City Gate Station Research Articles

Natural Gas Hydrate Prediction and Prevention Methods of City Gate Stations

During the process of distributing natural gas to urban users through city gate stations, hydrate is easy to form due to the existence of throttling effect which causes safety risks. To handle this problem, a program to quickly calculate hydrate prediction and prevention methods for city gate stations is developed. The hydrate formation temperature is calculated through the Chen–Guo model, and the Peng–Robinson equation of state combined with the balance criterion is used to analyze the water condensation in the throttling process. The Wilson activity coefficient model is used to calculate the mass fraction in the liquid phase of thermodynamic inhibitors for preventing hydrates. Considering the volatility of inhibitors, the principle of isothermal flash has been utilized to calculate the total injection volume of the inhibitor. Moreover, the effects of commonly used methanol and ethylene glycol inhibitors are discussed. In terms of safety and sustainability, the ethanol inhibitor, which is considered for the first time, exhibited better prevention and control effects under conditions with relatively high temperature and low pressure after throttling. Combined with the actual working conditions of a gate station, methanol has the best inhibitory effect, followed by ethylene glycol. From an economic point of view, the benefits of the gas phase of the inhibitor during the delivery of natural gas are obvious; therefore, the method of methanol injection is recommended for hydrate prevention. If the gas phase benefits of the inhibitor are not considered, the ethylene glycol injection method becomes more economical.

A technical-economic analysis and environmental impact assessment of power recovery at a city gate station: a case study

ABSTRACT In many countries, natural gas is dispatched to the end-users by using pipeline configurations with a high pressure. The gas pressure must be dwindled at city gate stations before ingression into the distribution network. The hydrocarbon turbo-expander can be applied in such stations and regains this pressure as electricity. The present study presents the techno-economic feasibility of power recovery at a city gate station in Iran. On the grounds of exiguity of economic data regarding application of turbo-expander to resuscitate power at city gate stations, this paper has tried to devote greater attention to the financial and economic dimensions. Technical analysis suggested two different capacities of 616.7 kW (option A) and 815.6 kW (option B). The financial analysis exposed that option A with a positive NPV of 71 thousand Euros, an IRR of 17.13%, a normal payback of 7.7 years and a discounted payback of 15 years is favorable while with a negative NPV of −191 thousand Euros, option B is not feasible. The investigation established that power price is the most predominant determinant while operating costs have negligible effect on the results. Furthermore, by inputting the cost-benefit analysis of CO2 dissemination into the economic model, it is transpired that with a positive NPV of 1009 thousand Euros and a significant IRR of 45% for option A, and a positive NPV of 406 thousand Euros and an acceptable IRR of 25% for option B, the two alternatives are more economically viable. Substantively, from the public standpoint, both options are more desirable compared to the private viewpoint.

An evaluation of the efficiency of odorant removal by sodium ferrate(VI) oxidation

Gas odorization of organosulfur is common used to detect a leakage of natural gas at city gate station. An effective method for solving breathing problem caused by smell of organosulfur needs to be understood. This study used the response surface methodology (RSM) based on central composite design (CCD) approach to deal with the design and analysis of the experiments. Effects of the reaction time range of 10–50 min, initial Na2FeO4 concentration range of 12.5–62.5 g/L, temperature range of 10–50 °C and pH range of 2–10 on the efficiency of odorant removal by Na2FeO4 oxidation were investigated to evaluate an applicability of the model. The results showed that using 50 g/L of Na2FeO4 concentration can remove 95% of the mixture organosulfur compounds with its concentration of 0.6 g/L over a reaction period of 40 min at pH 4 and 20 °C. The use of quadratic regression from RSM could be useful to predict the removal efficiency of odorant by Na2FeO4 oxidation. Optimizing the operating conditions of Na2FeO4 oxidation by CCD approach could be a promising method with possible applications in the removal of sulfur compounds from the remaining odorant in small containers. The findings of this study may contribute to an effective method of deodorizing organosulfur compounds in the natural gas containers.

