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Recirculating ash for maintaining soil and water quality
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What is soil?What is soil?
Mineral particlesOrganic material
AirWater
Root hair
Fungi mycelia
Bacteria
Soil organisms
0.2 mm
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Mineral soilMineral soil
Sorted soil
E.g.: Sediment
Formed through sedimentation in water
Unsorted soil
E.g.: Moraine
Formed through the retreat of inland ice
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Soiltype – an effect of external Soiltype – an effect of external factorsfactors
Podsol Cambisol
Climate
Type/size of mineral particles
Vegetation
Topography
Organisms
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Soil horizonsSoil horizons
Podsol Cambisol
The mor layer
Eluvial horizon
Illuvial horizon
Bedrock material
Topsoil
Subsoil
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Soil particles - negatively chargedSoil particles - negatively charged
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CEC and base saturationCEC and base saturation
CEC
Acidity
Base saturation level
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Cation exchangeCation exchange
The amount of acid substances increases in
the soil fluid
Soil
part
icle
s
Nutrients are forced out from
the soil participles into the soil fluid
Soil fluid
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Growth mediated acidification 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
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Mass balance account:
Weathering + Deposits = Harvest + Leakage
Risk of acidification if:
Weathering + Deposits < Harvest + Leakage
Mass balanceMass balance
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Deposition
Weathering Harvest
Leakage
Mass balanceMass balance
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Potential effects of acidificationPotential 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
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SLASH removalSLASH removal
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SLASH removal in SwedenSLASH removal in Sweden
Region x 1000 hectares/year average 1998-99
Northern Norrland
0,8
Southern Norrland
2,5
Svealand 6,7 Götaland 21,9 Whole country 31,9
Source: D1-polytax, The Forestry Commission
North
South
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Methods for SLASH removalMethods 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)
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SLASH treatment at the thermal SLASH treatment at the thermal power station?power station?
Unloading
Magnetic separator
Storage
Drying
Boiler
Bottom ash
Fly ash
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Unloading SLASHUnloading SLASH
Borås Energi
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Drier for forest chippingsDrier for forest chippings
By heating to 175° C, the moisture content is reduced from 50% to ~25%
Borås Energi
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FurnaceFurnace
Borås Energi
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Different boilers produce different Different boilers produce different
types of ashtypes of ash
Boiler Bottom ash (%) Fly ash (%)
Grid 70-80 20-30
Spread Stoker 40-50 50-60
CFB 10-20 80-90
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Fly- and bottom ashFly- and bottom ash
Bottom ash– pH 10-11– Poor lime effectiveness – Higher content Si and Al (sand)
Fly ash– pH 12-13– Good lime effectiveness – Higher content K and S (volatile) – Higher content heavy metals– Higher content unburnt organic material
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• Bottom ash from grid boilers • Fly ash from CFB and spread stoker boilers• Smaller stations often mix these
Recyclable ashRecyclable ash
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Difficulties with extracting energy from Difficulties with extracting energy from forest fuel/SLASHforest 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
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AshAsh
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What is 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)
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•Base cations and trace elements
•Heavy metals and possibly caesium•Nitrogen is lost during combustion
OxidesHydroxidesSulphatesChloridesSilicates
Carbonates
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Predominant compounds in ashPredominant compounds in ash
60 - 90 % of the content is oxides of Ca, K and Si
Ca 10 - 30 %
K 1 - 2 %
Mg 1 - 2%
P 1 %
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Recommended minimum contentRecommended minimum content
Nutrient substance
Minimum content (g/kg TS)
Ca 125
Mg 20
K 30
P 10
Zn 1
(Swedish National Forestry Board)
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Recommended upper limitsRecommended upper limits
Substance Max. cont. (mg/kg TS) Substance Max. cont.
(mg/kg TS)
B 500 Cr 100
Cu 400 Hg 3
Zn 7000 Ni 70
As 30 V 70
Pb 300 2
Cd 30 Cs-137 10 kBq/kg
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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 – adding water
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Ash treatment - hardeningAsh treatment - hardening
Self-hardening and sifting CompactingGranulation
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Hardening processHardening process
Oxides in the ash
Hydroxides and carbonates
Secondary mineral such as ettringit
Examples:
CaO Ca(OH)2 CaCO3
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Hardening means that: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
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Self-hardening in heaps - the most Self-hardening in heaps - the most common methodcommon method
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Ash treatment – crushing & sifting Ash treatment – crushing & sifting
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Ash treatment - pelletingAsh treatment - pelleting
T Claesson, 2004, Kalmar Högskola [University of Kalmar]
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Ash treatment - granulesAsh treatment - granules
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+ 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 pelletingGranulation and pelleting
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Ash analysisAsh analysis
Before treatment
Unburnt material
Nutrients and heavy metals
Possibly Caesium and PAH
After treatment Solubility rate
Texture and water content
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Ash recirculationAsh recirculation
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Ash quantities Ash quantities (tonnes/year)(tonnes/year)
Fraction \
Sector
Recyclable bio ash
Non-recyclable mixed ash
Other ash
Energy
80,000 120 – 160,000 264,000
Wood pulp and paper
98 – 130,000 130 – 170,000 -
The wood industry
100,000 - -
(Bjurström et al, ÅF)
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Ash recirculatedAsh recirculated in Sweden (2000)in Sweden (2000)
• 11,000 tonnes • 4,400 hectares• average 2.5 tonnes per hectare
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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 recirculationTarget of ash recirculation
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Swedish Forestry Protection ActSwedish 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])
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Ground spreadingGround spreading
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Ground spreadingGround spreading
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Rotating dishRotating dish
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Spreader unit for helicopterSpreader unit for helicopter
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What happens to the ash in the soil?What happens to the ash in the soil? S
oil p
artic
le
Lime effectiveness
Ca2+
Mg2+
K+
H+
------
H+
Ca2+
Al3+ Acid substance neutralised pH raised
ash
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Positive effects of spreading ashPositive 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.
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Risks when spreading ashRisks 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.
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Examples of damage to vegetationExamples of damage to vegetation
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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 Swedish recommendations for ash recirculationfor ash recirculation