WO2016119603A1 - Methane synthetic method and system - Google Patents

Abstract

The present invention provides a methane synthetic method, used for synthesizing underground gasified crude gas into methane. The method comprises: step 1: gasifying an underground coal layer by using an underground gasification furnace to produce crude gas; step 2: removing impurities in the crude gas by using a crude gas pretreatment system; step 3: calculating a required removal quantity of CO₂ in the crude gas and removing the corresponding quantity of CO₂ by using a low-temperature methanol washing device according to the required removal quantity of CO₂; and step 4: synthesizing the crude gas into methane by using a methanation device. According to the method for synthesizing crude gas into methanol and a methanol synthetic system in the present invention, carbon dioxide is absorbed by using a low-temperature methanol washing device, so that the content of carbon dioxide in the crude gas can be reduced, the composition of raw material gas for synthesizing the methanol is more stabilized, and the stable and safe methanol production using the methanation device can be ensured.

Description

Methane synthesis method and system Technical field

The invention relates to the technical field of methane production, in particular to a methane synthesis method and a methane synthesis system.

Background technique

As China's natural gas consumption continues to increase, the gap between natural gas supply and demand has expanded year by year. In order to solve the problem of domestic natural gas shortage and expand natural gas supply, in recent years, a number of coal-to-gas projects have been built in China, that is, coal is used as raw materials to produce waste gas by means of surface gasification. Due to the high cost of coal surface gasification, the continuous decline in natural gas prices has limited the development of coal surface gasification and natural gas technology to a certain extent. Underground coal gasification is a process in which coal in the ground is controlled to burn, and the combustible gas is generated by the heat and chemical action of coal. The biggest difference between this technology and traditional surface gasification is the use of underground gasification channels to replace traditional gasifiers, which greatly reduces equipment investment and saves huge coal mining, transportation, washing, and gas. Equipment such as chemical processing has the advantages of good safety, low investment and high efficiency.

Although the underground coal gasification process has great advantages in the preparation of raw material gas, due to the limitation of the control conditions of the underground gasification process, the waste gas component and gas production volume of the gasification furnace gasification fluctuate greatly, and the CO content fluctuation range is 0.67~ 6.06%, H ₂ content fluctuation range is 25.97～37.14%, single furnace gas production fluctuation range is 3000～5000Nm3/h), the waste gas produced by coal underground gasification process has a large carbon dioxide content, which causes the methanation synthesis section to change operating conditions. Large, directly affecting the stability of the synthesis reaction temperature and fluctuations in the gas composition of the export product, it is difficult to meet the needs of methanation industrialization, and limits the industrialization of underground gasification and natural gas technology.

The methanation technology uses CO and CO ₂ to react with H ₂ to completely convert to CH ₄ ; due to the different heat generated by CO and CO ₂ and H ₂ , the unstable waste gas component will be methanated in the methanation unit. The temperature of the methanation unit is different. On the one hand, the temperature-unstable methanation unit has great safety hazard; on the other hand, the temperature-unstable methane synthesis affects the activity or even failure of the nickel-based catalyst, which greatly affects methane. The yield of chemical synthesis.

Summary of the invention

The invention aims to provide a method for synthesizing methane and a methane synthesis system, which can reduce the carbon dioxide content in the waste gas to make the composition of the raw material gas for synthesizing methane more stable, and ensure the stable and safe production of methane by the methanation device.

In order to achieve the above advantages, the present invention provides a methane synthesis method for synthesizing underground coal gasification waste gas into methane, characterized in that the method comprises:

Step 1: The underground coal seam is gasified by an underground gasifier to generate waste gas;

Step 2: removing the particulate matter and tar in the waste gas by using a waste gas pretreatment system;

Step Three: calculating a shortage of gas in the required amount of CO ₂ removal, and removal of the amount of use of CO ₂ removing device Rectisol the corresponding amount of CO ₂ according to the needs;

Step 4: synthesizing the waste gas into methane using a methanation unit.

In an embodiment of a methane synthesis method of the present invention, in the third step, the method for calculating the amount of CO ₂ to be removed in the waste gas is to sample and analyze CO, H ₂ , and CO _{2 in the} waste gas. The volume fraction, according to the value of the methanation control coefficient a, calculate the amount of CO ₂ that needs to be removed in the low-temperature methanol washing device.

