A method for determining natural gas expansion pressure difference power generation technology under different working conditions

Abstract

The present invention relates to a method for determining natural gas expansion pressure difference power generation technology under different working conditions, which includes: combining the actual situation on site, sorting out the basic data in the pressure difference power generation process; determining the calculation data of the three expansion pressure difference power generation technologies, including the components and properties of natural gas; determining the physical parameters of natural gas, calculating the volume fraction of a component in the mixed gas, and converting the volume flow rate into mass flow rate; respectively calculating the mechanical work generated by the expander or consumed by the compressor of the three expansion pressure difference power generation technologies, the enthalpy difference between the inlet and outlet, and the difference between the inlet temperature and the outlet temperature; then calculating the temperature of the expander or compressor outlet of the three expansion pressure difference power generation technologies; drawing a schematic diagram of the ratio of an operable diverter; drawing a matching natural gas expansion pressure difference power generation technology screening chart under different gas source parameters; and using the chart to select a suitable natural gas expansion pressure difference power generation technology. The present invention can maximize the utilization of natural gas resources, reduce pollution and reduce costs.

Description

Technical field:

[0001] The present invention relates to the field of natural gas transportation technology, and in particular to a method for determining natural gas expansion pressure difference power generation technology under different working conditions. Background technology:

[0002] With the development of social economy, environmental issues are increasingly valued by countries around the world, and the energy consumption structure is gradually shifting towards low-carbon, environmentally friendly and sustainable development. Natural gas is one of the cleanest primary energy sources, and its proportion is increasing year by year. In the production and transportation process of natural gas, many links need to be depressurized, which causes a large amount of waste of pressure energy. During the depressurization process, a large amount of cold energy will be generated while the pressure drops, which may cause frost and even major safety accidents such as pipeline blockage. Therefore, it is urgent to study appropriate technologies to recycle this part of the lost pressure energy and cold energy to reduce resource waste and improve energy utilization.

[0003] At present, the research method for natural gas pressure energy power generation is to use an expander to replace the pressure reducing valve, generate electricity while recovering the pressure energy of the natural gas pressure reduction process, and recycle the cold energy generated in the process to maximize energy utilization. At present, the research on natural gas expansion pressure difference power generation technology in China has been quite mature, and a variety of technologies suitable for natural gas pressure energy recovery have been proposed, such as direct expansion pressure difference power generation technology, expansion pressure difference power generation-LNG production technology, and combined heat, cooling and power technology.

[0004] Natural gas expansion pressure difference power generation technology is different from general pressure difference power generation technology. The operating parameters can vary according to the specific equipment and application conditions. Most of them are based on specific natural gas qualities or specific operating parameters. Due to the different properties of natural gas sources in major domestic large gas fields, the production and operation of natural gas fields are complex and changeable. At the same time, there are many process flows for natural gas expansion pressure difference power generation. There will inevitably be a mismatch between power generation technology and operating conditions, resulting in economic problems such as low efficiency, low output, and high pollution, as well as serious safety problems such as system shock causing explosions. Therefore, it is urgent to find a method to screen out one or several more suitable natural gas expansion pressure difference power generation technologies for different gas source parameters.

[0005] In summary, based on the current research results and combined with the current production and operation mode of domestic gas fields, three technologies for generating electricity using natural gas pressure energy are summarized: direct single-stage expansion pressure difference power generation technology ("DS"), split-flow two-stage expansion pressure difference-LNG production power generation technology ("SDL"), and parallel expansion compression synergy-CNG production power generation technology ("PCC"). In view of the technical research on the above three types of expansion pressure difference power generation, it is urgent to find a method to determine the natural gas expansion pressure difference power generation technology under different working conditions, find out the power generation technology that matches it under the operating parameters of different gas sources, provide effective technical guidelines for production units related to gas fields, and have certain guiding significance for energy conservation and consumption reduction in related places such as gas gathering stations and processing plants, which will help promote the development of renewable energy and achieve the coordinated development of economy, environment and energy. Summary of the invention:

[0006] The purpose of the present invention is to provide a method for determining the natural gas expansion pressure difference power generation technology under different operating conditions. This method for determining the natural gas expansion pressure difference power generation technology under different operating conditions is used to solve the problem that the natural gas expansion pressure difference power generation process in the prior art is complicated, and there is a mismatch between the power generation technology and the operating conditions, which leads to economic problems such as low efficiency, low output, high pollution, and serious safety problems such as system shock causing explosion.

[0007] The technical solution adopted by the present invention to solve the technical problem is: the method for determining the natural gas expansion pressure difference power generation technology under different working conditions includes the following steps:

[0008] Step 1: Based on the actual situation on site, clarify the infrastructure construction and process characteristics of the three expansion pressure difference power generation technologies; sort out the basic data in the pressure difference power generation process, including expander, compressor, heat exchanger, and generator operating parameter data; the three expansion pressure difference power generation technologies are direct single-stage expansion pressure difference power generation technology DS, split-flow two-stage expansion pressure difference-LNG production power generation technology SDL and parallel expansion compression synergy-CNG production power generation technology PCC;

[0009] Step 2: Determine the calculation data of the three expansion pressure difference power generation technologies, including the composition and properties of natural gas, the inlet temperature, inlet and outlet pressures, and volume flow rate of the expander or compressor;

[0010] Step 3: Determine the physical properties of natural gas, including the specific heat capacity at constant pressure and gas density of the gas; calculate the volume fraction of a certain component in the mixed gas. Since the pressure and temperature of the gas change when it passes through the expander, the volume flow rate changes at all times. According to the molecular weight of the mixed gas and the volume fraction of a certain component in the mixed gas, the volume flow rates of the three expansion pressure difference power generation technologies are converted into mass flow rates;

[0011] Step 4: Calculate the mechanical work generated by the expander or consumed by the compressor of the three expansion pressure difference power generation technologies, the enthalpy difference between the inlet and outlet, and the difference between the inlet temperature and the outlet temperature; and then calculate the outlet temperature of the expander or compressor of the three expansion pressure difference power generation technologies;

[0012] Step 5: Repeat steps 1 to 4 to calculate the parameter results under different working conditions;

[0013] Step 6: Based on the parameter results calculated above under different working conditions, the power generation rules of the three expansion pressure difference power generation technologies are summarized. For the split-flow two-stage expansion pressure difference-LNG production power generation technology SDL and the parallel expansion compression synergy-CNG production power generation technology PCC, a schematic diagram of the operable splitter ratio is drawn to obtain the threshold range for the smooth operation of SDL and PCC;

[0014] Step 7: Arrange the parameter results under different working conditions calculated in step 5, and draw the natural gas expansion pressure difference power generation technology screening chart under different gas source parameters. The natural gas expansion pressure difference power generation technology screening chart consists of six sub-charts, each of which is composed of a black area, a gray area, and a white area connected in sequence from bottom to top, and each area is tilted upward from left to right;

[0015] Step 8: Use the natural gas expansion pressure difference power generation technology screening chart under different gas source parameters to select the appropriate natural gas expansion pressure difference power generation technology from PCC, SDL and DS. If two power generation technologies among PCC, SDL and DS are selected, calculate the liquid phase fraction and power generation to distinguish the most suitable technology under this condition; the liquid phase fraction q is calculated by the following formula:

[0016] q=ψ-ψ vap , where ψ is the compressibility factor of the entire mixture; ψ vap is the relative gas compressibility factor.

