Combustion of gaseous fuels, such as natural gas (methane), propane, butane, and hydrogen, involves the reaction of these gases with oxygen to produce heat, light, and combustion products. The combustion process of gaseous fuels exhibits several distinctive characteristics compared to solid or liquid fuels
1. COMBUSTION OF GASEOUS FUELS
BY
ER. T. AYISHA NAZIBA, DR. D. RAMESH, DR. S. PUGALENDHI
2. There are numerous factors which need to be taken into account when selecting a fuel for any give
application
Gas fuels are the most convenient require least amount of handling and simplest and most maintenance
free burner systems
Gas is delivered "on tap" via a distribution network and so is suited to a high population or industrial
density
However, large consumers do have gas holders and some produce their own gas
3. ADVANTAGE'S
Can be produced at a central location and clean gas can be distributed over a wide area
Greater control of variation in demand, conditions of combustion and nature of flame and heating atmosphere
possible
Greater economy by use of efficient heat exchange methods possible
Gaseous fuels require far less excess air for complete combustion
The supply of fuel gas and hence the temperature of furnace is easily and accurately controlled
The high temperature is obtained at a moderate cost by pre-heating gas and air with heat of waste gases of
combustion
They are directly used in internal combustion engine
They are free from solid and liquid impurities
They do not produce ash or smoke
They undergo complete combustion with minimum air supply
4. DRAWBACKS IN USING GASEOUS FUEL
They are readily inflammable
Its high specific volume results in displacement of air in a premixed combustion systems
Hence power produced with gaseous fuels is less when compared to solid and liquid fuels
Due to its high specific volume, gaseous fuel containers are much larger than those for liquid fuels
5. CLASSIFICATION OF GASEOUS FUELS
A. Fuels naturally found in nature
Natural gas
Methane from coal mines
B. Fuel gases made from solid fuel
Gases derived from coal
Gases derived from waste and biomass
Other industrial processes (Blast furnace gas)
C. Gases made from petroleum
Liquefied Petroleum gas (LPG)
Refinery gases
Gases from oil gasification
D. Gases from fermentation process
When deciding whether an alternative gas can be used in an appliance, three factors must be considered
For the same pressure drop, the heat release is roughly same
For the same air and fuel flows, the flame shape is same
For the same heat release conditions, are the pollutants within a specified tolerance
6. NATURAL GAS
Natural gas is obtained from deposits in sedimentary rock formations which are also sources of
oil.Naturally occurring gas is found in oil fields and coal fields
It is extracted from production fields and piped (at approx. 90 bar) to a processing plant where
condensable hydrocarbons are extracted from the raw product
May be found with (associated) or without (unassociated) crude oil
Contains 60 to 90% methane, rest are propane, butane, heavier and more complex hydrocarbons, carbon
dioxide and nitrogen, some helium
The quantities of the constituents vary but the principle component is methane
Other components include higher hydrocarbons which can be separated out as a condensate
7. Certain processes have to be carried out
Separation of liquid and gas: Liquid may be a hydrocarbon present in the gas well along with the gas
Dehydration: Water is corrosive and hydrates may form which will plug the flow. Water will also reduce
the calorific value of the gas
Desulfurization : Presence of hydrogen sulfide is undesirable. The gas is called sour. When the sulfur is
removed the gas is sweetened
8. Natural gas may be used as
Liquefied Natural Gas (LNG)
Compressed Natural Gas (CNG)
Natural gas when made artificially it is called substitute or synthetic or supplemental natural gas (SNG)
9. Natural gas has 90-95% methane with 0-4% nitrogen, 4% ethane and 1-2% propane
Methane is a green house gas with a global warming potential approximately 4 times that of carbon
dioxide
Its C/H ratio is lower than that of gasoline so its CO2 emissions are 22-25% lower (54.9 compared to 71.9
g CO2/MJ fuel)
Combustion of methane is different from that of liquid HC combustion since only C-H bonds are
involved
There are no C-C bonds involved so the combustion process is more likely to be complete thereby
producing less non-methane HC emissions
Optimal thermal efficiency occurs at = 1.3 -1.5
10. ADVANTAGES
Particulate emissions are very low relative to diesel fuel
Lower adiabatic flame temperature (~2240ºK) compared to gasoline (~2310ºK) due to its higher product
water content giving lower NOx
Operating under lean conditions will also lower peak combustion temperatures giving lower NOx
11. SYNTHETIC GASES
These are gases which are chemically made by some process
Increased interest presently in power generation due to the gasification properties of waste and biomass
12. MAIN METHODS OF SYNTHESIS – PRODUCER GAS
The gas is produced by blowing air and sometimes steam through an incandescent fuel bed (the process
is self heating)
The reaction with air is exothermic but insufficient air is added hence CO is produced
Steam addition results in the formation of hydrogen by the water gas reaction
This is endothermic and hence balances out the exothermic air reaction
Producer gas is low CV and is hence is only usually used on site
13. OIL GAS
This is the gas formed by the thermal cracking of crude oil
If oil is sprayed on to the heated refractory work, it cracks to form lower gaseous hydrocarbons
These depend entirely on the feed stock but calorific values can increase to as much as 25MJ/m3 but can
be as low as half of this
14. BLAST FURNACE GAS
Blast furnace gas is a by-product of blast furnaces that is generated when the iron ore is reduced with
