Читать книгу High-Performance Materials from Bio-based Feedstocks - Группа авторов - Страница 41

The production of syngas via biomass gasification has attracted a great deal of interest. However, this technology faces some challenges, the biggest one of which is excessive tar formation resulting in clogged up equipment. Consequently, the total cost of biomass‐derived syngas production is increased making it difficult to develop this process into industrial manufacturing. To overcome this drawback, tar removal technologies have been intensively researched with regard to their economic and environmental impacts. The catalytic thermochemical conversion using biochar catalysts has been reported as a capable technique for tar reforming, but currently still shows inferior performance compared to conventional metal‐supported catalysts. Particularly steam and CO₂‐treated biochars are very powerful catalysts for tar reforming. The biochar prepared by pyrolysis from several biomass sources was activated with 15 vol% H₂O mixed with argon or with CO₂ at 800 °C for a short time. The treated biochar catalysts increased the catalytic activity in the steam or CO₂ reforming of tar compared to a regular biochar. Treatment of the biochar resulted in an increased surface area and pore volume (both microporous and mesoporous), and high content of oxygenated functional groups [91, 92]. The CO₂ generated more micropores in the biochar, whereas conversely, steam created more mesopores, which adds further importance for tar reforming. Even though micropores showed a greater initial tar conversion, these are rapidly deactivated due to coke deposition in the pores [4, 91, 93]. The steam‐activated biochar had more oxygenated functional groups in the aromatic C–O forms, which are more active sites for tar reforming, than those treated by CO₂ [89, 92, 94]. Tar molecules were probably absorbed onto the unstable aromatic C–O structures on the biochar surface bringing about the transformation of tar into the gas products. When comparing the reforming gases, the treated biochar catalyzing the tar steam reforming was more effective than that in the CO₂ reforming [95]. The steam fed during the reforming reaction could produce additional oxygenated functional groups and aromatic C–O structures [92], which resulted in a gradual increase in catalytic performance. Therefore, steam was a promising agent for boosting and preserving the catalytic activity of biochar in tar reforming.

As discussed previously, raw biochar had slower rates of tar elimination than metal‐supported catalysts. Doping an active metal such as Ni and Fe onto the biochar surface is another favored approach for the development of catalytic biochar [96]. Kastner et al. prepared biochar by pyrolysis of pine bark, which was then impregnated with Fe [97]. Compared to the regular biochar, the Fe‐modified biochar catalyst possessed a lower surface area and pore volume as the Fe particles could hinder and shrink the biochar pores. Although the physical properties of the Fe‐modified biochar catalyst were not remarkable, this catalyst showed that the tar decomposition rate was increased and the activation energy was decreased by 47%. The tar conversion was significantly altered by the metal loading and the tar decomposition increased with higher metal content on the biochar. At a decomposition temperature of 900 °C, the biochar modified with 13% Fe had a tar conversion of 100%. The addition of alkali along with the metal impregnation on biochar was developed in order to further enhance the catalytic activity. The raw biomass material was impregnated with potassium ferrate prior to carbonization at 900 °C for two hours to obtain a K–Fe bimetallic catalyst‐supported biochar [98]. The K₂CO₃ was formed as an active site of this catalyst and showed a superior catalytic activity in cracking of biomass pyrolysis tar at a relatively low reaction temperature (600–700 °C). The active metal sites and pore structure were still maintained after the reaction. This catalyst also showed a high stability as no significant change in tar conversion was observed after five cycles.