Combustion and gasification characteristics of low-temperature ...

Jun. 09, 2025

Combustion and gasification characteristics of low-temperature ...

1 Introduction

Low-temperature pyrolytic semi-coke is a solid product after removing a number of volatiles from low-rank coal under low-temperature pyrolysis (500–600°C) and tar is separated out [1,2]. Part of this product has been applied to fields like coal gasification, ferroalloy smelting, and calcium carbide production; however, there is still a large quantity of semi-coke resources that require market consumption. In the field of iron-making, pulverized coal injection (PCI) in blast furnace replaces expensive and highly deficient metallurgical coke with relatively low-priced coal to reduce the coke ratio in the blast furnace during iron-making process, and thereby reduce pig iron cost [3,4]. With the continuously increasing blast furnace injection ratio, iron and steel enterprises have an increasing demand for anthracite. Moreover, anthracite reserves only occupy 10.9% of coal reserves in China with unceasingly prominent scarcity, which is then accompanied by rising price. Therefore, under the background of an increase in PCI ratio in blast furnace, seeking for new low-cost and high-quality injecting fuels such as biochar [5] and waste plastics [6] has always been a research emphasis of metallurgists. Using low price semi-coke as PCI fuel to replace expensive anthracite has been an important research orientation for optimizing blast furnace fuel structures, and the reduced production cost has attracted attention from metallurgists [7,8,9,10,11]. Semi-coke is a potential excellent blast furnace fuel by virtue of favorable transport performance, high calorific value, and no explosiveness [7,10]. However, compared with anthracite, the nature difference of semi-coke is considerable because of its instable quality; moreover, the fluctuation of its combustion performance is remarkable and hinders its application and promotion in blast furnace injection, because during semi-coke production, pyrolysis conditions will influence the semi-coke composition and structure and cause changes in its reactivity. Even if the same coal category is used as a pyrolytic raw material, the reactivity of prepared semi-coke will be critically different, and high pyrolysis degree is adverse to follow-up combustion of semi-coke [11,12]. Factors, such as devolatilization behavior, pore structure, specific surface area, and ordering degree of carbon lattice structure, will result in substantial loss of semi-coke reactivity [13,14,15]. Combustion reactivity in later phase of semi-coke under high temperature is closely related to semi-coke nature before combustion [16].

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The present industrialized pyrolytic processes generally use internal heating-type gas-carrier pyrolysis reactors. In such pyrolysis environments, the raw coal exists not in a pure N2 atmosphere but in mixed reducing gases such as CO, H2, and CH4. Moreover, pyrolysis atmosphere contents are different to a certain degree at various positions inside the furnace. The influences of pyrolysis conditions, such as pyrolysis temperature, heating rate, atmosphere pressure, and holding time, on semi-coke composition, structure, and reactivity [17,18,19] have been extensively studied; however, the effects of pyrolytic atmosphere on semi-coke reactivity remain controversial. Colette et al. [20,21] studied the influence of the coke-oven gas atmosphere on product distribution and semi-coke characteristics in fixed beds and found that semi-coke combustion characteristics were not eminently different under H2 and inert atmospheres. Liao et al. [22] indicated that the combustion reactivity of coal–coke-oven gas co-pyrolytic semi-coke is related to pyrolysis pressure and heating rate, and that low pyrolysis pressure and high heating rate contribute to semi-coke combustion reactivity. Zhong et al. [23] found that hydrogen-free radicals generated by H2 and CH4 could permeate semi-coke and influence its oxidizing reactivity. Thus, the influence of mixed atmospheres containing reducing gases on semi-coke nature and its reactivity requires further research.

During iron-making technology in blast furnace, PCI fuels experience processes, such as volatile extrusion and combustion and gasification of fixed carbon, within confined spaces in the tuyere and raceway region. Compared with the process of release and combustion of volatiles, char combustion and gasification are relatively slow (20 ms vs 1–4 s), and the time needed for a complete reaction of coal is primarily and jointly determined by char combustion and gasification time [24]. Combustion and gasification properties are highly important for the utilization ratio of fuels inside the furnace and the stable operation of the blast furnace because the combustion of atmosphere inside the hearth is gradually variational. In the front of the tuyere, generated coal gas components are different because of different combustion conditions at different positions along the hearth radius in front of the tuyere. O2 is sufficient in front of the tuyere and reacts with fuel combustion to generate a large quantity of CO2, O2 abruptly decreases and disappears, and CO2 rapidly rises to its maximum value. Therefore, the injected fuel first experiences atmospheric combustion with sufficient O2 and then experiences gasification under the atmosphere with a continuously rising CO2. However, many research on reactivity of PCI fuels only focuses on combustion reactivity [3,25,26] and neglects the importance of gasification reactivity on the consumption of unburned char; moreover, comparative studies on the two abovementioned subject matter are lacking.

In the present research, thermoanalysis was applied to comparatively study the combustion and gasification reactivity of semi-coke prepared under pyrolytic atmospheres containing different proportions of H2- and CH4-reducing gases. Moreover, the relationships between semi-coke composition/structure and combustion/gasification reactivity were obtained by analyzing the carbon chemical structures of different semi-coke, functional group analysis, and micropore structures to provide reference for further scientific and highly efficient application of semi-coke in PCI.

2 Experimental procedure

2.1 Experimental raw materials

Coal samples used in the experiment were typical low-rank coals from Sunjiacha coal mine in Shenmu region of Northern Shaanxi. Proximate analysis and element analysis of coals are illustrated in Table 1. Vertical-type pyrolyzing furnace was used to prepare semi-coke samples, and the pyrolysis system is shown in Figure 1. A total of 250 g samples with granularity within 20–40 mm was placed in a furnace and suspended on an electronic scale. A total of six pyrolytic atmospheres (respectively being (1) pure N2; (2) 10% CH4 in N2; (3) 20% CH4 in N2; (4) 10% H2 in N2; (5) 20% H2 in N2; and (6) 10% H2, and 10% CH4 in N2) were pumped into the reaction jar at a 0.6 L/min flow rate. Samples were heated to 600°C at a rate of 5°C/min and heat was preserved for 30 min, then nitrogen was pumped in for cooling in room temperature. After pyrolysis, semi-coke samples were extracted, labeled char1–char6, and preserved in a drying vessel for further property analysis. The detailed properties of the samples are summarized in Table 1.

