Fuels

Engineering Chemistry (revised edition)ISBN: 978-81-265-4475-2

Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Introduction

A fuel is defined as any substance used to produce heat or power by combustion. Any chemical process accompanied by the evolution of light and heat is called combustion. It is simply the reaction of substances with oxygen and converts chemical energy into heat and light.



Characteristics of a Good Fuel

1.It should ignite easily. The temperature of the fuel at which ignition starts and continues to burn without further addition of heat is called ignition temperature. It should be moderate for a good fuel. Very low ignition temperature leads to fi re hazard and very high ignition temperature disfavors the starting of fi re.

2.It should give out a lot of heat, that is, its specific heat should be high.

3.It should have low smoke and combustible matter such as ash. It should not give out harmful combustion products. This property depends on the nature of elements present in the fuel.

4.It should be inexpensive and readily available.

5.It should be easy to store and transport.

6.It should have low ash content. Ash reduces the calorific value of the fuel, causes hindrance to the flow of air and heat, reduces the specific heat and leads to unwanted disposable problems.



Calorific Value

Units

1.Mean British Thermal Unit (BTU)

2.Calorie (cal)

Gross and Net Calorific Values

1.Higher calorific value (HCV) or gross calorific value

2.Lower calorific value (LCV) or net calorific value



Determination of Calorific Value

Theoretically Determination

Dulong’s formula for calculating the calorific value from the chemical composition of the fuel may be written as follows:



Experimentally Determination

Bomb calorimeter

1.For calorific values of solid and liquid fuels

2.Known amount of fuel is burnt at constant volume

3.Temperature of surrounding water increases as heat is produced.

4.Quantity of heat and calorific values are calculated.



Boy’s calorimeter

1.Gas or volatile liquid burns at constant rate.

2. Water flowing at constant rate absorbs the heat produced.

3.Calorific value is calculated from volume of water, increase in temperature and volume of gas/liquid burnt.



Junker’s calorimeter

1.Control of rate of burning of gaseous/liquid fuel and water circulation is maintained.

2.The combustible products are released at nearly the atmospheric pressure.

3.Calorific value is calculated from amount of water passed, volume of gas burnt, the steady rise in temperature and mass of the condensed water flowing out.



Combustion

Combustion is a chemical process accompanied by evolution of light and heat.



Calculations

Calculation of theoretical air for combustion of a fuel requires the following points:

1.Percentage of oxygen in air by volume is 21% and 23.2% by weight.

2.Stoichiometric equations involved in combustion



Flue Gas Analysis

It comprises the gaseous products of combustion of fuel. Its analysis helps in finding out the correct quantity of air to be supplied in a furnace.

Orsat’s apparatus



Solid Fuels – Coal

Formed from dead plants buried for several million years.



Uses of Coal

1.As a primary fuel: Coal is used to produce steam through heat and combustion, which is again used for running turbines to generate electricity in power plants.

2.As a secondary fuel: The product of burning coal in the absence of air is of metallurgical importance. The byproducts are useful in making plastics, tar and synthetic fibers and also used making steel in industries.

Pulverized Coal

Pulverized coal generally refers to coal in powdered form obtained by crushing, grinding or pulverizing coal. As the surface area of pulverized coal is large, the volatile matter present in it comes quickly in contact with air and is released, enabling the combustion of fixed carbon. This increases the calorific value of the coal and enhances its quality. Engineering Chemistry (revised edition)

ISBN: 978-81-265-4475-2Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

The advantages of pulverized coal are as follows:

1.It provides easy transportation and storage.

2.Even high grade coal having high ash content, such as found in India, could be used satisfactorily.

3.It can easily undergo combustion in small percentage of excess air with less wastage of heat, and has high thermal efficiency.

4.It does not form clinkers (incombustible residue) and provides an oxidizing and reducing atmosphere inside the furnace for metallurgical purposes.

The disadvantages of pulverized coal are as follows :

1.It involves the extra cost of pulverizing and sieving.

2.Special types of burners are required for good mixing of air and fuel.

3.During the process of grinding, crushing and pulverizing, finely divided ash is generated in form of fly-ash causing air pollution and causing problems associated with its disposal.



Proximate and Ultimate Analyses of Coal

It includes the determination of moisture content, volatile matter, ash and fixed carbon. In this analysis, the data varies with the procedure adopted for study. The content of moisture, volatile matter and ash are experimentally determined, while that of fixed carbon is calculated.

Proximate Analysis

1.Moisture content: Lesser the moisture content, better is the quality of coal.



