Volatiles, combustible

A simplified model of PC combustion includes the following sequence of events (I) on entering the furnace, a PC particle is heated rapidly, driving off the volatile components and leaving a char particle (2) the volatile components burn independently of the coal particle and (3) on completion of volatiles combustion, the remaining char particle burns. Whue this simple sequence may be generally correct, PC combustion is an extremely complex process involving many interrelated physical and chemical processes. 

Elash point The lowest temperature at which vapors above a volatile combustible substance ignite in air when exposed to a source of ignition. 

Proximate analysis - a relatively low-cost analysis in which moisture content, volatile combustible matter, fixed carbon, and ash are determined. The fuel value of the sludge is calculated as the weighted average of the fuel values of its individual components. 

Rotary kiln systems usually have a secondary combustion chamber after the kiln to ensure complete combustion of the wastes. Airtight seals close off the high end of the kiln while the lower end is connected to the secondary combustion chamber or mixing cluimber. In some cases, liquid waste is injected into the secondary combustion chamber. The kiln acts as the primary chamber to volatilize and oxidize combustibles in the wastes. Inert ash is then removed from the lower end of the kiln. The volatilized combustibles exit the kiln and enter the secondary chamber where additional oxygen is available and ignitable liquid wastes or fuel can be introduced. Complete combustion of the waste and fuel occurs in the secondar> chamber. 

Foam—stable aggression of small bubbles of lower density than oil or water, which shows tenacious qualities in covering and clinging to vertical or horizontal surfaces. Foam flows freely over a burning liquid surface, forming a tough, air-excluding continuous blanket to seal volatile combustible vapors from access to air. 

Fuel, oxygen, and high temperature are essential for the combustion process. Thus, polyfluorocarbons, phosphazenes, and some composites are flame-resistant because they are not good fuels. Fillers such as alumina trihydrate (ATH) release water when heated and hence reduce the temperature of the combustion process. Compounds such as sodium carbonate, which releases carbon dioxide when heated, shield the reactants from oxygen. Char, formed in some combustion processes, also shields the reactants from a ready source of oxygen and retards the outward diffusion of volatile combustible products. Aromatic polymers, such as PS, tend to char and some phosphorus and boron compounds catalyze char formation aiding in controlling the combustion process. 

Char also shields the reactants from oxygen and in addition retards the outward diffusion of volatile combustible products. Aromatic polymers tend to char, and some phosphorus and boron compounds tend to catalyze char formation. 

Flash Point — This is defined as the lowest temperature at which vapors above a volatile combustible substance will ignite in air when exposed to a flame. Depending on the test method used, the values given are either Tag Closed Cup (C.C.) (ASTM D56) or Cleveland Open Cup (O.C.) (ASTM 093). The values, along with those given below, provide an indication of the relative flammability of the chemical. In general, the open cup value is about 10° to 15 °F higher than the closed cup value. 

Hartmann and associates (24G-28G) have conducted a great deal of experimental work on the combustion of dust dispersions. Explosions can be caused by particles as large as 700 microns. Many different dusts, including rosins, metal powders, and coal, have been investigated. Zirconium powder is the most explosive. Coal dust explosibility is closely associated with its volatile combustible content. Representative pressure rises as high as 75 pounds per square inch in an enclosed volume are reported. In all cases this maximum pressure is attained at mixture strengths well beyond stoichiometric. 

Condensed-Phase Mechanisms. The mode of action of phosphorus-based flame retardants in cellulnsic sy stems is probably best understood. Cellulose decomposes by a noncalalyzed route lo tarry depolymerization products, notably levoglucosan, which then decomposes to volatile combustible fragments such as alcohols, aldehydes, ketones, and hydrocarbons. However, when catalyzed by acids, the decomposition of cellulose proceeds primarily as an endothermic dehydration of the carbohydrate to water vapor and char. Phosphoric acid is particularly efficaceous in this catalytic role because of its low volatility (see Phosphoric Acids and Phosphales). Also, when strongly heated, phosphoric acid yields polyphosphoric acid which is even more effective in catalyzing the cellulose dehydration reaction. The flame-retardanl action is believed to proceed by way of initial phosphory lation of the cellulose. 

