Review of organic pollutants in coal combustion processes and control technologies

Abstract

Organic pollutants are acknowledged as one of the primary environmental hazards in the atmosphere, posing a significant threat to human health and the environment. This work provides a critical review of the recent research on organic pollutants from stationary coal-burning sources, including an overview of the effect of coal composition and coal source, the types of organics material in coal, the generation of organic pollutants during coal combustion, emission of organic pollutants, co-removal by air pollution control devices (APCDs), and the technologies used to remove organic pollutants from coal-fired power plants (CFPPs). Field sampling and analysis showed that the organic pollutants produced from coal combustion processes are mainly composed of volatile organic compounds (VOCs), polycyclic aromatic hydrocarbons (PAHs), and organic components in condensable particulate matter (CPM). The VOCs and CPM are mainly discharged into the atmosphere as part of the gas and particulate phases, respectively, whereas, PAHs are mainly concentrated in fly ash and bottom ash. The generation of organic pollutants during coal combustion is mainly affected by coal grade, temperature, heating rate, residence time, and pressure. Considering that the flue gas temperature and composition can vary from one CFPP location to another, the choice of the sampling methods is based on the specific needs of a CFPP. Because, the selection of a sampling method can significantly affect the final results and should be given special attention. Typical sampling methods for VOCs, PAHs, and CPM are summarized and their advantages and disadvantages are compared. The adsorption tube sampling method is more suitable for the sampling of VOCs because it is not limited by the volume and miscellaneous components of the flue gas. EPA Method 0010 and the dry impactor condensation method (EPA Method 202) are more suitable for sampling PAHs and CPM in flue gas due to their higher accuracy. The APCDs in CFPPs have organic pollutants co-removal abilities besides those for conventional pollutants. As a result, they can remove VOCs, PAHs, and CPM at efficiencies of 55.8–87.6 %, 74.7–89.8 %, and 36.3–81.5 %, respectively, with corresponding emission concentrations of 0.058–16.29 mg/m³, 0.42–43.3 μg/m³, and 5.9–65.1 mg/m³. Based on current publicly reported data, the VOCs, PAHs and CPM contents emitted by global coal-fired power plants are estimated to be 88.1 Gg, 6.76 Gg (2.22 Gg in flue gas and 4.54 Gg in fly ash), and 600 Gg (organic components: 328 Gg, inorganic components: 272 Gg), respectively. Among the many removal technologies reviewed: include improved selective catalytic reduction (SCR) catalysts; and an advanced oxidation process (AOP) combined with an SCR or with a wet flue gas desulphurization (WFGD) unit respectively are two potentially useful technologies for future use in CFPPs. The information collected and presented in this review will help in the formulation of air pollution control policies and the development and application of efficient removal technologies.