We’re often asked how different fuels compare. The most useful points of fuel comparison are: Length of burn time (density); Cost; and Heat Output.
Density
Generally speaking, the more dense a fuel is, the longer it will burn.
However, what they’re made of affects burn time as well. Bark is designed by nature not to burn. This is an adaptation to protect trees from wildfires. Even a low density briquette will burn longer if it contains a lot of bark.
For instance, Bark Overnight are at the lower end of the density scale. But because they are 100% dried bark they are capable of staying alight much longer than high density briquettes such as Blazers, Beech and Pini-kay. The latter will give much more heat over a shorter time period. Whilst having the potential to give a long ‘glow time’ because of their relative density.
This chart compares the density of various fuels in kg/m3.
We do not stock coal products, nor do we stock softwood or seasoned logs (only kiln dried logs). They are included in this chart for comparison purposes.
This chart compares the cost in pence per kilowatt hour (kWh) of the same range of fuels shown in the density chart above.
Price per kilowatt hour (kWh) is the best way to compare the cost of heat generated by different fuels, whether logs, coal, gas or electricity.
Don’t forget, moisture content has a big effect on energy released, so dry fuel is best.
- Seasoned logs have a high moisture content (mc) – 18 – 30%mc.
- Kiln dried logs are drier – 12%mc to 18%mc.
- Briquettes are driest, with a moisture content under 10%.
This combination of dryness and density means wood briquettes generate more heat than traditional logs.
Even though wood briquettes are marginally more expensive in terms of pence per kilo than logs, the extra heat they generate gives them a lower cost per kWh of heat generated. Given their greater density, they also last longer than all but the wettest of logs.
This scatter chart lets you see where any particular fuel sits in comparison to other fuels on a cost/density basis.
It is immediately clear that all forms of wood briquettes lay in a similar price range (pence per kWh) to fossil fuel coal products. Whilst coal products are denser and will burn longer, there are plenty of good reasons to look for sustainable alternatives.
Traditional logs are lower density and higher moisture content than wood briquettes. So they tend to have a higher cost per kWh than wood briquettes and a shorter burn time.
Burn time relates not only to density and moisture content, but also the amount of oxygen you allow into your stove. You tend to need more air to achieve a given heat level from traditional logs (because they contain more water) than wood briquettes, but anything will burn too fast if you allow too much air into the stove.
Where did we source this information and how do we calculate the density of a wood briquette?
A random sample of each briquette type is taken from stock (usually 3 or more).
The Volume is then calculated using simple mathematics (surface area x length); in the case of very complex and irregular shapes such as peat nuggets, water displacement can be used. This involves using a narrow container with a known quantity of water and dunking the nugget or briquette. The water will increase in level and the difference between this and the original level will reveal the volume of the briquette. Why a narrow container? because this will yield the maximum change in water level and can be read more accurately. A wide container on the other hand would not change much and would be very difficult to read. Some briquette shapes can be simplified, for instance a RUF briquette is a rectangular block with rounded edges. These edges are carefully removed to give a simple block.
Next, the briquette sample is weighed to obtain its mass. In the case of a nugget it is weighed prior to dipping in water since some of the water will be absorbed and alter the density reading.
Now we have the two pieces of information we requrie to calculate the density:
We make sure that all units of measurement are standardised, that is we use kilograms for mass and cubic metres for volume. Some units, like the water displacement, need to be converted from litres to cubic metres and mass measurements are taken in grams for accuracy before being converted to kilograms.
The Density of products we don’t sell have been taken from various sources on the web, and where values conflict an average has been taken.
Assumptions:
- The Calorific value of wood briquettes is around 4.9 kWh/kg. There is a reasonably linear relationship between moisture content and calorific value and briquettes are fairly predictable in that sense.
- All in all we feel the values obtained are a good representation of wood briquettes and acknowledge there will be a little variation from batches over time.
- The cost is a snap shot of current pricing and this can fluctuate over time.
- The density of logs depends on the large variation in moisture content depending on whether they are seasoned (air dried) or kiln dried, or freshly harvested.
