- Email: [email protected]
Electrofluid gasification of coal with nuclear energy
Fuel Processing Technology, 1 (1977/1978) 117--132 117 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
ELECTROFLUID GASIFICATION OF COAL WITH NUCLEAR ENERGY
A.H. PULSIFER and T.D. WHEELOCK Department of Chemical Engineering and Engineering Research Institute, Iowa State University, Ames, Iowa 50011 (U.S.A.)
(Received April 5th, 1977)
ABSTRACT The gasification of coal by reaction with steam requires addition of large amounts of energy. This energy can be supplied by a high4emperature nuclear reactor which is coupled to a fluidized bed gasifier either thermally or electrically via an electrofluid gasifier. A comparison of the economics of supplying energy by these two alternatives demonstrates that electrofluid gasification in combination with a high-temperature nuclear reactor may in some circumstances be economically attractive. In addition, a review of recent experiments in small-scale electrofluid gasifiers indicates that this method of gasification is technically feasible.
INTRODUCTION G a s i f i c a t i o n o f coal b y r e a c t i o n w i t h s t e a m is b a s e d u p o n t h e highly e n d o t h e r m i c c a r b o n - - s t e a m r e a c t i o n . I n m o s t o f t h e p r o p o s e d gasification p r o cesses, t h e e n e r g y f o r this r e a c t i o n is p r o v i d e d b y c o m b u s t i o n o f p a r t o f t h e coal fed to t h e gasifier. An a l t e r n a t i v e to c o m b u s t i o n is t h e use o f h e a t supplied b y a h i g h - t e m p e r a t u r e , gas-cooled n u c l e a r r e a c t o r ( H T G R ) . This alternative m a y be p a r t i c u l a r l y a t t r a c t i v e in areas w i t h l i m i t e d coal reserves o r high coal prices. S u c h a s i t u a t i o n exists n o w , f o r e x a m p l e , in t h e F e d e r a l R e p u b l i c o f G e r m a n y w h e r e coal prices are a b o u t US$ 9 / G c a l (US$ 2 . 3 0 / M M Btu) and t h e cost o f e n e r g y f r o m an H T G R is e s t i m a t e d to be a b o u t oneh a l f o f this. While this is n o t t r u e in t h e U n i t e d States t o d a y , it m i g h t be in the future. T h e g e n e r a t i o n o f p r o c e s s h e a t utilizing H T G R s has received o n l y limited s t u d y in t h e U n i t e d States. H o w e v e r , m o r e w o r k has b e e n d o n e o n this conc e p t in o t h e r c o u n t r i e s such as G e r m a n y , J a p a n and G r e a t Britain a n d t h e o p e r a t i o n o f small H T G R s has b e e n successfully d e m o n s t r a t e d . In p a r t i c u l a r , a 45 MW ( t h e r m a l ) H T G R w i t h a gas o u t l e t t e m p e r a t u r e o f 9 5 0 ° C ( 1 7 4 0 ° F ) has b e e n o p e r a t i n g in t h e F e d e r a l R e p u b l i c o f G e r m a n y f o r o v e r t w o y e a r s [ 1 ] . T h e successful o p e r a t i o n o f this n u c l e a r r e a c t o r is significant since a t e m p e r a t u r e level n e a r 9 5 0 ° C is t h e m i n i m u m n e e d e d f o r c a r r y i n g o u t coal gasification.
118 Additionally, coal gasification using nuclear heat has been investigated more extensively in the Federal Republic [2] than in the United States, although a preliminary evaluation of the concept has been made in this country [3,4]. The processes that have been proposed for coal gasification utilizing HTGR heat assume that thermal energy will be supplied directly by the nuclear reactor. The hot helium from an HTGR or intermediate heat exchanger would be circulated through a heat exchanger immersed in the fluidized bed gasifier (Fig. 1). When energy is supplied thermally by an HTGR, the gasification temperature is limited by the operating temperature of the HTGR. If this temperature is low, the gasification rate will be small and the gasifier volume large. An alternative, which allows the gasification temperature to be independent of the HTGR temperature, is to use an HTGR in conjunction with an electrofluid gasifier. In this case, the nuclear reactor is coupled electrically with a gasifier. Thermal energy from such a reactor is used to generate electricity and produce steam both of which are supplied to the gasifier (Fig. 2). While the direct use of heat is more efficient, electrofluid gasifiers could be used in conjunction with nuclear reactors operating at relatively low temperatures including boiling water reactors which are now commercially available. Furthermore, use of the electrofluid gasifier may be somewhat safer as the nuclear reactor and coal gasifier are not connected physically in any way. The development status and demonstrated technical feasibility of the
ELECTRICITY
_
q
200 °C
21 °C
~--~ 250 °C IY ~I ~TI~/~
y SNG l - HEATEXCHANGER
I~
I
2 - STEAMGENERATOR
I ~ ~
I
4 - GAS CLEANING
3 - GASIFIER X~ I 950 °C
,.
900 OC
5 - METHANATOR 6 - POWERSTATION 771 °C
Fig. 1, Coal gasification system using a thermally coupled HTGR to supply heat.
119
ELECTRICITY
f--540
co2, H2S ,~
~
:~H20
~
~ SNG
90( o
59O ° C 3 200 °C
~GR~
,
-- I
I\
1900 °C
I
i
~
l 2 3 4
-
HEAT EXCHANGER STEAM GENERATOR GASIFIER GAS CLEANING
6 - POWERSTATION
l
950 °C
Fig. 2. Coal gasification system using an electrically coupled HTGR to supply heat. electrofluid gasifier are reviewed here and a preliminary economic evaluation is presented which shows how the cost of gasifying coal in such a device coupled electrically to an H T G R compares with that of gasifying coal in a fluidized bed reactor which is coupled thermally with an HTGR. FEASIBILITY OF ELECTROFLUID GASIFICATION Tests conducted in bench-scale and pilot plant equipment have shown that it is technically feasible to gasify coal with steam in an electrofluid gasifier [5 r -11]. This type of gasifier is heated by passing an electric current through a conducting bed of fluidized coal or coal char particles. The fluidized bed serves as a resistor between electrodes immersed in it and heat is generated directly within the bed. This device is useful for carrying o u t high-temperature reactions which require substantial energy inputs such as the gasification of coal with steam. Also, it can provide high-temperature corrosion resistance with economical pressure containment since the gasifier can be built with a refractory, insulating lining inside of a steel shell. At Iowa State University, the gasification of coal char and lignite with steam has been demonstrated at elevated temperatures and atmospheric pressure with both 10.2-cm (4-in.) and 30.5-cm (12-in.) diameter electrofluid gasifiers [ 8 1 1 ] . Preliminary batch runs were carried out with the smaller unit and subsequent batch and continuous runs with the larger unit. A large
120 number of successful continuous runs were made with the larger unit at temperatures ranging between 700 and 1050°C (1300 and 1900 ° F). A variety of coal chars and electrodes were tested, and both single-phase and three-phase a.c. power were utilized. In many of these runs the operation proceeded smoothly for 10--20 h with no serious difficulty in controlling electric power or bed temperature. Steam conversions and gasification rates were adequate for operating temperatures above 870°C (1600 ° F). Some difficulty was experienced with electrode overheating which led to erosion of the electrodes or accumulation of slag on the electrode tips. Since the electrode configuration seemed at least partly responsible for the overheating problem, the electrode system was redesigned and the larger electrofluid gasifier was modified to accommodate the new electrode system. Whereas previously used electrodes were designed to produce a radial or transverse flow of current in the fluidized bed, the new electrodes were designed to produce a longitudinal flow. For the new design the walls of the gasifier needed to be electrically nonconducting. Hence, the lining was recast with a high-alumina refractory and the inner diameter of the gasifier was reduced to 25.4 cm (10 in.). The modified gasifier is illustrated in Fig. 3. With the new electrode arrangement the electrode surface area in contact with the fluidized bed of carbon particles was much greater than before [11]. The upper electrode consisted of nineteen 2.5-cm (1.0-in.) diameter by 53.3-cm (21-in.) long graphite rods all connected together and the lower electrode was a bed of carbon rings which also served as part of the steam distributor. The gap between the upper and lower electrodes could be varied between 5 and 28 cm (2 and 11 in.). The new electrode system apparently overcame the problem of overheating because in many hours of operation there was no noticeable erosion or slagging of the graphite electrodes [11]. Contact resistance between the electrodes and the fluidized bed appeared to be negligible, and the total electrical resistance of the system was nearly a linear function of the interelectrode gap and varied between 1 and 20 ohms depending upon the gap distance, temperature, and type of bed materials. Power input to the reactor was controlled easily by changing the interelectrode gap or the applied voltage. Reactor temperature was also easily controlled. Several successful runs were made with the m o d i f e d reactor at temperatures between 700 and 850°C (1300 and 1560°F) and using solids feed rates between 1.5 and 3.5 kg/h (3.3 and 7.7 lb./h). Coal char was fed in some of these runs and uncharred lignite in others. The coal char was impregnated with alkali for some of the runs. The addition of alkali resulted in reasonably high gasification rates even at the moderate temperatures which were employed. However, the largest gas production rate was obtained with lignite. All in all the operation proved quite satisfactory [11]. The gasification of coal char in electrofluid reactors has also been demonstrated at the Institute of Gas Technology [6,7]. Initial tests were conducted with a 15.2-cm (6-in.) diameter electrofluid gasifier which could be operated
121
106.7 cm
='
ELECTRODEHOLDER7
I
FEE0TDB:(,r i
STEELSHELL GASMAIN
87 3 cm
o, ~",P~e~
III
In
~°h~4-~
-
GRAPHITE ELECTRODES
FLUIDIZEDBED REFRACTORYLINER
4~ 5 cm i
l~
OVERFLOWTUBE CARSONRINGS
40.6 cm • THERMOCOUPLE WELL
I
\
I b
'~--STEAM INLET
Fig. 3. Bench-scale electrofluid gasifier used at Iowa State University. in e i t h e r b a t c h or c o n t i n u o u s m o d e s [ 6 ] . Over 50 runs were m a d e with this u n i t e m p l o y i n g pressures up to 70 atm, p o w e r inputs up to 100 kW, and t e m p e r a t u r e s b e t w e e n 920 and 1 0 4 0 ° C ( 1 6 9 0 and 1 9 0 0 ° F). T h e results s h o w e d the m e t h o d to be t e c h n i c a l l y feasible for t h e p r o d u c t i o n o f gas at 70 arm and 1 0 4 0 ° C ( 1 9 0 0 ° F). A m u c h larger unit was t h e n designed and built t o s u p p l y a h y d r o g e n - r i c h synthesis gas for the H Y G A S pilot plant [ 7 ] . T h e larger gasifier is 76.2 cm (30 in.) in d i a m e t e r and is c o n n e c t e d to a 2.25MW d i r e c t - c u r r e n t p o w e r s u p p l y which is c o n t r o l l e d b y an e l e c t r o n i c computer. This u n i t e m p l o y e s a c o n c e n t r i c e l e c t r o d e a r r a n g e m e n t with a single axial e l e c t r o d e w h i c h is 10.2 c m (4 in.) in diameter. This e l e c t r o d e is a solid c y l i n d e r m a d e o f T y p e 316 stainless steel. T h e operating feasibility o f t h e unit has been d e m o n s t r a t e d b y a series o f b a t c h tests in which coal char was gasified with steam at s u p e r - a t m o s p h e r i c pressure.
