Pulverized coal selective oxygen group alkylation modification method for ultralow nitrogen emission of fire coal

Abstract

The invention discloses a pulverized coal selective oxygen group alkylation modification method, which starts from fire coal, and changes the molecular configuration of pulverized coal through a mechanical force chemical coupling alkylation reagent selective modification method, thereby influencing the migration and conversion path of fuel nitrogen in raw coal, realizing the directional regulation and control of the nitrogen-containing product of the fire coal, enhancing the homogeneous and heterogeneous reduction of NOx, and improving the combustion efficiency. And finally, ultra-low nitrogen emission of the fire coal is realized.

Description

Technical Field

[0001] The present invention belongs to the technical field of pulverized coal modification, and particularly relates to a method for selectively alkylating oxygen groups of pulverized coal for ultra-low nitrogen emission of coal combustion. Background Art

[0002] China is rich in fossil fuel resources, but there is a current energy structure situation of "rich in coal, poor in oil, and scarce in gas". Fossil fuels, especially coal resources, occupy the most important main position in China's energy field. According to the data in the World Energy Statistical Yearbook released in 2024, currently China is the world's largest coal demand country, accounting for up to 56% of the world's total coal demand, and coal accounts for 55.3% in China's energy structure. These data indicate that coal will still be the main energy for China's development in the future for some time.

[0003] Nitrogen oxides (NO x ) are one of the main air pollutants generated during coal combustion. How to achieve ultra-low emission control of NO during coal combustion is an urgent problem to promote the sustainable development of China's economy and environment. x

[0004] The molecular configuration of coal mainly includes functional groups, pore structure, and microcrystalline morphology. By directionally regulating these key microscopic physical and chemical properties, the generation path of NO precursors during the thermal conversion of coal combustion can be effectively regulated, thereby affecting NO x release. The selective oxygen group alkylation modification of coal can directionally regulate its molecular configuration. However, current research mainly focuses on fields such as coal extraction, gasification, and liquefaction, and the migration mechanism of fuel nitrogen in coal after selective pretreatment has not been revealed. In addition, there is no report on the influence of coal molecular configuration on the migration path of fuel nitrogen during the thermal conversion process, and there is little discussion on the correlation between molecular configuration and NOx reduction and the effective method of NO x directional regulation. x

[0005] Currently, the commonly used technologies for controlling coal nitrogen emissions include staged combustion, flue gas recirculation, catalytic reduction of flue gas, etc. These technical methods control nitrogen emissions from the combustion system, so multiple steps are required during combustion, and even the combustion system needs to be redesigned and modified. The operation steps are cumbersome and the system is complex. There is no research directly starting from the structural properties of coal itself to attempt to reduce nitrogen emissions. Summary of the Invention

[0006] The main objective of the present invention is to overcome the shortcomings of the above-mentioned existing technologies for controlling nitrogen emissions from coal combustion. A method for selectively alkylating oxygen groups in pulverized coal to regulate the migration path of fuel nitrogen in coal combustion is proposed. Starting from the coal itself, the molecular configuration of pulverized coal is changed through a method of mechanical force chemical coupling and selective modification with alkylating reagents, thereby affecting the migration and transformation path of fuel nitrogen in raw coal, achieving directional regulation of nitrogen-containing products in coal combustion, and enhancing the homogeneous and heterogeneous reduction of NO x to ultimately achieve ultra-low nitrogen emissions from coal combustion.

[0007] The present invention is implemented by the following technical solutions:

[0008] A method for selectively alkylating oxygen groups in pulverized coal, comprising the following steps:

[0009] Step 1: Crush and grind raw coal, and perform mechanical force chemical modification on the pulverized coal to obtain modified pulverized coal;

[0010] Step 2: Add a polar aprotic solvent to the modified pulverized coal, introduce argon gas and stir;

[0011] Step 3: Add an alkaline solution to the mixed system, keep introducing argon gas and stir;

[0012] Step 4: Add an alkylating reagent to the mixed system and continue to stir;

[0013] Step 5: Add dilute hydrochloric acid to the mixed system to neutralize it, and remove the polar aprotic solvent in the system;

[0014] Step 6: Extract the mixed system, and use a corresponding solution to titrate to determine whether there are interfering heteroions in the extracted solution. Dry the extracted solution to obtain selectively alkylated oxygen group modified coal.

