Characterisation of biochar from maize residues produced in a self-purging pyrolysis reactor

Accepted Manuscript Characterisation of biochar from maize residues produced in a self-purging pyrolysis reactor Kiatkamjon Intani, Sajid Latif, Zebin Cao, Joachim Müller PII: DOI: Reference:

S0960-8524(18)30771-5 https://doi.org/10.1016/j.biortech.2018.05.103 BITE 20009

To appear in:

Bioresource Technology

Received Date: Revised Date: Accepted Date:

3 April 2018 28 May 2018 30 May 2018

Please cite this article as: Intani, K., Latif, S., Cao, Z., Müller, J., Characterisation of biochar from maize residues produced in a self-purging pyrolysis reactor, Bioresource Technology (2018), doi: https://doi.org/10.1016/j.biortech. 2018.05.103

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Characterisation of biochar from maize residues produced in a self-purging pyrolysis reactor

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Kiatkamjon Intani *, Sajid Latif, Zebin Cao, Joachim Müller

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University of Hohenheim, Institute of Agricultural Engineering, Tropics and Subtropics Group (440e), Stuttgart 70599, Germany

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Abstract

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Response surface methodology was used to optimise pyrolysis conditions to produce biochar from maize residues (cobs, husks, leaves and stalks).

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The aim was to obtain biochar with good potential as an additive for composting. Mathematical models were developed to explain the experimental

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responses of volatile matter content (VM), ash content (AC), pH and electrical conductivity (EC) to the operating parameters such as temperature,

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heating rate and holding time. The temperature had the most significant influence on biochar properties. AC, pH and EC significantly increased

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(p < 0.05) with increasing temperature, while the VM decreased. The holding time showed less effect on the responses, while the heating rate had

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insubstantial effect. Under the optimal conditions, the husk and leaf biochar had higher AC (11.42 and 26.55%), pH (10.96 and 11.51), and EC

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(12.37 and 6.79 mS/cm), but lower VM (7.38 and 8.39%) than those of cob and stalk biochar.

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Keywords:

Biochar; Characteristics; Composting; Maize residues; Self-purging pyrolysis

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Corresponding author: K. Intani E-mail address: [email protected] Tel: +49 (0)711 459 23114 Fax: +49 (0)711 459 23298

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1. Introduction Maize (Zea mays L.) residues are abundant, but still underutilised in the major maize production areas worldwide (Intani et al., 2016). For

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instance, China produced annually more than 200 million tons of maize straw (Chen et al., 2009). Maize residues can be used as an important

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feedstock for biochar production via slow pyrolysis because maize residue is an adequate, renewable and low cost biomass (Peterson and Jackson,

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2014). Biochar is a carbonaceous material produced via thermochemical conversion of biomass under oxygen-limited conditions. Slow pyrolysis

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has a lower reaction temperature (below 700 °C) and longer vapour residence time compared to fast pyrolysis. It is preferable to fast pyrolysis, when

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high biochar yield (up to 35%) is required (Sadhukhan et al., 2014). Converting maize residues to biochar can generate additional income for

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smallholder farmers in developing countries. The biochar can be used for environmental applications such as carbon sequestration, co-composting

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and heavy metals removal at a lower cost than other strategies (Sadhukhan et al., 2014; Sanchez-Monedero et al., 2018; Zhou et al., 2018). It can

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also be used for the production of solid fuel or activated carbon.

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In slow pyrolysis, the properties of the produced biochars depend on the differences in the biomass composition, such as the elemental

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constitute, the existence of contaminants, cellulose, hemicellulose and lignin content, and moisture content (Kloss et al., 2012). The characterisation

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of biochar generally include the proximate analyses, ultimate analyses, higher heating value (HHV), pH, electrical conductivity (EC), elemental

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analyses, cation exchange capacity (CEC) and the analysis of the microstructures (Mitchell et al., 2013). The feedstock and pyrolysis conditions,

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especially temperature and holding time were found to have a strong influence on the physicochemical properties of the biochar (Kloss et al., 2012;

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Steiner, 2016). Composting can be considered as a good alternative for the treatment of organic solid waste. In recent years, biochar has been used

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as an additive for the composting of organic waste. The incorporation of biochar showed promising improvement of the composting processes and

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compost quality (Sanchez-Monedero et al., 2018; Xiao et al., 2017). Subsequently, the high quality compost can be used to improve soil properties

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and fertility (Godlewska et al., 2017). Since the compost is intended for soil improvement, therefore the quality of biochar used in composting

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process should be suitable for soil application. For various environmental applications, biochar with appropriate properties is required. The valuable

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properties of biochar for composting process including porosity structure, chemical recalcitrance, large surface area and low content of toxic

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compounds (Sanchez-Monedero et al., 2018). Therefore, it is important to design and produce biochar with properties, which can fulfil the specific

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requirements for the application.

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The maize stover, which refers to leaves, husks, and stalks, was usually referred to as maize straw (Mullen et al., 2010). Maize cob and

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stover were the main maize residues, which had been frequently investigated. However, there are few studies focusing on the characteristic

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differences between biochars derived from different maize biomass fractions, such as cobs, husks, leaves, and stalks (Liu et al., 2014). In addition,

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nitrogen was widely used as the purging gas to maintain the oxygen-limited atmosphere, which significantly increased the cost of biochar

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production. The use of nitrogen as the medium to maintain an oxygen-free thermal cracking was also found to be the environmental impact hotspot

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in life cycle assessment studies (Gear et al., 2018). Farmers in rural areas do not have access to pure nitrogen, which is a barrier for the farmers to

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produce and use biochar. Moreover, the price of pure nitrogen gas would be too high for smallholder farmers. For example, nitrogen gas costs 0.42

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EUR per litre in Germany (Intani et al., 2016). Therefore, the need exists for the development of a low-cost biochar production system, which will

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enable the rural households to produce a quality biochar at a lower cost. The biochar yield from maize cobs, husks and leaves produced at different

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pyrolysis conditions using a self-purging pyrolysis reactor was reported in our previous study (Intani et al., 2016). Producing biochar without using

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nitrogen as purging gas (self-purging pyrolysis) can be considered as an alternative to reduce the production cost. The configurations of pyrolysis

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reactors without purging gas including ablative, auger screw, rotating cone and vacuum types were discussed with an in-depth analysis by

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Sadhukhan et al. (2014). It was found that each type of pyrolyser had advantages and disadvantages. One of the disadvantages of a self-purging

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pyrolysis reactor might be a negative effect on the quality of the biochar, i.e. the volatile matter content (VM) might increase drastically. To ensure

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the quality of the biochar, it is necessary to analyse the important characteristics of the biochar derived from a low-cost production system and

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simple equipment. Fundamental quality parameters of the biochar including ash content (AC), VM, pH and EC need to be carefully investigated.

