Formation and growth mechanisms of ultrafine particles in sludge-incineration flue gas
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Atmospheric particulate matter with diameter < 2.5 μm now makes up much of the air pollution in China, but it is the ultrafine particles (UFPs) with diameter < 90 nm that are of particular interest. This is because UFPs are strongly linked with human health for two reasons: they contain a variety of hazardous substances and they can deeply penetrate human respiratory systems. Therefore, scanning electron microscopy combined with X-ray dispersive energy spectrometry was used to characterize the morphology and surface texture, as well as the elemental composition of 60 UFPs. The UFPs was generated in a sewage sludge-incineration power plant in Zhejiang Province. This was done to determine the microstructure of the ultrafine particles and to follow the evolution of particle surface elemental composition with increasing particle size. Then, a comparison of the characteristic time for nucleation, condensation and coagulation was done to estimate the dominant mechanism. The results show that the UFPs have generally irregular shapes (cotton-like, irregular balls, sheets, etc.) and that they usually aggregate to form a mass. With increase in the size of a UFP, the mass fraction of the elements presents clearly changed: Na, K and Fe gradually decreased; while Ca, Si and Al, as well as the heavy metals Cu, Zn and Ni increased. Characteristic time estimation is a convenient and effective tool for identifying the predominant mechanisms during combustion. In this study, calculations of characteristic time were used to reveal a mechanism of vaporization, nucleation, condensation and coagulation, which drives the formation and growth of ultrafine particles.
KeywordsUltrafine particle Characteristic time Nucleation Condensation Coagulation
Sewage sludge causes serious environmental pollution because it contains some harmful and toxic substances, including viruses, bacteria, dioxins, non-biodegradable organic compounds and heavy metals [1, 2]. Incineration has been considered the most thorough, quick and economical way of sludge disposal due to its advantages on stabilization, volume reduction and resource recovery [3, 4]. Nevertheless, even though incineration has advantages of the reduction of sewage sludge volume and lower disposal costs. On account of almost 30% of the solids occur as incineration residues in the forms of fly ash and air pollution control (APC) residues , it is still not a perfect method to deal with sludge In recent decades, fly ash, which contains higher concentrations of toxic heavy metals [6, 7] and dioxin , has become an increasingly important issue . Especially revealed of the ultrafine particles (UFPs, diameter < 90 nm). Typically, UFPs account for about 90% of the total number–concentration of particles in the atmosphere [10, 11] and exhibited even more severe toxicity relative to larger particles of the same composition . The ultrafine particles are strongly linked with adverse health effects, including cardiovascular and pulmonary ailments and premature deaths in people with heart or lung disease . For these reasons, UFP air pollution has become a topic of great interest in terms of air quality harm, public health and global climate. Of particular interest, are the processes of driving formation of ultrafine particles.
Many scholars have carried out research on the physical and chemical properties of ultrafine particles. Abramesko and Tartakovsky  investigated the UFP air pollution inside diesel-propelled passenger trains in order to reduce train passenger exposure to these harmful particles. Buonanno  characterized the UFPs emitted at the stack of a waste-to-energy plant from a dimensional, chemical and morphological point of view. He found that the particle size number distribution at the stack presented a mode at about 90 nm, whereas the one measured before the fabric filter showed a higher value (about 150 nm). However, there is little information about the formation of ultrafine particles in the process of sludge incineration. Typically, particulates are characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). These technologies are widely employed for studying the microstructure, morphology and surfaces of solids, respectively. Through XRD it is possible to identify the microstructural state of particles (if the particle is crystalline or amorphous) and to identify the different crystalline phases. SEM offers the possibility to relate visually the morphology and size. XPS provides qualitative and semi-quantitative information regarding the chemical composition, oxidation state and bonding energy of the elements composing the particles .
Although conventional analytical techniques such as XRD, XPS, SEM, XRF, Raman spectroscopy, ICP-AES and ICP-MS allowed to evaluation the morphology and average concentration of trace elements in bulk samples, they do not provide information on the microstructure or chemical composition of a single ultrafine particle . Single-particle characterization can provide information on the evolution of size distribution and chemical composition of UFPs. Previous studies on UFPs have been focused on their classification based on size, concentration and chemical composition of whole particle masses, with relatively few details on the size, shape and chemical composition of individual particles.
