CN102911756B - A low-rank coal-to-methane process - Google Patents

Abstract

A low-rank coal-to-methane process is to pyrolyze low-rank coal to obtain pyrolysis semi-coke and pyrolysis gas containing tar. The pyrolysis gas containing tar is cooled and separated to obtain tar, water and pyrolysis gas; the pyrolysis semi-coke is sent to the methanation reactor, and the reactive pyrolysis semi-coke reacts with circulating hydrogen under high temperature and high pressure conditions The methanation gas is obtained, and the difficult-to-react pyrolysis semi-coke is sent to the gasification reactor, where it reacts with water vapor and oxygen to obtain the gasification gas. Methanation gas and gasification gas are used as heat sources to indirectly provide heat for the pyrolysis reaction of raw coal in the pyrolysis reactor. Pyrolysis gas and gasification gas are converted, decarbonized, and separated separately or mixed to obtain hydrogen-rich gas, which is heat exchanged, purified, and separated with methanation gas to obtain hydrogen-rich gas to provide hydrogen sources for methanation units . Using this process to produce natural gas for replacement has the advantages of high thermal efficiency, low hydrogen consumption, fast methanation reaction rate and the like.

Description

A low-rank coal-to-methane process

technical field

The invention relates to a process for producing methane from low-rank coal, in particular to a process for producing methane from low-rank coal through drying, pyrolysis, pyrolysis semi-coke methanation and methanation residue gasification.

technical background

my country's demand for natural gas is rising rapidly, and the supply gap is increasing day by day. It is estimated that the average annual growth rate of my country's natural gas demand will reach 15% before 2030, when my country's natural gas demand will reach 400-500 billion m3, of which about 50% will depend on imports. Insufficient natural gas supply will directly affect my country's economic construction and social development. my country is relatively rich in low-rank coal resources. Coal-to-natural gas is an important way to transform coal resources and a powerful means to realize natural gas supplementation.

The existing document CN102344841A discloses a "method for producing substitute natural gas by using coal-based synthesis gas". Hydrogen reacts with carbon monoxide and carbon dioxide to form methane, which is then condensed and separated to obtain natural gas with a methane volume percentage of more than 94%. Among them, the gas inlet temperature of the first-stage methanation reactor is 200-350°C, the pressure inside the reactor is 2-5MPa, the gas inlet temperature of the second-stage methanation reactor is 300-360°C, and the pressure inside the reactor is higher than that of the first-stage The internal pressure of the methanation reactor is 0.08-0.18MPa lower, and the catalyst is a nickel-based catalyst. This method is an indirect methanation process, and requires two-stage methanation reaction, and the process flow is long; the principle of this process is:

CO+3H ₂ →CH ₄ +H ₂ O (Ⅰ) CO ₂ +4H ₂ →CH ₄ +2H ₂ O (Ⅱ)

It can be seen from (I) that when one molecule of methane is generated, one molecule of hydrogen is wasted due to the formation of water. From (II), it can be seen that for the generation of one molecule of methane, two molecules of hydrogen are wasted due to the formation of water. This results in high hydrogen consumption in the system; in addition, the process heat is not coupled, resulting in energy waste.

The existing document CN102061182A discloses a "coal hydropyrolysis and gasification coupling method". The process flow of the method is as follows: firstly dry and preheat the raw coal with a particle size of ≤2mm, and after drying, the total moisture content is ≤8% ; The preheating temperature is 250-350°C, 60-80% of the raw coal enters the coal hydrogenation pyrolysis furnace for hydrogenation pyrolysis, the pyrolysis pressure is 1.0-3.0MPa, and the temperature is 550-680°C; the pyrolysis gas phase The product enters the purification and separation system; all the semi-coke produced by the coal hydrogenation pyrolysis furnace is sent to the coke hydrogenation gasification furnace for hydrogenation gasification, the gasification pressure is 1.0-3.0MPa, the temperature is 327-427℃, and the gasification gas phase product Separation system purification and separation; dry and preheated 20-40% raw coal and coke particles from the hydrogenation gasifier, oxygen and water vapor are gasified in the hydrogen-rich generator, and the reaction pressure is 1.0-3.0MPa , the temperature is 900-1500°C. This method needs to separate the dried and preheated raw coal according to different usage amounts, and the operation is cumbersome; the coupling of heat is not realized, and the energy loss is large; The hydromethanation rate is very low, resulting in a low overall methanation reaction rate.

