Biomass cogeneration: what it is and how it works

The term cogeneration refers to the combined production of electricity and thermal energy, ensuring better energy efficiency compared to separate productions.

Traditionally:

• electricity is produced in thermoelectric power plants, which release low-temperature thermal energy into the environment;
• thermal energy is produced with heating systems such as boilers that convert fuel energy into thermal energy.

In a cogeneration plant, this production is developed in a combined way from a single primary energy source, which can be a fuel such as natural gas, LPG, diesel, or biomass.

In this article, we will try to understand all the characteristics of a biomass cogenerator, the benefits it brings, and above all how to evaluate if it can represent the most suitable choice for your property.

What is a biomass cogenerator and how does it work?

A biomass cogenerator is a machine that produces electricity and thermal energy powered by organic matter, namely biomass.

Biomass is defined as “the biodegradable part of residual products from agriculture (including animal and vegetable substances) and forestry and related industries, as well as the biodegradable part of industrial and urban waste“.

Starting from the energy produced by engines powered by agricultural, forestry, wood chips according to different processes, a cogeneration plant compared to traditional ones, manages to reuse the dissipated heat, recovering it and thus producing thermal energy.

The cogeneration process originates from a fuel, in this case biomass, which powers a turbine, producing mechanical energy.

The mechanical energy produced generates:

• electricity, thanks to the conversion of mechanical energy by a generator;
• residual heat, which is recovered thanks to a heat exchanger. The recovered heat can be used for heating or DHW production.

The same principle is the basis for biomass micro-cogenerators, widely used in recent years in homes and public environments such as offices, hotels, restaurants, sports centers, because, also guaranteeing an independent electricity supply, it is an effective alternative to the normal heating system.

This results in on-site thermal energy production, ensuring energy and economic savings.

Diagram of a biomass cogenerator

How is biomass used in a cogeneration plant?

In a biomass cogeneration plant, biomass can be used in its raw form, if the burner directly processes dried, pressed, and shredded biomass, or it can be processed to produce biogas or vegetable oil.

Raw solid biomass is generally preferred only when it is directly available on-site. In its raw state, biomass has a low energy content per volume due to its complex composition and moisture content, resulting in significantly higher transportation costs and issues compared to other solutions.

Therefore, in a cogenerator, biomass is usually preferred only after a chemical-physical process that converts its energy carrier: through digestion or fermentation, biogas can be obtained, a mixture mainly composed of methane and CO2. Biogas can be further purified to obtain biomethane, consisting of about 95% methane, achieving a higher efficiency than biogas and comparable to natural gas.

Moreover, pressing and chemically extracting certain oily biomasses can also yield vegetable oil, which can be used as a fuel.

What are the main types of biomass cogenerators?

We have seen that a biomass cogenerator can be fueled by solid biomass, vegetable oil, biogas, or biomethane.

Let’s now analyze the main types of biomass cogenerators and their characteristics. Generally, we can have:

1. gas turbine cogenerator: in this system, biogas or biomethane is used to power a gas turbine acting as the primary engine. The rotational motion is transmitted to an electric generator for electricity production, while the exhaust gases are directed to a heat exchanger for thermal energy production;
2. steam turbine cogenerator: these can be back-pressure, where heat is recovered from the steam discharged from the turbine, or condensing, where heat is obtained from steam extracted at an intermediate stage of the turbine;
3. internal combustion engines (Diesel or Otto cycle): heat mainly comes from exhaust gases and engine cooling liquid;
4. combined cycle gas turbine/steam turbine plants.

Steam turbines and combined cycle gas turbine/steam turbine plants are mainly used for high-power industrial applications. Internal combustion engines and gas turbines are employed in both high-power plants and mini and micro-cogeneration systems.

Advantages and disadvantages of a biomass cogenerator

A biomass cogenerator allows utilizing the resources available in the area, which are always accessible and available, and by using waste materials, it helps reduce excessive waste production.

It also ensures lower primary energy consumption, reducing emissions and cutting energy transportation costs since energy is produced on-site.

However, it is important to note that the preliminary treatments to reduce the intrinsic moisture of biomass or the chemical-physical processes to convert it into biogas are quite expensive, requiring a certain amount of energy to carry them out.

Therefore, it is always necessary to evaluate the technical and economic feasibility beforehand and ensure long-term energy savings.