the industrial biogas power station. technology. definitions.
In an industrial/technological installation that decomposes biological/organic matter in the absence of air to produce biogas (i.e. anaerobic digestion, AD) we want to distinguish four different sub processes:
- Pretreatment processes of feedstock/entrants.
- Fermentation, anaerobic digestion, biogas production.
- Valorization/transformation of the biogas to produce electricity, heat and fuel.
- Post treatment of digestate.
Depending on the input material type and conditions different pretreatment steps are being considered with the aim to homogenize and liquefy the substrate.
- Mechanical sorting by machine or hand.
- Removal of inorganic substances, inert materials, impurities such as plastics, grit, glass, metals; this protects pumps and mixers later.
- Shredding to reduce size, e.g. straw of manure inlay, bones.
- Bio-extrusion, disintegration of organic matter on a cellular level, increase surface for bacterial attack; especially important for lignocellulosic matter, feathers.
- New developments include physical, thermal, chemical and biological combinations of pretreatment methods.
- Physical treatment: Shredding, crushing, extruding, pressurization, ultrasound.
- Thermal treatment: Heat exposure, steam exposure, e.g. for slaughterhouse waste.
- Chemical: Use of solvents and oxidants, acids and bases.
- Biological: Use of enzymes.
- Combinations: such as steam-explosion techniques.
- Different solid feed stock is dosed and weighed according to recipe in a charging hopper/funnel/recipient,
- Water and other liquids like recycled digestate or waste water and sludge is added in other mixing units, MultiMix® downstream to bring the total solids TS (dry matter DM) content to desired level.
- Pasteurization or sterilization of some feed stocks such as animal by-products might be obligatory to reduce pathogens (EU regulations usually refer to one hour at 70°C). Regularly any hygenization or sanitization process as this is also called would make use of the off-heat from the cogeneration (combined heat and power CHP). Read more on the Good Practice Guide: How to comply with the EU Animal By-Products Regulations.
fermentation. anaerobic digestion. AD.
These two terms are very much used interchangeably. They both refer to the biological degradation or decomposition of organic matter (larger molecules, such as sugars) into more basic compounds (such as acids, water, gazes).
It is bacteria that thrive in the absence of oxygen that carry out anaerobic digestion! We distinguish four stages; hydrolysis, acidogenesis, acetogenesis and methanogenesis. Each stage entails a different type or set of bacteria; the stages overlap and “co-exist in harmony” inside the fermenter in order to produce a stable process and a high production rate of biogas. Read anaerobic digestion.
What is most important in AD is stability: Biomass (bacteria) can and will adapt, and once biological equilibrium is found (i.e. the four stages and different sets of bacteria work hand-in-hand), all parameters such as feed stock recipe, temperature and pH-value (between 6.7-7.4) should be kept constant to avoid disruptions and the accumulation of undesirable intermediate substances. Instability can kill off any or many sets of bacterial biomass and lead to inhibition and the cessation of biological activity and hence biogas production. This is what should be avoided by all means.
Depending on the physical composition of inlay material and general economics, biogas plants will be designed differently.
Wet or dry fermentation/AD: Actually they both are wet, just some are wetter (TS<15%). In wet AD plants the substrate is either pumpable or made pumpable i.e. it is well shredded, liquefied, mixed and homogenized. In dry fermenters (TS>20%) substrate is less homogeneous; with higher DM content mixing/homogenization becomes difficult and energy intensive.
Continuous or discontinuous AD: Continuous refers to substrate being continuously fed and drained from the digester. Discontinuous or batch loading refers to the fermenter being loaded and emptied once and then again when biological degradation has terminated. Most industrial scale wet fermenters are continuously operated units. Many dry fermenters are discontinuous/batch or garage fermenters (TS>30%). Plug flow digesters are high solids, dry, laying down cylindrical digesters: Substrate is slowly mixed and continuously slowly moved forward.
Steered and non-steered AD: Regularly wet AD processes are constantly steered and mixed. Dry fermenters are often not steered as the steering of dry material is somewhat energy intensive.
Temperature, mesophilic (35°-40°C) or thermophilic (50°-65°C): Most wet fermenters these days are mesophilic; the process is easier to handle, more stable and less energy intensive. In thermophilic digestion the biological processes run much faster, up to 6 times faster than in mesophilic environment (foremost hand-shakes between acidogenesis and methanogenesis), errors in the handling of the bio-system escalate faster, with bigger consequences; thermophilic bacteria biomass is more prone to ammonia inhibition.
Dry fermenters often are thermophilic, and achieve higher gas yields at lower retention times. Thermophilic digestion in general also serves sanitization/pasteurization, killing off pathogens; this is achieved concurrently with the fermentation process. EU regulation makes hygenization obligatory for the digestion of animal by-products such as slaughterhouse waste SHW and the organic fraction of municipal solid waste OFMSW.
The mesophilic/thermophilic question is often whether the operator has the experience to handle a more delicate system to be able to reap the thermophilic benefits.
Heated and not-heated AD units: Most industrial fermenters are heated to keep the temperature and process stable and maximize biogas production. They are either centrally heated where the heated substrate is circulated by pumps and pipes back and forward to the fermentation tanks, or through heating pipes that are laid out along the inner walls of the tank.
