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Solar Energy 142 (2017) 224–230
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Demonstration of a prototype molten salt solar gasification reactor
Brandon J. Hathaway, Jane H. Davidson ⇑
Department of Mechanical Engineering, University of Minnesota, 111 Church St SE, Minneapolis, MN 55455, United States
a r t i c l e
i n f o
Article history:
Received 19 September 2016
Received in revised form 14 December 2016
Accepted 15 December 2016
Molten salt
a b s t r a c t
In the present work, a prototype molten salt solar gasification reactor was demonstrated at the 2.2 kW
scale using simulated concentrated solar radiation. The molten alkali carbonate salt offers the benefits
of improved heat transfer, catalysis of gasification, reduced production of tars, and thermal stability for
transient solar input. 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 at 1218 K. Based on an
energy balance on the reactor, we project efficiencies approaching 55% with future improvements to the
Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction
Solar thermochemical processes offer several routes to providing fuels and chemicals traditionally obtained from petroleum at
a greatly reduced carbon footprint. Among the various solar thermochemical processes, the use of solar energy to drive the gasification of carbonaceous materials is considered a near-term option
given the relatively mild process temperatures involved and the
potential for solar to fuel efficiency above 50%.
Considering cellulose as a surrogate feedstock representative of
biomass, the ideal stoichiometric gasification reactions are given in
Eqs. (1) and (2) for steam and carbon dioxide oxidizers,
diluted by excess CO2 and N2 nor contaminated by other combustion by-products, and there is no need for an economically and
energetically expensive oxygen plant.
With these motivating benefits, a number of solar gasification
reactors have been developed. Recent reviews of these efforts are
reported by Puig-Arnavat et al. (2013) and Piatkowski et al.
(2011) The assessment of solar gasification reactors is conventionally reported in terms of solar to fuel efficiency and carbon conversion. In the present work, we define the reactor efficiency in rate
form as the ratio of the lower heating value (LHV) of the useful
products to the energy input, which is the sum of the solar input
and the LHV of the carbonaceous feedstock:
n_ i LHVi
C6 H10 O5;ðsÞ þ H2 OðgÞ ! 6 ðCOðgÞ þ H2;ðgÞ Þ
greactor ¼ _ i¼fCO;H2 ;CH4 g
Q solar þ n_ feed LHVfeed
C6 H10 O5;ðsÞ þ CO2;ðgÞ ! 7 COðgÞ þ 5 H2;ðgÞ
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 solid feedstock is converted into a gaseous blend of carbon
monoxide and hydrogen, which is referred to as synthesis gas due
to its potential for subsequent synthesis of chemicals and fuels. The
process is endothermic, effectively allowing storage of solar energy
in the upgraded energy content of the synthesis gas relative to the
Based on chemical thermodynamics, all of the biomass is converted to useful synthesis gas by supplying the required thermal
energy at approximately 1200 K from a concentrated solar source.
By avoiding partial combustion of the feedstock, the product is not
⇑ Corresponding author.
E-mail address: [email protected] (J.H. Davidson).
0038-092X/Ó 2016 Elsevier Ltd. All rights reserved.
