STEAM PURITY CHALLENGES IN GEOTHERMAL POWER PLANTS: A CASE OF OLKARIA IAU POWER PLANT

Abstract

Steam purity refers to the amount of contaminants (solids, liquids or gases) that are present in steam, as opposed to steam quality which is a measure of the amount of moisture in steam. Steam purity is determined by a combination of factors including the chemistry of the produced geothermal fluid, the separation pressure(s), mechanical carry-over rate during the separation process and the effectiveness of passive and active steam washing processes. The Olkaria geothermal project currently has four conventional power plants; Olkaria I, Olkaria II, Olkaria IAU and Olkaria IV and wellhead units all generating a total of 533.3 MW. The construction of the 140 MW Olkaria V power plant is on course after its ground breaking in April, 2017.
The Olkaria I Additional Unit (Olkaria 1AU) power plant was commissioned in December 2014. The power plant hosts two condensing turbines each generating 70 MW of electricity. Sometime between February and March of 2015, numerous trips began affecting the Olkaria IAU power plant, with subsequent signs of wet steam and brine carry over being noticed in the steam lines. The first drain port analysis program for Olkaria I AU Unit IV and V was conducted between 27th February and 3rd March after water was observed in the steam line. Sampling was carried out for the drain ports from separator stations SE1, SE2, SE3, SN3, OW 32 and OW 35 to the scrubber at the power plant. A second drain port sampling and analysis program was initiated from 20th June to 21st June 2015. Also included in the sampling program were drain port samples from OW 727 separator station that supplies the Olkaria II power plant. The sampled condensate drain ports for SE1, SE2, SE3, SN3, OW 32 and OW 35 reported high concentrations of dissolved components. From the analysis, there was a carry- over of separated water into the steam phase. A root cause analysis was done to determine the cause of this brine carryover. A comparison was done with the purity tests carried out at the Eburru wellhead plant between 2012 and 2016 and in 2017.