Full Length ArticleSimulative prediction of ultrafine particulate matter formation by means of different pyrolysis models
Introduction
The formation of inhalable fine particles () is an undesirable side effect of various technical applications [1]. In particular, very fine particles () have a high lung damage potential because they have large residence times in the atmosphere and thus can accumulate in the human respiratory system [2], [3], [4]. An additional problem is the concentration of toxic trace elements (e.g. mercury or arsenic) bonded to the ultrafine particulate matter [5]. A significant source of anthropogenic fine particle emissions is the burning of solid fuels like coal or biomass [6], [7]. Coal is still one of the most important primary energy sources for electricity production worldwide [8]. PM emissions from coal-fired power plants in China amount to 3.81 million tons per year [9]. In addition, worldwide coal consumption is expected to increase due to a forecast in electricity consumption, particularly in developing countries [10]. The PM emission may become more severe, by increasing utilization of low-rank coals [11], [12]. Due to the high content of alkali and alkaline earth metals in raw lignite, the emission of can be increased by a factor of five compared to high-rank bituminous coal [12]. The reduction of emissions can either be achieved by a reduction of the primary particle formation or by an increased particle growth coming along with an improved deposition in the filter system [13], [14], [9]. An assessment of these measures requires basic understanding of reaction pathways and relevant parameters.
Coal combustion produces different types of particles with a wide range of sizes, morphologies and components due to different formation mechanisms, e.g. organic particles such as soot and inorganic particles such as ash. Previous numerical studies have developed a basic framework for the formation mechanisms of PM with different size ranges [15], [16], [17], [18]. It is assumed that submicron particles are formed during the entire process of coal combustion by an evaporation-nucleation-coagulation-agglomeration mechanism [19], [20], [21]. The conventional simulation of fine PM formation during coal combustion is mainly based on the redox reaction of minerals as proposed by Sarofim et al. [22], [23]. However, this model predicts the total yield of fine PM (mean values) without temporal and spatial resolution. In addition, detailed, size-resolved information on fine PM cannot be obtained with this model and particle dynamics are neglected. Various physical and chemical mechanisms are involved, such as the evaporation of volatile minerals, the redox reaction of refractory minerals or nucleation, coagulation, agglomeration and sintering [24], [23], [25]. The Population Balance Model (PBM), which is widely used in aerosol science and flame synthesis [26], [27], [28], [29], was used by Gao et al. [30] to simulate fine PM formation in the early stage of lignite combustion. The PBM is based on the General Dynamic Equation (GDE) of particles as formulated in [26]. By correctly defining a collision kernel in laminar flames, to describe the probability of a particle collision, the coagulation process of particles was simulated and the Particle Size Distribution (PSD) of fine PM was predicted. In addition, an important input for the PBM is the amount of minerals released. This release is closely related to the combustion behavior of coal particles in various combustion stages, including devolatilization, char combustion and char burnout [31], [13]. Therefore, an accurate description of coal devolatilization and combustion is relevant for determining the release rate and yield of PM. In the previous work by Gao et al. [30], the Fu Zhang Model (FZM) [32] was applied to determine the volatile release and finally calculate the ultrafine inorganic PM. The accuracy of the pyrolysis model to predict the PM formation has not been investigated by now. Therefore it is necessary to carry out further investigations in this area in order to gain a deeper insight into the PM formation.
The aim of this work is to investigate the influence of the devolatilization description within the PM formation simulation. For this purpose, the PBM is used to describe the PM formation of the early stage devolatilization. Li et al. [12] showed that the ultrafine particles smaller than 0.1 μm are mainly formed by nucleation/condensation of gas-phase minerals. Therefore, our simulations consider only the inorganic particle formation. The Fu Zhang model (empirical model, used in the previous work from Gao et al. [30]) [32] and the Chemical Percolation Devolatilization (CPD) model (sophisticated pyrolysis network model) [33], [34] are applied in this work to calculate the volatile release. To investigate whether the devolatilization model has an impact on predicting PM formation, the calculated PSD data with the two pyrolysis models are compared with experimentally obtained PSD data of a Hencken burner system.
Section snippets
Investigated experimental system
To validate numerical models for the prediction of PM-formation in solid fuel combustion, experimental data are relevant. Therefore Gao et al. [30] has performed experimental investigations with a flat flame (Hencken) burner. The Hencken burner (Fig. 1) enables similar particle heating rates (105 K s−1) as in common pulverized fuel burners [35]. The main advantage of a Hencken burner, however, is that it has a nearly homogeneous radial temperature distribution and a laminar gas flow [35]. This
Modeling
In this section the models to describe PM formation are presented. For this purpose, the description of the coal particle heating rate will be shown, which is decisive for the release of the volatiles and thus also for the PM formation [30]. Furthermore, the reaction mechanism of the ultrafine particulate matter and the volume-conserved discrete-sectional PBM will be presented. Finally, the differences of the pyrolysis models will be described in more detail.
Results and discussion
Fig. 5 shows the comparison of PSDs received from numerical simulations and experiments. The experiments were performed by Gao et al. [30]. For measuring the ultrafine particulate matter, a two-stage isokinetic sampling probe was used to collect the PM. Afterwards, the PSDs were measured by a scanning mobility particle sizer (SMPS). Because of the assumption of spherical particles in the simulations, the raw experimental data were corrected regarding their agglomerate sizes by applying the
Conclusion
In this study, the Population Balance Model (PBM) was applied for Particle Size Distribution (PSD) calculations in combination with two different pyrolysis models. To calculate the precursors of the PM, it is assumed that the precursor release is proportional to the volatile release. The volatile release calculated with the CPD model starts later than predicted in the FZM model. This leads in the PSDs, to a shift to smaller PM diameters caused by the reduced residence time of precursors (Na and
CRediT authorship contribution statement
Christian Axt: Conceptualization, Investigation, Writing - original draft. Stefan Pielsticker: Conceptualization, Methodology, Investigation, Writing - original draft. Thobias Kreitzberg: Writing - review & editing. Oliver Hatzfeld: Project administration. Qi Gao: Methodology. Shuiqing Li: Supervision. Reinhold Kneer: Writing - review & editing, Supervision.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors would like to thank the German Research Foundation (DFG) (Grant No. 392429716) and the National Natural Science Foundation of China (NSFC) (Grant No. 51761135126) for funding the joint Sino-German research project “Inorganic fine particulate matter formation during turbulent pulverized coal combustion”.
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