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Reaction rate analysis of borehole ‘in-situ’ gasification systems
REACTION RATE ANALYSIS OF BOREHOLE 'IN-SITU' GASIFICATION SYSTEMS I. McC. STEWART AND T. F. WALL
Department of Chemical Engineering, The University of Newcastle, N.S. W. 2308, Australia Performance is reported for quasi-steady state computational models of two zones of borehole type in-situ gasification systems. The plug-flow reduction zone is a close replica of real systems; the well-stirred reactor models some special systems and may be indicative of a wide range of ignition zone conditions. For both models it is postulated that seam moisture evaporation and coal devolatilization occur at some distance (inversely proportional to the advance rate) behind a reacting face of coke and that reaction of these products with coke occurs behind the face. This model produces an apparent reactivity for the system more strongly dependent on seam moisture and volatile content than on the chemical reactivity of the coke. Computations predict that gasification efficiency and combustion stability axe sensitive to an optimal combination of seam moisture and of blast moisture. These findings and the predicted gas analyses and efficiencies are generally in line with what published field information is available. The advantageous effect of high moisture content in the gas is to increase reaction rate (by increasing the concentration of a major reactant); this is balanced by the temperature drop due to added heat capacity. The overall effect is so dramatic as to warrant this type of analysis before completing design for field experiments. Laboratory testing on large samples of seam coal is advisable to test the model for detail and obtain better rate coefficients. It is predicted that high ratings and high gasification efficiencies are possible for oxygensteam gasification either in plug-flow or W.S.R. systems, particularly if preheat can be achieved.
Th e possibility of gasifying coal "in-situ,'" as an alternative to the costly and dangerous labour of m i n i n g has excited the minds of m e n since Siemens and later Ramsay in Great Britain and Men d el ce f in Russia proposed such a system in the later n i n e t e e n t h century. Russian experimental work c o m m e n c e d before 1930 and operations on a commercial scale, at 60-100 M W - - m o d e s t by current standards, have been continuous since about 1950. Foll o w i n g the " en erg y crisis" of the 1930's extensive experimental work was carried out up until about 1960 in the U n i t e d States, d o c u m e n t e d by U.S. Bureau of Mines; 1 in Great Britain, well d o c u m e n t e d by Sir Alex. G i b b and Partners 2 and in France and Poland. So far experimental and operational work has b e e n with air-blast to p r o d u c e a low C.V. gas w h i c h must compete with m i n e d coal as an energy source. For coal deposits u n e c o n o m i c a l or unworkable by conventional mining, e.g. very dirty seams, thick or stressed seams too deep 525
for opencast working, this still appears an attractive potential m e t h o d of w i n n i n g resources w h i c h w o u l d be otherwise unavailable. For this reason it has recently alitracted r en ew ed interest in the United States and Europe. However, now that the need for synthetic natural gas and l i q u i d fuels is clearly in sight, the proposition becomes very attractive. Th e major capital and operating cost in a synthesis project is the plant for gasification of the coal. If, as is generally estimated, the cost of "insitu" gasification is comparable with the cost of w i n n i n g the same a m o u n t of energy as coal, then "i n - si t u " gasification has a cost advantage comparable to the cost of the gasification plant. 3,5 This requires either oxygen-steam gasification or a cyclic "water-gas" type of operation, neither of w h i c h have been tried " i n - s i t u " for other than brief exploratory runs, Clearly a good u n d e r s t a n d i n g of the combustion and reduction regimes is necessary before
526
ENERGY PRODUCTION FROM COAL
very expensive field experiments are undertaken. Very few attempts have apparently b e e n made to analyse these regimes. Almost all of the field work reported has been devoted to the difficult tasks of securing linkage b e t w e e n boreholes, e s t a b l i s h i n g that ignition can b e achieved and stable c o m b u s t i o n controlled, and d r i l l i n g long large boreholes where required. The objective is to assess the significance of the chemical kineties and the relative importance of coal and blast characteristics and then, with the a i d of appropriate laboratory experiments to m o d e l more adequately a real system. The models s t u d i e d are most simply related to the simple British P5 system (Fig. l(a)). 2 Air was i n t r o d u c e d to a drive linking a set of parallel boreholes. These b u r n e d ' r a p i d l y to a tapered shape w h i c h then a d v a n c e d at more-or-less steady form. At the borehole spacing of about 20 m in a seam of 1.5 m height most of the tapered o p e n i n g is very wide; transport of oxygen from the bulk-stream I///111~111111///111111///I/
to the b u r n i n g faces is slow and it can be expected that overall surplus oxygen will remain until the w i d t h has become c o m p a r a b l e with the height. 4 Szekely 4,7 made an algebraic analysis of an e d d y - d i f f u s i o n controlled m o d e l of this wide oxidation zone and c o n f i r m e d this expectation. T h e result of this b e h a v i o u r is that the region available for the reduction reactions is small. As most of the area exposed to heat losses is located before reduction commences, the t e m p e r a t u r e range available to s u p p l y e n d o t h e r m i c reactions is iat a m i n i m u m . Doubts have b e e n expressed as to w h e t h e r kinetics will p e r m i t efficient gasification.
The Plug Flow Reduction Model
We therefore m a d e it our first task to examine the amount of reduction possible in such a zone if the gaseous products of complete combustion (without oxygen) are input to it. T h e range of expectable i n p u t conditions are considered, i.e. the height of seam, w i d t h of passage at c o m p l e t i o n of oxidation, e n t h a l p y of entering stream [after various levels of heat loss] and taper of passage of length L to an I111/1//I/11111111111111111111///I//, exit borehole (assumed at full seam height b u t width 0 . 3 / H . (Fig. 2a) Steady advance rate (quasi-steady state) conditions are assumed; strata losses and reaction heats are s u p p l i e d from the sensible heat of the gas stream conveered and radiated to the b o u n d i n g surfaces.
'r
. ,, .
.
.
The Well-Stirred Oxidation Model I
a
II
A particular d i s a d v a n t a g e of the system (l(a)) is that the greatest area of exposed coal surface is likely to be at the thin end of the coal w e d g e (the air entry point), where strata pressure and
b
~-.----
c
ZL 3
fl
Fro. 1. In-situ borehole gasification systems: (a) Drift and boreholes [ref. 2; p5 pl19], (b) Parallel borehole with reverse blast, (c) Boreholes normal to seam [ref. 2; p195], (d) Thick seam blind borehole [ref. 5]
a
b
FIG. 2. System models studied: (a) Plug flow reduction zone, (b) Jet stirred combustion zone
BOREHOLE 'IN-SITU' GASIFICATION SYSTEMS thermal stress will break and spread the coal. This extended surface area is not needed in an oxidation zone. It is clear that if c o m b u s t i o n could be completed in a compact high intensity zone, much more area would be available for reduction and c o m b u s t i o n products would enter this zone at a higher temperature. This may be achievable b y cyclic operation between parallel boreholes, oxidant b l o w n into one set and gaseous products removed from an adjacent borehole set (l(b)). This would place the region of broken coal in the reduction zone. Something of this apparently occurred in a Russian linked borehole system (l(c)) where a reverse flow stage occurs. "Blind-borehole" systems are of this type and some results are available. 2 A jet-stirred reactor has been proposed by A. D. Little Inc., 5 (Fig. l(d)), for extremely thick seam operations. We have therefore not yet tackled the elaborate three dimensional model needed for adequate analysis of the c o m b u s t i o n zone of Fig. l(a) but have given priority to examination of the conditions u n d e r which an intensive c o m b u s t i o n zone can be established as a jet stirred reactor. This model is also ealculated on a quasisteady state basis, i.e. by assuming that a regime has been reached where each part of the b u r n i n g face moves forward at a constant rate so that the shape of the zone is constant. For this to occur the shape must become broader than the spread of the jet from the borehole, the flow pattern will then be a jet with heavy recirculation. This is approximated as a well-stirred reactor (W.S.R.) This model is idealised in Fig. 2(b). For both models the critical assumptions are those relating to the nature and behaviour of the coal faces exposed to reacting gases and to the corresponding heat and mass transfer relations. These are examined in detail below. Burning Face Conditions
temperature, the relation is: T-
To+ L'=(T s-
where L ' = L w / ( C c + wCw) and a ' = k / p ( C c + w C w) It should be noted that for the steady advance of an infinite face the face heat flux requirement is simply the sum of the sensible heat content of dry coal and enthalpy of water vaponr (above liquid base) to face temperature and any reaction heat (positive if endothermal). In the reduction zone at the m a x i m u m advance rate, U, of about 0.1 x 10 -5 m / s e c the thickness of the coke zone from 900 to 1900~ is then about 1 in for a coal with 10% seam moisture and still over 30 eros for a b r o w n coal at 50% moisture (dry coal basis), with a comparable further distance to the drying zone. The face model is then of a b u r n i n g face of reasonably high temperature coke towards which volatiles a n d water vapour are percolating through shrinkage cracks. (Fig. 3). This pattern has b e e n confirmed in the field. 7,1 I n the reduction zone the m a x i m u m water vapour flow expected from a wet coal (water 0.5 times dry coal mass) is only 2.9 • 10 -3 n . m . / s e c on superficial area allowing, at 20% voids, several seconds contact time with reactive coke at from 900 to 1900~ considerable reaction can be expected. To estimate face temperature it is necessary to estimate the heat requirement of such reactions. A n y precise calculation for such a physically u n k n o w n a n d variable system is out of the question; we have quite arbitrarily assumed that half the seam water is deeomposed a n d that CO and CO z
I
rs
I.!
