A method for the synergistic removal of nitrogen oxides from low-temperature gasification tar in the indirect co-firing of biomass in coal-fired boilers

Abstract

This invention discloses a method for the synergistic removal of nitrogen oxides from low-temperature gasification tar in the indirect co-firing of biomass in coal-fired boilers. The method includes selecting suitable biomass, calculating the biomass feed rate based on the biomass co-firing ratio in the coal-fired boiler, and designing a fluidized bed gasifier of appropriate size. The biomass is fed into the fluidized bed gasifier at a certain feed rate for low-temperature gasification. The biomass is converted into low-temperature gasification products in the fluidized bed gasifier, and these products are directly transported to the coal-fired boiler under hot conditions to participate in combustion. The low-temperature gasification products form a localized reducing region inside the coal-fired boiler, reacting with nitrogen oxides generated from coal combustion inside the boiler, thereby providing heat while synergistically treating nitrogen oxide pollutants inside the coal-fired boiler. The method proposed in this invention for achieving high-temperature flue gas denitrification using biomass low-temperature gasification tar components has excellent denitrification performance and promising engineering application prospects.

Description

Technical Field

[0001] This invention relates to the field of biomass co-firing and flue gas purification technology in coal-fired boilers, and particularly to a method for the synergistic removal of nitrogen oxides from low-temperature gasification tar in the indirect co-firing of biomass in coal-fired boilers. Background Technology

[0002] Developing renewable alternative energy sources and reducing dependence on fossil fuels has become an important development direction in the global energy sector. Biomass, as the fourth largest energy source after coal, oil, and natural gas, accounts for approximately 10% of global primary energy consumption and has significant advantages such as wide availability, renewability, low sulfur and nitrogen content, and carbon neutrality. Rice and wheat straw are widely distributed throughout the country, making resource access convenient and providing a foundation for large-scale energy utilization. Studies show that every 2 tons of biomass can replace approximately 1 ton of standard coal. If efficient energy utilization is achieved, it is estimated that approximately 2 billion tons of CO2 emissions and 2 million tons of nitrogen oxides (NOx) can be reduced annually. X This includes emissions of 5.6 million tons of SO2. Therefore, the high-value utilization of biomass resources is of great strategic significance for alleviating the contradiction between energy supply and demand, reducing pollution from open burning of agricultural waste, and promoting the green and low-carbon transformation of the industrial sector.

[0003] Coal will remain the dominant energy source for a considerable period, and coal-fired boilers will continue to be crucial thermal energy equipment in industries such as power generation, building materials, metallurgy, and chemicals. However, the large amounts of nitrogen oxides produced during coal combustion have become a significant source of acid rain, photochemical smog, and regional air pollution. Meanwhile, NO... X The greenhouse effect potential of nitrogen oxides is far greater than that of carbon dioxide, posing significant threats to both the ecological environment and human health. Currently, NOx emissions from coal-fired boilers... X Control technologies mainly include low-NOx combustion, selective non-catalytic reduction (SNCR), and selective catalytic reduction (SCR). Among these, low-NOx burners typically only remove NOx. X Emissions controlled at 400–600 mg / m³ 3 While SNCR systems are simple, they are limited by the reaction temperature window, resulting in low ammonia utilization and susceptibility to ammonia escape and equipment corrosion. SCR denitrification efficiency can reach over 90%, but the catalyst is expensive, prone to poisoning and deactivation, and operating and maintenance costs increase significantly under high dust and high sulfur conditions. Therefore, developing a new, low-cost, high-efficiency in-situ denitrification technology that can be integrated with existing coal-fired boilers has significant engineering application value.

[0004] In recent years, biomass co-firing technology has received widespread attention due to its dual advantages of carbon reduction and pollution reduction. In particular, indirect co-firing technology can not only effectively avoid the problems of slagging, ash accumulation and high-temperature corrosion caused by direct co-firing, but also has the advantages of strong fuel adaptability, high co-firing ratio and small boiler modification. It has become one of the important technical routes for the low-carbon transformation of coal-fired boilers.

