Underground Coal Gasifcation: A new clean coal Technology

Shashank Pandey, Shricharan Arumugam and Rehan Arif Khan

Applied Petroleum Engineering - Upstream

University of Petroleum & Energy Studies

Dehradun, India.

Abstract— According to The International Energy Outlook

2013, the world energy consumption will grow by 56 percent

between 2010 to 2040. There is an urgent need to identify

alternative methods of extraction of energy from existing sources.

Underground Coal Gasification (UCG) is a clean coal technology

option suitable for the need of the hour .UCG is a technique to

gasify coal in-situ within the coal seam via a matrix of wells. Coal

is ignited using the rock mass as reactor while injecting steam

and gasifying agent which carries out the chemical reaction

whose end product is Syngas which is a mixture of methane,

carbon monoxide, hydrogen and carbon dioxide. It has been

stated that there are two methods of underground coal

gasification- the shaft method and the borehole method. It’ll be

able to provide cheap energy for the masses hence UCG is one

such source of energy which can immensely boost Energy

security of a nation. Syngas is readily useable fuel for thermal

power plants hence the technology does not require additional

specific infrastructure eventually making it economically

sustainable. This is a breakthrough technology as it provides an

efficient and reliable method to harness the unminable coal

resources. Since UCG operates on coal it is only apt to compare

its environmental performance with coal combustion and on that

parameter UCG stands out as a revolutionary technology because

its emission levels are closest to natural gas (the cleanest fossil

fuel known till date) however underground aquifer

contamination issues have been reported, which can be mitigated

by negative hydraulic gradient. There is a possible conflict with

Coal Bed Methane (CBM) developers but the technology has

enough scope in the future that it’ll carve out an independent

niche for itself.

Keywords—UCG; Syngas; Energy Security; aquifer; hydraulic

gradient; CBM

I. INTRODUCTION

About 100-400 million years ago trees and plants in huge swamp areas got buried deep below the earth and got converted into a carbonaceous substance which evolved to be the prime energy source for the modern world and came to be known as “Coal”.It has been used as a source of energy for nearly 3000years though it did not become the most sought after energy source until the beginning of the industrial revolution in 18

th century.

Although coal has been extensively been used and is still holds an important position in the world, it poses serious

environmental challenges. Mainly due to organic sulphur, a substance that is chemically bound to coal. Sulphur and nitrogen oxides produced as a result of combustion of coal are prime pollutants. Therefore clean coal technologies have been an area of interest from quite some time. Coal gasification is one such clean coal technology that bypasses conventional coal combustion process thus removing pollutants like SOx, NOx, particulates etc. from emissions which encompasses Underground Coal Gasification. UCG is a promising option for future use of un-worked and inaccessible coal reserves. At present Underground Coal Gasification (UCG) is the only feasible technology is the only feasible technology to harness energy from deep unminable coal seams in an economically and environmentally sustainable way. “Underground coal gasification (UCG) is a physico-chemical process of conversion of coal into gaseous energy source at the place of its occurrence. Through this process, techno-economically unviable coal resources can yield energy for present day use.” PSA (2007). As the name indicates UCG is a process to convert coal into combustible gas with the help of chemical reactions taking place in the rock bed deep down below the surface. The coal reacts with oxygen /air and steam to produce a mixture of Carbon monoxide (CO), Hydrogen (H2), Carbon dioxide (CO2) and Sulfur nitrogen oxides in traces called syngas.

II. UCG: TECHNOLOGY AND PROCESS OVERVIEW

UCG, via a matrix of wells allows for in situ coal seam

gasification. The coal is ignited and air is injected

underground to sustain fire, which is essentially used to

produce and transport combustible synthetic gas to surface.[1]

Essential uses of the synthetic gas produced are Industrial

heating, power generation or manufacture of hydrogen,

synthetic gas or other fuels. Compared to conventional mining

and surface gasification, 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 [2].

H2, CO, CO2, CH4, and H2S are the primary components of

UCG syngas. The pressures and temperatures of produced gas

are similar, at 30-50 bars for a 300-500 m deep seam, and 500-

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

1000 °C for bituminous coals[2]. Once product gas reaches the

surface it requires cleaning, either to be of sufficient purity for

use as a chemical feedstock for conversion to synthetic fuels.

A. Methodologies.

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

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

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

flushing cavity with steam and water to remove potential contaminants[3].

In the first step, from the surface to the coal seam and highly

permeable path within the coal seams are established between

these two well by drilling an injection and production well.

Prior to the gasification 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 gasification include CRIPs, long and

large tunnel gasification, and two-stage UCG [4-6].

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 fire propagates toward the production well as

shown in Fig. 4(b). During forward gasification, the flame

working face gradually moves to the outlet, making the dry

distillation zone shorter and shorter. At the time when forward

gasification is nearly complete, the reduction zone also

becomes shorter [7].

Fig. 4. Schematic views of the reverse and forward

combustion linking in UCG. (a) Reverse combustion linking.

(b) Forward combustion linking [5].

Flow of oxidant into the injection well is maintained until the

fire reaches the bottom of the injection well in the RCL or that

of the production well in the FCL. This outcome is

accompanied by a significant drop in the injection pressure

indicating creation of a low hydraulic resistance link between

the wells, which establishes 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 [8].

During the gasification process, the burning zone grows in the

upstream direction, in contrast to the gas flow 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 configuration, and

improves the overall efficiency of the UCG process [9].

Once a successful link has been established the

second step is ignited. The gasification step starts with ignition

of the coal and the injection of air or air enriched with oxygen.

Both permeable bed gasification and natural convection driven

surface gasification will occur. When the gas quality

deteriorates the injection well is burnt to allow injection

further upstream.

When a mixture of air or oxygen and steam is forced into

the coal seam through injection well and react chemically with

the coal, gasification occurs, generating a synthesis gas, which

is recovered through product well. At the surface the raw

product gas is cleaned for industrial uses [10]. As gasification

proceeds, an underground cavity is formed. Water from the

surrounding strata will enter the cavity and participate in the

gasification 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 gasifiers will be operated

simultaneously. Once the gasification operations in a section

of coal seam have finished, the third step is to return

environment back to its original state. This is achieved by

flushing the cavities with steam and/or water to remove

pollutants from 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

gasification [10]. 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 for thin, deep coal seams, replaces the vertical injector by a horizontal injector [8]. 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 new burn periodically along the horizontal wells [11]. The CRIP method was first tried successfully in early 1982 with a three-day trial, gasifying a 40-ton cavity. The injection pipe was then burned off and a second 10-ton cavity started. The original cavity cooled to 500

oC, and the second achieved the typical

operating temperature of 1000 oC. 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 (3-5 min) to initiate the combustion. An electric spark is generated for ignition of the liquefied petroleum gas (LPG) in the channel of the coal block near the mouth of the 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 [12].

