underground coal gasification from fundamentals to applications

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Review

Underground coal gasication: From fundamentals to applications

Abdul Waheed Bhutto a, Aqeel Ahmed Bazmi b,c, Gholamreza Zahedi b,*

a Department of Chemical Engineering, Dawood College of Engineering & Technology, Karachi, Pakistanb Process Systems Engineering Centre (PROSPECT), Chemical Engineering Department, Faculty of Chemical Engineering, Universiti Teknologi Malaysia, Skudai 81310,

Johor Bahru (JB), Malaysiac Biomass Conversion Research Centre (BCRC), Department of Chemical Engineering, COMSATS Institute of Information Technology, Lahore, Pakistan

a r t i c l e i n f o

Article history:

Received 4 April 2012

Accepted 10 September 2012

Available online 22 October 2012

Keywords:

Underground coal gasication

UCG kinetics

Gasier operation

Post-burn coal processing

Coal drilling

a b s t r a c t

Underground coal gasication (UCG) is a promising option for the future use of un-worked coal. UCG

permits coal to be gasied in situ within the coal seam, via a matrix of wells. The coal is ignited and air is

injected underground to sustain a re, which is essentially used to mine the coal and produce

a combustible synthetic gas which can be used for industrial heating, power generation or the manu-

facture of hydrogen, synthetic natural gas or diesel fuel. As compared with conventional mining and

surface gasication, UCG promises lower capital/operating costs and also has other advantages, such as

no human labor underground. In addition, UCG has the potential to be linked with carbon capture and

sequestration. The increasing demand for energy, depletion of oil, and gas resources, and threat of global

climate change have lead to growing interest in UCG throughout the world. The potential for UCG to

access low grade, inaccessible coal resources and convert them commercially and competitively into

syngas is enormous, with potential applications in power, fuel, and chemical production. This article

reviews the literature on UCG and research contributions are reported UCG with main emphasis given to

the chemical and physical characteristic of feedstock, process chemistry, gasier designs, and operating

conditions. This is done to provide a general background and allow the reader to understand the

inuence of operating variables on UCG. Thermodynamic studies of UCG with emphasis on gasier

operation optimization based on thermodynamics, biomass gasication reaction engineering andparticularly recently developed kinetic models, advantages and the technical challenges for UCG, and

nally, the future prospects for UCG technology are also reviewed.

2012 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .190

2. Underground coal gasification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .193

2.1. UCG for synthetic fuel production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193

2.2. Process overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193

2.2.1. Chemical processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195

2.2.2. Physical process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196

2.2.3. Effect of coal reactivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197

2.2.4. Gasifying agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197

2.2.5. Effect of pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198

2.2.6. Effect of heat loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198

2.2.7. Effect of temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198

2.2.8. Cavity growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199

2.2.9. Gas diffusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200

2.2.10. Velocity of combustion front . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201

2.2.11. Compositions of syngas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201

2.2.12. Optimization of UGC operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201

* Corresponding author. Tel.: 60 7 553583; fax: 60 7 5566177.

E-mail addresses:[email protected],[email protected](G. Zahedi).

Contents lists available atSciVerse ScienceDirect

Progress in Energy and Combustion Science

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m/ l o c a t e / p e c s

0360-1285/$e see front matter 2012 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.pecs.2012.09.004

Progress in Energy and Combustion Science 39 (2013) 189e214
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3. Thermodynamics of UCG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .201

3.1. Thermodynamic equilibrium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202

3.2. Carbon-oxygen steam equilibrium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202

3.3. Cold gas efficiency (hcg) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203

4. Kinetic studies of UCG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203

4.1. Single First Order Reaction model (SFORM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204

4.2. Distributed activation energy model (DAEM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204

4.3. Reactions of formation of selected gas products in coal pyrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205

4.4. Order of reaction and activation energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2074.5. Rate controlling step . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207

4.6. Chemical reaction rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207

5. Challenges for UCG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207

5.1. Suitable site selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207

5.2. Technical challenges for UCG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208

5.2.1. The major issues in the use of UCG technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208

5.2.2. Exploration of the UCG site . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208

5.2.3. Choice of a suitable drilling technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208

5.2.4. Environment and safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208

6. Advantages of underground coal gasification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209

6.1. Advantages of underground coal gasification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209

6.2. UGC challenge and promises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209

6.3. UCG-CCS concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209

6.4. Post-burn processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210

6.5. Economics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2107. Conclusions and future prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212

1. Introduction

Coal, a fossil fuel created from the remains of plants that lived

and died about 100e400 million years ago when parts of the Earth

were covered with huge swampy forests is classied as a nonre-

newable energy source because it takes millions of years to form.

Coal has been used as a source of energy for nearly 3000 years.

Although mined in Europe as early as the 13th century, it was not

a highly desirable fuel because of its toxic combustion products.Coal did notbecome an important source of fuel until the beginning

of the Industrial Revolution about 300 years later.

The coal industrys largest environmental challenge is removing

organic sulfur, a substance that is chemically bound to coal.

Traditional methods of burning coal produce emissions that can

reduce air and water quality. Clean coal technologies remove sulfur

and nitrogen oxides before, during, and after coal is burned, or

convert coal to a gas or liquid fuel. Fluidized Bed Combustion is

a clean coal technology which keeps both sulfur and nitrogen

oxides in check. Coal Gasication is another clean coal technology

bypasses the conventional coal burning process altogether by

converting coal into a gas. This method removes sulfur, nitrogen

compounds and particulates, before the fuel is burned, making it as

clean as natural gas. Coal was rst used in gas production duringthe late 18th century. Early production was used primarily for

lighting, but as gasication techniques improved, applications grew

wider. By the 19th century the conversion of coal to gas was a well-

established commercial process.

Globally, coal will still remain an indispensable source of

chemical feedstock and energy for a long period of time. New and

improvedprocesses for itsefcient and environmentally acceptable

use will be a steady challenge for coming generations of coal

scientists and for society to support the research required [1].

World energy policy is gripped by a fallacy d the idea that coal

is destined to stay cheap for decades to come. This assumption

supports investment in clean-coaltechnology and trumps serious

efforts to increase energy conservation and develop alternative

energy sources. Underground coal gasi

cation (UCG) is a promising

option for the future use of un-worked coal. UCGd may eventually

make marginal coal reserves accessible, but it will take time and

substantial investment to be commercialized on a large-scale[2].

Most current technologies of coal gasication such as entrainedow,uidized bed, and moving bed use a surface reactor for gasi-

cation. The main differences between these technologies relate to

the gas ow conguration, coal particle size, ash handling, and

process conditions [3]. An alternative for surface gasier is an

underground coal gasier. UCG is a is a combination of mining,exploitation and gasication that eliminates the need for mining

and can be used in deep or steeply dipping, unmineable coal seam

.UCG is an in situ technique to recover the fuel or feedstock value of

coal that is not economically available through conventional

recovery technologies. It has been regarded to be an important way

to utilize low-rank and unmineable coals. The international expe-

riences in the modeling and the experimental tests of underground

coal gasication (UCG) show that UCG process offers an attractive

option of utilizing unmineable coal[4e21]. Probably the strongest

appeal of underground coal gasication at present is its potential

value in exploiting marginal coal reserves that otherwise would

remain unrecoverable[22].

Coal reserves signicantly exceed those of oil and gas. Worlds

coal distribution on land is shown in Fig. 1. When coal resourcetotals is considered (including coal which it is uneconomic to

mine), it dominates the fossil fuel picture. Estimates of total world

coal resource (including unmineable coal) are usually stated in

trillions of tons rather than billions. Recent estimates of the total

remaining coal resource in the world quote a gure of 18 trillion

tons[23].

