Your choices so far:
1 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 | 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 process heat, the anaerobic digestion plant will most probably not be a major source for the energy distributed in the district heating system.
Large-scale CHP-production from digestible biomass can basically take place in two different ways, both using the raw biogas:
- The gas can be combusted in a steam boiler and a common steam cycle used for the electricity production. It may also be possible to use co-combustion with solid biomass in the same boiler unit. Boilers do not have a high gas quality requirement but it is recommended to reduce the H2S concentrations to values lower than 1 000 ppm which allows to maintain the dew point around 150 °C. It is advised to condense the water vapour in the raw gas prior to the burners and this will also remove a large proportion of the H2S, reducing the corrosion and stack gas dew point problems.
- The gas can be fed to a diesel engine adapted for the gas quality. Low-rpm, high power diesels are commercially available in different scales and have the advantage that the investment cost may be kept down. The utilisation of biogas in internal combustion engines is a long established and extremely reliable technology. Thousands of engines are operated on sewage works, landfill sites and biogas installations. The engine sizes range from 45 kW (which corresponds to approx. 12 kWel) on small farms up to several MW on large scale landfill sites. Gas engines have similar requirements for the gas quality as boilers, except that the H2S should be lower to guarantee a reasonable life-time of the engine. Otto engines designed to run on petrol are far more susceptible to hydrogen sulphide than the more robust diesel engines. For large scale applications (> 60 kWel) diesel engines are therefore standard. Occasionally, organic silica compounds in the gas can create abrasive problems. If so, they should be removed.
The water temperature obtained from the diesel engine application is in the low range for an efficient production of district cooling. If district cooling is desired it may be advisable to "top up" the water temperature a bit by direct firing of a side-stream of the biogas.
To maximise biogas yield a suitable mix of substrates should be used. The plant will in most cases have its natural home at the wastewater treatment plant where primary and biological sludge will be the base resource but residues from food manufacturing as well as biological sludge from other process industries, bio-waste from households and grease, fats and oils from restaurants and alike should also be considered as resources. As with all CHP processes, an upper limit for the plant size will be set by the requirements for heat in the district heating network.
Properly treated, the digestate is a valuable soil improvement agent containing not only nutrients and minerals but also organic carbon. Obviously, the quality of the digestate is strongly depending on the quality of the feedstock.