2. Volatile matter: Lesser the volatile matter, better is the rank of coal.

3. Ash content: Lower the ash content, better is the quality of coal.

4. Fixed carbon: Higher fixed carbon content, better is the quality of coal.



Ultimate Analysis

1.Carbon and hydrogen: Greater the percentage of carbon and hydrogen, better is the quality of coal.

2.Nitrogen: Presence of nitrogen is undesirable.



3. Sulphur: Presence of sulphur is highly undesirable.

4. Ash: Determined the same way as in proximate analysis.

5. Oxygen: Lower the percentage of oxygen, better is the coal.



Significance of Ultimate Analysis

1.Greater the percentage of carbon and hydrogen, better is the coal in quality and calorific value. However, hydrogen is mostly associated with the volatile matter and hence, it affects the use to which the coal is put to.

2.Nitrogen has no calorific value and hence its presence in coal is undesirable. Thus, a good quality coal has very little nitrogen content.

3.Sulphur is usually present to the extent of 0.5–3.0% and is derived from ores like iron pyrite and gypsum, etc., mines along with the coal. Sulphur, although contributes to the heating value of coal, on combustion it produces acids which have harmful effects of corroding the equipment and also causes atmospheric pollution.

4.Oxygen content decreases the calorific value of coal. High oxygen content coals are characterized by high inherent moisture content, low calorific value and low coking power. Moreover, oxygen is in combined form with hydrogen in coal and thus, hydrogen available for combustion is lesser than actually present.



Coke

Coke is obtained when coal is heated strongly out of contact with the air, the process is called carbonization or coking.

1.Low-temperature carbonization: 500 °C-700 °C; low temperature coke.

2.High-temperature carbonization: 900 °C-1100 °C; metallurgical coke.

Comparison of high and low temperature carbonization processes



Percentage analysis of a coke and the coal

Coke as Metallurgical Fuel

Metallurgical coke is preferred over coal due to the following reasons:

1.It is hard, porous and of good mechanical strength.

2.It does not contain much sulphur content.

3.It burns with a short flame.



Caking and Coking Coals

Coals that produce soft and plastic mass at around 400°C that re-solidifies to form a porous solid are called caking coals. Caking coals that produce solid product (coke) of useful grade are called coking coals. A good coking coal will produce a bright gray, strongly coherent, porous coke. All coking coals are cakingcoals but not vice versa.

High volatile coals are mixed with poor or non-coking low-volatile coals, to yield a denser, stronger coke. Where mixing is practiced, the mixture used depends on the coal used. Usually high- and low volatile coals are mixed to give a mixture having about 30% of volatile matter.



Coking Processes

Bee-Hive Oven



Otto-Hoffmann or Byproduct Oven



There are two methods of cooling the coke: dry and wet quenching:

1.Dry quenching: The insulated wall of the oven is removed, so that the inert gases like nitrogen present in the atmosphere are able to come in contact with the coke and cool it naturally over a period of time.

2.Wet quenching: Water is sprayed over the red hot coke for cooling it down. As a result of this, some gases escape from the hot surface of the coke in form of steam and cause pollution. This method also generates a large amount of coke dust.

Dry quenching is preferred over wet quenching due to the following reasons:

1.The method is dust free and dry without causing pollution to the environment.

2.The heated gases could be used to recover heat by circulating them through the boilers for steam generation.

3.The coke obtained is strong, dense and free from moisture.



Advantages of Byproduct oven.

1.The byproduct coke oven, the valuable byproducts are saved; whereas in the beehive oven, they are allowed to escape into the atmosphere.

2.The byproducts are well worth saving and have many important uses. Their recovery helps to make the process economical.

3.In the purification of the byproduct coke-oven gas by means of a liquid-contact process, pure sulphur can be obtained in a form much finer than the commercial precipitated sulphur.

4.This finely divided sulphur has been found to process superior qualities as a fungicide.

5.Among the other interesting and valuable byproducts recovered by the liquid-purification processes are the thiocyanates of ammonium, sodium, and calcium.



Biofuels

Biofuels are solid, liquid or gaseous fuels that are derived from living organisms and their waste matter.



Biomass

Waste material produced by living things There are methods of utilization of biomass in the form of energy.

1.Burn the biomass directly and obtain energy.

2.Convert the matter into ethanol and methanol or it is fermented anaerobically to obtain gaseous fuel, biogas.

Biodiesel

Produced from transesterification of vegetable oils that contain triglycerides.