Today, less than one sixth of the world s coal is economically

accessible. The chances of countries around the world choosing not

to use this coal resource are very low indeed but unless cleaner and

cheaper ways can be found to convert coal to gas or liquid fuels,coal

is unlikely to become an acceptable replacement for dwindling and

uncertain supplies of oil and natural gas. Underground coal gasi-

cation (UCG), taken on its own, offers the prospect of increasing the

world

s usable coal reserves by a factorof at least three.Fortunately,

A.W. Bhutto et al. / Progress in Energy and Combustion Science 39 (2013) 189e214190


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potential sites for UCG operations correspond to locations where

sites are plentiful for sequestering CO2 in geologic formations

underground. UCG also enhances the storage capacity of the coal

seam itself to store injected CO2. The generated gas, called syngas,

would be taken from the ground and the by-products separated

out. The CO2would then be returned downhole nearby.

The purpose of underground gasication of coal, regardless of

method used, is to obtain the energy contained in the fuel for use

on the surface, without mining in the usual sense of the term.

Underground gasication can be described as (1) a process where

coal, in place, is consumed by partial combustion with air, oxygen,

steam, or any combination of these to produce a low caloric value

gas (80e300 Btu per cu ft) or (2) a complete combustion process in

which air is used to produce a gas containing carbon dioxide,

nitrogen, and considerable thermal energy[25].

UCG also lowers the capital investment by eliminating the need

for specialized coal processing (transporting and stocking) and

gasication reactors. UCG has other advantages such as increased

Fig. 2. Current world-wide status of UCG technology: Map shows underground coal gasication (UCG) sites worldwide, including planned sites and prior pilot test sites, current

international UCG activities overlaying CO2 storage potential areas. Gray areas show potential areas for geological carbon storage [23,31,32].

Fig. 1. Worlds coal distribution black areas on land: Map excludes Antarctica, which contains large coal deposits but is not usable by international convention [24].

A.W. Bhutto e t al. / Progress in Energy and Combustion Science 39 (2013) 189e214 191


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work safety, no surface disposal of ash, low dust, and noise pollu-

tion. It can be operated at high pressure to increase the reaction

intensity and improve the efciency of the process. UCG is partic-

ularly advantageous for deep coal deposits and steeply dipping coal

seams since at these conditions less gas leakages to the surround-

ings and high pressures favor methane formation The successful

application of such a process would provide a low to medium BTU

gas (100e

300BTU/SCF), depending on whether air or an oxygenesteam mixture is used[26]. Composition and heating value of the

product gas depends on the thermodynamic conditions of the

operation as well as on the composition and temperature of the

gasifying agent employed.

In order to avoid potential environmental concerns, the reactor

cavity is operated at less than hydrostatic pressure, which brings

water into the gasication reactor in situ. As such, successful UCG

operation relies on the natural permeability of the coal seam to

transmit gases to and from the combustion zone, or on enhanced

permeability created through reversed combustion, an in-seam

channel, or hydro-fracturing[27].

The rst recorded proposal for UCG was by Siemens. Sir William

Siemens, a German scientist, was credited with rst suggesting

underground coal gasi

cation in 1868 [28], followed by

Mendeleyev 20 years later. No further work was done until the

1930s, when an experimental station was started in the Donetsk

coaleld in the then Soviet Union, to be followed by commercial

installations in 1940 [29]. John et al. [25] has given excellent

bibliography of the literature on UCG between 1945 and 60.

Underground gasication continued at a number of locations in the

Soviet Union until the late 1970s, with production of some

25,000 million Nm3 of gas from around 6.6 million tons of coal.

Some of the well-documented UCG operations are those at Angren-

Uzbekistan, Queensland-Australia, Alberta-Canada, Walanchabi

City-China, Majuba-South Aferica.

A commercial-scale UCG plant is still being operated in Angren,

Uzbekistan, where gas of an average heating value of 3.1e3.5 kJ/m3

is produced in an air-blown gasication process. The UCG gas

produced is fed into a power station which is situated adjacent to

the Yerostigaz operation in Angren. Yerostigaz has produced this

gas to generate power at the 400 MW power station at Angren.

Operators drill wells to inject air or oxygen that drives combustion

and gasication in situ, and to produce the coal gas to surface for

further processing, transport, or utilization. The most advanced

UCG operation is at Chinchilla in Queensland, Australia, where the

operator claims to be generating electricity from UCG product gas

at a highly competitive cost (1.5 US cents per kWh).In October 2008, Carbon Energy successfully produced syngas

from its unique UCG module based on the parallel controlled

retractable injection point (CRIP) method. The trial, which ran for

100 days, reached coal gasication rates of around 150 tons per day

and produced a high-quality syngas. Since then, Carbon Energy has

installed two more modules and constructed a 5-MW electric

power plant to be fed with syngas from Module 2. Module 1 is

being carefully decommissioned. Plans for scaling up to 25 MW of

electricity generation are under way, and a second project in

Queensland, known as the Blue Gum Energy Park, is also in the

early stages of planning.

Swan Hills Synfuels recently produced syngas from its pilot

project in Alberta, Canada. This project is the deepest UCG pilot

ever undertaken, at a depth of 1400 m, and is using the linearcontrolled retractable injection point method. The ENN Group Co.

Ltd. (a subsidiary of the Xinao company) produced syngas from

a pilot project in Walanchabi City, Inner Mongolia, China, for 26

months, gasifying more than 100,000 tons of coal. Although not

much information has been made available about this project, it is

known that there were initially seven injection and production

wells, which were rstred in October 2007 using air. ENN is now

in its fourth year of operation at the plant.

The Majuba UCG project has been producing syngas since

January 2007 and began delivering UCG syngas to core with coal

at the Majuba Power Station in late 2010. The project contributes

about 3 MW to the overall output of 650 MW from the electric

power station using the linked vertical well method. This project is

now the longest running UCG trial in the western world. Plans arein place to expand the facilities to 1200 MWe output, with 30% of

the plants fuel provided by syngas.

There have been over 50 UCG tests or pilot operations world-

wide. Trials were carried out at depths in excess of 500 m by

a European consortium (UK, Spain and Belgium) between 1992 and

1998 at Teruel in Spain.Table 1summarizes the history of the UCG

and Fig. 2 illustrates the current world-wide status of the

technology.

The potential for UCG to access low grade, inaccessible coal

resources and convert them commercially and, competitively into

syngas is enormous, with potential applications in power, fuel, and

chemical production. UCG research and development have been

conducted in several countries, including long-term commercial

operation of several UCG plants in the former Soviet Union.

Fig. 3. Potential development of UCG: Step 1: well drilling and link establishment. Step

2: coal seam ignition and commencement of gasication and step 3: site clean-up by

ushing cavity with steam and water to remove potential contaminants [19].



5/26

Information on UCG technology, however, is limited and there is

a lack of compact review articles in this area [33]. Most of the

current available literature on UCG emphasizes on its geological

implications, environmental concerns and numerical analysis,

modeling and simulation based on laboratory or pilot scale studies.However, and in spite of the signicance of all this, there is no

comprehensive review on UCG process description with emphasis

on its thermodynamic and kinetic studies. We believe that, in all

these respects, this is a timely contribution. We anticipate that this

review will promote research and development efforts, scale-up

of the gasication process, and large-scale implementation of

UCG in future.

In this article, research contributions are reported according to

the following sections:

In Section2, we reviewed the UCG with main emphasis given

to the chemical and physical characteristic of feedstock,

process chemistry, gasier designs, and operating conditions.

This is done to provide a general background and allow thereader to understand the inuence of operating variables on

UCG.

In Section3, we discussed thermodynamic studies of UCG with

emphasis on gasier operation optimization based on

thermodynamics.

