1 Formation and harm of flue gas pollutants from waste incineration
1. 1 The acid gas in the flue gas from acid gas incineration is mainly composed of SOX, NOX, HCl and HF, all of which come from the combustion of corresponding waste components. SOX is mainly composed of SO2, which is produced by incineration and oxidation of sulfur-containing compounds. NOX includes NO, NO2, N2O3, etc. It is mainly caused by the decomposition and transformation of nitrogen-containing compounds in garbage or the high temperature oxidation of nitrogen in the air during combustion. HCl comes from chlorides, such as NaCl and KCl in PVC, rubber, leather and kitchen garbage. The concentration of HCl gas in incineration flue gas is relatively high, often 400 ~ 1200 ppm. The concentrations of sulfur oxides and nitrogen oxides are relatively low.
[1] Therefore, HCl is the main pollution gas in waste incineration flue gas. Hydrogen chloride gas is harmful to human body. According to the calculation of the Global Pollution Emission Assessment Organization (GEIA), the amount of HCl gas emitted to the environment by domestic waste incineration in the world is as high as 2 18kg each year, which is equivalent to 0.42 kg HCl emitted to the atmosphere only by garbage incineration per person each year.
[2]HCl gas will cause high and low temperature corrosion to the heating surface and monitoring instrument of waste heat boiler, which will affect the safety of waste heat boiler and limit the increase of superheated steam parameters; The existence of HCl gas increases the dew point of flue gas, which leads to the increase of flue gas temperature and reduces the thermal efficiency of boiler.
[3] Chlorine source reacts with heavy metals under certain conditions to generate low-boiling metal chloride, which intensifies the volatilization of heavy metals, leads to the enrichment of heavy metals in fly ash and increases the toxicity of fly ash.
[4]HCl gas can promote the formation of chlorophenol, chlorobenzene, chlorobenzofuran and other "three-caused" organic compounds, and HCl produced after PVC pyrolysis is considered to promote the formation of polycyclic aromatic hydrocarbons (PAHs).
[5] Therefore, the effective removal of HCl gas is directly related to the safe and environmental protection operation of incineration system.
1.2 organic pollutants mainly refer to dioxin-like compounds, which are low in concentration in the environment but highly toxic and directly endanger human health. Their main components are polychlorinated dibenzo-dioxins and polychlorinated dibenzofurans. It is generally believed that garbage incineration is the main source of such compounds in the environment.
[6] There are two reasons for dioxin production in garbage incinerators: one is that the garbage itself contains a small amount of dioxin, and the other is that the incinerator produces dioxin during garbage combustion. There are three formation mechanisms.
[8]:( 1) High temperature synthesis. In the initial drying stage of garbage entering the incinerator, except water, low-boiling organic substances containing hydrocarbon components volatilize and react with oxygen in the air to generate water and carbon dioxide, forming a temporary anoxic state, so that some organic substances react with hydrogen chloride to generate dioxins;
(2) Ab initio synthesis. Dioxins are formed by ab initio synthesis. That is, at low temperature (250~350℃), macromolecular carbon (residual carbon) reacts with organic or inorganic chlorine in fly ash matrix to generate dioxins;
(3) precursor synthesis. Incomplete combustion and heterogeneous catalytic reaction on the surface of fly ash can form a variety of organic gas phase precursors, such as polychlorinated phenol and polyvinyl chloride. In the combustion process, precursor molecules generate dioxins through chemical reactions such as rearrangement, free radical condensation and dechlorination.
1.3 During the incineration of particulate matter and heavy metal waste, a large number of fine particulate matter will be generated. At the same time, the original particles in the garbage are lifted up by the airflow in the furnace and discharged with the incineration gas. The combustible components in garbage will form black smoke due to incomplete combustion, which contains a lot of carbon particles. The smaller the particles, the easier it is to enter the alveoli, and the greater the harm. Fine particles will contain heavy metals, such as chromium, copper, nickel, lead, zinc, manganese, antimony, cadmium, selenium and so on. Heavy metals such as chromium, cadmium, nickel, lead, selenium, etc. Among them, the harmful particles are mainly concentrated in particles smaller than 3 microns [10]. Therefore, while removing particulate matter, it also reduces the harm of heavy metals to a certain extent.
2 Smoke pollution control of waste incineration The pollutants produced by waste incineration come from waste components, and their forms and quantities are closely related to incineration conditions and purification systems. From the process of pollutant generation and discharge, the following measures can be taken to control the secondary pollution caused by garbage incineration.
2. 1 Controlling the generation of smoke pollutants According to the formation mechanism of smoke pollutants, the incineration conditions of garbage are controlled to keep the combustion in a good state, thus reducing the generation of harmful substances. With proper furnace and grate structure, garbage can be fully burned in the incinerator. The concentration of CO in flue gas is one of the indicators to measure the adequacy of garbage combustion. The lower the concentration of carbon monoxide, the more complete the combustion. The ideal carbon monoxide concentration indicator is less than 60 mg/m3. The flue gas outlet temperature of incinerator shall not be lower than 850℃, the residence time of flue gas in the furnace and secondary combustion chamber shall not be lower than 2 S, and the O2 concentration shall not be lower than 6%. The air volume, temperature and injection position of combustion-supporting air shall be reasonably controlled. Injecting CaCO3 or CaO into the furnace can reduce the high-temperature corrosion of chloride and sulfide on the high-temperature heating surface and the secondary pollution to the atmosphere. In the combustion process, the control conditions of NOX and dioxin are contradictory. Generally, in actual combustion operation, the generation of NOX should be considered on the basis of ensuring the full combustion of combustible components in garbage. The foreign treatment measure is to add a denitration device to the flue gas treatment system.
