Wood pellets

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Infobox on Wood pellets
Example of Wood pellets
Wood pellets.JPG
Facts
Origin -
Stowage factor (in m3/t) Approx. 1,5 m3/t (bulk)
Humidity / moisture See text
Ventilation See text
Risk factors See text

Wood pellets

Description / Shipment and storage / Risk factors

The manufacture and carriage of wood pellets have increased over the last few years because of their use as a non-fossil heating fuel. It is understood that the main countries of manufacture are North America and Scandinavia.

The following description of the manufacturing process is based on a process used in Canada but it is believed that the method of manufacture used elsewhere is similar. The pellets are produced totally from sawdust and wood shavings and do not contain any additives or binders. The sawdust and shavings are dried, and then milled into particles of up to approximately 2 mm particle size. The particles are then compressed approximately 3.5 times into pellets which are typically 10 - 20 mm long and 3-12 mm in diameter. The compression leads to an increase in temperature. The pellets typically have a moisture content of 4 - 8% and vary in colour from blond to brown depending on the types of wood used.

Due to transport movements and physical handling some breakage of the pellets occurs and this means that the material loaded aboard a ship for transport consists of pellets, pieces of broken pellets and wood dust.

The wood pellets are of course combustible and can be ignited by a range of ignition sources. In addition the dust associated with the pellets when dispersed and ignited can give rise to a dust explosion under appropriate conditions of containment. Stored bulk piles of wood pellets can self-heat in parts with high moisture contents and it is reported that this process can lead to the spontaneous combustion of the material after a long period of time.

In addition to the combustion hazards, wood pellets also undergo oxidation to produce carbon monoxide and carbon dioxide. In a closed space such as an unventilated ship’s hold, this can lead to a dangerous reduction in the oxygen concentration in the hold as well as the development of a dangerous concentration of carbon monoxide which is toxic (and flammable).

In a recent case a carbon monoxide concentration of approximately 1% was measured in a sealed cargo hold of a ship containing wood pellets some 18 days after the cargo was loaded. The oxygen concentration at this time was less than 1%. Emission rates for carbon monoxide from wood pellets of 100–885 mg/ton/day have been reported in the literature.

It is well known that carbon monoxide is produced when wood products are burned in reduced oxygen environments but the low temperature emission of the gas from wood products is rather unexpected. It has been suggested, that the gas is generated by the autoxidation of fats and Fatty Acids in the wood but the factors which promote the production have not been fully identified. The generation of carbon monoxide from stored rapeseed and stored wheat has also been reported.

With respect to the marine transport of wood pellets, the commodity has previously sometimes been classed as “Wood pulp pellets” which is entered in Appendix B of the BC Code. However wood pulp is not normally formed into pellets and the wood pellets are not pulp.

Furthermore the entry for “wood pulp pellets” while referring to oxygen depletion and the generation of carbon dioxide does not refer to the formation of carbon monoxide.

As a result a submission dated 1 July 2004 was made to the IMO Sub-Committee on Dangerous Goods, Solid Cargoes and Containers about the transport of wood pellets. The submission recommended a new entry in the BC Code for wood pellets in which reference was made to the hazard associated with the generation of carbon monoxide.

This submission was accepted and there is an entry for wood pellets in the 2005 Edition of the BC Code 2004. The hazard associated with oxygen depletion and the generation of carbon monoxide is now recognised by stevedores who routinely employ “gas doctors” to check spaces which contain or have contained wood pellets. It obviously also needs to be known by ship’s crews and others who may have need to enter a cargo hold which has, or had recently contained wood pellets.

Bulk Pellet Behaviour

Bulk pellets are prone to off-gassing and self-heating, which can be mitigated by procurement.

Numerous wood pellet mills are in development or under construction these days. With all the mills being constructed, and with most companies focusing on the pellet production and wood procurement processes, the storage of the finished product is often almost an afterthought.

However, resin in the form of sugars and organic compounds remains in the wood throughout the pellet production process; these compounds can begin to break down during the storage and shipping process, leading to dangerous off-gassing and self-heating. Companies can ensure the best product is delivered to their customers overseas by using enhanced procurement procedures and specific production techniques and by understanding the mechanics and physics of modern pellet storage management. All three of these areas must be taken into consideration when developing a facility. If not properly managed, off-gassing and pellet self-heating start with the procurement of the wood and continue throughout production, shipping, and storage.

Across the pellet industry, it is normal practice to procure the cheapest feedstock for pelletizing without compromising pellet quality. In addition to sawdust, pellet producers use tops, trimmings, and whole-tree chips to reduce overall operating costs because feedstock is the highest cost, normally averaging 40–50% of total production cost. The chemical composition of juvenile and mature wood differs. It is impractical to procure mature wood because lumber mills are competing for that material, so pulpwood is the main source for pellet mills.

A solution for reducing later off-gassing and self-heating currently being tested is aging or drying of the wood once it is cut. The rationale behind this idea is that wood tends to have variable moisture content throughout the year. Allowing the wood to age in the yard prior to processing would allow the wood to begin the natural drying process; therefore, lower drying temperatures would be needed to achieve 8–9.5% moisture content of fibre prior to pelletizing.

