Effects of Combustion Conditions on Formation Characteristics of

Sep 12, 2016 - In this study, pulverized coal bias ignition experiments were conducted in a 250 kW pilot-scale bias combustion simulator to investigat...
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Effects of Combustion Conditions on Formation Characteristics of Particulate Matter from Pulverized Coal Bias Ignition Guang Zeng,†,‡ Shaozeng Sun,† Heming Dong,† Yijun Zhao,*,† Zhenqi Ye,‡ and Lai Wei‡ †

School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, China Liaoning Electric Power Research Institute, Shenyang 110006, China

‡

ABSTRACT: In this study, pulverized coal bias ignition experiments were conducted in a 250 kW pilot-scale bias combustion simulator to investigate the effects of the combustion conditions on the particulate matter (PM) formation characteristics in a reducing atmosphere; the amount of PM10 was determined using an electrical low-pressure impactor. The particle size distributions of PM10 from pulverized coal bias ignition under various combustion conditions in a reducing atmosphere differ from those in the tail flue gas of a coal-fired boiler. Mainly PM0.38 is produced when the pulverized coal concentration (PCC) is 0.53, the lowest amounts of PM0.38 and PM0.38−10 are produced when the PCC is 0.4, and mainly PM0.38−10 is produced when the PCC is 0.33; the PCC significantly affects formation of PM0.5−0.794. As the bias concentration ratio (BCR) of the pulverized coal increases, the pulverized coal jets are prone to releasing viscous minerals, which easily coalesce into supercoarse particles of size greater than 10 μm; this decreases the formation of PM0.38 and PM0.38−10. The released viscous minerals are the main sources of PM0.38, and the BCR significantly affects PM0.205−0.5 formation. Volatiles and char burn most intensely at a primary air velocity (PAV) of 17 m/s, resulting in maximum production of PM0.38 and PM0.38−10. PM0.38 was mainly produced in the volatile combustion stage, and PM0.38−10 was mainly produced in the char combustion stage; the PAV significantly affects the formation of PM0.041−0.317. More PM10 with a larger fraction of ultrafine PM was produced at the optimum PAV, which was selected based on the ignition characteristics. OFS values and increased the proportion of fine-fragment mode. Zhou et al.9 studied the formation and control mechanisms of fine potassium-enriched particulates during combustion of different types of coal in a drop tube furnace (DTF). They observed that under O2/N2 conditions, as the combustion temperature increased, the transformation of potassium from potassium-rich coal to PM1 decreased, but volatile potassium was transformed to coarse particulates in increasing amounts. Carbone et al.10 explored the early processes of ultrafine ash formation for five different rank coals in a flow reactor at three oxygen concentration levels. They found that the oxygen concentration affected the size of nucleating particles and the preferential vaporization of some compounds over others through the char-burning temperature and local reducing properties of the gas environment; a high oxygen concentration promoted ultrafine particle formation. Zhang et al.11 investigated the effect of temperature on central-mode PM formation during the combustion of coals in a DTF at 1100 and 1300 °C and found that the central-mode PM concentration for coal A increased, whereas that for coal B decreased, with increasing temperature. This was attributed to different fragmentation and coalescence behaviors resulting from different mineral compositions of the two coals. Kazanc et al.12 investigated the physical characteristics of PM emissions from pulverized coal burning in air or in simulated oxy-fuel environments in a DTF; they used O2/N2 and O2/CO2 environments and observed trimodal ash particle size distributions (PSDs) with peaks in the submicron and

1. INTRODUCTION Coal is projected to remain an important source of energy for the foreseeable future. The total global consumption of coal is increasing, and China accounts for more than half of this consumption.1 The air pollution caused by coal burning in China has become an important factor and is restricting the sustainable development of China’s economy and society; it is also attracting attention from the international community, especially neighboring countries and regions. China’s dust emissions in 2014 were more than 17 million tons;2 particulate matter (PM) has become the primary pollutant affecting urban air quality. Fine particles have clear adverse effects on human health, and also affect atmospheric visibility, surface temperature, and precipitation.3−6 Coal-fired boilers are all equipped with efficient ash-removal equipment. The collection efficiency of the large-mass fraction, i.e., supermicron particles, in fly ash is high, but those of the large number of fine-particle fractions, especially submicron particles, are lower.7,8 The formation and control of coal-combustion particles needs to be further studied. Many factors affect the formation of coal-combustion particles and can be categorized as combustion conditions and coal quality characteristics. The aim of this study was to investigate the effects of combustion conditions on PM formation characteristics. The effects of combustion conditions such as combustion mode, combustion temperature, and combustion atmosphere have been widely studied. Fix et al.8 examined the effects of the oxygen-to-fuel stoichiometry (OFS) on the formation of ash particles. They found that the coarse mode is unaffected by OFS changes, but an OFS of 1.05 lowered the fraction of ultrafine ash compared with those obtained at higher © 2016 American Chemical Society

