Influence of coal co-firing on the particulate matter formation during pulverized biomass combustion

Influence of coal co-firing on the particulate matter formation during pulverized biomass combustion

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Journal of the Energy Institute xxx (2018) 1e9

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Influence of coal co-firing on the particulate matter formation during pulverized biomass combustion Q4

Xuebin Wang a, Zhongfa Hu a, Guogang Wang b, Xiaotao Luo c, Renhui Ruan a, Qiming Jin a, Houzhan Tan a, * a

MOE Key Laboratory of Thermo-Fluid Science and Engineering, School of Energy and Power Engineering, Xi'an Jiaotong University, 710049, Xi'an, China China IPPR International Engineering Company Limited, 100089, Beijing, China State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xi'an Jiaotong University, Xi'an, Shaanxi, 710049, China b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 March 2018 Received in revised form 8 May 2018 Accepted 21 May 2018 Available online xxx

Biomass is regarded as CO2-neutral, while the high contents of potassium and chlorine in biomass induce severe particulate matter emission, ash deposition, and corrosion in combustion facilities. Co-firing biomass with coal in pulverized-combustion (PC) furnaces is able to solve these problems, as well as achieve a much higher generating efficiency than grate furnaces. In this work, the particulate matter (PM) emission from biomass co-firing with coal was studied in an entrained flow reactor at a temperature of 1623 K simulating PC furnace condition. PMs were sampled through a 13-stage impactor, and their morphology and elemental composition were characterized by scanning electron microscopy and electron dispersive X-ray spectroscopy. SO2 emissions were measured to interpret the possibility of potassium sulfation during co-firing. Results show that PMs from the separated combustion of both biomass and coal present a bimodal particle size distribution (PSD). The concentration and size of finemode submicron particles (PM1.0) from biomass combustion are much higher than those from coal combustion because of the high potassium content in biomass. For the co-firing cases, with the coal ratio increasing from 0% to 50%, the PM1.0 yield is reduced by more than half and the PM1.0 size becomes smaller, in contrast, the concentration of coarse-mode particles with the size of 1.0e10 mm (PM1.0-10) increases. The measured PM1.0 yields of co-firing are lower than the theoretically weight-averaged ones, which proves that during the biomass and coal co-firing in PC furnaces, the vaporized potassium from biomass can be efficiently captured by these silicon-aluminate oxides in coal ash. In the studied range of coal co-firing ratio (50 wt.%), the chlorides and sulfates of alkali metals from biomass burning are the dominating components in PM1.0, and a certain amount of silicon is observed in PM0.1-1. The analysis of chemical composition in PM1.0, together with that of SO2 emission, indicates a marginal sulfation of alkali metal chloride occurring at high temperatures in PC furnaces. © 2018 Energy Institute. Published by Elsevier Ltd. All rights reserved.

Keywords: Biomass co-firing Fine particle Sulfation Coal High temperature

1. Introduction Increasing concerns and anxieties regarding climate change and global warming are prompting worldwide attention towards the utilization of biomass [1], because of its advantages regarding net CO2 emission and SOx/NOx reduction [2e4]. As a big agricultural country, China has an abundant biomass energy resource, which is of great potentials for reducing fossil fuel consumption and ensuring energysupply security [5,6]. However, large amounts of alkali and chlorine in biomass fuels, which are generally higher than those in coal, significantly result in severe problems of slagging, corrosion, gas pollutants and fine particles emissions in biomass combustion facilities [7e9].

* Corresponding author. E-mail address: [email protected] (H. Tan). https://doi.org/10.1016/j.joei.2018.05.003 1743-9671/© 2018 Energy Institute. Published by Elsevier Ltd. All rights reserved.

Please cite this article in press as: X. Wang, et al., Influence of coal co-firing on the particulate matter formation during pulverized biomass combustion, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.05.003

