A GUIDE TO THE SELECTION OF FILTER MEDIA FOR CEMENT BAGHOUSES 1

Cement baghouse filters are a vital part of the industrial separation process within the cement industry, therefore selecting the right media for them is of paramount importance. Although filter media normally accounts for a relatively small part of the total cost of the system they are in fact one of the most critical factors governing success.

Large quantities of airborne dust are produced during the winning of raw materials in quarrying and mining operations, and through crushing, kiln/clinker burning, cooling, milling, storage and packaging.

However, these emissions can be well controlled by closed systems, where the process air is passed through a fabric filter for dust separation before release to atmosphere. Collected dust may be returned to production processes or blended into other cement products.

During the baghouse filter process, dust is separated from the air stream by passing the dust laden air stream through some type of porous material, whilst retaining the dust particles on the surface of the material.

Filters can contain anything from a few, to many thousands of square metres of fabric which provide efficient dust separation by acting as a barrier to remove particulate. Therefore, amongst the many factors which must be taken into consideration in the filtration process, an important one is that the chosen filter media is capable of retaining solid particles but will still allow a gas to pass through.

Process dust emissions from kiln and mill processes, through use of high exhaust stacks, can have a greater impact over a much wider area, causing a health hazard and product loss. It is essential therefore that the filter media employed can successfully inhibit emissions of pollutant dusts into the environment in order to meet the stringent legislative control and scrutiny which is driving the demand for more efficient and effective filter media.

Factors determining selection of filter media

The following factors must be considered before determining the selection of filter media:

Temperature

Of all of the vital factors to be considered, operational temperature can probably be thought of as the first consideration, whatever the gas conditions may be. Filtration applications are sub-divided into two groups: low temperature filtration, ie. those that operate at local ambient temperatures, such as (milling, grinding, mixing and bagging operations; and high temperature filtration, where heat is induced or is part of the process, as with utility boilers, incinerators, dryers or kilns).

Adverse gas and chemical conditions

The ability of the fabric to resist degradation from expected levels of acids, alkalis, solvents and oxidising agents must be assessed at the operational temperatures of the filtration application.

Humidity

Resistance to Hydrolysis at expected temperature and humidity levels should be ascertained.

Dimensional stability

The filter media must be expected to resist shrinkage and stretching at the expected temperatures and gas conditions.

Type of filter unit

Here the cleaning method and pressure, filter bag construction and cage configuration must all be taken into account.

For retrofit

History of existing application should be explored.

What filter media is available?

Donaldson Membrane’s Tetratex® Microporous ePTFE membrane filter media has a proven track record as a very high efficiency, highly effective fabric filter media, with several different membrane types available to achieve, subject to various parameters, the desired performance: Tetratex Release, Tetratex, Tetratex High Efficiency and Tetratex Ultra High Efficiency. These high performance laminates supply superior dust cake release, low pressure drop, higher air flow, and high efficiency.

Tetratex membranes are manufactured from 100% PTFE polymer resin. They are chemically inert and thermally stable up to 325°C. The filtration membranes exhibit on average more than 100 million pores per cm2. The membranes are thermally bonded onto a range of substrates including woven, spun bonded and needlefelt materials which are utilised in pulse jet, reverse air and cartridge type dust collectors and filter baghouses.

Tetratex is a surface filtration membrane which acts as a primary dustcake, requiring no dust cake to act as a filter aid before or during operation. Tetratex preserves the substrate integrity by inhibiting dust particle penetration and restricting surface cake formation, maintaining optimum airflow and stable pressure drop.

This filter media vastly improves the performance of traditional filter fabrics which rely on the formation of a surface filter cake to capture fine dust particles, which can permit the migration of dust through the media, leading to high emission values and raised pressure drop.

Benefits of using Tetratex membrane media in cement baghouses

In general:

Long potential baglife

Stable pressure drop

Most resilient to process upsets

Unaffected by particulate size distribution and particulate chemistry

Lowest achievable emission rates

For kiln exhausts:

Reduced pressure drop

Increased air flow

Higher production throughput

Reduced power consumption

Power savings with variable speed fan motors

Reduced compressed air consumption

Longer bag life

Maintenance free operation

For clinker coolers:

High temperature media (Tetratex Fibreglass) reduces dilution air requirements or reduces heat exchanger load for increased cooler capacity

Higher airflow = increased cooler capacity = better kiln control

Reduced compressed air usage

For cement mills/raw mills:

Higher airflow = better sweep through mill/separator = increased cement production

Reduced pressure drop = longer bag life = ability to handle special type of high grade cement

Higher airflow = better drying in raw mill = improved production

For coal mills:

Reduced pressure drop = balanced airflow = better control

Less dust on bags = reduced risk of explosion

Reduced cleaning (demand pulse) = longer life

Lowering emissions and pressure drop

An example of the significant results that can be achieved through the correct selection of filter media is illustrated in recent case study – in which Donaldson Membranes’ Tetratex Release #5102 substantially increased production at a Turkish cement plant, where a production bottleneck at one of the mills was slowing down the cement process. A swift and cost-effective solution was required.

