Fuel Combustion and the Optimal Usage of Heat Energy

Fuel Combustion and the Optimal Usage of Heat Energy. Manufacture of Energy-Producing Installations

Many thanks to Brian Davis for translating the article (davisb@ecsd.net).

During combustion or heating, the combustible components of fuel are volatile gases and solid matter. The volatile gases are made up of carbon and hydrogen atoms and the solid matter is mainly carbon. Combustion represents a chemical reaction which produces heat during which a fuel is broken down into simpler materials by the combination of the fuel with a combustion agent which is usually oxygen from the air. When the temperature in the firebox reaches 300-350 °C, hydrogen ignites and starts burning. With the heat produced, it evaporates together with part of the carbon in the fuel, to form combustible gases called carbohydrates. Hydrogen during combustion turns into water vapour. However, the carbon is not yet burning as it ignites only at 700 °C. Therefore, thick black smoke is produced. This continues until the temperature reaches 700°C throughout the firebox. At this point, the combustion of carbon (soot) takes place and the smoke produced is no longer black and sooty and gradually disappears. However, even at this temperature, the combustion process is slow and inefficient because part of the combustible gases do not have time to burn completely. In order to ensure effective combustion, the temperature in the firebox should be at least 900 °C for wood and 1000°C for coal.

During efficient combustion, the output is carbonic acid from carbon, water vapour from hydrogen and nitrogen as a component of the air used for combustion which represents 4/5 of the output volume. Actually, due to the unequal mixing of the carbohydrate with air, air has to be supplied at a rate of 1.6 to 2.4 times the theoretical amount required. Therefore, there is always a surplus of air in the firebox that did not really take part in the combustion process plus the water vapours from water normally present in fuel. All these gases are called ballast gases, that is, they do not take part in combustion but only get heated from the combustion of carbon and hydrogen. In other words, they reduce the useful heat. The molecules of the above-mentioned gases are totally independent, that is, they are not coupled with each other.

It is possible to produce an effective combustion process and obtain maximum energy but use this heat inefficiently. Of course, it is preferable to completely extract the energy contained in fuel and use it effectively. Therefore, the total efficiency of a particular installation is made up of 1) the efficiency of energy extraction from the fuel and 2) the efficiency of heat usage. The combustion efficiency equals what percent of the total energy resource (eg. wood) can be transferred into heat during combustion. The question is, in what type of system? Fuel combustion in a bell shaped combustion chamber produces more energy than a forced-air system which has to deal with the negative influence of ballast gases!

In order to improve the efficiency of heat extraction, that is, burn fuel more efficiently, it is necessary to increase the temperature in the firebox and reduce the negative influence of ballast gases. Stoves operating on the principle of forced gas movement, including the counter gas flow designs, have been around for more than a hundred years and their design has not really changed since they were introduced. The designer’s goals were 1)the optimization of primary and secondary air supply and 2)the reduction of the amount of water in the fuel as well as 3)adapting their designs for various fireplaces and the improvement of electrical components. In order to ensure efficient combustion, it is necessary to supply more secondary air than is required theoretically so that the harmful carbonic oxide (the product of incomplete combustion) will combine with oxygen turning it into carbon dioxide. It is important to have good air distribution in the firebox as well as adequate mixing so that all the gases coming out of the firebox react with the air. Otherwise, we will have a case of “dirty” combustion. Then, however, the amount of ballast gases passing through a forced-air system leads to a reduction of efficiency. The design of forced-air installations are now so efficient that there is practically no room for improvement.

Currently, all power-assisted installations are built on the principle of forced gas movement. Heat accumulating furnaces are designed using the principle of counterflow. In boilers, pipes or tanks for a heat-transfer fluid are located in the firebox. Heating furnaces are also being built with the use of metal firebox inserts functioning on the principle of low-level combustion. Gas-generator boilers and furnaces have firebox walls with heat-extracting surfaces. In all these types of power installations, the gases that are formed as a result of combustion, including ballast gases, pass through some type of heat-transferring system. However, the ballast gases have a cooling effect, thus reducing the efficiency of any power-assisted installation. The fuel energy, that is heat produced by the combustion of carbon and hydrogen, is not used completely for heating because of the ballast gas. Therefore, in order to increase the efficiency of a power-assisted installation, it is essential to reduce the influence of ballast gases on the combustion process.

