Your choices so far:
1 Biomass (digestible sludge); 2 Fuel: gaseous
| 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 | 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) | Transport |
| Residual oils/fats etc | Local heating (ind. house) |
Renewable gaseous fuels can be produced mainly in two ways; either it can be produced by anaerobic digestion of a wet, organic, raw material or via thermal gasification.
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.
The formation of methane (CH4) is a biological process that occurs naturally when organic matter (biomass) decomposes in a humid atmosphere in the absence of air but in the presence of a group of natural microorganisms which are metabolically active, i.e. methane bacteria. In nature, methane is formed as marsh gas (or swamp gas), in the digestive tract in ruminants, in plants for wet composting, and in flooded areas and lands (rice fields).
In principle, all organic materials can ferment or be digested. However, only homogenous and liquid substrates should be considered for simple biogas plants: faeces and urine from cattle, pigs and possibly from poultry and the wastewater from toilets. When the plant is filled, the excrement has to be diluted with about the same quantity of liquid; if possible, the urine should be used. Waste and wastewater from food-processing industries are only suitable for simple plants if they are homogenous and in liquid form. The maximum gas production from a given amount of raw material depends strongly on the type of substrate.
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 the purified gas needs an addition of a bit of heavier hydrocarbons.
The raw biogas can also be used without any upgrading. In small scale applications, the gas may be burned directly in boilers for heat generation, though there will be a demand for a support burner for safety reasons. The raw gas can also be utilised in an adapted internal-combustion engine for CHP production. This is a common technique to provide low-temperature thermal energy as well as process electricity at municipal wastewater treatment plants and can also be applied to landfill gas.
If the wastewater treatment plant is big or the flow of landfill gas is big enough – or at dump sites aimed to take care of organic waste – upgrading and selling the gas may be a viable option.
The aim with the upgrading process is basically to remove everything but methane from the gas. This can be done in two different ways:
- Scrubbing the gas with water at an elevated pressure (some 10-30 bar) will make most of the gas components dissolve in the water. Methane does not dissolve in water to any major extent so what happens is that the methane content is increased up to about 90%. Cooling the gas to condense the water vapour will then bring the methane content up to about 95%.
- Pressurising the gas (again some 10-30 bar) in the presence of a suitable solid and selective absorber will absorb the contaminants in the absorber. When the pressure is again lowered, the enriched methane will leave the absorber volume while the absorbed contaminants are retained in the absorber until the pressure is low enough and they are finally given off.
With both processes, pressurised scrubbing as well as pressure-swing-absorption, there will be a loss of methane about 5% of the total through flow and the outgoing gas will have a methane content about 95% by volume. From the pressurised scrubber, the scrubber water will have to be cleaned prior to letting it into the municipal waste-water system. From the PSA-process, the contaminants are given of in gaseous form and may require gas cleaning.
Since the upgraded gas is available at an elevated pressure, it may be filled onto bottles and sold to individual households as well as it may be used as a car fuel or – after addition of a bit of heavier hydrocarbons – injected into the fossil gas pipeline system.
Both gas qualities – raw biogas as well as upgraded – can be delivered via pipelines but in the case of raw biogas it must be remembered that it contains large amounts of water and of acid and corrosive components.