Diesel VS Compressed Natural Gas

[mavericktim89]

Hi, I have recently changed from diesel to CNG for my Reheating furnace. We reheat billets in the furnace to be then rolled into construction steel bars. What I notice is that with the same amount of heat for both the fuel, CNG is faster than diesel to heat the raw material to the required temperature and the material is also quick to drop in temperature.

I have tried to explain to my superiors that the fuel doesn't play a part in how fast the material heats up and cools down, only the heat capacity of the material does. As long as I can supply the correct amount of fuel to achieve the targeted temperature then it doesn't matter if I use CNG or diesel, the heating properties will eventually be the same.

Is this true? Why does using CNG make the material loose heat faster than using diesel?

 

[EdStainless]

My guess is that it is related to your burners and the flame conditions.
The NG flame may be 'brighter' resulting is faster surface heating, but the interior of the bars may not be as hot as you think.
Have you changed soak times?
This is an issue in long products, everyone wants higher throughput, but turning down the fire rate and allowing a little more soak will improve uniformity and assure good heating.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[georgeverghese]

The actual flame temperature also depends on the air / fuel ratio.

After you changed from a diesel burner to a CNG burner, the flame profile may have changed, causing some local hot spot close to where the temp measurement is being taken ? While the flame may have been further away from the billet with the previous diesel burner, resulting in more even heating of the billet?

 

[LittleInch]

When you say "the same amount of heat" what do you mean? Same overall calorific value?

With the same amount of heat input, the temperatures should be the same, but I suspect you're not using an energy term here?

I'm with the others - sounds like the bars are not being heated all the way through to the same temperature as at the point you're measuring it ( presumably the outside) and that gas flame temperature is higher.

Only other thing is if it is faster, then maybe there is less stored energy in the lining of the furnace and hence it cools down quicker if left inside? No details of how you heat, where you measure temperature so not easy to think much further.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[racookpe1978]

Continuous-open furnace end doors, or do they cycle open and shut with each round?

 

[gruntguru]

The adiabatic flame temperature for diesel is higher than Methane but it is possible the diesel burners were set to a leaner AFR which will reduce the temperature. (Leaner mixture would result in a higher flue gas flow and greater heat loss.)

je suis charlie