Andreas Arlt

Processing of biowaste to biofuels - comparison of process chains for sewage sludge, organic and woody waste using a systems analysis approach

Karlsruhe: Forschungszentrum Karlsruhe 2003
(Wissenschaftliche Berichte, FZKA 6949)
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As organic waste amounts to 80 mio. MG dry mass per year and shows a high content of renewable biomass, the use of organic waste as a biofuel could considerably contribute to Germany's renewable energy supply and help meet the political goal to reduce 1990s CO2-emissions by 25 % in 2005. However, organic waste can hardly be used for energetic purposes in its original state. It has to be treated using a process chain consisting of technical components (dewatering, drying, chopping, shredding, etc.) and logistical components (transportation, collection, storage), requiring an enormous technological, economic and energetic effort. In the present study, the conversion of organic waste into biofuel was investigated for three different waste types: sewage sludge from municipal sewage treatment plants, separately collected biowaste from households, and woody waste from private gardens, municipal parks and landscape maintenance (hedges and trees, e.g. along country roads, between fields and on nature reserves). The goal of the study was to show which process technologies are available on the market, what costs are involved and how much energy is consumed to produce a storable, microbiologically inert bioful from organic waste. A systems analysis approach was chosen to conduct the study using complete cost calculations and mass and energy balances as tools. Calculations were based on the latest data collected from 200 different machine-producing companies and plant operators.

The results show that processing technologies are widely used on the market (exception: drying of biowaste and its co-fermentation with sewage sludge). The resulting costs for processing and disposal are comparable to other disposal alternatives, such as dumping or producing compost. Depending on plant size, costs vary between 100 and 170 €/MG dm for woody waste, 150 and 380 €/MG dm for sewage sludge and 400 and 800 €/MG dm for organic waste. The energy balance of the whole process chain is positive for all process chains investigated. The following amounts of energy can be produced by the energetic utilization of the biofuels (including the calorific value): for woody waste about 3.0 MWh primary energy (PE)/MG dm, for sewage sludge about 2.0 MWh PE/Mg dm and for biowaste about 1.3 MWh PE/Mg dm. If the available amount of these three waste types is used for energetic purposes alone, about 1.1 % of the nation's electricity demand can be covered and the emission of carbon dioxide can be reduced by 4.0 mio MG. Processing consumes however about 50 % of the theoretical energy content of the organic waste. The energetic demand should therefore be minimized as far as possible by directly connecting the drying and the thermochemical plants (combined heat and power concept), by widespread use of solar dryers, by the optimisation of the dewatering process and by the application of railbound logistic concepts. These energy saving technologies and their share in the market should be developed and supported accordingly. Potential negative effects of energetic utilization (mercury emissions) should be avoided by appropriate technologies (e.g. exhaust gas treatment).

Created: 20.02.2004   Last update: 20.02.2004   Comments to: