A comprehensive benefit evaluation method for a natural gas pressure differential power generation and ice making system

Abstract

The present invention discloses a comprehensive benefit evaluation method for a natural gas pressure differential power generation and ice-making system. The integrated natural gas pressure energy power generation and ice-making application system utilizes an expander to recover pressure energy for power generation, employs a brine ice-making method to recover cold energy, and preheats the exported natural gas via solar energy. This improves energy utilization efficiency and enables more efficient use of pressure energy. Comprehensive benefit evaluation indicators that consider the characteristics of the natural gas pressure energy utilization system are then proposed from four perspectives: production efficiency, equipment operation, economic benefits, and environmental protection. The AHP-anti-entropy weight method is used to calculate indicator weights, improving the reliability of the evaluation results and providing scientific guidance for reducing system operating costs and improving economic benefits.

Description

Technical Field

[0001] The present invention relates to the technical field of energy utilization, and more particularly to a comprehensive benefit evaluation method for a natural gas pressure difference power generation-ice making system. Background Art

[0002] With the country's increasing emphasis on reducing carbon emissions and growing energy demand, natural gas, as a highly efficient and clean energy source, is accumulating a growing share of the energy mix. Natural gas is typically transported via high-pressure, large-diameter, long-distance pipelines. Before delivery to end users, it undergoes pressure reduction at gas gate stations, where it contains a significant amount of recyclable pressure energy. Therefore, effectively utilizing the pressure energy contained in natural gas pipeline networks and analyzing and evaluating the operational benefits of comprehensive natural gas pressure energy utilization projects are crucial for achieving the "dual carbon" goals and improving energy efficiency.

[0003] Currently, common methods for recovering and utilizing pressure energy primarily focus on power generation and refrigeration. The fundamental principle behind utilizing natural gas pipeline pressure energy for power generation is to harness the mechanical energy generated by the expansion and decompression of high-pressure natural gas to drive a generator, which is then either supplied to the grid or used within the plant. Refrigeration uses cold energy primarily in ice making, chilled water air conditioning, natural gas liquefaction, and cryogenic pulverization. However, current research on pressure energy recovery focuses on a limited number of approaches. Systems that utilize the pressure and cold energy generated during the natural gas decompression process lack an integrated design, resulting in low energy efficiency. Furthermore, the preheating of the treated natural gas before it enters the downstream pipeline network has not been fully addressed. When evaluating the benefits of pressure energy utilization projects, existing evaluation methods only consider a single link of the pressure energy utilization system when establishing evaluation indicators, ignoring the assessment of system equipment operation and environmental protection. The evaluation indicators lack integrity, and there is little research on indicator calculation and evaluation methods. As a result, the evaluation weights of indicators are too subjective, increasing the influence of expert experience and preferences on the evaluation results, or only considering the changing trends of the indicator data itself, without combining the engineering background and industry status, resulting in inaccurate evaluation results, which cannot truly reflect the current operation status of the system and cannot provide clear directions and opinions for system upgrades and optimizations.

[0004] Therefore, how to achieve accurate evaluation of natural gas pressure energy utilization is an urgent problem that needs to be solved by those skilled in the art. Summary of the Invention

[0005] In view of this, the present invention provides a comprehensive benefit evaluation method for a natural gas pressure differential power generation and ice-making system. Specifically, the natural gas pressure energy power generation and ice-making integrated application system recovers pressure energy through a turbine expander to generate electricity, adopts a brine ice-making method to recover cold energy, and preheats the exported natural gas through solar energy, thereby improving energy utilization efficiency and enabling more efficient use of pressure energy. Then, operating benefit evaluation indicators that consider the characteristics of the natural gas pressure energy utilization system are proposed from four aspects: production efficiency, equipment operation, economic benefits, and environmental protection. The AHP-anti-entropy weight method is used to calculate the indicator weights, thereby improving the reliability of the evaluation results and providing scientific advice for reducing system operating costs and improving economic benefits.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] A comprehensive benefit evaluation method for a natural gas pressure differential power generation and ice making system comprises the following steps:

[0008] Step 1: Collect monitoring data from site monitoring instruments or operation logs, and combine Analyze the theory and calculate the evaluation index values ​​and corresponding evaluation scores of all evaluation indicators in the natural gas pressure difference power generation-ice making system;

[0009] Step 2: Establish a three-layer evaluation index system for the operation efficiency of natural gas pressure energy, which includes a target layer, a criterion layer, and an indicator layer. The criterion layer includes four evaluation criteria: production efficiency, equipment operation, economic benefits, and environmental protection. The indicator layer includes all evaluation indicators in the natural gas pressure differential power generation-ice making system. The criterion layer classifies all evaluation indicators in the indicator layer.

[0010] Step 3: Calculate the subjective weight and objective weight of each evaluation indicator under each evaluation criterion based on the classification results, and then use the combined weighting method to calculate the subjective weight coefficient and objective weight coefficient of each evaluation indicator. Use the subjective weight coefficient and objective weight coefficient to obtain the indicator combination weight of each evaluation indicator. According to the indicator combination weight of all evaluation indicators under each evaluation criterion, obtain the criterion combination weight.

[0011] The subjective and objective combined weighting method is used to determine the indicator weights, where the subjective weights are calculated using the hierarchical analysis method, and the objective weights are calculated using the anti-entropy weight method. The weight coefficients of each indicator are calculated in combination with the matrix theory, and the combined weights of the indicators are obtained.

[0012] Step 4: Determine the evaluation score calculation scheme for each type based on the classification results; obtain the index value-evaluation score data set according to the evaluation score calculation scheme;

[0013] Step 5: Use least squares fitting to obtain the scoring function for the indicator value-evaluation score data set, input the evaluation indicator value of each evaluation indicator into the scoring function, and obtain the corresponding evaluation score;

[0014] Step 6: Obtain the score of each evaluation criterion based on the evaluation score and the combined weight of each evaluation indicator; obtain the total evaluation score based on the score of each evaluation criterion and the combined weight of the criteria;

[0015] The score of each evaluation criterion is obtained by calculating the sum of the evaluation scores of all evaluation indicators corresponding to each evaluation criterion and multiplying it by the sum of the corresponding indicator combination weights;

[0016] Step 7: Classify the operating status according to the total evaluation score to obtain the evaluation results; divide it into four levels, each level corresponds to a set of scoring intervals, compare the total evaluation score with each set of scoring intervals, and evaluate it to the level of the corresponding scoring interval; determine the next upgrade and optimization plan based on the evaluation results.