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These parameters especially proximate analysis results can be used for the evaluation of biochar quality (Klasson, 2017). A detailed and systematic

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characterisation of biochar can also contribute to the establishment of database, which can be used for the reverse engineering of biochar (Morales et

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al., 2015).

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In the present study, the physiochemical characteristics of maize residues (cobs, husks, leaves and stalks) and their biochars were analysed.

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The objective of this work was to investigate the effects of different operating parameters (temperature, heating rate and holding time) of a self-

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purging pyrolysis on biochar properties. Response surface methodology was used to identify the optimal operating parameters to produce biochar,

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which has good potential as an additive in composting process, i.e. low VM, high AC, high pH and high EC.

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2. Materials and methods

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2.1 Biomass and biochar preparation

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The collection, preparation and analyses of maize residues and their biochars were carried out according to the procedures and standard

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methods as described by Intani et al. (2016). The pyrolysis of the maize residues was performed using a self-purging pyrolysis reactor. A detailed

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description of the pyrolysis process was also presented in our previous study (Intani et al., 2016). The biochars were ground by a coffee mill

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(CM3260, GRUNDIG Intermedia GmbH, Neu-Isenburg, Germany) and sieved to obtain a particle size less than 250 μm. The ground samples were

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stored in plastic containers until use.

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2.2 Characterisation of biochars

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The oven drying method for estimating moisture content (MC) was done based on the standard method of German Institute for

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Standardisation (DIN 51718, 2002). The determination of VM and AC of biochar was carried out according to the standard methods DIN 51720

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(2001) and DIN 51719 (1997), respectively. The fixed carbon content (FC) of biochar sample on a dry basis (wt.% db) was calculated by difference,

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as follows:

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FC  100  VM  AC

(1)

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The higher heating value (HHV) was estimated using the calorimeter (Parr 6100, Parr Instrument Company, Illinois, USA) according to the

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standard method DIN EN 14918 (2009). The determination of pH values was performed according to the standard method DIN ISO 10390 (2005),

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approximately 0.5 g biochar was mixed with 5 ml 0.01 mol/L CaCl2 solution (ratio: 1:10, w/v) and shaken for 1°h (Kloss et al., 2012; Van Zwieten 5

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et al., 2010). The pH values of the suspensions were electrometrically measured by a calibrated pH meter (HQ40D, Hach Company, Colorado,

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USA). As described previously by Kloss et al. (2012), the EC was measured in biochar water extract (1:10 w/v) using an EC meter (Cond 315i/SET,

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Xylem Analytics Germany Sales GmbH & Co. KG, Weilheim, Germany).

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For the ultimate and elemental analyses, the CHNS analysis was performed using an elemental analyser (EA3000, EuroVector, Pavia, Italy).

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Oxygen content was calculated by difference. Major mineral cations (Al, Ca, Fe, K, Mg, Mn, Na, Zn) and phosphorus were estimated through ICP-

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OES; while the trace elements (Cd, Co, Cr, Cu, Ni, Pb, Rb, Sr) were determined by ICP-MS. The scanning electron microscope (SEM) (DSM 940,

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Carl Zeiss AG, Oberkochen, Germany) was used to visualize the microscopic structures of the maize cob and its biochar according to the method

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described in our previous study (Intani et al., 2016).

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2.3 Design of experiments

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Statistically designed experiments were arranged using response surface methodology combined with Box–Behnken design (RSM-BBD).

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The experimental design, data analysis and model fitting were performed using Design Expert software (Version 9.0.4, Stat-Ease, Inc., Minneapolis,

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MN, USA). This experimental design was used to investigate the main effects and interactions of pyrolysis conditions (temperature, heating rate and

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holding time) on biochar properties (VM, AC, pH and EC) and to identify the optimal operating parameters to obtain the most suitable biochar for

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composting from each biomass fraction (cobs, husks, leaves and stalks). In total, 15 randomised experimental runs with three replicates at the centre

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points were generated for each biomass fraction. A detailed description of the experimental design was mentioned in the previous study (Intani et

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al., 2016).

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The mathematical representation of the full quadratic model is presented as:

Y   0   i X i   j X j   k X k   ij X i X j   ik X i X k   jk X j X k (2)

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  ii X i2   jj X 2j   kk X k2

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where Y represents the experimental responses including VM, AC, pH and EC of biochars, with Xi as temperature (°C), Xj as heating rate (°C/min),

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Xk as holding time (min) and β as regression coefficients.

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The relationships between the independent variables and each individual response were built using Eq. 2. Since the VM, AC, pH and EC

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were the important responses indicating the quality of biochar for remediation of contaminated soils and addition to composting pile (Klasson, 2017;

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Steiner, 2016), an optimal pyrolysis condition was predicted based on the targets of these four responses for each biomass fraction. The optimal

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pyrolysis condition was considered to produce biochar to utmost fulfil the targets of low VM, high AC, high pH and high EC. The optimal

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conditions and responses were predicted by the response surface model of each individual response using the Design-Expert software.

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2.4 Statistical analysis

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The statistical analyses were performed on the main characteristics of the maize biomass and biochar samples using general linear model

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(GLM) program. The differences among the means for each property at 95% confidence level were identified based on Fisher’s least significant

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difference test (LSD) using SAS program (Ver. 9.0, SAS Inst., Cary, NC, USA).

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The full quadratic models were fitted using Design Expert software and used for the prediction of the responses. Sliced plots of the quadratic response surface models for the VM, AC, pH and EC were created using MATLAB R2010a (The Mathworks Inc, Massachusetts, USA). The 7

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analysis of variance (ANOVA) and the prediction of the optimal pyrolysis conditions were performed using Design Expert. Statistical significance

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was tested at a significance level of 95%. The goodness of fit and the accuracy of the mathematical models were evaluated using the coefficient of

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determination (R2) and mean absolute percentage error (MAPE), respectively. In order to determine the optimal pyrolysis conditions, a numerical

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optimisation process with desirability function was used (Jung et al., 2016). The desirability value ranges from zero to one, whereas zero and one

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represent the least and the most desirable solution, respectively (Sáez-Bastante et al., 2015).

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3. Results and discussion

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3.1 Characteristics of the maize biomass and biochar samples

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The main characteristics of the maize biomass and biochar samples were presented in Table 1 and Fig. 1. Maize stalks had the highest MC

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and VM (10.11 and 75.29%) but lowest FC (20.11%). Among the biomass feedstock, the highest AC was found in leaves (9.49%). The lowest MC

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(6.66%) and volatile matter (67.78%) were found in husks and leaves, respectively. Among the biomass samples, cobs contained the lowest AC

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(1.54%) but highest FC content (25.51%). The highest FC content (85.20%) was found in cob biochar produced at 600 °C, which corresponded to

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the highest lignin content of 22.40% in cob biomass (Intani et al., 2016). In comparison to the biochemical composition of maize cobs, husks and

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leaves in our previous study (Intani et al., 2016), the stalk biomass contained 28.78% of cellulose, 23.27% of hemicellulose and 22.23% of lignin.

