Woodchips are a medium-sized solid material made by cutting, or chipping, larger pieces of wood.
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, restoration ecology, bioreactors for denitrification and mushroom cultivation. According to the different chemical and mechanical properties of the masses, the wood logs are mostly peeled, and the bark chips and the wood chips are processed in different processes. The process of making wood chips is called woodchipping and is done with a woodchipper.
The raw materials of wood chips can be pulpwood, wood plantations, waste wood and residuals from construction, agriculture, landscaping, logging, and sawmills and locally grown and harvested fuel crops.
A woodchipper is a machine used for reducing wood to smaller pieces. There are several types of woodchippers depending of the further processing of the woodchips. For industrial use, the woodchippers are large, stationary installations.
Pulp and paper industry
Wood chips used for chemical pulp must be relatively uniform in size and free of bark. The optimum size varies with the wood species. It is important to avoid damage to the wood fibres as this is important for the pulp properties. For roundwood it is most common to use disk chippers. A typical size of the disk is 2.0 – 3.5 m in diameter, 10 – 25 cm in thickness and weight is up to 30 tons. The disk is fitted with 4 to 16 knives and driven with motors of ½ - 2 MW. Drum chippers are normally used for wood residuals from saw mills or other wood industry.
Methods of conveyance
Woodchips are used primarily as a raw material for technical wood processing. In industry, processing of bark chips is often separated after peeling the logs due to different chemical properties.
Only the heartwood and sapwood are useful for making pulp. Bark contains relatively few useful fibres and is removed and used as fuel to provide steam for use in the pulp mill. Most pulping processes require that the wood be chipped and screened to provide uniform sized chips.
Woodchipping is also used to produce landscape and garden woodchips mulch. It is used for water conservation, weed control, reducing and preventing soil erosion, and for supporting germination of native seeds and acorns in habitat revegetation-ecological restoration projects. As the ramial chipped wood decomposes it improves the soil structure, permeability, bioactivity, and nutrient availability. Woodchips when used as a mulch are at least three inches thick.
Wood chips can be reprocessed into an extremely effective playground surfacing material, or impact-attenuation surface. When used as playground surfacing (soft fall, cushion fall, or play chip, as it is sometimes known), woodchips can be very effective in lessening the impact of falls from playground equipment. When spread to depths of one foot (30 centimeters) playground wood chips can be effective at reducing impacts in falls up to 11 feet (3 meters). Playground woodchip is also an environmentally friendly alternative to rubber type playground surfaces.
Woodchip piles at the edge of a field can inhibit nitrates from running off into water tiles. They are a simple measure for farmers to reduce nitrate pollution of the watershed without them having to change their land management practice. A 2011 study showed that most of the nitrate removal was due to heterotrophic denitrification. A 2013 experiment from Ireland 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 in the form of ammonium after one month. A January 2015 study from Ohio State University showed very low nitrogen gas, i.e. greenhouse gas emissions from nitrate transformation under the anaerobic conditions of the wood chip bioreactor. Scientists constructed a model for water flow and nitrate removal kinetics which can be used to design denitrification beds. It is unknown if other nutrients like phosphorus or pathogens are affected by the bioreactor as well.
Woodchips have been traditionally used as solid fuel for space heating or in energy plants to generate electric power from renewable energy. The main source of forest chips in Europe and in most of the countries have been logging residues. It is expected that the shares of stumps and roundwood will increase in the future. As of 2013 in the EU, the estimates for biomass potential for energy, available under current conditions including sustainable use of the forest as well as providing wood to the traditional forest sectors, are: 277 million m3, for above ground biomass and 585 million m3 for total biomass.
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 is particularly important when burning woodchip in small plants. Unfortunately there are not many standards to decide the fractions of woodchip. One standard 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 10–30% 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 tears 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 carbondioxide which was drawn in during the tree’s growth and stored in the wood is released into the atmosphere again. It is sustainable as long as crops are allowed to regrow; In most cases, biomass is not carbon neutral as wood is not regrown and the efficiency of biomass operations produce more pollutants than the processes they replace.
Waste and emissions
Compared to the solid waste disposal problems of coal and nuclear fuels, woodchip fuel's waste disposal problems are less grave; in a study from 2001 fly ash from wood chip combustion had 28.6 mg cadmium/kg dry matter. Compared to fly ash from burning of straw, cadmium was bound more heavily, with only small amounts of cadmium leached. It was speciated as a form of cadmium oxide, cadmium silicate (CdSiO3); authors noted that adding it to agricultural or forest soils in the long-term could cause a problem with accumulation of cadmium.
