How does a biomass power plant work?

What is biomass?

The word biomass describes materials originating from living animal or vegetable beings. In other words, all organic matter (living matter) from the animal and plant kingdom is obtained naturally or from artificial transformations.

All this matter is converted into energy by chemical processing.

Biomass energy ultimately comes from the sun. Plants absorb and store part of the solar energy that reaches the earth and animals in the form of food and energy. When organic matter stores solar energy, it also creates by-products that are not useful for animals or for making food but are useful for making energy from them.

Biomass was the most important energy source for mankind until the beginning of the Industrial Revolution, but its use declined as it was replaced by the massive use of fossil fuels.

Types of biomass

Biomass can be classified into three main groups:

Natural biomass: it is that which is produced in nature without human intervention.

Residual biomass: organic waste from human activities (e.g. municipal solid waste, MSW).

Produced biomass: these are energy crops, i.e. crop fields where a type of species is produced for the sole purpose of energy use.

Biomass to energy conversion

There are different ways to transform biomass into usable energy, but there are two that are most widely used today:

Thermochemical methods

It is the way to use heat to transform biomass. The materials that work best are those with lower humidity (wood, straw, husks, etc.). They are used for:

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

It is the most basic way to recover energy from biomass, from which hot gases are released to produce heat for use at home, in industry and to produce electricity.

• Pyrolysis: this is the decomposition of biomass using heat (about 500ºC) without oxygen. This process produces gases consisting of hydrogen, carbon, and hydrocarbon oxides, hydrocarbon liquids and carbonaceous solid residues. This process was used years ago to make charcoal.
• Gasification: this occurs when combustion takes place and different chemical elements are produced: carbon monoxide (CO), carbon dioxide (CO 2), hydrogen (H) and methane (CH 4), in different quantities. The gasification temperature can be between 700 and 1,500ºC and oxygen between 10 and 50%.

Depending on whether air or oxygen is used, two different gasification processes are created. On the one hand, gasogen or “lean gas” and on the other hand synthesis gas. The importance of the latter is that it can be transformed into liquid fuels (methanol and gasoline). For this reason, great efforts are being made to improve the gasification process with oxygen.

• Co-firing: consists of using biomass as an auxiliary fuel while coal is burned in the boilers. This process reduces coal consumption and CO2 emissions.

Biochemical methods

They are carried out using different microorganisms that degrade molecules. They are used for biomass with high moisture content. The most common are:

• Alcoholic fermentation: a technique that consists of the fermentation of carbohydrates found in plants to obtain an alcohol (ethanol) that can be used in industry.
• Methanic fermentation: anaerobic digestion (without oxygen) of biomass, where organic matter is decomposed (fermented) and biogas is created.

Biomass utilization systems

If we apply different technologies to the great variety of existing biomass, we can transform this energy for use in:

Thermal energy production

They are direct combustion systems. They are used to provide heat, which can be used directly for, for example, cooking food or drying agricultural products.

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

Biogas production

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

Biofuel production

They are an alternative to traditional transportation fuels and are unevenly developed in different countries. There are two types of biofuels:

• Bioethanol: substitutes gasoline. In the case of ethanol, and as regards raw material production, it is currently obtained from traditional crops such as cereal, corn, and beet.
• Biodiesel: its main application is aimed at replacing diesel. In the future, it will be used for varieties aimed at favoring energy production qualities.

Electricity production

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

What is a biomass power plant?

It is an industrial facility designed to generate electricity from biological resources. Thus, biomass power plants use renewable sources for the production of electrical energy.

Operation of a biomass power plant for electricity generation

The operating process of a biomass power plant is as follows:

Environmental impact of a biomass power plant

Biomass is the only energy source that provides a favorable CO2 balance, as long as the biomass is obtained in a renewable and sustainable way so that the consumption of the resource is slower than the Earth's capacity to regenerate itself. In this way, organic matter is able to retain more CO2 during its growth than it releases during combustion, without increasing the concentration of CO2.

Although the planet's existing energy potential would be sufficient to cover all energy needs, it cannot be used in its entirety, as it would require the large-scale exploitation of forest resources. This would make it impossible to keep consumption below the regenerative capacity, which would reduce the resulting net energy very considerably and lead to a depletion of these resources as well as to negative environmental effects.

The effects produced would be such as deforestation and a significant increase in CO2 emissions, which would imply a significant contribution to the deforestation process.