A biomass-fired boiler

Abstract

This utility model relates to the field of industrial boiler manufacturing technology, and in particular to a biomass-fired boiler and a method for reducing nitrogen oxide emissions. In the biomass-fired boiler, a front arch and a rear arch are located within the furnace. The grate is horizontally mounted below the furnace. The front arch and rear arch are respectively located on the inner sidewalls of the front and rear ends of the furnace. The boiler drum is mounted above the furnace. The horizontal angle of the inner sidewall of the front arch is 60–85°; the rear arch includes an inclined section and a straight section, with the horizontal angle of the inclined section being 8–15°. This design, on the one hand, helps to ensure a stable low-oxygen environment at the front of the furnace, providing suitable conditions for the continuous generation of reducing substances in the initial stage of biomass combustion; on the other hand, the inclined section stably guides the nitrogen oxide-containing flue gas from the main combustion zone and burnout zone to flow forward, forcing the nitrogen oxide-containing flue gas to remain sufficiently in the front arch area containing reducing substances, promoting efficient reduction reactions, and thus reducing reliance on additional denitrification equipment.

Description

Technical Field

[0001] This utility model relates to the field of industrial boiler manufacturing technology, and in particular to a biomass-fired boiler. Background Technology

[0002] The core of combustion efficiency and pollutant emission control in biomass boilers lies in the precise regulation of the flow and temperature fields within the furnace. The furnace arch, as a core structure guiding flue gas flow and enhancing heat transfer, directly determines the overall performance of the boiler through its rational design.

[0003] In terms of the current state of the industry, the structural design of traditional biomass-fired boilers has significant flaws, specifically manifested in the following ways: 1) Imbalance in the design of the front arch angle: If the inclination angle is too small, the high-temperature radiation range at the front of the furnace is insufficient, resulting in slow preheating and drying of the newly fed fuel. This not only increases the loss of incomplete combustion, but also makes it difficult to ensure the continuous generation of reducing substances (such as NH3 and HCN) in the early stage of biomass combustion (corresponding area of ​​the front arch) due to the unstable low-oxygen environment. If the angle is too large, it will compress the combustion space at the front of the furnace, causing the flame to concentrate and scour the front of the grate, which will not only destroy the stable atmosphere required for the reduction reaction, but also easily cause local overheating problems.

[0004] 2) Functional limitations of the rear arch structure: Traditional rear arches mostly adopt a single inclined design, and the flue gas return path is easily affected by load fluctuations and is unstable. Furthermore, it lacks a matching design with the front arch, thus failing to form a directional flow channel. It is difficult to effectively introduce the nitrogen oxide-containing flue gas generated in the main combustion zone and burnout zone into the reducing material area near the front arch, resulting in insufficient reduction reaction.

[0005] 3) Low utilization rate of reducing substances: Due to the lack of synergy between the front arch environment and the rear arch guiding function, the reducing substances naturally generated in the early stage of biomass combustion are not fully utilized, which makes it necessary to rely on additional denitrification equipment for nitrogen oxide emission control, significantly increasing the consumption of catalysts, reducing agents and system energy consumption, and restricting the balance between the economy and environmental protection of boiler operation.

[0006] In summary, technical personnel are urgently needed to solve the above problems. Utility Model Content

[0007] The purpose of this invention is to provide a biomass-fired boiler that optimizes the front and rear arch structures and utilizes the reducing substances generated at the front arch to reduce nitrogen oxides to nitrogen, thereby reducing the nitrogen oxide content in the flue gas and lowering denitrification costs.

[0008] This utility model relates to a biomass boiler, comprising a furnace, a grate, a boiler drum, a wind chamber, a front arch, and a rear arch; the grate is located directly below the furnace, while the front and rear arches are located within the furnace; the grate is horizontally mounted below the furnace; the wind chamber is located directly below the grate and maintains a predetermined gap with it; the front arch is inclinedly mounted on the inner wall of the front end of the furnace, with its lower end extending above the front end of the grate; the rear arch is inclinedly mounted on the inner wall of the rear end of the furnace, with its front end extending above the rear end of the grate; the boiler drum is mounted above the furnace and connected to the inner heating surface of the furnace. The inclination angle α between the inner wall of the front arch and the horizontal is controlled between 60° and 85°; the rear arch includes a sequentially connected inclined section and a straight section; the inclined section extends inclinedly from the end of the rear arch connected to the inner wall of the furnace towards the grate, and the inclination angle β with the horizontal is controlled between 8° and 15°; the straight section extends forward from the end of the inclined section away from the inner wall of the furnace and is horizontal.

