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Failing RTO Flies through EPA Test. Agent says, "Whatever you're doing, keep it up!"

“Whatever you’re doing, keep it up!”

You know your EPA test is going well when the EPA agent from the Air Pollution Control department looks at the numbers, and says, “Whatever you’re doing, keep it up.” I was asked to explain how an RTO that had barely passed its last EPA test (back in 2016) tested with a higher Destruction Rate Efficiency than it had in the past decade.

- In 2013 this RTO emitted 5.3lbs/hr, and barely passed with a 95.6% DRE. 

- In 2016 this RTO emitted 6.2lbs/hr, and passed with a paltry 95.2% DRE.

- Yesterday this RTO emitted 1.2lbs/hr, and averaged well over 97.76% DRE.

The EPA agent was stunned, and repeatedly asked me to explain the dramatic turnaround, and to keep doing what we're doing. This is what I wrote:

In response to yesterday’s EPA test, there were questions about what changes were made to the RTO and sources in order to achieve yesterday’s results. There are four main reasons why things went so well.

I. RTO Valve Retrofit: Shortly before the 2016 EPA test, we discovered that warping had occurred on the main valves, the result of decades of hard use. The valve deformation prevented the valve blades from properly seating against their seats. This allowed particulate flow by to occur, which negatively impacted the oxidizer’s Destruction Rate Efficiency. After the test, RHZ retrofitted the valves with a secondary seal. This seal successfully countered the effects of valve warping. It extended the life of the valves and was an integral part of yesterday’s successful test. The test confirmed the viability of RHZ’s design.

II. Improved Residence Time: We found the RTO’s inlet static pressure set point had been adjusted numerous times over the past twenty years to compensate for draw issues within the plant. These issues often had little to do with the RTO itself, but were the result of a variety of other things. At the beginning of the year the inlet static pressure set point was more than double its current value. By balancing the sources RHZ was able to operate the RTO at its original inlet static pressure set point. This reduced face velocities, improved thermal efficiency, and increased residence time. This resulted in a greatly improved Destruction Rate Efficiency. This also reduced the RTO’s utility consumption by around 50%.

III. Source Balancing: RHZ found that the changes to the RTO’s inlet static pressure set point were primarily driven by unbalanced sources, and an AMU issue, which has subsequently been resolved. The unbalanced sources created a variety of issues. Plant personnel would sometimes make incremental adjustments to sources to correct an issue that could have been better handled another way. Over time this created long-term issues. The unbalanced sources conservatively cost the facility around $500,000 in excess utility costs over the past decade. Some sources were found to be pulling over 6kSCFM in excess flow. The changes in the RTO’s inlet static set point compensated for unbalanced sources. RHZ balanced the sources and alerted plant personnel to non-balancing issues that were adversely affecting source draws, but which couldn’t be addressed through balancing alone, e.g. plugged source feeds & problematic VFDs. Balancing sources required a combination of mechanical and programming modifications. The plant’s sources were then tested under a variety of online conditions to ensure they satisfied all requirements.

IV. Media Replacement: This past April RHZ replaced the 40-cell monolith with two different styles of monolith. RHZ’s unique monolith configuration was designed to improve thermal efficiency, reduce the pressure drop across the media beds, while also reducing the cold face velocities in order to slow down the rate of media plugging and its resultant increase in delta-p. This increased residence time, which improved Destruction Rate Efficiency.

V. Valve Cycle Time Modification: This is a fundamental change, rather than an improvement, but it should be mentioned. This RTO was originally designed to operate with a 120 second valve cycle time to achieve its designed Destruction Rate Efficiency. This valve cycle time had been used since the unit was first commissioned, back in the 90s. It was the valve cycle rate used during all previous EPA tests. The 120 second valve cycle time reduces the number of valve changes, and subsequently the number of spikes in the outlet VOC concentration. With the improvements RHZ made to the RTO, we felt the oxidizer could still achieve a high DRE while reducing utility consumption. During the engineering test we tested out the 90 second valve cycle rate, and the RTO performed well. That success was repeated during yesterday’s EPA test. By operating with a reduced valve cycle time we are able to reduce the average outlet temperature by approximately 30F. This reduces gas consumption by over 1.3MCF/Hr. In addition to the gas savings, the draft fan VFD now operates at a lower speed, which has a positive impact on utility consumption. If the RTO operated with its original 120 second valve cycle time, it’s Destruction Rate Efficiency would have been even higher. The number of VOC spikes per hour would have decreased by 25%, lowering the average outlet concentration.

RHZ worked closely with the facility to improve the operation of their equipment, and the plant. The result of our collaboration speaks for itself. I would be more than happy to answer any questions.


Russ Friend

Site Engineer

RHZ Environmental Controls, L.L.C.