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Ultra-fine particulate matters (PMs) formation during air and oxy-coal combustion: Kinetics study
Applied Energy 218 (2018) 46–53
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Ultra-fine particulate matters (PMs) formation during air and oxy-coal combustion: Kinetics study Yanqing Niu, Bokang Yan, Siqi Liu, Yang Liang, Ning Dong, Shi'en Hui
T
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State Key Laboratory of Multiphase Flow in Power Engineering, Department of Thermal Engineering, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China
H I G H L I G H T S PMs formation during coal char combustion is studied using kinetics model. • Ultra-fine PMs formed in oxy-coal atmosphere have fewer number but larger size. • Ultra-fine PMs show increasing number density and decreasing size with increasing FGR. • Ultra-fine combustion advantages PMs removal. • Oxy-coal • Elevated Flue gas recirculation ratio disadvantages PMs removal.
A R T I C L E I N F O
A B S T R A C T
Keywords: Particulate matters (PMs) Flue gas recirculation (FGR) Coal char Combustion Kinetics
Although both air (21O2/N2) and oxy-coal (27O2/CO2) combustion are widely adopted in pulverized coal (PC) fired power plants, the formation mechanisms of ultra-fine PMs during PC char combustion under both atmospheres with various flue gas recirculation (FGR) ratios are still unclear. Moreover, conventional experimental measurement devices cannot provide detailed information on the formation and evolution of the size-number of ultra-fine PMs. Therefore, the formation of ultra-fine PMs during PC char combustion under both atmopspheres with and without FGR are studied by a self-developed Char Burning and Particulate Matters Kinetics model (CBPMK). The PC char shows similar burning temperature and thus ash vaporization rate under both atmospheres, whereas the vaporization amount under oxy-coal combustion atmosphere is lower than that under air combustion atmosphere due to the shortened burnout time caused by CO2 gasification reaction under the former. Consequently, during nucleation and condensation stages (before successive coalescence), both the particle size and number under air combustion atmosphere are higher than those under oxy-coal combustion atmosphere. However, after coalescence, the final particle shows fewer but larger size under oxy-coal combustion atmosphere due to the higher cohesion factor between the smaller sized nucleation particles, which improves particle collision and coalescence. Meanwhile, both mean size and number density of the nucleation particles decrease with increased FGR ratio under both air and oxy-coal combustion atmospheres, however, after coalescence the final PMs show increasing number density and decreasing size. As results, oxy-coal combustion advantages PMs removal through an ash collector, but elevated FGR ratio disadvantages PMs removal. Oxy-coal combustion with low FGR ratio should be recommended in PC thermal and power plants.
1. Introduction Together with sulfur oxide [1,2] and nitrogen oxide [3–6], particulate matters (PMs) emitted from pulverized coal (PC) combustion power plants play crucial role during the formation of haze and serious environmental pollution [5,7–11]. According to the most recent national air pollution emission standard for thermal power plants in China, PM emissions from coal power plant are limited to 30 mg/Nm3,
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and in some key regions they are restricted to 20 mg/Nm3 [12] and suggested to strive to 10 mg/Nm3 [13] or 5 mg/Nm3 in some provincial standards. The PMs capture and removal in flue gas is never 100% efficient during coal combustion, particularly of ultra-fine ash particles (smaller than 1.0 μm in aerodynamic diameter), which constitute most of the particles that discharge into atmosphere directly [14] or accumulate in the furnace through flue gas recirculation (FGR) and result in an increase in number density due to the failure of conventional filters
Corresponding author. E-mail address: [email protected] (S. Hui).
https://doi.org/10.1016/j.apenergy.2018.02.164 Received 19 December 2017; Received in revised form 7 February 2018; Accepted 24 February 2018 Available online 07 March 2018 0306-2619/ © 2018 Elsevier Ltd. All rights reserved.
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Nomenclature
Greek alphabet
Symbols
αc ρ η, η′ θ ν σ
d, d′ D, D′ f FGR I kB K Kn m MOn MOn−1 n N q P PC PMs r R S Sg T t Y ΔGv V
diameter (cm) diffusion coefficient (cm2/s) number fraction flue gas recirculation nucleation rate (#/m3/s) Boltzmann constant (1.38 × 10−23 J/K) cohesion factor (cm3/s)reaction equilibrium constant Knudsen number mass (g) metal oxides metal suboxides and metals number of mineral vapor molecule condensed on specified sized particle (#/m3) inclusion number in one coal char particle; or particle number concentration (#/m3) burning rate (g/cm2/s) pressure (atm, Pa) pulverized coal particulate matters radius (cm) ideal gas constant (8.314 J/mol/K) saturation degree of the vapors internal specific surface area (cm2/g C) temperature (K) coalescence time (s) ash content change in Gibbs' free energy for droplet formation per unit volume vaporization rate (mol/s)
condensation coefficient mass density (g/cm3) effectiveness factor porosity molecular volume (cm3) surface tension of liquid
Subscript a c cc CO CO2 eff eq g H2O i, j int MOn−1 MOn O2 s,l tot
ash carbon char core carbon monoxide carbon dioxide effective equilibrium gaseous steam serial numbers of different sized particles intrinsic kinetics metal sub-oxide or metal metal oxide or inclusion molecular oxygen solid, liquid total
Superscript eq s, s′
equilibrium surface
temperature drop tube furnace, where the synthetic chars with SiO2 inclusions were burned at 1873 K. They found that H2O in combustion atmosphere significantly enhanced the vaporization of SiO2 and considerably increased the yield of the ultra-fine PM, and the char burning temperature and gas properties surrounding the mineral inclusions seemed to be the primary influencing factors on SiO2 vaporization. In addition, low rank coals, which generally cause high combustion temperature, and a locally reducing atmosphere, which improves mineral reduction and subsequent vaporization in the char matrix, can enhance the formation of ultra-fine PMs [21,26]. In recent years, respecting of CO2 capture and storage (CCS) due to the more and more attention on environment pollution, oxy-coal combustion has been developed rapidly [1,27–29]. Sheng et al. [29] studied the impact of O2/CO2 combustion on ash particle formation in a drop tube furnace, and found that O2/CO2 combustion significantly affected the size distribution of submicron particles, O2/CO2 combustion at the same oxygen concentration shifted the size of the submicron mode center to smaller size and decreased the yield of the submicron particles in comparisons with air combustion. Under O2/CO2 atmospheres, the char burning temperature is lower than that under O2/N2 atmosphere with the same O2 content due to endothermic gasification reaction and low diffusion of O2 in CO2 [24,30], as a result, the number density of the ultra-fine PMs is higher, and the particle size is larger [22,31]. In addition, Morris et al. [10] studied the effects of various cleanup options prior to FGR on ash aerosol formation in a 37 kW down-fired pilot-scale combustor and found that the ultra-fine particle concentrations increased in the combustor despite flue gas treatment with fabric filters. The experiment results are consistent with the kinetics research using CBPMK, indicating that FGR caused a decrease in PMs size with increasing number density [9].
in removing the particles [10]. The emitted PMs from coal-fired furnace may be enriched with trace toxic compounds, which not only cause atmospheric haze but also penetrate into the lungs, consequently causing a number of diseases [15,16]; the accumulated PMs in furnace accelerate the abrasion of heating-surface and deposition formation on its surface, resulting in reduced boiler efficiency and operational safety [17,18]. Respecting of the formation of ultra-fine PMs during coal/char combustion, numerous experiments and kinetics modeling researches on the forming mechanisms and the effects of combustion temperature and atmospheres as well as coal types and FGR have been well documented. By an experiment conducted in a lab-scale drop-tube furnace, Jiao et al. [19] found that most ultra-fine PMs exhibited fractal structures due to the homogeneous nucleation of metallic vapors and/or their heterogeneous condensation on preexisting fine mineral grains during coal combustion with FGR. During coal char combustion, part of inorganic components (Si, Al, Fe, Ca, Na, etc.) in char matrix undergoes successive vaporization, homogeneous nucleation, heterogeneous condensation, coagulation, and coalescence to form ultra-fine PMs [7,9,20–22]. On basis of the abovementioned formation mechanisms of ultra-fine PMs, we developed a Char Burning and Particulate Matters Kinetics model (CBPMK) [9,22]. The modeling results showed that combustion temperature had a dominating positive effect on mineral vaporization, condensation, coagulation and coalescence, and subsequent ultra-fine PMs formation [21–23]. Consequently, the factors including high oxygen content, low ash content, and small sized char particles, which elevated the local char burning temperature [24], caused more and larger ultra-fine PMs [22,25]. Also, Xu et al. [7] studied the vaporization behavior of silica in wet recycle oxy-coal combustion conditions by modeling and experiments conducted in a high 47
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atmospheres with FGR is scarce, and the root mechanisms causing the differences of ultra-fine PMs formed under both atmospheres with and without FGR are unclear. Furthermore, during the formation of ultrafine PMs, existing experimental measurement instruments fail to
Although numerous studies on the formation of ultra-fine PMs during PC char combustion under traditional air atmosphere and oxycoal atmosphere have been well-documented, quantitatively research on ultra-fine PMs formation during PC char combustion under both
Fig. 1. Schematic flowchart of the CBPMK model [9].