[0005] However, in existing technologies, the low-temperature gasification process of biomass typically produces a high mass fraction of tar byproducts due to the high volatile content and loose structure of biomass. The tar yield can generally reach 5% to 20% of the feedstock mass. Tar has a complex composition, mainly consisting of phenols, aromatic hydrocarbons, and oxygen-containing heterocyclic compounds. It has high viscosity and easy condensation characteristics. Traditional processes usually regard it as a harmful byproduct and remove it through condensation, washing, catalytic cracking, or incineration. This not only increases the investment and operating costs of the gas purification system but also wastes the chemical energy in the tar, becoming one of the important bottlenecks restricting the large-scale promotion of biomass gasification coupled with coal combustion technology. Summary of the Invention

[0006] The purpose of this invention is to address the shortcomings of existing technologies by providing a method for the synergistic removal of nitrogen oxides from low-temperature gasification tar in coal-fired boilers that involves the indirect co-firing of biomass.

[0007] To solve the above-mentioned technical problems, the present invention provides the following technical solution: A method for the synergistic removal of nitrogen oxides from low-temperature gasification tar in the indirect co-firing of biomass in coal-fired boilers, characterized by comprising the following steps: S1: Select suitable biomass, calculate the biomass feeding rate based on the biomass co-firing ratio of the coal-fired boiler, and design a fluidized bed gasifier with appropriate size. S2: Biomass is fed into a fluidized bed gasifier at a certain feeding rate for low-temperature gasification. S3: Biomass is converted into low-temperature gasification products in a fluidized bed gasifier, and the low-temperature gasification products are directly transported to a coal-fired boiler under hot conditions to participate in combustion. S4: The low-temperature gasification products form a local reducing zone inside the coal-fired boiler, which reacts with the nitrogen oxides produced by coal combustion inside the boiler, thereby providing heat while simultaneously treating nitrogen oxide pollutants inside the coal-fired boiler.

[0008] Furthermore, in S4, the oxygen concentration in the locally reducing region is controlled at 1% to 5%.

[0009] Furthermore, in S2, the fluidized bed gasifier uses air as the gasification medium.

[0010] Furthermore, in S2, the air-to-biomass equivalence ratio is 0.10 to 0.25.

[0011] Furthermore, in S2, the gasification reaction temperature is 600℃~700℃.

[0012] Furthermore, in S2, the feeding rate is 4 t / h.

[0013] Furthermore, the biomass has a moisture content of less than 20%.

[0014] Furthermore, the low-temperature gasification products include combustible gas, gaseous tar, and residual carbon.

[0015] Furthermore, the gaseous tar includes olefins, aromatic hydrocarbons, ketones, alcohols, furans, and phenols.

[0016] Furthermore, the biomass includes rice husks.

[0017] Compared with the prior art, the beneficial effects of the present invention are: 1. This method directly uses the tar from the biomass low-temperature gasification products, which would otherwise be discarded, as a denitrification reducing agent for nitrogen oxides in coal-fired boilers, thereby realizing the resource utilization of waste and improving the overall energy utilization efficiency of the biomass gasification system.

[0018] 2. When tar is cracked in a high-temperature and low-oxygen environment, it produces a large number of reducing free radicals and small molecule hydrocarbon intermediates, which can undergo a highly efficient reduction reaction with nitrogen oxides. Under optimal conditions, the NO removal efficiency can reach more than 98%, thus effectively replacing and reducing the amount of traditional denitrification reducing agents such as urea or ammonia in the denitrification system of coal-fired boilers. This can significantly reduce the risk of ammonia escape and alleviate corrosion problems in preheaters, dust collectors and flue equipment. Attached Figure Description

[0019] Figure 1 This is a line graph showing the effect of temperature on NO reduction rate in Experiment 1 of Example 1; Figure 2 This is a bar chart showing the change in NO reduction rate as temperature increases in Experiment 1 of Example 1. Figure 3 This is a line graph showing the effect of oxygen concentration on NO reduction rate in Experiment 2 of Example 2; Figure 4 This is a bar chart showing the change in NO reduction rate as oxygen concentration increases in Experiment 2 of Example 2; Figure 5 This is a line graph showing the effect of residence time on NO reduction rate in Experiment 3 of Example 1; Figure 6 This is a bar chart showing the change in NO reduction rate with increasing residence time in Experiment 3 of Example 1. Figure 7This is a line graph showing the effect of initial NO concentration on NO reduction rate in Experiment 4 of Example 1. Figure 8 This is a bar chart showing the change in NO reduction rate as the initial NO concentration increases in Experiment 4 of Example 4. Figure 9 This is a line graph showing the effect of swirl number on NO reduction rate in Experiment 5 of Example 5; Figure 10 The bar chart shows the change in NO reduction rate as the swirl number decreases in Experiment 5 of Example 5. Detailed Implementation

[0020] To enhance understanding of the present invention, we will now describe it in further detail with reference to the accompanying drawings. These embodiments are for illustrative purposes only and do not constitute a limitation on the scope of protection of the present invention.

