Volume57 2015 CANADIANBIOSYSTEMSENGINEERING 3.23

Baishali Dutta1, Yvan Gariepy 2 and G.S. Vijaya Raghavan2*

1Eastern Cereal Oilseed Research Centre, Agriculture and Agri-Food Canada, Ottawa ON K1A 0C6 Canada 2Department of Bioresource Engineering, McGill University, Ste Anne de Bellevue QC H9X 3V9, Canada *Email: vijaya.raghavan@mcgill.ca http://dx.doi.org/10.7451/CBE.2015.57.3.23

Received: 2015 June 17, Accepted: 2015 November 27, Published: 2016 January 26.

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Dutta, B., Y. Gariepy and G.S.V. Raghavan. 2015. Effects of process parameters and selective heating on microwave pyrolysis of lignocellulosic biomass for biochar production. Canadian Biosystems Engineering/Le génie des biosystèmes au Canada 57: 3.23-3.32. Biochar has successfully emerged as a solid biofuel to address the concerns of greenhouse gas emissions. This research investigated microwave pyrolysis of maple wood in a laboratory-scale microwave pyrolysis reactor to study the effects of final pyrolysis temperature, holding time and selective heating on the biochar yield through microwave absorbers. A regression model was developed to predict the biochar yield as a function of pyrolysis temperature, holding time and doping ratio. The analysis indicated that microwave heating can fasten the process of pyrolysis conversion reactions and the yield of the pyrolysis products increased with increase in holding time and decrease in process temperature. On the other hand, variation in doping ratio did not have a significant effect on the biomass conversion to biochar. The biochar was analyzed through proximate analysis and differential scanning calorimetry (DSC) to determine its thermodynamic potential. A biochar sample can be characterized as a carbon-rich solid fuel with high fixed carbon content or residual matter but low volatile or volatile matter. The proximate analysis indicated that the highest residual matter (%) to volatile matter (%) ratio was obtained for the pyrolysis temperature of 290°C, holding time of 1 min and dope ratio of 24% while biochar produced at pyrolysis temperature of 250°C, holding time of 1 min and dope ratio of 32% had the highest energy in the DSC analyses. The regression model developed indicated that the predicted values for the exothermic energies were in good agreement to the observed values (P ≤ 0.05). Keywords. Biochar, Biomass, Pyrolysis, Microwaves, Thermodynamics, Proximate analysis. Le biochar est un biocarburant solide qu’a suscité beacoup d’interet pour répondre aux préoccupations des émissions de gaz à effet de serre. Cette recherche se concentre sur la pyrolise avec micro-ondes du bois d'érable dans un réacteur pyrolytique avec microondes en laboratoire pour étudier les effets de la température finale de pyrolyse, le temps de maintien et de chauffage sélectif sur le rendement de biochar par des absorbeurs de micro-ondes. Un modèle de régression a été développé pour prédire le rendement de biochar en fonction de la température de pyrolyse, le temps de maintien et le ratio de dopage. L'analyse a indiqué que le chauffage par micro-ondes peut accélererle réactions de conversion pyrolitique et le rendement en terme des produits pyrolytique augmenteen proportion à l'increment du temps de maintien et de diminution de la température du procédé. D'autre part, la variation de rapport de dopage n'a pas eu d'effet significatif sur la conversion de la

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biomasse en biochar. Le biochar a été analysée à travers l'analyse de proximité et la calorimétrie différentielle à balayage (DSC) pour déterminer sa potentielle thermodynamique. Un échantillon de biochar peut être caractérisé comme un combustible solide riche en carbone à haute teneur de carbone fixe ou de la matière résiduelle, mais faible teneur en matière volatile. L'analyse proximité a indiqué que le plus haut taux de matière résiduelle (en%) par rapport aux composantes volatiles est obtenu avec une température de pyrolyse de 330 ° C, temps de maintien de 5 min et rapport de dopage 32%, tandis que le biochar produit à la température de pyrolyse de 250 ° C, temps de 1 min et avec un rapport de dopage de 32% avait la plus haute d'énergie dans les analyses de DSC. Le modèle de régression développé a indiqué que les valeurs prévues pour les énergies exothermiques sont en accord avec les valeurs observées (P ≤ 0,05). Mots clés: Biochar, Biomasse, Pyrolyse, Micro-ondes, Thermodynamiques, Analyses immédiates.

