EDUARDO SAUTO,

GORCO, SPAIN, REVIEWS

DIFFERENT METHODS OF UPGRADING EXISTING

DEDUSTING INSTALLATIONS IN CEMENT PLANTS.

IntroductionDue to the legislation that will be modified in the near future (1Q17) on emissions to the atmosphere, many installations are planning urgent modifications, improvements and upgrades to the existing dedusting installations in cement plants, especially in the key installations, such as kiln gases, clinker coolers and mills.

Up to now these gases have been dedusted in many cases using precipitators, whose efficiency has been enough for the range of emissions demanded at present. Now, with the legal changes

that are to come, this range is not enough. It is necessary to reach the best technical solution available and this comes

with the use of bag filters.So we find plants with precipitators that are in a good shape, with materials in good

condition and well maintained, that are going to be

Reprinted from August 2016World Cement

out of use immediately; so how would it be possible to combine the evolution of the technology with the maximum exploitation of the existing installation?

Let us go back to the starting point; the approach we have is that it is mandatory to reduce the emissions below a level that is unachievable for a precipitator, so filter bags need to be installed. There are two options: the first one, and more evident, is to install a completely new bag filter, discarding the old ESP, which is normally an old piece of equipment and whose metal sheet is far from its best. The second option proposed is to use the prismatic chamber and hopper of the old ESP, and fill it with filtering bags to take advantage of the existing metal sheets and discharging elements, reusing them.

This second option is not always possible, because sometimes the casing of the ESP has a size that is OK for the gas flow in case of a precipitator, but is not large enough to install all the filtering m2 needed.

In this article both options are discussed to analyse the advantages and disadvantages of each one and help to reduce the initial doubt that appears when thinking of updating the dedusting installation.

Refitting the old precipitatorWhen starting to assess the refitting of the precipitator to a bag filter, the first thing to analyse is the capacity of the prismatic chamber to host bags. There are some precipitators with a very high height – 15 m – but this is not relevant for the bags because the maximum length recommended for these applications with high volumes would be 8 m. So the height, normally, is not the problem. Length and width of the precipitator will mark the dimensions of the bag plate and the maximum capacity allowed. If the result is viable and it is possible to install the needed m2, we can go on with the analysis.

Good points in the refitting are:

l Less investment cost. Using the prismatic chamber, the hopper and the discharge elements, the filter is much more economic than a completely new one. The global investment is also reduced, because there will be fewer new ducts (just connecting the new clean air channel with the existing clean air duct).

Weak points are:

l Isolation. Today, in applications as important as clinker kilns, whose maintenance intervals are about 12 months, it is necessary to have a security in the filter in case of emergency and/or bags failure. In cases of broken bags, it is very important to have the possibility of changing bags and doing maintenance work on the equipment while working, without shutting down the kiln. One broken bag in 3000 units will cause a reading of emissions higher than allowed. There are some precipitators that have several hoppers, and in this case, with a higher investment, and more technical complexity, it is possible to isolate them so that they (the

Figure 1. Side view of precipitator converted into bag filter.

Figure 3. New bag filter installed beside the old precipitator.

Figure 2. Cemex plant in Spain, refitted precipitators.

refitted filter) works as an off-line filter, but this is unusual.

l Design pressure. The precipitators hardly present an important deltaP across them, so the motorfans usually have a low design pressure. When using bags, they introduce more deltaP and so the pressure of the motorfan must be increased. This means that the prismatic chamber has to support a higher underpressure than in the case of the precipitator, usually higher than that of the design of the casing. So, to be in the range of security, the casing needs to be reinforced. These reinforcements could be installed outside or inside the chamber. If reinforced on the external side, the insulation must be taken out, increasing the cost of the project, while reinforcement of the internal side uses space that may be needed to host bags, so this point must be carefully analysed. There are some precipitators that work in positive pressure, with the fan upstream, so in this case the prismatic chamber is designed for so low a value that the reinforcements will be excessive and very expensive.

l Shutdown time. The shutdown of the kiln in case of refitting would take at least 3 – 4 weeks, counting on no unexpected problems and no need to reinforce in excess. This means a long shutdown time, especially when the shutdown of the kiln is short and the bottleneck of the installation are the works in the precipitator. If there are also unexpected problems, because the

sheet is corroded and in poor condition in some areas, the shutdown can take more time than expected.

