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Continuous duty solar coal gasification system using molten slag and direct-contact heat exchange
Solar Energy Vol. 34, No. 3, pp. 239-247, 1985
0038-092X/85 $3.00 + .00 © 1985 Pergamon Press Ltd.
Printed in the U.S.A.
CONTINUOUS DUTY SOLAR COAL GASIFICATION SYSTEM USING MOLTEN SLAG AND DIRECT-CONTACT HEAT EXCHANGE ADAM P. BRUCKNER Aerospace and Energetics Research Program, College of Engineering, FL-10, University of Washington, Seattle, WA 98195 (Received 2 August 1983; revision accepted 30 October 1984) Abstract--A new, very high temperature solar energy conversion technique which offers the potential
of increasing the throughput and decreasing the cost of solar thermal coal gasification is described. Feedgas (C02 or steam) is heated to very high temperatures in a direct-contact droplet heat exchanger using slag droplets melted in a solar central receiver. The heated feedgas is then reacted with coal in conventional reaction vessels. Thermal storage at the full thermodynamic potential of the solar receiver is an integral feature of this approach and permits operation of the gasifier at full capacity during periods of no sun. 1. INTRODUCTION The use of solar energy to supply process heat for coal gasification has been the subject of considerable interest in recent y e a r s [ l - 7 ] . By driving the endothermic reactions involved in coal gasification with solar thermal energy, the consumption of coal can be reduced by one-third to one-half for a given output of product gas. Solar coal conversion thus conserves this nonrenewable fuel resource and reduces gas stack cleanup requirements. In solar coal gasification the solar energy is stored in chemical bonds in the form of carbon monoxide, hydrogen and hydrocarbons, which are readily usable by our present energy infrastructure as storable fuels. An additional advantage of solar coal gasification is the fact that it eliminates the need for pure oxygen, which in a number of current gasification processes is reacted with coal char to supply adequate heat of reaction. Solar thermal gasification schemes that have been proposed or investigated to date have generally been of the moving bed or fluidized bed type with direct impingement of the solar flux on the pulverized c o a l [ l - 6 ] . These systems are unable to operate during periods of low or no insolation and suffer from the limitation of requiring windowed reaction chambers. The inability of these schemes to operate at night or during overcast periods severely restricts their duty cycle and thus reduces their throughput and significantly increases the costs of conversion. Windows reduce the available solar flux, suffer from thermal shock and are prone to optical degradation due to devitrification, abrasion and chemical etching. Alternative solar coal conversion schemes which eliminate the window have been proposed[3]. One of these is the fluidized bed gasification reactor in which the concentrated solar flux is absorbed on the walls of the reactor and heat is
conducted into a fluidized bed of coal, feedgas and product gas. This approach places severe requirements on the reactor walls, which must have high conductivity, high resistance to thermal shock and good chemical compatibility with the reaction zone environment. Generally, the wall material considerations limit the attainable temperature to about 1000°K and the system cannot operate during periods of no sun. The use of indirect heat exchange fluids such as helium or a molten salt has also been suggested[3]. Although the molten salt scheme offers the potential of thermal storage for continuous operation, both it and the gas-based systems are again temperature-limited by receiver wall material considerations. Another approach proposed recently[7] would heat the feedgases directly in a solar receiver and burn coal to provide process heat during periods of no sml. This hybrid approach must operate cyclically, is limited in peak temperature, and is in essence a solar-augmented rather than a pure solar gasification system. During the past several years at the University of Washington we have been investigating a novel, very high temperature solar energy conversion scheme[8-10] which has the potential for greatly enhancing the prospects of solar thermal coal gasification. The concept described in this paper involves the heating of feedgases to temperatures above 1500°K (well beyond the capability of conventional heat exchangers) in a direct-contact droplet heat exchanger using slag droplets melted in a solar central receiver. The heated feedgases are then reacted with pulverized coal in conventional reaction vessels. Thermal storage at the full thermodynamic potential of the solar receiver is an integral feature of our approach and permits operation of the gasifier at full capacity during periods of no sun. A schematic of a 10 MWth solar coal gasification pilot plant based on these principles is shown in
239
240
A. P. BRUCKNER SECONDARY CONCENTRATOR
SOLAR ,'
.
RECEIVER ~ l~I iI
'
MOLTENSLAG, 1800°K
i
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HELIOSTATS
il
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18000K SLAG ~O0°K
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_ PRODUCT r GAS "*"COAL
SLAG RETURN STEAM or C02 :i.:"
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740OK SLAG PARTICLES
GASIFIER
SLAG PARTICLE STORAGE
Fig. 1. Schematic of solar coal gasification system using molten slag as the heat transfer and storage medium. DHX denotes the direct-contact droplet heat exchanger. Figure 1. A glassy, synthetic slag (comprised, typically, of SiO2, CaO and MgO) is delivered to the top of a 30 MWth solar received cavity + as a uniform aggregate of small particles ( - 0 . 5 - lmm d i a l which are melted and heated to -1800°K by the concentrated solar flux. The resulting liquid slag flows into a thermal storage vessel and from there, by gravity feed, to the direct contact droplet heat exchanger (DHX)[8-11]. The DHX device consists of a vertical column into which the molten slag is sprayed as a shower of uniform-sized droplets. The droplets fall through the upward directed feedgas (COz or steam, or both), transferring their heat to the gas and changing phase into solid particles in the process. The slag particles are removed from the bottom of the heat exchanger through a rotary lockhopper and delivered back to the solar receiver or to a temporary storage bin. The heated feedgas exits at the top of the heat exchanger and is directed to the reactor vessel to combine with coal or char. During periods of normal insolation the thermal storage vessel is kept filled with molten slag at the peak temperature of the system. During nighttime hours or periods of inclement weather the molten slag in the storage vessel is drawn off to supply the DHX and thus continue heating the feedstock gases. Storage temperature remains constant at + A nominal daily insolation of 8 hr is assumed, resulting in a receiver duty cycle of 33%. Consequently, the solar receiver has been sized at 3 times the thermal input requirement of the gasifier to permit simultaneous charging of a 16 hr. storage vessel.
maximum cycle temperature even when in a low state of charge, and therefore a constant temperature is maintained at the inlet to the coal gasifier. The full thermodynamic potential of the sunlight in the receiver is thus available round-the-clock for gasification. Another advantage of the present approach, as compared to existing solar gasification schemes, is that the solar receiver does not require a window. Furthermore, the concept of the DHX can be used to advantage in the recovery of sensible heat from the hot, corrosive product gases which result from the coal conversion processes. In addition, since the solar receiver system is used to heat feedgases to very high temperatures rather than processing the coal directly, it can be easily integrated with existing gasification technology. These features of the proposed concept offer to markedly improve the throughput and reduce the cost of solar thermal coal gasification.~. The purpose of this paper is to examine the operational characteristics of a baseline design of the conceptual solar feedgas heating system. A few coal conversion processes are briefly reviewed which indicate the benefits of using the high temperature molten droplet heat exchanger approach to solar thermal coal gasification. This is followed by a discussion of the high temperature droplet material and recent droplet formation experiments. Each of :~ The system proposed here is not limited only to coal conversion. Other promising applications include the generation of high temperature process heat for chemicals production and high-efficiency electric power generation.
241
Continuous duty solar coal gasification system the components of the system is then described. No attempt at optimization or detailed design of the system has been made. Rather, the intent is to present a baseline design within which to examine the nature of the research and engineering problems that can be anticipated. 2. THE IMPACTOF HIGH TEMPERATURESON COAL REACTION CHEMISTRY
2.1 Rapid pyrolysis of coal in C02 A useful path for the production of hydrogen from coal is the reaction of coal with CO2 at high temperatures[12]: C + CO2 = 2CO - AH~, AHI = 168.7 kJ/g-mole,
(1)
CO + H20 = H2 + COz + AH2, (2)
Reaction (1) favors CO production at high temperatures (above 1000°K), whereas reaction (2) favors hydrogen production at low temperatures (below 1000°K). The high endothermicity of reaction (1) requires that the CO2 be preheated to very high temperatures in the DHX. The process could proceed in the following fashion: The finely powdered coal is injected into the reaction vessel following the DHX so that the most effective use can be made of the advantages of flash pyrolysis. The CO2 then reacts with the still active char and the high temperature tends to promote a favorable equilibrium to CO at an increased reaction rate. The fuel gas that leaves the system would consist of CO, various hydrocarbon fractions, small amounts of water vapor, and CO2. This largely CO mixture would be reacted with steam via the watergas shift reaction. The hydrogen which results is a high value product to be used either directly or for hydrogenation to produce liquid fuels. The CO2 which is separated from the H2 would be available for recycling through the DHX. 2.2 The effect of superheated steam The processes which involve the interaction of steam with coal or char are highly endothermic and in conventional gasifiers have been practicable only with the addition of oxygen or air to provide the heat of oxidation via: 1
C + ~O2 = CO + AH3, AH3 -- 111.5 kJ/g-mole
(2)[12]: C + H20 = CO + H2 - AH4,
followed by reaction of the CO with steam (watergas shift).
