COMBUSTION OF FUEL The burning of fuel in presence of air is known as combustion. It is a chemical reaction taking place between fuel and oxygen at temperature above ignition temperature. Heat is released during combustion process. 12:57:42 By: Prof K. M.Joshi, Assi. Professor, MED, SSAS Institute of Technology, Surat.
12:57:43 By: Prof. K. M. Joshi Introduction The substances taking part in combustion are called reactants and the substances produced, during combustion are called products of combustion. fuel + air = products of combustion + heat If the heat is released during the chemical process, it is called an exothermic reaction. When heal is absorbed from the surroundings during the chemical reaction, it is called an endothermic reaction.
Combustion Equations When combustion of fuel takes place, the constituents of fuel react with oxygen. The molecular mass of various constituents of fuel are given below : Constituent C 12 0 2 32 H 2 2 S 32 N 2 28 Molecular mass The relation between kg mass and kg mole of a substance is given by mass Number of moles = ------------------ molecular mass 12:57:43
12:57:43 By: Prof. K. M. Joshi Carbon : C + O 2 = CO 2 1 mole of C + 1 mole of 0 2 = 1 mole of C0 2... (1) 12 kg of C + 32 kg of 0 2 = 44 kg of C0 2 1 kg C + 8/3 kg 0 2 = 11/3 kg C0 2 Hydrogen : 2 H 2 + 0 2 = 2 H 2 0 2 moles of H 2 + 1 mole of 0 2 = 2 moles of H 2 0... (2) 4 kg H 2 + 32 kg 0 2 = 36 kg H 2 0
12:57:43 By: Prof. K. M. Joshi Sulphur : S + O 2 = So 2 1 mole of S + 1 mole of 0 2 = 1 mole of S0 2... (3) 32 kg S + 32 kg 0 2 = 64 kg S0 2 1 kg S + 1 kg 0 2 = 2 kg S0 2 Methane : CH 4 + 20 2 = C0 2 + 2H 2 0 1 mole CH 4 + 2 moles 0 2 = 1 mole C0 2 + 2 moles H 2 0... (4) 16 kg CH 4 + 64 kg 0 2 = 44 kg C0 2 + 36 kg H 2 0 1kg CH 4 + 4 kg 0 2 = 11/4 kg C0 2 + 9/4 kg H 2 0
These equations are helpful for calculating the amount of oxygen required to burn the fuel. Instead of supplying only oxygen, the fuel is generally burnt in presence of air which contains mainly O 2 and N 2 with small amount of CO 2. argon etc. N 2 and other gases in air' do not take part in chemical reaction. The composition of air is approximately as under : 0 2 N 2 Total air by volume 21% 79% 100% 1 mole 3.76 mole 4.76 mole by mass 23% 77% 100% 1 kg 3.35 kg 4.35 kg 12:57:43 By: Prof. K. M. Joshi
12:57:43 By: Prof. K. M. Joshi Mass fraction The ratio of mass of a constituent of a mixture or a compound to the total mass of a mixture or compound is called mass fraction. e.g. consider a gas CH 4, Molecular mass of CH 4 = 12 + 4 x 1 = 16 kg, in which C is 12 kg and H 2 is 4 kg.
12:57:43 Mole fraction The ratio of volume of gas in a mixture to the total volume of mixture is called the mole fraction. It is also equal to the ratio of no. of moles of gas in a mixture to the total no. of moles of mixture. e.g. for octane gas C 8 H 18, no. of moles of C is 8 and that of H 2 is 9. Total no. of moles = 8 + 9 = 17
12:57:43 Minimum Air Requirement The no. of moles of 0 2 required for complete combustion of 1 mole of fuel is known as the theoretical amount of 0 2 (molar basis). OR The mass of 0 2 required for complete combustion of 1 kg of fuel is known as the theoretical amount of 0 2 (mass basis). The mass of 0 2 and N 2 together (i.e. the air) required for complete combustion of 1 kg of fuel, is known as minimum air requirement. It is also called the theoretical or stochiometric amount of air. A mixture of theoretical air and fuel for complete combustion of fuel is called stochiometric mixture or chemically correct mixture.
