1. Introduction
Among global energy resources, coal and biomass rank first and third, respectively, all over the world, while oil is the second largest energy resource [
1]. Although the enormous energy generated by direct combustion of coal has positively accelerated the pace of human development, its leading role that includes global warming and air pollution has produced a negative influence on the environment [
2,
3]. As an independent thermochemical conversion process, pyrolysis technology is the initial stage step of combustion and reforming/gasification of coal [
4] and of great importance for clean utilization of coal and biomass [
5]. In addition, the research on pyrolysis products also has great development prospects.
The application of pyrolysis products is extensive and in line with the concept of sustainable development and green chemistry. The char is often used as an adsorbent [
6] to adsorb pollutants and a catalyst [
7] to catalyze experimental reactions. In addition, the solid product is also a soil conditioner to increase agricultural productivity. Pyrolysis oil is mainly utilized in the production of combustion heat, electricity production, fuel oil [
8], and chemicals. Although the calorific value of pyrolysis oil is only about half of that of fossil fuel oil, the liquid product has the advantage of being easy to handle, transport, and store.
In recent years, pyrolysis of coal and biomass has become an interesting topic in the field of energy and chemical industry, both home and abroad [
9,
10,
11,
12]. The main factors affecting the pyrolysis of the two are catalyst, pyrolysis temperature, heating rate, atmosphere, gas flow rate, residence time, etc. [
13]. Although a great number of investigations were accomplished on this pyrolysis behavior and probing deeply into the conditions in above fields, it is noteworthy that few articles laid emphasis on the atmosphere of pyrolysis and its conceivable effect. The surface structure, activity, and composition of char will be affected by the reaction gas, and the atmosphere surrounding char will also make a difference in the composition of tar. Therefore, it is of great significance to find out the pyrolysis mechanism of coal under various atmospheres. Moreover, some research results show that CO
2 can promote the pyrolysis of biomass and the condensation of volatiles [
14,
15,
16], while promoting tar cracking and inhibiting tar polymerization [
17].
Duan et al. [
18] investigated coal pyrolysis in CO
2 and predicted two dominating impacts on the progress of coal pyrolysis. CO
2 may change the yield and element of volatile compounds when they come from the pyrolysis of coal. Otherwise, CO
2 is involved in the gasification reaction of char, then SO
2/NOx precursors will form due to the participation of CO
2. Farrow et al. [
19] conducted the pyrolysis of sawdust and pinewood in a drop-tube furnace in both CO
2 and N
2 atmospheres at different temperatures. They found that the volatile yields can reach 99% in CO
2 and over 96% in N
2, which indicated that the biomass pyrolysis was actually a complete gasification process. Lee et al. [
20] focused on the susceptibility to CO
2 in pyrolysis of various biomasses and understood that CO
2 enhanced the thermal cracking of volatile organic compounds (VOCs) evolved from the thermal degradation of biomass. Hanaoka et al. [
21] investigated the gasification reaction of solid products under various atmospheres of N
2/CO
2/O
2 at 800 °C. The result indicated that solid and gaseous substance after reaction increased when the pressure of production rose. Choi et al. [
22] employed brown coal for pyrolysis in CO
2 and comprehended that the quantity of benzene derivatives in the tar became smaller, inversely CO in gaseous products gradually increased. The results illustrated that the carbon element will shift from condensable compounds to incondensable CO due to the aid of CO
2.
A large number of studies reported have been completed on the pyrolysis of coal and biomass [
23,
24]. Since the application of pyrolysis products has good prospects, the research on structural and compositional changes in char and pyrolysis oils under different atmospheres will help to enhance their commercial value and industrial application. As a result, the study of the impact of CO
2 on bituminous coal and Platanus sawdust pyrolysis will be a fairly necessary foundation.
