problems solved and not solved in ucg
TRANSCRIPT
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PROBLEMS SOLVED
AND
PROBLEMS NOT SOLVED I N
UCG
Robert
D.
Gunn
Un iv er s i ty o f Wyoming and the
Energy Research and Development Administration
Laramie Energy -Research Center
P. 0.
Box 3395, Un ive r s i t y S t a t i on
Laramie, Wyoming 82071
I
NTRODUCT
ION
Several d i f f e re n t p rocesses o f U C G (Underground Coal Gasif icat ion)
are be ing i nves t iga t ed i n f lo r th Amer ica . O f these, t he l i nked ve r t i c a l
w e ll proc ess, deve lop ed by th e Laramie Energy Research Center, has been
f i e l d tes ted most ex te ns ive ly and i s c loses t to eventual commerc ia l iza tion .
There i s , c o ns eq ue nt ly , s u b s t a n t ia l p a r t i c i p a t i o n i n f u r t h e r f i e l d
t e s t i n g o f t h e l i n k e d v e r t i c a l w e l l p ro ce ss o r mino r v a r i a t i o ns o f
i t .
P a r t i a l o r c omp le te i n d u s t r i a l p a r t i c i p a t i o n i s i n vo l ve d i n th e f i e l d
test ing programs
o f
th e Al be rt a Research Counci l , Texas A6M Un iv er si ty ,
and Texas U t i l i t i e s .
Problems, some so lv ed and
some
not solved, which ar e associated
w i t h U C G are d iscussed i n th i s work . D iscussion o f these problems out l ines
t he cu r r en t s t a t us of t he l ink ed ve r t ic a l w e l l p rocess. The purpose is
t o prov id e per sp ect iv e concerning what has been accomplished alre ady and
what remains y e t
t o
be done on the road t o commerc ia l izat ion o f UCG.
PROBLEMS SOLVED
1 .
Low Gas Q u a l i t y
An app r a i sal o f wo r ld - w ide r esea rch e f f o r t s i n
U C G
through
1971
showed
t ha t no f i e l d exper im en ts us ing a i r i n je c t i o n had cons i s t en t l y p roduced gas
wi th a heat ing va lue o f more than 4.7 -
5.1
MJ/m3 (120-130 Btu/scf).
most cases the gas hea ti ng values averaged l es s than 3.9 tlJ/m3 100 Btu /sc f )
I ) .
I n co nt ra st a l l exper iments conducted a t Hanna, Wyoming, l iave r es ul te d
i n heat ing values above
4.7
tlJ/m3
120 Bt u / sc f ) . Du r ing t he bes t con t r o l l ed
o f a l l o f th e Hanna experiment s, th e Phase I I lianna I te st , the gas
heat ing value averaged
6.7
MJ/m3
(171
B t d s c f ) a t p r o du c ti o n r a t es ex -
ceeding
215 ,000
m3/day
8
m i 1 1 i on sc f / day ) .
I n
The favorab le res u l t s a t Hanna stem f rom th ree we l l de f ined con d i t i ons :
1 .
Favorab le geo log ic a l cond i t ion s
2 ,
3). An impervious sh al e
over l ies the Hanna No.
1
coal seam. The seam i s r e l a t i v e l y th i ck , 9
m.
I t l i e s a t s u f f i c i e n t depth,
82-122
in, so t ha t gas leakage t o the su rf ac e
has not occurred.
A
s i n g l e a q u i f e r , o f ve ry l ow p r o d u c t i v i t y , o v e r l i e s
th e coa l seam.
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2. Subbituminous coal . Mathemat ical m d e l calc ula t io ns show th at
t he hea t i ng va lue o f gas p roduced f rom e i t h e r l i g n i t e o r b it um inous coa l
should be lower than the hea t ing value o f gas f rom subbi tuminous coal .
Gases produced by car bo niz at ion o f the coal make up a su bs ta nt ia l p a r t
of th e fuel gases produced by UCG. Subbituminous coa l has a h igh v o l a t i l e
content ; i n add i t ion , the carbon iza t ion gases are r i c h i n methane.
In boreholes and la rge channels, probably the most cr i t i c a l chemical
rea ct i on i s the steam-carbon rea ct i on
C
+
H20
->
C O
+
H 2
which requ ire s a lo ng residence t ime compared t o simple combustion. I n an
open borehole, o r a bo reho le p a r t i a l l y f i l l e d w i t h rubb le and l a rge p ieces
of coa l , there i s poor contac t between the so l id coa l char and the m ix ture
o f water vapor and h ot combustion gases.
I n con t ras t i n t he l i n ked ve r t i ca l w e l l p rocess , no open bo reho le
ex is ts between the a i r i n j ec t i on and gas produc t ion we l l s . Ins tead gases
permeate th rough the dr ied , p a r t i a l l y de vo la t i l i z ed coa l . Average
pa r t i c l e si ze , a t l e as t f o r t he Hanna
No. 1
Seam, i s on th e or de r o f one
mi l l im et er . Because ther e i s in t im ate contac t be tween gases and so l i d ,
the ga s i f i c a t io n reac t io ns are more ex tens ive ; and gas hea t ing va lues,
consequent ly, a r e h igher . B i tum inous coa l conta in s less v o l a t i l e mat ter
and, the refo re, produces a lower hea t ing value gas. L ig ni t e has a h igh
v o l a t i l e c on te nt , b u t on d e v o l a t i l i z a t i o n r e l a t i v e l y l i t t l e methane i s
produced and a lower qu al i ty gas i s obtained.
