2012 Pyrolysis for Biochar Purposes a Review to Establish Current Knowledge Gaps and Research Needs.

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Environmental Science & Technology

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Pyrolysis for biochar purposes: a review to establish

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current knowledge gaps and research needs

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Joan J. Manyà

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

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

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E-mail address: [email protected] address: [email protected]

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ACS Paragon Plus Environment

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Pyrolysis for biochar purposes: a review to establish

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current knowledge gaps and research needs

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Joan J. Manyà

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

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

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E-mail address: [email protected] address: [email protected]

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ABSTRACT

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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

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

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research. The present paper provides an updated review on two subjects: the available alternatives to produce biochar from fr om a biomass feedstock feed stock and the effect effec t of biochar addition to agricultural soils on soil properties and fertility. A high number of previous studies have highlighted the benefit of using

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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

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

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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:

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

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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

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

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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.

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Keywords:

Biochar; Pyrolysis; Soil fertility; Carbon sequestration.

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TOC/Abstract art

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Operating conditions

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Pyrolysis gas

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Biomass

Slow Pyrolysis

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Liquid fraction

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Charcoal (40-60% yield)

BIOCHAR USE

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1. INTRODUCTION

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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

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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

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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.

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

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mitigating global warming and as a strategy to manage soil health and productivity.4–8 In the most of cases, these studies are constrained by limited experimental data and are geographically limited.

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

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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

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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

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

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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

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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.11–14 These studies represent an initial step,

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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

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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

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

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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,

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

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2. THE BIOCHAR CONCEPT

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

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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

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

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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

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

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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

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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

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

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anthropogenic activity indicate millennial-scale stability. 15–16 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

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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

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

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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

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

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promising alternative to integrate carbon sequestration measures and renewable energy generation into conventional agricultural production.17

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3. BIOCHAR PRODUCTION

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

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

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

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Slow pyrolysis

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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

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charcoal yields,9,10,19–25 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

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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

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pronounced in the temperature range from 300 to 500 ºC. 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

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

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temperatures higher than 700 ºC does not seem appropriate to generate charcoals with potentially better adsorptive properties.29–31

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Pyrolysis or carbonization at elevated pressure (1.0–3.0 MPa) seems to improve the charcoal yield

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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

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

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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

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

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Pyrolysis pressure also produces an effect on the porosity of produced charcoals. Cetin and coworkers33 reported a slight decrease of the total surface area by increasing pressure during the

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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

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BET (Brunauer, Emmett and Teller) surface area of charcoals obtained by slow pyrolysis (at 13 K min –1 and at a peak temperature of 550 ºC) of miscanthus is reported (from 161.7 m2 g –1 at 0.1 MPa to 0.137 m2 g –1 at 2.6 MPa). The authors attributed this result to a clogging of the pores by tar

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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.

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Regarding the moisture content of the biomass feedstock, results obtained in previous studies 20–35 indicated that high moisture contents (in the range of 42–62%) can improve the yield of charcoal at elevated pressures. This finding makes certain agricultural residues, which are characterized by high

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

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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

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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

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contents (e.g., beech).

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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

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

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as a measure to reduce the ash content.39–43 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

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temperature. This assumption is consistent with experimental data reported in several research studies.44–45 Concerning the effect of the particle size on the charcoal yield, it seems reasonable to

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

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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

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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

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

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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

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contents.

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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

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

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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

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

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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

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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-

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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

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

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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

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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

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

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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

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

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critical and needs to be experimentally determined. Alternative processes

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Fast Pyrolysis

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Fast pyrolysis uses high heating rate (above 200 K min –1) and short vapor residence time (around 2 s). The peak temperature is usually set between 500 and 550 ºC in order to obtain the highest biooil yield.49–52 These operating conditions particularly favor the formation of liquid products (bio-

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

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charcoal yields from both cherry seeds and cherry seed shells obtained using two pyrolysis processes: a fixed bed reactor heated at 5 K min –1 and a fluidized bed reactor (fast pyrolysis). At a

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constant final temperature of 500 ºC, 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

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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 (3.10–21.6

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m2 g –1) 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 coworkers11 also observed a very small particle size for charcoals obtained from fast pyrolysis of

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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

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

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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

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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. 60–65 Di Blasi and co-

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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 ºC, of fir wood previously impregnated with an aqueous solution of KOH. A similar finding (an increase of the char yield of around 10%) was

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

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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

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charring reactions of primary volatiles.

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

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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

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

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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).