In Section 4, we reviewed coal gasication reaction engi-

neering and particularly we reviewed the recently developed

kinetic models.

Section5 has discussed challenges for UCG and the proposed

approach which has been implemented to overcome the

existing challenges.

Section6 summarized the advantages and limitations of UCG

and in Section 7, we provide concluding remarks and future

prospects for UCG technology.

2. Underground coal gasication

2.1. UCG for synthetic fuel production

UCG permits coal to be gasied in situ within the coal seam, viaa matrixof wells. The coal is ignited and air is injected underground

to sustain a re, which is essentially used to produce and transport

combustible synthetic gas to surface. This synthetic gas can be used

for industrial heating, power generation or the manufacture of

hydrogen, synthetic natural gas or other fuels. As compared with

conventional mining and surface gasication, UCG promises lower

capital/operating costs and also has other advantages, such as no

human labor underground for coal mining. In addition, UCG has the

potential to be linked with carbon capture and sequestration [34].

The increasing demand for energy, depletion of oil and gas

resources, and threat of global climate change have led to growing

interest in UCG throughout the world.

The primary components of UCG syngas are H2, CO, CO2, CH4,

and H2S. The pressures and temperatures of produced gas aresimilar, at 30e50 bars for a 300e500 m deep seam, and 500e

800 C outlet temperatures for sub-bituminous coals and up to

1000 C for bituminous coals. The product gas requires cleaning

once it has reached the surface, either to meet the specication for

input into a gas turbine (for electricity generation), or to be of

sufcient purity for use as a chemical feedstock for conversion to

synthetic fuels.

2.2. Process overview

UCG has been approached in many different ways. The old

technique to gasify the coal in situ uses two-vertically drilled wells

as the Injection and Production wells. The procedure consists of

three steps as shown in Fig. 3. In the

rst step an injection and

Table 1

History of the UCG[30].

Test site Country Year Coal type Seam thickness (m) Seam depth (m) Dipa (degrees) Coal gasied (t) Syngas cv (mj/m3)

Lisichansk Russia 1934e36 Bit 0.75 24 N/A N/A 3e4

Lisichansk Ukraine 1943e63 Bit 0.4 400 0 N/A 3.2

Gorlovka Russia 1935e41 N/A 1.9 40 N/A N/A 6e10

Podmoskova Russia 1940e62 SBB 2 40 0 N/A 6 with O2Bois-la-Dame Belgium 1948 A 1 N/A N/A N/A N/A

Newman Spinney UK 1949e

59 SBB 1 75 N/A 180 2.6Yuzhno-Abinsk Russia 1955e89 Bit 2-Sep 138 60 2 mt 9e12.1

Angren Uzbekistan 1965enow SBB 4 110 N/A Over 10 mt 3.6

Hanna 1 USA 73e74 HVC 9.1 120 0 3130

Hanna 2 USA 75e76 HVC 9.1 84 0 7580 5.3

Hoe Creek 1 USA 1976 HVC 7.5 100 0 112 3.6

Hanna 3 USA 1977 HVC 9.1 84 0 2370 4.1

Hoe Creek 2A USA 1977 HVC 7.5 100 0 1820 3.4

Hoe Creek 2B USA 1977 HVC 7.5 100 0 60 9.0

Hanna 4 USA 77e79 HVC 9.1 100 0 4700 4.1

Hoe Creek 3A USA 1979 HVC 7.5 100 0 290 3.9

Hoe Creek 3B USA 1979 HVC 7.5 100 0 3190 6.9

Pricetown USA 1979 Bit 1.8 270 0 350 6.1

Rawlins 1A USA 1979 SBB 18 105 63 1330 5.6

Rawlins 1B USA 1979 SBB 18 105 63 169 8.1

Rawlins 2 USA 1979 SBB 18 130e180 63 7760 11.8

Brauy-en-Artois France 1981 A 1200 N/A

Thulin Belgium 1982e84 SA 860 N/A

Centralia Tono A USA 84e85 SBB 6 75 14 190 9.7

Centralia Tono B USA 84e85 SBB 6 75 14 390 8.4

Haute-Duele France 1985e86 A 2 880

Thulin Belgium 1986e87 SA 6 860 157

Rocky Mountain 1A USA 87e88 SBB 7 110 0 11200 9.5

Rocky Mountain 1B USA 87e88 SBB 7 110 0 4440 8.8

El Tremedal Spain 1997 SBB 2 600

HVC High Vol Bit, Bit Bituminous, SBB Sub Bituminous, SA Semi-anthracite, A Anthracite.a Dip is the maximum angle between the inclined plane and the horizontal plane. Dip is always perpendicular to strike, and has both a compass direction and an angle.

Inclinometer is used to measure the amount of dip in degrees (a plane lying at along the horizontal as zero dip).



6/26

production well are drilled from the surface to the coal seam and

highly permeable path within the coal seams are established

between these two well.Prior to the gasication step a linkage path

is created between injector and producer. Several techniques can be

used for linking the wells, including the Reverse Combustion

Linking (RCL), Forward Combustion Linking (FCL), hydro-fracking,

electro-linking, explosive and in-seam linking. Other techniques

for the in situ gasication include CRIPs, long and large tunnel

gasication, and two-stage UCG[35e37].

The RCL is a method of linking which includes injection of an

oxidant into one well and ignition of coal in the other so that

combustion propagates toward the source of oxidant as shown in

Fig. 4(a).

In the course of the FCL coal is ignited in the injection well, and

the re propagates toward the production well as shown in

Fig. 4(b). During forward gasication, the ame working face

gradually moves to the outlet, making the dry distillation zone

shorter and shorter. At the time when forward gasication is nearly

complete, the reduction zone also becomes shorter[38].

Flow of oxidant into the injection well is maintained until the

rereaches the bottomof the injection wellin the RCL or that of the

production well in the FCL. This outcome is accompanied by

a signicant drop in the injection pressure indicating creation ofa low hydraulic resistance link between the wells, which estab-

lishes a low hydraulic resistance path between the two wells.

CRIP technique is suitable for thin, deep coal seams, replaces the

vertical injector by a horizontal injector [39]. During the gasica-

tion process, the burning zone grows in the upstream direction, in

contrast to the gas ow in the horizontal direction. This occurs by

cutting off or perforating the injection linear at successive new

upstream locations. The CRIP technique produces higher quality

gas, results in lower heat loss than the two-vertical well congu-

ration, and improves the overall efciency of the UCG process[40].

Once a successful link has been established the second step is

ignited. The gasication step starts with ignition of the coal and the

injection of air or air enriched with oxygen. Both permeable bed

gasication and natural convection driven surface gasication will

occur. When the gas quality deteriorates the injection well is burnt

to allow injection further upstream.

Gasication occurs when a mixture of air or oxygen and steam is

forced into the coal seam through injection well and react chemi-

cally with the coal, generating a synthesis gas, which is recovered

through product well. At the surface the raw product gas is cleaned

for industrial uses[20].

As gasication proceeds, an underground cavity is formed.

Water from the surrounding strata will enter the cavity and

participate in the gasication process leading to a drop in the local

water table. At some point, the coal in the vicinity of the injection

well will be exhausted and steps one and two will be repeated to

access fresh coal to sustain gas production. In the commercial

operations several underground gasiers will be operated simul-

taneously. Once the gasication operations in a section of coal seam

have nished, the third step is to return environment back to its

original state. This is achieved by ushing the cavities with steam

and/or water to remove pollutants from cal seams to prevent them

from diffusing into surrounding water aquifers. Over the time, the

water table will return to a level close to that existing prior to the

start of gasication[20]. The composition of the product gas from

UCG can very substantially depending on the injected oxidant used,

operating pressure and mass and energy balance of the under-ground reactor.

