Industrial energy systems waste more heat than most plant managers realise. In combustion-based processes, a significant portion of the energy input leaves the system as hot flue gas, carrying latent heat that conventional equipment simply exhausts to the atmosphere. Combining heat recovery with heat pumps addresses this directly, turning what was an unavoidable loss into a recoverable and usable energy source. This article builds from foundational concepts to practical application, guiding you through how flue gas scrubbers and heat pumps work together, where the combination delivers the greatest return, and what engineering decisions determine whether a system performs to its potential.

Whether you manage a district heating plant, a biomass facility, or an industrial drying process, the principles covered here apply to your operation. By the end, you will have a clear framework for assessing whether this technology configuration is appropriate for your site, and what factors your engineering team needs to evaluate before moving forward.

Mitä lämmön talteenotto ja lämpöpumput tarkoittavat teollisuudessa?

Heat recovery in an industrial context means capturing thermal energy that would otherwise be lost, typically from flue gases, exhaust steam, or process cooling streams, and returning it to a useful application. Heat pumps are thermodynamic machines that move heat from a lower-temperature source to a higher-temperature output, consuming electrical energy to do so but delivering more thermal energy than they consume.

These two technologies address different parts of the same problem. Heat recovery captures what is available in the flue gas stream. The heat pump then upgrades that recovered energy to a temperature level that is actually useful, for example high enough to feed a district heating network or a downstream process. Neither technology alone achieves what the combination can.

A useful analogy: think of heat recovery as collecting rainwater and the heat pump as the pump that lifts that water to a height where it can flow usefully through your system. The collection step and the elevation step are both necessary. In industrial settings, the combination is particularly relevant wherever combustion generates large volumes of flue gas, and wherever the recovered heat needs to be delivered at a controlled, elevated temperature.

Miten savukaasupesuri ja lämpöpumppu toimivat yhdessä?

A flue gas scrubber, specifically one using condensing technology, is the heat recovery device that makes this combination possible. As hot flue gases pass through the scrubber, the water vapour they contain is cooled below its dew point and condenses. This condensation releases latent heat, which is the energy originally used to evaporate that water during combustion. That latent heat is transferred to the scrubber’s cooling water circuit.

On its own, this cooling water circuit operates at a relatively low temperature, often in the range of 30 to 55 degrees Celsius depending on the process. For many district heating applications, this is too low to inject directly into the network. This is where the heat pump enters the system.

The heat pump’s role in the circuit

The heat pump takes the low-temperature heat from the scrubber’s cooling circuit and upgrades it, raising the output temperature to a level that the district heating network or downstream process can accept. The heat pump’s evaporator absorbs heat from the scrubber circuit, and its condenser releases that heat at the elevated temperature required by the application.

The critical engineering insight here is that the scrubber and heat pump must be designed as an integrated system, not as two independent units bolted together. The scrubber’s operating temperature range, the heat pump’s coefficient of performance, and the network’s return temperature all interact. For example, when district heating return temperatures rise during warmer months, the temperature differential driving condensation in the scrubber narrows, which reduces heat recovery efficiency unless the system has been specifically designed to compensate. A patented heat pump connection, such as the one used in Caligo Industria’s CSx HP configuration, maintains maximum heat recovery performance precisely under these conditions.

Energy flow through the combined system

To make the energy flow concrete, consider the following sequence in a biomass boiler application:

1. Combustion generates hot flue gases containing both sensible heat and latent heat in water vapour.
2. The flue gas scrubber cools the gases, condensing water vapour and capturing latent heat into the cooling water circuit.
3. The heat pump evaporator absorbs heat from the cooling water circuit.
4. The heat pump condenser delivers heat at the required output temperature to the district heating supply line or process heat exchanger.
5. The cooled flue gases, now cleaned of particulates and SO₂, are discharged through the stack.

This sequence recovers energy at two stages: the sensible heat drop as gases cool, and the latent heat released during condensation. Together, these can represent heat recovery of up to 35% of the original fuel energy input, translating directly into reduced fuel consumption and lower CO₂ emissions.

Missä sovelluksissa yhdistelmä tuottaa eniten hyötyä?

Building on the mechanics described above, the combination of a flue gas scrubber and heat pump delivers the greatest benefit in applications where three conditions align: high moisture content in the flue gas, a consistent heat demand at a controlled temperature, and a process where fuel savings translate directly into operating cost reduction.

The applications where this combination is most effective include:

• District heating networks supplied by biomass or waste-to-energy boilers, where the heat pump raises recovered heat to network supply temperatures and the system operates continuously across seasons.
• Sawmills and pellet production facilities, where drying processes generate large volumes of moisture-laden exhaust air that carries significant latent heat.
• Industrial drying processes in food manufacturing and other sectors, where continuous waste steam recovery reduces the thermal energy input required to maintain dryer temperatures.
• Combined heat and power plants where flue gas heat recovery supplements the electrical generation cycle and improves overall plant efficiency.