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The content of these three components was found to have significant effects on biochar properties (Kloss et al., 2012; Steiner, 2016).

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As shown in Table 1, the carbon (C) and nitrogen (N) content of the biochars ranged from 57.40 to 86.92% and 0.49 to 1.72%, respectively. The cob biochar contained the highest C content (72.58 to 86.92%), followed by the husk and stalk biochar. The leaf biochar, in contrast, had the

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lowest carbon content (57.40 to 63.55%). In addition, increasing temperature resulted in higher C content in the biochars. The leaf biochar had

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relatively higher N content (1.11 to 1.72%) than the other three biochars, which contained similar N content (0.49 to 0.81%). The higher operating

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temperature was found to have negative effect on the N content. Only in the leaves and its biochars, S could be detected and the concentration was

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not above 0.06%. Fig. 1 shows the H/C and O/C ratios of biomass and biochar samples. It was shown that all biochars produced at the temperature

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above 300 °C were successfully converted into C-rich materials, indicated by a H/C ratio less than 0.7 (EBC, 2012). In order to produce a biochar

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with a high potential for carbon sequestration, the pyrolysis temperature should be set above 450 °C. A biochar with a high stability should have the

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O/C ratio less than 0.2 (Intani et al., 2016; Spokas, 2010). There was a positive correlation between the operating temperature and C content of the

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biochars (see Table 1). This was in agreement with the results from Kloss et al. (2012) and Sadaka et al. (2014), who indicated that the C content of

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biochar (derived from wheat straw, spruce wood, poplar wood, and switchgrass) increased with the rising of pyrolysis temperature.

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High temperature intensified the loss of structural water from cellulose, hemicellulose, and lignin (Sadaka et al., 2014). The C and S contents

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of cob biochar were similar to the results of Mullen et al. (2010), who highlighted the importance of carbon in biochar. Biochar applied into

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composting pile can replace the C, N and some other plant nutrients after adding the compost into soils. Most importantly, carbon in biochar is

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exceedingly stable and might remain in the soil for hundreds to thousands of years (Mullen et al., 2010), which has a great benefit for carbon

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sequestration. The negative effect of pyrolysis temperature on the N content in the leaf biochar was in agreement with the study of Bagreev et al.

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(2001) on the sewage sludge biochar. They found that the organic N existed in the low-temperature produced biochar in the form of amine

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functionalities. The organic N was transformed to pyridine-like components at high temperature, which resulted in the decline of N in the biochar. A

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low N content in biochar presented less potential as an additional source of nutrient for plants.

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As shown in Table 2, Ca, K, Mg and P were four main ions with large quantity in the biochars. Leaf biochar contained the highest Ca and P

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content (15628 to16805 and 2225 to 2390 mg/kg, respectively), while the highest K (38338 to 43996 mg/kg) and Mg (14419 to16576 mg/kg) were

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found in stalk biochar. Nevertheless, cob biochar presented the lowest contents of Ca, K, Mg and P (293, 12584, 4298 and 705 mg/kg). The

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temperature showed various effects on the Ca, K, Mg and P content of each biochar. With increasing operating temperature (300 to 600 °C), Ca in

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stalk biochar, K and Mg in stalk and leaf biochar also increased. The lowest Ca contents were observed in cob, husk, and leaf biochar with the

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pyrolysis temperature of 450 °C. This trend existed also for the K and Mg in cob and husk biochar. In general, temperature had a negative impact on

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P content except for the leaf biochar, which obtained the lowest P content at 450 °C. In addition, the Fe content was strongly connected to the

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operating temperature. Especially at 600 °C, the Fe concentrations of all biochar were far higher than those produced at other two temperatures. In

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regard to the trace elements, the Cr and Ni contents were highly dependent on the maize biomass fractions and pyrolysis temperatures. Increasing

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operating temperature drastically increased the Cr and Ni content of all biochars, showing biochar under 600 °C obtained largely greater amount of

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Cr and Ni than the biochar under other two temperatures.

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The levels of alkali and alkaline elements (e.g. Na, K, Ca, and Mg) are most likely leading to high pH and EC (Singh et al., 2010), which can

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provide a liming effect to composting pile. Compared to the results from Mullen et al. (2010), the Mg and P contents of cob biochar from this study

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were higher (2150 and 4360 mg/kg, respectively); while the K and Ca were remarkably lower (43350 and 970 mg/kg, respectively). A few studies

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focused on biochar derived from other fractions of maize residues. However, the results of this study also indicated that some soluble plant

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nutrients, such as K and P, could be provided by the biochar derived from different fractions of maize residues. The negative effect of pyrolysis

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temperature on the P content could be explained by the volatilization at low temperature (even at 500 °C). In addition, the phenomenon of extremely

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high contents of Fe, Cr, and Ni of all biochars pyrolysed at 600 °C might originate from the walls of the pyrolysis reactor. The reactor was made of

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stainless steel, and at high temperature, a few cations might migrate from the stainless steel to the biochar due to corrosion, leading to an increase of

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Fe, Cr, and Ni content depending on pyrolysis temperature. These heavy metals might cause environmental problems.

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The SEM images of maize cob and cob biochar produced under various pyrolysis conditions showed that the structure of cob biochar was

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more heterogeneous and amorphous than those of cob biomass (Angın and Şensöz, 2014). In addition, the pores were clearly visible in the cob

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biochar. It can be seen that the pore size in cob biochar produced at 450 °C was larger than those of biochars produced at 300 and 600 °C.

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3.2 Volatile matter content of biochars

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As shown in Table 3, the experimental results of the VM of the biochars were obtained according to the RSM-BBD design matrix. The VM

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varied among biochar derived from different maize fractions (cobs, husks, leaves and stalks), which confirmed the strong influence of the biomass

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feedstock on the biochar properties (Klasson, 2017; Kloss et al., 2012; Steiner, 2016). In general, the VM of cob and stalk biochar were slightly

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higher than those of husk and leaf biochar. The VM of stalk biochar ranged from 13.56 to 33.47%, while the leaf biochar contained 7.84 to 30.12%

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of volatile matter.

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The four-dimensional response surface plots (Fig. 2) illustrated the interactive effects of the three operating factors on the VM of biochars

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produced from maize cobs, husks, leaves and stalks. The three surfaces in each plot represent the varying temperatures (i.e. 300, 450 and 600 °C).

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The effect of temperature could be clearly observed on the surface plots. Second order polynomial equations describing the relationship between the

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VM of the biochars and three operating factors were fitted without data transformation.

