Fuel oils calorific/heating values

Condensing boilers are now available for both gas- and oil-fired plant, the advantage of these being that the flue gases are further cooled down to below 100°C so that the latent heat available in the flue gas water vapor is recovered. The condensate has to be removed and the boiler capital cost is higher than for conventional plant. However, the boiler plant efficiency is increased to the order of 90 per cent, based upon the fuel gross calorific value. Where the flue gas exit temperatures are in excess of 200° C a further economy can be obtained by the provision of a spray recuperator in the case of gas and flue gas economizers for oil and coal. 

Because the function of a fuel is to produce heat, the calorific or heating value (ASTM D-240, IP 12) is one of the important fuel properties and a knowledge of this value is necessary in obtaining information regarding the combustion efficiency and performance of all types of oil-burning equipment. 

The calorific value (heat of combustion) of residual fuel oil (ASTM D-240, IP 12) is lower than that of lower-boiUng fuel oil (and other liquid fuels) because of the lower atomic hydrogen-to-carbon ratio and the incidence of greater amounts of less combustible material, such as water and sediment, and generally higher levels of sulfur. 

Density or specific gravity (relative density) is used whenever conversions must be made between mass (weight) and volume measurements. This property is often used in combination with other test results to predict oil quality, and several methods are available for measurement of density (or specific gravity). However, the density (specific gravity) (ASTM D-1298, IP 160) is probably of least importance in determining fuel oil performance but it is used in product control, in weight-volume relationships, and in the calculation of calorific value (heating value). 

The calorific value or heat of combustion or heating value of a sample of fuel is defined as the amount of heat evolved when a unit weight (or volume in the case of a sample of gaseous fuels) of the fuel is completely burnt and the products of combustion cooled to a standard temperature of 298 °K. Net calorific value assumes the water leaves with the combustion products without fully being condensed. Fuels should be compared based on the net calorific value (Table 2.55). The calorific value of coal varies considerably depending on the ash, moisture content and the type of coal, while calorific values of fuel oils are much more consistent. Table 2.55 shows that oil and gas have about 30% less specific CO2 emission than coal, based either on carbon content or energy output the only reason for this difference is the water content of the fuel and the energy loss due to water evaporation. Water condensation and heat recovery seems a way to increase the net efficiency and hence reduce emissions. 

The method of determining heats of combustion has been described above, for carbon. This method, with use of a bomb calorimeter, is the customary basis for determining the value of a fuel, such as coal or oil. A weighed sample of the fuel is placed in the bomb calorimeter, the bomb is filled with oxygen, and the fuel is burned. The fuel value or calorific value of the fuel is considered to be measured by its heat of combustion, and when large amounts of fuel are purchased the price may be determined by the result of tests in a bomb calorimeter. 

Gas. Gas has a high calorific value of more than 1+2,000 KJ/ NM. It is used as fuel for heating the pyrolysis kiln along with the recovered light oil. 

The heat of combustion (ASTM D-240, ASTM D-1405, ASTM D-2382, ASTM D-2890, ASTM D-3338, ASTM D-4529, ASTM D-4809, ASTM D-6446, IP 12) is a direct measure of fuel energy content and is determined as the quantity of heat liberated by the combustion of a unit quantity of fuel with oxygen in a standard bomb calorimeter. A high calorific value is obviously desirable in oil used for heating purposes. Calorific value does not, however, vary greatly in the range of paraffinic-type kerosene (ASTM D-240, IP 12). 

Natural gas (calorific value about 8,600 kcal/Nm ) is introduced via ports in the walls of the kiln. It burns with a longer, lazier flame than oil or coal and more readily produces medium reactivity lime. As it contains insignificant amounts of sulfur and produces no ash, the resulting lime is not contaminated by the fuel in any way. Moreover, natural gas does not readily produce smoke, so that the exhaust gases have a low opacity. The heat usages of gas-fired shaft kilns are generally about 1,150 kcal/kg. 

Select fuel type and its calorific value decide the air heating way, such as steam heater, electric heater, oil furnace, coal furnace. 

The determination of heats of combustion for fuels of all types is prescribed in detail by government regulation, since the calorific value or heat content of a particular coal, oil, or natural or manufactured gas determines its economic value. Combustion calorimeters or bomb calorimeters (usually operated adiabatically) are used for such measurements, whereby the sample is combusted in a calorimetric bomb (likewise standardized cf. Fig. 25) in pure oxygen at 3 10 Pa to a defined set of end products. Methods of measurement and interpretation are also standardized, and they have been simplified to such an extent that the entire procedure can be automated. 

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