RU2848593C1 - Shaft furnace for burning biofuels and peat - Google Patents

Abstract

FIELD: energy.

SUBSTANCE: shaft furnace, the walls of which are made of refractory material, contains a fuel sleeve and a loading hopper with fuel supplied from it to a vertical shaft, which ensures its supply to an inclined stepped grate, ending in a horizontal grate, under which there is a slag bunker with localised supply of primary air under the grates by means of air ducts equipped with individual dampers to regulate its flow. A feeder equipped with a fuel loader is installed between the fuel sleeve and the loading hopper. The slag bunker is designed as an air-cascade classifier with air supplied to its lower part and closed at the top by rotary grates that regulate the air flow at the outlet of the air-cascade classifier. When fully open, the rotary grates come into contact with a comb that removes slag build-up from the slotted openings.

EFFECT: complete combustion of fuel.

1 cl, 1 dwg

Description

The invention relates to devices for burning lump and milled peat, biofuels in the form of sawdust, shavings and chips, no more than 50 mm in size, and can find application in thermal power engineering.

Fixed-bed grate furnaces for burning solid fuels are known. They are used in low-power boilers, typically with manual maintenance and periodic fuel loading. These furnaces have a grate made of cast-iron grates supported by beams embedded in the brick walls. Air used for combustion is supplied under the grate via a special air duct. The grates have round or slit-shaped openings for the passage of air and the removal of slag and ash from the grate. Fresh portions of fuel are thrown in an even layer onto the grate through a loading window closed by a door [Golovkov S.I., Koperin I.F., Naidenov V.I. Energy Use of Wood Waste. - Moscow: Lesn. prom-t, 1987, p. 47].

Maintenance of manual grate furnaces requires significant amounts of heavy physical labor. Partial mechanization of manual furnace operation has been achieved by installing rotating or swinging grates. This simplifies one of the most labor-intensive furnace operations—cleaning the grate of slag [Styrikovich M.A., Katkovskaya K.Ya., Serov E.P. Boiler Units. Moscow-Leningrad State Power Publishing House, 1959, p. 75]. Unlike furnaces with rotating grates, where all slag and ash are removed from the grate after burnout, in furnaces with swinging grates, their periodic rocking loosens the focal residues and removes only the underlying, most burnt-out layer. The combustion process of the fuel layer above is not disrupted. Both manual and mechanized drives are used for rotating and swinging grates.

The operator's work can be made easier and the working conditions of the burning bed can be improved by mechanizing the fuel loading onto the grate using various fuel spreaders. In this case, a hopper is installed in front of the furnace, from which the fuel is fed to the fuel spreader, which then delivers it to the bed. Fuel spreaders used in practice are divided into mechanical, pneumatic, and pneumomechanical [Sidelkovsky L.N., Yurenev V.N. Boiler Installations of Industrial Enterprises: A Textbook for Universities. - 3rd ed., revised. - Moscow: Energoatomizdat, 1988, p. 117, Fig. 6.4]. Usually, several fuel spreaders are installed across the width of the furnace. However, mechanical and pneumatic spreaders provide an uneven distribution of fuel in terms of fractional composition along the length of the grate, which is undesirable. Pneumatic-mechanical throwers allow combining the advantages of mechanical and pneumatic ones and providing a more uniform distribution of fuel of different fractions along the length of the grate [Sidelkovsky L.N., Yurenev V.N. Boiler installations of industrial enterprises: Textbook for universities. - 3rd ed., revised. - M.: Energoatomizdat, 1988, p. 119, Fig. 6.5].

Shaft furnaces with an inclined fuel bed are widely used for burning wood (firewood). They consist of a lined shaft with inclined and horizontal grates installed in the lower part. Air for combustion of biofuel is supplied in zones through hatch doors, which allows the combustion process to be regulated depending on the moisture content of the fuel. Optimum vacuum in the shaft furnace is regulated by changing the position of the dampers in the gas tract and the performance of the smoke exhaust fan [Golovkov S.I., Koperin I.F., Naidenov V.I. Energy utilization of wood waste. - Moscow: Lesn. prom-t, 1987, p. 48, fig. 5].

Shaft-type furnaces with inclined fuel beds are also widely used for burning "fine" wood fuels such as sawdust, shavings, and wood chips. In these furnaces, biofuel is typically fed through a fuel hose, often a cascade-type, into a loading hopper, from which it flows through a vertical shaft onto a continuous, inclined, stepped grate. The grate consists of lateral, inclined cast-iron brackets with slots into which horizontal cast-iron bars are installed, forming a stepped grate. Under the force of gravity, the biofuel slides down the inclined, stepped grate onto the lower horizontal grates, where combustion is completed. The shaft-type furnace walls are made of refractory brick, which, during operation, generates powerful radiant flows that ensure the thermal preparation of the wet biofuel. Cleaning of the inclined grate steps and the horizontal grate is carried out through hatches located on the front wall of the furnace, using a special stoker's tool. Each of the air ducts for the zoned supply of primary air is equipped with an individual damper for regulating its flow rate [Golovkov S.I., Koperin I.F., Naidenov V.I. Energy utilization of wood waste. - Moscow: Lesn. prom-t, 1987, p. 48, Fig. 6].

