System for hydrogenation conversion of coal-fired flue gas CO2 into methanol under normal pressure and catalyst preparation method

Abstract

The invention provides a system for hydrogenation conversion of coal-fired flue gas CO2 into methanol under normal pressure and a catalyst preparation method. High-temperature flue gas led out from an outlet of a hearth of the coal-fired boiler is subjected to oxygen removal through the oxygen removal reactor and then returns to a boiler flue for continuous heat exchange, then the high-temperature flue gas is led out from the position between the first-stage air pre-heater and the second-stage air pre-heater and mixed with H2 according to a certain proportion to be led into the thermal catalytic reactor, CO2 and H2 are reduced into methyl alcohol through the Ni-Ga-Zr catalyst, and the methyl alcohol is recycled. And then the flue gas returns to the air pre-heater to continuously release heat, passes through a dust removal device and then is introduced into a low-temperature coal economizer, and the temperature of the flue gas is cooled to 70 DEG C or below, so that methanol is condensed. The used Ni-Ga-Zr catalyst is prepared through a coprecipitation method, and the used steps comprise stirring and dipping, adding of an alkaline solution for coprecipitation, ultrasonic aging, centrifugal washing, drying and calcining, H2 reduction and the like. According to the method, CO2 in the coal-fired flue gas can be reduced into methanol under normal pressure, a carbon capture and separation process is not needed, and the energy consumption is low.

Description

Technical Field

[0001] The present invention belongs to the field of CO2 catalytic conversion and utilization, and particularly relates to a system for converting coal-fired flue gas CO2 into methanol by hydrogenation under normal pressure and a method for preparing a catalyst. Background Art

[0002] With rising productivity and a growing demand for energy, the combustion of fossil fuels, particularly coal, has led to a continuous rise in atmospheric CO₂ concentrations, triggering a series of environmental issues such as the greenhouse effect. The conversion and utilization of CO₂ has become a hot topic of increasing concern. Methanol, a fundamental raw material for the chemical industry and a renewable, clean liquid fuel, can be used to produce organic compounds such as formaldehyde and methylamine. It can also be blended with gasoline and diesel to replace traditional fossil fuels. Converting CO₂ to methanol is highly attractive and will play a significant role in reducing CO₂ emissions and creating a new carbon cycle. Currently, the most common method for producing methanol is a thermal catalytic method, where pure CO₂ and H₂ are converted into methanol using a Cu-ZnO-Al₂O₃ catalyst at high pressures of 5-10 MPa. However, this method involves high pressures and energy consumption. A common approach to converting CO₂ from coal flue gas is to first separate the CO₂ from the flue gas through carbon capture, followed by hydrogenation of the pure CO₂ into various chemicals such as methanol through the aforementioned thermal catalytic process. However, the carbon capture process in this process is widely recognized as a highly energy-intensive process. Therefore, if the CO2 in the flue gas is directly hydrogenated into methanol without the carbon capture process, and the flue gas is then simply cooled to condense the methanol into a liquid and separated from the flue gas, a high-value carbon-containing product, methanol, can be obtained. This process does not require a specialized CO2 capture process and can achieve both high-value conversion and low-energy separation of CO2 from the flue gas in one fell swoop, offering advantages such as low energy consumption and a simple process. To implement this process, challenges that need to be addressed include the fact that the flue gas contains O2 (O2 molar concentration 3-5%), which reacts with H2 and is prone to explosion; and the need to develop an efficient catalyst for the conversion of CO2 and H2 to methanol at atmospheric pressure. Therefore, it is necessary to explore a system and a highly active catalyst capable of hydrogenating CO2 from flue gas into methanol at atmospheric pressure. Summary of the Invention

[0003] The present invention proposes a system for converting CO2 hydrogenation of coal-fired flue gas into methanol under normal pressure, characterized in that the system comprises a coal-fired boiler, a deoxygenation reactor, a reduction reactor, a thermal catalytic reactor and a methanol collection device. The high-temperature flue gas above 900°C drawn out from the tail of the furnace passes through the deoxygenation reactor, and is oxidized by Fe powder arranged therein to remove O2. At the same time, the oxidized Fe powder is fluidized and enters the reduction reactor, where it is reduced to Fe powder by H2 and then sent back to the deoxygenation reactor. The flue gas with oxygen removed is sent back to the boiler flue, and after flowing to the secondary air preheater, H2 is added according to a molar ratio of CO2 to H2 in the flue gas of 1:3, and then the mixture is passed into the thermal catalytic reactor. CO2 and H2 are reduced to methanol at 300°C-340°C by a Ni-Ga-Zr catalyst arranged therein, and the flue gas is sent back to the primary air preheater for further heat exchange. The low-temperature flue gas from the primary air preheater passes through a dust removal device and is passed into a low-temperature economizer for heat exchange, so that the flue gas is cooled to below 70°C to condense the methanol. At the same time, the present invention proposes a method for preparing a catalyst for converting CO2 hydrogenation from coal-fired flue gas into methanol under normal pressure, which is characterized by comprising the following steps:

[0004] (1) A mixture of Ni(NO3)2·6H2O, Ga(NO3)3·3H2O, and ZrCl4 was used as a precursor with an atomic molar ratio of Ni:Ga:Zr = 5:3:3. The precursor and deionized water were mixed and dissolved in a mass ratio of 1:4. Al2O3 powder (10% of the total mass of the precursor) was added to the solution and stirred at 30°C for 6 h at a stirring rate of 500 r / min.

