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 so far:
1 Biomass (solid)

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
1 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) Transport
Residual oils/fats etc Local heating (ind. house)

 

Regardless of the details in the photosynthesis process, the net effect is that carbon dioxide and water combine to store solar energy in the form of sugar which is then successively further transformed into cellulose, hemicellulose or other compounds. To transform carbon dioxide and water into glucose and cellulose, the plant needs use a number of enzymes, proteins and organic amino acids, chlorophyll being one of the most well-known actors.

The simplest way is to make direct use of the solid biomass originally formed by the photosynthesis. We – humankind – have used this fuel for cooking, boiling water, producing steel, glass, bronze and other utilities as well as for illumination since thousands of years and with modern process control technology we could well do so today.

At a global scale, the photosynthesis produces about 150 000 000 000 tons of dry cellulose every year, 150 billion tons. The harvesting and the use of this biomass for energy purposes does not in itself represent any contribution to the carbon circulation, but in case the harvesters or the transport system to supply the biomass to the end users makes use of fossil fuels – such as diesel oil for the trucks to transport the biofuel or to run the harvesters – then there will still be a net contribution to global warming also from biofuels.

Hence the use of biofuel is not completely carbon neutral but will be depending on the infrastructure.

The environmental concerns with biomass use tend to fall into four major questions:

It is obvious that the answers to these questions are to a great extent depending on local, regional, national and international policies and system strategies but it also depends on the choice of technology to be used.

The first priority as it comes to choice of biomass fractions suitable for local/regional biomass-based energy solutions should of course be to use such fractions that can be co-produced with food and with commercial crops. From that, it follows that large-scale monocultures should be avoided. The system solution – logistics as well as the choice of raw-material – shall simply be chosen so as to avoid competition and to promote co-production.

As the fourth question is concerned, it must be understood that the choice of process, direct combustion, gasification, high-temperature pyrolysis, low-temperature pyrolysis (torrefaction) or any other process, and the choice of process equipment must be apt to the biomass in question.

Being a product of the photosynthetic process, the dry matter content of herbaceous biomass resources are also typically composed of carbohydrate polymers (cellulose and hemicellulose), phenolic polymers (lignin) and, in lesser quantities, other substances, such as resins, fats and fatty acids, commonly known as extractives.

The distribution of cellulose, hemicellulose, lignin and extractives is most relevant in the case of liquid biofuels production and in cases when biochemical conversion processes such as anaerobic digestion and/or fermentation are applied. Lignin is the most difficult substance to digest and may, if present in too large amounts, cause problems or at least a drop in total conversion performance in anaerobic digesters while cellulose is not easily fermentable and may cause problems in the case of ethanol production plants

One key difference of herbaceous biomass and sewage sludge as compared to woody biomass is the lower weight percentage of lignin and the increased presence of cellulose and hemicelluloses, also fats in the case of sewage sludge. The lignin content of herbaceous resources typically ranges from 15% to a little over 20%.

Since lignin is less oxidized than hemicelluloses, it has a higher heating value and this typically translates to lower heating values of herbaceous biomass as compared to woody biomass or some agro-industrial residues, such as olive press cakes.

There will also be other differences between mainly herbaceous and woody biomass, such as the ash melting temperatures and the ash properties.

Provided that the combustion equipment is well-suited for the biomass in question and provided that the process control system is correctly tuned, the environmental impact from the thermal processing can be kept under close control and need not cause any worry.

However, inadequate equipment or process control, in combination with the varying quality with biomass fuels, may prove disastrous to the local environment and to the local air quality.