Parasitic heat consumption: Depending on the harshness of the climate up to 20% of all heat that is being produced in the cogeneration (combined heat and power CHP), up to 30% and more in the case of thermophilic fermentation, is auto-consumed by the digestion process itself to maintain the temperature of the substrate in the fermentation tanks.
Single or multi-stage fermentation: Sometimes the single-stage AD process is economically preferable; with the effect that substrate might not completely degrade. Alternatively different processes like hydrolysis, fermentation and post fermentation may occur in different stages, means in separate digestion units; that may result in greater overall biogas production. The question is whether the increased biogas output, achieved through longer total retention times, can pay for additional tanks and equipment or not.
Construction material: Digesters maybe constructed using building materials such as concrete, brick and mortar, stainless steel, even wood and plastic; all have advantages and disadvantages. If good concrete can be supplied concrete might be a cheap and satisfactory option.
The biogas forming in the fermenter contains amounts of hydrogen sulfide and ammonia. These aggressive chemicals attack unprotected components and concrete. For an installation to last through a 20 plus years lifetime, and for clean operations without biogas leaks to the atmosphere, only quality stainless steel is recommended. A light-weight steel structure is ideally modular and quickly installable, like the one we offer through our partner Weltec Biopower.
Insulation: Usually fermenters are insulated on the outside with a polystyrene cover to keep the heat inside. Additional coating protects the insulation against wind, weather and UV irradiation. Storage tanks for the digestate have no insulation, as the substrate is already degraded.
Co-digestion refers to the positive effects on biogas output and the supply of missing nutrients for microorganisms. Different (complementary) substrates generate a higher biogas yield in combination with each other; there is a synergy effect.
Gas storage: The produced biogas is being stored under a double membrane roof on top of the fermenter and post-fermenter. This “gazometer” is inflated with air by means of a blower to stabilize the tarp and counter weather influences (rain, wind); the gas inside is under slight pressure only. End digestate storage tanks might be covered as well to capture any residual gas production.
Additional gas storage: If needed additional storage capacity can be installed, which enables the biogas operator to shift his biogas usage by a couple of hours. As biogas is being produced constantly 24/7 it serves base load electricity production requirements, with storage this production can be shifted to the peak-times when prices are highest, or to times when electricity is needed in off-grid installations.
Security flare: The installation of a flare for security reasons is obligatory. The biological activity that produces the biogas cannot be turned off, thus any biogas that is produced needs to be burnt off even when there is no use for it, in the event of a cogeneration plant break down, or when maintenance work is being done, etc.
biogas. usage. valorization.
Constituencies of biogas: The wet freshly produced raw biogas is made up of 50%-60% methane CH4 and 35%-45% carbon dioxide CO2; it also contains water H2O and gases like hydrogen sulfide H2S, and others N2, O2, CO.
Dehumidification is an air dryer that takes out the moisture by chilling the gas; temperature is reduced in order to allow condensate to form and be removed, then bringing the gas back to ambient temperature.
Desulfurization is an chemical absorption technique that takes out the hydrogen sulfide H2S by means of an activated carbon filter.
Desulfurization and dehumidification are scrubbing processes that run before any biogas can be burnt in a combined heat and power plant or boiler/burner. This is vital as to keep engines and valves safe from corrosion; this increases engine life and reduces maintenance.
For the multiple and flexible usage of biogas please refer to our energy section.
post treatment. usage of digestate.
Medium retention time MRT: The substrate remains in the fermentation tank for a predefined period of time. In continuous systems this refers to the medium retention time MRT, a statistical measure. Total MRT can be anything between 20 and 120 days, depending on the digester system, quality and type of the substrate, pretreatment methods, and foremost economics; all effect the dimensioning of the installation, hence the MRT and the degree of degradation of the substrate at the point it exits the digester.
Digestate: Every matter that enters the fermentation tanks as substrate/feed stock/entrants will exit as digestate/residue of the digestion, with a slight reduction in volume of 10%-15%. The digestate is of higher liquid content then the feed stock; it consists of almost fully degraded organic matter, indigestible organic material such as lignocellulose plant stuff and dead mineralized bacteria/biomass.
Economics and mass balances may put the hygenisation/pasteurization step at this stage right after the last fermentation tank. This may make a lot of sense as the liquid fermentation residue is already at temperature and well homogenized. Off-heat from the cogeneration unit (combined heat and power CHP) would be used for sanitization at usually 70°C for one hour).
In the wet AD scenario the residue is pumped from the final fermentation tanks to the storage tanks or lagoons, which might be covered or open.
The digestate is pretty liquid and contains virtually the same nutriments, N, P, K and others, as before. The digestate constitutes a nutrient rich high quality liquid natural fertilizer and it may be directly applied for field spreading in agriculture.
A mechanical solids/liquids separation though is most common. By means of a screw filter press the solids fraction leaves this process as stackable matter (25%-30% TS content).
Composting, aeration of the solid fraction of the digestate for 2 months, giving rest, further enhances the end product. Upgrading steps might include mixing with mineral fertilizer or compost (combo-products). Pls. see our section fertilizer.
The liquid fraction represents nutrient rich irrigation water, a liquid fertilizer to be used in agriculture. Some liquid is frequently recycled back into the fermentation system.
Three questions need to be answered and their resolution usually determines the success of a biogas plant:
- Where do the entrants come from?
- Who uses the biogas or the derived energies produced?
- Where do the residues go?