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 benefit 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 a CRIP

maneuver, and between 10 and 20 such maneuvers are

expected during the course of a module’s 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 benefits

of lateral drilling for extending the life of wells and fixed

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 requirements at an

affordable cost is still under review [6]. The CRIP technique

produces higher quality gas, results in lower heat loss than the

two-vertical well configuration, and improves the overall

efficiency of the UCG process [9].

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

cyclically [13,14]. In the first stage, air is supplied to make

the coal burn and store heat to produce air gas; in the second

stage, steam is supplied to produce water gas. Only if

sufficient heat is stored in the first stage can the

decomposition reactions in the second stage run smoothly 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 [15]. During in situ coal

gasification remote sensing technique may be used for

mapping underground fracture systems, locating tunnels or

water-bearing strata and mapping burn fronts [16].

B. Chemical Process:

The study considers the quasi-steady burning of a carbon

particle which undergoes gasification at its surface by

chemical reactions, followed by a homogeneous reaction in

the gas phase. The main chemical processes occurring during

coal gasification are drying, pyrolysis, combustion and

gasification of the solid hydro-carbon. These processes occur

in all methods of coal gasification, whether conducted in

surface gasifiers or in situ. From the chemical and

thermodynamic point of view, the UCG process runs

analogically to gasification in the surface reactors [17]. The

most important chemical reactions taking place during

underground coal gasification are listed in Table 2.

Chemical reactions taking place during underground coal gasification.

Reaction equation Reaction DHo298 Equation

rate (Ri) (MJ/kmol) 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 O2/CO2

R5 285.1 (5)

2

H2 + 1

O2/H2O R6 -0.242 (6)

2

CO + H2O/CO + H2 R7 -0.041 (7)

Chemical reactions (1)to(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 final product gas consists of

hydrogen, carbon monoxide, carbon dioxide, methane and

nitrogen. 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 [17].

During in situ combustion of coal different processes of

vaporization (drying), pyrolysis, and combustion and

gasification of char take place collectively. The UCG process

has a zonal character and the main gasification 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 in Fig. 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 gasification

channel heterogenic reactions take place. Their rates are

determined by the diffusion and the accessible reaction area.

The major products of the reaction of oxygen with carbon in

the gasification area (oxidation zone) are carbon dioxide and

carbon monoxide [17].

Based on the differences in major chemical reactions, the

temperature, and the gas compositions, the gasification

channel can be divided into three zones: oxidization zone,

reduction zone and dry distillation zone as shown in Fig. 6

[18]. In the oxidization zone, the multi-phase chemical

reactions between oxygen contained in the gasification 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 oC to 1450

oC [19]. 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 [20].

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

With the O2 burning 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

oC to 1000

oC, and the length is

1.5E2 times that of the oxidation zone with its pressure being

0.01E0.2 Mpa [21]. 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 flow into

the destructive distillation and dry zone (2000C to 600

0C).

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 oC.

When the temperature is not higher than 300 0C, only small

amounts of paraffin hydrocarbon, water, and CO2 are

separated out. Over 300 oC, 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 350oC to 550

oC, a large proportion of tar oil is

separated out (500 oC at its peak) and a certain amount of

combustible gas is yielded. The hydrocarbon gas is given out

when the temperature stands at 450oC to 500

0C. As the

temperature of the coal seam continues to rise until it is over

550 0C, semi-coke remains begin to solidify and contract,

accompanied with the yield of H2, CO2, and CH4 [19-22].

The overall UCG process is strongly exothermic, and temperatures in the burn zone are likely to occasionally exceed 900

oC. Even after cooling (through conductive heat loss to

surrounding strata and convective heat loss to native groundwater), syngas typically flows through production wells at temperatures between 200

oC and 400

oC. 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 gasification

agent and air injection method to another. These three zones

move toward the outlet along the direction of the air flow,

which, in turn, ensures the continuous run of the gasification

reactions [18].

Figs. 6 and 7 illustrate different chemical regions of

gasification 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 specified 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 40-50% 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

gasified by the injected oxidant agents and exhausted gases

from the previous steps [24, 25]. The rates of the gaseous

phase reactions are determined mostly by the temperature and

concentration of the particular gaseous compounds.

Development of these reactions is frequently supported by the

catalytic influence of some chemical compounds, e.g. iron

oxides.

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

demonstrates the different regions[26].

C. Physical Process:

In the process of underground coal gasification (UCG), the

gas movement not only influences the concentration

distribution and movement of fluid in the burning zone

directly, but also restricts the diffusion of the gasification

agent in the whole gasifier Therefore; it eventually determines

the rate of chemical reaction between gas and solid, and the

process of burning and gasification. Evidently, Lanhe 2003

[27] suggested the study of moving patterns of fluid in the

gasifier should precede the study of the process of chemical

reaction, the moving patterns of agents, and the distribution

regularity of temperature fields near the flame working face.

In the process of underground coal gasification, under the

effect of high temperature, that a temperature field forms in

the coal layer to be gasified within the coal and rock mass,

which makes the coal and rock layers originally full of

stratification, joints, and fractures soften, 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 physic-mechanical

properties of the coal and rock mass.

In the process of underground coal gasification, a high

temperature field 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 seam’s

surface morphology. Hence, dramatic changes take place in

the physical and mechanical properties 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 coefficient 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 [28].

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 [21].

However, the small deformation of the solid skeleton still

produces a certain effect on the distribution of the temperature

field and seepage of underground water in the gasification

panel. Therefore, the deformation of the solid particle is not

negligible and can be regarded as compressible [29].

The coal rock is extended and deformed by the pore fluid

pressure. The fluid inside the pores affects the cracks inside

the skeleton of the coal rock and the pores’ opening and

closing; second, the relation between the stress and strain of

the coal rock is changed by the fluid in the pores, which in

turn changes the elastic modulus and compressive strength of

the coal rock [30-32]. The changes in the temperature field of

the coal seam are due mainly to the flame 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

coal seam drops.

