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Evolution of PM2.5 from biomass high-temperature pyrolysis in an entrained flow reactor
Journal of the Energy Institute xxx (2018) 1e9
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Evolution of PM2.5 from biomass high-temperature pyrolysis in an entrained flow reactor Yan Li a, Xuebin Wang a, *, Houzhang Tan a, **, Shengjie Bai a, Hrvoje Mikul ci c b, Fuxin Yang a a b
MOE Key Laboratory of Thermo-Fluid Science and Engineering, School of Energy and Power Engineering, Xi'an Jiaotong University, Xi'an, 710049, China Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb, Zagreb, 10002, Croatia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 May 2018 Received in revised form 9 July 2018 Accepted 9 July 2018 Available online xxx
In this study, wheat straw pyrolysis was conducted in an entrained flow reactor at 900e1300 C, and PM2.5 were sampled from the flue gas through a heated sampling system. Multi-phase PM2.5 including carbonaceous matter, potassium-containing particles, and ash particles, was separated and quantified using a thermogravimetric analyzer (TGA). The micro-morphologies and chemical compositions of these three phases were characterized by scanning electron microscopy (SEM), scanning transmission electron microscope (STEM), energy dispersive X-ray spectrometry (EDS), and X-ray diffraction (XRD). Results show that PM2.5 yields during biomass pyrolysis are in the range of 7e34 g/kg (dry-basis biomass) and increase with the increase of pyrolysis temperature. At low pyrolysis temperatures (900e1000 C), the carbonaceous matter is dominated by char-carbon. When the pyrolysis temperature increase from 1000 C to 1100 C, the production of soot is greatly enhanced and soot becomes dominant in PM2.5, and the amorphous morphologies of soot are replaced by the concentric graphitic layers. With the further increasing in pyrolysis temperature, soot particles become more spherical and onion-like. Above 1100 C, the KCl content in PM2.5 declines, which is because of the capture of KCl and the formation of lowmelting potassium aluminosilicates in large char particles. At 1300 C, the fragmentation of char particles is significantly strengthened, resulting in more ash in PM2.5. © 2018 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Keywords: Biomass pyrolysis PM2.5 Emission Soot Char
1. Introduction In view of biomass as CO2 neutral and renewable, there is a growing interest in the utilization of biomass fuels to substitute the fossil fuels and mitigate the greenhouse gas emissions. Many technologies have been developed for the utilization of biomass, however, its utilization process also generates a considerable amount of particulate matter (PM) [1,2]. The fine particles in biomass boilers can deposit on heating surfaces, inducing severe ash fouling and slagging, which further result in the heat transfer deterioration as well as the high temperature corrosion [3]. In addition, these fine particles from biomass utilization have also been reported to aggravate the impact on environment and public health [4,5]. The biological and environmental effects of PM from combustion sources are closely related to their physicochemical properties. Fine particles from biomass combustion have been reported multi-phase and different in shape, size and composition [6], in terms of the formation mechanism, which have been classified into four categories including soot, char, submicron fly ash, and particulate organic matter (POM) [7e11]. Soot is a carbonaceous substance formed in the fuel-rich atmosphere, which is composed of agglomerates of spherical particles with diameters in the range of 20e50 nm and is described as fractal clusters. The formation process of soot consists of a series of complex physical and chemical steps depending on the fuel types. For simple hydrocarbon fuels, C2H2 and PAHs are regarded as the precursors of surface growing [12]. For complicated solid fuels, soot is the one of the products from the secondary reactions of volatiles, especially, tar.
* Corresponding author. ** Corresponding author. E-mail address: [email protected] (X. Wang). https://doi.org/10.1016/j.joei.2018.07.019 1743-9671/© 2018 Energy Institute. Published by Elsevier Ltd. All rights reserved.
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The tar generated from biomass decomposition is easily to nucleate on the surfaces of pre-existing particles or to form POM during subsequent gas cooling. The char particles (>1 mm) are the major solid residuum of biomass devolatilization, which are released with the flue gas flow. The submicron fly ash (<1 mm) is produced by mineral vaporization and re-condensation (e.g., K, Na, S, Cl, P, and Zn) [13e15]. Due to the high content of K in biomass, K-containing species in atmospheric aerosols are the indicators of the contribution from biomass burning [16e18]. The yield and component of PM from biomass combustion depend on the combustion conditions such as furnace temperature, fuel type, oxygen concentration, air distribution, furnace type, etc. Under fuel-lean conditions (e.g., the large-scale biomass boiler), most soot and POM are burned out and the dominated components in fine particles are ash minerals; in contrast, under fuel-rich conditions (e.g., the biomass open burning, the house-hold stove), the dominated are the carbonaceous particles [7,19,20]. In view of the instability and complexity of POM, in this study, we focus on the formation of inorganic particles including soot, potassium-containing particles, and ash particles, while the formation of POM is not discussed. Pyrolysis is the first stage of the overall combustion process, and is also one of the most important biomass conversion technologies. The studies on the fine particle formation during biomass pyrolysis are essential for the control of PM emission and the improvement of the combustion efficiency. In general, few studies had focused on particle evolution mechanisms during biomass high-temperature pyrolysis. Henrik Wiinikka [21,22] characterized the submicron particles produced during oxygen blown entrained flow gasification of wood powder and investigated the influence of operating condition on particle formation. It was found that the soot particles in the real gasifier environment had less ordered nanostructure and higher reactivity as the temperature increased. The observation is in contrast with what is usually observed when using well-defined fuels. Qin et al. [23] and Trubetskaya et al. [24] investigated the influence of biomass constituents on soot formation, and the results showed that compared to cellulose and hemicellulose, lignin is more important for soot formation. Long et al. [25] studied the release of alkali and alkaline earth metals (AAEMs) during biomass pyrolysis, and proposed that the release of AAEMs were caused by the free radical attacking on bonds between the char matrix and AAEMs. Kurian et al. [26] reported that the PM10 emissions from the pyrolysis of oil sand asphaltenes presented an unimodal size distribution with the peak size at about 1 mm. In summary, most of the previous studies paid attention on the single component of fine particles. In this study, different contents in PM2.5, including carbonaceous matter (like soot and char), K-containing particles, and ash particles were separated and characterized respectively to investigate the evolution of these contents with reaction temperature. The wheat straw was pyrolyzed in an entrained flow reactor at 900e1300 C. The obtained results are helpful to deeply understand the formation mechanism of fine particles from biomass pyrolysis and combustion. 