Essay: Fuels – Classification – solid fuels

Fuels are materials such as coal, gas, or oil that is burned to produce heat or power. The various types of fuels like liquid, solid and gaseous fuels are available for firing in boilers, furnaces and other combustion equipments. The selection of right type of fuel depends on various factors such as availability, storage, handling, pollution and landed cost of fuel.
The knowledge of the fuel properties helps in selecting the right fuel for the right purpose and efficient use of the fuel.
Classification:
Fuels may broadly be classified in two ways, i.e. (a) according to the physical state in which they exist in nature as solid, liquid and gaseous and (b) according to the mode of their procurement as natural and manufactured.Some of the fuels are:
Class A – Wood, paper, cloth, trash, plastics.
Solid combustible materials that are not metals.
Class B – Flammable liquids: Gasoline, oil, grease, acetone.
Any non-metal in a liquid state on fire.
Class C – Electrical: Energized electrical equipment.
As long as it\’s “plugged in,” it would be considered a class C fire.
Class D – Metals: Potassium, sodium, aluminum, magnesium.
4.1.1 Coal – analysis of coal – proximate and ultimate analysis and their significance.
Coal is a natural fuel formed by the slow carbonization of vegetable matter buried under the earth some thousands of years ago. It is classified into four kinds based on the carbon content and the calorific value. They are:
1. Peat
2. Lignite
3. Bituminous Coal
4. Anthracite Coal
Peat:
It is the first stage of formation of coal from wood. It is brown, fibrous jelly-like mass. It contains 80-90% moisture. The composition of peat is C=57%; H=6%; O=35% Ash=2.5%. The calorific value of peat is 5400kcal/kg. It is a low grade fuel due to high water content. It is used as a fertilizer and packing material.
Lignite:
Lignite is immature form of coal. It contains 20-60% moisture. Air dried Lignite contains C=60-70% O=20%. It burns with a long smoky flame. The calorific value of lignite is 6500-7100 kcal/kg.
Uses:
1.It is used as a domestic fuel.
2.It is used as a boiler fuel for steam raising.
3.It is used in the manufacture of producer gas.
Bituminous Coal:
It is a high quality fuel. Its moisture content is 4%. Its composition is C=83%. O=10%, H=5% and N=2%. Its calorific value is 8500 kcal/kg.
Uses:
1. It is used in metallurgy.
2. It is used in steam raising.
3. It is used for making coal gas.
4. It is also used for domestic heating.
Anthracite Coal:
It is the superior form of coal. it contains C=92-98%, O=3%, H=3% and N=0.7%. It burns without smoke. It\’s calorific value is 8700 \\kcal/kg.
Uses:
1. It is used for steam raising and house hold purposes.
2. It is used for direct burning in boilers and in metallurgy.
3. It is used in thermal power plant.
4. It is used in coal tar distillation.
5. It is used in glass furnaces.
Analysis of coal
Proximate analysis
Proximate analysis includes the determination of moisture, volatile, ash and fixed carbon content.
(i) Determination of Moisture content: 1 g of finely powdered air dried sample is taken in a crucible and heated in an electrically heated hot air oven at 105°C-110°C for 1 hr. After heating, the crucible is taken out side, cooled in a dessicator and weighed.
(ii) Determination of Volatile matter: The dried sample left in the crucible along with the lid is heated in a muffle furnace at 950 ± 25°C for 7 minutes and then cooled in a desiccator and weighed.
(iii) Determination of Ash content: The residual sample after the two above experiments in the crucible is heated in the furnace at 750°C for 30 minutes without the lid. Then it is cooled in a desiccator and weighed.
(iv) Determination of Fixed carbon: The fixed carbon content can be determined indirectly by subtracting the percentage of moisture, volatile and ash content from 100.
Percentage of fixed carbon = 100 — % of (moisture+volatile matter + ash).
Significance:
a) Higher percentage of moisture content lowers the calorific value of coal. Hence lower the moisture content better the quality of coal.
b) Higher percentage of volatile matter reduces the calorific value of coal. Low volatile matter also reduces coking property of coal.
c) Ash being non combustible reduces the calorific value of coal. Ash deposition also causes problems in the furnace walls and the ultimate disposal of ash is also a problem.
d) Higher the percentage of fixed carbon, higher is the calorific value and better is the quality of coal.
