13
Biochar from Biomass and Waste W. Kwapinski C. M. P. Byrne E. Kryachko P. Wolfram C. Adley J. J. Leahy E. H. Novotny M. H. B. Hayes Received: 3 March 2010 / Accepted: 28 May 2010 / Published online: 12 June 2010 Ó Springer Science+Business Media B.V. 2010 Abstract There is an increasing realisation that biomass and organic wastes are valuable feedstocks for second generation biorefining processes that give rise to platform chemicals to substitute for dwindling petrochemical resources, and for pyrolysis processes that produce syngas, bio-oil, and biochar from biomass, organic wastes, and the biorefining residuals of the future. The experimental work described has focused on physical properties and compo- sitions of biochars produced from miscanthus (Miscan- thus 9 giganteus), willow (Salix spp) and pine (Pinus sylvestris) at 500°C and at 400, 500, and 600°C in the case of the miscanthus. Although the morphologies of the cell structures were maintained in the pyrolysis, the surface area of the miscanthus biochar was greatly increased by heating at 600°C for 60 min. Nuclear magnetic resonance spectra showed the disappearance of evidence for the car- bohydrate and lignin plant components as the pyrolysis temperature was raised, and the compositions of miscan- thus biochars after heating for 10 and for 60 min at 600°C were very similar and composed of fused aromatic struc- tures and with no traces of the aliphatic components in the starting materials. In greenhouse and growth chamber experiments the growth of maize (Zea mays L) seedlings was found to be inhibited by soil amendments with biochar from miscanthus formed at 400°C for 10 min, but stimu- lated by miscanthus char formed at 600°C for 60 min. In the course of discussion the relevance of the results obtained is related to the roles that soil amendments with biochar can have on soil fertility, carbon sequestration, on the emissions of greenhouse gases from soil, on fertilizer requirements, and on waste management. It is clear that biochar soil amendments can have definite agronomic and environmental benefits, but it will be essential to have clear guidelines for biochar production from various feedstocks and under varying pyrolysis parameters. It will be equally important to have a classification system for biochars that clearly indicate the product compositions that will meet acceptable standards. A case can be made for sets of standard biochars from different substrates that meet the required criteria. Keywords Biochar Á Biomass Á Waste Á Pyrolysis Á Thermal conversion Á Plant growth Á Carbon sequestration Introduction Soil amendment with biochar has attracted widespread attention because it increases the sequestration of carbon in soils and thereby decreases the amounts of CO 2 that enters the atmosphere [13]. Biochar is the carbon rich product obtained when biomass is thermally decomposed (pyroly- sed) in restricted air conditions at temperatures between 350 and 700°C[4]. The use of biochar as a soil amender is not a new concept. The Terra Preta da indio (TP), or the dark earth soils of the Amazon region are areas of very fertile W. Kwapinski Á C. M. P. Byrne (&) Á E. Kryachko Á P. Wolfram Á C. Adley Á J. J. Leahy Á M. H. B. Hayes Department of Chemical and Environmental Sciences, University of Limerick, Limerick, Ireland e-mail: [email protected] W. Kwapinski e-mail: [email protected] M. H. B. Hayes e-mail: [email protected] E. H. Novotny Embrapa Solos, Rua Jardim Bota ˆnico, Rio de Janerio 1024, Brazil 123 Waste Biomass Valor (2010) 1:177–189 DOI 10.1007/s12649-010-9024-8

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Page 1: Biochar from Biomass and Waste - Embrapaainfo.cnptia.embrapa.br/.../77675/1/Biochar-from-biomass-and-waste… · biochar can have on soil fertility, carbon sequestration, on the emissions

Biochar from Biomass and Waste

W. Kwapinski • C. M. P. Byrne • E. Kryachko •

P. Wolfram • C. Adley • J. J. Leahy •

E. H. Novotny • M. H. B. Hayes

Received: 3 March 2010 / Accepted: 28 May 2010 / Published online: 12 June 2010

� Springer Science+Business Media B.V. 2010

Abstract There is an increasing realisation that biomass

and organic wastes are valuable feedstocks for second

generation biorefining processes that give rise to platform

chemicals to substitute for dwindling petrochemical

resources, and for pyrolysis processes that produce syngas,

bio-oil, and biochar from biomass, organic wastes, and the

biorefining residuals of the future. The experimental work

described has focused on physical properties and compo-

sitions of biochars produced from miscanthus (Miscan-

thus 9 giganteus), willow (Salix spp) and pine (Pinus

sylvestris) at 500�C and at 400, 500, and 600�C in the case

of the miscanthus. Although the morphologies of the cell

structures were maintained in the pyrolysis, the surface

area of the miscanthus biochar was greatly increased by

heating at 600�C for 60 min. Nuclear magnetic resonance

spectra showed the disappearance of evidence for the car-

bohydrate and lignin plant components as the pyrolysis

temperature was raised, and the compositions of miscan-

thus biochars after heating for 10 and for 60 min at 600�C

were very similar and composed of fused aromatic struc-

tures and with no traces of the aliphatic components in

the starting materials. In greenhouse and growth chamber

experiments the growth of maize (Zea mays L) seedlings

was found to be inhibited by soil amendments with biochar

from miscanthus formed at 400�C for 10 min, but stimu-

lated by miscanthus char formed at 600�C for 60 min. In

the course of discussion the relevance of the results

obtained is related to the roles that soil amendments with

biochar can have on soil fertility, carbon sequestration, on

the emissions of greenhouse gases from soil, on fertilizer

requirements, and on waste management. It is clear that

biochar soil amendments can have definite agronomic and

environmental benefits, but it will be essential to have clear

guidelines for biochar production from various feedstocks

and under varying pyrolysis parameters. It will be equally

important to have a classification system for biochars that

clearly indicate the product compositions that will meet

acceptable standards. A case can be made for sets of

standard biochars from different substrates that meet the

required criteria.