D. Operating conditions:

The investigation by Perkins and Sahajwalla [33] has found

that the operating conditions that have the greatest impact on

cavity growth rate are temperature, water influx, pressure,

and gas composition in underground coal gasification. 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 [34].

Lanhe [35] while establishing the mathematical models on

the under-ground coal gasification in steep coal seams

according to their storage conditions and features of gas

production process concludes that numerical simulation on

the temperature field, concentration field and pressure field

is reasonable in the underground gasification of steep coal

seams on the experimental condition.

E. Thickness of Coal Seams:

UCG is influenced by several natural factors as described in Table 3.

Table 3 Classification criteria for UCG.

Criterion Characteristics/remarks

Coal type Any

Physicochemical properties of coal Recommended: high content of volatile

matter, low agglomerating capacity

or its lack, ash content < 50% by weight

Occurrence depth Profitability criterion

Bed thickness More than 1 m

Angle of inclination of coal bed Any

Type and tightness of rock mass Recommended: firmness and tightness

of rock mass, thickness and lithology

of rock massDoverburden in slightly

permeable layers (clays, silts, shale clays)

Hydrogeological conditions Recommended: lack of fissures, faults,

aquiferous layers, water reservoirs causing

water inflow

Deposit tectonics Recommended homogeneity of deposit

(lack of fissure, faults)

Quantity of resources Profitability criterion

Methane presence in the bed Causes gas hazard

Conditions of infrastructure Recommended lack of building development

Most UCG operations were carried out in more gas

permeable conditions of brown coal beds and younger

formations of hard coals. Generally, these deposits occurred

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

easily. Strongly swelling and coking coals have the tendency

to block gas flow through the coal bed, thus hindering the

course of the reaction. The gasification of beds 1 m thick or

more improves economics [36]. 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 (floor,

roof), thus having an adverse effect on the stability of the

underground gasification process. Eliot [37] 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

gasification station adopted oxygen-enriched blowing in the

coal seam, for which the thickness is less than 2 m [37].

When the thickness of coal layers is decreased or the intake

rate of water is increased, the CO2 content in the gas will rise

[37-38].

F. 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 orders of magnitude when

extrapolated to typical gasifier operating temperatures [33].

The coal intrinsic reactivity has a big impact on the

distributions in the gasifier and on the final 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 the final product gas calorific value.

G. Gasifying Agents:

Gasification under different atmospheres such as air, steam,

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

literature. In general, the gasifier atmosphere determines the

calorific 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

calorific value is produced. Adding steam changes carbon-

oxygen system balance to carbon-oxygen-steam system

balance in the combustion process. Oxygen-steam gasification

not only utilizes the surplus heat to improve the energy

efficiency 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 compositions and steam/oxygen ratios are shown in Fig.8

[39].

The experiment results show that pure-oxygen underground

coal gasification, 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 fluctuations. Therefore, it is required to adjust the

oxygen supplying volume so as to keep the stable proceeding

of gasification process. From Fig. 8, it can be seen that with

the rise in the steam/oxygen ratio, the volume of steam

increases, the H2 content in the coal gas improves, the CO

content drops, and the CH4 content is heightening a little [39].

Fig. 8. Gas composition variation with steam/oxygen (v/v)[39]

The syngas produced has a by UCG process has low calorific value approximately one-eighth of natural gas if air injection is used, and double this figure if oxygen injection is used. Oxygen-enriched steam forward gasification has remarkable effects on gas compositions. Under the testing environment, in pure oxygen gasification, the average rising rate for the temperature of the gasified coal seams is about 2.10

_C/h; in

the oxygen-enriched steam forward gasification phase, the high temperature field mainly concentrates around gasification

gallery, and the highest temperature in oxidation zone reaches over 1200

oC [40].

The air injected into a gasification channel is at a low speed,

the flame tends to propagate toward the injection point but, if

the air flow rate increases, the cavity tends to grow in the

downstream direction. It is also known that flame propagation

is faster when oxygen is used instead of air. This behavior is

also expected since oxygen-fed flames are hotter and have

higher reaction rates [41].

Saulov et al. [41] considered the limit of high temperatures,

high activation energy and a strong air flow. Under these

conditions the surface of the channel has two zones, cold and

hot. The temperature is insufficiently 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 conditions the

turbulent flame is fully controlled by diffusion and the

injection rate has no control over the flame 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 flame

position is affected by the air speed and becomes controllable.

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

operation time result in monotonic 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

significant reduction in the rate of growth of the cavity in the

forward direction [12].

H. Effect Of Pressure:

Pressure is known to positively impact the performance of

coal gasification [42]. At close to atmospheric pressure, the

gas calorific value is very low because of the kinetic

limitations of the gasification reactions. The changes in

operating pressure can perfect the underground gasification

process to a great extent. Under the cyclically changing

pressure condition, heat loss was obviously reduced, and heat

efficiency and gasification efficiency and the heat value of the

product gas are increased greatly. The underground gasifier

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

combustion and gasification conditions to a large extent,

markedly bettering the quality of the product gas and the

stability of gas production. Therefore, the large-scale

underground gasifier is a condition necessarily met by the

industrial production [22].

I. Effect Of Heat Loss:

Heat losses from underground coal gasification 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.

J. 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 field in the underground

gasifier and also leads to the occurrence of gasification

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

gasification. The patterns of variation for temperature field in

the gasifier are closely related to the nature of the gasification

agent, gasification modes, and the changes of cavity [21,40].

Under the pure oxygen gasification condition, the average

rising rate for the temperature of the gasified coal seams is

about 4.15 oC/h; in the oxygen-steam forward gasification

phase the high temperature field mainly concentrates around

loosening zones arising from the thermal explosions, and the

highest temperature in the oxidation zone approaches 1300 oC

[43]. Compared with forward gasification, the average

temperature in the gasifier for backward gasification is lowers

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

content while H2, CH4 and CO2 contents increases [22]. In thermal-explosion gasification method, under the pure

oxygen gasification condition, the average rising rate for the temperature of the gasified coal seams is about 4.15

oC/h; in

the oxygen-steam forward gasification phase, temperature field mainly concentrates around loosening zones arising from the thermal explosions, and the highest temperature in the oxidation zone approaches 1300

0C. Test data showed that the

forward oxygen-steam gasification with moving points can obviously improve the temperature conditions in the gasifier. During the backward oxygen-steam gasification, with the passage of time, the temperature of the gasification coal seams continuously increases, approaches stable little by little, and was basically the same with that of the forward gasification. Therefore, backward gasification can form new temperature conditions and improve the gasification efficiency of the coal seams.