[0015] In some embodiments, different coal ranks such as lignite, bituminous coal, and anthracite can be selected as the application objects for the raw coal. Since the selective alkylation of oxygen groups in coal is to replace the hydrogen atoms on the acidic centers (hydroxyl and carboxyl groups) in the coal macromolecule with alkaline ions to form coal polyanions, and then the carbon atoms of the halogenated hydrocarbon attack to complete the alkylation substitution, coal types with higher oxygen content, carboxyl group and hydroxyl group content are conducive to selective alkylation modification of oxygen groups.

[0016] The present invention uses raw coal as the object of mechanochemical modification and alkylation because the raw coal contains relatively more aromatic hydrocarbons, which can not only improve the alkylation degree and efficiency of the raw materials, but also improve the homogeneous and heterogeneous reduction efficiency of coal powder for NOx. In comparison, carbon-based solid waste contains a large amount of paraffins but a relatively low content of aromatic hydrocarbons, and the number and type of oxygen-containing functional groups in the solid waste cannot be guaranteed. These factors lead to a low efficiency of selective oxygen group alkylation. In addition, since solid waste is not conducive to the formation of bottom coke, the heterogeneous reduction efficiency of nitrogen-containing products is reduced. In addition, it is difficult to collect the modified products after the solid waste is modified, and the yield is low.

[0017] In some embodiments, in step 1, a pulverizing apparatus is used to weigh raw coal, pulverize and grind it to obtain coal powder.

[0018] In some embodiments, in step 1, the pulverizing apparatus includes but is not limited to a ball mill, a hammer mill, a jaw crusher, a roller mill, a shearing mill, a vibration mill, an air flow mill, and the like.

[0019] In some embodiments, in step 1, mechanical pulverization of the coal occurs through grinding, squeezing, shearing, or friction, which creates deformation or stress in the molecular or crystal structure. This leads to energy accumulation and the breaking or reforming of chemical bonds, thereby promoting the oxidation of the coal and increasing the number of oxygen groups in the coal. Mechanochemical pulverization can also produce different effects on coal of different particle sizes during the alkylation process due to differences in grinding time and speed.

[0020] In some embodiments, in step 1, the ground coal powder is directly collected without any screening to avoid the influence of component segregation as much as possible.

[0021] In some embodiments, in step 1, during the coal powder grinding process, adding appropriate additives can promote oxidative modification, increase the oxygen content and the number of hydroxyl and carboxyl groups in the coal powder, and thus facilitate subsequent selective oxygen group alkylation modification. Common additives include potassium permanganate, sodium hypochlorite, chromium oxide, ammonium persulfate and other reagents.

[0022] In some embodiments, in step 2, the addition of a polar aprotic solvent mainly serves the following functions: as a solvent to dissolve organic matter in the coal powder, thereby enhancing fluidity and improving reaction activity; a strongly polar solvent can enhance polar interactions in the alkylation reaction, thereby improving reaction efficiency and selectivity; and a polar aprotic solution can regulate the distribution of substances in the reaction system, stabilize intermediate products, inhibit side reactions, and improve reaction selectivity.

[0023] In some embodiments, in step 2, polar aprotic solvents that can be used include, but are not limited to, tetrahydrofuran (THF), dimethyl sulfoxide (DMSO), dimethylformamide (DMF), and the like.

[0024] In some embodiments, in step two, according to different coal ranks, the number of oxygen-containing functional groups in the pulverized coal is different, the solubility in polar aprotic solvents is different, and the feeding ratio of the pulverized coal to the polar aprotic solvent is dynamically adjusted. Generally speaking, for low-rank coal, the feeding ratio of the pulverized coal to the polar aprotic solvent is 1 g: 5 mL - 1 g: 15 mL; for high-rank coal, the feeding ratio of the pulverized coal to the polar aprotic solvent is 1 g: 15 mL - 1 g: 30 mL. If there is too much polar aprotic solvent, it may lead to a relatively low concentration of reactants, thereby reducing the reaction rate, and will increase the cost and the difficulty of post-treatment. If there is too little polar aprotic solvent, it may cause the pulverized coal to not be fully dispersed and dissolved, resulting in uneven reactions, and may also cause insufficient contact between the alkylating agent and the pulverized coal, reducing the efficiency of the alkylation reaction.

[0025] In some embodiments, in step two, the main function of introducing argon is to provide a protective atmosphere as an inert gas to avoid interference caused by reactions of other gases with substances in the reaction system.

[0026] In some embodiments, in step two, the main function of stirring is to fully mix the reactants, keep the substances in the solution system in sufficient contact, and improve the reaction activity and reaction rate.