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(qO2,int + qH2O,int + qCO2,int) denotes the external reaction rate after considering the embezzlements of ash with a fraction of Ya,cc and pores with a porosity of θc,cc on available external surface, whereas (ηρc,ccSgdcc/6)(qO2,int + qH2O,int + qCO2,int) represents the internal reaction rate within the char matrix. More detailed descriptions on the model can refer to previous paper [24].
provide inside information on the formation processes including the inclusion vaporization within char matrix, subsequent homogeneous nucleation, heterogeneous condensation, and coalescence. Therefore, in this study, to provide guidelines for practical operation, particular in the selection of combustion mode (conventional air combustion or oxy-coal combustion) and FGR ratio, the comparative studies of ultra-fine PM formation during PC char combustion under oxy-coal (27O2/CO2) and air atmospheres (21O2/N2) with various FGR ratios are conducted using the self-developed kinetics model CBPMK [9,22]. Moreover, the root causes for the differences of ultra-fine PMs formed under both combustion atmospheres with and without FGR are revealed by detailed comparisons on the time-dependent molecule number and size during vaporization, nucleation-condensation, and coalescence stages.
2.2. Ultra-fine PM formation kinetics model During coal char combustion, a small part of volatile alkali and alkali-earth elements (K, Na, Mg, and Ca) and refractory elements (Si, Al, and Fe) first vaporize and diffuse out of the char matrix in the form of suboxides or reduced metals. And then, they undergo re-oxidation, nucleation, condensation, coagulation, and coalescence, forming the ultra-fine ash particles [9,17,21,32,33].
2. Model descriptions (a) Vaporization submodel. As a primary hypothesis, all mineral inclusions are uniformly distributed as oxides throughout the char matrix and vaporized as suboxides or fully reduced metals according to R1 [34]. Once diffusing out of the char matrix, the suboxides and reduced metals are fastly reoxidized into metal oxides according to R2. As a simplified calculation, only SiO2 as a representative of various mineral inclusions in coal char is considered at present.
As illustrated in Fig. 1, the CBPMK model consists of char burning intrinsic kinetics model and PM formation kinetics model [9,22]. The char burning model (Submodel A) computes transient char properties (porosity, available surface area, density, and mass, etc.), ash distribution (ash film, ash dilution, and ash vaporization), char core atmosphere (CO/CO2 ratio and surrounding reacting gases), and char burning temperature during the burning lifespan of char particle. The PM formation model, which inherits above-mentioned temporal parameters from the char burning model and used to compute the formation, growth, and number-size distribution of the ultra-fine PMs, consists of mineral vaporization (Submodel B), homogeneous nucleation (Submodel C), heterogeneous condensation (Submodel D), and coalescence (Submodel E) four sub-models. The vaporization submodel (Submodel B) inherits the above-mentioned transient output parameters from the char kinetics model (Submodel B) and calculates the mineral vaporization rate and vaporized fraction. Once the saturation of the mineral vapors, S, is greater than 1.0, the vapor undergoes homogenous nucleation and generates solid particles (Submodel C). Then, the remaining vapors will condense on the particles, resulting in particle growth (Submodel D). Homogeneous nucleation and heterogeneous condensation occur simultaneously. When the char is burned out, both nucleation and condensation stop due to the termination of mineral vaporization. Finally, all existing particles undergo continuous coalescence in the flue gas. In the modeling begin, the input parameters include char particle size d, density ρ, porosity θ, ash content Y and inclusion content, combustion temperature T and gas properties (O2/CO2/N2), etc. As results, the timedependent output parameters such as char burning temperature, carbon conversion ratio, inclusion vaporization rate and ratio, ultra-fine PM size and number density during nucleation, condensation, and coalescence stages are obtained.
6
)(qO2 ,int + qH2O,int + qCO2 ,int )
MOn − 1(g) + O2 → MOn (s,l)
(R2)
V = η′NMOn 4πrMOn Deff
eq PMO n−1
RT
(2)
where η′ named as effectiveness factor is the ratio of the total vaporization rate of inclusions to the vaporization rate of NMOn isolated inclusions with a radius of rMOn in the char matrix; PMOn−1eq is partial pressure of the suboxide or reduced metal on the inclusion surface, which is calculated by Eq. (3) according to reaction (R1). eq PMO = K eq n−1
PCO PCO2
(3)
where Keq is the reaction equilibrium constant, which can be calculated according to the NASA thermodynamic database or the NSRDS-JANAF Thermochemical Tables [35]. (b) Homogeneous nucleation submodel. On basis of classical nucleation theory [36–38], the rate of nucleation, I, is calculated by Eq. (4).
I=
In the model, the initial char particle assumed to be spherical is ideally divided into plenty of equally spaced concentric shells during combustion. As an innovation point, besides of less amount of ash vaporization, some fraction of the ash components liberated from each burned shell is used to thicken the external ash film, and the rest is assumed to penetrate into the char core where ash and pore are distributed into the carbon matrix uniformly. Meanwhile, both oxidation and gasification reactions by CO2 and steam are comprehensively considered. The gross carbon conversion rate, qtot, is calculated according to Eq. (1).
ηρc,cc Sg d cc
(R1)
where MOn represents refractory oxides, and MOn−1 represents suboxides or reduced metals. Drawing lessons from gas diffusion in porous materials, the total mineral vaporization rate, V, is calculated according to Eq. (2).
2.1. Char burning intrinsic kinetics model
qtot = (1−Ya,cc−θc,cc +
MOn (s,l) + CO → MOn − 1 (g) + CO2
s 2σ 0.5v 2αC PMO MOn n
(2πm)0.5 (kB T )2
−16πσ 3 ⎞ exp ⎜⎛ 2⎟ ⎝ 3kB T ΔGv ⎠
(4)
s
where PMOn as the total partial pressure of the vapor molecules is the sum of the newly generated vapor molecules via vaporization-reoxidation reactions in local time step of numerical computation (as short as 10−6 s) and the remaining vapor molecules after nucleation-condensation during the previous time step. (c) Heterogeneous condensation submodel. Once the ultra-fine PMs are formed via nucleation or inherited from FGR, they will continue to grow through heterogeneous condensation and coagulation. According to the molecular gas dynamics theory, the rate of heterogeneous condensation of vapor molecule on the particle with a size of d′ is given by Eq. (5).
(1)
where η named as effectiveness factor denotes the internal reaction rate relative to the external burning rate. (1 − Ya,cc − θc,cc) 49
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2
s′
eq
corresponding coal properties analysis can refer to [40]. To access practical PC char combustion conditions in power station boilers, the PC char particle with a diameter of 45 μm is employed in the following kinetics modeling; The SiO2 inclusion size, combustion gas temperature in furnace, and furnace outlet temperature are assumed to be 16 μm, 1750 K, and 1150 K, respectively [9,37]; To maintain similar furnace temperature as that under air combustion atmosphere, the O2 content under oxy-coal combustion atmosphere is designed as 27 vol.% according to the subsequent modeling results. In addition, although FGR results in constant changes on the furnace gas compositions and temperature, these are ignored to provide a constant benchmark for kinetics modeling under different FGR conditions. Also, the break-up or fragmentation model is neglected.
⎧ πd′ (PMOn − PMOn ) , 2πmkB T ⎪ ⎪ 8πd′ 2 (P s′ − P eq )
K n≫ 1.0 ⎫ ⎪ ⎪ MOn MOn n= , K n≪ 1.0 ⎬ ⎨ 2πmkB T ⎪ ⎪ 2πd′D eq s′ ) − PMO ′ n (PMO 1 + Kn MO n n ⎪ , K n≈ 1.0 ⎪ kB T 1 + 1.71Kn + 1.333Kn2 ⎭ ⎩
(5)
where Knudsen number, Kn, is the ratio of the mean free path of vapor molecule to the nucleated particle; PMOs′ is the remaining vapor molecules pressure after a transient nucleation time step (10−6 s). The growth of particle size follows mass conservation. (d) Coalescence submodel. When the particles are formed through nucleation-condensation, they are assumed to pairwise collide in proportion to their number fractions first, and then the newly formed particles and the remaining particles that did not participate in the previous round of collisions pairwise collide again in the next time step according to their number fractions. The continued collisions cause increased particle size and decreased number density. The newly formed particle number concentrations, Ni,j, can be calculated by Eq. (6).
Ni,j =
3.1. Particle burning temperature and inclusion vaporization Fig. 2a shows the PC char combustion characteristics under both air and oxy-coal atmospheres. Due to the endothermicity of CO2 gasification reaction and low diffusion of O2 in CO2 [24,30,41], the char burning temperature under oxy-coal combustion atmosphere with the O2 content of 27.vol% is similar as that under air combustion atmosphere; However, the residence time is significantly shorter under oxyfuel combustion atmosphere because of the CO2 gasification reaction, which accelerates carbon conversion [30]. Fig. 2b shows the vaporization of inclusion minerals (taking SiO2 as an example) during PC char combustion under both air and oxy-coal atmospheres. Char combustion temperature exerts positive effects on the mineral vaporization rate and subsequent formation and growth of nano-particles under both O2/N2 and O2/CO2 atmospheres [22]. Thus, the ash vaporization rates are similar under both air and oxy-coal combustion atmospheres due to the similar char particle burning temperature (Fig. 2a). However, the vaporization amount or ratio under oxy-coal combustion atmosphere is lower than that under air combustion atmosphere due to the significant longer char burnout time in the latter.
