributions of NO along the furnace centerline under different conditions of coal combustion (cases 5−10).

10). Clearly, higher maximum temperatures (Figure 12) lead to higher NO productions. The high temperatures of the conventional combustion in the combustion zone produce extremely high NO formations, i.e., approximately 530 ppm for the air combustion at z = 840 mm and 1200 ppm for the oxycombustion at z = 1500 mm. In contrast, NO formations from four MILD combustion cases are generally less than 320 ppm. The NO formation is suppressed by the relatively low infurnace temperature under MILD combustion. Moreover, the centerline maximum temperature occurs at z = 470 mm (Figure 12), while the centerline maximum NO production takes place at z = 840 and 1500 mm (Figure 14), respectively, for air combustion and oxy-combustion. Namely, the maximum NO formation occurs at a location downstream from that of the maximum temperature because NO is always formed at a slow rate; see ref 53. Figure 15 shows the exhaust NO emissions under different combustion conditions (cases 5−10). Obviously, the conventional oxy-combustion (case 8) yields the highest NO emission (681 ppm), while both MILD oxy-combustion cases (cases 9 and 10) produce the lowest NO emissions (≈168 ppm).

Figure 13. Oxygen distributions along the furnace centerline under different conditions of coal combustion (cases 5−10).

5−10). The centerline oxygen concentrations are relatively higher because of faster mixing between the primary and secondary streams for both conventional combustion cases (cases 5 and 8) than those for the MILD cases. For example, at z = 840 mm, the oxygen concentration for the conventional cases is greater than 10 vol %, while it is less than 5 vol % for the MILD combustion. As a result, chemical reactions under

Figure 15. Exhaust NO emissions under different conditions of coal combustion (cases 5−10). 1532

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Moreover, the NO emissions from the two MILD oxycombustion cases (cases 9 and 10) account for only 24% of the flame oxy-combustion (case 8), while NO emissions from the two MILD air combustion cases (cases 6 and 7) are approximately 60% of that from the flame air combustion (case 5). Therefore, the NO reduction is relatively stronger in the MILD oxy-combustion than in the MILD air combustion. The NO formation mechanism is analyzed and discussed below. For the air combustion, it is well-known that the fuel NO route produces more than 80% of the total NO emission and the higher reaction temperature results in higher NO production.61,62 The low temperature of the MILD air combustion (cases 6 and 7) suppresses the fuel NO and thermal NO formations, and thus, the NO emission is reduced approximately 60% from the flame oxy-combustion case (case 5). In addition, the NO emission (213 ppm) of the MILD air combustion from burner III (case 7) is 15.5% lower than that (180 ppm) from burner II (case 6). It is thus obtained that, for the air combustion, the asymmetrical jet burner configuration (burner III) leads to larger recirculation (Figure 5) and stronger dilution of reactants by the flue gas, thus resulting in lower temperatures (Figure 12) and NO emissions, relative to the symmetrical burner configuration (burner II). For the oxy-combustion, the NO emission from the MILD oxy-combustion (cases 8 and 9) is only 25% of the NO emission from the conventional oxy-combustion (case 10). There are no thermal and prompt NO productions because of the replacement of N2 with the flue gas mixture (mainly CO2 and H2O).49 The NO emission derives mainly from the fuel NO route. For the present experiment, the relatively low maximum temperature from the two MILD oxy-combustion cases (cases 9 and 10) reduces the fuel NO production. Moreover, the NO contents from the recycled flue gas can be decreased in the combustion zone by the NO-reburning reaction.49 Therefore, although the maximal temperature of the MILD oxy-combustion from burner II (case 9) is nearly identical to that of the conventional air combustion (case 5), the NO emission of the former is significantly lower than that of the latter by 51%. In addition, for the MILD oxy-combustion, the NO emission by burner III (case 10) differs only slightly from that by burner II (case 9), while the difference enlarges considerably for the MILD air combustion. This can be explained here. It is wellknown that the fuel NO production increases as either the local temperature or the local oxygen concentration in the reaction zone increases. While Figure 13 suggests that the local oxygen concentration in the reaction zone from burner III (asymmetrical configuration; case 10) is higher than that from burner II (symmetrical configuration; case 9), the local temperature from burner III is lower than that from burner II (see Figure 12). Therefore, it does make sense that the NO emission of the former is only slightly higher than that of the latter. Figure 16 displays the exhaust coal burnout and CO emissions under different conditions of coal combustion (cases 5−10). As demonstrated, for the same oxidant (air or O2/CO2), the burnout of char is generally poorer for the MILD combustion than the conventional combustion. When detailed looks are taken for the furnace temperature (Figure 12), oxygen concentration (Figure 13), and coal burnout (Figure 16), it is found that the higher the peak temperature and oxygen concentration in the main reaction zone (z < 1500 mm), the higher the char burnout. A similar observation was made by Dally et al.12 for the sawdust MILD combustion. The short

Figure 16. Exhaust coal burnout and CO emissions under different conditions of coal combustion (cases 5−10).

residence time was thought as the main reason for the low burnout of char. In their experiment, Stadler et al.46 improved the char burnout by increasing the residence time for their coal MILD oxy-combustion. Moreover, both the volatile content and particle size also influence the char burnout. In the coal MILD air combustion experiment performed by Weber et al.,15 a high volatile coal (Guasare) was used and the char was nearly completely burnt out. Su et al.63 found that the high volatile of coal improves not only ignition but also the carbon burnout. Du et al.64 found that the coal burnout is increased by approximately 10−15% (depending upon the coal type) when the coal particle size is reduced approximately from 150−250 to 75−150 μm. Therefore, the low burnout issue under the coal MILD combustion should be resolved by increasing the char particle residence time or using the coal with higher volatile and smaller particle size. Figure 16 also demonstrates that exhaust CO emissions from all six cases are extremely low (