Tổng Quan UCG trên thế giới tính đến 2009

Ý tưởng Khí hoá than ngầm được đưa ra bởi những người anh em nhà Siemens ở cuối những năm 1800. Ý tưởng này đến như một gợi ý để khai thác than thải còn lại sau khi hoàn thành công tác khai thác mỏ hầm lò (Hurley 2008; Kostur and Blistanova, 2009). Hai mươi năm sau, nhà hoá học người Nga Dmitry Mendeleev phát triển khái niệm và thiết kế khá chi tiết hoạt động cho UCG (Burton et al.,năm 2009; Kostur và Blistanova, 2009; Hurley, 2008). Các bằng sáng chế Khí hoá than ngầm đầu tiên được cấp cho Betts ở Anh vào năm 1901 ( Hurley,2008). Việc thực hiện thực tế đầu tiên của khái niệm UCG được lên kế hoạch và bắt đầu bởi William Ramsay ở Anh vào năm 1912; Tuy nhiên các thí nghiệm tiến hành không thành công do chiến tranh thế giới lần thứ nhất nổ ra và gây ra cái chết cho Ramsey (Burton et al, 2009;. Hurley, 2008). Vào tháng 5 năm 1913 trên báo Tiến hóa Nga, Vladimir Lenin, trong khi lưu vong, xuất bản các bài viết đầu tiên về UCG dựa trên hiểu biết của Ramsay, tuyên bố lợi ích tiềm năng rất lớn của UCG cho Hội Mỏ bởi vì nó có thể xóa bỏ lao động trực tiếp trong khai thác hầm lò (Burton et al., 2009). Bài báo đặt một nền tảng tốt cho phát triển UCG trên thế giới (Burton et al, 2009;. Hurley, 2008; Bond, 2007; Walker, 2007). Thí nghiệm UCG đầu tiên được thực hiện vào năm 1920 tại Anh, thử nghiệm tiếp theo vào năm 1928 kéo dài trong khoảng 50 năm dẫn đến sự phát triển của kỹ thuật UCG (Hurley, 2008; Bond, 2007). UCG tiếp tục được thực hiện tại Mỹ trong cuộc khủng hoảng năng lượng của họ vào năm 1970. Một lượng tiền lớn được đầu tư vào kỹ thuật UCG cho phát điện và kết quả là trên 30 thí điểm bán công nghiệp được tiến hành ( Hurley,2008) . Mỹ hạ được giá thành khí đốt tự nhiên trong năm 1990, và sau đó UCG không được tiếp tục do thiếu nhân sự có kinh nghiệm.

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14 March 2009

This draft report has been produced by IEA Clean Coal Centre and is based on a survey and analysis

of published literature, and on information gathered in discussions with interested organisations andindividuals Their assistance is gratefully acknowledged It should be understood that the viewsexpressed in this report are our own, and are not necessarily shared by those who supplied theinformation, nor by our member countries

IEA Clean Coal Centre is an organisation set up under the auspices of the International EnergyAgency (IEA) which was itself founded in 1974 by member countries of the Organisation forEconomic Co-operation and Development (OECD) The purpose of the IEA is to explore means bywhich countries interested in minimising their dependence on imported oil can co-operate In thefield of Research, Development and Demonstration over fifty individual projects have been

established in partnership between member countries of the IEA

IEA Clean Coal Centre began in 1975 and has contracting parties and sponsors from: Australia,Austria, Brazil, Canada, China, Denmark, the European Commission, France, Germany, India, Italy,Japan, Poland, the Republic of South Korea, the Netherlands, New Zealand, Russia, Spain, SouthAfrica, Sweden, Thailand, the UK and the USA The Service provides information and assessments

on all aspects of coal from supply and transport, through markets and end-use technologies, toenvironmental issues and waste utilisation

Neither IEA Clean Coal Centre nor any of its employees nor any supporting country or organisation,nor any employee or contractor of IEA Clean Coal Centre, makes any warranty, expressed or

implied, or assumes any legal liability or responsibility for the accuracy, completeness or usefulness

of any information, apparatus, product or process disclosed, or represents that its use would notinfringe privately-owned rights

The report covers the potential for underground coal gasification (UCG) to increase the world’sresource of useable coal The technologies involving the drilling of injection and production wellsinto the coal seam are described, together with the methods for linking the wells With advances inmethods for directional drilling in-seam, new techniques for UCG have proved to be possible The test and trial work carried out in the former USSR, in China, Europe and the USA up to 2000 isdescribed, along with current efforts to commercialise the technology With the power of moderncomputers, the modelling of various aspects of the process has become possible, and current workshould facilitate the validation of some of these models Geological and hydrogeological issues arediscussed, as the single most important decision that will determine the technical and economicperformance of UCG is site selection

The report covers environmental issues, including carbon management, and discusses the options forthe use of the syngas formed The recent developments in Australia and South Africa are reviewed,together with other current proposals for trials in a wide range of countries including China, India,the UK and USA

2.2.5 Seam thickness2.2.6 Coal permeability2.2.7 Associated geopogical structures2.2.8 Developments on the surface

3.2.5 Using man-made excavations3.2.6 Advanced technologies3.3 Establishing underground linkages

3.3.1 Hydrofracturing and reverse combustion3.3.2 Directional drilling

3.4 Igniting the gasifier

3.5 Monitoring

3.6 Well design and operation

3.7 Operating with a CRIP

4 The main trials

5.13 USA

5.13.1 Wyoming5.13.2 Indiana5.14 Discussion and summary

5.14.1 Factors affecting the control of the reactor5.14.2 Resource utilisation efficiency

6 Geological and environmental issues

6.1 Exploration requirements

6.2 Site selection constraints

6.2.1 Geological and hydrological assessments6.3 Environmental impacts

6.4 Monitoring possibilities

6.4.1 Monitoring wells6.4.1 Managing ground deformation6.5 Regulatory frameworks

8 Syngas use

9 Carbon management

10 Key requirements during the next five years

10.1 Undertaking demonstration-scale projects

10.1.1 UCG economics10.2 Establishing a science and technology roadmap 10.3 Regulatory harmonisation

10.4 Improving the public perception of UCG

10.5 Meeting the skills shortage

11 Conclusions

12 References

List of Figures

2.1 Pie charts showing the world’s energy reserves and resources (GasTech, 2007)

2.2 Energy recovery comparison (Mallett, 2008)

3.1 Schematic of the processes involved in UCG (Ökten and Diddari, 1994; Chaiken and Martin,

1992; Beath and others, 2004)

3.2 The development of an UCG cavern/reactor (Perkins, 2005)]

3.3 The generic methods for UCG using drilled wells, as used in the US DOE trials (Beath and

Su, 2003)

3.4 Methods involving man-made excavations and conventional underground mining (Okten and

Didari, 1994; Beath and Su, 2004b)

