Microwave Drying Effect on Pyrolysis Characteristics and Kinetics of Microalgae

Abstract

Microalgae are one of the most potential biomass energy sources. An efficient drying method is important to development and utilization of microalgae. Microwave drying is receiving increasing attention because it is a rapid, high-efficiency, and economical method compared to conventional drying. Pyrolysis characteristics of microalgae (C. vulgaris) after conventional drying (drying at 105 °C for 20 h) and microwave drying (the microwave drying time of 20, 30, and 40 min) were investigated. The pyrolysis experiment of microalgae was carried out at the heating rates of 10, 20, and 40 °C·min⁻¹ in a thermogravimetric analyzer (TGA). And the bio-char after pyrolysis of C. vulgaris under different heating powers (conventional power of 2500 W and microwave power of 600, 1000, 1500, and 2250 W) were analyzed. Results show that comprehensive pyrolysis characteristic index (S) of C. vulgaris after microwave drying was higher than conventional drying; however, energy consumption and activation energy (E) after microwave drying were lower. For microwave drying, as microwave drying time increases, ignition temperature (T_i), final temperature detected as mass stabilization (T_f), reaction rate at the second peaks (R_p2), residual mass (M_r), and energy consumption increased, while average reaction rate (R_v) decreased. As the heating rate (β) increased, the T_i, T_f, R_p2, R_v, and S of C. vulgaris increased, while M_r decreased, and E firstly decreased and then increased. And except for microwave power of 600 W, as microwave power increased, the volatile content and the fixed carbon content of C. vulgaris bio-char decreased, and the ash was increased.

Introduction

In recent decades, on account of the increasing energy consumption and the requirement of searching energy substitutes for fossil fuels, more and more researches are being done for developing renewable energy resources [1]. Among all the renewable resources, biomass is a promising candidate to change the ways of energy production, establish a sustainable energy system and strengthen the protection of the environment [2]. Hence, biomass energy is increasingly recognized.

As one of the most potential biomass energy sources, microalgae have received considerable high attention [3, 4] because they have lots of advantages compared with other biomass energy crops [5]. For example, microalgae have fast growth rate and low land usage [6]. They can absorb large amounts of carbon dioxide during growing process and have high volume carbon abatement [7]. Microalgae are widely used in bio-fuels, biodiesel due to high oil content and high biomass production [8, 9]. Their development and utilization have been an important focus of biomass researchers.

However, the feasible technologies for the biomass utilization as fuel are challenging due to partly inferior fuel properties of biomass, such as high water and oxygen contents, low calorific value, and strong hygroscopicity [10]. Microwave drying is a promising pretreatment technology, which can improve the fuel properties and offer solutions to above issues [11]. Compared with conventional drying methods, the unique features of microwave drying indicate potential advantages, such as operating at lower temperatures and resulting in an expected energy saving, controlling the drying process automatically, reducing the drying time and increasing the energy efficiency, and avoiding potential safety and environmental problems [12, 13]. Hence, some researchers have paid attention to the microwave drying of biomass materials (pine wood sawdust [14], sweet sorghum bagasse [15], coffee grounds [16], etc.).

Pyrolysis, one of the most attractive thermal conversion technologies for biomass conversion, can decompose biomass into fuels to meet different energy needs [17]. At present, the researchers have studied pyrolysis characteristics of some biomass [18,19,20]. In addition, the researches on the pyrolysis behavior of microalgae have also received great attention. Zhao et al. [21] investigated bio-crude yield and composition by individual pyrolysis and co-pyrolysis of Isochrysis and Chlorella. Hu et al. [22] investigated the interaction effect of co-pyrolysis of oil shale and microalgae to produce syngas. Besides, some researchers investigated the co-pyrolysis characteristics and kinetics of microalgae with different fuels (wood [23], semi-anthracite coal [24], low-rank coal [25]). The above studies were carried out under the condition that the microalgae were pretreated by the traditional drying methods. However, the pyrolysis characteristics and kinetics of microalgae Chlorella vulgaris (C. vulgaris) after microwave drying pretreatment and the results compared with traditional drying have not been investigated yet.

