Presentation On Fuels and Combustion.pptx

Presentation On Fuels and Combustion.pptx

1. Presentation On Fuels & Combustion 1

2. What is a Fuel? Fuel is a substance that carries energy in chemical form and releases heat energy on combustion. The principal combustible elements of each fuel are carbon and hydrogen. Though sulphur is a combustible element too but its presence in the fuel is considered to be undesirable. Fuels have much higher energy densities than other ways of carrying energy. Fuels can be classified according to whether : They occur in nature called primary fuels or are prepared called secondary fuels . They are in solid, liquid or gaseous state. 2

3. Classification Natural or Primary Fuels 3

4. Classification Secondary or Prepared Fuels 4

5. 5 Coal Coal is the most common solid fuel. Coal is formed by the partial decay of plant materials accumulated million of years ago and further altered by the action of heat and pressure. Its main constituents are carbon, hydrogen, oxygen, nitrogen, sulphur , moisture and ash. Coal passes through different stages during its formation from vegetation. These stages are enumerated below: Solid Fuels

6. 6 Peat: It is the first stage in the formation of coal from wood. It contains huge amount of moisture and therefore it is dried for about 1 to 2 months before it is put to use. Lignites : It is the intermediate stages between peat and coal. It is also referred to as brown coal, is the lowest rank of coal and used almost exclusively as fuel for electric power generation Bituminous coal: It burns with long yellow and smoky flames and has high percentages of volatile matter. The average calorific value of bituminous coal is about 31350 kJ/kg. It may be of two types, namely caking or noncaking . Semi-bituminous coal: It is softer than the anthracite. It burns with a very small amount of smoke. It contains 15 to 20 per cent volatile matter and has a tendency to break into small sizes during storage or transportation.

7. 7 Semi-anthracite: It has less fixed carbon and less lustre as compared to true anthracite and gives out longer and more luminous flames when burnt. Anthracite: It is the highest rank; a harder, glossy, black coal used primarily for residential and commercial space heating. The calorific value of this fuel is high (35500 kJ/kg) and as such is very suitable for steam generation. Graphite: technically the highest rank, but difficult to ignite and is not so commonly used as fuel: it is mostly used in pencils and, when powdered, as a lubricant

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9. 9 Coke It consists of carbon, mineral matter with about 2% sulphur and small quantities of hydrogen, nitrogen and phosphorus. It is solid residue left after the destructive distillation of certain kinds of coals. It is smokeless and clear fuel and can be produced by several processes. It is mainly used in blast furnace to produce heat and at the same time to reduce the iron ore. Briquettes These are prepared from fine coal or coke by compressing the material under high pressure . Wood Charcoal It is obtained by destructive distillation of wood. During the process the volatile matter and water are expelled. The physical properties of the residue (charcoal), however depends upon the rate of heating and temperature.

10. Advantages & Disadvantages Of Solid Fuels: Advantages (a) They are easy to transport. (b) They are convenient to store without any risk of spontaneous explosion. (c) Their cost of production is low. (d) They posses moderate ignition temperature Disadvantages (a) Their ash content is high. (b) Their large proportion of heat is wasted. (c) They burn with clinker formation. (d) Their combustion operation cannot be controlled easily. (e) Their cost of handling is high. 10

11. Liquid Fuels Fuels that are in liquid form are called liquid fuels. Liquid fuels are complex mixture of hydrocarbon generally obtained from petroleum & its by product. Some of the liquid fuel are: petrol diesel kerosine 11

12. The advantages and disadvantages of liquid fuels are: Advantages: (a) They posses higher calorific value per unit mass than solid fuels. (b) They burn without dust, ash, clinkers, etc. (c) They are easy to transport through pipes. (d) Require less space for storage (e) Cleanliness Disadvantages (a) Costlier than solid fuels. (b) Requires costly storage tanks for storing liquid fuels. (c) There is a greater risk of fire hazards, particularly, in case of highly inflammable and volatile liquid fuels. (d) They give bad odour 12

