Biogas CHP Emissions: SCR Systems for NOx Control in Gas Engines

Biogas combined heat and power (CHP) engines are an established way to convert biogas from anaerobic digestion into useful electricity and heat. However, burning biogas in a reciprocating gas engine also produces exhaust emissions, including nitrogen oxides (NOx), carbon monoxide (CO) and other combustion products.

As emission standards have tightened, controlling these pollutants has become an increasingly important part of CHP plant design and operation. One of the principal technologies used to reduce NOx emissions is Selective Catalytic Reduction (SCR).

SCR systems inject a reducing agent into the hot exhaust gas and pass the resulting mixture through a catalyst. Under the correct operating conditions, the system converts much of the NOx into nitrogen and water before the exhaust gases are discharged to atmosphere.

This article explains how SCR works on biogas CHP engines, why NOx control matters, what operators need to consider when specifying and maintaining an SCR system, and how an early ETW Energietechnik installation demonstrated the technology in CHP service.

If you are looking for a broader introduction to generating electricity and useful heat from anaerobic digestion, see our guide to biogas CHP systems and energy utilisation.

Key Takeaways

• Biogas CHP engines are not emission-free. Combustion can produce nitrogen oxides (NOx), carbon monoxide (CO), residual hydrocarbons and other pollutants that may require control under an environmental permit.
• Selective Catalytic Reduction (SCR) is primarily used to reduce NOx emissions. It introduces a controlled reducing agent, usually based on urea or ammonia, into the exhaust before the gases pass through a catalyst.
• SCR performance depends on more than the catalyst. Exhaust temperature, reagent dosing, engine loading, combustion settings and catalyst condition all affect NOx reduction and the risk of ammonia slip.
• Biogas quality matters. Hydrogen sulphide, moisture, siloxanes and other contaminants can damage CHP equipment and may adversely affect downstream exhaust-treatment systems, making appropriate gas cleaning essential.
• Not every biogas CHP engine requires SCR. The requirement depends on plant size, engine type, emissions performance, location and the conditions imposed by the applicable environmental permit.
• Emissions control should be considered during CHP design. Allowing for gas treatment, monitoring, exhaust treatment and possible future SCR installation can be considerably easier and cheaper than retrofitting them later.
• High quoted CHP efficiency does not necessarily mean high useful energy utilisation. Recovered heat only provides a benefit when the AD plant or another nearby user has a practical demand for it.
• The 2018 ETW installations provide an early example of designing CHP for tighter NOx limits. The original guest-post case study is retained below as a historical example within this substantially updated guide.

What Emissions Are Produced by a Biogas CHP Engine?

A CHP gas engine converts the chemical energy in biogas into mechanical energy and then electricity. Heat is recovered from the engine cooling system and exhaust gases, allowing significantly more of the fuel's energy to be put to beneficial use than would normally be achieved by electricity generation alone.

But CHP does not mean emission-free combustion.

The exhaust from a biogas engine may commonly emit the following biogas CHP emissions:

• Nitrogen oxides (NOx) formed primarily during high-temperature combustion.
• Carbon monoxide (CO) resulting from incomplete combustion.
• Carbon dioxide (CO2) produced by combustion of methane and other combustible gases.
• Residual hydrocarbons, including small quantities of unburned methane where combustion is incomplete.
• Sulphur compounds, depending on the hydrogen sulphide content of the fuel and the effectiveness of upstream gas treatment.
• Other trace pollutants whose significance depends on the biogas composition, engine design and operating conditions.

The concentration and mass emission of each pollutant will depend on the engine, fuel quality, combustion settings, load profile and exhaust treatment system.

This is one reason why the emissions performance of a biogas CHP plant should be considered during design rather than treated as an issue to be addressed after the engine has been installed.

Why Is NOx an Important CHP Emission?

Nitrogen oxides are formed when nitrogen and oxygen react during combustion. Formation is strongly influenced by combustion temperature, oxygen availability and residence time within the engine cylinder.

NOx emissions matter because they contribute to poor air quality and can play a role in the formation of ground-level ozone and secondary particulate pollution.

Gas engines must achieve a compromise between efficiency, reliable combustion and low emissions. Engine combustion controls can reduce NOx formation, but where a sufficiently low emission limit cannot be achieved by primary engine controls alone, secondary exhaust-gas treatment may be required.

This is where SCR becomes important in reducing biogas CHP emissions.

What Is Selective Catalytic Reduction?

Selective Catalytic Reduction, or SCR, is an exhaust after-treatment process used to reduce nitrogen oxide emissions.