3 .
Cont ro l o f water in f l ux . Sov ie t da ta from f i e l d te s t s and com-
m erc ia l ope ra t i ons (4,
51,
mathematical model calculat ions (6,
7, 8 ) ,
and expe rime nta l r e s u l t s from Hanna, Wyoming, (9,
IO)
a l l v e r i f y t h at a
too h i gh w ater i n f l u x can produce a m ajor de t e r i o ra t i on o f gas q ua l i t y .
The phys ica l reasons f o r the de le t er io us e f fe c t o f water have been
discussed elsewhere (5,
6
8,
IO).
Ejost western Te r t i a r y coa l seams ar e
aq ui fe rs . The Hanna No. 1 coal seam, however, i s a re la t i v el y unproduct ive
aqu i fe r . There fore , i t i s r e l a t i v e l y easy t o a d ju s t a i r i n j e c t i o n r a t es
t o main ta in a near optimum a i r / water ra t io .
2. Decreasing Heating Value
In many f i e l d t es t s t he gas p roduced s ta r t ed i n i t i a l l y w i t h a
reasonab le heat ing va lue which then d ec l ined gr adua l l y t o unacceptab le
val ues . Two mechanisms a r e known wh ic h can cause th i s beh av io r:
1 .
Use o f boreholes. One method of coal g a si f ic a t i on inv olv es the
d r i l l i n g o f b o re ho le s t o c on ne ct t h e i n j e c t i o n and t h e p r o d u c ti o n w e l l .
The coa l i s ig n i t ed then and ga s i f ie d a long the le ngth o f the boreho le.
I n t h i s process the coal burns ra d ia l ly outward, and the borehole increases
i n siz e. As th e borehol e grows i n si ze , more gas by-passes th e coa l; and
the gas heat ing va lue dete r io r a tes cor responding ly .
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2 Hi gher wat er i nf l ux f or l arger burned ar eas. Si nce many coal
beds i n t he Vest ar e aqui f ers , wat er i nf l ux t ends t o i ncrease as mor e and
more sur f ace i s exposed by t he combust i on f r ont . I n addi t i on, f or l ar ger
burned out ar eas subsi dence occur s est abl i shi ng communi cat i on w t h
over l yi ng aqui f ers wi t hi n t he subsi dence zone.
Wt h an except i on di scussed l at er i n t hi s paper, a dr ast i c decl i ne
i n
gas heat i ng val ue has not occur r ed du r i ng t he Hanna f i el d t est s. The
maj or r eason i s t hat t he l i nked ver t i cal wel l pr ocess used at Hanna i s
not a borehol e met hod but a permeat i on met hod, t hat i s, i t i s essent i al l y
a packed bed pr ocess. Packed beds ar e wi del y used i n t he chem cal process
i ndustr i es . A pr i nci pl e, wel l known among pr ocess chem st s and engi neer s,
is t hat f or sat i sf act or y r esul t s channel i ng must be avoi ded in packed bed
equi pment such as chem cal r eact ors, l i qui d- l i qui d extr acti on col umns, and
di st i l l at i on t ower s. None
o
t he Hanna f i el d test s have yi el ded any
def i ni t e evi dence t hat open channel s have been cr eat ed.
Thermal data f r om i nst r ument ed obser vat i on wel l s 1 1 ) . f l ow r at e and
gas composi t i on i neasurement s
9 ,
121 , and mat hemat i cal model i ng
6 , 7)
have been used ext ensi vel y
i n
devel opi ng t he f or egoi ng descr i pt i on of t he
mechani cs of t he l i nked ver t i cal wel l pr ocess. A s mor e s l ear ned about
t he pr ocess, i t becomes i ncr easi ngl y cl ear t hat l i gni t e and subbi t um nous
coal pr opert i es ar e especi al l y amenabl e to UCG. Both t ypes of coal shr i nk
on heat i ng, and dr yi ng al one i ncr eases t he coal permeabi l i t y by about t wo
order s of magni t ude
1 3 ) .
t i s t hese pr oper t i es whi ch per m t r ever se
combust i on l i nki ng and a permeat i on t ype gasi f i cat i on pr ocess t o be used.
3 . Vari abi l i t y i n Gas Qual i t y and Gas Product i on Rat es
A
wi de var i abi l i t y i n gas qual i t y and pr oduct i on r ates has been
observed on an hour l y or dai l y basi s
i n
many f i el d exper i ment s. The need
f or a const ant gas f l ow r at e, however , pr esent s no real pr obl em t i s
r eadi l y achi eved w th a const ant ai r i nj ect i on r at e and w t h t he use of a
f l ow cont r ol val ve on t he pr oduct i on l i ne.
