High and Low Heating Values

Low heating value (LHV), The low heating value denoted LHV and expressed in J.kg and formerly called the net caloric value is the specific enthalpy of the combustion reaction of a liquid or solid fuel with air per unit mass of fuel excluding the latent heat of vaporization of liquid water. The temperature of the fuel before the combustion and that of the combustion products must be 298.15 K and 101.325 kPa, the water formed by the combustion must remain as vapor and the nitrogen must not be oxidized. 

Gaseous fuel Oxidant Adiabatic flame temp. (V°c) Ignition temp. (T/c) Burning velocity (Wm.s ) 

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The relationship between the high and low heating values of a fuel having the empirical chemical formula is given by Dulong s equations. Note that the negative sign indi-... 

For an industrial gaseous fuel (e.g., natural gas, syngas, producer gas and water gas) containing carbon monoxide, hydrogen, methane, ethane and acetylene, the high and low heating values can be calculated with the following two equations based on the volume or mole fraction of each gas ... 

The most important physical and chemical properties of petroleum and its derivatives are the specific gravity, the kinematic viscosity, the flash point, the high and low heating value and the C, H, N, S, and 0 content. The specific gravity of petroleum is measured at 60°F for historical reasons and usually expressed as a dimensionless index of density called the degree API ( API) from the American Petroleum Institute and defined as follows ... 

The very low water adsorption by Graphon precludes reliable calculations of thermodynamic quantities from isotherms at two temperatures. By combining one adsorption isotherm with measurements of the heats of immersion, however, it is possible to calculate both the isosteric heat and entropy change on adsorption with Equations (9) and (10). If the surface is assumed to be unperturbed by the adsorption, the absolute entropy of the water in the adsorbed state can be calculated. The isosteric heat values are much less than the heat of liquefaction with a minimum of 6 kcal./mole near the B.E.T. the entropy values are much greater than for liquid water. The formation of a two-dimensional gaseous film could account for the high entropy and low heat values, but the total evidence 22) indicates that water molecules adsorb on isolated sites (1 in 1,500), so that patch-wise adsorption takes place. 

The stability of the main boiler steam cycle has been excellent. The large openings that were made for the low Btu gas burners have not caused any disturbances in the water/steam circulation system. Furthermore, as regards the operation of the product gas burners, the product gas combustion has been stable even though the moisture content of the solid fuel has been mostly high and the heating value of the gas very low. The stability of the main boiler coal burners has been normal despite the fact that the product gas burners were integrated very close to the... 

Fig. 3. Relative values of some model parameters through Earth history. Both high and low heat-flow histories are indicated these are compared with the history of radiogenic heating.

Because of the complexity of the coalification processes, different measures are used to define different levels of rank (Table I) high moisture, low heating value, and nonagglomerating character of the coal define the rank (group) within the lignite and subbituminous classes and volatile matter (or fixed carbon) define the various groups of rank in the bituminous and anthracite classes. In addition to these properties, the reflectance of vitrinite, carbon content of the coal (dry, mineral matter free), and some other properties change proportionately as rank increases (Table I). 

Blast-Furnace Gas. The gas discharged from steel-mill blast furnaces is used at the mills in heating furnaces, in gas engines, and for steam generation. This gas is quite variable in quality but generally has a high carbon monoxide content and low heating value (Table 6.19). 

Whilst the Vicat test usually gives the higher values the differences are quite modest with many polymers (e.g. those of types A, B and C). For example, in the case of the polycarbonate of bis-phenol A (Chapter 20) the heat distortion temperatures are 135-140°C and 140-146°C for the high and low stress levels respectively and the Vicat softening point is about 165°C. In the case of an acetal homopolymer the temperatures are 100, 170 and 185°C respectively. With nylon 66 the two ASTM heat distortion tests give values as different as 75 and 200°C. A low-density polyethylene may have a Vicat temperature of 90°C but a heat distortion temperature below normal ambient temperatures. 

The heat given out in the combustion of propellants is readily measured by exploding them in special calorimetric bombs built to withstand the high pressures produced. The result thus obtained, however, does not necessarily give exactly the heat available under practical circumstances, as in the calorimetric bomb the products are cooled and can undergo reactions which have different equilibria at high and low temperatures. It is, therefore, usual for design purposes to obtain the calorimetric value of a propellant also by calculation. 

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