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Nov 28, 2017 - computational fluid dynamics (CFD), the PSD of ultrafine particles formed ..... make no big difference on the dynamic behaviors of Na d...
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Effect of CO2/H2O on the Incipient Ultrafine Particulate Matter Formation in Oxy-fuel Combustion of High-Sodium Lignite Qi Gao, Shuiqing Li,* Yang Xu, and Qiang Yao Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Energy and Power Engineering, Tsinghua University, Haidian District, Beijing 100084, People’s Republic of China ABSTRACT: In this work, the formation of incipient ultrafine particulate matter (PM) from high-sodium lignite combustion, under both dry and wet oxy-fuel conditions, is intensively investigated using an optically accessible flat-flame burner. The ambient temperature was set at 1500 K, and measurements were performed along the zones with the coal residence time of 0−40 ms. We simultaneously introduced thermophoresis sampling, dilution sampling, and phase-selective laser-induced breakdown spectroscopy (PS-LIBS) to characterize the morphologies, components, and particle size distributions (PSDs) of ultrafine PM as well as the behaviors of Na. The results indicate that incipient particles formed in CO2−O2 ambience are more abundant in Na and Si. The substitution of CO2 may accelerate the mass loss rate of coal in the early devolatilization stage, whereas the Nabased incipient particles form around a time of ∼10 ms that is similar to conventional N2−O2 ambience. In combination with the computational fluid dynamics (CFD), the PSD of ultrafine particles formed under the 70% CO2−30% O2 condition is successfully predicted by a population balance model (PBM)-based model. Finally, the effect of H2O on the ultrafine PM formation at the start of oxy-fuel char combustion is examined. The apparent increment of Si-based ultrafine particles caused by H2O addition in oxy-fuel conditions is theoretically interpreted.

1. INTRODUCTION With the crisis of global warming, high demands have been put forward on the emission of greenhouse gas CO2 from coal combustion processes.1 Oxy-fuel coal combustion, a concept of using pure O2 and partially recycled flue gas as the oxidant, is becoming one of the most promising technologies to control CO2 in coal-fired power plants.2,3 By replacement of N2 with CO2, a high concentration of CO2 can be achieved in the flue gas, which conduces to the following CO2 sequestration and capture. Because of the ambient changes, emphasis should be on two issues when retrofitting oxy-fuel power stations. One is the generation efficiency. As a result of the different physical properties between CO2 and N2, e.g., heat capacity and conductivity, it was proposed that similar characteristics of heat transfer can be obtained only when the mole fraction of O2 increases to around 30% in the oxy mode.4 The other is the concomitant changes of pollutant formation, including NOx, SOx, and particulate matter (PM). Notably, the PM formation mechanism in coal combustion not only determines particle emissions that cause severe atmospheric pollution but also relates operational concerns (ash deposition, fouling, and slagging) in the boiler. Therefore, comprehensive understandings on different properties of fine PM formation in oxy-fuel conditions are of significance to expand this advanced coal combustion technology. Bench-scale experiments in the once-through drop-tube furnace (DTF) have found that the yield of submicron particles (PM1) in oxy-fuel conditions is smaller than that in air conditions when the oxygen content was kept at 21%.5−7 Nevertheless, the increase of the oxygen content in oxy-fuel conditions significantly enhances the formation of submicron particles, approaching or surpassing that in air conditions at ∼30% O2.5−7 Differences were mostly attributed to the effects of ambient parameters on the coal combustion temperature and © XXXX American Chemical Society

the reaction of MOn + CO = MOn−1 + CO2 (M refers to Si, Al, Fe, etc.) governing the contribution of refractory minerals to PM1.8 However, in comparison to high-rank bituminous, lowrank lignite tends to be abundant in volatile minerals, typically alkali and alkaline earth metals (AAEM).9 The contribution of volatile minerals to PM1, especially ultrafine PM, becomes dominant via a different volatilization−nucleation−coagulation mechanism.10,11 Thus, further studies are still needed to clarify how the ambient conditions affect this process to predict the fine PM formation in oxy-fuel lignite combustion more reasonably. Traditional analyses on fine PM in coal combustion were centered on particle samples collected at coal burnouts by mechanical methods, e.g., dilution probe.5−7 The lack of process details and potential interference from intrusive sampling hinder the exact description of mineral particle formation. With the development of flame syntheses and soot studies,12,13 in situ diagnostics and in-flame sampling were established as powerful tools to detect elements, radicals, and aerosols in high-temperature flame fields with high resolution and low interference. Successful measurements have been performed on the dynamic behaviors of Na in the conventional coal combustion,10 and an extension to oxy-fuel conditions will help us well understand the atmosphere effect on the gas-toparticle conversion of minerals. In comparison to conventional conditions, the humidity in practical oxy-fuel conditions can be 2−3 times higher as a result Special Issue: 6th Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies Received: October 17, 2017 Revised: November 26, 2017 Published: November 28, 2017 A

