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Defination
Classification Of chemical Fuels
Characteristic of Fuels(Physical properties).
Various Uses of Fuels.
Fuels are all Those substance Which on
combustion give large Amount of heat.
IT contain carbon in main product.
Combution Reaction =
The First fuels
used By human is
WOOD
Fuels are Define as “Substance which Undergoes Combustion in the presence of air to produce a large amount of heat that can Be used Economically For industrial and Domestic PurPose”.
For E.g
Wood,Coal,
Kerosene,petrol,Diesel,Natural
gas(LPG)etc.
FUELS
Primary/Natural Secondary/Derived
LiquidEg Petroleum
Solid
e.g wood,caol
Gaseous
e.g Natural gas
Gaseous
e.g Coal gas
Liquid
Eg Alcohols
Solid
e.g Charcoal
High Calorific value.
Moderate Ignition Temperature.
Low Moisture Content.
Low Non-Combustible Matter Content.
Moderate velocity of combustion.
Product should not be Harmful.
Low cost.
Easy of Tranport.
Combustion Should be Easily Controllable.
Fuels play an important role in our
everyday life because they are used in
homes, transport and industry for
providing energy.
Domestic Usage:
Fuels like wood, coal, kerosene, cow
dung etc are used.
For Transport
Coal, diesel and petrol are used as fuel for
road, sea and air transport in automobiles
and locomotives.
For industry:
Fuels like coal and natural gas are used.
For Air Space Centre:
Specially prepared fuels like hydrazine
(Rocket fuels) [NH2-NH2] are used.
Definition:“The number of units of heat evolved during complete
combustion of unit weight of the fuel” is called as calorific value.
Calorific value can also defined as, “the number of parts of water which gets heated through 1°C by the heat evolved by the complete combustion of one unit weight fuel under the conditions such as:
• Whole of heat evolved is absorbed by water
• The product formed leave the system at atmospheric temperature & pressure.”
From both the above definitions, it is clear that a fuel, to be most useful, must possess high calorific value because the heat evolved by combustion of definite weight of fuel is directly related/proportional to the calorific value of the fuel.
Definition:Ignition temperature is defined as “minimum temperature to which a
substance must be heated before its burns spontaneously independent of the source of heat.”
E.g.: Ethanol has an ignition temperature of 425 C/798 F and flash point of 12 C/54 F.
Each fuel should be brought above its Ignition Temperature for starting the combustion process. The minimum ignition temperature at atmospheric pressure for some substances are:
o Carbon: 400°C
o Gasoline: 260°C
o Hydrogen: 580°C
o Carbon monoxide: 610°C
o Methane: 630°C
• The calorific value of solid fuels is expressed as British Thermal Units per Pound (B.T.U. per
1b) or Kilogram Centigram Unit per Kilogram (K.C.U. per kg.).
• A British Thermal Unit may defined as, the heat required to raise the temperature of one
pound of water from 60 F to 61 F.
• Calorie, a unit of heat, may be defined as, the heat required to raise the temperature of one
kilogram of water from 15 C to 16 C.
Taking both above definitions of these units, a correlation between them can be written as;
1 B.T.U = 2.252 k.cals = 252 cals
1 k.cal = 1000 cals
1 k.cal = 3.968 B.T.U.
The calorific value can be also expressed as Centigrade Heat Unit (C.H.U.) which is the amount
of heat required to raise temperature of 1 pound water through one degree centigrade. Thus,
1 k.cal = 2.2 C.H.U = 3.968 B.T.U.
Also 1 k.cal/kg = 1.8 BTU/1b
1 k.cal/m3 = 0.1077 BTU/ft3
1 BTU/ft3 = 9.3 k.cals/m3
Calorific values are of two types :
1. High or Gross Calorific Value.
2. Low or Net Calorific Value.
• High Calorific Value is defined as “the total amount of heat produced when one unit of the fuel has been burnt completely and the products of combustion have been cooled to 16 C or 60 F.”
• Low Calorific Value is defined as “the net heat produced when unit mass or volume of fuel is completely burnt and products of combustion are allowed to escape into the atmosphere.”