coke to metallic iron
It has a very low heating value, about 93 BTU/cubic foot because it consists of about 60 % nitrogen, 18-
20% carbon dioxide and some oxygen which are not flammable
The rest is mostly carbon monoxide which has a fairly low heating value
It is commonly used as a fuel within the steel works, but it can be used in boilers and power plants
equipped to burn it
15. CARBURETED WATER GAS
Water gas has still low CV for most purposes
Carbureted water gas is the result of combining the water gas and oil gas methods
Oil is sprayed into the hot water gas chamber to result in a good quality gas
The ratio of the two determines the quality
This was the method used to produce the "Town gas" of old and has largely been superseded by natural
gas in countries with an abundant supply
As supplies of natural gas diminish, however, it will become more important again
16. REFINERY GAS
Refinery gas is a mixture of gases generated during refinery processes which are used to process crude
oil into various petroleum products which can be traded or sold
The composition of refinery gas varies, depending on the composition of the crude it originates from
and the processes it has been subjected to
Common components include butanes, butylenes, methane, ethane and ethylene
Some products found in refinery gas are subject to controls as a result of programs which are designed
to address climate change
17. LIQUEFIED PETROLEUM GAS
Liquefied petroleum gas is one of the most common and an alternative fuels used in the world today
Liquefied petroleum gas is also called as LPG, LP Gas, or Auto gas
The gas is a mixture of hydrocarbon gases used as a fuel for various purposes
This is mainly used in heating appliances and vehicles and is replacing chlorofluorocarbons as an aerosol
propellant
It is also used as a refrigerant mainly to reduce damage to the ozone layer
LPG is a petroleum-derived product distributed and stored as a liquid in pressurized containers
LPG fuels have slightly variable properties, but they are generally based on propane (C3H8) or the less volatile
butane (C4H10)
Compared to other gaseous fuels, commercial propane and butane have higher calorific values (on a
volumetric basis) and higher densities
Both these fuels are heavier than air, which can have a bearing on safety precautions in some circumstances
18. COAL OR COKE OVEN GAS
It is also known as a town gas
It is obtained by the carbonization of coal and consists mainly of hydrogen, carbon monoxide and various
hydrocarbons
The quality of coal gas depends upon the quality of the coal used, temperature of the carbonization process
and the type of plant
It is very rich among combustible gases and is largely used in towns for street and domestic lighting and
heating
It is also used in furnaces and for running gas engines
Its calorific value is about 21,000 to 25,000 kJ/m3
These are cleaned, de-tarred and scrubbed and used as fuel. If coke is not required (coal gas), steam injection
at the end of the cycle reacts with the coke to form blue water gas
This reduces the CV of the gas produced but the thermal efficiency of conversion rises
19. MOND GAS
It is produced by passing air and a large amount of steam over waste coal at about 650°C
It is used for power generation and heating
It is also suitable for use in gas engines
Its calorific value is about 5850 kJ/m3
20. FLAMMABILITY LIMITS
Gaseous fuels are capable of being fully mixed (at a molecular level) with the combustion air
However, not all mixtures of fuel and air are capable of supporting, or propagating, a flame
Imagine that a region of space containing a fuel/air mixture consists of many small discrete (control) volumes
If an ignition source is applied to one of these small volumes, then a flame will propagate throughout the
mixture if the energy transfer out of the control volume is sufficient to cause ignition in the adjacent regions
Clearly the temperature generated in the control volume will be greatest if the mixture is stoichiometric, where
as if the mixture goes progressively either fuel-rich or fuel-lean, the temperature will decrease
When the energy transfer from the initial control volume is insufficient to propagate a flame, the mixture will
be nonflammable
This simplified picture indicates that there will be upper and lower flammability limits for any gaseous fuel, and
that they will be approximately symmetrically distributed about the stoichiometric fuel/air ratio
22. BURNING VELOCITY
The burning velocity of a gas-air mixture is the rate at which a flat flame front is propagated through its
static medium, and it is an important parameter in the design of premixed burners
A simple method of measuring the burning velocity is to establish a flame on the end of a tube similar to
that of a laboratory Bunsen burner
When burning is aerated mode, the flame has a distinctive bright blue cone sitting on the end of the
tube
The flame front on the gas mixture is traveling inwards normally to the surface of this cone
23. WOBBE NUMBER
This characteristic concerns the interchangeability of one gaseous fuel with another in the
same equipment
In very basic terms, a burner can be viewed in terms of the gas being supplied through a
restricted orifice into a zone where ignition and combustion take place
The three important variables affecting the performance of this system are the size of the
orifice, the pressure across it (or the supply pressure if the combustion zone is at ambient
pressure) and the calorific value of the fuel, which determines the heat release rate
If two gaseous fuels are to be interchangeable, the same supply pressure should produce the
same heat release rate
This ratio is known as the Wobbe number of a gaseous fuel and is defined as
3
0.5
Gross calorific value (MJ/m )
Relative density (air=1)