Table 1 Samples Proximate analysis (wt%, ad) Ultimate analysis (wt%, ad) M ad A ad V daf FC C H N O S Coal 3.83 7.53 37.06 55.97 71.97 4.08 0.93 10.21 0.20 char1 1.17 9.15 8.21 81.47 82.8 2.42 0.61 1.17 0.23 char2 1.38 9.99 8.04 80.59 83.71 2.02 0.38 1.38 0.20 char3 1.44 8.23 8.01 82.32 83.09 1.87 0.28 1.44 0.21 char4 1.40 9.01 8.49 81.10 82.81 2.24 0.55 1.34 0.16 char5 1.32 8.99 7.40 82.29 83.45 2.01 0.62 1.26 0.19 char6 1.23 8.40 7.11 83.26 83.02 1.95 0.51 1.31 0.17 Figure 1

2.2 Representation of semi-coke properties

Combustion and gasification reactivity of semi-coke with an experimental weight of 10 ± 0.1 mg was tested using STA449C thermal analyzer from German NESZCH company. Under atmosphere of air (combustion)/CO2 (gasification) and a flow rate of 50 mL/min, the temperature was elevated from room temperature to 1,000°C (combustion)/1,400°C (gasification) at a rate of 15°C/min, and weight change was synchronously recorded. For the quantitative comparison of semi-coke reactivity, the combustion reactivity index R c and gasification reactivity index R g were introduced [27,28]:

(1) R c = V rate T ignition , (2) R g = 0.5 t 0.5 ,

where V rate is the average reaction rate of semi-coke combustion, T ignition is the ignition point of semi-coke combustion determined through thermogravimetry-derivative thermogravimetry (TG–DTG) method [29], and t 0.5 is the time for carbon conversion rate α to reach 50%. α was determined using:

(3) α i = w 0 − w i w 0 − w ash ,

where w 0 is the initial semi-coke mass, w i is the semi-coke mass at any time, and w ash is the ash content mass in the semi-coke.

Carbon chemical constitution of semi-coke was measured through X-ray diffractometer (XRD) (X’Pert PROMPD) using Cu-Kα target at a scanning rate of 4°/min. Feature sizes of the microcrystalline structure of the semi-coke are represented by d 002, L a, and L c and were solved according to the Scherrer formula and Bragg equation [30]:

(4) d 002 = λ 2 sin θ 002 , (5) L c = 0.89 λ β 002 cos θ 002 ,

where d 002 is the distance between the single aromatic layers of the sample, L c is the microcrystal stack height perpendicular to the aromatic lamellas, θ 002 is the glancing angle, β 002 is the full width at half maximum of the diffraction peak, and λ is the wavelength at 0. nm of the incident X-ray.

Functional group of semi-coke was detected through Fourier-transform infrared spectrometer (FTIR) (German Bruker, Vector 22). Semi-coke samples were prepared using the KBr squashing technique, and test spectral range was 400–4,000 cm−1 with a resolution ratio of 4 cm−1. The sample spectra were obtained through scanning after deducting the blank KBr background. Aromaticity was derived using the formulas by Brown and Ladller [31]:

(6) f a = 1 − C / al C , (7) C / al C = [ ( H al / H ) ⋅ ( H / C ) ] / ( H al / C al ) ,

where C / al C is the content of aliphatic carbon, H/C is the ratio of hydrogen/carbon numbers, which can be solved through elemental analysis, H al / H is the proportion occupied by aliphatic hydrogen in total hydrogen, H al / C al is the carbon/hydrogen ratio in lipid groups and is taken as 1.8 for coal [31], and H al is the aliphatic hydrogen, which can be solved by dividing the integral area A al inside the wave band by the extinction coefficient a al ( a al is taken as 744 cm−1 for semi-coke), as shown in Eq. 8:

(8) H al = A al a al .

Physicochemical absorber (US Micromeritics, ASAP M+C) and N2 adsorption method were used to test the specific surface area and micropore structure of semi-coke, with a degasification temperature during the test at 200°C.

3 Results and discussion

3.1 Combustion/gasification reactivity

TG–DTG curves of combustion and gasification of semi-coke samples prepared under different pyrolytic atmospheres are shown in Figure 2, and semi-coke combustion and gasification characteristic parameters are presented in Table 2.

Figure 2 Table 2 Samples T max (°C) V max (°C min−1) T i (°C) T f (°C) R c (×103) T 0.5 (°C) R g (s−1 [×104]) char1 510 0. 447 574 2. 1,007 11. char2 509 0. 454 567 2. 1,016 10. char3 512 0. 447 559 1. 1,020 10. char4 515 0. 448 572 1. 1,028 9. char5 505 0. 450 579 1. 1,034 9. char6 501 0. 455 570 1. 1,048 8.

As shown in Figure 2a, under air atmosphere, six semi-coke samples started losing weight under 368°C or higher, which indicated that volatiles in semi-coke have started to decompose. Subsequently, weight loss rate increased, which suggested that the fixed carbon experienced a rapid combustion reaction. Weight loss basically ended under 570°C or so, which indicated semi-coke after-combustion. In the initial phase of rapid combustion of six semi-coke samples, differences between TG and DTG curves were not evident. In the phase of maximum weight loss rate, the maximum combustion rates of different semi-coke were distinctive, with the reaction rate of char1 being the fastest, followed by char2; moreover, minor differences existed in the reaction rates of char3–char6. In the late combustion phase, DTG curves of semi-coke no. 2–6 slightly advanced. The range of ignition temperature of the six semi-coke samples was from 447°C to 455°C, which indicated that the differences in ignition temperature in the various semi-coke samples were poor. To compare the semi-coke combustion reactivity values, the six curves were analyzed through combustion reactivity indexes in Eq. 1. After calculation, semi-coke combustion reactivity indexes were sorted in descending order: char6 > char5 > char4 > char3 > char2 > char1, which indicated that compared with the pyrolysis atmosphere of N2, adding reducing gases CH4 and H2 in the pyrolysis process of raw coal will reduce the combustion reactivity and gasification reactivity of semi-coke.

Figure 2b indicates that under a CO2 atmosphere, the six semi-coke samples experienced a volatile and slow gasification phase before 870°C or so, and a rapid gasification reaction happened under 870°C or so; moreover, the finishing temperature of gasification reaction was from 1,050°C to 1,100°C. The six semi-coke samples had noticeable differences in TG and DTG curves compared with char1 prepared under a nitrogen atmosphere; furthermore, the TG and DTG curves of char2–char6 prepared after adding reducing gases experienced retroposition, and the maximum reaction rate and reaction finishing temperature escalated. The semi-coke gasification reactivity indexes were arranged in descending order: char6 > char5 > char4 > char3 > char2 > char1. The results indicated that the addition of reducing gases in the pyrolysis phase of raw coal resulted in a clear degradation of gasification reactivity and inhibition of composite gas of H2; in addition, CH4 was the most recognizable, the influence of H2 was stronger than that of CH4, and the increased concentration of reducing gases increased its inhibitory effect.

To compare the combustion and gasification reactivity of different semi-coke, time-dependent changes in carbon conversion rates of different semi-coke are illustrated in Figure 3. At the same combustion reaction time, the difference in combustion conversion rates of the different semi-coke was minimal. From Figure 3b, at the same gasification reaction time, different semi-coke had observable differences in gasification reactivity. The time required by char1 to complete combustion and gasification was 700 s or so, and that of char2–char6 continuously increased; thus, this phenomenon became increasingly apparent during gasification. This indicated that the pyrolytic atmosphere conditions influenced the gasification reactivity at a higher degree than that of combustion reactivity. The semi-coke has favorable combustion reactivity; therefore, the after-combustion temperature was lower than 600°C, and the chemical reaction itself could be the restrictive link of combustion. The semi-coke gasification reaction temperature was higher than that of the combustion reaction; moreover, the chemical reaction itself proceeded rapidly and the diffusion of reactants and products should be the restrictive link of this gasification process. Emphasis will be placed on the factors influencing semi-coke chemical reactions, such as carbon chemical structure, functional group distribution, and micropore structural characteristics that influence diffusion.

