Energy Engineering Lecture

1. Gaseous fuels
Energy Engineering
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2. Introduction
Gaseous fuels are those which are burnt in gaseous state in air or oxygen to give heat for utilization in domestic/ commercial sector.
➢ A remarkable feature of gaseous fuels is absence of mineral impurities, consistency in quality and convenience and efficiency in
use.
➢ Some important gaseous fuels are
❑ Methane from coal mines i.e. CBM-coal bed methane
❑ Wood gas
❑ Gas from underground gasification of coal
❑ Natural gas
❑ Liquefied petroleum gas (LPG)
❑ Producer gas
❑ Water gas
❑ Blast furnace gas
❑ Coke oven gas
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3. 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
-From other industrial processes
(C) Gases made from petroleum
-Liquefied Petroleum gas (LPG)
-Refinery gases
-Gases from oil gasification
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4. Advantages of Gaseous Fuels
1. Can be produced at a central location and clean gas can be distributed
over a wide area.
2. Generally gases are free from ash.
3. Greater control of variation in demand, conditions of combustion
4. Complete combustion with no smoke.
5. Gaseous fuels require far less excess air for complete combustion.
6. Least amount of handling
7. Simplest burners systems
8. Burner systems require least maintenance
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5. Drawbacks in using gaseous fuel
1. Difficulty in storing.
2. Power produced with gaseous fuels is less when compared
to solid and liquid fuels.
3. Due to its high specific volume, gaseous fuel containers are
much larger than those for liquid fuels.
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6. 1. Natural Gas
❖ It is a mixture of paraffinic hydrocarbons, in which methane is the main constituent.
❖ After delivery from wells, it is processed to remove the solids.
❖ It is then treated for the recovery of gasoline and liquefied natural gas.
❖ When natural gas contains very less recoverable condensate (<15 gm/m3), it is
known as dry natural gas.
❖ If recoverable condensate is more (>50gm/m3), natural gas is termed as wet
natural gas.
❖ It is used as a fuel for cooking, domestic and industrial heating, process furnace
and boilers. It is also used for fertilizer production and as a source of C and H in
chemical industries
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Types of gaseous fuels

7. Natural gas
• Methane: 95%
• Remaining 5%: ethane, propane, butane, pentane, nitrogen, carbon dioxide, other
gases
• High calorific value fuel
• No sulphur
• Mixes readily with air without producing smoke or soot
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8. Natural Gas
❖ Origin is believed to be organic and not due to methanation of
carbon dioxide with hydrogen.
❖ May be found with (associated) or without (unassociated) crude
oil.
❖ The composition of a natural gas will vary according to where it
was extracted from, but the principal constituent is always
methane.
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9. Natural Gas
Certain processes have to be carried out.
1. Separation of liquid and gas. Liquid may be a hydrocarbon
present in the gas well along with the gas.
2. Dehydration. Water is corrosive and hydrates may form which
will plug the flow. Water will also reduce the calorific value of
the gas.
3. Desulfurization. Presence of hydrogen sulfide is undesirable.
The gas is called sour. When the sulfur is removed the gas is
sweetened.
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10. Natural Gas
There are some advantages also:
i. Particulate emissions are very low relative to diesel fuel.
ii. Lower flame temperature (~2240K) compared to gasoline (~2310K)
due to its higher product water content, giving lower NOx.
iii. Natural gas is a high calorific value fuel
iv. It mixes with air readily and does not produce smoke or soot
v. It is lighter than air and disperses into the air easily in case of leak
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11. Important Impurities
• Water
• Most gas produced contains water, which must be removed.
• Concentrations range from trace amounts to saturation.
• Sulfur species
• If the hydrogen sulfide (H2S) concentration is greater than 2 to 3%, carbonyl sulfide
(COS), carbon disulfide (CS2), elemental sulfur, and mercaptans may be produced.
• Naturally occurring radioactive materials
• Naturally occurring radioactive materials (NORM) [uranium and thorium] may also
present problems in gas processing.
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12. • Mercury
• Trace quantities of mercury may be present in some gases; levels reported
vary from 0.01 to 180 μg/Nm3.
• Because mercury can damage the heat exchangers used in cryogenic
applications, conservative design requires mercury removal to a level of 0.01
μg/Nm3
• Oxygen
• Some gas-gathering systems operate below atmospheric pressure. As a result
of leaking pipelines, open valves, and other system compromises, oxygen is
an important impurity to monitor.
• A significant amount of corrosion in gas processing is related to oxygen.
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13. Resources of Pakistan
Natural Gas
• Natural gas is used in domestic cooking
• As a raw material for fertilizer industry
• It is found in Sui, Attock and Kandhkot.
• Reserve in Pakistan 20.91 trillion cubic feet
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14. The World picture of Natural Gas
• Russia
• Iran
• Qatar
• Saudi Arabia
• United Arab Emirates
• United States
• Nigeria
• Algeria
• Venezuela
• Iraq
Six countries possess two thirds of the world’s gas reserves, with almost
half of the reserves located in Iran and Russia
Major gas producing countries:
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16. Benefits of Natural Gas over Coal and Oils
• Carbon dioxide production is 30 to 40% less in case of natural gas
as compared to oil and coal
• Nitrogen Oxides production is 20% less in case of natural gas as
compared to oil and coal
• Particulate formation is significantly less in gas
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17. Liquefied Petroleum Gas (LPG)
❖ LPG is a petroleum-derived product distributed and stored as a liquid in pressurized containers.
❖ LPG may be defined as those hydrocarbons, which are gaseous at normal atmospheric pressure, but may
be condensed to the liquid state at normal temperature, by the application of moderate pressures.
❖ Propane,butane and unsaturates, lighter C2 and heavier C5 fractions
❖ Hydrocarbons are gaseous at atmospheric pressure but can be condensed to liquid state
❖ C3 and C4 hydrocarbons are easily liquefied at room temperature with the application of low pressure.
❖ LPG vapor is denser than air: leaking gases can flow long distances from the source
❖ Generally, a mixture of about 80% butane and 20% propane is used for filling in LPG cylinders (also called
bottled gas).
Types of Gaseous Fuels
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18. LPG - Hazards & Precautions
• LPG vapor is denser than air, consequently, the vapor may flow along the
ground and into drains sinking to the lowest level of the surroundings and
be ignited at a considerable distance from the source of leakage.
• Escape of even small quantities of the liquefied gas can give rise to large volumes of
vapor / air mixture and thus cause considerable hazard.
• To aid in the detection of atmospheric leaks, all LPG’s are required to be
odorized. LPG is odorless, hence odorants like mercaptans or sulfides are
added to detect the leakage from cylinders by smell as mercaptans have
very pungent smell.
• There should be adequate ground level ventilation where LPG is stored.
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19. Water Gas
• It is a medium calorific value gas (about 2800 kcal/Nm3) comprising mainly of CO and H2. it is
prepared by the action of superheated steam on a bed of hot coke/coal at about 1000 °C as per
reaction
C + H2O → CO+ 2H2 H= 28 kcal/kmol
• This reaction is endothermic, so the coal cools down after a few minutes and the reaction
proceeds as
C + 2H2O → CO2 + 2H2 H=19000 kcal/kmol
• In order to avoid the above undesirable reaction, the current of steam is intermittently replaced
by a blast of air. The following reactions now occur
C+O2 → CO2 H = -97000 kcal/kmol
2C + O2 → 2CO H = -29000 kcal/kmol
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Types of Gaseous Fuels

