SMD – Self Mixing Digester
“There are no moving parts, thus nothing can get broken” 1. Introduction Self Mixing Digester (SMD) is a patented digester (up to 2400m³ volume) with two upright cylindrical tanks, one inside the other, without mixers. The only moving part is a gas valve situated in a wide gas pipe connecting the two chambers. By means of a gas valve, two communicating chambers and pressure generated by biogas production, an efficient mixing is reached. Conventional biogas plants work with different types of agitators in the fermenting chambers; they require additional electricity and must be maintained regularly. At SMD there are no mixers! Therefore: “There are no moving parts, thus nothing can get broken”. Figure 1.Scheme of Self Mixing Digester (SMD). 2. SMD – A four stage process 2.1 Non-operational state: In its non-operational state the gas valve, situated in a wide gas pipe connecting the main and secondary chambers, is open. This allows a balance in the fluid level between the two chambers. 2.2 Compressing period: Under operation the gas valve is closed. The biogas being produced can not escape; instead it is compressed and pushes the column in the main chamber downward. The displaced substrate moves into the secondary chamber via the four to eight static mixer jets situated at the bottom of the SMD. - approximately 110m³ of substrate are displaced
- >up to 450mbar of pressure in the main chamber
- the fluid levels in the main and secondary chambers vary approx. 4,5 meters
2.3 Resting period: When the preset pressure difference (450mbar) between the main and secondary chamber is reached, the so called “resting period” starts. This means that the digested substrate (also called effluent or digestate) will drain off the secondary chamber. The effluent runs off through a pipe into the follow up fermenter or storage tank / lagoon. 2.4 Mixing period: At the end of the resting period, the gas valve is opened by means of a pneumatic control. Within a fraction of second it will reach pressure equilibrium: 2.4.1 A huge crater is formed on the surface of the ferment in the secondary chamber. This destroys the surface scum. 2.4.2 In the next five to ten seconds, the ferment is flushed out of the secondary chamber into the main chamber. Substrate will flow through three different directions, at the same time: Substrate flow through short mixer columns: Digested material with a high proportion of starved bacteria is sprayed with high energy into the fresh undigested material in the main chamber, via the short mixer columns. Substrate flow through static mixer jets: At the same time, material is forced into the main chamber by the static mixer jets. The digested material is re-worked into the active ferment (approx. 50 - 75 m³ in five seconds via four to eight nozzles). Due to the high amount of energy released, the fluid column in the main chamber starts to rotate. This rotation facilitates the spraying of every square centimetre of the upper surface in the main chamber with the substrate from the secondary chamber via the short and long mixer columns. The surface scum is destroyed and re-worked into the ferment. Substrate flow through long mixer columns: In the ground sludge layers of a biogas unit there is a high concentration of starved bacteria, due to the insufficient nutrient supply. Part of this material is worked back to the top of the biomass via the long mixer columns and mixed into the undigested material in the main chamber. The surplus supply of nutrients causes a rapid multiplication of the quantity of bacteria. The production of biogas can already achieve a maximum in a very short period of time. Time duration from one mixing interval to the next depends on the input materials. Usually it varies between one and two hours, depending on the digestibility of the substrate. Input material with high energy will generate shorter mixing intervals. 3. Details / Advantages / Characteristics of the SMD 3.1 Accurate substrate flow It is possible to achieve a high biogas yield with the SMD-Digester over a shorter period of time due to the “accurate substrate flow”. The injected substrate follows specific paths through the SMD-Digester; no shortcuts are possible. Fresh input material is deposited onto the upper surface in the main chamber and worked into ferment by the mixing process. Laws of nature state that lighter material rises to the top; the biomass sinks to the bottom of the chamber only after it has been digested to a certain degree. Different materials demonstrate different characteristics in the time needed for this process, and in the SMD-Digester the available potential can be used more effectively. The ferment falls slowly to the bottom of the main chamber and then is forced through the static mixer jets. The ferment then moves slowly upwards into the secondary chamber, and the effluent can be drained off when reaching the highest point. A considerable advantage of the “accurate substrate flow” is that methane bacteria with a relatively slow growth rate are not flushed out of the fermenter. Instead, they are kept within the digester, and the high density of bacteria groups leads to fast digestion. 