[Stoves] RE Optimum Combustion

Tue Dec 12 10:58:39 CST 2023

Dear Willem

The conventional name for what you seek is a Lambda Controller.  Lambda (λ) is the conventional name for the total air demand, and Excess Air (EA) is the conventional name for air that is not used.

So EA- λ = the air used for combustion.

Interestingly, the air needed for combusting wood is quite a bit less than the among needed for LPG or kerosene because wood contains a lot of oxygen.  You can consider that there is enough oxygen in wood to convert almost all the hydrogen in wood to water vapour.  So the stoichiometric demand for oxygen to burn the little bit of hydrogen and all the carbon is roughly 24% less than for long chain hydrocarbons.  I mention this because many calculations assume that “excess air” at 100% means that the same amount of air was used for combustion. Not so.  If the excess air is 100%, we take that to mean 10.5% oxygen in the exhaust, but the amount of air actually needed to burn wood is only 74-76% of that, assuming you oxidize all the carbon.

For a TLUD making charcoal, the oxygen required is as little as 20% of the quantity we call 100% EA because most of the carbon is not involved.  Interesting, neh?  So if conventional EA is 100%, and it is actually 5 times the amount actually required, why isn’t it called 500% EA?  When designing a stove, if you want to create air supply channels, and the need is 20%, 100% EA is really 2 x that 20% by conventional formula.

In simple terms conventional EA is Measured value%/(21%-Measured value %).   This formula doesn’t work at all for a TLUD making char.

Because biomass is some homogeneous in composition, the air requirement is pretty constant as long as what you are burning is “wood” not “the volatiles”. The moisture content does not really change the air-fuel ratio.  If the fuel is really wet, it would lower the indicated oxygen concentration in the exhaust a bit but it would not change the actual air requirement per unit of dry fuel mass.

Lambda controllers are common in industrial equipment, but are they supposed to be observed, or control something?  Many times they control the secondary air supply.  The operator chooses a primary air supply (which regulates the burn rate) and the lambda controller maintains a standard EA value.  Many stoves would benefit from this. Most stove losses are from high excess air, not lost through the stove body which some people are a bit fanatical about insulating.  For an uninsulated high mass solid 500 litre stove, the loss to the body is about 18% of heat liberated from fuel. Reducing that to 13% makes little difference, when the chimney loss is still 50%. Get the EA correct and the stack loss can be brought down to 20-22%.  That is ~5 times the benefit of insulating the combustion chamber at none of the complexity and expense.

Given that these stoves are simple, let’s look for a low cost and reliable oxygen indicator and the operator can do the rest.

Best regards
Crispin

From: Stoves <stoves-bounces at lists.bioenergylists.org> On Behalf Of willem boers
Sent: Tuesday, December 12, 2023 2:33 AM

For the Kabuga Institutional stove (500 litres)  we look for a stoichiometric combustion whereby at all times, the O2 is just the right amount to ensure optimal combustion with no or limited emissions.

The reality is that each time, the combustion masters at the school will use a different type of wood with other moisture content and different firewood sizes. So, the O2 supply should be dynamic and cannot be set permanently.

I understand that it is common in the industrial combustion process to have a measuring/steering system to measure the correct O2 quantity regularly. Cooking with Kabuga is nearly industrial. Millions of students will receive school feeding, which must be cooked as energy efficient as possible. I estimate that one cooking kettle of 500 litres can feed, at best, 500 students. For a million students, we would need 2000 cooking kettles for starters.

What options are available at the market to measure the flue gasses while regulating the O2 supply? And ensure the highest efficiency with minimum emissions through an affordable, robust measuring/ steering system for a biomass institutional cook stove of 500 litres.

With kind regards,

Willem

Op 27-11-2023
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