Keywords Biochar � Biomass � Waste � Pyrolysis �Thermal conversion � Plant growth � Carbon sequestration

Introduction

Soil amendment with biochar has attracted widespread

attention because it increases the sequestration of carbon in

soils and thereby decreases the amounts of CO2 that enters

the atmosphere [1–3]. Biochar is the carbon rich product

obtained when biomass is thermally decomposed (pyroly-

sed) in restricted air conditions at temperatures between

350 and 700�C [4].

The use of biochar as a soil amender is not a new

concept. The Terra Preta da indio (TP), or the dark earth

soils of the Amazon region are areas of very fertile

W. Kwapinski � C. M. P. Byrne (&) � E. Kryachko �P. Wolfram � C. Adley � J. J. Leahy � M. H. B. Hayes

Department of Chemical and Environmental Sciences,

University of Limerick, Limerick, Ireland

e-mail: [email protected]

W. Kwapinski

e-mail: [email protected]

M. H. B. Hayes

e-mail: [email protected]

E. H. Novotny

Embrapa Solos, Rua Jardim Botanico, Rio de Janerio 1024,

Brazil

123

Waste Biomass Valor (2010) 1:177–189

DOI 10.1007/s12649-010-9024-8

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anthropogenic soils interspersed with relatively infertile

ferrisols close to rivers. There are several theories regard-

ing the processes that led to their formation. It is not clear

whether the soils resulted from intentional soil improve-

ment processes, or were the products of the agricultural and

household activities of indigenous populations [5]. The

dwelling places in the pre-Columbian past gave rise to

accumulations of plant and animal debris, as well as to

large amounts of ashes and of bonfire residues (charcoal),

and several chemical elements, such as P, Mg, Zn, Cu, Ca,

Sr, and Ba, which represent a geochemical signature of

human occupation [6]. The accumulation of such nutrient

residues is likely to be a major driver of the formation of

the fertile soils.

Large amounts of biochar derived carbon stocks remain

in these soils centuries and millennia after they were

abandoned. Typical Amazonian ferrisols have low fertility

and contain about 100 Mg carbon ha-1 m-1. The fertile

TP soils, though derived from similar mineral parent

materials, contain around 2.7 ± 0.5 times more organic

carbon (approximately 250 Mg ha-1 m-1) than the corre-

sponding ferrisols [4]. Carbon stored in soils in this way far

exceeds the potential sequestration of C in plant biomass

should bare soil be restocked with primary forest that

could, at maturity, be expected to have about

110 Mg C ha-1 above ground [7, 8].

There is increasing evidence indicating that biochar not

only increases the stable carbon stocks in a soil but also

increases nutrient availability beyond a fertiliser effect [9],

and is much more efficient at enhancing soil quality than

any other organic soil amendment [10]. Based on these

considerations the concept of Terra Nova has emerged, or

of soils whose properties and fertility would be enhanced

by modern variants of the management practises that cre-

ated the TP [11].

This communication reviews procedures for the pro-

duction and characterisation of biochars, and describes,

with some results, procedures that were utilised for the

preparation and characterisation of biochars. It reviews

aspects of how applications of biochar can influence pro-

cesses causing climate change, decrease emissions from

waste and of NOx from soil, and provides evidence to show

that although biochar can enhance seed germination and

plant growth there must be careful control of the prepara-

tion processes because some preparations can inhibit plant

growth.

Biochar Production and Characterisation

Pyrolysis is one of the many technologies that produce

energy from biomass and waste. The production of biochar,

a carbon-rich, solid, by-product [1] distinguishes pyrolysis

from the other technologies. The major pyrolysis products

are pyrolytic oil (bio-oil), synthesis gas with differing

energy values (syngas), and biochar [12, 13]. A range of

process conditions, such as the composition of the feed-

stock, temperature and heating rate can be optimized to

provide different amounts and properties of products.

Volatile products can be captured to provide energy, or

upgraded to specific chemical products (e.g. wood pre-

servative, meat browning, food flavouring, adhesives, etc.)

[1, 14].

Depending on the operational time and temperature,

there are three subclasses of pyrolysis. These are conven-

tional slow pyrolysis, fast pyrolysis, and flash pyrolysis

[15]. Yields of liquid products are maximised in conditions

of low temperature, high heating rate, and a short gas

residence time, whereas a high temperature, low heating

rate and long gas residence time would maximise yields of

fuel gas. Low operational temperatures and low heating

rates give maximum yields of biochar [16].

Feedstocks currently used on a commercial-scale, or in

research facilities include wood chip and wood pellets, tree

bark, crop residues (straw, nut shells and rice hulls), switch

grass, organic wastes including distillers’ grain, bagasse

from the sugarcane industry, olive mill waste, chicken lit-

ter, dairy manure, sewage sludge and paper sludge [11].

The particle sizes and moisture contents of feedstock are

important as wet feedstocks with large particle sizes will

require more energy for pyrolysis. Some of the carbon in

the feedstock may need to be burned to supply the energy

needed, and so less feedstock is converted to biochar and to

the other products. Maximising the production of biochar

relative to the mass of the initial feedstock is always at the

expense of bio-oil and gas production.

The studies described here have involved the pyrolysis

of willow (Salix species), miscanthus (9giganteus) and

pine (Pinus sylvestris) biomass and the characterisation of

the biochars formed.

