By Alexander Franklin, Ranheat Engineering Ltd.
Flue gas recirculation (FGR) is a critical technique in the world of biomass boilers, energy-from-waste systems, and industrial combustion. While it has been around for decades—first appearing in early steam boilers and internal combustion engines—its modern use has shifted towards reducing nitrogen oxide (NOx) emissions and improving combustion control.
This article is part of my series, “It Was Acceptable in the 80s,” where I explore biomass boiler innovations, looking at how technologies that were once standard practice have evolved to meet modern environmental and efficiency demands.
Flue gas recirculation is one such technology. It remains one of the most effective ways to control combustion temperatures, minimise NOx emissions, and optimise fuel burn—yet its implementation and effectiveness depend entirely on boiler and combustion chamber design.
So, what exactly is flue gas recirculation? And how does it apply to industrial biomass boilers? Let’s take a closer look.
At its core, flue gas recirculation (FGR) is a process where a portion of the exhaust gases from combustion is redirected back into the combustion chamber instead of being expelled immediately through the flue.
This reduces the oxygen content of incoming air, slowing down the combustion reaction and lowering peak flame temperatures. The benefits of this process include:
✅ Lower NOx emissions – Less oxygen in the combustion zone means fewer nitrogen oxides are formed.
✅ Better temperature control – Reduces thermal stress on the boiler and improves long-term durability.
✅ Less fouling and ash fusion – Lower peak temperatures prevent ash from melting and sticking to heat exchanger surfaces.
While flue gas recirculation is now commonplace in biomass combustion, it is also used in coal-fired power stations, natural gas systems, and even internal combustion engines to control combustion rates.
FGR was originally used in the early 20th century for a completely different reason than it is today. In early steam boilers and superheater systems, flue gas recirculation was used to maintain combustion gas flow rate at times of lower demand.
By recirculating gases back into the system, engineers could ensure a consistent flow of gases through the heat exchanger, preventing unintended heat imbalances and inefficient operation.
However, over time, the focus of combustion system design shifted towards reducing pollutants like NOx, and FGR became a key method for cutting emissions in modern biomass and fossil fuel systems.
To understand how FGR reduces NOx, we need to break down how nitrogen oxides form in a biomass boiler. There are two primary pathways for NOx formation:
1️⃣ Fuel NOx – Comes from nitrogen-based compounds that are naturally present in biomass or fossil fuels. This type of NOx is unavoidable and depends on the type of fuel burned.
2️⃣ Thermal NOx – Forms when airborne nitrogen (from the combustion air) is exposed to temperatures above 1000°C for extended periods. This is the primary target for NOx reduction strategies.
Flue gas recirculation works by:
Instead of using excess air to cool combustion (which increases emissions of CO and particulates), FGR provides a controlled way to slow the reaction while maintaining efficiency.
Aside from controlling NOx emissions, FGR provides several other benefits in biomass and energy-from-waste combustion systems:
If combustion temperatures get too high, ash from biomass fuels can melt and form sticky deposits on heat exchangers, reducing efficiency and increasing cleaning requirements.
By keeping temperatures below critical ash melting points, FGR helps minimise fouling and extend maintenance intervals.
Slowing down combustion reactions can improve the overall stability of the firebed, ensuring more even fuel burn and reducing the likelihood of localised overheating.
In some cases, controlling flue gas velocities can help increase overall boiler efficiency, allowing heat exchangers to operate more effectively without excessive thermal cycling.
While FGR is a highly effective tool, it isn’t always necessary. At Ranheat, we typically do not install FGR systems on smaller boilers like the WA 150 or MSU 300, as combustion temperatures at these scales are usually not high enough to create excessive NOx.
However, for larger biomass systems or installations burning nitrogen-rich fuels, flue gas recirculation is often essential for emissions compliance.
✔ Larger industrial biomass boilers (over 500kW)
✔ Combustion systems using nitrogen-rich fuels (MDF, chipboard, energy crops, waste-derived fuels)
✔ High-temperature combustion systems that risk thermal NOx formation
In many cases, FGR can be retrofitted to existing Ranheat boilers, allowing customers to improve emissions performance without requiring an entirely new system.
If your biomass boiler operates at high temperatures or burns nitrogen-rich fuels, flue gas recirculation is one of the best ways to:
✅ Reduce NOx emissions and meet MCPD compliance.
✅ Improve combustion control for stable, efficient operation.
✅ Minimise ash fouling and reduce maintenance downtime.
At Ranheat, we design tailored flue gas recirculation solutions for a range of industrial biomass boilers, helping businesses meet emissions targets while optimising efficiency.
Want to know if FGR is right for your biomass system?
📞 01604 750005
📧 [email protected]
🌐 www.ranheat.com
This article is part of our “It Was Acceptable in the 80s” blog series, where we explore how legacy combustion technologies have evolved to meet modern environmental and efficiency standards. Stay tuned for more insights from Ranheat Engineering Ltd.! 🚀