3.5 Texyn ‘Santa Barbara’ mining system (Tillman, 2008)

3.6 The super daisy shaft concept (Palarski, 2007)

3.7 The sequence of events in a UCG process using reverse combustion linking (Krantz and

Gunn, 1982)

3.8 Schematic views of reverse and forward combustion linking, and the shape of the cavity

formed in a constant ‘source-sink’ field (Blinderman and others, 2008b)

3.7 Linkage between holes at Podmoskovnaya Figure 21 from USSR history Gregg and others,

1976

3.8 Schematic views of reverse and forward combustion linking, and the shape of the cavity

formed in a constant ‘source-sink’ field (Blinderman and others, 2008b)

3.9 Directional drilling, the challenges underground with alternative trajectories (Jackson, 2003;

DTI, 2005)

3.10 Downhole drilling assemblies Figure 6 and Box 2 from DTI 2005

3.11 The progressive formation of new cavities as the CRIP is moved away from the production

well (Beath, 2004)]

4.1 Coal seam thickness and depth for the various field trials of UCG, and an indication of the

regions of interest for development (Perkins, 2005)

4.2 The linkage between holes at Podmoskovnaya (Gregg and others, 1976)

4.3 The CRIP device being inserted into the borehole and along the coal seam (DTI, 2004)4.4 The Spanish CRIP test, and the way the cavities develop (Beath and Davis, 2006)

4.5 Long tunnel large section in-seam gasification layout with the commonly used schematic

(Liang and Shimada, 2008)

5.1 Map showing the location of UCG activities, past and present (Friedmann, 2008)

5.2 UCG design process (Brand, 2008)

5.3 The Bloodwood Creek layout with parallel-holes CRIP for the 100 day trial (Mark and

Mallett, 2008)

5.4 The potential UCG sites near Velenje in the Šoštanj coalfield (Veber, 2003)

5.5 The proposed programme for the parallel demonstration and commercialisation of UCG at

Majuba (van der Riet and others, 2008)

5.6 Diagram of the Secunda UCG process showing the well matrix together with an elevation

(Brand, 2008)

6.1 Changes in the strata above a UCG reactor (Mallett, 2007)

5.2 Goafing behaviour

A) for a cavity larger than 100 m

B) for a cavity small enough not to break the Dolerite Sill on the surface (Brand, 2008)6.1 Changes in the strata above a UCG reactor (Mallett, 2008)

6.2 Goafing behaviour

A) for a cavity larger than 100 m

B) for a cavity small enough not to break the Dolerite Sill on the surface (Brand, 2008)7.1 An integrated UCG simulation (Kolar, 2008)]

8.1 UCG syngas composition from various trials (Mark, 2008)

8.2 Tomorrow’s syngas to products business (Puri, 2006)

10.1 Plans for UCG expansion to commercial scale (Mallett, 2007)

List of Tables

2.1 Methane content in coals at increasing depth (Sloss 2005)

4.1 Summary of past experience with UCG in the USSR (Beath and others, 2004)

4.2 Summary of experience with UCG in Europe (Beath and others, 2004; Burton and others,

2006)

4.3 Summary of government sponsored tests in the USA (Beath and Su, 2003; Burton and others,

2006)

4.4 Rawlins DOE test results

5.1 Summary of the principal Chinese UCG tests 1997 onwards (Beath and others, 2004; Feng

Chen, 2008)

5.2 The consortium partners for the HUGE project (Palarski, 2007)

5.3 Comparison of the resource utilisation efficiency of conventional mining and use with UCG

List of Abbreviations and Acronyms

CAER Centre of Applied Energy Research (University of Kentucky, USA)

CBM coal bed methane

CEPL Carbon Energy Pty Ltd

CRIP controlled retraction injection point

CSIRO Commonwealth Scientific and Industrial Research Organisation (Australia) CTL coal to liquids

CUMTB China University of Mining and Technology, Beijing

CV calorific value (heating value)

DOE Department of Energy (USA)

DTI Department of Trade and Industry (UK)

EIA environmental impact assessment

EPA Environmental Protection Agency

eUCG Ergo Exergy proprietary technology eUCGTM

FCL forward combustion linking

GAIL Gas Authority of India

HUGE Hydrogen oriented underground coal gasification for Europe

HV high volatiles

IGCC integrated gasification combined cycle

IIT Indian Institute of Technology (Mumbai)

IP intellectual property

IPPC Pollution Prevention and Control Regulations

LTLSTS long tunnel large section two stage (or LLTS)

MOU memorandum of understanding

MV medium volatiles

n/a not available

NEDO New Energy and Industrial Technology Development Organisation (Japan)NTPC National Thermal Power Corporation (in India)

OCGT open cycle gas turbine

ONGC Oil and Natural Gas Corporation

PDU process ddevelopment unit

PEDL petroleum exploration and development licence

R & D research and development

RCL reverse combustion linking

SDB steeply dipping bed

subbit subbituminous (coal)

UCG underground coal gasification

VWs vertical wells

WGS water gas shift (reaction)

3D three dimensional

1 Introduction

The concept of underground coal gasification (UCG) is simple It involves reacting (burning) coal insitu in a mixture of air/oxygen, possibly with some steam, to produce a syngas The steam may comefrom water which leaks into the underground cavity, from water already in the coal seam or fromsteam deliberately injected Gasification takes place at elevated temperatures with a stoichiometricshortage of oxygen, so the principal gases formed are hydrogen and carbon monoxide However,there are many other products, including carbon dioxide; hydrocarbons such as methane; tars; andcompounds such as hydrogen sulphide and COS arising from impurities in the coal The product mixcan vary widely depending on a number of factors The syngas produced can be used to produceelectric power or as a chemicals/liquid fuels feedstock

UCG offers the potential for using the energy stored in coal in an economic and environmentallysensitive way, particularly for deposits which are unmineable by conventional methods If UCG were

to be successfully developed and widely deployed, then the world's coal reserves might be revisedupwards by a substantial amount This is discussed in Chapter 2

The main method of achieving UCG involves a minimum of two boreholes (or wells) drilled into thecoal seam some distance apart, and connected by a link/channel through which gases can flow Theseholes may be vertical, or they can be inclined boreholes, partly drilled through the coal seam One ofthe holes, referred to as the injection borehole, is used to supply the gasifying agent (air, oxygenenriched air, or oxygen, possibly with added steam) The other is the production borehole (or well)through which the product gases are carried to the surface for treatment and use With some

production patterns the function of these wells is interchangeable, and from time to time, the

supply/injection well becomes the production well, and vice versa This may be to achieve the

linkage between them, or to smooth out the pattern of gasification in the (constantly changing)underground gasification chamber

During the past twenty years there have been significant advances in the techniques used for

directional drilling and in particular in-seam drilling This has been associated with the drillingneeded by the oil and gas industries, and with that used for recovering coal bed methane (CBM) TheUCG methods and in-seam drilling and well linkage techniques are covered in Chapter 3 whileChapter 4 includes an account of the main trials carried out in various parts of the world including theUCG work undertaken in China in abandoned mines, to recover some of the coal/energy left behind.This involves a somewhat different approach from the 'two boreholes' method, and one approach used

in China is based on man-made tunnels which form the gasification chamber.

