Pellet fuel

Pellet fuels are biofuels made from compressed organic matter or biomass. Pellets can be made from any one of five general categories of biomass: industrial waste and co-products, food waste, agricultural residues, energy crops, and virgin lumber. Wood pellets are the most common type of pellet fuel and are generally made from compacted sawdust and related industrial wastes from the milling of lumber, manufacture of wood products and furniture, and construction. Other industrial waste sources include empty fruit bunches, palm kernel shells, coconut shells, and tree tops and branches discarded during logging operations. So-called “black pellets” are made of biomass, refined to resemble hard coal and were developed to be used in existing coal-fired power plants. Pellets are categorized by their heating value, moisture and ash content, and dimensions. They can be used as fuels for power generation, commercial or residential heating, and cooking. Pellets are extremely dense and can be produced with a low moisture content (below 10%) that allows them to be burned with a very high combustion efficiency.

Further, their regular geometry and small size allow automatic feeding with very fine calibration. They can be fed to a burner by auger feeding or by pneumatic conveying. Their high density also permits compact storage and transport over long distance. They can be conveniently blown from a tanker to a storage bunker or silo on a customer’s premises.

A broad range of pellet stoves, central heating furnaces, and other heating appliances have been developed and marketed since the mid-1980s. In 1997 fully automatic wood pellet boilers with similar comfort level as oil and gas boilers became available in Austria. With the surge in the price of fossil fuels since 2005, the demand for pellet heating has increased in Europe and North America, and a sizable industry is emerging. According to the International Energy Agency Task 40, wood pellet production has more than doubled between 2006 and 2010 to over 14 million tons. In a 2012 report, the Biomass Energy Resource Center says that it expects wood pellet production in North America to double again in the next five years.

Pellets are produced by compressing the wood material which has first passed through a hammer mill to provide a uniform dough-like mass. This mass is fed to a press, where it is squeezed through a die having holes of the size required (normally 6 mm diameter, sometimes 8 mm or larger). The high pressure of the press causes the temperature of the wood to increase greatly, and the lignin plasticizes slightly, forming a natural “glue” that holds the pellet together as it cools.

Pellets can be made from grass and other non-woody forms of biomass that do not contain lignin: distiller’s dried grains (a brewing industry byproduct) can be added to provide the necessary durability. A 2005 news story from Cornell University News suggested that grass pellet production was more advanced in Europe than North America. It suggested the benefits of grass as a feedstock included its short growing time (70 days), and ease of cultivation and processing. The story quoted Jerry Cherney, an agriculture professor at the school, stating that grasses produce 96% of the heat of wood and that “any mixture of grasses can be used, cut in mid- to late summer, left in the field to leach out minerals, then baled and pelleted. Drying of the hay is not required for pelleting, making the cost of processing less than with wood pelleting.” In 2012, the Department of Agriculture of Nova Scotia announced as a demonstration project conversion of an oil-fired boiler to grass pellets at a research facility.

Rice-husk fuel-pellets are made by compacting rice-husk obtained as by-product of rice-growing from the fields. It also has similar characteristics to the wood-pellets and more environment-friendly, as the raw material is a waste-product. The energy content is about 4-4.2 kcal/kg and moisture content is typically less than 10%. The size of pellets is generally kept to be about 6 mm diameter and 25 mm length in the form of a cylinder; though larger cylinder or briquette forms are not uncommon. It is much cheaper than similar energy-pellets and can be compacted/manufactured from the husk at the farm itself, using cheap machinery. They generally are more environment-friendly as compared to wood-pellets. In the regions of the world where wheat is the predominant food-crop, wheat husk can also be compacted to produce energy-pellets, with characteristics similar to rice-husk pellets.

A report by CORRIM (Consortium On Research on Renewable Industrial Material) for the Life-Cycle Inventory of Wood Pellet Manufacturing and Utilization estimates the energy required to dry, pelletize and transport pellets is less than 11% of the energy content of the pellets if using pre-dried industrial wood waste. If the pellets are made directly from forest material, it takes up to 18% of the energy to dry the wood and additional 8% for transportation and manufacturing energy. An environmental impact assessment of exported wood pellets by the Department of Chemical and Mineral Engineering, University of Bologna, Italy and the Clean Energy Research Centre, at the University of British Columbia, published in 2009, concluded that the energy consumed to ship Canadian wood pellets from Vancouver to Stockholm (15,500 km via the Panama Canal), is about 14% of the total energy content of the wood pellets.

