1. Domestic waste incineration technology
Domestic waste incineration power generation technology mainly includes three links: garbage collection, incineration and power generation, and exhaust gas treatment. Among them, the garbage collection link is mainly to collect and transfer municipal garbage from various regions; Incineration power generation is to discharge and store domestic waste, and then remove the leachate through reasonable fermentation technology, and retain combustible waste as incineration power generation materials. At the same time, effective measures will be taken to treat the exhaust gas during the incineration process. During incineration, the boiler exhaust gas pipeline is the main way of exhaust gas emissions, which is first discharged into the purification system, and then discharged into the air environment after exhaust gas purification treatment to reduce exhaust gas pollution. The exhaust gas emitted contains not only dust and solid particles, but also a variety of acid gases such as NO2、SO2, HCl, etc.
Under the policy constraints of our country to basically achieve "zero landfill" of primary domestic waste in 2023, incineration methods will dominate for a long time and are the absolute main means to break the garbage siege. On the other hand, the newly announced policies such as national subsidy decline and garbage classification will have an important and far-reaching impact on the waste-to-energy industry. The new policy will force the market to make optimization and reform in terms of technology and management, and improve the profitability of waste-to-energy enterprises through technological progress, improving waste treatment charging policies, and improving the level of enterprise operation and maintenance.
2. Causes of acid gas formation in domestic waste incineration power plants
Biochemical waste contains a large amount of organic waste, such as tea residue, melon peel and core, eggshell, solid food, etc., as well as waste fruits, vegetable leaves, fish and shrimp produced by aquatic markets and fruit and vegetable markets, these substances contain protein, so there will be amino acids, that is, N elements, and some proteins also contain S elements, and during the incineration process, the exhaust gas will contain NOx and SO2。 In addition, gases such as HCl are produced during the incineration of waste plastics in household waste. In order to effectively control acid gas, it is necessary to first understand the causes of acid gas.
（1）SO2Causes. High temperature is the production of SO2Because domestic waste contains sulfur, it will react chemically with oxygen, and then form SO2。SO2It will seriously affect the ecological environment, and its discharge in the environment will make the surrounding rainwater appear acidic, that is, acid rain will appear, which will seriously damage the growth of surrounding plants and housing construction. At the same time, hydrogen sulfide undergoes a series of oxidation reactions under certain conditions, resulting in the formation of SO2, as in Equation (1).
2H2S+3O2＝2SO2+2H2O
S+O2+＝SO2
CL2+H2O+SO2＝SO2+2HCl
CXHYOZ+O2→CO2+H2O+SO2 （1）
(2) Causes of HCl. In domestic waste incineration, the organic matter contained in the garbage will contain chlorine, which will form HCl after incineration. The reaction formula is: CXHYClZ+O2→CO2+H2O+HCl+incomplete combustion. In addition, domestic waste contains inorganic substances, and its internal chlorine will be affected by external high temperature to produce a series of chemical reactions, and then form HCL, and its reaction formula is as shown in Equation (2).
2NaCL+4SiO2+Al2O3+H2O＝2HCl+Na2(SiO2)4 Al2O3
2NaCL+nSiO2++H2O＝2HCl+Na2(SiO2)n
(n=2 or 4)
2NaCL+SiO2+0.05•O2+H2O＝Na2SO4+2HCl（2）
HCl can seriously affect the surrounding environment because the gas can dissolve in water, causing acidification of water bodies, and can also irritate the human respiratory tract and threaten human health.
(3) Causes of NOx generation. Because domestic waste contains a large amount of organic matter, organic matter contains nitrogen, and during waste incineration, nitrogen will react chemically with oxygen to generate a large amount of NOx. These NOx will not only form acid rain, but also cause discomfort and threaten human health. The NOx generation reaction is equation (3).
CxHyNz+O2＝CO2+H2O+NOx （3）
3. Acid gas control measures
3.1 Wet acid removal technology
Using wet acid removal technology, the particulate matter in the exhaust gas is generally completely removed through the baghouse, and then transported to the wet smoke washing tower. First of all, after the exhaust gas enters the tower, the temperature of the exhaust gas gradually drops by spraying liquid, and when the saturation temperature is reached, it reacts chemically with the alkaline absorbent distributed in the filler void below to absorb acid gas.