Chapter 5 describes the ongoing efforts to establish the conditions for commercial devlopment Asthe bed and reactor conditions are quite variable, in order to supply a consistent feed of syngas to acommercial operation, it would almost certainly be necessary to operate a number of undergroundgasification 'reactors' in parallel Then the syngas products can be combined to ensure adequateuniformity in terms of both quantity and composition

Because the reactions take place underground and out of sight, control of key process parameters,such as temperature, is difficult The coal seam and surrounding rock form a huge heat sink with thereaction taking place largely in one part of the seam at a given time, spread along the whole of thelink between the two wells

Only a limited number of parameters can be either controlled or measured, and it can even be

difficult to determine where the reaction is taking place and what temperatures are reached, and as aresult, modelling has a substantial part to play in studying what is happening This is discussed inChapter 6 Modelling is also used in overall project assessment work and in designing the necessarysurface facilities Chapters 8 and 9 look at syngas use and carbon management respectively

In this report, the potential for UCG use is discussed, with an outline of the main technologies whichcan be applied, and a description of the outcome of test work The penultimate chapter, before theconclusions, looks at the key requirements during the next five years for the successful deployment ofUCG on a commercial scale Small scale trials are an essential preliminary step, following the

necessary exploration, but commercial-scale applications are considerably more challenging andthere is, as yet, virtually no relevant experience to build on

2 UCG potential

UGC is being looked at particularly for utilising unmineable coal deposits and deeper seams whichare not included in the proved reserves figures The amount of work carried out in deep seams is verylimited It can potentially be used in steeply dipping seams, and in coal deposits where the ash

content is so high that it precludes conventional extraction Nobody is looking at UCG in preference

to conventional mining where the coal can be extracted economically using established methods

Early studies suggest that the use of UCG could potentially increase world's coal reserves by as much

as 600 Gt (World Energy Council, 2007), which represents a 70% increase As discussed in thisreport, UCG is not easy to carry out without environmental impacts, and the inability to manage theseacceptably would reduce the amount of coal which can be utilised by this method However, even anincrease of 60 Gt (based on a conservative assumption that just 10% of the potential can be realised)would provide a significant amount of additional energy

2

In today’s climate, carbon management and the control of CO emissions are likely to become topicswhich may determine the future of individual UCG developments Carbon management is discussed

in Chapter 9

2.1 Coal reserves and resources

There is an important distinction between reserves and resources The geological resource is the totalendowment of coal in a particular area, and while there are some uncertainties, some geologistsbelieve that the world total is reasonably well known and assessed The ‘proved reserves’ are defined

as ‘the amount of the coal which would be economically recoverable using current technology’

Then in addition, there are ‘probable’ reserves and then ‘possible’ reserves Some commentators usethe terms proved, indicated and inferred reserves/resources; others describe the 'coal in place'

together with estimated additional amounts Added together, the three amounts (proved, indicated andinferred) are the total potential resource of a fossil fuel The figures for economically recoverablereserves are open to adjustment, in that as the relative price level increases, more of the resourcebecomes economically recoverable and thus becomes a proved reserve This adjustment is not,however, made on a regular or consistent basis The reserves figures commonly quoted by key

information providers such as BP (2007) and the World Energy Council (2007) remain much thesame from year to year even though coal prices and therefore what can be economically mined, may

change significantly.

The world's proved reserves, which are the amount of coal assessed as being economically

recoverable using current technology, were estimated to be just under 850 Gt at the end of 2005

(World Energy Council, 2007) The equivalent figure in the BP Statistical Review of World Energy

(BP, 2007) was 900 Gt The total resource which would include thin seams, deep seams and thosewhich are steeply dipping, could be anywhere between five and ten times this amount

Coal deposits located at or near the surface can be extracted by open pit methods at depths down to

100 or 200 m Underground mining of the deeper seams is possible at depths down to a little over

1000 m, although it gets increasingly expensive to ventilate and cool the deeper mines Extractioncosts increase at greater depths, and are proportionately higher for mining thinner seams

The accuracy of the reserves/resources figure for any country or coal basin depends on the amount ofdetailed exploration undertaken, and this is highly variable For the reserves amount, much depends

on people’s assessment of what is economic It is complicated by the fact that in spite of efforts tostandardise, different countries (and multi-national companies) use different methods and

conventions Some data are regarded as commercially sensitive and are therefore not published by thecompanies involved

Similar considerations apply to the world's reserves and resources of the other fossil fuels, oil andnatural gas The relation between the established reserves of the main fossil fuels and of the relativeresources is illustrated in Figure 2.1 where the predominance of coal is absolutely clear

[Figure 2.1 Pie charts showing the world’s energy reserves and resources (GasTech, 2007)]

In terms of energy content, the world's coal resources are vast, and are almost certainly much greaterthan those of the other fossil fuels, oil and gas However, only a fraction of the energy can be

recovered by conventional mining, some is recoverable in the form of coal bed methane (CBM)extraction, and considerably more would be recoverable if UCG is developed into a commercial-scaleprocess The order of magnitude of the different approaches is illustrated in Figure 2.2 which showsthe potential energy recovery in a particular location, from CBM, by using conventional mining(where this is possible) and from UCG The amounts of energy recoverable in practice will depend onmany different factors, many of which are discussed in this report If the coal can be extracted

economically by conventional mining methods, this will almost certainly be the preferred option in a

Figure 2.1 Pie charts showing the world’s energy reserves and resources (GasTech,

Oil

Oil Natural gas Coal

18 trillion tons

used in Australia, China, Germany, Russia, the UK and the USA (see Couch, 2006) Similarly, many

coal experts have developed an in-depth knowledge and understanding of their own coals and of theirgeological settings, but have limited knowledge of the coals which occur in other places Where coal

is mined, geological expertise is focused on its impact on conventional mine design, construction andoperation The behaviour of an UCG reactor/cavity will be quite different In addition there is

commonly little contact and technical understanding between those who mine coal and others whouse it Even at an academic level, there are few experts who can cut across the boundaries betweenthe geologists and mining engineers responsible for coal extraction and the chemical and processengineers largely responsible for its use

Coal rank

Coals of all ranks from lignite to anthracite can be gasified in surface gasifiers The tests on UCGhave included a range of coals of different ranks, however, it appears that the lower rank coals areeasier to process underground than the higher rank ones It seems likely that this is associated withtheir inherent permeability, and because they tend to shrink when heated, which enhances the linkagebetween the injection and production well The inherent moisture content of these coal is also high,

so that the product syngas may have a higher water content, but less steam injection may be needed

It has been suggested that some of the impurities in lower rank coals can act as catalysts and improvethe kinetics of the gasification (Burton and others, 2006) The lower rank coals also tend to be morereactive than higher rank ones

Figure 2.2 Energy recovery comparison (Mallett, 2008)

UCG Underground Coal seam gas

mining

Coal bed methane

Seams are highly variable in the amount of methane they may contain During conventional mining,the methane released can be a safety hazard, and the control of mine ventilation using large amounts

of air to prevent explosions, is a major consideration Methane emissions also arise from old and

abandoned mines CBM recovery is discussed in another IEA Clean Coal Centre report Coalbed

methane emissions - capture and utilisation (Sloss, 2005).

