The production of syngas using concentrated sunlight via solar chemical-looping reforming (SCLR) is an approach to produce sustainable transportation fuels. SCLR is a two-step thermochemical cycle that produces separate streams of syngas and high-purity H2. In the endothermic partial oxidation step, methane reacts with oxygen released from a metal oxide to produce syngas. In the exothermic water-splitting step, steam reacts with the reduced metal oxide to produce H2. Solar energy is stored in the products, upgrading the calorific content of the feedstock by up to 28%. Combustion of the products emits 41% less carbon per kilojoule than combustion of the productions from non-solar CLR. The cycle is thermodynamically favorable at 1273 K and 1000 suns, conditions easily achievable by commercial solar concentrated optics used currently for electricity production. Based on a thermodynamic analysis of the process, the solar-to-chemical efficiency for SCLR, defined as the net increase in the higher heating value of the gases divided by the solar thermal input to provide process heat and parasitic work for gas pumping and separation, can reach 54% with complete conversion of CH4 to syngas and H2O to H2.
While the net products and chemical energy requirements match that of solar steam reforming, decoupling the partial oxidation and H2O-splitting steps provides numerous advantages. The products of SCLR are separate streams of 2:1 H2:CO and high-purity H2 compared to the single stream of 3:1 H2:CO obtained from steam reforming. The 2:1 H2:CO syngas matches the desired composition for the synthesis of liquid hydrocarbon fuels and H2 is valuable as a fuel for electricity production via fuel cells or as a chemical feedstock in the synthesis of various chemical commodities such as ammonia. Furthermore, conventional steam reformers must operate with excess oxidizer (e.g. H2O/CH4 ≥ 3) to prevent catalyst deactivation by carbon deposition. With solar CLR, carbon deposition can be prevented by limiting the reduction extent of the metal oxide.
In this paper, we present measured performance for SCLR in a prototype reactor that utilizes cerium dioxide (ceria) as the oxygen carrier material. The reactor is operated in the University of Minnesota high flux solar simulator. The reactor comprises two sections: a solar receiver/reactor and a heat recuperator. Concentrated sunlight enters the cylindrical receiver/reactor cavity section through an open circular aperture and heats a sealed assembly of concentric alumina tubes, referred to as the reactive element. The annulus of the reactive element is filled with 336 g of ceria particles with a bed void fraction of 45%. The particles are 5 mm (length and diameter), 78% porous and have a specific surface area of 0.114 m2 g-1. The reactive element extends beyond the solar cavity to the heat recuperator. In the heat recuperator, the annulus and inner tube of the reactive element are filled with alumina reticulated porous ceramic that is 85-90% porous and has pore densities of 5 and 10 pores per inch in the inner tube and annulus, respectively.
The cycle is accomplished by alternating the gas feed to the reactive element between CH4 and H2O. During the endothermic step, CH4 flows through the inner tube, reverses direction at the closed end, and flows over the ceria particles in the annulus. Sensible heat from the product gases is transferred to the inlet flow in the heat recuperator. At the end of the partial oxidation step, the gas flow is switched automatically to H2O. The operating conditions (temperature, reactant feed rates, reaction times) are selected based on a thermodynamic process model of the reactor. The model predicts CH4 and H2O conversions and H2 and CO selectivities data from prior benchtop experiments.
Measured results include radiative input, transient temperatures of the solar cavity, reactive element, and inlet and outlet gases, upstream and differential pressures across the reactive element, and product gas composition. The axial temperature distribution along the top surface of the reactive element in the receiver/reactor and surface temperature of the reactor cavity are measured by alumina-sheathed type-B (Pt-30% Rh/Pt-6% Rh) thermocouple probes. The axial temperature distribution along the reactive element in the heat recuperator section and the gas temperatures at the inlet and outlet of the active reactive element are measured with type-K (Ni-Cr/Ni-Al) thermocouples. The high-flux solar simulator is controlled to obtain a cycle-averaged reactive element temperature of 1273 K. The radiative solar input is measured using a water-cooled black body calorimeter. The product gas composition is measured at 1 s intervals using Ramen laser gas spectroscopy. Data are evaluated to determine the solar-to-chemical efficiency of the reactor. The reactor demonstration provides an important proof of concept for SCLR and a comparison of measured efficiency to prior theoretical predictions of efficiency.
See more of this Group/Topical: 2017 International Congress on Energy