Biogas Heating Systems: Boilers, Direct Heat and When They Beat CHP

A biogas heating system can be one of the simplest and most efficient ways to use the gas produced by an anaerobic digestion plant. Instead of converting the biogas into electricity first, the gas is burned in a suitable boiler or process heater and its energy is used directly as heat.

That option is sometimes overlooked. Combined heat and power (CHP) has played such a prominent role in the development of anaerobic digestion that there can be a tendency to assume that a gas engine should be the default destination for biogas. More recently, biomethane upgrading has become another major option.

But neither CHP nor biomethane is automatically the best answer. If an AD plant has a substantial and dependable requirement for heat, particularly at or close to the site, direct use of biogas in a boiler may be simpler, cheaper and more energy-efficient.

This article explains how biogas boilers and heating systems work, where the heat can be used, what gas treatment may be required, and when direct biogas heating deserves to be considered alongside CHP and biomethane upgrading.

Key Takeaways

• A biogas boiler converts the chemical energy in biogas directly into useful heat. There is no intermediate electricity-generation stage.
• Direct heat can be particularly attractive where there is a large, reliable thermal demand close to the digester.
• Biogas boilers are generally mechanically simpler than reciprocating CHP engines. This can mean lower maintenance requirements, although burners, boilers and gas-treatment systems still require proper servicing.
• Gas quality matters. Moisture, hydrogen sulphide and other contaminants may need to be controlled according to the burner and boiler manufacturer's requirements.
• Heat demand should be measured rather than assumed. Seasonal or intermittent demand can leave substantial quantities of biogas without a useful outlet.
• CHP may be preferable where electricity has high value and recovered heat can also be used.
• Biomethane may be preferable where there is little local heat demand and a viable route exists for renewable gas.
• The best biogas heating system is therefore part of a wider biogas-utilisation decision.

What Is a Biogas Heating System?

In its simplest form, a biogas heating system takes gas from an anaerobic digester, conditions it as necessary and burns it in a boiler or other suitable combustion appliance. The resulting heat may be supplied as hot water, steam or, in some industrial applications, directly to a process.

The basic energy route is straightforward:

organic feedstock → anaerobic digestion → biogas → gas treatment → boiler or burner → useful heat.

This is different from CHP. A CHP engine first converts part of the biogas energy into mechanical power and electricity, while recovering some of the remaining energy as heat. With a biogas boiler, the objective from the outset is heat production.

That difference sounds simple, but it can have important consequences for capital cost, maintenance, efficiency and the amount of useful energy ultimately obtained from the gas.

How Does a Biogas Boiler Work?

A biogas boiler operates on broadly the same combustion principles as other gas-fired boilers, but the burner and fuel system must be suitable for biogas. Raw biogas has a lower calorific value than natural gas because a substantial proportion of it is normally carbon dioxide rather than methane.

The exact methane concentration varies with feedstock and digester performance. A boiler therefore needs to cope with the expected gas composition and pressure range, rather than being selected on the assumption that biogas behaves exactly like mains natural gas.

A typical installation may include:

• a biogas holder or other gas-storage provision;
• condensate removal;
• hydrogen sulphide treatment where necessary;
• gas filtration;
• a gas blower or booster;
• pressure regulation and safety valves;
• a biogas-compatible burner;
• the boiler itself;
• a hot-water or steam distribution system;
• controls and instrumentation; and
• a flare or alternative gas-management route for abnormal conditions.

The boiler should be considered as part of the complete gas system. Selecting a nominal boiler capacity without understanding the quantity, methane content and variability of the available biogas is a poor starting point.

Why Use Biogas Directly for Heat?

The strongest argument for direct heat is that it avoids an unnecessary energy conversion where electricity is not the main requirement. If what the site actually needs is heat, burning biogas efficiently in a boiler can be a very direct route from fuel to useful energy.

CHP has an important advantage when both electricity and heat are valuable. However, electricity generation in a reciprocating engine inevitably converts only part of the fuel energy into electrical output. The rest appears largely as heat and losses.