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The goodness of fit and accuracy of the mathematical models for the VM of the biochars were shown in Table 4. The effect of temperature

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and the second-order of temperature was highly significant for all biochars at p < 0.05. The temperature had negative effect on the VM in the

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biochars (Liu et al., 2014). This finding was in agreement with the results from a previous study by Antal Jr. and Grønli (2003), who emphasized

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that operating temperature predominantly controlled biochar quality, such as VM. It was found that an increase in temperature from 300 to 600 °C

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resulted in a substantial decrease in the VM in all biochars from 30 to 10%. This effect was compatible with the results of Rajkovich et al. (2011),

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who indicated that the VM of maize stover dropped by 28.38% (from 51.87 to 23.49%) as the heating temperature increased from 300 to 600 °C.

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The higher values from Rajkovich et al. (2011) were mainly ascribed to the different methods of determining VM. In their work, the standard

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method ASTM D1762-84 (2013) was used to estimate the proximate analyses, in which the VM was measured under 950 °C for 10 min instead of

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900 °C for 7 min according to DIN 51720 (2001). Biochar may consist of non-volatile component (relatively aromatic) and labile volatile

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component (relatively aliphatic). The non-volatile component is referred to a substance with high C and low O content, such as aromatic C. This

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non-volatile component is the resistant component in the biochar. In contrast, the labile volatile component contains lower C but higher O content,

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which is prone to be eliminated by a high pyrolysis temperature. Therefore, volatile matter, as the labile volatile component made of aliphatic C,

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carboxyl and carbohydrate, can be easily influenced by an increase in pyrolysis temperature (Peng et al., 2011).

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The heating rate showed negligible impact on the VM. An unfavourable installation of the ventilation of the muffle furnace could have

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caused the small difference among the three heating rates. There were a limited number of researchers, who studied the effect of the heating rate on

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the VM. Karaosmanoglu et al. (2000) investigated the impacts of heating rate on the characteristics of biochar derived from straws and stalks of

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rapeseed. It was found that the biochar produced at 10 °C/min had higher VM (6.52%) than those produced at 5 and 15 °C/min, with the values of

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6.05% and 4.95%, respectively.

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There was an insignificant negative correlation between the holding time and VM. However, the interaction term between temperature and

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holding time was significant for leaf and stalk biochars. In general, the volatile matter slightly decreased when the holding time increased from 30 to

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90 min except for the biochar from cob and stalk at high temperature (600 °C). In those cases, a longer holding time led to a slight increase in the

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VM. The negative effect of holding time was corresponding to the studies by Peng et al. (2011) and Sadaka et al. (2014). Peng et al. (2011)

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indicated that VM of rice straw biochar decreased with increasing holding time (from 2 to 8 h) at various pyrolysis temperatures. This negative

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correlation was also reported by Sadaka et al. (2014) in a study on switchgrass biochar.

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The R2 values were ≥ 0.9783, showing that the developed models in Table 4 could explain more than 97% of the variability in the VM. The accuracy of the mathematical models was evaluated by calculating MAPE using the observed and predicted values of the VM. The MAPE values

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were 4.24, 6.28, 2.82 and 3.74% for cob, husk, leaf and stalk biochars, respectively. The small MAPE values further confirmed good fitting of the

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models.

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3.3 Ash content of biochars

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Table 3 shows the experimental data of the AC in the biochars derived from cobs, husks, leaves and stalks produced under various pyrolysis

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conditions. The AC was dependent on the maize biomass fractions and pyrolysis conditions, which was similar to the case of the VM. The AC of

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cob biochar ranged from 3.04 to 4.79%, which were much lower than those of husk, leaf and stalk biochar. Among them, leaf biochar showed the

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highest AC (19.25 to 27.89%), followed by stalk and husk biochar (8 to 12%). The low AC could be attributed to less mineral elements in the cobs

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(Zhao et al., 2013), whereas the high AC in leaves was most likely ascribed to the accumulation of different inorganic components (Wang et al.,

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2013). Nevertheless, some elements, e.g. P, K, and S are likely to volatilise even at a temperature of 500 °C, which is far below the ashing

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temperature of 815 °C according to the standard method DIN 51719 (1997). This could probably lead to an underestimation of the AC of biochar

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from maize biomass fractions.

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Fig. 3 shows the interaction effect of temperature, heating rate and holding time on the AC of biochars produced from maize residues. The

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significant positive effect of temperature was clearly indicated in Fig. 3. The AC remarkably increased with the rising temperature. This was in

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agreement with the results of (Liu et al., 2014). Wang et al. (2013) also reported that an increase of pyrolysis temperature (500 to 700 °C) resulted in

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5% increase in the AC of maize straw biochar with 8 and 16 h of holding time, but resulted in a decrease in the AC of 0.6% with 4 h holding time.

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Their work also clearly showed the positive effect of the temperature on AC of biochar derived from other feedstock, such as bamboo, elm wood,

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rice straw. However, Rajkovich et al. (2011) found that the highest AC of maize stover biochar (17.60%) was produced at 500 °C among the

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biochar paralysed under a temperature range from 300 to 600 °C. An increase in pyrolysis temperature enhanced loss of organic components, which

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resulted in a high AC in biochar (Wang et al., 2013).

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Second order polynomial equations were generated to explain the relationship between the AC of the biochars and three operating variables.

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As shown in Table 4, the developed quadratic regression models were well fitting as indicated by the significant p-values for the coefficients of the

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models. It was evident that the temperature had a highly significant effect on the AC of the biochars, with p-values ≤ 0.0012. The temperature also

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showed a significant quadratic effect on the AC of all biochars.

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It was found that the holding time presented less significant influence on the AC of the biochars. The effect of the holding time was

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significant only on the AC of cob biochar. There was a positive effect of holding time on the AC in cob, husk and stalk biochars. In contrast, it

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showed a negative effect on the AC of leaf biochar. The results in this study were different from the study of Wang et al. (2013). In their study, the

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AC of a maize stalk biochar with 8 h holding time (25.6%) was lower than that of 4 and 16 h holding time, with the values of 30.0 and 28.8%,

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respectively. It was probably due to the various holding time ranges that led to the difference in the AC. Nonetheless, Sadaka et al. (2014) reported

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similar results to the present study, indicating the positive correlation between the holding time and AC in switchgrass biochar.

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The heating rate had negligible effects on the AC of the biochars. With the rising of the heating rate, the AC values maintained stable at the

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same temperature as illustrated in Fig. 3. According to the findings of Karaosmanoglu et al. (2000), the AC of biochars derived from stalk and straw

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of rapeseed showed no difference at 5 and 10 °C/min (15.31%), while the AC increased by merely 0.42% at 15 °C/min (15.73%). It could be

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concluded that the temperature and holding time were the main factors affecting the AC in the biochars, whereas the heating rate had negligible

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effect. As shown in Table 4, R2 values were ≥ 0.9245, which indicated that the models were well fitting. The accuracy of the models was further

233

confirmed by the small MAPE values of 1.26, 1.58, 2.06 and 1.87% for cob, husk, leaf and stalk biochars, respectively.