g
~
"~
z
~-
I .a.,g " |i
co2 o2 H20 co
.D I
I
T o = (T s -
To+ L') e x p ( - Ux/c~' )
Behind a b u r n i n g face of fixed carbon the coal is pyrolysed and the bed moisture evaporated. To assess the interactions of these components we use as a model a plane face, advancing at a steady rate by gasification of carbon. For homogeneous material the temperature falls exponentially with distance. For an infinite plane face T-
527
I
/
'
I
~%\"\\"'~
~H
2
I"~
T O) e x p ( - U x / a )
If there is a latent heat load the temperature gradient steepens and, above the evaporation
FIG. 3. Face model--temperature and reactant distribution.
528
ENERGY PRODUCTION FROM COAL
are produced in equal proportions. These assumptions are reasonable for the high moisture lignite but may underestimate CO p r o d u c t i o n for the b i t u m i n o u s coal. In the c o m b u s t i o n zone at atmospheric pressure, the expected face temperature gradient is very m u c h steeper and the validity of the model must be examined. The major i m p l i c a t i o n of the model is that the advance rate is determined by the reaction rate of only a portion of the fixed carbon of the coal. Carbon reacted at the face, Cf = fixed carbon ( C ' f ) less carbon reacted with seam moisture (fw) where f is the carbon requirement for water vapour reactions. The rate of advance in terms of dry coal mass is then R c / Cf if R c is the carbon reaction rate at the face. For a low rank coal with 45% fixed carbon a n d 50% moisture (dry coal basis) C i = 0 92 and the coal advance rate is 5 times the carbon reaction rate. This suggests that for "in-situ" gasification seam moisture has some advantages a n d that the low residual fixed carbon of low rank coals may be more significant than the coke reactivity.
and 1 at steam at temperatures from i 0 0 0 to 1600 K measured b y weight loss on measured lumps of high temperature coke. There is general agreement that oxidation rates fall off at high temperatures and that water vapour reduction is strongly inhibited by hydrogen, etc. In no way, however, can reliable coefficients be deduced for this specific application. We have therefore simply extrapolated these measured rate figures with the reservation that the significance of surface reaction rate must be critically examined in the results. Because of the expected heavy faee fissuring we have taken exposed coke area as 2.5 times the nominal plane surface a n d c o m b i n e d this area factor in the rate constant g i v i n g : - -
( 4 00) _
KRo 2 = 10.0exp
-
KRH20 = 1.03 x 10 a exp Kacoz = 0.31 x i 0 a exp
_
RT
( 48000 RT
]
( 4 8 9_ 0 _0 ) _ RT
Sensitivity tests are reported for the effect of changes in K R.
Face Reaction Rates Three gases, 0 2 , H 2 0 and CO 2 are assumed to react simultaneously at the carbon surface; i.e. Total rate of carbon gasification R e = S(Po2Ko2 + PH2oKHeo q-Pco2Kco2) where 1
1
1
Ko 2
KD
Kr~o2
Any real face will be rough. For mass and heat transfer only the simplest assumptions are warranted. Stanton n u m b e r has been taken at 0.014 (half the friction factor for a very rough duct,) giving h = 0.014 Cprh for heat transfer and k o = N / ~ p = 0.014 m / P M for coefficients based on n o m i n a l face area. For the well stirred reactor system, after some qualitative studies of expected steady b u r n i n g shape a n d jet recirculation patterns we assumed the characteristic velocity to be that due to a total flow of 2.25 jet flow at the mean cross section, giving K o = 0.378 rh (jet)/PMS [A sensitivity test is reported]. For the surface reaction rate coefficients K R the two problems are to estimate both suitable reaction rate coefficients in terms of external surface and also the external surface. We have used rate data b y Cruz 8 at 0.03 to 1 at oxygen
Heat Losses and Heat Transfer I n the real system the advancing face is of finite height b e t w e e n non-reacting roof a n d floor rocks, to w h i c h heat is lost by c o n d u c t i o n from b e h i n d the face, increasing the heat loss from the face itself, and by convection and radiation from the gas space. We have estimated the total heat loss by approximation developed b y Loison 9 who represented the gasification zone b y the shape of an isotherm for a steadily a d v a n c i n g line source at mid-seam height, giving for our range of interest H e a t / u n i t length, q' = 10.2k
(T s -
T o)
The a s s u m p t i o n in detail implies that the face and adjacent floor and roof are at a temperature Ts, a d v a n c i n g at a lineal velocity U and that roof a n d floor are m a i n t a i n e d at a temperature falling off very gradually for a distance back of several times the seam height. [If anything, it overestimated the losses to drtj strata in the systems we are studying]. Face and gas temperatures are related b y the heat-transfer required to supply losses a n d
BOREHOLE 'IN-SITU' GASIFICATION SYSTEMS surface reactions. Initial calculations predicted quite low temperature differences. We have assumed convection plus carbon-dioxide a n d water vapour radiation (using the Hadvig 1~ correlation) for the reduction zone and have assumed radiation from a black-flame in the c o m b u s t i o n zone.