That arrangement makes excellent sense when the electricity has a high value and the recovered heat has a genuine use. It is less convincing where the site mainly wants heat and would struggle to make productive use of the electrical output.

Where Can Biogas Heat Be Used?

Anaerobic digestion itself creates a useful thermal demand, but this is only one possible destination. Depending on the site, biogas heat can serve the digester, feedstock treatment, an industrial process or an external heat customer.

Digester Heating

Most commercial digesters need temperature control. Mesophilic and thermophilic digestion both depend on maintaining suitable operating conditions, and incoming feedstock can impose a substantial heating load, particularly during cold weather.

A biogas boiler can supply hot water to external heat exchangers or other digester-heating arrangements. This is a straightforward use because the heat demand arises from the process that produces the gas.

Feedstock Preheating

Heating incoming feedstock before it enters the digester can reduce the thermal shock and heat demand within the vessel. Where external heat exchangers are used, they also offer the practical advantage of being accessible for inspection and cleaning.

Pasteurisation

Food-waste and animal by-product treatment may require pasteurisation or hygienisation. Where the process requires hot water, a biogas boiler can provide some or all of that thermal energy, subject to the temperature and duty required.

Industrial Process Heat

This is potentially one of the most attractive applications. A food factory, dairy, brewery, distillery or other industrial operation may have a continuous demand for hot water or steam while also producing an organic residue suitable for anaerobic digestion.

In that situation the AD plant can become part of the site's energy system: residues produce biogas, and the biogas displaces some of the fossil fuel that would otherwise have been burned for process heat.

Digestate Processing

Heat may also be used for digestate concentration or drying. Care is needed here because drying can consume large amounts of thermal energy. The fact that heat is available does not automatically make drying economically or environmentally worthwhile.

Buildings and Space Heating

Offices, workshops, livestock buildings, greenhouses and other premises can sometimes use biogas-derived heat. The difficulty is often seasonality: a building that provides a useful winter heat load may require very little heat during summer, while the digester continues producing biogas throughout the year.

Hot Water or Steam?

The appropriate heating medium depends on the end use. Many AD applications require hot water rather than steam, particularly for digester heating, building heating and relatively low-temperature processes.

Industrial sites may require steam. A biogas-fired steam boiler can serve these applications, but the operating pressure, water treatment, boiler controls, statutory inspection requirements and operator competence all need to be considered.

It is therefore better to start with the heat user's requirement than with the boiler catalogue. Establish the required temperature, pressure, flow and operating profile first, and then select the equipment capable of meeting that duty.

How Efficient Is a Biogas Boiler?

A properly selected modern boiler can convert a high proportion of the fuel energy into useful heat. The precise efficiency depends on boiler design, return-water temperature, excess air, flue-gas temperature, burner control and operating load.

However, equipment efficiency is only part of the story. A boiler operating at high combustion efficiency is not creating useful value if the heat is subsequently dumped or supplied to a process that exists only to consume surplus energy.

That is the same principle we have discussed in relation to CHP: useful heat matters more than theoretically available heat. Current UK CHPQA guidance explicitly requires qualifying CHP heat outputs to be quantified and justified, which is a useful engineering principle even when assessing a direct biogas heating project.

Biogas Boiler Versus CHP

This is often the most useful comparison. Both technologies can use essentially the same renewable gas, but they produce different energy products and have different operating characteristics.

The comparison should not be reduced to a claim that boilers are more efficient than CHP or vice versa. They perform different jobs.

If a factory needs a large quantity of heat but little additional electricity, a boiler may make excellent sense. If an AD plant imports substantial electricity and can also use the engine heat, CHP may extract greater economic value from the same gas.

For the complete CHP case, see our Biogas CHP Systems: Benefits, Efficiency, Costs and Design guide.

When Can Direct Biogas Heat Beat CHP?

Direct heat deserves serious consideration when there is a large and reasonably continuous heat demand close to the digester. This is particularly true where the heat would otherwise be produced by burning a purchased fuel.