The calorific value of fuels is determined theoretically by Dulongformula or I.A. Davies formula.
According to Dulong, the calorific value of a fuel is the sum of the calorific
values of all the elements present . The calorific values of different elements
are given as under:
Calorific Value of C = 8080 cal/g
Calorific Value of H = 34500 cal/g
Calorific Value of S = 2240 cal/g
Thus, Dulong’s Formula is given as:
H.C.V. (G.C.V.) = 1/100 [8080C + 34500 (H – O/8) + 2240S]
where C, H, O & S are the % of C, H, O & S respectively. In this formula,
oxygen is assumed to be present in combination with hydrogen as water;
and:
L.C.V. (N.C.V.) = H.C.V. – 0.09H X 587
• The solid fuels are available in nature (primary fuels) and also
prepared artificially know as (secondary fuels).
• The common natural solid fuels are wood, peat, lignite and
coal.
• The artificial solid fuels are charcoal, coal, briquettes.
• The other industrial fuels are fossil coals, oil shales, furnace slags,
peat, boiler slags, anthracite etc.
• Coal is a combustible solid fuel. By and large, all the solid fuels are
formed in nature from cellulose, lignin, proteins, resins, fats and
waxes.
• All these raw materials, which are formed under the earth y the
burials of partially decomposed vegetation, undergo
fermentation liberating CH4, CO2 and H2 gas and form peat,
which is slowly converted into lignite and then by further pressure
and heat, anthracite is formed.
A) Banded Coals This type of coal is a variety of bituminous or sub-bituminous coal. These are generally formed from peat.
The structure of this type contains layers or bands of coal forming materials.
B) Splint Coals This is also variety of bituminous of sub-bituminous coal, with dull
lustre and greyish-black colour.
The bands of different coal forming materials are more irregular in this variety, as a result it breaks with irregular rough fracture.
There is no swelling on burning, but burns freely.
C) Cannal coals A variety of bituminous or sub-bituminous coal with compact fine
grained texture instead of bands.
The colour varies from dark grey to black; possesses high volatile matter, non-coking type, ignites easily and burns with luminous smoky flame.
D) Bog-head coals
A variety of bituminous or sub-bituminous coal similar to Cannel coal
in appearance as well as combustibility, but posses high content of
volatile matter and algal remains.
When subjected to distillation it gives high yield of tar and oil.
E) High and Low rank coals
High rank coals are further classified on basis of their percentage of
dry fixed carbon and volatile matter.
Example : Anthracite – There are three types of Anthracite:
Low rack coals are graded further on the basis of their moist B.T.U.,
indicating natural moisture
Meta-anthraciteWith minimum 92-98% fixed carbon
and maximum 2-8% volatile matter.
Anthracite87-91% fixed carbon, 9-13% volatile
matter.
Semi-anthracite 86% fixed carbon, 14% volatile matter.
Coal : A mined sample of coal contains the coal substance, intermixed with
mineral constituents such as kaolin, shale, chloride, sulphides, etc. The major constituents of coal are carbon, hydrogen and oxygen. The properties of coal depend upon these constituents.
There are hundreds of varieties of coal, depending upon its origin and chemical constituents of coal. The important types of coal are peat, lignite, bituminous and anthracite.
The conversion of wood into coal takes place progressively. Depending upon extent of transformation, coals are divided into 4 types (or grades or ranks):
During the progressive conversion from peat to anthracite, there is:
Increase in - Carbon percentage, calorific value, density, lustre, hardness, black colour intensity.
Decrease in - Moisture, volatile matter, % of N, H, O, S & Ash.
Peat is brown and fibrous in texture.
Freshly mined out peat contains large quantity of water as it is found in water logged
areas.
Air dried peat contains 15-25% moisture and it crumbles into powder during air
drying.
Calorific value of peat is about 5400 cal/gm.
It has low density.
It contains 57% C, 6% H, 35% O & 3-6% ash.
Uses Peat type of coal gets powdered during combustion, therefore it is used after
briquetting, as domestic and industrial fuel.
