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EC > Chemical Fuels > Flashcards

Chemical Fuels Flashcards

1
Q

Fuel

A

Any substance used to produce heat or power by combustion. Examples include coal, wood, oil, and gas.

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2
Q

Combustion

A

A chemical process that releases light and heat. Fuels undergo combustion when they react with oxygen.

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3
Q

Combustion Reaction

A

Fuel + O2 → CO2 + H2O + Heat. This is a general equation that represents the reaction between a fuel and oxygen to produce carbon dioxide, water, and heat.

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4
Q

Combustion Products

A

CO2 and H2O. These are the main products released during the combustion of fuels.

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5
Q

Heat Liberation

A

During combustion, heat is released due to the rearrangement of valence electrons in the atoms.

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6
Q

Fuel Classification (Origin)

A

Primary Fuels (Natural Fuels): These are naturally occurring fuels that can be used directly to generate heat or power without significant processing.
Secondary Fuels (Manufactured Fuels): These are derived from primary fuels through processing or refinement. They are often more convenient or efficient to use than primary fuels.

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7
Q

What are some examples of primary and secondary fuels?

A

Primary Fuels: Wood, coal, crude oil, natural gas, peat, lignite, anthracite
Secondary Fuels: Charcoal, coke, producer gas, petrol, diesel, LPG (Liquefied Petroleum Gas), ethanol, biodiesel

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8
Q

Characteristics of a Good Fuel

A
  • Have moderate ignition temperature
  • Have low moisture content
  • Available in bulk at low cost
  • Should not burn spontaneously
  • Should burn efficiently without releasing hazardous pollutants
  • Easy handling, transportation and storage
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9
Q

Calorie (cal)

A

The amount of heat required to raise the temperature of 1 gram of water by 1 degree Celsius.

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10
Q

Kilocalorie (kcal)

A

A unit of heat equal to 1000 calories. It is often used to express larger amounts of heat energy, particularly in food labeling.

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11
Q

British thermal unit (Btu)

A

The amount of heat required to raise the temperature of 1 pound of water by 1 degree Fahrenheit.

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12
Q

Centigrade heat unit (C.H.U)

A

The amount of heat required to raise the temperature of 1 pound of water by 1 degree Celsius. (Less commonly used unit)

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13
Q

Unit Conversions

A

1 kcal = 1000 cal = 3.968 Btu = 2.2 C.H.U.
1 Btu = 252 cal = 0.252 kcal

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14
Q

Calorific value

A

It is defined as the total quantity of heat liberated when a unit mass of a fuel is burnt completely

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15
Q

Gross Calorific Value (HCV) / Higher Heating Value (HHV)

A

The total amount of heat released when a unit of fuel is completely burned in oxygen and the combustion products are cooled to room temperature. This includes the latent heat released by condensing water vapor in the exhaust. HCV represents the maximum theoretical heat output from a fuel.

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16
Q

Net Calorific Value (NCV) / Lower Heating Value (LHV)

A

The net heat available when a unit of fuel is completely burned and the combustion products are allowed to escape without condensing the water vapor. NCV represents the usable heat output from a fuel, as the water vapor typically carries away some energy in real-world applications.

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17
Q

Bomb Calorimeter

A

A bomb calorimeter is a device used to measure the higher calorific value (HCV) of a fuel.

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18
Q

Principle of Boy’s calorimeter

A

Boy’s calorimeter relies on the principle that all the heat generated by burning a fuel at a constant rate is absorbed by a constant flow of water surrounding the combustion chamber. The heat absorbed by the water equals the heat released by the fuel.

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19
Q

Construction of boy’s calorimeter

A

Combustion chamber
Copper water tube surrounding the chamber (with thermometers T1 & T2 attached)
Burner inside the chamber
Gas tube delivering fuel at a constant rate and pressure (3-4 L/min for gases)

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20
Q

Working of boy’s calorimeter

A

Water flows steadily through the copper tube.

Initial (T1) and final (T2) water temperatures are measured.

Fuel burns in the chamber, transferring heat to the surrounding water.

Heated water is collected in a measuring jar.

The entire setup is insulated to minimize heat loss.

