Reduction of spontaneous combustion of coal

A composition for the prevention of spontaneous combustion of coal is described which is comprised of at least about 2 percent of polyethylene oxide and the balance water. Also described is a method for reducing the spontaneous combustion tendency of coal by contacting coal with the above composition and then drying the coal.

The expected decrease in the availability of premium fuels has focused 
attention on the resource potential of low-rank coals (sub-bituminous 
coals and lignites), which have a heating value of less than 13,000 BTU 
per pound. There are an estimated 485 billion tons of sub-bituminous coal 
and 478 billion tons of lignite in the United States, mostly in the 
continental states west of the Mississippi River. Increased production and 
utilization of low-rank western coals present many problems, both 
technical and economic. One serious problem associated with mining, 
transportation, and storage of low-rank coals is spontaneous combustion. 
Fires due to spontaneous combustion of coal may occur in the high wall of 
surface mines, on transporting the coal, or in storage piles of the coal, 
and present a potentially fatal hazard in underground mines. 
Spontaneous combustion, which is a common problem with some European and 
Soviet coals, has been the subject of research for over 100 years. This 
research has identified some of the important factors in spontaneous 
combustion as changes in the moisture content of the coal, air flow rate, 
particle size, temperature, pyrite content, geological factors, and mining 
practice. At present, however, there is no simple, universally applicable 
test for combustibility, no generally accepted index of combustibility, 
and no simple effective method of preventing spontaneous combustion. 
Prevention is normally based on suppressing the factors that favor 
spontaneous combustion, such as accumulation of fine coal particles, 
inefficient heat dissipation, and differences in the moisture content of 
the air and the coal. Control of spontaneous combustion is usually based 
on previous experience and includes mining practice to reduce risk and to 
detect incipient combustion before a fire occurs. 
The method of the present invention provides a simple and relatively 
inexpensive method for reducing the spontaneous combustion tendency of 
coal. The prevention methods currently in use include compaction of coal 
piles to reduce the oxygen in the space surrounding the coal particles, 
controlling the aging of the coal by reacting warm coal with air and then 
cooling it, and storing the coal in sealed containers. 
U.S. Pat. No. 2,184,621, patented Dec. 26, 1939, discloses a method for 
treating coal which decreases its tendency to degradate due to slacking or 
spontaneous ignition. The coal is treated with a crystallizable solution 
consisting of a paraffin wax and a liquid hydrocarbon. The coating is said 
to have sufficient fluidity to penetrate and seal the fissures and pores 
of the coal and prevent oxidation by the elements. However, the 
wax-hydrocarbon mixture must be applied hot and is not miscible with 
surface moisture. These factors may result in incomplete coverage which 
could allow oxidation and even combustion to occur. In the present 
invention, the polyethylene oxide solutions are completely miscible with 
surface moisture and should be applied at ambient temperature (greater 
than 0.degree. C.). This favors a more uniform and continuous coating and 
thus better resistance to oxidation. 
Another known means of preventing spontaneous combustion of coal is the 
spraying of latex rubber onto the coal. Neither this method nor the method 
described in the preceding paragraph is the same as or as effective as the 
use of polyethylene oxide solutions as contemplated in the present 
invention. 
SUMMARY OF THE INVENTION 
Briefly described, the invention is a composition for the prevention of 
spontaneous combustion of coal which is comprised of at least about 2 
percent polyethylene oxide and the balance water. Also described is a 
method for reducing the spontaneous combustion tendency of coal which 
comprises contacting coal with the afore-described composition, and then 
drying the coal. It is preferred that the composition be sprayed onto the 
coal. 
DETAILED DESCRIPTION OF THE INVENTION 
Polyethylene oxide is a crystalline, thermoplastic, water soluble polymer 
with the general formula HOCH.sub.2 (CH.sub.2 OCH.sub.2).sub.n CH.sub.2 OH 
or H(OCH.sub.2 CH.sub.2).sub.n OH. The end groups are said to be hydroxyl 
groups only in the case of the lower molecular weight species. Unlike most 
polymer systems, polyethylene oxide is commercially available in an 
extraordinarily wide range of molecule weights from ethylene glycol, 
diethylene glycol, and so on, up to polymers that have molecular weights 
many times greater than a million. The lower molecular weight members of 
the series with n up to about 130 (molecular weight from about 200 to 
about 6000) are generally known as polyethylene glycols while the higher 
members (molecular weight greater than 6500 up to 100,000 to several 
million) are known as polyethylene oxide, polyoxyethylene, or polyoxirane. 
The preferred polyethylene oxide polymers for use in the present invention 
have a molecular weight of at least about 200,000 and, theoretically, 
there is no maximum. 
The higher (polyethylene oxide) and lower (polyethylene glycol) molecular 
weight members of this series differ sufficiently in properties as to form 
two classes. The lower members range from relatively viscous fluids to 
wax-like solids while the higher members are true thermoplastics capable 
of being formed into tough, molded shapes. The property differences of 
these two classes are due principally to large differences in molecular 
weight and the relatively greater importance, therefore, of the end groups 
in the low molecular weight class. 
The polyethylene oxide polymers used in the present invention are made by 
conventional processes such as suspension polymerization or condensation 
of ethylene oxide. The composition of the present invention is prepared by 
dissolving the proper amount of polyethylene oxide in a measured amount of 
water. This may be accomplished by any conventional method, but I have 
found that simply mixing the polyethylene oxide in warm water (30.degree. 
C. to 70.degree. C.) is sufficient to provide the desired composition. 
As previously stated, the composition of the present invention is comprised 
of from at least about 2 percent polyethylene oxide and the balance water. 
If less than about 2 percent polyethylene oxide is used, then the 
spontaneous combustion potential is too high. Theoretically, there is no 
maximum but usually more than 20 percent is not necessary. 
The preferred method for treating coal to reduce its spontaneous combustion 
potential according to the present invention comprises spraying the 
above-described solution on the coal so that it completely covers the 
coal. Another preferred method comprises completely immersing coal in a 
solution of polyethylene oxide and water in the indicated concentration 
range. It is important that the coal is completely coated with the 
composition. Next, if the immersion method has been used, the polyethylene 
oxide solution is decanted from the coal. Any means of removing the 
solution from the coal may be used except water washing. Finally, the coal 
is exposed to ambient conditions to allow the liquid to evaporate for at 
least about 2 hours. 
It is theorized that the composition and method of the present invention 
provide the desired reduction in the spontaneous combustion tendency to 
sealing the coal from oxygen and sealing the volatile hydrocarbons in the 
coal, thereby preventing oxidation thereof.