Said

Where Q is the volume of waste gas,

The volume fraction of the corresponding component in the component of the waste gas,

For the low temperature methanol washing unit to remove the CO ₂ volume, a is the methanation control coefficient a.

In an embodiment of the methane synthesis method of the present invention, the methanation control coefficient a ranges from 2.9 to 3.1.

In an embodiment of the methane synthesis method of the present invention, the methane synthesis method further comprises: when the underground gasifier is insufficient in gas production, using a standby gasifier to gasify the underground coal seam to generate waste gas. step.

In an embodiment of the methane synthesis method of the present invention, the methane synthesis method further comprises: when the underground gasification furnace or the standby gasification furnace produces an excess of gas, excess waste gas enters the boiler The step of burning.

In an embodiment of the methane synthesis method of the present invention, the methane synthesis method further comprises the step of cooling the synthesized methane and then pouring it into the methane gas storage tank for storage.

In an embodiment of the methane synthesis method of the present invention, in the third step, the temperature of the methanol in the low-temperature methanol washing device ranges from -70 ° C to -10 ° C.

The present invention also provides a methane synthesis system for synthesizing underground coal gasification waste gas into methane, characterized in that the methane synthesis system comprises an underground gasification furnace, a waste gas pretreatment system, and a removal system. a low-temperature methanol washing device and a methanation device for carbon dioxide in a waste gas, wherein the underground gasifier is used for gasifying underground coal seam into waste gas, and the waste gas pretreatment system is used for removing particulate matter in the waste gas and a tar, the low-temperature methanol washing device for removing carbon dioxide from the waste gas, the methanation device for synthesizing the waste gas into methane, the waste gas pretreatment system and the underground gasification furnace and The low temperature methanol washing device is in communication, and the low temperature methanol washing device is in communication with the methanation device.

In an embodiment of the methane synthesis system of the present invention, the methane synthesis system further includes a second low temperature methanol washing device disposed between the low temperature methanol washing device and the methanation device, the second low temperature methanol The washing device is used for secondary removal of carbon dioxide from the waste gas.

In one embodiment of the methane synthesis system of the present invention, the methane synthesis system further includes a backup gasifier for gasifying the subterranean coal seam, the backup gasifier being in communication with the waste gas pretreatment system.

In an embodiment of the methane synthesis system of the present invention, the methane synthesis system further includes a heat exchanger for adjusting a temperature, the heat exchanger for reducing a temperature of the methanol in the methane or the low temperature methanol washing device .

In the method for synthesizing methane and methane synthesis system of the waste gas of the present invention, the low-temperature methanol washing device absorbs carbon dioxide, and the carbon dioxide content in the waste gas can be reduced to make the composition of the raw material gas for synthesizing methane more stable, and the methanation device is stable and safe. Production of methane.

DRAWINGS

Fig. 1 is a schematic view showing a methane synthesis system of a first embodiment of the present invention.

Figure 2 is a graph showing the equilibrium relationship of the solubility of different gases in methanol.

Fig. 3 is a schematic view showing a methane synthesis system of a second embodiment of the present invention.

Fig. 4 is a schematic view showing a methane synthesis system of a third embodiment of the present invention.

Fig. 5 is a schematic view showing a methane synthesis system of a fourth embodiment of the present invention.

detailed description

The detailed description of the specific embodiments, structures, features and functions of the present invention in accordance with the present invention are set forth in the accompanying drawings and claims.

Fig. 1 is a schematic view showing a methane synthesis system of a first embodiment of the present invention. Referring to FIG. 1, the methane synthesis system of the present embodiment is used for making waste gas into methane, and the methane synthesis system includes an underground gasification furnace 1, a standby gasification furnace 2, a waste gas pretreatment system 3, and a low temperature methanol washing device. , fine desulfurization system 5, methanation device 6, methane gas storage tank 7, oxygen storage tank 8, gasification agent mixing device 9, boiler 10.

