Method for preparing LNG (Liquefied Natural Gas) and co-producing liquid ammonia and nitrogen products by using raw gas

Abstract

The invention relates to a method for preparing LNG (Liquefied Natural Gas) and co-producing liquid ammonia and nitrogen products by using raw gas, aiming at solving the technical problems of poor utilization efficiency of the raw gas and low product diversity at present, and the technical scheme is as follows: the method comprises the following steps: collecting the raw gas by using a gas holder, primarily pressing the gas, purifying TSA, pressurizing the gas, converting, washing with methanol at low temperature, removing CO, performing cryogenic separation and synthesizing ammonia. Aiming at the characteristic that the content of H2, CH4 and N2 in the raw gas is high, gradient utilization of the raw gas components is achieved, an LNG product, a liquid ammonia product and a nitrogen product are produced in a combined mode, the ratio of hydrogen to nitrogen in the hydrogen and nitrogen product obtained after cryogenic separation can be effectively controlled to be 3: 1, the hydrogen and nitrogen product can be directly used as downstream synthetic ammonia feed gas, all nitrogen comes from nitrogen in the raw gas, and the nitrogen is recycled. And an air separation device is not needed to supplement nitrogen.

Description

Technical Field

[0001] The present invention belongs to the technical field of comprehensive utilization of coal resources and clean energy, and particularly relates to a method for preparing LNG by using waste gas from coal pyrolysis and co-producing liquid ammonia and nitrogen products. Background Art

[0002] Waste gas from coal pyrolysis is the core by-product of coal pyrolysis industries such as coking and semi-coke production. Its main components include hydrogen, carbon monoxide, methane, carbon dioxide, nitrogen, etc. Such gas has a relatively high calorific value, so it can be directly burned as fuel or used for power generation. However, due to the complex composition and high impurity content of waste gas from coal pyrolysis, its direct utilization efficiency is low and the pollution is serious. Moreover, whether used as industrial gas or for power generation, only the combustibility of waste gas from coal pyrolysis is utilized, and its chemical composition and properties are not fully utilized.

[0003] Replacing diesel / coal with LNG can reduce CO2 emissions, and its market demand continues to grow. Liquid ammonia is the core substance of chemical fertilizers, hydrogen energy carriers and chemical raw materials, and the global annual demand is large. To improve the value of waste gas from coal pyrolysis, the industry has tried to convert it into LNG or liquid ammonia. However, the current processes have problems such as complex processes, high energy consumption, low atom economy, resource waste, and low product purity, which limit their large-scale industrial application. Therefore, developing an efficient comprehensive utilization process for waste gas from coal pyrolysis to convert it into clean energy products with high added value has important economic and environmental significance.

[0004] Patent CN202420481828 "An apparatus for preparing LNG from waste gas from coal pyrolysis and co-producing hydrogen" discloses an apparatus for preparing LNG and hydrogen from waste gas from coal pyrolysis. The apparatus includes a pretreatment sub-unit, a conversion unit, a desulfurization and decarbonization unit, a methanation unit and a cryogenic separation unit. Through this apparatus, purified gas and products with high added value can be obtained. However, this solution only extracts methane and hydrogen, and does not reasonably utilize the effective components of waste gas from coal pyrolysis. The nitrogen component, which accounts for nearly 50% of waste gas from coal pyrolysis, cannot be processed. Patent CN201910411758 "A process method for producing chemicals and co-producing heat and power by hydrogen extraction from waste gas from coal pyrolysis" discloses a process system solution for efficient hierarchical and quality utilization of waste gas from coal pyrolysis. While producing high-value chemicals, the by-product waste gas is comprehensively utilized to achieve co-production of heat and power, which not only improves the utilization efficiency of waste gas from coal pyrolysis but also reduces pollutant emissions. However, this method has a complex process flow and does not effectively utilize components such as methane, hydrogen and nitrogen in waste gas from coal pyrolysis. Summary of the Invention

[0005] The object of the present invention is to solve the above problems and provide a method for preparing LNG by using waste gas from coal pyrolysis and co-producing liquid ammonia and nitrogen products.

