Pulverized coal combustion method for reducing nitrogen through pulverized coal modification and reburning

Abstract

The invention provides a pulverized coal combustion method for reducing nitrogen through pulverized coal modification and reburning, which belongs to the technical field of air staged combustion and comprises the following steps: distributing pulverized coal flow into main fuel flow and modified fuel flow; the modified fuel flow is introduced into an oxygen-poor gasification furnace for oxygen-poor combustion modification treatment, and a modified product containing active coke and reducing gas is generated; the main fuel flow is sprayed into a main combustion area of the pulverized coal combustion furnace for oxygen-deficient combustion; the modified product is sprayed into a reduction area located above the main combustion area through a four-corner tangential nozzle, and NOx in the flue gas is reduced; and over-fire air is sprayed into the over-fire area above the reduction area to complete combustion. Reducing gas molecules and active coke are generated through gasification modification treatment, the diffusion speed of the reducing gas molecules is high, and the reducing gas molecules and NOx in flue gas can be subjected to a homogeneous reduction reaction rapidly; the huge specific surface area and rich active sites of the active coke greatly promote the heterogeneous reduction reaction. The removal efficiency of pollutants such as NOx in the whole reduction area is remarkably and essentially enhanced.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of air staged combustion, and particularly relates to a coal powder combustion method for reducing nitrogen by coal powder modification and reburning. BACKGROUND

[0002] Air staged combustion (also known as two-stage combustion) is a core low nitrogen oxide (NOx) combustion control technology, and its core lies in that the combustion process is artificially divided into two stages with completely different physical and chemical environments: a main combustion zone (reduction zone) and a burnout zone (oxidation zone).

[0003] The reduction effect of air staged combustion mainly depends on the reducing intermediates (CO, H2, CmHn) and residual carbon generated by incomplete combustion of fuel under the oxygen-deficient environment of the main combustion zone. The concentration, activity (especially the content of CHi free radicals with strong reducing ability) and spatial distribution of these reducing agents are highly dependent on the quality of the fuel (such as volatile content, reaction activity) and the combustion conditions (such as local oxygen concentration, temperature, mixing) of the main combustion zone. For low-volatile coal, fuel with poor reaction activity or fuel with extremely high nitrogen content, the amount and activity of the reducing substances generated under the oxygen-deficient conditions of the main combustion zone are insufficient, which leads to a bottleneck in the reduction efficiency of NOx (especially fuel-type NOx), and it is difficult to reduce NOx emissions to an extremely low level.

[0004] In the main combustion zone of air staged combustion, two sometimes contradictory goals need to be considered at the same time: fuel ignition, stable combustion, preliminary burnout and the creation of an efficient NOx reduction environment. The temperature range (usually 1100-1400 DEG C) of the main combustion zone is not the most favorable temperature window for maximizing the NOx reduction efficiency (studies have shown that some reduction reactions have higher efficiency in the 900-1100 DEG C interval). High temperature is beneficial to the generation of reducing agents by pyrolysis, but it may also accelerate some reverse reactions. It is difficult to achieve independent and accurate control to achieve the optimal concentration and mixing state required for reducing NOx.

[0005] Although air staged combustion reduces the average temperature of the main combustion zone, in order to maintain combustion stability and burnout degree, the local peak temperature of the main combustion zone (especially the area near the burner) may still be very high (especially when high-quality coal is used), which provides conditions for the generation of thermal NOx. Its reduction of thermal NOx mainly relies on the overall reduction of oxygen concentration and average temperature, and the targeted control ability of local high temperature points is limited.

[0006] Under the oxygen-deficient environment of the main combustion zone, although part of the fuel nitrogen is converted into N2, the conversion efficiency is limited by the temperature, oxygen concentration distribution and residence time of the region. For coal with high fuel nitrogen content, even if air staged combustion is used, the absolute value of the initial fuel nitrogen conversion rate to NOx may still be high, which becomes an obstacle to further deep denitrification. SUMMARY

[0007] In view of the above problems of the prior art, the technical problem to be solved by the embodiment of the present application is to provide a coal powder combustion method for reducing nitrogen by coal powder modification and reburning.

