RU2087798C1 - Method of burning solid fuel and furnace for doing same - Google Patents

Abstract

FIELD: heat engineering. SUBSTANCE: method comprises supplying air into the combustion chamber at the mouth of the cold funnel with a velocity 10-24 m/s through the bottom blast nozzle. The device for bottom blast of the furnace is made up as a diffuser the outlet openings of which is in coincident with the mouth of the cold funnel. The inlet opening of the diffuser is at the nozzle of the device for bottom blast. The ratio of the inlet opening of the diffuser to the outlet opening is defined by a relationship presented in the invention description. EFFECT: enhanced efficiency. 3 cl, 4 dwg

Description

 The invention relates to heat engineering, and in particular to methods and furnaces with solid slag removal for burning solid fuel, and can be particularly successfully used when burning low-grade coarsely ground fuel.

 A known method of burning fuel [1] comprising supplying crushed fuel mixed with air through burners, supplying secondary air through air supply channels to the afterburner. The method is implemented in a furnace for burning solid fuel containing a combustion chamber and an afterburner located below it. The combustion chamber is equipped with burners mounted on its wall, and an inclined grate, installed in the lower part of the combustion chamber. The afterburner has two vertical walls, on each of which there are several protrusions tilted downward, and the protrusions of one wall are located opposite the gaps between the protrusions of the other wall. Each wall has channels for supplying secondary air. In addition, channels for supplying secondary air are also available at the bottom of the afterburner.

 When implementing this method, the fuel in a mixture with air is fed into the combustion chamber through the burner. The bulk of the fuel in the form of small particles burns in the exhaust pipe, and large and medium particles of fuel under the influence of gravity fall on the grate and then pour from the protrusion to the protrusion in the afterburner. Air is supplied through the channels to the afterburner; therefore, combustion of the fuel in the bed continues in the afterburner. Due to the slow movement of the fuel, as well as the uniform supply of air to the combustion zones through the channels, an effective afterburning of the fuel occurs.

 This method is successfully implemented in such a furnace if the fuel particles have a relatively small size and a relatively small dispersion of fuel particles in size. In the event that the fuel contains relatively large particles, they do not have time to burn out during movement along the ledges of the afterburner and are removed together with the ash. This is due to the fact that the combustion of fuel in the layer is slow due to the fact that the air washes the fuel particles mainly on one side. As a result, the fuel in the afterburner burns out unevenly, which can lead to crater burning and slagging of the structures of such an afterburner.

 Thus, for the successful implementation of this method in such a furnace, it is necessary to ensure the fulfillment of high requirements for the quality of fuel preparation, which leads to an increase in operating costs.

 A known method of burning solid fuel in a swirl furnace with solid slag removal [2] comprising supplying a fuel-air mixture to a combustion chamber through burners and a counter air supply to the combustion chamber through a lower blast device.

 The method is implemented in a swirl furnace with solid slag removal, containing a combustion chamber, on the wall of which there is a burner directed downward. The slopes of the walls in the lower part of the combustion chamber form a cold funnel with a slotted mouth. The furnace also contains a lower blast device, the nozzle of which is located directly below the mouth of the cold funnel.

 When implementing this method, the fuel is fed through the burner in a mixture with air. Toward this flow of the fuel-air mixture, air is supplied through the nozzle of the lower blast device along the ramp of the cold funnel of the combustion chamber. Since the nozzle of the lower blast device is located directly below the mouth of the cold funnel, the speed of the air flow in the region of the mouth of the cold funnel is equal to the speed of the air flow exiting the nozzle of the lower blast. The interaction of the flows in the combustion chamber causes the formation of two vortex and flare combustion zones. Small fractions of fuel are burned in the exhaust pipe (flare zone), and medium and large fractions as a result of gravity and inertia are lowered into the lower part of the furnace.

 Most of the relatively large particles of fuel have a soaring speed less than the air flow rate provided in this furnace in the area of the mouth of the cold funnel. This determines the rise of these particles in the vortex zone and their circulation in this zone. In the process of circulation, these particles lose moisture, crack and disintegrate into smaller particles, which are carried into the direct-flow part of the furnace, where they burn.