[0006] To solve the above technical problems, the technical solution adopted by the present invention is: A method for preparing LNG and co-producing liquid ammonia and nitrogen products from waste gas, comprising the following steps: Step 1) Collecting waste gas with a gas holder: Adjusting the gas volume fluctuation to ensure continuous and stable gas supply. After the waste gas leaves the gas holder, it is sent to the primary gas pressure device; Step 2) Primary gas pressure: The waste gas enters a compressor for primary pressure, and after the primary gas pressure, it is sent to the TSA purification device; Step 3) TSA purification: Using the temperature swing adsorption technology to remove tar and naphthalene impurities in the waste gas to meet the requirements of subsequent processes. The purified gas is sent to the gas booster device; Step 4) Gas boosting: The gas after TSA purification enters the gas booster device for further boosting to meet the process requirements of subsequent shift reactions and cryogenic separation devices; Step 5) Shift: Using the sulfur-tolerant shift process technology to make the CO in the waste gas react with steam to generate H 2 and CO 2 , and an oxygen removal reactor is set in front of the shift furnace to make the O2 in the gas react with H2 to generate water. The gas after the shift is sent to the low-temperature methanol washing device; Step 6) Low-temperature methanol washing: The low-temperature methanol washing device removes CO2, H2S, COS, and benzene in the waste gas through the high solubility of methanol in CO2, H2S, COS, and benzene. The gas after passing through the low-temperature methanol washing device is sent to the CO removal device; Step 7) CO removal: Using the temperature swing adsorption technology to remove CO in the gas. The gas after passing through the CO removal device is sent to the cryogenic separation device; Step 8) Cryogenic separation: Separating the gas into rich methane gas, hydrogen-nitrogen gas, and nitrogen at low temperature. Among them, the rich methane gas is sent to the LNG tank area as an LNG product, the hydrogen-nitrogen gas enters the ammonia synthesis device, and the nitrogen is sent out as a nitrogen product; Step 9) Ammonia synthesis: The ammonia synthesis device consists of an ammonia synthesis system, an ammonia synthesis gas compressor, an ammonia synthesis recycle gas compressor, and an ammonia ice machine. The hydrogen-nitrogen gas enters the ammonia synthesis device, and the hydrogen-nitrogen gas is boosted by the ammonia synthesis gas compressor. An ammonia synthesis reaction occurs in the ammonia synthesis tower. After heat recovery, cooling, and separation treatment, liquid ammonia is obtained and sold externally after being buffered in the liquid ammonia storage tank.

[0007] Furthermore, the gas holder in the step 1) adopts a dry rubber membrane gas holder or a dry oil-sealed gas holder, and the buffering time is 20 - 30 min.

[0008] Furthermore, the compressor in the step 2) adopts a screw compressor, a reciprocating compressor, or a centrifugal compressor, and is driven by an electric motor or a steam turbine. The gas pressure entering the compressor is 0 - 10 kPaG, and the gas pressure at the outlet after compression is 0.3 - 0.8 MPaG.

[0009] Furthermore, the TSA purification device in step 3) is divided into a temperature swing adsorption tar removal unit and a temperature swing adsorption naphthalene removal unit. The tar content in the gas at the outlet after TSA purification is < 1 mg / Nm3, and the naphthalene content is < 5 mg / Nm3.

[0010] Furthermore, the gas boosting device in step 4) uses a reciprocating compressor or a centrifugal compressor, which is driven by an electric motor or a steam turbine. The multi-stage compression technology is adopted in the boosting process, and the gas pressure after boosting reaches 2.0 - 4.0 MPaG.

[0011] Furthermore, the sulfur-tolerant shift process in step 5) uses a Co-Mo series catalyst, and the reaction temperature is controlled at 180 - 500 °C. The deoxygenation catalyst uses a Cu series catalyst. After the shift reaction, the CO content in the gas accounts for 0.2% - 0.5% of the total gas volume.

[0012] Furthermore, for the gas after low-temperature methanol washing in step 6), the impurity content indexes of tar and benzene are lower than 0.1 ppm, the total sulfide content index is lower than 0.1 ppm, and the CO2 content index is lower than 50 ppm.