Figure 3

3.2 XRD analysis of chars

Figure 4 shows the XRD spectrums of the six semi-coke samples. The C(002) peaks of samples char1–char6 sharpened, which indicated that the carbon microcrystalline structures of semi-coke prepared by adding reducing gases in the atmosphere were likely to be of the graphite state. X’Pert highscore analysis software, together with Eq. 4 and 5, was used to obtain the position of C(002) peak, lamellar spacing d 002 , and aromatic lamella stack thickness L c, and the results are listed in Table 3.

Figure 4 Table 3 Samples C(002) (°) d 002 (10−10 m) L c (10−10 m) L c (d 002) char1 24.83 3.58 12.09 2.26 char2 24.91 3.57 12.21 2.20 char3 25.01 3.56 12.26 2.32 char4 25.02 3.56 12.39 2.36 char5 25.03 3.56 12.50 2.39 char6 25.04 3.56 12.93 2.51

In Table 3, the differences in 2 θ angle and d 002 corresponding to the C(002) peaks of different semi-coke are unsatisfactory. However, the L c values of char1–char6 gradually increased, thereby indicating that the aromatic lamella stack thickness of the semi-coke gradually expanded. The number of carbon stack lamellas increased, which indicated the enhancement of the pseudo-crystalline phase degree of semi-coke samples and further indicated the enhancement of the carbon ordering degree in the semi-coke. Adding the reducing gases in semi-coke pyrolysis probably promoted the enhancement of the semi-coke graphitization degree because the hydrogen-free radicals generated by H2 and CH4 could permeate the semi-coke surface and would enhance the condensation of aromatic rings, thereby decreasing the number of available active sites [20,21,22].

Figure 5 shows the relationships of semi-coke L c with the combustion and gasification reactivity indexes. As shown in Figure 5, semi-coke carbon microcrystalline structure has identical influence rules on combustion reactivity and gasification reactivity, namely, with the enhanced ordering degree of carbon microcrystalline structure, decreased semi-coke reactivity indexes, and the certain linear relation of the two. In a study on the influence of heat treatment temperature and heating rate on coke reactivity, Lu et al. found [24] that from amorphous carbon, the carbon structure described by the aromaticity and crystallite size became highly systematized with the rise in heat treatment temperature and decline in heating rate, thus, semi-coke reactivity was degraded. With the increased proportion of reducing gases in the nitrogen atmosphere, the semi-coke carbon structure became highly ordered, which degraded semi-coke reactivity because when the size of the aromatic lamella increased, the ratio of the active marginal carbon atoms to the non-active carbon atoms in the cardinal plane will be reduced [32]. Moreover, when the arrangement of the aromatic lamellas became organized, the active carbon atoms bonded with the defects and the hetero atoms were reduced [21]. Both could degrade semi-coke combustion and gasification reactivity.

Figure 5

3.3 FTIR analysis of chars

Figure 6 shows the FTIR spectra of different semi-coke. To carry out specific analysis on the wave number region of semi-coke 4,000–400 cm−1, the entire infrared spectrum is divided into four parts [31] as follows: hydroxyl absorption peak (3,600–3,000 cm−1), aliphatic hydrocarbon absorption peak (3,000–2,700 cm−1), oxygen-containing functional group absorption peak (1,800–1,000 cm−1), and aromatic hydrocarbon absorption peak (900–700 cm−1). It can be seen from Figure 6 that the distribution of functional groups in different semi-coke samples is different. Compared with other semi-coke samples, there is an obvious hydroxyl absorption peak between char1 and char2, but char2 is mainly composed of free hydroxyl groups, char1 is mainly composed of phenol, alcohol, carboxylic acid, and hydroxyl in water, and there are obvious antisymmetric stretching vibrations of CH3 and CH2 in naphthenes or aliphatic groups. All the semi-coke samples had stretching vibration of S–H bond near 2,510 cm−1, but the vibration peak shape of char1 was not obvious. CH3 vibration peak exists in all samples near 1,440 cm−1, but obvious vibration peak exists in char1 and char2 near 1,590 cm−1. This is the vibration peak of aromatic C═C, and it is the skeleton vibration of benzene ring. char1–char6 have obvious characteristic peaks near 880 and 710 cm−1, but the peak intensity of char1 is significantly lower than that of other semi-focal points. char1 has obvious vibration peaks at 1,058 and 1,183 cm−1, which are stretching vibration peaks of Si–O–Si, Si═O, or Si–O–C. Oxygen-containing functional groups are obvious in char1. When the pyrolytic atmosphere contains H2 or CH4, removal of hydroxyls and oxygen-containing functional groups reduced content of unsaturated side chains, and elevated condensation degree of macromolecular network will be facilitated.

Figure 6

To further describe the differences in various semi-coke samples in the macromolecular structure, Eq. 6–8 were used to calculate and compare the aromaticity of semi-coke. Table 4 presents the obtained IR structural parameters. It can be seen from the table that when CH4 or H2 is added in the pyrolysis atmosphere, aliphatic hydrocarbons and carbon groups in the raw coal are more easily decomposed and precipitated, which is specifically reflected in the significant decrease in the contents of H al and H ar and the significant increase in f a of the samples. However, the influence of CH4 atmosphere and H2 atmosphere is different. Compared with H2 atmosphere, CH4 atmosphere has a more obvious effect on f a promotion.

Table 4 Samples H al (%) H al/H C al/C f a char1 0.789 0.326 0. 0. char2 0.559 0.277 0. 0. char3 0.497 0.266 0. 0. char4 0.412 0.184 0. 0. char5 0.37 0.184 0. 0. char6 0.359 0.184 0. 0.

Relationships of semi-coke aromaticity with combustion and gasification reactions are shown in Figure 7. There is no obvious relationship between the aromaticity and combustion and gasification reaction index, but char1 gasification reactivity and combustion reactivity are best, its corresponding f a also minimum, shows that N2 pyrolysis atmosphere compared to add CH4 and H2, can maintain the sample in a certain amount of reactive strong aliphatic group, inhibit samples influence the reactivity of the increase of aromatic carbon.

Figure 7

In summary, when CH4 or H2 is added in the pyrolysis atmosphere, aliphatic hydrocarbons and carbon groups in raw coal are more easily decomposed and precipitated, so as to improve the f a of semi-coke and reduce the reactivity of semi-coke. The influence of its functional groups is manifested in the fact that hydroxyl group, C═C, and oxygen-containing functional groups have a promoting effect on the improvement of reactivity, whereas the increase in S–H and aromatic hydrocarbon contributes to the improvement of aromaticity, thus reducing the reactivity of semi-coke.

