Combustion physical/chemical aspects

Seader j. D. and Einhorn I. N., Some physical, chemical, toxicological and physiological aspects of fire smokes, 16th Symposium (International) on Combustion, Institute, Pittsburgh, Pennsylvania, 1423-1445 (1976). 

There are two main varieties of carbon (i) crystalline (e.g., graphite and diamond), and (ii) amorphous. The amorphous variety consists of carbon blacks and charcoals. Carbon blacks are nonporous fine particles of carbon produced by the combustion of gaseous or liquid carbonaceous material (e.g., natural gas, acetylene, oils, resins, tar, etc.) in a limited supply of air. Charcoals are produced by the carbonization of solid carbonaceous material such as coal, wood, nut shells, sugar, synthetic resins, etc. at about 600 °C in the absence of air. The products thus formed have a low porosity, but when activated by air, chlorine, or steam, a highly porous material is produced this porous product is called activated charcoal. Chemically speaking carbon blacks and charcoals are similar, the difference being only in physical aspects. Carbon blacks find use in the rubber industry and in ink manufacture. An important use of charcoals is as adsorbents. 

This review paper is restricted to stirred vessels operated in the turbulent-flow regime and exploited for various physical operations and chemical processes. The developments in the field of computational simulations of stirred vessels, however, are not separated from similar developments in the fields of, e.g., turbulent combustion, flames, jets and sprays, tubular reactors, and multiphase reactors and separators. Fortunately, there is a strong degree of synergy and mutual cross-fertilization between these various fields. This review paper focuses on aspects specific to stirred vessels (such as the revolving impeller, the resulting strong spatial variations in turbulence properties, and the macroinstabilities) and on the processes carried out in them. 

It could be expected, that combustion reactions and possibly flames can be produced in such dense supercritical mixtures. Technical aspects of hydrothermal oxydation at moderate pressures have already been tested and discussed [7,8]. The study of combustion and flames in supercritical phases offers several possibilities 1. The variation of pressure over wide ranges should influence reaction mechanisms and flame characteristics because the density can be changed from low, gas-like, to high, liquid-like, values. 2. The variable temperature of the dense, fluid environment can have an influence on reactions and flames. 3. The chemical and physical character of this environment can be varied considerably, for example by using supercritical water as the major component, as in the present experiments. Certainly, the knowledge of transport coefficients of gases involved is desirable. For water the viscosity has been determined to... 

We discuss in this section four key aspects of heterogeneous reactions (1) theoretical and experimental structure and reactivity relationships (2) held measurements of relative and absolute PAH decay rates in near-source ambient air and during downwind transport (3) laboratory studies of the photolysis/photo-oxidation and gas-particle interactions with 03 and NOz of key 4- and 6-ring PAHs adsorbed on model substrates or ambient aerosols and (4) environmental chamber studies of the reactions of such PAHs associated with several physically and chemically different kinds of combustion-generated aerosols (e.g., diesel soot, wood smoke, and coal fly ash). Where such data are available, we also briefly consider some toxicological ramifications of these reactions. 

The combustion is an extremely complex process including many chemical and physical phenomena of transformation of matter. The need and desire to know and control this process urges man to study its various aspects. Organic polymers are but one example of the multitude of materials used by man. They possess peculiar features and properties which individually affect the material behavior in a critical fire situation. It is, therefore, important to study the flammability characteristics of polymeric materials and the factors affecting them. 

The following discussion shows how the chemical composition, rate of formation, and heat of combustion of the pyrolysis products are affected by the variations in the composition of the substrate, the time and temperature profile, and the presence of inorganic additives or catalysts. The latter aspect, however, is discussed in more detail in Chapter 14. Combustion may be defined as complex interactions among fuel, energy, and the environment. Consequently, the combustion process is controlled not only by the above chemical factors, but also by the physical properties of the substrate and other prevailing conditions affecting the phenomena of heat and mass transport. Discussion of this phenomenon is beyond the scope of this chapter. 

Once the physical prototype is created, it can be subjected to tests and evaluated under some or all aspects of the application environment. Mechanical, chemical, microstructural, electrical, visual, frictional, and combustive properties are usually relevant in some combination. Testing may also be divided into two categories simiflation testing in the laboratory, and service testing where the prototype is placed in actual use. 

Coal is a complex material and, not surprisingly, coal combustion is a complex science because of the variety of physical and chemical properties of coal (Chapters 8 and 9) (Field et al., 1967 Essenhigh, 1981 Morrison, 1986 Gay and Davis, 1987 Brill, 1993 Heitmann, 1993). In addition, it is not only the amount of energy available from coal combustion but also other aspects such as fuel handling, ash removal, emissions, and environmental control techniques that are of extreme importance (Littler, 1981 Reid, 1981 Slack, 1981). 

Lilley [7], The discussion here is limited primarily to the fluid dynamics aspects and aimed at alternative fuels with different physical and chemical properties than conventional fossil fuels. Here, the following three flow domains are of particnlar importance and can be distinguished in any multiphase combustion system flowfields connected with injection of fuel and air, flow regions dominated by free convection currents located far away from burners, and flow along the cooled walls of the combustion zone. Extensive reviews of the application of the classical turbulent combustion modeling methods have been presented for fossil fuels. For alternative fuels, it has been pointed out that there are major differences in the combustion of coal-derived liquids and shale oil in gas turbine combustors. Texts include Bartok and Sarofim [10] and Keating [11]. 

In this connection, it should be noted that, taking into account the basics of transport phenomena, chemistry and the results of a large body of research in this area, the above mentioned criterion of gasless combustion Pi (7) Pq 16) calls for serious argumentation. However, despite having the data on the physics of combustion, phase formation and the preparative-structural aspects of the combustion synthesis (interrelation of product characteristics with conditions of combustion), information about its detailed chemical mechanism(s) is still lacking. At the same time, the chemical mechanism defines in many respects other process characteristics and it is hardly (if ever) possible to control the synthesis without this understanding. The situation is made more complicated due to the fact that the synthesis and phase formation quite often coincide spatially and temporally n the combustion wave. Therefore, the sequence of phase transformations is usually termed as the chemical mechanism and the question how this or that phase of the reaction product was formed remains open. The direct dependence of the chemical... 

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