Modern flue gas scrubbers – significant improvements in the profitability of bioenergy heating plants

There are many reasons why flue gas scrubber technology should be used. One of the most important is the financial benefits it brings. An immeasurable amount of fully utilisable thermal energy vanishes into the sky through the chimneys of heating plants every day. A modern flue gas scrubber can recover any lost heat and offers effective particle filtration for heating companies.

Changes in the district heating market in Europe place many heating plant companies in a challenging position. There are several diverse reasons for these changes. Competition over heating customers has become significantly fiercer over the past ten years as competing forms of heating have become more popular. The fuel strategies for district heating production in different countries are tied to changes in energy reserves and production in the EU and to the entire global economy.

District heating networks are usually old and consumers have inadequate heat exchangers with poor rates of efficiency. In addition, upgrading production plants to meet the current energy efficiency and emission requirements often depends on municipal economies, which may delay or even prevent the completion of financially and environmentally productive investments.

In order to modernise district heating production, we must turn our eyes to financially reasonable small-scale investment projects that serve to increase the energy efficiency of heat production and limit the emissions of plants to cost-efficiently meet the current requirements.

Fuel oil is a financial burden for heating companies

The majority of heating companies still use heavy fuel oil. Fuel oil is consumed especially during periods of extreme cold, when primary production plants do not have sufficient capacity. In many cases, the additional need for heat leads to significant fuel oil consumption, even during shorter spells of cold weather. According to our experience, a typical energy volume of 3,500–6,000 MWh a year is produced using fuel oil in heating companies with solid fuel plants of 5–10 MW. Most fuel oil is consumed in cold weather during the winter.

In addition, heavy fuel oil is used to compensate for the stoppage and maintenance of solid fuel boilers. In summer, solid fuel plants run at the lower limits of their adjustment range where such plants are difficult to operate. The district heat required is then produced using oil or natural gas boilers.

Most heating companies are trying to stop the use of heavy fuel oil. Their reasons are financially and socially sensible: the use of heavy fuel oil is simply too expensive and has much higher carbon dioxide emissions that local biofuels.

Flue gas scrubbing and recovery of lost heat

A flue gas scrubber is a wet scrubber which was originally designed to reduce particle emissions from flue gases. Over time, its development has shifted to improving the recovery of lost heat from flue gases. The particle filtration and heat recovery properties of a traditional flue gas scrubber are based on two consecutive and connected processing stages. Flue gases are led through a scrubbing phase where the majority of small particles are removed. During the same phase, flue gases are cooled down to their wet temperature (60–70 °C).

After the scrubbing stage, flue gases are conveyed to a condenser where they release their thermal energy through condensation in water circulating against the current. Condensation takes place in filler layers (one or two layers) that act as heat transfer surfaces in the process. Circulated water (i.e. generated condensate) is led to the heat exchanger where the thermal energy transferred to the condensate is recovered in the district heating water. (See Figure 1.)

Figure 1. A simplified traditional scrubber process diagram

It is essential to reach the dew point in the condensation zone

The dew point refers to the temperature at which the relative humidity of the vapour contained by the flue gas is 100%. If the temperature falls below the dew point, the vapour contained by the flue gas begins to condensate, i.e. turn into water. When analysing the transfer of thermal energy, we can see that, in connection with different phase transitions of water, enthalpic changes are significantly higher than in temperature changes within a single phase. Therefore, there are significant transfers of thermal energy through water vaporisation and vapour condensation (2,350 kJ/kg).

However, the most important factor in heat recovery is that the temperature is below the dew point when the flue gas vapour condensates and the released thermal energy can efficiently access the circulated water and, ultimately, the heat exchanger.

On the other hand, if the temperature is significantly above the condensation point in the scrubber, the heat recovery capacity falls and the scrubber will, in the worst case scenario, act as an evaporator. To put it simply, the scrubber vaporises additional water into flue gases. (See Figure 2.)

Figure 2. Wet scrubber thermal energy transfer per fuel power unit as a function of flue gas temperature

Why does a normal flue gas scrubber not work in extreme cold?

Reaching the condensation point temperature in any situation is essential considering the scrubber’s heat recovery. Return district heating water is conducted to the secondary side of the scrubber’s heat exchangers. It cools the flue gases below the dew point temperature. Cooling works and the dew point can be achieved if the district heating network’s return water temperature remains clearly below the condensation point temperature of the flue gas.

During peak power periods, the aim is to use a high-quality (i.e. dry) fuel. The flue gases of a dry fuel are dry, meaning that the dew point is low. During peak loads, the return temperatures of district heating networks increase. The primary reason for this is that the heat exchangers used in households have poor efficiency ratios. They simply cannot transfer the heat required to households; instead, extra heat returns back to the heating plant.

When this excess heat is fed back to the flue gas scrubber’s heat exchangers, the dew point cannot be reached and heat recovery decreases significantly. In other words, the scrubber’s heat recovery capacity collapses. Any missing condensation also causes problems with sludge and clogging in the scrubber.

Significant improvements in heat recovery through a heat pump connection

Heat pumps have been used in industrial applications for decades in various recovery processes for waste heat. A heating plant process is a special case where a correct heat pump connection boosts the recovery process four- or eightfold compared with a traditional scrubber. The heat pump regulates the return temperature of district heating water led to the scrubber so that the condensation point can be reached regardless of the network load and other external conditions.

In a traditional scrubber, the return temperature forces the final flue gas temperature to be 3–5 °C higher than the return temperature. For example, if the return temperature is 55 °C, the minimum final temperature of flue gases can be 58 °C.

Using a heat pump connection, the return temperature of district heating water can be reduced by 20 °C, in which case, according to the previous example, the return temperature of district heating water would be 35 °C and the minimum final temperature of flue gases would be 38 °C.

However, the thermal energy of the return water is not lost in the cooling process. It is transferred to the condenser via the pump’s thermal substance and returned to the district heating network. The energy transferred using the heat pump simply passes the scrubber connection.

In more advanced applications, only the exact volume of the network’s return water needed by the scrubber is conducted, in which case the heat pump can be dimensioned optimally for the investment.

In the aforementioned example, additional cooling of 20 °C is significant considering the recovery of energy. If peat with a humidity of 35% was used as a fuel and the residual oxygen of the flue gases was 5% by volume, the flue gas dew point temperature would have been 57 °C. In such cases, a traditional scrubber cannot reach the dew point, which means that additional water is needed in the process.

The regulatory circuit and heat pump connection described above enable a significant increase in the scrubber’s heat recovery capacity and improve the heating plant’s energy efficiency, especially in extreme cold.

The use of heavy fuel oil can be reduced significantly, which guarantees a short payback time for investments. Currently, payback times of even 2–3 years have been reached in investments where heavy fuel oil has been replaced by a scrubber solution with a heat pump connection. (See Figure 3.)

Figure 3. The effect of a scrubber solution with a heat pump connector on a selected heating plant’s heat production duration curve

The original text was published in our 1/2014 Top Engineer magazine