1. Principles of Combustion by Prof. Reda I. Afify

3. Combustion 3 necessary components of combustion Fuel Oxygen Heat
Fuels are solids, liquids and gases
Solid or liquid fuels must be changed to a gas before they will burn.
Heat is required to change solids or liquids into gases.

4. Air
Oxygen (O2) is one of the most common elements on earth making up 20.9% of our air.
Most of the 79% of air is nitrogen, with traces of other elements.
Nitrogen reduces combustion efficiency by absorbing heat from the combustion of fuels and diluting the flue gases.
This nitrogen also can combine with oxygen (particularly at high flame temperatures) to produce oxides of nitrogen (NOx), which are toxic pollutants.

5. Carbon, hydrogen and sulphur in the fuel combine with oxygen in the air to form carbon dioxide, water vapor and sulphur dioxide, releasing 8084 kcals, kcals & 2224 kcals of heat respectively.
Under certain conditions, Carbon may also combine with Oxygen to form Carbon Monoxide, which results in the release of a smaller quantity of heat (2430 kcals/kg of carbon) Carbon burned to CO2 will produce more heat per pound of fuel than when CO or smoke are produced.

6. Oxygen is the key to combustion
Too much, or too little fuel with the available combustion air may potentially result in unburned fuel and carbon monoxide generation. A very specific amount of O2 is needed for perfect combustion and some additional (excess) air is required for ensuring complete combustion(good combustion). However, too much excess air will result in heat and efficiency losses.

7. Incomplete combustion
A combustion process is complete if all the carbon in the fuel burns to CO2, all the hydrogen burns to H2O, and all the sulphur (if any) burns to SO2
Incomplete combustion
The combustion process is incomplete if the combustion products contain any unburned fuel or components such as C, H2, CO, or OH.

8. Stoichiometric or theoretical air
Air-Fuel (AF) ratio
AF = m Air / m Fuel
Where:
m air = mass of air in the feed mixture
m fuel = mass of fuel in the feed mixture
Stoichiometric or theoretical air
The minimum amount of air needed for complete combustion of a fuel is called the stoichiometric or theoretical.

9. Excess air
Excess air is the amount of air in excess of the stoichiometric amount. The excess air needed to complete combustion.

10. Physical Combustion Requirements 3 (three) T’s of Combustion
Temperature high enough to ignite and maintain ignition of the fuel,
Turbulence or intimate mixing of the fuel and oxygen, and
Time sufficient for complete combustion.

11. Rules for combustion of oil
Heating oil to correct viscosity, it is necessary to heat it enough to get the desired viscosity
Atomize the oil completely to produce a fine uniform spray
Mix the air and fuel thoroughly
Introduce enough air for combustion, but limit the excess air to a maximum of 15%
Keep the burners in good condition

12. Oil Fuel Particles Size
The ability to burn fuel oil efficiently requires a high fuel surface area-to-volume ratio.
Experience has shown that oil particles in the range of 20 to 40 µm are the most successful.
Particles which are:
Bigger than 40 µm tend to be carried through the flame without completing the combustion process
Smaller than 20 µm may travel so fast that they are carried through the flame without burning at all

13. Burner
The burner is the device used to combust the fuel with an oxidizer (air) to convert the chemical energy in the fuel to thermal energy.
The burner also controls the correct amount of air used to burn the fuel.
The ratio of air to fuel has to be correct for complete combustion.

14. Pressure jet oil burners
A pressure jet burner is simply an orifice at the end of a pressurized tube.
Typically, the fuel oil pressure is in the range of 7 to bars.

15. Steam-atomizing oil burners
Steam at pressures in the range 5 to 10 bar is used
Jet of steam and oil are mixed either just inside or outside the burner through annular channels
Steam must be superheated or dry, if the steam is wet, cause spluttering of the flame at the burner tip, possibly extinguishing the flame and reducing the flame temperature or lengthening the flame.

16. Advantages
Used for all types of the oils and heavy oils at low preheating temperature of oil
High efficiency at low and medium loads rates
Low efficiency at high loads rates
Simplest design
Low cost

17. Optimizing Excess Air and Combustion
For complete combustion of every one kg of fuel oil kg of air is needed. Due to bad mixing excess air must be.
For optimum combustion, the real amount of combustion air must be greater than that required theoretically. Part of the stack gas consists of pure air, i.e. air that is simply heated to stack gas temperature and leaves the boiler through the stack.

19. Control of Air and Analysis of Flue Gas
Chemical analysis of the gases is an objective method that helps in achieving finer air control. By measuring carbon dioxide (CO2) or oxygen (O2) in flue gases by continuous recording instruments or Orsat apparatus
For optimum combustion of fuel oil, the CO2 or O2 in flue gases should be maintained at % in case of CO2 and 2-3% in case of O2.
A faster way to calculate the excess air is by using the figures 2 and 3, provided the percentage of CO2 or O2 in the flue gases have been measured.

20. Relation between CO2 & Excess Air for fuel oil

21. Relationship between residual oxygen and excess air for fuel oil

22. Combustion of Coal Fuel Burning Furnace
Fuel is burnt in a confined space called furnace.
The furnace provides supports and enclosure for burning equipment.
Solid fuels such as coal, coke, wood etc. are burnt by means of stokers where as burners are used to burn powdered (Pulverized) coal and liquid fuels.
Solid fuels require a grate in the furnace to hold the bed of fuel.

23. Types of Furnaces Grate fired furnaces Chamber fired furnaces
They are used to burn solid fuels. They may have a stationary or a movable bed of fuel.
Chamber fired furnaces
They are used to burn pulverized fuel, liquid and gaseous fuels.