AH2 = 32.8 kJ/g-mole.
gen is significant and adds to the burden of the capital investment. The use of air results in an offgas that is highly diluted with nitrogen and thus has a very low heating value. The separation of the nitrogen from this offgas imposes a significant cost penalty. The use of solar energy coupled with the molten droplet heat exchanger would provide a means of heating the steam to the 1500-1800°K regime and thus reduce or eliminate the oxygen required to make the reaction thermally self-sufficient. At high temperatures the production of Hz is strongly favored in an overall reaction that combines (1) and
(3)
to assure autothermic operation. The cost of oxy-
AH4 = 135.9 kJ/g-mole
(4)
with reaction (3) providing some of the energy to sustain reaction (4). To maintain constant temperature, 0.61 moles of 02 are required for each mole of H2 produced. However, at pressures below 20 atm reaction (4) is virtually complete at 1100°K[12], so that a temperature drop of 500°K would be acceptable if the droplet heat exchanger were to preheat the steam to 1600°K. In this case it can be shown that the O2/H2 molar ratio would be reduced to 0.4 for a stoichiometric feed, and to zero for 4.5 times excess steam.+ Recovery of the sensible heat of the excess steam would be required, and could also be effected by a separate droplet heat exchanger. Because excess oxygen promotes oxygenated hydrocarbons such as methanol, etc., which require further processing, an advantage of the elimination of oxygen in those processes which lead to liquefaction is increased selectivity in the final product gas. 3. THE SOLAR THERMAL SYSTEM
The principal components of the solar coal gasification system are identified in Fig. 1: the solar receiver, the thermal storage vessel, the droplet heat exchanger and the gasification reaction vessel. Not shown are the droplet heat exchangers proposed for extraction of sensible heat from the offgases. These would operate in a manner similar to the feedgas heater DHX, except that the heat transfer would proceed from the offgas to the droplets. The baseline pilot system has been sized to deliver 10 MWth to the feedgas. The thermal storage vessel has been sized for a 16-hr storage capability. The slag storage vessel, DHX and associated conduits, valves, droplet injectors, etc., must be designed with due regard for the physical and chemical stability of the materials in contact with the molten slag or the feedgas at the high temperatures + A similar computation for reaction (1) results in a requirement of 4.7 times excess CO2 for an O2/CO ratio of zero.
242
A. P. BRUCKNER
being considered. Much information relevant to this problem is already available from the highly developed technologies of the glass, ceramics, steel and chemical process industries[13, 14]. The slag and feedgas flow conditions at key points in each of the two flow loops are indicated in Fig. 1, For simplicity, heat losses from the conduits connecting the solar receiver, the storage tank and the DHX have been ignored. Also, fluid dynamic pressure drops in these same conduits have been ignored; these pressure drops were estimated to be much smaller than the hydrostatic pressure differences. Heat loss from the slag in transit from the DHX to the solar receiver has, however been included as a 40°K drop in the temperature of the slag.
3.1 The high temperature droplet material The droplet material to be heated in the solar receiver must have a melting point in the desired operating temperature range (-1600-1800°K) and should have relatively low viscosity and high surface tension to facilitate uniform droplet formation in the DHX. The material should also have low vapor pressure to avoid contamination of the feedgas in the DHX. Since the material is exposed to air in the solar receiver, it should be stable at high temperatures under oxidizing conditions. Finally, it should be low in cost. A number of common silicabased slag compositions meet these requirements. The system SiO2---CaO---MgO--A1203 is particularly attractive, due to its low viscosity[15], and the very low vapor pressure and high chemical stability of its components[16]. At present the slag composition which appears to be the most suitable as a droplet material is 50% SiO2, 30% CaO and 20% MgO, by weight[10]. The density of this composition in the molten state is ~2.9 g/croCI10], its viscosity varies from - 7 poise at 1650°K to ~2.1 poise at 1800°K[15], and its surface tension is - 4 2 0 dyne/cm over this temperature interval[17]. The specific heat, computed from data for its constituent oxides[18], varies from 1.09 kJ/ kg/°K at 700°K to 1.34 kJ/kg/°K at 1800°K. The vapor pressure of the slag is very low, even at the high temperatures considered. Evaporation occurs through dissociation of its constituent oxides, primarily that of SiO2 into SiO and 02. At 1800°K the partial pressures of SiO and O2 in a neutral atmosphere are only 1.15 × 10-6arm and 5.8 × 10-7atm, respectively[ 16]. We have carried out a series of high temperature experiments with slags of various compositions to study the droplet formation characteristics of thin jets of these materials and have successfully generated slag droplets of uniform size, in the 1 mm diameter range at temperatures of 17002000°K[19]. Figure 2 is a photograph showing solidified slag droplets of uniform size formed by the controlled breakup of a jet molten slag heated to 1810°K in an RF induction furnace. The slag was
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forced through a 0.5 mm dia 2.8 mm long molybdenum nozzle by a pressure difference of 0.9 atm. Uniform jet breakup was achieved by mechanically vibrating at 170 Hz the molybdenum crucible in which the slag was heated and the droplets were allowed to fall into a quenching bath of G.E. SF 9 6 - 1 0 0 silicon oil. The solidified droplets have a diameter of 1.3 mm, in good agreement with theoretical models of the breakup of laminar jets[20]. The extension of this droplet generation technique to a large array of orifices appears straightforward and is currently being investigated. 3.2 The solar central receiver One possible solar receiver configuration for heating the slag particles to the desired temperature is shown schematically in Fig. 3. The receiver is an
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Continuous duty solar coal gasification system insulated, refractory lined, vertical cylindrical cavity with its aperture at the top. The concentrated solar flux from the heliostats is reflected downward into the cavity by a secondary concentrator. The slag particles are delivered by a bucket conveyor to a manifold which surrounds the lip of the cavity and are introduced into the cavity as a uniformly distributed dispersion. The downward directed solar flux diffuses through the falling cloud of particles, heating them to the desired temperature. The melted slag is collected at the bottom of the cavity and flows out at a controlled rate through a high temperature slide valve. + An insulated refractory conduit directs the melt into the thermal storage tank situated at the base of the receiver structure. The essential requirement for the receiver is that the solar radiation be efficiently coupled to the slag particles and that scattering and reradiation losses be minimal. Because the slags considered for the absorbing medium are transparent over a sizeable portion of the solar spectrum, doping with an efficient absorber of solar radiation, such as iron or chromium oxides or finely divided carbon, will be required. An approximate analysis of the solar receiver has been carried out using a one-dimensional, twoflux radiative exchange model which includes scattering, absorption and emission by the particles[21, 22]. The model assumes that the concentrated solar flux illuminates the cavity aperture uniformly and diffusely. The side walls of the cavity are assumed to be perfectly reflecting to satisfy one-dimensionality. Conduction and convection losses to the air in the cavity have not been included. This assumption is valid provided the particles are not too small. The particles are assumed to be opaque, with a specular surface. The refractive index of the slag is 1.6123], resulting in a hemispherical emissivity of 0.89[24]. F o r opaque particles much larger than the wavelength of light the scattered light can be separated into two distinct components, reflection from the particle surface and diffraction. The diffracted component is confined to such small forward angles that it can be considered to be part of the undeviated beam and thus need not be included in scattering cross-section evaluations. The initial and final temperatures of the slag particles are 700°K and 1800°K, respectively. The specific heat of the slag was assumed to be 1.21 kJ/kg/ °K, the average value over this temperature range. The terminal velocity of the particles was computed assuming the air temperature in the cavity to be the average of the initial and final particle temperatures. Computations were carried out for two particle diameters: 0.5 and 0.7 mm and three values of average incident solar flux: !.0, 1.5 and 2.0 MW/m 2.
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These fluxes should be attainable with good heliostat and secondary concentrator design. Figure 4 shows plots of the particle temperature versus optical depth in the cavity. The optical depth, "r, is here defined as ~r = ( A + S ) x , where A and S are respectively the absorption and backscattering cross-sections per unit volume for diffuse radiation and x is the vertical distance from the cavity aperture. In each case most of the temperature rise occurs in a layer about one optical depth thick at the top of the cavity. The temperature increases more rapidly with optical depth as the incident flux decreases. This is because the final temperature is fixed and the receiver efficiency decreases as the solar flux decreases. Consequently, the mass flow of particles per unit area must decrease in proportion to the product of solar flux and efficiency to achieve the chosen temperature rise. The number of particles per unit volume and hence A + S, the total cross section per unit volume for absorption and backscattering, decreases in proportion to the mass flow per unit area. The geometrical distance corresponding to unit optical depth thus increases inversely as the product of incident flux and efficiency, i.e., the cloud of particles becomes more transparent. If the temperature is plotted as a function of distance from the aperture, the temperature rise is actually steeper for the higher fluxes, as would be expected. The curves of Fig. 4 are applicable to any particle size. For a given incident solar flux the particle size determines the optical depth and terminal velocity and thus the length of the receiver cavity. The diameter of the cavity is determined by the in* Such valves are widely used in the steel industry to control the flow of the molten metal from ladles and tun- (:ident solar flux and receiver efficiency. Table I dishes. lists geometrical and performance parameters for
A. P. BRUCKNER
244 Table 1. Solar Receiver Parameters. Incident
Particle
Receiver
Receiver
Efficiency
Flux (MW/m2)
Dia. (mm)
Dia. (m)
Length (m)
npR
1.0 1.0
0,5 0.7
7.9 7.9
10.2 21.3
0,61 0.61
0.4 0,4
1.5 1.5
0.5 0.7
5.7 5.7
8.4 17.5
0.79 0.79
0.6 0.6
nBB
cles at the top of the receiver cavity would automatically stop and an insulated clamshell shutter would close offthe cavity aperture to minimize heat loss.
