2012 pyrolysis for biochar purposes a review to establish current knowledge gaps and research needs

Environmental Science & Technology is published by the American ChemicalSociety. 1155 Sixteenth Street N.W., Washington, DC 20036Published by American Chemical Society. Copyright American Chemical Society.However, no copyright claim is made to original U.S. Government works, or worksproduced by employees of any Commonwealth realm Crown government in thecourse of their duties.

Critical ReviewPyrolysis for biochar purposes: a review to establish

current knowledge gaps and research needs.Joan Josep Many

Environ. Sci. Technol., Just Accepted Manuscript DOI: 10.1021/es301029g Publication Date (Web): 09 Jul 2012Downloaded from http://pubs.acs.org on July 10, 2012

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Pyrolysis for biochar purposes: a review to establish

current knowledge gaps and research needs

Joan J. Many

Thermo-chemical Processes Group (GPT), Aragn Institute of Engineering Research (I3A),

University of Zaragoza, Technological College of Huesca, crta. Cuarte s/n, E-22071 Spain.

E-mail address: [email protected]

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ABSTRACT

According to the International Biochar Initiative (IBI), biochar is a charcoal which can be applied to

soil for both agricultural and environmental gains. Biochar technology seems to have a very

promising future. Nevertheless, the further development of this technology requires continuing

research. The present paper provides an updated review on two subjects: the available alternatives to

produce biochar from a biomass feedstock and the effect of biochar addition to agricultural soils on

soil properties and fertility. A high number of previous studies have highlighted the benefit of using

biochar in terms of mitigating global warning (through carbon sequestration) and as a strategy to

manage soil processes and functions. Nevertheless, the relationship between biochar properties

(mainly physical properties and chemical functionalities on surface) and its applicability as a soil

amendment is still unclear and does not allow the establishment of the appropriate process

conditions to produce a biochar with desired characteristics. For this reason, it is highlighted the

need of enhancing the collaboration among researchers working in different fields of study:

production and characterization of biochar on one hand, and on the other, measurement of both

environmental and agronomical benefits linked to the addition of biochar to agricultural soils. In this

sense, when experimental results concerning the effect of the addition of biochar to a given soil on

crop yields and/or soil properties are published, details regarding the properties of the used biochar

should be well reported. The inclusion of this valuable information seems to be essential in order to

establish the appropriate process conditions to produce a biochar with more suitable characteristics.

Keywords: Biochar; Pyrolysis; Soil fertility; Carbon sequestration.

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TOC/Abstract art

Biomass Slow Pyrolysis

Pyrolysis gas

Liquid fraction

Charcoal (40-60% yield) BIOCHAR USE

Operating conditions

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1. INTRODUCTION

Concerns about climate change and food productivity have recently generated interest in biochar,

a form of charred organic matter which is applied to soil in a deliberate manner as a means of

potentially improving soil productivity and carbon sequestration.1 The idea of adding charcoal to

soil in order to increase its fertility is to be inspired by the ancient agricultural practices, by means of

which terra preta soils were created.2 These soils, which may occupy up to 10% of Amazonia,3 are

characterized by high levels of soil fertility compared to other soils where no organic carbon

addition occurred. Besides the potential of biochar to enhance the fertility of agricultural soils, its

apparent ability to increase the capacity of soil to retain water makes biochar a very promising

alternative in the current context of climate uncertainty.

A high number of recent studies have highlighted the benefit of using biochar in terms of

mitigating global warming and as a strategy to manage soil health and productivity.48 In the most of

cases, these studies are constrained by limited experimental data and are geographically limited.

This fact can be considered as expected because the complexity of the experimental tasks.

Biochar can be produced by several thermochemical processes: conventional carbonization or

slow pyrolysis, fast pyrolysis, flash carbonization, and gasification. Slow pyrolysis has the

advantage that can retain up to 50% of the feedstock carbon in stable biochar.8 Biomass pyrolysis

and gasification are well-known technologies for the production of biofuels and syngas. However,

commercial exploitation of biochar as a soil amendment is still in its infancy.2 Pyrolysis process and

its parameters (principally final temperature, heating rate, pressure, and residence time at the final

temperature) greatly condition the biochar production and quality. In addition to this, the intrinsic

nature of the biomass feedstock also interacts with the rest of variables in determining the properties

of the produced biochar.9,10

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The relationship between biochar properties and its potential to enhance agricultural soils is still

unclear and does not allow the establishment of the appropriate process conditions in order to

produce a biochar with desired characteristics.11 Several recent studies have been focused on

providing a characterization methodology of biochars.1114 These studies represent an initial step,

but further efforts are needed to perform soil tests in order to establish an appropriate formulation of

desired biochar properties.

The specific aim of the present study is to review and analyze the available published studies

related to biochar production, characterization, and its addition into agricultural soils. As a result of

this review process, the objective of the author is to highlight the research needs for this exciting

field of study. Among other potential research gaps, this paper focuses on the interaction between

biochar production and its potential applicability to agricultural soils. In this sense, the knowledge of

the effect of the operating conditions governing the pyrolysis process on the properties of the

resulting biochar (degree of aromaticity, cation exchange capacity...) for a given biomass feedstock,

seems to be necessary to facilitate future research on this topic.

2. THE BIOCHAR CONCEPT

Biochar is a carbon-rich, fine-grained, porous substance; which is produced by thermal

decomposition of biomass under oxygen-limited conditions and at relatively low temperatures (<

700 C).1,2 The definition adopted by the International Biochar Initiative (IBI) furthermore specifies

the need for purposeful application of this material to soil for both agricultural and environmental

gains.2 This fact distinguishes biochar from charcoal, which is used as a fuel for heat, as an

adsorbent material, or as a reducing agent in metallurgical processes.1

One of the interesting properties of biochar, that makes it attractive as a soil amendment, is its

porous structure, which is believed responsible for improved water retention and increased soil

surface area.2 Furthermore, the addition of biochar to soil has been associated with an increase of the

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nutrient use efficiency, either through nutrients contained in biochar or through physico-chemical

processes that allow better utilization of soil-inherent or fertilizer-derived nutrients.2 In addition to

the above-mentioned potentially beneficial effects, a key property of biochar is its apparent

biological and chemical stability. In fact, studies of charcoal from natural fire and ancient

anthropogenic activity indicate millennial-scale stability.1516 This property can allow biochar to act

as a carbon sink.2

According to the above-explained considerations, the conversion of biomass to long-term stable

soil carbon species can result in a long-term carbon sink, as the biomass removes atmospheric

carbon dioxide through photosynthesis.17 For this reason, the use of biochar can imply a net removal

of carbon from the atmosphere.1 Furthermore, three complementary goals can be achieved by using

biochar applications for environmental management: soil improvement (from both productivity and

pollution points of view), waste valorization (if waste biomass is used for this purpose), and energy

production (if energy is captured during the biochar production process). In light of this, the

production of biochar from agricultural residues and/or forest biomass appears to be a very

promising alternative to integrate carbon sequestration measures and renewable energy generation

into conventional agricultural production.17

3. BIOCHAR PRODUCTION

Biochar can be produced as a co-product from several different processes. The properties of a

given biochar strongly depend on each process characteristics and also on the material to which the

process is applied. In the next sections, several technologies currently in use or under development

are reviewed.