Received: May 27, 2016 Revised: August 16, 2016 Published: September 12, 2016 8691

DOI: 10.1021/acs.energyfuels.6b01280 Energy Fuels 2016, 30, 8691−8700

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Energy & Fuels

reaction environment of an actual furnace was provided. The airflow and pulverized coal bias ignition conditions can be adjusted flexibly and accurately in the PBCS to ensure stable operation during experiments and accurate adjustment of experimental conditions. Multiple-measurement instruments were used to investigate the ignition behavior of the pulverized coal jets. A sub-bituminous coal and its 1:1 blend with a bituminous coal were burned to study the effects of the combustion conditions of the pulverized coal concentration (PCC), bias concentration ratio (BCR) of the pulverized coal, and PA velocity (PAV) on the characteristics of PM formation from pulverized coal bias ignition. The results will be useful in the development of bias combustion technologies with low NOx and PM emissions, and economical boiler operation.

supermicron regions. The submicron particle yields in the effluents of all three coals were comparable, regardless of their ash contents, and the supermicron particle yields were nearly the same as the ash contents of the three coals. Pulverized coal shows different ignition behaviors under different combustion conditions, and the PM formation characteristics are different. Large amounts of pulverized coal are used in the tangential-fired utility boiler which are still widely developed and being modernized worldwide significantly. Low NOx bias combustion technologies have been developed for the tangential-fired utility boiler because of their environmental and economic benefits. An example is horizontal bias combustion technology, which is widely used in utility boilers in China. In this technology, the primary air (PA)/fuel stream is separated horizontally into fuel-rich and fuel-lean streams, which are then introduced separately into the furnace, to achieve strong ignition, NOx emission reduction, and slagging prevention.13,14 Stereo stage low NOx combustion technologies, which involve pulverized coal bias combustion and air stage combustion, are now being used for utility boilers to achieve even lower NOx emissions.15 However, if depth stage combustion is used, the excess air coefficient in the primary combustion zone will be much less than 1, and pulverized coal ignition will occur in an oxygen-deficient atmosphere. This inhibits NOx formation but decreases the combustion efficiency in the primary combustion zone.16,17 The combustion atmosphere and temperature then change, and the characteristics of PM formation also change. Although studies of the formation characteristics of coal-combustion particles have achieved considerable results, the PM formation mechanisms in different stages of actual pulverized coal combustion vary, and this needs to be studied. There are no reports of studies of the formation characteristics of PM in the primary combustion zone of a boiler using a stereo stage low NOx combustion technology. The formation characteristics of PM from pulverized coal bias ignition in a reducing atmosphere need to be studied in depth, to provide practical guidelines for the development of clean coal combustion technologies. In most experimental studies of coal-combustion particles, small or miniature benches equipped with external heating power supplies to maintain combustion are used because of the low flow rate of pulverized coal. In modern utility boilers of tangential-fired type, the combustion of pulverized coal jets is accomplished by injecting into a hot furnace. Sufficiently high heating rates for pulverized coal can be obtained using small or miniature benches, but they are still clearly different from those in an actual furnace, which are mainly achieved by convective heat transfer of high-temperature gas. Different heating modes affect the ignition characteristics; therefore, the results obtained using small or miniature benches are not fully applicable to industrial processes. In this study, hot experiments were performed in a 250 kW pilot-scale bias combustion simulator (PBCS). The intersection and ignition of six jets were achieved for the first time, as follows. From top to bottom, two jets of PA bias pulverized coal flow were injected into the primary combustion zone of the furnace, then mixed with two jets of high-temperature flue gas and ignited, and then two jets of secondary air (SA) gradually became involved in combustion, during the ignition process. The resultant momentum of the six jets was propagated down the central axis of the furnace, and the pulverized coal jets were not unaffected. The flow and heat transfer behaviors of the turbulent pulverized coal jets in this study were similar to those in engineering practice, and the

