On the other pages I have described the phenomenon of the flame in terms of the formation of poisonous pollution resulting from incomplete combustion. Theoretical considerations suggest that the ecological furnace should have separated combustion and heat collection sites. A two-way proposition of what to do to get a “zero-type” furnace, which burns cleanly and completely, is like that.
Behind the furnace in which solid fuel burns, place a well insulated afterburner chamber and use a static mixer in it. There are three key elements that determine if the afterburner will work well:
- high temperature, although not too extremely high to prevent NOX formation,
- good mixing,
- not too short residence time, i.e. proper volume of hot part of afterburner. The entire flame should end clearly within the hot part, this is where the mixer helps.
The mixer should be used according to the Reynolds number, which depends on the flow rate and dimensions. The required parameters, i.e. the temperature, mixing quality and post-combustion time of the afterburner depend very much on the temperature and composition of the mixture at its inlet, i.e. on the type of fuel and the design of the furnace. That is why it is difficult to give particulars. The temperature in the afterburner should be such that the combustion reaction can take place without a separate flame and its front, i.e. above 800-900 °C.
Experiments and current constructions are presented on the blog. The first conclusion from experiments is that the necessary condition is to ensure the correct ratio of fuel to air in order to ensure the correct afterburner temperature and at the same time to provide a sufficient amount of oxygen. The second conclusion is that even in a simple device, good combustion quality can be obtained, manifesting with sub-ppm levels of carbon monoxide.
The description of the formation of impurities presented on the other pages is quite theoretical. In particular, the discussion of impurities generated on the flame front is carried out on the example of a model laminar flame. Below, I tried to present the same subject from a more practical point of view, such as assessing whether a particular furnace and afterburner construction fulfills its role and possibly where its deficiencies lie.
Several phenomena in the flame occur at the same time, but above all it is a chemical reaction. In order for fuel and oxygen molecules to react, they have to meet. Will they react, this is question # 2, related to temperature, but first they have to meet, that is fuel and air must mix. Above is a movie with a classic chemical reaction between acid and base. The base is added dropwise to the acid. There is a dye in solution that is colorless in acid and pink in base. Imagine that acid is air, and the base is gas fuel. The area where the fuel is predominant is pink, i.e. soot will be formed there. Speech is here about yellow flames, because they cause the most trouble in terms of cleanliness of combustion. In the blue (premixed) flames there is an excess of air in every place from the beginning and combustion analysis is different.
At around 2:40, complete neutralization takes place and the sodium base becomes predominant. Water has a different behavior than gas, the movement is much more “tossed”. In order for the phenomena to look similar, we should observe such mixing with magnification. However, one can point out a characteristic and obvious feature. After adding the base, large unmixed pink clouds appear first, which gradually disappear leaving small fragments which then disappear completely. In case the added base was not homogeneous, unmixed in itself, just like a mixture of combustible gases in the furnace, it would be even more evident that the mixing and reaction take place gradually.
This is schematically illustrated in Fig. 1. The situation a moment after adding a new portion of reagent is shown on the left and after some time, when only small remains are unmerged is on the right. At this point, we can return from the analogy with the acid and the base to the combustion of gaseous fuel. In contrast to the reaction in the beaker, the combustion reaction only takes place at a high temperature. If there is no proper temperature, the reaction will not take place despite mixing the reactants. At the beginning, on the left, the temperature is guaranteed, because the reaction takes place and a lot of heat is released. The key question is the temperature on the right. It is these leftovers that are a potential source of pollution in the exhaust. Basically, a small portion of fuel burns well, as shown by an example of any small flame. However, the fragments of fuel on the right are most often the remnants of a burnt larger portion (which is symbolized by dotted lines in the picture). In their immediate surroundings there is a reduced oxygen concentration and combustion takes place slowly. The amount of heat released may not cover losses. This is the situation analogous to the place C in the photo of the candle (Figure 4) on the page about the flame front . So what’s the temperature here? It depends on many factors. Above all, on the heat loss rate. The maximum temperature reached in the intense combustion phase (right) and the rate of its decrease depend on the heat losses. Sometimes you can find the argument that a clean burning of leftovers requires high temperature. This statement is not correct. First of all, the combustion reaction takes place already over 850 °C, and secondly, it is not about the temperature of the main part of the flame at place A, but about the temperature of the remaining parts at place B.
In practice, the combustion reaction takes place in the stream. Point A is at its beginning and point B is the further point of the stream. We will recognize this in practically every flame. Either we are dealing with laminar tongues, like in the previously analyzed candle, or mixing occurs more turbulently and we have a scheme of large areas A becoming small B remains. The cooling “tail” of the laminar tongue of the candle can be considered as a single residue of type B surrounded by combustion products layer, as described at the bottom of the flame front page. For a large flame, the mixing takes place more turbulently and places of type B become more or less separated from the main combustion area, as shown in Fig. 2. Conclusion in this case is very similar. The formation of pollutants depends on the ratio between oxygen diffusion rate which limits combustion reaction and heat production on one side and the rate of cooling on the other side.
We can easily recognize the scheme “main part A / leftovers B” in the flame of automatic feed coal burner above.
It should be noted that in the construction of lower combustion boilers, we often deal with pre-mixed flames (they can be called “partly blue”). Heavy hydrocarbons disintegrate into lighter by passing through a layer of hot carbon. They mix with air and carbon monoxide from partial oxidation of carbon. The flame then has a mixed yellow-violet color. It is difficult to find a well-made photograph of such a flame showing the A / B scheme, but also these flames have their own “tips”, endings and leftovers, which we analyze here. The video of the new MDM (Polish boiler maker) burner test shows residues of type “place B” coming from the main area of the burner. They are surrounded with cold steel side walls.The radiative cooling rate of premixed flames is much smaller but anyway this setup makes unnecessary pollution increase.
So let’s come back to the key question: what is the temperature of leftovers in place B in Figure 1? Is the amount of heat generated when burning these small residues in a low oxygen environment enough to cover losses and maintain the temperature needed to complete the combustion reaction? This is a decisive factor in the purity of combustion (of course, a properly constructed furnace or boiler). Practical observations indicate that the addition of air at the end of the flame does not necessarily improve combustion. On the contrary, the cold air extinguishes the burning remains and causes a bigger smoke. One method is a general, overall raising of the flame temperature in part A. Then, firstly, the gases have a higher initial temperature, and secondly place B is heated up from place A by radiation. This is shown in Figure 3 in the left picture.
It is a working method, but unreliable. As you move away from the main part of the flame, the temperature drops quickly. In practice, it may turn out that a good high temperature must prevail in the main part of the flame for good afterburning. This is probably the source of the view that it is necessary to achieve clean combustion. Another method is shown on the right side of the drawing. To maintain the high temperature of place B, it should be insulated thermally. Both of these methods complement each other properly, you need a certain temperature reserve at the start, and then you need to carefully isolate the cooling remnants.
In Polish constructions one can often encounter the error presented in Fig. 4. When the boiler / burner is operated with full power, the flame flows out of the insulated area and passes to the part where the heat exchanger is. This of course causes an unnecessary deterioration in the quality of the combustion.
The following video comes from my friend’s afterburner tests and illustrates the issue. The settings were not optimal and you can see broken fragments of type “B” escaping from the hot area and even from the afterburner. They are clearly visible in the slow motion (video settings in the lower right corner). Such fragments cooled in a cold environment will cause an increase in the level of pollution compared to a situation in which the entire flame would be in the red area. In this example, it is easy to understand that the quality of the mixing, as well as the initial composition of the flame entering the afterburner, is important for achieving clean combustion.