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Woodchips are small- to medium-sized pieces of wood formed by cutting or chipping larger pieces of wood such as trees, branches, logging residues, Tree stump, , and wood waste.
Woodchips may be used as a biomass solid fuel and are raw material for producing wood pulp. They may also be used as an organic mulch in gardening, landscaping, and ecosystem restoration; in bioreactors for denitrification; and as a substrate for mushroom cultivation.
The process of making woodchips is called wood chipping and is done using a Woodchipper. The types of woodchips formed following chipping is dependent on the type of wood chipper used and the material from which they are made. Woodchip varieties include: forest chips (from forested areas), wood residue chips (from untreated wood residues, recycled wood and off-cuts), sawing residue chips (from sawmill residues), and short rotation forestry chips (from energy crops).
Forestry operations provide the raw materials needed for woodchip production. Almost any tree can be converted into woodchips, however, the type and quality of the wood used to produce woodchips depends largely on the market.
Softwood species, for instance, tend to be more versatile for use as woodchips than hardwood species because they are less dense and faster growing.
It has been promoted for use in habitat restoration projects. As the ramial chipped wood decomposes it improves the soil structure, permeability, bioactivity, and nutrient availability of the soil.
A 2013 experiment showed that after 70 days of startup, a woodchip pile loaded with liquid pig manure at 5 L/m2/day removed an average of 90% of nitrate after one month. However, if the environmental conditions do not support complete denitrification, undesirable greenhouse gas such as nitrous oxide gas and methane could be produced.
The newer fuel systems for heating use either woodchips or wood pellets. The advantage of woodchips is cost, the advantage of wood pellets is the controlled fuel value. The use of woodchips in automated heating systems, is based on a robust technology.
The size of the woodchips, moisture content, and the raw material from which the chips are made are particularly important when burning wood chips in small plants. Unfortunately, there are not many standards to decide the fractions of woodchip. However, as of March 2018, The American National Standards Institute approved AD17225-4 Wood Chip Heating Fuel Quality Standard. The full title of the standard is: ANSI/ASABE AD17225-4:2014 FEB2018 Solid Biofuels—Fuel Specifications and classes—Part 4: Graded wood chips. One common chip category is the GF60 which is commonly used in smaller plants, including small industries, villas, and apartment buildings. "GF60" is known as "Fine, dry, small chips". The requirements for GF60 are that the moisture is between 10 and 30% and the fractions of the woodchips are distributed as follows: 0–3.5mm: <8%, 3.5–30mm: <7%, 30–60 mm: 80–100%, 60–100 mm: <3%, 100–120 mm: <2%.
The energy content in one cubic metre is normally higher than in one cubic metre wood logs, but can vary greatly depending on moisture. The moisture is decided by the handling of the raw material. If the trees are taken down in the winter and left to dry for the summer (with teas in the bark and covered so rain can't reach to them), and is then chipped in the fall, the woodchips' moisture content will be approximately 20–25%. The energy content, then, is approximately 3.5–4.5kWh/kg (~150–250 kg/cubic metre).
Coal power plants have been converted to run on woodchips, which is fairly straightforward to do, since they both use an identical steam turbine heat engine, and the cost of woodchip fuel is comparable to coal.
Solid biomass is an attractive fuel for addressing the concerns of the energy crisis and climate change, since the fuel is affordable, widely available, close to carbon neutral and thus climate-neutral in terms of carbon dioxide (CO2), since in the ideal case only the carbon dioxide which was drawn in during the tree's growth and stored in the wood is released into the atmosphere again.
Like coal, wood combustion is a known source of mercury emissions, particularly in northern climates during winter. The mercury is both gaseous as elemental mercury (especially when wood pellets are burned) or mercury oxide, and solid PM2.5 particulate matter when untreated wood is used.
When wood burning is used for space heating, indoor emissions of 1,3-butadiene, benzene, formaldehyde and acetaldehyde, which are suspected or known carcinogenic compounds, are elevated. The cancer risk from these after exposure to wood smoke is estimated to be low in developed countries.
Certain techniques for burning woodchips result in the production of biochar – effectively charcoal – which can be either utilised as charcoal, or returned to the soil, since wood ash can be used as a mineral-rich plant fertilizer. The latter method can result in an effectively carbon-negative system, as well as acting as a very effective soil conditioner, enhancing water and nutrient retention in poor soils.
Woodchips are less expensive than cord wood, because the harvesting is faster and more highly automated. Woodchips are of greater supply, partly because all parts of a tree can be chipped, whereas small limbs and branches can require substantial labor to convert to cord wood. Cord wood generally needs to be "seasoned" or "dry" before it can be burned cleanly and efficiently. On the other hand, woodchip systems are typically designed to cleanly and efficiently burn "green chips" with very high moisture content of 43–47% (wet basis). (see gasification and woodgas)
If woodchips are harvested as a by-product of sustainable forestry practices, then this is considered a source of renewable energy.
Theoretically, whole-tree chip harvesting does not have as high a solar energy efficiency compared to short rotation coppice; however, it can be an energy-efficient and low-cost method of harvesting.
In 2006, prices were US$15 and US$30 per wet-ton in the northeast. Vermont Heat Research – An Experimental Wood Chip Furnace
In the 20 years leading up to 2008, prices have fluctuated between US$60–70/oven-dry metric ton (odmt) in the southern states, and between US$60/odmt and US$160/odmt in the Northwest. First quarter wood chip costs up almost 50% in western US, but pulpmills in the US South experienced only small upward price adjustments
The University of New Brunswick operates a wood chip burning furnace system to supply heat to the university, several industrial buildings, an apartment complex and a hospital. Usage of wood chips for heat is low in Quebec due to low hydroelectricity rates but a small town is using wood chips as an alternative to road salt for icy roads. EMC3 Technologies started producing wood chips coated with magnesium chloride in November 2017 for the town and has claimed it maintains traction in -30 degrees Celsius compared to regular road salt at -15 degrees Celsius. In Ontario, wood chip operations include a college in Brockville, a few secondary schools in Northern Ontario as well as a chip-fired boiler at the National Forestry Institute in Petawawa. In the late 1980s, the Ontario provincial government in conjunction with the federal government subsidized building three co-generation plants next to sawmills. The first one was constructed in 1987 in Chapleau followed by a plant built in Cochrane in 1989 and the largest one in Kirkland Lake which was built in 1991.
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