Comparison of four technical routes for the utilization of fly ash building materials

Fly ash refers to the flue gas purification system of domestic waste incineration facilities, the trap and the bottom ash settled at the bottom of the flue and chimney. In 2022, 930 domestic waste incineration plants have been built in China, and the actual domestic waste incineration power generation enterprises in China will process 2.81×10⁸t of domestic waste, and the per capita domestic waste incineration treatment capacity will be about 199 kg/a, of which the amount of fly ash generated will be about 2%~5% (grate furnace) or 8%~13% (circulating fluidized bed) of the waste incineration amount, that is, the national fly ash production will be about 664×10⁴t~between 1543×10⁴t. Fly ash contains dioxins, as well as heavy metals such as Pb, Cd, and Hg.

There are two main treatment and disposal methods of fly ash in China: landfill after solidification and stabilization and resource utilization after harmless treatment. There are two main types of curing and stabilization technologies: cement curing method and chemical agent method. According to the Technical Specifications for Pollution Control of Fly Ash from Domestic Waste Incineration (Trial) (HJ 1134-2020), fly ash can be disposed of in landfills, but the cost of rigid landfill treatment under the new standard is too high, and the location of flexible hazardous waste landfills is difficult. In addition, to promote the construction of "zero-waste cities", adhere to the principle of "reduction, recycling, and harmlessness", and require the maximum reduction of landfill volume, so the disposal of fly ash landfill is not a long-term solution and is difficult to sustain.

The appearance of fly ash is gray-white or dark gray powder, which has the characteristics of uneven particle size, low moisture content, high porosity and large specific surface area, and more than 85% of its particle size is concentrated in 20~125μm. The composition of fly ash includes CaO, SiO₂, Fe₂O₃, Al₂O₃ and other compounds, accounting for 50%~60%; Salts such as NaCl and KCl account for 20%~30%. Therefore, the main components of fly ash are CaO, SiO₂, Fe₂O₃, Al₂O₃, etc., which are similar to the composition of building materials such as ceramics and cement, and have the potential for building materials utilization. The utilization of fly ash as a building material can not only facilitate the safe treatment and disposal of fly ash, but also reduce the consumption of natural raw materials for building materials.

1. Washing-cement kiln co-disposal to produce cement

Fly ash contains more than 40% CaO and substances such as SiO₂, Fe₂O₃, and Al₂O₃, which is close to ordinary Portland cement. Cement is produced from limestone, clay mixtures and other materials, so fly ash can replace limestone to produce cement. Cement kiln has the characteristics of high temperature, long residence time, large heat capacity, alkaline environment, no waste residue discharge, etc., it is an ideal way for solid waste disposal, which can completely decompose dioxins in fly ash at high temperature, and can solidify most of the harmful heavy metals in fly ash at the same time, but the curing effect of heavy metals with high volatility such as Hg, Ba and Pb is poor. At present, the domestic water washing-cement kiln co-disposal of fly ash technology has been relatively mature and has been industrialized, but this technology path depends on the existing cement kiln production line and is not suitable for areas without cement kilns. In addition, the adoption of this process will also be affected by the amount of cement that can be consumed in the market.

2. High-temperature sintering produces ceramsite

As an artificial lightweight aggregate, ceramsite can be used to replace natural river sand to prepare light aggregate concrete and light mortar with its advantages of light weight and high strength, so fly ash sintering ceramsite has a huge market prospect. Fly ash contains SiO₂ and Al₂O3, which can be co-treated with solid wastes such as sludge and construction residue to produce ceramsite. High-temperature sintering is to use a temperature lower than the melting point of fly ash (about 900~1100°C) to heat fly ash, and under high-temperature sintering, melting and bonding between solid particles occur to form a dense and stable sintered body with a certain strength, and the involatile heavy metals are solidified in the dense mineral lattice of the sintered body. The advantage of this type of technology is that it can produce building materials required for engineering construction, and can replace sand and gravel; In addition, the technology can also co-dispose of other solid wastes, fly ash can be added in a certain proportion, which needs to be well controlled to ensure product quality and eliminate environmental pollution in the ceramsite production process.

3. High-temperature melting produces vitreous

High-temperature melting technology has the advantages of good reduction effect, low secondary emission, high thermal efficiency and high slag quality, and is one of the most promising technologies in fly ash treatment. CaO, SiO₂ and Al₂O₃ in fly ash are good raw materials for the production of glass-ceramic. High-temperature melting is the use of high temperatures above 1200°C to completely decompose the dioxins in fly ash, and then quickly cool down and cool to form molten glass, with the help of the dense Si-O tetrahedral structure of the glass body, to achieve the stabilization of heavy metals. The formed glass body has good mechanical and thermal properties, and can be used to produce high value-added building materials such as glass-ceramic. However, the investment cost and energy consumption of high-temperature melting equipment are high, and the requirements for process conditions and flue gas purification system are strict, which restricts its large-scale promotion. In addition, this technology has high requirements for process conditions and flue gas cleaning systems, especially the final fate of the formed vitrified products needs to be comprehensively considered.

4. Low-temperature pyrolysis-washing to produce raw materials for building materials

Compared with high-temperature sintering/melting technology, low-temperature pyrolysis-water washing technology has lower resource consumption and can be disposed of without relying on other industries, which is the latest disposal process of fly ash building materials. Low-temperature pyrolysis refers to the volatilization and decomposition of organic pollutants from the fly ash matrix through a low-temperature thermal decomposition reaction below 500°C in an anoxic or oxygen-free atmosphere, with an efficiency of up to 99% for dioxin removal. The advantages of low-temperature pyrolysis technology can be completed without relying on other industries, focusing on the removal of sensitive components such as dioxins in fly ash, the overall equipment investment and operating expenses are low, and the market prospect is relatively broad, but the disadvantage is that the final destination and use of the treatment products of the washing liquid need to be comprehensively considered.