Biogas CHP Design: 7 Factors for Selecting and Sizing a CHP System

August 15, 2026 Biogas Equipment No Comments

Biogas CHP design involves much more than selecting a gas engine with an electrical output that appears to match the expected biogas production from an anaerobic digestion plant.

A successful combined heat and power (CHP) installation must match the quantity and quality of the available biogas, the site's electrical demand, the genuinely useful heat demand, operating hours, maintenance requirements, grid connection and environmental constraints.

Getting those relationships wrong can leave an AD plant with an oversized engine, wasted heat, excessive flaring, poor availability or disappointing financial returns.

Getting them right allows CHP to convert biogas into useful electricity and heat while supporting the energy requirements of the anaerobic digestion process itself.

This guide examines seven of the most important engineering considerations when selecting and sizing a CHP system for a biogas plant.

For a broader introduction to the technology, see our main guide to biogas CHP systems and energy utilisation.

Biogas CHP Design: Key Takeaways

Do not size a biogas CHP engine from digester capacity alone. CHP selection should be based on a realistic annual and seasonal biogas-production profile.
Methane flow matters more than total biogas flow. The energy available to the CHP engine depends strongly on methane concentration as well as gas volume.
Match the engine to the site's electrical requirements. Electricity used behind the meter may have a different value from electricity exported to the grid.
Quantify genuinely useful heat. Heat that can technically be recovered from an engine should not automatically be treated as valuable useful heat.
Specify biogas cleaning around the engine manufacturer's requirements. Hydrogen sulphide, moisture, siloxanes and other contaminants can materially affect engine reliability and maintenance costs.
Design for maintenance and downtime. Biogas production does not conveniently stop whenever the CHP engine needs servicing.
Include emissions and permitting from the start. Engine selection, exhaust treatment, stack design and environmental limits can affect the entire CHP layout.
Compare CHP with alternative uses of biogas. Biomethane upgrading may sometimes provide a better use of the gas, particularly where there is little demand for surplus CHP heat.

What Is a Biogas CHP System?

A biogas combined heat and power system normally consists of a reciprocating gas engine coupled to an electrical generator together with equipment that recovers heat from the engine.

Biogas produced by anaerobic digestion is conditioned to the quality required by the engine and then burned to produce mechanical power.

The generator converts that mechanical power into electricity.

At the same time, thermal energy can be recovered from sources including:

engine jacket cooling water;
lubricating-oil cooling;
charge-air cooling; and
the engine exhaust gases.

At an anaerobic digestion plant, some of that heat can usually be returned to the process to maintain the required digester temperature. Depending on the plant, additional heat may be used for pasteurisation, feedstock preparation, drying, buildings or an external heat customer.

The fundamental design challenge is therefore to match one variable biological fuel source to two different energy outputs: electricity and heat.

The 7 Most Important Biogas CHP Design Factors

1. Establish the Realistic Biogas and Methane Production Profile

The starting point for CHP sizing should be the amount of usable energy that the anaerobic digestion process is realistically expected to produce.

This should not simply be the highest theoretical biogas yield quoted for the feedstock.

Actual biogas production depends on factors including:

feedstock composition;
feedstock quantity;
dry-matter and volatile-solids content;
biodegradability;
organic loading rate;
hydraulic and solids retention time;
digester temperature;
process stability;
seasonal feedstock variation; and
plant availability.

The biogas CHP design should therefore consider expected gas production over time, not simply a single average or maximum figure.

A CHP unit selected against an optimistic peak biogas-production figure may spend much of its operating life at part load or repeatedly starting and stopping.

Conversely, an undersized engine may leave substantial quantities of biogas unused and potentially require frequent flaring.

Size from Methane Energy, Not Simply Cubic Metres of Biogas

Biogas volume by itself is not a sufficient basis for CHP sizing.

The useful fuel component is principally methane.

Two digesters producing the same volume of biogas can therefore provide different amounts of fuel energy if their methane concentrations differ.

The biogas CHP design calculation should establish the expected:

biogas flow rate;
methane concentration;
methane flow rate;
fuel energy input;
expected electrical efficiency; and
expected electrical output.

Allowance should also be made for realistic variation in gas composition.

Engine suppliers should confirm the acceptable methane range, calorific value and other fuel-quality requirements for the proposed biogas CHP design package.

