volatiles calculation & estimation

Hello xxxx,

    Assuming you do not have an alkali bypass and you have sufficient kiln inlet oxygen, almost all of the SO3 in petroleum coke (up to 99%) will leave the kiln in the clinker, either as alkali sulphates, calcium sulphate anhydrite or a mixture of both, depending on your alkalis/sulphur balance. However, if insufficient alkalies are present to convert all of the SO3 present into alkali sulphates, blockages and rings will be prevalent in the preheater and kiln due to an excessive sulphur recirculation cycle. Alkali-rich minerals such as feldspars may need to be added to the raw mix to ensure that the molar alkali/SO3 balance is controlled to ~1.

    If there is a significant amount of chloride in the raw mix (>0.1%) then the molar balance of alkali/(SO3 +2*chloride) should be controlled to 1.0. This is because chloride will preferentially combine with potassium to form KCl instead of K2SO4. Therefore enough alkalis need to be present to balance both chloride and SO3.

    Almost all of the sulphur emissions in a cement kiln are due to reduced sulphur compounds in the kiln feed, not sulphur in the fuel. Sulphides (eg pyrites), elemental sulphur, organic sulphur compounds etc in the kiln feed will oxidise between 300 - 1000 deg C to SO2 and leave the kiln via the stack.

    On the other hand, sulphur in the fuels will be oxidised in the burning zone to SO2 or even SO3 and then react with alkalis and CaO present in the preheater/kiln inlet and ultimately leave the kiln with the clinker as sulphates. However for this to happen, the kiln inlet must have sufficient oxygen present (>2%) . ie  K2O + SO2 + 1/2 O2 --> K2SO4     When using petcoke this point is critically important. It is absolutely essential that there is enough oxygen present in the kiln inlet gases and that the sulphur content is completely balanced with alkalis to prevent the escape of significant amounts of SO2 through the preheater an out the stack.

    Another factor is the burnability of raw mix. Hard to burn raw mixes require hotter burning zone temperatures in order to control free lime. This higher temperature volatilises more alkalis and SO3 from the kiln charge. This is particularly true of white cement clinker which, because of its very low liquid phase, requires much higher temperatures to facilitate clinker mineral reactions and control free lime.

    The worst case example for SO2 emissions is a kiln producing white cement clinker which is also firing petcoke. In this case SO2 emissions can only be controlled by ensuring that the molar alkali/SO3 balance is about 1.1,  and that enough excess oxygen is present in the kiln inlet gasses to keep the hotmeal SO3 content below 2% (preferrably below 1.5%). The actual amount of excess oxygen required may vary depending on the kiln but certainly more than 2-3% would be necessary. (sometimes values as high as 4-6% are required)

    Hot meal enrichment has previously been discussed in the forum in these threads;-

http://www.cemnet.com/Forum/thread/108042?page=1

http://www.cemnet.com/Forum/thread/108234/re-sulphates-of-alkali.html

     But the bottom line is that any calculations you use to estimate the SO3 content of clinker, hot meal or bypass dust all require the analysis of every input and output stream in the kiln. So, rather than estimate the SO3 by calculation, it is far better and easier to just sample and test the clinker, hotmeal or bypass dust directly.

Regards.

Ted.