[0005] (2) Place the stirred solution in an ultrasonic machine and sonicate for 1 hour to obtain a uniform dispersion;

[0006] (3) Adding NaOH solution to the homogenized solution to adjust the pH to 9, and aging the solution for 2 h to obtain a precipitate;

[0007] (4) The precipitate was washed twice with deionized water by centrifugation at 4000 r / min for 5 min, and then dried in a drying oven at 100°C for 12 h;

[0008] (5) After drying, the solid was calcined by placing it in a muffle furnace and calcining it at 500°C for 2 h at a heating rate of 5°C / min to obtain a bulk Ni-Ga-Zr catalyst, which was then placed in a tubular furnace and calcined again at 800°C in a N2 atmosphere for 2 h at a heating rate of 10°C / min;

[0009] (6) tableting and crushing the calcined catalyst, and screening the sieve to a pore size of 60-80 mesh;

[0010] (7) The catalyst was then reduced in a H2 atmosphere at 500°C for 2 h to obtain the Ni-Ga-Zr catalyst required for the hydrogenation of CO2 into methanol at normal pressure.

[0011] The deoxygenation reactor and the reduction reactor adopt a dual fluidized bed reactor structure connected in series. The dual fluidized bed reactor consists of two cross-connected fluidized bed reactors, both of which are equipped with cyclone separators and return legs. The return material of each fluidized bed flows to the furnace of the other fluidized bed. One fluidized bed serves as a deoxygenation reactor and the other as a reduction reactor. The deoxygenation reactor is fed with high-temperature flue gas from the furnace of a coal-fired boiler, which contains a certain amount of oxygen (generally 3-5% by volume), and the reduction reactor is fed with H2. An oxidation reaction between Fe powder and O2 occurs in the deoxygenation reactor, which absorbs and removes oxygen from the flue gas; a reduction reaction between iron oxide and H2 occurs in the reduction reactor, which reduces the iron oxide to Fe powder, thereby realizing the recycling of Fe powder. The temperature of the two reactors is controlled at 800-900°C by the cooling system on the surface of the deoxygenation reactor and the reduction reactor to prevent Fe powder from sintering.

[0012] The thermal catalytic reactor is arranged after the secondary air preheater. The flue gas temperature at the outlet of the secondary air preheater is 200-300℃. It adopts a fixed bed structure with an insulation layer on the surface. An H2 flow meter is installed at the inlet of the fixed bed to adjust the molar ratio of CO2 and H2 in the flue gas to 1:3. Ni-Ga-Zr catalyst is placed in the bed. The bed has a built-in cooling water pipe system and is equipped with a temperature sensor to ensure the reaction temperature is 300℃-340℃.

[0013] The beneficial effect of the present invention is that the CO2 in the coal smoke is directly hydrogenated into methanol without going through a carbon capture process, and then the flue gas is simply cooled to condense the methanol into a liquid and separated from the flue gas, so that a high-value carbon-containing product, methanol, can be obtained. This process does not require a special CO2 capture process, and can achieve high-value conversion and low-energy separation of CO2 in coal flue gas in one fell swoop, with the advantages of low energy consumption and simple process. In addition, the high-temperature flue gas above 900°C drawn out from the furnace outlet can directly meet the reaction temperature of the Fe powder oxidation reaction and deoxygenation after passing it into the deoxygenation reactor, without the need for additional heating. The Fe powder inside the deoxygenation reactor can undergo an oxidation reaction with O2 to remove O2 in the flue gas, avoid contact between O2 and H2 in the subsequent hydrogenation process, and avoid the risk of explosion. After the Fe powder is oxidized to Fe oxide, it can be reduced to Fe powder by H2 in the reduction reactor and fluidized and circulated back to the deoxygenation reactor, forming an Fe powder cycle for repeated use, saving resources and costs. The Ni-Ga-Zr catalyst used has high activity and good selectivity for methanol at normal pressure, does not require reaction gas pressurization, can avoid the high energy consumption problem under high pressure, and is simple to prepare and easy to operate. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 Schematic diagram of a system for converting CO2 hydrogenation from coal-fired flue gas into methanol at atmospheric pressure. The following symbols are used: 1. Boiler; 2. Deaerator; 3. Cyclone separator; 4. Return leg; 5. Reduction reactor; 6. Secondary air preheater; 7. Thermal catalytic reactor; 8. Primary air preheater; 9. Dust removal device; 10. Low-temperature economizer; 11. Methanol tank; 12. Chimney.