Gas Storage Can Help – But It Does Not Correct Fundamental Oversizing

Biogas storage provides useful buffering between biological gas production and CHP consumption.

A gas holder can accommodate short-term fluctuations, allowing an engine to operate more steadily than would otherwise be possible.

However, storage capacity is finite.

It should not be used to disguise a fundamental mismatch between average gas production and CHP capacity.

If the digester continuously produces more gas than the CHP installation can consume, the gas holder will eventually fill. If the engine continuously consumes gas faster than the digester produces it, the holder will eventually empty.

In the biogas CHP design world, storage is therefore a balancing component rather than a substitute for correct CHP sizing.

2. Match CHP Electrical Output to the Site Load and Export Strategy

The next consideration is what will happen to the electricity generated.

An AD plant itself consumes electricity for equipment such as:

feedstock reception and preparation;
pumps;
mixers;
depackaging equipment;
macerators and grinders where used;
gas blowers;
digestate separation;
ventilation;
control systems; and
other auxiliary equipment.

This internal electricity consumption is commonly referred to as the plant's parasitic load.

Electricity generated by the CHP unit may therefore be divided between:

electricity consumed by the AD facility itself;
electricity used by another activity on the same site; and
surplus electricity exported to the grid.

These outputs may have different economic values.

Electricity that replaces imported grid electricity can be worth more to the operator than electricity exported at a lower tariff.

Consequently, CHP sizing should consider the site's actual electrical load profile as well as total annual electricity consumption.

CHP Schematic Diagram — CHP Flow Chart courtesy USDA https://energy.gov

Do Not Forget CHP Auxiliary Loads

The CHP installation itself also consumes electricity.

Pumps, gas boosters, ventilation fans, cooling systems, control panels and gas-treatment equipment all require power.

The electrical output quoted for the generator should therefore be distinguished from the net electrical benefit available to the wider site or grid.

Grid Connection Can Become a Design Constraint

Where surplus electricity will be exported, the available grid connection can influence CHP capacity.

The local network operator may impose export limitations or require reinforcement, protection systems or other works.

A technically attractive CHP capacity may therefore not be economically attractive if connecting it to the electricity network requires major expenditure.

Grid discussions should begin early rather than after the CHP package has been selected.

3. Establish the Genuine Useful Heat Demand

This is one of the most important and most frequently misunderstood parts of CHP design.

A CHP supplier can calculate how much heat is technically recoverable from an engine.

That does not mean the project has a use for all of it.

The design team should establish:

how much heat the digesters require;
at what temperatures the heat is required;
how demand varies seasonally;
whether pasteurisation requires additional heat;
whether buildings or other site processes can use recovered heat;
whether an external heat customer exists;
the distance to that customer;
the heat-distribution losses; and
what fuel or energy source the CHP heat will genuinely replace.

The last point is particularly important.

UK Combined Heat and Power Quality Assurance (CHPQA) methodology distinguishes useful heat from heat that is merely produced or recoverable. Qualifying heat must be capable of being quantified and justified.

For engineering and economic assessment, the appropriate question is therefore not:

“How much heat does this engine produce?”

It is:

“How much of that heat will genuinely displace heat that would otherwise need to be supplied?”

Our article on the advantages and disadvantages of combined heat and power examines this issue in more detail.

Heat Temperature Matters as Well as Heat Quantity

Thermal energy is useful only if it is available at a suitable temperature for the intended application.

Engine jacket water can be well suited to digester heating and many low-temperature applications.

Some industrial processes require higher temperatures.

Higher-grade heat may potentially be recovered from the exhaust, but doing so requires appropriate heat-exchanger design and consideration of exhaust-gas conditions.

A design should therefore develop a heat balance by both quantity and temperature level.

Consider Seasonal Heat Demand

An AD plant may require more heat during cold winter weather than during summer.

An external heat customer may show even greater seasonal variation.

For example, a greenhouse, office building or district-heating system may have a strong winter demand but limited summer demand.

Meanwhile, biogas production may continue throughout the year.

Annual totals can therefore be misleading. Monthly, weekly or even hourly load profiles may be needed for larger projects.

4. Specify the Required Biogas Cleaning

Biogas is a variable biological fuel rather than a tightly controlled pipeline gas.

Contaminants can significantly affect CHP reliability.

