1. The concept of flue gas whitening
About 5-10 years ago, there was a lot of exhaust smoke but little haze; after years of treatment, there is now less black smoke and yellow smoke but more haze, and white smoke mainly produced by wet desulfurization is common. The white smoke emitted from the chimney mixes with the cold ambient air. During the cooling process, the water vapor contained in it is saturated and condenses. The condensed water mist droplets refract and scatter the light, making the smoke plume appear white or gray, which is called "wet smoke". "Feather", commonly known as "big white smoke", smoke whitening refers to the removal of large white smoke. The source of white smoke is moisture. The saturated water vapor in the flue gas is produced by absorbing heat and contains a lot of latent heat. Therefore, the key to whitening the flue gas is to control the temperature and humidity of the discharged flue gas.
2. The purpose and function of flue gas whitening
Since flue gas dewhitening has just started, there are different understandings of why flue gas dewhitening occurs. My opinion is: the minimum requirement is to achieve smoke-free smoke all year round, which can solve visual pollution and reassure the general public; it can also reduce the impact on flights and highway transportation; at the same time, there should be at least four other Function:
①It is beneficial to remove smog;
②Water saving;
③Energy saving;
④ Treat wastewater;
Although the industry has different opinions on whether flue gas dewhitening can help remove haze, its contribution to haze removal, water saving, and energy saving have been initially verified by performance, and will be discussed separately below.
2.1. Remove smog
The density of water vapor in flue gas is 0.804kg/m3, which is lighter than air. It rises easily after leaving the chimney. After mixing with ambient low-temperature air and cooling, the water vapor is saturated and condenses into water droplets. The density of water droplets is 1000kg/m3, and the particles are extremely fine. , water droplets that are much denser than air float and accumulate in the atmosphere to form fog. Dense fog affects flight and highway traffic. It can be blown away by wind, rain, and snow or condense and land. In calm weather, dense fog is like a big cover. , dissolved particulate matter precipitates itself, and it also absorbs particulate matter from other pollution sources, blocks its diffusion, and accumulates in the fog layer to form smog. This is the cause and process of frequent smog in our country. Flue gas dewhitening based on dehumidification can remove smog and reduce foggy weather. What is particularly important is that it can achieve compliance with standards and ultra-low near-zero emissions of dust, sulfur dioxide, nitrogen oxides, heavy metals, and organic pollution components at a coordinated and low-cost basis. , transforming environmental protection from a pure investment into a benefit.
2.2. Water saving
Based on the following method, it can be estimated that my country’s annual anthropogenic emissions of water into the atmosphere exceed 16 billion tons.
1) Coal power and thermal power industry: For every ton of standard coal burned, the wet flue gas takes away 1 ton of moisture and the circulating water
Cooling Tower drains 5 tons of water. The coal power and thermal power industry discharges more than 12 billion tons of moisture into the atmosphere every year.
2) Steel industry: For every ton of steel produced, 1.5 tons of water are emitted, and the entire industry emits about 1.5 billion tons of water every year.
3) Coal coking industry: For every ton of coke wet-quenched, 0.5 tons of water are emitted. Both wet desulfurization and dry desulfurization of coke oven flue gas emit water. The total coal coking industry emits more than 200 million tons of water into the atmosphere every year.
4) Natural gas combustion: For every cubic meter of natural gas burned, 2 cubic meters and 1.55 kilograms of water vapor will be produced. The country emits more than 300 million tons of water every year.
In addition, there are cement, building materials, non-ferrous metals, petrochemical and other industries.
The 16 billion tons of water artificially discharged into the atmosphere is indeed a small amount compared with the amount of water circulation in nature. However, this is an increase based on the normal circulation of nature. Especially since my country's economic development is uneven and the emission intensity in developed areas is It is more than 10 times the national average, which is extremely uneven. my country's national conditions of huge energy consumption and emissions determine that when doing flue gas dewhitening in China, we must pay full attention to water conservation. Water is like haze when it comes to the sky, but when it is recovered, it is a resource. Especially for water-scarce enterprises in the west, recycling water is more meaningful. Atmospheric moisture is an important unconventional water source that cannot be ignored in our country.
2.3. Energy saving
Using flue gas dewhitening based on dehumidification, assuming that 70% of the exhaust steam is recovered, the country can recover heat equivalent to more than 10 billion tons of water vapor every year, which can be used for heating, heating of mineral powder and coal powder, which has considerable energy-saving benefits. .
2.4. Treatment and utilization of wastewater
Flue gas dewhitening not only does not produce new wastewater, but also helps to treat the original difficult-to-treat wastewater. Wet desulfurization wastewater, coking wastewater, and concentrated brine produced by comprehensive sewage treatment are all wastewaters that are difficult to treat and require high investment and treatment costs. Conventional treatment methods based on membrane treatment are difficult to achieve "zero discharge" or low-cost "zero discharge". "Emission", after using flue gas dewhitening, it blocks the transfer of pollutants in the flue gas from water to the atmosphere, uses the waste heat of the flue gas and residual oxygen to pyrolyze and oxidize, harmlessly digest the organic components of COD in the wastewater, and absorb and dilute Ammonia nitrogen can be concentrated by spray evaporation and dried to remove dissolved salts, which can greatly reduce wastewater treatment investment, especially the cost of treatment.
To sum up, flue gas dewhitening is an effective means to achieve ultra-low emissions, save energy, remove haze, save water, and treat and utilize wastewater.
January 16, 2024
1. The concept of flue gas whitening