Ni f j + Nj fi 1 + (Ni f j + Nj fi ) Ki,j t
(6)
where i and j denotes different sized particles, N and f denotes the particle number and number fraction during locating collision time step t. The coalescence time step is as short as 10−5 s in avoid of secondary coalescence that makes simulation calculation complex and may result in error. More detailed descriptions can refer to previous paper [9,22]. 3. Modeling results and discussions In our previous studies [22,24], the char burning characteristics (including time-dependent char burning temperature and carbon conversion rate) and ultra-fine PMs forming characteristics (including the vaporization rate and ratio of different inclusions as well as the transient number-size distribution of ultra-fine PMs during PC char combustion) predicted by the kinetic models were compared with the experimental data provided by Murphy and Shaddix from Sandia [39] and Neville et al. from MIT [21]. The well agreements between the kinetic modeling results and the experimental data promise the CBPMK model to predict the evolution of ultra-fine PMs during PC char combustion with or without FGR. In this study, Huangling lignite (a typical coal used in Chinese power plants), with an overall ash content of 13.6 wt.% and a SiO2 content of 42.3 wt.%, was used as a coal representative. The
3.2. Comparisons of ultra-PMs formation under air and oxy-coal combustion atmospheres Fig. 3 shows the particle mean size and number density of ultra-fine PMs generated after undergoing nucleation-condensation (before subsequent coalescence) and coalescence during PC char combustion under both air (21O2/N2) and oxy-coal (27O2/CO2) atmospheres with and without FGR. Before coalescence both the particle size and number under air combustion atmosphere are higher than those under oxy-coal combustion atmosphere. That is due to higher ash vaporization amount
a) Particle burning temperature
b) Ash vaporization
Fig. 2. Effect of air combustion (21O2/N2) and oxy-coal combustion (27O2/CO2) on particle combustion characteristics and ash vaporization.
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a) Particle mean size
b) Number density
Fig. 3. The particle mean size and number density of ultra-fine PMs after nucleation-condensation (before coalescence) and coalescence stages during PC char combustion under air (21O2/N2) and oxy-coal (27O2/CO2) atmospheres.
of ultra-fine PMs generated after undergoing nucleation-condensation (i.e., before coalescence) during PC char combustion under both air and oxy-coal atmospheres. At a certain FGR ratio, both particle number density and size under air combustion atmosphere are higher due to higher inclusion vaporization ratio and similar vaporization rate compared to those under oxy-coal combustion (Fig. 2). With increasing FGR ratio both particle size and number density decrease under both atmospheres due to dilution effect. This is consistent with the results from previous research for the same PC char particles with a diameter of 100 μm, and both quantities decrease with increasing FGR ratio [9]. Fig. 5 presents an explanation for the decreasing trends of both particle size and number density through detailed transient parameters comparisons during PC char combustion with FGR of 0.30 and without FGR under air atmosphere. As deviations of temperature and gas composition from FGR are ignored, the vaporization rate is the same for both FGRs. During the initial burning stage, compared to without FGR the nuclei during PC char combustion with an FGR of 0.30 are larger due to the dilution effect. FGR results in a low degree of vapor saturation (lower vapor concentration, magenta1 lines ④), and consequently larger nuclei (black lines ②, and negative values in the embedded sub-illustration) with fewer amounts (blue lines ③). However, although the nuclei show smaller sizes without FGR, they adsorb more vapor molecules through condensation (olive lines ⑤) due to high condensation rate on smaller nuclei (Eq. (5)), resulting in slightly larger particle sizes after undergoing condensation (red lines ①, and positive values in the embedded sub-illustration). These trends (more and larger particles without FGR) continue to carbon conversion ratio of approximate 0.4. Noted, during the very early char burning stage, due to significantly low vapor saturation with FGR the particles are obviously larger than those formed without FGR, but the number densities are so low that can be ignored. Although the accumulated nucleation particles during PC char combustion without FGR are slightly larger than those formed with FGR, higher number densities consume more vapor molecules through condensation (olive lines ⑤). When the carbon conversion ratio is above 0.4, the accumulated vapor concentration (magenta lines ④) during PC char combustion with FGR overpasses that without FGR, consequently, resulting in more and slightly smaller nuclei formation with FGR (black lines ②, and reduced negative values in the embedded sub-illustration). During condensation stage, the particles formed with FGR grow quickly and show slightly larger size (red lines ①, and negative values in the
in the air atmosphere (Fig. 2b), which improves particle formation through nucleation and growth through condensation [22]. After undergoing successive coalescence, the particle shows fewer amounts but larger size under oxy-coal combustion atmosphere. Smaller sized particle generated after undergoing nucleation and condensation stages under oxy-coal combustion possesses higher Knudsen number, Kn, and thus higher cohesion factor between each other, Ki.j (Eq. (6)), consequently improving particle collision and coalescence and resulting in fewer but larger particle. The results indicate that oxy-coal atmosphere advantages particle removal through an ash collector. Meanwhile, it can be seen that compared to without FGR, during PC char combustion with FGR the ultra-fine PMs sizes are smaller in both nucleation-condensation and coalescence stages, and the particle number density with FGR during nucleation-condensation stage is also lower than that without FGR; However, after undergoing coalescence, the final particle number density with FGR is higher compared to without FGR. Detailed but systematic disclosures on the effect of FGR on ultra-fine PMs formation during PC char combustion under air and oxy-coal atmospheres with various FGR ratios are performed as below. 3.3. Effect of FGR on ultra-PMs formation during air and oxy-coal combustion Fig. 4 shows the effect of FGR ratio on the number-size distribution
Fig. 4. Effect of FGR ratio on the number-size distribution of ultra-fine PMs generated after undergoing nucleation-condensation during PC char combustion under air and oxycoal atmospheres.
1 For interpretation of color in Fig. 5, the reader is referred to the web version of this article.
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Fig. 5. Transient parameters comparisons during PC char combustion with and without FGR under air atmosphere, the embedded sub-illustration shows the size difference of the particle formed after nucleation and condensation with and without FGR.
therefore, studied by a self-developed Char Burning and Particulate Matters Kinetics model (CBPMK), which couples of a char burning intrinsic kinetics model and complete ultra-fine PMs formation mechanisms consisting of ash mineral vaporization, homogenous nucleation, heterogeneous condensation, and coalescence four sub-models. The modeling results show that the PC char shows similar burning temperature and thus similar ash vaporization rate under both air and oxy-coal atmospheres, whereas the vaporization amount under oxy-coal combustion atmosphere is lower than that under air combustion atmosphere due to the high carbon conversion rate caused by CO2 gasification reaction under oxy-coal atmosphere and thus shortened burnout time. Consequently, the higher ash vaporization amount under air combustion atmosphere, which improves particle formation through nucleation and growth through condensation, leads to larger PMs with higher number density after undergoing nucleation and condensation stages (before successive coalescence). However, after undergoing successive coalescence, the particles generated under oxy-coal combustion atmosphere show larger size with fewer amounts due to the higher cohesion factor between the smaller sized nucleation particles, which improves particle collision and coalescence. Meanwhile, with increased FGR ratio, both mean size and number density of the nucleation particles decrease under both air and oxy-coal combustion
embedded sub-illustration) due to the less amount of accumulated particles compared to without FGR. These trends continue to carbon conversion ratio of approximate 0.8. Meanwhile, it can be seen that compared to the particles formed before the carbon conversion ratio of approximate 0.4, the total particle number formed between carbon conversion ratios of 0.4–0.8 is lower. When the carbon conversion ratio is above 0.8, the vapor evaporation rate accesses to zero (Fig. 2b), and the particles formed without FGR are obviously larger than those formed with FGR (red lines ②) due to the significantly decrease of vapor concentration (magenta lines ④). Thus, for the whole char burnout stage, the mean particle diameter and number density without FGR are higher compared to those with FGR. Similar results are presented under oxy-coal combustion atmosphere. Fig. 6 shows the effect of FGR ratio on the number-size distribution of ultra-fine PMs generated after undergoing coalescence during pulverized coal char combustion under both air and oxy-fuel combustion atmospheres. With increasing FGR ratio, the mean particle size after coalescence also decrease due to the dilution effect, which hinders particle coalescence by diffusion and collision. Particle number density follows mass conservation. In addition, due to the lower vaporization ratio under oxy-coal combustion atmosphere compared to that under air combustion atmosphere (Fig. 2b), less and smaller sized of the ultrafine PMs are formed after undergoing nucleation-condensation under oxy-coal combustion atmosphere (Fig. 4), whereas subsequently resulting in less but larger sized particles after undergoing coalescence due to the higher cohesion factor between the smaller nucleation particles, which improves particle collision and coalescence. Thus, to mitigate PM emissions during PC combustion, oxy-coal combustion is a better choice compared to conventional air combustion. Meanwhile, under both air and oxy-coal combustion atmospheres, low FGR ratio should be recommended. 4. Conclusions Considering the intensive concern on the emission of particulate matters (PMs) during pulverizeds coal (PC) char combustion and the extensive adoption of both oxy-coal combustion (27O2/CO2) and air combustion (21O2/N2) in PC power plants, as well as the inherent defects of conventional measurements that cannot provide detailed information on the formation and evolution of ultra-fine PMs, the formation of ultra-fine PMs during PC char combustion under both atmospheres with and without flue gas recirculation (FGR) are,
Fig. 6. Effect of FGR ratio on the number-size distribution of ultra-fine PMs generated after undergoing coalescence during PC char combustion under both air and oxy-coal atmospheres.