[0009] As a further improvement to the technical solution disclosed in this utility model, the front arch and the rear arch extend in opposite directions within the furnace, forming a throat in the cross-sectional contraction area of ​​the furnace; and the minimum cross-sectional width of the throat is 35% to 50% of the maximum cross-sectional width of the furnace.

[0010] As a further improvement to the technical solution disclosed in this utility model, the length ratio of the front arch, throat and rear arch in the direction of grate travel is 1:1.1:2.1.

[0011] As a further improvement to the technical solution disclosed in this utility model, the ratio of the horizontal projection length of the inclined segment to the length of the straight segment is 1.2 to 1.5:1; and the length of the straight segment is 66 to 83% of the horizontal projection length of the inclined segment.

[0012] As a further improvement to the technical solution disclosed in this utility model, the biomass boiler also includes a primary air duct, a secondary air duct, and a tertiary air duct. The primary air duct enters from the refractory side of the front arch, extends into the interior of the furnace after a bend, and its air outlet corresponds to the front end area of ​​the grate. The secondary air duct enters from the side wall of the furnace, extends to the middle of the furnace after a bend, and its air outlet faces the main combustion zone in the middle section of the grate. The tertiary air duct has an upward inclined structure with an inclination angle controlled between 5 and 15°. It extends forward and upward from the upper side wall of the rear arch to the space above the rear arch. The thrust generated by the inclined air supply, combined with the guiding effect of the rear arch, forms a forward airflow force to push the NOx-containing flue gas to the front arch area.

[0013] As a further improvement to the technical solution disclosed in this utility model, the turning angle of the primary air duct is 90-135°, and the vertical distance between the air outlet and the front end of the grate is 100-300mm.

[0014] As a further improvement to the technical solution disclosed in this utility model, the radius of curvature at the turning point of the secondary air duct is 1.5 to 2 times its own diameter; a guide plate is provided at the end of the secondary air duct; and the angle between the guide plate and the axis of the secondary air duct is 30 to 60°.

[0015] As a further improvement to the technical solution disclosed in this utility model, the diameter of the tertiary air duct is 1.2 to 1.8 times that of the primary air duct and 1.1 to 1.5 times that of the secondary air duct.

[0016] In practical applications, the biomass-fired boiler disclosed in this utility model can achieve at least the following beneficial technical effects, specifically: 1) The inclined design of the front arch, with reasonable control of the horizontal inclination angle, not only helps to ensure the formation of a stable low-oxygen environment in the front of the furnace, providing suitable conditions for the continuous generation of reducing substances (such as NH3, HCN, etc.) in the early stage of biomass combustion, but also avoids insufficient preheating due to too small an angle or compression of the combustion space caused by too large an angle; furthermore, it can also enhance the preheating and ignition effect of new fuel by optimizing its radiation range, thereby improving combustion efficiency and laying the foundation for subsequent reduction reactions. 2) The rear arch adopts a segmented design of "inclined section + straight section", which can stably guide the nitrogen oxide-containing flue gas in the main combustion zone and burnout zone to flow back forward. Combined with the horizontal extension structure of the straight section, this facilitates the formation of a directional flow channel, forcing the nitrogen oxide-containing flue gas to stay in the front arch area containing reducing substances for a sufficient period of time, promoting the efficient reduction reaction, thereby reducing the dependence on additional denitrification equipment and taking into account both the economy and environmental protection of boiler operation. On the other hand, it helps to reduce flue gas flow resistance and path fluctuations, solving the defects of unstable flue gas return and poor matching with the front arch in the traditional single inclined rear arch. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of this utility model or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this utility model. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 This is a front view of the biomass boiler disclosed in this utility model.

