Oxidation and Spontaneous Combustion

However, coal reacts with ambient oxygen, even at ambient temperatures and the reaction is exothermic (Chapter 12). If the heat liberated during the process is allowed to accumulate, the rate of the earlier reaction increases exponentially and there is a further rise in temperature. When this temperature reaches the ignition temperature of coal, the coal ignites (spontaneous ignition—the onset of an exothermic chemical reaction and a subsequent temperature rise within the combustible material, without the action of an additional ignition source) and starts to burn (spontaneous combustion). 

Generally, self-ignition occurs when the thermal equilibrium between the two counteracting effects of heat release due to the oxidation reaction and heat loss due to the heat transfer to the ambient is disturbed. When the rate of heat production exceeds the heat loss, a temperature rise within the material will consequently take place including a further acceleration of the reaction. 

The temperature at which the coal oxidation reaction becomes self-sustaining and at which spontaneous combustion occurs varies generally depending on the type (nature and rank) of coal and the dissipation (or lack thereof) of the heat. For low-quality coal and where the heat retention is high, the coal starts burning at temperatures as low as 30°C-40°C (86°F-104°F). 

The tendency of a coal to heat spontaneously in storage is primarily dependent upon the tendency of the coal to oxidize. This in turn is closely correlated with (1) coal rank (the higher the rank, the lower the tendency to oxidize), (2) the size consistency or distribution of the coal in the pile 

Oxidation is an exothermic reaction and, since the rate of a chemical reaction increases for each 10°C (18°F), the reaction will generate heat at a faster rate than can be dissipated or expelled from the stockpile by natural ventilation. Hence, the temperatnre will rise to appoint where spontaneous ignition occnrs and combustion ensues. 

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Li, S.R. 2008. Experimental study on performance that carbon dioxide inhibits coal oxidation and spontaneous combustion. Xi an Xi an University of Science and Technology. 

Guney, M. 1968. Oxidation and spontaneous combustion of coal. Colliery Guardian, 216 105, January-February, London, U.K. 

The mechanism of influence of NO2 on the oxidation and spontaneous combustion of hydrocarbons, primarily at low pressures, was discussed in detail in [13]. For the slow oxidation of methane, as in the case of other alkanes, addition of NO2 was demonstrated to shorten or even eliminate (starting from a certain amount) the induction period, causing no changes in the qualitative and quantitative composition of the oxidation products. For the oxidation of a 15% CH4—85% air mixture at T = 480—510 °C and P = 300 Torr in the presence of a small (1.37%) NO2 additive, the heat-release curve featured two peaks [175], the first of which, according to the authors, is associated with the formation of formaldehyde, whereas the second, with its decomposition. This explanation is difficult to accept, because in the absence of NO2, the formation and decomposition of formaldehyde also occur, but no double peak is observed. A double exothermic peak in the oxidation of methane in the presence of NO2 was observed in [176] and for the oxidation of propane in [177]. 

There are nine classes of dangerous goods, with divisions of some classes. The classes are explosive flammable and non-flammable non-toxic gases flammable liquids flammable solids and spontaneously combustible materials oxidizing substances and organic peroxides toxic and infectious substances radioactive and fissile materials corrosive substances and miscellaneous. 

The principal disadvantage of absorption bleaching is the problem of disposal of spent bleaching clay. Oil absorbed on the clay is exposed to air and is generally too oxidized to recover. Furthermore, spontaneous combustion of the oil-laden clay is a possibiUty in a landfill. Incineration of the spent clay along with sohd municipal waste to recover otherwise wasted energy is an attractive possibiUty. 

Like many other combustible Hquids, self-heating of ethyleneamines may occur by slow oxidation in absorbent or high-surface-area media, eg, dumped filter cake, thermal insulation, spill absorbents, and metal wine mesh (such as that used in vapor mist eliminators). In some cases, this may lead to spontaneous combustion either smoldering or a flame may be observed. These media should be washed with water to remove the ethyleneamines, or thoroughly wet prior to disposal in accordance with local and Eederal regulations. 

NOC1 is a powerful oxidizing agent, and causes expins when mixed with reducing substances. For instance, an expln occurs when it is mixed with an equal quantity of hydrogen. When powdered As or Sb is introduced into gaseous NOG, spontaneous combustion occurs. An expl reaction was reported when it was sealed in a tube with a residue of acet in the presence of Pt catalyst (Ref 2)... 

The rates of gas—liquid reactions are surface area dependent. Hence in the spontaneous combustion of oil impregnating fibrous thermal insulation on hot equipment, oxidation is facilitated by the large exposed surface area and, since the dissipation of heat is restricted, the temperature can rise until the oil ignites spontaneously. 

The rates of gas-solid reactions are surface area dependent, so finely- vided metals, coal etc. may be prone to oxidation leading to spontaneous combustion. A combustible dust will burn much more rapidly than the bulk sold, and if dispersed in air cause a dust explosion (refer to Table 6.2). 

Iron (II) oxide and especially that made by reducing the other oxides, combusts spontaneously if it is heated to 200°C. It also strongly catalyses the combustion of carbon in air. This behaviour can explain the spontaneous inflammable property of the products of burning iron oxalate, which contain this oxide and carbon. When they are placed on the hand and thrown into the air, they form very spectacular showers of sparks. It combusts in contact with liquid oxygen in the presence of carbon. 

There is a very spectacular reaction with the charring of lead tartrate. When this residue is freshly prepared, it combusts spontaneously in air by throwing up showers of sparks. When it is rubbed on a sheet of paper, or put in the hand and thrown, the reactions of this residue are more violent. It is thought that this is due to the catalysis by lead oxide of the combustion of carbon formed, which is very porous. 

Methyldichlorosilane (CH3SiHCl2) combusts spontaneously in the presence of potassium permanganate, lead oxide and dioxide, copper (II) oxide and silver oxide, even when they are in an atmosphere of inert gas. 

Dunn., B. W., Bur. Expl. Rept. 13, Pennsylvania, Amer. Rail Assoc., 1920 A veterinary preparation containing the oxidant and reducant materials (possibly with some additional combustibles) ignited spontaneously. 

Many fires are initiated as a result of auto-oxidation, referred to as spontaneous combustion. Some examples of auto-oxidation with a potential for spontaneous combustion include oils on a rag in a warm storage area, insulation on a steam pipe saturated with certain polymers, and filter aid saturated with certain polymers (cases have been recorded where 10-year-old filter aid residues were ignited when the land-filled material was bulldozed, allowing auto-oxidation and eventual autoignition). 

Ignition - The process of starting a combustion process through the input of energy. Ignition occurs when the temperature of a substance is raised to the point at which its molecules will react spontaneously with an oxidizer and combustion occurs. 

Many scenarios involving spontaneous combustion involve a combination of materials exposed to sufficient air, often in an insulating situation that prevents heat from a slow oxidation reaction to dissipate and thus results in a self-heating situation. Examples include activated carbon exposed to a high concentration of organic vapors and cotton or cellulose materials contaminated with oil. These combination scenarios can be... 

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