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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

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

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pyrolysis.70–72 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 1–2 MPa, and a flash fire

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

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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

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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

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

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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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 mchar  % FC    m 100  % ash   bio 

y FC  

(1)

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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

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

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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

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

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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 min –1 up to 450 ºC with no soaking time) 27 and

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flash carbonization at 1.0–1.5 MPa.72 The results obtained for leucaena wood, oak wood, and

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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

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

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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

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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.

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Gasification

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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 ºC).18,74–76

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

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processed), the process involves partial combustion of the fuel to generate a combustible gas with a low heating value of 3.5–10.0 MJ Nm –3,75 which can be used as a fuel for boiler, gas turbine or gas

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

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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.

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Charcoal yield from gasification is very low (5–10%)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

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

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application of this kind of biochars to soil. 77 For all of the reasons mentioned above, it seems clear that conventional gasification systems,

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

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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

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also known as physical activation process and is widely used to produce activated carbons from biomass feedstocks for adsorption and catalysis purposes.79–84 The percentage of burn-off (usually ranged from 30% to 55%)80 and the activation temperature (which is kept constant at 700–850 ºC)80

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are the key operating parameters during the gasification step.

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Table 2 lists the activation conditions and the textural properties of several activated charcoals, which have been collected from some published works.80,83–87 In all of these previous studies, the

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surface area (S BET ) 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 N 2 gas adsorbed at a relative pressure of

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0.99 to liquid volume of nitrogen; and the micropore volume (V 0) was determined according to the Dubinin–Radushkevich method88 (see detailed explanation of textural characterization in section 5).

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

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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

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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.

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

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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

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

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

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4. EFFECTS OF BIOCHAR ON SOIL QUALITY

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

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Soil properties

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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

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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

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130 to 153 m2 g −1 as the biochar concentration increased from 0 to 20 g kg−1. An increased surface area might also benefit the overall sorption capacity and the native microbial communities of soils. 89

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

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

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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

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

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was absent. Certain studies have reported high cation exchange capacity (CEC) values for biochar 2,94 (consistently higher than that of whole soil, clay or soil organic matter 2), probably due to its

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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

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

43 44 45 46 47 48 49

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

50 51

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

52 53 54 55 56

negatively affects the quality of surface and groundwater.95 Experimental results obtained from previous studies concerning the effects of biochar addition on

57 58

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

6 7 8 9 10

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

11 12

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

13 14 15 16 17 18 19

that biochar samples with higher liming values (measured in percentage of CaCO 3 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

20 21 22 23 24 25 26

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

27 28

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

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Soil processes Biochar stability on the environment

34 35

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

36 37 38 39 40 41 42

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,

43 44 45 46 47 48 49

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

50 51

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

52 53 54 55 56

decomposition was negligible compared to abiotic degradation. 101 Fresh biochar surfaces are commonly hydrophobic and have negative surface changes.90,103

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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,

6 7 8 9 10 11 12

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

13 14 15 16 17 18 19

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

20 21 22 23 24 25 26

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 Smernik 105 observed, for biochar produced from red pine wood, an inversely proportional relationship between

27 28

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

29 30 31 32 33

workers106 found that increasing the peak temperature from 350 to 600 ºC (during slow pyrolysis of corn residues and oak wood) produced a decrease in the carbon loss for mixtures of biochar and pure

34 35

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

36 37 38 39 40

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

41 42

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

43 44 45 46 47 48 49

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

50 51

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

52 53 54 55 56 57 58

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 (N 2O and CH 4) emissions

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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

13 14 15 16 17 18 19

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

20 21 22 23 24 25 26

soil (hapludand) when biochar derived from municipal biowaste was applied (at a rate of 180 t ha –1). In line with this, Zhang and co-workers109 showed that the total N 2O emissions from a hydroagric stagnic anthrosol were decreased by 40%–51% and by 21–28% when biochar (produced by slow

27 28

pyrolysis of wheat straw at 350–550 ºC) was added at a rate of 40 t ha−1 compared to the control

29 30 31 32 33

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

34 35

flux).

36 37 38 39 40

On the other hand, wide variations in the rates of CH 4 emissions from soils amended with biochar have been reported in the literature. Xiong and co-workers111 observed that CH4 emissions depended

41 42

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

43 44 45 46 47 48 49

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

50 51

organic carbon is more important than CEC as driving factor controlling CH 4 production.

