Design of a Screw Based Feed System for a Molten Salt Solar Gasification Reactor

Abstract

The objective of this study is to design a feed system to deliver carbonaceous feedstock into a solar gasification reactor that uses a ternary blend of carbonate salts at 1200 K as the reaction medium. The feed system must continually and reliably provide 3-15 g/min of microcrystalline cellulose to a 12.7 mm (1/2″) NPT port on the reactor and minimize the use of gas, as excess purge gas increases the sensible heating requirements of the reactor and can potentially create undesirable flow patterns within the molten salt. A screw based feed system was chosen for its ability to accurately provide material over the range of target feed rates. Advanced Manufacturing Systems provided a screw based extrusion system, and in this study an injector to transport feed from the screw system to the reactor was designed. The injector was then fabricated and tested in an apparatus that simulates important aspects of the reactor. The injector was designed through a combination of numerical and experimental studies. A numerical model was developed to simulate a porous plug advancing through a cylindrical injector into a molten salt environment. The 2-D axisymmetric model solved the species conservation and energy equations to yield transient temperature, composition, and morphology (porosity) distributions. The model was implemented for feed rates of 3-15 g/min and initial plug porosities between 0.1 and 0.5. The model determines, for a given mass flow rate, what inlet radius would keep the cellulose plug at a porosity low enough throughout the feed system to ensure the interface between the molten salt and cellulose is located within the reactor. An 8 mm radius was found to meet this criterion for all flow rates and initial porosities considered. The injector must therefore include a taper from the shaft diameter of 19 mm at the end of the screw to the 16 mm injector diameter at the outlet of the feed system. An experimental study of 3-D printed prototypes determined a 0.020 m long pipe with an 8.6° taper would allow feedstock to move freely through the injector. The final part was fabricated of stainless steel from these specifications. This part was tested with the screw feed system in an apparatus in which 200 µm diameter microcrystalline cellulose particles were fed to a water reservoir with the same hydrostatic pressure as the molten salt in the reactor. An additional design feature was developed to prevent water flow into the injector when the feed system is not in operation. This feature is intended to be used as the reactor is heated to operating temperature before feed begins. A combination of a perforated foil boundary and purge gas was shown to prevent leaking from the water reservoir for over 3 hours. Cellulose was fed into the reservoir at an estimated rate of 9 g/min. A plug of cellulose was able to advance through the boundary into the water reservoir, but after 170 seconds, feed was blocked. The blockage is attributed to the absorption of water by the cellulose. Further study is required to determine if this behavior will repeat in a molten salt environment. However, using a screw based feed system appears to be a valid approach to provide cellulose to a molten salt based solar gasification reactor, as numerical simulations show that cellulose can advance fast enough to prevent molten salt backflow and tests of the feed system demonstrated that cellulose can advance through the injector into a liquid environment.