Materials and Methods

In the first set of experiments 250 g samples of the

miscanthus, pine and willow were chipped (maximum size

1 cm) and heated in the absence of air to 500�C for 10 min

in a 1 dm3 capacity pyrolyser. The pyrolysed materials

were then rapidly moved to an ambient temperature zone,

still in a nitrogen atmosphere, and allowed to cool. In the

second set of experiments 250 g samples of the miscanthus

chips were heated for 10 min at 400�C and for 60 min at

600�C. The data for the yields of biochar obtained (on a dry

weight basis) from the three substrates; the elemental

analyses (Elementar); BET surface area (Gemini Microm-

eritics apparatus); and high heating value (HHV) deter-

mined using a Paar calorimeter (CEN TS 14918) are given

178 Waste Biomass Valor (2010) 1:177–189

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in Table 1. Morphologies of biochars were observed using

Scanning Electron Microscopy (SEM) (CaryScope

Jcm5700).

Solid-state 13C NMR experiments were carried out using

a VARIAN INOVA spectrometer at 13C and 1H frequen-

cies of 100.5 and 400.0 MHz, respectively. A JACKOB-

SEN 5 mm magic-angle spinning double-resonance probe

head was used for Varied-Amplitude Cross-Polarisation

(CP) experiments at spinning frequencies of 13 kHz.

Typical cross-polarisation times of 1 ms, acquisition times

of 13 ms, and recycle delays of 500 ms were used. The

cross-polarisation time was chosen after variable contact

time experiments, and the recycle delays in CP experi-

ments were chosen to be five times longer than the longest1H spin–lattice relaxation time (T1H) as determined by

inversion-recovery experiments.

Results

The yields of biochar obtained from the different sources

are given in Table 1. The pyrolysis temperature typically

changes the yield and properties of biochar. With

increasing temperature, the yield of biochar commonly

decreases but the carbon concentration increases [17]. That

trend is evident for the data in Table 1.

The data for Miscanthus (Table 1) show the very great

increase in surface area for the Miscanthus biochar as the

temperature and time of heating were increased. The SEM

pictures (Fig. 1) show that the plant cellular morphologies

were maintained in the biochars following heating of the

miscanthus and willow feedstocks for 10 min at 500�C.

The HHV data indicate the relative values as fuels of the

different biochar products. There were little differences in

the values for willow, miscanthus and pine biochars pro-

duced under the same conditions (at 500�C for 10 min). The

data for miscanthus indicate that the more ‘mature’ biochar

formed at 600�C for 60 min was a slightly better fuel.

The NMR spectrum of the miscanthus used as a feed-

stock for biochar production in this study is shown in

Fig. 2. This spectrum is similar to that obtained for

miscanthus by Brosse et al. [18] and is dominated by car-

bohydrate signals in the resonance region 60–110 ppm,

representing the contributions from cellulose and hemi-

celluloses, and the resonance at 105 ppm is assigned to the

anomeric C. Contributions from lignin are indicated by the

resonance at 56 ppm (methoxyl) and at 142–164 ppm

(O-aromatic) [5]. There are also some aliphatic contributions

with signals from alkyl C groups (0–46 ppm), probably

Table 1 Yields and some properties of biochars from different substrates and reaction conditions

Biochar source Willow Pine Miscanthus Miscanthus Miscanthus

Pyrolysis time, min 10 10 10 10 60

Temperature, �C 500 500 500 400 600

Yield of char, wg.% 25.0–26.2 22.2–22.5 25.9–26.2 29.4–31.2 19.8–20.2

Surface area, m2 g-1 1.41–1.55 1.93–2.15 1.65–1.95 1.4–1.7 50.9–51.1

HHV, MJ kg-1 31.0–31.5 30.4–31.3 29.9–30.7 29.4–30.3 31.5–32.5

C, wt.% 79.9 81.4 76.3 74.8 85.1

H, wt.% 3.34 3.38 4.26 4.33 2.40

N, wt.% 0.97 0.47 0.40 0.39 0.55

Fig. 1 SEM of A miscanthus biochar and B willow biochar

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attributable to cutin and waxes, and the 20–25 ppm can be

attributed to terminal methyl groups.13C NMR is one of the most promising techniques for

the determination and characterisation of biochar in soils

[19]. The 13C NMR spectra for the biochars formed from

miscanthus by heating at different temperatures and time

intervals (Fig. 3) are vastly different from that for the

feedstock. The spectral patterns show that the structural

compositions of the biochars change during pyrolysis. The

spectra for the miscanthus biochar at 400, 500 and 600�C

are directly comparable because the pyrolysis time was the

same (10 min) for both reactions. Both spectra show

significant aliphatic composition [3] at the 25–35 ppm

resonance, and for terminal methyl at *15 ppm. Raising

the pyrolysis temperature to greater than 400�C gave rise to

a loss of aliphatic-C moieties and a transformation of C

compounds to mostly poly-condensed aromatic-C type

structures dominated by a peak at 130 ppm [3]. There is

still some evidence in the spectrum at 400�C, and to a

lesser extent in that at 500�C for contributions from lignin

with a peak at 56 ppm indicative of methoxyl and a

shoulder at *150 ppm typical of the O-aromatic func-

tionalities in lignin. When the temperature was raised to

600�C the lignin signals disappeared. Increased heating

also resulted in a significant peak shift in the aryl region

from 131 to 126 ppm [3]. An up field shift of the aryl peak

to 126 ppm is typical of charred residues attributable to

polycyclic aromatic structures [5]. That indicates that fused

aromatic structures were formed with low H (see Table 1)

and O substituents [20]. These changes signify the trans-

formations of labile compounds into environmentally

recalcitrant forms. Recognisable fragments of feedstock

biopolymers (lignin signals) in the chars produced at 400

and 500�C would indicate that the reaction time was

insufficient for complete conversion to char. On the other

hand, the char produced at 600�C in 10 min and 60 min

contained no recognisable feedstock components, indicat-

ing complete conversion to char.

