Pyrolysis Characteristics and Kinetics of Phoenix Tree Residues as a Potential Energy

By using a thermogravimetric analyser under argon atmosphere, the pyrolysis process and the kinetic model of phoenix tree residues (the little stem, middle stem, and leaf) at a 30 °C min−1 heating rate and the phoenix tree mix at three different heating rates (10 °C min−1, 30 °C min−1, and 50 °C min−1) were examined. The catalyst and the co-pyrolysis samples were at a 30 °C min−1 heating rate. The catalysts were Na2CO3, ZnCl2 and CaO in a mass fraction of 5 %. The experimental results revealed that the phoenix tree residues pyrolysis process consisted of three stages: dehydration stage, main pyrolysis stage, and the slow decomposition of residues. As the heating rate increased, the pyrolysis characteristic temperature of the phoenix tree grew, there was a backward-shift of the pyrolysis rate curve, and the mass loss rate gradually increased. The phoenix tree residues’ activation energy changed throughout the whole pyrolysis process, and the pyrolysis temperature ranges of the three main components (cellulose, hemicellulose, and lignin) existed in overlapping phenomenon. As compared to the little stem, middle stem, and leaf, the phoenix tree mix was more likely to be pyrolysed under the same heating rate. Different catalysts had a different impact on the pyrolysis: ZnCl2 moved the start point of the reaction to the lower temperatures, but did not speed up the reaction; Na2CO3 speeded up the reaction without changing the start point of the reaction; CaO speeded up the reaction, moved the start point of the reaction to higher temperatures.


Introduction
Biomass energy is a kind of chemical energy that is stored in biomass, which is transferred from light energy by photosynthesis.As an ideal renewable energy, bioenergy consumption ranks fourth worldwide in the most consumed sources of energy (about 14 % of the total consumption) and ranks just behind fossil oil, natural gas, and coal. 1 Indeed, biomass has been predicted to account for up to 40 % of the world's energy consumption by the year 2020.In fact, plant yield alone is already 20 times that of mineral energy, 2 which is indicative of the abundant reserves of biomass energy.Furthermore, the classic energy consumptions have costly disadvantages: namely, the negative impact of global warming caused by greenhouse gases, the energy crisis of non-renewable fossil energy, external dependence, security coefficients of energy supply, and so on.Therefore, reasonable utilization of biomass energy is of great interest to the world.
4][5][6][7] The technique is not only applicable to the combustion and gasifying process of biomass but also to the transfer of biomass into gas, liquid, and solid forms of high energy density.Generally, biomass pyrolysis is a process in which gas (biomass gas), liquid (bio-oil), and solid (biochar) 8 are gained as products from the biomass through thermal degradation under oxygen-free or limited oxygen conditions.][11][12]

Experimental samples
The phoenix tree residues (the little stem, middle stem, leaf, and tree mix) in the Nanjing Area were chosen as the experimental samples.Every sample was ground to 830 μm -1500 μm size particles, dried for 24 h under 105 °C, and then stored in a drying vessel.

Experimental conditions
A simultaneous thermal analyser STA449F3 produced by the German Netzsch Company was the instrument used in this study.High-purity argon gas was used as a protection gas with a flow of 10 ml min −1 , and the heating rates were 10 °C min −1 , 30 °C min −1 , and 50 °C min −1 in the pyrolysis of the tree mix, and 30 °C min −1 in the other samples.In all the experiments, the final temperature was 900 °C.The catalysts were Na 2 CO 3 , ZnCl 2 , and CaO in the mass fraction of 5 %.

