Composition of in situ burn residue as a function of weathering conditions

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Abstract

Troll B crude oil was weathered under Arctic conditions with different ice coverage: open water, 50% ice and 90% ice. Samples (100 mL) were taken during the experiment and tested for ignitability in a burning cell. From each burning a residue sample was taken for analysis. The burning process removed the light compounds eluting before C13. No effect from the prior weathering time or the different ice coverage was seen in the burn residue composition. The content of selected Poly Aromatic Hydrocarbons (PAHs) was determined and it was noted that the concentration of PAHs with more than 4 rings were increased. The source origin of the PAHs was investigated by use of relative ratios of PAH isomers and indicated that some formation of PAHs was additionally taking place during burning.

Highlights

► Burn residue composition is not influenced by weathering time or ice coverage. ► The burning process removed the light compounds before C13. ► Concentration of PAHs with more than 4 rings were increased in residue. ► Some formation of PAHs was taking place during burning.

Introduction

In situ burning (ISB) of oil spills is a method with large potential, especially for oil spills in Arctic ice-filled waters. Experiments with ISB in recent years have shown removal efficiencies of up to 90% or even higher in some cases (Brandvik and Faksness, 2009, Buist, 2003, Buist et al., 1999, Guenette and Sveum, 1995, Guenette, 1997). The removal effectiveness and time window for ISB depends on oil type, weathering conditions and the thickness of the oil layer. Some concern however, is related to the effects seen from the burn residue, but the literature that exists is sparse. The Newfoundland Oil Burn Experiment (NOBE) from 1994 addressed issues related to ISB, where one part was to study the aquatic toxicity to the organisms in the water column (Daykin et al., 1994). The toxicity from the ISB residue was found to be minor and not beyond the effect already seen from the oil spill (Blenkinsopp et al., 1997, Daykin et al., 1994). Gulec and Holdway (1999) tested the toxicity of laboratory burned oil and found that the toxicity related to ISB was less than that was found by use of dispersant. However, the literature is sparse, few oil types has been used and therefore, as pointed out by Blenkinsopp et al. (1997), there is a need to test more oil types to see if the trend is general.

Few studies have characterized the burn residue (Holland-Bartels and Pierce, 2011); however it is essential background knowledge to be able to evaluate the effects of ISB operations. Li et al. (1992) burned 7.6–19 m3 crude oil, and found that the burn residue had the same gas chromatographic (GC) profile as the fresh crude, but there was a notable depletion in the lighter components. Lin et al. (2005) burned crude oil and diesel oil in marshes (laboratory experiments) and found that the burning decreased the total targeted alkanes and polycyclic aromatic hydrocarbons (PAHs). The concentration of alkanes heavier than C22 and C26 for diesel and crude oil, respectively, and PAHs with 4 or more rings was increased in the residue. This is explained by lower removal rates, decrease in oil volume and, for the PAHs, generation of PAHs during the burning (pyrogenic) (Lin et al., 2005). Small-scale burnings (1.25 L) of Statfjord crude oil showed a decrease in all the PAHs on the list of U.S. Environmental Protection Agency (1998) and an increase in the pyrogenic compounds (compounds generated during the burn) due to the reduction in the total amount of oil during burning (Garrett et al., 2000). Garrett et al. (2000) also showed that the alkane C30 was almost constant in the residue; hence the temperature in the residue had not exceeded 450 °C. Trudel et al., 1996, Buist et al., 1997 studied small-scale burns of oil pools on water with 8 different oils as a function of oil thickness; two of the oils were weathered by aeration. The residue was characterized by fractionating into three ranges dependent on boiling point. The low boiling point fraction (<204 °C) was completely removed and the middle boiling fraction (204–538 °C) was reduced. The higher boiling fraction (>538 °C) was increased. The residue contained some solids and deviated from the fresh oil.

In spite of the research already conducted with regard to the composition of the residue, there is still a lack of knowledge and need for more research. This is emphasized in the report by Holland-Bartels and Pierce (2011) who recommend that there is a need for a better characterization of the residue from ISB. The purpose of this study was to establish knowledge regarding the influence on the weathering degree (time) and weathering conditions (ice) on the residue. The residue samples were from a series of burns with Troll B crude oil, weathered at Arctic conditions, with different ice conditions and for different periods of time. The residue was analyzed by Gas Chromatography/Flame Ionization Detector (GC/FID) and Gas Chromatography/Mass Spectra Detector (GC/MS).

Section snippets

Methods

Troll B crude oil was used for the experiments. Troll B crude oil has origin from the Troll field in the northern part of the North Sea. Troll B is high in naphthenic components (cyclic and branched saturated hydrocarbons). This is caused by microorganisms that have degraded the linear hydrocarbons in the reservoir, yielding a very low paraffinic content and a relatively high content of naphthenic components. The naphthenic Troll B has a low pour point due to the low content of wax (naphthenic

Ignitability of samples

An overview of the weathering time in the flume and the ignitability of the samples for the three experiments are shown in Table 2. The ignitability of the oil changes with time and at a certain point the oil becomes “not ignitable”, thus no burn residue will be produced.

For the open water experiment the oil was ignitable until 20 h of weathering, thereafter the oil was not ignitable within the standard ignition procedure. The weathering processes are slowed down with less energy in the system

Conclusion

The analysis of the burn residue showed that all the light compounds with a boiling point up to 230 °C were removed during the burning and that the samples that could not be ignited had a less significant depletion of light compounds. Regarding the PAHs the result showed that the content of 2 ring PAHs were reduced and the concentration of 4–6 ring PAHs were increased in the burn residue samples. The source origin of the PAHs was investigated and indicated a formation of 3 ring PAHs during

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