For the reasons given above, field emission scanning electron microscopy (FE-SEM) combined with X-ray dispersive energy spectrometry (EDS) was used to characterize UFPs. At high magnification, FE-SEM offers a quick and easy way of analyzing particular matter (PM) down to 10 nm in diameter, and can provide brilliant images of the particle morphology, shape, size and roughness. Chemical information as well, can be acquired for individual particles using energy dispersive spectroscopy (EDS) . In this work, the UFPs were not present individually, but were aggregated to form larger particles. For this reason, particles of < 2.5 μm were obtained by sieving, and more than 50 images of PM2.5 were obtained. From these, the morphology and chemical composition of 60 UFPs adsorbed to larger particles were analyzed using SEM–EDS to characterize the evolution of UFPs.
In addition, quantitative simulation of UFP formation was provided using mathematical models. Kulmala et al.  used measurements during nucleation episodes of evolving size distributions down to 3 nm to calculate the apparent source rate of 3 nm particles and the particle growth rate. Wu and Biswas  compared the characteristic times of nucleation and condensation, to determine the fate of metallic species in a combustion environment. In such cases, there are many processes of particle formation and growth occurring simultaneously due to the high temperatures of the combustion environment (e.g., nucleation, condensation, coagulation and so on). For better understanding of them, it was necessary to identify the relative importance of these mechanisms in the combustion environment by comparing their characteristic times.
Materials and methods
The particles used in these experiments were collected in a sewage sludge-incineration power plant in Zhejiang Province, China. The particles were collected from bag filters, which are very common at sludge-incineration plants. First, the samples were randomly collected over 3 days. The mass of each sample was 5 kg to guarantee a representative particle sample. Then according to the centrifugal principle, an air classifier was used to separate particles of different size. The rotation speed of the induced-draft fan was adjusted to regulate the size of the internal negative pressure traction of the equipment. This guaranteed the fineness and precision of the grading treatment, and we obtained a fine powder of approximately 2.5 μm. Compared with the use of sizing screens, high-precision air classification equipment causes minimal damage to the original particles; thus, the accuracy of the final classification was improved significantly.
Composition and phase analysis of the particles
Main components of a ultrafine particles (wt%)
Microstructure and surface composition analysis of the particle
In this investigation, the morphology, crystal structure, surface topography and chemical composition of 60 UFPs were investigated using a Nova NanoSEM450 FE-SEM. The electron microscope resolution was 2 nm and the magnification was ~ 7 × to ~ 500,000 ×. The FE-SEM was fitted with a backscatter electron detector that not only gave visual imaging magnification up to 500,000 ×, but could also distinguish electron density in the sample. That is to say, heavier elements appeared brighter and the lighter elements appeared darker. Having the FE-SEM fitted with EDS-detectors provides the option of scanning an area or a spot for elemental composition. The atomic content of particles fixed on a substrate can in this way be analyzed by focusing the electron beam on an isolated particle and detecting the resulting X-ray emissions . Thus, the images of the particles and the specific sizes of 60 UFPs were obtained. The surface chemical composition of individual particles was obtained using EDS, and the elements detected included oxygen, sodium, potassium, iron, calcium, silicon, aluminium, sulphur and trace elements.
Results and discussion
Ultrafine particles characteristics
The main chemical components of the PM2.5 were determined by X-ray fluorescence spectrometer (XRF) and normalization methods. The results from Table 1 show that the PM2.5 collected was mainly composed of the elements C, O, Fe, Al, Ca, Si, S and P. The particles contained more sulphur because, in the process of sludge incineration, the sludge was mixed with coal to improve its calorific value. The high calcium content in the particles may be because, to control the acid gas generated during burning, a certain amount of lime was added. The XRD results regarding the PM2.5 fraction showed that the main components were silicon dioxide and iron oxide; followed by calcium sulphate. Aluminosilicates were present in lower proportion, and occasionally, aluminium dioxide was also found.
Next, the SEM–EDS was used to provide detailed image information on the morphology and surface texture of individual particles, as well as the surface elemental composition of the UFPs. As noted by Vassilev and Vassileva , SEM is one of the best and most widely used techniques for the chemical and physical characterization of fly ash.