[0017] The beneficial effect of the above technical solution is to realize comprehensive and comprehensive evaluation by using the four evaluation criteria of the criterion layer, which solves the problem that the operating benefits and system deficiencies of the pressure energy utilization system cannot be accurately evaluated due to the one-sided evaluation indicators in the current pressure energy utilization system evaluation field (only economic evaluation or only focusing on system operation stability), and can well reflect the operation status of the pressure energy recovery and utilization system; according to the evaluation index value, considering the data characteristics, industry standards, expected operation standards and other factors of the indicator, the evaluation score corresponding to the indicator value is determined; for example, if the calculated value of the device failure rate indicator is 40%, the evaluation score is 60 points.

[0018] Preferably, the criterion layer includes four evaluation criteria: production efficiency, equipment operation, economic benefits and environmental protection. Among them, the evaluation indicators under production efficiency include the output of the expansion generator set, the expansion machine Efficiency, heat exchanger The evaluation indicators of equipment operation include equipment utilization coefficient, device failure rate and production operation impact; the evaluation indicators of economic benefits include pressure energy power generation benefit, cold energy ice making benefit, annual operation and maintenance cost and investment payback period; the evaluation indicators of environmental protection include CO2 emission reduction, SO2 emission reduction, NO x Emission reduction and noise compliance rate.

[0019] Preferably, the AHP weight calculation method is used to construct a hierarchical model, calculate the subjective weight value, and divide the three-level evaluation index system of the comprehensive benefit of natural gas pressure energy into three levels, namely, the target layer Z, the criterion layer N, and the indicator layer Y; the four macro indicators of the criterion layer are production efficiency N1, equipment operation N2, economic benefit N3, and environmental protection N4; the evaluation indicators under the criterion layer constitute the indicator layer; the specific process of calculating the subjective weight value of each evaluation indicator is as follows:

[0020] Step 311: The criterion layer contains four evaluation criteria, and the number of evaluation indicators under the i-th criterion evaluation criterion is n i (i=1,2,3,4), the jth (j=0,1,2,…,n i ) indicators can be used Y ij To express it, the hierarchical model is expressed as:

[0021]

[0022] Step 312: Use AHP weight calculation to obtain the relative importance of the evaluation indicators. To avoid the inaccuracy of the qualitative results, the relative importance of the evaluation indicators is compared pairwise, and the comparison results are quantified using a proportional scale to obtain the weighted quantified value of each evaluation indicator. The judgment matrix is ​​constructed based on the weighted quantified value. The weighted quantified value between each factor is expressed as a gk (g,k=1,2,3,4) means, where a gk =1 / a kg ;

[0023] Step 313: Normalize the judgment matrix by column using the sum-product method to obtain the eigenvector;

[0024] Step 314: Perform consistency check on the feature vector to obtain the subjective weight value of each evaluation indicator under each evaluation criterion;

[0025] Step 315: The target layer evaluates the pressure energy utilization system.

[0026] The optimization method uses the anti-entropy weight method to calculate the objective weight value. The specific process is as follows:

[0027] Step 321: Use the indicator preprocessing method to non-dimensionalize the evaluation indicators, reduce the differences between the evaluation indicators, and calculate the correlation coefficients between the evaluation indicators. The correlation coefficient between the jth evaluation indicator and its optimal indicator value under the i-th evaluation criterion of the four evaluation criteria of production efficiency, equipment operation, economic benefit and environmental protection is expressed as:

[0028]

[0029] Where: is the indicator Yij The normalized value is the jth (j=1, 2, ..., nth) value under the i-th evaluation criterion. i ) evaluation indicators are used as Y ij To express; is the evaluation index Y ij The optimal value after normalization; is the evaluation index Y ij The optimal index value; τ∈[0,1] is the resolution coefficient, which is set to 0.5; the optimal index value can be determined based on industry guidelines, or parameters such as design power and power generation during construction, as well as expert opinions;

[0030] Step 322: Obtain a preprocessing index evaluation matrix based on the correlation coefficient, expressed as:

[0031]

[0032] Step 323: Calculate the anti-entropy and indicator weight of each indicator based on the preprocessing indicator evaluation matrix. The expression is:

[0033]

[0034]

[0035]

[0036] Where, α i (j) represents the correlation coefficient between the jth evaluation indicator and the optimal indicator value of the indicator under the i-th evaluation criterion; n i is the number of evaluation indicators of the i-th evaluation criterion in the hierarchical model; h ij is the anti-entropy of each indicator, is the objective weight value of the evaluation index determined based on the anti-entropy weight method.

[0037] Preferably, the specific process of the combined weighting method is:

[0038] Step 331: Calculate the subjective weight coefficient and the objective weight coefficient according to the subjective weight value and the objective weight value respectively. The expressions are:

[0039]

[0040] Among them, δ and ψ represent the relative importance of subjective weight and objective weight respectively; δ ij and ψ ij They represent the subjective weight relationship coefficient and objective weight coefficient of the jth evaluation indicator under the i-th evaluation criterion respectively; is the subjective weight value; is the objective weight value;

[0041] Step 332: Calculate the indicator combination weight of the evaluation indicator based on the subjective weight coefficient and the objective weight coefficient, expressed as:

[0042]

[0043] Step 333: Sum the indicator combination weights of all evaluation indicators included in each evaluation criterion to obtain the criterion combination weight of each evaluation criterion.

[0044] Preferably, the assessment is divided into four levels, namely, very low level, lower level, medium level and higher moisture, and the corresponding scoring intervals are less than 30 points, between 30-60 points, between 60-80 points and greater than 80 points.