234

3.4 pH value and electrical conductivity of biochars

235

The experimental results of the pH values and the EC of biochars derived from maize biomass fractions (cobs, husks, leaves and stalks) were

236

listed in Table 3. The pH and EC values varied among the biochars derived from different fractions and pyrolysis conditions. In general, pH values

237

of leaf biochar were the highest (9.42 to 11.54), followed by husk and stalk. Cob biochar presented the lowest pH values, with a range of 7.86 to

238

9.21. Husk biochar presented the highest EC with the values ranging from 4.34 to 13.18 mS/cm. The EC of stalk biochar was lower than husk

239

biochar but higher than leaf biochar. Cob biochar (0.89 to 4.09 mS/cm) showed extremely low EC compared to the other biochars.

240

The overall interaction effect of temperature, heating rate and holding time on the pH and EC values of biochars was presented in Fig. 4 and

241

Fig. 5, respectively. It was evident from the changes on the colour-map that the temperature had the most influential effect on the pH and EC values.

242

The predominant effect of the temperature on leaf and stalk biochars was observed in Fig. 4c and Fig. 4d, while this effect on the EC of husk and

243

stalk biochars was presented in Fig. 5b and Fig. 5d. The full quadratic models were established to describe the relationship between the pH and EC

244

values and the three operating parameters. The models were evaluated for a goodness of fit, as described in Table 4.

245

The p-values indicated that the model parameters were significant for all biochars at p < 0.05. The effect of temperature was significant. The

246

second-order of temperature was also significant (p < 0.05) for the pH of cob, husk and leaf biochars, while it was significant for the EC of cob, leaf

16

247

and stalk biochars. The temperature presented a positive effect on the pH and EC values of the biochars. The heating rate and holding time had

248

relatively small effects on the pH and EC values. The effect of holding time was significant only for pH of the leaf biochar and the EC of the cob

249

biochar. The interaction terms were not significant for the pH, but the interaction term between temperature and holding time was significant for the

250

leaf biochar.

251

The R2 values for both pH and EC values of all biochars were ≥ 0.9474 (see Table 4), which implied that the developed models were in a

252

promising goodness of fit. The small MAPE values of 0.53, 0.98, 0.28 and 1.07% for the pH, and 2.84, 8.89, 1.08 and 2.18% for the EC indicated

253

that the models were able to accurately predict the pH and EC values of the biochars.

254

The AC and ash constituents of a biochar, to a large extent, determine the pH and EC of the biochar. In other words, higher ash and mineral

255

contents lead to higher pH and EC. The pH is more dependent on the alkali metals (e.g. Na and K); whereas, the alkali-earth metals, such as Ca and

256

Mg, are more likely to affect the EC. The pH and EC of the biochar are largely dependent on the feedstock and pyrolysis conditions, these findings

257

were in agreement with the results from a previous study by Luo et al. (2014). As shown in Table 3, the leaf biochar contained the highest AC and

258

had the highest pH, followed by husk and stalk biochar with the pH approximately from 9.0 to 10.90. The lowest pH was detected in cob biochar,

259

which corresponded to the lowest AC in cob biomass. However, the correlation between AC and EC was not conclusive. The highest EC was found

260

in husk biochar, followed by stalk and leaf biochar, with the values of 4.14 to 11.14 mS/cm and 4.67 to 6.79 mS/cm, respectively. Cob biochar had

261

the lowest EC value. The elemental analyses did not support the explanation that the alkali-earth metals (Ca and Mg) affected the EC. For example,

262

the leaf biochar (under conditions of 600 °C, 10 °C/min, 60 min) contained higher Ca and Mg contents than those of the husk biochar, but the husk

17

263

biochar had higher EC than that of the leaf biochar. The reason could be that K also strongly influenced the EC. In addition, lack of replicates of

264

measuring elemental composition might generate the inconsistency.

265

Operating temperature was the most significant factor influencing pH and EC of the biochars (Lee et al., 2013). Increasing temperature

266

significantly increased the pH and EC. These findings were in consistent with Luo et al. (2014), who found that the pH of maize straw increased

267

from around 5.5 to 10 with increasing temperature from 200 to 500 °C, while the EC roughly increased from 0.8 to 1.2 mS/cm. Wagner and

268

Kaupenjohann (2014) indicated that the pH of maize-derived biochar produced at 750 °C for 2 h was 9.3, which was similar to the results of the

269

present study. In addition, Singh et al. (2010) also reported the positive effects of temperature on pH and EC of Eucalyptus saligna wood biochar.

270

According to a previous study by Bagreev et al. (2001), the drastic increase of pH with increasing pyrolysis temperature was due to the

271

dehydroxylation reactions.

272

Due to the problem with the ventilation of the muffle furnace used in this study, the heating rate had a negligible effect the pH and EC. A

273

few previous studies focused on the effect of heating rate on the pH and EC of biochar. The holding time showed significant effect on the EC of cob

274

biochar and the pH of leaf biochar. In general, extending the holding time slightly increased the pH and EC of the biochars from all maize biomass

275

fractions; while Wang et al. (2013) found that extending the holding time (from 4 to 16 h) had no effect on the pH of maize straw biochar.

276

3.5 Optimal pyrolysis conditions

277

The target criteria for identifying the optimal operating parameters to produce biochar for composting improvement were low VM, high AC,

278

high pH and high EC. According to a previous study, the VM should be below 20% as described in Table 5 (Brewer et al., 2011). The AC limit was

18

279

set at 50%, although many researchers think that the limit was too low (IBI, 2015). A high AC indicated that the biochar had low C content, which

280

led to a low potential for carbon sequestration. On the other hand, high AC could provide plant nutrients such as K, Ca, Mg and other micronutrients

281

(Brewer et al., 2011). A pH greater than neutral was considered as a positive property of biochar, but the pH value should not be above 10 (EBC,

282

2012). It was found that biochar with EC value of ≤ 2.0 mS/cm had negligible effects from a salt stress on seed germination (Hoekstra et al., 2002).

283

Allaire et al. (2015) reported that the negative effect of high EC level (i.e. salinity) on plant growth was not evident, when the biochar was mixed

284

with soil or compost.