[0027] In some embodiments, in step three, the main functions of adding the alkaline solution are as follows: strong alkaline compounds can provide a large number of hydroxide ions, promote the deprotonation reaction to form coal polyanions, and enhance the alkylation reaction activity; alkaline compounds can help the alkylating agent to dissociate and activate, improve the reaction rate and reaction efficiency, and help inhibit the formation of side reactions.

[0028] In some embodiments, in step three, according to different coal ranks, the number of oxygen-containing functional groups in the pulverized coal is different, and the reaction activity with the alkaline solution is different, so the amount of the alkaline solution can be dynamically adjusted. In addition, the activity of the alkylating agent is also different, which will affect the amount of the alkaline solution used. A highly active alkylating agent requires an increase in the amount of the alkaline solution to promote the alkylation reaction. For low-rank coal, a relatively large number of oxygen-containing functional groups make it easier to react with alkaline reagents. The molar ratio of the pulverized coal to the alkaline reagent in the alkaline solution is 1: 0.2 - 1: 0.5. For high-rank coal, the structure is dense and the reaction activity is relatively low. The molar ratio of the pulverized coal to the alkaline reagent in the alkaline solution is 1: 0.5 - 1: 1. Adding too much alkaline reagent will cause side reactions (such as hydrolysis and elimination reactions) to occur, reducing the yield of the target product, and increasing the difficulty of post-treatment and the cost. Adding too little alkaline reagent may lead to incomplete reactions, a decrease in the reaction rate, or an inability to effectively stabilize the intermediate product, resulting in a decrease in the reaction yield.

[0029] In some embodiments, in step three, the alkaline solutions that can be used include, but are not limited to, inorganic bases (such as NaOH, KOH, LiOH) and organic bases (such as tetrabutylammonium hydroxide).

[0030] In some embodiments, in step four, the alkylating agents include, but are not limited to, halogenated hydrocarbons such as methyl iodide, 1-bromobutane, 1-bromopentane, 1-bromoheptane, 1-bromodecane, 2-bromobutane, 2-bromopentane, 3,3-dimethyl-1-bromobutane, 1-bromo-4-methylpentane, etc. The carbocation of the halogenated hydrocarbon attacks the coal polyanion to complete the alkylation modification.

[0031] In some embodiments, in step four, during the selective alkylation process, the content and quantity of oxygen groups in the pulverized coal have a great impact on the alkylation process. Low-rank coal contains more oxygen-containing functional groups and is more likely to react with the alkylating agent. For low-rank coal, the molar ratio of pulverized coal to alkylating agent is 1:0.5 - 1:1. For high-rank coal, the molar ratio of pulverized coal to alkylating agent is 1:1 - 1:1.5. Adding too much alkylating agent may lead to side reactions (such as over-alkylation), reducing the yield of the target product, increasing costs and waste liquid emissions. Adding too little alkylating agent may lead to incomplete reactions, resulting in a decrease in reaction rate and reaction yield.

[0032] In some embodiments, in step five, the role of adding dilute hydrochloric acid until neutral is to neutralize residual alkaline substances, such as hydroxide ions, etc., reduce impurities in the product, and thus purify the alkylated coal product.

[0033] In some embodiments, in step five, the polar aprotic solvent in the solution can be removed by distillation, for example, separated using a rotary evaporator.

[0034] In some embodiments, in step six, the extraction solvent can be a 50% aqueous methanol solution, and the sample is extracted using a Soxhlet extractor to obtain the product.

[0035] In some embodiments, in step six, silver nitrate solution titration can be used to check for the presence of halide ions, and precipitation methods, etc., can be used to detect whether the alkaline reagent has been removed completely (such as using sodium tetraphenylborate to detect tetrabutylammonium ions).

[0036] On the other hand, the present invention provides a selectively oxygen-group alkylated coal obtained by the above-mentioned method for selectively oxygen-group alkylation modification of pulverized coal.

[0037] Alkylation is an important organic synthesis method that grafts alkyl functional groups onto carbon and oxygen atoms in organic molecules. Its chemical mechanism is as follows: hydrogen atoms on acidic centers (hydroxyl and carboxyl groups) are replaced by alkaline ions to form coal polyanions, and then the coal polyanions are attacked by carbon atoms in halogenated hydrocarbons to complete alkylation substitution.

[0038] Coal alkylation is the primary method to reduce the aromatic layer spacing and intermolecular forces. Therefore, through selective oxygen group alkylation reaction, the strong hydrogen bond interaction between macromolecules can be changed under mild conditions, while the damage to the coal structure is very small, which helps to improve the thermal conversion efficiency of coal.