 Due to the relatively high speed of the air flow in the region of the mouth of the cold funnel, some medium and small particles captured by larger ones and therefore falling into the region of the mouth of the cold funnel acquire too much momentum, penetrate the vortex of the furnace and the stream of fuel-air mixture leaving burners (burner jet), and are carried out into the direct-flow part of the furnace with high speed. The time spent in the furnace is reduced, they do not have time to burn and are thrown out of the furnace, reducing its efficiency and having a harmful effect on the environment. In addition, due to the relatively high air velocity in the region of the mouth of the cold funnel, small fuel particles in the vortex zone acquire a relatively high speed, which leads to a relatively high wear of the furnace elements interacting with these particles. A decrease in the air velocity in the region of the slotted mouth of the cold funnel leads to the fact that the largest fuel particles can no longer be held by the flow of the lower blast in the combustion chamber (since their speed will be higher than the speed of the air leaving the nozzle of the lower blast device), and consequently, they fail, without having time to burn out, through the slotted mouth of a cold funnel into a slag chest, thereby increasing the heat loss from the mechanical underburning of the fuel with a failure.

 The aggravation of the fractional composition of the fuel leads to the appearance of larger particles, to keep in suspension which have to further increase the speed of the air leaving the nozzle of the lower blast device, and, therefore, exacerbates the problems associated with the wear of the furnace elements and the removal of unburned fuel particles.

 Thus, a decrease in the lower blast velocity in such a furnace leads to an increase in the number of unburnt large fuel particles in the failure, and an increase in this speed leads to an increase in the wear rate of the furnace elements and an increase in fuel loss with entrainment.

 Known vortex furnace with solid slag removal [3] containing a combustion chamber with downward slit burners located on the wall of the chamber, with a cold prismatic funnel having a slotted mouth. The funnel is formed by the slopes of the walls of the lower part of the combustion chamber. The swirl furnace also contains a lower blast device made in the form of a parallelepiped-shaped failure chamber located under the mouth of a cold funnel and having an air nozzle located in the lower part of this chamber.

 In this furnace, fuel is fed into the combustion chamber through slot burners. As a result of the interaction of the air stream supplied through the afterburner of the hole by means of an air nozzle located in the lower part of the latter with the flow of the fuel-air mixture exiting the burners, a swirling movement of flue gases with multiple circulation of fuel particles forms in the lower part of the combustion chamber. Particles of fuel rotating in peripheral zones constantly interact with the side walls of the combustion chamber. On the one hand, this leads to an increase in the wear rate of screen tubes in these zones, especially when particles, having acquired a significant impulse in the high-speed air flow in the lower part of the afterburning chamber of a failure, are able to freely enter the combustion chamber. On the other hand, the loss of speed by fuel particles after interacting with the wall of the combustion chamber leads to a slowdown of the vortex movement of gases in the peripheral zones due to additional energy costs for the acceleration of these particles. This leads to a decrease in the intensity of the process of afterburning of fuel in these zones compared to the central ones. Therefore, there is an accumulation of unburned fuel in the peripheral zones of the combustion chamber. Overloading the peripheral zones of the furnace with unburned fuel leads to increased wear of screen tubes in these zones, i.e. to reduce the reliability of the combustion equipment. In this case, oxygen in the central zones is used much less efficiently, which leads to an increase in fuel loss with flue gases. If the furnace contains two burners located on opposite walls in different halves, the process of accumulation of fuel in the peripheral zones, and hence a decrease in the intensity of the afterburning of the fuel, will be aggravated by centrifugal forces arising from the formation of additional swirling movement of flue gases in such furnaces having a vertical axis.

 The basis of the present invention is the creation of such a method of burning fuel in a swirl furnace with solid slag removal, which would ensure classification of fuel particles by size in the device of the lower blast and thereby provide a more complete combustion of fuel.

