Natural gas 1

X1

Natural gas
Wan Azelee Wan Abu Bakar and Rusmidah Ali
Department of Chemistry, Universiti Teknologi Malaysia,
Skudai, Johor, Malaysia

1. Introduction
This chapter contains a description of background of natural gas: what exactly natural gas
is?, how it is formed and how it is found in nature; history of natural gas: a brief history and
development of modern natural gas; resources: how much abundance, where to find and
what is the composition of natural gas; Uses: application and the important of energy
source; natural gas versus environment: emission from the combustion of natural gas;
natural gas technology: role of technology in the evolution of the natural gas industry;
Purification of crude natural gas: various technologies used to convert sour to sweet natural
gas; synthesis of artificial natural gas: methanation reaction.

2. Background of Natural Gas

A mixture of gaseous hydrocarbons occurring in reservoirs of porous rock (commonly sand
or sandstone) capped by impervious strata. It is often associated with petroleum, with
which it has a common origin in the decomposition of organic matter in sedimentary
deposits. Natural gas consists largely of methane (CH4) and ethane (C2H6), with also
propane (C3H8) and butane (C4H10)(separated for bottled gas), some higher alkanes (C5H12
and above) (used for gasoline), nitrogen (N2) , oxygen (O2), carbon dioxide (CO2), hydrogen
sulfide (H2S), and sometimes valuable helium (He). It is used as an industrial and domestic
fuel, and also to make carbon-black and chemical synthesis. Natural gas is transported by
large pipelines or (as a liquid) in refrigerated tankers. Natural gas is combustible mixture of
hydrocarbon gases, and when burned it gives off a great deal of energy. We require energy
constantly, to heat our homes, cook our food, and generate our electricity . Unlike other
fossil fuels, however, natural gas is clean burning and emits lower levels of potentially
harmful byproducts into the air. It is this need for energy that has elevated natural gas to
such a level of importance in our society, and in our lives.
Natural Gas is a vital component of the world's supply of energy. It is one of the cleanest,
safest, and most useful of all energy sources. Despite its importance, however, there are
many misconceptions about natural gas. For instance, the word 'gas' itself has a variety of
different uses, and meanings. When we fuel our car, we put 'gas' in it. However, the
gasoline that goes into your vehicle, while a fossil fuel itself, is very different from natural
gas. The 'gas' in the common barbecue is actually propane, which, while closely associated
and commonly found in natural gas, is not really natural gas itself. While commonly

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grouped in with other fossil fuels and sources of energy, there are many characteristics of
natural gas that make it unique. Below is a bit of background information about natural gas,
what exactly it is, how it is formed, and how it is found in nature

2.1 History of Natural Gas

Naturally occurring natural gas was discovered and identified in America as early as 1626,
when French explorers discovered natives igniting gases that were seeping into and around
Lake Erie. The American natural gas industry got its beginnings in this area. In 1859,
Colonel Edwin Drake (a former railroad conductor who adopted the title 'Colonel' to
impress the townspeople) dug the first well. Drake hit oil and natural gas at 69 feet below
the surface of the earth.

Fig. 1. A Reconstruction of ‘Colonel’ Drake’s First Well in Titusville, Pa (Source: API)

Most in the industry characterize this well (Fig.1) as the beginning of the natural gas
industry in America. A two-inch diameter pipeline was built, running 5 and ½ miles from
the well to the village of Titusville, Pennsylvania. The construction of this pipeline proved
that natural gas could be brought safely and relatively easy from its underground source to
be used for practical purposes.
In 1821, the first well specifically intended to obtain natural gas was dug in Fredonia, New
York, by William Hart. After noticing gas bubbles rising to the surface of a creek, Hart dug a
27 foot well to try and obtain a larger flow of gas to the surface. Hart is regarded by many as
the 'father of natural gas' in America. Expanding on Hart's work, the Fredonia Gas Light
Company was eventually formed, becoming the first American natural gas company.
In 1885, Robert Bunsen invented what is now known as the Bunsen burner (Fig.2). He
managed to create a device that mixed natural gas with air in the right proportions, creating
a flame that could be safely used for cooking and heating. The invention of the Bunsen
burner opened up new opportunities for the use of natural gas in America, and throughout

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the world. The invention of temperature-regulating thermostatic devices allowed for better
use of the heating potential of natural gas, allowing the temperature of the flame to be
adjusted and monitored.

Fig. 2. A Typical Bunsen Burner (Source:DOE)

Without any way to transport it effectively, natural gas discovered pre-world war II was
usually just allowed to vent into the atmosphere, or burnt, when found alongside coal and
oil, or simply left in the ground when found alone.
One of the first lengthy pipelines was constructed in 1891. This pipeline was 120 miles long,
and carried natural gas from wells in central Indiana to the city of Chicago. However, this
early pipeline was very rudimentary, and did not transport natural gas efficiently. It wasn't
until the 1920's that any significant effort was put into building a pipeline infrastructure.
After World War II welding techniques, pipe rolling, and metallurgical advances allowed
for the construction of reliable pipelines. This led to a post-war pipeline construction boom
lasting well into the 60's, creating thousands of miles of pipeline in America.
Once the transportation of natural gas was possible, new uses for natural gas were
discovered. These included using natural gas to heat homes and operate appliances such as
water heaters and oven ranges. Industry began to use natural gas in manufacturing and
processing plants. Also, natural gas was used to heat boilers used to generate electricity. The
transportation infrastructure made natural gas easier to obtain, and as a result expanded its
uses.

