Lesson 3. Fuels and its classification


This section briefly describes the main features of fuels. Energy from the Sun is converted into chemical energy by photosynthesis. But, as we know, when we burn dried plants or wood, producing energy in the form of heat and light, we are releasing the Sun’s energy originally stored in that plant or in that wood through photosynthesis. We know that, in most of the world today, wood is not the main source of fuel. We generally use natural gas or oil in our homes, and we use mainly oil and coal to heat the water to produce the steam to drive the turbines for our huge power generation systems.

These fuels - coal, oil, and natural gas - are often referred to as fossil fuels. The various types of fuels (like liquid, solid and gaseous fuels) that are available depend on various factors such as costs, availability, storage, handling, pollution and landed boilers, furnaces and other combustion equipments.

The knowledge of the fuel properties helps in selecting the right fuel for the right purpose and for the efficient use of the fuel. Laboratory tests are generally used for assessing the nature and quality of fuels.

Fuel when burnt produces heat. Thus, the substances classified as fuel must necessarily contain one or several of the combustible elements: carbon, hydrogen, sulphur, etc. In the process of combustion, the chemical energy of fuel is converted into heat energy. To utilize the energy of fuel in most usable form, it is required to transform the fuel from its one state to another, i.e. from solid to liquid or gaseous state, liquid to gaseous state, or from its chemical energy to some other form of energy via single or many stages. In this way, the energy of fuels can be utilized more effectively and efficiently for various purposes.

Fuel is any material that stores potential energy in a form that can be practicably released and used as heat energy. The concept originally applied solely to those materials storing energy in the form of chemical energy that could be released through combustion, but the concept has since been also applied to other sources of heat energy such as nuclear energy (via nuclear fission or nuclear fusion), as well as releases of chemical energy through non-combustion oxidation (such as in cellular biology or in fuel cells). The heat energy released by many fuels is harnessed into mechanical energy via an engine. Other times the heat itself is valued for warmth, cooking, or industrial processes, as well as the illumination that comes with combustion. Fuels are also used in the cells of organisms in a process known as cellular respiration, where organic molecules are oxidized to release un-usable energy. Hydrocarbons are by far the most common source of fuel used by humans, but other substances, including radioactive metals, are also utilized. Fuels are contrasted with other methods of storing potential energy, such as those that directly release electrical energy (such as batteries and capacitors) or mechanical energy (such as flywheels, springs, compressed air, or water

in a reservoir).


The fuel can be classified into three type’s mainly liquid, solid, and gaseous on the bases of their physical state.


Liquid fuels like furnace oil and LSHS (low sulphur heavy stock) are predominantly used in industrial applications. The various properties of liquid fuels are given below:

3.2.1.a DENSITY

Density is defined as the ratio of the mass of the fuel to the volume of the fuel at a reference temperature of 15°C. Density is measured by an instrument called a hydrometer. The knowledge of density is useful for quantitative calculations and assessing ignition qualities. The unit of density is kg/m3.


This is defined as the ratio of the weight of a given volume of oil to the weight of the same volume of water at a given temperature. The density of fuel, relative to water, is called specific gravity. The specific gravity of water is defined as one. Since specific gravity is a ratio, it has no units. The measurement of specific gravity is generally made by a hydrometer. Specific gravity is used in calculations involving weights and volumes. The specific gravity of various fuel oils is given in Table 3.1:

 Table 3.1  Specific gravity of various fuel oils (adapted from Thermax India Ltd.).




Oil L.D.O

(Light Diesel Oil)


Furnace oil



(Low Sulphur

Heavy Stock)

Specific Gravity

0.85 - 0.87

0.89 - 0.95

0.88 - 0.98




The viscosity of a fluid is a measure of its internal resistance to flow. Viscosity depends on the temperature and decreases as the temperature increases. Any numerical value for viscosity has no meaning unless the temperature is also specified. Viscosity is measured in Stokes / Centistokes. Sometimes viscosity is also quoted in Engler, Saybolt or Redwood. Each type of oil has its own temperature - viscosity relationship. The measurement of viscosity is made with an instrument called a Viscometer. Viscosity is the most important characteristic in the storage and use of fuel oil. It influences the degree of pre-heating required for handling, storage and satisfactory atomization. If the oil is too viscous, it may become difficult to pump, hard to light the burner, and difficult to handle. Poor atomization may result in the formation of carbon deposits on the burner tips or on the walls. Therefore pre-heating is necessary for proper atomization.