The underground gasification furnace 1 and the waste gas pretreatment system 3, the gasification agent mixing device 9, and the boiler 10 are connected through a pipeline; the standby gasification furnace 2 and the gasification agent mixing device 9 and the boiler 10 are connected through a pipeline; The treatment system 3 is connected to the underground gasification furnace 1, the low temperature methanol washing device 4 through a pipeline; the low temperature methanol washing device 4 and the waste gas pretreatment system 3, the fine desulfurization system 5, and the gasification agent mixing device 9 are connected through a pipeline; The system 5 is connected to the low temperature methanol washing device 4 and the methanation device 6 through a pipeline; the methanation device 6 is connected to the fine desulfurization system 5 and the methane gas storage tank 7 through a pipeline; the methane gas storage tank 7 and the methanation device 6 are connected through a pipeline; The oxygen storage tank 8 and the gasification agent mixing device 9 are connected through a pipeline; the gasification agent mixing device 9 is connected to the underground gasification furnace 1, the standby gasification furnace 2, the low temperature methanol washing device 4, and the oxygen storage tank 8 through a pipeline; The boiler 10 is connected to the underground gasification furnace 1, the backup gasification furnace 2, and the gasification agent mixing device 9 through a pipe. There are valves between the underground gasifier 1 and the boiler 10, such as a shut-off valve and a solenoid valve; a valve is provided between the backup gasifier 2 and the boiler 10.

The underground gasifier 1 is used to gasify underground coal seams into waste gas, which is mainly composed of CO, CO ₂ , H ₂ and H ₂ S. The backup gasifier 2 vaporizes the underground coal seam into waste gas when the waste gas produced by the underground gasification furnace 1 is insufficient. The waste gas pretreatment system 3 is used for removing particulate matter and tar from waste gas, specifically after filtering dust, tar and other impurities. The low-temperature methanol washing device 4 is used for removing excess acid gases such as CO ₂ and H ₂ S in the waste gas. These acid gases are unfavorable for production, and the sulfides cause poisoning of the catalyst in the downstream production, and must be taken off. Divide and recycle. Therefore, the core of low-temperature methanol washing technology is acid gas removal technology.

The low-temperature methanol washing device 4 is used for absorbing acid gases such as CO ₂ , H ₂ S, COS, and the low-temperature methanol washing device 4 uses cold methanol as an absorption solvent, and uses methanol at a low temperature such as -10 ° C to -70 ° C for acid gases such as The excellent solubility of CO ₂ , H ₂ S, COS and the like, and the removal of the acid gas in the raw material gas are a physical absorption method. The theoretical basis and basic principle of the low temperature methanol washing purification process are:

Raoul's law and Henry's law are two basic laws for studying the equilibrium of any gas and liquid phase. The gas and liquid phase equilibrium of the absorbed gas in methanol also conforms to these two basic laws.

Raoul's law: The vapor pressure of a solvent in a solution is equal to the product of the vapor pressure of the pure solvent and its mole fraction.

which is

P _A ——————Vapor pressure of solvent in mixed solution

—————Vapor pressure of pure solvent

X _A —————— mole fraction of solvent

Let the molar fraction of the solute be X _B because X _A =1-X _B , so

That is, the fraction of the solvent vapor pressure drop in the solution is equal to the mole fraction of the solute.

Henry's Law: In a constant temperature and equilibrium state, the solubility of a gas in a solution is proportional to the equilibrium pressure of the gas.

P _B =KX _B

P _B ——————The equilibrium pressure of the gas

X _B —————— The mole fraction of the gas in the solution

K——————Henry coefficient

Experiments have shown that if the solute obeys Henry's law in a dilute solution, the solvent must obey Raul's law.

Low-temperature methanol washing is the use of methanol at low temperatures (-70 ° C ~ -10 ° C), high pressure conditions, CO ₂ , H ₂ S has a higher absorption capacity, the effective components of the synthesis gas CO, H ₂ Lower solubility. That is, methanol as an absorption solvent has a high selectivity to the absorbed gas.

Henry's Law can be used in a variety of expressions for the convenience of material balance during use. For example, the relationship between the solubility of a gas in a liquid and the equilibrium partial pressure of the gas can be expressed as C _A ^* = HP _A . The equilibrium partial pressure of the solute gas above the liquid surface and the molar fraction of the solute in the solvent liquid can be expressed as P _A ^* =EX _A .

