FAMSYSTEMS designs, manufactures and install ceramic filters in association with a UK design engineering company. The ceramic elements used are manufactured by Brightcross, a UK company that has applied all the latest material technologies to ensure trouble free operation of the filters.

The operation of the ceramic filters is described in more detail below. They are used specifically for the filtering of dust at higher temperatures. In many cases they are used together with a dry scrubbing system to remove chemical pollutants from the air.

This makes them suitable for many applications such as incinerators, metallurgical furnaces, many kiln operations and other processes where hot dusty gas is generated.

Besides the high operating temperatures, the ceramic filters also ensure very high removal efficiencies, with typical dust outlet concentrations of below 10 mg/Nm³.

This allows the plant to operate in terms of the international standards, even for plants installed in built-up areas (a problem incurred regularly with incinerators)

GAS QUALITY

In the design of any air pollution abatement system, the first aspect that needs to be investigated is the type of gases that have to be cleaned.

There are many aspects that have to be specified such as dust quantity, dust density and dust size distribution. Although the size and quantity of the dust will affect our design velocities in the filter, it can be confirmed that a ceramic filter can handle a very wide range of dust sizes. As can be seen from the detailed description, in the case of ceramic filters, the dust layer that is established on the outside of the ceramic filter elements actually does the filtering.

If the ceramic filter is to be designed in association with a dry scrubbing system, it is important to specify the chemicals in the flue gas. In the case of an incinerator the average and the maximum figures have to be specified. It is a proven fact for example that in medical waste incinerators the HCl content in the flue gas will tend to vary extensively during operation. Figures of average around 500ppm but peaks well above 3000ppm have to be assumed. It is these fluctuations that make operation to the standards more difficult to ensure that the filter operates to international standards.

As the authorities are now looking at instant peak values and no longer only averaged values, which can easily be manipulated, the importance of the correct design of the gas cleanup system becomes even more crucial.

The correct outlet gas specifications have to be defined from the beginning. This does depend to some extent on the process involved. As an example for incinerators the following standards would be applied to South African companies:

1. DEAT Schedule 39
2. EU Council Directive 94/67/EC of December 16, 1994  

CHEMICAL	UNITS	DEAT SCHEDULE	EU 94/67/EC
Total dust Total organic carbon Hydrochloric acid Hydrogen Fluoride Sulphur dioxide Nitrogen Oxides Carbon Monoxide Cadmium and Thallium Mercury Other heavy minerals Dioxin and furans	Mg/Nm3 Mg/Nm3 Mg/Nm3 Mg/Nm3 Mg/Nm3 Mg/Nm3 Mg/Nm3 Mg/Nm3 Mg/Nm3 Mg/Nm3 ngTE/Nm3	180 (50) N/A 30 30 25 150 N/A 0.05 0.05 0.5 N/A	10 10 10 1 50 N/A 50 0.05 0.05 0.5 0.1

Table 1 : Allowable levels based on current legislation

Two points require some comments:

• The bracketed figure of 50 Mg/Nm³ has become the norm and in design of new plant this figure should be used to ensure that the plant will comply with the latest requirements and will be able to operate at these levels for the life of the plant.
• The EU specification confirms that the percentages given above are to be related to the actual gases leaving the incinerator and any dilution air used for cooling or other purposes has to be excluded from the air volume used in the concentration calculation. This increases the required efficiency significantly, as dilution air of up to 200% can be used for the proper control of the plants.

CERAMIC FILTER DESCRIPTION

The operation of a ceramic filter is best described when comparing it to a standard bag filter, except that a solid ceramic element is used instead of a flexible bag.

The filter arrangement is shown on figure 1. Gas enters at a point below the plenum plate where the ceramic elements are supported. The dirty gas flows upwards through the candles and the dust settles on the candle surface.

Using a standard blow back system based on compressed air, the dust on the filter elements is dislodged and falls in a hopper from where it is removed.