285

All experimental responses were weighed at the same level, therefore the VM, AC, pH and EC were given the same level of importance. The

286

results in Table 5 show that the optimal temperatures were relatively high (584.11 to 600.00 °C). There was no consistent trend in the optimal

287

heating rate; while the optimal holding time ranged from 66.80 to 90 min. The cob biochar from the optimal pyrolysis conditions (588.42 °C,

288

11.09 °C/min, 90 min) would have the VM of 11.61%, AC of 4.75%, pH of 9.14 and EC of 4.05 mS/cm. Under the optimal conditions (600.00 °C,

289

5.00 °C/min, 90 min), the husk biochar showed higher pH and EC (10.96 and 12.37 mS/cm, respectively) and lower contents of ash and volatile

290

matter (11.42 and 7.38%, respectively). The leaf biochar at the optimal conditions (600.00 °C, 15.00 °C/min, 79.37 min) had the highest AC and pH

291

values (26.55% and 11.51, respectively). The leaf biochar had the volatile matter of 8.39% and the EC of 6.79 mS/cm. Under the optimal conditions

292

(584.11 °C, 15.00 °C/min, 66.80 min), the stalk biochar showed higher values than cob biochar with the AC of 12.10%, VM of 15.00%, pH of 10.89

293

and EC of 10.96 mS/cm. The lowest AC, pH and EC were found in the cob biochar; while the husk biochar contained the lowest VM. As shown in

19

294

Table 5, the desirability values for all biochars were ≥ 0.9485. This indicated that the optimal conditions obtained by the numerical optimisation

295

process were highly desirable.

296

The findings of this study clearly indicated that the temperature and holding time were the main influential factors on the VM, AC, pH and

297

EC of the biochars. The interaction effect of the temperature and holding time is presented in Fig. 6. The differences in the properties among the

298

biochars from various biomass feedstocks were easily identified using the three-dimensional plots.

299

In general, the biochar is supposed to have the abilities to neutralise the compost due to its alkaline nature (Czekała et al., 2016), return

300

nutrients to soil, increasing soil CEC, and contain low volatile matter (< 20%) (Brewer et al., 2011). Deenik et al. (2010) reported that compared

301

with untreated soil, the soil amended by partially carbonised macadamia nut shell biochar (with high volatile matter of 22.5%) obtained lower yields

302

from lettuce and maize. While the soil amended by more carbonised biochar (with low volatile matter of 6.3%) had no negative impact on plant

303

growth, which could remarkably increase the yield when applied together with N fertilizer. The negative effects of biochar could be ascribed to the

304

high volatile matter in partially carbonised biochar, which contained the phenolic components. The microbial activity was stimulated by the

305

phenolic components, resulting in less availability of inorganic N (Manyà, 2012). The phenol was considered as a phytotoxic compound (Wang et

306

al., 2002), which may directly inhibit the growth of plant roots. Furthermore, the decomposition of volatile matter by microorganisms would

307

enhance microbial respiration with increasing the net release of CO2 (Mitchell et al., 2013). Higher AC has the potential to provide more mineral

308

nutrients, which can be directly adsorbed by plants (Steiner, 2016; Wang et al., 2013). Yuan and Xu (2011) suggested that soil pH was positively

309

related to the biochar pH. Moreover, biochar with high EC can be selected as soil supplement to remedy the mineral nutrients loss (Luo et al., 2014).

20

310

The high optimal temperatures for the biochar production were corresponding to results above, which indicated that operating temperature strongly

311

influenced the VM, AC, pH and EC. Increasing temperature drastically enhanced the AC, pH and EC and remarkably reduced the VM (Fig. 6). The

312

inconsistent trend of heating rates was most likely due to the problem of the muffle furnace. Longer holding time (66.80 to 90 min) was also

313

compatible with the results above.

314

In this study, it was found that the husk and leaf biochar from the optimal pyrolysis conditions were more appropriate for composting due to

315

the lower VM. Moreover, husk biochar presented highest EC and leaf biochar contained the highest AC. However, Rajkovich et al. (2011)

316

suggested that the quantity of ash might not affect the short-term maize growth. Both, beneficial mineral nutrients and adverse salts (e.g. Na) may

317

exist in ash. Moreover, it was unnecessary to apply high pH biochar into alkaline soils or compost, which might lead to an adverse effect on the crop

318

growth (Chan and Xu, 2009). Therefore, different soils and crops needed different biochar with appropriate properties in order to improve soil

319

fertility and crop performance (Van Zwieten et al., 2010). The optimal pyrolysis conditions and corresponding responses were estimated by the

320

developed models; further experimental analyses are required to verify the accuracy of the predictions and ensure the quality of the biochar.

321

4. Conclusions

322

It was found that the husk and leaf biochar were more desirable for composting with the optimal conditions of 600 °C, 5 °C/min, 90 min and

323

600 °C, 15 °C/min, 79 min, respectively. The cob biochar contained the highest C content (72.58 to 86.92%), which increased with rising pyrolysis

324

temperature. Nitrogen content was found to be higher in leaf biochar (1.11 to 1.72%). Based on those findings, it can be concluded that maize husk

325

and leaf were suitable to be converted into biochar at 600 °C for composting. The results could be useful for establishing a biochar database.

21

326 327

Acknowledgements This work was supported by the Food Security Center of Universität Hohenheim, the foundation fiat panis, the German Academic Exchange

328

Service (DAAD), the Federal Ministry of Economic Cooperation and Development (BMZ) and the German Federal Ministry of Education and

329

Research (BMBF). This research is the result of the project BiomassWeb WP 5.1 (Project No. 031A258F).

330

Appendix A. Supplementary data

331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349

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Figures

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Fig. 1. H/C and O/C ratios of biomass and biochar samples, including maize cobs (MC), husks (MH), leaves (ML) and stalks (MS) and biochars

430

produced at different temperatures (300, 450 and 600 °C). Different letters indicate significant differences in H/C (A-H) and O/C (a-j) ratios at p-

431

value < 0.05. Dashed lines are the upper limit of 0.7 for H/C ratio and 0.2 for O/C ratio, indicating degree of carbonisation in biochar.

432

27

433

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434 435

Fig. 2. Sliced plot of the quadratic response surface models for the volatile matter content (VM) of biochars produced from maize (a) cobs, (b)

436

husks, (c) leaves and (d) stalks.

29

437

30

438

439

31

440

Fig. 3. Sliced plot of the quadratic response surface models for the ash content (AC) of biochars produced from maize (a) cobs, (b) husks, (c) leaves

441

and (d) stalks.

442

32

443

444

33

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Fig. 4. Sliced plot of the quadratic response surface models for the pH values of biochars produced from maize (a) cobs, (b) husks, (c) leaves and

446

(d) stalks.

447

34

448

449 450

Fig. 5. Sliced plot of the quadratic response surface models for the electrical conductivity (EC) of biochars produced from maize (a) cobs, (b) husks,

451

(c) leaves and (d) stalks. 35

452

36

453

37

454 455

Fig. 6. Three-dimensional plots of the interaction effect of temperature and holding time on (a) volatile matter content (VM), (b) ash content (AC),

456

(c) pH and (d) electrical conductivity (EC) of biochars derived from maize residues.

38

457 458

39

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Tables

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Table 1 Main characteristics of the maize biomass and biochar samples produced at 300, 450 and 600 °C.