Q1

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To relieve the consequent furnace operational problems, biomass co-firing with coal was proposed and proved to be feasible [10,11]. It was also regarded as one of the most efficient and economical approaches for the utilization of biomass because it could utilize the existing large and expensive infrastructure burning fossil-fuel with the lowest cost required. Therefore, there have been many investigations on biomass cofiring, mainly focusing on the efficiency [12], flame stability [13], gas pollutant emission [14] and ash deposition [15] in past decades. However, few studies have focused on the particulate matter emission in biomass co-firing. Naiema et al. [16] investigated the effect of co-firing biomass on the emissions of totally suspended particulates (TSP) within a full-scaled circulating fluidized bed boiler, observing that €m et al. [17] co-firing 50% coal significantly reduced the TSP by 90% and led to the enrichment of potassium in the bottom ash. Fagerstro conducted a similar research on co-firing with ash, observing that the addition of peat ash reduced the fine particle emissions by capturing potassium into the bottom ash. The significant reduction of fine particles with coal fly ash addition was also observed in a fullscale experiments by Damoe et al. [18]. It is widely proven that the fine particles from biomass combustion are formed through the volatilization of volatile elements of alkali, chlorine and sulfur, following by the gas phase reactions, homogenous nucleation and heterogeneous condensation [8,19e21]. When cofiring with coal or its ash, the higher content of silico-aluminate oxides in coal ash is effective to interact with the releasing alkali vapors and form aluminosilicates and silicates [22e24], thereby portioning into ultra-micrometer particles and reducing the sub-micrometer particles emission. Furthermore, biomass co-firing with coal containing higher sulfur content was found to significantly decrease deposition rate and relieve chlorine-induced corrosion and slagging [25]. At high temperatures, the gaseous alkali chlorides, released from biomass, is readily transformed into alkali sulfates via the sulfation process [26,27]. Together with alkali chlorides vapors, these alkali sulfates vapors are also the precursors of the fine particles. The presence of alkali sulfates in the flue gas would advance the onset of nucleation (mainly K2SO4), thus leading to a longer residence time for condensation [19,21]. This indicated that the sulfation process of alkali species had effects on the aerosol formation in biomass co-firing. In our previous research on ash deposition within a lab-scale furnace and a full-scale furnace, it was found that most of the alkali chloride had been converted into alkali sulfate in the flue gas before deposition, and the major pathways of alkali sulfation and the role of aerosols formation were proposed [28]. The importance of alkali sulfation to aerosol formation was confirmed by Glarborg and Marshall [26] and Hidiyarti et al. [29] and they developed models for the gaseous sulfation of alkali chloride. Jimenez et al. [30] carried out an investigation on the effect of sulfation by adding SO2 into the biomass, considering the oxidation of sulfur as a rate limiting step in alkali sulfate formation. With SO2 addition, more potassium sulfates were observed in the sub-micrometer particles, indicating the promotion of sulfation during combustion. A similar result was also observed in a field study on the effect of sulfur oxides on the aerosol formation [31]. Considering the significant role of sulfation process in fine particle emission during biomass combustion, the investigations on biomass co-firing with coal containing high sulfur is still lacking. Therefore, it is still necessary to study the effect of coal co-firing and its sulfation on the particulate matter emission in biomass combustion. In this study, biomass co-firing with coal was investigated within an entrained flow reactor at 1623 K, aiming to demonstrate its effect on the PM emission. The particle matters were collected of 13 stages with sizes from nano to micro meters with a coupling analysis on gaseous SO2 emissions on-line. The microstructure and composition of sampled particle matters were further analyzed, and the effect of the blending ratio was mainly considered. Further analysis on the sulfation process of potassium species on PM formation was conducted. 2. Experimental methods 2.1. Fuel properties Wheat straw and coal (Huangling bituminous coal) used in the study were both from Shaanxi Province, China. The proximate and ultimate analysis of the fuel samples are shown in Tables 1 and 2, respectively. It can be observed that the coal has a higher sulfur content than the biomass. Additionally, the alumina content in coal ash are much higher than that in biomass ash as shown in Table 2, which may have a significant effect on alkali capture during combustion. The coal adopted was grinded and sieved with the diameter below 105 mm, while the biomass was below 1 mm. Both the biomass and coal were dried at 105  C for 2 h before using. 2.2. Drop tube furnace system An entrained flow reactor was adopted to study the SO2 and PM emissions behavior of biomass co-firing with coal. A schematic of the entrained flow reactor system is shown in Fig. 1. The furnace is heated by silicon molybdenum with a maximum temperature of 1700  C. The Table 1 Properties of biomass and coal used in this study. Fuel type

Biomass Coal

Proximate analysis (wt.%, ad)

Ultimate analysis (wt.%, ad)

