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Recovered energy generation using an organic Rankine cycle system.

Article Excerpt
INTRODUCTION

There are more than 1200 natural gas compressor stations in the U.S. interstate natural gas pipeline network. The compressor stations maintain the flow of natural gas across the country (Figure 1). The very largest stations, designated by the larger circles on this figure, can move up to 4.6 billion [ft.sup.3] (130 million m3) of natural gas per day (U.S. DOE 2007).

[FIGURE 1 OMITTED]

Almost all of the pipeline compressor stations are fueled by a small fraction of the gas flowing through the pipeline. Although the older and smaller compressor stations may be powered by reciprocating engines or electric compressors, most of the 300 larger-scale stations are equipped with combustion turbines to drive centrifugal compressors. These larger stations represent more than 57% of the installed capacity (US DOE 2007). At almost all of these locations, the high-temperature turbine exhaust gas stream, still containing about 70% of the energy from the combustion process, is discharged to the environment. It is very difficult to find customers for this energy in the form of heat because most of these larger pumping stations are in very remote locations. However, electricity can be economically transported over long distances. If the turbine exhaust waste heat could be converted to electricity, significant improvements in overall energy efficiency would be achievable.

As long ago as 1979, studies were made to investigate technology capable of transforming this wasted heat into electricity (General Electric 1979). With the increase in fuel and electricity prices, the economic case for harvesting this wasted energy has become even more compelling. A Rankine cycle power system is often used to transform thermal energy into electrical energy. The most familiar Rankine systems include these four steps: (1) use thermal energy (in a boiler) to turn water into steam; (2) send the steam through a turbine, which in turn drives an electric generator; (3) condense the steam back into water by discharging the remaining thermal energy in the steam to the environment; and (4) pump the water back to the boiler. Such steam Rankine systems, powered by either coal or nuclear sources, provide most of the electricity generated in the United States.

Natural gas turbines are also used to generate electricity. While many gas turbine-generators operate as simple Brayton cycle systems, they have the potential for higher generating efficiency by combining the Brayton cycle with a Rankine cycle. In a combined-cycle system, the steam systems are powered by the exhaust from large gas-fired combustion turbines.

It is thus natural to consider steam Rankine systems as a possible method to recover the waste heat from the compressor station combustion turbines. Four bottoming steam systems were installed at pipeline compressor stations between 1968 and 1970 (General Electric 1979). Another such system was installed in the early 1980s near San Francisco (Tateosian and Roland 1983). However, by 2006, there were only three Rankine heat recovery systems in use at natural gas compressor stations. One of these three was initially constructed to work with a water/steam cycle, but freezing problems in Alberta, Canada, led to the installation of a system using an organic working fluid in 1999 (FERC 2006).

The organic Rankine cycle (ORC) is not new. Ships using acetone as motive fluid in piston engines were in service for some time between Europe and the Amazons in the late nineteenth century. A small solar turbine was operating in Libya in the 1930s. A clear theoretical analysis leading to criteria for fluid selection in a modified Rankine cycle was developed between 1958 and 1985, and related technology has been commercialized since 1965 (Bronicki and Schochet 2005; Tabor and Bronicki 1961; Tabor and Bronicki 1962; Bronicki 1972; Bronicki 1981; Bronicki 1988). As of March 2008, 2500 recovered energy generation (REG) systems for geothermal, solar, and heat recovery were in operation.

In an ORC, another working fluid, typically one chosen because it has a lower freezing temperature and other desirable properties, goes through the same four-step Rankine cycle process as a steam system. In the REG, the use of a recuperator and an intermediate heat transfer fluid widens the choice of the working fluid to optimize the heat-to-power efficiency and provides additional safety for the system operations. This intermediate fluid (thermal oil) is used in the waste heat oil heater (WHOH) to capture the waste heat from the turbine exhaust gas. The thermal oil selection is based upon low operating pressures, stability, and a low freezing point. The hot thermal oil from the WHOH is fed into the vaporizer and preheater of the REG where its thermal energy is transferred to the Rankine cycle's working fluid. Many compounds such as chlorofluorocarbons, ammonia, and hydrocarbons can be used to match the Rankine cycle to the level of heat available. Pentane was selected for this recuperated cycle and offers a good match for many industrial waste heat streams due to its thermodynamic properties and limited environmental impact.

OBJECTIVE

The objective of the market transformation project described in this paper was to demonstrate the technical and economic feasibility of capturing thermal energy from a gas turbine driving a natural gas pipeline compressor by using a REG system based on a modified ORC that produces electricity with no additional fuel and near-zero emissions.

APPROACH

Four identical REG plants were designed, manufactured, and installed, and are owned and operated...

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