In the process of coal gasification, the changes of the

temperature in the coal seam are due mainly to the heat

transfer medium of the flame working face, which corresponds

to a source of heat [28]. In the process of underground coal

gasification, the temperature of coal seams around the

gasification 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 [21, 28]. King and Ertekin [44] 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 gasification theory, the temperature above

1000 oC indicates a high-speed diffusion of the water

decomposition 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

0C 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 components, mainly hydrogen. The oxygen stage was therefore continued to achieve temperatures in the range between 1100 and 1200

oC.

According to the simulated calculation results [35], with the

increase of the length for the gasification channel, the heating

value of the gas improves. However, behind the reduction

zone, it increases with a smaller margin. The influence of the

temperature field 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 concentration field for the

gas compositions is larger than that of calculated value.

The underground gasification of a large quantity of coal at

temperatures higher than 1000 oC results in the typically

argillaceous overburden rocks overlying the coal becoming

thermally affected. Most of thermal reactions in argillaceous

rocks are endothermic.

K. Cavity Growth:

As the coal gasification 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 influence 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 gasification channel during UGC are of vital

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

geological formation [45]. Due to upward growth the cavity

eventually reaches the interface between the coal seam and the

overburden. From that point onwards the development of the

cavity can be strongly influenced 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 sufficiently large underground cavity. In

this early stage, cavity growth is unconstrained by roof

interactions. Once a stable temperature field is attained, steam

is introduced in the cavity for gasification of the coal in order

to obtain the combustible product gases [7]. The shape and

rate of growth of this cavity will strongly impact other

important phenomena, such as reactant gas flow patterns,

kinetics, temperature profiles, and so on [12]. The cavity size

at any given time depends on the rate of coal consumption and

its shape depends on the non-ideal flow patterns inside the

cavity.

The cavity shape is almost symmetric around the injection

well. The cavity evolution behind the injection well (i.e.

backward length) is less than the height (in the vertical

direction) and the width at 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 convective flux of the reactant gases in

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

contributes to the additional growth of the cavity in this

direction. The observed final cavity dome that is associated

with a long outflow channel is nevertheless nearly

hemispherical in shape. Fig. 9 is a schematic of the final

cavity shape, indicating the vertical, forward, backward and

transverse directions as defined here. Fig. 9. Schematic diagram defining forward length, backward length, height and width of

the final cavity[12].

The temperature at the cavity roof is in the range of 950-1000 0C whereas the floor temperature varies between 650 and 700

oC. The volume of the cavity increases progressively with coal

consumption and thermo mechanical spalling, if any, from the

roof. As the cavity growth is irregular in three dimensions, the

flow 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 radiation, spalling, water intrusion from

surrounding aquifers, several chemical reactions, and other

geological aspects [36]. 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 [4,33,35].

Perkins and Sahajwalla [33] 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.[7] 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 identified

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 flow field inside the

cavity, and other effects such as thermo mechanical spalling of

the coal. Chen et al. [45] 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

and the mining geology conditions of the burial for the coal

seams, two kinds of gasification channels can be formed in the

gasification panel; namely, free channel without solid phase

and the percolation patterned porous loose channel. In the

longitudinal (or radial) direction, the free channel can be

divided into three zones (Fig. 10), i.e., free flowing zone,

reaction zone and the coal seams zone.

Fig. 10. Gasification channels in coal seems[46].

The gas phases flow 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 flowing of the gas phases. In the coal seams zone, part of

the heat in the 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 gasification process.

The cavity growth directly impact on the coal resource

recovery and energy efficiency 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 gasification 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 gasifier. Where the roof is strong and fails to break, or

where the broken ground is blocky and poorly consolidated,

some fluid reactants will by-pass the coal and the reactor

efficiency could decline rapidly. In general, as depth increases,

conditions should become increasingly favorable to gasifier

development with a lower risk of bypass problems occurring,

except possibly in strong roof conditions [47].

L. Gas Diffusion:

In the process of combustion and gasification for the coal

seams in the gasifier, 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 [21], under the low temperature condition, the

overall velocity of the combustion and gasification process is

mainly determined by the dynamics conditions of the chemical

reactions; under the high temperature condition, the overall

velocity of combustion and gasification 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 field test of underground

gasification and model experiment, the temperature within the

gasifier (the vicinity of the flame working face, in particular)

is very high.

Moreover, considering the movement conditions for the

fluid, we can conclude that the convection diffusion for gas is

the significant factor influencing the process of the

underground gasification. Under the condition of high

temperature, molecular diffusion results from the existence of

concentration gradient, temperature gradient and pressure

gradient [48].

While studying the basic features of convection diffusion for

the gas produced in underground coal gasification, on the

basis of the model experiment, through the analysis of the

distribution and patterns of variation for the fluid

concentration field in the process of the combustion and

gasification of the coal seams within the gasifier, Lanhe[48]

established the 3-D non-linear unstable mathematical models

on the convection diffusion for oxygen. Same study concludes

that oxygen concentration is in direct proportion to its distance

from the flame working face, i.e. the longer its 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

[48].

During UCG processes, the surrounding rock acting as the

furnace walls will be affected by high temperature, and its

mechanical properties will change with the increased

temperatures. At the same time, stress and displacement will

happen among rocks due to the high temperature. Gasifier

instability would result in steam interruption, and incomplete

contact between gasification 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 influences the speed

of the gas front movement more significantly than the

temperature increase that is almost negligible. Nevertheless,

for all tested conditions CO2 appears at the distance of the few

hundred meters after some years only. The direct

proportionality of the effective permeability coefficient to the

effective squared mean pore radius was confirmed [49].

M. Velocity of Combustion Front:

In packed bed gasification, the combustion front moves

slowly down the bed parallel to the flow of gases. Hot

combustion gases always have intimate contact with the

unburned coal ahead of the combustion zone until the fire

breaks through to the production well. In channel gasification,

the combustion zone moves outward at nearly right-angles to

the flow 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 the wave

remains unchanged, the processes occurring at each

temperature level in the moving wave remain unchanged in

time, and an apparent steady-state or 10seudo-steady-state

condition prevails. Under these conditions in a one-

dimensional system, it is possible to transform the

mathematical model to a moving coordinate system which

converts partial differential to ordinary differential equations,

a major simplification of the problem. This transformation is

[50]:

n ¼ x E vt

where:

x ¼ fixed spatial

coordinate

t ¼ time v ¼ velocity of thermal wave or combustion front n ¼ 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.

