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

Frans Peeters peetersfrans at telenet.be
Fri Jan 8 07:13:50 CST 2016


Andrew ,

    You have better air 22% ? then we have ....20% means 10 vol air for 1
mol propan .

-----Oorspronkelijk bericht-----
Van: Stoves [mailto:stoves-bounces at lists.bioenergylists.org] Namens
ajheggie at gmail.com
Verzonden: vrijdag 8 januari 2016 11:39
Aan: Discussion of biomass cooking stoves
Onderwerp: Re: [Stoves] venturi system -ratios of air and gas?

[Default] On Fri, 8 Jan 2016 00:31:19 +0100,"Boll, Martin Dr."
<boll.bn at t-online.de> wrote:

> A happy New Year to all stovers!
>
>In a venturi system like of a propane gas-burner the driving gas has
pressure about 30 to 50 millibar.
>- What ratio have the volumes of gas and air, to make our admired blue
flame, as it does in gas-flames?

As you thought it is a bit more complicated, the stoichiometric (chemically
exact amounts to react for complete combustion) mass ratio of methane (same
principle for propane but numbers are simpler) and oxygen is about 4:1 but
as you have to allow for the other constituents of air, principally
nitorgen, this means you actually have to supply about 17 kg of air for
every 1 kg of gas.

Volume ratio is  different but can be calculated by knowing that each mole
of gas occupies the same volume. From the mass calculation with molar values
rounded to intergers.


CH4+2O2+7N2=CO2+2H20+7N2

So 1 volume of gas reacts with 9 volumes of air to give 10 volumes of
combustion products at standard temperaure and pressure, of course as you
are aware after the flame the temperature has increased to 2000C so the
volumes are much larger.

In practice a little more air is supplied to keep the flame clean.

>-What ratio(range) must have air and woodgas , to get the same blue flame?

The blueness is a feature of the gas and air being premixed.

The stochiopmetric mass ratio for wood is about 5 parts air to 1 part dry
wood, because wood is already partially oxidised. This relates to about 4.4
M3 of air to 1 kg of wood. In practice much more air is used, this higher
need for excess air is one reason a wood flame is never going to get as hot
as a gas flame.

Who volunteers to check my workings?

Andrew

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