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Study on Technology of Domestic Waste Incineration Power Generation and Prevention Measurement of Waste Gas Pollution

According to current application effects, we can see that the waste incineration power generation could not only solve the urban domestic waste, but also use the heat generated by the garbage combustion to generate electricity to release part power supply pressure of the city. Therefore, the waste incineration power generation has become a key project in the city construction. But during the process of waste incineration power generation, due to technical defects, the harmful gases and dust generated after waste incineration have not been properly treated. These problems in waste incineration power generation processes greatly limit the application of urban waste incineration power generation technology.

1.The principle of waste incineration power generation

Waste incineration power generation means that the garbage is sent to an incinerator through the feed inlet and fully burned at a temperature of 800° C to 1000° C, during the incineration process, large of heat will be generated, then heat & feed water to superheated steam, then lead to the steam turbine, the high temperature and high pressure steam converted the heat energy to power energy after the steam turbine work, so as to achieve power generation. The flue gas and dust produced during the incineration will purified by the dust-cleaning equipment and discharged into the atmosphere through the chimney, the slag and ash etc will be collected for comprehensive utilization or landfill. The volume of garbage after incineration is reduced by as much as 80%-90%, which reduced the weight by 70%. Therefore in addition of less volume and weight, garbage incineration also as many other advantages, such as small occupied area, waste heat power generation, thorough and effective elimination of bacteria and the waste residue could be used as building materials. Based on above advantages, “The National Environmental Protection 13TH Five-Year Plan” proposed the waste treatment system technology as the primary consideration for domestic waste disposal in the future. The plan pointed out that advanced domestic waste treatment equipment must be used to reduce the discharge of pollutants to improve the waste treatment level. With the strong support of national policies, waste incineration power generation technology develops quickly and will become the main method of waste disposal soon in China.

2.Waste gas analysis in waste incineration power generation

2.1 Compositions of waste gas after waste incineration

The main compositions of waste incineration is four non-hazardous substances including N2, O2, CO2 and H2O, this is almost 99% volume of the flue gas. Due to the uncontrollable waste composition and the variability of the combustion process, there is still about 1% harmful pollutants in the incineration flue gas, it mainly includes as following:

(1)Particulate matter, such as inert oxides, metal salts, incomplete combustion products, etc.;

(2)Acid gases, such as NOx, SOx, HCl and HF, etc.;

(3)Heavy metal, mainly Hg, Pb, Cd, Cr, Zn etc elementary substance and oxides;

(4)Residual organics, including incompletely combusted organics and reaction products, such as aromatic polycyclic derivatives, hydrocarbon compounds, unsaturated hydrocarbon compounds, dioxins, etc.;

2.2 Limit of environmental capacity

There will be a lot of pollutants produced in the incineration process of municipal wastes. “Standard for pollution control on the municipal solid waste incineration” in China sets out the discharge limits of 10 pollutants, including CO, soot, SO2, HCl, NOx, and Hg. Dioxins and other substances. According to “Standard for pollution control on the municipal solid waste incineration” (GB18485-2014), the maximum limit of dioxin is 0.1 ngTEQ/m3, but in most first-tier city such as Shanghai, Beijing and Guangzhou, the monitored contains of dioxin in the air has reached the maximum limit, even some exceed the limit.

The current atmospheric load has reached the limit it can withstand. If new municipal waste incineration plants are added to these first-tier cities, they will greatly increase the residents; exposure to serious environmental risks. So far, many cities are facing the atmosphere capacity problems, although there will be EIA report before the implementation of the project, the potential source of danger still need to get more attention and focus.

3.Strengthening pollution prevention

Flue gas purification is the key measure for effective control and scientific treatment of smoke pollution after the flue gas of waste incineration power generation plant discharged. In current work of flue gas purification in China’s garbage  power plants, the most commonly used combination process is “SNCR denitrification + semi-dry deacidification + dry lime injection + activated carbon adsorption + bag filter” which to achieve successive treatment of flue gas discharged of waste incineration. Following analyze the principle of different purification process:

3.1 SNCR denitrification

The operating data of the waste incineration plant in China shows that the NOx emission value can be within 400mg/Nm3 as long as the combustion temperature and oxygen are well controlled, but this cannot meet the latest requirements of the national standard. In order to further reduce the NOx content in the flue gas, selective non-catalytic reduction (SNCR) can be used to remove nitrogen oxides. The SNCR method is to spray ammonia solution into the flue gas at a temperature 800-1100°C, the ammonia radical generated by the decomposition of ammonia water will react with NOx, reduce to N2, H2O, and CO2 to achieve NOx removal.

3.2 Semi-dry deacidification

The volume of flue gas at the outlet of the incinerator that goes through the rotary spray drying deacidification reaction tower can ensure to achieve the predetermined removal efficiency, and also meet the requirement of evaporating lime slurry into dry powder. The flue gas containing acid gases at the outlet of the incinerator will do deacidification after entering the reaction and cool down. The lime slurry delivered by the pulping system is atomized into droplets with a particle size of about 120 to 200 μm by a double-phase flow fixing nozzle at the top of the tower, these fine droplets are in full contact with the acid gas, and removes most of the acid gases in the flue gas after a series chemical reactions. During the reaction process, the droplets absorbs heat from the flue gas and continuously evaporates water, combined with the unique design of the reaction tower, the high-temperature flue gas in the town allows the slurry droplet to dry during the drop process and fully evaporates the water before reaching the bottom of the town. The formation of solid reactions will be discharged from the bottom of the tower.

The remaining gaseous pollutants in the flue gas are removed by further reaction with incompletely reacted Ca(OH)2 when it passes through the filter bag. In addition, due to the decrease of the smoke temperature, some of toxic organic matter and heavy metals in the flue gas can also be removed by the adsorption of condensed or adsorbed dust.

3.3 Dry lime injection

The system has following two functions: pre-spray and dry deacidification. Each outlet of dry powder silo is set with dosing device and clogging alarm device, and can independently feed materials, the dosing device controls the amount of slaked lime added, and is sprayed into the outlet of the reaction tower via a Venturi injector. The air from the injection fan blows the slaked lime discharged from the feeding device into the flue between the deacidification tower and the baghouse, and reacts with the sour gas SOx, HCl, etc. In the flue gas. After reacting with the slaked lime, the flue gas enters the bag filter with fly ash and various dusts.

3.4 Activated carbon adsorption

Due to the production of dioxins during the waste incineration process, in order to better remove the heavy metals and dioxins, activated carbon is injected into the the flue before the precipitator, and activated carbon absorbs heavy metals and dioxins to ensure their emissions reach the national emission standards. The activated carbon has a large specific surface area, so as long as the activated carbon and the flue gas mix evenly to get a sufficient contact time, it could reach the required purification efficiency. After activated carbon is injected into the flue pipe, it will start to adsorb the heavy metal pollutants such as  dioxin and Hg, but they do not reach saturation. They then enter the bag filter along with the flue gas and are adsorbed on the surface of the filter bag. The flue gas on the surface of the bag is in full contact with the purpose of removing heavy metals and dioxins from the flue gas.

3.5 Bag filter

The system uses a low-pressure jet pulse bag filter to collect smoke from the flue gas. Dust-laden flue gas enters the box through the air inlet at the lower part of the dust-removal chamber, and the purified gas enters the upper box through the filter bag mouth and is discharged from the air outlet. The airflow is then turned back and forth, and the dust is trapped on the outlet surface of the filter bag through a filter bag with a metal frame inside it to purify the gas. The purified gas enters the clean room in the upper part of the filter bag chamber and is collected in the air outlet pipe. With the continuous operation of the dust collector, when the dust on the surface of the filter bag reached a certain thickness, the resistance of the gas passing through the filter material increases, the air permeability of the bag decreases, and the inner wall of the bag is blown with the pulse air flow, the powder layer on the outer surface of the cloth bag is pressed. After falling down, the dust layer fell into the ash bucket and the filter bag resumed filtration.

In summary, waste incineration power generation can not only properly handle the waste, but also generate electricity by using the waste resources, which reflects the effective use of resources. While in the process of waste incineration and power generation, the more important thing is the waste treatment, power generation is only a part of the reuse of energy in the waste disposal process. Therefore, waste incineration power generation plants must update the technology and contribute to realize the waste harmless and resourceful target.

No category Apr 26,2018 HNAC Comments are closed

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3 Responses

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May 06,2017

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May 06,2017

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