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21
Q

Calorific Value Calculation

A

Temperature rise of the water
Mass of water flowing through the calorimeter
Mass of the fuel burned

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22
Q

What is Coal?

A

Coal is a fossil fuel formed from the buried remains of plants that decomposed over millions of years under high pressure and temperature.

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23
Q

Coal Formation Stages

A

(a) Biochemical (Peat Stage): Plant matter undergoes decomposition by microorganisms.
(b) Chemical (Metamorphism): Peat deposits buried deep underground lose moisture and volatile components due to high temperature and pressure.

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24
Q

Coal Classification (by Rank)

A

Coals are ranked based on their degree of coalification (transformation from wood). This ranking is primarily determined by carbon content, which increases, and oxygen/nitrogen content, which decreases, as wood transforms into coal.
Wood > Peat > Lignite > Bituminous Coal > Anthracite

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25
Q

Examples of Solid Fuels

A

Wood, peat, lignite, coal, and charcoal

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26
Q

Coalification Process

A

Wood -> Peat -> Lignite -> Bituminous Coal -> Anthracite (increasing carbon content)

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27
Q

Proximate Analysis

A

A method to assess coal quality by determining its moisture, volatile matter, ash, and fixed carbon content. Data can vary slightly depending on the specific procedure used.

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28
Q

How is Moisture Content Measured?

A

Heat a weighed coal sample at 105-110°C for 1 hour in an oven.
Cool the sample in a desiccator (dries with desiccant) and weigh again.
Calculate moisture content as the percentage of weight loss.

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29
Q

Why is Low Moisture Content Desirable?

A

High moisture content in coal is undesirable because it:
* Increases transportation costs.
* Lowers heating value by absorbing heat for evaporation.
* Can hinder combustion and extinguish fires in furnaces.

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30
Q

What is volatile matter?

A

Volatile Matter: Gaseous and liquid hydrocarbons released when coal is heated without oxygen.

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31
Q

How is Volatile Matter Measured?

A

Heat a moisture-free coal sample in a covered crucible at 950°C for 7 minutes.
Calculate volatile matter content as the percentage of weight loss.

32
Q

What does High Volatile Matter Indicate?

A

High volatile matter content:
* Means a larger portion of the coal burns as gas, producing long flames and smoke.
* May require additional air (secondary air) for complete combustion.
* Is desirable for coal gas production (becomes gas and tar products).

33
Q

How is Ash Content Measured?

A

Heat the residue left after removing volatile matter at 700°C for 30 minutes without a cover.
Calculate ash content as a percentage of the original coal weight.

34
Q

Why is Low Ash Content Desirable?

A

High ash content:
* Reduces the heating value of the coal.
* Can restrict airflow in furnace grates, hindering combustion efficiency.
* Leads to clinker formation (molten ash that traps coal particles), wasting fuel.

35
Q

How is Fixed Carbon Calculated?

A

Fixed carbon content is determined by subtracting the sum of moisture, volatile matter, and ash content from 100%.

36
Q

What does High Fixed Carbon Content Indicate?

A

Higher fixed carbon content generally translates to:
* Greater heating value of the coal.
* Better coal quality for applications requiring high heat output.

37
Q

Ultimate Analysis

A

A method for determining the elemental composition of coal, providing a more comprehensive picture compared to proximate analysis.

38
Q

Elements Analyzed in ultimate analysis

A

Carbon (C): Main component of coal, influencing its heating value.
Hydrogen (H): Contributes to heating value, though to a lesser extent than carbon.
Sulfur (S): Undesirable due to air pollution concerns when coal is burned.
Nitrogen (N): Minor component, but important for some industrial processes.
Oxygen (O): Determined indirectly by difference (100% minus the sum of other elements).

39
Q

How are Carbon and Hydrogen Measured?

A

A coal sample is burned in a combustion tube with excess oxygen. The analysis often happens alongside bomb calorimetry for heating value determination.

40
Q

How is Nitrogen Measured?

A

The Kjeldahl method is commonly used:

The sample is digested with sulfuric acid and a catalyst to convert nitrogen to ammonia.
The ammonia is then distilled and absorbed in an acid solution.
The unused acid is titrated with sodium hydroxide to determine the amount of ammonia, and consequently, nitrogen.

