Coal and red mud co-combustion method for simultaneous desulfurization and denitrification and boiler system

Abstract

The present application belongs to the field of clean combustion of coal and collaborative disposal of industrial solid waste, and discloses a coal and red mud collaborative denitration and desulfurization clean combustion method and a boiler system, aiming to solve the technical problems of high cost of denitration and desulfurization of traditional coal-fired boiler and difficulty in utilization of red mud. Through a double-furnace series structure and a staged air distribution system, a partitioned atmosphere is constructed in the furnace, realizing in-situ integration of denitration in the reduction zone and desulfurization in the oxidation zone. The strength of the reduction atmosphere is enhanced by relying on atomized water to strengthen the water gas reaction, and the material residence time and the catalyst life are prolonged by cooperating with the countercurrent contact of the porous particles of red mud and the two-stage circulation. Meanwhile, the iron oxides in the red mud are in-situ reduced and magnetized in the furnace, and a high-purity ferroferric oxide product is obtained through crushing and magnetic separation at the furnace bottom. The present application realizes multiple goals of in-situ low-cost clean combustion of coal, large-scale consumption of red mud and high-value resource utilization, and brings environmental and economic benefits.

Description

Technical Field

[0001] This invention specifically relates to the field of clean coal combustion technology, and in particular to a clean combustion method and boiler system for coal red mud co-processing denitrification and desulfurization. Background Technology

[0002] Red mud is a large-scale industrial solid waste generated during the production of alumina from bauxite. my country is the world's largest alumina producer, with annual red mud production exceeding 100 million tons. By the end of 2025, the cumulative stockpile of red mud nationwide is estimated to exceed 1.6 billion tons. Red mud is highly alkaline, has a fine particle size, and a complex composition, making its comprehensive utilization difficult. Its large-scale stockpiling not only occupies land resources for a long time but also poses significant environmental risks. However, surveys show that while relevant enterprises have invested heavily in research and development for the resource utilization of red mud and have achieved some results at the pilot-scale level, they generally face bottlenecks such as high processing costs, low product added value, and limited market acceptance. Particularly in the recovery of iron from red mud, although the current utilization rate of iron from red mud in my country is relatively high, it still faces practical difficulties such as low iron recovery rates, high costs, and poor economic benefits. As a large-scale, highly alkaline solid waste produced by the aluminum industry, red mud stockpiling occupies land, poses environmental risks such as leakage and dust, and traditional resource recovery pathways have low utilization rates and low added value.

[0003] In the field of flue gas pollutant control, with the continuous improvement of national requirements for air quality, emission standards for coal-fired boilers are becoming increasingly stringent. Currently, the main technical challenge plaguing the industry lies in the fact that in traditional processes, desulfurization and denitrification facilities operate independently, requiring the separate addition of desulfurizing agents (such as limestone) and denitrification reducing agents (such as ammonia or urea). This reliance on expensive commercial denitrification catalysts leads to high equipment investment, high operating costs, large land area requirements, and complex byproduct treatment. The economic pressure of adding independent desulfurization and denitrification devices is particularly pronounced for the numerous and widespread small and medium-sized coal-fired boilers, necessitating the development of more intensive, efficient, and low-cost integrated flue gas treatment technologies.

[0004] Currently available technologies for red mud denitrification and desulfurization mostly focus on catalyst modification and preparation, external reactors, simple addition in a single furnace, low-temperature desulfurization, or chemical looping combustion. These technologies fail to achieve low-cost denitrification and desulfurization of flue gas to meet emission standards while simultaneously utilizing large quantities of industrial solid waste (red mud) and recovering high-value iron resources. These technological routes generally suffer from insufficient denitrification and desulfurization efficiency, inadequate utilization of red mud, and low resource value. This invention aims to address these technical deficiencies and achieve a synergistic unity between clean coal combustion, large-scale red mud utilization, and high-value resource recovery. Summary of the Invention

