CN1873285A - Method and equipment for enriching and storing and transporting coalbed gas by using hydrate - Google Patents

Abstract

The invention proposes a method and device for enriching, storing and transporting coalbed methane by utilizing hydrate. The technical solution of this method is to make use of the difference in the partition coefficient between methane (CH ₄ ) and other components in the gas phase and hydrate phase in coalbed methane, and to make the CH ₄ in coalbed methane rich in the hydrate phase by controlling the formation conditions of hydrates. set. The generated slurry or solid hydrate can be directly used for the storage and transportation of coalbed methane, and can also be stored and transported in the form of compressed gas after decomposing. The hydrate generating device of the present invention can be configured according to the output of coalbed methane, and can be designed as a vehicle-mounted or ship-mounted mobile form, which is suitable for the characteristics of scattered coalbed methane wells and small-scale single-well mining. The invention adopts the heat pump principle and the method of connecting three towers in series to recover waste heat and cold capacity, which can improve the energy utilization rate of the system.

Description

A method and device for coalbed methane enrichment, storage and transportation using hydrate

technical field

The invention proposes a method and device for enriching, storing and transporting coalbed methane by utilizing hydrate.

technical background

Coalbed methane is an unconventional natural gas that is mainly in the state of adsorption, generated and stored in coal-measure strata, and its composition is basically the same as that of conventional natural gas (mainly CH ₄ , also containing N ₂ , CO _{2 ,} etc., and CBM Due to different types, the content of each component has certain changes), it can be mixed with conventional natural gas, and can be used as high-quality energy and chemical raw materials equal to conventional natural gas. According to the latest evaluation, my country's onshore coalbed methane resources buried below 2,000 meters are 31.46 trillion cubic meters, equivalent to 45 billion tons of standard coal or 31 billion tons of oil. The development and utilization of this high-quality clean energy can optimize my country's energy structure to a certain extent, improve coal mine safety production conditions, reduce greenhouse gas emissions and protect the atmospheric environment. Compared with conventional natural gas, coalbed methane has a wide distribution area and low gas pressure. It has the problems of low gas production per well and short service life. There is no economical collection, storage and transportation method, and large-scale mining is more difficult. In addition, some coalbed methane needs to be enriched to increase its utilization value due to its high N ₂ and CO ₂ content and low calorific value.

Hydrate is an ice-like clathrate formed by small molecule gas (N ₂ , CO ₂ , CH ₄ , C ₂ H ₆ , C ₃ H _{8 ,} etc.) and water under certain temperature and pressure conditions. things. It is an ideal solid solution, and the theoretical gas storage capacity of hydrate is 1:164, that is, under standard conditions, 1 unit volume of natural gas hydrate can hold 164 times the volume of natural gas, and the energy density is relatively high. Under the same conditions, different gases have different tendencies to form hydrates. By controlling the hydrate formation conditions, the components in the mixed gas can be separated. Utilizing the gas storage characteristics of hydrates and the ability to separate mixed gases for gas storage and transportation and gas separation has attracted extensive attention. At present, the hydrate gas storage and transportation or gas separation technology reported in the literature is mainly used for natural gas and refinery gas, and the technology of coalbed methane enrichment, storage and transportation using hydrate has not been reported yet.

Under the same temperature and pressure conditions, among the main components of coalbed methane, _N2 is the most difficult to form hydrates, _CH4 and _CO2 have similar ability to form hydrates and the formation conditions are relatively mild, as shown in Figure 1. Acid gases such as CO ₂ and H ₂ S contained in coalbed methane can be removed by acid gas scrubbers. In view of this, the present invention proposes a method and device for enriching, storing and transporting coalbed methane using hydrate.