[0017] The calculation method of the specific constant pressure heat capacity of the gas in step 3 of the above scheme is:

[0018] Specific heat capacity at constant volume at low pressure:

[0019]

[0020] In the formula, is the specific heat capacity of the gas mixture at low pressure, kJ / (kmol·k); R is the gas constant, R=8.314kJ / (kmol·K);

[0021] Specific heat capacity at constant volume under high pressure:

[0022]

[0023]

[0024] In the formula,

[0025]

[0026]

[0027]

[0028] f w =0.37464+1.54226ω-0.26992ω 2

[0029]

[0030] Where, p is the inlet pressure, MPa; T is the inlet temperature, K; P c is the outlet pressure, MPa; T c is the critical temperature of the gas, which is a constant, T c =20K; ρ is the gas density, kg / m 3 ; R is the gas constant, R = 8.314 kJ / (kmol·K); T r is the relative temperature of the gas, T r =T / T c ; ω is the eccentricity factor, which is determined according to the gas composition and has a value of 2.335;

[0031] Calculate C v The value of and integration:

[0032]

[0033] C v Substitute the calculated results into the above C p In the calculation formula, the specific constant volume heat capacity C is obtained p The value of .

[0034] The method for calculating the volume fraction of a component in the mixed gas in step 3 of the above scheme is:

[0035] λ i =Z i / Z

[0036] In the formula, λ i is the molar fraction of the i-th component, dimensionless, expressed as a decimal; Z i is the molar number of the ith component, and Z is the sum of the molar numbers of all gas components.

[0037] The method for converting the volume flow rate of the three expansion pressure difference power generation technologies into mass flow rate in step 3 of the above scheme is:

[0038] Q m =Q v ρ

[0039]

[0040]

[0041] In the formula, Q m is the mass flow rate, kg / day; Q v is the volume flow rate, m 3 / day; ρ is density, kg / m 3 ; M is the average molecular weight of the mixed gas, kg / mol; R is the gas constant, 0.08206; N i is the molecular weight of component i; i is the molar fraction of i in the mixed gas.

[0042] Specific method of step 4 of the above scheme:

[0043] Calculate mechanical work;

[0044]

[0045] Where W is the mechanical power generated by the expander or the mechanical work consumed by the compressor, kW·h; k is the natural gas isentropic index, which is 1.33; p is the inlet pressure, MPa; P c is the outlet pressure, MPa; Q m is the mass flow rate at the expander or compressor inlet, kg / day;

[0046] Obtaining the enthalpy difference between the inlet and outlet of the expander or the compressor according to the mechanical work generated by the expander or consumed by the compressor obtained above;

[0047]

[0048] Where ΔH is the enthalpy difference between the inlet and outlet of the expander or compressor, kJ / kg; Q m is the mass flow rate of natural gas entering the expander or compressor, kg / day; ρ is the gas density, kg / m 3 ;

[0049] Based on the above enthalpy difference, the difference between the inlet temperature and the outlet temperature of the expander or compressor can be calculated;

[0050] ΔT=T2-T1=C p ×ΔH

[0051] Where ΔT is the difference between the inlet temperature and the outlet temperature of the expander or compressor, K; T1 is the inlet temperature of the expander or compressor, K; T2 is the outlet temperature of the expander or compressor, K; Cp is the specific heat capacity of natural gas at constant pressure, kJ / (kg·K);

[0052] Calculate the temperature at the outlet of the expander or compressor of the three power generation technologies;

[0053] T2=T1-ΔT

[0054] The heat exchanger outlet temperature calculation formula is:

[0055] c h ×m h ×(T 2,h -T 1,h )=c c ×m c ×(T 2,c -T 1,c )

[0056] In the formula, c h is the specific heat capacity at constant pressure of the heat source, kJ / (kg·K); c c is the specific heat capacity at constant pressure of the cold source, kJ / (kg·K); m h is the mass of the heat source, kg; m c is the mass of the cooling source, kg; T 1,h is the inlet temperature of the heat source, K; T 2,h is the outlet temperature of the heat source, K; T 1,c is the inlet temperature of the cold source, K; T 2,c is the outlet temperature of the cold source, K; where, c h and c c The value is obtained directly when the natural gas is delivered from upstream.

[0057] The specific method of step seven in the above scheme is:

[0058] According to the outlet pressure P of the pressure difference power generation system out The value is divided into six sub-plates, and the outlet pressure of the six sub-plates is 0 <P out ≤1Mpa, 1 <P out ≤2Mpa, 2 <P out ≤3Mpa, 3 <P out ≤4Mpa, 4 <P out ≤5Mpa, 5 <P out ≤6Mpa, the horizontal and vertical coordinates of each sub-diagram are determined by the inlet pressure P of the pressure difference power generation system. in and inlet temperature T inDifferent pressure difference power generation technologies have their applicable inlet pressure and inlet temperature. According to the applicable scope of the three pressure difference power generation technologies, the sub-diagram is divided into three areas: black area, gray area, and white area. Among them, the bottom area of ​​each sub-diagram is the black area, which represents the working range of DS, the middle area of ​​each sub-diagram is the gray area, which represents the working range of SDL, and the upper area of ​​each sub-diagram is the white area, which represents the working range of SDL and PCC.

[0059] The regional boundaries of the three pressure difference power generation technologies in the plate are determined by the inlet pressure and inlet temperature applicable to the three pressure difference power generation technologies calculated in steps one to five. The black area, gray area and white area are determined and drawn through the regional boundaries to complete the drawing of a sub-plate. The drawing process of other sub-plates is similar to this, and all sub-plates are drawn one by one.

[0060] In step eight of the above scheme, the specific method for determining the natural gas expansion pressure difference power generation technology by using the natural gas expansion pressure difference power generation technology screening chart under different gas source parameters is:

[0061] For the operating parameters of a pressure difference power generation system, the operating parameters include inlet pressure, inlet temperature and outlet pressure. Select a sub-diagram that meets the conditions according to its outlet pressure. When the inlet pressure and inlet temperature of a pressure difference power generation system fall in the white area of ​​the selected sub-diagram, both PCC and SDL power generation technologies are available; when the inlet pressure and inlet temperature of a pressure difference power generation system fall in the gray area of ​​the selected sub-diagram, select the PCC power generation technology; when the inlet pressure and inlet temperature of a pressure difference power generation system fall in the black area of ​​the selected sub-diagram, select the DS power generation technology.

