Anaerobic Digestion Heat Exchangers: Types, Uses, Fouling and Heat Recovery

Anaerobic digestion heat exchangers perform one of the less glamorous but most important jobs on a biogas plant: moving heat from where it is available to where the process actually needs it.

They can heat incoming feedstock, maintain digester temperature, recover heat from hot digestate, transfer heat from CHP engines, provide energy for pasteurisation and, in some installations, support evaporation or digestate concentration.

Heat exchangers are therefore not an optional refinement added to sophisticated AD plants. In many modern installations, they are a central part of the process design.

The challenge is that anaerobic digestion does not always present heat exchangers with an easy fluid to handle. Sludge, slurry and digestate can be viscous, fibrous, abrasive and prone to fouling. A heat exchanger that performs beautifully on clean water may block rapidly when asked to handle thick digestate containing fibres, grit and suspended solids.

This guide explains the main types of heat exchanger used at anaerobic digestion plants, where they fit into the process, what causes fouling and blockage, and what designers should consider when selecting equipment.

Key Takeaways on the Topic of Anaerobic Digestion Heat Exchangers

• Heat exchangers are used throughout modern AD plants. Applications include digester heating, feedstock preheating, pasteurisation, CHP heat recovery and digestate processing.
• The fluid being heated matters as much as the required heat duty. Thick sludge and digestate can foul or block equipment designed for clean liquids.
• Tube-in-tube, shell-and-tube and spiral heat exchangers are commonly used for sludge and slurry applications. Plate heat exchangers can be excellent for cleaner liquid circuits but may be unsuitable for fibrous or heavily contaminated streams.
• Counter-current flow generally provides more efficient heat transfer than parallel flow. It allows a greater useful temperature approach between the two fluids.
• Fouling is often the real design constraint. Access for cleaning, tube diameter, flow velocity and solids characteristics can be more important than achieving the smallest theoretical exchanger area.
• Heat recovery can materially reduce energy demand. Hot digestate or pasteurised material can be used to preheat incoming feedstock rather than allowing that energy to be lost.
• CHP heat is only valuable if it reaches a genuine heat demand. Heat exchangers are the equipment that make that transfer possible.
• The best heat exchanger is not necessarily the one with the highest quoted heat-transfer coefficient. Reliability, cleanability, pressure drop and resistance to blockage are equally important in AD service.

What Does a Heat Exchanger Do?

A heat exchanger transfers thermal energy from one fluid to another without normally allowing the two fluids to mix.

At an anaerobic digestion plant, a hot-water circuit might pass heat from a CHP engine to digestate circulating through a separate pipe. Alternatively, hot pasteurised digestate may transfer heat to cold incoming feedstock before the two streams move on to different stages of the process.

The principle is simple. The engineering difficulty lies in obtaining enough heat transfer without creating excessive pressure loss, fouling, sedimentation or blockage.

That is particularly important because many AD process fluids behave nothing like water. A digestate containing fibres, grit, plastics contamination or high suspended solids may require a very different exchanger from one used for a clean glycol circuit.

Why Anaerobic Digesters Need Heat

Anaerobic digestion is temperature-sensitive. The microbial populations responsible for methane production perform best within a controlled operating range, so commercial digesters normally require some form of heating.

Mesophilic digesters commonly operate in the region of the mid-30s °C, while thermophilic processes operate at higher temperatures. The exact setpoint depends on the process design rather than a single universal figure.

Heat is required because energy is continually lost through tank walls, roofs, pipework and other surfaces. Incoming feedstock may also arrive well below digester temperature, particularly during winter, creating an additional thermal load.

The heat exchanger is the link between the source of thermal energy and the material that needs heating.

Where Does the Heat Come From?

At an AD plant, useful heat may come from several sources.

CHP Engine Heat

Where biogas is burned in a combined heat and power unit, considerable thermal energy is available from the engine cooling circuits and exhaust gases.

Hot water from the engine can be circulated through a heat exchanger to warm digester sludge or another process stream. Exhaust heat can also be recovered into a hot-water circuit through a suitable exhaust-gas heat exchanger.

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

Biogas Boilers

Where biogas is used directly for heat, a boiler can produce hot water or steam for the AD process. A separate heat exchanger then transfers that energy to feedstock, digestate or another process stream.

This can be a particularly straightforward arrangement where heat rather than electricity is the main energy requirement. Our article on Biogas Heating Systems: Boilers, Direct Heat and When They Beat CHP discusses that option in more detail.

