The calorific value H i (formerly lower calorific value H u ) is at a combustion maximum usable amount of heat , which is not a condensation of the gas contained steam is, based on the amount of used fuel (in distinction to the calorific value H s , the is therefore greater than the calorific value).
The calorific value is therefore the measure for the specifically usable amount of heat without the condensation enthalpy of the water vapor. The calorific value says nothing about the rate of combustion . The calorific value of the explosive TNT is only a quarter of the value of wood.
The physical quantity
The calorific value is given as a mass-related calorific value, for example in kilojoules per kilogram , kilojoules per gram or kilojoules per ton . In the case of hydrous fuels such as biomass or waste, a distinction is made as to whether the values relate to the total mass including water content ( raw calorific value ), or whether the anhydrous mass serves as a reference value ( calorific value, anhydrous ). In the literature (especially in waste management), calorific values are often related to the water-containing fuel, whereas calorific values are often related to the anhydrous fuel, without this being apparent from the unit kJ / kg.
With the help of the density of the fuel, the mass-related calorific value can also be converted into a volume-related calorific value , for example in kilojoules per liter or in kilojoules per cubic meter . Energy data in kWh are also common in building services , i.e. in kWh / l for heating oil or in kWh / m³ for gas.
The symbol for the calorific value is H i . The «i» stands for Latin inferior ("lower"). H u as well as kJ / m N ³ with the indicated unit of measurement for the normal volume for gases are no longer in accordance with the standard.
Technical / commercial simplification
In Germany, technically and commercially, the calorific value is often given in hard coal units and internationally in the dimensionless oil unit (ÖE). Other mass and volume-related units of comparison are also used in tables: kilograms of oil units (kgÖU), tons of oil units (toe), cubic meters of oil units (m³ÖE) and liquid US gallons of oil units (US.liq.gal.ÖU).
Calorific value and calorific value
To determine the heat of combustion , a dried substance is burned under pressure in a calorimeter with excess oxygen . This creates gaseous carbon dioxide as combustion products and water as condensate (which is liquid under the pressure conditions). These values are referenced to 25 ° C as standard in tables.
- The calorific value is identical to the absolute amount of the standard enthalpy of combustion Δ V H ° of general thermodynamics, given with a negative sign . In terms of heating technology, this means that the water content (from product moisture residues, supply air moisture and the oxidized hydrogen atoms in the fuel) is not in vapor form in this calculation, but in liquid form before and after combustion. The term condensing technology for heating systems also refers to this: The enthalpy of evaporation bound in the water vapor is also used effectively. For heating purposes, the calorific value (more precisely: the upper calorific value) is the better characteristic value, because when the (lower) calorific value is used, apparently physically nonsensical degrees of utilization of over 100% can occur due to the fact that the evaporation enthalpy of the water is not taken into account.
- The calorific value of a substance cannot be determined directly by experiment. The calorific value refers to a combustion that only produces gaseous combustion products. For the calculation, the evaporation enthalpy of the water is subtracted from the calorific value, provided hydrogen atoms are contained in the fuel, so the calorific values of such fuels are approx. 10% below their calorific values.
- Example: The enthalpy of vaporization of water is 45.1 kJ / mol (0 ° C), 44.0 kJ / mol (25 ° C) or 40.7 kJ / mol at 100 ° C (see also heat of vaporization ).
In the case of gaseous substances, the calorific value is related to the volume at 101.325 kPa and 0 ° C ( standard conditions ). The specification is then made in kilojoules per standard cubic meter as kJ / m³ iN, where «iN» means "in standard condition". The difference between the calorific value and the calorific value is higher with gaseous fuels than with other materials, because here, in contrast to heating oil or even wood (only 4%), the hydrogen content is very high.
The calorific value is also taken into account when billing heating energy. However, it is related to 0 ° C by energy suppliers. Then the calorific value of the gases is about 10% higher per volume because of the higher gas density (i.e. higher energy density ).