CRIP technique,is suitable forthin, deep coal seams,replaces the

vertical injector by a horizontal injector [39]. The CRIP method

requires two horizontal wells drilled along a coal seam. One is near

the top of the seam and the other near the bottom. The bottom

(injection) well is lined with metal pipe. The upper well is the

production well. As pyrolysis proceeds, the burn cavity moves

toward the base of the wells, progressively exposing more and

more of the injection pipe. At an appropriate time, the pipe is

melted or burned off and a new period of pyrolysis begins. In effect,

the old problems of well plugging are circumvented by simply

starting a newburn periodically along the horizontal wells [41]. The

CRIP method was rst tried successfully in early 1982 with a three-

day trial, gasifying a 40-ton cavity. The injection pipe was thenburned off and a second 10-ton cavity started. The original cavity

cooled to 500 C, and the second achieved the typical operating

temperature of 1000 C. The average heating values of the product

gases were between 265 and 277 Btu per standard cubic foot.

Burning is started by pyrophoric silane and propane gases. The

silane ignites upon encountering the oxygen in the burn cavity and

burns long enough to subsequently ignite the propane, which is

injected into the well. The propane actually ignites the coal in the

cavity. At a suitable time, the propane is shut off and the pyrolysis

sustains itself. This method has proved reliable since its adoption.

Burning can also be started by passing LPG through the injection

well for a short period of time (3e5 min) to initiate the combustion.

An electric spark is generated for ignition of the liqueed petro-

leum gas (LPG) in the channel of the coal block near the mouth ofthe injection well. Once coal is ignited, the LPG supply is stopped

and oxygen is continuously passed through the channel created in

the coal block until the completion of the experiment[42].

CRIP technique uses a combination of conventional and direc-

tional drilling to drill the process wells. First, the vertically-drilled

Production Well is drilled until it intersects the coal seam. Then

the vertical section of the Injection Well is drilled to a pre-

determined depth, after which directional drilling is used to

deviate the hole and drill along the coal seam until it intersects the

Production Well. This technique enables the injection point (i.e. the

end of the coiled tubing) to be retracted back along the coal seam,

which is of benet because it allows for fresh coal to be accessed

each time the syngas quality drops as a result of cavity maturation.

Retraction of the injection point along the coal seam is known as

Fig. 4. Schematic views of the reverse and forward combustion linking in UCG. (a)

Reverse combustion linking. (b) Forward combustion linking [36].



7/26

a CRIP maneuver, and between 10 and 20 such maneuvers are ex-

pected during the course of a modules lifetime. Directional drilling

is a proven technology in the oil and gas industry.

The in-seam drilling of coal seams has been part of coal

exploitation since at least the 1950s. Underground steering of

boreholes made its commercial entrance in the oil and gas industry

around 1990, when operators established the benets of lateral

drilling for extending the life of wells and xed drilling platforms

and for reaching inaccessible locations. Nowadays directional

drilling has become common for coal bed methane (CBM) and

enhanced CBM applications; there are specialist drilling companies

around who supply services to CBM operators. The focus to-date

has been on reducing costs. UCG has a tighter requirement on

accuracy. The ability of directional drilling to meet these require-

ments at an affordable cost is still under review [37]. The CRIP

technique produces higher quality gas, results in lower heat loss

than the two-vertical well conguration, and improves the overall

efciency of the UCG process[40].

Two-stage UCG is a technique of supplying air and steam

cyclically[10,43]. In the rst stage, air is supplied to make the coal

burn and store heat to produce air gas; in the second stage, steam is

suppliedto produce water gas. Only if sufcient heat isstored in the

rst stage can the decomposition reactions in the second stage runsmoothly and the water gas with high heating value be ensured.

Meanwhile, the degree of the coal layer decomposition and the

production volume of the gas are totally determined by the

temperature distribution in the coal layers[44].

During in situ coal gasication remote sensing technique

may be used for mapping underground fracture systems,

locating tunnels or water-bearing strata and mapping burn

fronts[45].

2.2.1. Chemical processes

The study considers the quasi-steady burning of a carbon

particle which undergoes gasication at its surface by chemical

reactions, followed by a homogeneous reaction in the gas phase.

The main chemical processes occurring during coal gasication aredrying, pyrolysis, combustion and gasication of the solid hydro-

carbon. These processes occur in all methods of coal gasication,

whether conducted in surface gasiers or in situ. Fromthe chemical

and thermodynamic point of view, the UCG process runs analogi-

cally to gasication in the surface reactors [46]. The most important

chemical reactions taking place during underground coal gasica-

tion are listed inTable 2.

Chemical reactions (1)e(4) take place on the wall plane of the

coal seams (heterogeneous reactions), while (6) and (7) reactions

occur at the gaseous stage (homogeneous reactions).

In addition to these listed, reactions involving nitrogen and

sulfur are also important. The nal product gas consists of

hydrogen, carbon monoxide, carbon dioxide, methane and

nitrogen. Composition and heating value of the product gasdepends on the thermodynamic conditions of the operation as well

as on the composition and temperature of the gasifying agent

employed[46].

Duringin situ combustion of coal different processes of vapor-

ization (drying), pyrolysis, and combustion and gasication of char

take place collectively. The UCG process has a zonal character and

the main gasication reactions occur both in the solid and the

gaseous phases as well as on their boundaries. Qualitative

description of phenomena at the UCG cavity wall is explained

inFig. 5.

In the solid phase mainly the pyrolysis and the drying processes

take place. Along with the migration of the gaseous product of the

thermal decomposition through the pores and slots of the solid

phase, various homo- and heterogenic reactions occur. The rates of

these processes depend mostly on the temperature. On the phase

boundary in the gasication channel heterogenic reactions take

place. Their rates aredetermined by the diffusion and the accessible

reaction area. The major products of the reaction of oxygen with

carbon in the gasication area (oxidation zone) are carbon dioxide

and carbon monoxide[46].

Based on the differences in major chemical reactions, the

temperature, and the gas compositions, the gasication channelcan be divided into three zones: oxidization zone, reduction zone

and dry distillation zone as shown in Fig. 6[21]. In the oxidization

zone, the multi-phase chemical reactions between oxygen con-

tained in the gasication agent and the carbon in the coal seam

occur, producing heat and making the coal seam very-hot. The

coal seams become incandescent with temperature ranging

from 900 C to 1450 C [47]. Inherent water plays a role in coal

oxidation, affecting oxygen transport within coal pores and

participating in the chemical reactions during the oxidation

process. Details of chemical reactions involving water have not yet

been elucidated[48].

With the O2burning up gradually, the air stream gets into the

reduction zone. In the reduction zone H2O(g) and CO2 are

reduced to H2 and CO under the effect of high temperature,when they meet with the incandescent coal seams. The

temperature ranges from 600 C to 1000 C, and the length is

1.5e2 times that of the oxidation zone with its pressure being

0.01e0.2 MPa [49]. Additionally, under the catalytic action of

coal ash and metallic oxides, a certain methanation reaction

occurs [Eq. (4)]. The above endothermic reactions cause the

temperature at the reduction zone to drop until it is low enough

to terminate the reduction reactions.

After the endothermic reactions in the reduction zone, the

gas current temperature drops, and then it begins to ow into

the destructive distillation and dry zone (200Ce600 C).

The main physical changes for coal with high water content

are dewatering and cracking, as well as absorption and

contraction of the coal, when the temperature is below 100

C.When the temperature is not higher than 300 C, only small

amounts of parafn hydrocarbon, water, and CO2 are separated

out. Over 300 C, the slow chemical changes take place,

accompanied with a light polymerization and depolymerization.

