Energy Recovery in Natural Gas Compressor Stations Taking Advantage of Organic Rankine Cycle: Preliminary Design Analysis

Abstract

Gas compressor stations represent a huge potential for exhaust heat recovery. Typical installations consist of open cycle configurations with multiple gas turbine units, usually operated under part-load conditions during the year with limited conversion efficiency. At least, one of the installed unit serves as back-up to ensure the necessary reserve power and the safe operation of the station. Organic Rankine Cycle (ORC) has been proven as an economical and environmentally friendly solution to recover waste heat from gas turbines, improving the overall energy system performance and reducing the CO2 emissions. In this context, taking as reference typical gas compressor stations located in North America, the paper investigates the potential benefit of ORC application, as bottomer section of gas turbines, in natural gas compression facilities. Thus, ORC converts gas turbines wasted heat into useful additional power that can be used inside the compression facility reducing the amount of consumed natural gas and, consequently, the environmental emissions, or directed to the grid, thus furthermore earning economic benefits. Different case studies are examined with reference to two typical compressor station size ranges: a “small-medium” and a “medium-high” size range. Two different gas turbine models are considered according to most common manufacturers. Typical gas compressor stations and integrated cycle configurations are identified. Based on Turboden experience in development and production of ORCs, specific design options and constraints, layout arrangements and operating parameters are examined and compared in this study, such as the use of an intermediate heat transfer fluid, the type of organic fluid, the influence of superheating degree and condensation temperature values. Emphasis is given on thermodynamic performance of the integrated system by evaluating thermal energy and mechanical power recovery. Several key performance indexes are defined such as, the ORC power and efficiency, the specific power recovery per unit of compression power, the integrated system net overall power output and efficiency, the ORC expander and heat exchangers size parameters, the carbon emission savings, etc. The performed comparison of various configurations shows that: (i) the energy recovery with ORC can be remarkable, adding up to more than 35% of additional shaft power to the compression station in the best configuration; (ii) the ORC condensation temperature value has a significant impact on the ORC bottomer cycle and on the integrated system performance; (iii) in case of Cyclopentane, keeping the same ORC cycle operating parameters, the max specific power recovery is achieved in the direct configuration case, (iv) the bottomer cycle size can be reduced with the use of a refrigerant fluid (R1233zd(E)), compared to hydrocarbon fluids; (v) the max environmental benefit can be up to 120 kg CO2/h saved per MW of installed compression power.

Optimum Organic Rankine Cycle Design for the Application in a CHP Unit Feeding a District Heating Network

“…The condensation pressure depends on the working fluid, and it is bound by the cold source temperature. In this study, the cooling medium temperature was assumed equal to 25 • C as representative of a medium climate condition [24]. Considering a reasonable temperature difference between organic fluid saturation temperature and cooling medium temperature inlet equal to 15 • C, the organic fluid condensation temperature was set equal to 40 • C for all considered fluids, resulting in the condensation pressure values in Table 6 (see also Figure 7).…”

Feasibility of ORC application in natural gas compressor stations

“…The arrangement and technical details of the bottoming cycles are not provided in this reference and power size is only focused to values in the range 5-6 MW. Figure 1 shows, according to Authors preliminary investigations [3], NG CS facilities taking advantage of ORC technology as bottomer cycle for WHR.…”

“…The main contributions of this work, with respect to the above discussed references and as complementary to Authors preliminary investigations [3,22] can be summarized as follows:…”

“…ORC cycle is considered subcritical and regenerated, according to the current state of the art of ORCs for WHR. Thus a heat exchanger (REG in Figure 2 depicted by the Authors in [3,22].…”

“…Grid-type CSs, which often have more variability in NG flow, generally have multiple compressor units in each station to provide for greater flexibility and efficiency in compression [2]. A NG CS facility, described in details by Authors in [3], in addition to compressors and driver units, includes, as main components, gas scrubbing and liquid removal devices for NG cleaning, aftercooler heat exchanger for gas cooling before injection into the mainline, pipes, valves, control and emergency shutdown systems for unmanned monitoring. Indeed, NG CSs are usually remoted controlled and operated and are designed with enough horsepower to meet firm contractual requirements.…”