It is used for soil conditioning.
It can be used for steam raising, thermal insulation, packing, gas purification and
some times for power generation.
It is intermediate stage between peat and black coal.
It is brownish black and more compact than peat.
It contains 45-50% volatile matter and burns with long flame.
It has Calorific value of 6000-6700 cal/gm.
It cantains 65-70% C, 5% H, 20% O & 10-15% ash.
Uses After briquetting it is used as domestic and industrial fuel.
Lignite is used for making ' producer gas ‘
It can be used for power generation.
These coal burns with smoky yellow flame and are dark grey to black. They contains 70-90% C and therefore they are classified as:
A) Sub-bituminous coal: This coal has characters between lignite and bituminous coal.
It is harder and denser than lignite.
It is grey black and has dull waxy lustre.
It contains 70-75% C and large volatile matter upto 35-40%.
It has Calorific value of 7000 cal/gm.
It is non-caking coal.
Uses This coal is used for domestic and industrial purposes.
B)Bituminous coal: This coal has banded or laminated structure with alternate bright and dull layers.
It has cubical fracture.
It is black, dense and hard.
It contains 75-80% carbon.
It has Calorific value of 8000-8500 cal/gm.
Uses It is used most widely for domestic and industrial purposes.
It is used for steam generation and power generation.
C) Semi-bituminous coal: It has characteristics between bituminous and anthracite.
It has low volatile matter and 75-85% C.
It has low caking property.
It has Calorific value of 8400 cal/gm.
Uses Making of coke, high temperature heatings, coal gas for tar and chemicals.
This is highest rank or grade of coal.
It has calorific value about 8700 cal/gm.
It has 92-98% C.
It contains very low volatile matter, ash and moisture.
It is highly lustrous, black and hard coal.
Uses Being costly coal, it is used for specific industrial purposes.
It is used as metallurgical fuel.
It is used for making electrodes.
It is used for high temperature heating.
• The liquid fuels are generally the products obtained from petroleum refining.
• The main constituents of crude or raw petroleum are paraffin, naphthalene and aromatic hydrocarbons. The concentration of all there vary.
• The characteristic features of the liquid fuels are,
a) Liquid fuels possess low flash and fire point.
b) The calorific value of liquid fuels is generally very high.
c) The viscosity of liquid fuel is very low at ordinary temperature.
d) The moisture and sulphur content of liquid fuels is low.
Crude Petroleum Oils Petroleum or crude oil is the main source of almost all liquid fuels used
now and a large number of petrochemicals such as plastics, rubbers, fibres, organic chemicals, hydrogen etc. can be manufactured from crude oil fraction.
It has negligible percentage of ash and moisture and has minute quality of Sulphur. It has very high calorific value such as 40,000 kJ/kg.
Origin of petroleum As per modern theory, petroleum is formed from buried debris of plants and
animals (organic matter), under favorable condition.
The burial of the organic matter during volcano, upheavals in earth surface should take place along with large quantity of water and under a dome of hard, impervious rock.
The anaerobic bacteria, higher temperature and radioactive substances enables degradation of organic matter, in the presence of water, under the dome, to form highly alkane rich matter as crude oil.
The anaerobic bacteria take out oxygen atoms from cellulose, protein, oil molecules and forms alkane rich crude oil.
For example:
Petroleum, commonly known as rock oil or mineral oil, is obtained from nature, under the earth, in the form of deeply coloured highly viscous liquid.
It contains a large number of different individual chemicals ranging from methane to asphalt.
Average elemental composition of crude petroleum is:
C = 80 to 87% H = 11 to 15%
S = 0.1 to 3% O = 0.1 to 0.9%
N = 0.4 to 0.9%
Petroleum contains following types of compounds:
Open chain alkanes: Both straight chain and branched chain alkanes are present in crude oil.
Cycloalkanes: Crude oil contains cycloalkanes like cyclopentane, cyclohexane and their alkyl substituted products.
Aromatics: In all the crude oil benzene and alkyl substituted benzenes are upto 2%.
