Your choices:
1 Local heating (ind. house); Comfortable indoor climate; 2 Biomass (solid)
| 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 |
| 2 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 | 1 Local heating (ind. house) |
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. Properly prepared the moisture content in the firewood should be about 20% and then will the single log represent approximately 4 kWh of thermal energy.
With old domestic boilers, the walls of the combustion chamber were cold, with a temperature close to that of the water. Hydrocarbons and tar released from the wood logs will then escape from the combustion chamber and may cause severe air quality problems.
The most modern wood-log boilers are designed for downwards combustion and lined with ceramics. 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 very low 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 saves a lot of manual work since it does not require a continuous feed as did the old boilers.
Pellets consist mainly of stem wood either from conifer trees or from broadleaf trees, with or without bark. They will have a diameter of 6, 8, 10 or 12 mm, a particle density for the individual pellet exceeding 1 100 kg/m3 and they will be mechanically robust. They will also have a low moisture content, typically about 12%, corresponding to an energy content (heating value) about 5 kWh of thermal energy per kg.
Since wood pellets thus have a smooth surface, a uniform shape and relatively constant properties as a whole, they lend themselves well to automatic feeding and firing systems. 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 are commercially available as off-the-shelf units. The reliability with the commercial systems is high and the need for maintenance is low.
In case a pellet system replaces old oil- or gas fired systems it is crucial to check the status of the chimney. Pellet firing will result in larger flue gas volumes and the chimney must be able to cope with that. In case a pellet burner is mounted in an old boiler the flue gas temperatures may also become higher.
The heating system may be designed mainly in two different ways – with or without an accumulator tank and this is of special importance for single-family houses and applies basically only for wood-log firing.
- Without an accumulator tank, the water volume is comparatively small and the heating system has a low thermal mass or thermal inertia. Thus, the water will cool down rapidly and the system will require frequent energy input. This means that firing must almost be continuous during cold periods – but at the same time, the boiler must not be overloaded. Hence such a system will require feeding with a new log at short time intervals, 10-15 minutes. Also; if there is no accumulator tank and if the boiler is overloaded, the capacity of the water to take store the energy released may be insufficient and boiling may occur.
- If the system is equipped with an accumulator tank, and if it is properly designed, will the accumulator be able to store all the heat released from one or two full batches in the boiler. The system will then have the built-in capacity to deliver heat at an even rate during a 24 h period while the temperature in the accumulator successively drops until it is again re-charged by firing the boiler with one or two full batches of firewood. The accumulator will thus save work – since the boiler needs be fired only once per day, preferably during the evening but then it will need one or two full batches. Also will this type of firing, using the full capacity of the boiler combustion chamber but not overloading, produce the smallest possible emissions of harmful, unburned hydrocarbons to the ambient air.
Wood-log firing will not be an alternative in larger buildings. As the pellet burners grow large (in the case of larger buildings, that is), 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 called grate-firing as is a technology employed from thermal powers about 50 kW and up. So for larger buildings it would be recommended to replace the old gas- or oil-fired boiler completely and install a new boiler specifically aimed for pellets.
In applications where the thermal load exceeds about one MW, such as large shopping malls, or greenhouses or hotels, chips firing becomes a serious alternative. Again, the technology used will be grate-firing. Opposed to pellets, chips are a raw fuel with a higher ash content, lower heating value and variable quality. Thus the fuel- and ash handling systems must be carefully designed to cope with these properties.
The installation of chips-fired units in single buildings in the outskirt of a city or a municipality may well serve as a starting point for an integrated district heating system. As several buildings get equipped with modern water-borne systems and boilers, the capacity of which is not fully used on warm days, co-operation between the house owners will naturally evolve and it will be natural to connect the individual heating systems to each other.