The Benefits of Wood Ash from a Wood Boiler or Pellet Boiler
Tarm Biomass® recently attended the third New Hampshire Boiler Operator’s Workshop. The workshop was generously hosted by EMD Millipore in Jaffrey, NH and sponsored by the New Hampshire Wood Energy Council. About 50 people attended what proved to be a very interesting morning workshop.We were particularly surprised and impressed by a presentation made by Shelagh Connelly, President of Resource Management, Inc. of Holderness, NH. Resource Management manages ash disposal for several large wood burning power plants and has made a practice of sampling, sorting and mixing ash as necessary to create a high value, resource rich soil amendment. Old Yankees have known for centuries that wood ash contains beneficial nutrients. Many rural home owners scatter wood ashes on their lawns or in their gardens for its fertilizing benefits. Larger wood burning systems, whether they burn wood chips or wood pellets also create ash, but at larger scales.
What happens when a site is burning 305,000 tons of whole tree wood chips in a year? What about a wood pellet boiler burning 80 tons of wood pellets in a year? How much ash is produced and can it also be used as a fertilizer? Are there any risks?
Whole tree wood chips contain about 3% ash on average. Ash usually is predominantly noncombustible mineral. However different combustion processes yield different ash consistencies. For instance, older, inefficient boilers may produce ash with carbon content that is 60% of the “ash” by weight. Ash that originates directly at the primary combustion zone and falls into a collector is called bottom ash. This ash tends to be higher in carbon content and is generally courser than ash that is collected from the exhaust stream. Many boilers use cyclones, bag filters, or electrostatic precipitators to collect ash and fine particulates before they exit the exhaust stack. Ash resulting from these capture mechanisms is generally very fine and dusty and contains relatively low carbon content.Small, high efficiency thermal boilers leave almost zero carbon in the ash. Pellet boilers typically use fuel that contains less than 1 % ash. Even the bottom ash from these boilers is almost carbon free. Wood pellet boilers burning 80 tons of premium wood pellet fuel each year will produce about 3/4 of a ton of ash, or enough to mostly fill a pickup truck bed.
Depending on what is in wood ash, the ash has different values. Ash that is high in carbon content is useful for mixing with manures and organic composts to add nutrients and to control odor. Ash that is more pure and contains primarily inorganic minerals is great for spreading on farm fields, but would not help with odor. One ton of wood ash contains approximately 600 pounds of lime, 200 pounds of calcium, 75 pounds of potassium, and 20 pounds of magnesium. The lime component tends to be faster acting that standard pulverized lime.
Some wood ash is contaminated with harmful materials, such as heavy metals, in excess of regulated safe levels. High contaminant levels generally stem from coated construction and demolition debris or from mixing of natural wood with other non-pure fuels. As one example, even contamination with commonly available molybdenum grease used in processing equipment can cause contamination levels of molybdenum in ash above acceptable levels.
Because the purity of ash is an important health consideration, and to learn what nutrients an individual ash stream contains, testing should always be performed prior to use. States regulate the use of wood ash, especially when commerce and/or large quantities are involved. It is advisable to check local regulations for wood ash and to think about how ash will be handled prior to making a biomass boiler purchasing decision.
Wood ash is commonly recognized as an organic fertilizer and has value. Often the best arrangements for disposing of moderate quantities of ash are made with local farmers. This is just another example of the local benefits of wood as a renewable fuel source.