[0039] In addition, the alkylation process can also increase the hydrogen-donating groups in coal macromolecules, thereby effectively inhibiting the coupling reaction during pyrolysis and promoting the progress of pyrolysis. During the alkylation modification process, due to the steric hindrance effect of alkyl functional groups and the aromatic carbon hydrogenation reaction, etc., the configuration of coal macromolecules changes, and at the same time, a large number of electron transfers, free radical generation, ether bond and weak C-C bond cleavage occur. Therefore, the nitrogen-containing structure and morphology in coal will inevitably change, which will affect the migration paths of volatile nitrogen and coke nitrogen during the coal thermal conversion process.

[0040] On the other hand, with the addition of more free radical groups, the graphite sheets are attacked to generate more dangling bonds, the interfacial potential energy increases, and the aromatic sheets tend to be miniaturized and curled, making it easier to form a turbostratic structure, which will increase the activity of the lattice structure. Therefore, the change in the configuration of alkylated coal molecules can not only promote the release of reducing gases during pyrolysis, but also strengthen the homogeneous and heterogeneous reduction of NO x by pulverized coal.

[0041] To sum up, by selectively oxygen group alkylation to orientedly regulate the molecular configuration of coal combustion, improve the reactivity of pulverized coal, regulate the migration and transformation of fuel nitrogen, and promote the homogeneous and heterogeneous reduction of NO x by pulverized coal, the ultra-low nitrogen emission of coal combustion can be achieved.

[0042] Selective oxygen group alkylation modification is accompanied by a large number of electron transfers, free radical generation, ether bond and weak C-C bond cleavage, which leads to the change of coal molecular configuration and further induces the change of the fuel nitrogen migration path during the coal thermal conversion process, and then affects the release of nitrogen-containing products. Due to factors such as mechanical force collision and extrusion during the pulverization process of pulverized coal, the structure is deformed, and the oxygen content, the number of hydroxyl and carboxyl groups in pulverized coal are significantly increased, which is conducive to the subsequent oxygen group alkylation reaction. Therefore, using mechanochemistry-coupled selective oxygen group alkylation modification can orientedly regulate the local microenvironment around fuel nitrogen and the molecular configuration of pulverized coal, affect the conversion path and efficiency of fuel nitrogen in coal combustion to nitrogen-containing products such as HCN and NH3, enhance the homogeneous and heterogeneous reduction of NO x and reduce nitrogen emissions.

[0043] The present invention has at least the following technical effects:

[0044] 1. Achieve the directional regulation of the molecular configuration of pulverized coal: During alkylation modification, the smaller pores of alkylated coal increase relative to raw coal, and the proportion of larger pores decreases. The proportions of carbon and hydrogen elements in alkylated coal increase, and alkyl functional groups replace the hydrogen atoms on carboxyl and hydroxyl groups, increasing steric hindrance. In addition, the phase transfer catalytic reaction causes the long fatty chains to break or fall off, reducing the average length of the fatty chains, and also disrupts the delocalized state of π electrons in the aromatic ring by breaking hydrogen bonds and van der Waals forces, thereby inducing the opening of weakly stable aromatic rings to form fatty chains.

[0045] 2. Achieve the directional regulation of the microcrystalline morphology of pulverized coal: The average aromatic layer span (L a ) of alkylated coal decreases, the standard deviation (σ1) of the adjacent distribution of the first-layer carbon atoms increases, the preferred orientation parameter (q) decreases, the standard deviation (σ3) of the aromatic layer gap increases, the average stacking height (L c ) decreases, the average layer gap (a3) increases significantly, and the average number of stacked layers ( <n>) Decrease.

[0046] 3. Achieve directional regulation of the conversion paths of NH3 and HCN: After alkylation modification, the release of NH3 is inhibited, while the release of HCN is promoted. The conversion rate of NH3 in alkylated modified coal decreases, while the conversion rate of HCN increases. After alkylation modification, the interlayer spacing parameters a3 and σ3 of the aromatic stacking increase, and the average number of aromatic stacking layers decreases, promoting the exposure of nitrogen-containing heterocycles and strengthening the attack of active free radicals on nitrogen-containing heterocycles, which is beneficial to the formation of HCN. After alkylation, the aromatic stacking size L a , L c and the decrease in the degree of disorder q indicate the breakage of large-span aromatic sheets, resulting in the reduction of the microcrystalline size, which can induce the transformation of N-Q embedded in the aromatic layer into N-5 or N-6. N-5 and N-6 are prone to directly form HCN under the attack of abundant CH or H free radicals during pyrolysis, thus dominating the increase in HCN and the decrease in NH3 after alkylation.