INTRODUCTION

The past century has seen average surface temperature increase of 1.3 degrees on the Earth and has been projected to be raised by an additional 3.2 to 7.2 degrees over the 21st century by the Intergovernmental Panel on Climate Change (IPCC 2007). This temperature increase has been attributed to a rise in carbon dioxide and other greenhouse gases released from the burning of fossil fuels, deforestation, agriculture and other industrial processes (Schahczenski and Hill 2009).

Biomass, as an energy source, has two striking characteristics of being the only renewable organic resource and having the ability to capture carbon dioxide in the atmosphere by photosynthesis. Among the various kinds of biomass, woody biomass is the most popular in terms of its application as an energy source, in the form of firewood or charcoal. It is, however, difficult to use firewood or charcoal as an alternative fuel for commercial equipment and industrial processes where fossil fuels, in particular oil, are used at present. Thus it becomes necessary to develop a technology, which leads to conversion of biomass to a more suitable form (Demirbas 1999).

In general, the thermochemical conversion of biomass leads to the formation of biochar in the absence (or under reduction) of oxygen. Biochar is a 2,000 year-old practice that converts agricultural waste into a soil enhancer that

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can hold carbon, boost food security and discourage deforestation. Biochar can potentially play a major role in the long-term storage of carbon. It is used in sequestration of carbon in soil and thereby reducing carbon dioxide levels in the atmosphere through uptake by plants. Biochar increases the fertility, water retention capability of the soil as well as increasing the rate of mineral delivery to roots of the plants (Wu and Abdullah 2009; Lehmann et al. 2003). The co-production of biochar and bioenergy can help in combating global climate change by displacing fossil fuel use and by sequestering carbon in stabilized soil carbon pools (Lehmann 2007).

Recently, there has been a surge on finding alternate methods of efficient pyrolysis techniques for different biomass sources. One of the methods proven to have measured up to good efficiency standards is the use of microwave or microwave assisted pyrolysis methods to form biochar and other useful volatiles (Fernández et al. 2011). However, the concept of using microwave radiation to carry out pyrolysis of biomass for the production of biochar is still in its nascent stages. The inherent nature of dielectric heating could be taken advantage of in this process that would lead to higher yields of pyrolytic products. Microwave heating benefits from the fact that it is very efficient at only heating the material, which is targeted wherein, microwave chamber walls and exterior surfaces are not directly heated (Gaunt 2012; Dai et al. 2010). Moreover, the microwave process has very short residence times compared with conventional techniques, thus improving efficiency and also reducing the process time from hours to minutes (Orsat et al. 2007).

Most of the research on microwave pyrolysis has been focussed on the maximization of bio-oil or syngas production such as in the case of Menéndez et al. (2004) which consisted of the use of microwave pyrolysis of sewage sludge to analyse the different gas fractions. Specific research activities on microwave pyrolysis of biomass include pyrolysis of fir pine wood sawdust (Wang et al. 2009), corn stover (Yu et al. 2007), rice straw (Huang et al. 2008), fir sawdust (Guo et al. 2006), biomass (Wan et al. 2009), coffee hulls (Domínguez et al. 2007) and wheat straw (Budarin et al. 2009).

Many researchers have concluded that in order to attain the optimum pyrolysis conditions for product maximization addition of microwave susceptible doping agent is necessary (Al-Sayegh et al. 2010; Robinson et al. 2010), which has been found to have significant impact on the pyrolysis conditions and the products formed. The advantages of the use of a doping agent are to absorb maximum microwave radiation in the initial phase and then to help sustain the pyrolysis process and final temperatures (Domínguez et al. 2008). Salema and Ani (2011) investigated the effect of microwave irradiation on oil palm biomass pyrolysis and found that char could be used as a microwave absorber to initiate the pyrolysis reaction and enhance the heating process. In our previous research, we found through simulations and experimental

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trials that the doping of the biomass helped to provide a more efficient heat transfer compared to non-doped samples. In the assessment of doping agents of char and graphite, char doping yielded better heat transfer compared to graphite doping, as it resulted in optimal temperatures for maximization of biochar production (Dutta et al. 2015).

Therefore, the current study holds interest to investigate the performance of char as a microwave absorber to initiate the pyrolysis reaction and its ability to enhance the heating process for microwave pyrolysis of wood biomass. The objective of this study was to investigate the effect of variation in the microwave absorber to biomass ratio on the yield of biochar and heating characteristic through the temperature profiles of maple wood biomass under microwave radiation. Further, the effects of the pyrolysis process parameters on the biochar yields and fuel properties of biomass samples were determined by using statistical design techniques, proximate analysis and Differential Scanning Calorimetry (DSC).

