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Feasible Technologies for the Control of Medical Waste Incineration Flue Gas Pollution
Origin：Hefan Hit： Time：2020-02-13
The key to promoting the disposal of medical waste by incineration is to effectively treat the pollutants contained in the incinerated flue gas. According to the main components of incineration of flue gas from medical waste incineration, Hefan Environment summarized feasible treatment technologies and pointed out the relevance of process routes.
As a special hazardous waste, the hazards of medical waste include the spread of disease and environmental pollution. Incineration and non-incineration methods are two major types of medical waste disposal at this stage. The incineration method is suitable for a variety of medical wastes, and the process equipment is relatively mature and widely used. Chemical or microwave disinfection, high-temperature cooking, etc. are non-incineration methods, and although they are rarely used, they are considered to have the most potential for development. However, the disadvantages of the non-incineration method are obvious. Firstly, the scope of application to medical waste is narrow, and secondly, the treatment efficiency is lower than that of the incineration method. Based on the specific conditions in China and the technology and equipment used in the centralized medical waste disposal plants that have been built in various places, the incineration method is still preferred at this stage. The advantage of the medical waste incineration method is that it can effectively decompose long-chain organic matter and kill pathogenic bacteria; the heat released by combustion can be used to generate electricity. Based on the main components of pollutants in the flue gas of medical waste incineration, this paper summarizes the feasible treatment technologies for various types of pollutants, and provides a reference for the selection of pollutant treatment process routes in the preparation stage of medical waste incineration plants.
1.Incineration of flue gas pollutants
The physical composition of medical waste: organic matter such as cotton swabs, paper, body tissues, etc. account for about 80%; inorganic matter such as metals, glass, etc. account for about 10%; other such as waste pharmaceuticals and drugs account for about 10%. In view of the above medical waste components, it can be judged that the pollutants contained in the incineration exhaust gas mainly include: acid gases (hydrogen chloride, sulfur dioxide, sulfur trioxide, nitrogen oxides), soot, dioxins and heavy metals.
2.Incineration smoke control measures
2.1 Source control
The proportion of plastic products in medical waste is quite large. Its main component contains polyvinyl chloride, which is the main source of acid gases and dioxins in incineration smoke. Plastic products are renewable resources, strengthen the early management of medical waste, conduct separate collection, joint storage and transportation and disposal supervision, sterilize, sterilize and reuse the separated collected plastic, and directly incinerate the remaining waste. This not only reduces the waste of resources, but also reduces the chlorine content in the medical waste involved in the incineration, thereby reducing the generation of dioxins and acid gases, and reducing the difficulty of subsequent flue gas treatment.
2.2 Medical waste incineration system
Medical waste incineration treatment system mainly includes: feeding device, incinerator, combustion air supply, waste heat utilization, flue gas purification, ash and slag treatment, etc. The incinerator is divided into the first combustion chamber and the second combustion chamber, which is the key equipment of the technology. The materials are dried and pyrolyzed in the first combustion chamber, and completely burned in the second combustion chamber. The incineration flue gas purification system generally includes the following processes: quenching, acid removal, adsorption and dust removal.
2.3 Acid gas control measures
To remove acid-containing substances in the flue gas (mainly hydrogen chloride), wet, semi-dry or dry methods can be used. In the wet method, the absorbent is formulated into a solution or a slurry, and then sprayed into a large number of purification equipment such as a sieve plate tower, a spray tower, and a packing tower. At this time, the acidic pollutants in the flue gas are neutralized and absorbed to remove them. Absorbents often use caustic soda or quicklime. The dry method is to spray a dry powder absorbent into a flue or other reactor, and the acidic substances in the flue gas react with the absorbent, and then sent to the dust removal device as a whole, and the reactants and other particles in the flue gas are removed together. The working principle of the semi-dry method is similar to that of the wet method. The injection amount of the liquid absorbent should be strictly controlled, which is based on the powdery product that can be evaporated by high-temperature flue gas.
The semi-dry flue gas deacidification process has the advantages of both dry and wet processes, with less energy consumption, simple equipment, and more domestic applications. The semi-dry deacidification equipment mainly includes a quench tower and a semi-dry deacidification tower.
The incineration flue gas first enters the quench tower, and its main function is to rapidly cool the flue gas and reduce the generation of dioxins in the temperature range of 300 ~ 500 ° C. After the heat exchange equipment, the smoke temperature is reduced to about 550 ° C, and water is sprayed into the quench tower as a direct cooling medium. When the flue gas passes through the quench tower, it comes into contact with the atomized water, which makes the heat transfer speed faster and reduces the smoke temperature quickly. To about 200 ° C, the reaction time controlling this process is less than 2s. Flue gas and water mist are vaporized during the mixing and falling process, and no sewage is generated at the bottom of the tower.
After passing through the quench tower, the flue gas enters the semi-dry deacidification tower, the purpose of which is to remove acid gases from the flue gas. The inlet smoke temperature is about 200 ° C and the outlet smoke temperature is about 170 ° C. Most of the acid gases in the flue gas are removed by spraying absorbent. Taking lime slurry as an example, this reaction is divided into two steps. First: in a semi-dry deacidification tower, the flue gas to be treated is fully mixed with droplets of lime slurry, and acid gases such as hydrogen chloride and alkalinity in the slurry droplets. The components undergo a chemical reaction; the second stage: the heat carried by the flue gas evaporates the water in the slurry mist droplets, so that the lime and reaction products in the slurry become solid particles, in the lower part of the tower and in the subsequent flue gas processing equipment, and Gaseous pollutants continue to react, and the overall pollutant purification effect has improved.