[0015] Figure 2 The invention relates to a method for preparing a catalyst for converting CO2 hydrogenation from coal-fired flue gas into methanol under normal pressure.

[0016] Figure 3 This is a comparison chart of the catalytic effects of Ni-Ga-Zr catalysts at different temperatures. DETAILED DESCRIPTION

[0017] The present invention proposes a system for converting CO2 hydrogenation from coal-fired flue gas into methanol under normal pressure, which is described below with reference to the accompanying drawings.

[0018] like Figure 1 In the system shown, CO2 hydrogenation of coal-fired flue gas is converted into methanol under normal pressure. The high-temperature flue gas of more than 900°C drawn from the outlet of the furnace of boiler 1 is passed into deaerator 2. The Fe powder arranged therein undergoes an oxidation reaction at 900°C to remove O2 from the flue gas. The flue gas with O2 removed and the oxidized Fe powder flow through cyclone separator 3 together. In cyclone separator 3, Fe oxides are separated and sent to reduction reactor 5 through return legs 4. The flue gas returns to the flue from the top of the cyclone separator to continue heat exchange. In the reduction reactor 5, Fe oxides are reduced to Fe powder by H2 and then fluidized into cyclone separator 3, separated and returned to deaerator reactor 2 through return legs 4. The temperature of the two reactors is controlled at 8 by the cooling system on the surface of the deaerator and reduction reactor. 00-900℃ to prevent Fe powder from sintering; the flue gas from which oxygen is removed is extracted when it flows through the outlet of the 6th secondary air preheater, and H2 is added according to the molar ratio of CO2 and H2 in the flue gas of 1:3, and then the mixture is passed into the 7th thermal catalytic reactor. In the 7th thermal catalytic reactor, CO2 and H2 are reduced to methanol at 300℃-340℃ by the Ni-Ga-Zr catalyst arranged in the bed, and then the flue gas is sent back to the 8th primary air preheater for further heat exchange. After flowing through the outlet of the 8th primary air preheater, the low-temperature flue gas is passed into the 9th dust removal device for dust removal, and then passed into the 10th low-temperature economizer for further heat exchange, and the flue gas temperature is cooled to below 70℃ to condense the methanol. The condensed methanol is collected and stored in the 11th methanol tank, and then the flue gas continues to flow to the chimney 12 for discharge.

[0019] Figure 2 This is a method for preparing a catalyst for converting CO2 hydrogenation from coal-fired flue gas into methanol under normal pressure, the specific steps are:

[0020] (1) Ni(NO3)2·6H2O, Ga(NO3)3·3H2O, and ZrCl4 were used as precursors. The precursors were weighed according to the atomic molar ratio of Ni:Ga:Zr = 5:3:3. 2.47g Ni(NO3)2·6H2O, 1.3g Ga(NO3)3·3H2O, and 1.19g ZrCl4 were weighed and added to 20ml deionized water in sequence and mixed and dissolved. 0.5g Al2O3 powder was also weighed and added to the solution. The mixture was stirred and dried at 30°C at a rate of 500 r / min for 6h.

[0021] (2) The stirred solution was placed in an ultrasonic machine and sonicated for 1 hour to obtain a homogeneous dispersion; NaOH solution was added to the homogeneous dispersion to adjust the pH to 9, and then the solution was aged for 2 hours to obtain a precipitate;

[0022] (3) The precipitate was washed twice with deionized water by centrifugation for 5 min at a speed of 4000 r / min. After centrifugation, it was placed in a drying oven at 100°C for 12 h.

[0023] (4) Calcination is performed after drying. First, it is placed in a muffle furnace and calcined at 500°C for 2 hours with a heating rate of 5°C / min. After calcination, it is placed in a tubular furnace and calcined again at 800°C in a N2 atmosphere with a heating rate of 10°C / min.

[0024] (5) tableting the calcined catalyst to obtain a catalyst having a certain particle size distribution, with a sieve aperture of 60-80 mesh;

[0025] (6) The catalyst is then placed in a H2 atmosphere at 500°C for 2 hours to reduce the oxides therein to active metals or active component phases, thereby obtaining the Ni-Ga-Zr intermetallic compound catalyst required for the hydrogenation of CO2 into methanol at normal pressure.