[0008] To solve the above technical problem, the present application provides the following technical scheme: a coal powder combustion method for reducing nitrogen by coal powder modification and reburning, comprising:

[0009] dividing the coal powder flow into a main fuel flow and a modified fuel flow;

[0010] passing the modified fuel flow into a lean-oxygen gasification furnace for lean-oxygen combustion modification treatment to generate modified products containing active coke and reducing gas;

[0011] injecting the main fuel flow into a main combustion zone of a coal powder combustion furnace for hypoxic combustion;

[0012] injecting the modified products into a reduction zone above the main combustion zone through a four-corner tangential circle nozzle to reduce NOx in the flue gas;

[0013] injecting a burnout air into a burnout zone above the reduction zone to complete the combustion.

[0014] As a further improved scheme: the excess air coefficient in the main combustion zone is controlled at 0.8-0.95.

[0015] As a further improved scheme: the lean-oxygen gasification furnace fully mixes the coal powder with the split air through a cyclone nozzle.

[0016] As a further improved scheme: the temperature of the lean-oxygen gasification furnace is controlled at 950-1050℃.

[0017] As a further improved scheme: the adjustable baffle in the adjustable coal powder distributor controls the coal powder flow and the distribution of the primary air.

[0018] As a further improved scheme: the reducing gas includes CO and H4.

[0019] As a further improved scheme: the excess air coefficient in the lean-oxygen combustion is 0.5-0.6.

[0020] Compared with the prior art, the present application has the beneficial effects that: the reducing gas molecules and active coke are generated by the gasification modification treatment, the reducing gas molecules have a fast diffusion speed and can rapidly react with NOx in the flue gas in a homogeneous phase reduction reaction; the huge specific surface area and rich active sites of the active coke greatly promote the heterogeneous reduction reaction. This significantly and essentially enhances the removal efficiency and depth of NOx and other pollutants in the entire reduction zone.

[0021] The modified product itself has higher energy density (gas) and reactivity, and the combustion / gasification process in the reduction zone is faster and more complete, thereby improving the energy utilization efficiency and carbon conversion rate of the system as a whole. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 A flowchart of a coal combustion method for reducing nitrogen by coal powder modification and reburning;

[0023] In the figure: 1, air energy water heater; 2, solar water storage tank; 3, support plate; 4, first mounting bracket; 5, transparent water tank; 6, first communication pipe; 7, first output pipe; 8, second communication pipe; 9, serpentine heat collecting pipe; 10, dust collecting bag; 11, third communication pipe; 12, bottom plate; 13, fourth communication pipe; 14, solar panel; 15, second mounting bracket; 16, gas collecting pipe; 17, second output pipe; 18, moving groove; 19, spring; 20, sliding bar; 21, water inlet pipe; 22, power storage box; 23, air inlet hole; 24, air inlet pipe; 25, filter; 26, dust removal assembly; 261, rotating shaft; 262, mounting cavity; 263, motor; 264, fan blade; 265, dust removal pipe. DETAILED DESCRIPTION

[0024] The technical solutions of the present application will be further described in detail below in conjunction with specific embodiments.

[0025] The embodiments of the present application are described in detail below, and examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference signs represent the same or similar elements or elements having the same or similar functions throughout. The embodiments described below by referring to the drawings are exemplary and are only used to explain the present application, and cannot be understood as a limitation of the present application.

[0026] Please refer to Figure 1 In one embodiment, a coal combustion method for reducing nitrogen by coal powder modification and reburning, comprising:

[0027] Dividing the coal powder flow into a main fuel flow and a modified fuel flow;

[0028] Passing the modified fuel flow into a lean-oxygen gasifier for modification treatment to generate a modified product containing active coke and reducing gas;

[0029] Spraying the main fuel flow into a main combustion zone of a coal combustion furnace for oxygen-deficient combustion;

[0030] Spraying the modified product into a reduction zone located above the main combustion zone through a four-corner tangential circular nozzle to reduce NOx in the flue gas; and spraying burnout air into a burnout zone above the reduction zone to complete combustion.

[0031] The excess air ratio in the oxygen-poor combustion is 0.5-0.6. The excess air ratio in the main combustion zone is controlled at 0.8-0.95.