52 53 54 55 56 57 58

Rondon and co-workers112 reported that application of wood-derived biochar at a rate of 20 t ha−1 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 ha−1 compared to the

6 7 8 9 10

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

11 12

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

13 14 15 16 17 18 19

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 CH 4 emission has been reported for soils with different chemical and physical properties111 and under different water

20 21 22 23 24 25 26

regimes. In any case, the precise mechanism behind the soil CH 4 emissions still remains unclear. Sorption of hydrophobic organic compounds (HOCs)

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

27 28

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

29 30 31 32 33 34 35

negative effect can be a function of the chemical and structural properties of the contaminant (e.g., molecular weight and hydrophobicity),114–116 as well as of the surface area, pore size distribution, and functionality on surface of the biochar.114,115

36 37 38 39 40

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

41 42

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

43 44 45 46 47 48 49

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

50 51

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

52 53 54 55 56 57 58

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-workers 118

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

6 7 8 9 10

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 ºC) exhibited a very low

11 12

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

13 14 15 16 17 18 19

of surface alkyl functionalities (C–H), which were detected when biochars were pyrolyzed at a peak temperature below 400 ºC, on the hydrophobicity of the analyzed biochars. These apparently contradictory results suggest that further studies focused on analyzing the hydrophobicity of

20 21 22 23 24 25 26

biochars are required. Other sources of soil contamination

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

27 28

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

29 30 31 32 33 34 35

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

36 37 38 39 40

However, McHenry17 suggested that high levels of biochar addition (> 250 t ha –1) are needed to potentially contaminate soil, surface water and crops. Obviously, this topic needs further assessment

41 42

in future studies.

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

50 51

to 1000 ºC) from pitch pine wood, exhibited PAHs concentrations ranged from 3 to 16 μg g –1 (with

52 53 54 55 56 57 58

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 μg g –1) 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

6 7 8 9 10

some soils.90 Soil productivity

11 12

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

13 14 15 16 17 18 19

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

20 21 22 23 24 25 26

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

27 28

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

29 30 31 32 33 34 35

of soil nutrient availability. 5,126–129 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

36 37 38 39 40 41 42

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

43 44 45 46 47 48 49

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

50 51

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

52 53 54 55 56 57 58

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 ha –1 on several plant species. However, biochar addition at higher rates (> 100 Mg ha –1) seemed

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to inhibit plant growth. On the other hand, a later study conducted by Steiner and co-workers 132

6 7 8 9 10

showed that biochar application (at a rate of 11 Mg ha –1) significantly improved plant growth for a highly weathered Central Amazonian upland soil fertilized with NPK (in comparison to the effect of

11 12

the same rate of NPK-fertilizer without biochar).

13 14 15 16 17 18 19

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,

20 21 22 23 24 25 26

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

27 28

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

29 30 31 32 33

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

34 35

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

36 37 38 39 40

explain the negative effect of biochar for these soil types was proposed by Lehmann and coworkers:126 the available nutrients applied with biochar in this type of soils are not limiting, the CEC

41 42

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

43 44 45 46 47 48 49

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-

50 51

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

52 53 54 55 56

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

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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

6 7 8 9 10

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

11 12

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

13 14 15 16 17 18 19

derived biochar on soil properties and production function. Regarding the biochar characteristics, Peng and co-workers observed that increasing both peak temperature (from 250 ºC to 450 ºC) and soaking time (from 2 to 8 hours) obviously decreased the biochar yield and volatile matter content

20 21 22 23 24 25 26

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

27 28

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

29 30 31 32 33 34 35

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 Hawaii 137 showed that

36 37 38 39 40

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

41 42

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

43 44 45 46 47 48 49

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

50 51

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

52 53 54 55 56 57 58

with the fertilized control. Both biochars were obtained from macadamia nut shells by means of a flash carbonization process at different peak temperatures: 430 ºC for the high (225 g kg –1) VM biochar and 650 ºC for the low (63 g kg –1) VM biochar. Deenik and co-workers,137 who conducted a

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series of short-term (4–6 weeks) greenhouse experiments and laboratory incubations, observed the

6 7 8 9 10 11 12

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

13 14 15 16 17 18 19

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

20 21 22

important as the soil type.

23 24

5. BIOCHAR CHARACTERIZATION REQUIREMENTS

25 26 27 28 29 30 31

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

32 33 34 35 36 37 38

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,

39 40

the International Biochar Initiative has published guidelines138 to provide standardized information

41 42 43 44 45

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

46 47

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

48 49 50 51 52

enhancement properties. In the next sections, information concerning the analytical methods used to measure biochar properties is given.

53 54

Proximate and elemental analysis

55 56 57 58 59

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

6 7 8 9 10 11 12

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

13 14 15 16 17 18 19

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

20 21 22 23 24 25 26

combustion with a pure oxygen atmosphere.139 Inorganic fraction characterization

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

27 28

materials:139 the low-temperature ashing (LTA) in an oxygen plasma at 100–150 ºC and the

29 30 31 32 33

medium-temperature ashing (MTA) in air a 600 ºC. Suárez-García and co-workers140 suggested the use of both isolation techniques to securely identify the inorganic constituents of a given sample.