2. EXPERIMENTAL SECTION 2.1. Experimental Setup. In this study, experiments were performed in a PBCS, which was fabricated at the National Engineering Laboratory for Reducing Emissions from Coal Combustion in Harbin Institute of Technology, China. A schematic diagram of the PBCS system is shown in Figure 1; the main components are a furnace, bias pulverized coal transportation system, air supply system, hightemperature flue gas generation system, steam generation system, tail flue gas system, measurement and sampling system, and operationmonitoring system. The main design parameters are listed in Table 1. The furnace is columnar type, and the upper part is the primary combustion zone. The bias pulverized coal transportation system consists mainly of two variable frequency adjustable screw feeders, two PA ducts of equal dimensions, and a bias pulverized coal burner. Pulverized coals from two silos, after continuous stable bias feeding, are separately mixed with the corresponding PA to form two bias pulverized coal jets, which are then injected into the furnace through two equal-dimension rectangular burner nozzles, to achieve bias feeding. Air from a forced draft fan (FDF), after heating in an air preheater, enters the PA and SA windboxes for temperature regulation by hot/ cold air mixing and is finally injected into the furnace as PA and SA. The PA is used to carry pulverized coal and provide the oxygen required for ignition; the SA replenishes oxygen sequentially and maintains a constant stoichiometry in the reaction between air and coal. A high-temperature flue gas generated by complete gas combustion passes through a special pipe into the furnace; it creates an ignition environment approximating that of an actual boiler, ignites the bias pulverized coal jets, and maintains stable combustion. The flue gas from the furnace finally enters the tail flue gas ducts, and successively passes through the air preheater, flue gas treatment apparatus, induced draft fan (IDF), and chimney into the atmosphere. A steam generation system is included in the PBCS system to regulate the moisture content in the pulverized coal jets. Measurement holes are arranged vertically from top to bottom around the furnace; one side of the hole is for determining the radial temperature and gas components, as well as ash collection, and the other side is for obtaining flame spectra and images. 2.2. Experimental Measurements. In pulverized coal ignition experiments, the ignition characteristics are reflected by changes of temperature in the axial direction in the primary combustion zone of the furnace.18 A measurement hole placed between the two nozzles of the bias pulverized coal burner at the top of the furnace was used for inserting different thermocouples into the furnace along the central axis, to measure different gas temperature in the axial direction in the primary combustion zone, which will be called as axial temperature for short in this paper. The axial temperature in each predetermined location of primary combustion zone was measured by 2 mm diameter Inconel Type-K armored thermocouple. An electrical low-pressure impactor (ELPI; Dekati, Finland) sampling and collection system was placed between the air preheater and flue gas treatment apparatus of the PBCS, as shown in Figure 2. Because the PM concentration in the emissions before the flue gas treatment apparatus is similar to that formed in the furnace, changes in its concentration can be used to determine the PM formation characteristics in the experiments. 8692

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Figure 1. Schematic diagram of PBCS system.

Table 1. Main Design Parameters of PBCS parameter

value

rated total thermal power rated coal fired thermal power rated gas fired thermal power coal feeding flow rate range gas feeding flow rate range PA/SA temperature permissible temperature of furnace heating rate of pulverized coal jets single section furnace body length primary combustion zone height furnace inner diameter

250 kW 200 kW 50 kW 3−60 kg/h 2−10 N m3/h 85/250 °C 0−1500 °C 105 °C/s 640 mm 1280 mm 800 mm

The sampling probe nozzles were chosen based on isokinetic sampling before the tests. During the tests, dust leaving the sampling probe first went through a cyclone separator to remove PM10+, entered two stage diluters mixed with pure air. The resulting diluted and cooled PM was introduced into the ELPI, and PM of size no more than 10 μm (PM10) was classified into 12 size fractions by the ELPI. The PM10 cutter, sampling pipe, and first stage diluter were heated and insulated to ensure that condensable PM was not separated out; the second stage diluter was kept at room temperature to ensure that the ELPI working temperature was less than 45 °C. 2.3. Experimental Coal. Sub-bituminous Trafigura coal from Indonesia and its 1:1 blend with bituminous White Haven coal from Australia were used in the experiments. The sub-bituminous coal was used in variable PCC experiments, and the blended coal was used in variable BCR and variable PAV experiments. The proximate and

Figure 2. Schematic diagram of ELPI sampling and collection system. ultimate analytical data for the raw coal and pulverized coal are presented in Table 2, and the ash property analysis results are presented in Table 3. The pulverized coal used in the study had a full PSD, and the pulverized coal fineness was R75 = 16% (i.e., the surplus after 75 μm sieving was 16%). 2.4. Experimental Methods. The furnace had a reducing atmosphere during the experiments, and the air-to-coal stoichiometry 8693

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thermal power of 200 kW; the output of each high-temperature flue gas generator was gradually reduced to a thermal power of 25 kW, and the total thermal power of the PBCS was 250 kW. Data were acquired only if the PBCS was operating in a continuous and stable state that could be repeated, to ensure data accuracy and repeatability. Axial temperature data were acquired once per minute for 10 min, and the average value was used as the axial temperature in that experiment, the deviation range between measured temperature data and its average value was 0−11 °C which was caused by the normal fluctuation of coal combustion. PM10 data were acquired once per second for 3 min, and the average value was used for that PM10 set; five sets of PM10 data were acquired for each experiment, and the average value was used, the deviation percentage between measured PM10 data and its average value was 0−1% which was caused by the normal concentration variation of fly ash in the flue gas flow.