Gasification 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. 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

[31]. At constant steam/oxygen ratio gas compositions

remained stable [51].

N. UGC Optimization:

Underground gasification 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 flexibility, as input flow has been shown to have a direct

relationship to production flow, with little effect on product

gas quality. The power output from the gasifier could be

rapidly increased or reduced by increasing or decreasing the

O2 flow rate. Although elevated depth and pressure are not

pre-requisites for a high quality gas, the benefit is in higher

mass flows and hence greater efficiency of energy

transmission to the surface. The energy output of a UCG

system depends on the flow rate of gaseous products and the

heat value of the gas mixture. The volume flow of the product

gas is typically four times the injection flow so the limiting

factor is the dynamic resistance of the production well. The

mass flow capability of a well is proportional to input

pressure. Increasing well depth increases the product gas

density and pressure. The mass flow gain due to pressure

increase exceeds the frictional loss due to increased borehole

length. Increasing the diameter of production tubing also

raises the limiting flow rate. Increasing the diameter of

production tubing, or the number of production wells, also

raises the limiting flow rate [47]. Information on the process

conditions must be constantly monitored and updated as the

gasification process moves forward. The ideal temperatures of

above ground coal gasification are about 1000 oC, however, it

may or may not be possible to achieve these temperatures in

UCG, primarily because of the lack of control on water influx

and reactant gas flow patterns [36]. Blinderman et al. [5] Has

used intrinsic disturbed flame equations to determine the key

parameters of the RCL process. Wang et al.[52] performed

field trial with various operational maneuvers, such as

implementing controlled moving injection points, O2-enriched

operation and variation of operational pressure to ensure the

gas flow comparatively controllable and hence improve

efficiency of heat and quality of the production syngas.

Lawrence Livermore National Laboratory (LLNL) is

evaluating commercial computational fluid dynamics (CFD)

code to model cavity gas flow and combustion in two and

three dimensions. Fig. 11 [53] shows a typical cavity

configuration at a mid-to-late stage of a linked vertical well

module. Nitao et al. [53] has provided the details of models

and simulators. It will be more useful to couple the UCG

process models with full scale process simulator so that the

entire process can be modeled at once, rather than

sequentially.

III. UCG: TECHNOLOGY RELEVANCE TO INDIAN

PERSPECTIVE

Coal is the single largest energy source for India with total

estimated reserves of about 293 billion tones i.e. roughly 8% of

world’s total coal reserves [54]. It accounts for 55% of total

primary commercial energy production. 75% of produced coal

in India is consumed in the power sector. In addition other

industries like steel cement fertilizer chemicals and other small

scale industries account for the usage of rest of it. The total

recoverable coal reserves have been estimated to be only about

15.6% (43 billion tones) leaving about 250 billion tones

unminable. If India wants to be energy secure it is of extreme

importance to utilize our prime energy source to the fullest.

India, the third largest economy in terms of Purchase Power

Parity (PPP) with a net Gross Domestic Product (GDP) of 6776

Billion USD is growing at an average rate of 7.5 % since the

last 15 years [55]. The current total energy demand of India is

approximately 700 million tons of oil equivalents (MMTOE)

making it fourth largest consumer of energy of the world after

United States, China and Russia. About 65% of this energy

requirement is met indigenously which makes India one of the

largest energy importers of the world [56]. Thus the Indian

Govt. faces formidable challenge of coping with substantial

energy crisis and supply of affordable energy to the masses.

And Underground Coal gasification is the answer to India’s

energy problems as it has many advantages over conventional

mining. The relevance of this technology to Indian perspective

can be understood by comparing Indian coal to that already

used in test/plot plants globally.

A. Indian Coals:

India has a huge coal reserve but most of it is non-coking coal

comprising semi-bituminous and ignites (a brown variety of

tertiary Indian coal that has relatively less energy/heating vale).

Coal which when heated in the absence of air forms coherent

beads, free from volatiles, with a strong and porous mass called

coke, is called coking coal. Coals which do not have coking

properties, are non-coking coals. Moreover most of the minable

coal of India is concentrated in small patch of Eastern India

(Bihar, Jharkhand and West Bengal).

B. Coal Depth and Thickness:

The coal occurrence at various depth levels in India is

categorized in Table 9.

Table 9 Indian coal reserves at various depths (in million tonnes) [57]

Depth (m) Proved

Indicate

d Inferred Total

%

Total

reserve reserve reserve reserve

Reserv

e

0–300 54 627.35 54 242.51 20 519.91 129 389.77 62.74

300–600 18 929.82 25 694.76 17 384.94 62 009.52 30.07

600–1200 1560.58 9141.99 4137.64 14 840.21 7.19

Total 0–

1200 75 117.75 89 079.26 42 042.49 206 239.50 100

A total of 62.74% of the coal deposits lie at a depth of 0–300

m, 30.07% at 300–600 m and 7.19% coal is at a depth of 600–

1200 m.[58]. At many places like Madhya Pradesh, West

Bengal, Maharashtra and Assam both the coal seam depth(

>300m) and thickness( >2m) are suitable for UCG but still it

has been found that UCG pilot plants can be setup in Madhya

Pradesh and West Bengal only and not in Maharashtra and

Assam because of the less availability of coal in terms of

quantity.

C. Lignite Depth and Thickness:

Gujarat, Rajasthan and Tamil Nadu have huge lignite reserves that are suitable for Underground Coal Gasification. Gujarat has nearly 65 billion tonnes lignite at a depth of 700-1700m

and thickness varying from 5-15m. Going by the U.K. standards these characteristics are ideal for UCG pilot plants.

45

Chinchilla 40

Sasti-Rajura

35

30

25

20

15

10

5

0 Ash VM Moisture FC

Fig. 4. Comparison of properties of coals from Sasti-Rajura and Chinchilla.