General Assumptions and Method For each model the objective is to compute carbon gasification rate, strata losses, and gas and surface temperatures for a range of coal properties, bed moisture contents and blast conditions, for the reduction zone model, inlet gas is the products of stoichiometric combustion at coal plus blast enthalpy less specified strata losses. C o m p u t a t i o n proceeds by finite difference steps in gas temperature for each of which carbon reaction rate, losses, and change of gas composition per u n i t surface are calculated; thence surface temperature is recalculated and the surface area required for the specified temperature step. For the W.S.R. c o m b u s t i o n zone model, reaction rate is calculable at a specified surface temperature a n d assumed gas composition. Gas composition is then iterated to obtain a mass balance. From this i n p u t and output enthalpies and conduction heat losses are calculable as functions of temperature. Ignition a n d stable b u r n i n g conditions are obtained from the two temperatures at which energy i n p u t and output rates coincide. Coal properties selected for computation
Test Calculations at 10% loss 20% H 2 0
Seam heights m
Blast nma/sec
1.5
1.4
1.5
2.0
The major variables investigated in the computational program were the strata losses in the c o m b u s t i o n zone, the water vapour content of the c o m b u s t i o n gases, the height of seam, the l e n g t h o f the reduction zone, the coal type a n d the relative influence of mass transfer and chemical reaction rate. All calculations were made for the tapering reduction zone, typical of a borehole system, with low entry mass velocity. Typical profiles of temperature and cold gas efficiency (The proportion of the heating value of the coal (net)recovered in the product gases, w h e n cooled)
529
TABLE I Coal properties: Calculated to dry coal basis. Bituminous Sough 2
Brown Angren 2,6
0.064 0.37 0.51 0.14
0.52 0.50 0.36 0.14
7475 , 0.74 0.05 0.09 0.49
5692 0.66 0.03 0.17 0.18
Moisture Volatile H.C. F. Carbon Ash Gross cal. value Kcal / kg C H O Cf
correspond to coals for which some field date is obtainable (Table I). The blast volume reported here corresponds to the upper level of reported practice. Results a n d Discussion
1. Plug Flow Reduction System A reasonable check on the magnitude of the results is possible from day 90 of the P5 results (Ref. 2 pp142-152) where a plan of b u r n o u t zone as established by test drilling enables a reasonable estimate to be made of the area available for reduction. The comparison is as f o l l o w s : - -
Equivalent face length m min25
max35
40
Gas CV net K cal/m a
Cold gas efficiency %
490-540
45Z55
630
49
against path length are shown in Figures 4 and 5 for b i t u m i n o u s coal and b r o w n coal. Strata losses for the c o m b u s t i o n zone only have been taken to bracket the expected range at 20% (which is higher than reported for most complete gasification systems) and zero (which is equivalent to 10% strata loss together with 10% sensible heat recovery as blast preheat). The effect on reduction rate and overall coldgas efficiency is striking. The results indicate that a reasonable efficiency is barely obtainable with the 40 m of equivalent face readily obtainable i n a bore-
530
ENERGY PRODUCTION FROM COAL f
I
r
5o
10
,g
s e.5
.=_
.u
25 "~
1
g, 2500
0
~.,2ooa
240(
-
200{
m m m.
~.151
1600
[
10
|
210 Length,lL]
30
40
m
FIG. 4. Plug flow reduction; bituminous coal: 1) Blast 20% moisture, net combustion losses 20%; seam 1.5m, 2) Blast 20% moisture, net combustion losses 20%; seam 3.0m, 3) Dry air, net combustion losses 0%; seam 1.5m, 4) Dry air, net combustion losses 20%; seam 1.5m hole system. Some increase in performance is expectable with greater available area b u t not enough to warrant expensive fracturing system s. The most significant conclusion is that, for this system, mass transfer control is p r e d o m i nant d o w n to about 1400 K with the corollaries that high gas flow rates m a y be advantageous [which are c o n f i r m e d in practice, Ref. 2 pp189 and 193]. Steam a d d i t i o n to the blast p r o d u c e s high reduction rates because of the higher water-vapour partial pressure, but in the reverse direction it increases the heat capacity of the gas stream giving lower temperatures entering the c o m b u s t i o n zone. For the b i t u m i nous coal there is very little difference in c o m p u t e d p e r f o r m a n c e between dry air a n d the very high steam content of 20%. Ref. (2) quotes Russian figures s h o w i n g an improvement in gas quality and efficiency b y steam addition for a very dry seam. For the h i g h moisture b r o w n coal the effect of a d d e d moisture is deleterious. T h e balance appears to be more critical a n d at a different level from the conventional gas producer. At the low p r e d i c t e d temperatures for brown-coal a c o n s i d e r a b l e control b y chemical
1200
I 1O
[ 20
I 30
40
L,m
FIG. 5. Plug flow reduction; brown coal: 1) Blast 20% moisture, 10% losses, seam 1.5 m, 2) Dry air; 0% losses, seam 1.5 m, 3) Dry air; 10% losses, seam 1.5m, 4) Dry air; 10% losses, seam 1.5 and K R = 3 times K R used previously. reaction rate w o u l d be expected. T h e computations used the same hard-coke reactivity for both coals. Curve 4 of Figure 5 shows the comparatively small effect, b y c o m p a r i s o n with curve 3, of a threefold increase in a s s u m e d reactivity. For brown-coal installations reference(6) claims higher calorific value and efficiency than we computed. H o w e v e r these claims are for complete systems and will include the contribution of distillation products from coal s u r r o u n d i n g the burn-out zone. 2. Well-Stirred Reactor A representative set of results are shown in Table II. The sensitivity of this model to the moisture contents of coal and blast is very striking. At dry blast a n d dry b i t u m i n o u s coal will only sustain stable c o m b u s t i o n at a coinparatively low rating (low blast r a t e / c o a l face surface area) and at high residual oxygen content. At a h i g h moisture content in the blast stable operation is indicated at high ratings, with ignition at a lower temperature and considerable p r o d u c t i o n of h y d r o g e n and carbonmonoxide at the stable condition. Under these conditions the reaction rate is entirely sus-
BOREHOLE 'IN-SITU' GASIFICATION SYSTEMS
531
TABLE II W.S.R. with reacting wall [ DA--dry air BLAST ]A20--80% air 20% H20 2 mS/sec at 1 at ~080--20% oxygen 80% H20 300K from borehole ~067--33% oxygen 67% HeO of 0.35 m diameter \Coal properties as Table I. Temperature Ignition Burning K K
Dry cool rate Kg/sec
Combustible (CO & H2) %
Varying blast conditions 1640 1900 No ignition 1625 2025 1440 1735
0.155 -0.223 0.365
Oxygen 7.2% -6.17 10.12 13.6 13.0 13.1 30.6
Coal
Blast
Coal face m 2
Sough
DA DA A20 080
100 25 25 25
Angren
DA A20 080 067
25 25 25 25
<1200 1430 1300 <1200
1860 1590 1530 1700
0.424 0.207 0.415 0.840
Sough (bit)
A20 A20 A20
Varying Rating 10 1790 25 1625 100 1440
2055 2025 1990
0.214 0.223 0.230
Angren Brown
DA DA DA
10 25 100
1900 1860 1790
0.387 0.424 0.457
1600 < 1200 < 1200
tained by the face reactions of CO,z and H 2 0 with carbon at temperatures where rate is p r e d o m i n a n t l y mass transfer controlled. This is apparent from the shape of the rate curves and has been checked b y sensitivity tests in which reduction of the mass transfer coefficient to one half of the normal value produced a larger change in computed performance than reduction of reaction rate constants b y a factor of ten. O n the other hand, the high moisture coal with its heavy latent-heat debit will only just ignite u n d e r W.S.R. conditions if the blast has an appreciable moisture load, with corres p o n d i n g l y increased heat capacity. The b r o w n coal shows a higher apparent reactivity than the b i t n m i n o u s in lower ignition temperatures and greater production of CO and H 2 at lower b u r n i n g temperatures. As in the plug flow model the same carbon reactivity has been assumed; the difference is due to the effect of the higher volatile and moisture content which, in our model, reduces the proportion of carbon to be reacted at the face. The general behaviour is in line with qualitative reports of Russian experience in Ref. 2, both with regard to the improvements by
4.70 6.17 7.16 10.1 13.6 16.8