Excess air In practice, the combustion of fuel is never complete with the theoretical amount of air because of imperfect mixing of fuel and air. Mixture of fuel and air is never homogeneous. So, to ensure the complete combustion of fuel- usually the actual air supplied is more than the theoretical air required for complete combustion of fuel. The difference of actual air supplied and the theoretical or stochiometric air required for complete combustion of fuel is called excess air. By: Prof. K. M. Joshi
Air Fuel Ratio 12:57:43
12:57:43 If actual AF ratio is more than stochiomelric AF ratio, mixture of air and fuel is said to be weak mixture or lean mixture. If actual AF ratio is less than stochlometric AF ratio, mixture is said to be a rich mixture. Mixture strength or equivalence ratio is defined as the ratio of stochlometric AF ratio to actual AF ratio. If mixture strength is more than 100%, it represents a rich mixture. If mixture strength is less than 100%. it represents a weak mixture.
Calculation for minimum air requirement per kg of fuel Consider 1 kg of fuel. It contains x 1 kg of carbon. x 2 kg of hydrogen, x 3 kg of sulphur and x kg of oxygen. The amount of oxygen required for complete combustion can be calculated using eq. (1), (2) and (3). 1 kg carbon requires 8/3 kg of O 2 x 1 kg carbon requires ( 8/3 X x 1 ) kg of O 2 Similarly x 2 kg of hydrogen requires 8 x 2 kg of O 2 x 3 kg of sulphur requires x 3 kg of O 2 Total oxygen required = 8/3 x 1 + 8 x 2 + x 3 kg x kg oxygen is already presents in the fuel. 12:57:43
12:57:43 x 1 x 2 x 3 x x 1 x 2 x x 3 x 1 x 2 x x 3
12:57:43 Conversion of volumetric analysis into mass analysis or gravimetric analysis Multiply the percentage volume of each constituent by their respective molecular masses. It gives the proportionate mass of each constituent in the flue gas. Add these proportionate masses of all constituents to get total mass. Divide the mass of each constituent by total mass and express it as percentage. This gives the mass analysis of flue gas.
12:57:43 Calorific Value Of Fuel The amount of heat energy produced by combustion of unit mass or volume of fuel is called its calorific value (CV). For the solid and liquid fuels, unit mass is considered and the unit of CV will be kj/kg. For gaseous fuel, unit volume is considered under NTP and the unit of CV will be kj/m 3. Most of the fuels contain hydrogen. During combustion process. H 2 combines with 0 2 and forms steam (water vapour). If this water vapour is condensed at constant temperature, large amount of heat is released.
12:57:43 On account of this, two types of calorific values are defined. (1) Higher Calorific Value (HCV) : The higher calorific value is defined as the total heat liberated by combustion of unit mass of fuel when the water vapour formed by combustion is completely condensed at constant temperature releasing its latent heal. (2) Lower Calorific Value (LCV) : The lower calorific value of fuel is defined as the net heat liberated by combustion of unit mass of fuel when the water vapour formed by combustion exists completely in vapour phase.
Determination of calorific value of fuel by bomb calorimeter 12:57:43
Calorific value of solid and liquid fuels can be determined with the help of bomb calorimeter. The basic principle used for determination of calorific value of fuel is that the known quantity of fuel is burnt and the heat energy liberated is transferred to a medium of known mass and sp. heat and rise in temperature of that medium is measured. Construction : The calorimeter C consists of a thick walled bomb B made of stainless steel. It has a capacity of about 650 c.c. and it is designed to with stand high pressure upto 200 atmosphere. The non return oxygen valve V and a release valve U are connected to a bomb at the top.
The crucible made of silicon or quartz is carried on support ring R which can slide on insulated pillars P. The fuse wire W is passed though the slots kept in pillars. The fuse wire is connected to electrical circuit. The sensitive thermometer T is used to measure the temperature of water filled in the calorimeter. Stirrer is provided in the water to stir the water. Working : A pillet of solid fuel whose calorific value is to be determined is prepared and 1 gm of this pillet is kept into the crucible. A known quantity of water (approximately 2500 c.c.) is filled around the bomb inside the calorimeter. The crucible and the calorimeter are weighed before starting the experiment.
12:57:47 The oxygen is admitted until a pressure of about 25 atmosphere. The stirring of water is continued throughout the experiment and the temperature readings are taken every minute. After five minutes when the equilibrium sets, the fuel is ignited. The temperature of water rises quickly due to heat energy released by the combustion of fuel. After the temperature has reached its maximum value, it again starts falling due to heat transfer losses to the surroundings. At the end of experiment, the release valve is opened so that the pressure inside the bomb reduces to atmospheric pressure.