Biochar as a Fertiliser and Soil Ameliorant

By improving on existing best management practices,

biochar can play a major role in expanding options for

sustainable soil management. Biochar amendments will not

only improve soil productivity, but also decrease the

environmental impact of agriculture on soil and water

resources. Biochar should therefore not be seen as an

Fig. 2 13C DP/MAS NMR

spectrum of miscanthus

Fig. 3 13C DP/MAS NMR spectra of biochar resulting from the

pyrolysis of miscanthus at various temperatures and durations

180 Waste Biomass Valor (2010) 1:177–189

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alternative to existing soil management, but as a valuable

addition that facilitates the development of sustainable land

use [10]. Biochar can act as a soil conditioner and as a

fertiliser. It will enhance plant growth by supplying and

retaining nutrients, and by improving soil physical and

biological properties [8]. However, it has been reported

that biochar can have negative as well as positive effects on

plant growth.

Materials and Methods

Greenhouse Experiments

The preparation, and some properties of biochars, prepared

at 500�C for 10 min from willow, pine and miscanthus are

have been described, and relevant data are given in

Table 1. Biochars were mixed at rates of 1% and 5% with a

clay loam soil and cropped to maize (Zea mays L.). Six

maize seeds, steeped in water for 2 h, were planted per pot

(13 cm pot diameter). Eight replications for each experi-

ment were incubated in the greenhouse during May 2009.

Biochar was not added to the control. Pots were watered to

field capacity and allowed to drain. Subsequently the pots

were watered (50 cm3) every fourth day. After 10 days the

weakest three plants were removed from each pot. Whole

plant and leaf growth were monitored every 3 days.

Growth Chamber Experiments

A shallow calcareous brown earth loam soil (20% clay,

pH 4.9) from the Kinvara series in the Burren area

(N.Clare-S. Galway, Ireland) was amended with biochar

(5 wg.%). Two types of biochar were prepared from

miscanthus chips (max. size 1 cm) in a lab-scale pyrolyser

(1 dm3) at: (a), 400�C for 10 min; and (b), 600�C for

60 min. Ten maize (Zea mays L.) seeds were planted per

pot (13 cm pot diameter). Four replications for each

experiment were incubated in the growth chamber. The

pots were watered to field capacity and allowed to drain.

The system was incubated at 25�C in a growth chamber for

21 days and artificial light was provided for 13 h/day. The

pots were watered (50 cm3) every fourth day. After

10 days the weakest five plants were removed from each

pot. Twenty-one days after planting all plants were cut at

soil level, weighed, and oven dried at 60�C until a constant

weight was attained.

Microbial Experiments

Tissues of plant roots were examined to determine fungal

presence by staining with Chlorazol Black [21].

Results

Seedling emergence in the greenhouse was first observed

after 15 days in the cases of the controls; however, at the

same time plant stems were 10 cm in height and had three

leaves per stem in the biochar amended pots. The best

growth was in pots amended with 5 wt% miscanthus bio-

char (Fig. 4 and Table 2). Biochar from willow and pine

gave similar growth (though less than that for miscanthus).

The difference in growth in the control and char-amended

soils became less with time. Better growth was obtained for

higher amendments in the cases of the willow and pine

biochars, but even relatively small amendments (1 wt.%)

gave growth results similar to those at higher concentra-

tions in the cases of the miscanthus biochars. The green-

house experiments showed that the biochar amendments

had the best effects in the first stages of germination and

plant growth.

Microbiological analyses showed greater enrichments of

microorganisms on the root tissue of the plants grown in

soils amended with biochar. Most of the root surface from

soil amended with miscanthus biochar was covered with

large fungal colonies whereas colonies were barely visible

on roots from the control pots (Fig. 5).

When plant roots were taken from the control and char

amended soils it was seen that the roots proliferated where

biochar enrichments occurred (Fig. 5).

The data in Table 3 for the growth chamber experiment

indicate that favourable plant responses were not obtained

for all of the biochar samples. Best growth was obtained for

the biochar heated at 600�C for 60 min. The product from

heating for 10 min at 400�C suppressed plant growth

measured after 21 days. Material from the lower temper-

ature and shorter heating time had a lower surface area, and

a lower C content (indicating lesser transformation to sta-

ble biochar product) than for the product at the higher

temperature.

Fig. 4 Growth of plants in greenhouse experiments after 21 days in

soil amended with 1 and 5 wg.% biochar from willow, pine, and

miscanthus

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Discussion and Considerations of Implications

of Biochar Applications to Soil

It is important to establish the preparation criteria that give

rise to biochars that will have optimum value when added

to soil. Properties that will influence biochar applications

for promoting plant growth and the sequestration of carbon

in soil will include:

– nutrients content, and composition and availability;

– elemental composition, ash content, and volatility;

– capacity to adsorb a wide range of organic and

inorganic molecules;

– surface area, porosity, and particle size; and

– bulk density.

A classification system for biochar is needed for

potential users and researchers. Various characterisation

techniques can be utilised in order to establish a set of

standards needed to develop a classification system.

The two properties of biochar that are most important

for carbon sequestration and for soil amendment are: (1),

a high resistance to microbial degradation; and (2), an

ability to retain nutrients that will equal or surpass that of

other forms of soil organic matter [1]. The ability of

biochar to store C and improve soil fertility will depend

on its physical and chemical properties. Properties of

biochar relevant to soil fertility and carbon sequestration

can be varied in the pyrolysis process and through the

choice of feedstock [3].