In order to simplify the calculation, it is assumed in the specification and examples of the present invention that the coalbed methane has been washed with pressurized water, and the gas entering the hydrate generation system is a mixed gas of CH ₄ and N ₂ . The gas phase-hydrate phase equilibrium composition diagram of CH ₄ and N ₂ mixed gas under different pressure and temperature conditions is shown in Figure 2. Attached Figure 2 shows. Hydrates show a good enrichment effect on _CH4 and _N2 components in coalbed methane.

Contents of the invention

The present invention relates to a method and device for enriching, storing and transporting coalbed methane by utilizing hydrate, and its conceptual flow chart is shown in Figure 3 . The present invention is characterized by a gas compression-expansion system designed using the heat pump principle, a hydrate generation system with 3 towers connected in series, and relatively mild operating conditions (controlling the temperature of hydrate formation to be 260-285K and the pressure to be 2-10MPa).

Among them, the gas compression-expansion system is composed of a single compressor or multiple compressors in series coaxially, a heat exchanger, a pressurized water washing device, a dehydrator, and an expansion turbine. A heat exchanger is set between every two compressors to reduce the gas temperature, and the heat is recovered for use by other parts of the hydrate generation device. The above equipments are all standard equipments, and the specific types can be selected according to the design during implementation.

When the gas compression-expansion system adopts three-stage compression, it can achieve 27 times isothermal pressurization of coalbed methane. Generally speaking, the wellhead pressure of coalbed methane is 0.2-0.6MPa, and the pressure of compressed coalbed methane can reach 5.4-16.2MPa. By adjusting the expansion ratio, the temperature and pressure in the hydrate generation tower can be adjusted and controlled. If the wellhead pressure of CBM is relatively high, the number of compression stages can be reduced accordingly according to the pressure requirement.

The hydrate generation system is composed of three hydrate generation towers 1, 2 and 3 connected in series, and the working states are hydrate generation state, intermediate cooling state and unloading state respectively. The low-temperature coalbed methane generated by the gas compression-expansion system enters tower 1 from the bottom, and the pressure of the hydrate generation tower is adjusted and controlled so that it is higher than the equilibrium pressure corresponding to the hydrate formation temperature, and the gas contacts with water to form hydrate during the ascent . After the gas without forming hydrate is discharged from the top of tower 1, it enters tower 2 from the bottom through the pipeline, and the water in tower 2 is initially cooled, and at the same time, the gas is further formed into hydrate, and the remaining tail gas with _N2 as the main component is discharged from the tower 2 top discharge. Part of the tail gas is returned to the hydrate generation system after being pressurized by the third-stage compressor, and part of the tail gas can be used as fuel. When the hydrate in tower 1 is completely formed, the working state of tower 1 is switched from the state of hydrate generation to the state to be unloaded. Tower 2 is switched to the state of hydrate formation, and tower 3 is switched to the intermediate cooling state. Such a design is conducive to making full use of the cold energy in the low-temperature coalbed methane.

In order to promote the formation of hydrates from coalbed methane, the hydrate generation tower is specially designed (as shown in Figure 3): (1) The hydrate generation tower is divided into several chambers by partitions (or wire mesh); ( 2) When partitions are used, each tower chamber can be used as an independent hydrate generation unit, and the tower chambers are connected to each other through the bubble caps set on the partitions. The setting and design of the partition can refer to the common tower equipment in the chemical industry; (3) The purpose of using wire mesh is to continuously disperse the air bubbles and improve the gas-liquid heat transfer and mass transfer efficiency; (4) A cooling plate is installed outside the hydrate formation tower (5) If the generated hydrate needs to be decomposed in the tower, a heating coil can be added in the hydrate generating tower to use the heat recovered by the gas compression-expansion system for hydration decomposition of matter.

