Recirculating ash for maintaining soil and water quality

What is soil?What is soil?

Mineral particlesOrganic material

AirWater

Root hair

Fungi mycelia

Bacteria

Soil organisms

0.2 mm

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

Soiltype – an effect of external Soiltype – an effect of external factorsfactors

Podsol Cambisol

Climate

Type/size of mineral particles

Vegetation

Topography

Organisms

Soil horizonsSoil horizons

Podsol Cambisol

The mor layer

Eluvial horizon

Illuvial horizon

Bedrock material

Topsoil

Subsoil

Soil particles - negatively chargedSoil particles - negatively charged

CEC and base saturationCEC and base saturation

CEC

Acidity

Base saturation level

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

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

Mass balance account:

Weathering + Deposits = Harvest + Leakage

Risk of acidification if:

Weathering + Deposits < Harvest + Leakage

Mass balanceMass balance

Deposition

Weathering Harvest

Leakage

Mass balanceMass balance

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

SLASH removalSLASH removal

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

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)

SLASH treatment at the thermal SLASH treatment at the thermal power station?power station?

Unloading

Magnetic separator

Storage

Drying

Boiler

Bottom ash

Fly ash

Unloading SLASHUnloading SLASH

Borås Energi

Drier for forest chippingsDrier for forest chippings

By heating to 175° C, the moisture content is reduced from 50% to ~25%

Borås Energi

FurnaceFurnace

Borås Energi

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

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

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

Recyclable ashRecyclable ash

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

AshAsh

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)

•Base cations and trace elements

•Heavy metals and possibly caesium•Nitrogen is lost during combustion

OxidesHydroxidesSulphatesChloridesSilicates

Carbonates

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 %

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)

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

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

Ash treatment - hardeningAsh treatment - hardening

Self-hardening and sifting CompactingGranulation

Hardening processHardening process

Oxides in the ash

Hydroxides and carbonates

Secondary mineral such as ettringit

Examples:

CaO Ca(OH)2 CaCO3

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

Self-hardening in heaps - the most Self-hardening in heaps - the most common methodcommon method

Ash treatment – crushing & sifting Ash treatment – crushing & sifting

Ash treatment - pelletingAsh treatment - pelleting

T Claesson, 2004, Kalmar Högskola [University of Kalmar]

Ash treatment - granulesAsh 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 pelletingGranulation and pelleting

Ash analysisAsh analysis

Before treatment

Unburnt material

Nutrients and heavy metals

Possibly Caesium and PAH

After treatment Solubility rate

Texture and water content

Ash recirculationAsh recirculation

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)

Ash recirculatedAsh recirculated in Sweden (2000)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 recirculationTarget of ash recirculation

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])

Ground spreadingGround spreading

Ground spreadingGround spreading

Rotating dishRotating dish

Spreader unit for helicopterSpreader unit for helicopter

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

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.

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.

Examples of damage to vegetationExamples 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 Swedish recommendations for ash recirculationfor ash recirculation