Pyrolysis of biofuel

Pyrolysis of biofuel in the bell and combustion of its product in the system of "Free gas movement".

Not so much time has passed since our harmful politics of cheap energy sources was a dominant policy in the economy of our state. As a result the country rich in forests and peat, having a tremendous waste reserves of their treatment has been using and still uses as fuel irreplaceable fuel (gas, oil, coal), their deposits being limited and their price is constantly going up. In the fifties there was a great success made in the USSR in gas extraction from solid fuel, in industrial extraction and use of producer gas. However, due to the above-mentioned politics of the state the organizations that were dealing with gas extraction were disbanded, many achievements have been lost, and researchers, engineers and workers possessing the necessary knowledge have gone.

Why do we speak about gas extraction from solid fuel? When burning wood, peat, and waste from timber industry, especially with high moisture content it is impossible to get high temperatures whereas while gas burning extracted from this fuel such temperatures could be reached. It is possible to evaporate moisture from the gas, which is ballast in this case, and it is not difficult to heat the gas before burning it. Besides, while gas burning less amount of excess air is necessary in comparison with the amount needed in case of lump fuel due to which the combustion temperature increases, and as a result, the energy available in the fuel is extracted to a larger extent. It is easier to introduce automation into the process of fuel burning. There is a possibility of burning solid fuel almost as comfortably and efficient as burning gas or diesel fuel using natural possibilities of nature, which is natural and expedient. It shall be taken into consideration that Russia signed Kyoto protocol, according to which the priority in the fuel balance is given to replaceable kinds of fuel at set annually quotas for mineral fuel that will be reduced each year. In this connection energo-chemical technology will start a new life.

While fuel heating a separation takes place into its volatile part and solid remnants, which is called coke. Coke of all kinds of fuel consists mainly of carbon. Carbon is also present in volatile part. If the flame is cooled by a metal sheet, soot, which is also carbon, will appear on its surface. The volatile part consists of carbon and hydrogen and is called hydrocarbon. If in these two matters oxygen from the air is added at high temperature, combustion will take place, and during combustion they turn into other matters, also into gas.

If fuel is being heated without access of oxygen of the air steam and gases will be produced and coke rich in carbon will remain (charcoal, if wood is used as fuel). This process is called destructive ( dry) distillation (or pyrolysis) . Pyrolysis of essential part of wood takes place at temperature of 200-500 °Ñ. Already at 200 °Ñ ÑÎ, ÑÎ2 gases are produced. Thermal decomposition of wood takes place on the whole with calorification, i.e. this process is an exothermal one, due to that heating of wood mass becomes easier. Êislitsyn A.N. (2). The composition of products of pyrolysis is complex and varied. The volatile matters (mixture of gas and steam) consist of gases, representing a compound of carbon, hydrogen and oxygen, etc. (ÑÎ, ÑÎ2, ÑÍ4, Ñ2Í4, N2, Î2, Í2), vapors of resin, acetic acid (ÑÍ3ÑÎÎÍ), water steam, etc. Among these gases carbon dioxide (ÑÎ2), a product of complete combustion of carbon, is not capable of further combustion, and is therefore a harmful mixture on pyrolysis gas. Nitrogen (N2), is a simple gaseous matter not capable of combustion and is a ballast. The highest amount of gases is produced by wood and peat.

Resin is a mixture of various compound matters. It can be liquid, viscous and sometimes solid matter at room temperature. Resin consists of carbon, hydrogen, oxygen, etc. Resin can burn with a great amount of calorification. During heating the resin evaporates forming gaseous matter-resin vapors.

The amount and features of gaseous and vaporous products of distillation of different kinds of fuel are different. Vapors of resin, acetic acid, gases and coke received as a result of pyrolysis can be burnt. During their combustion a great calorification takes place, much greater in comparison with heat spent for heating the fuel. The combustion processes can be expressed by chemical equations, showing in which proportions and in what way separate matters correlate. In the right part of the equation a calorific effect of reaction is shown, i.e. the amount of heat produced during reaction. Ginsburg D.B. (1) gives equations of combustion of certain matters. The matters indicated are in the content of exhaust products resulting from reaction during wood heating in a closed space.