However, the boost capabilities of these shaft furnaces are limited by the need to maintain primary air velocities below critical values to ensure dense bed stability. Furthermore, they are very sensitive to changes in biofuel quality, its flowability, and the operating time before shutdown for cleaning of the inclined and horizontal grates. As moisture content decreases, small wood fuel rolls to the bottom of the grate, making it impossible to optimally distribute primary air along the grate. Excessive fuel thickness at the bottom of the inclined grate and on the horizontal grate, causing increased aerodynamic drag in these areas, limits the ability to supply air to these combustion zones. These circumstances lead to increased heat loss due to chemical and mechanical incomplete combustion. Failure to properly flow fuel along the grate reduces the aerodynamic density of the biofuel feed units into the shaft furnace and can cause combustion in the loading hopper and the lower portion of the fuel hose. It's also important to note the difficulty of removing slag from the grate steps, especially from the horizontal portion of the grate, which limits the use of these furnaces for burning contaminated biofuels. These phenomena are regularly observed in boilers equipped with inclined-bed shaft furnaces, leading to reduced efficiency and reliability of the boilers and heating systems.

We have adopted this shaft furnace as a prototype.

The problem addressed by the invention is the creation of a more efficient shaft furnace with an inclined bed for burning lump and briquetted peat with low ash content (up to 10.0% on a dry weight basis), as well as biofuels in the form of sawdust, shavings and chips, no more than 50 mm in size, with reduced losses from chemical and mechanical incomplete combustion and the absence of slagging of the horizontal grate and increased aerodynamic density of the fuel supply units.

This is achieved by the fact that the shaft furnace, the enclosures of which are made of refractory material, has a fuel hose and a loading funnel with fuel supply from it into a vertical shaft, which ensures its supply to an inclined stepped grate, ending with a horizontal grate under which a slag bunker is located, with a zoned supply of primary air under grate bars With the help of air ducts equipped with individual dampers for regulating its flow rate, a feeder equipped with a fuel loader is installed between the fuel hose and the loading hopper, the slag bin is made in the form of an air-cascade classifier with air supplied to its lower part and closed at the top with rotary grates that regulate the air flow at the outlet of the air-cascade classifier, when the rotary grates are fully opened, they contact the comb, ensuring the removal of slag build-up from the slit-shaped openings.

Fig. 1 shows the proposed firebox, a longitudinal section, and a local section A-A.

A furnace for burning low-ash lump and briquetted peat, as well as biofuels in the form of sawdust, shavings, and chips no larger than 50 mm, comprises a combustion chamber 1 whose side walls, ceiling 2, and front wall 3 are made of refractory material. Fuel enters a feeder 4 from a fuel hose 5. A vertical shaft 6 is formed in the ceiling 2 of the combustion chamber 1, into which fuel enters from a loading hopper 7. A feeder 4, equipped with a loader 8 that performs reciprocating movements, continuously fills the loading hopper 7 and the vertical shaft 6 with fuel. Under the action of gravity, the fuel slides down an inclined stepped grate 9 onto rotating grates 10 with slit-shaped openings 11. Rotary grates 10 are located above the air-cascade classifier 12, into the lower section of which air is supplied via air duct 13. This air, when the rotary grates 10 are opened, separates and returns fine particles and unburned fuel particles from the combustion residue to the inclined stepped grate 9. Furthermore, the air flow directed above the surface of the burning fuel layer toward its initial section on the inclined stepped grate 9 ensures active mixing of the gas flows, reducing chemical and mechanical underburning of the fuel and intensifying its ignition. When the rotary grates 10 are in the closed (horizontal) position, air passes through their trapezoidal slit-shaped openings 11, ensuring the combustion of combustible substances in the combustion residues. The upper guide wall 14 of the air-cascade classifier 12 deflects the gas flow toward the inclined stepped grate 9, activating the mixing of the gas flows. To clear the openings 11 of the rotary grates 10 of slag buildup, they are fully opened, whereby the projections 15 of the comb 16 enter the slit-shaped openings 11 of the rotary grates 10, ensuring their cleaning. Pieces of slag and foreign impurities that have entered the fuel accumulate in the collection bin 17 of the air-cascade classifier 12, from where they are periodically removed. Primary air is supplied to the inclined stepped grate 9 on a zone-by-zone basis via air ducts 18 equipped with individual control devices (dampers). Ash and slag that have fallen from the inclined stepped grate 9 onto the dividing partitions 19 are removed through hatches 20 located on the front wall 3. Air flow to the air-cascade classifier 12 is regulated by damper 21. When burning fuel with a relative humidity of over 45% per working mass, air is supplied to the shaft furnace after the air preheater to ensure efficient fuel combustion.