[0047] 4. Achieve directional regulation of NO x generation: By directionally regulating the molecular configuration, microcrystalline morphology and fuel nitrogen migration and conversion paths of pulverized coal, the homogeneous and heterogeneous reduction of NO x during the combustion of alkylated coal is enhanced, the NO x reduction effect is enhanced, and the NO x emission is reduced, thus achieving ultra-low nitrogen emission. The total reduction amount of NO in alkylated coal can reach 30%-60%. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 is the FTIR transmission spectrum of raw coal and alkylated coal in Example 1 of the present invention;

[0049] Figure 2 shows the evolution of aliphatic carbon structure based on NMR spectrum;

[0050] Figure 3 shows the evolution of aromatic carbon structure based on NMR spectrum;

[0051] Figure 4 shows the evolution of the release of flash pyrolysis gas-phase products;

[0052] Figure 5 shows the evolution of the conversion rates of NH3 and HCN in raw coal and alkylated coal;

[0053] Figure 6 shows the homogeneous and heterogeneous reduction amounts of NO by raw coal and alkylated coal char;

[0054] Figure 7 shows the change in NO concentration during the direct reduction process at different temperatures. DETAILED DESCRIPTION OF THE INVENTION

[0055] Example 1

[0056] A method for modifying pulverized coal by selective oxygen group alkylation includes the following steps:

[0057] Use a pulverizing device to pulverize and grind 1 - 10 g of raw coal and add it to a two-necked flask. Pour 30 - 300 ml of tetrahydrofuran (THF) solution into the two-necked flask, continuously introduce argon into the flask at a rate of 500 ml / min, and stir magnetically at room temperature for 10 - 60 min. Slowly add 3 - 30 ml of aqueous tetrabutylammonium hydroxide solution (1.54 mol / L) to the flask, continue to stir under argon protection for 1 - 4 h, then dropwise add 4.5 - 45 mmol of alkylating agent to the flask, and continue to stir the mixed solution for 36 - 144 h. Then slowly add dilute hydrochloric acid until the solution is neutral, and use a rotary evaporator to remove the excess THF. Use a Soxhlet extractor to extract the sample, titrate to check whether there are impurity ions in the liquid after Soxhlet extraction. The coal sample after Soxhlet extraction is vacuum dried at 80 - 120 °C for 2 days.

[0058] After mechanical force pulverization coupled with selective oxygen group alkylation modification, the specific changes in alkylated coal are as follows:

[0059] The elemental proportion changes:

[0060] Compared with raw coal, the proportions of carbon and hydrogen elements in alkylated coal increase because alkyl functional groups replace the hydrogen atoms on carboxyl and hydroxyl groups. At the same time, the introduced carbon and hydrogen atoms also indirectly lead to a decrease in the proportions of oxygen, nitrogen, and sulfur elements.

[0061] The surface chemical structure changes:

[0062] After alkylation modification, in Fourier transform infrared spectroscopy ( Figure 1 ) it shows that the peak intensities of aliphatic hydrocarbons (2800 - 3000 cm -1 ), C - O bonds (1675 - 1750 cm -1 ), and A / R - CH3 (1350 - 1400 cm -1 ) increase significantly, indicating that alkyl functional groups have successfully replaced the hydrogen atoms on carboxyl or hydroxyl groups. However, after alkylation modification, -CH2 decreases and -CH3 increases because H radicals in the phase transfer catalytic reaction can attack -CH2, thus promoting the formation of -CH3. In addition, the -CH2 / -CH3 of alkylated coal is lower than that of raw coal because the phase transfer catalytic reaction during alkylation causes the long aliphatic chains to break or fall off, reducing the average length of the aliphatic chains.

[0063] The carbon skeleton structure changes:

[0064] Such as Figure 2 As shown, the aliphatic carbon (fal) of alkylated coal is significantly greater than that of raw coal because the alkylating reagent replaces the hydrogen atoms on the carboxyl or hydroxyl groups in the raw coal. In addition, there is generally no significant difference in the methyl and methylene groups (falH) between the raw coal and the alkylated coal. Therefore, the increase in alkylated fal is attributed to the accumulation of non-protonated aliphatic carbon caused by the saturated butyl chains.

