Your choices:
1 Transport; 2 Biomass (digestible sludge); Fuel: gaseous
| 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) | 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) | 1 Transport |
| Residual oils/fats etc | Local heating (ind. house) |
Production of biogas through anaerobic digestion (AD) of animal manure and slurries as well as of a wide range of digestible organic wastes, converts these substrates into renewable energy and offers a natural fertiliser for agriculture. At the same time, it removes the organic fraction from the overall waste streams, increasing this way the efficiency of energy conversion by incineration of the remaining wastes and the biochemical stability of landfill sites.
Various types of feedstock can be used for the production of biogas: animal manure and slurries, crop residues, organic wastes from dairy production, food industries and agro industries, wastewater sludge, organic fraction of municipal solid wastes, organic wastes from households and from catering business as well as energy crops. Biogas can also be collected, with special installations, from landfill sites.
One main advantage of biogas production is the ability to use "wet biomass" types as feedstock, all characterised by moisture content higher than 60-70% (e.g. sewage sludge, animal slurries, flotation sludge from food processing etc.). Besides crop residues, all kinds of agricultural residues, damaged crops, unsuitable for food or resulting from unfavourable growing and weather conditions, can be used to produce biogas and fertiliser.
The cell walls in land-based biomass will consist mainly of cellulose, hemicellulose and lignin while most types of algae and kelp will not have cell walls. Since lignin is the constituent having the highest heating value, the result becomes that dry, land-based biomass generally will have a higher heat content than dry aquatic biomass. Thus the harvesting of e.g. kelp for use as a solid fuel is not of major interest.
Aquatic biomass is instead suitable for biochemical conversion processes such as anaerobic digestion, since the low content of lignin makes it readily digestible. Therefore, the recent years have seen an increasing interest in the use of marine biomass as a substrate for anaerobic digestion, thus increasing the resource base significantly.
The raw biogas will consist, depending on substrate, of 50 – 65% methane in a water-saturated gas where carbon dioxide is the main other component. There will also be corrosive components in the gas such as hydrochloric acid, hydrogen sulphide and other gaseous compounds. These contaminants can be removed either in pressurised scrubbers or in pressure-swing-absorption units to yield a gas with a methane content exceeding 95%, ready to be used as a car fuel. For injection into the fossil gas pipeline system as substitute-natural-gas the purified gas needs an addition of a bit of heavier hydrocarbons and this is also common practice for the car fuel quality.