34 35

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

36 37 38 39 40 41 42

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

43 44 45 46 47 48 49

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

50 51

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

52 53 54 55 56 57 58

exchangeable P) in biochar were found to range widely as a function of the feedstock, with values between 1.0–58.0 and 2.7–480 g kg –1, 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

6 7 8 9 10

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

11 12

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

13 14 15 16 17 18 19

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

20 21 22 23 24 25 26

typically used. From the results corresponding to the N 2 adsorption isotherms, the specific surface area based on the equation of Brunauer, Emmett and Teller (S BET ); can be determined.88 From the same adsorption isotherms and adopting the Dubinin–Radushkevich method, the micropore volume

27 28

(V 0) can be calculated.88 Furthermore, the volume of mesopores (V me) can be estimated from the

29 30 31 32 33

isotherm as the difference between the volume of N2 adsorbed at a relatively pressure of 0.95 and the value of V 0.144 On the other hand, the narrow micropore volume (W 0; pore width below 0.7 nm)

34 35

can be estimated from the CO2 adsorption isotherms assuming the Dubinin–Radushkevich

36 37 38 39 40 41 42

method.145 Regarding the morphological characterization, Scanning Electron Microscopy (SEM) is commonly used to analyze the char particle structure and surface topography.2,11

43 44 45 46 47 48 49

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

50 51

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

52 53 54 55 56 57 58

groups (change in the O–H stretch peak around 3400 cm –1, which dominates the feedstock’s spectrum).11 Assignment of other spectral peaks of interest for biochar samples, including the aliphatic C–H stretch at 3000–2860 cm –1, the aromatic C–H stretch around 3060 cm –1, and the

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various aromatic ring modes at 1590 and 1515 cm –1, was proposed by Sharma and co-workers.146

6 7 8 9 10 11 12

The peaks characteristic of the carbonyl groups should appear in the range 1660–1725 cm –1. The exact position of the peaks depends on whether the carbonyl groups are in conjunction with the aromatic ring (position below 1700 cm –1) or not (position above 1700 cm –1).146

13 14 15 16 17 18 19

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

20 21 22 23 24 25 26

the carbon and oxygen forms on the biochar surface. For the C1s spectrum, different binding energies are assigned to C–C, C=C, C–H, C–O, C=O, and COO stretches; whereas for the O1s spectrum, signal peaks at different binding energies can be attributed to O=C and O–C stretches. 101

27 28

Aromatic character

29 30 31 32 33

Solid-state

13

C

Magic Angle Spinning (MAS) Nuclear Magnetic Resonance (NMR) is

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

34 35

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

36 37 38 39 40

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

41 42

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

43 44 45 46 47 48 49

aromatic structure. Freitas and co-workes148 reported

13

C 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 ºC, these authors observed two main resonance lines,

50 51

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

52 53 54 55 56

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

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

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produced at higher peak temperatures (390–605 ºC), the authors showed a progressive development

6 7 8 9 10

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

11 12

aliphatic groups (broad line around 30 ppm).

13 14 15 16 17 18 19

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

20 21 22 23 24 25 26

biochar rapidly increases when peak temperature is above 400 ºC. In addition, the authors also reported that proportion of aromatic C detected was similar for both CP and DP techniques for all charcoals.

27 28 29 30 31

6. CONCLUSIONS

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

32 33 34 35 36 37 38

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

39 40

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

41 42 43 44 45

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

46 47

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

48 49 50 51 52

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,

53 54

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

55 56 57 58 59

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

6 7 8

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

9 10

●

Among the operating conditions of the slow pyrolysis process, the peak temperature

11

seems to be the most important parameter affecting the characteristics of biochar

12 13 14

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

15 16

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

17 18

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

19 20

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

21 22 23

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

24 25

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

26 27 28

●

Although slow pyrolysis or carbonization is the process commonly used to produce

29 30

biochar, because of the high charcoal yields obtained, other technologies cannot be

31 32

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

33 34

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

35 36 37

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

38 39

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

40 41

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

42 43

biochar.

44 45 46 47 48 49

●

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

50 51

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

52 53 54 55 56

agent at a final temperature of 700–850 ºC) or an additional carbonization step (under an inert atmosphere at a temperature of 850–1000 ºC). In both cases (but especially in the

57 58

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

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30


Page 31 of 56


1 2 3 4

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

5 6 7

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

8 9

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

10 11

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

12 13 14

from a given biomass feedstock.

15 16 17

●

18

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

19 20

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

21 22 23

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

24 25

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

26 27

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

28 29 30 31 32 33

●

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

34 35

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

36 37 38 39 40

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.