[0065] As Figure 3 (a) shows that the aromatic carbon (fa) and the sp2-hybridized carbon of the aromatic ring (fa’) in the alkylated coal decrease significantly, indicating that the aromaticity of the alkylated coal decreases significantly because of the introduction of alkyl functional groups and the ring-opening of the weak aromatic rings. As Figure 3 (b) shows that the sum of the aromatic bridgehead carbon (faB) and the aromatic branched carbon (faS) in the alkylated coal is less than that in the raw coal, indicating that the lateral span of the aromatic layers in the alkylated coal decreases because the alkylation reaction destroys the van der Waals forces and hydrogen bond interactions in the coal macromolecules, which further leads to the breaking of some large-span aromatic layers into small aromatic fragments.

[0066] Alkylated coal will reduce the molar fraction of aromatic bridging carbon (xb) and the ratio of aromatic bridging carbon to surrounding carbon (xbp) in the carbon skeleton, indicating that both the size and the average condensation degree of the aromatic rings will decrease because some weak large-span aromatic layers will break into small aromatic structures during the alkylation reaction.

[0067] In summary, compared with the raw coal, the oxygenated aliphatic carbon of the alkylated coal is greater, but the oxygenated aromatic carbon is smaller, indicating that the phase-transfer catalytic reaction can disrupt the delocalization state of π electrons in the aromatic rings by breaking hydrogen bonds and van der Waals forces, thereby inducing the ring-opening of weakly stable aromatic rings to form aliphatic chains.

[0068] The characteristics of flash pyrolysis change:

[0069] As Figure 4 (a) shows that after alkylation, the aliphatic hydrocarbons released by pyrolysis increase significantly, while the aromatic hydrocarbons decrease significantly. This is mainly attributed to the fact that the grafted alkyl groups significantly promote the release of aliphatic hydrocarbons. Moreover, the attack of free radicals on weak aromatic hydrocarbons during the alkylation reaction leads to the formation of new aliphatic chains, which can also promote the generation of aliphatic hydrocarbons.

[0070] As Figure 4 (b) shows that the oxygen-containing products of the alkylated coal are significantly lower than those of the raw coal because the carboxyl and hydroxyl groups in the raw coal are replaced by butyl groups, which induces the formation of more stable esters and ethers, further hindering the generation of oxygen-containing products during the pyrolysis of the alkylated coal. Compared with the raw coal, the alkylated coal has a higher relative content of nitrogen- and sulfur-containing substances. This is because active free radicals may attack the nitrogen- and sulfur-containing heterocycles during alkylation, further inducing the transformation of high-stability heterocycles into low-stability structures.

[0071] The pore structure changes:

[0072] During alkylation modification, the smaller pores of alkylated coal increase relative to raw coal, and the proportion of larger pores decreases. This is because the grafted alkyl side chains can support a part of the micropores in the coal to form smaller mesopores during the electron transfer process, while the penetration of the alkylating agent into the micropores through the larger mesopores destroys the structure of the larger mesopores.

[0073] The microcrystalline morphology changes:

[0074] The average aromatic layer span (L a ) decreases because the alkylation reaction destroys the intermolecular interaction forces and induces the fracture of some large-span aromatic layers into small aromatic structures. The standard deviation (σ1) of the adjacent distribution of the first-layer carbon atoms in alkylated coal increases because the alkylation reaction grafts alkyl side chains to the edges of the aromatic sheets, and the introduction of side chains provides more edge defects. The preferred orientation parameter (q) of alkylated coal decreases, while the standard deviation (σ3) of the aromatic layer gap increases because the alkyl side chains grafted by the alkylation reaction will interfere with the stacking edges, thereby affecting the uniformity of the interlayer distribution of the aromatic layers and making them more disordered. The average stacking height (L c ) decreases, the average layer gap (a3) increases significantly, and the average number of stacked layers ( <n>Decrease, because the alkylating agent penetrates into the interstices to produce a swelling effect, causing the aromatic layers on the upper and lower surfaces of some aromatic stacks to break free from the constraints.

[0075] The conversion paths of fuel nitrogen to NH3 and HCN change:

[0076] After alkylation modification, the release of NH3 is inhibited while the release of HCN is promoted. On the one hand, this is because the ability of alkylated coal to provide free radicals during pyrolysis is enhanced. On the other hand, it is due to the influence of the microcrystalline morphology of alkylated coal.