XC ¼
nCO þ nCO2 þ nCH4 nCO2 ;in
Reactors can be categorized generally by batch or continuous
fuel production. The packed bed reactors developed at Lawrence
Livermore Laboratory (Gregg et al., 1980; Taylor et al., 1983) and
the Paul Scherrer Institute and ETH Zurich (Piatkowski et al.,
2009; Wieckert et al., 2013) are batch reactors. These reactors
achieved efficiencies from 15% to as high as 35% (Wieckert et al.,
2013) but exhibited issues with condensable tars or other secondary products in the product gas stream which, in addition to
unreacted material remaining within the reactor, resulted in
B.J. Hathaway, J.H. Davidson / Solar Energy 142 (2017) 224–230
area [m2]
flux concentration ratio [–]
molar enthalpy [J mol1]
standard insolation [1000 W m2]
lower heating value [J mol1]
number of moles [mol]
molar flow rate [mol s1]
heat transfer rate [W]
temperature [K]
carbon conversion [–]
volume or mole fraction [–]
emissivity [–]
efficiency [–]
stoichiometric oxidizer ratio [–]
Stefan-Boltzman constant [5.67 108 W m2 K4]
apparent value
absorption efficiency
carbon conversions from 27 to 73% (Wieckert et al., 2013). Additionally, the presence of residual ash retarded gasification of the
remaining feedstock in the case of the directly absorbing reactors
(Gregg et al., 1980). The entrained flow (Melchior et al., 2009), vortex flow (Z’Graggen et al., 2007, 2006), and fluidized bed (Cho et al.,
2015; Taylor et al., 1983) reactors are continuous reactors. At
1500 K, these reactors yielded a low-tar product stream and
achieved rapid rates of conversion. However, they require significant amounts of carrier/fluidization gases to deliver the feedstock
and maintain the feed entrainment or bed fluidization. Thus, efficiencies were low, 0.5–10%, despite high levels of conversion. Most
recently Kruesi et al. (2014) demonstrated a new two-zone reactor
design consisting of a drop-tube pyrolysis zone and a trickle bed
gasification zone. The reactor achieved efficiencies up to 21% with
carbon conversion near 90%.
In the present work, we characterize a new solar reactor in
which the pyrolysis and gasification reactions of Eq. (2) are carried
out in a ternary blend of molten alkali carbonate salts and operation is continuous. The molten salt offers several advantages over
operation in a gaseous environment. The relatively high effective
thermal conductivity of the salt (0.87 W m1 K1) allows for rapid
transfer of heat to the feedstock. The thermal capacity of the salt
(1840 J kg1 K1) provides stable operation through solar transients (Hathaway et al., 2013b), and the catalytic activity of the
alkali metal cations accelerates carbon gasification and encourages
tar cracking reactions (Adinberg et al., 2004; Iwaki et al., 2004; Jin
et al., 2005; Ratchahat et al., 2015). Improved rates and yields from
gasification reactions have been demonstrated at the bench-scale
for coal (Matsunami et al., 2000; Yoshida et al., 1999) and a variety
of biomass materials, including wastepaper, wood waste, corn
stover, switchgrass, and perennial blends (Adinberg et al., 2004;
Hathaway et al., 2011, 2013a; Iwaki et al., 2004; Jin et al., 2005).
In a crucible heated in an electric furnace, gasification in the molten salt increased the overall gas yield from cellulose by up to 25%,
doubled the rates of pyrolysis rates, and increased the rates of carbon gasification tenfold compared to gasification in a gaseous
media (Hathaway et al., 2011). The reactivity index, defined as
the inverse of twice the time needed to reach 50% carbon conversion, increased by as much as 600% for corn stover. Condensable tar
measured value
pertaining to the reactor aperture
pertaining to the reactor cavity
Carbon Monoxide
Carbon Dioxide
pertaining to the solid feedstock
gaseous phase
general index variable
inlet mass flow
convection and conduction losses to the ambient
outlet mass flow
radiative losses to the ambient
thermal efficiency of the solar reactor
carbon remaining within the reactor
solid phase
amount of oxidizer predicted for stoichiometric reaction
was reduced by 77% (Hathaway et al., 2013a). Additionally, the
improved reaction rates have been found to be insensitive to the
presence of ash in the melt at concentrations up to 20 wt.%
(Trilling, 1977).
The performance of the molten salt reactor is demonstrated in
an indoor high-flux solar simulator for carbon dioxide gasification
of cellulose at 1218 K. Data include temperatures within the reactor, product gas composition, carbon conversion, and solar efficiency. The data are interpreted to identify areas for future
improvement in the reactor.
2. Experimental
2.1. Reactor prototype
The nominal 3 kW prototype reactor (Fig. 1), absorbs concentrated radiation to drive steam or CO2 gasification of biomass in
a ternary eutectic blend of molten alkali-carbonate salts at
1200 K and near-atmospheric pressure. The reactor is a concentric
cylinder arrangement. The inner cylinder is a cavity receiver with a
front aperture. The outer cylinder bounds an annular volume
which contains the molten salt and reacting biomass. It is constructed of Inconel X-750 alloy to provide strength and resistance
to creep as well as resistance to corrosion by the molten alkali carbonate salts (Coyle et al., 1986).
