Plant installations Fqi
AN/Biothane process (Figure 4.45) The AN/Biothane process was probably the first process to be developed for bio waste fermentation.
In this process the substrate is prepared, then carefully piled up in the percolation reactor and sprayed with a little water, which percolates through the biomass and removes all water-soluble substances. The wash water, containing all the dissolved substances, is roughly separated and fermented in an Biogas from Waste" href="/biogas-from-waste/b-br-brs-x.html">UASB-digester.
Prethane/Rudad-Biopaq process or ANM process (Figure 4.46) In the 1980s, this process was implemented twice on a large industrial scale (about 3000 Mga-1) for the treatment of bio waste (vegetable matter, slaughter waste, dung). In Germany it was called the ANM process, while in the Netherlandes it was named the PrethaneTM/RudadTM-BiopaqTM process.
4.5 Plants with separation of non-hydrolyzable biomass | 307 Biogas
Air injection Figure 4.46 Prethane™/Rudad™-Biopaq™ process.
Two additional plants were implemented in Germany in the years 2000/2001 on the basis of the meanwhile gained experiences. Both plants were based on a two-stage process with an initial mechanical preparation stage. Their output amounts to 25 000 Mg a-1 oftotal waste produced, since bio waste cannot be separately collected.
By the means of a rotating drum screen (sieve cutting of 140 mm), coarse particles with a high heating value and biologically degradable fine particles are separated, and metals are removed. About 96% of the degradable organic substances are removed by sieving with a 140 mm sieve and transferred to the percolator.
The fine particles in the percolation reactor are continuously circulated and strongly agitated for 2-4 days while being sprayed with about 2 m3 Mg-1 wash water from the top (related to the percolator supply) and cyclically blown through by air from the bottom (V* « 8.6m3/(m3.h)). In this way, soluble organic substances are removed from the biomass and organic compounds are hydrolyzed or acidified. Alternatively, the waste is suspended in water to give 10% of dry matter and is anaerobically hydrolyzed in a bioreactor at 35 °C up to a certain targeted degree of hydrolysis. The remaining solids are dehydrated by means of a screw press.
The total quantity of solution accumulating in the press and percolator is cleaned of sand and fibrous materials and injected into the fermentation reactor, e.g., into a UASB reactor, where the organic components are degraded for 2-6 days.
Part of the water flowing out of the reactor is re-used for percolation. From the other part, the nitrogen is removed in a NO.-reduction plant. Afterwards, this water can be used as industrial water.
IMK process86) In the IMK process, the acidification is done aerobically. The liquid with the dissolved substances is squeezed out and fermented anaerobically. For the separation of fine particles from the liquid stream, hydraulic cyclones are used.
The process is suitable for the utilization of all types of bio waste.
- Figure 4.47 Dry anaerobic composting process.
|
Plant capacity |
Mga-1 |
30000 |
|
Residence time |
days | |
|
For methanation |
10-20 | |
|
For rotting |
ca. 10 | |
|
Spec. waste water accumulation |
L Mg-1 Input |
340 |
|
Spec. electrical energy consumption |
kWh Mg-1 Input |
64 |
|
Personnel requirement |
Persons |
6 |
|
Investment costs |
Mio US$ |
14 |
|
Operation costs |
US$ Mg-1 Input |
90 |
Dry Anaerobic Composting (Figure 4.47 and Table 4.23) The DRANCO or Dry Anaerobic Composting of manure (Anacom) or ATF processes are one-stage processes in which substances dissolved during water percolation are methanized.
All three processes are very similar: The substrate preparation is performed by sieving and shredding of the waste. Afterwards, the particles smaller than 40 mm are heated up to the fermentation temperature of 55 °C and the substrate is moistened to DM = 25-45% in a special conditioning unit. The next step is a singlestage thermophilic fermentation in a vertical cylindrical bioreactor (residence time in the fermenter: 15-30 days). Fresh biomass is continuously loaded from the top of the reactor. It slowly flows to the bottom almost in a plug flow current. At the bottom, the biomass is continuously removed by a combination of floor scraper and discharging screw. While the biomass flows through the reactor, it is sprayed with circulation water.
