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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.
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 completecombustion of unit weight of the fuel” is called as calorificvalue. Calorific value can also defined as, “the number ofparts of water which gets heated through 1°C by the heatevolved by the complete combustion of one unit weight fuelunder 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 bemost useful, must possess high calorific value because the heatevolved by combustion of definite weight of fuel is directlyrelated/proportional to the calorific value of the fuel.
Definition: Ignition temperature is defined as “minimum temperature to which asubstance must be heated before its burns spontaneously independent of thesource of heat.”E.g.: Ethanol has an ignition temperature of 425 C/798 F and flash point of12 C/54 F.Each fuel should be brought above its Ignition Temperature for starting thecombustion process. The minimum ignition temperature at atmospheric pressure forsome substances are:o Carbon: 400°Co Gasoline: 260°Co Hydrogen: 580°Co Carbon monoxide: 610°Co 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 amountof 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 calorificvalues of all the elements present . The calorific values of different elementsare given as under: Calorific Value of C = 8080 cal/g Calorific Value of H = 34500 cal/g Calorific Value of S = 2240 cal/gThus, 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: With minimum 92-98% fixed carbon Meta-anthracite and maximum 2-8% volatile matter. 87-91% fixed carbon, 9-13% volatile Anthracite matter. Semi-anthracite 86% fixed carbon, 14% volatile matter. Low rack coals are graded further on the basis of their moist B.T.U., indicating natural moisture
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 ofsuch a storage of petroleum varies from few hundreds to fewthousands of feet below the surface of the earth.It is surrounded by layers of natural gas, under the earth. Inshort, the crude petroleum thus formed floats upon a layer of saltwater and is surrounded by layer of natural gas, deep below theimpervious rock. Mining of oil is done by drilling holes inearth’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 petroleumpiped out with pressure.
Crude oil coming from the petroleum wellsconsists of a viscous, dark coloured frothingmixture of solid, liquid and gaseoushydrocarbons containing sand and water insuspension. In order to make it into a marketableproduct, the oil is made free from impurities likewater, dissolved salts, sulphur etc. The process by which petroleum is made freefrom impurities and separated into various usefulfraction having different boiling points andfurther treated to remove undesirabletendencies and to impart specific properties tothem 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 emulsionof oil and salt water. The process of removing oil from water consists inallowing the crude to flow between two highly charged electrodes.The colloidal water droplets aggregates to form large drop, whichseparate out from the oil. To remove the persistent impurities, colouretc., various fractions are passed over adsorbents like Kieselgure clayetc. and the resultant oils are generally pure.(2) REMOVAL OF HARMFUL IMPURITIESNaCl and MgCl2 can corrode the refining equipment and can causescale formation in the heating pips. Hence special care should betaken to remove them. Modern techniques of electrical desaltingand dehydration are developed for this purpose. Then oil is treatedwith copper-oxide. The reaction occurs with sulphur compound,which result in the formation of copper sulphide (a solid), which isthen removed by filtration.
(3) FRACTIONAL DISTILLATION Fractional distillation is a combination of distillationand rectification. Rectification process consists of counterflow contacting of the vapour formed in distillation with theliquid obtained by condensation of vapuor. For effectiverectification in distillation column, it is essential to see thatan ascending flow of vapour (formed due to the heatsupplied at the bottom section) meets the descending flowof 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 andbubble tower. In pipe still the crude petroleum is heatedand is fractionated in bubble tower.
Bubble tower consists of horizontal stainless steel trays or plates atshort distances. Each tray is provided with a small chimney covered withloose cap though which vapour rises up. These small chimneys are coveredwith loose bubble caps.Crude oil is piped through a pipe still where it is heated to about 400*C andthe vapours are the introduced near the bottom of the bubble tower. As thevapour move upwards, the 3 higher boiling fractions condense at lowerplats and only the lower boiling fractions move to higher plates. The vapourare allowed to pass up through higher plates via bubble caps. The heaviercomponent having high boiling fraction condense and the condensateflows down to the next lower tray through the down comers. The vapourwhich condenses give out the latent heat of condensation to the liquid inthe tray, from which more volatile components moves up the column.This process of condensation and vapourization takes place many timecausing separation of the fractions according to their boiling points. Thushigher boiling fractions condense at the lower parts of the column and lowerboiling fractions condense at the higher parts of the column. Thus the crudeoil is fractionated into different fractions depending on their boiling rangesand are collected at different heights in the column. In this way mixture ofvapours and liquids of different boiling points are separated. Thisfractionation gives uncondensed gases, gasoline, kerosene and gas oilfraction.
Cracking is a process of converting heavy oil withhigher molecular weight hydrocarbons to the oil with lowermolecular weight hydrocarbon which is known as gasoline. Thus, heavy oil is heated at a high temperatureunder pressure or in the presence of catalyst to obtaingasoline.For example: C10H22 C5H12 + C5H10 (Decane) n-Pentane Pentene
There are two methods of cracking Thermal cracking Catalytic crackingliquid phase vapour Fixed bed Moving bed or phase
Liquid phase thermal cracking:By this method, any type of oil can be cracked. In this method, theoil is pumped into the coil kept at 420°C-550°C under a pressure of15-100 kg/cm2. As the temperature increases, a better quality ofgasoline is produced. The octane value of this gasoline is low, i.e. 65-70. Therefore, it is mixed with higher octane value gasoline and thenused I engine. Vapour phase thermal cracking:In this method, the heavy oil is heated in the heater at 400°C toconvert it into the vapours and then these vapous are passed to thereaction chamber which is maintained at 600°C-650°C and under apressure of 10-20 kg/cm2. At this stage, the higher hydrocarbons areconverted into lower hydrocarbons easily and the octane value topetrol is usually 75-80.
