Bringing greater efficiency to small-scale LNG with multi-section integrally geared compressors HANDLE THE PRESSURE

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1 Bringing greater efficiency to small-scale LNG with multi-section integrally geared compressors Tushar Patel, Marketing Manager for Atlas Copco Gas and Process. For more information, please contact the author at or visit HANDLE THE PRESSURE

2 Bringing greater efficiency to small-scale LNG with multi-section integrally geared compressors HOW IS THE LNG MARKET CHANGING? While lower natural gas prices have created challenges for large-scale LNG liquefaction plants, they are opening up opportunities for small to midsize counterparts. From small-scale LNG (SSLNG) projects in the U.S. spurred on by shalegas development to early adopters in Europe and Asia, many operators are taking advantage of the benefits of going small scale. SSLNG applications have spread out among fuel markets, with their reach ranging from transportation hubs for long-haul vehicles, busses, trains and shipping fleets to remote commercial and residential heating networks. Other applications include cost-effective, stand-alone power generation in remote locations, peak shaving plants and even virtual pipeline systems to deliver energy to off-grid areas. Because the gas volume is being reduced 600 times, SSLNG poses an attractive alternate to CNG since it enables less frequent refueling. Many operators are taking advantage of the benefits of going small scale CHALLENGE Although SSLNG is helping to expand the options for LNG s availability and scope beyond what the bigger counterparts can provide, it is important to understand that small-scale projects have a different set of technical and logistical requirements than larger facilities. At the same time, the technologies used in larger plants often can t be efficiently scaled down. This includes the compressors running the refrigeration cycle. SOLUTION Integrally geared centrifugal compressors (IGCs) offer the fast deployment, scalability and redeployment capability that SSLNG projects require in order to respond quickly to market demand and maximize ROI. Design efficiencies such as standardized aerodynamics and a lowmaintenance seal design contribute to a lower initial investment and lower operating costs. Operational efficiencies such as intercooling and inlet guide vanes (IGVs) allow operators to make full use of agile plant deployment. And when combined with the efficiency gains inherent to the multi-section technology employed by IGCs, these efficiencies promote significant energy savings, as well. Bringing greater efficiency to small-scale LNG with multi-section integrally geared compressors 2

3 Design efficiency STANDARDIZED AERODYNAMICS FOR THE MIXED-REFRIGERANT CYCLE The most common liquefaction process for SSLNG liquefaction trains those with a capacity of less than 1 MTPA is the single mixed-refrigerant cycle (SMR). SMR uses various hydrocarbons (ethylene, methane, and i-butane and / or i-pentane) in place of the pure hydrocarbon refrigerants used in the discrete cooling steps of a cascade refrigerant cycle. In basic terms, SMRs condense a hydrocarbon gas mixture to a liquid and then use cooling through its vaporization to chill natural gas close to its boiling point of -162 C (-260 F). The final phase change into LNG is often done with a valve utilizing the Joule-Thomson effect. There are several proprietary SMR processes on the market. They typically employ a single pressure mixed-refrigerant cycle (i.e., the refrigerant is compressed to its highest pressure before being sent to a series of condensers and separators and finally to the heat exchanger to chill LNG). To boost efficiency, other SMR processes vaporize the mixed-refrigerant in two steps (a high-pressure and low-pressure stage), which can be driven by either two compressors or preferably a single compressor with two separate stage groupings. Depending on the specific proprietary process, ammonia and propane precooling and / or subcooling might be added, along with fractionation of the refrigerant early in the cycle. (Figure 1). Choosing which process is right for a specific plant usually requires that a balance is struck between the initial capital expenditure and long-term efficiency savings, in short Lifecycle Cost Analysis. Hot-tocold startup time especially for plants running intermittently and targeted capacity might also be deciding factors. LNG NG Separator Separator LP MR HP MR Separator Figure 1. Example of a process flow with interstage gas cooling Although there is some variety in the application of the SMR concept, from the standpoint of compression technology the specific processes are quite similar. They employ a mixed hydrocarbon refrigerant with a molecular mass ranging from 32 to 37 kg / kmol. This rather narrow range of compression requirements allows for standardization of the compressor s aerodynamic design. Bringing greater efficiency to small-scale LNG with multi-section integrally geared compressors 3