Biogas

The advantages of using gobar gas over heating cattle dung directly in dried state are as follows:

1.The gobar gas produces more useful energy.2.It provides high device efficiency.3.It is free from dust and smoke, and environment-friendly.4.It is used as a domestic fuel as well as an illuminant.Engineering Chemistry (revised edition)


Anaerobic digestion tank



Semi-Solid Fuels – Some Recent Advances

The term semi-solid fuel in modern context refers to non-volatile substances that are environmentally safe and produce no hazardous waste on burning. These have long shelf-life and their ignition can be easily started and stopped. The conventional solid and liquid fuels are now being used as formulations in semi-solid form to circumvent the problems associated with their use in the regular form.

The use of coal is associated with many environmental problems, starting from its excavation from mines to gaseous and fly ash emissions. To overcome these problems, developing countries are transforming coal into gaseous or liquid fuel formulations or converting it into low ash and low sulphur varieties. For example, a solvent-refined, semi-solid form of coal has been prepared by suspending pulverized coal in a solvent and treating it with hydrogen gas at high temperature and pressure. The product compares well with high grade anthracite in combustion properties, is free from ash and has high calorific value of 16000 BTU per pound.



Liquid fuels – Petroleum

1.Petroleum is made from the remains of plants and animals buried millions of years ago.

2.It is a non-renewable resource.

3.It contains straight or cycloparaffins.

4.Olefins

5.Aromatics

6.Other organic compounds containing N, O, S.



Composition

Petroleum is a dark, greenish brown, viscous liquid that is found underground. It comprises hydrocarbons such as:

1.Straight paraffins or cycloparaffins such as methane, ethane, propane, butane, isobutane, pentane, hexane.

2.Olefins such as ethylene, butene, isobutene and acetylene, butadienes.

3.Aromatics such as benzene, naphthalene, cyclohexane, methyl cyclopentane.

4.Some organic compounds containing nitrogen, oxygen and sulphur.



Production from Refining of Crude Oil

The petroleum obtained by mining is viscous and dark colored liquid. Due to the presence of sulphur, it has an unpleasant smell. It also contains impurities of sand, brine or sea water. Hence it is called crude oil.

The important steps involved are:

1.Fractional distillation to give various fractions.

2.Conversion of less desirable fractions to valuable products by processes like cracking.

3.Treatment of fractions to remove undesirable substances.



Fractional Distillation



Fractions of petroleum



Cracking

Cracking is decomposition of high molecular weight compounds (with high boiling points) to low molecular weigh compounds with low boiling points).



Fixed bed catalytic cracking



Fluidized-bed catalytic cracking



The fluidized bed cracking has the following advantages over fixed-bed cracking:

1.Better contact with the feed and the catalyst, enabling uniform temperature and efficient heat transfer.

2.The catalyst can be regenerated and used again for the cracking process.



Catalytic Reforming

Reforming is a process of converting low octane naphthas into high octane gasoline.



Knocking

Knocking in Spark Ignition Engines and Octane Number

Petrol is used in spark ignition engines. The rapid compression of the fuel-air mixture heats the engine, and it detonates without the spark being passed. This causes a violent jerk to the piston giving a metallic sound called knocking.

Octane number is the percentage volume of isooctane in the isooctane-heptane mixture that matches the knocking characteristics of the fuel being tested is called the octane number.

Molecular structure affects the octane number.



Knocking in Compression Ignition Engines and Cetane Number

Diesel is used in compression ignition engines.Cetane number represents the spontaneous ignition temperature of a particular diesel fuel. It is the percentage of cetane present in a mixture of cetane and alpha-methylnaphthalene which matches the fuel under test in ignition property.



Power Alcohol and Synthetic Petrol

Power AlcoholEthyl alcohol is used as additive to motor fuels. When blended with petrol at concentrations of 5–10%, it is called power alcohol. The addition of alcohol to petrol increases its octane number.

Manufacture of Ethanol

1.By fermentation

2.Using Molasses as raw material



The advantages of power alcohol are as follows:

1.Ethyl alcohol is a good antiknocking agent and power alcohol has octane number 90, while that of petrol is 65.

2.Any moisture content present is absorbed by alcohol.

3.Ethyl alcohol contains oxygen atoms, which help for complete combustion of power alcohol and the polluting emissions of CO, hydrocarbon, particulates are largely reduced.

4.Power alcohol is cheaper than petrol.



The disadvantages of power alcohol are as follows:

1.Ethyl alcohol has calorific value 7000 cal/g much lower than that of petrol which has 11500 cal/g. However, this problem could be overcome by using a specially designed engine with higher compression ratio.