2.2 How Natural Gas is Formed

Millions of years ago, the remains of plants and animals decayed and built up in thick
layers. This decayed matter from plants and animals is called organic material –a compound
that capable of decay or sometime refers as a compound consists mainly carbon. Over time,

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the mud and soil changed to rock, covered the organic material and trapped it beneath the
rock. Pressure and heat changed some of this organic material into coal, some into oil
(petroleum), and some into natural gas – tiny bubbles of odorless gas. The main ingredient
in natural gas is methane, a gas (or compound) composed of one carbon atom and four
hydrogen atoms, CH4 . It is colorless, shapeless, and odorless in its pure form.
In some places, gas escapes from small gaps in the microscopic plants and animals living in
the ocean rocks into the air; then, if there is enough activation energy from lightning or a
fire, it burns. When people first saw the flames, they experimented with them and learned
they could use them for heat and light. The formation of natural gas can be explained
starting with microscopic plants and animals living in the ocean.
The process began in amillions of years ago, when microscopic plants and animals living in
the ocean absorbed energy from the sun, which was stored as carbon molecules in their
bodies. When they died, they sank to the bottom of the sea. Over millions of years, layer
after layer of sediment and other plants and bacteria were formed.
As they became buried ever deeper, heat and pressure began to rise. The amount of pressure
and the degree of heat, along with the type of biomass (biological materials derived from
living organisms), determined if the material became oil or natural gas. More heat produced
lighter oil. At higher heat or biomass made predominantly of plant material produced
natural gas.
After oil and natural gas were formed, they tended to migrate through tiny pores in the
surrounding rock. Some oil and natural gas migrated all the way to the surface and escaped.
Other oil and natural gas deposits migrated until they were caught under impermeable
layers of rock or clay where they were trapped. These trapped deposits are where we find
oil and natural gas wells today where drilling process was conducted to obtain the gas.
In a modern technology, machines called "digesters" is used to turn today's organic material
(plants, animal wastes, etc.) into synthetic natural gas (SNG). This replaces waiting for
thousands of years for the gas to form naturally and could overcome the depletion of
natural resources. The conventional route for SNG production is based on gasification of
biomass to produce synthesis gas and then the subsequent methanation of the synthesis gas
turn it to synthesis natural gas. Woody biomass contain 49.0% carbon and 5.7% hydrogen
that can be converted to 76.8% methane, CH4.

2.3 How Natural Gas is Obtained

Now imagine how to obtain the invisible treasure? That's the challenge face by geologist
when exploring for natural gas. Sometimes there are clues on the earth's surface. An oil
seeps is a possible sign of natural gas below, since oil and gas are sometimes found together.
Geologists also have sensitive machines that can "sniff" surface soil and air for small
amounts of natural gas that may have leaked from below ground.
The search for natural gas begins with geologists who locate the types of rock that are
known to contain gas and oil deposits. Today their tools include seismic surveys that are
used to find the right places to drill wells. Seismic surveys use echoes from a vibration
source at the Earth's surface (usually a vibrating pad under a truck built for this purpose) to
collect information about the rocks beneath. They send sound waves into the ground and
measure how fast the waves bounce back. This tells them how hard and how thick the
different rock layers are underground. The data is fed into a computer, which draws a

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picture of the rock layers. This picture is called a seismogram. Sometimes, it is necessary to
use small amounts of dynamite to provide the vibration that is needed.
The next task are taken by scientists and engineers who explore a chosen area by studying
rock samples from the earth and taking measurements. If the site seems promising, drilling
begins. Some of these areas are on land but many are offshore, deep in the ocean. Once the
gas is found, it flows up through the well to the surface of the ground and into large
pipelines. Some of the gases that are produced along with methane, such as butane and
propane, are separated and the other sour gases such as carbon dioxide and hydrogen
sulfide are cleaned at a gas processing plant (normally called as sweetening process). The
by-products, once removed, are used in a number of ways. For example, propane and
butane can be used for cooking gas.
Because natural gas is colorless, odorless and tasteless, mercaptan (a sulfur-containing
organic compound with the general formula RSH where R is any radical, especially ethyl
mercaptan, C2H5SH) is added before distribution, to give it a distinct unpleasant odor (like
that of rotten eggs). This serves as a safety device by allowing it to be detected in the
atmosphere, in cases where leaks occur.
Most of the natural gas consumed in the United States is produced in the United States.
Some is imported from Canada and shipped to the United States in pipelines. Increasingly
natural gas is also being shipped to the United States as liquefied natural gas (LNG).

