Fuels and Combustion Lec.8

Introduction Any source of heat energy is termed as fuel fuel means a substance composed of carbon and hydrogen (hydrocarbon). The fossil fuels have been classified according to : (1)occurrence (and preparation) (2)the state of aggregation. 1- Occurrence:

2-The state of aggregation Calorific Value The calorific value of a fuel is defined as the amount of heat liberated by burning unit mass or volume of the fuel and indicates the heat content of the fuel.

Characteristics of an Ideal Fuel 1- It should possess high calorific value. 2- It should have low moisture content. 3- It should be readily available in bulk to low cost. 4- It should possess moderate ignition temperature. 5- Its velocity of combustion should be moderate. 6-It should be easy to transport without any danger 7-Its products of combustion should not be harmful. 8-- Its cost of storage in bulk should be low. 9- It should burn in air with efficiency without much smoke. 10-Its combustion should be easily controllable. 11- In case of solid fuel, the size should be uniform so that combustion is regular.

Combustion of fuel  COMBUSTION: It is generally described as "rapid oxidation" of any material. Any substance which combines readily and rapidly with oxygen under favorable conditions. The most convenient source of oxygen supply is that of the atmosphere. Air is composed of approximately 21% oxygen and 79% nitrogen and other inert gases by volume.  Engineer Should Ensure that Fuel is Burnt efficiently :- Fuel is expensive, and inefficient combustion means expensive wastage. Inefficient combustion results in pollution of the atmosphere with noxious gases.

1-Combustion of Carbon Complete combustion of carbon occurs when carbon is burnt with a sufficient supply of oxygen: C + O 2 CO 2 M. wt 12 2x16 44 Moles Combustion of Hydrogen The combustion of hydrogen also gives off heat and, as is noted by the equation, the product is water. H 2 +1/2 O 2 H 2 O M. wt Moles 1 1/2 1 3-Combustion of Sulphur S + O 2 SO 2 M. wt Moles 1 1 1

Theoretical Air The right amount of air were supplied for complete combustion known as theoretical amount of air. Composition of Air: By weight: 23% O 2 : 77% N 2 By moles: 21% O 2 : 79% N 2 For example:  if 1 Kg of O 2 : (mass) O 2 : air 23 : 100 Kg 1Kg : ?? Air theoretical= (1x100)/23 Kg  if 1m 3 of O 2 : (volume) O 2 : air 21 :100 m 3 1m 3 ?? Air theoretical= (1x100)/21 m 3

Example (2) Calculate the mass of air required /Kg of fuel if 20% excess air is used for complete combustion. The fuel contains 85% C, 15% H2. Also calculate the % composition by mass of dry product. Assume air contains 20% O2 & 80% N2. Answer Basis 1Kg of fuel i.e.: C=0.85 Kg H 2 =0.15 Kg

Example (3) Theoretical quantity of air required for complete combustion of 1 liter fuel (gaseous fuel), 2.5L at STP [760 mm Hg, 0 0C]. For actual combustion 20% excess air used, determine volume of air used at 750 mm Hg& 270C.