Contents of the invention

The technical problem to be solved by the present invention is the problem of large hydrogen consumption and low heat energy utilization rate in the hydrogenation methane process, and provides a low-rank coal methane process.

The measures taken by the present invention to solve the problems referred to above are a kind of technique of low rank coal to methane, and its described technique is to carry out according to the following steps:

(1) Dry the 3-10mm low-rank coal at 110-200°C to reduce the moisture content to 5-8% (wt%), then send it to the pyrolysis reactor, and dry it at 300-700°C and normal pressure Perform pyrolysis under the same conditions to obtain pyrolysis semi-coke and pyrolysis gas containing tar;

Then the pyrolysis semi-coke is sent to the methanation reactor, and the reaction is carried out at a temperature of 750-930°C and a pressure of 3-5Mpa. The reactive part of the pyrolysis semi-coke will generate C+ _2H with the gasification agent H2 ₂ → CH ₄ reaction to generate reaction gas rich in CH ₄ , and the remaining methanation residue from the reaction is sent to the gasification reactor for pressurized gasification;

(2) The output of the pyrolysis gas containing tar is cooled, separated, and purified to obtain tar and pyrolysis gas, and then the pyrolysis gas is subjected to CO conversion and gas separation to obtain methane, carbon dioxide, and hydrogen-rich gas, of which Methane is output as product, and hydrogen-rich gas is used as cycle gas I;

(3) The 500-800°C methanation gas output from the methanation reactor is sent to the pyrolysis reactor for indirect heat exchange with the coal material, and the raw coal is pyrolyzed; the methanation gas output from the pyrolysis reactor Then exchange heat with circulating hydrogen and cold water respectively to obtain cold methanation gas at 30-150 °C, and then obtain synthetic natural gas and circulating gas II after purification and separation;

(4) The 500-700°C gasification gas in the pyrolysis reactor conducts indirect heat exchange with the coal material, and after purification, conversion, and decarburization, the cycle gas III is obtained;

(5) After mixing the recycle gas I, recycle gas II and recycle gas III obtained in the above steps, recycle hydrogen is obtained, and the recycle hydrogen is sent to the methanation reactor as a gasification agent after preheating.

Based on the above-mentioned process for producing methane from low-rank coal, further,

The low-rank coal is lignite, long-flame coal or non-caking coal.

The pyrolysis gas in step (2) and the gasification gas in step (4) may be mixed first, and then purified and transformed.

The tar yield in the pyrolysis product gas containing tar is 5-12% (wt%) of the raw coal sent to the pyrolysis reactor.

After the pyrolysis semi-coke is sent into the methanation reactor for reaction, the conversion rate of carbon in the pyrolysis semi-coke is 35-60%.

Realize a kind of low-rank coal-to-methane process provided by the present invention, compared with the prior art, its advantages and positive effects are:

First, the main reaction of methane production in this process is C+2H ₂ →CH ₄ , and the hydrogen consumption of the system is low.

The second is that the absorption and exothermic reactions of this process are coupled, and the thermal efficiency of the system is higher.

The third is that the properties of the product gases of different units are very different. In this process, the product gases of different units do not mix with each other, and the gas treatment is highly targeted, the processing load is small, and the efficiency is high.

Fourth, this process can realize hydrogen self-sufficiency.

Description of drawings

Fig. 1 is a process flow chart of the present invention for producing methane from low-rank coal.

Detailed ways

A low-rank coal-to-methane process provided by the present invention can be further described through the description of the drawings and specific examples, rather than limiting the process of the present invention.

