Addressing the threat posed by climate change represents one of

the most pressing challenges

facing humanity.

Carbon exists as carbon dioxide in the atmosphere and constitutes

about 0.04% of the atmosphere. In the recent past, it has gained

a lot of attention as a greenhouse gas, as it has potential to

influence the climate pattern of the world.

Anthropogenic activities like industrialization, deforestation,

forest degradation and burning of fossil fuel, has caused an

increase in the level of carbon in the atmosphere and disrupted

the global carbon cycle.

With all the carbon dioxide pumped into the atmosphere from the

various human activities, the planet would have been overheated

rapidly if not for the nature’s mechanism of sequestering the

carbon from the atmosphere and storing it in its reservoirs like

the oceans, forests and soils.

The nature’s way of sequestering carbon from the atmosphere is a

process of achieving balance of carbon dioxide levels in

atmosphere and maintaining the global carbon cycle, and this

cycle has been happening billions of years. However, humans have

greatly disturbed this balance with various activities like

combustion of fossil fuels and change in land-use patterns such

as deforestation. About 60% of the observed global climate change

is attributable to this increasing carbon dioxide concentration

in the atmosphere (Kuimi T. Vashum and S. Jayakumar).

Efforts to control the climate change mainly focus on emission

controls and on removal of carbon dioxide from the atmosphere.

Carbon dioxide is believed to be the most important anthropogenic

greenhouse gas causing global warming and climate change (IPCC

2007). Most attention has been focused on CO as it is believed to

contribute more than half of the increase in the global

temperature in the next 100 years.

The most important global instrument linked to mitigating adverse

impacts of carbon dioxide and other greenhouse gases is the Kyoto

Protocol to the United Nations Framework Convention on Climate

Change, which entered into force in February 2005. The protocol

is a legally binding international agreement that commits

industrialized countries to reduce emissions of greenhouse gases

by 2008-2012 and promote sustainable development (Sardu Bajracharya,

2008).

The Global Carbon Cycle

Paula Nicole Butcher et al stated that there are 750 Giga ton of

Carbon in the atmosphere, the majority of which is in the form of

carbon dioxide (CO2), a slightly smaller amount (560 Gt) is

contained in the living biomass of the terrestrial biosphere.

The soils of the earth are estimated to contain between 1,400 and

1,700 Gt of Carbon. The C in terrestrial ecosystems is in a

reduced organic form. Organic Carbon (representing 40-50% of

organic material) exists in many different forms, and has turn-

over times of less than a year to over 1,000 years (Schlesinger,

1997). Most terrestrial Carbon is stored in the vegetation and

soils of forests. 99.9% of the Carbon present in the world's

biota is represented by vegetation; animals are a negligible

Carbon reservoir. Fossil fuel reserves such as coal, oil and gas

are estimated to contain about 5,000 Gt of Carbon, representing

the second largest reservoir on earth. Dissolved inorganic

Carbon in the ocean is the largest near-surface pool (38,000 Gt),

and is more than 50 times greater than the atmospheric C pool.

The oceans typically control the atmospheric C because

interchange between the two pools tends toward equilibrium.

However, the relatively slow rate of ocean circulation dictates

that equilibrium between the atmosphere and ocean may take

hundreds of years.

Figure 1.1. Annual C exchange (Gt) between the major global

reservoirs.

The following are the mitigating options suggested by Agrawala et

al., 1995

CARBON SEQUESTRATION

Carbon sequestration refers to the storage of carbon in a stable

solid form. It occurs through direct and indirect fixation of

atmospheric CO.

Direct soil carbon sequestration occurs by inorganic chemical

reactions that convert CO2 into soil inorganic carbon compounds

such as calcium and magnesium carbonates. Direct plant carbon

sequestration occurs as plants photosynthesize atmospheric CO2

into plant biomass.

Subsequently, some of this plant biomass is indirectly

sequestered as soil organic carbon (SOC) during decomposition

processes.

Carbon sequestration is simply the process of keeping CO2 out of

the atmosphere where, according to most scientific studies, it

contributes to the Greenhouse effect, which causes global

warming.

Carbon dioxide (CO2) capture and storage (CCS) is a process consisting of the

separation of CO2 from industrial and energy-related sources, transport to a storage

location and long-term isolation from the atmosphere.

The net reduction of emissions to the atmosphere through CCS depends on the fraction

of CO2 captured, the increased CO2 production resulting from loss in overall efficiency

of power plants or industrial processes due to the additional energy required for

capture, transport and storage, any leakage from transport and the fraction of CO2

retained in storage over the long term (IPCC 2005).