2.4 How Natural Gas is Stored and Delivered

Natural gas is normally produced far away from the consumption regions, therefore they
requires an extensive and elaborate transportation system to reach its point of use. The
transportation system for natural gas consists of a complex network of pipeline, designed to
quickly and efficiently transport natural gas from the origin to areas of high natural gas
demand. Transportation of natural gas is closely linked with its storage since the demand of
the gas is depend on the season.
Since natural gas demand is greater in the winter, gas is stored along the way in large
underground storage systems, such as old oil and gas wells or caverns formed in old salt
beds in western country. The gas remains there until it is added back into the pipeline when
people begin to use more gas, such as in the winter to heat homes. In Malaysia, and other
tropical country, gas is supplied throughout the year, therefore it was storage in a large tank
in the processing plant, either in Bintulu, Sarawak, or at Kertih, Terengganu.
Three major types of pipeline available along the transportation route, the gathering system,
the interstate pipeline and the distribution system. The gathering system consists of low
pressure, low diameter pipelines that transport raw natural gas from the wellhead to the
processing plant. In Malaysia, the natural gas is transported from oil rig offshore to the
processing plant at Petronas Gas Berhad at Kertih, Terengganu, and Bintulu LNG Tanker,
Sarawak. Since Malaysia natural gas and other producing country contain high sulfur and
carbon dioxide (sour gaseous) it must used specialized sour gas gathering pipe. Natural wet
gas from the wellhead contain high percentage of water therefore it will react with sour
gaseous to form acids, which are extremely corrosive and dangerous, thus its transportation
from the wellhead to the sweetening plant must be done carefully. The topic will be
discussed in depth in the treatment and processing of natural gas.
Pipeline can be classified as interstate or intrastate either it carries natural gas across the
state boundary (interstate) or within a particular state (intrastate). Natural gas pipelines are

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subject to regulatory oversight, which in many ways determines the manner in which
pipeline companies must operate. When the gas gets to the communities where it will be
used (usually through large pipelines), the gas is measured as it flows into smaller pipelines
called mains. Very small lines, called services, connect to the mains and go directly to homes
or buildings where it will be used. This method is used by rich country such as in the United
State, Canada or European country, such as United Kingdom, France etc.
The used of pipeline for natural gas delivery is costly, therefore some countries prefer to use
trucks for inland delivery. Using this method the natural gas should be liquefied to
minimize the size of the tanker truck. In certain country, the natural gas is transported by
trucks tankers to the end users. For example in Malaysia the natural gas was transported as
Liquefied Natural Gas (LNG) using tanker trucks to different state in peninsular of Malaysia
and in East Malaysia. The gas was supplied by Petronas Gas Berhad, at Kertih, Terengganu
while in east Malaysia, Sabah and Sarawak, the gas was supplied by Bintulu Plant. The
natural is exported by large ships equipped with several domed tanks.
When chilled to very cold temperatures, approximately -260°F, natural gas changes into a
liquid and can be stored in this form. Because it takes up only 1/600th of the space that it
would in its gaseous state, Liquefied natural gas (LNG) can be loaded onto tankers (large
ships with several domed tanks) and moved across the ocean to deliver gas to other
countries. When this LNG is received in the United States, it can be shipped by truck to be
held in large chilled tanks close to users or turned back into gas to add to pipelines. The
whole process to obtain the natural gas to the end user can be simplified by the diagram
shown in Fig. 3.

Fig. 3. Natural gas industry. Image (source: Energy Information Administration, DOE)

2.5 What is the Composition of Natural Gas

Natural gas, in itself, might be considered a very uninteresting gas - it is colorless, shapeless,
and odorless in its pure form. Quite uninteresting - except that natural gas is combustible,

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and when burned it gives off a great deal of energy. Unlike other fossil fuels, however,
natural gas is clean burning and emits lower levels of potentially harmful byproducts into
the air. We require energy constantly, to heat our homes, cook our food, and generate our
electricity. It is this need for energy that has elevated natural gas to such a level of
importance in our society, and in our lives.
Natural gas is a combustible mixture of hydrocarbon gases. While natural gas is formed
primarily of methane, it can also include ethane, propane, butane and pentane. The
composition of natural gas can vary widely, but below is a chart outlining the typical
makeup of natural gas before it is refined.

Chemical Name Chemical Formula Percentage (%)

Methane CH4 70-90%

Ethane C2H6
Propane C3H8 0-20%
Butane C4H10
Carbon Dioxide CO2 0-8%
Oxygen O2 0-0.2%
Nitrogen N2 0-5%
Hydrogen sulphide H2S 0-5%
Rare gases A, He, Ne, Xe trace

Table 1. Typical composition of Natural Gas

In its purest form, such as the natural gas that is delivered to your home, it is almost pure
methane. Methane is a molecule made up of one carbon atom and four hydrogen atoms,
and is referred to as CH4. Malaysia producing sour natural gas. Before purification process,
Malaysia’s natural gas is consists of several gaseous and impurities. The chemical
composition of Malaysia natural gas before it is being refined is shown in Table 2.

Chemical Name Chemical Formula Percentage (%)

Methane CH4 40-50%

Ethane C2H6 5-10%
Propane C3H8 1-5%
Carbon Dioxide CO2 20-3-%
Hydrogen sulphide H2S 0-1%
Table 2. Chemical composition in crude natural gas provided by Bergading Platform
offshore of Terengganu, Malaysia.

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2.6 How Much Natural Gas is there

There is an abundance of natural gas in North America, but it is a non-renewable resource,
the formation of which takes thousands and possibly millions of years. Therefore,
understanding the availability of our supply of natural gas is important as we increase our
use of this fossil fuel. This section will provide a framework for understanding just how
much natural gas there is in the ground available for our use, as well as links to the most
recent statistics concerning the available supply of natural gas.
As natural gas is essentially irreplaceable (at least with current technology), it is important
to have an idea of how much natural gas is left in the ground for us to use. However, this
becomes complicated by the fact that no one really knows exactly how much natural gas
exists until it is extracted. Measuring natural gas in the ground is no easy job, and it involves
a great deal of inference and estimation. With new technologies, these estimates are
becoming more and more reliable; however, they are still subject to revision.

