Problems Solved and Not Solved in UCG

8/10/2019 Problems Solved and Not Solved in UCG

1/12

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

70


8/12

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

71


9/12

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

7 2


10/12

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.

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(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

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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.

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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,

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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

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E. Humphrey, Practical

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Eastwood,

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E. ,

D . F.

Moore,

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6. King,

W. J.

Lanum, J. K. Ea st la ck ,

and J. W. Jennings, Real Time Process Mon ito r i ng

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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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Problems Solved and Not Solved in UCG

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