Dynamic Advanced Exergetic, Exergoeconomic, and Environmental Analyses of a Hybrid Solar City Gate Station

Abstract The use of solar energy to preheat natural gas before a city gate station (CGS) for reducing fuel consumption and environmental emissions is investigated in a real CGS. All analyses are conducted with a 1-h time-step throughout the entire year so that seasonal climate changes are accounted for precisely. A thermodynamic analysis of the hybrid system is performed with TRNSYS and verified with THERMOFLEX so as to ensure reliability. In addition, dynamic exergetic, exergoeconomic, and exergoenvironmental analyses for the proposed system are carried out. A life cycle assessment (LCA) based on Eco-indicator 99 is performed using SIMA PRO to compute the environmental impacts for each component of the system. The exergetic, exergoeconomic, and environmental analyses are performed in Engineering Equation Solver (EES) software. To perform the transient exergetic, exergoeconomic, and environmental analyses, the results of the thermodynamic analysis from TRNSYS are automatically imported into the EES code. The advanced exergetic, exergoeconomic, and exergoenvironmental analyses are performed to better determine components that have high potentials for improving the system; potentials are considered based on the exergy destruction, exergetic cost of destruction, and environmental impacts associated with exergy destruction.

Thermodynamic assessment and proposal of new configurations of an indirect water bath heater for a City Gate Station (a case study)

This paper deals with thermodynamic modeling and proposal new configurations of an indirect water bath heater used in a city gate station (CGS) to preheat the natural gas (NG). In the transmission pipeline of natural gas before reducing NG pressure, it is necessary preheating to prevent hydrate formation. In the traditional systems, an indirect water bath heater is used for this purpose. Thermodynamic modeling of the proposed systems has been done in EES software, and an effective analysis based on proposed new configurations is conducted. The results indicate that using exhaust combustion products to preheat the combustion air can increase the thermal efficiency of the system from 46.36% (for base case) to 73.84%. In addition, with the use of an ORC to utilize the exhaust waste thermal energy, the system efficiency of 55.27% with an electrical output of 79 kW can be achieved. A comparison between configurations indicates that the second configuration (preheating combustion air) with 78.97 kg/h has the lowest emission rate.

Investigation of Operational Scenarios to Mitigate CO2 Emission and Natural Gas Consumption in City Gate Stations (CGSs)

City gate stations (CGSs) are the main pressure reduction stations of natural gas (NG) transmission lines where enormous decrease of NG pressure results in significant temperature drop. Therefore, high pressure gas is preheated by means of NG fuel water bath heaters (WBHs) to prevent downstream hydrate formation. In this research, the effect of different operational scenarios was investigated by means of process simulation to enhance the combustion efficiency of WBHs which consequently causes lower CO2 emission into the atmosphere. In this regard, WBH of Minudasht CGS in the north of Iran with nominal capacity of 30,000 m3/h was considered as a case study and process enhancement was carried out with the goal of lower CO2 emission and the NG fuel consumption. On the basis of obtained results, it was demonstrated that the NG preheating and air preheating by means of energy of hot stack gases are the most effective scenarios which can result in around 65 and 50 tons lower CO2 emission, respectively. In addition, these scenarios have the merit of saving more than 33,000 and 25,000 m3 lower NG fuel consumption annually, respectively.

Energy, exergy and economic analysis of utilizing the supercritical CO2 recompression Brayton cycle integrated with solar energy in natural gas city gate station

The pressure of natural gas stream must be reduced in city gas station. The natural gas has to be preheated before pressure reduction takes place usually through throttling valves. In conventional city gas stations, the natural gas is preheated by indirect water bath heaters, which burn a large amount of the natural gas as fuel. In this study, the rejected heat from a supercritical carbon dioxide recompression cycle using solar energy is recovered for preheating the natural gas. The novel design of this system generates uniform electricity as well as preheats the natural gas in city gate station. The proposed system is simulated for Birjand city gas station as a case study, and a thorough techno-economic analysis is performed in Engineering Equation Solver for evaluating the system performance. The results of this study demonstrate that parabolic trough collectors with 25 rows are the most efficient solar system while the annual average of thermal and exergy efficiency of the system is 0.56 and 0.41, respectively. The exergetic analysis of the system shows that the highest average exergy destruction takes place in the throttling valve and the second highest in the solar collectors. Also, the total amount of fuel saving is estimated at 4.87 million cubic meters annually and the net power output is equal to 2.86 MW. From the economic point of view, the value of the payback period is estimated 4 years and, based on the net present value method, after 8 years, the initial investment could be returned.

