- How long-term work is possible
- Manufacturing material
- Sizing and power
- Assembling a two-chamber firebox
- Heat exchanger for pyrolysis boiler
- Optional equipment
Technically, the most advanced boilers are considered, in which complete combustion of fuel occurs with the maximum possible absorption of the released heat. Since schemes of this kind are available to a wide range of people, we will try to figure out how to independently manufacture a pyrolysis boiler for large portions of fuel.
How long-term work is possible
In the open air, even partially damp firewood burns very quickly – literally in 1–1.5 hours. The reason for this is the free access of oxygen – it is absent in the closed boiler furnace, a portion of the incoming oxygen is dosed with the help of the blowing damper, and combustion is less intense.
One of the main problems of this method of burning organic fuel was its ability to “burn” even without oxygen. At high temperatures, pyrolysis occurs – the thermal decomposition of solid fuel into volatile gaseous compounds. Oxygen is not needed for this process; it is enough to heat the bookmark to 400–500 ° С. In this case, colossal losses of calorific value occur – the most energetically valuable component of coal or firewood is simply carried out by the residual draft into the chimney, not having time to burn out completely.
Wood-fired pyrolysis boiler: 1 – blower; 2 – chamber for loading fuel and gasification; 3 – heat exchanger; 4 – combustion chamber; 5 – afterburner chamber; 6 – chimney
In the design of every modern solid fuel boiler, an additional air supply must be provided for the afterburning of the evolved gases. In this case, the intensity and rate of combustion of the bookmark is regulated not by the volume of supplied oxygen, but by the heating temperature of the fuel. Indeed, if you warm up the entire bookmark at once, then combustible gases will be released very quickly and you will have to forget about the long-term operation of the boiler. However, if the solid fuel is heated in separate portions, its gradual decomposition in the generator chamber and efficient complete combustion in the second section of the furnace is possible. In this case, the flow of gases is inverted, they move from top to bottom under the action of the thrust created by the ejection.
The design of the pyrolysis boiler for long-term burning: 1 – solid fuel pyrolysis; 2 – combustion chamber; 3 – lower door; 4 – secondary air supply; 5 – primary air supply; 6 – upper loading door; 7 – gasification chamber; 8 – smoke exhauster
Pyrolysis boilers are characterized by an elevated working area temperature. Combustion does not occur in the gasification chamber; however, the return heat flow is capable of heating the walls to 500–600 ° С. The bottom of the gas generating compartment is exposed to the greatest temperature effect – it is this part that comes into contact with flammable gases and experiences a serious thermal load. It is recommended that the bottom of the bookmark chamber be made in the form of a cast iron grate or a special refractory product with a thin slot or a number of small holes.
The main difficulty in the independent manufacture of boiler equipment lies in the choice of a suitable steel grade that can be processed at home without special equipment. The most suitable in this regard are considered to be steels of the austenitic and austenite-ferritic class with moderate chromium and nickel content. Examples of grades of such steels are 12X18H9T, 08X22H6T or AISI 304.
The technology of welding such metals is recognized as moderately complex, but reproducible in artisanal conditions using arc welding with covered electrodes without a protective medium. The main factor deteriorating the quality of the welded structure is the formation of hot and cold cracks due to a high temperature difference in a relatively small linear section of a metal product..
To eliminate the negative factors of thermal effects, the following technological methods are used:
- Cutting parts with a smooth feed of the cutting tool, which eliminates overheating of the edges.
- Limiting the density of the welding current by 20-25% compared with structural steel, welding in soft modes.
- Puddle temperature limitation, high speed multi-pass seam without lateral vibrations.
- Correct cutting of the joined edges in accordance with GOST 5264 and their cleaning with a metal brush.
- Lining under the seam of the metal heat sink, forging the seam during cooling.
And, of course, the correct choice of the content of alloying additions in the electrode rod should be made in order to ensure the ferrite content in the weld structure of the order of 5–8%. Recommended for use are electrodes of the TsT-15 and TsT-16 brands, as well as special electrodes 6816 MoLC or ROST 1913.
After the structures are welded, it is recommended that they be initially annealed at a temperature of at least 700 ° C for 2.5–3 hours. It is enough to load the inside of the welded body with coal and ignite the fuel, providing a weak forced blowing. Before annealing, it is advisable to pickle the welding seams with a special paste corresponding to the steel grade used..
Sizing and power
Before proceeding with the manufacture of a pyrolysis boiler, it is necessary to calculate the dimensions of the furnace chambers and additional compartments. As the initial data, the required heating power is taken, which is determined taking into account the efficiency of a home-made boiler of the order of 75–80%. At home, you can make solid fuel boilers with a capacity of up to 20-25 kW, more efficient units require the use of heat-resistant steels of considerable thickness, which are difficult to weld at home.