[0013] Furthermore, the adsorbent for CO removal in step 7) includes a silica gel-based adsorbent, an activated carbon adsorbent, and a CuO-ZnO adsorbent filled in layers in sequence. The operating temperature of the adsorption tower for temperature swing adsorption of CO removal is 40 - 220 °C. After CO removal, the CO content in the gas is lower than 30 ppm.

[0014] Furthermore, in step 8), the ratio of hydrogen to nitrogen in the hydrogen-nitrogen gas is 3:1. The mixed refrigeration process using N2 and a C1 - C5 hydrocarbon mixture as the circulating refrigerant is adopted to separate LNG, hydrogen-nitrogen gas, and nitrogen at a temperature of -169 °C.

[0015] Furthermore, in step 9), the ammonia synthesis gas compressor and the ammonia synthesis recycle gas compressor can be separately set or combined to use one centrifugal compressor. After the hydrogen-nitrogen gas is boosted by the ammonia synthesis gas compressor, the pressure reaches 11.0 - 17.0 MPaG. Any high-efficiency ammonia synthesis catalyst can be used as the ammonia synthesis catalyst.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. The present invention realizes the cascade utilization of the components of raw coal gas, co-produces LNG products, liquid ammonia products, and nitrogen products. The hydrogen-nitrogen ratio in the hydrogen-nitrogen gas product obtained after cryogenic separation can be effectively controlled at 3:1, which can be directly used as the raw material gas for downstream ammonia synthesis. All the nitrogen comes from the nitrogen in the raw coal gas, and there is no need to use an air separation unit to supplement nitrogen; 2. The present invention improves the utilization efficiency of raw coke oven gas, and has the advantages of advanced and reliable process flow, low comprehensive energy consumption, environmental friendliness, and high economic benefits; 3. In view of the characteristics of high H2, CH4, and N2 contents in raw coke oven gas, the present invention provides a new way for the utilization of raw coke oven gas, getting rid of the original situation of extensive emission and low-value utilization of raw coke oven gas; 4. The LNG product prepared by the present invention meets the index requirements of lean liquid products specified in "Liquefied Natural Gas" GB / T38753-2020; the liquid ammonia product prepared meets the first-class product index requirements in "Liquid Anhydrous Ammonia" GB / T536-2017; the nitrogen product prepared meets the index requirement that the nitrogen purity is greater than 99.9%. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a schematic process flow diagram of the present invention; DETAILED DESCRIPTION OF THE INVENTION

[0018] The present invention will be further described below with reference to the drawings and embodiments. Example 1

[0019] The raw coke oven gas produced as a by-product by a semi-coke plant is 50000 Nm3 / h, with a temperature of 40 °C and a pressure of 10 kPaG. The total content of dust and tar in the raw coke oven gas: 220 mg / Nm3, the hydrogen sulfide content: 600 mg / Nm3, the organic sulfur content: 100 mg / Nm3, and its chemical composition is shown in Table 1:

