Environment & Safety

Trace Elements

 

The main potential risk of fly ash is contamination of groundwater during infrastructure applications. In the framework of investigating this issue, many chemical analyses were performed on coal ash. A summary of the analyses is presented in the following table.
 
Concentration of Trace Elements in Coal and Coal Bottom Ash and Fly Ash (parts per million – ppm)

 
Ag
As
B
Ba
Cd
Co
Cr
Cu
Coal
< 1
5
49
170
0.47
6.1
15
16
Fly Ash
5
23
225
1780
0.9
45
147
77
Bottom Ash
2
6
100
1560
0.2
31
144
43
Sedimentary Rock*
0.1
10
130
600
0.3
20
90
60

 

Hg
Mn
Mo
Ni
Pb
Se
V
Zn
Coal
0.2
43
3.3
14
11
2.8
22
53
Fly Ash
< 0.2
540
13
100
67
7
197
114
Bottom Ash
0.2
350
4
69
18
3
127
47
Sedimentary Rock*
0.4
800
2
70
25
0.5
120
150

* Average in the earth’s crust, Wedepohl

Source: Geological Survey of Israel.

 

The refractory trace elements (non-volatiles), Ba, Cr, Mn and V, remain in the solid phases of the inorganic material during combustion. Their concentrations in the bottom ash are slightly lower than in the fly ash because a portion of them is nevertheless volatilized from the bottom ash and precipitated afterwards on the surface of fly ash particles in the cold zone of the exhaust gas exit from the furnace. The elements that evaporate both from the fly ash and bottom ash at the combustion temperature of a power plant, As, Hg, Mo, Pb, Se, Zn, partially precipitate at the exit of the furnace on the fly ash particles and enrich the fly ash with high concentrations of these elements, relative to the bottom ash.
 
In order to determine the migration potential of ions from the ash to the environment and to aquifers, extraction tests were performed using the stringent American method (dissolution in acid) and the following results were obtained:
 
Concentrations of Trace Elements in the Extraction Solutions of Coal Fly Ash and Bottom Ash Compared with the Standards for Maximum Concentrations in Drinking Water (parts per billion – ppb)

 
Ag
As
B
Ba
Cd
Co
Cr
Cu
Fly Ash
1
175
7465
3810
1.3
24
455
15
Bottom Ash
0.7
14
775
735
0.4
8.9
8.8
7.8
Drinking Water Standard
10
50
-
1000
5
-
50
1400

 

Hg
Mn
Mo
Ni
Pb
Se
V
Zn
Fly Ash
0. 7
275
270
140
3.4
83
765
65
Bottom Ash
< 0.3
495
11
30
3.8
5.5
2.3
173
Drinking Water Standard
1
500
-
50
10
10
-
5000

Source: Geological Survey of Israel.

 

The concentrations of all the trace elements in the extraction solutions of bottom ash are lower than the values permitted in the drinking water standard. Extraction in water, which is more representative of reality, would have resulted in even lower values. Indeed, bottom ash is defined as an inert material.
 
The quantity of trace elements that appear as cations (Cd for example) that is likely to be released from the fly ash is minimal. Moreover, the cation absorption and fixation system that exists in the ground reduces the chance for pollution of water, vegetation and the environment from these elements.
 
On the other hand, results of laboratory tests indicated a non-negligible level of release of anions and oxi-anions from fly ash, of which the prominent elements are B, Cr, and Se, which is partially enhanced by mixing with soil. The difference in the behavior of the cations and anions can be attributed to the fact that the ash has more than one buffer system, that is, even after dissolution of all the calcium oxide by prolonged leaching, the pH value does not drop below ~8.

Monitoring of Pollutants in Infrastructure Applications

The monitoring of groundwater that has continued for a few years in the vicinity of a field test of construction of an infrastructure from bottom ash to the extent of tens of thousands of tons in a sandy area (water permeable), indicated stability with regard to the concentration of trace elements. In other words, no impact of bottom ash leachates on groundwater was detected.
 
The findings at a plot of land for coal fly ash leachate monitoring, which was installed by the Soil, Water and Environment Institute of the Vulcani Center with a road barrier made from coal fly ash, and prolonged monitoring of the chemical and mineral composition of the ash in this barrier, which was performed by the Geological Institute, indicate the imperviousness of the ash to water on one hand, and chemical and mineral stability of the ash for a prolonged period on the other hand, leading to the conclusion that under paving conditions (controlled wetting and compaction), there is no leaching of pollutants from the fly ash.

Monitoring of Pollutants in Agricultural Applications

In the framework of development of the use of coal bottom ash, instead of tuff, as a bed for the growth of plants and/or as flooring for animal pens, the pollutants concentrations were tested in the agricultural product that was obtained from growth on an ash bed: in vegetables, spices and fruit – at the Faculty of Agriculture of the Hebrew University; and in the milk of cows and tissue of cows and chickens – at the Residues Laboratory of the Veterinary Institute. The concentrations that were obtained were lower than the maximum permitted values based on the requirements of the Food Authority of the Ministry of Health, and they were similar to the concentrations that were obtained in the control groups that were grown on regular beds for comparison.

Environmental Guidelines for the Use of Coal Ash in Infrastructures

Based on studies that were performed in Israel and on world-wide knowledge, the environmental quality and health authorities established guidelines for application of the use of coal ash in infrastructures, subject to the requirements in the coal ash work specifications that were intended to create an engineering separation between the ash and the environment, as follows:

  • Maximum permitted levels of polluting elements in “Usable Ash”
  • In areas having no ground water sensitivity – unlimited use.
  • In areas having ground water sensitivity – subject to local testing.
  • In marine construction – with engineering separation by accepted means between the ash and water.
  • Bottom Ash – unlimited, except in areas in which the concentration of boron in the groundwater is within the sensitive range for growing plants.
  • Bottom ash in small quantities (agricultural growth beds, casings of underground facilities, etc.) – unlimited within a defined quantitative framework.

Furthermore, the water leachates from construction materials containing more than 50% coal ash must meet the drinking water standard.