41
Q

How is Sulfur Measured?

A

Sulfur analysis often leverages the washings from a bomb calorimeter experiment used for heating value determination.

The washings contain sulfur as sulfate.
Barium chloride is added to precipitate the sulfate as barium sulfate (BaSO4).
The BaSO4 precipitate is filtered, ignited (heated to high temperatures), and weighed.
The weight of the BaSO4 correlates to the amount of sulfur in the coal.

42
Q

Benefits of Ultimate Analysis

A

Provides a more detailed picture of coal composition compared to proximate analysis.
Allows for a better understanding of coal behavior during combustion and potential environmental impacts.
Useful for selecting appropriate coal for specific applications.

43
Q

Importance of Liquid Fuels

A

Almost all combustion engines run on liquid fuels.

44
Q

Main Source of Liquid Fuels

A

Petroleum (also called rock oil or mineral oil)

45
Q

What is Petroleum?

A

Petroleum is a complex mixture of hydrocarbons.
It also contains small amounts of organic compounds with oxygen, nitrogen, and sulfur.
Trace amounts of metallic constituents are also present.

46
Q

What is refining of petroleum?

A

The process of refining involves the following stages:
> (a) Separation of water(cottrell’s process)
> (b) Removal of harmful sulphur compounds
> (c) Fractional distillation

47
Q

Cracking of Petroleum

A

The process of breaking down large hydrocarbon molecules in petroleum into smaller, more useful ones.

48
Q

Why Crack Petroleum?

A

Fractional distillation separates by boiling point, but doesn’t create enough gasoline (high demand fuel). Cracking allows us to get more gasoline from the crude oil.

49
Q

Products of Cracking

A

Smaller hydrocarbon molecules, including:

Gasoline (primary product)
Liquefied petroleum gas (LPG) used for cooking and heating
Alkenes (used to make plastics)

50
Q

Types of Cracking

A

There are two main types of cracking:

Thermal Cracking - Uses high temperatures to break down the molecules (older method)
Catalytic Cracking - Uses a catalyst (a substance that speeds up the reaction) for more efficient cracking and specific product control.

51
Q

Vapor Phase Thermal Cracking

A

Cracks vaporized oil at 600-650°C and low pressure (10-20 Kg/cm²).
Yield: 50-60%
Gasoline has better anti-knock properties but lower stability compared to liquid phase cracking.

51
Q

Liquid Phase Thermal Cracking

A

Cracks heavy oil/gas oil stock at 475-530°C and 100 Kg/cm² pressure.
Yield: 50-60%
Octane rating: 65-70 (good, but not ideal)

52
Q
A
53
Q

Catalytic Cracking

A

Uses a catalyst (silica-alumina or zeolite) to crack at lower temperatures and pressures.
Significantly improves gasoline quality and yield.

54
Q

Types of Catalytic Cracking

A

Fixed bed catalytic cracking
Fluid/Moving bed catalytic cracking (also known as fluid catalytic cracking)

55
Q

Fixed Bed Catalytic Cracking

A

Catalyst is in the form of granules/pellets in towers.
Oil is vaporized, passed through the catalyst bed, and cracked at 425-450°C and 1.5 kg/cm² pressure.
Yields around 40% gasoline.
Requires catalyst regeneration due to carbon build-up.

56
Q

Fluid/Moving Bed Catalytic Cracking (FCC)

A

Catalyst is a fine powder that flows with the oil vapors in the reactor.
Cracking occurs at higher temperatures (~530°C) and pressures (3-5 kg/cm²) than fixed bed.
A cyclone separator removes catalyst from the cracked vapors before fractionation.
Carbon builds up on the catalyst, requiring regeneration at 600°C with air.

57
Q

Knocking in Spark-Ignition Engines

A

Knocking is when unburned fuel-air mixture explodes prematurely in the combustion chamber, causing a knocking sound.

58
Q

Knocking Tendency of Hydrocarbons

A

Straight-chain hydrocarbons knock more readily than branched-chain, cycloalkanes, and aromatics.

