**3. Results and discussion**

*Applications of Biochar for Environmental Safety*

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

*<sup>C</sup>* = \_

into the reactor.

of known injected species.

average values and then taken as the results.

spectrometer (SEM-EDX, Carl Zeiss, Germany).

model to determine the sample's surface area and pore volume.

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

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):

> *C*<sup>2</sup> − *C*<sup>1</sup> 1 − *C*<sup>2</sup>

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):

Tar yield = \_\_\_\_\_\_\_\_\_\_\_

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

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

The biochar's particle morphology and surface composition were measured by an EVO18 scanning electron microscope coupled to an energy dispersive X-ray

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

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

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

*C* × *M*Tar solution *M*biomass

(1)

(2)

up to volume with a mixture of chloroform and methanol (4:1, v/v).

**52**

### **3.1 Homogeneous conversion of biomass tar**

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

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

**Figure 3.** *Tar yield during homogeneous conversion at 500–900°C.*

### **Figure 4.**

*GC-MS analysis of biomass tar in Ar at 500–900°C.*

good thermal stability, such as toluene, indene, and naphthalene, among others. Increasing the temperature resulted in either a gradual reduction or a complete removal of tars containing branched or heteroatom compounds, and polycyclic aromatic hydrocarbons (PAHs) were gradually formed. For the biomass tar homogeneous conversion, the aromatic ring structure has higher thermal stability than that of the non-aromatic structures. Specific tar components decompose into small molecular gases and C1–C5 hydrocarbons, while there is evidence for the promotion of aromatic rings as a function of increasing temperature. H-abstraction, C2H2 addition (HACA), and cyclodehydrogenation are the mechanisms typically responsible for such processes [49, 50]. Performing the reactions at the mid-temperature range (700–800°C) results in aromatic conversion with oxygen and substituents. Thermal decomposition [51] and additional reactions convert short-chain hydrocarbons (C1–C5) into compounds containing unsaturated double and triple bonds that gradually increase in concentration by the acetylene addition reaction. The aromatic components can also be polymerized by dehydrogenation. Further increasing the temperature to 900°C results in the relative content of PAHs, such as naphthalene, phenanthrene, and anthracene, to increase the above conversion pathway yielding highly stable aromatic hydrocarbons.

GC-MS analysis during H2O and CO2 homogeneous conversion at 700–900°C can be seen in **Figure 5**. At 700–900°C, H2O and CO2 have a degree of influence on the conversion of tar. The degree of tar homogeneous conversion in the presence of either a H2O or CO2 atmosphere was significantly higher than that of the thermal decomposition in Ar. PAH concentration was low. The results show that H2O and CO2 have obvious effects on the transformation of aromatics, especially PAHs [52]. The free radical theory is used to explain the homogeneous transformation of tar. The formation of aromatic radicals in the polymerization of aromatic hydrocarbons is considered to be the key to the reaction. The continuous polymerization process is considered to be the main pathway [53–56]. Thermal decomposition is a method of generating free radicals through thermal breaking of bonds. The free radicals generated by the original tar form different final products by reacting with different free radicals produced as a function of the atmosphere. The presence of H2O and CO2 promoted the formation of free radicals with H/O/OH moieties. The influence of temperature is mainly reflected in the promotion of the decomposition reaction caused by free radicals [57]. CO2 is a pure oxygen donor. **Figure 6** shows that the active oxygen atoms used for oxidative decomposition of hydrocarbons and intermediate

**55**

**Figure 6.**

**Figure 5.**

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

products are mainly produced by the reaction CO2 + e× → CO + O·+ e. Active OH free radicals can be formed by replacing the hydrogen atoms in the hydrocarbons with oxygen atoms. Increasing the content of CO2 is helpful to inhibit the cyclization of aromatics. The addition of CO2 promotes the formation of free radicals such as O, which can further react with hydrocarbon groups. The oxidation reaction of active oxygen atoms with hydrocarbons forms CO, H2O, and other products. The oxidative cracking process of tar is initiated, and the polymerization process of aromatic

*Homogeneous conversion of tar in Ar, H2O, and CO2 at 500–900°C [52].*

*GC-MS tar analysis during (a) 15 vol.% H2O and (b) 29 vol.% CO2 homogeneous conversion at 700–900°C.*

