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Fig. 4 - Chemical reaction
between gas constituents
while passing through the
catalytic filter wall (example
SCR DeNOx reaction)

will typically be chosen for its excellent filtration
properties and the ease of transporting reaction
products, like calcium sulfate. However, remov-
al efficiency is not solely limited to the acid gas
components like SO , SO , HCl or HF accord-
3
2
ing to below reactions, but also helps to control
metal and heavy metal oxides as a precursor of
Brønsted acids, like Se, Ar, Cr, Pb or Hg, and
in combination with pulverized activated carbon
(PAC) to a very low level.

Ca(OH) + SO → CaSO + H_2O Depending on the design of the cement plant, Table 1 - NOx and
4
2
3
Ca(OH) + 2HCl → CaCl + 2H O different exhaust flows are leaving the cement ammonia emission
2
2
2
Ca(OH) + 2HF → CaF + 2H O kiln with a requirement for dust separation, like regulation in different
2 2 2
the main exhaust of the kiln (position 1 in Fig. 5) geographies
The application – adding to the stack, the chlorine bypass (position 2 in
Fig. 5) and the clinker cooler (position 4 in Fig.
functionality 5). Fabric filter baghouses have been the most
common technology installed for highly efficient
The decades of experience of using ceramic product and raw material recovery in clinker
filtration for pollution control have driven signifi- cooler and main kiln exhaust. As already earlier
cant advancements in the technology. One of the described both gas streams have to be condi-
newest developments is within the ceramic filter tioned in order not to exceed the limit operation
(candle) itself, with filter lengths up to 6 m (20 temperature of the fabric filter bags.
ft). This development has enabled the technolo-
gy to be even more cost efficient when used on Since the clinker, driven by the product proper-
large gas volume flows, such as in the exhaust ties, has to be indirectly cooled by an air cooler
of a cement kiln. Additionally, these longer filter (position 5 in Fig. 5), utilizing ceramic filters at
elements also help with a filter house design, re- increased filtration temperature instead of bag
ducing footprint in constrained space conditions filters, can be highly attractive, with a return of
when required on an existing site. investment (ROI) from saving power from elimi-
Although local regulations still wildly vary, there nation of the cooler. Since the first introduction
is a notable trend that is requiring higher levels of the high temperature ceramic filtration tech-
of reductions that exceeds the capability of cur- nology in Europe in 2015 (Italcementi), the de-
rently installed technologies. Taking NOx regula- velopment of longer filter elements has resulted
tions as an example, it is evident that in areas of in a number of additional installations of 6 m
high industrial activity (and air pollution) regula- non-catalytic ceramic filter elements for clinker
tions are requiring more stringent emission limits, recovery in Europe.
as shown in Table 1. Based on the example of
Germany, emission regulation changes triggered The bag filter in the kiln main exhaust (position
the installation of 23 SCR based systems since 1 in Fig. 5) is another location for raw materi-
2013, when new emission standards came into al recovery where ceramic filtration can provide
effect. added benefit. Depending on the source of raw

Impiantistica Italiana - Marzo-Aprile 2022 71 71