﻿ Heat of combustion#Lower heating value

# Heat of combustion#Lower heating value

The calorific value Hi (formerly net calorific value Hu) is the maximum usable at a combustion quantity of heat, in which there will be no condensation of the water vapor contained in the exhaust gas based on the amount of the fuel used ( in distinction to the combustion value, which therefore is greater than the calorific value ). The calorific value is colloquially called imprecise " energy content " or " energy value ".

The calorific value is thus the measure of the specific tax per unit useable heat without heat of condensation. The calorific value says nothing about the rate of combustion. Thus, the heating value of the explosive TNT is only a quarter of the value of wood.

• 7.1 Solid fuels ( at 25 ° C)
• 7.2 Liquid fuels (at 25 ° C)
• Gaseous fuels 7.3 (at 25 ° C)
• 7.4 Conversion factors according to calorific value calorific value and vice versa according to German Energy Saving Ordinance

## The physical size

Is specified, the heating value than mass-based heating value, for example, in kilojoules per kilogram in kJ / kg, grams or ton. For water- containing fuels such as biomass or waste is necessary to distinguish whether the values ​​of the total mass including water content relate ( Rohheizwert ), or whether the anhydrous mass serves as a reference value ( calorific value, anhydrous). In the literature (especially in waste management ) calorific values ​​are often based on the water-containing fuel, calorific values ​​, however, often on the dry anhydrous fuel, without this being from the unit kJ / kg apparent.

With the aid of the density of the fuel of the mass-related calorific value can also be converted into a volume-related calorific value, ie for example in kJ per liter / liter or per cubic meter in kJ / m³. Are also common figures in kWh for heating oil that is in kWh / l or for gas in kWh / m³ in building technology.

The symbol for the calorific value Hi. The "i" stands for the Latin inferior ( "lower "). Hu as well kJ / mN ³ with indexed unit of measurement for the normal volume of gases are no longer the standard.

## Technical / commercial simplification

In Germany, technically and commercially, the heating value is often expressed in units of coal and internationally via the dimensionless oil equivalent ( dE ). In Table works, other mass - and volume-related units of comparison are used: kilograms of oil equivalent ( kgÖE ) tons of oil equivalent ( tÖE ), cubic meters of oil equivalent (m³ dE ) and liquid U.S. gallon of oil equivalent ( US.liq.gal.ÖE ).

## Calorific value and Calorific value

To determine the heat of combustion, a dried substance with excess oxygen is burned in a pressurized calorimeter. Are formed as by products of combustion of gaseous carbon dioxide and water as condensate (which at the pressure conditions is liquid ). These values ​​are by default relative in Table works to 25 ° C.

• The calorific value is identical to the absolute value of the specified standard enthalpy with negative sign ΔVH ° of the general thermodynamics. Spoken Heiztechnisch means that the water content is present ( from product moisture remains, supply air and from the oxidized hydrogen atoms in the fuel derived ) in this calculation does not vaporous, but before and after combustion in liquid form. Referred to by the expression condensing boilers for heating systems: Here, the bound in water vapor or " wasted " energy is used for heat production. For heating purposes is the calorific value of the characteristic value better, because when using the heating value can arise physically nonsensical efficiencies over 100 percent.
• The calorific value of a substance can not be directly determined experimentally. The calorific value refers to a combustion formed in the only gaseous combustion products. For the calculation is therefore the focal value, if hydrogen atoms are contained in the fuel, deducted the enthalpy of vaporization of water, so are the calorific values ​​of these fuels is about 10 % below their calorific values ​​.

For the gaseous components referring to the calorific value of volume at 101.325 kPa and 0 ° C ( standard conditions). The information is then in kilojoules per standard cubic meter as kJ / m³ STP, said, " iN " " in standard condition" means. The difference between heating value and energy value for gaseous fuels higher than other fabrics, since, in contrast to heating oil or even wood (only 4%), the hydrogen content is very high.

The calorific value is also reflected in the billing of heating energy. However, it is obtained from energy suppliers to 0 ° C. Then the calorific value of gases ( higher energy density so ) per volume is due to the higher gas density again about 10 % higher.

• 55.5 MJ / kg at 25 ° C - 55.6 MJ / kg at 0 ° C (on mass)
• 36.3 MJ / m³ at 25 ° C - 39.9 MJ / m³ at 0 ° C ( by volume)

## Calculation of calorific value and Calorific value

Conventional fuels such as oil or coal are mixtures of substances whose elemental composition is mostly known from analyzes. With approximate formulas of the calorific value of such mixtures for technical applications can be calculated with sufficient accuracy from the composition.