Table 2. Proximate and Ultimate Analyses of Coal sub-bituminous items

raw coal

bituminous

pulverized coal

raw coal

blended coal

pulverized coal

raw coal

pulverized coal

6.80 27.96 11.78 53.46

18.90 29.03 12.06 40.01

14.75 31.79 8.42 45.04

67.06 3.92 8.89 1.24 0.31

53.23 3.47 10.77 1.05 0.54

59.42 3.81 12.20 1.05 0.37

25.74

20.23

22.66

proximate analysis (wt %, as received) Mar 28.10 22.7 9.70 Var 32.61 35.63 25.45 Aar 4.99 5.06 19.13 FCar 34.30 36.61 45.72 ultimate analysis (wt %, as received) Car 48.89 51.77 57.56 Har 3.45 3.7 3.48 Oar 12.91 15.5 8.63 Nar 0.9 0.85 1.19 Sar 0.76 0.42 0.31 net calorific value (MJ/kg, as received) NCV 18.43 19.58 22.03

3. RESULTS AND DISCUSSION 3.1. Effect of PCC on PM Formation Characteristics. The PCC is the mass ratio of pulverized coal to PA in the pulverized coal jets.19,20 The axial temperatures of the primary combustion zones at different PCCs are shown in Figure 3.

Table 3. Analysis of Ash Composition items

sub-bituminous

bituminous

blended coal

SiO2(%) Al2O3(%) Fe2O3(%) CaO(%) MgO(%) SO3(%) TiO2(%) K2O(%) Na2O(%) P2O5(%) MnO2(%) deformation T (°C) soften T (°C)

33.62 20.25 15.28 8.32 4.25 8.46 1.78 1.15 3.07 0.31 0.09 1070 1129

55.72 22.54 10.38 4.35 1.00 2.08 1.60 1.10 0.59 0.05 0.06 1251 1360

44.67 21.395 12.83 6.335 2.625 5.27 1.69 1.125 1.83 0.18 0.075 1160 1244

Figure 3. Axial temperatures at various PCCs.

was 0.75. Experiments were performed under three types of combustion conditions, i.e., variable PCC, variable BCR, and variable PAV; different cases were studied for each set of conditions. The variable PCC experiments were performed using PCC = 0.33, 0.4, and 0.53, with BCR = 4 and PAV = 17m/s; the variable BCR experiments were performed using BCR = 1:1, 3:1, 4:1, and 5:1, with PCC = 0.33 and PAV = 17m/s; and the variable PAV experiments were performed using PAV = 14, 17, 22, and 25 m/s, with PCC = 0.33 and BCR = 4. The main parameters for each set of experiments are shown in Table 4. The experiments were performed as follows. The IDF and FDF were started up, the high-temperature flue gas generation systems on both sides were put into service, and each side was kept running at a thermal power of 50 kW. Bias pulverized coal was then fed into the system at an appropriate flow rate. Until ignition became stable, the bias pulverized coal flow rate was increased until it corresponded to a

The figure shows that there is little difference among the trends in the axial temperatures at different PCCs, but in the range in this study, the combustion temperature of the pulverized coal increased gradually and the combustion became more intense with increasing PCC. This is because after the highconcentration pulverized coal jets were injected into the furnace, more volatiles were released, and the volatile content per unit volume of pulverized coal jet increased; therefore, the ignition temperature decreased and the combustion temperature increased. The mass and number concentration PSDs of PM10 at different PCCs are shown in Figures 4 and 5. The number

Table 4. Experimental Parameters coal type

coal flow rate (kg/h)

pulverized coal fineness R75 (%)

coal moisture content (%)

PAV (m/s)

BCR

PCC (kg/kg)

sub-bituminous sub-bituminous sub-bituminous blended coal blended coal blended coal blended coal blended coal blended coal blended coal