For example, suppose an industrial site already operates boilers throughout the year and also has organic residues suitable for AD. Producing biogas and using it directly in a suitable boiler may allow fossil gas consumption to be reduced without installing a reciprocating engine, generator, grid connection and CHP heat-recovery system.

There are fewer energy conversions and potentially fewer moving parts to maintain. The economics can consequently be attractive even though there is no electricity revenue.

But the key phrase is reliable heat demand. If that demand disappears for several months each year, the utilisation problem returns.

When Is CHP Likely to Be Better?

CHP becomes more compelling where electricity has a high on-site value and there is also a useful destination for the recovered heat. Anaerobic digestion plants themselves can fit this profile because pumps, mixers, depackaging systems and other equipment consume electricity while the digester needs heat.

A well-matched CHP system can therefore displace purchased electricity and boiler fuel simultaneously.

CHP may also be preferable where there is a viable electricity export arrangement but no single thermal load large enough to justify consuming all the biogas in a boiler.

Our separate article on Combined Heat and Power Advantages and Disadvantages looks more closely at that decision.

What About Biomethane?

There is now a third major option that cannot sensibly be ignored. Instead of burning raw biogas locally, the gas can be upgraded by removing much of its carbon dioxide and contaminants to produce biomethane.

Where an appropriate connection and sufficient scale exist, biomethane can be injected into the gas grid. It can also be used in other applications such as suitable transport or industrial fuel markets.

This changes the direct-heat decision. A site with only a modest local heat demand may obtain greater value by upgrading its gas rather than installing a boiler simply because a heat use can be found.

The wider comparison is covered in our Biogas Utilisation: CHP, Biomethane, Heat and the Best Uses for Biogas guide.

Biogas Quality for Boilers and Burners

Biogas does not have the same composition as pipeline natural gas. Its methane content is lower, carbon dioxide content is higher, and untreated gas can contain hydrogen sulphide, moisture and other contaminants.

A boiler installation therefore needs a fuel specification agreed with the equipment supplier. Gas treatment should be designed to meet that requirement rather than based on a generic assumption that every biogas boiler needs identical cleaning equipment.

Hydrogen Sulphide

Hydrogen sulphide (H2S) is one of the most important contaminants. It is toxic and corrosive, and combustion converts sulphur compounds into products that can contribute to corrosion in boilers, flues and downstream equipment.

The acceptable H2S concentration depends on the equipment, materials, operating temperatures and manufacturer's requirements. Where treatment is needed, biological, chemical, activated-carbon and other desulphurisation methods are available.

Moisture and Condensate

Raw biogas leaving a digester is normally saturated or close to saturated with water vapour. As the gas cools in pipework, condensate forms.

Gas lines should therefore be designed with suitable falls, condensate traps and drainage. Allowing liquid water to collect in gas pipework can cause operational and safety problems.

Siloxanes

Siloxanes are particularly associated with sewage sludge gas and some waste-derived biogases. Their significance depends on the feedstock and combustion equipment.

Where they are expected, gas analysis should establish whether treatment is necessary rather than assuming either that they are harmless or that expensive removal is automatically required.

Can an Existing Natural-Gas Boiler Burn Biogas?

Sometimes an existing boiler can be adapted, but it should never simply be assumed that natural gas and biogas are interchangeable.

The lower calorific value of raw biogas means a greater gas volume is required to deliver the same thermal input. Burner capacity, gas pressure, flame stability, controls, valve sizing and combustion-air arrangements may therefore need alteration.

Materials must also be compatible with the expected contaminants. The boiler and burner manufacturers should be involved in determining whether conversion is practical and what modifications are required.

In some industrial installations, a dual-fuel arrangement can be valuable. The boiler can use biogas when available and switch to natural gas or another permitted fuel when gas production is insufficient or the AD system is unavailable.

Do Biogas Boilers Need a Backup Fuel?