In this paper, the effect of different microwave drying time (20, 30, and 40 min in a microwave oven at the power of 600 W) on the pyrolysis of C. vulgaris was investigated and the results were compared with that of conventional drying for 20 h at 105 °C in an electrical oven. And pyrolysis characteristics of C. vulgaris were studied at the heating rates of 10, 20, and 40 °C·min⁻¹ in a thermogravimetric analyzer (TGA). The comprehensive pyrolysis characteristic index and the energy consumption were calculated and analyzed, and kinetics analysis was carried out finally.

Materials and Methods

Materials

The feedstock used in this study was provided by the Jiangmen Yue Jian Biotechnologies Co, Ltd. (Guangdong Province, China). The ultimate analysis, proximate analysis, and lower heating values of C. vulgaris were determined through Vario EL-II chons elemental analyzer (Elementar Analysen systeme Gmbh, Germany), MA260S electronic balance (Shanghai Second Balance Instrument Factory, Shanghai, China), and Parr 6300 oxygen bomb calorimeter (PARR instrument company, America) correspondingly. The results are shown in Table 1.

Table 1 The ultimate analysis, proximate analysis, and lower heating values of C. vulgaris

Experimental Procedure and Methods

The samples of C. vulgaris were dried in two different methods. The microwave oven of COWB-L produced by Wan Cheng microwave Co., Ltd. in Guangzhou city was chosen. From the past experimental research, it can be found that the oven temperature reached to 200 °C when the microwave power was 750 W at heating time of 2000 s [26], and C. vulgaris started pyrolysis at this temperature [27]. In order to dry the sample and avoid the sample decomposition, the microwave power of 600 W was a suitable choice. Some part of samples was dried respectively for 20, 30, and 40 min in microwave oven. The other samples were at 105 °C for 20 h in an electric furnace of SXZ-5-12 with 2500 W electric output power. The amount of each drying was 30 g. The drying samples were pulverized finely and sieved with a mesh size of less than 200 μm and then held in desiccators.

The high temperature thermal decomposition of C. vulgaris was evaluated using TGA of American TA Q500 to study their pyrolysis behavior. Samples (10 ± 0 .1 mg) which were loaded into the ceramic pan of the TGA were heated from room temperature to 900 °C at the heating rates of 10, 20, and 40 °C·min⁻¹ under a high purity N₂ (99.99%) flow rate of 100 ml·min⁻¹. The experiment was repeated three times to ensure the reliability of experimental data.

Comprehensive Pyrolysis Characteristic Index

Comprehensive pyrolysis characteristic index was used to characterize the properties of C. vulgaris under different drying time and different methods. And the comprehensive characteristic index was proportional to the pyrolysis characteristics of the fuel. The comprehensive pyrolysis characteristic index (S) was represented as follows [28]:

$$ S=\frac{R_{\mathrm{max}}{R}_{\mathrm{ave}}}{T_{\mathrm{i}}^2{T}_{\mathrm{f}}} $$

(1)

Where R_max (%·min⁻¹) and R_ave (%·min⁻¹) refer to the maximum and average mass loss rates respectively; T_i (°C) is the ignition temperatures and T_f (°C) is the burnout temperatures.

Kinetic Model

An overall kinetic model which has been developed to describe the rate of degradation or conversion can be expressed in the following form:

$$ \frac{d\alpha}{d t}=k(T)f\left(\alpha \right) $$

(2)

Where t (min) is time, T (K) is the absolute temperature, and α is the conversion degree.

f(α) is a function depending on the reaction mechanism which is expressed as [29]:

$$ f\left(\alpha \right)={\left(1-\alpha \right)}^{\mathrm{n}} $$

(3)

where n is the reaction order, α is defined as:

$$ \alpha =\frac{m_{\mathrm{i}}-{m}_{\mathrm{t}}}{m_{\mathrm{t}}-{m}_{\infty }} $$

(4)

where m_i (g) is the initial mass of the sample, m_t (g) is the mass of the sample at time t (min), and m_∞ (g) is the final mass of the sample in the reaction [30].

k(T) is a constant depending on the temperature rate, which has been usually described by the Arrhenius expression:

$$ k=A\exp \left(-\frac{E}{RT}\right) $$

(5)

Where A (min⁻¹) is the pre-exponential factor, E (kJ/mol) is the activation energy, R (kJ/(mol·K)) is the gas constant. The combination of Eq. (2) and (5) gives:

$$ \frac{d\alpha}{d t}=A\exp \left(-\frac{E}{RT}\right){\left(1-\alpha \right)}^{\mathrm{n}} $$