blast moisture addition for dry coals and, the deleterious effect of water seepage in brown coal installations. These differences show up more strikingly in this combustion model than in the reduction model. The effect of our face model is so great that experimental checks are desirable. At the highest rates tabulated (which at 10 m 2do represent as small and compact a c o m b u s t i o n zone as we can imagine) the rate of face advance is a little under 2 • 10 * c m / s e c , which would require a temperature gradient close to the face of about 600K per cm. It is a real question whether the material could tolerate such gradients or whether, for such very high ratings the hot zone would intermittently flake off the face exposing cold surface. At lower ratings, particularly for b i t u m i n o u s coal, the model appears feasible. The effect of rating, i.e. available surface for a given blast rate, is not nearly so significant; a ten-fold increase in available surface shows a drop in the required ignition temperature but, particularly for the b i t u m i n o u s coal, only a modest drop in stable b u r n i n g temperature. There is, however, with increased comb u s t i o n zone area an increase of 10-20% in
532
ENERGY PRODUCTION FROM COAL
carbon c o n s u m p t i o n with production of combustible gases. This limited sensitivity of rati n g i s largely a r e s u l t of the conditions a s s u m e d for the model in w h i c h a constant shape is assumed for the reaction zone with the result that the gas velocities (and transfer coefficients) vary inversely with the surface area. However we b e l i e v e that this is a realistic assumption. The calculated effect of seam height has not been tabulated as it is very small w h e n c o m p a r e d on the basis of equal total surface area. However in a field situation the p l a n dimensions of a zone clear of obstructions will d e p e n d on roof behaviour. The expected surface area available will then be proportional to seam height, with the performance improvement discussed above. As an indication of the order of m a g n i t u d e of the rates reported the m a x i m u m rate tabulated is 2.83 x 10-3 g m / e m 2 s e c . Free b u r n i n g vertical w o o d e n slabs of less than 1 m h i g h are reported as b u r n i n g at 2.8 x 10 -a g m / c m % e c in cold surroundings. IX F i e l d trials reported for b l i n d - b o r e h o l e experiments in Ref. (2) ( p l i 3 ) show that, at least in the early stages, the system was p r e d o m i nantly a jet stirred reactor with velocities comparable to our basis. A highly p r e h e a t e d blast of dry air gave stable operation w i t h b i t u m i n o u s coal over a range of surface ratings comparable with our tabulations. Gas was somewhat richer t h a n we calculate b u t obviously i n c l u d e d a considerable p r o p o r t i o n of excess (unsteady state) volatile matter. Results do show that a considerable degree of water and C O 2 reduction can be aehieved in a W.S.R. system. Further computations indicate that an oxygen-steam blast p r e h e a t e d to 500~ (e.g. b y regenerative cyclic operation) could gasify coal at 0.1 M W / m 2 with 60% cold gas efficiency in an open W.S.R. alone. For any b o r e h o l e system stability a n d compactness of the c o m b u s t i o n zone is essential for efficient operation. In m a n y layouts the oxidation zone m a y be p r e d o m i n a n t l y p l u g flow. Ignition c o n d i t i o n s for pure p l u g - f l o w oxidation are very d e p e n d e n t on the d e t a i l e d geometry adjacent to the ignition area. However any real system of a high velocity jet issuing into an irregular volume must i n c l u d e m a n y turbulent reeirculation zones and c o u l d be expected to b e h a v e in a manner qualitatively related to this s i m p l e W.S.R. model. Conclusions
of a plug-flow t a p e r i n g reduction zone w i t h evaporation, distillation and some carbon reaction b e h i n d the face predicts performance generally in line with reported field experience. T h e computations i n d i c a t e : that the system is largely mass-transfer controlled that performance is sensitive to an optimal balance b e t w e e n seam and blast moisture contents that it is difficult to account for the extent of reduction reported in field trials without use of the face model p r o p o s e d that adequate area for reasonable gasification performance with cold-air blast is obtainable with conventional borehole systems, b u t that very h i g h gasification efficiency is possible in such a system with an oxygen-steam blast, and that any steps to increase c o m b u s t i o n zone exit temperature are advantageous. (if) C o m p u t a t i o n of a c o m b u s t i o n zone as a reacting wall W.S.R. shows that the effect of moisture balance on ignition stability is very marked and suggests that this type of c o m p u t a t i o n is an excellent tool to study optimal blast moisture content, and that an oxygen-steam blast with preheat could p r o d u c e synthesis gas at high efficiency in a jet stirred reactor alone; a considerable proportion of c o m b u s t i b l e gas can be achieved from air or air-steam. (iii) C o m p u t e d heat loss by c o n d u c t i o n to s u r r o u n d i n g rock appear generally in line w i t h field reports for overall systems. We have p r o b a b l y overestimated the losses to dry roof and floor. H o w e v e r the computation does not predict the very poor b e h a v i o u r of thin seams generally attributed to this cause. It appears that the real explanation is more complex a n d could be partly due to the greater ease w i t h w h i c h a low face can be blanketted b y fallen material. (iv) The face m o d e l used indicates that the effect of seam moisture and volatile content on overall reaction rate is more significant than that of carbon reactivity. Calculations indicate that at very h i g h surface ratings the p h y s i c a l condition of the face m a y necessitate some modification to this model. It w o u l d be advisable to make laboratory experiments on reasonably large coal lumps in "in-situ" state to verify these points a n d d e d u c e numerical data for both mass transfer a n d reaction rate coefficients.
(i) C o m p u t a t i o n of the reduction zone of a borehole " i n - s i t u " gasification system in terms
Better kinetic information to permit extrapolation of l u m p - c o k e data to high pressure
B O R E H O L E 'IN-SITU" GASIFICATION SYSTEMS c o n d i t i o n s is n e c e s s a r y for t h e d e s i g n of f u t u r e h i g h p r e s s u r e systems. T h e w h o l e p r o b l e m of e s t i m a t i o n of mass transfer c o e f f i c i e n t s in this t y p e of system r e q u i r e s f u r t h e r attention.
Acknowledgments We are grateful to the National Coal Research Advisory Committee for a grant to assist this program, and to Mr. S. S. Chin, research assistant, for computational work.
REFERENCES 1. CAeV,J. P., ET AL. U.S Bureau of Mines R1 4164, 4808, 5367, 5605, 5666, 5808, 5830, 6042 (19471962).
533
2. GIBB, A. ANDPARTNERS,"The Underground Gasification of Coal" Pitman 1964. 3. STEWART,I. McC., "'The Changing Technology of Fuel"--Conf. Inst. Fuel Aust. Membership, Adelaide 1974, pp16.1-16.10. 4. SZEKELY,J. AND MAROUDAS,N. G. Trans.I.Ch.E., 44, 1966, T3. 5. NnoKUrINI,R. M., Buss, C., WATSOSW., Chem. Tech. April 1974. 6. Data sheet from "'Podzemgas," U.S.S.R., 1975. 7. WARNErl,F. E., SZEKELY,J., "Underground Gasification of Coal" I.Chem.E. Symposium Series 2, 1965. 8. CRuz, I. B., Ph.D. Thesis, Univ of N.S.W., 1967. 9. LoIsoN, R., Jl.Inst.F., 26, 1963, p255. 10. HOa'rEL, H. C., SAROFIM,A. "Radiative Transfer" McGraw-Hill 1967 p234. 11. HSIANG, CnENG KING; "Fifteenth Symposium (International) on Combustion" p243, The Combustion Institute 1975.
COMMENTS R. F. Chaiken, U.S. Bureau of Mines, USA. Do you think that it will be possible to maintain the well stirred geometry in an actual coal seam? My feeling is that in reality the opening at the point of air injection will grow at a faster rate due to the vicinity of maximum oxygen concentration. This was indeed the way the Newman Spinney burn (P-5) proceeded.
Authors" Reply. We have computed reaction rates on the basis of a section which has burned out in the manner which Dr. Chaiken suggests, i.e., beyond the boundary of the oxidant jet. We have assumed that all surfaces are exposed only to a recirculating flow of combustion products. This may well retain a stable shape; however, it will probably not advance at the same rate as the reduction zone. We expect that cyclic operation would be necessary.
