BOILER FUEL COMBUSTION

Combustion

 

                             The oxidation of fuel or chemical Combustion with oxygen to produces light and  heated is called combustion.in fuel mainly hydrogen and carbon are presents. During combustion, this carbon and hydrogen react with oxygen to for carbon dioxide and water vaporous. 

For Combustion Following Three Things Are Required:

1. Fuel.
2. Oxygen.
3. Three Ts.

1. Fuel :

                The Main constituents of fuel are carbon and hydrogen. So fuel is called as hydrocarbon. In fuels the energy is stored in the form of chemical energy that can be released as heat by combustion or oxidation. 

2. Oxygen :

                  For Combustion process, oxygen is obtained  from air. Air Contains  79% nitrogen and 21% Oxygen by volume or 77% nitrogen and 23% Oxygen by weight. As the nitrogen Percentage  is more in air. So large amount of air is required to get the required amount of oxygen. This nitrogen does not take a part in combustion but is required to be heated during combustion.

3. Three Ts :

                          Efficient combustion of any fuel depends on it  chemical and physical characteristics, and how well it is mixed with combustion air. three important factors - Time, Temperature, Turbulence - control the completeness of combustion and influence the design of boiler equipment and operating practices.

• TIME - 

  When a fuel is being burned, it is important that sufficient time is available so that the fuel burns completely. 100% combustion means that the fuel is fully oxidized and full oxidation of the carbon, hydrogen and other combustible elements has taken place.

• TEMPERATURE - 

  During the combustion, if the temperature is not sufficiently high, fuel will take some time to ignite thus increasing the time of the combustion. This will affect the heat output. Hence it is very important to maintain correct temperature which ensures that fuel is quickly burnt releasing the complete energy.

• TURBULENCE - 

  If the fuel and air are mixed in swirling paths, instead of each flowing in streamlined paths, combustion will be greatly improved because the mixing of fuel and air is more complete. The proper amount of air for a given amount of fuel means nothings if the two are not mixed.

               As it can be seen the 3Ts discussed above impact the process of combustion and can significantly affect the efficiency if not monitored closely.

               At oil burning plants, the oils burned must be heated on their way to the burner. This accomplishes two tasks. First the oil flows more readily when heated; secondly it atomizes better.

 

 

COMBUSTION CONTROL HAS 3 OBJECTIVES

  1.  To balance continuously the heat input with heat demand. This is known as

   ‘RATE OF FIRING CONTROL’.

  2.   To maintain this balance using minimum amount of fuel and air. This is known as

           ‘COMBUSTION EFFICIENCY CONTROL’.

  3.  To maintain safe conditions at all times in the furnace.

 

 

 

BENEFITS OF PROPER AUTOMATIC COMBUSTION CONTROL SYSTEM ARE

  1.Constant steam pressure is maintained resulting in better efficiency to both steam generating plant & steam receiving plant.

  2.Savings in fuel due to increased efficiency.

  3.Increase in life of equipment's and pressure parts.

  4.When more than one boiler is connected to common  steam header proper load distribution between boilers will be achieved.

 

 

Several types of combustion controls exist for solid, liquid and gaseous fuels. 

  Four of these are given below.

  1.  Parallel adjusted positioning type with no metering of fuel and air used in package boilers.

  2.  Parallel adjusted with metering of fuel/air (Used in industrial process with constant loads).

3.  Parallel adjusted metering of fuel with fuel limiting (used in process with wild swings and gas/oil as fuel).

  4.  Any one of the above with manual or automatic oxygen trim (Applied when optimum performance is required).

 

                          Control of the combustion process involves  the addition of the proper amounts of fuel and air to the furnace in correct proportions. 

 

 

EXCESS AIR CONTROL

                         Satisfactory Combustion will not be achieved by supplying only STOICHIOMETRIC AIR (Theoretical minimum quantity air for complete combustion).

  EXCESS AIR REQUIREMENT DEPENDS ON

   1.   TYPE OF FUEL

  2.   COMBUSTION EQUIPMENT

  3.   SYSTEM DESIGN

 

ADDITIONAL AIR MAY BE REQUIRED FOR CONSIDERATIONS SUCH AS

     1. Prevention of slagging in furnace

     2. Requirement of higher steam temperature at partial loads 

     3. Prevention of explosion possibilities at startup and low loads by keeping air rich  mixture.

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