Kalmar Energi’s Moskogen CHP plant provides about 90% of the district heating consumed in the city of Kalmar Credit: Kalmar Energi

Flue gas condensing and combustion air humidification can benefit a CHP plant. Daniel Jedfelt, Risto Eteläaho and Tarja Korhonen describe how these processes help to optimize operations over a yearly cycle at a wood-burning facility in Sweden.

Various fees, taxes and incentives facing power plant owners have given them an increasing interest in the efficient operation of their facilities in general. Even though these stick-and-carrot stimuli vary from country to country, the trend is towards the better use of renewable resources. One way to achieve this is to improve the economics of operating a plant, which depend mainly on the energy sold compared to operating costs.

Swedish utility Kalmar Energi has turned to flue gas condensing and combustion air humidification to optimize the operation of its biomass-fired CHP plant in Moskogen. Experience there has shown the company how the latter process enhances the heat recovery from the former. But when is their employment economically justified, what are the operating hours of these processes at Moskogen and how do they affect the plant’s capacity profile?

The CHP plant

Kalmar Energi has been operating Moskogen since 2009. It comprises a 90 MWth bubbling fluidized bed (BFB) boiler that produces 30 MW of electrical power and 85 MW of district heat. Bark, forest residue and wood chips are the main fuels, and its total output is about 400 GWh of district heating and 130 GWh of electricity per year, providing about 90% of the district heating consumed in the city of Kalmar.

Figure 1. The direct correlation between the moisture content of the fuel and the moisture level in the flue gas and its dew point temperature
Figure 2. The variability in the return temperature of the district heating water at the Moskogen CHP plant over a year

When biomass or other high-moisture fuel are fired, it is quite common to boost the efficiency of a plant by using flue gas condensing systems, which can help to raise the overall plant efficiency to 110% or more.

Figure 3. The heat recovery potential as the temperature of the district heating return water at the Moskogen CHP plant changes

The heat effect of flue gas condensing depends on the fuel moisture content and the temperature of the return water in the district heating. High fuel moisture content and low return temperature of the district heating water enable a high heat effect, which makes flue gas condensing very interesting in an economic sense.

Figure 1 shows how a greater content of moisture in the fuel means the flue gas contains more moisture and has a higher dew point. The process of condensing flue gas produces hot water at a dew point temperature that again depends mainly on the fuel moisture content.

Biomass-fired boilers produce flue gas that contains plenty of heat (mainly latent heat) because the moisture content of the fuel is relatively high. As the flue moisture is in a vapour form, it has high enthalpy, which is measured in kJ/kg.

Energy is released when the water vapour condenses, a process that occurs when the temperature of the flue gas falls to that of the water dew point, in other words when the relative humidity is 100%. About 1 m3 of condensed water per hour corresponds to 1 MW of recovered heat.

In the case of biomass- fuelled plants the temperature of the hot water produced in the flue gas condensing process is typically 65–70ºC.

Although a CHP plant commonly uses this hot water to raise the temperature of the return water in the district heating system, there can be other consumers of the heat, such as large-scale industrial processes. So the higher the temperature of the water produced by the flue gas condensing process, the more energy can be transferred to the district heating system.

Figure 2 shows how a typical district heating system’s return water temperature varies according to the heat demand in the network. A high return temperature limits heat transfer from the flue gas condensate to the district heating water.

In the case of Kalmar, the high temperature of the return water in summer occurs when the CHP plant is shut down and the heat plant provides hot water to the Moskogen plant. Figure 3 shows the approximate heat recovery potential at Moskogen as the temperature of the return water changes.

A combustion air humidifier uses waste heat from the flue gas to raise the temperature of the combustion air, which allows additional moisture to transfer into it. After the humidifier, the relative moisture content of the combustion air can reach 100%.

Figure 4 shows how the addition of moisture to the combustion air – which increases the flue gas moisture content – increases the condensing heat effect.

Combustion air humidification is an effective method to increase heat production. However, its use requires some optimization of the operation of a CHP plant because the increase in the condensing heat effect can reduce the electrical effect.