Natural Gas Resource Category As of January 1, 2007(Trillion Cubic Feet)

Nonassociated Gas
Undiscovered 373.20
Onshore 113.61
Offshore 259.59
Inferred Reserves 220.14
Onshore 171.05
Offshore 49.09
Unconventional Gas Recovery 644.92
Tight Gas 309.58
Shale Gas 267.26
Coalbed Methane 68.09
Associated-Dissolved Gas 128.69
Total Lower 48 Unproved 1366.96
Alaska 169.43
Total U.S. Unproved 1536.38
Proved Reserves 211.09
TOTAL NATURAL GAS 1747.47
Table 3. Natural Gas Technically Recoverable Resources (Source: Energy Information
Administration - Annual Energy Outlook 2009)

A common misconception about natural gas is that we are running out, and quickly.
However, this couldn't be further from the truth. Many people believe that price spikes, seen
in the 1970's, and more recently in the winter of 2000, indicate that we are running out of
natural gas. The two aforementioned periods of high prices were not caused by waning
natural gas resources - rather, there were other forces at work in the marketplace. In fact,

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there is a vast amount of natural gas estimated to still be in the ground. In order to
understand exactly what these estimates mean, and their importance, it is useful first to
learn a bit of industry terminology for the different types of estimates.
The EIA provides classification system for natural gas resources. Unconventional natural
gas reservoirs are also extremely important to the nation's supply of natural gas.
Below are three estimates of natural gas reserves in the United States. The first (Table 3),
compiled by the Energy Information Administration (EIA), estimates that there are 1,747.47
Tcf of technically recoverable natural gas in the United States. This includes undiscovered,
unproved, and unconventional natural gas. As seen from the table, proved reserves make
up a very small proportion of the total recoverable natural gas resources in the U.S.
The following table includes an estimate of natural gas resources compiled by the National
Petroleum Council (NPC) in 1999 in its report Natural Gas - Meeting the Challenges of the
Nation's Growing Natural Gas Demand. This estimate places U.S. natural gas resources
higher than the EIA, at 1,779 Tcf remaining. It is important to note that different
methodologies and systems of classification are used in various estimates that are
completed. There is no single way that every industry player quantifies estimates of natural
gas. Therefore, it is important to delve into the assumptions and methodology behind each
study to gain a complete understanding of the estimate itself.

1992 NPC Study 1999 NPC Study

As of Jan 1, 1991 As of Jan 1, 1998
Lower 48 Resources
Proved Reserves 160 157
Assessed Additional Resources 1135 1309
Old Fields (Reserve Appreciation) 236 305
New Fields 493 633
Nonconventional 406 371
Total Remaining Resources 1295 1466

Alaskan Resources
Proved Reserves 9 10
Assessed Additonal Resources 171 303
Old Fields (Reserve Appreciation) 30 32
New Fields 84 214
Nonconventional 57 57
Total Remaining Resources 180 313

Total U.S. Remaining Resources 1475 1779

Table 4. U.S. Natural Gas Resources (Trillion Cubic Feet) ( Source: National Petroleum
Council - Meeting the Challenges of the Nation's Growing Natural Gas Demand, 2007)

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Below (Table 5) is a third estimate completed by the Potential Gas Committee. This estimate
places total U.S. natural gas resources at just over 1,836 Tcf. This estimate classifies natural
gas resources into three categories: probable resources, possible resources, and speculative
resources, which are added together to reach a total potential resource estimate. Only this
total is shown below.

Total Potential
Resource
Traditional Resources
Lower 48 States
Total Lower 48 1479.6
Alaska
Onshore 94.432
Offshore 99.366
Total Alaska 193.831

Total Traditional 1,673.4

Coalbed Methane 163.0

Total United States 1,836.4

Table 5. Potential Natural Gas Resources of the U.S. (Trillion Cubic Feet) (Source: Potential
Gas Committee - Potential Supply of Natural Gas in the United States, 2009)

There are a myriad of different industry participants that formulate their own estimates
regarding natural gas supplies, such as production companies, independent geologists, the
government, and environmental groups, to name a few. While this leads to a wealth of
information, it also leads to a number of difficulties. Each estimate is based on a different set
of assumptions, completed with different tools, and even referred to with different
language. It is thus difficult to get a definitive answer to the question of how much natural
gas exists. In addition, since these are all essentially educated guesses as to the amount of
natural gas in the earth, there are constant revisions being made. New technology,
combined with increased knowledge of particular areas and reservoirs mean that these
estimates are in a constant state of flux. Further complicating the scenario is the fact that
there are no universally accepted definitions for the terms that are used differently by
geologists, engineers, accountants, and others.
Natural gas has been discovered on all continents except Antarctica. World natural gas
reserves total approximately 150 trillion cu m (5.3 quadrillion cu ft). The world's largest
natural gas reserves, totaling, 50 trillion cu m (1.9 quadrillion cu ft) are located in
Russia. The second-largest reserves, 48 trillion cu m (1.7 quadrillion cu ft), are found in
the Middle East. Vast deposits are also located in other parts of Asia, in Africa, and in

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Australia. Natural gas reserves in the United States total 5 trillion cu m (177 trillion cu ft).
In Asia-Oceania, natural gas reserves total 12.6 trillion cu m (Table 6). Malaysia has the
14th largest gas reserves as at January 2008. As at January 2008, Malaysia's gas reserves
stood at 88.0 trillion standard cubic feet (tscf) or 14.67 billion barrels of oil equivalent,
approximately three times the size of crude oil reserves of 5.46 billion barrels.

