Coal Ash - The Grey Resource

Introduction

Since the operation of the first coal fired power plant in Hadera in 1982, coal ash began to be produced as a residue of coal combustion and as a byproduct of the production of electricity. The coal that is imported to Israel contains between 4% - 15% ash by weight, depending on the source of the coal, and averages about 10% - 12% depending on the overall composition of the variety of coals purchased.

The main use of coal in Israel is for generation of electricity, both currently and apparently also in the future. Therefore, the centers of its production are the existing coal-fired power plants - "Orot Rabin" in Hadera, and "Rutenberg" in Ashkelon, and the plants that are planned to be built at these sites. The Coal Ash Administration that was established in 1993 with the objective of coordinating the effort at a national level to solve the problem of ash accumulation at the power plants, chose to deal with the problem by advancing the use of coal ash as a resource having economic value, in various sectors, as is done in most of the developed countries in the world.

This site is intended to provide information regarding coal ash characteristics, beneficial uses of coal ash, both currently and in the future, in construction, infrastructures, industry and agriculture in Israel, to provide information on environmental quality issues, and to describe what is being done in this area to prevent damage to the environment and to improve it.

Attached is an article submitted to AshTech2006 Conference in Birmingham.
The article presents the coal ash utilization development process in Israel and focuses on uses in the construction sector and on the Israeli fly ash quality in that context. The article refers also to unique Israeli bottom ash uses in agriculture.

Source and description

Coal ash is the mineral residue that is obtained as a byproduct of the combustion of coal for the production of electricity. Two types of coal ash are obtained:

  • Fly ash, which constitutes 85% - 90% of the overall ash, is a fine, light gray powder made up of glassy spheres from sub-micron to more than 100 microns in size, (98% smaller than 75 microns; 70% - 80% smaller than 45 microns). The material has a specific gravity between 1.9 - 2.4, a bulk density of about 0.8 - 1 ton per cubic meter and a maximal density (modified) of 1,000 - 1,400 kg per m3. The specific surface area of fly ash varies between 2,000 to 6,800 cm2 per gram. Fly ash contains cenospheres - hollow spherical particles having an especially low bulk density of 0.4 - 0.6 ton per cubic meter, which constitute up to 5% of the ash weight and are suitable to be utilized for special industrial applications.

  • Bottom Ash, which constitutes about 10% - 15% of the overall ash, has an appearance similar to dark gray coarse sand, and its particles are clusters of micron-sized granules, up to 10 mm in diameter (60% - 70% smaller than 2 mm. 10% - 20% smaller than 75 microns).It has a bulk density of about 1 ton per cubic meter and a maximal density (modified) of 1,200 - 1,500 kg per m3.

Along with coal ash gypsum (FGD - Flue Gas Desulfurization) is obtained as a result of removal of sulfur from exhaust gases, it is also one of the group of Coal Combustion Products, CCP's, or Coal Combustion Byproducts, CCB's. In Israel, coal ash is produced at the "Orot Rabin" Power Station in Hadera and at the "Rutenberg" Power Station in Ashkelon. FGD is now produced only at Rotenberg. The coal is imported to Israel from various sources: mainly from South Africa, Colombia, Australia and Indonesia. Certain quantities are also imported occasionally from the United States, Poland, and China. On average, the coal that is combusted in Israel contains 10% coal ash.

Process.

Fly ash particles are swept along with the exhaust gases and are collected in electrostatic precipitators before they reach the stack. The ash is stored in silos in a dry state. From the silos, it is transferred in trucks to be used in industry as a pozzolanic additive in cement and concrete batch plants, or as a filler in various products. When the silos are full, it is conditioned by moistening (to 22% moisture) and sent for intermediate storage in open piles at the power stations or outside of them until it is utilized as a filler material in roads and infrastructure works. Transport of the moistened ash is done in semi-trailer trucks that are covered with canvas to prevent creation of dust.
The bottom ash falls into a water pool at the bottom of the boiler and is removed by a conveyor belt to storage facilities and from there to intermediate storage piles while in a water-saturated state. In the piles, the water content decreases to about 20%.

Composition

The ash is mainly composed of silica, alumina, and smaller quantities of oxides of iron, magnesium, calcium and other elements. It practically contains almost all the known elements, including toxic elements in trace quantities, which, to the extent of the concentration of their availability, may sometimes demands preventive environmental care. Furthermore, the ash contains radioactive elements in small quantities, but their concentration in ash is relatively larger than their concentrations in most natural rocks and soil. The ash also contains unburned carbon residue that gives it its dark color. The carbon content, as measured by a loss on ignition test, LOI, is commonly found in Israeli coal ash to be in the range of 3% - 6%.

Major Elements (Concentration in %)

 
Fly ash
Bottom ash
 
Low
High
Low
High
SiO2
38
70
40
65
Al2O3
17
35
18
30
Fe2O3
2
16
3
14
CaO
0.6
13
1
6
MgO
0.4
5
0.4
3
Na2O
0.1
2
0.2
1.3
K2O
0.5
3
0.7
1.6
SO3
0.4
5
0.5
4
TiO2
0.04
1.6
0.3
0.7

Characteristics

The ash is characterized by physical (lightweight, small spherical particles, hardness) and chemical (cement-like) properties that provide it with an economic value as a raw material in many applications.

Coal Fly Ash Coal Fly Ash (Magnification 20,000 X) Coal Bottom Ash Coal Bottom Ash, Raw and Classified by Size