MATERIAL AND METHODS Sample preparation The biomass used in this study was maple wood of 25 mm diameter and 33 mm length with a moisture content of ≃ 7% determined in a convective oven through standard ASTM D4442 – 07 methods. The maple wood was then doped with char of lignocellulosic origins (willow wood) to the desired dope to biomass volumetric ratio. The thermal properties of the char dope have been presented in Dutta et al. (2013). The length of the char used as a dope was kept constant at 28 mm and only the diameter of the char dope was varied to attain the required volumetric doping ratio according to the experimental conditions presented in Table 1 (adapted from Dutta et al. 2015). Microwave Pyrolysis Experimental set up The microwave pyrolysis of maple wood was carried out in a custom-built microwave pyrolysis unit as shown in the schematic in Fig.1. A grounded K-type thermocouple was inserted into the reactor through the gaseous product release tube to monitor the biomass temperature continuously throughout the experiment. The thermocouple was inserted in a cylindrical copper shield of 5 mm diameter and 14 mm length to prevent signal interruption by microwaves (Ramaswamy et al. 1998). The wood sample was subjected to microwave heating at 2.45 GHz frequency and 300 W with a heating rate of 30°C/min (Fig. 2) and heated to the desired temperatures according to the pyrolysis conditions presented in Table 1. The microwave source was cycled on and off intermittently to maintain the specific pyrolysis temperature (°C) for the holding time (min) according to the experimental conditions. The details of the microwave pyrolysis process with a photograph of the experimental set up have been presented in Dutta et al. (2015). After pyrolysis, the solid char was removed and weighed to analyze the yield of the product.

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Biochar characterization Proximate analysis The biochar products were analyzed through a proximate analysis of the char using modified ASTM methods (McClaughlin 2010) in a Barnstead Thermolyne 48000 Furnace. A response surface optimization analysis was carried out to evaluate the optimum conditions for the maximization of biochar yields as well as to find the highest residual carbon content from the biochar samples. The modified standard set of ASTM procedures intended for the characterization of solid fuels which can be applied to charcoal that is intended for burning and such testing yields appropriate measurements, as they relate to the burning of charcoal as a fuel (McLaughlin et al. 2009). Differential scanning calorimetry TA Instruments Q100 Differential Scanning Calorimeter (NewCastle, DE, USA) operated with the TA Instruments Q100 DSC 7.0 Build 244 software was used to measure the exothermic enthalpy of the biochar samples. The samples were first placed in aluminium pans (2 mg pan-1) and then hermetically sealed.

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The pans were transferred to the instrument pan holder made to equilibrate to 0°C. These pans were then heated from 0°C to 550°C at a constant rate of 50°C/min. An empty pan was used as a reference. The calorimetric study also maintained the central composite experimental design matrix as shown in Table 1 taking the average for the 6 central points (Run 1) and of the fresh maple wood (control) for a temperature range of 0°C to 550°C with a heating rate of 50°C min-1. The objective of this investigation was to compare the exothermic enthalpy of the biochar samples produced from microwave pyrolysis and carry out a response surface optimization to produce biochar with highest exothermic energy and properties of a soil biofuel under the given optimum pyrolysis conditions.

RESULTS AND DISCUSSION Biochar yields The experimental results were interpreted using a response surface design technique in which the influence of the three experimental variables and their interactions on the response variable of biochar yield was investigated. Based on the experimental results, the following biochar yields were obtained from the microwave pyrolysis of the maple wood decreased with increasing temperature as shown in Fig. 3. This trend has been corroborated in several studies (Mašek et al. 2013; Yu et al. 2010) and has been attributed either to greater primary decomposition of the wood at higher temperatures or to secondary decomposition of the char. The parametric relations concluded in the current investigation were also consistent with previous studies of cellulose and lignocellulosics (Şensöz and Can 2002; Valenzuela-Calahorro et al. 1987).

A regression model for the biochar yields in terms of the pyrolysis process parameters of temperature, holding time and doping ratio was derived. The observed biochar yields ranged between 35 to 72% under the microwave pyrolysis conditions with the % relative error to the predicted values as shown in Fig. 3. The corresponding

Fig. 1. Schematic of the microwave pyrolysis setup.

Fig. 2. Microwave pyrolysis of maple wood with heating rate of 30 °C/min at 300 W with a desired reaction temperature of 330 °C.

Table 1. Experimental conditions with a three factorial central composite design.