The receiver cavity is cylindrical with a 150 mm length, 100 mm
inside diameter and 3 mm wall thickness. Concentrated solar radiation enters the cavity through an open aperture of 50 mm diameter (left side of Fig. 1(a)). The aperture was sized to intercept
3 kW of incident thermal radiation at a nominal average flux of
1500 kW m2. Based on Monte-Carlo ray tracing radiative
exchange simulations of the cavity and accounting for the spectral
variations of reflectance of Inconel X-750 (Touloukian and DeWitt,
1979), the apparent absorptivity of the cavity receiver is 98% at
1200 K (Hathaway et al., 2012). The far end (right side in Fig. 1
(a)) of the cavity is not in contact with the molten salt. To minimize
absorption on this surface, the rear end-cap is coated with alumina,
applied using a plasma spray, to achieve a diffuse solar weighted
hemispherical reflectivity of 80% as measured by UV–VIS
B.J. Hathaway, J.H. Davidson / Solar Energy 142 (2017) 224–230
Product gas outlet
radiation shield and
insulation mount
Rear insulation
Molten salt
Salt drain
Feed auger
Fig. 1. Isometric views of the gasification reactor with (a) side-cutaway to reveal key internal features and (b) axial-cutaway at the injector plane to illustrate feed delivery
system and exit disengagement diffuser. Insulation material not shown. Concentrated radiation enters the central cylindrical cavity receiver through the aperture. The
annular gap between the receiver and the outer reactor housing is filled with molten salt. Feed is delivered from the bottom and product gas exits at the top.
spectrophotometry over wavelengths from 250 to 2500 nm. A
water-cooled stainless steel radiation shield with a conical frustum
of 45° half-angle is attached to the front flange of the receiver/reactor. This angle was selected to slightly exceed the 37.5° half-angle
of convergence of the incoming radiation from the University of
Minnesota high-flux solar simulator (Krueger et al., 2013).
The housing, which surrounds the inner cavity receiver, has a
length of 159 mm. The housing mates with the front flange and
the rear end of the cavity, creating a sealed 108 mm ID by
188 mm OD annular volume of 3.0 L to hold the molten salt. Heat
is transferred from the cavity surface to the feed material via convection and radiation within the salt. The flow field within the salt
melt is driven by the combination of natural convection due to the
hot inner absorber walls and cooler outer housing walls and forced
convection from motion of gases injected at the feed inlet as well
as those generated from the reacting feed. In the development of
the prototype, a coupled CFD/FEA analyses of the cavity was performed to ensure adequate heat transfer from the receiver cavity
to the molten salt and to avoid mechanical failure due to thermal
stresses (Hathaway, 2013; Hathaway et al., 2014). For the reactor
demonstration, temperatures were measured within the salt melt
using Type-K (Ni-Cr, Ni-Al) thermocouples (±9 K) at three positions: 10 mm below the top surface of the melt, and two positions
centered 20 mm from the cavity or annulus walls on either side of
the annulus at the midplane of the receiver/reactor. The temperature of the surface of the cavity was measured with two Type-K
thermocouples placed at the midplane on the annulus-side. Feedstock enters through a vertically-aligned port on the bottom of
the reactor and product gases exit from the top port. A drain port
is located at the bottom rear of the reactor. All external surfaces
of the reactor are insulated with a 51 mm thick layer of refractory
ceramic fiber blanket with a thermal conductivity of 0.25 W m1 K1 and emissivity of 0.3 to reduce losses (Unifrax I LLC, 2010).
The feedstock delivery system, shown in Fig. 1(b), consists of a
wedge-shape hopper, auger-driven feed delivery passage, and a
particle entrainment injector. The hopper holds up to 2000 g of
solid feed. In the present study, the feedstock is ash-free microcrystalline cellulose (C6H10O5) sieved to a particle size of 0.5 mm.
The cellulose particles exit the hopper and are moved to the injector via a horizontal auger metering conveyor. Within the injector,
the solid feedstock is entrained in a vertical flow of oxidizer gas
through a 3 mm diameter passage exiting into the molten saltfilled annulus at the bottom of the reactor. The oxidizer gas, which
is CO2, is fed through the injector along with the cellulose to prevent back-flow of salt into the injector passage. The oxidizer flow
rate was selected to provide sufficient velocity for feedstock
entrainment within the injector. The flow rate of feedstock is controlled through the rotational speed of the delivery/metering auger
and the average feed rate is determined through measured mass
change in undelivered feed before and after operation.