3.3 The energy storage system Absorbing energy directly into the droplet material in the solar central receiver leads to a storage 2.0 0.5 4.7 7.2 0.85 0.7 method that is easily integrated with the feedgas 2.0 0.7 4.7 15.1 0.85 0.7 heating cycle. Energy is stored by piping the molten slag to a large, insulated, cylindrical storage vessel npR - slag particle receiver nBB - equivalent blackbody surface located directly below the solar receiver. A reserve of high temperature molten slag is thus available, permitting the coal gasifier to be on line continuthe various particle sizes and solar fluxes consid- ously, including nighttime hours and periods of ered. F o r particles much smaller than - 0 . 5 mm, cloud cover. For a temperature drop of 1100°K in convective heat transfer to the air in the cavity be- the 10 MWth DHX a storage capacity of 16 hr recomes significant and for particles larger than - 0 . 7 quires 4.31 × l05 kg, i.e., - 1 5 0 m 3, of the slag commm the increased terminal velocity and transpar- position discussed earlier. ency of the particle cloud result in excessive cavity The storage vessel must be lined with a refraclengths. The mass flow of particles is 22.5 kg/sec tory such as fused-cast a - a l u m i n a (99.3% A1203), in all cases. which is highly resistant to attack by basic slags[25The receiver efficiency (i.e., fraction of incident 28]. Corrosion of dense refractories by glasses and solar energy retained in the slag) is of particular slags proceeds most rapidly at the junction of the interest. Note that in all cases the efficiency, ~qeR, surface of the melt and the refractory wall[25]. The of the present concept exceeds the efficiency, "qBs, slag corrosion tests of Miller and Shott[28] showed of a solid blackbody surface heated to the same that at the melt line the corrosion of s - a l u m i n a by peak temperature by the equivalent solar flux. The a slag of basicity 1.0+ (same basicity as the slag reason for this enhanced efficiency is that the ra- proposed here) proceeds at the rate of - 1 . 5 mm per diation emitted by the hottest particles deep inside day at a temperature of 1800°K. Corrosion of the the receiver is trapped by the cooler particles near totally submerged parts of their test samples prothe receiver aperture. Thus, the effective emission ceeded at a much slower rate. It should be noted temperature of the cloud of particles is less than the that the level of the melt line in the storage tank peak temperature and reradiation losses are re- would continuously change over any given 24-hr duced significantly. The backscattering from the period because at any given instant the tank is either particles (this loss mechanism is included in the charging or discharging. Thus, over a significant computation of receiver efficiency) is very small, portion of a charge-discharge cycle much of the amounting only to 1.4% of the incident solar en- tank liner would be free of slag contact. This is expected to result in considerably lower corrosion ergy. As noted earlier, the present receiver geometry rates than if the melt line were stationary. It is difrequires the use of a secondary mirror. This re- ficult at this time to predict with any certainty what flector is needed to redirect the focused solar en- the liner lifetime would be, however, it is estimated ergy downward and to provide terminal concentra- that up to a 2-year service life may be attainable tion to increase the solar flux incident on the with a 30 cm thick fused-cast ~t-alumina liner. receiver cavity to -> 1 MW/m z. It has been pointed When the liner wears out, the storage vessel would out[3] that the reflective losses incurred with a be relined in much the same way as glass melting secondary mirror can be used to produce steam for tanks are relined in the glass manufacturing industhe gasification process. Similarly, if the feedgas is try. COz the " l o s t " energy can be used to preheat the Figure 5 illustrates a sketch of one possible storgas prior to injection into the droplet heat exchan- age tank design, showing its principal components ger. In this manner the secondary mirror becomes and dimensions. Floor and wall insulation is pronearly 100% efficient. vided by insulating firebrick. Sufficient insulation The start-up and shut-down procedures for the is provided to limit thermal losses to no more than solar receiver would be relatively straightforward. 1% of the total storage capacity over the rated storIn a cold start the receiver cavity would be pre- age period of 16 hr. Since the roof does not come heated by the concentrated solar flux in the absence into contact with the molten slag, it does not need of particle injection. Additional preheating in the a slag resistant liner. The roof could be constructed vicinity of the slide valve could be provided by gas combustion, if necessary, to prevent plugging of the + Basicity is defined as the ratio of the sum of the valve orifice. In the case of loss of insolation due weight fractions of CaO and MgO to the sum of the weight to overcast or nightfall, the injection of slag parti- fractions of SiOz and A1203.
245
Continuous duty solar coal gasification system
tact permits operation at temperatures in the 1500°K-1800°K range, well beyond the capabilities of conventional tube-type heat exchangers. The DHX circumvents many of the material limitations of conventional heat exchangers and does away with complicated plumbing systems and their potential for catastrophic single point failure. .0LTEN SLAG The DHX consists of a single, large, insulated cylindrical vessel[8-11]. One possible configuration is illustrated in Fig. 6. Molten slag from the storage vessel flows through an insulated refractory conduit via gravity feed and is injected into the heat ~ " "+" ]" ~ " '" ~' ('Notti scale) exchanger through a molybdenum nozzle plate I . I n l e t conduit, 50 cm I.D. pierced with a multitude of small holes. As noted 2. Refractory fiber insulation, 80 cm earlier, by suitably vibrating the nozzle plate or 3. Fused-cast ~-alumina, 30 cm pulsing the feed pressure at the appropriate fre4. Low temp. insulating f i r e b r i c k , 50 cm quency the resulting thin streams of slag can be 5. High temp. insulating f i r e b r i c k , 30 cm (side) made to break up quickly into a series of uniform lO0 cm (bottom) droplets. Uniformity of droplet size is essential to 6. Concrete foundation efficient operation of the DHX. Droplets that are 7. Outlet conduit, 25 cm I.D. too small would be lofted out of the DHX and con8. Steel outer shell taminate downstream components, whereas dropFig. 5. Molten slag storage vessel. The internal dimensions lets that are too large would not transfer all their are 8.5 m dia.× 3.1 m high; this includes 0.25 m freeboard available sensible heat and would tend to overtake above fill level. Slag volume above exit port level is smaller droplets and cause agglomeration prob150 m 3, sufficient for 16 hr. thermal storage. lems. Atmospheric pressure feedgas is uniformly inentirely of insulating silica-alumina refractory fiber troduced through inlet manifolds at the bottom and suspended from a roof cover plate. The slag exit proceeds upward through the droplet shower. Heat port is located above the floor level to provide a transfer between the two media occurs primarily by buffer layer of slag which protects the floor from convection. Although thermal radiation diffuses excessive thermal shock and erosion during initi- downward from the hot to the cold end of the DHX, its effect on the overall heat transfer is relatively ation of tank fill. Several gas-fired combustor inlets would be integrated into the storage tank ceiling to preheat the MOLTEN SLAG tank in the case of a dead cold start, and thus avoid INLET thermal shock damage to the walls. The exhaust ~ . a-ALUMINA stack could be the slag inlet conduit. In this manner preheating of this conduit and the solar receiver cavity could also be accomplished at the same time. A dump tank located below the storage vessel would receive the entire contents of the storage tank in case of an emergency failure at any point in the slag flow loop. The slag will form thermally stressed, solid filaments by quick cooling in water FALLING _ _ --INSULATION flooded into the dump tank. These filaments can be DROPLETS ~STEEL SHELL easily shattered with a mechanical crusher so that the resulting fines can be recycled to the solar receiver at start-up. Normally, this tank would be used as a solids storage bin, i.e., during periods of PARTICLE_ .":::.:( J.:-;.: (J: : ' - G A S no insolation the solidified slag beads removed from COLLECTORS"--. ~ : . K ~ ) . : : ~ ~DISTRIBUTOR the D H X would be stored in this tank instead of being returned to the solar receiver. The tank would ~COLD ~ , . .- . . . . ~ INLET be suitably insulated to maintain the beads at their D H X exit temperature over the rated storage period GAS MANIFOLD of 16 hr.
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3.4 The droplet heat exchanger (DHX) The ability of the droplet heat exchanger to transfer heat between the material heated in the solar receiver and the desired feedgas in direct con-
GAS
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Fig. 6. High temperature droplet heat exch~ger.