3.2 Dry acid removal technology
Using dry acid removal technology, quicklime powder is usually sprayed in the absorption tower, so that the quicklime powder can be in contact with the acid gas in a large area, so as to absorb the acid gas and obtain a good acid removal effect. However, this method tends to have low acid removal efficiency, with HCL removal rates of about 60% and SO2 removal rates of only about 30%. In order to effectively improve the acid removal efficiency, it is necessary to moderately increase the amount of absorbent used in the application.
3.3 Semi-dry acid removal technology
The use of semi-dry acid removal technology is mainly under the application of high-efficiency atomizer, spraying slaked lime slurry in the absorption tower, and the spraying direction can be sprayed down from the top of the tower or upwards from the bottom of the tower, so as to promote the sprayed slaked lime slurry to contact a large area with the exhaust gas, and efficiently absorb the solid particles and acid gas in the exhaust gas. The semi-dry acid removal technology has a good atomization effect in the application, and the exhaust gas can be in relatively full contact with the absorbent liquid, which not only helps to reduce the temperature of the exhaust gas, but also effectively absorbs the acid gas in it and removes the solid particles distributed in it. After the slaked lime slurry is sprayed, the water contained in it will evaporate to reduce the amount of wastewater generated. In the application of semi-dry acid removal technology, the device involved is relatively simple, the amount of wastewater generated is relatively small, and the operating cost is not high, which can obtain a good acid removal effect. The semi-dry acid removal technology has a high acid gas removal rate, and the hydrogen chloride removal rate can exceed 9 0 % when used with cloth bags.
3.4 Low nitrogen combustion technology
At present, foreign low nitrogen combustion technology is relatively mature, especially the air graded combustion technology is highly effective, many developed countries have also developed low NOx burners, our country attaches more importance to the application of small investment and convenient installation and transformation control measures, in recent years a variety of low nitrogen burners, such as Tsinghua radial nitrogen concentration low NOx burners, Harbin Institute of Technology radial nitrogen concentration low NOx burners, etc., the application effect is remarkable. The low-NOx burners in the application stage include low-NOx pre-ignition chamber burners, hybrid booster burners, separated flame burners, self-recirculating burners, etc. Air graded combustion technology is mainly divided into one or even multiple stages of adding air, adding part of the air to the combustion area, making the excess air coefficient a<1, creating a fuel-rich area, and simultaneously creating a reducing atmosphere, so as to inhibit the generation of NOx and simultaneously reduce the generated NOx. After that, add enough air to the rear combustion area to make A>1 to create a low-fuel area to improve combustion efficiency.
3.5 Flue gas denitrification technology
At present, in the process of waste-to-energy incineration, because of the application of low-nitrogen combustion technology and "3T" technology, the secondary pollutants in the incineration treatment are effectively controlled, and the application of SNCR technology can use ammonia as a reaction reducing agent, and an efficient and fully automatic denitrification system in the incinerator can be established in the incinerator, which can reduce NOx emissions again and make the final discharge meet the relevant standards. In addition to SNCR, flue gas denitrification technology also has SCR, etc., and various industries have also developed some new flue gas denitrification technologies, such as activated carbon adsorption method, liquid absorption method, microbial method, electron beam method, etc.
3.6 New acid removal process
In addition to the above technologies, new acid removal processes such as NHD removal process and low-temperature methanol washing process can also be applied. Among them, the NHD removal process is a low-energy, new purification process, the main components used are made of polyethylene glycol dimethyl ether mixture, with small volatility and low vapor pressure, and the solvent does not need to be washed and recycled, the essence is to absorb acid gas by physical methods; The low-temperature methanol washing process is a physical absorption method that continuously reduces the circulation volume as the operating pressure continues to increase, and can remove HCl and SO2, NO and other acid gases. The flue gas treatment system of the domestic waste incineration plant is a combination of multiple technologies. After practical research, it was found that the SO2 removal rate was more than 90% and the HCl removal rate was as high as 99% when using dry and semi-dry deacidification techniques.
epilogue
In order to make full use of domestic waste, waste-to-energy plants need to pay close attention to taking effective technical measures to treat acid gas, reduce related gas emissions, alleviate the huge pressure on domestic waste faced by cities, and promote the development of environmental protection.