(source:http://www.uspowerpartners.org/Topics/SECTION5_HEAD-

CarbonSequestration_files/image002.jpg)

FORMS OF CARBON SEQUESTRATION

According to Jeff Daniels, there are at least three potential

means of sequestering CO2:

1. Oceanic Sequestration: Dumping the CO2 into the ocean depths.

2. Terrestrial Sequestration: Binding the CO2 in plants.

3. Geologic Sequestration: Burying the CO2 deep within the earth.

Oceanic Sequestration

Pumping CO2 into the deep ocean basins (350-3000 meters), where

it is anticipated it may form lakes of liquid, supercritical, or

solid hydrates. The thinking on this disposal scenario is that it

would stabilize in the ocean depths, or slowly dissolve into the

ocean waters. This option has been under study for several years,

but there are many potential environmental downsides to its

implementation, and it is not a high priority research focus at

this time.

Overview of ocean storage concepts: In “dissolution type” oceanstorage, the CO2 rapidly dissolves in the ocean water whereas in“lake type” ocean storage, the CO2 is initially a liquid on thesea floor (Courtesy IPCC, 2005).

Ocean storage potentially could be done in two ways:

i. By injecting and dissolving CO2 into the water column

(typically below 1,000 meters) via a fixed pipeline or a

moving ship

ii. By depositing it via a fixed pipeline or an offshore

platform onto the sea floor at depths below 3,000 m, where

CO2 is denser than water and is expected to form a “lake”

that would delay dissolution of CO2 into the surrounding

environment (see Figure …..). Ocean storage and its

ecological impacts are still in the research phase.

.

TERRESTRIAL SEQUESTRATION

Terrestrial sequestration consists of storing CO2 in soils and

vegetation near the earth’s surface. Tree-plantings, no-till

farming, wetlands restoration, land management on grasslands and

grazing lands, fire management efforts, and forest preservation.

More advanced research includes the development of fast-growing

trees and grasses and deciphering the genomes of carbon-storing

soil microbes. NETL’s Program efforts in the area of terrestrial

sequestration include a focus on increasing carbon uptake on

mined lands and quantifying sequestration benefits of growing

biomass for power generation.

These activities complement research into afforestation and

agricultural practices.

Claudio O. Delang and Yu Yi Hang (2004) suggested that Slowing

deforestation, combined with an increase in forestation and other

management measures could improve forest ecosystem productivity,

which would conserve or sequester significant quantities of

carbon: forests store 57% more carbon per hectare than agro-

forests and 86% more than pastures.

Furthermore, slowing land-use changes and expanding forest areas

could conserve approximately 2.9 and 6.5 Pg of carbon per year,

respectively.

Also that, Soil is another very important carbon sink. The amount

of carbon in soil is a function of soil-forming factors,

including climate, relief, organisms, parent materials, and time.

Organic matter (OM) plays a critical role in storing carbon in

soils. Soil organic matter (SOM) is a mixture of animal and plant

residues (at any stage of decomposition), living and decaying

microbial tissue and heterotrophic biomass, and relatively

resistant humic substances. The SOM turnover time is relatively

long at a global average of 26 years.

Forest Carbon Reservoir

Forests form an integral part for scientific research as the

“forest carbon reservoir” has a dynamic relationship with the

climate system. Forests behave as a “carbon sink”, sequestering

atmospheric carbon into biomass. According to Kyoto protocol, one

of the mitigation strategies for reducing the greenhouse gases in

the atmosphere is increasing the terrestrial sink for CO2. The

uptake of CO by plants is referred to as gross primary

productivity (GPP). At the same time the forest also acts as

“carbon source” by releasing carbon into the atmosphere through

processes such as respiration. During respiration, half of the

GPP is respired with remainder referring to net primary

productivity (NPP), which is the total production of biomass

matter (Sardu Bajracharya, 2008).

Under Kyoto Protocol, forests are considered important for their

unique role as carbon sinks because they are capable of capturing

and storing carbon dioxide from the atmosphere. According to

Sardu Bajracharya (2008), each time there is a forest growth of 2

cubic meters of wood; roughly 1 ton of carbon of the air is

captured.

Forests act as carbon sink by increasing above ground biomass

through increased forest cover and by increased level of soil

organic carbon content.

Biomass is defined as “mass of all organic matter per unit area

at particular time (reported in g/m2 or kg/ha)”.

The above-ground biomass (AGB) is described by IPCC Guidelines

for National Gas Inventories (2006) as “all biomass of living

vegetation, both woody and herbaceous, above the soil including

stems, branches, bark, foliage, bark and stumps”. Forest biomass

represents the largest terrestrial carbon sink and accounts for

approximately 90% of all living terrestrial biomass (Dixon et al.

1994; Tan et al. 2007).

Geologic Sequestration

Geologic sequestration consists of capturing CO2 from stationary

sources, like a power

plant, and injecting it into the subsurface.

Three types of geological formations that have received extensive

consideration for the geological storage of CO2: oil and gas

reservoirs, deep saline formations and unminable coal beds.