122 ELECTRICAL CHARACTERISTICS OF THE ELECTROFLUID GASIFIER The electrical characteristics of an electrofluid gasifier are unique and deserve special mention. In general the characteristics are those of a timevarying resistor whose resistance depends on a n u m b e r o f system parameters. Although the resistance fluctuates because o f the bubbling which occurs in fluidized beds, the time-averaged resistance is constant for a given system at steady state. The total interelectrode resistance can be divided into contact resistances between the electrodes and the fluidized bed and the resistance o f the bed itself. Experimental measurements have shown that cont act resistance varies inversely with electrode surface area and depends greatly on the electrode material [ 1 1 - - 1 3 ] . Thus the cont act resistance between a silicon carbide electrode and fluidized bed o f calcined coke at room t e m p e r a t u r e was found to be very large while the contact resistance between a graphite electrode and the same bed was negligible. The cont act resistance for a stainless-steel electrode was also appreciable but not nearly so large as that for the silicon carbide electrode. The c o n t a c t resistance for a brass electrode was relatively small but greater than for a graphite electrode. Where the cont act resistance is significant as in the case of stainless-steel or silicon carbide electrodes, it depends upon the gas velocity, current density, and nature o f the bed material as well as the electrode material. Thus the cont act resistance generally rises with an increase in gas velocity with the greatest change occurring at the minimum fluidization velocity. Moreover, the c o n t a c t resistance is inversely proportional to the current density raised to some fractional power. This power was found to be 0.5 for a stainless-steel electrode immersed in a fluidized bed o f calcined coke at r oom temperature. F u r t h e r m o r e the c o n t a c t resistance seems to be proportional to the resistivity o f the fluidized bed although the relationship is not a simple one. Current flow within a gas fluidized bed seems to be along continuous chains o f conducting particles, with some arcing taking place between particles at higher applied voltages. The conductivity o f these chains and, hence, the bed conductivity is affected by gas velocity, temperature, particle size, and the inherent conductivity of the individual particles [ 1 1 , 1 3 , 1 4 ] . Although the evidence concerning the effect o f current density on the conductivity and/ or resistivity of fluidized beds is somewhat cont radi ct ory, it appears that at lower levels of current density the resistivity is i ndependent o f this p a r am e t e r whereas at higher levels it is somewhat d e p e n d e n t on it. At higher levels of current density, the t e n d e n c y is for the resistivity to decrease as the current density is increased [ 11]. However, the effect of current density on bed resistivity is much less p r o n o u n c e d than the effect of this parameter on electrode c o n t a c t resistance. When the bed resistivity is i ndependent o f current density and the resistivity is uniform t h r o u g h o u t a fluidized bed, it is possible to apply the basic laws o f electromagnetic fields to analyze and predict the electrical character-
123
istics of conducting fluidized beds [13,15]. The results of such an analysis is a field diagram which shows equipotential and current flow lines that divide the bed into increments of equal voltage drop or equal current flow (Fig. 4). From the information contained in a field diagram and a knowledge of the bed resistivity, it is possible to predict the total bed resistance. In addition the spacing of the equipotential lines provides an indication of the voltage gradient. For example, at the b o t t o m corners of the center electrode in the system shown in Fig. 4, the lines pinch together indicating that the gradient tends to become infinite at these points. Large voltage gradients are conducive ~ NONCONDUCTING SURFACE
ELECTRODE
i
u
U--CURRENT FLOW LINES \ EQUIPOTENTIAL LINES - -
k
NONCONDUCTING SURFACE
Fig. 4. Electric~ field diagram for conducting fluidized bed located between concentric electrodes.
124 to arcing. Since the same a m o u n t of heat is generated within each of the bed segments b o u n d e d by the equipotential and current c o n t o u r surfaces, it is possible to see where most o f the heat will be generated within the system. Thus for the system shown in Fig. 4 the equithermal increments are concentrated around the center electrode so that much of the heat is generated in the region surrounding the electrode.This c o n c e n t r a t i o n could lead to o v e r heating o f the electrode. Of course, another cause o f electrode overheating is a large contact resistance between the electrode and the bed. ECONOMIC EVALUATION The commercial use of electrofluid gasifiers to produce SNG ( synthetic natural gas) will depend ultimately on the costs associated with this m e t h o d relative to the costs associated with other methods. In order to compare the economic merits o f gasifiying coal by means o f an electrofluid gasifier against gasifying it by means of a gas-heated reactor, the cost o f producing SNG by each m e t h o d was estimated. In each case an H T G R was chosen as the primary source o f thermal energy. The costs were based on 1975 prices and on European conditions since most of the work on the utilization o f process heat from HTGRs has been done there. The basis for the cost estimate and the results are presented below. Basis for cost estimate Costs were estimated for the processes shown in Figs. 1 and 2 based on the parameters in Table 1. It was assumed that an H T G R capable of supplying 3000 MW (thermal) and operating at 950°C (1740° F) would be available. This is the m a x i m u m operating t e m p e r a t u r e which is likely to be realized in the near future. Gas compositions and p r o d u c t i o n rates were based on the use of a German high volatile, bituminous coal (Table 1). Basic gasification data for this coal have been determined in small-scale laboratory experiments at Bergbau-Forschung [ 16] and are presented in Table 1. The raw gas from the gasifier was assumed to have the following (moisture-free) composition [17] : 59% H 2 , 2 0 % CO, 18% CO2 and 3% CH4. In estimating process energy requirements, only the energy needed for the gasification reactions and steam generation was taken into account and energy losses and coal preheating were ignored. However, energy recovery from the h o t gases and m e t h a n a t o r was also ignored. Complete conversion of the carbon in the coal was assumed, while a steam conversion o f 50% was used. The operating conditions for the gas-heated plant are summarized in the first column o f Table 2. Material and energy balances for this process were based on those presented by Wiegand et al. [ 1 7 ] . Since only the high-temperature energy from the nuclear reactor can be used in the gasifier, part o f the released nuclear energy is available for ot her purposes such as generation o f
125
TABLE 1 Coal properties and basic parameters used for cost estimate Proximate analysis of coal, wt.% Volatile matter (maf) Ash (mf) Moisture
37 6 4
Ultimate analysis of coal (maf), wt.% Carbon Hydrogen Oxygen Nitrogen Sulfur
82 5 8 2 3
Lower heating value of coal kcal/kg Btu/lb.
