纤维素低温炭化特性

Features of low temperature carbonization of cellulose

为了解纤维素在低温下焦炭的生成及演变过程，在固定床上开展了慢速热解实验，采用元素分析以及二维相关红外光谱技术对焦炭特性进行了分析。研究发现，纤维素慢速热解的分解主要集中于250～360℃。250～300℃时纤维素的炭化以脱水为主，且在炭化初期，分子间氢键断裂生成自由羟基，并使纤维素的大分子结构松散。而随着温度的升高，分子内氢键断裂以及羟基脱水，使得焦炭中生成大量的羰基、烯烃双键以及环醚结构。300℃时吡喃环开环及糖苷键断裂后，含双键及羰基的脂肪烃进一步发生分子重排、缩聚及芳环化而生成苯环、芳基烷基醚等结构。300～460℃的炭化以脱氧反应为主，此时焦炭中脂肪烃的含量逐渐降低，而芳香环含量增加。＞460℃时的炭化以脱氢反应为主，此时焦炭中存在缩合程度较高的芳烃结构。

The slow pyrolysis of cellulose was conducted in a fixed bed reactor at a temperature ranging of 250—550℃. The formation and evolution characteristics of residual char was investigated by elemental composition analysis and two-dimensional infrared correlation spectroscopy. The inter- and intra-molecular hydrogen bonding networks in cellulose were identified and the change of hydrogen bonding patterns during the initial stage of pyrolysis was investigated. The decomposition of cellulose during slow pyrolysis was mainly focused on 250—360℃with a maximum decomposition rate at about 300℃. The predominant reaction of cellulose carbonization at 250—300℃ was dehydration. Furthermore, the O（3）H＂O（5） intra-molecular H-bonds were found to be the most sensitive to temperature, and it changes before that of the methylene groups. However, it changes after that of O（2）H＂O（6） intra-molecular and O（6）H＂O（3） inter-molecular H-bonds. In the initial stage of carbonization, the cleavage of O（6）H＂O（3） intra-molecular H-bonds resulted in the formation of free hydroxyl groups and the tight cellulose structure was loosen up. With temperature rising, the cleavage of intra-molecular H-bonds and the subsequent dehydration generated large amount of carbonyl, double bonds and＆nbsp;cyclic ethers in the char. At 300℃, the ring-opening of pyran structure together with the cleavage of glycosidic bond gave rise to the drastic degradation of cellulose. As a result, substantial amount of aliphatic hydrocarbons with double bonds and carbonyl functional groups were generated in the char. These groups were further underwent molecular rearrangement, condensation and aromatization and resulted in the formation of aromatic ring and aryl alkyl ethers. In 300—460℃, the predominant reaction of cellulose carbonization was deoxygenation, such as decarbonylation and decarboxylation. Meanwhile, the content of aliphatic hydrocarbons in char decreased gradually and that of the aromatic structure increased. The predominant carbonization reaction shifted from deoxygenation toward dehydrogenation as temperature exceeded 460℃, such as demethylation and demethylenation. The aromaticity of char increased with temperature and a highly condensed aromatic structures emerged in the char.