CASE STUDY | Fine-tuning catalyst renewal, saving more than $190.000 per year

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Monitoring catalyst degradation leads to $190.000 savings a year

Case study

The catalyst layers of a Selective Catalytic Reduction (SCR) installation must be timely renewed to ensure proper reduction, avoid ammonia contamination of the fly ash, and prevent unplanned outages. Yet, for economic reasons it is equally important that the catalyst layers are used to their full potential. The case of the Zolling coal-fired power plant illustrates the importance of thorough monitoring and analyzing.

The 502 MW gross/472 MW net capacity power plant at Zolling near Munich, Germany, was built in 1990 and was one of the first coal-fired plants in Europe to be equipped with an SCR deNOx installation. It includes two parallel reactors with three catalyst layers each. No sootblower equipment is needed because exhaust gases and NOx are quite evenly distributed over the reactor layers.

Catalyst potential too low

Nevertheless, some SCR problems occurred during Zolling’s first decade of operation. “In the first 60,000 hours operational, catalyst layers were replaced rather late,” says ENGIE Laborelec Senior Expert Frédéric Mercier, who recently engaged in analyzing and monitoring
the plant’s SCR installation. “Measurements made on catalyst samples showed that the catalyst potential had twice gone below the minimal level. This was a costly affair, because it deteriorated the quality of the resulting fly ash due to ammonia slip and it provoked an unplanned outage at least once.”

Facts and Figures

Saving more than $190.000 per year

The Zolling plant has made a significant saving every year since it cancelled its OEM catalyst layer replacement program and adopted ReNOx Laborelec’s, backed up by monitoring. The three-yearly replacement schedule advised by the OEM would have cost $420.000 each year. ReNOx Laborelec’s monitoring program, which costs about $50.000, allows catalyst layer lifetime to be extended from 4.5 years to 10 years, corresponding to an average annual replacement cost of $180.000. This means that Zolling saves around $190.000 per year.

Not too early, not too late

That’s why ReNOx Laborelec proposed to more closely observe the evolution of the catalyst potential and postpone renewal as close as possible to reaching the minimal potential. “We continue to regularly measure and analyze catalyst samples,” elaborates Mercier. “Based on that, we develop a catalyst degradation curve, which is extrapolated to assess the time the SCR installation will reach the minimal potential. This allows us to carefully plan catalyst renewal activities in line with outage plans. In this Zolling case, we were able to reassure the operator that the current catalyst layers will perform well at least until the next planned outage. Money was saved!”
ReNOx Laborelec performs catalyst sampling, monitoring and analysis at coal, gas, and biomass power plants as well as at glass, cement, and metals production sites and waste treatment facilities.

A waste of money

The plant operator subsequently decided to accelerate the pace of catalyst renewal to always ensure a catalyst potential above the minimal level. Yet out of prudence, they overcompensated. “The measurement data on samples taken at 80,000 hours operational and beyond show much higher catalyst potential levels,” says Mercier.
“The data also show that from that moment on catalyst layers have been replaced long before the minimal potential was reached. Now this is a waste of money, because it is clear that the layers still had great catalytic potential at the time they were replaced.”

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$420k per year saved thanks to global SCR deNOx performance improvement

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How sound O&M improves SCR deNOx performance - $420k saved

Case study

Systematic monitoring of the Selective Catalytic Reduction (SCR) installation at a coal-fired power plant in Thailand revealed various opportunities for improvement. This led to targeted SCR equipment upgrades as well as the finetuning of a number of operations and maintenance processes to dramatically increase SCR performance. “The case demonstrates the need for a more comprehensive approach to managing SCR performance.”

The 502 MW gross capacity coal-fired boiler, in operation since 2012, is equipped with two parallel high-dust SCR deNOx reactors in a 2+1 layer configuration. The plant’s EPC contractor initially equipped each reactor with two honeycomb catalyst layers. Arrays of sonic horns (five per layer per reactor) were put in place to periodically clean the layers so that they could continue to perform well until end-of-life, defined as a rather limited 16,000 hours operational.

