A multi-chamber furnace system with controllable temperature and atmosphere

Abstract

A temperature and atmosphere controllable multi-chamber furnace system comprises a rake furnace, an oxygen supply system and a flue gas circulation system. The furnace body is divided into a preheating zone, a variable atmosphere main combustion zone and a cooling zone from top to bottom, and each zone is provided with an independent oxygen-enriched burner and monitoring instruments. Oxygen-enriched air is provided through the oxygen supply system, and the ratio of oxygen in the primary air and the secondary air to the circulating flue gas is dynamically adjusted by combining the flue gas recirculation technology, so as to accurately control the oxygen partial pressure and the temperature in each region. The system supports flexible switching of multiple atmospheres such as oxidation, neutral and reduction, and can adapt to various processes such as roasting, reduction, incineration and activated carbon regeneration. The present application has the advantages of accurate temperature gradient control, flexible atmosphere adjustment, efficient waste heat recovery, clean emission and the like, and is suitable for continuous heat treatment production.

Description

Technical Field

[0001] This invention relates to the field of industrial continuous heat treatment equipment, and in particular to a multi-hearth furnace system that combines oxygen-enriched combustion and flue gas recirculation, suitable for multi-stage roasting, reduction, carbonization or sintering processes. Background Technology

[0002] Multihearth furnaces, as a type of industrial furnace with simple structure and flexible operation, are widely used in various industries, including metal oxide reduction, metal ore roasting, activated carbon regeneration, and sludge / waste / garbage incineration. In metal oxide reduction, multihearth furnaces are used to process materials such as iron oxide, generating metals or low-valence oxides through high-temperature reduction reactions. However, challenges exist in controlling and ensuring uniformity of the high-temperature reducing atmosphere. In metal ore roasting, multihearth furnaces are used for the oxidative roasting of sulfide ores to extract metals or remove sulfur. However, precise control of the temperature gradient and oxidizing atmosphere within the furnace is difficult, often leading to incomplete roasting or excessive energy consumption. In activated carbon regeneration, multihearth furnaces restore the adsorption performance of activated carbon through high-temperature pyrolysis and gasification processes. However, uneven heating of the material and inaccurate atmosphere control result in low regeneration efficiency. In sludge / waste / garbage incineration, multihearth furnaces are used to treat waste with high moisture content or complex compositions. However, significant problems such as large flue gas emissions, low thermal efficiency, and insufficient pollutant control increase the burden of environmental governance.

[0003] In existing technologies, Chinese patent CN119979797A proposes a direct reduction method using a gasification multi-hearth furnace. This method utilizes CO and H2 as reducing agents, reacting them with metal oxides in a multi-hearth furnace to generate pure metal and reducing agent gas. This method improves gas-solid contact efficiency, enhancing the uniformity and efficiency of the reduction reaction. Compared to hydrogen-based vertical shaft furnaces, it offers advantages such as energy saving, simple operation, and cleaner production. However, this technology does not address the issues of uneven gas distribution, inaccurate temperature control potentially leading to localized overheating or incomplete reduction, affecting product quality, or the potential fire or explosion risks posed by high concentrations of H2 and CO operating in a high-temperature multi-layer furnace. Chinese patent CN118291752A proposes a multi-hearth furnace roasting device, which improves roasting efficiency and the uniformity of the oxidation reaction by optimizing the furnace structure and airflow distribution. This device employs pneumatic conveying technology, solving the problems of excessively high molybdenum concentrate concentration and insufficient roasting. However, its potential problems include high dust generation and high energy consumption. Excessive dust can affect product quality and increase the load on dust removal systems, while high energy consumption reduces the economic efficiency of the process. Chinese patent CN116146987A proposes a sludge incineration system that separates high-carbon and low-carbon components in sludge using gravity separation equipment, which are then burned separately in fluidized beds and cyclones to improve incineration efficiency and heat recovery. However, the complex equipment introduced by this system significantly increases production costs. The high manufacturing, installation, and maintenance costs of the gravity separation equipment, fluidized beds, and cyclones may limit its application in small and medium-sized enterprises. Furthermore, the coordinated operation of multiple devices increases operational complexity and may lead to decreased operational stability, particularly posing technical challenges in multi-device control and maintenance. The feasibility of practical industrial implementation requires further verification. Chinese patent CN114191892A proposes a multi-hearth furnace activated carbon regeneration system. Its main advantage lies in the annular secondary combustion chamber encasing the furnace body on the outer wall. This effectively treats unburned components (such as CO and volatile organic compounds) in the flue gas through secondary combustion, improving flue gas cleanliness and reducing pollutant emissions. Furthermore, the annular structure enhances the furnace's insulation performance, reducing heat loss and improving thermal efficiency. However, under high-temperature conditions, the annular combustion chamber may face material durability issues, such as thermal stress concentration or corrosion, leading to increased maintenance frequency. The control system needs to precisely adjust the air-fuel ratio in the secondary combustion chamber to ensure complete combustion and low emissions, which increases operational complexity and reliance on advanced control systems, potentially affecting process stability.

[0004] The aforementioned technical problems reflect common bottlenecks in the application of multi-hearth furnaces across various industries: First, temperature and atmosphere are difficult to control precisely, leading to uneven heating of materials, low process efficiency, or unstable product quality; second, the large volume of flue gas to be treated means that traditional flue gas treatment systems fail to effectively recover waste heat or reduce emissions, increasing energy consumption and environmental pressure. These problems limit the widespread adoption of multi-hearth furnaces across industries. Summary of the Invention

[0005] To address the aforementioned problems, this invention proposes a multi-hearth furnace system with controllable temperature and atmosphere. The system utilizes the inherent characteristics of segmented combustion within the multi-hearth furnace and the staged combustion of oxygen-enriched combustion to achieve atmospheric changes within the furnace. An oxygen-enriched combustion zone is added in the middle of the furnace body, and each layer is equipped with an independent flue gas recirculation pipe, effectively solving the problems of difficult temperature and atmosphere control and large flue gas volume in multi-hearth furnace applications.

[0006] This invention is achieved through the following technical solutions.

[0007] The present invention discloses a multi-hearth furnace system with controllable temperature and atmosphere, comprising a rake furnace, an oxygen supply system, and a flue gas recirculation (FGR) system.

[0008] The furnace body of the rake furnace is divided into a feeding zone and a preheating zone, a variable atmosphere main combustion zone, a cooling zone, and a discharge zone from top to bottom.

[0009] The preheating zone, variable atmosphere main combustion zone, and cooling zone are each uniformly equipped with oxygen-enriched burners of the same structure on the side walls of the furnace body. A flue gas analyzer I and a thermocouple I are installed at the outlet of the preheating zone; a flue gas analyzer II and a thermocouple II are installed in the variable atmosphere main combustion zone; a flue gas analyzer III and a thermocouple III are installed in the cooling zone; and a flame detector is also installed in the variable atmosphere main combustion zone.

[0010] Each of the aforementioned oxygen-enriched burners is connected to a gas pipe channel, a coaxial annular primary air channel, and an outer annular secondary air channel. The gas pipe channel is equipped with a gas flow valve and a gas flow meter; the coaxial annular primary air channel is equipped with a primary air flow meter and a primary air valve; and the outer annular secondary air channel is equipped with a secondary air flow meter and a secondary air valve.

[0011] The primary air duct is connected to the pressure reducing valve of the oxygen supply system through an oxygen flow meter and an oxygen valve, and the oxygen flow meter and oxygen valve control the oxygen flow rate of the primary air in the oxygen-enriched burner; and is connected to the fan of the flue gas circulation system through an FGR flue gas flow meter and an FGR flue gas flow valve, and the FGR flue gas flow rate of the primary air in the oxygen-enriched burner is controlled by the FGR flue gas flow meter and FGR flue gas flow valve.

