(389b) Syngas Production Via Solar Chemical-Looping Reformation of Methane in a Fixed-Bed Reactor

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.