Effect of geometrical parameters on thermal efficiency and fuel consumption of heaters of a natural gas pressure reduction station using Sobol statistical sensitivity analysis

AbstractThe main objective of natural gas pressure reduction station or city gate stations (CGSs) is to adjust gas pressure within a standard range for home usage. CGSs usually utilize heaters for pressure tuning, which generally consume a considerable amount of energy. In this paper, the effect of six influential parameters, namely, the length of gas‐coil, the diameter of gas‐coil, the radius of heater shell, length of heater shell, fire tube diameter, and length, on the amount of fuel consumption and thermal efficiency is investigated using Sobol statistical sensitivity analysis. The results show that gas‐coil length and diameter are not effective in fuel consumption and thermal efficiency. In contrast, increasing the heater shell radius, heater shell length, and fire tube radius has the same effect as they cause a linear increase in fuel consumption. According to results taken from Sobol analysis, they similarly have an influence of 19%–23% on fuel consumption and 21%–22% on thermal efficiency. Finally, it was revealed that the effect of fire tube length on fuel consumption and thermal efficiency of the heater was 37% and 35%, respectively.

Optimal working conditions of various city gate stations for power and hydrogen production based on energy and eco-exergy analysis

In this study, the feasibility of producing power and hydrogen from the waste heat of different City Gate Stations (CGSs) is investigated to select the optimal working conditions. A thermodynamic model is developed for a proposed system combined of the CGS station, the Rankin cycle and the extended hydrogen production cycle. Initially, six CGS stations are simulated based on energy, exergy-economic and environmental analysis and then a comparative study is conducted between different stations. The results of numerical modeling show that the Mashhad-old station with 5315 kW and 31.062 ton/year has the highest amount of power and hydrogen production among other stations, respectively. It is also observed that, it is more economic to increase the input gas pressure in order to increase the production rate. In addition, optimal working conditions are determined based on the two important optimization factors of the hydrogen production rate and SUCP (sum unit cost of the product) using genetic algorithm optimization technique. The results of multi-objective optimization indicate that Gonbad, Gorgan and Mashhad-old stations, where the inlet gas mass flow rate is in the range of 8–9 kg/s, are the optimum stations.

Thermal modeling of indirect water heater in city gate station of natural gas to evaluate efficiency and fuel consumption

In this paper, a comprehensive model of indirect water heater for a pressure reduction station of natural gas has been presented. Effiiency and fuel consumption of heater have been estimated by applying energy and mass conservation equations. Three terms of energy including heat absorbed by natural gas, heat transferred to environment from stack and heat loses to environment from heater body have been evaluated. Thermal properties of natural gas has been calcualated using AGA8 state equation. Therefore, the model is a function of climate, heater geometry and natural gas characteristics including temperature, pressure and its composition. The results are compared with data recorded from real station in Iran (Arak, Kheriabad and Malekabad stations) and very good agreement has been observed. Reducing temperature and increasing pressure of inlet gas, thickening glass wool insulation and rising fire tube radius led to an increase in heater’s thermal efficiency from 20 to 60% and consequently fuel consumption of heater was reduced. Finally, choosing the sutible geometery yeilded a higher heater efficincy with a raise from 44% to 70%.

