*3.2.4 BET analysis of biochar during H2O/CO2 activation and tar reforming*

The biochar samples' N2-absorption/desorption isotherms at 77 K during H2O/ CO2 activation and biomass tar reforming are shown in **Figure 9**. Compared with those of the original pyrolysis biochar, the pore systems of H2O/CO2-activated biochar samples and that from tar H2O reforming over H2O-activated biochar were better developed. Conversely, the other conditions exhibited pore structures that were somewhat blocked, especially for reforming over pyrolysis biochar in Ar and for the CO2-activated biochar.

To further investigate the microphysical structures of the biochar samples, their BET surface properties were evaluated and are presented in **Table 4**. The unactivated pyrolysis biochar presented a BET surface area of 195.35 m2 /g and a pore volume of 0.0999 cm3 /g. Activation by H2O and CO2 increased the BET surface area to 307.45 and 237.71 m2 /g, respectively. The biochar's porous structure enabled efficient tar adsorption, resulting in a good residence time of the tar reacting with the catalyst [46]. **Table 5** also shows that the concentration of micropores (<2 nm), mesopores (2–50 nm), and macropores (>50 nm) varied between the samples. Thus, the ratio of micropores (<2 nm) to mesopores and macropores (>2 nm) (SMic./SExt.) was employed. The H2O-activated biochar showed a lower value of this ratio (2.28) than that of the CO2-activated biochar (4.57) indicating that activation/ gasification under a CO2 atmosphere produced a higher relative micropore content, whereas under an H2O atmosphere mesopores were favored. This may be explained by considering that H2O removes carbon atoms from the particle's interior, enlarging open micropores and opening closed micropores, promoting the formation of mesopores. Meanwhile, CO2 causes changes in the biochar surface that create more micropores. According to Klinghoffer et al. [63], the higher biochar surface area was the main reason for better catalyst performance, but pore size distribution also affected its activity, and evidence of diffusion limitations in microporous biochar was observed. Elsewhere, it has been confirmed that mesopores significantly

### **Figure 9.**

*N2-absorption/desorption isotherms at 77 K for biochar obtained from different conditions: (A) in Ar over pyrolysis biochar; (B) in Ar over H2O-activated 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.*

*Applications of Biochar for Environmental Safety*

*3.2.3 SEM-EDX analysis of biochar*

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).

As shown in **Figure 8(a)**, the unactivated biochar particles' surfaces showed more, larger (40 × 60 μm) hill-like structures than the surfaces of the activated biochars. H2O and CO2 activate the biochar via C + H2O → CO + H2 and C + CO2 → 2CO, respectively. However, its larger size meant activation by CO2 was limited to the surface of biochar, resulting in small structures (15 × 15 μm), which can be seen in **Figure 8(c)**. However, as illustrated by the structure shown in **Figure 8(b)**, H2O, as well as H/O/OH radicals, was able to alter the surface morphology (creating structures of 20 × 20 μm) and infiltrate into the particle's carbon matrix to produce new larger pore structures from the inside out. According to Wu et al. [62], interactions between radicals and metal species take place on the surface of internal pores or inside the char matrix. **Table 4** shows little change in the biochar's internal metal (K, Ca, Mg, Fe, and Al) contents before and after H2O/CO2 activation. Thus, the effect of H2O/CO2 may be more focused on changing the distribution of metal species within the biochar samples. As we reported previously [43], the effect of K in biochar on tar reforming is stronger than that of Ca and other species. Thus, the surface content and distribution of K were studied, as shown in **Figure 8(a)–(c)**. The surface content of K significantly increased from 0.18% in the unactivated biochar to 0.35% in the H2O-activated biochar and 0.21% in CO2-activated biochar. In addition, an obvious enrichment occurred on the surface of H2O-activated biochar. Klinghoffer et al. [63] reported that during thermal treatment the metal species migrated to the biochar surface, some of which formed clusters that then acted as an active site for catalytic reactions. During H2O/CO2 activation, an increase in surface O content occurred alongside the migration of AAEM species from the interior of the particles to the surface, forming metal-carbon complexes. The redox properties of these metal-carbon complexes may have had implications for the biochar's catalytic properties. Also, highly dispersed metal species in a highly porous carbon matrix could have effectively acted as active adsorption sites that

*SEM-EDX analysis of biochar samples. (a) Pyrolysis biochar, (b) H2O activated biochar, and (c) CO2*

**58**

**Figure 8.**

*activated biochar.*

enhance catalytic activity by allowing the penetration of macromolecules, facilitating their adsorption on the catalyst surface [64–66]. Thus, biochar used for catalytic tar reforming should ideally possess a high surface area and high mesoporosity (i.e., a small SMic./SExt. value).