3.4 Effects of pore structure of chars on reactivity

Figure 8 shows the pore structural distribution of the six semi-coke samples. Figure 8a shows that micropores below 2 nm and mesopores at 2–50 nm contributes to the main specific surface area. Independent addition of H2 or CH4 to the pyrolytic atmosphere increased the specific surface area of semi-coke micropores, and the micropore-specific surface areas of char2 and char3 added with CH4 gas were significantly enlarged. Differences in specific surface area between semi-coke mesopores were visible, and the independent addition of H2 or CH4 in pyrolytic atmosphere reduced the specific surface area of the semi-coke mesopores. Mesopore-specific surface areas of char3 and char4 added with H2 gas decreased more evidently than those of char2 and char3 added with CH4 gas, and the specific surface areas of mesopores added with both H2 and CH4 were mostly reduced. Mesopores contributed to the main pore volume and the order of pore volumes of the different semi-coke was consistent with rule of specific surface area. The difference between micropores in terms of pore volume was not obvious, and pore volumes 50 nm above pores were small. The above results showed that the main pore structural types of semi-coke were micropores and mesopores; moreover, the influence of the regularity of the pyrolytic atmosphere on micropores was not strong, but the pyrolytic atmosphere could reduce the quantity of mesopores. This is because, with an elevated pyrolysis degree, the generation of micropores, which dominated the semi-coke-specific surface area, was reduced. Moreover, the CH4 and H2 in the pyrolytic atmosphere reacted with macromolecular side chains in the coal during pyrolysis to improve the yield and precipitation rate of pyrolytic gases [33], which then further boosted the development and growth of micropores toward mesopores, as well as the cross-linking and combination of mesopores. As a result, the pore-specific surface area and pore volume were reduced.

Figure 8

Figures 9 and 10 show the relationships between semi-coke-specific surface area and combustion and gasification reactivity indexes. Combustion and gasification reactivity indexes presented weak linear correlations with micropore-specific surface area and pore volume. However, they have a favorable linear correlation with mesopore volume (i.e., as mesopore-specific surface area and pore volume increased, the combustion and gasification reactivity were improved) because compared with homogeneous reaction, as heterogeneous reaction processes, the semi-coke combustion and gasification contained two important features, namely diffusion of reactant molecules and reaction interface conditions. Semi-coke pore structure not only provided a diffusion channel for oxygen/carbon dioxide molecules that is required by combustion and gasification but also provided a large specific surface area for gas analysis and solid contact during gas–solid heterogeneous reaction [28]. Therefore, the developed pore structure of semi-coke improved their combustion and gasification reactivity.

Figure 9 Figure 10

Semi-coke combustion and gasification reactivity were closely related to the ordering degree of carbon chemical structure and micropore structure; moreover, they had a certain relationship with the distribution of functional groups. For the convenience of semi-coke application in iron-making, the pyrolysis temperature, holding time, heating rate, and other conditional parameters during pyrolysis should be reasonably regulated to counterbalance the adverse effects of reducing gases in the pyrolytic atmosphere on the semi-coke reactivity, so that they meet blast furnace PCI requirements.

Pyrolysis process and characteristics of products from sawdust ...

Yang, H., Huang, L., Liu, S., Sun, K., and Sun, Y. (). "Pyrolysis process and characteristics of products from sawdust briquettes," BioRes. 11(1), -.

Abstract

The pyrolysis of briquettes made from biomass is an available and economic technological route for the production of briquette charcoal, but by-products (tar and gas) cannot be brought into full utilization, leading to the waste of resources and the addition of environmental concerns. Temperature is the most important parameter that affects the distributions and properties of briquette charcoal. This work investigated the three kinds of products of the pyrolysis of sawdust briquette in a fixed bed across a wide temperature range (250 to 950 °C). The purpose of this experiment was to study the pyrolysis process and the properties of the resulting products (briquette charcoal, liquid, and gas) of sawdust briquettes and explore the optimum operating temperature to generate good quality briquette charcoal, liquid, and gaseous products simultaneously. According to the results, the optimum pyrolysis temperature range was 450 to 650 °C, for which the briquette charcoal produced within this range had the highest calorific value (2,9.14 to 30.21 MJ/kg). Meanwhile, the liquid product is considered to be useful for liquid fuels or valuable chemical materials, and the low heating value of the gaseous product was 11.79 to 14.85 MJ/Nm3 in this temperature range.

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Pyrolysis Process and Characteristics of Products from Sawdust Briquettes

Hua Yang,b Li Huang,b Shicai Liu,a,b* Kang Sun,a,b* and Yunjuan Sun a,b

Keywords: Sawdust briquette; Pyrolysis; Temperature; Briquette charcoal; Product characteristics

Contact information: a: Research Institute of New Technology of Forest, Chinese Academy of Forestry (CAF), Beijing , China; b: Institute of Chemical Industry of Forestry Products, Chinese Academy of Forestry (CAF), Nanjing , PR China;

* Corresponding author: and .

INTRODUCTION

It is generally accepted that fossil fuels throughout the world are both vulnerable in the short term and limited over the long term; therefore, worldwide searches for sustainable and renewable energy resources have been explored to meet a considerable part of the energy demand in the future. Increasing attention has been focused on biomass resources because they can be converted to multifarious fuels and chemicals. Environmental problems, such as air pollution and greenhouse gas emissions, also play a vital role in the utilization of renewable biomass resources, as biomass is considered to be environmentally friendly and CO2-neutral (Chen et al. . The utilization of biomass has great potential for reducing the dependence on fossil fuels and alleviating the burden of environmental degradation (Abnisa and Wan Daud ).

Biomass resources are abundant and widely dispersed in China. They are primarily present in five forms: wood and forestry residues, agricultural residues, animal manure, municipal waste, and wastewater resources. And the total quality of each are , 728, , 155, and Mt (million tons), respectively (Shen et al. b). In the past few years, a variety of biomass feedstocks, including forest and agricultural residues, have been selected as subjects of research; however, a majority of them have been used without pretreatment and are unsuitable for the large-scale industrialization use in the energy conversion process because they have lower energy density and higher transportation and storage costs than fossil fuels. Additionally, large reaction equipment is needed when original biomass materials are utilized directly. These characteristics of biomass materials are the bottlenecks for its industrial utilization, and it is crucial to convert loose agricultural and forestry residues into high-density and high-value solid materials of uniform size before utilization (Khardiwar et al. ). The biomass briquette technique provides a promising solution to the effective utilization of biomass resources. This technique can be defined as a compression molding process for converting low-bulk density biomass into solid fuels with high energy density.

Presently, interest in smokeless briquettes as fuel has been growing because of their high density, high calorific value, and small amount of environmental pollution. Briquette charcoal has been given attention as a potential substitute for charcoal and coal in developing countries. The pyrolysis of briquette is an available technological route for the production of briquette charcoals. No binders are added in this technology, which reduces processing costs; however, by-products such as tar and gas cannot be brought into full utilization, leading to the waste of resources and environmental concerns. It is very crucial to investigate the manufacturing process to ensure that the three products (briquette charcoal, tar, and gas) can be used simultaneously.