[0062] The present invention has the following beneficial effects:

[0063] 1. The present invention combines the current production and operation mode of domestic gas fields, comprehensively considers the working characteristics of three expansion pressure difference power generation technologies, and proposes a method for determining natural gas expansion pressure difference power generation technology under different working conditions. First, the basic data of the three power generation technologies, including inlet and outlet pressures, inlet temperatures, volume flow rates, etc., are collected, sorted, and calculated to determine the physical properties of natural gas, including the constant pressure specific heat capacity of the gas, gas density, etc., and the mechanical work generated by the expander or consumed by the compressor is calculated respectively. The enthalpy difference between the inlet and outlet of the expander or compressor is obtained according to the obtained mechanical work generated by the expander or consumed by the compressor, and then the difference between the inlet temperature and the outlet temperature of the expander or compressor can be calculated according to the above enthalpy difference, so as to obtain the outlet temperature. According to the parameter calculation results, the power generation laws of the three pressure difference power generation technologies are summarized, and a schematic diagram of the operable diverter ratio is drawn. The above parameter calculation results are sorted out, and a matching natural gas expansion pressure difference power generation technology determination chart is drawn to determine the expansion pressure difference power generation technology under different working conditions. Since multiple pressure difference power generation technologies may be matched under the same working condition, the liquid phase fraction and power generation are finally calculated to screen out the most suitable technology under specific working conditions.

[0064] 2. Nowadays, there is still waste of natural gas energy and over-reliance on traditional energy, which cannot meet the needs of reducing carbon emissions, reducing pollution and saving energy. The differences in different gas sources lead to instability in the operation of expansion pressure difference power generation technology, causing system shocks, increasing economic costs and environmental pollution. The present invention can maximize the use of natural gas resources, improve energy utilization efficiency, effectively solve the above problems, contribute to the sustainable development of resources and economy, provide theoretical support for the energy conservation and consumption reduction work of relevant units, and have certain guiding significance for energy conservation and consumption reduction in relevant places such as gas gathering stations and processing plants. .

[0065] 3. The present invention is used to match the pressure difference power generation technology under different gas source operating parameters. For the situation where multiple pressure difference power generation technologies may be matched under the same working condition, the liquid phase fraction and power generation will be calculated to select the most suitable technology under the working condition. Description of the drawings:

[0066] Figure 1 This is a process flow chart of direct single-stage expansion pressure difference power generation technology;

[0067] Figure 2 This is the process flow chart of the split-flow two-stage expansion pressure difference-LNG production and power generation technology;

[0068] Figure 3 Parallel expansion and compression synergy - CNG production and power generation technology process flow chart;

[0069] Figure 4 It is the maximum split ratio diagram of the split-flow two-stage expansion pressure difference-LNG production and power generation technology with different incoming gas pressures;

[0070] Figure 5 It is a schematic diagram of the ratio of the parallel expansion and compression synergy-CNG production and power generation technology that can operate the splitter;

[0071] Figure 6 Screening plate for natural gas expansion pressure difference power generation technology. Specific implementation method:

[0072] The present invention will be further described below in conjunction with the accompanying drawings:

[0073] The method for determining the natural gas expansion pressure difference power generation technology under different working conditions includes the following steps:

[0074] Step 1: Based on the actual situation on site, clarify the infrastructure construction and process characteristics of the three pressure difference power generation technologies; sort out the basic data in the pressure difference power generation process, including expander, compressor, heat exchanger, and generator operating parameter data; the three expansion pressure difference power generation technologies are direct single-stage expansion pressure difference power generation technology DS, split-flow two-stage expansion pressure difference-LNG production power generation technology SDL and parallel expansion compression synergy-CNG production power generation technology PCC;

[0075] Step 2: Determine the calculation data of the three expansion pressure difference power generation technologies, including the composition and properties of natural gas and the parameters such as the expander or compressor inlet temperature, inlet and outlet pressures, and volume flow rate;

[0076] Step 3: Determine the physical properties of natural gas, including the specific heat capacity at constant pressure and gas density; calculate the volume fraction (molar fraction) of a component in the mixed gas. Since the pressure and temperature of the gas change when it passes through the expander, the volume flow rate changes at all times. According to the molecular weight of the mixed gas, the volume fraction (molar fraction) of a component in the mixed gas, and other parameters, convert the volume flow rates of the three expansion pressure difference power generation technologies into mass flow rates;

[0077] Step 4: Calculate the mechanical work generated by the expander or consumed by the compressor of the three expansion pressure difference power generation technologies, the enthalpy difference between the inlet and outlet, and the difference between the inlet temperature and the outlet temperature; further, calculate the outlet temperature of the expander or compressor of the three expansion pressure difference power generation technologies; determine the appropriate pressure difference power generation technology according to the process characteristics of the three pressure difference power generation technologies;

[0078] Step 5: Repeat steps 1 to 4 to calculate the parameter results under different working conditions;

[0079] Step 6: Based on the above parameter calculation results, summarize the power generation rules of the three expansion pressure difference power generation technologies. For the split-flow two-stage expansion pressure difference-LNG production power generation technology ("SDL") and the parallel expansion compression synergy-CNG production power generation technology ("PCC"), draw a schematic diagram of the operable splitter ratio. Only within this threshold can it operate smoothly;

[0080] Step 7: Arrange the above parameter calculation results and draw a natural gas expansion pressure difference power generation technology screening chart under different gas source parameters; the chart is based on the outlet pressure of the pressure difference power generation system (P out ) is divided into six sub-plates (0 <P out ≤1Mpa,1 <P out ≤2Mpa, 2 <P out ≤3Mpa, 3 <P out ≤4Mpa, 4 <P out ≤5Mpa, 5 <P out ≤6Mpa), the horizontal and vertical coordinates of each sub-diagram are determined by the inlet pressure of the pressure difference power generation system (P in ) and inlet temperature (T in ), different differential pressure power generation technologies have their applicable inlet pressures and inlet temperatures. According to the applicable scopes of the three differential pressure power generation technologies, the sub-diagram can be divided into three areas: black, gray, and white. Among them, the area below each sub-diagram is black, representing the working range of DS, the area in the middle of each sub-diagram is gray, representing the working range of SDL, and the area above each sub-diagram is white, representing the working range of SDL and PCC. The applicable inlet pressures and inlet temperatures under the three differential pressure power generation technologies calculated in steps one to five determine the regional boundaries of the three technologies in the diagram; the operating parameters (inlet pressure, inlet temperature, outlet pressure) of an existing differential pressure power generation system are first calculated according to The outlet pressure selects the sub-diagram that meets the conditions. If the inlet pressure and inlet temperature coordinates of the pressure difference power generation system fall in the black area below the sub-diagram, the direct single-stage expansion pressure difference power generation technology ("DS") is selected; if the coordinates are in the gray area in the middle of the sub-diagram, the parallel expansion compression synergy-CNG production power generation technology ("PCC") is selected; if the coordinates are in the white area above the sub-diagram, both the parallel expansion compression synergy-CNG production power generation technology ("PCC") and the split-flow two-stage expansion pressure difference-LNG production power generation technology ("SDL") can be selected; similarly, the drawing process of other sub-diagrams is similar to this, and all sub-diagrams are drawn one by one and sorted and summarized;

[0081] Step 8: Use the natural gas expansion pressure difference power generation technology screening chart under different gas source parameters to select the appropriate natural gas expansion pressure difference power generation technology from PCC, SDL and DS. If two power generation technologies among PCC, SDL and DS are selected, calculate the liquid phase fraction and power generation to distinguish the most suitable technology under this condition; the liquid phase fraction q is calculated by the following formula:

[0082] q=ψ-ψ vap , where ψ is the compressibility factor of the entire mixture; ψ vap is the relative gas compressibility factor.

