ucg explained

8/12/2019 UCG Explained

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This information sheet provides a technical description of the Underground Coal

Gasification (UCG) process, the key parameters involved, factors in site selectionand the operational influences on gas quality.

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UCG SERIES

GTL SERIES

ENVIRONMENT SERIES

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UCG Explained02 UCG Series

The UCG process

UCG is the process of the gasification

of coal in-situ to produce a synthesis

gas (syngas). The operating life of a

UCG operation can be broadly broken

down into four steps:

1. Well construction and linkage:

Wells are drilled into the coal to

allow for oxidant injection and

product gas extraction. The wells

are linked or extended to form an

in-seam channel to facilitate oxidant

injection, cavity development and

syngas flow.

2. Ignition: The coal seam isdried and then ignited.

3. Gas production: Syngas is produced

through combustion and gasification

reactions. Combustion produces

heat, carbon dioxide and some

syngas (through partial combustion).

Gasification reactions then take

place, involving heat and carbon

dioxide from combustion, pressure,

steam and carbon from the coal.

The syngas flows from the

gasification zone, through

constructed or formed horizontalchannels, to the gas production

well where it flows to the surface

for treatment.

4. Decommissioning: Once all the

available coal has been extracted

as a gaseous product, the

gasification process is shut

down according to known

and demonstrated shut

down procedures.

Overview of the UCG Process

Figure 1: Overview of the UCG process


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Well construction

and linkage

Wells are constructed into the coal

seam. Construction varies depending

on whether a well is used in service as a

production well. In this case, it must be

constructed to withstand hot gases and

the effects of heating and cooling.

Linking the wells is necessary to ensure

a flow path between the injection and

production wells. Linking assists the

development of the cavity and the

collection of product gas.

Linkage can be achieved in a

number of ways:

1. In-seam directional drilling:

This involves developing a horizontal

drill hole in the coal seam betweenthe two wells.

2. Artificial fracturing:

This involves pressurising the

coal, by using either air or water, to

crack the coal between the wells.

3. Reverse combustion:

This involves igniting the

seam and forming a linkage by

combusting a channel between

wells. The flow is then reversed

and gasification commences.

Linc Energy has trialled various

methods and is progressing with

in-seam directional drilling as the

preferred method for achieving linkage.

Figure 4: Cavity Growth and Operating Temperature (standard two well

configuration) (Linc Energy 2009)

Figure 3: Gas Velocity Profile (Linc Energy 2009)

Figure 2:Directional Drilling (in-seam)

Ignition

Once the wells have been linked, the coal seam is partially dried. This is done by

blowing air through the injection well until the location of ignition is sufficiently dry.

The coal is then ignited, using any one of a variety of ignition methods.

In Seam Drill Head


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Gasification and gas production

Following ignition, oxidants are injected and the

conversion of coal through gasification occurs by:

1. Oxidation and /or combustion reactions

2. Reduction

3. Pyrolysis, producing gas,

oils, char and vaporised tars.

Air (21 per cent oxygen), oxygen enriched air or pure

oxygen can be used as the oxidant in the process.

Using pure oxygen (or oxygen enriched air) results in a

higher temperature gasification reaction. The result isdifferent production gas composition and volumes. The

differences mainly relate to nitrogen, which is injected as

an inert component when air is the oxidant of choice. The

oxidant chosen will depend on economic considerations,

including the end use of the gas. During the UCG process,

exothermic (releasing heat) combustion reactions supply

the energy required by endothermic (absorbing heat)

reduction reactions.

The UCG process can be roughly divided into zones, with

the oxidation or reduction zone near the oxidant injection

point. This is followed by a gasification zone and pyrolysis

zone where the coal is exposed to temperature as a result

of radiant heat and hot gases passing over the coal.

Combustion (oxidation) stage reactions

Combustion of the coal generates heat (i.e. an exothermic

reaction) and other gases which are utilised in reactions

which occur in later stages. The combustion reactions are:

C + O CO+ heat (complete combustion)

C + O CO + heat (partial combustion)

CO + OCO

+ heat

The gasification process will progressively consume the

coal and create a cavity. The cavity will over time expandin the direction of the flow of gases, namely towards

the production well. The lateral extent of combustion is

controlled by the quenching associated with inward flowing

groundwater.

The rate at which groundwater flows into the process is

governed by many factors, the main one being operating

pressure.

Reduction stage reactions

After the oxygen in the process is utilised during the

combustion stage and reducing conditions prevail, then

reduction reactions take place utilising the heat

from the combustion stage. These reactions include:

C + HO + heat H+ CO

C + CO+ heat 2CO

These reactions are heterogenous, meaning they are gas/

solid reactions (gas and coal reactions).