DOI: 10.1021/acs.energyfuels.7b03189 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 1. Setup of the experimental system (A, PS-LIBS; B, dilution sampling; and C, thermophoresis sampling).

of the flue gas recirculation.14 It was reported that the increase of the H2O content promoted the amount of submicron particles during oxy-fuel combustion, which was attributed to the enhanced Si release from reactions between SiO2 and H2O/ H2.15,16 However, the reason seems to be more complex because H2O also influences the combustion process of coal particles. Partially replacing CO2 with H2O will probably cause changes in the combustion of coal particles,17 whose surface temperature determines the release of mineral precursors. Intensively, the multi-effects of H2O should be declared on the fine PM formation from the initial stage of oxy-fuel coal combustion. This work aims to investigate the incipient formation of ultrafine PM in the oxy-fuel lignite combustion. On the basis of an optically accessible Hencken flat-flame burner, systematical measurements of ultrafine PM formed in the early combustion stage were conducted, including mineral dynamics, particle morphology, particle component, and particle size distribution (PSD). Further, discussions were focused on the effect of CO2 and H2O on the incipient particle formation in oxy-fuel conditions.

Table 1. Operation Conditions of Different Cases (L/min) case 80% 80% 70% 60% 56%

N2−20% O2 CO2−20% O2 CO2−30% O2 CO2−30% O2−10% H2O CO2−30% O2−14% H2O

CO

O2

N2

3.9 5.3 5.1 1.4

8.9 9.7 12.8 14.8 15.1

21.0

CO2

CH4

18.7 15.9 17.4 17.4

1.4 1.9

device (ICCD) camera (Princeton Instruments PI-MAX4), and an operating computer. The wavelength of the laser was set as 532 nm at 10 Hz, and that of the collector was tuned to around 589 nm for Na diagnostics. For the PS-LIBS measurement, we selected a laser intensity of 8.8 mJ/pulse according to our previous work.10 2.3. Dilution Sampling. A two-stage N2 dilution sampler was used to collect particles for the PSD measurements, as shown in Figure 1B. Samples after collection by the probe were first separated by a PM1 cutter, and submicron particles were delivered into the scanning mobility particle sizer (SMPS, DMA model 3085 and CPC model 3776, TSI, Inc.). The dilution ratio was set as ∼90, and the measurement time for each run was fixed at 135 s.18 2.4. Thermophoresis Sampling. On the basis of the principle of thermophoresis force, ultrafine particles formed along the coal combustion were collected by a self-designed thermophoresis sampler,10 as shown in Figure 1C. Fast sampling can be completed with the aid of the high-speed gas cylinder assembled in the sampler, guaranteeing little disturbance to the flow field and good protection of the sampler units.10 Transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS) analyses were subsequently performed on samples on the grid to detect the particle morphologies as well as components. 2.5. Coal Properties. The proximate and low-temperature ash (LTA) analyses of the lignite and bituminous used in this work are shown in Table 2. Particularly, the Zhundong lignite owns a large amount of Na in minerals, and the high-ash-fusion (HAF) bituminous is typically abundant in Si- and Al-containing minerals. All coal particles were first dried and then screened to 65−74 μm before every combustion experiment.