[0021] A specific embodiment of a method for the synergistic removal of nitrogen oxides from low-temperature gasification tar used in the indirect co-firing of biomass in coal-fired boilers includes the following steps: S1: Rice husks are selected as a suitable biomass raw material, and the moisture content of the rice husks is below 20%. A fluidized bed gasifier of appropriate size is designed.

[0022] S2: Based on the biomass co-firing ratio of the coal-fired boiler, rice husks are fed into a fluidized bed gasifier at a feed rate of 4 t / h for low-temperature gasification. The fluidized bed gasifier uses air as the gasification medium, and the air-to-rice husk equivalent ratio is controlled at 0.10~0.25, with a gasification reaction temperature of 600℃~700℃.

[0023] S3: Biomass is converted into low-temperature gasification products in a fluidized bed gasifier. The low-temperature gasification products include combustible gas, gaseous tar and residual carbon. The low-temperature gasification products are directly transported to a coal-fired boiler under hot conditions to participate in combustion. S4: The proportions of olefins, aromatic hydrocarbons, ketones, alcohols, furans, and phenols in gaseous tar are 13.04%, 23.30%, 9.27%, 5.59%, 6.92%, and 24.94%, respectively. These reducing components provide heat inside the coal-fired boiler and form a local reducing zone with an oxygen concentration of 1% to 5%. The reducing components react with nitrogen oxides produced by coal combustion inside the boiler, thereby synergistically treating nitrogen oxide pollutants inside the coal-fired boiler while providing heat.

[0024] The nitrogen oxide reduction performance test was conducted on the gaseous tar in the above embodiments to explore the response law of biomass gaseous tar to different oxygen concentrations under high temperature and low oxygen conditions, and to analyze the influence of oxygen concentration in the reducing zone inside the coal-fired boiler on the NO reduction rate.

[0025] Reaction conditions: Toluene concentration 100 g / Nm3 The initial NO concentration was 1000 ppm, the CO2 concentration was 25%, the temperature of the reduction reaction zone was 850℃, the oxygen concentration was 1-5%, and the residence time was 3 s. The results are shown in Table 1.

[0026] Table 1: Effect of oxygen concentration on NO reduction rate.

[0027] NO concentration in the outlet gas 126 ppm 14 ppm 508 ppm 643 ppm 822 ppm NO reduction rate 87.40% 98.60% 49.19% 35.69% 17.80% As shown in Table 1, the optimal oxygen concentration is around 2%, which can maximally reduce the nitrogen oxides produced by coal combustion.

[0028] To further investigate the response of biomass gaseous tar to key factors such as temperature, oxygen concentration, initial NO concentration, residence time, and swirl number under high temperature and low oxygen conditions, and to analyze the reduction performance of different operating parameters on NO reduction in coal-fired boilers, toluene was selected as a tar simulant for the experiment.

[0029] Experiment 1: The effect of temperature in the reducing zone inside a coal-fired boiler on the NO reduction rate.

[0030] Reaction conditions: Toluene concentration 100 g / Nm 3 The initial NO concentration was 1000 ppm, the CO2 concentration was 25%, the reaction temperature was 750–950 ℃, the oxygen concentration was 1%, 2%, 3%, 4%, 5%, the residence time was 3s, and the vortex number was 0.80.

[0031] Experimental results are as follows Figure 1 , 2 As shown, the reduction efficiency of toluene for NO increases significantly with increasing temperature. At 750 °C, the NO reduction efficiency is below 20% under all oxygen concentration conditions; when the temperature rises to 850 °C, the reduction efficiency increases rapidly; it reaches its highest value at 900 °C, where the NO reduction efficiency reaches 95.24% under 1% oxygen concentration. When the temperature further increases to 950 °C, the increase in reduction efficiency slows down or even decreases slightly.

[0032] During the experiment, a small amount of yellowish-brown deposits were observed on the reactor outlet filter membrane at temperatures above 900 °C, indicating that some toluene underwent polymerization to form carbon black precursors. The results suggest that there is an optimal temperature window for the reduction of NO by toluene, with the preferred reaction temperature being 850–900 °C.