Ultimate Analysis: It refers to determination of weight percentage of carbon, hydrogen, nitrogen, oxygen and sulfur.
(i)Carbon and Hydrogen: A known amount of coal sample is taken and burnt in a current of O2 in combustion apparatus whereby CO2 and H2O are formed. CO2 and H2O are absorbed by previously weighed tubes containing KOH and anhydrous CaCl2. The increase in weight gives the C and H content as follows:
C + 02 → CO2
H2 + 1/2 O2 → H2O
2KOH + CO2 → K2CO3 + H2O
CaCl2 + 7H2O → CaCl2 . 7H2O
Increase in weight of KOH tube % of C= × 12/44 × 100.
Weight of coal taken.
Increase in wight of CaCl2 tube % of H = × 2/18 × 100.
(ii) Nitrogen: About 1 g of accurately weighed coal sample is taken in a long necked flask along with conc. H2SO4, K2SO4 and heated. Then it is treated with excess of NaOH and the liberated NH3 is absorbed in known excess of standard acid solution. The excess acid is back-titrated with standard NaOH solution. From the volume of acid consumed, N content is calculated as follows:
 
(iii) Sulphur: While determining the calorific value of a coal sample in a bomb calorimeter, the S in the coal is converted to sulphate. Finally, the washings containing sulphate is treated with dilute HCl and BaCl2 solution which precipitates BaSO4 which is filtered in a sintered glass crucible, washed with water and heated to a constant weight.
(iv) Oxygen content = 100 – % of (C + H + S + N).
Significance:
a) Higher percentage of C and H increases the calorific value of coal and hence better is the coal.
b) Higher the percentage of O2 lower is the calorific value and lower is the coking power. Also O2 when combined with hydrogen in the coal, hydrogen available for combustion becomes unavailable.
c) S although contributes to calorific value is undesirable due to its polluting properties as it forms SO2 on combustion.
problem
The ultimate analysis of a coal sample indicates Carbon = 84%, Sulphur = 1.5%, Nitrogen = 0.6%, Hydrogen = 5.5% and Oxygen = 8.4%. Let us calculate the GCV.
Solution:
Given data:
C = 84%
S=1.5%
N = 1.6%
H = 5.5%
O= 8.4%
4.2 Liquid fuels – petroleum and its refining – cracking – types – fixed bed catalytic cracking.
Petroleum is the naturally available liquid fuel. It is a dark greenish-brown viscous oil found deep in earth\’s crust. It is composed of various hydrocarbons with small amount of other organic compounds as impurities.
Refining of Petroleum: The process of purification and separation of various fractions present in petroleum by fractional distillation is called refining of petroleum. Refining is done in oil refineries.
Cracking:
Cracking is a process by which the hydro carbons of high molecular mass are decomposed into hydrocarbons of low molecular mass by heating in the presence or absence of a catalyst. Generally, aluminum silicates are used as catalyst.
Example:
Gas oil and Kerosene contain hydrocarbons of high molecular mass and boiling. They are unsuitable as fuel in automobiles. Hence they are decomposed into hydrocarbons of low molecular mass and low boiling point.
Fluidized (moving) bed catalytic cracking:
Principle:
In this the finely divided catalyst is kept agitated by cracking oil, so that it can be handled like a fluid system. One of the advantages of the FCC is that it gives very good contact between oil and catalyst. Therefore high yield of petrol is obtained.
Conditions: Feed stalk : Vapours of heavy oil fraction .
Catalyst : Al2O3 + SiO2(Alumina+ silicon dioxide)
Temperature : 530oC
Pressure : Little above the normal pressure.
Yield : Very high, usually 10 gallons per day.
The finely divided catalyst is fluidized by the upward passage of the feed stalk in the cracking chamber. Cracked vapors are continuously withdrawn from the cracking chamber and fed into fractionating column where it gets separated into gas and gasoline. The uncracked oil may be cracked in the second stage of cracking process. The spent catalyst is continuously withdrawn from the bottom of the cracking chamber and transported into regenerators by a stream of air in which the carbon deposited on a catalyst is burnt off using hot air. The regenerated catalyst is mixed with fresh feed stalk and returned to cracking chambers.