Department of Chemical Engineering, The University of Newcastle, N.S. W. 2308, Australia Performance is reported for quasi-steady state computational models of two zones of borehole type in-situ gasification systems. The plug-flow reduction zone is a close replica of real systems; the well-stirred reactor models some special systems and may be indicative of a wide range of ignition zone conditions. For both models it is postulated that seam moisture evaporation and coal devolatilization occur at some distance (inversely proportional to the advance rate) behind a reacting face of coke and that reaction of these products with coke occurs behind the face. This model produces an apparent reactivity for the system more strongly dependent on seam moisture and volatile content than on the chemical reactivity of the coke. Computations predict that gasification efficiency and combustion stability axe sensitive to an optimal combination of seam moisture and of blast moisture. These findings and the predicted gas analyses and efficiencies are generally in line with what published field information is available. The advantageous effect of high moisture content in the gas is to increase reaction rate (by increasing the concentration of a major reactant); this is balanced by the temperature drop due to added heat capacity. The overall effect is so dramatic as to warrant this type of analysis before completing design for field experiments. Laboratory testing on large samples of seam coal is advisable to test the model for detail and obtain better rate coefficients. It is predicted that high ratings and high gasification efficiencies are possible for oxygensteam gasification either in plug-flow or W.S.R. systems, particularly if preheat can be achieved.
Th e possibility of gasifying coal "in-situ,'" as an alternative to the costly and dangerous labour of m i n i n g has excited the minds of m e n since Siemens and later Ramsay in Great Britain and Men d el ce f in Russia proposed such a system in the later n i n e t e e n t h century. Russian experimental work c o m m e n c e d before 1930 and operations on a commercial scale, at 60-100 M W - - m o d e s t by current standards, have been continuous since about 1950. Foll o w i n g the " en erg y crisis" of the 1930's extensive experimental work was carried out up until about 1960 in the U n i t e d States, d o c u m e n t e d by U.S. Bureau of Mines; 1 in Great Britain, well d o c u m e n t e d by Sir Alex. G i b b and Partners 2 and in France and Poland. So far experimental and operational work has b e e n with air-blast to p r o d u c e a low C.V. gas w h i c h must compete with m i n e d coal as an energy source. For coal deposits u n e c o n o m i c a l or unworkable by conventional mining, e.g. very dirty seams, thick or stressed seams too deep 525
for opencast working, this still appears an attractive potential m e t h o d of w i n n i n g resources w h i c h w o u l d be otherwise unavailable. For this reason it has recently alitracted r en ew ed interest in the United States and Europe. However, now that the need for synthetic natural gas and l i q u i d fuels is clearly in sight, the proposition becomes very attractive. Th e major capital and operating cost in a synthesis project is the plant for gasification of the coal. If, as is generally estimated, the cost of "insitu" gasification is comparable with the cost of w i n n i n g the same a m o u n t of energy as coal, then "i n - si t u " gasification has a cost advantage comparable to the cost of the gasification plant. 3,5 This requires either oxygen-steam gasification or a cyclic "water-gas" type of operation, neither of w h i c h have been tried " i n - s i t u " for other than brief exploratory runs, Clearly a good u n d e r s t a n d i n g of the combustion and reduction regimes is necessary before
526
ENERGY PRODUCTION FROM COAL
very expensive field experiments are undertaken. Very few attempts have apparently b e e n made to analyse these regimes. Almost all of the field work reported has been devoted to the difficult tasks of securing linkage b e t w e e n boreholes, e s t a b l i s h i n g that ignition can b e achieved and stable c o m b u s t i o n controlled, and d r i l l i n g long large boreholes where required. The objective is to assess the significance of the chemical kineties and the relative importance of coal and blast characteristics and then, with the a i d of appropriate laboratory experiments to m o d e l more adequately a real system. The models s t u d i e d are most simply related to the simple British P5 system (Fig. l(a)). 2 Air was i n t r o d u c e d to a drive linking a set of parallel boreholes. These b u r n e d ' r a p i d l y to a tapered shape w h i c h then a d v a n c e d at more-or-less steady form. At the borehole spacing of about 20 m in a seam of 1.5 m height most of the tapered o p e n i n g is very wide; transport of oxygen from the bulk-stream I///111~111111///111111///I/
to the b u r n i n g faces is slow and it can be expected that overall surplus oxygen will remain until the w i d t h has become c o m p a r a b l e with the height. 4 Szekely 4,7 made an algebraic analysis of an e d d y - d i f f u s i o n controlled m o d e l of this wide oxidation zone and c o n f i r m e d this expectation. T h e result of this b e h a v i o u r is that the region available for the reduction reactions is small. As most of the area exposed to heat losses is located before reduction commences, the t e m p e r a t u r e range available to s u p p l y e n d o t h e r m i c reactions is iat a m i n i m u m . Doubts have b e e n expressed as to w h e t h e r kinetics will p e r m i t efficient gasification.
The Plug Flow Reduction Model
We therefore m a d e it our first task to examine the amount of reduction possible in such a zone if the gaseous products of complete combustion (without oxygen) are input to it. T h e range of expectable i n p u t conditions are considered, i.e. the height of seam, w i d t h of passage at c o m p l e t i o n of oxidation, e n t h a l p y of entering stream [after various levels of heat loss] and taper of passage of length L to an I111/1//I/11111111111111111111///I//, exit borehole (assumed at full seam height b u t width 0 . 3 / H . (Fig. 2a) Steady advance rate (quasi-steady state) conditions are assumed; strata losses and reaction heats are s u p p l i e d from the sensible heat of the gas stream conveered and radiated to the b o u n d i n g surfaces.
'r
. ,, .
.
.
The Well-Stirred Oxidation Model I
a
II
A particular d i s a d v a n t a g e of the system (l(a)) is that the greatest area of exposed coal surface is likely to be at the thin end of the coal w e d g e (the air entry point), where strata pressure and
b
~-.----
c
ZL 3
fl
Fro. 1. In-situ borehole gasification systems: (a) Drift and boreholes [ref. 2; p5 pl19], (b) Parallel borehole with reverse blast, (c) Boreholes normal to seam [ref. 2; p195], (d) Thick seam blind borehole [ref. 5]
a
b
FIG. 2. System models studied: (a) Plug flow reduction zone, (b) Jet stirred combustion zone
BOREHOLE 'IN-SITU' GASIFICATION SYSTEMS thermal stress will break and spread the coal. This extended surface area is not needed in an oxidation zone. It is clear that if c o m b u s t i o n could be completed in a compact high intensity zone, much more area would be available for reduction and c o m b u s t i o n products would enter this zone at a higher temperature. This may be achievable b y cyclic operation between parallel boreholes, oxidant b l o w n into one set and gaseous products removed from an adjacent borehole set (l(b)). This would place the region of broken coal in the reduction zone. Something of this apparently occurred in a Russian linked borehole system (l(c)) where a reverse flow stage occurs. "Blind-borehole" systems are of this type and some results are available. 2 A jet-stirred reactor has been proposed by A. D. Little Inc., 5 (Fig. l(d)), for extremely thick seam operations. We have therefore not yet tackled the elaborate three dimensional model needed for adequate analysis of the c o m b u s t i o n zone of Fig. l(a) but have given priority to examination of the conditions u n d e r which an intensive c o m b u s t i o n zone can be established as a jet stirred reactor. This model is also ealculated on a quasisteady state basis, i.e. by assuming that a regime has been reached where each part of the b u r n i n g face moves forward at a constant rate so that the shape of the zone is constant. For this to occur the shape must become broader than the spread of the jet from the borehole, the flow pattern will then be a jet with heavy recirculation. This is approximated as a well-stirred reactor (W.S.R.) This model is idealised in Fig. 2(b). For both models the critical assumptions are those relating to the nature and behaviour of the coal faces exposed to reacting gases and to the corresponding heat and mass transfer relations. These are examined in detail below. Burning Face Conditions
temperature, the relation is: T-
To+ L'=(T s-
where L ' = L w / ( C c + wCw) and a ' = k / p ( C c + w C w) It should be noted that for the steady advance of an infinite face the face heat flux requirement is simply the sum of the sensible heat content of dry coal and enthalpy of water vaponr (above liquid base) to face temperature and any reaction heat (positive if endothermal). In the reduction zone at the m a x i m u m advance rate, U, of about 0.1 x 10 -5 m / s e c the thickness of the coke zone from 900 to 1900~ is then about 1 in for a coal with 10% seam moisture and still over 30 eros for a b r o w n coal at 50% moisture (dry coal basis), with a comparable further distance to the drying zone. The face model is then of a b u r n i n g face of reasonably high temperature coke towards which volatiles a n d water vapour are percolating through shrinkage cracks. (Fig. 3). This pattern has b e e n confirmed in the field. 7,1 I n the reduction zone the m a x i m u m water vapour flow expected from a wet coal (water 0.5 times dry coal mass) is only 2.9 • 10 -3 n . m . / s e c on superficial area allowing, at 20% voids, several seconds contact time with reactive coke at from 900 to 1900~ considerable reaction can be expected. To estimate face temperature it is necessary to estimate the heat requirement of such reactions. A n y precise calculation for such a physically u n k n o w n a n d variable system is out of the question; we have quite arbitrarily assumed that half the seam water is deeomposed a n d that CO and CO z
I
rs
I.!
