Final Exam (Dec.): Combustion & Boiler (with solution)
- Due No Due Date
- Points 14
- Submitting on paper
- Available after Dec 19, 2017 at 8:55am
A steam boiler for a CHP plant is fired with wet biomass woodchips and operates only during the cold season (for a total of 4000 hours per year). The owners plan to install flue gas treatment equipment for reducing the NOx emissions; this requires an estimation of the total amount of flue gas and of the inert nitrogen gas mass flow per year; as well as a precise calculation of the thermal efficiency of the boiler.
Hint: Start with an independent combustion calculation (complete combustion) followed by boiler efficiency estimation via the indirect method.
The following is given:
Rated heat input capacity of furnace QB = 50 MWth (fuel energy input rate)
Higher heating value of fuel on dry basis HHVdry = 19 MJ/kg
Flue gas temperature in the stack Tgas = 400 oC
Operational hours per standard year 4000 h/a
Air excess for combustion (air factor) m = 1.4 (assume dry technical air)
Fuel (biomass) composition on dry basis, mass-%:
C 48 %
H2 10 %
O2 28 %
N2 find it from the other given data! %
S 2 %
Ash 6 %
Moisture content in fuel on total basis (wet mass-%): 50 %
Additional input parameters for the boiler efficiency estimation:
CO (carbon monoxide) in dry flue gas COdry = 900 ppm at normal conditions
Unburned carbon in residual ash 10 %
Specific heat of ashes Cpash = 0.9 kJ/kgK
LHV of unburned carbon in ash LHVC = 30 MJ/kg
LHV of unburned CO in gases LHVCO = 12.634 MJ/m3n
The list of partial answers below shows the necessary calculation steps:
|
|
Sub-task |
Units |
Points |
|
1) |
% Carbon content C on total wet fuel basis |
% |
0.25 |
|
2) |
% Hydrogen content H2 on total wet fuel basis |
% |
0.25 |
|
3) |
% Oxygen content O2 on total wet fuel basis |
% |
0.25 |
|
4) |
% Nitrogen content N2 on total wet fuel basis |
% |
0.25 |
|
5) |
% Sulfur content S on total wet fuel basis |
% |
0.25 |
|
6) |
% Ash content on total wet fuel basis |
% |
0.25 |
|
7) |
Stoichiometric Oxygen needed for combustion |
mol/kgfuel |
1 |
|
8) |
Total final flue gas amount, mols per kg fuel |
mol/kgfuel |
1 |
|
9) |
Lower Heating Value of the wet fuel (LHVwet) |
MJ/kg |
1 |
|
10) |
Real flue gas volume flow at stack temperature |
m3/s |
1.5 |
|
11) |
Nitrogen (N2) mass flow in flue gas (103 t/a) |
kton/year |
1.5 |
|
12) |
Average specific heat value for CO2 |
J/mol.K |
0.2 |
|
13) |
Average specific heat value for H2O |
J/mol.K |
0.2 |
|
14) |
Average specific heat value for N2 |
J/mol.K |
0.2 |
|
15) |
Average specific heat value for SO2 |
J/mol.K |
0.2 |
|
16) |
Average specific heat value for O2 |
J/mol.K |
0.2 |
|
17) |
Fraction of outgoing ash per kg fuel (not in %) |
- |
0.25 |
|
18) |
% energy loss due to unburned soot in ashes |
% |
1 |
|
19) |
% thermal energy loss with stack temperature |
% |
1 |
|
20) |
% energy loss due to CO in flue gas |
% |
1 |
|
21) |
% thermal energy loss due to residual ash heat |
% |
1 |
|
22) |
% radiation loss from boiler walls (from graph) |
% |
1 |
|
23) |
Boiler efficiency result, in percent |
% |
0.25 |
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