Review articleCritical review of condensable particulate matter
Introduction
Fossil fuels (coal, petroleum, and natural gas) account for about 86% of the world’s primary energy consumption. Particulate matter (PM) emitted by fossil fuel combustion is the main cause of air pollution such as haze [1], [2]. Condensable particulate matter (CPM) is a material that is gaseous at flue gas temperature before discharge but is formed as a particulate substance after dilution and cooling in the plume. Filterable particulate matter (FPM) is another type of particulate that is commonly known as soot. Total particulate matter (TPM) is composed of CPM and FPM. With the rapid development of FPM control technology, the emission concentration of FPM is considerably decreasing [3], [4], making the discharge of CPM an issue that merits attention [5]. At present, people are focusing on CPM because its characteristics make it difficult to remove. However, only a few studies have explored CPM, and relevant knowledge about CPM is limited. For this reason, we collected relevant literature and conducted a review on the status of CPM research.
Corio and Sherwell [6] found that CPM accounts for about 76% of the total PM10 emitted by coal-fired boilers and about 50% of the total PM10 emitted by oil- and natural gas-fired boilers. After testing the CPM emission of three coal-fired boilers (300–1000 MW), Pei [7] found that the average CPM concentration in exhaust gas is 21.2 ± 3.5 mg/m3 (i.e. about 25 mg/Nm3), accounting for 51% of the TPM. Existing data demonstrated that the emission concentration of CPM and its proportion of emissions are very large, so CPM’s potential harm to the environment cannot be ignored.
At present, no country in the world has set limits on CPM emissions. Existing regulations only focus on FPM emissions. For example, the US Environmental Protection Agency (EPA) stipulates that the emission limit of particulate matter in coal-fired power plants is 20 mg/Nm3 [8]. The EU stipulates that the particulate emission limit of coal-fired power plants is 30 mg/Nm3 [9]. China is a large coal-consuming country. China stipulates that the particulate emission limit for key areas is 20 mg/Nm3 [10]. In the past, people’s main efforts were focused on the control of FPM due to the huge amount of FPM emissions. As a result, FPM control technology rapidly developed. Nowadays, coal-fired power plants in China are implementing ultra-low-emission reform [11], which makes FPM emissions reach advanced levels below 5 mg/Nm3. After the resolution of FPM emissions, people’s attention began to shift to CPM. China’s Ministry of Science and Technology released a key national R&D project about the “Causes and Control of Air Pollution” in 2016. This project put forward to solve the key technology of CPM control [12], which is a research direction that will be developed in the future.
The authors reviewed the research topic of CPM to help researchers gain a systematic understanding of CPM. This review provides researchers with detailed information on the concept of CPM, its characteristics, test methods, emissions, and composition. The emission concentration of CPM in coal combustion is much higher than that in other types of fuel combustion [6]. Current CPM research has mainly focused on coal-fired sources. Therefore, this critical review concentrates on coal-fired stationary sources.
Section snippets
Basic cognition of CPM
As a result of the limited research attention to CPM in the past, there is a lack of understanding about CPM. There are also some misunderstandings in people’s cognition toward CPM. Therefore, we begin with an in-depth explanation of the definition of CPM.
Methods for measuring CPM and improvements
Accurate measurements are the basis of the in-depth study on CPM. The first CPM test method was originally proposed by the EPA, and other methods are basically improved versions of it. The principle of these methods is to condense gaseous CPM to CPMP for sampling and analysis.
In general, the sampling temperature of the method is set to ≤30 °C to simulate the temperature of the flue gas outlet (the condensing temperatures of different sampling devices vary). This setting can substantially ensure
Characteristics of CPM
In previous literatures, there is a lot of research about CPM’s characteristics. In this section, the size and morphological characteristics of CPM, emission concentration, and composition are reviewed to enrich the understanding of the characteristics of CPM.
Control of CPM
The emission concentration of CPM exceeds that of FPM and is the focus of particle control in the next step. The literature on CPM control is lacking. On the basis of the understanding of the characteristics of CPM and the references on existing methods for controlling gaseous pollutants, we propose two possible directions for the development of CPM control technologies. First, if the gaseous CPM could be condensed before discharge by a cooling process, it might be removed by the traditional
Conclusions
With the gradual decrease in the emission concentration of FPM, the proportion of CPM in TPM has gradually increased. The removal of CPM before discharge cannot be easily achieved with traditional dedusting devices. Therefore, the problem of CPM emission from stationary sources has attracted considerable attention. This article is a critical review of CPM. It summarizes CPM from the aspects of concept, formation mechanism, test methods, emission, composition, and morphological characterization.
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Declarations of interest
None.
Acknowledgements
The authors gratefully acknowledge the support from the National Key R&D Program of China (2017YFB0603202), the Key R&D Program of Shandong Province, China (2016GGX104009), and the Natural Science Foundation of Shandong Province, China (ZR2018MEE033).
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