From the measurement methods of high calorific value and low calorific value, we can know that low calorific value is the heat obtained by subtracting the vaporization heat of water (water produced by the combustion of water and hydrogen in coal) from high calorific value. We generally think that the moisture in the flue gas exists in the form of steam and leaves the boiler system, so this should be fully considered when calculating the heat loss of boiler flue gas.
Let's see how ASME calculates the heat loss of water (or steam) in flue gas. Water (or steam) in flue gas consists of four parts:
(1) Moisture from fuel;
(2) Water generated by the combustion of hydrogen in fuel;
(3) Moisture brought by air;
(4) atomizing steam or blowing steam.
According to ASME standard, the heat loss of the above four items is calculated as follows: LMF = FM× (h-HRW) (1).
LH=8.936×fH×(H-HRW) (2)
LmA = WmA′WA′×(H-HRV)(3)
Lz = wz/wfe× (h-HRV) (4) where
H —— steam enthalpy and steam partial pressure at exhaust temperature;
HRV refers to the enthalpy of saturated steam at reference temperature;
HRW refers to the enthalpy of saturated water at the reference temperature;
LMA- The moisture and heat loss caused by air can be obtained from the output item of ASME module and used to calculate the boiler loss.
WMA '- air humidity, in pound/pound.
Wa '- dry air volume per pound of fuel, in pounds per pound. It can also be obtained from the output term of ASME module to calculate boiler loss, that is, C 1-WAI-LHV.
Obviously, ASME believes that the water brought in by the fuel and the water generated by the combustion of hydrogen in the fuel leave the boiler system as saturated water at the reference temperature, and release the latent heat of vaporization before leaving. This is probably the physical intention of ASME standard to use high calorific value.
If the low calorific value is used to calculate the furnace efficiency, the heat losses of the water brought by the fuel and the water generated by the combustion of hydrogen in the fuel are calculated as l ′ MF = FM× (h-HRV) (5) respectively.
L ′ h = 8.936× FH× (h-HRV) (6), that is, the total heat loss of water vapor in flue gas should be the sum of L ′ MF, L ′ H, LmA and LZ. Therefore, when calculating the furnace efficiency, the total loss term c 1- loss-interior in the module can no longer be used directly, but the self-made c 1- loss -LHV calculation module can be used.
However, in the heat loss calculation module of ASME, both H and HRV are intermediate calculation processes, and their data are not directly output. It is difficult to establish a new mathematical model to directly calculate H and HRV. Therefore, we studied the ASME heat loss calculation module carefully, and finally found a relatively simple conversion method, that is, by solving the value of (H-HRV) and then calculating the values of L ′ MF and L ′ H, thus solving the problem of using low calorific value in ASME standard calculation module.
(h-HRV) can be solved by Formula (3) to calculate the heat loss of moisture brought by air.
In this way, (H-HRV) in Formula (3) can be directly solved as the only unknown number. Then according to the following formula: "c 1- loss-lhv" = "c1-loss-inside"-"lmf"-"LH"+"l 'mf"+"l" "h, the total loss of the boiler is calculated, and then the boiler efficiency is calculated.