Uncovering the True Cost

Over the past decade, the importance of filtration in the overall cost of cement production in the US has increased substantially. The Cement MACT regulations have increased the importance of filtration by reducing dust emissions requirements ten-fold. These low requirements must be monitored and maintained on a continuous basis. At the same time, demand on existing plants has increased significantly. Older plants that could not be economically upgraded to meet the new regulations were shuttered while demand for clinker in the US increased steadily. Today, a poorly operating filter significantly impacts a cement plant’s production reliability, maintenance costs, operating costs, and energy use. Evonik’s P84^® fibre has been used by cement plants around the world for over 20 years to cost-effectively improve the performance of cement plant filter units and reduce the total cost of ownership of pulse-jet fabric filters.

Filtration – essential to cement plants

Operational principal of a pulse-jet

Pulse-jet fabric filters have been adopted at cement plants due to their low overall operating costs, small footprint, and low emissions compared to electro-static precipitators and reverse air baghouses. Pulse-jet units have two sides: a dirty side and a clean side separated by a tube sheet. From the tube sheet, filter bags supported by cages prevent dust from flowing to the clean side of the filter, protecting the environment.

As the dust accumulates on the dirty side of the bag, the pressure drop across the bag increases. Upon reaching a predetermined pressure setpoint, a blast of compressed air cleans the dust from the bags. When the unit is operating properly, pressure drop is low and consistent, pulsing frequency is low and steady, and bag life is long.

Maintaining and operating filtration units properly is critical to the success of cement plants. Poor performing filter units impact plant operation, maintenance costs and energy use. During the life of a filter unit, cement plants make many changes to increase production, switch fuels, improve product quality, and meet new and evolving environmental requirements. Too often, the original filter was not designed to meet these new conditions, generating extra maintenance and production costs due to inadequate performance of the original filter media. Fortunately, a smarter filter media can frequently be used to significantly reduce the cost of ownership of an existing unit. A cement plant will typically operate two categories of pulse-jet fabric filters: process filters (kiln-raw mill, clinker cooler, coal and clinker mill) and dedusting filters. The continuous and efficient operation of process filters is paramount to the overall cement plant production as shutting down for unplanned maintenance will have a negative impact on the entire plant’s profitability.

Standard filter media

The most common membrane-coated filtration material for cement plants is ePTFE membrane laminated on a woven fibreglass base, typically known as ‘glass-membrane’. When first installed, due to the small pore sizes, glass-membrane filter media captures more particulate matter, particularly PM2.5, and prevents dust penetration.¹ When applied under ideal conditions, glass-membrane filter bags can last for 5 years in a pulse-jet fabric filter. Unfortunately, many cement plants do not operate under ideal conditions over the long-term. After installation, plants will change fuels, increase production rates, change raw materials, change product mix, or enhance environmental controls. Plants around the world are switching to lower cost energy sources like refuse derived fuels, while also reducing the environmental impact of the cement plant. Unburned hydrocarbons from refuse derived fuels can blind glass-membrane bags. These changes in plant operation lead to damaged bags causing increased pressure drop, increased pulsing frequency, and increased emissions. Ultimately, reduced bag-life is observed, requiring unplanned maintenance or even unplanned shutdowns.

Advantages of specialist filter media

P84 filter media allows cement plants to experience the benefits of a robust non-woven felt and superior filtration efficiency. Due to their unique, irregular multi-lobal shape, P84 fibres have the capacity to retain a permanent dust cake on the surface of the filter media, thereby meeting the same emissions criteria as a glass-membrane filter, but with a more robust and porous media that can withstand harsher operating conditions, such as increased air-to-cloth ratio, finer particles, and increased dust loading. After 12 months of service life, P84 filter media prevents emissions of fine (PM2.5) particles better then glass-membrane filter media.¹

P84 filter media can be constructed in a variety of ways to cost-effectively take advantage of its unique qualities. P84 fibre can withstand operating and surge temperatures typical of cement kiln baghouses, up to 500°F for short-term temperature excursions. For cement kiln/raw mill baghouses, P84 dirty and clean side batts with PTFE scrim is commonly used.

In lower temperature applications, cement plants can take advantage of P84’s unique surface-oriented dust capture by applying a cap layer of P84 to a standard non-woven fabric. This stratification of fibres creates an effective filter media.

Assessing the total cost of ownership of filter units

Production reliability

Cement plants in the US must continuously meet strict emissions limits imposed by the US EPA. A single failed bag can require corrective action, including a kiln shutdown if the kiln baghouse or clinker cooler baghouse compartments cannot be isolated for on-line bag replacement. During a 4-day unplanned outage, the shutdown of a kiln producing 125 tph would experience production losses of 12 000 t. At US$115/t of clinker, this would equate to a revenue loss of US$1.4 million. Even a small reduction in the productivity of a clinker grinding mill can be significant. When pressure drop across the bags exceeds the capacity of the fan, production rates can be reduced. Over the course of a year, a 10% reduction in production for a 15 tph ball mill would be 12 000 tpy, equalling an annual loss of revenue of US$1.4 million.

Maintenance costs

Cement plants are operating lean and maintenance time for bag replacement can be considerable. Maintenance staff required to conduct bag changes are taken away from other necessary projects around the plant. Labour costs associated with frequent bag replacement can add up over time. For example, labour costs for bag replacement in a clinker cooler can be equal to bag costs over a 2 year bag replacement cycle. A typical cost for a 3 man team dye testing a clinker cooler every other week at 8 hours per test would cost US$31 000, approximately the same as a complete set of standard m-aramid filter bags.

References

^1. HARFMANN, P. and RUOFF, G., ‘Filter media today and future requirements’, Technical Textiles, vol. 51, no. 2, (2008), pp. 86 – 89.

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