41 42 43 44 45 46 47 48 49 50

ACKNOWLEDGEMENTS

The author would like to thank Prof. Clara Martí 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

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suggestions aimed at improving the paper.

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manure and charcoal amendments. Plant Soil 2003, 249, 343–357.

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Chicken manure biochar as liming and nutrient source for acid Appalachian soil. J. Environ. Qual. 2011, in press; DOI 10.2134/jeq2011.0124.

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[128] Rondon, M.; Lehmann, J.; Ramirez, J.; Hurtado, M. Biological nitrogen fixation by common

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beans (Phaseolus vulgaris L.) increases with bio-char additions. Biol. Fertil. Soils 2007, 43, 699– 708. [129] Steiner, C.; Glaser, B.; Teixeira, W. G.; Lehmann, J.; Blum, W. E. H.; Zech, W. Nitrogen

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retention and plant uptake on a highly weathered central Amazonian Ferralsol amended with compost and charcoal. J. Plant Nutr. Soil Sci. 2008, 171, 893–899. [130] Liang, B.; Lehmann, J.; Solomon, D.; Sohi, S.; Thies, J. E.; Skjemstad, J. O.; et al. Stability of

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Long term effects of manure, charcoal and mineral fertilization on crop production and fertility on a highly weathered Central Amazonian upland soil. Plant Soil 2007, 291, 275–290.

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et al. Effects and fate of biochar from rice residues in rice-based systems. Field Crops Res. 2011, 121, 430–440.

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sludge biochar and bioavailability of metals in production of cherry tomato (Lycopersicon

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esculentum). Chemosphere 2010, 78 , 1167–1171.

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straw-derived biochar: Characteristics and its effects on soil properties of an Ultisol in southern China. Soil Tillage Res. 2011, 112, 159–166.

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a strategy to sequester carbon and increase yield in durum wheat. Eur. J. Agron. 2011, 34, 231–238. [137] Deenik, J. L.; McClellan, T.; Uehara, G.; Antal, M. J.; Campbell, S. Charcoal volatile matter content influences plant growth and soil nitrogen transformations. Soil Sci. Soc. Am. J. 2010, 74,

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[145] Plaza, M. G.; Pevida, C.; Martín, C. F.; Fermoso, J.; Pis, J. J.; Rubiera, F. Developing almond

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shell-derived activated carbons as CO2 adsorbents. Sep. Purif. Technol. 2010, 71, 102–106. [146] Sharma, R. K.; Wooten, J. B.; Baliga, V. L.; Lin, X.; Geoffrey Chan, W.; Hajaligol, M. R. Characterization of chars from pyrolysis of lignin. Fuel 2004, 83, 1469–1482.

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[147] Gao, Y.; Wang, X. H.; Yang, H. P.; Chen, H. P. Characterization of products from hydrothermal treatments of cellulose. Energy 2012, 42, 457–465. [148] Freitas, J. C. C.; Bonagamba, T. J.; Emmerich, F. G. Investigation of biomass and polymer-

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based carbon materials using 13C high-resolution solid-state NMR. Carbon 2001, 39, 535–545.

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[149] McBeath, A. V.; Smernik, R. J.; Schneider, M. P. W.; Schmidt, M. W. I.; Plant, E. L. Determination of the aromaticity and the degree of aromatic condensation of a thermosequence of

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[150] Lehmann, J.; Silva, J. P.; Rondon, M.; Silva, C. M.; Greenwood, J.; Nehls, T.; et al. Slashand-char –a feasible alternative for soil fertility management in the central Amazon?. In Proceedings

41 42

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48 49 50 51 52 53 54 55 56 57 58 59 60

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1 2 3 4 5

Table 1. Charcoal yields obtained from slow pyrolysis of several biomass feedstocks

6 7 8 9

Reference

Biomass

Composition

10

Pressure (MPa)

11 12 13

Alder wood

14 15

Dried with air Lignin: 24.8% Ash: 1.1%

0.1

16

1.0

17 18 19 20 21

Birch wood

22 23


0.1

24 25 26

1.0

Antal et al. (2000)27

27 28

Oak wood

29


0.1


0.1


0.1

Dried with air Lignin: 40.0% Ash: 0.8% Moisture: 16% Lignin: 18.1% Ash: 1.3% Moisture: 29% Moisture: 56.9%

0.4 1.0 3.3

30

1.0

31 32

Pine wood

33 34 35

1.0

36 37

Spruce wood

38 39 40 41

Macadamia nut shells

42 43 44 45

Antal et al. (1996)46

Eucalyptus wood

46 47 48

1.0

1.0

49 50 51

Olive husk

52 53 54 55

Demirbas (2001)