7,000 12,600
Kinetic parameters (ref. 16) Activation energy, cal/g mol Btu]lb. coal Frequency factor, 1]min
31,800 57,200 50,000
Basic parameters (ref. 17) Heat of reaction, kcal/kg coal Btu/lb. coal SNG yield, m 3/kg coal ft. 3/lb. coal Steam conversion, % Process water used, kg/kg coal Fluidized bed density, kg/m 3 lb/ft. 3 Power generation efficiency, % Stream factory, h/annum Higher heating value of SNG kcal/m 3 Btu/ft. 3
1,400 2,800 0.88 14.1 50 3.2 200 12.5 39 8,000 8,400 944
electricity. While t h e p r o p o r t i o n o f n u c l e a r energy used in the gasification process rises as t h e H T G R t e m p e r a t u r e increases, o n l y 36% o f the e n e r g y is used f o r gasification with an H T G R o p e r a t i n g t e m p e r a t u r e o f 9 5 0 ° C. It was assumed t h a t t h e rest o f t h e available e n e r g y w o u l d be c o n v e r t e d t o electricity with a g e n e r a t i o n e f f i c i e n c y o f 39%. T h e gasifier t e m p e r a t u r e is t h e n 7 7 1 ° C ( 1 4 2 0 ° F ) and the t o t a l coal t h r o u g h p u t is 208 t o n n e s / h ( 2 2 9 t o n s / h ) . The design o f a gas-heated gasifier w h i c h provides t h e n e e d e d heat e x c h a n g e r area within a reasonable v o l u m e has been described elsewhere [ 1 8 ] . Several gasifiers based o n this design w o u l d be n e e d e d with a t o t a l effective gasific a t i o n v o l u m e o f 1 3 9 9 m 3 ( 4 9 4 0 8 ft. 3 ). With an electrofluid gasifier, all o f the energy f r o m the H T G R w o u l d be used t o generate steam and electricity w h i c h are used in t h e gasifier. With a 3 0 0 0 MW H T G R and a gasifier t e m p e r a t u r e o f 9 0 0 ° C ( 1 6 5 0 ° F ) , 371 t o n n e s / h (409 t o n s / h ) o f coal are gasified ( s e c o n d c o l u m n o f Table 2). A t o t a l gas-
126
TABLE 2 Operating conditions used for cost estimates Gasifier type Gas-heated
Electrofluid
Combination
HTGR outlet temp., °C °F
950 1740
950 1740
950 1740
Gasifier temp., °C °F
771 1420
900 1650
771 and 900 1420 and 1650
208 229
371 409
1399 49408
477 16844
Coal throughput, tonnes/h tons/h Gasification volume, m 3 ft. 3 m3 ft. 3
441 486 1399 } gas-heated 49408 286 } electrofluid 1010b
SNG produced, 1000 m 3/h MM ft. 3/h
183 6.46
326 11.5
388 13.7
Total power generated, MW
755
638
380
Net power for sale, MW
755
0
0
ifier v o l u m e o f 477 m 3 ( 1 6 8 4 4 ft. 3 ) is required w h e n t h e o p e r a t i n g t e m p e r a ture is 9 0 0 ° C ( 1 6 5 0 ° F).
Gas production costs T h e t o t a l p l a n t i n v e s t m e n t including all facilities e x c e p t t h e H T G R and i n t e r m e d i a t e h e a t e x c h a n g e r was e s t i m a t e d to be US$ 6 7 1 . 4 million f o r t h e gas-heated r e a c t o r process, while the e s t i m a t e f o r e l e c t r o f l u i d gasification was US $ 7 8 3 . 8 million (first and s e c o n d c o l u m n s of T a b l e 3). In m a k i n g these e s t i m a t e s , the c o s t of the t w o t y p e s o f gasifiers per u n i t of gasification v o l u m e was a s s u m e d to be the s a m e US $ 9 4 . 3 X 103 / m 3 (US$ 2.67 X 1 0 3 / f t . 3 ). T h e gas-heated gasifier with its h e a t e x c h a n g e r is m o r e c o m p l e x t h a n the e l e c t r o f l u i d gasifier. On the o t h e r h a n d , electrical c o n t r o l e q u i p m e n t is r e q u i r e d with the e l e c t r o f l u i d process. Also, the c o s t o f all e q u i p m e n t associated w i t h S N G p r o d u c t i o n was e s t i m a t e d to be d i r e c t l y p r o p o r t i o n a l to coal t h r o u g h p u t , while the costs o f the electrical p o w e r g e n e r a t o r a n d s t e a m g e n e r a t o r w e r e a s s u m e d to be p r o p o r t i o n a l , respectively, to the a m o u n t s o f p o w e r and s t e a m p r o d u c e d . F o r an S N G p l a n t utilizing gas-heated r e a c t o r s a n d coal at US$ 2 0 / t o n , the e s t i m a t e d fixed capital i n v e s t m e n t , w o r k i n g capital, and a n n u a l p r o d u c t i o n costs are s h o w n in T a b l e s 4- 6, respectively. T h e c o s t o f p r o d u c i n g S N G b y b o t h m e t h o d s is s h o w n as a f u n c t i o n o f coal c o s t in Fig. 5. In e s t i m a t i n g t h e c o s t o f p r o d u c i n g S N G , a c o s t o f US$ 4 / G c a l ( U S $ 1 / M M Btu) was used for the c o s t o f t h e r m a l e n e r g y supplied b y an H T G R . This c o s t includes all
127
TABLE 3 Capital cost of gasification plants in millions of US dollars Unit
Gasifier type Gas-heated
Electrofluid
Combination
SNG production Steam generation Power generation
466.4 24.0 181.0
603.2 27.2 153.4
843.8 24.0 105.4
Total plant investment
671.4
783.8
973.2
TABLE 4 Fixed capital investment for SNG plant based on gas-heated reactors Component
Cost (US$)
Total plant investment Interest during construction (9% × 2 years) Startup costs, 10% of direct operating cost
671,400,000 120,900,000 18,400,000
Fixed capital investment
810,700,000
TABLE 5 Working capital required for SNG plant based on gas-heated reactors Item
Value (US$)
Coal supply, 60 days at US$ 20/ton Materials and supplies, 0.9% of total plant investment Receivables less payables, US$ 20/Gcal (US$ 5/MM Btu)
6,600~000
13,800,000
Total working capital
26,400,000
6,000,000
the costs associated with the H T G R and i n t e r m e d i a t e heat e x c h a n g e r and is based o n d a t a published b y S c h u l t e n et al. [19] o f t h e Federal Republic o f G e r m a n y . While this cost seems low based o n the c u r r e n t e c o n o m i c s o f nuclear reactors, it is t h e cost c u r r e n t l y being used for feasibility estimates in t h e Federal Republic. Since a significant q u a n t i t y o f electricity is generated when H T G R h e a t is used directly, t h e cost o f SNG p r o d u c e d in a p l a n t utilizing gas-heated reactors d e p e n d s s t r o n g l y o n the credit for electrical energy. SNG costs for t h e gas-heated r e a c t o r process are p r e s e n t e d in Fig. 5 f o r t w o values o f electrical energy, 1 ~/kWh and 2 C/kWh.
128 TABLE 6 Annual production costs for SNG plant based on gas-heated reactors Stream factor = 8000 h/annum. Component
Annual cost (US$)
Raw material, coal at US$ 20/ton Utilities water, 40 ~t/m3 (1.1 ~t/ft. 3 ) heat from HTGR, US$ 4/Gcal (US$ 1/MM Btu) Labor, operating and maintenance 800 men at US$ 20,000/a Supervision and administration 80% of labor Supplies, 30% of labor plus 1.5% of total plant investment Taxes and insurance 2.7% of total plant investment Total direct operating cost Depreciation and interest 15.6% of total fixed investment Interest on working capital, 9%
36,600,000
126,500,000 2,400,000
Total production cost
312,600,000
2,100,000 83,200,000 16,000,000 12,800,000 14,900,000 18,100,000 183,700,000
DISCUSSION
Based on the costs shown in Fig. 5, electrofluid gasification is competitive with the direct use of HTGR heat when the value of the by-product electrical energy is 1.8 C/kWh or less under the assumptions used in making the cost estimates. Since the by-product electricity from the gas-heated reactor process would have to be sold at base load prices, it is improbable that the selling price would be greater than 1.8 C/kWh. Any changes in or differences between the costs of the two types of gasifiers will affect the economic comparison between them. Since the gasheated reactor process requires more gasification volume, its economics are more strongly affected by changes in gasifier cost. For example, if the cost per unit of gasification volume is increased to U S $ 1 4 1 . 5 X 103/m 3 (US$ 4.01 X 103/ft. 3 ), the cost of SNG produced by this process is increased by US$1.2/Gcal (US$ 0.3/MM Btu). However, the cost of gas produced by the electrofluid gasifier method is increased by only US$ 0.2/Gcal (US$ 0.05/MM Btu). Obviously, any decrease in gasifier cost tends to favor the use of the gas-heated reactor. Electrofluid gasifiers would be attractive for gasification of unreactive coals where a higher temperature would reduce the gasification volume required. Another alternative which might be attractive under some conditions is a gasification plant employing both gasheated and electrofluid gas generators (Fig. 6). In such a plant, the hot helium from the gas-heated gasifiers would
129
223 21 -
:E
J 19
-
~o~
~_
~
~
~
75 -
1
L
l
20
30
40
/
COAl. COST, S/ton Fig. 5. SNG p r o d u c t i o n costs as a f u n c t i o n o f coal cost. C o m b i n a t i o n system i n c l u d e s b o t h types o f gasifiers.
ELECTRICITY
540
-oc~ 1
I ~ 250~
8 2 ~ IX~ I 900 °C[ <
7~, I
I
c02, H2S
- HEATEXCHANGER STEAMGENERATOR GAS-HEATEDGASIFIER TO
950 °C
I
GASCLEANING 6 - METHANATOR 7 - POWERSTATION
Fig. 6. Coal gasification system using b o t h thermally and electrically coupled H T G R to provide heat.