 3.2.1.d FLASH POINT

The flash point of a fuel is the lowest temperature at which the fuel can be heated so that the vapour gives off flashes momentarily when an open flame is passed over it. The flash point for furnace oil is 66 °C.

 3.2.1.e POUR POINT

The pour point of a fuel is the lowest temperature at which it will pour or flow when cooled under prescribed conditions. It is a very rough indication of the lowest temperature at which fuel oil is ready to be pumped.


Specific heat is the amount of calories needed to raise the temperature of 1 kg of oil by 10C. The unit of specific heat is kcal/kg0C. It varies from 0.22 to 0.28 depending on the oil specific gravity. The specific heat determines how much steam or electrical energy it takes to heat the oil to a desired temperature. Light oils have a low specific heat, whereas heavier oils have a higher specific heat.


The calorific value is the measurement of heat or energy produced, and is measured either as gross calorific value or net calorific value. The difference is determined by the latent heat of condensation of the water vapour produced during the combustion process. Gross calorific value (GCV) assumes all vapour produced during the combustion process is fully condensed. Net calorific value (NCV) assumes the water leaves with the combustion products without fully being condensed. Fuels should be compared based on the net calorific value. The calorific value of coal varies considerably depending on the ash, moisture content and the type of coal while calorific value of fuel oils is much more consistent. The typical GCVs of some of the commonly used liquid fuels are given below:

 Table 3.2. Gross calorific values for different fuel oils (adapted from Thermax India Ltd.)

Fuel Oil                   Gross Calorific Value (kCal/kg)

Kerosene                                 - 11,100

Diesel Oil                                - 10,800

L.D.O                                      - 10,700

Furnace Oil                            - 10,500

LSHS                                      - 10,600


 3.2.1.h SULPHUR

The amount of sulphur in the fuel oil depends mainly on the source of the crude oil and to a lesser extent on the refining process. The normal sulphur content for the residual fuel oil (furnace oil) is in the order of 2 - 4 %. Typical figures for different fuel oils are shown in Table 3.3.

 Table 3.3. Percentages of sulphur for different fuel oils (adapted from Thermax India Ltd.)

Fuel oil                                                        Percentage of Sulphur

Kerosene                                                              0.05 - 0.2

Diesel Oil                                                            0.05 - 0.25

L.D.O                                                                   0.5 - 1.8

Furnace Oil                                                          2.0 - 4.0

LSHS < 0.5


The main disadvantage of sulphur is the risk of corrosion by sulphuric acid formed during and after combustion, and condensation in cool parts of the chimney or stack, air pre-heater and economizer.

 3.2.1.i ASH CONTENT

The ash value is related to the inorganic material or salts in the fuel oil. The ash levels in distillate fuels are negligible. Residual fuels have higher ash levels. These salts may be compounds of sodium, vanadium, calcium, magnesium, silicon, iron, aluminium, nickel, etc. Typically, the ash value is in the range of 0.03 - 0.07 %. Excessive ash in liquid fuels can cause fouling deposits in the combustion equipment. Ash has an erosive effect on the burner tips, causes damage to the refractories at high temperatures and gives rise to high temperature corrosion and fouling of equipments.


Carbon residue indicates the tendency of oil to deposit a carbonaceous solid residue on a hot surface, such as a burner or injection nozzle, when its vaporizable constituents evaporate. Residual oil contains carbon residue of 1 percent or more.


The water content of furnace oil when it is supplied is normally very low because the product at refinery site is handled hot. An upper limit of 1% is specified as a standard.

Water may be present in free or emulsified form and can cause damage to the inside surfaces of the furnace during combustion especially if it contains dissolved salts. It can also cause spluttering of the flame at the burner tip, possibly extinguishing the flame, reducing the flame temperature or lengthening the flame. STORAGE OF FUEL OIL

It can be potentially hazardous to store furnace oil in barrels. A better practice is to store it in cylindrical tanks, either above or below the ground. Furnace oil that is delivered may contain dust, water and other contaminants. The sizing of the storage tank facility is very important. A recommended storage size estimate is to provide for at least 10 days of normal consumption. Industrial heating fuel storage tanks are generally vertical mild steel tanks mounted above the ground. It is prudent for safety and environmental reasons to build bund walls around tanks to contain accidental spillages. As a certain amount of settlement of solids and sludge will occur in tanks over time, tanks should be cleaned at regular intervals: annually for heavy fuels and every two years for light fuels. Care should be taken when oil is decanted from the tanker to the storage tank. All leaks from joints, flanges and pipelines must be attended to at the earliest. Fuel oil should be free from possible contaminants such as dirt, sludge and water before it is fed to the combustion system.



