**2.3 Homogeneous/heterogeneous reforming of biomass tar**

As shown in **Figure 2**, a two-stage fluidized bed/fixed bed reactor was used for the investigation of the homogeneous conversion and heterogeneous reforming of biomass tar over biochar. The inner diameter of the reactor was 37 mm. The reactor is divided into two layers by four quartz frits. The upper layer is fixed bed, while the lower is fluidized bed. The heights of upper and lower layers are 30 and


**Table 2.**

*Primary metal contents of pyrolysis rice husk biochar samples.*

**Figure 2.**

*Schematic diagram of a two-stage fluidized bed/fixed bed reactor for the homogeneous conversion and heterogeneous reforming of biomass tar.*

**51**

**Table 3.**

reforming over biochar.

*Mechanism of In-Situ Catalytic Cracking of Biomass Tar over Biochar with Multiple Active Sites*

130 mm, respectively. For the homogeneous conversion of tar, the lower fluidized bed reactor was heated to 500°C, with the temperature increased to 500–900°C for the upper fixed bed reactor (without catalyst). The silica sand with the weight of 60 g was pre-loaded into the bottom stage of the quartz reactor followed by Ar purging (1.0 L/min carrier gas and 1.5 L/min fluidizing gas) before heating the desired temperature. Once stabilization of the temperatures was achieved, the H-form rice husk was injected into the fluidized bed through the water-cooled pipes at a feeding rate of 100 mg/min. Simultaneously, for the CO2 or H2O separate treatment, the atmosphere was switched to CO2 (29 vol.%) or H2O (15 vol.%) through a dedicated gas injection system located in between the lower and upper reactors as shown in **Figure 2**. Pure Ar gas was also injected into the dedicated gas injection system, to balance the system, at rates of 1.03/0.92/0.82/0.75/0.68 L/min for reaction temperatures of 500/600/700/800/900°C, respectively, to maintain constant residence times for each reforming temperature. For 15 vol.% H2O, steam injection was achieved by feeding a metered amount of water through a high-performance liquid chromatography (HPLC) pump into the heated zone of the reactor where the water was evaporated into steam. De-ionized H2O was injected at rates of 0.34/0.30/0.28 ml/min along with 0.40/0.36/0.33 L/min of balanced Ar for the 700/800/900°C reaction temperatures, respectively. CO2 was injected through the dedicated gas injection system at rates of 0.82/0.75/0.68 L/min to achieve 29 vol.% for 700/800/900°C reaction temperatures, respectively. The temperature was held for 10 min for each reaction. Reactions were terminated by switching the atmosphere to

For the heterogeneous reforming of biomass tar over biochar activated by H2O or CO2, the activation of biochar was carried out for 10 min in the fixed-bed zone in a 15 vol.% H2O or a 29 vol.% CO2 atmosphere with no supplemental H-form rice husk added to the fluidized-bed zone. This was followed by another 10 min at 800°C in an Ar atmosphere, to maintain the same total reaction time (20 min) as the tar-reforming conditions. Details of the five experimental conditions (A–E) are shown in **Table 3**. The experiments involved three pyrolysis experiments: tar reforming (A) in Ar with unactivated pyrolysis biochar; (B) in Ar over H2Oactivated biochar; and (C) in Ar over CO2-activated biochar. In (B) and (C), a 10-min activation of the biochar was first carried out with the activated biochar then used for 10 min of tar reforming in an Ar atmosphere at 800°C, while H-form biomass was also fed to the reactor. In addition, two gasification experiments were carried out: tar reforming in (D) a 15 vol.% H2O atmosphere over H2O-activated biochar and in (E) a 29 vol.% CO2 atmosphere over CO2-activated biochar. In (D) and (E), both atmospheres were kept constant for 20 min even though the period was evenly divided into a biochar-activation stage, which was followed by tar

**Conditions No. Conditions of biomass tar reforming over biochar at 800°C**

B Biomass tar reforming in Ar over H2O-activated biochar C Biomass tar reforming in Ar over CO2-activated biochar

E Biomass tar CO2 reforming over CO2-activated biochar

Pyrolysis A Biomass tar reforming in Ar over pyrolysis biochar

Gasification D Biomass tar H2O reforming over H2O-activated biochar

*Experimental conditions investigated for tar reforming over biochar.*

*DOI: http://dx.doi.org/10.5772/intechopen.91380*

argon and removing the reactor out of the furnace.