Comprehensive techno-economic and environmental sensitivity analysis and multi-objective optimization of a novel heat and power system for natural gas city gate stations

In the present study, a novel combined heat and power system is proposed to use in natural gas city gate stations. The proposed cogeneration system is used for power generation and satisfying heating requirements of the natural gas preheating in city gate stations. The system is composed of a regenerative gas turbine cycle, a heat exchanger and a Kalina cycle integrated with a city gate station heater. A comprehensive sensitivity analysis and single- and multi-objective optimizations are applied to investigate the system performance. The analysis is based on energetic, exergetic, environmental and economic standpoints which includes all important effective parameters (compressor presser ratio rCom, gas turbine inlet temperature TGT, ammonia base concentration XNH3, condenser pinch point temperature difference ΔTPP,Con, ambient temperature Tamb and inlet NG pressure PNG,in). Also, in addition to the energetic and exergetic efficiencies, a new performance criterion is defined as the system’s annual costs to incomes ratio (this new function covers economic and environmental criteria simultaneously). The outputs of sensitivity analysis reveal that, rCom affects significantly on all of the three performance criteria; also, Tamb and PNG,in has the most impact on energetic efficiency. The exergetic analysis at the base system conditions shows that the combustion chamber has the maximum contribution in total exergy destruction with a share of 54.96%. This share is reduced by 3.65% in multi-objective optimization mode. Also, comparing the outputs of the base system and the multi-objective optimization mode for the proposed cogeneration system represents 30.72%, 3.48% and 10.55% improvement in energetic efficiency, exergetic efficiency and system’s annual costs to incomes ratio, respectively.

Energy Harvesting by a Novel Substitution for Expansion Valves: Special Focus on City Gate Stations of High-Pressure Natural Gas Pipelines

A countless amount of energy has been wasted in all kinds of expansion valves (EV) in industries. In fact, EVs, including regulators, throttling valves, capillary tubes, etc., have been used to intentionally reduce the potential of carrier fluid. City gate stations (CGS) have been recognized as one of the important points with high potential for energy harvesting due to its function for regulating natural gas (NG) pressure by EV. In this study, Tesla turbine (TT) is introduced as a new candidate for substitution of EV, particularly those that have been employed in CGS on high-pressure NG pipelines, as well as those applications in which high-potential fluid must be reduced to a low-potential state to form a complete thermodynamic cycle or to be used at end-user equipment. Although harvesting energy is one of the hottest fields of science and engineering, there are few traces of research on using a TT as an alternative for EVs, even for the industries possessing high-pressure lines. This numerical experiment intends to show the capability of TT as a robust candidate for substituting regulation valves through investigating thermohydrodynamic characteristics of the turbulent high-pressure compressible NG flow through a TT under different operation conditions. This study, with the objective of managing the exploitation of resources, can be considered as one step forward toward reinforcing economic and environmental pillars of sustainable development. It is also found that the generated power by TT can support the 285 7W LED simultaneously, or it is equivalent to 84.4 m2 area of the solar panel (150 W, 15.42% efficiency) for the climate condition of Toronto, Canada.

Energetic, exergetic, environmental and economic assessment of a novel control system for indirect heaters in natural gas city gate stations

The energy consumption and greenhouse gases emissions in natural gas city gate stations are important issues in the natural gas industry. In order to improve efficiency, have a cleaner environment and achieve economic benefits, the present study aims to propose an optimal system for the indirect water bath heaters in natural gas city gate stations. The optimization procedure is carried out by designing a control system to gain an eligible discharge temperature for the heater based on the gas entry conditions to the city gate station. The controller calculates the temperature of hydrate formation in terms of passing gas pressure and gives this information to the torch of the heater for regulating fuel consumption. A comprehensive study is accomplished based on energy, exergy, environment and economic analysis for different pressure reduction stations. The results indicate that employing the proposed system decreases the amount of fuel consumption and greenhouse gases emissions along with increasing system efficiency. Analyzing the results reveals that using the proposed system leads to a maximum of 28.54% relative increment in the heater efficiency compared to the conventional system (at this condition, the heater efficiency of the conventional and proposed system is η = 36.12% to η = 46.43%, respectively). Furthermore, with choosing a heater with a capacity of 100,000 SCMH, it is possible to reduce the pollutants emissions and total costs down to 142.6 tons per year and 3,671,000 $ per year, respectively.