Comparison of liquid phase and vapour phase thermal crackingSr. Liquid Phase Thermal Vapour Phase CharacteristicsNo. Cracking Thermal Cracking Cracking Moderate (420- 1 600-650°C temperature 550°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) Pre-requirement for Any type of heavy oil Oil has to be 5 process can be used vaporised readily 6 Time required Comparatively more Comparatively less
Catalytic CrackingFixed bed catalytic cracking: In this method, vapours of the heavy oil are heated in the presence ofcatalyst 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, thecatalyst used are crystalline alumina-silicate, bentonite, bauxite and zeolites. Thereaction 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) andallowed to pass into fractionating tower, where gasoline fraction is collected. Theoctane value of this gasoline is about 80-85. During the cracking, free carbon is alsoformed which deposits on catalyst, then the flow of vapours of heavy oil is passedover the second set of reaction chamber and the catalyst is earlier chamber isregenerated by burning the carbon deposits with the help of air and reused.
Moving bed catalytic cracking:It is also called fluidised bed catalytic cracking principle. In this method, a fine powder of catalyst is circulated throughthe cracker along with the vapours of heavy oil (higher hydrocarbon).The catalyst accelerates and directs the cracking efficiently to formgasoline and other lower hydrocarbons. For example:C18H38 C10H22 + C8H16n-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.
Comparison of fixed bed and moving bed catalytic crackingSr. Fixed bed catalytic Moving bed catalytic CharacteristicNo. cracking cracking Chamber reaction1 425°C-540°C 550°C-570°C temperature2 Pressure 1.5 kg/cm2 Around 1 kg/cm23 Octane value 80-85 85-90
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.
Comparison between thermal cracking and catalytic crackingSr. Thermal Cracking Catalytic CrackingNo. Heavy oils are cracked by simply Heavy oils are cracked using small quantity of 1 heating them under high catalyst. temperature and pressure. It is of two types: Liquid and vapour 2 It is of two types: Fixed bed and moving bed. phase. Temperature and Pressure is of high Temperature and pressure is of low range due range as: to catalyst: (a) Liquid Phase: (a) Fixed bed: 3 T: 420-450°C, P: 15-100 kg/cm2 T: 425-540°C, P: 1.5 kg/cm2 (b) Vapour Phase: (b) Moving bed: T: 600-650°C, P: 15-20 kg/cm2 T: 550-570°C, P: very low Octane value of product ranges from 4 Octane value of product ranges from 80-90. 60-80. Yield % of gasoline is low (app. 50- 5 Yield % of gasoline is low (app. > 75%). 60%). Efficiency is low and not commonly 6 Efficiency is high and used in modern refineries. used in refineries. 7 Initial and operating cost is high. Initial cost is high but operating cost is low.
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 .
TransesterificaionTransesterificaion is the process of converting one ester to anotherester.A molecule of oil or fat is the trimester of glycerol and threemolecules of long chain carboxylic acids. This triester is convertedinto 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 Liquid HybridPropellants Propellants Propellants
Any solid propellant consists of two parts:• An oxidizer• A fuel or a reducer.In solid propellants, the fuel and oxidizer components areprepared separately and are then mixed together. This isbecause the oxidizer is in powder form and the fuel is a fluid ofvarying consistency. They are then blended together and pouredinto the rocket case under carefully controlled conditions. Inaddition to fuel and oxidizer, some other compounds are addedto increase the efficiency of the propellants.
The oxidizer is ammonium per chlorate (NH4ClO4) (69.93 %). The fuel is a form of powdered aluminum (16 %).
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 moredifficult to handle than solid propellants they require separate oxidizerand fuel tanks. Lightweight pumps and injectors are used to spray thepropellants into the combustion chamber. The combustion of liquidpropellants can be controlled easily by controlling the rate at which thepumps spray the liquid into the combustion chamber. Shutting off thepumps completely can easily stop combustion. Thus controlling,stopping and starting the combustion is very easy when liquidpropellants are used. In order to start the combustion process, sparkplugs, igniters, explosives are used.
Liquid propellants used in launch vehicles can be classified into:• Petroleum• Cryogenic propellants• Hypergolic propellants
In a cryogenic propellant the fuel and the oxidizer are in theform of very cold, liquefied gases. These liquefied gases arereferred to as super cooled as they stay in liquid form eventhough they are at a temperature lower than the freezingpoint. Thus we can say that super cooled gases used as liquidfuels are called cryogenic fuels. These propellants are gasesat normal atmospheric conditions. But to store thesepropellants aboard a rocket is a very difficult task as theyhave very low densities. Hence extremely huge tanks will berequired to store the propellants. Thus by cooling andcompressing them into liquids, we can vastly increase theirdensity and make it possible to store them in large quantitiesin smaller tanks. Normally the propellant combination used isthat of liquid oxygen and liquid hydrogen, Liquid oxygenbeing the oxidizer and liquid hydrogen being the fuel. Liquidoxygen 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 ).