Types and Construction of Biogas PlantZeeshan Syed Faheemzeesy497@student.liu.se

IntroductionBiogas has to become an energy source in future because of vulnerability of fossil fuel.

So, like other advanced scientific fields, biogas production also needs very carefully manufactured infrastructure so that it could be fully exploited for maximum production of biogas.

Types of Biogas PlantsOn-farm or small scale biogas plantsSmaller in sizeLow-costEasy sustainability for one household energy needs

Industrial or Full-scale biogas plantsWorks Centrally Farmers of surrounding area are supposed to support this plant in terms of input slurry

On-farm/Small-Scale Plant

Floating Cover Digesters

Fixed Cover Digesters

Plastic Cover Digesters/Balloon Digesters

Floating Cover Digesterscommonly made from brick and cement

covers with a floating steel cylinder with an open bottom

This cylinder moves up with the higher gas production and moves down in the opposite case

Constant Volume Plant

Advantages/Disadvantages of Floating Cover DigestersWorks on constant gas pressure principle Changing volume and gas volume is quite visible due to a visible rise in the drum

Cost of floating drum construction is high Requires continuous maintenance like paints etcShort life due to corrosion of metallic drum

Floating Cover Digesters

Fixed Cover DigestersAlso made from brick and cement

Pressure increases with the production of gas and expels some digester liquid into the compensation compartment

Pressure of digester decreases when gas consumption is higher which again lets the digester liquid expel but this time in reverse action

Advantages/Disadvantages ofFixed Cover DigestersLow construction cost No moving parts are involvedNo rusting metal parts are involved, relatively long life

Temperature of fixed cover plants keeps relatively lowIts construction is recommended at only those places where construction work can be supervised by experienced biogas technicians

Fixed Cover digesters

Plastic Cover/Balloon DigestersLong cylindrical bag made of rubber or polythene or PVC materials

Usually half buried in the ground having one inlet at one end and one outlet at other end

Bag swells like a balloon with the production of gas

Pipe outlet for gas

Advantages/Disadvantages of Plastic Cover/Balloon DigestersLow cost, easy to transport type of plantEasy to maintain e.g. cleaning, emptying et c.Relatively high temperature can be maintained in this type of plant

Life of this plant is short e.g. 5 years maximumIts vulnerable to be damaged easily

Plastic Cover/Balloon Digesters

Designing a Biogas PlantShape and Load Bearing CapacitiesMain digester usually constructed underground in a on-farm biogas plant

Internal and external forces can damage the plant if the shape is not appropriate

According to experience, domed or spherical shape is the best shape which can bear maximum load hence proved long life

Shapes and Load Bearing CapacitiesLoad bearing capacity of different shapes

Bottom SlabBottom slab has to bear the full load of digester shell and digester materials

It should be hard rather than soft because a rigid shell distributes the weight better than a soft slab

Again spherical shape is recommended for base slab but conical is also recommended because of easier excavation

Steel ring enforcement on the outer edge of the bottom slab is also recommended but not necessary, depends on cost

Bottom SlabDesigns of bottom slabs

Constructing Spherical ShellIts radius should be the same from the first layer of bricks

The cemented material (mortar) should be of fine particles

Gaps should be filled carefully so that there would be no chance of leakage

This shell should be water tight

Constructing Spherical Shell

Other Considerationsfor Spherical Shell ConstructionThe ratio of the digester volume and gasholder volume should be 6:1

In case of a floating drum plant, the roof of the gas drum should be slightly sloping

Drum guidance system should be well built otherwise it can be jammed or broken up

Other Considerationsfor Spherical Shell ConstructionHeight of compensating and composting tanks does matter in a fixed cover digester

The height of the base of compensating tank and composting tank should not be more or less than the level of slurry in the digester

if its too low then there are chances to get exposed to the oxygen

Important CalculationsCalculation of digester loadingDigester volume (VD) = 4800 liters (4.8 m) Retention time (RT) = 80 daysDaily amount of fermentation slurry (Sd) = 60 kgProportion of organic matter = 5 %R = 5x60/100 x 4.8 = 0.625 kg/m3/day

Important CalculationsDigester VolumeThe digester volume VD (liters) = Sd (liters/day) x RT (days)If,30 liters dung + 30 liters water = 60 liters fermentation slurryDaily supply (Sd) = 60 litersRetention time (RT) = 80 daysDigester volume (VD) = 60 liters/day x 80 days = 4800 1iters (4.8 m)

Important CalculationsRetention time calculationRT (days) = VD (liters) / Sd (liters/day)Digester volume (VD) = 4800 litersDaily supply (Sd) = 60 liters/dayRetention time (RT) = 4800 liters / 60 liters/day = 80 days

Important CalculationsDaily amount of feed calculationSd (liters/day) = VD (liters) / RT (days)Digester volume (VD) = 4800 litersRetention time (RT) = 80 daysDaily fermentation slurry requirement (Sd) = 4800 l / 80 days = 60 liters/day

Full-Scale Biogas PlantsSome extra components which small-scale plants dont have

Engine driven stirrer

Heating system

Some pumps to put the material in and out the digester

Valve, Pipes etc.

Types of Full-Scale PlantMainly two types of digestersHorizontal DigestersUpright Standard Digesters

Sizes vary in great rangeCan be of 50 m3 or 100 m3 or in lager plants it can be of 400 m3 to 5000 m3 in size

Horizontal Biogas Plants Smaller in size, made of steel

Standard volume is between 50 m3 and 150 m3

Hydraulic retention time usually is 40 to 50 days depending on the input material

The material in these types of plants first heated by the heater installed inside the digester

After desired heat this material then mixed with mixing arms

Horizontal Biogas PlantsDifferent parts of horizontal digester

Upright Standard PlantsCan be made of either concrete or steel

Steel plants are usually made of glass coated prefabricated sheets to avoid corrosion

Very big in size

Usually range between 1500 m3 to 5000 m3

Height varies between 15 to 20 meters

Diameter varies between 10 to 18 meters

Upright Standard PlantsInput material either can be preheated before entering the digester

Or hot water pipelines are installed inside the digester to provide required heat

Hydraulic retention time is usually 20 days

Engine driven mixer is installed on the roof

Upright Standard PlantsUpright standard plant where input material is heated before hand

Upright Standard PlantsUpright standard plant

ConclusionSmall scale or on-farm biogas plants are more civil engineering oriented

These require less money and can provide relatively more benefit in terms of energy

Some methods, like fixed dome plant, can last longer life without major maintenance hence cheapest way to get energy

ConclusionConstruction of full scale plants on the other hand is more mechanical engineering oriented

It involves some moving parts and containers which require proper mechanical engineering designs

Construction of small scale biogas plants is well described in literature, on the other hand full scale plants construction is not well described, though structure and typed are given

Thank You