Development of a Molten Salt Reactor for Solar ... · Solar gasification has the benefits of maximizing the yield of synthesis gas from gasification of biomass and other carbonaceous

Energy Procedia 49 ( 2014 ) 1950 – 1959

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1876-6102 © 2013 J.H. Davidson. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).Selection and peer review by the scientifi c conference committee of SolarPACES 2013 under responsibility of PSE AG. Final manuscript published as received without editorial corrections. doi: 10.1016/j.egypro.2014.03.207

SolarPACES 2013

Development of a molten salt reactor for solar gasification of biomass

B. J. Hathawaya, D. B. Kittelsona, J. H. Davidsona,* aDept. of Mechanical Engineering, University of Minnesota, 111 Church St SE, Minneapolis MN 55455, USA

Abstract

Solar gasification has the benefits of maximizing the yield of synthesis gas from gasification of biomass and other carbonaceous feed stock and storing solar energy in chemical form. The University of Minnesota has developed a 3kWth prototype solar gasification reactor in which biomass is converted to synthesis gas within a molten carbonate salt. The salt serves as a catalyst for gasification, ensures effective transfer of heat to the reactants, and provides thermal storage to ensure steady operation. This paper describes the reactor concept and provides an overview of the design process. © 2013 The Authors. Published by Elsevier Ltd. Selection and peer review by the scientific conference committee of SolarPACES 2013 under responsibility of PSE AG.

Keywords: solar; thermochemistry; gasification; molten salt; biomass; reactor design

1. Introduction

With the ever growing demand for liquid fuels and the looming peak in traditional petroleum production, developing new routes of liquid fuel production is becoming increasingly important. Gasification of solid fuels to produce synthesis gas is an important first step in many proposed biomass-to-liquids (BTL) or coal-to-liquids (CTL) fuel production pathways. The process of gasification is endothermic, requiring energy input, and in the conventional approach to gasification, this energy is derived from the partial combustion of a portion of the feedstock or the product synthesis gas [1]. An alternative approach to gasification, which avoids partial combustion,

* Corresponding author. Tel.: +1-612-626-9850; fax: +1-612-625-6069.

E-mail address: [email protected]

© 2013 J.H. Davidson. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).Selection and peer review by the scientific conference committee of SolarPACES 2013 under responsibility of PSE AG. Final manuscript published as received without editorial corrections.

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B.J. Hathaway et al. / Energy Procedia 49 ( 2014 ) 1950 – 1959 1951

is the use of concentrated solar energy to drive the gasification reactions. Compared to conventional synthesis gas, solar-derived synthesis gas is ideal for use in BTL or CTL processes due to the following benefits:

maximized yield of synthesis gas per unit mass of feed consumed synthesis gas free of combustion byproducts such as CO2 or NOx solar energy storage via endothermic gasification reactions in readily useable chemical form

The challenge is to develop efficient solar receiver/reactors to carry out the process in a clean and continuous manner suitable for coupling with a downstream fuel production process. In this paper, an overview of the design process used to develop a 3 kWth prototype solar gasification reactor is presented. The prior developments in the area of solar gasification are discussed with emphasis on the operational issues that have arisen and the potential for use of molten alkali carbonate salts to address these issues.

Nomenclature

C Solar concentration ratio D Dimensionless cavity-aperture diameter ratio DA Dimensionless annulus-cavity diameter ratio da Diameter of aperture, m dc Diameter of cavity cylindrical wall, m di Diameter of inner annulus surface, m do Diameter of outer annulus surface, m hfeed Enthalpy of feedstock, J/g hsg Enthalpy of synthesis gas, J/g hi,i/o Enthalpy of gas species ‘i’ entering (i) or exiting (o) the reactor, J/g I Standard insolation, 1000 W/m2 kw Thermal conductivity of cavity wall, W/m2-K L Dimensionless cavity length-diameter ratio l Length of cavity and housing, m LHVfeed Lower heating value of feedstock material, J/kg LHVsg Lower heating value of synthesis gas, J/kg

feedm Feedstock mass flow rate, kg/s sgm Synthesis gas flow rate, kg/s

2Nm Injector gas flow rate, kg/s absQ Net thermal power absorbed by the cavity, W lossQ Rate of energy lost to the ambient environment by conduction/convection, W

reradref,Q Power lost from the cavity through reflection and thermal emission, W solarQ Incident solar power at the reactor aperture, W

wrad,q Net heat flux through the cavity absorber wall, W/m2 r Radial position, m Tb Bulk temperature for convection calculations, K Tsur Temperature of the ambient surroundings, K Tw,o Temperature of the outer cavity wall, K z Axial position, m εsur Emissivity of the ambient surroundings ηabs Cavity absorption efficiency ηsolar Reactor solar efficiency