Temp (°C) Time (min) Dope ratio (%) Pattern

250 1 16 −−− 250 1 32 −−+ 250 5 16 −+− 250 5 32 −++ 250 3 24 a00 290 3 24 0 290 3 24 0 290 3 24 0 290 3 24 0 290 3 24 0 290 3 24 0 290 5 24 0A0 290 3 16 00a 290 3 32 00A 290 1 24 0a0 330 5 32 +++ 330 5 16 ++− 330 1 16 +−− 330 3 24 A00 330 1 32 +−+

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analysis of variance (ANOVA) for the regression model is presented in Table 2a. The model indicated that the predicted biochar yield had a good correlation to the model with the correlation coefficient of determination of R2 = 0.89 (P ≤ 0.01) and low root mean square error (RMSE) of 4.61% (Table 2b). The predicted biochar yields with the corresponding experimental values are presented in Fig. 4 (a). The high correlation coefficient of determination (Figure 4), implied that the reduced quadratic regression model can be used to explain the pyrolysis reaction, and

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the biochar yield variation of 35.6 – 71.3% was attributed to the independent variables of reaction time, reaction temperature, doping ratio and their interaction. The corresponding analysis of variance (ANOVA) is presented in Table 2a. However, only pyrolysis temperature was found to be the significant factor in determining biochar yields. The predicted optimum levels of the experimental variables of pyrolysis reaction were obtained by applying the regression analysis to the model. The overall % relative error of the measured biochar yields with the

Fig. 3. Yields of biochar produced from the microwave pyrolysis of maple wood with the % Relative error from the predicted values obtained through the regression model (P ≤ 0.01).

Fig. 4. Regression analysis of actual vs. predicted biochar yields with R2 = 0.89 and RMSE = 4.61% (P ≤ 0.05).

Table 2. (a) Analysis of variance of regression model for biochar yields obtained through microwave pyrolysis of maple wood; (b) goodness of fit of the model.

(a) Source DF Sum of

squares Mean square F ratio P > F

Model 9 1635.9 181.77 8.5676 0.001 Error 10 212.16 21.216 Total 19 1848.1

(b) Summary of fit (b) R2 0.89 Root Mean Square Error (RMSE) 4.61 Mean of Response 45.1 Observations 20.0

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predicted value was < 0.5% with the central replicates (290°C, 3 min and 24%) indicating a % relative error of ≤ 5.0%.

The desirability plots are used to indicate if the desirability of the response variable (maximize or minimize) could be increased by increasing any of the factors and is especially useful when there are multiple responses. The highest predicted biochar yield derived through the regression model was 72.6% at the pyrolysis temperature of 250°C, holding time of 3.4 min and dope ratio of 16% with a desirability of 0.92 as shown in Fig. 4 (b). On the other hand, the maximum yield of biochar from the experimental results was 71.3% for pyrolysis conditions of pyrolysis temperature of 250°C, holding time of 1 min and dope ratio of 16%, presenting a relative error of -3.4% with the predicted highest yield obtained from the model. Moreover, the pyrolysis temperature of 250°C, holding time of 1 min and dope ratio of 32% also produced a high yield of 65.9%. The variation of the dope ratio (16- 32%) did not affect the pyrolysis process significantly as shown through the regression analysis by the insignificant model terms (p > 0.50). Although our previous investigations have shown that doping did enhance biochar yield compared to the non-doped samples (Dutta et al. 2013), the results of the current analysis indicated that the variation in the doping ratio did not have a significant effect on the biochar yield. The response surface optimization for the results of the biochar yields are represented by the surface and contour plots for the pyrolysis conditions in Fig. 5. It was evident from the response surface plots that the treatments with lower temperatures produced higher yields of biochar which confirms the findings of a number of studies (Di Blasi 1996; Domínguez et al. 2007; Mašek et al. 2013; Dutta et al. 2011). Validation of Regression Model

microwave pyrolysis.

Fig. 5. Response surface plot of biochar yield for microwave pyrolysis from regression model (a) Y: Temperature; X: Time; (b) Y: Temperature; X: Dope ratio; (c) Y: Time; X: Dope ratio.

Table 3. Regression model validation: Comparison of experimental and predicted biochar yields.