24

0.1

56 57 58 59 60


Beech wood


Heating conditions 2 K min –1 up to 450 ºC followed by 60 min of soaking at 450 ºC 2 K min –1 up to 450 ºC followed by 240 min of soaking at 450 ºC 6 K min –1 up to 450 ºC with no soaking time 2 K min –1 up to 450 ºC followed by 60 min of soaking at 450 ºC 2 K min –1 up to 450 ºC followed by 240 min of soaking at 450 ºC 6 K min –1 up to 450 ºC with no soaking time 2 K min –1 up to 450 ºC followed by 60 min of soaking at 450 ºC 6 K min –1 up to 450 ºC with no soaking time 2 K min –1 up to 450 ºC followed by 60 min of soaking at 450 ºC 6 K min –1 up to 450 ºC with no soaking time 2 K min –1 up to 450 ºC followed by 60 min of soaking at 450 ºC 6 K min –1 up to 450 ºC with no soaking time

10 K min –1 up to 450 ºC followed by 15 min of soaking at 450 ºC

10 K min –1 up to 377 ºC with no soaking time 10 K min –1 up to 477 ºC with no soaking time 10 K min –1 up to 577 ºC with no soaking time 10 K min –1 up to 377 ºC with no soaking time 10 K min –1 up to 477 ºC with no soaking time 10 K min –1 up to 577 ºC with no soaking time

Charcoal yield (% w/w)

Number in Fig. 3

30.5

1

29.8

2

35.9

3

28.8

4

32.1

5

34.6

6

31.2

7

39.8

8

32.1

9

35.2

10

32.2

11

37.5

12

40.5 44.4 51.0

13 14 15

41.8

16

42.2 46.1

17 18

39.7

19

36.8

20

33.3

21

29.7

22

26.2

23

24.7

24

48


Page 49 of 56


1 2 3 4 5

Table 1 (continued)

6 7 8 9

Reference

Biomass

Composition

10

Pressure (MPa)

11 12 13

24

Demirbas (2001) Corncob

14


0.1

15 16 17 18 19 20 21 22 23 24

Karaosmanoglu et al. (2000)47

Straw and stalks of rapeseed plant


0.1

Heating conditions 10 K min –1 up to 377 ºC with no soaking time 10 K min –1 up to 477 ºC with no soaking time 10 K min –1 up to 577 ºC with no soaking time 5 K min –1 up to 400 ºC with no soaking time 5 K min –1 up to 500 ºC with no soaking time 5 K min –1 up to 600 ºC with no soaking time 5 K min –1 up to 700 ºC with no soaking time

Charcoal yield (% w/w) 26.0

Number in Fig. 3 25

23.2

26

21.5

27

39.4

28

35.6

29

32.6

30

29.6

31

25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

49



Page 50 of 56

1 2 3 4 5

Table 2. Activation conditions and textural properties of several activated charcoals

6 7 8 9

Reference

10 11

Biomass Charcoal feedstock production

Oxidizing agent

Oak wood

12 13 14 15 16 17

Zhang et al. (2004)80

Corn hulls

Fast pyrolysis at 500 ºC

CO2

18 19

Corn stover

20 21 22 23 24 25

Plaza et al. (2009)84

Olive stones

Skodras et al. (2007)85

Pine wood Oak wood Olive seed waste

26 27 28 29 30 31 32 33 34 35 36 37

Demiral et al. (2008)86

Olive bagasse

Lua and Gia (2009)87

Oil palm shell

38 39 40 41 42

Slow pyrolysis up to 600 ºC

Fast pyrolysis at 800 ºC Slow pyrolysis up to 500 ºC Slow pyrolysis up to 400 ºC

CO2

Activation temperature (ºC) 700 700 800 800 700 700 800 800 700 700 800 800 Not activated 800 800 800

Burn-off (%)

(m2 g –1)

(cm3 g –1)

V 0 /V

31.8 41.8 43.0 51.4 32.3 37.0 44.4 45.2 41.5 41.7 42.6 50.2

642 644 845 985 977 902 1010 975 660 432 712 616

0.270 0.245 0.321 0.379 0.335 0.328 0.435 0.379 0.282 0.182 0.285 0.234

0.657 0.606 0.534 0.592 0.376 0.399 0.521 0.557 0.577 0.546 0.519 0.555

–

43

n. a.

n. a.