*Mechanism of In-Situ Catalytic Cracking of Biomass Tar over Biochar with Multiple Active Sites DOI: http://dx.doi.org/10.5772/intechopen.91380*

130 mm, respectively. For the homogeneous conversion of tar, the lower fluidized bed reactor was heated to 500°C, with the temperature increased to 500–900°C for the upper fixed bed reactor (without catalyst). The silica sand with the weight of 60 g was pre-loaded into the bottom stage of the quartz reactor followed by Ar purging (1.0 L/min carrier gas and 1.5 L/min fluidizing gas) before heating the desired temperature. Once stabilization of the temperatures was achieved, the H-form rice husk was injected into the fluidized bed through the water-cooled pipes at a feeding rate of 100 mg/min. Simultaneously, for the CO2 or H2O separate treatment, the atmosphere was switched to CO2 (29 vol.%) or H2O (15 vol.%) through a dedicated gas injection system located in between the lower and upper reactors as shown in **Figure 2**. Pure Ar gas was also injected into the dedicated gas injection system, to balance the system, at rates of 1.03/0.92/0.82/0.75/0.68 L/min for reaction temperatures of 500/600/700/800/900°C, respectively, to maintain constant residence times for each reforming temperature. For 15 vol.% H2O, steam injection was achieved by feeding a metered amount of water through a high-performance liquid chromatography (HPLC) pump into the heated zone of the reactor where the water was evaporated into steam. De-ionized H2O was injected at rates of 0.34/0.30/0.28 ml/min along with 0.40/0.36/0.33 L/min of balanced Ar for the 700/800/900°C reaction temperatures, respectively. CO2 was injected through the dedicated gas injection system at rates of 0.82/0.75/0.68 L/min to achieve 29 vol.% for 700/800/900°C reaction temperatures, respectively. The temperature was held for 10 min for each reaction. Reactions were terminated by switching the atmosphere to argon and removing the reactor out of the furnace.

For the heterogeneous reforming of biomass tar over biochar activated by H2O or CO2, the activation of biochar was carried out for 10 min in the fixed-bed zone in a 15 vol.% H2O or a 29 vol.% CO2 atmosphere with no supplemental H-form rice husk added to the fluidized-bed zone. This was followed by another 10 min at 800°C in an Ar atmosphere, to maintain the same total reaction time (20 min) as the tar-reforming conditions. Details of the five experimental conditions (A–E) are shown in **Table 3**. The experiments involved three pyrolysis experiments: tar reforming (A) in Ar with unactivated pyrolysis biochar; (B) in Ar over H2Oactivated biochar; and (C) in Ar over CO2-activated biochar. In (B) and (C), a 10-min activation of the biochar was first carried out with the activated biochar then used for 10 min of tar reforming in an Ar atmosphere at 800°C, while H-form biomass was also fed to the reactor. In addition, two gasification experiments were carried out: tar reforming in (D) a 15 vol.% H2O atmosphere over H2O-activated biochar and in (E) a 29 vol.% CO2 atmosphere over CO2-activated biochar. In (D) and (E), both atmospheres were kept constant for 20 min even though the period was evenly divided into a biochar-activation stage, which was followed by tar reforming over biochar.