As the gas progresses through the process, homogenous

reactions (gas phase only) take place until the gas reaches

its equilibrium composition. Water vapour present in the

process promotes the water-gas-shift (WGS) reactionthat contributes significantly to the H/CO balance. Key

chemical reactions during this stage include:

CO + HO H+ CO (WGS reaction)

CO + 3HCH4+ HO (methanation reaction)

The equilibrium gas composition is dictated by

temperature, pressure, the amount of water vapour present,

and the composition of the gas, once the heterogenous

reactions are complete. As the gaseous product has

enough residence time to reach equilibrium in the generator,

it is fairly easy to predict the composition of the producedgas given a specific set of operating conditions.

Pyrolysis

As the coal loses its moisture it undergoes pyrolysis

(thermal decomposition) at temperatures close to 400C:

Coal CH4+ HO + Hydrocarbons + Tars + Volatile gases

Depending on the temperature of the process,

hydrocarbons and tars will either be consumed in the

process or be entrained in product gas where they

condense and are separated at the surface and can be

reused or reprocessed into valuable by-products.Figure 5:Linc Energy 3D Cavity Growth Model


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ortant disclaimer:Information contained in this information sheet is provided for information only and Li nc Energy makes no warranties as to its accuracy and completeness. Use of information contained in this information sheet is at the sole risk

e user. Linc Energy has made reasonable efforts to ensure that information in this i nformation sheet is accurate at the time of its compilation, however there may be inadvertent errors or omissions for which Linc Energy apologises. To the extent

mitted by law, Linc Energy accepts no responsibility for any loss, damage, cost or expense whatsoever incurred by any person as a result of any use of or error or omission in or relating to, the information contained in this information sheet.


Related information sheets

UCG, GTL and the Environment

Modern Practices in UCG

About Linc Energy

Linc Energy is a globally focused,

diversified energy company with astrong portfolio of coal, oil and gas

deposits. Linc Energys purpose is

to unlock the value of its resources

to produce energy to fuel the future.

A public company, Linc Energy is

the global leader in UCG, delivering

synthesis gas for commercially viable

energy solutions (electricity, transport

fuels and oil production), via gas turbine

combined cycle power generation,

Gas to Liquids processing andEnhanced Oil Recovery.

Syngas composition

The composition of syngas produced

will ultimately dictate what the gas can

be used for.

Calorific value will be important for

power generation and the H/CO

ratio will be relevant for chemical or

petrochemical applications.

Syngas will contain differing

proportions of CO, H, CO, N

, CH4,

water and gaseous hydrocarbons,

depending on various factors,

including:

1. The oxidant used: Due to the

presence of nitrogen, air will result in

lower gasification temperatures and

more inert gas dilution. The decision

about whether to use oxygen or

air as the oxidant is ultimately a

financial one.

2. Water influences: The rate at which

groundwater (or introduced water)

contributes to the gasification

process ultimately dictates

the hydrogen concentration inthe gas. This is influenced by

coal permeability, overburden

permeability, natural or induced

fracturing, coal moisture, hydrostatic

pressure, and operating pressure of

the cavity.

3. Coal quality (meaning reactivity, ash

content and structural properties):

The ideal coals for UCG shrink and

fall apart when heated. The break

into smaller particles provides a

larger surface area for reactions totake place. This includes most of the

lower rank coals.

4. Operating temperature and

pressure: With increasing pressure,

more methane and COis produced,

while the yield of Hand CO drops.

There are however, efficiency and

economic advantages to operating

gasification at high pressure.

Site selection

The main factors to consider for the

selection of a UCG site are:

Coal properties: Chemical nature,

structure, depth and thickness

Hydrogeology: Groundwater plays

an integral part of the UCG process

because it supplies water for the

gasification reactions, and the

hydrostatic pressure serves to contain

the process. Operating the process

below the hydrostatic pressure ensures

there is movement of water towards

the cavity, as well as movement of gastowards the production well

Geology: Good structure and low

permeability of rock immediately

overlying the coal is favourable to

limit subsidence and provide a seal

between the coal and overlying strata.

Decommissioning

Shutting down the gasification process

and ensuring the spent gasification

chamber does not contribute to

groundwater contamination is a criticalpart in the lifecycle of a UCG operation.

Decommissioning a UCG site involves a

number of key principles:

While the process is still hot, allow

groundwater to flow into the

cavity to generate steam.

This ensures any residual tars

or liquid hydrocarbons that may

have condensed on the walls are

remobilised as gas and flow

through wells to the surface fortreatment or use

The groundwater inflows

quench the process

The cavity is pumped out and flushed

until the water is clean (usually once

or twice).

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