Your choices:
1 Transport; 2 Fuel: gaseous; Biomass (digestible sludge)
| 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 | Electricity |
| Biomass (solid) | Electricity | Process cooling (< 0 °C) |
| Geothermal | 2 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) | 1 Transport |
| Residual oils/fats etc | Local heating (ind. house) |
If a gaseous transportation fuel is desired, the only raw material is digestible sludge.
During the digestion process, hydrochloric acid as well as hydrogen sulphide and ammonia are formed and they will to a certain extent be present in the raw gas. The biogas will leave the digester at the digester temperature, typically 35-40 °C and since the digestion requires the presence of water, the water content in the gas will be some 5-7%. Since the gas will also contain corrosive and water soluble components, cooling the gas may become problematic.
Biogas can easily be up-graded either by pressurised scrubbing where water is used to wash out the contaminants and to raise the methane content to 90-95% or in a pressure-swing-absorption (PSA) process yielding a similar quality. Since the fossil gas distributed throughout Europe in pipelines (natural gas) consists to the main part of methane but with a bit of heavier hydrocarbons (3-5%) in it, the addition of a little bit of LPG to the upgraded biogas makes it a copy of the fossil gas and it may then be injected into the pipeline system for distribution as sng-quality (substitute natural gas) biogas. Obviously a strict quality control will be necessary prior to injection, but given that, there will be no major problems.
Sng-quality biogas is an alternative fuel first for Otto engines. The energy efficiency is generally equal to that of gasoline engines, but lower compared with modern diesel engines.
Gasoline/petrol vehicles converted to run on gas tend to suffer because of the low compression ratio, resulting in a reduction about 10-15% of delivered power while running on gas.