Knoxcheck checks NOx

A catalyst-activity testing tool developed with funding from the US Department of Energy offers a new way to manage the selective catalytic reduction systems that are used in power plants to control nitrogen oxides (NOx) emissions.

The commercially-available tool will help plant operators to more cost-effectively comply with NOx emissions regulations, including the new US Clean Air Interstate Rule.

Most of America's energy systems rely on combustion processes. A drawback of combustion is the formation of NOx - a group of highly reactive gases that form when fuel is burned at high temperatures and which contribute to smog, acid rain, and global warming. Selective catalytic reduction (SCR) systems control NOx emissions by injecting ammonia or urea into flue gas in the presence of a catalyst, converting NOx into nitrogen and water.

Over time, the ability of the SCR catalyst to convert NOx degrades, and the catalyst surface can also become clogged with fly ash. Additionally, SCR systems contain multiple catalyst layers, and each layer's activity can decay at different rates, requiring layer-specific maintenance. To restore SCR performance, power plant maintenance staff can either replace a layer of used catalyst with fresh catalyst, add an additional layer of fresh catalyst, or do both.

Detecting catalyst deactivation is key for predicting remaining catalyst life, and the timing of maintenance is critical for catalyst management. Replacing a catalyst too soon means not getting the full life out of the catalyst and results in higher operating costs. Replacing it too late compromises NOx reduction, which may lead to reducing plant output or purchasing costly emissions credits to avoid penalties for non-compliance with NOx regulations.

The new Knoxcheck Online Catalyst Activity Test System, which was developed by Fossil Energy Research Corporation (FERCo) of Laguna Hills, California, monitors catalyst activity in an SCR system without the need to shut down the unit to obtain catalyst samples.

When tested on a full-scale operating SCR reactor at Southern Company's Plant Gorgas Unit 10, the new system produced results similar to those obtained from traditional laboratory testing of catalyst activity. Linked with EPRI's CatReact catalyst management software, the Knoxcheck technology helped Southern Company make proactive decisions about the operation and maintenance of the SCR system and for overall NOx compliance on the unit.

Prior to the development of the Knoxcheck system, the traditional way to analyse catalyst activity was to physically enter the SCR system during a generating unit outage and remove a sample of catalyst from each layer of the SCR, then take it to a laboratory for analysis. To comply with implementation of the Clean Air Interstate Rule, which requires deep reductions in NOx emissions in Eastern states by 2009, plant operators are moving toward year-round operation of their SCR systems. This will mean fewer and less convenient opportunities for shut-down, removal of catalyst samples, and catalyst maintenance.

Using the in situ system in conjunction with catalyst management software allows power plant owners to optimize planning, which could save money by minimising costs for NOx-related outages, load reductions, and other items required for NOx compliance. For example, avoiding a 5-day unit outage for SCR work, and the corresponding need to purchase replacement power at $20 per megawatt-hour during that outage, could save up to $1.2m on a 500MW unit.