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The knowledge of the explosion limits of biogases in air is necessary to define explosion protection measures. Biogases from agricultural plants vary however in their composition, so that for each gas composition the explosion limits would have to be measured. Frequently the explosion limits of biogas are therefore referred in the safety-relevant literature only to the methane fraction of the gas mixture, without indicating this additionally. This leads to the fact that the explosion ranges for the total mixture consists of methane, carbon dioxide and further residual gases are falsely specified.
For this reason a simple method is presented in the following, which show, how the explosion limits of a biogas can be determined from its methane content using an explosion diagram of methane/carbon dioxide/air system. The explosion diagram was measured according to the German Standard DIN 51649-1.
The presented calculation method supplies however correct results only if the nitrogen fraction in the biogas does not exceed approx. 5 % by volume.
The paper discusses a new method to approximate the sometimes missing apex point of the explosion limit curves of flammable substances with diluents in air. The base of the new method is to vary the frame points of the co-monotonic Splines using the de Casteljau algorithm. We show several examples for flammable/inert/oxidizing gas containing systems - selected by the program TRIANGLE - where the method was applied. Due to the definition of the frame Splines it can be stated that the new method never restricts the explosion range around the apex and shifts the explosion limit curve into the direction of higher inert gas concentrations. This means that the new method can correct the highly "cut down nose" of the co-monotonic Splines and gives a more safer explosion range of these systems.