Fluorine in coal

Coal and fly ash are currently monitored to control air and ground water pollution, to evaluate mass balance in coal production and as indirect control of emission (based on the differences in contents between coal and fly ash). Some areas sufTer from a severe pollution by emission of fluorine to the atmosphere and its consequent transport into the ground water. Chlorine is responsible for burner corrosion and has to be determined to assess the risk monitoring of these elements under good quality control implies that CRMs of coal and fly ash should be available. A new coal reference material (CRM 460) has hence been prepared with the objective to certify both F and Cl [13,14] as described below, only fluorine could be certified, owing to an in-homogeneity detected for Cl. 

---

Table X. Fluorine in Coal from the Lower Kittanning Coalbed, Centre County, Penn.

Table XI. Fluorine in Coal from an Uncorrelated Coalbed, Mahoska County, Iowa...

The Determination of Fluorine. A coal sample is decomposed by ignition in a combustion bomb containing Na3C03 solution under 24 atm oxygen. After combustion, the bomb washings are diluted to a known volume and an aliquot is taken to determine fluorine by the standard Orion specific ion procedure. 

Fluorine. Tables X, XI, and XII show the results obtained for fluorine in the coal-washing study with coals from Pennsylvania, Iowa, and West Virginia. In all three coals, the fluorine appears to be associated with the mineral matter in the coal. Therefore, removal of the higher specific gravity fractions would lower the fluoride content of the remaining coal. 

Commonly associated with limestones, fluorine-rich granites and associated ore deposits. May he present in coal combustion b3fproducts, fly ash. 

These probes have told us a great deal about such molecules in coal. The F NMR and ESR experiments bracket the time scales for motion for the molecules we have examined as lying between sec and 10 sec. Further, the Tj measurements of fluorinated molecules in coal reveal several nonequivalent relaxation times. This nonequivalence in motion appears related to nonequivalent positions for these molecules in the coal structure. The fact that CgDg molecules can spin in their plane but not tumble suggests that they are sterically confined, perhaps between planar elements of the coal structure. The fact that C Dg mole-... 

Thus, bromination (which can be achieved by shaking powdered coal with bromine water in carbon tetrachloride or in chloroform) is also believed to involve addition and substitution reactions the products can contain up to 20% w/w bromine. A similar concept has been invoked for the interaction of iodine with coal but the reaction of fluorine with coal will ultimately yield a mixture of fluorocarbon oils. 

A cell for the Hall—H roult process is shown in Fig. 4.1, and a block diagram of the whole process is shown in Fig. 4.2. Cell design is determined largely by the need to contain molten cryolite at high temperatures and to withstand attack by molten aluminium and also by sodium and fluorine formed as minor products at the cathode and anode respectively. Hence the cell is a strong steel box lined first with alumina to act as a refractory, thermal insulator and then with carbon. In fact the base of the tank is lined with prebaked carbon blocks which are inlaid with steel bars to reduce their electrical resistance and which act as current carriers to the molten aluminium cathode. The sides are lined with partially graphitized anthracite in coal tar pitch. The process is then run so that there remains a layer of solid cryolite and alumina at the sides of the cell and a soHd crust on the surface. This acts as a further barrier to corrosion and also to reduce the heat loss from the cell. The cell also has facilities for the periodic addition of alumina through the crust and for the removal of aluminium metal by suction. It is hooded with an extractor... 

Treatment of coal with chlorine or bromine results in addition and substitution reactions. At temperatures up to 600°C chlorinolysis produces carbon tetrachloride, phosgene, and thionyl chloride (73). Treatment with fluorine or chlorine trifluoride at atmospheric pressure and 300°C can produce large yields of Hquid products. 

When carbon is in the impure form of soot or coal, it combusts in contact with fluorine and then can detonate spontaneously. If the carbon is very pure, the ignition is delayed. 

Gonska H, Griepink B, Colombo A, Muntau H (1984) The Certification of the Contents of Arsenic, Cadmium, Chromium, Cobalt, Fluorine, Manganese, Mercury, Nickel, Lead and Zinc in a Coal BCR No. 40. European Commission Report EUR 9473 EN, Community Bureau of Reference, Brussels,... 

Under atmospheric conditions, a low rank coal (0.5-1 mm particle size) from Spain was burned in a fluidized bed reactor at seven different temperatures (50 °C increments), beginning at 650 °C. The combustion experiment was also conducted at different amounts of excess oxygen (5 to 40%) and different flow rates (700 to 1,100 L/h). At 20% excess oxygen and a flow rate of 860 L/h, the amount of fluorine emitted ranged from 850.7 ng/kg at 950 °C to 3,632.8 ng/kg at 750 °C. The greatest amount of PAHs emitted were observed at 750 °C (Mastral et al., 1999). 

Specific Gravity Fraction % Coal in Fraction Fluorine Concentration (w/g)... 

Clearly the oil is a much cleaner fuel than the original coal, from the point of view either of the environmentalist or of the plant engineer concerned with fouling of steam and superheater tubes. The sulfur contents of the oils are 0.1-0.2%, which is acceptable, but the nitrogen contents are about 0.6%, which may cause undesired NOx emissions. Some of the more toxic elements, (mercury, selenium, fluorine, and cadmium) have not yet been determined in oil. It is not clear what will be done with the solid residue whether it will be disposed of as waste or whether its small carbon content, typically 20-50% depending on the... 

Goldhaber, M.B., Hatch, J.R., Pashin, J.C. et al. (1997) Anomalous arsenic and fluorine concentrations in Carboniferous coal, Black Warrior Basin, Alabama evidence for fluid expulsion during Alleghanian thrusting Abstracts with Programs. The Geological Society of America, 29(6), 51. 

---