From being useless to being full of treasures, the rise of “fly ash”

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In the cold winter, northern cities have started to burn hot furnaces one after another, and the towering chimneys of thermal power plants are spewing out bursts of white mist. We all know that thermal power plants generate heat by burning coal, and then generate electricity or heating. However, there are still a lot of non-combustible substances such as clay minerals in coal. They are dehydrated, decomposed, softened and melted at high temperatures, and then rapidly cooled and solidified to form fly ash particles. These particles vary in size, ranging from a few microns to hundreds of microns. In the dry ash discharge system, they are accurately captured by the electrostatic precipitator or bag filter in the flue, fall into the ash hopper and embark on a journey of transportation and storage; or in the wet ash discharge system, they are melted into slurry with water and flow into the ash pool for further disposal.

Fly ash storage, source network

my country is a large coal-producing country, using coal as the basic fuel for electricity production. According to statistics, the emission of fly ash reached 125 million tons in 1995, about 150 million tons in 2000, and will reach 300 million tons by 2010; the output of fly ash was 580 million tons in 2013, and the annual output will reach 749 million tons (another saying is 865 million tons) by 2023. Looking back ten years ago, these fly ashes were mainly stored in open-air piles, forming a large number of fly ash yards. Once these fly ashes become "willful", dust will rise everywhere, polluting the atmosphere; discharging them into water systems will cause river siltation, and the toxic chemicals in them will also cause harm to humans and organisms.

Annual production of fly ash, self-made

In fact, fly ash is mainly composed of Al₂O₃, SiO₂, Fe₂O₃, CaO, TiO₂, MgO, Na₂O, etc. Under the microscopic view, it can be seen that the surface has a smooth glass structure. This amorphous glass has high chemical internal energy and potential activity. Since the beginning of the 20th century, research on the comprehensive utilization of fly ash has begun at home and abroad. It was not until the end of World War II that coal gradually became the main fuel in European countries. However, the environmental pollution caused by its combustion and the depletion of mineral resources have seriously emerged, which strongly stimulated people to study and develop fly ash application technology. Its application scope is becoming more and more extensive, involving many fields such as building materials, construction, road construction, chemical industry, environmental protection, agriculture, backfill, high value-added products, etc.

In the construction field: fly ash accounts for 45% of the total utilization in construction, and the main products are fly ash cement (more than 30% admixture), concrete, ordinary cement, wall materials, silicate load-bearing blocks and small hollow blocks, sintered ceramsite, sintered bricks, etc. Due to its volcanic ash activity, fly ash can be made into cement admixtures or solid waste cementitious materials to replace cement.

In the field of environmental protection: mainly based on the adsorption characteristics and specific components of fly ash for the treatment of wastewater and waste gas. For example, polyaluminium chloride, aluminium sulfate and other flocculants prepared from fly ash have the characteristics of low cost and high efficiency in sewage treatment, and have broad application prospects.

In the agricultural field: The particle shape of fly ash makes it easy to absorb, and fly ash is rich in a large number of trace elements that are beneficial to crops. Therefore, fly ash can provide nutrients for crops and promote their growth.

In the road field: fly ash can be used for laying the road base and subbase. Fly ash is mixed with lime, gravel, etc., and a stable road structure layer is formed after compaction. This is because the chemical components in fly ash can react with lime to form a gelling material with certain strength and stability, which enhances the bearing capacity of the road. For example, in the construction of secondary roads in some cities, fly ash mixed materials are used, which not only effectively utilizes fly ash, but also reduces the cost of road construction.

According to reports, a large number of fly ash resource utilization demonstration projects have been put into operation in Inner Mongolia, Hebei, Ningxia, Shanxi and other places in my country, which has greatly promoted the comprehensive utilization of fly ash. At present, the comprehensive utilization level of fly ash in my country is as high as about 80%, which is one of the most widely used and widely used industrial solid wastes. Fly ash has truly changed from a useless power plant waste to a popular commodity.

Quote:

1. "2022-2023 Bulk Industrial Solid Waste Comprehensive Utilization Industry Development Report"

2. " 2022-2026 Fly Ash Industry Development Research Report "

3. "2024-2030 China Fly Ash Industry Development Trends and Investment Opportunities Analysis Report" Zhiyan Consulting Research

4. "Research Report on the Development and Operation Status and Investment Strategy of China's Fly Ash Industry from 2024 to 2030" by China Economic Industry Research Institute

5. Shi Yaqian, Guan Yushan, Ge Weizhe, et al. Value-added utilization of fly ash as building materials: latest technology and future prospects[J]. Journal of China Coal Society, 2024, 49(06): 2860-2875.

6. Wang Liping, Li Chao, Cao Kun, et al. Research progress on the application of fly ash in wastewater treatment[J]. New Chemical Materials, 2024, 52(S2):71-75+82.

7. Yan Peiyu. Mechanism of fly ash in the hydration process of composite cementitious materials[J]. Journal of the Chinese Ceramic Society, 2007(S1):167-171.

8. Lei Rui, Fu Dongsheng, Li Guofa, et al. Research progress on comprehensive utilization of fly ash[J]. Clean Coal Technology, 2013, 19(03): 106-109.1

9. Liu Mengru, Yang Yadong, Yang Sujie, et al. Study on the current status of comprehensive utilization of fly ash resources[J]. Chemical Minerals and Processing, 2021, 50(04): 45-48.

Author: Cui Yanfa, Key Laboratory of Science and Technology for Solid Waste Building Materials Utilization in Hebei Province, solid waste utilization expert.

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