A controlled quantity of reducing agent is injected into the exhaust stream upstream of a catalyst. In many CHP applications the reagent is an aqueous urea solution. Under suitable temperature conditions, the reagent provides ammonia which reacts selectively with NOx across the catalyst.

The principal end products are nitrogen and water.

In simplified terms, the process consists of four stages:

1. The CHP engine produces hot exhaust gas containing NOx.
2. A carefully controlled quantity of urea solution or another ammonia-based reagent is introduced into the exhaust stream.
3. The reagent is mixed with the exhaust gas before entering the catalyst.
4. Within the catalyst, chemical reactions convert a large proportion of the NOx into nitrogen and water.

The principle is straightforward, but achieving consistently low biogas CHP emissions in practice requires good control of temperature, reagent dosing, mixing, catalyst condition and engine operation.

Why SCR Control Is More Than Simply Injecting Urea

Successful SCR operation depends on supplying approximately the right amount of reducing agent for the quantity of NOx entering the system.

Too little reagent can result in insufficient NOx removal.

Too much can result in excess ammonia passing through the catalyst. This is known as ammonia slip.

Modern systems therefore use automatic controls linked to engine operating conditions and, where specified, NOx measurement before or after the catalyst. The objective is to achieve the required NOx reduction without excessive reagent consumption or unacceptable ammonia emissions.

The catalyst must also operate within an appropriate temperature window. If the exhaust temperature is too low, conversion efficiency may fall. Excessively high temperatures may shorten catalyst life or damage the catalyst material.

Biogas Quality Can Affect Biogas CHP Emissions Performance

Biogas is not the same as pipeline-quality natural gas.

Raw biogas from an anaerobic digester typically contains methane and carbon dioxide together with varying quantities of water vapour, hydrogen sulphide and trace contaminants. Depending on the feedstock, siloxanes and other compounds may also be present.

These contaminants matter for several reasons.

Hydrogen sulphide (H2S) is corrosive and can contribute to sulphur-related emissions and deposits. It may also adversely affect downstream equipment and catalyst performance.

Water vapour can cause condensation and corrosion if gas cooling and condensate management are inadequate.

Siloxanes, which may occur particularly in biogas derived from sewage sludge and some waste streams, can form hard silica-containing deposits during combustion. These deposits can damage engines, foul heat-transfer surfaces and cause problems within downstream exhaust systems.

For these reasons, gas cleaning should not be considered separately from the CHP engine and its emissions-control equipment. The required degree of gas treatment should be established from the actual biogas composition and the specifications of the engine and exhaust-treatment suppliers.

Effective gas conditioning can improve engine reliability while helping protect the exhaust catalyst and associated equipment.

Does Every Biogas CHP Engine Need SCR?

No.

The need for SCR depends on the applicable environmental permit, engine size and type, installation date, fuel, operating regime, local air-quality considerations and the emissions performance that can be achieved without secondary abatement.

Emission requirements also vary between countries and jurisdictions.

In England and Wales, for example, combustion plant may fall within the environmental permitting requirements applying to Medium Combustion Plant (MCP) and, where applicable, Specified Generators. The Medium Combustion Plant regime applies to qualifying combustion units such as engines, turbines and boilers, with requirements determined by factors including rated thermal input, fuel and plant status.

Current Environment Agency standard rules distinguish between natural gas, biogas and other gaseous fuels and set specific emission limit values for qualifying plant.

For example, current rules for certain new or qualifying biogas-fired medium combustion plant engines include emission limits for NOx and sulphur dioxide. However, operators should not assume that a generic published limit automatically represents the limit applicable to their own installation. The environmental permit and regulator's requirements for the individual plant remain decisive.

Current Environment Agency guidance on permitting can be found at Medium combustion plant: apply for an environmental permit.

CHP Emissions Should Be Considered During Plant Design

It can be expensive and technically awkward to discover after commissioning that an engine cannot achieve the required emissions performance.

An AD developer considering CHP should therefore establish at an early stage:

• the proposed engine's rated thermal and electrical output;
• expected biogas composition and variability;
• manufacturer fuel-quality requirements;
• expected NOx and CO emissions at representative loads;
• applicable environmental-permit conditions;
• whether primary engine controls can achieve the required limits;
• whether oxidation catalysts, SCR or other secondary treatment will be required;
• reagent storage and consumption requirements;
• space and access requirements for the exhaust-treatment equipment;
• stack height and dispersion requirements;
• emissions-monitoring requirements; and
• whole-life maintenance and catalyst replacement costs.

This is part of the wider task of properly matching the CHP plant to the anaerobic digestion facility. Our separate article on combined heat and power system design considerations discusses the broader engineering issues involved.

SCR Catalyst Fouling, Ageing and Maintenance

An SCR catalyst is not necessarily a fit-and-forget component.