4.3 Knocking – octane and cetane rating
Octane Number of a fuel
Octane number of a fuel is a measure of its ability to resist knocking. The knocking characteristics of petrol are described by the octane number. Higher the octane number lower is the knocking tendency & better is the quality of petrol. The octane number is an arbitrary factor. Isooctane when used as a fuel found to have zero knocking hence its octane number is taken as 100. But when n-heptane was used as a fuel it had maximum knocking hence its octane number is taken as zero.
For a petrol sample under test whose octane number is to be determined, it is compared with reference standards of isooctane and n-heptane prepared at different ratios( 90:10; 80:20, 75:25 etc) and the knocking characteristics of these is determined under same conditions as that of the sample under test. Suppose the knocking characteristics of the fuel is same as that of 80 :20 mixture, the octane number of the fuel is 80. Therefore the octane number is defined as the percentage by volume of isooctane present in a standard mixture of isooctane and n – heptane which has the same knocking characteristics as the petrol under test.
Cetane Number of a fuel: Cetane number of a fuel is a measure of its ability to resist knocking. The knocking characteristics of Diesel are described by the Cetane number, Cetane number is defined as the percentage by volume of Cetane(or hexa-decane,) present in a standard mixture of cetane and α- methyl naphthalene which has the same knocking characteristics as the diesel fuel under test.
Generally diesel fuels with cetane numbers of 70-80 are used.
Petrol: The followings are the substances added to petrol in order to prevent knocking in I.C. Engines.
Ex: TEL – Tetra Ethyl led.
TML – Tetra Methyl led.
MTBE – Methyl Tertiary Butyl Ether.
Leaded Petrol:
The petrol containing TEL or TML as anti knocking agents is called leaded petrol. TEL or TML are the very good anti knocking agents but has some disadvantages as follows:
a) After combustion lead is deposited as lead oxide on piston and engine walls it leads to mechanical damage.
b) Lead is a poisonous air pollutant.
c) It spoils the catalyst used in catalytic converter.
Unleaded Petrol:
The petrol, which contains anti-knocking agent other than lead, is known as unleaded petrol.
Ex: MTBE is used, as an anti-knocking agent in place of TEL or TML and the petrol is known as unleaded petrol.
Knocking: The explosive combustion of petrol and air mixture produces shock waves in I.C. engine, which hit the walls of the cylinder and piston producing a rattling sound is known as knocking.
Mechanism of Knocking
Beyond a particular compression ratio the petrol mixture suddenly burns into flame. The rate of flame propagation increases from 20 to 25m/s to 2500m/s, which propagates very fast, producing a rattling sound.
The activated peroxide molecules decomposes to give number of gaseous products which produces thermal shock waves which hit the walls of the cylinder and piston causing a rattling sound which is known as knocking.
The reactions of normal and explosive combustion of fuel can be given as follows taking ethane as an example:
Prevention of Knocking ( Anti-knocking agents )
The substances added to gasoline to control knocking are called anti-knocking agents. Usually organometallic compounds are added to gasoline.
The followings are the substances prevent knocking in I.C. Engines.
TEL – Tetra Ethyl led(leaded petrol).
TML – Tetra Methyl led.
MTBE – Methyl Tertiary Butyl Ether(unleaded petrol).
4.3.1 Synthetic petrol, Bergius and Fischer-Tropsch’s process:
Synthesis of petrol by Fischer-Tropsch process: (Indirect conversion of coal)
This method involves the following steps:
Production of water gas: Water gas (CO+H2) is obtained by passing steam over white hot coal.
b) The water gas is mixed with hydrogen and the mixture is purified by passing through Fe2O3 and then passed into a mixture of Fe2O3 +Na2CO3.
Fe2O3 is used to remove H2S.
Fe2O3 + Na2CO3 is used to remove organic sulphur compounds.
Production of synthesis gas: Water gas obtained above is freed from dust, H2S and organic Sulfur compounds and blend water gas with hydrogen to form synthesis gas (CO + 2H2).
c) Hydrogenation of carbon monoxide: the Synthesis gas (CO + 2H2) is compressed to 5-10 atm pressure and admitted into a catalytic reactor containing the catalyst (mixture of cobalt (100 parts), thoria (5 parts) and magnesia (8 parts)). The reactor is heated to about 250°C. Hydrogenation, reactions takes place to form saturated and unsaturated hydrocarbon. These mixture of saturated and unsaturated hydrocarbons are passed through a fractionating column for separation of petroleum fractions. It is produced as a result of polymerisation.
