5 replies to this topic

[sheiko]

Hi,

I do know how to calculate the thermal efficiency of a fired-heater when the fuel is only in gas state (considering the total heat input is the sum of the fuel gas LHV, the sensible heat by air and by the fuel gas entering the burners...).

Now I was wondering how to calculate the thermal efficiency when burners are fed with both fuel gas and fuel oil?
Do we simply have to consider the latent heat of vaporization of the liquid fuel oil components when calculating the total heat input?

FYI, I have developped a spreadsheet in the case of fuel gas and I would like to do the same in the mixed case...

Thanks for your inputs.

[Flame]

Net input = Fuel gas LHV + Fuel Oil LHV + Fuel sensible heat+Atomising steam sensible heat. (All in kcal/kg of fuel)

Useful heat = Net input - heat lost
Heat lost = Flue gas heat lost (i.e heat content in flue gas while raising to stack exit temperature from a standard temperature of 15.6 deg C.) + Radiation losses. (All in kcal/kg of fuel)

Thermal efficiency (in terms of LHV) = Useful heat x 100 /Net heat input

Only difference in your case will be accurate estimation of kg of flue gas per kg of fuel and flue gas components for specific heat calculation. This can be easily done if fuel gas composition & in case of fuel oil, C/H ratio, wt %Sulfur & wt% inerts are known.


For more learning, details can be found out in the Annexure of API standard 560.

Hope the above information is useful.

Thanx

[kkala]

I do know how to calculate the thermal efficiency of a fired-heater when the fuel is only in gas state (considering the total heat input is the sum of the fuel gas LHV, the sensible heat by air and by the fuel gas entering the burners...).
Now I was wondering how to calculate the thermal efficiency when burners are fed with both fuel gas and fuel oil?
Do we simply have to consider the latent heat of vaporization of the liquid fuel oil components when calculating the total heat input?....

Mentioned latent heat of vaporization has to be considered (as heat consumed, not input), if the reported heat of combustion of liquid fuel is referred to gas state. This heat of combustion is usually referred to liquid state (e.g. LHV of fuel oil = 9600 kcal/kg), thus heat of liquid fuel vaporization should not be considered, as it has been already "considered" in the given heat of combustion.
Post of Flame describes thermal efficiency by "heat loss method". In steam boiler guarantee test runs, we applied "heat loss method, abbreviated test" for the efficiency calculation. Ambient temperature during runs is usually different to 15.6 oC, so some "correction" should be made.
It is also pointed out that API recommends using HHV of fuel; but we say in requisition that efficiency should be based on the LHV of fuel, which means that H2O from combustion remains in gaseous state (the two "efficiencies" are different in value).
It is finally pointed out that (at least in our understanding) efficiency may not be identical in meaning from vendor to vendor. So we additionally ask the vendor to guarantee fuel consumption under normal conditions and specific climatic data. Penalties are based on this guaranteed fuel consumption, not on efficiency.

[sheiko]

Mentioned latent heat of vaporization has to be considered (as heat consumed, not input), if the reported heat of combustion of liquid fuel is referred to gas state. This heat of combustion is usually referred to liquid state (e.g. LHV of fuel oil = 9600 kcal/kg), thus heat of liquid fuel vaporization should not be considered, as it has been already "considered" in the given heat of combustion.

Thanks kkala. But how can I know if the reference state of the fuel oil LHV is gas or liquid? Shall it be mentionned in the source where I take the values?

[kkala]

how can I know if the reference state of the fuel oil LHV is gas or liquid? Shall it be mentionned in the source where I take the values?

Yes, the reference state should be mentioned in the source reporting the heat of combustion. Look for example at Perry, 5th edition (1973), Table 3-203 (Heats of combustion), where state is specified for any substance considered. For some of them two different values of heat of combustion are given; e.g. for n-hexane (C6H14) Perry's table gives a LHV of 10737.2 kcal/kg if gas, versus 10650.0 kcal/kg if liquid (at 25 oC reference temperature). The difference of 87.2 kcal/kg is the heat of vaporization of n-hexane, found in another table as 87.50 kcal/kg (25 oC, Perry 5th edition, table 3-172).

It is pointed out that there are tables reporting heat of combustion without clarifying the state of the substance (e.g. look at wikipedia, "heat of combustion"). You could verify the value through the combustion reaction, considering heats of formation (distinction between gas and liquid is clearer there).
Nevertheless you can guess reference state in several cases. Reference conditions are 25 oC and atmospheric (or vapor) pressure, so most probably state of the substance under these conditions has been considered. For instance, fuel oil LHV must refer to liquid fuel oil.
In general heat of vaporization of hydrocarbons is small compared to water, so error is not great by neglecting it (less than 1% of heat of combustion in case of n-hexane).

Edited by kkala, 10 April 2011 - 02:50 AM.

[breizh]

kostas &sheiko
You may be inrested with this calculator ( Heat values of fuels)

http://hydrogen.pnl....?canprint=false

Breizh