[0083] The specific method of using the natural gas expansion pressure difference power generation technology screening chart under different gas source parameters to determine the natural gas expansion pressure difference power generation technology is as follows: for the operating parameters of a certain pressure difference power generation system, the operating parameters include inlet pressure, inlet temperature and outlet pressure, select a qualified sub-chart according to its outlet pressure, when the inlet pressure and inlet temperature of a certain pressure difference power generation system fall in the white area of ​​the selected sub-chart, both PCC and SDL power generation technologies are available; when the inlet pressure and inlet temperature of a certain pressure difference power generation system fall in the gray area of ​​the selected sub-chart, select PCC power generation technology; when the inlet pressure and inlet temperature of a certain pressure difference power generation system fall in the black area of ​​the selected sub-chart, select DS power generation technology.

[0084] The second embodiment of the present invention:

[0085] In the present invention, the direct single-stage expansion pressure difference power generation technology is replaced by DS; the split-flow double-stage expansion pressure difference-LNG production power generation technology is replaced by SDL; the parallel expansion compression synergy-CNG production power generation technology is replaced by PCC;

[0086] Step 1: Based on the actual situation on site, clarify the infrastructure construction and process characteristics of the three pressure difference power generation technologies;

[0087] like Figure 1 As shown, DS includes No. 1 expander, flow meter, generator, inlet manifold, flow branch pipe, outlet manifold;

[0088] This technology has the characteristics of simple process flow, skid-mounted equipment, easy installation, small footprint, convenient operation, and low requirements for manpower and material resources. Although under some process parameters, this technology will cause the temperature of the expanded gas to be too low, and the water analysis will freeze or even cause ice blockage, etc., but no heating device is set in this technology. The reason is that when the above situation occurs, the cold energy must be recycled. At this time, DS is not used, and the next two technologies are selected instead.

[0089] like Figure 2As shown, the SDL includes a splitter, a No. 1 expander, a No. 2 expander, a heat exchanger, a generator, an inlet manifold, a flow branch pipe, and an outlet manifold;

[0090] SDL technology is based on DS, with a new turbine expander added. It uses secondary expansion to cool the natural gas again, reducing the temperature of the natural gas to below the water dew point so that it can be liquefied to produce LNG as a by-product. This technology recycles the pressure energy of the natural gas while taking into account the cold energy, thus maximizing energy utilization.

[0091] like Figure 3 As shown, the PCC includes a splitter, a No. 1 expander, a compressor, a heat exchanger, a generator, an inlet manifold, flow branches, and an outlet manifold.

[0092] The technological features of this power generation technology are: while recycling and utilizing the natural gas pressure energy to generate electricity, it also utilizes the cold energy generated during the natural gas pressure reduction process, thereby improving energy utilization, and produces a by-product CNG, thereby increasing the economic benefits of the product.

[0093] Organize the basic data in the pressure difference power generation process, including expander, compressor, heat exchanger, and generator operating parameter data;

[0094] Step 2: Determine the calculation data for the three expansion pressure difference power generation technologies, including the composition and properties of natural gas and parameters such as expander or compressor inlet temperature, inlet and outlet pressures, and volume flow rate.

[0095] Step 3: Determine the physical properties of natural gas, including the specific heat capacity at constant pressure and gas density; calculate the volume fraction (molar fraction) of a component in the mixed gas. Since the pressure and temperature of the gas change when it passes through the expander, the volume flow rate changes at all times. According to the molecular weight of the mixed gas, the volume fraction (molar fraction) of a component in the mixed gas, and other parameters, convert the volume flow rates of the three expansion pressure difference power generation technologies into mass flow rates;

[0096] The PR equation is used to calculate the gas density and the specific heat capacity at constant pressure of the gas;

[0097] First calculate the gas density:

[0098]

[0099] Among them, a(T)=a(T c )α(T r ,ω)

[0100]

[0101]

[0102]

[0103] f w =0.37464+1.54226ω-0.26992ω 2

[0104] Where, p is the inlet pressure, MPa; T is the inlet temperature, K; P c is the outlet pressure, MPa; T c is the critical temperature of the gas, which is a constant, T c =20K; R is the gas constant, R = 8.314 kJ / (kmol·K); α is the temperature function; T r is the relative temperature of the gas, T r =T / T c ; v is the specific volume of gas m 3 / kmol; ω is the eccentricity factor, which is determined according to the gas composition and has a value of 2.335; the gas specific volume v can be calculated by the above formula;

[0105] ρ=1 / v

[0106] ρ is the gas density, kg / m 3 .

[0107] Calculate the specific heat capacity of a gas at constant pressure:

[0108]

[0109] In the formula, is the specific heat capacity at constant pressure of the gas mixture at low pressure, kJ / (kmol·k); is the specific heat capacity at constant pressure of component i at low pressure, kJ / (kmol·k);

[0110]

[0111] In the formula, B i ,C i ,D i ,E i ,F i are the constant values ​​of pure gas, determined according to the gas composition. Taking CH4 as an example, the specific data are as follows: B i =2.39, C i =-22.18×10 4 , D i =57.40, E i =-372.79×10 11 , F i =85.50×10 12 , which is a constant value; T is the inlet temperature, K; y i is the relative molecular mass of the gas.

[0112] In the calculation at low pressure, the gas mixture can be regarded as an ideal gas, and the value of the specific heat capacity at constant volume can be obtained by using the Mayer formula;

[0113]

[0114] In the formula, is the specific heat capacity of the gas mixture at low pressure, kJ / (kmol·k); R is the gas constant, R=8.314kJ / (kmol·K);

[0115] Under high pressure, the specific heat capacity at constant pressure and specific heat capacity at constant volume of real gas are quite different from those of ideal gas. The specific heat capacity at constant volume under high pressure is derived from thermodynamic analysis.