After tar reforming in the Ar atmosphere, the biochar samples' BET surface area and pore volume markedly decreased. This was especially the case for tar reforming in Ar over the pyrolysis biochar and the CO2-activated biochar where the BET surface areas fell to 3.78 and 4.07 m2/g, respectively. According to the findings of Hosokai et al. [19], the decrease in surface area was attributed to tar forming coke deposits on the biochar's surface. In the Ar atmosphere, tar was mainly decomposed via coking [CmHn (aromatic compounds) = CmHx(coke) + (*n* − *x*)/2 H2]. Thus, the biochar's activity could have fallen with a decrease in the biochar's surface area and/or pore volume caused by coke deposition. This implies that when some tar molecules reacted with the biochar they were absorbed in a way that yielded a condensed-phase product (coke) that remained on the biochar surface.

However, with the gasification agents, especially H2O, the relatively high BET surface area and pore volume of biochar were maintained following the tar-reforming reactions (see **Table 5**). This indicated that the tar was not reformed directly to give gaseous products but instead involved the intermediate formation of coke, which was subsequently gasified by H2O/CO2. El-Rub and Kamel [59] suggested that tars can be adsorbed onto the active sites of biochar particles. Adsorbed tar and coke molecules can be catalytically reformed to give CO and H2 by steam and dry CO2 thermochemical reactions, regenerating the pore structure. Meanwhile, free radicals that enter polymerization reactions and coke on biochar surfaces can be formed from tar decomposition. For the H2O-activated biochar, the BET surface area only decreased a little following reforming (to 268.52 m2 /g) and the SMic./SExt. value remained 2.30. Given that these conditions also reformed the greatest portion of the tar, the existence of gasification agents, especially H2O, appeared to stimulate and maintain catalytic activity by continually creating and regenerating pore structures in the biochar.


**61**

**Figure 10.**

investigation [24].

bon conversion.

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

The elemental contents (C, O, K, and Ca) at the surface of the biochar samples

The surface AAEM content remained high. For example, 2.12 atomic% K in H2O and 1.83 atomic% in CO2. This was similar for the surface O content (34.01 atomic% in H2O and 32.07 atomic% in CO2). A biochar with a higher O content appeared to favor the retention of AAEM species, with O likely serving as a link between the AAEM species and the char matrix [69]. In addition, the results of Wu et al. [70] suggest that adding H2O was likely to have eliminated more tar, while the presence of CO2 induced the formation of OH, H, and O radicals, which increase hydrocar-

*XPS analysis of biochar samples from different conditions: (A) in Ar over pyrolysis biochar; (B) in Ar over H2O-activated 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.*

are shown in **Figure 10**. Samples taken the following tar reforming in (A) Ar over pyrolysis biochar, (B) Ar over H2O-activated biochar, and (C) Ar over CO2 activated biochar showed that the H2O/CO2 activation of biochar played an important role in maintaining the biochar's active sites, such as surface O-containing functional groups and AAEM species (especially K and Ca) and improved its tar-reforming performance. According to Du et al. [67], XPS revealed the evolution of AAEM species and char structures, and concentrations of AAEM species agreed well with surface atomic O concentrations. Similar results were obtained in **Figure 10**, where Ar reforming over H2O-activated biochar yielded a biochar with a higher surface content of O (16.25 atomic%), K (0.80 atomic%), and Ca (0.45 atomic%) than the samples from Ar reforming with the CO2-activated biochar and the pyrolysis biochar. Abundant O-containing groups on the biochar surface can form acidic centers that can combine with biomass tar precursors, which have negatively charged π-electron systems and activate thermal cracking reactions [61]. For tar reforming in Ar, more carboxylic (O═C▬O)/carbonyl (C═O) groups and fewer aromatic (C▬C/C═C) groups were formed on the H2O/CO2-activated biochar surface. Franz et al. [68] investigated the effects of O-containing groups, particularly carboxylic and carbonyl groups, on the adsorption of dissolved aromatics on ash-free activated carbon. They found that the adsorption mechanism was influenced by the surface functional group's properties, especially its ability to hydrogen-bond, and through its activating/deactivating influence on the tar's aromatic ring. As shown in **Figure 5** for conditions (D) and (E), the existence of the gasification agents during tar reforming over H2O/CO2-activated biochar helped to limit coke formation on the biochar surface, likely by continually creating and regenerating surface active sites. This finding was consistent with that of a previous