Methods for storing CO2 in deep underground geologicalformations. Two methods may be combined with the recovery ofhydrocarbons: EOR (2) and ECBM (4). See text for explanation ofthese methods (IPCC Special Report on Carbon Dioxide Capture andStorage, 2005).

CARBON CAPTURE AND STORAGE (CCS) CAPTURE SYSTEM

There are different types of CO2 capture systems: post-

combustion, pre-combustion and oxy-fuel combustion (Figure ……..).

CO2 capture systems (adapted from BP, 2004: Statistical Review of World Energy. Http:\www.bp.com)

The concentration of CO 2 in the gas stream, the pressure of the

gas stream and the fuel type (solid or gas) are important factors

in selecting the capture system.

Post-combustion capture of CO2 in power plants is economically

feasible under specific conditions. It is used to capture CO2

from part of the flue gases from a number of existing power

plants. Separation of CO2 in the natural gas processing industry,

which uses similar technology,

operates in a mature market.

Pre-combustion capture

Pre-combustion capture involves reacting a fuel with oxygen or

air and/or steam to give mainly a ‘synthesis gas (syngas)’ or

‘fuel gas’ composed of carbon monoxide and hydrogen. The

carbon monoxide is reacted with steam in a catalytic reactor,

called a shift converter, to give CO2 and more hydrogen. CO2 is

then separated, usually by a physical or chemical absorption

process, resulting in a hydrogen-rich fuel which can be used in

many applications, such as boilers, furnaces, gas turbines,

engines and fuel cells.

The technology required for pre-combustion capture is widely

applied in fertilizer manufacturing and in hydrogen production.

Although the initial fuel conversion steps of pre-combustion are

more elaborate and costly, the higher concentrations of CO2 in

the gas stream and the higher pressure make the separation easier

(Kelly et al, 2005).

Oxy-fuel combustion is in the demonstration phase and uses high

purity oxygen. This results in high CO concentrations in the gas

stream and, hence, in easier separation of CO2 and in increased

energy requirements in the separation of oxygen from air

Schematic representation of capture systems: Fuels and productsare indicated for oxy-fuel combustion, pre-combustion (includinghydrogen and fertilizer production), post-combustion andindustrial sources of CO2 (including natural gas processingfacilities and steel and cement production) (source IPCC, 2005).

LOCAL HEALTH, SAFETY AND ENVIRONMENT RISKS OF CCS

The local risks associated with CO2 pipeline transport could be

similar to or lower than those posed by hydrocarbon pipelines

already in operation.

For existing CO2 pipelines, mostly in areas of low population

density, accident numbers reported per kilometer pipeline are

very low and are comparable to those for hydrocarbon pipelines. A

sudden and large release of CO would pose immediate dangers to

human life and health, if there were exposure to concentrations

of CO2 greater than 7–10% by volume in air. Pipeline transport of

CO through populated areas requires attention to route selection,

overpressure protection, leak detection and other design factors.

No major obstacles to pipeline design for CCS are foreseen.

With appropriate site selection based on available subsurface

information, a monitoring programme to detect problems, a

regulatory system and the appropriate use of remediation methods

to stop or control CO releases if they arise, the local health,

safety and environment risks of geological storage would be

comparable to the risks of current activities such as natural gas

storage, EOR and deep underground disposal of acid gas.

Natural CO2 reservoirs contribute to the understanding of the

behavior of CO2 underground. Features of storage sites with a low

probability of leakage include highly impermeable

caprocks, geological stability, absence of leakage paths and

effective trapping mechanisms.

There are two different types of leakage scenarios:

(1) abrupt leakage, through injection well failure or leakage up

an abandoned well, and

(2) gradual leakage, through undetected faults, fractures or

wells. Impacts of elevated CO2 concentrations in the shallow

subsurface could include lethal effects on plants and subsoil

animals and the contamination of groundwater. High fluxes in

conjunction with stable atmospheric conditions could lead to

local high CO2 concentrations in the air that could harm animals

or people. Pressure build-up caused by CO2 injection could

trigger small seismic events.

While there is limited experience with geological storage,

closely related industrial experience and scientific knowledge

could serve as a basis for appropriate risk management, including

remediation. The effectiveness of the available risk management

methods still needs to be demonstrated for use with CO2 storage.

If leakage occurs at a storage site, remediation to stop the

leakage could involve standard well repair techniques or the

interception and extraction of the CO2 before it would leak into

a shallow groundwater aquifer. Given the long timeframes

associated with geological storage of CO2, site monitoring may be

required for very long periods.

Adding CO to the ocean or forming pools of liquid CO2 on the

ocean floor at industrial scales will alter the local chemical

environment. Experiments have shown that sustained high

concentrations of CO2 would cause mortality of ocean organisms.

CO2 effects on marine organisms will have ecosystem consequences.

The chronic effects of direct CO2 injection into the ocean on

ecosystems over large ocean areas and longtime scales have not

yet been studied ().