246
A. P. BRUCKNER
small due to the opacity of the droplet cloud. Gas and droplet flows near the end planes are two-dimensional[11], however, these regions comprise a relatively small fraction of the chamber height, thus leaving the majority of the chamber to exhibit primarily one-dimensional flow and heat transfer behavior. The molten slag droplets are injected into the heat exchanger at 1800°K. Feedgas inlet and exit temperatures are 500°K and 1600°K respectively. The relative mass flow rates of the slag and feedgas were chosen to match their thermal capacities at the conditions prevailing at the top of the DHX. (Any suitable mass flow ratio may be chosen to tailor the temperature profiles or heat exchanger effectiveness to specific needs.) A 10 MWth DHX requires a mass flow of slag of 7.5 kg/sec and a corresponding mass flow of steam of 3.8 kg/sec or a mass flow of CO2 of 7.4 kg/sec. For a given feedgas the size of the DHX is strongly dependent on the droplet size and on the choice of droplet injection velocity and gas outlet velocity. Parameteric studies have been carried out for a variety of DHX applications and operational regimes[ 11]. Table 2 summarizes the results of computations relevant to the present application for droplet sizes of 0.5 and 0.7 mm, which correspond to the particle sizes used in the analysis of the solar receiver. Due to the low gas velocities and low particle loading (droplet volume fraction --10 -3 ) the pressure drop between the bottom and top of the DHX is less than 0.2% of the inlet gas pressure for both the steam and CO2 DHX and both droplet diameters. The droplets cool to a non-sticky solid state (740°K) during their transit down the heat exchange column. No heat of fusion is released due to the amorphous nature of the slag. A series of concentric troughs at the bottom of the DHX collect the solidified slag particles and direct them to a rotary lockhopper underneath, which removes the particle aggregate from the DHX. Conveyors deliver the aggregate back to the solar receiver to continue the cycle or deposit it in the solids storage vessel during nighttime or overcast periods. The vaporization of the slag in the DHX has been estimated assuming that the feedgas leaving the device is saturated with SiO vapor and 02 at the maxTable 2. DHX Parameters. Feedgas
Particle Die (mm)
Ut (m/see)
0p/0t
%/Ut
Diameter of DHX(m)
Length of DHX(m)
Steam
0.5
3.58
1.2
0.8
3.5
4.g
Steam
0.7
5.89
0.4
0.6
3.2
10.7
CO2
0.5
3.90
1.2
0.8
3.0
5.0
CO2
0.7
5.87
0.4
0.6
2.B
11.4
Ut - droplet terminal velocity at top of DHX Un droplet injecti . . . . locity U~ gas velocity at top of DHX
Table 3. Flow Rates Through Gasifier (Metric Tons/day) Feedgas
Coal 1
CO
H2
(H20) 3332
40.3
94.1
6.7
(CO2) 6423
30.8
143
__3
iCoal assumed to be 100% carbon, for simplicity. U.S. coals have 50-85% carbon content.
As mined, typical
2Only 18% is actually consumed; rest is recyclable. 31f followed by water-gas s h i f t reaction, 10.2 Tons/day of H2 and 113 tons/day of CO2 are produced.
imum temperature (1800°K) of the slag. The resuiting loss of SiO2 is -1.3 × 10-5 kg/sec. Compared to the mass flow rates of CO2 or steam the contamination of the feedgas is very small ( - 1 . 5 3 parts per million). The loss of feedgas resulting from removal of the particle aggregate at the bottom of the heat exchanger by the rotary lockhopper can be shown to be only ~10 -4 of the mass flow of the gas through the DHX. 3.5 The coal gasifier As noted earlier, the reactor in which the coal is gasified by the hot feedgas is of conventional design, making it possible to integrate the present system with existing gasification technology. The feedgas enters the reactor at 1600°K and the product gas exits at 1100°K. The heat of reaction is provided by the sensible heat of the excess feedgas. It has already been shown that excess steam in the amount of 4.5 times stoichiometric, is required for the C + H20 reaction. The excess CO2 required for the C + CO2 reaction is 4.7 times stoichiomettic. Based on these figures and the steam and CO2 flow rates calculated in Section 3.4 for the 10 MW DHX, the consumption of coal and feedgas and the production of syngas can be easily computed. The results are shown in Table 3. The steam and CO2 flow rates include the excess, which is recyclable. The product gases carry considerable sensible heat, which can be recovered as discussed below. 3 . 6 0 f f g a s heat recovery The sensible heat carried by the corrosive and erosive offgas produced in the coal gasification reactor can be recovered by a second droplet heat exchanger operating in reverse mode. In this device cold particles are introduced at the top and the hot offgas at the bottom. The cooled offgas is removed at the top of the DHX and is available for further processing. The particle material would be chosen to have a melting temperature below that of the incoming offgas temperature. Carbonate salts such as Li2CO3, Na2CO3 and K2CO3, or eutectics thereof, may be suitable for this purpose[29]. The molten material would be removed through a suitable manifold at the bottom of the DHX and used to preheat the CO2 or generate steam for the main DHX. Particulate matter, such as ash, present in the offgas would be largely removed by the falling
Continuous duty solar coal gasification system shower of droplets in much the same manner as in liquid scrubbers. Insoluble matter adhering to the droplets would be skimmed and/or settled out depending on the relative densities of the contaminants and droplet material. Downstream offgas cleanup requirements would thus be reduced, resulting in further system cost reductions. 4. CONCLUSIONS A high-temperature, indirect solar coal gasification scheme, incorporating thermal energy storage for continuous operation, has been presented. The approach is based on the direct heating and melting of a uniform dispersion of slag particles in a windowless solar receiver and the transfer of the sensible heat of the slag to the desired feedgas in a direct-contact droplet heat exchanger. Thermal energy storage is achieved by storing the molten slag at 1800°K in an insulated refractory vessel. The full thermodynamic potential of the solar receiver is available to operate the coal gasifier at full capacity during nighttime and other periods of no sun. The capacity of the proposed system for continuous operation at temperatures well above those of other proposed solar coal conversion systems gives the present system the potential for increased syngas throughput at lower cost. Acknowledgements--The author is indebted to A. Hertzberg, Y. K. Rao, D. W. Bogdanoff and A. T. Mattick for helpful comments and discussions. The work was supported in part by the Solar Energy Research Institute, under subcontract No. XP-0-9371-1. REFERENCES
1. Proceedings of Solar Fuels Workshop, Solar Thermal Test Facilities Users Association, Albuquerque, NM, (SERI/9020-3) (1979). 2. W. H. Beattie and J. A. Sullivan, "Flash Pyrolysis and Gasification of Coal Through Laser Heating," Proc. 15th IECEC, Seattle, WA, 637-641 (1980). 3. D. W. Gregg, W. R. Aiman, H. H. Otsuki and C. B. Thorsness, "Solar Coal Gasification," Solar Energy 24, 313 (1980). 4. D. W. Gregg, R. W. Taylor, J. H. Campbell, J. R. Taylor and A. Cotton, "Solar Gasification of Coal, Activated Carbon, Coke and Coal and Biomass Mixtures," Solar Energy 25, 353 (1980). 5. W. R. Aiman, C. B. Thorsness and D. W. Gregg, "Plant Design and Economic Analysis for Solar Coal Gasification," Proc. Annual Meeting of Solar Thermal Test Facilities Users Association, STTFUA/8113, Pasadena, CA, 168-184 (1981). 6. R. W. Taylor, R. Berjoan and J. P. Coutures, "Solar Gasification of Carbonaceous Materials," Solar Energy 30, 513 (1983). 7. V. K. Mathur, R. W. Breault and S. Lakshmanan, "Coal Gasification Using Solar Energy," Solar Energy 30, 433 (1983). 8. D. Shaw, A. P. Bruckner and A. Hertzberg, "A New Method of Efficient Heat Transfer and Storage at Very High Temperatures," Proc. 15th 1ECEC, Seattle, WA, 125-132 (1980).
247
9. A. P. Bruckner and A. Hertzberg, "A New Method for High Temperature Solar Thermal Energy Conversion and Storage," Proc. Annual Meeting of Solar Thermal Test Facilities Users Association, SSTFUA/ 81-13, Pasadena, CA, 186-197 (1981). 10. A. P. Bruckner and A. Hertzberg, "High Temperature Integrated Thermal Energy Storage System for Solar Thermal Applications," SERI Report No. SERI/STR231-1812, (1982). 11. W. J. Thayer, K. M. Sekins and A. P. Bruckner, "Heat Transfer Performance and Flow Studies of High Temperature Droplet/Gas Counterflow Heat Exchangers," presented at the 21st AiChE/ASME National Heat Transfer Conference, Seattle, WA, July 24-28, 1983. Published in Heat Exchangers for TwoPhase Applications, ASME HTD Vol. 27, New York, 159-167 (1983). 12. H. H. Lowry, ed., Chemistry of Coal Utilization, Wiley, New York, Suppl. Vol. (1963). 13. W. D. Kingery, K. H. Bowen and D. R. Uhlmann, Introduction to Ceramics, Wiley, New York, (1976). 14. J. H. Chesters, Refractories, The Iron and Steel Institute, London, (1973). 15. E. T. Turkdogan and P. M. Bills, "A Critical Review of Viscosity of CaO---MgO---AI203--SiO2 Melts," Ceramic Bull. 39, 682 (1960). 16. "'Engineering Property Data on Selected Ceramics: Volume III, Single Oxides," Report No. MCIC-HB 07-Vol. Ill, Metals and Ceramics Information Center, Battelle Columbus Laboratories, Columbus, OH, (1981). 17. C. K. Lyon, "Calculation of the Surface Tensions of Glasses," J. Am. Ceram. Soc. 27, 186 (1944). 18. O. Kubaschewski and E.LL. Evans, Metallurgical Chemistry, Pergamon Press, London 306-321 (1956). 19. P. H. Benda, "'Experimental Investigation of Droplet Formation with High Temperature Molten Slags." M.S. Thesis, University of Washington (1983). 20. J. P. Anno, The Mechanics of Liquid Jets, D. C. Heath & Co., Lexington, MA (1977). 21. J. C. Chen and K. C. Chen, "Analysis of Simultaneous Radiative and Conductive Heat Transfer in Fluidized Beds," Chem. Eng. Commun. 9,255 (1981). 22. P. S. Mudgett and L. W. Richards, "Multiple Scattering Calculations for Technologyy," Appl. Opt. 10, 1485 (1971). 23. M. Huggins, "Silicate Glasses--Calculation of Densities, Refractive Indices, and Dispersions from Glass Composition," Ind. & Eng, Chem. 32, 1433 (1949). 24. R. Siegel and J. R. Howell, Thermal Radiation Heat Transfer, McGraw-Hill, New York, 103-105 (1981). 25. E. Vago and C. E. Smith, "The Corrosion of Refractory Materials by Molten Glass," Proc. 7th International Congress on Glass, Technical Paper TP-848-15, (1965). 26. C. B. Clark, "Petrology of the Reactions of Fused Cast Alumina Refractories with Metals and Slags," J. Metals 18, 1047 (1966). 27. R. W. Brown and K. H. Sandmeyer, "Fused Cast Refractories for the Chemical Process Industry," Preprint 32B, Symposium on Glass and Related Materials, Part II, Materials Conference, Philadelphia, PA, (1968). 28. W. A. Miller and W. L. Shott, "Corrosion of Refractories by Blast Furnace Slags," Am. Ceram. Sot:. Bull. 47, 648 (1968). 29. G. J, Janz, C. B. Allen, N. P. Bansal, R. M. Murphy and R. P. T. Tomkins, "Physical Properties Data Compilations Relevant to Energy Storage. II. Molten Salts: Data on Single and Multi-Component Salt Systems," NSRDS-NBS 61, Part H, U.S. Department of Commerce, National Bureau of Standards (1979).