Experimental investigation of the increasing thermal efficiency of an indirect water bath heater by use of thermosyphon heat pipe

Natural gas must be preheated prior to pressure reduction in city gate stations. Indirect water bath heaters are employed in city gate stations for preheating, consume a considerable amount of natural gas for the process. This type of heater has a low efficiency therefore a significant amount of energy is wasted. Due to the high capacity of thermosyphon heat pipe, its utilization in city gate stations heater were investigated experimentally in this paper. For this purpose, a heater set-up was manufactured and its thermal efficiency was calculated. The thermosyphon heat pipe were then designed, manufactured, and utilized between the fire tube and the gas tube. Further the type of working fluid and the range of filling ratio were discussed and the most effective state was suggested. Moreover, the thermal efficiency of the heater in the presence of thermosyphon heat pipe was investigated. The obtained results showed that the thermal efficiency of the heater improved up to 13% with the addition of thermosyphon heat pipe. The most effective state of thermosyphon heat pipe was associated with the water as working fluid with 20% filling ratio in the front route and methanol as working fluid with 30% filling ratio in the back route of the fire tube.

Effect of various twisted-tape designs on the thermal and environmental performance of line-heaters in city gate stations

A city gate station is a point in natural gas transmission pipeline at which the pressure of gas flow should be reduced significantly. For preventing gas hydrate forming in the gas flow, the gas is heated before the throttling process. This is accomplished via a line-heater. The major problem of the heater is its low energy efficiency (below 50%), leading to wasting a big portion of the combustion released energy through the chimney. As such heaters burn natural gas, this low efficiency causes them to release a large amount of greenhouse gas. This study investigates the effects of using various twisted-tape designs with different pitch lengths and the number of twists on the thermal and environmental performance of line-heater. The proposal is numerically analyzed for a line-heater in a real city gate station. The results show that a larger number of twists and shorter pitches better enhance the efficiency so that twisted-tapes with the total numbers of 17 and 68 twists improve the average Nusselt number by 12.5% and 36.5%, respectively. These heat transfer enhancement figures are associated with preventing a total annual of 24 and 50 mil. tonnes of equivalent carbon dioxide being released by the case study heater.

Optimization of city gas network: a case study from Gujarat, India

A natural gas flow network consists of city gate station, district regulator, pipelines, compressors and valves. The performance of such a complex system cannot be studied by analysing individual constituents. Simultaneous performance of integrated system can be expressed using non-linear equations to simulate and forecast operational behaviour. The operational behaviour is subject to various alternative operational conditions also. Nodal analysis is performed to calculate the flow capacity of each network segment and the pressure at each node. Two conditions are evaluated, namely, keeping flow capacity unknown or by making nodal pressure primary unknown. The resulting set of governing equations are then solved simultaneously. Genetic algorithm method is applied to solve the desired target unknown. Compressor governs the energy supply required to transport gas from one end to another. The compressor equation is also incorporated in city gas network simulation. The simulations are performed by straight gas ducts and complex gas ducts. Boundary conditions are dependent on the form in which gas duct is fed. It is assumed that the gas duct takes gas from City Gate Station at x = 0 and supplies the gas to a consumer located at x = L according to a time variable curve demand Qn (L, t). The capacity of the compressor is also changed and it is observed that the demand is usually periodic in nature. Flow demand at individual node of a gas pipeline network is calculated. Pressures at various nodes are also calculated and matched with original pressure at nodes. Percentage deviation error suggest a range between 0 and 1.02%. The aim of this paper is to optimise certain parameters of a gas transmission pipeline network namely flow rate, mass, cost and pressure. This optimization is performed to gas transmission pipeline network of Gandhinagar, district, where Gujarat State Petronet Limited (GSPL) and Sabarmati Gas Limited (SGL) are the major players. The network which has been chosen for pipeline network consists of 10 CNG stations and 10 DRS.