Henry's law applies to insoluble and less soluble gases. For easily soluble and more soluble gases, it can only be used in low liquid phase concentrations, and it must be noted that the molecular state of the solute in the gas phase and in the solution should be the same; For mixed gas, when the pressure is not large, Henry's law can be applied to each gas separately, and does not affect each other. However, when the partial pressure exceeds the applicable range of any one of the gases, the force between the points is added. Large, at this time, various solute gases should reduce their solubility to each other. Henry's law cannot be fully applied.

Absorption is the process of applying a liquid to absorb a gas. Usually used to absorb one or several components from a gas, The purpose of gas separation is achieved. The basic principle is to realize the difference in the solubility of each component in the gas mixture in the solvent by gas-liquid mass transfer.

The physical absorption method is to achieve the purpose of absorption by the difference in solubility of the acid gas in the solvent. The relationship between the minimum required agent flow rate Wmin, the total amount of raw material gas V, the feed gas pressure P, and the dissolution coefficient λi of the removed component is as follows:

Wmin=V/(P×λi)

It can be seen from the above formula that the minimum required solvent flow rate Wmin is independent of the concentration of the pre-removal component, and increases with decreasing pressure, and the economical efficiency of the device increases as the pressure increases. It can be seen from Fig. 2 that the dissolution coefficient λ is mainly affected by temperature and rises with decreasing temperature, thereby determining the optimum operating temperature of the methanol absorption process at -70 ° C to -10 ° C.

In industrial production, the concentration of actual gases often exceeds the range applicable to Henry's law. In the low-temperature methanol washing process, the solubility of each component in the shift gas in methanol is beyond the range applicable by Henry's law. The equilibrium relationship can be determined by experimental methods, but the basic idea of Henry's law is applicable. Figure 2 is a graph showing the equilibrium relationship of the solubility of different gases in methanol. Referring to Figure 2, the solubility of CO ₂ in methanol is much larger than that of inert gases such as H ₂ , N ₂ , CO under the same conditions. Therefore, when the mixed gas containing the above components is washed with methanol under pressure, there is only a small amount of inertness. The gas is absorbed by methanol, and during the depressurization regeneration process, the H ₂ and N ₂ gases are first desorbed at a higher pressure. On the other hand, the solubility of H ₂ , N ₂ , CO and the like in methanol is temperature. The change is not sensitive, and the solubility of H ₂ decreases as the temperature decreases, so methanol washing is suitable for low temperature absorption.

In methanol, H ₂ S has greater solubility than CO ₂ . The solubility of H ₂ S is 6 times higher than that of CO _{2 at} low temperature. The absorption rate of methanol to H ₂ S is much higher than that of CO ₂ . Therefore, when the gas contains both H ₂ S and CO ₂ , the methanol first absorbs H ₂ S. Similarly, during the depressurization regeneration process, the regeneration pressure can be appropriately controlled to desorb a large amount of CO ₂ , and the H ₂ S remains in the solution, and then the two acid gases can be supported by gas stripping, distillation, vacuum, and the like. They were separately recovered to obtain high concentrations of CO ₂ and H ₂ S, respectively. See Table 1 for the relative solubility of various gases in methanol at -40 °C.

Table 1: Relative Solubility of Various Gases in Methanol at -40 ° C

gas	Solubility of gas / solubility of H2	Solubility of gas / solubility of CO2
H2S	2540	5.9
COS	1555	3.6
CO2	430	1.0
CH4	12
CO	5

N2	2.5
H2	1.0

See Table 2 for the solubility of CO ₂ in methanol at different temperature and pressure. Table 2: Solubility of CO ₂ in methanol at different temperature and pressure

Methanol dissolved in CO ₂ , H ₂ S, COS, first depressurized and desorbed CO ₂ , then desorbed H ₂ S by temperature, desorbed CO ₂ , H ₂ S methanol and used as solvent for CO ₂ , H ₂ S, COS Removal of acid gas for recycling.

The low-temperature methanol washing process has the advantages of low power consumption, low steam consumption, low solvent price, and low operating cost. In particular, the desulfurization has a high degree of purification and is very advantageous for methane production.