The major components of biomass are cellulose, hemi-

cellulose and lignin. The proportions of these three com-

ponents determine the ratios of volatile carbon (in bio-oil

and gas) and stabilised carbon (biochar) in the pyrolysis

products. Fast pyrolysis favours feedstocks with high cel-

lulose and hemicellulose contents as bio-oil and gas are the

main products, and with lesser yields of biochar. Feed-

stocks with high lignin contents produce the highest bio-

char yields when pyrolysed at moderate temperatures

(approx. 500�C) [17, 22, 23]. Slow pyrolysis yields high

quantities of biochar, and therefore favours feedstocks with

high lignin contents. In the future, selection of feedstock

may be dictated by the desired balance between pyrolysis

Table 2 Growth of maize seedlings (as a percentage of the controls where biochar was not added) in soils amended with biochars (5 wt.%) in

greenhouse experiments

Biochar source Control Willow Pine Miscanthus

Yield after 21 days (as growth % of control) 100 (±9.12) 393 (±33.9) 402 (±32.2) 437 (±40.3)

Yield after 28 days (as growth % of control) 100 (±6.51) 128 (±9.82) 135 (±9.41) 153 (±11.9)

Biochar was made for 10 min in 500�C. (±standard deviation)

Fig. 5 Root tissues A covered

with fungal colonies and B roots

from control pots

Table 3 Mass of maize seedlings after 21 days of growth (as a

percentage of the control where biochar was not added) from soils

amended with miscanthus biochar (3 wt.%) (±standard deviation)

Preparation method Control 400�C, 10 min 600�C, 60 min

Yield of dry matter

(as wt.% of control)

100 (±16.1) 76.6 (±7.00) 165 (±19.8)

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products (gas, oil and biochar), and whether or not slow

pyrolysis is being used [11].

Solid state 13C NMR provides an excellent tool for the

characterization of biochars. The spectra in Fig. 3 allow

determination of the optimum conditions for the prepara-

tion of chars with different compositions. These spectra

and the elemental analyses data (Table 1) give indications

of the extents to which the biomass feedstocks are con-

verted to the fused aromatic structures. Such structural

arrangements may be considered to be optimum for long

term C sequestration, and may be an ideal biochar char-

acteristic for the sequestration of soil C for millennia.

Biochars produced at higher temperatures do not provide

ion exchange functional groups due to dehydration and

decarboxylation. That would of course limit its usefulness

in retaining soil nutrients by ion exchange mechanisms.

As seen in Table 1, biochars produced at the higher

temperatures have high surface areas, and will have high

porosities. Such porosity will allow the retention of water,

enhance the soil sorption properties [24–27], and provide

refuge for microorganisms.

Earlier studies have shown that biochar with appropriate

surface areas and pore sizes will provide a refuge for

Arbuscular mycorrhizal fungi (AMF) [13]. The addition of

biochar to soils has been found to promote higher coloni-

zation rates of host plant roots by AMF [28]. These fungi

form a symbiotic association with plant roots, effectively

extending the roots and enabling the uptake of additional

plant nutrients. In return the plant provides the organic

energy that the fungi need. Recent research suggests that

AMF might be an important component of the soil organic

pool, in addition to facilitating carbon sequestration by

stabilizing soil aggregates [29]. Glomalin, a glycoprotein

containing 30-40% carbon, is produced by all AMF [30]. It

has been found to be involved in the aggregation and sta-

bilization of soil particles.

Higher nutrient retention and nutrient availability

properties have been found after charcoal additions to soil

[24]. These properties could arise from the oxidation of

peripheral aromatic groups. However, that would be a long

term effect. It is reasonable to conclude that the prolifer-

ation of microbial species in the environs of biochar will

give rise to significant amounts of biomass which will be

transformed to humic substances with high ion exchange

properties.

Significance of Biochar for the Sequestration of Carbon

The global carbon cycle is made up of flows and pools of

carbon in the Earth’s system. The important pools of car-

bon are terrestrial, atmospheric, ocean, and geological. The

carbon within these pools has varying lifetimes, and flows

take place between them all. Carbon in the active carbon

pool moves rapidly between pools [1]. In order to decrease

carbon in the atmosphere, it is necessary to move it into a

passive pool containing stable or inert carbon. Biochar

provides a facile flow of carbon from the active pool to the

passive pool.

Biomass (non biochar) materials have varying degrees

of resistance to biodegradation to CO2 and water. The more

readily biodegradable components of the biomass decom-

pose quickly over one to 5 years, the more stable compo-

nents transform over decades to centuries, and the most

refractory components can last for several centuries to a

few thousand years. The organic matter in long term

grassland or forest reaches a steady state that will not

change substantially until the management is changed.

Then there will be a slow decline in the organic C content

until eventually a much lower steady state is reached [31].

The carbon released in that way must be regarded as fossil

carbon. Additions of organic matter during cultivation will

lessen the losses of indigenous organic matter. However, it

is estimated that less than 10–20% of the added carbon will

remain in agricultural soils after 5–10 years (depending on

the carbon quality and the environment) [8]. The net result

is that the bulk of carbon added as biomass is rapidly

released back to the atmosphere as CO2.

Adding biochar to soils can be regarded as a means of

sequestering atmospheric CO2 [10]. The conversion by

pyrolysis of biomass organic matter to biochar significantly

increases the recalcitrance of the carbon in biomass.

Because biochar is inherently stable in soil, applications of

biochar inevitably gives rise to far more soil C sequestra-

tion than would result from applications of biomass of

similar C contents [8]. Biochar can thus be considered to

harness the natural carbon cycle.

About four times more organic carbon is stored in the

Earth’s soil than in atmospheric CO2. The annual uptake of

CO2 by plants through photosynthesis, 120 Gt CO2–C

[32], is 8 times greater than anthropogenic greenhouse gas

(GHG) emissions at this time [10]. Switching even a small

amount of the carbon cycling between the atmosphere and

plants into a much slower biochar cycle would make a

worthwhile difference to atmospheric CO2 concentrations.

It is estimated, for example, that diverting as little as 1% of

net annual plant uptake into biochar would mitigate almost

10% of current anthropogenic carbon emissions [10]. That

does not take into consideration the additional multiplier

effects that lock up additional carbon, such as increased

plant growth, increased root biomass, and increased soil

microbial biomass.

