User:Milton Beychok/Sandbox: Difference between revisions

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The heat of combustion is traditionally measured with a [[Calorimeter#Bomb calorimeters|bomb calorimeter]]. It may also be calculated as the difference between the [[Standard enthalpy change of formation|heat of formation]] (Δ<sub>f</sub>H<sup>0</sup>) of the products and reactants.
The heat of combustion is traditionally measured with a [[Calorimeter#Bomb calorimeters|bomb calorimeter]]. It may also be calculated as the difference between the [[Standard enthalpy change of formation|heat of formation]] (Δ<sub>f</sub>H<sup>0</sup>) of the products and reactants.


== Common expressions for the heats of combustion ==
== Common expressions for the heats of combustion of fuels ==


The heat of combustion of a [[fuel]] is commonly referred to as the ''heating value'' or the ''caloric value'' and defined as the  
The heat of combustion of a [[fuel]] is commonly referred to as the ''heating value'' or the ''caloric value'' and briefly defined as the amount of heat released when a unit amount of the fuel is completely combusted. The heating value is a characteristic of each fuel.
amount of heat released when an amount of the fuel is completely combusted. The heating value is a characteristic of each fuel.


The heating value of a fuel may be categorized as either the ''higher heating value'' (HHV) or the ''lower heating value'' (LHV). The HHV is also known as the ''gross heating value'' (GHV) or the ''gross caloric value'' (GCV) and the LHV is also known as the ''net heating value'' (NHV) or the ''net caloric value'' (NCV).
The heating value of a fuel may be categorized as either the ''higher heating value'' (HHV) or the ''lower heating value'' (LHV). The HHV is also known as the ''gross heating value'' (GHV) or the ''gross caloric value'' (GCV) and the LHV is also known as the ''net heating value'' (NHV) or the ''net caloric value'' (NCV).


To be more specifically defined, the HHV is the amount of heat released when a unit amount of fuel at a given initial temperature (usually 20 °C or 25 °C) is completely combusted at stoichiometric conditions and constant pressure with the products of combustion being cooled back to the initial temperature and any water vapor in the products being condensed. Condensing any water in the combustion products during the determination of the HHV means that the HHV includes the heat of vaporization (H<sub>v</sub>) of the water produced. Stoichiometric combustion means that the combustion products do not contain any oxygen (i.e., there was no excess of combustion air during the combustion).


The LHV is similarly defined except that any water in the combustion products is not condensed and remains as a vapor. Thus, the LHV does not include the heat of vaporization of the water produced.


The heat of combustion for [[fuel]]s is expressed as the HHV, LHV, or GHV:
The relation between the HHV and the LHV may be simply expressed as:


* The quantity known as [[higher heating value]] ('''HHV''') (or ''gross calorific value'' or ''gross energy'' or ''upper heating value'') is determined by bringing all the products of combustion back to the original pre-combustion temperature, and in particular condensing any vapor produced.  This is the same as the thermodynamic heat of combustion since the [[enthalpy]] change for the reaction assumes a common temperature of the compounds before and after combustion, in which case the water produced by combustion is liquid.
:LHV = HHV – H<sub>v</sub>
* The quantity known as [[lower heating value]] ('''LHV''') (or ''net calorific value'') is determined by subtracting the [[heat of vaporization]] of the water vapor from the higher heating value.  This treats any H<sub>2</sub>O formed as a vapor. The energy required to vaporize the water therefore is not realized as heat.
* '''Gross heating value''' (see '''AR''') accounts for water in the exhaust leaving as vapor, and includes liquid water in the fuel prior to combustion.  This value is important for fuels like [[wood]] or [[coal]], which will usually contain some amount of water prior to burning.


Most applications which burn fuel produce water vapor which is not used, and thus wasting its heat content. In such applications, the lower heating value is the applicable measure. This is particularly relevant for [[natural gas]], whose high [[hydrogen]] content produces much water. The gross calorific value is relevant for gas burnt in [[condensing boiler]]s which condense the water vapor produced by combustion, recovering heat which would otherwise be wasted.
Fuel gases and fuel liquids usually contain little, if any, water. However, raw solid fuels like coal, wood or peat do contain significant amounts of water. Coal, in particular, also contains significant amounts of minerals that form ash when combusted.
 