Ñ+Î2=ÑÎ2+7940 kcal/kg Ñ, or (33190 kJ/kg);

Í2+1/2Î2=Í2Î+2579 kcal/nm3 Í2, or (10780 kJ/nm3);

ÑÎ+1/2Î2=ÑÎ2+3018 kcal/nm3 ÑÎ, or (12615 kJ/nm3);

ÑÍ4+2Î2= ÑÎ2+2Í2Î+8555 kcal/nm3 ÑÍ4, or (35760 kJ/nm3)

Ñ2Í4+3Î2=2ÑÎ2+2Í2Î+14107 kcal/nm3 Ñ2Í4 (ethylene), or (58967 kJ/nm3)

For comparison the calorific value of wood of 30% moisture makes:

Qä=12600 kJ/kg=12,6ÌJ/kg=3011 kcal/kg.

The amount and features of volatile part and solid remainder significantly depend on the method of heat supply, on temperature and speed of pyrolysis. It is usually performed in the appliances with external heating or in the appliances with internal heat-transfer agent Levin E.D. (8) gives a table 2.32, in which he shows the composition and the amount of gases received while external heating of wood in a closed space at temperature of 400-700 °Ñ. Oxidation reactions take place due to oxygen evolved during disintegration of organic mass of fuel subjected to pyrolysis process. If the temperature of pyrolysis process is increased, the yield of gases will increase, mainly due to the increase of amount of carbon dioxide (item 6), hydrogen (item7), methane (item8), and the yield of solid matters will decrease, item2. The amount of carbon dioxide (item3) will get lower, as well as nitrogen (item 9), undesirable mixtures in the gas of dry sublimation. Its calorific value gets higher and its density goes down (items 10, 11, 12).

The yield and the composition of gases. Table 2.32 > At high temperatures the dilution of gas is weaker and it is squeezed to a greater extent. At temperature of 700°C hydrostatic (gravitational) head is very high.

At temperature above 400 °C the formation of carbon dioxide (ÑÎ2) is not the product of reaction of oxidation but a result of interaction of water vapors with red-hot coal. As a result of such an interaction hydrogen is also formed. In this case we speak about external heating of raw material in a closed space.

The same result will be achieved during pyrolysis of fuel in the bell, heated from the outside. Let’s view the diagram shown in Fig.À. We will point out one more important feature of the system of "free gas movement". When hot gases pass through the bell two forces specifying the condition of the system exert force on the gas flow. One force is the pipe draught (natural or artificial), the other one is gravitational (thermal) head. Impact of pipe draught (rarefaction) is becoming less in each above section, and thermal head (pressure) is being increased. In the lower zone of the bell the total constituent of these pressures creates rarefaction (depression) specified by the preset value of the pipe draught. At a certain height these forces shall be equal and there will be no depression. High pressure arises above this point. This feature shall be considered during fuel pyrolysis in the bell heated from outside. When the fuel subjected to pyrolysis is placed in the bell heated from outside the temperature in it and hydrostatic pressure increases due to formation of products of pyrolysis. The outside air that significantly worsens the calorific value of the pyrolysis products does not come to the pyrolysis zone from the bottom, as it will be displaced by the vapor-steam products of pyrolysis, which are streaming from high-pressure zone to depression zone.

The process of pyrolysis and the yield of products depend on the features of raw material and the mode of pyrolysis process. The first factor includes type of wood availability of bark and rot, the size of wood pieces and its moisture content. Rot badly reduces the yield and the quality of the products of pyrolysis (approximately by 1.5-2 times). The quality of charcoal worsens. The size of wood pieces subjected to pyrolysis has a great influence on the yield of liquid products and duration of process. Excessive moisture results in coal yield increase and to decrease of yield of resin, gases and acids. Wood relative moisture change within 13-20% does not exert influence on the yield of the products and is optimum. Kislitsyn A.N. (2), section 3.4.2.