At Hanna var i at i ons i n gas heat i ng val ues on t he or der of 5 to 10
percent have been obser ved at a si ngl e wel l on a dai l y basi s. Thi s f al l s
w t hi n the accept abl e l i m ts f or the f i r i ng of l arge boi l ers . F o i a
commerci al oper at i on, however , many pr oduct i on wel l s woul d be i n use
si mul t aneousl y and the var i abi l i t y
i n
t he gas
composi t i on^
woul d tend t o
average out . t
i s
al so not ed t hat gas vari abi l i t y has been mr e ext r eme
i n the bor ehol e or st r eam ng methods of UCG.
4. Low Thermal Col d Gas) Ef f i ci ency
n
t hi s work t her mal ef f i ci ency i s def i ned as t he upper heat i ng
val ue of dr y gas and l i qui ds pr oduced di vi ded by t he heat i ng val ue of t he
coal consumed. Consi st ent wi t h t hi s def i ni t i on, sensi bl e heat i s not
i ncl uded nor i s t he l at ent heat of any water vapor i n t he gas.
The i nst r ument at i on used dur i ng t he Hanna f i el d t ests per m t s a
accur at e det er m nat i on of t he t hermal ef f i ci ency. These ef f i ci enci es
are the hi ghest ever r ecor ded. The Phase
I I
Hanna
I I
t est achi eved an
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e f f i c i en c y o f
89
percent f o r t he en t i r e 25 days o f t he t e s t du r ing wh ich
2300 tonnes (2500 tons)
o f
coal were consumed.
Such h ig h e f f i c i en c i es are r ea d i ly achieved under good opera t ing
The mn y fe e t o f ear th ov er l y i ng and under ly ing the coal
ond i t ions .
seam prov ide exc e l le n t insu la t io n . In t h i c k coa l seams, there fore , the
LiCG process operates nea r ly ad ia ba t i ca l l y . Most of the thermal energy
released f rom th e combust ion o f co al
char and a i r must be produced a t t he
su r f ace i n t he f or m o f sens i b le and l a t en t heat and i n the heat i ng va lue
o f th e gas produced, i.e., chemical heat. The se ns ib le hea t i s a les s
convenient form o f energy because i t can be t rans por te d on ly over very
shor t d is tances .
I n the boreho le or s t reaming method o f
U G
a subs tan t ia l p o rt i on o f
. t he t o t a l energy re leased appears a t the sur face i n t he f o rm o f sens ib l e
the
hot
combust ion gases by-pass the coal and a consi derabl e p or t i on o f
heat. In permeat ion processes on ly a small
po r t io n o f th e energy goes
i n t o sensi b le heat. The combust ion gases in t i ma te ly contac t the coal ,
and most o f t he sensi b le heat i s used up fo r the h ig hl y endothermic
steam-char re ac ti on which produces a comb ust ibl e gas.
A
number
o f
cond i t ions can lead t o lower thermal e f f i c i en c i es as
we l l as lower gas he at i ng values.
1 .
Thin coal seams. A l a r g e r p o r t i o n o f t h e energy i s l o s t to t he
surrounding rock format ions.
2. Very h ig h ash coal (over 50 percent ) .
A
s u b s t a n t i a l p o r t i o n o f
th e thermal energy i s taken up by the ash.
3 . Low a i r in je c t io n ra tes . Gas res idence t i m e underground i s
longer, and a l a rger por t io n o f t he energy i s l o s t to the surroundings .
Very low a i r f l ow ra tes a ls o r es u l t i n lower reac t i on zone tempera tures.
4. Gas channeling . Th is r es ul ts i n poor co nt ac t between gases and
coa l .
5. Too hi gh water in f l ux . Vapo r iza t ion o f the water uses up much
o f th e av ai la bl e thermal energy.
6 .
Gas leakage.
The mathematical model mentioned
i n
t h i s paper can be used t o qu an t i fy
i nd i v idua l e f f e c t s l i s t e d above.
syne r g i s t i c i n f l uence o f two o r more o f t hese e f f ec t s ac t i ng s im ul t aneous ly .
More de t a i l e d d i scuss ions o f t he d i s t r i bu t i on o f ene rgy du r ing t he U G
process have been repo rte d f o r t he Hanna f i e l d te st s (5, 6,
7,
10).
5. Low Resource Recovery
It
can a l so be used t o qua nt i f y th e
I n the borehol e or streamin g method o f UC6, th e combust ion f r o n t tends
t o t ra ve l down the boreho le ra ther rap id ly and t o b reak th rough to th e pro-
duc t i on we l l .
below acceptable levels . Cnder these c i rcumstances, a larg e po r t io n o f the
Once th i s occurs the gas qu a l i t y de ter i o ra tes very rap i d l y
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coa l
i s
l i k e l y t o be by-passed, and energy recovery i s low.
I n
a l l t es ts o f
th e l i nk ed ve r t i c a l we l l process a t Hanna, Wyoming, th e combustion zone
advanced along a broad f ro nt , and m s t o f the coa l i n p lace was consumed.
