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<div>Dear all,
<div><br>
</div>
<div>Crispin has done a good job at explaining the operation of a Bunsen burner, but it should be noted this approach works with any gaseous fuel. For a particular fuel under a particular operating condition, the size of the injector (a.k.a. "tiny hole") determines
the level of air entrainment, and thus the stoichiometry (whether the flame is premixed, nonpremixed or partially premixed). The optimal burner design is determined using very well established criteria. It is not true that "propane should generally not be
premixed but natural gas should be". It will depend on the situation and application.</div>
<div><br>
</div>
<div>It seems pertinent to clarify some terminology:</div>
<div>- A nonpremixed flame has no oxidiser mixed with the fuel prior to issuing from the burner exit plane.</div>
<div>- A flame is only premixed if the equivalence ratio is within the flammability limits.</div>
<div>- A flame is partially premixed if some air is added to the fuel, but it is outside the flammability limits.</div>
<div><br>
</div>
<div>Any flame operates equally well in any combustion mode. The difference is the burner design may not be well suited to a particular application. It is not possible to classify a flame without knowing the exact operating conditions.</div>
<div><br>
</div>
<div>Perhaps it is also worth noting that equivalence ratio is determined on a mass-basis (volumetric-based comparisons need to be made carefuly), and also that the adiabatic flame temperature of natural gas and propane are almost identical.</div>
<div><br>
</div>
<div>Regards,</div>
<div>Paul</div>
<div><br>
</div>
<div><br>
</div>
<br>
<br>
<br>
-------- Original message --------<br>
From: Crispin Pemberton-Pigott <crispinpigott@outlook.com> <br>
Date: 09/01/2016 8:11 AM (GMT+09:30) <br>
To: 'Discussion of biomass cooking stoves' <stoves@lists.bioenergylists.org> <br>
Subject: Re: [Stoves] venturi system -ratios of air and gas? <br>
<br>
<br>
</div>
<font size="2"><span style="font-size:10pt;">
<div class="PlainText">Dear Paul<br>
<br>
If you examine a propane cooker, the gas comes from a tiny hole in the jet<br>
and is fed through a lot of small holes into the air. There are a myriad<br>
flames each small in diameter and the length varies from short to long<br>
depending on the gas flow rate. The gas burns on the 'outside' only with a<br>
hollow core, the core being unburned propane (or more likely, a blend of<br>
propane and butane). <br>
<br>
Propane has a high energy content per litre compared with natural gas and is<br>
fairly dense. It has 2.4 times the energy per litre. When it burns it is<br>
hotter and tends to be able to 'get enough air' by diffusion.<br>
<br>
Natural gas is usually pre-mixed in order to get the air (and nitrogen)<br>
mixed well before ignition in order to shorten the flame. The gas-air mix is<br>
ejected through small holes such that the velocity of the gas coming out is<br>
higher than the flame speed. That is why the flame can't/doesn't leap into<br>
the burner holes and explode inside the premix tube. <br>
<br>
Natural gas burns well in air, but the flame would be long and lazy if put<br>
into a propane burner. It doesn't have to be a wonderful and balanced<br>
premix, with 'just enough' air - it will also burn with different mixes. If<br>
you fiddle with the air adjustment plate on a gas cooking stove it won't<br>
suddenly 'stop working' it will just have a different CO level due to the<br>
way the flame interacts with the stove and pot. If you change altitude, the<br>
plate has to be adjusted, and for a really big difference in altitude it<br>
needs a different burner. Notice the plate and the altitude rating on the<br>
side of a water heater, furnace or stove. <br>
<br>
Regards<br>
Crispin<br>
<br>
<br>
<br>
Natural gas has a lower heat content <br>
-----Original Message-----<br>
From: Stoves [<a href="mailto:stoves-bounces@lists.bioenergylists.org">mailto:stoves-bounces@lists.bioenergylists.org</a>] On Behalf Of<br>
Paul Medwell<br>
Sent: Friday, January 08, 2016 14:10<br>
To: Discussion of biomass cooking stoves <stoves@lists.bioenergylists.org><br>
Subject: Re: [Stoves] venturi system -ratios of air and gas?<br>
<br>
Dear Crispin,<br>
<br>
I wonder if you can explain why a propane (C3H8) flame is not likely to be<br>
premixed, but a natural gas (CH4) flame probably is?<br>
<br>
Regards,<br>
Paul<br>
<br>
<br>
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