20.0 40.0 50.0

613 909 1079

0.242 0.364 0.436

0.840 0.833 0.868

n. a. (activation time: 2.5 h)

897

0.340

0.557

684

0.220

0.489

800

n. a. (activation time: 3 h)

1690

0.700

0.778

850

n. a. (activation time: 0.5 h)

718

0.315

0.851

900

n. a. (activation time: 1 h)

1183

0.310

0.449

140

0.092

0.948

176

0.090

0.783

800

n. a. (activation time: 0.5 h) n. a. (activation time: 1 h) n. a. (activation time: 1.5 h) n. a. (activation time: 2 h)

269

0.129

0.872

352

0.178

0.918

900 H2O–CO2 mixture

Steam

Steam

43 44 45 46 47 48 49 50 51 52 53

Nowicki and Pietrzak (2010)83

Pine sawdust pellets

Slow pyrolysis up to 800 ºC

CO2

S BET

V 0

54 55 56 57 58 59 60

50


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1 2 3 4 5

Table 3. Cation exchange capacity of soils amended with biochar

6 7 8 9 10

Biochar –1

CEC (cmol kg –1)

(g kg )

Available K –1

(cmol kg )

Available Ca –1

(cmol kg )

Available Mg –1

pH

(cmol kg )

11

Soil: terra preta from Amazonia; biochar: from incomplete combustion of organic material 5

12 13 14

1.4

3.20

0.239

2.58

0.330

Not reported

11.9

28.9

0.026

15.4

1.71

Not reported

15 16 17 18

Soil: ferralsol from Amazonia; biochar: from incomplete combustion of black locust wood 150

19 20

0

5.40

2.81

1.48

0.88

Not reported

135

29.0

25.8

1.7

1.0

Not reported

21 22 23 24

Soil: ferralsol from Amazonia; biochar: from incomplete combustion of black locust wood 126

25 26

0

5.40

2.81

1.48

0.88

5.14

100

28.5

25.8

1.71

0.97

5.89

27 28 29 30

Soil: red ferrosol (ferralsol 151) from New South Gales (Australia); biochar: from slow pyrolysis of

31

paper mill wastes 96

32 33 34 35 36 37 38

0

4.03

0.11

1.23

0.30

4.20

20

10.5

0.66

8.87

0.67

5.93

Soil: calcarosol (carcisol 144) from Victoria (Australia); biochar: from slow pyrolysis of paper mill

39

wastes96

40 41 42 43 44

0

31.0

2.07

21.7

6.23

7.67

15

29.0

2.23

20.3

6.10

7.67

45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

51



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1 2 3 4 5

Table 4. Summary of studies assessing the impact of biochar on crop yield

6 7 8 9 10 11

Reference Glaser et al. (2002)5

12 13 14 15 16

Steiner et al. (2007)132

17 18 19 20 21 22

Van Zwieten at al. (2010)96

23 24 25

Soil type

Biochar

Crop

Response

A weathered xanthic ferralsol from Central Amazonia (Brazil) A weathered xanthic ferralsol from Central Amazonia (Brazil) A ferralsol (acidic soil) from New South Gales (Australia)

From secondary forestry wood. Addition rate: 67.2 and 135.2 Mg ha –1.

Rice and cowpea

From secondary forestry wood. Addition rate: 11.0 Mg ha –1.

Rice and sorghum

At a rate of 67.2 Mg ha –1 biomass increased by 20% (rice) and 50% (cowpea) compared to control (no biochar). At 135.2 Mg ha –1 biomass cowpea increased by 100%. Stover and grain yields increased by 29% and 73%, respectively; compared to control treatment (only inorganic fertilizer).

From mixtures of paper mill wastes and wood chips pyrolyzed at 550 ºC. Addition rate: 10.0 Mg ha –1.

Wheat, soybean and radish

26 27 28 29 30 31 32 33 34 35 36 37

Van Zwieten at al. (2010)96

A carcisol (alkaline soil) from Victoria (Australia)

From mixtures of paper mill wastes and wood chips pyrolyzed at 550 ºC. Addition rate: 10.0 Mg ha –1.

Wheat, soybean and radish

Major et al. (2010)125

A clay-loam ferralsol from the oriental savanna of Colombia

Commercial wood charcoal. Addition rate: 20.0 Mg ha –1.

Maize

38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

Wheat: biochar improved yield by a factor of 1.3. When biochar was applied together to inorganic fertilizer, yields increased by a factor of 2.4 compared to using fertilizer alone. Soybean: biochar improved yield by a factor of 1.4 in the presence of fertilizer. Radish: dry biomass production was significantly increased by a factor of 1.5–2 both in the presence and absence of fertilizer. Biochar significantly increased both pH and CEC and reduced Al availability. Wheat: yields were reduced by a factor of 2 both in the presence and absence of inorganic fertilizer. Soybean: biochar improved yield by a factor of 1.3 in the presence of fertilizer. Radish: biochar addition in the absence of fertilizer increased biomass production by a factor of 1.5. However, in the presence of fertilizer, yields were reduced by a factor of 2. Effect for 4 years (2003–2006). Maize grain yield did not significantly increase in the first year, but increases over the control were 28, 30 and 140% for 2004, 2005 and 2006, respectively.