For example, Fi gu re
1
shows th e we l l la yo ut f o r Phases and
l l
o f t he
Hanna
I I
experiment. Wel ls 5, 6, 7, and
8
a r e p r o du c t io n and a i r i n j e c t i o n
w e l l s . L e t t e r s A t o 0 i nd i ca te i ns trum en ted obse rva t i on w e l l s w i t h
the rm coup les a t seve ra l l eve l s w i t h i n t he coa l seam. W i th t he t he rm al
data
i t
i s p o s s i b l e t o t r a c k t h e pr og re ss
o f
t h e combustion zone. These
data show tha t th e combust ion f r on t burned through a l l we l l s w i t h i n the
60
foo t square pa t t e r n except we l l
K. I t
i s conc luded , t he re fo re , t h a t
t he a rea l sweep e f f i c i e nc y i s w e l l ove r
80
percent .
The square w e l l p at te rn shown i n Figur e
1
conta ined
4170
tonnes
4600
tons) o f coa l . Mate r ia l ba lance ca lc u l a t io ns based on the carbon content
o f produced gases show t ha t abo ut 6070 tonnes
6690 tons) o f coa l were
consumed 9 , 1 4 . Obviously cons iderab le bur n ing occur red outs ide th e
square pat t e rn .
I n
fa ct , th e combustion zone burned through
t o
w e l l
A
b u t
n o t t o w e l l
N
on t h e o p p os i t e s i d e o f t h e p a t t e r n . A t the same time cores
o f coal taken near burned ou t reg ions have shown no re a l ev idence o f p a r t i a l
u t i l i z a t i o n o f coa l . i .e ., cored coa l samples i nd i ca te no subs tan t i a l
ca rbon i za t i on
15).
I t
i s
i n f e rr e d , t h e re f o re , t h a t p r a c t i c a l l y a l l co al
contac ted by the combust ion f ro n t i s comple te ly qas i f ie d .
A two dimens ional mathem atical model developed f o r U C G shows reasonable
agreement w i t h f i e l d performance determined by thermal measurements and
m ate r i a l ba lance ca l cu l a t i ons
16).
When work on t h i s model i s completed,
i t w i l l
be poss ib le to pr ed ic t the shape o f the combust ion zone
for
any
g i ven w e l l pa t t e rn .
6.
O vera l l P rocess E f f i c i en cy
The ove r a l l p rocess e f f i c i en cy i s de f i ned he re as t he upper hea t i ng
va lue o f d r y gas and l i qu id s p roduced d i v i ded by t he hea t i ng va lue o f t he
coal consumed pl us a l l energy consumed on s i t e f o r
gas
compression,
u t i l i t i e s , etc.
A l l
t e s t s w i t h t h e l i n k e d v e r t i c a l w e l l p ro ce ss a t Hanna,
Wyoming, have shown t h a t
U C G
i s an e f f i c i e n t m ethod o f ene rgy recove ry IO).
Typ ica l l y about
I4
percent
o f
th e energy produced i s consumed f o r gas
compression and oth er purposes. Most o f th e energy consumpt ion i s f o r
gas compression. Th ere for e, th e
1 4
percent f ig ur e can be gre at l y reduced
by o p t i mi z i n g t h e s i z e o f w e l l
c a s i n g and s ur f a c e p i p i n g and u t i l i z i n g
ef f i c i en t a i r compression equipment .
f o r a w e l l spac ing o f
18 m
60 fee t ) , p ressure losses ar e on ly 0.7-2.0
N/m2
1-3 p si ) even a t a i r i n j e c t i o n r at es o f 120,000 m3/day 4.5 m i l l i o n s cf /d ay ).
Thus, v e r y l i t t l e e ne rg y i s l o s t i n f o r c i n g a i r t hr ou gh t h e c o a l seam
because o f t he g re a t pe r m eab i l i t y o f l i g n i t e and subbi tuminous coal a f t e r
dry i ng and de vo la t i l i za t i on by reverse combust ion. Overa l l p rocess e f -
f i c i en c i es range from
65
t o
74
p e rc e nt f o r t h e l i n k e d v e r t i c a l w e l l t e s t s
cond ucted a t Hanna, Wyoming IO).
Pressure measurements show that
7. Control of Combustion Front
In a permeat ion t ype method o f
U C G
such as t he l i nke d ve r t i c a l w e l l
p rocess , con t ro l o f t he d i r e c t i o n and ra te o f p rog ress o f t he com bust ion
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f ront
i s ach ieved through se le c t io n o f the pa t t e r n fo r p roduc t ion and
i n j e c t i o n w e l l s and th ro ug h c o n t r o l o f t h e a i r i n j e c t i o n r a t e . A two
dimensional mathematical model descri bed by Jennings e t a l . IO) has been
used to pre d i c t loca t ion and shape o f the combust ion zone w i th s a t i s fa c to ry
accuracy. The theory requ i res f u r t he r ve r i f i ca t i on w i t h m u l t iw e l l pa t t erns .