[0012] The secondary air duct is connected to the pressure reducing valve of the oxygen supply system via an oxygen flow meter and an oxygen valve, and the oxygen flow meter and oxygen valve control the oxygen flow rate of the secondary air in the oxygen-enriched burner; it is also connected to the fan of the flue gas circulation system via an FGR flue gas flow meter and an FGR flue gas flow valve, and the FGR flue gas flow rate of the secondary air in the oxygen-enriched burner is controlled by the FGR flue gas flow meter and FGR flue gas flow valve.

[0013] The flue gas recirculation system includes a secondary combustion chamber, an economizer, a bag filter, a plate heat exchanger, and an FGR fan; these components are connected sequentially. A pressure sensor, a flue gas analyzer I, and a thermocouple I are installed at the front end of the secondary combustion chamber. The return flue gas from the plate heat exchanger is cooled to 150–300°C before being delivered to the FGR fan.

[0014] A CO2 channel is also provided at the bottom of the rake furnace. The CO2 channel is connected to the CO2 source through a CO2 flow valve and a CO2 flow meter. The CO2 flow valve and CO2 flow meter control the CO2 flow.

[0015] A flue gas analyzer I, thermocouple I, and pressure sensor are installed at the upper flue of the preheating zone. A flue gas analyzer II, thermocouple II, and flame detector are installed at the top flue of the variable atmosphere main combustion zone. A flue gas analyzer III and thermocouple III are installed before the flue gas collection pipe in the cooling section. The thermocouples are selected as type S thermocouples, with a temperature resistance of over 400°C, and the pressure sensor is a differential pressure sensor with a range of 0–1000 Pa.

[0016] Flue gas analyzer I is used to analyze O2, CO2, and H2O; flue gas analyzer II is used to analyze O2, CO, CO2, H2, and NO. x Flue gas analyzer III is used to analyze O2.

[0017] The working process of the multi-hearth furnace system with controllable temperature and atmosphere described in this invention is as follows:

[0018] The material enters the preheating zone of the furnace from the feeding zone, where it comes into full contact with the counter-current flue gas. Oxygen-enriched burners are evenly distributed along the sidewalls of the preheating zone. The material is then pushed by the rake arm system into the variable atmosphere main combustion zone, where oxygen-enriched burners are also evenly distributed along the sidewalls, creating oxidizing, neutral, and reducing combustion environments within the furnace. The material continues to be pushed by the rake arm system into the cooling zone, where oxygen-enriched burners are also evenly distributed along the sidewalls. Under the purging of cooling flue gas from the flue gas recirculation system, the furnace temperature drops to a specified temperature, and the combusted material falls to the bottom of the furnace and is discharged from the discharge zone. The flue gas from the preheating zone, variable atmosphere main combustion zone, and cooling zone converges at the top of the furnace into the main flue gas duct. The flue gas then sequentially enters the secondary combustion chamber, economizer, bag filter, and plate heat exchanger. The treated flue gas is then recirculated through the primary and secondary air channels of the oxygen-enriched burners in the preheating zone, variable atmosphere main combustion zone, and cooling zone via the flue gas recirculation system, dynamically adjusting the temperature and atmosphere within each zone.

[0019] The multi-hearth furnace system with controllable temperature and atmosphere described in this invention includes the following atmosphere and temperature control process:

[0020] The atmosphere and temperature inside the rake furnace are controlled by a two-stage linkage of total circulation ratio K and zoned flow distribution. First, the total amount of recirculated flue gas available for adjustment in each zone is determined by setting the overall system circulation ratio K. Then, at the zone level, flue gas is distributed from this total amount to the preheating zone, variable atmosphere main combustion zone, and cooling zone as needed through valves and flow meters to meet the local oxygen partial pressure and temperature targets of each zone. The total circulation ratio K is controlled between 0.60 and 0.80, and the proportion of recirculated flue gas in each zone, k, can be independently adjusted within the range of 0–85%. The proportion of recirculated flue gas allocated to the preheating zone (k_preheating zone), variable atmosphere main combustion zone (k_main combustion zone), and cooling zone (k_cooling zone) can be adjusted between 5–30%, 60–85%, and 0–20%, respectively.

[0021] The total cycle ratio K is:

[0022] K = G / B

[0023] Where G is the volumetric flow rate of the recirculated flue gas, and B is the total volumetric flow rate of the flue gas at the furnace outlet.

[0024] The proportion of recirculated flue gas (k) in each zone is:

[0025] k = g / G

[0026] Where g is the volumetric flow rate of the recirculated flue gas returning to the designated area, and G is the volumetric flow rate of the recirculated flue gas.

[0027] The preheating zone operates in a neutral or weakly oxidizing atmosphere, where the circulating flue gas allocated to the preheating zone accounts for 5–30% of the recirculated flue gas G (k_preheating zone); the oxygen partial pressure of the primary air is controlled at 18–25%, and the oxygen supply accounts for 0.15–0.25% of the total oxygen supply; the oxygen partial pressure of the secondary air is controlled at 25–30%. The ratio of oxygen to FGR flue gas in the secondary air is adjusted to maintain the over-oxygen coefficient λ at 1.0–1.05 and the temperature at 200–400℃, ensuring a slow temperature rise of the material and avoiding local overheating.

[0028] Under oxygen-enriched combustion conditions, the variable atmosphere main combustion zone allows for flexible switching between oxidizing, neutral, and locally reducing atmospheres by adjusting the ratio of oxygen to FGR flue gas in the secondary air. The furnace temperature is adjustable within the range of 200–1200°C. The circulating flue gas allocated to the variable atmosphere main combustion zone accounts for 60–85% of the recirculated flue gas G (k_main combustion zone). The primary air consists of a mixture of oxygen and FGR flue gas, with its oxygen partial pressure controlled at 18–25%, providing 0.15–0.25% of the total oxygen supply for mixing with the fuel gas and achieving stable combustion. The secondary air is the main oxygen supply channel, with an adjustable oxygen partial pressure of 5–45%, and mixes with FGR flue gas to achieve different atmospheres. The secondary air oxygen supply accounts for 0.75–0.85% of the total oxygen supply, and its oxygen partial pressure and flow rate are adjusted by regulating the ratio of oxygen to FGR flue gas.

[0029] In the variable atmosphere main combustion zone, under oxidizing atmosphere mode, primary air and fuel gas are mixed and combusted. The oxygen partial pressure of the primary air is controlled at 18–25%, and the oxygen supply of the primary air accounts for 0.15–0.25% of the total oxygen supply. The oxygen partial pressure of the secondary air is controlled at 30–45%. The ratio of oxygen to FGR flue gas in the secondary air is adjusted to maintain the excess oxygen coefficient λ at 1.05–1.30. The oxygen concentration at the furnace outlet is maintained at 2–5% to ensure complete fuel combustion and maintain an oxidizing atmosphere.

[0030] In the variable atmosphere main combustion zone, under neutral atmosphere mode, the primary air oxygen partial pressure is controlled at 18–25%, and the primary air oxygen supply accounts for 0.15–0.25% of the total oxygen supply; the secondary air oxygen partial pressure is controlled at 25–30%. The ratio of oxygen in the secondary air to FGR flue gas is adjusted to maintain the excess oxygen coefficient λ at 0.95–1.05, the temperature at 800–900°C, and the O2 concentration at the furnace outlet at 1–2%, in order to maintain a near-neutral combustion atmosphere.