It is possible to turn the combustible part of solid fuel complete into gaseous fuel using oxygen of air, water steam and carbon dioxide or their mixtures at high temperature in the appliances with internal heat-transfer agent. This process is called gasification, ànd the gas received – producer gas. The devices in which producer gas is produced are called generators. Usually gas generator represents a pit into which fuel is charged from above and draught is supplied from the bottom. At the top the gas generators are supplied with charging device, with the help of which fuel is dropped into gas generator. At the bottom it is equipped with fire-grate under which draught is supplied to be distributed along the section of gas generator.

The calorific value of producer gas is lower in comparison with gases of destructive distillation. (1)

Thus it is possible to burn in energy installation:
Fuel directly; The question has been viewed in detail in the article ”Basics for boiler design…” … in our site.
Producer gas received during the process of gas extraction; This question is not the subject of this article.
Resin vapors, acetic acid, coke and burning gasses being the product of the pyrolysis process. Gaseous and liquid products, i.e. resin vapors, acetic acid and gases can be supplied to combustion zone through burning hot carbon. In this case reduction of carbon dioxide into carbon oxide takes place, as well as reduction of water steam into hydrogen, which are burnt together with carbon (coke).
Summing up the above said, one can say that an optimal energy installation is an installation performing the following functions and meeting the following requirements:
Wood or its derivatives shall be subjected to destructive (dry) distillation in external tank heated without access of air;
Ensure, if necessary, preliminary drying of rawwood;
The source of external heating shall be the initial raw material subjected to pyrolysis, the products of pyrolysis shall be subjected to combustion.
The temperature of pyrolysis shall be maintained at 600-700î Ñ and higher. At this temperature the yield, quality and the composition of pyrolysis products are preferable.
There shall be a possibility to adjust combustion capacity without significant reduction of energy installation efficiency.
There shall be a possibility of combustion both of single fuel batch and a possibility of continuous combustion of fuel i.e. the fuel shall be suitable as an energy source of periodic and constant action.
Conditions shall be provided for temperature increase of reaction of combustion of pyrolysis products.
Energy installation to solve these tasks shall be designed for the system of “free gas movement” taking into consideration of claim “The stove’s lower level and the firebox are combined to form a single space creating a lower bell”. I will try to explain the gist of the claim. Here we speak about combustion of fuel in a firebox located in the bell and about optimal use of heat energy liberated during this process. The firebox is located in the bell that can be different both from the point of view of its design and the principle of fuel combustion. However the common feature is that besides the holes for exhaust of gases on top it is necessary to make a dry joint (slot). In other words the firebox is united with the bell from the point of view of the height (convective system). Such design ensures a better combustion of pyrolysis products as well as separation of flows of hot and cold gases. The latter being heavy are extracted from the lower zone of the bell without cooling it. The convective system serves as a tool for the use of liberated heat energy, which can be used for water heating in the boiler, heater with internal and external heating and for accumulation of energy, etc. The design of energy installation in accordance with this principle makes it possible to build them in various forms and dimensions. This system possesses a number of unique features, which are not inherent in other systems of gas flow. A detailed information is given in the articles " Once again about the system…” and "Basics for the design of boilers…"
Fig.1
There are several principles of firebox design for fuel combustion. For example, Ìyakelya K. (4) quotes the following principles shown in Fig.1: The principle of top combustion-1, the principle of bottom combustion-2, the principle of inverse combustion -3. In figures the supply of primary air is marked by letter Ð, supply of secondary air is marked by letter S. All of them can be used for combustion of single batch of fuel, the second and the third ones – for continuous combustion of fuel when fuel addition doesn’t influence much on the combustion process. A common feature of all the three principles is that when combustion power is changed due to reduction of air supply the efficiency of energy installation drops disproportionately. In other words, the possibility to adjust heat productivity without reduction of efficiency is limited. Besides, much more excess air is needed, by 1,6-2,4 times as compared with theoretically needed amount. A great amount of ballast gases goes through the firebox. These are water steams and nitrogen, a certain amount of oxygen that has not taken part in combustion, water steam from evaporation of water present in the fuel. All these gases do not take part in the combustion process but are only heated due to heat of carbon and hydrogen combustion. They don’t require energy for heating.