As indicated above, the main difference is the fact that the elements are solid and do not flex. This has certain operational and design implications:

• The dust layer on the filter element is not dislodged completely and only excess dust is removed. This is in stark contrast when using flexible bags, where the flexing of the bag will clean the surface completely. As a layer of dust is maintained on the ceramic element this dust on the surface and not the element itself achieves the filtering process. The ceramic filter is thus capable of handling most dusts with varying size distributions (a specific advantage when dealing with very fine dust). Another advantage of this fact is that this dust layer also protects the element from abrasion and wear. The disadvantage is that the pressure drop for a ceramic filter will be higher than a bag filter and some extra power will be required from the fan.
• The support system of the filter elements is crucial and must be done correctly to ensure the trouble-free operation of a ceramic filter plant. The individual support method shown in figure 1 ensures optimum life of the filter elements and reduces the risk of dust penetration though the support joints.

Another advantage is that ceramic elements can resist temperatures over 1000 ⁰C. The operational temperature of the filter is determined by the metal casing. This allows one to work well above dew temperatures and ensure that no acids condense in the filter, requiring special materials and causing operational problems by blinding the filter elements.

The higher the temperature the easier the operation becomes .

CERAMIC FILTER PROCESS

To apply ceramic filters to the cleaning of flue gases, we need to look at three factors: the removal of the dust, the removal of chemicals and the possible existence of dioxins.

The overall process is shown on Figure 2

The various features of this process are:

• The use of dilution air in this case has no process reasons and is normally done purely to allow the use of cheaper materials of construction for the ducting and cooler.
• The cooler can be of various designs including a reboiler. Internationally the use of reboilers allows some of the energy to be reused and sold to associate companies or processes in the form of steam.
• As the filter only removes dust, it is necessary to react the chemicals in the dry state and form salts in the gas stream so that these can be collected on the filter elements. As the pollutants can vary in concentration and identity, a number of absorbents can be used.

Some of the typical sorbents used are lime, bicarbonate sometimes with other additives. Typical results on sorbent removal are 95% for SO₂ and over 99% for HCl.

• The absorbent enters the gas stream and reacts with the chemicals present. The salts formed are also a dry solid and can be collected in the filter. It should be part of each design to take into account the specific reaction temperature of the absorbent used.
• The filter is a standard ceramic filter designed for the specific gas flow, including dilution air, and the chosen operating temperature. The area of the filter is increased slightly to accommodate the extra dust load due to the addition of a dry sorbent.
• One of the advantages of ceramic filters not mentioned above is that they do not burn. When working with incinerators and a dry system it is always possible to have sparks entering the filter. Using bagfilters this danger is significant and the burning down of such a filter will affect the operation of the plant and result in possible closure.

CERAMIC ELEMENTS

The most crucial part of any filter design is the ceramic filter itself. Being a hard fibre element, the incorrect design and /or material will result in high replacement rates.

FAMSYSTEMS is proud to be able to offer the Brightcross elements. This range consists of a number of element sizes and materials, which ensure good operational characteristics.

The elements incorporate all the experiences made over the last 10 years. Initially it was thought that one material and one size could offer a solution to all problems.

It has become clear that this is not the case and unfortunately some failures in the field have confirmed this.

It has also become a proven fact during this time that with the correct choice of element (size and material, as well as support system), ceramic filters enable many processes to operate today a high efficiency levels and lower pollution levels.

At this stage two main element sizes are being used:

• The 65 mm diameter element either 1000 or 1250mm long
• The 125mm diameter element with a length of 1500mm.

The latter can only be achieved with the specially designed Brightcross elements.

SERVICE OFFERED

FAMSYSTEMS offers the design, South African manufacture and erection for plants on a worldwide basis.

Using the South African manufacture basis, prices can be competitive with most other parts of the world, without reducing quality of workmanship.

FAMSYSTEMS internal design and quality control systems ensure that this high standard of quality is maintained throughout.

Filters are designed as per requirement and can be supplied for small test or lab furnaces to large industrial applications.