The most important commonly encountered constituents include:

hydrogen sulphide (H2S);
water vapour and condensate;
siloxanes;
particulates;
ammonia in some applications; and
other trace compounds depending on the feedstock.

Hydrogen Sulphide

Hydrogen sulphide is corrosive and can contribute to lubricant deterioration, deposits and damage to engine and exhaust-system components.

The acceptable concentration should be established from the engine manufacturer's fuel specification.

Gas desulphurisation may use biological, chemical, adsorption or other treatment processes depending on the inlet concentration and required outlet quality.

Moisture and Condensate

Raw biogas is normally saturated or close to saturated with water vapour when it leaves the digester.

Cooling the gas causes condensation.

Pipework should therefore be designed with suitable gradients, condensate traps and drainage arrangements so that water cannot accumulate or be carried into the engine.

Siloxanes

Siloxanes can be particularly significant in biogas produced from sewage sludge and some waste feedstocks.

During combustion they can form hard silica-containing deposits on engine components, spark plugs, valves, turbochargers and exhaust equipment.

Where siloxanes are present, appropriate gas analysis and removal equipment should be considered.

The required treatment system should ultimately be based on measured or reasonably predicted gas composition and the CHP manufacturer's warranty requirements, not on a generic assumption about biogas quality.

5. Design for Engine Availability, Maintenance and Redundancy

A CHP engine operating for thousands of hours each year requires planned maintenance.

It will also eventually require major overhaul.

That creates a particular problem at an anaerobic digestion plant:

the biological process continues producing biogas while the engine is unavailable.

The design must therefore answer a simple question:

What happens to the gas when the CHP engine stops?

Possible provisions include:

biogas storage;
a second CHP engine;
a standby boiler capable of burning biogas;
biomethane upgrading where installed;
another legitimate gas user; and
an appropriately designed emergency flare.

One Large CHP Engine or Two Smaller Engines?

This is an important design decision.

One larger engine may have advantages including:

lower capital cost per unit of installed electrical capacity;
simpler pipework and controls;
fewer engines to maintain; and
potentially higher electrical efficiency.

Two smaller units may provide:

greater redundancy;
better turndown;
greater flexibility as gas production varies;
continued generation while one engine is being serviced; and
better matching to seasonal or changing gas output.

There is no universal answer.

The decision should be based on gas-production variability, maintenance strategy, availability requirements, engine efficiency at different loads and the financial consequences of downtime.

Do Not Design for Continuous 100% Availability

Annual energy and revenue projections should allow realistically for planned servicing, major overhaul and unplanned downtime.

A financial model that assumes an engine will generate at full rated output for 8,760 hours every year is not a realistic operating model.

Availability assumptions should be justified using the proposed engine technology, maintenance contract and operating experience.

6. Include Emissions, Noise and Permitting in CHP Selection

Biogas may be renewable, but burning it in an engine produces combustion emissions.

Potential pollutants include:

nitrogen oxides (NOx);
carbon monoxide (CO);
unburned hydrocarbons, including methane;
sulphur compounds; and
other trace pollutants depending on fuel composition and combustion conditions.

Environmental requirements can influence:

engine selection;
combustion settings;
gas-treatment requirements;
oxidation-catalyst requirements;
Selective Catalytic Reduction (SCR);
stack height;
monitoring provisions; and
site layout.

These matters should therefore be addressed during design rather than treated as accessories to be added after the engine has been ordered.

Our specialist article on biogas CHP emissions and SCR systems for NOx control explains this subject in detail.

Noise and Vibration

Reciprocating engines, exhausts, ventilation systems and cooling equipment can all create significant noise.

Where sensitive receptors are nearby, acoustic design may require:

engine acoustic enclosures;
exhaust silencers;
acoustic ventilation louvres;
low-noise cooling equipment;
building attenuation; and
careful positioning of plant.

Vibration isolation and foundations should also follow the CHP manufacturer's requirements.

7. Assess Whole-Life Economics – Not Just CHP Purchase Price

The cheapest CHP package to purchase is not necessarily the cheapest to own.

A proper comparison should consider whole-life cost and value.

Capital expenditure may include:

CHP engine-generator package;
gas treatment;
gas booster equipment;
heat-recovery equipment;
thermal distribution;
electrical switchgear;
transformers;
grid connection;
emissions-control equipment;
stack;
acoustic treatment;
building or container;
civil works; and
professional and permitting costs.

