How to calculate the furnace temperature of a Boiler???

 

               How to calculate the furnace temperature of a Boiler???

The furnace temperature of a boiler depends on various factors such as fuel type, combustion efficiency, and heat transfer.

The total heat released by fuel during combustion is not completely utilized. Some heat is taken out by water vapour which is produced during combustion of hydrogen. Such heat value taken by considering heat taken away by water vapor is called NCV or LCV.

LCV = HCV – (9 X H2% X 586), Where H2 = Hydrogen% in fuel and 586 is latent heat of steam

Boiler furnace efficiency is a measure of how effectively the combustion process converts fuel energy into useful heat. It is typically calculated using the combustion efficiency method or the direct and indirect methods.

Example-1

A coal fired Boiler with lower calorific value 4200 kcal/kg is burnt in a Boiler with air fuel ration 6:1. Neglect the ash generated, calculate the maximum temperature attained in the furnace of the Boiler.

Assume the total heat generated by combustion of coal is given to the production of combustion. Consider the average specific heat of flue gas 0.24 kcal/kg & Boiler atmospheric temperature 30 deg C.

Given data;

L.C.V of fuel: 4200 kcal/kg

Specific heat of fuel: 0.24 kcal/kg

Boiler area temperature: 30 deg C

Furnace temperature = Heat released by combustion

No losses have been considered

Mg X Cpg X (t2-t1) = 1 X LCV

Mg X Cpg X (t2-t1) = 1 X 4200

Where, Mg = Mass of flue gas in kg

Cpg = Specific heat of flue gas

T2 = Maximum furnace temperature

(7+1) X 0.24 X (t2-30) = 4200

3.2 X t2 -57.6 = 4200

Furnace temperature t2 = 1330.5 deg C

Example-2:

A Biomass fired Boiler with GCV  2200 kcal/kg is burnt in a Boiler with air fuel ratio 3.5:1, Neglect the ash generated, calculate the maximum temperature attained in the furnace of the Boiler.

Assume the 70% of total heat generated by combustion of fuel is given to the products of combustion & 30% is losses. Consider the average specific heat of flue gas 0.24 kcal/kg & Boiler atmospheric temperature 32 deg C.

The details of the ultimate analysis of the fuel is;

Carbon, C: 23%

Oxygen, O2: 22%

Sulphur, S = 0%

Hydrogen, H2: 3.2%

Moisture, M: 50%

 

Given data;

GCV of the fuel: 2200 kcal/kg

LCV = HCV – (9 X H2% X 586), Where H2 = Hydrogen% in fuel and 586 is latent heat of steam

LCV = 2200-(9 X 3.2% X 586) = 2031.2 kcal/kg

Air to fuel ratio: 3.5:1

Combustion efficiency = 70%

Specific heat of flue gas, Cpg = 0.24 kcal/kg

Boiler area temperature t1 = 32 deg C

Therefore, we have;

Mg X Cpg X (t2-t1) = 1 X LCV

Where t2 = Furnace temperature to be attained

(3.5+1) X 0.24 X (t2-32) = 1 X 2031.2 X 70%

1.08 X t2 – 34.56 = 1421.84

Furnace temperature t2 = 1348.51 deg C

Example-3:

An oil-fired Boiler with a Lower calorific value (LCV) 10200 kcal/kg is burnt in a oil fired Boiler of capacity 100 TPH. The ratio of air fuel is 18:1, neglect the ash generation & heat loss in combustion, calculate the maximum temperature attained in the furnace of the Boiler.

Given data;

LCV of the fuel: 10200 kcal/kg

Air to fuel ratio: 18:1

Combustion efficiency :100

Specific heat of flue gas, Cpg = 0.24 kcal/kg

Boiler area temperature t1 = 30 deg C

Therefore, we have;

Mg X Cpg X (t2-t1) = 1 X LCV

Where t2 = Furnace temperature to be attained

(18+1) X 0.24 X (t2-30) = 1 X 10200

Furnace temperature, t2 =2266.84 deg C

 

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