Based on what has been stated above, for biochar to act

as a carbon sink it must remain in the soil for a very long

time and it must have a high level of resistance to bio-

logical and chemical degradation. Radiocarbon dating of

TP soils indicate origins of 500 to 7,000 years BP [33, 34].

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Mean residence times of 1300–2600 years for biochar from

Australian savannah soils were estimated by matching

annual production of biochar from the savannah soils with

measured carbon stocks for soils in Northern Australian

woodlands [35]. At a mean annual temperature of 10�C the

mean residence time of biochars was estimated to be

1,335 years [36]. Although there is some uncertainty about

the decomposition rates of biochar in soils, the great age of

biochars found in soil studies and in archaeological sites

provides evidence for its stability [34].

Biochar creates a real increase in terrestrial carbon

stocks because it gives rise to a stable form of biomass

carbon and significantly delays its decomposition [37].

Lehmann [38] estimates that 20% of all the carbon in

biomass could be captured by conversion to biochar.

Theoretically that could amount to 24 9 109 tonnes of

carbon, or 88 9 109 tonnes of CO2 annually. However, in

practical terms much of that biomass would not be avail-

able for conversion to biochar and the actual potential

would depend on the ability to access biomass feedstocks

in an economically viable and environmentally responsible

manner [37]. It is estimated that 10% of the annual US

fossil-fuel emissions could be captured if biochar was

produced using existing forest residues, or crop wastes, or

fast growing biomass established on idle cropland [38].

As already mentioned, the ability of biochar to store

carbon and improve soil fertility will depend on its physical

and chemical properties, and these can be varied in the

pyrolysis process, or through the choice of feedstock.

Hayes et al. [39] reviewed the potential feedstocks avail-

able in Ireland for biorefining technologies. These feed-

stocks include wastes and biomass residues (biodegradable

municipal solid waste (MSW), poultry litter, spent mush-

room compost (SMC), straws, forest residues, and sawmill

residues that potentially could be utilised for biochar

production.

Converting biomass carbon to biochar carbon enables up

to 50% of the initial carbon to be sequestered in a relatively

stable form. The proportion of the carbon in the biomass

which is converted to biochar is highly dependent on the

type of feedstock [8]. Downie et al. [40] examined biochar

production from different feedstocks (wastes, sludge and

wood) and estimated a 63% retrieval of the biomass carbon

in the biochar. These values, along with measurements

made in the laboratory, were used to estimate the potential

carbon sequestration of biochar produced from the avail-

able wastes and residues in Ireland (Fig. 6). These calcu-

lations do not evaluate emissions associated with the whole

life cycle of biochar production but they aim to give an

insight into the potential of biochar. Approximately 2.4%

of Ireland’s 2007 GHG emissions, 1.67 Mt of CO2, could

be sequestered by the biochar produced from slow pyro-

lysis of wastes and residues available under Scenario 1.

That uses the minimum available straw resources, the

minimum collectable forest residues, all available biode-

gradable MSW, poultry litter, SMC, and the available

sawmill residues. In Scenario 2, when all the maximum

available straw resources for energy purposes and the

maximum collectable forest residues are included for bio-

char production, the amount of CO2–eq that can be

sequestered increases to 2.25 Mt of CO2 or 3.25% of the

total GHG emitted in 2007. When all straw resources,

including those currently used for animal bedding and soil

amendments, are utilised, with the maximum collectable

forest residues, Scenario 3 becomes operational. The

resulting biochar would enable the sequestration of

2.79 Mt of CO2 or 4.03% of the total GHG (CO2–eq)

emitted in 2007.

Biochar for a More Appropriate Utilisation of Waste

Wastes and residues from animal, crop, and forest pro-

duction are valuable resources that can be used directly and

indirectly for energy, including pyrolysis energy and bio-

char [41]. In addition to providing energy and biochar, the

pyrolysis process will decrease the volume and the weight

of the waste material [10]. Many waste streams offer

economic opportunities for energy recovery particularly

when a significant reliable source of feedstock is generated

at a specific location. Biochar as a waste management

strategy decreases GHG emissions associated with tradi-

tional strategies. Land filling of organic waste, e.g. garden

waste, results in the release of significant quantities of

methane. Anaerobic digestion of animal wastes release

methane and nitrous oxide, and these gases are 25 and 298

times, respectively, more potent as GHGs than CO2.

Fig. 6 The potential percentage of Irish 2007 GHG emissions that

could be sequestered by the biochar produced from slow pyrolysis of

varying amounts of available wastes and residues

184 Waste Biomass Valor (2010) 1:177–189

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Management strategies that avoid these emissions can

therefore contribute significantly to mitigation of climate

change [37].

The production of biochar rather than composting is

more effective in locking up carbon. Carbon contained in

compost will be released by microbial transformations

within 10–20 years [37]. In contrast, the carbon seques-

tered in the pyrolysed wastes would be stable in the soil,

and also simultaneous emissions of GHGs, such as meth-

ane, would be decreased [27].

The combination of biorefining and pyrolysis can pro-

vide profitable industries that will decrease waste disposal

costs and provide cost effective energy services that profit

agriculture and industry [42]. Substrates with significant

carbohydrate contents, including the lignocellulose com-

ponents of straws, waste wood and forestry thinnings,

provide appropriate materials for biorefining operations.

One example is the production by acid hydrolysis under

conditions of high temperature and pressure of levulinic

acid, furfural, and formic acid from the hexose components

and furfural from the pentoses of hemicelluloses of bio-

mass and of lignocellulosic and other waste materials.