Some seams have enough methane to be economically recoverable independent of whether the coalitself will be mined conventionally, for example where the deposit is dipping or disturbed CBMrecovery has been carried out successfully on a considerable scale in both Australia and the USA, and

is being actively considered or applied on a smaller scale in a number of other countries The mainmethod used is via vertical wells and the use of hydrofracturing to establish paths in the coal tofacilitate methane release when the water is pumped out

The success of a CBM project depends principally on the accessibility of the methane The highest

concentrations of methane are generally found in deeper coal seams, see Table 2.1.

The release of methane gas from coal in order to allow its collection occurs only when the

pressure is sufficiently reduced by removing ground water from porous, fractured coal formations In

an effort to increase the quantity of methane gas removed from coal seams, fluids are forced into theformation through a well at very high pressures to hydraulically fracture the coal seams Sand

particles in the hydraulic fluid prop up the widened and newly created fractures in the coal allowingmore methane gas to escape after much of the hydraulic fluid and ground water have been pumpedout the well(s) The drilling techniques established to facilitate CBM recovery can be useful for someUCG applications

[Table 2.1 Methane content in coals at increasing depth (Sloss 2005)]

2.2.1 Variability

As a crude illustration of the variability of the coals currently used in different parts of the world, therange of properties/characteristics can be expressed as follows:

• a lower heating value from 5 MJ/kg to 28 MJ/kg;

• ash content can go from 1% to 50%;

• similarly, moisture content can vary from 5% to 65%;

Table 2.1 Methane content in coals at increasing depth (Sloss, 2005)

• sulphur content can range from virtually nothing to as much as 10%;

• the age of a given deposit can be from 350 My to as little as 2 My, meaning that the degree of

coalification is highly variable

In addition, the geological setting of the different coal deposits is highly variable with deposits atdifferent depths, inclination, seam thickness, and involving different interactions with adjacentstructures and formations

The relevance of this to UCG is that trials and tests have so far only looked in any depth, at a

relatively narrow range of coals and geological settings, and the results obtained have been quitescattered, site-specific, and in many cases inconclusive This will be discussed further in Chapters 4

and 5 People have too readily generalised from the claim “this is what happened here in this

particular context” to “this is what happens in coal” without recognising coal’s variability, nor the

differences in geology and hydrology In addition, unlike conventional mining and coal usage wherethe expertise is split between two quite separate technical communities, UCG requires an integratedmulti-disciplinary approach to the production of syngas and its use

2.2.2 Coal seam properties

Coal is composed of distinct organic entities called macerals, and lesser amounts of inroganic

minerals Some impurities such as Ca, Na, N and S are attached to the organic coal structure of thecoal Others have been deposited as discrete mineral particles carried by the wind, or arising from amarine environment The macerals and minerals occur in distinct associations called lithotypes, and acoal seam can consist of a number of lithotype layers Coal properties are heavily dependent on theage of the deposit, generally described in terms of its rank

Some seams lie in horizontal or only gently inclined strata, while others have been folded over bygeological action and lie at a steep angle Seam thickness can vary from just a few centimetres up toamounts of more than 100 metres Only some of the coal which has been formed can be minedeconomically In the deeper bituminous coal deposits, the extracted seams commonly have a

thickness of between 1 m and 4 m The coal seams frequently lie in a sequence representing

successive periods when the peat-like deposits were formed There can be as many as twenty or thirtydifferent seams in a sequence Of these seams, only two or three are likely to be mineable by

conventional methods There are substantial deposits at depths below 1000 m which cannot generally

be extracted by conventional mining methods as the temperatures and pressures in the seam would be

too high If UCG could be established as a viable method for recovering the energy from such seamsthen the potential is enormous

2.2.3 Coal seam properties affecting UCG

Key features which affect the potential use of UCG are:

• the nature of the coal (and in particular its rank and reactivity, together with its ash, moisture,

sulphur and methane contents);

• the deposit/seam depth, thickness and inclination;

• seam continuity and its physical strength;

• the permeability of the seams, based on the pore structure and the presence of cleats (or

cracks);

• the associated geological structures, and particularly that of the roof materials and of the

hydrogeology of the area involved If there is an impermeable seal between the coal seam andthe surface, then the environmental implications are very different when compared with thosefor a roof consisting of permeable shales or sandstones;

• developments on the surface Carrying out UCG will be much easier under land which is

undeveloped and where a small amount of subsidence is likely to be acceptable

It is too simplistic to say that almost any coal apart from the swelling/caking coals can be gasifiedunderground While that may be true in principle, the practicalities associated with an economicdevelopment mean that initially UCG is likely to proceed in seams that are broadly deep enough toeliminate the risk of gas leakage, but shallow enough to facilitate economic drilling Coals whichswell on heating and can be suitable for coke production are likely to be unsuitable for UCG as thelinkages between the injection and production wells can be blocked off Trials in higher rank coalsincluding anthracites have been relatively few, as have trials in thick brown coal seams Considerablymore work would be required to establish the conditions for a commercial UCG operation in suchcoals

2.2.4 Seam depth

Another feature affecting the use of UCG and its application is the deposit/seam depth, as this affectsthe hydrostatic pressure in the seam and the operating range for the pressure in the undergroundreactor cavity In shallow seams where the hydrostatic pressure is minimal, increases in the reactorchamber pressure are likely to result in increased gas leakage In deeper seams the hydrostatic

pressure will be considerably higher It may be necessary to operate at below this pressure to ensurethat water flows into the reactor chamber rather then out of it This is to reduce the chances of watercontamination, although at greater depths this is less of an issue since many of the aquifers arealready saline The hydrostatic pressure increases with depth at about 0.01 MPa/m for fresh water or0.012 MPa/m for a saturated saline aquifer This means that at 100 m depth the hydrostatic pressure

is some 1 MPa, and at 1000 m depth it increases to a little over 10 MPa These differences have asignificant affect on the permissible operating conditions and thus on the composition of the syngasformed

