*3.2. From an Environmental Perspective*

The measured SO2 emission factors with a scrubber and HFO fuel are lower than when the engine use LSFO, 84% to 96% lower from high to low engine load. Significant differences in emissions between the use of HFO combined with a scrubber, and LSFO, are observed for a number of gases (Table 2). There is a significant reduction of the total hydrocarbon (THC) concentration over the scrubber and specific emissions downstream of the scrubber are lower than at LSFO combustion for all engine loads tested. Emissions of CO and NOX are however lower for LSFO combustion than for HFO combustion combined with the scrubber.

Average specific PM mass emissions at combustion of LSFO with 0.1% sulphur are lower than PM emissions downstream of the scrubber when using HFO. Comparisons at 75% (LSFO) and 76% (downstream) engine loads indicate emissions are 59% lower at LSFO combustion. At engine loads of 50% (LSFO) and 48% (downstream) the specific PM mass emission is 36% lower.

Compared to previous studies on particle emissions from low sulphur fuels, our results are in the lower end. PM emissions increase with sulphur content of fuel due to the sulphate content of particles. However, the correlation between particle emissions and sulphur content in fuel is weak for fuels with sulphur content below <0.5%. Emission factors derived for fuels with sulphur contents in the interval 0–0.5% give a mean value for the PMtot emission factor of 0.2 g/kWh [2,8,11,14,20–27], compared to 0.12–0.17 g/kWh in our study. Other characteristics of fuel that impact particle formations include density, since a high density will increase combustion thermal efficiency, and viscosity, due to its impact on atomization during combustion. The low sulphur fuel oils will have higher density, increasing combustion efficiencies and oxidation of particles, and higher viscosity, which cause higher particle formation compared to distillate oils (Gysel et al). It is further difficult to relate how differences between engines and measurement setups influence the results. In two published studies, emissions from low-sulphur fuel oil and distillate oils have been tested on the same engine; Gysel et al., tested an MGO and a low sulphur (≤0.5%) fuel oil in the same engine and observed three times higher PM emissions from the latter [27]. Sulphur contents were 0.005% in the MGO and 0.009% in the LSFO. Winnes et al. observed similar differences in particle emissions when testing an MGO with 0.1% fuel sulphur content and a LSFO with 0.5% sulphur in two engines [20].

Specific emissions of both BC and EC downstream of the scrubber seem little influenced by engine load. Although some BC is removed by the scrubber, the specific emissions are higher than those from LSFO combustion by approximately 1.5 to 4 times.

PAH emissions are in our tests lower at LSFO combustion than at HFO combustion downstream of the scrubber, at each comparable engine load. The share of different PAH species is similar for all loads. Naphthalene is the most abundant and accounts for around 50% or more at all trials. The two- and three ring PAHs constitute between 78% and 91% of total PAHs, which is close to, but higher than, the 76% concluded as a typical value for diesel engine exhausts reported by [28]. The emissions of heavier PAHs, which can be represented by the EPA PAH-7 priority carcinogenic species, are at all occasions higher for LSFO than downstream of the scrubber. For the PAH-7 species an increase in emissions with decreasing engine load is observed at the LSFO tests. Specific emissions of US Environmental Protection Agency's PAH-16 and PAH-7 for tests at LSFO combustion and HFO combustion downstream of the scrubber are compared in Figure 5a,b, respectively.