Asphaltenes: All the crude oils contain the small amount of polycondensed aromatic solids as colloidal dispersion in crude oil.
Resins: These are the polymeric substances. They are gummy and are lower molecular weight polymers.
Petroleum gets formed collected under the earth. The depth of such a storage of petroleum varies from few hundreds to few thousands of feet below the surface of the earth.
It is surrounded by layers of natural gas, under the earth. In short, the crude petroleum thus formed floats upon a layer of salt water and is surrounded by layer of natural gas, deep below the impervious rock. Mining of oil is done by drilling holes in earth’s crust and sinking pipe up to the oil bearing rocks.
Due to the hydrostatic pressure exerted by natural gas, surrounding to the stock of petroleum helps to get petroleum piped out with pressure.
Crude oil coming from the petroleum wells consists of a viscous, dark coloured frothing mixture of solid, liquid and gaseous hydrocarbons containing sand and water in suspension. In order to make it into a marketable product, the oil is made free from impurities like water, dissolved salts, sulphur etc.
The process by which petroleum is made free from impurities and separated into various useful fraction having different boiling points and further treated to remove undesirable tendencies and to impart specific properties to them is broadly called „REFINING OF PETROLIUM.‟
The Refining of Petroleum is done in the following stages:
(1) SEPARATION OF WATER (COTTRELL’S PROCESS)
The crude oil from the oil well is an extremely stable emulsion of oil and salt water. The process of removing oil from water consists in allowing the crude to flow between two highly charged electrodes. The colloidal water droplets aggregates to form large drop, which separate out from the oil. To remove the persistent impurities, colouretc., various fractions are passed over adsorbents like Kieselgure clay etc. and the resultant oils are generally pure.
(2) REMOVAL OF HARMFUL IMPURITIES
NaCl and MgCl2 can corrode the refining equipment and can cause scale formation in the heating pips. Hence special care should be taken to remove them. Modern techniques of electrical desalting and dehydration are developed for this purpose. Then oil is treated with copper-oxide. The reaction occurs with sulphur compound, which result in the formation of copper sulphide (a solid), which is then removed by filtration.
(3) FRACTIONAL DISTILLATION
Fractional distillation is a combination of distillation and rectification. Rectification process consists of counter flow contacting of the vapour formed in distillation with the liquid obtained by condensation of vapuor. For effective rectification in distillation column, it is essential to see that an ascending flow of vapour (formed due to the heat supplied at the bottom section) meets the descending flow of liquid (formed due to cold spraying in the top section). These principles are used in the “FRACTIONTING TOWER”widely used in petroleum refining.
Fractionating Tower (in figure) consists of a pipe still and bubble tower. In pipe still the crude petroleum is heated and is fractionated in bubble tower.
Bubble tower consists of horizontal stainless steel trays or plates at short distances. Each tray is provided with a small chimney covered with loose cap though which vapour rises up. These small chimneys are covered with loose bubble caps.
Crude oil is piped through a pipe still where it is heated to about 400*C and the vapours are the introduced near the bottom of the bubble tower. As the vapour move upwards, the 3 higher boiling fractions condense at lower plats and only the lower boiling fractions move to higher plates. The vapourare allowed to pass up through higher plates via bubble caps. The heavier component having high boiling fraction condense and the condensate flows down to the next lower tray through the down comers. The vapourwhich condenses give out the latent heat of condensation to the liquid in the tray, from which more volatile components moves up the column.
This process of condensation and vapourization takes place many time causing separation of the fractions according to their boiling points. Thus higher boiling fractions condense at the lower parts of the column and lower boiling fractions condense at the higher parts of the column. Thus the crude oil is fractionated into different fractions depending on their boiling ranges and are collected at different heights in the column. In this way mixture of vapours and liquids of different boiling points are separated. This fractionation gives uncondensed gases, gasoline, kerosene and gas oil fraction.
Fractionating tower
Portion of fractionating
tower
Cracking is a process of converting heavy oil with higher molecular weight hydrocarbons to the oil with lower molecular weight hydrocarbon which is known as gasoline.