2. Materials and Methods
2.1. Sample Preparation
The sample of coal (NSC) was bituminous coal whose standard substance number was GBW11109 (n) produced in Shandong Metallurgical Research Institute (Jinan, China). The raw coal was naturally dried and crushed, then all passed through a 0.2 mm (80 mesh) sieve, and its particle size was less than 0.2 mm. The Platanus sawdust (Sipailou Campus, Southeast University, Nanjing, China) was selected as the sample of biomass (PS). The raw material was ground into 150–180 μm (80–100 mesh), then dried under vacuum drying oven at 333 K for 48 h to remove moisture. The proximate and ultimate analyses on a dry basis of the materials are exhibited in
Table 1.
2.2. Tube Furnace Pyrolysis
The fast pyrolysis of NSC and PS was carried out at 500 °C under N
2 and CO
2 atmosphere in a tube furnace. The schematic diagram of the pyrolysis device is displayed in
Figure 1. The initial side of the device was a gas supply unit that contained nitrogen and carbon dioxide tank, then it was connected to the assembled quartz tube through a gas pipe. The gas pipe was equipped with a pressure gauge for real-time pressure detection to avoid dangerous situations, such as overpressure. The mass flow controller ensured that the flow rate of gas was 500 mL/min and the air in the tube was discharged for 10 min. The quartz tube was 1 m long and has an inner diameter of 60 mm. Two quartz boats were placed inside to accommodate the sample. The quartz tube located horizontally in the tube groove inside the tube furnace, and the quartz tube can be electrically heated when the upper and lower cases of tube furnace were touched. The tube furnace can control the heating time and set the reaction temperature. The heating rate was set to 100 K/min, the char residence time was 20 min, and the gas residence was 3 min. It can also reflect the temperature and heating in the box in real-time during operation. The other end of the quartz tube was connected to the condensing tube. The two ports on the side of the condensing tube were connected to the freezing fluid circulation device. The inside of the device was used for cooling ethanol, and the condensation temperature was set to −10 °C. The outlet of the condensing pipe was used for receiving the condensed pyrolysis oil, and the generated gas was discharged. After that, the discharged gas passed through the drying pipe and entered the gas-sampling bag, then the gas exited into the atmosphere after the treatment.
2.3. Physicochemical Analysis
FEI Inspect F50 scanning electron microscopy (SEM) (Thermo Fisher Scientific Inc., Waltham, MA, USA) was employed to examine the aperture structure of chars. ASAP 2020M (Micromeritics Inc., Norcross, GA, USA) was performed to obtain the results of specific surface area, pore diameter and pore volume for BET test. N2 adsorption isotherms and mercury porosimetry were conducted at 77.7 K. EURO EA3000 elemental analyzer (Eurovector Inc., Pavia, Italy) was used to explore the constituents of char and tar which utilized combustion to obtain hydrogen, carbon, sulfur, nitrogen contents, and confirm the oxygen content by mass difference.
2.4. Infrared Analysis of Char
Nicolet iS 10 FT-IR spectrometer (Thermo Scientific Inc., Waltham, MA, USA) was employed to investigate functional groups of the char. The range shown in the spectra was from 4000 to 700 cm−1, and the results were obtained from the average of multiple experimental data.
2.5. NMR Analysis of Tar
AVANCE III HD 600MHz NMR spectrometer (Bruker Inc., Zurich, Switzerland) was used to acquire NMR spectral results of tar produced under different atmospheres. The samples, that weighed 100 mg, were dissolved in 500 μL deuterated dimethyl sulfoxide (DMSO-d
6). The solvent presented seven peaks in
13C NMR spectra and the middle of peaks was at 39.51 ppm. The operation parameters were set to 1500 scans and 3 s pulse delay [
25,
26].
For 1H NMR, the tars were collected to dissolve in deuterated dimethyl sulfoxide (DMSO-d6) equally. The solvent presented five peaks and the middle of peaks was at 2.54 ppm. The operation parameters were set to 16 transients and 5 s pulse delay. All NMR results were processed by MestReNova v11.0 (Mestrelab Research, Santiago De Compostela, Spain).