| Title | Geothermal Steam Purity Modelling – Theory and Practice |
|---|---|
| Authors | T.D. Mills and B.G. Lovelock |
| Year | 2020 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | Geothermal steam purity, modelling, chloride ion, silica, silica distribution |
| Abstract | Geothermal steam purity modelling is considered by some to be an esoteric exercise. Two experienced practitioners in geothermal process engineering and geothermal chemistry seek to demystify steam purity modelling for the next generation of practitioners. This is done by explaining the basic principles used (along with key assumptions) and how modelling can assist steam separation and scrubbing pipeline design. Verified by drain-pot sampling, it provides an accurate measure of the overall efficiency of steam line scrubbing and gives confidence in the level of steam purity at the turbine - a key parameter operators are interested in. Steam purity modelling considers initial steam separation performance and quantifies heat and mass transfer within steam lines in discrete stages, corresponding to each successive condensate removal point as well as final demisting prior to admission to the power plant. Chloride ion is a representative dissolved impurity that is non-volatile and remains entirely with the liquid phase in each stage; it is present in steam lines due to minor amounts of brine carryover. The removal of slightly volatile silica can also be accounted for. Heat loss from pipelines can produce significant amounts of steam condensate which is involved in solution/mixing within each stage. If the condensation can be reasonably estimated in any section, then it can be used as a dilution-tracer to calculate the purity of the total steam flow in that section. Assumptions about turbulent mixing of steam and brine/condensate and attainment of chemical equilibrium in each stage are explained, along with guidance on acceptable scrubbing velocity criteria, and the effectiveness of condensate removal at drop pots. Insights that may be gained from steam purity modelling are discussed, and “proof” of the validity of modelling is provided by comparing modelling predictions with generalised (i.e. non-specific) geothermal steam system data. |