Annex 35 Complementary information to source category 4a Cement Production
Overview of recent revisions
The emission factors for this source category have not been revised or otherwise changed. Additional guidance has been introduced on classifying sources within this category, estimating activity rates, and on data quality aspects.
Derivation of emission factors
Release to Air
Emissions to air in terms of PCDD/PCDF produced per unit production will be influenced by the concentration of the PCDD/PCDF in the flue gas and the amount of gas produced per unit production. A larger volume of flue gas is generated in wet kilns per unit output than in dry kilns. Modern kilns produce between 1,500 and 2,500 m³ per ton of clinker (BREF 2010).
The SINTEF study is based on more than 1,700 PCDD/PCDF measurements from the early 1990s until recently. It summarizes emissions to air from wet and dry kilns and from plants operating on fossil fuels and “natural” raw materials and plants utilizing alternative raw materials and alternative fuels. These alternative fuels and raw materials, including wastes, were co-fired to the main burner, to the rotary kiln inlet or the preheater/precalciner. In many countries, this is usual practice. The vast majority of the data reported have PCDD/PCDF concentrations far below 0.1 ng TEQ/m³; emissions from dry kilns may be slightly lower than those from wet kilns. Emissions in this range correspond to emission factors below 0.05 μg TEQ/t of cement. These plants were considered BAT and the emission factor represents class 4. However, the SINTEF study also includes some old data – from U.S.A. – which had an emission of up to 25.8 ng TEQ/m³, which corresponds to an emission factor of 16.7 μg TEQ/t of cement (SINTEF 2006).
Very low concentrations of PCDD/PCDF were found in the sampling campaign in Thailand at a cement plant utilizing the dry process. During normal operation (lignite/petroleum coke and full load), the stack emissions were all below 0.02 ng TEQ/Nm³ and as low as 0.0001 ng TEQ/Nm³; the means were 0.0105 ng TEQ/m³ and 0.0008 ng TEQ/m³ for the normal operation conditions and 0.003 ng TEQ/Nm³ and 0.0002 ng TEQ/Nm³ for the tests performed with substitute secondary fuels, respectively. The resulting emission factors were at a mean 0.02 and 0.001 μg TEQ/t of clinker for the normal operation and 0.005 and 0.003 μg TEQ/t of clinker in the case of co-firing alternative fuels/wastes. Thus, all test results were far below the orientation value of 0.1 ng TEQ/Nm³. The results demonstrated that the addition of tires and/or liquid hazardous waste had no effect on the emission results keeping in mind that the dry cement kiln process employed in the cement plant is state-of-the-art technology and the plant is well-managed (UNEP 2001, Fiedler et al. 2002).
Concentration of PCDD/PCDF in the flue gases seems to be influenced by the temperature of the dust collection device. Low temperatures (<200°C) seem to indicate that typical concentrations will be under 0.1 ng TEQ/Nm³, temperatures over 300°C increase the likelihood of finding higher emissions, typical concentrations would be 0.3 ng TEQ/Nm³ and above. In some cases much higher emissions may be found. These seem to be linked to high dust collector temperatures, high levels of organic matter in the raw materials and may be linked to use of certain wastes under inappropriate conditions.
For the purpose of this Toolkit, an average emission factor of 5 μg TEQ/t of cement is applied for old kilns and with dust collectors operating at temperatures above 300°C (derived from old US data) for class 2. An average emission factor of 0.6 μg TEQ/t of cement is applied where the dust collector is between 200 and 300°C (class 3). An emission factor of 0.05 μg TEQ/t of cement is applied for modern plants where dust collector temperature is held below 200°C (class 4). Since there are no measured PCDD/PCDF data available for shaft kilns, no emission factor could be calculated and provisionally, class 1 has been incorporated for this technology and the same emission factor assigned as for the old wet kilns.
Release to Water
Releases to water are not expected. However, if effluents are identified these should be noted and the origin in the process described.
Release to Land
Some residues may be spread on land, in some cases the use of cement kiln dust to increase alkalinity and add lime has been reported. Any use of cement kiln dust (CKD) in this manner should be noted.
Release in Products
Releases in the cement product are expected to be small since the product has been exposed to very high temperatures.
Release in Residues
It should be mentioned that the dusts collected in air pollution control systems, typically electrostatic precipitators (ESP) or cyclones, mainly consist of raw materials fed into the kiln (at the end of the secondary burner). The remainder of the dust consists of emissions from the kiln that has passed the hot zone. Typically, the dusts from the ESPs/cyclones or bagfilters are re-introduced into the kiln.
In cases where solid residues from flue gas cleaning equipment are not recycled into the kiln, an initial estimate of release of PCDD/PCDF in CKD would be based on the assumption that approximately 30 kg of CKD per ton of clinker (0.03% of clinker production) is generated. This value is based on a report that gave 0.4 million tons CKD from 13.5 million tons of clinker/cement production (Dyke et al. 1997).
Concentrations of PCDD/PCDF in the CKD are expected to vary and a range of concentrations from 0.001 to 30 ng TEQ/kg has been reported for UK kilns (Dyke et al. 1997), 1-40 ng TEQ/kg were summarized for German tests (SCEP 1994). SINTEF (2006) and BREF (2010) report an average value of 6.7 ng I-TEQ/kg CKD, from 90 samples taken from wet and long dry kilns, from clinker cooling, from by-pass and ESPs in dry suspension preheater kilns.