Thus, heavy oil is heated at a high temperature under pressure or in the presence of catalyst to obtain gasoline.
For example:
C10H22 C5H12 + C5H10
(Decane) n-Pentane Pentene
There are two methods of cracking
Thermal cracking
liquid phase or
vapour phase
Catalytic cracking
Fixed bed Moving bed
Liquid phase thermal cracking:
By this method, any type of oil can be cracked. In this method, the oil is pumped into the coil kept at 420°C-550°C under a pressure of 15-100 kg/cm2. As the temperature increases, a better quality of gasoline is produced. The octane value of this gasoline is low, i.e. 65-70. Therefore, it is mixed with higher octane value gasoline and then used I engine.
Vapour phase thermal cracking:
In this method, the heavy oil is heated in the heater at 400°C to convert it into the vapours and then these vapous are passed to the reaction chamber which is maintained at 600°C-650°C and under a pressure of 10-20 kg/cm2. At this stage, the higher hydrocarbons are converted into lower hydrocarbons easily and the octane value to petrol is usually 75-80.
Sr. No.
CharacteristicsLiquid Phase Thermal
CrackingVapour Phase
Thermal Cracking
1Cracking
temperatureModerate (420-
550°C)600-650°C
2 Pressure High (15-100 kg/cm2) Low (15-20 kg/cm2)
3 Yield percentages 50-60% -
4 Octane rating 65-70 >70 (75-80)
5Pre-requirement for
processAny type of heavy oil
can be usedOil has to be
vaporised readily
6 Time required Comparatively more Comparatively less
Comparison of liquid phase and vapour phase thermal cracking
Catalytic Cracking
Fixed bed catalytic cracking:
In this method, vapours of the heavy oil are heated in the presence of
catalyst due to which a better yield of petrol is obtained.
In this method, heavy oil is vaporised by heating in an electrical heater.
Then the vapours are passed over a series of trays containing catalyst. Generally, the
catalyst used are crystalline alumina-silicate, bentonite, bauxite and zeolites. The
reaction chamber is maintained at 425°C-540°C and under a pressure of 1.5 kg/cm2.
The cracked gases are taken out from the top of the reaction chamber (cracker) and
allowed to pass into fractionating tower, where gasoline fraction is collected. The
octane value of this gasoline is about 80-85. During the cracking, free carbon is also
formed which deposits on catalyst, then the flow of vapours of heavy oil is passed
over the second set of reaction chamber and the catalyst is earlier chamber is
regenerated by burning the carbon deposits with the help of air and reused.
Fixed bed catalytic cracking
Moving bed catalytic cracking:
It is also called fluidised bed catalytic cracking principle.
In this method, a fine powder of catalyst is circulated through
the cracker along with the vapours of heavy oil (higher hydrocarbon).
The catalyst accelerates and directs the cracking efficiently to form
gasoline and other lower hydrocarbons. For example:
C18H38 C10H22 + C8H16
n-octadecane n-decane Octene
Process:
In this method, a mixture of heavy oil and catalyst is heatedin the still to convert the heavy oil into vapours. There vapoursalong with hot catalyst are brought to the cracker. The cracker ismaintained at a temperature of 550-570°C and atmosphericpressure. In the cracker, the vapours of the heavy oil and hotcatalyst come in intimate contact with each other and thebreaking of higher molecular weight hydrocarbons into lowerhydrocarbon (Gasoline) takes place. The low boiling hydrocarbonsmove up to the top of the cracker are passed through the cycloneseparator while the catalyst remains in the cracker.
These cracked gases are further passed through thefractionating column to have three major fractions: Gasoline,middle oil & heavy oil. The gasoline is further cooled and purified toremove the impurities of sulphur, unsaturated hydrocarbons andcolouring matter, if present. The catalyst performs two functions: (1)To get a better quality gasoline during cracking process & (2) tocarry heat during process.
