According to PTC (Power Test Code) 1.4 ASME (American Standard of Mechanical Engineering) there are two methods for boiler efficiency calculation namely: input output method and heat loss method. The following below is further explanation of the boiler efficiency calculation methods:
- Input-output method
η = Output / Input
Output is defined as the heat that is absorbed by working fluid. Input is defined as the chemical heat of fuel plus the heat that is added to working fluid, air, gas and other fluid circuit in which the fluid circuit across thin layer.
- Heat Loss Method
Heat loss method is the method that is used to calculate the percentage amount of heat which is not useful. This method is very effective used in an attempt to find potential savings of boiler with heat balance.
There are several factors act as the source of heat loss combustion. These factors will be used to calculate boiler efficiency based on heat loss method. The source of heat loss can be seen as following below:
Heat loss due to dry flue gas is the heat loss which is contained in dry flue gas such as CO2, O2 and N2. These gases are one of causes of heat loss in combustion process. The high temperature of flue gas coming out from boiler affect amount of heat loss.
The weight and heat loss of dry flue gas of combustion of coal can be calculated as following formula:
Wdfg = mCO2 + mSO2 + mO2 + mN2
Eloss dfg = Wdfg x Cp x (To - Ts)
Wdfg = weight of dry flue gas (kg/kg coal)
Eloss dfg = heat loss in dry flue gas (kJ/kg)
To = temperature of dry flue gas coming out from boiler
Ts = temperature of surrounding air
There are two sources of steam contained in flue gas. It is steam from burning H2 and steam from fuel moisture coupled with steam present in combustion air.
- The calculation formula of heat due to steam from burning H2 is:
WH2O = 9 x H2
Eloss H2 = WH2O x (hg - hf)
WH2O = weight of water content (kg/kg coal)
Eloss-H2O = amount of heat loss due to steam from burning process of hydrogen (kJ/kg)
Hg = enthalpy of steam at the temperature of flue gas coming out from boiler
hf = enthalpy of water at surrounding air temperature.
hg and hf can be seen in Table 1 below.
|Table 1: Saturated Water and Saturated Steam|
- Steam from fuel moisture and combustion air
Calculation of heat loss due to steam from fuel moisture and combustion air: can be seen in the following formula:
Eloss-H2O = (WH2O-coal + WH2O-air) x (Hg - Hf)
Eloss-H2O = amount of heat loss due to steam from fuel moisture and combustion air
The calculation is assumed level of excess air is provided about more 20% and combustion that occurs only produces unburned carbon about 1% of carbon content in fuel. Calculation of heat loss due to unburned carbon can be seen as following formula:
Eloss-UC = WUC x HHVcarbon
WUC = weight of unburned carbon (kg/kg)
HHVcarbon = highest heating value (kJ/kg)
Eloss-UC = amount of heat loss due to unburned carbon
The amount of heat loss due to radiation depends on the rate of heat which is released in combustion furnace. Based on the ABMA chart (American Boiler Manufacturers Association), at big heat release rate, the amount of heat loss due to radiation tends to constant. The heat radiation that occurs is estimated 0.2% calorific value of fuel.
- Uncountable heat loss
There are several sources that can cause heat loss in combustion, where the value is too complicated to be calculated certainly. This heat loss is called as uncountable heat loss. The amount of uncountable heat loss is about 0.3% of calorific value of fuel.
- Heat losses in the boiler if the heat is used only for warming fluid only.
Total heat loss that may occur in boiler system can be calculated as formula below, so boiler efficiency calculation can be determined.
Eloss total = Eloss-dfg + Eloss-H2 + Eloss-H2O + Eloss-UC + Eradiation + Euncountable