Effect of the water washing pretreatment on biomass pyrolysis in CO2 atmosphere

The alkali metals have limited the large-scale industrial promotion of biomass gasification technology. The water washing pretreatment technology can effectively remove alkali metals from biomass and reduce the problem of the slag fouling. This work investigated the effects of washing temperature and washing time on the fuel properties of Salix psammophila, and analyzed the composition distribution of the leachates, in order to provide reference for the harmless treatment of the leachates. The effect of water washing on the pyrolysis of Salix under N2 and CO2 atmospheres was studied on a fixed-bed experimental platform. The results show that the water washing pretreatment effectively remove alkali metals from Salix and reduce the slagging inclination. The leachates contain inorganic ions such as Ca, K, Na required for plant, as well as organic components that are harmful to the environment. The organic components are mainly aromatic compounds, alkanes and ketones, acids, and alcohols. Under the conditions of 30°C-3h, the aromatics account for 16.51% and the alkanes account for 74.22%. With the increase of washing time and washing temperature, the organic matter content in the leachates increases, the proportion of alkanes decreases, and the proportion of aromatics increases. The pyrolysis experiment shows that under N2 atmosphere, the water washing increases syngas production and reduces tar production. Under the conditions of 30°C-12h, the production of CO, CH4, H2, and CO2 reach the maximum values, increasing by 5.24%, 26.81%, 28.75%, and 8.84%, respectively. The tar yield decreases by 35.04%. In CO2 atmosphere, CO increases by 169.65% compared to N2 atmosphere. After the washing pretreatment, the CO production decreases slightly, and the CH4 production increases by 0.45% to 6.54%. The removal of alkali metals, especially K, reduces the reactivity of char and CO2 and gas production. After the washing pretreatment, the tar yield decreases. This is manifested as an increase in the proportion of alkanes and other compounds, and a decrease in the proportion of polycyclic aromatic hydrocarbons.