Pellet production: A greener biofuel

  

1. What Are Pellets?

According to the European Environment Agency (EEA), 87% of the urban population of the European Union (EU) is exposed to air pollution levels that exceed the limits established by the World Health Organization (WHO). The conclusions of the latest report (2019) could not be more categorical: for the EEA , "air pollution is the single greatest environmental risk to the health of European citizens. It reduces life expectancy and contributes to the appearance of serious diseases such as heart conditions, respiratory problems, and cancer”.

Heating emissions aggravate urban pollution. (EFE)

We are all concerned about the impact our daily activities have on pollution and we want to know how to do our bit to combat the climate emergency. A sustainable and efficient option is to opt for a pellet stove for heating, but what are pellets ?

Pellets are residues from forest cleaning and wood industries that are crushed and turned into shavings. Then, after being subjected to a drying process to reduce the humidity level, they are pressed into small cylinders.

The pellet, as biofuel, is clean, easy to handle, takes up little space and allows self-feeding in stoves and boilers, allowing autonomous operation for hours. In addition, it emits fewer particles into the atmosphere than a wood stove, which is beneficial in the fight against pollution.

2. Advantages of using pellet stoves compared to other alternatives

a. Less polluting

Some have reservations because, despite the fact that biomass is a renewable energy, stoves are still polluting. It is true that combustion pollutes, but it is also true that pellet stoves are increasingly efficient, safer and cleaner.

Ocu.org compared the emissions from a state-of-the-art pellet stove, a wood stove, and a cassette fireplace in the lab. The test was carried out under real conditions of use, considering the entire combustion cycle. These are the results:

In the table above, it can be seen that modern pellet stoves pollute much less overall, since they emit less dust particles and carbon monoxide.

Furthermore, if we take into account that, in certain countries, they replace the traditional chimney (without cassette or combustion chamber), the leap in safety, pollution and energy efficiency is much greater than what is reflected in the table. In reality, wood pellets are a suitable fuel, especially for forest countries , since it is derived from forest biomass and does not add more greenhouse gases and the CO2 emitted is the same as that absorbed by the tree.

b. Safer

In safety matters, it should be noted that pellet stoves are like a closed wood-burning fireplace. Pellets, unlike diesel and gas, are difficult to ignite and do not explode, making them safer.

Pellet stove (BroPro)

c. More effective

Pellet stoves have yields that range between 85% and 95% with respect to PCI (calorific value), while wood stoves, thanks to the latest improvements such as ventilation or improved insulation, reach efficiency levels close to 75% also on the PCI. This means that a smaller amount of pellet fuel will give us the same useful heat as more kilograms of firewood.

d. Same price as a wood stove

In reference to the price of fuel, pellets have a higher price than firewood per kilogram. Although to take stock of the final cost it is necessary to take energy efficiency into account, since the final price of kWh of heat is the most important. In this sense, both prices are balanced.

3. Pellet production process

a. Industrial pellet production process

1. Grinding and drying

The pellet production process begins with the grinding of non-commercial or industrially usable wood into shavings, to then proceed to its drying, which reduces the humidity of the biomass from around 50% to values close to 15% (the final product must have a moisture content of less than 10%). This prevents overweight in processing and reduces the elasticity of the material. Considering the overall process, drying is one of the stages with the highest energy consumption.

Wood logs and pellets (caloryfrio.com)

2. Cooling and sieving

Then the chips are pelletized, using rotary machines that press them against a sieve with circular holes, and cutting the resulting "noodles" to the desired length. Finally, cooling and sieving is carried out, in which the chips and loose particles that have been strained are filtered and introduced again into the process.

In the conventional production of pellets, biomass is used without any pre-treatment. Although biomass pelleting improves its density, it is possible to further improve the density of the fuel and its calorific value. In this sense, treatments have been developed, mainly using steam, to obtain products with higher performance.

b. Homemade process

The home-grown pellet making process requires home equipment with prices starting at $4,000 and includes a chipper, shredder, and pelletizer.

The process consists of four phases:

● The refining of the thickest wood logs to be able to introduce them into the crusher.

● The crushing of said trunks, branches, and green wood.

● The drying of the shavings.