The fine desulfurization system 5 is used to further remove sulfides from the waste gas. The methanation unit 6 is used to methanize the waste gas to produce methane methanation. The main reaction is CO+3H ₂ ≒CH ₄ +H ₂ O; CO ₂ +4H ₂ ≒CH ₄ +2H ₂ O, and Q is set. Waste gas volume, Q _CO ,

The volume of the corresponding component in the components of the waste gas,

It is the volume fraction of the corresponding component in the components of the waste gas.

The volume of the CO ₂ component that is the methanation reaction,

The volume fraction of the CO ₂ component of the methanation reaction,

For the low temperature methanol washing unit to remove the CO ₂ volume, a is the methanation control coefficient.

Therefore, the highest methane production is:

which is

In the case of the methanation control coefficient a is defined as:

inferred:

When a is 3, the methanation reaction is the most sufficient, and the volume of CO ₂ is the highest. At this time, the volume of CO ₂ removed by the low-temperature methanol washing device is:

Table 3 shows the ratio of m to m when the yield is different for a, and the ratio is defined as the methane conversion rate η. It can be seen from the above reaction formula that when H _{2 is} constant, when there is CO in the waste gas, there is no CO ₂

a>3, CO is completely converted to methane, H ₂ has remaining: CO complete reaction η _small = a / 3, a < 3: CO complete reaction η _small = 3 / a;

When there is CO _{2 in the} waste gas, there is no CO.

a>3: CO _{2 is} completely converted to methane H ₂ has residual CO ₂ complete reaction η _large = (a+1) / 4; when a < 3: H _{2 is} completely converted to methane CO ₂ remains, H ₂ complete reaction _large [eta] = 4 / (a + 1) ; [eta] is not the same specific _small α, η _large values, see Table III:

Table 3: List of methane conversion ratio η

In addition, the methanation control coefficient can also be used.

It is shown that the most methanation reaction at the β=4 is the highest methane production.

The methane gas storage tank 7 is a container for storing methane gas. The oxygen storage tank 8 is a container for storing oxygen. Compounding means 9 gasifying agent for gasifying agent prepared by mixing carbon dioxide with oxygen gasification of underground coal seams using gasification agents may be used Rectisol apparatus 4 desorbed CO _2. The boiler 10 is used to consume excess waste gas produced by the underground gasification furnace 1 and the backup gasification furnace 2, and the boiler 10 can burn waste gas to generate electricity, output power or heat.

The specific process of the methane synthesis system of this embodiment is:

Step A1: The underground gasification furnace 1 gasifies the underground coal seam as waste gas;

Step A2: When the local gasifier 1 is insufficient in gas production, the standby gasifier 2 may be activated and the underground coal seam is gasified as waste gas;

Step B: the waste gas enters the waste gas pretreatment system 3 to remove dust, tar and other impurities from the waste gas;

Step C1: Before the waste gas enters the low-temperature methanol washing device 4, sample and analyze the volume fraction of the waste gas occupied by CO, H ₂ and CO ₂ , and calculate the CO _{2 to} be removed by the low-temperature methanol washing device 4 according to the value of the methanation control coefficient a. the amount

At the same time, the waste gas is cooled and pressurized, so that the temperature of the waste gas is close to the temperature of the methanol in the low temperature methanol washing device 4;

Step C2, controlling the methanol flow rate in the low temperature methanol washing device 4

Removing CO ₂ and adjusting the methanation control coefficient a of the waste gas;

Step D, the waste gas enters the fine desulfurization device 5 to remove impurities such as residual organic sulfur;

Step E, the waste gas enters the methanation unit 6 to synthesize methane;

Step F: The methane gas is cooled and compressed, and then stored in the methane gas storage tank 7.

In step G, when the local lower gasifier 1 or the backup gasifier 2 produces an excessive amount of gas, the excess waste gas can enter the boiler 10 to be combusted.

The specific gas production volume of the underground gasification furnace 1 is 8000 Nm ³ /h. For the CO ₂ removal ratios of CO, H ₂ and CO ₂ components, please refer to Table 4.

Table 4: List of methane conversion ratio η

1) When the gasification furnace 1 excess of the local gas production, gas production such as underground gasification furnace 1 is 10000Nm ³ / h, the shortage of the remaining mixed gas 2000Nm ³ / h with a shortage of gas generated spare gasification furnace 2 into the boiler 10, Used as a raw material gas of the boiler 10;

2) When the gas production in the local gasifier 1 is insufficient, if the gas production of the underground gasifier 1 is only 6000 Nm ³ /h, the backup gasifier 2 is supplemented with 2000 Nm ³ /h of waste gas.