Hot Digestate or Process Streams

Heat recovery does not always require an external fuel source. Material leaving a hot process may itself contain energy worth recovering.

For example, hot digestate leaving a pasteurisation process can transfer heat to colder incoming material. The outgoing stream cools while the incoming stream warms, reducing the net heat that must be supplied by the boiler or CHP system.

This is often referred to as thermal regeneration.

Main Uses of Heat Exchangers at AD Plants

1. Digester Heating

This is perhaps the most familiar application. Digestate or sludge is circulated from the digester through an external heat exchanger and returned at a controlled higher temperature.

The heating medium is normally hot water from a CHP unit, boiler or another thermal source.

External heat exchangers offer an important maintenance advantage over heating coils buried inside a digester: they can be inspected, isolated and cleaned without entering the vessel.

2. Feedstock Preheating

Cold feedstock can consume a surprising amount of thermal energy, particularly at plants processing high daily throughput.

Preheating incoming material before it enters the digester reduces the heat duty within the main vessel and can give tighter temperature control.

The heat may come directly from a hot-water circuit or, more efficiently, from a warmer outgoing process stream.

3. Pasteurisation

Some waste streams require pasteurisation or hygienisation before or after digestion according to the applicable regulatory regime.

Heat exchangers can be used to bring the material to the required temperature and, importantly, to recover part of that heat afterwards.

Rather than taking cold feedstock directly to pasteurisation temperature and then dumping all the heat from the treated material, a regenerative exchanger can use the outgoing hot stream to warm the incoming cold stream.

That reduces the amount of new heat that needs to be supplied.

4. Sludge-to-Sludge Heat Recovery

Where one sludge stream is hotter than another, direct heat recovery between the two can be attractive. This is particularly relevant in temperature-phased digestion and some sludge-treatment systems.

The two streams remain physically separate while heat passes through the exchanger wall.

The benefit is straightforward: energy already paid for once is used again.

5. Digestate Concentration and Evaporation

Surplus heat can sometimes be used to evaporate water from digestate, thereby reducing liquid volume and increasing solids concentration.

Heat exchangers are important in these systems because digestate is difficult to handle. Its solids content, viscosity and tendency to foul increase as it becomes more concentrated.

This means exchanger design becomes progressively more important as water is removed.

6. Thermal Hydrolysis and High-Temperature Treatment

Some sludge-treatment processes use much higher temperatures than conventional anaerobic digestion. Thermal hydrolysis, for example, uses heat and pressure to break down sludge structure before digestion.

Heat recovery between hot treated sludge and colder incoming material can substantially reduce the net energy demand of such systems.

Internal Heating Coils Versus External Heat Exchangers

Early and simple anaerobic digesters often rely on heating pipes or coils fitted inside the digester. Hot water passes through these pipes and transfers heat directly through the pipe wall into the digesting material.

The arrangement is mechanically simple, but maintenance can be awkward. If the heating surface becomes coated with deposits, inspection and cleaning may require lowering the digester level or taking the vessel out of service.

External heat exchangers take a different approach. Sludge is pumped out of the digester, through the exchanger and back again.

This adds pumps, pipework and controls, but it offers much easier access for maintenance. It also allows the heat exchanger to be sized and configured independently from the digester vessel itself.

Neither arrangement is universally superior. The decision depends on digester geometry, sludge characteristics, circulation requirements, maintenance philosophy and available space.

Tube-in-Tube Heat Exchangers

Tube-in-tube, sometimes called concentric-tube, heat exchangers are widely associated with sludge heating because they can provide a relatively large flow passage for the difficult process fluid.

One fluid flows through the inner tube while the heating or cooling medium flows through the surrounding annulus.

For digestate service, the sludge side can be designed with generous bore sizes and smooth bends to reduce the risk of blockage.

The technology is relatively straightforward and can be arranged in modular sections. That can be useful where equipment must be installed in an existing plant room with restricted access.

Advantages of Tube-in-Tube Exchangers

• large sludge flow passages can be provided;
• relatively tolerant of suspended solids;
• straightforward mechanical construction;
• can be designed for mechanical cleaning;
• modular configurations are possible; and
• well suited to external digester circulation loops.

Possible Limitations

• large installations can require considerable floor space;
• heat-transfer area per unit volume may be lower than compact exchanger types;
• high-viscosity sludge can create significant pumping requirements; and
• poorly designed bends or low-flow areas can still accumulate solids.

