If our cement kiln feed includes slag or fly ash, how does the IPCC say to treat its lime and its carbon?
Two corrections. Lime (CaO) from slag or fly ash is subtracted first, because it releases no process carbon dioxide (CO2): 4% slag CaO in a 65% CaO clinker takes the factor from 0.51 to 0.48. In the Tier 3 equation, fly ash carbon can be set to zero if its heat is under 5% of fuel heat.
Correction 1: lime that did not come from limestone
The Tier 1 default assumes clinker is 65 percent CaO, all from carbonate: 0.65 / 0.5603 = 1.160 tonnes of calcium carbonate, x 0.4397 = 0.51 tonnes of CO2 per tonne of clinker. With the 2 percent kiln dust correction that is 0.52.
If a plant gets a significant share of its CaO from a non-carbonate source such as steel slag or fly ash, the guide says that part should first be subtracted. Its example: 4 percent of the CaO in a 65 percent CaO clinker is from slag, so carbonate CaO is 61 percent and the factor is 0.48, before the kiln dust correction, so do not read it against the 0.52 default.
Same rule at other lime contents
- 60 percent CaO, all from carbonate: 0.47 per the guide. 67 percent: 0.53.
- My arithmetic for the slag example: 0.61 / 0.5603 x 0.4397 = 0.479. Adding the 2 percent kiln dust correction gives about 0.49, a figure the guide does not print.
Correction 2: carbon left in fly ash
The guide says fly ash and other raw materials may carry carbon residues that give extra CO2 when burned. It says these emissions typically are not accounted for in the Energy sector, and that too few data exist to give a default value.
In the Tier 3 equation, a term for carbon bearing non fuel materials (tonnes used x fraction of carbon x emission factor) can be set to zero if the heat from that carbon is less than 5 percent of total heat from fuels. The guide's good practice sentence about more than 5 percent names kerogen, and a note to the equation extends the 5 percent test to kerogen or other carbon, which includes fly ash carbon. Where it contributes more than 5 percent, good practice is to include it. If such materials are used extensively, the guide asks compilers to check whether the emissions are already counted in the Energy sector.
What this does not cover
Fly ash or slag added at the grinding stage never goes through the kiln, so neither correction applies to it. The page on fly ash allocation covers what a power plant passes on with it. The Marmara page covers the same slag lime point for a raw meal.
Does the fly ash carbon pass the 5 percent test
- Kiln fuel heat: 3,500 megajoules (MJ) per tonne of clinker
- Heat from carbon in fly ash feed: 60 MJ per tonne of clinker
- Share: 60 / 3,500 = 1.7 percent, under 5 percent, so the term can be set to zero
- Limit for this kiln: 5 percent of 3,500 = 175 MJ
The 3,500 and 60 MJ are example numbers. The 5 percent test is the guide's. Get the plant's real figures from fuel records and the lab.
What Senseible does here
Senseible reads fuel invoices and electricity bills into monthly records, so kiln fuel and mill power are on file for each month. Clinker chemistry and cement type records stay with your plant laboratory and production log.
- CO2 per tonne of clinker
- Fly ash from a power plant: carbon allocation
- Kiln dust and bypass dust correction
- Slag in raw meal, Marmara
- Free calculators
Last reviewed 2026-11-05. Figures are from the 2006 IPCC Guidelines. Check whether your buyer or reporting scheme asks for a different edition of the IPCC guidance, and use your own plant measurements where you have them.