2.The output of the power generated is reduced up to 35%.

3.It has high surface tension and its atomization is difficult, especially at lower temperature, thereby causing starting trouble.

4.It may undergo oxidation reaction to form acetic acid, which corrodes engine parts.

5.As it contains oxygen atoms, the amount of air require for complete combustion is less, therefore, the carburetor and engine need to be modified.



Synthetic Petrol

Synthetic petrol is a mixture of alkanes with composition resembling that of petrol, obtained artificially from coal.

Bergius Process



Fischer–Tropsch Process



Gaseous Fuels

The ease and flexibility of application of gaseous fuels give them advantages over solid or liquid fuels



Liquefied Petroleum Gas

Liquefied petroleum gas (LPG) is obtained as a byproduct during cracking of heavy oil or from natural gas. It contains hydrocarbons that are highly volatile but are easily liquefiable under high pressures. The components that form LPG are n-butane, isobutane, butylenes and propane with traces of propylene and ethane. It is generally used as a domestic fuel for cooking and in industry. Though it is cheaper fuel and knock resistant, yet, it has limited uses as a motor fuel owing to its following disadvantages over gasoline:

1.Has a faint odor and its leakage almost goes unnoticed.

2.Pressure needs to be monitored for effective handling.

3.Has low octane number and high load sensitivity.

4.Has high compression ratio.

5.Less responsive towards blending unlike power alcohol.Engineering Chemistry (revised edition)


Natural Gas

The origin of natural gas is closely associated with that of petroleum; it is always found in or near the petroleum fields. Because of this fact, it is not available over large parts of the country, although pipelines are being constantly extended to furnish gas to large-consuming centers, where it may be used either alone or mixed with artificial gas.

Natural gas is a cheap and convenient fuel consisting of methane and other saturated hydrocarbons. Its calorific value is 12000–14000 kcal/m2. The composition of a natural gas is as follows:

Methane = 85%, Ethane = 8%, Propane = 4%, Butane = 1.5% and Hydrocarbons = 1.5%

About 14% of the gas produced is used for making carbon black. It is also used as a fuel for industrial and is domestic constituent.



Compressed Natural Gas

This gaseous fuel is compressed natural gas (CNG) which mainly contains methane. It is obtained by applying high pressure, generally, 1000 atm to reduce the original volume at standard atmospheric pressure to less than 1%. A steel cylinder can fill 15 kg of this fuel gas with a volume of 20 m3 at a pressure of 1 atm. CNG is used as automobile fuel, especially, in cars and its use is increasing progressively. Its advantages are as follows:

1.It is a safer fuel since its ignition temperature is higher than conventional IC engine liquid fuels.

2.Combustion of CNG produces lesser number of pollutants.

However, its disadvantages are as follows:

1.It requires a lot of space for storage and hence is not suitable for smaller vehicles.

2.It has high ignition temperature 550°C and requires more air for ignition.



Coal Gas

Made up of hydrogen, methane and carbon monoxide.



Producer gas

Formed from the incomplete combustion of a solid fuel (coal or coke) in a partly closed generator.



Water Gas



Rocket Propellants

Rocket propellants: A chemical mixture (fuel + oxidizer) burned to produce thrust in rockets.



The chemical propellants are classified into solid and liquid propellants.

1.Solid propellants: These consist of a fuel, usually a hydrocarbon and an oxidizer which contains a large percentage of oxygen. These substances are mixed so as to produce a solid of desired chemical or physical characteristics. The finished product is called a grain or stick. One or more grains constitute a charge.

2.Liquid propellants: These possess many advantages over solid propellants. They are more versatile and the engine using them can be checked and calibrated more easily. However, unlike solid propellants, the engine using liquid propellant is quite delicate and cannot withstand any rough handling. Liquid propellants may be monopropellants or bipropellants.



Explosives

A material which is capable of producing an explosion by its own energy. It can be a single compound or a mixture of compounds.

1.Chemical explosives: Compounds containing –NO2, -ONO and –NHNO2 on detonation release gases.

Examples: nitroglycerin, trinitrotoluene (TNT), octogen (HMX), pentaerythritol (PETN) and nitrocellulose.

2.Mechanical explosives: Materials whose explosion is based on physical methods.

Examples: nuclear explosives.



Preparation of Explosives

1.Lead azide

2.Nitrocellulose

3.PETN



4. TNT (2,4,6-trinitrotoluene):



5. Picric acid

6. Nitroglycerine



Fuels

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