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Coal is used as a major source of energy throughout the world. In 2015 coal supplied 29% of energy world-wide; despite increases in use of renewables, the share of coal is expected to still be 24% by 2035. Since global energy consumption is expected to increase by 30% by 2035, the actual amount of coal consumed per year will increase from 3840 million tons oil equivalent (mtoe) in 2015 to 4032 mtoe in 2035. In 2015, China was the world’s largest coal consumer (50% of global demand, or 1920 mtoe), and is expected to remain so, accounting for 47% of global coal demand in 2035 (1876 mtoe). India’s coal consumption is expected to more than double between 2015 and 2035 (from 407 to 833 mtoe) to feed its power sector, although the United States’ demand will fall by half (from 396 mtoe in 2015 to 198 in 2035). provides global coal fly ash production and utilization. Fly ash utilization is only 1/4th of the total production. The top producers, India and China have less than 50% utilization rate, while Denmark, Italy and Netherlands have 100% fly ash utilization rate.

Burning coal produces coal combustion residuals (CCR), or byproducts, which include fly ash, bottom ash, boiler slag, flue-gas desulfurization residues, and fluidized bed combustion ash. Over 70% of waste coal ash is categorized as fly ash (FA), fine particulates captured by particulate control equipment, ranging in size from 0.5 μm to 300 μm. In 2015, fly ash utilization rates were 70% for China, 43% for India, and 53% for the US. This leaves substantial potential for increased utilization. Re-use of fly ash can decrease disposal volumes and costs, as well as replace non-renewable or expensive resources.

Physical properties:

Fly ash consists of fine, powdery particles predominantly spherical in shape, either solid or hollow, and mostly amorphous in nature. In general, the specific gravity of coal ashes lies around 2.0 but varies to a large extent, from 1.6-3.1. This variation is due to a combination of several factors such as particle shape, gradation, and chemical composition. Based on the grain size distribution fly ashes can be classified as sandy silt to silty sand. Particularly, Indian coal ashes are predominantly of silt-size, with some clay-size fraction. Fly ash has high specific surface area and low bulk density. The amount of unburned carbon and iron impact the color of fly ash, which can vary from orange to deep red, brown, or white to yellow.

Fly ash consists of fine, powdery particles predominantly spherical in shape, either solid or hollow, and mostly amorphous in nature. In general, the specific gravity of coal ashes lies around 2.0 but varies to a large extent, from 1.6-3.1. This variation is due to a combination of several factors such as particle shape, gradation, and chemical composition . Based on the grain size distribution fly ashes can be classified as sandy silt to silty sand. Particularly, Indian coal ashes are predominantly of silt-size, with some clay-size fraction . Fly ash has high specific surface area and low bulk density. The amount of unburned carbon and iron impact the color of fly ash, which can vary from orange to deep red, brown, or white to yellow .

Since 316 individual minerals and 188 mineral groups are recognized in fly ash, it is one of the most complex materials in terms of characteristics. However, all fly ash includes substantial amounts of silicon dioxide (SiO2) (both amorphous and crystalline), aluminum oxide (Al2O3) and calcium oxide (CaO), the main mineral compounds in coal-bearing rock strata. Depending on pH value and calcium/sulfur ratio, fly ashes are classified as acidic ash (pH 1.2 up to 7), mildly alkaline ash (pH 8–9), and strongly alkaline ash (pH 11–13).

Source: https://www.sciencedirect.com/science/article/pii/S2214509518303735