Slow pyrolysis

Conventional carbonization or slow pyrolysis processes, in which a relatively long vapor

residence time and a low heating rate are the key process parameters, have been used to generate

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charcoal from many years ago.18 As a result of many relatively recent studies focused on increasing

charcoal yields,9,10,1925 several variables and factors that play a critical role during the pyrolysis

process have been identified; among these are peak temperature, pressure, vapor residence time, and

moisture content.19

The peak temperature is the highest temperature reached during the process.19 As a general rule,

the charcoal yield decreases as temperature increases. However, an increase of the peak temperature

results in an increase of the fixed-carbon content in biochar.19,26,27 This increase is especially

pronounced in the temperature range from 300 to 500 C. In addition, the peak temperature has

influence on surface area and pore size distribution (both properties generally related to specific

adsorptive properties) of charcoals. Khalil28 reported very low surface areas for charcoals (from a

wide variety of biomass feedstocks) pyrolyzed at temperatures near 550 C. However, setting peak

temperatures higher than 700 C does not seem appropriate to generate charcoals with potentially

better adsorptive properties.2931

Pyrolysis or carbonization at elevated pressure (1.03.0 MPa) seems to improve the charcoal yield

as a consequence of the increase of the vapor residence time within the solid particle. This effect,

which results in a substantial increase of the secondary charcoal production (as a consequence of the

decomposition of vapors onto the solid carbonaceous matrix), is magnified when the gas flow

through the particle bed is small.19 Furthermore, it should be kept in mind that the energy demand of

the pyrolysis process is closely related to the production of charcoal by primary (endothermic) and

secondary (exothermic) reactions.20,32 In line with this, an increase of the charcoal produced by

secondary reactions can significantly reduce the amount of energy required to sustain the process.

Pyrolysis pressure also produces an effect on the porosity of produced charcoals. Cetin and co-

workers33 reported a slight decrease of the total surface area by increasing pressure during the

pyrolysis of several biomass feedstocks (radiata pine, eucalyptus wood, and sugarcane bagasse).

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However, in a recent study conducted by Melligan and co-workers,34 a dramatic decrease of the

BET (Brunauer, Emmett and Teller) surface area of charcoals obtained by slow pyrolysis (at 13 K

min1 and at a peak temperature of 550 C) of miscanthus is reported (from 161.7 m2 g1 at 0.1 MPa

to 0.137 m2 g1 at 2.6 MPa). The authors attributed this result to a clogging of the pores by tar

deposits as a consequence of the high pressure. In addition to this, Melligan and co-workers also

reported that chars formed at high pressure had more extended fused aromatic structures, reflected

also in the higher carbon contents, than those obtained at atmospheric pressure.

Regarding the moisture content of the biomass feedstock, results obtained in previous studies2035

indicated that high moisture contents (in the range of 4262%) can improve the yield of charcoal at

elevated pressures. This finding makes certain agricultural residues, which are characterized by high

moisture contents, particularly attractive for biochar purposes. In addition to the moisture effect, it

must also be taken into account that the charcoal yield from a given biomass feedstock is influenced

by its inherent composition (holocellulose, lignin, extractives, and inorganic matter). In this sense,

pyrolysis of biomass species with high lignin contents can produce higher charcoal yields.19,36 An

increase of the charcoal production was also observed by Di Blasi and co-workers37 when pyrolyzed

extractive-rich woods (e.g., chestnut) instead of another wood varieties with lower extractives

contents (e.g., beech).

Special attention has been focused on discussing the influence of the inorganic matter on pyrolysis

product distribution. During biomass pyrolysis, inorganic matter, especially alkali and alkali earth

metals, catalyses biomass decomposition and char forming reactions.38 Several researchers have

obtained lower charcoal yields when the biomass feedstock was pre-treated with hot water (at 80 C)

as a measure to reduce the ash content.3943

Additional process variables that might affect charcoal yields are the soak time at peak

temperature and the particle size. Regarding the first one, Antal and Gronli19 stated that the soaking

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time has no effect on the charcoal yield because pyrolysis kinetics is primarily governed by

temperature. This assumption is consistent with experimental data reported in several research

studies.4445 Concerning the effect of the particle size on the charcoal yield, it seems reasonable to

assume that an increase in particle size leads to higher charcoal yields. As the particle size is greater,

the rate of diffusion of the volatiles through the char decreases and, consequently, the formation of

additional char by means of secondary reactions should be expected.9,10,23

Table 1 reports several experimental data from the literature for slow pyrolysis of different

biomass feedstocks under various operating conditions.24,27,46,47 A qualitative examination of the

data reported in Table 1 appears to confirm the effects of some variables (peak temperature,

pressure, and both moisture and lignin contents) on the charcoal yield. In this sense, and in

agreement with the considerations previously mentioned in this section, the charcoal yield is favored

by increasing pressure and/or decreasing peak temperature. Moreover, greater charcoal yields were

obtained, under identical operating conditions, for biomass samples with high moisture and/or lignin

contents.

To reach a more conclusive interpretation of the experimental data showed in Table 1, normalized

principal components analysis (NPCA) was applied to the same data using the Rcmdr package in R

(version 2.14.2). NPCA is a robust statistical technique, the purpose of which is to reduce the

complexity of the multivariate data into the principal components space and then choose the first

principal components that explain most of the variation in the original variables.10,42,48 The

following variables, the values of which are listed in Table 1, were selected for the principal

component analysis: pressure, peak temperature, heating rate, soaking time, lignin content, moisture

content, and ash content. Figure 1 shows both score and loading plots obtained for the three first

principal components (derived from the correlation matrix), which explained 67% of the total

variance. From the analysis of the results displayed in Figure 1, some considerations can be

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outlined: (a), the first principal component is partially due to pressure (data points with a large x-

coordinate correspond to experiments performed under elevated pressure); (b); pressure and char

yield are highly correlated, as can be seen from the loading plot (Figure 1a) (c), it seems that the

peak temperature is associated with the second principal component because the value of the y-

coordinate increases as the peak temperature raises; (d), the second principal component is also

partially explained by soaking time, but no correlation between this variable and char yield is

observed; (e) the third principal component seems to be related to the intrinsic properties of the

biomass feedstock (in this case, lignin and moisture contents).