Flue gas quenching and semi-dry deacidification can also be performed simultaneously. The rapid cooling method using an alkali-containing aqueous solution is sprayed (a heat transfer method with high heat transfer efficiency). First, the sodium hydroxide solution is atomized into extremely small droplets by a high-efficiency atomizing nozzle, and then heat exchange is directly performed with the flue gas. Water droplets quickly turn into water vapor, and the temperature of the flue gas can be reduced to 160 to 200 ° C within a short period of time (≤1s), thereby preventing heterogeneous catalysis of "flying ash" in the middle temperature section, preventing the dioxin from being generated again, and Neutralizes most acidic pollutants.
In order to improve the deacidification efficiency, some process routes are followed by rapid cooling and semi-dry deacidification, and then add one step of dry deacidification, spray slaked lime powder into the flue, and continue to react with the remaining acid substances in the flue gas. Because the flue gas still contains a part of water vapor after quenching or semi-dry deacidification, the deacidification effect is better by this method.
Nitrogen oxides in incineration flue gas are also common acidic pollutants. It is a source of control to control the generation of combustion by controlling the combustion conditions, and then the end treatment is considered. Nitrogen oxide removal technologies include selective non-catalytic reduction, ammonia reduction, and flue gas recycling.
2.4 Smoke control measures
Incineration smoke includes smoke and fly ash particles generated during combustion. Fumes generally refer to solid particulate aerosols generated during incineration. It is a condensate of gaseous substances formed after the molten substance has evaporated and cooled. Fly ash can be considered as a finer ash discharged with the combustion smoke. The collection of large particles of fly ash (5 ~ 50mm) produced by incineration can use a sedimentation chamber and a cyclone dust collector. The most commonly used to collect small particles of fly ash is a bag filter, with a dust removal efficiency of 99.9%.
2.5 Dioxin control measures
Dioxins are generated during the incineration process and the benzene ring and chloride ions in the flue gas are combined at a certain temperature under incomplete combustion conditions, and the contents are relatively high in the flue gas and fly ash. There are two main ways to reduce dioxin emissions. One is to control the material combustion process and cut off its generation conditions: According to relevant regulations, a medical waste incinerator needs to set up a second combustion chamber, the combustion temperature is maintained above 850 ℃, and the flue gas A dwell time of 2 s or more is a necessary condition to effectively reduce the formation of dioxins.
The low-temperature resynthesis of dioxins in the flue gas can be controlled by the aforementioned quench tower. The other is to purify the combustion flue gas, which can be activated carbon adsorption or bag filter to capture.
There are mainly activated carbon methods for removing the generated dioxins, including fixed bed adsorption technology, powder injection adsorption technology, and selective catalytic reduction. At present, activated carbon injection adsorption technology is mainly used in China: spray activated carbon powder, and use activated carbon particles to absorb organic gas more strongly to further remove dioxins in flue gas.
Dioxins are often attached to fly ash particles, and the bag filter removes the dioxins by capturing 99.9% of the particles. Dioxins are characterized by higher boiling points and lower gasification pressures. When the temperature of the flue gas is between 150 and 280 ° C, dioxin exists in the form of fine particles. The membrane bag filter is coated with an organic polymer on the surface of the conventional cloth bag, so that the filter bag forms an initial filtration layer. Membrane bag dust collector can effectively capture dioxin. It is recommended to replace the common bag filter with a film bag filter.
2.6 Heavy metals
When medical waste is incinerated, the flue gas also contains some heavy metals such as substances containing mercury, chromium, arsenic, copper, lead and so on. They mainly exist in the form of various salts and oxides in the flue gas and fly ash. At present, there are no separate measures for heavy metals. They can be used in various ways with other methods Synergistic removal of pollutants.
2.7 Medical waste incinerator flue gas treatment process route
According to the above-mentioned treatment measures for various pollutants in the incinerator flue gas and the technical parameters of the incinerator, the commonly used process route is:
Quenching, semi-dry deacidification → dry deacidification → spraying activated carbon powder → coated bag dust collector → discharge It should be seen that the various treatment measures in the process route do not exist independently, but are organic whole, such as quenching and semi-dry The process of deacidification can be carried out successively or simultaneously, and the cooling medium is different. While the bag filter traps particulate matter, it also traps heavy metals and dioxins. In the process selection and design, the connection and synergy between the combined technologies should not be ignored, and their comprehensive effects should be considered in a unified manner.
Based on China's current situation, incineration is still the mainstream technology for medical waste disposal. Proper disposal of hazardous substances in incineration flue gas has become the key to medical waste incineration disposal. The disadvantages of the incineration method are mainly that the composition of the incineration flue gas is complex, and the types of pollutants contained in the incineration method are large. Achieving compliance with discharge standards is a basic condition, and preparations should be made to meet emission standards that may be tightened at any time. The study of feasible technologies for the treatment of medical waste incineration flue gas is of great significance for carrying out demonstration projects in the industry, comprehensive promotion and then improving the overall technical level of the industry.