In addition to changes in hydrostatic pressure with depth, the strata will increase in temperature,although this effect has much less impact The temperature gradient varies in different places, but inQueensland, Australia, values of just over 2ºC/100m are quoted, so that the strata at 1000 m deep will

be some 20-25ºC hotter than that at 50 m deep

Most current developments are looking at seams deeper than 150 m which would minimise the risk ofuncontrolled gas losses and of shallow aquifer contamination In terms of the costs of UCG, theexploitation of deeper seams involves considerably higher drilling costs, although any nearby

aquifers are more likely to contain saline (non-potable) water thus reducing any risk of unacceptablecontamination

Compared with conventional mining, UCG can be more readily used in coal seams that are faulted orsteeply dipping It might also be used where there are irregular intrusions into a seam which wouldmake mining difficult With UCG it should be possible to identify parts of the seam which can beeconomically exploited where conventional mining would be either uneconomic or even virtuallyimpossible

2.2.5 Seam thickness

Current developments seem to be mainly in seams around 5 to 10 m thick where there are substantialquantities of coal to be gasified but where cavity collapse would probably not result in unacceptableground movement and subsidence

If UCG is used in thick seams of >20 m it would seem that there is a greater risk of unexpected cavitycollapse as it is not easy to forecast where the reactions will take place, and whether they will movevertically or horizontally A great deal more work is required to assess its application in thick seams

UCG has been proposed for use in thin seams less than 2 m thick, but while a syngas can be

produced, there will be many questions relating to the potential economics of such an operation Thepotential heat losses through the roof can be considerable, leading to low thermal efficiency andlower product gas quality

2.2.6 Coal permeability

The cleats/cracks in coal are associated with the geological stresses exerted on the seam as it issubject to massive ground movement and may even become folded over during the process Stressescan result in the formation of tight closely-spaced fractures within the coal seam whose presence is ofsignificance during UCG, and in relation to the release (and recovery) of CBM In general, the higherrank coals and those in deeper seams have low/lower permeabilities This will have an impact on theappropriate technology for making the necessary underground linkages to facilitate controlled UCG

2.2.7 Associated geological structures

The various layers situated above the coal will have a direct influence on whether UCG is possible in

a particular location This will include the roof materials above the seam, and other layers right up tothe surface It also includes the hydrogeology of the area involved Choosing appropriate places fordevelopment involves not only understanding the geology but also appreciating the potential effects

of the presence of a moving reactor in the coal seam with reaction temperatures above 1000ºC Therewill be a pattern of the collapse of the roof into the cavity, with resultant movement and cracking inthe strata above, depending on its properties and strength

2.2.8 Developments on the surface

In conventional mining it is common practice to leave certain parts of the seam undisturbed so as toprevent damage to surface buildings and installations A pillar of coal (sometimes quite a large pillar)will be left in place around the base of a mine shaft and other pillars will be left to prevent or

minimise subsidence damage to surface buildings and to installations like railway lines Pillars mayalso be left in order to prevent rivers and canals from subsiding

UCG activities can similarly result in surface subsidence, and this effect may restrict the number ofplaces where it will be practical to carry it out In addition, UCG will generally best be undertakenwhere the land (on the surface) is fairly open, undeveloped and of relatively low value Using some

UCG methods, the landscape will be punctured by a whole network of boreholes which are movedprogressively across the surface as gasification of the underground seam proceeds Other methodsusing inclined and in-seam boreholes may be much less intrusive, but subsidence effects will be muchthe same All developments will require the provision of surface installations to supply the feed gases(including possibly oxygen and steam), to facilitate ignition of the underground seam, and to cleanand use the syngas produced (either for power generation or chemicals/liquid fuels production).Developments will also require appropriate administration and laboratory buildings.

3 UCG technologies

UCG requires a multi-disciplinary integration of knowledge from exploration, geology,

hydrogeology, drilling, and of the chemistry and thermodynamics of gasification reactions in acavity in a coal seam (DTI, 2004)

It also requires an understanding of the nature of different coals, their variability, and how they lie indifferent geological settings Coal deposits are far more variable than those of the other fossil fuels,gas and oil, and this has major impact on their exploitation

A number of techniques have been established to facilitate UCG, and four main methods are

providing the basis for current industry developments In addition there has been work in China torecover energy from abandoned mines and using man-made underground tunnels There are a number

of technical overviews of the status of UCG technologies, which include:

2001) prepared by the UK DTI;

written for the CSIRO, Australia; and

currently being updated by the LLNL, Livermore, CA, USA

In this Chapter, the basics which lie behind the technology are discussed, while the experience gainedfrom work in the USSR, USA, China and in Europe up to the 1990s is assessed in Chapter 4 Currentdevelopments in Australia, Russia, Europe, China and the USA, and proposals for the use of UCG in

a variety of countries are discussed in Chapter 5 Since there was some activity at the end of the1990s, it is difficult to create a precise cut-off point, but generally work undertaken before 2000 isdiscussed in Chapter 4, while that since, much of which is ongoing and with a view to commercialdevelopment, is discussed in Chapter 5

The concepts behind UCG are simple, but controlling the reaction and producing a syngas withconsistent quality and quantity with minimal environmental impact can be difficult to achieve

There are key differences between surface and underground gasification:

• on the surface the reactions are contained in a fixed vessel where the temperature and

pressure can be measured and controlled with considerable precision, as can the feed of coaland oxidants Under these conditions the quantity and composition of the products can beaccurately predicted and controlled; whereas

• underground, the shape and location of the reaction zone will be continually and

progressively changing and it is not possible to measure or control the operating conditions inthe same way During UCG there will be burn-out, caving and thermal deformation in thesurrounding rock formations The reaction zone will be moving into different parts of thedeposit, and this movement will not be precisely predictable and controllable Some of thegaseous products may escape from the reactor zone into the strata, and the influx of watermay or may not be controllable

Historical overview

The early history of UCG was slow and inauspicious Sir William Siemens, a German scientist, iscredited with the first suggestion to gasify coal underground in 1868 At about the same time, inRussia, Dmitriy Mendeleyev, suggested the idea of controlling and directing spontaneous

underground coal fires, including the idea of drilling injection and production wells (Olness andGregg, 1977)

The first patent recorded for UCG was issued in 1909 in the UK to an American, A G Betts Over thenext several years, Sir William Ramsey promoted and expanded upon Betts’ idea, culminating inplans for a first trial experiment underground This work obtained financing but never actuallyhappened, because of Ramsey’s death and the outbreak of the FirstWorld War (Ergo Exergy, 2005)

