1 With the development of the global economy, the prevention and control of atmospheric volatile organic compounds (VOCs) has become a serious challenge in the field of industry and power generation. The VOCs are bio-accumulative, carcinogenic, teratogenic, and mutagenic, which are important inducements that cause complex atmospheric pollution, induce haze, and photochemical smog. It was found that the composition of VOCs from coal combustion was mainly composed of hydrocarbons, benzene and its derivatives, phenols, esters, and polycyclic aromatic hydrocarbons. The VOCs concentration in the complex coal combustion flue gas is low, while the components are complex, the total amount is large, and the toxicity is high, posing a major threat to the ecological environment. There is still a lack of effective control methods, and it is urgent to develop efficient control technologies for VOCs from coal combustion. In view of the existing control methods on industrial VOCs emission, the adsorption control and photocatalytic degradation have attracted much attention, but due to the complex characteristics of VOCs emissions from coal combustion, related technologies need to be improved.
2 The use of biomass to prepare carbon-based adsorbents (biochar) is an important approach for the low-carbon use of renewable energy. It is widely used in coal combustion due to its wide range of raw materials, strong controllability of physicochemical properties, and high adaptability of target adsorbents. Applying it to the purification of coal combustion flue gas has become a research hotspot in recent years. Nano TiO2, which is non-toxic, harmless, non-corrosive, low cost, high photocatalytic activity and wide application range, can be used for photocatalytic oxidation degradation of industrial, indoor, coal combustion VOCs.
3 Based on the development potential of biochar adsorption and TiO2 photocatalytic oxidation on the effective degradation of VOCs, we proposed the concept of “biochar-nano-TiO2 functionalized cross-linking”. We found that non-polar small VOCs molecules were trapped and enriched in the developed micropores of biochar, while the polar molecules and non-polar macromolecules were adsorbed and distributed in the meso/macropores of biochar. In addition, the nano TiO2 could generate hotogenerated electron-hole pairs, forming reducing electrons (e-) and oxidizing holes (h+). H+ could convert H2O to hydroxyl radicals (•OH), which are highly oxidizing, and are used to oxidatively degrade VOCs. O2 could react with photogenerated electrons to generate superoxide anion radicals (•O2-) with high activity. The improved photocatalytic performance of nano TiO2 could be attributed to the effective separation of electron-hole pairs and lower charge transfer resistance. Biochar could adsorb VOCs around TiO2 particles, increase the local concentration to improve its catalytic efficiency, hinder photoelectron-hole recombination, and effectively avoid the free or volatilization of intermediate products. Modified nano-TiO2 photocatalysis under the sunshine promoted the adsorption of VOCs from coal combustion by biochar, and prolonged the saturated adsorption time of biochar to VOCs, while biochar could be in-situ regenerated when the adsorption was saturated. We have achieved a sustainable process of hierarchical adsorption, pore channel enrichment, transport diffusion, photocatalytic oxidation, and in-situ regeneration with biochar-nano-TiO2 functionalized cross-linking for VOCs reduction from coal combustion.