**Abstract**

Biomass tar is the bottleneck in the development of efficient utilization of biomass syngas. The in-situ catalytic cracking biomass tar with multi-active biochar is investigated in a two-stage fluidized bed-fixed bed reactor. It indicates that adding H2O or CO2 is found to improve the homogeneous and heterogeneous cracking of biomass tar. Activation of biochar by H2O or CO2 impacted the morphology of biochar surface and distribution of metal species. H2O or CO2 affects the creation and regeneration of pore structures, influencing the biochar structure and dynamical distribution of alkali and alkaline earth metal species (AAEMs), which ensure enough surface active sites to maintain the catalytic activity of biochar. The tar cracking into low-quality tar or small-molecule gas may be catalyzed by K, while the combination of tar with biochar would be promoted by Ca. The volatilizations of K and Ca, due to their reaction with volatiles, are to a large extent in accordance with their valences and boiling points. The subsequent transformation from the small aromatic ring systems to the larger ones occurs due to the volatile-biochar interaction. During tar cracking over biochar, K and Ca act as the active sites on biochar surface to promote the increase of active intermediates (C▬O bonds and C▬O▬K/Ca).

**Keywords:** biochar, tar, catalytic cracking, AAEM species

## **1. Introduction**

Tar is a generic term comprising all organic compounds present in syngas except for gaseous hydrocarbons. Tars can condense to more complex structures in pipes, filters, or heat exchangers of downstream equipment and processes, which may cause mechanical breakdown of the entire system [1]. For biomass gasification, the allowable limit for tar in the producer gas is less than 5 mg/Nm3 for a direct-fired gas turbine [2], and for some fuel synthesis processes, the contents of tar and ammonia are required to be <0.1 mg/Nm3 and <10 ppm, respectively [3–5], in order to protect the catalysts and downstream equipment and to improve the overall efficiency and economics. The tar mixture is classified into five classes by Padban [6]: undetectable, heterocyclic, light aromatic hydrocarbons (LAHs), light polyaromatic hydrocarbons (LPAHs), and heavy polyaromatic hydrocarbons (HPAHs). The removal of biomass tar is one of the main challenges for the biomass gasification industry [7, 8]. Catalytic cracking is a known method for the efficient removal of biomass tar [9–12]. Biochar, as a product of pyrolysis and gasification of biomass, is a relatively

cheap catalyst with high activity in tar heterogeneous cracking [13–20]. During tar catalytic cracking over biochars, even after the loss of catalytic activity through coking, the biochar samples can still be directly combusted, so as to recover the chemical energy of catalyst, thus avoiding any reprocessing as a result of deactivation.

In addition to the analysis of model tar compounds [21–23], studies of biomass tar over biochar mainly discuss the reforming of real tar from raw materials [24, 25]. However, the AAEM species (e.g., Na, K, Mg, and Ca) in raw biomass play an important role as the "cross points" during tar formation. The chemical bonds between AAEM species and the carbon matrix are repeatedly breaking and reforming. This process promotes the production of gaseous products from the fatty acid tar and a degree of small aromatic compounds. Simultaneously, larger aromatic ring compounds (≥5 aromatic ring system) are formed within the biochar structure [26, 27]. The presence of AAEM species can inhibit the release of volatile matter (especially for biomass tar)—even the strong interaction between volatile materials and biochar will affect tar composition, leading to the catalytic conversion of the real tar components before contacting with the catalyst, which misleads mechanistic studies of subsequent heterogeneous reforming over biochar catalyst.

The formation (e.g., 500–700°C) and thermal decomposition (e.g., 700–900°C) of tar during the gasification process are an extremely complex multistep reaction [28–32], which involves not only homogeneous conversion, but also heterogeneous reforming. H2O and CO2 are two important reforming agents [33] in the biomass gasification industry. Studying the influence of H2O and CO2 on tar homogeneous transformation and heterogeneous reformation is valuable to better understand the analysis of the tar complex gas-solid phase reaction. However, there is still less research on separate discussion between homogeneous conversion and heterogeneous reforming of biomass tar over biochar. Although there are reports detailing the influence of H2O and CO2 on tar during the gasification process [34–39], they were mainly focused on the single concentration of reforming agent (15 vol.% H2O or pure CO2 atmosphere). There has yet to be detailed a complete understanding of the influence of H2O and CO2 on the homogeneous conversion and heterogeneous reforming over biochar as a function of biomass tar evolution.