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Both HHV and LHV can be expressed in terms of AR (all moisture counted), MF and MAF (only water from combustion of hydrogen).  AR, MF, and MAF are commonly used for indicating the heating values of coal:
Both HHV and LHV can be expressed in terms of AR (all moisture counted), MF and MAF (only water from combustion of hydrogen).  AR, MF, and MAF are commonly used for indicating the heating values of coal:
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* '''MF''' (Moisture Free) or '''Dry''' indicates that the fuel heating value has been measured after the fuel has been dried of all inherent moisture but still retaining its ash forming minerals.
* '''MF''' (Moisture Free) or '''Dry''' indicates that the fuel heating value has been measured after the fuel has been dried of all inherent moisture but still retaining its ash forming minerals.
* '''MAF''' (Moisture and Ash Free) or '''DAF''' (Dry and Ash Free) indicates that the fuel heating value has been measured in the absence of inherent moisture and ash forming minerals.
* '''MAF''' (Moisture and Ash Free) or '''DAF''' (Dry and Ash Free) indicates that the fuel heating value has been measured in the absence of inherent moisture and ash forming minerals.
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==Heating values of some common fuels==
==Heating values of some common fuels==

Revision as of 03:10, 23 September 2008

The heat of combustion (ΔHc0) is the energy released as heat when a compound undergoes complete combustion with oxygen. The chemical reaction for the combustion is typically that of a hydrocarbon fuel reacting with oxygen derived from atmospheric air to form carbon dioxide, water and heat. It may be quantified with these units:

  • energy/mole of fuel (such as kJ/mol or Btu/lb-mol)
  • energy/mass of fuel (such as MJ/kg or Btu/lb)
  • energy/volume of fuel (such as MJ/m3 or Btu/ft3)

The heat of combustion is traditionally measured with a bomb calorimeter. It may also be calculated as the difference between the heat of formation (ΔfH0) of the products and reactants.

Common expressions for the heats of combustion of fuels

The heat of combustion of a fuel is commonly referred to as the heating value or the caloric value and briefly defined as the amount of heat released when a unit amount of the fuel is completely combusted. The heating value is a characteristic of each fuel.

The heating value of a fuel may be categorized as either the higher heating value (HHV) or the lower heating value (LHV). The HHV is also known as the gross heating value (GHV) or the gross caloric value (GCV) and the LHV is also known as the net heating value (NHV) or the net caloric value (NCV).

To be more specifically defined, the HHV is the amount of heat released when a unit amount of fuel at a given initial temperature (usually 20 °C or 25 °C) is completely combusted at stoichiometric conditions and constant pressure with the products of combustion being cooled back to the initial temperature and any water vapor in the products being condensed. Condensing any water in the combustion products during the determination of the HHV means that the HHV includes the heat of vaporization (Hv) of the water produced. Stoichiometric combustion means that the combustion products do not contain any oxygen (i.e., there was no excess of combustion air during the combustion).

The LHV is similarly defined except that any water in the combustion products is not condensed and remains as a vapor. Thus, the LHV does not include the heat of vaporization of the water produced.

The relation between the HHV and the LHV may be simply expressed as:

LHV = HHV – Hv

Fuel gases and fuel liquids usually contain little, if any, water. However, raw solid fuels like coal, wood or peat do contain significant amounts of water. Coal, in particular, also contains significant amounts of minerals that form ash when combusted.

Re-write this:

Both HHV and LHV can be expressed in terms of AR (all moisture counted), MF and MAF (only water from combustion of hydrogen). AR, MF, and MAF are commonly used for indicating the heating values of coal:

  • AR (As Received) indicates that the fuel heating value has been measured with all moisture and ash forming minerals present.
  • MF (Moisture Free) or Dry indicates that the fuel heating value has been measured after the fuel has been dried of all inherent moisture but still retaining its ash forming minerals.
  • MAF (Moisture and Ash Free) or DAF (Dry and Ash Free) indicates that the fuel heating value has been measured in the absence of inherent moisture and ash forming minerals.

