A novel High-Gravity Advanced Oxidation Process (HiGee-AOP) using basic H2O2 solution as a liquid oxidizing reagent was examined for enhanced NOx removal efficiency in this study. At ambient temperature and optimal conditions, the process achieved a 99% removal of the inlet 1000 ppm NO. Spectroscopic experiments and radical-quenching tests indicated that the nucleophilic hydroperoxyl anion (-OOH) dissociated from H2O2 was the strongest reactive oxygen species (ROS) responsible for NO oxidation in alkaline H2O2, producing peroxynitrite (ONOO-) as detected by Fluorescence Spectroscopy. A mass transfer model coupling the reaction kinetics and gas diffusion under high liquid-film renewal conditions was established to simulate the HiGee-AOP process for NO absorption, and it achieved a model-prediction accuracy of within 10% of experimental data. It is speculated that the improved NO removal efficiency by the HiGee-AOP/alkaline H2O2 process stems from the enhanced mass transfer of NO in the heterogeneous reaction system provided by HiGee and the strong affinity of OOH to NO. Oxidation of NO by OOH may proceed first to form the [NO center dot center dot center dot OOH](-) intermediate due to a strong nucleophile-electrophile interaction, which is followed by an electron transfer within the intermediate producing ONOO- and ultimately as nitrate.

The combustion of fossil fuels has resulted in rapidly increasing emissions of nitrogen oxide (NO x ), 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 NO x 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 SO 4 •− . 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 SO 4 •− 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

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.