Let, m f = mass of fuel CV = calorific value of fuel m c = mass of calorimeter C C = sp. heat of calorimeter m w = mass of water C w = sp. heat of water δt = rise in temperature of water and calorimeter Heat released from combustion of fuel = heat gained by calorimeter and water m f x CV = m c C C δt + m w Cw δt When other quantities are known, CV of fuel can be calculated. 12:57:47 By: Prof. K. M. Joshi
12:57:47 By: Prof. K. M. Joshi Junkers gas calorimeter
12:57:47 Construction : Junkers gas calorimeter consists of a combustion chamber surrounded by water jacket. A gas pipe line is connected with a burner kept in combustion chamber. A gas flow meter and pressure regulator are provided in a gas pipe line. Thermometers are used to measure the temperature of water at inlet and outlet. Condensate from the gases is collected in condensate pot.
12:57:48 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 the cold water flowing through water jacket. The gases are cooled upto room temperature as far as possible, so that entire heat released from the combustion may be absorbed by circulating water. The temperature of cooling water at inlet and outlet and exit gas temperature are measured. Mass flow rate of cooling water is also measured. Volume flow rate of gas is converted to STP condition.
12:57:48 By: Prof. K. M. Joshi
12:57:48 Enthalpy Of Reaction During the constant pressure process, the heat energy released by combustion of fuel at STP is called enthalpy of reaction. It is also known as heating value or heat of reaction or calorific value of fuel at constant STP. It is denoted by Q p or H 0. Applying 1st law of thermodynamics to combustion process at constant pressure,
12:57:48 By: Prof. K. M. Joshi
Enthalpy Of Formation 12:57:49 It is assumed that the reactants C and O 2 and the product CO 2 both enter and leave the combustion chamber at STP.
The chemical energy of fuel carbon is 393520 kj/kg mole. Above reaction is exothermic, so the amount of heat energy transferred from system to surroundings is equal to 393520 kj/kg mole which is equal to chemical energy of carbon. Enthalpy datum for all naturally occuring stable elements is assigned as zero enthalpy, e.g. the stable form of gases oxygen, hydrogen and nitrogen are respectively O 2, H 2 and N 2 while the natural form of carbon is. graphite. All these elements are assigned zero enthalpy of formation. So. the change in enthalpy of above chemically reactive system at STP is [0-393520 ] = - 393520 kj/kg mole Negative value indicates that heat energy is released in forming the compound from its stable elements. A positive value will Indicate that the heat energy is absorbed in formation of compound from its stable elements. 12:57:49 By: Prof. K. M. Joshi
12:57:49 Adiabatic Flame Temperature Adiabatic flame temperature is defined as the theoretical temperature attained by the products of combustion in an adiabatic process assuming complete combustion. If the combustion chamber is completely insulated, the chemical energy released during combustion will be utilized to raise the temperature of products of combustion alone and no heat will be transferred to the surroundings. In this situation, the combustion process is said to be adiabatic.
12:57:49 In actual practice, the actual flame temperature is less than adiabatic flame temperature due to following reasons : (1) Heat transfer to the surroundings since the system can not be made perfectly insulated. (2) Combustion is never complete. (3)At high temperatures, the gases may not be stable, they may dissociate and absorb energy internally from the system, thus reducing the flame temperature. Factors affecting adiabatic flame temperature; (1) Heat losses from combustion chamber (2) Incomplete combustion of fuel (3) Dissociation of gases (4) Excess or deficient air
As adiabalic flame temperature is the maximum temperature in combustion chamber, it helps in selecting the material for combustion chamber and design of combustion chamber. Adiabatic flame temperature can be reduced by increasing the air fuel ratio. It is lower if excess air is supplied, since the total heat of reaction of fuel is utilized by more no. of moles of products of combustion. The adiabalic flame temperature is also lower with deficient air as heat of reaction is not fully released since 0 2 available is not sufficient to burn the entire fuel. Adiabatic flame temperature is maximum with stochiometric air. 12:57:50 By: Prof. K. M. Joshi
Equation for adiabatic flame temperature Consider a perfectly insulated combustion chamber. The reactants are supplied at P 0, T 1 and the products leave at P 0, T 2. According to definition, T 2 is adiabaltic flame temperature. From steady energy equation, with negligible changes in KE and PE. Where H p = enthalpy of products and H R = enthalpy of reactants As combustion chamber is completely insulated, Q = 0 No work is done. So W = 0 12:57:50
As the products and reactants are respectively at temperatures T 2 and T 1, their enthalpy is more than enthalpy of formation by the amount equal to difference between enthalpy at that temperature and enthalpy at STP. Using this equation, adiabatic flame temperature T 2 can be calculated with the help of chemical reaction equation, enthalpy of formation table and enthalpy of gas table. 12:57:50 By: Prof. K. M. Joshi