*3.1. Scrubber emission reduction e*ffi*ciency*

The scrubber efficiently reduces emissions of sulphur dioxide in the exhaust gases. The SO2 emission factor at HFO tests is reduced >99% over the scrubber, at all engine loads.


**Table 2.** Specific emissions at the different tests. Coefficients of variation (cov) at continuous measurements are presented in italics. MCR = Maximum continuous

(PAHs); n.d. = no data; b.d.l. = below detection limit.

Around 1%–8% of the sulphur oxides that form during the combustion in a diesel engine are sulphur trioxide SO3 [17]. SO3 will react rapidly with water in the exhaust gas to form gaseous H2SO4. Condensed H2SO4 can cause corrosive damage in locations with low temperatures in the cylinder and the exhaust channel and levels are preferably kept at a minimum. The temperature at which the H2SO4 condenses depends on the concentration of SO3 and H2O in the gas. In a wet scrubber, the exhaust is rapidly cooled to below the acid dew point; the hot exhaust in our measurements were approximately 250 ◦C and reduced to 20 ◦C, over the scrubber. Since the rate of cooling exceeds that of H2SO4 gas absorption in the scrubber fluid, it has been suggested that sub-micron H2SO4 particles are formed [18]. A reduction of these particles in the scrubber is dependent on mass-transfer through Brownian di ffusion. As a removal mechanism, it is not e fficient enough to remove all H2SO4 from the exhaust gas [18].

The concentrations of gas phase sulphuric acid and sulphur trioxide (H2SO4/SO3) in our tests were significantly reduced over the scrubber. At 76% engine load there is a removal of 78% of the H2SO4/SO3. At the two lower engine loads the reduction is less, 61%–63%. The observed reduction efficiency for SO2 of over 99% for all loads indicates that the scrubber is less e fficient in removing H2SO4/SO3 than in removing SO2. The measured H2SO4/SO3 concentrations are higher than the SO2 concentrations in the tests downstream of the scrubber. Metals such as V and Ni, present in the exhaust, can act as catalysts for the oxidation of SO2 to SO3, which can be a reason for the higher SO3 levels at combustion of HFO [19].

There is also a reduction of the CO concentration over the scrubber. At lower engine loads this is more pronounced. We saw minor di fferences in the emissions of NOX upstream and downstream of the scrubber at low engine loads. These di fferences are, however, most likely more related to the di fferent engine loads than to scrubbing of NOX. The engine load is around 32% at tests upstream of the scrubber and 41% at tests downstream of the scrubber.

Furthermore, specific PM emissions at HFO combustion are reduced over the scrubber. Reductions are approximately 34% at 76% engine load, and 42% at 48%–49% engine load. These reductions are in line with a central value of PM reductions in a joint analysis of previous studies, although values to compare with are few and results vary significantly.

As presented in Table 2, both BC and EC measurement indicate that there are reductions over the scrubber. BC results are not as consistent as EC results, although also these indicate a removal rate that increase at low engine loads.

Particle number (PN) concentrations were measured only at tests on HFO. Measurements were conducted using a thermodenuder (TD) to remove volatile species. By removing a majority of the volatiles with the TD (some organic matter can still remain at 300 ◦C), the large uncertainties coupled to the nucleation of volatile particles during the sampling dilution process is eliminated. The fraction of the solid particles that remains downstream of the exhaust gas scrubber can therefore be argued to give a more reliable value on cleaning e fficiency than total particles. No clear reducing e ffect on solid PN concentration by the scrubber is seen, see Table 2, although the size distribution is changed. Size distributions of solid particle number concentration are presented in Figure 3.

Recalculating particle numbers in the di fferent size channels to particle mass indicates higher specific emissions downstream than upstream of the scrubber at 75-76% engine load. This can possibly be explained as salt formation during the scrubber process. The increase in mass despite the loss in EC over the scrubber further emphasizes that there might be an addition of solids during the process. There is no indication of increases in solid particle mass at tests at the lower engine loads, see Table 3.

While upstream of the scrubber the thermodenuder tests indicate large part of particles being volatile (50–85% by number), measurements downstream of the scrubber give comparable number concentrations of particles with and without the thermodenuder. This could be due to a high hydrophilic content of the particles, which would cause them to react with, and be removed by, the scrubber liquid during the scrubbing process to a large extent. Additional explanations include that as the exhaust temperature decreases in the scrubber stack, the volatile compounds condensate on any available

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surface, such as water droplets and solid particles, and the effect of the reheating of the cold sample in the dilution system.

**Figure 3.** Particle number concentrations of solid particles measured in diluted exhaust gas upstream and downstream of the scrubber at different engine loads. A thermodenuder is used to remove volatile particles; DR = dilution ratio.

**Table 3.** Particle mass calculated from particle number size distribution for the size range 5.6–532 nm. Engine loads during trials are indicated. Cov of emission factors (EF) in parenthesis.


Calculated from measured PN concentration in different size channels of the EEPS.

There was a significant decrease of total particle number over the scrubber (Table 2). At the highest engine loads tested (75% and 76% MCR) the reduction was 79% and at the medium engine loads tested (48% and 49% MCR) the reduction was 82%. The lowest engine loads are not fully comparable since one test is run on 32% MCR and the other on 41% MCR, but the measurements indicate a significant reduction also at low loads.

There is also a reduction of the total PAH-16 concentrations over the scrubber at all engine loads, Table 2 and Figure 4a,b. The specific emissions of total PAH-16 are in most cases higher at lower engine loads.

**Figure 4.** Specific emissions of US Environmental Protection Agency's (EPA) PAH-16 (**a**) and PAH-7 (**b**) at HFO combustion upstream and downstream of the scrubber.