The product gases exit the salt from the upper surface of the
melt, and then flow upward through the outlet port on the top of
the housing into a particle disengagement region. In the conical
disengagement region, the diameter of the flow passage increases
from 18 mm to 74 mm. The disengagement diameter was selected
to provide a gas velocity that would limit particle elutriation, or
loss, to those having achieved >90% conversion (Zenz and Weil,
1958). The velocity required was calculated assuming gasification
proceeds as a volumetric reaction, resulting in the formation of
an internally porous char particle, with porosity increasing along
with the extent of reaction. Overall particle size was assumed to
remain at 0.5 mm, as no information was available a-priori on
the morphological changes to the cellulose particles during
The salt is a ternary eutectic blend containing lithium, potassium, and sodium carbonate (Table 1). This blend was selected
for several reasons including the reduced melting point (670 K as
compared to 970 K without lithium carbonate), reduced corrosion
Table 1
Composition and properties of salt blend at 1200 K (Janz, 1967; Janz and Lorenz,
1961; Makino et al., 1992).
Composition [wt.%]
Material thermal conductivity
Effective thermal conductivity
Absorption coefficient
Specific Heat capacity
Melting point
32% Li2CO3
33% Na2CO3
35% K2CO3
0.75 W m1 K1
0.87 W m1 K1
8890 m1
1842 J kg1 K1
670 K
1680 kg m3
B.J. Hathaway, J.H. Davidson / Solar Energy 142 (2017) 224–230
of stainless steel surfaces when lithium is present (Coyle et al.,
1986), and evidence of superior catalysis from lithium cations
(Jin et al., 2005). The effective thermal conductivity of the salt is
enhanced at the operating temperature of 1200 K due to participation with radiation. Using a diffusion approximation for the radiative contribution to conductivity within the salt, the effective
thermal conductivity is 0.87 W m1 K1, 15% greater than simple
conduction (Modest, 2003).
2.2. Procedure
The experimental facility is illustrated in Fig. 2. The reactor was
positioned horizontally on-axis with the solar simulator lamp
array. 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
(Krueger et al., 2013). The total power delivered to the aperture
of the reactor was measured using a water-cooled black body
calorimeter of the same aperture diameter.
Prior to the test, the reactor body is heated by electric resistance
heaters to 720 K. Then carbon dioxide flow through the feedstock
injector is initiated at 4770 lmol s1 and solid pre-mixed salt chips
are introduced through the outlet port to the annular volume to
provide a salt load of 2.5 kg, corresponding to 1.5 L of liquid. The
liquid volume is 50% of the annular volume, but the gas holdup, or increase in apparent volume of a bubble-laden liquid,
increases the volume by 29%. After salt loading is complete, the
outlet gas handling connections are installed, electric heaters are
turned off, and the reactor is heated by the solar simulator. Onsimulator heating continues without feed delivery until the salt
is at 1200 K. Input power was adjusted as needed to stabilize temperatures and prevent any single temperature measurement from
exceeding a limit temperature of 1240 K. Feedstock delivery is
implemented over the course of two step changes of 120-s duration to ensure normal operation prior to setting the final feed rate
of 73 mg s1. During feeding, a 34 lmol s1 flow of nitrogen is provided to maintain a slightly elevated pressure in the hopper and
prevent backflow through the feed auger. The relative flow rates
of carbon dioxide and feedstock result in a stoichiometric oxidizer
45 kWe High Flux
Solar Simulator
ratio of / = 11. The stoichiometric oxidizer ratio is the ratio of the
actual oxidizer provided to the amount required for the ideal stoichiometric reaction of Eq. (2).
While operation with a value of / > 1 is required for the current
pneumatic feed injection approach, it presents two drawbacks.
About 200 W of additional sensible heating are required for the
non-participating portion of the CO2 flow, and gas velocities are
higher throughout the reactor, resulting in lower gas-phase residence times and increased likelihood of particle elutriation.