8.
Equipment Re1 i a b i 1 i y
Equipment f a i l u re s have severely p lagued research on sur f ace coal
ga s i f i ca t io n processes. Th is has not been t r ue w i th U C G (19). The hi gh
leve l o f equipment dep endab i l i t y i n U C G re su l t s from two cond i t i ons , the
g r ea t s i m p l i c i t y o f t h e su rf ac e i n s t a l l a t i o n s r e qu i re d and t he r e l a t i v e l y
low
temperatures o f gases produced.
9.
Lack o f P red i c tab i 1
i
y
A f requent complaint has been that U C G i s h i g h l y u n p r e d i c t ab l e ;
there fo re , re l i ab le eng ineer ing des ign was not poss ib le present ing a
major obs tac le t o commerc ia l i za t ion o f
i n
s i t u c o a l g a s i f i c a t i o n. I n t h e
past th is has undoubtedly been t rue , bu t th e res u l ts f rom the l a t es t te s t
a t Hanna s t rong l y i nd i ca te t ha t t he p roblem i s c l ose t o so lu t i on .
Although
U C G
i s no t ye t ready fo r commerc ia l i za t ion , tha t t ime i s
approaching rapid ly.
A t
the present , unders tand ing o f the phys ica l and
chemical mechanisms control l ing
U C G
i s f a r more complete than o f many
compet ing coal gasi f icat ion processes.
has resu l ted from three developments: ex tens i ve ins t rumenta t ion o f f i e l d
exper iments , a v a i l a b i l i t y o f computers la rg e and small
(20) ,
and the
development o f soph is t i c a ted models capab le o f p r ed ic t i ng accura te ly f i e l d
t e s t performance.
Th is g re at l y increased unders tanding
IO.
S i t e S p e c i f i c i t y
The very fav orab le re su l ts obta in ed f rom
U C G
f i e l d t es t s a t H anna,
Wyoming, have no t been du pl ic at ed anywhere e l s e
i n
the world.
I t
might
be concluded that success i s sp ec i f i c t o the Hanna s i te . Th is i s no t the
case, however. Most o f th e parameters ess ent ia l t o successful
U C G
have
been id e nt i f i ed through the use o f mathemat ical models and o f massive
amounts o f data acqui red du r i ng f ou r yea rs o f f i e l d t es t i ng .
a number
o f
f avorab le f ac to rs have con t r i bu ted g r ea t l y t o success ful t es t s
a t Hanna, Wyoming; sev era l o f these f a c t o r s have been discuss ed al re ad y
( r e f e r t o i t em 1 . Low Gas Qu al i t y) . These fa ct o rs , however, a re by no
means unique to the Hanna coal f i e l d b ut occ ur i n many i f n o t m s t areas
o f th e West.
Undoubtedly
PROBLEMS NOT SOLVED
No
at tempt i s made here t o d iscuss a l l research problems which remain
unsolved because,
even wi t h proven processes, new problems freq ue nt ly
ar i se. Instead problems which remain unsolved ar e c la s si f i e d as one o f
th ree types as a bas is fo r d iscuss ion.
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C r i t i c a l problems. These ar e problems which,
i f
not reso lved favor -
ably,
w i l l
have a major harmful
impact on the c ommerc ia l i za t io n o f
U C G .
Only
t w o
problems o f t h i s type a re known, subsidence and excessive water
i n f l u x .
No n -c ri t i ca l problems. These ar e problems which can have a major
economic impact, bu t whi ch
w i l l
no t prevent commerc ia l izat ion even
i f
no
favorab le so lu t i on i s found. Uncer ta in ty concern ing maximum we l l spac ing
i s such a problem.
Developmental problems. These a r e problems which re qu ir e ap pl ic at io n
o f of f - the -s hel f technology, o r are problems which may requ i re new tech-
nology b ut w i l l no t have a major economic impact on th e process. Gas
clean.-up i s such a problem.
1 1.
Subsidence
Subsidence i s proba bly th e most important s i ng le ob sta cle t o com-
m e r c i a l i z a t i o n o f UCG. Because o f f i s ca l l i m i t a t i on s , t he t e s t s a t Hanna
have been l i m i t e d
to
two and fou r w e l l pa t t e rns w i t h
60
fo ot spacing. Wi th
t h i s spacing no subsidence has been observed a t t he sur face, a l though
subsur face cav ing o f t he
roof has occur red d i r e c t l y over areas o f burned
o u t c o al .
When l a r g e r
UCG
pat t erns a re used, subsidence o f the sur fac e
w i
1 1
occu r i nev i t ab l y . A t many l oca t i ons i n t he w es te rn s ta tes t h i s i s no t an
insurmountable problem. Even w i t h ext ens ive subsidence, the sur fac e i s
less d is t urbe d than
i t
would be by s t r i p m in ing.
There are, however, thr ee m j o r problems associated
w i t h
subsidence:
1. D i s r u p t i o n o f o v e r l y i n g a q u i f er s .
A
v er y s e n s i t iv e p o l i t i c a l
i ss ue i n a r i d r e g io n s.