[0031] In the variable atmosphere main combustion zone, under reducing atmosphere mode, the primary air oxygen partial pressure is controlled at 18–25%, and the primary air oxygen supply accounts for 0.15–0.25% of the total oxygen supply. The secondary air oxygen partial pressure is initially set at 15–20%, and gradually reduced to 5–10% during operation. Simultaneously, the FGR flue gas fraction in the secondary air is increased by an equal amount to maintain a constant total gas flow and stabilize the furnace pressure. The excess oxygen coefficient λ is controlled at 0.7–0.9, and the temperature of the variable atmosphere main combustion zone is maintained at 850–950°C. Preferably, by injecting excess fuel locally into the oxygen-enriched burner, the local λ in the variable atmosphere main combustion zone is set to 0.6–0.9, and the local flame temperature is set to 1100–1200°C. This promotes the gasification reaction of carbon and carbon dioxide, as well as the reaction of carbon with water vapor, thereby forming a reducing mixture containing 2–5% CO and <1% H2 within the variable atmosphere main combustion zone. At this point, the local oxygen partial pressure is controlled at 0.3–1.0%, while the overall furnace outlet atmosphere remains weakly oxidizing (O2 > 0.8%). To ensure combustion stability and system safety, an external secondary combustion chamber is connected to the top of the furnace for complete combustion of the reducing gases containing CO and H2 discharged from the variable atmosphere main combustion zone under an oxygen-enriched atmosphere, ensuring sufficient oxidation of the flue gas and safe emission.

[0032] The cooling zone uses FGR flue gas from the flue gas recirculation system as the cooling medium to cool the high-temperature materials after reaction in the variable atmosphere main combustion zone. The FGR flue gas, pressurized by the FGR fan, is sent to the bottom of the furnace through oxygen-enriched burner channels located within the cooling zone. The recirculated flue gas allocated to the cooling zone accounts for 10–15% of the recirculated flue gas G (k_cooling zone), and is injected in a low-oxygen or oxygen-free manner. It comes into counter-current contact with the high-temperature materials, achieving convective and radiative cooling. The outlet flue gas temperature of the cooling zone is controlled at 100–200℃.

[0033] The temperature and atmosphere regulation modes in the preheating zone and the variable atmosphere main combustion zone under neutral or oxidizing conditions follow the following principle: Firstly, the ratio of oxygen to FGR flue gas in the secondary air is changed to increase or decrease the oxygen partial pressure in the secondary air. When adjusting the oxygen partial pressure in the secondary air alone fails to achieve the desired temperature, or when flame stability decreases, temperature control is further achieved by adjusting the FGR flue gas ratio allocated to that zone. When heating is required, the recirculation flow is reduced; when cooling is required, the recirculation flow is increased, thus regulating the flame temperature through the gas specific heat absorption effect. When the k_main combustion zone is increased to the set upper limit (exceeding 70–85%) and the temperature is still higher than the target value, that is, when any temperature measurement point exceeds 1000℃ for 30 consecutive seconds or the heating rate is greater than 15℃ / min, the total circulation ratio K of the system is gradually increased within the capacity range of the FGR fan to further increase the dilution and heat absorption of the cold flue gas; if the total circulation ratio K has reached the upper limit and is still insufficient, the inert gas (CO2) passage is opened, and the oxygen flow in the secondary air is reduced by the same amount. Under the premise of keeping the total flow fluctuation of the system less than ±5%, the waste heat is further diluted and absorbed to achieve rapid cooling.

[0034] The temperature and atmosphere regulation mode in the reduction mode follows these principles: First, the temperature and reducing intensity of the variable atmosphere main combustion zone are rapidly adjusted by adjusting the local rich fuel injection quantity; after the rich fuel adjustment, the proportion of FGR flue gas allocated to this zone (k_main combustion zone) or the oxygen partial pressure in the secondary air is reduced as a fine adjustment means; in the cooling mode, the opposite is true, first reduce the rich fuel injection, and then dilute and absorb heat by increasing the proportion of FGR flue gas allocated to this zone (k_main combustion zone) and reducing the oxygen partial pressure in the secondary air mixture; the entire process should be based on a safe closed loop of CO, H2 and outlet O2, and when CO or H2 exceeds the limit or outlet O2 exceeds the safe range, a retreat strategy should be implemented immediately. First, the inert gas CO2 passage is opened, and CO2 is rapidly introduced into the furnace. While maintaining a relatively constant total system flow, this quickly dilutes the concentration of combustible gases in the furnace, inhibiting further accumulation of CO and H2 and reducing the risk of explosion. Simultaneously, neutral or lightly oxidizing conditions are restored by reducing or cutting off the supply from the rich fuel nozzle and moderately increasing the secondary air oxygen partial pressure by 2-5%. If the CO and H2 concentrations still exceed the limits, the CO2 flow rate is further increased until it completely replaces the oxygen in the secondary air, achieving inert protection until the system stabilizes or is safely shut down. This ensures combustion stability and safety, preventing runaway combustion or equipment damage caused by a reducing atmosphere. The cooling zone allows for independent control of the amount of cooling flue gas returning to this zone, controlling the rate of temperature increase or decrease.

[0035] The oxygen supplied by the oxygen supply system is a mixture of Ar and O2, wherein the volume fraction of O2 is not less than 95%.

[0036] The beneficial effects of this invention are as follows:

[0037] (1) Strong compatibility with multiple processes: One furnace body can switch between multiple processes such as activated carbon regeneration, waste incineration, and ore roasting, reducing equipment investment.

[0038] (2) Temperature gradient is precisely controllable: the temperatures of the preheating zone, the variable atmosphere main combustion zone and the cooling zone are independently adjustable to meet the segmented temperature requirements of different processes.

[0039] (3) Flexible atmosphere switching: The oxygen and circulating flue gas channels in the oxygen-enriched burner are jointly regulated to support dynamic adjustment of oxidizing, reducing, neutral and micro-oxidizing atmospheres.

[0040] (4) High-efficiency waste heat utilization: The circulating flue gas is reused in the cooling zone to realize energy recovery and reduce fuel consumption.

[0041] (5) Environmentally friendly: NO is suppressed through flue gas recirculation and oxygen-enriched combustion. x It generates and reduces flue gas emissions, meeting environmental emission standards. Attached Figure Description

[0042] Figure 1This is a schematic diagram of a multi-hearth furnace system with controllable temperature and atmosphere according to the present invention.