Mad

Aad

Vad

FCad

Cad

Had

Oad

Nad

Sad

Clad

3.88 6.80

6.01 13.59

72.10 30.25

18.01 49.36

43.92 65.67

4.47 3.95

40.98 8.6

0.44 0.85

0.30 0.54

0.25 e

Table 2 The composition of biomass ash. Component

SiO2

Al2O3

Fe2O3

MgO

CaO

Na2O

K2O

SO3

Biomass (wt.%) Coal (wt.%)

36.22 34.61

3.97 17.21

2.72 13.35

7.17 3.12

11.06 21.03

0.41 0.91

15.43 1.61

2.5 5.26

Please cite this article in press as: X. Wang, et al., Influence of coal co-firing on the particulate matter formation during pulverized biomass combustion, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.05.003

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Fig. 1. Drop tube furance and gas-particle sampling system.

temperatures were measured by Pt-Rh-Pt thermocouples (accuracy ±1  C). The reactor tube is 60 mm in diameter and 3000 mm in length. Three zones of temperature controlling system were adopted to obtain a long constant-temperature region of 1500 mm. The fuels are fed by a micro-scale spiral feeder with a stable rate ranging from 1 to 4 g min1. All the tests were carried out at 1 atm and a temperature of 1623 K. In our tests, the air ratio was generally controlled at 1.2, and the total supply air flow rate was kept stable in the range of 13.0e13.6 L/min depending on the fuel feeding rate, keeping the residence time of ~3 s. 2.3. Sampling and characterizing methods A GASMET DX4000 Fourier transform infrared (FTIR) gas analyzer (Temet Instruments Oy, Helsinki, Finland) was used to record the concentration of SO2 in the flue gas on line. A ceramic probe was inserted from the bottom for gas sampling, and all the sampling line was heated above 180  C to avoid condensing. The filter before gas analyzer was used to remove the ash particle and keep the gas analyzer clean. The PM sampling system consists of a water-cooled probe, a Dekati cyclone (Model SAC-65), a Dekati low pressure impactor (DLPI) composed of 13 collection stages with size from nano to micro meter scale, and a vacuum pump (Leybold SogevacSV 25) with a standard flow rate of 10 L/min. Pure nitrogen was introduced into the PM probe for quenching, dilution and isokinetic sampling, in order to minimize the interaction amongst the particles but also forcing the nucleation of vaporized materials. After each experiment the collected substrates were carefully placed into a weighing bottle, and then they were dried in a dryer and analyzed by a high-precision microbalance with accuracy of ±0.001 mg (Sartorius M2P, German) to obtain the particle size distribution. Scanning electron microscopyeelectron dispersive X-ray spectroscopy (SEMeEDS, JEM-2100F, JEOL, Japan) was employed to analyze the PM collected and determine its microstructure and elemental content distribution. 3. Result and discussion 3.1. Particulate matter emission of biomass and coal combustion alone The average mass-based particle size distributions of PM10 from the combustion of biomass and coal alone, are depicted in Fig. 2. It can be seen that PM10 follows a bimodal size distribution, with a fine mode and a coarse mode, which is similar to the results reported in the previous research [8,20]. For biomass combustion alone, the fine mode peak was located at 397 nm, while the coarse mode peak at about 6.64 mm. For coal combustion, however, the fine mode peak was located at a smaller size of 274 nm, while the coarse mode peak remained the same size as that from biomass combustion. Further data on PM emission in Fig. 2 reveals a much higher PM1.0 but lower PM1.0~10 emissions generated from biomass combustion. The higher PM1.0 yield from biomass combustion is mainly attributed to the much higher content of alkali and chlorine in biomass fuels as shown in Tables 1 and 2. During combustion, much larger amounts of alkali and chlorine in Please cite this article in press as: X. Wang, et al., Influence of coal co-firing on the particulate matter formation during pulverized biomass combustion, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.05.003

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Fig. 2. . Fine particle emission of biomass or coal combustion.

