Co-gasification of coal and biomass: synergistic effects on gasification reactivity induced by inherent organic components

It is widely recognized that the synergistic effects during co-gasification process of coal and biomass are caused by the inherent alkali/alkaline earth metals (AAEM). However, the compositions of biomass and coal are complex, with main components being not only AAEM, but also aromatic, aliphatic, and heteroatom-containing compounds. Therefore, whether these organic components can also result in synergistic effects during co-gasification is necessary to be investigated. In this work, to reveal the reaction characteristics of organic components in co-gasification process, two raw materials with little content of ash were selected. The gasification behaviors of coal and biomass were studied and the influences of blending ratio and gasification temperature were mainly examined. The results showed that gasification reactivity of pure biomass was the strongest and its carbon conversion reached 100% within 33 min. With the increase of biomass ratio, gasification reactivity of mixtures was gradually enhanced, which could be ascribed to active functional groups, porous structures, and incompact macromolecular structures of biomass. At high temperature, the gasification reactivity difference was lessened among different samples. As for coal, the reactivity index at 800°C was 0.012. With the increase of biomass blending ratio from 25% to 100%, the reactivity index slowly went up to 0.023 at maximum. When temperature rose to 900°C, the reactivity index was enlarged nearly eight times compared with that of 800°C. At temperatures tested, the shrinking core mode was appropriate to describe gasification process, especially for coal and coal/biomass mixtures. Kinetic analysis showed the theoretical activation energies were all higher than the experimental values, indicating that the interaction between coal and biomass diminished activation energy. The synergy index suggested that gasification reactivity of coal was indeed enhanced during co-gasification process. However, a high-temperature environment might cover up the positive roles of biomass and the synergistic effect was hence weakened. In conclusion, the synergistic effects during co-gasification process could be induced by the organic components in coal and biomass.