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Furnace Fired HRSGs

Turbine exhaust gases have typically 13 to 15 % volume of oxygen available,which can be utilized to fire additional fuel and thus improve the efficiency of the system.Supplementary firing is a very efficient process and can easily double or quadruple the steam generation compared to unfired mode at a fuel to steam efficiency of more than 100 %he efficiency of supplementary firing is nearly 100 %;the reasons for this are discussed at other pages of my web site.
This article describes a furnace fired O-type boiler capable of firing up to 2300 F.It consists of a completely water cooled furnace,about 12 ft long,sized to contain the flame,followed by a bare tube screen section,a two-stage superheater with interstage attemperation,a convection bank  with bare/finned tubes and a horizontal gas flow economizer. Four 6 in pipes located at the end of convection bank act as downcomers.The performance of the unit is shown in Table below in unfired and fired modes.A Coen burmer, firing natural gas, was used.This is located at the furnace front wall. Steam drum and mud drum sizes are 48  and 30 in respectively.The unit is externally insulated.In addition the main stack and a diverter valve with bypass stack were provided.The unit is in operation at a paper plant in Monterrey,Mexico since 1999.
 
   item  Unfired mode  Fired mdoe
 exhaust gas flow,lb/h  187,789 187,789
exhaust gas temperrature,F  979   979
 firing temperature,F  979  1969
 gas temperature lvg furnace,F  970  1890
 gas temperature to superheater,F  922  1747
 gas temperature to evaporator,F  871  1576
 gas temperature to economizer,F  494  548
 gas temperature lvg stack,F  353  314
 burner fuel,MM Btu/h (lhv)  0  59.42
 total gas pressure drop,in wc  8.2  9.45
 steam pressure,psig  430  430
  steam flow,lb/h  27,600  80.500
 steam temperature,F  590  590
  feed water temperature,F   225  225
 water temperature to evap,F   446  364
                 [ exhaust gas analysis -% volume: co2=2.9,h2o=5.6,n2=75.8,o2=14.5 .]

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