3.1.3. Effect of catalyst on CO, HC, PM and SO2 emissions
Table 3 reports the results concerning emissions of the other
main pollutants in the gas phase at the P1–P3 sampling points of
the combined abatement system, that is, points related to the catalyst
section. In this case, the regular diesel fuel (10 ppm S) was
employed. The catalyst shows typical features of an oxidation catalyst
in that CO is fully oxidised: no CO was detected after the catalyst
in any of the experiments. The elimination of HC and PM was
somewhat less efficient under the present reaction conditions. An
important aspect, which is in line with NOx conversion above discussed,
is that all the conversion is essentially achieved over the
first catalyst bed. This indicates that much higher GHSV could be
employed with comparable conversion levels. Dimension and price
of the catalyst are fundamental if extension of the present technology
to main power engines is to be considered.
When the high sulphur fuels are considered, CO is fully converted
over the catalyst (data not reported), whereas HC conversion
depends on the temperature, reversible deactivation of the
catalyst being observed below 350 _C if compared to sulphur-free
fuel (Fig. 9). Above 400 _C comparable activities are observed both
in the presence and absence of sulphur in the fuel. This is consistent
with a SO2 adsorption/desorption model in which above
400 _C the catalysts is effectively desulphated under the exhaust
conditions [56].
It is well known that DOC promotes SO2 oxidation [39]. Fig. 10
reports the data obtained for fuel with sulphur content of 0.4% S.
As would be expected, given the significant NO oxidation activity,
very significant SO2 oxidation was also observed. This can be
attributed to SO2 oxidation to SO3. SO3 was not directly measured,
accordingly its formation was calculated from the disappearance of
SO2.
In the case of the 2.0% S fuel, the high sulphur content of the fuel
means that the base levels of SO2 are much higher. Once the engine
load was increased to 50% load, a sulphuric acid mist was generated
which did not allow completion of the tests. This was not
the case for the fuel containing 0.4% S. As can be seen in Fig. 10,
conversion of SO2 to SO3, that is, production of SO3, remained
below 100 ppm throughout these tests. In the case of the higher
sulphur content, as already stated, the formation of the acid mist
occurred on increasing the engine load to and above 50%. It is interesting
to note that at 18%, while the amount of SO2 formed is, as
expected, high, conversion to SO3 remains low due to the low
temperature of the catalyst and less than 100 ppm of SO3 were
produced. On increasing the engine load to 50%, the amount of conversion
increases dramatically SO3 (>100 ppm) suggesting this
value as a limit under the scrubber working conditions used.