130 be used to generate steam and electricity for electrofluid gasification. Based on the assumptions used in previous calculations, such a plant could gasify an additional 233 tonnes/h (257 tons/h) of coal employing an electrofluid gasifier with a volume of 286 m 3 (10106 ft. 3 ) at 900°C (1650°F) (third column of Table 2). The total coal throughput of this plant would then be 441 tonnes/h (486 tons/h). The total capital cost of this plant ~,vas estimated to be US$ 973.2 million (third column of Table 3), giving the SNG costs shown in Fig. 5. Needless to say, this alternative has the highest thermal efficiency of any of the processes, 57% as compared to 54% and 53% for the gas-heated reactor and electrofluid processes, respectively. The thermal efficiency corresponds to the ratio of the energy recovered as SNG or electric power to the energy supplied as coal or heat from the HTGR. The combined use of both types of gasifiers would be attractive if the selling price of electricity is low. Also if conditions are such that high carbon conversions are not achieved in the gas-heated reactors, the residue could be gasified at a higher temperature in an electrofluid reactor. In summary, electrofluid gasification coupled with an HTGR appears attractive when compared to direct use of thermal energy from the reactor under several circumstances. These include a low selling price for the byproduct electricity, low coal reactivity or high gasifier cost. It should be noted that as the HTGR temperature is raised the a m o u n t of energy which needs to be converted to electricity in the gas-heated reactor process is significantly reduced. This increases the overall efficiency of the process and reduces the cost significantly, thus making the process relatively more attractive compared to electrofluid gasification. CONCLUSIONS The results of the preliminary cost estimate which are presented here show that coal gasification in reactors coupled electrically with an HTGR may compete economically with gasification in reactors coupled thermally with an HTGR. Electrical coupling offers several advantages over thermal coupling. Thus with electrical coupling the gasifier temperature is independent of the nuclear reactor temperature and is not limited by the latter. By employing higher gasifier temperatures, faster rates of reaction are obtained and greater gasifier productivity is realized. Also with electrical coupling the gasifier design is much simpler than with thermal coupling and obtaining suitable materials of construction, except possibly for the electrodes, is less of a problem since only refractory materials are required to be in contact with the hot reaction mixture. In addition electrical coupling provides greater flexibility than thermal coupling with regard to both plant siting and operation since by-product electrical power is not produced. Moreover, the thermal efficiency of a gasification process employing electrical coupling is only slightly less than one employing thermal coupling at reasonable HTGR temperatures. A particularly attractive alternative would be a gasification plant utilizing
131 an H T G R in c o m b i n a t i o n with b o t h d i r e c t and electrically h e a t e d gasifiers. Such a p l a n t w o u l d have t h e highest t h e r m a l e f f i c i e n c y o f t h e alternatives c o n s i d e r e d and m o s t p r o b a b l y w o u l d p r o d u c e gas f o r t h e l o w e s t price. Furt h e r m o r e , t h e o p e r a t i o n o f this p l a n t w o u l d o f f e r m o r e flexibility t h a n a p l a n t c o n t a i n i n g o n l y gasifiers using d i r e c t heating. F o r e x a m p l e , t h e electrofluid gasifier c o u l d b e used t o gasify high ash residue, or s o m e o f the electrical p o w e r m i g h t be sold w i t h m o r e p o w e r being supplied to e l e c t r o f l u i d gasifier w h e n d e m a n d f o r p o w e r was low. T h e t e c h n i c a l feasibility o f t h e e l e c t r o f l u i d gasifier which c o u l d be c o u p l e d electrically t o a n u c l e a r r e a c t o r has b e e n d e m o n s t r a t e d in b o t h b e n c h and p i l o t p l a n t scale e q u i p m e n t . In a d d i t i o n t h e i m p o r t a n t electrical and p h y s i c a l characteristics o f t h e s y s t e m have b e e n d e t e r m i n e d so t h a t larger units m a y b e designed with s o m e degree o f c o n f i d e n c e . In light o f t h e s e e n c o u r a g i n g p r e l i m i n a r y results o n e c a n see t h a t a m o r e detailed and c o m p r e h e n s i v e design s t u d y and m o r e definitive cost e s t i m a t e are n o w in o r d e r t o p r o v i d e f u r t h e r c o n f i r m a t i o n . This s t u d y should consider m o r e t h a n o n e t y p e o f n u c l e a r r e a c t o r and c o n d i t i o n s in t h e U n i t e d States as well as E u r o p e . F u r t h e r m o r e , it should also c o m p a r e t h e m e t h o d s o f gasification d e s c r i b e d h e r e w i t h o t h e r m e t h o d s being d e v e l o p e d w h i c h d o n ' t utilize n u c l e a r energy. ACKNOWLEDGEMENTS This p a p e r was p r e p a r e d while o n e o f t h e a u t h o r s (A.H.P.) was on-leave at B e r g b a u - F o r s c h u n g , Essen, G . F . R . T h e h e l p f u l discussions w i t h Drs. Feistel and V a n H e e k and Mr. Wiegand a b o u t t h e use o f n u c l e a r h e a t f o r coal gasification and t h e financial s u p p o r t o f t h e A l e x a n d e r v o n H u m b o l d t F o u n d a t i o n d u r i n g this t i m e are g r a t e f u l l y a c k n o w l e d g e d . T h e e x p e r i m e n t a l w o r k o n electro fluid gasification at I o w a S t a t e University was s u p p o r t e d b y t h e E n g i n e e r i n g R e s e a r c h I n s t i t u t e t h r o u g h f u n d s m a d e available b y E R D A und e r c o n t r a c t No. E ( 4 9 - 1 8 ) - 4 7 9 .
REFERENCES 1 Kr~mer, H. The high temperature reactor in the Federal Republic of Germany -- Present situation, development progress and future aspects. International Symposium on GasCooled Reactors with Emphasis on Advanced Systems, J~lich, Germany, Oct. 13 17, 1975. 2 Schulten, R., J~lntgen, H. and Teggers, H., 1976. Nukleare Prozessw~rme f~]r die Kohlenvergasung. Gl~]ckauf, 112: 14. 3 Quade, R.N. and McMain, A.T. Nuclear energy for coal gasification. Symposium on Clean Fuels From Coal, Sept. 10--14, 1973, Institute of Gas Technology, Chicago, Ill. 4 Quade, R.N. and Woebcke, H.N. HTGR for coal gasification/liquefaction. BNES International Conference, London, Nov. 1974. 5 Pulsifer, A.H. and Wheelock, T.D., 1974. Observations of coal processing in an electrofluid reactor. J. Inst. Fuel, 47: 235. 6 Kavlick, V.J., Lee, B.S. and Schora, F.C., 1971. Electrothermal coal char gasification. AIChE Symposium Series No. 116, 67 : 228.
132
7 HYGAS: 1972 to 1974. Pipeline gas from coal hydrogenation, (IGT Hydrogasification Process). R&D Report No. 110: Interim Report No. 1, Institute of Gas Technology, Chicago, Ill., prepared for ERDA, Washington, D.C., 1975.. 8 Pulsifer, A.H., Knowlton, T.M. and Wheelock, T.D., 1969. Coal char gasification in an elec trofluid reactor. Ind. Eng. Chem. Process Des. Develop., 8: 539. 9 Beeson, J.L., Pulsifer, A.H. and Wheelock, T.D., 1970. Coal char gasification in a continuous electrofluid reactor. Ind. Eng. Chem. Process Des. Develop., 9: 460. 10 Beeson, J.L., Pulsifer, A.H. and Wheelock, T.D., 1974. Hydrogen from coal char in a continuous elecgrofluid reactor. Ind. Eng. Chem. Process Des. Develop., 1 3 : 1 5 9 . 11 Pulsifer, A.H. and Wheelock, T.D. Coal processing by electrofluidics. Final Report, Engineering Research Institute, Iowa State University, Ames, Iowa, prepared for Office of Fossil Energy, ERDA, Washington, D.C., 1975. 12 Chen, T.P. Yuan, E. and Pulsifer, A.H. Bed and contact resistances in an electrically conducting fluidized bed. Amer. Inst. Chem. Eng. Meeting, Washington, D.C., Dec. 1974. 13 Pulsifer, A.H. and Wheelock, T.D. Coal processing by electrofluidics. Interim Report No. 3, Iowa State University, Ames, Iowa, prepared for Office of Coal Research, Dept. of the Interior, Washington, D.C., 1974. 14 Jones, A.L. and Wheelock, T.D., 1968. The electrical resistivity of fluidized carbon particles: Determination of resistivity by the four terminal method. I. Chem. E. (London) Symposium Series No. 30, p. 174. 15 Knowlton, T.M., Pulsifer, A.H. and Wheelock, T.D., 1973. Prediction of fluidized bed resistance using field theory. AIChE Symposium Series No. 128, 69: 94. 16 Van Heek, K.H., J~Jntgen, H. and Peters, W., 1973. Fundamental studies on coal gasification in the utilization of thermal energy from nuclear, high-temperature reactors. J. Inst. Fuel, 46: 249. 17 Wiegand, D., Van Heek, K.H. and Jilntgen, H. The significance of the HTR temperature for the economic use of nuclear heat for coal gasification. BNES International Conference, London, Nov. 1974. 18 JUntgen, H., Van Heek, K.H., D~]rrfeld, R. and Feistel, P.P. Kinetics, heat transfer and engineering aspects of coal gasification with steam using nuclear heat. BNES International Conference, London, Nov. 1974. 19 Schulten, R., Von der Decken, C.B. and Barnert, H. Nuclear water splitting by heat from the pebble bed HTR. BNES International Conference, London, Nov. 1974.