In May 1913, Lenin published an article in Pravda calling UCG "one of the great triumphs of

technology", and praising its social significance because of the elimination of hard mining labor TheSoviet UCG programme was later championed by Joseph Stalin for the same reason Some of thedetails of the technical aspects of the Soviet programme are discussed in Chapter 4 and as most of thework was undertaken before the break-up of the Soviet Union, there are several references to theUSSR The programme was significantly downsized and lost its momentum in the 1960s, when largereserves of natural gas and oil were discovered in Russia By 1996 (in the post Soviet era), when thelast Russian UCG plant was shut down, the UCG plants in Russia and other countries of the FSU hadconsumed over 15 Mt of coal The plant at Angren, now in Uzbekistan, is still in operation The totalSoviet effort has far exceeded the combined efforts of those of other countries both in terms of

numbers of tests and of the amount of coal gasified (Burton and others, 2006) Test work and trials inboth Europe and in the USA have explored a wider range of conditions than were tried in the SovietUnion, and some sophisticated models have been developed

Between the years 1944 to 1959, a shortage in energy resulted in new interest for UCG in WesternEuropean coal mining countries Tests were carried out in Czechoslovakia, France (in Morocco),Italy, and Poland (Curl, 1979) The boreholes method was tested in the United Kingdom, on the sites

of Newman Spinney and Bayton (1949-1950) A few years later, a first attempt was made to develop

a commercial pilot plant: the P5 Trial in Newman Spinney (1958-1959) During the 1960s, allEuropean work was stopped, principally due to an abundance of energy and to low oil prices

European work was restarted during the 1980s, with trials in Belgium and France, and culminated inthe UCG test at El Tremedal in Spain which was undertaken jointly by Spain, the UK and Belgium,and supported by the European Commission The two short tests there took place in 1997 There hasbeen considerable interest in Europe in the gasification of deep seams with higher rank (bituminous)coals, compared with much other work which has been using coals of lower rank which are morenaturally permeable

In the USA, a UCG programme was initiated in 1972, which built upon Russian experience, as well

as trying out new techniques More than thirty field tests were carried out (Burton and others, 2006),and at the end of the programme in 1989, the technology was thought to be ready for commercialdemonstration Unfortunately, in terms of development, because the price of natural gas in the 1990swas low, no commercial demonstrations took place, and the skills developed have become dilutedsince most of the people involved in the US programme have either moved on to other jobs, or haveretired

The largest on-going programme is being conducted by China, and includes at least sixteen trials,mainly in abandoned mines and/or developing their long tunnel two stage technology based on man-made gasification chambers (discussed in Sections 4.4 and 5.4), although their most recent trialinvolves the two-borehole method There are active sites where tests are currently being undertaken

in Australia, China and South Africa, see Chapter 5 Feasibility studies and assessments are under

way in a range of countries

3.1 UCG chemistry

During gasification, the in-seam coal is heated by hot gases to a very high temperature and is

consumed by oxidation reactions Then in a region where the oxygen content is depleted, the

gasification reactions take place A flame front is initiated within the passage linking the injectionand production wells, and as the gases pass through the various reactions approach equlibrium

conditions before they leave via the production well at temperatures which will probably be between

200 and 300ºC Figure 3.1 is a schematic showing the various stages using the basic Russian

methodology, with vertical wells Figure 3.2 shows the changes in conditions through the gasificationchamber right through to the final clean-up of the cavity when the useful coal has been reacted

[Figure 3.1 Schematic of the processes involved in UCG (Ökten and Diddari, 1994; Chaiken and

Martin, 1992; Beath and others, 2004)]

[Figure 3.2 The development of an UCG cavity/reactor (Perkins, 2005)]

In UCG the main source of the necessary heat is coal (carbon) combustion As the coal is heated(throughout the length of the combustion chamber and the linkage routes), the coal starts to lose themoisture held in the pore structure, and then undergoes pyrolysis at temperatures above 400ºC, duringwhich hydrogen-rich volatile matter is released, together with tars, phenols and hydrocarbon gases.Simultaneously, at higher temperatures, the char is gasified, releasing gases, tar vapours and solidresidues Details of the different processes of gasification, the reactions taking place and the

chemistry involved can be found in the IEA Coal Research report Understanding coal gasification,

Kristiansen (1996) The dominant gasification reactions are those of partial oxidation of the charwhich produces a syngas consisting mainly of hydrogen and carbon monoxide At the same timevarious impurities in the coal which may either be organically bound or in the form of discrete

particles held in the matrix will also be reacting Many coals have a significant pore structure withadsorbed gases and in particular methane, and this will be released and will contribute to the

reactions taking place along the reactor zones

The basic reactions can be generalised as:

Figure 3.1 Schematic of the processes involved in UCG (Ökten and Diddari, 1994;

Chaiken and Martin, 1992; Beath and others, 2004)

Figure 3.2 The development of an UCG cavity/reactor (Perkins, 2005)

• C + H º CH (+ heat)

2The heat needed to promote the formation of the H +CO syngas comes principally from the oxidation

Minerals which are naturally present in the coal may preferentially catalyse some of the reactions

of H may be advantageous For the downstream production of chemicals and liquid fuels, the gasifierneeds to produce a syngas with a heating value broadly in the range 10.5-16 MJ/m The syngas could3also be used as the fuel gas for an IGCC unit or for producing a substitute natural gas (SNG)

The conditions are dependent on:

• the rank of the coal involved;

• the properties of the mineral matter and other impurities present;

• on the temperatures achieved underground, together with the reaction chamber pressure;

• the size and shape of the connecting links, the shape of the developing cavity, and the

distance to the production well;

• the nature of the injection gas, and on whether it is air or oxygen-enriched air and on whether

it carries any steam with it;

• where the UCG reaction zone is below the water table, water will flow into the reaction zone

from surrounding formations

Once the reaction has been established, the product gas composition can be modified to an extent bychanging the feed rate and/or composition of the injection gas, and the pressure in the combustionzone To maintain an inflow of water this needs to be maintained at a level below the hydrostaticpressure In most situations the aim will be to establish relatively stable conditions so that there will

be consistency for periods of several days from an individual production channel

Coal properties are important, and coal rank can be significant in terms of permeability, reactivity,water content and structural strength In addition, if there is a high ash content there is likely to be aproportionately smaller surface area of char available to be gasified to CO A primary property ofconcern is whether the coal swells or shrinks on heating If it swells it could block the passageslinking the injection and production wells The ideal coals for UCG shrink and fall apart when

heated, and this includes most of the lower rank coals These will not block the necessary channelsthrough the seam, and the breakup into smaller particles can provide a large surface area for thevarious reactions to take place While there have been tests with higher rank bituminous coals, andeven with anthracites, most have been for only short periods

The coal seam depth will have a considerable effect on which technologies and techniques are usedfor establishing the connections between the injection and production wells The proposed use of thesyngas will determine whether air-blown or oxygen enriched systems are used

