Every child drawing a house knows that there is a chimney in a true house and smoke is coming from the chimney. This is a page for those who would like to change this picture.
I described here the phenomenon of the flame in the aspect of the formation of pollutants coming from incomplete combustion. I must note that I have skipped many issues. I focused on the understanding and description of those phenomena that are key to incomplete combustion and which have not been adequately highlighted so far. I recommend starting reading from the pages “flame front”, “radiation” and “practical notes”. On these three pages I presented the main mechanisms leading to incomplete combustion along with suggestions on how to prevent it. The description is far from exhausted. However, I hope that according to the Polish proverb, “for a wise head two words is enough” it will be useful anyway. I encourage you to comment, supplement, ask questions and correct errors.
I am a theoretician, but together with a friend, we test the ideas described here. Relationship in the “news” section.
Flame is a chemical reaction that occurs in gas. This reaction requires high temperatures to take place. At the same time, combustion produces heat. So the reaction taking place in the flame creates itself the environment necessary for its further course. The flame’s environment is usually much colder than it. In those places of the flame in which the reaction slows down, the heat release decreases and the temperature drops quickly. When the temperature drops, the reaction ceases completely and the leftovers are left unburned. That is smoke. This phenomenon usually occurs in places with insufficient oxygen supply. Due to the poorly controllable nature of the flame, it is very difficult to get rid of such places completely. However, it can be remedied by not letting the reagents cool down quickly. Speaking figuratively, in a typical furnace the flame gets cold. In its viewpoint our environment is a frost -1000 °C. It can not finish its action because it is freezing quickly. Therefore, a simple way to improve the quality of combustion is to accurately insulate the place where the flame cools down, and to mix hot residues with a slight excess of air. In this way we will create conditions for thorough burning up poisonous leftovers. The furnaces designed using this method achieve very good flue quality results.
More scientific explanation
The aim of this webpage is not to design a combustion device that would approach ideal. It is about how to apply a present day knowledge about combustion and flames to low and mid range power, low budget devices that can use solid fuels like biomass and coal.
Broadly speaking chemical reactions in a flame can occur either in regions where oxygen is present or where it is not present. In the first case in the chemical equilibrium state (at the flame temperature) there are no carbon compounds other than its oxide and dioxide, the first one in a very small amount. So why do we observe a broad spectrum of carbon based pollutants? Explanation presented here can be summarized by figure 4 at page “flame front”. Some of no oxygen zones cool down before they mix with sufficient amount of oxygen. Soot and hydrocarbons which are present there remain unoxidized. It seems that there exist a simple solution for this problem. Why do not mix reactants without letting them to cool down? In other words use a thermal oxidizer without an additional heat source. Experimental observations of simple devices show that this idea works as expected. The usual measure of carbon derivative pollution is the level of its oxide. Sub-ppm CO level can be achieved in a low budget, low power, garage device. I have found that similar method was already used with good results by some people in their niche constructions like “batch box rocket stove” or other rocket stove derivatives, just without deeper scientific explanation. So why do not use this technique more broadly?
There is a common believe that we are facing the following alternative. If we want to completely burn all hydrocarbon residues then we need high temperatures and good oxygen supply and then an increased level of nitrogen oxides appears. And if we create low temperature flame and also want to partially remove fuel based nitrogen, then we are unable to oxidize carbon compounds. This alternative is well supported by experience. However most of typical combustion devices create a flame which is surrounded by cold side walls. The observed anticorrelation comes in my opinion from the way how the combustion reaction organizes itself in a much colder environment. Typical situation is presented on the left part of figure 3 on page “summary” and such setup creates above-mentioned anticorrelation between NOx and carbon based pollutants. In general, however, the nitrogen reactions and equilibrium of carbon compounds with oxygen are two different issues which can be decoupled and managed separately.
The combustion reaction is highly temperature dependent. And on the other side it releases the heat. So it creates highly nonlinear system. The phenomenon called flame is an excitation of that system. Most of traditional combustion devices crucially rely on this excitation with hope of creation of such environment that would lead the reaction to the end. The proposed insulated afterburner method does not rely on the excitation mechanism. It is the aspect it has in common with with flameless combustion burners. However in a flameless burner particular conditions are created to inhibit the excitation. My proposal is to simply do not care about that. There is a border condition which must be preserved which is to keep temperatures below those that would allow thermal nitrogen oxides to be created (high temperatures create also material difficulties). And the idea is simple, mix reactants in an elevated temperature like in usual chemistry reaction. Just brute force method. It works very well. In the experimental part we did not measure nitrogen oxides but their emission should be moderate like in any other moderate temperature combustion. One is free to design earlier parts of the combustion process in the way that it would allow more careful reduction of fuel nitrogen and the design will be not bound by worrying about incomplete carbon compounds combustion.
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