The removal amount of CO ₂ in low-temperature methanol washing is related to the waste gas component, the methanation control coefficient a, and the waste gas production. Since the waste gas component and the waste gas output will fluctuate, the removal amount of CO ₂ should be adjusted in time.

Fig. 3 is a schematic view showing a methane synthesis system of a second embodiment of the present invention. Referring to FIG. 3, the structure and principle of the methane synthesis system of the present embodiment are similar to those of the first embodiment, and the difference between the two is:

The waste gas synthesis methane of the present embodiment further includes a methane purification device 11, a heat exchanger 12, and a carbon dioxide compressor 13. The methane purification device 11 is in communication with the methanation device 6 and the heat exchanger 12; the heat exchanger 12 is connected to the methane gas storage tank 7, the oxygen storage tank 8, the gasification agent mixing device 9, and the methane purification device 11; the carbon dioxide compressor 13 It is in communication with the low-temperature methanol washing device 4 and the gasifying agent mixing device 9. The oxygen storage tank 8 is in communication with the low temperature methanol washing device 4 and the heat exchanger 12.

The methane purifying device 11 is for purifying methane gas. The heat exchanger 12 is for exchanging heat between the liquid oxygen stored in the oxygen storage tank 8 and the methane gas from the methane purification device 11, so that the liquid oxygen can be converted into normal temperature oxygen, and the methane gas is lowered to the liquid to enter the methane gas storage tank. 7. The liquid oxygen in the oxygen storage tank 8 is also used to provide the required cooling capacity for the acid gas removal section of the low temperature methanol scrubber unit 4.

The carbon dioxide compressor 13 is used to raise the pressure of the CO ₂ gas desorbed by the low-temperature methanol washing device 4 to 0.5 MPa or more, and then to the gasifying agent mixing device 12. After pressurization, the CO ₂ is mixed with the normal temperature oxygen from the heat exchanger 12 or the low temperature methanol washing device 4 in the gasification agent mixer 12, adjusted to the desired oxygen concentration, and then sent to the underground gasification furnace 1 and the backup gasification furnace 2 The gasification agent required for the gasification reaction is provided for the underground gasifier.

The other parts of the methane synthesis system of this embodiment are the same as those of the methane synthesis system of the first embodiment, and will not be described again.

Fig. 4 is a schematic view showing a methane synthesis system of a third embodiment of the present invention. Referring to FIG. 4, the structure and principle of the methane synthesis system of the present embodiment are similar to those of the first embodiment, and the difference between the two is:

The methane synthesis system of the present embodiment includes only the underground gasification furnace 1, the waste gas pretreatment system 3, the low temperature methanol washing device 4, the methanation device 6, the methane gas storage tank 7, and the second embodiment also includes the second low temperature methanol washing. Device 41. The second low temperature methanol washing device 41 is disposed between the low temperature methanol washing device 4 and the methanation device 6. The methane synthesis method using the methane synthesis system of the present embodiment includes the following steps:

Step A1: The underground gasification furnace 1 gasifies the underground coal seam as waste gas;

Step B: the waste gas enters the waste gas pretreatment system 3 to remove dust, tar and other impurities from the waste gas;

Step C1: Before the waste gas enters the low-temperature methanol washing device 4, sample and analyze the volume fraction of the waste gas occupied by CO, H ₂ and CO ₂ , and calculate the low-temperature methanol washing device 4 according to the value of the methanation control coefficient a=2.9-3.1. In addition to the amount of CO ₂ , at the same time, the waste gas is cooled and pressurized, such as a temperature of -30 ° C to -60 ° C, a pressure of 0.5 to 5 Mpa, so that the temperature of the waste gas is close to the methanol temperature of the low temperature methanol washing device 4 - 30 ° C to - 60 ° C;

Step C2, controlling the methanol flow rate in the low temperature methanol washing device 4 to remove part of the CO ₂ , such as 60% to 95% of the amount of CO ₂ removed;

Step C3: Before the waste gas enters the second low temperature methanol washing device 41, the volume fraction of the waste gas occupied by CO, H ₂ and CO _{2 is} sampled and analyzed, and the amount of CO ₂ required to be removed by the second low temperature methanol washing device 41 is calculated.