4 Because impeller blade geometry and other aerodynamics do not need to be fully customized to each process, CAPEX and manufacturing time is reduced; the compressor is delivered faster for speedier commissioning, at a lower initial investment. Considering that LNG projects are inherently capital-intensive and that the liquefaction process can account for upward of 50% of the total project costs lower investment costs for the compressor can go a long way toward plant viability. REDUCED MAINTENANCE Taking advantage of the gas-tight characteristics of dry gas seal technology and integrally geared compressor design, an Atlas Copco mixedrefrigerant (MR) compressor requires only a single external-facing dry gas seal which comes standard with the machine. The option of using tandem dry face seals is also available; this allows for minimization of the seal leakage and also improves machine reliability. IGCs offer comparably low maintenance requirements and a high reliability of 99.8%. FLEXIBLE PLANT DESIGN AND RAPID SETUP Because integral-gear technology is the most compact compressor design in the industry, a fourstage Atlas Copco MR compressor can fit onto a small skid measuring 10 m by 6 m (32'9" by 19'8"). (Figure 2). The skid can accommodate a motor, instrumentation and peripherals like lube oil, seal support and control systems. It is simple to install and can be easily incorporated into modular plant design for rapid setup and commissioning. Internal carbon ring and labyrinth seals minimize seal gas flow and prevent oil mist from entering the chamber. The seal design helps reduce mixed refrigerant leakage by up to 50%, saving about $ in refrigerant top-ups per year, according to case study data. IGCs offer comparably low maintenance requirements and a high reliability of 99.8%. Not only do IGCs deliver superb, continuous uptime, they also help simplify servicing. The gearbox casing allows for easy access in case of bearing replacements and for effective health monitoring of gears and pinions. Apart from this, maintenance does not differ greatly from standard single-shafts designs. Figure 2. Atlas Copco MR compressor Bringing greater efficiency to small-scale LNG with multi-section integrally geared compressors 4

5 Energy efficiency MULTI-SECTION SPEED OPTIMIZATION Integral-gear technology is based on a central (integral) gearbox inside which a main bull gear drives several separate pinons. (Figure 3). These pinions supply rotational power to compression stages that are paired sequentially into stage groups of two. Figure 3. Atlas Copco integrally geared compressor s gearbox One of the primary benefits of integrally geared compressors is the ability to set the optimal speed for each two-stage grouping From a mechanical and aerodynamic perspective, one of the primary benefits of integrally geared compressors is the ability to set the optimal speed for each two-stage grouping during the design process. This optimal speed is, in turn, determined by the rotor and aerodynamic properties of the impeller, diffuser and volute the parts of the machine that compress the gas. This speed is translated into the gear ratio between the bull gear and the corresponding pinion for its stage group. This type of stage optimization is not possible on a single-shaft machine, where all stages are arranged in one, or sometimes two, housings and are run off the main shaft at the same speed. Instead, a singleshaft compressor relies on a compromise: The design attempts to set as many stages as close to their respective optimal speeds as possible. But the discrepancies between the optimal and actual rotational speed at each stage are compounded, leading to reduced efficiency for the entire compressor: More energy is required to drive the machine, requiring more power. Bringing greater efficiency to small-scale LNG with multi-section integrally geared compressors 5

6 As an additional benefit, the improved aerodynamics that IGCs can obtain at each stage further enhanced by optional intercooling typically translate into higher per-stage pressure ratios. The result is that fewer compression stages are necessary to reach a target outlet pressure. This means lower material and manufacturing costs, which, in turn, help keep the price tag of an IGC below a comparable singleshaft, in-line compressor. BOOSTING EFFICIENCY WITH INTERCOOLING In addition to stage-speed optimization, an integrally geared compressor design also supports intercooling, which is optionally installed between the stages on the Atlas Copco MR compressor. By comparison, intercooling is more complicated, and costly in a single-shaft in-line compressor, requiring that the housing be divided into sections. Intercooling is essential for high isentropic efficiency, a measurement of the deviation of the actual compression process from an ideal, theoretical compression process. Due to thermodynamic principles, the temperature of a gas rises when it is put under pressure. This requires an increased amount of work and thus more energy in the next compression stage to further compress the gas. By cooling the gas between stages, the compression more closely mirrors an ideal isentropic process because fluid friction is reduced. CASE STUDY 1 Generally, compressors requiring gas or electric drives delivering up to 25 MW ( hp) are good fits for integralgear technology. In a reference case study, a non-intercoolerequipped Atlas Copco MR compressor delivering a mass flow of Nm3 / h (36,388 scfm) for a refrigerant with a mole weight of kg / kmol saw a power savings of 3,2% versus a base single-shaft compressor design when running at full availability. At the local, very competitive electricity price of roughly $0,06 / kwh, this resulted in a cost savings of nearly $ per year. CASE STUDY 2 In our case study, intercooling further increased efficiency by an additional 6% and contributed to a total power savings of around 9,3%. Running at 100% availability, this could save the equivalent of $ in power costs per year. Bringing greater efficiency to small-scale LNG with multi-section integrally geared compressors 6