The practicalities of UCG are dependent on the overall geology of a coal deposit, including its rank,and the seam depth, inclination, and thickness It also depends on the detailed geology of the

surrounding strata, and in particular the nature and strength of the rock formations above and possiblyimmediately below the seam, and the presence of aquifers During UCG there can be substantial localthermal impacts which will affect the various layers near the reaction chamber which moves throughthe seam as the reaction proceeds As gasification proceeds, an underground cavity is formed Thepossible influx of water into the reaction chamber and gas leakage into the surrounding rock (whichmay be permeable) are factors which will affect the efficacy and applicability of UCG in particularstrata Water entering the cavity may form steam and participate in the gasification process This mayalso lead to a drop in the local water table, but over time, when the gasification is complete, the water

to be constantly monitored since by observing changes it may be possible to deduce what is

happening underground

The operating pressure is a key parameter The optimal pressure is probably the value at which thewater inflow into the cavity is controlled at an acceptable level This means that the reactions areconfined within the coal seam and that, generally, the gasification products do not escape into thesurrounding strata In deeper coal seams, pressure balance considerations require higher pressures tocontrol the influx of water, but this may be accompanied by increased losses of product gas If thecoal in deep seams is covered by an impermeable membrane of hard rock, then a reasonable balancebetween water influx and gas losses may be achieved However, deep seams with high coal or

overburden permeability pose a potential problem in that the required pressure may be accompanied

by unacceptable gas loses

Both temperature and pressure affect the chemistry of gasification, and therefore the composition ofthe product syngas For above ground gasification, the ideal temperature is probably in the

1000-1200ºC range, but it may not be possible to achieve this for much or any of the residence timeunderground because of the gas flow patterns, or because of too much local water influx

In the reaction pattern illustrated in Figure 3.1, the high temperature zone is initially near the bottom

of the injection well, although during the life of the coal seam section it will move along towards thesyngas product exit well Thus initially, there will be a higher proportion of unreacted pyrolysisproducts in the syngas, including tars and phenols As the main gasification chamber gets nearer tothe production well, the exit gas temperature will tend to increase, and the gas composition willchange This can be adjusted by changing the composition of the injected gas

Once gasification operations in a section of the coal seam are finished the area needs to be returned tosomething like its original state in terms of the environment This can be achieved by flushing itthrough with steam and/or water to remove any pollutants from the coal seam to prevent them from

diffusing into surrounding aquifers The ‘Clean Cavern’ concept for the end of a trial or productionrun has only been tested on a few occasions.

3.2 UCG methods

The basic UCG concept uses two boreholes, one for the injection of oxidants and the other for theremoval of the product gas, see Figure 3.1 which shows one commonly used UCG layout The

oxidants react with the coal in a series of pyrolysis and gasification reactions to form carbon

monoxide, hydrogen, methane, carbon dioxide and a variety of minor constituents The transport ofthe gases between the inlet and outlet boreholes and their temperature and pressure controls thereactions As shown in Figure 3.2, there are three stages in the life of an underground reactor In thefirst, the permeable linkages are established between the wells In the second the coal seam is

gasified, and in the third the cavity is shut down and flushed through with steam and water, removingmany of the more toxic byproducts which might otherwise remain

Younger coals such as lignite and brown coals may have sufficient permeability to enable a

satisfactory connection between wells to be created over short distances (20 to 50 m), but most oldercoals are too compact or variable to rely on the natural fissures as pathways for UCG and the linkagehas to be artificially 'encouraged'

UCG development has largely been concerned with:

• establishing methods to enhance the connection between the wells/boreholes in the coal with

the intention of increasing the distance between adjacent holes, and/or reducing the timetaken to establish the links (which can be several days, or even weeks);

• for some of the methods, establishing the techniques for drilling accurate in-seam boreholes;

• establishing methods for igniting the coal in the underground reactor;

• controlling the process and the product syngas quality;

• ensuring a satisfactory level of resource recovery from the coal seam for a given

assessment

It is difficult/impossible to simulate many of the UCG conditions in a laboratory so pilot tests have to

be carried out in the coal seams where large-scale exploitation is being considered Trials are

expensive, as is the necessary prior exploration, and the results are not always easy to assess (DTI,2004) The current position is that some successful trials have been carried out, notably at Majuba inSouth Africa, Chinchilla and Bloodwood Creek in Australia, and at Gonggou in China The nextstage in each case is to scale-up the operation to an intermediate demonstration-scale stage beforeexpansion to commercial scale This may take several years to achieve

The mid-1990s onward has seen a resurgence of interest in UCG throughout the coal-producingworld, and recent trials have established that viable solutions to the inseam connection problem can

be achieved in some places Recent increases in the value of energy will have helped to promote theprospects for UCG

Broadly, four generic methods for carrying out UCG using drilled boreholes/wells have evolved.They are:

• vertical wells linked by hydrofracturing and/or reverse combustion;

• vertical wells linked by an in-seam borehole;

• using a Controlled Retraction Injection Point (CRIP) to move the place where the oxidant is

introduced;

• in steeply dipping seams

The wells are normally cased and sealed so that they do not interfere with the aquifers above thetarget seam and there is no gas exit path around the well on the outside The discussion below usesmaterial taken from Beath and Su (2003a/b) and Beath and others (2004), and the methods areillustrated in Figure 3.3 These methods were all used in the trials which took place in the USA in the1970s and 80s, and are discussed in Section 4.3

Figure 3.3 The generic methods for UCG using drilled wells, as used in the US DOE trials

(Beath and Su, 2003)

In addition, ways of extracting energy from abandoned mines, and using man-made undergroundtunnels, have been developed in China which are discussed in Section 3.2.5 Some of the early work

in both China and the USSR, looked at ways of developing UCG from existing mine workings

Figure 3.3 The generic methods for UCG using drilled wells, as used in the US DOE

trials (Beath and Su, 2003)

3.2.1 Using vertical wells

This technique has been widely used and relies on two vertical wells, linked underground by using either high pressure air or water and/or by reverse combustion, to open up an internal pathway in

the coal seam to facilitate gas flow between the wells This is based on technology developed

originally in the USSR, which has been used at Chinchilla in Australia and Majuba in South Africa.The techniques provide the basis for the eUCG proprietary technology used by Ergo ExergyTMTechnologies Inc Reverse combustion and hydrofracturing are discussed below in the paragraphs

entitled Establishing underground linkages in Section 3.3.

In a typical arrangement, see Figure 3.3(1), the basic link is between two holes, one for injection the

oxidant (with air, oxygen-enriched air and/or steam) and the other for carrying the product syngas tothe surface for treatment and use For demonstration or production-scale operations, a series of holesare used which are drilled in parallel rows, and this is discussed further in Section 3.3.1

The technique is best suited to use in relatively shallow seams, probably in those <300 m deep, andwhere there is a clear and fairly level area on the surface Typically the spacing between the holes isfrom 20 to 30 m, although the first Linc Energy test at Chinchilla reportedly used distances of up to

60 m The spacing will be dependent on the coal properties, and in particular the ease or otherwise ofestablishing the necessary linkages It will also depend on the moisture content of the coal as thesyngas composition will be affected by the combination of inherent moisture and the water that flowsinto the reactor cavity

The method has been thoroughly tried and tested over many years, mainly in the USSR, but itsapplication to any new UCG prospect is subject to a considerable degree of trial and error It is thebasis of the eUCG technology used by Ergo Exergy Extensive exploration and pilot testing will berequired to assess the potential behaviour of the coal at a new site and the ease of making linkagesbetween the holes Before a commercial-scale operation can be costed and set up with any confidence

a great deal of work is necessary including extended in-seam trials

3.2.2 Using in-seam boreholes

Advances in directional drilling which have provided the basis for the construction of correctlypositioned in-seam boreholes are discussed in Section 3.3.1 This is shown in Figure 3.3(2) where the

in-seam hole provides the link between vertical holes.