2.
Estab l i shment o f communicat ion w i th ov er ly in g a qu i f e rs th rough
subsidence and consequent f lo od in g o f th e combustion zone.
3. Gas leakage t o aqu i fe rs and pos sib ly
to
the sur face.
O nl y f u t u r e f i e l d t e s t s w i t h l a r g e p a t t e r n s c an d e te rm in e t o what
ex tent the fo rego ing harmfu l e f fec ts can be m in im ized.
O f
course, i f t h e e f f e c t s o f subsidence shou ld prove in to le ra b l e i n
a g i ve n s i t u a t i o n ,
i t
c o u l d be a vo id ed e n t i r e l y b y u t i l i z i n g sm a ll i s o l a t e d
burn pa t t e rns . Th i s w ou ld be p ra c t i ca l o n l y
i f
the roc k overburden had
s u f f i c i e n t s t r u c t u r a l s t r e ng t h as
i t
does a t Hanna.
i n
an unf or tu nat e red uct i on i n the amount o f recoverab le coa l .
I t w ou ld re su l t a l so
12. Excessive Water In f l u x
V i rg in c oa l i n the Hanna No. 1 seam has low per me ab i l i t y and i s a very
unproduct i ve aqu i fe r . For t h i s reason,
i t
i s p o s s ib l e t o m a i nt a in a n e a rl y
optimum w a te r /a i r r a t i o
(moles water produced from th e co al seam/moles a i r
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i n j ec ted ) a t r easonab le a i r i n j e c t i o n ra tes . Th i s was t r ue f o r Phases
I
and
I I
o f th e Hanna
I I
experimen t. B o th o f t hese t es t s i nvo l ved on l y two w e l l s
spaced 16
m
apa rt f o r Phase
I
and
18 m
ap ar t f o r Phase
I I
Both
the heat i ng va lue o f gas produced and the thermal e f f i c i en cy o f
th e pro cess were much lower f o r Phase
l l
t han f o r t he prev ious
tw
t e s t s .
F i e l d d at a 9, IO) and ca lc u l a t io ns w i t h the mathemat ica l model 8) b o t h
con f irm ed tha t t h e de te r i o r a t i ng res u l t s ob ta i ned i n t he Phase
l l
t e s t
r e s u l t e d fr om an e x ce s si ve i n f l u x o f w at e r. P h y s i ca l l i m i t a t i o n s o f t h e
a i r i n j e c t i o n system p re v en t ed a d ju s tm e nt o f t h e w a t e r / a i r r a t i o .
Phase
I l l
i nvo lv ed a fou r we l l t e s t pa t te r n . Thus, the reac t io n zone
was exposed t o a much
la rg er area o f water drainage f rom th e coal seam.
Als o the lar ger burn area may have promoted gre at er c avi ng of th e roof and
communication w i th an ov er ly in g aq ui f er .
E xcess ive w a ter i n f l ux can be con t ro l l e d
i n
four ways:
1 . Use of dewater ing wel ls .
2. Carefu l p ressure con t ro l .
3.
Adj us tmen t o f t h e a i r i n j e c t i o n r a t e .
4.
G a s i f i c a t i o n i n an up d i p d i r e c t i o n .
The degree o f success t h a t can be achieved wi t h these c o nt ro l measures can
o n l y b e p ro ve n w i t h t h e u se o f l a r g e w e l l p a t t e r n s i n f u t u r e t e s t s .
13.
Maximum Wel l Spa cing and Depth
Fact ors a f f e c t i n g maximum we1
1
spacing and depth a re l a r ge l y con jec tu ra l
and have not been i nv est iga ted i n f i e l d te sts . Maximum depth a t which the
process i s workable i s an impor tant in d i ca to r o f t he amount o f coa l t h a t may
b e s u i t a b l e f o r UCG. Maximum we l l spacing i s impor tant because the dr i l l i n g
and com p let i on o f w e l l s
i s a m aj or c o s t i t e m i n t h e o p e r a t i o n
o f
a
U C G
pro j ec t . Ne i ther i s a c r i t i c a l prob lem, however. There ar e vas t depos i ts
o f c o al a v a i l a b l e a t d ep th s a l r e a dy t e s t e d s u c c e s s f u l l y w i t h U C G . Economic
s t u d i es i n d i c a t e t h a t
U C G
even wi t h t he cl os e spac ing used a t Hanna, Wyoming,
may be compet i t i ve a l re ady w i t h some in t r as ta te natura l gas pr i c es 21).
14.
Bituminous Coal
I t has been emphasized ea r l i e r t h a t t he l i nk ed ve r t i ca l w e l l p rocess i s
a permeat ion method and t ha t t h i s fa ct has been res pon sib le f o r much o f the
success o f the Hanna tes ts. L i g n i t e and subbi tuminous coal sh r in k on
dry i ng and carbon iza t io n . Th is perm i ts the use o f reverse combustion
l ink ing, and the es tab l i shment o f a permeat ion process dur ing fo rward ga s i f i -
c a t i o n . A t t h i s t i m e i t i s n o t c e r t a i n t h a t t h e l i n k e d v e r t i c a l w e l l p ro ce ss
can be used succes s fu l l y i n eas tern b i tum inous coa l wh ich swel l s on heat ing .