Performance can deteriorate because of ageing, thermal damage, contamination or physical fouling. Deposits may restrict the available catalyst surface or increase exhaust-system backpressure.

The significance of catalyst contamination depends strongly on the fuel, upstream gas cleaning, engine operation and catalyst formulation.

Operators should therefore monitor trends rather than waiting until an emission test fails.

Potential warning signs include:

• increasing NOx emissions downstream of the catalyst;
• increasing reagent consumption for the same engine load;
• increasing ammonia slip;
• increased exhaust backpressure;
• changes in exhaust temperature;
• visible deposits or contamination found during inspection; and
• failure to achieve the permitted emission limit during scheduled monitoring.

Cleaning may be possible in some circumstances, but catalyst cleaning should only be undertaken by a competent specialist using a method suitable for the catalyst and the contaminants involved.

Eventually, catalyst replacement may be necessary.

Why SCR Does Not Eliminate the Need for Good Engine Operation

It would be a mistake to regard SCR as a substitute for good combustion control.

The CHP engine should first be correctly specified, commissioned and maintained. Ignition timing, air-fuel ratio, engine load and other combustion parameters affect both efficiency and emissions.

An engine operating badly can impose unnecessarily high loads on the SCR system.

A sound emissions-control strategy therefore normally combines:

• appropriate engine selection;
• good-quality biogas treatment;
• correct combustion settings;
• preventive engine maintenance;
• appropriate exhaust after-treatment; and
• periodic emissions monitoring.

That whole-system approach is particularly important at anaerobic digestion facilities because biogas composition and flow can vary with feedstock, biological performance and digester loading.

“What Is CHP? | US EPA” from www.epa.gov and used with no modifications.

SCR, CHP Efficiency and the Wider Energy Balance

The purpose of a CHP system is not simply to generate electricity. Its major advantage is that heat which would otherwise be rejected can also be recovered and used.

Modern CHP packages can achieve high overall efficiencies where both the electricity and recovered heat are put to useful purposes.

But quoted CHP efficiency figures need to be interpreted carefully.

A CHP engine may be capable of recovering a large proportion of the fuel energy as electricity plus heat, but this does not mean that all of the heat is necessarily useful to the AD plant.

Some heat may be required to maintain digester temperature. Additional heat may be used for pasteurisation, feedstock processing, drying or other site activities. Where there is no suitable demand for the remaining heat, it may ultimately have to be rejected.

Consequently, developers should distinguish between recoverable heat and usefully utilised heat.

For a fuller discussion of when CHP makes economic and technical sense, see our article on the advantages and disadvantages of combined heat and power.

Historical Case Study: ETW's Early SCR-Equipped CHP Plants in Duisburg

Historical note: This article was originally published in March 2018 following information supplied about SCR-equipped CHP installations delivered by German CHP specialist ETW Energietechnik. It was subsequently expanded in 2022 and has now been substantially rewritten to provide a broader technical explanation of biogas CHP emissions and SCR technology while preserving this historically interesting example.

At the time of the original report, ETW Energietechnik had delivered CHP plants incorporating a urea-injection SCR exhaust-gas cleaning system for Stadtwerke Duisburg in Germany.

The project comprised three CHP units using MWM TCG 2020 V12 gas engines, each providing approximately 999 kW of electrical output.

Two units were installed at the Duisburg-Mitte and Duisburg-Süd sites and were reported as operating by the end of 2017, with a third unit planned for Duisburg-Süd.

The project was notable because SCR technology was being integrated into the CHP packages in anticipation of more stringent emissions requirements.

ETW reported that the installations combined SCR exhaust treatment with a high level of heat recovery. A second exhaust-gas heat-exchanger stage and recovery of heat from the engine cooling circuit formed part of the overall energy-utilisation concept.

The original project specifications targeted substantially reduced emissions of NOx, carbon monoxide and formaldehyde compared with the older German requirements applying when the project was conceived.

Designing for Future Emission Requirements

One particularly useful lesson from the Duisburg project remains relevant today.

ETW advocated considering future SCR requirements when the CHP installation was first designed, rather than waiting until tighter emission standards made retrofit unavoidable.

That principle remains sound engineering practice.

Even where SCR is not immediately required, an operator may consider whether the exhaust arrangement can economically accommodate future secondary abatement. Allowing appropriate space, access, duct geometry, electrical capacity and control interfaces during initial construction can make a later upgrade much easier.

This is particularly relevant because CHP installations can remain operational for many years, during which environmental standards, permit requirements and local air-quality expectations can change.