37 37 37 32 32 32 32 32 32 32

16 16 16 16 16 16 16 16 16 16

22.7 22.7 22.7 14.75 14.75 14.75 14.75 14.75 14.75 14.75

17 17 17 17 17 17 17 14 22 25

4 4 4 1 3 5 4 4 4 4

0.33 0.40 0.53 0.33 0.33 0.33 0.33 0.33 0.33 0.33

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than that in the tail flue gas of a boiler with complete combustion, and this was the main reason that cause our study results different from other study in number and mass concentration PSDs of PM10. Figures 4 and 5 also show that the PCC significantly affects formation of PM of sizes between 0.5 μm (ELPI stage 6) and 0.794 μm (ELPI stage 7); the PSD for PM10 formation at PCC = 0.4 is the lowest, the PSD for PM10 formation at PCC = 0.53 is the highest before 0.317 μm (ELPI stage 5), and the PSD for PM10 formation at PCC = 0.33 is the highest after 0.317 μm. In this study, these changes were quantitatively evaluated by dividing the PSDs of PM10 into PM0.38 and PM0.38−10, where PM0.38 represents ultrafine particles,21 and calculating the accumulated concentrations of PM0.38 and PM0.38−10 in different experiments. The number and mass concentrations of PM0.38 and PM0.38−10 at different PCCs are shown in Figures 6 and 7. Figure 3 shows

Figure 4. Number concentration PSDs of PM10 at various PCCs.

Figure 6. Number and mass concentrations of PM0.38 at various PCCs.

Figure 5. Mass concentration PSDs of PM10 at various PCCs.

concentration decreases as the particle size increases, with no obvious peak. The mass concentration increases as the particle size increases, with a peak at 0.205 μm (ELPI stage 4). The PSD of the PM emission of a coal-fired boiler is generally trimodal, with three clear peaks.21 Analysis shows that the number and mass concentration PSDs of PM10 from pulverized coal bias ignition at different PCCs in a reducing atmosphere differ from those in the tail flue gas of a coal-fired boiler; the same conclusion was reached based on other experiments in this study. The pulverized coal combustion process consists of two stages: volatile combustion and char combustion. In the volatile combustion stage, a large amount of volatiles are released and form a local reducing atmosphere in the vicinity of the char particles, abundant carbon black precursors are formed by dehydro-condensation, and ultimately primary spherical carbon black particles of size 20−50 nm are formed. Normally, most of the carbon black formed in a coal-fired boiler is oxidized in the char combustion stage, and only a small amount is transformed into chain or flocculent soot aggregates through coagulation and condensation.22 In our experiments, the pulverized coal jets were ignited in a reducing atmosphere and stayed a short time in the high-temperature region. Therefore, most of the carbon black was retained in the flue gas as single particles of mature carbon black or soot aggregates of small aerodynamic diameter due to the different combustion atmosphere compared to conventional combustion, reflecting the PSD characteristics of the PM in the primary combustion zone of a boiler using pulverized coal bias combustion rather

Figure 7. Number and mass concentrations of PM0.38−10 at various PCCs.

that when the PCC is 0.53, ignition is strong, and the number and mass concentrations of PM0.38 are highest. When the PCC is 0.4, ignition is medium, and the number and mass concentrations of PM0.38 and PM0.38−10 are lowest; when the PCC is 0.33, ignition is weak, and the number and mass concentrations of PM0.38−10 are highest. Analysis shows that, with increasing PCC to 0.53, the oxygen concentration around the coal particles decreased and the reducing atmosphere strengthened. This was conducive to the transformation of refractory elements to 8695

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Energy & Fuels suboxides,23 and the formation of ultrafine PM0.38 was accelerated. Also, the increasing temperature on the coal particle surfaces contributed to the formation of carbon black,22 mineral vaporization, and char fragmentation,24,25 which further accelerated the formation of PM0.38. In contrast, for PCC = 0.4, the reducing atmosphere around the coal particles weakened and the temperature on the particle surfaces decreased; vaporization of minerals to PM0.3823,24 and fragmentation of chars and minerals to PM0.38−1025 were suppressed. Therefore, the final amounts of PM0.38 and PM0.38−10 formed were both lowest. When the PCC was 0.33, the reducing atmosphere around the coal particles continued to weaken and the temperature on the particle surfaces continued to decrease. As reported, under the same combustion conditions, a decrease in the combustion temperature around the particle surfaces shortens the time for nucleation and coagulation of fine particles,26 and an increased oxygen concentration around the particle surfaces prolongs the time for nucleation and coagulation of fine particles.27,28 Analysis shows that when pulverized coal was bias ignited at different PCCs in a reducing atmosphere, the combustion temperature difference was small, as shown in Figure 3; consequently, an increase in the oxygen concentration around the particle surface predominates over a decrease in the combustion temperature. This causes the time for nucleation and coagulation to increase, the nucleated and coagulated particles become larger, and the number of submicron particles (0.38−1 μm) increases. The data in Table 5 show that, when

Figure 8. Axial temperatures at various BCRs.

