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EFFECT OF GAS ANALYSIS ON BOILER PERFORMANCE  by V.Ganapathy
Flue gas analysis plays an important role in the performance of steam generators.A large volume of water vapor in particular affects the boiler duty or steam production through higher specific heat.Convective and non-luminous heat transfer coefficients are impacted by high thermal conductivity, ,specific heat and partial pressure of water vapor.Hydrogen,present in reformed gas stream in hydrogen plants,has a high specific heat and thermal conductivity,see table below. Also,a high gas pressure results in a much higher gas mass velocity,thus increasing the heat transfer coefficient and the associated heat flux. The heat transfer coefficient inside the tubes in reformed gas boilers can be 6 to 8 times that of regular flue gases at atmospheric pressure.In reformed gas boilers, the heat flux limits the gas mass velocity rather than gas pressure drop due to the large gas pressures. Presence of sulfur dioxide or trioxide or hydrogen chloride can cause low temperature corrosion problems as discussed  in the article on Acid corrosion concerns.Hydrogen chloride gas can also cause corrosion problems at the superheater if the tube wall temperatures are above 850 F.Hence gas analysis is a very important aspect of boiler design.

Table of Flue Gas Analysis
 
% volume  CO2 H2O N2 O2 SO2 Hcl H2 CH4 CO Press,atm
sulfur combustion     80 9 11         1
Gas turbine exhaust 3 7 75 15           1
Flue gas-nat gas 8 18 71 3           1
Flue gas-oil 12 12 73 3           1
Reformed gas 6 36         46 3 9 20-60
Incineration -fume 5 8 77 4   6       1
 [ unfired turbine exhaust gas anlaysis is shown above. If steam or water is injected into the turbine,the % volume water vapor will increase.]
 
 Table showing effect of gas properties on Heat Transfer inside and outside tubes
Heat Transfer Coefficients inside Tubes 
The heat transfer coefficient inside tubes is given by the expression:  
NU=0.023Re0.8Pr0.4 where  
NU=hidi/12k  
Re=15.2w/(dim 
Pr =m Cp/k  
NU=Nusselt Number  
Re=Reynolds Number  
Pr=Prandtl Number  
Cp=gas specific heat,Btu/lbF  
m =viscosity,lb/fth  
k=thermal conductivity,Btu/fthF  
Simplifying we have:  
hi=2.44Cw0.8/di1.8  
Where C=(Cp0.4k0.6/m 0.4 ) 
hi=inside heat transfer coefficient,Btu/ft2hF  
w=flow per tube,lb/h 
di=tube inner dia,in
Heat Transfer outside Tubes(convection) 
Though there are several correlations,the following is used to show the effect of the variables on heat transfer: 
NU=0.35Re0.6Pr0.33 
Re=Gd/12m 
NU=hod/12k 
Simplifying: 
ho=0.945G0.6F/d0.4  
where F=(k0.67Cp0.33/m 0.27 
In addition to convective heat transfer coefficient,non-luminous radiation plays a role. This is a function of triatomic gases such as water vapor,carbon dioxide and sulfur dioxide. 
ho=outside convective heat transfer coefficient,Btu/ft2hF 
G=mass velocity,lb/ft2
do=tube outer dia,in
 { for more explanation of the terms or computation of  gas mixture properties or the effect of pressure on gas properties,heat transfer,please see my books]
 C and F values were evaluated for various gas streams and are shown below at an average temperature of 1000 F.
 
Gas Properties for different gas streams
Gas  Cp m K C F
Nat gas flue gas 0.298 0.0832 0.0325 0.2131 0.132
Fuel oil flue gas 0.288 0.0840 0.032 0.2075 0.129
Sulfur gases 0.250 0.0858 0.0298 0.1863 0.1167
Gas turbine exhaust 0.2767 0.087 0.0321 0.2018 0.1263
Incineration gases 0.2753 0.085 0.0313 0.200 0.124
Reformed gas 0.639 0.0677 0.0867 0.307 0.175
 
It may be seen that presence of hydrogen or water vapor increases the heat transfer coefficients both inside and outside tubes. The boiler duty will also be different due to variations in gas specific heats for the same gas temperature drop. Hence one should not neglect the effect of gas analysis while evaluating duty or steam generation. 3 to 10 % variations are likely if the analysis is guessed. The error can be higher as the average gas temperature increases or if non-luminous radiation effects are considered.For example between natural gas combustion products and typical incineration gases,for a drop of say 1000 F,the ratio of duty=0.298/0.2753=1.053![ duty = gas flow x specific heat x gas temperature drop]

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