As a preliminary conclusion, it can be stated that predicting the charcoal yield as a function of

both operating conditions and biomass properties is still difficult despite the large number of studies

published to date. It seems clear that increasing pressure and decreasing peak temperature enhances

the yield of charcoal. Nevertheless, the effect of the intrinsic properties (holocellulose and lignin

contents, moisture, amount and composition of the mineral matter) of the biomass feedstock on

the charcoal yield (as well as on the chemical and textural characteristics of produced charcoals) is

critical and needs to be experimentally determined.

Alternative processes

Fast Pyrolysis

Fast pyrolysis uses high heating rate (above 200 K min1) and short vapor residence time (around

2 s). The peak temperature is usually set between 500 and 550 C in order to obtain the highest bio-

oil yield.4952 These operating conditions particularly favor the formation of liquid products (bio-

oil), but inhibit the formation of charcoal.18 Duman and co-workers,53 in a recent study, compare the

charcoal yields from both cherry seeds and cherry seed shells obtained using two pyrolysis

processes: a fixed bed reactor heated at 5 K min1 and a fluidized bed reactor (fast pyrolysis). At a

constant final temperature of 500 C, the charcoal yield decreased from 27% to 18% for cherry seeds

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(from 28% to 17% for cherry seed shells) when using the fluidized bed reactor instead of the fixed

bed one.

Concerning the physical properties of charcoals obtained by fast pyrolysis, Boateng,54 Brewer and

co-workers,11 and Mullen and co-workers55 reported relatively low BET surfaces areas (3.1021.6

m2 g1) for chars formed from switch grass and corn stover in a fluidized bed reactor. This result is

expected because of the short residence time of solid particles.56 In addition to this, Brewer and co-

workers11 also observed a very small particle size for charcoals obtained from fast pyrolysis of

switch grass. This fact is mainly due to the small particle size of the feedstock (averaging around 1

mm) usually required in fast pyrolysis systems, and, probably, to the hypothesis that fast

devolatilization might create very fragmented char structures.57 From a chemical composition point

of view, charcoals obtained at high heating rates are characterized by high oxygen content50 and low

calorific value,53 probably as a result of the relatively short particle residence time.

In the last years, increasing attention has been focused on upgrading the composition and qualities

of the bio-oil product by means of the addition of a catalyst (in-situ upgrading).58,59 Taking into

account the catalytic effect of alkaline and alkaline earth metals on the pyrolysis of biomass, several

researchers measured the effect of impregnating a given biomass feedstock with a potassium,

sodium or magnesium salt on both product distribution and composition.6065 Di Blasi and co-

workers63 observed a substantial increase of the char yield (from 19% to 30% in weight basis)

during the fast pyrolysis, at a peak temperature of 527 C, of fir wood previously impregnated with

an aqueous solution of KOH. A similar finding (an increase of the char yield of around 10%) was

reported by Wang and co-workers65 during the pyrolysis of pine wood particles physically mixed

with potassium carbonate. In both cases, the increase of the charcoal yield occurred at the expense of

the liquid-phase organic products, as a consequence of the catalytic enhancement of the secondary

charring reactions of primary volatiles.

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Nevertheless, the in-situ upgrading of the bio-oil product is usually performed using catalysts

based on mesoporous aluminosilicate materials (Al-MCM-41) or microporous HZSM-5 zeolites.66

69 Zhang and co-workers68 reported a decrease of the char yield (from 23.2% to 20.1%) when

corncob samples were catalytically pyrolyzed using a HZSM-5 zeolite. The bio-oil yield also

decreased in the presence of catalyst (from 33.9% to 13.7%). However, the collected liquid after

catalytic fast pyrolysis exhibited interesting properties for its use as transport oil (lower oxygen

content and higher heating value than the bio-oil collected without catalyst).

Flash carbonization

The flash carbonization (FC) process has been developed at the University of Hawaii (UH) under

the leadership of Professor Michael J. Antal. This process is a novel procedure by which biomass

can be converted to charcoal in a more efficient way than conventional carbonization or slow

pyrolysis.7072 A canister containing a packed bed of a given biomass feedstock is placed within a

pressure vessel. Air is used to pressurize the vessel to an initial pressure of 12 MPa, and a flash fire

is ignited at the bottom of the packed bed. After a few minutes, air is delivered to the top of the

packed bed and biomass is converted to charcoal. The total reaction time is less than 30 min and the

temperature profile of the packed bed is conditioned by several factors: biomass feedstock, moisture

content of the feedstock, heating time, and the total amount of air delivered.72 In any case, the flame

front moves up the packed bed, causing the middle and top temperatures to successively increase,

until reaching values near 600 C.

Using the FC process, Nunuora and co-workers70 reported high fixed-carbon yields (in the range

28%32%) for two types of biomass feedstocks (corncob and macadamia nut shells). The fixed

carbon yield (yFC) (which is a better index than the charcoal yield, because the yFC takes into account

the chemical composition of the produced charcoal) is defined as follows:

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ashFC

mmy

bio

charFC %100

% (1)

where %FC and %ash denote the percentage of fixed-carbon contained in the charcoal and the

percentage of ash in the feedstock, respectively.27 The ratio between mchar (dry mass of produced

charcoal) and mbio (dry mass of feedstock) correspond to the charcoal yield.

The thermochemical equilibrium value of the fixed-carbon content can be useful for comparison

purposes. These theoretical values can be calculated using the STANJAN software,73 as a function

of the elemental biomass composition, final temperature, and pressure. Figure 2 shows a comparison

of the attainment of the theoretical yield of fixed carbon for four biomass feedstocks (leucaena

wood, oak wood, macadamia nut shells, and corncob), which were pyrolyzed using two different

processes: slow pyrolysis at 1.0 MPa (heating at 6 K min1 up to 450 C with no soaking time)27 and

flash carbonization at 1.01.5 MPa.72 The results obtained for leucaena wood, oak wood, and

macadamia nut shells were similar in terms of fixed-carbon retained in the charcoal. However, a

substantial increase of the yFC value (reaching 100% of its theoretical maximum value) is deduced

for the corncob samples when they were carbonized using the FC process. In other words, it is

possible to retain, in the charcoal, the maximum amount of fixed carbon from corncob samples

using the FC process. Taking into account the elemental analysis of corncob samples (43% of C)72

and the yFC value obtained using the flash carbonisation process (28%),72 it can be determined that

65% of the carbon initially contained in the feedstock was transformed to carbon in the charcoal. In

addition to this, the FC process seems to be a very interesting option, because the reaction times are

very short (< 30 min) compared to the slow pyrolysis process.