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

*3.2.5 XPS analysis of biochar during biomass tar reforming*

### **Table 5.**

*BET properties of biochar samples during activation and tar reforming.*

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

### *3.2.5 XPS analysis of biochar during biomass tar reforming*

*Applications of Biochar for Environmental Safety*

decreased a little following reforming (to 268.52 m2

**area**

**(m2**

*BET properties of biochar samples during activation and tar reforming.*

**Conditions BET surface** 

(i.e., a small SMic./SExt. value).

surface.

the biochar.

(A) Pyrolysis and Ar reforming

(B) H2O activation and Ar reforming

(C) CO2 activation and Ar reforming

(D) H2O activation and H2O reforming

(E) CO2 activation and CO2 reforming

enhance catalytic activity by allowing the penetration of macromolecules, facilitating their adsorption on the catalyst surface [64–66]. Thus, biochar used for catalytic tar reforming should ideally possess a high surface area and high mesoporosity

After tar reforming in the Ar atmosphere, the biochar samples' BET surface area and pore volume markedly decreased. This was especially the case for tar reforming in Ar over the pyrolysis biochar and the CO2-activated biochar where the BET surface areas fell to 3.78 and 4.07 m2/g, respectively. According to the findings of Hosokai et al. [19], the decrease in surface area was attributed to tar forming coke deposits on the biochar's surface. In the Ar atmosphere, tar was mainly decomposed via coking [CmHn (aromatic compounds) = CmHx(coke) + (*n* − *x*)/2 H2]. Thus, the biochar's activity could have fallen with a decrease in the biochar's surface area and/or pore volume caused by coke deposition. This implies that when some tar molecules reacted with the biochar they were absorbed in a way that yielded a condensed-phase product (coke) that remained on the biochar

However, with the gasification agents, especially H2O, the relatively high BET surface area and pore volume of biochar were maintained following the tar-reforming reactions (see **Table 5**). This indicated that the tar was not reformed directly to give gaseous products but instead involved the intermediate formation of coke, which was subsequently gasified by H2O/CO2. El-Rub and Kamel [59] suggested that tars can be adsorbed onto the active sites of biochar particles. Adsorbed tar and coke molecules can be catalytically reformed to give CO and H2 by steam and dry CO2 thermochemical reactions, regenerating the pore structure. Meanwhile, free radicals that enter polymerization reactions and coke on biochar surfaces can be formed from tar decomposition. For the H2O-activated biochar, the BET surface area only

remained 2.30. Given that these conditions also reformed the greatest portion of the tar, the existence of gasification agents, especially H2O, appeared to stimulate and maintain catalytic activity by continually creating and regenerating pore structures in

> **Pore volume**

Pyrolysis biochar 195.35 0.0999 170.46 24.89 6.85 H2O-activated biochar 307.45 0.1745 213.83 93.62 2.28 CO2-activated biochar 237.71 0.1330 195.02 42.69 4.57