[0045] Preferably, the natural gas pressure difference power generation-ice making system comprises: a pressure regulating station, a pressure energy power generation system and a cold energy ice making system;

[0046] The pressure regulating station includes a pretreatment device, a preheating device, and a pressure regulating device arranged in sequence; the pretreatment device is connected to a high-pressure pipeline network for transporting natural gas, and the natural gas is transmitted to the pressure regulating device and the pressure energy power generation system through the preheating device; the pressure regulating device regulates the pressure and transmits the pressure to the downstream pipeline;

[0047] The pressure energy power generation system includes a throttling pressure regulating valve, an expander, a gear box, a coupling, and a generator, which are arranged in sequence. The throttling pressure regulating valve is connected to the expander; one output of the expander is connected to the gear box to convert pressure energy into mechanical energy; the gear box and the coupling are connected to the generator to drive the generator to generate electricity;

[0048] The cold energy ice-making system includes a heat exchanger, a brine pool, a water storage tank, a solar water heating system and a heater; another output of the expander is connected to the heat exchanger, and the low-temperature natural gas after expansion and cooling enters the heat exchanger to exchange heat with ethylene glycol. The heat exchanger and the brine pool exchange heat to achieve ice making, and the refrigerant is transported to the water storage tank through a throttling and pressure-stabilizing valve; the refrigerant with a lowered temperature after heat exchange in the heat exchanger continuously provides cold capacity for the brine in the brine pool, the brine pool and the water storage tank exchange heat through a water injection system, and the brine after heat exchange is exchanged with water in the water injection system; the solar water heating system exchanges heat with the water storage tank, and the natural gas passing through the water storage tank is then transported to the downstream pipeline through the heater.

[0049] Preferably, the pretreatment device includes a dehydration device and a dehydrogenation device; the pressure regulating device is composed of a throttling pressure-stabilizing valve; the preheating device is an electric heater, and the electric heater, dehydration device and dehydrogenation device are used to avoid the formation of natural gas hydrates and achieve safe and stable operation of the system.

[0050] Preferably, the expander is a turbine expander generator. When natural gas enters the turbine expander generator, the volume expansion of the natural gas releases pressure energy, which drives the turbine expander generator to rotate. The pressure energy is converted into mechanical energy, which is transmitted to the generator through the gear box and coupling, driving the coaxial generator to generate electricity.

[0051] Preferably, the heat exchanger is filled with ethylene glycol as a refrigerant, which exchanges heat with the incoming low-temperature natural gas, and the refrigerant continuously provides cold energy for the brine in the brine pool; the ice-making raw water in the water storage tank exchanges heat with the brine in the brine pool through the water injection system.

[0052] Through the above technical solutions, it can be seen that compared with the existing technology, the present invention discloses a comprehensive benefit evaluation method for a natural gas pressure difference power generation-ice making system, wherein the natural gas pressure energy power generation-ice making integrated application system realizes the maximum utilization of pressure energy, effectively solves the problem of low pressure energy utilization rate of existing stations, and the cold energy utilization system has a high energy recovery rate. When exchanging heat through the water storage tank, the solar water heating system is used to preliminarily heat the natural gas, and then it is heated by the electric heater before entering the downstream pipeline network, which reduces the use of electricity and improves the safety of the pipeline network. The entire system provides a basis for the planning and construction of a new pressure regulating station pressure energy utilization system. The comprehensive benefit evaluation method constructs a comprehensive benefit evaluation index system for the comprehensive utilization of pressure energy. These indicators are comprehensive and targeted, accurately reflecting the current status and existing deficiencies of the pressure energy utilization system from the perspectives of operational, economic, and public welfare characteristics. They fully consider the economic and environmental benefits of pressure energy utilization, and meet the requirements of relevance, importance, feasibility, and timeliness. They embody the operational philosophy of "energy conservation and emission reduction, quality and efficiency improvement, and safety first." While highlighting the park's current shortcomings, they also provide effective guidance on the next steps for optimization and key areas of research. The AHP+anti-entropy weighting method for determining indicator weights in the comprehensive benefit evaluation combines the advantages of both subjective and objective weights. Based on expert experience and the actual operational characteristics of the pressure energy utilization stations, the final weights derived from the subjective and objective weight coefficients accurately reflect the current focus of the park's development. The weight distribution is accurate and appropriate, demonstrating the characteristics of "overview, focus on the present, and look to the future." BRIEF DESCRIPTION OF THE DRAWINGS

[0053] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are merely embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying any creative work.

[0054] Figure 1The accompanying drawing is a flow chart of the natural gas pressure energy power generation and ice making integrated application system provided by the present invention;

[0055] Figure 2 The accompanying drawing is a schematic diagram of comprehensive benefit evaluation indicators provided by the present invention;

[0056] Figure 3 The accompanying drawing is a schematic diagram of the criterion layer weights provided by the present invention. DETAILED DESCRIPTION

[0057] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0058] The embodiment of the present invention discloses a comprehensive benefit evaluation method for a natural gas pressure differential power generation and ice making system, comprising the following steps:

[0059] S1: Taking into account the utilization of both pressure energy and cold energy, an integrated natural gas pressure differential power generation and ice-making application system was established. To improve the efficiency of cold energy utilization, a two-stage heat exchange method was used to recover the cold energy carried by the reduced-pressure natural gas. Electric heaters and dehydration and dehydrogenation devices were also introduced to avoid the formation of natural gas hydrates and ensure safe and stable operation of the system. The entire solution maximized the utilization of natural gas pressure energy.

[0060] S2: Based on The natural gas pressure energy potential calculation model is established by analyzing the theory. and specific pressure The analysis can evaluate the available pressure of the pressure regulating station;

[0061] S3: Establish a three-tiered evaluation index system for the operational benefits of natural gas pressure energy, consisting of a target layer (Z), a criterion layer (N), and an indicator layer (Y). The criterion layer includes four evaluation criteria: production efficiency, equipment operation, economic benefits, and environmental protection. Each criterion contains multiple specific evaluation indicators. Together, these evaluation indicators form the indicator layer, which is comprehensive and can well reflect the operation of the pressure energy recovery and utilization system.

[0062] S4: The indicator weights are established by combining subjective and objective weights. The subjective weights are determined by the hierarchical analysis method, and the objective weights are determined by the anti-entropy weight method. Combined with the basic idea of ​​matrix theory, the weight coefficients of each indicator are obtained, and finally the indicator combination weights are obtained. The indicator combination weights of all evaluation indicators under each evaluation criterion are summed to obtain the corresponding criterion combination weights.