[0043] Among them, 1 is the oxygen supply system, 1-1 is the pressure reducing valve, 2 is the rake arm system, 3 is the feeding zone, 4 is the preheating zone, 5 is the variable atmosphere main combustion zone, 6 is the cooling zone, 7 is the discharge zone, 8 is the oxygen-enriched burner in the preheating zone, 9 is the oxygen-enriched burner in the variable atmosphere main combustion zone, and 10 is the oxygen-enriched burner in the cooling zone; 81 is the gas pipeline channel in the preheating zone, 82 is the primary air channel in the preheating zone, 83 is the secondary air channel in the preheating zone, 81-1 is the gas flow valve I, 81-2 is the gas flow meter I, 82-1 is the primary air oxygen flow valve I, 82-2 is the primary air oxygen flow meter I, 83-1 is the secondary air oxygen flow valve I, and 83-2 is the secondary air oxygen flow meter I. 2-3 is primary air FGR flue gas flow valve I; 82-4 is primary air FGR flue gas flow meter I; 83-3 is secondary air FGR flue gas flow valve I; 83-4 is secondary air FGR flue gas flow meter I; 82-5 is primary air flow meter I; 82-6 is primary air valve I; 83-5 is secondary air flow meter I; 83-6 is secondary air valve I; 91 is the gas pipe passage for the variable atmosphere main combustion zone; 92 is the primary air passage for the variable atmosphere main combustion zone; 93 is the secondary air passage for the variable atmosphere main combustion zone; 91-1 is gas flow valve II; 91-2 is gas flow meter II; 92-1 is primary air oxygen flow valve II; 92-2 is primary air oxygen flow valve II. Gas flow meter II; 93-1 is secondary air oxygen flow valve II; 93-2 is secondary air oxygen flow meter II; 92-3 is primary air FGR flue gas flow valve II; 92-4 is primary air FGR flue gas flow meter II; 93-3 is secondary air FGR flue gas flow valve II; 93-4 is secondary air FGR flue gas flow meter II; 92-5 is primary air flow meter II; 92-6 is primary air valve II; 93-5 is secondary air flow meter II; 93-6 is secondary air valve II; 1001 is cooling zone gas pipe passage; 1002 is cooling zone primary air passage; 1003 is cooling zone secondary air passage; 1001-1 is gas flow valve III; 100 1-2 is a gas flow meter III; 1002-1 is a primary air oxygen flow valve III; 1002-2 is a primary air oxygen flow meter III; 1003-1 is a secondary air oxygen flow valve III; 1003-2 is a secondary air oxygen flow meter III; 1002-3 is a primary air FGR flue gas flow valve III; 1002-4 is a primary air FGR flue gas flow meter III; 1003-3 is a secondary air FGR flue gas flow valve III; 1003-4 is a secondary air FGR flue gas flow meter III; 1002-5 is a primary air flow meter III; 1002-6 is a primary air valve III; 1003-5 is a secondary air flow meter III; 1003-6 is a secondary air valve III.11 is the CO2 channel; 11-1 is the CO2 flow valve; 11-2 is the CO2 flow meter; 12 is the secondary combustion chamber; 13 is the economizer; 14 is the bag filter; 15 is the plate heat exchanger; 16 is the FGR fan; 17-1 is the flue gas analyzer I; 17-2 is the flue gas analyzer II; 17-3 is the flue gas analyzer III; 18-1 is the thermocouple I; 18-2 is the thermocouple II; 18-3 is the thermocouple III; 19 is the pressure sensor; 20 is the flame detector. Detailed Implementation

[0044] The present invention will be further described in conjunction with the accompanying drawings and through the following embodiments.

[0045] The embodiments described below are merely some, not all, of the embodiments of the present invention. The specific embodiments described herein are only for explaining the present invention and are not intended to limit the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0046] like Figure 1 As shown in the figure, the multi-hearth furnace system with controllable temperature and atmosphere described in this embodiment includes a rake furnace, an oxygen supply system 1, and a flue gas circulation system.

[0047] The furnace body of the rake furnace is divided into a feeding zone 3 and a preheating zone 4, a variable atmosphere main combustion zone 5 and a cooling zone 6 and a discharge zone 7 from top to bottom.

[0048] The preheating zone 4, the variable atmosphere main combustion zone 5, and the cooling zone 6 are each uniformly equipped with oxygen-enriched burners 8, 9, and 10 of the same structure on the side walls of the furnace body. The preheating zone 4 is equipped with a flue gas analyzer I 17-1 and a thermocouple I 18-1; the variable atmosphere main combustion zone 5 is equipped with a flue gas analyzer II 17-2 and a thermocouple II 18-2; the cooling zone 6 is equipped with a flue gas analyzer III 17-3 and a thermocouple III 18-3; and the variable atmosphere main combustion zone 5 is also equipped with a flame detector 20.

[0049] The preheating zone oxygen-enriched burner 8 is connected to a gas pipe channel 81, a coaxial annular primary air channel 82, and an outer annular secondary air channel 83. The gas pipe channel 81 is equipped with a gas flow valve I81-1 and a gas flow meter I81-2; the coaxial annular primary air channel 82 is equipped with a primary air flow meter I82-5 and a primary air valve I82-6; and the outer annular secondary air channel I83 is equipped with a secondary air flow meter I83-5 and a secondary air valve I83-6.

[0050] The primary air duct 82 is connected to the pressure reducing valve 1-1 of the oxygen supply system 1 via oxygen flow meter Ⅰ82-2 and oxygen valve Ⅰ82-1, and the oxygen flow meter Ⅰ82-2 and oxygen valve Ⅰ82-1 control the oxygen flow rate of the primary air in the oxygen-enriched burner Ⅰ8; and is connected to the fan 16 of the flue gas circulation system via FGR flue gas flow meter Ⅰ82-4 and FGR flue gas flow valve Ⅰ82-3, and the FGR flue gas flow meter Ⅰ82-4 and FGR flue gas flow valve Ⅰ82-3 control the FGR flue gas flow rate of the primary air in the oxygen-enriched burner 8;

[0051] The secondary air duct 83 is connected to the pressure reducing valve 1-1 of the oxygen supply system 1 via oxygen flow meter Ⅰ83-2 and oxygen valve Ⅰ83-1, and the oxygen flow meter Ⅰ83-2 and oxygen valve Ⅰ83-1 control the oxygen flow rate of the secondary air in the oxygen-enriched burner 8; and is connected to the fan 16 of the flue gas circulation system via FGR flue gas flow meter Ⅰ83-4 and FGR flue gas flow valve Ⅰ83-3, and the FGR flue gas flow meter Ⅰ83-4 and FGR flue gas flow valve Ⅰ83-3 control the FGR flue gas flow rate of the secondary air in the oxygen-enriched burner 8.

[0052] The oxygen-enriched burner 9 is connected to a gas pipe channel 91, a coaxial annular primary air channel 92, and an outer annular secondary air channel 93. The gas pipe channel 91 is equipped with a gas flow valve II 91-1 and a gas flow meter II 91-2; the coaxial annular primary air channel 92 is equipped with a primary air flow meter II 92-5 and a primary air valve II 92-6; and the outer annular secondary air channel 93 is equipped with a secondary air flow meter II 93-5 and a secondary air valve II 93-6.

[0053] The primary air duct 92 is connected to the pressure reducing valve 1-1 of the oxygen supply system 1 via oxygen flow meter II 92-2 and oxygen valve II 92-1, and the oxygen flow meter II 92-2 and oxygen valve II 92-1 control the oxygen flow rate of the primary air in the oxygen-enriched burner 9; and is connected to the fan 16 of the flue gas circulation system via FGR flue gas flow meter II 92-4 and FGR flue gas flow valve II 92-3, and the FGR flue gas flow meter II 92-4 and FGR flue gas flow valve II 92-3 control the FGR flue gas flow rate of the primary air in the oxygen-enriched burner 9;

[0054] The secondary air duct 93 is connected to the pressure reducing valve 1-1 of the oxygen supply system 1 via oxygen flow meter Ⅱ93-2 and oxygen valve Ⅱ93-1. The oxygen flow meter Ⅱ93-2 and oxygen valve Ⅱ93-1 control the oxygen flow rate of the secondary air in the oxygen-enriched burner 9. It is also connected to the fan 16 of the flue gas circulation system via FGR flue gas flow meter Ⅱ93-4 and FGR flue gas flow valve Ⅱ93-3. The FGR flue gas flow meter Ⅱ93-4 and FGR flue gas flow valve Ⅱ93-3 control the FGR flue gas flow rate of the secondary air in the oxygen-enriched burner 9.

[0055] The oxygen-enriched burner 10 is connected to a gas pipe channel 1001, a coaxial annular primary air channel 1002, and an outer annular secondary air channel 1003. The gas pipe channel 1001 is equipped with a gas flow valve III1001-1 and a gas flow meter III1001-2; the coaxial annular primary air channel 1002 is equipped with a primary air flow meter III1002-5 and a primary air valve III1002-6; the outer annular secondary air channel 1003 is equipped with a secondary air flow meter III1003-5 and a secondary air valve III1003-6.