2. Material and methods 2.1. Feed material The biomass used in this study is the wheat straw from Baoji district of Shaanxi province, China. The proximate, ultimate analyses and ash composition of fuel samples are shown in Table 1. The ash compositional analysis was performed by an X-ray fluorescence (XRF) instrument. Prior to the XRF analysis, 1 g biomass samples were heated in a muffle furnace at 550 C for 7 h, following the procedures in the Chinese standard GB/T 28731-2012. The fuel particles were pulverized and sieved into 100 mm, and then dried at 90 C in oven for 12 h before use. 2.2. Experimental setup The pyrolysis experiments were conducted in an entrained flow reactor system as shown in Fig. 1. It is comprised of a fluidized-bed feeding unit, gas supply unit, reactor and furnace heating unit, and solid product sampling unit. The corundum tube reactor is 1200 mm in length and 54 mm in diameter. The furnace is electrically heated by a silicon-molybdenum resistance element to a maximum temperature of 1500 C. The temperatures are measured by Pt-Rh-Pt thermocouples (accuracy, ±1 C). Three zones of temperature controlling yield a constant-temperature region of 500 mm long, as shown in Fig. 1. The pyrolysis temperatures studied at in this work were 900, 1000, 1100, 1200, and 1300 C. A micro-scale fluidized-bed feeder was used to feed biomass particles into the reactor through a water-cooled feeding probe. The feeding rate was 0.22 g/min. Biomass particles were carried by the primary nitrogen flow of 1 L/min, and the secondary nitrogen flow of 3 L/min was introduced from the surrounding. The gas residence time in the isothermal zone was 3e4 s in the studied temperature range. As shown in Fig. 1, all of the gas and solid products were sampled using a hot-gas diluted probe, which fulfilled the requirement of the PM2.5 cyclone (URG-2000-30ENS-1, URG) on the operation flow at 10 L/min, by introducing additional hot dilution gas of 6 L/min. Through the PM2.5 cyclone, the coarse particles larger than 2.5 mm were removed and collected from the flue gas. The fine particles (PM2.5) after cyclone were collected by a quartz or metal filter with a diameter of 47 mm. The PM2.5 sampling unit was heated up to ~230 C to avoid the significant condensation of POM. The main components of the PM2.5 contain ash-embedded char fragments, soot and condensed volatile inorganic particles. The sampling process at each condition was conducted in duplicate to ensure the repeatability.
Table 1 Proximate analysis, ultimate analysis, and ash composition of the biomass used in this study. Proximate Analysis (wt.%) Mad 7.66
Ad 9.93
Vd 74.40
Ultimate Analysis (wt.%) FCd 15.67
Cdaf 45.53
Hdaf 5.26
Ndaf 0.78
Ash composition (wt.%) Odaf 48.15
St,daf 0.28
K2O 12.56
Fe2O3 2.35
CaO 8.01
SiO2 12.52
Cl 3.44
S 1.02
Please cite this article in press as: Y. Li, et al., Evolution of PM2.5 from biomass high-temperature pyrolysis in an entrained flow reactor, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.07.019
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Fig. 1. The (a) schematic of the entrained flow reactor and (b) temperature profiles.
2.3. Chemical and physical analyses of PM2.5 samples Morphologies and elemental compositions of PM2.5 samples were determined using SEM-EDS (SU8010, Hitachi, Japan), and highresolution STEM (FEI Tecnai G2 F30, 300 kV, Hillsboro, USA). Samples used for STEM analysis were first separated from the quartz substrate and then treated by ultrasonic extraction using dichloromethane solvent for 5 min. After centrifugalizing three times, the particles were deposited on copper grid. A detailed procedure on the pre-treatment was outlined in the literature [27]. The crystalline constituents of the PM2.5 were characterized by using a PANalytical X'pert diffract meter. It was analyzed by using Cu Ka radiation in the range of 10e80 (wavelength 1.5406 Å) with the scanning step of 0.05 /s. The main mineral phase peaks were matched with the PDF data of stand substance peak by adopting Jade 6.0 software. PM2.5 samples collected by the metal filters were analyzed by a thermogravimetric analyzer (TGA, Netzsch STA-449C, Germany). In TGA measurements, 5 mg sample in a corundum crucible was heated from 35 C to 1000 C at a rate of 10 C/min. The atmosphere was 10 vol.% O2: 90 vol.% N2. With this approach, different phases in PM2.5, such as moisture, volatiles, fixed carbon, KCl, and residual ash, can be quantified. 3. Results and discussion 3.1. PM2.5 yields PM2.5 yields from biomass pyrolysis in the temperature range of 900e1300 C are shown in Fig. 2, and the original appearances of PM2.5 samples on quartz substrates are compared in Fig. 1. The total yield of PM2.5 increased with increasing temperature, accompanied by the deepening of appearance color. A sharp increase in PM2.5 yield from 12.8 g/kg dry biomass (db) to 27.3 g/kg (db) was observed when the pyrolysis temperature increases from 1000 C to 1100 C. The yield of PM2.5 produced at 1300 C reached 33 g/kg (db). In addition, through comparing the PM2.5 yields in this study with previously reported data [23,24] (also presented in Fig. 2), It can be found that the yields of PM2.5 emissions were in a wide range (7e90 g/kg (db)), depending on different biomass origins. The wood produces more fine particles than herbaceous biomass, which is probably because of the different constitutes between the two biomasses. Woody biomass has higher lignin contents but lower alkali metal contents than herbaceous biomass. Researches indicated that lignin tends to form more aromatics through guaiacol- and syringol-groups during biomass pyrolysis [27,28], thus creating more soot particles. In contrast, high potassium content can catalyze the decomposition of soot precursor tar, resulting in less soot formation for herbaceous biomass [29]. 3.2. Distributions of different components in PM2.5 From the XRD patterns of PM2.5 (Fig. 3), the strong signal of KCl could be observed, which indicates the significant amount of potassium salts in all PM2.5 samples. The broad reflection around 25 appeared at high temperatures (1200e1300 C) is assigned to the turbostratic structure of graphene layers in soot, corresponding to the 002 reflection of graphite. As the temperature increased, this refection became Please cite this article in press as: Y. Li, et al., Evolution of PM2.5 from biomass high-temperature pyrolysis in an entrained flow reactor, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.07.019
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Fig. 2. The PM2.5 yields of biomass pyrolysis at varied temperatures from 900 C to 1300 C.