[0020] The gas volume of the raw coke oven gas entering the project boundary is 50000 Nm 3 / h, and the following steps are carried out: Step 1) Collect the raw coke oven gas with a gas holder: Prepare a dry rubber membrane gas holder with a volume of 30000 m3, an operating pressure of 0.005 MPaG, adjust the gas volume fluctuation to ensure continuous and stable gas supply, and the raw coke oven gas is sent to the gas primary pressure device after buffering by the gas holder; Step 2) Gas primary pressure: The gas primary pressure device uses 2 screw compressors, 2 in operation and 0 in standby, driven by an electric motor. After the gas is pressurized to 0.6 MPaG, it is sent to the TSA purification device; Step 3) TSA purification: The TSA purification device is divided into a temperature swing adsorption tar removal unit and a temperature swing adsorption naphthalene removal unit. The temperature swing adsorption technology is used to remove impurities such as tar and naphthalene in the raw coke oven gas to meet the requirements of the subsequent sections. The tar content in the gas after TSA purification < 1 mg / Nm3, the naphthalene content < 5 mg / Nm3, and the purified gas is sent to the gas booster device; Step 4) Gas boosting: The gas boosting device uses a centrifugal compressor, 1 on 0 standby, driven by a motor, and the boosting process uses multi-stage compression technology to further boost the pressure to 2.5MPaG to meet the process requirements of subsequent shift reaction and cryogenic separation devices; Step 5) Conversion: Use Co-Mo catalyst to carry out sulfur-resistant conversion process technology to make CO in raw gas react with water vapor to generate H2 and CO2. The reaction temperature is controlled at 220-340℃. A deoxygenation reactor is set in front of the conversion furnace. Use Cu catalyst to make O2 in coal gas react with H2 to generate water. The CO content in the converted gas accounts for 0.2% of the total gas volume and is sent to the low-temperature methanol washing device. Step 6) Low-temperature methanol washing: The low-temperature methanol washing device removes CO2, H2S, COS and benzene from the raw gas through the high solubility of methanol in CO2, H2S, COS and benzene. The impurity content index of tar and benzene in the gas after the low-temperature methanol washing device is less than 0.1ppm, the total content index of sulfides such as H2S and COS is less than 0.1ppm, and the CO2 content index is 30ppm. The gas washed with low-temperature methanol is sent to the CO removal device; Step 7) CO removal: The adsorbent in the CO removal device includes silica gel adsorbent, activated carbon adsorbent and CuO-ZnO adsorbent filled in layers in sequence. The CO in the gas is removed by temperature swing adsorption technology. The operating temperature is 40°C for adsorption and 120°C for desorption. The CO content in the gas after CO removal is 10ppm. The gas after the CO removal device is sent to a cryogenic separation device. Step 8) Cryogenic separation: The cryogenic separation device uses a mixed refrigeration process of N2 and C1-C5 hydrocarbon mixture as the circulating refrigerant to achieve the separation of LNG, hydrogen nitrogen and nitrogen at a temperature of -169°C. The methane-rich gas is used as the LNG product with a yield of 3.1t / h and is sent to the LNG tank area. The recovery rate of methane in the LNG product is 99%. The ratio of hydrogen to nitrogen in the hydrogen nitrogen is 3:1 and enters the ammonia synthesis unit. Nitrogen is sent out as a nitrogen product with a yield of 15534Nm3 / h. Step 9) Ammonia synthesis: The ammonia synthesis unit is composed of an ammonia synthesis system, an ammonia synthesis gas compressor, an ammonia synthesis circulating gas compressor and an ammonia ice machine. The ammonia synthesis gas compressor and the ammonia synthesis circulating gas compressor are separately provided, and the machine type is a reciprocating compressor driven by a motor. Hydrogen and nitrogen enter the ammonia synthesis unit, and the ammonia synthesis gas compressor pressurizes the hydrogen and nitrogen to 11.0 MPaG. Ammonia synthesis reaction occurs in the ammonia synthesis tower. The ammonia synthesis catalyst adopts a DNCA type high-efficiency ammonia synthesis catalyst. After heat recovery, cooling and separation treatment, liquid ammonia product is obtained with an output of 8.0 t / h, which is sold after being buffered in a liquid ammonia storage tank.

[0021] The gas holder in step 1) can also be a dry oil-sealed gas holder, and the buffer time can be any value within 20 - 30 minutes.

[0022] The compressor in step 2) can also be a reciprocating compressor or a centrifugal compressor, and can also be driven by a steam turbine. The gas pressure entering the compressor can be any value within 0 - 10 kPaG, and the gas pressure at the outlet after compression can also be any value within 0.3 - 0.8 MPaG.

[0023] The gas boosting device in step 4) can also be a reciprocating compressor, driven by a steam turbine, and the gas pressure after boosting can also be any value within 2.0 - 4.0 MPaG.

[0024] The reaction temperature control in step 5) can also be any value within 180 - 500 °C, and the CO content in the gas after conversion can also be any value within 0.2% - 0.5%.

[0025] The CO2 content index in step 6) can also be any value lower than 50 ppm.

[0026] The operating temperature of the adsorption tower for removing CO by temperature swing adsorption in step 7) can also be any value within 40 - 220 °C, and the CO content in the gas after CO removal can also be any value lower than 30 ppm.