Shell-and-Tube Heat Exchangers

Shell-and-tube exchangers contain multiple tubes within an outer shell. One fluid flows through the tubes while the other passes around them.

They are widely used throughout industry because they can handle high temperatures and pressures and can be built in many configurations.

In anaerobic digestion they may be used for hot-water circuits, sludge heating, process heat recovery and exhaust-heat applications.

For sludge service, tube diameter and access for cleaning become critical. A compact exchanger with small tubes may offer good theoretical heat transfer but become a maintenance problem if fibres or solids repeatedly obstruct the passages.

Spiral Heat Exchangers

Spiral heat exchangers use two flow passages wound around a central core. The geometry creates a single continuous passage for each fluid and can be attractive for sludge and wastewater applications.

One reason is that the flow path can maintain comparatively good velocity through the exchanger, reducing low-flow zones where solids tend to settle.

Spiral exchangers can also provide a compact footprint. This can be valuable when retrofitting equipment into an existing wastewater or AD plant where space is limited.

They are not immune to fouling, despite promotional claims sometimes implying otherwise. The important question is how easily the exchanger can be opened, inspected and cleaned when fouling eventually occurs.

Plate Heat Exchangers

Plate heat exchangers offer very high heat-transfer rates in a compact package. They consist of multiple thin plates creating alternating flow channels for the hot and cold fluids.

They work extremely well with clean liquids and are often used on secondary hot-water circuits.

They require more caution where raw feedstock, fibrous sludge or thick digestate passes directly through the plates. Narrow channels can block, and solids may accumulate in areas where the passage geometry is unsuitable.

Wide-gap plate exchangers are available for more difficult fluids, but the decision should be based on actual particle size, solids content, viscosity and cleaning requirements.

For this reason, plate exchangers should not be rejected categorically for AD service, but nor should their impressive clean-water performance be assumed to translate directly to digestate.

Scraped-Surface Heat Exchangers

Highly viscous or fouling products may justify a scraped-surface exchanger. In these units, rotating or reciprocating elements continually remove material from the heat-transfer surface.

This helps maintain heat transfer where ordinary static surfaces would rapidly develop an insulating layer.

The disadvantage is greater mechanical complexity, power consumption and maintenance. Consequently, scraped-surface equipment tends to be justified where the process fluid is sufficiently difficult that simpler exchanger designs cannot remain clean.

Which Type of Heat Exchanger Is Best for Anaerobic Digestion?

There is no universal best design.

The answer depends on what is flowing through it.

A designer should therefore start with the process fluid and required duty, not with a preferred heat-exchanger technology.

Fouling: The Problem That Often Determines the Design

In a clean laboratory calculation, heat exchanger performance is largely a matter of surface area, temperature difference and heat-transfer coefficients. In a real AD plant, fouling can dominate the entire design.

Material can accumulate on heat-transfer surfaces and act as insulation. As the layer grows, heat transfer falls and pressure drop may increase.

Fouling can result from:

• fibrous feedstocks;
• grease and fats;
• mineral scaling;
• struvite and other precipitates;
• biological solids;
• grit;
• foreign material; and
• thermal degradation of material on hot surfaces.

The effect is gradual in some plants and alarmingly rapid in others.

Why Small Passages Can Be a Problem

Increasing velocity and turbulence can improve heat transfer, which encourages designers toward smaller flow passages. Unfortunately, smaller passages also increase blockage risk.

This is a classic AD design compromise.

A heat exchanger intended for clean water can achieve excellent performance using narrow channels. Put fibrous food-waste slurry through the same geometry and it may fail operationally despite looking excellent in the original thermal calculation.

The exchanger must therefore be designed around the largest credible solids, fibre characteristics and viscosity of the actual process stream.

Flow Velocity Matters

Low flow velocity encourages solids to settle and allows stagnant layers to form at the heat-transfer surface. Higher velocity increases turbulence and can help keep the exchanger cleaner.

But higher velocity also increases pressure loss and pumping energy.

It may also increase wear where abrasive grit is present.

The optimum is consequently a balance: enough velocity to maintain useful heat transfer and limit deposition, without imposing excessive pumping cost or erosion.

Pressure Drop and Pumping Energy

A heat exchanger is not genuinely energy-efficient if achieving its thermal performance requires excessive pumping power.

High-viscosity digestate can require substantial pressure to move through pipework and exchanger passages. A seemingly modest change in solids concentration can materially alter pumping behaviour.

The exchanger supplier should therefore provide pressure-drop calculations using representative fluid properties rather than water values.

