There are three categories of emissions from cement manufacturing process:
1- Gaseous emissions:
These emissions include include NOx, SOx, CO, CO2, H2S, VOCs, dioxins, furans.(1)
It has estimated that cement production originates about 5% of global manmade CO2 emissions; producing a ton of cement requires 4.7 million BTU of energy and generates nearly a ton of CO2
The direct CO2 emissions of cement occur through a chemical process called calcination. Calcination occurs when limestone, which is made of calcium carbonate, is heated, breaking down into calcium oxide and CO2. This process accounts for ~50% of all emissions from cement production. Indirect emissions are produced by burning fossil fuels to heat the kiln. Kilns are usually heated by coal, natural gas, or oil, and the combustion of these fuels produces additional CO2 emissions, just as they would in producing electricity. This represents around 40% of cement emissions. Finally, the electricity used to power additional plant machinery, and the final transportation of cement, represents another source of indirect emissions and account for 5-10% of the industry’s emissions.(2)
2- Dust emissions:
Suspended particles in the air which are generated from material handling activates ;( from quarry to cement dispatch)
There are different methods to control such dust according to the source and locations; filters, fog cannons, water sprays,…
3- Cement Kiln By-Pass Dust (CKD)
During kiln operation a portion of hot gases is by-passed to prevent the accumulation of volatile alkalis which increase coating requires Shut-down. The ratios of sulfur(i), alkalis & chloride(ii) are playing role;
As a result of combustion, the sulfur of the raw mix and of the fuel evaporates in the burning zone as SO2, which in the kiln atmosphere combines with alkali and oxygen, generating vaporized alkali sulfate, which condense on raw mix particles in colder kiln zones as well as in the preheater. With the exception of small part which is carried away by the kiln dust, the alkali sulfate returns with the kiln feed in the burning zone, and due to its low volatility leave the kiln with the clinker.
If the SO3 content is insufficient to combine all alkalis, the circulation of the medium volatile alkali carbonate or of the high volatile alkali chloride begins so far as the alkali carbonate are not bound by the clinker phases they evaporate again in the burning zone.
In the preheater, an excess SO2 is able to react with CaCO3, and return to the kiln as CaSO4. Then in the burning zone, it decompose again and thus increase the SO2 circulation of the kiln gas. However, a certain part will appear in the clinker as undecomposed CaSO4.
The presence of alkali in the raw mix in excess of the a mount which already combined by the sulfur contained in the raw mix, allows the use of high sulfur fuels without emitting noteworthy quantities of SO2 by the kiln exit gases.
The alkali sulfate combined by the clinker is of advantage for the early strength of the cement. On the other hand, higher sulfur contents can result in increased SO2 emission with the exit gases, choking of suspension preheater, as well as formation of kiln coating rings.
Chloride react with the alkalis in the rotary kiln, forming alkali chloride, which leave the rotary kiln with the gases, and condense in the preheater. They return into the kiln with the raw mix, but un-like the alkali sulfates, they vaporizing almost completely in the burning zone. Since they condense quantitatively in the preheater, they circulate between burning zone and preheater, until increasing coating requires shut-down of the kiln operation. To prevent this phenomenon, part of kiln gases (up to 10 or 25%), are bypassed and do not enter the preheater. As is known from practice, bypassing of kiln gases is necessary at chloride content of more than 0.015% Cl in the raw mix.
(1)International Journal of Geology, Agriculture and Environmental Sciences Volume – 2 Issue – 3 Jun 2014 Website: www.woarjournals.org/IJGAES A Review On Environmental and Health Impacts Of Cement Manufacturing Emissions Shraddha Mishra, Dr. Nehal Anwar Siddiqui Health, Safety & Environment, University of Petroleum & Energy Studies, Dehradun, Uttarakhand, India https://www.woarjournals.org/admin/vol_issue1/upload%20Image/IJGAES021313.pdf
[(2)Emissions from the Cement Industry by Madeleine Rubenstein, The Earth Institute, Columbia University]
[(3)&(4)cement data book, duda, 2nd edition]