Allowing the wood to dry naturally before debarking and chipping would reduce chemical breakdown during chip pile storage as well. Spontaneous heating in wood chip and sawdust piles is caused by oxidation of unsaturated fatty acids and other extractives. Mills that acquire feedstock and unload and chip it without storing it first are putting chips with average moisture content of 40–48% into their piles, which will increase the chances of both spontaneous heating and chemical breakdown prior to processing.

Hazardous off-gassing

Pellet mills throughout the world use different methods to process feedstock once it is chipped. For instance, one mill may produce three-quarter-inch pine chips that move through a single-pass dryer and into a storage silo until a hammermill pulverizes the strand to a smaller size that is sometimes determined by the end-user contract for particle distribution for co-firing. Another mill may produce half-inch, mixed hardwood chips that pass through a hammermill, then a triple-pass dryer, and then another hammermill to further reduce the strand size to meet the specs of the end-user contract. With each process the wood passes through, the strand structure is altered.

Research on the effects of processing on the condition of chips shows that the chemical composition changes as a function of refining actions.

Drying temperature is correlated strongly with the volatile organic compounds (VOCs) emitted from stored pellets, along with pellet self-heating. The major constituents of VOCs emitted from wood pellets are aldehydes, some of which are upper airway irritants. The drying gas temperature is the only significant factor for aldehydes/ketone emissions. Some believe that high drying temperature removes more of the VOCs from the wood prior to pelletizing; however, others believe that high drying temperature actually opens the cellular structure, which, once pelletized, begins to emit more VOCs than what would have been emitted otherwise. High drying temperatures are used to maintain high throughput while reducing moisture content. Lower drying temperatures usually have lower throughput, but the wood emits fewer VOCs and could possibly have reduced VOC emission and self-heating in storage.

An analysis of VOCs emitted from fresh and stored Norway spruce and Scots pine during storage found that unsaturated fatty acids are the leading raw materials of emitted VOCs. Aldehydes such as pentanal and hexanal are major constituents of the off-gas, but the amount and composition of emitted substances is affected by drying temperature of the raw material and self-heating of pellet stocks. Spruce and stored pine sawdust contain less fatty acid, which should generate fewer aldehydes. The study’s authors hypothesized that it might be possible to reduce emissions from pellets in storage by optimizing the drying temperature and other process parameters such as wood aging and raw material mixes.

Wood pellet storage and shipping have come under the spotlight since two fatal accidents plus serious injuries in 2002 and 2006 while off-loading pellets shipped overseas from British Columbia to Europe. A typical shipping vessel can carry 30,000 tonnes of pellets at a time, and a typical port storage facility can store as much as 100,000 tonnes in a single facility.

Temperature effects

All biomass gradually decomposes over time, releasing toxic and oxygen-depleting gases such as carbon monoxide (CO), carbon dioxide (CO2), and methane (CH4). Emissions from wood pellets during storage comprise one-carbon compounds such as CO, methanol, formic acid, and formaldehyde, as well as multi-carbon aldehydes such as hexanal and pentanal. The oxidation of Fatty Acids and other components in the wood is the likely cause. The oxidation processes occur below room temperature but are accelerated by elevated temperature.

In a conventional biomass composting system, CH4 generation is usually associated with anaerobic decomposition of biomass, whereas CO2 likely is generated from the thermal oxidation of aerobic degradation products. A high temperature favours a high CO/CO2 ratio. As the temperature rises, both CH4 and CO2 emissions increase, with CH4 generation favoured over CO2 at higher temperatures.

Minimizing problems

The problem with pellets heating up or off-gassing is noticed during shipping or bulk storage. Many different types of storage facilities exist for wood pellets, from cylindrical silos like those used to store grains to warehouse-type facilities in which pellets are loaded by conveyor and removed by a loader. Extensive research has been conducted on the storage of grains in different types of vessels in different climates; however, very little research has been conducted on wood pellet storage. What has been done has been in closed or sealed containers because of costs and other limiting factors on testing. Research to date indicates that ambient temperature is the number one factor for off-gassing and self-heating. Nothing can be done to control ambient temperature, so work must be done to control the temperature within the environment where the pellets are stored. This can be done in several ways but must be considered first prior to building a facility, and not as an afterthought.

Aeration, also known as active, mechanical, low-volume, or forced ventilation, can be defined as the forced movement of ambient air through bulk product for improvement of storability. The objectives of pellet aeration can be best described as: cooling pellets, equalizing temperatures throughout the pellets, preventing biological heating in damp pellets, circulating off-gasses, and removing odours created by off-gasses. Pellet aeration also helps prevent moisture migration and headspace water condensation in humid climates. Rates of chemical deterioration are very slow and sometimes insignificant at low temperatures, and increase significantly with each 10ºC increase in temperature. Therefore, maintaining low temperatures in the pellets is essential.