The following example is intended to illustrate the invention and not to 
limit it in any way. 
EXAMPLE 
The polyethylene oxide used in this example was Union Carbide POLYOX.RTM. 
WSR-1105 and has an approximate molecular weight of 900,000 and a 5 
percent solution viscosity of 800 to 17,600 centipoises at 25.degree. C. 
Solutions of one percent, two percent, three percent, and four percent 
polyethylene oxide in water were prepared by mixing the appropriate amount 
of polyethylene oxide in the appropriate amount of warm (50.degree. C.) 
water. Ten grams of lignite coal was added to each solution and the 
mixtures were stirred vigorously for 15 minutes. After the mixing, the 
polyethylene oxide solution was decanted and the coal was allowed to dry 
in the open air at ambient temperature for 24 hours. 
Separate samples of each of the separately treated coals were exposed to 
combustion conditions at the varying temperatures and the time it took for 
combustion to occur was recorded. 
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Seconds to Combustion 
% PEO 330.degree. C. 
340.degree. C. 
350.degree. C. 
360.degree. C. 
370.degree. C. 
380.degree. C. 
390.degree. C. 
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0(Control) 
53 33 -- 25 18.5 16 12 
1 47 37 37 30 25 -- -- 
2 no comb. 
55 41 30 25 -- -- 
3 71 45 33 25 18.5 -- -- 
4 48 33 20 12 7 -- -- 
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I have developed a method for predicting the spontaneous combustion 
tendency of coal. A thermo-gravimetric analysis was done on each sample by 
placing it in a Perkin-Elmer TGS-II and causing pure oxygen to flow 
through the furnace chamber at 30 cubic centimeters per minute. The 
samples were then heated to the test temperature at 320.degree. C. per 
minute and the time until combustion occurred was recorded. This procedure 
was repeated for each coal sample at a variety of temperatures. The 
resulting data for each sample was fitted into the Arrhenius Relationship 
and plotted against a logarithmic time base. The resulting line was then 
extrapolated to near ambient temperature (25.degree. C.) and the expected 
life span noted. The relative spontaneous combustion potential is defined 
as one divided by the logarithm (base 10) of the expected life span. 
The relative spontaneous combustion potential has a direct relationship to 
spontaneous combustion. In other words, the lower the number, the less 
likely combustion will occur under ambient conditions. The control sample 
had a relative combustion potential of 0.125. The sample treated with one 
percent PEO had a relative combustion potential of 0.164. The sample 
treated with 2 percent PEO had a relative combustion potential of 0.108. 
Finally, the sample treated with 4 percent PEO had a relative combustion 
potential of 0.074. Thus, it can be seen that treating the coal with a 
solution of polyethylene oxide of a concentration greater than 2 percent 
reduces the relative spontaneous combustion potential and therefore 
decreases the spontaneous combustion tendency of the coal.