Gasification

Gasification is a thermochemical process by which a carbonaceous feedstock (coal, biomass, or a

mixture of both) is converted into a non-condensable gas at high temperatures (> 800 C).18,7476

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When air is employed as the gasification agent (the most common case when biomass wastes are

processed), the process involves partial combustion of the fuel to generate a combustible gas with a

low heating value of 3.510.0 MJ Nm3,75 which can be used as a fuel for boiler, gas turbine or gas

engine. The quality of the producer gas is improved when other oxidizing agents are used (i.e.,

steam, carbon dioxide, or a mixture of oxygen and steam). The producer gas obtained in this way is

rich in carbon monoxide and hydrogen and its field of applicability is wide: chemical synthesis, fuel

cell feed, hydrogen production, etc.

Charcoal yield from gasification is very low (510%)11,76 because of the high operating

temperature and the partial oxidizing atmosphere. In addition, the produced char from gasification

systems can exhibit a high concentration of metals and minerals depending of the ash content and

composition of the feedstock. This fact may imply potential safety concerns with regard to the

application of this kind of biochars to soil.77

For all of the reasons mentioned above, it seems clear that conventional gasification systems,

whose purpose is to maximize the gas product fraction, are not the best option to generate biochar

for soil amendment. However, a partial and controlled gasification process using air, steam or CO2

as the oxidizing agent; may be an interesting way to improve the textural properties of a given

charcoal, which has previously been obtained by a pyrolysis process.30,78 The gasification step is

also known as physical activation process and is widely used to produce activated carbons from

biomass feedstocks for adsorption and catalysis purposes.7984 The percentage of burn-off (usually

ranged from 30% to 55%)80 and the activation temperature (which is kept constant at 700850 C)80

are the key operating parameters during the gasification step.

Table 2 lists the activation conditions and the textural properties of several activated charcoals,

which have been collected from some published works.80,8387 In all of these previous studies, the

surface area (SBET) was calculated using the BET equation from the N2 adsorption data; the total

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pore volume (V) was estimated by converting the amount of N2 gas adsorbed at a relative pressure of

0.99 to liquid volume of nitrogen; and the micropore volume (V0) was determined according to the

DubininRadushkevich method88 (see detailed explanation of textural characterization in section 5).

Although a direct comparison of the results reported in Table 2 is difficult, because of the

different experimental conditions (for both pyrolysis and activation processes) and biomass

feedstocks used in each study, it is pertinent to note that both the surface area and the micropore

volume are affected by the conditions of activation. Nevertheless, the choice of the best set of

activation conditions will depend on the experimental results obtained for a specific biomass

charcoal. In other words and as has been previously noted for the pyrolysis charcoal yield, the nature

of the precursor (biomass) has a great influence on the properties of the resulted activated carbons.

Despite the positive benefits linked to the production of valuable porous materials from biomass

feedstocks, it must be kept in mind that as the conversion of the fixed-carbon (during the

gasification or activation step) becomes greater; the carbon sequestration potential of the biochar

becomes smaller. For this reason, it will be interesting to see if a compromise between the textural

properties and the fixed-carbon yield can be reached for biochar purposes.

4. EFFECTS OF BIOCHAR ON SOIL QUALITY

In this section, the effects of the addition of biochar on soil properties, processes, and functions

are reviewed.

Soil properties

The addition of biochar into soil can alter soil physical properties such as structure, pore size

distribution, bulk density, and texture. This fact brings important implications for soil aeration,

water holding capacity, plant growth, and soil workability.89,90 It is reported that biochar application

into soil could increase the overall surface area of the soil91 and consequently, could improve soil

water retention89 and soil aeration.92 Laird and co-workers93 reported that the specific surface area of

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a typical very deep loam soil (Clarion soil map unit 1138B, from Iowa State, USA) increased from

130 to 153 m2 g1 as the biochar concentration increased from 0 to 20 g kg1. An increased surface

area might also benefit the overall sorption capacity and the native microbial communities of soils.89

However, experimental evidence of such mechanisms is very scarce at present.

The soil water retention is determined by the distribution and connectivity of pores in the soil-

matrix, which is largely regulated by soil particle size (texture), combined with structural

characteristics (aggregation) and the soil organic matter (SOM) content.90 The soil aggregation can

be improved by addition of biochar, as a consequence of the related interactions with SOM,

minerals, polymers from micro activity, clay, and microbiological activity. All of these reasons may

explain the data of Glaser et al.,5 who reported an increase in the water hold capacity of 18% for

anthrosols (man-made tropical soils) rich in biochar in comparison to adjacent soils in which biochar

was absent.

Certain studies have reported high cation exchange capacity (CEC) values for biochar2,94

(consistently higher than that of whole soil, clay or soil organic matter2), probably due to its

negative surface charges94. This fact can enable biochar to act as a binding agent. Nevertheless, the

addition to a given soil of a biochar with a high CEC does not necessarily imply an increase in the

cation exchange capacity of the soil, which results in an enhancement of the ability of the soil to

adsorb and retain cations (e.g.; Mg2+, Ca2+, K+, and NH4+). The CEC of a given soil indicates how

well some nutrients (cations) can be bound to the soil, and, therefore; available for plant uptake. An

increase in nutrient retention also results in decreased leaching losses below to the effective rooting

zone. Leaching of nutrients from soils decreases soil fertility, promotes soil acidification and

negatively affects the quality of surface and groundwater.95

Experimental results obtained from previous studies concerning the effects of biochar addition on

the CEC of soil are given in Table 3. In the most of cases, the incorporation of biochar into soil

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increased both pH and CEC. However, this improvement in soil quality was not observed for a high

pH soil tested by Van Zwieten and co-workers96 (a loamy calcisol from a vineyard in Victoria,

Australia). In addition, Van Zwieten and co-workers observed differences in soil properties when

these soils were amended with different biochar samples. More in detail, these researchers observed

that biochar samples with higher liming values (measured in percentage of CaCO3 equivalent)

showed a better ability to increase pH of an acid soil (ferralsol). Partially in line with this, Yuan and

co-workers94 also tested the effect of biochar incorporation on the pH of an acidic soil (ultisol from