[0005] This invention aims to overcome the shortcomings of existing technologies and provide a coal-red mud co-processing denitrification and desulfurization clean combustion method and boiler system. By constructing a dual-furnace series structure and a graded air distribution system, a zoned atmosphere is created within the furnace, achieving in-situ integration of denitrification in the reduction zone and desulfurization in the oxidation zone. The intensity of the reducing atmosphere is enhanced by atomized water to strengthen the water-gas reaction. In conjunction with the countercurrent contact of porous red mud particles and two-stage circulation, the material residence time and catalytic life are extended. At the same time, iron oxides in the red mud are reduced and magnetized in situ within the furnace. After crushing and magnetic separation at the bottom of the furnace, high-purity iron tetroxide is obtained. Ultimately, multiple goals are achieved, including efficient and clean combustion of coal, low-cost flue gas treatment to meet standards, large-scale disposal of bulk industrial solid waste, and high-value recovery of iron resources.

[0006] To achieve the above objectives, the present invention provides a coal-red mud co-processing denitrification and desulfurization clean combustion method and boiler system, wherein the core reaction unit is composed of a left reduction furnace 1, a cyclone separator 2 and a right oxidation furnace 3 connected in series; the left reduction furnace 1 is divided from bottom to top into a foam bed zone 1-1, a dense phase reduction zone 1-2 and a fixed bed zone 1-3; the bottom of the furnace is provided with an atomizing water nozzle 1-7 and a primary air inlet 1-4; the middle and lower part is provided with a coal feed port 1-6 and a secondary air inlet 1-5; and the top is provided with an aerial red mud feed port 1-8. The upper outlet of the cyclone separator 2 is connected to the right oxidation furnace 3, and the bottom outlet is connected to the dense phase reduction zone 1-2 of the left reduction furnace 1. The top of the right oxidation furnace 3 is equipped with a tertiary air inlet 3-1, and its outlet is connected in sequence to the high-temperature dust collector 6, the economizer 4 and the air preheater 5. The high-temperature dust collector 6 is equipped with a mixing and pulverizing device 8 after the ash hopper, and its outlet is connected to the tertiary air inlet 3-1 at the top of the right oxidation furnace 3. The bottom ash discharge pipe of the left reduction furnace 1 is connected to the crushing and magnetic separation device 7, forming a complete system of reaction, circulation, heat exchange, dust removal and resource recovery.

[0007] The system employs a precise graded air distribution method, strictly controlling the total air volume proportionally: 40% primary air and 20% secondary air are sent to the left reduction furnace 1 to create a strongly oxygen-deficient reducing atmosphere, while the remaining 40% tertiary air is sent to the right oxidation furnace 3 to create an oxygen-rich oxidizing atmosphere. This ensures that the denitrification and desulfurization reactions proceed stably within their respective optimal atmosphere and temperature ranges. Coal is fed into the foam bed zone 1-1 of the left reduction furnace 1 through the coal feed inlets 1-6 at the furnace front. Under a high-temperature fluidized state of 700-900℃, it reacts with atomized water injected from the bottom atomizing water nozzles 1-7 to produce carbon monoxide and hydrogen, significantly enhancing the intensity of the nitrogen oxide reducing atmosphere. Red mud, pre-formed into porous coarse particles with a particle size of 1-2mm, is uniformly added as a catalyst through the airborne red mud feed inlets 1-8 at the top of the furnace. It forms a countercurrent contact with the rising reduction flue gas, and under the catalytic action of components such as iron, chromium, and manganese, it reduces nitrogen oxides in the flue gas to nitrogen, while simultaneously reducing ferric oxide to magnetite, achieving in-situ magnetization.

[0008] Cyclone separator 2 separates unreacted coarse red mud particles and returns them to the left reduction furnace 1, forming the main circulation of coarse red mud catalyst particles. Fine particles collected by high-temperature dust collector 6 are further mixed with red mud by mixing and pulverizing device 8 to produce 325-mesh fine particles, which are then fed into the top of the right oxidation furnace 3 via tertiary air inlet 3-1, forming a secondary circulation of fine red mud reactant particles. These two circulation stages work together to improve red mud utilization and denitrification and desulfurization stability. The alkaline components such as calcium and sodium in the red mud fine particles entering the right oxidation furnace 3 with the tertiary air convert sulfur dioxide into stable sulfates under oxygen-rich conditions, achieving high-temperature dry desulfurization. The reaction temperature is controlled within a reasonable range to avoid decomposition of desulfurization products or coking in the furnace. The ash discharged from the bottom of the furnace is crushed and separated by crushing and magnetic separation device 7 to produce high-purity ferric oxide. The tailings can be further used in building materials or roadbed materials, achieving full-scale resource utilization of red mud.