That depends on the importance of the heat load. If maintaining digester temperature is essential, relying on a single biogas boiler without considering failure, low gas production or maintenance would create an unnecessary process risk.

A backup boiler or dual-fuel capability can provide resilience. This becomes particularly important where the same heating system serves pasteurisation or an industrial process that cannot simply stop whenever biogas production falls.

The opposite problem also needs consideration: what happens when biogas production continues but the heat user stops?

Gas storage can accommodate short-term imbalance, but a gas holder is not normally a long-term energy store. The plant may need an alternative gas user or, ultimately, an appropriately designed flare.

Biogas Boilers and Seasonal Heat Demand

Seasonality is one of the easiest problems to underestimate. A greenhouse, office complex or district-heating customer may look like an excellent heat sink during winter but consume much less energy during warm weather.

The digester, meanwhile, continues producing biogas. Indeed, its own heating requirement may also fall in summer.

A feasibility study should therefore use a monthly, and preferably more detailed, heat-demand profile rather than simply comparing annual biogas energy with annual heat consumption. Two figures can match perfectly on an annual basis while being badly mismatched hour by hour or season by season.

Can Heat Storage Help?

Thermal storage can help smooth differences between the timing of heat production and heat consumption. A hot-water accumulator can, for example, allow a boiler to run steadily while the heat demand varies over shorter periods.

Storage is particularly useful for intermittent loads such as batch pasteurisation. Heat can accumulate before the batch begins and then be drawn down rapidly when required.

However, ordinary hot-water storage is not a practical solution to a large seasonal mismatch. It can shift energy over hours and sometimes days; it does not usually solve the problem of having surplus biogas heat throughout an entire summer.

Biogas Heating for the Digester Itself

Maintaining digester temperature is one of the most dependable uses of biogas heat, but the heat-transfer arrangement deserves proper design. Uneven heating can affect biology, while fouling can reduce heat-transfer efficiency.

Common approaches include internal heating pipes, heating elements associated with the vessel wall, and external heat exchangers through which digestate is circulated.

External heat exchangers have a practical attraction because they can be accessed for maintenance without entering the digester. Their design must nevertheless account for the difficult rheology of digestate, including suspended solids, fibres and the potential for fouling or blockage.

Direct injection of hot water into the digester is generally less attractive where it unnecessarily dilutes the digestate. Indirect heat transfer allows temperature to be controlled without adding water simply for heating purposes.

Should Surplus Biogas Be Burned Just to Avoid Flaring?

A boiler can certainly provide a useful secondary gas outlet and may reduce flaring. But there is an important distinction between avoiding flaring by supplying useful heat and simply burning the gas in a boiler while dumping the resulting heat.

The first creates energy value. The second is little more than moving the point at which the gas is combusted.

If surplus biogas occurs regularly, the underlying reason should be investigated. The CHP may be undersized, the heat demand may have been overestimated, the upgrading plant may lack capacity, or gas production may simply have exceeded the original design assumptions.

Biogas Heating System Costs

It is not sensible to give a universal installed cost or payback period for a biogas boiler. The old version of this article suggested that RHI-supported systems could repay capital within 12 months; that claim is now obsolete and was too broad even when the RHI was open.

The real installed cost may include much more than the boiler itself:

• burner and boiler package;
• biogas treatment;
• gas booster and controls;
• pipework;
• hot-water or steam distribution;
• heat exchangers;
• thermal storage;
• building or container;
• flue and emissions provisions;
• civil and electrical works;
• instrumentation;
• backup heating;
• design and commissioning; and
• ongoing maintenance.

The economic benefit is equally site-specific. The important question is what fuel or energy purchase the biogas heat displaces and how consistently it does so.

The RHI Has Closed to New Applicants

Older discussions of biogas heating often emphasised the UK Renewable Heat Incentive. That is now historical information rather than a reason for installing a new biogas boiler.

The Great Britain Non-Domestic RHI closed to new applicants in 2021. Existing qualifying installations may continue under their applicable scheme arrangements, but a new project should not base its business case on joining the former RHI.