g
~
"~
z
~-
I .a.,g " |i
co2 o2 H20 co
.D I
I
T o = (T s -
To+ L') e x p ( - Ux/c~' )
Behind a b u r n i n g face of fixed carbon the coal is pyrolysed and the bed moisture evaporated. To assess the interactions of these components we use as a model a plane face, advancing at a steady rate by gasification of carbon. For homogeneous material the temperature falls exponentially with distance. For an infinite plane face T-
527
I
/
'
I
~%\"\\"'~
~H
2
I"~
T O) e x p ( - U x / a )
If there is a latent heat load the temperature gradient steepens and, above the evaporation
FIG. 3. Face model--temperature and reactant distribution.
528
ENERGY PRODUCTION FROM COAL
are produced in equal proportions. These assumptions are reasonable for the high moisture lignite but may underestimate CO p r o d u c t i o n for the b i t u m i n o u s coal. In the c o m b u s t i o n zone at atmospheric pressure, the expected face temperature gradient is very m u c h steeper and the validity of the model must be examined. The major i m p l i c a t i o n of the model is that the advance rate is determined by the reaction rate of only a portion of the fixed carbon of the coal. Carbon reacted at the face, Cf = fixed carbon ( C ' f ) less carbon reacted with seam moisture (fw) where f is the carbon requirement for water vapour reactions. The rate of advance in terms of dry coal mass is then R c / Cf if R c is the carbon reaction rate at the face. For a low rank coal with 45% fixed carbon a n d 50% moisture (dry coal basis) C i = 0 92 and the coal advance rate is 5 times the carbon reaction rate. This suggests that for "in-situ" gasification seam moisture has some advantages a n d that the low residual fixed carbon of low rank coals may be more significant than the coke reactivity.
and 1 at steam at temperatures from i 0 0 0 to 1600 K measured b y weight loss on measured lumps of high temperature coke. There is general agreement that oxidation rates fall off at high temperatures and that water vapour reduction is strongly inhibited by hydrogen, etc. In no way, however, can reliable coefficients be deduced for this specific application. We have therefore simply extrapolated these measured rate figures with the reservation that the significance of surface reaction rate must be critically examined in the results. Because of the expected heavy faee fissuring we have taken exposed coke area as 2.5 times the nominal plane surface a n d c o m b i n e d this area factor in the rate constant g i v i n g : - -
( 4 00) _
KRo 2 = 10.0exp
-
KRH20 = 1.03 x 10 a exp Kacoz = 0.31 x i 0 a exp
_
RT
( 48000 RT
]
( 4 8 9_ 0 _0 ) _ RT
Sensitivity tests are reported for the effect of changes in K R.
Face Reaction Rates Three gases, 0 2 , H 2 0 and CO 2 are assumed to react simultaneously at the carbon surface; i.e. Total rate of carbon gasification R e = S(Po2Ko2 + PH2oKHeo q-Pco2Kco2) where 1
1
1
Ko 2
KD
Kr~o2
Any real face will be rough. For mass and heat transfer only the simplest assumptions are warranted. Stanton n u m b e r has been taken at 0.014 (half the friction factor for a very rough duct,) giving h = 0.014 Cprh for heat transfer and k o = N / ~ p = 0.014 m / P M for coefficients based on n o m i n a l face area. For the well stirred reactor system, after some qualitative studies of expected steady b u r n i n g shape a n d jet recirculation patterns we assumed the characteristic velocity to be that due to a total flow of 2.25 jet flow at the mean cross section, giving K o = 0.378 rh (jet)/PMS [A sensitivity test is reported]. For the surface reaction rate coefficients K R the two problems are to estimate both suitable reaction rate coefficients in terms of external surface and also the external surface. We have used rate data b y Cruz 8 at 0.03 to 1 at oxygen
Heat Losses and Heat Transfer I n the real system the advancing face is of finite height b e t w e e n non-reacting roof a n d floor rocks, to w h i c h heat is lost by c o n d u c t i o n from b e h i n d the face, increasing the heat loss from the face itself, and by convection and radiation from the gas space. We have estimated the total heat loss by approximation developed b y Loison 9 who represented the gasification zone b y the shape of an isotherm for a steadily a d v a n c i n g line source at mid-seam height, giving for our range of interest H e a t / u n i t length, q' = 10.2k
(T s -
T o)
The a s s u m p t i o n in detail implies that the face and adjacent floor and roof are at a temperature Ts, a d v a n c i n g at a lineal velocity U and that roof a n d floor are m a i n t a i n e d at a temperature falling off very gradually for a distance back of several times the seam height. [If anything, it overestimated the losses to drtj strata in the systems we are studying]. Face and gas temperatures are related b y the heat-transfer required to supply losses a n d
BOREHOLE 'IN-SITU' GASIFICATION SYSTEMS surface reactions. Initial calculations predicted quite low temperature differences. We have assumed convection plus carbon-dioxide a n d water vapour radiation (using the Hadvig 1~ correlation) for the reduction zone and have assumed radiation from a black-flame in the c o m b u s t i o n zone.