- Example: calorific value methane CH 4
- 55.5 MJ / kg at 25 ° C - 55.6 MJ / kg at 0 ° C (based on mass)
- 39.8 MJ / m³ at 25 ° C - 39.9 MJ / m³ at 0 ° C (based on volume) ( see table: Gaseous fuels (at 25 ° C) )
Calculation of calorific value and calorific value
Common fuels such as crude oil or coal are mixtures of substances whose elemental composition is mostly known from analyzes. With approximation formulas , the calorific value of such mixtures of substances can be calculated with sufficient accuracy from the composition for technical applications.
There is also a determination of the calorific value according to Dulong .
Solid and liquid fuels
With solid and liquid fuels, the calorific value and calorific value are calculated from the proportions of combustible substances . The mass percentages of carbon, hydrogen, nitrogen, sulfur, oxygen and water are divided by 100 in the total mass including the water content (only those proportions that are not in the form of water count for the mass proportions of hydrogen and oxygen).
Calorific value (based on the total mass):
Calorific value (based on the total mass):
Calorific value (based on the anhydrous fuel):
Calorific value (based on the anhydrous fuel):
When converting between calorific value and calorific value, it must be taken into account that the water resulting from the hydrogen content and the water already contained in the fuel are in gaseous form for calorific value (at 25 ° C), but in liquid form for calorific value (at 25 ° C ). Therefore, the enthalpy of evaporation of water at 25 ° C of 2.441 MJ / kg is included in the conversion:
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 materials and the end products of the combustion reaction on the other. It is calculated using the energy balance formula:
- Starting temperature × heat capacity of the starting materials + calorific value = end or combustion temperature × heat capacity of the end products.
The heat dissipation to the environment is neglected ( adiabatic consideration). Uninvolved, but present substances must be taken into account: For example, it makes a difference whether magnesium burns in air , where combustion temperatures of around 2,000 ° C are reached, or in pure oxygen . When burning in pure oxygen, no uninvolved substances such as nitrogen need to be heated.
In most cases, an adiabatic view that does not take the reaction speed into account is unsuitable . A block of wood only burns on the surface and the heat is released into the environment over time. In contrast, wood flour reacts explosively with air ( dust explosion ).
Calorific value and nominal heat load / boiler efficiency
- The greatest heat load to which a heat generator is set and which must not be exceeded is indicated on the nameplate .
- The same applies to the smallest heat load , i.e. the amount of fuel that has to be added according to its calorific value and must not fall below.
- The nominal heat load lies in between and is the amount of fuel supplied during a measurement in constant continuous operation with nominal heat output .
- The ratio of nominal heat output to nominal heat load is the boiler efficiency .
1 MJ / kg = 1000 kJ / kg; 1 MJ = 0.27778 kWh or 1 kWh = 3.6 MJ
Solid fuels (at 25 ° C)
|fuel||Calorific value (in MJ / kg)||Calorific value (in MJ / kg)||Calorific value (in kWh / kg)|
|air-dry wood , barley grains , paper , peat||*||14.4-15.8||4-4.4|
|Straw (absolutely dry), wheat grains , hemp briquettes||*||16.7-17.2||4.7-4.8|
|Wood pellets , olive pits , wood briquettes||*||18-18.7||4.8-5.0|
|Raw lignite , sulfur||9.3-10||8-9.3||2.2-2.6|
Lignite briquettes , lignite dust ,
dry liquor ( DDGS )
Hard coal , various types, hard coal , coke , hard coal dust
Charcoal , lignite coke , petroleum coke ,
old tires / old rubber , carbon (graphite)
|Phosphorus , magnesium||25.0-25.2||25.0-25.2||7th|
- (*) currently not known
Liquid fuels (at 25 ° C)
(in MJ / kg)
(in MJ / kg)
(in kWh / kg)
(in kg / dm³)
|Diesel , heating oil EL||45.4||42.6||11.8||0.820-0.845|
|Biodiesel||40 ( RME ) (2)||37||10.2||0.86-0.9|
|Heating oil S (heavy)||42.3||40.0||11.0||0.96-0.99|
|Used fat (1)||*||36||10||*|
- (*) currently not known
- (1) Used fat are esters of long-chain fatty acids (mostly C18) with glycerine (e.g. rapeseed oil).