In forced-gas installations, there is no good place for the heat exchanger that provides for maximum efficiency. If boiler heat exchangers are placed inside the firebox, the temperature goes down! In other words, the conditions for fuel combustion worsen. If the firebox dimensions are increased to fit a heat exchanger, the energy of the gas flow is diluted and the temperature again decreases. Besides, the efficiency of firebox inserts and boilers also depend on the heat exchange value through the walls of the firebox and the walls of the water heat exchanger. Heat exchangers do not really allow for optimal fuel combustion. So, as we increase heat emission efficiency, we reduce the efficiency ratio of energy extracted from fuel. Heat exchangers that are placed inside the firebox (cold core) decrease the temperature inside, thus worsening the conditions of fuel combustion.

Free Gas Movement

The combustion of fuel and the exchange of heat in a ‘free gas movement’ heat generator follows a patented formula which states ”The stove’s lower level and the firebox are combined to form a single space creating a lower bell”. This formula provides for a dry joint (a 3 mm crevice) between the fireplace and the bell. The firebox may be different both with regard to its design and principle of fuel combustion. There can be variations in the principles of upper combustion, lower combustion, return combustion or gas generation. Any type of fuel can be used for combustion.

The formula focuses on the fuel combustion in the firebox located in the bell and the optimum use of extracted heat energy. The main goals are to 1) obtain maximum heat during fuel combustion 2) maximize the volume of the heat obtained 3) design the heat generator to meet functional requirements and ensure optimum heat emission.

One of the remarkable features of the bell should be pointed out. “If hot gases are transferred through the lower zone of the bell, it will accumulate its heat and radiate it through the walls or heat exchanger placed inside the bell”. That is, when passing through the bell, the gas flow is distributed according to the temperature of its components.

In a multilayer firebox as shown in Figure 1, the fuel gasification occurs mainly in the solid fuel layer. This is also where the gaseous products of combustion are mixed with ballast gases coming into the firebox. In this area, part of the cold ballast gases, being heavier, pass through the crevice into the lower part of the first bell. The combustion zone is mostly located above the layer in the firebox where the combustion of exhaust gases takes place due to the supply of secondary air into this area. To improve gasification of fuel, the supply of primary air should be limited, especially when there are still a lot of glowing coals in the firebox.

High-temperature combustion is possible only by ensuring fuel gasification, and that can only be guaranteed by increasing the temperature in the firebox and creating an oxygen-rich environment. Only high temperatures are capable of quickly heating the fuel to activate the gasification process. High-temperature combustion is also possible as long as there is no cold core or separated hot and cold gas flows. Another way of increasing temperatures in the firebox may be regeneration, that is, heating the incoming air by recycling the exhaust gases.

In heat generators built in accordance with the above-mentioned formula, all the conditions for complete fuel combustion are easily accomplished, namely, optimum supply of primary and secondary air, good mixing of air with the fuel, high firebox temperatures and proper design of the firebox. The combustion should complete within the firebox and not past the catalyst or beyond the dry joint. Failure to satisfy even one of these conditions will lead to the incomplete combustion of fuel. These conditions are satisfied by the proper distribution of hot and cold gases, the absence of low temperatures of the firebox core and regeneration, that is, the heating of combustion air using exhaust gases or a combustion catalyst.

One of the requirements of efficient combustion is the proper supply of secondary air. It can be supplied from the ash-pit in two ways; through the crevice around the firebox door (Fig1, #5) or under/through the catalyst with a small amount through the dry joint. In the first case, the secondary air supply, caused by chimney draft, passes over the fuel layer (being cold) into the dry joint, ensuring oxidation of carbohydrates. Part of the excess air and ballast gases that haven’t taken part in the reaction are discharged at the lower part of the bell. In this case, turbulent flow is ensured since, during combustion, the hot gases tend to flow upward. In the second case, the air passes through the a crack in the firebox wall (Fig1, #8) under or into the catalyst and also into the dry joint through the crevice (Fig1, #9), being heated on the way. The combustion catalyst ensures turbulence of the outlet flow and high temperature, which is especially important during the end-stages of combustion when the concentration of fuel and oxidant is low. These are separated by the combustion product making their interaction difficult. In other words, it ensures complete combustion. In the same way, the gas flow passing through the dry joint also gets oxidized. The amount of secondary excess air doesn’t make a large impact on the power installation efficiency.

