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Chemical tests - Sulphur, chlorine and nitrogen content

To assess biofuel quality it is of high importance to know the concentration of sulphur, chlorine and nitrogen in biofuels. This is particularly true for biomass conversion processes. Contents of sulphur and chlorine in biofuels are of relevance for corrosion and fouling, for emissions of SOx, HCl and PCDD/F as well as for aerosol formation. The concentration of nitrogen causes NOx emissions. Standards defining analytical methods for the determination of these elements are not available for solid biofuels so far. Hence, different approaches and procedures are in use. The individual laboratory methods and system devices applied are originally designed for the analysis of coal samples, whereby certain deviations result.

Concerning sample preparation, for S, Cl and N analysis a particle size of < 1 mm is sufficient in most cases. Smaller particles increase the repeatability but also increase the risk of contaminations with metals (i.e. from the inner materials of the used mills). The reproducibility of chemical analyses is usually improved when larger sample amounts are used (e.g. 1 g for the determination of S and Cl). Furthermore, in laboratory practise it should be especially considered that (i) the samples are always be within the calibration, and (ii) if low nitrogen concentrations are to be analysed, an increase in sample amount may improve the result.

Besides this, the method evaluation for element analysis led to the conclusions and recommendations given below. Basically, except for the Kjeldahl method, all of the identified most promising methods such as IC ("ion chromatography"), ICP ("inductively coupled plasma") and titration as well as the automated analysers are recommended for standardisation. They are described in detail in the corresponding best practice guideline and draft standards.

Chlorine and sulphur determination:
Sample combustion in an oxygen bomb and the quantification of sulphate and chloride in the receiving solution is currently the best method. It enables the application of procedures presently standardised on European level.

For chlorine, the determination applying the method of water-soluble chlorine led to similar results compared to the bomb combustion method (at least for all untreated biofuels investigated). For samples characterised by a high ash content even higher values were obtained.

In general, the required repeatability and reproducibility can only be obtained when strictly abided to the standard procedures. Solid biofuels that are characterised by low concentrations of sulphur and chlorine are difficult to analyse. Since currently the reproducibility is not satisfactorily, there is more need on research and method improvement. Coupled with this, the INAA method (¿irradiation neutron activation analysis¿) may be of interest for further scientific investigations, as for instance absolute values can be obtained. Furthermore, this method could contribute to improve and validate analytical methods.

Nitrogen determination
From the comparison of the obtained results for automated analysers (different and same brands and types) and for the Kjeldahl method the following was revealed.

With respect to certain types or brands of automated analysers available no systematic deviations were found. However, there were differences between the participating laboratories (persons operating the systems). Thus, for the obtained results the operation and especially the calibration is critical. Nevertheless, automated analysers are suitable for the nitrogen determination in solid biofuels and thus, involved in respective draft standardised. Since recognising comparable results for different designs of automated analysers, no specific system design is recommended in the draft standard. However, the applied apparatus should meet the functional requirements. No influence of moisture content on the nitrogen concentration could be found. Hence, it is not necessary to used dried samples that are difficult to handle since they are in general very hygroscopic.

Against the standardised methods developed so far, the work has generally revealed that there is further need on improvement with regard to the test methods, the lack of reference material and laboratory experiences as well as on own laboratory practice. Final recommendations involve to further investigating the chemical fuel concentrations of bromine and iodine (especially e.g. for recovered fuels, fruity biomass and seaweed).

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