ELECTROFLUID GASIFICATION OF COAL WITH NUCLEAR ENERGY
A.H. PULSIFER and T.D. WHEELOCK Department of Chemical Engineering and Engineering Research Institute, Iowa State University, Ames, Iowa 50011 (U.S.A.)
(Received April 5th, 1977)
ABSTRACT The gasification of coal by reaction with steam requires addition of large amounts of energy. This energy can be supplied by a high4emperature nuclear reactor which is coupled to a fluidized bed gasifier either thermally or electrically via an electrofluid gasifier. A comparison of the economics of supplying energy by these two alternatives demonstrates that electrofluid gasification in combination with a high-temperature nuclear reactor may in some circumstances be economically attractive. In addition, a review of recent experiments in small-scale electrofluid gasifiers indicates that this method of gasification is technically feasible.
INTRODUCTION G a s i f i c a t i o n o f coal b y r e a c t i o n w i t h s t e a m is b a s e d u p o n t h e highly e n d o t h e r m i c c a r b o n - - s t e a m r e a c t i o n . I n m o s t o f t h e p r o p o s e d gasification p r o cesses, t h e e n e r g y f o r this r e a c t i o n is p r o v i d e d b y c o m b u s t i o n o f p a r t o f t h e coal fed to t h e gasifier. An a l t e r n a t i v e to c o m b u s t i o n is t h e use o f h e a t supplied b y a h i g h - t e m p e r a t u r e , gas-cooled n u c l e a r r e a c t o r ( H T G R ) . This alternative m a y be p a r t i c u l a r l y a t t r a c t i v e in areas w i t h l i m i t e d coal reserves o r high coal prices. S u c h a s i t u a t i o n exists n o w , f o r e x a m p l e , in t h e F e d e r a l R e p u b l i c o f G e r m a n y w h e r e coal prices are a b o u t US$ 9 / G c a l (US$ 2 . 3 0 / M M Btu) and t h e cost o f e n e r g y f r o m an H T G R is e s t i m a t e d to be a b o u t oneh a l f o f this. While this is n o t t r u e in t h e U n i t e d States t o d a y , it m i g h t be in the future. T h e g e n e r a t i o n o f p r o c e s s h e a t utilizing H T G R s has received o n l y limited s t u d y in t h e U n i t e d States. H o w e v e r , m o r e w o r k has b e e n d o n e o n this conc e p t in o t h e r c o u n t r i e s such as G e r m a n y , J a p a n and G r e a t Britain a n d t h e o p e r a t i o n o f small H T G R s has b e e n successfully d e m o n s t r a t e d . In p a r t i c u l a r , a 45 MW ( t h e r m a l ) H T G R w i t h a gas o u t l e t t e m p e r a t u r e o f 9 5 0 ° C ( 1 7 4 0 ° F ) has b e e n o p e r a t i n g in t h e F e d e r a l R e p u b l i c o f G e r m a n y f o r o v e r t w o y e a r s [ 1 ] . T h e successful o p e r a t i o n o f this n u c l e a r r e a c t o r is significant since a t e m p e r a t u r e level n e a r 9 5 0 ° C is t h e m i n i m u m n e e d e d f o r c a r r y i n g o u t coal gasification.
118 Additionally, coal gasification using nuclear heat has been investigated more extensively in the Federal Republic [2] than in the United States, although a preliminary evaluation of the concept has been made in this country [3,4]. The processes that have been proposed for coal gasification utilizing HTGR heat assume that thermal energy will be supplied directly by the nuclear reactor. The hot helium from an HTGR or intermediate heat exchanger would be circulated through a heat exchanger immersed in the fluidized bed gasifier (Fig. 1). When energy is supplied thermally by an HTGR, the gasification temperature is limited by the operating temperature of the HTGR. If this temperature is low, the gasification rate will be small and the gasifier volume large. An alternative, which allows the gasification temperature to be independent of the HTGR temperature, is to use an HTGR in conjunction with an electrofluid gasifier. In this case, the nuclear reactor is coupled electrically with a gasifier. Thermal energy from such a reactor is used to generate electricity and produce steam both of which are supplied to the gasifier (Fig. 2). While the direct use of heat is more efficient, electrofluid gasifiers could be used in conjunction with nuclear reactors operating at relatively low temperatures including boiling water reactors which are now commercially available. Furthermore, use of the electrofluid gasifier may be somewhat safer as the nuclear reactor and coal gasifier are not connected physically in any way. The development status and demonstrated technical feasibility of the
ELECTRICITY
_
q
200 °C
21 °C
~--~ 250 °C IY ~I ~TI~/~
y SNG l - HEATEXCHANGER
I~
I
2 - STEAMGENERATOR
I ~ ~
I
4 - GAS CLEANING
3 - GASIFIER X~ I 950 °C
,.
900 OC
5 - METHANATOR 6 - POWERSTATION 771 °C
Fig. 1, Coal gasification system using a thermally coupled HTGR to supply heat.
119
ELECTRICITY
f--540
co2, H2S ,~
~
:~H20
~
~ SNG
90( o
59O ° C 3 200 °C
~GR~
,
-- I
I\
1900 °C
I
i
~
l 2 3 4
-
HEAT EXCHANGER STEAM GENERATOR GASIFIER GAS CLEANING
6 - POWERSTATION
l
950 °C
Fig. 2. Coal gasification system using an electrically coupled HTGR to supply heat. electrofluid gasifier are reviewed here and a preliminary economic evaluation is presented which shows how the cost of gasifying coal in such a device coupled electrically to an H T G R compares with that of gasifying coal in a fluidized bed reactor which is coupled thermally with an HTGR. FEASIBILITY OF ELECTROFLUID GASIFICATION Tests conducted in bench-scale and pilot plant equipment have shown that it is technically feasible to gasify coal with steam in an electrofluid gasifier [5 r -11]. This type of gasifier is heated by passing an electric current through a conducting bed of fluidized coal or coal char particles. The fluidized bed serves as a resistor between electrodes immersed in it and heat is generated directly within the bed. This device is useful for carrying o u t high-temperature reactions which require substantial energy inputs such as the gasification of coal with steam. Also, it can provide high-temperature corrosion resistance with economical pressure containment since the gasifier can be built with a refractory, insulating lining inside of a steel shell. At Iowa State University, the gasification of coal char and lignite with steam has been demonstrated at elevated temperatures and atmospheric pressure with both 10.2-cm (4-in.) and 30.5-cm (12-in.) diameter electrofluid gasifiers [ 8 1 1 ] . Preliminary batch runs were carried out with the smaller unit and subsequent batch and continuous runs with the larger unit. A large
120 number of successful continuous runs were made with the larger unit at temperatures ranging between 700 and 1050°C (1300 and 1900 ° F). A variety of coal chars and electrodes were tested, and both single-phase and three-phase a.c. power were utilized. In many of these runs the operation proceeded smoothly for 10--20 h with no serious difficulty in controlling electric power or bed temperature. Steam conversions and gasification rates were adequate for operating temperatures above 870°C (1600 ° F). Some difficulty was experienced with electrode overheating which led to erosion of the electrodes or accumulation of slag on the electrode tips. Since the electrode configuration seemed at least partly responsible for the overheating problem, the electrode system was redesigned and the larger electrofluid gasifier was modified to accommodate the new electrode system. Whereas previously used electrodes were designed to produce a radial or transverse flow of current in the fluidized bed, the new electrodes were designed to produce a longitudinal flow. For the new design the walls of the gasifier needed to be electrically nonconducting. Hence, the lining was recast with a high-alumina refractory and the inner diameter of the gasifier was reduced to 25.4 cm (10 in.). The modified gasifier is illustrated in Fig. 3. With the new electrode arrangement the electrode surface area in contact with the fluidized bed of carbon particles was much greater than before [11]. The upper electrode consisted of nineteen 2.5-cm (1.0-in.) diameter by 53.3-cm (21-in.) long graphite rods all connected together and the lower electrode was a bed of carbon rings which also served as part of the steam distributor. The gap between the upper and lower electrodes could be varied between 5 and 28 cm (2 and 11 in.). The new electrode system apparently overcame the problem of overheating because in many hours of operation there was no noticeable erosion or slagging of the graphite electrodes [11]. Contact resistance between the electrodes and the fluidized bed appeared to be negligible, and the total electrical resistance of the system was nearly a linear function of the interelectrode gap and varied between 1 and 20 ohms depending upon the gap distance, temperature, and type of bed materials. Power input to the reactor was controlled easily by changing the interelectrode gap or the applied voltage. Reactor temperature was also easily controlled. Several successful runs were made with the m o d i f e d reactor at temperatures between 700 and 850°C (1300 and 1560°F) and using solids feed rates between 1.5 and 3.5 kg/h (3.3 and 7.7 lb./h). Coal char was fed in some of these runs and uncharred lignite in others. The coal char was impregnated with alkali for some of the runs. The addition of alkali resulted in reasonably high gasification rates even at the moderate temperatures which were employed. However, the largest gas production rate was obtained with lignite. All in all the operation proved quite satisfactory [11]. The gasification of coal char in electrofluid reactors has also been demonstrated at the Institute of Gas Technology [6,7]. Initial tests were conducted with a 15.2-cm (6-in.) diameter electrofluid gasifier which could be operated
121
106.7 cm
='
ELECTRODEHOLDER7
I
FEE0TDB:(,r i
STEELSHELL GASMAIN
87 3 cm
o, ~",P~e~
III
In
~°h~4-~
-
GRAPHITE ELECTRODES
FLUIDIZEDBED REFRACTORYLINER
4~ 5 cm i
l~
OVERFLOWTUBE CARSONRINGS
40.6 cm • THERMOCOUPLE WELL
I
\
I b
'~--STEAM INLET
Fig. 3. Bench-scale electrofluid gasifier used at Iowa State University. in e i t h e r b a t c h or c o n t i n u o u s m o d e s [ 6 ] . Over 50 runs were m a d e with this u n i t e m p l o y i n g pressures up to 70 atm, p o w e r inputs up to 100 kW, and t e m p e r a t u r e s b e t w e e n 920 and 1 0 4 0 ° C ( 1 6 9 0 and 1 9 0 0 ° F). T h e results s h o w e d the m e t h o d to be t e c h n i c a l l y feasible for t h e p r o d u c t i o n o f gas at 70 arm and 1 0 4 0 ° C ( 1 9 0 0 ° F). A m u c h larger unit was t h e n designed and built t o s u p p l y a h y d r o g e n - r i c h synthesis gas for the H Y G A S pilot plant [ 7 ] . T h e larger gasifier is 76.2 cm (30 in.) in d i a m e t e r and is c o n n e c t e d to a 2.25MW d i r e c t - c u r r e n t p o w e r s u p p l y which is c o n t r o l l e d b y an e l e c t r o n i c computer. This u n i t e m p l o y e s a c o n c e n t r i c e l e c t r o d e a r r a n g e m e n t with a single axial e l e c t r o d e w h i c h is 10.2 c m (4 in.) in diameter. This e l e c t r o d e is a solid c y l i n d e r m a d e o f T y p e 316 stainless steel. T h e operating feasibility o f t h e unit has been d e m o n s t r a t e d b y a series o f b a t c h tests in which coal char was gasified with steam at s u p e r - a t m o s p h e r i c pressure.
