Marking Filtration as a Top Priority for Peaker Plants

Submitted by Greg Neneman || Valin Corporation
For a power plant to run as efficiently as possible, proper filtration is a critical part of the equation.

The benefits are clear, but for one reason or another, filtration can sometimes be less of a priority among plant operators and maintenance personnel than it really should be. Furthermore, the evolution of the modern plant is such that filtration has become even more important with the more frequent utilization of peaker plants.

As more plants have been equipped to generate electricity through solar energy during the day, they are now relying more on peaker power during the time when the sun goes down.  At these times when the solar energy is depleted, the peaker plant gets fired up to supplement the required energy.

The nature of peaker plant use leads to a need for proper filtration.  If ignored, the consequences can be far more costly.  Although peaker plants are designed to fire up relatively quickly, they can also be more expensive to run.  Additionally, because there are many areas of the country that rely on peaker power during certain times, keeping the peaker plant up and running is absolutely paramount.  Thus, ensuring the peaker plants are sufficiently filtering the ammonia becomes a top priority.

The guidelines set forth by the Environmental Protection Agency (EPA) must be adhered to by any power producing plant in the country that burns fuel.  One of the main objectives of the EPA is to regulate air pollutants.  In fact, the EPA determines the maximum allowable levels of NOx that can be released into the air.  When a plant burns coal, natural gas or solid waste, there is a level of NOx that will inevitably be released.

For a power plant to run as efficiently as possible, proper filtration is a critical part of the equation.

How is this Combatted?

By spraying aqueous ammonia into the combustion zone of the furnace through a fine spray nozzle, NOx levels can ultimately be reduced.  The aqueous ammonia creates chemical reactions that produce extremely high temperatures.

The process, however, is a bit more complicated than most realize.  In order to begin the process, one must spray ammonia directly into the flue gas with the anticipation that it must completely evaporate before reaching the catalyst.  This is a critical piece associated with this process because if the droplets of ammonia do not evaporate, it will inevitably bind with the catalyst and thus, not react to the gas.  The only way to ensure the ammonia droplets do, in fact, evaporate is to keep them as fine as possible through strategic filtration. Additionally, any time ammonia is being used in an industrial process such as this, one must be 100 percent certain that there is no aqueous ammonia leaking out.  This is strictly prohibited by the EPA.

As the filtration of the aqueous ammonia is so critical in this process, it stands to reason the process has a fair amount of nuance.  There are several factors that must be considered before selecting, sizing and installing filtration into this specific industrial process.

The first consideration is the sizing of the filter and the desired micron or pore size.  An improperly sized filter can sometimes be as ineffective as no filter at all.  How does one go about sizing a filter in this type of situation?  There are three elements that should be analyzed.  The first is the distance to the peaker units from the bulk storage tank.  Secondly, the number of peakers at the facility can have an effect, and finally, the flow required to the ammonia system for the peaking units needs to be understood.

Once this information is collected, it will help guide the decision as whether it is optimal to use a sintered instrument filter or a pleated cartridge filter.  Pleated cartridge filters are simply process filters designed to filter contaminates that are a constant in a process system.  Sintered instrument filters, however, are used more as a protective mechanism to catch anomalous particles.  They both have their respective benefits.  For example, based on their intended design, pleated cartridge filters will have more surface area than a sintered instrument filter.  On the other hand, sintered instrument filters have the benefits of their size, saving a tremendous amount of space.

In addition to the benefits, one must consider the needs of the process.  For example, if the process dictates a pleated cartridge filter, the material of the filters must be properly specified.  If the ammonia service requirement is 19 to 29 percent, a material must be used to meet this.

When the time comes to specify a material to use for the filter, a complete understanding of the media earmarked for running through it is critical. The material must be able to hold up over time.  If it doesn’t, and the operator is unaware of the failure, the filter may as well not even be there.  And of course, if the filter ceases to exist in a process that includes aqueous ammonia, there can be dire consequences. The issue of filter disintegration is even more prevalent when discussing such volatile substance like aqueous ammonia.  For this particular challenge, it can often be a sound decision to use a type of polypropylene, a material known to hold up well over time. The material selected needs to meet the aggressive nature of aqueous ammonia.

The wetted materials should also be considered for this type of application.  Once again, in order to best account for possible disintegration caused by the aqueous ammonia, it’s important to pick materials that will hold up.  A good choice for wetted housing and gasket/seal materials is often 316 stainless steel and EPDM or Teflon encapsulated materials.

Cleaning Up the Curveball

In addition to selecting proper filter and housing material for a filter system that has the capacity to handle the flow, there is also the issue of the bulk ammonia cleanliness.  In the power plant configuration being discussed, depending on the actual number of peaker units on site, there is typically a bulk ammonia tank along with other pieces of equipment.  The cleanliness of the bulk ammonia in the tank can throw the proverbial “curve-ball” into the filtration process.  Specifically, the inconsistency in the cleanliness of the ammonia cannot be overlooked.  It is often recommended to have multiple filtration stages in the process.  A good recommendation is to place a housing with a meshed strainer basked at a point between the method of bulk delivery and the ammonia storage tank to catch any unforeseen large obstructions.  Due to the aggressive nature of the ammonia it is recommended that these filters are added to a regular PM schedule.

Additionally, it can be preferred to add filtration at the recirculation loop.  There are processes that utilize a standard filtration system at a specified micron-level along with another “pre-filtration” system downstream of the outlet side of the bulk tank.  This additional filtration is often specified at roughly a third of the micron size as the original.  Furthermore, some plants use an additional filtration stage at an even smaller micron level.  This multi-stage approach to filtration has been known to be very effective.

Due to the nature of aqueous ammonia and its importance into an evolving power generation industry, proper filtration is critical.  Failure to properly filter aqueous ammonia can lead to dangerous leaks and unscheduled, costly shutdowns of plants. In addition to understanding size requirements, limitations and the nature of the media, a multi-stage filtration process may be the most optimal approach.

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