2. EXPERIMENTAL SECTION 2.1. Hencken Burner. As shown in Figure 1, the combustion experiments were conducted in a flat-flame laminar burner termed as Hencken burner. Detailed information on the burner construction can be found in ref 10. For conventional cases, we used CO as the main fuel gas and a mixture of N2 and O2 as the oxidant gas. As for oxy-fuel cases, the oxidant gas was alternated by the mixture of CO2 and O2. In addition, we adopted a mixture of CO and CH4 in different ratios as the fuel gas to achieve dry and wet oxy-fuel conditions. A uniform hot ambience for coal combustion can be created by the product gas, and the temperature was set around 1500 K. Detailed operation conditions of different cases are listed in Table 1. The coal particle stream was injected from the center of the burner, with a feeding rate of 0.085 g/ min and an average speed of ∼1.5 m/s. 2.2. Phase-Selective Laser-Induced Breakdown Spectroscopy (PS-LIBS). The laser system is shown in Figure 1A, composed of an Nd:YAG laser generator (Solo PIV), an intensified charge-coupled B

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be attributed to a high-temperature reaction between solidphase minerals (mainly Si in this work) and post-formed CO as

Table 2. Proximate and LTA Analyses of the Zhundong Lignite and HAF Bituminous Zhundong lignite fixed carbon volatile matter ash HHV (MJ/kg) SiO2 Al2O3 Fe2O3 CaO MgO TiO2 SO3 K2O Na2O

SiO2 + CO = SiO + CO2

HAF bituminous

Proximate Analysis (wt %, Dry Basis) 63.54 30.58 5.88 21.89 LTA Analysis (wt %, Ash Basis) 14.5 3.96 4.85 39.65 3.48 0.36 25.97 0.69 7.48

(4)

The gas-phase product, SiO, will then be oxidized to form fine SiO2 particles. The feasibility of this has been validated by experimental measurements21 and acknowledged by subsequent studies on ultrafine/submicron PM formation in different coal combustion conditions.22,23 In the existence of H2O, the following reactions may also happen, promoting the formation of the gas-phase Si compound.16

55.52 24.11 20.37 25.26 56.83 26.17 6.98 2.68 0.7 1.2 2.1 1.02 0.2

(1)

Here, φ stands for the moles of O2 consumed per mole of C, representing the extent of char oxidation. As a result of a switch from the conventional condition to the dry/wet oxy-fuel condition, the following reactions are added: C + CO2 = 2CO

(2)

C + H 2O = CO + H 2

(3)

The kinetic as well as thermodynamic data for reactions 2 and 3 are obtained from refs 17 and 20. All reaction rates obey the Arrhenius law, with parameters tabulated in Table 3. For the Table 3. Reaction Parameters in Char Combustion Simulations (Arrhenius Form A exp(−E/RT)[gas]n, with Data Obtained from the Literature17,19,20) reaction

A (mol1−n m−3n−2 s−1)

E (kJ/mol)

n

1 2 3

1.78 × 104 9.24 × 104 7.68 × 102

175.6 283 233

0.3 0.54 0.65

(5)

SiO2 + H 2 = SiO + H 2O

(6)

With reference to refs 21, 24, and 25, chemical equilibrium is hypothesized on the above reactions to calculate the partial pressure of products, with the equilibrium constant computed using the surface temperature of the char particle. It should be noted that, in reaction 6, the partial pressure of product H2O is assumed to equal that of product SiO. Whereas, the partial pressure of CO2 in reaction 4 is estimated with the average pressure between product CO2 (equal to product SiO) and surface CO2 of the char particle, considering the high CO2 concentration in oxy-fuel conditions. In reaction 5, we assume an instant decomposition of SixO2x−y(OH)2y to form SiO in high-temperature ambiences. 3.3. Formation of Ultrafine PM in the Early Lignite Combustion. PSDs of ultrafine PM are calculated with the model proposed in our previous work,18 which involves processes of mineral release (mainly Na and Si in the early stage of high-sodium lignite), decomposition, oxidation, surface reaction, and particle coagulation. Mineral precursors after release from coal particles will instantly decompose and then be oxidized, forming monomer particles to coagulate with each other. In this work, we assume the same amount of Na (33% water soluble) and Si (100% organically bonded) released during coal devolatilization in both conventional and oxy-fuel conditions.18 Moreover, the particle coagulation process is simulated on the basis of a sectional population balance model (PBM). In addition, the volatile combustion in different conditions is simplified as C2H2 combustion and simulated by the computational fluid dynamics (CFD) calculation, which provides profiles of the O2 concentration as boundary conditions for PM simulation.