Reforming of Petrol:
Reforming is a thermo catalytic process carried out to improve the octane number of petrol by bringing about changes in the structure of hydrocarbons. It involves a molecular rearrangement of hydrocarbons without any change in the number of carbon atoms to form new compounds It is usually brought about by passing the petroleum fraction at about 500oC over platinum coated on aluminum catalyst in the presence of hydrogen. The changes in structure could be isomerization, cyclization or aromatization.
Isomerization: Straight chain hydrocarbons with low octane number are converted to branched hydrocarbons having high octane number.
Cyclization: Straight chain hydrocarbons with low octane number are converted to cyclic compounds having high octane number.
Aromatization: Cyclic compounds with low octane number undergo dehydrogenation to form aromatic compounds having high octane number.
Dehydrogenation: In dehydrogenation cyclo alkanes gets converted to aromatic hydrocarbon.
Manufacture of synthetic petrol (Bergius process)
Coal is ground and made into a paste with heavy recycle oil and a catalyst like tin oleate. The paste is sent along with H2 at 250-350 atm pressure into a converter which is maintained at 450°C—500°C temperature. The un-reacted coal is filtered-off and the liquid product distilled. Hydrogen combines with coal to form saturated hydrocarbons which decompose at high temperature yielding low-boiling hydrocarbons. The crude oil is fractionated to get (i) gasoline (ii) middle oil and (iii) heavy oil which is recycled. Middle oil is hydrogenated in vapour phase with catalyst to yield more gasoline. Yield of gasoline is 60% of the coal dust.
Bergius process
4.4 Gaseous fuels – constituents, characteristics and applications of natural gas
Gaseous fuels occur in nature, besides being manufactured from solid and liquid fuels. 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.
Types of gaseous fuel
The following is a list of the types of gaseous fuel:
1. Fuels naturally found in nature:
– Natural gas
– Methane from coal mines
2. Fuel gases made from solid fuel:
– Gases derived from coal.
– Gases derived from waste and biomass.
– From other industrial processes (blast furnace gas)
3. Gases made from petroleum:
– Liquefied Petroleum gas (LPG)
– Refinery gases
– Gases from oil gasification
4. Gases from some fermentation process:
Advantages
Gaseous fuels due to erase and flexibility of their applications, possess the following advantages over solid or liquid fuels:
(a) They can be conveyed easily through pipelines to the actual place of need, thereby eliminating manual labour in transportation.
(b) They can be lighted at ease.
(c) They have high heat contents and hence help us in having higher temperatures.
(d) They can be pre-heated by the heat of hot waste gases, thereby affecting economy in heat.
(e) Their combustion can readily by controlled for change in demand like oxidizing or reducing atmosphere, length flame, temperature, etc.
(f) They are clean in use.
(g) They do not require any special burner.
(h) They burn without any shoot or smoke and ashes.
(i) They are free from impurities found in solid and liquid fuels.
Disadvantages:
(a) Very large storage tanks are needed.
(b) They are highly inflammable, so chances of fire hazards in their use is high.
Natural gas:
The natural gas is said to be a fossil fuel formed during layers of buried plants, gases and animals are exposed to intense heat and pressure over thousands of years. Therefore the energy that the plants originally obtain from the sun is stored in the form of chemical bonds in natural gas.
Characteristics of Natural Gas
Such as crude oil, the natural gas is an energy source based on hydrocarbon chains, but the composition of natural gas is generally different than the composition of crude oil. The natural gas is primarily composed of methane, even though some natural gas deposits additionally contains substantial fractions of other hydrocarbon gases or liquids like ethane and propane. Most of the gas deposits also contain impurities like sulfur or other carbon compounds that should be separated prior to the gas being injected into transmission or distribution pipelines. The gas deposits that consists primarily of methane are called as dry gas deposits, while those with larger fractions of other hydrocarbons are known as wet or rich gas deposits.
Unlike oil, the natural gas is essential to its transportation system without pipelines, there is no economical way to get large quantities of gas to market. Though, the natural gas pipelines generally require to be dedicated assets. Using oil or petroleum product pipelines to move the natural gas is not really possible, and moving other products in natural gas pipelines is not possible without completely re-purposing the pipeline. This is an asset specificity and complementarity between natural gas and the pipeline transportation infrastructure has been a significant factor in the development of the natural gas market. Every has little use without the other.