 The basis of the present invention is also the task to create such a furnace with solid slag removal, in which the device of the lower blast would be made in such a way as to provide a relatively low air flow rate in the mouth of the cold funnel and thereby reduce the wear of the elements of the combustion chamber, while maintaining relatively high air flow in the nozzle area of the lower blast device.

 The problem is solved in that in the known method of burning solid fuel in a swirl furnace with solid slag removal, containing a combustion chamber with a downward-directed burner, a cold funnel formed by the slopes of the walls of the lower part of the combustion chamber and a lower blast device with a nozzle located under the mouth of the cold funnel, comprising supplying the fuel-air mixture downwardly inclined through the burner and oncoming air supply to the combustion chamber through a lower blast device according to the invention for classifying fuel about the dimensions in the device of the lower blast in the area of the mouth of the cold funnel provide a speed of 10-25 m / s.

The problem is also solved by the fact that in a vortex furnace with solid slag removal, containing a combustion chamber with a downward-facing burner placed on the chamber wall, with a prismatic cold funnel having a slotted mouth formed by slopes of the walls of the lower part of the combustion chamber, as well as a lower blast device with a nozzle located under the mouth of the cold funnel for supplying air to the combustion chamber, according to the invention, the lower blast device between the mouth of the cold funnel and the nozzle is made in the form of expanding the air flow of the prismatic diffuser, the outlet of which is aligned with the mouth of the cold funnel, and the inlet is located in the nozzle area of the lower blast device, while the ratio of the areas of the inlet and outlet of the diffuser is determined by the ratio:
^And F: F ⁱⁿ (1-K ₁₎ W _GV : (K ₁ • K ₂ • W _in )
where F ^{in the} area of the outlet of the diffuser, m ²
F ^and diffuser inlet area, m ²
K ₁ 0.3-0.5 coefficient determining the proportion of air passing through the lower blast device, of the total amount of air required to burn 1 kg of fuel;
K ₂ 3-5 coefficient determining the ratio of the quantities of movement of the air flow from the burners and the flow of the lower blast;
W _m.v. target flow rate of the fuel-air mixture exiting the burner of the combustion chamber, m / s;
W _{in the} estimated velocity of the fuel particle in advance of a predetermined size, m / s.

 Due to the implementation of the diffuser expanding along the air flow, the air velocity decreases along this flow, as a result of which in each region of the diffuser the speed of this flow becomes equal to the speed of the movement of fuel particles of a certain size. Thus, particles of a certain size, i.e. In accordance with the invention, in the diffuser of the lower blast device, particle size is classified.

 Due to the implementation of the furnace in this way and determining the ratio of the areas of the inlet and outlet sections of the diffuser of the lower blast device, in accordance with the method, a relatively low air flow rate is provided in the region of the mouth of the cold funnel and thereby the wear of the furnace elements is reduced. At the same time, a relatively high air flow rate in the region of the nozzle of the lower blast device is maintained, which ensures that the largest fuel particles soar in the air stream.

 Since the velocity of the air flow in the region of the mouth of the cold funnel is relatively low, the medium and small particles of fuel picked up by the air flow of the lower blast have a relatively long residence time in the combustion chamber, and, therefore, more complete burnout of these particles.

 Due to the fact that the fuel is in suspension in the entire volume of the furnace, including the lower blast device, more complete combustion of the fuel occurs, since the oxidizer has access to fuel particles from all sides.

 It is advisable to make the diffuser of the lower blast device in such a way that one of its walls in the upper part of the diffuser is a continuation of the slope of the wall of the combustion chamber on which the burner is located, the opposite wall would have the same orientation, in the lower part these walls passed vertically, and the distance between two other walls would increase along the air flow. The wall at the top of the diffuser should overlap in plan the outlet at the bottom of the diffuser. With this embodiment of the diffuser, accidental ingress of large unprepared (i.e., not warmed up, not dried) fuel particles into the combustion chamber at a relatively high speed, which can occur if some particle gets too strong at the bottom of the diffuser an impulse that will allow it to fly by inertia into the combustion chamber. In the indicated embodiment of the diffuser, such an accelerated particle will be inhibited by the inclined upper wall of the upper part of the diffuser, and only particles whose velocity of rotation will be less than the air flow velocity in the region of the mouth of the cold funnel will enter the combustion chamber, relatively small particles, prepared for combustion. When one of the walls of the diffuser is continued by the ramp of the cold funnel, the oncoming air flows from the burners and from the lower blast device will be directed towards each other, but with a certain displacement along the height of the furnace, which ensures the formation of a vortex flow and fuel circulation in the combustion chamber.