3. THEORETICAL METHODOLOGY 3.1. Surface Temperature of the Burning Char Particle. The char combustion process is assumed as a quasistatic process in our calculation, with the heat generation from the reaction equal to the heat loss from particle convection and radiation at each combustion time. For conventional conditions, the reaction used in this work is shown as follows:19 C + φO2 = 2(1 − φ)CO + 2(φ − 0.5)CO2

xSiO2 + y H 2O = SixO2x − y (OH)2y

4. RESULTS AND DISCUSSION 4.1. Ultrafine PM Properties and Na Dynamic Behaviors. Figure 2 illustrates the morphologies and components of ultrafine PM from combustion of different coals in the 80% N2−20% O2 and 70% CO2−30% O2 conditions, respectively. Samples were collected at a position corresponding to the residence time of ∼10 ms, which is just in the stage of coal devolatilization. Component information was obtained by scanning the effective area of the TEM grid with several particles in the view field. It can be seen that ultrafine particles initially generated from lignite mostly exist in spherical or short-chain shapes. While for bituminous, long-chain agglomerates constitute the majority of particles emerging in the view field. The differences of particle morphology can partly result from the different chemical structures between lignite and bituminous. In general, bituminous often possesses a high C−H ratio and consists of big clusters abundant in aromatic

mass transfer process, transport of all gas species inside the char particles follows the Knudsen diffusion, while that in the environmental space follows molecular diffusion. At every time step, the calculated surface temperature of the char particle determines a carbon consumption rate. In the next time step, the diameter of the char particle is updated on the basis of the consumption rate, and then a new surface temperature will be computed. The initial diameter of the char particle is set as 70 μm based on experiments. 3.2. Formation of the Gas-Phase Si Compound during Char Combustion. As proposed by Quann et al.,8 the formation of submicron particles during coal combustion can C

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Figure 2. Morphologies and components of ultrafine PM in the early combustion stage.

hydrocarbons. Comparatively, lignite owns a large quantity of aliphatic hydrocarbons in the organic structure, mainly acting as side chains.26 As reported in the work of Fletcher et al.,26 bituminous yields a larger amount of tar in the total volatiles than lignite in the coal devolatilization stage. Thus, there exists a high tendency of soot formation from bituminous combustion because coal-derived soot mainly comes from the secondary reaction of tar under high temperatures.27 These soot particles probably contribute to the long-chain agglomerates observed in the case of oxy-fuel bituminous combustion, as shown in Figure 2. Besides, the existence of the Fe element in the agglomerates further enhances their soot possibility because Fe ions tend to interact with carbonaceous matter during the formation of soot in flames.28 As for other elements, Si is detected in particles for all cases because it is one of the fundamental elements in coal minerals. However, Na is only found in particles for cases of lignite combustion, probably as a result of its abundance in lowrank coals. Noteworthy, the ambient alteration from 80% N2− 20% O2 to 70% CO2−30% O2 does not influence the existence of Na in ultrafine particles formed in the early devolatilization stage. In addition, very few amounts of Mg and Al can be detected in particles, and Mo comes from the background material of the grid skeleton with no relevance to particle samples. To reveal the dynamic behaviors of Na, as found in incipient particles from Zhundong lignite, we particularly conducted the PS-LIBS diagnostics on particle-phase Na in both conventional and oxy-fuel lignite combustion. Results are illustrated in Figure 3, and all intensities of signals have been normalized by the maximum value in each case. To compare to coal particle behaviors, thermogravimetric analyses (TGAs) on Zhundong lignite in ambiences of N2 and CO2 were also performed, as displayed in the inset of Figure 3. TGA curves indicate that, under a heating rate of 10 K/min, the mass loss of lignite shows similar trends between N2 and CO2 ambience in temperatures lower than 990 K. Nevertheless, a sharp mass decrease happens in CO2 ambience when the temperature increases up to 990 K, which is probably caused by the gasification reaction of carbon by CO2. Eventually, the mass loss percentage of coal particles is only about 35% in N2 but becomes as high as 90% in CO2. It can be reasonably deduced that the influence of the gasification

Figure 3. Particle-phase Na in the early stage of lignite combustion.

