Your choices so far:
1 Electricity; 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) | 1 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) |
It must be stressed that though electricity may be considered the most important product, electricity production from fuels and combustibles should never ever take place without selling the heat too. Even though the most highly valued product may be the electricity, the size off the plant should not be chosen to maximise electricity production but to fit the market for heat. The total economy for the plant will be strongly depending on the possibilities to sell all the heat produced – or to use all the heat for district cooling production.
The second factor of importance when dimensioning this type of plants is the availability of fuel. For best environmental performance, the fuel to each individual boiler unit should be as uniform as possible. And then, for highest efficiency, the boiler units should be as big as possible. Sometimes these demands can become contradictory. Assume a situation where there is agro-fuel available enough for 50 MW and there is also forest-fuel available to an equal amount. In this case it may be a better solution to have two boiler units, each 50 MW, even if they are placed side-by-side, than to build one 100 MW unit for co-firing.
Given this, solid biomass is best used in combined heat-and-power processes in which the fuel is first combusted in a steam boiler, the steam runs through a turbine to produce electricity and the turbine outlet is cooled by district heating water so as to produce district heating.
Using this type of system, total efficiencies well above 90% are common, 95% in case the plant has a thermal power about 100 MW or more and even if the fuel has moisture contents exceeding 40% by weight. If the fuel has higher moisture contents, then a flue-gas condenser should be included in the system to further raise the energy transferred to the district heating system.
The energy input with the fuel will then be approximately split to 5-10% losses, 30-38% electricity and the remaining energy as district heating.