Formation of Oxalate (and Phosphate) in the LP Saturated Steam of a Combined Cycle Plant During a Period of High Phosphonate-Based Antiscalant Dosage into a RO Feedwater – Part 3: Why the Contamination Source Proved Elusive

ABSTRACT

We previously reported observing oxalate (C₂O₄^2–) along with phosphate (PO₄^3–) almost exclusively in the low-pressure (LP) saturated steam of a combined cycle power plant at a time when an atypically high amount of the phosphonate (PN)-based antiscalant was being dosed into the reverse osmosis (RO) feedwater. The coincidence of these events would point to the overdosed PN as the source, having passed through the RO and the demineralizing mixed beds unremoved into the heat recovery steam generator (HRSG) feedwater and further downstream to the LP drum and saturated steam sections, where the operating temperature and pressure conditions would favor the hydrothermal breakdown to C₂O₄^2– and PO₄^3–. With phosphorous (P) as a surrogate measure for the PN registering at only 1.4 ± 0.6 µg ⋅ L^–1 in the mixed bed product water however, we could not provide evidence for this source/pathway as the way in which these undesired anions were formed in the LP saturated steam section. In this current paper, however, we provide a systematic analysis of PN passage through the water treatment plant into the water/steam cycle using total organic carbon (TOC) as the surrogate measure for the PN. With various organic compounds represented by the TOC measurement, there were however breaks in some of the correlation trends, particularly between the TOC in the HRSG makeup tank and TOCs at locations immediately downstream within the water/steam cycle, i.e., the condensate extraction pump, LP economizer inlet, and the LP drum, as well as with the C₂O₄^2– and PO₄^3–. This obscurity was largely due to occasional "dumping" of condensate return water into the makeup tank which contained various kinds of organics, including volatile ones. Finally, we have shown the underestimation of PN as P to be the primary reason for the elusiveness of confirming the overdosed PN in the RO feedwater as the source and pathway into the water/steam cycle. We have done this by providing laboratory evidence that this PN underestimation is at least, in part, due to the formation of insoluble iron(III) phosphate (FePO₄) during the acid digestion, which prevented it being measured by inductively coupled plasma mass spectroscopy (ICP-MS), and propose the potential for in-line and in-vessel deposition of PN (in the presence of a high iron concentration) as another reason for the underestimation of PN because it eluded sampling in the first place.

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