Your choices:
1 Electricity; 2 Electricity; 3 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 | 2 Electricity |
| 3 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) |
Large-scale CHP-production from solid biomass in CHP or tri-generation plants is done by combusting the solid biomass in a steam boiler, superheating the steam to 540-580 °C at a steam pressure 140-160 bar and then expanding the steam through a steam turbine. At the turbine outlet, the cooling is then performed by district heating water and the district heating water is either distributed or used for the production of district cooling, depending on the customer demands.
The electricity efficiency is determined to a great extent by the superheating temperature and pressure of the steam. This is in turn limited by 1) the corrosive properties of the steam itself and 2) the presence of corrosive components in the combustion gases. Agricultural fuel qualities and -waste as well as household waste, may contain significant amounts of sulphur and chlorine and hence be so corrosive as to limit the superheating temperatures. In contrast, wood residues do normally exhibit extremely low contents of sulphur and chlorine so that higher electricity efficiencies may be obtained with wood fuels and, consequently, these fuel qualities should not be co-combusted in the same boiler unit.
Solid biomass may well be co-combusted with coal in large-scale pulverised fuel boilers. The prerequisite is that the solid biomass has been pre-treated so that it may be milled in the coal mills. One will not want to install separate mills and a separate fuel feeding system for the biomass. To improve grindability, the biomass can be pelletized or torrefied depending on which is the cheaper.
If the latter option – co-firing with coal – is used, then the total efficiency of the biomass use will be limited to the total efficiency of the coal-power plant. That means that the total energy lost from the biomass feed will be approximately 60% while, if it is instead used in modern CHP or tri-generation plants the losses will amount only to about 10% or less. However, any replacement of coal in power generation will reduce the emissions of greenhouse gases, so even though this option is inefficient from an energy point of view it will still be efficient from a climate point of view.