(6)

Define the heating rate β (°C/min) as:

$$ \beta =\frac{dT}{dt} $$

(7)

By substituting this into Eq. (6), it is transformed to:

$$ \frac{d\alpha}{d T}=\frac{A}{\beta}\exp \left(-\frac{E}{RT}\right)f\left(\alpha \right) $$

(8)

An integration function of Eq. (8) is shown as below:

$$ g\left(\alpha \right)={\int}_0^{\alpha}\frac{d\alpha}{f\left(\alpha \right)}=\frac{A}{\beta }{\int}_{T_0}^T\exp \left(-\frac{E}{RT}\right) dT $$

(9)

Where g(α) is the integrated form of f(α), T₀ (°C) is the initial temperature.

Equation (9) is integrated by using approximation method [31], and then is converted to:

$$ \ln \left[\frac{g\left(\alpha \right)}{T^2}\right]=\ln \left[\frac{AR}{\beta E}\left(1-\frac{2 RT}{E}\right)\right]-\frac{E}{RT} $$

(10)

And the term 2RT/E is much less than 1 for the thermal decomposition of polymer materials. Hence, Eq. (10) can be presented as follows:

$$ \ln \left[\frac{g\left(\alpha \right)}{T^2}\right]=\ln \frac{AR}{\beta E}-\frac{E}{RT} $$

(11)

Where

$$ g\left(\alpha \right)=\Big\{{\displaystyle \begin{array}{c}-\ln \left(1-\alpha \right)\\ {}\frac{1-{\left(1-\alpha \right)}^{1-\mathrm{n}}}{1-n}\end{array}}\kern3em {\displaystyle \begin{array}{c}n=1\\ {}n\ne 1\end{array}} $$

(12)

And then a plot of $\ln \left[\frac{g\left(\alpha \right)}{T^2}\right]$ against 1/T should result in a straight line with the slope of –E/R and the intercept of $\ln \frac{AR}{\beta E}$. A straight line can be obtained in the figure, and apparent activation energy E (kJ/mol) and the frequency factor A (min⁻¹) can be calculated.

Results and Discussion

Effect of Drying Time and Drying Method on Pyrolysis

The thermogravimetric (TG) and differential thermogravimetric (DTG) curves of C. vulgaris pyrolysis at β = 20°C·min⁻¹ in N₂ atmosphere after different microwave drying time (20, 30, and 40 min) and conventional drying for 20 h were shown in Fig. 1(a) and (b) respectively. TG and DTG curves revealed three degradation steps common to all pyrolysis processes. DTG curves indicated main devolatilization stages more clearly.

Fig. 1

a TG curves of microalgae pyrolysis in N₂ atmosphere at β = 20 °C·min⁻¹after different microwave drying time and conventional drying for 20 h. b DTG curves of microalgae pyrolysis in N₂ atmosphere at β = 20 °C·min⁻¹after different microwave drying time and conventional drying for 20 h

The first stage (room temperature to around 180 °C) was associated with a slight weight loss due to the loss of water and some light volatile compounds [32]. The water lost here may be the extracellular water which has not been completely removed during the drying process [33]. The second stage (180 to 600 °C) was the main pyrolysis stage, during which most of the sample weight was lost due to the decomposition of volatiles, such as carbohydrates, proteins, and lipids [31]. This is because 180 °C is mainly associated with the degradation of protein and soluble polysaccharide whereas temperature peaks (305 and 340 °C) would correspond to the degradation of crude cellulose in the cell wall, other insoluble polysaccharides and crude lipid [34]. In the third stage (the temperature higher than 600 °C), the decomposition of solid residual (char or other carbonaceous matters) contributed to the slight weigh loss.

From Fig. 1(a) and (b), the TG or DTG plots of C. vulgaris pyrolysis after different microwave drying time and conventional drying were very similar, which were basically in a line, but after 400 °C the lines had a little separation. When the temperature was more than 400 °C in TG curves, the separation was enlarged and some obvious differences can be found. Observing three TG curves of different microwave heating time, as the drying time increases, the TG curve moved to a higher temperature. This is the reason that, as the drying time increases, more moisture was lost in the sample, resulting in a higher densification degree of the sample [5], which made thermal decomposition slightly difficult. And the TG or DTG curves of the microwave drying for 20 min and that for 30 min were very close to each other, which demonstrates that 20 min is a suitable microwave drying time for C. vulgaris. The microwave drying for 20 and 30 min had closely similar curves, so microwave drying for 20 min had the same effect with 30 min. But in comprehensive consideration of heating time and energy consumption, microwave drying for 20 min is a better choice.