Table 5. Number and Mass Concentration Fractions for PMs at Various PCCs PM0.38−10/PM10 (%) PM0.38−1/PM10 (%) PM1−10/PM10 (%) PCC (kg/kg)

number

mass

number

mass

number

mass

0.33 0.40 0.53

0.65 0.51 0.49

96.9 96.6 96.3

0.49 0.35 0.33

3.11 2.34 2.54

0.16 0.16 0.16

93.8 94.2 93.9

Figure 9. Number concentration PSDs of PM10 at various BCRs.

the PCC was 0.3, the fraction of PM0.38−1/PM10 was largest, but the amounts of PM1−10/PM10 were basically unchanged with changing PCC. These results indicate that more PM0.38−1 was produced when PCC was 0.3, i.e., the PCC significantly affects PM0.5−0.794 formation. 3.2. Effects of BCR on PM Formation Characteristics. The BCR is the bias concentration ratio of pulverized coal in the fuel-rich stream to that in the fuel-lean stream.19,20 The axial temperatures in the primary combustion zone at different BCRs are shown in Figure 8; the trends in axial temperatures at different BCRs are similar, and the axial temperatures start to increase at 120 mm. The rate of the temperature rise increases, the final combustion temperature increases, and combustion becomes more intense with increasing BCR. The mass and number concentration PSDs of PM10 at different BCRs are shown in Figures 9 and 10. When the BCR is 1:1 or 3:1, the number concentration decreases as the particle size increases, with no obvious peak. The number concentration at BCR = 4:1 decreases with increasing particle size, with an inflection at 0.794 μm. The number concentration at BCR = 5:1 decreases with increasing particle size, with an inflection at 0.5 μm. The mass concentration increases with increasing particle size, with a peak at 0.205 μm. Analysis shows that the number and mass concentration PSDs of PM10 from pulverized coal bias ignition at different BCRs in a reducing atmosphere differ from those in the tail flue gas of a coal-fired

Figure 10. Mass concentration PSDs of PM10 at various BCRs.

boiler. Figures 9 and 10 show that the PSDs for PM10 formation gradually become smaller, and the BCR significantly affects formation of PM between 0.205 and 0.5 μm. The number and mass concentrations of PM0.38 and PM0.38−10 at different BCRs are shown in Figures 11 and 12. The number and mass concentrations of PM0.38 and PM0.38−10 decrease as the BCR increases. Figure 8 suggests that with increasing BCR, the concentration of pulverized coal on the fuel-rich side gradually increased, the ignition of pulverized coal intensified, and the ignition zone in the fuel-rich side was severely oxygen deficient, resulting in a higher combustion temperature and 8696

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slagging; therefore, the PM PSDs shifted to larger size ranges compared with those in a previous study.33 Consequently, the formation of PM0.38 and PM0.38−10 decreased with increasing BCR. The differences among the percentage mass concentrations at BCR 1:1 and 5:1 of particles classified into various sizes in the ELPI are shown in Figure 13. The formation of PM of size

Figure 11. Number and mass concentrations of PM0.38 at various BCRs.

Figure 13. Percentage variations in mass concentrations of ELPIclassified particle sizes between BCR 1 and 5.

0.20−0.5 μm decreased most. Analysis suggests that viscous minerals, which were easily released, were probably mainly distributed in the range 0.205−0.5 μm, and the amount decreased greatly because of adhesion to the surfaces of coarse particles. The BCR therefore significantly affects the formation of PM0.205−0.5. At present, the correlation between the PM formation mechanism and low NOx combustion in a pulverized coal boiler with bias combustion is unknown. However, based on the results of our study, we speculate that for the primary combustion zone in a pulverized coal boiler using stereo stage low NOx combustion technology, PM10 formation will decrease and PM10+ formation will increase, because of strengthening of ignition and the reducing atmosphere. The combustion process in the burnout zone becomes more important, because it is necessary to burn off coarse particles that are produced by coalescence in the primary combustion zone and to prevent excessive secondary formation of fine particles. This needs to be further studied. 3.3. Effects of PAV on PM Formation Characteristics. The PAV is the velocity of primary air at the nozzles of a pulverized coal burner. The PAV is an important design parameter for a pulverized coal burner. Selection of an inappropriate PAV causes abnormal ignition, which directly affects PM formation. The axial temperatures in the primary combustion zone at different PAVs are shown in Figure 14. The axial temperature increased at a higher rate when the pulverized coal jets entered the furnace at 120−300 mm. Analysis suggests that the volatile combustion stage of pulverized coal ignition occurred mainly at 120−300 mm and the char combustion stage mainly occurred after 300 mm. The PAV greatly affected the axial temperature; the initial rates of increase of the axial temperature were higher at 120−240 mm when the PAV was 14 or 17 m/s, and the rates of increase at 240−300 mm were higher when the PAV was 17 or 22 m/s; the volatiles burned most intensely when the PAV was 17 m/s. The final combustion temperature was highest and most stable when the PAV was 17 m/s, which means that the