Pellet standards
Pellets conforming to the norms commonly used in Europe (DIN 51731 or Ö-Norm M-7135) have less than 10% water content, are uniform in density (higher than 1 ton per cubic meter, thus it sinks in water)(bulk density about 0.6-0.7 ton per cubic meter), have good structural strength, and low dust and ash content. Because the wood fibres are broken down by the hammer mill, there is virtually no difference in the finished pellets between different wood types. Pellets can be made from nearly any wood variety, provided the pellet press is equipped with good instrumentation, the differences in feed material can be compensated for in the press regulation.. In Europe, the main production areas are located in south Scandinavia, Finland, Central Europe, Austria, and the Baltic countries.

Pellets conforming to the European standards norms which contain recycled wood or outside contaminants are considered Class B pellets. Recycled materials such as particle board, treated or painted wood, melamine resin-coated panels and the like are particularly unsuitable for use in pellets, since they may produce noxious emissions and uncontrolled variations in the burning characteristics of the pellets.

Standards used in the United States are different, developed by the Pellet Fuels Institute and, as in Europe, are not mandatory. Still, many manufacturers comply, as warranties of US-manufactured or imported combustion equipment may not cover damage by pellets non-conformant with regulations. Prices for US pellets surged during the fossil fuel price inflation of 2007–2008, but later dropped markedly and are generally lower on a per-BTU basis than most fossil fuels, excluding coal.

Regulatory agencies in Europe and North America are in the process of tightening the emissions standards for all forms of wood heat, including wood pellets and pellet stoves. These standards will become mandatory, with independently certified testing to ensure compliance. In the United States, the new rules initiated in 2009 have completed the EPA regulatory review process, with final new rules issued for comment on June 24, 2014. The American Lumber Standard Committee will be the independent certification agency for the new pellet standards.

Wood pellets for industrial use
Wood pellets are traded in five size classes (see table) and two water content classes M10 and M15 with a maximum of 10 and 15% water content. For ash content (A), mechanical strength (DU), fines content (F) and bulk density (BD) and for sulfur (S), nitrogen (N) and chlorine content (Cl) are also classes defined. The calorific value Q and a possible content of additives must, the ash softening temperature DT should be specified. The bulk density must be at least 600 kg / m³, for the other aforementioned properties no limit values are given.

magnitude diameter length Water content Class water content
D06 6 mm ± 1.0 mm 3.15 mm to 40 mm, 1% up to 45 mm M10 Max. 10%
D08 8 mm ± 1.0 mm 3.15 mm to 40 mm M15 Max. 15%
D10 10 mm ± 1.0 mm 3.15 mm to 40 mm
D12 12 mm ± 1.0 mm 3.15 mm to 50 mm
D25 25 mm ± 1.0 mm 10 mm to 50 mm

Wood pellets for non-industrial use
For non-industrial use only sizes D06 and D08 are traded in water content class M10. The bulk density must be at least 600 kg / m³, the fines content must not exceed 1%, the additive content not more than 2%. There are three property classes. Class A1 and A2 pellets are made from freshly harvested wood or chemically untreated wood residues, in the case of A1 from low ash and nitrogen content material, at A2 with slightly higher ash and nitrogen content (eg solid wood pellets, forest residue wood or nitrogen) barks). Class B pellets may also consist of industrial residual wood and chemically untreated used wood and may have a higher ash and nitrogen content. In addition, a minimum heating value and a minimum strength are specified in each case. The strength test according to EN 15210 includes a 10-minute treatment in a rotating box with baffle plate. Before and after this treatment, the sample is sieved through a perforated plate with 3.15 mm diameter holes, only the retained material of the first sieve is tested. For the second screening, at least the stated mass fraction must be retained.

property Class A1 Class A2 class B
ash content A0.7 Max. 0.7% A1.2 Max. 1.2% A2.0 Max. 2.0%
nitrogen content N0.3 Max. 0.3% N0.5 Max. 0.5% N1.0 Max. 1.0%
strength DU97.5 minute 97.5% DU97.5 minute 97.5% DU96.5 minute 96.5%
calorific value Q16.5 > 16.5 MJ / kg Q16.5 > 16.5 MJ / kg Q16.5 > 16.5 MJ / kg

For sulfur, chlorine and heavy metal contents limit values are specified. For ash melting behavior, the temperature at the beginning of the shrinkage (SST), softening temperature (DT), hemisphere temperature (HT) and flow temperature (FT) should be indicated.