Burning fuel in the lower firebox goes through a secondary burn in the bell. Thus, the degree of energy extraction from the fuel increases. The ballast gases, being cold and heavy, do not go up and through the dry joint. The largest part of the fuel energy is extracted in the first bell but in the upper bell, the combustion energy is increased to a higher level and the gasses continue to burn there until the temperature is at least somewhat higher than the temperature of exhaust gases from the lower bell, that is, until it is absorbed by the heat exchanger. Optimum use of the exhaust energy is realized due to the use of the “double-bell” system. The system is very flexible and provides an opportunity to adjust the design for various purposes. The bell may have any form and volume. A boiler, heat exchanger, steam generator, retort, heat-accumulating device providing a 24-hour heat output or other equipment may be installed inside of it. Unlike a forced-gas system, this system provides fuel combustion conditions matching the needs of the heat exchanger, that is, the heat emission can increase without adversely affecting combustion efficiency.

On the basis of this theory, new design principles have been developed for the charcoal burning installation and gas-generator boiler. Complete control of the pyrolysis process at every stage is ensured, as well as preparation of the initial raw fuel.

Fig.1 is a diagram of a power installation built in accordance with the formula: ”The stove’s lower level and the firebox are combined to form a single space creating a lower bell”. The three views A, B and C show the firebox, lower bell and the upper bell. Heat exchangers, such as a boiler heater, can be fitted into the lower bell (see view B). The firebox consists of an ash-pit with a fire-grate over it, the combustion space (Fig1, #1), the catalyst (Fig1, #2), the space over the catalyst (Fig1, #3), and the upper part which is provided with holes connecting to the lower bell. On the front of the firebox, there is crevice (Fig1, #5) which supplies secondary air from the ash-pit. A heat recycler (Fig1, #6) can be fitted in the lower part of the sidewall. The rear part of the firebox is provided with a dry joint (a 3-cm wide crevice). The sidewalls of the firebox are provided with a chamber (Fig1, #8) through which secondary air is transferred from the ash-pit via openings under the fire-grate or under or through the catalyst. The rear part of the chamber is provided with a 5 mm crevice (Fig1, #9) for supply of secondary air into the dry joint.

A baking chamber, steam generator for a sauna or other equipment may be inserted into the space over the catalyst (Fig1, #3). The firebox in the diagram is shown with symmetrical outlets into a symmetrical bell. The firebox may have an asymmetrical form and possess outlets into the asymmetrical bell. This makes it possible to modularize the firebox to create various models for mass production.

Compare The picture “Compare” demonstrates combustion of recently cut wood in the fire box of a boiler with forced gas movement in the system of free gas movement (as per the article “Combustion of fuel in the bell and optimum use of heat energy”). In this case it is clearly seen in what way wood gasification and combustion of pyrolysis gases takes place. The boiler operates without blast.

In the photo it is easily seen that in a high temperature field wood heating is uniform (pyrolysis takes place). One can also notice that thermal conductivity of wood across the grain is considerably higher than thermal conductivity of wood in flatting, which is proved by the character of carbonization of the log end (over circumference) and the lack of carbonization on the end surface.

Baking chamber of baking oven may be inserted into the space over catalyst-3, a steam generator (in steam sauna furnace) and other equipment. The firebox in the diagram is shown with symmetrical outlets into symmetrical bell. The firebox may have an asymmetrical form and have outlets into an asymmetrical bell. This makes it possible to unify the firebox for various models of various purpose stoves and create modular stoves of factory manufacture.

The bell-shaped combustion chanbers may have multiple forms and can be built from many different materials. Different stove equipment can be included according to the customer’s request. For example, it is possible to mass-produce a firebox from heat-resistant concrete which can then be used in various installations, for example, a baking stove, a sauna, a heating boiler, a multilayer stove or a stove combining several of these functions. If we take firebox A and insert it into bell B shown in Fig.1, we can obtain the following stoves that are also capable of using electricity: