The effect of sulphur addition on engine-out exhaust gas composition
is shown in Fig. 5. Except for SO2 concentration, which
obviously depends on sulphur content in the fuel, all the other
components follow similar trends independently of the fuel used,
suggesting that the engine combustion regimes are substantially
unaffected by addition of sulphur to the fuel. Thus, all the modifications
of the exhaust composition after the catalyst and after the
scrubber are directly linked to the performance of the system
rather than to any variation in the engine regime.
Table 2 summarises the data collected for NOx species for the
three fuel compositions tested at the various loads (temperatures),
measured at the P1, P3 and P4 sampling points, that is before-catalyst,
after-catalyst and after-scrubber, respectively. The NO concentration
is the difference between NOx and NO2 values. It
should be noted that the data obtained with normal diesel fuel differ
from the other two in that the scrubber was not employed and
the tests were was mainly intended to check the baseline engine
emissions and the functioning of the catalyst. In addition, a limited
number of measurements were performed at P2 (compare Fig. 7
below) to investigate the ‘‘progress’’ of conversions across the
two catalyst beds.
The conversions observed between P1 and P3 represent the
overall efficiency of the catalyst. Of particular interest in this context
is the ability of the catalyst to convert NO to NO2 for potential
removal through absorption in the scrubber section. The overall
behaviour of the catalyst with regard to the conversion of NOx species
and conversion of NO to NO2 over the catalyst is illustrated in
Fig. 6. Analysis of the data indicates that the catalytic behaviour
may be divided into three regimes (vide infra). Within this framework,
the effect of increasing sulphur content in the fuel on the catalytic
activity is also evidenced. The three catalytic regimes
observed for regular diesel fuel (Fig. 6a) are as follows: