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  • Writer's picture@ Cynthia Adina Kirkwood

Portugal: How Wood-Pellet Stoves Really Work

Updated: Dec 10, 2023


Pellet stoves are the cleanest solid-fuel residential heating appliances, reported the U.S. Department of Energy. Yet, the source of the pellets is an integral factor in determining the ecological sensitivity of the heating system.

In Portugal, for example, wood pellets are a mix of wood waste and trees, mostly pine, according to an environmental organization. In British Columbia, Canada, pellets are made entirely from the residuals of sustainably managed forests, according to the forestry industry.

However, a recent investigation found that century-old trees in old-growth forests of Romania and Slovakia were being felled and turned into wood pellets for customers in France, Italy and Poland. Due to its findings, in September 2022, the European Union Parliament called for an end to subsidies for burning trees and to “phase down” the share of trees counted as renewable energy in EU targets, reported The Guardian (September 14, 2022).

Pellet Stoves

A pellet stove produces very little ash or smoke, much less than a wood stove, that sometimes needs cleaning only weekly as compared to daily. It burns so efficiently that little creosote builds up in the flue, posing less of a fire hazard.

The key components of a pellet stove are a hopper, auger, burn pot, heat exchanger and fan. The hopper is where the pellets are stored; the auger, a feeder device like a long screw or rod, moves the pellets from the hopper to the burn pot, where the pellets are ignited and burned to create heat, according to Pellet Kings, a heating shop in the United Kingdom. The heat exchange is a series of metal tubes or plates that absorb the heat of the fire and transfer it to the air that is either blown into the room or directed elsewhere by a fan.

A pellet stove often can connect to a house’s existing pipes and radiators for central heating.

How quickly pellets are fed to the burn pot determines the heat output, according to the U.S. Office of Energy Saver, of the Department of Energy. Some models can be automated by a smartphone.

Pellet stoves, which are certified by the U.S. Environmental Protection Agency (EPA), are likely to be 70 percent to 83 percent efficient. However, new catalytic wood stoves, which allow combustion gases to burn at lower temperatures, thereby cleaning the exhaust gas, can burn as efficiently.

Depending on the size of the home, pellet stoves require refueling every few hours to once a day. However, with these top-fed stoves, the bagged pellets can be difficult to carry and hoist up. Bags weigh 15 kilograms in Portugal (33) pounds) and 18 kilograms (40 pounds) in the United States. Yet, they are less cumbersome than wood logs, and they take up far less storage space.

Most pellet stoves’ exteriors – except glass doors – stay relatively cool while operating, reducing the risk of accidental burns. However, the glass door is in the front and near the floor, making it a hazard to toddlers and young children -- as are wood-burning stoves --unless using a fireguard.

Pellet stoves run on electricity. The auger and the burn pot’s igniter, for example, are electrically powered parts. What happens in electric power outages? The stove would fill with smoke which likely would escape into the room. Stove owners who experience electricity loss for days at a time usually have a recourse.

“Most people have battery backup. . . Very easy to run a heater with solar and battery backup (or a generator),” said Tassie Pellet Heater Owners in Australia.

Pellet stoves are complex and have expensive parts that will need to be replaced at some time, according to the U.S. Office of Energy Saver.

According to heating experts, generally, the longevity of a pellet stove is 15 to 20 years compared to a wood stove’s 20 to 25 years.

Pellet stoves have heating capacities that range between 8,000 btu (British thermal units) and 90,000 btu per hour. Many wood stoves have btu ratings over 100,000.

“Though their btu ratings typically hover below the 50,000 mark, making them seem less potent than their wood-burning counterparts, they play a different game. Pellet stoves excel in delivering a consistent heat output. Their fuel – compressed wood pellets – burns at a predictable rate, allowing these stoves to maintain a stable temperature over extended periods,” reported a forestry website in the United States.

Some owners complain of the hum of the fan and the sound of the augur dropping pellets into the burn pot, while others call the sounds relaxing white noise.



Pellet stoves burn compacted pellets, usually made of wood. However, some models also can burn pellets of other organic materials, such as nutshells, corn kernels and small woodchips. Pellets should be kept in a dry storage area.

In a bag of best-quality pellet fuel, there should be less than 4 ounces (113 grams) of dust throughout and at the bottom of an 18-kilogram (40-pound) bag, reported the U.S. Office of Energy Saver. Excessive dirt and dust can form clinkers in the stove.

The smell of the pellets, as they are emptied from their bag into the stove’s hopper, is of the wood and its sawdust, according to heating experts and manufacturers.

In Portugal, pellet certification is not mandatory, but it is a first step available to manufacturers to ensure greater quality control and provide an efficient product with a very small ecological footprint, according to Deco, (Associação Portuguesa para a Defesa do Consumidor), a consumer protection organization in Portugal, who, on its page of how to choose a heating system in Portugal, where maintaining thermal comfort in homes can be a challenge, reported:

“A certified product includes an image with the letters EN/plus and PT followed by three digits that indicate the certificate number, unique for each producer, The EN/plus certification includes three quality classes (A1, A2 and B).”

The EN/plus certification, established in 2011 in Europe, is now international with more then 1,200 companies, according to its website.

