Using Renewable Fuels
Under this heading, you would normally consider wood and wood products, although Hydrogen might well be considered one day if economical storage and refilling became available. Other possible materials to consider in compressed briquette form are more local in origin, and include farming biomass and even coffee grounds;
The two major fuel considerations for us are mass energy yield and storage energy density, although price, ash content and soot/tar yields can become important.
Material Aproximate Energy yield per kg Aproximate Volume per kW/Hr
|
Wood (20% moisture) |
4kW/Hr |
1.0 litres |
|
Wood Briquettes |
5kW/Hr |
0.2 litres |
|
Lump charcoal |
8kW/Hr |
0.6 litres |
|
Coal (for reference) |
8kW/Hr |
0.1 litres |
Notice that even the smallest of our boats uses a 20kW fire, so a long summer day steaming could use somewhere between a 25kg bag of coal (20 litres bunker space), to 50kg of logs, occupying more than 200 litres.
Wood
Like coal, both wood and wood products have a multistage burning process, so a fire bed can’t be expected to behave the same with fresh or part used fuel. The real problem for our boats using bare logs is the bunker volume occupied, 12 times more than for the same coal.
Wood products
At the time of writing, Briquettes are particularly good value in many countries. Featuring minimum moisture, 5 x the energy volume density of logs, and with hardly any smoke or ash. They don’t like getting wet as they rapidly swell, but dipping in paraffin wax around 100 deg C makes them water resistant and can be used to bind loose sawdust. Sadly, vegetable waxes don't work well here.
Now, you may be looking at those 20ft Leylandii garden trees in a new light, but the chipping machine, the drying process, and the briquette press you will need, all use a considerable quantity of energy (10 HP minimum), normally obtained from oil fuel; https://www.charcoalbriquettemachine.com
Charcoal
A single stage burning fuel, without smoke or ash, but in lumpwood form it occupies 3 times the volume of wood briquettes for the same energy yield.
Charcoal briquettes have at least twice the density of lump charcoal, but those sold for barbeques seem to feature huge quantities of grate clogging ash, which may be acceptable for open cooking grills, but not boiler grates.
This is a worthwhile area of research though, considering the quest for coal substitutes and the growing popularity of quite large Restaurant grills.
However, the difficulty of putting charcoal fires out is well known to all barbequers, and restriction of the ashpan air alone will produce quantities of poisonous Carbon Monoxide until the fuel is exhausted. This alone may rule charcoal out if your boiler is enclosed.