NOx attenuation in flue gas by •OH/SO4•--based advanced oxidation processes.

The combustion of fossil fuels has resulted in rapidly increasing emissions of nitrogen oxide (NOx), which has caused serious human health and environmental problems. NO capture has become a research focus in gas purification because NO accounts for more than 90% of NOx and is difficult to remove. Advanced oxidation processes (AOPs), features the little secondary pollution and the broad-spectrum strong oxidation of hydroxyl radicals (•OH), are effective and promising strategies for NO removal from coal-fired flue gas. This review provides the state of the art of NO removal by AOPs, highlighting several methods for producing •OH and SO4•-. According to the main radicals responsible for NO removal, these processes are classified into two categories: hydroxyl radical-based AOPs (HR-AOPs) and sulfate radical-based AOPs (SR-AOPs). This paper also reviews the mechanisms of NO capture by reactive oxygen species (ROS) and SO4•- in various AOPs. A HiGee (high-gravity) enhanced AOP process for improving NO removal, characterized by intensified gas-liquid mass transfer and efficient micro-mixing, is then proposed and discussed in brief. We believe that this review will be useful for workers in this field. Graphical abstract.