 The implementation of the device of the lower blast in the form of a diffuser allows you to redistribute the fuel between the peripheral and central zones of the combustion chamber. This is because fuel particles that enter the lower blast device from the peripheral zones move down the inclined walls of the diffuser, shifting to its center, and after appropriate thermal preparation are carried out to the central zones of the combustion chamber. Thus, due to this design of the lower blast device, there is a constant process of fuel transfer from the peripheral zones of the combustion chamber overloaded with the solid phase to the central ones rich in oxygen, which on the one hand eliminates the wear of the side screens of the combustion chamber and, on the other hand, increases the efficiency of the swirl furnace.

 A vortex furnace may contain two burners located on opposite walls in different parts of the furnace, divided by a plane passing through the vertical axis of the furnace perpendicular to the opposite walls, and equipped with a second lower blast device, both lower blast devices are also located in different parts of the furnace, and the diffuser wall in the upper part of each device of the lower blast is a continuation of the slope of the wall on which the burners are located in this part of the furnace.

 In FIG. 1 shows a cross section of a furnace made in accordance with the invention; in FIG. 2, section AA in FIG. one; in FIG. 3 is a cross-sectional view of a furnace with burners placed on opposite walls, in FIG. 4 a section BB in FIG. 3.

 The solid slag vortex furnace in accordance with the invention comprises a combustion chamber 1, in the wall of which a downward-facing burner 2 is installed. A burner 3 is located under the burner 2, also located in the wall of the combustion chamber 1. Wall 1a and the opposite wall 1b of chamber 1 (Fig. 1) pass vertically in the upper part, and obliquely in the lower part. Two other opposite walls 1B and 1D (Fig. 2) extend vertically. The inclined parts of walls 1a and 1b form slopes 1d and 1e and, together with walls 1c and 1d, form a cold funnel 4 of a prismatic shape with a slotted mouth 5 in the lower part of the combustion chamber 5. A nozzle 6 of a lower blast device 7 is located under the mouth 5 of a cold funnel 4 and expands into the direction from the nozzle 6 to the mouth 5 of the cold funnel 4 is the diffuser 8. In the upper part of the diffuser 8, its wall 8a is a continuation of the slope 1e of the wall 1a of the combustion chamber 1, on which the burner 2 is located. The opposite wall 8b in the upper part of the diffuser 8 has the same orientation. In the lower part of the diffuser 8, the walls 8a and 8b extend vertically, and the distance between the other two walls 8b and 8g increases in the direction from the nozzle 6 to the mouth 5 of the cold funnel 4. The wall 8b of the upper part of the diffuser 8 overlaps in plan the outlet of the lower part of the diffuser 8.

 In FIG. 3 shows a vortex furnace 1, containing two burners 9 and 10 located on opposite walls 11 and 12 in different parts of the furnace, mentally separated by a plane passing through the vertical axis of the furnace perpendicular to these walls 11 and 12. The furnace is equipped with one lower blast device 13 and a second the device of the lower blast 14. Each of these devices 13, 14 is located in its part of the furnace. The design of these lower blast devices is similar to that described above. The walls of the diffusers 13 and 14 are a continuation of the slopes 12a and 11a of the walls 12 and 11, respectively, on which the burner 9 or 10 are located in the corresponding part of the furnace.

