Biomass reforming processes in hydrothermal media

Abstract

While hydrothermal technologies offer distinct advantages in being able to process a wide variety of biomass feedstocks, the composition of the feedstock will have a large effect on the processing employed. This thesis characterizes the role of two major components of biomass, salts and proteins, that if dealt with intelligently can create valuable byproducts, but if dealt with inefficiently can cause processes to fail or run sub-optimally. In supercritical-water, most ionic species exhibit only sparing solubility. Thus, in the supercritical-water gasification of biomass, a supercritical-water reverse-flow vessel (RFV) may be employed to precipitate the salts from the biomass before the biomass is brought into contact with catalysts. However, these vessels are subject to blockage, and flow characterization within the vessels has largely been unknown. Therefore, the technique of neutron radiography is introduced to the supercritical-water processing field in order to characterize the behavior within these RFVs. This allows for non-invasive, in situ, real-time measurements of precipitation phenomena within thick-walled vessels. Two methods that provide complimentary information were developed to observe the precipitation behavior of salts from supercritical water. In the first method ("normalphase" radiography), the salts have a lower attenuation coefficient than the continuous 1H2O phase. This allowed for the visualization of major blockages, buildups of salts at the walls, and flow pattern changes. By using the strongly attenuating 1H2O form of water, fluid density changes resulting from flow pattern changes were also apparent.

(cont.) In the second method ("reverse-phase" radiography), strongly attenuating salts, such as those containing boron, were employed in a continuous phase of D2O, which has a weak neutron attenuation coefficient. In this manner, the onset of precipitation events could be seen with finer resolution. The neutron radiographical method was also employed to perform an elegant 1H2Oin-D2O tracer study of the fluid mechanics within the RFV, in which a stream of water that spans its critical temperature undergoes a reversing flow pattern. The flow was found to penetrate deep into the vessel and to mix thoroughly with the contents of the vessel, presumably because of the strong buoyant forces generated by heat at the walls. In the absence of wall heating, the flow was observed to reverse at a point much nearer to the top of the vessel. Also, as compared to comparable conditions at subcritical pressures, the supercritical jet was found to be more diffuse and to penetrate the vessel at a lower velocity. A jet entrainment model was derived to describe the flow within the vessel. Due to the near-critical conditions in the vessel, the model was developed without the Boussinesq approximation or the density deficiency formulation commonly applied to such models. The model was solved numerically and was found to provide excellent agreement with the neutron radiographical data for the two cases studied: an isothermal case and a case in which the temperatures spanned the critical point of water. In the isothermal limit, the model accurately predicted the jet reversal point.

(cont.) In the case with temperatures that spanned the critical point, the model accurately predicted that the jet would not fully reverse before reaching the bottom of the vessel. The model also predicted the characteristic decay time of a tracer within the vessel with near-perfect accuracy. Proteins are a second constituent of biomass that require special treatment in the development of hydrothermal processes. Industrial and research laboratory reports have indicated that the presence of proteins may create processing conditions that lead to equipment fouling and reduced gasification yields in hydrothermal processing. In studies on the interaction between glycine and glucose as model compounds at 250C and 10 MPa, strong kinetic and qualitative evidence is presented that a Maillard-type reaction occurs in hydrothermal processing. This reaction, which is very common at the lower temperatures encountered in food and medicinal chemistry, is known to lead to the formation of polymeric material that may be desirable for color and flavor generation in food processing, but is generally undesirable in hydrothermal processing, since these polymers will act to foul process equipment. Glucose and glycine were found to strongly influence the reaction pathway of the other compound, and the resulting reactor effluent had UV absorbances typical of Maillard reaction products. Compounds with the same functional groups, a primary amine and a carbonyl group, were substituted for glucose and glycine and were found to have similar effects.

(cont.) Also, the degradation pathway of glucose was found to be altered, with the significant product hydroxymethylfurfural being suppressed with the addition of glycine. The reactive form of glucose is the acyclic, aldehydoglucose form. Glucose can also exist as an unreactive, cyclic form, which dominates at room temperature, making up over 99.99% of the aqueous equilibrium composition. However, this equilibrium is unknown at elevated temperatures and pressures. A preliminary computational chemistry study was undertaken to predict this equilibrium under hydrothermal conditions. The acyclic form was predicted to rise to be of comparable prevalence to the cyclic forms. The study indicates that this was caused by two main effects, a change in the relative stability imparted by the decreased dielectric constant of water, and by an entropic effect in which the much larger number of conformers that are possible in the acyclic state become more energetically populated at higher temperatures.