"Garbage sorting, starting from me." An easy-to-understand slogan, with magical background music, is played in an infinite loop next to the garbage can, and each garbage throwing is a spiritual "baptism". Uncles and aunts in the community came together as volunteers on duty in front of the barrel, tirelessly explaining the classification method to everyone.
Maybe you've also been confused: Why do we want to promote garbage sorting? Here, we will talk about the importance of garbage classification from the final stage of garbage disposal to the disposal of garbage incineration fly ash (referred to as fly ash).
Domestic waste incineration to generate electricity
In 2020, China's domestic waste removal volume reached 235 million tons, and "the largest landfill in China will be filled" has been on Weibo hot search, triggering extensive discussions about domestic waste landfill and other disposal methods. If domestic garbage is allowed to be landfilled on the outskirts of the city, the "garbage siege" will no longer be a fictional scene in the science fiction movie "Robot Story", but will become a thorny problem faced by most cities in real life.
Figure 1 Stills of Robot Story (Image from the Internet)
In order to break through the "garbage siege" dilemma, developed countries such as Europe, the United States, Japan and South Korea, as well as developing countries such as China, have vigorously developed domestic waste incineration power generation technology. Large-scale reduction of the volume of garbage through incineration, and the energy in the garbage is converted into electricity. In 2020, the proportion of domestic waste incineration in China is 45%, and a total of 1202 incinerators are in commercial operation, which can burn 580,000 tons of garbage per day. Take a family of three as an example, per capita produces 1 kilogram of garbage per day, the whole family can produce more than 1 ton of garbage in 1 year, and 1 ton of garbage incineration will generate 300 kWh of electricity, which is enough for the family's electricity demand for 2-3 months.
Domestic waste incineration power generation must comply with the national standard "GB 18485-2014 domestic waste incineration pollution control standards", and there are strict requirements for waste conditions, operating parameters and pollutant discharge in the incineration process. The modern waste incineration power plant has realized the closed type of all aspects of waste disposal, and the plant area and vicinity are clean and odorless. In addition, the leachate can be extracted from biogas after biochemical fermentation, and the residual liquid is returned to the furnace chamber for high temperature treatment.
Figure 2 Schematic diagram of domestic waste incineration power plant (picture from the network)
Waste incineration fly ash toxicity
Since there is such a good disposal technology, is it possible to "worry-free" garbage? Of course not. Incineration power generation still cannot perfectly solve the "last mile" problem of waste disposal, that is, the safe disposal of fly ash. Garbage combustion and downstream flue gas purification will inevitably produce a certain amount of fly ash. For example, the proportion of fly ash produced by the mainstream grate waste incineration technology is about 3 to 5% of the amount of waste entering the furnace. According to the provisions of the national standard "Hazardous Waste Identification Standard Leaching Toxicity Identification" (GB5085.3-2007), due to its leaching toxicity exceeding the standard, fly ash is listed in the National Hazardous Waste Directory (2021 Edition) with the category HW18. Therefore, countries around the world have clear and strict regulations for the disposal of fly ash. Due to the sharp increase in the amount of garbage disposal, it is expected that China's annual output of fly ash will produce tens of millions of tons at the end of the "14th Five-Year Plan". The disposal of fly ash has become an urgent pain point problem to be solved in the construction of green water and green mountains.
Fly ash, median particle size 18.6 μm, finer than flour, the color is grayish white, there is a pungent odor, the main components are calcium chloride hydroxyl, calcium sulfate, calcium hydroxide, calcium carbonate, sodium chloride and potassium chloride. Judging from the main ingredients, it seems that there is no need to "talk about gray change". In fact, fly ash is rich in a large number of inorganic and organic toxic substances: toxic heavy metals such as zinc, copper, nickel, cadmium, chromium, lead, mercury, etc., of which heavy metal cadmium is one of the ten major environmental pollution events in the twentieth century, the culprit of Tomito bone pain in Japan; non-metallic toxic elements and arsenic, which often appear in costume TV series, that is, arsenic oxide; in addition, the surface of fly ash is also attached to dioxin highly toxic organics, which are 900 times more toxic than arsenic, and dioxins are difficult to biodegrade. It can exist in nature for a long time, and may even be enriched into the human body with the biological chain, causing chronic poisoning and even carcinogenic mutations in human organs. Agent Orange, one of the main components of dioxins, was heavily used by the U.S. military during the Vietnam War, causing millions of Vietnamese children to deform or suffer from diseases such as cancer.
Figure 3 Fly ash and local enlargement 1000 times photo
Waste incineration fly ash disposal technology
Therefore, for the hazardous waste such as fly ash, the disposal method must be extremely careful and careful. At present, the international mature commercial disposal technology mainly includes chelation stabilization - cement curing, cement kiln collaborative disposal. In addition, there are other technologies under development, such as plasma melting technology.
Chelation stabilization-cement curing technology uses organic chelating agents to construct a "cage" to limit the migration of harmful elements in ash, while cement adds a layer of "wall" to the "cage". The disposal of fly ash meets the "Domestic Waste Landfill Pollution Control Standards" and can enter the domestic waste landfill partition landfill. The advantages of this technology are simple operation, low investment and low disposal costs. At present, about 90% of the country's fly ash adopts this technology for disposal. However, the technology has not really realized the harmlessness of garbage fly ash, and with the passage of time, the continuous invasion of rain and wind, the risk of precipitation of harmful substances in the future still exists. In addition, the disposal still needs to occupy the landfill space with a serious shortage of "balance", which is not in line with the vision of future fly ash resource disposal.
