C2+ Measurement Solutions for Turbine Control Using an X-STREAM Enhanced Analyzer0 pages

Application Note

Power

C2+ Measurement Solutions for Turbine Control
Using an X-STREAM Enhanced Analyzer
Background
Industrial power markets covering multiple energy sources
require rapid control in order to optimize plant performance and
prevent costly turbine damage. Lower emission requirements
and varying fuel sources due to the increase in unconventional
natural gas sources, such as shale, have created a higher priority on
maintaining turbine efficiency. Although the plants gas sources are
predominantly made up of methane, extensive variations in higher
hydrocarbons (C2+) typically exist. In fact, in some cases, over
18 % variation has been recorded with fluctuations from 10% to
16 % within one minute. In addition, 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.

X-STREAM XE
Typical Power Plant Turbine

Application
Methane and higher hydrocarbons like ethane and propane
behave differently during combustion. Particularly for combined
cycle turbines with lower emissions, variability of fuel sources
presents challenges for control and optimization. Although a gas
chromatograph can provide all needed measurements, speed of
response is sometimes not sufficient for such control.
A process gas analyzer (PGA) with a specific configuration of optical
benches and filters provides continuous measurement and the rapid
response necessary. 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 may be adequate to prevent turbine damage and
optimize performance. If the variation of C2+ in the fuel sources are
known, the error can be greatly minimized.

Table 1 - Typical Gas Composition in Natural Gas Power Plants
Component

Formula

Concentration

Methane

CH4

70–90 %

Ethane

C2H6

Propane

C3H8

0–20 %

Butane
Carbon Dioxide
Oxygen
Nitrogen

C4H10
CO2
O2
N2

0–8 %
0–0.2 %
0–5 %

Hydrogen Sulphide
Rare Gases

H2S
Ar, He, Ne, Xe

0–5 %
Trace

Solution
With a 0–100 % CH4 NDIR bench measuring 7.85 µm, and a 0–25 %
C2H6 NDIR bench measuring 6.6 µm, we designed a solution which
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. Best accuracy is achieved in
Natural Gas mixtures with CH4 as a major component, C2+ up to
20 %, 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 non-dispersive infrared (NDIR) technology can also be added
to the X-STREAM Enhanced analyzer configuration.
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.