Results and discussion
The pyrolysis process of the phoenix tree residues Fig. 1 illustrates the thermogravimetric (TG) curves as well as the differential thermogravimetric (DTG) curves of the pyrolysis process of the phoenix little stem, middle stem, leaf, and tree mix at a heating rate of 30 °C min −1 .
As temperature increased, the samples' major components (such as cellulose, hemicellulose, and lignin) produced gases under a series of chemical reactions, and the TG-DTG curves revealed the distribution of gas and solid pyrolysis products.As determined from the samples' pyrolysis experiences, pyrolysis consisted of three distinct stages: the dehydration stage (stage I), the main pyrolysis stage (stage II), and the slow decomposition of residues (stage III).In the first stage, the room temperature range was 165-200 °C, and the TG curves had a small mass loss.Furthermore, the DTG mass loss rate exhibited only little changes because of the moisture evaporation in the samples.The little stems' water loss peaks showed up at 120 °C, and other samples showed up slightly earlier, at 110 °C.Stage II, ending at 505-550 °C, was the major stage in the pyrolysis process, where the samples decomposed and different volatiles spread out by heating under an anoxia atmosphere.Also in this stage, up to 60-70 % of the samples' mass loss occurred with rapidly decreasing TG curves and evident peaks in the DTG curves.More precisely, the peaks for the little and middle stem are shown at 365 °C, and the rest show at 350 °C.These are the results from the complete reactions of three components in the samples. 16In the final stage, stage III, the samples formed loose porous carbon because of the diffusing volatiles, so this stage can also be called the carbonization stage.Both TG and DTG curves changed slowly as caused by lignin decomposition 17 and the long duration of carbonization.

Impact of the heating rate on the pyrolysis
The heating rate is one of the most important factors in biomass pyrolysis, particularly as it can influence the pyrolysis rate in a different temperature region, in products distribution, or in the biomass stay time.Fig. 2 shows the DTG curves of the tree mix under three different heating rates.The relative characteristic pyrolysis temperatures of the samples are summarized in Table 1.For the water evaporation rate peaks of the DTG curves in stage I, the maximum dehydrate rate is marked as V 1 , and the related peak temperature is T 1 .As for the mass loss rate peaks in stage II, the maximum mass loss rate is called V 2 , and the related peak temperature is T 2 .
As seen in Table 1 and Fig. 2, as the heating rate increases, all the characteristic temperatures T of the tree mix pyrolysis grow with the backward move of the pyrolysis rate curves.This indicates that the higher the heating rate, the stronger the influence of diffusion on the temperature gradients, thus leading to heating transfer lag and increased T values.In addition, the amount the T value increases becomes lower at the higher heating rates.For example, the T 2 values at 30 °C min −1 and 50 °C min −1 are the same, which reveals lower impact of an increasing heating rate on the heating transfer lag.
Fig. 3 shows heating rate's influence on the mass loss rate.
It can be seen that the mass loss rate V gradually accelerates under the higher heating rates, and V 2 changes more easily than does V 1 .Normally, higher heating rates allow the biomass to proceed through pyrolysis under a higher temperature, thus resulting in an accelerated pyrolysis.

Impact of the different catalysts on the pyrolysis
Catalysts can move the start point of the pyrolysis to the low temperature direction, or speed up the reaction, thus improve the efficiency of the pyrolysis.9][20][21] Three different kinds of catalyst were chosen in this paper: they are carbonate Na 2 CO 3 , alkaline metallic oxide CaO, and chlorine salt ZnCl 2 .Fig. 4 shows the DTG curves with different catalysts at the heating rate of 30 °C min −1 .As shown in Fig. 4, when contrasted to the phoenix tree mix pyrolysed at the heating rate of 30 °C min −1 , ZnCl 2 moves the start point of the reaction to the lower temperatures, but does not speed up the reaction; Na 2 CO 3 speeds up the reaction without changing the start point of the reaction; CaO speeds up the reaction, but it moves the start point of the reaction to the higher temperatures.