Because
of
t h e l a r g e p o p u l a t i on o f t h e e a s t e rn s t a t e s ,
i t
i s i mp or ta nt t o
t e s t t he v i a b i l i t y o f
U C G
i n eas te rn coa l . However, t h i s i s no t c l assed as
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a c r i t i c a l p ro bl em, t h a t i s , a pro bl em c r i t i c a l t o c om me rc ia li za ti on o f UCG.
Regardless
o f
t he outcome o f eastern tes ts,
UCG
remains a workable process
i n l i gn i t e and subbi tuminous coa l.
15
Gas Clean-up
Gas trea tment i s c l a s s i f i e d as an unsolved problem because
i t
has not
been at tempted o r demonstrated i n the f i e l d . Gas analyses, however, in di -
c a te t h a t o n l y e x i s t i n g t ec hn ol og y i s r e q u i r e d
for
gas c lean-up which i s
primari ly a developmental problem.
Coal gas f ro m coke ovens o r Lur gi g as i f ie rs con tain s heavy t ar s and
much pa r t i c u l a t e mat ter . Ex tens ive and r e l a t i v e l y expensive c lean-up i s
requ i red fo r these gases, and the h ig h ly v iscous coa l ta rs tend to p lug
valves o r ot her equipment.
In
contrast gas f rom U C G i s much cle ane r. The condensed l i q u i d s cause
fewer problems tha n ty pi ca l coal t ar s because
o f
t he d i f f e re n c e i n t h e i r
phys i ca l p rope r t i es . The l i qu id s f r om
U C G
have a l ow v i sc os i t y s im i l a r
t o t h a t o f o i l s . None
o f
the mater ia l has a b o i l i n g po in t above 780
K
(950 F ) . A l m s t a qua r te r o f t he more t yp i ca l coa l t a r de r i ved f rom the
lab ora tor y ca rbo niz at i on o f Hanna No. 1 co al was composed o f resid ue wi t h
a b o i l i ng p o in t above 810
K l O O O o F)
(17) .
Pa rt ic u la te conce ntra t ion s and composi tions have been re por ted as wel l
as t race meta l ana lyses (18). Dur ing fo rward combust ion pa r t i c u l a t e load ing
has var ied f rom
0.05
t o
0.90
gm/m3. About
1/2
to
2 /3
w ei gh t f r a c t i o n o f
t h e pa r t i c u l a t e m a t te r co l l ec ted f a l l s i n t he submicron range. A na lyses
i n d i c a t e t h a t
i t
cons i s t s o f p a r t i a l l y ca rbon i zed coa l ,and coa l char .
Su l fu r i s p roduced i n ' th e fo rm o f hydrogen s u l f i d e and no su l f u r
dioxide has been measured.
much more eas i l y f rom th e gas than s u l f u r d i ox id e.
Hydrogen su lf id e, o f course, can be scrubbed
Gas p ro du ct io n temperatures u su al ly range between 510-590 K (450-600 F).
Thus, h i gh temperature c lean-up i s not needed, and e xi s t i n g technology
appears t o be adequate f o r gas tr eatme nt.
SUMMARY AND CONCLUSIONS
Fi f t ee n major techn ic a l p roblems assoc ia ted w i th
U C G
have been discussed.
Ten problems have been la rg el y solved, f i v e remain unsolved. O f t h e f i v e ,
i t
i s be l ie ved t h a t on l y two, subsidence and excess ive water in f l ux , can
p resen t po te n t i a l l y ma jo r obs tac les t o com merc ia l i za t i on o f UCG. The Laramie
Energy Research Center has had v i r t u a l l y no f i e l d exper ience 'w i th e i th er
problem because t hey become major ones onl y wi t h lar ge wel l pa tte rn s which
have y e t t o be f i e l d t e s t ed .
should determine wi th in the next few years i f these two problems can be
reso lved favorab ly .
However, proposed la rg e area f i e l d experiments
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ACKNOWLEDGMENT
Dennis F is ch er and Mike Boyd o f the Laramie Energy Research Center
have reviewed th e manusc ript and pr ov id ed many he lp fu l comments.
1 .
2.
3.
4.
5.
6.
7.
8.
9.
IO.
1 1 .
REFERENCES
A r thu r D. L i t t l e , I n c . ,
A
Curren t Appr ai sal o f Underground Coal
Gasi f icat ion, p.
38,
76, BuMines Rept. C-73671, 1971.
Campbell, G .
G . , C .
F. Brandenburg, and R. M. Boyd, Prel iminary
Ev al ua ti on o f Underground Coal Ga s i f ic a t i o n a t Hanna, Wyo., BuMines,
TPR 82, Oct.,
1974.
Schrider, L.
A.,
and J.
W.
Jennings,
SPE
P r e p r i n t
4993,
SOC.
Petroleum
Eng. Annual Fa1
1
Mtg., Oct. 6-9, 1974.
