Combustion of gaseous fuels - its characteristics

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

21. APPARATUS TO TEST FLAMMABILITY LIMITS

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

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