reaction will be enhanced under an ultrahigh heating rate of 105 K/s in flat-flame experiments as a result of a fast reach to high temperature for coal particles. However, the mass loss differences of lignite in conventional and oxy-fuel conditions make no big difference on the dynamic behaviors of Na during coal devolatilization in 1500 K. As shown in Figure 3, whether replacing N2 with CO2 (from 80% N2 to 80% CO2) or changing the CO2 content (from 80 to 70% CO2), particlephase Na always increases around 10 ms and presents similar formation trends, implying similar behaviors of Na release and gas-to-particle conversion between conventional and dry oxyfuel conditions. Specifically, in comparison to the cases of 80% N2−20% O2 and 70% CO2−30% O2, the slope of increasing curve between 7 and 13 ms in the case of 80% CO2−20% O2 shows a slight decrease. It is probably caused by the slowdown of Na release from coal particles with a low combustion temperature in the 80% CO2 case. Further, as pointed in our previous work,10 the characteristic time when particle-phase Na starts to increase implies an essential physical connection among coal devolatilization, Na gas-to-particle conversion, and ultrafine PM formation. Hence, an identical mechanism of volatilization−nucleation−coagulation fit for Na-based ultrafine PM formation can be inferred in both N2−O2 and CO2−O2 ambiences. D

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Energy & Fuels 4.2. Number PSD of Ultrafine PM. Figure 4 displays the number PSDs of ultrafine particles sampled at the residence

PSD data is performed for a better comparison to simulations because all particles are assumed as spheres in our PBM-based modeling.18 Calculated and corrected PSDs are shown in solid and dashed lines in Figure 4, respectively. It can be seen that simulation results present a good match with corrected data on the distribution shape and peak positions. The peaks of simulated PSDs are located at 21.41 nm for the 70% CO2−30% O2 case and 24.18 nm for the 80% N2−20% O2 case, which well-predicted the differences between 23.93 nm (CO2−O2) and 26.30 nm (N2−O2) of the corrected data. Notably, the satisfied simulations further validate that the formation of ultrafine PM in the early stage of high-sodium lignite combustion follows a common mechanism of volatilization− nucleation−coagulation, even though the combustion condition changes from N2−O2 to CO2−O2. 4.3. Ultrafine PM Formation in Wet Oxy-fuel Conditions. Analyses in sections 4.1 and 4.2 are mainly centered on ultrafine PM formed in dry oxy-fuel conditions (CO2−O2 system). In this section, the effect of steam in wet oxy-fuel conditions (CO2−O2−H2O system) on the incipient formation of ultrafine particles is intensively discussed. Particularly, we conducted three cases in the Hencken burner, with different H2O contents of 0, 10, and 14% and collected particle samples at the residence time of ∼20 ms, corresponding to the start of the char combustion stage in 1500 K ambience. Figure 5 illustrates the TEM pictures of samples, where we can find more ultrafine particles emerge with an increasing H2O substitution for CO2 in ambience. Further, total particles and particles less than 30 nm were counted in the same view field of different cases. As shown in Figure 5, the particle increment is probably due to the increase of small nascent particles, which are mainly composed of Si based on the EDS result, while Cu comes from the content of the background. It indicates that the existence of H2O may enhance the formation of Si-based ultrafine particles at the start of char combustion. Previous studies have found an increased ultimate yield of submicron particles from coal when H2O was introduced in dry oxy-fuel (CO2−O2) cases.15,16 An explanation was attributed to additional reactions between SiO2 and

Figure 4. Experimental and calculation results of number PSDs of ultrafine PM (exp., experimental data, cor., corrected data, and cal., calculation results).

time of ∼40 ms in the 70% CO2−30% O2 and 80% N2−20% O2 conditions. Specifically, the distribution of particles in CO2−O2 ambience was measured in this work, and that in N2− O2 ambience referred to our published work for comparison.18 It indicates that the PSDs both exhibit unimodal, with the peak in the oxy-fuel condition standing at a slightly smaller size than that in the conventional condition. The distribution peak in the 70% CO2−30% O2 case is located at the aerodynamic diameter of 33.34 nm, and that in the 80% N2−20% O2 case is located at the aerodynamic diameter of 38.46 nm.18 The maximum value of particle number concentration in both cases stays around 109 cm−3. On the basis of the aforementioned analysis, we particularly apply the same formation mechanism of ultrafine PM proposed for the early stage of conventional lignite combustion18 to simulate the number PSD in the dry oxy-fuel case. In addition, some environmental parameters for PM formation, e.g., oxygen concentration profile along volatile combustion, are obtained from external CFD calculations. Meanwhile, an agglomerate correction on raw experimental

