What is biogas and how is it produced?
Biogas is produced when microorganisms break down organic matter in the absence of oxygen. Behind that simple definition is a biological and technical process that requires careful control.
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TOTAL BAU BLOG
Technical articles on biogas
Clear technical guides to biogas production, mixing and treatment, plant equipment, biomethane, digestate and CHP energy.
Biogas
From the fundamentals to an investment decision
12 articles
Reception and dry feeding
Preparation and wet feeding
Which feedstocks can be used to produce biogas?
A suitable feedstock must be biodegradable, available in predictable quantities and compatible with the specific plant. A carefully selected mixture often gives the best result.
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The four stages of anaerobic digestion
Methane production is not a single chemical step. It results from the work of several groups of microorganisms through hydrolysis, acidogenesis, acetogenesis and methanogenesis.
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Main components of a biogas plant
Each plant is adapted to its feedstocks and intended use, but most share the same functional areas: reception, digestion, gas, energy and digestate.
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Substrate feeding at a biogas plant: five different approaches
A comparison of dry feeding, active mixing and loosening, and wet feeding of solids into a recirculation stream.
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Reception and dry feeding
Huning substrate feeding systems: RDS, Compakt and moving floor
Three configurations cover different capacities and ways to discharge, loosen and transfer solid material.
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Reception and dry feeding
Trioliet Solomix: stationary mixers for substrate feeding
One, two or three vertical augers mix solid components before controlled transfer to the next process stage.
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Reception and dry feeding
BioG BioFeeder – feeding long-fibre and heterogeneous feedstocks
The system is designed for consistent delivery of straw, farmyard manure, catch crops and other fibrous materials.
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Preparation and wet feeding
Vogelsang PreMix: preparing and wet feeding solid substrates
One compact unit separates heavy objects, mixes, cuts fibres and pumps the prepared suspension.
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Preparation and wet feeding
Börger Powerfeed: enclosed feeding into a recirculation stream
Biomass is compacted, trapped air is expelled and the material enters the pipeline to the selected digester in a controlled manner.
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Preparation and wet feeding
WANGEN BIO-MIX 200: mixing and pumping demanding substrates
Solid substrates are mixed with liquid in the intake chamber and then pumped to one or more digesters.
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Preparation and wet feeding
WELTEC MULTIMix: size reduction, homogenisation and wet feeding
The system prepares different solid and waste feedstocks for controlled introduction into a liquid process stream.
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How are electricity and heat produced from biogas?
The most common energy route for biogas is CHP: an engine drives a generator, while heat from the engine and exhaust gases is recovered for use.
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Biogas and biomethane – what is the difference?
Biogas is the raw gas mixture from a digester. Biomethane is the upgraded gas with a high methane content and a wider range of uses.
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Digestate – by-product or valuable organic fertiliser?
Digestate retains much of the nutrient content of the input feedstocks. Its value depends on input quality, treatment, storage and proper application.
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Benefits of biogas plants for farms
Biogas can bring together the management of organic residues, energy production and nutrient recycling, but only with a secure feedstock base and sound organisation.
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How should a biogas plant's site and size be chosen?
The site and capacity must reflect actual flows of feedstock, energy and digestate. A larger plant is not necessarily a better project.
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Biogas plant safety – fire, explosion and toxic gas hazards
Biogas is flammable and some of its components can be toxic. Safety must therefore be built into the design, equipment, controls and daily operating procedures.
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Economic viability of a biogas plant – costs, revenue and key risks
The selling price of energy alone does not determine viability. Feedstock, plant availability, on-site consumption, digestate, financing and contractual risks are all decisive.
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Options overview
How can biogas be used after the digester? An overview
A comparison of direct heat, turbines, fuel cells, trigeneration and other on-site biogas conversion routes.
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Heat and steam
Direct biogas use for heating, drying and steam generation
Where heat is more valuable than electricity, biogas can be used in boilers, dryers and industrial processes.
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Turbine and generator
Biogas microturbines and gas turbines
Turbines offer a compact rotating system for electricity and useful high-temperature heat.
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Electricity and heat without conventional combustion
Biogas fuel cells: an alternative to a conventional CHP engine?
Fuel cells can generate electricity and useful heat from biogas, but require much cleaner gas and have different economics from gas engines.
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Electricity, heat and cooling
Biogas trigeneration: electricity, heat and cooling
Adding an absorption chiller can use CHP heat to supply cooling in summer.
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Grid, transport and industry
How can biomethane be used after upgrading?
Biomethane can be injected into the grid, compressed or liquefied for transport, used in industry or converted into hydrogen and other products.
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Grid and settlement
Biomethane injection into the public gas grid
From gas quality and pressure to metering, odorisation and connection: what a reliable delivery station needs.
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Transport and logistics
Bio-CNG and Bio-LNG – two ways to use biomethane in transport
Compressed and liquefied biomethane differ in energy density, infrastructure, transport and suitable vehicle duty cycles.
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Industrial energy and feedstocks
Biomethane in industry – heat, hydrogen, methanol and chemical feedstocks
Biomethane can replace fossil gas for process heat or serve as a feedstock for hydrogen, synthesis gas and methanol.
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Biogenic carbon and Power-to-X
Biogenic CO₂ from biomethane production – sale, use and methanation
Separated CO₂ may become a product for industry and cultivation, or a feedstock for producing additional renewable methane.