0038-092X/85 $3.00 + .00 © 1985 Pergamon Press Ltd.
Printed in the U.S.A.
CONTINUOUS DUTY SOLAR COAL GASIFICATION SYSTEM USING MOLTEN SLAG AND DIRECT-CONTACT HEAT EXCHANGE ADAM P. BRUCKNER Aerospace and Energetics Research Program, College of Engineering, FL-10, University of Washington, Seattle, WA 98195 (Received 2 August 1983; revision accepted 30 October 1984) Abstract--A new, very high temperature solar energy conversion technique which offers the potential
of increasing the throughput and decreasing the cost of solar thermal coal gasification is described. Feedgas (C02 or steam) is heated to very high temperatures in a direct-contact droplet heat exchanger using slag droplets melted in a solar central receiver. The heated feedgas is then reacted with coal in conventional reaction vessels. Thermal storage at the full thermodynamic potential of the solar receiver is an integral feature of this approach and permits operation of the gasifier at full capacity during periods of no sun. 1. INTRODUCTION The use of solar energy to supply process heat for coal gasification has been the subject of considerable interest in recent y e a r s [ l - 7 ] . By driving the endothermic reactions involved in coal gasification with solar thermal energy, the consumption of coal can be reduced by one-third to one-half for a given output of product gas. Solar coal conversion thus conserves this nonrenewable fuel resource and reduces gas stack cleanup requirements. In solar coal gasification the solar energy is stored in chemical bonds in the form of carbon monoxide, hydrogen and hydrocarbons, which are readily usable by our present energy infrastructure as storable fuels. An additional advantage of solar coal gasification is the fact that it eliminates the need for pure oxygen, which in a number of current gasification processes is reacted with coal char to supply adequate heat of reaction. Solar thermal gasification schemes that have been proposed or investigated to date have generally been of the moving bed or fluidized bed type with direct impingement of the solar flux on the pulverized c o a l [ l - 6 ] . These systems are unable to operate during periods of low or no insolation and suffer from the limitation of requiring windowed reaction chambers. The inability of these schemes to operate at night or during overcast periods severely restricts their duty cycle and thus reduces their throughput and significantly increases the costs of conversion. Windows reduce the available solar flux, suffer from thermal shock and are prone to optical degradation due to devitrification, abrasion and chemical etching. Alternative solar coal conversion schemes which eliminate the window have been proposed[3]. One of these is the fluidized bed gasification reactor in which the concentrated solar flux is absorbed on the walls of the reactor and heat is
conducted into a fluidized bed of coal, feedgas and product gas. This approach places severe requirements on the reactor walls, which must have high conductivity, high resistance to thermal shock and good chemical compatibility with the reaction zone environment. Generally, the wall material considerations limit the attainable temperature to about 1000°K and the system cannot operate during periods of no sun. The use of indirect heat exchange fluids such as helium or a molten salt has also been suggested[3]. Although the molten salt scheme offers the potential of thermal storage for continuous operation, both it and the gas-based systems are again temperature-limited by receiver wall material considerations. Another approach proposed recently[7] would heat the feedgases directly in a solar receiver and burn coal to provide process heat during periods of no sml. This hybrid approach must operate cyclically, is limited in peak temperature, and is in essence a solar-augmented rather than a pure solar gasification system. During the past several years at the University of Washington we have been investigating a novel, very high temperature solar energy conversion scheme[8-10] which has the potential for greatly enhancing the prospects of solar thermal coal gasification. The concept described in this paper involves the heating of feedgases to temperatures above 1500°K (well beyond the capability of conventional heat exchangers) in a direct-contact droplet heat exchanger using slag droplets melted in a solar central receiver. The heated feedgases are then reacted with pulverized coal in conventional reaction vessels. Thermal storage at the full thermodynamic potential of the solar receiver is an integral feature of our approach and permits operation of the gasifier at full capacity during periods of no sun. A schematic of a 10 MWth solar coal gasification pilot plant based on these principles is shown in
239
240
A. P. BRUCKNER SECONDARY CONCENTRATOR
SOLAR ,'
.
RECEIVER ~ l~I iI
'
MOLTENSLAG, 1800°K
i
'
HELIOSTATS
il
' ~i
',,,\,,.11//2
16 hr THERMAL STORAGE VESSEL
18000K SLAG ~O0°K
S
_ PRODUCT r GAS "*"COAL
SLAG RETURN STEAM or C02 :i.:"
5O
740OK SLAG PARTICLES
GASIFIER
SLAG PARTICLE STORAGE
Fig. 1. Schematic of solar coal gasification system using molten slag as the heat transfer and storage medium. DHX denotes the direct-contact droplet heat exchanger. Figure 1. A glassy, synthetic slag (comprised, typically, of SiO2, CaO and MgO) is delivered to the top of a 30 MWth solar received cavity + as a uniform aggregate of small particles ( - 0 . 5 - lmm d i a l which are melted and heated to -1800°K by the concentrated solar flux. The resulting liquid slag flows into a thermal storage vessel and from there, by gravity feed, to the direct contact droplet heat exchanger (DHX)[8-11]. The DHX device consists of a vertical column into which the molten slag is sprayed as a shower of uniform-sized droplets. The droplets fall through the upward directed feedgas (COz or steam, or both), transferring their heat to the gas and changing phase into solid particles in the process. The slag particles are removed from the bottom of the heat exchanger through a rotary lockhopper and delivered back to the solar receiver or to a temporary storage bin. The heated feedgas exits at the top of the heat exchanger and is directed to the reactor vessel to combine with coal or char. During periods of normal insolation the thermal storage vessel is kept filled with molten slag at the peak temperature of the system. During nighttime hours or periods of inclement weather the molten slag in the storage vessel is drawn off to supply the DHX and thus continue heating the feedstock gases. Storage temperature remains constant at + A nominal daily insolation of 8 hr is assumed, resulting in a receiver duty cycle of 33%. Consequently, the solar receiver has been sized at 3 times the thermal input requirement of the gasifier to permit simultaneous charging of a 16 hr. storage vessel.
maximum cycle temperature even when in a low state of charge, and therefore a constant temperature is maintained at the inlet to the coal gasifier. The full thermodynamic potential of the sunlight in the receiver is thus available round-the-clock for gasification. Another advantage of the present approach, as compared to existing solar gasification schemes, is that the solar receiver does not require a window. Furthermore, the concept of the DHX can be used to advantage in the recovery of sensible heat from the hot, corrosive product gases which result from the coal conversion processes. In addition, since the solar receiver system is used to heat feedgases to very high temperatures rather than processing the coal directly, it can be easily integrated with existing gasification technology. These features of the proposed concept offer to markedly improve the throughput and reduce the cost of solar thermal coal gasification.~. The purpose of this paper is to examine the operational characteristics of a baseline design of the conceptual solar feedgas heating system. A few coal conversion processes are briefly reviewed which indicate the benefits of using the high temperature molten droplet heat exchanger approach to solar thermal coal gasification. This is followed by a discussion of the high temperature droplet material and recent droplet formation experiments. Each of :~ The system proposed here is not limited only to coal conversion. Other promising applications include the generation of high temperature process heat for chemicals production and high-efficiency electric power generation.