The operation of the furnace for burning lump and briquetted peat with low ash content, biofuels in the form of sawdust, shavings and chips, no more than 50 mm in size, is carried out as follows.

Fuel from fuel hose 5 is lowered into feeder 4, where it is moved by loader 8 into loading hopper 7. From there, under the force of gravity, the fuel descends into vertical shaft 6 and then slides down inclined stepped grate 9 onto rotating grates 10 with slit-shaped openings 11. During this movement, the fuel is dried, its organic matter undergoes thermal decomposition, followed by ignition of volatile substances and ignition and combustion of the coke residue. To supply the oxidizer for these processes, primary air is supplied zone by zone under inclined stepped grate 9 via air ducts 18, equipped with individual control devices. Air is supplied under the rotary grates 10 by means of an air-cascade classifier 12. When the rotary grates 10 are in the closed (horizontal) position, air passes through their openings 11 of a slit-shaped trapezoidal cross-section, ensuring the combustion of combustible substances in the focal residues, while the upper guide wall 14 of the air-cascade classifier 12 deflects the gas flow towards the inclined stepped grate 9, activating the mixing of the gas flows. To clear the openings 11 of the rotary grates 10 of slag build-up, they are fully opened, whereby the projections 15 of the comb 16 enter the slit-shaped openings 11 of the rotary grates 10, ensuring their cleaning. Pieces of slag and foreign impurities that have entered the fuel accumulate in the collection bin 17 of the air-cascade classifier 12, from where they are periodically removed. Ash and slag that have fallen from the inclined stepped grate 9 onto the dividing partitions 19 are removed through hatches 20 located on the front wall 3. Air flow to the air-cascade classifier 12 is regulated by damper 21.

The optimal distribution of primary air between zones is determined during industrial and operational testing and depends on the type of fuel being burned, its thermal properties, and particle size distribution. Completion of the fuel burnout process and the formation of residual fuel occurs primarily on the rotating grates 10, through whose openings 11 the primary air passes. To remove the residual fuel, the rotating grates 10 open, and the residual fuel enters the air-cascade classifier 12, where it is aerodynamically separated: fine fractions and particles of unburned fuel are returned to the inclined stepped grate 9 for combustion, while slag pieces and foreign impurities contained in the fuel are lowered into the collection bin 17, from where they are periodically removed. The upper guide wall 14 of the air-cascade classifier 12 deflects the gas flow toward the inclined stepped grate 9, activating the mixing of the gas flows, reducing chemical and mechanical underburning of the fuel, and intensifying its ignition. Thus, air entering the air-cascade classifier 12 through the air duct 13, when the horizontally positioned rotating grates 10 are closed, functions as primary air. Further, due to the profile of the upper guide wall 14, it moves toward the inclined stepped grate 9, ensuring the afterburning of combustible components rising from the fuel layer, which enables the organization of the second combustion stage in the combustion process. To ensure high completeness of combustion of combustible fuel components, the ceiling 2, front wall 3, and side walls of the combustion chamber 1 are made of refractory material.

This two-stage layered combustion system ensures highly complete combustion of combustible fuel components and reduced nitrogen oxide and carbon monoxide emissions. Research experience with shaft furnace boilers combusting various biofuels and with various designs allows us to predict the efficient combustion of sod and briquetted peat with low ash content (up to 10.0% dry matter), as well as biofuels in the form of sawdust, shavings, and wood chips no larger than 50 mm, in the proposed combustion device. Furthermore, the use of this combustion device creates the preconditions for increasing the average gross operating efficiency of the boiler by at least 1.5% by reducing heat losses due to chemical and mechanical incomplete combustion of the fuel. It also enables operation with excess air at the furnace outlet (α _t = 1.20-1.25), reduces nitrogen oxide emissions by 10-20% and carbon monoxide emissions by 15-30%, and extends the life of combustion chamber components and the fuel supply system. When burning low-moisture fuels, the walls of the air-cascade classifier are water-cooled; however, the necessity of this measure is determined during industrial and operational activities.

Claims (1)

A shaft furnace, the enclosures of which are made of refractory material, having a fuel sleeve and a loading funnel with fuel supply from it into a vertical shaft, ensuring its supply to an inclined stepped grate, ending with a horizontal grate, under which a slag bin is located with a zoned supply of primary air under the grates by means of air ducts equipped with individual dampers for regulating its flow rate, characterized in that between the fuel sleeve and the loading funnel a feeder is installed, equipped with a fuel loader, the slag bin is made in the form of an air-cascade classifier with air supply to its lower part and closed from above by rotary grates, regulating the air flow at the outlet of the air-cascade classifier, when the rotary grates are fully opened, they contact a comb, ensuring the removal of slag build-up from the slit-shaped openings.