[0077] The NH3 conversion rate of alkylated modified coal decreases ( Figure 5 (a)), while the HCN conversion rate increases ( Figure 5 (b)). This is because a large number of alkyl chains are grafted onto the coal molecular structure during the alkylation reaction, promoting the release of CH and H free radicals during pyrolysis. Further, the free CH free radicals can directly combine with N free radicals to form HCN. In addition, CH free radicals can also attack nitrogen-containing heterocycles, thus promoting the release of HCN.

[0078] After alkylation modification, the interlayer gap parameters a3 and σ3 of the aromatic stack increase, and the average number of aromatic stack layers decreases, promoting the exposure of nitrogen-containing heterocycles, strengthening the attack of active free radicals on nitrogen-containing heterocycles, and being conducive to the formation of HCN. However, the increase in a3 and σ3 indicates an increase in the aromatic layer gap and an increase in smaller pores, which is conducive to the formation of structural defects, thereby strengthening the adsorption of defects on HCN and hindering the release of HCN. Therefore, it can be inferred that after alkylation, a3, σ3, σ1 and <n>The evolution did not dominate the increase in HCN. However, after alkylation, the aromatic stacking size L a , L c and the decrease in disorder q indicate that there are breaks in the large-span aromatic sheets, leading to a decrease in the microcrystalline size, which can induce the transformation of the N-Q embedded in the aromatic layer into N-5 or N-6. N-5 and N-6 are prone to directly form HCN under the attack of abundant CH or H radicals during pyrolysis, thus dominating the increase in HCN and the decrease in NH3 after alkylation.

[0079] Enhance the homogeneous and heterogeneous reduction of pulverized coal to NO x , and significantly reduce NO x emissions:

[0080] Compared with raw coal, alkylation modification enhances the homogeneous and heterogeneous reduction ability of pulverized coal to NO x , and the direct reduction effect of alkylated pulverized coal on NO x significantly increases. The total reduction of NO x by alkylated modified pulverized coal is dominated by heterogeneous reactions. There is a synergistic promotion mechanism between the homogeneous and heterogeneous reduction of NO x , and the proportion of homogeneous and heterogeneous synergistic reduction increases with the increase in temperature.

[0081] As Figure 6 shown, the homogeneous reduction of NO by the pyrolysis gas of alkylated coal is significantly stronger than that of the corresponding raw coal, and the homogeneous reduction effect is enhanced by 2-4 times. On the one hand, it is because the grafted alkyl side chains promote the release of pyrolysis reducing gases, and on the other hand, it is because the microcrystalline stacking size in alkylated modified pulverized coal decreases, the stacking layer spacing and structural defects increase, which promotes heat transfer and mass transfer during pyrolysis and the escape of pyrolysis gas. The heterogeneous reduction effect of alkylated coal is also significantly stronger than that of the corresponding raw coal, because it enhances the diffusion of NOx in the pores and increases the number of reactive sites, thus enhancing the heterogeneous reduction of NO.

[0082] As Figure 7 shown, the total reduction amount of NO by alkylated coal varies depending on the alkylation functional groups, and the decrease range compared with raw coal is 30%-60%.

[0083] Therefore, by carrying out mechanochemical coupling selective oxygen group alkylation modification on raw coal, the microenvironment around fuel nitrogen, the molecular structure of pulverized coal, the microcrystalline morphology, etc. can be directionally regulated, so as to effectively control the conversion path of fuel nitrogen to NH3 and HCN, enhance the homogeneous and heterogeneous reduction of pulverized coal to NO x , and significantly reduce NO x emissions, and then achieve ultra-low nitrogen emissions. Specific examples:

[0085] Select high-volatile Inner Mongolia bituminous coal.

[0086] It was ground using a high-energy ball mill (Retsch PM100, Germany) under an air atmosphere. Approximately 150 g of raw coal was weighed and placed into a grinding jar (500 ml), and grinding balls with a diameter of φ20 mm were added to the grinding jar at 1 / 3 of its volume. Then, the grinding rate was set from 300 rpm / min to 500 rpm / min.

[0087] To minimize the influence of compositional segregation as much as possible, the pulverized coal after grinding was directly collected without any screening.