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

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

**Figure 5.** *GC-MS tar analysis during (a) 15 vol.% H2O and (b) 29 vol.% CO2 homogeneous conversion at 700–900°C.*

### **Figure 6.**

*Applications of Biochar for Environmental Safety*

*GC-MS analysis of biomass tar in Ar at 500–900°C.*

good thermal stability, such as toluene, indene, and naphthalene, among others. Increasing the temperature resulted in either a gradual reduction or a complete removal of tars containing branched or heteroatom compounds, and polycyclic aromatic hydrocarbons (PAHs) were gradually formed. For the biomass tar homogeneous conversion, the aromatic ring structure has higher thermal stability than that of the non-aromatic structures. Specific tar components decompose into small molecular gases and C1–C5 hydrocarbons, while there is evidence for the promotion of aromatic rings as a function of increasing temperature. H-abstraction, C2H2 addition (HACA), and cyclodehydrogenation are the mechanisms typically responsible for such processes [49, 50]. Performing the reactions at the mid-temperature range (700–800°C) results in aromatic conversion with oxygen and substituents. Thermal decomposition [51] and additional reactions convert short-chain hydrocarbons (C1–C5) into compounds containing unsaturated double and triple bonds that gradually increase in concentration by the acetylene addition reaction. The aromatic components can also be polymerized by dehydrogenation. Further increasing the temperature to 900°C results in the relative content of PAHs, such as naphthalene, phenanthrene, and anthracene, to increase the above conversion

GC-MS analysis during H2O and CO2 homogeneous conversion at 700–900°C can be seen in **Figure 5**. At 700–900°C, H2O and CO2 have a degree of influence on the conversion of tar. The degree of tar homogeneous conversion in the presence of either a H2O or CO2 atmosphere was significantly higher than that of the thermal decomposition in Ar. PAH concentration was low. The results show that H2O and CO2 have obvious effects on the transformation of aromatics, especially PAHs [52]. The free radical theory is used to explain the homogeneous transformation of tar. The formation of aromatic radicals in the polymerization of aromatic hydrocarbons is considered to be the key to the reaction. The continuous polymerization process is considered to be the main pathway [53–56]. Thermal decomposition is a method of generating free radicals through thermal breaking of bonds. The free radicals generated by the original tar form different final products by reacting with different free radicals produced as a function of the atmosphere. The presence of H2O and CO2 promoted the formation of free radicals with H/O/OH moieties. The influence of temperature is mainly reflected in the promotion of the decomposition reaction caused by free radicals [57]. CO2 is a pure oxygen donor. **Figure 6** shows that the active oxygen atoms used for oxidative decomposition of hydrocarbons and intermediate

pathway yielding highly stable aromatic hydrocarbons.

**54**

**Figure 4.**

*Homogeneous conversion of tar in Ar, H2O, and CO2 at 500–900°C [52].*

products are mainly produced by the reaction CO2 + e× → CO + O·+ e. Active OH free radicals can be formed by replacing the hydrogen atoms in the hydrocarbons with oxygen atoms. Increasing the content of CO2 is helpful to inhibit the cyclization of aromatics. The addition of CO2 promotes the formation of free radicals such as O, which can further react with hydrocarbon groups. The oxidation reaction of active oxygen atoms with hydrocarbons forms CO, H2O, and other products. The oxidative cracking process of tar is initiated, and the polymerization process of aromatic

hydrocarbons is also inhibited. H2O not only promotes tar cracking conversion but also inhibits the polymerization reaction. This is related to the higher activity of free radical formation being a more active reformer in the conversion of tar. H2O and CO2 have similar oxidation capacities. The difference between the two is mainly reflected in the product—H2O produces higher numbers of H free radicals than CO2 [58]. O and OH free radicals can be formed by ionization of H2O (H2O → H + OH) in the presence of steam. The fracture of OH can form new H and O free radicals. The H/O/ OH atoms in the gas phase exist in radical form. According to the free radical mechanism, the primary constituents of the biomass are broken into activated tar fragments at high temperatures. A large number of H/O/OH free radicals will combine with activated tar fragments before tar polymerization.

As shown in **Figure 6**, the conversion of the tar homogeneous transformation process is considered to be a two-stage process. The first stage involves the decomposition and transformation of the active heteroatom-containing groups in the tar, along with the decomposition of dealkylated side chains, hydrocarbon molecular cyclization, and aromatization reactions. The products include low-chain aliphatic hydrocarbons, oxygen-containing small molecular gases, and single-ring aromatic hydrocarbons. The second stage is the reforming of tar components; the dehydrogenation of cyclization products; the addition of acetylene; and the growth, recombination, and isomerization of aromatics. The two processes constitute the basis of the biomass tar homogeneous reaction. In the presence of different reforming agents (H2O or CO2), the atmosphere can promote or inhibit tar pyrolysis conversion, thus influencing the composition of the final tar. The addition of H2O and CO2 can promote the generation of active free radicals such as O, OH, H, and so on. These free radicals can react with the active free tar fragments generated from the first stage of thermal decomposition demonstrating the importance of the H2O and CO2 reforming agents in the homogeneous conversion of biomass tar.

