By-pass minimisation

This weeks blog is a short discussion relating to the use of gas by-pass systems for the removal of volatile materials such as alkalis, sulphates and chlorides from the cement manufacturing process. The challenges that these systems present are discussed as well as some of the simple steps that can be taken to address these challenges.

As most "cementies" know, the principle of the by-pass system is to extract some of the gas from the kiln back-end/riser area of the preheater and precalciner system. The volatile materials are mostly in the gas phase at the temperature at which the gases are extracted and these gases are rapidly cooled by quench air, causing the volatiles to condense on the dust that is also extracted from the system with the gas. The amount of gas that is extracted should be sufficient to allow enough of the volatile materials to be extracted from the system so that the process can be operated without sulphate or chloride blockages in the lower stages of the pyro-processing system (or in the case of alkalis, sufficient alkalis to be removed to reach the desired final clinker quality).

The use of a by-pass system results in an elevated fuel consumption which is made up to the following four elements:

  1. The loss of the heat from the process of the hot gases from the kiln which could have been used in the preparation of the meal going to the kiln
  2. The heat which is carried out of the process in the dust that leaves with the bypass gas
  3. The energy that has been used to heat and partially decarbonate the dust that leaves the process
  4. The heat that is drained from the burning zone when the volatiles are evaporated, which is then released when the volatiles condense.

In addition to this, the use of a by-pass results in the challenge of disposing of the by-pass dust, which by its very nature is rich in volatiles. The high volatile dust is very fine and fluffy and is very easily lifted in the wind causing fugitive dust issues. The dust also contains metals which can be leached out of the dust and in many countries is a controlled substance. 

It is essential for the cement manufacturer to keep a close control over the by-pass usage to ensure that it is operating at the correct level and that it is not being over-used - resulting in significant waste of fuel, power and raw materials. A few simple checks that can be completed are listed below:

  1. Check the raw materials inputs to see if they are the same as when the plant was designed and establish why the by-pass was installed - was it for alkalis, sulphate or chloride? Have the raw materials changed since the plant was commissioned - allowing the by-pass level to be reduced? Check on the alkali sulphate molar ratio to see whether it is within an acceptable range where volatilisation shouldn't be an issue?
  2. Check whether the amount of by-pass losses are close to what they should be for the process - this is by a heat and mass balance of the process and adding up all of the elements of by-pass loss. The industry standard for a precalciner kiln is 2.5 kcal additional fuel consumption/percentage of by-pass used. This assessment can indicate whether excessive levels of dust are leaving the system with the by-pass gas.
  3. Check the burning conditions in the kiln to minimise volatilisation in the burning zone - is the burner pipe centrally positioned up the axis of the kiln, is the kiln back end oxygen analyser working correctly and is there sufficient oxygen in to avoid reducing conditions and is the raw meal residue at the correct level to avoid excessive combinability temperature?

These simple checks can have surprising results that can lead to an action plan to reduce the by-pass usage and therefore a reduction in fuel consumption, an increase in throughput and a reduction in the quantity of by-pass dust generated