Let’s build energy installation, Fig.2 in accordance with above-given claim. Let’s locate firebox 2, Fig 1 in the bell, using the principle of bottom combustion.
Designation in Fig.2 is as follows: 1 – opening for fuel loading; 2 - capacity for fuel pyrolysis (pyrolysis reactor); 3 –opening for gas exhaust from the firebox; 4 - firebox; 5 – dry joint; 6 - combustion catalyst (fire clay grate); 7 – water boiler; 8 – ash-pit; 9 – regeneration system heater (supply of heated air by exhaust gases, P è S, i.e. supply of primary and secondary air); 10 – exhaust gases opening from the bell; 11 – convective system (bell).
Such design of energy installation as a whole and that of firebox is used to receive and to combust the products received during wood distillation, (carbonization) i.e. resin vapors, acetic acid, coke and burning gases. Gaseous and liquid products of carbonization: resin vapors, acetic acid and gases are supplied to combustion zone through red-hot carbon (charcoal for wood). In this case carbon dioxide is reduced into carbon oxide, ànd water steams into hydrogen, which are burnt together with the carbon (coke).
In working condition coke received during pyrolysis process is subjected to combustion, its calorific value is 2,6 times higher than the calorific value of wood. Primary air-P and secondary air –S in heated condition (heated in heater by the exhaust gases from the lower zone of the bell) is supplied to combustion area. High temperature is created in the firebox due to regenerative method; combustion catalyst in the form of fire clay grate making a gas mixture and creating heat, also due to separation of flows of hot and cold gases. Such installations are called power-chemical installations.
It shall be pointed out that wood-burning boilers with traditional method of fuel combustion (using principle of top combustion), with the regenerator and combustion catalyst device designed on the principle of "free gas movement" show wonderful results due to high temperature produced in the firebox. A metal circle shown in Fig.3 having been in the firebox of wood-burning stove built on the principle of "free gas movement" started to ruin under the influence of high temperature due to scale formation.
Hot gases heat the walls of pyrolysis generator where the temperature gets higher and wood carbonization of wood forming vapors, gases and coke. Exothermic reaction takes place with additional calorification making the heating of fuel massive easier. With temperature increase the pressure on pyrolysis generator also gets higher and pyrolysis products are coming to depressurized zone of fuel combustion where they burn significantly increasing the power of combustion and temperature in the firebox. The process of combustion of carbon and pyrolysis products takes place over the whole thickness of layer, in which high temperature develops. Sosnin Ju. P., Bukharkin E.N. (9, page. 58) indicate that combustion of pure carbon in the layer is provided with a feature of self-regulation. This means that the quantity of carbon burnt will correspond to the quantity of oxidant (air); therefore during constant air consumption the quantity of the fuel burnt will be also constant. The change in air supply will lead to reduction or to an increase of heat productivity, while the value of excess air ratio remains stable. The same can be said, in my opinion, about combustion of carbon with pyrolysis products. Hence an important conclusion can be made. In this case there appears a possibility to regulate combustion power due to the change of draft-draught without significant reduction of energy installation efficiency. In the system of "free gas movement" the excess air that hasn’t react as well as ballast vapors and gases being cold, are extracted through the dry joint and lower zone of the bell without cooling energy installation. There is also a possibility to adjust combustion power without reducing the efficiency of energy installation to the change of combustion area, which is achieved by changing the dimension À shown in Fig. 4. Power combustion at optimum mode can be adjusted automatically by installing temperature gauge and transducer of the composition of exhaust gases, using actuation mechanisms exerting influence upon draft-draught and changing the size of combustion area. This makes it possible to optimize supply of primary and secondary air on different stages of combustion and to automate the combustion process. Heater of regeneration system 9 can ensure the adjustable supply of heated air in a natural way or by means of forced supply.