Pyrolysis has been considered one of the most promising techniques for utilizing biomass (Maschio et al. ) and is the initial step for other thermochemical conversion processes, such as combustion and gasification (Elmay et al. ). Biomass pyrolysis is defined as the technique of converting lignocellulosic materials into liquid products, along with non-condensable gases and solid chars, by heating in the absence of oxygen or in reduced air. Pyrolysis can convert biomass feedstock to high-energy density bio-oil, solid char, and gas with high conversion ratio (Demirbas ), allowing the products’ distributions and qualities to be regulated depending on operating parameters (such as temperature, heating rate, or catalyst).

The solid products (briquette charcoal) can be used as a substitute for coals and traditional charcoals for some applications in chemistry, metallurgy, environmental protection, barbecuing, and living fuel (Mwampamba et al. ). The liquid product is not only an important chemical material but also a potential fuel oil. The gaseous product is a kind of biogas applied in heating and electricity generation.

The pyrolysis temperature is the most important parameter affecting the pyrolysis behaviors and product distribution of solid, liquid, and gas (Guizani et al. ). In this work, briquettes made of Chinese fir sawdust were chosen as the pyrolysis object, and pyrolysis tests were performed in a fixed bed across a wide range of temperatures (250 to 950 °C). The yields and characteristics of the three products were analyzed in detail. The aim of this study was to evaluate the pyrolysis process and the physical and chemical properties of the pyrolysis products and explore the optimum operating temperature for generating briquette charcoal, liquid, and gaseous products simultaneously. These investigations provide a comprehensive understanding of the pyrolysis characteristics of biomass briquettes and benefit both the design and optimization of the thermochemical conversion process. In addition, this technology expands the utilization of biomass briquettes and accelerates the industrial process of the large-scale utilization of biomass resources.

EXPERIMENTAL

Materials

Sawdust briquettes were collected from a briquette manufacturer in the FuJian province of China. The sawdust (15% of moisture content) was pressed in the screw extrusion molding machine at 175 oC. The prepared products are bar briquettes of 30 mm diameter and 100 mm length. The physical properties such as unit density, shatter resistance, and resistance to water penetration were tested with different methods.

Unit density

The unit density of bar briquette can be obtained by weighting its quality and calculating its volume based on its diameter and length (measured by Vernier caliper) (Lajili et al. ). The following equation was used to calculate the bulk density of bar briquette,

(1)

where ρ (g/cm2), m (g), d (cm) and l (cm) represent the bulk density, quality, diameter, and length respectively.

Shatter resistance

The shatter test was aimed to obtain the percentage of weight loss of briquettes after falling action. A certain quality (about 500 g) of briquettes were placed in a suitable plastic bag. The plastic bag would be dropped freely upon the cement ground at a height of 2 m. Repeat this operation 2 times and then weigh the briquette. The weight loss was calculated by the following equation,

Percent weight loss (%) = (m1–m2)/m1 (2)

Shatter resistance (%) = 100-percent weight loss (3)

where m1 (g) and m2 (g) represent the weight of briquette before and after being dropped, respectively.

Resistance to water penetration

The percentage of water absorbed by an individual briquette was tested when the briquette was immersed into water. Each briquette was weighted and then immersed in distilled water at 25 oC for 5 s. It was taken out and the water was wiped off the surface. Finally, the briquette was weighted. The resistance to water penetration was determined by the following formula,

Water gain by briquette = (m2–m1)/m1 (4)

Resistance to water penetration (%) = 100- water gain by briquette (%) (5)

where m1 (g) and m2 (g) represent the weight of dry briquette and weight of wet briquette, respectively.

Each test described above was repeated several times, and an average value was obtained to represent the physical characteristic. The unit density, shatter resistance, and resistance to water penetration of sawdust bar briquette were 1.20 g/cm3, 99.02%, and 94.05%, respectively.

The briquettes were cut into uniform size cubes of 20 mm for use in a pyrolysis reactor. Proximate and ultimate analyses of the samples were performed to understand the compositions and estimate the potential of the materials for producing bioenergy The materials were dried at 105 oC for the determination of moisture according to ASTM E871-82. The dried materials were stored in a desiccator for further studies. The evaluations of ash and volatile matter content on a dry basic were conducted according to the standard methods ASTM D-84 () and ASTM E872-82 (), respectively. The fixed carbon was calculated as the difference. The chemical compositions of the samples were determined for the C, H, N, and S contents using a PE- elemental analyzer (PerkinElmer Corporation, USA). The oxygen content was calculated as the difference. The higher heating values (HHV) of the samples were measured using an oxygen bomb type-calorimeter Parr (Parr Instrument Company, USA). The basic characteristics of the sawdust briquettes are given in Table 1, similar to reports for Chinese fir sawdust presented by other researchers (Wu and Qiu ).

Table 1. Proximate and Ultimate Analysis of Sawdust Briquettes

*By difference

Methods

The pyrolysis experiments with the sawdust briquettes were carried out in a fixed-bed reactor made of quartz with a length of 500 mm and an internal diameter of 30 mm. The quartz reactor was equipped with an inert gas (N2) supplied from above and a volatile outlet from below to connect the condensing system. The reactor was externally heated by an electric furnace, and a Pt-Rh-Pt thermocouple was placed inside the bed to control the temperature. A movable feeding tank made of stainless steel mesh was used to rapidly move the samples into the heated center of the reactor at the start of pyrolysis. The volatiles produced during pyrolysis were cooled through a condensing system consisting of two U-tubes located in a cold trap maintained at below 0 °C.

The pyrolysis experiments were conducted at eight different temperatures, ranging from 250 to 950 °C, to study the effect of temperature on the pyrolysis process and product characteristics of the sawdust briquettes. Purified nitrogen at a flow rate of 100 mL/min was continuously flushed into the reactor to provide an inert atmosphere and purge volatiles produced during pyrolysis from the reactor. Once the reactor was preheated to the target temperature, the feeding tank that the 5-g samples had been placed in was moved rapidly into the heated center of the reactor. It took about 1 to 2 s to introduce the feeding tank into the bed. The sample was maintained there for 30 min to remove all volatiles. The device was then allowed to cool naturally, and the N2 flow remained to prevent solid char from being oxidized. When room temperature was reached in the reactor, the yield of the solid product, namely sawdust briquette charcoal, was calculated according to the overall weight loss of the feeding tank and materials after reaction. The yield of liquid product was determined by weighing the two U-tubes before and after the experiments. The gaseous yield was determined by the difference from a mass balance. All experiments were carried out at atmospheric pressure. Each pyrolysis experiment was repeated three times, and the reported results are the averages of three experiments to eradicate discrepancies.

Analysis of Properties of Pyrolysis Products

The proximate and ultimate analyses of the briquette charcoal obtained at various pyrolysis temperatures were conducted following the same procedures used for material characterizations. The FTIR spectra of the briquette charcoals were recorded in transmission mode between and 400 cm-1 using a Magna-IR550 Fourier transform infrared spectrometer (Nicolet, USA) with a KBr compression method. The surface morphologies of the briquette charcoal were observed by scanning electron microscopy (SEM-S-, Hitachi, Japan).