Step C4, controlling the calculation of the methanol flow rate in the second low-temperature methanol washing device 41 to remove CO ₂ , adjusting the methanation control coefficient a of the waste gas, and achieving high removal precision by secondary removal of carbon dioxide in the waste gas;

Step E, the waste gas enters the methanation unit 6 to synthesize methane;

Step F: The methane gas is cooled and compressed, and then flows into the methane gas storage tank 7 for storage.

The other parts of the methane synthesis system of this embodiment are the same as those of the methane synthesis system of the first embodiment, and will not be described again.

Fig. 5 is a schematic view showing a methane synthesis system of a fourth embodiment of the present invention. Referring to FIG. 4, the structure and principle of the methane synthesis system of the present embodiment are similar to those of the third embodiment, and the difference between the two is:

The methane synthesis system of the present embodiment is not provided with a second low temperature methanol washing device 41, and the low temperature methanol washing device 4 is disposed between the waste gas pretreatment system 3 and the methanation device 6, the underground gasification furnace 1 and the waste gas pretreatment system 3 Connected, the waste gas pretreatment system 3 is connected to the underground gasification furnace 1, the low temperature methanol washing device 4, and the low temperature methanol washing device 4 is connected to the low temperature methanol washing device 4 and the methanation device 6. When the methane synthesis system of this embodiment is operated, methane can be synthesized as follows:

Step A1: The underground gasification furnace 1 gasifies the underground coal seam as waste gas;

Step B: the waste gas enters the waste gas pretreatment system 3 to remove dust, tar and other impurities from the waste gas;

Step C2, controlling the methanol flow rate in the low temperature methanol washing device 4 to remove all CO ₂ ;

Step C5: Before the waste gas enters the methanation unit 6, sample and analyze the volume fraction of the waste gas occupied by CO, H ₂ and CO ₂ , calculate the volume of CO _{2 to} be replenished according to the methanation control coefficient a=2.7 to 3.3, and add CO ₂ adjusts the methanation control coefficient a.

Step E, the waste gas enters the methanation unit 6 to synthesize methane;

Step F: The methane gas is cooled and compressed, and then flows into the methane gas storage tank 7 for storage.

The other parts of the methane synthesis system of the present embodiment and the methane synthesis system of the third embodiment have the same structure and principle, and are not described herein again.

In other embodiments of the invention, the methane synthesized by the methanation unit may be piped or otherwise delivered to the gas plant, i.e., the methane gas storage tank may not be provided.

In summary, the methane synthesis method of the present invention has at least the following advantages:

1. In the methane synthesis method of the present invention, the low-temperature methanol washing device absorbs carbon dioxide, and the carbon dioxide content in the waste gas can be reduced to make the composition of the raw material gas for synthesizing methane more stable, and the methanation device can stably and safely produce methane.

2. In an embodiment of the methane synthesis method of the present invention, the carbon dioxide is absorbed by a low-temperature methanol washing device, and the volume fraction of CO, H ₂ and CO ₂ in the waste gas is analyzed by sampling, and the value of the methanation control coefficient a is calculated. The amount of CO ₂ that needs to be removed in a low temperature methanol washing unit

The methanation control coefficient a of methanation synthesis is ensured, which makes the composition of the raw material gas for synthesizing methane more stable, and ensures the stable and safe production of methane by the methanation device, which has the following advantages:

(1) The solvent has a strong absorption capacity for acid gases such as CO ₂ , H ₂ S, and COS at a low temperature, and the solution has a small circulation amount and low power consumption.

(2) The solvent does not oxidize, does not degrade, and has good chemical and thermal stability.

(3) The quality of the purified gas is good and the purification degree is high.

(4) The solvent does not blister.

(5) It has the characteristics of selectively absorbing CO ₂ , H ₂ S, and COS, and can be separately removed and regenerated.

(6) The solvent is cheap and easy to obtain.

3. In an embodiment of the methane synthesis method of the present invention, a standby gasification furnace is provided, and when the local gasification furnace is insufficient in gas production, the standby gasification furnace operates to gasify the underground coal seam as waste gas, thereby ensuring the production of waste gas. The gas volume is stable.

