At present, there are still many difficulties in VOCs governance. For example, the emission sources are small and scattered, involving many industries. The VOCs emitted by various industries are various in variety and complex. Even in the same industry, if the raw material composition and production process are different, the types and properties of VOCs emitted are different. And the concentration is not the same. Therefore, the required technical route is different.
China's treatment of volatile organic waste gas is mainly divided into four stages:
The first stage, 2000 years ago, the treatment of local irritating or recycled value of exhaust gas, only the irritating exhaust gas through the water spray absorption treatment;
In the second stage, from 2001 to 2005, the exhaust gas was collected and treated in an organized manner, and the main treatment method of the exhaust gas was water and alkali spray two-stage absorption treatment;
The third stage, after 2005, strengthen the source control, improve the centralized collection of exhaust gas, and then strengthen the treatment by activated carbon adsorption on the basis of the second stage;
In the fourth stage, after 2007, the whole process of volatile organic waste gas is gradually processed.
At this stage, China's oxidation control technology for industrial volatile organic waste gases mainly includes: photocatalytic oxidation, direct combustion, catalytic combustion technology and RTO.
Among them, the RTO technology developed on the basis of the combustion method, because of simple operation, less operation and maintenance, high removal efficiency of volatile organic compounds, generally more than 95%, has become one of the main technologies of organic waste gas treatment in China. .
RTO, the full name RegenerativeThermal Oxidizer, Chinese translation "regenerative thermal oxidation combustion", listening is very complicated, isn't it? Don't worry, and listen to Xiaobai slowly. First of all, we can extract three core information from the name - heat storage, thermal oxidation, and combustion. It can be said that this is the purification and concentration of RTO working principle.
Thermal oxidation and combustion are well understood. Organic compounds (VOCs) react with oxygen at a certain temperature to form CO2 and H2O, and emit a certain amount of heat in the oxidation reaction process. Students who have studied chemistry can see this in seconds. What is the key to "heat storage"? How to store heat?
This mysterious heat storage source benefits from the regenerator in the RTO. If the heat recovery rate of the regenerator is higher, indicating that the higher the heat accumulated, the heat required to oxidize the exhaust gas is correspondingly lower, and the fuel can be consumed with little or no fuel during the treatment, and the concentration is higher. It is even possible to export heat to the secondary heat recovery.
Therefore, the heat recovery rate is one of the key indicators here, and it changes with the material and shape of the regenerator.
Earlier, the low-order version of the heat storage was the TO (gas incinerator) in the rivers and lakes. The air preheater was plate or tube type, the heat recovery rate was about 50% domestically, and the largest in Germany was 85%.
Now, the advanced version of the heat storage is RTO, and the air preheater is replaced by a ceramic packed bed, which heats the exhaust gas to above 700 °C, so that the VOC in the exhaust gas is oxidized and decomposed into CO2 and H2O, and the high temperature gas flow generated by oxidation. The ceramic regenerator is used to heat up and store heat, and is used to preheat the organic exhaust gas that is subsequently entered, thereby saving the treatment technology of exhaust gas heating fuel consumption.
Understand the working principle of RTO, and then understand the family of RTO, it is much simpler. The current common forms of processing VOCs are: two-room RTO, three-chamber RTO and rotating RTO, which can be designed into five-room RTO, seven-room RTO and other structural forms according to requirements.
The N-room RTO uses the two-room RTO and three rooms as an example, and so on:
The working principle of two-room RTO
Fresh air is used to replace the organic exhaust gas at the start of the work, and the regenerator is heated to a certain temperature by the burner. Since the regenerator has an extremely high heat storage property, it takes a certain time to heat from a cold RTO to a certain high temperature and to reach a normal temperature distribution.
In normal operation, one of the regenerators has stored heat in the previous operating cycle, and the organic exhaust gas first enters the regenerator from the bottom, and the exhaust gas is preheated through the regenerator bed to near the combustion temperature, while the heat storage is performed. The body is gradually cooled at the same time.
The preheated exhaust gas enters the top combustion chamber, and after the organic matter is oxidized in the combustion chamber, it enters the other regenerator as a high-temperature purification gas; at this time, the heat of the purified gas is transmitted to the regenerator, and the regenerator bed gradually It is heated, and the purified gas is cooled and discharged. When the cooled regenerator is cooled to an acceptable temperature level, the direction of the airflow should be switched, ie the first cycle is completed.
After switching the flow, the organic waste gas enters the regenerative chamber that has been heated, and the purified gas after the reaction transfers the heat to the regenerator that was cooled in the previous cycle, as described above, completing the second cycle.
Three-room RTO working principle
The regenerator of the three-chamber RTO operates simultaneously: when the first regenerator is in a stage where it is cooled and the exhaust gas is preheated (cold cycle), the second regenerator is in the process of being heated by the purified gas (thermal cycle) ), while the third regenerator is flushing (washing cycle). Therefore, after one cycle, the exhaust gas always enters the regenerator that discharges the purified gas in the previous cycle, and the regenerator that originally entered the exhaust gas is flushed with the purified gas (or air), and the residual unreacted exhaust gas is sent. It is returned to the reaction chamber for oxidation and then discharged together with the purge gas from the rinsed regenerator.
In rotating RTO, the improved core is still the regenerator. The rotary RTO is mainly composed of a combustion chamber, a ceramic packed bed and a rotary valve. A filter plate is arranged in the regenerator of the rotating RTO, and the regenerator bed is divided into several separate sectors. The exhaust gas enters the preheating zone from the bottom through the air inlet distributor, so that the gas temperature is preheated to a certain temperature and then enters the top combustion chamber and is completely oxidized.
The purified high temperature gas leaves the oxidation chamber, enters the cooling zone, transfers heat to the regenerator, and the gas is cooled and discharged through the gas distributor. The ceramic regenerator in the cooling zone absorbs heat and stores a large amount of heat (used to heat the exhaust gas in the next cycle).
In order to prevent unreacted exhaust gas from entering the purifying gas outlet with the rotation of the regenerator, a sector is provided as a flushing zone before the regenerator is rotated to the purifier outlet zone.
By the rotation of the regenerator, the regenerator is periodically cooled and heated while the exhaust gas is preheated and cooled by the purifier. This is constantly alternating.