Chemical Composition of Natural Gas
The natural gas is primarily composed of methane, that also contains ethane, propane and also heavier hydrocarbons. It contains small amounts of nitrogen, carbon dioxide, hydrogen sulphide and also trace amounts of water.
LNG COMPOSITION (Mole Percent)
Source Methane Ethane Propane Butane Nitrogen
Alaska 99.72 0.06 0.0005 0.0005 0.20
Algeria 86.98 9.35 2.33 0.63 0.71
Baltimore Gas & Electric 93.32 4.65 0.84 0.18 1.01
New York City 98.00 1.40 0.40 0.10 0.10
Sand Diego Gas & Electric 92.00 6.00 1.00 – 1.00
Application:
The gas district cooling and cogeneration is the production of electricity and chilled water for the cooling system in an office building. The application of gas cooling can be seen in both Thailand and overseas for instance, the new Tokyo national airport in Japan, Kuala Lumpur International Airport and Suvarnabhumi Airport, Thailand.
The natural gas will be used for cooking in replace of LPG in hotels, hospitals, restaurants and residences. The gas will be used with all types of stoves, ovens, grills, and rice cookers.
Natural gas is used for producing the hot water and steam in hotels, the laundry services, the sterile process at hospitals and household residence.
4.4.1 LPG and CNG
CNG:
Stored in a high-pressure container (usually at 3000 to 3600 psi). It is used mainly as an alternative fuel for internal combustion engines (such as automobile engines). It generates low hydrocarbon emissions but a significant quantity of nitrogen oxide emissions.
LPG:
Liquefied petroleum gas or liquid petroleum gas (LPG or LP gas) also referred to as simply propane or butane are flammable mixtures of hydrocarbon gases used as fuel in heating appliances, cooking equipment and vehicles.
It is increasingly used as an aerosol propellant and a refrigerant replacing chlorofluorocarbons in an effort to reduce damage to the ozone layer. When specifically used as a vehicle fuel it is often referred to as autogas.
4.5 Analysis of flue gas by Orsat’s apparatus – Numerical Problems.
ORSAT method
The mixture of gases like CO2, CO, O2 etc., coming out from the combustion chamber is called as flue gas.
The amount of flue gases like CO2, CO, O2 etc.,can be estimated by using Orsat’s apparatus on the absorption principle. The gases like CO2, CO and O2 are absorbed by KOH, alkaline pyro-gallon and ammoniacal cuprous chloride solutions respectively.
Analysis of these gases gives an idea about the complete combustion of fuel or not.
i. If CO is high in the flue gas shows incomplete combustion of the fuel and short supply of oxygen.
ii. If CO2 and O2 are high in the flue gas shows complete combustions of the fuel and excess supply of oxygen.
It consists of a horizontal tube having three way stop cock at one end. The end of three way stop cock is connected to a U tube containing fused CaCl2 to remove moisture in the gas. The end of the tube is connected with a graduated burette. The burette is surrounded by a water jacket in order to keep the temperature of gas constant. The lower end of the burette is connected by a water reservoir by means of rubber tube. The level of the water in burette can be raised or lowered by raising or lowering the reservoir. The middle of the horizontal tube is connected with 3 bulbs (A, B and C) for absorbing flue gases as follows:
i. Bulb ‘A’ containing KOH solution and it absorbs only CO2.
ii. Bulb ‘B’ containing alkaline pyro-gallon solution and it absorbs only O2.
iii. Bulb ‘C’ containing ammoniacal cuprous chloride solution and it absorbs CO.
Working of Orsat Apparatus:
Now the three way stop cock is opened and the burette is filled with water by raising the water reservoir to remove air from the burette. Then the flue gas is taken in the burette up to 100 cc by raising and lowering the reservoir. The 3 way stop cock is now closed.