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atmospheres, whereas after undergoing coalescent, the final PMs show increasing number density and decreasing size. Therefore, oxy-coal combustion is advantaged for PMs removal through an ash collector, but elevated FGR ratio disadvantages PMs removal. Oxy-coal combustion with low FGR ratio should be recommended in practice.
[18] [19]
Acknowledgements
[20]
The present work was supported by National Natural Science Foundation of China (Grant number 51776161). Special thanks to Christopher Shaddix for his assistance with the code at Sandia.
[21]
[22]
References
[23]
[1] Gimeno B, Artal M, Velasco I, Blanco ST, Fernandez J. Influence of SO2 on CO2 storage for CCS technology: evaluation of CO2/SO2 co-capture. Appl Energy 2017;206:172–80. [2] Qi GJ, Wang SJ. Experimental study and rate-based modeling on combined CO2 and SO2 absorption using aqueous NH3 in packed column. Appl Energy 2017;206:1532–43. [3] Niu Y, Shang T, Zeng J, Wang S, Gong Y, Hui S. Effect of pulverized coal preheating on NOx reduction during combustion. Energy Fuels 2017;31:4436–44. [4] Ahn SY, Go SM, Lee KY, Kim TH, Seo SI, Choi GM, et al. The characteristics of NO production mechanism on flue gas recirculation in oxy-firing condition. Appl Therm Eng 2011;31:1163–71. [5] Zhuang Y, Pavlish JH. Fate of hazardous air pollutants in oxygen-fired coal combustion with different flue gas recycling. Environ Sci Technol 2012;46:4657–65. [6] Tan H, Niu Y, Wang X, Xu T, Hui S. Study of optimal pulverized coal concentration in a four-wall tangentially fired furnace. Appl Energy 2011;88:1164–8. [7] Xu YS, Liu XW, Zhou ZJ, Sheng L, Wang C, Xu MH. The role of steam in silica vaporization and ultrafine particulate matter formation during wet oxy-coal combustion. Appl Energy 2014;133:144–51. [8] Beer JM. Combustion technology developments in power generation in response to environmental challenges. Prog Energy Combust Sci 2000;26:301–27. [9] Niu Y, Liu X, Wang S, Hui Se, Shaddix CR. A numerical investigation of the effect of flue gas recirculation on the evolution of ultra-fine ash particles during pulverized coal char combustion. Combust Flame 2017;184:1–10. [10] Morris WJ, Yu D, Wendt JOL. A comparison of soot, fine particle and sodium emissions for air- and oxy-coal flames, with recycled flue gases of various compositions. Proc Combust Inst 2013;34:3453–61. [11] Dong HJ, Dai HC, Dong L, Fujita T, Geng Y, Klimont Z, et al. Pursuing air pollutant co-benefits of CO2 mitigation in China: a provincial leveled analysis. Appl Energy 2015;144:165–74. [12] Ministry of Environmental Protection of the People's Republic of China, General Administration of Quality Supervision, Inspection and Quarantine of the People's Republic of China. National Standards of People’s Republic of China: emission standard of air pollutants for thermal power plants, GB 13223-2011; 2012. [13] National Development and Reform Commission, Ministry of Environmental Protection of the People's Republic of China, National Energy Administration. Transformation and upgrading action program of energy conservation and emission reduction for coal-based power. Beijing; 2014. [14] McElroy MW, Carr RC, Ensor DS, Markowski GR. Size distribution of fine particles from coal combustion. Science 1982;215:13–9. [15] Fernandez A, Davis SB, Wendt JOL, Cenni R, Young RS, Witten ML. Public health Particulate emission from biomass combustion. Nature 2001;409:998. [16] Meng J, Liu JF, Guo S, Huang Y, Tao S. The impact of domestic and foreign trade on energy-related PM emissions in Beijing. Appl Energy 2016;184:853–62. [17] Niu YQ, Tan HZ, Hui SE. Ash-related issues during biomass combustion: Alkali-
[24] [25]
[26]
[27]
[28] [29] [30]
[31] [32] [33]
[34] [35] [36]
[37]
[38] [39] [40]
[41]
53
induced slagging, silicate melt-induced slagging (ash fusion), agglomeration, corrosion, ash utilization, and related countermeasures. Prog Energy Combust Sci 2016;52:1–61. Niu Y, Tan H, Ma L, Pourkashanian M, Liu Z, Liu Y, et al. Slagging characteristics on the superheaters of a 12 MW biomass-fired boiler. Energy Fuels 2010;24:5222–7. Jiao FC, Chen JA, Zhang LA, Wei YJ, Ninomiya Y, Bhattacharya S, et al. Ash partitioning during the oxy-fuel combustion of lignite and its dependence on the recirculation of flue gas impurities (H2O, HCl and SO2). Fuel 2011;90:2207–16. Okumura Y, Hanaoka T, Sakanishi K. Effect of pyrolysis conditions on gasification reactivity of woody biomass-derived char. Proc Combust Inst 2009;32:2013–20. Neville M, Quann R, Haynes B, Sarofim A. Vaporization and condensation of mineral matter during pulverized coal combustion. Symposium (international) on combustion. Elsevier; 1981. p. 1267–74. Niu Y, Wang S, Shaddix CR, Hui Se. Kinetic modeling of the formation and growth of inorganic nano-particles during pulverized coal char combustion in O2/N2 and O2/CO2 atmospheres. Combust Flame 2016;173:195–207. Fix G, Seames W, Mann M, Benson S, Miller D. The effect of combustion temperature on coal ash fine-fragmentation mode formation mechanisms. Fuel 2013;113:140–7. Niu Y, Shaddix CR. A sophisticated model to predict ash inhibition during combustion of pulverized char particles. Proc Combust Inst 2015;35:561–9. Fix G, Seames WS, Mann MD, Benson SA, Miller DJ. The effect of oxygen-to-fuel stoichiometry on coal ash fine-fragmentation mode formation mechanisms. Fuel Process Technol 2011;92:793–800. Wen C, Xu M, Yu D, Sheng C, Wu H, Pa Zhang, et al. PM10 formation during the combustion of N-2-char and CO2-char of Chinese coals. Proc Combust Inst 2013;34:2383–92. Ortiz C, Valverde JM, Chacartegui R, Benitez-Guerrero M, Perejon A, Romeo LM. The Oxy-CaL process: a novel CO2 capture system by integrating partial oxy-combustion with the calcium-looping process. Appl Energy 2017;196:1–17. Yan JY. Carbon capture and storage (CCS). Appl Energy 2015;148:A1–6. Sheng C, Li Y. Experimental study of ash formation during pulverized coal combustion in O2/CO2 mixtures. Fuel 2008;87:1297–305. Hecht ES, Shaddix CR, Geier M, Molina A, Haynes BS. Effect of CO2 and steam gasification reactions on the oxy-combustion of pulverized coal char. Combust Flame 2012;159:3437–47. Sheng C, Li Y, Liu X, Yao H, Xu M. Ash particle formation during O-2/CO2 combustion of pulverized coals. Fuel Process Technol 2007;88:1021–8. Jia Y, Lighty JS. Ash particulate formation from pulverized coal under oxy-fuel combustion conditions. Environ Sci Technol 2012;46:5214–21. Gao Q, Li S, Yang M, Biswas P, Yao Q. Measurement and numerical simulation of ultrafine particle size distribution in the early stage of high-sodium lignite combustion. Proc Combust Inst 2017;36:2083–90. Quann R, Sarofim A. Vaporization of refractory oxides during pulverized coal combustion. Symposium (international) on combustion. Elsevier; 1982. p. 1429–40. Stull DR, Prophet H. JANAF Thermochemical tables(NSRDS-NBS37). Washington, D.C.; 1971. McNallan M, Yurek G, Elliott J. The formation of inorganic particulates by homogeneous nucleation in gases produced by the combustion of coal. Combust Flame 1981;42:45–60. Neville M. Formation of inorganic submicron particles under simulated pulverized coal combustion conditions [PhD dis—sertation]. Cambridge: Massachusetts Institute of Technology; 1982. Zhang R, Khalizov A, Wang L, Hu M, Xu W. Nucleation and growth of nanoparticles in the atmosphere. Chem Rev 2011;112:1957–2011. Murphy JJ, Shaddix CR. Combustion kinetics of coal chars in oxygen-enriched environments. Combust Flame 2006;144:710–29. Luo R, Fu JP, Li N, Zhang YF, Zhou QL. Combined control of secondary air flaring angle of burner and air distribution for opposed-firing coal combustion. Appl Therm Eng 2015;79:44–53. Li T, Niu Y, Wang L, Shaddix C, Løvås T. High temperature gasification of high heating-rate chars using a flat-flame reactor. Appl Energy 2017.
