Your choices:
1 Fuel: solid; Biomass (solid); 2 Comfortable indoor climate
| What is your resource? | What do you want to deliver? | What is the service the customer wants? |
| Biomass (digestible sludge) | District cooling | 2 Comfortable indoor climate |
| Biomass (fermentable sludge) | District heating | Electricity |
| 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 | 1 Fuel: Solid | Process heat (> 1000 °C) |
| Wind | Local cooling (ind. house) | Transport |
| Residual oils/fats etc | Local heating (ind. house) |
Pellet firing systems for domestic use in single-family houses generally fall into two different categories, namely
- Burners to be mounted in hot-water boilers
- Self-contained stoves for air heating, containing combustion chamber, burner and day storage tank assembled. For some units the day tank might be separate while quite often all the parts are assembled in one physical unit
Both types of system solutions are commercially available as off-the-shelf units. The reliability with the commercial systems is high and the need for maintenance is low.
The household market for wood pellets is rapidly expanding and pellets may be bought in bags as well as in bulk in most places over Europe.
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, which is the case in for example apartment houses or other larger buildings, will typically be upwards-burning. Pellets are then fed to the burner from below, using a screw feeder, and enter into a burner-cup where they are combusted. The ash is successively pushed up by the pellets and fall out of the cup over its rim. Combustion air is introduced through holes and slots in the double-mantled burner cup.
As the heat demand grows even larger, 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. This is the most common when the thermal power exceeds some 500 kW.
The characteristics of pellet heating plants is that because of the uniformity of the fuel, the fuel feeding as well as the combustion control may be rather simple. Hence, the investment cost may become relatively low. However, the pelletizing process is costly. Therefore, the combination becomes a low fixed cost and a high variable cost, a combination that makes the use of wood pellets economically favourable in small scale (typically less than 1 MW thermal). In larger scales, biomass used for energy production aimed for comfort and tap-water heating should preferably not be upgraded.