高热值板栗壳基生物质炭的制备

姜可茂， 程朝歌， 冉 敏， 吕永根,2, 吴琪琳,2

(1. 纤维材料改性国家重点实验室， 上海201620；2. 东华大学 材料学院， 上海201620)

1 Introduction

Biomass is abundant as natural resources[1]. The direct burning of biomass causes environmental pollution and waste of resources. Biomass can be converted into biochar via pyrolysis[2]. Under appropriate condition, chestnut shells can be pyrolyzed to biochar[3]. Several studies on the carbonization of different biomass sources have been reported, including pistachio shells, mangosteen shells, switch grass, human hair,typha orientalis, and other biomass[4-8]. The chemical structure and major organic components in biomass are extremely important for producing derived fuels and chemicals[9-12]. In addition, biochar characteristics are influenced by treatment temperature, holding time, pyrolysis atmosphere, et al[13-15].

The sulfuric acid and urea solution are effective catalytic systems. Levoglucosan can be decomposed to combustible products in the later stage of pre-oxidation, which reduces the yield and the strength of the biochar. In the catalytic system, sulfuric acid makes the pyrolysis reaction of pre-oxidation in advance and reduces the production of levoglucosan. Urea promotes the decomposition of levoglucosan product (L-glucose) and inhibits the polymerization of tar[16]. Simultaneously, the addition of urea slows down the pyrolysis reaction. Thus high quality biochar is obtained with these catalytic systems[17].

The potential to utilize biochar for various applications depends on its properties. According to the innate advantages of chestnut shellsat carbon content and calorific value, chestnut shells-based biochar with a high calorific value had been prepared through catalytic pre-oxidation and carbonization[18, 19]. The pyrolysis behaviors and the transmutation of both the structure and properties during the carbonization were investigated[20].

2 Experimental

2.1 Material and equipment

The pyrolysis of chestnut shells was carried out in a thermo-gravimetric(TG) analysis apparatus (NETZSCH STA 409C). Samples were analyzed at heating rates of 5 and 10 ℃/min from room temperature to 900 ℃ under nitrogen (99.999%) flow at 30 mL/min. The total mass was 10 mg for each run. Fourier transform infrared (FTIR) spectra of the samples were recorded on a Nicolet NEXUS-670 spectrometer.Elemental analysis (C, H, O and S) was investigated on a Vario EL Ⅲ Microanalyzer. All specimens for SEM observation were coated with a thin layer of gold in a sputter-coating unit prior to observation to avoid charging in JSM-5600LV microscope. The pore size distribution of biochar was determined by N2adsorption (Micromeritics ASAP 2020) under 77.3 K. The specific surface area was determined by the Brunauer-Emmett-Teller (BET) method.

The calorific values of samples were measured by the ASTM bomb calorimeter method using an IKA Calorimeter C 7000 model instrument. The calorific values were calculated using the following equation:

Q=E(t-t0)/G

(1)

WhereQis the calorific values of samples, t is endpoint temperature,t0is initial temperature(20 ℃) andGrefers to the mass of samples.

2.2 Carbonization

Fig. 1 illustrates the steps involved in the preparation based on the raw chestnut shells. The raw chestnut shells were first immersed in the catalyst solution containing sulfuric acid and urea solution. Shells were taken out after reaching equilibrium and dried in an oven at 60 ℃ for 24 h. Afterwards, the pre-treated shells were pre-oxidized from room temperature to 250 ℃ at a rate of 5 ℃/min in a furnace in air. Nitrogen was then infused into the furnace to prevent oxidization reaction, and the pre-oxidized shells were carbonized till 750 ℃.

Fig. 1 A schematic illustration for preparing chestnut shells-based biochar.

3 Results and discussion

3.1 Decomposition

As seen in Fig. 2, the TG curves for the chestnut shells present a reversed S shape.