Where rheology is uncertain, obtaining actual viscosity or pumping data can be valuable.

Counter-Current Versus Parallel Flow

In a parallel-flow exchanger, the hot and cold fluids enter at the same end and travel in the same general direction. Their temperatures progressively approach one another.

In a counter-current exchanger, the fluids move in opposite directions.

Counter-current flow normally provides a greater average temperature-driving force and allows the cold outlet to approach the hot inlet temperature more closely.

For heat recovery and regeneration, this can make a substantial difference. It is one reason counter-current arrangements are widely favoured where the objective is maximum heat recovery.

Heat Exchanger Sizing

Heat exchanger sizing starts with the required heat duty.

At its simplest:

Heat duty = mass flow × specific heat capacity × required temperature rise.

But that is only the beginning.

The designer also needs to know:

• hot-side inlet and outlet temperatures;
• cold-side inlet and required outlet temperatures;
• flow rates;
• specific heat capacities;
• solids content;
• viscosity;
• particle and fibre characteristics;
• allowable pressure drop;
• likely fouling rate;
• materials compatibility; and
• cleaning requirements.

Using an unrealistically optimistic fouling allowance can result in an exchanger that meets the specification when clean but fails to provide the required duty after a short period in service.

Do Not Forget the Winter Heat Balance

Digester heat demand changes through the year. Cold incoming feedstock and greater heat loss from tanks can make winter the critical design condition.

A heat exchanger sized only around annual average conditions may therefore fail to maintain digester temperature during cold weather.

The thermal model should include the lowest credible feedstock temperature and appropriate ambient conditions, together with realistic CHP or boiler availability.

If the main heat source is CHP, the designer should also consider what happens when the engine is down for maintenance. A backup boiler may still be needed even when there is normally ample CHP heat.

Heat Recovery from CHP Engines

A biogas engine converts only part of its fuel energy into electricity. Much of the remainder appears as heat.

Heat can normally be recovered from the engine water circuits and, where required, from the exhaust.

The resulting hot-water circuit can supply heat exchangers elsewhere on the AD plant.

This separation is useful because the CHP manufacturer’s water circuit remains clean while the dirty digestate passes through equipment specifically designed for sludge.

The CHP heat exchanger effectively creates a boundary between the engine package and the biological process.

Why Useful Heat Is More Important Than Heat Recovered

Recent articles in this series have repeatedly returned to one point because it genuinely matters: recovering heat is not the same as using it well.

A CHP engine can produce large quantities of hot water, and a heat exchanger can transfer that heat very efficiently. But if the receiving process does not need the energy, the installation has not created useful thermal value.

For an AD plant, good examples of genuine heat demand include maintaining digester temperature, replacing boiler fuel in pasteurisation or supplying an industrial process that would otherwise consume another energy source.

This distinction is discussed further in our article on the advantages and disadvantages of combined heat and power.

Pasteurisation and Thermal Regeneration

Pasteurisation is one of the applications where good heat-exchanger design can make a particularly large difference.

Imagine cold feedstock entering a pasteurisation process while an equal quantity of hot treated material leaves. Without heat recovery, energy must be supplied to heat the incoming stream and the outgoing material then cools with much of that energy wasted.

A regenerative exchanger allows the two streams to exchange heat.

The hot outgoing material preheats the cold incoming material before final heating. At the same time, the outgoing stream is cooled.

The result is a lower net heating requirement.

This is often more important than simply choosing a boiler with a higher nominal efficiency because it reduces the heat demand of the process itself.

Heat Exchangers and Digestate Evaporation

Digestate concentration is attractive where reducing liquid volume can lower storage or transport requirements. However, evaporating water requires substantial energy.

Using genuine surplus heat can improve the case, particularly if heat would otherwise be rejected.

As concentration increases, though, the digestate becomes more difficult to pump and more prone to fouling. The heat exchanger therefore needs to cope not only with the original digestate but also with the changing physical properties of the increasingly concentrated stream.

Claims about dramatic percentage reductions in digestate volume should always be treated in context. Removing large amounts of water is technically possible, but the energy balance, ammonia management, nutrient retention and final disposal route determine whether it makes sense.

Heat Exchanger Materials

Material selection depends on both sides of the exchanger.

Stainless steel is widely used because of its corrosion resistance, but specifying stainless steel by name alone is not sufficient. Different grades have different resistance to chlorides, sulphides and other aggressive constituents.

Carbon steel may be suitable in some services and more economical, particularly on clean closed hot-water circuits, but corrosion allowances and water chemistry need consideration.