[0032] In the present embodiment, the primary air system is:

[0033] Primary air fan→primary air pipeline→coal mill→pulverized coal pipeline→(main fuel branch)→main burner primary air inlet

[0034] Carrying the pulverized coal of the main fuel to spray into the main combustion zone of the furnace.

[0035] Primary air fan→primary air pipeline→coal mill→pulverized coal pipeline→(gasification agent branch)→pulverized coal modified gasification furnace.

[0036] Secondary air system: air supply fan→main air duct→secondary air box→secondary air inlet (located in the main combustion zone)→sprayed into the main combustion zone of the furnace.

[0037] Overfire air system: OFA fan→OFA air duct→OFA nozzle in the upper part of the furnace→sprayed into the overfire zone in the upper part of the furnace.

[0038] Fuel supply system (solid pulverized coal flow):

[0039] Coal bunker→coal feeder→coal mill (mixed, dried and transported with primary air)→pulverized coal distributor→(main fuel flow)→main fuel pulverized coal pipeline→main burner primary air inlet→carrying the pulverized coal to spray into the main combustion zone of the furnace.

[0040] (modified fuel flow)→modified fuel pulverized coal pipeline→pulverized coal modified gasification furnace (feed inlet).

[0041] Modified fuel gasification system: pulverized coal modified gasification furnace→high-temperature reducing synthesis gas pipeline→furnace reduction zone nozzle (located above the main combustion zone, below the OFA nozzle)→sprayed into the reburning zone of the furnace (to reduce the NOx generated in the main combustion zone).

[0042] Flow splitting mechanism: adjustable pulverized coal distributor at the outlet of the pulverized coal bunker, with an adjustable baffle inside, to control the pulverized coal flow and the distribution of primary air through a servo motor. To ensure the operation of the pulverized coal combustion furnace and the gasification furnace under different loads.

[0043] Main combustion path: split pulverized coal→straightly sent to the primary air pipeline of the pulverized coal combustion furnace. Gasification branch path: split pulverized coal→gasification furnace feed inlet.

[0044] Gasification furnace cavity (size: Φ1m x 4.5m, material: 310S stainless steel + silicon carbide lining). The coal powder and the split air are mixed by the cyclone nozzle while prolonging the residence time, so that the uniformity of solid-gas distribution is > 95%. The gasification furnace temperature control: 950-1050℃. The oxygen-poor combustion occurs in the furnace (the excess air coefficient in the oxygen-poor combustion is 0.5-0.6), and as many reducing gas and active coal coke as possible are generated under the condition of ensuring the sustainable combustion in the furnace.

[0045] Reaction mechanism in the main combustion zone:

[0046] Fuel oxidation (incomplete): fuel (C, H, N) + insufficient O2→ CO + H2+ CH4 / C m H n (unburned hydrocarbon) + HCN / NH3 (nitrogen-containing intermediate) + coke;

[0047] NOx reduction: NO + CO / H2 / CH4 / C m H n / HCN / NH3→N2+CO2 / H2O;

[0048] Fuel nitrogen conversion: fuel nitrogen (N-fuel)→ HCN / NH3→ NO;

[0049] Thermal NOx: N2+ O2→ NO;

[0050] Air gasification reaction mechanism in the coal powder gasification modification furnace

[0051] Oxidation exothermic: C + O2→ CO2;

[0052] Boudouard reaction: CO2+ C→ 2CO;

[0053] Water gas reaction: C + H2O→ CO + H2, water from coal moisture.

[0054] The reducing gas and the active coal coke are sprayed into the coal powder combustion furnace by the four-corner tangential circle method to occur the reduction reaction:

[0055] 2CO + 2NO→ 2CO2+ N2;

[0056] 2H2+ 2NO→ 2H2O + N2;

[0057] 2C + 2NO→ 2CO + N2;

[0058] Please refer to Figure 1 , in an embodiment, the oxygen-poor gasification furnace mixes the coal powder and the split air by the cyclone nozzle.

[0059] The oxygen-poor gasification furnace temperature is controlled at 950-1050℃.

[0060] The reducing gas includes CO and H4.