Experimental and numerical investigation of odorant dispersion in natural-gas pipelines

One of the important factors in natural-gas pipeline networks is the optimal concentration of odorant injection. The purpose of this study is to investigate the behavior of odorant dispersion along the natural gas pipelines. A numerical modeling analysis has been conducted on odorant injection at the City Gate Station (CGS) of Shahrekord distribution network in Iran and validated with experimental data of a real gas pipeline at pressure and temperature of 1586 to 1723 kPa and 0- $30 {}^{\circ}{\rm C}$ , respectively. Navier-Stokes, concentration equations and k- $\varepsilon$ turbulence model have been used for numerical modeling of the turbulent and incompressible fluid flow. The results showed that the low pressure or high temperature of the gas as well as the increase in the pipe length decrease the odorant concentration at the gas pipeline. In addition, odorant concentration decreases with high gas velocity and low initial concentration of odorant injection. Finally, by curve fitting through the obtained result, a correlation, with an average error of 3%, was proposed to calculate the final odorant concentration in natural-gas pipelines. This correlation is based on the temperature, pressure and velocity of natural gas, initial concentration and duration of odorant injection, length and diameter of the pipe as input parameters.

Design and implementation of an expert system for periodic and emergency control under uncertainty: A case study of city gate stations

Safety analysis is essential in the natural gas transmission industry to guarantee effective hazard identification and to prevent the failure of components in advance. The overall aim of this research is to introduce a new hybrid expert system and fuzzy logic to monitor city gate stations as one of the most vital installations of gas distribution networks. The proposed model utilizes fuzzy if-then rules based on multiple experts' opinions to study the compound interrelations between the mechanical and physical elements of city gate stations. The presented expert system accounts for uncertainty associated with the experts’ judgments by fuzzy sets theory. The expert system is implemented in an object-oriented platform and is programmed with C#. The validity of the expert system is confirmed using simulation experiments through a case study of city gate stations.

Performance enhancement of water bath heater at natural gas city gate station using twisted tubes

Abstract Natural gas is transported from producing regions to consumption regions by using transmission pipelines at high pressures. At consumption regions, the pressure of natural gas is reduced in city gate stations (CGSs). Before the pressure reduction process, the temperature of natural gas is increased usually by using a water bath heater, which burns natural gas as fuel, to protect against freezing of natural gas. These types of heat exchangers have a low efficiency and consume a lot of fuel to generate the required heat. In the current study, the twisted configuration of the heating coil is proposed and investigated to enhance the heat transfer through a water bath heater with a nominal capacity of 1000 m3·h−1. Firstly, the implementation procedure is validated with data collected from the CGS of Qaleh-Jiq (located in Golestan province of Iran). A very good agreement is achieved between the obtained results and the real data. Then, three different twist ratios are considered to examine the twisting effects. The proposed technique is evaluated in the terms of velocity, temperature, and pressure variations, and the results are compared with the conventional case, i.e. straight configuration. It is found that both the heat transfer rate and the pressure drop augment as the twist ratio is raised. Finally, it is concluded that the twisted tubes can reduce the length of the gas coil by about 12.5% for the model with low twist ratio, 18.75% for the model with medium twist ratio, and 25% for the model with high twist ratio as compared to the straight configuration.

MILP model for integrated expansion planning of multi‐carrier active energy systems

In growing economic energy systems, the interdependency of various energy infrastructures has led to a change in countries' policies in their expansion planning of energy networks. In this study, a mixed-integer non-linear programming (MINLP) model is proposed for expansion planning of the multi-carrier systems including electricity and gas distribution networks. The optimal planning determines the best location, time, and alternative for network assets in order to minimise investment costs and reduce losses. Also, as another distinctive feature of this study, given the integration of electricity and gas distribution networks and the complexity of the problem, a new MILP model using linearisation methods is presented. In this planning model, several types of alternative plans and a set of candidates for the new placements or increase of the capacity of transformers, feeders, distributed generation, gas pipelines, and city gate stations are considered. The proposed MILP model provides the convergence of the problem to a global optimum response using the powerful commercial software. In addition, a solution based on Benders decomposition algorithm is finally proposed to reduce the solution time of the problem. Finally, the efficiency of the proposed model is evaluated by means of some simulation results.