Figure 12. Number and mass concentrations of PM0.38−10 at various BCRs.

stronger reducing atmosphere. These factors should favor formation of submicron particles, but the experimental results showed the opposite; this has not been reported for previous studies. Analysis suggests that this is because the viscous minerals released from the sub-bituminous coal in the blended coal acted as a binder for the melting and coalescence of included minerals, and the melting and coalescence of these minerals formed coarse particles.29,30 In this study, pulverized coal ignition occurred in a reducing atmosphere, and therefore the burnout rate was poor and residual chars with large amounts of unburned carbon were produced; with increasing BCR, the reducing atmosphere in the ignition zone strengthened, which would lower the ash melting point.31 Because the melting point of ash from the sub-bituminous coal was very low, as shown in Table 3, the ashes melted and coalesced easily, and the viscous characteristics of the included minerals were further enhanced; therefore, more coarse PM was produced. The released viscous minerals, which were the main sources of PM0.38, became adhered to the surfaces of the coarse particles.32 It has been reported that the PSDs of PM10 from a pulverized coal boiler with horizontal bias combustion shifted to a larger size compared with those for PM10 from conventional combustion.33 In this study, pulverized coal bias combustion was performed in a reducing atmosphere, which corresponds to combustion in the primary combustion zone of a boiler, without combustion in the burnout zone. There was a tendency to coalescence into supercoarse particles of size >10 μm, which easily caused 8697

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concentration decreases with increasing particle size, without an obvious peak. The number and mass concentration PSDs of PM10 from pulverized coal bias ignition at different PAVs in a reducing atmosphere differ from those in the tail flue gas of a coal-fired boiler. Figures 15 and 16 show that the effects of different PAVs on the PM10 PSDs differ significantly. The size distributions at 14 and 17 m/s are similar, and the size distributions at 22 and 25 m/s are similar. This is because of different flow characteristics at different air velocities. The PAV also significantly affects formation of PM between 0.041 μm (ELPI stage 1) and 0.317 μm. The number and mass concentrations of PM0.38 and PM0.38−10 at different PAVs are shown in Figures 17 and 18. Figure 14. Axial temperatures at various PAVs.

char burned most intensely when the PAV was 17 m/s; char combustion at 22 m/s also occurred intensely. The mass and number concentration PSDs of PM10 at different PAVs are shown in Figures 15 and 16. The figures

Figure 17. Number and mass concentrations of PM0.38 at various PAVs.

Figure 15. Number concentration PSDs of PM10 at various PAVs.

Figure 18. Number and mass concentrations of PM0.38−10 at various PAVs.

Figure 17 shows that the number and mass concentrations of PM0.38 are highest when the PAV is 17 m/s, followed by 14, 22, and 25 m/s. Analysis suggests that this is caused by differences among the ignition characteristic at different PAVs. After the pulverized coal jets are injected into the furnace, volatiles are released and ignited, and then the char is burned.22 Figure 14 suggests that before 240 mm in the furnace, at PAVs of 14 and 17 m/s, which are relatively small, more heat is absorbed within the same distance. The volatiles are released more intensely, the ignition standoff distance of the pulverized coal jets is smaller, and the combustion temperature increases faster. The stronger reducing atmosphere caused by the high

Figure 16. Mass concentration PSDs of PM10 at various PAVs.

show that when the PAV is 14 or 17 m/s, the number concentration decreases with increasing particle size, without an obvious peak. The number concentration increases with increasing particle size, with a peak at 0.129 μm (ELPI stage 3), when the PAV is 22 or 25 m/s. The mass concentration increases with increasing particle size, with a peak at 0.205 μm, when the PAV is 14 or 17 m/s. When the PAV is 22 or 25 m/s, the mass 8698

