(389c) Solar Gasification in a Molten Salt Reactor for Continuous Production of Syngas

The design and operation of a concentrated solar reactor to gasify biomass in a ternary eutectic blend of alkali metal carbonate salts are presented. Solar gasification has numerous advantages over conventional gasification. The use of concentrated solar energy as the source of process heat eliminates the need for partial combustion of the feedstock. The product synthesis gas is not diluted by excess CO₂ and N₂ nor contaminated by other combustion by-products. There is no need for an economically and energetically expensive oxygen plant that otherwise is necessary for oxygen-based partial combustion. The energetic value of the product is greater than that of the feedstock—the difference is equal to the storage of solar energy in the fuel. The benefits of the molten salt solar gasifier in comparison to other solar gasifiers are the thermal mass of the salt allows continuous operation during solar transients, provides rapid heat transfer, catalyzes the gasification reaction, and reduces the amount of tars and other contaminants in the product gas stream for an operating temperature of 1200 K. In prior work, utilizing cellulose as feedstock and carbon dioxide as oxidizer, the reactor achieved a solar efficiency of 30% and converted 47% of the carbon in a continuous process. The efficiency at 1218 K was comparable to the highest achieved in other solar gasifiers operated at higher temperature, but the pneumatic feed system was prone to blockage and required a stoichiometric oxidizer ratio of eleven. The use of excess carbon dioxide increased the sensible heating requirement and exacerbated the entrainment of fine char particles in the product gas stream prior to complete conversion.

In the present study, we present a model of an improved volumetric pumping feed system and provide operational data for gasification of cellulose in the improved reactor. The feed system consists of a single fluted screw within a straight barrel, similar to that used for polymer extrusion systems. The feed enters the reactor as a densified cylinder rather than as loose powder, encouraging the formation of larger char particles which have a reduced tendency to be entrained by the product gases leaving the reactor. The design of the feed delivery system is based in part on a numerical model of the transient heat transfer and kinetics of pyrolysis and gasification of the feedstock material during transport.

The reactor was operated for CO₂ gasification of cellulose in the University of Minnesota’s high flux solar simulator. The simulator consists of seven radiation units, each composed of a 6.5 kW xenon short arc lamp close-coupled to a custom reflector in the shape of a truncated ellipsoid of revolution. The total power delivered to the aperture of the reactor was measured using a water-cooled black body calorimeter of the same aperture diameter. The feedstock is ash-free microcrystalline cellulose (C₆H₁₀O₅) sieved to a particle size of 0.5 mm. Temperatures of the solar cavity and within the salt melt are measured using Type-K thermocouples. The product gas composition is measured using a Raman Laser Gas Analyzer. The downstream gas lines and HEPA filter are analyzed for residual secondary products. Tars are separated from solids using a solvent wash and the particle size distribution of the solid material is measured using laser diffraction.

The reactor performance is quantified by the solar to fuel efficiency defined in rate form as the ratio of the lower heating value of the useful products and the sum of the solar input and the LHV of the carbonaceous feedstock. Carbon conversion is given by the ratio of the net gaseous carbon released from the reactor to the feedstock carbon delivered to the reactor. The measured species flow rates are compared to the equilibrium flow rates predicted using Gibbs free energy minimization. The data demonstrate the improvements in performance for the reactor with the new feed system in terms of longer operation without blockage, and higher solar efficiency.