Gregg, D . W., R. W. H i l l , and D.
U.
Olness, An Overview of t he Sov iet
E f f o r t i n Underground G as i f i ca t i o n o f Coal , Lawrence Livermore Lab.,
UCRL-52004, Jan. 29, 1976.
Gunn,
R.
D., D.
W.
Gregg, and
D.
L. Whitman, A Theore t i ca l A na l ys i s
o f Soviet In S i t u Coal Ga si f i ca t i on F i e l d Tests, Second Annual
Underground Coal G a s i f i c a t i o n Symp., Morgantown, W. Va., Aug.
10-12,
1976.
Gunn, R. D. , and D. L. Whitman, An I n S i t u Coal Gas if ic at io n Model
(Forward Mode) f o r F e a s i b i l i t y Studi es and Design, LERC/RI-76/2,
U.
S.
E.R.D.A.,
Feb.. 1976.
Gunn,
R.
D., D . D. Fischer, and D . L. Whitman, The Ph ys ic al Beha vio r
o f Forward Combustion i n the Underground Ga si f i ca t i on o f Coal , SPE
P repr i n t 6149,
SOC.
Petr ole um Eng. Annual F a l l Mtg., Oct.
3-6, 1976.
Gunn,
R. D., D.
L. Whitman, and
D. D.
Fischer , paper presented a t
Am.
Nuclear SOC Mtg., Energy Mi ne ra l Recovery Research, Golden, Colo.,
Apr. 12-14, 1977.
Fischer , D. D., C. F. Brandenburg, S .
B.
King, R.
M.
Boyd, and
H.
L.
Hutchinson, Status o f the Linked Ve rt i ca l Wel l Process i n Underground
Coal Ga si fi ca ti on , Second Annual Underground Coal G a s if i c a t i o n Symp.,
Morgantown, W. Va., Aug.
10-12,
1976.
Fischer ,
D. D., J.
E. Boysen, and
R.
D. Gunn, An Energy Balance f o r
t he Second Underground Coal G a s i f i c a t i o n Experiment, Hanna, Wyoming,
presented a t the
1977
N at i ona l Meet ing o f
SME
o f AIME, At la nt a, Ga.,
Mar. 6-8, 1977.
Beard, S. G., and R.
P.
Reed, Some In si g h ts f ro m Temperature Measurements
on Recent Underground Coal Ga si f ic at io n Experiments, Second Annual
Underground Coal Ga s i f i c a t i o n Symp., Morgantown,
W.
Va., Aug. 10-12 1976.
73
-
8/10/2019 Problems Solved and Not Solved in UCG
11/12
12. Brandenburg,
C .
F., R.
P.
Reed,
R .
M. Boyd,
D.
A.
Northrop, and
J.
W.
Jennings,
SPE
P r e p r i n t
5654,
SOC Pet rol eu m Eng. Annual F a l l
Mtg., Da ll as , Tex., Sept. 28-Oct. 1 , 1975.
1 3 . Jennings,
J.
W., I n i t i a l Resu l ts - -Coal Permeabi l i t y Tes ts Hanna,
Wyoming, EPRl Gr ant No. R P 542-1, Q u a r t e r l y Rept., Feb., 1976.
14. Brandenburg, C . F., D. D. Fischer , D. A. Northrop, and L. A. Schr ider ,
Results and Status o f the Second Hanna I n S i t u Coal Ga si f i ca t i on
Experiment, Second Annual Underground Coal G a s i f i c a t i o n Symp.,
Morgantown, W. Va., Aug. 10-12, 1976.
15. Boyd,
R.
M.,
Post Burn Ana ly sis Techniques Appl i ca bl e t o Underground
Coal Gas
i f
a t o n
,
i
i d.
16. Jenn ings , J.
W., R. D.
Gunn,
C.
F. Brandenburg, and
D. L.
Whitman,
SPE Pr ep r i n t 6181, SOC Petr ole um Eng. Annual F a l l Mtg., New Orle ans ,
La., Oct. 3-6.
1976.
17. King, S. B., P re l im ina ry U C G Tar Analyses, Second Annual Underground
Coal G a s i f i c a t i o n Symp., Morgantown,
W.
Va., Aug. 10-12, 1976.
18. Fischer,
D. D.,
Moni tor i ng o f Emiss ions f rom an I n S i t u Coal Ga s i f i -
cat io n Exper ment ,
i i
d.
19. Campbell,
G. G.,
T. E. Sterner, and
A.
E. Humphrey, Practical
Cons iderat ions i n Des ign ing an
U C G
Test , ib id.
20.
Eastwood,
D.
E. ,
D . F.
Moore,
S .
6. King,
W. J.
Lanum, J. K. Ea st la ck ,
and J. W. Jennings, Real Time Process Mon ito r i ng
i n
Underground Coal
G a s i f i c a t i o n, i i d.
21. Moll, A. J., The Economics o f Underground Coal Ga si fi ca ti on , ib id .
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8/10/2019 Problems Solved and Not Solved in UCG
12/12
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