56 57 58 59 60

52


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1 2 3 4 5

Table 4 (continued)

6 7 8 9 10 11

Reference

Soil type

Biochar

Hossain et al. (2010)134

A luvisol from Sydney (Australia)

From sewage sludge pyrolysed at 550 ºC. Addition rate: 10.0 Mg ha –1.

Cherry tomato

Haefele et al. (2011)133

An anthraquic gleysol from Laguna (Philippines)

From rice husk partially burned in a combustion chamber Addition rate: 0.413 Mg ha –1.

Rice


A humic nitisol from Siniloan (Philippines)


Rice


A gleyic acrisol from Ubon (Thailand)


Rice

Peng et al. (2011)135

A typical From rice straw ultisol from pyrolyzed at low southern China heating rate and at peak temperatures below 450 ºC. Addition rate: 240 Mg ha –1.

Maize

Vaccari et al. (2011)136

A silt loam soil (with a sub-acid pH of 5,2) from the region of Tuscany (Italy)

Durum wheat

12 13

Crop

14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

A commercial charcoal obtained from coppiced woodlands (beech, hazel, oak, and birch). Addition rate: 30–60 Mg ha –1.

Response Application of biochar improves the production of cherry tomatoes by 64% above the control soil conditions. The yield of production was found to be at its maximum when biochar was applied in combination with an inorganic fertilizer. Effect for 4 years (2005–2008). Application of carbonized rice husks increased total organic carbon, total soil N, the C/N ratio, and available P and K. Biochar application decreased rice yields, especially in the first few seasons after application. For the entire period evaluated, rice yield decreased by 2.5% relative to control conditions. Effect for 4 years (2005–2008). Application of carbonized rice husks increased total organic carbon, total soil N, the C/N ratio, and available P and K. For the entire period evaluated, rice yield increased by 8.9%. Effect for 4 years (2005–2008). Application of carbonized rice husks increased total organic carbon, total soil N, the C/N ratio, and available P and K. For the entire period evaluated, rice yield increased by 8.7%. The effected of biochar amendment application on the 40-day maize dry matter production was marked: in the absence of NPK fertilizer, biomass production increased by 64%. In the presence of fertilizer, biomass production increased by a factor of 3 compared to using fertilizer alone. Effect for 2 years (2009–2010). Biochar addition significantly increased grain production with respect to the control. On average, the grain yield increase ranged from 28% to 39%. No significant differences were observed between biochar rate treatments of 30 and 60 Mg ha –1.

53



Page 54 of 56

1 2 3 4 5

Caption for figures

6 7 8

Figure 1. Score and loading plots for the three first principal components: (a), second versus first

9 10

principal component; (b); third versus second principal component.

11 12 13 14 15 16 17

Figure 2. Percentage of the theoretical value of yFC for four biomass feedstocks: leucaena wood

(LW), oak wood (OW), macadamia nut shells (MS), and corncob (CC). The white bars correspond to the results obtained using a slow pyrolysis process, whereas the gray bars indicate the results

18 19

obtained when a FC process was used.

20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

54


Page 55 of 56


1 2 3 4 5 6

Coefficient PC1

(a)

7

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

4

8 9

0.6

soaking time

10 11

0.4

2

12

lignin

13 14 15 16 17 18

0.2

2 C P t 0.0 n e i c i f f -0.2 e o C

char yield 2 C P

0

pressure

19

moisture

ash

20

-2

21

-0.4

22 23

heating rate

24 25

-0.6

peak temp.

-4

26

-2

0

27

2

4

PC 1

28 29 30 31

(b)

32 33

Coefficient PC2 -0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

34

0.6

35

lignin

2

36

heating rate

37

0.4

38 39 40

43 44

3 C P

pressure

45 46

3 C P t 0.0 n e i c i f f -0.2 e o C

0

41 42

0.2

peak temp.

-2

47

ash

soaking time

char yield

-0.4

48 49

moisture

50 51 52 53 54

-0.6

-4 -4

-2

0

2

4

PC 2

55 56 57

Figure 1. Score and loading plots for the three first principal components: (a), second versus first

58 59 60

principal component; (b); third versus second principal component. 55


2012 Pyrolysis for Biochar Purposes a Review to Establish Current Knowledge Gaps and Research Needs.

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