Gaseous Fuels

Gaseous fuels are those which

are burnt in gaseous sate in air

or oxygen to provide heat

Some Types of Gaseous Fuels Natural gas Liquefied Petroleum Gas (LPG) Producer Gas Coal Gas Gobar Gas Blast Furnace Gas Water Gas Refinery Gas Hydrogen Acetylene

Liquefied petroleum gas (LPG)

Liquefied petroleum gas (LPG or LP

Gas) is a mixture of hydrocarbon

gases used as a fuel in heating

appliances and vehicles, and

increasingly replacing

chlorofluorocarbons as a refrigerant

to reduce damage to the ozone layer.

Liquefied petroleum gas (LPG)

Varieties of LPG bought and sold

include mixtures that are primarily

propane, mixtures that are primarily

butane, and the more common,

including both propane and butane

depending on the season—in winter

more propane, in summer more

butane.

Liquefied petroleum gas (LPG) Propylene and butylenes are usually also

present in small concentration. A powerful odorant, ethanethiol, is added so that leaks can be detected easily. LPG is usually derived from fossil fuel sources, being manufactured during the refining of crude oil, or extracted from oil or gas streams as they emerge from the ground.

Liquefied petroleum gas (LPG)

At normal temperatures and pressures, LPG will evaporate. Because of this, LPG is supplied in pressurized steel bottles. In order to allow for thermal expansion of the contained liquid, these bottles are not filled completely; typically, they are filled to between 80% and 85% of their capacity.

Liquefied petroleum gas (LPG) The pressure at which LPG becomes liquid, called

its vapour pressure, likewise varies depending on composition and temperature.

for example, it is approximately 220 kilopascals (2.2 bar) for pure butane at 20 °C (68 °F)

and approximately 2.2 megapascals (22 bar) for pure propane at 55 °C (131 °F). LPG is heavier than air sp. Gr. 1.9 (80 % butane 20 % propane)

Liquefied petroleum gas (LPG) The ratio between the volumes of the vaporised gas and the liquefied gas

varies depending on composition, pressure and temperature, but is typically around 250:1. The pressure at which LPG becomes liquid, called its vapour pressure, likewise varies depending on composition and temperature; for example, it is approximately 220 kilopascals (2.2 bar) for pure butane at 20 °C (68 °F), and approximately 2.2 megapascals (22 bar) for pure propane at 55 °C (131 °F). LPG is heavier than air, and thus will flow along floors and tend to settle in low spots, such as basements. This can cause ignition or suffocation hazards if not dealt with.

LPG is the lowest carbon emitting hydrocarbon fuel available in rural areas, emitting 19 per cent less CO2 per kWh than oil, 30 per cent less than coal and more than 50 per cent less than coal- generated electricity distributed via the grid.

LPG burns cleanly with no soot and very few sulfur emissions, posing no ground or water pollution hazards.

Large amounts of LPG can be stored in bulk tanks and can be buried underground if required. Alternatively, gas cylinders can be used.

LPG has a typical specific calorific value of 46.1MJ/kg compared to 42.5MJ/kg for diesel and 43.5MJ/kg for premium grade petrol (gasoline).

Water gasWater gas is a synthesis gas, containing carbon monoxide and hydrogen. It is a useful product but requires careful handling because of the risk of carbon monoxide poisoning. The gas is made by passing steam over red-hot coke:

C + H2O → CO + H2The reaction is endothermic so the coke must be continually re-heated to keep the reaction going. At low temperature the following reaction takes place

C + 2H2O → CO2 + 2H2

This is usually heated by alternating the steam stream with an air stream. Following exothermic reactions take palce

C + O2 → CO2

2C + O2 → 2CO

Carburetted water gas.

Water gas has a lower calorific value than coal gas so the calorific value can be boosted by passing the gas through a heated retort into which oil is sprayed. The resulting mixed gas is called carburetted water gas.

Producer Gas It comprises mainly of CO & N2. It is produced in a furnace called producer, by

blowing air or a mixture of air and steam through hot bed of solid fuels ( coal / coke ).

Producer gas made from coal or coke has the following composition & properties:

CO = 20 – 30 %

CH4 = 0 – 3 %

H2 = 11 – 20 %

CO2 = 4 – 6 %

N2 = 46 – 55 %

C.V. = 1250 – 1550 kcal/Nm3

Sp. Gr. = 0.85 – 0.90

Combustion Air Requirement = 1 – 1.3 Nm3/Nm3 of producer gas

Reactions in Gas Producer

Air-Carbon ReactionsC + O2 CO2

C + CO2 2CO Steam-Carbon Reactions

C + H2O CO + H2

C + 2H2O CO2 + 2H2

CO + H2O CO2 + H2

Methanation ReactionC + 2H2 CH4

Fuels for Producer Gas Manufacture Any solid carbonaceous fuel like

Wood-waste Peat Coke Coals of all ranks

Uses of Producer Gas In furnace of glass melting In open hearth furnace of steel

making In Coke oven heating In internal combustion (IC) engines

Gobar Gas Gobar gas is obtained by the

fermentation of gobar (cattle dung) in the absence of air.

The refuse material can still be used as a fertilizer.

It consists mainly CH4 and CO2. Two main products of gobar gas plant

are fuel gas and manure. A farmer with 5 or more cattle can

install a gobar gas plant.

Gobar Gas Typical composition & properties are shown

below: CH4 = 60 % CO2 = 30 % H2 = 9.5% N2 = 0.5% H2S & O2 = Trace

Ignition Temperature = 650 °C Octane Rating= 110 C.V. = 5400 kcal/Nm3

Explosion Limit in Air = 5 – 15 % Air/Gas Ratio for Complete Combustion=10 : 1

Blast Furnace Gas It is by-product of the iron blast furnace It is produced by the partial combustion of coke and

partial reduction of iron ore

Different reactions take place at different temperatures (for details see the book)

Yield: Typical Composition: CO = 22-25 %

CO2 = 16-20 % H2 = 4-5% N2 = 51-55 % O2 = 0.2- 0.5%

Blast Furnace Gas

Properties: It is very poisonous due to CO Low calorific value (800 -900 kcal/m3) Higher dust content (may choke the burner) High specific gravity (1.3 -1.4) Lower theoretical flame temperature (1450 C)

as compared to other fuel gases Wider explosion limits (37-71%) so more

danger of explosion

Wood Gas Wood gas is obtained either by

carbonization of wood in metal retorts or by gasification of wood.

It is a medium C.V. gas not of much commercial interest

It can be used in engines, stoves and furnaces by mixing with their proper fuels.

Theoretical air and Air-Fuel Ratio (a) Calculate the amount of air required for

theoretically complete combustion of coal with the composition:

C: 82 % H-6%, O2- 4% Ash- 8%(b) Calculate the amount of air required for the

complete combustion 100 m3 of Blast Furnace Gas of the following composition (by volume%)

CO2=17 CO= 22.1 H2 = 4.9 N2 = 55.8 O2=0.2(c) Calculate the volume of products of combustion

for part (b)(d) Calculate the volume of products of combustion

for part (b) if 10 % excess air is used

Combustion calculations Example: Octane is burnt with 10 %

excess air. Calculate (a) Air/Fuel Ratio by weight (b) Air/Fuel ratio by volume (c) Weight of dry exhaust gas formed per

unit weight of fuel (d) Volume of exhaust gas at 1 atm and

260 C per unit weight of fuel Sp. Gr. Of Octane may be assumed as 0.7

Combustion Calculations Example Nos. 5-38 (Self Study)

5, 6, 7, 8, 9,10, 11, 12,13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32,33, 34, 35,36, 37, 38