Organic pollutant removal from marine diesel engine off-gases under electron beam and hybrid electron beam and wet scrubbing process

Flue gas was generated from a tugboat ‘Orkāns’ equipped with two two-stroke diesel engines in Riga Shipyard. Sulphur-free fuel was used. Flue gas compositions were measured in three main points: at the inlet of the irradiation unit, at the outlet after irradiation unit and at the outlet of the plant after wet-scrubbing treatment. Two different types of flue gas analysers were used: Kane Quintox flue gas analyser (Kane Int. Limited, UK) at the inlet and outlet of the treatment plant and Land Lancom series II portable gas analyser (AMETEK Land, UK) after the irradiation unit. Concentration of the following elements of gas composition were determined by flue gas analysers: sulphur dioxide (SO₂), nitrogen monoxide (NO), nitrogen dioxide (NO₂), oxygen (O₂), carbon dioxide (CO₂), carbon monoxide (CO) and hydrocarbons (CxHy). The temperature-values of flue gas before irradiation, after irradiation and after wet-scrubbing treatment were 133°C, 85°C and 34°C, respectively. It was measured by using thermocouples type K manufactured by Czaki (Poland). Flue gas velocity was measured by Testo 452 anemometer produced by Testo, Germany. Flue gas of 4915 Nm³/h emitted from tugboat ‘Orkāns’ was treated by deploying a mobile accelerator unit WESENITZ-II [5]. Also, 3 m³ seawater containing 3.3 g/dm³ NaClO₂ oxidant was used as a wet scrubber solution. Temperature of the scrubbing solution increased from 27°C to 29°C after scrubbing flue gas. Organic pollutants’ removal from ship emissions have been studied under two processes, EB and EB hybrid wet scrubbing process. The residence time of flue gas in the irradiation zone and the wet-scrubber were 0.112 s and 2.2 s, respectively. Gaseous organic pollutants, mainly VOCs, were collected at three different sampling points: before irradiation vessel, after irradiation vessels and after wet-scrubber unit (Fig. 1). They were collected with glass sampling bottles, tedlar bags, Coconut Shell Charcoal (CSC) sorbents (SKC Inc., USA) and XAD-2 sorbents (SKC Inc., USA) according to USEPA Method 18 [6], and a scheme of the sampling system is presented in Fig. 2.

Fig. 1

Fig. 2

Fuel and combustion condition are the two main factors that influence the flue gas composition. In this pilot test, composition of flue gas emitted from diesel engine consists of SO₂, NO_x, CO, CO₂, particulate matter (PM), hydrocarbons etc., and this composition is similar to the composition of flue gas emitted from heavy fuel combustion [7]. SO₂, NO_x, CO, CO₂, O₂ and total hydrocarbons were directly measured with portable flue gas analysers. For gaseous organic pollutants, glass sampling bottles and tedlar bags were used to collect very volatile compounds having high concentration. Further, adsorbents were used to concentrate the sample for the low concentration compounds, thus lowering the detection limit of these compounds [6]. A GCMS-QP5050 (Shimadzu company, Japan) analyser was used for analysis. Standard solutions, such as AK-102.0-NAS-10X standard, M-502-REG and AK-101AA-ARO standard were used for making calibration curves. All these standard solutions were ordered from AccuStandard company, USA. For the off-gases sampled using Tedlar bags and glass bottles, 500 microliter sampling gas was directly injected into the GC-MS analyser. For the off-gases adsorbed by the sorbents, 10 ml CH₃OH (HPLC purity) and CH₃OH/CH₂Cl₂ (1:1, HPLC purity) were used to extract VOCs from CSC and XAD-2 sorbents, respectively. The solid-free extraction solutions were obtained by using syringe filters to separate extraction solution from sorbents. They were concentrated to 0.2 ml using a micro-extractor under continuously blowing high-purity Ar (Argon-X5OS PRM, air products, Poland). Then, 1 μl concentrated solution was injected into the GC-MS for analysis. HP-5MS column (30 m × 0.25 mm ID × 25 μm, Agilent J&W, USA) was used. The analytical conditions of GC-MS were as follows: 40°C held for 1 min, increased to 60°C at 1°C/min, then increased to 280°C at 5°C/min, column flow was 1.2 ml/min, split ratio was 100:1 when liquid sample was injected and the injection temperature was 250°C while the interface temperature was 280°C; solvent cutting time was 2 min. Electron ionization was applied and Wiley library was used for reference mass spectra.