Regeneration of exhausted catalyst:
Catalyst gets deactivated due to thedeposition of free carbon on catalyst. Thedeactivated catalyst is taken from the bottomof the cracker and brought into regeneratorwhere it is heated to about 500°C in thepresence of hot air to burn carbon dioxidewhich is taken out from the top of thegenerator. The regenerated catalyst in hotcondition is taken down to the vapours of heavyoil and re-circulated in the cracker.
Sr. No.
CharacteristicFixed bed catalytic
crackingMoving bed catalytic
cracking
1Chamber reaction
temperature425°C-540°C 550°C-570°C
2 Pressure 1.5 kg/cm2 Around 1 kg/cm2
3 Octane value 80-85 85-90
Comparison of fixed bed and moving bed catalytic cracking
The cracking reaction can be carried out at lower temperature
and pressure.
The cracking is specific in nature and can give proper quality
of gasoline.
The octane value of gasoline is higher by catalytic process,
hence better for petrol engine.
The process can be better controlled than the thermal process.
The product contains less sulphur compounds.
The percentage of gum or gum forming compounds is very
low.
Sr. No.
Thermal Cracking Catalytic Cracking
1Heavy oils are cracked by simply
heating them under high temperature and pressure.
Heavy oils are cracked using small quantity of catalyst.
2It is of two types: Liquid and vapour
phase.It is of two types: Fixed bed and moving bed.
3
Temperature and Pressure is of high range as:
(a) Liquid Phase:T: 420-450°C, P: 15-100 kg/cm2
(b) Vapour Phase:T: 600-650°C, P: 15-20 kg/cm2
Temperature and pressure is of low range due to catalyst:
(a) Fixed bed:T: 425-540°C, P: 1.5 kg/cm2
(b) Moving bed:T: 550-570°C, P: very low
4Octane value of product ranges from
60-80.Octane value of product ranges from 80-90.
5Yield % of gasoline is low (app. 50-
60%).Yield % of gasoline is low (app. > 75%).
6Efficiency is low and not commonly
used in refineries.Efficiency is high and used in modern refineries.
7 Initial and operating cost is high. Initial cost is high but operating cost is low.
Comparison between thermal cracking and catalytic cracking
Chemical nature:
Chemically biodiesel is the methyl esters of long chain carboxylic acids.
Biodiesel is obtained by transesterification of vegetable oil or animal fat with methyl alcohol using sodium metal or sodium methoxide, as catalyst .
Transesterificaion
Transesterificaion is the process of converting one ester to another
ester.
A molecule of oil or fat is the trimester of glycerol and three
molecules of long chain carboxylic acids. This triester is converted
into methyl ester of the fatty acids by the following reaction:
Vegetable oil/Animal fat
1. Filter the cheap or waste vegetable oil/fat.
2. Heat it at 110ᵒC with stirring to remove any water from it.
3. Prepare sodium methoxide from sodium metal and methanol. Add the sodium
methoxide about 2% by weight to the vegetable oil or fat.
4. Add methanol about 20% stirring for 30 minutes.
5. Cool and mix sufficient water, stir well. The glycerol and soap dissolve in water
phase.
6. Separate the water insoluble phase (biodiesel) from water phase.
7. Add antioxidant to the biodiesel to avoid it to become gummy due to oxidation
and polymerization.
8. Biodiesel can be obtained from various vegetable oils like soyabeen oil, palm
oil, groundnut oil , cottonseed oil, mustard oil, sunflower oil etc.
Biodiesel can be used as good fuel for diesel engines but generally it is used as its 20% mixture with diesel.
Biodiesel is cheaper.
It has high cetane numbers 46 to 54 and high C.V. of about 40kJ/gm.
It is regenerative and environment friendly.
It does not give out particulate and CO pollutants.
It has certain extent of lubricity.
It is clean to use biodiesel in diesel engines.
A propellant is a chemical which is used in the production of
energy and pressurized gas that is used to create movement
of a fluid or to generate propulsion of a vehicle or projectile
or other object.