3.2 No mixers in the digester The aim of a biogas plant is efficient production and extraction of biogas. Since biogas will be produced throughout the digester and must rise upward toward the top of the chamber, conventional units require the use of mixers. The SMD-Digester uses a different operation system to overcome this necessity; like carbon dioxide in a shaking bottle of fizzy water, at SMD the biogas is freed and rises by itself to the top through the periodic compression and expansion of substrate. Conventional mixer units create shear forces within the ferment. These forces destroy a large portion of the flakes, which have formed clusters of different bacterial strains. The highly sensitive equilibrium state of these bacteria, which aids in gas production, will be destroyed and, therefore, reduce efficiency. However, this is not the case for the SMD-Digester; as soon as the ferment leaves the upper stress zone of the main chamber, there are no more shear forces. The nutritional requirements and, thus, the life function of the flakes, are secured by the transportation of dissolved matter and ions within the watery solution. The constant compression and expansion of the ferment allows cavities to form, through which nutrients can reach the bacteria. The flakes are able to grow uninterrupted, and very high process stability can be achieved. 3.3 Constant Temperature An important factor to gain a high yield of biogas is constant temperature. Just as important is the smooth heating of the liquid manure. In the SMD the constant temperature is maintained with approximately 8.000m of integrated wall and floor heating. With such a large surface area, it is possible to have a low supply temperature. Through smooth warming of the substrate, the bacterial strains can develop in the material and, thus, increase the biological effect of the system. Other biogas plants with external heat exchangers must be heated to above their optimal temperature for efficient digestion. Therefore, the heat losses of the unit must be balanced. If additional heating is required, active ferment must be pumped through the heat exchangers. Only very few bacteria can survive such a dramatic rise in temperature and, consequently, the whole process, --from hydrolysis to methane production-- must restart from zero. In some other digesters, the heat exchangers are fixed to the wall. Thus, the digester might show a poor heat transfer, due to the fact that thin surfaces require high temperatures to stay warm. High temperatures destroy the bacteria. The heating pipe will become overgrown with fibrous matter, which reduces heat transmission and efficiency; therefore, heat transfer process will become worse over time. The chamber must then be emptied and the heating pipe cleaned. A loss of earnings is guaranteed. 3.4 Extraction of ground sludge Due to the fermenting process, the sediments will fall to the bottom. Over the years the sediments layers will grow, decreasing the volume of the digester, thereby decreasing the biogas production. Consequently, the operator of the biogas plant must stop to remove the sediments. Thanks to the sludge drain at SMD-Digester, the extraction of ground sludge and grit can be done without stopping fermentation. This enables SMD Digester to work with complicated feedstock, such as poultry slurry, some residues or soiled energy crops which usually carry sand or grit into the digester. 4. References.BIOGAS PLANT BERLIN: Specification: Pre dump: Monolithic reinforced concrete tank Volume = 450 m³ Digester: Self Mixing Digester (SMD), no mixer inside Monolithic reinforced concrete tank with wall and floor heating Volume = 2.400 m³ Follow up fermenter: Monolithic reinforced concrete tank Desulfurisation: External biological desulfurization CHP-Unit: 537 kW elect. Input Substrates: cow and pig manure, energy crops Start-up: 2006 * More references available upon request. 5. Feasibility Studies: We also offer to develop a feasibility study for your biogas plant with a complete study of: feedstock materials (availability and energy content), plant design, financial analysis, etc. 6. Contact: For any questions or requestes for further information, do not hesitate to contact us. We invite you to visit our biogas plants references; we look forward to meeting with you soon. Dipl.-Agrar-Wirtschaftsing. Burkhard Meiners AgroEnergien Brunner Str. 18 D-26316 Varel, Germany Mobil: +49 179-2031052 Tel: +49 4453-985800 E-mail:
This e-mail address is being protected from spambots, you need JavaScript enabled to view it
|
