Stack Losses – General Methodology
Calculating boiler stack losses consists of six steps. The calculations described below form the basis of the Boiler Efficiency Calculator. Since stack losses depend on fuel composition‚ firing conditions and flue gas temperature more detailed descriptions of the steps are available for five fuel types.
- Natural Gas
- Light Fuel No. 2 Oil
- Light Fuel Oil (Diesel) Low Sulphur
- Heavy Fuel No. 6 Oil
- Heavy Fuel No. 6 Oil Low Sulphur
Step 1: Obtain and convert measurements
Determine the maximum capacity rating of the boiler unit in lb/h for steam or millions of Btu/h for high temperature water generators.
Measure:
- gas temperature (FGT), °F or °C
- combustion air temperature (CAT)‚ F or °C
- O_{2} in flue gas, % by volume
- actual unit output, lb/h or millions of Btu/h
If FGT and CAT are measured in °C, convert to °F.
If O_{2} is measured on a wet gas basis, convert to dry gas basis as described for each fuel.
Determine the CO_{2} in flue gas, % by volume, dry gas basis, from the relationship to measured O_{2}‚ as described for each fuel. Express the actual unit output as a percentage of the maximum capacity rating
Step 2: Calculate dry flue gas (DG), lb/lb fuel
The ASME formula for wt of dry flue gas is:
DG‚ lb/lb fuel‚ | = | 11CO_{2} + 8O_{2} + 7(N_{2} + CO) | x [( lb C burned) | + 0.375S] |
3(CO_{2} + CO) | ( lb fuel ) |
Step 3: Calculate excess air
The value of excess air is not needed for calculation of stack losses‚ but it is the most common parameter of burner performance. In the following calculations CO is assumed to be zero‚ because the maximum amount allowable under emissions regulations has a very small impact on efficiency calculations. For the same reason‚ it is assumed that all the carbon in the fuel is burned.
The ASME formula for excess air‚ (EA)‚ % by volume‚ when CO is negligible‚ is:
EA‚ % by volume | = | 100 x O_{2dry } | ||
0.2682N _{2} – O_{2dry } |
Step 4. Calculate dry flue gas loss (L_{DG} )‚ % of fuel input
Dry flue gas loss is the (sensible) heat energy in the flue gases due to the flue gas temperature.
The ASME formula for L_{DG} ‚ % of fuel input‚ is:
L_{DG} ‚ % of fuel input | = | [DG x C_{p} x (FGT – CAT)] | x | 100 |
HHV |
Step 5: Calculate latent heat loss (LH)‚ % of fuel input
Flue gas loss due to moisture represents the (latent) energy in the steam in the flue gas stream due to the water produced by the combustion reaction being vaporized from the high flue gas temperature. In the process of combustion the hydrogen content of the fuel is converted to H_{2}O‚ which normally leaves the stack as water vapor‚ carrying with it the latent heat of vaporization‚ as well as sensible heat proportional to the stack temperature.
The ASME formula for LH‚ % of fuel input is:
L_{H}‚ % of fuel input=9H_{2} x [(enthalpy of vapor at 1 psia & FGT) – (enthalpy of liquid at CAT)] x | 100 | |||
HHV |
Step 6: Calculate stack losses (L_{s} )‚ % of fuel input
This is the sum of Steps 4 and 5.
L_{S} ‚ % of fuel input = L_{DG} + L_{H} | ||||
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