Natural Gas Characteristics

What is Natural GasNatural gas is a fossil fuel derived from hydrocarbons produced thousands of years ago when plant andanimal materials were buried beneath layers of soil and rock. It is mainly composed of methane (CH4), acolourless, odourless and non-toxic substance. Methane is also produced naturally from sources such asswamps and ruminants (e.g. cattle and sheep).

BC Gas’ natural gas comes primarily from wells located in northeastern BC. After removal of thecontaminants, natural gas is only slightly odorous. As a safety measure, an odourant called mercaptan (asulphur-based compound) is added to the gas before it is fed into the distribution system. Only traceamounts of odourant (approximately 10mg/m3 or 4ppm) are needed to give natural gas its familiar odour.

Typical Analyses and Heating Values of Natural GasGas heating value in BC varies with the geographic location. It varies from a higher heating value (HHV)of 39.21 MJ/m3 (1052.8 Btu/cf) to 37.55 MJ/m3 (1008.2 Btu/cf).

Notes:1. Since values in the table represent only the average, maximum and minimum for values in each

column, the sum of natural gas constituents does not add to 100%.

2. Natural gas is analyzed at several points throughout the transmission system. From daily samples themonthly and yearly averages are calculated.

Natural Gas Characteristics and Combustion

COMPOSITION – % VOLUME

Carbon RelativeMethane Ethane Propane Butane Dioxide Nitrogen Density HHV HHV

CH4 C2H6 C3H8 C4H10 CO2 N2 air=1 MJ/m3* Btu/cf†

Average 95.52 2.627 0.441 0.136 0.40 0.74 0.580 38.58 1035.8Maximum 98.86 5.135 1.522 0.155 0.64 1.21 0.591 39.21 1052.8Minimum 93.33 0.225 0.028 0.017 0.01 0.43 0.563 37.55 1008.2

* At 15˚C, 101.325 kPa, dry (Metric Standard Conditions)† At 60˚C, 14.73 psia, dry (Imperial Standard Conditions)

Natural Gas Utilization

Natural gas, due to its unique molecular structure, is the cleanest burning fossil fuel. The efficiency andemissions from burning natural gas will vary depending on the application. Equipment-specific data shouldbe used for energy calculations.

Typical conventional boilers efficiencies:Industrial gas boiler 75 – 85%Commercial gas boiler 65 – 80%

Combustion characteristics of natural gas:Ignition point 590 to 650˚C (1100 – 1200˚F)Flammability Limits 4.3 – 15% (% volume natural gas in air)Theoretical flame temperature 1940˚C (3525˚F)Maximum flame velocity (in air) 0.30 m/s (1 ft/s)†Specific volume (15˚C, 101.325 kPa) 1.443 m3/kg (23 cf/lb)Relative density (Specific gravity) 0.58 (average, relative to air)Boiling point (of liquified natural gas) -162˚C (-260˚F)Combustion products CO2, H2O, heat, trace of NO2 and SO2

†Calculated from natural gas sample with higher heat value of 38.6 MJ/m3.

Air for natural gas combustion:Theoretical (stoichiometric) requirement forcomplete combustion 10/1 (approximate, air/gas volume ratio)Specific volume (dry, 15˚C, 101.325 kPa) 0.816 m3/kg (13.07 cf/lb)Air composition (% by volume) O2=20.99, N2=78.03, Inerts=0.98

Combustion with excess airIn practice, combustion is hardly ever carried out in stoichiometric conditions. Some industrial burnersmay operate at air/gas ratios which are extremely close to the theoretically correct value, but the majority ofburners require a measure of air in excess of the stoichiometric quantity to ensure combustion is carriedthrough to completion.

Typical excess air:For gas boiler (power) burners: 5 – 20% (by volume)For atmospheric burners: 20 – 30%

Combustion of a Unit Volume

Combustion is a chemical reaction combining fuel and oxygen to produce heat and combustion products.Atmospheric air contains 21% oxygen (by volume) and is the most convenient O2 source. Stoichiometriccombustion conditions are those where the relative fuel and air quantities are the theoretical minimumneeded to produce complete combustion.

The combustion reaction can be represented by way of a chemical equation for each constituent present inthe fuel:

Calculation of Stoichiometric Combustion Products From One Volume (m3 or cf)From a sample with HHV = 38.6 MH/m3 (1036.4 Btu/cf) which can be considered a typical average.

Hence for stoichiometric combustion of the above natural gas sample we have:Theoretical air requirement: 9.727 m3 of air/m3 of gasWet combustion products: 1.032 + 2.023 + 7.869 = 10.744 m3/m3 of gasDry combustion products: 1.032 + 7.689 = 8.721 m3/m3 of gas

Note: All volumes at standard conditions 15˚C, 101.325 kPa, dry.

Methane CH4 + 2 O2 —> CO2 + 2 H2O

Ethane C2H6 + 3.5 O2 —> 2 CO2 + 3 H2O

Propane C3H8 + 5 O2 —> 3 CO2 + 4 H2O

Butane C4H10 + 6.5 O2 —> 4 CO2 + 5 H2O

Pentane C5H12 + 8 O2 —> 5 CO2 + 6 H2O

Hexane C6H14 + 9.5 O2 —> 6 CO2 + 7 H2O

Constituent Stoichiometric Combustion Products

O2/Air CO2 H2O N2

m3 m3 m3 m3 m3

CH4 0.95830 x 2 = 1.917 x 1 = 0.958 x 2 = 1.917

C2H6 0.02575 x 3.5 = 0.090 x 2 = 0.052 x 3 = 0.077

C3H8 0.00446 x 5 = 0.022 x 3 = 0.013 x 4 = 0.018

C4H10 0.00166 x 6.5 = 0.011 x 4 = 0.007 x 5 = 0.008

C5H12 0.00040 x 8 = 0.003 x 5 = 0.002 x 6 = 0.002

C6H14 0.00004 x 9.5 = 0.000 x 6 = 0.000 x 7 = 0.000

CO2 0.00208 — 0.002 —

N2 0.00738 — — — 0.007

O2 = 2.043

N2 = O2 x 3.76 = —> — — — — — — — — — — — — — ——> 7.682

Air = O2 x 4.76

1.000 Air 9.727 1.032 2.023 7.689

Combustion of One Gigajoule

Stoichiometric Combustion Products From One Gigajoule of Natural GasFor typical average natural gas with higher heat value or 38.6 MJ/m3. All volumes at standard conditions:15˚C, 101.325 kPa, dry.

Natural Gas Billing

Natural Gas SalesIn Canada and the USA gas is sold on a higher heating value (HHV) basis. The billing is either in energyor “standard” volume units. In British Columbia and Alberta the SI metric unit “gigajoule” is used forbilling while in the rest of Canada gas is sold in “standard” cubic metres. Western Europe (including UK)now sells gas by the kilowatt hour. The SI metric “standard” m3 of gas is at 15˚C and 101.325 kPa, dry.

The US gas industry uses Btu, its multiples (e.g. therm) and also volumetric measure — the “standard”cubic foot. From the six different Btu’s presently in use, only one is defined exactly with the help of a joule— the Btu IT (International Table). In the US it is the sales contract, or government regulations whichspecify the Btu to be used in a particular transaction. The “standard” reference conditions were establishedin 1969 at 14.73 psia and 60˚F. However, many state laws prescribing other pressure and temperature baseconditions for fuel gas measurements still exist.

Product per Gigajoule Volume Mass Density Mass (Weight)

m3/GJ kg/m3 kg/GJ

Natural Gas and Combustion Air

Natural gas, one gigajoule 25.9 x 0.6928 = 17.9

Combustion air, dry, per GJ 252 x 1.225 = 308.7

Combustion Products

CO2 per GJ 26.7 x 1.861 = 49.8

N2 per GJ 199.2 x 1.185 = 236

H2O per GJ 52.4 x 0.762 = 40

Total Combustion Products

Wet combustion products per GJ 278.4 x 1.170 325.8

Dry combustion products per GJ 225.9 — 285.8

SO2 per GJ: (Calculated from the average sulphur content of 20 mg/m3 or 15 ppm) approx. 1g SO2/GJ

Definitions and Conversions

SI Metric Energy Units1. The SI unit of quantity of heat, energy and

work is the joule (J), which is equal to anewton metre (N•m), and to a watt second(W•s).

2 The joule provides one coherent unit tosupersede a large number of traditional units:Btu, therm, calorie, ft.-lbf, hp-h, etc.

3. The preferred multiples for commercial gastransactions on an energy basis are themegajoule (MJ) and gigajoule (GJ).

4. The kilowatt hour is widely used as a measureof electric energy. The accepted symbol inNorth America is “kWh”, but theinternational symbol is “kW•h”.

Selected SI Prefixes(Also apply to earlier metric systems)

Heating Value Definition• Higher Heating Value (HHV)

Gross Heating ValueTotal Heating ValueGross Calorific ValueTotal Calorific Value

These terms are equivalents. HHV is a term inthe metric SI and is now used more frequently.The HHV is the quantity of heat produced bythe combustion of a unit (volume, mass oramount of substance) of gas in air under constantpressure, after cooling of the combustionproducts to the initial temperature of the gas andair, and after condensation of the water vapourcreated by the combustion to the liquid state.

• Lower Heating Value (LHV)Net Heating ValueNet Calorific Value

Lower (net) heating value is the higher (gross)heating value minus the latent heat ofvaporization of the water vapour formed by thecombustion of the hydrogen in the fuel. A “ruleof thumb” for natural gas is HHV=LHV x 1.11.

PREFIX SYMBOL MULTIPLYING FACTOR

exa E 1 000 000 000 000 000 000 = 1018

peta P 1 000 000 000 000 000 = 1015

tera T 1 000 000 000 000 = 1012

giga G 1 000 000 000 = 109

mega M 1 000 000 = 106

kilo k 1 000 = 103

Conversions

Energy

1 Btu (International Table) = 1.055 056 kJ 1 GJ = 947.82 MBtu IT ASME, ASTM, equipment manufacturers

1 Btu (59˚F, 15˚C) = 1.054 80 kJ 1 GJ = 948.05 MBtu59 AGA, US Government

1 Btu (60.5˚F) = 1.054 615 kJ 1 GJ = 948.21 MBtu60.5 CGA, Canadian Government

1 Btu (thermochemical) = 1.054 35 kJ 1 GJ = 948.45 MBtu th ASHRAE

1 kilowatt hour = 3.6 MJ 1 GJ = 277.78 kW•h Electric Energy and European Gas Industry

Power and Heat Flow

1 kilowatt (kW) = 3 412 BtuIT/h = 859.85 kcal IT/h

1 boiler horsepower = 33 475 Btu IT/h = 9.81 kW

1 ton refrigeration = 12 000 BtuIT/h = 3.52 kW

Corporate Head Office

1111 West Georgia Street,

Vancouver, B.C.

Canada V6E 4M4

Web Address: www.bcgas.com 8qrk-1000 07-11-01

For more information and technical assistance,please contact Bill Hennessey, Transportation andMarketing Services, (604) 592-7916.

This information herein is a guide only. BC Gas is not responsible forthe accuracy and cannot accept any legal responsibility for any errorsthat may be made.