Biomass

1. What is biomass?

The word biomass describes the material coming from living animal or vegetable beings. That is to say, all the organic matter (living matter) from the animal and vegetable kingdom or coming from artificial transformations.

All the matter becomes energy if it undergoes a chemical processes.

The energy of the biomass comes in the last instance from the Sun. Vegetation absorbs and stores part of the solar energy that reaches the Earth and the animals in the shape of food and energy. When the organic matter stores solar energy, it also creates sub-products which are not used for animal or manufacture food but to make energy with them.

The biomass was the most important source of energy for mankind until the beginning of the industrial revolution, but it started to decrease as it became replaced by the massive use of fossil fuel.

2. Types of biomass

The biomass can be classified into three main groups:

• Natural biomass: that produced by nature without human intervention.
• Residual biomass: the organic waste coming from human activity (urban solid waste, RSU, for example).
• Produced biomass: these are energy crops, that is to say, farms where a species is grown with the sole purpose of it being used for the generation of energy.

Would you like to know more? Access the interactive game on the types of biomass.

3. Conversion of biomass into energy

There are different ways of transforming biomass into energy for it to be used, but two of them are the ones most commonly used today:

Thermo-chemical methods

It is the way of using heat to transform biomass. The materials that work the best are the ones with less humidity (wood, hay, shells, etc.). They are used for the following:

• Combustion: when we burn biomass with a great deal of air (20-40% higher than the theoretical) at a temperature of between 600 and 1,300ºC.

This is the most basic method to recover the energy from biomass, from which hot gas comes out to generate heat and then use it at home, in industries and to generate electricity.

• Pyrolysis: the decomposition of the biomass using heat (at about 500ºC) without oxygen. Using this process gas is obtained, made up of hydrogen, carbon oxides and hydrocarbons, hydro carbonate liquid and carbon solid waste. This process has been used for many years to obtain charcoal.
• Gasification: when there is combustion and different chemical elements are produced: carbon monoxide (CO), carbon dioxide (CO₂), hydrogen (H) and methane (CH₄), in different amounts. The temperature of the gasification can be between 700 and 1,500ºC and the oxygen is between 10 and 50%.
• Depending on whether air or oxygen is used, two different gasification processes are created. On the one hand, the gasogene or “poor gas” and on the other synthesis gas. The importance of this is that it can be transformed into a liquid fuel (methanol and petrol). That is why great efforts are being made that tend towards improving the gasification process with oxygen.
• Co-combustion: consists of the use of biomass as a back up fuel while the combustion of coal takes place in the boilers. With this process the combustion of coal and the CO₂ emissions are reduced.

Biochemical methods

They are carried out using different micro organisms that degrade the molecules. They are used for biomass with a high content of humidity. The most usual are the following:

• Alcoholic fermentation: technique that consists of the fermentation of carbohydrates which are in the plants and in which an alcohol (ethanol) is produced, which can be used for industry.
• Methane fermentation: this is the anaerobic digestion (without oxygen) of biomass, where the organic matter decomposes (ferments) and biogas is created.

4. Biomass exploitation systems

If we add different technology to the existing large variety of biomass we can transform this energy in order to use in the:

Production of thermal energy

These are direct combustion systems. They are used to generate heat, which can be directly used to, for example, cook food or to dry agricultural products.

They can also be used to make steam for the industry or to generate electricity.

The drawback, however, is the contamination.

Production of biogas

The purpose is to create fuel, mainly methane, which is very useful for thermal applications for the livestock or agricultural sector, supplying electricity and heat.

Production of bio fuel

These are an alternative to the traditional transport fuel and have a different degree of development in different countries. There are two types of bio fuel:

• Bioethanol: replaces petrol. In the case of the ethanol, and with regards to the production of raw material, it is currently obtained from traditional crops such as wheat, corn and beetroot.
• Bio-diesel: Its main application is addressed to the replacement of diesel. In the future it will be used for varieties designed to encourage the quality of energy production.

Production of electricity

Electricity can be generated by combustion or gasification and power of up to 50MW can be obtained.

5. What is a biomass power station?

It is an industrial installation designed to generate electricity using biological resources. Therefore, the biomass power stations use renewable sources for the generation of electricity.

Operation of a electric generation biomass power station

The operation process of a biomass electricity power station is the following:

• In the first place, the main fuel of the installation and the forestry waste are stored in the power station. There their bulk is reduced, if necessary.
• Then, they are taken to a fuel preparation building, where it is classified according to their size and finally are taken to the corresponding warehouses.
• Then they are taken to the boiler for their combustion; this makes the water of the boiler pipes become steam due to the heat.
• The water circulating along the boiler pipes come from the supply tank, where it is preheated by means of a heat exchanger with the combustion gases which are even slower coming from the boiler itself.
• In the same way as in other conventional thermal power stations, the steam generated in the boiler goes to the steam turbine which is linked to the electricity generator, where the electricity that will be transported by means of the corresponding lines is generated.
• The steam becomes liquid in the condenser, and from here it is again taken to the supply tank thus completing the main water-steam circuit of the power station.

6. Environmental impact of a biomass power station

The biomass is the only source of energy with a favourable CO₂ balance, providing that the biomass is obtained in a renewable and sustainable way, in such a way that the consumption of the resource is slower than the capacity of the Earth to regenerate. Thus, the organic matter is able to retain more CO₂ during its growth than that freed up in its combustion, without increasing the CO₂ concentration. 

Although the existing potential of energy in the planet would be sufficient to cover all the energy needs, this cannot be used entirely, as it would require the exploitation of the forestry resources on a great scale. This would make it impossible to maintain the consumption below the regeneration capacity, which would very considerably reduce the resulting net energy and would lead to an exhaustion of said resources which would give rise to negative environmental effects.

The effects produced would be such as deforestation and outstanding increasing of the CO₂ emissions, what would imply a contribution to climate change.