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.
Automated handling of solid fuel
Unlike the smooth, uniform shape of manufactured wood pellets, woodchip sizes vary and are often mixed with twigs and sawdust. This mixture has a higher probability of jamming in small feed mechanisms. Thus, sooner or later, one or more jams is likely to occur. This reduces the reliability of the system, as well as increasing maintenance costs. Despite what some pellet stove manufacturers may say, researchers who are experienced with woodchips, say they are not compatible with the 2 inch (5 cm) auger used in pellet stoves.
Micro combined heat and power
Wood is occasionally used to power engines, such as steam engines, Stirling engines, and Otto engines running on woodgas. As of 2008, these systems are rare, but as technology and the need for it develops, it is likely to be more common in the future. For the time being, wood can be increasingly used for heating applications. This will reduce the demand for heating oil, and thereby allow a greater percentage of fuel oil to be used for applications such as internal combustion engines, which are less compatible with wood based fuel and other solid biomass fuels. Heating applications generally do not require refined or processed fuels, which are almost always more expensive.
Comparison to other fuels
Woodchips are similar to wood pellets, in that the movement and handling is more amenable to automation than cord wood, particularly for smaller systems. Woodchips are less expensive than wood pellets and are theoretically more energy efficient than pellets, because less energy is required for manufacturing, processing, and transportation; however, this assumes that they are consumed in an appropriately designed burner, and as of 2008, these are mostly only available in large systems designed for commercial or institutional use, which have been very successful in terms of performance, cost, reliability, and efficiency.
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 much labor convert to cord wood. Cordwood 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 through sustainable forestry practices, then this is considered a source of renewable energy. On the other hand, harvesting practices, such as clearcutting large areas, are highly damaging to forest ecosystems.
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 some cases, this practice may be controversial when whole-tree harvesting may often be associated with clear cutting and perhaps other questionable forestry practices.
Woodchip biomass does not have the waste disposal issues of coal and nuclear power, since wood ash can be used directly as a mineral-rich plant fertilizer.
Forest fire prevention
Woodchip harvesting can be used in concert with creating man made firebreaks, which are used as barriers to the spread of wildfire. Undergrowth coppice is ideal for chipping, and larger trees may be left in place to shade the forest floor and reduce the rate of fuel accumulation.
Market products, supply and demand
Currently, domestic or residential sized systems are not available in products for sale on the general market. Homemade devices have been produced, that are small-scale, clean-burning, and efficient for woodchip fuels. Much of the research activity to date, has consisted of small budget projects that are self-funded. The majority of funding for energy research has been for liquid biofuels.
"Wood chip costs usually depend on such factors as the distance from the point of delivery, the type of material (such as bark, sawmill residue or whole-tree chips), demand by other markets and how the wood fuel is transported. Chips delivered directly to the (powerplant) station by truck are less expensive than those delivered ... and shipped by railcar. The range of prices is typically between US$18 to US$30 per (wet)-ton delivered."
In several well wooded European countries (e.g. Austria, Finland, Germany, Sweden) wood chips are becoming an alternative fuel for family homes and larger buildings due to the abundant availability of wood chips, which result in low fuel costs. The European Union is promoting wood chips for energy production in the EU Forest action plan 2007–2011. The total long term potential of wood chips in the EU it is estimated to be 913 million m3.
After a long period of negative scores, the demand of wood chip for paper manufacturing started increasing again. Starting in the last quarter of 2013, orders for printing paper and card board increased before the consumption tax increase then by weakening yen, import of papers like copy paper decreases and export of paper increases, which stimulate paper production in Japan. Softwood chip prices from the United States increased by 12% compared to October 2013 and softwood chip prices from Australia increased by 7%.
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- VTHR Green wood Chip Furnace
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- Woodchip price factors for a power Generating Station in Burlington, VT, US
- Vermont Heat Research – An Experimental Wood Chip Furnace
- First quarter wood chip costs up almost 50% in western US, but pulpmills in the US South experienced only small upward price adjustments
- "Good development for the Japanese wood chip market". ITTO. Fordaq S.A. 15 May 2014.
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