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Etamax: Driving with biogas from biowaste

The utilization of vegetable biomass for the recovery of bioenergy - power, heat or fuels - plays an exceptional role as a sustainable alternative to conventional energy carriers. Biogas, a mix of energetically usable methane and carbon dioxide, is created during the anaerobic digestion of organic matter. In conjunction with the combined heat and power generation, biogas generation is considered as a technology with a high net energy yield and a high CO2 avoidance potential.

This project has focused its activities on easily fermentable, low-in-lignocellulose, wet biomass - low-cost biowaste and residual algal biomass which constitute no competition for the production of foodstuffs - with a combined, modular process under maximum energy recovery. The aim is the complete conversion into biogas and the simultaneous closing of all materials cycles. The main focus is on the regional creation and utilization of bio-methane. The biogas is to be purified by separation of the carbon dioxide with membrane technology so that the bio-methane can be used as fuel for vehicles which are operated on compressed natural gas (CNG). The potential of biomass for the creation of biogas has so far been insufficiently exploited in general and hardly at all in vehicles. To ensure that a digestion plant can convert the different substrates as efficiently as possible into biogas, the process technology for the individual substrates is specifically adapted by means of a flexible multi-substrate high-load digestion plant. Only by this the substrate can be converted into methane with the maximum degree of efficiency. During the investigations unsorted market waste from the Stuttgart central market was provided, disintegrated and digested. Our investigations showed that the fluctuations in the composition of the substrate could only be compensated by means of intelligent process control: by adding substrate portions of different digestion stages via several pre-digestion tanks, we can ensure a continuous biogas production with few variations despite the great fluctuations in the substrates. The investigations in the pilot plant enabled us to determine successfully permissible values such as the minimum detention period and maximum volume load and parameters for the ideal feed of different substrate compositions. In a high-load digestion process developed by the Fraunhofer IGB the solid matter biowaste fractions which are low in lignocellulose are almost completely converted into biogas within a hydraulic retention time of only 8.5 days. During the first year the process parameters were determined in a digestion plant at pilot plant scale (2 x 30-l-reactors) at the Fraunhofer IGB. The two-stage pilot plant produced 850 l of biogas from central market waste per total volatile solids (TVS). In terms of the existing reactor volume this equates to an average of 190 l biogas per day at a volume load of 7 g TVS/ld. Based on those results the demonstration plant scale (2 x 3.5 m³) was designed. Additional wet, low-lignocellulose biomass for multi-substrate high-load digestion is contributed by means of residual algal biomass. Energy recovery from algal biomass is already possible in a highly efficient manner today thanks to a photobioreactor platform developed by the Fraunhofer IGB. The algae in the reactors grow to high cell densities using only sunlight as an energy source and carbon dioxide as a carbon source plus inorganic nitrogen and phosphate. The aim is to find robust algae which grow rapidly in flue gas, and in the changing seasonal light and temperature conditions in central Europe. EtaMax utilizes carbon dioxide which is created as a co-product during the digestion process and during the combustion of biogas as a source for the cultivation of algae. Current investigations have shown that the inorganic nutrients necessary for growth are contained in sufficient quantities in the filtrate of the digestion plant and can be used for the cultivation of algae. Expensive nutrients will therefore not be necessary. For small amounts of digestion residues which cannot be further decomposed in an anaerobic environment, the catalyst-supported hydrothermal gasification under high pressure and high temperature is examined. Here the same products are created as during the digestion process: carbon dioxide and methane. The joint factor of all individual components to be utilized is the minimum energy input for the implementation of the individual tasks. In the end of 2011 the findings determined at pilot plant scale were converted to a demonstration plant on the site of the EnBW combined heat and power station in Stuttgart-Gaisburg where they will be tested. In a future full scale plant it would be possible to generate 300,000 cubic meters of methane gas per year from the municipal biowaste from the city of Stuttgart. After purification it can be used as vehicle fuel for a small fleet of collection vehicles which run on natural gas. This would also be beneficial for local air quality.

Copyright:	© European Compost Network ECN e.V.
Quelle:	Orbit 2012 (Juni 2012)
Seiten:	8
Preis:	€ 8,00
Autor:	Dr.-Ing. Ursula Schließmann
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