New bag filterAssembling a totally new installation has many advantages, such as the possibility of designing it to fit the present or future needs exactly, without being conditioned by the available space or the reuse of elements.

Strong points:

l Specific design for the application, technically and economically optimising the equipment.

l Isolated design for maintenance purposes and off-line cleaning. The bag filter can be isolated and divided totally independently into chambers, usually in 8 or 10 bodies, so that one chamber can be isolated from the rest to do all the maintenance works (changing bags, etc) without generating a bottleneck in the kiln installation.

l Mechanical erection is usually easier, because precipitators are usually installed at high levels with difficult access for cranes and lifting equipment, and now the new installation can be located in easy access areas.

l Reduced shutdown time. Normally it is possible to install most of the equipment prior to the shutdown, including dirty and clean air ducts, and during the shutdown the only work needed is to

Bag fi ltersPneumatic conveying

ATEX installationsComplete dedusting plants Cement millCement separator

Clinker coolerCement Kiln

All over the world

GORCO S.A. LEIOA (Vizcaya) Spain

+34 944635244 / gorco@gorco.es

Layout.indd 1 14/06/2016 14:25

Reprinted from August 2016World Cement

connect the ducts, which can last even less than two days.

Weak points:

l The dust discharge, as the equipment is new and installed in a different area than the precipitator, is usually more complicated and may require long screw conveyors with a high capacity and sometimes even bucket elevators and disc conveyors, which make the installation more expensive than the other alternative.

l Economically it is more expensive than refitting, due to the quantity of new elements that appear and to its mechanical erection.

Case Studies Refitting clinker kiln precipitator:Manufactured for a Cemex plant in Spain. It is about the dedusting of and old cement kiln that had two precipitators in parallel working in positive pressure. The volume of gases of the kiln in direct mode was

about 55 000 Nm3/h at 450 – 500˚C, so it had to be cooled down with ambient air to acceptable values for the chosen bags, approx. 180 – 200˚C. The final volume in this case was 350 000 m3/h and the two casings of the precipitators were refitted with two bag filters with a total filtering surface of 5564 m2, with fiber glass and PTFE membrane bags, 4500 mm length. In this case, as the precipitator was not designed to resist the new design underpressure of the fan (formerly was working in positive pressure), the works for the internal reinforces were very complicated and required important improvements.

New bag filterIn the Italcementi cement plant (now belonging to the Heidelberg group) in Arrigorriaga, Spain, the upgrade and reduction of emissions in the kiln was proposed. The existing installation was formed of two precipitators in parallel and a third one in series after them. In this case one of the most important parameters for the project was to minimise the shutdown of the kiln. So the new bag filter was located in the small area that remained between the precipitator and the raw materials store. All the mechanical erection was done while the plant was running and during the shutdown of a few days the connection between ducts (dirty and clean) was done. Figure 3 shows, on the right, the new bag filter connected to the ducts and on the left, the precipitator just disconnected. In this installation the existing motorfan was reused with a new impeller (needed due to the increase of pressure needs). Also modified was the gas cooling tower, using Lechler equipment to adapt it to the new range of temperatures. The resultant bag filter, with bag quality fiber glass with PTFE membrane and 6 m long, was designed isolated to allow maintenance while working, adding another important advantage over the old precipitator.

These examples, indicate that different starting points require different solutions, and even when the best solution is usually the completely new installation (the simple fact of maintenance while working is worth the investment), sometimes refitting can be interesting.

ConclusionIt is not possible to obtain a clear and definitive conclusion, because there are cases in which what prevails is the location of the precipitator and the economical terms, while in other cases what prevails is the minimum shutdown time and the possibility of maintenance works while working. Generally, the preference and recommendation is for new equipment, which means optimised equipment, but it is true that there are cases in which refitting remains the best option.

GORCO suggests studying both cases and deciding according to the above mentioned parameters.

Figure 5. New bag filter in substitution of old precipitator in Steetley Dolomite (Lhoist group).

Figure 4. Italcementi (Heidelberg group) plant, new bag filter for kiln.