The DTG curves of C. vulgaris pyrolysis were shown in Fig. 1(b) after different pretreatments. There were two obvious peaks found between 250 and 400 °C, and there was a slight bulge between 400 and 500 °C. There are no obvious shoulder peaks in DTG curves of C. vulgaris after drying; it is different from the pyrolysis behavior of C. vulgaris which have not been dried reported in literatures [35, 36]. This is because the material content is different in different microalgae species and the drying pretreatment may cause the peak temperature to overlap.

The pyrolysis characteristic parameters of C. vulgaris from TG and DTG curves after two drying methods were shown in Table 2. The maximum reaction rate occurred at the temperature of the second peak (T_p2). As microwave drying time increases from 20 to 40 min, T_i of C. vulgaris increased from 140 to 145 °C. T_f, reaction rate at the second peaks (R_p2) and residual mass (M_r) of C. vulgaris increased from 769 to 788 °C, 8.32 to 8.43%·min⁻¹ and 27.9 to 29.1% respectively, but R_v of C. vulgaris decreased from 2.27 to 2.23%·min⁻¹ and the comprehensive pyrolysis characteristic index (S) decreased from 1.25 to 1.13%²/min²/°C³. It can be seen that as the drying time increases, T_i, T_f, R_p2 and M_r increased, while R_v decreased, S of C. vulgaris at 20 min drying time was maximum. Compared with the pyrolysis characteristic parameters of conventional drying method, T_i of the microwave drying was lower, while R_v and R_p were higher. T_f and M_r of the microwave drying for 20 and 30 min were lower than those of conventional drying, but T_f and M_r of 40 min were higher than that of conventional drying. And the S of the microwave drying was higher than the conventional drying. This is because the water molecule is polar and absorbs microwave energy more readily than other components; hence, the biomass by microwave drying has a larger surface area and more inner paths [28]. It indicated that the microwave drying can improve the pyrolysis characteristics of microalgae.

Table 2 Pyrolysis characteristic parameters of C. vulgaris at β = 20 °C·min⁻¹ after two drying methods

Effect of Heating Rate on the Pyrolysis of C. vulgaris by Microwave Drying

In Table 2, the reaction temperature interval (T_d) (T_d = T_f − T_i) of microwave drying for 20, 30, and 40 min were 629, 632, and 643 °C, respectively. A larger reaction interval is more helpful for pyrolysis characteristics analysis. In addition, the longer the drying time, the less moisture content in the sample, the influence of moisture on the pyrolysis is reduced. In order to minimize the effects of water molecules as much as possible, the pyrolysis of C. vulgaris after microwave drying for 40 min was investigated in different heating rate (β). Figure 2 (a) and (b) show TG and DTG curves of C. vulgaris pyrolysis at β = 10, 20 and 40 °C·min⁻¹ after microwave drying for 40 min. As shown in Fig. 2(a), as the heating rate increases, the TG curve shifted slightly toward higher temperature and the maximum weight loss rate also increased. In addition, the value of peak increased with enhancing heating rate in DTG curves. The main reason is that as the heating rate increases, more thermal energy facilitated better heat transfer between the surroundings and the insides of the samples [37].

Fig. 2

a, b TG and DTG curves of microalgae pyrolysis at different βs in N₂ atmosphere for microwave drying for 40 min

Table 3 shows the characteristics parameters of C. vulgaris at different heating rates. As shown in Table 3, when heating rate increases, T_i, T_p, and T_f increased because the increased heating rate leads to a temperature lag of decomposition, which shows that increasing the heating rate can delay pyrolysis and gasification reaction [38]. As the heating rate increases, R_p and R_v also increased, while M_r decreased; the main reason is that it would provide more energy to material at a higher heating rate. Since there is enough energy at 40 °C·min⁻¹, the decomposition phase does not appear to be graded [27]. Hence the DTG curve had only one peak at 40 °C·min⁻¹. Moreover S of C. vulgaris at heating rate of 40 °C·min⁻¹ was maximum, which indicated that increasing heating rate can enhance pyrolysis characteristics of C. vulgaris after microwave drying.