The “ENplus” and “EN B” labels, trademarks of the European Biomass Association, are awarded through the European Pellet Councilfrom national associations of pellets to companies producing, trading or transporting pellets. The requirements for pellets for the seals ENplus-A1, ENplus-A2 and EN-B correspond in principle to the corresponding qualities of standard ISO 17225-2, however, the ash softening temperature should additionally be specified. In addition, the entire chain is recorded to the end customer and appropriate storage and transport conditions prescribed. Permanent return samples (except delivery in 15kg sacks) must be taken and traceability of all deliveries to the manufacturer must be ensured. In addition, annual reviews are carried out by the certifying body with the producers, the trade is checked every three years.

With the quality mark “DINplus”, mark of the DIN CERTCO mbH, is another established certification on the market. In addition to the requirements of the standard DIN EN ISO 17225-2, there are additional requirements for the product. The fines may not exceed 0.5% and not as in the standard of 1%. Annual reviews of the producers are carried out.

Typical properties of wood pellets for small combustion plants
Wood pellets have the following properties:

Energy density of about 4.8 kWh / kg (17,000 kJ / kg), 2 t of pellets contain the energy of about 1000 l of fuel oil (equivalent)
Bulk density of around 650 kg / m³
Water content of less than 10%
Ash content below 0.5% for high quality pellets
Other important characteristics and quality features are the diameter and length of the pellets, the content of certain elements (sulfur, chlorine), the abrasion resistance, the raw materials used and others.

Manufacturing method
The raw material (sawdust and chips from sawmills) is compressed into rods of a few millimeters in diameter under high pressure without addition of agglomerating agent.

The energy required for production depends on the moisture content of the raw material. It is between 1.3% and 2.7% of the calorific value.

Since the end of 2009 the production plants, starting to run out of sawdust, are converting roundwood of small diameters (coniferous) with a high final cost (extra cost of about 10 euros per tonne of pellets) and sometimes difficult to debark, which will not be lacking. to pose quality problems modifying upwards the gray energy consumed to produce it.

Still marginally, some producers produce fuel pellets that are substitutable for wood but made with other raw materials. We can cite :

The vine shoots.
The coffee grounds , dried, compacted, which provides heating value slightly above the timber .

Wood pellets, in particular freshly made, are chemically active and can deplete the atmosphere of the oxygen required to sustain life. Wood pellets can also emit large quantities of the poisonous carbon monoxide. Fatal accidents have taken place in private storerooms and onboard marine vessels. When handled, wood pellets give off fine dust which can cause serious dust explosions.

Pellet stove operation
There are three general types of pellet heating appliances, free standing pellet stoves, pellet stove inserts and pellet boilers. Pellet stoves “look like traditional wood stoves but operate more like a modern furnace. [Fuel, wood or other biomass pellets, is stored in a storage bin called a hopper. The hopper can be located on the top of the appliance, the side of it or remotely.] A mechanical auger [automatically feeds] the pellets into a burn pot, where they are incinerated at such a high temperature that they create no vent-clogging creosote and very little ash or emissions… “Heat-exchange tubes”: Send air heated by fire into room… “Convection fan”: Circulates air through heat-exchange tubes and into room… The biggest difference between a pellet stove and … a woodstove, is that, inside, the pellet stove is a high-tech device with a circuit board, a thermostat, and fans—all of which work together to [regulate temperature and] heat your space efficiently.”

A pellet stove insert is a stove that is inserted into an existing masonry or prefabricated wood fireplace. See Fireplace insert

Pellet boilers are standalone central heating and hot water systems designed to replace traditional fossil fuel systems in residential, commercial and institutional applications. Automatic or auto-pellet boilers include silos for bulk storage of pellets, a fuel delivery system that moves the fuel from the silo to the hopper, a logic controller to regulate temperature across multiple heating zones and an automated ash removal system for long-term automated operations.

Pellet baskets allow a person to heat their home using pellets in existing stoves or fireplaces.

Fuel costs
Since the pellets are produced from co-products from the sawmill industry, the production is related to the construction industry, the general economy (packaging timber) and the accumulation of round wood (wood from harmful storm or beetle calamities) together..

