Use of Stable Isotope Ratios to Determine the Origin of Coke Formed in Gas Turbines

Coke deposits can form in compression equipment on natural gas transmission lines. These deposits are a result of incomplete combustion due to poor fuel gas quality or incorrect equipment design and/or operation. There is an isotopic “signature” which is carried over from the fuel gas into the coke if incomplete combustion occurs, allowing the origin of the coke deposit to be identified. The use of a continuous flow gas chromatograph isotope ratio mass spectrometer (GC/IRMS) provides a convenient method for determining the isotopic composition of the components in the fuel gas. These ratios can then be used to identify if a correlation exists with the ¹³C/¹²C ratio of the coke sample. Previous projects in 1999 and 2000 demonstrated that the coke deposits found within gas turbines could be closely related to the incomplete combustion of the fuel gas. One outcome of that work was determining the value for having a good understanding of stable isotope signatures for gas entering the pipeline system. In 2000, fuel gas samples were obtained from various areas in the Alberta TransCanada collection system, their isotope ratios were measured and then used to produce coke. This work substantiated previous project work, reinforcing the isotopic relationship between coke and the source material. Current work is to produce a model to quantitatively relate the various components of the source material to coke deposits. This model could then be used as a tool to enhance the performance of the gas turbines. Detailed investigation into the sensitivity of stable carbon isotope measurements for identifying the source of coke deposits for a complete range of TransCanada fuel gas mixtures is presented.