D. Previous studies for UCG in India :

UCG studies were undertaken in the 1980s in India as

National Projects. Three regions were studied namely Mehsana in Gujarat (deeper lignite 500–1700 m), Merta Road in Rajasthan (shallow lignite 100–200 m) and Bihar (now Jharkhand) (bituminous coal). [59]

The Oil and Natural Gas Corporation of India found substantial coal reserves at depths greater than 600m while drilling for hydrocarbons. These coal seams are supposed to contain more than 350 billion tonnes of coal. Due to subsurface conditions like temperature, depth, geo hydrology, high gas content there can be no other method of exploitation of this reserve other than underground coal gasification. The quantum of energy generated from Gujarat blocks only is estimated to be 15000 billion cubic meters of natural gas equivalent. In addition to it the prime by-product of this process has the potential to be utilized as a stimulant for Enhanced Oil Recovery for oilfields in vicinity. The relevance of UCG in Indian perspective with particular reference to Gujarat reserves of about 65 billion tonnes was first considered in January, 1981. It was decided that setting up UCG facility at Kalol, Gujarat as worthwhile and seeking technical help from foreign countries was also considered. After a gap of 20 years ONGC India has again undertaken site selection for UCG pilot studies with technical support from SIM Russia [59]. Five coal blocks were studied out of which four were rejected due to hydrological reasons, basalt envelope around the reserves and discontinuous blocks. National thermal power corporation (NTPC), India, presented the cost estimation study of an UCG–IGCC power plant at a workshop held at Kolkata, India in 2006 [60]. The conclusion of the study that a standard UCG power plant

with Integrated Gasification combined cycle technology

would be about Rs. 640 crore and the cost of energy would be

Rs. 3.6 kW/hr., but this cost be can be cut down apparently by

applying UCG on large and good quality coal reserves.

E. India UCG policy:

There is no dedicated policy for Underground Coal

Gasification in India as of now but government approach is

such that we are certainly going to witness set of new rules

and guidelines regarding this vital energy generation method.

The present policies do not allow Underground coal

gasification as the end use of coal blocks allotted to various

companies. The Ministry of Coal would amend the Coal

Mines (Nationalization) Act of 1976. After sorting out all the

technology and related issues by the interested industry, coal

blocks can be allotted based on the data available with the

Coal Mining and Planning Development Institute after

notification by the ministry. [61].

The draft coal vision 2025 encompasses underground coal

gasification as an integral part of Indian energy programme

Though development of UCG is at initial stage, there has to be

a provision to discuss public perception based issues in later

stages and to educate the general masses about the benefits

and requirement of this resurrected technology. It should be

noted that a detailed study conducted in the UK for public

perception issues indicated the importance of the local public

opinion for such projects [2].

The time is right for Indian scientific community to emanate

extensive scientific research and experiments, and mathematic

modelling and simulations of Indian coal along with site

specific field trials.

IV. ADVANTAGES OF UCG

UCG offers significant advantages both in terms of ecological

and economical over traditional coal mining and gasification

methods, which is indeed of utmost importance for a rapid

developing country like India.

A. Ecological Benefits:

UCG permits lower emissions and air pollution

because gasification in UCG is underground thus it

reduces environment management costs.

Suspended Particulate Matter (SPM) is generated at

half the rate of surface equivalent technologies and

most of the particulates generated are confined to

subsurface.

There is a substantial decrease in noise and visual

impact on the surface due to this process.

Since UCG is applied on deeper coal seams and

according to DTI-UK standard a vertical separation

of 100m or above is to be maintained between nearest

aquifer and UCG rock bed reactor site, hence there is

a low risk of groundwater contamination.

Methane (naturally occurring over coal beds) is

recovered in this process, unlike the conventional

coal technologies which helps in reduced Greenhouse

Gas emissions.

Amount of water required in this technology is very

less in comparison to other coal technologies as water

is used as a reactant only and there is no need of coal

washing in this process.

B. Economical Benefits:

Unlike conventional gasification facilities there is no

need to procure surface gasifiers or build ash and slag

management facilities hence UCG aids in reduction

of Capital Expenditure.

Storage, transportation and preparation of coal is not

required in this technology which helps in reduction

of operating costs.

Since it is a clean technology environment

management costs are saved.

There is no risk of supply disruption, providing clear

advantages in secure fuel supplies.

Syngas can be piped directly to the end user,

reducing the need for rail/road infrastructure and the

expenditure associated thereof.

Additional power generation infrastructure is not

required as the produced syngas can be directly

utilized by conventional gas based thermal power

plants.

C. Collateral Benefits:

1. UCG-CCS Integration: CBM development

If the greenhouse gas (GHG) emission reductions sought by

government and world climate bodies are to be met, then

modern technical solutions are to be employyed. Moost

scientific reviews promote CO2 capture and storage(CCS) as

the most promising technology to lower fossil fuel

emissions.UCG already incorporates many advantages over

traditional coal extraction methods making it cleaner;

combining UCG and CCS wouuld offer even greater reduction

in emissions.

UCG in combination with CCS is recognized as potential

route to carbon abatement from coal. Report suggests that

UCG in combination with CO2 injection into adjacent coal

seams to enhance Coal Bed Methane (CBM) is a potentially

attractive option. Recent tests in Australa have shown that

considerable quantities of Methane can be extracted using

UCG and at a lower cost than CBM.

2. UCG-CCS Integration: Enhanced Oil Recovery

CO2 generated from UCG operations can be sequestrated into

mature oilfield which will help in optimizing crude oil

production from ageing fields. Injecting carbon dioxide

creates pressure in reservoir which iincreases the drive force

directing oil trough the production tubing to rise upto the

surface. CO2 increases the sweep efficiency of the productiion

operation and hence is on of the most suitable stimulant for

enhanced oil recovery. It will be dual benefit as problem of

waste/emission management will be resolved and increased oil

production would strengthen the nation by pushing it step

closer to energy security.

V. CHALLENGES AND MITIGATION

1. Groundwater Contamination: Though there are rules

and guidelines regarding the suitability of coal seams

for UCG, sometimes there is a risk of groundwater

contamination. Such risks and challenges are

mitigated by applying negative hydraulic gradient.

This principle is still is a concept and research is

going on[1].

A model of the action of negative hydraulic gradient.[1].

2. Ground Subsidence: Another challenge in

development of UCG as the pioneer energy

generation technology is ground subsidence. Though

subsidence is a phenomenon that is common with all

forms of mineral and coal extraction industry, current

knowledge and state of the art practices can eliminate

these risks. The primary control is depth

standardization. Ground subsidence risks are reduced

manifold when the coal seams to be gasified in-situ

are at depths greater than 200m. Subsidence can

further be reduced by using alternative UCG designs

with narrow cavities because roof collapses only

when the reactor cavity widens more than what is

estimated.