Most pellet fuels have a moisture content of 5% to 10%, rendering them more efficient than well-seasoned firewood, which has about 20 percent, according to the U.S. Office of Energy Saver.

According to naturally wood, of Forestry Innovation Investment, of British Columbia, Canada:

“To create wood pellets, producers remove moisture from incoming wood fiber, grind the fiber into dust, and compress the dust into small cylinders – pellets – typically with a 6-millimeter (.2 inches) or 8-millimeter (.3 inches) diameter, and a length of up to 40 millimeters (1.6 inches).

“Heat is applied in this process which causes lignin – a natural polymer found in wood – to act as a glue to hold the compressed particles together. The result is a dry, highly compressed and high energy-value product that can be easily handled and transported efficiently over very long distances.”

Origin of Pellets in Portugal

The consumer group, Deco, reported that pellets “are made from leftover leaves, sawdust and wood chips”.

However, in Annual Barometer: Wood Pellet Industry in Portugal: 2022, Zero (Associação Sistema Terrestre Sustentável), an environmental organization, reported:

“Pine, (specifically, pinheiro bravo or maritime pine), is the main species used by pellet producers and the vast majority of the feedstock is in the form of whole trees, from final cuts or thinning operations. Secondary sources such as sawmill residues represent a much smaller share.”

However, smaller mills tend to be associated with sawmills and use higher quantities of wood residues, or secondary feedstock. Larger mills are much more dependent on roundwood, or primary feedstock.

Of the 26 mills, 10 are smaller-scale producers of less than 40,000 tons per year. Larger mills may have both EN/plus and the SBP (Sustainable Biomass Program for large-scale industrial production) certifications. Smaller mills have only the EN/plus.

The environmental group, Zero, warned of the decline in pine stands, closely associated with forest fires.

Portugal consumed 57 percent more pine than the estimated productive capacity of pine stands in 2021. Pellet production accounted for one-fifth of pine consumption, though it represented only 3 percent of the export value of pine products. Therefore, pellets are a low-value but high-impact commodity.

More than 1.5 million tons of wood were used to produce pellets by the 26 pellet mills in Portugal. Almost half of the pellets were exported to power stations in northern Europe (United Kingdom, Denmark and The Netherlands), with Drax power station near Selby, North Yorkshire, in the U.K., being the single biggest consumer (23 percent), reported Annual Barometer.


Just over half of Portugal’s pellet production was used for industrial and residential heat production, which included pellet sales in Portugal (38 percent) and exports to Spain (16 percent).

Zero called for increased transparency in pellet production and sourcing of wood; a moratorium on increased pellet production; ending public finance for pellet production, and increased public investment in pine forest management.


Origin of Some Pellets in Northern Europe

In protected areas of Romania and Slovakia, companies are clear-cutting forests and grinding up hundred-year-old trees for the manufacturing of pellets in the name of renewable energy, reported The New York Times (September 7, 2022).

Pellets are shipped across Western Europe helping countries reach their renewable-power commitments.

“None of this is illegal. In fact, it is encouraged by green-energy subsidies. But in reality, burning wood can be dirtier than burning coal,” reported The Times.

Burning wood was never supposed to be the cornerstone of the European Union’s green energy strategy.

When the EU began subsidizing wood burning more than a decade ago, it was viewed as a quick boost for renewable fuel and an incentive to move homes and power plants away from coal and gas. Chips and pellets were marketed as a way to turn sawdust waste into green power.

Those subsidies gave rise to a booming market. Wood is Europe’s largest renewable energy source, far ahead of wind and solar, reported The Times.

Europe is the biggest international market for wood pellets, according to Deutsche Welle (January 4). To meet the rising demand, 20 percent are imported from North America, while most comes from Eastern Europe, Scandinavia and the Baltic states.

The EU spent €13 billion in bioenergy subsidies in 2020, down from €17 billion the previous year, according to the European Commission reported The Guardian. Non-governmental organizations claimed that most subsidies go to power plants but could be better used to support clean technology such as heat pumps.

“Evidence is mounting that Europe’s bet on wood to address climate change has not paid off,” said The Times. “Forests in Finland and Estonia, for example, once seen as key assets for reducing carbon from the air, are now the source of so much logging that government scientists consider them carbon emitters.”

In the European Parliament in September 2022, representatives voted to end subsidies for “primary woody biomass”, or healthy trees logged for fuel. Trees cut down for fire protection or road safety may continue to benefit from renewable energy subsidies, repotted The Guardian.

The vote set the stage for negotiations between the MEPs (Members of the European Parliament) and the European Union’s member states.

The European Union is seeking to accelerate its green transition and end dependence on Russian fossil fuels . . . fast. MEPS have voted for 45 percent of energy to come from renewable sources by 2030.

History of Pellets

Wood pellet technology heated up during the oil energy crisis in the 1970s, reported the European Pellet Council. Sweden, due to its prominent timber industry, desire for increased energy independence and commitment to environmental conservation, designed the first operational plants in Mora and Vargarda in the early 1980s.

At this time, wood pellet production was based on animal feed pellet manufacturing.

With the recovering price of oil, the wood pellet alternative became less compelling until the 1990s. Increasing worry about global warming has led wood pellets to be considered an alternative for fossil fuels in Europe.

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