Overcoming Turbocharger Limitations for Low BMEP Two-Stroke Large Bore Gas Engines: A Technical Analysis and Case Study

Abstract

Abstract This paper reviews the unique challenges of implementing emissions upgrade on low Brake Mean Effective Pressure (BMEP) natural gas engines. A large population of the low BMEP gas engines are still utilized in natural gas compressor stations nationwide. Reducing NOx emissions of these engines requires leaner air-to-fuel ratio operation. The typical engine upgrade utilizing a “pure” turbocharged approach is a challenge for low BMEP engines due to poor overall efficiencies of turbochargers at relatively low pressure ratios needed. Previous papers have reported on the use of pure turbocharging and noted performance deficiencies. Other papers have dealt with turbocharging in series with the reciprocating scavenger. Neither approach proved successful, as the units either had a very limited turn down, or suffered fuel efficiency penalties that can no longer be tolerated, given today concerns for Green House Gas emissions. Current strict emission regulations and the need of pipeline companies to utilize their low BMEP engine fleet, necessitates revisiting of the low BMEP upgrade challenge. One of the main merits of these low BMEP units is the operation flexibility. They provide pipeline companies the ability to accommodate wide range of gas demand. This paper first offers a thermodynamic analysis to demonstrate that the exhaust energy of the low BMEP engine is not sufficient to properly match the required compression energy of the turbocharger compressor wheel for a broad operating range. This means the required boost pressure can’t be achieved with a self-sustaining Turbo across a “normal” operating range. The paper then offers a technical analysis of the various viable options to upgrade low BMEP engines. Using a Cooper GMV-TF, a very common engine type in this class as a benchmark, several options are investigated, starting from a series turbocharging with the exiting on engine blower/piston scavenger to supercharging with electrical motor driven blower. It also investigates the option of using an external gas burner in the exhaust stream. The paper closes with a successful case study using electric driven blower in two different compressor station sites.