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Ultra-fine particulate matters (PMs) formation during air and oxy-coal combustion: Kinetics study Yanqing Niu, Bokang Yan, Siqi Liu, Yang Liang, Ning Dong, Shi'en Hui
T
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State Key Laboratory of Multiphase Flow in Power Engineering, Department of Thermal Engineering, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China
H I G H L I G H T S PMs formation during coal char combustion is studied using kinetics model. • Ultra-fine PMs formed in oxy-coal atmosphere have fewer number but larger size. • Ultra-fine PMs show increasing number density and decreasing size with increasing FGR. • Ultra-fine combustion advantages PMs removal. • Oxy-coal • Elevated Flue gas recirculation ratio disadvantages PMs removal.
A R T I C L E I N F O
A B S T R A C T
Keywords: Particulate matters (PMs) Flue gas recirculation (FGR) Coal char Combustion Kinetics
Although both air (21O2/N2) and oxy-coal (27O2/CO2) combustion are widely adopted in pulverized coal (PC) fired power plants, the formation mechanisms of ultra-fine PMs during PC char combustion under both atmospheres with various flue gas recirculation (FGR) ratios are still unclear. Moreover, conventional experimental measurement devices cannot provide detailed information on the formation and evolution of the size-number of ultra-fine PMs. Therefore, the formation of ultra-fine PMs during PC char combustion under both atmopspheres with and without FGR are studied by a self-developed Char Burning and Particulate Matters Kinetics model (CBPMK). The PC char shows similar burning temperature and thus ash vaporization rate under both atmospheres, whereas the vaporization amount under oxy-coal combustion atmosphere is lower than that under air combustion atmosphere due to the shortened burnout time caused by CO2 gasification reaction under the former. Consequently, during nucleation and condensation stages (before successive coalescence), both the particle size and number under air combustion atmosphere are higher than those under oxy-coal combustion atmosphere. However, after coalescence, the final particle shows fewer but larger size under oxy-coal combustion atmosphere due to the higher cohesion factor between the smaller sized nucleation particles, which improves particle collision and coalescence. Meanwhile, both mean size and number density of the nucleation particles decrease with increased FGR ratio under both air and oxy-coal combustion atmospheres, however, after coalescence the final PMs show increasing number density and decreasing size. As results, oxy-coal combustion advantages PMs removal through an ash collector, but elevated FGR ratio disadvantages PMs removal. Oxy-coal combustion with low FGR ratio should be recommended in PC thermal and power plants.
1. Introduction Together with sulfur oxide [1,2] and nitrogen oxide [3–6], particulate matters (PMs) emitted from pulverized coal (PC) combustion power plants play crucial role during the formation of haze and serious environmental pollution [5,7–11]. According to the most recent national air pollution emission standard for thermal power plants in China, PM emissions from coal power plant are limited to 30 mg/Nm3,
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and in some key regions they are restricted to 20 mg/Nm3 [12] and suggested to strive to 10 mg/Nm3 [13] or 5 mg/Nm3 in some provincial standards. The PMs capture and removal in flue gas is never 100% efficient during coal combustion, particularly of ultra-fine ash particles (smaller than 1.0 μm in aerodynamic diameter), which constitute most of the particles that discharge into atmosphere directly [14] or accumulate in the furnace through flue gas recirculation (FGR) and result in an increase in number density due to the failure of conventional filters
Corresponding author. E-mail address: [email protected] (S. Hui).
https://doi.org/10.1016/j.apenergy.2018.02.164 Received 19 December 2017; Received in revised form 7 February 2018; Accepted 24 February 2018 Available online 07 March 2018 0306-2619/ © 2018 Elsevier Ltd. All rights reserved.
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Nomenclature
Greek alphabet
Symbols
αc ρ η, η′ θ ν σ
d, d′ D, D′ f FGR I kB K Kn m MOn MOn−1 n N q P PC PMs r R S Sg T t Y ΔGv V
diameter (cm) diffusion coefficient (cm2/s) number fraction flue gas recirculation nucleation rate (#/m3/s) Boltzmann constant (1.38 × 10−23 J/K) cohesion factor (cm3/s)reaction equilibrium constant Knudsen number mass (g) metal oxides metal suboxides and metals number of mineral vapor molecule condensed on specified sized particle (#/m3) inclusion number in one coal char particle; or particle number concentration (#/m3) burning rate (g/cm2/s) pressure (atm, Pa) pulverized coal particulate matters radius (cm) ideal gas constant (8.314 J/mol/K) saturation degree of the vapors internal specific surface area (cm2/g C) temperature (K) coalescence time (s) ash content change in Gibbs' free energy for droplet formation per unit volume vaporization rate (mol/s)
condensation coefficient mass density (g/cm3) effectiveness factor porosity molecular volume (cm3) surface tension of liquid
Subscript a c cc CO CO2 eff eq g H2O i, j int MOn−1 MOn O2 s,l tot
ash carbon char core carbon monoxide carbon dioxide effective equilibrium gaseous steam serial numbers of different sized particles intrinsic kinetics metal sub-oxide or metal metal oxide or inclusion molecular oxygen solid, liquid total
Superscript eq s, s′
equilibrium surface
temperature drop tube furnace, where the synthetic chars with SiO2 inclusions were burned at 1873 K. They found that H2O in combustion atmosphere significantly enhanced the vaporization of SiO2 and considerably increased the yield of the ultra-fine PM, and the char burning temperature and gas properties surrounding the mineral inclusions seemed to be the primary influencing factors on SiO2 vaporization. In addition, low rank coals, which generally cause high combustion temperature, and a locally reducing atmosphere, which improves mineral reduction and subsequent vaporization in the char matrix, can enhance the formation of ultra-fine PMs [21,26]. In recent years, respecting of CO2 capture and storage (CCS) due to the more and more attention on environment pollution, oxy-coal combustion has been developed rapidly [1,27–29]. Sheng et al. [29] studied the impact of O2/CO2 combustion on ash particle formation in a drop tube furnace, and found that O2/CO2 combustion significantly affected the size distribution of submicron particles, O2/CO2 combustion at the same oxygen concentration shifted the size of the submicron mode center to smaller size and decreased the yield of the submicron particles in comparisons with air combustion. Under O2/CO2 atmospheres, the char burning temperature is lower than that under O2/N2 atmosphere with the same O2 content due to endothermic gasification reaction and low diffusion of O2 in CO2 [24,30], as a result, the number density of the ultra-fine PMs is higher, and the particle size is larger [22,31]. In addition, Morris et al. [10] studied the effects of various cleanup options prior to FGR on ash aerosol formation in a 37 kW down-fired pilot-scale combustor and found that the ultra-fine particle concentrations increased in the combustor despite flue gas treatment with fabric filters. The experiment results are consistent with the kinetics research using CBPMK, indicating that FGR caused a decrease in PMs size with increasing number density [9].
in removing the particles [10]. The emitted PMs from coal-fired furnace may be enriched with trace toxic compounds, which not only cause atmospheric haze but also penetrate into the lungs, consequently causing a number of diseases [15,16]; the accumulated PMs in furnace accelerate the abrasion of heating-surface and deposition formation on its surface, resulting in reduced boiler efficiency and operational safety [17,18]. Respecting of the formation of ultra-fine PMs during coal/char combustion, numerous experiments and kinetics modeling researches on the forming mechanisms and the effects of combustion temperature and atmospheres as well as coal types and FGR have been well documented. By an experiment conducted in a lab-scale drop-tube furnace, Jiao et al. [19] found that most ultra-fine PMs exhibited fractal structures due to the homogeneous nucleation of metallic vapors and/or their heterogeneous condensation on preexisting fine mineral grains during coal combustion with FGR. During coal char combustion, part of inorganic components (Si, Al, Fe, Ca, Na, etc.) in char matrix undergoes successive vaporization, homogeneous nucleation, heterogeneous condensation, coagulation, and coalescence to form ultra-fine PMs [7,9,20–22]. On basis of the abovementioned formation mechanisms of ultra-fine PMs, we developed a Char Burning and Particulate Matters Kinetics model (CBPMK) [9,22]. The modeling results showed that combustion temperature had a dominating positive effect on mineral vaporization, condensation, coagulation and coalescence, and subsequent ultra-fine PMs formation [21–23]. Consequently, the factors including high oxygen content, low ash content, and small sized char particles, which elevated the local char burning temperature [24], caused more and larger ultra-fine PMs [22,25]. Also, Xu et al. [7] studied the vaporization behavior of silica in wet recycle oxy-coal combustion conditions by modeling and experiments conducted in a high 47
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atmospheres with FGR is scarce, and the root mechanisms causing the differences of ultra-fine PMs formed under both atmospheres with and without FGR are unclear. Furthermore, during the formation of ultrafine PMs, existing experimental measurement instruments fail to
Although numerous studies on the formation of ultra-fine PMs during PC char combustion under traditional air atmosphere and oxycoal atmosphere have been well-documented, quantitatively research on ultra-fine PMs formation during PC char combustion under both
Fig. 1. Schematic flowchart of the CBPMK model [9].