Absorption of Gases on Bulbs:
i)Absorption of CO2:
The stopper of the bulb ‘A’ is opened and the flue gas is allowed to pass by raising the water reservoir. CO2 present in flue gas is absorbed by KOH. This process is repeated several times by raising and lowering the water reservoir until the volume of burette becomes constant. The decrease in volume of burette indicates the volume of CO2 in 100 cc of the flue gas. Now the stopper of the bulb ‘A’ is closed.
ii)Absorption of O2:
The stopper of the bulb ‘B’ is opened and the flue gas is allowed to pass. O2 present in the flue gas is absorbed by alkaline pyrogallol. This process is repeated several times until the volume of burette becomes constant. The decrease in volume of flue gas in burette indicates the volume of O2. Now the stopper is closed.
iii)Absorption of CO:
The stopper of the bulb ‘C’ is opened and the flue gas is allowed to pass. ‘CO’ present in the flue gas is absorbed by ammoniacal cuprous chloride solution. This process is repeated several times until the volume of burette becomes constant. The decrease in volume of flue gas in burette indicates the volume of CO. The remaining gas in the burette after the absorption of CO2, O2 and CO is taken as nitrogen.
The % of N2 = [ 100 – (% of CO2 + % of O2 + % of CO)]
Flue Gas Analysis
4.6 Combustion – Definition, Calorific value of fuel – HCV , LCV
Combustion:
Combustion is the rapid chemical combination of oxygen with the combustible elements of a fuel, resulting in the production of heat. Combustion is accomplished by mixing fuel and air at elevated temperatures. The air supplies oxygen which unites chemically with the carbon, hydrogen and a few minor elements in the fuel to produce heat. Steam has been generated from the burning of a variety of fuels.
The heating value of a fuel may be determined either by a calculation from a chemical analysis or by burning a sample in a calorimeter.
In the former method the calculation should be based on an ultimate analysis which reduces the fuel to its elementary constituents of carbon, hydrogen, oxygen, nitrogen, sulphur, ash and moisture to secure a reasonable degree of accuracy. A proximate analysis which determines only the percentage of moisture, fixed carbon, volatile matter and ash without determining the ultimate composition of the volatile matter cannot be used for computing the heat of combustion with the same degree of accuracy as an ultimate analysis but estimates may be based on the ultimate analysis that are fairly correct.
Calorific Value
Calorific Value (CV) is a measure of heating power and is dependent upon the composition of the gas. The CV refers to the amount of energy released when a known volume of gas is completely combusted under specified conditions.
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 being 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 that 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 are much more consistent.
Higher calorific value of a fuel portion is defined as the amount of heat evolved when a unit weight (or volume in the case of gaseous fuels) of the fuel is completely burnt and the products of combustion cooled to the normal conditions (with water vapor condensed as a result). The heat contained in the water vapor must be recovered in the condensation process.
Lower calorific value of a fuel portion is defined as the amount of heat evolved when a unit weight (or volume in the case of gaseous fuels) of the fuel is completely burnt and water vapor leaves with the combustion products without being condensed.
Principle:
The gross calorific value of a solid or liquid fuel when burnt in excess air or oxygen is calculated. The heat liberated is absorbed into surrounding water and copper calorimeter.
Construction: The bomb calorimeter consists of a cylindrical steel vessel (bomb A) with an airtight screw lid and an inlet valve B, for pumping oxygen. The bomb has a platinum crucible with a loop of wires or electrical ignition coil for initial combustion of fuel. The bomb is kept in a rectangular copper vessel (calorimeter) containing weighed amount of water. The equipment is provided with a mechanical stirrer for uniform heat distribution and a Beckmann thermometer to note the temperature. Calorimeter is enclosed in a jacket to minimize the heat exchange with surroundings.
Working: A known mass of the fuel is made into a pellet and taken in the crucible and oxygen is passed through the inlet valve. A known mass of water is taken in the calorimeter and is closed with the lid. The initial temperature of water is noted.The fuel is ignited by passing current through the coil. The heat released due to burning of fuel is absorbed by water. The temperature of water rises. The final temperature of water is noted on the thermometer.
Calculation
Let mass of fuel = mkg
Mass of water = w1kg
Water equivalent of calorimeter = w2 kg
Initial temperature of water = t10C
Final temperature of water = t20C
Specific heat of water (s) = kJ kg-1 0C-1
NCV can be calculated if the % of H2 in the fuel is known.
