5.4 Design and construction of plant components
Biogas plants of simple design consist of the following main components:
Depending on the available building material and type of plant under construction, different variants of the individual components are possible.
Table 5.8: common substrate mixing ratios (Source: OEKOTOP, compiled from various sources)
5.4.1 Mixing pit
In the mixing pit, the substrate is diluted with water and agitated to yield a homogeneous slurry.
The useful volume of the mixing pit should amount to 1.5-2 times the daily input quantity. A rock or wooden plug can be used to close off the inlet pipe during the mixing process. A sunny location can help warm the contents before they are fed into the digester in order to preclude thermal shock due to the cold mixing water. In the case of a biogas plant that is directly connected to animal housing, it is advisable to install the mixing pit deep enough to allow installation of a floating gutter leading directly into the pit. Care must also be taken to ensure that the low position of the mixing pit does not result in premature digestion and resultant slurry formation. For reasons of hygiene, toilets should have a direct connection to the inlet pipe.
5.4.2 Inlet and outlet
The inlet (feed) and outlet (discharge) pipes lead straight into the digester at a steep angle. For liquid substrate, the pipe diameter should be 10-15 cm, while fibrous substrate requires a diameter of 20 - 30 cm. Plastic or concrete pipes are preferred.
- Both the inlet pipe and the outlet pipe must be freely accessible and straight, so that a rod can be pushed through to eliminate obstructions and agitate the digester contents;
- The pipes should penetrate the digester wall at a point below the slurry level. The points of penetration should be sealed off and reinforced with mortar.
- The inlet pipe ends higher than the outlet pipe in the digester in order to promote more uniform throughflow. In a fixed-dome plant, the inlet pipe defines the bottom limit of the gasholder, thus providing overpressure relief.
- In a floating-drum plant, the end of the outlet pipe determines the digester's slurry level.
The digester of a biogas plant must accommodate the substrate and bacterial activity, as well as fulfill the following structural functions:
- accept the given static forces
As a rule, the digesters of simple biogas plants are made of masonry or concrete. Such materials are adequately pressure-resistant, but also susceptible to cracking as a result of tensile forces.
The following forces act on the digester:
- external active earth pressures (pE), causing compressive forces within the masonry
Thus, the external pressure applied by the surrounding earth must be greater at all points than the internal forces (pE > pW). For the procedure on how to estimate earth force and hydrostatic forces, please refer to chapter 10.1.4.
Round and spherical shapes are able to accept the highest forces—and do it uniformly. Edges and corners lead to peak stresses and, possibly, to tensile stresses and cracking. Such basic considerations suggest the use of familiar cylindrical and dome designs allowing:
- inexpensive, material-sparing construction based on modest material thicknesses
The dome foundation has to contend with the highest loads. Cracks occurring around the foundation can spread out over the entire dome, but are only considered dangerous in the case of fixed-dome plants. A rated break ring can be provided to limit cracking.
The first step of building the plant consists of defining the plant level line with a taut string. All important heights and depths are referred to that line.
The pit for the biogas plant is excavated by hand in the shape of a cylindrical shaft. The shaft diameter should be approx. 2 x 50 cm larger than that of the digester. If the soil is adequately compact and adhesive, the shaft wall can be vertical. Otherwise it will have to be inclined. The overburden, if reusable, is stored at the side and used for backfilling and compacting around the finished plant.
The foundation slab must be installed on well-smoothed ground that is stable enough to minimize settling. Any muddy or loose subsoil (fill) must be removed and replaced by sand or stones. The bottom must have the shape of a shallow inverted dome to make it more stable and rigid than a flat slab. Quarrystones, bricks and mortar or concrete can be used as construction materials. Steel reinforcing rods are only necessary for large plants, and then only in the form of peripheral ties below the most heavily burdened part, i.e. the dome foundation.
The dome of the biogas plant is hemispherical with a constant radius. Consequently, the masonry work is just as simple as for a cylinder and requires no falsework. The only accessory tool needed is a trammel.
The dome masonry work consists of the following steps:
- finding and fixing the centerpoint of the dome radius in relation to the level line
- layer-by-layer setting of the dome masonry, with the bricks set in mortar, positioned and aligned with the aid of the trammel and tapped for proper seating
- in the upper part of the dome - when the trammel is standing at a steeper angle than 45°, the bricks must be held in place until each course is complete. Sticks or clamps with counterweights can be used to immobilize them.
