When it comes to industrial heating using biomass, one of the most important—and most often misunderstood—factors is moisture content. Whether you’re operating biomass boiler kilns in the UK or Ireland, selecting the right drying method and understanding how wood fuel behaves in combustion is key to achieving high energy efficiency and reliable operation. At Ranheat, as a UK biomass boiler manufacturer, we’ve seen firsthand how moisture can make or break the performance of a biomass system.
This article explores biomass boiler kilns, the differences between dry basis and wet basis moisture percentages, and how drying methods—from batch kiln drying to continuous processing—impact fuel performance. If you’re running or planning a biomass boiler kiln project in the UK or Ireland, read on.
Why Moisture Content Matters in Biomass Boilers
Let’s get one thing straight: you can’t burn water. The classic phrase we use in the industry is:
“Jesus could only walk on water—even he couldn’t burn it.”
Moisture in wood has a huge impact on calorific value, combustion stability, and even maintenance schedules. The higher the biomass moisture content, the less energy you’ll get per tonne of fuel. Below are calorific values from UK government data based on wet basis moisture content:
At 10% moisture: 4.68 MWh/tonne
At 25% moisture: 4.09 MWh/tonne
At 45% moisture: 2.60 MWh/tonne
At 60% moisture: 1.70 MWh/tonne
That means wood at 60% moisture has nearly 60% less usable energy than wood at 10%. If you’re running a biomass boiler kiln, this difference is critical.
Dry Basis vs. Wet Basis Moisture – Know the Difference
This is an area that’s commonly misunderstood and occasionally misrepresented. We’ve even had cases where other suppliers have lost customers money by not explaining the difference clearly.
Wet basis measures moisture as a percentage of the total weight of the wet wood.
Dry basis measures moisture as a percentage of the dry material’s weight.
Always clarify what basis a supplier or report is using—especially when designing biomass boiler kiln systems or assessing fuel loads.
Effects of Moisture on Combustion Efficiency
When wood has too much moisture:
The combustion reaction slows down.
The fire chamber must be larger to burn more material at once.
Heat exchanger efficiency drops as water-laden gases reduce flue temperatures.
There’s a risk of flue gas condensation, especially in non-condensing biomass boiler designs.
Condensation is a serious issue. Water in the flue gases can mix with ash, forming concrete-like deposits inside the ductwork. This affects flue gas recirculation systems, which are critical in larger boilers to control temperature and reduce NOx.
Kiln Drying vs. Air Drying Wood for Biomass
There are two main approaches to drying fuel wood: air drying and kiln drying.
Air Drying
Low cost, simple, but very slow.
Relies on natural airflow and weather.
Suitable for low-throughput operations.
Final moisture content may be inconsistent.
Kiln Drying
Fast and precise, ideal for fuel prep.
Uses gas, electric, steam, or biomass heat.
Kills insects and fungi in timber.
Requires more space, infrastructure, and energy input.
At Ranheat, we support kiln-drying setups powered by our own biomass boilers—closing the loop with your own waste fuel supplying heat for drying more fuel or timber.
Batch Kilns vs. Continuous Dryers
Most timber kilns in the UK and Ireland are batch kilns, especially in the furniture and joinery sector. But for biomass fuel drying, continuous systems often make more sense:
Batch kilns are ideal for drying valuable timber.
Continuous kilns or dryers are better for wood fuel—less precision needed.
Systems can include moving floor dryers or rotating drum dryers.
For systems between 150kW and 2.2MW, bin-based drying with forced air can be the best option.
If you’re setting up a biomass boiler kiln in the UK or Ireland, our engineers can help match the dryer style to your fuel type and site layout.
Moisture Levels Change with the Seasons
Don’t forget: biomass moisture content varies throughout the year. Freshly felled timber or green offcuts can have up to 60% moisture, while air-dried material might reach 20–25% if left long enough.
That’s why it’s essential to size your boiler and combustion chamber with a margin. Our Ranheat systems are designed to cope with seasonal variation, with optional upgrades like flue gas recirculation, precombustion chambers, and variable-speed fuel feeding.
The Ranheat Approach – Practical Biomass Engineering
We lost a tender in Scotland once because of a misunderstanding over moisture content units—after installation, the customer found out their fuel wouldn’t work as expected. It cost them a rework of the whole system.
Our goal at Ranheat is to partner with you on your biomass project, not just supply a boiler. We help you answer the right questions:
What is the average moisture content of your fuel?
Is it chips, shavings, offcuts, or logs?
Will you air-dry, kiln-dry, or do both?
Do you need batch or continuous processing?
These aren’t just technical details—they affect everything from system sizing to long-term cost and emissions compliance.
Conclusion
Whether you’re installing biomass boiler kilns in the UK or Ireland, or upgrading fuel drying at your factory, understanding biomass moisture content is critical. The drying method, combustion system design, and boiler integration must all work together.
At Ranheat, as a specialist UK biomass boiler manufacturer, we provide complete systems tailored to your process—so you can turn wood waste into reliable, efficient, and sustainable heat.
Interested in biomass kilns or fuel drying solutions?
Get in touch with Ranheat Engineering:
Phone: 01604 750005
Email: [email protected]
Website: www.ranheat.com