Applications with dry flue gases, low or intermittent heat demand, or very low fuel costs relative to electricity costs will see a reduced benefit from the heat pump component. The economics of the heat pump depend on the coefficient of performance, which in turn depends on the temperature lift required. The larger the temperature difference between the scrubber cooling circuit and the required output temperature, the more electrical energy the heat pump consumes, and the longer the payback period.

Mitkä tekijät vaikuttavat järjestelmän mitoitukseen?

Correct sizing of a combined heat recovery and heat pump system requires a detailed understanding of the specific process parameters at your facility. There is no universal configuration. The variables that most directly affect system design are the flue gas volume and moisture content, the required output temperature, the district heating or process return temperature, and the load profile across the operating year.

Flue gas moisture content

Moisture content in the flue gas determines how much latent heat is available for recovery. Biomass fuels with high water content, such as wet wood chips or bark, produce flue gases rich in water vapour and therefore offer the greatest latent heat recovery potential. Drier fuels or natural gas produce less water vapour, reducing the available latent heat and shifting the balance of recovery toward sensible heat alone.

Return temperature and its seasonal variation

The return temperature of the district heating network or process cooling circuit is one of the most important sizing parameters, and also one of the most commonly underestimated. A system sized for a winter return temperature of 40 degrees Celsius may lose significant efficiency in summer when return temperatures rise to 60 degrees or more. The heat pump must be sized to maintain performance across the full seasonal range, not just at the design-point condition.

Heat demand profile

Systems with flat, continuous heat demand, such as industrial processes running around the clock, offer the best conditions for heat pump integration because the capital cost is spread across maximum operating hours. Systems with highly variable or seasonal demand require careful economic modelling to confirm that the heat pump’s contribution justifies its capital and electrical operating costs.

Electrical energy cost

Because the heat pump consumes electricity to upgrade recovered heat, the ratio of electricity cost to fuel cost is a key economic variable. In markets where electricity is expensive relative to biomass or waste fuel, the heat pump’s operating cost must be weighed carefully against the fuel savings it enables. In markets with low-cost renewable electricity, the economics typically favour a larger heat pump with a higher temperature lift.

Näin rakennat toimivan lämmön talteenottojärjestelmän

Applying the concepts covered above, the following framework describes how to approach the design and implementation of a combined heat recovery and heat pump system in a structured, methodical way.

Step 1: Characterise your flue gas stream. Before any equipment is specified, measure or calculate the flue gas volume, temperature, moisture content, and composition at your process outlet. These parameters define the maximum recoverable energy and determine whether condensing technology is viable. If the flue gas dew point is too low, condensation-based recovery may not be effective without process modifications.

Step 2: Define your heat demand requirements. Establish the required output temperature, the load profile across the year, and the return temperature conditions you will encounter across all seasons. This step is often where projects encounter surprises: the summer return temperature may be significantly higher than the winter value, and the system must be designed for both.

Step 3: Assess the heat pump temperature lift. Calculate the difference between the scrubber cooling circuit temperature and the required output temperature. This temperature lift determines the heat pump’s coefficient of performance and therefore its electrical energy consumption. A smaller lift means a more efficient heat pump; a larger lift means higher electricity costs per unit of heat delivered.

Step 4: Model the annual energy balance. Use your load profile and efficiency figures to calculate the annual heat recovery, annual electricity consumption, and net fuel savings. This analysis should cover the full operating year, not just the peak condition, to give an accurate picture of economic performance.

Step 5: Engage in a consultative engineering review. The interactions between scrubber performance, heat pump sizing, and network conditions are complex enough that a detailed consultative process with your equipment supplier is essential before finalising the design. The right configuration for a pellet dryer operating at 85 degrees Celsius is not the right configuration for a district heating plant managing seasonal return temperature swings. We work through these parameters with our customers before any equipment recommendation is made, because an undersized or incorrectly configured system will underperform for its entire operational life.

Step 6: Specify integrated controls and automation. A combined scrubber and heat pump system requires coordinated control logic that manages the interaction between the two units across varying operating conditions. Specifying integrated automation from the outset, rather than connecting two independently controlled systems after installation, significantly reduces commissioning complexity and improves long-term operational stability.

Understanding emissions control technology alongside heat recovery gives a complete picture of what condensing flue gas systems can achieve, since cleaning and energy recovery happen simultaneously within the same unit.

If you are evaluating whether a combined heat recovery and heat pump system is appropriate for your facility, the most productive next step is a technical conversation with an engineering team that has designed and commissioned these systems across a range of industrial applications. Contact us to discuss your process parameters and we will work through the configuration options with you.