Woody Biofuels: Past, Present and Future

Agricultural Experiment Station SP702-A Biofuels Initiative Woody Biofuels: Past, Present and Future A short summary of the many ways that wood has been, is and could be used as a fuel: Firewood, charcoal, pellets, briquettes, hog fuel, black liquor, gasification, bio-oil and ethanol. Adam Taylor, Assistant Professor Forestry, Wildlife & Fisheries ood – and all other plant Charcoal: From a cleaner for absorbing odors, color or other biomass – is ultimately the impurities from air or water. product of photosynthesis biofuel to a cleaner W Charcoal is formed when wood in living plants. The sun’s energy Hog fuel: Waste to power wood is combined with carbon dioxide is heated in the absence of oxygen. processing (CO ) and water to form simple sug- In its simplest form, charcoal is 2 created when a wood fire is smoth- A major use of wood in the ars. These sugars are then convert- United States is for the production ed biochemically in trees to form ered: as the wood is heated by the fire – but starved of oxygen – a dry, of heat and electricity at industrial wood. While wood is a remarkably locations. Wood fuels are dominant durable and potentially long-lasting black, lightweight charcoal is left behind. Charcoal can be burned energy sources for energy-intensive biomaterial, the sunlight energy can wood processing operations such as be released (along with the CO and later in fireplaces or stoves. Char- 2 the kiln drying of lumber. water) when we want to use wood coal produces a relatively clean and hot fire that is useful for cooking. Wood slabs and bark can be as a fuel. ground up (‘hogged’) to make a fuel. This fact sheet briefly describes Charcoal is a preferred cooking fuel in many regions. The main Sawdust and planer shavings can be some of the major uses of wood as used directly. The quality of a hog a fuel. disadvantage of traditional charcoal production is that most of the or wood waste fuel is a function of wood’s original energy is lost in the its density, moisture (water) content Firewood: The original conversion process. and other factors, but simply put, biomass fuel Charcoal can be generated almost any wood-processing waste Wood ‘fires’ can be as simple from trees, wood chips and even can be used for fuel. In the past, it was and small as a campfire, as large as sawdust. The charcoal ‘briquettes’ common for sawmills to burn their a pulp-mill boiler or as sophisticat- used for barbequing are made from wastes simply to get rid of them. Now, ed as a fast pyrolysis unit. In each compressed sawdust that has been however, it is understood that the case, however, the sunlight energy converted to charcoal. Charcoal can energy content of those residues is too that was captured by photosynthe- also be refined and used in a vari- valuable to waste. Mills that don’t have sis and concentrated and stored in ety of products such as filters and the need for wood energy for their the wood of trees can be put to use crayons. ‘Activated charcoal’ refers own operations will sell their wastes as a biofuel. to charcoal with a high surface to pulp mills or lumber kiln-drying Wood has been used as a source area. Activated charcoal is useful operations that can use the fuel. of heat for warmth and cooking throughout history. Despite the widespread use of wood for other purposes and the dominance of fossil fuels for energy today, fuel remains the main use of wood in the Woody residues such as this world. More than 50 percent of the hog fuel are the largest source trees harvested globally are used for of renewable energy used in the firewood. United States. Wood is also the largest renewable energy source in the United States. Some of the major ways that wood is used for fuel are described below. Biofuels Initiative 2 Cogeneration: Getting heat and electricity from wood fuel Using wood fuels in boilers is not limited to powering wood-processing plants. Other industries or institutions that use steam or hot water for processing or heating can be fueled by burning wood processing waste. In addition to using heat from burning wood waste to generate hot water or steam, combined- heat-and-power, or ‘cogen’, boilers can be used to simultaneously generate electricity. This electricity may then be used on-site or sold back to the electrical grid. Pellets: Firewood in a different form Fluffy (low-density) wood wastes such as sawdust can also be formed into small shapes such as Pulp and paper making uses woody biomass as a source of both fuel and pellets or briquettes, providing an easy-to-handle fuel. Wood pellets raw material. are formed using machinery that compresses sawdust so much that broken down and a solid charcoal the wood sticks together – no glue