RES-chains training material:

The aim was to identify sustainable renewable energy source chains (RES-Chains) to encourage sustainable development within the South Baltic Region. The training material aimed to describe the connections between renewable energy sources and customers.

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Your choices:
1 Biomass (digestible sludge);   2 Transport;   3 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 3 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) 2 Transport
Residual oils/fats etc Local heating (ind. house)

 

If the primary aim with the biogas plant is to produce transportation fuel, then the gas has to be upgraded and the best thing is then to upgrade to the quality demand set by the fossil gas pipeline network.

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.

This is one major option at municipal-scale biogas plants such as digesters in connection to wastewater treatment plants or dump sites designated for organic waste fractions.

The aim with the upgrading process is basically to remove everything but methane from the gas. This can be done in two different ways:

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.

The gas finally obtained may be sold as a car fuel, it may be injected into the fossil gas pipeline system or it may be bottled and sold as cooking gas to households.