122 ELECTRICAL CHARACTERISTICS OF THE ELECTROFLUID GASIFIER The electrical characteristics of an electrofluid gasifier are unique and deserve special mention. In general the characteristics are those of a timevarying resistor whose resistance depends on a n u m b e r o f system parameters. Although the resistance fluctuates because o f the bubbling which occurs in fluidized beds, the time-averaged resistance is constant for a given system at steady state. The total interelectrode resistance can be divided into contact resistances between the electrodes and the fluidized bed and the resistance o f the bed itself. Experimental measurements have shown that cont act resistance varies inversely with electrode surface area and depends greatly on the electrode material [ 1 1 - - 1 3 ] . Thus the cont act resistance between a silicon carbide electrode and fluidized bed o f calcined coke at room t e m p e r a t u r e was found to be very large while the contact resistance between a graphite electrode and the same bed was negligible. The cont act resistance for a stainless-steel electrode was also appreciable but not nearly so large as that for the silicon carbide electrode. The c o n t a c t resistance for a brass electrode was relatively small but greater than for a graphite electrode. Where the cont act resistance is significant as in the case of stainless-steel or silicon carbide electrodes, it depends upon the gas velocity, current density, and nature o f the bed material as well as the electrode material. Thus the cont act resistance generally rises with an increase in gas velocity with the greatest change occurring at the minimum fluidization velocity. Moreover, the c o n t a c t resistance is inversely proportional to the current density raised to some fractional power. This power was found to be 0.5 for a stainless-steel electrode immersed in a fluidized bed o f calcined coke at r oom temperature. F u r t h e r m o r e the c o n t a c t resistance seems to be proportional to the resistivity o f the fluidized bed although the relationship is not a simple one. Current flow within a gas fluidized bed seems to be along continuous chains o f conducting particles, with some arcing taking place between particles at higher applied voltages. The conductivity o f these chains and, hence, the bed conductivity is affected by gas velocity, temperature, particle size, and the inherent conductivity of the individual particles [ 1 1 , 1 3 , 1 4 ] . Although the evidence concerning the effect o f current density on the conductivity and/ or resistivity of fluidized beds is somewhat cont radi ct ory, it appears that at lower levels of current density the resistivity is i ndependent o f this p a r am e t e r whereas at higher levels it is somewhat d e p e n d e n t on it. At higher levels of current density, the t e n d e n c y is for the resistivity to decrease as the current density is increased [ 11]. However, the effect of current density on bed resistivity is much less p r o n o u n c e d than the effect of this parameter on electrode c o n t a c t resistance. When the bed resistivity is i ndependent o f current density and the resistivity is uniform t h r o u g h o u t a fluidized bed, it is possible to apply the basic laws o f electromagnetic fields to analyze and predict the electrical character-
123
istics of conducting fluidized beds [13,15]. The results of such an analysis is a field diagram which shows equipotential and current flow lines that divide the bed into increments of equal voltage drop or equal current flow (Fig. 4). From the information contained in a field diagram and a knowledge of the bed resistivity, it is possible to predict the total bed resistance. In addition the spacing of the equipotential lines provides an indication of the voltage gradient. For example, at the b o t t o m corners of the center electrode in the system shown in Fig. 4, the lines pinch together indicating that the gradient tends to become infinite at these points. Large voltage gradients are conducive ~ NONCONDUCTING SURFACE
ELECTRODE
i
u
U--CURRENT FLOW LINES \ EQUIPOTENTIAL LINES - -
k
NONCONDUCTING SURFACE
Fig. 4. Electric~ field diagram for conducting fluidized bed located between concentric electrodes.
124 to arcing. Since the same a m o u n t of heat is generated within each of the bed segments b o u n d e d by the equipotential and current c o n t o u r surfaces, it is possible to see where most o f the heat will be generated within the system. Thus for the system shown in Fig. 4 the equithermal increments are concentrated around the center electrode so that much of the heat is generated in the region surrounding the electrode.This c o n c e n t r a t i o n could lead to o v e r heating o f the electrode. Of course, another cause o f electrode overheating is a large contact resistance between the electrode and the bed. ECONOMIC EVALUATION The commercial use of electrofluid gasifiers to produce SNG ( synthetic natural gas) will depend ultimately on the costs associated with this m e t h o d relative to the costs associated with other methods. In order to compare the economic merits o f gasifiying coal by means o f an electrofluid gasifier against gasifying it by means of a gas-heated reactor, the cost o f producing SNG by each m e t h o d was estimated. In each case an H T G R was chosen as the primary source o f thermal energy. The costs were based on 1975 prices and on European conditions since most of the work on the utilization o f process heat from HTGRs has been done there. The basis for the cost estimate and the results are presented below. Basis for cost estimate Costs were estimated for the processes shown in Figs. 1 and 2 based on the parameters in Table 1. It was assumed that an H T G R capable of supplying 3000 MW (thermal) and operating at 950°C (1740° F) would be available. This is the m a x i m u m operating t e m p e r a t u r e which is likely to be realized in the near future. Gas compositions and p r o d u c t i o n rates were based on the use of a German high volatile, bituminous coal (Table 1). Basic gasification data for this coal have been determined in small-scale laboratory experiments at Bergbau-Forschung [ 16] and are presented in Table 1. The raw gas from the gasifier was assumed to have the following (moisture-free) composition [17] : 59% H 2 , 2 0 % CO, 18% CO2 and 3% CH4. In estimating process energy requirements, only the energy needed for the gasification reactions and steam generation was taken into account and energy losses and coal preheating were ignored. However, energy recovery from the h o t gases and m e t h a n a t o r was also ignored. Complete conversion of the carbon in the coal was assumed, while a steam conversion o f 50% was used. The operating conditions for the gas-heated plant are summarized in the first column o f Table 2. Material and energy balances for this process were based on those presented by Wiegand et al. [ 1 7 ] . Since only the high-temperature energy from the nuclear reactor can be used in the gasifier, part o f the released nuclear energy is available for ot her purposes such as generation o f
125
TABLE 1 Coal properties and basic parameters used for cost estimate Proximate analysis of coal, wt.% Volatile matter (maf) Ash (mf) Moisture
37 6 4
Ultimate analysis of coal (maf), wt.% Carbon Hydrogen Oxygen Nitrogen Sulfur
82 5 8 2 3
Lower heating value of coal kcal/kg Btu/lb.
