Your choices so far:
1 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 | 1 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 | Local heating (ind. house) |
The use of solid biomass for climate control can be large-scale, using primarily forest residues or agricultural by-products such as straw in CHP or tri-generation plants and providing district heating, district cooling and electricity to the customers. In this scale also sorted, solid and clean waste fractions may be included – depending on legal aspects.
It can be intermediate-scale using upgraded solid fuels like briquettes or dry wood chips and endings from wood manufacturing industries and producing heat for a major shopping mall, a hospital, a school or an individual office building or alike.
And it can be small-scale using wood-logs or pellets in single-family houses or straw or other agricultural residues in the case of a large enough farm.
Generally speaking, woody biomass tend to have ash properties that are "nicer" – higher melting points, less aggressive, smaller amounts – than agricultural biomass. Also, agricultural fuels tend to have lower heating values and comparatively low densities which means that the volumetric throughputs become high. Therefore, agricultural fuels should best be fired in separate units specially designed to cope with these ash properties and with a proper fuel feed system and should not be mixed with wood fuels.
So if a large-scale district heating plant is planned for, based on local fuels including both agricultural and silvicultural qualities, then the plant layout should include two separate boiler units. During procurement, it is important to specify the anticipated fuel quality variations to the boiler manufacturer so as to avoid future operational problems. Boilers can be, and are, designed to handle fuels of different quality but there is a limit and to make full use of the cheapest – i.e. the most un-specified – fuel qualities the boiler will have to be equipped with fuel handling systems apt for the purpose and with advanced combustion control. If not, not only efficiency but also environmental performance will suffer.
In the intermediate scale –like the supply of a hospital or a large office building, sports centre or shopping mall – will the investment cost for the boiler often be a hindrance. Therefore, the boiler will become simpler and not be able to handle variations in fuel quality. Thus, for these applications, the demands on the fuel quality to stay within narrow limits will be strict. So the boiler will become relatively cheaper but the fuel will become relatively more expensive – unless for example chipped, carpentry-dry endings or briquettes from saw-dust and cutter shavings from some nearby wood manufacturing industry can be contracted.
For single-family houses the demands become even more extreme and for best environmental performance pellets are the recommended choice. However, for economic reasons, also wood-log firing is used in this scale. Provided proper treatment of the wood-log fuel, provided modern, down-firing boilers, provided heating systems equipped with proper-sized accumulator tanks and provided the house-owners have been properly trained, such systems may also have acceptable environmental performance.
For sufficiently large farms, for riding schools and similar cases there is also the option to acquire specially designed straw-fired boilers.