Challenging conservative OEM guidelines

In 2014, the plant operator called upon ReNOx Laborelec to monitor the SCR installation. “It was a wise decision, because we have seen that only a few plants make the most out of their SCR installations,” says ENGIE Laborelec Senior Expert Frédéric Mercier. “An independent investigation could uncover a range of opportunities to improve performance and reduce costs.” “In this case, the first thing we brought under scrutiny was the catalyst layer’s limited established lifetime of only two to three years.

This was clearly based on OEM guidelines, following a theoretical model where the layer’s initial deNOx potential deteriorates fairly rapidly over time until it reaches the minimal potential required for the given primary NOx. But this assumption is generally too conservative.” “According to our experience, catalyst layers are capable of performing well for a much longer period of time, often four or five years, provided they are operated under favorable conditions.”

Addressing design issues

Additional investigations were needed to confirm whether the catalyst replacement cycle could be stretched to save costs. “We needed to get a better view of the actual catalyst state and its performance,” confirms Mercier, “so we carried out detailed SCR inspections during an outage in 2015. Unfortunately, we found that large areas of the catalyst layers were severely plugged. Not only that, the sonic horns were severely plugged too, which is fairly uncommon.” “To fully understand how this had happened, we analyzed the related operations and maintenance data. This revealed several design issues. For example, the choice of honeycomb catalysts and the way they were designed significantly contributed to the plugging issue. And the sonic horns provided insufficient acoustic energy to keep the reactor clean. We therefore launched programs to install custom pluggingresistant plate catalysts and upgrade the sonic horn system.”

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Balancing the combustion

The apparent design weaknesses, the ReNOX Laborelec engineers suspected that the poor catalyst performance and reliability also stemmed from unsatisfactory operating conditions. Mercier: “The limited catalyst lifetime surely couldn’t just be due to design issues. So, we carried out air-flow measurements and analyzed the boiler’s combustion data. This revealed significant imbalances in the combustion flow and temperature. For example, there was a big difference between the left and right primary NOx, by as much as twice the good practice guideline.”

“The imbalances identified meant that some SCR areas were being systematically overcharged, raising the required minimal deNOx potential in these areas and thus reducing catalyst lifetime. Once the plant operators understood this, they asked us to optimize the combustion by tuning secondary air flow rates and control loops. The balance is much better now.”

Adjusting to the changed coal mix

Note that operating conditions must be closely monitored on a permanent basis. “Any change in the plant’s process can significantly impact the catalyst,” says Mercier. “For example, in 2016 we learned that the plant’s coal mix source had changed, making for increased ash, sodium, potassium and sulfur concentrations. Our analysis showed that this increased boiler slagging and fouling and resulted in higher temperatures at the catalyst layers, leading to more rapid degradation. We therefore advised that the boiler cleaning program should be improved and intensified accordingly.”

Tuning NH3 injection

ReNOx Laborelec also analyzed and improved the operation of the SCR reactors themselves. “Note that a sufficient quantity of NH3 should be injected in all the catalyst areas to allow maximum completion of the NOx + NH3 >>> N2 + H2O reaction,” explains Mercier. “Our measurements showed that there was a problem.”

“We analyzed the NOx distribution at the reactor outlets and found that there were areas with low NH3 injection but high NOx, as well as areas with high NH3 injection but low NOx. Both negatively impact performance: the first leads to incomplete deNOx, the other produces performancedegrading NH3 slip. We therefore tuned the ammonia
injection grid of both reactors to achieve a more even distribution and better equilibrium between the open and closed control valves.”

Global improvement in performance leads to savings of ~ $420k per year compared with OEM approach

NH3: tuning the ammonium injection grid leads to more homogeneous NOx distribution (from 23 to 12.5 RMS) and a significant reduction in the NH3 slip (reduction of 1.3 vpm), with a positive impact on catalyst lifetime.
Combustion tuning leads to significantly reduced unbalances, which also contributes to extending lifetime.
Unbalance left/rightGood practiceBefore tuningAfter tuning
Total secondary air [kg/s]<10~203-7
FEGT [°C]<10~102-7
O<sub>2</sub> [%w]<0.5~10.3 - 0.7
Primary NOx [ppm]<10~20<5
Pressure drop is reduced from values above 10 mbar/layer using the OEM catalyst to values below 2 mbar/layer when using the custom ReNOx Laborelec catalyst design in combination with an improvement by ReNOx Laborelec of O&M practices concerning the use of sonic horns.