Venturi Burner by Louis Katz , all-weather paint stick on plywood, torched.

Nothing in this post should be considered safety information. A lot of it is just what I think, not having read it anywhere in a format I could really understand. Other information I am consolidating.

Atmospheric burners are not understood particularly well by ceramic artists. I am going to try and clarify some things about them. I have not found any wonderful texts about them, most of what I know has been gleaned from catalogs that used to provide much more information than can be found now. While this is almost exclusively Eclipse® and Pyronics® catalogs others have entered into the mix.  To simplify things I am only going to consider Natural gas, an impure methane that is generally delivered through pipes to homes and businesses. I live in the US. So its possible that something I say will only be applicable here,,, but I can’t think what that would be.

Entrained Air
An atmospheric burner with a venturi tube is a device whose function is to efficiently use the kinetic energy of gas coming out of an orifice to carry air with it. The air that it carries through the burner is called “entrained air” also “primary air”. 

Orifice
The orifice that the gas flows out of is called the orifice, sort of a truism. The size of the orifice and the pressure the gas is under determines how many BTUs, calories, or cubic feet of gas are flowing into the burner. Orifices that are properly made and drilled create less turbulence when the gas exits the orifice and prevents loss of kinetic energy. This consequently increases the amount of air that will or can be entrained. We will discuss this further below in several places.

Primary Air
Primary air is the air entrained in the burner. The air coming into the kiln around the end of the burner is secondary air.  I guess much of this is redundant. It seemed needed. 

Flame Retention Ring
These are devices on the kiln end of the venturi tube whose job is to efficiently mix the gas and air, create a faster area for the mixed gas and air to flow through, and often provide a small amount of slower gas around the circumference to act as a pilot for the flame so that it will always light near the Retention Ring.  The faster speed is so that the mixed gas and air is moving out quicker than it is burning back into the burner. If the speed in the retention ring is too slow then you get back burning, burning inside the venturi tube. This creates soot and other troubles.

Methane – CH4

How Atmospheric burners, a mixer head where the orifice and air come, a venturi or straight tube, and flame retention ring, are sized.

The exact amount of oxygen (from air) needed to burn a molecule of methane, CH4, into carbon dioxide and water is CH4 +2O2 = CO2 + 2H20,, two molecules of O2. How much is this by volume or weight is not really important. But this ratio, we will call Neutral Combustion.

If it were only the gas we were concerned with then burners would be sized by the size of the gas orifice. You would not use a burner, and just pump gas into the kiln. All the air would be brought in with the chimney and the kiln would need to be kept at a higher negative pressure to do this. But there are two problems with this. One is that any leaks in the kiln will bring larger amounts of air in, and if they are not near the gas source then they will cool those areas, and keep them oxidizing more than anyone would need them to be. The second problem is that you need mixing of gas and air.  Gas heated in the absence of enough oxygen will produce soot, pure or nearly pure carbon. [This is hard on people contributing to heart disease, lung disease and cancer along with particulate pollution and other environmental effects.] Soot can be very slow to burn. There may be times when you want soot such as carbon trapping glazes but for the most part it is usually just wasted fuel. Assuming that the mixing of gas and air before heating is good, then this happens when the oxygen is less than 1/2 of the amount needed. CH4+O2 = C +2 H2O. In general this is a good way to waste fuel. In small gas kilns some of this burns in the chimney, but some does not. Unfortunately Carbon does not reduce carbon dioxide to monoxide, at least not easily as far as I know. I suspect that if you heat carbon and carbon dioxide hot enough you will get carbon monoxide forming,,, but I do not know enough to be sure of this. Perhaps a chemist will chime in.

[I recently found a copy of a a catalog for the  now unavailable single state Low Pressure Injectors from Eclipse. It gives numbers for 30-50% entrainment. This does not fit my memory or understanding.  Eclipse Atmospheric Injectors, Bulletin 650, 1/8/2015]

The charts that I have seen for burners being sold generally state an assumed amount of entrained air. For example; These numbers may assume that some percentage of the air needed for neutral combustion will be entrained by the burner. That is, a little over 1/2 the air needed to completely burn the gas will be entrained by the burner (or mixed in at the burner tip as secondary air). You will get some CO2 from this air, but most of the carbon will leave as CO which will still burn if more air, more O2, is provided. You only need or want a little CO for reduction. Too much and your kiln will not climb in temp and you will waste time, fuel, money. Too little will be talked about under mixing a few paragraphs down. 