2 H + ½ O2 →H2O
If the fuel contains x% hydrogen, NCV of the fuel is calculated as follows:
2 atoms of hydrogen produces one molecule of water
2g of hydrogen produces 18 g of water
i.e., x g of hydrogen produces 9 x g of water
x % hydrogen (9/100) x g of water = 0.09 x g of water
NCV = GCV – latent heat of steam formed
= GCV – 0.09 × % of H2 ×latent heat of steam
= GCV – 0.09 × % of H2 × 587 cal/g (OR )
NCV = GCV – 0.09 × % of H2 ×2454 kJ kg-1
Problems
1. Let us calculate the gross calorific value and net calorific value of a sample of coal. 0. 5g of which when burnt in a bomb calorimeter, raised the temperature of 1000g of water from 293K to 296.4K. The water equivalent of calorimeter is 350 g. The specific heat of water is 4.187 kJ kg-1K-1, latent heat of steam is 2457.2 kJ kg-1. the coal sample contains 93% carbon, 5% hydrogen and 2% ash.
Solution:
Given:
m = Mass of the fuel = 0.5 g
w1= Mass of water taken = 1000 g
w2= Water equivalent of calorimeter = 350 g
t1 = Initial temperature of water = 293 K
t2= Final temperature of water = 296.4 K
S = Specific heat of water = 4.187 kJ kg-1K
Formula to be used:
NCV (solid fuel) = GCV – latent heat of steam formed.
= GCV – (0.09 ×% of H2) ×latent heat of steam
NCV (solid fuel) = GCV – latent heat of steam formed.
= GCV – (0.09 ×% of H2) ×latent heat of steam
= 38437 kJ kg-1- (0.09 ×5) ×2457.2 kJ kg-1
= 38437 kJ kg-1– 1106 kJ kg-1
= 37,331 kJ kg-1
2.Let us calculate the gross and net calorific value of a coal sample from the following data obtained from bomb calorimeter experiment.
Solution:
Given:
(i) Weight of coal (m) = 0.73 g.
(ii) Weight of water taken in calorimeter (w1) = 1500 g
(iii) Water equivalent of calorimeter (w2 ) = 470 g
(iv) Initial temperature (t1) = 25oC
(v) Final temperature (t2) = 27.3oC
(vi) Percentage of Hydrogen in coal sample = 2.5%
(vii) Latent heat of steam = 587 Cal/g
Formula to be used:
NCV (solid fuel) = GCV – latent heat of steam formed.
= GCV – (0.09 ×% of H2) ×latent heat of steam
Net calorific value = HCV – Heat released by the condensation of steam
= GCV – 0.09 × % H2× Latent heat of steam
= 25988 – 0.09 x 2.5 × 2454
= 25435.85 kJ kg-1
4.7 Determination of calorific value by Junker’s gas calorimeter
Principle of OperationThe gas calorimeter works on the Junker\’s principle of burning of a known volume of gas and imparting the heat with the maximum efficiency for steadily flowing water and finding out the rise in temperature of a measured volume of water. The formula is:
Calorific Value of Gas X Volume of Gas = Volume of water X Rise in Temperature
It is then used to determine the Calorific Value of the Gas.
Construction :
Junkers gas calorimeter consists of a combustion chamber surrounded by water jacket.
Then, a gas pipe line is connected with a burner kept in combustion chamber.
A gas flow meter and the pressure regulator are provided in a gas pipe line.
Thermometers are used to measure the temperature of water at inlet and outlet.
Then, condensate from the gases is collected in condensate pot.
Working :
The gas whose calorific value is to be measured is supplied through a pipeline to the gas burner where it is burnt.
The flow rate of gas is measured by a flow meter. The pressure of gas is measured by a manometer attached to the pressure regulator.
The heat produced by combustion of gas is absorbed by cold water which is flowing through water jacket. The gases are cooled upto room temperature as much as possible, thus the entire heat released from the combustion may be absorbed by circulating water.
The temperature of cooling water at the inlet and the outlet ,the exit gas temperature are measured. Mass flow rate of cooling water is also measured. Volume flow rate of gas is converted to STP condition.
CVg=( Vw x ℓw x CPw x ΔT ) / Vg xℓg
Where,
ℓw is the density of water .
Vw is the volume of water collected in litres .
CPw is the specific heat of water .
ΔT is the change in temp. of water.
Vg is the volume of gas burnt in litres .
ℓg is the density of the gas burnt .
4.7.1 Theoretical calculation of Calorific value by Dulong’s formula – Numerical problems on combustion.
If both hydrogen and oxygen are present, it could also be assumed that all the oxygen is combined with 1/8 of its weight of hydrogen to form water. This fraction is then deducted from the hydrogen content of the fuel in the calculation. Therefore for a fuel containing carbon, hydrogen, oxygen and sulphur, the calorific value of the fuel is given by Dulong Formula.