[0056] The primary air duct 1002 is connected to the pressure reducing valve 1-1 of the oxygen supply system 1 via oxygen flow meter Ⅲ1002-2 and oxygen valve Ⅲ1002-1. The oxygen flow meter Ⅲ1002-2 and oxygen valve Ⅲ1002-1 control the oxygen flow rate of the primary air in the oxygen-enriched burner 10. It is also connected to the fan 16 of the flue gas circulation system via FGR flue gas flow meter Ⅲ1002-4 and FGR flue gas flow valve Ⅲ1002-3. The FGR flue gas flow meter Ⅲ1002-4 and FGR flue gas flow valve Ⅲ1002-3 control the FGR flue gas flow rate of the primary air in the oxygen-enriched burner 10.

[0057] The secondary air duct 1003 is connected to the pressure reducing valve 1-1 of the oxygen supply system 1 via oxygen flow meter Ⅲ1003-2 and oxygen valve Ⅲ1003-1. The oxygen flow rate of the secondary air in the oxygen-enriched burner 10 is controlled by oxygen flow meter Ⅲ1003-2 and oxygen valve Ⅲ1003-1. It is also connected to the fan 16 of the flue gas circulation system via FGR flue gas flow meter Ⅲ1003-4 and FGR flue gas flow valve Ⅲ1003-3. The FGR flue gas flow rate of the secondary air in the oxygen-enriched burner 10 is controlled by FGR flue gas flow meter Ⅲ1003-4 and FGR flue gas flow valve Ⅲ1003-3.

[0058] The flue gas recirculation system includes a secondary combustion chamber 12, an economizer 13, a bag filter 14, a plate heat exchanger 15, and an FGR fan 16; the secondary combustion chamber 12, economizer 13, bag filter 14, plate heat exchanger 15, and FGR fan 16 are connected in sequence. At the front end of the secondary combustion chamber 12, a pressure sensor 19, a flue gas analyzer I 17-1, and a thermocouple I 18-1 are installed. The return flue gas from the plate heat exchanger 15 is cooled to 150–300°C before being delivered to the FGR fan 16.

[0059] A CO2 channel 11 is also provided at the bottom of the rake furnace. The CO2 channel 11 is connected to the CO2 source through a CO2 flow valve 11-1 and a CO2 flow meter 11-2. The CO2 flow valve 11-1 and the CO2 flow meter 11-2 control the CO2 flow.

[0060] The material enters the preheating zone 4 from the feeding zone 3, where it comes into full contact with the counter-current flue gas. Oxygen-enriched burners 8 are evenly distributed along the sidewalls of the preheating zone. The material is then pushed by the rake arm system 2 into the variable atmosphere main combustion zone 5, where oxygen-enriched burners 9 are evenly distributed along the sidewalls, creating oxidizing, neutral, and reducing combustion environments within the furnace. The material continues to be pushed by the rake arm system 2 into the cooling zone 6, where oxygen-enriched burners 10 are also evenly distributed along the sidewalls. After being purged by the cooling flue gas in the flue gas circulation system, the furnace temperature drops to a specified temperature, and the burned material falls to the bottom of the furnace and is discharged from the discharge zone 7. The flue gas from the preheating zone 4, the variable atmosphere main combustion zone 5, and the cooling zone 6 converges at the top of the furnace into the main flue gas duct. The flue gas then sequentially enters the secondary combustion chamber 12, the economizer 13, the bag filter 14, and the plate heat exchanger 15. The treated flue gas is circulated through the primary air passage 92 and secondary air passage 93 in the oxygen-enriched burners of the preheating zone 4, the variable atmosphere main combustion zone 5, and the cooling zone 6 via the flue gas circulation system, thereby dynamically adjusting the temperature and atmosphere within the zones.

[0061] Example 1

[0062] Municipal solid waste incineration.

[0063] This embodiment is designed for municipal solid waste with high moisture content and low calorific value (such as wet kitchen waste, wet paper towels, and fruit and vegetable waste). It uses natural gas as fuel and employs oxygen-enriched combustion and flue gas recirculation (FGR) technology to regulate the furnace temperature and atmosphere. The furnace body, from top to bottom, consists of a preheating zone, a variable atmosphere main combustion zone, and a cooling zone. Each zone has an independent oxygen-enriched burner, allowing for separate control of oxygen supply and circulating flue gas flow. The total recirculation ratio K is 0.65, and the proportion of circulating flue gas allocated to each zone is k_preheating zone = 15%, k_main combustion zone = 75%, and k_cooling zone = 10%. This parameter design ensures a stable airflow channel and appropriate flue gas volume within the furnace, achieving multi-zone temperature stratification and atmosphere isolation.

[0064] During the ignition and heating stages, the FGR fan 16 is first activated to form a basic flue gas passage, maintaining a slight positive pressure inside the furnace. During the initial heating stage, due to the material's high moisture content requiring significant drying, the FGR flue gas allocated to the preheating zone (k_preheating zone) is temporarily increased to 30%. The oxygen-enriched burner 8 in the preheating zone is ignited. Its primary air mixture consists of oxygen and circulating flue gas, with an oxygen partial pressure of 21%, and the primary air supply accounts for 20% of the total oxygen supply. The secondary air mixture has an oxygen partial pressure of 25%, and the FGR flue gas accounts for 0.70–0.80% of the circulating flue gas in the preheating zone (k_preheating zone). By adjusting the secondary air oxygen partial pressure and the ratio of circulating flue gas, the excess oxygen coefficient λ is stabilized at 1.0–1.05 to maintain a neutral to weakly oxidizing atmosphere and prevent localized overheating or accumulation of unburned components. After ignition, the flame gradually stabilizes and the temperature rises slowly. The temperature in the preheating zone can reach 250°C in about 30 minutes, providing a stable heat source for subsequent combustion in the variable atmosphere main combustion zone.

[0065] After combustion stabilizes in the preheating zone, the oxygen-enriched burner 9 in the variable atmosphere main combustion zone is started. The oxygen partial pressure of the primary air mixture in the variable atmosphere main combustion zone is 21%, and the oxygen supply in the primary air accounts for 20% of the total oxygen supply; the oxygen partial pressure of the secondary air is 30%, and the proportion of FGR flue gas in the circulating flue gas of the variable atmosphere main combustion zone (k_main combustion zone) is 0.65–0.75. The oxygen-enriched burner 10 in the cooling zone is not ignited temporarily; only the circulating flue gas after heat exchange is introduced to create a low-temperature airflow channel at the cold end.

[0066] When thermocouple II18-2 in the variable atmosphere main combustion zone detects a temperature exceeding 850°C, the feeding system 3 is activated, continuously feeding waste into the upper layer of the furnace. The proportion of circulating flue gas allocated to each zone returns to its default value: 15% in the preheating zone, 75% in the variable atmosphere main combustion zone, and 10% in the cooling zone. The material moves downwards under the push of the rake arms of the rake furnace 2, sequentially passing through the drying and evaporation in the preheating zone, the strong oxidizing combustion in the variable atmosphere main combustion zone, and the cooling in the cooling zone. The furnace temperature gradually decreases along the height, maintaining approximately 350°C, 900°C, and 180°C respectively.

