January 16, 2014

C2+ Measurement Solutions for Turbine Control Using an Advanced Flameproof Process Gas Analyzer

By Dr. Michael Kamphus

To optimize plant performance and prevent expensive turbine damage, rapid control is critical for industrial power markets covering multiple energy sources. Maintaining turbine efficiency is an even higher priority today because of lower emission requirements and varying fuel sources due to the increase in unconventional natural gas sources, such as shale. Plant gas sources are primarily composed of methane, but typically there are extensive variations in higher hydrocarbons (C2+) as well. In fact, in some cases, over 18 percent variation has been recorded with fluctuations from 10 percent to 16 percent within one minute. Turbines have to be more flexible and need to present the above criteria over a wide range of loads because gas turbine power plants are often used only for peak loads.

Methane and higher hydrocarbons like ethane and propane behave differently during combustion. The variability of fuel sources can sometimes pose challenges for control and optimization, especially for combined cycle turbines with lower emissions. While a gas chromatograph can provide all the required measurements, the speed of response it provides is sometimes not sufficient for effective control.

The solution can often be a process gas analyzer (PGA) with a specific configuration of optical benches and filters that provide continuous measurement and the rapid response that’s required. In addition, an approximate calorific value (BTU) can be provided as long as the higher hydrocarbons are low in content. If this is not the case, the BTU value will be underestimated but never overestimated. Such values cannot be used for custody transfer but they may be adequate to prevent turbine damage and optimize performance. If the operator knows the variation of C2+ in the fuel sources, then that will substantially minimize the error.

With a 0–100 percent CH4 NDIR (non-dispersive infrared) bench measuring 7.85 μm, and a 0–25 percent C2H6 NDIR bench measuring 6.6 μm, an ideal solution has been designed that provides a high selectivity for CH4 against C2H6, C3H8 and C4H10 as well as CO2.

The C2H6 bench measures C2H6, C3H8 and C4H10 with response factors of 1.0, 1.0, 1.1 and gives a low response from CH4, internally corrected by cross compensation. The best accuracy is achieved in natural gas mixtures with CH4 as a major component, C2+ up to 20 percent, and low water content.

Combination with Other Measurements
Since CO2 content also varies in fuel sources, this measurement may be combined with the hydrocarbon values for better control and calorific value calculations. Carbon dioxide measurement utilizing optical NDIR technology can also be added to the configuration of today’s advanced process gas analyzers.

A calculation of the Wobbe Index is also possible utilizing a calculated relative density from the concentration values or by utilizing an external density signal. Calorific value calculations might require integration of additional channels. As with the BTU calculation, this is not for custody transfer but aids in rapid adjustments to operations, if fluctuations occur.

XSTREAM System Configuration
Methane (C1) and Ethane plus (C2+) measurements for natural gas power applications often take place in hazardous areas. Therefore a flameproof analyzer, such as the Rosemount Analytical X-STREAM Enhanced analyzer (to the right), is recommended.

Sample Conditioning
The figure below shows a flow diagram of a typical analyzer system. Sample extraction, transport, and conditioning occurs using Rosemount Analytical Sample System Modules, which include a flanged sample probe, heated pressure reduction station, sample filtration, and sample flow control.


Process Optimization
To prevent costly turbine damage or process shutdowns, it’s critical that plants conduct continuous measurement analysis with the rapid response necessary to monitor the component effectively. The Rosemount Analytical X-STREAM Enhanced multi-stream flameproof analyzer with optical benches and filters is ideally suited to help address these specific challenges to improve process performance.

To learn more about the Rosemount Analytical X-STREAM Enhanced analyzer and how it can help you improve your process performance, Click HERE.