VI. CONCLUSION AND FUTURE PROSPECTS

UCG no doubt is the most suitable technology present today

for utilization of almost all inaccessible and unable coal

reserves. It is an old technology and has been proven to come

good on all what is expected of it. The only thing is evolve

this to create a new power generation mechanism which will

create an energy self-sufficient India. The government must

take steps to recognize this as potential pathway to energy

security and take appropriate measures which include making

robust UGC policy, attract corporate sector for investment into

the concerned energy sector. The technology is promising and

has enough scope in future. A ready testimonial to this

technology exists in the name of underground coal seam fires.

Underground coal fires are common all over world. Most of

them are severe and out of control. One such fire has burned in

the Jharia coal field of India for nearly a century. Leave aside

the amount of coal that has already burnt, reports suggest that

is if the burning continues at the present rate it will continue

for more than centuries to come. The need of the hour is to

find or discover some or the other way/technology that can

help in utilization of this burning coal. Underground Coal

Gasification is the first step and the foundation towards

development of required technology. There is a requirement to

utilize the heat of burning coal to carry on the chemical

reaction to gasify coal by limiting the heat to the required

value.

VII. REFERENCES

[1] Bhutto AW,Bazmi AA, Zahedi G.Underground Coal Gasification:from Fuundamental to

Applications.2013.Progress in Energy and Combustion Science.Elsevier

[2] Shackley S, Mander S, Reiche A. Public perceptions of underground coal

gasification in the United Kingdom. Energy Policy 2006;34(18):3423E33

[3] Perkins G, Sahajwalla V. A mathematical model for the chemical reaction of a semi-

infinite block of coal in underground coal gasification. Energy & Fuels 2005;19(4):1679E92. http://dx.doi.org/10.1021/ef0496808.

[4] Biezen ENJ, Bruining J. An integrated 3D model for underground coal gasi-fication. SPE Annual Technical Conference and Exhibition, 22E25 October 1995; Dallas, Texas: Society of Petroleum Engineers; 1995. [5] Blinderman MS, Saulov DN, Klimenko AY. Forward and reverse combustion linking in underground coal gasification. Energy 2008;33(3):446E54. [6] Roddy DJ, Younger PL. Underground coal gasification with CCS: a pathway to decarbonising industry. Energy & Environmental Science 2010;3(4):400E7 [7] Yang L, Zhang X, Liu S, Yu L, Zhang W. Field test of large-scale hydrogen manufacturing from underground coal gasification (UCG). International Journal of Hydrogen Energy 2008;33(4):1275E85.

[8] Hill RW, Shannon MJ. The controlled retracting injection point (CRIP) system: a

modified stream method for in situ coal gasification. Berkeley, CA: Lawrence Livermore National Laboratory (LLNL) Report; 1981. Contract No.: UCRL-85852 [9] Thorsness CB, Hill RW, Britten JA. Execution and performance of the CRIP process during the rocky mountain I UCG field test. Berkeley, CA: Lawrence Livermore National Laboratory (LLNL) Report; 1988. Contract No.: UCRL-98641. [10] Perkins G. Mathematical modelling of underground coal gasification. The University of New South Wales; 2005 [11] Haggin J. Key tests set for underground coal gasification. Chemical & Engi-neering News 1983;61(29):15E9. http://dx.doi.org/10.1021/cen-v061n029. p015 [12] Daggupati S, Mandapati RN, Mahajani SM, Ganesh A, Mathur DK, Sharma RK, et al. Laboratory studies on combustion cavity growth in lignite coal blocks in the context of underground coal gasification. Energy 2010;35(6):2374E86 [13] Yang LH, Yu L, Liang J. In: The coal gasification in the discarded mines and comprehensive use of its product gas, vol. 16. Min World; 1995. p. 20. [14] Yang L. The dynamic temperature field of two-stage underground coal gasification (UCG). Energy Sources, Part A: Recovery, Utilization, and Environmental Effects 2006;28(7):667E80. http://dx.doi.org/10.1080/ 009083190951438. [15] Guo C. In: Review of mathematical simulation study on the underground coal gasification, vol. 15. Mining World; 1994. p. 3E5 [16] Daily W, Lytle J. Geophysical tomography. Journal of Geomagnetism and Geoelectricity 1983;35(11E12):423E42 [17] Stanczyk K, Kapusta K, Wiatowski M, Swiadrowski J, Smolinski A, Rogut J, et al. Experimental simulation of hard coal underground gasification for hydrogen production. Fuel 2012;91(1):40E50. [18] Yang L, Liang J, Yu L. Clean coal technologyDstudy on the pilot project experiment of underground coal gasification. Energy 2003;28(14):1445E60. [19] Wu RY. Coal gasification. Xuzhou: China University of Mining and Tech-nology Press; 1988. p. 68E73. [20] Wang H, Dlugogorski BZ, Kennedy EM. Coal oxidation at low temperatures: oxygen consumption, oxidation products, reaction mechanism and kinetic modelling. Progress in Energy and Combustion Science 2003;29(6):487E513. [21] Yang LH, Song DY. Study on the method of seepage combustion in under-ground coal gasification. Xuzhou, China: China University of Mining and Technology Press; 2001. [22] Yang LH. A review of the factors influencing the physicochemical charac-teristics of underground coal gasification. Energy Sources, Part A: Recovery, Utilization, and Environmental Effects 2008;30(11):1038E49. http://dx.doi. org/10.1080/15567030601082803. [23] Couch G. Underground coal gasification. London, United Kingdom: IEA Clean Coal Centre; 2009. 01/07/2009. Report No.: Contract No.: CCC/151. [24] Campbell JH. Pyrolysis of subbituminous coal as it relates to in situ gasifi-cation. Part 1. Gas evolution. [383 to 1273/sup 0/K in argon]. NTISLLNL; 1976. UCRL-52035(Pt.1) United StatesThu Mar 24 09:14:52 EDT 2011Dep, ERA-01-018449; EDB-76-051202English [25] Merrick D. Mathematical models of the thermal decomposition of coal: 1. The evolution of volatile matter. Fuel 1983;62(5):534E9 [26] Seifi M, Chen Z, Abedi J. Numerical simulation of underground coal gasifi-cation using the CRIP method. The Canadian Journal of Chemical Engineering 2011. http://dx.doi.org/10.1002/cjce.20496. [27] Lanhe Y. Numerical simulation on three-dimensional nonlinear and unstable seepage of fluid in underground coal gasification. Fuel Processing Technology 2003;84(1E3):79E93. [28] Yang L. Model and calculation of dry distillation gas movement in the process of underground coal gasification. Numerical Heat Transfer, Part B: Fundamentals 2003;43(6):587E604. http://dx.doi.org/10.1080/713836314. [29] Yang L. Thermophysical models of underground coal gasification and FEM analysis. Numerical Heat Transfer, Part B: Fundamentals 2007;52(5):449E70. http://dx.doi.org/10.1080/10407790701443883. [30] Huang T. Study on the coupling effect of seepage-stress-temperature of the cracked rock. Chinese Journal of Rock Mechanics and Engineering 2002;21:77 [31] Guo KL, Kong XQ, Chen CN. Calculation heat transfer. Hefei: China University of