[0009] This invention achieves integrated in-situ reduction denitrification and oxidation desulfurization within the boiler through a dual-furnace series-connected zoned reaction and graded air distribution, significantly simplifying the flue gas treatment system. Utilizing atomized water to enhance the water-gas reaction and generate high-concentration reducing gas, it significantly improves boiler denitrification efficiency, eliminating the need for reducing agents such as ammonia or urea, and preventing ammonia escape and secondary pollution. A two-stage circulation mode is used: red mud is processed into 1-2mm porous coarse particles and fed back into the right oxidation furnace via counter-current contact; and red mud is processed into 325-mesh fine particles and returned to the right oxidation furnace. This results in a long material residence time and stable catalytic activity, enabling large-scale and efficient utilization of red mud. The iron component in the red mud is reduced to magnetite (Fe3O4) in-situ within the furnace, and high-value-added products are obtained through magnetic separation. These products are then utilized on a large scale during the circulation and catalytic process, significantly improving the resource utilization rate and economic efficiency of solid waste. The main reaction temperature in the right oxidation furnace is controlled between 700℃ and 900℃, and the desulfurization products exist in the form of stable sulfates, without decomposition or resulfurization, eliminating the risk of coking and high-temperature corrosion, ensuring stable and reliable system operation. Attached Figure Description

[0010] Figure 1 This invention relates to a coal-red mud co-processing denitrification and desulfurization clean combustion method and boiler system diagram.

[0011] The diagram is marked as follows: 1. Left reduction furnace; 1-1 Bubble bed zone; 1-2 Dense phase reduction zone; 1-3 Fixed bed zone; 1-4 Primary air inlet; 1-5 Secondary air inlet; 1-6 Coal feed port in front of the furnace; 1-7 Atomizing water nozzle; 1-8 Air red mud feed port; 2. Cyclone separator; 3. Right oxidation furnace; 3-1 Tertiary air inlet; 4 Economizer; 5. Air preheater; 6. High temperature dust collector; 7 Crushing and magnetic separation device; 8 Mixing and pulverizing device. Detailed Implementation

[0012] The following text, in conjunction with the appendix Figure 1This document provides a detailed description of specific embodiments of the present invention. Those skilled in the art can fully implement the technical solution based on this content, satisfying the full disclosure requirements of the patent law and effectively supporting the scope of protection of the claims.

[0013] Unless otherwise defined, the technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0014] The boiler system of this invention adopts a series integrated structure of a left reduction furnace 1, a cyclone separator 2, and a right oxidation furnace 3. All denitrification, desulfurization, combustion, and resource recovery reactions are completed in situ inside the boiler, without relying on external denitrification and desulfurization towers. The system has a compact structure and stable operation. The system consists of a left reduction furnace 1, a cyclone separator 2, a right oxidation furnace 3, an economizer 4, an air preheater 5, a high-temperature dust collector 6, a crushing and magnetic separation device 7, and a mixing and pulverizing device 8. The left reduction furnace 1 is arranged from bottom to top as follows: a foam bed zone 1-1, a dense phase reduction zone 1-2, and a fixed bed zone 1-3. Atomizing water nozzles 1-7 and a primary air inlet 1-4 are installed at the bottom of the furnace. A coal feed port 1-6 and a secondary air inlet 1-5 are opened in the middle and lower part of the furnace. An aerial red mud feed port 1-8 is set at the top. The outlet of the left reduction furnace is directly connected to the flue gas inlet of the cyclone separator 2. The lower material leg of the cyclone separator connects back to the upper part of the left reduction furnace, realizing the main circulation of coarse particles of red mud catalyst. The flue gas outlet of the cyclone separator is connected to the inlet of the right oxidation furnace 3. The right oxidation furnace is equipped with a tertiary air inlet 3-1. The furnace outlet is sequentially connected to a high-temperature dust collector 6, an economizer 4, and an air preheater 5, achieving efficient dust removal and stepwise cooling of flue gas. A mixing and pulverizing device 8 is installed after the ash hopper of the high-temperature dust collector 6. The collected fine particles are mixed with red mud through the mixing and pulverizing device 8 to form 325-mesh fine particles, which are then sent to the top of the right oxidation furnace 3 through the tertiary air inlet 3-1, forming a secondary circulation of fine particles of red mud reactants. The ash discharge pipe at the bottom of the left reduction furnace 1 is connected to a crushing and magnetic separation device 7. After treatment, the ash residue is used to recover ferric oxide products, and the tailings can be sold for comprehensive utilization.