more obvious, demonstrating the further development of a crystalline phase. Besides soot, a certain amount of SiO2 was observed at 1000 C, while it almost disappeared at 1100 C and 1200 C. The quantification of different phase composition in PM2.5 was performed in a TGA reactor. The mass loss curves are shown in Fig. 4, where two steps of mass losses are observed. The first mass loss is in the temperature range of 400e500 C, which is ascribed to the oxidation of carbonaceous matter. These carbonaceous matter includes carbon in soot and carbon in char, named as soot-C and char-C, respectively. Because both soot and char are carbonaceous substances, it is difficult to separate the two by oxidation. However, since char usually contains inorganic ash, the content of char in PM2.5 can be roughly estimated by the ash content. The second mass loss starts around 800 C, which coincides with the evaporating temperature of KCl. Considering the strong signals of KCl in XRD patterns, we ascribe the second mass loss into the evaporating of KCl. After the heating and oxidation of PM2.5 samples in TGA, a certain amount of solid residue was obtained, that consisted of the refractory ash and the condensed minerals with an evaporating temperature above 1000 C. The measured elemental compositions of the ash residual
Fig. 3. XRD spectra of PM2.5 samples.
Fig. 4. TG and DTG curves of PM2.5 samples.
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at different pyrolysis temperatures are compared in Table 2. Main elements in the residue are Si, Ca, K, Al, Mg, S, among which Si, K content is about 35 wt.% and 12 wt.%, respectively, while a negligible amount of Cl is observed. This indicates that partial K is chemically captured by the silicate network [30,31]. By means of the quantification method of TGA, percentage contents and yields of the three components (carbonaceous matter, KCl, and ash) in PM2.5 were obtained in Table 3 and Fig. 5. For carbonaceous matter, its yield increased with the increasing temperature. In low temperature range of 900e1000 C, the percentage and yield of carbon were relatively stable. In terms of the high ash percentages (40.04e46.87 wt.%) in this temperature range, it is deduced that below 1000 C the carbonaceous matter contains a large amount of char-C. This conclusion can be further verified by the SEM observation in later discussion. When the pyrolysis temperature further increased from 1000 to 1100 C, the yield of carbonaceous matter showed a fast increase, from 11.6 to 24.8 g/kg (db), along with its percentage increased from 41.46 wt.% to 51.43 wt.%, and the percentage of ash decreased from 40.04 wt.% to 21.86 wt.%. This indicates a large amount of soot was formed at 1000e1100 C. Above 1000 C, the further increase of temperature significantly promotes the formation of soot. When the pyrolysis temperature was beyond 1200 C, the percentage of carbon was stable at ~70 wt.% while that of ash was 10e20 wt.%. This indicates that at higher temperatures, soot became dominant in PM2.5. KCl is another non-negligible component in PM2.5 from biomass pyrolysis, as shown in Table 3, its percentage content was in the range of 7.34e23.47 wt.% depending on the temperature. Fig. 5 shows that the KCl yield first increased from 0.52 g/kg (db) to 5.82 g/kg (db) with temperature increasing from 900 C to 1100 C, then dropped to 2.88 g/kg (db) with temperature further increasing to 1300 C. The increase of KCl yield at low temperatures can be well understood because the increased temperature favors the evaporation of KCl. However, when the temperature increased to higher than 1100 C, the KCl content started to decrease. The reasons involved the following: (1) The reaction between KCl and aluminosilicates is enhanced by the temperature (2KCl þ Al2O3$2SiO2 þ 4SiO2 þ H2O / KAlSi3O8 þ HCl) [13]. (2) The produced potassium aluminosilicates have a melting temperature around 1150 ± 20 C, which causes ash fusion in biomass fuel particles. As a result, partial inner channels in biomass fuel particles are blocked, suppressing the diffusion and release of volatiles (including potassium and other mineral vapors) through the pyrolytic charash shell [32]. (3) The melted surfaces strengthen their collision capture capabilities for fine KCl particles [33], thus KCl particles are more distributed in coarse particles (PM2.5þ) than in PM2.5. In view of the similar release mechanism to KCl, the profile of ash yield versus temperature showed the similar trend. Note that the ash yield started to increase when temperature increased from 1200 C to 1300 C. Seen from XRD patterns, a small amount of SiO2 appeared in
Table 2 Elemental analyses of solid residues of PM2.5 in TGA reactor. T (oC)
Composition (wt.%) Na
Mg
Al
Si
P
S
K
Ca
Fe
Cu
Zn
1000 1100 1200
1.63 2.22 4.33
6.39 10.01 7.05
5.48 5.85 4.31
35.13 34.54 30.59
2.97 4.66 3.60
3.64 2.93 6.63
13.86 9.90 15.55
22.80 23.34 19.84
5.87 4.25 6.13
0.82 1.14 1.03
0.76 1.17 0.94
Table 3 Percentage contents of three phases in PM2.5 samples. T (oC)
Carbonaceous matter (wt.%)
KCl (wt.%)
Ash (wt.%)
900 1000 1100 1200 1300
39.37 41.46 51.43 68.53 70.50
7.34 17.20 23.47 16.94 8.60
46.87 40.04 21.86 13.65 20.75
Fig. 5. Yields of three phases in PM2.5.
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Fig. 6. SEM images of char particles in PM2.5 (*: the cracked cenospheres are more likely to break by mechanical force during the SEM sample preparation process).
1300 C samples. This is due to the intensified fragmentation of char particles at higher temperatures, producing more fine fragments. The included ash leads to the increase of ash content in PM2.5.
3.3. PM2.5 morphologies 3.3.1. Char particles The general morphologies of PM2.5 are showed in Fig. 6. Although the cyclone can separate majority of the coarse particles, quite a few supermicron particles with geometric diameters larger than 2.5 mm can be still observed in the fine particles due to the limited separation efficiency. Those are pyrolytic char particles. The morphology of char is strongly influenced by the biomass origin (including organic constitutes and the minerals) and the combustion conditions (such as temperature, heating rate and pressure). As seen from Fig. 6, PM2.5 formed at 900 C was predominated by supermicron char particles. When temperature increased to 1100 C and 1300 C, due to the large formation of soot, char particles are coated with densely packed soot particles. In this experiment, two types of char particles were identified, the irregular shape particles and the spherical shape particles. The former was produced from the fragmentation of char particles, while the latter was closely associated with the eutectic minerals with low melting temperatures in char particles, which mainly contain CayKySiyAl as indicated by the EDS results in Fig. 7. Trubetskaya et al. [34] proposed that in addition to plant cell constituents, the potassium silicates is the main mineral matter that significantly affects the formation of these near-spherical wheat straw chars.
Fig. 7. Elemental compositions of spherical char particles at three temperatures.
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Fig. 8. STEM images of mature and immature soot particles.
Increasing temperature gives rise to a higher fraction and a faster release rate of volatiles, thus promoting the swelling extent of the liquid metaplast, creating large internal cavities in char particles. As a result, char particles are more likely to collapse and produce more fragments [35,36]. Fig. 6 shows that as the temperature increased, the produced char particles had undergone a more severe transformation. The char particles at higher temperatures showed a smaller size and a more melting surface. At 900 C, a small amount of KCl crystal was gently adhered to the surface of the char particles. When the temperature rose to 1100 C, the amount of adhered particles increased significantly with an obvious melting state and they were fused with the large particles. When the temperature reached 1300 C, the fusion became more intense, forming a smoother surface of the char particles. This indicates that the trapping effect, both chemically and physically, of char particles on KCl particles is enhanced with the increasing temperature. The observation in Fig. 6 corresponds to the results in Table 3. 3.3.2. Soot particles Soot is one of the products from the secondary reactions of biomass volatiles at high temperatures. The formation mechanism of soot is complicated, involving hundreds of steps, but can be summarized into four stages: the nucleation from gaseous hydrocarbons, the particle growth by surface reactions, the particle coagulation, and the agglomeration [37]. STEM images of soot particles formed at different temperatures are showed in Fig. 8. Similar to soot particles from the combustion of small-molecule hydrocarbon fuels, the soot particles formed during biomass pyrolysis also consist of the agglomerates of nanometer spherules. These nanometer spherules exhibited a concentric arrangement of graphitic layers, roughly parallel and equidistant, forming an onion-like microtexture. The diameter of spherules varied from 10 to 180 nm, dominantly in the range of 20e40 nm. With the increase of temperature, the soot particles became more spherical and more onion-like. At 1000 C (Fig. 8a), an amorphous structure of soot with disordered inner fringes is observed, which is because a lower temperature cannot sufficiently drive the growth and the reorientation of carbon lamellas [38]. When the temperature was beyond 1100 C, a more mature graphitic structure is formed (Fig. 8b,c). Particles originated at 1000 C are asphaltene precursors rather than the carbonaceous soot. 3.3.3. Submicron KCl particles Biomass contains a considerable amount of potassium, which is easy to release as potassium vapor and then condense to form KCl particles. In this study, a large number of particles with regular crystal structures are observed by STEM, as shown in Fig. 9, the sizes of these particles are uniform and in the range of 100e200 nm. Significant signals of potassium and chlorine are found in the EDS spectrograms, therefore, it can be concluded that these crystal particles are KCl. Moreover, we can observe some amorphous liquid-like substances coated on the surface of KCl particles at 1000 C, and they disappeared at 1300 C. These amorphous substances are considered as the relatively heavy species of the pyrolytic tar. Light species with boiling points lower than 230 C were removed from PM2.5 by heating the sampling unit, however, the heavy fractions with boiling points higher than 230 C could still condense to form POM. As the temperature increased, the tar decomposed gradually. This agrees with the previous research which reported that the tar generated during biomass pyrolysis completely decomposed at 1200 C [39,40].
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Fig. 9. STEM images and EDS spectrogram of KCl particles.
4. Conclusion The distribution and evolution of different phases in PM2.5 from biomass pyrolysis at 900e1300 C were investigated in an entrained flow reactor. On the basis of morphology and composition analyses, the main conclusions are as follows: (1) The overall PM2.5 yields during biomass pyrolysis were in the range of 7e34 g/kg (db) and generally increased with increasing temperature. In the low temperature range of 900e1000 C, the carbonaceous matter content was stable at about 40 wt.%, dominated by char-carbon. When the temperature increased from 1000 C to 1100 C, the content of carbonaceous matter in PM2.5 significantly increased because of the formation of soot particles, which became dominant in PM2.5. (2) The soot graphitization started at 1100 C, when matured soot particles with concentric graphitic layers were observed. The diameter of soot particle varied from 10 nm to 180 nm, with majority in the range of 20e40 nm. With the increase in temperature, the soot particles became more spherical and more onion-like. (3) The release of potassium salt in PM2.5 increased initially with the temperature. When the temperature was above 1100 C, char particles underwent aggravated melting due to the formation of low-melting potassium aluminosilicates, resulting in the decline of KCl. When the temperature increased to 1300 C, the fragmentation of char particles were significantly strengthened, producing more PM2.5 from the char fragments, thereby increasing the ash content in PM2.5. Acknowledgements This study was supported by the National Key Research and Development Plan of China (No. 2016YFB0600605) and National Natural Science Foundation of China (No. 51676157). Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.joei.2018.07.019. References [1] S. Akagi, R.J. Yokelson, C. Wiedinmyer, M. Alvarado, J. Reid, T. Karl, et al., Emission factors for open and domestic biomass burning for use in atmospheric models, Atmos. Chem. Phys. 11 (2011) 4039e4072. [2] M.O. Andreae, P. Merlet, Emission of trace gases and aerosols from biomass burning, Global Biogeochem. Cycles 15 (2001) 955e966. [3] X. Wang, H. Tan, Y. Niu, M. Pourkashanian, L. Ma, E. Chen, et al., Experimental investigation on biomass co-firing in a 300MW pulverized coal-fired utility furnace in China, Proc. Combust. Inst. 33 (2011) 2725e2733. [4] A. Winquist, J.J. Schauer, J.R. Turner, M. Klein, S.E. Sarnat, Impact of ambient fine particulate matter carbon measurement methods on observed associations with acute cardiorespiratory morbidity, J. Expo. Sci. Environ. Epidemiol. 25 (2015) 215e221.
