Control of NOx Emissions in a Titanium Sheet Metal Pickling Bath

Project Scope

A large stainless steel and titanium pickling and finishing company had an issue with visible NOx fumes discharging out of one of their Midwest facilities. The plant had been tasked with the elimination of the fumes that were being generated from their various pickling baths they utilize. Visible threshold limits for NOx gas are generally 300-400 ppm depending on weather conditions. One of the facilities pickling baths is particularly aggressive and pickles a continuous titanium sheet on a coiler machine and has twice the residence time of other treatments in turn generating high levels of NOx. This can overwhelm the stack gas scrubber, causing NOx emission levels to rise well over 600 ppm with orange plumes emanating from the plant which are visible to the public.

The company’s environmental department reached out to USP Technologies (USP) for assistance. Initial discussions with USP confirmed successful implementation of their hydrogen peroxide (H2O2) technology to pickling baths can specifically inhibit NOx formation. USP had done prior work for stainless steel finishers and it was deemed the technology was transferrable to titanium processing.

Technology

On a mass basis, the H2O2 demand is a composite of that due to NO2 and NO, with the typical 50:50 ratio producing a theoretical demand of approximately 1.0 parts H2O2 per part NOx.

HNO₂ + H₂O<sub”>2 <sub”>→ HNO₃ + H₂O Wt. ratio: 0.4

2NO + 3H₂O_{2 →} 2HNO₃ + 2H₂O Wt. ratio: 1.7

As shown in the reactions above, oxidation of NOx by H₂O₂ in-bath, produces nitric acid, thereby allowing recovery and reuse of a critical process reagent.

Solution

After initial testing, a full-scale demo was put in place for several months on the coiler bath at the plant. USP’s turn-key supply scope included 50 percent technical grade H₂O₂, a 3,000 gallon double walled bulk storage and an automated feed system. The turn-key system was solely maintained by USP. The program also included USP’s ChemWatch™ – advanced control system with remote telemetry, allowing tank monitoring for inventory and pump systems control and analysis.

The tank and pump system are located in the middle of the coil storage area. Since the delivery connection is outside of the building, the system employs a remote delivery alarm system. Results shown in Figure 1 demonstrates how after an initial high dose of H₂O_{2 to treat the NOx that had already built up in the bath, the NOx emissions were effectively controlled with a steady feed rate of 5 gph.}

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