Stabilization Of Coal

Abstract

A process for stabilizing dried lignitic and subbituminous coal against spontaneous combustion which comprises treating said dried coal at about 175.degree. to about 225.degree. C. with oxygen in an amount of from about 0.5 percent to about 8 percent by weight of said coal and rehydrating the oxygen treated coal with water in an amount of from about 1.5 percent to about 6 percent by weight of said oxygen treated coal.

Description

Dried lignitic and subbituminous coals are unstable to storage in that they are subject to air oxidation and combust spontaneously. Thus, in order to enhance storage stability special treatments must often be used. For example, coal piles are often arranged in a particular manner to obtain safe storage; e.g., thin layers which are compacted with sloping sides at a maximum angle of 14.degree., smooth final surfaces, and top surface continually smoothed as coal is removed from the top only. Other approaches to prevent spontaneous combustion during storage involves chemical treatment of the coal; e.g., coating the coal with petroleum products and their emulsions, spraying with calcium bicarbonate or aqueous hydroquinone or amines. Such treatments, however, are either not completely effective or are excessively expensive for a low priced commodity such as coal.

Numerous investigations have been made on the oxidation of coal with an objective toward improving its stability. One such study concluded that use of oxygen to form oxidized zones on the surface and in the interior of the mass of brown coals imparted protection against further oxidation (Szadeczky-Kardoss, E., Ungar., Z. Berg u. Huettenwessen, 77, 241-7, 253-60, 1944 ). My experimental work confirms this effect with oxygen treatment, but I have found this technique to be an incomplete solution to the storage stability problem. In my experiments I find that this oxygen treatment delays somewhat the time until spontaneous combustion might occur, but the treatment does not greatly extend the safe storage period.

I have now found a means to significantly improve the storage stability of dried lignitic and subbituminous coal and this is accomplished, in accord with my invention, by the process which comprises treating said dried coal at a temperature between about 175.degree. C. and about 225.degree. C. with oxygen in an amount of from about 0.5 percent to about 8 percent by weight of said coal and rehydrating the oxygen treated coal with water in an amount of from about 1.5 percent to about 6 percent by weight of said coal.

The coals used in the process of the invention will be, as indicated, lignitic and subbituminous coals and will include North Dakota lignite, Powder River subbituminous coal, Wyodak coal, and the like. Such coals usually contain from about 20 to 40 percent water as they come from the mine, but for use in subject process they will first be dried to a water level of about 1 percent to about 10 percent by weight and this is readily done simply by flowing hot flue gas through the coal. The process of the invention is applicable to coal of any size, but will, of course, be of most value with the smaller sizes since they have the greater surface area and are most subject to oxidation.

In carrying out the process of the invention it is important to control the amount of oxidation since I have found that excessive oxygen actually decreases the stability of the dried coal. The process of the invention requires that the oxygen treatment be controlled so that its uptake is between about 0.5 percent and about 8 percent by weight of the coal treated. As a guide in carrying out this controlled oxidation it should be understood that an oxygen uptake of 1 percent corresponds to about 1 pound of oxygen per 100 pounds of coal and thus, the process of the invention will use from about 0.5 pounds to about 8 pounds of oxygen per 100 pounds of coal treated. It will be also understood that air is a suitable source of oxygen and that pure oxygen, while operable, is somewhat expensive for use in the process.

The oxygen treatment may be carried out by any conventional technique which will distribute the oxygen over the surface of the coal. In a preferred method, the dried coal is treated with air in a rotary drum burner, the temperature of treatment being from about 175.degree. C. to about 225.degree. C. Not all of the oxygen in the air will be reactive with the coal since about one-half of the available oxygen will be carried out of the burner. This is readily compensated for by adjusting air flow, drum rotation speed, and coal volume in accord with known techniques available to the skilled art worker. The oxygen level of the gas used for the oxidation treatment should be at least about 10 percent and as the oxidation proceeds about 45 percent of the oxygen is converted to water, about 45 percent to CO.sub.2, and about 10 percent to CO.

The rehydration of the coal with water is readily accomplished by a spraying technique, although when smaller amounts are treated, the coal particles may be simply agitated with the required amount of water to effect distribution on the surface. As indicated, the amount of water used will be between about 1.5 percent and about 6 percent by weight of the oxygen treated coal. In a preferred technique the hot coal emerging from the oxygen treating step is cooled to a temperature between about 150.degree. and about 50.degree. C. by heat exchange with the hot gas oxygen supply, and then the cooled coal is sprayed with water. Empirical techniques are easily employed in the selection of type of spray nozzle, water pressure, time of spray, etc. in order to place from about 1.5 percent to about 6 percent water on the coal.

After the water treatment the stabilized coal is handled, transported and stored by conventional methods and prolonged storage stability of the treated coal is evident.

In order to more fully illustrate the invention the following examples are given:

Test Procedure

Dried Wyodak coal of 14 mesh size is placed in a stirred container equipped with an oxygen inlet, an outlet which is connected in series to a drying tube, a CuO catalyzed converter at 800.degree. C. to convert CO to CO.sub.2, an Ascarite tube for CO.sub.2 absorption, and then connected through a pump to the input for oxygen recycle. The stirred coal is heated to 200.degree. C. as oxygen is passed through the system and recycled. The amount of oxygen used is measured with appropriate gas flow meters.

The oxidized coal is cooled and wetted with the desired amount of water, thorough distribution being obtained by agitation. Then the rehydrated coal is placed in a Dewar flask fitted at the bottom with a sparging tube to pass oxygen through the coal and equipped with a temperature measuring device. The coal (450g.) in Dewar flask (70 mm. I.D.) is about 200 mm. in depth and oxygen is passed through it at 63.degree. C. at a rate of 190 ml/min. The time for combustion of the coal to occur is noted as shown by the sudden temperature increase and is taken as a measure of stability.

The following tables illustrate the effects of the process:

TABLE I

Ex. no. Coal treatment Hours to combustion __________________________________________________________________________ 1. No oxygen, No water 1.6 2. 1% O.sub.2, No water 6.0 3. 7% O.sub.2, No water 4.8 4. 8% O.sub.2, No water 4.5 5. No oxygen, 6% water 4.3 6. 4% O.sub.2 + 1.5% water 9.0 7. 4% O.sub.2 + 3% water Never combusts

It is evident from the above data that oxygen treatment alone and water treatment alone are both ineffective to prevent combustion. It is also evident that increased oxygen treatment alone is actually harmful in that time to combustion is decreased. The oxygen treatment plus rehydration, however, gives significant protection against combustion. In Example 7, the temperature of the coal mass actually dropped at the twelfth hour of the combustion test, thus indicating that combustion would never occur under the test conditions.

The following examples shown in Table II are further illustrative of the invention:

TABLE II

Ex. no. Coal Treatment Hours to Combustion __________________________________________________________________________ 8. No oxygen, 6% water 4.3 9. 4% O.sub.2 +6% water Never combusts 10. 7% O.sub.2 + 6% water Never combusts 11. 8% O.sub.2 + 6% water Never combusts

It is clear from the above data that the process of the invention provides a means to prevent spontaneous combustion of coal during storage and makes a valuable contribution to the art.

Claims

The invention claimed is:

1. A process for stabilizing dried lignitic and subbituminous coal against spontaneous combustion which comprises treating said dried coal at about 175.degree. to about 225.degree. C. with oxygen in an amount of from about 0.5 percent to about 8 percent by weight of said coal and rehydrating the oxygen treated coal with water in an amount of from about 1.5 percent to about 6 percent by weight of said oxygen treated coal.

2. The process of claim 1 where the amount of water is from about 3 percent to about 6 percent.