Wherever pellets are bought and sold, the talk turns to calorific value — how much heat comes out of burning one kilogram. And yet a curious thing happens. For one and the same pellet, the figure written on the papers often differs from one document to the next. One says 4,800; another says 4,300. No one has lied, no one has measured wrong. How does this come about?
Two calorific values
First, something to know. There are, in fact, two calorific values: the gross and the net.
When wood burns, the hydrogen within it meets oxygen and becomes water. This water is born as hot vapour. But when that vapour cools and gathers back into droplets, it gives up the heat it held. Water takes heat away as it evaporates, and returns heat as it condenses — that is its nature.
the net value is what is left after taking that heat out.
Gross calorific value is measured inside a sealed vessel in a laboratory. The vapour has nowhere to escape, so in the end it all cools to water, and that heat too enters the count. The net calorific value, by contrast, is the figure with that returned heat subtracted. Why subtract it? Because a real power plant does not wait for that vapour to turn to water.
The heat that flies up the flue
Picture a real boiler. Where does the hot vapour from the furnace go? Before it cools into water, it escapes up the flue as hot steam. And the heat that vapour held flies off with it.
So the gross value measured in the lab is always higher than the heat a real boiler can actually grasp. That heat, which would be returned on condensing, vanishes up the flue in the real world. This is why what a power plant truly cares about is the net value — because the net value is the heat that can actually be used.
This is the first divide. For the very same pellet, recorded as gross it yields a high number, recorded as net a lower one. Not knowing which basis is meant, one sets the two numbers side by side and makes a false comparison.
Water sways the number
The second divide is moisture. In an earlier story, we said a freshly cut tree is half water. Even a well-dried pellet holds some moisture still.
This moisture sways the figure greatly. If there is much water in a pellet, part of the heat must first be spent warming and evaporating that water when it burns. And more water means, by that much, less wood to burn. So the more moisture a pellet holds, the less heat it can actually deliver.
This is why, when recording calorific value, the basis must always be stated. Whether it is an 'as-received' figure — measured with the moisture still in — or a 'dry-basis' figure — measured with the moisture fully removed — the number changes again. Measured dry, it naturally yields a higher number, since the water that got in the way is gone.
And so four numbers appear
Let us gather it up. Gross or net is one axis; as-received or dry-basis is another. Combine the two, and from one and the same pellet, four calorific figures can appear.
| As-received | Dry basis | |
|---|---|---|
| Gross | middle | highest |
| Net | lowest | middle |
All four cells are the calorific value of the same pellet. This is exactly why the figure differed from document to document. No one was wrong — they simply held up different rulers. And the value a power plant actually grasps is the lowest, the bottom-left cell: the net calorific value, as received.
What truly matters is the heat that arrives
So those who have long worked in this trade never call a pellet good or bad from a single calorific figure. They always ask back:
And as-received, or dry basis?"
Only once these are matched do they set two pellets side by side. In the end, what the buyer wants to know is one thing: when this pellet is shipped across the sea and fed into the plant's furnace, how much heat is actually grasped?
Calorific value is not merely a number. It is a number that takes on meaning only when you ask, alongside it, by what ruler it was measured. To choose a good pellet is not to hunt for the highest number, but to weigh numbers measured by the same ruler.
The reason the calorific value of one pellet differs from paper to paper is that there are several rulers. First, the gross value adds in the heat the combustion vapour returns on condensing, but in a real boiler that vapour flies up the flue, so the net value — with that heat subtracted — is the heat actually usable. Second, the more moisture a pellet holds, the more heat is lost evaporating that water, and the figure changes again by whether it is 'as-received' or 'dry-basis.' Combine these two axes and one pellet yields four numbers. So calorific value must always be compared by matching "gross or net, as-received or dry basis."