241
Continuous duty solar coal gasification system the components of the system is then described. No attempt at optimization or detailed design of the system has been made. Rather, the intent is to present a baseline design within which to examine the nature of the research and engineering problems that can be anticipated. 2. THE IMPACTOF HIGH TEMPERATURESON COAL REACTION CHEMISTRY
2.1 Rapid pyrolysis of coal in C02 A useful path for the production of hydrogen from coal is the reaction of coal with CO2 at high temperatures[12]: C + CO2 = 2CO - AH~, AHI = 168.7 kJ/g-mole,
(1)
CO + H20 = H2 + COz + AH2, (2)
Reaction (1) favors CO production at high temperatures (above 1000°K), whereas reaction (2) favors hydrogen production at low temperatures (below 1000°K). The high endothermicity of reaction (1) requires that the CO2 be preheated to very high temperatures in the DHX. The process could proceed in the following fashion: The finely powdered coal is injected into the reaction vessel following the DHX so that the most effective use can be made of the advantages of flash pyrolysis. The CO2 then reacts with the still active char and the high temperature tends to promote a favorable equilibrium to CO at an increased reaction rate. The fuel gas that leaves the system would consist of CO, various hydrocarbon fractions, small amounts of water vapor, and CO2. This largely CO mixture would be reacted with steam via the watergas shift reaction. The hydrogen which results is a high value product to be used either directly or for hydrogenation to produce liquid fuels. The CO2 which is separated from the H2 would be available for recycling through the DHX. 2.2 The effect of superheated steam The processes which involve the interaction of steam with coal or char are highly endothermic and in conventional gasifiers have been practicable only with the addition of oxygen or air to provide the heat of oxidation via: 1
C + ~O2 = CO + AH3, AH3 -- 111.5 kJ/g-mole
(2)[12]: C + H20 = CO + H2 - AH4,
followed by reaction of the CO with steam (watergas shift).
AH2 = 32.8 kJ/g-mole.
gen is significant and adds to the burden of the capital investment. The use of air results in an offgas that is highly diluted with nitrogen and thus has a very low heating value. The separation of the nitrogen from this offgas imposes a significant cost penalty. The use of solar energy coupled with the molten droplet heat exchanger would provide a means of heating the steam to the 1500-1800°K regime and thus reduce or eliminate the oxygen required to make the reaction thermally self-sufficient. At high temperatures the production of Hz is strongly favored in an overall reaction that combines (1) and
(3)
to assure autothermic operation. The cost of oxy-
AH4 = 135.9 kJ/g-mole
(4)
with reaction (3) providing some of the energy to sustain reaction (4). To maintain constant temperature, 0.61 moles of 02 are required for each mole of H2 produced. However, at pressures below 20 atm reaction (4) is virtually complete at 1100°K[12], so that a temperature drop of 500°K would be acceptable if the droplet heat exchanger were to preheat the steam to 1600°K. In this case it can be shown that the O2/H2 molar ratio would be reduced to 0.4 for a stoichiometric feed, and to zero for 4.5 times excess steam.+ Recovery of the sensible heat of the excess steam would be required, and could also be effected by a separate droplet heat exchanger. Because excess oxygen promotes oxygenated hydrocarbons such as methanol, etc., which require further processing, an advantage of the elimination of oxygen in those processes which lead to liquefaction is increased selectivity in the final product gas. 3. THE SOLAR THERMAL SYSTEM
The principal components of the solar coal gasification system are identified in Fig. 1: the solar receiver, the thermal storage vessel, the droplet heat exchanger and the gasification reaction vessel. Not shown are the droplet heat exchangers proposed for extraction of sensible heat from the offgases. These would operate in a manner similar to the feedgas heater DHX, except that the heat transfer would proceed from the offgas to the droplets. The baseline pilot system has been sized to deliver 10 MWth to the feedgas. The thermal storage vessel has been sized for a 16-hr storage capability. The slag storage vessel, DHX and associated conduits, valves, droplet injectors, etc., must be designed with due regard for the physical and chemical stability of the materials in contact with the molten slag or the feedgas at the high temperatures + A similar computation for reaction (1) results in a requirement of 4.7 times excess CO2 for an O2/CO ratio of zero.
242
A. P. BRUCKNER
being considered. Much information relevant to this problem is already available from the highly developed technologies of the glass, ceramics, steel and chemical process industries[13, 14]. The slag and feedgas flow conditions at key points in each of the two flow loops are indicated in Fig. 1, For simplicity, heat losses from the conduits connecting the solar receiver, the storage tank and the DHX have been ignored. Also, fluid dynamic pressure drops in these same conduits have been ignored; these pressure drops were estimated to be much smaller than the hydrostatic pressure differences. Heat loss from the slag in transit from the DHX to the solar receiver has, however been included as a 40°K drop in the temperature of the slag.
3.1 The high temperature droplet material The droplet material to be heated in the solar receiver must have a melting point in the desired operating temperature range (-1600-1800°K) and should have relatively low viscosity and high surface tension to facilitate uniform droplet formation in the DHX. The material should also have low vapor pressure to avoid contamination of the feedgas in the DHX. Since the material is exposed to air in the solar receiver, it should be stable at high temperatures under oxidizing conditions. Finally, it should be low in cost. A number of common silicabased slag compositions meet these requirements. The system SiO2---CaO---MgO--A1203 is particularly attractive, due to its low viscosity[15], and the very low vapor pressure and high chemical stability of its components[16]. At present the slag composition which appears to be the most suitable as a droplet material is 50% SiO2, 30% CaO and 20% MgO, by weight[10]. The density of this composition in the molten state is ~2.9 g/croCI10], its viscosity varies from - 7 poise at 1650°K to ~2.1 poise at 1800°K[15], and its surface tension is - 4 2 0 dyne/cm over this temperature interval[17]. The specific heat, computed from data for its constituent oxides[18], varies from 1.09 kJ/ kg/°K at 700°K to 1.34 kJ/kg/°K at 1800°K. The vapor pressure of the slag is very low, even at the high temperatures considered. Evaporation occurs through dissociation of its constituent oxides, primarily that of SiO2 into SiO and 02. At 1800°K the partial pressures of SiO and O2 in a neutral atmosphere are only 1.15 × 10-6arm and 5.8 × 10-7atm, respectively[ 16]. We have carried out a series of high temperature experiments with slags of various compositions to study the droplet formation characteristics of thin jets of these materials and have successfully generated slag droplets of uniform size, in the 1 mm diameter range at temperatures of 17002000°K[19]. Figure 2 is a photograph showing solidified slag droplets of uniform size formed by the controlled breakup of a jet molten slag heated to 1810°K in an RF induction furnace. The slag was
@ Q%
I
III 1111111111111
I
III III /1111IIIIII
MM123456789123456789 123456789112345 Fig. 2. Solidified slag droplets formed by uniform breakup of a 0.5 mm dia slag jet at 1810°K.
forced through a 0.5 mm dia 2.8 mm long molybdenum nozzle by a pressure difference of 0.9 atm. Uniform jet breakup was achieved by mechanically vibrating at 170 Hz the molybdenum crucible in which the slag was heated and the droplets were allowed to fall into a quenching bath of G.E. SF 9 6 - 1 0 0 silicon oil. The solidified droplets have a diameter of 1.3 mm, in good agreement with theoretical models of the breakup of laminar jets[20]. The extension of this droplet generation technique to a large array of orifices appears straightforward and is currently being investigated. 3.2 The solar central receiver One possible solar receiver configuration for heating the slag particles to the desired temperature is shown schematically in Fig. 3. The receiver is an
WATERORI COLDCO 2 ~ S E C O N DCO ANCR ENTR Y ATOR
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I
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I CONCENTRATED FLUX
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t
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~
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SLAG PARTICLE FEED
~'
ii!i!i!iiiiiiiiiiiiliiiiiii . -..........-..........
..............-........... •............-....,.... ...............-.........., ...........-.........-....,
:!:!:!:!:!:!:!:!:!:!:i:i:i:
::::::::::::::::::::::::::::
RECEIVER
FALLING i!i!iiiiiiiiiiiiiiiiiiii!i! .............. "~--"CAVITY PARTICLES-- ~i~iii!i!~!~!i!i!! i!i!!i!ii~i{iiiii{ii~ii!i!i
....,..........-........ MOLTEN~ S '~ 'L-A ---S GILIDE~CONDUITOVERSIZE VALVE hI~r'~FREEJETOF MOLTENSLAG Fig. 3. Solar receiver for heating slag particles
\
~, \
Continuous duty solar coal gasification system insulated, refractory lined, vertical cylindrical cavity with its aperture at the top. The concentrated solar flux from the heliostats is reflected downward into the cavity by a secondary concentrator. The slag particles are delivered by a bucket conveyor to a manifold which surrounds the lip of the cavity and are introduced into the cavity as a uniformly distributed dispersion. The downward directed solar flux diffuses through the falling cloud of particles, heating them to the desired temperature. The melted slag is collected at the bottom of the cavity and flows out at a controlled rate through a high temperature slide valve. + An insulated refractory conduit directs the melt into the thermal storage tank situated at the base of the receiver structure. The essential requirement for the receiver is that the solar radiation be efficiently coupled to the slag particles and that scattering and reradiation losses be minimal. Because the slags considered for the absorbing medium are transparent over a sizeable portion of the solar spectrum, doping with an efficient absorber of solar radiation, such as iron or chromium oxides or finely divided carbon, will be required. An approximate analysis of the solar receiver has been carried out using a one-dimensional, twoflux radiative exchange model which includes scattering, absorption and emission by the particles[21, 22]. The model assumes that the concentrated solar flux illuminates the cavity aperture uniformly and diffusely. The side walls of the cavity are assumed to be perfectly reflecting to satisfy one-dimensionality. Conduction and convection losses to the air in the cavity have not been included. This assumption is valid provided the particles are not too small. The particles are assumed to be opaque, with a specular surface. The refractive index of the slag is 1.6123], resulting in a hemispherical emissivity of 0.89[24]. F o r opaque particles much larger than the wavelength of light the scattered light can be separated into two distinct components, reflection from the particle surface and diffraction. The diffracted component is confined to such small forward angles that it can be considered to be part of the undeviated beam and thus need not be included in scattering cross-section evaluations. The initial and final temperatures of the slag particles are 700°K and 1800°K, respectively. The specific heat of the slag was assumed to be 1.21 kJ/kg/ °K, the average value over this temperature range. The terminal velocity of the particles was computed assuming the air temperature in the cavity to be the average of the initial and final particle temperatures. Computations were carried out for two particle diameters: 0.5 and 0.7 mm and three values of average incident solar flux: !.0, 1.5 and 2.0 MW/m 2.