General Assumptions and Method For each model the objective is to compute carbon gasification rate, strata losses, and gas and surface temperatures for a range of coal properties, bed moisture contents and blast conditions, for the reduction zone model, inlet gas is the products of stoichiometric combustion at coal plus blast enthalpy less specified strata losses. C o m p u t a t i o n proceeds by finite difference steps in gas temperature for each of which carbon reaction rate, losses, and change of gas composition per u n i t surface are calculated; thence surface temperature is recalculated and the surface area required for the specified temperature step. For the W.S.R. c o m b u s t i o n zone model, reaction rate is calculable at a specified surface temperature a n d assumed gas composition. Gas composition is then iterated to obtain a mass balance. From this i n p u t and output enthalpies and conduction heat losses are calculable as functions of temperature. Ignition a n d stable b u r n i n g conditions are obtained from the two temperatures at which energy i n p u t and output rates coincide. Coal properties selected for computation
Test Calculations at 10% loss 20% H 2 0
Seam heights m
Blast nma/sec
1.5
1.4
1.5
2.0
The major variables investigated in the computational program were the strata losses in the c o m b u s t i o n zone, the water vapour content of the c o m b u s t i o n gases, the height of seam, the l e n g t h o f the reduction zone, the coal type a n d the relative influence of mass transfer and chemical reaction rate. All calculations were made for the tapering reduction zone, typical of a borehole system, with low entry mass velocity. Typical profiles of temperature and cold gas efficiency (The proportion of the heating value of the coal (net)recovered in the product gases, w h e n cooled)
529
TABLE I Coal properties: Calculated to dry coal basis. Bituminous Sough 2
Brown Angren 2,6
0.064 0.37 0.51 0.14
0.52 0.50 0.36 0.14
7475 , 0.74 0.05 0.09 0.49
5692 0.66 0.03 0.17 0.18
Moisture Volatile H.C. F. Carbon Ash Gross cal. value Kcal / kg C H O Cf
correspond to coals for which some field date is obtainable (Table I). The blast volume reported here corresponds to the upper level of reported practice. Results a n d Discussion
1. Plug Flow Reduction System A reasonable check on the magnitude of the results is possible from day 90 of the P5 results (Ref. 2 pp142-152) where a plan of b u r n o u t zone as established by test drilling enables a reasonable estimate to be made of the area available for reduction. The comparison is as f o l l o w s : - -
Equivalent face length m min25
max35
40
Gas CV net K cal/m a
Cold gas efficiency %
490-540
45Z55
630
49
against path length are shown in Figures 4 and 5 for b i t u m i n o u s coal and b r o w n coal. Strata losses for the c o m b u s t i o n zone only have been taken to bracket the expected range at 20% (which is higher than reported for most complete gasification systems) and zero (which is equivalent to 10% strata loss together with 10% sensible heat recovery as blast preheat). The effect on reduction rate and overall coldgas efficiency is striking. The results indicate that a reasonable efficiency is barely obtainable with the 40 m of equivalent face readily obtainable i n a bore-
530
ENERGY PRODUCTION FROM COAL f
I
r
5o
10
,g
s e.5
.=_
.u
25 "~
1
g, 2500
0
~.,2ooa
240(
-
200{
m m m.
~.151
1600
[
10
|
210 Length,lL]
30
40
m
FIG. 4. Plug flow reduction; bituminous coal: 1) Blast 20% moisture, net combustion losses 20%; seam 1.5m, 2) Blast 20% moisture, net combustion losses 20%; seam 3.0m, 3) Dry air, net combustion losses 0%; seam 1.5m, 4) Dry air, net combustion losses 20%; seam 1.5m hole system. Some increase in performance is expectable with greater available area b u t not enough to warrant expensive fracturing system s. The most significant conclusion is that, for this system, mass transfer control is p r e d o m i nant d o w n to about 1400 K with the corollaries that high gas flow rates m a y be advantageous [which are c o n f i r m e d in practice, Ref. 2 pp189 and 193]. Steam a d d i t i o n to the blast p r o d u c e s high reduction rates because of the higher water-vapour partial pressure, but in the reverse direction it increases the heat capacity of the gas stream giving lower temperatures entering the c o m b u s t i o n zone. For the b i t u m i nous coal there is very little difference in c o m p u t e d p e r f o r m a n c e between dry air a n d the very high steam content of 20%. Ref. (2) quotes Russian figures s h o w i n g an improvement in gas quality and efficiency b y steam addition for a very dry seam. For the h i g h moisture b r o w n coal the effect of a d d e d moisture is deleterious. T h e balance appears to be more critical a n d at a different level from the conventional gas producer. At the low p r e d i c t e d temperatures for brown-coal a c o n s i d e r a b l e control b y chemical
1200
I 1O
[ 20
I 30
40
L,m
FIG. 5. Plug flow reduction; brown coal: 1) Blast 20% moisture, 10% losses, seam 1.5 m, 2) Dry air; 0% losses, seam 1.5 m, 3) Dry air; 10% losses, seam 1.5m, 4) Dry air; 10% losses, seam 1.5 and K R = 3 times K R used previously. reaction rate w o u l d be expected. T h e computations used the same hard-coke reactivity for both coals. Curve 4 of Figure 5 shows the comparatively small effect, b y c o m p a r i s o n with curve 3, of a threefold increase in a s s u m e d reactivity. For brown-coal installations reference(6) claims higher calorific value and efficiency than we computed. H o w e v e r these claims are for complete systems and will include the contribution of distillation products from coal s u r r o u n d i n g the burn-out zone. 2. Well-Stirred Reactor A representative set of results are shown in Table II. The sensitivity of this model to the moisture contents of coal and blast is very striking. At dry blast a n d dry b i t u m i n o u s coal will only sustain stable c o m b u s t i o n at a coinparatively low rating (low blast r a t e / c o a l face surface area) and at high residual oxygen content. At a h i g h moisture content in the blast stable operation is indicated at high ratings, with ignition at a lower temperature and considerable p r o d u c t i o n of h y d r o g e n and carbonmonoxide at the stable condition. Under these conditions the reaction rate is entirely sus-
BOREHOLE 'IN-SITU' GASIFICATION SYSTEMS
531
TABLE II W.S.R. with reacting wall [ DA--dry air BLAST ]A20--80% air 20% H20 2 mS/sec at 1 at ~080--20% oxygen 80% H20 300K from borehole ~067--33% oxygen 67% HeO of 0.35 m diameter \Coal properties as Table I. Temperature Ignition Burning K K
Dry cool rate Kg/sec
Combustible (CO & H2) %
Varying blast conditions 1640 1900 No ignition 1625 2025 1440 1735
0.155 -0.223 0.365
Oxygen 7.2% -6.17 10.12 13.6 13.0 13.1 30.6
Coal
Blast
Coal face m 2
Sough
DA DA A20 080
100 25 25 25
Angren
DA A20 080 067
25 25 25 25
<1200 1430 1300 <1200
1860 1590 1530 1700
0.424 0.207 0.415 0.840
Sough (bit)
A20 A20 A20
Varying Rating 10 1790 25 1625 100 1440
2055 2025 1990
0.214 0.223 0.230
Angren Brown
DA DA DA
10 25 100
1900 1860 1790
0.387 0.424 0.457
1600 < 1200 < 1200
tained by the face reactions of CO,z and H 2 0 with carbon at temperatures where rate is p r e d o m i n a n t l y mass transfer controlled. This is apparent from the shape of the rate curves and has been checked b y sensitivity tests in which reduction of the mass transfer coefficient to one half of the normal value produced a larger change in computed performance than reduction of reaction rate constants b y a factor of ten. O n the other hand, the high moisture coal with its heavy latent-heat debit will only just ignite u n d e r W.S.R. conditions if the blast has an appreciable moisture load, with corres p o n d i n g l y increased heat capacity. The b r o w n coal shows a higher apparent reactivity than the b i t n m i n o u s in lower ignition temperatures and greater production of CO and H 2 at lower b u r n i n g temperatures. As in the plug flow model the same carbon reactivity has been assumed; the difference is due to the effect of the higher volatile and moisture content which, in our model, reduces the proportion of carbon to be reacted at the face. The general behaviour is in line with qualitative reports of Russian experience in Ref. 2, both with regard to the improvements by
4.70 6.17 7.16 10.1 13.6 16.8