Ambient air temperature, solar radiation, atmospheric weather changes that result in major barometric pressure fluctuations, and storage structure parameters affect the transfer of heat within the stored product. South walls in the northern hemisphere and north walls in the southern hemisphere intercept most of the solar radiation, so rectangular bins should be placed with their longer axes running north to south to help keep pellets cooler. Depending on the thermal absorptivity and emissivity of the structural material and the surface temperature, solar radiation may result in heat gain or loss from stored pellets. From the aspect of bin wall temperatures and product warming from solar radiation, insulated galvanized steel and galvanized steel are the first and second worst materials to use for bin walls.

The easiest and most economical way to control pellet temperatures and off-gassing may be to control it within the storage facility itself. The initial capital of building a storage facility that has aeration will be significantly cheaper in the long run than having port union stevedores constantly moving product to disperse heat within piles, like some wood chip management practices. By managing temperature, both pellet off-gassing and self-heating are managed, ensuring staff safety and pellet quality.

Safety & prevention for bulk pellet handling

Wood pellets have been handled in bulk since the late 1980's in Scandinavia, but it was not until the mid-1990s when safety concerns were raised for pellets stored, handled, and transported in large quantities from Canada to Europe. The initial concern was in maintaining the mechanical and chemical integrity of pellets during months of containment in poorly ventilated spaces such as silos and cargo holds of large ocean vessels.

Unexpected fatal accidents in the Port of Rotterdam in 2002 and the Port of Helsingborg in 2006 brought safety to the forefront. Subsequent fatal accidents in Finland and Germany accentuated the issue even further. Carbon-monoxide intoxication has killed five and caused severe brain injury to others.

The accident in 2002 initiated serious research in Canada, followed by Sweden and Austria, to understand better the root causes of these accidents. The research revealed that wood pellets in bulk rapidly generate large amounts of highly toxic carbon monoxide, as well as carbon dioxide and small amounts of methane gas.

Carbon dioxide and methane are not toxic but act as simple asphyxiants, meaning they displace oxygen in confined spaces, which can cause suffocation. Carbon monoxide blocks the transport of oxygen by the red blood cells, rapidly causing oxygen starvation, particularly in the brain, which is highly dependent on a continuous supply of oxygen. In an atmosphere of depleted oxygen in combination with the presence of carbon monoxide, the body responds with hyperventilation, which increases the intake of carbon monoxide, quickly causing unconsciousness. Many other woody products such as green lumber and other biotic substances such as agricultural products have similar characteristics and can also cause fatal accidents.

Normally, we associate carbon monoxide build-up and oxygen depletion with wood smouldering and inadequate combustion of kerosene, gasoline, and other fuels. Without combustion, carbon monoxide and other gases are generated by the decomposition of some of the cellular building blocks in the biomass. This also generates self-heating, which can escalate to the point of open fire if the process is left alone long enough and sufficient oxygen is present. The heat generation is particularly pronounced in material with higher moisture content such as wood chips, but also occurs in dry wood pellets.

Because of these issues, the Wood Pellet Association of Canada (WPAC) developed Material Safety Data Sheets (MSDSs) for bulk and bagged pellets that provide advice related to the off-gassing issue, including formulas for predicting the amount of off-gassing and oxygen depletion. Additional information such as safe storage design and handling of pellets in large bulk is found in The Pellet Handbook, which has substantial information on material safety, developed through research at the University of British Columbia. Published by Earthscan, the text can be purchased through WPAC at www.pellet.org . One of the key recommendations is that all personnel working in areas where large amounts of pellets are handled and stored should at all times be equipped with a well-maintained combined oxygen/carbon-monoxide meter. Using only one or the other could easily generate a false sense of safety.

Precautions are gradually being implemented, but much more work is needed to promote safe design and operating procedures. The pressure is on by insurance underwriters to have the wood pellet industry comply with proven rules and recommendations, and a certification process is under development to introduce compliance-driven incentives for the industry, including regular audits, similar to those used in many other industries.

Application

Pellet fuels are heating fuels made from compressed biomass. Wood pellets are the most common type. A form of wood fuel, wood pellets are generally made from compacted sawdust or other wastes from sawmilling and other wood products manufacture. Other woody biomass sources include palm kernel shell, coconut shell, and whole-tree removal or tree tops and branches leftover after logging and which otherwise help replenish soil nutrients. As well grasses can also be pelletized, creating grass pellets. Pellets are manufactured in several types and grades as fuels for electric power plants, homes, and other applications in between. 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 rational transport over long distance. They can be conveniently blown from a tanker to a storage bunker or silo on a customer's premises.

Wood pellets are primarily used as a fuel and have a calorific (heat) value of around 5 MWh/metric tonne (18 GJ/metric tonne) which is approximately half that of fuel oil. In North America the wood pellets are used in wood pellet stoves and fireplaces. In Europe, particularly Scandinavia, the bulk of the pellets produced are used as fuel in central heating stations supplying heat for entire communities or even entire cities.

See also: http://www.canadianbiomassmagazine.ca/index

Sources used (which should be mentioned on cargohandbook.com)
Part(s) of the above article was produced by the Carefully to Carry Committee - the UK P&I Club's advisory committee on cargo matters.