Why Pellets
Global warming potential, sustainablility, and lifecycle emissions concerns
The climate impact of wood pellets is disputed[by whom?], though biomass fuels, including wood pellets (and other wood fuels, such as regular cordwood) produced using best practices from sustainably-managed forests, fuel crops, or other forms of biomass waste are generally recognized as having far lower net lifecycle carbon dioxide emissions than fossil fuel equivalents, to the order of 98% fewer emissions.[2] However, it must be emphasized that if best practices and sustainable biomass management is not instituted, carbon emissions can exceed those of natural gas combustion.[2]
Others counter that the source sawdust would not otherwise have contributed to greenhouse gases[dubious ], and that burning fuel pellets releases a large amount of CO2 into the air. Secondary emissions related to pellet manufacture also have a carbon impact, with the release of carbon involved in the harvesting, manufacturing, and process of transport to the end-user for these pellets and other biomass fuels (known as grey energy) - though with appropriate practices, such as the use of biodiesel for transport fuel and to power harvesting equipment, along with solar thermal energy in wood drying facilities, this can be reduced to a minimum. Indeed, it is undoubtedly the case that pellet combustion releases carbon dioxide, but the use of pellet fuels is argued to have a low net lifecycle impact because the carbon dioxide released is ultimately consumed in the regrowth of the trees or biomass products used to produce the pellets; this process takes approximately 15 years (for trees) and as little as 1 season (for crop-based biomass) to sequester the carbon released by their combustion. Thus, although there is a short term radiative forcing effect from biomass combustion, there is a generally very low long term radiative forcing effect from biomass combustion if best practices for sustainability are followed.[2]
This is because wood/plant biomass is made up of mostly carbon (and water); carbon came from the carbon dioxide absorbed from the atmosphere by the tree or grass while it grew through the process of photosynthesis, and the carbon returns to the atmosphere when the wood is either burned or left to decompose. It is true that in combustion, most of the carbon joins with oxygen and returns to the atmosphere as carbon dioxide, with a GWP100 of 1. But - if the wood is instead left to decompose through natural processes, the carbon still partially returns to the air - but in the form of the gas methane, which has a GWP100 of 21 - thus causing 21 times more damage to the climate over 100 years than the same quantity of CO2 does over 100 years. Only part of the decaying wood is released as methane; other parts stay within the soil and often improve it substantially over time. This criticism of the climate aspect of wood decay does not apply if the wood is used in a process that fixates the carbon, such as in building materials, or, in the case of sawdust, in particle board, as these media sequester carbon unless or until they are allowed to decay or are consumed by fire.
Some[who?] argue that the speed of return matters; the release of methane is, for example, over 5 years, the release of CO2 is over five minutes if burned. Residence time also matters - and the methane of wood decay is not absorbed by the oceans or by growing biomass, as is carbon dioxide. However, one undoubted source of potential radiative forcing from wood pellets is if inefficient combustion technologies are used to burn them, and this results in black carbon emissions.
Organizations like the Sierra Club cautiously support use of biomass, though they are opposed to what might be termed wanton or reckless use of biomass, expressing concern about biomass that is grown without regard to techniques for sustainable forestry or agriculture, such as the use of unsustainable fertilizers, the wholesale removal of tree roots, which fixate carbon, or unsustainable harvesting is used, such as clearcutting, leading to erosion. They warn that if unsustainable land use practices are instituted in the production of biomass fuels, this may aggravate problems with global warming.[3]
The United States forest products industry, with apparent justification, fears that if massive use of wood as a fuel is instituted, forestry will no longer be able to be sustainably managed, and may result in ecological devastation. This is especially the case if wood is extensively used to generate electricity, though this would probably not be in the form of wood pellets.[4]
Still, the combustion of sustainably managed and harvested wood or biomass is generally considered to represent one of the best practical and available means for sustainable use of low net-carbon combustion for economic sectors that require the use of combustion. The medium of wood or biomass pellets as a form of energy transfer provide a fungible, mass-producible, commerce-ready product capable of putting sustainable biomass combustion technology to work for the modern-day consumer - and, in many cases, wood/biomass pellets are less expensive and nearly as convenient as other fuel sources.