In the meantime, appropriate amounts of volatile and oil-like

liquid are separated out, which take on a gelatinous state

afterward. When the temperature of the coal seam rises to

350Ce550 C, a large proportion of tar oil is separated out

(500 C at its peak) and a certain amount of combustible gas is

yielded. The hydrocarbon gas is given out when the temperature

stands at 450Ce500 C. As the temperature of the coal seam

continues to rise until it is over 550 C, semi-coke remains begin

to solidify and contract, accompanied with the yield of H2, CO2,

and CH4 [47,50].

Table 2

Chemical reactions taking place during underground coal gasication.

Reaction equation React ion

rate (Ri)

DHo298

(MJ/kmol)

Equation

number

C O2/CO2 R1 393.8 (1)

C CO2/2CO2 R2 162.4 (2)

C H2O/H2 CO R3 131.4 (3)

C 2H2/CH4 R4 74.9 (4)

CO 1

2O2/CO2

R5 285.1 (5)

H2 1

2O2/H2O

R6 0.242 (6)

CO H2O/CO H2 R7 0.041 (7)



8/26

The overall UCG process is strongly exothermic, and tempera-

tures in the burn zone are likely to occasionally exceed 900 C. Even

after cooling (through conductive heat loss to surrounding strata

and convective heat loss to native groundwater), syngas typically

ows through production wells at temperatures between 200 C

and 400 C. Around the burn zone, the high buoyancy of hot syngas

relative to groundwater will tend to lead to large pores getting

invaded with bubbles of syngas, which will heat the groundwater

and turn it into steam. A dynamic interface between steam and hot

groundwater will develop around the UCG burn zone, in which

steam will mix with the syngas[23].

Passing through these three reaction zones, the gas with the

main combustible compositions of CO, H2 and CH4 is formed, whose

proportion of contents varies from one gasication agent and air

injection method to another. These three zones move toward theoutlet along the direction of the airow, which, in turn, ensures the

continuous run of the gasication reactions[21].

Figs. 6 and 7illustrate different chemical regions of gasication

of coal in situ. In the drying zone, surface water in the wet coal is

vaporized at temperatures above the saturation temperature of

seam water at a specied pressure, which makes the coal more

porous. The dried coal undergoes the pyrolysis process upon more

heating in the next phase. During pyrolysis, coal loses about 40e

50% of its dry weight as low molecular weight gases, chemical

water, light hydrocarbons and heavy tars, and after evolving the

volatile matters, a more permeable solid substance called char will

be combusted and gasied by the injected oxidant agents and

exhausted gases from the previous steps [51,52]. The rates of the

gaseous phase reactions are determined mostly by the temperature

and concentration of the particular gaseous compounds. Develop-

ment of these reactions is frequently supported by the catalytic

inuence of some chemical compounds, e.g. iron oxides.

2.2.2. Physical process

In the process of underground coal gasication (UCG), the gas

movement not only inuences the concentration distribution and

movement ofuid in the burning zone directly, but also restricts

the diffusion of the gasication agent in the whole gasier.

Fig. 5. Qualitative description of phenomena at the UCG cavity wall [16,19].

Fig. 6. Division of gasication channel into three zones: oxidization zone, reduction

zone and dry distillation zone[21].

Fig. 7. Thermal wave propagation through coal seam during in situ gasication which

demonstrates the different regions[3].



9/26

Therefore, it eventually determines the rate of chemical reaction

between gas and solid, and the process of burning and gasication.

Evidently, Lanhe 2003[15]suggested the study of moving patterns

ofuid in the gasier should precede the study of the process of

chemical reaction, the moving patterns of agents, and the distri-

bution regularity of temperature elds near the ame working

face.

In the process of underground coal gasication, under the effect

of high temperature, that a temperature eld forms in the coal layer

to be gasied within the coal and rock mass, which makes the

coal and rock layersdoriginally full of stratication, joints, and

fracturesdsoften, melt, cement, and solidify. Accordingly, the

internal molecular structure is rearranged and reorganized, which

leads to qualitative changes of organizational structure and

morphological appearance. Hence, obvious changes take place in

the physicomechanical properties of the coal and rock mass.

In the process of underground coal gasication, a high

temperature eld comes into being in the coal body under the high

temperature, which makes the coal seam, full of layers and joints

and interstices, soften, melt, glue, and solidify. Under the high

temperature, the internal molecular structure reorganizes, which

completely changes the coal seams surface morphology. Hence,

dramatic changes take place in the physical and mechanical prop-erties of the coal body. As a result, its corresponding physical and

mechanical properties are no longer constants, but functions of

temperature. The differences in the heat expansion coefcient

among coal grains and anisotropy generate new cracks, whose

extension leads to the connected net structure. Thus, all these

improve the connectivity of the pore passageway and increase the

seepage pressure of the dry distillation gas[53].

Research indicates that, under the non-isothermal condition,

the densities of the solid media and pore water are greatly affected

by the temperature and pressure [49]. However, the small defor-

mation of the solid skeleton still produces a certain effect on the

distribution of the temperature eld and seepage of underground

water in the gasication panel. Therefore, the deformation of

the solid particle is not negligible and can be regarded ascompressible [9].

The coal rock is extended and deformed by the pore uid

pressure. The uid inside the pores affects the cracks inside the

skeleton of the coal rock and the pores opening and closing;

second, the relation between the stressand strain of the coal rock is

changed by the uid in the pores, which in turn changes the elastic

modulus and compressive strength of the coal rock [54e56]. The

changes in the temperatureeld of the coal seam are due mainly to

the ame working face. When the temperature in the coal seam

rises, the desorption rate of the dry distillation gas in the coal seam

accelerates. The free dry distillation gas content in the coal

increases. The mass of the dry distillation gas which participates in

the seepage increases too. On the other hand, with the rise of the

temperature, the amount of absorbed dry distillation gas in the coalseam drops.

2.2.2.1. Operating conditions. The investigation by Perkins and

Sahajwalla[18]has found that the operating conditions that have

the greatest impact on cavity growth rate are temperature, water

inux, pressure, and gas composition in underground coal gasi-cation. In this section, the effect of operating conditions and coal

properties, namely, coal reactivity, operating pressure, heat loss,

and the type of oxidant used are investigated [16]. Lanhe [13]

while establishing the mathematical models on the under-

ground coal gasication in steep coal seams according to their

storage conditions and features of gas production process

concludes that numerical simulation on the temperature eld,

concentration

eld and pressure

eld is reasonable in the

underground gasication of steep coal seams on the experimental

condition.

2.2.2.2. The thickness of coal layers. UCG is inuenced by several

natural factors as described inTable 3. Most UCG operations were

carried out in more gas permeable conditions of brown coal beds

and younger formations of hard coals. Generally, these deposits

occurredat shallower depths, down to 300 m, and ignited relatively

easily. Strongly swelling and coking coals have the tendency to

block gas ow through the coal bed, thus hindering the course of

the reaction. The gasication of beds 1 m thick or more improves

economics [57]. Beds that are thinner than 0.5 m are not considered

suitable for UCG.

In the process of UCG, the burning area and gas are not only

cooled down through heat exchange but a part of the heat is also

lost into the coal seam and surrounding rocks (oor, roof), thus

having an adverse effect on the stability of the underground gasi-cation process. Eliot [58] suggested that when the thickness of

coal seam is smaller than 2 m, the cooling action with a dramatic

change for surrounding rocks affects the heat value of coal gas

considerably. As for comparatively thin coal seam, enhancing the

blowing velocity or oxygen-enriched blowing can improve the

heating value of gas. In the former Soviet Union, Lischansk under-ground gasication station adopted oxygen-enriched blowing in

the coal seam, for which the thickness is less than 2 m[58]. When

the thickness of coal layers is decreased or the intake rate of water

is increased, the CO2content in the gas will rise [58,59].

