Identification of Compressor Load-Step Effects on NOx Emissions and Efficiency, 03-9337Printer Friendly Version
Inclusive Dates: 07/15/02 - 11-18-02
Background - SwRI conducted a program to investigate integrated engine and compressor controls entitled "Integrated Engine and Compressor Controls." The projects were funded by the Gas Machinery Research Council (GMRC) and Pipeline Research Council International (PRCI) under SwRI's annual GMRC research contract. The objective of the program was to investigate the feasibility of implementing advanced and integrated controls on a compressor unit to enhance efficiency, reduce emissions, and extend the structural life of the unit. During the conduct of this program, a phenomenon was observed that resulted in engine fuel efficiency and NOx emissions being affected by compressor load-step when the unit was operated at constant load. An SwRI-funded Quick-Look Internal Research Project was initiated to obtain additional data to explain the physics that caused this affect and propose solutions to the gas compressor industry.
Approach - A Clark TLA-6 inline six-cylinder, turbocharged and aftercooled two-stroke natural gas fueled integral compressor engine was utilized for this project. The engine was coupled to four double-acting integral compressors via the crankshaft. Testing was conducted in a manner similar to the previous project, but with additional instrumentation that included compressor and power cylinder pressure transducers for crank-angle-resolved measurement in each cylinder. These dynamic pressure measurements provided the additional data to diagnose the root cause of the shift in NOx emissions with load step, and compressor imbalance.
Accomplishments - Data analysis indicated no significant effect of load step at the rated engine speed and load, as observed in the previous project. Detailed analysis of the crank-angle resolved power cylinder pressure data confirmed that no step effect should have been observed. However, data analysis at reduced speeds and/or loads indicated clear shifts in the NOx emissions and fuel efficiency with load step. The data acquired at the unbalanced Load Step #2 confirmed higher NOx emissions as observed in the previous project. The power cylinder pressure-based data did not show a cylinder to cylinder deviation as initially expected, and in fact suggested that the NOx emissions shift could be correlated to variations in the mean horsepower. Horsepower deviated slightly between load steps due to difficulties in setting compressor suction pressure precisely for an exact match of the horsepower.
The data acquired in this QLIR program indicates that the two-stroke integral compressor engine is highly sensitive to small deviations in horsepower. Based on these results, additional load-step investigations have been made part of a follow-on program currently funded by the Department of Energy, PRCI, and GMRC.