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(qO2,int + qH2O,int + qCO2,int) denotes the external reaction rate after considering the embezzlements of ash with a fraction of Ya,cc and pores with a porosity of θc,cc on available external surface, whereas (ηρc,ccSgdcc/6)(qO2,int + qH2O,int + qCO2,int) represents the internal reaction rate within the char matrix. More detailed descriptions on the model can refer to previous paper [24].
provide inside information on the formation processes including the inclusion vaporization within char matrix, subsequent homogeneous nucleation, heterogeneous condensation, and coalescence. Therefore, in this study, to provide guidelines for practical operation, particular in the selection of combustion mode (conventional air combustion or oxy-coal combustion) and FGR ratio, the comparative studies of ultra-fine PM formation during PC char combustion under oxy-coal (27O2/CO2) and air atmospheres (21O2/N2) with various FGR ratios are conducted using the self-developed kinetics model CBPMK [9,22]. Moreover, the root causes for the differences of ultra-fine PMs formed under both combustion atmospheres with and without FGR are revealed by detailed comparisons on the time-dependent molecule number and size during vaporization, nucleation-condensation, and coalescence stages.
2.2. Ultra-fine PM formation kinetics model During coal char combustion, a small part of volatile alkali and alkali-earth elements (K, Na, Mg, and Ca) and refractory elements (Si, Al, and Fe) first vaporize and diffuse out of the char matrix in the form of suboxides or reduced metals. And then, they undergo re-oxidation, nucleation, condensation, coagulation, and coalescence, forming the ultra-fine ash particles [9,17,21,32,33].
2. Model descriptions (a) Vaporization submodel. As a primary hypothesis, all mineral inclusions are uniformly distributed as oxides throughout the char matrix and vaporized as suboxides or fully reduced metals according to R1 [34]. Once diffusing out of the char matrix, the suboxides and reduced metals are fastly reoxidized into metal oxides according to R2. As a simplified calculation, only SiO2 as a representative of various mineral inclusions in coal char is considered at present.
As illustrated in Fig. 1, the CBPMK model consists of char burning intrinsic kinetics model and PM formation kinetics model [9,22]. The char burning model (Submodel A) computes transient char properties (porosity, available surface area, density, and mass, etc.), ash distribution (ash film, ash dilution, and ash vaporization), char core atmosphere (CO/CO2 ratio and surrounding reacting gases), and char burning temperature during the burning lifespan of char particle. The PM formation model, which inherits above-mentioned temporal parameters from the char burning model and used to compute the formation, growth, and number-size distribution of the ultra-fine PMs, consists of mineral vaporization (Submodel B), homogeneous nucleation (Submodel C), heterogeneous condensation (Submodel D), and coalescence (Submodel E) four sub-models. The vaporization submodel (Submodel B) inherits the above-mentioned transient output parameters from the char kinetics model (Submodel B) and calculates the mineral vaporization rate and vaporized fraction. Once the saturation of the mineral vapors, S, is greater than 1.0, the vapor undergoes homogenous nucleation and generates solid particles (Submodel C). Then, the remaining vapors will condense on the particles, resulting in particle growth (Submodel D). Homogeneous nucleation and heterogeneous condensation occur simultaneously. When the char is burned out, both nucleation and condensation stop due to the termination of mineral vaporization. Finally, all existing particles undergo continuous coalescence in the flue gas. In the modeling begin, the input parameters include char particle size d, density ρ, porosity θ, ash content Y and inclusion content, combustion temperature T and gas properties (O2/CO2/N2), etc. As results, the timedependent output parameters such as char burning temperature, carbon conversion ratio, inclusion vaporization rate and ratio, ultra-fine PM size and number density during nucleation, condensation, and coalescence stages are obtained.
6
)(qO2 ,int + qH2O,int + qCO2 ,int )
MOn − 1(g) + O2 → MOn (s,l)
(R2)
V = η′NMOn 4πrMOn Deff
eq PMO n−1
RT
(2)
where η′ named as effectiveness factor is the ratio of the total vaporization rate of inclusions to the vaporization rate of NMOn isolated inclusions with a radius of rMOn in the char matrix; PMOn−1eq is partial pressure of the suboxide or reduced metal on the inclusion surface, which is calculated by Eq. (3) according to reaction (R1). eq PMO = K eq n−1
PCO PCO2
(3)
where Keq is the reaction equilibrium constant, which can be calculated according to the NASA thermodynamic database or the NSRDS-JANAF Thermochemical Tables [35]. (b) Homogeneous nucleation submodel. On basis of classical nucleation theory [36–38], the rate of nucleation, I, is calculated by Eq. (4).
I=
In the model, the initial char particle assumed to be spherical is ideally divided into plenty of equally spaced concentric shells during combustion. As an innovation point, besides of less amount of ash vaporization, some fraction of the ash components liberated from each burned shell is used to thicken the external ash film, and the rest is assumed to penetrate into the char core where ash and pore are distributed into the carbon matrix uniformly. Meanwhile, both oxidation and gasification reactions by CO2 and steam are comprehensively considered. The gross carbon conversion rate, qtot, is calculated according to Eq. (1).
ηρc,cc Sg d cc
(R1)
where MOn represents refractory oxides, and MOn−1 represents suboxides or reduced metals. Drawing lessons from gas diffusion in porous materials, the total mineral vaporization rate, V, is calculated according to Eq. (2).
2.1. Char burning intrinsic kinetics model
qtot = (1−Ya,cc−θc,cc +
MOn (s,l) + CO → MOn − 1 (g) + CO2
s 2σ 0.5v 2αC PMO MOn n
(2πm)0.5 (kB T )2
−16πσ 3 ⎞ exp ⎜⎛ 2⎟ ⎝ 3kB T ΔGv ⎠
(4)
s
where PMOn as the total partial pressure of the vapor molecules is the sum of the newly generated vapor molecules via vaporization-reoxidation reactions in local time step of numerical computation (as short as 10−6 s) and the remaining vapor molecules after nucleation-condensation during the previous time step. (c) Heterogeneous condensation submodel. Once the ultra-fine PMs are formed via nucleation or inherited from FGR, they will continue to grow through heterogeneous condensation and coagulation. According to the molecular gas dynamics theory, the rate of heterogeneous condensation of vapor molecule on the particle with a size of d′ is given by Eq. (5).
(1)
where η named as effectiveness factor denotes the internal reaction rate relative to the external burning rate. (1 − Ya,cc − θc,cc) 49
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2
s′
eq
corresponding coal properties analysis can refer to [40]. To access practical PC char combustion conditions in power station boilers, the PC char particle with a diameter of 45 μm is employed in the following kinetics modeling; The SiO2 inclusion size, combustion gas temperature in furnace, and furnace outlet temperature are assumed to be 16 μm, 1750 K, and 1150 K, respectively [9,37]; To maintain similar furnace temperature as that under air combustion atmosphere, the O2 content under oxy-coal combustion atmosphere is designed as 27 vol.% according to the subsequent modeling results. In addition, although FGR results in constant changes on the furnace gas compositions and temperature, these are ignored to provide a constant benchmark for kinetics modeling under different FGR conditions. Also, the break-up or fragmentation model is neglected.
⎧ πd′ (PMOn − PMOn ) , 2πmkB T ⎪ ⎪ 8πd′ 2 (P s′ − P eq )
K n≫ 1.0 ⎫ ⎪ ⎪ MOn MOn n= , K n≪ 1.0 ⎬ ⎨ 2πmkB T ⎪ ⎪ 2πd′D eq s′ ) − PMO ′ n (PMO 1 + Kn MO n n ⎪ , K n≈ 1.0 ⎪ kB T 1 + 1.71Kn + 1.333Kn2 ⎭ ⎩
(5)
where Knudsen number, Kn, is the ratio of the mean free path of vapor molecule to the nucleated particle; PMOs′ is the remaining vapor molecules pressure after a transient nucleation time step (10−6 s). The growth of particle size follows mass conservation. (d) Coalescence submodel. When the particles are formed through nucleation-condensation, they are assumed to pairwise collide in proportion to their number fractions first, and then the newly formed particles and the remaining particles that did not participate in the previous round of collisions pairwise collide again in the next time step according to their number fractions. The continued collisions cause increased particle size and decreased number density. The newly formed particle number concentrations, Ni,j, can be calculated by Eq. (6).
Ni,j =
3.1. Particle burning temperature and inclusion vaporization Fig. 2a shows the PC char combustion characteristics under both air and oxy-coal atmospheres. Due to the endothermicity of CO2 gasification reaction and low diffusion of O2 in CO2 [24,30,41], the char burning temperature under oxy-coal combustion atmosphere with the O2 content of 27.vol% is similar as that under air combustion atmosphere; However, the residence time is significantly shorter under oxyfuel combustion atmosphere because of the CO2 gasification reaction, which accelerates carbon conversion [30]. Fig. 2b shows the vaporization of inclusion minerals (taking SiO2 as an example) during PC char combustion under both air and oxy-coal atmospheres. Char combustion temperature exerts positive effects on the mineral vaporization rate and subsequent formation and growth of nano-particles under both O2/N2 and O2/CO2 atmospheres [22]. Thus, the ash vaporization rates are similar under both air and oxy-coal combustion atmospheres due to the similar char particle burning temperature (Fig. 2a). However, the vaporization amount or ratio under oxy-coal combustion atmosphere is lower than that under air combustion atmosphere due to the significant longer char burnout time in the latter.
