Your choices:
1 Electricity; Electricity; 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 | 1 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) |
Unless for example a municipal-sized digester is used for local electricity production in a CHP-plant, large-scale electricity production from digestible sludge can be accomplished in different ways:
- In case the power station is close to where the biogas is produced, the raw biogas can be piped to the power station and used as it is. A large-scale, fossil-fuel fired, condensing power station will be equipped with advanced combustion control and flue-gas cleaning equipment and the acid components in the raw biogas, hydrogen sulphide, hydrogen chloride and others, will not cause any severe environmental problems.
- If the power station is situated at a far distance from where the biogas is produced, the gas will have to be upgraded at the production site and it will have to be doped with a bit of heavier hydrocarbons so as to be accepted as a substitute gas from the operator of the fossil gas pipeline network. The gas will then be injected into the fossil gas pipeline grid and can be contractually extracted at any point in the grid.
There are a few differences between these two ways:
- In the first case, the actual gas produced will be combusted and used for the electricity production. In this case, any fluctuation in gas flow or quality will immediately be reflected in the combustion process in the power station. At the same time, the power production unit will most likely be small. To cope with variable fuel gas flow and -quality, the combustion unit will have to have fairly sophisticated control equipment and it will also have to be equipped with flue-gas cleaning systems. Hence, the investment cost is likely to be high and this type of installations may not be too small. This contradicts the use of locally produced biogas.
- In the second case, the gas actually fired in the power station will only be in contractual balance with the gas fed to the grid – in will not physically be the same gas. Hence, any variations in gas flow or -quality will be evened out over the contractual periods and the power plant will not be affected at all. Even if the power plant is small, this will save investment cost in the electricity production unit – but additional investments for upgrading will have to be done at the gas production plant.
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.
For large-scale production plants, biogas of sng-quality is available from the gas grid.
For local use, such as a farm of sufficient size, raw biogas can be used 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%.