Coal is classified into three major types; anthracite, bituminous, and lignite. However, there is no clear demarcation between them. Coal is further classified as semi-anthracite, semi-bituminous, and sub-bituminous. Anthracite is the oldest coal from a geological perspective. It is a hard coal composed mainly of carbon with little volatile content and practically no moisture. Lignite is the youngest coal from a geological perspective. It is a soft coal composed mainly of volatile matter and moisture content with low fixed carbon. Fixed carbon refers to carbon in its free state, not combined with other elements. Volatile matter refers to those combustible constituents of coal that vaporize when coal is heated. The common coals used in for example Indian industry are bituminous and sub-bituminous coal. The chemical composition of coal has a strong influence on its combustibility. The properties of coal are broadly classified as physical properties and chemical properties.


Physical properties of coal include the heating value, moisture content, volatile matter and ash. The chemical properties of coal refer to the various elemental chemical constituents such as carbon, hydrogen, oxygen, and sulphur.


There are two methods to analyze coal: ultimate analysis and proximate analysis. The ultimate analysis determines all coal component elements, solid or gaseous and the proximate analysis determines only the fixed carbon, volatile matter, moisture and ash percentages. The ultimate analysis is determined in a properly equipped laboratory by a skilled chemist, while proximate analysis can be determined with a simple apparatus. (It may be noted that proximate has no connection with the word “approximate”).

 i. Measurement of moisture

The determination of moisture content is carried out by placing a sample of powdered raw coal of size 200- micron size in an uncovered crucible, which is placed in the oven kept at 108 +2 °C along with the lid. Then the sample is cooled to room temperature and weighed again. The loss in weight represents moisture.

 ii. Measurement of volatile matter

A fresh sample of crushed coal is weighed, placed in a covered crucible, and heated in a furnace at 900 + 15 oC. The sample is cooled and weighed. Loss of weight represents moisture and volatile matter. The remainder is coke (fixed carbon and ash).

 iii. Measurement of carbon and ash

The cover from the crucible used in the last test is removed and the crucible is heated over the Bunsen burner until all the carbon is burned. The residue is weighed, which is the incombustible ash. The difference in weight from the previous weighing is the fixed carbon. In actual practice Fixed Carbon or FC derived by subtracting from 100 the value of moisture, volatile matter and ash.

iv. Proximate analysis

The proximate analysis indicates the percentage by weight of fixed carbon, volatiles, ash, and moisture content in coal. The amounts of fixed carbon and volatile combustible matter directly contribute to the heating value of coal. Fixed carbon acts as a main heat generator during burning. High volatile matter content indicates easy ignition of fuel. The ash content is important in the design of the furnace grate, combustion volume, pollution control equipment and ash handling systems of a furnace.

v. Fixed carbon

Fixed carbon is the solid fuel left in the furnace after volatile matter is distilled off. It consists mostly of carbon but also contains some hydrogen, oxygen, sulphur and nitrogen not driven off with the gases. Fixed carbon gives a rough estimate of the heating value of coal.

    vi. Volatile matter

Volatile matters are the methane, hydrocarbons, hydrogen and carbon monoxide, and incombustible gases like carbon dioxide and nitrogen found in coal. Thus the volatile matter is an index of the gaseous fuels present. A typical range of volatile matter is 20 to 35%. Volatile matter:

  1. Proportionately increases flame length, and helps in easier ignition of coal

  2. Sets minimum limit on the furnace height and volume

  3. Influences secondary air requirement and distribution aspects

  4. Influences secondary oil support

vii. Ash content

Ash is an impurity that will not burn. Typical range of ash is  5% to 40%. Ash-

  1. Reduces handling and burning capacity.

  2. Increases handling costs.

  3. Affects combustion efficiency and boiler efficiency.

  4. Causes clinkering and slagging.

 viii. Moisture content

Moisture in coal must be transported, handled and stored. Since it replaces combustible matter, it decreases the heat content per kg of coal. Typical range is 0.5 to 10%. Moisture-

  1. Increases heat loss, due to evaporation and superheating of vapour

  2. Helps to a certain extent with binding fines

  3. Aids radiation heat transfer


 ix. Sulphur content

Typical range is 0.5 to 0.8% normally. Sulphur-

  1. Affects clinkering and slagging tendencies

  2. Corrodes chimney and other equipment such as air heaters and economizers

  3. Limits exit flue gas temperature

 x. The ultimate analysis indicates the various elemental chemical constituents such as carbon, hydrogen, oxygen, sulphur, etc. It is useful in determining the quantity of air required for combustion and the volume and composition of the combustion gases. This information is required for the calculation of flame temperature and the flue duct design etc.