Operating costs may include:

routine servicing;
lubricating oil;
spark plugs and other consumables;
gas-treatment media;
SCR reagent where applicable;
replacement catalysts;
major engine overhaul;
monitoring;
insurance; and
operator time.

Against those costs should be set realistic values for:

electricity used on site;
electricity exported;
useful heat genuinely displacing another energy source; and
any applicable support mechanism or other project-specific income.

Financial sensitivity analysis should test what happens if gas production is lower than forecast, electricity prices change, maintenance costs rise or the expected external heat customer uses less heat than anticipated.

A Simple Biogas CHP Sizing Sequence

A preliminary CHP design can be approached in the following order:

Estimate realistic annual and seasonal feedstock throughput.
Predict realistic biogas production and methane concentration.
Convert methane production into available fuel-energy input.
Develop a gas-production profile rather than relying only on an annual average.
Establish the site's electrical load profile and likely export.
Establish the site's useful heat demand by quantity, temperature and time.
Compare suitable CHP engine sizes and their part-load performance.
Establish the gas-cleaning requirements for each engine option.
Allow for maintenance downtime and determine what happens to biogas during outages.
Check grid, planning, emissions, noise and permitting constraints.
Prepare whole-life financial comparisons and sensitivity tests.
Compare CHP with alternative biogas-utilisation options before making the final selection.

This sequence helps prevent a common design error: selecting the CHP engine first and then attempting to make the rest of the AD plant fit around it.

CHP Sizing Example: Why the Largest Engine May Not Be the Best Choice

Consider an AD plant where predicted biogas production could theoretically support a 1 MW electrical CHP unit at maximum gas output.

It may be tempting simply to specify the 1 MW engine.

But suppose further analysis shows that:

average gas production is substantially below the predicted maximum;
feedstock availability varies seasonally;
the site itself normally consumes only 300 kW of electricity;
grid export is constrained;
digester heating uses only part of the recoverable heat; and
there is no reliable external heat customer.

Under those circumstances, the largest technically possible engine may not provide the best project.

A smaller engine operating for longer periods at an efficient load – or two smaller modular units – might provide better utilisation, availability and financial performance.

The correct decision requires modelling the complete system rather than maximising a single nameplate capacity.

Should a Biogas Plant Use CHP or Upgrade to Biomethane?

Historically, CHP was an obvious utilisation route for many AD plants because it allowed biogas to generate renewable electricity while supplying the digester's heat requirement.

Today, it should not automatically be assumed to be the best option for every new project.

Biogas can instead be upgraded by removing carbon dioxide and contaminants to produce biomethane.

Depending on the project and regulatory framework, biomethane may be:

injected into a gas distribution network;
compressed for vehicle fuel;
supplied to industrial users; or
used in other applications requiring a higher-methane gas.

CHP may remain particularly attractive where there is a strong on-site electrical demand and a substantial year-round use for recovered heat.

Biomethane upgrading may deserve closer consideration where:

surplus CHP heat would largely be wasted;
electricity export has limited value;
a suitable gas-grid connection is available;
the plant is large enough to justify upgrading equipment; or
the market value of biomethane is stronger.

The correct comparison should consider the complete energy balance and whole-life economics of each option.

CHPQA and Good Quality CHP

In the UK, the Combined Heat and Power Quality Assurance (CHPQA) programme provides a formal methodology for assessing CHP quality.

The assessment is significant because it does not treat every unit described as CHP as equally efficient.

The methodology considers fuel input, power output and useful heat output and applies a Quality Index (QI) to assess scheme performance.

This reinforces an important principle for AD developers:

Good CHP design is not demonstrated merely by installing heat-recovery equipment. The electricity and heat must form part of an efficient, properly utilised energy system.

For small CHP schemes of up to 2 MW, current CHPQA guidance describes fuel used, electricity generated and heat supplied as the three fundamental sets of information needed to assess whether a scheme qualifies as Good Quality CHP.

Current official guidance is available from the UK Government CHPQA guidance notes.

Common Biogas CHP Design Mistakes

Many disappointing CHP projects can be traced to a relatively small number of assumptions made during development.