Figure 4 Schematic diagram of fly ash chelation stabilization-cement curing process (picture from the network)
Cement kiln collaborative disposal technology uses the high temperature environment of the cement production system to achieve the detoxification of harmful elements and dioxin organic matter in fly ash. Since the cement production process has strict requirements for chlorine and alkali metals in raw materials, and alkali metal chlorine salts are one of the main components of fly ash, the fly ash needs to be desalinated before entering the kiln. The water washing solution obtains sodium chloride and potassium chloride by evaporating crystallization. The material temperature, flue gas residence time, gas disturbance and excess air coefficient in the kiln meet the international common "3T +1E" principle, harmful elements and minerals are cured at a high temperature of more than 1400 °C, dioxin organic matter is completely decomposed, and calcium-based minerals are transformed into cement products.
However, the technology has obvious geographical limitations, such as Shenzhen, Shanghai and other places do not have cement production lines; at the same time, the chlorine in the furnace raw materials should not be higher than 0.04%; and the proportion of washed fly ash mixing is usually not higher than 4%, too much blending will affect the quality of cement and production stability. In addition, the energy consumption of salt evaporation crystallization in the washing liquid is high, and the crystalline salt treatment technology is still immature.
Figure 5 Schematic diagram of the process flow of cooperative disposal of fly ash by cement kiln
At present, the R&D team at home and abroad is committed to completely solving the technical problems of fly ash disposal, of which plasma melt disposal is one of the research hotspots. This technology uses plasma torch to produce high-temperature plasma to achieve high-temperature melting of fly ash, in the process of complete decomposition of dioxin organics, high-temperature vitrification of harmful elements, low toxicity of vitrified slag leaching, and 75% reduction of fly ash capacity. It can be said that plasma melting technology can achieve fly ash reduction, harmlessness and resource utilization. However, this technology has problems such as high power consumption, short electrode service life, and corrosion and shedding of heat-resistant materials, resulting in extremely high equipment investment and operating costs. Domestic related research is still in the semi-industrial experimental research stage.
Figure 6 Schematic diagram of fly ash plasma melt disposal process (picture from the network)
Based on the internationally accepted "3T+1E" principle, the Institute of Engineering Thermophysics of the Chinese Academy of Sciences has developed a fly ash fluidization melt disposal technology. This technology can realize the high-temperature melting of fly ash and flux, form molten glassy slag, directly "seal" the harmful elements in the fly ash in the glass, and the toxicity test of harmful substances leached in the slag is extremely low; dioxin organic compounds are completely decomposed at high temperature, and the concentration of dioxin organic matter in the flue gas is far lower than the national standard. This technology simplifies the fly ash pretreatment process, eliminating the need for washing and granulation, while the key energy consumption problems of melt disposal are effectively controlled. This technology is expected to solve the problem of waste incineration fly ash disposal.
The importance of domestic garbage sorting
No matter how advanced the fly ash disposal technology is, it will inevitably consume a lot of energy and labor costs, and there are also certain environmental risks. Therefore, in the face of increasingly large fly ash production, we need to reduce the amount and poison from the source, and do a good job in sorting domestic waste.
The basic principles of domestic garbage classification are the separation of recyclables from non-recyclables; the separation of combustibles from non-combustibles; the separation of dry garbage from wet garbage; and the separation of toxic and harmful substances from general substances. Taking Beijing as an example, domestic garbage is mainly divided into four categories, that is, recycling garbage, kitchen waste, other garbage and hazardous garbage.
Figure 7 Domestic garbage classification poster (picture from the Internet)
Different types of waste disposal methods are very different. Among them, recyclable garbage is classified and recycled as much as possible, turning waste into treasure, saving resources; kitchen waste contains a large amount of organic matter, high moisture content, generally take sanitary landfill and biochemical treatment, the use of microbial degradation, can be converted into organic fertilizer utilization; hazardous waste disposal needs to be particularly cautious, generally by professional qualification companies centralized disposal; and other garbage has a certain calorific value, can be disposed of by incineration, make full use of the energy of combustibles to generate electricity.
It can be seen that after the domestic garbage classification is carried out from the source, other garbage is separated from the other three types of garbage, which will greatly reduce the amount of incineration treatment, and the amount of fly ash will also be significantly reduced. In this way, we not only recover resources from the source, but also to a certain extent avoid the large amount of resource consumption of fly ash disposal.
Therefore, we call on everyone to carry out domestic garbage classification from the source, so that each kind of garbage has its own most suitable "destination", recycle the available substances in the garbage as much as possible, reduce the amount of garbage incineration, and minimize the difficulty and cost of fly ash treatment. Let us work together with technical personnel to work hard for the cause of green water and green mountains in the motherland, and guard the blue sky and white clouds, birds and flowers that you and I cherish, and the laughter of children.
Source: Institute of Engineering Thermophysics, Chinese Academy of Sciences