Example 1

Implement the technology of a kind of low-rank coal methane production provided by the present invention, take the technological process of producing 1,100,000,000 cubic meters of methane as an example, and its described specific technology is to carry out according to the following steps:

In this embodiment, lignite is used as raw material, and the basic coal quality analysis of the lignite used is shown in Table 1. 17,800 tons/day lignite raw coal was crushed, sieved to 3-6mm, and dried and dehydrated at 150°C to obtain dry lignite with a moisture content of 8% (wt%). The amount of dry lignite in the pyrolysis reactor is 16,000 tons/day. Dried lignite stays in the pyrolysis reactor at a pyrolysis temperature of 600°C for 1.5 hours, and the products obtained are pyrolysis semi-coke and pyrolysis gas containing tar, of which the output of pyrolysis semi-coke at 600°C is 10,000 tons /day, the pyrolysis gas is cooled and separated to obtain about 1900 tons/day of low-temperature fuel-type tar, and the amount of the purified pyrolysis gas is 2.4 million Nm3 of pyrolysis gas. The composition and volume content are shown in Table 2.

Pyrolysis semi-coke at 600°C is fed intermittently from the top of the methanation reactor, and circulating hydrogen is continuously fed from the bottom of the methanation reactor. The reaction temperature in the methanation reactor is 800-830°C, and the reaction pressure is 5MPa. Under these conditions , the carbon in the pyrolysis semi-coke reacts with hydrogen to generate methane. Since there is still a small amount of oxygen in the pyrolysis semi-coke, a small amount of CO, CO ₂ and H ₂ O will be generated. Because the side chain groups in the pyrolysis semi-coke There are different carbon-containing structures such as groups, aromatic ring structures, and aromatic layer structures, so there are parts that are easy to hydrogenate and difficult to hydrogenate in pyrolysis semi-coke, and the reactivity of the difficult-to-hydrogenate part is poor and the reaction rate is slow. The methanation reactor only converts the easy hydrogenation regardless of the carbon structure. In this example, the carbon conversion rate of the pyrolysis semi-coke in the methanation reactor is controlled to 41.64%, and the remaining unreacted part is sent to the gasification reaction as the methanation residue device. The output of methanation gas is 11.12 million Nm3/day, and its composition and volume content are shown in Table 3.

The high-temperature methanation residue is intermittently fed into the gasification reactor, and undergoes gasification reaction with water vapor and oxygen at 1000-1200°C and 3MPa pressure, so that the carbon in the methanation residue is almost completely utilized and gasified to generate gas The yield is 9.46 million Nm3, and its composition and volume content are shown in Table 4.

Methanation gas and gasification gas are both high-temperature and high-pressure gases. These two gases are sent to different heating tubes in the pyrolysis reactor, so that the high-temperature gas is used as a heat source to exchange heat indirectly with the coal material, and the gas that exits the pyrolysis reactor There are three paths in total, which are pyrolysis gas, methanation gas and gasification gas. These three gases are in different pipelines, so they can be processed separately. The pyrolysis gas produced from the pyrolysis reactor is firstly cooled, separated to remove dust, tar and water, then purified, transformed, and finally gas separated to obtain methane-rich gas I, carbon dioxide and the remaining hydrogen-rich gas. Hydrogen gas is used as the cycle gas.

The methanation gas that exits the pyrolysis reactor is first exchanged with circulating hydrogen and cold water respectively, and after purification, gas membrane separation is performed to obtain methane-rich gas 2 and hydrogen-rich gas 2, and hydrogen-rich gas 2 is used as cycle gas II.

The gasification gas that exits the pyrolysis reactor is first subjected to heat exchange, purification, and conversion, and then decarburized to reduce the CO ₂ concentration to below 0.3% to obtain cycle gas III.

Cycle gas I, cycle gas II and cycle gas III are mixed to obtain mixed cycle hydrogen, which is then sent to the methanation reactor as a gasification agent after heat exchange with the methanation gas.

The above process produced a total of 3.7974 million Nm3/day of CH ₄ , or 1.139 billion Nm3/year. The system produced a total of 7.3013 million Nm3/day of H ₂ in the whole process (including 488,400 Nm3 generated in the pyrolysis reaction and 4,189,800 Nm3 generated in the gasification reaction , CO transformation produces 2.6232 million Nm3), which is greater than the system consumption of H ₂ 6.82 million Nm3/day (consumption in the methanation reactor), and the system can achieve hydrogen self-sufficiency.

