Method for removing sulfur in coal by adopting DMF (Dimethyl Formamide) dissolution

Abstract

The invention relates to a method for removing sulfur in lignite by adopting DMF (Dimethyl Formamide) swelling. The problems that an existing coal desulfurization technology is high in cost and energy consumption and complex in process or the desulfurization rate needs to be improved can be solved. According to the method, DMF is used as an efficient swelling solvent, molecules of DMF permeate into a three-dimensional macromolecular network structure of lignite, interaction force (such as hydrogen bonds) among the molecules is weakened, the coal body is expanded, and a pore structure is opened, so that sulfur-containing minerals (especially inorganic sulfur) wrapped in a coal matrix are exposed; then, under the conditions of heating and stirring, sulfur removal is promoted. The method comprises the following steps: mixing a pretreated lignite sample with DMF (Dimethyl Formamide) according to a specific mass ratio, carrying out swelling desulfurization treatment under heating and condensation reflux conditions, and finally carrying out solid-liquid separation to obtain desulfurized coal. The method is easy to operate, mild in condition and low in cost and energy consumption, the highest removal rate of sulfur in the lignite can reach 83.05%, and remarkable economic and environmental benefits are achieved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of coal cleaning, and particularly relates to a method for removing sulfur in lignite by DMF swelling. BACKGROUND

[0002] Coal, as a basic energy source and important chemical raw material in China and even in the world, plays an indispensable role in ensuring energy security and promoting economic development. However, coal generally contains sulfur elements (the content is usually between 0.5% and 10%), which will be converted into sulfur oxides (SOx) in the process of combustion or gasification and other conversion processes, which is one of the main culprits of serious environmental problems such as acid rain and haze. According to statistics, more than 50% of global anthropogenic SO2 emissions are derived from coal combustion. With the increasingly stringent environmental regulations, for example, the “Coal-fired Power Plant Air Pollutant Emission Standards” (GB 13223-2011) of China puts forward a stringent requirement that the SO2 emission concentration should be less than 35 mg / m 3 Therefore, efficient desulfurization (i.e., “pre-combustion desulfurization”) before coal utilization has become a key technical link to realize clean and efficient utilization of coal.

[0003] The existing coal pre-combustion desulfurization technologies can be mainly divided into physical methods, chemical methods, biological methods and some new desulfurization methods. The physical methods (such as gravity separation and flotation) mainly remove inorganic sulfur in the form of pyrite, but they are powerless to fine-grained embedded sulfides and organic sulfur. The chemical methods (such as alkali treatment and oxidation method) usually have harsh reaction conditions, may damage coal quality, and have high reagent cost and large consumption. The biological method has a long processing period and is limited in industrial application.

[0004] In recent years, new desulfurization technologies represented by solvent extraction have attracted widespread attention. For example, ionic liquids and deep eutectic solvents are used for selective desulfurization due to their strong designability and low volatility. Patent CN110835571A discloses a mixed solvent desulfurization method of ionic liquid and non-polar supercritical fluid. Although this method has low energy consumption, the high cost of ionic liquid itself limits its large-scale industrial application. Patent CN108753408B uses phase transfer catalytic oxidation method, which has certain desulfurization effect on large particle coal, but the highest desulfurization rate is only 31.0%, and the desulfurization effect rate needs to be further improved. In addition, supercritical fluid hydrogenation (such as patent CN110835569B) desulfurization method can remove organic sulfur and inorganic sulfur at the same time, but it needs expensive high-pressure reaction equipment; microwave-assisted (such as patents CN108728201B and CN116943853A) method, the former has high energy consumption and is difficult to scale up, and the latter can improve the removal rate of inorganic sulfur in coal, but its process flow is relatively complex; boiler method (such as CN120059817B), this method can desulfurize while saving coal, cleaning ash, and removing coke, etc., but its initial investment cost is high, and the system complexity increases; Fenton reagent oxidation coal desulfurization method (CN112358900A) can efficiently and environmentally remove sulfur, but its processing cost is high, it has an impact on coal quality, the reaction conditions are relatively harsh, and the operation process is complex.

[0005] In summary, the existing coal desulfurization technologies generally have one or more of the following problems: low desulfurization efficiency, high cost of reagents or equipment, complex operation process, high energy consumption, or certain degree of damage to coal quality. Especially for lignite, which is a low-rank coal with loose structure, high moisture content, and complex macromolecular network structure, how to remove the sulfur closely combined with the coal matrix in an economical, mild, and efficient way is a technical problem that needs to be solved in the field. SUMMARY

[0006] The purpose of the present application is to overcome the shortcomings of the prior art, and to provide a new lignite desulfurization method with simple operation, mild conditions, low cost and energy consumption, and high desulfurization efficiency.