Figure 5. TEM−EDS results of ultrafine PM in oxy-fuel cases with different H2O contents. E

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promoting the formation of Si-based ultrafine particles, as experimentally observed in Figure 5. Above deductions are further validated by a statistical source analysis of SiO in our calculations. In the range of experimental settings, Figure 7 illustrates the contributions to SiO formation

H2O/H2 that improve the vaporization of SiO2 and subsequent formation of ultrafine particles.16 However, as a result of a faster gasification rate of carbon with H2O than CO2,17 char particles may possess a low combustion temperature when CO2 is partly replaced by H2O in oxy-fuel conditions. The decrease of the char particle temperature will slow down all reactions related to the vaporization of SiO2, both thermodynamically and kinetically, inhibiting the formation of ultrafine particles. To clarify the detailed effect of steam on char combustion as well as particle formation, a simulation on the surface temperature of char combustion is first conducted. Six cases are chosen, with three in N2−O2 ambiences and the others in CO2−O2 ambiences. The content of O2 is kept at 20% in the conventional condition and 30% in the oxy-fuel condition, while that of H2O is set as 0, 10, and 50% for different cases in each ambience. As illustrated in the inset of Figure 6, our

Figure 7. Contribution of different reactions to the SiO formation at the start of char combustion.

from different reactions at the first time step of calculation, which corresponds to the maximum SiO concentration on char surfaces. First, despite similar combustion temperatures of the char particle in 80% N2−20% O2 and 70% CO2−30% O2 conditions, the SiO surface concentration in the conventional case is about 1 order of magnitude larger than that in the oxyfuel case. The reason is that reaction 4 is highly suppressed in the CO2-abundant ambience by the shift of chemical balance. As a result of reduced SiO from reaction 4 in CO2−O2 ambience, SiO formed from reactions 5 and 6 becomes dominant after H2O is introduced into the atmosphere. On the basis of our calculation, the SiO yield from reactions 5 and 6 takes up 68.7% of the total SiO formed at the start of char combustion in the 56% CO2−30% O2−14% H2O condition, while only 10.5% in the 66% N2−20% O2−14% H2O condition. Even in the case of 5% H2O, this fraction can be as large as 57.6% in the 65% CO2−30% O2−5% H2O condition. That is to say, the increment of SiO as a result of the H2O addition in N2−O2 ambience is too limited to compensate for the reduction caused by the decreasing combustion temperature, whereas that in CO2−O2 ambience dominates the SiO generation and further enhances the formation of Si-based ultrafine particles.

Figure 6. Effect of the H2O content on the combustion temperature and SiO concentration on the char surface.

calculation well predicts the reported similar surface temperatures of ∼2000 K for 70 μm char particles combusted in 80% N2−20% O2 and 70% CO2−30% O2 conditions.29 Moreover, it indicates that the maximum surface temperature decreases dramatically from 2012 to 1846 K when the H2O content in N2−O2 ambience increases from 0 to 50%. Likewise, a decrease of 101 K from 2036 to 1935 K occurs in CO2−O2 ambience under the same addition of H 2 O. Further, the SiO concentration on the char surface was computed for different cases and, hereinafter, taken as a ruler evaluating the ability of ultrafine particle formation from char combustion. Figure 6 displays the calculated SiO mole fraction on the char surface, XSiO_surface, as a function of the residence time of coal particles. It is demonstrated that the increase of H2O in N2−O2 ambience significantly decreases the SiO concentration at the start of char combustion, which mainly results from the reduction of the surface temperature during char combustion. Specifically, initial SiO on char surfaces decreases by an order of magnitude from 13.3 ppm in the 80% N2−20% O2−0% H2O condition to 1.8 ppm in the 30% N2−20% O2−50% H2O condition. However, the reverse situation happens when it comes to CO2−O2 ambience, in which the increment of H2O improves the surface concentration of SiO. Although the surface temperature of char particles also decreases, the initial SiO concentration in CO2−O2 ambience increases from 0.5 to 1.6 ppm as the H2O content increases from 0 to 50%. Reasonably, it can be inferred that the enhancement of SiO by the H2O addition probably prevails over the inhibition caused by the temperature decrease in oxy-fuel conditions, eventually