DOI: 10.1021/acs.energyfuels.6b01280 Energy Fuels 2016, 30, 8691−8700

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Energy & Fuels

characteristics of PM formation from pulverized coal bias ignition in a reducing atmosphere. The main conclusions are as follows. (1). The number and mass concentration PSDs of PM10 from pulverized coal bias ignition under different combustion conditions in a reducing atmosphere differed from those in the tail flue gas of a coal-fired boiler; the curves had at most one peak. (2). Combustion became more intense with increasing PCC. At a PCC of 0.53, the number and mass concentrations of PM0.38 were highest; at a PCC of 0.4, the number and mass concentrations of PM0.38 and PM0.38−10 were lowest; and at a PCC of 0.33, the number and mass concentrations of PM0.38−10 were highest. The PCC therefore significantly affects PM0.5−0.794 formation. (3). With increasing BCR of the pulverized coal, combustion became more intense, and the pulverized coal jets tended to release viscous minerals that easily coalesced into supercoarse particles (>10 μm). The number and mass concentrations of PM0.38 and PM0.38−10 decreased with increasing BCR. The released viscous minerals are the main sources of PM0.38; therefore, the BCR significantly affects PM0.205−0.5 formation. For the primary combustion zone in a pulverized coal boiler using stereo stage low NOx combustion technology, PM10 formation will decrease and that of PM10+ will increase; the combustion process in the burnout zone becomes more important. (4). At various PAVs, the volatile combustion stage of pulverized coal ignition for pulverized coal jets injected into the furnace mainly occurred at 120−300 mm and the char combustion stage mainly occurred after 300 mm. When the PAV was 17 m/s, the volatiles and char burnt most intensely, and formation of PM0.38 and PM0.38−10 were both maximum; PM0.38 was mainly produced in the volatile combustion stage, and PM0.38−10 was mainly produced in the char combustion stage. The PAV therefore significantly affects PM0.041−0.317 formation. More PM10 with a larger fraction of ultrafine PM will be produced if the best PAV is selected based on the ignition characteristics. The other bias combustion conditions also need to be considered for achieving control of PM formation.

concentration of volatiles and the higher temperature favor formation of ultrafine PM0.38.22−25 Between 240 and 300 mm in the furnace, the turbulence intensities at PAV 17 and 22 m/s are stronger, with better entrainment; therefore the axial temperature continues to rise at a higher rate, which results in production of more PM0.38. A PAV of 17 m/s produced most PM0.38 because it gave the most intense combustion of volatiles, but a PAV of 25 m/s produced the least PM0.38 because the high velocity caused unstable combustion and a lower combustion temperature. Figure 18 shows that the number and mass concentrations of PM0.38−10 were highest when PAV was 17 m/s, followed by 22, 14, and 25 m/s. Analysis suggests that, after the pulverized coal jets entered the furnace beyond 300 mm, for a PAV of 17 m/s, which gave the most intense volatile matter combustion and excellent entrainment, the char burned more intensely, which produced more fragmentation,24 and the largest amount of PM0.38−10 was produced; however, less PM0.38−10 was formed at 14 and 25 m/s because of unstable combustion. Figures 14, 17, and 18 show that most PM0.38 was formed within 300 mm of the pulverized coal jets entering the furnace but PM0.38−10 was mainly formed beyond 300 mm. This means that with increasing residence time of pulverized coal in the furnace, PM0.38 formation decreased significantly, but PM0.38−10 formation increased significantly, as previously reported.34 This indicates that PM0.38 is mainly formed in the volatile combustion stage and PM0.38−10 is mainly produced in the char combustion stage during pulverized coal ignition. Figure 14 shows that the differences among the axial temperatures at different PAVs before 300 mm are greater than those after 300 mm. We speculate that the effect of different PAVs on the volatile combustion stage is greater than that on the char combustion stage, and therefore PM0.38 formation differs greatly at different PAVs; that is, the PAV significantly affects the formation of PM0.041−0.317. In this study, the ignition characteristics of pulverized coal were best when the PAV was 17 m/s, but as shown in Table 6, Table 6. PM10 Concentrations and PM1/PM10 Ratios at Various PAVs PM10 concentration PAV (m/s) 14 17 22 25

3

number (/cm ) 9425804.306 12639946.64 3889589.024 3419970.55

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fraction of PM1/PM10 3

mass (mg/cm ) 597.7804176 708.8266009 615.5634488 503.7738702

number (%) 99.3 99.4 98.2 98.3

mass (%)

AUTHOR INFORMATION

Corresponding Author

3.8 4.0 3.9 3.9

*Tel: +86-451-86413231. Fax: +86-451-86412528. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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the formation of PM10 with a larger fraction of submicron particles occurred at this PAV. This suggests that at the best PAV selected according to the ignition characteristics of pulverized coal bias combustion, which guides the design of pulverized coal burners, more PM10 and more ultrafine PM are produced in the ignition region. Other bias combustion conditions such as the PCC and BCR therefore also need to be considered to achieve control of particle formation. Furnace control measures such as mixed combustion with other coals or additives should also be considered.

ACKNOWLEDGMENTS The authors acknowledge financial assistance from the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (51421063). The authors are grateful to Prof. Du Qian for invaluable advice. Technical assistance with experiments and analysis from the National Engineering Laboratory for Reducing Emissions from Coal Combustion of China is acknowledged.

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4. CONCLUSIONS A sub-bituminous coal from Indonesia and its 1:1 blend with a bituminous coal from Australia were burned in a 250 kW PBCS to investigate the effect of combustion conditions on the

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DOI: 10.1021/acs.energyfuels.6b01280 Energy Fuels 2016, 30, 8691−8700