## **3.2 Heterogeneous reforming of biomass tar over biochar**

### *3.2.1 Biomass tar reforming*

As shown in **Figure 7**, the highest proportion of bio-tar was reformed (including homogeneous and heterogeneous phases) in the 15 vol.% H2O atmosphere over H2O-activated biochar (D). The proportion of tar reformed in the 29 vol.% CO2 atmosphere over CO2-activated biochar (E) was also considerably higher than results for reforming in an Ar atmosphere (A, B, and C). This illustrates that the presence of a gasification agent (H2O/CO2) greatly promotes in-situ reforming of nascent bio-tar over biochar. Under pyrolysis conditions, the homogeneous transformation of biomass tar was mainly based on secondary reactions (i.e., tar thermal cracking at 800°C), yielding a conversion efficiency of 70.86%. The ability of H2O/ CO2 activation to improve biochar reactivity was also clearly observed. In the Ar atmosphere, the highest proportion of tar was reformed over the H2O-activated biochar (B, 20.08%), followed by that over the CO2-activated biochar (C, 19.01%), while the lowest conversion was for the (unactivated) pyrolysis biochar (A, 17.41%). El-Rub and Kamel [59] and Chen et al. [60] studied biochar's catalytic activity for tar reforming using a fixed char bed. They concluded that in an inert atmosphere, the tar molecules were mainly adsorbed on biochar active sites and converted into larger polyaromatic molecules and coke via a series of dehydrogenation, cyclization, and condensation reactions. Differences between the unactivated and H2O/ CO2-activated biochars may be attributed to differences in inherent catalytic AAEM species (such as K and Ca) and the biochars' physiochemical structures [61], as discussed later.

**57**

**Table 4.**

**Figure 7.**

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

The presence of a gasification agent further improved the homogeneous reform-

*Proportion of tar reformed under different conditions: (A) in Ar over pyrolysis biochar, (B) in Ar over H2Oactivated biochar, (C) in Ar over CO2-activated biochar, (D) in 15 vol.% H2O over H2O-activated biochar, and* 

The metal contents of the biochar samples are shown in **Table 4**. Apart from K, there was little difference (±0.02 wt.%) observed for the metals between the biochar samples. During the H2O/CO2 activation of biochar, K appears to have been released from the biochar, decreasing from 1.12 wt.% in the pyrolysis biochar to 1.06 wt.% when activated by H2O and 1.09 wt.% in the CO2-activated biochar.

**Biochar samples Metal species content (wt.% in biochar)**

H2O-activated biochar 1.06 0.10 0.16 0.06 0.02 CO2-activated biochar 1.09 0.09 0.15 0.05 0.03

**K Mg Ca Fe Al**

ing of the bio-tar (72.62 and 71.57% with H2O and CO2 present, respectively). The homogeneous transformation of biomass tar in the gas phase was in a certain extent in H2O/CO2 gasification condition, which was in broad agreement with the results obtained by Wang et al. [17] and Min et al. [24]. Besides, in the gasification conditions, H2O and CO2 further improved the biochar's catalytic reactivity for the heterogeneous reforming process with the proportion reformed increasing from 20.08% (B) to 26.85% (D) with H2O and from 19.01% (C) to 22.17% (E) with CO2. According to Min et al. [24], the reforming of tar molecules over biochar may be activated in two ways. First, in the gas phase, nascent tar (volatiles) contains abundant free radicals that would react with extant tar molecules to form activated tar fragments. Second, tar molecules and biochar may be activated by H2O/CO2 during the chemisorption of tar on the biochar surface with reactions between tar and H2O/CO2 adsorbed on the biochar's active sites then leading to further reforming reactions. Overall, gasification agents (H2O and CO2) improved and maintained the system's ability to carry out homogeneous and heterogeneous reforming of biomass tar.

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

*3.2.2 H2O/CO2 activation of biochar*

*Biochar samples' metal-content analysis.*

*(E) in 29 vol.% CO2 over CO2-activated biochar.*

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

### **Figure 7.**

*Applications of Biochar for Environmental Safety*

activated tar fragments before tar polymerization.

hydrocarbons is also inhibited. H2O not only promotes tar cracking conversion but also inhibits the polymerization reaction. This is related to the higher activity of free radical formation being a more active reformer in the conversion of tar. H2O and CO2 have similar oxidation capacities. The difference between the two is mainly reflected in the product—H2O produces higher numbers of H free radicals than CO2 [58]. O and OH free radicals can be formed by ionization of H2O (H2O → H + OH) in the presence of steam. The fracture of OH can form new H and O free radicals. The H/O/ OH atoms in the gas phase exist in radical form. According to the free radical mechanism, the primary constituents of the biomass are broken into activated tar fragments at high temperatures. A large number of H/O/OH free radicals will combine with