biomass fuels are generally released into the flue gas as gaseous alkali chlorides, which undergoes a series of processes including homogeneous nucleation and heterogeneous condensation, thereby dominating the PM1.0 formation [20,21]. In order to clarify the difference in the PM emission between biomass and coal combustion, SEM-EDS was employed to analyze the images and elemental compositions of PMs. Note that the particles were collected by aluminum foil, elements of alumina and oxygen were ignored and excluded. Table 3 presents the SEM images and elemental compositions of the particles from biomass and coal combustion, for the fine particle and coarse particle modes respectively. For biomass combustion, only K, Cl and little S were found in the sub-micrometer particles, and these species appeared as cubic crystals. The stoichiometric correlation between K and 2S þ Cl indicates that the K, Cl and little S exist as KCl and K2SO4, which are the dominant constituents in the sub-micrometer particles generated from biomass combustion. The results were similar to previous research [32,33]. However, the ultra-micrometer particles are dominated by refractory elements including Mg, Ca, Si, and Fe, and also contain K, S, and Cl, which is quite different with the sub-micrometer particles. These ultra-micrometer particles appeared as smooth spheres of molten materials, which is consistent with previous research [20]. The highly different compositions and microstructures between the sub-micrometer and ultra-micrometer particles as shown in Table 3, are ascribed to different formation mechanisms [8]. The formation of sub-micrometer particles starts with the volatilization of inorganic elements (mainly K, S and Cl) from the fuel followed by the subsequent nucleation and condensation during the cooling stage of flue gas [10,38]. While The ultra-micrometer particles are formed through char/ash fragmentation and retain the properties of the mineral matters. For coal combustion, the sub-micrometer particles are mainly composed of alkali, alkaline earth, sulfur, phosphorus and little silicon and iron, which is quite different with that from biomass combustion as shown in Table 3. In addition to volatile alkali, sulfur and phosphorus, a small portion of alkaline earth metals (calcium and magnesium) can be reduced under reducing atmosphere during char combustion and thereby easily volatilized to the flue gas [34]. Meanwhile, a portion of refractory silicon and iron can also be gasified under reducing atmosphere of char particles [35]. These vapors released from coal particles, then go through homogeneous nucleation and heterogeneous condensation, thus portioning in PM1.0 particles. Similar to the sub-micrometer particles, the ultra-micrometer particles are also consist in alkali, alkaline earth, sulfur, phosphorus, silicon and iron. The differences between them are their contents of silicon and iron and microstructures. This is mainly due to the different formation mechanisms during combustion. The ultra-micrometer particles are formed through char/ash fragmentation and retain the properties of the mineral matters, thereby containing higher silicon and iron contents [36]. However, in contrast to that from biomass combustion, the ultra-micrometer particles appear as sphere with rough surface. This can be presumably ascribed to the much higher alumina contents in coal ash as shown in Table 2. During combustion, the refractory species in ash particles experience coalescence and form aluminosilicates and silicates. Previous research have proven that the aluminosilicates own much higher melting points than silicates [24]. Due to lack of alumina in biomass, those dominant refractory mineral matters in ultra-micrometer particles mainly exist as silicates which were fused in high temperatures. However, for coal combustion, the ultra-micrometer particles contain more aluminosilicates, which may not be fused during combustion. 3.2. PSD and elemental composition of PM1.0 during biomass and coal co-firing The average mass-based particle size distributions of PM1.0 during biomass and coal co-firing with different ratios are depicted in Fig. 3. As presented in Fig. 3, all the PSDs of PM1.0 from biomass co-firing with coal follow the unimodal. With increase of coal blending ratio from 0% to 50%, co-firing with coal obviously shifted the peak towards smaller scale with the value from 400 nm to 150 nm. Furthermore, it can be observed that the total quantity of fine particles was obviously reduced by coal blending, indicating a significant PM1.0 reduction. A full-scale study on the coal fly ash addition showed the coal fly ash significantly reduced the mass-load of sub-micrometer particles emission and shifted the peak towards smaller scale [18], which is consistent with present study. In order to investigate the effect of co-combustion on the PM1.0 emission, the experimental and mass-based averaged calculative PM1.0 yields are compared for all the three blending ratios. Basing on the assumption that no interaction among the mineral matters occurs during co-combustion, the mass-based averaged calculative PM1.0 yield are obtained as follows: Please cite this article in press as: X. Wang, et al., Influence of coal co-firing on the particulate matter formation during pulverized biomass combustion, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.05.003

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Table 3 The morphology by SEM and elemental composition by EDS of sub-micrometer and ultra-micrometer from biomass or coal combustion. Type

