Your choices so far:
1 Biomass (solid); 2 Local heating (ind. house)
| What is your resource? | What do you want to deliver? | What is the service the customer wants? |
| Biomass (digestible sludge) | District cooling | Comfortable indoor climate |
| Biomass (fermentable sludge) | District heating | Electricity |
| 1 Biomass (solid) | Electricity | Process cooling (< 0 °C) |
| Geothermal | Fuel: Gaseous | Process heat/steam (50 - 150 °C) |
| Sunshine | Fuel: Liquid | Process heat (150 - 1000 °C) |
| Water | Fuel: Solid | Process heat (> 1000 °C) |
| Wind | Local cooling (ind. house) | Transport |
| Residual oils/fats etc | 2 Local heating (ind. house) |
There are three main alternatives for single-house heating with solid biofuel, namely wood fuel. These are log-wood firing, wood pellets and wood chips.
Firewood consists mainly of stem wood either from conifer trees or from broadleaf trees, with or without bark. It will have a cross sectional measure about 10 cm and a length adapted to the boiler where it is used, typically 40-80 cm. A single firewood log will then have a dry substance weight about 1 kg and with a moisture content of 20 % will the single log represent approximately 14-15 MJ or about 4 kWh of thermal energy. The single log will also be comparatively big, compared to the internal size of the combustion chamber, that is.
Supposing the boiler has a thermal capacity of 20 kW then means that no more than five wood logs may be burnt during any one hour to avoid overloading the boiler. Overload, as well as too low load, will cause emission of unburned hydrocarbons through the chimney. Hence, with a continuous firing, there is a need to supply one new log to the boiler every 10-15 minutes if the emission of hydrocarbons shall be avoided.
The most modern wood-log boilers are designed for downwards combustion. In these, the gases from the coldest log will pass down, through the bed of already burning and glowing material, so that the gas is maintained at a high temperature throughout combustion, leading to even lower emissions of hydrocarbons. These boilers are also designed for batch firing, so that a specific load of logs are input, ignited and allowed to burn out completely. Hence a modern boiler save a lot of manual work since it does not require a continuous feed as did the old boilers.
Installing a wood pellet system in a single family house with a central heating system includes installing a boiler with a pellet burner and a pellet store.
For the boiler, it is important that the fireplace is large enough to contain the flames from the pellet burner. Typically, the flames from wood pellet combustion are larger than those from oil or gas firing and the fireplace thus needs be bigger. In case the fireplace is too small, the flames will be extinguished close to the walls, resulting in hydrocarbon and soot emissions.
In some cases, a boiler originally aimed for oil firing can be used also with pellet burners but in most cases it is recommendable to acquire a boiler designed for pellets. Such boilers will also be designed to accommodate a reasonably large amount of ash, thus prolonging the intervals for ash removal. Finally, designated pellet boilers will also be designed to simplify the ash removal.
For pellets in larger buildings different types of burners are the main alternative, in the higher segment (above approximately 100 kW) complemented by grate-firing.
Pellet burners aimed for domestic applications (typically less than 25 kW) may basically be of two different kinds, namely forward-burning with a horizontal flame or upwards-burning with a vertical flame.
As the thermal power and the burners grow larger, the air velocity through the burner will be increased and forward-burning burners will run into problems simply because pellets will start to roll and blow out of the burner because of the high gas velocity. Hence, pellet burners for thermal output larger than about 50 kW will typically be upwards-burning.
As the burners grow large, there becomes a problem to acquire a reasonably uniform pellet bed across the cross-section of the burner cup and it also becomes difficult to make the combustion air penetrate the pellet bed from the outer edge towards the centre. Hence, as the scale (the thermal load) increases, another technology tends to be preferred. The pellets are then pushed onto a perforated grid, a grate, through which air is supplied from underneath.
From a thermal point of view will "larger buildings" include most anything from smaller office buildings with a heating demand of maybe 30-50 kW and all the way up to large shopping malls or greenhouses where the thermal power demand may rise well above 1000 kW during cold days. The lower end of the segment is touching the domestic scale and pellets are the recommended fuel but in the high end wood chips become an attractive alternative.
The most complicated but most flexible technology is the moving grate, the preferred technology in many scales or if fuel flexibility is paramount. The moving grate resembles a flight of stairs but each step slides back and forth with an adjustable period. Because of the sliding motion, the fuel is successively pushed down the stair. The "steps" are perforated so that air can penetrate from below – though this is omitted in the schematic. With some designs, the fuel movement may be completely horizontal though the most common is to have an inclination. The geometry of the steps – length and height – in combination with the amplitude and period of the sliding motion, will mainly determine the amount of tumbling in the fuel bed as well as the fuel particle residence time while the almost horizontal arrangement of the bed as such will minimize toppling.