1952 B.J. Hathaway et al. / Energy Procedia 49 ( 2014 ) 1950 – 1959

1.1. Prior work on solar gasification

Prior research on solar gasification has demonstrated multiple successful approaches [2-10], but there have been recurrent issues that highlight areas for improvement through enhanced heat transfer, reduced formation of tar and ash, and improved stability during solar transients. Nonuniform temperature distributions have been observed in packed beds [2-3,9] due to insufficient heat transfer, while weak absorption of radiation by biomass has led to large temperature gradients observed in fluidized beds [5,10]. Within some fluidized beds, poor heat transfer coupled with the necessary high flow rates of fluidizing gas led to solar to chemical conversion efficiencies of only 8 to 10% [5]. Direct irradiation in aerosol flow reactors has been successful for highly absorbing feedstock such as chars and petroleum coke, but similar operation with biomass is not expected given the inherent low absorptivity of most biomass materials [6-7].

The process of biomass gasification has the potential to produce secondary products, including tars, ash, and unreacted char. The buildup of residual ash can increase radiative losses as well as insulate the reaction zone [2-3, 8-9]. Tars produced as a byproduct of uncatalyzed gasification can condense on optical surfaces [2,4]. Packed beds are especially prone to tar production during startup [2,9]. Platinum automotive catalyst particles have been used in an attempt to crack and gasify these tars with moderate success [5]. Operation at ~1500 K reduced tars to 1% to 2% of the mass of feedstock [8]; however deviation from design conditions (a drop in temperature to 1273 K) led to rapid production of tar and unreacted char [11].

1.2. Molten carbonate salts for gasification

A promising technology with potential to resolve the aforementioned issues of poor heat transfer, transient sensitivity, and secondary product contamination is the use of molten carbonate salts as a combined heat transfer media and catalyst within the reactor. The benefits of these salts arise from their relatively high thermal conductivity (0.75 W/m-K) and heat capacity (1842 J/kg-K), and catalytic activity of the alkali metal cations lithium, sodium, and potassium. Originally utilized for the autothermal gasification of coal [12], the potential thermal and chemical benefits of molten salts for solar gasification were first described by Epstein [13], and subsequently investigated by several groups [14-18]. In a molten salt solar gasification reactor, the salt is melted and heated to 1200-1250 K. Biomass and steam are introduced to the salt melt where flash pyrolysis and gasification reactions take place. The operating temperature is based on the thermodynamic equilibrium of the system of steam gasification of cellulose as illustrated in Fig. 1. When the reactor temperature is at 1200 K, carbon conversion is >98% with the product gas distribution consisting of H2 and CO as desired.

Fig. 1. Thermodynamic equilibrium product distribution as a function of temperature for the steam gasification of cellulose: C6H10O5+H2O


In recent work by the authors [19,20], justification is provided for utilizing a molten blend carbonate salts as a reaction medium in order to create a uniform temperature field within a solar reactor, to retain the solid and condensable byproducts including ash, and to convert the tarry byproducts into useful product gas. The benefits of the molten salt were verified in an electric furnace for cellulose, activated carbon, switchgrass, corn stover, and a blend of perennial plants. A two fold increase in the rate of pyrolysis and a tenfold increase in the rate of gasification compared to pyrolysis and gasification in an inert gaseous environment were demonstrated for cellulose and carbon [19]. For the biomasses studied [20], molten salt increased the total useful syngas production by up to 25%, and increased the reactivity index five fold. Secondary products, in the form of condensable tar, were reduced by 77%.