Morphology

Elemental composition

Biomass, sub-micrometer

Biomass, ultra-micrometer

Coal, sub-micrometer

Coal,ultra-micrometer

PM1:0; Cal ¼ xPM1:0 Coal þ ð1  xÞPM1:0; Biomass where x represents the blending ratio of coal and its value were taken as 0.1, 0.3 and 0.5, respectively. Fig. 4 shows the comparison of experimental and mass-based averaged calculative PM1.0 yields and PM1.0 reduction efficiency (relative difference between the experimental and mass-based averaged calculative PM1.0) of different coal blending ratios. As presented in Fig. 4, when below 50%, with the increasing coal blending ratio, the PM1.0 yield decreases while the PM1.0 reduction efficiency by coal blending increases. It is well-known that the formation of PM1.0 starts with the release of inorganic vapor when the fuel particle is fed into the furnace, followed by a series of processes including gas phase reactions, homogeneous nucleation and heterogeneous condensation [19,20]. During co-combustion of biomass and coal, the alkali in biomass particles are quickly released into the flue gas. The alkali-containing vapor species (mainly in the form of KCl and KOH) in the flue gas would then interact with silico-aluminate oxides in ash via R1-4 [22,24], forming aluminosilicates and silicates and portioning in PM1.0þ. The interactions between them are efficient to capture alkali-containing vapor species, thereby resulting in the reduction of fine particles. With the increase of coal blending ratio, the silico-aluminate oxides in fly ash increases and promotes the capture of alkali-containing vapors. Furthermore, the significant decrease of alkali-containing vapors in the flue gas may weaken the homogeneous and heterogeneous condensation [37], consequently leading to a significant left-shift of the PM1.0 PSD with coal co-firing as shown in Fig. 3. 2KOH(g) þ SiO2(s) / K2O$SiO2(s) þ H2O(g)

(R1)

2KOH(g) þ Al2O3$2SiO2(s) / K2O$Al2O3$2SiO2(s) þ H2O(g)

(R2)

Please cite this article in press as: X. Wang, et al., Influence of coal co-firing on the particulate matter formation during pulverized biomass combustion, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.05.003

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Fig. 3. Mass-based PSD of sub-micrometer particles (PM1.0) in co-firing.

Fig. 4. Comparison on PM1.0 yield between experimental and calculated amounts and the reduction efficiency of PM1.0 for variety of coal blending ratio.

2KCl(g) þ 3SiO2(s) þ H2O(g) / K2O$3SiO2(s) þ 2HCl(g)

(R3)

2KCl(g) þ Al2O3$2SiO2(s) þ H2O(g) / K2O$Al2O3$2SiO2(s) þ 2HCl(g)

(R4)

Fig. 5 shows the elemental composition of sub-micrometer particles within the size ranges of 0.029e0.097 mm and 0.163e0.633 mm with co-firing ratios below 50%. As shown in Fig. 5, the sub-micrometer particles from the co-combustion are dominated by K, Cl, S and Na, which is consistent with previous research [30]. Besides, a comparable amount of silicon is observed in the sub-micrometer particles within the size range of 0.163e0.633 mm. Furthermore, the mole ratios between (K þ Na) and (2S þ Cl) in the sub-micrometer particles generated from co-combustion are ~1.0, which is similar to that from biomass combustion alone. It indicates the dominant presence of alkali chlorides and sulfates in the sub-micrometer particles regardless of coal co-firing. This may be presumably ascribed to the presence of chlorine from biomass. Previous research on PM formation from biomass combustion proved the dominant role of chlorine [38,39]. During co-combustion of biomass and coal, the presence of chlorine from biomass particles facilitates the formation of stable gaseous alkali chlorides at high temperatures [40,41], which undergo nucleation/condensation and thereby dominating PM1.0 formation. Additionally, the stable gaseous alkali chlorides would be simultaneously sulfated by SO2 or SO3 [27,42], forming alkali sulfates and portioning in PM1.0. Increased sulfur content in sub-micrometer is observed with the increasing in coal ratio, which will be further discussed later. 3.3. PSD and elemental composition of PM1.0~10 during biomass and coal co-firing Fig. 6 presents the average mass-based PSDs of PM1.0~10 under different ratios. As presented in Fig. 6, the PSDs of PM1.0~10 from biomass co-firing with coal are similar and all follow the unimodal with the peak at 6.64 mm. With the increase of coal blending ratio from 0% to 50%, a slight increase of PM1.0~10 emission can be observed. Fig. 7 shows the effect of coal blending ratio on the elemental chemical compositions of PM1.0~10 within the size range of 0.98e1.64 mm and 2.4e10 mm under different co-firing ratios. Unlike the submicrometer particles, the ultra-micrometer particles generated from co-combustion are dominated by the refractory elements of Fe, Si, Ca and Mg, accounting for ~70% of the total mass. The ultra-micrometer particles are formed via ash/char fragmentation together with the interaction among those refractory elements [36]. Generally they retain the major mineral matters in the combusted fuel, and consist Please cite this article in press as: X. Wang, et al., Influence of coal co-firing on the particulate matter formation during pulverized biomass combustion, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.05.003

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Fig. 5. . Inorganic element contents of sub-micrometer particles (PM1.0) under different co-firing ratios.

