VALMETIndustry News

Increasing efficiency for Combined Heat & Power plants and paper mills

Process efficiency can be significantly increased by utilizing flue gas condensing to recover heat: a source of energy that typically goes up in smoke. Text Lari-Matti Kuvaja


lue gases contain plenty of (mainly latent) heat, especially when the fuel’s moisture content is high.

After combustion, the moisture from the fuel is in vapour form in the flue gas, with high enthalpy (kJ/kg). In a normal combustion process, this energy is not utilized, and all of it literally goes up in smoke. Typically, CHP plant efficiency has been close to 90%. Overall power plant efficiency is calculated using the lower heating value of the fuel – the energy lost in flue gas moisture is not taken into consideration. Although a little deceptive – the fuel moisture has a very significant effect – this efficiency calculation method has become the norm in the

energy industry. Almost all the energy from

the flue gas is released when the water vapor condenses into liquid form. Condensation occurs as the flue gas reaches the dew point of water, where the relative humidity is 100%. The higher the moisture content (the higher the dew point) and the lower the district heating water return temperature, the greater the heat recovery. By adding a combustion air humidifier into the mix with the condenser, we can achieve even greater heat recovery. The flue gas temperature leaving the stack may even be reduced to 35 °C, which is roughly 100 °C less than in a conventional process. The feasibility of flue gas

condensing becomes apparent when the power plant’s total

condensing efficiency is calculated. An increase of 24.7 MW in the district heating energy produced increases total efficiency to 110%. This defiance of thermodynamics of course stems from the usage of the lower heating value in the calculation.

Short payback time District heating demand fluctuates with the need for heating, i.e. the weather. This means that the maximum potential cannot be utilised all year round. For this reason, the most likely profit lines have been highlighted in green (see chart). However, if the district heating return temperature remains nearly constant throughout the year, it may be possible to operate with the two highest profit lines.

The calculation can be made

by using either the cost of fuel (generating savings) or selling price (generating revenues). A conservative estimate of EUR 20 per MWh (low-cost biomass) in district heating savings and 5,000 operating hours suggests we can achieve savings of roughly EUR 2.5 million per year, meaning a typical payback time of less than two years.

Recovered heat to replace steam consumption Flue gas condensing can be used to increase the efficiency of various processes in which it is possible to utilise low temperatures. Potential heat sources are fresh and white- water heating in paper machines, and heating of boiler water and

Heat recovery by flue gas condensing 12 Forest Bioenergy Review Summer 2019

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