[0063] S5: By classifying the evaluation indicators into three types: enhanced, reduced, and interval, a calculation method for the evaluation scores corresponding to each indicator is established. After obtaining multiple sets of indicator values ​​and corresponding scores, a scoring function is obtained through least squares fitting. The evaluation indicator values ​​of each evaluation indicator are input into the scoring function to obtain the corresponding evaluation score;

[0064] S6: Obtain the score of each evaluation criterion based on the evaluation score corresponding to each evaluation indicator and the indicator combination weight; obtain the total evaluation score based on the score of each evaluation criterion and the criterion combination weight;

[0065] The score of each evaluation criterion is obtained by calculating the sum of the evaluation scores of all evaluation indicators corresponding to each evaluation criterion and multiplying it by the sum of the corresponding indicator combination weights;

[0066] S6: Divide the total evaluation score into four levels, and determine the level of system operation effect by comparing the scoring intervals of the total evaluation score to obtain the final evaluation result.

[0067] The comprehensive benefit evaluation method of the present invention can also be used to evaluate the benefits of other pressure energy utilization systems, identify system deficiencies, and provide suggestions to support system upgrades and optimizations.

[0068] Example

[0069] In one embodiment, the natural gas pressure difference power generation and ice making system is specifically as follows:

[0070] The system uses a turbine expansion generator to recover pressure energy to generate electricity, and the cold energy generated during the pressure reduction process is exchanged through a refrigerant, and then the refrigerant is exchanged with water to make ice. At the same time, electric heaters and dehydration and dehydrocarbonization devices are introduced to avoid the formation of natural gas hydrates and achieve safe and stable operation of the system. The present invention integrates the power generation and ice-making system into an integrated design, recovering pressure energy and cold energy at the same time, greatly improving the energy utilization efficiency. In addition, the system innovatively uses brine ice-making method to recover cold energy through secondary heat exchange, which improves the recovery efficiency of cold capacity. At the same time, a solar water heating system is added at the outlet to use clean energy to heat the outgoing natural gas, which not only reduces the use of electricity, but also pre-cools the raw water for ice making, achieving multiple goals at one stroke. The entire solution maximizes the utilization of natural gas pressure energy. The system flow chart is as follows: Figure 1 The system consists of the following three parts:

[0071] (1) Traditional pressure regulating station

[0072] Traditional natural gas pressure regulating stations utilize an isenthalpic throttling system. The pressure regulating system consists of a pretreatment unit (dehydration and dehydrogenation), a preheating unit, and a throttle valve. Before entering the high-pressure pipeline network for transportation, the extracted natural gas undergoes a rough treatment process to remove most impurities such as water, hydrogen sulfide, and carbon dioxide. Before entering the pressure regulating station for pressure reduction, the high-pressure natural gas also passes through a dehydration unit and a dehydrogenation unit. This prevents the temperature of the water and hydrocarbons in the natural gas from dropping below their dew point after adiabatic throttling, potentially forming natural gas hydrates and compromising operational safety. The heated natural gas then passes through a throttle valve for pressure reduction before entering the downstream pipeline network.

[0073] (2) Pressure energy power generation system

[0074] A branch is drawn out from the traditional pressure-regulating pipeline network, and the natural gas with heavy hydrocarbons and water removed is first stabilized by a throttling pressure-stabilizing valve to keep the front pressure at the expander inlet stable. The stabilized natural gas enters the turbine expander, and the volume expansion of the natural gas releases pressure energy, which drives the expander to rotate. The pressure energy is converted into mechanical energy, which is transmitted to the generator through the gearbox and coupling, driving the coaxial generator to generate electricity.

[0075] (3) Cold energy ice making system

[0076] When natural gas undergoes adiabatic expansion (Q = 0) and performs work (W > 0) within a turbine expander, its thermodynamic energy decreases (U < 0), lowering its temperature and achieving a cooling effect. This solution utilizes the relatively mature brine ice-making method. After expansion and cooling, the low-temperature natural gas enters a heat exchanger where it exchanges heat with ethylene glycol (50%). The glycol temperature drops to approximately -19°C, while the natural gas temperature rises to approximately -10°C. The cooled ethylene glycol continuously provides cooling energy to the brine (29.4%) in the brine tank, maintaining a constant temperature of approximately -10°C. The heated natural gas still has usable cooling energy, which is then exchanged with the ice-making raw water in the water storage tank. This heat exchange reduces the water temperature to below 10°C, while the natural gas temperature rises to approximately -5°C. The ice-making raw water in the water storage tank then exchanges heat with the cold brine through a water injection system, cooling the water and forming ice cubes, effectively recovering the natural gas's cooling energy. Since the natural gas outlet temperature is higher than the standard gas supply temperature (>5°C), the natural gas is preliminarily heated by the solar water heating system when passing through the water storage tank, and then heated by heater 2 before entering the downstream pipeline network.

[0077] Calculation model of available natural gas pressure energy potential:

[0078] The energy of pipeline high-pressure natural gas mainly has two forms, namely potential energy and kinetic energy, which are respectively reflected by pressure and temperature. Analytical theory, specific enthalpy of natural gas By temperature and specific pressure Composition can be expressed as:

[0079] e x,h =e x,T +e x,P (9)

[0080]

[0081]

[0082] Where: e x,h is the specific enthalpy of natural gas kJ / kg;e x,T is the specific temperature of natural gas kJ / kg;e x,P is the specific pressure of natural gas kJ / kg; is the inlet specific enthalpy of natural gas, kJ / kg; is the specific enthalpy of natural gas outlet, kJ / kg; T0 is the ambient temperature, K; c p is the specific heat capacity of natural gas at constant pressure, kJ / (kg-K); T1 is the natural gas inlet temperature, K; T2 is the natural gas outlet temperature, K; R g is the gas constant of natural gas, kJ / (kg-K); p1 is the inlet absolute pressure, MPa; p2 is the outlet absolute pressure, MPa.

