Jonas Oldin, Mats Eriksson and Alexia Yiakoumi, Höganäs Bjuf AB, Sweden and Peter Trane, Skamol A/S, Denmark, Discuss the use of alkali and chloride-resistant refractory bricks and strong, acid-resistant insulating bricks to save energy.
Finding ways to reduce fuel consumption, and solving corrosion problems related to the increased use of alternative fuels are top priorities for the cement industry. Producers focus on reducing the volmue of magnesite bricks in rotary klins and instead choose other solutions that can be used in combination with insulating bricks. Using gunning material as hot-face lining, combined with an insulating lining, often leads to reduced energy efficiency.
Chemicals attacts in the form of alkali penetration are unavoidable in cemnt production. The worst damage occurs in the lower cyclone stages - in the riser ducts, rotary klin inlets and even i the calciners. Alkali and acid vapours infiltrate the refractory lining and attack the binding phase at temperatures as low as 600 - 700oC, thus endangering the lining. When there gases penetrate the insulating lining behind the refractories, the consequences are even more profound. Chloride and sulfur, combined with condensing steam, form acids that corrode the anchors. At higher temperatures, corrosive vapours can penetrate higher up into the cyclone system, amking the use of alkali and chloride-resistant refractories a necessity.
Today, energy and environmental issues are becoming more and more important. In order to better address these problems, Höganä Bjuf has established a close cooperation with Skamol A/S to design a solution that protects the overalle construction from corrosion and still minimises energy consumption. It consists of a selection of alkali and chloride-resistant refractory bricks and strong, acid-resistant insulating bricks.
By combining the right materials, the new design could not only sace energy, but also prevent corrosion because the bricks are alkali and chloride resistant. The design is based on hot face bricks for acidic atmospheres produced by Höganäs Bjuf and acid-resistant insulating bricks provided by Skamol. In order to avoid corrosion of the steel shell, there are two different approaches to the design of the refractory and insulating lining:
Moveing the acid drew point temperature inside the insulating lining sets high demands for the materials used. They should be extremely acid resistant and especially resistant to sulfuric acid, hydrochloric and nitric acid, which are the most typical types of acid here.
The main challenges in the preheater tower abd the connecting ducts are also alkali attacks, buildups, and abrasion, where the brick lining is demolished inside the cyclons. Alkali and acid vapours infiltrate the lining and attack the binding phase at temperatures as low as 600 - 700oC. At higher temperatures corrosive vapours can penetrate higher into the cyclone system, where they attack the refractory lining and generate pressure within the structure- This can cause refractory spalling in order to relive stress-
To minimise these damaging effect, Högan¨s Bjuf designed a number of hot-face bricks:
As back-up insulating lining, the following insulating bricks could be used:
The danish Moler and Diatomaceous earth insulation bricks by Skamol A/S are used as back-up lining. They have low content in weight percent of acid soluble oxides(~4.3%) to provide acid resistance. Tests haven shown 91.8 - 93.5% acid resistant results.
Given that there are five cyclones in the preheater tower, the recommendation is to move the acid dew point temperature outside the steel shell in cyclone stages1, 4 and 5. This is to avoid acid gas condensation inside the construction. For this to be possible, no insulating lining is necessary.
For cyclone stages 2 and 3, it can be valuable to use insulation in order to move the dew point temperature inside the steel shell, between the insulating lining and the steel shell itself, thereby providing ventilation. Viking 330 bricks can be as refractory lining combined with Super 1100E calcium silicate insulating boards.
When there are problems with buildups, Alsic 500 can provide a solution because it contains SiC, which reduces alkali attacks and buildups are not able to accumulatte on the surface of this brick.
All suspension beams should be painted with acid-proof primer to prevent rust.
The riser ducts recycle heated air from lower cyclone stages and from the rotary klin, thus improving thermal efficiency and reducing the fuel cocts of a cement plant. In the lower, hotter part of the preheater system, chemical influences are the main problem and can affect both straight and curved duct sections. Riser ducts that are connected to calciners often suffer from buildup problems, whereas riser ducts that are connected to cyclones are usually affected by buildups caused by a venturi effect.This occurs mostly in the curved sections of the duct. The optimal lining design depends here on the specific shape of the riser duct and og thermal and chemical influences, but wide variety of brick solutions, prefabricated elements and casables in combination with insulation can all be used.
For rotary klins, it is important to create high-quality refractory solutions in order to minimise the impact on the surroundings and on the environment.
Today, saving energy is a key issue for the cement industry. Many rotary kilns do not have an insulating lining. Anticipating an increase in prices for fossil fuels, an energy savings design has been developed for thermal insulation in all zones of the klin. Most of the savings can be achieved in the safety/inlet zone, and smaler, but still essential, savings can be achieved in the upper and lower transition zone. Combined with the right installation techniques for double layer linings, the optimal lining design allows the brick lining to serve for amny years. Most often, rotary kilns are used in order to burn waste materials but the widespread use of alternative fuels causes problems for the refractories installed. The lower transition zone, burning zone and upper transition zone are usually lined with basic bricks (magnesite bricks) and, especially in these zones, a higher refractoriness is required along with high alkali and thermal shock resistance and significant resistance to clinker liquid phase corrosion.
The use of high-sulfur fuels, combined with poor combustion engineering, can lead to a higher sulfate compound volatilisation, ring formations and buildups. A number of factors can cause coating to disappear completely, with a resulting tendency for the brick to became weak and friable due to thermomechanical fatigue.
Höganäs and Skamol have also designed hot-face bricks to minimise these effects:
As back-up insulating lining, the following insulating bricks were designed:
A combination of this design and the right installation technique can provide significant energy saving and at the same time minimise samage such as corrosion, ovality and cracks on the steel shell. This design also minimise the twisting of the lining, meaning increased safety and a longer liftetime.
Exsample 1: Semi-dry cemnt rotary kiln1
Examlpe 2: dry cement rotary kiln1
Of course each rotary klin is uniquw, so these examples should only be used as a guideline.
Abrasion resistance and alkali resistance are both important in tertiay air duct refractories. High alkali and sulfur centent can occur in the inlet area.
In TAD, different qualities could be used depending on the process.
For hot-face brick lining:
For back-up insulating lining:
For shell protection, Coro Tex pro or acid-proof primer is recommeneded due to the risk of acid attacks on the steel shell.
The increasing use of alternative fuels has a significant impact on the lifetime of the lining, as well the lifetime of the steel casing. In order to reduce the damaging effects of using alternative fuels, alternative solutions must be implemented regarding the refractories and the backup insulating lining. A wide variety of traditional methods have been attempted in the hope od finding a solution to the problem, but have been shown to have no vital effect. The solutions presented here are all well-tested, can reduce the installation time significantly and prolong the lifetime of the lining.
1. Comparing insulated and non-insulated klins.