7,000 12,600
Kinetic parameters (ref. 16) Activation energy, cal/g mol Btu]lb. coal Frequency factor, 1]min
31,800 57,200 50,000
Basic parameters (ref. 17) Heat of reaction, kcal/kg coal Btu/lb. coal SNG yield, m 3/kg coal ft. 3/lb. coal Steam conversion, % Process water used, kg/kg coal Fluidized bed density, kg/m 3 lb/ft. 3 Power generation efficiency, % Stream factory, h/annum Higher heating value of SNG kcal/m 3 Btu/ft. 3
1,400 2,800 0.88 14.1 50 3.2 200 12.5 39 8,000 8,400 944
electricity. While t h e p r o p o r t i o n o f n u c l e a r energy used in the gasification process rises as t h e H T G R t e m p e r a t u r e increases, o n l y 36% o f the e n e r g y is used f o r gasification with an H T G R o p e r a t i n g t e m p e r a t u r e o f 9 5 0 ° C. It was assumed t h a t t h e rest o f t h e available e n e r g y w o u l d be c o n v e r t e d t o electricity with a g e n e r a t i o n e f f i c i e n c y o f 39%. T h e gasifier t e m p e r a t u r e is t h e n 7 7 1 ° C ( 1 4 2 0 ° F ) and the t o t a l coal t h r o u g h p u t is 208 t o n n e s / h ( 2 2 9 t o n s / h ) . The design o f a gas-heated gasifier w h i c h provides t h e n e e d e d heat e x c h a n g e r area within a reasonable v o l u m e has been described elsewhere [ 1 8 ] . Several gasifiers based o n this design w o u l d be n e e d e d with a t o t a l effective gasific a t i o n v o l u m e o f 1 3 9 9 m 3 ( 4 9 4 0 8 ft. 3 ). With an electrofluid gasifier, all o f the energy f r o m the H T G R w o u l d be used t o generate steam and electricity w h i c h are used in t h e gasifier. With a 3 0 0 0 MW H T G R and a gasifier t e m p e r a t u r e o f 9 0 0 ° C ( 1 6 5 0 ° F ) , 371 t o n n e s / h (409 t o n s / h ) o f coal are gasified ( s e c o n d c o l u m n o f Table 2). A t o t a l gas-
126
TABLE 2 Operating conditions used for cost estimates Gasifier type Gas-heated
Electrofluid
Combination
HTGR outlet temp., °C °F
950 1740
950 1740
950 1740
Gasifier temp., °C °F
771 1420
900 1650
771 and 900 1420 and 1650
208 229
371 409
1399 49408
477 16844
Coal throughput, tonnes/h tons/h Gasification volume, m 3 ft. 3 m3 ft. 3
441 486 1399 } gas-heated 49408 286 } electrofluid 1010b
SNG produced, 1000 m 3/h MM ft. 3/h
183 6.46
326 11.5
388 13.7
Total power generated, MW
755
638
380
Net power for sale, MW
755
0
0
ifier v o l u m e o f 477 m 3 ( 1 6 8 4 4 ft. 3 ) is required w h e n t h e o p e r a t i n g t e m p e r a ture is 9 0 0 ° C ( 1 6 5 0 ° F).
Gas production costs T h e t o t a l p l a n t i n v e s t m e n t including all facilities e x c e p t t h e H T G R and i n t e r m e d i a t e h e a t e x c h a n g e r was e s t i m a t e d to be US$ 6 7 1 . 4 million f o r t h e gas-heated r e a c t o r process, while the e s t i m a t e f o r e l e c t r o f l u i d gasification was US $ 7 8 3 . 8 million (first and s e c o n d c o l u m n s of T a b l e 3). In m a k i n g these e s t i m a t e s , the c o s t of the t w o t y p e s o f gasifiers per u n i t of gasification v o l u m e was a s s u m e d to be the s a m e US $ 9 4 . 3 X 103 / m 3 (US$ 2.67 X 1 0 3 / f t . 3 ). T h e gas-heated gasifier with its h e a t e x c h a n g e r is m o r e c o m p l e x t h a n the e l e c t r o f l u i d gasifier. On the o t h e r h a n d , electrical c o n t r o l e q u i p m e n t is r e q u i r e d with the e l e c t r o f l u i d process. Also, the c o s t o f all e q u i p m e n t associated w i t h S N G p r o d u c t i o n was e s t i m a t e d to be d i r e c t l y p r o p o r t i o n a l to coal t h r o u g h p u t , while the costs o f the electrical p o w e r g e n e r a t o r a n d s t e a m g e n e r a t o r w e r e a s s u m e d to be p r o p o r t i o n a l , respectively, to the a m o u n t s o f p o w e r and s t e a m p r o d u c e d . F o r an S N G p l a n t utilizing gas-heated r e a c t o r s a n d coal at US$ 2 0 / t o n , the e s t i m a t e d fixed capital i n v e s t m e n t , w o r k i n g capital, and a n n u a l p r o d u c t i o n costs are s h o w n in T a b l e s 4- 6, respectively. T h e c o s t o f p r o d u c i n g S N G b y b o t h m e t h o d s is s h o w n as a f u n c t i o n o f coal c o s t in Fig. 5. In e s t i m a t i n g t h e c o s t o f p r o d u c i n g S N G , a c o s t o f US$ 4 / G c a l ( U S $ 1 / M M Btu) was used for the c o s t o f t h e r m a l e n e r g y supplied b y an H T G R . This c o s t includes all
127
TABLE 3 Capital cost of gasification plants in millions of US dollars Unit
Gasifier type Gas-heated
Electrofluid
Combination
SNG production Steam generation Power generation
466.4 24.0 181.0
603.2 27.2 153.4
843.8 24.0 105.4
Total plant investment
671.4
783.8
973.2
TABLE 4 Fixed capital investment for SNG plant based on gas-heated reactors Component
Cost (US$)
Total plant investment Interest during construction (9% × 2 years) Startup costs, 10% of direct operating cost
671,400,000 120,900,000 18,400,000
Fixed capital investment
810,700,000
TABLE 5 Working capital required for SNG plant based on gas-heated reactors Item
Value (US$)
Coal supply, 60 days at US$ 20/ton Materials and supplies, 0.9% of total plant investment Receivables less payables, US$ 20/Gcal (US$ 5/MM Btu)
6,600~000
13,800,000
Total working capital
26,400,000
6,000,000
the costs associated with the H T G R and i n t e r m e d i a t e heat e x c h a n g e r and is based o n d a t a published b y S c h u l t e n et al. [19] o f t h e Federal Republic o f G e r m a n y . While this cost seems low based o n the c u r r e n t e c o n o m i c s o f nuclear reactors, it is t h e cost c u r r e n t l y being used for feasibility estimates in t h e Federal Republic. Since a significant q u a n t i t y o f electricity is generated when H T G R h e a t is used directly, t h e cost o f SNG p r o d u c e d in a p l a n t utilizing gas-heated reactors d e p e n d s s t r o n g l y o n the credit for electrical energy. SNG costs for t h e gas-heated r e a c t o r process are p r e s e n t e d in Fig. 5 f o r t w o values o f electrical energy, 1 ~/kWh and 2 C/kWh.
128 TABLE 6 Annual production costs for SNG plant based on gas-heated reactors Stream factor = 8000 h/annum. Component
Annual cost (US$)
Raw material, coal at US$ 20/ton Utilities water, 40 ~t/m3 (1.1 ~t/ft. 3 ) heat from HTGR, US$ 4/Gcal (US$ 1/MM Btu) Labor, operating and maintenance 800 men at US$ 20,000/a Supervision and administration 80% of labor Supplies, 30% of labor plus 1.5% of total plant investment Taxes and insurance 2.7% of total plant investment Total direct operating cost Depreciation and interest 15.6% of total fixed investment Interest on working capital, 9%
36,600,000
126,500,000 2,400,000
Total production cost
312,600,000
2,100,000 83,200,000 16,000,000 12,800,000 14,900,000 18,100,000 183,700,000
DISCUSSION
Based on the costs shown in Fig. 5, electrofluid gasification is competitive with the direct use of HTGR heat when the value of the by-product electrical energy is 1.8 C/kWh or less under the assumptions used in making the cost estimates. Since the by-product electricity from the gas-heated reactor process would have to be sold at base load prices, it is improbable that the selling price would be greater than 1.8 C/kWh. Any changes in or differences between the costs of the two types of gasifiers will affect the economic comparison between them. Since the gasheated reactor process requires more gasification volume, its economics are more strongly affected by changes in gasifier cost. For example, if the cost per unit of gasification volume is increased to U S $ 1 4 1 . 5 X 103/m 3 (US$ 4.01 X 103/ft. 3 ), the cost of SNG produced by this process is increased by US$1.2/Gcal (US$ 0.3/MM Btu). However, the cost of gas produced by the electrofluid gasifier method is increased by only US$ 0.2/Gcal (US$ 0.05/MM Btu). Obviously, any decrease in gasifier cost tends to favor the use of the gas-heated reactor. Electrofluid gasifiers would be attractive for gasification of unreactive coals where a higher temperature would reduce the gasification volume required. Another alternative which might be attractive under some conditions is a gasification plant employing both gasheated and electrofluid gas generators (Fig. 6). In such a plant, the hot helium from the gas-heated gasifiers would
129
223 21 -
:E
J 19
-
~o~
~_
~
~
~
75 -
1
L
l
20
30
40
/
COAl. COST, S/ton Fig. 5. SNG p r o d u c t i o n costs as a f u n c t i o n o f coal cost. C o m b i n a t i o n system i n c l u d e s b o t h types o f gasifiers.
ELECTRICITY
540
-oc~ 1
I ~ 250~
8 2 ~ IX~ I 900 °C[ <
7~, I
I
c02, H2S
- HEATEXCHANGER STEAMGENERATOR GAS-HEATEDGASIFIER TO
950 °C
I
GASCLEANING 6 - METHANATOR 7 - POWERSTATION
Fig. 6. Coal gasification system using b o t h thermally and electrically coupled H T G R to provide heat.