The authors found experimentally that the proposed method is implemented in this design if a certain ratio of the areas of the inlet and outlet openings of the diffuser is maintained:
^And F: F ⁱⁿ (1-K ₁₎ W _GV : (K ₁ • K ₂ • W _in )
where F ^{in the} area of the outlet of the diffuser, m ²
F ^and diffuser inlet area, m ²
K ₁ 0.3-0.5 coefficient determining the proportion of air passing through the lower blast device, of the total amount of air required to burn 1 kg of fuel;
K ₂ 3-5 coefficient determining the ratio of the quantities of movement of the air flow from the burners and the flow of the lower blast;
W _m.v. target flow rate of the fuel-air mixture exiting the burner of the combustion chamber, m / s;
W _{in the} estimated velocity of the fuel particle in advance of a predetermined size, m / s.

The coefficient K ₁ is selected depending on the fractional composition of the fuel burned and the design features of a particular furnace. So, this coefficient K ₁ increases if the fuel has a significant amount of large particles. At the same time, the coefficient K ₁ decreases if the cross section of the furnace approaches a square.

If the coefficient K ₁ is less than 0.3, it is impossible to keep the fuel entering the lower blast device in suspension. Possible loss of unburned fuel with a failure, as well as a blockage of the device of the lower blast.

The coefficient K ₂ is selected depending on the reactivity of the fuel and the sail of its particles.

In the event that the coefficient K ₂ is less than 3, heat losses with fuel entrainment will not increase unacceptably, since the burner jet will not have enough momentum to return the fuel lifted by the lower blast jet to the lower part of the combustion chamber.

In the event that the coefficient K ₂ is greater than 5, too high speeds of the burner jet can lead to wear of the pipes of the opposite wall of the combustion chamber or its slagging due to the flare.

Speed W _m.v. the flow of the fuel-air mixture exiting the burner is selected so that the flame of the burning fuel does not reach the opposite wall and is usually from 20 to 30 m / s.

The estimated speed of the fuel particle in advance of a predetermined size is determined by known methods, for example, as described in [4]
For example, if the estimated speed of W soaring _into fuel particles with a size of about 30 mm is 22 m / s, and the specified flow rate of the fuel-air mixture is set to 25 m / s, the coefficients K ₁ and K ₂ are 0.4 and 4.0, respectively , the ratio of the areas F ^and : F ^{in the} inlet and outlet of the diffuser will be 0.43. In this case, the air flow velocity in the area of the mouth of the cold funnel will be 21.3 m / s with the speed usually used in the region of the nozzle exit section of the specified device 50 m / s.

 Vortex furnace, made in accordance with the invention, operates as follows.

Coarse fuel mixed with air is fed through burner 2 to furnace 1 with a design speed of W _g.v. , which is defined as described above. At the initial start-up, the fuel is ignited using a standard ignition-protective device 3. In this case, mainly small fuel particles light up. At the same time, air is supplied through the nozzle 6 of the lower blast device. As a result of the interaction of the two streams indicated in FIG. 1 by arrows 15 and 16, a vortex zone is formed in the furnace. 17. Under the influence of the vortex flow of this vortex zone, the fuel approaches the mouth 5 of the cold funnel 4. In conditions of a large amount of air coming from the lower blast device, the fuel burns actively. In this case, burning particles, the speed of which is less than the speed of the air flow and the cold funnel 4 coming out of the mouth 5, return to the torch root, ensuring its reliable and stable ignition.

 Those fuel particles, the speed of which is greater than the speed of the air leaving the mouth of a cold funnel, under the influence of gravity fall into the device of the lower blast. As mentioned above, the cross-sectional area of the diffuser increases along the flow from the nozzle 6 of the lower blast device 7 to the mouth 5 of the cold funnel 4. Accordingly, the air flow rate also changes (decreases). In each cross section of the diffuser, the air flow rate corresponds to the speed of the fuel particle of a certain size. Therefore, in the device of the lower blasting is the classification of fuel particles by size. All fuel particles are in suspension. In this case, the processes of heating and drying the fuel, the release of volatiles and their ignition. As a result of heating, some particles crack, become lighter, their speed of rotation decreases, and they rise to the upper part of the diffuser. In addition, the particles lose moisture and volatile elements, as a result of which they also become lighter and rise to the upper part. The duration of these processes (drying, cracking, and volatilization) depends on the size of the particles. These processes continue until the particles rise up to the removal to the furnace and complete burnout.

