Your choices so far:
1 Biomass (digestible sludge); 2 Electricity
| What is your resource? | What do you want to deliver? | What is the service the customer wants? |
| 1 Biomass (digestible sludge) | District cooling | Comfortable indoor climate |
| Biomass (fermentable sludge) | District heating | 2 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 | Local heating (ind. house) |
In the odd case when the biogas producer can find no use whatsoever for the heat produced, stand-alone electricity production may be an alternative. However, since electricity production from fuels or combustibles should never be a stand-alone activity but should always be combined with heat production, this is only the very last alternative.
There will be three major routes for this production:
- The most obvious one, and the cheapest and most established in smaller scales, is to use an IC (internal combustion) engine with a generator attached to the engine shaft. Attainable electricity production efficiencies will be about 25 – 30% in the small scale.
- A more expensive alternative is to use a steam boiler in combination with a steam turbine. Using a screw expander, saturated steam may be used and the total investment is kept at the lower range for steam cycles, but the price paid is the electricity efficiency which will be less than 20%. However, when the expected production approaches or exceeds about 0.5 MWel, the steam cycle will generally become a better alternative than the IC-engine. To raise the efficiency in steam cycles, the steam must be superheated. This will significantly increase the investment cost but with increasing pressure and temperature the efficiency may be raised.
- The most expensive alternative is to use a gas turbine. To avoid corrosion in the turbine, the biogas should first be upgraded. The advantage with the gas turbine is the short start-up-time but the price paid is a low efficiency, some 25 – 30%. The gas turbine may be an alternative in case the electricity supply from the grid is unreliable and the electricity production is mainly for emergency cases.
Since, in this case, the full cost for the electricity production must be carried by the electricity produced, the electricity will become expensive and generally speaking it will be too expensive to be sold to the national grid. Hence, the scale of the electricity production unit, and hence the technical system solution, shall be set primarily to supply the local use by the producer.