The effects of reforming agent concentration and reaction temperature on the tar homogeneous conversion and heterogeneous reforming over biochar were investigated in a two-stage fluidized bed/fixed bed reactor. The H-form biomass samples (with little AAEM species) were used to provide the real tar components, which effectively inhibited the tar-AAEM interactions in gas phase during H2O/ CO2 homogeneous conversion and prevented any secondary catalytic effects of AAEM species from the volatilization of raw materials on the biochar catalyst surface. The analysis of biochar structures examined with Raman spectroscopy to comprehensively elucidate the changes of biochar catalyst structure after the H2O and CO2 heterogeneous reforming of biomass tar. In addition to the measurement of tar yields, GC/MS spectroscopy was used to characterize the detailed structural features of tar [40], so as to understand the molecular biomass tar transformation pathway and the coupling mechanism (e.g., collaboration and interaction effects) between the biochar structure and the AAEM species during tar reforming.

### **2. Experiment**

### **2.1 Material preparation**

Biomass (rice husks) obtained from the Wu Chang area in Harbin, Heilongjiang Province, China, was used in the experiments. The samples were dried overnight

**49**

**Figure 1.**

**Table 1.**

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

at 105°C, pulverized, and sieved to obtain a fraction with particle sizes between 0.15 and 0.25 mm. The proximate and ultimate analyses data [41] for the rice husk samples are listed in **Table 1**, which could be used to characterize the composition of biomass, grasping its reaction characteristics and application value (*M*: moisture, *A*: ash, *V*: volatile, *FC*: fixed carbon; *C*: carbon, *H*: hydrogen, *O*: oxygen, *N*: nitro-

The H-form rice husk was used as the raw material to supply real biomass tar. The raw pyrolysis biochar was mixed with an aqueous solution of 0.2 M H2SO4 in an acid solution:sample mass ratio of 30:1 and stirred in an argon atmosphere for 24 h. The slurry was filtered and washed with deionized water until the filtrate pH was constant (pH ≈ 7). After drying, the acid-washed sample is termed as the

Pyrolysis biochar was used as the catalyst for biomass tar reforming. The set-up to pyrolysis biochar comprises a quartz reactor and a standard muffle furnace, as

The quartz tray (red tray) with 5.0 g raw rice husk was placed into the quartz reactor. Along with the reactor cover, the quartz reactor was placed into the muffle furnace. At room temperature, the air in the reactor was displaced by Ar at a rate of 2.0 L/min for 30 min. Pyrolysis was performed at a slow-heating rate of 10°C/ min up to a final pyrolysis temperature of 700°C with 70 min. Thereafter, with the temperature of turn-off furnace back to room temperature, the door of the muffle furnace was opened, and the reaction quenched by removing the reactor from the furnace. Ar gas was passed continuously through the reactor to prevent oxidation during cooling. The pyrolyzed biochar was removed from the reactor and stored at 4°C. Ar gas is supplied through a gas pipe (400 mm, long) into the porous distributor (with a diameter of 120 mm) and fed from the bottom of the quartz reactor filling the entire reactor. The upper cover acts as a partial seal under the action of its own gravity; however, with increasing internal gas volume produced as a function

**Sample Proximate analysis (wt.%) Ultimate analysis (wt.%)**

Rice husk 6.86 17.00 60.92 15.22 37.35 4.40 34.05 0.20 0.14

*M***ad.** *A***ad.** *V***ad.** *FC***ad.** *C***ad.** *H***ad.** *O***ad.(diff)** *N***ad.** *S***t,ad.**

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

gen, *S*: sulfur).

H-form char.

shown in **Figure 1**.

**2.2 Biochar catalyst preparation**

*Note: diff. = by difference, ad. = air dry basis.*

*Proximate and ultimate analyses of rice husk samples.*

*Biochar catalyst preparation using a muffle 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*

at 105°C, pulverized, and sieved to obtain a fraction with particle sizes between 0.15 and 0.25 mm. The proximate and ultimate analyses data [41] for the rice husk samples are listed in **Table 1**, which could be used to characterize the composition of biomass, grasping its reaction characteristics and application value (*M*: moisture, *A*: ash, *V*: volatile, *FC*: fixed carbon; *C*: carbon, *H*: hydrogen, *O*: oxygen, *N*: nitrogen, *S*: sulfur).

The H-form rice husk was used as the raw material to supply real biomass tar. The raw pyrolysis biochar was mixed with an aqueous solution of 0.2 M H2SO4 in an acid solution:sample mass ratio of 30:1 and stirred in an argon atmosphere for 24 h. The slurry was filtered and washed with deionized water until the filtrate pH was constant (pH ≈ 7). After drying, the acid-washed sample is termed as the H-form char.