[0061] In the prior art, when fuel staging (such as reburning technology) is used, the original solid coal powder is directly injected into the third combustion zone (reduction zone) without treatment in an attempt to reduce pollutants such as NOx. In the present application, the part of the coal powder to be divided into the reburning / reduction zone is subjected to an innovative pre-gasification modification treatment before entering the staged combustion / gasification process. This treatment process (such as partial gasification or rapid pyrolysis under the control of specific temperature, atmosphere and residence time) is not a simple physical separation or crushing, but a substantial transformation of the chemical structure and morphology of the coal powder before it enters the coal powder combustion furnace. As a result, the material delivered to the reduction zone is no longer the original coal powder with relatively slow reaction and limited reduction capacity, but is transformed into active coke rich in high-activity reducing gas components (such as carbon monoxide CO, hydrogen H2, methane CH4, etc.) and with high reactivity and developed pore structure.

[0062] Significant improvement in reaction rate and reduction efficiency: the diffusion speed of the reducing gas molecules is fast, which can quickly react with NOx in the flue gas; the large specific surface area and rich active sites of the active coke greatly promote the heterogeneous reduction reaction. This significantly and essentially enhances the removal efficiency and depth of pollutants such as NOx in the entire reduction zone.

[0063] Optimization of combustion / gasification process: the modified product itself has higher energy density (gas) and reactivity, and its combustion / gasification process in the reduction zone is faster and more complete, which improves the energy utilization efficiency and carbon conversion rate of the system as a whole.

[0064] Foundation for system stability: the fast and controllable reaction of high-activity material in the reduction zone reduces the problems of unburned carbon and reaction lag, which is conducive to the stable control of the entire furnace working condition (especially the reduction zone).

[0065] Coal powder primary air diversion device for adapting to variable load and main combustion zone stability guarantee (key support): the fuel staging system in the prior art has a fixed diversion ratio or limited adjustment range and slow response, which is difficult to adapt to the frequent load change requirements in actual operation.

[0066] It will be obvious to a person skilled in the art that the application is not limited to the details of the foregoing exemplary embodiments and can be implemented in other concrete forms without departing from the spirit or essential characteristics of the application. The embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the application being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein and no

[0067] Furthermore, it should be understood that although the description is made on the basis of the embodiments, not every embodiment contains only one independent technical solution, and the description is made in this way only for the sake of clarity, and a person skilled in the art should consider the description as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by a person skilled in the art.

Claims

1. A pulverized coal combustion method for reducing nitrogen by pulverized coal modification reburning, characterized by, The application relates to a coal combustion method and device. The pulverized coal flow is divided into a main fuel flow and a modified fuel flow; The modified fuel flow is sent into a lean-oxygen gasifier for lean-oxygen combustion modification treatment to generate modified products containing active coke and reducing gas; The main fuel flow is injected into a main combustion zone of a pulverized coal combustion furnace for oxygen-deficient combustion; The modified products are injected into a reduction zone above the main combustion zone through a four-corner tangential circle nozzle to reduce NOx in the flue gas; Combustion is completed by injecting a burnout air into a burnout zone above the reduction zone.

2. The coal combustion method for reducing nitrogen by coal powder modification reburning according to claim 1, characterized in that, The excess air coefficient in the main combustion zone is controlled to be 0.8-0.

95.

3. The method for reducing nitrogen emissions through coal powder modification and reburning according to claim 1, characterized in that, The lean-oxygen gasifier fully mixes the pulverized coal with the split air through a cyclone nozzle.

4. The coal combustion method of claim 1, wherein the coal is modified by coal powder. The temperature of the lean-oxygen gasifier is controlled to be 950-1050 DEG C.

5. The coal combustion method with coal powder modification reburning for nitrogen reduction according to claim 1, characterized in that, The pulverized coal flow and the distribution of the primary air are controlled through an adjustable baffle in the adjustable pulverized coal distributor.

6. The coal combustion method with coal powder modification reburning for nitrogen reduction according to claim 1, characterized in that, The reducing gas includes CO and H4.

7. The coal combustion method of claim 5, wherein the coal is modified by coal powder. The excess air coefficient in the lean-oxygen combustion is 0.5-0.6.

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