- (2) Biodiesel is an ester of long-chain fatty acids (mostly C18) with methanol (e.g. rapeseed oil methyl ester).
- (3) Gasoline-benzene mixture (gasoline) in the most commonly used mixture of "6 parts gasoline and 4 parts benzene"
Gaseous fuels (at 25 ° C)
|fuel||Calorific value (in MJ / kg)||Calorific value (in MJ / kg)||Calorific value (in MJ / m³) (4)||Calorific value (in MJ / m³) (4)||Calorific value (in kWh / m³) (4)|
|Furnace gas (1)||1.5-2.1||1.5-2.1||2.5-3.4||2.5-3.3||0.695-0.917|
|Town gas (2)||18.21||16.34||19… 20||17… 18||4.72-5.00|
|Natural gas (3)||36… 50||32… 45||35… 46||31… 41||8.6-11.4|
- Source: Basics of gas technology
- (1) Blast furnace gas consists of (2… 4)% hydrogen , (20… 25)% carbon monoxide and (70… 80)% inert gases (carbon dioxide, nitrogen).
- (2) Town gas consists of (19… 21)% methane , 51% hydrogen, (9… 18)% carbon monoxide and (10… 15)% inert gases.
- (3) Types of natural gas:
- Natural gas "L" consists of approx. 85% methane, 4% ( ethane , propane , butane , pentane ) and 11% inert gases.
- Natural gas "H" (North Sea) consists of approx. 89% methane, 8% (ethane, propane, butane, pentane) and 3% inert gases.
- Natural gas "H" (CIS countries) consists of approx. 98% methane, 1% (ethane, propane, butane, pentane) and 1% inert gases.
- (4) Volume-related information relates to the normal volume under normal conditions (0 ° C and 101325 Pa)
Conversion factors calorific value to calorific value
and vice versa according to German EnEV
|fuel||Calorific value → calorific value
(calorific value → calorific value)
|Natural gas , ethanol||1.11 (0.901)|
|Propane , paraffin||1.09 (0.917)|
|Butane , gasoline , heating oil , biodiesel , wood||1.08 (0.926)|
|Diesel , vegetable oil , lignite briquettes||1.07 (0.935)|
|Heavy fuel oil||1.06 (0.943)|
|Hard coal briquettes||1.02 (0.980)|
- EN 437: 2003 Test gases - Test pressures - Appliances categories ; German: DIN EN 437: 2003-09 test gases - test pressures - device categories and ÖNORM EN 437: 1994-05-01 devices for operation with fuel gases - test gases - test pressures and device categories
- This European standard also introduces the symbols H i for the calorific value and H s for the calorific value in the interests of international harmonization
- DIN 5499 calorific value and calorific value, terms (January 1972)
- DIN 51900 Determination of the calorific value with the bomb calorimeter and calculation of the calorific value
- Part 1 General information, basic devices, basic procedures (April 2000)
- Part 2 method with isoperibolic or static-jacket calorimeter (May 2003)
- Part 3 procedure with adiabatic jacket (July 2004)
- DIN 1340 Gaseous fuels and other gases, types, components, use (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, calorific value, density, relative density and Wobbe index of gases and gas mixtures (March 1997)
- DIN 51612 testing of liquid gas; Calculation of the calorific value (June 1980)
- DIN 51854 Testing of gaseous fuels and other gases; Determination of the ammonia content (September 1993)
- DIN V 18599 Energetic evaluation of buildings - Calculation of useful, final and primary energy requirements for heating, cooling, ventilation, hot water and lighting
- Wobbewert , condition number , gas energy , parameters of the effect of a fuel
- Flue gas loss , a measure of the efficiency of a heating system
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- gases, calorific values
- DIN V 18599 supplement 1: 2010-01