Principle Of Boiler Combustion

Principle of Boiler Combustion

Introduction • A knowledge of the principle of combustion is utmost important for boiler or gas turbine operator for two (2) main reasons : 1. 2.

So that furnace / combustor explosions are avoided So that boilers / GT are operated with maximum efficiency

The aim of this lecture is to promote an awareness of the conditions which lead to a furnace explosion, the steps which must be taken to avoid furnace explosions, an understanding of the combustion process, and practices necessary for efficient & prudent boiler / GT operation.

Definitions • Combustion is the controlled generation of useful heat by the rapid chemical combination of oxygen with the combustible elements of a fuel. • Explosion is the uncontrolled, extremely rapid, chemical combination of a fuel with oxygen. It results in a rapid expansion of combustion gases which leads to a rapid increase of furnace pressure.

Furnace Explosions • Statistics Furnace explosions are much more frequent than people generally realise. Although the majority of furnace explosions occur in coal fired boilers, the same conditions leading to an explosion can occur in an oil or natural gas fired boilers. Studies of furnace explosion on a world basis have indicated that the causes have been : 5% undetermined 21% malfunction of equipment 74% operating errors The high “operating error” indicates the need for sound operating procedures at all times. Operating conditions at the time of the explosions are the most important – the above study indicated also that 90% of the explosions occurred during the boiler starting sequence.

Conditions for Explosions • In a boiler, ignition is provided as the fuel and air are brought together so that there is a progressive burning of fuel and a flow of combustion products to the chimney. The mixing of air & fuel is carried out by the burner. • Furnace Explosions result from the ignition of unburnt fuel and oxygen after they have accumulated in the boiler, and not in the burner itself as could be expected. Before the accumulation can explode, air & fuel must be present in certain ratios. When the ratio of the explosion has been reached an ignition source is necessary, e.g. hot metal, hot refractory, hot gases, stray zones of combustion.

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• 90% of explosions occurs during start-up can be further broken down into : 44% improper ignition 24% supplying ignition energy to furnace full of combustion mixture 14% fuel trip with fuel rich furnace 8% main fuel interruption 5% main air interruption 5% improper fuel preparation

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• 1.

The following notes give correct procedures for each case above : Improper Ignition Fire has three ingredients fuel (3)

air (1)

ignition (2) These three ingredients can be sequenced six different ways.

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• 1.2.3. • 1.3.2. • 2.1.3. • 2.3.1

This sequence is the correct one & light off should give no trouble In THIS case an accumulation will occur before ignition and an explosion will result This is providing ignition before purge and an explosion could occur The fuel could not burnt without air and would accumulate until the introduction of air in the final stage. At this point the mixture would be ignited and an explosion would result. Similar to sequence 1.3.2. Similar to sequence 2.3.1.

• 3.1.2. • 3.2.1 Note : Unless Fuel Is Supplied LAST, AN Explosion is Likely Air : Ignition : Fuel is the only safe sequence

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2. Supplying ignition energy to furnace full of combustible mixture The furnace must be purge : a) Before light-off b) After failure of burners to ignite c) After flame off d) If there is any appreciable delay between the first purging and burner lighting Purging is usually designed to supply the furnace with an air volume which is 4 – 5 times the boilers’ total combustion gas volume. For this purpose a minimum air flow is necessary for a certain time.

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3) Fuel Rich Mixture Whenever a fuel rich condition occurs – FUEL MUST BE REDUCED (Or Even Tripped). Air must NOT be Increased. 4&5) Main Fuel and Air Interruption This condition can occur at any time. It is only serious if the majority of the burners is affected – a complete flame-out is then likely If air trips, fuel supply to all burners must be tripped as in (3) above. This may not guarantee that no explosion will occur, as purging will supply air to the fuel already in the furnace

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• If fuel trips, an air rich mixture will develop which, depending on the mixture, could still be explosive In both cases the boiler must be purged as soon as possible after fuel is tripped. 6) Improper Fuel Preparation The condition of the fuel as it leaves the burner determines to a large extent its ability to ignite rapidly and burn stably. If fuel temperature is low, ignition may be difficult and burning rate may be low, leading to fuel-rich conditions

Furnace Combustion • The mere fact that oxygen is brought into the presence of a combustion substance does not mean that combustion will automatically follow : Fuel & Air require : Temperature – ignition temperature must be reached before combustion can start Turbulence - necessary during mixing for air & fuel to be intimately mixed Time – time is required for complete burning to occur

Ignition Temperature Ignition Temperature

Is the minimum temperature at which the fuel will chemically combine with oxygen. No combustion can occur bellow this temperature. • Ignition temperature range from 470oF for sulphur , 800925oF for coal, to 1200oF for methane (natural gas). • When combustion has started, the heat evolved in the oxidation of the combustible matter will maintain sufficiently high temperature for further ignition.

The Appearance of Combustion The flame - is a mass of intensely heated gas. It luminosity is due to the heating-to-incandescence of the as-yet unburnt particles of combustion matter present in these heated gases. The higher the temperature of these gases, the whiter the flame. If it were possible for combustion of any fuel to be completed instantaneously, there would be no visible flame, since the byproducts (CO & H2O) are invisible. Visible flame , then in evidence of incomplete or not-yet achieved combustion. Short flame – mean rapid & complete combustion Long flame – delayed or imperfect combustion Flame Temperature Range : 1000oF – a dark red flame to 3000oF for a dazzling white flame

Fuel The most common elements & compounds which combine with oxygen and liberate heat are : - Carbon - Hydrogen - Sulphur - Hydrocarbon (compounds of carbon & hydrogen) Fuel are made up of various combinations of these elements and compounds Fuel compositions (with combustible underlined) are given as follows :

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• • • • • • • • • • • •

Carbon Hydrogen Oxygen Nitrogen Sulphur Moisture CO2 Ash Methane Ethane Propane Others Calorific Value

» Coal (% wt)

Fuel Oil (% wt)

Gas (% wt)

37 2 11 1 1 31 17 6300 Btu/lb

81.76 12.4 1.2 1.7 3 18,700

1.5 4.5 82 7 3.8 0.6 22,300

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• Calorific value is the quantity of heat (in BTU) liberated during complete combustion of 1 lb of the fuel. Two calorific values are usually quoted : HCV and LCV or (LHV : low heating value) • HCV is the figures usually quoted and it differs from LCV in that it includes the heat necessary to convert water vapour into water liquid – produced by burning hydrogen. • LCV = HCV – latent heat value

Air • Air has the following major components : » % Volume

– Oxygen 21% – Nitrogen 79%

% Weight

23.2% 76.8%

In all combustion reaction, only oxygen is necessary – nitrogen is present but plays no part in the reaction. 2 C + O2 = 2 CO + 4350 BTU/lb heat 2CO + O2 = 2CO2 + 10 240 BTU/lb heat C + O2 = CO2 + 14 590 BTU/lb heat 2H2 + O2 = 2H2O + 61 340 BTU/lb heat CH4 + 2O2 = CO2 + 2H2O + 23 860 BTU/lb heat

Weight of Air for Complete Combustion Knowing the composition of a fuel, the molecular weights of the components and the equation of combustion for each of the constituents, it is possible to determine how much oxygen is needed to completely burn each pound of the fuel. From this oxygen amount, we can derive the quantity of air is required to burn one pound of fuel. For each fuel, there is a quantity of air is required to theoretically completely burn one pound of that fuel. For fuel oil, 13.7 lb of air are theoretically required to completely burn one lb fuel oil.

Practical Combustion