Heating values of some common fuels

Table 1: Higher Heating Value (HHV) Of Various Common Fuels
Fuel  Phase   Molecular 
Weight		 kJ/mol  MJ/kg   MJ/m3   Btu/lb   Btu/ft3 
Hydrogen [1] gas 2.016 285.84 141.79 12.75 60,986 324
Methane [1][2] gas 16.043 890.31 55.50 39.72 23,870 1,009
Ethane [1][2] gas 30.069 1,559.88 51.88 69.59 22,313 1,768
Propane [1][2] gas 44.096 2,220.05 50.35 99.05 21,654 2,516
Butane [1][2] gas 58.122 2,878.52 49.53 128.43 21,301 3,263
Ethanol [3] liquid 46.068 1,375.01 29.85 12,837
Gasoline [3] liquid 110 5,013.47 45.58 19,603
Kerosene [4] liquid 178 8,084.99 45.42 19,536
Diesel oil [4] liquid 225 10,124.99 45.00 19,355
Coal [5] solid 25.58 11,002
Wood (dry) [6] solid 21.14 9,093
Peat (dry) [7] solid 22.09 9,500
-- The gas temperature and pressure for the values of MJ/m3 are 0 °C and 101.325 kPa.
-- The gas temperature and pressure for the values of Btu/ft3 are 60 °F and 14.696 psia.
-- LPG is marketed as propane or butanes or a mixture of propane and butanes. 
-- Natural gas, after removal of impurities and natural gas liquids (NGL), is essentially pure methane.
Table 2: Lower Heating Value (HHV) Of Various Common Fuels
Fuel  Phase   Molecular 
Weight		 kJ/mol  MJ/kg   MJ/m3   Btu/lb   Btu/ft3 
Hydrogen [1] gas 2.016 241.83 119.96 10.79 51,596 274
Methane [1] gas 16.043 802.32 50.01 35.80 21,511 909
Ethane [1] gas 30.069 1,427.84 47.49 63.70 20,424 1,618
Propane [1] gas 44.096 2,044.00 46.35 91.19 19,937 2,317
Butane [1] gas 58.122 2,658.45 45.74 118.61 19,673 3,013
Ethanol [3] liquid 46.0684 1,241.66 26.95 11,593
Gasoline [3] liquid 110 4,675.00 42.50 18,280
Kerosene [8] liquid 178 7,519.05 42.24 18,169
Diesel oil [8] liquid 225 9,395.99 41.76 17,961
Coal [8] solid 24.429 10,507
Wood (dry) [8] solid 20.09 8,639
Peat (dry) [8] solid 20.65 8,883
-- The gas temperature and pressure for the values of MJ/m3 are 0 °C and 101.325 kPa.
-- The gas temperature and pressure for the values of Btu/ft3 are 60 °F and 14.696 psia.
-- LPG is marketed as propane or butanes or a mixture of propane and butanes. 
-- Natural gas, after removal of impurities and natural gas liquids (NGL), is essentially pure methane.

Sources of additional heating values

  • Robert C. Weast (Editor) (1975). Handbook of Chemistry and Physics, 56th Edition. CRC Press. ISBN 0-87819-455-X. 

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 Perry, R.H. and Green, D.W. (Editors) (1997). Perry's Chemical Engineers' Handbook, 7th Edition. McGraw Hill. ISBN ISBN 0-07-049841-5.  Cite error: Invalid <ref> tag; name "Perry" defined multiple times with different content
  2. ↑ 2.0 2.1 2.2 2.3 NIST Chemistry WebBook
  3. ↑ 3.0 3.1 3.2 3.3 Heating Values of Hydrogen and Fuels U.S. Department of Energy
  4. ↑ 4.0 4.1 Average of various sources
  5. ↑ There are a great many different coals. The values given here are of a single, specific bituminous coal on an "as received" basis which includes the ash and inherent moisture content of the coal.
  6. ↑ Table 1: Higher heating values for wood (oven dried) The values in this table are the average of oven-dried woods from 29 different species of trees, as listed in this publication of the U.S. Forest Service Laboratory.
  7. ↑ Thermal and Catalytic Upgrading in a Fuel Context: Peat, Biomass and Alkenes Thesis by Christina Hornell, Chemical Engineering Dept., Royal Institute of Technology, Stockholm, Sweden, 2001
  8. ↑ 8.0 8.1 8.2 8.3 8.4 Estimated the difference between LHV and HHV
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