ABSTRACT

Starting in 2007 the regulations in the Netherlands state that renewable fuel has to be added to fossil diesel in the context of energy and climate policy. At present, the statutory rate designed to be 7%. Despite the sympathetic image of the use of bio-components, it also has important disadvantages with relation to corrosion for underground steel storage tanks (USTs) which are uncoated and therefore vulnerable for corrosion. This paper describes the results from a current study in which historical data of 1,401 USTs were analyzed for average maximum pit depths over the years 2003 till 2016. Furthermore tank sludge and wall samples from two tanks were analyzed using Next Generation Sequencing to identify microorganisms, to test the hypothesis that MIC has played a role in the formation of the pitting corrosion defects in USTs. The results showed that since the addition of biodiesel the average pit depth increased. The microbial data showed that MIC was likely involved in the formation of the pits and that sampling location is extremely relevant for analysis of MlC. The data also revealed that analysis on the most common MIC suspected microorganisms like sulphate reducing bacteria might not be a sufficient target for USTs.

INTRODUCTION

With the increasing concern about the environment and the understanding that vehicles that use diesel partially produced from vegetable oils or animal vet, called biodiesel, burns cleaner than fossil diesel, biodiesel is considered a good and more sustainable alternative. The use of biodiesel leads to a less harmful exhaust emissions. National regulations are adjusted towards sustainability. Consequently, in the Netherlands, starting in 2007, regulation state that renewable fuel has to be added to fossil fuel in the context of energy and climate policy. Properties of biodiesel are different than properties of pure fossil fuels. One of these differences is that they have the ability to absorb more water. Water can enter the storage tanks mainly from three different sources: infiltration, temperature affected solubility, and condensation.1 Infiltration can occur for example during rainy periods or during fuel loading of the tanks. The temperature is also a source of water accumulation since warmer diesel can hold more water, resulting in the excess of water when the temperature of the diesel drops. Finally, condensation is a source of moisture in fuel tanks, originated from the aeration side connecting to the atmosphere, which results in condensation every time the atmosphere temperature falls down below dew point. The increased amount of water in diesel increases the risks for chemical corrosion as well as the risks for MIC. The difference between chemical corrosion and MIC is that chemical corrosion due to the presence of water is slow and in general homogeneous. MIC is in general fast and local and therefore difficult to predict. Especially in the higher segments since less turbulence is expected due to the location of the filling point which is at the lowest point. Without the effect of turbulence, microorganisms have the ability to adhere to the surface of the tank wall and to grow in time resulting in a firm biofilm. This biofilm is ideal for MIC related microorganisms since it provides an unique and protected environment in which they can live and thrive, leading to severe corrosion defects.

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