Order of knocking tendency (low to high): Aromatics < Cycloalkanes < Olefins < Branched-chain alkanes < Straight-chain alkanes

59
Q

Antiknock Agents

A

Additives mixed with gasoline to reduce knocking and increase octane rating.

60
Q

Examples of Antiknock Agents

A

Tetra-ethyl lead (toxic - no longer commonly used)
MTBE (methyl tertiary butyl ether)
MMT (Methyl cyclopentadienyl manganese tricarbonyl)
Ferrocene
Iron pentacarbonyl
Toluene
Isooctane

61
Q

Octane Rating

A

Higher octane rating indicates greater resistance to knocking.
Defined as the percentage of iso-octane in a mixture with n-heptane that matches the knocking characteristics of the test fuel.

62
Q

Cetane Number (Diesel Fuel)

A

Defined as the percentage of cetane in a mixture with α-methyl naphthalene that matches the ignition characteristics of the test fuel.
Cetane (n-hexadecane) is a reference fuel for cetane number.

63
Q

Reforming

A

Process to improve the anti-knock characteristics of gasoline by changing the molecular structure of its components.

Achieved by increasing volatility (smaller molecules) or converting straight-run gasoline components into:
Iso-paraffins
Olefins
Aromatics
Aromatics from naphthenes (cyclic hydrocarbons)

64
Q

Reforming vs. Cracking

A

Cracking: Converts heavier oils into gasoline-like molecules.

Reforming: Converts existing gasoline components into higher-octane molecules.

65
Q

Types of Reforming

A

Thermal Reforming: Uses high temperatures and pressure (no catalyst).
Catalytic Reforming: Uses a catalyst (platinum on alumina) for better control and yield.

66
Q

Thermal Reforming

A

Carried out at 500-600°C and 85 atm pressure.
Rapid cooling needed to avoid excessive gas formation.
May involve some cracking and dehydrogenation/dehydrocyclization reactions (formation of aromatics).
Also converts n-alkanes to branched-chain alkanes.

67
Q

Catalytic Reforming

A

Achieves higher octane rating (90-95) compared to thermal reforming (65-80).
Uses a platinum catalyst on alumina at 460-530°C and 35-50 atm pressure (fixed-bed or fluidized-bed).
Key reactions include:
Dehydrogenation (removal of hydrogen)
Dehydrocyclization (formation of aromatic rings)
Hydrocracking (breaking down molecules with hydrogen)
Isomerization (rearranging carbon skeletons)

68
Q

Combustion

A

Exothermic chemical reaction releasing heat and light, often with a significant temperature increase.

69
Q

Ignition Temperature

A

The minimum temperature required for a substance to ignite and sustain burning without further external heat.

70
Q

Flammable Gas Concentration Limits

A

Gaseous fuels only ignite within a specific range of concentration in the fuel-air mixture. This range is called the ignition range.

Lower flammability limit (LFL): Minimum concentration for combustion. Below this limit, the mixture is too lean to burn.
Upper flammability limit (UFL): Maximum concentration for combustion. Above this limit, the mixture is too rich to burn.

71
Q

Ignition Range Examples

A

Hydrogen: 6% to 71% concentration in air
Methane: 6% to 13% concentration in air
Petrol Vapors: 2% to 4.5% concentration in air

72
Q

Producer Gas

A

A mixture of combustible gases (CO, H2) and non-combustible gases (N2, CO2) made by reacting steam or air with a carbonaceous fuel (coal or coke).

73
Q

Composition of Producer Gas (Example)

A

27% Carbon Monoxide (CO)
12% Hydrogen (H2)
0.5% Methane (CH4) (increases to 3% with coal)
5% Carbon Dioxide (CO2)
55% Nitrogen (N2)

74
Q

Heating Value of Producer Gas

A

Around 5,000 kJ/m³

75
Q

Applications of Producer Gas

A

Industrial fuel:
Firing coke ovens and blast furnaces (iron and steel manufacturing)
Cement and ceramic kilns
Mechanical power: Gas engines
Reducing agent: Metallurgical operations

76
Q

Water gas (blue gas)

A

Applications:
(i) Used as a source of hydrogen gas.
(ii)Used as fuel gas
(iii)Used as an illuminating gas

EC (10 decks)

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