As it is pointed out, the optimum relative moisture of raw material for pyrolysis is 13-20%. Therefore raw material shall be preliminary dried. Usually it is done in dryers, which normally operate on the principle of blowing of raw materials with stack gases with their exhaust into the atmosphere together with evaporated moisture. In the system of “free gas movement" with the horizontal layout of consecutive bells heating in each horizontal section of the system is not uniform and the first bell receives more energy of the hot gases than the next following one. Figure 5 features a diagram of preliminary drying of raw material. In the left part of the diagram there is an energy installation shown in Fig.2. To the right there is a place where initial raw material that will be dried is stored. Conditionally cold exhaust gases from energy installation thanks to draught come to the right bell where they heat and dry the raw material. The bells can also be mounted in vertical direction, one over the other. An example of this layout may be a dryer of cascade-and-gutter type with fire-bars through which ash doesn’t penetrate, used as gutters of system of V.V. Pomerantsev. This original and efficient technical solution was elaborated by the authors and developed until the level of normative documentation. The gist of the method is that the gutters-fire-bars are made in the form that ensures the material flow angle, which is significantly higher than the natural slope. The particles of the materials are moving without shifting relative each other. When transferring from one gutter to another one the upper and the lower surfaces in the material layer are replaced. As a result all particles are treated by hot heat-transfer agent which is supplied under the fire-bars practically in the same way. The material will dry evenly. (10)
Granulated wood fuel (energy pellets), briquetted biological fuel, peat, wastes of timber industry, wood, etc. can be used as fuel for pyrolysis. Each type of fuel has its own peculiarities that shall be considered while designing of elements of energy installation. Fig. 6 features a diagram of fuel charging, conventional (each type of fuel has its own preparation technology and its own charging diagram). A common condition for all fuel charging diagrams during continuous fuel combustion is the necessity to avoid gas burst through charging opening and providing the condition when its addition doesn’t influence significantly on the process of combustion.
Application of energy installation and methods of its use can be different, as well as its design.
It can be used for combustion of a single batch of fuel, i.e. be heat-accumulative or for continuos fuel combustion; for local heating of an apartment, a house or a group of objects. A way of heat transfer to the consumer. As heat-transfer agent the following matters can be used:
Air when radiator heating is used (with heat exchanger inside the stove; with location of furnace in the ventilation chamber; with location of furnace with the integrated heat exchanger in the ventilation chamber);
Water when water heating is used;
Heat emission can be carried out directly through the surface of energy installation and is made of heat irradiation and convective heat from a large surface.
Depending on these various factors the questions concerning adjustment of heat emission of energy installation can be solved differently. Some methods of adjustment are described in the article “Basics for design of boilers...” on our site.
Some of the material given in the article is theory. I have a unique possibility of immediately applying into life new ideas and solutions on energy installations made of brick using wood burning. This is possible while the question of financing of works is solved. There are a lot of customers wishing to build our stoves and boilers, having a wide recognition. We have a group of high skilled experts capable of performing jobs with high quality We have the necessary materials and fixtures needed for energy installations. This is possible due to the fact of having a theory of energy installation design in the system of "free gas movement", we have a group of associates. We do not need productive capacity, we have a large experience of building stoves and boilers and their operation; our works enjoy great popularity. However, I don’t have enough finance to conduct independent laboratory tests of energy installations, to test them and to bring a number of installations we‘ve built up to the mark.
To my great regret I am not provided with an opportunity on energy installations in metal. In this field we have no finance, no industrial capacity, no experts and no experience in construction. However, we’ve accumulated a lot of theoretical studies, which are approved in practice, on the basis of which hundreds of energy installations of various application have been designed and which have no analogues in world literature. This article is written on the basis of this theory. I have no doubt that energo-chemical installations built on the basis of his theory is a new step forward in the use of replaceable bioenergy. I regard the present article as continuation of work specified in the article “ Proposals for research ”
I express my gratitude to Professor Ryzhkov A.F., who holds a chair of “Thermoelectric power stations” and “Energy saving” from the faculty of heating engineering of the Ural State Polytechnic University USTU-UPI, Doctor of Science, Corresponding Member of Engineering Sciences for the assistance in preparation and writing of this article.
List of literature used:

Kuznetsov I. V. , Ekaterinburg, tel: +7 (343) 3077303 e-mail: igor@stove.ru; http://stove.ru
10/10/2004 © Igor Kuznetsov "Kuznetsov's stoves"