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Evolution of PM2.5 from biomass high-temperature pyrolysis in an entrained flow reactor Yan Li a, Xuebin Wang a, *, Houzhang Tan a, **, Shengjie Bai a, Hrvoje Mikul ci c b, Fuxin Yang a a b
MOE Key Laboratory of Thermo-Fluid Science and Engineering, School of Energy and Power Engineering, Xi'an Jiaotong University, Xi'an, 710049, China Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb, Zagreb, 10002, Croatia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 May 2018 Received in revised form 9 July 2018 Accepted 9 July 2018 Available online xxx
In this study, wheat straw pyrolysis was conducted in an entrained flow reactor at 900e1300 C, and PM2.5 were sampled from the flue gas through a heated sampling system. Multi-phase PM2.5 including carbonaceous matter, potassium-containing particles, and ash particles, was separated and quantified using a thermogravimetric analyzer (TGA). The micro-morphologies and chemical compositions of these three phases were characterized by scanning electron microscopy (SEM), scanning transmission electron microscope (STEM), energy dispersive X-ray spectrometry (EDS), and X-ray diffraction (XRD). Results show that PM2.5 yields during biomass pyrolysis are in the range of 7e34 g/kg (dry-basis biomass) and increase with the increase of pyrolysis temperature. At low pyrolysis temperatures (900e1000 C), the carbonaceous matter is dominated by char-carbon. When the pyrolysis temperature increase from 1000 C to 1100 C, the production of soot is greatly enhanced and soot becomes dominant in PM2.5, and the amorphous morphologies of soot are replaced by the concentric graphitic layers. With the further increasing in pyrolysis temperature, soot particles become more spherical and onion-like. Above 1100 C, the KCl content in PM2.5 declines, which is because of the capture of KCl and the formation of lowmelting potassium aluminosilicates in large char particles. At 1300 C, the fragmentation of char particles is significantly strengthened, resulting in more ash in PM2.5. © 2018 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Keywords: Biomass pyrolysis PM2.5 Emission Soot Char
1. Introduction In view of biomass as CO2 neutral and renewable, there is a growing interest in the utilization of biomass fuels to substitute the fossil fuels and mitigate the greenhouse gas emissions. Many technologies have been developed for the utilization of biomass, however, its utilization process also generates a considerable amount of particulate matter (PM) [1,2]. The fine particles in biomass boilers can deposit on heating surfaces, inducing severe ash fouling and slagging, which further result in the heat transfer deterioration as well as the high temperature corrosion [3]. In addition, these fine particles from biomass utilization have also been reported to aggravate the impact on environment and public health [4,5]. The biological and environmental effects of PM from combustion sources are closely related to their physicochemical properties. Fine particles from biomass combustion have been reported multi-phase and different in shape, size and composition [6], in terms of the formation mechanism, which have been classified into four categories including soot, char, submicron fly ash, and particulate organic matter (POM) [7e11]. Soot is a carbonaceous substance formed in the fuel-rich atmosphere, which is composed of agglomerates of spherical particles with diameters in the range of 20e50 nm and is described as fractal clusters. The formation process of soot consists of a series of complex physical and chemical steps depending on the fuel types. For simple hydrocarbon fuels, C2H2 and PAHs are regarded as the precursors of surface growing [12]. For complicated solid fuels, soot is the one of the products from the secondary reactions of volatiles, especially, tar.
* Corresponding author. ** Corresponding author. E-mail address: [email protected] (X. Wang). https://doi.org/10.1016/j.joei.2018.07.019 1743-9671/© 2018 Energy Institute. Published by Elsevier Ltd. All rights reserved.
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The tar generated from biomass decomposition is easily to nucleate on the surfaces of pre-existing particles or to form POM during subsequent gas cooling. The char particles (>1 mm) are the major solid residuum of biomass devolatilization, which are released with the flue gas flow. The submicron fly ash (<1 mm) is produced by mineral vaporization and re-condensation (e.g., K, Na, S, Cl, P, and Zn) [13e15]. Due to the high content of K in biomass, K-containing species in atmospheric aerosols are the indicators of the contribution from biomass burning [16e18]. The yield and component of PM from biomass combustion depend on the combustion conditions such as furnace temperature, fuel type, oxygen concentration, air distribution, furnace type, etc. Under fuel-lean conditions (e.g., the large-scale biomass boiler), most soot and POM are burned out and the dominated components in fine particles are ash minerals; in contrast, under fuel-rich conditions (e.g., the biomass open burning, the house-hold stove), the dominated are the carbonaceous particles [7,19,20]. In view of the instability and complexity of POM, in this study, we focus on the formation of inorganic particles including soot, potassium-containing particles, and ash particles, while the formation of POM is not discussed. Pyrolysis is the first stage of the overall combustion process, and is also one of the most important biomass conversion technologies. The studies on the fine particle formation during biomass pyrolysis are essential for the control of PM emission and the improvement of the combustion efficiency. In general, few studies had focused on particle evolution mechanisms during biomass high-temperature pyrolysis. Henrik Wiinikka [21,22] characterized the submicron particles produced during oxygen blown entrained flow gasification of wood powder and investigated the influence of operating condition on particle formation. It was found that the soot particles in the real gasifier environment had less ordered nanostructure and higher reactivity as the temperature increased. The observation is in contrast with what is usually observed when using well-defined fuels. Qin et al. [23] and Trubetskaya et al. [24] investigated the influence of biomass constituents on soot formation, and the results showed that compared to cellulose and hemicellulose, lignin is more important for soot formation. Long et al. [25] studied the release of alkali and alkaline earth metals (AAEMs) during biomass pyrolysis, and proposed that the release of AAEMs were caused by the free radical attacking on bonds between the char matrix and AAEMs. Kurian et al. [26] reported that the PM10 emissions from the pyrolysis of oil sand asphaltenes presented an unimodal size distribution with the peak size at about 1 mm. In summary, most of the previous studies paid attention on the single component of fine particles. In this study, different contents in PM2.5, including carbonaceous matter (like soot and char), K-containing particles, and ash particles were separated and characterized respectively to investigate the evolution of these contents with reaction temperature. The wheat straw was pyrolyzed in an entrained flow reactor at 900e1300 C. The obtained results are helpful to deeply understand the formation mechanism of fine particles from biomass pyrolysis and combustion. 