Properties of Natural Gas

Knut-Olav Færaas

Natural gas – different names

Naturgass: Energy rich gas found in underground oild fields, natural gas fieldsand in coal beds. Natural gas is normally found in a mix with heavier gases and liquids. The heavier components are separated on the oil rig or at a processingterminal.

LNG: Liquefied Natural Gas. Natural gas is cooled to liquid form at -162oC. Thisis done to facilitate bulk transport and storage. By doing this we are able to store 600 times the energi in the same volume compared to gas at standards conditions.

CNG: Compressed Natural Gas. Natural gas is compressed by increase ofpressure 2-300 Bar, also to facilitate bulk transport and storage. CNG has 2-300 times the energy in the same volume compared to gas at standard conditions.

Tabell 3.1 Sentrale tekniske data for metan og etan

Gasstype Kjemisk

formel

Typisk andel i

naturgass, %

Kritisk

temperaturoC

Brennverdi,

nedre, kWh/kg

Antennelseste

mperatur, oC

Eksplosjonsgre

nser i luft,

vol.%

Metan CH4 85-95 -83 13.9 645 5.1-13.5

Etan C2H6 5-15 32 13.2 530 3.1-11.7

Tabell 3.1 Sentrale tekniske data for metan og etan

Gasstype Kjemisk

formel

Typisk andel i

naturgass, %

Kritisk

temperaturoC

Brennverdi,

nedre, kWh/kg

Antennelseste

mperatur, oC

Eksplosjonsgre

nser i luft,

vol.%

Metan CH4 85-95 -83 13.9 645 5.1-13.5

Etan C2H6 5-15 32 13.2 530 3.1-11.7

Gasstype Kjemisk

formel

Typisk andel i

naturgass, %

Kritisk

temperaturoC

Brennverdi,

nedre, kWh/kg

Antennelseste

mperatur, oC

Eksplosjonsgre

nser i luft,

vol.%

Metan CH4 85-95 -83 13.9 645 5.1-13.5

Etan C2H6 5-15 32 13.2 530 3.1-11.7

GasstypeGasstype Kjemisk

formel

Kjemisk

formel

Typisk andel i

naturgass, %

Typisk andel i

naturgass, %

Kritisk

temperaturoC

Kritisk

temperaturoC

Brennverdi,

nedre, kWh/kg

Brennverdi,

nedre, kWh/kg

Antennelseste

mperatur, oC

Antennelseste

mperatur, oC

Eksplosjonsgre

nser i luft,

vol.%

Eksplosjonsgre

nser i luft,

vol.%

MetanMetan CH4CH4 85-9585-95 -83-83 13.913.9 645645 5.1-13.55.1-13.5

EtanEtan C2H6C2H6 5-15 5-15 3232 13.213.2 530530 3.1-11.73.1-11.7

Properties of natural gas components

Natural gass?

> Whats missing? What are the molecules called?

H

H H

HH H

HH

HH

Important properties

> Combustible gas = combustible when mixed withair

> Propane is combustible in a 2-10% mix with air.

> Natural gas is combustible in a 4-14% mix with air.

> The cleanest of the fossil fuels

> Odourless and colourless

> The gas is originally odourlessand is for safety precautionsadded scent(Ethylmerkapthan).

Natural gas properties

> Is the most applicable energy source today

> The cleanest fuel available today

> Able to compete with other energy sources

> Non-toxic – Lighter than air

> In Norway natural gas can quickly replace otherenergy sources which have greater impact onthe environment.

> Is a very safe and environmentally sound energysource

Different phases of substances

Example: ice – water - steam

> Normal state – phase at normal pressureand temperature

> Energy gases are normally in gas phase

> A gas is not stable by form or volume

Phase transitions

Drifter

Energigasser

Fast form

Væske

Gass

-

Boiling point

> Boiling for a gas is the phase transition from

liquid to gas phase

Kokepunkt -162 °°°°CFordampningsvarme

Kokepunkt 100 °°°°CFordampningsvarme540 kcal/kg (2,26 MJ/kg)

Kokepunkt -42 °°°°CFordampningsvarme90 kcal/kg (0,4 MJ/kg)

LNG/naturgass VannPropan

Kokepunkt -162 °°°°CFordampningsvarme

Kokepunkt 100 °°°°CFordampningsvarme540 kcal/kg (2,26 MJ/kg)

Kokepunkt -42 °°°°CFordampningsvarme90 kcal/kg (0,4 MJ/kg)

LNG/naturgass VannPropan

Vapor pressure - methane

> Vapor pressure is the pressure of a vapor in equilibrium with its non-vapor phases.

> Methane has a steaper vurve than C3 and C4

> What is the pressure at –125°C?

Damptrykkurve for metan

0

5

10

15

20

25

30

35

40

-170 -160 -150 -140 -130 -120 -110 -100 -90

Temperatur, grd C

Metn

ing

str

ykk,

bar

Mass and density

> Mass is the weight of all molecules in a

substance.

> Density is the mass devided by the volume

Fig. Samme antall molekyl (samme masse) – tetthet per volumenhet vil endres

Relative density / Specific gravity

> Relative density is the ratio of the density relative to the density ofair or water

> Gas phase (temperature 15°C and atmospheric pressure)> Air = 1> Propane = 1,55> Natural gas = 0,6

> Liquids> Water = 1> LPG = 0,54> LNG = 0,45

Explosion limits

Eksplosjonsgrenser for gasser i luft

Acetylen

Nafta

Butan

Propan

Etan

Metan

0 20 40 60 80 100 120

1

2

3

4

5

6

Volumprose nt gass i luft [%] UEL propane 10%

Butane 12%

Methane 14%

LEL propane 2%

Butane 2%

Methane 4%

1 m3 Propane gas

Oxygen 22%

Nitrogen 78%25 m3 Exhaust

3 m3 CO2

4 m3 Water vapor24 m3 Air

19 m3 Nitrogen

27 kWh

Combustion