A guide to the very real benefits of a modern clean burning wood stove

Advanced modern log burners such as the ones produced by RAIS, Piazzetta, Brunner or Harrie Leenders are stylish and beautifully designed products but when it comes to wood burning, beauty has to be much more than skin deep. The external lines of a stove are only part of the story and to be truly world class, a stove must be designed to function.

In order to understand the benefits of clean burn and the challenges for the stove designer, we have to look at the science behind the combustion of logs. Wood itself is composed of 70-80% volatile hydrocarbons (tars and oils) and 20-30% fixed carbon (burnt as charcoal). There is also a very small mineral content (left behind as ash after burning)

The combustion or oxidization of wood involves three processes, pyrolysis (also known as gasification), gas combustion and char burn. Wood ignites at around 200⁰C and the volatiles within the wood start to vaporise. They are initially given off as smoke but as the temperature in the firebox rises above 400⁰C, the smoke ignites and is replaced by long yellow flames which start just above the surface of the wood. At this stage, heat starts to be produced which raises the temperature further. Most of the heat energy is generated during this part of the process. The final stage is char burn where oxidisation of pure carbon continues after all the volatiles are exhausted and just charcoal remains. This stage of combustion is gentler and produces less heat but takes place at the highest temperature. Glowing embers can have a temperature of up to 1000⁰C.

The stove has to be carefully designed in order to control and optimise the three stages of a complex chemical reaction. The ideal design will;

  • Supply just the right quantity of oxygen to sustain combustion.
  • Provide enough turbulence above the fuel to thoroughly mix the gases with the oxygen.
  • Allow sufficient time under the right conditions for the volatile gases to burn completely.
  • Heat up very quickly after lighting to ignite the volatiles and then sustain a high temperature to completely oxidise all constituents of the fuel.

The most efficient wood burners oxidise virtually all the fuel in the wood which in practical terms means;

  • Very little smoke is given off (known as particulate matter).
  • The largest amount of heat is produced for every kg of wood (high efficiency).
  • Virtually no unburnt material left behind in the firebox (the pure ash content of wood is only around 1%).

So to summarise, a modern clean burning stove will save money on fuel, keep the chimney clear and minimise air pollution.