2.2.3. Effect of coal reactivity

The chemical reactivity of the coal is potentially very important

for UCG. The reported intrinsic reactivities of low rank coals differ

by up to 4 ordersof magnitudewhen extrapolated to typical gasier

operating temperatures[18]. The coal intrinsic reactivity has a big

impact on the distributions in the gasier and on the nal product

gas. In particular, high reactivity favors the production of methane

via the char-H2 reaction. Because this reaction is exothermic, the

increased reactivity for this reaction can lead to big changes in thenal product gas caloric value.

2.2.4. Gasifying agents

Gasication under different atmospheres such as air, steam,

steam-oxygen, and carbon dioxide has been reported in the liter-

ature. In general, the gasier atmosphere determines the caloric

Table 3

Classication criteria for UCG.

Criterion Characteristics/remarks

Coal type Any

Physicochemical properties of coal Recommended: high content of volatile

matter, low agglomerating capacityor its lack, ash content < 50% by weight

Occurrence depth Protability criterion

Bed thickness More than 1 m

Angle of inclination of coal bed Any

Type and tightness of rock mass Recommended: rmness and tightness

of rock mass, thickness and lithology

of rock massdoverburden in slightly

permeable layers (clays, silts, shale clays)

Hydrogeological conditions Recommended: lack ofssures, faults,

aquiferous layers, water reservoirs causing

water inow

D eposit tec tonic s Recommend ed homogeneity of deposi t

(lack ofssure, faults)

Quantity of resources Protability criterion

Methane presence in the bed Causes gas hazard

Conditions of infrastructure Recommended lack of building development



10/26

value of the syngas produced. When one uses air as the gasifying

agent, a syngas with low heating value is obtained. This is mainly

due to the syngas dilution by the nitrogen contained in air.

However, if one uses steam or a combination of steam and oxygen,

a syngas with a medium caloric value is produced. Adding steam

changes carbon-oxygen system balance to carbon-oxygen-steam

system balance in the combustion process. Oxygen-steam gasi-

cation not only utilizes the surplus heat to improve the energy

efciency of the process, but also increases the gas production

volume per ton of coal and lowers the oxygen consumption volume

per ton of coal. The changing relationships between gas composi-

tions and steam/oxygen ratios are shown in Fig. 8[60].

The experiment results show that pure-oxygen underground

coal gasication, the water in the coal seams, or the leaching water

on the roof can be used to produce water gas. However, because

water evaporation consumes heat, and it is impossible to control

steam volume, gas compositions often present the wide uctua-

tions. Therefore, it is required to adjust the oxygen supplying

volume so as to keep the stable proceeding of gasication process.

FromFig. 8, it can be seen that with the rise in the steam/oxygen

ratio, the volume of steam increases, the H2content in the coal gas

improves, the CO content drops, and the CH4 content is heightening

a little[60].The syngas produced has a by UCG process has low caloric

value approximately one-eighth of natural gas if air injection is

used, and double this gure if oxygen injection is used. Oxygen-

enriched steam forward gasication has remarkable effects on gas

compositions. Under the testing environment, in pure oxygen

gasication, the average rising rate for the temperature of the

gasied coal seams is about 2.10 C/h; in the oxygen-enriched

steam forward gasication phase, the high temperature eld

mainly concentrates around gasication gallery, and the highest

temperature in oxidation zone reaches over 1200 C[61].

The air injected into a gasication channel is at a low speed, theame tends to propagate toward the injection point but, if the air

ow rate increases, the cavity tends to grow in the downstream

direction. It is also known that ame propagation is faster whenoxygen is used instead of air. This behavior is also expected since

oxygen-fed ames are hotter and have higher reaction rates[62].

Saulov et al. [62] considered the limit of high temperatures, high

activation energy and a strong air ow. Under these conditions the

surface of the channel has two zones, cold and hot. The tempera-

ture is insufciently high in the cold zone to initiate reactions,

while in the hot zone any oxygen on the surface reacts instantly.

Since the activation energy is high, these zones are separated only

by a very small distance. The overall reaction rate is determined by

the rate of diffusion of oxygen to the hot zone, while the oxygen

concentration on hotwalls is essentially zero. Under such condi-

tions the turbulent ame is fully controlled by diffusion and the

injection rate has no control over theame position. Combustion of

coal begins with devolitalization reactions at low temperatures and

can be cooled by the air stream. If these reactions play a noticeable

role in initiating the rest of the oxidation process or in the overall

energy balance, the ame position is affected by the air speed and

becomes controllable.

When other factors are the same, increases in ow rate and

operation time result inmonotonic increases in all the dimensions

of the cavity, and its volume. However, when the distance between

the injection and production wells is increased, the overall cavity

volume decreases, due to signicant reduction in the rate of growth

of the cavity in the forward direction[42].

2.2.5. Effect of pressure

Pressure is known to positively impact the performance of coal

gasication[63]. At close to atmospheric pressure, the gas caloric

value is very low because of the kinetic limitations of the gasi-

cation reactions. The changes in operating pressure can perfect the

underground gasication process to a great extent. Under thecyclically changing pressure condition, heat loss was obviously

reduced, and heat efciency and gasication efciency and the heat

value of the product gas are increased greatly. The underground

gasier with a long channel and big cross-section could improve

the combustion and gasication conditions to a large extent,

markedly bettering the quality of the product gas and the stability

of gas production. Therefore, the large-scale underground gasieris

a condition necessarily met by the industrial production[50].

2.2.6. Effect of heat loss

Heat losses from underground coal gasication are not easy to

estimate. If the cavity remains completely in the coal seam, then

heat losses to the surrounding strata will probably be small and can

be ignored. However, as the overburden is progressively exposed,irreversible heat loss to the surrounding will increase. It is not easy

to estimate this heat loss, because if the overburden undergoes

spalling, some of the energy used to heat it to cavity temperatures

may be recovered through preheating of the injected gas. The heat

loss mechanisms can probably be more easily investigated using

a dynamic model, in which cavity growth and heat loss are esti-

mated as functions of time, simultaneously.

2.2.7. Effect of temperature

The process of UCG is virtually one of a self-heat balance. The

heat produced by coal combustion contributes to the establishment

for ideal temperature eld in the underground gasier and also

leads to the occurrence of gasication reactions and, eventually, the

generation of gas.Temperature is a key factor in determining the continuous and

stable production in the process of underground coal gasication.

The patterns of variation for temperature eld in the gasier are

closely related to the nature of the gasication agent, gasication

modes, and the changes of cavity [8,49,61,64,65]. Under the pure

oxygen gasication condition, the average rising rate for the

temperature of the gasied coal seams is about 4.15 C/h; in the

oxygen-steam forward gasication phase the high temperature

eld mainly concentrates around loosening zones arising from the

thermal explosions, and the highest temperature in the oxidation

zone approaches 1300 C[6]. Compared with forward gasication,

the average temperature in the gasier for backward gasication is

lower [61]. The drop of temperature results in a decrease in CO

content while H2, CH4and CO2contents increases[50].Fig. 8. Gas composition variation with steam/oxygen (v/v)[60].



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In thermal-explosion gasication method, under the pure

oxygen gasication condition, the average rising rate for the

temperature of the gasied coal seams is about 4.15 C/h; in the

oxygen-steam forward gasication phase, temperatureeld mainly

concentrates around loosening zones arising from the thermal

explosions, and the highest temperature in the oxidation zone

approaches 1300 C. Test data showed that the forward oxygen-

steam gasication with moving points can obviously improve the

temperature conditions in the gasier. During the backward

oxygen-steam gasication, with the passage of time, the tempera-

ture of the gasication coal seams continuously increases,

approaches stable little by little, and was basically the same with

that of the forward gasication. Therefore, backward gasication

can form new temperature conditions and improve the gasication

efciency of the coal seams.