[0019] Figure 2 This is a side view of the biomass boiler disclosed in this utility model.

[0020] 1-Furnace chamber; 2-Grate; 3-Boiler drum; 4-Wind chamber; 5-Front arch; 6-Rear arch; 61-Inclined section; 62-Straight section. Detailed Implementation

[0021] The present invention will be further described in detail below with reference to specific embodiments. Figure 1 , Figure 2 The diagram shows the structure of the biomass boiler disclosed in this utility model. It is mainly composed of several parts, including a furnace 1, a grate 2, a boiler drum 3, an air chamber 4, a front arch 5, and a rear arch 6. The front arch 5 and rear arch 6 are located in the combustion chamber of the furnace 1, providing a stable environment for fuel combustion. The grate 2 is horizontally installed directly below the furnace 1, serving as a carrier and conveyor for the biomass fuel. The air chamber 4 is located directly below the grate 2, maintaining a predetermined gap with it, and provides the basic airflow for combustion. The boiler drum 3 is installed directly above the furnace 1 and connected to the inner heating surface of the furnace 1, undertaking the functions of steam generation and steam-water separation.

[0022] Depend on Figure 1 As clearly shown, the front arch 5 is inclinedly set on the inner wall of the front end of the furnace 1, with its lower end extending above the front end of the grate 2. The angle α between the inner wall and the horizontal line is strictly controlled between 60° and 85°, which can be flexibly adjusted within this range according to the volatile matter content of the biomass fuel (a higher angle for high volatile matter fuels and a lower angle for low volatile matter fuels). This inclined front arch design, with reasonable control of the horizontal angle, serves two purposes: firstly, it facilitates the formation of a stable low-oxygen environment in the front of the furnace 1, thus providing suitable conditions for the continuous generation of reducing substances (such as NH3 and HCN) in the early stages of biomass combustion; secondly, it effectively prolongs the residence time of high volatile matter in the biomass fuel within the furnace 1, ensuring that it is fully decomposed into reducing components in the low-oxygen environment, rather than directly burned, laying the foundation for the subsequent in-furnace reduction of nitrogen oxide-containing flue gas.

[0023] The rear arch 6 is inclinedly positioned on the inner rear wall of the furnace 1, with its front end extending above the rear end of the grate 2. It employs a segmented structural design, comprising a series of connected inclined sections 61 and straight sections 62. The inclined sections extend inclinedly from the connection point between the rear arch 6 and the inner wall of the furnace 1 towards the grate 2, with the angle β between the inclined sections and the horizontal line controlled between 8 and 15°. This serves two purposes: firstly, it stably guides the nitrogen oxide-containing flue gas from the main combustion zone and burnout zone to flow forward back. Combined with the horizontal extension of the straight section 62, this facilitates the formation of a directional flow channel, forcing the nitrogen oxide-containing flue gas to remain sufficiently in the front arch area containing reducing substances, promoting efficient reduction reactions and reducing reliance on additional denitrification equipment. Secondly, it reduces the flow resistance and path fluctuations of the flue gas to a certain extent.

[0024] To ensure sufficient mixing of nitrogen oxide-containing flue gas and reducing substances, thereby reducing nitrogen oxide emission concentrations, as a further optimization of the above technical solution, the front arch 5 and the rear arch 6 extend in opposite directions within the furnace 1, incidentally forming a throat (not shown in the figure) in the cross-sectional contraction area of ​​the furnace 1. The minimum cross-sectional width of the throat is 35%–50% of the maximum cross-sectional width of the furnace 1, and is designed according to fuel particle size differences: for large-particle fuels (≥30mm), the throat width is 45%–50% to reduce flow resistance; for small-particle fuels (≤10mm), the throat width is 35%–40% to enhance swirl and improve mixing efficiency. Thus, on the one hand, by utilizing the cross-sectional contraction effect of the throat, the swirling intensity of the nitrogen oxide-containing flue gas in furnace 1 is greatly enhanced, causing the nitrogen oxide-containing flue gas to form a violent turbulent mixture with the reducing substances generated in the front arch area, which greatly improves the contact probability and reaction efficiency between the two; on the other hand, the throat structure prolongs the residence time of the nitrogen oxide-containing flue gas in the reduction reaction zone, providing sufficient time for the nitrogen oxides (NOx) to fully react with reducing components such as NH3 and HCN, thereby significantly improving the reduction conversion rate.

