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Understanding Boiler Stack Losses

What are stack losses?

Constant combustion takes place inside a boiler furnace. In order to burn the fuel, oxygen is required. Hence, the burner is supplied with air. Air normally has 21% Oxygen, 78% Nitrogen and moisture content depending upon the location. The fuel which is burnt inside is composed of hydrocarbons mainly. After combustion, Carbon dioxide and water vapor are formed due to combustion of carbon and hydrogen as follows-

C+ O2=CO2

2H2 + O2 = 2H2O

 Air comes inside a boiler at ambient temperature, i.e. around 25- 40 Degree Celsius depending upon the location.  After combustion, combustion products, i.e. Water vapor, Carbon dioxide and other gases depending on fuel composition are formed. These flue gases heat the water and then leave the system through a Chimney. These flue gases are at high temperature and carry a large amount of heat (m*cp* ?t). This heat energy is taken from the fuel being burnt and along with the flue gases it escapes un utilized. These losses are termed as stack losses.

Excess Air and stack losses

From the equations mentioned above, it can be seen that one molecule of oxygen is required for the complete combustion of one unit carbon to form one molecule of carbon dioxide. Similarly, one molecule of Oxygen is required for combustion of two Hydrogen molecules.  So, there is a fixed ratio of Oxygen which should be supplied in order to achieve complete combustion. In reality, the perfect mixing of fuel and Oxygen never takes place and if we supply just the necessary amount Oxygen (air), due to improper mixing, complete combustion of Caron does not take place. This results in partial combustion of the fuel as follows–

2C + O2 = 2CO

It can be seen that due to insufficient amount of Oxygen, instead of Carbon dioxide, Carbon monoxide is formed. This process releases significantly less amount of heat as compared to the previous one. Hence, additional Oxygen should be supplied in order to achieve complete combustion of Carbon. This is achieved by sending in some quantity of excess air.

One more reason to supply excess air is, there are always fluctuations taking place during combustion. Oxygen content of air changes seasonally. Apart from that, in case of increase in boiler load, oxygen starvation might take place. Considering all these parameters, it is always recommended to allow certain buffer or excess air to pass in.

Heat carried per hour by stack gases can be calculated by-

Q= m*Cp*?t

Where,

m= mass flow rate of flue gases (kg/hr.)

Cp= Specific heat capacity of flue gases

?t =   Difference between inlet and outlet temperatures of flue gases.

Stack losses are directly proportional to mass flow rate of flue gases and the difference between inlet and outlet temperatures of flue gases. Mass flow rate of flue gases and the outlet temperature of flue gases should always be monitored to keep a check on stack losses.

Controlling mass flow rate of flue gases

For efficient combustion, we need to supply a certain amount of excess air. Only required amount of excess air should be supplied. Depending upon the type of fuel which is being used and burner type, standard percentage of allowable excess should be found out and these values should be followed strictly.

Controlling outlet temperature of flue gases

A common trend observed is, as boiler is operated continuously, the measured outlet temperature of flue gases increases. After operation for a few months, significant rise in flue gas temperature and hence stack losses is observed. The reason behind this is, as a boiler is operated, scales are formed on the boiler tubes. As the thickness of the scales increases, the coefficient of heat transfer goes down. As a result, heat from flue gases is not transferred to the water side and it is carried away by the flue gases. The best way to tackle the situation is monitoring flue gas temperature just at the boiler outlet and cleaning the tubes when it goes beyond acceptable limits. By treating feed water, the frequency of cleaning can be brought down.

Heat recovery from flue gases

Heat which is being carried away by flue gases can be recovered up to a certain limit. This reduces the temperature of flue gases and saves the cost of cooling the gases before leaving them into the atmosphere. The easiest way of recovering heat from flue gases is to pass them through a pre heater. Pre heater heats the air going inside the furnace. This elevated temperature of air results in reduced value of   ?t hence reduces the stack losses.

Dew point condensation

Depending upon the type of fuel being used, acidic vapors can be present in the flue gases (like SO2). If the temperature drops below the dew point for these acidic vapors, corrosion of metal walls takes place. This puts a restriction on up to what level heat from flue gases should be recovered.

Installing a pre heater system and constantly monitoring excess oxygen and flue gas temperature can help to reduce the stack losses and keep them minimal.