On the pellet market there has been a strong increase in supply and demand over the past few years with varying time lag. After an initially quite high price after market introduction in the late 1990s, a phase of relatively low prices followed by 3.50 cents / kWh in Germany from 2002 to 2005. This was followed by several months of high pellet prices of more than 5 cents / kWh in winter 2006 / 07 because of supply bottlenecks. Since 2007, the manufacturers have further expanded their capacities, so that the commercial value has fallen to a level between about 3.50 and 4.50 cents / kWh.

Cost comparison
To estimate the efficiency of a pellet heating system, apart from the fuel costs, the specific costs of storage and incineration must be taken into account. In particular, the lower specific calorific value requires a higher storage volume. A pellet supply, based on the specific calorific value, should therefore be at least 10% cheaper than a storage of heating oil or at least 20% cheaper than the supply of fuel gas free of storage costs. In Austria, the cost advantage of pellets compared to heating oil has not always been between 40 and 56% since 2010.

Price development
Until the spring of 2004, the price of pellets was about the same as the heating oil price and about 30% cheaper than natural gas. Thereafter, the price of pellets rose only moderately, while the price of heating oil and natural gas rose sharply. On the basis of Austrian prices, the heating costs savings at the end of 2005 were 40 to 50% compared to oil. The price fluctuated between higher prices in winter and low in summer.

From the summer of 2006 there was no decline for the first time, but a continuous increase in prices. The price for DIN plus pellets in Germany in July 2006 averaged € 206 per tonne. In Austria, the price in the autumn of 2006 increased to up to 250 € and has settled in December at 255 €.

Due to the extremely mild winter of 2006/07, but especially after the windstorm by the winter storm Kyrill on 18./19. January 2007 and the following oversupply of wood prices began to fall significantly again until the spring of 2007 to an average of € 185 per tonne, and stabilized by the massive expansion of production capacity with 180 to 200 € to autumn.

In 2008, the pellet price in Austria had not exceeded the value of € 200 per ton and in mid-2008 was between € 155 and € 175.

In 2010, the annual average price for a delivery of 5 tonnes of bulk goods within a radius of 50 km in Germany was € 228.45 per tonne and increased to € 241.41 per tonne in 2011. With a calorific value of 4.9 kWh / kg, this corresponds to 4.66 ct (2010) or 4.93 ct (2011) per kWh. In June / July, the price per tonne is up to 10% lower.

In December 2012, wood pellets in Germany cost an average of € 256.24 per tonne. The price in Austria in September 2014 was € 243.2 per tonne, in Switzerland it was € 391.68 / tonne, which corresponds to € 324.23 per tonne.

The price development in Austria compared to natural gas and wood pellets can be observed objectively using the Austrian price indices of the products: In January 2013, the Austrian gas price index stood at 143.75 compared to the base value January 2006; H. the price of gas rose 1.44 times over the past seven years (see gas price evolution and for the explanation of the usual price fluctuations). The pellet price index stood at 136.80 in September 2014, d. H. The price of pellets increased by 1.37 times.

Energy output and efficiency
The energy content of wood pellets is approximately 4.7 – 5.2 MWh/tonne (~7450 BTU/lb).

High-efficiency wood pellet stoves and boilers have been developed in recent years, typically offering combustion efficiencies of over 85%. The newest generation of wood pellet boilers can work in condensing mode and therefore achieve 12% higher efficiency values. Wood pellet boilers have limited control over the rate and presence of combustion compared to liquid or gaseous-fired systems; however, for this reason they are better suited for hydronic heating systems due to the hydronic system’s greater ability to store heat. Pellet burners capable of being retrofitted to oil-burning boilers are also available.

The importance of wood pellets has steadily increased in recent years in Germany and in Europe. In 1999, for example, only 800 pellet heating systems were installed in residential buildings in Germany. The number rose to 27,000 by 2004, to about 100,000 in 2008 and 180,000 in 2013. In 2016, just under 422,000 pellet heating systems are installed in Germany.

In Austria, 7,000 pellet boilers were in operation in 2000, and in 2012 more than 100,000. Worldwide, pellet production increased from 2.5 million tonnes in 2002 to 23 million tonnes in 2012.

Air pollution emissions
Emissions such as NOx, SOx and volatile organic compounds from pellet burning equipment are in general very low in comparison to other forms of combustion heating. A recognized problem is the emission of fine particulate matter to the air, especially in urban areas that have a high concentration of pellet heating systems or coal or oil heating systems in close proximity. This PM2.5 emissions of older pellet stoves and boilers can be problematic in close quarters, especially in comparison to natural gas (or renewable biogas), though on large installations electrostatic precipitators, cyclonic separators, or baghouse particle filters can control particulates when properly maintained and operated.