Kinetics analysis of phoenix tree residue pyrolysis
The mass loss ratio of samples can be expressed by the Arrhenius kinetics equation: ( Conversion x at t moment is: ( In this equation, m o is the initial mass of samples, m f is the mass of samples after pyrolysis, m(t) is the sample mass at moment t, k o is the pre-exponential factor, E is the apparent activation energy, R is the gas constant (8.314J mol −1 K −1 ), T is the absolute temperature, and f(x) is the solid conversion function model relying on the reaction mechanism and types. 22,23E, k o , and f(x) are called the kinetic triplet, and the driving goal behind studying kinetics is determining the kinetic triplet.
After replacing Eq. ( 1) with the heating rate q = dT/dt (K s −1 ) and f(x), which can be expressed by the most simplistic first order mechanism reaction equation f(x) = 1 − x, it can be obtained that: (3) Integrate Eq. ( 3), and obtain: (4) The right side of the equation above can only produce an approximate solution, not an analytical solution.
In this paper, the Agrawal-Sivasuburamanian integration methods, 24 one of the most effective methods for the precise evaluation of temperature, is applied to evaluate the above equation.The result is: (5) where<<1, so Eq. ( 5) can be simplified as: The curve can be drawn according to the pyrolysis data, and both the apparent activation energy E and pre-exponential factor k o can be solved by the straight slope and intercept.
The kinetics parameters of the phoenix tree residues pyrolysis at different stages as deduced by the above mentioned methods are summarized in Table 2.Although this kinetics analysis is divided into several parts based upon the three stages of pyrolysis, it is still hard to describe the whole pyrolysis process, particularly due to the changing activation energy and the overlapping temperature ranges of the three components pyrolysis.In addition, in comparison to other samples, the tree mix has a relatively smaller activation energy and pre-exponential factor at the same heating rate of 30 °C min −1 , which allows for an easier pyrolysis of the tree mix.The kinetics parameters of the phoenix tree residues pyrolysis with different catalysts at the main pyrolysis stage as deduced by the above mentioned methods are summarized in Table 3.According to the activation energy E and pre-exponential factor k o at the same heating rate, compared with these parameters of free-catalyst, ZnCl 2 moves the start point of the reaction to the lower temperatures, Na 2 CO 3 does not change the start point of the reaction, CaO moves the start point of the reaction to the higher temperatures.

Conclusions
Throughout this study, a number of conclusions were established; they are as follows: 1) The phoenix tree residues pyrolysis process consists of three separate stages: the dehydration stage, the main pyrolysis stage, and the slow decomposition of residues.
In the main pyrolysis stage, the competition reactions of cellulose, hemicellulose, and lignin occurs, and the mass loss ratio rises to 60-70 %.
2) As the heating rate increases, the characteristic temperature of the tree mix pyrolysis grows higher, the weight loss rate accelerates, and the pyrolysis rate curves move backwards.However, with these higher heating rates, the increasing amount of the characteristic temperature becomes smaller.
3) According to the curves and parameters of pyrolysis, different catalysts have different impact on the pyrolysis: ZnCl 2 moves the start point of the reaction to the lower temperatures, but does not speed up the reaction; Na 2 CO 3 speeds up the reaction without changing the start point of the reaction; CaO speeds up the reaction, it moves the start point of the reaction to the higher temperatures.
4) The activation energy of the phoenix residues pyrolysis changes in the whole stages, and the pyrolysis temperature ranges of the three main components (cellulose, hemicellulose, and lignin) exist in overlapping phenomenon.
5) As compared to the little stem, middle stem, and leaves, the tree mix is more likely to be pyrolysed at the same heating rate, partly due to its relatively smaller activation energy and pre-exponential factor.

List of abbreviations and symbols
Popis kratica i simbola

Table 1 -
Maximum dehydration rate temperature T 1 and the maximum mass loss rate temperature T 2 of tree mix under different heating rates

Table 3 -
Kinetics parameters of the phoenix tree residues pyrolysis with different catalysts at the heating rate of 30 °C min−1 E is apparent activation energy, k o is pre-exponential factor, r 2 is correlation coefficient.E je prividna energija aktivacije, k o predeksponencijalni faktor, r 2 je korelacijski koeficijent.