### **Table 3.**

*Experimental conditions investigated for tar reforming over biochar.*

*Applications of Biochar for Environmental Safety*

of muffle furnace temperature, some gaps between the upper cover and the reactor allow the release of gases under internal positive pressure. The volatile matters formed during the volatilization of biomass were rapidly dispersed away from the reactor, carried by Ar gas, so as to ensure an inert atmosphere inside the reactor. The

As shown in **Figure 2**, a two-stage fluidized bed/fixed bed reactor was used for the investigation of the homogeneous conversion and heterogeneous reforming of biomass tar over biochar. The inner diameter of the reactor was 37 mm. The reactor is divided into two layers by four quartz frits. The upper layer is fixed bed, while the lower is fluidized bed. The heights of upper and lower layers are 30 and

Origin biochar 0.03 1.44 0.09 0.14 0.03 0.05 H-form biochar 0.00 0.02 0.01 0.01 0.01 0.01

**Na K Mg Ca Al Fe**

*Schematic diagram of a two-stage fluidized bed/fixed bed reactor for the homogeneous conversion and* 

metal contents of the origin and H-form biochar are listed in **Table 2**.

**Biochar Primary metal contents (wt.%)**

*Primary metal contents of pyrolysis rice husk biochar samples.*

**2.3 Homogeneous/heterogeneous reforming of biomass tar**

**50**

**Figure 2.**

**Table 2.**

*heterogeneous reforming of biomass tar.*

### **2.4 Sampling and analysis of biochar and tar**

The biomass tar compounds were trapped in two gas bottles, connected in series, and filled with a mixture of HPLC-grade chloroform and methanol (4:1, v/v), as shown in **Figure 2**. The bottles were placed in a brine ice bath (≤0°C). After the reaction, the total solution was transferred to a 200 mL volumetric flask and made up to volume with a mixture of chloroform and methanol (4:1, v/v).

The tar yield was determined by evaporating the solvents and water at 35°C for 4 h. The tar is thus experimentally defined as the material soluble in the chloroform/methanol (4:1, v/v) solvent mixture not being evaporated (with the solvents) at 35°C within 4 h [24, 25, 42]. The residues in the solvents themselves (i.e., blank) and the biomass moisture content were considered in the tar yield calculation. The equation used for tar concentration in the solution is shown as follows Eq. (1): *<sup>C</sup>* = \_

$$\mathbf{C} = \frac{\mathbf{C}\_2 - \mathbf{C}\_1}{\mathbf{1} - \mathbf{C}\_2} \tag{1}$$

where *C* is the concentration of tar; *C*1 is the concentration of the mixed solution residue (blank experiment); and *C*2 is the concentration of residue in tar solution.

The equation used to determine tar yield is shown as follows Eq. (2):

$$\text{Tar } \mathbf{yield} = \frac{\mathbf{C} \times \mathbf{M}\_{\text{Tar solution}}}{\mathbf{M}\_{\text{biomass}}} \tag{2}$$

where *C* is the concentration of tar in the solution; *M*Tar solution is the total mass of tar collected in the solution; and *M*biomass is the feed quality of the H-form rice husk into the reactor.

The samples were analyzed using an Agilent Gas Chromatography Mass Spectrometer (GC-MS) instrument (6890 series GC with a 5973 MS detector) with a capillary column (DB-5 ms; length 30 m, internal diameter 0.25 mm, film thickness 0.5 μm). The sample solution (5 μL) was injected into the injection port, set at 260°C, with a split ratio of 80:1. The column was operated in constant-flow mode using 2.0 mL/min of helium as the carrier gas. The column temperature was initially maintained at 35°C for 3 min, then increased to 260°C at a heating rate of 10°C/min, and then maintained at 260°C for 5 min. Mass spectra were acquired after a 4-min solvent delay [21]. The chromatogram peaks were identified by comparison with the standard spectra of compounds in the National Institute of Standards and Technology library (NIST) and/or from the retention times/spectra of known injected species.