One of the most common methods used to determine percent composition and empirical formulas, particularly for compounds containing carbon and hydrogen, is combustion analysis. In this method, a compound of unknown composition is burned with oxygen to produce the volatile combustion products C02 and H20, which are separated and weighed by an automated instrument called a gas chromatograph. Methane (CH4), for instance, burns according to the balanced equation... 

In fires, the polymeric materials are consumed by the flaming combustion which is a gas-phase process. Thus, the polymer must degrade to yield volatile combustible species to fuel the conflagration. To begin, this chapter first considers the various processes by which pure polymer systems degrade. 

Petroleum coke calcining is a process whereby green or raw petroleum coke is thermally upgraded to remove associated moisture and volatile combustible matter (VCM) and to otherwise improve critical physical properties, e.g., electrical conductivity and real density (JL ) The calcining process is essentially a time-temperature function the most important variables to control are heating rate, VCM to air ratio and final temperature. To attain the calcined coke properties necessary for its end use by the amorphous carbon or graphite industries, the coke must be heat treated to temperatures of 1200-1350°C (2200-2500°F), or higher, to refine its crystalline structure. 

At about the same time, Lavoisier was preoccupied with the disappearance of diamonds, which had caused a sensation since Jean Darcet s (1725-1801) report to the Academy in May of 1768. Public demonstrations continued intermittently until April of 1772. Early in 1772, to settle the dispute among several contending interpretations for this phenomenon, the Academy set up a committee consisting of Macquer, Cadet, and Lavoisier. They examined three hypotheses—volatilization, combustion, and decrepitation—respectively favored by Darcet, Macquer, and Cadet. The answer hinged on the experimental control of air in the process. Volatilization or vaporization did not require the presence of air combustion and decrepitation would require the chemical or the mechanical role of air in the process, respectively. Darcet had... 

The rotational speed and angle at which it is positioned control the residence time of the solid in the kiln. Normally solid waste is converted into CO, particulate matter, or ash. For complete oxidation of flue gases and particulate matter, the kiln is also provided with a secondary combustion chamber. The volatilized combustibles exit the kiln and enter the secondary chamber where a complete oxidation tube is placed. 

The formation of NO2 shows a maximum value around 800°C. It is only about 0.4% of the total nitrogen inlet for wood volatile combustion. The formation of NO occurs from NO via... 

Therefore it is necessary to mandate good practice in using the Turbo Stove. In the volatile combustion mode CO is a minor hazard because if the flame should go out, the copious smoke warns the operator to re-ignite the Are or move the stove outside. However, in the charcoal combustion mode the CO is odorless and could pose a health hazard. 

Increased Char/Reduced Volatiles Theories. Most of the evidence relating to the mechanism of fire retardancy in the burning of wood indicates that retardants alter fuel production by increasing the amount of char and reducing the amount of volatile, combustible... 

The effectiveness of various compounds by TEA (Table VI) can be compared to the effectiveness determined by TG (Table VII) (56). Except for a few compounds, such as NaCl, NaHS04, and Na2C03, the ranking of the effectiveness of various fire retardants by the two different methods agree—high effectiveness numbers by TG correspond to high effectiveness numbers by TEA and vice versa. This is as expected if we assume the mechanism for fire retardancy is to increase the amount of char produced and lower the amount of volatile combustible products. 

Volatile combustion is a gas-phase homogeneous reaction, so there is no mass transport limitation between phases. There is, however, a mass transfer resistance due to gas-phase mixing of combustible gases and oxidizers. In general, volatile combustion is much faster than the pyrolysis process, which in turn is much faster than the char gasification process. 

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