A steam power plant generating 500MW of electrical power employs a natural circulation boiler which supplies a steam at pressure 150 bars and temperature of 5500C. The condenser pressure is 0.05 bar.The turbine, mechanical and generator efficiencies are 87%, 98% and 99% respectively. The boiler uses pulverized coal having a calorific value of 26 MJ/kg and yields 92% efficiency. The feed water passing through the feed heaters enters the boiler at 160oC. The risers of the furnace are 55 meter high while the downcomers are placed outside the furnace for producing natural circulation. The quality of steam at the top of the riser is 12% and minimum exit velocity of mixture leaving the risers and entering the drum is 1.4 m/s. The dimensions of the riser tubes are 65 mm OD and 3mm wall thickness while the dimensions of downcomers are 185 mm OD and 8 mm thick. Assume no pressure drop and heat loss to the risers. Work out the following:
(i) Sketch the layout of natural circulation boiler unit showing furnace, drum, risers, downcomers, superheater, reheater and economizers and show the process on T-s diagram.
(ii) The generation of rate of steam ignoring the amount of steam bled off to feed heaters in kg/s.
(iii) The rate of fuel flow required in kg/s.
(iv) The evaporation factor
(v) The circulation ratio
(vi) The number or riser tubes
(vii) The number of downcomers
(viii) State reasons for selecting fewer in number and bigger in diameter as down comers while more in number and smaller in diameter as riser tubes in natural circulation boiler.
Take the following properties:
h1 = 3448.6 kJ/kg, h2s=1987 kJ/kg,
hf3 = 137.8 kJ/kg, hf5=675.5 kJ/kg,
rriser, top=396 kg/m3
rdowncomer,inlet = 603 kg/m3
Where suffixes 1, 2, 3, 4 and 5 denote boiler exit (or turbine inlet) turbine outlet, condenser exit, pump inlet and boiler inlet respectively.