[0026] Figure 3 The figure shows the comparison of the catalytic effects of Ni-Ga-Zr catalysts at different temperatures. The literature has reported that Ni-Ga bimetallic catalysts can be used to catalyze CO2 to methanol. The present invention proposes a Ni-Ga-Zr trimetallic catalyst and conducts a comparative experiment on the effect of CO2 hydrogenation to methanol at normal pressure (1 atmosphere). Figure 3 As shown, Ni-Ga catalyst is the base catalyst, Ni-Ga-Zr xThis is a trimetallic catalyst with the addition of Zr (x represents the atomic molar ratio of Ni:Ga:Zr = 5:3:x). Compared with the Ni-Ga catalyst without Zr, the methanol yield is significantly improved after the addition of Zr. The best effect is achieved when Ni:Ga:Zr = 5:3:3, with the methanol yield reaching 3 times that of the Ni-Ga catalyst. This shows that the catalyst provided by the present invention has a good effect of catalyzing CO2 hydrogenation to methanol at normal pressure. At the same time, Figure 3 Tests show that the Ni-Ga-Zr catalyst has a catalytic effect in the range of 240-360°C, with the best reaction temperature being 300-340°C.

Claims

1. A system for converting CO2 hydrogenation of coal-fired flue gas into methanol under normal pressure and a catalyst preparation method, characterized in that: The system includes a coal-fired boiler, a deoxidation reactor, a reduction reactor, a thermal catalytic reactor and a methanol collection device. The high-temperature flue gas above 900°C drawn out from the tail of the furnace passes through the deoxidation reactor and is oxidized by the Fe powder arranged therein to remove O2. At the same time, the oxidized Fe powder enters the reduction reactor through fluidization and is reduced to Fe powder by H2 and then sent back to the deoxidation reactor. The flue gas with oxygen removed is sent back to the boiler flue. After passing through the secondary air preheater, H2 is added according to the molar ratio of CO2 to H2 in the flue gas of 1:3, and then the mixture is passed into the thermal catalytic reactor. CO2 and H2 are reduced to methanol at 300°C-340°C by the Ni-Ga-Zr catalyst arranged therein. The flue gas is sent to the primary air preheater for further heat exchange. The low-temperature flue gas coming out of the primary air preheater passes through the dust removal device and is passed into the low-temperature economizer for heat exchange, and the flue gas is cooled to below 70°C to condense the methanol. The preparation method of a catalyst for converting CO2 hydrogenation of coal-fired flue gas into methanol at normal pressure is characterized by comprising the following steps: (1) A mixture of Ni(NO3)2·6H2O, Ga(NO3)3·3H2O, and ZrCl4 was used as a precursor with an atomic molar ratio of Ni:Ga:Zr = 5:3:

3. The precursor and deionized water were mixed and dissolved in a mass ratio of 1:

4. Al2O3 powder (10% of the total mass of the precursor) was added to the solution and stirred at 30°C for 6 h at a stirring rate of 500 r / min. (2) Place the stirred solution in an ultrasonic machine and sonicate for 1 hour to obtain a uniform dispersion; (3) Adding NaOH solution to the homogenized solution to adjust the pH to 9, and aging the solution for 2 h to obtain a precipitate; (4) The precipitate was washed twice with deionized water by centrifugation at 4000 r / min for 5 min, and then dried in a drying oven at 100°C for 12 h; (5) After drying, the solid was calcined by placing it in a muffle furnace and calcining it at 500°C for 2 h at a heating rate of 5°C / min to obtain a bulk Ni-Ga-Zr catalyst, which was then placed in a tubular furnace and calcined again at 800°C in a N2 atmosphere for 2 h at a heating rate of 10°C / min; (6) tableting and crushing the calcined catalyst, and screening the sieve to a pore size of 60-80 mesh; (7) The catalyst was then reduced in a H2 atmosphere at 500°C for 2 h to obtain the Ni-Ga-Zr catalyst required for the hydrogenation of CO2 into methanol at normal pressure.

2. The system according to claim 1, characterized in that The deoxidation reactor and the reduction reactor adopt a dual fluidized bed reactor structure; the dual fluidized bed reactor is composed of two cross-connected fluidized bed reactors, both of which are equipped with cyclone separators and return legs. The return material of each fluidized bed flows to the furnace of the other fluidized bed. One fluidized bed serves as a deoxidation reactor and the other as a reduction reactor. The temperature of the two reactors is controlled at 800-900°C by the cooling system on the surface of the deoxidation reactor and the reduction reactor to prevent Fe powder from sintering.

3. The system according to claim 1, characterized in that The thermal catalytic reactor adopts a fixed bed structure and is arranged after the secondary air preheater. The flue gas at the outlet of the secondary air preheater is at 200-300℃. Ni-Ga-Zr catalyst is placed in the bed of the thermal catalytic reactor. The bed has a built-in cooling water pipe system and is equipped with a temperature sensor to ensure that the bed reaction temperature is between 300℃ and 340℃.

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