Temp

(°C)

Time

(min)

Dope

ratio (%)

Exp

Yield%

Predicted

yield %

%

Error

250 3 24 65.60 70.03 6.33

290 5 24 51.28 54.47 5.85

290 3 24 66.16 61.29 -7.95

290 1 24 55.51 60.18 7.77

330 3 24 42.09 47.30 11.0

Mean 4.60

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occurs at the expense of the carbon content in the biochar sample. Research in relation to biomass combustion has shown that feedstock containing more silica in their ash content have relatively high slagging tendencies compared to the hardwood biomass, which have been reported to contain more alkali metals. Furthermore, contamination by sand or soil during biomass collection enhances this tendency. It has been shown by researchers that char from switchgrass and corn stover would inherently have three challenges of high overall ash content, high silica content, and contamination by soil compared to traditional charcoals for use as fuels (Brewer et al. 2009). Differential Scanning Calorimetry The experimental results of the DSC study for the different doped biochar samples are presented in Fig. 6. The highest exothermic enthalpy of 188.31J/g was found to be at the pyrolysis temperature, holding time and dope ratio of 250°C, 1 min and 32% respectively. A regression analysis was also carried out in order to compare the predicted and observed biochar exothermic enthalpy (Fig. 7 (a)). The

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Biochar properties as a fuel Proximate analysis A biochar sample can be characterized as a carbon-rich solid fuel with high fixed carbon content (residual matter % or RM %) but low volatile matter content (volatile matter % or MM %) (Şensöz and Can 2002; Boateng 2007). The results of the proximate analysis indicating the volatile matter (%), residual matter, ash (%) and moisture contents of the maple wood and its biochar component obtained through varying treatments are presented in Table 4. Results indicated that the biochar obtained treatments at 290°C had a higher ratio of RM to MM in comparison to all other treatments. The highest RM% with comparatively low MM% was found for pyrolysis temperature of 290°C, holding time 1 min and doping ratio of 24% with 32.94% MM and 59.14% RM. The treatment also had the lower ash% value of 7.92%. In turn, the treatment at 250 °C, holding time of 1 min and doping ratio of 32% had the highest MM% of 97.20 and low RM% of 2.25 but had the lowest ash% of 0.55. The presence of higher ash contents

Fig. 6. Enthalpy of exothermic energy (J/g) of biochar produced from the microwave pyrolysis of maple wood with the % Relative error from the predicted values obtained through the regression model (P ≤ 0.05).

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corresponding analysis of variance (ANOVA) is presented in Table 5 (a). This comparison showed that the experimental values had a good correlation to the model with R2 and RMSE of 0.90 (P ≤ 0.05) and 16.87(J/g) respectively (Table 5b). The significant parameters obtained from the ANOVA (P ≤ 0.05) are provided in Table 5 (c) in which the parameters with p ≥ 0.05 were rejected based on the regression model. The predicted variables of pyrolysis reaction were optimized with the significant factors for the exothermic energy. The predicted maximized exothermic energy was found to be 191.05 J/g at the optimized pyrolysis temperature of 250°C, holding time of 1 min and dope ratio of 32% with a

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desirability of 0.91 as shown in Fig. 7 (b). These optimal conditions were found to be in good agreement with the observed results.

Hence a combined regression analysis for exothermic energy (J/g), RM: MM ratio and volatile matter (%) was also carried out which demonstrated that temperature and the quadratic term of dope ratio were significant factors in both (Table 6). This in turn suggests that the variation of

Fig. 7. Regression analysis of Actual vs. Predicted exothermic energy (J/g) of maple wood biochar treatments with R2 = 0.84 and RMSE = 15.5 (J/g) (P ≤ 0.05). Fig. 8. Optimum pyrolysis parameters for

maximization of exothermic energy (J/g) and volatile matter (%) content of microwave pyrolysis maple wood biochar treatments through the regression model (P ≤ 0.05).

Table 4. Proximate analysis of maple wood and biochar samples obtained through microwave pyrolysis.

Temp

(°C)

Time

(min)

Dope

ratio (%) Ash%

Volatile

matter

(MM) %

Residual

matter (RM)

%

MC% Ratio of

RM/MM

330 5 32 5.640 53.68 40.68 4.14 0.76

290 5 24 15.50 48.68 35.82 3.42 0.74

250 1 16 5.480 91.09 3.430 4.13 0.04

330 5 16 10.64 53.37 36.00 2.27 0.67

330 1 16 11.11 84.12 4.770 1.18 0.06

330 3 24 2.800 56.19 41.01 6.47 0.73

290 3 16 18.25 64.30 17.45 2.87 0.27

250 1 32 0.550 97.20 2.250 3.60 0.02

290 3 32 3.050 74.12 22.83 5.89 0.31

330 1 32 10.18 66.68 23.14 1.13 0.35

250 5 16 3.550 68.51 27.94 6.41 0.41

250 5 32 13.33 77.34 9.330 2.79 0.12

290 1 24 7.920 32.94 59.14 1.82 1.80

250 3 24 1.020 80.43 18.55 3.85 0.23

290 3 24 11.64 65.84 33.00 2.10 0.30

Wood

2.050 75.26 22.68 7.24 0.50

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fuel from different biomass sources.

