SD Vision - Halyps Cement

24
Monselice cement plant: electrostatic precipitator for dedusting gases
to the catalytic system for NOx abatement
reaction among these compounds and the
nitrogen oxides to form elementary nitrogen
and water. This method, known as SNCR
(Selective Non-Catalytic Reduction), has
some advantages, including that of being
easily applicable also to existing plants,
especially dry process ones.
However, it also has drawbacks, including
that of releasing ammonia from the stack
(ammonia slip) when, with the intent to
minimize NOx emissions as much as possible,
reagent is added at higher rates than those
required by the reaction stoichiometry.
The simultaneous adoption of the abovedescribed
techniques allows cutting the
emissions from a cement kiln to 500 mg/
Nm 3 dry at 10% oxygen. Other industries
and the energy sector in particular have
been using another “secondary method”
for some time, namely the injection of
aqueous ammonia solution into the gas
stream at temperatures of around 350°C. In
this case, the reaction can take place only in
the presence of a catalyst, hence the name
of the method known as SCR (Selective
Catalytic Reduction). The catalyst consists
essentially of vanadium and titanium
oxides, mixed in appropriate proportions
and so arranged as to obtain hollow bricks
that maximize the surface/volume ratio
while minimizing the amount of catalyst
per volume unit of treated gas. In a cement
kiln, the temperature of gases exiting the
preheater is between 300°C and 350°C:
so, extending sic et simpliciter this kind
of technique to cement kilns would seem
feasible. For some time, attempts in this
direction have been made with unsuccessful
results, not so much in terms of reducing
emissions, but rather for the negative effects
on kiln performance, which showed disadvantages
in terms of energy consumption,
steady operating conditions and production
levels. Actually, the amount of dust in the
gases from the preheater is higher than the
amount measured in the hot gas circuit of
a coal boiler. Such a high amount of dust
clogs the narrow channels of the catalyst
requiring continuous cleaning by means
of high-pressure air jet cleaning, resulting
in higher energy consumption. Despite
cleaning, it is impossible to run the plant
correctly because of the frequent stops and
lower production levels.
Italcementi Group also wanted to verify the
real effi cacy of a catalytic system and did it
in a particular context: a Lepol kiln, characterized
by ammonia emissions from the raw
materials of some tens of milligrams per cubic
meter, was equipped with a catalytic system
very similar to the above mentioned ones,
in which, however, nitrogen oxides were
used to abate ammonia. The system has
proved to be very effi cient, also considering
that the dust contained in the gas was only
a few grams per cubic meter, thus demonstrating
that the only real hindrance to a
correct operation of the catalytic system
on a cement kiln is that of reducing the
concentration of dust transported by the
gas to be treated to values compatible
with the catalyst. After being discussed
and analyzed at length, the innovative idea
proposed by the Group Technical Center
was considered valid and fi nally patented.
This consisted in the installation of a technological
electrostatic precipitator between
the preheater’s exhaust gas outlet and the
inlet of the system (catalytic reactor) for
reducing dust to values which are certainly
at least compatible with the best operating
conditions required by the catalyst. The
electrostatic precipitator must work at
medium to high temperatures, as occurs
for similar plants treating emissions from
linker coolers. No specially high effi ciency is
needed since the dust concentration value
required downstream of the electrostatic
precipitator is still of the order of grams per
cubic meter. This solution will be applied to
all the new lines that are being designed for
Italian plants and for which the authorisation
process is in progress. The adoption
of this technique is expected to provide a
continuous NOx emission value of around
200 mg/Nm 3 under standard reference
conditions while ensuring continuity of
operation, equal throughput, reasonable
electricity consumption levels and, last
but not least, optimal preservation of the
catalyst. The emission level associated with
this technique corresponds to the value
specifi ed in the latest BAT document for
the cement industry, which updates the
2001 version. Among the BATs for nitrogen
oxides, the new document also indicates the
SCR technique, which, however, requires
further studies before its fi nal application.
New techniques for SO 2
emissions reduction
Though the problem of sulfur oxide
emission cannot be considered typical of
the cement industry, there are however
cases where the sulfi des present in the
raw materials may generate SO 2 emissions.
In fact, the sulfur added in the process is
oxidized at temperatures that are too low
to make its assimilation possible by reaction
with the alkaline materials present in the
kiln. It is well known, however, that sulfur