[0025] As Figure 1 , Figure 2 As shown, the front arch 5, throat, and rear arch 6 are distributed collaboratively in the direction of grate 2 travel to optimize the spatial layout and flue gas flow path within the furnace 1. The front arch 5, with a length set to a baseline value of 1, serves primarily to create a low-oxygen space for the full release of volatile components from biomass fuel, forming a stable reducing atmosphere within a limited length. This provides sufficient reducing substances for the subsequent reduction reaction of nitrogen oxide flue gas, rather than simply meeting initial combustion requirements. The throat length is set at a ratio of 1.1, preserving the cross-sectional contraction effect to enhance flue gas swirl intensity while avoiding a surge in local resistance by appropriately extending the contraction section. Ultimately, this ensures that the nitrogen oxide-containing flue gas guided by the rear arch 6 and the reducing substances generated by the front arch 5 form a forced mixture, providing an ideal turbulent environment for the reduction reaction. The rear arch 6 extends with a length ratio of 2.1 to complement its segmented design (inclined section 61 + straight section 62). Specifically, the inclined section 61, with an inclination angle of 8-15° and sufficient length, ensures stable recirculation of high-temperature flue gas from the main combustion zone and burnout zone; the straight section 62, with its horizontally extended structure, serves as a guide, providing a buffer space for the recirculation of nitrogen oxide-containing flue gas through length matching, avoiding airflow turbulence caused by sudden path changes, and thus directing the nitrogen oxide-containing flue gas into the throat and front arch 5 area. The front arch 5 area can form a closed-loop flow field of "generating reducing substances → transporting nitrogen-containing flue gas → forced mixing reaction", ultimately significantly improving the nitrogen oxide reduction efficiency.

[0026] It is also important to note that the length ratios of the front arch 5, the throat, and the rear arch 6 correspond to the angle design of the front arch 5 and the inclined section 61. Through the coordinated control of spatial dimensions and flow field characteristics, the temperature and oxygen concentration field distributions within the furnace 1 are highly compatible with the stepped combustion characteristics of biomass. This ensures a reducing environment in the front zone to suppress NOx formation while simultaneously improving boiler thermal efficiency through thorough combustion in the rear zone. This achieves efficient energy utilization while keeping nitrogen oxide emissions below permissible levels. As a further optimization of the above technical solution, the ratio of the total horizontal projection length of the inclined section 61 to the length of the straight section 62 is set to 1.2 to 1.5:1 (the horizontal projection length of the inclined section 61 accounts for 40% to 50%). This provides a sufficient path for the recirculation of high-temperature flue gas and incandescent carbon particles, ensuring that the nitrogen oxide-containing flue gas from the main combustion zone and burnout zone can be stably introduced into the front arch 5 area. The straight section 62, with a matching length (accounting for 55% to 65% of the total length of the rear arch 6), undertakes the guiding function, avoiding both excessive length leading to sluggish flue gas flow and excessive shortness causing flow interruption, thus providing structural protection for the "conveying nitrogen oxide-containing flue gas" stage. Furthermore, the ratio of the horizontal projection lengths of the straight section 62 and the inclined section 61 (approximately 1:1.2 to 1.5), through the flow field connection between the two, ensures that the overall structure of the rear arch 6 can stably guide the heat from the middle and rear to the main combustion zone, maintaining combustion intensity, while also smoothly guiding the nitrogen oxide-containing flue gas to the front arch area containing reducing substances. More importantly, the segmented proportional design of the rear arch 6, together with the angle and throat structure of the front arch 5, forms a three-dimensional synergy: the front arch 5 generates sufficient reducing substances through its angle and length design, the rear arch 6 achieves directional transport of nitrogen oxide-containing flue gas through its segmented proportions, and the throat strengthens the forced mixing of the two through a contraction effect, ultimately achieving the environmental protection goal of meeting standards without additional denitrification equipment.