Anhui province, China). In that study, authors reported that the liming effects of the biochars

produced from several crop straws (canola, corn, soybean, and peanut) increased with the rise of the

peak pyrolysis temperature, the value of which seems to affect both the alkalinity and the form of

alkalis of a given biochar, as recently suggested by Hossain and co-workers.97

Soil processes

Biochar stability on the environment

The stability of biochar in soil is a key parameter in order to evaluate the potential of using

biochar as a CO2 sequestration tool. Current evaluations of the age of black carbon particles from

anthropogenic activity (and from natural fire events) indicate great stability of (at least) a significant

component of biochar, ranging from several thousands to hundreds of years.90,98,99 Nevertheless,

freshly-made biochar is not an inert material and can be oxidized in the short term by contact with

strong chemical oxidants at high temperatures.100,101 In soil, biochar can be degraded by both

photochemical and microbiological processes, as reported in a relatively small number of short-term

incubation studies.101,102 From these experimental results, it was also deduced that biological

decomposition was negligible compared to abiotic degradation.101

Fresh biochar surfaces are commonly hydrophobic and have negative surface changes.90,103

Nevertheless, biochar in the soil environment can probably be oxidized over time resulting in a

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probable accumulation of carboxylic functionalities in the surface of biochar particles.104 This fact,

which depends at least on the biochar characteristics and the environmental conditions, could

enhance the interactions between biochar and other soil components, such as organic and mineral

matter.101,104

The influence of biochar properties (i.e., particle size, pore size distribution and surface

chemistry) on the both short-term and long-term carbon loss (mineralization) of biochar remains still

unclear. Hamer and co-workers102 reported that biochar obtained from corn stover and rye was

mineralized more rapidly than that produced from wood, indicating a certain role of the biomass

type in the stability of biochar (influence of H/C, O/C, and C/N ratios). Baldock and Smernik105

observed, for biochar produced from red pine wood, an inversely proportional relationship between

the pyrolysis peak temperature and the carbon loss by mineralization. Recently, Nguyen and co-

workers106 found that increasing the peak temperature from 350 to 600 C (during slow pyrolysis of

corn residues and oak wood) produced a decrease in the carbon loss for mixtures of biochar and pure

sand incubated for 1 year. An increase of the pyrolysis peak temperature results in a greater degree

of aromaticity of the biochar and, consequently, in a greater chemical recalcitrance. Furthermore,

these researchers also observed that the remaining carbon for mixtures of biochar from corn residue

and sand (at a given peak temperature) was lower than that of mixtures composed by biochar from

oak wood. Besides the biomass feedstock and the pyrolysis peak temperature, the stability of

biochar also depends on environmental conditions and soil type. Nguyen and co-workers16,106

reported strong influences of both water regime (saturated or unsaturated conditions) and

temperature on the mineralization of biochar; whereas Qayyum and co-workers observed different

carbon mineralization rates of a wheat straw-derived biochar for three types of soils (ferralsol.

topsoil lixisol, and subsoil lixisol). The last authors reported the lowest mineralization rate for the

ferralsol soil type.107

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Greenhouse gas (N2O and CH4) emissions

Under anaerobic conditions N2O is emitted from soil through denitrification, a microbially

facilitated process of NO3 reduction that may ultimately produce N2. In addition, nitrifying bacteria

generally involved in conversion of N2 to ammonium can simultaneously promote denitrification.2

The addition of biochar in a given soil can decrease the availability of N for denitrification and,

consequently, reduce the total N2O emissions. Yanai and co-workers108 showed, in a short-term

laboratory chamber experiment, a significant decrease in N2O emissions from a wetted volcanic ash

soil (hapludand) when biochar derived from municipal biowaste was applied (at a rate of 180 t ha1).

In line with this, Zhang and co-workers109 showed that the total N2O emissions from a hydroagric

stagnic anthrosol were decreased by 40%51% and by 2128% when biochar (produced by slow

pyrolysis of wheat straw at 350550 C) was added at a rate of 40 t ha1 compared to the control

treatments with and without N-fertilizer, respectively. Similar findings were also reported by

Sarkhot and co-workers110 for a dairy manure-derived biochar (26% reduction in cumulative N2O

flux).

On the other hand, wide variations in the rates of CH4 emissions from soils amended with biochar

have been reported in the literature. Xiong and co-workers111 observed that CH4 emissions depended

on the properties of soil. These authors measured the CH4 emissions during the flooded season for

two Chinese anthrosols with different CEC and organic carbon content. After analyzing

experimental results (in which the highest CH4 emissions was measured for the soil type with a

highest organic C content) and as a preliminary conclusion, Xiong and co-workers stated that soil

organic carbon is more important than CEC as driving factor controlling CH4 production.

Rondon and co-workers112 reported that application of wood-derived biochar at a rate of 20 t ha1

remarkably increased the annual methane sink in an acidic tropical soil. In contrast to this finding,

Zhang and co-workers109 reported that CH4 emissions were increased by 34% and 41% in a

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hydroagric stagnic anthrosol amended with wheat straw-derived biochar at 40 t ha1 compared to the

treatments with and without N-fertilizer, respectively. As suggested by Zhang and co-workers, labile

components of biochar could be decomposed and become the predominant source of methanogenic

substrates, thus promoting CH4 production. In line with this and according to Van Zwieten and co-

workers113, the source and chemical properties of biochar might also have an influence on CH4

yield. In addition, and as has mentioned before, wide variation of soil CH4 emission has been

reported for soils with different chemical and physical properties111 and under different water

regimes. In any case, the precise mechanism behind the soil CH4 emissions still remains unclear.