The ultimate analysis of coal gives data useful for the theoretical calculation of calorific value. It is based on the known gross calorific values of combustible constituents of coal. Dulong\’s formula for theoretical calculation of calorific value is given as:
where the values 8,080, 34,500 and 2,240 are higher calorific values of carbon, hydrogen and sulphur respectively and C, H, 0 and S represent the percent content of carbon, hydrogen, oxygen and sulphur respectively as determined by ultimate analysis. Since oxygen if present in the fuel will exist in combined form with hydrogen (fixed hydrogen) and since eight parts of oxygen combine with one part hydrogen, the total mass of hydrogen in the fuel available for providing heat by combustion is less by one eighth of mass of oxygen in the fuel. The net calorific value will be calculated as per the equation.
Problems:
1. Let us calculate the volume of air required for complete combustion of 1 m3 of gaseous fuel having the composition:
CO = 46%, CH4 = 10%, H2 = 4%, C2H4 = 2%, N2 = 1% and the remaining being CO2.
Solution:
Given:
1 m3 of gaseous fuel
CO = 46%
CH4 = 10%
H2 = 4%
C2H4 = 2%
N2 = 1%
remaining being CO2
1. 1 m3 of fuel contains:
4/100= 0.04 m3 of H2.
10/100 = 0.10 m3 of CH4.
l/100 = 0.01 m3 of N2.
46/100 = 0.46 m3 of CO.
2/100 = 0.02 m3 of C2H4.
2. N2 and CO2 are non-combustible constituents they do not undergo combustion.
3. The combustion equation is written as follows:
(a) H2 + 1/2 O2 → H2O
1 vol 0.5 vol.
1 m3 of H2 requires 0.5 m3 of O2.
0. 04 m3 of H2 requires = 0.5 × 0.04/1 = 0.02 m3 of O2.
(b) CH4 + 2O2 → CO2 + 2H2O
1 vol 2 vol.
1 m3 of CH4 requires 2 m3 of O2.
0. 10 m3 Of CH4 requires = 0.10 × 2/1 = 0.2 m3 of O2.
(c) CO + 1/2 O2 → CO2
1 vol 0.5 vol.
1 m3 of CO requires 0.5 m3 of O2.
0.46 m3 of CO requires = 0.46 × 0.5/1 = 0i23 m3 of O2.
(d) C2H4 + 3O2 → 2CO2 + 2H2O
1 vol 3 vol.
1 m3 of C2H4 requires 3 m3 of O2.
0.02 m3 of C2H4 requires = 0.02 × 3/1 = 0.06 m3 of O2.
Total volume of O2 required = 0.02 + 0.2 + 0.23 + 0.06
= 0.51 m3 of O2.
We know that, 21 m3 of O2 is supplied by 100 m3 of air.
0.51 m3 of O2 is supplied by = 100 × 0.51/21 = 2.428 m3 of air.
Amount of oxygen required my 100 m3 of fuel = 2.428 m3 of air.
A fuel contains C = 75%; H = 4%; O = 5%; S = 7% remaining ash. Let us calculate the minimum quantity of air required for complete combustion of 1kg of fuel.
Solution:
Given:
Weight of the fuel = 1 kg
Weight of C in the fuel = 0.75 kg
Weight of H in the fuel = 0.04 kg
Weight of O in the fuel = 0.05 kg
Weight of S in the fuel = 0.07 kg
(i) CO + O2 CO2
12 kg of carbon requires 32 kg of oxygen.
0.75 kg carbon requires = (32/12) x 0.75 = 2 kg.
(ii) H2 + 1/2 O2 H2O
2 kg of hydrogen requires 16 kg of oxygen.
0.04 kg hydrogen requires = (16/2) x 0.04 = 0.32 kg.
(iii) S + O2 SO2
32 kg of carbon requires 32 kg of oxygen.
0.07 kg carbon requires = (32/32) x 0.07 = 0.07 kg.
Total weight of oxygen required = 2 + 0.32 + 0.07 = 2.39 kg.
But weight of oxygen already present = 0.05 kg.
Actual weight of oxygen required = 2.39-0.05 = 2.34 kg.
Weight of air required = 2.34 x (100/23) = 10.17 kg of air.