[0067] After stable operation, the temperature of the variable atmosphere main combustion zone is maintained within the range of 850–950°C. If the furnace temperature monitored by thermocouple II18-2 deviates from the set temperature, the total secondary air flow ratio is kept constant, and the oxygen and FGR flue gas flow ratio in the secondary air of the oxygen-enriched burner 9 is adjusted first. If the furnace temperature is higher than the set value of 950°C, the opening of the FGR flue gas flow valve II93-3 is increased first, while the secondary air oxygen flow valve II93-1 is moderately reduced to decrease the oxygen flow in the secondary air, thereby reducing the local oxygen supply intensity and lowering the flame temperature by reducing the combustion rate. If the temperature is still higher than the target value, the k_main combustion zone can be temporarily increased to increase the FGR flue gas in the variable atmosphere main combustion zone for dilution and cooling. After the furnace temperature returns to the set value, the default FGR flue gas circulation ratio is restored in each zone of the system. Under stable combustion conditions, the O2 concentration monitored by the outlet flue gas analyzer II17-2 should be maintained at 2–5%, and the CO concentration should be lower than 300 ppm. If the outlet O2 concentration is higher than 5%, it indicates that combustion is biased towards oxidation. The opening of the secondary air oxygen flow valve II93-1 can be appropriately reduced by 3-5%, or the FGR flue gas flow valve II93-3 can be increased to increase the circulating flue gas flow in the secondary air. If the outlet CO concentration increases, it indicates incomplete combustion in some areas. The secondary air oxygen partial pressure should be slightly increased by 2-3% until the flame color returns to a stable blue-yellow. The oxidizing atmosphere and temperature level are regulated by the flow ratio of oxygen to FGR flue gas in the secondary air channel, and the excess oxygen coefficient λ in this area is maintained at 1.10-1.15. The furnace outlet oxygen concentration is stable at 3-5%, ensuring complete combustion and clean exhaust. When the overall system temperature is too high or the local oxygen partial pressure continues to rise, i.e., when the above adjustments are ineffective, the FGR fan speed 16 should be increased to gradually increase the total system circulation ratio K, such as from 0.65 to 0.70, to enhance dilution and overall temperature regulation capabilities, thereby ensuring combustion stability and compliance with flue gas emission standards.

[0068] The operational results of this embodiment show that the oxygen concentration at the furnace outlet monitored by flue gas analyzer I17-1 remains at 3–5%, the CO content is below 50 ppm, and the ash burnout rate reaches over 99%. The furnace temperature fluctuation range is controlled within ±20°C, indicating stable combustion, high thermal efficiency, and clean flue gas. This oxidation condition is particularly suitable for the stable combustion and waste heat utilization of municipal solid waste with high moisture content, and can operate continuously while achieving energy recovery.

[0069] Example 2

[0070] Oxidative roasting of tungsten ore.

[0071] This embodiment is used for the oxidative roasting process of tungsten concentrate, using natural gas as fuel. Oxygen-enriched combustion and flue gas recirculation are employed to achieve zoned furnace temperature control and efficient oxidation of sulfides. The tungsten concentrate includes raw materials such as wolframite (FeMnWO4) or scheelite (CaWO4). The roasting objective is to oxidize the sulfides to tungsten oxide (WO3) and remove impurities. The furnace body is sequentially equipped with a preheating zone, a variable atmosphere main combustion zone, and a cooling zone. Each zone is equipped with an independent oxygen-enriched burner to maintain the corresponding oxidizing atmosphere and temperature gradient. The total recirculation ratio K is set to 0.60, and the proportion of recirculated flue gas allocated to each zone is k_preheating zone = 15%, k_main combustion zone = 70%, and k_cooling zone = 15%.

[0072] During the ignition and heating stages, the FGR fan 16 is first activated to form a basic flue gas passage, maintaining a slight positive pressure inside the furnace. In the initial heating stage, due to the material's high moisture content requiring significant drying, the proportion of FGR flue gas allocated to the preheating zone (k_preheating zone) is temporarily increased to 20% to accelerate moisture evaporation and furnace preheating. Subsequently, the oxygen-enriched burner 8 in the preheating zone is activated. Its primary air mixture consists of oxygen and circulating flue gas, with an oxygen partial pressure of 21%, and the primary air supply accounts for 20% of the total oxygen supply. The secondary air mixture has an oxygen partial pressure of 25%, and the FGR flue gas accounts for 0.65–0.8% of the circulating flue gas in the variable atmosphere main combustion zone (k_main combustion zone). By adjusting the secondary air oxygen partial pressure and the circulating flue gas ratio, the excess oxygen coefficient λ is stabilized at 1.0–1.05, maintaining a neutral to weakly oxidizing atmosphere in the preheating zone and preventing localized overheating or accumulation of unburned components. After ignition, the flame gradually stabilizes and the temperature rises slowly. The preheating zone temperature reaches 250°C in about 30 minutes, providing a stable heat source for subsequent combustion in the variable atmosphere main combustion zone. The uniform furnace wall temperature and stable heat flow in the preheating zone indicate that the system is ready for ore feeding. The circulating flue gas ratios in each zone are restored to default values, and then the oxygen-enriched burner 9 in the variable atmosphere main combustion zone is started. The primary air mixture consists of oxygen and FGR flue gas, with an oxygen partial pressure of 21% and an oxygen supply accounting for 20% of the total oxygen supply. The secondary air oxygen partial pressure is 30%, and the FGR flue gas in the secondary air accounts for 0.70–0.80% of the circulating flue gas in the variable atmosphere main combustion zone (k_main combustion zone), with the excess oxygen coefficient λ controlled at around 1.10. The temperature in the variable atmosphere main combustion zone then rises to 700–800°C, forming a stable oxidation zone.

[0073] Once the furnace temperature stabilizes, dried tungsten concentrate is continuously fed in. The proportions of circulating flue gas allocated to each zone are restored to their default values: 15% in the preheating zone, 70% in the variable atmosphere main combustion zone, and 15% in the cooling zone. The ore passes through three sections sequentially from the top: the preheating zone is used for moisture evaporation and surface oxidation reactions, with the temperature maintained at 250–400°C; the variable atmosphere main combustion zone is the main reaction zone, with the temperature maintained at 800–900°C, where the oxidizing atmosphere promotes the oxidation of sulfides to SO2 and its release; the cooling zone temperature is controlled below 200°C to prevent product re-oxidation and agglomeration. At this point, the oxygen concentration at the furnace outlet stabilizes at 2–4%, and the SO2 concentration is approximately 1000–1500 ppm, indicating complete sulfide oxidation.

[0074] After stable operation, the temperature of the variable atmosphere main combustion zone is maintained within the range of 700–900°C. When the temperature of the variable atmosphere main combustion zone exceeds 900°C, the opening of the secondary air FGR flue gas flow valve II93-3 is adjusted first, and the k_main combustion zone is appropriately increased by 10–15% to utilize the specific heat absorption effect of the flue gas to reduce the flame temperature. When the temperature is below 800°C, the circulating flue gas flow is reduced by about 5% to enhance the combustion reaction and increase the temperature. If adjusting the recirculation flue gas ratio alone cannot stabilize the atmosphere, the oxygen flow valve II93-1 in the secondary air is adjusted by changing the flow ratio of oxygen in the secondary air to the circulating flue gas, and the oxygen partial pressure in the secondary air is appropriately reduced by 2–3%, or the oxygen partial pressure is increased by about 3% when the temperature is low, to maintain a stable oxidizing atmosphere in the furnace. If the O2 concentration at the furnace outlet is higher than 5%, it indicates excessive oxygen supply. The oxygen partial pressure in the secondary air should be reduced by 3–5% first, and the recirculation flow should be kept constant to prevent over-oxygen combustion from causing temperature overshoot. In the variable atmosphere main combustion zone, the recirculated flue gas flow rate accounts for 65–75% of the total recirculated flow rate, which can ensure sufficient oxidation of the material while controlling combustion temperature fluctuations within ±20°C. The cooling zone is not ignited; only recirculated flue gas cooled by heat exchange is introduced, with a recirculation ratio of approximately 10–20% of the total recirculated flow rate. This recirculated gas absorbs residual heat and lowers the material temperature to below 200°C, thus completing the cooling stage after roasting.