7 Operational efficiency IGVS / DGVS FOR IMPROVED PROCESS CONTROL Let s examine cases where air-cooled condensers are used: When ambient temperature rises, the refrigerant condensing temperature will also rise, while pressure increases in the cycle. To maintain the same level of cooling, the system will require increased flow at higher pressures. In order to effectively control mass flow and power consumption, two control types are incorporated into IGCs, either variable inlet guide vanes or variable diffuser guides, according to process characteristics and specifications. For liquefaction trains that primarily operate at 80% capacity and above (that s mostly the case for water-cooled condensers), IGVs provide excellent efficiency, helping to regulate spikes and troughs in inlet pressure so that the compressor can deliver a constant mass flow. For processes that see frequent start-up and turndowns, and where high adjustability to fluctuations in LNG output is required, variable diffuser guide vanes provide a great answer. Through aerodynamic enhancements such as IGVs and DGVs, overall compressor efficiency can routinely reach around 82 84%. Further boosting process control, the Atlas Copco MR compressor segregates its four stages into two groupings. This is particularly attractive for MR plants running processes that vaporize their refrigerants at two different levels of pressure (a high-pressure and low-pressure stage). The segregation allows for each stage grouping to be independently controlled with IGVs and DGVs and for precise control when, for example, refrigeration must be adjusted to match increased or decreased LNG throughput. Through aerodynamic enhancements such as IGVs and DGVs, overall compressor efficiency can routinely reach around 82 84%. When integral-gear technology s inherent efficiency gains are added to the savings that IGV / DGVs and intercooling provide, total energy savings reach 13 14%. Bringing greater efficiency to small-scale LNG with multi-section integrally geared compressors 7

8 Conclusion Having the experience of delivering mixed-refrigerant compressors for over 20 SSLNG projects, we are keenly aware of customer requirements for a more standardized, CAPEX-friendly compressor approach that takes into account the agile and flexible nature of the SSLNG landscape. Our aim is to offer an option that not only is economically attractive upfront, but that continues to save money through higher efficiency and lower maintenance throughout the entire project lifecycle. With our first applications now running for three to four years, we continue to receive very positive feedback from key market players. In these discussions, we find that integral-gear technology is solidifying its reputation as a reliable and efficient compression solution offering a very attractive CAPEX. To learn more about Atlas Copco Gas and Process compressors, visit Atlas Copco is a world-leading provider of sustainable productivity solutions. The Group serves customers with innovative compressors, vacuum solutions and air treatment systems, construction and mining equipment, power tools and assembly systems. Atlas Copco develops products and services focused on productivity, energy efficiency, safety and ergonomics. The company was founded in 1873, is based in Stockholm, Sweden, and has a global reach spanning more than 180 countries. In 2015, Atlas Copco had revenues of BSEK 102 (BEUR 10.9) and more than employees. Gas and Process is a division within the Atlas Copco Group s Compressor Technique business area. It designs, develops, manufactures and maintains turbocompressors, positive displacement compressors, and expansion turbines. In addition, Gas and Process offers a matching range of aftermarket products. The Division s solutions are used in oil and gas and chemical / petrochemical processes, power generation, renewables, and the industrial-gases sector. The divisional headquarters is located in Cologne, Germany, and the production centers are in the United States, China, and India. Bringing greater efficiency to small-scale LNG with multi-section integrally geared compressors 8