4. In an embodiment of the methane synthesis method of the present invention, the waste gas enters the methanation unit to sample and analyze the volume fraction of the waste gas occupied by CO, H ₂ and CO ₂ , and when the gas composition of the waste gas changes, methane The synthesis system can adjust the removal amount of CO ₂ in time to ensure the stable and safe production of methane by the methanation unit.

The above is only a preferred embodiment of the present invention, and is not intended to limit the scope of the present invention. Although the present invention has been disclosed in the above preferred embodiments, it is not intended to limit the present invention. The skilled person can make some modifications or modifications to the equivalent embodiments by using the above-disclosed technical contents without departing from the technical solutions of the present invention. Any simple modifications, equivalent changes, and modifications made to the above embodiments in accordance with the technical spirit of the present invention are still within the scope of the present invention.

Claims (9)

1. A method for synthesizing methane for synthesizing underground coal gasification waste gas into methane, characterized in that the method comprises:

   Step 1: Using underground gasifier (1) to gasify underground coal seam to generate waste gas;

   Step 2: pretreating the waste gas;

   Step 3: calculating the amount of CO ₂ to be removed in the waste gas, and removing the amount of CO ₂ according to the need to remove the corresponding amount of CO ₂ using a low temperature methanol washing device (4);

   Step 4: synthesizing the waste gas into methane using a methanation unit (6).
2. The method for synthesizing methane according to claim 1, wherein in the third step, the method for calculating the amount of CO ₂ to be removed in the waste gas is to sample and analyze CO, H ₂ and CO _{2 in the} waste gas. Volume fraction, according to the value of the methanation control coefficient a, calculate the amount of CO ₂ that needs to be removed in the low-temperature methanol washing device (4)

   

   Said

   

   Where Q is the volume of waste gas,

   

   The volume fraction of the corresponding component in the component of the waste gas,

   

   The volume of CO ₂ removed for the low temperature methanol scrubber, a is the methanation control coefficient a.
3. The method for synthesizing methane according to claim 2, wherein the methanation control coefficient a ranges from 2.9 to 3.1.
4. The method for synthesizing methane according to claim 1, wherein the methane synthesis method further comprises: when the underground gasification furnace (1) is insufficient in gas production, using a standby gasification furnace (2) to gas the underground coal seam The steps to generate waste gas.
5. The method for synthesizing methane according to claim 4, wherein the methane synthesis method further comprises: when the underground gasification furnace (1) or the standby gasification furnace (2) produces an excess of gas, excess The waste gas enters the step of burning the boiler (10).
6. A methane synthesis system for synthesizing underground coal gasification waste gas into methane, characterized in that the methane synthesis system comprises an underground gasification furnace (1), a waste gas pretreatment system (3), and is used for taking off In addition to the low-temperature methanol washing device (4) and the methanation device (6) for carbon dioxide in the waste gas, the underground gasifier (1) is used for gasifying underground coal seam into waste gas, and the waste gas pretreatment system ( 3) for removing particulate matter and tar in the waste gas, the low temperature methanol washing device (4) is for removing carbon dioxide from the waste gas, and the methanation device (6) is used for The gas is synthesized into methane, the waste gas pretreatment system (3) is in communication with the underground gasification furnace (1) and the low temperature methanol washing device (4), the low temperature methanol washing device (4) and the methane The device (6) is connected.
7. A methane synthesis system according to claim 6 wherein said methane synthesis system further comprises a second low temperature methanol wash disposed between said cryogenic methanol scrubbing unit (4) and said methanation unit (6) The device (41), the second low temperature methanol washing device (41) is used for secondary removal of carbon dioxide in the waste gas.
8. The methane synthesis system according to claim 6, wherein said methane synthesis system further comprises a standby gasification furnace (2) for gasifying underground coal seams, said standby gasification furnace (2) and said waste The gas pretreatment system (3) is connected.
9. A methane synthesis system according to claim 6 wherein said methane synthesis system further comprises a heat exchanger (12) for adjusting the temperature, said heat exchanger (12) for reducing said methane or The temperature of methanol in the low temperature methanol washing device (4).

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