Science and Technology Press; 1988. p. 84. [32] Zheng SH, Zhu WS. Theoretical analysis of the coupling effect of seepage damages of the cracked rock. Chinese Journal of Rock Mechanics and Engi-neering 2001;20:156 [33] Perkins G, Sahajwalla V. A numerical study of the effects of operating conditions and coal properties on cavity growth in underground coal gasi-fication. Energy & Fuels 2006;20(2):596E608. http://dx.doi.org/10.1021/ ef050242q. [34] Perkins G, Sahajwalla V. Steady-State model for estimating gas production from underground coal gasification. Energy & Fuels 2008;22(6):3902E14. http://dx.doi.org/10.1021/ef8001444. [35] Lanhe Y. Study on the model experiment and numerical simulation for underground coal gasification. Fuel 2004;83(4E5):573E84. [36] Burton E, Friedmann J, Upadhye R. Best practices in underground coal gasification. Livermore, CA: Lawrence Livermore National Laboratory; 2006. Contract W-7405-Eng-48 Contract No.: Contract W-7405-Eng-48. [37] EliotMA. Coalutilizationchemistry. BeijingChemicalIndustry Press; 1991. p. 283. [38] Yang LH, Song QY, Li YJ. Project of UCG. Xuzhou: China University of Mining and Technology Press; 2001. p. 116E23. [39] Yang LH, Zhang X, Liu S. Underground coal gasification using oxygen and steam. Energy Sources, Part A: Recovery, Utilization, and Environmental Effects 2009;31(20):1883E92. http://dx.doi.org/10.1080/ 15567030802462531. [40] Yang LH, Zhang X, Liu S. Characteristics of temperature field during the oxygen-enriched underground coal gasification in steep seams. Energy Sources, Part A: Recovery, Utilization, and Environmental Effects 2009;32(4): 384E93. http://dx.doi.org/10.1080/15567030802611897. [41] Saulov DN, Plumb OA, Klimenko AY. Flame propagation in a gasification channel. Energy 2010;35(3):1264E73. [42] Wall TF, Liu G-S, Wu H-W, Roberts DG, Benfell KE, Gupta S, et al. The effects of pressure on coal reactions during pulverised coal combustion and gasifi-cation. Progress in Energy and Combustion Science 2002;28(5):405E33. [43] Yang LH, Zhang X, Liu SQ. The temperature features of the thermal-explosion oxygen-steam underground coal gasification in approximately horizontal coal seams.

Energy Sources, Part A: Recovery, Utilization, and Environmental Effects 2011;33(16):1483E92. http://dx.doi.org/10.1080/15567030903397933.

[44] King GR, Ertekin TM. Review of methane-related mathematical models (part one):

experiment and the model on balanced absorption. Coal Layer Gas 2000;25:13.

[45] Chen L, Hou C, Chen J, Xu J. A back analysis of the temperature field in the

combustion volume space during underground coal gasification. Mining Science and Technology (China) 2011;21(4):581E5

[46] Yang L. Three-Dimensional unstable non-linear numerical analysis of the underground coal gasification with free channel. Energy Sources, Part A: Recovery,

Utilization, and Environmental Effects 2006;28(16):1519E31. http://dx.doi.org/10.1080/009083190929785.

[47] Creedy DP, Garner K, Holloway S, Jones N, Ren TX. Review of underground coal gasification technological advancements. UK DTI Cleaner Coal Tech-nology Transfer Programme; 2001. COAL R211/Pub URN 01/1041 Contract No.: COAL R211/Pub URN 01/1041

[48] Lanhe Y. Three-dimensional non-linear numerical analysis on the oxygen concentration field in underground coal gasification. Fuel Processing Tech-nology

2004;85(15):1605E22.

[49] Solcová O, Soukup K, Rogut J, Stanczyk K, Schneider P. Gas transport through

porous strata from underground reaction source; the influence of the gas kind, temperature and transport-pore size. Fuel Processing Technology

2009;90(12):1495E501

[50] Gunn RD, Krantz WB. Underground coal gasification: development of theory,

laboratory experimentation, interpretation, & correlation with the hanna field tests. Wvoming 82071. University of Wyammg; 1987. DE$AS20E 80LC10442

[51] Yang L. Coal properties and system operating parameters for underground coal

gasification. Energy Sources, Part A: Recovery, Utilization, and Environmental Effects

2008;30(6):516E28. http://dx.doi.org/10.1080/15567030600817142.

[52] Wang GX, Wang ZT, Feng B, Rudolph V, Jiao JL. Semi-industrial tests on enhanced underground coal gasification at Zhong-Liang-Shan coal mine. Asia-Pacific

Journal of Chemical Engineering 2009;4(5):771E9. http://dx.doi. org/10.1002/apj.337.

[53] Nitao JJ, Camp DW, Buscheck TA, White JA, Burton GC, Wagoner JL, et al.

Progress on a new integrated 3-D UCG simulator and its Initial Application. International Pittsburgh Coal Conference; Pittsburgh, PA, United States; 2011.

[54] U.S. Energy Administration Report-India.2013.

[55] IMF: International Monetary Fund. 2013.

[56] IEA: International Energy Agency, Mckinsey Analysis Report, 2013

[57] Acharyya SK. Coal and lignite resources of India—an overview. Geological Society of

India, 2000.

[58] : Chandra D, Singh RM. Text book of coal: Indian context. Tara Book Agency India, 2000

[59] Conference paper. Underground coal gasification and underground coal mining for bright

and secure future. In: National seminar, New Delhi, 30–31 January, 2006

[60] NTPC (National Thermal Power Corporation). Economics of power generation with UCG.

UGC Workshop at Kolkata, India, November 2006. [61] Chemical Weekly. Coal blocks allotment for underground gasification to be notified soon, 20 February 2007. P. 144