### **2.2 Biochar catalyst preparation**

*Applications of Biochar for Environmental Safety*

cheap catalyst with high activity in tar heterogeneous cracking [13–20]. During tar catalytic cracking over biochars, even after the loss of catalytic activity through coking, the biochar samples can still be directly combusted, so as to recover the chemical energy of catalyst, thus avoiding any reprocessing as a result of deactivation. In addition to the analysis of model tar compounds [21–23], studies of biomass tar over biochar mainly discuss the reforming of real tar from raw materials [24, 25]. However, the AAEM species (e.g., Na, K, Mg, and Ca) in raw biomass play an important role as the "cross points" during tar formation. The chemical bonds between AAEM species and the carbon matrix are repeatedly breaking and reforming. This process promotes the production of gaseous products from the fatty acid tar and a degree of small aromatic compounds. Simultaneously, larger aromatic ring compounds (≥5 aromatic ring system) are formed within the biochar structure [26, 27]. The presence of AAEM species can inhibit the release of volatile matter (especially for biomass tar)—even the strong interaction between volatile materials and biochar will affect tar composition, leading to the catalytic conversion of the real tar components before contacting with the catalyst, which misleads mechanistic

studies of subsequent heterogeneous reforming over biochar catalyst.

reforming over biochar as a function of biomass tar evolution.

The effects of reforming agent concentration and reaction temperature on the tar homogeneous conversion and heterogeneous reforming over biochar were investigated in a two-stage fluidized bed/fixed bed reactor. The H-form biomass samples (with little AAEM species) were used to provide the real tar components, which effectively inhibited the tar-AAEM interactions in gas phase during H2O/ CO2 homogeneous conversion and prevented any secondary catalytic effects of AAEM species from the volatilization of raw materials on the biochar catalyst surface. The analysis of biochar structures examined with Raman spectroscopy to comprehensively elucidate the changes of biochar catalyst structure after the H2O and CO2 heterogeneous reforming of biomass tar. In addition to the measurement of tar yields, GC/MS spectroscopy was used to characterize the detailed structural features of tar [40], so as to understand the molecular biomass tar transformation pathway and the coupling mechanism (e.g., collaboration and interaction effects) between the biochar structure and the AAEM species during tar reforming.

Biomass (rice husks) obtained from the Wu Chang area in Harbin, Heilongjiang Province, China, was used in the experiments. The samples were dried overnight

The formation (e.g., 500–700°C) and thermal decomposition (e.g., 700–900°C) of tar during the gasification process are an extremely complex multistep reaction [28–32], which involves not only homogeneous conversion, but also heterogeneous reforming. H2O and CO2 are two important reforming agents [33] in the biomass gasification industry. Studying the influence of H2O and CO2 on tar homogeneous transformation and heterogeneous reformation is valuable to better understand the analysis of the tar complex gas-solid phase reaction. However, there is still less research on separate discussion between homogeneous conversion and heterogeneous reforming of biomass tar over biochar. Although there are reports detailing the influence of H2O and CO2 on tar during the gasification process [34–39], they were mainly focused on the single concentration of reforming agent (15 vol.% H2O or pure CO2 atmosphere). There has yet to be detailed a complete understanding of the influence of H2O and CO2 on the homogeneous conversion and heterogeneous

**48**

**2. Experiment**

**2.1 Material preparation**

Pyrolysis biochar was used as the catalyst for biomass tar reforming. The set-up to pyrolysis biochar comprises a quartz reactor and a standard muffle furnace, as shown in **Figure 1**.

The quartz tray (red tray) with 5.0 g raw rice husk was placed into the quartz reactor. Along with the reactor cover, the quartz reactor was placed into the muffle furnace. At room temperature, the air in the reactor was displaced by Ar at a rate of 2.0 L/min for 30 min. Pyrolysis was performed at a slow-heating rate of 10°C/ min up to a final pyrolysis temperature of 700°C with 70 min. Thereafter, with the temperature of turn-off furnace back to room temperature, the door of the muffle furnace was opened, and the reaction quenched by removing the reactor from the furnace. Ar gas was passed continuously through the reactor to prevent oxidation during cooling. The pyrolyzed biochar was removed from the reactor and stored at 4°C. Ar gas is supplied through a gas pipe (400 mm, long) into the porous distributor (with a diameter of 120 mm) and fed from the bottom of the quartz reactor filling the entire reactor. The upper cover acts as a partial seal under the action of its own gravity; however, with increasing internal gas volume produced as a function


**Table 1.**

*Proximate and ultimate analyses of rice husk samples.*

**Figure 1.** *Biochar catalyst preparation using a muffle furnace.* 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 metal contents of the origin and H-form biochar are listed in **Table 2**.