The weight loss increases slowly, becomes severe and then levels off with increasing temperature. In case of a heat rate of 5 ℃/min,weight loss increases from 100-200 ℃ at a relatively slow rate, which is mainly due to the evaporation of H2O[21]. Then a considerable loss, almost 50%, appears from 200-340 ℃, which is the most intense range for pyrolysis of chestnut shells. At 220 ℃, a large amount of hemicellulose decomposes, and the connection between cellulose, hemicellulose and lignin are destroyed, which result in the breakdown of the intercellular layer[22]. With the increase of temperature, the reaction rate and the weight loss rate are enhanced. The maximum loss rate appears around 260 ℃, according to the DTG curves.The results show that the cellulose in chestnut shells begins to decompose massively to levoglucosan and tar. Lignin is difficult to decompose, and the decomposition is almost across the whole pyrolysis[23]. The weight loss is 15% from 340-900 ℃ andthe total loss reaches 63%, with a solid residue of 37%. The decomposition of lignin and the formation of volatile gases occur, cellulose undergoes decarbonylation and crosslinking reactions to form an aromatic ring structure and is converted to a small amount of charcoal composed of graphite microcrystals[24]. The analysis of the change in weight loss and the rate of weight loss with increasing temperature shows that the decomposition of this material takes place mainly in the range of 200-340 ℃.Compared with the TG curve of 5 ℃/min, the curve of 10 ℃/min shifts to the right and the maximum weight loss rate increases. The results indicate that the heating rate has a great influence on the pyrolysis rate.

3.2 Chemical transformation

The chemical transformations during conversion of raw materials into biochar products by means of hydrothermal carbonization were examined. Along with the elimination of CO2and other carbon-containing compounds, a large amount of C atoms are removed from the structure of chestnut shells.At the same time, there is an obvious reduction in the oxygen content (Fig. 3). It is also observed that a temperature increase from 250 to 750 ℃ causes a decrease in the O/C and H/C atomic ratios, suggesting an increase in the carbon yield. Thus, the carbonization leads to an increase in carbon content from 47.4 wt.% for raw shells to 88.2 wt.% for the biochar samples under pyrolysis at 750 ℃. This is mainly due to the reaction between the chestnut shells and the catalyst,which broadens the pyrolysis zone and promotes the formation and stabilization of carbon[17].

Fig. 3 Element content changing trend with temperatures.

Sulfur and ash contents play an important role in the performance of fuel. Sulfur and ash contents of the chestnut shells-based biochar with a high calorific value are less than those of the first-level clean coal (Table 1), which offers significant environmental benefits and alternative energy sources.It can be seen from Table 1 that the specific surface area of the product increases gradually with increasing temperature, but the specific surface area is still very small. The pores are basically large pores, which illustrates that carbonization has a little effect on formation of micropores.

Table 1 Sulfur content, ash content and textural properties of the samples.

Fig. 4 FTIR spectra of chestnut shells treated at different temperatures.

3.3 Morphology

The microstructure of samples in pre-oxidation and carbonization have been studied by SEM (Fig. 5). As the temperature increases, the holes on surface shrink and the pores in the cross section become large, while the network structure becomes regular. The network structure of cross section emerges and the surface becomes turtle shell-shaped with progress of heating. The pyrolysis contributes to these structural changes during the carbonization. The cellulose glycoside bond has been basically broken. A large amount of volatile leads to the deformation and the fractural morphology also changes greatly.

Fig. 5 SEM images: cross section of(a) raw chestnut shells and (b-d) heat-treated shells at (b) 250 ℃, (c) 550 ℃, and (d) 750 ℃, out surface of (e) raw chestnut shells and (f-h) heat-treated shells at (f) 250 ℃, (g) 550 ℃, and (h) 750 ℃.

3.4 Combustion performance

The calorific values were calculated by the equation (1). The gross calorific value of raw chestnut shells is 17.46 MJ/kg. In the stage of pre-oxidation (below 250 ℃), the gross calorific value increases slowly, mainly associated with the removal of moisture. In the carbonization stage, it grows rapidly to 35.48 MJ/Kg (Fig. 6).The calorific value is higher than the first-level cleaned coal of 30.00 MJ/Kg, indicating a good practical value. Due to its pore structure and high fixed carbon content, biochar of chestnut shells has a higher calorific value than the raw material and first-level cleaned coal.

Compared with the first-level cleaned coal, the calorific value of the chestnut shells-based biochar is higher, and its ash and sulfur contents are lower, indicating a lower emission of pollution gases. The results demonstrate that the as-prepared biochar is a promising candidate as an alternative fuel in electricity and heating fields.

Fig. 6 The gross calorific values of samples at different temperatures.

4 Conclusions

We have presented a procedure for obtaining a biochar product by carbonization of chestnut shells. With the help of catalysts, the chestnut shells-based biochar with a yield of 44.31% and gross calorific value of 35.48 MJ/Kg can be obtained at 750 ℃, whose calorific value is higher and sulfur and ash contents are lower than the first-level cleaned coal. Therefore, the biochar is a promising candidate for fuel.

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