Liquid fuels produced via the steam hydrogasification of biomass feedstock followed by downstream gas to liquid processes appears to be a cost effective approach to replace fossil fuels, to decrease the dependence of imported oil and to decrease greenhouse gas emissions. A critical technical obstacle of using biomass feedstock effectively in many gasification processes, including steam hydrogasification, is the need to prepare a high carbon concentrated (i.e., high energy) slurry that can be introduced into a pressurized reactor in a cost effective and energy efficient manner. Conventional dry feeding systems, such as lock-hopper and pressurized pneumatic transport, are complex, unreliable, and operationally expensive. An extra carrier gas is needed in some instances or vibrators to avoid fluctuations in the quantity of feed introduced into the reactor. Slurry feeding is a simpler, reliable, and inexpensive method of transporting and pressurizing the feedstock into gasification reactors and has been demonstrated in commercial scale application using 100% coal. Biomass slurries have less energy density compared to coal and require high carbon content in order to be efficiently gasified. The hygroscopic and hydrophilic nature of biomass results in a significantly reduced amount of carbon in pumpable slurries. The main goal of this thesis was to develop, design, and implement a Hydro-Thermal Pretreatment (HTP) process that will result in a pumpable biomass slurry with high energy and carbon content for use in a commercial scale, pressurized steam hydrogasification reactor.
The first objective of this thesis was to design, build and evaluate the performance of a laboratory scale HTP process. Four carbonaceous feedstocks (coal-wood-water, wood-water, wood-biosolids and wood-manure) were hydrothermally treated using a procedure developed as part of this thesis. The viscosity, flow and energy content were determined under various experimental conditions including: particle size; initial composition of feedstocks (carbon/water ratio), thermal input (time and temperature of the heating process) and head space gas composition as a consequence of heating. The rheology properties and the settling velocities of the particles for the resultant slurries were evaluated before and after HTP. An empirical model was developed to simulate the rheology properties. The modeling work was necessary to assist with predicting flow behavior of the slurry for commercial applications. The carbon balance for the cumulative gas, liquid and solid phases of the feedstock slurry after HTP was analyzed to determine the carbon recovery in the slurries and found to be greater than 98% recovery. Finally, the heating value of pretreated wood particles was determined to estimate the energy recovery in the slurries which was also found to be in excess of 98%.
The second objective of this thesis focused on obtaining a better understanding of the chemical mechanism of the formation of the biomass slurry as a consequence of the HTP. Four mechanisms and analytical methods were utilized to assist in the explanation: (1) Surface charge alternation, zeta potentials of raw wood particle and pretreated wood particle were analyzed and compared; (2) Particle shrink, Scanning Electron Microscopy (SEM) comparison of wood particles before and after HTP were compared to visualize this effect of HTP; (3) Free bulk water release from biomass microstructure, both SEM observation and liquid-solid distribution of slurry were used to confirm porous site generation and free water release into bulk phase.
The third objective of this thesis focused on the design and scale up of HTP at the lab scale basis, a demonstration scale basis and a commercial scale basis. The lab scale and demonstration scale HTP process was designed and tested in our laboratory. Feedstocks production rates were targeted at 90g/hr and 8kg/hr on a wet basis for lab and demonstration scales. Mass and energy balance of both processes were performed based on experimental data. An ASPEN Plus simulation of a commercial scale HTP process was done using a production rate of 16,700kg/hr on a wet basis. Comparison of economy and energy efficiency was performed between biomass gasification with or without HTP process using the ASPEN Plus results.