[0015] During system operation, materials are stably fed into four channels—coal, porous red mud particles, atomized water, and staged air—at designated points and parameters. Coal is continuously fed into the upper part of the bubble bed zone 1-1 in the left reduction furnace through coal feed ports 1-6, with the material falling at the center of the primary air, allowing the coal particles to quickly enter a fluidized state. Red mud is pre-crushed, screened, and granulated into porous particles of 1 to 2 mm, which are evenly sprayed into the fixed bed zone through the top aerial red mud feed port 1-8. Moving from top to bottom, it forms a countercurrent contact with the upward-flowing reduction flue gas, prolonging the gas-solid contact time and improving the catalytic reaction efficiency. Atomized water is sprayed into the bubble bed zone 1-1 in the form of high-pressure droplets through atomized water nozzles 1-7 at the bottom of the furnace. The droplet size is controlled between 50 and 200 μm to ensure full contact with the high-temperature coal particles and rapid vaporization. Primary, secondary, and tertiary air are supplied by independent fans, and the total air volume ratio is strictly controlled to 40% primary air, 20% secondary air, and 40% tertiary air. An oxygen-deficient reducing atmosphere is formed in the left reduction furnace 1, and an oxygen-rich oxidizing atmosphere is formed in the right oxidation furnace 3, providing stable atmospheric conditions for the zoned reaction.

[0016] In the left reducing furnace 1, coal primarily undergoes gasification and semi-coke combustion, with incomplete combustion occurring. The excess air coefficient in the furnace is below 0.8. Coal particles entering the foam bed zone 1-1 form a boiling fluidized state with the primary air. Atomized water droplets, injected through atomizing nozzles 1-7, vaporize instantaneously within a temperature range of 700 to 900°C, reacting with carbon in the coal to produce carbon monoxide and hydrogen, ensuring a continuous and stable production of reducing gas and significantly enhancing the reducing atmosphere. Coal volatiles precipitate and partially decompose in the dense phase reducing zone 1-2, forming a strong reducing atmosphere together with the water gas products. Semi-coke further reacts and releases heat in the dense phase reducing zone 1-2 to maintain the furnace temperature. Unburned semi-coke and red mud particles enter the cyclone separator 2 with the flue gas. Coarse particles are separated and returned to the dense phase reducing zone 1-2, while fine particles enter the right oxidation furnace 3 with the flue gas for complete combustion. The excess air coefficient in the right oxidation furnace 3 is higher than 1.15. The tertiary air replenishes the oxygen, allowing the unburned components to burn completely and release heat, maintaining the temperature of the right oxidation furnace 3 and providing the optimal environment for the desulfurization reaction.

[0017] The denitrification process is completed in situ within the left reduction furnace 1, following an ammonia-free reduction denitrification route. This eliminates the consumption of urea and ammonia, thus avoiding the risk of ammonia escape and secondary pollution. The red mud particles contain transition metal oxides such as ferric oxide, chromium oxide, and manganese oxide, which possess excellent catalytic reduction activity within the 700-900℃ range. As a catalyst, these oxides efficiently reduce nitrogen oxides in the flue gas to nitrogen under a strong reducing atmosphere, while simultaneously reducing ferric oxide to magnetite, achieving in-situ magnetization within the furnace and laying the foundation for subsequent magnetic separation and recovery. The porous red mud particles come into countercurrent contact with the reducing gas, resulting in high mass transfer efficiency and a complete reaction. Cyclone separator 2 returns unreacted coarse particles to the left reduction furnace 1, achieving catalyst recycling and ensuring long-term stable denitrification performance.