Proven reserves Annual production Reserve to product

3 3 (years)
(Tm ) (Gm )
Australia 2.5 34.5 72.5
China 1.5 32.6 46.0
India 0.8 28.4 28.2
Indonesia 2.6 70.6 36.8
Malaysia 2.1 50.3 41.8
Others 3.1 85.3 36.3
Total 12.6 301.7 41.8
Table 6. Proven reserves and Annual production, Asia-Oceania. (Taken from BP Statistical
Review, 2003)

Most of this gas reserves are located at offshore Peninsular Malaysia, Sarawak and Sabah.
The Malaysian natural gas reserves are as shown in Figure 4 [4].

Fig. 4. Malaysian Natural Gas Reserve (Taken from Oil and Gas Exploration and
Production-Reserves, Costs, Contract, 2004)

Currently, Malaysia is a net exporter of natural gas and is the third largest exporter after
Algeria and Indonesia. In 2001, the country exported 49.7% of its natural gas production to
the Republic of Korea and Taiwan under long-term contracts. The other 50.3% of Malaysia
natural gas was delivered to the gas processing plants.

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2.7 Uses of Natural Gas

For hundreds of years, natural gas has been known as a very useful substance. The Chinese
discovered a very long time ago that the energy in natural gas could be harnessed, and used
to heat water. In the early days of the natural gas industry, the gas was mainly used to light
streetlamps, and the occasional house. However, with much improved distribution channels
and technological advancements, natural gas is being used in ways never thought possible.
There are so many different applications for this fossil fuel that it is hard to provide an
exhaustive list of everything it is used for. And no doubt, new uses are being discovered all
the time. Natural gas has many applications, commercially, in your home, in industry, and
even in the transportation sector! While the uses described here are not exhaustive, they
may help to show just how many things natural gas can do.
According to the Energy Information Administration, total energy (Fig. 5) from natural gas
accounts for 23% of total energy consumed in the developing countries, making it a vital
component of the nation's energy supply.

Fig. 5. Total Energy Consumed in the U.S. - 2007 (Source: EIA - Annual Energy Outlook
2009)

Natural gas is used across all sectors, in varying amounts. The pie chart below (Fig. 6) gives
an idea of the proportion of natural gas use per sector. The residential sector accounts for the
greatest proportion of natural gas use in the most of the developing countries, with the
residential sector consuming the greatest quantity of natural gas.

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Fig. 6. Natural Gas Use By Sector (Source: EIA - Annual Energy Outlook 2009)

Commercial uses of natural gas are very similar to electric power uses. The commercial
sector includes public and private enterprises, like office buildings, schools, churches, hotels,
restaurants, and government buildings. The main uses of natural gas in this sector include
space heating, water heating, and cooling. For restaurants and other establishments that
require cooking facilities, natural gas is a popular choice to fulfill these needs.
According to the Energy Information Administration (EIA), as of the year 2003, the
commercial sector consumes about 6,523 trillion Btu's of energy a year (aside from electrical
system losses), most of which is required for space heating, lighting, and cooling. Of this
6,523 trillion Btu, about 2,100 trillion Btu (or 32.2%) are supplied by natural gas.

Fig. 7. Commercial Energy Use (Source: EIA Major Fuel Consumption by End Use, 2003.)

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Natural gas space and water heating for commercial buildings is very similar to that found
in residential houses. Natural gas is an extremely efficient, economical fuel for heating in all
types of commercial buildings. Although space and water heating account for a great deal of
natural gas use in commercial settings, non-space heating applications are expected to
account for the majority of growth in natural gas use in the commercial sector. Cooling and
cooking represent two major growth areas for the use of natural gas in commercial settings.
Natural gas currently accounts for 13 percent of energy used in commercial cooling, but this
percentage is expected to increase due to technological innovations in commercial natural
gas cooling techniques. There are three types of natural gas driven cooling processes. Engine
driven chillers use a natural gas engine, instead of an electric motor, to drive a compressor.
With these systems, waste heat from the gas engine can be used for heating applications,
increasing energy efficiency. The second category of natural gas cooling devices consist of
what are called absorption chillers, which provide cool air by evaporating a refrigerant like
water or ammonia. These absorption chillers are best suited to cool large commercial
buildings, like office towers and shopping malls. The third type of commercial cooling
system consists of gas-based desiccant systems (Fig. 8). These systems cool by reducing
humidity in the air. Cooling this dry air requires much less energy than it would to cool
humid air.

