Environment & Safety

Free Crystalline Silica (SiO2)

 

Coal contains free crystalline silica in the form of quartz (SiO2), and therefore, the ash that is produced from it contains small quantities of quartz. Tests that were performed at the Geologic Institute indicated that the concentration of quartz in coal ash is in the range of 3% – 5%, and it decreases with particle size.
 
Quartz is liable to cause cancerous lung diseases in workers that are consistently exposed to it for extended periods. Nevertheless, there is no evidence of these impacts with regard to workers exposed to coal ash. The reason for this is that the fiber-related impacts of quartz are attributed to reactions on the surface of the particles, but the low concentration of respirable quartz in fly ash (0.1% – 0.2%), as well as the fact that its particles are embedded in the glassy ash particles and their surfaces are hidden, explain the absence of findings of silicosis and other quartz impacts in epidemiological studies of workers exposed to coal fly ash.
 
The workplace safety regulations (industrial hygiene, public health and people who work with harmful dust) define crystalline silicon dioxide as a material that causes the generation of harmful dust, while also applying this definition to coal ash. Dust, including harmful dust, is defined in the regulations as a material that may penetrate the respiratory system, and the following maximum permitted industrial values were established (for exposure for 8 hours):
 

Harmful Substance
Crystalline Silicon Dioxide (SiO2)
Respirable dust that can enter the lungs (smaller than 7 microns)
0.1 mg/m3
Total Suspended Dust (smaller than 10 microns)
0.3 mg/m3

 

Quartz in bottom ash

The fraction of respirable particles in bottom ash is only 0.4% and some of them do not become suspended due to their being trapped inside the coarser particles, and therefore the concern regarding harmful impacts of coal bottom ash is much lower.
 
After screening bottom ash and removal of most of the particles smaller that 2 mm. with the objective of designating it for use in agricultural growth beds, the concentration of quartz remaining in the fraction that poses a respiratory risk (smaller than 10 microns) is less than 0.1%.
 
Based on a conservative theoretical calculation, assuming that the depth of the layer of coal bottom ash in an agricultural growth bed that dried due to exposure to air with no disruption of the surface of the layer from which there is a possibility of creating dust is 1 mm, the potential concentration of quartz whose source is coal bottom ash dust, having particles smaller than 10 microns, is two orders of magnitude lower than the maximum permitted value.
 
However, a calculation based on an analysis that was performed at the Geologic Institute of the findings of dust monitoring that was performed by the Life Sciences Research Company at a time when screened coal bottom ash was spread as an agricultural growth bed on field sections, indicated the following results:
 

Dust Fraction
Concentration (mg/m3)
Monitoring Result
Standard Requirement
Crystalline Silicon Dioxide, environmental
< 0.001
≤ 0.3
Crystalline Silicon Dioxide, Industrial
< 0.2 X 10-7
≤ 0.1

 
Reinforcement of these findings was obtained as a result of suspended dust tests that were performed by the Life Sciences Research Company and by the Environmental Services Company during various works with coal bottom ash in the construction of infrastructure, manufacture of blocks and dispersal as an agricultural growth bed in fields and greenhouses.
 
The conclusion to be drawn from these assessments is that coal ash, and especially bottom ash, can be regarded as an inert material whose dust that is created is a nuisance. However, due to the difficulty of differentiating between an impact caused by the quartz in the ash and lung functional problems as a result of the behavior of the ash as a nuisance dust, the industrial health specialists in Israel preferred to be stringent and included coal ash in the harmful dust regulations.
 
Based on the findings above, a recommendation was developed by a panel of experts that advises the National Coal Ash Board, to remove coal bottom ash from being subject to the regulations for harmful dust.
 

Quartz in fly ash

Dr. Yaacov Nathan from the Geological Survey of Israel (GSI), together with Dr. Ariel Metzger from the Israel Elecdtric Corporation, Dr. Asher Pardo from Tel Aviv University and Michael Dvorachek from GSI, led in 2008 a study aimed at monitoring fly ash dust generated at the “Orot Rabin” Power Station in Israel. The authors investigated both in the inhalable and respirable fractions, and examined the crystalline silica (quartz) concentration in the dust using different methodologies (Dijmap software for a semi-quantitative chemical analysis, X-Ray diffraction and Scanning Electron Microscopy observations).
 
The study produced two major findings:

  1. The content of quartz in coal ash is considerably lower than that calculated from its concentration in the inorganic fraction of the coal. This is due to the formations of new silicates during combustion, i.e., due to chemical reactions that occur at the temperature of 1500oC that exists in the boilers, mainly with aluminum and sometime also with calcium and/or iron oxides. Because of their relatively larger surfaces, the fine quartz particles have a higher reactivity and thus a larger fraction of these fine particles reacts with the alumino-silicate phases, often together with CaO or Ca(OH)2 particles, to form new phases in the boiler.
  2. Similarly to what was found in other studies (e.g., Meij, 2003), most (60-80%) of the fine-sized particles (less then 10μm) become coated during combustion by an alumino-silicate layer and assume a spherical shape, so that the free quartz in the respirable fraction does not exceed 1% of the total bulk. The net result is a loss of biological activity, which is practically exclusive to the angular quartz surfaces, especially in the respirable fraction which is able to penetrate the lungs and damage them.

As a consequence of the above-mentioned work, the Israeli Safety and Health Administration has decided to remove fly ash from the scope of the Harmful Dust Bill, in line with the regulations of most other countries (e.g., the Netherlands, UK, Germany and the USA), where coal fly ash is considered to be a nuisance dust and not a toxic (or “harmful”) dust, thus assigning the fly ash the same limit value as for nuisance dust.