[0007] To achieve the above object, the present application provides a method for removing sulfur from lignite by swelling in N,N-dimethylformamide (DMF). The method is based on the following core technical idea: lignite has a complex three-dimensional cross-linked macromolecular network structure, in which a large number of sulfur-containing minerals (mainly inorganic sulfur) are wrapped in closed or semi-closed pores. N,N-dimethylformamide (DMF) is a strong polar aprotic solvent, which has excellent penetration and swelling ability for coal macromolecules. When lignite is soaked in DMF, DMF molecules can effectively penetrate into the macromolecular chains of coal, and by destroying and weakening the intermolecular non-covalent bonding force (such as hydrogen bond, π-π stacking, etc.), the coal matrix is significantly swollen, and the internal cross-linked network structure becomes relaxed, so that the originally physically embedded closed pores are opened. This process exposes the inorganic sulfur particles that were previously difficult to contact with the reaction medium, creating extremely favorable conditions for subsequent separation and removal of inorganic sulfur from the coal matrix under mild physical conditions (such as heating and stirring).

[0008] The technical scheme adopted by the present application specifically includes the following steps: (1) taking lignite raw coal, crushing and sieving it to a predetermined particle size range to obtain a pretreated coal sample; (2) mixing the pretreated coal sample with DMF in a specific mass ratio in a reaction vessel (such as a round-bottom flask); (3) placing the reaction vessel containing the mixture in a heating and stirring device (such as a constant-temperature magnetic stirring heating jacket) to perform swelling desulfurization reaction at a set temperature and stirring rate, while connecting a condensation reflux device above the reaction vessel to prevent DMF from being lost by volatilization during heating; (4) after the reaction is completed, the obtained mixture is separated by solid-liquid separation operation (such as vacuum filtration) to separate the solid product. The solid product is washed with a detergent (such as deionized water) to remove adsorbed and residual DMF, and then dried to obtain clean desulfurized lignite.

[0009] Compared with the prior art, the present application has the following remarkable beneficial effects: High desulfurization efficiency: through the swelling effect of DMF, the exposure and accessibility of sulfur (especially inorganic sulfur) are significantly improved, and efficient desulfurization can be achieved under mild conditions. Experiments show that the highest dry basis desulfurization rate can reach 82.52%.

[0010] 2. Simple process and easy operation: the entire process only involves conventional chemical unit operations such as crushing, mixing, heating reflux and filtration washing, without the need for high-pressure equipment or complex processes, and is easy to realize industrialization.

[0011] 3. Low cost and good economy: the main reagent DMF is a relatively low-priced, widely available common chemical solvent. In addition, through condensation reflux and subsequent distillation recovery, DMF can be recycled, further reducing operating costs.

[0012] 4. Mild reaction conditions and low energy consumption: the reaction can be carried out efficiently at normal pressure and relatively low temperature (such as 120℃), avoiding the high temperature and high pressure conditions commonly seen in the prior art, significantly reducing energy consumption.

[0013] 5. Little impact on coal quality: the method mainly uses physical swelling to expose and remove inorganic sulfur, and the chemical action is mild, which can maximize the maintenance of the organic matter structure and calorific value of the coal. BRIEF DESCRIPTION OF DRAWINGS

[0014] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed in the embodiment description will be briefly introduced as follows.

[0015] Figure 1 The process flow diagram of the DMF swelling desulfurization method provided in the embodiments of the present application is shown in the figure. Figure 2 The heating, stirring and condensation reflux device used in the embodiments of the present application is shown in the figure. DETAILED DESCRIPTION

[0016] The technical solutions of the present application will be described in detail below with reference to the drawings and specific embodiments, so as to facilitate the understanding and implementation of the present application by those skilled in the art. It should be noted that the described embodiments are only a part of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0017] In the description of the present application, unless otherwise explicitly specified and limited, the terms "mounting", "connection" and "connection" should be understood broadly, for example, it can be fixedly connected, or it can be detachably connected, or integrally connected; it can be mechanically connected, or it can be electrically connected; it can be directly connected, or it can be indirectly connected through an intermediate medium, or it can be connected inside two elements. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0018] REFERENCE Figure 1 The present application provides a process flow of a method for removing sulfur from lignite by DMF swelling. The method mainly includes: step S101, raw coal pretreatment; step S102, mixing of coal sample and DMF; step S103, heating, stirring and condensation reflux reaction; step S104, product post-treatment.

[0019] Reference Figure 2 A typical laboratory-scale implementation device is shown. The device includes a round-bottom flask 1 for containing a mixture of coal sample and DMF. The round-bottom flask 1 is placed in a constant-temperature magnetic stirring heating jacket 2, which provides heat and uses the internal magnetic stirring function to make the mixture uniformly heated and contacted. The mouth of the round-bottom flask 1 is connected to a condenser tube 3, the jacket of which is connected to cooling water, which is used to condense and return the DMF vapor evaporated during heating to the flask 1, thereby maintaining the stability of the amount of solvent in the reaction system.

[0020] The present application is described in detail below through specific examples. The raw coal used in the experiment is Zhaotong lignite from Yunnan, and its total sulfur content is determined as a reference value.

