Doğal gaz basınç düşürme istasyonlarından elektrik üretiminin Çorlu-Kayseri ve Yalova RMS-A istasyonları için incelenmesi
Investigating power generation from natural gas pressure reduction using Çorlu Kayseri̇ and Yalova RMS-Astations
Yazar: ÖZER DENİZ
Danışman: YRD. DOÇ. DR. GÜRŞAT ALTUN
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Petrol ve Doğal Gaz Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2014
Konu: Petrol ve Doğal Gaz Mühendisliği = Petroleum and Natural Gas Engineering

Özet:
Gazların daha az enerji sarfiyatı ile daha ekonomik bir şekilde taşınabilmeleri için özgül hacimlerinin küçültülmeleri gerekir. Gazların özgül hacimlerinin küçültülmesi ise basınçlandırılarak ya da sıvılaştırılarak yapılabilir. Günümüzde doğal gazın taşınabilmesi için kullanılan iki ana metod vardır. Bunlar sıvılaştırılmış doğal gaz (LNG) ve yüksek basınç altında boru hattı ile taşıma metodlarıdır. Sıvılaştırılmış doğal gaz metodu boru hattının olmadığı yerlerde ve deniz aşırı taşımalarda kullanılır. Boru hatları ile taşıma metodu ise genellikle karasal alanda yapılmasına rağmen, zaman zaman denizaltından da yapılmaktadır. Uzun mesafe taşıma metodu her ne olursa olsun tüketiciye temin edilecek doğal gaz, düşük basınçlı şehir içi doğal gaz dağıtım hatlarıyla yapılır.
Ülkemizde üretilen ve ithal edilen doğal gaz, BOTAŞ iletim şebekesi vasıtasıyla 50 – 75 bar basınç altında şehir giriş istasyonları olarak tanımladığımız RMS-A tipi istasyonlarda 18 – 25 bar basınca düşürülerek şehir doğal gaz dağıtım şebekesine ulaştırılır. Bu istasyonlarda yaklaşık 50 bar basınç düşürme işlemi, kullanılabilir enerjiyi üretmeden gerçekleşir. Bu problem termodinamik açıdan incelendiğinde, klasik basınç düşürme işlemleri yerine genleşme türbini (turbo-expander) kullanarak elektrik üretiminin yapılmasını, hatta küçük ölçekli sıvılaştırılmış doğal gaz (LNG) ünitesinin çalıştırılmasını mümkün kılabilir.
Genleşme türbini vasıtasıyla elektrik üretimi, enerji kaynaklarının kısıtlılığı ve çevresel değerler dikkate alındığında hızla önem kazanmaktadır. Bu yolla yapılacak sistemin kapasitesi, giriş basıncı, giriş sıcaklığı, debi ve çıkış basıncına bağlı olarak birkaç yüz kW'dan birkaç MW'a kadar değişebilir. Bu konuda Amerika, İngiltere, İtalya, Çekoslovakya ve Rusya gibi ülkelerde çeşitli çalışmalar ve araştırma amaçlı uygulamalar yapılmıştır. San Diego (California), Memphis (Tennessee) ve Hamilton (New Jersey) ilk uygulamalar arasındadır, (Bloach ve Soares, 2001).
 Bu teknolojinin, Türkiye'deki iletim ve dağıtım sisteminin ve ekonomik koşullarının dikkate alınarak kullanılması durumunda potansiyel elektrik üretimi ve bir kazanç elde edilebileceği analizi bu çalışmanın temelini oluşturmaktadır.

Abstract:
Specific volume of gas has to be decreased in order to transmit it with more economical way as well as diminishing energy consumption. Specific volume of gas can be reduced with liquefaction or compression. Currently, there are two different methods to transmit natural gas. These methods are liquefied natural gas (LNG) and transmission through pipeline under high pressure. Liquefied natural gas method is used in locations in which there is no available pipeline or overseas transportations. Transmission with pipelines is mostly constructed at terrestrial zones, although submarine pipeline implementations are rarely needed. No matter what the long distance transmission method is, natural gas for the end user must be distributed through low pressure inner-city pipelines. The natural gas produced domestically or being imported by licensed governmental/private companies is transmitted to local gas distribution network by reducing the high pressures from 50-75 bar to moderate pressures 18-25 bar at city gate stations, so called RMS-A.
This approximately 50 bar pressure reduction process is carried out via regulator at the RMS-A type stations without recovering useful energy. If the problem is analyzed with respect to thermodynamic principles, the power generation by operating a turbo expander or a small scale LNG unit is possible to reduce line pressure without using or along with conventional regulators. It should not be forgotten that the turbo expander system is installed parallel to regulator system and will operate between the predetermined design flow rates.
Power generation through an expansion turbine comes into prominence quickly when considering limited energy sources and environmental concerns. The capacity of this kind of power generation system may vary from a few hundred kW to several MW depending on the system design parameters such as inlet and outlet pressures, inlet and outlet temperatures, mass flow rate, and system efficiency, thermodynamic properties of natural gas, etc.  Some applications on power generation issue including research oriented works have been carried out in the USA, England, Italy, Czech Republic, Slovakia, Russia, Iran, Pakistan, and Bangladesh. Early applications of turbo expander systems are performed in San Diego (California), Memphis (Tennessee), Hamilton (New Jersey), (Bloach and Soares, 2001). Nowadays, widespread application of turbo expanders is very common all around the World, (Bloach ve Soares, 2001).
The main objective of this study is to determine and demonstrate the potential profitability of power generation from turbo expanders if this technology is used in RMS-A stations in Turkey.