The product gas is diluted and cooled by 2690 lmol s1 of nitrogen gas to prevent further gas-phase reactions and to reduce residence time to the Raman Laser Gas Analyzer (RLGA). The gas flow
is then cooled to sub-ambient temperature in a water-jacketed
condenser after which a 168 lmol s1 sampling side stream is
passed through a HEPA filter to ensure any condensable tars or
moisture are removed prior to gas analysis. The rate of gas production of each species, i, is calculated using the known molar flow of
nitrogen for reference, assuming it remains inert.
n_i ¼
n_ N2
Using the measured gas production rates, the reactor performance is quantified in terms of the reactor efficiency of Eq. (3)
and the carbon conversion given in Eq. (4).
The downstream gas lines and HEPA filter are analyzed for
residual secondary products. All components are washed with
methanol to dissolve any tars and rinse out solid particulate present. The used methanol is run through a glass fiber filter to separate solid particulate from dissolved tars. The solvent is then
evaporated to allow measurement of any residual tars, while the
filter is massed to measure solids collected.
3. Results
Simulated solar power and temperatures of the reactor surfaces
are plotted in Fig. 3(a) from t = 0 s when the flow of cellulose into
V_ CO2 ;actual
V_ CO ;stoich
To Vent
Raman Laser
Gas Analyzer
Thermocouple Placement Detail:
Front and Side Cross Sections
Fig. 2. Schematic diagram of the facility for characterization of the reactor.
B.J. Hathaway, J.H. Davidson / Solar Energy 142 (2017) 224–230
the molten salt was initiated until the test was terminated at
t = 1420 s. Material flows into and out of the reactor are shown
in Fig. 3(b). The feed rate was increased in steps and then held
steady after t = 240 s at 73 mg s1. Input power was decreased to
2.6 kW at t = 380 s and to 2.2 kW at t = 820 s to avoid exceeding
the temperature limit of 1240 K. The salt temperature at the top
of the reactor stabilized at 1230 K, while temperatures at the
mid-plane of the reactor were about 10 degrees cooler. The higher
temperature measured at the top of the salt cavity is not representative of the actual salt melt temperature, but rather attributed to
the presence of gas pockets that slowed heat removal from the top
portion of the cavity and allowed direct radiative exchange
between the cavity surface and the thermocouple probe. From
t = 400 s to 1420 s, the temporally and spatially averaged salt temperature was 1218 K. At t = 1420 s, the test was terminated due to
an interruption in feedstock delivery attributed to blockage in the
injector, reflected by the fluctuations in temperature and an
observed rise in injector pressure during operation. Despite this
issue, the test provides meaningful initial measurement of performance of the reactor and demonstrates the advantages of the molten salt.
The material flows in Fig. 3(b) show yield rates of CO, H2 and
CH4 increasing concordant with increasing feed delivery rate, while
the outlet flow of CO2 decreases from nearly equal to the inlet flow
rate of 4770 lmol s1 to 3360 lmol s1 as feedstock is converted
into char. The peak rate of production of useful fuel (the sum of CO,
H2, and CH4 flow rates) is 3560 lmol s1 at t = 400 s. The fuel pro-
Salt, Midplane
Cavity, Midplane
Solar Power [kW]
Temperature [K]
Salt, Top
Time [s]
duction rate declined slightly during operation, potentially caused
by feed residue building up in the injector. However, the composition of the product gas remained stable throughout the
Using the time averaged gas flow rates and solar power levels
from t = 400 s to 1420 s to quantify reactor performance, the average reactor efficiency is 30% and the average carbon conversion is
47%. The measured efficiency is on par with the highest values previously reported for a batch reactor (Wieckert et al., 2013) and
higher than the maximum 10% and 21% reported for continuous
vortex flow (Z’Graggen et al., 2007) and two-zone (Kruesi et al.,
2014) reactors. Carbon conversion was lower than the 73–90%
reported in prior reactor publications, highlighting a focus for
future refinement efforts (Kruesi et al., 2014; Wieckert et al., 2013).