462 463 464 465 466

Sample

Proximate analysis (wt.% db) MC VM AC

FC

Ultimate analysis (wt.% db) C H N

Cobs CB300

7.85b ± 0.11 1.48g ± 0.04

72.95b ± 1.09 33.91d ± 0.47

S

O

25.51j ± 1.08 63.06g ± 0.54

46.92k ± 0.30 72.58f ± 0.38

6.08a ± 0.05 3.74c ± 0.16

0.61g ± 0.02 0.76e ± 0.03

-

44.86b ± 0.29 18.23hi ± 0.06 19.78ef ± 0.51 28.47cd ± 0.19

CB450

1.10h ± 0.28

14.55gh ± 1.52 3.94l ± 0.07

81.51b ± 1.57

83.76b ± 0.46

2.45f ± 0.05

0.65fg ± 0.03 -

9.01ij ± 0.48

31.49a ± 0.12

CB600

2.04de ± 0.04

10.46j ± 0.22

4.33k ± 0.03

85.20a ± 0.23

86.93a ± 0.52

1.29i ± 0.08

0.49h ± 0.01

-

6.79l ± 0.58

31.67a ± 0.07

Husks

6.66c ± 0.28

74.24a ± 0.14

2.97m ± 0.02

22.79k ± 0.14

44.96l ± 0.32

6.02a ± 0.07

0.49h ± 0.02

45.57a ± 0.29

17.86i ± 0.19

HB300

1.79f ± 0.08

28.99f ± 0.23

7.78i ± 0.23

63.23g ± 0.18

68.29g ± 0.13

3.48d ± 0.04 0.80e ± 0.03

-

19.09f ± 0.13

27.97de ± 0.12

HB450

1.95ef ± 0.04

15.24g ± 0.06

10.36f ± 0.12

74.40e ± 0.06

77.87c ± 0.30

2.17g ± 0.05 0.62fg ± 0.02 -

8.91j ± 0.31

29.27b ± 0.06

HB600

1.98def ± 0.27 11.51j ± 0.45

11.28d ± 0.20 77.21c ± 0.29

78.24c ± 0.35

1.23i ± 0.03

8.58j ± 0.37

28.55cd ± 0.07

Leaves

7.69b ± 0.09

67.78c ± 0.76

9.49g ± 0.16

22.73k ± 0.78

43.68m ± 1.16 5.82b ± 0.13 0.95d ± 0.11

0.06a 39.88d ± 0.91

17.78i ± 0.95

LB300

1.46g ± 0.16

30.12e ± 0.31

20.41c ± 0.13

49.47i ± 0.29

57.40j ± 0.45

3.23e ± 0.05

0.04a 17.41g ± 0.51

23.14g ± 0.07

LB450

1.28gh ± 0.02

13.19i ± 0.11

27.89a ± 0.10

58.92h ± 0.14

63.19i ± 0.47

1.90h ± 0.07 1.28b ± 0.02

0.05a 7.53k ± 0.47

23.60g ± 0.19

LB600

1.43g ± 0.03

7.84k ± 0.47

26.30b ± 0.16 65.86f ± 0.60

63.55i ± 0.22

0.93j ± 0.03

1.11c ± 0.03

0.06a 7.84k ± 0.22

23.26g ± 0.16

Stalks

10.11a ± 0.14

75.29a ± 1.35

4.60j ± 0.06

20.11l ± 1.41

47.42k ± 0.20

5.98a ± 0.02

0.26i ± 0.02

41.75c ± 0.16

18.51h ± 0.28

SB300

0.68i ± 0.18

33.36d ± 0.13

8.40h ± 0.17

58.24h ± 0.19

67.02h ± 0.12

3.49d ± 0.03 0.81e ± 0.03

-

20.04e ± 0.13

27.03f ± 0.14

SB450

2.07de ± 0.20

14.02hi ± 0.09

11.13d ± 0.20 74.85de ± 0.29 75.70e ± 0.42

10.66h ± 0.45

28.66bc ± 1.04

SB600

2.20d ± 0.13

13.56hi ± 0.16

10.90e ± 0.01

2.11g ± 0.07 0.64fg ± 0.02 1.27i ± 0.04 0.65fg ± 0.02 -

9.67i ± 0.17

27.52ef ± 0.05

1.54n ± 0.01 3.03m ± 0.14

75.54d ± 0.16

76.74d ± 0.20

0.66f ± 0.02 1.72a ± 0.03

-

*

Calorific value HHV (MJ/kg)

MC = Moisture content, VM = Volatile matter content, AC = Ash content, FC = Fixed carbon content. C = Carbon, H = Hydrogen, N = Nitrogen, S = Sulphur, O = Oxygen, HHV = Higher heating value. CB, HB, LB and SB stand for cob, husk, leaf and stalk biochar, produced at 300, 450 and 600 °C. * Calculated by difference. a-n Different letters indicate significant differences within the same column at p-value < 0.05. 40

467

Table 2

468

Mineral content (mg/kg) and trace element content (mg/kg) of the maize biomass and biochar samples produced at operating temperature of 300,

469

450 and 600 °C (values based on dry matter).

470 471

Feedstock T (°C) Al Ca Fe K Mg Mn Na P Zn Cd Cobs 97.5 285 61.9 6383 373 8.0 18.9 430 33.1 <0.15 CB300 300 15.7 293 53.7 12584 4298 8.9 9.4 705 53.0 <0.10 CB450 450 22.5 182 87.5 9998 3401 4.5 11.2 363 27.2 <0.10 CB600 600 26.4 185 609.1 11522 3911 14.7 14.9 271 38.2 <0.10 Husks 82.8 2285 77.5 9209 1061 29.4 41.2 552 25.3 <0.15 HB300 300 70.8 3440 129.6 20869 8126 51.1 73.1 1014 43.9 <0.10 HB450 450 126.0 1781 165.4 19762 7223 24.8 48.9 652 29.7 <0.10 HB600 600 173.0 2802 283.4 29448 10808 33.9 72.3 925 39.0 <0.10 Leaves 200.0 11477 304.0 7479 2081 127.0 52.1 1422 25.3 <0.15 LB300 300 357.0 15628 462.1 13276 9754 186.0 54.6 2225 38.6 0.25 LB450 450 534.0 15280 488.0 14896 10237 167.0 65.1 2095 31.7 0.20 LB600 600 725.0 16805 639.2 15896 11142 178.0 104.4 2390 20.5 <0.10 Stalks 77.0 2306 72.8 9350 1064 29.2 49.9 562 25.0 0.15 SB300 300 31.3 4888 59.4 38338 14419 18.2 22.1 1336 24.9 0.11 SB450 450 42.7 5472 78.4 43874 16463 14.4 29.6 1082 20.0 <0.10 SB600 600 60.6 5673 241.5 43996 16576 19.7 33.7 1021 25.5 <0.10 CB, HB, LB and SB stand for cob, husk, leaf and stalk biochar, produced at 300, 450 and 600 °C.