130 be used to generate steam and electricity for electrofluid gasification. Based on the assumptions used in previous calculations, such a plant could gasify an additional 233 tonnes/h (257 tons/h) of coal employing an electrofluid gasifier with a volume of 286 m 3 (10106 ft. 3 ) at 900°C (1650°F) (third column of Table 2). The total coal throughput of this plant would then be 441 tonnes/h (486 tons/h). The total capital cost of this plant ~,vas estimated to be US$ 973.2 million (third column of Table 3), giving the SNG costs shown in Fig. 5. Needless to say, this alternative has the highest thermal efficiency of any of the processes, 57% as compared to 54% and 53% for the gas-heated reactor and electrofluid processes, respectively. The thermal efficiency corresponds to the ratio of the energy recovered as SNG or electric power to the energy supplied as coal or heat from the HTGR. The combined use of both types of gasifiers would be attractive if the selling price of electricity is low. Also if conditions are such that high carbon conversions are not achieved in the gas-heated reactors, the residue could be gasified at a higher temperature in an electrofluid reactor. In summary, electrofluid gasification coupled with an HTGR appears attractive when compared to direct use of thermal energy from the reactor under several circumstances. These include a low selling price for the byproduct electricity, low coal reactivity or high gasifier cost. It should be noted that as the HTGR temperature is raised the a m o u n t of energy which needs to be converted to electricity in the gas-heated reactor process is significantly reduced. This increases the overall efficiency of the process and reduces the cost significantly, thus making the process relatively more attractive compared to electrofluid gasification. CONCLUSIONS The results of the preliminary cost estimate which are presented here show that coal gasification in reactors coupled electrically with an HTGR may compete economically with gasification in reactors coupled thermally with an HTGR. Electrical coupling offers several advantages over thermal coupling. Thus with electrical coupling the gasifier temperature is independent of the nuclear reactor temperature and is not limited by the latter. By employing higher gasifier temperatures, faster rates of reaction are obtained and greater gasifier productivity is realized. Also with electrical coupling the gasifier design is much simpler than with thermal coupling and obtaining suitable materials of construction, except possibly for the electrodes, is less of a problem since only refractory materials are required to be in contact with the hot reaction mixture. In addition electrical coupling provides greater flexibility than thermal coupling with regard to both plant siting and operation since by-product electrical power is not produced. Moreover, the thermal efficiency of a gasification process employing electrical coupling is only slightly less than one employing thermal coupling at reasonable HTGR temperatures. A particularly attractive alternative would be a gasification plant utilizing
131 an H T G R in c o m b i n a t i o n with b o t h d i r e c t and electrically h e a t e d gasifiers. Such a p l a n t w o u l d have t h e highest t h e r m a l e f f i c i e n c y o f t h e alternatives c o n s i d e r e d and m o s t p r o b a b l y w o u l d p r o d u c e gas f o r t h e l o w e s t price. Furt h e r m o r e , t h e o p e r a t i o n o f this p l a n t w o u l d o f f e r m o r e flexibility t h a n a p l a n t c o n t a i n i n g o n l y gasifiers using d i r e c t heating. F o r e x a m p l e , t h e electrofluid gasifier c o u l d b e used t o gasify high ash residue, or s o m e o f the electrical p o w e r m i g h t be sold w i t h m o r e p o w e r being supplied to e l e c t r o f l u i d gasifier w h e n d e m a n d f o r p o w e r was low. T h e t e c h n i c a l feasibility o f t h e e l e c t r o f l u i d gasifier which c o u l d be c o u p l e d electrically t o a n u c l e a r r e a c t o r has b e e n d e m o n s t r a t e d in b o t h b e n c h and p i l o t p l a n t scale e q u i p m e n t . In a d d i t i o n t h e i m p o r t a n t electrical and p h y s i c a l characteristics o f t h e s y s t e m have b e e n d e t e r m i n e d so t h a t larger units m a y b e designed with s o m e degree o f c o n f i d e n c e . In light o f t h e s e e n c o u r a g i n g p r e l i m i n a r y results o n e c a n see t h a t a m o r e detailed and c o m p r e h e n s i v e design s t u d y and m o r e definitive cost e s t i m a t e are n o w in o r d e r t o p r o v i d e f u r t h e r c o n f i r m a t i o n . This s t u d y should consider m o r e t h a n o n e t y p e o f n u c l e a r r e a c t o r and c o n d i t i o n s in t h e U n i t e d States as well as E u r o p e . F u r t h e r m o r e , it should also c o m p a r e t h e m e t h o d s o f gasification d e s c r i b e d h e r e w i t h o t h e r m e t h o d s being d e v e l o p e d w h i c h d o n ' t utilize n u c l e a r energy. ACKNOWLEDGEMENTS This p a p e r was p r e p a r e d while o n e o f t h e a u t h o r s (A.H.P.) was on-leave at B e r g b a u - F o r s c h u n g , Essen, G . F . R . T h e h e l p f u l discussions w i t h Drs. Feistel and V a n H e e k and Mr. Wiegand a b o u t t h e use o f n u c l e a r h e a t f o r coal gasification and t h e financial s u p p o r t o f t h e A l e x a n d e r v o n H u m b o l d t F o u n d a t i o n d u r i n g this t i m e are g r a t e f u l l y a c k n o w l e d g e d . T h e e x p e r i m e n t a l w o r k o n electro fluid gasification at I o w a S t a t e University was s u p p o r t e d b y t h e E n g i n e e r i n g R e s e a r c h I n s t i t u t e t h r o u g h f u n d s m a d e available b y E R D A und e r c o n t r a c t No. E ( 4 9 - 1 8 ) - 4 7 9 .
REFERENCES 1 Kr~mer, H. The high temperature reactor in the Federal Republic of Germany -- Present situation, development progress and future aspects. International Symposium on GasCooled Reactors with Emphasis on Advanced Systems, J~lich, Germany, Oct. 13 17, 1975. 2 Schulten, R., J~lntgen, H. and Teggers, H., 1976. Nukleare Prozessw~rme f~]r die Kohlenvergasung. Gl~]ckauf, 112: 14. 3 Quade, R.N. and McMain, A.T. Nuclear energy for coal gasification. Symposium on Clean Fuels From Coal, Sept. 10--14, 1973, Institute of Gas Technology, Chicago, Ill. 4 Quade, R.N. and Woebcke, H.N. HTGR for coal gasification/liquefaction. BNES International Conference, London, Nov. 1974. 5 Pulsifer, A.H. and Wheelock, T.D., 1974. Observations of coal processing in an electrofluid reactor. J. Inst. Fuel, 47: 235. 6 Kavlick, V.J., Lee, B.S. and Schora, F.C., 1971. Electrothermal coal char gasification. AIChE Symposium Series No. 116, 67 : 228.
132
7 HYGAS: 1972 to 1974. Pipeline gas from coal hydrogenation, (IGT Hydrogasification Process). R&D Report No. 110: Interim Report No. 1, Institute of Gas Technology, Chicago, Ill., prepared for ERDA, Washington, D.C., 1975.. 8 Pulsifer, A.H., Knowlton, T.M. and Wheelock, T.D., 1969. Coal char gasification in an elec trofluid reactor. Ind. Eng. Chem. Process Des. Develop., 8: 539. 9 Beeson, J.L., Pulsifer, A.H. and Wheelock, T.D., 1970. Coal char gasification in a continuous electrofluid reactor. Ind. Eng. Chem. Process Des. Develop., 9: 460. 10 Beeson, J.L., Pulsifer, A.H. and Wheelock, T.D., 1974. Hydrogen from coal char in a continuous elecgrofluid reactor. Ind. Eng. Chem. Process Des. Develop., 1 3 : 1 5 9 . 11 Pulsifer, A.H. and Wheelock, T.D. Coal processing by electrofluidics. Final Report, Engineering Research Institute, Iowa State University, Ames, Iowa, prepared for Office of Fossil Energy, ERDA, Washington, D.C., 1975. 12 Chen, T.P. Yuan, E. and Pulsifer, A.H. Bed and contact resistances in an electrically conducting fluidized bed. Amer. Inst. Chem. Eng. Meeting, Washington, D.C., Dec. 1974. 13 Pulsifer, A.H. and Wheelock, T.D. Coal processing by electrofluidics. Interim Report No. 3, Iowa State University, Ames, Iowa, prepared for Office of Coal Research, Dept. of the Interior, Washington, D.C., 1974. 14 Jones, A.L. and Wheelock, T.D., 1968. The electrical resistivity of fluidized carbon particles: Determination of resistivity by the four terminal method. I. Chem. E. (London) Symposium Series No. 30, p. 174. 15 Knowlton, T.M., Pulsifer, A.H. and Wheelock, T.D., 1973. Prediction of fluidized bed resistance using field theory. AIChE Symposium Series No. 128, 69: 94. 16 Van Heek, K.H., J~Jntgen, H. and Peters, W., 1973. Fundamental studies on coal gasification in the utilization of thermal energy from nuclear, high-temperature reactors. J. Inst. Fuel, 46: 249. 17 Wiegand, D., Van Heek, K.H. and Jilntgen, H. The significance of the HTR temperature for the economic use of nuclear heat for coal gasification. BNES International Conference, London, Nov. 1974. 18 JUntgen, H., Van Heek, K.H., D~]rrfeld, R. and Feistel, P.P. Kinetics, heat transfer and engineering aspects of coal gasification with steam using nuclear heat. BNES International Conference, London, Nov. 1974. 19 Schulten, R., Von der Decken, C.B. and Barnert, H. Nuclear water splitting by heat from the pebble bed HTR. BNES International Conference, London, Nov. 1974.