Off-gases (or flue gas) before and after treatment (EB or EB with scrubber) collected with glass sampling bottles and Tedlar bags were directly analysed. A GC-MS spectrum of off-gases before irradiation is shown in Fig. 3. Seven unidentified peaks (due to the lack of standards or database of reference mass spectra in the library) were recorded in the flue gas. After EB or EB was combined with the wet scrubber treatment, no peak was detected.

Fig. 3

CSC and XAD-2 sorbents were used to collect the flue gas samples before and after treatment. Quantities of 106 L, 68 L and 112 L of flue gas were sampled at the three different sampling points (before EB, after EB, after EB with scrubber), respectively. The results are presented in Fig. 4. Table 1 lists organic compounds, which were identified by the GC-MS analyser, eluting at different retention time (RT). Note that “+” in Table 1 means organic compounds detected by the GC-MS analyser; and “−” means organic compounds below detection limit. From Fig. 4a and Table 1, it is seen that the identified organic compounds from ship emission include: nitro compounds (nitropropanone, 3-nitro-1-butene), esters (methyl butyrate, butoxyethoxyethyl acetate, (hexadecanoic acid, methyl ester), dibutyl phthalate, (octadecanoic acid, methyl ester)), aliphatic hydrocarbons (C_xH_2x+2, x = 12, 15–18, 20–21), toluene and chlorotoluene. Most organic compounds are removed from flue gas after the application of the EB treatment (see Fig. 4b). Chlorotoluene and nitro compounds are removed completely from gas phase; however, some aliphatic compounds (dodecane, hexadecane and octadecane) with high concentration (Table 3) still exist in the gas phase; butoxyethoxyethyl acetate and dibutyl phthalate were still detected by the GC-MS analyser. After the application of EB and wet scrubber treatment (see Fig. 4c), only toluene, hexadecane, octadecane and dibutyl phthalate were present in the gas phase, while other organic compounds are completely removed.

Fig. 4

To capture other organic pollutants which were not effectively adsorbed from flue gas by CSC sorbents, XAD-2 sorbent was connected after the CSC sorbents. Analytical results of the GC-MS were presented in Fig. 5 and Table 2. It is seen that 2,2-dimethoxypropane, methyl octanoate, octanoic acid and toluene were detected in the flue gas emitted from ship (see Fig. 5a and Table 2). After EB treatment (Fig. 5b), 2,2-dimethoxypropane (RT = 2.184 min) and methyl octanoate (RT = 31.208) were removed from flue gas. Then, after EB with wet scrubber treatment, only toluene (RT = 3.817 min) was present in the flue gas.

Fig. 5

We summarized the analytical results obtained from CSC and XAD-2 sorbents, and the removal efficiency of the organic pollutants after EB and EB with wet scrubber process are presented and listed in Table 3. For these organic compounds which we had standard solutions for making calibration curve, their concentrations are given in Table 3. For other organic compounds, only removal efficiency was given based on their area counting given by the GC spectrum. It is seen that the toluene concentration was relatively high, around 1.04 mg/m³ in the flue gas arising from ship emission. Concentration of aliphatic hydrocarbons varies from 9.60 μg/m³ (n-eicosane) to 57.96 μg/m³ (hexadecane). After EB treatment, most VOCs were removed from gas phase. When degradation efficiency for aliphatic hydrocarbons is considered, the compounds with lower carbon chain have higher removal efficiency, namely 79.59% for dodecane and 48.57% for octadecane. Removal efficiencies of butoxyethoxyethyl and octanoic acid were 89.7% and 57.8%, respectively. However, only 7.5% dibutyl phthalate was removed from flue gas after EB treatment. Eicosane and heneicosane were not detected after EB treatment due to their very low concentration in the flue gas. For those compounds with relatively high concentration, (e.g., toluene, hexadecane, octadecane and dibutyl phthalate) their removal efficiency greatly increased after EB with wet-scrubbing process, when compared to EB treatment alone, from 70.8% to 83.2% for toluene, 54.71% to 97.79% for hexadecane, 48.57% to 92.01% for octadecane and from 7.50% to 86.3% for dibutyl phthalate. Toluene concentration in flue gas emitted from ships decreased from 1.04 mg/m³ to 0.175 mg/m³ after EB with wet-scrubbing process.