Biochar stability assessed by chemical methods

Christophe Naisse1, Marie Alexis1, Alain Plante2, Katja Wiedner3, Christopher Carcaillet4, Cyril Girardin1, Bruno Glaser3, Irène Criscuoli1,5, Alessandro Pozzi6, Cornelia Rumpel1

1Bioemco – UPMC / CNRS, Thiverval‐Grignon, France2University of Pennsylvania, Philadelphia, USA3Martin‐Luther‐University, Halle, Germany4University of Montpellier 2, France5Foxlab – Fondazione Edmund Mach, Trento, Italy6Advanced Gasification Technology, Cremona, Italy

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Chemical stability of organic materials

Methods used to assess the reactivity of organic materials :

• Acid dichromate oxidationPowerful oxidant used to (1) isolate black carbon from soilBird & Gröcke, 1997 ; Certini et al., 2011

(2) evaluate chemical resistance of charcoal Rumpel et al., 2007 ; Ascough et al., 2008a ; Knicker, 2010 ; Calveilo Prereira et al., 2011

• Acid hydrolysis (HCl)Commonly used to determine the stable fraction of soil organic matterPaul et al., 1997 ; Rovira & Vallejo, 2002 ; Plante et al., 2006

Hydrolysis resistant fraction of soil is more stable compared to the bulk SOM and may contain charcoal, with 14C ages of several hundred years (Rumpel et al., 2008)

Carbon resistant to acid dichromate oxidation could mirror the degree of biochar aromatization (Calveilo Pereira et al., 2011)

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Objectives

• What is the chemical reactivity of biochars producedby  gasification  (GS)  and  hydrochars produced by hydrothermal carbonization (HTC)?

• Is the chemical reactivity of GS and HTC different fromthat of very old charcoals?

• Does physical weathering of  GS  and  HTC  influence their chemical reactivity?

Acid hydrolysis and dichromate oxydation were conducted on biocharsand hydrochars produced from the same feedstocks

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Technologie Tp (°C) Liquid (%) Char (%) Gaz (%)

Pyrolysis slow 400‐600 30 35 35

Gasification(Gs) 1200 5 10 85

Hydrothermal carbonisation(HTC) 230 5‐20 50‐80 2‐5

Chars used in this study

Hydrothermal carbonisationFuertes et al. (2009)

GasificationBrewer et al. (2009)

Holocene coals:‐Ca 2000 years‐Ca 100 years‐(pyrolysis)

=> Produced with same 3 feedstocks (Poplar wood, Wheat straw & Olive residues)

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Methods

Acid hydrolysis

24Time (h)

After 24 h : filtration  wash Freeze‐dry

HCl 6M at 110°C

Weigh & CHN analysis

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Acid dichromate oxydation

Methods

Sample in reactionNo sample

Time (h)1260

Biochar

Feedstock

Ox.Ox.

After 2 hours

Time (h)2 4 12

Each 2 hours

Ox.

Red.

K2Cr2O7changed

After 11 hours all feedstock samples were completely oxidised

K2Cr2O7 0.3M at 80°C

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Olive residuesPoplar woodWheat straw

: Feedstocks: HTC: GS

Van Krevelen Diagram

Gasification • GS‐Biochars are highly condensed

Feedstocks

• Feedstocks are mainlycomposed of carbohydrates

HTC

• Hydrochars are  intermediatebetween feedstocks and GS‐biochars

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Holocene charcoals

Olive residuesPoplar woodWheat straw

: Feedstocks: HTC: GS

Ca 2000 years Ca 100 years

• Holocene charcoals are more heterogeneousthan fresh biochar & hydrochars

Van Krevelen Diagram

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Resistance to acid hydrolysis

HTC containing aliphatic & aromatic compounds are  similarly resistantas GS containing highly aromatic carbon

Feedstock < HTC = GS

Release of O‐ and N‐ functional groups 

degradation of small molecules(fatty acids, proteins and polysaccharides)

Why ?

Gasification

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HTC ‐ biocharFeedstockWheat

OliveGS ‐ biocharPoplar

100

80

40

20

Acid hydrolysis

Feedstock

HTCGS

Acid hydrolysis may not  be specific enough to  evaluate stabilityrelated to highly condensed aromatic C 

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Ca 2000 years

Ca 100 years

Resistance to acid dichromate oxidation

Does the reactivity correspond  to elemental composition ?

• GS > HTC• Slight difference due to feedstocks

Production Conditions > feedstock properties

Reactivity for holocene charcoals:

100 yrs<2000 yrs

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= 0.88***

100

80

40

20

60

C remaining

(%)

Resistance to acid dichromate oxidation

GS

HTC

2000 years

100 years

Acid dichromate may be a  good method to  analyse  the degree of condensation of aromatic material

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150 °C – 10 min => Desorption of Volatile organic carbon (Spokas, 2011)

3 x wetting / drying (60°C)

3 cycles

3 x freezing‐ thawing

Artificial weathering of industrial chars

GS HTC

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Artificial weathering of biochar

Corn GS seem to be very sensitive to physical weathering

Gasification (GS)

Hydrochars (HTC)

Poplar wood Corn silage Corn silage1° Medite

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ymposiu

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Carbon remaining after dichromate oxidation

Physical weathering increases the chemical reactivity of biochars

Poplar wood Corn silage Corn silage

Gasification (GS)

Hydrochars (HTC)

Biochar

Weathered Biochar / Hydrochar

Hydrochar

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Laboratory incubations during 36 days

Artificial weathering did not change biological reactivity

0%

20%

40%

60%

80%

100%

Fresh 

Corn Silage

C remaining

 after incuba

tion (%

)Biochars (GS) Hydrochars (HTC)Feedstocks

Weathered Biochar / Hydrochar

Corn Silage Corn Silage

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Conclusions

• Different  feedstocks  marginally  influenced  the  chemical reactivity of biochars

• Acid dichromate oxidation may be a suitable method to assess the  chemical  reactivity of biochars  related  to  their degree of condensation

• The  chemical  reactivity of holocene  chars may  increase with time of exposure in soil

Artificially  weathered  biochars  showed  higher  chemical reactivity

Some  correspondance  between  chemical  and  biological reactivity

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Thank you for your attention

Acknowledgements:

1° Mediterra

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