Your choices:
1 Electricity; 2 Electricity; 3 Biomass (digestible sludge)
| What is your resource? | What do you want to deliver? | What is the service the customer wants? |
| 3 Biomass (digestible sludge) | District cooling | Comfortable indoor climate |
| Biomass (fermentable sludge) | District heating | 2 Electricity |
| 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) |
When digestible sludge is the source of energy, logistics will prevent the establishment of large plants. Rather, the plant size will be limited by the local availability of the biomass than from the potential market. So though a local district heating network may benefit from the demand for electricity, the anaerobic digestion plant will most probably not be a major source for the energy produced.
Electricity production from biogas can basically be done in two different ways depending on scale. For applications like these it will usually not be feasible to upgrade the gas prior to use.
- The raw biogas is used in an internal combustion engine – basically a modified car or ship engine – with a generator connected to it. The cooling water from the engine is used for district heating, maybe with an extra temperature boost for cold winter days. Such an extra temperature boost can be achieved in an external combustion chamber again fired with the raw biogas and maybe with a pilot flame. A system like this will exhibit a limited flexibility with respect to the ratio between produced electricity and produced heat. Electricity production may amount to at the most 20-30% of the biogas input and down to 5-15% in small-scale applications and the total efficiency can be up to about 80%.
- The raw biogas is fired in a steam boiler. Depending on state law there may be restrictions to the use of such a low-quality gas in open combustion chambers so that a pilot flame, like an LPG or natural-gas-flame, must be available for security reasons. The steam is superheated to the desired temperature and pressure and is then allowed to expand through a steam turbine. The outlet steam is condensed in a heat exchanger and the condensation heat is delivered to the district heating system. In this case, the temperature boost for cold winter conditions may be achieved as a turbine bypass of steam. This system solution is flexible with respect to the ratio of produced electricity to produced heat. The maximum electricity production may amount to about 30-35% of the biogas input during summer conditions (low district heating temperatures), 25-30% of the biogas input during winter conditions but can also be deliberately set as low as 0 if all steam bypasses the turbine so that only heat is produced. The total efficiency can be 85-90%.
So the cost distribution between gas production and electricity production becomes different and the total cost associated with the two solutions will have to be carefully investigated before any decisions are taken.