13. PETROLEUM The single largest source of liquid fuel is petroleum . Petroleum has originated probably from organic matter like fish and plant life etc., by bacterial action or by their distillation under pressure and heat. It is mainly composed of gaseous, liquid and solid hydrocarbons together with small amount of organic compounds containing oxygen nitrogen and Sulphur. 13

14. Refinery processes Distillation: Continuous, Atmospheric, and Vacuum. Cracking: Thermal, Catalytic and Hydro. Reforming: Thermal, Catalytic and Hydro. Polymerization Alkylation Isomerization Hydrogenation 14

15. OIL Refining crude oil: Based on boiling points, components are removed at various layers in a giant distillation column. The most volatile components with the lowest boiling points are removed at the top. 15

16. GASEOUS FUELS These are the fuel that are in gaseuos phase. Gaseous fuel are also hydrocarbon which are derived from petrolium reserve. Most common gaseous fuel is natural gas(methane is the main component). Some of the gaseous fuels are : Natural Gas Coal gas Sewer gas Producer gas,etc. 16

17. 17 Coal gas Mainly consists of hydrogen, carbon monoxide and hydrocarbons. It is prepared by carbonisation of coal. It finds its use in boilers and sometimes used for commercial purposes . Gaseous Fuels Natural gas Natural gas is a hydrocarbon, which means it is made up of compounds of hydrogen and carbon The main constituents of natural gas are methane (CH 4 ) and ethane (C 2 H 6 ). It has calorific value nearly 21000 kJ/m 3 . Natural gas is used alternately or simultaneously with oil for IC engines.

18. 18 Producer gas It results from the partial oxidation of coal, coke or peat when they are burnt with an insufficient quantity of air. Producer gas has lower heating value than other gaseous fuels, but it can be manufactured with relatively simple equipment; it is used mainly as a fuel in large industrial furnaces It is obtained from sewage disposal vats in which fermentation and decay occur. It consists of mainly marsh gas (CH 4 ) and is collected at large disposal plants. It works as a fuel for gas engines which in turn drive the plant pumps and agitators. Sewer gas

19. Analysis of a Fuel 19 Proximate Analysis % moisture, % volatile matter, % ash and % fixed carbon Ultimate Analysis %C, %H, %S, %O, % ash & %N

20. 20 ANALYSIS OF A FUEL A sample of a fuel is analyzed on one of the bases: Ultimate analysis and Proximate analysis . An accurate chemical analysis by mass of the important elements in the fuels is called the ultimate analysis , the element usually included being carbon, hydrogen, nitrogen and sulphur . The proximate analysis gives the percentages of inherent moisture, volatile matter, and combustible solid (called fixed carbon). The fixed carbon is found as a remainder by deducting the percentages of the other quantities .

21. The advantages and disadvantages of gaseous fuels Advantages (a) They are clean in use. (b) They do not require any special burner. (c) They burn without any shoot, or smoke and ashes. (d) They are free from impurities found in solid and liquid fuels. (e) Economy in fuel and more efficiency of furnace operation Disadvantages (a) Very large storage tanks are needed. (b) They are highly inflammable, so chances of fire hazards in their use is high. 21

22. 22 COMBUSTION Combustion is an oxidation process and is usually exothermic (i.e. releases the chemical (or bond) energy contained in a fuel as thermal energy). This combustion is usually performed using air because it is freely available, although other oxidants can be used in special circumstances. In a combustion chamber proportionate masses of air and fuel enter where the chemical reaction takes place, and then the combustion products pass to the exhaust.