**/g) (cm3**

/g) and the SMic./SExt. value

**/g) (m2**

**External pore > 2 nm**

**/g)**

**SMic./SExt.**

**Micro pore < 2 nm**

3.78 0.0070 3.14 0.65 4.86

117.53 0.0693 78.28 39.25 1.99

4.07 0.0074 2.12 1.95 1.09

268.52 0.1512 187.08 81.44 2.30

54.31 0.0346 42.98 11.33 3.79

**/g) (m2**

**60**

**Table 5.**

The elemental contents (C, O, K, and Ca) at the surface of the biochar samples are shown in **Figure 10**. Samples taken the following tar reforming in (A) Ar over pyrolysis biochar, (B) Ar over H2O-activated biochar, and (C) Ar over CO2 activated biochar showed that the H2O/CO2 activation of biochar played an important role in maintaining the biochar's active sites, such as surface O-containing functional groups and AAEM species (especially K and Ca) and improved its tar-reforming performance. According to Du et al. [67], XPS revealed the evolution of AAEM species and char structures, and concentrations of AAEM species agreed well with surface atomic O concentrations. Similar results were obtained in **Figure 10**, where Ar reforming over H2O-activated biochar yielded a biochar with a higher surface content of O (16.25 atomic%), K (0.80 atomic%), and Ca (0.45 atomic%) than the samples from Ar reforming with the CO2-activated biochar and the pyrolysis biochar. Abundant O-containing groups on the biochar surface can form acidic centers that can combine with biomass tar precursors, which have negatively charged π-electron systems and activate thermal cracking reactions [61]. For tar reforming in Ar, more carboxylic (O═C▬O)/carbonyl (C═O) groups and fewer aromatic (C▬C/C═C) groups were formed on the H2O/CO2-activated biochar surface. Franz et al. [68] investigated the effects of O-containing groups, particularly carboxylic and carbonyl groups, on the adsorption of dissolved aromatics on ash-free activated carbon. They found that the adsorption mechanism was influenced by the surface functional group's properties, especially its ability to hydrogen-bond, and through its activating/deactivating influence on the tar's aromatic ring. As shown in **Figure 5** for conditions (D) and (E), the existence of the gasification agents during tar reforming over H2O/CO2-activated biochar helped to limit coke formation on the biochar surface, likely by continually creating and regenerating surface active sites. This finding was consistent with that of a previous investigation [24].

The surface AAEM content remained high. For example, 2.12 atomic% K in H2O and 1.83 atomic% in CO2. This was similar for the surface O content (34.01 atomic% in H2O and 32.07 atomic% in CO2). A biochar with a higher O content appeared to favor the retention of AAEM species, with O likely serving as a link between the AAEM species and the char matrix [69]. In addition, the results of Wu et al. [70] suggest that adding H2O was likely to have eliminated more tar, while the presence of CO2 induced the formation of OH, H, and O radicals, which increase hydrocarbon conversion.

### **Figure 10.**

*XPS analysis of biochar samples from different conditions: (A) in Ar over pyrolysis biochar; (B) in Ar over H2O-activated 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.*

### *3.2.6 GC-MS analysis of biomass tar reforming over biochar*

The biomass tar reformed in the gas phase (without involving biochar) in Ar at 800°C was mainly composed of aromatic tar compounds, owing to the secondary thermal cracking of in-situ biomass tar [52], compared with tar from H-form rice husk pyrolysis in fluidized bed at 500°C. Thermal cracking cannot completely convert tars [71]. Regarding the biochar catalyzed reactions, defined as the net tar loss owing to exposure to the biochar (i.e., the amount remaining after subtracting the amount of tar destroyed by vapor-phase cracking upstream and downstream of the biochar bed from the total change in tar amount during thermal treatment) [16], no new tar compounds were observed. According to Yao et al. [72], absent biochar, the gasification agent has a larger effect on the reforming of large aromatic ring systems (e.g., ≥2 fused benzene rings) than on smaller and isolated aromatics. However, here, biomass tars with a single aromatic ring or more than one ring structure were catalytically reformed over the various biochars. The conversion rates of tar compounds for the tar reformed without biochar in Ar at 800°C can be seen in **Figure 11(a)** and **(b)**. These figures also show that H2O/CO2 notably enhanced in-situ reforming of both large and small aromatic ring systems in biomass tar. For the experiments in Ar (conditions (A), (B), and (C)), individual tar conversion rates were improved by activation by H2O/CO2. For example, phenylethyne conversion increased from 42.27% over pyrolysis biochar to 77.43% over H2O-activated biochar and to 49.93% over CO2-activated biochar. However, the magnitude of the improvement was limited because of coke formation on the active sites, which deactivated the biochar. Thus, continuously supplying gasification agents (H2O and CO2 in conditions D and E, respectively) made more complete biomass tar conversion possible. For example, reforming in 15 vol.% H2O over H2O-activated biochar saw conversion rates of tars with both single aromatic ring structures (e.g., phenylethyne and benzofuran) and multiring structures (e.g., 1-methy-naphthalene, 2-methy-naphthalene, and phenanthrene) reach almost 100%. Although tar conversion was not completed in the 29 vol.% CO2 atmosphere, it was also notably higher than for reforming in Ar over CO2-activated biochar. As mentioned above, the gasification agent directly affected gas-phase tar reforming reactions [72], and it is likely that H2O/CO2 indirectly affected tar destruction by influencing the biochar structure and distribution of AAEM catalysts during the reaction by helping to ensure enough active sites on the biochar surface to maintain its catalytic activity.