Each closed course is inherently stable and therefore need not be held in place any longer. The mortar should be sufficiently adhesive, i.e. it should be made of finely sieved sand mixed with an adequate amount of cement.
Table 5.9: Mortar mixing ratios (Source: Sasse, 1984)
Table 5.10: Suitability tests for rendering/mortar sands (Source: Sasse, 1984)
Mortar consisting of a mixture of cement, sand and water is needed for joining the bricks and rendering the finished masonry. Biogas plants should be built with cement mortar, because lime mortar is not resistant to water.
The sand for the mortar must be finely sieved and free of dust, loam and organic material. That is, it must be washed clean.
Special attention must be given to the mortar composition and proper application for rendering, since the rendering is of decisive importance with regard to the biogas plant's durability and leaktightness. Ensure that:
- trowelling is done vigorously (to ensure compact rendering)
Crack-free rendering requires lots of pertinent experience and compliance with the above points. Neither the rendering nor the masonry is gaslight and therefore has to be provided with a seal coat around the gas space (cf. chapter 5.4.4).
Basically, there are three different designs/ types of construction for gasholders used in simple biogas plants:
- integrated floating drums
Most floating-drum gasholders are made of 2 - 4 mm-thick sheet steel, with the sides made somewhat thicker than the top in order to counter the higher degree of corrosive attack. Structural stability is provided by L-bar bracing that simultaneously serves to break up surface scum when the drum is rotated.
A guide frame stabilizes the gas drum and keeps it from tilting and rubbing on the masonry. The two equally suitable types used must frequently are:
- an internal rod & pipe guide with a fixed (concrete-embedded) cross pole (an advantageous configuration in connection with an internal gas outlet)
- external guide frame supported on three wooden or steel legs (cf. fig. 5.7).
For either design, it is necessary to note that substantial force can be necessary to turn the drum, especially if it is stuck in a heavy layer of floating scum. Any gasholder with a volume exceeding 5 or 6 m³ should be equipped with a double guide (internal and external).
All grades of steel normally used for making gasholders are susceptible to moisture-induced rusting both inside and out. Consequently, a long service life requires proper surface protection consisting of:
- thorough derusting and desoiling.
The cover coats should be reapplied annually. A well-kept metal gasholder can be expected to last between 3 and 5 years in humid, salty air or 8-12 years in a dry climate.
Materials regarded as suitable alternatives to standard grades of steel are galvanized sheet metal, plastics (glass-reinforced plastic/ GRP, plastic sheeting) and ferrocement with a gaslight lining. The gasholders of waterjacket plants have a longer average service life, particularly when a film of used oil is poured on the water seal to provide impregnation.
In a fixed-dome plant the gas collecting in the upper part of the dome displaces a corresponding volume of digested slurry. The following aspects must be considered with regard to design and operation:
- An overflow must be provided to keep the plant from becoming overfilled.
- The gas outlet must be located about 10 cm higher than the overflow in order to keep the pipe from plugging up.
- A gas pressure of 1 mWG or more can develop in the gas space, Consequently, the plant must be covered with enough earth to provide an adequate counterpressure; special care must be taken to properly secure the entry hatch, which may require weighing it down with 100 kg or more.
The following structural measures are recommended for avoiding or at least limiting the occurrence of cracks in the dome (cf. fig. 5.23):
- For reasons of static stability, the centerpoint of the dome radius should be lowered by 0.25 R (corresponding to bottom center of the foundation). This changes the geometry of the digester, turning it into a spherical segment, i.e. flatter and wider, which can be of advantage for the plant as a whole.
- The foot of the dome should be made more stable and secure by letting the foundation slab project out enough to accept an outer ring of mortar.
- A rated break/pivot ring should be provided at a point located between 1/2 and 2/3 of the minimum slurry level. This in order to limit the occurrence or propagation of cracks in the vicinity of the dome foot and to displace forces through its stiffening/ articulating effect such that tensile forces are reduced around the gas space.
