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Coal furnace, and pressure

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Andy

Mon Mar 31 2014, 03:39AM

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2Spoons

Mon Mar 31 2014, 04:03AM

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Andy wrote ...

What can I expect from 600C mildsteel in relation to tensile strength, going by gut 10mm plate, with angle iron strength supports.

Steel at 600C, pressurized to 90psi? A loud bang.

Forced air + coal can easily go over 1000C, and is the classic way to smelt iron (search cupola furnace). Mild steel begins to lose strength over 400C, is at half strength around 600C, and getting floppy by 1000C.
You only need a couple of psi from a blower. Keep your coal burner separate from your ethanol. Run the hot exhaust gases through heat exchanger pipes to heat the liquid.

Going by gut is going to get you killed. This stuff has to be properly engineered, with appropriate safety factors.

Andy

Mon Mar 31 2014, 04:58AM

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Tha

Dr. Slack

Mon Mar 31 2014, 08:06AM

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I wouldn't start from pressures or velocity, that's really the tail wagging the dog. Start from mass flow.

You can compute total energy in (mass * temperature * heat capacity), then add to it the combustion energy of the coal + O2 in the air, which gives you the total energy out. We will of course assume no heat gets lost to the furnance, because we are considerig the steady state, the warmup phase will obviously be heating the lining and the coal up. As the combustion gasses will always be >100C, some of the combustion heat due to any hydrocarbon contaminants int he coal is exitting as the latent heat of steam, but as a first approximation you can neglect those. There is a difference between heat capacity of gases at constant pressure, and at constant volume, which if you are going into detail you would need to consisder. I suspect that your pressure changes across any practical furnace are going to be small, so use the constant pressure figures. Does 100% of your incoming O2 burn? This may restrict the amount of heat that burning coal adds to the airflow.

Now you've got massflow and delta energy, worry seperately about acheiving that with a combination of velocity, pressures, airflow resistance due to cross sections. Don't forget that with a 600C rise, the gas volume will increase at least 3 fold due to heating as it passes across the coal, so to a first approximation, assuming constant cross section, your gas flow resistance will be dominated by the region downstream of the coal.

It depends exactly what you're trying to do, how fast you want the gas flowing. If it's heat, then slow sounds an easier engineering proposition. If you're making a jet engine, then a heat source in a gas stream does add kinectic energy to the output flow, and operating at the highest temperatures and pressures is key to getting best efficiency out of the heat engine.

BigBad

Mon Mar 31 2014, 04:48PM

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FWIW at 90 psi the outflow will choke, anything above 45 psi will probably do so, so you may well get a supersonic exhaust. Why would you want to run your furnace at such high pressure anyway?

klugesmith

Mon Mar 31 2014, 07:33PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
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Maybe Andy is making a coal and air-fired rocket engine.
I see that Dr Slack hinted at that.

Note that most coal is burned to boil water & generate steam at enormous temperatures and pressures. But I bet the combustion takes place at absolute pressures not much higher than 1 atmosphere. Refer to Babcock & Wilcox, who have made high efficiency boilers for about 150 years.

Andy

Mon Mar 31 2014, 09:50PM

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Steve Conner

Mon Mar 31 2014, 09:57PM

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Yes, there is no need to use compressed air, it just adds expense and danger.

(We love expense and danger on this forum, but there has to be at least a token scientific justification for it.

)

In relation to the original question: If the inlet pressure is 90psi, then combusting fuel inside the chamber won't raise the outlet pressure above 90psi. The reason is that if the pressure inside the chamber exceeded the inlet pressure, it would try to force its way out of the inlet hole as well as the outlet. It would stop the fresh air getting in and limit the combustion until the pressure fell below 90psi again.

In a jet engine, the heat added by the fuel doesn't increase the pressure in the combustion chamber. It just expands the air to a greater volume.

2Spoons

Mon Mar 31 2014, 10:29PM

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Are you serious about the MHD thing? Because that needs a plasma to work, which means flame temperatures , not 600C. You'd be better off using a turbine driving an ordinary generator.

Andy

Mon Mar 31 2014, 10:58PM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
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