[0018] The desulfurization process is completed within the right oxidation furnace 3, employing a high-temperature dry oxidation desulfurization route. The desulfurization products are stable and do not revert to sulfur. Alkaline components such as calcium oxide and sodium oxide in the red mud enter the right oxidation furnace 3 with the flue gas. Under oxygen-rich conditions and with the temperature controlled between 700℃ and 900℃, they react with sulfur dioxide to form stable sulfates. The intermediate product, sulfite, is completely oxidized, preventing high-temperature decomposition and the re-release of sulfur dioxide. The reaction temperature is strictly controlled within a reasonable range, below the sulfate decomposition temperature and below the ash melting point, ensuring desulfurization efficiency while preventing coking and corrosion of the furnace. This desulfurization method uses red mud as the desulfurizing agent and reactant, eliminating the need for additional desulfurizing agents such as limestone, thus treating waste with waste and significantly reducing operating costs.

[0019] The flue gas enters the high-temperature dust collector 6, economizer 4, and air preheater 5 sequentially through the outlet of the right oxidation furnace 3, and is discharged after the temperature drops to 150-180℃. The fine fly ash collected by the high-temperature dust collector 6 contains unreacted red mud, semi-coke, and magnetic components. It is mixed with the supplemented red mud through the mixing and pulverizing device 8 to form 325-mesh fine particles, which are then sent to the top of the right oxidation furnace 3 through the tertiary air inlet 3-1, forming a secondary circulation of red mud reactant fine particles. The main circulation of coarse red mud catalyst particles in the cyclone separator 2 and the secondary circulation of fine red mud reactant particles in the high-temperature dust collector 6 form a two-stage circulation system, which significantly extends the residence time of red mud in the system, improves the catalytic utilization rate and denitrification and desulfurization effect, and reduces material emissions and waste.

[0020] High-temperature ash slag, mainly composed of ferric oxide, silicates, and sulfates, is continuously or periodically discharged from the bottom of the left reduction furnace 1. The crushing and magnetic separation unit 7 consists of a crushing device and a magnetic separation device. The ash slag first enters the crushing device, where agglomerates are crushed to below 1mm, improving the efficiency of subsequent magnetic separation. The crushed material enters the magnetic separation device, where strongly magnetic ferric oxide is efficiently separated under a magnetic field strength of 1500 to 3000 Gs, yielding a high-purity ferric oxide product that can be directly sold as a magnetic material or ironmaking raw material. The non-magnetic tailings, after reaction, have reduced alkalinity and stabilized and solidified heavy metals, making them suitable for preparing building materials and roadbed materials, achieving full material resource utilization of red mud.

[0021] This system is equipped with multiple key process control points, which are linked and adjusted by the instrumentation and automatic control systems to ensure long-term stable operation. The total air volume is precisely controlled according to the ratio of 40% primary air, 20% secondary air, and 40% tertiary air, with an error not exceeding ±2%. The water-gas reaction temperature in the foam bed zone 1-1 is controlled between 700 and 900℃, with priority given to stabilizing at 850℃. The main reaction temperature in the left reduction furnace 1 is maintained between 700 and 900℃. The coarse particle size of the red mud is strictly controlled to be a porous structure of 1 to 2 mm. The atomizing water pressure is controlled between 0.4 and 0.8 MPa to ensure atomization effect. The magnetic field strength of the crushing and magnetic separation device is set between 1500 and 3000 Gs to ensure that the recovery rate of ferric oxide is not less than 85%.

[0022] The above description is merely an application example of the present invention and does not limit the scope of protection of the present invention. Any equivalent structures made using the content of this specification and drawings, directly or indirectly applied to related technical fields, are similarly included within the patent protection scope of the present invention. The right oxidation furnace 3, economizer 4, and air preheater 5 can be arranged in different ways depending on site conditions: a side-by-side layout is suitable for situations with sufficient site width; a stacked layout is suitable for situations with compact sites; and an integrated layout combines the three into one unit, further reducing the footprint. The cross-sections of the left reduction furnace 1 and the right oxidation furnace 3 can be circular, square, or polygonal, without shape restrictions; baffles, baffle columns, and other baffle components can be added inside the furnace as needed to form an overall baffle effect, enhance flue gas disturbance and gas-solid contact mixing, and improve reaction efficiency. The primary, secondary, and tertiary air intake methods can be direct intake, tangential intake, or combined intake, all of which can achieve zoned atmosphere construction and efficient reaction. The high-temperature dust collector 6 is not limited to a bag filter; it can also be a two-stage cyclone dust collector, a cartridge dust collector, or other high-efficiency dust removal equipment. All of these can achieve fine particle collection and recycling without affecting the technical effect of the present invention.