1800
1
243 .
0
~
16oo /~2"° MW/zm l'Yl
~
1400 o ~ 12oo I 7-
lOOO
800
600
1
I
l
I
I
2 3 OPTICALDEPTH,"I~
I
I
4
5
Fig. 4. Temperature of slag particle dispersion vs. optical depth, ~, in solar receiver, A, B and C denote the minimum optical depth to achieve 1800°K for incident solar fluxes of 1.0, 1.5 and 2.0 MW/m 2, respectively.
These fluxes should be attainable with good heliostat and secondary concentrator design. Figure 4 shows plots of the particle temperature versus optical depth in the cavity. The optical depth, "r, is here defined as ~r = ( A + S ) x , where A and S are respectively the absorption and backscattering cross-sections per unit volume for diffuse radiation and x is the vertical distance from the cavity aperture. In each case most of the temperature rise occurs in a layer about one optical depth thick at the top of the cavity. The temperature increases more rapidly with optical depth as the incident flux decreases. This is because the final temperature is fixed and the receiver efficiency decreases as the solar flux decreases. Consequently, the mass flow of particles per unit area must decrease in proportion to the product of solar flux and efficiency to achieve the chosen temperature rise. The number of particles per unit volume and hence A + S, the total cross section per unit volume for absorption and backscattering, decreases in proportion to the mass flow per unit area. The geometrical distance corresponding to unit optical depth thus increases inversely as the product of incident flux and efficiency, i.e., the cloud of particles becomes more transparent. If the temperature is plotted as a function of distance from the aperture, the temperature rise is actually steeper for the higher fluxes, as would be expected. The curves of Fig. 4 are applicable to any particle size. For a given incident solar flux the particle size determines the optical depth and terminal velocity and thus the length of the receiver cavity. The diameter of the cavity is determined by the in* Such valves are widely used in the steel industry to control the flow of the molten metal from ladles and tun- (:ident solar flux and receiver efficiency. Table I dishes. lists geometrical and performance parameters for
A. P. BRUCKNER
244 Table 1. Solar Receiver Parameters. Incident
Particle
Receiver
Receiver
Efficiency
Flux (MW/m2)
Dia. (mm)
Dia. (m)
Length (m)
npR
1.0 1.0
0,5 0.7
7.9 7.9
10.2 21.3
0,61 0.61
0.4 0,4
1.5 1.5
0.5 0.7
5.7 5.7
8.4 17.5
0.79 0.79
0.6 0.6
nBB
cles at the top of the receiver cavity would automatically stop and an insulated clamshell shutter would close offthe cavity aperture to minimize heat loss.
3.3 The energy storage system Absorbing energy directly into the droplet material in the solar central receiver leads to a storage 2.0 0.5 4.7 7.2 0.85 0.7 method that is easily integrated with the feedgas 2.0 0.7 4.7 15.1 0.85 0.7 heating cycle. Energy is stored by piping the molten slag to a large, insulated, cylindrical storage vessel npR - slag particle receiver nBB - equivalent blackbody surface located directly below the solar receiver. A reserve of high temperature molten slag is thus available, permitting the coal gasifier to be on line continuthe various particle sizes and solar fluxes consid- ously, including nighttime hours and periods of ered. F o r particles much smaller than - 0 . 5 mm, cloud cover. For a temperature drop of 1100°K in convective heat transfer to the air in the cavity be- the 10 MWth DHX a storage capacity of 16 hr recomes significant and for particles larger than - 0 . 7 quires 4.31 × l05 kg, i.e., - 1 5 0 m 3, of the slag commm the increased terminal velocity and transpar- position discussed earlier. ency of the particle cloud result in excessive cavity The storage vessel must be lined with a refraclengths. The mass flow of particles is 22.5 kg/sec tory such as fused-cast a - a l u m i n a (99.3% A1203), in all cases. which is highly resistant to attack by basic slags[25The receiver efficiency (i.e., fraction of incident 28]. Corrosion of dense refractories by glasses and solar energy retained in the slag) is of particular slags proceeds most rapidly at the junction of the interest. Note that in all cases the efficiency, ~qeR, surface of the melt and the refractory wall[25]. The of the present concept exceeds the efficiency, "qBs, slag corrosion tests of Miller and Shott[28] showed of a solid blackbody surface heated to the same that at the melt line the corrosion of s - a l u m i n a by peak temperature by the equivalent solar flux. The a slag of basicity 1.0+ (same basicity as the slag reason for this enhanced efficiency is that the ra- proposed here) proceeds at the rate of - 1 . 5 mm per diation emitted by the hottest particles deep inside day at a temperature of 1800°K. Corrosion of the the receiver is trapped by the cooler particles near totally submerged parts of their test samples prothe receiver aperture. Thus, the effective emission ceeded at a much slower rate. It should be noted temperature of the cloud of particles is less than the that the level of the melt line in the storage tank peak temperature and reradiation losses are re- would continuously change over any given 24-hr duced significantly. The backscattering from the period because at any given instant the tank is either particles (this loss mechanism is included in the charging or discharging. Thus, over a significant computation of receiver efficiency) is very small, portion of a charge-discharge cycle much of the amounting only to 1.4% of the incident solar en- tank liner would be free of slag contact. This is expected to result in considerably lower corrosion ergy. As noted earlier, the present receiver geometry rates than if the melt line were stationary. It is difrequires the use of a secondary mirror. This re- ficult at this time to predict with any certainty what flector is needed to redirect the focused solar en- the liner lifetime would be, however, it is estimated ergy downward and to provide terminal concentra- that up to a 2-year service life may be attainable tion to increase the solar flux incident on the with a 30 cm thick fused-cast ~t-alumina liner. receiver cavity to -> 1 MW/m z. It has been pointed When the liner wears out, the storage vessel would out[3] that the reflective losses incurred with a be relined in much the same way as glass melting secondary mirror can be used to produce steam for tanks are relined in the glass manufacturing industhe gasification process. Similarly, if the feedgas is try. COz the " l o s t " energy can be used to preheat the Figure 5 illustrates a sketch of one possible storgas prior to injection into the droplet heat exchan- age tank design, showing its principal components ger. In this manner the secondary mirror becomes and dimensions. Floor and wall insulation is pronearly 100% efficient. vided by insulating firebrick. Sufficient insulation The start-up and shut-down procedures for the is provided to limit thermal losses to no more than solar receiver would be relatively straightforward. 1% of the total storage capacity over the rated storIn a cold start the receiver cavity would be pre- age period of 16 hr. Since the roof does not come heated by the concentrated solar flux in the absence into contact with the molten slag, it does not need of particle injection. Additional preheating in the a slag resistant liner. The roof could be constructed vicinity of the slide valve could be provided by gas combustion, if necessary, to prevent plugging of the + Basicity is defined as the ratio of the sum of the valve orifice. In the case of loss of insolation due weight fractions of CaO and MgO to the sum of the weight to overcast or nightfall, the injection of slag parti- fractions of SiOz and A1203.