The power of the boiler and the duration of its operation are determined by the volume of the gasification chamber. Without taking into account efficiency, the calorific value of most common types of wood is about 4–5 thousand kcal / kg, which approximately corresponds to 4–4.5 kWh of thermal power. These values are only applicable for wood with a moisture content not exceeding 25%. The essence of the calculation is simple – determine the required instantaneous power and multiply it by the number of hours of operation. It is worth remembering that pyrolysis boilers, even of perfect designs, have a maximum operating time of no more than a day, and self-produced units should be counted on for a maximum of 12-15 hours of continuous burning.
The volume of the bookmark chamber is determined at the rate of 2 liters for each kilogram of firewood. To the resulting value, you need to add about 30%, because in the pyrolysis boiler they use non-chopped chocks that cannot be stacked close. The size of the combustion chamber of gases must be at least 30–40% of the volume of the gasification chamber. The most advantageous is the structure of the boiler, in which two chambers are located one above the other, have the same shape, but differ in height..
Assembling a two-chamber firebox
It is better to choose a hot-rolled sheet with a thickness of at least 8 mm, ideally 10–12 mm, as a material for the manufacture of the chamber walls. The thicker the metal, the more difficult the welding process, however, a structure made of too thin steel is guaranteed to lead and twist in unpredictable directions. That is why among the parts from which the boiler is assembled, there should not be small elements with an aspect ratio of more than 2: 1.
The basis of a two-chamber firebox is the outer side walls. They are common to both chambers and are connected by means of a front wall in which two rectangular openings are made for the doors. The bottom hole is intended for servicing the combustion chamber, its height should be about 120–150 mm, width – at least 300 mm, the hole is located with an indent of 150 mm from the bottom edge. The upper hole is intended for loading the gasification chamber, the larger it is, the better, the hole should be no closer than 100 mm to the top of the chamber. The firebox is closed from below and behind with solid sheets, which are cut out according to the outer dimensions of the combustion chamber, but are not welded until the assembly of the internal parts is completed. The boiler is covered from above with a sheet of nominal section.
An example of the dimensions of a pyrolysis boiler
The gasification and combustion chambers will be separated by a solid plate, the width of which corresponds to the internal distance between the walls, and the length is 400 mm less. In the rear part of the slab, a one-piece partition is welded vertically, which separates the loading chamber along its entire height; a hole 50 mm wide and 400–600 mm long is cut in the center along the horizontal part. The assembled L-shaped partition is not welded until the assembly of the heat exchanger is completed.
Heat exchanger for pyrolysis boiler
The best configuration for a homemade pyrolysis boiler heat exchanger would be a water jacket for the lower chamber and chimney. This is not the most efficient type, however, the production of your own honeycomb heat exchanger will cause inevitable difficulties either with finding pipes of the corresponding steel grade, or with welding dissimilar parts..
The assembly of the heat exchanger parts is carried out at the stage when the bottom of the boiler, the front panel and two side walls are welded. Welding access is provided from the rear of the boiler. The first step is to install the top baffle of the shirt. This is a rectangular plate along the inner width of the firebox and 200 mm less than the depth of the combustion chamber. On the sides of the slab, remove two rectangular fragments 100 mm wide so that two protrusions with a length of 200 mm remain in the front part of the slab. The resulting part is welded to the walls and front panel flush with the lower edge of the opening of the combustion chamber door. In this case, the cutouts in the partition form channels for circulation between the lower zone and the side walls of the heat exchanger.
The inner walls of the jacket are made along the edge of the flow channels, have the height of the combustion chamber and are adjacent to the front panel. From above they are covered with two strips 100 mm wide.
The length of the heat exchanger does not reach the rear wall of the boiler about 200 mm, and by about the same distance the side channels protrude beyond the L-shaped partition between the chambers. When it is installed, it remains only to form the double walls of the chimney duct, cut out its outlet, fix the back wall of the boiler and cut in the threaded fittings for connection to the heating pipeline. The return flow is cut in one of the front lower corners of the jacket, the feed is cut at any highest point of the chimney jacket.
Please note that the combustion chamber is bounded on all sides by a water jacket, except for the baffle with the gasification chamber. This is necessary to transfer heat, which ensures thermal decomposition of the fuel. In this case, not the whole bookmark will be warmed up at once, but only its layers adjacent to the heated walls.
Unfortunately, pyrolysis boilers are not non-volatile. Due to the reverse flow of gases, forced airflow is required. For models with power up to 15 kW, it is realized with a blower fan, which is mounted on the lower door. In this case, replenishment of the load during combustion is impossible..
More powerful boilers are equipped with a fan-exhaust fan, which is installed on the upper wall of the body at the outlet of the chimney duct. At the same time, the appearance of a reverse draft is excluded and the gasification chamber door can be opened without consequences even during combustion..
Particular attention should be paid to the temperature of the coolant inside the jacket. After the boiler reaches the mode, it should not be less than 60 ° С to prevent the formation of condensate. This problem is solved by installing an automatic recirculation unit that mixes water from the supply to the return. It also requires the installation of a safety group for closed heating systems and a main circulation pump.