The supplier should know the expected:

• chloride concentration;
• pH;
• hydrogen sulphide exposure;
• cleaning chemicals;
• maximum temperature; and
• other corrosive constituents.

This is particularly important where clean-in-place chemicals will periodically be stronger than the normal process fluid.

Cleaning a Digester Heat Exchanger

Every exchanger handling sludge should be designed on the assumption that it will eventually need cleaning.

Possible methods include:

• flushing at high velocity;
• reverse-flow cleaning;
• chemical cleaning;
• clean-in-place systems;
• mechanical rodding;
• opening removable covers; and
• removing exchanger sections for workshop cleaning.

The appropriate method depends on the type of deposit.

Fibrous blockages need a different response from mineral scale. Chemical cleaning that dissolves carbonate scale will not necessarily remove a mat of plastic fibres or hair.

Good access is therefore one of the most valuable features a heat exchanger can have.

Design for Isolation and Maintenance

An exchanger that cannot be isolated easily can turn a routine cleaning job into a plant shutdown.

Useful design provisions include:

• isolation valves on both process and heating circuits;
• bypass pipework where continuity is important;
• drain and flushing connections;
• lifting space and maintenance access;
• removable bends or covers;
• pressure and temperature measurement; and
• provision for clean-in-place equipment where appropriate.

These items may appear unimportant on a process-flow diagram, but they often determine how pleasant or unpleasant the equipment is to operate for the next twenty years.

Monitoring Heat Exchanger Performance

Fouling often reveals itself gradually.

The most useful indicators include:

• increasing pressure drop;
• declining outlet temperature;
• greater hot-water flow needed to achieve the same duty;
• increased pumping power;
• larger temperature differences than expected; and
• more frequent process temperature alarms.

Recording inlet and outlet temperatures on both sides of the exchanger allows operators to see whether heat-transfer performance is changing over time.

Pressure gauges or transmitters across the dirty side can be equally valuable because an increasing differential pressure often provides early warning of blockage or deposition.

What About Maceration Before the Heat Exchanger?

Where fibrous feedstock is being pumped through an exchanger, upstream size reduction may reduce the risk of blockage. Macerators are sometimes installed specifically to protect pumps, valves and heat exchangers.

That does not mean that every AD stream should be aggressively shredded. Equipment selection should consider the feedstock, unwanted contamination and the possible consequences of producing smaller plastic fragments.

The aim is to provide a pumpable, manageable feedstock, not simply to make every particle as small as possible.

Heat Exchanger Efficiency Versus System Efficiency

A manufacturer may quote an impressive heat-transfer coefficient or thermal effectiveness. Those figures are useful, but they describe the exchanger, not necessarily the whole heating system.

System efficiency also depends on:

• pumping electricity;
• pipe heat losses;
• boiler or CHP efficiency;
• fouling;
• temperature control;
• thermal storage;
• maintenance downtime; and
• whether the recovered heat actually has a useful destination.

A larger exchanger with lower pressure drop and better fouling tolerance may therefore outperform a theoretically more compact unit over a full operating year.

Questions to Ask a Heat Exchanger Supplier

1. What sludge solids content has the proposed exchanger handled successfully?
2. What maximum particle and fibre size can pass through it?
3. What process-fluid viscosity has been used for the design?
4. What pressure drop is expected when clean?
5. How much additional pressure drop has been allowed for fouling?
6. What fouling factor has been used in the thermal calculation?
7. What minimum flow velocity is recommended?
8. Can the exchanger be cleaned mechanically?
9. Can it be cleaned in place?
10. How quickly can it be opened for inspection?
11. What materials are used on the sludge side?
12. Are those materials compatible with the proposed cleaning chemicals?
13. What is the guaranteed heat duty under the specified process conditions?
14. Does the guarantee apply to actual digestate properties or only water-test conditions?
15. What instrumentation is recommended?
16. How much pumping power is expected?
17. What spare parts should be held on site?
18. Can maintenance be carried out without stopping the entire AD process?

Common Anaerobic Digestion Heat Exchanger Mistakes

• sizing from clean-water data;
• underestimating digestate viscosity;
• ignoring fibres and foreign material;
• choosing very small passages solely to maximise heat-transfer coefficient;
• failing to allow for fouling;
• providing inadequate cleaning access;
• ignoring pressure drop and pumping power;
• assuming annual-average heat demand represents winter conditions;
• failing to provide backup heat during CHP downtime;
• valuing all recovered heat as useful heat;
• ignoring corrosion from process chemistry;
• installing an exchanger without isolation and bypass arrangements; and
• selecting equipment on purchase price rather than whole-life operating reliability.