2. Reactor design

2.1. Concept

The basic geometry of the prototype molten salt solar gasification reactor is illustrated in Fig. 2 a. The design consists of two concentric cylinders, with the inner cylinder acting as a blackbody absorber containing an aperture on one end, and the outer cylinder housing the salt melt that fills the annulus. Feedstock will be delivered to the salt melt in the annulus along with a gasifying agent such as steam or carbon dioxide and undergo gasification within the melt. The concentric cylinder geometry reduces the thermal resistance between the molten salt and the cavity receiver [21] and is relatively simple to manufacture. The high thermal emissivity of the salt melt (absorption coefficient around 8900 1/m) and the incompatibility of the materials available for windows preclude a direct receiver/reactor design. The geometry is suitable for use either in a horizontal orientation for tower-top operation, or in a vertical orientation for a beam-down optical system.

a b

Fig. 2. The geometry of the UMN molten salt solar gasification reactor: (a) simplified dimensioned form of the full reactor and (b) detailed view of the cavity and the associated boundary conditions that describe the cavity receiver for modeling purposes

2.2. Material selection

Compatibility with molten salts has been an issue complicating material selection for both nitrate and carbonate salt energy systems [22-23]. The two primary requirements are resistance to corrosion in a molten carbonate salt and elevated temperature strength and creep resistance at 1200 K. Molten alkali metal salts are corrosive to most metals. However because of the risks of thermal shock and large thermal stresses, and the relatively low thermal conductivity of ceramics, metals remain the most appropriate material of construction. Nickel-based superalloys offer the best resistance to corrosion compared to other stainless and carbon steels [23-24]. Inconel alloy 600 is the best performing material out of those examined. Of the Inconel series of alloys, the alloy with the greatest strength, resistance to creep and relaxation at 1200 K is Inconel alloy X-750 (UNS N07750) [25]. It is based on the formulation for alloy 600 with the addition of aluminum and titanium to enable precipitation hardening. With similar compositions, the corrosion behaviors of alloys 600 and X-750 are expected to be similar. The pertinent

CL

)(dd

ow,bw TThrTk

0dd

zT

CI

0wrad,dd

w qk rT

10

sur

surT0

dd

zT

c

0wrad,dd

w qk zT

0wrad,dd

w qk zT


physical properties of the alloy X-750 selected for the design are a density of 8430 kg/m3, a thermal conductivity of 29 W/m2-K, and a specific heat capacity of 639 J/kg-K.

2.3. Cavity sizing

The reactor prototype is designed for a 3 kW nominal solar power and operation at a concentration ratio of 1530 suns consistent with concentrating characteristics of tower-style concentrating solar receiver. The design aperture size of da = 0.05 m is specified by

CIdQ 2a4solar (1)

A parametric study was carried out using Monte Carlo Ray Tracing (MCRT) to predict the radiative exchange between a solar concentrating system and the cavity receiver [26]. The parameters considered are the geometric ratios D=dc/da and L=l/dc, the surface material of the cavity (either oxidized Inconel alloy or a plasma-spray coating of alumina), and the intensity of convection on the outside of the cavity, represented by the convection coefficient h. The cavity was modeled as a right cylinder with a finite wall thickness and thermal conductivity kw, insulation at the two flat ends of the cylinder, and exposure to a convective environment around the curved cylinder wall (Fig. 2 b.). The incident radiation was assumed to be C = 1530 suns with a diffuse angular distribution limited to the cone angle of θc = 37°, equal to that obtained in Minnesota’s high flux simulator.

The primary output quantities are the absorption efficiency, defined in eq. (2) as the ratio of net absorbed radiation to the incident solar power, and the temperature and flux distributions along the cavity walls.

solar

absabs Q

Q (2)

The best absorption efficiency was obtained by maximizing the cavity-aperture diameter ratio D and cavity length-diameter ratio L within the confines of any restrictions on size and operating temperature. Cavities with aspect ratios or cavity/aperture diameter ratios less than 1.5 are less efficient, and exceed the temperature limits of Inconel. However, cavities with increasingly large values of D and L have very localized regions of high net flux with much of the cavity wall having “cold” regions with near zero net flux values. In considering the impact of surface material properties, Inconel was shown to be superior to alumina and was selected for the cavity. An average 5 percentage point improvement in absorption efficiency was predicted for Inconel due to improved solar absorption and reduced thermal emission. The oxidized Inconel layer acts as a selective surface. The cavity dimensions were set with relative parameter values of D = 2 and L = 1.5, resulting in a cavity of 10cm inner diameter and length of 15cm and a predicted absorption efficiency > 82% for a range of convective boundary conditions within the salt.

