Advanced heat recovery for district heating: what, why and how?

Advanced heat recovery for district heating plants can lower fuel consumption, improve efficiency and boost heat and power generation. What’s more, integrating heat pump technology also helps to future-proof plants ready for carbon capture – a key process in meeting ever stricter emissions limits. Learn more about the drivers behind heat recovery investments, the available technologies, and the benefits they are already delivering.

District heating plant owners and operators are seeking new ways to achieve increasingly ambitions efficiency and emission reduction targets. Heat recovery from flue gas is a highly effective method with a short payback time. Today, many plants employ advanced technologies like combustion air humidifiers and integrated heat pumps alongside flue gas condensing technology. Combining these allows plant owners and operators to achieve significant gains in plant efficiency, evidenced by low flue gas temperatures in the stack.

Why should energy plants invest in heat recovery?

Energy plants stand to gain in many different ways when they invest in efficiency-enhancing heat recovery technology. Here are six key reasons to invest: 

1. Additional revenue – plants can produce more energy using the same assets.
2. Production flexibility – heat recovery is easy to adjust according to energy and heat demand, improving plant responsiveness.
3. Lower emissions – wet flue gas cleaning is a highly effective way to reduce emissions.
4. Carbon-capture readiness – heat recovery technology future-proofs installations against tightening carbon limits by creating optimal conditions for carbon capture in the stack.
5. Fulfil regulatory obligations and support government objectives – optimizing plant efficiency supports compliance with regulations and supports climate-related governmental initiatives. 

What are the key drivers for investments in heat recovery?

The key drivers for investments in heat recovery for energy plants include:

• market-driven factors like the price of fuel, district heat and electricity and the cost of CO2 emissions
• plant-specific conditions like annual operating hours, process conditions like district heating return temperature, and the availability of steam, hot water, or other heat sources.
• business-related factors like the potential for additional electricity production and heat sales.

What heat recovery technologies are available today? There are several different technologies available today for flue gas heat recovery. These include: established methods like flue gas scrubbers and tube condensers, which are essentially different ways to achieve same goal, offering gains of up to 30% more energy from same amount of fuel, and combustion air humidifiers and heat pumps, which can be integrated into the process, including as a retrofit, to boost flue gas heat recovery; these methods can increase heat recovery from flue gas by as much as 30–50%.

How much heat could energy plants recover?

The district heating water return temperature can have a significant effect on the feasibility of the chosen heat recovery concept. Performance in terms of the percentage of heat that can be recovered is quite different between the different combinations of technologies.

In one project that Valmet has been involved in, a biomass plant in Finland producing district heat, adding a combustion air humidifier enabled a 16% increase in heat recovery, and adding an absorption heat pump enabled an additional 19% gain in heat recovery. The total heat recovery from flue gas at this plant is up to 69 MW, and stack temperatures as low as 12°C have been recorded, indicating highly efficient plant operation.

What types of plants can flue gas heat recovery be used for?

The typical application for flue gas heat recovery systems is solid fuel or solid biomass firing plants, but there are many other applications too, including combined cycle gas turbine (CCGT) plants and plants with boilers fired by natural gas. Electricity production can be boosted by 10–20% when turbines are operating in condensing mode and using a heat pump driven by low-pressure steam.

How does heat recovery improve the feasibility of carbon capture processes?

Plants can integrate efficient emissions reduction and heat recovery in one step with a flue gas scrubber as an ideal solution for carbon capture pre-treatment. This type of setup brings down flue gas temperatures to a very low level, eliminating the need for separate direct quench or direct contact cooling technology in the carbon capture process.

By integrating an absorption or compression heat pump as well, plants can recover the heat needed to regenerate the solvent in the carbon capture process. This improves the efficiency of the carbon capture step by reducing the need for steam or other external energy sources.

What are the key benefits of heat recovery for energy plants?

Combined flue gas cleaning and condensing is now the standard in the Nordics, and interest is growing globally. The key benefits of heat recovery for energy plants are:

• Profitability: 30% less fuel required, producing same amount of energy with less fuel or utilizing same amount of fuel to produce more energy.
• Increased capacity: 30% more heat or 12% more power.
• Future-proofing: heat recovery systems provide ideal pre-treatment conditions for carbon capture processes.
• Overall process optimization: Plant processes can be adjusted not only to enhance energy production capacity but also to optimize the plant’s water balance by enabling water to be recovered for various applications.