[0027] To enhance combustion efficiency and precisely control oxygen concentration distribution to suppress nitrogen oxide generation, biomass boilers are also equipped with primary air ducts, secondary air ducts, and tertiary air ducts (not shown in the figure). The primary air duct enters from the refractory side of the front arch 5, then extends into the interior of the furnace 1 after a bend, with its outlet corresponding to the front end of the grate 2, providing initial oxygen to the fuel preheating zone to meet the requirements for volatile matter release. The secondary air duct enters from the side wall of the furnace 1, then extends to the middle of the furnace 1 after a bend, with its outlet facing the main combustion zone in the middle section of the grate 2, enhancing turbulent mixing and supplementing the oxygen required for combustion. The tertiary air duct is an upwardly inclined structure with an inclination angle controlled between 5 and 15°, extending forward and upward from the upper side wall of the rear arch 6 to the space above the rear arch 6. The thrust generated by the inclined air supply, combined with the guiding effect of the rear arch 6, forms a forward airflow force, pushing NOx-containing flue gas to the area of ​​the front arch 5.

[0028] As a further optimization of the above technical solution, the turning angle of the primary air duct should be controlled between 90° and 135°, and the vertical distance between the air outlet and the front end of the grate 2 should be 100-300mm (100-200mm for fuel with a particle size ≤20mm; 200-300mm for fuel with a particle size >20mm). This helps to accurately guide the air supply to the core area of ​​the fuel preheating zone, ensuring that the air temperature is not excessively lost during transmission, ensuring sufficient generation of reducing substances, and reducing the adverse effects of the air supply process on the backflow of nitrogen oxide flue gas in the forward arch 5 direction.

[0029] Furthermore, the radius of curvature at the bend of the secondary air duct should be 1.5 to 2 times its own diameter, and its end should be equipped with a guide plate (not shown in the figure) at an angle of 30 to 60° to the axis. The diameter of the tertiary air duct should be 1.2 to 1.8 times that of the primary air duct and 1.1 to 1.5 times that of the secondary air duct. In the actual operation of biomass boilers, thanks to the design of the radius of curvature of the secondary air duct, airflow resistance and local eddies are reduced, avoiding a sudden increase in local oxygen concentration; and the guide plate can evenly disperse the air supply to the main combustion zone, effectively reducing the adverse effects of the air supply process on the backflow of nitrogen oxide flue gas towards the front arch 5, suppressing local high temperature phenomena, and helping to reduce the generation rate of thermal NOx. Furthermore, the diameter ratio of the tertiary air duct is adapted to the air volume requirements, enhancing the flue gas swirl, and working with the rear arch 6 to guide the NOx-containing flue gas to the front arch 5 area, utilizing the reducing substances in the front area to achieve the reduction of nitrogen oxide-containing flue gas.