[0033] Experiment 2: The effect of oxygen concentration in the reducing zone inside a coal-fired boiler on the NO reduction rate.

[0034] Reaction conditions: Toluene concentration 100 g / Nm 3The initial NO concentration was 1000 ppm, the CO2 concentration was 25%, the reaction temperature was 750–950 ℃, the oxygen concentration was 1%, 2%, 3%, 4%, 5%, the residence time was 3s, and the vortex number was 0.80.

[0035] Experimental results are as follows Figure 3 , 4 As shown, under the same temperature conditions, the NO reduction efficiency generally decreases with increasing oxygen concentration. Specifically, the NO reduction efficiency reaches its highest level of 95.24% at 900 ℃ and 1% oxygen concentration; the reduction efficiency decreases most significantly when the oxygen concentration increases from 1% to 2%; and the NO reduction efficiency continues to decrease when the oxygen concentration exceeds 3%.

[0036] The results showed that a lower oxygen concentration was beneficial for the cracking of toluene to produce reducing free radicals and small molecule active components, thereby improving the NO reduction efficiency; the preferred oxygen concentration range was 1% to 2%.

[0037] Experiment 3: The effect of toluene residence time in the reducing zone inside a coal-fired boiler on NO reduction rate.

[0038] Reaction conditions: Toluene concentration 100 g / Nm 3 The initial NO concentration was 1000 ppm, the CO2 concentration was 25%, the reaction temperature was 850 ℃, the oxygen concentrations were 1%, 2%, 3%, 4%, and 5%, the residence time was 1–5 s, and the vortex number was 0.80.

[0039] Experimental results are as follows Figure 5 , 6 As shown, the NO reduction efficiency increases with increasing residence time under all oxygen concentration conditions. Specifically, the NO reduction efficiency reaches 91.98% at 1% oxygen concentration and 5s residence time; under 1%–4% oxygen concentration conditions, the reduction efficiency increases most significantly when the residence time increases from 2s to 3s; and under 5% oxygen concentration conditions, the increase is greatest when the residence time increases from 3s to 4s.

[0040] The results showed that extending the residence time was beneficial for the toluene cracking products to fully contact with NO and undergo a reduction reaction, with the preferred residence time being 3–5 s.

[0041] Experiment 4: The effect of the initial NO concentration in the reducing zone inside a coal-fired boiler on the NO reduction rate.

[0042] Reaction conditions: Toluene concentration 100 g / Nm 3The CO2 concentration was 25%, the reaction temperature was 850 ℃, the oxygen concentrations were 1%, 2%, 3%, 4%, and 5%, the residence time was 3 s, and the vortex number was 0.80. The initial NO concentrations were 200 ppm, 400 ppm, 600 ppm, 800 ppm, and 1000 ppm.

[0043] Experimental results are as follows Figure 7 , 8 As shown, under the same oxygen concentration conditions, the NO reduction efficiency generally decreases with increasing initial NO concentration. Specifically: when the NO concentration is 200 ppm and the oxygen concentration is 1%, the reduction efficiency reaches 99.50%; when the NO concentration increases to 1000 ppm, the reduction efficiency remains above 80%; under the conditions of 2%–5% oxygen concentration, the highest reduction efficiency is achieved at a NO concentration of 400 ppm.

[0044] The results show that the toluene reduction system used in this invention has the best adaptability to NO concentrations of 200–400 ppm and is highly matched with the NO concentration range of cement decomposition furnace flue gas.

[0045] Experiment 5: Effect of vortex number on NO reduction rate.

[0046] Reaction conditions: Toluene concentration 100 g / Nm 3 The CO2 concentration was 25%, the initial NO concentration was 1000 ppm, the reaction temperature was 850 ℃, the oxygen concentrations were 1%, 2%, 3%, 4%, and 5%, the residence time was 3 s, and the swirl numbers were 1.00, 0.80, and 0.67.

[0047] Experimental results are as follows Figure 9 , 10 As shown, the NO reduction efficiency gradually increases with the increase of the swirl number. Specifically, when the oxygen concentration is 1% and the swirl number is 1.00, the reduction efficiency reaches 90.37%. Under low oxygen concentration conditions (1%–2%), the change in swirl number has the most significant impact on the NO reduction efficiency. Under 5% oxygen concentration conditions, the reduction efficiency only increases significantly when the swirl number increases to 1.00.