is Wood chemicals, such as lignin and hemicelluloses, are dissolved, fuel remains. During this process, used. These pellets are then burned much of the solid wood is vapor- in boilers or small household heat- leaving behind the cellulose fibers that will be used for making writing ized into potentially flammable ing stoves. Wood pellets are clean- gases. In traditional charcoal paper, tissues, etc. Only about 50 burning, with a low water content making, these gases escape, wast- because they are made from dry percent of the wood is recovered as ing much of the potential wood sawdust, making the burning pro- usable pulp. However, the mixture energy. In other pyrolysis systems, cess more efficient. Pellets can be of used pulping chemicals and dis- these gases can be captured and continuously dispensed into stoves solved wood components is not dis- used. The pyrolysis process can using hoppers and automated feed carded. This ‘black liquor’ is burned be adjusted to maximize the gases screws. in special furnaces at the mill to produced and minimize the solid Pellets and other compressed- char products. wood-waste products, such as recover the pulping chemicals so they can be re-used. The processing In wood gasification, pyrolysis briquettes, are simply a convenient gases are captured and burned. form of firewood – a new take on of black liquor also releases heat when the wood that is dissolved in The energy from burning these the traditional biofuel. wood gases can be used to power the liquor is burned. This heat is boilers or even to operate internal- Black liquor: Wood fuel for converted to steam, which is used combustion engines. During World making paper for pulp and paper-making process- War II, wood-gas-fired cars and Wood pulp and paper produc- ing. Black liquor-derived steam is trucks were built in response to tion uses low-grade trees and saw- also used to generate electricity. shortages of gasoline and diesel. mill residues. Papermaking also re- The burning of black liquor In bio-oil production, pyroly- quires lots of energy, especially for in the Kraft pulping process is the sis gases are condensed, forming drying the paper sheet. Much of the largest use of wood as a biofuel in a brown liquid ‘oil’ that can be burned in furnaces. Bio-oil has energy for this processing comes the United States. about the same fuel value of etha- from wood components removed in Pyrolysis, gasification and nol (a common gasoline additive), the pulping process. and it can be burned in boilers The Kraft pulping process is the bio-oil: Liquid and gaseous that use heating oil. Bio-oil cannot most common method of isolat- fuels from solid wood yet be used as a fuel for vehicles. ing pulp fibers from wood. In Kraft As was described above, if Bio-oil is a complex mixture of pulping, wood chips are mixed with wood is heated in the absence of chemicals, and current research chemicals under heat and pressure. oxygen (called pyrolysis), it gets is investigating how to purify and Biofuels Initiative 3 make use of those components for wood substance) is pure glucose. 3. Wood is a relatively high-den- value-added products. However, this sugar is bonded sity fuel that can be harvested in special ways in wood, and is and stored for relatively long Ethanol: Using wood as a raw protected by the lignin and other periods of time without de- material for fuel production substances in the wood. In order composing. Wood is commonly used as for the glucose to be available for 4. In addition to being a source a fuel with little or no modifica- attack by the yeast and conversion of renewable raw materials to ethanol, the wood must first be for the production of carbon- tion. Firewood is simply cut, dried 1 and burned. Hog fuels are waste broken down. This breakdown can neutral fuels , forests provide materials from wood processing be achieved in various ways, using many other products and ben- that are used like firewood. Even heat and chemicals or enzymes. efits, such as wildlife habitat the pyrolysis products such as The technology for the break- and recreational opportunities. wood gas and bio-oil are in essence down of wood to fermentable burning of wood – the difference is sugars and ethanol production is Summary: Wood is good in the control over the combustion being continually refined, but wood as a biofuel! process. may one day provide a significant Wood is a concentrated form The concept of making ethanol source of raw material for manu- of stored sunlight (solar energy). or other liquid fuels from wood facturing fuels. Wood has a num- This energy can be released and is different from traditional wood ber of advantages as a biomass raw used as a fuel.

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