23. 23 THEORETICAL AIR AND EXCESS AIR The minimum amount of air that supplies sufficient oxygen for the complete combustion of all the carbon, hydrogen, and any other elements in the fuel that may oxidize is called the theoretical air . When complete combustion is achieved with theoretical air, the products contain no oxygen . The amount of air actually supplied may also be expressed in terms of percent excess air. The excess air is the amount of air supplied over and above the theoretical air. Thus 150 percent theoretical air is equivalent to 50 percent excess air. As per theoretical basis there is a minimum amount of air which is required by the fuel to burn completely, but always, air in excess is used because whole of air supplied for combustion purposes does not come in contact with the fuel completely and as such portion of fuel may be left unburnt .

24. 24 Combustion Equations Combustion of hydrogen 2H 2 + O 2 → 2H 2 O Hydrogen reacts with oxygen to form steam or water . Two molecules of hydrogen react with one molecule of oxygen to give two molecules of steam or water, 2 volumes H 2 + 1 volume O 2 → 2 volumes H 2 O The H 2 O may be liquid or a vapor depending on whether the product has been cooled sufficiently to cause condensation. It will be noted from equation that the total volume of the reactants is 2 volumes H 2 + 1 volume O 2 = 3 volumes . The total volume of the product is only 2 volumes . There is therefore a volumetric contraction on combustion.

25. 25 2H 2 + O 2 → 2H 2 O The proportions by mass are obtained by using atomic weights as follows 2(2 × 1) H 2 + 2 × 16 O 2 → 2(2 × 1 + 16) H 2 O 4 kg H 2 + 32 kg O 2 → 36 kg H 2 O 1 kg H 2 + 8 kg O 2 → 9 kg H 2 O Since the oxygen is accompanied by nitrogen if air is supplied for the combustion, then this nitrogen should be included in the equation. As nitrogen is inert as far as chemical reaction is concerned, it will appear on both sides of the equation. With one mole of oxygen there are 79/21 moles of nitrogen, hence equation becomes

26. 26 Combustion of Carbon Complete combustion of carbon to carbon dioxide C + O 2 → CO 2 Including the nitrogen By Volume By Mass 1 volume of C + 1 Volume of O 2 + volume of N 2 1 Volume of CO 2 + volume of N 2  

27. 27 The incomplete combustion of carbon The incomplete combustion of carbon occurs when there is an insufficient supply of oxygen to burn the carbon completely to carbon dioxide. 2C + O 2 → 2CO Including the nitrogen ,

28. 28 By Mass

29. 29 If a further supply of oxygen is available then the combustion can continue to completion By Mass

30. 30 Combustion of Sulphur S + O 2 → SO 2 Including the nitrogen By Volume By Mass

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32. 32 Stoichiometric Air-fuel (A/F) Ratio Stoichiometric mixture of air and fuel is one that contains just sufficient oxygen for complete combustion of the fuel. A weak mixture is one which has an excess of air. A rich mixture is one which has a deficiency of air. The percentage of excess air is calculated as

33. 33 A coal sample gave the following analysis by weight, Carbon 85 %, Hydrogen 6 %, Oxygen 6 %, the remainder being incombustible. Determine minimum weight of air required per kg of coal for chemically correct composition. Example 1 Composition of 1 kg of coal: Carbon (C): 85% of 1 kg = 0.85 kg Hydrogen (H): 6% of 1 kg = 0.06 kg Oxygen (O): 6% of 1 kg = 0.06 kg Incombustible: 3% of 1 kg = 0.03 kg Calculate the oxygen required for the complete combustion: For Carbon: C + O 2 → CO 2 12 kg of carbon requires 32 kg of oxygen. Therefore, 0.85 kg of carbon requires : . Solution

34. ×0.85=2.267 kg of oxygen For Hydrogen: 2H 2 + O 2 → 2H 2 O 4 kg of hydrogen requires 32 kg of oxygen. Therefore, 0.06 kg of hydrogen requires: ×0.06=0.48 kg of oxygen Total oxygen required : 2.267 kg+0.48 kg=2.747 kg Account for the oxygen already present in the coal: Oxygen in coal: 0.06 kg Net oxygen required: 2.747 kg−0.06 kg=2.687 kg Convert the oxygen requirement to air requirement: Since air contains approximately 23.3% oxygen by weight, the weight of air required is:  kg =11.53 kg Therefore, the minimum weight of air required per kg of coal for chemically correct combustion is approximately 11.53 kg   34