Global warming
There is uncertainty to what degree making heat or electricity by burning wood pellets contributes to global climate change, as well as how the impact on climate compares to the impact of using competing sources of heat. Factors in the uncertainty include the wood source, carbon dioxide emissions from production and transport as well as from final combustion, and what time scale is appropriate for the consideration.

A report by the Manomet Center for Conservation Sciences, “Biomass Sustainability and Carbon Policy Study” issued in June 2010 for the Massachusetts Department of Energy Resources, concludes that burning biomass such as wood pellets or wood chips releases a large amount of CO2 into the air, creating a “carbon debt” that is not retired for 20–25 years and after which there is a net benefit. In June 2011 the department was preparing to file its final regulation, expecting to significantly tighten controls on the use of biomass for energy, including wood pellets. Biomass energy proponents have disputed the Manomet report’s conclusions, and scientists have pointed out oversights in the report, suggesting that climate impacts are worse than reported.

Until ca. 2008 it was commonly assumed, even in scientific papers, that biomass energy (including from wood pellets) is carbon neutral, largely because regrowth of vegetation was believed to recapture and store the carbon that is emitted to the air. Then, scientific papers studying the climate implications of biomass began to appear which refuted the simplistic assumption of its carbon neutrality. According to the Biomass Energy Resource Center, the assumption of carbon neutrality “has shifted to a recognition that the carbon implications of biomass depend on how the fuel is harvested, from what forest types, what kinds of forest management are applied, and how biomass is used over time and across the landscape.”

In 2011 twelve prominent U.S. environmental organizations adopted policy setting a high bar for government incentives of biomass energy, including wood pellets. It states in part that, “iomass sources and facilities qualifying for (government) incentives must result in lower life-cycle, cumulative and net GHG and ocean acidifying emissions, within 20 years and also over the longer term, than the energy sources they replace or compete with.”

The wood products industry is concerned that if large-scale use of wood energy is instituted, the supply of raw materials for construction and manufacturing will be significantly curtailed.

Due to the rapid increase in popularity since 2005, pellet availability and cost may be an issue. This is an important consideration when buying a pellet stove, furnace, pellet baskets or other devices known in the industry as Bradley Burners. However, current pellet production is increasing and there are plans to bring several new pellet mills online in the US in 2008–2009.

The cost of the pellets can be affected by the building cycle leading to fluctuations in the supply of sawdust and offcuts.

Per the New Hampshire Office of Energy and Planning release on Fuel Prices updated on 5 Oct 2015, the cost of #2 Fuel Oil delivered can be compared to the cost of Bulk Delivered Wood Fuel Pellets using their BTU equivalent: 1 ton pellets = 118.97 gallon of #2 Fuel Oil. This assumes that one ton of pellets produces 16,500,000 BTU and one gallon of #2 Fuel Oil produces 138,690 BTU. Thus if #2 Fuel Oil delivered costs $1.90/Gal, the breakeven price for pellets is $238.00/Ton delivered.

Usage by region

Usage across Europe varies due to government regulations. In the Netherlands, Belgium, and the UK, pellets are used mainly in large-scale power plants. The UK’s largest power plant, the Drax power station, converted some of its units to pellet burners starting in 2012; by 2015 Drax had made the UK the largest recipient of exports of wood pellets from the US. In Denmark and Sweden, pellets are used in large-scale power plants, medium-scale district heating systems, and small-scale residential heat. In Germany, Austria, Italy, and France, pellets are used mostly for small-scale residential and industrial heat.

The UK has initiated a grant scheme called the Renewable Heat Incentive (RHI) allowing non-domestic and domestic wood pellet boiler installations to receive payments over a period of between 7–20 years It is the first such scheme in the world and aims to increase the amount of renewable energy generated in the UK, in line with EU commitments. Scotland and Northern Ireland have separate but similar schemes. From Spring 2015, any biomass owners—whether domestic or commercial—must buy their fuels from BSL (Biomass Suppliers List) approved suppliers in order to receive RHI payments.