Claims

1. A method for clean combustion of coal and red mud through synergistic denitrification and desulfurization, characterized in that, In-situ implementation within a dual-furnace series structure, including: Graded air distribution: Primary air is introduced into the foam bed zone (1-1) of the left reduction furnace (1), and secondary air is introduced into the dense phase reduction zone (1-2), with the total air volume accounting for 40% of the primary air and 20% of the secondary air; tertiary air is introduced into the right oxidation furnace (3), with the air volume accounting for 40%; In-situ denitrification in the reduction zone: Atomized water reacts with coal in the foam bed zone (1-1) to produce carbon monoxide and hydrogen, with coarse red mud particles as the catalyst; In-situ desulfurization in the oxidation zone: The alkaline components in the fine red mud particles react with the oxygen added by the tertiary air in the right oxidation furnace (3) to generate sulfate, and the fine red mud particles are the reactants; Two-stage circulation of red mud particles: The main circulation is the cyclone separator (2) separating coarse red mud particles and returning them to the left reduction furnace (1), and the secondary circulation is the high-temperature dust collector (6) collecting fine red mud particles and returning them to the right oxidation furnace (3). The ash and slag discharged from the bottom of the furnace are crushed and magnetically separated to recover ferric oxide products.

2. The combustion method according to claim 1, characterized in that, The water-gas reaction temperature is controlled at 700 to 900°C, and the reaction temperature of the right oxidation furnace (3) is controlled at 700 to 900°C.

3. The combustion method according to claim 1, characterized in that, The coarse red mud particles are porous particles with a diameter of 1 to 2 mm, and the fine red mud particles are particles with a diameter of 325 mesh.

4. The combustion method according to claim 1, characterized in that, Cyclone separator (2) separates coarse red mud particles and returns them to the dense phase reduction zone (1-2) of the left reduction furnace (1). The fine red mud particles collected by the high temperature dust collector (6) and the red mud enter the mixing and pulverizing device and are then sent into the right oxidation furnace (3) by tertiary air.

5. A dual-furnace series boiler system for implementing the method of claim 1, characterized in that, include: Left reduction furnace (1), cyclone separator (2), right oxidation furnace (3), economizer (4), air preheater (5), high temperature dust collector (6), bottom crushing and magnetic separation device (7), and mixing and pulverizing device (8); The cross-sections of the left reduction furnace (1) and the right oxidation furnace (3) are circular, square, or polygonal. Each air inlet adopts a direct air intake, tangential air intake, or a combination of air intake methods; Fluctuation-inducing components can be installed inside the furnace.

6. The dual-furnace series boiler system according to claim 5, characterized in that: The left reduction furnace (1) consists of, from bottom to top, a primary air inlet (1-4), an atomizing water nozzle (1-7), a foam bed zone (1-1), a coal feed port in front of the furnace (1-6), a dense phase reduction zone (1-2), a secondary air inlet (1-5), a fixed bed zone (1-3), and an aerial red mud feed port (1-8). The upper outlet of the cyclone separator (2) is connected to the right oxidation furnace (3), and the bottom outlet is connected to the dense phase reduction zone (1-2) of the left reduction furnace (1). The right oxidation furnace (3) is provided with a tertiary air inlet (3-1) at the top. The mixing and pulverizing device (8) is connected to the ash hopper of the high-temperature dust collector (6) at its inlet and to the tertiary air inlet (3-1) at its outlet. The crushing and magnetic separation device (7) is connected to the ash discharge port of the left reduction furnace (1).

7. The dual-furnace series boiler system according to claim 5, characterized in that, The right oxidation furnace (3), economizer (4) and air preheater (5) are arranged in any of the following layouts: side by side, stacked on top of each other, or integrated.

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