Sorption of hydrophobic organic compounds (HOCs)

Biochar incorporation into soil can enhance the sorption capacity of soils towards hydrophobic

organic compounds (such as PAHs and pesticides).90 Previous studies have indicated that this

negative effect can be a function of the chemical and structural properties of the contaminant (e.g.,

molecular weight and hydrophobicity),114116 as well as of the surface area, pore size distribution,

and functionality on surface of the biochar.114,115

The influence of the textural properties of biochar on sorption capacity was analyzed in previous

studies,116,118 in which researchers observed a strong (and expected) effect of the pyrolysis peak

temperature on sorption capacity for biochars obtained from wood and wheat residues. As the

pyrolysis peak temperature increases, produced biochars exhibits a greater surface area (and a

greater micropore volume) and a lower oxygen content (lower O/C ratio). Taking into account that

the O/C ratio of a given biochar is a potential indicative of both its hydrophilicity and polarity, an

increase of the pyrolysis peak temperature probably causes a decrease in polar surface groups, and

consequently, a reduction of the biochar affinity for water molecules. As a consequence of both the

increase of pore surface area and the decrease of water affinity, the sorption capacity of biochars is

expected to increase with the pyrolysis final temperature, as observed by Chun and co-workers118

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and Wang and co-workers.116 In contrast to these results, Kinney and co-workers119 observed that

biochars obtained from several feedstocks (magnolia leaves, apple wood chips, and corn stover) by

slow pyrolysis (at a peak temperature ranged from 400 to 600 C) exhibited a very low

hydrophobicity. Kinney and co-workers also reported a statistically significant effect of the presence

of surface alkyl functionalities (CH), which were detected when biochars were pyrolyzed at a peak

temperature below 400 C, on the hydrophobicity of the analyzed biochars. These apparently

contradictory results suggest that further studies focused on analyzing the hydrophobicity of

biochars are required.

Other sources of soil contamination

This section is focused on the potential for soil contamination linked to some component of

biochar, such as heavy metals and PAHs. Despite the fact that this type of contamination can lead to

severe public health problems, relatively little attention has been focused on this issue.90

Biochar produced from pyrolysis of some organic wastes, such as sewage sludge and tannery

residue, generally retains high levels of heavy metals (e.g., chromium, cooper, nickel, and zinc).97,120

However, McHenry17 suggested that high levels of biochar addition (> 250 t ha1) are needed to

potentially contaminate soil, surface water and crops. Obviously, this topic needs further assessment

in future studies.

Otherwise, it seems clear that biomass pyrolysis at peak temperatures above 700 C could

generate heavily condensed PAHs.121,122 Brown and co-workers123 reported that several biochar

products, which were obtained by slow pyrolysis at different peak temperatures (ranging from 450

to 1000 C) from pitch pine wood, exhibited PAHs concentrations ranged from 3 to 16 g g1 (with the highest value at the highest peak temperature). Brown and co-workers also analyzed the PAHs

content of a natural biochar (charred pine from a prescribed burn area), showing that this value (28

g g1) was slightly higher than that measured for synthetic biochars. This preliminary finding could

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suggest that PAHs levels in biochar can be often comparable (or even lower) than those found in

some soils.90

Soil productivity

An increase in soil fertility is the most frequently reported benefit linked to adding biochar to

soils. Most of the published studies to date have been conducted for tropical soils.90 In tropical or

sub-tropical environments, soil fertility tends to be poor due to rapid mineralization of soil organic

matter, the low cation exchange capacity (CEC) of the tropical soils (which is usually due to their

clay content and mineralogy), and the low nutrient contents.5 Moreover, the use of inorganic

fertilizers in these types of soils has certain drawbacks, the most important of which are the high

cost of continuous applications of fertilizers and their low efficiency in highly weathered soils.124,125

Some previous studies reported that biochar addition in several tropical soils resulted in an increase

of soil nutrient availability.5,126129 In the short-term basis, the direct nutrient additions with the

added biochar (e.g., K, P, and Ca, which are present in the inorganic fraction of biochar) seems to be

responsible for short-term enhancement of soil fertility.126 Regarding the long-term effect of biochar

on nutrient availability, it depends on an appropriate increase of both CEC and surface

oxidation.125,130 Previous investigations, which have been focused on analyzing both CEC and pH

evolution over time, reported an increase in both variables as biochar addition time increased.101

Currently published studies considering the effect of biochar addition on crop yield were

generally performed on small scale and sometimes, without considering the environmental

conditions, under which a decrease of the biochar content in the soil can occur through

decomposition (when temperature raises), leaching, or erosion.98 Glaser and co-workers131 reviewed

a substantial number of earlier studies, which were conducted for tropical soils during the 1980s and

1990s. These studies reported positive impacts of biochar additions at a low application rate of 0.5

Mg ha1 on several plant species. However, biochar addition at higher rates (> 100 Mg ha1) seemed

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to inhibit plant growth. On the other hand, a later study conducted by Steiner and co-workers132

showed that biochar application (at a rate of 11 Mg ha1) significantly improved plant growth for a

highly weathered Central Amazonian upland soil fertilized with NPK (in comparison to the effect of

the same rate of NPK-fertilizer without biochar).

Table 4 presents examples of experimental studies focused on investigating the response of crops

to biochar application. As can be deduced from the data reported in Table 4, the effect of biochar

depends on several factors including the soil type, the addition rate, and the kind of crop. Moreover,

an interaction between biochar and fertilizer addition is generally observed. In this sense, and as

argued above, the fertility of tropical and sub-tropical soils (such as acidic ferralsols and nitisols)

seems to substantially improve by biochar treatment,5,96,125,132,133 especially when biochar was

applied together to inorganic fertilizers.96,134,135 However, Van Zwieten and co-workers96 reported

significant decreases in wheat and radish biomass production for a high pH calcisol. Negative

impacts on crop yield were also observed by Haefele and co-workers133 for rice growth in a gleysol

(which had a high CEC and base saturation and high N, P, and K availability). A mechanism to

explain the negative effect of biochar for these soil types was proposed by Lehmann and co-

workers:126 the available nutrients applied with biochar in this type of soils are not limiting, the CEC

is very high already, and water stress does not occur; nevertheless, the high C/N ratio of biomass

probably limits N availability (from both soil and inorganic fertilizer), causing a decrease of grain

yield.

From Table 4, it is also important to highlight the promising results reported by Vaccari and co-

workers136 regarding the yield in durum wheat for a silt loam soil (with a pH of 5.2) under the

Mediterranean climate conditions. These preliminary results, which should be confirmed in further

studies, could indicate that the positive effect of biochar addition on soil production is also possible

for other soils than ferralsols in tropical environments.

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Unfortunately, very little information is available in the literature with respect to the influence of

both biochar properties and pyrolysis conditions on plant growth. Nevertheless, a recent study

conducted by Peng and co-workers135 reveals some interesting findings. These authors analyzed the

effect of both the pyrolysis peak temperature and the soaking time at this temperature for rice straw-

derived biochar on soil properties and production function. Regarding the biochar characteristics,

Peng and co-workers observed that increasing both peak temperature (from 250 C to 450 C) and

soaking time (from 2 to 8 hours) obviously decreased the biochar yield and volatile matter content

but increased the C, K, and P contents. In addition, volatile matter, O, H, and aliphatic functional

groups decreased at the expense of aromatic C as peak temperature and soaking time increased. As a

result, the biochar stability and its liming effect increased with pyrolysis peak temperature and

residence time. Nevertheless, and interestingly, no significant effects of pyrolysis conditions on the

CEC of the tested soil (a highly weathered ultisol from southern China) and the maize yield were

observed by Peng and co-workers.