[0083] The evaluation indicators in step three include:

[0084] The present invention proposes evaluation indicators from four aspects: production efficiency, equipment operation, economic benefits and environmental protection, such as Figure 2 As shown:

[0085] (1) Evaluation indicators of production capacity efficiency

[0086] Expansion generator output:

[0087] The power generation performance of the expander is easily affected by fluctuations in natural gas pressure, flow, and temperature. The calculation formula is as follows:

[0088]

[0089] Where: P te is the output of the expansion generator set, kW; Q is the hourly flow rate of natural gas, Nm 3 / h; ρ is the density of natural gas, kg / m 3 ; C pis the isobaric specific heat capacity of natural gas in kJ / (kg·k); T is the natural gas inlet temperature in K; P1 is the natural gas inlet absolute pressure in MPa; P2 is the natural gas outlet absolute pressure in MPa; k is the natural gas adiabatic index; η T is the expander efficiency; η g is the generator efficiency;

[0090] expander efficiency:

[0091] In use When analyzing the efficiency of an expander during power generation, the expander and generator are generally considered as a single system. The calculation formula is as follows:

[0092]

[0093] Where: PE For expander efficiency; is the natural gas mass flow rate, kg / s; is the specific enthalpy of natural gas at the expander inlet, kJ / kg; is the specific entropy of natural gas at the expander outlet, kJ / (kg-K); is the specific entropy of natural gas at ambient conditions, kJ / (kg-K);

[0094] heat exchanger efficiency:

[0095] The heat exchanger is a key device for recovering the cold energy carried by the natural gas after pressure reduction. The temperature of the natural gas increases in the heat exchanger. Increase. The calculation formula is as follows:

[0096]

[0097]

[0098]

[0099]

[0100]

[0101] Where: he For heat exchanger efficiency; The natural gas entering and leaving the heat exchanger kW; The ethylene glycol entering and leaving the heat exchanger kW; For the natural gas at the heat exchanger outlet kW; For the natural gas exported by the expander kW; is the specific enthalpy of natural gas at the heat exchanger outlet, kJ / kg; is the specific enthalpy of natural gas at ambient conditions, kJ / kg; is the specific entropy of the natural gas at the heat exchanger outlet, kJ / (kg-K); is the mass flow rate of ethylene glycol, kg / s; are the specific enthalpies of ethylene glycol at the inlet and outlet of the heat exchanger, kJ / kg; are the specific entropies of ethylene glycol at the inlet and outlet of the heat exchanger, kJ / (kg-K);

[0102] Annual ice production:

[0103] Ice making is an important part of the comprehensive utilization system of pressure energy. The output of ice cubes not only reflects the effect of cold energy utilization, but also brings production efficiency. The calculation formula is as follows:

[0104]

[0105] Where: L im is the annual ice production of the system, tons; is the daily ice making capacity, tons / day; t im is the working time of the ice making system in a year, days; im The ice retention rate in the ice making system after deducting the loss caused by transportation;

[0106] Solar collector heat collection capacity:

[0107] The heat collected by the solar collector can be expressed as follows:

[0108] φ sc =λ sc I sr B oa (20)

[0109] Where: φ sc is the heat collected by the solar collector, kW; sc is the heat conversion efficiency; I sr is the solar radiation intensity, kW / m2; B oa is the effective light receiving area of ​​the collector, m2;

[0110] Expander power generation voltage fluctuation:

[0111] The expander power generation is affected by the pressure and flow of the upstream gas source and the gas load, and the voltage value will also change continuously. The voltage fluctuation calculation formula is as follows:

[0112]

[0113] Where: ε um is the voltage fluctuation value; U max 、U min are the two adjacent maximum and minimum values ​​on the voltage RMS curve, V;

[0114] (2) Equipment operation indicators

[0115] Equipment utilization factor:

[0116] The equipment utilization coefficient is related to the actual working time of each key equipment in the pressure energy recovery system. The calculation formula is as follows:

[0117]

[0118] Where: eo is the equipment operation coefficient; n is the number of pressure energy recovery core equipment; t d The planned working time of a single device, days; t i is the actual working time of the i-th device, days;

[0119] Device failure rate:

[0120] The calculation formula is as follows:

[0121]

[0122] Where: df is the device failure rate; T fs T is the downtime caused by the failure, days; pw Plan working time for the system, days;

[0123] Impact on production operations:

[0124] The production operation impact (σ∈(0,1)) is used to evaluate the impact of the pressure difference power generation system on the safe and stable operation of the normal oil and gas production process and the operation and maintenance workload due to problems such as sealing, heating, gas liquid and connection with the original system.

[0125] (3) Economic benefit indicators

[0126] The pressure energy recovery system involves investment and modification of the original pipeline. The cost recovery period and the economic effect evaluation after the modification are particularly important.

[0127] Benefits of pressure energy generation:

[0128] The benefits of pressure energy power generation include electricity savings for self-use at the site and revenue from selling surplus electricity to the grid. The calculation formula is as follows:

[0129]

[0130] Where: M ne The economic benefit of pressure energy power generation, yuan; A og The grid electricity price for the station is RMB / kW-h; A ug is the on-grid electricity price of surplus power generated by pressure energy, RMB / kW-h; The power consumption of the pressure energy power station, kW-h; The amount of electricity used for Internet access;

[0131] Benefits of cold energy ice making:

[0132]

[0133] Where: M im is the annual income from ice making, yuan; is the unit price of ice, yuan / ton;

[0134] Annual operation and maintenance costs:

[0135] The operation and maintenance costs of the pressure energy power generation and ice making system mainly include the maintenance costs of equipment such as the expander, excitation system, and background system; the replacement costs of wearing parts such as lubricating oil, fuses, circuit breakers, filters, and bearings; and the electricity costs generated by the lubricating oil pump, exhaust fan, and PLC control cabinet.

[0136] Payback period:

[0137] The shorter the payback period, the better the economic efficiency of the project. The calculation formula is as follows:

[0138]

[0139] Where: T ir is the investment payback period, years; Z ve is the total investment of the system, RMB; M ai is the annual economic benefit of the system, RMB; M om The system operation and maintenance cost is RMB.

[0140] (4) Environmental protection indicators

[0141] Natural gas pressure energy is a clean energy source. Its recycling can improve energy efficiency, reduce the use of coal-fired power generation, lower carbon emissions, and protect the environment.

