Ignition spontaneous

Factors that can contribute to the spontaneous ignition of solids are many  

These processes listed here are just some of the factors that can cause and promote spontaneous ignition. 

Common examples of spontaneous ignition have included moist haystacks, oily cotton rags and mounds of low-grade coal from mining operations. Many have experienced the 

Fundamentals of Fire Phenomena James G. Quintiere 2006 John Wiley Sons, Ltd ISBN 0-470-09113-4 

A less expensive accident attributed to wood fiberboard in 1950 brought the subject of spontaneous ignition to the forefront of study. This is vividly described in the opening of an article by Mitchelle [2] from the NFPA Quarterly, New light on self-ignition  

Substances that catch fire spontaneously in air without an ignition source are called pyrophoric. These include several elements— white phosphorus, the alkali metals (group lA), and powdered forms of magnesium, calcium, cobalt, manganese, iron, zirconium, and aluminum. Also included are some organometallic compounds, such as ethyllithium (LiC2H5) and phenyllithium (LiQHj), and some metal carbonyl compounds such as iron pentacarbonyl, Fe(CO)5. Another major class of pyrophoric compounds consists of metal and metalloid hydrides, including lithium hydride, LiH  

The heat generated from this reaction can be sufficient to ignite the hydride so that it bums in air  

Some compounds with organometallic character are also pyrophoric. An example of such a compound is diethylethoxyaluminum  

Many mixtures of oxidizers and oxidizable chemicals catch fire spontaneously and are called hypergolic mixtures. Nitric acid and phenol form such a mixture. 

4 MATERIAL PARAMETERS CONTROLLING THE IGNITION OF SOLID FUELS 3.4.1 Spontaneous Ignition 

The appropriate boundary conditions need to be included before a solution can be achieved. The solution to this problem remains complex, therefore, simplifications are necessary. 

The first approach was developed by Semenov, who treated the system as a lumped control volume V which results in the following equation  

FIGURE 3.2 Schematic of the conditions leading to spontaneous ignition. The horizontal axis is the system temperature, and the vertical axis is the rate of heat generation/loss. 

A more elaborated approach is that of Frank-Kamenetski who relaxed the assumption of homogeneous solid temperature allowing conduction within the solid. A similar eigenvalue analysis will lead again to a critical ignition temperature (Tc) and the ambient temperature required for ignition to occur (T0). Nevertheless, the ambient temperature, given that conduction of heat from the core to ambient is allowed, becomes a function of the volume of the solid, and hence, for each 7 0 a critical volume, Vc is obtained. 

Strongly linked to reactivity is the tendency of coal to ignite spontaneously. The adsorption of gas on the coal surface is linked to the release of adsorption enthalpy resulting in a local temperature increase. The self-insulation of the coal bulk can in turn cause heat accumulation. If the adsorbed gas is oxygen, which is usually present in ambient air, oxidation reactions can start at ambient conditions (30-40 °C). Whether these silent oxidation processes result in ignition (=350°, depends on the following variables [76]  

If such coals are intended for a gasification project, special care must be talcen in storage and also during sample handling in the laboratory. In addition to ignition, the release of reaction products (e.g., CO2, CO, or sulfur compounds) should be kept in mind. A first hint is usually the detection of weathered macer-als indicating certain reactivity toward oxygen. 

If the coal is stored by complete immersion in water, then this would not be the case. It appears, however, that in spite of the exothermic valne of the oxidation of pyrite, the pyrite may only play a minor part in the oxidation of coal bnt, on occasion, it has been suggested that pyrite oxidation will contribute to the spontaneous ignition of coal. 

Spontaneons combnstion, or self-heating, of coal is a naturally occurring process caused by the oxidation of coal. The self-heating of coal is dependent on a nnmber of controllable and uncontrollable factors. Controllable factors include close managanent in the power plant, of coal storage in stockpiles, silos/bunkers, and mills and managanent during coal transport. Uncontrollable factors include the coal itself and ambient conditions. 

The self-heating of coal is due to a number of complex exothermic reactions. Coal will continue to self-heat provided that there is a continuous air supply and the heat produced is not dissipated. The property of coal to self-heat is determined by many factors, which can be divided into two main types properties of the coal (intrinsic factors) and environment/storage conditions (extrinsic factors). Self-heating results in degradation of the coal by changing its physical and chemical characteristics, factors that can seriously affect boiler performance. 