[0088] 5 g of pulverized coal was added to a two-necked flask, and 150 ml of tetrahydrofuran (THF) solution was added. Argon was continuously introduced into the flask at a rate of 500 ml / min, and magnetic stirring was carried out at room temperature for 30 min; 14.6 ml of aqueous tetrabutylammonium hydroxide solution (1.54 mol / L) was slowly dropped into the flask, and stirring was continued for 2 h under argon protection; thereafter, 22.5 mmol of alkylating agent was added dropwise to the flask, and the mixed solution was stirred for 72 h; then, dilute hydrochloric acid was slowly added until the solution was neutral, and THF was removed at 60 °C using a rotary evaporator; 50% aqueous methanol solution was used as the extraction solvent, and extraction was carried out using a Soxhlet extractor for 6 days; the liquid after Soxhlet extraction was titrated with silver nitrate to check for the presence of halide ions, and sodium tetraphenylborate was used to detect the presence of tetrabutylammonium ions; the pulverized coal after Soxhlet extraction was vacuum dried at 100 °C for 2 days to obtain selectively oxygen-group alkylated modified pulverized coal.

[0089] Within the scope of this example, for Inner Mongolia raw coal, it was found that after selective oxygen-group alkylation modification, the decrease in the aromatic stacking dimensions La and Lc of alkylated coal led to the conversion of N-Q embedded in aromatic carbon to N-5 or N-6, which was prone to generating HCN under the attack of abundant CH or H free radicals during pyrolysis, thus dominating the increase in alkylated HCN. In addition, the increase in a3 and σ3 promoted the diffusion of HCN and NH3, but the increase in σ1 strengthened the adsorption of HCN and NH3 by structural defects. Selective oxygen-group alkylated modified pulverized coal directionally regulated the molecular configuration and microcrystalline morphology, changed the surrounding environment of fuel nitrogen, affected the migration and transformation path of fuel nitrogen, enhanced the homogeneous and heterogeneous reduction of NOx, and significantly reduced NOx emissions. When the temperature was 1170 °C, the total reduction amount of NO in alkylated coal had the largest increase, and the decrease range compared with raw coal was 31.57% - 61.03%, achieving ultra-low nitrogen emissions from coal combustion.< / n> < / n> < / n>

Claims

1. A method for selectively alkylating oxygen groups of pulverized coal, comprising the following steps: Step 1: Crush and grind raw coal, and perform mechanochemical modification on the pulverized coal to obtain modified pulverized coal; Step 2: Add a polar aprotic solvent to the modified pulverized coal, introduce argon and stir; Step 3: Add an alkaline solution to the mixed system, keep introducing argon and stir; Step 4: Add an alkylating reagent to the mixed system and continue to stir; Step 5: Add dilute hydrochloric acid to the mixed system to neutralize it, and remove the polar aprotic solvent in the system; Step 6: Extract the mixed system, and use a corresponding solution to titrate to determine whether there are interfering heteroions in the extracted solution, and dry the extracted solution to obtain selectively oxygen-group alkylated modified coal.

2. The method for selectively alkylating and modifying oxygen groups of pulverized coal according to claim 1, wherein, The raw coal is selected from at least one of lignite, bituminous coal, and anthracite.

3. The method for selectively alkylating and modifying oxygen groups of pulverized coal according to claim 1, wherein In Step 1, the mechanochemical modification is to cause deformation or stress in the molecular or crystal structure of the pulverized coal by means of grinding, extrusion, shearing, or friction.

4. The method for selectively alkylating and modifying oxygen groups of pulverized coal according to claim 1, wherein, In Step 1, during the pulverized coal grinding process, an additive is added, and the additive is selected from at least one of potassium permanganate, sodium hypochlorite, chromium oxide, and ammonium persulfate.

5. The method for selectively alkylating and modifying oxygen groups of pulverized coal according to claim 1, wherein, In Step 4, the alkylating reagent is selected from at least one of methyl iodide, 1-bromobutane, 1-bromopentane, 1-bromoheptane, 1-bromodecane, 2-bromobutane, 2-bromopentane, 3,3-dimethyl-1-bromobutane, and 1-bromo-4-methylpentane.

6. The method for selectively alkylating and modifying oxygen groups of pulverized coal according to claim 1, wherein, In Step 5, the distillation method is used to remove the polar aprotic solvent in the solution.

7. The method for selectively alkylating and modifying oxygen groups of pulverized coal according to claim 1, wherein, In Step 6, the extraction solvent is a 50% aqueous methanol solution.

8. The method for selectively alkylating and modifying oxygen groups of pulverized coal according to claim 1, wherein, In Step 6, silver nitrate solution titration is used to check for the presence of halide ions.

9. The method for selectively alkylating and modifying oxygen groups of pulverized coal according to claim 1, wherein, In Step 6, the precipitation method is used to detect whether the alkaline reagent has been removed completely.

10. Selectively oxygen-group alkylated modified coal obtained by the method for selectively alkylating oxygen groups of pulverized coal according to any one of claims 1-9.

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