[0142] CO2 emission reduction:

[0143] This indicator refers to the reduction in CO2 emissions from coal-fired power generation by generating clean electricity through pressure difference power generation. The calculation formula is as follows:

[0144] R c =P te ·Oc (27)

[0145] Where: R c is the CO2 emission reduction, kg; O c The amount of CO2 emissions reduced for each kilowatt-hour of electricity saved, kg / kW-h;

[0146] SO2 emission reduction:

[0147] This indicator refers to the reduction in coal-fired power SO2 emissions from clean electricity generated through differential pressure power generation. The calculation formula is as follows:

[0148] R s =P te ·O s (28)

[0149] Where: R s is the SO2 emission reduction, kg; O s The amount of SO2 emissions reduced per kilowatt-hour of electricity saved, kg / kW-h;

[0150] NO x Emission reduction:

[0151] This indicator refers to the reduction of coal-fired power NOx by generating clean electricity through pressure difference power generation. x Emissions are calculated as follows:

[0152] R n =P te ·O n (29)

[0153] Where: R n NO x Emission reduction, kg; O n For every kilowatt-hour of electricity saved, NO x Emissions, kg / kW-h;

[0154] Noise compliance rate:

[0155] After the expansion power generation equipment is put into use, it will generate noise, which will affect the normal work of the operators and is the root cause of various accidents. The noise compliance rate calculation formula is as follows:

[0156]

[0157] Where: ε vo is the noise compliance rate; T un T is the time when the noise exceeds the upper limit, in hours; all is the total monitoring time, hours.

[0158] Combined weighting method:

[0159] (1) AHP weight calculation

[0160] AHP is a subjective weight calculation method. The pressure energy comprehensive utilization evaluation system is divided into three levels, namely the target layer (Z), the criterion layer (N) and the indicator layer (Y); the four macro indicators of the criterion layer are production efficiency N1, equipment operation N2, economic benefit N3 and environmental protection N4; the micro indicators under the criterion layer constitute the indicator layer, and the number of micro indicators under the i-th criterion layer factor is n i (i=1,2,3,4), n i Satisfies formula (26), so the jth (j=0, 1, 2, ..., nth) i ) indicators available Y ij To express; after the hierarchical structure model is established, the judgment matrix can be constructed to calculate the indicator weights of the criterion layer and the indicator layer:

[0161]

[0162] In order to avoid the inaccuracy of qualitative results when constructing the judgment matrix, the comparison results between indicators are quantified using the scale table shown in Table 1. The obtained criterion layer judgment matrix is ​​shown in Table 2 below. The weight quantification value between each factor is expressed as a gk (g,k=1,2,3,4) means, where a gk =1 / a kg ;

[0163] Table 1 Ratio scale table

[0164] Factor i vs. factor j Quantized value Equally important 1 Slightly important 3 Strong and important 5 Strongly important 7 Extremely important 9 The middle value of two adjacent judgments 2，4，6，8

[0165] Table 2 Criteria layer judgment matrix

[0166]

[0167]

[0168] Using the sum-product method to normalize the columns, the eigenvector W of the judgment matrix can be obtained z =[w z1 ,w z2 ,w z3 ,w z4 ],in

[0169] Finally, the weights of each evaluation criterion at the criterion level can be obtained by performing consistency checks on equations (24) and (25), including the weights of production efficiency, equipment operation, economic benefits, and environmental protection.

[0170]

[0171]

[0172] Where: max is the maximum eigenvalue among the eigenvectors of the judgment matrix; n is the order of the judgment matrix; the RI value is obtained from Table 3;

[0173] Table 3 RI value table of AHP

[0174] n 1 2 3 4 5 6 7 8 RI 0 0 0.52 0.89 1.12 1.26 1.36 1.41

[0175] Table 4 Evaluation index judgment matrix

[0176]

[0177] When CR≤0.1, the consistency test is passed. Otherwise, the scale between the elements of the judgment matrix needs to be adjusted to adjust the CI value. The weights of the evaluation indicators under each evaluation criterion can be calculated by the above method. The judgment matrix is ​​shown in Table 4. Finally, the subjective weight value of each indicator is obtained.

[0178] (2) Anti-entropy weight method

[0179] The anti-entropy weight method is an objective weight determination method. By improving the entropy weight method, the uncertainty of the measurement is reduced and the stability of the weight value is improved. The entropy value originated from thermodynamics and was later introduced into information theory to represent the degree of disorder in the system. The specific calculation steps are as follows:

[0180] 1) Index preprocessing

[0181] Different indicators under different evaluation modules can be divided into enhanced, reduced, and interval types because of their different physical meanings. The larger the enhanced indicator value, the better, the smaller the reduced indicator value, the better, and the interval indicator scores the highest at a certain value in the middle. If different types of indicators are evaluated under the same data target, the results will lack accuracy. The present invention uses an indicator preprocessing method to process data indicators dimensionlessly and reduce the differences between indicators. i The correlation coefficient between the jth indicator and its optimal indicator under each evaluation module is shown in formula (26);

[0182]

[0183] Where: is the indicator Y ij Normalized value; is the indicator Y ij The optimal value after normalization; is the indicator Y ij The optimal value of ; τ∈[0,1] is the resolution coefficient, which is 0.5;

[0184] The preprocessing index evaluation matrix is ​​obtained as

[0185]

[0186] 2) Determine anti-entropy and weights

[0187]

[0188]

[0189]

[0190] Where: h ij is the anti-entropy of each indicator, The subjective weight value of each indicator is obtained by determining the indicator weight based on the anti-entropy weight method.

[0191] (3) Portfolio empowerment

[0192] The relative importance of subjective weight and objective weight varies with the change of indicators. Combining the basic idea of ​​matrix theory, δ and ψ are used to represent the relative importance of subjective and objective weights respectively, and the subjective and objective weight relationship coefficient δ of each indicator is obtained. ij and ψ ij , and finally get the combined weight of the indicators;

[0193]

[0194] The subjective weight value obtained using AHP And the objective weight value obtained by anti-entropy weight method Combining the subjective and objective weight relationship coefficients, the final weight value of the indicator is obtained as shown in formula (8).

[0195]

[0196] Determined rating ranges:

[0197] When the score is less than 30 points, the level of the evaluation score is "extremely low level, obvious problems"; when the score is between 30-60 points, the level of the evaluation score is "low level, urgent need for improvement"; when the score is between 60-80 points, the level of the evaluation score is "medium level, needs optimization"; when the score is greater than 80 points, the level of the evaluation score is "high level, good development".