Co <0.15 0.04 0.09 1.13 <0.15 0.09 0.15 0.43 <015 0.20 0.25 0.50 0.15 0.07 0.10 0.59

Cr 8.42 2.98 9.33 118.00 5.89 4.41 8.15 34.90 8.26 5.86 7.35 27.30 6.40 1.78 5.29 55.70

Cu 5.81 5.75 2.50 3.84 5.89 9.53 5.75 9.56 12.10 21.00 17.90 11.00 5.62 10.60 6.14 6.61

Ni 2.61 1.07 2.99 35.20 1.29 1.98 3.53 11.70 1.96 2.03 2.57 10.20 1.24 1.00 2.34 18.00

Pb 0.50 0.59 0.31 0.15 0.40 0.22 0.25 0.22 0.53 1.02 1.17 0.49 0.42 0.18 0.70 0.15

Rb 0.97 2.03 1.10 1.09 1.02 2.33 1.90 2.99 0.90 1.59 1.87 1.99 0.98 3.50 3.51 3.43

Sr 1.06 1.12 0.52 0.52 4.34 7.46 3.66 6.18 19.50 37.30 35.20 36.80 4.25 11.90 10.30 11.20

41

472

Table 3

473

Experiment results of the volatile matter content (VM), ash content (AC), pH and electrical conductivity (EC) of biochars produced from maize

474

residues.

475 476 477 478

Pyrolysis conditionsa T HR HT

VM (wt.% db)b CB HB LB

300 300

5 10

60 30

30.92 29.29 26.48 29.51 34.13 29.00 30.12 33.47

3.04 8.12 3.04 7.78

300

10

90

28.57 25.47 26.29 27.88

300

15

60

450

5

450

pH (-) CB HB

LB

SB

EC (mS/cm) CB HB LB

20.45 9.75 20.41 8.40

7.86 9.09 7.93 9.11

9.72 9.42

9.31 8.92

0.90 6.00 0.89 4.34

4.67 4.80 4.88 4.14

3.28 8.49

19.25 8.13

8.19 9.37

9.66

9.39

0.96 5.80

4.72 4.60

29.01 23.15 27.31 27.34

3.13 9.06

19.52 8.68

8.21 9.84

9.58

9.60

0.96 6.46

4.71 4.86

30

13.32 13.34 13.96 15.53

4.06 10.15 24.68 11.11

8.95 10.21 10.58 10.14

2.51 8.82

5.27 8.45

5

90

12.72 12.57 12.25 16.06

4.08 10.38 25.27 11.83

8.96 10.53 10.87 10.04

2.90 9.04

5.22 8.92

450

10

60

15.30 15.24 13.19 14.02

3.94 10.36 27.89 11.13

8.92 10.13 10.73 10.11

2.48 7.73

5.48 8.51

450

10

60

14.13 11.95 12.39 14.41

4.26 10.78 24.14 10.23

9.01 10.47 10.62 10.36

2.71 12.76 5.39 9.10

450

10

60

13.65 12.90 13.09 15.77

4.17 10.15 26.24 11.32

8.89 10.64 10.78 10.49

2.72 8.96

450

15

30

13.74 11.90 13.25 16.14

3.87 10.44 26.14 10.82

8.90 10.30 10.58 10.12

2.55 10.79 5.31 8.27

450

15

90

13.90 13.77 13.52 16.82

4.29 10.96 25.79 10.60

8.93 10.45 10.76 10.32

3.06 9.57

600

5

60

11.90 7.88

9.08

15.04

4.33 11.18 26.35 11.88

9.03 10.67 11.33 10.29

3.73 10.95 6.41 10.25

600

10

30

10.46 11.80 7.84

13.56

4.33 11.28 26.30 10.89

9.06 10.45 11.16 10.77

3.68 12.73 6.02 10.02

600

10

90

10.63 9.70

8.29

15.83

4.79 11.63 25.06 12.34

9.21 10.86 11.54 10.70

4.09 13.18 6.79 11.14

600

15

60

11.77 8.63

7.94

15.62

4.48 11.17 27.09 11.98

8.96 10.63 11.41 10.89

3.75 12.20 6.50 10.99

SB

AC (wt.% db) CB HB LB

SB

SB

5.17 9.45 5.64 8.82

a

Pyrolysis conditions include temperature (T), heating rate (HR) and holding time (HT). The operating conditions were temperature of 300, 450, 600 °C, heating rates of 5, 10, 15 °C/min and holding time of 30, 60, 90 min. b db = dried basis. CB, HB, LB and SB stand for cob, husk, leaf and stalk biochar, respectively.

42

479

Table 4

480

The goodness of fit and accuracy of the mathematical models for volatile matter content

481

(VM), ash content (AC), pH and electrical conductivity (EC) of biochars produced from

482

maize cobs, husks, leaves and stalks (see Eq. 2). Response

VM

483

Biochar

R2

adjusted R2

CB 0.9900 0.9719 HB 0.9783 0.9394 LB 0.9970 0.9917 SB 0.9847 0.9573 AC CB 0.9831 0.9528 HB 0.9765 0.9341 LB 0.9245 0.7887 SB 0.9605 0.8895 pH CB 0.9826 0.9514 HB 0.9504 0.8612 LB 0.9968 0.9910 SB 0.9483 0.8552 EC CB 0.9959 0.9884 HB 0.9474 0.8292 LB 0.9854 0.9591 SB 0.9909 0.9746 CB, HB, LB and SB stand for cob, husk, leaf and stalk biochar, respectively.

MAPE

4.24 6.28 2.82 3.74 1.26 1.58 2.06 1.87 0.53 0.98 0.28 1.07 2.84 8.89 1.08 2.18

484

43

Table 5 Optimal pyrolysis conditions, corresponding responses and recommended values. Cob

Husk

Leaf

Stalk

biochar

biochar

biochar

biochar

Temperature (°C)

588.42

600.00

600.00

584.11

-

Heating rate

11.09

5.00

15.00

15.00

-

90.00

90.00

79.37

66.80

-

11.61

7.38

8.39

15.00

< 20% (Brewer et al.,

Biochar

Recommended value

(°C/min) Holding time (min) Volatile matter content (wt.%) Ash content

2011) 4.75

11.42

26.55

12.10

≤ 50% (IBI, 2015)

9.14

10.96

11.51

10.89

7 ≤ pH ≤ 10 (EBC,

(wt.%) pH

2012) Electrical

4.05

12.37

6.79

10.96

≤ 2.0 mS/cm

conductivity

(Hoekstra et al.,

(mS/cm)

2002)

Desirability

0.9653

0.9625

0.9485

0.9608

-

44

Highlights 

Maize cobs, husks, leaves and stalks were pyrolysed at 300, 450 and 600 °C.



Pyrolysis temperature had the most significant effect on biochar quality.



Prediction models of biochar quality were developed for the four biomass fractions.



Optimal pyrolysis conditions were identified to produce biochar for cocomposting.



Biochars derived from husk and leaf biomass were preferable for cocomposting.

45

Graphical abstract

46