Fig. 4 compares the time-averaged measured species flow rates
to the equilibrium flow rates predicted at 1 atm and 1218 K using
Gibbs free energy minimization. Carbon monoxide was produced
at a rate 51% slower than predicted for equilibrium, while carbon
dioxide was released at a rate 11% faster. A small flow of
336 lmol s1 of methane was also observed while equilibrium
predicts only trace amounts. In agreement with the calculated carbon yield value, the excess carbon dioxide and methane were not
sufficient to account for the low level of carbon monoxide present
given the inlet flow of carbon from feedstock and oxidizer.
The amount of hydrogen present in the methane produced
accounts for the lower than equilibrium hydrogen production.
These observations indicate that initial pyrolysis of hydrogencontaining cellulose reached completion. However, the measured
47% carbon yield indicates that the gasification of carbon char
was not complete.
Incomplete char gasification is supported by the presence of
carbonaceous particulate collected downstream from the particle
disengagement diffuser. The particle size distribution of this material was measured using laser diffraction and is presented in Fig. 5.
The mean particle size was 37 lm with >99% of the collected particles having size less than 500 lm. This distribution indicates that
at least some of the partially reacted feed material broke down into
particles sufficiently small to allow passage through the particulate
disengagement diffuser. The amount of char collected was roughly
3 g, but based on a mass balance we infer that additional carbon
was entrained in the unfiltered exhaust flow. The use of larger feed
particles may result in larger char particles with decreased tendency for elutriation from the reactor outlet at the expense of a larger volume fraction of char present in the salt melt. On the other
hand, smaller feed particles offer faster reaction rates and thus
CO2 (Out)
Avg Gas Flow Rate [std L min-1]
Gas Flow Rate [μmol s-1]
CO2 (In)
Time [s]
Fig. 3. Transient performance of the reactor. (a) Thermal behavior with temperatures of the salt melt and cavity walls referenced to the left ordinate and incident
radiative power at the aperture is referenced the right ordinate. (b) Molar species
flow rates during the reactor test.
Fig. 4. Comparison of product gas yield rates to equivalent values based on
equilibrium from 400 s to 1400 s.
B.J. Hathaway, J.H. Davidson / Solar Energy 142 (2017) 224–230
The absorption efficiency gabs is a function of the cavity temperature, apparent emissivity of the cavity, and the flux intensity at
the aperture.
Volume Density [%]
gabs ¼ ea 1 3
Parcle Size [μm]
Fig. 5. Particle size volume distribution for material collected downstream of the
particle disengagement diffuser.
shorter required residence times, potentially enabling full conversion. A tradeoff for obtaining smaller feed particles is increased
feedstock pre-treatment energy costs. The mass of condensed tars
collected from downstream surfaces and filters were below detectable limits of the mass balance.
4. Discussion
The characterization of the reactor demonstrates the ability of
the molten salt process to yield high efficiency. We expect higher
efficiency and improved reliability with redesign of the feedstock
delivery system and potentially internal features of the reactor
upstream of the gas outlet. A feed system that does not rely on
pneumatic entrainment is desired to allow oxidizer flow to be
reduced to a stoichiometric level, which will eliminate 200 W
of sensible heating losses, while also reducing the outlet gas velocity and thus elutriation of unreacted material from the reactor.
Other internal features will be considered to enhance the wetting
of the char particulate by the salt and/or to increase the solids residence time to allow the char to achieve near complete conversion
as was observed in earlier molten salt gasification experiments.
To put in perspective the gain in efficiency expected with an
improved reactor system, an energy balance on the reactor is
applied using the reactor design conditions with two assumption
differing from the demonstration conditions. First, the char is
assumed to remain in the reactor until complete conversion is
achieved, and second, the carbon dioxide feed rate is reduced to
a stoichiometric oxidizer ratio of 1. The steady-state energy balance on the reactor given in Eq. (7) is applied to determine the
desired material throughput for a specified input solar power at
the aperture.
0 ¼ Q_ solar Q_ rad Q_ loss þ
n_ i;in hi;in n_ j;out hj;out
The first term on the right hand side is the design incident solar
power at the aperture.