The total amount of metal species in the biochar samples was quantified by employing a previously established procedure [43]. Using a microwave system (Ethos 1, Milestone, Sorisole, Italy), the sample (0.1 g) was digested in a 1:3:8 (v/v/v) mixture of 40% HF, 30% H2O2, and 65% HNO3 at 200°C for 60 min. The metal species content was then quantified by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). Three measurements were conducted with the average values and then taken as the results.

The biochar's particle morphology and surface composition were measured by an EVO18 scanning electron microscope coupled to an energy dispersive X-ray spectrometer (SEM-EDX, Carl Zeiss, Germany).

Biochar samples were set for at least 24 h to displace the reaction gas within the pore structure with air. N2-adsorption isotherms were then obtained at −196°C (ASAP 2020M, Micromeritics Instrument Crop, USA) and analyzed by the BET model to determine the sample's surface area and pore volume.

XPS analysis was used to evaluate the characteristics of surface elements in biochar. This was performed using a K-Alpha spectrometer (Thermo Fisher Scientific)

**53**

**Figure 3.**

*Mechanism of In-Situ Catalytic Cracking of Biomass Tar over Biochar with Multiple Active Sites*

equipped with monochromatic Al Kα X-rays at 1486.6 eV. To exclude effects on the binding energies caused by changing the sample during measurements, the data were corrected by a linear shift with the maximum peak of the C1s binding energy of the adventitious carbon corresponding to 284.6 eV. The surface's elemental condition was analyzed using the number of escaped electrons from the char surface at a depth of 1–10 nm, according to the findings in our previous investigations [44, 45].

The homogeneous conversion of tar mainly refers to the initial pyrolysis tar experiences during a series of decomposition and polymerization processes under ambient conditions (heat and atmosphere). Tar yield during the Ar, H2O, and CO2 homogeneous conversion experiments performed at 500–900°C can be seen in **Figure 3**. In the presence of an Ar-only atmosphere, the tar yield decreased gradually as a function of increasing temperature from 500 (26.18%) to 900°C (6.38%). Temperature in the Ar-only experiments has a greater influence between 500 and 700°C. Further increasing the temperature to 700–900°C results in increased biomass decomposition, thus lowering tar yields. Thermal decomposition is considered to be the main factor in the conversion of tar [46, 47]. As shown in **Figure 3**, in 15 vol.% H2O and 29 vol.% CO2, the effects of H2O and CO2 on the homogeneous transformation of biomass tar over biochar are significant. At the same temperature, the tar reforming effect of 15 vol.% H2O is significantly higher than that of 29 vol.% CO2. In 15 vol.% H2O, the tar yield decreased from 6.95% at 700°C to 3.56% at 900°C. In 29 vol.% CO2, the tar yield decreased from 7.99% at 700°C to 5.01% at 900°C. For the higher temperatures, 700–900°C are required for H2O and CO2 to influence tar homogeneous transformations, while for the lower temperatures, 500

As shown in **Figure 4**, it can be seen that at lower temperatures (500–600°C), the majority of the biomass tar still comprises components based on the primary biomass tar containing oxygen and substituent compounds, such as levoglucosan and dimethoxymethane. However, when subjecting the biomass to higher temperatures (700–900°C), most of the primary pyrolyzed tar gradually transforms [48]. The tar composition seems to be mainly composed of aromatic compounds having

*DOI: http://dx.doi.org/10.5772/intechopen.91380*

**3.1 Homogeneous conversion of biomass tar**

and 600°C are not in the gasification thermal range.

*Tar yield during homogeneous conversion at 500–900°C.*

**3. Results and discussion**

equipped with monochromatic Al Kα X-rays at 1486.6 eV. To exclude effects on the binding energies caused by changing the sample during measurements, the data were corrected by a linear shift with the maximum peak of the C1s binding energy of the adventitious carbon corresponding to 284.6 eV. The surface's elemental condition was analyzed using the number of escaped electrons from the char surface at a depth of 1–10 nm, according to the findings in our previous investigations [44, 45].