Common mistakes include:

sizing the engine from maximum theoretical biogas production;
assuming constant methane concentration;
ignoring seasonal feedstock variation;
valuing all recovered heat as useful heat;
failing to establish the required heat temperature;
underestimating parasitic electrical loads;
assuming unlimited electricity export;
under-specifying H2S or siloxane removal;
assuming near-100% engine availability;
failing to provide a gas-management strategy during maintenance;
considering emissions control too late;
underestimating noise;
comparing CHP units only on capital price; and
failing to compare CHP with biomethane upgrading.

Questions to Ask a CHP Supplier

Before selecting a biogas CHP package, developers should obtain clear answers to questions including:

What methane concentration range can the engine tolerate?
What are the maximum permitted H2S and siloxane concentrations?
What gas pressure and temperature are required?
What electrical efficiency is guaranteed at full and part load?
What useful heat outputs are available and at what temperatures?
What is the minimum continuous operating load?
How rapidly can output be modulated?
What are the scheduled service intervals?
When is a major overhaul expected?
What availability can realistically be guaranteed under the maintenance agreement?
What are the guaranteed NOx and CO emissions?
What exhaust after-treatment is required?
What auxiliary electrical consumption should be allowed for?
What remote monitoring is included?
What warranty conditions apply to biogas quality?
What happens to the warranty if gas contaminants exceed those limits?

Answers should be incorporated into the design and financial model rather than left in supplier literature that is never reconciled with the AD process design.

Biogas CHP Design 7 Factors for Selecting and Sizing a CHP System- Text - Featured image.

Frequently Asked Questions About Biogas CHP Design

How do you size a CHP engine for a biogas plant?

CHP sizing should start with a realistic profile of biogas production and methane concentration. The available methane energy is then compared with suitable engine fuel consumption, electrical demand, heat demand, part-load performance, maintenance requirements and grid constraints. Sizing solely from maximum predicted biogas flow can result in an oversized engine.

Should a CHP engine be sized for peak biogas production?

Not necessarily. An engine sized for a short-duration peak may operate inefficiently or intermittently during the rest of the year. The optimum capacity depends on the shape of the gas-production profile, gas storage, part-load performance and the value of the electricity and heat produced.

Is one large CHP engine better than two small engines?

Not always. One larger engine may offer lower capital cost and potentially higher electrical efficiency, while two smaller engines can offer better redundancy, turndown and maintenance flexibility. The correct choice depends on the individual AD plant.

How much heat does a biogas CHP engine produce?

The amount depends on the engine and its electrical efficiency. Manufacturers normally provide heat-balance data showing recoverable heat from cooling circuits and exhaust gases. The important design question is not merely how much heat can be recovered, but how much can genuinely be used at the required temperature.

What gas cleaning does a biogas CHP engine need?

The requirements depend on the raw biogas composition and the engine manufacturer's specification. Treatment commonly includes moisture and condensate removal and may require hydrogen sulphide and siloxane removal. Gas analysis should be used to establish the appropriate treatment system.

What happens to biogas when the CHP engine is being serviced?

The design needs a defined gas-management strategy. Depending on the installation this may include gas storage, another CHP engine, a biogas boiler, another gas user or an emergency flare. Simply assuming that gas production will stop is normally unrealistic.

Does all CHP heat count as useful heat?

No. Heat being technically recoverable does not necessarily make it useful. There should be a genuine demand for the heat at the available temperature and time. UK CHPQA guidance specifically requires qualifying useful heat to be quantified and justified.

Is CHP always the best way to use biogas?

No. CHP is one utilisation route. Biomethane upgrading should also be considered, particularly where useful heat demand is limited or the economics of gas-grid injection or another biomethane market are stronger.

Conclusion: Design the CHP Around the AD Plant – Not the Other Way Around

The most important principle in biogas CHP design is that the engine is only one component of a larger energy system.

The digester determines how much biogas is available. Gas composition determines how much fuel energy that biogas contains and what treatment it requires. The site determines the value of the electricity. Real heat users determine the value of the thermal output. Maintenance and permitting determine how much of the theoretical production can actually be achieved.

A well-designed CHP system brings those factors together.

The objective should therefore not be to install the largest engine that the predicted biogas flow can theoretically support.

It should be to select the CHP configuration that provides the best practical match between biogas production, electrical demand, useful heat demand, reliability, environmental compliance and whole-life economics.