In principle, however, masonry, mortar and concrete are not gaslight, with or without mortar additives. Gastightness can only be achieved through good, careful workmanship and special-purpose coatings. The main precondition is that the masonry and rendering be strong and free of cracks. Cracked and sandy rendering must be removed. In most cases, a plant with cracked masonry must be torn down, because not even the best seal coating can render cracks permanently gaslight.
Some tried and proven seal coats:
- multilayer bitumen, applied cold (hot application poses the-danger of injury by burns and smoke nuisance); solvents cause dangerous/explosive vapors. Two to four thick coats required.
- bitumen with aluminum foil: thin sheets of overlapping aluminum foil applied to the still-sticky bitumen, followed by the next coat of bitumen.
- plastics, as a rule epoxy resin or acrylic paint; very good but expensive.
- paraffin, diluted with 2 - 5% kerosene heated to 100 °C and applied to the preheated masonry. The paraffin penetrates deep into the masonry, thus providing an effective (deep) seal. Use kerosene/gas torch to heat masonry.
In any case, a pressure test must be performed before the plant is put in service (cf. chapter 7.1).
Table 5.11: Quality ratings for various dome-sealing materials (Source: OEKOTOP)
Gasholders made of plastic sheeting serve as integrated gasholders (cf. chapter 5.3.3: earth pits), as separate balloon/bag-type gasholders and as integrated gas-transport/ storage elements.
For plastic (sheet) gasholders, the structural details are of less immediate interest than the question of which materials can be used. Table 5.12 (p. 74) surveys the relative suitability of various commercial grades of plastic sheeting.
Differentiation is made between:
- low-pressure, wet and dry gasholders (10 - 50 mbar) Basically, these gasholders are identical to integrated and/or plastic (sheet) gasholders. Separate gasholders cost more and are only worthwhile in case of substantial distances (at least 50-100 m) or to allow repair of a leaky fixed-dome plant.
- medium- or high-pressure gasholders (8 - 10 bar/200 bar)
Neither system can be considered for use in small-scale biogas plants. Even for large-scale plants, they cannot be recommended under the conditions anticipated in most developing countries. High-pressure gas storage in steel cylinders (as fuel for vehicles) is presently under discussion. While that approach is possible in theory, it would be complicated and, except in a few special cases, prohibitively expensive. It would also require the establishment of stringent safety regulations.
Table 5.12: Properties of plastic sheeting - gasholder suitability ratings (Source: UTEC 1985)
5.4.5 Gas pipe, valves and fittings
The following types of gas pipes are in use:
Galvanized steel water supply pipes are used most frequently, because the entire piping system (gas pipe, valves and fittings) can be made of universally applicable English/U.S. Customary system components, i.e. with all dimensions in inches. Pipes with nominal dimensions of 1/2" or 3/4" are adequate for small-to-midsize plants of simple design and pipe lengths of less than 30 m. For larger plants, longer gas pipes or low system pressure, a detailed pressure-loss (pipe-sizing) calculation must be performed (cf. chapter 10.2).
Table 5.13: Gas-pipe pressure losses (Source: OEKOTOP)
1 Velocity of flow in the pipe
When installing a gas pipe, special attention must be paid to:
- gastight, friction-type joints
- line drainage, i.e. with a water trap at the lowest point of the sloping pipe in order to rule out water pockets
- protection against mechanical impact.
Some 60% of all system outages are attributable to defective gas pipes. For the sake of standardization, it is advisable to select a single size for all pipes, valves and fittings.
Valves and fittings
To the extent possible, ball valves or cock valves suitable for gas installations should be used as shutoff and isolating elements. Gate valves of the type normally used for water pipes are conditionally suitable. Any water valves used must first be checked for gastightness.
A U-tube pressure gauge is quick and easy to make and can normally be expected to meet the requirements also of a fixed-dome system.
The task of running a fixed-dome system can be made easier by installing a spring-loaded pressure reducing valve that guarantees a constant (adjustable) supply pressure.
If at all possible, the water trap should operate automatically. However since fixed-dome systems need a high water seal, often amounting to more than 1 m WG, the use of condensate collector with a manually operated drain valve is advisable.
As a rule, the water trap also functions as a flashback chamber. If deemed necessary, a gravel trap can be installed for added safety.
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