The collected liquid product was extracted with dichloromethane, and the dichloromethane-soluble fraction, which contained the most compounds in the liquid product, was used for gas chromatography-mass spectroscopy (GC-MS) analysis. The main organic components in the pyrolysis oil were qualitatively analyzed by GC-MS, performed on an Agilent A/C (USA) instrument equipped with a HP-5 quartz capillary column (length, 30 m; film thickness, 0.25 μm; internal diameter, 0.25 mm). The column temperature was retained at 50 °C for 2 min, then was increased to 280 °C at a rate of 5 °C/min, and then was kept constant for 20 min. The carrier gas was helium, and the sweep rate was maintained at 1.6 mL/min. The temperature of the injector was 250 °C, and the injector split ratio was 10:1. The NIST mass spectra library provided the foundation for the identification of each peak in the chromatograms.

The gaseous compounds from the pyrolysis of the sawdust briquette were analyzed using a gas chromatograph GC- (Shimadzu Company, Japan) equipped with a Shin Carbon ST100/120 micro-packed column and a capillary column. A TCD (thermal conductivity detector) was applied to detect inorganic compounds (H2, O2, N2, CO, and CO2), and a FID (flame ionization detector) was used for determination of CH4, C2H6, C2H4, and C2H2. The gaseous components were qualitatively determined by a contrast with standard gas, and quantitatively determined using the single point external standard. The heat values of the gases were calculated according to the volume fraction of the combustible compounds.

RESULTS AND DISCUSSION

Products Distribution

The yield distribution of pyrolysis products (solid, liquid, and gas) has a close association with pyrolysis temperature (Shadangi and Mohanty ). The weight percentages of the products obtained at different temperatures (250 to 950 °C) from the pyrolysis of the sawdust briquette are given in Fig. 1. The figure shows the connection between the yield distribution of the products and pyrolysis temperature. The yield of the briquette charcoal decreased from 63.40 to 20.90 wt.% as the temperature was increased from 250 to 650 °C. After that, the decrease in solid yield with increase in temperature was not distinct. In other words, the depolymerization of the raw material or the release of volatile matters is primarily focused on the lower temperatures in the range of 250 to 650 °C (Burhenne et al. ). The yield of the liquid product increased gradually in the temperature range of 250 to 450 °C and reached a maximum of 52.28% at 450 °C, and then decreased from 52.28% to 16.95% when the temperature increased sequentially from 450 to 950 °C. The trend of variation in weight percentage of gaseous yield with increasing pyrolysis temperature was incremental under the scope of the temperature investigated. A more substantial growth in gaseous yield was observed when the temperature was above 450 °C. This phenomenon results from secondary reactions such as cracking and rearrangement of the unstable volatile matters and the solid residue at higher temperatures (> 450 °C), leading to the formation of incondensable gas (Morf et al. ; Şensöz and Angın ). The temperature range of 450 to 650 °C thus should be the better choice for pyrolysis temperature, taking the yields of the three types of products into consideration.

Fig. 1. The yield distribution of sawdust briquette pyrolysis as a function of temperature

In the temperature range of 450 to 650 °C, the yield of the briquette charcoal was above 20%, ensuring the production of the major product (Bhattacharya et al. ). The temperature of 450 °C maximized the production of the pyrolysis liquid, and temperature of above 450 °C favored the yield of the gaseous product. According to the practical situation in the selection of obtaining high yield of liquid or gaseous products, an appropriate operating temperature can be determined.

Properties of the Briquette Charcoal

Basic analysis of the briquette charcoal

The basic properties of the briquette charcoal created at different pyrolysis temperatures are shown in Table 2. Particularly, the proximate analysis and higher heating value (HHV) are important indexes for measuring the quality of the solid biomass fuel. With the pyrolysis temperature increasing from 250 to 650 °C, the volatile content in the briquette charcoal decreased from 62.80% to 15.92% and the fixed carbon content increased from 33.40% to 75.43%; however, the change was not obvious at temperatures higher than 650 °C. The ash content was the non-volatile substance and non-combustible material in the charcoal (Angın and Şensöz ) and its content showed an increasing tendency due to the release of the volatile matters with increase in pyrolysis temperature. The HHVs of the briquette charcoals prepared at different pyrolysis temperatures are also shown in Table 2. The HHVs of the charcoal obtained between 250 and 950 °C were 22.98 to 30.21 MJ/kg, much higher than that of sawdust briquette. The HHVs first increased and then decreased with the increase in temperature, and the turning point, representing a maximum value, was between 550 and 650 °C. The HHVs produced at 550 °C and 650 °C were 30.21 MJ/kg and 29.30 MJ/kg, respectively; these values are approaching the HHV of soft coal (29 MJ/kg) (Amutio et al. ). From the ultimate analyses, with the increasing of pyrolysis temperature, the carbon content increased from 63.89% at 250 °C to 88.05% at 950 °C. Inversely, the hydrogen and oxygen contents of the solid charcoal reduced gradually with increasing temperature. These findings suggest that dehydrogenation reactions occurred during the charring process and the degree of carbonization became stronger as the temperature increased.

Table 2. The Basic Properties of the Sawdust Briquette Charcoal at Different Pyrolysis Temperatures on Dry Basic

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The basic properties of the briquette charcoals obtained in the temperature range of 450 to 650 °C were compared to the properties of the raw materials (sawdust briquette) and traditional wood charcoal, which originated from some softwood species (Demirbas et al. ; Mwampamba et al. ).These comparisons are presented in Table. 3.

Table 3. A Comparison of Properties between the Briquette Charcoal and those of Briquette and Traditional Wood Charcoal

The fixed-carbon and ash contents of the briquette charcoal were higher than those of briquette because of carbonization. The HHV of the briquette charcoal reached approximately 30 MJ/kg, which was greater than the sawdust briquette by about 52%. This indicates that pyrolysis of briquette improved its fuel quality. The briquette charcoal had a similar amount of volatile content, but slightly lower fixed carbon contents than those of traditional wood charcoal. The ash content of briquette charcoal was a little higher than that of traditional wood charcoal. This may be due to a small amount of mechanical impurities (such as dust) having been mixed in briquette fuels when loose sawdust was converted into briquette by compressing and molding. The heating value of briquette charcoal prepared in the optimum temperature range is slightly lower when compared to traditional wood charcoal because of slightly lower fixed carbon for briquette charcoal but reached the value of commercial briquette charcoal (Hirunpraditkoon et al. ). These results make it possible to conclude that sawdust briquette charcoal has potential for being an alternative for traditional wood charcoal.