### **Figure 11.**

*Biomass tar conversion rates (based on tar observed following the treatment without biochar in Ar at 800°C) for different reforming conditions: (A) in Ar over pyrolysis biochar; (B) in Ar over H2O-activated 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.*

**63**

**Figure 12.**

the biochar catalyst.

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

The heterogeneous reforming mechanism of the biomass tar over biochar and in the presence of the H2O and CO2 reforming agents at 800°C is shown in **Figure 12**. H2O and CO2 dissociate in space to form a large number of H/O/OH radicals, which play an important role in the tar-biochar reforming reaction. Biomass tar, through the biochar layer, is adsorbed onto the acid-base active sites (oxygen-containing functional groups and AAEM catalysts). The attraction effect of the carbon-rich biochar matrix invokes an electron pair shift in the tar molecules (relatively small mass), which promotes the tar molecules to break at high temperatures. According to the free radical theory [73], the tar adsorbed on the catalyst surface will catalytically crack to form the corresponding free radicals. The chemical reaction between these free radicals may permit new products. H2O and CO2 act as the reforming agents in the biochar carbon matrix, resulting in the fragmentation of the smaller aromatic rings. The empty active sites, formed by bond cleavage, were gradually occupied by H/O/OH radicals, forming active groups such as O-containing functional groups. In the presence of H2O and CO2, a significant amount of H/O/ OH radicals in the vicinity ingress into the biochar carbon structure. The catalytic elements, such as AAEM species migrate at different rates and transformation from the carbon matrix onto the gas-solid interface or the gas phase undergoes, as shown in **Figure 12**. As the AAEM species are bonded with the C element on the biochar surface by the O element [21], H2O and CO2 react with these C elements on the biochar surface resulting in AAEM-O bond cleavage followed by precipitation. The valence state of Ca results in a stronger bonding interaction with the biochar when compared with K. Additionally, Ca migration and precipitation are more difficult than K. When tar adsorbs then cleaves the AAEM-O bond and functional group bond on the biochar surface, an aromatic fragmented radical is formed when other free radicals are encountered. At the same time, active AAEM species in the vicinity will continue to occupy active sites on the tar fragment groups, thereby inhibiting their secondary polymerization. At the same time, the H/O/OH radicals are exchanged to the AAEM species, which increases the possibility for the reforming of tar macromolecules. After the reaction, gas and light tars (CnHy/CO/H2) were formed, thus realizing the H2O or CO2 heterogeneous reforming of biomass tar over

*Heterogeneous reforming mechanism of biomass tar over biochar in the presence of H2O and CO2 at 800°C [52].*