[0198] Example 2

[0199] (1) Taking the comprehensive benefit evaluation of a natural gas purification plant as an example, the plant's residual pressure power generation device uses the pressure difference between the purified gas pressure and the downstream pipeline network to generate electricity. The process used is a centripetal turbine expander. The pressure difference power generation is used for the purification plant's own use, and the cold energy generated by the natural gas pressure reduction is used to make ice for export. The relevant basic parameters are shown in Table 5.

[0200] Table 5 Purification plant related parameters

[0201]

[0202]

[0203] (2) From the four aspects of production efficiency, equipment operation, economic benefits, and environmental protection, the detailed evaluation indicators under each indicator criterion are calculated using formulas (4)-(22).

[0204] (3) The evaluation index system is divided into three levels: target level (Z), criterion level (N) and indicator level (Y). Figure 2 As shown in Figure 1, a judgment matrix is ​​constructed, and the relative importance of each indicator is compared pairwise using the proportional scale table in Table 1. Then, the eigenvector of the judgment matrix is ​​obtained by column normalization using the sum-product method, and the AHP subjective weight is calculated using formulas (24)-(25).

[0205] (4) The objective weight of the indicator is determined by using the anti-entropy weight method. The parameters of each evaluation indicator in the past three years are calculated using the obtained parameters. The indicators are divided into three types: enhanced, reduced, and interval types. The indicators are dimensionlessly preprocessed using formula (26), and the weight values ​​of the indicators are calculated using formulas (27)-(30).

[0206] (5) Combining the subjective weight and the objective weight, using δ and ψ to represent the relative importance of the subjective and objective weights, respectively, to obtain the subjective and objective weight relationship coefficient δ of each indicator ij and ψ ij , and finally get the combined weight of the indicators. As shown in Table 6, the weight of the criterion layer is Figure 3 shown.

[0207] Table 6 Evaluation index combination weights

[0208]

[0209]

[0210] (6) Analysis of weighted results.

[0211] According to the results in Table 6, equipment operation (0.3095) and production efficiency (0.2816) have the largest weights and are the most important aspects of the operational benefit evaluation of the pressure energy utilization system in this purification plant. Domestically, there is a lack of mature and stable operational experience for natural gas pressure energy utilization, and unified industry standards have yet to be established. When recycling pressure energy, it is particularly important for purification plants to increase power generation and ice production, reduce equipment failure rates (0.1134), and maintain stable equipment operation. Environmental protection accounts for 0.2295, with carbon dioxide emissions reduction (0.0807) being the most important component. Southwest China is rich in natural gas, and reducing carbon emissions through pressure energy recovery is a key means of achieving the "dual carbon" goals. Economic benefits (0.1397) have the lowest weight, indicating that the economic benefits of the comprehensive pressure energy utilization system are not currently a priority for this purification plant. Resolving the integration issues between the pressure energy equipment and the original system and ensuring safe and stable operation are the top priorities.

[0212] (7) Determine the evaluation score.

[0213] The evaluation index calculation method proposed in Step 3 can be used to calculate the values ​​of each indicator based on actual data. Substituting these values ​​into the established scoring function yields the evaluation scores for each indicator. Combined with the indicator weights determined using the subjective and objective weight determination method in Table 6, the final scoring results for the natural gas pressure energy comprehensive utilization system can be obtained, as shown in Table 7.

[0214] Table 7 Comprehensive evaluation scores

[0215]

[0216]

[0217] Evaluation Results Analysis. Table 7 shows that since its official commissioning in 2019, scores for all indicators have significantly improved, with equipment operation rising from a low level (47.04) to a medium level (72.81), representing the most significant improvement. Production quality (63.79) has seen minimal improvement and has remained at a low level for a long time. This is primarily due to the fact that the purification plant began implementing pressure reduction operations in 2020 to support upstream gas mines in maximizing gas field capacity. Consequently, the residual pressure power generation unit did not operate at full capacity, resulting in overall economic benefits (60.49) falling short of expectations and slowing improvement in the score. Furthermore, with the gradual improvement in the efficiency of differential pressure power generation and ice production, the system's role in reducing carbon emissions and utilizing clean energy has become increasingly evident, leading to significant improvement in the environmental protection score (64.09). However, due to the high noise levels during operation, which impacted both park production and surrounding residents, this criterion's score fell below expectations, remaining at a medium level. This evaluation result is consistent with the purification plant's initial operational status, demonstrating the correctness and effectiveness of the comprehensive pressure energy utilization benefit evaluation system.

[0218] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. Reference can be made to the common and similar parts between the various embodiments. For the devices disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the description is relatively simple, and the relevant parts can be referred to the method description.

