Natural Gas Blending for Turbine Optimization

Gas Turbines (GT) require fuels of sufficient heating value to meet firing requirements, within environmental emission limits.  Poor quality fuel sources are often high in nitrogen.  Blending with high quality fuel to achieve required heating value is preferred over costly scrubbing to remove (volumetrically reduce) nitrogen. 

Blending station(s) utilize either Heating Value (BTU or Metric) or Wobbe Index as the measurement of the heating value to blend the gases such that the resulting fuel will meet the firing requirements of gas turbines and environmental emissions limits.  The COSA Xentaur Corp. (CXC) 9610 analyzer determines the composition of the natural gas in real time. The analyzer outputs, as well as other instrumentation parameters, were brought into the plant’s DCS for operator monitoring and control of the gas blending stations.

The discussed example data is specific to a GE Frame gas turbine but applies through the industry.  A full detailed discussion is contained within COSA ISA Power Division publication XXYY.

Application Challenges

GAS TURBINE LIMITS

Although the ultimate cost control goal is to maximize use of low Heating Value gas, the amount of nitrogen introduced to the GTs must be controlled to limit NOx emissions.

In addition to the potential of generating excessive NOx emissions, fuel gases with large percentages of inert gases such as nitrogen will have a ratio of rich-to-lean flammability limits less than that of natural gas. Low flammability ratios may cause the GT to experience problems maintaining stable combustion over the full operating range of the turbine. Therefore, a gas blending system is required to ratio the two sources of gas to prevent combustion instability and the generation of excessive NOx emissions. The intent is to burn as much of the Low Heating Value gas as possible since it is available at a more attractive price than High Btu gas.

A total nitrogen content of 15% is the maximum nitrogen limit recommended by GE.  Table X specifies GE’s allowable limits for fuel properties and constituents. This sets the upper limits for the design project.  Source gases can range from 30 % nitrogen in low heating value feeds to 2 % or less nitrogen in high heating value feeds.

COSA's Solution – Calorimeter

The COSA 9610 BTU Analyzer’s measuring principle
is based on the analysis of the oxygen content in the flue gas after combustion of the sample. A continuous gas sample is mixed with dry air at a precisely maintained constant ratio, which depends on the BTU range of the gas to be measured.

The fuel air mixture is oxidized in a combustion furnace in the presence of a catalyst at 800°C, and the oxygen concentration of the combusted sample is measured by a zirconia oxide cell. The residual oxygen provides an accurate measurement for the Combustion Air Requirement of the sample gas, which can be correlated accurately to the Wobbe Index of the gas.

CXC 9610 provides the data needed to blend turbine fuels to optimum heating value while controlling emissions.

Additionally, the CXC 9610, with real time analysis over extended calibration range is able to control and optimize a multiple blending stations vs. single blending systems.

The benefits from utilizing multiple blending systems include:

• Gas Turbines, although designed to the same or similar specifications may require the blend to be unit specific.
• Operational availability of all units is of utmost importance. The operation of any of the units may be limited by its combustion or NOx characteristics. Individual blending allows individual unit control
• A central blending system represents a single point of failure that could require all units to revert to Hi Btu gas, curtailing use of the low cost Low Btu gas.
• A multiple blend system offers the ability to tune each unit and its blend depending upon changing unit characteristics, fowling, or de-rating due to some process problem (i.e., turbine vibration, generator limiting, unit specific problems, etc.). Self-tuning and Neural Network may be utilized later to maximize unit flexibility and operability.

The CXC 9610 met the application requirements of

• Real-time outputs, digital and analog of heating value and Wobbe index
• Low maintenance requirements
• Hazardous areas installation.