The working status and operation process of each chamber of the hydrate formation tower after being filled with cooled water are as follows. 1. Hydrate formation state: The low-temperature coalbed methane enters the tower chamber a from the bottom of the hydrate formation tower, and passes through each layer of partitions (or screens) one by one from bottom to top. Another hydrate generating tower in an intermediate cooling state. After the generation of slurry or solid hydrate in tower chamber a is complete, switch the coalbed gas inlet valve so that it enters from the lower part of tower chamber b. Other tower chambers (c, d, e, ...) all operate according to this. When all the tower chambers are filled with hydrates, all the intake valves are closed, and the working state is switched to the unloading state. Hydrate decomposition or unloading operations can be performed as required. The coalbed gas entering the tower enters from the bottom of each tower chamber and is distributed through the bubble cap. The gas-liquid contact is enhanced through gas stirring, and the contact time between gas and water is prolonged, which can increase the formation rate of hydrate. In order to promote the formation of hydrates, the accelerators described in patents 01130135.X and 00130866.1 can also be selected. There is no need to adopt mechanical stirring measures to enhance gas-liquid contact in the hydrate generation tower, which can simplify the structure of the tower.

2. Intermediate cooling state: the low-temperature gas discharged from the top of the hydrate formation tower enters from the bottom of the tower and passes through each tower chamber in turn, so that the water temperature in each tower chamber gradually decreases. If the temperature still cannot meet the requirement of hydrate formation, a cooling medium can be passed into the cooling coil to further reduce the temperature. After the temperature condition required for hydrate formation is reached, the working state can be switched to the hydrate formation state, and the coalbed methane can be fed into the hydrate formation operation.

3. To-be-unloaded state: If coalbed methane is transported in the form of hydrate, the hydrate-generated tower in the state to be unloaded can be replaced by a spare water-formation tower. If the coalbed methane is transported in the form of compressed gas, the heating medium can be passed into the heating coil and decompressed to decompose the hydrate to separate the enriched coalbed methane, which is loaded into a gas cylinder for transportation.

If the single-stage coalbed methane enrichment process composed of three towers in series still cannot meet the enrichment requirements, multi-stage enrichment can be set up, and the hydrate generated in the previous enrichment process is analyzed before entering the next enrichment process , to increase the enrichment degree of CH ₄ in coalbed methane. The number of stages set in the coalbed methane enrichment system can be determined according to the equilibrium composition diagram of the CH ₄ -N ₂ mixed gas gas phase-hydrate phase shown in Figure 4 .

In addition, it should be noted that the acid gas washing device provided in the present invention is a pressurized water washing device, and this method can reduce the content of acid gases such as CO ₂ and H ₂ S contained in the coal bed gas to less than 1%. The valves involved in the concept flow of the present invention are all two-way valves, which can be switched between two different pipelines according to different requirements.

Aiming at the characteristics of coalbed methane production, the present invention can be designed as a vehicle-mounted or ship-borne mobile system suitable for different coalbed methane production. If it is transported in the form of hydrate, multiple sets of parallel hydrate generating towers can be set up on the vehicle-mounted or ship-borne platform. After all hydrates are completely formed, they are transported to the processing center for centralized treatment or packaging. If it is transported in the form of compressed gas, it can be configured as a hydrate generation system + a gas cylinder transport vehicle, and the gas cylinder transport vehicle is used to connect the coalbed methane well and the user.

Description of drawings

Fig.1 Equilibrium temperature-pressure diagram of CH ₄ , CO ₂ , N ₂ , H ₂ S hydrate

Fig.2 Equilibrium composition-temperature diagram of CH ₄ (1)-N ₂ (2) mixed gas gas phase-hydrate phase at different pressures

Figure 3 is a conceptual flow chart of utilizing hydrates for coalbed methane enrichment, storage and transportation designed by the present invention

Figure 4 Equilibrium composition diagram of CH ₄ (1)-N ₂ (2) mixed gas gas phase-hydrate phase (P=3MPa)