Table 1 Lignite coal quality analysis (received basis %)

moisture Ash C h o N S 17 19.49 40.03 2.84 19.53 0.79 0.32

Table 2 Composition and content of pyrolysis gas

components CH ₄ CO H ₂ O ₂ CO ₂ C ₂ H ₄ C ₂ H ₆ N ₂ Content (v%) 24.52 10.52 20.35 0.43 40.15 1.02 2.48 0.53

Table 3 Composition and content of methane-generated gas

components CH ₄ CO CO ₂ C _m H _n H ₂ H ₂ S NH ₃ Content (v%) 27.64 2.01 1.26 1.35 66.37 0.18 1.19

Table 4 Composition and content of gasification gas

components H ₂ CO CH ₄ CO ₂ O ₂ N ₂ H ₂ S Content (v%) 44.29 25.06 1.43 28.89 0.13 0.05 0.15

Example 2

Implement a process for producing methane from low-rank coal provided by the present invention, a process flow with an annual output of 600 million cubic meters of methane. Its described specific technique is carried out according to the following steps:

In this embodiment, long-flame coal is used as raw material, and its specific analysis is shown in Table 5. The long-flame coal with a particle size of 5-10mm is fed into the pyrolysis reactor, and the feeding amount is 5000 tons/day. The residence time of long-flame coal in the pyrolysis reactor is 2.5 hours. After pyrolysis, 650°C pyrolysis semi-coke and pyrolysis gas containing tar are obtained. The output of 650°C pyrolysis semi-coke is 4028 tons/day. After cooling and separating the pyrolysis gas, about 273 tons/day of low-temperature fuel-type tar can be obtained. The purified pyrolysis gas is 500,000 Nm3 of pyrolysis gas. The composition and volume content are shown in Table 6.

The pyrolysis semi-coke at 650°C is fed intermittently from the top of the methanation reactor, and the circulating hydrogen is continuously fed from the bottom of the methanation reactor. The reaction temperature in the methanation reactor is 760-800°C, and the reaction pressure is 4MPa. Under these conditions , the carbon in the pyrolysis semi-coke reacts with hydrogen to generate methane. In this example, the carbon conversion rate of the pyrolysis semi-coke in the methanation reactor is controlled to 38%, and the remaining unreacted part is sent to the gasification reaction as the methanation residue device. The output of methanation gas is 7.46 million Nm3/day, and its composition and volume content are shown in Table 7.

The high-temperature methanation residue is intermittently fed into the gasification reactor, and undergoes gasification reaction with water vapor and oxygen at 1000-1200°C and 3MPa pressure, so that the carbon in the methanation residue is almost completely utilized and gasified to generate gas The yield is 7.61 million Nm3, and its composition and volume content are shown in Table 8.

Methanation gas and gasification gas are both high-temperature and high-pressure gases. These two gases are sent to different heating tubes in the pyrolysis reactor, so that the high-temperature gas is used as a heat source to exchange heat indirectly with the coal material, and the gas that exits the pyrolysis reactor There are three paths in total, which are pyrolysis gas, methanation gas and gasification gas. These three gases are in different pipelines, so they can be processed separately. The pyrolysis gas produced from the pyrolysis reactor is first cooled, separated to remove dust, tar and water, then purified, transformed, and finally gas separated to obtain methane-rich gas 1, carbon dioxide and the remaining hydrogen-rich gas. Hydrogen gas is used as cycle gas I.

The methanation gas that exits the pyrolysis reactor is first exchanged with circulating hydrogen and cold water respectively, and after purification, gas membrane separation is performed to obtain methane-rich gas 2 and hydrogen-rich gas 2, and hydrogen-rich gas 2 is used as cycle gas II.

The gasification gas that exits the pyrolysis reactor is first subjected to heat exchange, purification, and conversion, and then decarburized to reduce the CO ₂ concentration to below 0.3% to obtain cycle gas III.

Cycle gas I, cycle gas II and cycle gas III are mixed to obtain mixed cycle hydrogen, which is sent to the methanation reactor as a gasification agent after heat exchange with the methanation gas.

The above process produced a total of 2.08 million Nm3/day of CH ₄ , or ₆₂₄ million Nm3/year (according to 300 days). 3.68 million Nm3 generated by the methanation reaction, and 1.8 million Nm3 generated by CO conversion), which is greater than the system consumption of H ₂ 4.3 million Nm3/day (consumption in the methanation reactor), and the system can achieve hydrogen self-sufficiency.