Uncertainty in the availability and transportation of fuel necessitates storage and subsequent handling. Storing coal has its own disadvantages like build-up of inventory, space constraints, deterioration in quality and potential fire hazards. Other minor losses associated with the storage of coal include oxidation, wind and carpet loss. A 1% oxidation of coal has the same effect as 1% ash in coal. Wind losses may account for nearly 0.5 – 1.0 % of the total loss. The main goal of good coal storage is to minimize carpet loss and the loss due to spontaneous combustion. Formation of a soft carpet, comprising of coal dust and soil, causes carpet loss. On the other hand, if the temperature gradually rises in a coal heap, then oxidation may lead to spontaneous combustion of coal stored. Carpet losses can be reduced by:

  •  Preparing a hard solid surface for coal to be stored

  • Preparing standard storage bays of concrete and brick

 In industry, coal handling methods range from manual and conveyor systems. It would be advisable to minimize the handling of coal so that further generation of fines and segregation effects are reduced. The preparation of coal prior to feeding into the boiler is an important step for achieving good combustion. Large and irregular lumps of coal may cause the following problems:

Poor combustion conditions and inadequate furnace temperature.

Higher excess air resulting in higher stack loss.

Increase of unburnts in the ash.

Low thermal efficiency. GASEOUS FUEL

Gas fuels are the most convenient because they require the least amount of handling and are used in the simplest and most maintenance-free burner systems. Gas is delivered "on tap" via a distribution network and so is suited for areas with a high population or industrial density. However, large individual consumers do have gasholders and some produce their own gas. TYPES OF GASEOUS FUEL

  • The following is a list of the types of gaseous fuel:

  • Fuels naturally found in nature:

  • Natural gas

  • Methane from coal mines

  • Fuel gases made from solid fuel

  • Gases derived from coal

  • Gases derived from waste and biomass

  • From other industrial processes (blast furnace gas)

  • Gases made from petroleum

  • Liquefied Petroleum gas (LPG)

  • Refinery gases

  • Gases from oil gasification

  • Gases from some fermentation process                                                               

Gaseous fuels in common use are liquefied petroleum gases (LPG), Natural gas, producer gas, blast furnace gas, coke oven gas etc. The calorific value of gaseous fuel is expressed in Kilocalories per normal cubic meter (kCal/Nm3) i.e. at normal temperature (20 0C) and pressure (760 mm Hg).


Since most gas combustion appliances cannot utilize the heat content of the water vapour, gross calorific value is of little interest. Fuel should be compared based on the net calorific value. This is especially true for natural gas, since increased hydrogen content results in high water formation during combustion.

 2.3.3 LPG

LPG is a predominant mixture of propane and butane with a small percentage of unsaturated (Propylene and Butylene) and some lighter C2 as well as heavier C5 fractions. Also propane (C3H8), Propylene (C3H6), normal and iso-butane (C4H10) and Butylene (C4H8) are included in the range of LPG. 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. Although they are normally used as gases, they are stored and transported as liquids under pressure for convenience and ease of handling. Liquid LPG evaporates to produce about 250 times volume of gas.

 LPG vapour is denser than air: butane is about twice as heavy as air and propane about one and a half times as heavy as air. Conseque ntly, the vapour 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. In still air vapour will disperse slowly. Escape of even small quantities of the liquefied gas can give rise to large volumes of vapour / air mixture and thus cause considerable hazard. To aid in the detection of atmospheric leaks, all LPG’s are required to be odorized. There should be adequate ground level ventilation where LPG is stored. For this very reason LPG cylinders should not be stored in cellars or basements, which have no ventilation at ground level.


Methane is the main constituent of natural gas and accounting for about 95% of the total volume. Other components are: Ethane, Propane, Butane, Pentane, Nitrogen, Carbon Dioxide, and traces of other gases. Very small amounts of sulphur compounds are also present. Since methane is the largest component of natural gas, generally properties of methane are used when comparing the properties of natural gas to other fuels.

 Natural gas is a high calorific value fuel requiring no storage facilities. It mixes with air readily and does not produce smoke or soot. It did  not contains sulphur. It is lighter than air and disperses into air easily in case of leak.

Last modified: Monday, 3 February 2014, 5:37 AM