[0030] Furthermore, this utility model also discloses a method for reducing nitrogen oxide emissions, comprising the following steps: S1. Biomass fuel travels from the front to the rear on grate 2, passing through the fuel preheating zone, main combustion zone, and burnout zone in sequence. During this process, the radiant heat of the front arch 5 is used to ignite the newly entering fuel, while the temperature at the front of the furnace is controlled at 600-800℃ to avoid a surge in thermal NOx due to local overheating; the high-temperature flue gas containing nitrogen oxides in the rear of the furnace 1 is guided to the front arch 5 through the rear arch 6, and mixed with reducing substances. S2. Air of 15-20% of the total air volume and 80-120°C is sent to the fuel preheating zone through the primary air duct. It is precisely applied to the front section of grate 2 to provide basic oxygen for volatile matter to be released. At the same time, a low-oxygen environment is formed in the preheating zone, which is conducive to the sufficient generation of reducing substances. S3. Supplies of 40-45% of the total air volume and at a temperature of 80-120℃ are delivered to the main combustion zone through secondary air ducts, reaching the middle section of grate 2. After being dispersed by guide plates, the supplied air is thoroughly mixed with volatiles to control the temperature of the main combustion zone to not exceed 1300℃, thereby reducing the formation of thermal NOx. S4. Air supply of 15-20% of the total air volume and 80-120°C is delivered to the space above and forward through the tertiary air duct, covering the area above the rear arch 6. In conjunction with the rear arch 6, the NOx-containing flue gas is directed to the area of ​​the front arch 5, where reducing substances in the front area are used to reduce nitrogen oxides. At the same time, air supply of 20-25% of the total air volume and 80-120°C is supplied to the combustion zone (rear end of grate 2) through the air chamber 4, providing sufficient oxygen for the combustion of unburned materials. Since the air chamber 4 is located below grate 2, it avoids interfering with the low-oxygen environment in the area of ​​the front arch 5. Throughout the process from S1 to S4, the contraction effect of the throat enhances the flue gas swirl, prolongs the residence time of nitrogen oxides in the region containing reducing substances, and fully utilizes the reduction reaction, thereby significantly reducing nitrogen oxide emissions.

[0031] Note: The total air supply volume of primary air duct (15-20%), secondary air duct (40-45%), tertiary air duct (15-20%) and air chamber (20-25%) is 100%. In actual operation, it can be dynamically adjusted within the above range according to the fuel characteristics to ensure total balance.

[0032] It is known that in actual operation, the combustion characteristics of biomass fuel (such as volatile matter release rate and burnout time) are closely related to the temperature of furnace 1, specifically: When the load is reduced, the outlet temperature of furnace 1 is easily lower than 800℃. At this time, the fuel preheating and drying speed is slowed down, the volatile matter is not released sufficiently, which leads to an increase in incomplete combustion loss and difficulty in generating sufficient reducing substances. When the load increases, the outlet temperature of furnace 1 may exceed 1000℃. The high temperature environment will exacerbate the formation of thermal nitrogen oxides (NOx) (and thermal NOx increases exponentially with increasing temperature).

[0033] In view of this, the present invention adds dynamic control logic for the supply air temperature in the method for reducing nitrogen oxide emissions to adapt to the fluctuation of furnace outlet temperature with load, specifically as follows: During the supply air steps S2 to S4, a real-time monitoring link for the outlet temperature of furnace 1 is simultaneously added (achieved through thermocouples or non-contact temperature sensors evenly arranged across the outlet cross-section of furnace 1), and the supply air temperature of the primary air duct, secondary air duct, and tertiary air duct is dynamically adjusted based on the monitored values: When the temperature at the furnace outlet 1 is detected to be below 800℃, the primary air duct temperature is simultaneously increased to 110-120℃, the secondary air duct temperature to 100-110℃, and the tertiary air duct temperature to 100-110℃. Specifically, the air supplied through the primary air duct is given priority for temperature increase, which enhances the heat supply to the fuel preheating zone, accelerates the drying of new fuel and the release of volatiles, and, in conjunction with the radiative ignition effect of the front arch 5, facilitates the sufficient generation of reducing substances. The air supplied through the secondary and tertiary air ducts is simultaneously heated to maintain the combustion intensity in the main combustion zone and burnout zone, ensuring that the high-temperature flue gas containing nitrogen oxides guided by the rear arch carries sufficient heat.

[0034] When the furnace outlet temperature exceeds 1000℃, the primary air duct temperature is maintained at 100-110℃, the secondary air duct temperature drops to 80-90℃, and the tertiary air duct temperature drops to 90-100℃. This targeted cooling via the secondary air duct effectively suppresses temperature rise in the main combustion zone, helping to avoid exceeding the critical value for thermal NOx formation (1300℃). This synergizes with the flue gas recirculation guided by the inclined section 61, reducing the duration of the high-temperature zone. Furthermore, the cooling via the primary air duct prevents localized pre-ignition in the preheating zone due to excessively high air temperature. The moderate cooling via the tertiary air duct ensures oxygen supply to the burnout zone while avoiding disruption of the reducing atmosphere in the burnout zone above the rear arch 6, ensuring that NOx-containing flue gas fully participates in the reduction reaction when guided to the front arch 5 area.

