The sources of several major air pollutants can be categorized into three main areas: ① fuel combustion; ② industrial production processes; and ③ transportation. The first two categories of pollution sources are collectively referred to as stationary sources, while vehicles—such as motor vehicles, trains, and airplanes—are classified as mobile sources. Statistics in China on four widely distributed and highly prevalent pollutants—particulate matter, SO2, NOx, and CO—show that atmospheric pollutants generated by fuel combustion account for the largest share, at 70%, whereas industrial production and motor vehicles each account for 20% and 10%, respectively. Today, the editors from Source and Environmental Protection will introduce you to a commonly used flue gas treatment method for stationary pollution sources—the catalytic combustion process for flue gas purification.

I. Catalytic Combustion Wastewater Purification Process Flow

For catalytic combustion, different emission scenarios and various types of exhaust gases call for different process flows. However, regardless of the specific process flow adopted, all such systems consist of the following process units: ① Pre-treatment of exhaust gas. To prevent blockage of the catalyst bed and catalyst poisoning, the exhaust gas must undergo pre-treatment before entering the bed to remove dust, liquid droplets, and catalyst poisons from the gas stream.

② Preheating device. The preheating device includes an exhaust gas preheater and a catalyst burner preheater. Since all catalysts have a specific catalytic activity temperature—referred to as the ignition temperature for catalytic combustion—it is essential to raise both the exhaust gas temperature and the bed temperature to the ignition temperature before catalytic combustion can take place. Therefore, a preheating device must be installed. However, in cases where the exhaust gas itself already has a relatively high temperature—for example, in drying exhaust gases from enameled wires, insulating materials, or baked coatings—where the temperature can reach over 300℃, a preheating device is not necessary.

Catalytic combustion

The heat source for the preheater can be flue gas or electric heating; currently, electric heating is more commonly used. Once the catalytic reaction has started, the exhaust gases should be preheated as much as possible using the recovered reaction heat. In cases where the reaction heat is substantial, a waste-heat recovery system should also be installed to conserve energy. The temperature of the heat source used to preheat the exhaust gases generally exceeds the catalyst's active temperature. To protect the catalyst, the heating device should be kept at a fixed distance from the catalytic combustion unit; this arrangement also helps ensure a uniform distribution of exhaust-gas temperature. From the perspective of requiring preheating, the catalytic combustion method is best suited for the continuous purification of exhaust gases. If the exhaust gas is discharged intermittently, not only will each preheating process consume energy, but the reaction heat itself cannot be recovered and utilized, leading to significant energy waste. This point should be carefully considered during both the design and selection stages.

II. Commonly Used Equipment for Catalytic Combustion Exhaust Gas Purification

Common catalytic combustion devices include regenerative catalytic combustion units, which can handle organic waste gas concentrations ranging from 100 to 1,000 mg/m³. Their uniquely designed, highly efficient and advanced heat-exchange system ensures effective recovery of the heat generated during combustion, giving them outstanding advantages in the field of purifying large-volume, low-concentration organic waste gases. Other types of catalytic combustion devices include catalytic burners and plasma-assisted catalytic burners. A typical structure of a catalytic combustion purifier is shown here: on a base material, a specific process is used to deposit an intermediate layer, creating a catalyst surface that allows gas molecules to pass through freely. The active substances used as catalysts are precious metals such as platinum, rhodium, and palladium. These metals are deposited onto the intermediate layer, enabling them to maintain extremely high activity even at relatively low combustion temperatures, effectively oxidizing harmful substances. These highly active precious metals can simultaneously convert multiple harmful components present in the waste gas.