Frequently Asked Questions About Anaerobic Digestion Heat Exchangers

Why do anaerobic digesters need heat exchangers?

Heat exchangers transfer heat from CHP engines, boilers or hot process streams into feedstock or digestate. They are commonly used to maintain digester temperature, preheat feedstock, recover process heat and support pasteurisation.

What type of heat exchanger is best for digestate?

There is no universal best type. Tube-in-tube and spiral exchangers are commonly considered because they can provide relatively generous flow passages. The correct choice depends on solids content, fibres, viscosity, required duty, pressure drop and cleaning requirements.

Can a plate heat exchanger handle digestate?

Some can, particularly wide-gap designs handling relatively well-screened material, but conventional narrow-channel plate exchangers may block on fibrous or high-solids digestate. The actual process fluid should determine the selection.

How is an anaerobic digester normally heated?

Common arrangements include internal heating coils or external recirculation through a heat exchanger. External exchangers are attractive because they can normally be isolated and maintained without entering the digester.

Can CHP heat be used to heat a digester?

Yes. This is one of the most common uses of recovered CHP heat. Hot water from the engine cooling circuit can transfer energy through a heat exchanger to circulating digester sludge.

Can heat be recovered from digestate?

Yes. Where outgoing digestate is warmer than incoming feedstock, a sludge-to-sludge exchanger can recover part of that energy and reduce the amount of new heat required.

Why do sludge heat exchangers block?

Common causes include fibres, grit, plastics, high solids content, mineral scaling and low flow velocity. Narrow passages, sharp bends and poor cleaning access can make the problem worse.

How can heat-exchanger fouling be detected?

Increasing pressure drop, falling heat duty and changing inlet-to-outlet temperature differences are common indicators. Regular monitoring helps identify deterioration before the exchanger becomes severely obstructed.

Is counter-current flow better than parallel flow?

For most heat-recovery duties, counter-current flow provides a greater effective temperature difference and can achieve closer temperature approaches, making it generally preferable for efficient thermal regeneration.

Can surplus CHP heat be used for digestate drying or evaporation?

Potentially, yes. However, evaporation requires substantial energy and concentrated digestate can be difficult to handle. The complete thermal and economic balance should be checked before assuming that surplus heat makes evaporation automatically worthwhile.

Conclusion: In AD, Cleanability Can Matter More Than Compactness

Heat exchangers are fundamental to efficient anaerobic digestion because they allow thermal energy to be moved from CHP engines, boilers and hot process streams to the parts of the plant that actually need it.

The thermal calculation itself is rarely the hardest part. The real challenge is transferring heat reliably through a fluid that may be thick, fibrous, abrasive and prone to fouling.

That is why successful AD heat exchanger selection goes beyond asking how many square metres of heat-transfer area are required. Designers need to ask how the sludge will flow, what might block the passages, how the equipment will be cleaned, what pressure the pumps must provide and whether maintenance can be carried out without stopping the process.

A compact exchanger with excellent clean-fluid performance is of little value if operators have to dismantle it every few weeks. Conversely, a somewhat larger exchanger with generous passages, sensible velocities and good access may prove far more efficient over its working life.

The best heat exchanger is therefore the one that continues to transfer the required heat after it has spent months and years handling the real feedstock and digestate produced by the plant.

Further Reading on the Topics of Anaerobic Digestion Heat Exchangers

• Biogas Utilisation: CHP, Biomethane, Heat and the Best Uses for Biogas – deciding how best to use the gas produced by an AD plant.
• Biogas CHP Systems: Benefits, Efficiency, Costs and Design – our main CHP pillar guide.
• Biogas CHP Design: Selecting and Sizing a CHP System – detailed CHP design and heat-demand considerations.
• Biogas Heating Systems: Boilers, Direct Heat and When They Beat CHP – direct use of biogas for heat.
• Combined Heat and Power Advantages and Disadvantages – including the distinction between recoverable and genuinely useful heat.
• Evaporation Techniques for Digestate and Sludge Management – where heat recovery and exchanger design become particularly important.

Heat exchanger selection is process-specific. Actual solids content, viscosity, particle characteristics, fouling tendency, temperature, chemistry and required heat duty should be established before equipment is specified.

[Published in October 2017. Updated and rewritten August 2026.]