[0027] The ammonia synthesis gas compressor and the ammonia synthesis recycle gas compressor in step 9) can also be combined into one centrifugal compressor. The pressure of the hydrogen-nitrogen gas after being boosted by the ammonia synthesis gas compressor can also be any value within 11.0 - 17.0 MPaG, and the ammonia synthesis catalyst can also be any other high-efficiency ammonia synthesis catalyst. Example 2

[0028] The difference between Example 2 and Example 1 lies in: The raw coal gas produced as a by-product by a certain semi-coke plant is 115000 Nm3 / h, with a temperature of 60 °C and a pressure of 5 kPaG. The total content of dust and tar in the raw coal gas is 300 mg / Nm3, the hydrogen sulfide content is 700 mg / Nm3, and the organic sulfur content is 150 mg / Nm3. Its chemical composition is shown in Table 2:

[0029] The gas holder in step 1) is a dry oil-sealed gas holder with a volume of 50000 m3 and an operating pressure of 0.0035 MPaG; The raw coal gas primary pressure device in step 2) uses 1 centrifugal compressor, 1 in operation and 0 in standby, driven by a steam turbine, and the raw coal gas is boosted to 0.3 MpaG by the raw coal gas primary pressure. The naphthalene content in the gas purified by TSA in step 3) is 3 mg / Nm3; The gas boosting device in step 4) is driven by a steam turbine, and the gas is boosted to 3.0 MPaG by gas boosting; The reaction temperature in step 5) is controlled at 220 - 310 °C, and the CO content in the gas after conversion accounts for 0.5% of the total gas volume; The CO2 content index in the gas after passing through the low-temperature methanol washing device in step 6) is 40 ppm; The operating temperature in step 7) is adsorption at 40 °C and desorption at 150 °C, and the CO content in the gas after CO removal is 20 ppm; The methane recovery rate in the LNG product in step 8) is 98.5%, the output of the LNG product obtained at the outlet of the cryogenic separation device is 8.7 t / h, and the nitrogen product at the outlet of the cryogenic separation device is 36735 Nm3 / h; In step 9), the ammonia synthesis gas compressor boosts the hydrogen-nitrogen gas to 11.0 - 17.0 MPaG, the ammonia synthesis catalyst uses the AMOMAX type high-efficiency ammonia synthesis catalyst, and the output of the liquid ammonia product obtained is 18.6 t / h.

[0030] Other technical features are the same as those in Example 1.

Claims

1. A method for preparing LNG and co-producing liquid ammonia and nitrogen products using raw coal gas, characterized in that: The steps include: Step 1) Collect raw gas with a gas holder: adjust the gas volume fluctuation to ensure continuous and stable gas supply. After the raw gas leaves the gas holder, it is sent to the gas primary pressure device; Step 2) Gas initial compression: The raw gas enters the compressor for initial compression and is sent to the TSA purification device after the initial compression; Step 3) TSA purification: Use temperature swing adsorption technology to remove tar and naphthalene impurities in the raw gas to meet the requirements of subsequent processes, and the purified gas is sent to the gas booster; Step 4) Gas pressurization: The gas purified by TSA enters the gas pressurization device for further pressurization to meet the process requirements of the subsequent shift reaction and cryogenic separation device; Step 5) Conversion: The sulfur-resistant conversion process technology is used to make the CO in the raw gas react with water vapor to generate H2 and CO2. A deoxygenation reactor is set in front of the conversion furnace to make the O2 in the gas react with H2 to generate water. The converted gas is sent to the low-temperature methanol washing device; Step 6) Low-temperature methanol washing: The low-temperature methanol washing device removes CO2, H2S, COS and benzene from the raw gas through the high solubility of methanol in CO2, H2S, COS and benzene. The gas after the low-temperature methanol washing device is sent to the CO removal device; Step 7) CO removal: The CO in the gas is removed by temperature swing adsorption technology, and the gas after the CO removal device is sent to a cryogenic separation device; Step 8) Cryogenic separation: Separate the gas into methane-rich gas, hydrogen-nitrogen gas and nitrogen at low temperature, wherein the methane-rich gas is sent to the LNG tank farm as the LNG product, the hydrogen-nitrogen gas enters the ammonia synthesis unit, and the nitrogen is sent out as the nitrogen product; Step 9) Ammonia synthesis: The ammonia synthesis unit is composed of an ammonia synthesis system, an ammonia synthesis gas compressor, an ammonia synthesis circulating gas compressor and an ammonia ice machine. Hydrogen and nitrogen gases enter the ammonia synthesis unit and are pressurized by the ammonia synthesis gas compressor. Ammonia synthesis reaction occurs in the ammonia synthesis tower. Liquid ammonia is obtained after heat recovery, cooling and separation treatment, and is buffered in a liquid ammonia storage tank before being sold.