[0116]

[0117]

[0118] In the formula,

[0119]

[0120]

[0121]

[0122] f w =0.37464+1.54226ω-0.26992ω 2

[0123]

[0124] Where, p is the inlet pressure, MPa; T is the inlet temperature, K; P c is the outlet pressure, MPa; T c is the critical temperature of the gas, which is a constant, T c =20K; ρ is the gas density, kg / m 3 ; R is the gas constant, R = 8.314 kJ / (kmol·K); T r is the relative temperature of the gas, T r =T / T c ; ω is the eccentricity factor, which is determined according to the gas composition and has a value of 2.335;

[0125] Substituting this into the formula for specific heat capacity at constant volume under high pressure and integrating it, we can calculate C v The value of

[0126]

[0127] Cv Substitute the calculated results into the above C p The specific constant volume heat capacity C can be obtained from the calculation formula p The value of .

[0128] Calculate the volume fraction (molar fraction) of a component in a gas mixture;

[0129] λ i =Z i / Z

[0130] In the formula, λ i is the molar fraction of the i-th component, dimensionless, usually expressed as a decimal; Z i is the molar number of the ith component, and Z is the sum of the molar numbers of all gas components.

[0131] Since the pressure and temperature of the gas change when it passes through the expander, the volume flow rate changes all the time. The volume flow rates of the three pressure difference power generation technologies are converted into mass flow rates according to parameters such as the molecular weight of the mixed gas and the volume fraction (molar fraction) of a component in the mixed gas.

[0132] Q m =Q v ρ

[0133]

[0134]

[0135] In the formula, Q m is the mass flow rate, kg / day; Q v is the volume flow rate, m 3 / day; ρ is density (ideal state), kg / m 3 ; p is the absolute pressure under working conditions, atm (pressure conversion is performed in units of one atmosphere); M is the average molecular weight of the mixed gas, kg / mol; R is the ideal gas constant, 0.08206; T is the working temperature, K; N i is the molecular weight of component i; i is the molar fraction of i in the mixed gas, which has been calculated by the above formula.

[0136] Step 4: Calculate the mechanical work generated by the expander or consumed by the compressor of the three expansion pressure difference power generation technologies, the enthalpy difference between the inlet and outlet, and the difference between the inlet temperature and the outlet temperature; further, calculate the outlet temperature of the expander or compressor of the three expansion pressure difference power generation technologies;

[0137] Calculate the mechanical work consumed by the three power generation technologies respectively;

[0138]

[0139] Where W is the mechanical power generated by the expander or the mechanical work consumed by the compressor, kW·h; k is the natural gas isentropic index, which is 1.33; p is the inlet pressure, MPa; P c is the outlet pressure, MPa; Q m is the mass flow rate at the expander or compressor inlet, kg / day. That is to say, under different power generation technologies, the gas flow rate entering each flow branch and the pressure at the expander or compressor outlet are artificially set.

[0140] The enthalpy difference between the inlet and outlet of the expander or compressor can be obtained according to the mechanical work generated by the expander or consumed by the compressor obtained above;

[0141]

[0142] Where ΔH is the enthalpy difference between the inlet and outlet of the expander or compressor, kJ / kg; Q m is the mass flow rate of natural gas entering the expander or compressor, kg / day; ρ is the density of natural gas entering the expander or compressor, kg / m 3 .

[0143] Based on the above enthalpy difference, the difference between the inlet temperature and the outlet temperature of the expander or compressor can be calculated;

[0144] ΔT=T2-T1=C p ×ΔH

[0145] Where ΔT is the difference between the inlet temperature and the outlet temperature of the expander or compressor, K; T1 is the inlet temperature of the expander or compressor, K; T2 is the outlet temperature of the expander or compressor, K; C p is the specific heat capacity of natural gas at constant pressure, kJ / (kg·K), which has been calculated using the formula in step three.

[0146] Calculate the temperature at the outlet of the expander or compressor of the three power generation technologies respectively;

[0147] T2=T1-ΔT

[0148] The heat exchanger outlet temperature calculation formula is:

[0149] c h ×m h ×(T 2,h -T 1,h )=c c ×m c ×(T 2,c -T 1,c )

[0150] In the formula, c his the specific heat capacity at constant pressure of the heat source, kJ / (kg·K); c c is the specific heat capacity at constant pressure of the cold source, kJ / (kg·K); m h is the mass of the heat source, kg; m c is the mass of the cooling source, kg; T 1,h is the inlet temperature of the heat source, K; T 2,h is the outlet temperature of the heat source, K; T 1,c is the inlet temperature of the cold source, K; T 2,c is the outlet temperature of the cold source, K; where, c h and c c The value can be obtained directly when the natural gas is delivered from upstream.

[0151] Step 5: Repeat steps 1 to 4 to calculate the parameter results under different working conditions;

[0152] According to the inlet temperature, inlet pressure and outlet pressure, the outlet temperature is calculated by adopting the iterative cycle method, and the step size is set to a constant value.

[0153] Step 6: Based on the above parameter calculation results, summarize the power generation rules of the three pressure difference power generation technologies. For the two power generation technologies of SDL and PCC, draw a schematic diagram of the operable diverter ratio. Only within this threshold can it operate smoothly;

[0154] According to the inlet temperature and outlet pressure of the expander, determine the respective maximum split ratios at different inlet pressures;

[0155]

[0156]

[0157] T b =T / T1

[0158] In the formula, β max is the maximum split ratio, dimensionless; γ is the specific heat ratio of the gas, which can be calculated by the PR equation in step 3; T b Temperature ratio, dimensionless; T is the inlet temperature, K; T1 is the outlet temperature, K;

[0159] β min =(P in -P out ) / P in

[0160] In the formula, β min is the minimum split ratio, dimensionless; P in is the inlet pressure, MPa; P out is the outlet pressure, MPa.

[0161] Step 7: Arrange the above parameter calculation results and draw the matching natural gas expansion pressure difference power generation technology to determine the plate.

[0162] Step 8: When the inlet pressure and inlet temperature of a differential pressure power generation system fall in the white area of ​​the sub-diagram, both PCC and SDL power generation technologies are available. The liquid fraction and power generation will be calculated to distinguish the most suitable technology under this condition.

[0163] The calculation of liquid fraction can adopt the so-called liquid fraction model, among which the commonly used models include the Soave-Redlich-Kwong (SRK) model;

[0164] q=ψ-ψ vap

[0165]

[0166]

[0167]

[0168]

[0169] f=0.48+1.574ξ-0.176ξ 2

[0170]

[0171] Where q is the liquid fraction; ψ is the compressibility factor of the entire mixture; ψ vap is the relative gas compressibility factor; V is the molar volume; A, A vap ,B,B vap is the parameter obtained from the SRK equation, which is related to the outlet temperature and outlet pressure; R is the ideal gas constant, 0.08206; T1 is the outlet temperature, K; P c is the outlet pressure, MPa; f is the relative function of the convection law; ξ is the second-order bond deviation factor, which is set according to the properties of the substance and is taken as 0.254; u is the correction parameter; T is the inlet temperature, K; T c is the critical temperature of the gas, which is a constant, T c =20K;

[0172] Based on the mechanical work calculated above, the power generation of the expander and the power consumption of the compressor can be calculated;

[0173]

[0174] Where W fd is the power generation of the expander, kW·h; η1 is the mechanical efficiency of the expander, %; W hdis the power generation of the compressor, kW·h; η2 is the mechanical efficiency of the compressor, %.