For the history of the scheme, including the important lessons about proving genuinely useful heat, see Renewable Heat Incentive (RHI): What It Was, Why It Closed and Lessons for Biogas CHP.

Biogas Boiler Emissions

Burning renewable biogas is still combustion, so emissions must be considered. Depending on the fuel composition, burner and operating conditions, these can include nitrogen oxides, carbon monoxide and sulphur-related pollutants.

Good combustion control is important for both efficiency and emissions. Environmental permitting requirements should be established for the particular installation rather than assuming that a renewable fuel is automatically exempt from emissions controls.

Where the alternative is CHP, the emissions characteristics are different because a reciprocating engine operates under different combustion conditions. Our specialist article on Biogas CHP Emissions and SCR Systems explains the engine side of this subject.

Heat Networks and Nearby Customers

A neighbouring heat customer can transform the economics of an AD project, but only if the heat can be delivered economically and the demand is dependable.

Heat is not as easy to transport as electricity or gas. Insulated flow and return pipework is expensive, pumps consume electricity, and thermal losses increase with distance.

Before assuming that a nearby factory, housing development or leisure centre will provide a valuable heat market, establish:

• its actual annual heat consumption;
• the hourly and seasonal demand profile;
• required flow temperature;
• distance from the AD plant;
• existing boiler capacity;
• required supply reliability;
• the value of the fuel being displaced;
• the cost of the heat network; and
• the commercial commitment of the heat customer.

A prospective customer expressing interest is not the same thing as a bankable long-term heat offtake agreement.

Could a Hybrid System Be Better?

The utilisation decision does not always have to be CHP or boiler or biomethane. Some sites benefit from a combination.

For example, a CHP engine might provide the normal base electrical and thermal load while a biogas boiler uses additional gas during periods of high heat demand. Alternatively, a boiler may provide backup heat while most of the gas is upgraded to biomethane.

Solar thermal, heat pumps, waste-heat recovery and thermal storage can also form part of the wider heating system. What matters is that the technologies complement one another rather than being added simply because each is individually described as renewable.

Biogas Boiler, CHP or Biomethane: Which Should You Choose?

A useful first screening exercise is to start with the site's energy demand rather than with a preferred technology.

This is only a screening guide. Capital cost, gas quality, plant scale, energy prices, grid capacity, reliability requirements and regulatory conditions can change the result.

Questions to Ask Before Installing a Biogas Heating System

1. How much biogas will actually be available throughout the year?
2. What methane concentration can be expected?
3. How variable will the gas production be?
4. What contaminants are present?
5. What gas quality does the burner manufacturer require?
6. How much heat does the site genuinely need?
7. What is the hourly and seasonal heat-demand profile?
8. At what temperature and pressure is the heat required?
9. What fuel will the biogas replace?
10. How far is the heat user from the digester?
11. Will a hot-water system or steam boiler be required?
12. Is thermal storage worthwhile?
13. What happens when heat demand falls?
14. What happens if the boiler fails?
15. What happens if biogas production falls?
16. What emissions and permitting requirements apply?
17. Would CHP produce greater overall value?
18. Would upgrading the gas to biomethane be a better long-term use?

Common Biogas Heating System Mistakes

• sizing the boiler from theoretical maximum biogas production;
• ignoring methane concentration;
• assuming an annual heat demand means a continuous heat demand;
• underestimating seasonal variation;
• assuming a natural-gas burner will automatically operate correctly on biogas;
• ignoring hydrogen sulphide and condensate;
• forgetting the need for backup heat;
• having no plan for surplus biogas when the heat load disappears;
• installing a long heat main before securing the heat customer;
• valuing dumped heat as useful energy;
• using obsolete RHI assumptions in the financial model; and
• failing to compare direct heat with CHP and biomethane.

Frequently Asked Questions About Biogas Heating Systems

Can biogas be used for heating?

Yes. Biogas can be burned in a suitably designed boiler or burner to produce hot water, steam or process heat. The gas may require treatment to meet the combustion equipment manufacturer's specification.

