Tackling Corrosion

Dip tubes used in pre-heater cyclones have a perennial problem of shorter life than expected. Chlorine induced high temperature corrosion kinetics in the preheater cyclone reduces the service life of dip tube material.

Portland cement is manufactured in a rotary kiln. The rotation of the kiln causes the raw material to gradually pass along and undergo chemical reaction from where it enters the cooler. The molten clinker gets cooled from -1300-?C to 100-?C in the cooler over a period of time. Before entering into the kiln, the blended raw material enters the pre-heater and pre-calciner, where it is preheated and calcined to around 900-?C at various stages of cyclone and calciner. The preheater and pre-calciner use the hot gases coming from the kiln outlet and through the tertiary air duct. The raw meal fed at the top of the pre-heater tower is passed through a series of cyclone and calciner. Hot gas from the kiln is admitted from bottom to top and in the process heat gets transferred efficiently from hot gas to the raw material according to the counter current principle. A typical flow diagram of preheater cyclone is given in figure 1.
Figure1: Process flow diagram of pre heater cyclone in cement plant.

40-60 per cent of the feed material is calcined at the precalciner before it enters to the rotary kiln. Alternative fuels are fired in the calciner system where it generates deleterious gases such as SOx, NOx, and Chlorides. Dip tubes are used to improve the separation and thermal efficiency of a cyclone and the deleterious gas from the pre-calciner comes in contact with dip tubes causing severe corrosion. Dip tube failures are found occurring in several cement plants and various corrosion resistant alloys have been tried to enhance the service life of the same. The corrosion is caused by Chlorine and Sulphur that are introduced to the pyro-system as impurities in the fuel and the product of reaction formed by alkali, Sulphur and Chlorine creates severe corrosion in the dip tube, thereby reducing the service life.

Figure 2: Failed central tube sample received
from cement plant.

A detailed failure investigation was carried out by collecting samples (refer figure 2) from several cement plants. The relative quantities of element of cast samples were determined by scanning electron microscope (SEM) equipped with Energy dispersed X-ray (EDX) elemental spectrometer. SEM-EDX analysis(refer to figure 3) revealed the presence of high level of Chlorine in the base metal.

Test results showed that Chlorine gas had penetrated through the protective Chromium oxide layer and has lead to the corrosion of base metal. These results were verified with the data collected from the cement plant.

Figure 3: SEM-EDX analysis of failed sample

The concentration limit of HCl in the exhaust gas is 10mg/Nm3 .However the analysis showed the presence of high amount of chlorine in the fuels which was then converted to HCl gas during combustion.

Gas containing Cl2 and HCl will cause direct corrosion by accelerating the oxidation of the metal alloy. When the dip tube is exposed to an oxidising atmosphere at high temperature, the metal will gradually oxidise to the stable oxide and form an oxide scale adjacent to the metal.

Figure 4: HCI in the flue gas

This oxide layer is smooth and dense, providing a barrier for further diffusion of oxygen. But chlorine has the ability to penetrate the protective oxide layer, through pores and cracks to the scale metal interface, where it reacts with the dip tube and creates severe corrosion.

Figure 5: Hydrogen chloride in the flue gas

The FLSmidth R&D centre has carried out intensive research work and is in the process of developing (refer to figure 6) a new corrosion resistant alloy, which offers greater benefits to the customer, in terms of product life and performance.

Figure 6: New corrosion resistant alloy

(This article has been authored by Biju Karakkunnummal, Metallurgist, FLSmidth Pvt. Ltd).