FTIR analysis

FTIR spectra of the raw materials and briquette charcoals obtained at various pyrolysis temperatures are shown in Fig. 2. The spectra can be applied to characterize the changes of various chemical functional groups in the solid charcoal with increasing pyrolysis temperature. The main typical functional groups in the raw materials were as follows: a wide peak at to cm-1 for the O-H Stretching vibration, a peak at to cm-1 caused by the stretching vibration of alkyl C-H, a peak at cm-1 caused by C=O stretching vibration, two peaks at cm-1 and cm-1corresponding to C=C vibration in aliphatic and aromatic structures, respectively, a peak between and cm-1 caused by C-O-(C)/(H) stretching vibrations and O-H deformation vibration, and a peak between 750 and 500 cm-1 caused by C-H bending vibration. These major absorption bands in the FTIR spectrum of sawdust briquette represented the typical structures of lignin, cellulose, and hemicellulose, which are the main constituents of biomass resources. These major chemical groups in the raw materials are very non-stable and can be easily ruptured.

Fig. 2. FTIR spectra of the raw material and charcoal produced at various temperatures

From the FTIR spectra it is apparent that the type and content of the chemical groups in the raw materials decreased gradually as the pyrolysis temperature increased. The shape of the spectra for the briquette charcoal obtained at 250 °C was similar to that of the raw material but the intensities of the peaks became weak (Chen et al. ), which indicated that some of the chemical bonds, O-H, C=O, and C-O, began to break (Gu et al. ). The peak at around cm-1 was not observed at temperatures 350 °C and above, indicating the rupture of the methylene group (-CH2-) in hemicellulose at the lower temperature (Shaaban et al. ). Vibrations of peaks detected at cm-1 (C=O) and cm-1 (C-O-) became weak at 250 °C and disappeared at 350 °C, leading to the formation of some small molecular gases, such as CO and CO2. When the raw materials were pyrolyzed at 450 to 550 °C, the degradation of lignin was prominent. The peaks of C=C band vibration ( to cm-1) and aromatic C-H deformation (810 to 750 cm-1) provided evidence for the presence of an aromatic structure (Yuan et al. ). When the pyrolysis temperature was above 650 °C, most of the volatile compounds were released and the charcoal became more and more thermally stable. The spectrum of the charcoals obtained at 750 to 950 °C were more simplified and invariable, in which few peaks remained including some representative peaks of aromatic rings. These FTIR characteristics of the materials and charcoals in the experimental temperature range (250 to 950 °C) indicate that the pyrolysis reactions of sawdust briquette mainly occur at temperatures below 650 °C.

Scanning electron microscopy (SEM) analysis

The comparisons of some of the SEM images of the briquette charcoals obtained at typical temperatures were analyzed to illustrate the evolution of the surface morphologies with increasing pyrolysis temperature. The SEM photos of the charcoals obtained at 250, 450, 650, and 850 °C are shown in Fig. 3. The briquette was prepared from the loose sawdust by mechanical pressing at certain temperature and pressure. An irregular laminated structure of the charcoal surface can be observed in Fig. 3(a), and this feature became increasingly less clear with increasing temperature, almost vanishing by Fig. 3(d). These suggest that the release of volatile compounds may promote the shrinkage of the internal structure of charcoal as temperature is increased, leading to a compact texture. There were almost no pores on the surface of the charcoal generated at the lower temperatures (Fig. 3(a) and Fig. 3(b)) and some pores emerged at 650 oC (Fig. 3(c)). But the phenomenon that the majority of pores on surface disappeared and only a few smaller pores existed was observed at 850oC (Fig. 3(d)). This suggested charcoal may melt and deform, leading to pores shrinking or even closing at higher temperature (Wang et al. b). Meanwhile, a smooth morphology of charcoal was obtained. These phenomena described are similar to those of sawdust char reported by another work (Zhang et al. ).

Properties of the Pyrolysis Oil

Bio-oil originates from the degradation of lignocellulosic biomass resources, and it is well known to be a complex mixture that contains various kinds of molecular compounds of different sizes (Zhang et al.). The complexity of a pyrolysis oil composition makes it very difficult to identify all of its compounds. In the aspect of qualitative and quantitative analysis of pyrolysis oil, a “good enough” indication of the chemical composition is provided by GC-MS analysis (Jung et al. ). The results of GC-MS analyses of the pyrolysis oil from the pyrolysis of the sawdust briquette at different temperatures are shown in Table 4.

Fig. 3. The SEM photos of the briquette charcoal obtained at different pyrolysis temperature (×magnification)

The values quoted below represent the related GC-MS area percent (%). The types and contents of compounds in the pyrolysis oil varied greatly with temperature. The organic compounds identified in the pyrolysis oil obtained at low temperatures (250 to 650 °C) mainly included many oxygenated compounds such as aldehydes, ketones, furans, and phenols. Nevertheless, the major compounds in the pyrolysis oil obtained at higher temperatures (750 to 950 °C) were aromatics with a single ring (benzene and indene) and polycyclic aromatic hydrocarbons (PAHS).

Cellulose and hemicellulose in biomass are mainly degraded in the temperature range of 250 to 450 °C (Yang et al. ). The glycosidic linkages between the monomer units of cellulose and hemicellulose and some functional groups on the branched chains become reactive and rapidly rupture, leading to the formation of different unstable intermediates and functional groups (Van de Velden et al. ). These intermediate compounds undergo different reactions, such as dehydration, decomposition, oxidation, and secondary reactions (Collard and Blin ), which contribute to the formation of aldehydes, ketones, furans, etc. As shown in Table 4, the major aldehydes and ketones obtained in this temperature range were 1,2-cyclopentanedione, 3-methyl-1,2-cyclopentanedione, and 2-methyl-2-cyclopentene-1-one. The major furans were furfural, 2(5h)-furanone, and 2-furanmethanol. These furan ring compounds can be converted from pyran rings through contraction and cyclization reactions (Shen and Gu ; Branca et al. ); pyran rings form the main-chain structure of cellulose and hemicellulose.

The most abundant compounds in the pyrolysis oil of the sawdust briquette were phenolic compounds, which are most likely to have been derived from the degradation of lignin owing to its molecular structure (Wang et al. c). Lignin is a phenol polymer composed of three phenylpropane units of guaicyl(G), p-hydroxyphenyl(H), and syringyl(S), and the main decomposition reactions happen over a large temperature range of 200 to 450 °C (Wang et al. a). As shown in Table 4, the major phenolic compounds in the pyrolysis oil can be classified into phenolic ethers and alkylated phenols. The phenolic ethers were guaiacol, 4-methyl guaiacol, 4-ethylguaiacol, 2-methoxy-4-vinylphenol, eugenol, 2-methoxy-4-propyl-phenol, 2-methoxy-4-(1-propenyl)-, vanillin, (e)-isoeugenol, acetovanillone, 2-propiovanillone, homovanillyl alcohol, and homovanillic acid. The alkylated phenols consisted of phenol, o-cresol, 4-methylphenol, 2,4-dimethylphenol, 3,4-dimethylphenol, 1,2-benzenediol, 4-methyl-1,2-benzenediol, and 2,4-di-tert-butylphenol. The accumulated area percentages of all phenolic compounds in the pyrolysis oil obtained from sawdust briquette at all the temperatures tested are shown in Fig. 4.