blast moisture addition for dry coals and, the deleterious effect of water seepage in brown coal installations. These differences show up more strikingly in this combustion model than in the reduction model. The effect of our face model is so great that experimental checks are desirable. At the highest rates tabulated (which at 10 m 2do represent as small and compact a c o m b u s t i o n zone as we can imagine) the rate of face advance is a little under 2 • 10 * c m / s e c , which would require a temperature gradient close to the face of about 600K per cm. It is a real question whether the material could tolerate such gradients or whether, for such very high ratings the hot zone would intermittently flake off the face exposing cold surface. At lower ratings, particularly for b i t u m i n o u s coal, the model appears feasible. The effect of rating, i.e. available surface for a given blast rate, is not nearly so significant; a ten-fold increase in available surface shows a drop in the required ignition temperature but, particularly for the b i t u m i n o u s coal, only a modest drop in stable b u r n i n g temperature. There is, however, with increased comb u s t i o n zone area an increase of 10-20% in
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ENERGY PRODUCTION FROM COAL
carbon c o n s u m p t i o n with production of combustible gases. This limited sensitivity of rati n g i s largely a r e s u l t of the conditions a s s u m e d for the model in w h i c h a constant shape is assumed for the reaction zone with the result that the gas velocities (and transfer coefficients) vary inversely with the surface area. However we b e l i e v e that this is a realistic assumption. The calculated effect of seam height has not been tabulated as it is very small w h e n c o m p a r e d on the basis of equal total surface area. However in a field situation the p l a n dimensions of a zone clear of obstructions will d e p e n d on roof behaviour. The expected surface area available will then be proportional to seam height, with the performance improvement discussed above. As an indication of the order of m a g n i t u d e of the rates reported the m a x i m u m rate tabulated is 2.83 x 10-3 g m / e m 2 s e c . Free b u r n i n g vertical w o o d e n slabs of less than 1 m h i g h are reported as b u r n i n g at 2.8 x 10 -a g m / c m % e c in cold surroundings. IX F i e l d trials reported for b l i n d - b o r e h o l e experiments in Ref. (2) ( p l i 3 ) show that, at least in the early stages, the system was p r e d o m i nantly a jet stirred reactor with velocities comparable to our basis. A highly p r e h e a t e d blast of dry air gave stable operation w i t h b i t u m i n o u s coal over a range of surface ratings comparable with our tabulations. Gas was somewhat richer t h a n we calculate b u t obviously i n c l u d e d a considerable p r o p o r t i o n of excess (unsteady state) volatile matter. Results do show that a considerable degree of water and C O 2 reduction can be aehieved in a W.S.R. system. Further computations indicate that an oxygen-steam blast p r e h e a t e d to 500~ (e.g. b y regenerative cyclic operation) could gasify coal at 0.1 M W / m 2 with 60% cold gas efficiency in an open W.S.R. alone. For any b o r e h o l e system stability a n d compactness of the c o m b u s t i o n zone is essential for efficient operation. In m a n y layouts the oxidation zone m a y be p r e d o m i n a n t l y p l u g flow. Ignition c o n d i t i o n s for pure p l u g - f l o w oxidation are very d e p e n d e n t on the d e t a i l e d geometry adjacent to the ignition area. However any real system of a high velocity jet issuing into an irregular volume must i n c l u d e m a n y turbulent reeirculation zones and c o u l d be expected to b e h a v e in a manner qualitatively related to this s i m p l e W.S.R. model. Conclusions
of a plug-flow t a p e r i n g reduction zone w i t h evaporation, distillation and some carbon reaction b e h i n d the face predicts performance generally in line with reported field experience. T h e computations i n d i c a t e : that the system is largely mass-transfer controlled that performance is sensitive to an optimal balance b e t w e e n seam and blast moisture contents that it is difficult to account for the extent of reduction reported in field trials without use of the face model p r o p o s e d that adequate area for reasonable gasification performance with cold-air blast is obtainable with conventional borehole systems, b u t that very h i g h gasification efficiency is possible in such a system with an oxygen-steam blast, and that any steps to increase c o m b u s t i o n zone exit temperature are advantageous. (if) C o m p u t a t i o n of a c o m b u s t i o n zone as a reacting wall W.S.R. shows that the effect of moisture balance on ignition stability is very marked and suggests that this type of c o m p u t a t i o n is an excellent tool to study optimal blast moisture content, and that an oxygen-steam blast with preheat could p r o d u c e synthesis gas at high efficiency in a jet stirred reactor alone; a considerable proportion of c o m b u s t i b l e gas can be achieved from air or air-steam. (iii) C o m p u t e d heat loss by c o n d u c t i o n to s u r r o u n d i n g rock appear generally in line w i t h field reports for overall systems. We have p r o b a b l y overestimated the losses to dry roof and floor. H o w e v e r the computation does not predict the very poor b e h a v i o u r of thin seams generally attributed to this cause. It appears that the real explanation is more complex a n d could be partly due to the greater ease w i t h w h i c h a low face can be blanketted b y fallen material. (iv) The face m o d e l used indicates that the effect of seam moisture and volatile content on overall reaction rate is more significant than that of carbon reactivity. Calculations indicate that at very h i g h surface ratings the p h y s i c a l condition of the face m a y necessitate some modification to this model. It w o u l d be advisable to make laboratory experiments on reasonably large coal lumps in "in-situ" state to verify these points a n d d e d u c e numerical data for both mass transfer a n d reaction rate coefficients.
(i) C o m p u t a t i o n of the reduction zone of a borehole " i n - s i t u " gasification system in terms
Better kinetic information to permit extrapolation of l u m p - c o k e data to high pressure
B O R E H O L E 'IN-SITU" GASIFICATION SYSTEMS c o n d i t i o n s is n e c e s s a r y for t h e d e s i g n of f u t u r e h i g h p r e s s u r e systems. T h e w h o l e p r o b l e m of e s t i m a t i o n of mass transfer c o e f f i c i e n t s in this t y p e of system r e q u i r e s f u r t h e r attention.
Acknowledgments We are grateful to the National Coal Research Advisory Committee for a grant to assist this program, and to Mr. S. S. Chin, research assistant, for computational work.
REFERENCES 1. CAeV,J. P., ET AL. U.S Bureau of Mines R1 4164, 4808, 5367, 5605, 5666, 5808, 5830, 6042 (19471962).
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2. GIBB, A. ANDPARTNERS,"The Underground Gasification of Coal" Pitman 1964. 3. STEWART,I. McC., "'The Changing Technology of Fuel"--Conf. Inst. Fuel Aust. Membership, Adelaide 1974, pp16.1-16.10. 4. SZEKELY,J. AND MAROUDAS,N. G. Trans.I.Ch.E., 44, 1966, T3. 5. NnoKUrINI,R. M., Buss, C., WATSOSW., Chem. Tech. April 1974. 6. Data sheet from "'Podzemgas," U.S.S.R., 1975. 7. WARNErl,F. E., SZEKELY,J., "Underground Gasification of Coal" I.Chem.E. Symposium Series 2, 1965. 8. CRuz, I. B., Ph.D. Thesis, Univ of N.S.W., 1967. 9. LoIsoN, R., Jl.Inst.F., 26, 1963, p255. 10. HOa'rEL, H. C., SAROFIM,A. "Radiative Transfer" McGraw-Hill 1967 p234. 11. HSIANG, CnENG KING; "Fifteenth Symposium (International) on Combustion" p243, The Combustion Institute 1975.
COMMENTS R. F. Chaiken, U.S. Bureau of Mines, USA. Do you think that it will be possible to maintain the well stirred geometry in an actual coal seam? My feeling is that in reality the opening at the point of air injection will grow at a faster rate due to the vicinity of maximum oxygen concentration. This was indeed the way the Newman Spinney burn (P-5) proceeded.
Authors" Reply. We have computed reaction rates on the basis of a section which has burned out in the manner which Dr. Chaiken suggests, i.e., beyond the boundary of the oxidant jet. We have assumed that all surfaces are exposed only to a recirculating flow of combustion products. This may well retain a stable shape; however, it will probably not advance at the same rate as the reduction zone. We expect that cyclic operation would be necessary.