Detailed ways

Example 1

In this embodiment, the present invention is used for coalbed methane with relatively high CH content (CH _{85 %} , CO ₂ 5%, N ₁₀ %), pure water is used as the medium, and for simplified calculation, it is assumed that the pressurized water washing device has Remove all _CO2 , control the hydrate formation temperature to 273K, and the formation pressure to 3MPa. The single-stage enrichment effect of the present invention on coalbed methane under the operating conditions can be obtained from the accompanying drawing 2, see the table below for details. gas composition CBM composition, %(V/V) to be enriched After pressure washing After enrichment exhaust _{CH4CO2N2 _ _} 85510 89.5010.5 95.005.0 82.0018.0

Example 2

In this embodiment, the present invention is used for coalbed methane (CH ₄ 65%, CO ₂ 5%, N ₂ 30%) with lower CH ₄ content, and pure water is used as the medium. For simplified calculation, it is assumed that the pressurized water washing device has Remove all _CO2 , control the hydrate formation temperature to 273K, and the formation pressure to 4MPa. The single-stage enrichment effect of the present invention on coalbed methane under the operating conditions can be obtained from the accompanying drawing 2, see the table below for details. gas composition CBM composition, %(V/V) to be enriched After pressure washing After enrichment exhaust _{CH4CO2N2 _ _} 65530 68.4031.6 83.0017.0 50.0050.0

Example 3

In this embodiment, the present invention is applied to coalbed methane with higher _CO2 content ( _CH4 65%, _CO2 15%, _N2 20%), pure water is used as the medium, and for simplification of calculation, it is assumed that the pressurized water washing device has Remove all _CO2 , control the hydrate formation temperature to 277K, and the formation pressure to 5MPa. The single-stage enrichment effect of the present invention on coalbed methane under the operating conditions can be obtained from the accompanying drawing 2, see the table below for details. gas composition CBM composition, %(V/V) to be enriched After pressure washing After enrichment exhaust _{CH4CO2N2 _ _} 651520 76.5023.5 91.508.5 72.0028.0

Example 4

This embodiment is to apply the present invention to coalbed methane with low CH ₄ content (CH ₄ 50%, N ₂ 50%), use pure water as the medium, control the hydrate formation pressure to 3MPa, adjust the formation temperature, and adopt two stages For the enrichment operation, it can be obtained from Fig. 4 that the present invention can enrich the CH ₄ content in the coal bed gas to more than 90% (V/V).

Claims (8)

1. A method for utilizing hydrates to enrich, store and transport coalbed methane, characterized in that the methane (CH ₄ ) in the coalbed methane is in the form of hydrates in the hydrate generating device by controlling the conditions for the formation of hydrates The nitrogen (N ₂ ) in the coalbed methane is separated in the form of gas. If the content of acid gas (CO ₂ , H ₂ S, etc.) Standard pressure water washing device. 2. The conditions for forming hydrates according to claim 1, characterized in that the hydrate formation temperature is controlled to be 260-285K, and the formation pressure is 2-10 MPa. 3. The hydrate generating device according to claim 1, characterized in that a gas compression-expansion system and a hydrate generating system are provided. 4. The gas compression-expansion system according to claim 3, characterized in that it is a heat pump system composed of single-stage or multi-stage compression and one-stage expansion, and a heat exchanger is arranged between each compressor to recover waste heat. 5. The hydrate generating system as claimed in claim 3, characterized in that the three hydrate generating towers are connected and switched. 6. The hydrate generating tower as claimed in claim 5, characterized by the special design of the hydrate generating tower. 7. The coalbed methane enrichment and storage method described in claims 1, 2, 3, 4, 5, 6 is characterized in that the method can be configured according to the gas production of a single well of coalbed methane, and when the gas production is low, it can be The expansion turbine of the gas compression-expansion system is eliminated, and the cooling capacity required for hydrate formation can be provided by the refrigeration equipment installed outside the hydrate formation tower, and the standard refrigeration device can be selected. 8. The hydrate generating device described in claims 1, 2, 3, 4, 5, and 6 can also be used for enrichment, storage and transportation of light hydrocarbon components in natural gas, refinery gas, and coke oven gas.

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