[0075] After stable operation, the oxidation reaction was complete, resulting in a WO3 content greater than 99% and a sulfur content less than 0.02%. The roasted products were uniform in color and loosely granulated, without sintering or pulverization. The SO2 concentration in the flue gas was stable, indicating a reasonable distribution of the oxidizing atmosphere. Throughout the operation, the atmosphere in each zone was coupled and controlled through primary air combustion stabilization, secondary air oxygen regulation, and FGR flue gas cooling. No complex automatic system was required; stable operation was achieved solely through flow meter and valve adjustments.

[0076] Example 3

[0077] Activated carbon regeneration.

[0078] This embodiment is used for the regeneration of waste activated carbon. The regeneration process creates a localized reducing atmosphere in the center of the furnace to promote the decomposition and removal of adsorbed organic impurities, while preventing excessive oxidation of the carbon surface. The furnace body is divided into a preheating zone, a variable atmosphere main combustion zone, and a cooling zone, all equipped with oxygen-enriched burners. The total circulation ratio K is 0.75, and the proportion of circulating flue gas allocated to each zone is k_preheating zone = 15%, k_main combustion zone = 75%, and k_cooling zone = 10%.

[0079] During the ignition and heating phase, the FGR fan 16 is first activated to form a basic flue gas passage, maintaining a slight positive pressure inside the furnace, with k_preheating zone at 15%. The oxygen-enriched burner 8 in the preheating zone is ignited. Its primary air mixture consists of oxygen and circulating flue gas, with an oxygen partial pressure of 21% and the primary air supply accounting for 20% of the total oxygen supply. The secondary air mixture has an oxygen partial pressure of 25%, and the circulating flue gas fraction accounts for 0.65–0.8 of the total circulating flue gas in the variable atmosphere main combustion zone. By adjusting the secondary air oxygen partial pressure and the ratio of circulating flue gas, the excess oxygen coefficient λ is stabilized at 1.0–1.05. This ensures the flame temperature in the preheating zone remains stable at approximately 300°C.

[0080] Once the temperature and atmosphere in the preheating zone stabilize, the oxygen-enriched burner 9 in the variable atmosphere main combustion zone is activated, entering the core stage of activated carbon regeneration. The total system circulation ratio K is maintained at 0.75, with k_main combustion zone at 75%, to provide stable dilution and temperature control capabilities. This stage operates in a reducing atmosphere mode, aiming to achieve gentle oxidation of the activated carbon surface and regeneration of its internal structure by controlling the local oxygen partial pressure and the proportion of circulating flue gas. The primary air oxygen partial pressure of the oxygen-enriched burner 9 in the variable atmosphere main combustion zone is 21%, supplying 20% ​​of the total system oxygen supply, used to stabilize combustion and maintain flame morphology. The secondary air oxygen partial pressure is initially set at 15–20%, with FGR flue gas in the secondary air accounting for 0.80–0.90% of the total circulating flue gas in the variable atmosphere main combustion zone. As the temperature and reaction atmosphere change, the secondary air oxygen partial pressure is gradually reduced to 8% over 30–40 minutes, while simultaneously increasing the FGR flue gas fraction in the secondary air to maintain a constant overall gas flow rate and furnace pressure. At this point, the overall oxygen coefficient λ of the system is maintained at 0.8, the furnace temperature is stabilized at 900°C, and a stable reducing atmosphere is formed.

[0081] To enhance the gasification and surface reactivation of carbonaceous materials, a small amount of fuel gas (10–15% of the total fuel) is added to the central part of the variable atmosphere main combustion zone, reducing the local excess oxygen coefficient λ to 0.7 and raising the local flame temperature to 1150°C. This high-temperature zone promotes the gasification reaction of carbon and carbon dioxide (C + CO2 → 2CO) and the reaction of carbon and water vapor (C + H2O → CO + H2), forming a reducing mixture containing 2–4% CO and 0.5–1.0% H2 within the variable atmosphere main combustion zone. Flue gas analyzer II17-2 monitors the local oxygen partial pressure in the variable atmosphere main combustion zone, maintaining it at approximately 0.3–1.0%, while flue gas analyzer I17-1 monitors the overall furnace outlet atmosphere as weakly oxidizing (O2 ≈ 0.8%), ensuring combustion stability and system safety.

[0082] During operation, increase the fuel injection rate by 5–10% when the temperature is too low or the reducing power is insufficient; immediately reduce the injection rate when the temperature exceeds the limit or the CO / H2 concentration exceeds the standard, and simultaneously increase the proportion of FGR flue gas in the secondary air by 5–10%. When adjusting the rich fuel alone cannot stabilize the atmosphere, adjust the oxygen to circulating flue gas ratio in the secondary air to finely adjust the oxygen volume fraction of the secondary air mixture by ±2%. If the distribution ratio of recirculated flue gas in the main combustion zone of the variable atmosphere has reached the upper limit and the temperature is still higher than 950°C, the total system circulation ratio K can be temporarily increased to 0.78 within the allowable range of the blower to absorb waste heat and reduce the flame temperature.

[0083] The cooling zone is not ignited; only recirculated flue gas, cooled by heat exchange, is introduced. The recirculation rate is approximately 10% of the total recirculation flow, used to absorb residual heat and reduce the material temperature to below 200°C, thus completing the cooling stage after roasting. During cooling, precise control of temperature and atmosphere is achieved by adjusting the speed of the FGR fan 16 and the opening of the primary air FGR flue gas flow valve Ⅲ1002-3 or the secondary air FGR flue gas flow valve Ⅲ1003-3 in the oxygen-enriched burner 10 of the cooling zone. If the outlet O2 concentration is detected to exceed 0.5%, the recirculation flue gas flow rate is immediately and appropriately increased, or a short-term switch to CO2 gas purging is made to restore a weak reducing atmosphere.

[0084] Under these conditions, the specific surface area recovery rate of activated carbon can reach over 95%, the CO concentration is maintained at 0.5–1%, and no knocking or fluctuations are observed in the flue gas. Through staged oxygen supply and a high circulation rate, a localized reducing atmosphere can be effectively formed in the variable atmosphere main combustion zone, achieving a balance between carbon regeneration and combustion safety.