The risk of spontaneous combustion during final preparation such as in silos/bunkers and mills also presents concerns in some cases. Properties that influence the propensity of coal to self-heat include volatile content, coal particle size, rank, heat capacity, heat of reaction, the oxygen content of coal, and pyrite content. The propensity of coal to self-heat and spontaneously combust tends to increase with decreasing rank. Thus, lignites and subbituminous coals are more prone to spontaneous combustion than bituminous coals and anthracites. 

The temperature of coal increases due to self-heating until a plateau is reached, at which the temperature is temporarily stabilized. At this point, heat generated by oxidation is used to vaporize the moisture in the coal. Once all the moisture has been vaporized, the temperature increases rapidly. On the other hand, dry material can readily ignite following the sorption of water—dry coal in storage should not be kept in a damp place because this can promote self-heating. Therefore, it is recommended that dry and wet coal be stored separately. 

Activated carbon is flammable, and the dust is toxic by inhalation. When some carbon-based materials, such as activated carbon or charcoal briquettes, are in contact with water, an oxidation reaction occurs between the carbon material, the water, and pockets of trapped air. The reaction is exothermic, which means heat is produced 

Charcoal briquettes are a dangerous fire risk. They may undergo spontaneous ignition when they become wet. However, this is a slow process, and the heat generated must be confined as it builds up and ignites. 

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Clock-type induction periods occur in the spontaneous ignition of hydrocarbon-oxygen mixtures [2], in the setting of concrete and the curing of polymers [3]. A related phenomenon is the induction period exhibited... 

Griffiths J F 1986 The fundamentals of spontaneous ignition of gaseous hydrooarbons and related organio oompounds Adv. Chem. Phys. 64 203-303... 

Fuel Quantity of fuel per Gf" Flammability limit ia air, vol % gas Lower Higher Maximum flame speed, cm/s Spontaneous ignition temperature, °C Ignition d -jC energy, m ... 

Phosphoms(III) oxide is slowly oxidized in the air, but when heated above 70°C, it can spontaneously ignite as a result of disproportionation to elemental phosphoms. Above 2I0°C, the oxide decomposes into phosphoms and phosphoms tetroxide ... 

The low autoignition temperature of benzaldehyde (192°C) presents safety problems since benzaldehyde can be ignited by exposure to low pressure steam piping, for example. Benzaldehyde may also spontaneously ignite when soaked into rags or clothing or adsorbed onto activated carbon (13). 

Fig. 2. Slow oxidation, spontaneous ignition, and explosion as a function of pressure and temperature variations in hydrocarbon mixtures (1).

Cool Flames. Under particular conditions of pressure and temperature, incomplete combustion can result in the formation of intermediate products such as CO. As a result of this incomplete combustion, flames can be less exothermic than normal and are referred to as cool flames. An increase in the pressure or temperature of the mixture outside the cool flame can produce normal spontaneous ignition (1). 

Iron ennecarbonyl (di-iron nonacarbonyl) [15321-51-4] M 363.7, m 100 (dec). Wash with EtOH and Et20 and dry in air. Sublimes at 35° at high vacuum. Dark yellow plates stable for several days when kept in small amounts. Large amounts, especially when placed in a desiccator spontaneously ignite in a period of one day. It decomposes in moist air. It is insoluble in hydrocarbon solvents but forms complexes with several organic compounds. [J Am Chem Soc 72 1107 7950 Chem Ber 60 1424 1927. ]... 

The nickel is pyrophoric and must be kept moist to prevent spontaneous ignition. 

Chemical Reactivity - Reactivity with Water. Decomposes slowly but the reaction is not hazardous Reactivity with Common Materials Corrodes metals slowly. If mixed with combustible materials or finely divided metals, mixture can spontaneously ignite or become unstable by friction Stability During Transport Sable Neutralizing Agents for Acids and Caustics Not pertinent Polymerization Not pertinent Inhibitor of Polymerization Not pertinent. 

Not to be Used Water, foam, carbon dioxide, or halogenated hydrocarbons Special Hazards of Combustion Products No data Behavior in Fire Reacts violently with water, forming flammable and explosive hydrogen gas. This product may spontaneously ignite in air Ignition Temperature No data Electrical Hazard Not pertinent Burning Rate Not pertinent. 

Adiabatic induction time Induction period or time to an event (spontaneous ignition, explosion, etc.) under adiabatic conditions, starting at operating conditions. 

Spontaneous ignitions also possible (e.g. MEOH plus K)... 

Warning Depending on the ratio of hydrogen to halogen in a given molecule, addition of an interhalogen to that substance in the absence of solvent can result in spontaneous ignition or worse. 

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