Pellets are widely used in Sweden, the main pellet producer in Europe, mainly as an alternative to oil-fired central heating. In Austria, the leading market for pellet central heating furnaces (relative to its population), it is estimated that 2⁄3 of all new domestic heating furnaces are pellet burners. In Italy, a large market for automatically fed pellet stoves has developed. Italy’s main usage for pellets is small-scale private residential and industrial boilers for heating.

In 2014 in Germany the overall wood pellet consumption per year comprised 2,2 mln tones. These pellets are consumed predominantly by residential small scale heating sector. The co-firing plants which use pellet sector for energy production are not widespread in the country. The largest amount of wood pellets is certified with DINplus and these are the pellets of the highest quality. As a rule, the pellets of lower quality are exported.

New Zealand
The total sales of wood pellets in New Zealand was 3–5,000 tonnes in 2003. Recent construction of new wood pellet plants has given a huge increase in production capacity.

United States
Some companies import European-made boilers. As of 2009, about 800,000 Americans were using wood pellets for heat. It is estimated that 2.33 million tons of wood pellets will be used for heat in the US in 2013. The US wood pellet export to Europe grew from 1.24 million ton in 2006 to 7 million ton in 2012, but forests grew even more.

Other uses

Horse bedding
When small amounts of water are added to wood pellets, they expand and revert to sawdust. This makes them suitable to use as a horse bedding. The ease of storage and transportation are additional benefits over traditional bedding. However, some species of wood, including walnut, can be toxic to horses and should never be used for bedding.

In Thailand, rice husk pellets are being produced for animal bedding. They have a high absorption rate which makes them ideal for the purpose.

Wood pellets are also used to absorb contaminated water when drilling oil or gas wells.

Wood pellet grills have gained popularity as a versatile way to grill, bake, and smoke. The size of the pellets makes it useful for creating a wood fired grill that still controls its temperature precisely.

Advantages and disadvantages

Pellets have several advantages over wood:

It is a renewable and easily accessible resource due to the use of waste from other industries.
Being reused material, it is a cheaper fuel.
It is easier to make the stoves programmable so that they turn on or off automatically.
By filling the space better and having greater apparent density, they occupy less than the trunks or branches and fit in any container of any shape.
They generate an appreciable amount of ashes of vegetable origin and non-toxic that can be used as fertilizer or mineral supplement of animals. These ashes are rich in calcium and potassium.
As added benefits they are usually cheaper than traditional fuels such as diesel and produce less pollutants (SOx and dioxins).

It also takes more place than diesel, so more space is needed to store it or replenish stocks more times.
There are pellet distributors nationwide (Spain) that can ensure the supply of pellets by contract. Space is a problem that needs to be weighed.
As in all combustion, waste is created, which implies a certain appreciable amount of ash that, depending on the type of pellets that burn, varies in volume. The combustion of the pellets also generates soot, which is emitted into the atmosphere in the form of microparticles, fouling the humeros (it is necessary to remove them) and increasing the environmental filth.
The production of wood waste in the industry is limited, so that an extended consumption can result in the start of the use of crushing wood destined to the board market and cause the price of the pellet to rise.
If the pellets pass through several feeding augers, they dissolve a little, which creates sawdust that sometimes obstructs the feeding of the boiler.
The combustion of the pellets requires a greater consumption of air, which means that greater ventilation is required in the biomass boiler room than in a boiler room for gas, diesel or other fuel, which is not a major drawback in boiler rooms, but very important in individual stoves that work in the room to be heated, since they require more cold air inlet, which also cools the premises.
There are several types of pellets, according to their origin (olive, poplar, city tree pruning, etc.) and the geographical area, which means that the calorific value varies and therefore pellets of lesser power can be sold at the same time. price than those of greater power. This produces helplessness for the consumer, who finally wants to buy heat, not masses of fuel. Although to solve this there are different classifications of pellets according to their quality (EN plus) which ensure the value of the calorific value of the product, among other characteristics.

Regarding other traditional forms of biomass such as wood or wood chips, it also has disadvantages:

Sometimes you have access to firewood at a very low price or free.
Chips have a lower energy density. They are not pressed nor have a uniform grain. They can gather almost all the advantages of the pellets (automation, energy use…) and chips are much easier to produce from waste pruning or other wood waste. Only one suitable crusher is necessary and the pressing process is avoided.
This need for added processing of the pellets makes them more expensive to manufacture economically and energetically. On the other hand, the uniform granulometry of the pellets makes the automatic feeding of the boiler easier.

Source from Wikipedia