Apparent composition analysis of ultrafine particles
Figure 4a shows the mass fractions of alkali metals: Na and K gradually decreased with increase in the particle size from the initial fractions (1.5%) to the last (0.2%). Elemental iron showed the same obvious trend, decreasing from the initial (37%) to the final (18%) condition (Fig. 4b). However, the mass fractions of the main elements Ca, Si and Al gradually increased with increase in the particle size. There were some calcium-rich particles of size 71.69 nm. The calcium-rich particles usually showed more roundness than other particles . There were also Si-rich particles (95.47 nm and 139.8 nm), which were characterized by a prismatic, regular morphology typically observed in crystalline materials . Figure 4c, d shows the mass fractions of heavy metals. The fractions of Cu, Zn and Ni gradually increased with increase in particle size, while the mass fraction of Pb, Cd, Cr and As tended to be constant. The heavy metal mass fraction was very low: < 1.5%.
Many scholars have carried out researches on the formation mechanism of fly ash. Seames  proposed a tri-modal particle size distribution that included a submicron fume region to describe the particle formation. Wang et al.  considered that PM1 is formed from vaporization and then subsequent nucleation and accumulation of material. In more detail, Li et al.  found that UFPs mostly formed by vaporization and nucleation of mineral metals associated with the volatiles. These gas-phase minerals came mostly from two sources: the volatilization of mineral components present in water-soluble or ion-exchangeable form (Na, Cl, etc.), and vaporized gas from mineral inclusions through a reduction reaction as MOn (solid) + CO = MOn-1 (vapor) + CO2. In this case, MOn and MOn-1 refer to the refractory oxides (SiO2, CaO, etc.) and the corresponding volatile sub-oxides (SiO) or metal vapous (Ca), respectively [27, 28, 29]. It can be seen from Fig. 4 that, with increase in particle size, the mass fractions of the volatile alkali metal elements Na and K gradually decreased. This indicates that the initially occurring Na and K nucleation was homogeneous. This was followed by Fe, which provided heterogeneous nucleation of secondary nanoscale particles. Then, secondary nanoscale particles were formed by coagulation, heterogeneous condensation and collision. During this process, elemental Si, Ca and Al condensed to form nanoscale particles, so the mass fractions of Si, Ca and Al gradually increased.
With regard to heavy metal concentrations, the elements with higher boiling temperature presented higher concentrations at lower diameters. It disclosed that incomplete evaporation during combustion followed by consequent condensation of semi-volatile compounds on solid nuclei . From Fig. 4a, c, d, the concentration of the mass fractions of trace heavy metal elements is significantly higher in these UFPs, than it was for the alkali metals. The specific process by which trace heavy metal elements transform from the vapor phase to the condensed phase is important in assessing the toxicity of waste products.
Characteristic times for UFP formation and growth
The calculations indicated that the critical diameter for homogeneous nucleation is dp = 0.419 nm, and that the nucleation characteristic time is 0.1268 ms. This presented that the nucleation process occurs first and that the process is very quick. Once the UFPs form, the vapor caused rapid growth of the submicron particles by condensation, and simultaneously, the particles collided with each other to form particle aggregated of about 1 nm. As Fig. 5 shows, the characteristic time of condensation is shorter than the time of coagulation when the particle size is < 1 nm. Then, as the size of the particles increases, coagulation becomes the dominant mechanism of particle growth. The characteristic time of condensation gradually increased because the increase of the specific surface area leads to difficulty for condensation of vapor onto the particles. However, the coagulation coefficient increases with the square of the particle number concentration and increases non-linearly with decreasing particle size . This is because the characteristic time of coagulation gradually decreases with increasing particle size.
The morphology and surface chemical composition of 60 UFPs were characterized using SEM–EDS. The particles were usually irregular spheres, and the submicron particles (PM1) resulted from coalescence of UFPs. The mass fractions of heavy metals (Cu, Zn and Ni) were higher than those of alkali metals (Na and K), indicating that the UFPs have high heavy metal toxicity and must be properly handled.
With increase of the particle size from 10.17 to 155.9 nm, the mass fractions of Na, K and Fe gradually decreased, while the main elements Ca, Si and Al, as well as the heavy metals Cu, Zn and Ni, gradually increased. This indicates that, during the process of particle formation, the initial part of the sequence involves Na and K nucleation, followed by Fe nucleation. Then, the elements Si, Ca and Al, and parts of the heavy metal elements, condense onto the nanoscale particles.
According to the calculation of characteristic times, the nucleation process occurs initially, then rapid growth of the submicron particles to about 1 nm occurs by condensation. When the particle size is > 1 nm, coagulation becomes the dominant mechanism of particle growth.
The authors acknowledge support by the National Natural Science Foundation of China in the form of a research grant (No. 51576134) and the National Key R&D Program of China (2017YFC0703100).
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