*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*

The heterogeneous reforming mechanism of the biomass tar over biochar and in the presence of the H2O and CO2 reforming agents at 800°C is shown in **Figure 12**. H2O and CO2 dissociate in space to form a large number of H/O/OH radicals, which play an important role in the tar-biochar reforming reaction. Biomass tar, through the biochar layer, is adsorbed onto the acid-base active sites (oxygen-containing functional groups and AAEM catalysts). The attraction effect of the carbon-rich biochar matrix invokes an electron pair shift in the tar molecules (relatively small mass), which promotes the tar molecules to break at high temperatures. According to the free radical theory [73], the tar adsorbed on the catalyst surface will catalytically crack to form the corresponding free radicals. The chemical reaction between these free radicals may permit new products. H2O and CO2 act as the reforming agents in the biochar carbon matrix, resulting in the fragmentation of the smaller aromatic rings. The empty active sites, formed by bond cleavage, were gradually occupied by H/O/OH radicals, forming active groups such as O-containing functional groups. In the presence of H2O and CO2, a significant amount of H/O/ OH radicals in the vicinity ingress into the biochar carbon structure. The catalytic elements, such as AAEM species migrate at different rates and transformation from the carbon matrix onto the gas-solid interface or the gas phase undergoes, as shown in **Figure 12**. As the AAEM species are bonded with the C element on the biochar surface by the O element [21], H2O and CO2 react with these C elements on the biochar surface resulting in AAEM-O bond cleavage followed by precipitation. The valence state of Ca results in a stronger bonding interaction with the biochar when compared with K. Additionally, Ca migration and precipitation are more difficult than K. When tar adsorbs then cleaves the AAEM-O bond and functional group bond on the biochar surface, an aromatic fragmented radical is formed when other free radicals are encountered. At the same time, active AAEM species in the vicinity will continue to occupy active sites on the tar fragment groups, thereby inhibiting their secondary polymerization. At the same time, the H/O/OH radicals are exchanged to the AAEM species, which increases the possibility for the reforming of tar macromolecules. After the reaction, gas and light tars (CnHy/CO/H2) were formed, thus realizing the H2O or CO2 heterogeneous reforming of biomass tar over the biochar catalyst.

### **Figure 12.**

*Heterogeneous reforming mechanism of biomass tar over biochar in the presence of H2O and CO2 at 800°C [52].*

*Applications of Biochar for Environmental Safety*

*3.2.6 GC-MS analysis of biomass tar reforming over biochar*

The biomass tar reformed in the gas phase (without involving biochar) in Ar at 800°C was mainly composed of aromatic tar compounds, owing to the secondary thermal cracking of in-situ biomass tar [52], compared with tar from H-form rice husk pyrolysis in fluidized bed at 500°C. Thermal cracking cannot completely convert tars [71]. Regarding the biochar catalyzed reactions, defined as the net tar loss owing to exposure to the biochar (i.e., the amount remaining after subtracting the amount of tar destroyed by vapor-phase cracking upstream and downstream of the biochar bed from the total change in tar amount during thermal treatment) [16], no new tar compounds were observed. According to Yao et al. [72], absent biochar, the gasification agent has a larger effect on the reforming of large aromatic ring systems (e.g., ≥2 fused benzene rings) than on smaller and isolated aromatics. However, here, biomass tars with a single aromatic ring or more than one ring structure were catalytically reformed over the various biochars. The conversion rates of tar compounds for the tar reformed without biochar in Ar at 800°C can be seen in **Figure 11(a)** and **(b)**. These figures also show that H2O/CO2 notably enhanced in-situ reforming of both large and small aromatic ring systems in biomass tar. For the experiments in Ar (conditions (A), (B), and (C)), individual tar conversion rates were improved by activation by H2O/CO2. For example, phenylethyne conversion increased from 42.27% over pyrolysis biochar to 77.43% over H2O-activated biochar and to 49.93% over CO2-activated biochar. However, the magnitude of the improvement was limited because of coke formation on the active sites, which deactivated the biochar. Thus, continuously supplying gasification agents (H2O and CO2 in conditions D and E, respectively) made more complete biomass tar conversion possible. For example, reforming in 15 vol.% H2O over H2O-activated biochar saw conversion rates of tars with both single aromatic ring structures (e.g., phenylethyne and benzofuran) and multiring structures (e.g., 1-methy-naphthalene, 2-methy-naphthalene, and phenanthrene) reach almost 100%. Although tar conversion was not completed in the 29 vol.% CO2 atmosphere, it was also notably higher than for reforming in Ar over CO2-activated biochar. As mentioned above, the gasification agent directly affected gas-phase tar reforming reactions [72], and it is likely that H2O/CO2 indirectly affected tar destruction by influencing the biochar structure and distribution of AAEM catalysts during the reaction by helping to ensure enough active sites on the biochar surface to maintain its catalytic activity.

*Biomass tar conversion rates (based on tar observed following the treatment without biochar in Ar at 800°C) for different reforming conditions: (A) in Ar over pyrolysis biochar; (B) in Ar over H2O-activated biochar; (C) in Ar over CO2-activated biochar; (D) in 15 vol.% H2O over H2O-activated biochar; and (E) in 29 vol.% CO2*

**62**

**Figure 11.**

*over CO2-activated biochar.*
