Calorific value of wood pellets

Calorific value in plain terms

When people talk about the “calorific value” of pellets, they usually mean the net calorific value (The amount of heat released by complete combustion of a unit mass of fuel when the water in the flue gas remains as vapour (net calorific value, NCV).)The amount of heat released by complete combustion of a unit mass of fuel when the water in the flue gas remains as vapour (net calorific value, NCV).: the amount of heat released by complete combustion of a kilogram of fuel. For wood pellets it is given in megajoules per kilogram (MJ/kg) or kilowatt-hours per kilogram (kWh/kg). The method of determination is described in ISO 18125^[17].

Net calorific value is a resulting parameter: it depends above all on how much water and ash the fuel contains, and to a lesser extent on the wood species. That is why it only makes sense together with the basis on which it is reported — more on that below.

Gross versus net calorific value

Technical documents use two quantities that are easy to confuse:

• gross calorific value (The amount of heat from complete combustion when the water vapour in the flue gas condenses and releases its heat (gross calorific value, GCV).)The amount of heat from complete combustion when the water vapour in the flue gas condenses and releases its heat (gross calorific value, GCV). — also includes the heat released by the water vapour in the flue gas when it condenses. The vapour comes from the water in the fuel and from the hydrogen that is part of the wood;
• net calorific value — assumes that the water vapour leaves the boiler uncondensed, so this heat is not counted. It is always lower than the gross calorific value.

In a typical boiler the flue gas leaves for the chimney hot, and the water vapour does not condense in it. That is why the quality requirements for wood pellets — in ENplus and in the Polish regulation — refer precisely to the net calorific value as received^[1],[36]. When comparing offers, check that both figures refer to the same quantity: the gross calorific value looks “better”, but it is not comparable with the net value.

MJ/kg and kWh/kg — conversions

Both units describe the same thing. 1 kWh = 3.6 MJ, so a value in MJ/kg is divided by 3.6 to obtain kWh/kg. Kilowatt-hours are convenient when comparing pellets with electricity or gas, or when calculating the cost of a unit of energy.

The ENplus minimum is 4.6 kWh/kg, which the document equates with 16.5 MJ/kg^[1]. The exact conversion gives 16.5 / 3.6 = 4.58 kWh/kg, so 4.6 is a rounded value. Per tonne: 16.5 MJ/kg means 16,500 MJ, or about 4,580 kWh of energy in the fuel.

Requirements: ENplus, ISO and Polish regulation

Note that the minimum is the same for all classes. Class A1 therefore does not guarantee a higher calorific value than A2 or B — the differences between the classes concern mainly ash, durability and chemical composition^[1]. The ISO values come from the draft standardData require verification: The ISO 17225-2 values were read from the FDIS draft; they need to be confirmed in the published standard..

Dry basis versus as received — the formula

Laboratories often report the net calorific value on two bases: the dry basis (A result recalculated to fuel with the water removed.)A result recalculated to fuel with the water removed. and the as-received basis (A result for the fuel as delivered, including its water.)A result for the fuel as delivered, including its water.. The dry-basis value is always higher, because it leaves the water out. The boiler, however, burns the fuel together with its water, so the as-received value is what counts. It is calculated with the formula^[44]:

Q_ar = Q_d × (100 − M) / 100 − 0.02443 × M

• Q_ar — net calorific value as received, MJ/kg;
• Q_d — net calorific value on a dry basis, MJ/kg;
• M — moisture content as received, %;
• 0.02443 — correction for the heat of vaporisation of water at 25 °C, in MJ/kg per 1% of moisture.

The formula comes from the VTT report, which states that it is consistent with ISO 18125^[44],[17]. Only the final result is converted to kWh/kg (by dividing by 3.6).

Try it with the figures from your own report:

What the bag may state

The ENplus document ST 1003 sets out what must and what may appear on a bag of certified pellets. Fuel properties are voluntary information. If the producer states them, they must be equal to the ENplus threshold values or stricter, and backed by laboratory results within the scheme. The calorific value may be stated only as received — never on a dry basis^[2].

What calorific value depends on

Moisture content has the greatest effect — every per cent of water reduces the share of wood in the fuel mass and also takes heat for evaporation. We show detailed calculations on the moisture content page.

Ash does not burn, so the more of it there is, the less combustible matter a kilogram of fuel contains. At the levels typical of A1 pellets this effect is small, but in fuel with bark or mineral contamination it becomes noticeable.

Wood species matters less than is often thought. In Finnish measurements, the net calorific value of dry stemwood was 19.31 MJ/kg for Scots pine, 19.05 MJ/kg for Norway spruce and about 18.6–18.7 MJ/kg for downy birch. Softwoods have slightly higher values thanks to their higher lignin and resin content^[44]. On a dry, ash-free basis (Dry, ash-free basis (daf) — a result that refers only to the combustible part of the fuel.)Dry, ash-free basis (daf) — a result that refers only to the combustible part of the fuel., the FAO handbook gives typically 19.2 MJ/kg for softwood and 19.0 MJ/kg for hardwood^[45]. The differences between species are therefore of the order of a few per cent — similar to the effect of a few extra percentage points of moisture. More on species in the comparison softwood or hardwood pellets.