[0175] Example:

[0176] In order to make the above contents of the invention more clearly understandable, the natural gas source of a gas field in southwest China is taken as the research object, and the expansion pressure difference power generation technology screening method under different working conditions is described in detail as follows:

[0177] Step 1: Based on the actual situation on site, clarify the infrastructure construction and process characteristics of the three pressure difference power generation technologies; sort out the basic data in the pressure difference power generation process, including the expander, compressor, heat exchanger, and generator operating parameter data. The specific parameters are shown in Table 1;

[0178] Table 1 Operating parameters of three power generation technologies

[0179]

[0180]

[0181] Step 2: Determine the calculation data of the three expansion pressure difference power generation technologies, including the composition and properties of natural gas and the expander or compressor inlet temperature, inlet and outlet pressures, volume flow and other parameters; the specific parameters are shown in Table 2:

[0182] Table 2 Composition of natural gas

[0183]

[0184] Total flow ≥100×10 4 Nm 3 / d; the incoming gas pressure (i.e. the expander inlet pressure) is between 1 and 30 MPa; the incoming gas temperature (i.e. the expander inlet temperature) is between 15 and 60°C; the outlet gas pressure (i.e. the expander outlet pressure) is between 0 and 6 MPa; the compressor outlet pressure is between 20 and 30 MPa;

[0185] In order to make the whole calculation process clearer, the calculation is taken as an example with an inlet pressure of 19.6MPa, an expander outlet pressure of 3MPa, a compressor outlet pressure of 20MPa, and an inlet temperature of 30°C.

[0186] Step 3: Determine the physical properties of natural gas, including the specific heat capacity at constant pressure and gas density; calculate the volume fraction (molar fraction) of a component in the mixed gas. Since the pressure and temperature of the gas change when it passes through the expander, the volume flow rate changes at all times. According to the molecular weight of the mixed gas, the volume fraction (molar fraction) of a component in the mixed gas, and other parameters, convert the volume flow rates of the three expansion pressure difference power generation technologies into mass flow rates;

[0187] The gas density ρ is calculated by PR equation = 122.451 kg / m 3 ; Specific heat capacity of gas at constant volume C p =2.4kJ / (kmol·K); calculate the volume fraction (molar fraction) of a component in the mixed gas. The specific parameters are shown in Table 3:

[0188] Table 3 Mole fraction of gas components

[0189]

[0190] Since the pressure and temperature of the gas change when it passes through the expander, the volume flow rate changes all the time. The volume flow rates of the three pressure difference power generation technologies are converted into mass flow rates according to parameters such as the molecular weight of the mixed gas and the volume fraction (molar fraction) of a component in the mixed gas.

[0191] The average molecular weight of the gas component is calculated to be 16.47 g / mol, and the volume flow rate at 20°C and 1 atm is 2300000 m 3 / day, substituting the data into the above formula, the density of the gas before entering the expander is: 0.685kg / m 3 , so the mass flow rate is: 1575514.129kg / day.

[0192] Step 4: Calculate the mechanical work generated by the expander or consumed by the compressor of the three expansion pressure difference power generation technologies, the enthalpy difference between the inlet and outlet, and the difference between the inlet temperature and the outlet temperature; further, calculate the outlet temperature of the expander or compressor of the three expansion pressure difference power generation technologies; the specific parameters are shown in Table 4:

[0193] Table 4 Parameters of three power generation technologies

[0194]

[0195] According to the process characteristics of the three pressure difference power generation technologies, DS technology is not suitable for power generation under this condition.

[0196] Step 5: Repeat steps 1 to 4 to calculate different parameter results under various working conditions;

[0197] According to the inlet temperature, inlet pressure and outlet pressure, the outlet temperature is calculated by adopting the iterative cycle method, and the step length is set to a constant value;

[0198] According to the parameter thresholds set above (incoming air pressure 1-30MPa, incoming air temperature 15-60℃, outlet air pressure 0-6MPa), the outlet temperature is calculated by an iterative cycle method. The specific example is as follows: first control the incoming air pressure to 1MPa, the inlet temperature to 15℃, and the outlet pressure to increase from 0-6MPa in sequence with a step size of 0.1, and use the above formula to calculate the outlet temperature; continue to change the incoming air pressure and the inlet temperature, and the outlet pressure still increases in sequence, for example: the incoming air pressure is 1MPa, the inlet temperature is 15.1℃, and the outlet pressure is 0-6MPa, and the result is obtained by cyclic calculation; until the threshold spaces of the three parameters are traversed, the incoming air pressure is 1-30MPa, the incoming air temperature is 15-60℃, and the outlet air pressure is 0-6MPa, the calculation is stopped.

[0199] Step 6: Based on the above parameter calculation results, summarize the power generation rules of the three pressure difference power generation technologies. For the two power generation technologies of SDL and PCC, draw a schematic diagram of the operable diverter ratio. Only within this threshold can it operate smoothly;

[0200] For DS, the natural gas temperature T for the expander in , the natural gas pressure p flowing into the expander, the natural gas flow rate Q flowing into the expander 总 , the natural gas pressure out of the expander P c etc. operating parameters, calculate the expander outlet temperature and other parameters, and summarize their changing rules;

[0201] The change of the inlet and outlet temperature difference of the expander is only affected by the pressure difference and has nothing to do with the flow rate.

[0202] As the pressure difference gradually increases, the temperature difference also gradually increases, but the growth rate gradually slows down;

[0203] When the outlet pressure is determined, as the pressure difference gradually increases, the growth rate of power generation gradually slows down;

[0204] For the case where the pressure difference between the front and rear is low and the outlet temperature of the expander can meet the normal transportation conditions and no cold recovery is required, DS can be used. However, for the case where the pressure difference between the front and rear is high and cold recovery is required, the following two expansion pressure difference power generation technologies are required.

[0205] Based on the range of pressure and temperature changes, a large amount of data was calculated for SDL, and the following power generation rules were summarized:

[0206] Each working condition has a situation where the outlet fluid liquid fraction is the largest. At this time, the most LNG is generated and the flow fraction flowing into the heat exchanger is the lowest. The split ratio at this time is defined as the maximum split ratio. If the split ratio is higher than the maximum split ratio, the process cannot run smoothly.

[0207] Under certain working conditions, the maximum split ratio has nothing to do with the total flow rate, that is, no matter how much the total incoming gas flow is, the maximum split ratio remains unchanged;

[0208] Under certain working conditions, there is a minimum incoming gas pressure P min , for all pressure conditions below this pressure, no matter how the splitter ratio is changed, the liquid phase fraction is too small or the entire process cannot be successfully operated, making the technology unreasonable;

[0209] The expander inlet temperature and outlet pressure are specified, and the respective maximum split ratios at different inlet pressures are determined. The specific parameters are shown in Table 5:

[0210] Table 5 Maximum split ratio for different incoming gas pressures

[0211]

[0212] like Figure 4 As shown, based on the calculated data drawn into an image, it can be seen intuitively that when the incoming gas pressure is between 10.1 and 19.8 MPa, as the incoming gas pressure increases, the maximum split ratio shows a decreasing trend.

