Several factors need to be considered when trying to get an effective, clean and economical burn, and by no means are they all obvious. Solid fuel most likely means wood. Domestic bituminous ‘house’ coal sales were prohibited during 2023 in the UK and manufactured smokeless coal is only of minor interest. Also, this means a stove – burning solid fuel in an open fire is another matter entirely.
Log size
The optimum log diameter for efficient burning in most stoves is about 125mm/5”. With around 3/4 of the fuel locked up in a piece of wood, it has to heat up and then vaporise off the surface as flammable gas. The rest of the fuel is mainly fixed carbon or charcoal. This carbon is what causes the surface of burning wood to glow – once the flames are gone, it becomes glowing embers. A large log has a small surface area relative to its volume, so the flammable gases are given off slowly. As a result, the burning temperature falls and less of the gases burn, which causes inefficient burning and increased emissions.
The critical temperature inside the stove, the liner and chimney is >114ºC. This is the condensate point of the gases produced during combustion. Below this, the gases will turn back into liquids, tars and solids (soot). As these liquids and compounds are acidic, they will cause iron, steel, stainless and brick to corrode. The solution burns hot and keeps the gasses as gas for as much of the evacuation process as possible. In reality there is usually a condensate band in most chimney liners near the terminal, as most flues drop to around 500C near the end of a standard 9m run.
Dryness
The fuel must be dry. According to Woodsure, the UK’s quality assurance body, it must be below 20%. This means the fire will not have to boil off the excess moisture and results in more heat for the room and less condensate in the appliance and flue. Some sources suggest about 81% of the calorific value of the fuel is harvestable at 20%, lower moisture content usually means more heat. This is best checked with a moisture meter.
Appliances
Multi-fuel stoves benefit from leaving a bed of ash in the grate. This slows wood consumption by gently restricting the air flow. Most wood stoves will remain hot the morning after a burn, while most multi-fuel appliances will not. This is purely down to greater airflow in a multi-fuel appliance – the payoff is generally that multi-fuel stoves are easier to light. In the same vein, DEFRA-approved appliances with their constant airflow tend to burn hotter (and cleaner) and are easier to light.
Storage
Wood stored outside rarely achieves much below 20% moisture content. If stored outside, the optimum storage solution is a south facing open log store, neatly stacked. Failing this, old pallets make an ideal base for wood storage as stacking rather than piling allows optimal airflow (and hence drying). A tarpaulin or board to keep the rain off is recommended. A couple of visual checks to note – dry firewood will be grey with cracks along the edges and much lighter than wet firewood. Wood typically contains about 4-5kWh per kg.
In the next part of this series on solid fuel we will be considering carbon and climate change and comparing wood burning to other heating methods.
Image supplied by OFTEC
