Net calorific value

Net calorific value (net heat of combustion at constant pressure) heat produced by combustion of a unit quantity of a solid or liquid fuel when burned, at a constant pressure of 1 atm, under conditions such that all the water in the products remains in the forms of vapor. 

Calorific value The measure of the heating capacity of a fuel, usually expressed as the available heat resulting from the complete combustion of that fuel in kj kg or kj nr Gross calorific value includes the heat of condensation of the water vapor in a hydrogen fuel net calorific value excludes this. 

The high hydrogen/carbon ratio of gas means that the quantity of water vapor in the products of combustion is greater than most other fossil fuels. The latent heat of this cannot be released in conventional appliances leading to a low net/gross ratio of calorific value of 90 per cent. (It is normal practice to quote gross CV in Europe net CV is often used. If net CV is quoted, efficiencies of over 190 per cent are possible.)... 

The calorific value is constantly monitored, and it is a condition of the Gas (Declaration of Calorific Value) Regulations 1972 that alterations in the declared calorific value (CV) are publicly made known. It is customary to quote the gross (upper) CV rather than the net (lower) CV. The difference between the two represents the heat contained in the latent heat of vaporization of the water vapor in the products of combustion that can only be recovered in condensing appliances. 

H = heat input rate (W) based on the net calorific value of the fuel,... 

Temperature Oxygen at Net energy Calorific value Energy in gas... 

There are three basic classifications (11 total calorific value types. (2) net calorific value types, and (3) inferential types. Net calorific value is less than the total calorific value by an amount equal to the latent heat of vaporization of the water formed during combustion. A net calorific value instrument uses a means that give results more nearly related to the net value. Thus, these types arc affected by gas composition and musl be calibrated for the gas to be tested. Inferential-type instruments depend upon such characteristics as flame appearance, maximum flame temperature, specific gravity, or gas analysis as indicative of the calorific value. 

The calorific value is the heat produced by the combustion of a unit quantity of coal in a bomb calorimeter with oxygen and under a specified set of conditions (ASTM D-121 ASTM D-2015 ASTM D-3286 ISO 1928). For the analysis of coal, the calorific value is determined in a bomb calorimeter either by a static (isothermal) method or by an adiabatic method, with a correction made if net calorific value is of interest. The unit is calories per gram, which may be converted to the alternate units (1.0 kcal/kg = 1.8 Btu/lb = 4.187 kJ/kg). 

The calorific value is a direct indication of the heat content (energy value) of the coal and represents the combined heats of combustion of the carbon, hydrogen, nitrogen, and sulfur in the organic matter and of the sulfur in pyrite and is the gross calorific value with a correction applied if the net calorific value is of interest. 

The experimental conditions require an initial oxygen pressure of 300 to 600 psi and a final temperature in the range 20 to 35°C (68 to 95°F) with the products in the form of ash, water, carbon dioxide, sulfur dioxide, and nitrogen. Thus, once the gross calorific value has been determined, the net calorific value... 

In either form of measurement (ASTM D-2015 ASTM D-3286), the calorific value recorded is the gross calorific value. The net calorific value is calculated from the gross calorific value [at 20°C (68°F)] by making a suitable subtraction (= 1030 Btu/lb = 572 cal/g = 2.395 MJ/g) to allow for the water originally present as moisture as well as the moisture formed from the coal during the combustion. The deduction, however, is not equal to the latent heat of vaporization of water [1055 Btu/lb (2.4 MJ/g) at 20°C (68°F)] because the calculation is made to reduce from the gross value at constant volume to a net value at constant pressure for which the appropriate factor under these conditions is 1030 Btu/lb (2.395 MJ/g). 

ISO 540. Determination of the Fusibility of Ash High-Temperature Tube Method. ISO 1928. Determination of Gross Calorific Value and Calculation of Net Calorific Value. 

Net calorific value This is the measurement of the actual available energy per unit volume at standard conditions, which is always less than the gross calorific value by an amount equal to the latent heat of vaporization of the water formed during combustion. 

In Europe and in other countries where the metric system is used, natural gas calorimeters are calibrated in megajoule units, and in the United States in Btu units. The output of the calorimeter may represent the gross calorific value (sometimes referred to as upper heating value or gross heating value), the net calorific value (sometimes referred to as lower heating value or net heating value), or the Wobbe index. 

The heat flow rate (Q) of a gaseous fuel is calculated as the product of its volumetric flow rate at standard conditions (V0) and its calorific value (CV). The Wobbe index (WI) measures the ratio between the net CV and the square root of specific gravity (SG). With orifice-type flow sensors, the advantage of detecting the WI is that it eliminates the need to separately measure the specific gravity this is because the product of the WI and orifice pressure drop results in a constant times the heat flow rate (KxQ), without requiring a separate measurement of SG. 

The thermal combustion properties measured in the test are related to the flammability characteristics of the material.5155 For example, the heat release temperature from method A approximates the surface temperature at ignition (Section 14.3.2.1). The net calorific value from method B approximates the net heat of combustion measured in an oxygen bomb calorimeter. 

Although Jerusalem artichoke tops need extensive drying, they can be a useful source of biomass for direct combustion. Trials in Lithuania have confirmed Jerusalem artichoke s suitability as an energy crop. The bulk density of tops harvested in autumn was 78 kgm3, compared to 65 kgm3 in the spring, while the average net calorific value of dry biomass harvested in autumn was 18.0... 

The gas phase composition is shown in Table 10.23. It consists mainly of carbon oxides and consequently has a very low net calorific value. 

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