20. • Thus due to the exothermic reaction, the T of carbon again rises and
when the T increases 1000 °C, air entry is stopped and steam is again
passed.
• Thus in modern gas plants, steam and air are blown alternatively.
• The period of steam blow (cold blow) is usually 4 minutes while the
period of air blow (hot blow) is very short (about 1-2 minutes). The
duration of these periods are adjusted in such a way that the
maximum yield of water gas is obtained.
• It is used as a fuel in furnaces. Normally it is enriched by adding
hydrocarbon gas, and the mixture is called carburetted water gas
which has high C.V.
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21. Synthesis Gas
• Syngas, or synthesis gas, is a fuel gas mixture consisting primarily of
Water gas (a mixture of carbon monoxide and hydrogen) and very often
some carbon dioxide.
• Production methods include:
• Steam reforming of natural gas or liquid hydrocarbons to produce hydrogen
• The gasification of coal or biomass
• Waste-to-energy gasification facilities
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Types of Gaseous Fuels

22. Syngas production
• When used as an intermediate in the large-scale, industrial synthesis of
hydrogen, it is produced from natural gas (via the steam reforming
reaction) as follows:
𝐶𝐻4 + 𝐻2𝑂 → 𝐶𝑂 + 3𝐻2
• In order to produce more hydrogen from this mixture, more steam is
added and the water gas shift reaction is carried out:
𝐶𝑂 + 𝐻2𝑂 → 𝐶𝑂2 + 𝐻2
• The hydrogen must be separated from the CO2 to be able to use it.
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23. Producer Gas
• It comprises mainly of CO and N2 and is produced by blowing air or a
mixture of air and steam through hot bed of solid fuels (coke/coal).
• Air is passed over the red-hot carbonaceous fuel and carbon
monoxide is produced.
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Types of Gaseous Fuels

24. Producer gas constituents
• Under ideal conditions:
• Carbon Monoxide (34.7%)
• Nitrogen (65.3%)
• Under normal conditions:
• Carbon Dioxide (5-15%)
• Carbon Monoxide (15-30%)
• Methane (2-4%)
• Hydrogen (10-20%)
• Water (6-8%)
• Nitrogen (45-60%)
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25. Blast Furnace Gas
• Blast furnace gas (BFG) is a by-product of blast furnaces that is generated when the iron ore is
reduced with coke to metallic iron.
• It consists of:
• Nitrogen (51-55%)
• Carbon dioxide (16-20%)
• Oxygen (0.2-.5%)
• Carbon monoxide (22-25%)
• Hydrogen (4-5%)
• Very poisonous gas due to the presence of CO in the gas. Blast furnace gas pipes and vessels
should be leak-proof.
• It has low calorific value.
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Types of Gaseous Fuels

26. Coke Oven Gas
• It is produced during high temperature carbonization of coking coal.
• The gas mainly consists of hydrogen (50-60%), methane (23-25%) and
a small percentage of carbon monoxide (7-9%), carbon dioxide (2-4%)
and nitrogen (2-3%).
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Types of Gaseous Fuels

27. Calorific value
• Fuel should be compared based on the net calorific value (NCV), especially
natural gas
Typical physical and chemical properties of various gaseous fuels
Fuel
Gas
Higher Heating
Value kCal/Nm3
Air/Fuel
ratio m3/m3
Flame
Temp oC
Flame
speed m/s
Natural
Gas
9350 10 1954 0.290
Propane 22200 25 1967 0.460
Butane 28500 32 1973 0.870
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