35. 35 A sample of dry anthracite has the following composition by mass: C 90 %; H 3 %; O 2.5 %; N 1 %; S 0.5 %; ash 3 % Calculate The stoichiometric A/F ratio; The A/F ratio and the dry and wet analysis of the products of combustion by mass and by volume when 20 % excess air is supplied . Example 2 Solution Let's calculate the required values step-by-step for the given sample of dry anthracite: 1. Determine the stoichiometric A/F ratio: Given the composition by mass: Carbon (C): 90% Hydrogen (H): 3% Oxygen (O): 2.5% Nitrogen (N): 1% Sulfur (S): 0.5% Ash: 3%

36. Assuming 1 kg of anthracite: Mass of C = 0.90 kg Mass of H = 0.03 kg Mass of O = 0.025 kg Mass of N = 0.01 kg Mass of S = 0.005 kg Mass of ash = 0.03 kg Stoichiometric Oxygen Requirement: For Carbon: C+O 2 →CO 2 0.90 kg C×  O 2 = 2.40 kg O 2 CO 2 produced due to burn 0.9 kg of Carbon = ×0.9 = 3.3 kg For Hydrogen: 2H 2 +O 2 →2H 2 O 0.03 kg H× O 2 = 0.24 kg O 2 ​ H 2 O produced due to burn 0.03 kg of Hydrogen = 0.03 ×9 = 0.27 kg For Sulfur: S+O 2 →SO 2 0.005 kg S×  O 2 = 0.005 kg O 2 SO 2 produced due to burn 0.005 kg of Sulphur = 0.005 ×2 = 0.01kg   36

37. Total Oxygen Requirement: Total O 2 required = 2.40 + 0.24 + 0.005 = 2.645 kg O 2 Net oxygen requirement, after subtracting the oxygen already present in the anthracite: 2.645−0.025 = 2.62 kg O 2 Stoichiometric Air Requirement: Since air contains 23.3% oxygen by weight, the weight of air required is: = 11.24 kg air Stoichiometric A/F ratio: A/F ratio = 11.24 kg air / 1 kg fuel 2. Calculate the A/F ratio and the dry and wet analysis of the products of combustion by mass and by volume when 20% excess air is supplied: A/F Ratio with 20% Excess Air: Air required with 20% excess: 11.24 kg air×1.20 = 13.488 kg air Combustion Products: Moles of Combustion Products: CO 2 : 0.90 kg C× = 0.075  mol  CO 2 H 2 O: 0.03 kg H× = 0.015  mol  H 2 O SO 2 : 0.005 kg S×   = 0.00015625  mol  SO 2   37

38. Dry Analysis by Volume: Excess O 2 : 13.488 kg air×0.21 O 2 /kg air = 2.83248 kg O 2 = = 0.088515  mol  O 2   N 2 in air: 13.488 kg air×0.79 N 2 /kg air = 10.65552 kg N 2 = = 0.380198  mol  N 2 Total moles of dry products: 0.075 + 0.00015625 + 0.088515 + 0.380198 = 0.54386925 Mole fraction of each component: CO 2 : = 0.1342 SO 2 : = 0.00028 O 2 : = 0.1627 N 2 : = 0.6991 Wet Analysis by Volume: Adding water vapor to the total moles: Total moles = 0.54386925 + 0.015 = 0.55886925   38

39. Mole fraction of each component (including water vapor): CO 2 : = 0.1342 SO 2 : = 0.00028 O 2 : = 0.1583 N 2 : = 0.6803 H 2 O: = 0.0268   39

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