[0035] Finally, it should be noted that when the outlet temperature of furnace 1 is below 800℃, the air volume supplied through the primary air duct is increased by 2-3 percentage points from the original 15-20%, while the air volume supplied through the air chamber is reduced by 2-3 percentage points to maintain the balance of the total air volume. This, combined with the increased air supply, ensures sufficient generation of reducing substances. When the outlet temperature of furnace 1 is above 1000℃, the proportion of air volume supplied through the primary, secondary, and tertiary air ducts remains unchanged. Only the air temperature is controlled to suppress high temperatures, thereby effectively avoiding combustion instability caused by air volume fluctuations.

[0036] The above description of the disclosed embodiments enables those skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A biomass-fired boiler, comprising a furnace, a grate, a boiler drum, a wind chamber, a front arch, and a rear arch; the grate is disposed directly below the furnace, while the front arch and the rear arch are disposed within the furnace; the grate is horizontally mounted in the lower middle part of the furnace; the wind chamber is located directly below the grate and maintains a predetermined gap with it; the front arch is inclinedly disposed on the inner wall of the front end of the furnace, and its lower end extends above the front end of the grate; the rear arch is inclinedly disposed on the inner wall of the rear end of the furnace, and its front end extends above the rear end of the grate; the boiler drum is mounted above the furnace and connected to the inner heating surface of the furnace, characterized in that, The inner wall of the front arch is tilted at an angle α of 60 to 85° with respect to the horizontal. The rear arch includes a series of inclined sections and a straight section. The inclined section extends inclinedly from the end of the rear arch that connects to the inner wall of the furnace towards the grate, and the tilt angle β is controlled at 8 to 15° with respect to the horizontal. The straight section extends forward from the end of the inclined section away from the inner wall of the furnace, and is horizontal.

2. The biomass-fired boiler according to claim 1, characterized in that, The front arch and the rear arch extend in opposite directions within the furnace, forming a throat in the cross-sectional contraction region of the furnace; and the minimum cross-sectional width of the throat is 35% to 50% of the maximum cross-sectional width of the furnace.

3. The biomass-fired boiler according to claim 2, characterized in that, The length ratio of the front arch, the throat, and the rear arch in the direction of grate travel is 1:1.1:2.

1.

4. The biomass-fired boiler according to claim 2, characterized in that, The ratio of the horizontal projection length of the inclined segment to the length of the straight segment is 1.2 to 1.5:1; and the length of the straight segment is 66 to 83% of the horizontal projection length of the inclined segment.

5. The biomass-fired boiler according to any one of claims 2-4, characterized in that, It also includes a primary air duct, a secondary air duct, and a tertiary air duct; the primary air duct enters from the refractory side of the front arch, extends into the interior of the furnace after a bend, and its air outlet corresponds to the front end area of ​​the grate; the secondary air duct enters from the side wall of the furnace, extends to the middle of the furnace after a bend, and its air outlet faces the main combustion zone in the middle section of the grate; the tertiary air duct has an upward inclined structure with an inclination angle controlled between 5 and 15°, extends forward and upward from the upper side wall of the rear arch to the space above the rear arch, and forms a forward airflow force through the thrust generated by the inclined air supply, combined with the guiding effect of the rear arch, to push the NOx-containing flue gas to the front arch area.

6. The biomass-fired boiler according to claim 5, characterized in that, The turning angle of the primary air duct is 90-135°, and the vertical distance between the air outlet and the front end of the grate is 100-300mm.

7. The biomass-fired boiler according to claim 5, characterized in that, The radius of curvature at the bend of the secondary air duct is 1.5 to 2 times its own diameter; a guide plate is provided at the end of the secondary air duct; and the angle between the guide plate and the axis of the secondary air duct is 30 to 60°.

8. The biomass-fired boiler according to claim 5, characterized in that, The diameter of the tertiary duct is 1.2 to 1.8 times that of the primary duct and 1.1 to 1.5 times that of the secondary duct.

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