Two-stage scrubbing

At Moskogen, an electrostatic precipitator removes particles from the flue gas coming from the BFB. The flue gas then passes to a two-stage condensing scrubber. A first washing process occurs in a spray stage, with a packed bed performing the final cleaning. The scrubber removes oxides of sulphur, ammonia slip and any remaining particles.

Condensation takes place in the scrubber, where the condensate is pumped over a packed bed layer. In the packed bed, heat from the water-saturated flue gas is transferred to the condensate. The condensate is continuously pumped through a set of plate heat exchangers to transfer the heat to the district heating return water – its temperature after the heat exchangers is typically 60ºC.

Figure 4. The temperature of the returning district heating water affects the condensing heat effect, with and without the use of a humidifier

After the flue gas condenser, the district heating water flows through the turbine condensers and is heated to the actual temperature set point of the departing district heating water, which typically ranges from 85ºC to 100ºC.

Operational profile

Moskogen typically starts up for the heating season in September, when heat demand is high enough for the minimum load operation of the boiler. The plant is in operation through the entire heating season, and typically shuts down in early June, when heat demand is too low for operation at minimum load.

In winter, the heat effect from flue gas condensing amounts to nearly 30% of the total plant heat effect. Figure 5 shows how the plant’s production of electricity and district heat varied between the plant’s startup in 2009 and summer 2012. The figure also displays the change in the district heat effect from flue gas condensing over the same period.

Moskogen employs combustion air humidification most of the time, but not in early autumn and late spring. The CHP plant operates for about 260 days annually, with the duration of the scheduled summer shutdown in the region of 100 days.

Optimizing operations

The most important variable for heat and electricity production in a CHP plant is the heat demand in the district heating system, which in turn depends on the weather. At ambient air temperatures of 0ºC or lower, the heat demand load is high enough to allow full operation of the CHP plant, the flue gas condenser and the air humidifier.

This heat demand decreases at higher ambient temperatures, when the CHP cannot run at full load. In this situation there are alternative ways to optimize the energy production of the plant.

The most important variables are the price achieved for the electricity sold and the cost of fuel. If the ratio of the two is high enough, it becomes profitable to limit flue gas condensing and keep up steam production in the boiler. This enables full electricity production, even though the plant’s full heat output potential cannot be delivered to the district heating system. Moskogen is also equipped with a hot water accumulator to enable shorter term optimization between heat and electricity production.

The limitation of heat recovery in the flue gas condensing process is carried out in steps. The first step involves turning off the air humidifier. In the second, condensate flow from the condensing scrubber to the heat exchangers is reduced and the minimum heat output from the flue gas condensing is determined on the basis of the maximum temperature of the packed bed layer or the minimum condensate flow of about 5 m3/h.

The minimum condensate flow is maintained to avoid the concentration of solids in the scrubber. It is also possible to stop the condensate flow to the heat exchangers, but then the scrubber consumes expensive city water for cooling and makeup, so this is done only at minimum load just before plant shutdown. This strategy avoids starting up other boilers that use more expensive fuels, such as dry wood powder.

Planning of the operation takes place on a weekly basis. Plans are reviewed daily and adjusted for weather and ambient temperature.

In summary

Thus in winter, the CHP plant is operated at a full load with flue gas condensing and combustion air humidification in full operation. At this time of year flue gas condensing produces about 28% of the plant heat effect. Without combustion air humidification the additional heat effect would be slightly lower at 22%.

At the end of the heating season, the operation of flue gas condensing and combustion air humidification is adjusted. When summer approaches and heat demand in the district heating system decreases towards the minimum load of the boiler, flue gas condensing is limited. The first step is to turn off the air humidifier, then reduce the condensate flow.

Daniel Jedfelt is operation manager at Kalmar Energi Värme AB, Sweden, Risto Eteläaho is product manager at BFB Metso Power Oy, Finland and Tarja Korhonen is product manager, Environmental Systems, at Metso Power Oy, Finland. www.metso.com

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