These components provide excellent platform chemicals

for the production of a wide variety of products, including

liquid fuels and fuel additives such as ethyllevulinate,

gammavalerolactone (GVL), and methyltetrahydrofuran

(MTHF) [43]. In the cases of lignocellulose materials the

residuals from the biorefining process will amount to about

50% of the starting materials. These can be pyrolysed to

give syngas, bio-oil, and biochar.

Biochar and Decreases in Nitrous Oxide and Methane

Emissions from Soil

There are limited studies to indicate that biochar can sig-

nificantly decrease nitrous oxide and methane emissions

from soil. In 2004 agriculture contributed 42% of the total

of the 8% GHG emissions attributable nitrous oxide [44].

Nitrogen fertilisers, organic fertilisers, nitrogen fixation by

bacteria, the urine and the excreta of grazing animals are

the major sources of nitrous oxide emissions from soil.

Methane made up about 14% of global GHG emissions in

2004 [44], and methane emissions from soil are associated

with anaerobic conditions, typical of those found in wet-

lands, paddy fields, and landfills.

Additions of biochar to the soil have been shown to

decrease nitrous oxide emissions [45], and some by up to

50% [46]. Biochars from municipal biowaste also caused a

decrease in emissions of nitrous oxide in laboratory

soil chambers [47]. Additions of 15 g kg-1 of soil to a

grass sward and 30 g kg-1 of soil to a soil cropped with -

soybeans completely suppressed methane emissions [46].

Such information emphasises the need for further stud-

ies to aid the development of biochar as a tool for

decreasing non-CO2 GHG emissions from soil. More

research is needed to understand the interactions between

biochar, site specific soil, climatic conditions, and man-

agement practises that alter the sink capacity of soils.

In addition to climate change mitigation, additional

complementary and often synergistic objectives may

motivate biochar applications for environmental manage-

ment. Soil improvement, waste management, and energy

production, will individually, or in combination have either

a social or a financial benefit, or both [10].

Role of Biochar in Decreasing Atmospheric CO2

and Fertiliser Requirements

The increased plant growth from the fertiliser effect will

increase carbon sequestration in the growing biomass and

the organic carbon in soil as the result of increased root

growth and microbial proliferations, and increased addi-

tions to soil of plant residues. Because biochar increases

the soil’s nutrient retention capacity, it can increase the

efficiency with which fertiliser is used. This will decrease

emissions from artificial fertiliser, both in terms of those

associated with the manufacture and the uses of fertilisers.

Research has demonstrated that the use of biochar plus

chemical amendments (N, P, K fertiliser and lime) on

average doubled grain yield over four harvests compared

with the use of fertiliser alone [48]. Another study found

that biochar plus chemical fertiliser increased growth of

winter wheat and several vegetable crops by 25–50%

compared with chemical fertilization alone [49]. Lehmann

and Joseph [50] have presented a case study that examines

the production of biochar from poultry litter in West Vir-

ginia, USA. Biochar was sold for US $480 per tonne at the

farm gate for application to soybean crops and hay fields.

Since the poultry litter biochar is rich in nutrients, 20% less

nitrogen fertiliser was required, and no applications of

phosphorus and potassium were needed. Such savings

increase the economic value of biochar. For example, in

Ireland approximately 320 k tonnes of nitrogen in syn-

thetic fertiliser is used annually [51]. The average price of

one tonne of nitrogen is approximately €1,800 (based on

calculations from [52]), and so the annual spend on syn-

thetic nitrogen fertiliser in Ireland amounts to €576 million.

However, poultry litter biochar could provide up to 20% of

the total amount of nitrogen fertiliser needed saving

farmers €115 million annually. Biochar could also be for-

tified with nitrogen, for example in a composting step, by

mixing with poultry litter. It will be important to carry out a

detailed study of the availability to plant needs of the

nutrients in biochar. In the making of biochar, nitrogen

may be sorbed to the biochar in an available form or

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immobilized in the heterocyclic structures characteristic of

biochar. Detailed 15N NMR studies are needed to fully

ascertain the nature of the nitrogen associated with biochar,

and hence its nutrient capabilities. The limited evidence we

have suggests that biochar will prevent the leaching of

nutrients, and allow these to be available to plants.

The fundamental mechanisms of biochar functions in

soil are poorly resolved and consequently our abilities to

predict the different roles that biochar can have in the soil

environment are inadequate [27]. Research is also needed

to determine the most suitable biochars for different soil

applications, and the most suitable application rates. For

example, some studies [53, 54] have shown that additions

of biochar can temporarily decrease the soil fertility. One

such study found that amendment of soil with biochar may

in some situations lead to low nitrogen availability to crops

[53] while another study [54] found that it increased

nitrification.

Modern agriculture is dependent on phosphorus (P)

derived from phosphate rock. This is a non-renewable

resource and current global reserves may be depleted in

50–100 years. While P demand is projected to increase, the

expected global peak in P production is predicted to occur

around 2030. Although the exact timing of peak P pro-

duction might be disputed, it is widely acknowledged

within the fertiliser industry that the quality of the

remaining phosphate rock is decreasing and production

costs are increasing [55]. Thus it is important to optimise

the use of P. Several studies have demonstrated enhanced P

uptake by plants in the presence of biochar but very little

work has focused on the variety of mechanisms through

which biochar may directly or indirectly influence the

phosphorus cycle [56]. Pyrolysis can greatly enhance P

availability by breaking bonds resulting in soluble phos-

phorus salts associated with the biochar. In addition, bio-

char modifies soil pH and exchange capacity and the

general consensus is that it makes phosphorus more

available in the soil.

It is claimed that biochar might be fortified with nitro-

gen in an energy production process by flue gas scrubbing

[25]. Should that be so significant environmental benefits

could arise for such applications of biochar in power

generation.