2. Material and methods 2.1. Feed material The biomass used in this study is the wheat straw from Baoji district of Shaanxi province, China. The proximate, ultimate analyses and ash composition of fuel samples are shown in Table 1. The ash compositional analysis was performed by an X-ray fluorescence (XRF) instrument. Prior to the XRF analysis, 1 g biomass samples were heated in a muffle furnace at 550 C for 7 h, following the procedures in the Chinese standard GB/T 28731-2012. The fuel particles were pulverized and sieved into 100 mm, and then dried at 90 C in oven for 12 h before use. 2.2. Experimental setup The pyrolysis experiments were conducted in an entrained flow reactor system as shown in Fig. 1. It is comprised of a fluidized-bed feeding unit, gas supply unit, reactor and furnace heating unit, and solid product sampling unit. The corundum tube reactor is 1200 mm in length and 54 mm in diameter. The furnace is electrically heated by a silicon-molybdenum resistance element to a maximum temperature of 1500 C. The temperatures are measured by Pt-Rh-Pt thermocouples (accuracy, ±1 C). Three zones of temperature controlling yield a constant-temperature region of 500 mm long, as shown in Fig. 1. The pyrolysis temperatures studied at in this work were 900, 1000, 1100, 1200, and 1300 C. A micro-scale fluidized-bed feeder was used to feed biomass particles into the reactor through a water-cooled feeding probe. The feeding rate was 0.22 g/min. Biomass particles were carried by the primary nitrogen flow of 1 L/min, and the secondary nitrogen flow of 3 L/min was introduced from the surrounding. The gas residence time in the isothermal zone was 3e4 s in the studied temperature range. As shown in Fig. 1, all of the gas and solid products were sampled using a hot-gas diluted probe, which fulfilled the requirement of the PM2.5 cyclone (URG-2000-30ENS-1, URG) on the operation flow at 10 L/min, by introducing additional hot dilution gas of 6 L/min. Through the PM2.5 cyclone, the coarse particles larger than 2.5 mm were removed and collected from the flue gas. The fine particles (PM2.5) after cyclone were collected by a quartz or metal filter with a diameter of 47 mm. The PM2.5 sampling unit was heated up to ~230 C to avoid the significant condensation of POM. The main components of the PM2.5 contain ash-embedded char fragments, soot and condensed volatile inorganic particles. The sampling process at each condition was conducted in duplicate to ensure the repeatability.
Table 1 Proximate analysis, ultimate analysis, and ash composition of the biomass used in this study. Proximate Analysis (wt.%) Mad 7.66
Ad 9.93
Vd 74.40
Ultimate Analysis (wt.%) FCd 15.67
Cdaf 45.53
Hdaf 5.26
Ndaf 0.78
Ash composition (wt.%) Odaf 48.15
St,daf 0.28
K2O 12.56
Fe2O3 2.35
CaO 8.01
SiO2 12.52
Cl 3.44
S 1.02
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Fig. 1. The (a) schematic of the entrained flow reactor and (b) temperature profiles.
2.3. Chemical and physical analyses of PM2.5 samples Morphologies and elemental compositions of PM2.5 samples were determined using SEM-EDS (SU8010, Hitachi, Japan), and highresolution STEM (FEI Tecnai G2 F30, 300 kV, Hillsboro, USA). Samples used for STEM analysis were first separated from the quartz substrate and then treated by ultrasonic extraction using dichloromethane solvent for 5 min. After centrifugalizing three times, the particles were deposited on copper grid. A detailed procedure on the pre-treatment was outlined in the literature [27]. The crystalline constituents of the PM2.5 were characterized by using a PANalytical X'pert diffract meter. It was analyzed by using Cu Ka radiation in the range of 10e80 (wavelength 1.5406 Å) with the scanning step of 0.05 /s. The main mineral phase peaks were matched with the PDF data of stand substance peak by adopting Jade 6.0 software. PM2.5 samples collected by the metal filters were analyzed by a thermogravimetric analyzer (TGA, Netzsch STA-449C, Germany). In TGA measurements, 5 mg sample in a corundum crucible was heated from 35 C to 1000 C at a rate of 10 C/min. The atmosphere was 10 vol.% O2: 90 vol.% N2. With this approach, different phases in PM2.5, such as moisture, volatiles, fixed carbon, KCl, and residual ash, can be quantified. 3. Results and discussion 3.1. PM2.5 yields PM2.5 yields from biomass pyrolysis in the temperature range of 900e1300 C are shown in Fig. 2, and the original appearances of PM2.5 samples on quartz substrates are compared in Fig. 1. The total yield of PM2.5 increased with increasing temperature, accompanied by the deepening of appearance color. A sharp increase in PM2.5 yield from 12.8 g/kg dry biomass (db) to 27.3 g/kg (db) was observed when the pyrolysis temperature increases from 1000 C to 1100 C. The yield of PM2.5 produced at 1300 C reached 33 g/kg (db). In addition, through comparing the PM2.5 yields in this study with previously reported data [23,24] (also presented in Fig. 2), It can be found that the yields of PM2.5 emissions were in a wide range (7e90 g/kg (db)), depending on different biomass origins. The wood produces more fine particles than herbaceous biomass, which is probably because of the different constitutes between the two biomasses. Woody biomass has higher lignin contents but lower alkali metal contents than herbaceous biomass. Researches indicated that lignin tends to form more aromatics through guaiacol- and syringol-groups during biomass pyrolysis [27,28], thus creating more soot particles. In contrast, high potassium content can catalyze the decomposition of soot precursor tar, resulting in less soot formation for herbaceous biomass [29]. 3.2. Distributions of different components in PM2.5 From the XRD patterns of PM2.5 (Fig. 3), the strong signal of KCl could be observed, which indicates the significant amount of potassium salts in all PM2.5 samples. The broad reflection around 25 appeared at high temperatures (1200e1300 C) is assigned to the turbostratic structure of graphene layers in soot, corresponding to the 002 reflection of graphite. As the temperature increased, this refection became Please cite this article in press as: Y. Li, et al., Evolution of PM2.5 from biomass high-temperature pyrolysis in an entrained flow reactor, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.07.019
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Fig. 2. The PM2.5 yields of biomass pyrolysis at varied temperatures from 900 C to 1300 C.