Ni f j + Nj fi 1 + (Ni f j + Nj fi ) Ki,j t
(6)
where i and j denotes different sized particles, N and f denotes the particle number and number fraction during locating collision time step t. The coalescence time step is as short as 10−5 s in avoid of secondary coalescence that makes simulation calculation complex and may result in error. More detailed descriptions can refer to previous paper [9,22]. 3. Modeling results and discussions In our previous studies [22,24], the char burning characteristics (including time-dependent char burning temperature and carbon conversion rate) and ultra-fine PMs forming characteristics (including the vaporization rate and ratio of different inclusions as well as the transient number-size distribution of ultra-fine PMs during PC char combustion) predicted by the kinetic models were compared with the experimental data provided by Murphy and Shaddix from Sandia [39] and Neville et al. from MIT [21]. The well agreements between the kinetic modeling results and the experimental data promise the CBPMK model to predict the evolution of ultra-fine PMs during PC char combustion with or without FGR. In this study, Huangling lignite (a typical coal used in Chinese power plants), with an overall ash content of 13.6 wt.% and a SiO2 content of 42.3 wt.%, was used as a coal representative. The
3.2. Comparisons of ultra-PMs formation under air and oxy-coal combustion atmospheres Fig. 3 shows the particle mean size and number density of ultra-fine PMs generated after undergoing nucleation-condensation (before subsequent coalescence) and coalescence during PC char combustion under both air (21O2/N2) and oxy-coal (27O2/CO2) atmospheres with and without FGR. Before coalescence both the particle size and number under air combustion atmosphere are higher than those under oxy-coal combustion atmosphere. That is due to higher ash vaporization amount
a) Particle burning temperature
b) Ash vaporization
Fig. 2. Effect of air combustion (21O2/N2) and oxy-coal combustion (27O2/CO2) on particle combustion characteristics and ash vaporization.
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a) Particle mean size
b) Number density
Fig. 3. The particle mean size and number density of ultra-fine PMs after nucleation-condensation (before coalescence) and coalescence stages during PC char combustion under air (21O2/N2) and oxy-coal (27O2/CO2) atmospheres.
of ultra-fine PMs generated after undergoing nucleation-condensation (i.e., before coalescence) during PC char combustion under both air and oxy-coal atmospheres. At a certain FGR ratio, both particle number density and size under air combustion atmosphere are higher due to higher inclusion vaporization ratio and similar vaporization rate compared to those under oxy-coal combustion (Fig. 2). With increasing FGR ratio both particle size and number density decrease under both atmospheres due to dilution effect. This is consistent with the results from previous research for the same PC char particles with a diameter of 100 μm, and both quantities decrease with increasing FGR ratio [9]. Fig. 5 presents an explanation for the decreasing trends of both particle size and number density through detailed transient parameters comparisons during PC char combustion with FGR of 0.30 and without FGR under air atmosphere. As deviations of temperature and gas composition from FGR are ignored, the vaporization rate is the same for both FGRs. During the initial burning stage, compared to without FGR the nuclei during PC char combustion with an FGR of 0.30 are larger due to the dilution effect. FGR results in a low degree of vapor saturation (lower vapor concentration, magenta1 lines ④), and consequently larger nuclei (black lines ②, and negative values in the embedded sub-illustration) with fewer amounts (blue lines ③). However, although the nuclei show smaller sizes without FGR, they adsorb more vapor molecules through condensation (olive lines ⑤) due to high condensation rate on smaller nuclei (Eq. (5)), resulting in slightly larger particle sizes after undergoing condensation (red lines ①, and positive values in the embedded sub-illustration). These trends (more and larger particles without FGR) continue to carbon conversion ratio of approximate 0.4. Noted, during the very early char burning stage, due to significantly low vapor saturation with FGR the particles are obviously larger than those formed without FGR, but the number densities are so low that can be ignored. Although the accumulated nucleation particles during PC char combustion without FGR are slightly larger than those formed with FGR, higher number densities consume more vapor molecules through condensation (olive lines ⑤). When the carbon conversion ratio is above 0.4, the accumulated vapor concentration (magenta lines ④) during PC char combustion with FGR overpasses that without FGR, consequently, resulting in more and slightly smaller nuclei formation with FGR (black lines ②, and reduced negative values in the embedded sub-illustration). During condensation stage, the particles formed with FGR grow quickly and show slightly larger size (red lines ①, and negative values in the
in the air atmosphere (Fig. 2b), which improves particle formation through nucleation and growth through condensation [22]. After undergoing successive coalescence, the particle shows fewer amounts but larger size under oxy-coal combustion atmosphere. Smaller sized particle generated after undergoing nucleation and condensation stages under oxy-coal combustion possesses higher Knudsen number, Kn, and thus higher cohesion factor between each other, Ki.j (Eq. (6)), consequently improving particle collision and coalescence and resulting in fewer but larger particle. The results indicate that oxy-coal atmosphere advantages particle removal through an ash collector. Meanwhile, it can be seen that compared to without FGR, during PC char combustion with FGR the ultra-fine PMs sizes are smaller in both nucleation-condensation and coalescence stages, and the particle number density with FGR during nucleation-condensation stage is also lower than that without FGR; However, after undergoing coalescence, the final particle number density with FGR is higher compared to without FGR. Detailed but systematic disclosures on the effect of FGR on ultra-fine PMs formation during PC char combustion under air and oxy-coal atmospheres with various FGR ratios are performed as below. 3.3. Effect of FGR on ultra-PMs formation during air and oxy-coal combustion Fig. 4 shows the effect of FGR ratio on the number-size distribution
Fig. 4. Effect of FGR ratio on the number-size distribution of ultra-fine PMs generated after undergoing nucleation-condensation during PC char combustion under air and oxycoal atmospheres.
1 For interpretation of color in Fig. 5, the reader is referred to the web version of this article.
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Fig. 5. Transient parameters comparisons during PC char combustion with and without FGR under air atmosphere, the embedded sub-illustration shows the size difference of the particle formed after nucleation and condensation with and without FGR.
therefore, studied by a self-developed Char Burning and Particulate Matters Kinetics model (CBPMK), which couples of a char burning intrinsic kinetics model and complete ultra-fine PMs formation mechanisms consisting of ash mineral vaporization, homogenous nucleation, heterogeneous condensation, and coalescence four sub-models. The modeling results show that the PC char shows similar burning temperature and thus similar ash vaporization rate under both air and oxy-coal atmospheres, whereas the vaporization amount under oxy-coal combustion atmosphere is lower than that under air combustion atmosphere due to the high carbon conversion rate caused by CO2 gasification reaction under oxy-coal atmosphere and thus shortened burnout time. Consequently, the higher ash vaporization amount under air combustion atmosphere, which improves particle formation through nucleation and growth through condensation, leads to larger PMs with higher number density after undergoing nucleation and condensation stages (before successive coalescence). However, after undergoing successive coalescence, the particles generated under oxy-coal combustion atmosphere show larger size with fewer amounts due to the higher cohesion factor between the smaller sized nucleation particles, which improves particle collision and coalescence. Meanwhile, with increased FGR ratio, both mean size and number density of the nucleation particles decrease under both air and oxy-coal combustion
embedded sub-illustration) due to the less amount of accumulated particles compared to without FGR. These trends continue to carbon conversion ratio of approximate 0.8. Meanwhile, it can be seen that compared to the particles formed before the carbon conversion ratio of approximate 0.4, the total particle number formed between carbon conversion ratios of 0.4–0.8 is lower. When the carbon conversion ratio is above 0.8, the vapor evaporation rate accesses to zero (Fig. 2b), and the particles formed without FGR are obviously larger than those formed with FGR (red lines ②) due to the significantly decrease of vapor concentration (magenta lines ④). Thus, for the whole char burnout stage, the mean particle diameter and number density without FGR are higher compared to those with FGR. Similar results are presented under oxy-coal combustion atmosphere. Fig. 6 shows the effect of FGR ratio on the number-size distribution of ultra-fine PMs generated after undergoing coalescence during pulverized coal char combustion under both air and oxy-fuel combustion atmospheres. With increasing FGR ratio, the mean particle size after coalescence also decrease due to the dilution effect, which hinders particle coalescence by diffusion and collision. Particle number density follows mass conservation. In addition, due to the lower vaporization ratio under oxy-coal combustion atmosphere compared to that under air combustion atmosphere (Fig. 2b), less and smaller sized of the ultrafine PMs are formed after undergoing nucleation-condensation under oxy-coal combustion atmosphere (Fig. 4), whereas subsequently resulting in less but larger sized particles after undergoing coalescence due to the higher cohesion factor between the smaller nucleation particles, which improves particle collision and coalescence. Thus, to mitigate PM emissions during PC combustion, oxy-coal combustion is a better choice compared to conventional air combustion. Meanwhile, under both air and oxy-coal combustion atmospheres, low FGR ratio should be recommended. 4. Conclusions Considering the intensive concern on the emission of particulate matters (PMs) during pulverizeds coal (PC) char combustion and the extensive adoption of both oxy-coal combustion (27O2/CO2) and air combustion (21O2/N2) in PC power plants, as well as the inherent defects of conventional measurements that cannot provide detailed information on the formation and evolution of ultra-fine PMs, the formation of ultra-fine PMs during PC char combustion under both atmospheres with and without flue gas recirculation (FGR) are,
Fig. 6. Effect of FGR ratio on the number-size distribution of ultra-fine PMs generated after undergoing coalescence during PC char combustion under both air and oxy-coal atmospheres.
