Mid-term Exam: Combustion & Boiler (with solution)

Q1.  Combustion and Boiler  (14 points)

 A boiler is fired with locally mined high-sulphur lignite coal and produces steam for an industrial factory operating only during usual work hours (a total of 2000 hours per year) and thus having permission to run without flue gas treatment facilities. The owners are obliged to report to the national regulatory authorities the volumetric flow of flue gases at rated conditions; the mass flow of sulphur dioxide emitted per one year of nominal operation; as well as the value for the thermal efficiency of the boiler.

The solution involves a combustion calculation (complete combustion) followed by the boiler efficiency estimation via the indirect method.

The following is given:

Rated thermal capacity of the boiler          Q_B = 8 MW_th (fuel energy input rate in boiler)
Higher heating value of fuel on dry basis   HHV_dry = 22 MJ/kg
Flue gas temperature in the stack               T_gas = 250 ^oC 
Total operational hours per year                  2000  h/a
Air excess for combustion (air factor)          m = 1.2  (assume dry technical air) 

Fuel composition on dry basis, mass-%:

C         45 % 
H2      15 % 
O2       find it from the other given data! % 
N2       3 % 
S          6 % 
Ash     20 % 

Moisture content in fuel on total basis (mass-%):  20 %

Additional input parameters for the boiler efficiency estimation:
CO (carbon monoxide) in dry flue gas         CO_dry = 900 ppm at normal conditions
Unburned carbon in residual ash                 7 % 
Specific heat of ashes                                  Cp_ash = 0.9 kJ/kgK
LHV of unburned carbon in ash                   LHV_C = 30 MJ/kg 
LHV of unburned CO in gases                     LHV_CO = 12.634 MJ/m³_n