Claims

1. A multi-hearth furnace system with controllable temperature and atmosphere, characterized in that, Includes rake furnace, oxygen supply system and flue gas recirculation (FGR) system; The furnace body of the rake furnace is divided into a feeding zone, a preheating zone, a variable atmosphere main combustion zone, a cooling zone, and a discharge zone from top to bottom; The preheating zone, variable atmosphere main combustion zone, and cooling zone are each uniformly equipped with oxygen-enriched burners of the same structure on the side walls of the furnace body. Each oxygen-enriched burner is connected to a gas pipe channel, a coaxial annular primary air channel, and an outer annular secondary air channel; the gas pipe channel is equipped with a gas flow valve and a gas flow meter; the coaxial annular primary air channel is equipped with a primary air flow meter and a primary air valve; the outer annular secondary air channel is equipped with a secondary air flow meter and a secondary air valve; The primary air duct is connected to the pressure reducing valve of the oxygen supply system through an oxygen flow meter and an oxygen valve, and the oxygen flow meter and oxygen valve control the oxygen flow rate of the primary air in the oxygen-enriched burner; and is connected to the fan of the flue gas circulation system through an FGR flue gas flow meter and an FGR flue gas flow valve, and the FGR flue gas flow rate of the primary air in the oxygen-enriched burner is controlled by the FGR flue gas flow meter and FGR flue gas flow valve. The secondary air duct is connected to the pressure reducing valve of the oxygen supply system through an oxygen flow meter and an oxygen valve, and the oxygen flow meter and oxygen valve control the oxygen flow rate of the secondary air in the oxygen-enriched burner; it is also connected to the fan of the flue gas circulation system through an FGR flue gas flow meter and an FGR flue gas flow valve, and the FGR flue gas flow rate of the secondary air in the oxygen-enriched burner is controlled by the FGR flue gas flow meter and FGR flue gas flow valve. The flue gas recirculation system includes a secondary combustion chamber, an economizer, a bag filter, a plate heat exchanger, and an FGR fan; the secondary combustion chamber, economizer, bag filter, plate heat exchanger, and FGR fan are connected in sequence; at the front end of the secondary combustion chamber, a pressure sensor, a flue gas analyzer I, and a thermocouple I are installed; the return flue gas from the plate heat exchanger is cooled to 150–300°C before being sent to the FGR fan; A CO2 channel is also provided at the bottom of the rake furnace. The CO2 channel is connected to the CO2 source through a CO2 flow valve and a CO2 flow meter. The CO2 flow valve and CO2 flow meter control the CO2 flow. A flue gas analyzer I, thermocouple I, and pressure sensor are installed at the upper flue of the preheating zone; a flue gas analyzer II, thermocouple II, and flame detector are installed at the top flue of the variable atmosphere main combustion zone. A flue gas analyzer III and a thermocouple III are installed before the flue gas collection pipe in the cooling zone. The atmosphere and temperature of the system are controlled by a two-level linkage of total circulation ratio K and zoned flow distribution. The total cycle ratio K is: K = G / B Where G is the volumetric flow rate of the recirculated flue gas, B is the total volumetric flow rate of the flue gas at the furnace outlet, and K is controlled within 0.60–0.80; The proportion of recirculated flue gas (k) in each zone is: k = g / G Where g is the volumetric flow rate of the recirculated flue gas returning to the designated area, G is the volumetric flow rate of the recirculated flue gas, and the control range of the proportion k of the recirculated flue gas in the preheating zone, the variable atmosphere main combustion zone, and the cooling zone are 5–30%, 60–85%, and 0–20%, respectively; The preheating zone operates in a neutral or weakly oxidizing atmosphere, with the primary air oxygen partial pressure controlled at 18–25% and the oxygen supply accounting for 0.15–0.25% of the total oxygen supply; the secondary air oxygen partial pressure is controlled at 25–30%; the ratio of oxygen to FGR flue gas in the secondary air is adjusted to maintain the over-oxygen coefficient λ at 1.0–1.05 and the temperature at 200–400℃.

2. The multi-hearth furnace system with controllable temperature and atmosphere according to claim 1, characterized in that, Under oxygen-rich combustion conditions, the variable atmosphere main combustion zone can flexibly switch between oxidizing atmosphere, neutral atmosphere and local reducing atmosphere by adjusting the ratio of oxygen in the secondary air to FGR flue gas, and the furnace temperature can be adjusted within the range of 200–1200°C. In the oxidizing atmosphere mode, primary air and fuel gas are mixed and combusted. The oxygen partial pressure of primary air is controlled at 18–25%, and the oxygen supply of primary air accounts for 0.15–0.25% of the total oxygen supply. The oxygen partial pressure of secondary air is controlled at 30–45%. The ratio of oxygen in secondary air to FGR flue gas is adjusted to maintain the excess oxygen coefficient λ at 1.05–1.30, and the oxygen concentration at the furnace outlet is maintained at 2–5%. In neutral atmosphere mode, the oxygen partial pressure of primary air is controlled at 18–25%, and the oxygen supply of primary air accounts for 0.15–0.25% of the total oxygen supply; the oxygen partial pressure of secondary air is controlled at 25–30%; the ratio of oxygen in secondary air to FGR flue gas is adjusted to maintain the excess oxygen coefficient λ at 0.95–1.05, the temperature at 800–900°C, and the O2 concentration at the furnace outlet at 1–2%. In reducing atmosphere mode, the oxygen partial pressure of primary air is controlled at 18–25%, and the oxygen supply of primary air accounts for 0.15–0.25% of the total oxygen supply. The oxygen partial pressure of secondary air is initially set at 15–20% and gradually reduced to 5–10% during operation. At the same time, the FGR flue gas integral number in secondary air is increased by an equal amount. The oxygen coefficient λ is controlled at 0.7–0.9, and the temperature is maintained at 850–950°C. A reducing mixture containing 2–5% CO and <1% H2 can be formed through partial fuel surplus injection from the oxygen-enriched burner.

3. The multi-hearth furnace system with controllable temperature and atmosphere according to claim 1, characterized in that, The cooling zone uses FGR flue gas from the flue gas recirculation system as the cooling medium, which is injected in a low-oxygen or oxygen-free manner and comes into countercurrent contact with the high-temperature material to cool the material. The outlet flue gas temperature of the cooling zone is controlled at 100–200℃.

4. The multi-hearth furnace system with controllable temperature and atmosphere according to claim 1, characterized in that, The temperature and atmosphere regulation modes in the preheating zone and the variable atmosphere main combustion zone under neutral or oxidizing conditions follow these principles: First, the ratio of oxygen in the secondary air to FGR flue gas is changed to increase or decrease the partial pressure of oxygen in the secondary air. If adjusting the partial pressure of oxygen in the secondary air alone fails to achieve the desired temperature or causes a decrease in flame stability, the temperature is further controlled by adjusting the ratio of FGR flue gas allocated to that zone. When the circulating flue gas ratio k in the main combustion zone increases to the set upper limit of 70–85%, and the temperature is still higher than the target value (i.e., any temperature measurement point exceeds 1000℃ for 30 consecutive seconds or the heating rate is greater than 15℃ / min), the total system circulation ratio K is gradually increased within the FGR fan's capacity range to further increase the dilution and heat absorption of the cold flue gas. If the total circulation ratio K has reached its upper limit and is still insufficient, the inert gas CO2 passage is opened, while the oxygen flow rate in the secondary air is reduced by the same amount. This further dilutes and absorbs residual heat while maintaining a total system flow fluctuation of less than ±5%, achieving rapid cooling. The temperature and atmosphere regulation mode in reduction mode follows these principles: First, rapid adjustment of the temperature and reducing intensity of the main combustion zone in the variable atmosphere is achieved by adjusting the local rich fuel injection quantity. After rich fuel adjustment, fine-tuning is achieved by reducing the proportion of FGR flue gas allocated to that area or lowering the oxygen partial pressure in the secondary air. In cooling mode, the opposite is true: first, rich fuel injection is reduced, then the proportion of FGR flue gas allocated to that area is increased, and the oxygen partial pressure in the secondary air mixture is lowered to dilute and absorb heat. The entire process uses CO, H2, and outlet O2 as a safety closed loop; if CO or H2 exceeds limits or outlet O2 exceeds the safe range, a retreat strategy is immediately implemented. First, open the inert gas CO2 passage and rapidly introduce CO2 into the furnace. While maintaining a relatively constant total system flow, quickly dilute the concentration of combustible gases in the furnace, inhibit further accumulation of CO and H2, and reduce the risk of explosion. At the same time, restore neutral or light oxidation conditions by reducing or cutting off the supply of rich fuel nozzles and moderately increasing the oxygen partial pressure of secondary air by 2-5%. If the concentration of CO and H2 still exceeds the standard, further increase the CO2 flow until it completely replaces the oxygen in the secondary air to achieve inert protection until the system stabilizes or the furnace is safely shut down. The amount of cooling flue gas returning to the cooling zone can be controlled separately to control the heating or cooling rate.

5. The multi-hearth furnace system with controllable temperature and atmosphere according to claim 1, characterized in that, The oxygen supplied by the oxygen supply system is a mixture of Ar and O2, wherein the volume fraction of O2 is not less than 95%.

Images