Sprucing Up Wood Waste
Wood chips are commonly used as a feedstock in the biomass industry-but what processes must waste wood undergo before it is suitable for use? How is contaminated wood treated to meet standards and obey regulations? Biomass Magazine investigates the old and new processes some companies are using to clean and recycle waste wood.
As Germany strove to develop a more efficient industrial method of wood-to-ethanol production, the process found its way to the United States during World War I. Although a lull in lumber production hindered the development of the technology, a small but significant amount of research continued.
What was acceptable decades ago, though, is no longer. Concerns have risen regarding the use of wood for fuel-primarily over the environmental effects of removing too many trees-and more pertaining to this day and age, the releasing of toxins from burning or processing contaminated wood. These types of treated wood often contain plastics, nails and other metals that are problematic for landfills because of low density, large volumes and extremely slow decomposition rates. As massive landfills are being filled up and closed, interest has developed in how more of this waste can be utilized. Although most are aware of the benefits and the ways plastic, aluminum and paper are recycled, little is known about the less commonly considered recyclable-waste wood.
Recycling Wood 101
The benefits of recycling wood are numerous, including preserving trees, protecting habitat, prolonging the lifespan of landfills, reducing the need for new landfills, maintaining air quality, providing cleaner energy and fuel, and reducing soil erosion. According to the U.S. EPA, there are now more than 500 wood processing facilities throughout the United States. Fortunately, that number is increasing as the number of landfills available for waste disposal has gone down from 8,000 in 1988 to less than 1,800 in 2006-a year in which nearly 14 million tons of wood waste was generated prior to recycling. The process by which wood waste could go through to be reutilized varies depending on the intended use.
Refurbishing is a common process for recycling wood. Rather than creating something new, this is simply the act of repairing and cleaning a broken wood product such as a crate or pallet. Mueller Pallets LLC in Sioux Falls, S.D., repairs, cleans and manufactures thousands of wood pallets each day. The company accepts unwanted wood from landfills, contractors, construction companies, waste disposal companies, and tree service providers and turns it into a variety of recycled wood products.
Processing, which entails the cleaning and grinding of waste wood, requires first that foreign materials piled with the wood are removed. This may be done manually or with machinery. A common method of removing these objects that many facilities use involves water and a conveying system. The waste wood is dumped into tanks containing water; the nonwood materials do not contain buoyant properties and will sink to the bottom of the tanks. This not only separates the two, but cleans existing dirt from the wood. After being removed from the tank, the wood is moved onto a conveyer belt to be inspected and separated into painted or treated and nontreated wood piles, since some company's permits do not allow them to process treated wood. The wood they are permitted to treat is exposed to large magnets to extract nails and metal pieces, and then transferred to a mill to be ground into wood chips. In the last step, the wood chip material is re-exposed to to magnets to remove any remaining metal.
Manufacturing takes the recycling process one step further by creating a new product with the cleaned and processed wood chips. A majority of the time, the wood will be ground into particle board or a similar product. If the wood chips are going to be used as compost, they are put through another grinding process to reduce particle size.
Cleaned wood chip particles may also be used for incineration to generate energy. Mueller Pallets has a contract to supply up to 350 tons of wood chips per day to fuel the boiler in Poet LLC's ethanol plant in Chancellor, S.D.
Though these processes utilize a large amount of wood, a great quantity of treated or contaminated wood remains-mainly because of chemicals such as creosote that many companies do not have the technology or permits to process.
The Creosote Controversy
Creosote is a complex mixture of a hazardous nature that may serve as a fungicide, insecticide and sporicide in wood protection treatments. Despite being classified as a possible human carcinogen by the U.S. EPA, creosote remains the second most widely used wood preservative in the United States-primarily for treatment of industrial products such as railroad ties and utility poles. According to the Agency for Toxic Substances and Disease Registry, there are more than 300 identifiable chemicals in creosote, but as many as 10,000 more may be in the mixture. There is strong evidence of harm resulting from improper safety precautions, disposal and use. Because creosote is a restricted-use pesticide that can only be applied by certified applicators or someone under their direct supervision, it is not available for sale to or use by homeowners. However, the EPA has acknowledged in reports that some creosote treated wood such as in railroad ties are used outdoors in home landscaping, and that creosote does, in fact, have the capabilities of having negative health effects on animals, humans and the environment. On the contrary, a definite plus to creosote is that the chemical significantly prolongs the lifespan of wood, thus reducing the need to harvest new wood.
Since more companies than ever before are gaining permits to accept creosoted wood to recycle for biomass, opponents and environmentalists want to know what is being done to ensure the toxins are being properly removed-and if it is really environmentally safe. Enerkem, a leading producer of cellulosic biofuels, has developed a new technology to remove contaminates such as creosote from wood using a gasification and catalysis process.
Enerkem, founded in 2000, is headquartered in Montreal. The company has operated a pilot plant in Sherbrooke, Québec, since 2003-testing a new gasification and catalytic synthesis technology.
The unique factor Enerkem possesses is the ability to process certain types of demolition wood-such as decommissioned power poles and creosote treated wood-that most corporations cannot, because of permit limitations preserving air quality and the environment. Enerkem has recently partnered with GreenField Ethanol, Canada's leading ethanol producer, and is in the midst of constructing the company's first commercial-scale plant in Westbury, Québec, along with a series of other projects in Canada.
Enerkem is colocated with a saw mill that recycles the middle part of the decommissioned power poles into construction wood, such as 2x4s. The remaining treated portions containing impurities cannot be recycled into construction wood. These pole residues are transformed into wood chips by the saw mill and transferred to Enerkem to be converted into ethanol.
In the first step, the wood chips or other feedstocks are dried, sorted and shredded to be stored in a container that is connected to the gasifier by a front-end feeding system capable of handling fluffy material, without the need to pelletize. Slurries or liquids may also be fed into the gasifier through appropriately designed injectors. The carbonaceous materials, such as biomass treated with creosote, are converted into a synthetic gas-consisting mostly of carbon monoxide and hydrogen-through a chemical gasification process.
The gasification process is carried out using air as a partial oxidation agent or using oxygen-enriched air, with the oxygen level tailored to the desired composition of the synthetic gas. The presence of steam at a specific partial pressure is also necessary. The gasifier operates at low severities, temperatures of approximately 700 degrees Celsius (1,292 degrees Fahrenheit) and below 10 units of atmospheric pressure. During the gasification process, part of the creosote is broken down, forming a portion of the product syngas.
Other traces of impurities are captured as residues in the form of a neutralized ash, or through a wastewater treatment system for effective disposal through the syngas cleaning system. At this stage, the gas is effectively cleaned and conditioned for use with existing catalysts. This is accomplished through a sequential conditioning system, which includes cyclonic removal of inerts, secondary carbon and tar conversion, heat recovery units, and reinjection of tar/fines into the reactor. The gas that is produced at the end of the cleaning process is ready for conversion into liquid fuel and end products which Enerkem says meet all requirements as either industrial grade products or fuel additives.
The Westbury plant is scheduled to begin production in the fall of 2008, and will have the capacity to produce more than 1.3 MMgy. The company is currently negotiating with other facilities to take their treated wood, Enerkem tells Biomass Magazine.
The number of companies that are striving to develop new technologies to utilize less commonly used but accumulating wastes, such as creosote treated wood, is on the rise. As the renewable fuels race continues-so does the recycling race.
Anna Austin is a Biomass Magazine staff writer. Reach her at firstname.lastname@example.org or (701) 738-4968.