5. CONCLUSION In this paper, effects of oxy-fuel conditions on the incipient formation of ultrafine PM during high-sodium lignite combustion are examined. Different from typical bituminous, ultrafine particles formed in the early stage of oxy-fuel lignite combustion are less soot-like. PS-LIBS diagnostics and EDS analysis both verify that the switch from N2−O2 to CO2−O2 has a limited mechanistic impact on the dynamic behaviors of Na and its existence in initially formed ultrafine particles, despite the distinctly different mass loss of coal in N2 and CO2 hot ambiences. Unimodal PSDs of ultrafine PM collected at ∼40 ms are obtained, with peaks located at 33.34 nm in the 70% CO2−30% O2 case and 38.46 nm in the 80% N2−20% O2 case, respectively. Moreover, good PSD predictions by our PBMbased model interpret that the distribution difference probably F

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(18) Gao, Q.; Li, S.; Yang, M.; Biswas, P.; Yao, Q. Proc. Combust. Inst. 2017, 36 (2), 2083−2090. (19) Graham, K. A. Submicron ash formation and interaction with sulfur oxides during pulverized coal combustion. Ph.D. Dissertation, Massachusetts Institute of Technology (MIT), Cambridge, MA, 1991. (20) Smith, K. L.; Smoot, L. D.; Fletcher, T. H.; Pugmire, R. J. Devolatilization Rate Processes and Products. The Structure and Reaction Processes of Coal; Springer: Boston, MA, 1994; pp 209−323, DOI: 10.1007/978-1-4899-1322-7_5. (21) Senior, C. L.; Panagiotou, T.; Sarofim, A. F.; Helble, J. J. Formation of ultrafine particulate matter from pulverized coal combustion. Proceedings of the 219th American Chemical Society National Conference; San Francisco, CA, March 26−30, 2000; pp U671−U672. (22) Zhang, L.; Ninomiya, Y.; Yamashita, T. Fuel 2006, 85 (10−11), 1446−1457. (23) Sheng, C.; Lu, Y.; Gao, X.; Yao, H. Energy Fuels 2007, 21 (2), 435−440. (24) Tso, S. T.; Pask, J. A. J. Am. Ceram. Soc. 1982, 65 (9), 457−460. (25) Kirkorian, O. H. Thermodynamics of the silica-steam system. Proceedings of the Symposium on Engineering with Nuclear Explosives; Las Vegas, NV, Jan 14−16, 1970; CONF-700101(Vol. 1). (26) Fletcher, T. H.; Kerstein, A. R.; Pugmire, R. J.; Solum, M. S.; Grant, D. M. Energy Fuels 1992, 6 (4), 414−431. (27) Fletcher, T. H.; Ma, J.; Rigby, J. R.; Brown, A. L.; Webb, B. W. Prog. Energy Combust. Sci. 1997, 23 (3), 283−301. (28) Zhang, J.; Megaridis, C. M. Symp. (Int.) Combust., [Proc.] 1994, 25 (1), 593−600. (29) Maffei, T.; Khatami, R.; Pierucci, S.; Faravelli, T.; Ranzi, E.; Levendis, Y. A. Combust. Flame 2013, 160 (11), 2559−2572.

comes from the precursor sources (e.g., oxidation process) but not the particle coagulation. Further, it can be reasonably foreseen that the difference of final ultrafine PM yields, resulting from different vaporization amounts of refractory minerals between conventional and oxy-fuel conditions, will be suppressed by the dominant contribution of volatile minerals in high-sodium lignite combustion. Incipient formation of ultrafine PM is sensitive to the H2O content in oxy-fuel conditions. Introducing H2O into oxy-fuel ambience enhances the formation of nascent Si-based particles from exactly the start of char combustion. Although the combustion temperature of char particles decreases as a result of the H2O addition, the SiO increment from reactions with H2O/H2 takes up such a large portion in oxy-fuel conditions that prevails over the SiO reduction caused by the temperature decrease.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86-10-62773384. E-mail: [email protected]. ORCID

Shuiqing Li: 0000-0001-6278-5956 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is mainly funded by the National Natural Science Foundation of China (NSFC−NSF Joint Program 51561125001) and the National Key Research and Development Program of China (Grant 2016YFC0203705). Special thanks are due to Prof. Reinhold Kneer at RWTH Aachen University, Aachen, Germany, for useful discussion.

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DOI: 10.1021/acs.energyfuels.7b03189 Energy Fuels XXXX, XXX, XXX−XXX