Recirculating ash for maintaining soil and water quality

What is soil? Mineral particles Organic material Air Water Root hair Fungi mycelia Bacteria Soil organisms 0.2 mm

Mineral soil Sorted soil E.g.: Sediment Formed through sedimentation in water Unsorted soil E.g.: Moraine Formed through the retreat of inland ice

Soiltype – an effect of external factors PodsolCambisol Climate Type/size of mineral particles Vegetation Topography Organisms

Soil horizons PodsolCambisol The mor layer Eluvial horizon Illuvial horizon Bedrock material Topsoil Subsoil

Soil particles - negatively charged

CEC and base saturation CEC Acidity Base saturation level

Cation exchange The amount of acid substances increases in the soil fluid Soil particles Nutrients are forced out from the soil participles into the soil fluid Soil fluid

Growth mediated acidification Soil fluid Roots The absorption of positively charged nutrients > the absorption of negatively charged nutrition The absorption of nutrients is acidified Acidification becomes permanent if the biomass is harvested

Mass balance account: Weathering + Deposits = Harvest + Leakage Risk of acidification if: Weathering + Deposits < Harvest + Leakage Mass balance

Deposition Weathering Harvest Leakage Mass balance

Potential effects of acidification Acid groundwater with increased levels of e.g. aluminium and cadmium Leakage of acid substances and heavy metals such as aluminium into lakes and watercourses Leaching of nutrients from the soil Effect on terrestrial flora Effect on future stand growth

SLASH removal

SLASH removal in Sweden Source: D1-polytax, The Forestry Commission North South

Methods for SLASH removal Deneedling in small heaps, chipping by forwarders in clearings Deneedling in small heaps, forwarder collects the twigs together into piles, piles chipped Unchipped SLASH driven out, chipping at terminals such as thermal power stations Chipped or unchipped green SLASH driven out (not deneedled)

SLASH treatment at the thermal power station? Unloading Magnetic separator Storage Drying Boiler Bottom ash Fly ash

Unloading SLASH Borås Energi

Drier for forest chippings By heating to 175° C, the moisture content is reduced from 50% to ~25% Borås Energi

Furnace

Different boilers produce different types of ash BoilerBottom ash (%)Fly ash (%) Grid Spread Stoker CFB

Fly- and bottom ash Bottom ash – pH – Poor lime effectiveness – Higher content Si and Al (sand) Fly ash – pH – Good lime effectiveness – Higher content K and S (volatile) – Higher content heavy metals – Higher content unburnt organic material

Bottom ash from grid boilers Fly ash from CFB and spread stoker boilers Smaller stations often mix these Recyclable ash

Difficulties with extracting energy from forest fuel/SLASH Difficult to achieve an even supply of fuel Variable fuel quality Uncertain origin of the content of chipped fuel Imposes strict demands on monitoring and control Co-burning with peat or oil

Ash

What is ash? Ash from SLASH contains: the bulk of the nutrients originally found in the SLASH (not nitrogen). carbon heavy metals polyaromatic hydrocarbons (PAH)

Base cations and trace elements Heavy metals and possibly caesium Nitrogen is lost during combustion Oxides Hydroxides Sulphates Chlorides Silicates Carbonates

Predominant compounds in ash % of the content is oxides of Ca, K and Si Ca % K1 - 2 % Mg1 - 2% P1 %

Recommended minimum content Nutrient substance Minimum content (g/kg TS) Ca125 Mg20 K30 P10 Zn1 (Swedish National Forestry Board)

Recommended upper limits Substance Max. cont. (mg/kg TS) Substance Max. cont. (mg/kg TS) B500Cr100 Cu400Hg3 Zn7000Ni70 As30V70 Pb3002 Cd30Cs kBq/kg

Technology Rotating concrete mixers, or with knives to break up the aggregate. Horizontal cylinder with a rotating shaft where shovels/blades have been fitted (similar to a helical conveyer). Paddle mechanism where mixing is carried out with shovels or blades. Ash treatment – adding water

Ash treatment - hardening Self-hardening and sifting Compacting Granulation

Hardening process Oxides in the ash Hydroxides and carbonates Secondary mineral such as ettringit Examples: CaO Ca(OH) 2 CaCO 3

Hardening means that: A reduced solubility rate and reduced risk of damage to soil and vegetation The ash is less soluble The pH of the ash is lowered The particle size increases

Self-hardening in heaps - the most common method

Ash treatment – crushing & sifting

Ash treatment - pelleting T Claesson, 2004, Kalmar Högskola [University of Kalmar]

Ash treatment - granules

+ A more unified, homogeneous product + A more stable product + The products are easier to spread - involve heavy investment - higher operating costs - Only small quantities are produced today. Granulation and pelleting

Ash analysis Before treatment Unburnt material Nutrients and heavy metals Possibly Caesium and PAH After treatment Solubility rate Texture and water content

Ash recirculation

Ash quantities (tonnes/year) (Bjurström et al, ÅF)

Ash recirculated in Sweden (2000) 11,000 tonnes 4,400 hectares average 2.5 tonnes per hectare

By 2010 the annual area where forest fuels are harvested is equal to the annual area that receives wood ash. These areas shall, in longer terms, conform Target of ash recirculation

Swedish Forestry Protection Act "When fertilisation is undertaken in forests, for vitalisation, compensation or the spreading of biocides, this should be carried out so that damage to the environment is avoided or limited." (Provisions of the Forestry Protection Act § 30 SKSSF 1993:2 [For.Com. Statute Book])

Ground spreading

Rotating dish

Spreader unit for helicopter

What happens to the ash in the soil? Soil particle Lime effectiveness Ca 2+ Mg 2+ K+K+ H+H H+H+ Ca 2+ Al 3+ Acid substance neutralised pH raised ash

Positive effects of spreading ash Reduced leakage of acid water and water containing aluminium into lakes and watercourses. Compensation for the nutrition and lime effects lost due to the removal of SLASH. The continual removal of forest fuel may increase - the use of fossil fuels may diminish The cycle of nutrients is completed. Ash becomes a resource and not waste. Resistance to soil acidification increased.

Risks when spreading ash Increased amount of traffic with an increased risk of vehicle damage to soil and roots. Poorly stabilised ash products can cause damage to vegetation. Stable ash products can cause scorching of tree trunks. In some poorer areas, there may be a risk of reduced accretion since the availability of N is reduced. If the wrong sort of ash is recycled, there is a risk of build-up of heavy metals and other toxic substances in the soil.

Examples of damage to vegetation

Apply wood ash... in thinned stands where forest fuels has been harvested or where harvest is planned that originates from forest fuel with a maximum of 3 t per ha and 10 years interval in or adjacent to valuable habitats in or adjacent to undisturbed wetlands on cultural objects where harvest of forest fuels is not recommended Do NOT apply wood ash... Swedish recommendations for ash recirculation