The rest of the air needed is supplied by secondary air coming in around the flame retention ring. The O2 in this air burns the CO into CO2.

Why do you get more BTUs with higher pressures?

Lets just say you have a cubic meter, or cubic yard if you insist, of gas. I find it easier to image this with more gas especially when it is at a very low pressure, say 1cm water column, or 1 inch water column if you prefer. The gas, coming out of the orifice will come out slowly and will have little kinetic energy in the stream. In fact it will only have the kinetic energy used to compress it. If you use way more energy to compress the gas you will also need a smaller orifice if the gas is going to come out at the same rate. But having put more energy in you get more energy out. This energy, in part transfers to the air coming into the burner, in fact this energy is what draws air into the burner. I find it easier to visualize with the idea that the gas coming out the orifice blows air out of the burner, the air then needs to be replaced so more is drawn in. High pressure, small orifice, entrains more air because there is more “blow”, more kinetic energy, in the gas. More air means that keeping to 65% of what you need for full combustion, means you can use more gas. More gas means even more kinetic energy. Higher pressures increase the capacity of the burner. However they also increase turbulence. At some point the amount entrained in the burner no longer increases much as the pressure increases. When you get to the point where the air that can be entrained drops below 65% of that needed you have reached the maximum practical pressure of the burner.  Eclipse and Pyronics used to release good charts on their burners that mostly made this clear if you studied the numbers. 

Negative pressure at the burner port.

If  kiln is hot and the damper open you can get a slight decrease in pressure at the burner port. Assuming that the burner is placed properly in relation to the port, this decrease in pressure will allow more air to be pushed (or pulled if you like) through the burner,,, . I think of this lower pressure as sucking air through the burner. [The sucking or pulling is an easier model but is not technically correct.]  Lets assume that you make those ports too big. In order to get a given volume through them you need only a little pressure difference. As they get smaller, the pressure in the ports is going to fall. This increases the capacity of the burners because it decreases the pressure at the burner head. There needs to be enough space around them for the requisite secondary air. [100%-65%=35%]. But often these ports are made too large if the goal is maximizing the capacity of the burners so that you can heat faster.

Further the flame retention ring acts in respect to the burner port similar to the gas in the burner and carries secondary air into the kiln using its kinetic energy.

Mixing

In order for the gas to burn it has to be in contact with the oxygen in the air. If it gets hot without the air it won’t burn. If it gets hot with only a little air you will get soot. The most important mixing happens before it burns, in the flame retention ring. Burners made without them have the potential to waste a lot of fuel as soot . CH4+O2 = C +2 H2O. Interestingly higher gas pressure should produce more turbulence in a given amount of time but the gases also move through the burner body quicker. I think that you end up with better mixing, but I am not sure.  It does appear that you get better mixing with the secondary air.

About 68 percent of the heat in methane is released burning it to carbon monoxide. Knowing this makes it clear why firing closer to neutral is quicker. I like to think if firing with too little air, firing in too much reduction, as being like paddling upstream with a small paddle.

Bad mixing, too little primary air, soot, once the kiln is quite hot,  can make determining if you are reducing difficult. The soot can be burning off in the flue making flame and looking like reduction. The same can happen out spyholes. You can close down one burner’s primary air and get soot, have the kiln oxidize and have flame at the spy holes (bungs) and at the flue.

While we are on it, the flame does not come out of the kiln unless there is oxygen that is not yet combined coming out as well. The flames we normally see from uncombusted gases coming out from the kiln are hollow and start where they come in contact with fresh air. Paying attention to this, especially in wood or oil kilns can save a lot of heartache and trouble.

Heat is released when all the necessities for combustion are met, heat, O2, fuel and mixing. Well I suppose they are not in the same place unless they are mixed, but that is a fine point. The sooner good mixing takes place the sooner the heat is released. Since primary air mixes at the burner, secondary air released later in the kiln chamber. Up to a point this can be used to control temp in different parts of the kiln. 

Mixing is always imperfect. As you approach neutral combustion you should assume that some parts of the kiln are in oxidation, some reduction, as well as some parts effectively neutral. Likely with methane there is always some water gas reaction. Who knows how long hydrogen can survive in a mixed atmosphere.  There is too much I don’t know.

Wood Combustion

Part of this dynamic and discussion seems very important in wood kilns, particularly long ones. “This is what I see happening.”” This seems in part more like conjecture.” I am going to encode sentences with how certain I am of them. Sentences with fairly high certainties will have one period. Sentences where I am pretty sure two periods.. Less certain, three periods… just a working theory, four…. These are of course approximate. Some things might be substantiated by reading, some by experience, some just because they make sense.