ETW Energietechnik delivered the first combined heat and power plants with SCR equipped exhaust gas cleaning system in 2018. A Duisburg municipal utility CHP plant was equipped with this new, urea injection-based system “SCR” exhaust gas cleaning system.

Watch the video below which introduces this subject.

ETW Energietechnik and Current CHP Technology

ETW Energietechnik remains active in CHP and biogas technology. The company currently supplies gas-engine CHP systems as well as biomethane upgrading equipment.

Its current CHP technology includes solutions for compliance with exhaust-emission requirements and SCR catalyst technology incorporating NOx monitoring and control.

For current information from the original post contributor, visit ETW Energietechnik's CHP and biogas technology page.

Biogas CHP, Biomethane or Another Energy Route?

CHP is only one way to obtain value from biogas.

At many modern anaerobic digestion plants, an alternative is to upgrade raw biogas to biomethane by removing carbon dioxide and contaminants. The biomethane may then be injected into a gas network or used as a vehicle or industrial fuel where the necessary infrastructure and regulations allow.

The best option depends on factors including:

• electricity value;
• availability and value of useful heat;
• grid connection;
• biogas production rate;
• biomethane incentives or market value;
• gas-grid availability;
• capital and operating costs; and
• the site's long-term energy strategy.

Our main biogas CHP guide explains the role of CHP within an anaerobic digestion plant in more detail.

Frequently Asked Questions About Biogas CHP and SCR

What does SCR stand for in a CHP plant?

SCR stands for Selective Catalytic Reduction. It is an exhaust-gas treatment process primarily used to reduce nitrogen oxide emissions from combustion equipment such as gas engines.

What does SCR remove from CHP exhaust gas?

SCR is principally used to reduce NOx. A reducing agent, usually based on ammonia or urea, reacts with nitrogen oxides across a catalyst to form mainly nitrogen and water.

Can SCR be used on a biogas engine?

Yes. SCR can be applied to reciprocating engines burning biogas where the engine exhaust conditions are suitable and additional NOx reduction is required. The complete system must be designed around the engine, exhaust temperature, fuel quality and applicable emission limits.

Is AdBlue used in stationary CHP plants?

Aqueous urea solutions based on the same general principle as automotive AdBlue can be used as the reagent for stationary SCR systems. The dosing and control equipment, however, forms part of a purpose-designed industrial exhaust-treatment system.

Does a biogas CHP plant need an environmental permit?

That depends on the plant size, location, fuel, regulatory jurisdiction and the other activities undertaken at the site. In England and Wales, qualifying medium combustion plant and specified generators are subject to environmental permitting requirements. Anaerobic digestion facilities may also require environmental permits for the wider waste-treatment activity.

Can H2S in biogas damage a CHP engine or catalyst?

High hydrogen sulphide concentrations can contribute to corrosion, deposits, lubricant degradation and problems in downstream equipment. Engine and catalyst suppliers normally specify fuel-quality requirements, and appropriate H2S removal should be provided where needed.

Does an SCR system make a CHP plant emission-free?

No. SCR is primarily a NOx-control technology. A CHP plant continues to produce combustion gases, including carbon dioxide, and may produce CO, residual hydrocarbons and other pollutants. Different pollutants require different prevention, combustion-control or exhaust-treatment measures.

Conclusion: Treat Emissions Control as Part of the CHP System

A well-designed biogas CHP installation should be considered as an integrated energy and emissions-control system rather than simply an engine connected to a generator.

The engine must suit the expected biogas supply. The gas should be conditioned to an appropriate quality. Recoverable heat should have a genuinely useful destination. Exhaust emissions must comply with the applicable permit, and any SCR system must be properly specified, controlled and maintained.

The early ETW installations reported here are interesting because they demonstrated the value of anticipating stricter emission requirements rather than designing only for the minimum standard applicable at the time.

That principle is even more relevant today.

For further information within our CHP series, read:

• Biogas Utilisation: Combined Heat and Power Units for Efficient Energy Conversion – our main overview of CHP for anaerobic digestion.
• Combined Heat and Power Systems: CHP Design Considerations – engineering considerations when selecting and designing a CHP installation.
• Combined Heat and Power Advantages and Disadvantages – when CHP makes sense and where its limitations should be recognised.

[Originally published on 24 March 2018. Major update and completely rewritten August 2026.]

[This article contains historical information originally supplied in connection with ETW Energietechnik's CHP installations. It has been substantially revised and expanded to provide current technical information. Environmental requirements vary according to location, plant size, fuel, operating regime and permit conditions. Operators should confirm the requirements applicable to their individual installation with the appropriate regulator and equipment suppliers.]

For more information on Combined Heat and Power Plants with SCR, visit the CHP page on their website.