[0021] Example 1 (1) Step S101: Take Zhaotong lignite raw coal from Yunnan, crush it with a crusher, and screen it with a standard sieve to collect coal samples with a particle size of 60-100 mesh. The drying degree does not affect the desulfurization effect, so this step can be omitted.

[0022] (2) Step S102: Accurately weigh 10g of dry coal sample obtained in step (1), and add 20g of DMF (i.e. the mass ratio of coal to DMF is 1:2) into a 500mL round-bottom flask 1.

[0023] (3) Step S103: Place the round-bottom flask 1 containing the mixture into the constant-temperature magnetic stirring heating jacket 2, set the heating temperature to 120℃, and start the magnetic stirring. Connect the condenser tube 3 and pass cooling water, and perform heating and stirring reaction for 4 hours.

[0024] (4) Step S104: After the reaction is completed, stop heating and stirring. After the mixture cools to room temperature, perform solid-liquid separation using a vacuum filtration device. Wash the filter cake with enough deionized water until there is no DMF odor in the filtrate. Dry the washed filter cake in a 60℃ oven to a constant weight to obtain a desulfurized coal sample.

[0025] Determine the total sulfur content of the coal samples before and after treatment (according to GB / T 214-2007), and calculate the air-dry base desulfurization rate as 82.08% and the dry base desulfurization rate as 80.56%.

[0026] Examples 2-7 (investigating the effect of temperature) The same steps as in Example 1 are used, except that the heating temperature in step (3) is changed. The specific parameters and results are shown in Table 1 below: Table 1 From the data in the above table, in the range of 50℃ to 200℃, the method shows very high desulfurization efficiency. Considering the desulfurization effect and energy consumption, 50℃ is a preferred reaction temperature, at which more than 80% of dry basis desulfurization rate can be obtained.

[0027] Example 8-12 (influence of time) The same procedure as in Example 1 was adopted, only the reaction time in step (3) was changed. The specific parameters and results are shown in Table 2 below: Table 2 From the data in the above table, in the range of 0.5 hours to 5 hours of reaction time, the desulfurization rate reaches a high level. From the perspective of economic benefits, 1 hour is a more preferred reaction time range.

[0028] Example 12-14 (influence of the mass ratio of coal to DMF) The same procedure as in Example 1 was adopted, only the mass ratio of coal to DMF in step (2) was changed. The specific parameters and results are shown in Table 3 below: Table 3 From the data in the above table, the mass ratio of coal to DMF has a significant influence on the desulfurization effect. In the range of 1:1 to 1:4, increasing the amount of DMF can effectively improve the desulfurization rate, which may be because more solvent is conducive to more complete swelling and mass transfer. When the ratio reaches 1:3 to 1:4, the dry basis desulfurization rate exceeds 81%, and the highest can reach 82.52%. Considering the desulfurization effect and solvent cost, 1:3 to 1:4 is a preferred mass ratio range.

[0029] In summary, the present application provides a method for removing sulfur from lignite by swelling with DMF. By systematically studying the influence of reaction temperature, reaction time, mass ratio of coal to solvent and other key process parameters on the desulfurization effect, the preferred process conditions of the present application are determined. The present method uses common chemical solvent DMF, under normal pressure and relatively mild conditions, through a simple and efficient process, realizes efficient desulfurization of lignite, solves the problems of cost, energy consumption and efficiency in the prior art, and shows great potential for industrial application and good economic and social benefits.

[0030] It should be understood that the application of the present application is not limited to the Yunnan Zhaotong lignite in the above examples, and should also be applicable to other types of lignite or low-rank coal with similar structure. In addition, those skilled in the art can fine-tune or optimize the above process parameters, or combine the present method with other desulfurization technologies (such as ultrasonic assistance), which are improvements and changes without departing from the core idea of the present application, and should fall within the protection scope of the present application.

Claims

1. A method for removing sulfur from lignite by swelling with DMF amine, characterized in that, The method comprises uniformly mixing lignite raw coal with N,N-dimethylformamide and heating for at least 0.5 hours, condensing the volatilized gas in the mixture, then separating the solid and liquid in the mixture, and washing and drying the solid product to obtain desulfurized lignite.

2. The method of claim 1, wherein, The particle size of the lignite raw coal ranges from 60 mesh to 100 mesh.

3. The method of claim 1, wherein, The lignite raw coal does not need to be dried before being mixed with N,N-dimethylformamide.

4. The method of claim 1, wherein, The lignite raw coal is mixed with DMF at a mass ratio of 1:1 to 1:

4.

5. The method of claim 1, wherein, The heating time is 0.5 hours to 5 hours.

6. The method of claim 1, wherein, The heating temperature is 50°C to 200°C.

7. The method of claim 1, wherein, The solid-liquid separation is performed by vacuum filtration, and the solid product is washed with deionized water to remove residual DMF.

8. The method of claim 1, wherein, The lignite raw coal is Yunnan Zhaotong lignite.

Images