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atmospheres, whereas after undergoing coalescent, the final PMs show increasing number density and decreasing size. Therefore, oxy-coal combustion is advantaged for PMs removal through an ash collector, but elevated FGR ratio disadvantages PMs removal. Oxy-coal combustion with low FGR ratio should be recommended in practice.
[18] [19]
Acknowledgements
[20]
The present work was supported by National Natural Science Foundation of China (Grant number 51776161). Special thanks to Christopher Shaddix for his assistance with the code at Sandia.
[21]
[22]
References
[23]
[1] Gimeno B, Artal M, Velasco I, Blanco ST, Fernandez J. Influence of SO2 on CO2 storage for CCS technology: evaluation of CO2/SO2 co-capture. Appl Energy 2017;206:172–80. [2] Qi GJ, Wang SJ. Experimental study and rate-based modeling on combined CO2 and SO2 absorption using aqueous NH3 in packed column. Appl Energy 2017;206:1532–43. [3] Niu Y, Shang T, Zeng J, Wang S, Gong Y, Hui S. Effect of pulverized coal preheating on NOx reduction during combustion. Energy Fuels 2017;31:4436–44. [4] Ahn SY, Go SM, Lee KY, Kim TH, Seo SI, Choi GM, et al. The characteristics of NO production mechanism on flue gas recirculation in oxy-firing condition. Appl Therm Eng 2011;31:1163–71. [5] Zhuang Y, Pavlish JH. Fate of hazardous air pollutants in oxygen-fired coal combustion with different flue gas recycling. Environ Sci Technol 2012;46:4657–65. [6] Tan H, Niu Y, Wang X, Xu T, Hui S. Study of optimal pulverized coal concentration in a four-wall tangentially fired furnace. Appl Energy 2011;88:1164–8. [7] Xu YS, Liu XW, Zhou ZJ, Sheng L, Wang C, Xu MH. The role of steam in silica vaporization and ultrafine particulate matter formation during wet oxy-coal combustion. Appl Energy 2014;133:144–51. [8] Beer JM. Combustion technology developments in power generation in response to environmental challenges. Prog Energy Combust Sci 2000;26:301–27. [9] Niu Y, Liu X, Wang S, Hui Se, Shaddix CR. A numerical investigation of the effect of flue gas recirculation on the evolution of ultra-fine ash particles during pulverized coal char combustion. Combust Flame 2017;184:1–10. [10] Morris WJ, Yu D, Wendt JOL. A comparison of soot, fine particle and sodium emissions for air- and oxy-coal flames, with recycled flue gases of various compositions. Proc Combust Inst 2013;34:3453–61. [11] Dong HJ, Dai HC, Dong L, Fujita T, Geng Y, Klimont Z, et al. Pursuing air pollutant co-benefits of CO2 mitigation in China: a provincial leveled analysis. Appl Energy 2015;144:165–74. [12] Ministry of Environmental Protection of the People's Republic of China, General Administration of Quality Supervision, Inspection and Quarantine of the People's Republic of China. National Standards of People’s Republic of China: emission standard of air pollutants for thermal power plants, GB 13223-2011; 2012. [13] National Development and Reform Commission, Ministry of Environmental Protection of the People's Republic of China, National Energy Administration. Transformation and upgrading action program of energy conservation and emission reduction for coal-based power. Beijing; 2014. [14] McElroy MW, Carr RC, Ensor DS, Markowski GR. Size distribution of fine particles from coal combustion. Science 1982;215:13–9. [15] Fernandez A, Davis SB, Wendt JOL, Cenni R, Young RS, Witten ML. Public health Particulate emission from biomass combustion. Nature 2001;409:998. [16] Meng J, Liu JF, Guo S, Huang Y, Tao S. The impact of domestic and foreign trade on energy-related PM emissions in Beijing. Appl Energy 2016;184:853–62. [17] Niu YQ, Tan HZ, Hui SE. Ash-related issues during biomass combustion: Alkali-
[24] [25]
[26]
[27]
[28] [29] [30]
[31] [32] [33]
[34] [35] [36]
[37]
[38] [39] [40]
[41]
53
induced slagging, silicate melt-induced slagging (ash fusion), agglomeration, corrosion, ash utilization, and related countermeasures. Prog Energy Combust Sci 2016;52:1–61. Niu Y, Tan H, Ma L, Pourkashanian M, Liu Z, Liu Y, et al. Slagging characteristics on the superheaters of a 12 MW biomass-fired boiler. Energy Fuels 2010;24:5222–7. Jiao FC, Chen JA, Zhang LA, Wei YJ, Ninomiya Y, Bhattacharya S, et al. Ash partitioning during the oxy-fuel combustion of lignite and its dependence on the recirculation of flue gas impurities (H2O, HCl and SO2). Fuel 2011;90:2207–16. Okumura Y, Hanaoka T, Sakanishi K. Effect of pyrolysis conditions on gasification reactivity of woody biomass-derived char. Proc Combust Inst 2009;32:2013–20. Neville M, Quann R, Haynes B, Sarofim A. Vaporization and condensation of mineral matter during pulverized coal combustion. Symposium (international) on combustion. Elsevier; 1981. p. 1267–74. Niu Y, Wang S, Shaddix CR, Hui Se. Kinetic modeling of the formation and growth of inorganic nano-particles during pulverized coal char combustion in O2/N2 and O2/CO2 atmospheres. Combust Flame 2016;173:195–207. Fix G, Seames W, Mann M, Benson S, Miller D. The effect of combustion temperature on coal ash fine-fragmentation mode formation mechanisms. Fuel 2013;113:140–7. Niu Y, Shaddix CR. A sophisticated model to predict ash inhibition during combustion of pulverized char particles. Proc Combust Inst 2015;35:561–9. Fix G, Seames WS, Mann MD, Benson SA, Miller DJ. The effect of oxygen-to-fuel stoichiometry on coal ash fine-fragmentation mode formation mechanisms. Fuel Process Technol 2011;92:793–800. Wen C, Xu M, Yu D, Sheng C, Wu H, Pa Zhang, et al. PM10 formation during the combustion of N-2-char and CO2-char of Chinese coals. Proc Combust Inst 2013;34:2383–92. Ortiz C, Valverde JM, Chacartegui R, Benitez-Guerrero M, Perejon A, Romeo LM. The Oxy-CaL process: a novel CO2 capture system by integrating partial oxy-combustion with the calcium-looping process. Appl Energy 2017;196:1–17. Yan JY. Carbon capture and storage (CCS). Appl Energy 2015;148:A1–6. Sheng C, Li Y. Experimental study of ash formation during pulverized coal combustion in O2/CO2 mixtures. Fuel 2008;87:1297–305. Hecht ES, Shaddix CR, Geier M, Molina A, Haynes BS. Effect of CO2 and steam gasification reactions on the oxy-combustion of pulverized coal char. Combust Flame 2012;159:3437–47. Sheng C, Li Y, Liu X, Yao H, Xu M. Ash particle formation during O-2/CO2 combustion of pulverized coals. Fuel Process Technol 2007;88:1021–8. Jia Y, Lighty JS. Ash particulate formation from pulverized coal under oxy-fuel combustion conditions. Environ Sci Technol 2012;46:5214–21. Gao Q, Li S, Yang M, Biswas P, Yao Q. Measurement and numerical simulation of ultrafine particle size distribution in the early stage of high-sodium lignite combustion. Proc Combust Inst 2017;36:2083–90. Quann R, Sarofim A. Vaporization of refractory oxides during pulverized coal combustion. Symposium (international) on combustion. Elsevier; 1982. p. 1429–40. Stull DR, Prophet H. JANAF Thermochemical tables(NSRDS-NBS37). Washington, D.C.; 1971. McNallan M, Yurek G, Elliott J. The formation of inorganic particulates by homogeneous nucleation in gases produced by the combustion of coal. Combust Flame 1981;42:45–60. Neville M. Formation of inorganic submicron particles under simulated pulverized coal combustion conditions [PhD dis—sertation]. Cambridge: Massachusetts Institute of Technology; 1982. Zhang R, Khalizov A, Wang L, Hu M, Xu W. Nucleation and growth of nanoparticles in the atmosphere. Chem Rev 2011;112:1957–2011. Murphy JJ, Shaddix CR. Combustion kinetics of coal chars in oxygen-enriched environments. Combust Flame 2006;144:710–29. Luo R, Fu JP, Li N, Zhang YF, Zhou QL. Combined control of secondary air flaring angle of burner and air distribution for opposed-firing coal combustion. Appl Therm Eng 2015;79:44–53. Li T, Niu Y, Wang L, Shaddix C, Løvås T. High temperature gasification of high heating-rate chars using a flat-flame reactor. Appl Energy 2017.