In contrast to the Anacom process, in the ATF process the substrate is moistened to a DM concentration of 35-50%, so that no additional fresh water has to be added for the percolation step.
Prior to the rotting, the digested sludge is dehydrated by means of a screw press.
- Figure 4.48 Four big size bioreactors for batch dry fermentation.87'
Dry fermentation process in a stack In well-developed dry fermentation plants, gas -tight buildings serve as bioreactors, in which either piled up biomass-8) or biomass that has been filled into containers-9 is subjected to the fermentation process. The building is under slight overpressure and well heat-insulated. The doors are specifically equipped with special sealing lips to prevent oxygen from entering.
To achieve a throughput of 1000-1200 Mg a-1, four drivable containers ofabout 100 m3 volume each are necessary.
Fresh and fermented substrate in equal quantities are piled up together in layers.
Liquid is circulated over the piled-up biomass in a two-hour cycle through two distributors. The liquid percolates through the biomass, warms it up to 35 °C, and brings nutrients along, so that biogas forms. A double perforated floor in the reactor serves for the collection of drained off percolate.
The percolate is heated up in a separate tank with integrated heat exchanger or is conveyed through a tubular heat exchanger integrated in the floor. A floor heating system supplied with the exhaust heat of the CHP engines is enough to keep the substrate coming into the tank at a temperature of approx 65 °C after an initial cooling to 40 °C.
The formed biogas is withdrawn. The sulfur content of the biogas is very low at 10mgm-3. Desulfurization is thus not necessary in most cases.
The fermentation lasts for 2-4 weeks, depending on the substrate. The methane content in the biogas is relatively high at 80%.
A few days before the fermentation process is completed, the percolation is interrupted. The water is allowed to percolate off. Afterwards, the bioreactor is back-flushed with engine exhaust gas in order to avoid the development of an explosive
gas mixture. The biomass is dried out by injection of air through the perforated floor. Thus, the complicated afterfermentation under the roof is no longer required. The fermenter is then emptied and reloaded with fresh biomass.
Since gas generation and gas composition are not constant during a batch process, at least three bioreactors have to be run in parallel and out of phase.
Fermentation channel process (Figure 4.49) In this process, the biomass is placed in wire metal box wagons which are transferred into the channel by means of gas-tight sluices. The boxes are then pulled through the closed, completely gas -tight channel filled with liquid (bacteria and nutrients). The biomass remains in the channel until the methanation is finished. Two channels can be passed through sequentially to isolate the processes of hydrolysis and methanation.
Aerobic-Anaerobic-Aerobic process (3-A process) (Figure 4.50) The 3A process enables savings to be made in transportation costs, and includes three procedures which take place sequentially in the same bioreactor. First, the solids are subjected to an aerobic decomposition and dissolution of organic matters. In the second procedure, the biomass undergoes an anaerobic fermentation process followed by
- Wire metal box wagons connected via chains Figure 4.49 Channel process.
Biomass
Aerobic process 20 days, thermophilic conditions
Anaerobic process Biogas
40 days, mesophilic conditions ^
Aerobic process 20 days, thermophilic conditions
Anaerobic process Biogas
40 days, mesophilic conditions ^
Aerobic process 20 days, thermophilic conditions
^ Compost an aerobic rotting. One disadvantage to be mentioned here is the release of emissions during the composting, e.g., climatic gases and substances with an unpleasant smell.
Usually the first two procedures take 60-70 days. It is possible to improve and accelerate it by inoculating the biomass using methanogenics from the rumen.90' In this way the aerobic pre- fermentation is reduced to 4 days and the anaerobic fermentation occurs in 15 days only. Between the two processes, the mass is inactivated by increasing the pH value to pH > 7.2.
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