• Wood pellets promise to be a lower emissions alternative to coal.winter coal image by Rick Smith from Fotolia.com
  
  Renewable energy is a goal that more and more organizations are looking to tap in to, and one source of renewable energy is burning biomass in the form of wood pellets or cellulose pellets. These pellets are pressed out of sawdust or are grown from fast growing woody grasses and plants -- typically willow -- that are ground up and compacted. Wood pellets have the advantage that they can be mixed in with coal, and reduce the total amount of coal used to generate power. They also have some drawbacks.
  
  

Energy Density

• One of the reasons why coal is an effective fuel is due to its density -- its nearly pure carbon, and has a density of anywhere from 1100 kilograms per cubic meter to 830, depending on the type of coal. Wood pellets are effectively the same material that particle board is made from, and has a density of about 240 to 500 kilograms per cubic meter. What this means is that for a given volume of coal, the coal will weigh anywhere from three to five times more -- which means that to get the same energy out of a given volume of fuel, you'll need anywhere from three to five times as many wood pellets as you would coal. (This assumes that wood pellet fuel burns at the same temperature as coal, which is not always the case.)
  
  

Transportation Issues

• In addition to needing more fuel per unit of energy released, the wood chips also require more fuel to transport to the power plant. Most coal fired power plants are built close to coal mines to minimize the fuel use, and coal transport was one of the major drivers of the industrial revolution -- and is still the single most dominant freight used on railways the world over. By comparison, the lumber yards that process waste silage and wood scrap into wood pellets aren't conveniently located near to power plants. The current estimate is that the benefit of using waste biomass for power plants is negated if the total distance from the wood pellet source to the power plant exceeds 50 km (30 miles).
  
  

Sourcing Issues and Fuel Costs

• The driver for using wood pellets is not to save money, but to reduce the amount of non-renewable CO2 emitted into the atmosphere; by burning material that would otherwise go into compost heaps and landfills as sawdust, the appeal is to kill two birds with one stone -- remove a waste material and reduce fossil fuel CO2 emissions. In practice, by creating a market for wood pellets as fuel, a number of these benefits are being eroded by the law of supply and demand. Because wood pellets cost more per pound than coal does, there's an incentive to cut down trees to make wood pellets for the profit motive, causing a number of concerns about the sourcing of this alternate fuel.