 It should be noted that the speed of movement of particles relative to the walls of the diffuser is small, the particles mainly oscillate at a very low speed, which leads to a relatively small wear of the walls of the diffuser. In addition, since the air flow rate, and, consequently, the speed of the fuel particles in the region of the mouth of the cold funnel is also relatively small, the wear of the furnace elements is also relatively small. Both of these circumstances lead to increased reliability of the furnace. The relatively high air flow rate in the region of the lower blast nozzle ensures the whirling of the largest fuel particles and does not reduce the reliability of the furnace, since the furnace can be made quite durable in the lower part.

 In the case as shown in FIG. 3 and 4, when two burners 9 and 10 and two lower blast devices 13 and 14 are used in the furnace, the operation of the furnace is carried out in a similar way. When using such a device, a more complete combustion of the fuel is ensured, a uniform temperature field is ensured, which prevents slagging and thereby the reliability of such a furnace is further improved.

 Thus, the proposed method, implemented in the proposed furnace, provides increased efficiency as a result of more complete combustion of fuel, and improved environmental characteristics of the furnace, as well as reduced wear of the elements of the inner surface of the furnace and increase its reliability.

Claims (4)

 1. The method of burning solid fuel in a swirl furnace with solid slag removal, containing a combustion chamber with a downward-directed burner, a cold funnel formed by the slopes of the walls of the lower part of the combustion chamber and a lower blast device with a nozzle located under the mouth of the cold funnel, including a fuel-air supply with a downward inclination mixture through the burner and counter flow of air into the combustion chamber through the lower blast device, characterized in that for classifying fuel particles by size in the lower blast device in the region STI mouth funnel provide cold speed of 10 24 m / s. 2. Vortex furnace with solid slag removal, containing a combustion chamber, at least one downward directed burner, a prismatic cold funnel formed by slopes of the walls of the lower part of the combustion chamber having a slotted mouth, and a lower blast device with a nozzle located under the mouth of a cold funnel , for supplying air to the combustion chamber, characterized in that the lower blast device between the mouth of the cold funnel and the nozzle is made in the form of a prismatic diffuser expanding along the air flow, the output about the aperture of which is aligned with the mouth of the cold funnel, and the inlet is located in the area of the nozzle of the lower blast device, while the ratio of the areas of the inlet and outlet of the diffuser is determined by the ratio
F ^and F ⁱⁿ (1 K ₁ ) W _{g . in} (K ₁ • K ₂ • W _in ),
where F ^{in the} area of the outlet of the diffuser, m ² ;
F ^{and the} area of the inlet of the diffuser, m ² ;
K ₁ 0.3 0.5 a coefficient that determines the proportion of air passing through the lower blast device, of the total amount of air required to burn 1 kg of fuel;
K ₂ 3 5 coefficient determining the ratio of the momentum of the air flow from the burners to the flow of the lower blast;
W _{g . at a} given flow rate of the air-fuel mixture leaving the burner of the combustion chamber, m / s;
W _{in the} estimated speed of the soaring of the fuel particles of a predetermined size, m / s.  3. The furnace according to claim 2, characterized in that it contains at least two burners placed on opposite walls in different parts of the furnace, divided by a plane passing through the vertical axis of the furnace perpendicular to the opposite walls, and provided with a second lower blast device, the lower blast are located in different parts of the furnace, and the wall of the diffuser in the upper part of each device of the lower blast is a continuation of the slope of the wall on which the burner is located in this part of the furnace.  4. A furnace according to claims 2 and 3, characterized in that one of its walls in the upper part of the diffuser is a continuation of the slope of the wall of the combustion chamber on which the burner is located, and the opposite wall of the diffuser has the same orientation, in the lower part of the diffuser these walls extend vertically and the distance between the other two walls of the diffuser increases along the air flow, and the wall of the upper part of the diffuser overlaps in plan the outlet of the lower part of the diffuser.

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