[0048] The results showed that increasing the swirl number enhanced the mixing effect between toluene vapor and high-temperature flue gas, promoted toluene cracking and the formation of active intermediates, increased the probability of contact between toluene and NO, and thus enhanced NO reduction capacity. The preferred swirl number was 0.80–1.00.

[0049] Therefore, directly introducing biomass low-temperature gaseous tar, which is traditionally removed as a harmful byproduct, into the high-temperature reduction zone of a coal-fired boiler allows it to simultaneously undergo thermal decomposition and NO reduction during co-combustion. XIn-situ reduction can not only convert low-value-added tar into high-value byproducts with denitrification functions, eliminating the need for tar purification and reducing the operating costs of the gasification system, but also reduce the dependence of SNCR or SCR systems on denitrification media such as ammonia and catalysts. This significantly reduces the cost of nitrogen oxide removal from coal-fired boilers and improves the overall economic and environmental benefits of biomass gasification coupled with coal-fired power plants, demonstrating significant engineering promotion value and promising industrial application prospects. The above specific embodiments are only for illustrating the technical concept and structural features of the present invention, and are intended to enable those skilled in the art to implement them. However, the above content does not limit the scope of protection of the present invention. Any equivalent changes or modifications made in accordance with the spirit and essence of the present invention should fall within the scope of protection of the present invention.

Claims

1. A method for the synergistic removal of nitrogen oxides from low-temperature gasification tar in the indirect co-firing of biomass in coal-fired boilers, characterized in that, Includes the following steps: S1: Select suitable biomass, calculate the biomass feeding rate based on the biomass co-firing ratio of the coal-fired boiler, and design a fluidized bed gasifier with appropriate size. S2: Biomass is fed into a fluidized bed gasifier at a certain feeding rate for low-temperature gasification. S3: Biomass is converted into low-temperature gasification products in a fluidized bed gasifier, and the low-temperature gasification products are directly transported to a coal-fired boiler under hot conditions to participate in combustion. S4: The low-temperature gasification products form a local reducing zone inside the coal-fired boiler, which reacts with the nitrogen oxides produced by coal combustion inside the boiler, thereby providing heat while simultaneously treating nitrogen oxide pollutants inside the coal-fired boiler.

2. The method for co-removing nitrogen oxides from low-temperature gasification tar in the indirect co-firing of biomass in a coal-fired boiler, as described in claim 1, is characterized in that: In S4, the oxygen concentration in the locally reducing region is controlled at 1% to 5%.

3. The method for co-removing nitrogen oxides from low-temperature gasification tar in the indirect co-firing of biomass in a coal-fired boiler, as described in claim 1, is characterized in that: In S2, the fluidized bed gasifier uses air as the gasification medium.

4. The method for co-removing nitrogen oxides from low-temperature gasification tar in indirect co-firing of biomass in coal-fired boilers according to claim 3, characterized in that: In S2, the air-to-biomass equivalence ratio is 0.10 to 0.

25.

5. The method for co-removing nitrogen oxides from low-temperature gasification tar in the indirect co-firing of biomass in a coal-fired boiler, as described in claim 4, is characterized in that: In S2, the gasification reaction temperature is 600℃~700℃.

6. The method for co-removing nitrogen oxides from low-temperature gasification tar in indirect co-firing of biomass in coal-fired boilers according to claim 1, characterized in that: In S2, the feeding rate is 4 t / h.

7. The method for co-removing nitrogen oxides from low-temperature gasification tar in the indirect co-firing of biomass in a coal-fired boiler according to claim 1, characterized in that: The biomass has a moisture content of less than 20%.

8. The method for co-removing nitrogen oxides from low-temperature gasification tar in indirect co-firing of biomass in coal-fired boilers according to claim 1, characterized in that: The low-temperature gasification products include combustible gas, gaseous tar, and residual carbon.

9. The method for co-removing nitrogen oxides from low-temperature gasification tar in the indirect co-firing of biomass in a coal-fired boiler, as described in claim 8, is characterized in that: The gaseous tar includes olefins, aromatic hydrocarbons, ketones, alcohols, furans, and phenols.

10. The method for co-removing nitrogen oxides from low-temperature gasification tar in the indirect co-firing of biomass in a coal-fired boiler according to claim 1, characterized in that: The biomass includes rice husks.

Images