Final Exam (Jan): Combustion & Boiler (with solution)

  • Due No Due Date
  • Points 14
  • Submitting on paper
  • Available after Jan 11, 2018 at 8:55am

Q1.  Combustion and Boiler  (14 points)

A steam boiler for a CHP plant is fired with half-dried wood logs and operates only during a short cold season (a total of 3000 hours per year). The owners plan to install flue gas treatment equipment for reducing the emissions; this requires an estimation of the total amount of flue gases and of the free oxygen contained in the gas mass flow; 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 thermal input capacity of boiler          QB = 40 MWth (fuel energy input rate)
Higher heating value of fuel on dry basis      HHVdry = 18 MJ/kg
Flue gas temperature in the stack                Tgas = 100 oC 
Operational hours per standard year            3000  h/a
Air excess for combustion (air factor)           m = 1.5  (assume dry technical air) 

Fuel (biomass) composition on dry basis, mass-%:  
C          46 % 
H2        9 % 
O2        29 % 
N2        find it from the other given data! %   
S          2 % 
Ash      8 %  
Moisture content in fuel on total basis (wet mass-%):  30 %

Additional input parameters for the boiler efficiency estimation:
CO (carbon monoxide) in dry flue gas          COdry = 500 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)

Oxygen mass flow remaining in flue gas (103t/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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