Fig. 8. A Desiccant Unit Atop the Park Hyatt Hotel, Washington D.C. (Source: National
Renewable Energy Laboratory, DOE)

Another area of growth in commercial natural gas use is in the food service industry.
Natural gas is an excellent choice for commercial cooking requirements, as it is a flexible
energy source in being able to supply the food service industry with appliances that can
cook food in many different ways. Natural gas is also an economical, efficient choice for
large commercial food preparation establishments. New developments such as
Nontraditional Restaurant Systems, which provide compact, multifunctional natural gas

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appliances for smaller sized food outlets such as those found in shopping malls and
airports, are expanding the commercial use of natural gas. These types of systems can
integrate a gas-fired fryer, griddle, oven, hot and cold storage areas, and multiple venting
options in a relatively small space - providing the ease and efficiency of natural gas cooking
while being compact enough to serve small kiosk type establishments.
In addition to traditional uses of natural gas for space heating, cooling, cooking and water
heating, a number of technological advancements have allowed natural gas to be used to
increase energy efficiency in commercial settings. Many buildings, because of their high
electricity needs, have on-site generators that produce their own electricity. Natural gas
powered reciprocating engines, turbines, and fuel cells are all used in commercial settings to
generate electricity. These types of 'distributed generation' units offer commercial
environments more independence from power disruption, high-quality consistent
electricity, and control over their own energy supply.
Another technological innovation brought about is combined heating and power and
combined cooling, heating and power systems, which are used in commercial settings to
increase energy efficiency. These are integrated systems that are able to use energy that is
normally lost as heat. For example, heat that is released from natural gas powered electricity
generators can be harnessed to run space or water heaters, or commercial boilers. Using this
normally wasted energy can dramatically improve energy efficiency.

Natural gas fired electric generation, and natural gas powered industrial applications, offer
a variety of environmental benefits and environmentally friendly uses, including:
 Fewer Emissions - combustion of natural gas, used in the generation of electricity,
industrial boilers, and other applications, emits lower levels of NOx, CO2, and
particulate emissions, and virtually no SO2 and mercury emissions. Fig. 9 shows a
picture of emissions from Industrial Smokestacks (Source: EPA). Natural gas can be
used in place of, or in addition to, other fossil fuels, including coal, oil, or
petroleum coke, which emit significantly higher levels of these pollutants.
 Reduced Sludge - coal fired power plants and industrial boilers that use scrubbers
to reduce SO2 emissions levels generate thousands of tons of harmful sludge.
Combustion of natural gas emits extremely low levels of SO2, eliminating the need
for scrubbers, and reducing the amounts of sludge associated with power plants
and industrial processes.
 Reburning - This process involves injecting natural gas into coal or oil fired boilers.
The addition of natural gas to the fuel mix can result in NOx emission reductions of
50 to 70 percent, and SO2 emission reductions of 20 to 25 percent.
 Cogeneration - the production and use of both heat and electricity can increase the
energy efficiency of electric generation systems and industrial boilers, which
translates to requiring the combustion of less fuel and the emission of fewer
pollutants. Natural gas is the preferred choice for new cogeneration applications.
 Combined Cycle Generation - Combined cycle generation units generate electricity
and capture normally wasted heat energy, using it to generate more electricity.
Like cogeneration applications, this increases energy efficiency, uses less fuel, and
thus produces fewer emissions. Natural gas fired combined cycle generation units
can be up to 60 percent energy efficient, whereas coal and oil generation units are
typically only 30 to 35 percent efficient.

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16 Natural Gas

 Fuel Cells - Natural gas fuel cell technologies are in development for the generation
of electricity. Fuel cells are sophisticated devices that use hydrogen to generate
electricity, much like a battery. No emissions are involved in the generation of
electricity from fuel cells, and natural gas, being a hydrogen rich source of fuel, can
be used. Although still under development, widespread use of fuel cells could in
the future significantly reduce the emissions associated with the generation of
electricity.
 Essentially, electric generation and industrial applications that require energy,
particularly for heating, use the combustion of fossil fuels for that energy. Because
of its clean burning nature, the use of natural gas wherever possible, either in
conjunction with other fossil fuels, or instead of them, can help to reduce the
emission of harmful pollutants.

Fig. 9. Emissions from Industrial Smokestacks (Source: EPA)

3. Purification of Natural Gas

Gas processing of acidic crude natural gas is necessary to ensure that the natural gas
intended for use is clean-burning and environmentally acceptable. Natural gas used by
consumers is composed almost entirely of methane but natural gas that emerges from the
reservoir at the wellhead contains many components that need to be extracted. Although,
the processing of natural gas is less complicated rather than the processing and refining of
crude oil, it is equal and necessary before it can be used by end user.
One of the most important parts of gas processing is the removal of carbon dioxide and
hydrogen sulfide. The removal of acid gases (CO2, H2S and other sulfur components) from
natural gas is often referred to as gas sweetening process. There are many acid gas treating
processes available for removal of CO2 and H2S from natural gas. These processes include
Chemical solvents, Physical solvents, Adsorption Processes Hybrid solvents and Physical
separation (Membrane) (Kohl and Nielsen, 1997).