Pyrolysis of wood produces many products. At higher temperatures these include H2 (hydrogen molecules), CH4 (Methane), CO (Carbon Monoxide) , and CO2(Carbon Dioxide) and H2O (water). Charcoal becomes mostly just carbon and ash as pyrolysis progresses. As it becomes more pure it burns more and more only on the surface. 
Before coming into contact with added air, some of the methane and water is going to go through the water gas reaction, CO+ H2O = CO2 + H2  .. This does not really change what is in the mix, just the proportions of it.

So, for me the easiest to start with is the Hydrogen. It is the easiest to burn.. It has the lowest flash point and the highest affinity for oxygen of all the common products of pyrolysis.. It burns first.. Like carbon monoxide its affinity for oxygen makes it an agent for reduction. But these two ingredients appear to have different properties of reduction of glazes.. This  appears to create some of the vagaries of wood fire.

Because of hydrogen’s high affinity for oxygen and its tendancy to burn quicker and easier it is the first to leave the stream. Because of this, it often is not likely to affect the clay. It disappears too quickly by becoming water. But one of its properties differentiates it from carbon monoxide. It is more soluble in glass than carbon monoxide.. Where carbon monoxide either needs to reduce a glaze only on the surface or only before it is melted, hydrogen can effectively penetrate the surface of the glass and reduce materials after they have melted.. This appears to be the mechanism that creates the salmon colored flashing often sought in wood and soda firing.. How this relates to other mysticism relating to the miraculous Avery Kaolin I don’t know. I am certain that Alan Watts must have done a lecture on Avery…

That salmon color seems to require a small amount of volitalized alkali metal. You can get it with both potassium carbonate and sodium carbonate is vapor kilns. It is not strictly a soda color. But introduction of water into a reducing stream of gas appears to increase the amount of salmon color.. I first heard of this from Mac McClanahan in the early 1980’s and then I tested it. It seemed true. Further use of it in my work, and my students has made me more certain..  The year after testing it I was a resident at The Archie Bray and read about the water gas reaction being used to reduce iron oxide in brick and lowering the maturing temperature of pavering block in Archie’s old library. I became a believer.

The next fuel in the mix is CH4. Given enough oxygen this just burns to water and carbon dioxide, just like it would coming from a pipe.  The Hydrogen generally goes to water first but given some existing water some of it becomes hydrogen. This is the part of this that makes me squirm and wonder if the model is correct. Some of it if mixed with air poorly or if the oxygen mix is too low becomes soot and water.  This is part of the reason that firebox design  in wood kilns seems critical. Methane burning with suffient oxygen has a blue color.

Carbon monoxide, just as in gas kilns appears to be the main reduction agent lingering in part, if you are firing in reduction, until it comes out from the kiln. The yellow orange flame at the flue or spy holes, if not the result of sodium flare, appears to be the carbon monoxide burning. Oxy -Hydrogen flames have a similar color but are much weaker in light. This can keep things confused I think. I believe that this color is more towards red, but also has a small component of blue…

It can be really difficult to determine the source of a flame color. It is important in learning to do this to evaluate the hue as this changes by source. There are several different fuels with yellow orange flames.

In a kiln, you only see flame where there is enough combustion to create enough light. I suspect that the water-gas reaction also produces light…

This brings us to carbon, soot, C, is of all of these fuels the hardest to burn. The first part of this is that it just needs a higher temperature to combust. I believe that Cardew states that this is 650˚C and I have seen other reference to 660˚C..  Cold air can extinguish burning carbon.. It also produces less heat per gram. The third is that it tends to clump when it is produced and can only mix with air on its outside layer. It has the ability to lengthen flames if the kiln has sufficient oxygen. Carbon may have an important role in evening the temperature in long single chamber kilns….But soot leaving the kiln is wasted fuel.

I wonder if having multiple stoke ports along with its obvious use in evening temperature also contributes to a bigger distribution of salmon color as it puts the hydrogen closer to more of the ware….

Burning charcoal that has lost its volatiles requires hot air. Since it only burns on the surface it goes away slowly. Maintaining the heat is critical to burn charcoal and this is one of the important aspects of Bourry box kilns. The do this by putting the location to burn charcoal after the production of methane and hydrogen and after where these gases start to burn.

Archie Bray Foundation Front showing Louis’ soda fired bottles and stack of Gail Busch’s in the window. Brick were fired in the brickyard.