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Determination of biogenic fraction in solid and liquid fuel by the 14C method

An increased level of CO2 concentration in the atmosphere is a direct consequence of intense fossil-fuel combustion over a last century. Search for alternative energy sources and reduction of emission of CO2 from fossil fuels by production of energy from biogenic material are topics of intense current studies. The “environmentally kind politics” of the European Union stimulates the production and use of biofuels by lower excise and income tax relief. In this context there is a need for control of producers and dealers of various types of fuels and independent determination of the biogenic fraction in fuel. Here, the term “biogenic” refers to a compound produced in natural processes by living organisms but not fossilized or derived from fossil resources. The European Standard EN 15440:2011 specifies three normative methods for the determination of the biogenic fraction in solid recovered fuel (a mixture of biogenic and non-biogenic substances): the selective dissolution in a hydrogen peroxide/sulphuric acid mixture, the manual sorting method, and the method based on 14C content. The 14C method of the biogenic fraction determination is based on different 14C content of biogenic and fossil compounds: bio-fuels reflect the 14C activity of the atmosphere during the growth period due to the use of atmospheric CO2 for the photosynthesis, while fossil fuels do not contain 14C. Therefore, a simple linear mixing curve with well-defined end members can be used to determine the fractions of biogenic and fossil carbon within a product. The 14C method is suitable for samples of all types of fuel, both solid and liquid. Any of the measuring technique for 14C content, widely used in the radiocarbon dating laboratories, may be used: (i) benzene synthesis and measurement by liquid scintillation counter (LSC), (ii) absorption of CO2 in a scintillation cocktail, followed by LSC measurement, or (iii) conversion to graphite and measurement by accelerator mass spectrometry (AMS) technique. All these techniques are accurate and precise, but are also time-consuming and expensive, so there is a need for simple, fast, reliable, sensitive, accurate, and not expensive technique. When liquid fuel is concerned, a direct measurement of fuel mixed with an appropriate scintillator by LSC is possible. The advantages of direct measurement are fast sample preparation and low analysis cost, but there are still some problems before the method can be standardized, such as higher uncertainty and the problem of colour. This paper will compare advantages and disadvantages of the 14C techniques used for determination of the biogenic fraction of various types of fuel, present the techniques used in the Zagreb Radiocarbon Laboratory and discuss some results obtained for both solid and liquid fuel.

14C; biogenic fraction; fuel; liquid fuel

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