NEWS RELEASE August 2019
Determining True Cost for Dry Scrubbing through Coal Fired Power Plant Decisions
Dry scrubbing is one of the options used by coal fired power plant operators to capture SO2, sulfuric acid mist and other acid gases. 44I Coal Fired Power Plant Decisions provides a forum for end users and suppliers to determine which pollution control options are best for a specific plant. Because of lack of water, low SO2 concentrations, weak gypsum market demand, and other factors a dry rather than wet scrubbing solution may be the best choice.
The choice is made more complex by the need to address multiple pollutants. Dry scrubbing is effective in removing SO3 whereas this is not true of wet scrubbers. With the addition of activated carbon, dry scrubbers can also remove mercury. There are multiple dry scrubber designs. Some are better with higher sulfur levels. So the fuel becomes one variable in the pursuit of the lowest total cost of ownership solution.
There are multiple dry scrubbing technologies. One is the spray dryer absorber (SDA). This utilizes the same principle used in manufacturing instant coffee or powdered soft drinks. A slurry is sprayed into a vessel through which the hot gas flows. The liquid evaporates. The acid gas reacts with the absorbing particles. The particles are then captured in a fabric filter.
The circulating fluid bed (CFB) scrubber functions by passing the acid gas through a fluidized bed of lime particles. Circulating Dry Scrubbers (CDS) entrain and then separate the particles. Lime or sodium particles are injected into the flue gas duct with Dry Sorbent Injection (DSI) systems. The catalytic filter uses the DSI principle ahead of a ceramic filter medium with embedded catalyst for both particulate control and NOx reduction.
There are a number of materials, fibers, media configurations and element designs which are utilized. The collection media in the fabric filter has a major impact on cost. The optimum gas velocity can vary depending on the media selected. If the velocity can be doubled then the size of the unit can be halved. Instead of 4000 bags for a big installation maybe only 2000 would be needed.
On the other hand, the cost of ownership is significantly affected by the bag life. If bags last one year rather than four or five years, then bag costs become a significant part of the total expense. The energy cost is also a factor. Energy consumption increases in direct proportion to velocity. It also increases in direct proportion to the thickness of the dust cake on the filter media.
In the case where only particulate is to be captured it is best to establish a semi-permanent cake on the bags and then pulse off the new cake. In accounting terms this is LIFO (last in first out). However, for dry scrubbing it is best to remove the reacted gypsum and retain the fresh lime (FIFO). The industry has not formally addressed FIFO vs LIFO and needs to do so.
The need to maximize acid gas capture and the resultant substantial increase in particulate loading affects the choice of fibers and filter media. The type of cleaning (reverse air or pulsing with compressed air) also determines the selection of the lowest true cost medium.
The fuel or product being calcined or treated also impacts the medium. If a high sulfur fuel is burned, the costs of dry scrubbing are comparatively higher than if a low sulfur fuel is burned. Various fibers react differently to various combinations of acid gases, temperature and humidity. One fiber may handle SO2 and HF in relatively humid conditions at 300 F whereas another cannot.
Temperature resistance is important for several reasons. One is that a fiber which can withstand the temperature excursions will have a longer life than one which is dependent on more perfect operation of the system. Another consideration is the potential to recover heat.
An alternative to glass and polymeric resins is a ceramic fiber matrix. Elements can contain embedded catalysts. Dry sorbent injection ahead of the ceramic media can be utilized to provide removal of dust and acid gases while reducing NOx. The resultant clean hot gas at 600 F or higher can then be directed to an efficient heat exchanger and most of the potential energy recovered.
The material, fiber design, media construction and filter element shape all have to be designed to address the unique requirements of the application and the technology being employed.
The two major fiber types are glass and polymers.
|Type of Fiber||Glass||Polymers|
|Construction||Woven generally but also some non-woven||Non-woven
Performance varies both in terms of temperature, abrasion and chemical resistance.
Max Continuous Temperature
There are differences in laminates with multiple non-woven layers being employed with varying support materials and designs. Fiber shape is also a variable.
How complex are the decisions: There are at least 1500 combinations to assess.
But there are also site specific considerations such as cost of electricity, reagents, pollution limits, plant life, existing equipment. So these can present another 100 variables resulting in 150,000 factors.
There are mountains of information available on all the different variables. Determining which information will help select the lowest total cost of ownership product is a daunting task. The answer is “shared responsibility and collaboration”. Organizations focused on components such as filter media or reagents can contribute but only if there is a clear path on how to do so. The Dry Scrubber Users Group which is focused on the subject in the broadest terms can be the catalyst to help bring other organizations into what can be titled a True Cost program. The program can provide access to the needed evidence for validation of lowest total cost of ownership. Conferences and exhibitions can provide a forum for actual validation of supplier claims. The categories include component related conferences as well as industry conferences.
Shared responsibility and collaboration are best driven by suppliers who are convinced that their product has the lowest true cost and are willing to help provide all the evidence pro & con to support their position. Coal Fired Boiler Decisions has a Dry Scrubber Decision Guide, a monthly FGD newsletter and another newsletter just on Fabric Filters. Information has been compiled for this service for 45 years. Collaboration with publishers and conference organizers includes dry scrubbing tour maps and networking at exhibitions.
Coal Fired Power Plant Decisions is being offered free of charge to utility operators. Suppliers are able to participate with just a $1600/yr subscription. For more information click on 44I Coal Fired Power Plant Decisions