This is sometimes called a salt barrier, since its ability to remove particles is largely related to salt compounds. Together, these very small particles constitute the greatest proportion by weight - about 70% - of all the particles that the condenser is not able to remove.

A wet electrostatic precipitator (WESP)

The salt barrier acts as a wet electrostatic precipitator that works in combination with the condenser. All large particles are removed by the condenser, which functions much like a very efficient scrubber. This reduces the load on the WESP, which only needs to process small particles of a well-defined size. Particles can be removed to a level of < 1 mg/nm3. See also Performance below.

Completely saturated gas

The condenser has another important function in this respect. Cooling the gas to its saturation level enables the electrostatic precipitator to always to process completely saturated gas. This is essential for wet electrostatic precipitators. The other alternatives for humidifying the gas will never produce completely saturated gas. If the condenser can also cool the gas to low temperatures, the gas volume is reduced dramatically, compared with a dry electrostatic precipitator. The equipment can thus be made smaller and produced at significantly lower cost. Lower gas volumes and superior efficiency with respect to the WESP's ionization capacity result in significantly lower electricity requirements than for dry applications, while ensuring that the degree of separation is extremely high. 

Operational reliability

Except for the ionization electronics and a pump, the salt barrier is a completely static component. It is completely maintenance-free, meaning that 100% reliability can be guaranteed.


Svensk Rökgasenergi has a complete portfolio of equipment for various degrees of dust separation. Furans, hydrocarbons, phosphorous, chloride compounds, blue haze and other compounds can be eliminated.

Electrostatic precipitator as a humidifier

The electrostatic precipitator can be supplemented at low cost so that it also functions as a humidifier.

After the flue gas has been cooled by the condenser, still contains a considerable amount of energy. This energy
can be reclaimed in the condenser by humidifying and pre-heating the combustion air, thus further reducing the temperature of the emitted flue gas. This temperature is often significantly lower than the district heat return.


A large number of water pipes with small holes are placed between the flue gas pipes. Through these holes, the flue gas is sprinkled with water under relatively high pressure. The water is vaporized on impact with the flue gas pipe and broken down into fine droplets that are easily absorbed by a non-saturated gas.

The water used for humidification consists of softened condensate.

For efficient humidification, the combustion air should be taken from the top of the boiler, where the temperature is high. The gas is fed to the electrostatic precipitator, where it immediately becomes saturated and reaches the wet gas temperature. The high temperature is reduced in that the gas absorbs water. This process takes place at the wet gas temperature and does not involve any energy exchange.

Thereafter, the air passes over the pipe surfaces heated by the flue gases and the temperature increases. Directly after the row of pipes, the gas is once again sprinkled with water and the temperature is reduced at the same time as the gas becomes saturated with water. The air is again heated, and the process continues in a gradual upward spiral on the gas's saturation curve. At this point, the combustion air has a significantly higher energy content that can be reclaimed in the condenser.

Operational reliability

In this process, the flue gas and the combustion air are completely separated, thus avoiding all problems with leakage flows between the gases. (Compare this with rotating exchangers, for example.) The moving parts, which are a pump, a fan and the softening filter, are the only components in the system subject to damage. 

System solutions

After the combustion air undergoes humidification, the gas, which is saturated, is heated to avoid condensation in the system.

In the collection vessel, a slightly positive pressure is maintained that is regulated by a frequency-controlled fan via a pressure sensor in the vessel. From the vessel, ducts lead to each air intake fan. The vessel's cover is pre-regulated by a magnetic lock, and if an accident occurs, the voltage to the magnetic lock is cut and the cover falls. Conventional operation can continue without disturbing combustion.

Operations with electrostatic precipitator but condenser disconnected

It is possible to disconnect the condenser from the district heating network while the electrostatic precipitator and the boiler are in operation. The condenser then functions solely as a conventional scrubber. This solution is often applied when the load is so low that boiler operation with the condenser connected is difficult while it is at the same time possible to clean the flue gas completely via the electrostatic precipitator.

An outdoor air cooler has sufficient capacity to enable the flue gas to be saturated in the scrubber (i.e. the condenser), as well as to cool the gas so that it can be condensed.

Operation with oversize chip boiler during low-load periods

The above solution is also applicable when the power requirements are less than the chip boiler's minimum load level and oil boilers would otherwise need to be brought on line.

The valves to the condenser are opened and the air-filled battery is allowed to cool the district heat network to the point where the boiler's operation becomes stable. In this situation, the energy that the boiler has produced is thrown away simply to continue operation.

It is easy to compare this intentionally generated heat loss with the operating cost for oil-generated energy, a calculation that often results in an advantage for chip-only boiler operation.

System solution with condenser, or condenser equipped with a humidifier

 The process with a humidifier can be described as follows:

Without humidification, the condenser's energy is extracted by changing
the energy state of the flue gas from A to B. For humidification, energy is required to humidify the air.

This energy is taken from the flue gas, which is cooled from state B to C. The condensate is used as liquid for humidification.

The now more humid and pre-heated combustion air generates an output flue gas that contains more energy than without humidification, meaning that state A1 is higher in energy than A.

The flue gas now contains more energy and can generate greater heat recovery in the condenser, which is shown here as an elevated return temperature to the boiler. 

The energy content between B and C corresponds to an elevated energy state after the boiler.

It is thus the case that A1-A=B-C, meaning that the energy that is taken
up in the humidification stage B-C is equal to the change in energy that
takes place after the boiler, i.e. A1-A.

As a curiosity, it may be noted that the humidification stage acts as a condenser, since the change in state of the gas from B to C creates condensation according to the laws of conservation.