User:Milton Beychok/Sandbox
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
The heating value or calorific value of a substance, usually a fuel or food, is the amount of heat released during the combustion of a specified amount of it. The calorific value is a characteristic for each substance. It is measured in units of energy per unit of the substance, usually mass, such as: kcal/kg, kJ/kg, J/mol, Btu/m³. Heating value is commonly determined by use of a bomb calorimeter.
The heat of combustion for fuels is expressed as the HHV, LHV, or GHV:
- 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.
- 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 H2O 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 boilers which condense the water vapor produced by combustion, recovering heat which would otherwise be wasted.
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
| 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. | |||||||
| 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.
- Perry, R.H. and Green, D.W. (Editors) (1997). Perry's Chemical Engineers' Handbook, 7th Edition. McGraw Hill. ISBN ISBN 0-07-049841-5.
- Selected Values of Heats of Combustion and Formation of Organic Compounds Containing the Elements C, H, N, O, P and SEugene S. Domalski, Chemical Thermodynamics Data Center, National Bureau of Standards, Washington, D.C., J. Phys. Chem. Ref. Data, Vol. 1, No. 2, 1972
- Lower and Higher Heating Values of Hydrogen and Fuels Hydrogen Analysis Resource Center, U.S. Department of Energy
References
- ↑ 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 - ↑ 2.0 2.1 2.2 2.3 NIST Chemistry WebBook
- ↑ 3.0 3.1 3.2 3.3 Heating Values of Hydrogen and Fuels U.S. Department of Energy
- ↑ 4.0 4.1 Average of various sources
- ↑ 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.
- ↑ 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.
- ↑ 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
- ↑ 8.0 8.1 8.2 8.3 8.4 Estimated the difference between LHV and HHV