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3.1 Various Technologies Used to Convert Sour to Sweet Natural Gas

According to previous research done by Hao et al. (2002), there are ways to upgrading the
low quality natural gas with selective polymer membranes. The membrane processes were
designed to reduce the concentrations of CO2 and H2S in the natural gas pipeline
specifications. However, this technique incurs high cost and low selectivity towards toxic
gas separation. This technique also needs further development because the performance of
membrane depends upon the specific characteristics of flue gas composition, and the
specific features of the separation (i.e. large volumetric flow rate, low pressure source, high
temperature, and the relative low commodity value of H2S and CO2) (Rangwala, 1996).
Another method of H2S removal and one that leaves the CO2 in the natural gas is called the
Iron Sponge process. The disadvantage of this is that it is called a batch-type function and is
not easily adapted to continuous operating cycle. The Iron Sponge is simply the process of
passing the sour gas through a bed of wood chips that have been impregnated with a special
hydrated form of iron oxide that has a high affinity for H2S. Regeneration of the bed incurs
excessive maintenance and operating costs, making this method inconsistent with an
efficient operating program. If there are any real advantages in using this process, it is fact
that CO2 remains in the gas, thereby reducing the shrinkage factor which could be
significant for very large volumes with an otherwise high CO2 content (Curry, 1981).
Chemical absorption processes with aqueous alkanolamine solutions are used for treating
gas streams containing CO2. They offer good reactivity at low cost and good flexibility in
design and operation. However, depending on the composition and operating conditions of
the feed gas, different amines can be selected to meet the product gas specification
(Mokhatab et al., 2006). Some of the commonly used alkanolamine for absorption
desulfurization are monoethanolamine (MEA), diethanolamine (DEA), triethanolamine
(TEA), diglycolamine (DGA), di-isopropanolamine (DIPA) and methyldiethanolamine
(MDEA). MDEA allows the selective absorption of H2S in the presence of CO2 but can be use
effectively to remove CO2 from natural gas in the present of additives (Salako and
Gudmundsson, 2005).
In the other hand, CO2 can be removed from natural gas via chemical conversion
techniques. Catalysts for CO2 methanation have been extensively studied because of their
application in the conversion of CO2 gas to produce methane, which is the major component
in natural gas (Wan Abu Bakar et al., 2008a). Usually, the catalysts are prepared from the
metal oxide because of the expensiveness of pure metal. This process can increase the purity
and quality of the natural gas without wasting the undesired components but fully used
them to produce high concentration of methane (Ching Kuan Yong, 2008).

3.2 Synthesis of Artificial Natural Gas: Methanation Reaction

Methane (CH4) gas was formed from the reaction of hydrogen gas and carbon dioxide gas
through methanation process by reduction reaction as in Equation 1.1 below:-

CO2 (g) + 4H2 (g) → CH4 (g) + 2H2O (l) (1.1)

This reaction is moderately exothermic, Ho = -165 kJ/mol. In order for this method to be
effective, a suitable catalyst must be applied to promote selectively CO2 methanation
because of the main side product under this reaction also will be form (Eq 1.2), which
obviously should be avoided. Thus, high selectivity of the catalyst in promoting CO2

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methanation is paramount importance. In Equation 1.2, carbon monoxide produced by this

reaction can also be used to form methane by reaction with hydrogen.

CO2 (g) + H2 (g) → CO (g) + H2O (l) (1.2)

CO (g) + 3H2 (g) → CH4 (g) + H2O (l) (1.3)

3.2.1 Mechanism of Methanation Reaction

Mechanism of methanation reaction has been studied a long time ago. A lot of researcher
agreed that methanation process involves Langmuir-Hinshelwood (LH) mechanism to
support the reaction process between active species and catalyst surface.
For the simplest possible reaction, methanation process can be described as follows:

CO2 + S CO2(ads) ( 1.4)

H2 + S H2(ads) ( 1.5)

CO2(ads) + H2(ads) CH4(ads) + H2O(ads) ( 1.6)

CH4(ads) CH4(desorp) + S ( 1.7)

H2O(ads) H2O(desorp) + S ( 1.8)

Where S = Catalyst surface; ads = adsorbed species on the catalyst surface; desorp =
desorbed species from catalyst surface.

According to Equation 1.4, carbon dioxide is reacting with the catalyst surface, (S) by
chemisorptions and creates an active species that adsorbed onto catalyst surface. This is
followed by hydrogen compound that also react with catalyst surface by chemisorptions
and adsorbed onto catalyst surface as an active species. Both active species than react each
other to produce products that is methane and water. Finally, ( Equation 1.7 & 1.8 ) both
products were dissociated from the catalyst surface.

4. Catalysts Used in Methanation Reaction

Metal oxide supported catalysts have been widely used in research for investigating the CO
and CO2 methanation reaction. Depending on the metal used and the reaction conditions, a
variety of products may be formed including methane. However, fewer researches on the
catalyst for in-situ reactions of CO2 methanation and H2S desulfurization have been carried
out. In fact, there is also presence of H2S in real natural gas. Therefore, H2S should be
considered in invention of methanation catalyst, since it could cause poisoning of the nickel
catalyst (Wan Abu Bakar et al., 2008b). As been said by Xu et al. (2003), a good methanation
catalyst is physically durable and reducible at temperature not more than 300oC with high
performance ability and these properties should retained in the catalyst while in use with a
life span up to 10 years.

