Formation of Oxalate (and Phosphate) in the LP Saturated Steam of a Combined Cycle Plant from High Phosphonate-Based Antiscalant Dosage into a RO Feedwater – Part 1: Operational Evidence

ABSTRACT

We provide evidence for the almost exclusive detection of oxalate (C₂O₄^2–) and phosphate (PO₄^3–) in the low-pressure (LP) saturated steam during an eight-month period involving commissioning and commercial operation of a new triple-pressure circuit combined cycle plant with a feed-forward LP drum. This interval coincided with a period in which the antiscalant dosage into the reverse osmosis (RO) feedwater in the water treatment plant, which supplies the heat-recovery steam generator (HRSG) makeup water, was atypically high (≥ 50 mg ⋅ L^–1) compared to the recommended 2–4 mg⋅ L^–1. Based on the relative concentrations of C₂O₄^2– and PO₄^3–, two different phenomena or conditions have been identified for the formation of these analytes: a) volatile carryover of a lower molecular weight degradation product of the original phosphonate from the LP drum into the LP saturated steam during normal operations, where hydro(steam)thermal breakdown into the two analytes occurs; b) conditions and/or activities associated with LP steam bypass of the steam turbine into the air-cooled condenser, which includes steam attemperation with condensate water. The latter is a less significant contamination pathway, but the condition is marked by a C₂O₄^2–/PO₄^3– molar ratio > 1 (1.65 ± 0.36) in the LP saturated steam and low yet detectable levels of these analytes in both the LP drum and the LP superheated steam. In contrast, in the former case the C₂O₄^2–/PO₄^3– molar ratio is < 1 (0.48 ± 0.23) and almost complete absence of the analytes in the water/steam samples from the other two locations can be observed. A further difference is observed in this case between the commissioning period and the commercial operating period, with the latter period exhibiting higher iron (Fe) content in the LP saturated steam, which is accompanied by an Fe-dependent mechanism favoring extra C₂O₄^2– formation over PO₄^3– during this period. After reducing the antiscalant injection to about a fifth of the original dose rate, both C₂O₄^2– and PO₄^3– disappeared from the LP saturated steam, and of course from the LP drum and LP superheated steam. To date, since reducing the antiscalant dosage, these anions, especially C₂O₄^2–, have barely been detectable by ion chromatography even in the LP saturated steam. Yet, very puzzlingly, the concentrations of total phosphorus (TP) and total organic carbon (TOC) in the RO permeate and the demineralizing bed product water were very low during the period, suggesting a different pathway of the phosphonate into the HRSG water/steam cycle than slippage through the RO and demineralizing treatments.

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