Professionals in the oil and gas industry are continually dealing with energy expressions. The Wobbe Index may be a frequent reference, but most people can’t quote you the Gross BTU of propane off the top of their heads or give you a precise definition of Inferior Heating Value. That’s why the Analytic Experts at Rosemount Analytical are bringing you a Review of Gas Energy Expressions in today’s blog. Check it out, save it, print it. It’s your handy reference guide.
Gross Heating Value (also referred to as higher heating value [HHV]): The heating value (Btu) produced by combustion at constant pressure with the following conditions:
(a) a volume of one cubic foot
(b) 60° Fahrenheit
(c) reference base pressure
(d) with air and gas having the same temperature and pressure
(e) recovered heat from the water vapor formed by combustion
Net Heating Value (also referred to as lower heating value [LHV]): The heating value produced under conditions similar to gross heating value conditions excepting the amount of heat potentially recovered from the water vapor produced at combustion. Net heating value is always less than gross heating value.
The Relationship of Gross Heating Value and Net Heating Value
- The hydrocarbons combine with oxygen during combustion and these reactions provide the heat. When the hydrogen combines with oxygen, it forms water in a gaseous or vapor state at the high temperature of the combustion. The resulting formation of water is mostly carried away with the other products of combustion in the exhaust gases from the equipment where the gases are combusted (calorimeter, boiler, furnace, etc.). When the exhaust gases cool, the water will condense out and transform into a liquid state and release heat, known as latent heat, which is wasted in the atmosphere. The heating value of a fuel may be expressed as a gross value or a net value.
- The gross heating value includes all of the heat released from the fuel, including any carried away in the water formed during combustion.
- The net heating value excludes the latent heat of the water formed during combustion.
- The differences between gross and net heating values are typically 10% for natural gases, solid and liquid fuels.
- There are a few fuels that contain little or no hydrogen (for example, blast furnace gas, high-temperature cokes and some petroleum cokes). In these cases there will be negligible differences between gross and net heating values.
- The net calorific value of a process stream gas is the total heat produced by complete stoichiometric combustion, less the heat needed to evaporate the water present in the gas or produced during its combustion.
Relative Density: The ratio of the density of a gas to the density of dry air under the same pressure and temperature conditions (it’s sometimes referred to as specific gravity), relative to 1.0000 (air).
CARI value: Combustion air requirement index, the amount of air required for complete stoichiometric combustion of the fuel. CARI directly relates to Wobbe. It is the most common value used for air/fuel control.
Wobbe Index: The ratio of the gross heating value of a gas to the square root of the relative density of the gas (WI = Hv /Ö RD).
- Wobbe Index is a measure of the amount of energy delivered to a burner via an injector (orifice). The energy input is a linear function of Wobbe index.
- Two gases differing in composition but having the same Wobbe Index will deliver the same amount of energy for any given injector/orifice under the same injector pressure.
- In natural gas appliances, the gas flow is restricted by passing it through an orifice (hole). The Wobbe Index is useful because for any fixed orifice size and gas pressure, any gas compositions that have the same Wobbe number will deliver the same amount of heat energy or expressed as interchangeability of varying gas compositions.