Q_ solar ¼ ICAap
The reactor was designed to operate at an average flux concentration, C, of 1500 suns at the aperture of area Aap , providing a solar
input of 3 kW. The term I represents the standard flux intensity of
1000 W m2. This value of solar input power represents power
delivered at the aperture to the reactor. It does not include optical
losses of a hypothetical concentration field or the high-flux simulator used in the present study. The second term is the rate of
losses due to reflected and emitted radiation from the receiver
Q_ rad ¼ Q_ solar ð1 gabs Þ
rT 4cav
The target reactor temperature is 1200 K to achieve >99% conversion of carbon into gaseous species at equilibrium as observed
in earlier molten salt gasification trials (Hathaway et al., 2011).
We assume negligible difference in temperature between the cavity and the salt for the purposes of this estimate. The modeled
absorption efficiency at these conditions is 90%. The third term
represents conduction/convection losses from the reactor body to
the ambient, which were calculated to be 1.8 kW during the prototype demonstration at the slightly higher operating temperature of
1218 K. The same rate of losses is assumed here for a conservative
performance estimate.
The last two terms represent the enthalpy flows corresponding
to reactants entering the reactor and products exiting the reactor.
The two assumptions mentioned earlier regarding feed conversion
and reduced oxidizer flow impact the energy balance via these two
terms. Inlet components include the cellulose feedstock and the
carbon dioxide oxidizer, while outlet species include CO, H2, and
trace (<1%) amounts of H2O and CO2 in a distribution corresponding to chemical equilibrium. The design feed rate of cellulose for
operation at 3 kW is set by iteratively solving Eqs. (7)–(10) assuming a stoichiometric oxidizer ratio of unity and enforcing chemical
equilibrium. Equilibrium was calculated for the inlet flow of feedstock and oxidizer at isothermal and isobaric conditions of 1200 K
and 1 bar using Gibbs free energy minimization. A feedstock flow
rate of 155 mg s1 produces 6520 lmol s1 of carbon monoxide
and 4640 lmol s1 of hydrogen. At these conditions the projected
reactor efficiency, based on Eq. (3) and assuming full feed conversion at the target feed rate determined from the energy balance, is
55%, higher than other solar reactors demonstrated to-date and
consistent with the results of prior bench-scale molten salt gasification experiments (Hathaway et al., 2011, 2013a).
5. Conclusion
The benefits of utilizing molten carbonate salts as a reaction
media in a prototype solar gasification reactor have been demonstrated during continuous CO2 gasification of cellulose in an indoor
solar simulator. At 1218 K the reactor achieved a 30% solar efficiency and converted 47% of the feedstock carbon into product
gases. The demonstrated efficiency is comparable to the highest
achieved in other solar gasifiers that were operated at higher temperature. This encouraging result is due to catalysis and rapid heat
transfer provided by the molten salt. Carbon conversion was lower
than reported for other solar gasifiers due to the fact that fine char
particles with a mean diameter of 37 lm exited the reactor in the
product gas stream before conversion. The loss of char could be
due to a number of factors including entrainment in the flow of
CO2 from the pneumatic feed injector or inadequate wetting of
the particulate by the carbonate salts.
An energy balance suggests efficiency can reach 55% if the loss
of char fines is eliminated. Our current plans are to redesign the
inlet to the reactor to reduce the molar flow rate carbon dioxide
(or steam in the case of steam gasification) from a stoichiometric
oxidizer ratio of 11 to a value close to 1. We plan to replace the
pneumatic injection system with a mechanical feed system.
Although in prior work, we explored the fluid mechanics in the
molten salt based on natural convection, it will be beneficial to
consider the more complex fluid mechanics of the particle and
gas interaction in the molten salt. It may be beneficial to add inter-
B.J. Hathaway, J.H. Davidson / Solar Energy 142 (2017) 224–230
nal features to the reactor to encourage particle wetting and/or discourage elutriation.
Development of the prototype gasification reactor was funded
by a grant from the Initiative for Renewable Energy and the Environment (IREE). The authors would like to acknowledge the contributions of Amey Kawale to the design of the feed system and Jasper
Adamek-Bowers to the design of the particle disengagement section of the reactor. We also thank Professor David Kittelson for
his contributions towards the design of the prototype reactor.
We thank Dr. Peter Krenzke, Dr. Stephen Sedler, Jasper AdamekBowers and Amey Kawale for their assistance during experimental
testing of the reactor.
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