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In an oxidizing atmosphere, reactions, which involve the combustion of the organic matter in the biochar samples, are exothermic in their thermodynamic nature. Joseph et al. 2009 observed in a similar study with DSC that the oxidation of the sample begins at around the temperature of 200°C and the weight loss conversion from char to ash was complete by the time the samples reached temperatures of 510°C. Similar results have been observed in the current study with the DSC curves ending at around 535°C. Further studies which combine the DSC results to weight loss should be investigated for the determination of the inorganic and organic impurities in the biochar.

CONCLUSION The effects of pyrolysis conditions such as temperature, time and dope ratio on the biochar yields and fuel properties of biomass samples were investigated in this study by using response surface techniques. Results showed that the biochar yield decreased with increasing pyrolysis temperature while variation in doping ratio and holding time did not have a significant effect on the biochar yields. The maximum predicted biochar yield for microwave pyrolysis was optimized to be at the pyrolysis temperature of 250°C, holding time of 3.4 min and doping ratio of 16%. The regression analysis showed strong correlation between the experimental and predicted results with R2 of 0.89 (P ≤ 0.01). The DSC analysis to compare the exothermic energy of the biochar samples indicated that the highest exothermic enthalpy of 188.31J/g was at the pyrolysis temperature, holding and dope ratio of 250°C, 1 min and 32% respectively. The regression analysis for the DSC analysis indicated that the predicted and observed results had a good correlation with a high R2 value of 0.90 (P ≤ 0.05) and corresponded well with the optimization results through the response surface methods. The proximate analysis indicated the relationship between the chemical characteristics of the biochar obtained with

Table 5. (a) Analysis of Variance of Regression Model for exothermic energy of biochar obtained from microwave pyrolysis of maple wood; (b) Goodness of fit of the model; (c) Significant parameter estimates obtained from Analysis of Variance of Regression Model for exothermic energy of biochar obtained from Microwave Pyrolysis of maple wood (P ≤ 0.05).

(a) Source DF Sum of

squares Mean square

F ratio P > F

Model 9 13261.29 1473.48 5.17 0.04 Error 5 1423.77 284.75 Total 14 14685.06

(b) Summary of fit

R2 0.90

Root Mean Square Error (RMSE) 16.87

Mean of Response 119.3

Observations (n) 15

(c) Term Value Std error

t Ratio P >|t|

Time (s)*Dope ratio (%) -19.99 5.97 -3.35 0.02*

Temp (C)*Dope ratio (%) -17.25 5.97 -2.89 0.03*

Temp (C)(250,330) -14.37 5.34 -2.69 0.04* Dope ratio (%)*Dope ratio (%) 27.29 10.52 2.59 0.05*

Dope ratio (%)(16,32) 9.50 5.34 1.78 0.14

Time (s)(1,5) 9.06 5.34 1.70 0.15

Temp (C)*Time (s) 7.23 5.97 1.21 0.28

Temp (C)*Temp (C) 6.31 10.52 0.60 0.58

Time (s)*Time (s) 3.94 10.52 0.37 0.72 * describes the significant factors for P ≤ 0.05

Parameter Volatile matter %

Enthalpy of exothermic energy (J/g)

Temp(250,330) 0.01* 0.02* Time(1,5) 0.05* 0.09 Dope ratio(16,32) 0.81 0.08

Temp*Time 0.96 0.22 Temp*Dope ratio 0.28 0.01*

Time*Dope ratio 0.48 0.00*

Temp*Temp 0.06 0.89 Time*Time 0.03* 0.91 Dope ratio*Dope ratio 0.05* 0.04*

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the pyrolysis conditions through the ratio of residual to volatile matter. The design of a microwave-assisted pyrolysis reactor for the optimal performance in terms of biochar yields was experimentally validated in a custom-built lab-scale unit in this study with proximate analysis as well as calorimetry analysis strengthened the concept of biochar as a solid fuel.

ACKNOWLEDGEMENTS The authors would like to gratefully acknowledge the financial support of NSERC (Natural Sciences and Engineering Research Council), Canada, Le Fonds de recherche du Québec - Nature et technologies (FQRNT) and Schulich Canada Graduate Scholarships.

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