This can either be, as shown, a link between a series of vertical holes which operate in the same way

as those described in Section 3.2.1, or in some circumstances it may be preferable to utilise the seam hole to inject the oxidant gases In both cases the use of an in-seam hole to provide the primarylink between the oxidant injection point and the product well is going to be a matter of the balance ofadvantage/overall cost, compared with using the vertical wells method using hydrofracturing andreverse combustion to establish the necessary linkages

in-3.2.3 Using a Controlled Retraction Injection Point (CRIP)

The third generic method uses a moveable ignition device known as a CRIP to determine the location

of the underground reaction This would normally start at a point close to the production well whichcarries the syngas to the surface, and as the coal is burned away, the injection point is graduallywithdrawn along the seam to provide a continuous and consistent supply of syngas CRIP was

pioneered in the USA by the LLNL during trials in the 1980s It was used in the Spanish trial in 1998

(see Chapter 4), and is the basis for the current Bloodwood Creek trial by CEPL in Queensland, Australia (see Chapter 5) In-seam drilling is sufficiently well established in some countries that holes

up to 1.5 km long can be drilled, but this expertise is not available everywhere (except possibly fromhigh cost contractors who are brought in as specialists)

Again, there are two variants using this technique, see Figure 3.3(3) and 3.3(4) In the first, the

injection well is the in-seam borehole, while the production well is a vertical hole drilled to interceptthis The CRIP is progressively withdrawn along the seam as the coal nearer the production wellbecomes exhausted

The second arrangement is known as ‘knife-edge or parallel-holes CRIP’ In this, two in-seam holesare drilled in parallel and then turned near the end to meet at a point, maybe as much as 500-700 mfrom where they started Where they intercept, a vertical well is drilled, which is used to facilitate theignition of the underground gasification This well is then sealed during the rest of the UCG

operation When operating, one of the in-seam holes is the injection well, where the CRIP is situated.The other is the production well, carrying the syngas product to the surface This method is currently

being used in the Bloodwood Creek trial in Australia (see Section 5.1.2), where the 30 m wide coal

panel being used could last for up to five years before it is exhausted It thus involves a higher

start-up cost that the other CRIP arrangement, with two long in-seam holes, but once started should

involve a lower operating cost, and be more flexible

The use of a CRIP has two potential advantages over other methods, first it can provide much greatercontrol over where the gasification reaction takes place, and second it will probably prove to be moreapplicable in operations at greater depth There are, of course, possible issues if the CRIP device getsjammed underground, but the in-seam hole will probably be lined with a metal or plastic sleeve tominimise this possibility The hole/casing size will be restricted by the overall cost of the in-seamdrilling

The economics and practicality of the CRIP techniques are sensitive to the oxidant gas used, due tothe different gas volumes that must be injected When using air, there is an energy dilution effect inthe gasification because of the need to heat the nitrogen present which means that more air must beadded to maintain the reactions In view of the potential drilling costs and the use of somewhatsmaller holes, parallel-holes CRIP is likely to be used initially with a highly oxygen-enriched feed,possibly with some steam, and that is what is being used at Bloodwood Creek

3.2.4 In steeply dipping seams

For gasifying a dipping seam (with >30º dip), a vertical injection well and an angled production wellcan be used The injection well should feed into the lower part of the seam while the production wellcan be near the top of the seam It is relatively easy to establish the linkage between the wells because

of the tendency of the reactor cavern to grow upwards After ignition, a cavity will grow until itprogresses up to where the production well is situated At this stage it is either necessary to put in anew injection well lower down, or to withdraw the production well up the seam

Disadvantages to this technique are the specific seam characteristics required which limits the

amount of accessible coal, and as the seam dips, drilling costs increase for exploiting the lower parts

of the seam In addition, the operating pressure increases with the depth of the coal being used It hasbeen tested both in the USSR, and during the US DOE programme at Ralwins

3.2.5 Using man-made excavations

Several methods for UCG are based around conventional mining procedures, with shafts, drifts and

roadways There are several variations of these procedures, which have been used in China and the

USSR Some early European trials also used sites involving existing mines The site is usually

developed from mine workings using man-made galleries and links while areas of the mine are sealedoff to prevent gas leakage The different methods are illustrated in Figure 3.4, but are not now beingactively developed, apart from the Chinese ‘long tunnel large section’ system

[Figure 3.4 Methods involving man-made excavations and conventional underground mining

(Okten and Didari, 1994; Beath and Su, 2004b)]

The chamber gasification technique is an extension of conventional mining where gasifiers replace working coal faces (Ökten and Didari, 1994) In China it is referred to as undersurface gasification

(Creedy and others, 2003) The gasifiers consist of either shortwalls, longwalls or room-and-pillarsystems The gasifier chambers are sealed off before being brought into operation, while the mineroadways provide the access route for the pipes carrying both injection air and the product syngas.Operations take place at relatively low pressures, to minimise the risk of leakage Hence the syngasproduct commonly has a heat value from 4-6 MJ/m The techniques can be used to recover some of3the remaining coal from a partly exhausted mine Underground access is maintained at all stages ofthe operation

In one procedure, the permeable links in a gasification chamber are created using explosives ( see

Figure 3.4A) In another method, linking holes are drilled underground across a coal seam from one

roadway to another ( see Figure 3.4B) The gasifiers are controlled independently from underground

to ensure optimum performance

A Soviet development, generally applied in steeply dipping seams, is called the stream method see

Figure 3.4C The injection and product holes were drilled into and along the coal seam and areconnected at the bottom by a mined roadway The flame was initiated in the connecting channel andgradually spread along its entire length The flow had to be reversed from time to time so as to get aroughly horizontal burn (Gregg and others, 1976) As the burn developed upwards, more coal fell intothe reaction zone Further panels were prepared for use once an area became exhausted Using thestream method, the Soviets encountered significant flow distribution problems associated with roofcollapse, and at some periods, severe gas leakage through cracks created by subsidence

In China, considerable use has been and is being made of the long tunnel, large section, two-stage (LLTS) method, where the gasifier is constructed using mining methods, but is subsequently

Figure 3.4 Methods involving man-made excavations and conventional underground

mining (Okten and Didari, 1994; Beath and Su, 2004b)