Biochar for Energy Production

Biochar products are often gasified to provide energy, or

used in high value products such as activated carbon [11,

57]. The addition of biochar to soil, instead of using it as a

fuel, decreases the energy efficiency of pyrolysis bioenergy

production. However, the decreases in the emissions

associated with biochar additions to soil would be greater

than those that would be attained had the biochar been used

as a fuel [13]. Biochar can contribute significantly to

securing a future supply of green energy [10] because it can

effectively retain nutrients and promote an environment in

the soil that will enhance plant growth.

Role of Biochar in Waste Management

In the future materials that are now regarded as organic

wastes are likely to be seen as resources for biorefining and

bioenergy initiatives. In many agricultural and forestry

production systems, waste is produced in significant

amounts from crop residues such as: (i), forest residues

(logging residues, dead wood, excess saplings, pole trees);

(ii), mill residues (lumber, pulp, veneers); (iii), field crop

residues; or (iv), urban wastes (yard trimmings, site

clearing, pallets, wood packaging) [10]. Animal and crop

wastes, for example, can present significant environmental

burdens that lead to the pollution of ground and surface

waters. Thus, strategies that will utilise such materials are

ecologically and economically attractive. All of these res-

idues contain significant amounts of carbohydrate that

provide the raw materials for biorefining processes [43],

and those with high lignin contents will enhance the yields

of residuals that, on pyrolysis can be expected to yield

syngas, bio-oil, and high yields of biochar [10, 41, 58, 59].

Such wastes can provide a significant reliable source of

feedstock generated at a single point location [60] thereby

decreasing the energy used in transportation, and decreas-

ing methane emissions that would result when such mate-

rials are landfilled.

Long Term Prospects for Biochar and Issues

to be Addressed

Biochar can be regarded as an important ‘wedge’, con-

tributing to an overall portfolio of strategies for climate

change [10]. However, considerations must be given to

uses for biorefining and for biochar production of biomass

resources that would otherwise fulfil ecosystem services or

human needs. Possible conflicts have to be considered

between producing energy and biochar versus food as a

consequence of any future adoption on a large scale of

biochar technologies [61]. Such considerations need not

apply when wastes and non-food biomass crops and resi-

dues are used.

Biochar has the potential to have enormous economic

and environmental benefits in comparison to current con-

ventional methods for the utilisation of biomass and waste

materials, such as combustion, co-firing of biomass, land

filling, and incineration in the case of wastes. Biochar

should be seen as a part of an integral approach to the

utilisation of biomass and waste materials and it will have

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an important role in the emerging second generation bior-

efining processes. In these processes, acid or enzymatic

hydrolysis will give rise to platform chemicals and bio-

refinary residuals. The biorefinary residuals, when sub-

jected to pyrolysis will give biochar, bio-oil and gas, all of

which have economic and environmental value. It may not

be profitable to grow biomass merely for the production of

pyrolysis products. However, biomass for second genera-

tion biorefining, and the pyrolysis of the biorefinery

residuals would appear to have a viable economic future.

Technological issues for biochar production need to be

resolved, such as the refining of methods for the production

of biochar from different biomass and waste feedstocks, the

transportation of biochar and its application to soil,

avoiding unacceptable dust formation, environmental, or

health hazards [10]. Most products used in agriculture and

in the industrial sector must conform to a standard, and it

would be highly advantageous to develop a single frame-

work for the assessment and comparison of different

biochar scenarios for their net carbon benefit and socio-

economic impacts [11]. The multitude of possible factors

influencing biochar properties and the array of changes in

relevant properties make it necessary to have an acceptable

classification of biochars [62]. Not all feedstocks are

equally attractive for biochar production. A ranking of the

feedstocks in terms of their suitability for biochar pro-

duction and guidelines for production of the biochars

would aid proper planning of feedstock utilisation and the

development of a biochar industry.

Conclusions

There are many reasons why biochar technology should be

promoted. Biochar has significant potential as a tool for

climate change mitigation because it can increase stable

soil carbon stocks and increase soil carbon sequestration,

while also decreasing atmospheric CO2 concentrations. In

addition to sequestering carbon in the soil, biochar has the

potential to act as a fertiliser and as a soil improver. The

experimental results presented here have shown that bio-

char amendments significantly influenced seed germination

and the subsequent plant growth. Conclusive mechanisms

for the growth promoting or inhibiting effects of the bi-

ochars cannot be put forward at this stage. Elemental

analyses indicate that biochar per se is unlikely to provide

the nutrients needed for plant growth, and there is lack of

information about the associations of plant nutrients with

biochar that are available for plants. There is abundant

evidence to suggest prolific microbial associations with soil

biochar, and the biomass from these will transform to soil

humic substances whose nutrient holding capacities are

well recognised.

The fact that seed germination and plant growth pro-

motion was effectively the same for applications of 1 wt%

of miscanthus biochar compared to 5 wt% would suggest

that the effects were not concentration dependent, and may

well arise from growth promoting (hormonal) substances

sorbed on the biochar. However, experimental results

presented here have shown that, depending on the prepa-

ration conditions, biochar can inhibit as well as promote

plant growth. Thus it will be important to determine to

what extents the growth promotion or inhibition effects are

attributable to sorbed materials and to structural properties

such as surface areas, pore structures, and microbial

associations.

It has become clear that sets of standards will be needed

with regard to acceptable compositions of biochars that can

be used when the products are added to soils. These stan-

dards should define the process conditions that will give

rise to products from different substrates that will provide

biochars that will endure in the soil environment and

benefit plant growth.

Acknowledgements We acknowledge financial support of Science

Foundation Ireland under Grant Number 06/CP/E007, and Geof 833,

Enterprise Ireland under ‘‘Competence Centre for Biorefining and

Bioenergy’’ Grant Number CC/2009/1305C, and the European

Community’s Seventh Framework Programme (FP7/2007-2013)

under grant agreement no: 227248.

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