In another interesting study, Deenik and co-workers at the University of Hawaii137 showed that

partially carbonized biochar containing a relatively high volatile matter (VM) content produced

lower yielding plants in biochar-amended soil compared with soil not treated with biochar. The poor

yield in the high-VM biochar amended soils could be due to an inhibition of N availability (the

authors attributed this effect to the presence of phenolic compounds in the volatile matter, which

stimulated microbial activity leading to a reduction of inorganic N). In contrast, more fully

carbonized biochar with low-VM content did not produce a negative effect on plant growth, and

when it was combined with N fertilizer, there was a significant improvement in crop yield compared

with the fertilized control. Both biochars were obtained from macadamia nut shells by means of a

flash carbonization process at different peak temperatures: 430 C for the high (225 g kg1) VM

biochar and 650 C for the low (63 g kg1) VM biochar. Deenik and co-workers,137 who conducted a

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series of short-term (46 weeks) greenhouse experiments and laboratory incubations, observed the

above-mentioned effects for two types of Hawaiian soils: an andosol (a volcanic soil) and an

uncultivated, highly weathered and extremely acid ultisol. The results obtained for the tested ultisol

are clearly in disagreement with many other studies,5,96,125,132,135 in which positive effects on plant

growth of biochar addition to acid tropical soils are reported. Nevertheless, it should be noted that

not all biochars will exhibit the same effects for a given soil type. In other words, the negative

results reported by Deenik and co-workers137 only suggest that the quality of biochar is at least as

important as the soil type.

5. BIOCHAR CHARACTERIZATION REQUIREMENTS

Taking into account that the form of carbon (aromatic or nonaromatic C) present in biochar is

believed to be related to the stability of this material on soil, a key aspect of determining the

potential of a given charcoal for biochar purpose may be the ability to characterize its surface

chemistry.11 However, additional properties should be considered in order to preliminary evaluate

the potential of a given biochar. These properties can be physical (e.g.; specific surface area and

morphology) or chemical (such as proximate and elemental analysis and mineral content). Recently,

the International Biochar Initiative has published guidelines138 to provide standardized information

regarding the characterization of biochar materials and to assist in achieving more consistent levels

of product quality. These Biochar Guidelines identify three categories of tests for biochar: test A for

basic utility properties, test B for toxicant assessment, and test C for advanced analysis and soil

enhancement properties. In the next sections, information concerning the analytical methods used to

measure biochar properties is given.

Proximate and elemental analysis

The proximate analysis yields the weight fractions of moisture, volatile matter (VM), ash, and

fixed carbon (FC). There are standardized methods for performing a proximate analysis (ASTM,

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ISO, DIN and BS). These standards are very similar in nature except for slight differences in the

operating conditions (temperature and soaking time) used to quantify the volatile matter content. As

was mentioned in previous sections, the volatile matter content is negatively correlated with peak

temperature and, according to earlier studies,137 a high value of this parameter could indicate a low

potential of a given biochar for soil amelioration purposes.

Regarding the elemental analysis, the weight percentage of carbon, hydrogen, nitrogen and sulfur

are usually determined using analytical devices, the operation of which is based on the complete

combustion with a pure oxygen atmosphere.139

Inorganic fraction characterization

Two techniques are generally applied to isolate the inorganic fraction of carbonaceous

materials:139 the low-temperature ashing (LTA) in an oxygen plasma at 100150 C and the

medium-temperature ashing (MTA) in air a 600 C. Surez-Garca and co-workers140 suggested the

use of both isolation techniques to securely identify the inorganic constituents of a given sample.

Once the inorganic fraction has been isolated, several analytical techniques can be applied to

characterize the inorganic species: Inductively Coupled Plasma Atomic Emission Spectroscopy

(ICP-AES), X-ray fluorescence (XRF), and X-ray diffraction (XRD). ICP-AES is able to determine

the absolute concentration of inorganic elements (Al, Ca, Fe, K, P, Mg, Si...).141 XRF spectrometry

is useful to determine the ash compositions in terms of weight fraction of oxides140 and XRD can be

used to identify the crystalline minerals in ash.141

Both exchangeable K and P are important parameters that can partially establish the capability of

biochar to supply nutrients to soil in a short-term basis. These contents (exchangeable K and

exchangeable P) in biochar were found to range widely as a function of the feedstock, with values

between 1.058.0 and 2.7480 g kg1, respectively.142 These ranges are somewhat wider than those

reported in the literature for typical organic fertilizers.90 Nevertheless and according to Joseph and

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co-workers,143 the role of high-ash biochars is still unknown and experimental data are needed in

order to determine the effect of the ash on soil properties on the medium and long-term basis.

Textural characterization and morphology

As has already been mentioned in the earlier sections, both the specific surface area and pore size

distribution depend mainly on two factors: the nature of the biomass feedstock and the pyrolysis

operating conditions (especially, peak temperature). To experimentally determine the textural

parameters of a biochar sample, adsorption of N2 at 77 K and adsorption of CO2 at 273 K are

typically used. From the results corresponding to the N2 adsorption isotherms, the specific surface

area based on the equation of Brunauer, Emmett and Teller (SBET); can be determined.88 From the

same adsorption isotherms and adopting the DubininRadushkevich method, the micropore volume

(V0) can be calculated.88 Furthermore, the volume of mesopores (Vme) can be estimated from the

isotherm as the difference between the volume of N2 adsorbed at a relatively pressure of 0.95 and

the value of V0.144 On the other hand, the narrow micropore volume (W0; pore width below 0.7 nm)

can be estimated from the CO2 adsorption isotherms assuming the DubininRadushkevich

method.145

Regarding the morphological characterization, Scanning Electron Microscopy (SEM) is

commonly used to analyze the char particle structure and surface topography.2,11

Surface functionality

Surface functionality can be investigated by means of Fourier Transform Infrared (FTIR)

spectroscopy. The FTIR spectra of both biomass feedstock and biochars obtained at different

pyrolysis peak temperatures are useful to analyze the gradual loss of lignocellulosic functional

groups (change in the OH stretch peak around 3400 cm1, which dominates the feedstocks

spectrum).11 Assignment of other spectral peaks of interest for biochar samples, including the

aliphatic CH stretch at 30002860 cm1, the aromatic CH stretch around 3060 cm1, and the

