BioCycle April 2006, Vol. 47, No. 4, p. 76
Investment in the biowaste sector is increasing, prompted by legislative drivers and a growing interest in energy recovery and treated by-products.
OPERATION of a successful biogas plant depends on a positive interaction between three components: the raw material, the operator, and the technology. For operators, it is beneficial to use the broadest possible spectrum of biowaste (separated organic residuals) since this offers the greatest scope for the generation of energy-rich substrates with high gas yield. But a biogas plant running on food residues, animal by-products and other organic wastes places high demands on the operator.
Because the type, consistency, quality and amount of raw material can change on a daily basis, the facility planner must be flexible in plant design from the outset. Only with this flexibility can the operating personnel control processes optimally for each raw material. Furthermore, contaminants such as packaging, stones, ceramics, metals or sand can cause technical problems in fermentation (biogas production). As a result, selective treatment stages are needed to produce a more uniform substrate.
By using the right technologies, heavily-contaminated market waste and expired food can be treated in a fermentation plant alongside less-contaminated biowaste and food residues. The main contaminants and treatment strategies are compiled in Table 1. The treatment strategies include technologies available from a variety of vendors. The specific treatment train described in this article reflects technologies used by Komptech. The Komptech biogas production technology utilizes a wet fermentation process (versus anaerobic digestion).
SHREDDING AND SEPARATION ALTERNATIVES
The first stage in treating contaminated organic waste is shredding – used for coarse preshredding and tearing open packaging. Waste then can be dissolved or pressed to separate the fermentation substrate. As indicated in Table 1, the dissolving process is suitable for most types of biowaste, including heavily contaminated biowaste. In contrast, the pressing process is particularly well-suited to the treatment of separately collected organic household waste, but is not suitable for expired food because the associated packaging material cannot be pressed properly.
There are two options for the wet phase of separation, and preparing the biowaste for fermentation:
Dissolving biowaste and use of a wet screen: The shredded waste is transported in batches into a round, closed container where it is diluted with process water and agitated well. The resulting shear forces separate the contaminants from the substrate. Unwanted heavy materials sink and are discharged through a sluice to a special hopper. The liquid and lighter materials (10 percent solid content) arrive at a wet screen and oversized particles such as fibers and packaging residues are pressed out. The remaining liquid is fed into a hydrocyclone for sand separation. This process helps to maximize biogas production, with 70 to 80 percent weight of the initial input material (depending on the quality of the material) being suitable for fermentation. This approach is economical for a system input of >15,000 metric tons/year.
Pressing biowaste: In this technique, the input material is first homogenized by a mixing unit using mixing screws. After homogenization, a spiral press separates the material into a liquid and a solid phase. If needed, a screening step also can be used prior to pressing to separate plastics and larger contaminants. Between 30 and 50 percent weight of the input material is yielded as press liquid, containing approximately 15 percent solids. The liquid substrate is fermented and the residue is processed in a composting facility. This approach offers greater scope for combined fermentation and composting and is economical for a system input of >10,000 metric tons/year.
SANITIZATION, FERMENTATION AND ENERGY EXTRACTION
After mechanical treatment, the conditioned substrate is stored in a collector tank, which acts as a buffer storage for consistent fermenter feed. The tank also allows the introduction of liquid input materials such as fat and oil, which require no preparation. From the collector tank, the substrate is homogenized using an agitator and then pumped into the sanitization unit. The substrate is heated with backflow of already sanitized material and by water from a cogeneration unit. The heat expended can be regained by heat exchangers thus reducing the energy demands of the system. After this phase, the sanitized substrate is introduced into the fermenter.
The aim of fermentation is the efficient microbiological conversion of substrate into biogas and fertilizer substrate. For this to occur, the material must remain homogeneous. A propeller agitator rotates the yielded substrate regularly so that sufficient liquid-gas separation can take place and no layers form. Settling sediment may be pumped off from the reactor if needed. Residence time in the fermenter is at least 15 days. Fermentation generates a raw biogas and a liquid residue containing nitrogen, phosphorus and stabilized organic substances that is suitable for agricultural use.
The raw biogas is stored in gas tanks, then further treated to maximize the efficiency of energy conversion. The sulphur water content of the biogas can be reduced to less than 50 ppm using a biological desulphurization unit. This offers a two-fold benefit for plant operation – it sets a strict level for emissions that is well within current European regulations, and the useful working life of the gas motor is increased significantly (an economic benefit).
After desulphurization, the biogas passes to the dehydration phase under its own weight, where the gas is condensed by cooling. It is then fed via a compressor into the gas motor, within a carefully controlled environment. In the cogeneration units, combined heat and power plants (CHPs), the generator is driven, generating electricity, which is then fed into the grid of the energy providers via a transformer.
MARKETS AND OPERATIONS
Due to rising oil prices and a growing interest in exploring renewable energy, biogas production from biowaste is attracting increasing attention and investment in Europe and elsewhere. A growing number of plants – in Germany, Sweden and Austria in particular -reflect the potential of this technology as an alternative energy source. Financial incentives in Europe currently offer biogas plants a price per produced electric energy unit that is higher than the market price for energy from fossil resources. This enhances the competitive potential of this technology. There is also competition for substrate.
The biowaste fermentation plant operated by Marchfelder Bioenergie GmbH near Vienna, Austria was launched in 2006. With an annual capacity of 15,000 metric tons, it can process a variety of biowaste such as material from the meat processing industry, separately collected biowaste from households and food waste contained in steel barrels with a capacity of up to 200 liters. The barrels are shredded and the contaminants are separated. The plant uses the dissolver treatment process from Komptech. This provides automatic discharge of the contaminants and produces a pumpable, homogenous material stream for the associated wet fermentation process. At full capacity the plant generates 360 kW of power, which is then fed into the public grid. Annually, around 10,000 m3 of liquid fertilizer from the plant will be used for agricultural purposes.
Joachim Hirtenfellner is with KOMPTECH GmbH in Frohnleiten, Austria.
April 21, 2006 | General
Wet Separation, Fermentation For Biowaste (Austria)
BioCycle April 2006, Vol. 47, No. 4, p. 76