NATURAL GAS PROPERTIES

The properties of natural gas can be divided in Physical and Chemical properties. 

While the physical properties describe the physical aspect of the fuel, its state, composition, weight, colour and odour, the chemical properties describe the characteristics of the fuel's combustion properties. 

Physical Properties

Natural gas is gaseous at any temperature over -161 C (-258 F). Since that is a very cold temperature, we normally consider natural gas as a gas. Natural gas boils at atmospheric pressure and a temperature of -161 C, exactly like water turns into a gas (steam) at +100 C. Because of this property, natural gas is transmitted and stored as a gas, even when compressed at 200 Bar (3,000 psi) inside an automobile cylinder. The volume of natural gas is measured in cubic metres (m3) or cubic feet (cu.ft. or cf); its flow in m3/hr or cu.ft./hr or cfh. 

For automotive purposes, the fuel is measured by weight (in kilograms) when sold. Although the measurement
of kilograms is very accurate, the conversion to Rupees/litre is only meant to give an approximate idea as to
the quantity of useful energy compared to gasoline pumped into the vehicle. It is not to be taken as an
accurate means of measurement. 

In its pure state, natural gas is odourless, colourless, and tasteless. For safety reasons, however, an odourant called Mercaptan is added, so that any leak can be easily detected because of the typical smell.

The composition of natural gas is never constant. However, Methane is by far the largest component, its presence accounting for about 95% of the total volume. Other components are: Ethane, Propane, Butane, Pentane, Nitrogen, Carbon Dioxide, and traces of other gases. Very small amounts of sulphur compounds are
also present. Since methane is the largest component of natural gas, we generally use the properties of methane when comparing the properties of natural gas to other fuels.

Methane is a simple hydrocarbon, a substance consisting of carbon and hydrogen. There are many of these compounds, and each has its own number of carbon and hydrogen atoms joined together to form a particular hydrocarbon gas or fuel gas. 

Methane is a very light fuel gas. If we increase the number of hydrogen and carbon atoms, we have
progressively heavier gases, releasing more heat - therefore more energy - when ignited. For this reason the heat content of butane, for instance, is greater then that of propane, and propane has more energy than methane per unit of volume. 
Specific gravity of a gas is defined as the weight of a given volume gas compared to the weight of the same amount of air at the same temperature and pressure, where air weight is taken 
as reference (= 1). 

Specific gravity of air = 1.00

Specific gravity of methane = 0.55

Specific gravity of natural gas = typically 0.60

Specific gravity of propane = 1.56

Specific gravity of butane = 2.00

This means that natural gas will rise if escaping, thus dissipating from the site of a leak. This important characteristic makes natural gas safer than most fuels. 

Natural gas does not contain any toxic component, therefore there is no health hazard in handling of the fuel. Heavy concentrations, however, can cause drowsiness and eventual suffocation. 

Chemical Properties

The air-to-fuel ratio (AFR) indicates the amount of air relative to the amount of fuel used in combustion. The minimum amount of air relative to fuel for complete combustion is called the stoichiometric ratio. The stoichiometric ratio for natural gas (and most gaseous fuels) is normally indicated by volume; it is not to be confused with the weight ratio often indicated for gasoline (14.7:1). 

The air to natural gas (stoichiometric) ratio by volume for complete combustion is 9.5:1 to 10:1. This ratio is
not exact because of the slight variations in fuel composition and engine configuration. 

Ideally, complete combustion means the total oxidation of the fuel without residual pollutants. 

The figure shows how two cubic meters of oxygen oxidize one cubic metre of methane to create one cubic
metre of carbon dioxide and two cubic metres of water vapour. In reality, air is not made up of just oxygen (nitrogen being 70% of it) and natural gas consists of more than just methane (see composition). As well,
other chemical reactions occur spoiling the perfect model of 9.5:1.

Combustion

In the engine we want to achieve a rapid combination of oxygen and fuel (oxidation) so as to create a release
of heat (combustion). Combustion is the result of the ignition of air-fuel mixture after accumulation in a combustion chamber, resulting in the release of heat. The heated air expansion drives the pistons which
creates torque which is converted to rotary energy delivered to the wheels. Depending on efficiency, approximately 15 to 30% of the heat created is converted to torque and the remaining portion wasted.

The ingedients for combustion are often shown using the classic combustion triangle with fuel, oxygen, and ignition each forming a side. Triangle A is exactly the same as triangle B; however, in an Internal Combustion engine we refer to triangle B more often, since we know that the heat release is initiated by a spark plug. An ignition temperature of 1100 F to 1200 F or 593 C to 649 C is required to initiate combustion. The range of flammability is the upper and lower percentage of a gas in an air-fuel mixture within which the mixture can
burn or explode. The range of flammability is determined by the Lower Explosive Limit (LEL) and the Upper Explosive Limit (UEL). 

For natural gas, the LEL is 4%, while the UEL is 14%. It means that a natural gas mixture ignites within a
range of 25:1 to 7:1 air-to-fuel ratio by volume. By comparison, a propane mixture ignites within a range 2%
LEL to 10% VEL. 

Burning speed is the speed at which flame travels through an air-fuel mixture. Burning speed is also called ignition velocity or flame velocity. 

Hydrogen gas itself is the fastest with 2.8 metres/second (m/s) at atmospheric pressure. 

Natural gas has a very slow flame velocity: only 0.290 m/s at its highest. 

An air-natural gas mixture of 0 to 4% is too lean to burn, thus the burning speed is zero. A mixture of 15 to
100% is too rich, so the burning speed is again zero. Only when we enter the range of flammability (4 to 14%)
can good combustion efficiency be achieved.

At the bottom part of this range, flame velocity is low, but it increases rapidly with richer mixtures, reaching
its peak around Stoichiometric. Flame velocity falls off again when the mixture gets richer, from 11 to 14% gas. 

Ignition velocity of other gases vary with air-gas mixture in the same way. Peak flame velocities occur in stoichiometric mixtures. The percentage of these gases in air is of course different than natural gas. Burning speed increases when air-fuel mixture is heated. 

The energy contents of a gas is the amount of British Thermal Units (Btu) per unit of volume at the same pressure and temperature. 

A Btu is the amount of heat required to increase the temperature of one pound of distilled water 1 F at 70 F. 

The more carbon and hydrogen atoms in the molecule of a hydrocarbon-based fuel, the higher its energy content. Natural gas has an energy contents of about 1000 Btu per cubic foot at atmospheric pressure. By comparison, propane is 2500 Btu per cubic foot and butane 3200 per cubic foot. 

Natural gas has a very high research octane number, approximately 130. By comparison, propane is approximately 105, and gasoline 92 to 94 at best. This means that a higher compression ratio engine can be used with natural gas than gasoline. Indeed, many race cars use the high octane rating of natural gas to give them more power. 

The pressure of a gas is the force being exerted by its molecules against the walls of a container. Pressure is measured as a force per unit area; this pressure is equal in all directions (Pascal's Law). The pressure of a gas
is determined by the number of molecules of the gas existing inside the container. The more molecules
existing per unit volume, the greater the pressure, since there are more molecules colliding with the
container's walls. The pressure will also increase by reducing the volume of the container, while the amount of gas remains the same (i.e.: during the compression stroke). 

Flow is the motion of a gas from a higher to a lower pressure zone. There is resistance to flow, as there is resistance to electricity when it passes through a wire. As the molecules of the gas move through a pipe, some lose a certain amount of their energy because of the friction against the sides of the pipe. This creates a drop
in flow. It is important to remember this concept when we deal with the analysis of fuel delivery of carburation equipment.