Can a normal gas boiler run on biogas?

Not necessarily. Raw biogas has a lower calorific value and different composition from natural gas. Burner capacity, controls, gas pressure and materials may need modification, so compatibility should be confirmed with the equipment manufacturer.

Is a biogas boiler more efficient than CHP?

If the only useful energy required is heat, direct combustion can avoid the conversion losses associated with electricity generation. But CHP produces the additional high-value product of electricity, so the better option depends on the site's actual electricity and heat demands.

Can a biogas boiler heat the anaerobic digester?

Yes. Digester heating is one of the most straightforward uses of biogas-derived heat and can be supplied through internal, wall-mounted or external heat-exchange systems.

Does biogas need to be cleaned before burning it in a boiler?

Some treatment is usually required, particularly condensate management, and hydrogen sulphide may need to be reduced. The required treatment depends on the raw gas composition and the burner and boiler specification.

Can biogas produce steam?

Yes. A suitably designed biogas-fired boiler can produce steam for industrial processes. The steam pressure, boiler-water treatment, statutory requirements and burner design must all be appropriate to the application.

Can biogas replace natural gas in an industrial boiler?

Potentially, yes, either wholly or partly. Because raw biogas contains less energy per cubic metre than natural gas, the burner and gas train need to accommodate a greater volumetric gas flow for an equivalent heat input.

Is direct biogas heating cheaper than CHP?

It can involve less complex equipment and avoid reciprocating-engine maintenance, but there is no universal answer. CHP also creates electricity, which may have substantial value. Whole-life economics should be compared for the particular site.

What happens to biogas when no heat is needed?

Short-term gas storage can absorb some imbalance. For longer periods the plant needs another utilisation route or ultimately a flare. This is why seasonal heat demand should be examined carefully before choosing direct heating as the principal gas use.

Can biogas heating still receive the Renewable Heat Incentive?

The Great Britain Non-Domestic RHI is closed to new applicants. Existing accredited installations may continue under their applicable arrangements, but new projects should be assessed using current energy values and support mechanisms.

Conclusion: Sometimes the Simplest Use of Biogas Is the Best

Biogas does not have to pass through a generator before its energy becomes valuable. Where a site has a strong and reliable demand for heat, a well-designed biogas boiler can provide a relatively simple route from anaerobic digestion to useful renewable energy.

That simplicity can be particularly attractive at industrial sites where boilers are already consuming fossil fuel. Instead of producing electricity and then searching for somewhere to use the CHP heat, it may make more sense to use the biogas directly for the energy service the site actually needs.

But direct heating has its own limitation: heat is difficult to transport and demand is often seasonal. A boiler that has no useful summer load does not solve the biogas-utilisation problem.

The right question is therefore not, “Should an AD plant have a biogas boiler?” It is, “What useful energy does this site need, and which route extracts the greatest practical value from the available biogas?”

Sometimes the answer will be CHP. Sometimes it will be biomethane. And where there is a substantial nearby heat demand, the answer may be the simpler option: burn the biogas efficiently and use the heat directly.

Further Reading

• Biogas Utilisation: CHP, Biomethane, Heat and the Best Uses for Biogas – the main decision guide for choosing what to do with AD biogas.
• Biogas CHP Systems: Benefits, Efficiency, Costs and Design – our main guide to producing electricity and useful heat from biogas.
• Biogas CHP Design: Selecting and Sizing a CHP System – detailed engineering considerations for CHP selection.
• Combined Heat and Power Advantages and Disadvantages – when CHP works well and when its limitations become important.
• Renewable Heat Incentive (RHI): What It Was, Why It Closed and Lessons for Biogas CHP – the history of renewable heat support and the importance of genuinely useful heat.

Biogas heating-system design is site-specific. Gas production and composition, heat demand, equipment specifications, emissions requirements, safety, maintenance and alternative utilisation routes should be assessed for each project.

[Published December 2019. Updated and rewritten August 2026.]