In the process of coal gasication, the changes of the tempera-

ture in the coal seam are due mainly to the heat transfer medium of

the ame working face, which corresponds to a source of heat[53].

In the process of underground coal gasication, the temperature of

coal seams around the gasication channel rises along with the

conducted heat. When the coal surface is heated by the hot gas or

the neighboring incandescent coal, its temperature distribution

expands toward the coal grains or the interior of the coal seam,which inevitably results in the thermal effects of absorption,

desorption, and seepage movement of dry distillation gas stored in

the coal seam [49,53,66,67]. King and Ertekin [68] study shows that

under non-isothermal conditions, either the absorption-desorption

process or the permeation-expansion process is linked to the

temperature.

According to the gasication theory, the temperature above

1000 C indicates a high-speed diffusion of the water decomposi-

tion reaction constituting the fundamental process for the

production of a hydrogen rich gas in the course of the UCG steam

stage. On the other hand, the temperature drop below 700 C

slowed down the reaction speed considerably. For these reasons,

special attention was paid to keeping parameters preferable for the

production of gas with a high content of the combustible compo-nents, mainly hydrogen. The oxygen stage was therefore continued

to achieve temperatures in the range between 1100 and 1200 C.

According to the simulated calculation results [13], with the

increase of the length for the gasication channel, the heating value

of the gas improves. However, behind the reduction zone, it

increases with a smaller margin. The inuence of the temperature

eld on the heating value for the gas is noticeable. Due to the effect

of temperature, in high temperature zone, the change of the

measured value of the concentrationeld for the gas compositions

is larger than that of calculated value.

The underground gasication of a large quantity of coal at

temperatures higher than 1000 C results in the typically argilla-

ceous overburden rocks overlying the coal becoming thermally

affected. Most of thermal reactions in argillaceous rocks are

endothermic.

2.2.8. Cavity growth

As the coal gasication reaction precedes a cavity consisting of

coal, char, ash, rubble, and void space, is created underground. The

size of the cavity formed during UCG impacts directly the economic

and environmental factors crucial to its success. Lateral dimensions

inuence resource recovery by determining the spacing between

modules, and ultimate overall dimensions dictate the hydrological

and subsidence response of the overburden. The exact shape and

size of the gasication channel during UGC are of vital importance

for the safety and stability of the upper parts of the geological

formation[69]. Due to upward growth the cavity eventually rea-

ches the interface between the coal seam and the overburden. From

that point onwards the development of the cavity can be strongly

inuenced by the interaction of the gas mixture with the over-

burden. At the start of the UCG process, typically, the exothermic

coal combustion reaction is required in order to create a sufciently

large underground cavity. In this early stage, cavity growth is

unconstrained by roof interactions. Once a stable temperature eld

is attained, steam is introduced in the cavity for gasication of the

coal in order to obtain the combustible product gases [38]. Theshape and rate of growth of this cavity will strongly impact other

important phenomena, such as reactant gas ow patterns, kinetics,

temperature proles, and so on [42]. The cavity size at any given

time depends on the rate of coal consumption and its shape

depends on the non-ideal ow patterns inside the cavity.

The cavity shape is almost symmetric around the injection well.

The cavity evolution behind the injectionwell (i.e.backward length)

is less than the height (inthe verticaldirection) and thewidthat the

injection point (in the transverse direction). The forward length of

the cavity (i.e. distance from injection well to the end point of the

cavity dome in the forward direction) is larger than the height and

the backward length. The convectiveuxof the reactantgases inthe

forward direction (i.e. toward the production well) contributes to

the additional growth of the cavity in this direction. The observednal cavity dome that is associated with a long outow channel is

nevertheless nearly hemispherical in shape.Fig. 9is a schematic of

thenal cavity shape, indicating the vertical, forward,backward and

transverse directions as dened here. The temperature at the cavity

roof is in the range of 950e1000 C whereas the oor temperature

varies between 650 and 700 C.

The volume of the cavity increases progressively with coal

consumption and thermomechanical spalling, if any, from the roof.

As the cavity growth is irregular in three dimensions, the ow

pattern inside the UCG cavity is highly non-ideal. The complexity

increases further because of several other processes occurring

simultaneously, such as heat transfer due to convection and radi-

ation, spalling, water intrusion from surrounding aquifers, several

Fig. 9. Schematic diagram de

ning forward length, backward length, height and width of the

nal cavity[42].



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chemical reactions, and other geological aspects [57]. Several

mathematical models have been developed considering the UCG

cavity as either a packed bed or a free channel Most of the existing

models consider the UCG cavity as a rectangular or cylindrical

channel[13,16,18,19,35,70e75].

Perkins and Sahajwalla [18] predicted cavity growth rate

between 1.6 and 5 cm/h using their mathematical model which

links linear cavity growth rates to reactivity and mass transport

properties. Daggupati et al. [38] measured the linear, vertical

growth rate of 1.1 cm/h (obtained using the measured cavity

heights at different times, with the other operating conditions

being the same).

The cavity volume is directly proportional to the coal

consumption whereas the shape depends on the relative rates of

growth taking place in each of the four identied representative

directions. While the coal consumption is governed by the extent or

rate of reaction that takes place in the cavity reactor, the growth in

each individual direction is a function of the complex reactant gas

ow eld inside the cavity, and other effects such as thermo

mechanical spalling of the coal. Chen et al. [69] has developed

model to calculate the temperature distribution in the vertical

direction, and the combustion volume.

According to the physical and chemical properties of coal andthe mining geology conditions of the burial for the coal seams, two

kinds of gasication channels can be formed in the gasication

panel; namely, free channel without solid phase and the percola-

tion patterned porous loose channel. In the longitudinal (or radial)

direction, the free channel can be divided into three zones (Fig.10),

i.e., free owing zone, reaction zone and the coal seams zone. The

gas phases ow under the condition of wall plane of the channel

continuously exchanging heat, consuming or producing certain

compositions. At the same time, the homogeneous reactions also

occur to the gas phases. In the reaction zone, the oxidation, reduced

reactions and the pyrolysis reactions of the coal occur. The heat

transfer to the gas phases, the consumption and production of the

compositions can be regarded as the boundary conditions for the

owing of the gas phases. In the coal seams zone, part of theheat inthe reaction zone loses in the coal seams mainly in the form of the

heat conduction, making the dry and distillation of the coal seams.

Therefore, we can observe the characteristics of the gas phase

moving and establish the control equation set of the free channel

gasication process.

The cavity growth directly impact on the coal resource recovery

and energy efciency and therefore the economic feasibility. Cavity

growth is also related to other potential design considerations

including avoiding surface subsidence and groundwater

contamination.

Installation of well pairs (injection and production wells) is

costly and therefore it is desirable to gasify the maximum volume of

coal between a well pair. As gasication proceeds, a cavity is formed

which will extend until the roof collapses. This roof collapse is

important as it aids the lateral growth of the gasier. Where the

roof is strong and fails to break, or where the broken ground is

blocky and poorly consolidated, some uid reactants will by-pass

the coal and the reactor efciency could decline rapidly. In

general, as depth increases, conditions should become increasingly

favorable to gasier development with a lower risk of bypass

problems occurring, except possibly in strong roof conditions[76].

2.2.9. Gas diffusion

Inthe process of combustion and gasication for the coal seams

in the gasier, the major reactions are multi-phase reactions. At

each stage of multi-phase reactions, the gas state reactant spreads

to the surface of the solid state reaction by the diffusion method.

