(404e) Effect of Metallic Compounds Addition on the Decrease in Tar Yield during Pyrolysis of Biomass

In order to utilize efficiently wood biomass, it is desired that a high-calorific carbonaceous residue in high yield is recovered with suppression of tar evolution. We have developed a new pyrolysis method using metallic compounds for this purpose. The concept lies in the promotion of intermolecular cross-linking reaction in cellulose by interacting metallic compounds with functional groups of biomass. Biomass with several kinds of metallic compounds was pyrolyzed at a constant heating rate in a N₂ gas stream. Hydroxides of alkali and alkali earth metal caused an increase in char yield at a temperature above 400°C. The weight loss profiles were classified into two groups; one shifted to lower temperature (Group I), and the other shifted to higher temperature (Group II) as compared with original profile. In the case of Group II such as CaOH, a large amount of OH groups was remained in the char. In the case of Group I such as NaOH and KOH, however, the amount of OH groups in the char started to decrease at the temperature as low as 200°C. Especially, sodium hydroxide was the most effective reagent to suppress tar formation as well as increase char yield. The tar yield was successfully decreased by nearly zero level. This is because crystal structure of cellulose was distorted by the interaction between OH functional groups in biomass and NaOH. Such an interaction due to deliquescency of NaOH promoted dehydration reaction in cellulose, and the cross-linking structure formed before breakage of glycoside linkage at around 250°C prevented small fragment from volatilization. We have also pyrolyzed several kinds of cellulose with NaOH so as to investigate quantitatively the effect of metallic compound on the degree of cross-linking. The char yield at 200°C decreased, on the contrary, increased at 500°C monotonously with the increase in the amount of NaOH added. The increase in char yield at 500°C correlated well in linear cross-linking index determined from the char yield at 200°C. Thus, it was clarified that the addition of metallic compounds strongly affected on the pyrolysis behavior at low temperature. The method will be an attractive method for an efficient conversion of biomass with suppressing tar evolution.