Waste heat boiler efficiency 2

[21121956]

Hi everybody

In our power plant we have waste heat boilers, using the exhaust gases from Diesel engines.
The inlet gas flow to the boiler is 30.5 kg/s at 386ºC, and the outlet conditions are 30.5 kg/s at 311ºC.

Each boiler delivers 3760 kg/h of saturated steam at 7 barg, 170ºC and the water in the feeder tank is at 90ºC.
I want to know how to calculate the efficiency of such boilers.

Can you help me in this subject, please?

 

[25362]

The thermal efficiency would be calculated as the ratio of the enthalpy output in the steam (over that of the feed water) to the enthalpy input from the exhaust combustion gases.

From steam tables, the difference in enthalpy between sat. steam at 170^oC and water at 90^oC is 570.9 kcal/kg.

The heat supplied by the hot gas depends on its composition. I'll assume its specific heats are 0.275 and 0.27 kcal/(kg.^oC), respectively, for the indicated temperatures. Heat content is affected by the gas composition and temperature.

Heat supplied: 30.5*(0.275*386-0.27*311) = 676.5 kcal/s
Heat recovered: (3760[÷]3600)*570.9 = 596.2 kcal/s

The efficiency = 596.2[÷]676.5 = 88.1%

Please note this estimate is only approximate since it is based on the assumptions above.

 

[21121956]

Thanks 25362(Chemical) for your help.

 

[21121956]

Hello 25362 (Chemical):

I have been looking for (without success) tables where appear typical values for the Specific Heat of combustion gases for engines burning Bunker C fuel oil.

Once again, could you give a hand on this subject?

Thanks a lot¡

 

[TBP]

The engine manufacturer should be able to provide the data you need.

 

[athomas236]

There are well known equations for the specific heats of the basic components such as O2, N2 etc, the problem is then to know the composition of the gases.

Once you know the composition, the specifice heat of the gas can be calculated from the equations for the basic components.

If all else fails the specific heat of air will be a reasonable approximation.

athomas236

 

[quark]

If you know the percentage of basic components, go to

http://www.hrsgdesign.com

-----> Calculators -----> Gas Properties.

 

[25362]

Tabulated values of specific heats, kcal/kg, of flue gas components, are as follows,

^oC O2 N2 CO H2O CO2 SO2 air

100 0.223 0.250 0.250 0.450 0.220 0.159 0.241
200 0.230 0.252 0.253 0.462 0.238 0.171 0.245
300 0.238 0.256 0.258 0.475 0.255 0.180 0.250
400 0.245 0.261 0.264 0.491 0.268 0.188 0.255
500 0.251 0.267 0.270 0.506 0.278 0.193 0.261

Tabulated enthalpies, kcal/kg, of the flue gases from the combustion of an oil containing mass% as follows: C, 82.8; H, 12.19; O, 2.15; and S, 2.8, are:

300 Celsius 400 Celsius

no excess air 78.8 106
28.5% xs air 77.6 105.5

By interpolation, for 311 and 386 Celsius, we'd get a difference of 20.4 and 20.9 kcal/kg for the two excess air conditions, respectively. If we take, for example, a value of 20.9 kcal/kg, the heat input would be:

30.5*20.9 = 637.5 kcal/s, bringing the thermal efficiency to

596.2[÷]637.5 = 93.5%​

 

[sailoday28]

25362 (Chemical) specific heat units??
I assume at constant pressure and per degreE K or C

 

[fzacamw]

Hi, everybody:

The efficiency calculated using the method provided by 25362 results in the ratio between the heat received in the water-steam cycle to the heat actually lost from the hot gases. The difference between is because of heat losses through the boiler envelope. However, this does not give a proper idea of the total efficiency of a waste heat boiler, as it does not take into account the maximum available heat it could have recovered. i.e., it does not account for the power lost in the hot gases exhausting into the atmosphere.

A better idea of the waste heat recovery efficiency is got by comparing the heat actually received by the water-steam cycle with the maximum available heat it could get from the hot gases, i.e. with the enthalpy change in the gases in case they left the boiler at ambient temperature. The cooler the hot gases leave the boiler, the better the efficiency.

 

[25362]

Sailoday28, you are absolutely right. Specific heats expressed in kcal/(kg.^oC). You may have noticed I gave enthalpies in Kcal/kg further down in the same posting.

CazamFW, how do you intend to get the outgoing gases to a lower temperature than the incoming boiler feed water ?

 

[fzacamw]

Hi, 25362

You are right to remark that the maximum heat transfer could be limited by the temperature of the cold medium, that in many applications could be the feed water. Thus, the ratio of heat received at the water-steam cycle to the heat delivered by the gases in case they left at water temperature would be an interesting consideration.
In some installations it is worth to use a condensate preheater bank at the end of the exhaust gases pass to preheat the condensate upstream of the deaerator, increasing the efficiency of the heat recovery.

 

[21121956]

Thanks to all of you for the valuable contribution giving answers to my questions. Sincerely, thanks ¡