Gas Turbine Efficiency Boosting by Real-Time Gas Analysis

Closed-loop control of fuel/air mixture equivalence ratio in gas turbine becomes today more important for energy industries. The mass flow of fuel is well metered but there is no easy way to establish the accurate equivalence ratio without fast on-line measurement of gas composition and Wobbe index. Reduction of emissions is another indisputable requirement, which can be only achieved by closed-loop control of combustor in a range of equivalence ratio avoiding increase of CO and NOx.

Using the unconventional natural gas sources, such as shale and biogas,requires turbine control to become more flexible to withstand with significant fuel composition variations. The premixed mode of combustion, which enables to reduce flame temperature and consequently NOx pollutions, requires even more accurate and faster determination of the equivalence ratio.

For gas turbines with high pressure ratio compressors it is important to correctly establish the required fuel pressure by varying the ratio of fuel injected over the total fuel mass flow, in order to reach lower levels of NOx through control of the flame temperature and the combustion efficiency.

Following the market demand, modern gas turbines design allows continuous operation for wide Wobbe index range of 21 MJ/m3 by measuring the fuel composition and automatically adapting the control parameters for the actual measured values. This advanced concept allows to achieve maximum possible efficiency and to minimize emissions for wide range of gas composition.

Traditionally, gas chromatography was used to measure gas composition and calorific value of fuel gas. However, speed of response which is reachable with this well-proven technology is not sufficient for effective closed-loop control. Most modern turbines can only withstand variations of ± 5% in Wobbe Index, which has fast and significant variations in premixed mode of combustion. Therefore, gas chromatography is not capable to prevent turbine damage or process shutdown, as a result of varying flame dynamics caused by fast changes in Wobbe index.

The solution for this problem comes from the spectrometry, which provides real-time gas analysis with 1 second update rate. This technology has the same analysis capabilities as chromatography, but doesn’t require any carrier gases and continuous maintenance.

MOD-1022 Natural Gas Analyzer is a real-time hydrocarbon gas analyzer based on Tunable Filter Spectroscopy. Calibrated for natural gas composition monitoring and heating value analysis, this package is optimized for gas-phase product analysis containing C1– C6 alkane gases as typically found in natural gas pipeline and LNG. Calorific Value (CV) and Wobbe Index (WI) are calculated as per ISO 6976:1995, to establish the accurate equivalence ratio by closed-loop control of combustor avoiding increase of CO and NOx.

Additional measurements which can be provided as option includes the following:

• Concentration of Hydrogen Sulfide (H2S) content in natural gas
• Hydrocarbon Dew Point (HDP) is a temperature at which higher hydrocarbons condense
• Water Dew Point (WDP) is the temperature at which water condenses out

Modcon will assume total systems responsibility and provides complete, integrated turbine optimization system from initial engineering through manufacturing, testing, installation, configurations and field start-up.