David:

Our kiln is 3000 tpd, with off line PC and 5 stages PH, our fuel is natural gas, H2S content in the gas is 2%.

SO3 content in our kiln feed is around 0.4% and in our hot meal reach sometimes to  3% which increase the molarity ratio of SO3/ alkali to 1.8. My question is why does SO3 increase in the hotmeal more than others like alkalis?

Hello David,

SO3 becomes excessively enriched in the hotmeal due to reducing conditions in the back end of the kiln. The lack of oxygen caused by these conditions means that there is not enough O2 to form alkali sulphates.

The formation of sulphates is important since they have a much lower volatility compared to chlorides and can leave the kiln with the clinker more easily, hence removing SO3 from the kiln recirculation cycle.

There is also the decomposition of CaSO4 that lalbatros mentioned.  If excessive amounts of CO are present (up to 10%) CaSO4 will decompose at temperatures as low as 900 deg C, according to the following equation;-

CO + CaSO4 --> CaO + CO2 + SO2

So, the lower the amount of oxygen in the kiln inlet, the larger the amount of CO and the higher the degree of sulphur recirculation,  up to a maximum which will depend on total sulphur and alkali inputs, CO content in the kiln gases and back-end temperature.

David:

if the reason is the used gas fuel, is there any method to calculate it? 

Sulphur in the fuel can play a large role in SO3 recirculation. Quite often there is as much SO3 input from raw materials as there is from the fuel, and with 2% H2S in your natural gas, I would suspect that this is also true in your case.

Firstly, I would do as lalbatros has suggested and calculate the SO3/Alkali balance for your total inputs (eg raw materials, fuels, waste streams etc)  Try not to use kiln feed for this because normally kiln feed samples contain a significant percentage of recirculating dust which contains high amounts of alkalis, sulphur and chloride. It is best to use the analysis of the raw materials, or that of fresh raw meal before the dust is added.

Regards,

Ted.

 HeyTed Krapkat ;

Thank you very much for your complete answer ... you are great, you have given me the exact answer.

Thanks again to you and to Lalbatros. I am following your answers in this forum and believing that you both are real experts in cement ndustry.

Best Regards