Table 3 The characteristics parameters of C. vulgaris at different heating rates

Kinetics Analysis

To obtain the kinetic parameters, the TG-DTG information and a non-linear regression scheme were used in fitting. According to Eqs. (11) and (12), a plot of $\ln \left[\frac{g\left(\alpha \right)}{T^2}\right]$ against 1/T should be a straight line, and fitting lines could be obtained by the Origin 9.0 software. From fitting straight lines, the slopes of –E/R and intercepts of $\ln \frac{AR}{\beta E}$ could be obtained, and then the apparent activation energy (E) and frequency factor (A) could be calculated respectively. Comparing the correlation coefficient (R²) at different n. n is the reaction order and the value of n which is from 1.0 to 3.0 are used to calculate g(α) in Eq. (11), and then according to Eq. (11), a plot of ln[g(α)/T²] against 1/T can fit a straight line with the slope of –E/R and the intercept of ln(AR/βE), increasing the n values, the different fitting straight lines have the different correlation coefficients (R²s). R² of microwave drying for 20, 30, and 40 min was maximum at n = 2.3 and R² of conventional drying was maximum (0.97587) at n = 2.5, but R² of conventional drying at n = 2.3 also reached a relatively high value (0.968768). Hence, n of 2.3 was chosen in the next analysis. The linear fitting results (see Figs. 3, 4, 5, 6), and the calculated E and A of C. vulgaris pyrolysis at β = 20 °C·min⁻¹ by different drying pretreatment are listed in Table 4.

Fig. 3

Fitting result of ln[g(α)/T²] versus 1/T for C. vulgaris pyrolysis at n = 2.3 by conventional drying

Fig. 4

Fitting result of ln[g(α)/T²] versus 1/T for C. vulgaris pyrolysis at n = 2.3 by microwave drying for 20 min

Fig. 5

Fitting result of ln[g(α)/T²] versus 1/T for C. vulgaris pyrolysis at n = 2.3 by microwave drying for 30 min

Fig. 6

Fitting result of ln[g(α)/T²] versus 1/T for C. vulgaris pyrolysis at n = 2.3 by microwave drying for 40 min

Table 4 Kinetic parameters of C. vulgaris pyrolysis by different drying methods and different drying time

As shown in Table 5, it can be seen that R² between predicted and experimental values is 0.968768–0.986347, which indicates that the kinetic parameter data were reliable.

Table 5 Energy consumption of different drying time and different drying methods

From Table 5, comparing with the microwave drying, E and A occurred at conventional drying for 20 h were the maximum. This result indicates that microwave drying pretreatment of microalgae reduced the activation energy required for the pyrolysis process. This is reason that microwave drying pretreatment increases the temperature of the sample and enhances the release of phenolic compounds from the matrix to make sample structure tend to be simple, causing them more accessible to extraction [39].

As the microwave drying time increases from 20 to 40 min, E firstly decreased from 68.01 to 67.49 kJ/mol and then increased from 67.49 to 68.12 kJ/mol. This indicates that microwave drying had a time demarcation point for the activation energy of microalgae thermal decomposition, when this point was exceeded, more volatiles and fixed carbon were preserved as the microwave drying time increased leading to activation energy increased during pyrolysis. In general, a reaction with lower E needs a lower reaction temperature or shorter reaction duration [32]. Compared with microwave drying for 20 and 40 min, the value of E occurred at microwave drying for 30 min was the minimum one.

Energy Consumption

The energy consumption is existed during drying process because of the high moisture content of microalgae after harvest. The effective energy consumed for the drying process includes two parts: one is utilized to increase the sample temperature and the other is used for water evaporation. Energy consumption during a drying process was calculated in this section.

The total energy consumption of the drying system during drying process could be obtained [40] by Eq. (13):

$$ Q={P}^{\ast }t $$

(13)

Where Q (J) is total energy imported to the drying system, P (W) is output power, and t (min) is drying time.