more obvious, demonstrating the further development of a crystalline phase. Besides soot, a certain amount of SiO2 was observed at 1000 C, while it almost disappeared at 1100 C and 1200 C. The quantification of different phase composition in PM2.5 was performed in a TGA reactor. The mass loss curves are shown in Fig. 4, where two steps of mass losses are observed. The first mass loss is in the temperature range of 400e500 C, which is ascribed to the oxidation of carbonaceous matter. These carbonaceous matter includes carbon in soot and carbon in char, named as soot-C and char-C, respectively. Because both soot and char are carbonaceous substances, it is difficult to separate the two by oxidation. However, since char usually contains inorganic ash, the content of char in PM2.5 can be roughly estimated by the ash content. The second mass loss starts around 800 C, which coincides with the evaporating temperature of KCl. Considering the strong signals of KCl in XRD patterns, we ascribe the second mass loss into the evaporating of KCl. After the heating and oxidation of PM2.5 samples in TGA, a certain amount of solid residue was obtained, that consisted of the refractory ash and the condensed minerals with an evaporating temperature above 1000 C. The measured elemental compositions of the ash residual
Fig. 3. XRD spectra of PM2.5 samples.
Fig. 4. TG and DTG curves of PM2.5 samples.
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at different pyrolysis temperatures are compared in Table 2. Main elements in the residue are Si, Ca, K, Al, Mg, S, among which Si, K content is about 35 wt.% and 12 wt.%, respectively, while a negligible amount of Cl is observed. This indicates that partial K is chemically captured by the silicate network [30,31]. By means of the quantification method of TGA, percentage contents and yields of the three components (carbonaceous matter, KCl, and ash) in PM2.5 were obtained in Table 3 and Fig. 5. For carbonaceous matter, its yield increased with the increasing temperature. In low temperature range of 900e1000 C, the percentage and yield of carbon were relatively stable. In terms of the high ash percentages (40.04e46.87 wt.%) in this temperature range, it is deduced that below 1000 C the carbonaceous matter contains a large amount of char-C. This conclusion can be further verified by the SEM observation in later discussion. When the pyrolysis temperature further increased from 1000 to 1100 C, the yield of carbonaceous matter showed a fast increase, from 11.6 to 24.8 g/kg (db), along with its percentage increased from 41.46 wt.% to 51.43 wt.%, and the percentage of ash decreased from 40.04 wt.% to 21.86 wt.%. This indicates a large amount of soot was formed at 1000e1100 C. Above 1000 C, the further increase of temperature significantly promotes the formation of soot. When the pyrolysis temperature was beyond 1200 C, the percentage of carbon was stable at ~70 wt.% while that of ash was 10e20 wt.%. This indicates that at higher temperatures, soot became dominant in PM2.5. KCl is another non-negligible component in PM2.5 from biomass pyrolysis, as shown in Table 3, its percentage content was in the range of 7.34e23.47 wt.% depending on the temperature. Fig. 5 shows that the KCl yield first increased from 0.52 g/kg (db) to 5.82 g/kg (db) with temperature increasing from 900 C to 1100 C, then dropped to 2.88 g/kg (db) with temperature further increasing to 1300 C. The increase of KCl yield at low temperatures can be well understood because the increased temperature favors the evaporation of KCl. However, when the temperature increased to higher than 1100 C, the KCl content started to decrease. The reasons involved the following: (1) The reaction between KCl and aluminosilicates is enhanced by the temperature (2KCl þ Al2O3$2SiO2 þ 4SiO2 þ H2O / KAlSi3O8 þ HCl) [13]. (2) The produced potassium aluminosilicates have a melting temperature around 1150 ± 20 C, which causes ash fusion in biomass fuel particles. As a result, partial inner channels in biomass fuel particles are blocked, suppressing the diffusion and release of volatiles (including potassium and other mineral vapors) through the pyrolytic charash shell [32]. (3) The melted surfaces strengthen their collision capture capabilities for fine KCl particles [33], thus KCl particles are more distributed in coarse particles (PM2.5þ) than in PM2.5. In view of the similar release mechanism to KCl, the profile of ash yield versus temperature showed the similar trend. Note that the ash yield started to increase when temperature increased from 1200 C to 1300 C. Seen from XRD patterns, a small amount of SiO2 appeared in
Table 2 Elemental analyses of solid residues of PM2.5 in TGA reactor. T (oC)
Composition (wt.%) Na
Mg
Al
Si
P
S
K
Ca
Fe
Cu
Zn
1000 1100 1200
1.63 2.22 4.33
6.39 10.01 7.05
5.48 5.85 4.31
35.13 34.54 30.59
2.97 4.66 3.60
3.64 2.93 6.63
13.86 9.90 15.55
22.80 23.34 19.84
5.87 4.25 6.13
0.82 1.14 1.03
0.76 1.17 0.94
Table 3 Percentage contents of three phases in PM2.5 samples. T (oC)
Carbonaceous matter (wt.%)
KCl (wt.%)
Ash (wt.%)
900 1000 1100 1200 1300
39.37 41.46 51.43 68.53 70.50
7.34 17.20 23.47 16.94 8.60
46.87 40.04 21.86 13.65 20.75
Fig. 5. Yields of three phases in PM2.5.
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Fig. 6. SEM images of char particles in PM2.5 (*: the cracked cenospheres are more likely to break by mechanical force during the SEM sample preparation process).
1300 C samples. This is due to the intensified fragmentation of char particles at higher temperatures, producing more fine fragments. The included ash leads to the increase of ash content in PM2.5.
3.3. PM2.5 morphologies 3.3.1. Char particles The general morphologies of PM2.5 are showed in Fig. 6. Although the cyclone can separate majority of the coarse particles, quite a few supermicron particles with geometric diameters larger than 2.5 mm can be still observed in the fine particles due to the limited separation efficiency. Those are pyrolytic char particles. The morphology of char is strongly influenced by the biomass origin (including organic constitutes and the minerals) and the combustion conditions (such as temperature, heating rate and pressure). As seen from Fig. 6, PM2.5 formed at 900 C was predominated by supermicron char particles. When temperature increased to 1100 C and 1300 C, due to the large formation of soot, char particles are coated with densely packed soot particles. In this experiment, two types of char particles were identified, the irregular shape particles and the spherical shape particles. The former was produced from the fragmentation of char particles, while the latter was closely associated with the eutectic minerals with low melting temperatures in char particles, which mainly contain CayKySiyAl as indicated by the EDS results in Fig. 7. Trubetskaya et al. [34] proposed that in addition to plant cell constituents, the potassium silicates is the main mineral matter that significantly affects the formation of these near-spherical wheat straw chars.