Following a careful evaluation of the mill’s production system, Donaldson Membranes recommended the use of Tetratex Release #5102, a 100% Polyester Needlefelt Filter Media employing the economical, high performance Tetratex Release® ePTFE Membrane, for 2,500 filter bags, to be manufactured and installed by German specialist Fasse Filter.

An inspection of the mill’s cement process revealed that the emission rate and pressure drop of the filterbags was too high. The existing filterbags were made from a standard Polyester Needlefelt, with a high air permeability of 100-250 l/dm2min, weighing 450-500 g/m2. The parameters of the filter system were a flow rate of 176500m3/h, with a differential pressure of 197mmwg, cleaning pressure of 6 bars with a time-controlled system.

An order was agreed for Fasse Filter to supply and supervise the installation of 2,500 replacement filterbags, with a snapring top, each manufactured from Tetratex Release #5102 filter media.

After the new filterbags had been in operation for four weeks, substantial improvements were recorded. The flow rate had increased to 179500m3/h, whilst the differential pressure had decreased to 131mmwg and the cleaning pressure could be reduced to 5 bars.

The plant manager was delighted to report that Donaldson Membranes and Fasse Filter had succeeded in reducing the pressure drop by a third, eliminating all visible emissions and removing the bottleneck, thereby increasing production and profits at the plant.

Environmentally-friendly ESP replacement

Donaldson Membranes was chosen as the supplier of filter media for bag filters as part of the overhaul of the dry process line at HeidelbergCement’s Lixhe plant.

HeidelbergCement, a world-leading producer of cement, concrete and building materials, shut down Lixhe’s wet process kiln and carried out a complete modernisation of the dry process production line in order to: increase capacity from 3400 tpd to 4200 tpd; maximise the use of the most profitable secondary fuels; and to meet environmental protection standards.

As part of the overhaul and in order to reduce dust emissions, two bag filters were erected for the dedusting of the kiln exhaust gases and of the cooler gases; NOx emissions have been decreased by the installation of a multistage combustion system on the preheater.

Previously kiln exhaust gases and part of the cooler exhaust gases used to dry raw materials in the rotary dryer were dedusted in an electrostatic precipitator. This electrostatic precipitator, with dust emissions of 50 mg/Nm3, was clearly not sufficient to accommodate the higher gas flow and to satisfy the more and more stringent environmental regulations. Therefore a new bag filter was necessary to operate as a kiln filter.

Tetratex ePTFE membranes have become a significant market player as the fabric and technology of choice in the replacement of electrostatic precipitators (ESP) with baghouse filters in the kiln de-dusting process, therefore Donaldson Membrane’s recommendation was Tetratex #6255 with its requirements for lower pressure compressed air and less frequent cleaning due to its exceptional dust cake release properties, whilst giving: near zero particle emissions; increased bag life; and lower maintenance costs.

The bag filter, supplied by Redecam, encompasses 7680 bags, height 4.5 meter; diameter 145mm, guaranteed for 5 years. It was manufactured in Tetratex #6255 Woven Glass fabric with Tetratex Ultra High Efficiency ePTFE membrane, suitable for continuous service at temperatures of up to 2600C. The bag filter was installed after the raw materials dryer in order to dedust kiln gases and part of the cooler gases when the dryer is in operation – and was built behind the casing of the electrostatic precipitator which has been retained as a settling chamber. In order to protect the filter against high temperatures a conditioning tower has been added in the circuit to cool down the gases in case of lack of raw materials in the dryer

The new bag filter has been in operation since July 2001 and the bags are still performing well. The emissions have been reduced drastically from 50 mg/Nm3 to below 3 mg/Nm3); the guaranteed pressure drop is 80 mm, although the actual pressure drop is somehow higher due to higher gas flow. The bag filter has provided a dramatic reduction of dust (and Hg ) emissions, and the lifetime guarantees of the bags have been maintained.

Today the kiln is generally operated at a throughput between 4400 and 4600 tons/day (5-10% above nominal). The exit gases temperature may be regulated between 350°C and 420°C and the pressure drop of the system is as low as 340-370 mm WG.

Tetratex #6255 is an ideal media for fabric filters employed on kiln dedusting applications and also provides a solution for high temperature applications.

Tetratex #6255 Woven Glass fabric with Tetratex Ultra High Efficiency ePTFE membrane is suitable for continuous service of up to 2600C. As such it is the obvious choice for Pulse Jet bag filters employed to filter dust laden gases from a major cement plant’s kilns and raw mills, using an off-line cleaning system, where the usual operating temperature ranges from 220 το 240°Χ.

The highly effective offline bag cleaning system is designed to reduce pressure drop and compressed air consumption whilst extending bag life and decreasing maintenance needs. Donaldson Membranes Tetratex #6255 greatly enhanced the process, requiring lower pressure compressed air and less frequent cleaning due to its exceptional dust cake release properties, whilst giving near zero particle emissions.

Unbeatable control technology

As all case studies illustrate, the type of solution recommended depends upon the problem which needs to be solved and the application’s environment – but there is no doubt that for applications employing fabric filter media, the best available control technology for the capture of fine and difficult particulate is recognised as ePTFE membranes.