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various aromatic ring modes at 1590 and 1515 cm1, was proposed by Sharma and co-workers.146

The peaks characteristic of the carbonyl groups should appear in the range 16601725 cm1. The

exact position of the peaks depends on whether the carbonyl groups are in conjunction with the

aromatic ring (position below 1700 cm1) or not (position above 1700 cm1).146

X-ray photoelectron spectroscopy (XPS) can also be used for surface analysis.16,101,147 The XPS

wide-scan spectrum usually shows the presence of two main peaks in C (C1s) at around 285 eV and

O (O1s) at around 530 eV. The spectra of high resolution XPS of C1s and O1s are used to quantify

the carbon and oxygen forms on the biochar surface. For the C1s spectrum, different binding

energies are assigned to CC, C=C, CH, CO, C=O, and COO stretches; whereas for the O1s

spectrum, signal peaks at different binding energies can be attributed to O=C and OC stretches.101

Aromatic character

Solid-state 13C Magic Angle Spinning (MAS) Nuclear Magnetic Resonance (NMR) is

commonly used for making quantitative comparisons without recurring to the procedure of taking

peak ratios. Rather, each resonance peak can be quantified in relation to the total resonance

intensity, giving therefore the relative abundance of individual molecular groups.146 As mentioned

before, the aromatic character of the produced biochar seems to be directly correlated to the value of

the pyrolysis peak temperature: as peak temperature increases it is expected to show a higher

aromatic structure. Freitas and co-workes148 reported 13C Cross Polarization (CP) MAS NMR

spectra for biochars obtained by pyrolysis of rice hulls at different peak temperatures. For the

biochars obtained at a peak temperature of 300 C, these authors observed two main resonance lines,

around 130 ppm (broader) and 148 ppm, associated with non-oxygenated and oxygenated aromatic

carbons, respectively. Simultaneously and for the same biochar samples, Freitas and co-workers147

observed broad resonance around 31 ppm, probably associated with aliphatic chains, and the

development of a small signal near 208 ppm, ascribed to ketone groups. Regarding the biochars

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produced at higher peak temperatures (390605 C), the authors showed a progressive development

of a well-defined aromatic resonance, centered at 125 ppm, which occurs simultaneously with the

attenuation of the signals corresponding to oxygenated aromatic carbons (around 150 ppm) and

aliphatic groups (broad line around 30 ppm).

Recently, McBeath and co-workers149 analyzed both cross polarization (CP) and direct

polarization (DP) spectra for chestnut wood-derived biochars pyrolyzed at different peak

temperatures. Results from the work of McBeath and co-workers indicated that aromaticity of

biochar rapidly increases when peak temperature is above 400 C. In addition, the authors also

reported that proportion of aromatic C detected was similar for both CP and DP techniques for all

charcoals.

6. CONCLUSIONS

The present review highlights the need for greater collaboration among researchers working in

different fields of study: production and characterization of biochar on one hand, and on the other,

measurement of both environmental and agronomical benefits linked to the addition of biochar to

agricultural soils. In this sense, when experimental results concerning the effect of the addition of

biochar to a given soil on crop yields and/or soil properties are published, details about the

properties of the used biochar should be well reported. These details include the biomass feedstock

and its composition (elemental and proximate analysis, holocellulose and lignin contents, and

mineral matter characterization), the process chosen for the biochar production and the detailed

operating conditions of which (peak temperature, soaking time, heating rate...), and information

concerning the properties of the used biochar (ultimate and proximate analysis, specific surface area,

pore size distribution, organic character...). The inclusion of this valuable information seems to be

essential in order to establish the appropriate process conditions to produce a biochar with more

suitable characteristics.

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In addition to the general consideration outlined above, several research gaps and issues have

been identified through this literature review. These research priorities are listed below:

Among the operating conditions of the slow pyrolysis process, the peak temperature seems to be the most important parameter affecting the characteristics of biochar

product. An increase of peak temperature seems to lead to the generation of biochars

with higher aromatic character and fixed carbon and higher porosity. At the current state

of the art, this fact seems to be positive regarding the stability of the carbon in the

biochar and the enhancement of nutrient retention of a given biochar-amended soil.

Further studies analyzing the effect of pyrolysis peak temperature on both biochar

stability and nutrient retention (CEC) are required to confirm this preliminary trend.

Although slow pyrolysis or carbonization is the process commonly used to produce biochar, because of the high charcoal yields obtained, other technologies cannot be

underestimated. In this sense, in-situ catalytic fast pyrolysis can be an interesting option

to simultaneously produce a bio-oil with enhanced properties and a biochar at an

acceptable yield. On the other hand, developing innovative process, such as the flash

carbonization process, would be a key priority for the research community in order to

improve both the productivity and the quality (fixed carbon yield) of the produced

biochar.

The specific surface area and the micropore volume of a given biochar obtained after pyrolysis can be substantially enhanced through an activation step. This secondary

activation process can be a gasification step (physical activation by using an oxidizing

agent at a final temperature of 700850 C) or an additional carbonization step (under an

inert atmosphere at a temperature of 8501000 C). In both cases (but especially in the

physical activation process), the benefit of improving biochar porosity (and,

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consequently, the potential of the biochar to improve the soil water retention and soil

aeration) is accompanied by a loss of carbon retention and sequestration capacity. For

this reason, further investigations would be required to reach a compromise between the

desired textural properties and the carbon sequestration potential for a biochar obtained

from a given biomass feedstock.

Despite the fact of that the form of carbon (aromatic or nonaromatic C) present in biochar is believed to be related to the stability of this material on soil, the influence of

additional properties (physical and chemical) on the stability of the biochar placed in

soil remains still unclear and further studies, in which the effect of environmental

conditions (i.e., water regime) on biochar stability can be measured, are needed. As

mentioned before, the properties of the tested biochars must be reported in these studies.

Very little information is now available regarding the influence of both biochar properties and pyrolysis conditions on plant yield. Consequently, further research

studies, at the field scale, focused on analyzing the effect of a given biochar, obtained

under a given set of operating conditions, on the biomass yield of a given plant in a

given type of soil will be crucial to gain knowledge on this topic.

ACKNOWLEDGEMENTS

The author would like to thank Prof. Clara Mart for useful remarks and comments in the field of

soil science. The author also wishes to thank reviewer #2 for his detailed comments and helpful

suggestions aimed at improving the paper.

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