Annual data representing year 2013 from Çorlu, Kayseri and Yalova RMS-A stations have been used for designing power generation system to achieve the goal in each aforementioned city-gate stations. The tabulated raw data was processed using Excel spreadsheet program to determine maximum and minimum volumetric flow rates in hourly basis. Then, the data in hourly basis was converted into monthly and annually bases. Afterwards, design flow rate of turbo expander that could be operated at various volumetric flow rates was initially determined using the annual consumption rate. The variable operation rate of turbine is selected to be in the range of 50% higher and 40% lower than the design flow rate as informed by a manufacturer. Applying this preselected variable flow rates of turbo expander to raw data, net operating period of turbine in terms of hours per year were determined for the each examined RMS-A stations. 
The main components of the turbo expander system are turbine, gear box and generator, and boiler and heat exchanger. The other assumed constant parameters used in the design process are as follow; pressure at turbine outlet is 19 bar, temperature at turbine outlet is 5 oC (by using minimum dew point of 0 oC based on the technical specification by BOTAŞ), temperature at turbine inlet before preheating is 20 oC, natural gas composition is 100% methane, combined efficiency of boiler and heat exchanger is 0.80, combined efficiency of gear box and generator is 0.85. In addition, the process in the turbine is assumed to be adiabatic, reversible, and isentropic.
The average inlet pressure at the turbine in each city-gate stations was calculated using the approach of weighted mean flow rate. Design of turbo expander system relies on conservation of mass and energy principles on a control volume. Applying the second law of thermodynamics, the minimum required natural gas temperature at the inlet of turbine can be calculated by considering the outlet temperature and pressure constraints.
Heat energy necessary for increasing the temperature of natural gas before entering the turbine, so called preheating requirement, is determined from the temperature difference between the calculated gas temperature at the inlet of turbine and the average gas inlet temperature at RMS-A station. The required heat energy is provided by burning some of the natural gas from the system. Power generation from a turbo expander strongly depends on the isentropic efficiency of a turbine and the mass flow rate of natural gas.
In this study, the power generation from the turbine and the required preheating energy were calculated with different isentropic efficiencies ranging from 0.70 to 0.85 and different design flow rates of natural gas for Çorlu, Kayseri, and Yalova RMS-A stations.
Results reveal that unless provided from external sources, the preheating energy cost for increasing the natural gas temperature is approximately 50% of the total income obtained from the generated electricity sales. In the analyses, it is assumed that natural gas cost is 0.06746081 TL/kWh, and selling price of electricity is 0.105 US$/kWh, and exchange rate of US $ to TL is 2.1019.
Net power generation from a turbo expander has a strong relationship with the pressure ratio at inlet and outlet, the design flow rate, and the isentropic efficiency. However, the pressure ratio is not a controllable parameter. The required preheating energy similar to total income from the turbo expander increases with increasing isentropic efficiency of turbine.
Results also indicate that considering the consumption rate in year 2013, 1.26 MW of installed capacity is sufficient for Çorlu RMS-A station with the design flow rate of 30,000 Nm3/hr. Similarly, 3.57 MW of installed capacity is sufficient for Kayseri RMS-A station with the design flow rate of 85,000 Nm3/hr, and 1.412 MW of installed capacity is sufficient for Yalova RMS-A station with the design flow rate of 35,000 Nm3/hr. The design flow rates using 2013 data are adequate to operate the turbo expanders 6069 hours in Çorlu, 5009 hours in Kayseri, and 8370 hours in Yalova.
Net annual revenues from the stations yield that $225,071 from Çorlu RMS-A, $500,898 from Kayseri RMS-A, and $347,069 from Yalova RMS-A. The revenues do not consider operating costs.