Gas diffusion mainly has two kinds: molecular diffusion and

convection (eddy) diffusion. The process of the combustion for coal

seams depends on the gas diffusion features and the dynamic

characteristics for the chemical reactions. According to the

diffusion-dynamic theory for combustion [49], under the low

temperature condition, the overall velocity of the combustion andgasication process is mainly determined by the dynamics condi-

tions of the chemical reactions; under the high temperature

condition, the overall velocity of combustion and gasication

process mostly depends on the speed for oxygen to diffuse from the

main current to the carbon surface and the velocity of its product

diffusing from the carbon surface to the main current. Seeing from

the circumstances of the eld test of underground gasication and

model experiment, the temperature within the gasier (the vicinity

of the ame working face, in particular) is very high.

Moreover, considering the movement conditions for the uid,

we can conclude that the convection diffusion for gas is the

signicant factor inuencing the process of the underground

gasication. Under the condition of high temperature, molecular

diffusion results from the existence of concentration gradient,temperature gradient and pressure gradient[14].

While studying the basic features of convection diffusion for the

gas produced in underground coal gasication, on the basis of the

model experiment, through the analysis of the distribution and

patterns of variation for the uid concentrationeld in the process

of the combustion and gasication of the coal seams within the

gasier, Lanhe[14]established the 3-D non-linear unstable math-

ematical models on the convection diffusion for oxygen. Same

study concludes that oxygen concentration is in direct proportion

to its distance from the ame working face, i.e. the longer its

Fig. 10. Gasi

cation channels in coal seems[11].



13/26

distance, the higher the oxygen concentration; otherwise, the

lower.

In the vicinity of the combustion zone, due to the very high

temperature, the oxygen is almost exhausted in the reaction with

carbon; in loosening zone, the oxygen concentration drops to a very

low point where it almost approaches zero; in dropped out zone,

owing to the comparatively low temperature, the drop of the

oxygen concentration is slow[14].

During UCG processes, the surrounding rock acting as the

furnace walls will be affected by high temperature, and its

mechanical properties willchange with the increased temperatures.

At thesame time, stressand displacement will happenamong rocks

due to the high temperature. Gasier instability would result in

steam interruption, and incomplete contact between gasication

agents and coal. Two mechanisms can play a role in a gas transport

through the porous stratum above the gas source, viz. diffusion and

permeation. The diffusion driving force is the composition gradient

(expressed through gas component mole fractions); the driving

force for permeation is the total pressure gradient.

It was found that the pressure increase inuences the speed of

the gas front movement more signicantly than the temperature

increase that is almost negligible. Nevertheless, for all tested

conditions CO2appears at the distance of the few hundred metersafter some years only. The direct proportionality of the effective

permeability coefcient to the effective squared mean pore radius

was conrmed[77].

2.2.10. Velocity of combustion front

In packed bed gasication, the combustion front moves slowly

down the bed parallel to the ow of gases. Hot combustion gases

always have intimate contact with the unburned coal ahead of the

combustion zone until the re breaks through to the production

well. In channel gasication, the combustion zone moves outward

at nearly right-angles to the ow of air and combustion gases.

During UCG a thermal wave is formed which gradually travels

through the coal bed toward the gas production well. The shape of

the thermal wave tends to change very little. Since the shape of thewave remains unchanged, the processes occurring at each

temperature level in the moving wave remain unchanged in time,

and an apparent steady-state or psuedo-steady-state condition

prevails. Under these conditions in a one-dimensional system, it is

possible to transform the mathematical model to a moving coor-

dinate system which converts partial differential to ordinary

differential equations, a major simplication of the problem. This

transformation is[78]:

n x e vt

Where

x xed spatial coordinatet time

v velocity of thermal wave or combustion front

n coordinate system moving with frontal velocity v

When the physical properties of coal tend to vary widely over

short distances even in a single coal seam making the task of

modeling such as UCG process very complex. Gasication of typical

9 m seam of sub-bituminous coal proceeds at a rate of 0.3e0.6m/

day consuming all the coal in a swath 12 to 15 m wide for a well

spacing of approximately 18 m.

2.2.11. Compositions of syngas

The precise proportions of the various component gases in any

particular syngas mixture are a function of quality and rank of

coal, seam depth, steam: oxygen ration and oxygen injection rate

and other parameter discussed in Section 2. Compositions of

syngas from a variety of coals as reported in literature reveals

component fractions in the following ranges [8,18,26,79e81].

At constant steam/oxygen ratio gas compositions remained

stable[8].

H2:11e35%; CO: 2e16%; CH4: 1e8%; CO2: 12e28%; H2S:0.03e

3.5%.

2.2.12. Optimization of UGC operation

Underground gasication cannot be controlled to the same

extent as a surface process as the coal feed cannot be processed. The

UCG process can be operated with stability and exibility, as input

ow has been shown to have a direct relationship to production

ow, with little effect on product gas quality. The power output

from the gasier could be rapidly increased or reduced by

increasing or decreasing the O2 ow rate. Although elevated depth

and pressure are not pre-requisites for a high quality gas, the

benet is in higher mass ows and hence greater efciency of

energy transmission to the surface. The energy output of a UCG

system depends on the ow rate of gaseous products and the heat

value of the gas mixture. The volume ow of the product gas is

typically four times the injection ow so the limiting factor is the

dynamic resistance of the production well.The mass ow capability

of a well is proportional to input pressure. Increasing well depth

increases the product gas density and pressure. The massow gain

due to pressure increase exceeds the frictional loss due to increased

bore hole length. Increasing the diameter of production tubing also

raises the limiting ow rate. Increasing the diameter of production

tubing, or the number of production wells, also raises the limiting

ow rate [76]. Information on the process conditions must

be constantly monitored and updated as the gasication

process moves forward. The ideal temperatures of above ground

coal gasication are about 1000 C, however, it may or may not be

possible to achieve these temperatures in UCG, primarily because

of the lack of control on water inux and reactant gas ow

patterns[57].Blinderman et al. [36,82] has used intrinsic disturbed ame

equations to determine the key parameters of the RCL process.

Wang et al. [83] performed eld trial with various operational

maneuvers, such as implementing controlled moving injection

points, O2-enriched operation and variation of operational pres-

sure to ensure the gas ow comparatively controllable and hence

improve efciency of heat and quality of the production syngas.

Lawrence Livermore National Laboratory (LLNL) is evaluating

commercial computational uid dynamics (CFD) code to model

cavity gas ow and combustion in two and three dimensions.

Fig. 11 [84] show a typical cavity conguration at a mid-to-late

stage of a linked vertical well module. Nitao et al. [84] has

provided the details of models and simulators. It will be more

useful to couple the UCG process models with full scale processsimulator so that the entire process can be modeled at once, rather

than sequentially.

3. Thermodynamics of UCG

The gasication performance is controlled by both of kinetic and

thermodynamic factors. The thermodynamic properties are, by

denition, point functions of the gasication process, indicating the

conditions of a system at equilibrium, regardless of the reaction

path followed in attaining equilibrium or the time required. On the

other hand, the kinetics of a reacting system denes a particular

sequence of reaction paths, as well as the rates at which the

chemical changes take place.



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3.1. Thermodynamic equilibrium

In gasication, both homogeneous and heterogeneous reactions

occur simultaneously in a complex reacting system [85]. As

underground coal gasication processes are thermally auto-

balanced, at given constant pressure and initial enthalpy, the

equilibrium state is reached when;

dS 0 at constantp; H (8)

Therefore, at equilibrium, when conditions of constant pressure

and enthalpy are applied, the total entropy is at maximum. Some of

the processes are at specic pressure and temperature, exothermic

or endothermic. Constraining the unit to constantTandp, we nd

that;

dG dSg (9)

and at equilibrium under these conditions, the following equation

mu

underground coal gasification from fundamentals to applications

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