As shown in Table 4, comparing with conventional drying method, the energy consumption in microwave drying system had greater reduction. As can be seen from the results of the previous discussion, microwave drying and conventional drying have a substantially identical pyrolysis characteristic, even three microwave drying performances (20, 30, and 40 min) were all slightly better than the conventional drying performance. Compared with conventional drying, microwave drying method saved time and energy. The reason is that the direction of heat and mass transfer is consistent during microwave drying process, which can promote the evaporation of water and shorten the drying time, however in conventional drying method, the heat is transferred to surface by convection and then to the interior by conduction, therefore the direction of heat transfer is opposite to that of mass transfer and thus the resistance of water removal is increased [32]. Hence, microwave drying is a technically and economically pretreatment method for microalgae pyrolysis.

From Table 5, different drying time have different energy consumptions. As drying time increased from 20 to 40 min, total energy consumption increased from 0.72 to 1.44 MJ, and energy consumed per gram increased from 0.024 to 0.048 MJ/g. The energy consumption of microwave drying for 40 min was twice as much as 20 min. Hence, the suitable microwave drying time not only saves time and energy, but also improves the pyrolysis properties of microalgae. From “Effect of drying time and drying method on pyrolysis” and “Kinetics analysis,” microwave drying for 20 min is the largest S, and 30 min is the least E. E of microwave drying for 20 min (68.01 kJ/mol) is slightly larger than E of microwave drying for 30 min (67.49 kJ/mol), S of microwave drying for 20 min (1.25 × 10⁶%²/min²/°C³) was larger than S of microwave drying for 30 min (1.19 × 10⁶%²/min²/°C³). The physical order of magnitude of S is larger than the physical order of magnitude of E. And as drying time increases from 20 to 40 min, total energy consumption increased from 0.72 to 1.44 MJ; it can be seen that as drying time increased, the energy consumption increased. Comprehensively considering the results in “Effect of drying time and drying method on pyrolysis” and “Kinetics analysis” and the energy consumption, microwave drying for 20 min is a better choice.

Bio-Char

Bio-char is a product from pyrolysis and has high carbon content. Bio-char can be used not only as a fuel but also as an underground carbon sink. Due to its molecular structure, it is quite stable both chemically and biologically, which means that it can remain stable in soil for 100 or even 1000 years [41]. Thus, production bio-char by pyrolysis technique helps to reduce the amount of carbon dioxide in the atmosphere. Moreover, bio-char has a highly porous structure, and the addition of bio-char to soil could improve water retention and increase the surface area of the soil, improving the efficiency of nutrient use [42].

The bio-chars after pyrolysis of C. vulgaris to a constant weight under different heating powers (conventional power of 2500 W and microwave power of 600, 1000, 1500, and 2250 W) were analyzed; the weight of each sample was 30 g. Their proximate analysis was showed in Table 6.

Table 6 Proximate analysis of bio-char produced by pyrolysis under different heating power

From Table 6, volatile, ash, and fixed carbon with a microwave power of 600 W were very close to those in Table 1. It agreed that C. vulgaris did not start pyrolysis when the microwave power was 600 W [26]. As the microwave power increases, the volatile content and the fixed carbon content of C. vulgaris bio-char decreased, and the ash increased. Bio-char had the highest fixed carbon and lowest ash at microwave power of 1000 W. And the fixed carbon content of bio-char with a microwave power of 1000 W was higher than that of conventional power of 2500 W, and the ash content was lower. Thus, bio-char produced by C. vulgaris with a microwave power of 1000 W could be highly beneficial for carbon sequestration.

Conclusions

The influence of different drying methods and drying time on the C. vulgaris pyrolysis by the TGA was studied. Results showed that there were three stages in the C. vulgaris pyrolysis after microwave drying pretreatment. Compared with the pyrolysis characteristic parameters of C. vulgaris by conventional drying, T_i of the microwave drying was lower, while R_v, R_p, and S were higher. T_f and M_r of the microwave drying for 20 and 30 min were lower than conventional drying, but T_f and M_r of 40 min were higher than it. At microwave drying for 40 min, as the heating rate increases, T_i, T_p, T_f, R_p2, R_v, and S increased, while M_r decreased, and E firstly decreased and then increased. For microwave drying, as microwave drying time increases, T_i, T_f, R_p2, M_r, and energy consumption increased, while R_v decreased. Microwave drying for 20 min was the largest S and minimal energy consumption, and 30 min was the least E and A. Hence, microwave drying for 20 min is a better choice. And bio-char was the highest fixed carbon and lowest ash at microwave power of 1000 W.