2. The method for preparing LNG and co-producing liquid ammonia and nitrogen products by using raw coal gas according to claim 1, characterized in that: The gas holder in step 1) adopts a dry rubber membrane gas holder or a dry thin oil seal gas holder, and the buffer time is 20 to 30 minutes.

3. The method for preparing LNG and co-producing liquid ammonia and nitrogen products by utilizing raw coal gas according to claim 1, characterized in that: The compressor in step 2) is a screw compressor, a reciprocating compressor or a centrifugal compressor, driven by a motor or a steam turbine, the pressure of the gas entering the compressor is 0-10 kPaG, and the pressure of the gas at the outlet after compression is 0.3-0.8 MPaG.

4. The method for preparing LNG and co-producing liquid ammonia and nitrogen products by utilizing raw coal gas according to claim 1, characterized in that: The TSA purification device in step 3) is divided into a temperature swing adsorption detarring unit and a temperature swing adsorption denaphthalene unit. The tar content in the outlet gas after TSA purification is less than 1 mg / Nm3, and the naphthalene content is less than 5 mg / Nm3.

5. The method for preparing LNG and co-producing liquid ammonia and nitrogen products by utilizing raw coal gas according to claim 1, characterized in that: The gas boosting device in step 4) adopts a reciprocating compressor or a centrifugal compressor, which is driven by a motor or a steam turbine. The boosting process adopts a multi-stage compression technology, and the gas pressure after boosting reaches 2.0-4.0 MPaG.

6. The method for preparing LNG and co-producing liquid ammonia and nitrogen products by utilizing crude gas according to claim 1, characterized in that: The sulfur-resistant conversion process in step 5) uses a Co-Mo catalyst, the reaction temperature is controlled at 180-500°C, the deoxidation catalyst uses a Cu catalyst, and the CO content in the converted gas accounts for 0.2%-0.5% of the total gas.

7. The method for preparing LNG and co-producing liquid ammonia and nitrogen products by utilizing raw coal gas according to claim 1, characterized in that: In the step 6), the impurity content indexes of tar and benzene in the gas after low-temperature methanol washing are lower than 0.1 ppm, the total content index of sulfides is lower than 0.1 ppm, and the CO2 content index is lower than 50 ppm.

8. The method for preparing LNG and co-producing liquid ammonia and nitrogen products by using raw coal gas according to claim 1, characterized in that: The adsorbent for CO removal in step 7) includes silica gel adsorbent, activated carbon adsorbent and CuO-ZnO adsorbent filled in layers in sequence. The operating temperature of the adsorption tower for CO removal by temperature swing adsorption is 40-220°C, and the CO content in the gas after CO removal is less than 30ppm.

9. The method for preparing LNG and co-producing liquid ammonia and nitrogen products by using crude gas according to claim 1, characterized in that: In the step 8), the ratio of hydrogen to nitrogen in the hydrogen-nitrogen gas is 3:1, and a mixed refrigeration process using a mixture of N2 and C1-C5 hydrocarbons as a circulating refrigerant is used to achieve separation of LNG, hydrogen-nitrogen gas and nitrogen at a temperature of -169°C.

10. The method for preparing LNG and co-producing liquid ammonia and nitrogen products by using raw coal gas according to claim 1, characterized in that: The ammonia synthesis gas compressor and the ammonia synthesis circulating gas compressor in step 9) are respectively provided or combined into one centrifugal compressor, the pressure of the hydrogen and nitrogen gases after being pressurized by the ammonia synthesis gas compressor reaches 11.0-17.0 MPaG, and the ammonia synthesis catalyst uses any high-efficiency ammonia synthesis catalyst.

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