[Stoves] venturi system -ratios of air and gas?

[Paul Medwell]

Dear all,

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.

It seems pertinent to clarify some terminology:
- A nonpremixed flame has no oxidiser mixed with the fuel prior to issuing from the burner exit plane.
- A flame is only premixed if the equivalence ratio is within the flammability limits.
- A flame is partially premixed if some air is added to the fuel, but it is outside the flammability limits.

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.

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.

Regards,
Paul





-------- Original message --------
From: Crispin Pemberton-Pigott <crispinpigott at outlook.com>
Date: 09/01/2016 8:11 AM (GMT+09:30)
To: 'Discussion of biomass cooking stoves' <stoves at lists.bioenergylists.org>
Subject: Re: [Stoves] venturi system -ratios of air and gas?


Dear Paul

If you examine a propane cooker, the gas comes from a tiny hole in the jet
and is fed through a lot of small holes into the air. There are a myriad
flames each small in diameter and the length varies from short to long
depending on the gas flow rate. The gas burns on the 'outside' only with a
hollow core, the core being unburned propane (or more likely, a blend of
propane and butane).

Propane has a high energy content per litre compared with natural gas and is
fairly dense. It has 2.4 times the energy per litre. When it burns it is
hotter and tends to be able to 'get enough air' by diffusion.

Natural gas is usually pre-mixed in order to get the air (and nitrogen)
mixed well before ignition in order to shorten the flame. The gas-air mix is
ejected through small holes such that the velocity of the gas coming out is
higher than the flame speed. That is why the flame can't/doesn't leap into
the burner holes and explode inside the premix tube.

Natural gas burns well in air, but the flame would be long and lazy if put
into a propane burner. It doesn't have to be a wonderful and balanced
premix, with 'just enough' air - it will also burn with different mixes. If
you fiddle with the air adjustment plate on a gas cooking stove it won't
suddenly 'stop working' it will just have a different CO level due to the
way the flame interacts with the stove and pot. If you change altitude, the
plate has to be adjusted, and for a really big difference in altitude it
needs a different burner. Notice the plate and the altitude rating on the
side of a water heater, furnace or stove.

Regards
Crispin



Natural gas has a lower heat content
-----Original Message-----
From: Stoves [mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of
Paul Medwell
Sent: Friday, January 08, 2016 14:10
To: Discussion of biomass cooking stoves <stoves at lists.bioenergylists.org>
Subject: Re: [Stoves] venturi system -ratios of air and gas?

Dear Crispin,

I wonder if you can explain why a propane (C3H8) flame is not likely to be
premixed, but a natural gas (CH4) flame probably is?

Regards,
Paul


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