24. Furnace shape and size depends upon the following factors:
Type of fuel to be burnt.
Type of firing to be used.
Amount of heat to be recovered.
Grate area required.
Ash fusion temperature.
Flame length.
Amount of steam to be produced and its conditions.
Pressure and temperature desired.
Amount of excess air to be used.

25. Furnace Construction Simply furnace walls consists of:
interior face of refractory material such as fireclay, silica, alumina, kaolin, …
intermediate layer of insulating materials such as magnesia
exterior casing made up of steel sheet.

26. Insulating materials reduce the heat loss from furnace but raise the refractory temperature.
Smaller boilers used solid refractory walls but they are air cooled.
In larger units, bigger boilers use water cooled furnaces.

27. Water walls
Water from the boiler is made to circulate through these tubes which connect lower and upper headers of boiler.
The provision of water walls is advantageous due to following reasons:
These walls provide a protection to the furnace against high temperatures.
They avoid the erosion of the refractory material and insulation.
The evaporation capacity of the boiler is increased.

28. Multi-Layer Insulation

29. Various Water Walls Arrangement.
The tubes are attached with the refractory materials on the inside or partially embedded into it.
Various Water Walls Arrangement.

30. To burn fuels completely, the burning equipment should fulfill the following conditions:
The flame temperature in the furnace should be high enough to ignite the incoming fuel and air. Continuous and reliable ignition of fuel is desirable.
For complete combustion the fuel and air should be thoroughly mixed by it.
The fuel burning equipment should be capable to regulate the rate of fuel feed.

31. To burn fuels completely, the burning equipment should fulfill the following conditions:
To complete the burning the fuel remains in the furnace for sufficient time.
The fuel and air supply regulated to achieve the optimum air fuel ratios.
Coal firing equipment should have means to hold and discharge the ash.

32. Methods of Fuel firing
The solid fuels are fired into the furnace by the following methods:
Hand firing
Mechanical firing

33. Mechanical firing (Stokers)
Stoker fired boilers use sized coal and hence require less excess air. Also in these systems primary air is supplied below the grate and
secondary air is supplied over the grate to ensure complete combustion.

34. (a) Chain Grate Stoker Advantages
The operation and maintenance cost is low.  
Disadvantages
Initial cost of this stoker is high.

35. (b) Spreader Stoker

36. Spreader Stoker Advantages Its operation cost is low.
A wide variety of coal can be burnt easily.
A thin fuel bed on the grate is helpful in meeting the fluctuating loads.
The fuel burns rapidly and there is little coking with coking fuels.
Ash under the fire is cooled by the incoming air and this minimizes clinkering.

37. Spreader Stoker Disadvantages
The spreader does not work satisfactorily with varying size of coal.
the coal burns in suspension and due to this fly ash is discharged with flue gases

38. Pulverized fuel firing
Coal is pulverized (powdered) to increase its surface exposure thus permitting rapid combustion.

39. Pulverized fuel firing
Advantages
The system is simple and cheaper.
There is direct control of combustion from the pulverizing mill.
Coal transportation system is simple.

40. Burning of pulverized coal has some problems
particle size of coal used in pulverized firing is limited to microns,
the generation of high temp. about (1650 oC) in the furnace creates number of problems like
slag formation on super heater,
formation of SO2 and NOX in large amount.

41. Fluidized bed combustion
The fuel and inert material dolomite are fed on a distribution plate and air is supplied from the bottom of distribution plate.
The air is supplied at high velocity so that solid feed material remains in suspension condition during burning.

42. Fluidized bed combustion
Fluidized bed combustion in which turbulence is created leads to mixing of air and fuel resulting in further reduction of excess air.
The molten slag is tapped from the top surface of the bed.
Dolomite to reduce the emission of SO2

43. Various advantages of FBC system are:
FBC system can use any type of low grade fuel including municipal wastes and therefore is a cheaper method of power generation.
It is easier to control the amount of SO2 and NOX, formed during burning. Low emission of SO2 and NOX during combustion. SO2 emission is nearly 15% of that in conventional firing methods.
There is a saving of about 10% in operating cost and 15% in the capital cost of the power plant.

44. Combustion of Gas
The stoichiometric ratio for natural gas (and most gaseous fuels) is normally indicated by volume.
The air to natural gas (stoichiometric) ratio by volume for complete combustion vary between 9.5:1 to 10:1
Natural gas is essentially pure methane, CH4. Its combustion can be represented as follows:
CH4 +2O2 = CO2 + 2H2O
So for every 16 kgs of methane that are consumed, kgs of carbon dioxide are produced.
Note: (Remember that the atomic weights of carbon, oxygen and hydrogen are 12, 16 and 1, respectively.)

45. Combustion of Gas

46. Excess air levels from good combustion practice

47. Draft System
The function of draft in a combustion system is to exhaust the products of combustion into the atmosphere.
The draft can be classified into two types namely
Natural and
Mechanical Draft.

48. Draft System Natural Draft
It is the draft produced by a chimney alone. It is caused by the difference in weight between the column of hot gas inside the chimney and column of outside air of the same height and cross section.

49. Draft System Mechanical Draft
It is draft artificially produced by fans. Three basic types of drafts that are applied are:
Forced draft
Induced draft
Balanced draft

50. Forced draft:
Radial airfoil (centrifugal) or variable pitch (axial) fans are preferred for FD service.

51. Induced Draft:
An airfoil centrifugal fan is typically used.

52. Balanced Draft:
Most modern boilers are balanced draft.