● Pellet production.

i. Wood tuning

It is necessary to make the pieces of wood finer using mechanical tools such as axes or specific hydraulics for this purpose, to be able to introduce them into the shredder (since domestic ones do not usually accept branches of more than 5-6 cm in diameter).

ii. Grinding

To obtain a good pellet, it would be ideal to use a grind made with pruning remains and any type of wood. A pellet produced from virgin wood shavings will have a higher calorific yield than one made from sawdust, as well as being more resistant. The pieces of branches and wood must be crushed with a woodchipper that reduces them to 2-3 cm chips. It is necessary to carry out another crushing that produces 8mm pieces, since this is the correct size to be able to pass them through the pelletizer.

iii. Drying

Drying can be done between the first and second crushing by just leaving the chips out in the open on a sunny day, no tools needed.

iv. Compression

For the final phase of the manufacture of pellets, a pelletizer is needed. It is a machine that, through a mechanical process, compresses the wood shavings. Ideally, only pruning remains and clean wood should be introduced in the pelletizer, not sawdust.

Sawdust and pellets (argentinaforestal.com)

There are two large types of wood, with or without resin. The most common woods, such as beech, birch, poplar, and oak, can be mixed with conifers such as pine, fir, cedar, etc., which are resinous woods, must be added in a 40% percentage to ensure perfect compaction, since the resin acts as a binder, and to obtain grains of pellets of at least 2 cm from the pellet machine.

Whether the wood is resinous or not, the use of a binder is recommended, and it can be natural, like corn or oil residues, or a specific binder for the production of pellets that is added in percentages of 0.5% and 2 %, depending on the type of wood used.

Bags of pellets (argentinaforestal.com)

4. Types of pellets and qualities

As we already know, not all pellets are the same, since different types of wood, pruning remains, sawdust, etc. are used. for its manufacture. These are the different types of pellets classified according to quality.

Calorific power of the pellet (AgriEuro.es)

1. Class A1. Wood of forest origin and wood residues with no chemical treatment and low ash, nitrogen and chlorine content.
2. Class A2. Pellets with more ash, nitrogen, and chlorine than the previous one.
3. Class B. Allows the use of wood that comes from recycling.

5. The pellet and boiler market

Graph and map of pellet production in the EU (avebiom.org)

The EU produced 18.1 million tonnes of pellets in 2020, making it the world's leading producer . Production grows stably and between 2019 and 2020 it increased by 5%. Germany remains the largest producer in the European Union, and the Czech Republic registered a notable increase of 21.5% in 2020.

Regarding consumption, it increased by 7% in the world compared to 2019, reaching 19.3 million tons. The EU-27 remains the world's largest consumer of pellets, with the Italian residential sector leading the way with a total consumption of 3.4 million tons per year.

Wood shavings and pelletized branches (argentinaforestal.com)

If you are curious about how the pellet market will evolve worldwide, you can consult this market projection until 2027.

6. Social projects related to pellets

Social Solver always has the social nature of engineering in mind, and, for this reason, we include in this article several social projects in emerging countries. Pellet offers an opportunity to recover waste and other elements, and, at the same time, it is a sustainable activity that is less polluting than other alternatives. We have selected a couple of examples that illustrate its value.

CONGO

Bavon Mubake and Sylvestere Bin Kyuma organize charcoal for organic cooking in Bukavu. REUTERS/Crispin Kyalangalilwa

In the eastern forests of the Democratic Republic of the Congo, a social initiative has sprung up that employs elderly people to collect cardboard waste, stalks, and corn leaves to later soak the mixture, dry it, grind it into a powder that is complemented with charred sawdust. Afterwards, everything is pressed until obtaining the pellets that are used as fuel for cooking, since they can be burned safely.

With limited access to electricity, most people in Bukavu cook with "makala," or charcoal, slow-burning pieces of wood felled from the nearby national park, home to the endangered eastern lowland gorilla.

CHILE

In Valdivia, Chile, the Nutrition course of the Santo Tomás University has promoted the project called "Preparation of pellets from organic waste from the school feeding program (PAE)".

The Project Director, Massiel Salazar Garrido, states that this initiative encourages the use of unconventional raw materials for the production of pellets. These correspond to organic waste that is eliminated mainly in homes and landfills without an alternative use. This is how this experimental pilot project contributes in the medium and long term to diversify the energy matrix through an alternative of solid biofuels (pellets) and to mitigate air pollution due to the massive use of firewood.