Element budgets of forest biomass combustion and ash fertilisation – A Danish case-study

Harvest of forest biomass for energy production may lead to a significant export of nutrients from the forest. Ash spreading and recycling of nutrients from wood chip combustion to the forest has come into focus as a means for counteracting the nutrient export. This study was carried out to examine the retention of various elements in the different ash fractions and utilise the nutrient recovery to evaluate the fertiliser quality of the examined ash. The mass and element flux of wood chips, bottom ash, cyclone fly ash and condensation sludge at Ebeltoft central heating plant was studied over a four day period in spring 2005. On average, 19 ton wood chips (dry weight) were combusted each day. The combustion of the wood chips produced 0.70% ash and sludge (dry weight). The ash and sludge dry matter was distributed as 81% fly ash, 16% bottom and residual grate ash and 3% sludge solid phase. Substantial amounts of nutrients were retained in the fly ash (P, Ca, Mg, Mn and Cu have a recovery higher than 60% and K, S and Fe have a recovery higher than 30%). The recovery of elements in the bottom ash was smaller. The added recovery of the usable fractions of ashes (both fly ash and bottom ash) exceeded 75% for the nutrients P, Ca, Mn and Mg. Both these ash fractions should be considered for fertilisation.

► On average, 19 ton wood chips (dry weight) were combusted each day.
► The combustion of the wood chips produced 0.70% ash and sludge (dry weight).
► The ash and sludge dry matter was distributed as 81% fly ash, 16% bottom and 3% sludge solid phase.
► Substantial amounts of nutrients were retained in the fly ash (P, Ca, Mg, Mn and Cu).
► The recovery of elements in the bottom ash was smaller than that in the fly ash.