Wood vs. pellets - what to burn?
The heating season in Poland lasts roughly half a year. During this period, usually between October and April, owners of single-family houses and farms unplugged to the proverbial "thread" face a difficult choice - what to burn? The options are many. Some fuels tempt with their efficiency and low price, others with their low emissions and ease of sourcing. In this comparison, we will take two popular natural heating materials - wood and pellets - under the microscope.
Wood and pellets - characteristics
Wood is a fuel that hardly needs to be introduced to anyone. It is the oldest fuel known to man, used continuously for hundreds of thousands of years in every latitude. Wood owes its flammability to cellulose - a molecule that consists of bonds between carbon, hydrogen and oxygen. There are thousands of species of trees, and therefore varieties of wood. Some make better and others inferior fuel. However, they all grow through photosynthesis, which requires carbon dioxide and sunlight. In the process of combustion, wood releases the energy that the tree has taken from the sun. Like almost every other fuel on Earth, wood is merely a carrier of solar energy.
Pellet is a wood-based fuel that comes in pellet form. It is produced from wood waste, wood chips, sawdust, straw cuttings or sunflower shells. The initial material is fed into a machine called a pelletizer, where the biomass is compressed and processed to form cylindrical pellets no more than 1 cm long. The weight of the finished pellets is about 70 percent of the initial biomass.
Performance
The calorific value of wood varies depending on its moisture content. Another key characteristic is the density of the wood. It is worth remembering that, in general, deciduous trees are denser than conifers, so they are better suited for fuel. For freshly cut wood, the calorific value settles around 7.2 MJ/kg. The wood, after a year of storage, reaches about 12.2 MJ/kg. For well-dried wood, this figure can reach 14.4 MJ/kg.
The calorific value of a good quality pellet is 19 MJ/kg, which is 30 percent higher than for the best grade of wood. This may seem paradoxical, given that the base for pellet production is usually wood. However, in the process of pelletizing biomass, the pellets gain in energy content relative to the starting material through drying and pressing. Of course, this ratio can fluctuate depending on the type of pellet: it will be slightly different for sawdust pellets and different for sunflower husk pellets.
Environmental impact
From the point of view of the CO2 cycle in nature, both wood and pellets are zero-carbon fuels. When they are burned, about as much carbon dioxide enters the atmosphere as the tree from which the wood (or wood-based fuel) comes absorbed from the atmosphere during the growth period. Burning wood can be considered environmentally friendly, as long as the origin of our fuel is known to us and legal.
When it comes to the emission of harmful substances into the atmosphere, things look a little worse. Burning wood is harmful because the resulting smoke is a mixture of gases and fine particles (also known as particulate pollution, particulate matter or PM). In addition, smoke from burning wood contains benzene, formaldehyde and acrolein. The most dangerous components of the smoke are fine particles, also known as fine particulate matter or PM 2.5. They exacerbate symptoms in asthma sufferers, increase the risk of heart attacks and can cause chronic lung disease. It is estimated that burning 15 kg of wood generates as much PM 2.5 in the air as burning 7,500 cigarettes.
In the case of pellets, the harmfulness of the gases released into the atmosphere depends largely on the quality of the fuel. A good pellet should burn 99 percent and leave behind little ash and smoke. Smoke from pellets, like that emitted by burning wood, contains particles that are dangerous to health, but when pellets are burned, smoke emissions are much lower than when burning wood. Pellets, thanks to the pressing process, are far less moist and sticky than raw wood, so they smoke far less.
The situation gets dangerous when wood waste with an admixture of glues, varnishes or binders is used to make pellets. If you throw parts of old furniture or fragments of plywood or OSB into the pelletizer, you will get contaminated pellets, the combustion of which will release fumes of melted glue or varnish that are dangerous to health. For this reason, it is worth taking care of a reliable source of fuel or responsible production of pellets, even for your own use.
It is worth noting that pellets are burned in a sealed boiler equipped with a special pellet burner, while wood is often burned in open fireplaces inside the house. In this respect, people who burn wood may be at greater risk of exposure to harmful substances.
However, it is worth remembering that in terms of air pollution, both fuels are far less harmful than popular fossil fuels - coal or fuel oil.
Read more on this topic: Is fuel pellets environmentally friendly?
Wood or pellets - which pays off?
Comparing the economic advantages of these two fuel materials is problematic, since both can be sourced outside the traditional cycle. There are many ways to obtain wood: it can be bought from a depot, bought back from organizations responsible for logging, or obtained from other sources. The price of wood at the depot varies between 200 and 700 zlotys per cubic meter, depending on the species of tree and the period of storage. Wood prices from the forestry division in 2023 oscillated between PLN 124 and PLN 282 per spatial meter.
In the case of pellets, the issue is also complex. The price of pellets is about PLN 1,000 per ton (let us add at this point that about 20 kg of pellets are used to heat a 100 m² area for 24 hours). A cost-effective alternative is to produce pellets yourself from biomass, wood waste or shells. For farms generating large quantities of wood pulp, sawdust, straw cuttings or sunflower shells, such a solution is an attractive form of self-sufficiency. With a pelletizer, it is possible to produce valuable fuel from waste that would have to be disposed of anyway.
For farms producing their own fuel, the one-time expense in purchasing a pelletizer is an investment that will pay for itself after a short time. In-house pellet production also allows independence from supply chains or speculation on fuel prices.
Summary
Pellet firing appears to be an improved version of wood firing. The processing of biomass in a pelletizer allows the full energy potential of plant-based fuel to be realized. Wood will always enjoy the title of the most elegant fuel. It is irreplaceable in elegant fireplaces, where it pleases the eye with its bright flame, or in smokehouses or restaurants, where its smoke conquers the taste of traditional dishes. It has a cultural value that is not threatened by any substitute. However, in a sealed boiler, where efficiency and reliability are more important than a pleasant smell, pellets are a firm favorite. It is a fuel created to provide energy, and in this field it remains unrivaled.
