Your choices:
1 Local heating (ind. house); Comfortable indoor climate; 2 Biomass (digestible sludge)
| What is your resource? | What do you want to deliver? | What is the service the customer wants? |
| 2 Biomass (digestible sludge) | District cooling | 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 | Fuel: Solid | Process heat (> 1000 °C) |
| Wind | Local cooling (ind. house) | Transport |
| Residual oils/fats etc | 1 Local heating (ind. house) |
A common situation in agricultural areas is that the individual farm has access to digestible biomass and then the option to use this resource for individual house heating and maybe also for electricity production is an obvious one. Depending on the size of the farm, the total resource and the actual production at the farm, the heat may also be used for drying of cereals or hay, for pasteurization of milk or for any other purposes.
In case heat production is the main aim of the biogas digester, then it must be remembered that the raw biogas is a low-grade fuel with variable properties.
The simplest way to make use of the raw biogas for heat production, and especially so if the temperatures desired are at a maximum 80 °C (tap water production and comfort heat) and up to 150 °C (hygienization, pasteurization and alike) is to use the raw biogas to replace fossil gas firing directly in the hot-water boiler.
It must then be remembered, though, that the raw biogas will have a variable quality and that – for safety reasons – there should be a fossil gas pilot flame in the boiler to guarantee a stable ignition of the biogas. Hence, the use of biogas will not make the farm completely independent of fossil fuel.
Since the raw biogas contains hydrogen chloride, this will also be present in the boiler gas atmosphere. There will also be other compounds in the furnace atmosphere, the presence of which are directly connected to the biogas composition. This will call for attention with respect to any corrosion problems in the boiler heat transfer sections and in the chimney.
If also electricity is desired, then CHP technology should be used. In the specific case of biogas, this may be accomplished on a single-farm scale. This is unique for biogas as compared to the other biomass sources, and is based on the fact that the output from a digester is a gas with a sufficiently high quality to be used in an (adopted) internal combustion engine.
The investment cost for such a unit is low enough to make CHP-production feasible in single-farm scale so that the individual farm may use the hot cooling water from the engine for comfort heat and that the electricity may be used locally or sold onto the low-voltage electricity grid.