VOCs exhaust gas regenerative thermal oxidation treatment method

At present, the organic waste gas discharged from the petrochemical, light industry, plastics, printing and other industries is treated as a direct-fired incinerator and a regenerative thermal oxidizer (called RTO). The ceramics of the regenerative thermal oxidizer can store the heat of the burner, when the temperature of the ceramic exceeds the ignition point of the organic waste gas, the hot honeycomb ceramic can ignite the organic exhaust gas even if there is no fire in the furnace. The regenerative thermal oxidizer has the advantages of low energy consumption, good safety and wide application range, and is a promising VOCs gas treatment method.

1. VOCs waste gas treatment status

Volatile Organic Compounds (VOCs) are the most common pollutants emitted by petrochemical processes and various industries that use organic solvents, such as paint, printing, pharmaceutical, and coal chemical industries. Most of these compounds have irritating odors and toxicity. Some have been classified as carcinogens; most VOCs are flammable and explosive, posing a threat to the safety of production.

Due to the dangers of VOCs, many countries have enacted laws regulating the emissions of VOCs. In the United States and Europe, around the 1990s, emission requirements were imposed on all places where organic solvents were used. In 1990, the United States increased its emission standards and classified 189 pollutants in industrial production as toxic pollutants, most of which were VOCs. China's "Air Pollution Prevention and Control Law" requires purification of toxic gases produced in industrial production, and recycling of flammable gases; China's "Integrated Emission Standards for Air Pollutants" (GB16297-1996) stipulates the emission standards for 33 volatile organic compounds, and most of the other volatile organic compounds are treated as non-methane hydrocarbons, and specifies uniform emission standards.

1. Regenerative oxidation treatment method

Thermal oxidation is a method of applying thermal oxidation and catalytic oxidation techniques to destroy organic matter in emissions. The Regenerative Thermal Oxidizer (RTO) differs from other thermal oxidation technologies currently used in the country in that it uses ceramic or other high-density inert material beds to absorb and store heat from the exhaust gases, and then heat, the release of cold inlet gas instead of shell-and-tube heat transfer between the two fluids, RTO can achieve more than 98% heat recovery efficiency.

The working principle of the regenerative thermal oxidizer is: after the organic exhaust gas is heated by the preheating chamber, it enters the combustion chamber and is incinerated at a high temperature (heating to 800 ° C), so that the organic matter is oxidized into carbon dioxide and water, and then stored in another regenerator, the stored heat is used to preheat the newly entered organic waste gas, and the furnace temperature is kept stable by periodically changing the direction of the gas flow.

Typical regenerative thermal oxidation treatment processes are as follows: the integrated heat storage incineration process (one room), as shown in Figure 1 [2]; the thermal storage split communication Platform incineration process (two rooms), such as Figure 2 [2]; integrated heat storage process (three rooms), as shown in Figure 3 [3].

3 RTO features

3.1 High exhaust gas treatment and high efficiency

RTO should be used to treat organic waste gas with a concentration of 2-8g/m³, for low calorific value gases (such as ethyl acetate), the concentration can reach 12g/m³, which is especially suitable for incineration of difficult-to-decompose components [3]. Compared to other processing technologies (such as heat exchange thermal oxidation), the main advantage of RTO is that the heat recovery rate can reach 98%, while other systems can only achieve a heat recovery rate of about 70%. The high heat recovery rate significantly reduces the amount of supplemental fuel used, thereby saving operating costs. In particular, the industrial gas with a large amount of processing and a low organic content has a more remarkable effect.

The RTO's high heat recovery efficiency also has the advantage of providing a higher thermal oxidation temperature if a higher Destroyed Removal Efficiency (DRE) is to be guaranteed. This makes it easy to handle difficult-to-decompose organic matter, while the increase in system operating costs is small. DRE of 98% to 99% is typical of these systems. For RTO systems, achieving a very high preheat temperature in the packed bed causes the thermal oxidation process to actually take place in the bed, and the preheating temperatures of these systems are typically significantly higher than the autoignition temperatures of most organics.

3.2 Energy saving, environment friendly, easy to maintain

Application results show that for a given size of RTO, the use of structured packing has the following advantages over the use of loose packing: (1) higher thermal efficiency can reduce annual fuel costs (60% to 65%); (2) lower pressure drop reduces the annual electricity cost (14% to 78%); (3) the downtime and maintenance costs are also reduced due to reduced clogging capacity and improved cleaning capacity. However, structured packing is more expensive to manufacture and install, and the cost per unit volume is 5 to 10 times that of conventional bulk packing. Therefore, the application of the standard filling and filling costs is 5% to 20% higher than the investment cost of applying bulk packing.

4.RTO application in China

At present, most of the exhaust gas incinerators used in the domestic insulating material industry and the CCL production industry are direct-fired incinerators. Although the cost of such incinerators is low, fuel consumption is large. Today, as fuel prices continue to soar, saving fuel and reducing costs are issues that every company attaches to. In fact, as long as the regenerator (honeycomb ceramic) is added to the direct-fired incinerator, fuel saving can be achieved. After adding the regenerator in the direct-fired exhaust gas incinerator, it not only plays the role of heat storage, but also plays the role of a second fire source, when the burner is "extinguished", the organic exhaust gas hits the hot honeycomb, when it is ceramic, it will burn on fire. The cost of converting a direct-fired exhaust gas incinerator into a regenerative waste gas incinerator is very low, therefore, the direct-fired exhaust gas incinerator used in the domestic insulating material industry and the CCL production industry can be modified, and it is necessary to carry out transformation. .

Conclusion

The regenerative thermal oxidation treatment technology has obvious advantages over the previous direct-fired incineration treatment technology, the technology is very mature abroad, but due to cost reasons, it has not been popularized in China, and the concept of regenerative heat is only in a few industries. In the context of the current surge in energy prices, the organization's efforts to research, develop and promote the use of this technology can not only save energy and reduce environmental pollution, but also obtain considerable economic benefits and significant social benefits.