Rheological Study of Comingled Biomass and Coal Slurries with Hydrothermal Pretreatment†

Abstract

Gasification of comingled biomass and coal feedstock is an effective means of reducing the net life cycle greenhouse gas emissions in the coal gasification process while maintaining its inherent benefits of abundance and high-energy density. However, feeding a comingled biomass and coal feedstock into a pressurized gasification reactor poses a technical problem. Conventional dry feeding systems, such as lock hoppers and pressurized pneumatic transport, are complex and operationally expensive. A slurry formation of comingled biomass and coal feedstock can be easily fed into the gasification reactor but, in normal conditions, only allows for a small portion of biomass in the mixture. This is a consequence of the hydroscopic and hydrophilic nature of the biomass. The College of Engineering Center for Environmental Research and Technology (CE-CERT) at the University of California, Riverside, has developed a process producing high solid content biomass−water slurry using a hydrothermal pretreatment process. In this paper, the systematic investigation of the rheological properties (e.g., shear rate, shear stress, and viscosity) of coal−water slurries, biomass−water slurries, and comingled biomass and coal−water slurries is reported. The solid particle size distribution in the slurry and the initial solid/water ratio were investigated to determine the impact on shear rate and viscosity. This was determined using a rotational rheometer. The experimental results show that larger particle size offers better pumpability. The presence of a high percentage of biomass in solid form significantly decreases slurry pumpability. It is also shown that the solid loading of the biomass−water slurry can be increased to approximately 35 wt % with viscosity of less than 0.7 Pa·s after the pretreatment process. The solid loading increased to approximately 45 wt % when the biomass is comingled with coal.