Fig. 6. Mass-based PSD of ultra-micrometer particles (PM1.0-10) in co-firing.

Fig. 7. . Inorganic element contents of ultra-micrometer particles (PM1.0-10) under different co-firing ratios.

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Fig. 8. Comparison between the experimental and calculative SO2 emission.

of aluminosilicates and/or silicates following by coalescence. Further data in Fig. 7 shows that, the K and Cl contents in the particles within size range of 0.98e1.64 mm decreased while the S content almost remained unchanged with the increasing in the coal blending ratio. 3.4. Further discussion on the synergetic effect of sulfation on PM formation in co-firing Note that increasing sulfur content in the sub-micrometer particles with the increasing coal blending ratio was observed in Fig. 5. This indicates that more alkali sulfates are formed with coal co-firing, revealing the possible existence of the sulfation process in biomass cofiring with coal. In order to provide further evidence on the possible existence of sulfation, the SO2 emission under different co-firing ratios was analyzed. Fig. 8 presents the comparison between the experimental and mass-averaged SO2 emission. It can be observed that the experimental SO2 concentration in co-firing was not obviously but slightly lower than the calculative values. This indicates the existent but marginal effect of sulfation in co-firing. During combustion, most of alkali and chlorine in biomass are easily released into the flue gas in the form of alkali chlorides, which can be transformed into alkali sulfates via the reaction R5-6. With coal adding, almost all of the sulfur in coal is oxidized into SO2, leading to a higher SO2 concentration as shown in Fig. 8. A portion of SO2 would be further transformed into SO3, which has been proven to be the ratelimiting step of the sulfation process [26,42]. However, the oxidation of SO2 to SO3 is not favorable at high temperatures (eg. 1623 K in the present study) [42]. Nonetheless, some alkali chlorides would be sulfated into alkali sulfates, thereby resulting in higher alkali sulfates vapors content in the flue gas. The weakening-intensified alkali sulfates vapors and alkali chlorides vapors left then experience nucleation/ condensation, forming fine particles [8,19] and result in a bit higher sulfur content but lower chlorine content in the sub-micrometer particles as shown in Fig. 5. 2KCl(g) þ SO2(g) þ H2O(g) þ 1/2O2(g) / K2SO4(g)þ2HCl(g)

(R5)

2KCl(g) þ SO3(g) þ H2O(g) / K2SO4(g) þ 2HCl(g)

(R6)

4. Conclusions 1) The combustion of biomass or coal alone produces a bimodal particle size distribution. In comparison with coal combustion, the larger and higher PM1.0 for biomass combustion is because of its high potassium content in biomass. 2) The co-combustion of biomass and coal significantly reduces the PM1.0 yield and shifts the PM1.0 towards smaller scale. The intensified capture of potassium by silicon-aluminate oxides in coal ash is responsible for the PM1.0 reduction. However, the co-combustion shows little impacts on the PM1-10 PSD, with only a small increasing in particle concentration. 3) In the studied range of coal co-firing ratio 50 wt.%, the chlorides and sulfates of alkali metals from biomass burning are the dominated components in PM1.0, while a certain amount of silicon is observed in PM0.1-1. The analysis of chemical composition in PM1.0, together with the SO2 emission, indicates a marginal sulfation of alkali metal chloride occurring at high temperatures in PC furnaces. Acknowledgements The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (Nos. 51676157 and 5161101654), the Key Projects of Shaanxi Natural Science Foundation (No. 2017JZ010), and the Fundamental Research Funds for the Central Universities. Please cite this article in press as: X. Wang, et al., Influence of coal co-firing on the particulate matter formation during pulverized biomass combustion, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.05.003

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Please cite this article in press as: X. Wang, et al., Influence of coal co-firing on the particulate matter formation during pulverized biomass combustion, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.05.003