[0213] By calculating a large amount of PCC data, the following power generation rules are summarized:

[0214] Under certain incoming air temperature, pressure and flow rate, when the net power consumed by the compressor is equal to the net power generated by the expander, the splitter ratio is certain, and can be added or multiplied with the change of flow rate. The flow ratio (expander / compressor) at this time is specified to be the minimum split ratio, because if the flow entering the expander is further reduced, the generated electric energy will be reduced, which is not enough to compensate for the power consumed by the compressor, which is not worth the loss;

[0215] For each given working condition, the flow rate into the expander is continuously increased. When the flow rate increases to a certain extent, the entire process cannot be successfully operated. The main reason is that the temperatures of the cold and hot fluids are too close or cross. The ratio of the splitter at this time is specified as the maximum split ratio, that is, the ratio of the maximum flow rate into the expander to the flow rate into the compressor under the premise of ensuring the successful operation of the process;

[0216] Specify the expander inlet temperature and outlet pressure, calculate the minimum split ratio, maximum split ratio and the difference between the two at different inlet pressures, and the specific parameters are shown in Table 6:

[0217] Table 6 Maximum and minimum split ratios at different pressure points

[0218]

[0219] like Figure 5As shown, an image is drawn based on the calculated data. The range between the maximum and minimum flow split ratios is the operable flow splitter ratio. For intuitive representation, Figure 5 the shaded area in [Figure] represents all operable conditions.

[0220] Step 7: Organize the above parameter calculation results and draw a matching determination chart for the natural gas expansion differential pressure power generation technology;

[0221] According to the ranges of the inlet pressure, inlet temperature, and outlet pressure set in Step 2, through a large number of calculations, the expansion differential pressure power generation technologies applicable to different natural gas operating condition ranges are obtained. After organizing and summarizing, the following operating condition applicability table is obtained, and the specific parameters are shown in Table 7 - 12:

[0222] Table 7 Applicability Table of Expansion Differential Pressure Power Generation Technology for Outlet Pressure 0 < Pout ≤ 1 MPa

[0223]

[0224] Table 8 Applicability Table of Expansion Differential Pressure Power Generation Technology for Outlet Pressure 1 < Pout ≤ 2 MPa

[0225]

[0226] Table 9 Applicability Table of Expansion Differential Pressure Power Generation Technology for Outlet Pressure 2 < Pout ≤ 3 MPa

[0227]

[0228]

[0229] Table 10 Applicability Table of Expansion Differential Pressure Power Generation Technology for Outlet Pressure 3 < Pout ≤ 4 MPa

[0230]

[0231] Table 11 Applicability Table of Expansion Differential Pressure Power Generation Technology for Outlet Pressure 4 < Pout ≤ 5 MPa

[0232]

[0233] Table 12 Applicability Table of Expansion Differential Pressure Power Generation Technology for Outlet Pressure 5 < Pout ≤ 6 MPa

[0234]

[0235]

[0236] In order to find the matching technology under different working conditions, combined with the working conditions (inlet temperature, outlet pressure, inlet pressure) of the three pressure difference power generation technologies obtained by the above calculation, according to Table 7-12, the natural gas expansion pressure difference power generation technology screening chart under different gas source parameters is drawn, such as Figure 6 As shown; this chart can determine the pressure difference power generation technology that should be used currently based on the inlet pressure, inlet temperature and outlet pressure of the pressure difference power generation system.

[0237] Step 8: When the inlet pressure and inlet temperature of the differential pressure power generation system fall in the white area of ​​the sub-diagram, both PCC and SDL power generation technologies are available. The most suitable technology under this specific condition is determined based on the liquid fraction and power generation.

[0238] Taking the above-mentioned parameter settings as an example (inlet pressure is 19.6MPa, expander outlet pressure is 3MPa, compressor outlet pressure is 20MPa, and inlet temperature is 30°C), by comparing the chart, it can be determined that SDL and PCC power generation technologies are suitable under this condition.

[0239] The most suitable technology under specific working conditions is further selected according to the liquid phase fraction and power generation. The specific parameters are shown in Table 13:

[0240] Table 13 Liquid fraction and power generation at different pressure differences

[0241]

[0242] By comparison, the liquid fraction of SDL is 73.39% (≥60%, the minimum liquid fraction of this technology), which meets the recycling requirements. In addition, the power generation of SDL at this time is greater than that of PCC. Comprehensive analysis shows that SDL is the optimal power generation technology under this condition.

[0243] In summary, the above are only preferred specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field within the transportation scope disclosed by the present invention and the technical solution according to the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for determining natural gas expansion pressure difference power generation technology under different working conditions, characterized in that The steps include: Step 1: Based on the actual situation on site, clarify the infrastructure construction and process characteristics of the three expansion pressure difference power generation technologies; sort out the basic data in the pressure difference power generation process, including expander, compressor, heat exchanger, and generator operating parameter data; the three expansion pressure difference power generation technologies are direct single-stage expansion pressure difference power generation technology DS, split-flow two-stage expansion pressure difference-LNG production power generation technology SDL and parallel expansion compression synergy-CNG production power generation technology PCC; Step 2: Determine the calculation data of the three expansion pressure difference power generation technologies, including the composition and properties of natural gas, the inlet temperature, inlet and outlet pressures, and volume flow rate of the expander or compressor; Step 3: Determine the physical properties of natural gas, including the specific heat capacity at constant pressure and gas density of the gas; calculate the volume fraction of a certain component in the mixed gas. Since the pressure and temperature of the gas change when it passes through the expander, the volume flow rate changes at all times. According to the molecular weight of the mixed gas and the volume fraction of a certain component in the mixed gas, the volume flow rates of the three expansion pressure difference power generation technologies are converted into mass flow rates; Calculation method of specific heat capacity of gas at constant pressure: Specific heat capacity at constant volume at low pressure: In the formula, is the specific heat capacity of the gas mixture at low pressure, kJ / (kmol·k); R is the gas constant, R=8.314kJ / (kmol·K); Specific heat capacity at constant volume under high pressure: In the formula, f w =0.37464+1.54226ω-0.26992ω 2 Where, p is the inlet pressure, MPa; T is the inlet temperature, K; P c is the outlet pressure, MPa; T c is the critical temperature of the gas, which is a constant, T c =20K; ρ is the gas density, kg / m 3 ; R is the gas constant, R = 8.314 kJ / (kmol·K); T r is the relative temperature of the gas, T r =T / T c ; ω is the eccentricity factor, which is determined according to the gas composition and has a value of 2.335; Calculate C v The value of and integration: C v Substitute the calculated results into the above C p In the calculation formula, the specific constant volume heat capacity C is obtained p The value of Step 4: Calculate the mechanical work generated by the expander or consumed by the compressor of the three expansion pressure difference power generation technologies, the enthalpy difference between the inlet and outlet, and the difference between the inlet temperature and the outlet temperature; and then calculate the outlet temperature of the expander or compressor of the three expansion pressure difference power generation technologies; Step 5: Repeat steps 1 to 4 to calculate the parameter results under different working conditions; Step 6: Based on the parameter results calculated above under different working conditions, the power generation rules of the three expansion pressure difference power generation technologies are summarized. For the split-flow two-stage expansion pressure difference-LNG production power generation technology SDL and the parallel expansion compression synergy-CNG production power generation technology PCC, a schematic diagram of the operable splitter ratio is drawn to obtain the threshold range for the smooth operation of SDL and PCC; Step 7: Arrange the parameter results under different working conditions calculated in step 5, and draw the natural gas expansion pressure difference power generation technology screening chart under different gas source parameters. The natural gas expansion pressure difference power generation technology screening chart consists of six sub-charts, each of which is composed of a black area, a gray area, and a white area connected in sequence from bottom to top, and each area is tilted upward from left to right; Step 8: Use the natural gas expansion pressure difference power generation technology screening chart under different gas source parameters to select the appropriate natural gas expansion pressure difference power generation technology from PCC, SDL and DS. If two power generation technologies among PCC, SDL and DS are selected, calculate the liquid phase fraction and power generation to distinguish the most suitable technology under this condition; the liquid phase fraction q is calculated by the following formula: q=ψ-ψ vap , where ψ is the compressibility factor of the entire mixture; ψ vap is the relative gas compressibility factor.