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Options overview
CHP waste heat: where it comes from and how it can be used
Heat from CHP at two temperature levels can serve heating, drying, process heat and cooling or generate additional electricity.
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Heat networks
Heat networks: heating settlements and buildings with CHP heat
A local heat network can connect CHP to homes, schools, farms and commercial premises, but demand and distance determine the economics.
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Product drying
Dryers using CHP heat: grain, wood, biomass and other products
Surplus heat can dry grain, maize, wood and biomass, creating an additional service or product at the plant.
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Digestate and drying
Digestate drying: from separated solids to a dry product
A separator divides digestate into liquid and solid fractions; surplus CHP heat can dry the solids into material better suited to further processing.
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Pelletising and fertiliser
Digestate pelletising: turning dried digestate into fertiliser
After drying, digestate can be conditioned and pelletised into a more uniform product for easier transport, application and potential sale.
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ORC and electricity
ORC: generating additional electricity from CHP heat
ORC uses exhaust heat and other engine heat streams to generate additional electricity without additional biogas consumption.
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Industrial heat
Industrial use of CHP heat: hot water, process heating and steam
Food processing and other industries can use CHP heat for hot water, washing, preheating, drying and part of their steam demand.
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Heat pumps
Heat pumps: raising the temperature of CHP waste heat
An industrial heat pump can raise the temperature of CHP cooling or return heat to a level useful for a more demanding process.
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Trigeneration
Trigeneration: producing cooling from CHP heat
An absorption chiller uses CHP hot water to produce chilled water for storage, food processing, process cooling or air conditioning.
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Greenhouses and farms
Greenhouses and agriculture: using CHP heat throughout the year
Greenhouses, poultry houses, farms and agricultural dryers can use CHP heat across different seasons and temperature requirements.
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Mixing overview
Mixing in a biogas digester: why it matters and what it needs to achieve
An overview of mixing in a digester: homogenisation, temperature control, prevention of sediment and floating layers, and contact between fresh substrate and active biomass.
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Submersible mixers
Submersible mixers: flexible mixing in digesters and secondary digesters
A submersible mixer can be repositioned on its guide rail and directed towards areas needing more flow, which is useful when the fill level varies.
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Long-shaft and side-entry
Long-shaft and side-entry mixers: strong flow through a large digester
The drive remains accessible from outside, while the shaft and propeller enter the tank and create strong horizontal flow through a large volume of biomass.
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Central mixers
Central and vertical slow-speed mixers: large propellers at low rpm
A large, slow-speed propeller can move a substantial volume of biomass and create vertical circulation at a relatively low rotational speed.
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Paddle mixers
Paddle mixers: viscous material and floating layers
Large slow-moving paddles move viscous material and can be particularly useful where floating or settled layers form in a digester.
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External recirculation
Digester mixing without an immersed mixer: biomass and biogas recirculation
An external system of pumps, pipework and nozzles can mix digester contents by recirculating biomass and biogas, without a submerged motor inside the tank.
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Fibrous substrates
Straw, grass and manure: mixing fibrous and heavy substrates
Long fibres, high dry matter content and heterogeneous structure can cause wrapping, floating layers and high torque, requiring a different mixing approach.
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System selection
Choosing a digester mixer: volume, dry matter, geometry and energy use
Start with the tank geometry, substrate characteristics, feeding method and maintenance requirements before deciding on the mixer type and power rating.
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Gas-treatment overview
Biogas cleaning: which contaminants must be removed, and how clean must the gas be?
Raw biogas contains moisture, H₂S, CO₂ and trace compounds. The required treatment depends on whether the gas will feed a CHP unit, be upgraded to biomethane, supply a fuel cell or be made into Bio-LNG.
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Microaeration and H₂S
Digester microaeration – biological H₂S removal before biogas leaves the digester
Controlled addition of a small amount of air or oxygen can promote biological sulphide oxidation within the digester and reduce the load on downstream equipment.
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Additives and chemical control
Iron salts and additives: reducing H₂S during digester feeding
Iron salts can bind sulphides in the liquid phase before they enter the biogas. Dosing can be integrated with substrate intake and feeding.
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Activated carbon and polishing
Activated carbon – final removal of H₂S, siloxanes and VOCs from biogas
Activated carbon is often the final polishing stage, but different contaminants require different carbon grades and impregnation.
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Drying and condensate
Biogas cooling and drying: removing moisture and condensate
Cooling the gas condenses water vapour; a separator and drains remove the condensate before the compressor, engine, filters or upgrading system.
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Scrubbers and desulphurisation
Biological and chemical scrubbers for high-volume biogas desulphurisation
At higher gas flow rates or H₂S loads, an external scrubber can remove most of the sulphur using liquid regeneration or biological oxidation.
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Trace compounds and polishing
Siloxanes, VOCs, terpenes and ammonia – often overlooked contaminants
H₂S is not the only concern. Traces of siloxanes, VOCs, terpenes and ammonia can cause deposits, corrosion and problems for engines, membranes and catalysts.
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Upgrading to biomethane
From biogas to biomethane: membranes, PSA, water wash and amine scrubbing
Upgrading separates CO₂ and raises the methane concentration. Membranes, PSA, water wash and amine scrubbing differ in their operating requirements, energy use and process characteristics.
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Gas quality for each end use
CHP, fuel cells or Bio-LNG: how clean must the gas be for each end use?
Requirements differ: a CHP engine, fuel cell, gas grid and Bio-LNG plant require different gas specifications. Design the treatment chain backwards from the intended end use.
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