As shown in **Figure 6**, the conversion of the tar homogeneous transformation process is considered to be a two-stage process. The first stage involves the decomposition and transformation of the active heteroatom-containing groups in the tar, along with the decomposition of dealkylated side chains, hydrocarbon molecular cyclization, and aromatization reactions. The products include low-chain aliphatic hydrocarbons, oxygen-containing small molecular gases, and single-ring aromatic hydrocarbons. The second stage is the reforming of tar components; the dehydrogenation of cyclization products; the addition of acetylene; and the growth, recombination, and isomerization of aromatics. The two processes constitute the basis of the biomass tar homogeneous reaction. In the presence of different reforming agents (H2O or CO2), the atmosphere can promote or inhibit tar pyrolysis conversion, thus influencing the composition of the final tar. The addition of H2O and CO2 can promote the generation of active free radicals such as O, OH, H, and so on. These free radicals can react with the active free tar fragments generated from the first stage of thermal decomposition demonstrating the importance of the H2O and

CO2 reforming agents in the homogeneous conversion of biomass tar.

As shown in **Figure 7**, the highest proportion of bio-tar was reformed (including homogeneous and heterogeneous phases) in the 15 vol.% H2O atmosphere over H2O-activated biochar (D). The proportion of tar reformed in the 29 vol.% CO2 atmosphere over CO2-activated biochar (E) was also considerably higher than results for reforming in an Ar atmosphere (A, B, and C). This illustrates that the presence of a gasification agent (H2O/CO2) greatly promotes in-situ reforming of nascent bio-tar over biochar. Under pyrolysis conditions, the homogeneous transformation of biomass tar was mainly based on secondary reactions (i.e., tar thermal cracking at 800°C), yielding a conversion efficiency of 70.86%. The ability of H2O/ CO2 activation to improve biochar reactivity was also clearly observed. In the Ar atmosphere, the highest proportion of tar was reformed over the H2O-activated biochar (B, 20.08%), followed by that over the CO2-activated biochar (C, 19.01%), while the lowest conversion was for the (unactivated) pyrolysis biochar (A, 17.41%). El-Rub and Kamel [59] and Chen et al. [60] studied biochar's catalytic activity for tar reforming using a fixed char bed. They concluded that in an inert atmosphere, the tar molecules were mainly adsorbed on biochar active sites and converted into larger polyaromatic molecules and coke via a series of dehydrogenation, cyclization, and condensation reactions. Differences between the unactivated and H2O/ CO2-activated biochars may be attributed to differences in inherent catalytic AAEM species (such as K and Ca) and the biochars' physiochemical structures [61], as

**3.2 Heterogeneous reforming of biomass tar over biochar**

*3.2.1 Biomass tar reforming*

**56**

discussed later.

*Proportion of tar reformed under different conditions: (A) in Ar over pyrolysis biochar, (B) in Ar over H2Oactivated biochar, (C) in Ar over CO2-activated biochar, (D) in 15 vol.% H2O over H2O-activated biochar, and (E) in 29 vol.% CO2 over CO2-activated biochar.*

The presence of a gasification agent further improved the homogeneous reforming of the bio-tar (72.62 and 71.57% with H2O and CO2 present, respectively). The homogeneous transformation of biomass tar in the gas phase was in a certain extent in H2O/CO2 gasification condition, which was in broad agreement with the results obtained by Wang et al. [17] and Min et al. [24]. Besides, in the gasification conditions, H2O and CO2 further improved the biochar's catalytic reactivity for the heterogeneous reforming process with the proportion reformed increasing from 20.08% (B) to 26.85% (D) with H2O and from 19.01% (C) to 22.17% (E) with CO2. According to Min et al. [24], the reforming of tar molecules over biochar may be activated in two ways. First, in the gas phase, nascent tar (volatiles) contains abundant free radicals that would react with extant tar molecules to form activated tar fragments. Second, tar molecules and biochar may be activated by H2O/CO2 during the chemisorption of tar on the biochar surface with reactions between tar and H2O/CO2 adsorbed on the biochar's active sites then leading to further reforming reactions. Overall, gasification agents (H2O and CO2) improved and maintained the system's ability to carry out homogeneous and heterogeneous reforming of biomass tar.

## *3.2.2 H2O/CO2 activation of biochar*

The metal contents of the biochar samples are shown in **Table 4**. Apart from K, there was little difference (±0.02 wt.%) observed for the metals between the biochar samples. During the H2O/CO2 activation of biochar, K appears to have been released from the biochar, decreasing from 1.12 wt.% in the pyrolysis biochar to 1.06 wt.% when activated by H2O and 1.09 wt.% in the CO2-activated biochar.


### **Table 4.** *Biochar samples' metal-content analysis.*

Because of their valence states [43], other metal species like Ca bond to the biochar more strongly (i.e., at two or more sites) than K (which only bonds at one site).