In rockets and aircraft, propellants are used to produce a gas that can be directed through a nozzle, thereby producing
thrust. In rockets, rocket propellant produces an exhaust and
the exhausted material is usually expelled under pressure
through a nozzle. The pressure may be from a compressed
gas, or a gas produced by a chemical reaction. The exhaust
material may be a gas, liquid, plasma, or, before the
chemical reaction, a solid or liquid.
In this method of propulsion techniques, propellants used are basically chemicals, which produces high amount of energy
on burning. Depending upon the physical state of the
propellant used, they can be classified as:
Propulsion using solid propellants: - Here solid propellants are used to propel the rocket When the solid fuel is ignited, it
burns along the walls of the combustion chamber. As
discussed earlier, solid fuels have perforation. This is to
increase the surface area and eventually to increase the
thrust produced by the rocket. As the combustion proceeds,
the perforation shape changes into a circle. This provides
high thrust initially and thrust lowers during the middle of the
flight.
Types of Propellant
Solid Propellants
Liquid Propellants
Hybrid Propellants
Any solid propellant consists of two parts:
• An oxidizer
• A fuel or a reducer.
In solid propellants, the fuel and oxidizer components are
prepared separately and are then mixed together. This is
because the oxidizer is in powder form and the fuel is a fluid of
varying consistency. They are then blended together and poured
into the rocket case under carefully controlled conditions. In
addition to fuel and oxidizer, some other compounds are added to increase the efficiency of the propellants.
The oxidizer is ammonium per chlorate
(NH4ClO4) (69.93 %).
The fuel is a form of powdered aluminum
(16 %).
SOLID PROPELLANTS
Homogeneous Solid Propellants
Simple Base Homogeneous
Solid Propellants
Double Base Homogeneous
Solid Propellants
Composite Solid Propellants
Liquid propellants are nothing but rocket propulsion fuels in liquid state.
They are made up of 2 parts:
• An oxidizer &
• A fuel.
Both the oxidizer and fuel are in liquid form. Liquid propellants are more
difficult to handle than solid propellants they require separate oxidizer
and fuel tanks. Lightweight pumps and injectors are used to spray the
propellants into the combustion chamber. The combustion of liquid
propellants can be controlled easily by controlling the rate at which the
pumps spray the liquid into the combustion chamber. Shutting off the
pumps completely can easily stop combustion. Thus controlling,
stopping and starting the combustion is very easy when liquid
propellants are used. In order to start the combustion process, spark
plugs, igniters, explosives are used.
Liquid propellants used in launch vehicles
can be classified into:
• Petroleum
• Cryogenic propellants
• Hypergolic propellants
Liquid oxygen and
Liquid hydrogen
In a cryogenic propellant the fuel and the oxidizer are in the form of very cold, liquefied gases. These liquefied gases are referred to as super cooled as they stay in liquid form even though they are at a temperature lower than the freezing point. Thus we can say that super cooled gases used as liquid fuels are called cryogenic fuels. These propellants are gases at normal atmospheric conditions. But to store these propellants aboard a rocket is a very difficult task as they have very low densities. Hence extremely huge tanks will be required to store the propellants. Thus by cooling and compressing them into liquids, we can vastly increase their density and make it possible to store them in large quantities in smaller tanks. Normally the propellant combination used is that of liquid oxygen and liquid hydrogen, Liquid oxygen being the oxidizer and liquid hydrogen being the fuel. Liquid oxygen boils at 297oF and liquid hydrogen boils at 423oF.
Hybrid propellants are those propellants, which are a mixture of solid and liquid propellants. In these propellants, one of the two components (oxidizer and fuel) is solid (usually fuel) whereas the other is liquid (usually oxidizer). In a hybrid propellant rocket engine, the liquid part is injected into the solid part. Thus the storage chamber of the solid part acts as the combustion chamber. In a hybrid rocket the oxidizer flows down the perforation (see solid propellants) (This is not a part of the site tour) in the solid fuel grain and reacts with the solid fuel. This produces the hot exhaust gases required to produce thrust. This process can be seen in the following image: In many hybrid motor designs, the oxidizer is pressurized liquefied nitrous oxide (N2O) while the fuel is cellulose (C6H10O5 ).