245
Continuous duty solar coal gasification system
tact permits operation at temperatures in the 1500°K-1800°K range, well beyond the capabilities of conventional tube-type heat exchangers. The DHX circumvents many of the material limitations of conventional heat exchangers and does away with complicated plumbing systems and their potential for catastrophic single point failure. .0LTEN SLAG The DHX consists of a single, large, insulated cylindrical vessel[8-11]. One possible configuration is illustrated in Fig. 6. Molten slag from the storage vessel flows through an insulated refractory conduit via gravity feed and is injected into the heat ~ " "+" ]" ~ " '" ~' ('Notti scale) exchanger through a molybdenum nozzle plate I . I n l e t conduit, 50 cm I.D. pierced with a multitude of small holes. As noted 2. Refractory fiber insulation, 80 cm earlier, by suitably vibrating the nozzle plate or 3. Fused-cast ~-alumina, 30 cm pulsing the feed pressure at the appropriate fre4. Low temp. insulating f i r e b r i c k , 50 cm quency the resulting thin streams of slag can be 5. High temp. insulating f i r e b r i c k , 30 cm (side) made to break up quickly into a series of uniform lO0 cm (bottom) droplets. Uniformity of droplet size is essential to 6. Concrete foundation efficient operation of the DHX. Droplets that are 7. Outlet conduit, 25 cm I.D. too small would be lofted out of the DHX and con8. Steel outer shell taminate downstream components, whereas dropFig. 5. Molten slag storage vessel. The internal dimensions lets that are too large would not transfer all their are 8.5 m dia.× 3.1 m high; this includes 0.25 m freeboard available sensible heat and would tend to overtake above fill level. Slag volume above exit port level is smaller droplets and cause agglomeration prob150 m 3, sufficient for 16 hr. thermal storage. lems. Atmospheric pressure feedgas is uniformly inentirely of insulating silica-alumina refractory fiber troduced through inlet manifolds at the bottom and suspended from a roof cover plate. The slag exit proceeds upward through the droplet shower. Heat port is located above the floor level to provide a transfer between the two media occurs primarily by buffer layer of slag which protects the floor from convection. Although thermal radiation diffuses excessive thermal shock and erosion during initi- downward from the hot to the cold end of the DHX, its effect on the overall heat transfer is relatively ation of tank fill. Several gas-fired combustor inlets would be integrated into the storage tank ceiling to preheat the MOLTEN SLAG tank in the case of a dead cold start, and thus avoid INLET thermal shock damage to the walls. The exhaust ~ . a-ALUMINA stack could be the slag inlet conduit. In this manner preheating of this conduit and the solar receiver cavity could also be accomplished at the same time. A dump tank located below the storage vessel would receive the entire contents of the storage tank in case of an emergency failure at any point in the slag flow loop. The slag will form thermally stressed, solid filaments by quick cooling in water FALLING _ _ --INSULATION flooded into the dump tank. These filaments can be DROPLETS ~STEEL SHELL easily shattered with a mechanical crusher so that the resulting fines can be recycled to the solar receiver at start-up. Normally, this tank would be used as a solids storage bin, i.e., during periods of PARTICLE_ .":::.:( J.:-;.: (J: : ' - G A S no insolation the solidified slag beads removed from COLLECTORS"--. ~ : . K ~ ) . : : ~ ~DISTRIBUTOR the D H X would be stored in this tank instead of being returned to the solar receiver. The tank would ~COLD ~ , . .- . . . . ~ INLET be suitably insulated to maintain the beads at their D H X exit temperature over the rated storage period GAS MANIFOLD of 16 hr.
GAS HOTGAS MAN z~ J" ~ -~~~,.. ~OUTLET
INJECTOR PLATE
ii!i
V~
~:~
3.4 The droplet heat exchanger (DHX) The ability of the droplet heat exchanger to transfer heat between the material heated in the solar receiver and the desired feedgas in direct con-
GAS
KHOPPER
PARTICLE AGGREGATE OUTLET
Fig. 6. High temperature droplet heat exch~ger.
246
A. P. BRUCKNER
small due to the opacity of the droplet cloud. Gas and droplet flows near the end planes are two-dimensional[11], however, these regions comprise a relatively small fraction of the chamber height, thus leaving the majority of the chamber to exhibit primarily one-dimensional flow and heat transfer behavior. The molten slag droplets are injected into the heat exchanger at 1800°K. Feedgas inlet and exit temperatures are 500°K and 1600°K respectively. The relative mass flow rates of the slag and feedgas were chosen to match their thermal capacities at the conditions prevailing at the top of the DHX. (Any suitable mass flow ratio may be chosen to tailor the temperature profiles or heat exchanger effectiveness to specific needs.) A 10 MWth DHX requires a mass flow of slag of 7.5 kg/sec and a corresponding mass flow of steam of 3.8 kg/sec or a mass flow of CO2 of 7.4 kg/sec. For a given feedgas the size of the DHX is strongly dependent on the droplet size and on the choice of droplet injection velocity and gas outlet velocity. Parameteric studies have been carried out for a variety of DHX applications and operational regimes[ 11]. Table 2 summarizes the results of computations relevant to the present application for droplet sizes of 0.5 and 0.7 mm, which correspond to the particle sizes used in the analysis of the solar receiver. Due to the low gas velocities and low particle loading (droplet volume fraction --10 -3 ) the pressure drop between the bottom and top of the DHX is less than 0.2% of the inlet gas pressure for both the steam and CO2 DHX and both droplet diameters. The droplets cool to a non-sticky solid state (740°K) during their transit down the heat exchange column. No heat of fusion is released due to the amorphous nature of the slag. A series of concentric troughs at the bottom of the DHX collect the solidified slag particles and direct them to a rotary lockhopper underneath, which removes the particle aggregate from the DHX. Conveyors deliver the aggregate back to the solar receiver to continue the cycle or deposit it in the solids storage vessel during nighttime or overcast periods. The vaporization of the slag in the DHX has been estimated assuming that the feedgas leaving the device is saturated with SiO vapor and 02 at the maxTable 2. DHX Parameters. Feedgas
Particle Die (mm)
Ut (m/see)
0p/0t
%/Ut
Diameter of DHX(m)
Length of DHX(m)
Steam
0.5
3.58
1.2
0.8
3.5
4.g
Steam
0.7
5.89
0.4
0.6
3.2
10.7
CO2
0.5
3.90
1.2
0.8
3.0
5.0
CO2
0.7
5.87
0.4
0.6
2.B
11.4
Ut - droplet terminal velocity at top of DHX Un droplet injecti . . . . locity U~ gas velocity at top of DHX
Table 3. Flow Rates Through Gasifier (Metric Tons/day) Feedgas
Coal 1
CO
H2
(H20) 3332
40.3
94.1
6.7
(CO2) 6423
30.8
143
__3
iCoal assumed to be 100% carbon, for simplicity. U.S. coals have 50-85% carbon content.
As mined, typical
2Only 18% is actually consumed; rest is recyclable. 31f followed by water-gas s h i f t reaction, 10.2 Tons/day of H2 and 113 tons/day of CO2 are produced.
imum temperature (1800°K) of the slag. The resuiting loss of SiO2 is -1.3 × 10-5 kg/sec. Compared to the mass flow rates of CO2 or steam the contamination of the feedgas is very small ( - 1 . 5 3 parts per million). The loss of feedgas resulting from removal of the particle aggregate at the bottom of the heat exchanger by the rotary lockhopper can be shown to be only ~10 -4 of the mass flow of the gas through the DHX. 3.5 The coal gasifier As noted earlier, the reactor in which the coal is gasified by the hot feedgas is of conventional design, making it possible to integrate the present system with existing gasification technology. The feedgas enters the reactor at 1600°K and the product gas exits at 1100°K. The heat of reaction is provided by the sensible heat of the excess feedgas. It has already been shown that excess steam in the amount of 4.5 times stoichiometric, is required for the C + H20 reaction. The excess CO2 required for the C + CO2 reaction is 4.7 times stoichiomettic. Based on these figures and the steam and CO2 flow rates calculated in Section 3.4 for the 10 MW DHX, the consumption of coal and feedgas and the production of syngas can be easily computed. The results are shown in Table 3. The steam and CO2 flow rates include the excess, which is recyclable. The product gases carry considerable sensible heat, which can be recovered as discussed below. 3 . 6 0 f f g a s heat recovery The sensible heat carried by the corrosive and erosive offgas produced in the coal gasification reactor can be recovered by a second droplet heat exchanger operating in reverse mode. In this device cold particles are introduced at the top and the hot offgas at the bottom. The cooled offgas is removed at the top of the DHX and is available for further processing. The particle material would be chosen to have a melting temperature below that of the incoming offgas temperature. Carbonate salts such as Li2CO3, Na2CO3 and K2CO3, or eutectics thereof, may be suitable for this purpose[29]. The molten material would be removed through a suitable manifold at the bottom of the DHX and used to preheat the CO2 or generate steam for the main DHX. Particulate matter, such as ash, present in the offgas would be largely removed by the falling
Continuous duty solar coal gasification system shower of droplets in much the same manner as in liquid scrubbers. Insoluble matter adhering to the droplets would be skimmed and/or settled out depending on the relative densities of the contaminants and droplet material. Downstream offgas cleanup requirements would thus be reduced, resulting in further system cost reductions. 4. CONCLUSIONS A high-temperature, indirect solar coal gasification scheme, incorporating thermal energy storage for continuous operation, has been presented. The approach is based on the direct heating and melting of a uniform dispersion of slag particles in a windowless solar receiver and the transfer of the sensible heat of the slag to the desired feedgas in a direct-contact droplet heat exchanger. Thermal energy storage is achieved by storing the molten slag at 1800°K in an insulated refractory vessel. The full thermodynamic potential of the solar receiver is available to operate the coal gasifier at full capacity during nighttime and other periods of no sun. The capacity of the proposed system for continuous operation at temperatures well above those of other proposed solar coal conversion systems gives the present system the potential for increased syngas throughput at lower cost. Acknowledgements--The author is indebted to A. Hertzberg, Y. K. Rao, D. W. Bogdanoff and A. T. Mattick for helpful comments and discussions. The work was supported in part by the Solar Energy Research Institute, under subcontract No. XP-0-9371-1. REFERENCES
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