The situation with greenhouse gases from composting and digestion plants for biowaste
Gewitra company is carrying out R&D project on determining gaseous emissions from different types of large scale treatment plants for biowaste in Germany.
Arrangements to optimise / minimise process emissions to air are Guidelines: 'Good practice of composting'. They are well known by operators but sometimes superior necessities swing a decision. On principle for a well done process the material characteristics should have structure (high porosity) and a water content: of max. 65-70 %, the C/N ratio should be 25-35 to avoid ammonia emissions. Important process parameters are water content: 50-60%, O2-supply, turning cycles (intensive phase 1-2 times per week, phase of declining activity 0.5 times per week). Windrow profile: height max. 2.50 m (active aeration), height max. 1.50 m (passive aeration), best available technology must be decided case-by-case.
Depending on the rotting milieu there is an opposed formation of CH4 (anaerobic) and N2O (aerobic) within the biological process. It is a principle that minimisation of the CH4 and N2O emissions to air is the result of the right material characteristics and the right process parameters for the entire time of aerobic treatment. Because there is no end-of-pipe technology to reduce CH4 and N2O in the waste gas afterwards, like scrubber and biofilter.
Arrangements for the emission control with acid scrubber and biofilter are shown to the components. For methane (CH4) there is only very less / no reduction in biofilters at suitable air loads > 50 m³/m³*h. For N2O and NO there are no reductions, but rather new generation due to NH3 degradation in biofilter. For Non-Methane Volatile Organic Compounds (NMVOC) there are normally good reductions of easily degradable compounds (~ 80%) at well operating biofilters and suitable air loads < 100 m³/m³*h. Ammonia (NH3) has a high deposition rate in biofilter, accordingly to new generation of N2O and NO, declining pH value -as a result of nitrification- reinforces accumulating NH4. Acid scrubber (H2SO4) precipitates NH3 > 90%, mostly necessary after anaerobic step. Ammonium sulfate from acid scrubbers could be used as fertilizer in agriculture.
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