Sour gas processing plant GHG emissions
Figure 2 shows the various streams and available measurement points and data commonly used to estimate the annual GHG emissions from a sour gas processing plant. It shows numerous locations of measurement points for stream volumetric flow rates (Q) and periodic sample and analysis concentration data (C) for various natural gas constituents like methane, ethane, etc. Most of these are combusted into CO2.
Most of the volume-measured gas plant streams are associated with relatively dry (moisture-free) streams, other than the wet acid gas and certain wet fuel gas streams. The wet acid gas stream, which contributes in the order of half of the facility CO2 emissions because of the removed sour gas CO2 content, may contain up to about 8 vol.% moisture. Using a wet flow rate and a dry gas CO2 content to estimate the released CO2 mass rate will result in a larger than true release rate value.
The commonly used, and jurisdictionally accepted, estimation optional approach of using wet acid gas volume rates and dry compositional acid gas values results in over-estimated gas plant facility GHG emissions.
Conclusions
These two examples show various scientific pitfalls associated with conforming to dictated jurisdictional and institutional guidelines on calculating and estimating GHG emissions. The uncertainty is so large that an informed scientist would have serious difficulties in ascertaining whether real emission reductions have occurred.
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Global institutions and various regulatory jurisdictions need to become less zealous about setting numerous detailed guidelines as to how to calculate GHG emissions. They need to fully recognize the value of independent scientific insight versus dictated broad approaches that are not scientifically defensible. Otherwise, GHG emission verification efforts will have no real reduction merits, other than tracking the monetary credit and penalty transactions.
Kurt Hansen, M.Sc., P. Eng., has been an environmental consultant since 1974 and contributes regularly to ES&E. This article appeared in ES&E’s September/October 2015 issue.