2.2 flue gas purification treatment flue gas purification system is the key to pollution control of municipal solid waste incineration, and the emission concentration of various pollutants after flue gas purification should meet the national standard GWKB3-2000. Flue gas purification generally consists of deacidification, dust removal and activated carbon adsorption. Semi-dry process/dry process+bag filter is widely used at home and abroad, in which deacidification technology is the core of flue gas purification system of waste incineration.
2.2. 1 deacidified acid gases HCl, SOx and HF are mainly removed by wet, dry or semi-dry neutralization and absorption of alkaline substances such as Ca(OH)2 and NaOH. Among them, the wet process has high efficiency, which can reach more than 97%, but it is easy to cause secondary pollution by discharging a lot of sewage. Dry process has no sewage discharge, but the removal efficiency is only 60%~70%. Semi-dry technology has high removal efficiency (up to about 90%), less chemical consumption and no sewage discharge, and is the main applicable technology for flue gas deacidification. The semi-dry deacidification device is generally arranged in front of the dust collector, and mainly includes a feeding system, a mixing system and a reaction system. The deacidifying agent CaO generates powdered Ca(OH)2 in the feeding system, then enters the mixing system to be fully mixed with flue gas and a small amount of water, and finally enters the reaction system in the form of spray. Acidic components such as HCl, SOx and HF are absorbed to generate neutral and dry solid particles, which will enter the next purification system with the flue gas. The main reaction is: 2hcl+ca (oh) 2 = CaCl2+2h2o (1) SO2+ca (oh) 2 = caso3+H2O.
2.2.2 The dust collector is the terminal equipment of the flue gas purification system, and the national standard GB 18485-200 1 stipulates that the dust collector of the domestic waste incinerator must adopt the bag dust collector. Bag filter can not only collect ordinary particles, but also collect aerosols with a diameter of ≤0.5 micron condensed by volatile heavy metals or their chlorides, sulfates or oxides, and also collect organic pollutants such as dioxins adsorbed on ash or activated carbon particles. In the bag dust removal system, the bag is made of fibers of different materials, and the tail gas is filtered to achieve the purpose of dust removal and dioxin adsorption. Dust particles accumulate on the surface of filter cloth to form a dense thin layer, so the dust removal rate of bag filter is generally high. Due to the limitation of heat resistance of cloth bag materials, the tail gas temperature must be controlled at about 250℃, which is lower than the temperature of dioxin re-synthesis.
2.2.3 Activated carbon adsorption At present, activated carbon adsorption is mainly used to treat dioxins in flue gas treatment of garbage incineration at home and abroad. Activated carbon can not only adsorb dioxins, but also effectively remove heavy metals and other substances. Due to the large specific surface area of fly ash, it has a strong adsorption effect on dioxins, resulting in a high concentration of dioxins in fly ash, which usually accounts for about 70% of the total dioxin emission during incineration.
[1 1] and most of the heavy metals (>: 70%) remain in the slag, only Hg and Cd volatilize at high temperature, enter the fly ash and be discharged with the incineration flue gas.
[12] In order to improve the removal rate of dioxins and heavy metal pollutants in flue gas, the following methods can be adopted.
[13]( 1) Reducing the residence time of flue gas in the temperature range of 200~350℃ is beneficial to reduce the regeneration of dioxin pollutants, and controlling the flue gas temperature at the inlet of dust collector below 200℃ is beneficial to the removal of organic matter and heavy metal pollutants, that is, adopting "temperature control" in design and operation;
(2) Between the reaction tower and the dust collector, activated carbon or porous adsorbent is sprayed into the flue gas through the powder mixer, which can adsorb dioxin and heavy metal pollutants, and then collected by the bag dust collector.
Conclusion Waste incineration can maximize the reduction, harmlessness and resource utilization of domestic waste, and has a good application prospect. However, incineration inevitably brings secondary pollution, especially the pollution of incineration flue gas composed of fly ash, acid gas, dioxins and heavy metals. The prevention and control of secondary pollution of waste incineration flue gas is an indispensable part of waste incineration system. Appropriate flue gas purification technology should be adopted to effectively control the emission of pollutants.
The harm of the so-called garbage incineration power plant to life actually depends on the operation of the power plant. Just talking about the furnace, if the furnace temperature is about 800℃, it is no problem to bypass the generated flue gas, but domestic power plants basically start the furnace with garbage. In that case, the smoke will be greatly polluted if it bypasses. There are also some power plants, in order to save operating costs, smoke passes through chimneys at night, which is a case in China. If all power plants are in strict accordance with the requirements, garbage incineration power plants will do little harm to life. Take Japanese power plants as an example. Other people's power plants can be built in urban dense areas.