Fig. 4. The total amount of phenolic compounds and PAHS in the pyrolysis oil at different temperatures

The total area percentages of phenolic compounds reached a maximum between 350 and 450 °C, which was consistent with the highest degradation rate of lignin – between 350 and 400 °C (Wang et al. a). For temperatures higher than 450 °C, the related area percent of phenolic compounds decreased with increase in temperature. This may be attributed to the second reaction and release of unstable volatile compounds during the pyrolysis process.

Table 4. Composition of Pyrolysis Oil Identified by GC/MS Produced at Various Temperatures

One main characteristic of the phenolic compounds in the pyrolysis oil obtained at various temperatures was that the phenolic ethers were formed at low temperatures (250 to 650 °C) while the alkylated phenol compounds formed at high temperatures (450 to 950 °C). This may be attributed to the instability of phenolic ether compounds, leading to the methoxy groups in the ortho position of hydroxyl groups being substituted by -OH, -CH3, or –H groups (Shen et al. a). The pyrolysis oil obtained in the temperature range of 350 to 650 °C is considered to be valuable as it has important chemicals owing to its high content of phenolic compounds. For example, Guaiacol can be used for the synthesis of vanillin, and Phenol for the synthesis of phenol-formaldehyde resin adhesives (Effendi et al. ).

One obvious phenomenon of pyrolysis oil was the important concentration of polycyclic aromatic hydrocarbons (PAHS) at higher temperature (750 to 950 °C). The major PAHS presented in pyrolysis oil were naphthalene, biphenyl, acenaphthylene, anthracene, fluorene, and their derivatives. The changes of total area percent of PAHS with temperature increasing are also presented in Fig. 4. The related percentages of PAHs in pyrolysis oil increased from 40.63% at 750 °C to approximately 68.31% at 950 °C because of the deoxygenation and aromatization reactions of primary and secondary volatile components (Yu et al. ).

Properties of the Gas Product

The main components of non-condensable gas from the pyrolysis process of the sawdust briquette were CO2, CO, H2, and CH4, and other minor compounds of C2H6, C2H4, and C2H2 were also detected. The variations in volume fraction of each pyrolysis gas component at different temperatures are given in Fig. 5. The fraction of each non-methane hydrocarbon compound was less than 1% of the total volume of the gaseous product, so C2H6, C2H4, and C2H2 were categorized as C2 hydrocarbon compounds in this research. The lower heating value (LHV) of the gas product was calculated according to the following formula (Gil-Lalaguna et al. ),

LHV(gas) (MJ/Nm3) = ∑(xi *LHVi) (6)

where LHVi (MJ/Nm3) and xi represent the lower heating value and the volume fraction of each component, respectively. The smallest heating value of C2H2 among non-methane hydrocarbon compounds was regarded as that of C2.

As shown in Fig. 5, the volume fractions of the gas components was highly correlated with the pyrolysis temperature. At low temperatures of 250 to 350 °C, CO2 and CO were the main components, which may be attributed to the fragmentation of the unstable structural chains or linkage with intermediate functional groups like carbonyl and carboxyl (Park et al. ). The volume fraction of CO2 was reduced dramatically from 47.10 to 14.20% when the temperature increased from 350 to 750 °C, which resulted from the dilution effect of H2 and CH4 rapidly being released at this temperature range. Meanwhile, a relatively small change was presented for the fraction of CO (around 41%). For high temperatures (T > 750 °C), a continuous increase in H2 volume fraction and slower increase in other gases (CH4 and C2) were observed. The production of CO at a higher temperature is derived from some of the oxygenated function groups being converted and remaining in the solid residue, such as ether linkages and hydroxyls (Liu et al. ). The production of CH4 was brought by the cracking of methyl or methoxyl groups in the residues and volatile compounds (Collard and Blin ). The volume fraction of H2 increased rapidly from 4.47% to 23.68% when the temperature increased from 450 to 950 °C. The formation of H2 was mostly attributed to the dehydrogenation reaction during the charring process (Collard and Blin ).

Fig. 5. Gaseous compositions of the sawdust briquette pyrolysis at different temperatures

A decrease in the fraction of CO2 and a simultaneous increase in the productions of H2, CH4, and C2led to the LHV of the gaseous product increasing from 6.75 to 16.89 MJ/Nm3 when the temperature increased from 250 to 950 °C. A slow increase was observed when the temperature was above 750 °C; therefore, the optimal operating temperature should not exceed 750 °C with the purpose of more economy. In the temperature range of 450 to 650 °C, the lower heating value of the gaseous product ranged between 11.79 and 14.85 MJ/Nm3, suggesting that the gaseous product from sawdust briquette pyrolysis can be used as a syngas. A good quality gaseous product with a low proportion of non-combustible components is directly available for generating power for civil and industrial usage. In this experiment, however, the percentage of CO2 in the gas obtained from sawdust briquette was very high; therefore, a gas purification step, namely the abstraction of CO2 would be needed to improve the quality of the gaseous product.

CONCLUSIONS

Pyrolysis of sawdust briquette in a fixed bed was investigated across a broad range of temperatures (250 to 950 °C). The optimum operating temperature range was 450 to 650 °C considering the yields and properties of three kinds of products.
Increasing the pyrolysis temperature decreased the yield of briquette charcoal and increased the production of the gaseous product. The liquid yield firstly increased and then decreased as the temperature increased, reaching a maximum of 52.28% at 450 °C.
In the temperature range of 450 to 650 °C, the basic properties of briquette charcoal were comparable with wood charcoal and commercial briquette charcoal. FTIR spectrum of the briquette and charcoal indicated that the pyrolysis reaction of sawdust briquette mainly occurred when the temperature was below 750 °C. The surface of the briquette charcoal became more and more smooth and compact as the temperature increased.
At a low temperature (250 to 650 °C), the organic compounds identified from the pyrolysis oil mainly included many oxygenated compounds such as aldehydes, ketones, furans, and phenols. Nevertheless, the major compounds in the pyrolysis oil obtained at higher temperatures (750 to 950 °C) were aromatics with a single ring (benzene and indene) and polycyclic aromatic hydrocarbons (PAHS). The pyrolysis oil generated at 450 to 650 °C contained a large amount of phenolic compounds, such as guaiacol, 4-methyl guaiacol, 2-methoxy-4-vinylphenol, and (e)-isoeugenol, which are considered to be valuable chemical substances.
The main components of the gaseous product were CO2, CO, H2, CH4, and C2. Increasing the temperature decreased the volume fraction of CO2 and increased the H2, CH4, and C2productions. The volume fraction of CO was not greatly affected by temperature. The low heating value of the gaseous product increased by increasing the temperature, and ranged between 11.79 and 14.85 MJ/Nm3 for temperatures of 450 to 650 °C.

ACKNOWLEDGMENTS

This work was financially supported by the Special Fund for Fundamental Research (CAFINTK04) and province cooperation project (SY15).

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Article submitted: September 30, ; Peer review completed: December 19, ; Revised version received and accepted: January 6, ; Published: January 26, .