Cement manufacturing plants have laboured to maintain optimal O2 levels in their manufacturing process, while maintaining enough O2 for combustion. Optimising O2 in the kiln exit gas is important for a few reasons, but most importantly, it ensures complete combustion and the efficient operation of the kiln.
All plants want to operate as efficiently as possible. The fact is, if a kiln is operating with excess O2, it is operating inefficiently. The goal is to have enough O2 for 100% combustion. However, too much O2 can actually cool the process, so there is an optimal balance.
Many popular refractory linings make use of SiC-based products because of their non-wetting properties, low thermal expansion, and high strength. But SiC materials are not necessarily the best choice in reduced O2 conditions. The non-wetting properties of SiC depend on having sufficient oxidation to form a protective skin. A reduced O2 environment interferes with this oxidation, at least partially compromising one of the key benefits of SiC as a refractory material. This has side effects in terms of alkali resistance, which in turn affects the service life of not only the refractory lining but also other structures, such as the shell and metallic anchors. It also affects the susceptibility of the lining to build-up of dust and other particulates, which was a principal reason for using non-wetting material in the first place.
Alkali penetration and buildup
The major factor that contributes to lower service life is the presence of alkali vapours and salts in the cement making process. Alkalis are introduced into the system through feed raw materials or fuels. They are vaporised in the kiln preheating and calcining zones and flow back with the gas stream toward the preheater tower. These alkalis precipitate on dust particles and attach themselves to refractory linings in the riser area and lower vessels. The infiltration of these alkali compounds alters the surface density of the refractory lining, which increases the risk of cracking and thermal shock. The increased densification also greatly increases the potential for spalling. The net effect of these issues is the reduced life of the lining due to corrosion and mechanical degradation.
Clearly, protection against corrosion caused by alkalis is a key concern. SiC-based liner materials achieve this protection by so-called ‘glassy phase coating’ that liberates silica and carbon through intense oxidation. The carbon forms a protective skin, while the free silica fills in porous holes, contributing to alkali resistance. But herein lies the problem. To achieve these non-wetting properties requires sufficient O2 for oxidation to occur. In a reducing environment, the lower O2 levels may actually prevent this from occurring when SiC material is used. That contributes to susceptibility to alkalis, leading to corrosion of those materials and reduced service life for the lining.
Alkalis are not the only source of corrosion. In the cement industry, many plants use materials that cause or contribute to the build-up of dust and particulates on the lining surface. This is exacerbated by the presence of alkali vapours and salts. When these particulates build up in areas that restrict airflow, it can have extremely undesirable effects and even cause plant shutdown. Costs of US$500 000 and more are common in these scenarios.
To further complicate the issues, many plants have shifted to the use of alternative fuels for cost-saving reasons. These can introduce more chemicals into the process mix that contribute to build up. Again, materials with non-wetting properties have historically been used to combat these effects.
Other drawbacks of SiC-based products
Of course there are also unintentional conditions that can lead to reducing conditions, in particular air ‘inleakage’. For many reasons, such as placement of a kiln gas analyser with respect to kiln exit gases and faulty door seals, it is possible to introduce undesirable process effects. The presence of additional O2 may be falsely detected by the control system, leading to increased fuel and thereby reducing the O2 content at the bottom of the riser.
Another drawback to SiC-based castables is higher thermal conductivity. This leads to higher shell temperatures, process heat loss, and potentially premature metallic anchor failure. Additional insulation can be used to mitigate these effects, but that increases the heat held in the hot face lining, resulting in deeper penetration of alkali compounds.
Since reduced O2 levels interfere with SiC-containing materials’ ability to oxidise and produce the protective glassy phase surface, what alternatives are available to mitigate those effects? An alternative material for monolithics is alumina-zirconia-silica (AZS). AZS exhibits substantially increased resistance to alkali corrosion. Results from alkali tests have shown that products with as little as 14% zirconia are almost completely resistant to alkali penetration under both oxidising and reducing conditions. Another benefit is that zirconia-containing products have lower thermal conductivity than SiC. Products using virgin zirconia grain have been shown to outperform recycled zirconia grain.
AZS-based products are noted for their superior corrosion resistance. Adding more zirconia increases toughness and decreases alkali penetration. Long used in glass and steel making for exactly this reason, AZS products also bring these desirable qualities to the cement-making process.
HarbisonWalker International (HWI) has worked with customers that have experienced poor results with SiC-based liners due to inleakage. In these instances, false readings led to a reducing environment that causes accelerated corrosion of the SiC liner, typically over a 3 − 4 month period. In these situations, replacement of the linings with AZS-based materials resulted in an improved process and increased service life for the lining.
In other examples, HWI has supplied AZS products in many greenfield installations, as specified by OEMs, to help fight build up and alkali attack. With HWI’s help, these customers significantly increased daily clinker production using the company’s ASZ products. In several cases, some of that material remained in service for more than 4 years.
Whether the motivation is environmental impact, operating efficiency, reduced cost, or increased service life, a reducing environment for cement refractories is more than a possibility. While SiC-based products are widely popular and ideal in several applications, there are clear shortcomings that are at cross-purposes with these goals in cement-making refractories. AZS monolithics provide practical alternatives that contribute to these goals and mitigate the drawbacks of SiC.
Read the article online at: https://www.worldcement.com/special-reports/24122018/mitigating-production-drawbacks/