Pre-Coating Recommendations: Start-up of a Coal-Fired Boiler With ePTFE Membrane Filter Bags

Precoat is a chemically inert light density powder that is injected into a baghouse to establish uniform porous dust cake on the filter bags. The pre-coating of a fabric filter is a preventive measure to protect the filter media from moisture and other upset conditions that may occur within the start-up of a coal fired boiler. The use of No. 2 Fuel Oil adds an additional area of caution, especially when utilizing any of the ePTFE membrane media in the baghouse.

Micronics Service crew overseeing precoat process to optimize baghouse performance.

ePTFE membrane is especially sensitive to Volatile Organic Compounds (VOCs) that can be generated by the burning of the No. 2 Fuel Oil, or the incomplete combustion of that oil. The VOCs actually penetrate the membrane and blind or block the pores off by condensing within the membrane structure. The damage that can be caused to the filter bags tends to be unrecoverable.

As a preventative measure, one of the simplest protocols is to precoat the filter bags whenever one is starting up on fuel oil. It is highly recommended that pre-coat be injected if there is any possibility of the filter surface being without some fly ash on it. The nature of the ePTFE membrane is that it will tend to shed the collected material more effectively than non-membrane and, as a consequence, the risk of some unprotected surface is higher with ePTFE laminates.

Appropriate selection of pre-coat material is critical, as application can impact the long-term viability and performance of the selected filter material. Areas that we consider when evaluating potential precoat materials include:

–      particle size distribution

–      chemistry of the precoat material

–      cake propagation characteristics

Particle Size Distribution

The particle size characteristics evaluated are mean particle size and the fine percentage of the distribution. While neither is a real concern from a blinding standpoint as compared to a conventional fabric, it is a factor regarding long-term baghouse pressure drop levels. Once fines are introduced into a fabric filter system – whatever the cleaning method employed – they are extremely difficult to get out of the system. Even with off-line cleaning, a disappropriate percentage of fines will remain in the bag mass and will bias the permeability of the generated filter cake to the lower end, which leads to a higher initial pressure drop.  While this will happen eventually, the use of the precoat filter aid will delay the impact of the introduction of the fines.

Chemistry of the Precoat

The chemistry of the precoat is critical. We want a material that will mitigate potential free moisture and condensable acid gases that can be generated as the unit is ramped up in load, both with oil firing as well as coal combustion. Except in specific cases, a basic material with good reactivity is the preferred precoat material.

Cake Propagation Characteristics

The third factor to be considered in precoat selection are the cake propagation characteristics of the material, both in the injected state and after it meets the gas stream. We are looking for a material that will remain ‘dry” and will not compact on the filter surface. We prefer a material that will absorb any moisture and will not generate an overly cohesive cake that may be difficult to remove from the filter surface.

Recommendations

Here are recommendations – in order of preference – for selection of a precoat filter aid for use in a coal-fired boiler baghouse with ePTFE membrane filter bags:

• A commercially available pre-coat material such as PreKote® Surface Pretreatment – The advantages of this specific aluminum silicate product include being chemically inert, having an engineered particle size distribution, absorbency of moisture and combustion products, and an exceptionally low bulk density. The low bulk density allows the material to be transported more easily to the filter surface, even in designs that try to minimize dust loading to the filters. Typical loading is 42 lbs. per 1000 ft² of filter area.
• Limestone – We recommend a limestone product that has a CaO percentage of not less that 90% and 80% minimum passing 325 mesh. Typical loading is 80 lbs. to 90 lbs. per 1000 ft² of filter area. The higher requirement is due to the higher bulk density of the material.
• Dry Fly Ash or a Mixture of Limestone and Fly Ash – Utilizing straight fly ash may not be viable due to the subsequent low mean particle size of the ash and the normally higher percentage of fines in these types of ash than one would expect out of a straight pulverized coal fired boiler. The use of this fly ash may be viable in a mixture with pulverized limestone, meeting the specifications listed above. A 70:30 mix – Limestone to Ash – may provide a viable distribution that would provide the precoat protection required. This precise ratio would need to be finetuned to provide an appropriate distribution. As with the pulverized limestone, we would recommend 80 lbs. to 90 lobs. per 1000 ft² of filter area.

Micronics technical experts are always happy to work with you in optimizing your filter media selection and dust collector performance.

Contact us to see how Micronics can help with particulate control on coal-fired boilers in the power generation industry.