2.4. Annulus sizing

The remaining free parameter of the reactor is the annulus size, represented by the annulus diameter ratio DA=do/dc. Initial estimates of reactor performance demonstrated that minimizing the external surface area is an important part of maximizing the solar to chemical efficiency of the reactor; however, too thin of an annulus will stagnate the convection of salt and risk overheating the walls of the cavity. Therefore an optimum annulus size was sought by studying the worst-case of natural convection between concentric cylinders.

Prior studies of natural convection between concentric cylinders examined the effect of stagnation for thin annuli, but did not consider the effect of endothermic chemical reactions within the annulus [27]. A parametric study of varying values of DA was conducted using computational fluid dynamics (CFD) including a volumetric energy sink, representing a well-mixed reacting material. The inner cylinder thermal boundary condition was determined by coupling the MCRT radiative exchange simulation with the CFD simulation. Based on the results, we selected DA = 1.5, resulting in an annulus of 10.6 cm I.D. and 16 cm O.D. For testing in the high flux solar simulator at the University of Minnesota, we also desired that the annulus hold enough salt so that the temperature does not drop more than 40 K over a one minute shut-off. This requirement is met with approximately 2.5 kg of salt. The annulus holds about 2.6 kg of salt (taking into account a 1cm void region above the salt melt).


2.5. Structural sizing

The design was refined for manufacturability and operability resulting in the dimensions and bolted-flange style design with inlet and outlet ports as depicted in Fig. 3. The inner cavity cylinder is welded to a flange at the front and contains threaded bolt holes at the rear. The outer cylinder has a front and rear flange. The two cylinders are sealed together with an outer bolt circle at the front flange and an inner bolt circle at the rear flange. Pipe thread ports are available for feed delivery or thermocouple insertion at the bottom and sides of the reactor. Flanged tube connections are available at the rear for draining salt from the cavity, and on top for removal of product gas from the reactor. The front of the cavity flange features the aperture while the rear features a solid endcap.

Fig. 3. Isometric and side-cutaway views of the prototype molten salt solar receiver / reactor are shown. The design consists of concentric cylinders with bolted flange connections. Ports 8 and 9 are not shown but mirror ports 6 and 7 on the opposite side of the reactor.

The bolts that hold the reactor cavity and housing together at the front and rear flanges were selected to support the total force that results from this differential thermal expansion of the inner and outer cavity walls. The inner cylinder walls are in a compressed stress state, while the outer housing walls are in tension. The thicknesses of the inner and outer cylinders, mating flange surfaces, and selection of bolts were determined via coupled structural and thermal finite element analysis. Thermal boundary conditions were determined from the CFD computations and the structural boundary conditions were formulated assuming the rear surfaces of the cavity cylinder and the housing are fixed together. The temperature distribution and stress state within the cylinder walls, flanges, and bolts were determined in a parametric study of wall thickness. The temperature distribution used to determine the forces induced due to differential thermal expansion of the two cylinders as well as the resultant von Mises stresses for the final design geometry are depicted in Fig. 4. The maximum stresses are observed in the portion of the bolt closest to the central axis of the reactor as well as where the inner cylinder joins the front flange.


Fig. 4. Structural simulation results displaying (a) temperature distribution and (b) von Mises stress resulting from differential thermal expansion of the inner and outer reactor cylinders.

In general, thermal stresses are minimized as the cylinder walls become thinner. However, the stresses in the bolts that secure the front flange become very large due to bending when the flanges are thin and less rigid. Therefore, a thin cylinder wall of 3.2 mm was selected based on the minimum size suitable for welding as recommended by the reactor manufacturer. Bolts of nominal diameter 6.4 mm were selected so that the resulting maximum stress was below the range where permanent deformation due to creep would occur [25]. The flanges are 6.4 mm thick to provide sufficient stiffness to avoid the flanged seal from being broken due to thermal deformation while also reducing bending stress in the bolts to an acceptable level.