2. The method for determining the natural gas expansion pressure difference power generation technology under different working conditions according to claim 1 is characterized by: The method for calculating the volume fraction of a component in the mixed gas in step 3 is: λ i =Z i / WITH In the formula, λ i is the molar fraction of the i-th component, dimensionless, expressed as a decimal; Z i is the molar number of the ith component, and Z is the sum of the molar numbers of all gas components.

3. The method for determining the natural gas expansion pressure difference power generation technology under different working conditions according to claim 2 is characterized in that: The method for converting the volume flow of the three expansion pressure difference power generation technologies into mass flow in step 3: Q m =Q v ρ In the formula, Q m is the mass flow rate, kg / day; Q v is the volume flow rate, m 3 / day; ρ is gas density, kg / m 3 ; M is the average molecular weight of the mixed gas, kg / mol; R is the gas constant, 0.08206; N i is the molecular weight of component i; i is the molar fraction of i in the mixed gas.

4. The method for determining the natural gas expansion pressure difference power generation technology under different working conditions according to claim 3 is characterized by: The specific method of step 4 is as follows: Calculate mechanical work; Where W is the mechanical power generated by the expander or the mechanical work consumed by the compressor, kW·h; k is the natural gas isentropic index, which is 1.33; p is the inlet pressure, MPa; P c is the outlet pressure, MPa; Q m is the mass flow rate at the expander or compressor inlet, kg / day; Obtaining the enthalpy difference between the inlet and outlet of the expander or the compressor according to the mechanical work generated by the expander or consumed by the compressor obtained above; Where ΔH is the enthalpy difference between the inlet and outlet of the expander or compressor, kJ / kg; Q m is the mass flow rate of natural gas entering the expander or compressor, kg / day; ρ is the gas density, kg / m 3 ; Based on the above enthalpy difference, the difference between the inlet temperature and the outlet temperature of the expander or compressor can be calculated; ΔT=T2-T1=C p ×ΔH Where ΔT is the difference between the inlet temperature and the outlet temperature of the expander or compressor, K; T1 is the inlet temperature of the expander or compressor, K; T2 is the outlet temperature of the expander or compressor, K; C p is the specific heat capacity of natural gas at constant pressure, kJ / (kg·K); Calculate the temperature at the expander or compressor outlet; T2=T1-ΔT The heat exchanger outlet temperature calculation formula is: c h ×m h ×(T 2,h -T 1,h )=c c ×m c ×(T 2,c -T 1,c ) In the formula, c h is the specific heat capacity at constant pressure of the heat source, kJ / (kg·K); c c is the specific heat capacity at constant pressure of the cold source, kJ / (kg·K); m h is the mass of the heat source, kg; m c is the mass of the cooling source, kg; T 1,h is the inlet temperature of the heat source, K; T 2,h is the outlet temperature of the heat source, K; T 1,c is the inlet temperature of the cold source, K; T 2,c is the outlet temperature of the cold source, K; where, c h and c c The value is obtained directly when the natural gas is delivered from upstream.

5. The method for determining the natural gas expansion pressure difference power generation technology under different working conditions according to claim 4 is characterized in that: The specific method of step seven is: According to the outlet pressure P of the pressure difference power generation system out The value is divided into six sub-plates, and the outlet pressure of the six sub-plates is 0 <P out ≤1Mpa, 1 <P out ≤2Mpa, 2 <P out ≤3Mpa, 3 <P out ≤4Mpa, 4 <P out ≤5Mpa, 5 <P out ≤6Mpa, the horizontal and vertical coordinates of each sub-diagram are determined by the inlet pressure P of the pressure difference power generation system. in and inlet temperature T in Different pressure difference power generation technologies have their applicable inlet pressure and inlet temperature. According to the applicable scope of the three pressure difference power generation technologies, the sub-diagram is divided into three areas: black area, gray area, and white area. Among them, the bottom area of ​​each sub-diagram is the black area, which represents the working range of DS, the middle area of ​​each sub-diagram is the gray area, which represents the working range of SDL, and the upper area of ​​each sub-diagram is the white area, which represents the working range of SDL and PCC. The regional boundaries of the three pressure difference power generation technologies in the plate are determined by the inlet pressure and inlet temperature applicable to the three pressure difference power generation technologies calculated in steps one to five. The black area, gray area and white area are determined and drawn through the regional boundaries to complete the drawing of a sub-plate. The drawing process of other sub-plates is similar to this, and all sub-plates are drawn one by one.

6. The method for determining the natural gas expansion pressure difference power generation technology under different working conditions according to claim 5 is characterized in that: The specific method for determining the natural gas expansion pressure difference power generation technology by using the natural gas expansion pressure difference power generation technology screening chart under different gas source parameters in step eight is: For the operating parameters of a pressure difference power generation system, the operating parameters include inlet pressure, inlet temperature and outlet pressure. Select a sub-diagram that meets the conditions according to its outlet pressure. When the inlet pressure and inlet temperature of a pressure difference power generation system fall in the white area of ​​the selected sub-diagram, both PCC and SDL power generation technologies are available; when the inlet pressure and inlet temperature of a pressure difference power generation system fall in the gray area of ​​the selected sub-diagram, PCC power generation technology is selected; When the inlet pressure and inlet temperature of a pressure difference power generation system fall in the black area of ​​the selected sub-diagram, the DS power generation technology is selected.

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