Table 5 Long-flame coal analysis table (received basis %)

moisture Ash C h o N S 3.6 3.76 75.66 4.42 11.12 1.19 0.25

Table 6 Composition and content of pyrolysis gas

components CH ₄ CO H ₂ O ₂ CO ₂ C ₂ H ₄ C ₂ H ₆ N ₂ Content (v%) 22.72 17.03 26.77 0.21 30.63 0.87 1.43 0.34

Table 7 Composition and content of methane-generated gas

components CH ₄ CO CO ₂ C _m H _n H ₂ H ₂ S NH ₃ Content (v%) 25.37 2.15 1.14 1.74 68.47 0.11 1.02

Table 8 Composition and content of gasification gas

components H ₂ CO CH ₄ CO ₂ O ₂ N ₂ H ₂ S Content (v%) 48.41 22.56 1.03 27.77 0.10 0.02 0.11

The feature of the pyrolysis reactor in this process is that the high-temperature and high-pressure methanation gas and the high-temperature and high-pressure gasification gas exchange heat with the coal material indirectly, and the methanation gas, gasification gas and pyrolysis gas interact with each other. Isolation, which makes it easier to deal with each part of the gas, the load is small and the efficiency is high. Moreover, the above-mentioned three-way gas can exchange heat with the circulating hydrogen after it comes out of the pyrolysis reactor, which improves the heat utilization rate and reduces energy waste. The coal utilization rate of this process is high, the final C conversion rate of raw coal is above 95%, and the system can realize hydrogen self-sufficiency. In addition, the tar yield can be adjusted according to market demand.

Claims (5)

1. A low-rank coal methane process, its described process is carried out according to the following steps: (1) Dry the 3-10mm low-rank coal at 110-200°C to reduce the moisture content to 5-8% (wt%), then send it to the pyrolysis reactor, and dry it at 300-700°C and normal pressure Perform pyrolysis under the same conditions to obtain pyrolysis semi-coke and pyrolysis gas containing tar; Then the pyrolysis semi-coke is sent to the methanation reactor, and the reaction is carried out at a temperature of 750-930°C and a pressure of 3-5Mpa. The reactive part of the pyrolysis semi-coke will generate C+ _2H with the gasification agent H2 ₂ → CH ₄ reaction to generate reaction gas rich in CH ₄ , and the remaining methanation residue from the reaction is sent to the gasification reactor for pressurized gasification; (2) The pyrolysis gas containing tar is output from the pyrolysis reactor, and after cooling, separation and purification, tar and pyrolysis gas are obtained, and then the pyrolysis gas is subjected to CO conversion and gas separation to obtain methane, carbon dioxide and Hydrogen-rich gas, in which methane is output as a product, and hydrogen-rich gas is used as cycle gas I; (3) The 500~800°C methanation gas output from the methanation reactor is sent to the pyrolysis reactor for indirect heat exchange with the raw coal to pyrolyze the raw coal; the methanation gas output from the pyrolysis reactor is The gas is then exchanged with circulating hydrogen and cold water to obtain cold methanation gas at 30-150°C, which is purified and separated to obtain synthetic natural gas and circulating gas II; (4) The gasification gas at 500-700 °C output from the gasification reactor is sent to the pyrolysis reactor for indirect heat exchange with the raw coal, and then purified, transformed, and decarbonized to obtain cycle gas III; (5) After mixing the recycle gas I, recycle gas II and recycle gas III obtained in the above steps, recycle hydrogen is obtained, and the recycle hydrogen is sent to the methanation reactor as a gasification agent after preheating. 2. The low-rank coal-to-methane process as claimed in claim 1, wherein said low-rank coal is lignite, long-flame coal or non-caking coal. 3. The low-rank coal-to-methane process as claimed in claim 1, wherein the pyrolysis gas in step (2) and the gasification gas in step (4) are mixed first, and then purified, transformed, and separated . 4. The low-rank coal-to-methane process as claimed in claim 1, wherein the tar yield is 5-12% (wt%) of the low-rank coal sent to the pyrolysis reactor. 5. The low-rank coal-to-methane process according to claim 1, wherein after the pyrolysis semi-coke is reacted in the methanation reactor, the conversion rate of carbon in the pyrolysis semi-coke is 35-60%.

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