Further still, there is a calorific value determination according to Dulong.

### Solid and liquid fuels

For solid and liquid fuels to calculate heating and calorific value of the shares of combustible materials. Here are the percentage divided by 100 mass fractions of carbon, hydrogen, nitrogen, sulfur, oxygen and water in the total mass including water content. ( For the mass fractions of hydrogen and oxygen include only those components that are not present in the form of water. )

Calorific value (based on the total composition):

Calorific value (based on the total composition):

Calorific value (based on the dry anhydrous fuel):

Calorific value (based on the dry anhydrous fuel):

When converting between the combustion and heating value must be taken into account that the resulting from the hydrogen content of water and the water already contained in the fuel during the heating value gaseous form (at 25 ° C), the calorific value but in liquid form ( at 25 ° C ). Therefore, the enthalpy of vaporization of water flowing at 25 ° C of 2.441 MJ / kg in the conversion of a:

### Gas mixtures

For gas mixtures, the calculation on the hydrogen gas and the main hydrocarbons enters. The etc. are the mole fractions of the components with the empirical formulas given in brackets.

Energy:

Calorific value:

## Calorific value and combustion temperature

The combustion temperature depends on the calorific value on the one hand and on the heat capacity of both the starting substances and the final products of the combustion reaction, on the other hand. It is calculated using the energy balance formula:

Here, the heat transfer to the surroundings is negligible ( adiabatic approach). Bystanders, but present materials are necessarily to be considered with: There is for example a difference whether magnesium burns in air, firing temperatures of about 2,000 ° C are reached, or in pure oxygen. During combustion in pure oxygen must not uninvolved substances, such as nitrogen, are heated with.

For the same reason you used for oxyacetylene welding acetylene and pure oxygen, so that temperatures of about 3,000 ° C are reached.

Most adiabatic analysis is not appropriate, the reaction rate can be ignored. As a burning block of wood only at the surface and the heat is released over time into the environment. In contrast, wood flour reacts with air explosive (dust explosion).

## Calorific value and net heat input / boiler efficiency

The calorific value of a source of heat supplied to the fuel amount in kW ( kJ / s) is the heat load.

• The largest heat load, set to a heat source and must not be exceeded, is indicated on the nameplate.
• Similarly, the smallest heat load, ie the amount of fuel that needs to be fed according to their calorific value at least, and must not be exceeded.
• The nominal heat load in between and is the case of a measurement at constant power supplied at rated thermal output quantity of fuel.
• The ratio of nominal heat output to the rated heat load is the boiler efficiency.

## Tables

1 MJ / kg = 1000 kJ / kg; 1 MJ = 0.27778 kWh or 1 kWh = 3.6 MJ

### Gaseous fuels (at 25 ° C)

• Natural gas, " L" is from about 85% methane, 4% ( ethane, propane, butane, pentane) and 11% inert gases.
• Natural gas "H" ( North Sea ) is composed of about 89 % of methane, 8 % ( ethane, propane, butane, pentane ) and 3% inert gases.
• Natural gas "H" ( CIS ) consists of approximately 98 % methane, 1 % ( ethane, propane, butane, pentane ) and 1% inert gases.

## Norms and Standards

• EN 437:2003 Test gases - Test Pressures - Appliances categories; German: DIN EN 437:2003-09 Calibration gas mixtures - Test pressures - equipment categories and ÖNORM EN 437:1994-05-01 devices for use with fuel gases - Calibration gas mixtures - Proofs and equipment categories This Euro norm leads also in terms of international harmonization, the symbols for the calorific value Hi and Hs for the calorific value a
• DIN 5499 calorific value and heat value terms ( January 1972 )
• DIN 51900 Determination of calorific value by the bomb calorimeter and calculation of net calorific Part 1 Principles, apparatus, methods (April 2000)
• Part 2 procedure with isoperibolem or static jacket calorimeter (May 2003)
• Part 3 process with adiabatic jacket ( July 2004)
• DIN 1340 Gaseous fuels and other gases, types, components, using ( December 1990)
• DIN 1871 Gaseous fuels and other gases - density and other volumetric quantities ( May 1999)
• DIN 51857 Gaseous fuels and other gases - Calculation of calorific value, density, relative density and Wobbe index of pure gases and gas mixtures (March 1997)
• DIN 51612 Testing of liquefied petroleum gas; Calculation of net calorific value ( June 1980)
• DIN 51854 Testing of gaseous fuels and other gases; Determination of ammonia content ( September 1993 )
• DIN V 18599 Energy performance of buildings - Calculation of the net, final and primary energy demand for heating, cooling, ventilation, domestic hot water and lighting
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