[0219] The above description of the disclosed embodiments is intended to enable one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be readily apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention is not limited to the embodiments shown herein but is intended to conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A comprehensive benefit evaluation method for a natural gas pressure differential power generation and ice making system, characterized in that: The following steps are involved: Step 1: Collect monitoring data from site monitoring instruments or operation logs, and combine the monitoring data with Analyze the theory and calculate the evaluation index values ​​of all evaluation indicators in the natural gas pressure difference power generation-ice making system; Step 2: Establish a three-tiered evaluation index system for the comprehensive benefits of natural gas pressure energy, including a target layer, a criterion layer, and an indicator layer. The criterion layer includes four evaluation criteria: production efficiency, equipment operation, economic benefits, and environmental protection. The indicator layer includes all evaluation indicators in the natural gas pressure differential power generation and ice making system. The criterion layer classifies all evaluation indicators in the indicator layer. Step 3: Calculate the subjective weight value and objective weight value of each evaluation indicator based on the classification results, and then use the combined weighting method to calculate the subjective weight coefficient and objective weight coefficient of each evaluation indicator. Use the subjective weight coefficient and objective weight coefficient to obtain the indicator combination weight of each evaluation indicator. According to the indicator combination weight of all evaluation indicators under each evaluation criterion, obtain the criterion combination weight; Step 4: Determine the evaluation score calculation scheme for each type based on the classification results; according to the evaluation score calculation scheme, combine the evaluation index value and the corresponding evaluation score to obtain the index value-evaluation score data set; Step 5: Use least squares fitting to obtain the scoring function for the indicator value-evaluation score data set, input the evaluation indicator value of each evaluation indicator into the scoring function, and obtain the corresponding evaluation score; Step 6: Obtain the score of each evaluation criterion based on the evaluation score and the combined weight of each evaluation indicator; obtain the total evaluation score based on the score of each evaluation criterion and the combined weight of the criteria; Step 7: Classify the operating status according to the total evaluation score to obtain the evaluation results; The natural gas pressure difference power generation and ice making system includes: a pressure regulating station, a pressure energy power generation system and a cold energy ice making system; The pressure regulating station includes a pretreatment device, a preheating device, and a pressure regulating device arranged in sequence; the pretreatment device is connected to a high-pressure pipeline network for transporting natural gas, and the natural gas is transmitted to the pressure regulating device and the pressure energy power generation system through the preheating device; the pressure regulating device regulates the pressure and transmits the pressure to the downstream pipeline; The pressure energy power generation system includes a throttling and pressure-stabilizing valve, an expander, a gear box, a coupling, and a generator arranged in sequence, wherein the throttling and pressure-stabilizing valve is connected to the expander; one output of the expander is connected to the gear box; the gear box and the coupling are both connected to the generator; The cold energy ice-making system includes a heat exchanger, a brine pool, a water storage tank, a solar water heating system and a heater; another output of the expander is connected to the heat exchanger, which exchanges heat with the brine pool and transmits the heat to the water storage tank through a throttling and pressure-stabilizing valve; the brine pool and the water storage tank exchange heat through a water injection system; the solar water heating system exchanges heat with the water storage tank, and the natural gas passing through the water storage tank is then transmitted to the downstream pipeline through the heater.

2. The comprehensive benefit evaluation method of a natural gas pressure difference power generation-ice making system according to claim 1 is characterized in that: The evaluation indicators under the capacity efficiency include the output of the expansion generator set, the output of the expander Efficiency, heat exchanger The evaluation indicators of equipment operation include equipment utilization coefficient, device failure rate and production operation impact; the evaluation indicators of economic benefits include pressure energy power generation benefit, cold energy ice making benefit, annual operation and maintenance cost and investment payback period; the evaluation indicators of environmental protection include CO2 emission reduction, SO2 emission reduction, NO x Emission reduction and noise compliance rate.

3. The comprehensive benefit evaluation method of a natural gas pressure difference power generation-ice making system according to claim 1, characterized in that: In step 3, the AHP weight calculation method is used to construct a hierarchical model to calculate the subjective weight values ​​of the evaluation indicators in the indicator layer corresponding to each evaluation criterion in the criterion layer. The specific process is as follows: Step 311: The criterion layer contains four evaluation criteria, and the number of evaluation indicators under the i-th evaluation criterion is n i (i=1,2,3,4), the hierarchical model is expressed as: Step 312: Using AHP weight calculation to obtain the relative importance of the evaluation indicators, performing pairwise comparisons of the relative importance of the evaluation indicators, and quantifying the comparison results using a proportional scale to obtain the weighted quantified values ​​of each evaluation indicator. A judgment matrix is ​​constructed based on the weighted quantified values. Step 313: Normalize the judgment matrix by column using the sum-product method to obtain the eigenvector; Step 314: Perform a consistency check on the feature vector to obtain the subjective weight value of the evaluation index under each evaluation criterion.

4. The comprehensive benefit evaluation method of a natural gas pressure difference power generation-ice making system according to claim 3 is characterized in that: In step 3, the anti-entropy weight method is used to calculate the objective weight value. The specific process is as follows: Step 321: Use the indicator preprocessing method to non-dimensionalize the evaluation indicators, reduce the differences between the evaluation indicators, and calculate the correlation coefficient between the evaluation indicators. The correlation coefficient between the j-th evaluation indicator and its optimal indicator value under the i-th evaluation criterion is expressed as: Where: is the evaluation index Y ij The normalized value is the jth (j=1, 2, ..., nth) value under the i-th evaluation criterion. i ) evaluation indicators are used as Y ij To express; is the evaluation index Y ij The optimal value after normalization; is the evaluation index Y ij The optimal index value; τ∈[0,1] is the resolution coefficient, which is 0.5; Step 322: Obtain a preprocessing index evaluation matrix based on the correlation coefficient, expressed as: Step 323: Calculate the anti-entropy and indicator weight of each indicator based on the preprocessing indicator evaluation matrix. The expression is: Where, α i (j) represents the correlation coefficient between the jth evaluation indicator and the optimal indicator value of the indicator under the i-th evaluation criterion; n i is the number of evaluation indicators of the i-th evaluation criterion in the hierarchical model; h ij is the anti-entropy of each indicator; is the objective weight value of the evaluation index determined based on the anti-entropy weight method.

5. The comprehensive benefit evaluation method of a natural gas pressure difference power generation-ice making system according to claim 4, characterized in that: The specific calculation process of the combined weighting method is: Step 331: Calculate the subjective weight coefficient and the objective weight coefficient according to the subjective weight value and the objective weight value respectively. The expressions are: Among them, δ and ψ represent the relative importance of subjective weight and objective weight respectively; δ ij and ψ ij They represent the subjective weight coefficient and objective weight coefficient of the jth evaluation indicator under the i-th evaluation criterion respectively; is the subjective weight value; is the objective weight value; Step 332: Calculate the indicator combination weight of the evaluation indicator based on the subjective weight coefficient and the objective weight coefficient, expressed as: Step 333: Sum the indicator combination weights of all evaluation indicators included in each evaluation criterion to obtain the criterion combination weight of each evaluation criterion.

6. The comprehensive benefit evaluation method of a natural gas pressure difference power generation-ice making system according to claim 1, characterized in that: The assessment is divided into four levels: extremely low level, lower level, medium level and high water content, and the corresponding scoring ranges are less than 30 points, between 30-60 points, between 60-80 points and greater than 80 points.

7. The comprehensive benefit evaluation method of a natural gas pressure difference power generation-ice making system according to claim 1, characterized in that: The pretreatment device includes a dehydration device and a dehydrogenation device; the pressure regulating device is composed of a throttling pressure stabilizing valve.

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