Fig. 7. Elemental compositions of spherical char particles at three temperatures.
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Fig. 8. STEM images of mature and immature soot particles.
Increasing temperature gives rise to a higher fraction and a faster release rate of volatiles, thus promoting the swelling extent of the liquid metaplast, creating large internal cavities in char particles. As a result, char particles are more likely to collapse and produce more fragments [35,36]. Fig. 6 shows that as the temperature increased, the produced char particles had undergone a more severe transformation. The char particles at higher temperatures showed a smaller size and a more melting surface. At 900 C, a small amount of KCl crystal was gently adhered to the surface of the char particles. When the temperature rose to 1100 C, the amount of adhered particles increased significantly with an obvious melting state and they were fused with the large particles. When the temperature reached 1300 C, the fusion became more intense, forming a smoother surface of the char particles. This indicates that the trapping effect, both chemically and physically, of char particles on KCl particles is enhanced with the increasing temperature. The observation in Fig. 6 corresponds to the results in Table 3. 3.3.2. Soot particles Soot is one of the products from the secondary reactions of biomass volatiles at high temperatures. The formation mechanism of soot is complicated, involving hundreds of steps, but can be summarized into four stages: the nucleation from gaseous hydrocarbons, the particle growth by surface reactions, the particle coagulation, and the agglomeration [37]. STEM images of soot particles formed at different temperatures are showed in Fig. 8. Similar to soot particles from the combustion of small-molecule hydrocarbon fuels, the soot particles formed during biomass pyrolysis also consist of the agglomerates of nanometer spherules. These nanometer spherules exhibited a concentric arrangement of graphitic layers, roughly parallel and equidistant, forming an onion-like microtexture. The diameter of spherules varied from 10 to 180 nm, dominantly in the range of 20e40 nm. With the increase of temperature, the soot particles became more spherical and more onion-like. At 1000 C (Fig. 8a), an amorphous structure of soot with disordered inner fringes is observed, which is because a lower temperature cannot sufficiently drive the growth and the reorientation of carbon lamellas [38]. When the temperature was beyond 1100 C, a more mature graphitic structure is formed (Fig. 8b,c). Particles originated at 1000 C are asphaltene precursors rather than the carbonaceous soot. 3.3.3. Submicron KCl particles Biomass contains a considerable amount of potassium, which is easy to release as potassium vapor and then condense to form KCl particles. In this study, a large number of particles with regular crystal structures are observed by STEM, as shown in Fig. 9, the sizes of these particles are uniform and in the range of 100e200 nm. Significant signals of potassium and chlorine are found in the EDS spectrograms, therefore, it can be concluded that these crystal particles are KCl. Moreover, we can observe some amorphous liquid-like substances coated on the surface of KCl particles at 1000 C, and they disappeared at 1300 C. These amorphous substances are considered as the relatively heavy species of the pyrolytic tar. Light species with boiling points lower than 230 C were removed from PM2.5 by heating the sampling unit, however, the heavy fractions with boiling points higher than 230 C could still condense to form POM. As the temperature increased, the tar decomposed gradually. This agrees with the previous research which reported that the tar generated during biomass pyrolysis completely decomposed at 1200 C [39,40].
Please cite this article in press as: Y. Li, et al., Evolution of PM2.5 from biomass high-temperature pyrolysis in an entrained flow reactor, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.07.019
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Y. Li et al. / Journal of the Energy Institute xxx (2018) 1e9
Fig. 9. STEM images and EDS spectrogram of KCl particles.
4. Conclusion The distribution and evolution of different phases in PM2.5 from biomass pyrolysis at 900e1300 C were investigated in an entrained flow reactor. On the basis of morphology and composition analyses, the main conclusions are as follows: (1) The overall PM2.5 yields during biomass pyrolysis were in the range of 7e34 g/kg (db) and generally increased with increasing temperature. In the low temperature range of 900e1000 C, the carbonaceous matter content was stable at about 40 wt.%, dominated by char-carbon. When the temperature increased from 1000 C to 1100 C, the content of carbonaceous matter in PM2.5 significantly increased because of the formation of soot particles, which became dominant in PM2.5. (2) The soot graphitization started at 1100 C, when matured soot particles with concentric graphitic layers were observed. The diameter of soot particle varied from 10 nm to 180 nm, with majority in the range of 20e40 nm. With the increase in temperature, the soot particles became more spherical and more onion-like. (3) The release of potassium salt in PM2.5 increased initially with the temperature. When the temperature was above 1100 C, char particles underwent aggravated melting due to the formation of low-melting potassium aluminosilicates, resulting in the decline of KCl. When the temperature increased to 1300 C, the fragmentation of char particles were significantly strengthened, producing more PM2.5 from the char fragments, thereby increasing the ash content in PM2.5. Acknowledgements This study was supported by the National Key Research and Development Plan of China (No. 2016YFB0600605) and National Natural Science Foundation of China (No. 51676157). Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.joei.2018.07.019. References [1] S. Akagi, R.J. Yokelson, C. Wiedinmyer, M. Alvarado, J. Reid, T. Karl, et al., Emission factors for open and domestic biomass burning for use in atmospheric models, Atmos. Chem. Phys. 11 (2011) 4039e4072. [2] M.O. Andreae, P. Merlet, Emission of trace gases and aerosols from biomass burning, Global Biogeochem. Cycles 15 (2001) 955e966. [3] X. Wang, H. Tan, Y. Niu, M. Pourkashanian, L. Ma, E. Chen, et al., Experimental investigation on biomass co-firing in a 300MW pulverized coal-fired utility furnace in China, Proc. Combust. Inst. 33 (2011) 2725e2733. [4] A. Winquist, J.J. Schauer, J.R. Turner, M. Klein, S.E. Sarnat, Impact of ambient fine particulate matter carbon measurement methods on observed associations with acute cardiorespiratory morbidity, J. Expo. Sci. Environ. Epidemiol. 25 (2015) 215e221.
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Please cite this article in press as: Y. Li, et al., Evolution of PM2.5 from biomass high-temperature pyrolysis in an entrained flow reactor, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.07.019