Images of the fabricated reactor are shown in Fig. 5. The reactor was manufactured from Inconel Alloy X-750 and was given triple heat treatment (AMS 5668) after welding and prior to finishing machining to obtain a precipitation hardened state. The dark grey surfaces are the oxide layer resulting from the heat treatment. The front view (Fig. 5 a) shows the aperture of 5cm diameter and the top mounted outlet for synthesis gas. The side view (Fig. 5b) shows additional ports for insertion of thermocouple probes (probes not pictured). The lower rear flanged connection is used for draining molten salt from the reactor. The white material visible between the flanges in both images is a woven alumina gasket sheet. Rear mounting feet are shown in these photos and in actual testing similar front mounting feet will also be attached to the front flange bolts. Holes visible on the front and rear housing flanges are for attaching insulation and radiation splash shields to the reactor. During operation the reactor is insulated with 100mm of refractory insulation.

The bottom ports are used for feedstock delivery. As pictured the front port is plugged and the rear is open for attachment of the feedstock injector (not shown). The feedstock delivery system consists of a straight vertical passage of 4mm diameter, which is joined in a tee configuration by a horizontal 10mm tubing passage. A flow of injection gas (nitrogen or recycled synthesis) along with a gasifying agent such as steam or carbon dioxide is delivered to the vertical passage at a sufficient velocity for pneumatic transport of the feedstock. Feedstock is delivered from a pressurized hopper via a motor driven screw conveyor along the 10mm tube, where the feed joins the vertical passage and is entrained by the flowing gases and delivered into the molten carbonate salt.


Fig. 5. Photographs of the finished reactor viewed from (a) front and (b) left side. Feed is delivered through an injector (not shown) that is threaded into the rear bottom port. Product gases exit the reactor from the top flanged tube connection (shown capped).

3. In progress

Evaluation of the reactor is on-going in the University of Minnesota high flux solar simulator [28]. The testing aims to determine the performance of the reactor in terms of the product distribution at various operation conditions as well as the steady state solar efficiency of the reactor. Testing conditions will cover temperatures in the range of 1200 K to 1225 K, injector gas flow rates of 6 SLPM with either 25% CO2 or 25% H2O as oxidizer and 75% N2. Feed delivery rates were selected based on an thermodynamic analysis of the reactor system. The reactor can be described by the following steady state energy balance during operation:

)(0 o,Ni,NNsgsgsteamsteamfeedfeedlossreradref,solar 222hhmhmhmhmQQQ (3)

The relative magnitude of reflected and re-radiated power to solar power was determined in the cavity design section while the injector gas flow rates are known along with the assumption that the injector gas reaches the same temperature as the salt melt. Assuming equilibrium products are obtained as predicted in Fig. 1, we can calculate the expected steady feedstock delivery rates for an assumed range of losses to the ambient environment of 10% to 20% of the 3 kW design input power, resulting in feedstock delivery rates between 8 g/min and 11 g/min. We therefore anticipate to achieve solar efficiencies of 40% to 55% based on the endothermic heat of complete pyrolysis and steam gasification of cellulose, 9.05 kJ/g, using the definition of solar efficiency given by:

solarfeedfeed

sgsg

LHVLHV

Qmm

solar (4)


4. Summary

An overview of the motivation behind and the design of a molten salt solar biomass gasification reactor are presented. The desire to create a thermally stable reactor which can minimize the production secondary products at moderate temperatures drives the use of molten carbonate salts as a heat transfer medium and catalyst.

Numerical simulation of the various physical processes allowed parametric studies to be used to guide the selection of various design parameters. The inner cylinder, acting as a blackbody receiver for the solar energy, is optimized in size, shape, and surface material based on a simulation of radiative exchange between the cavity and a concentrated radiation source. The outer cylinder is sized to allow for sufficient heat transfer from the hot cavity walls in the worst-case scenario of pure natural convective motion while minimizing the external surface area of the reactor. A structural analysis of the reactor allows for the determination of wall and bolt sizes to finalize design in the form to be manufactured. Testing of the reactor is on-going.

Acknowledgements

This study was supported by the University of Minnesota Initiative for Renewable Energy and the Environment.

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