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4.1 Nickel Oxide Based used in Methanation Catalysts

The methanation of carbon dioxide on Ni catalysts was studied in detail by fewer
researchers because of the theoretical significance and possible practical application of this
reaction. The methanation activity of Ni/Al2O3 catalyst depended intimately on the surface
chemical state of Ni and different active phases formed from the reduction of different
nickel species in the oxidated states. Nickel oxides appeared in Ni/Al2O3 in two forms prior
to reduction as “free” and “fixed oxide”, and formed large and small crystallites,
respectively, when reduced (Zielinski, 1982). Studied done by Rodriguez et al. (2001)
showed that NiO catalyst has ability to gives higher catalytic activity with higher methane
formation due to the malformation sites which converted to active sites on the surface of
nickel oxide. This property is important as reference to construct excellent catalysts for CO2
conversion
Previously, it was shown that nickel particles change their morphology during catalytic
reactions by cluster growth processes and that part of the active clusters are lifted from the
support due to carbon deposition and carbon whisker formation (Czekaj et al., 2007). Early
study by Douglas et al., (2001) found that Ni catalysts are promising catalysts since they are
active and more resistant to sulfur poisoning thus high dispersion of Ni and is expected to
be used in catalytic reaction that proceeds at relatively low temperature (Takahashi et al.,
2007). Moreover, Inui (1996) claimed that NiO has a bimodal pore structure, which will
enhance the higher activity for CO2 methanation. A bimodal pore structure was found to be
beneficial to catalyst preparation and methanation rate (Inui, 1979) which will serve as an
optimum pore size for the adsorption of both the reactants. Therefore, Ni based catalyst are
commonly used as catalysts in hydrogenation and hydrogenolysis reaction.
Aksoylu and Onsan (1997) reported 5.5 × 10-5 % of CH4 was produced at 250oC over the Ni/
Al2O3 catalyst prepared by conventional impregnation method at 350oC for 3 hours under
reduction environment. They also investigated the 15%-Ni/Al2O3 prepared by
coprecipitation method for methanation of carbon dioxide. The result achieved 30% of
conversion with 99.7% selectivity towards methane at 510 K (Aksoylu et al., 1996). Some
previous research was only focused on conversion of CO2 without mentioned the yielded of
CH4. Similarly to Chang et al. (2003) who had investigated CO2 methanation over NiO
supported on rice husk ash-Al2O3 and SiO2-Al2O3 which had been synthesized by
impregnation method and calcined at 500oC. At reaction temperature of 400oC, there were
30% conversion of CO2 over the rice husk ash-Al2O3 supported catalyst, while only 5%
conversion of CO2 over the SiO2-Al2O3 catalyst.
Moreover, Ni/SiO catalyst prepared by conventional impregnation method was also
studied by Shi and Liu, (2009). The sample was treated by glow discharge plasma for 1 hour
and followed by calcinations thermally at 500oC for 4 hours. Such prepared catalyst presents
smaller metal particles (17.5 and 7.9 nm) and higher conversion of CO at 400oC around 90%
for methantion reaction. However, Ni/SiO2 catalyst prepared by a sol gel process showed
better quality when compared to the Ni/SiO2 catalyst prepared by conventional
impregnation (Tomiyama et al., 2003). Thus, Takahashi et al. (2007) investigated the bimodal
pore structure of Ni/SiO2 prepared by the sol-gel method of silicon tetraethoxide and nickel
nitrate in the presence of poly(ethylene oxide) (PEO) and urea.
They found that the catalyst shows steady activity which around 30-40% without decay
within the reaction period until 240 min with total flow rate of 360 cm3/min. The
performance of the catalyst influenced strongly by Ni surface area rather than the presence

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of macropores. As been shown that, nickel oxide can be prepared through various methods
such as wetness impregnation, co-precipitation, sol gel method, ion-exchange, adsorption,
deposition-precipitation and else. These preparation methods are, however very
complicated and difficult to control except for wetness impregnation method. Therefore,
most of the work published has focused on the use of impregnation technique for their
catalyst preparation.
Research done by Liu et al. (2008) on the removal of CO contained in hydrogen-rich
reformed gases was conducted by selective methanation over Ni/ZrO2 catalysts prepared
by conventional wetness impregnation method. The catalyst achieved CO conversion of
more than 96% and held a conversion of CO2 under 7% at temperature range 260oC-280oC.
The results showed that only methane was observed as a hydrogenated product.
Furthermore, the maximum of CO2 conversion was found by Perkas et al. (2009) which
achieved about 80% at 350oC on the Ni/meso-ZrO2 catalyst. Around 100% selectivity to CH4
formation was obtained at the same reaction temperature. This catalyst was prepared by an
ultrasound-assisted method and testing with gas hourly space velocity (GHSV) of 5400 h-1
at all temperatures. They also reported that none modified mesoporous Ni/ZrO2 catalyst
and with the Ni/ZrO2 modified with Ce and Sm did not effect the conversion of CO2.
Previous work by Sominski et al. (2003), a Ni catalyst supported on a mesoporous yttria-
stabilized-zirconia composite was successfully prepared by a sonochemical method using
templating agent ofvsodium dodecyl sulfate (SDS). However, the result is not as good as the
catalyst that had been obtained by Perkas et al.
In a research done by Rostrup-Nielsen et al. (2007), supported nickel catalyst containing 22
wt% Ni on a stabilized support was exposed to a synthesis gas equilibrated at 600oC and
3000kPa for more than 8000h. The CO2 conversion is 57.87% while methane formed is
42.76%. The research showed that at 600oC, loss of active surface area proceeds via the atom
migration sintering mechanism. The methanation reaction is structure sensitive and it was
suggested that atomic step sites play the important role as the active sites of the reaction.
High temperature methanation may play a role in manufacture of substitute natural gas
(SNG). The key problem is resistance to sintering, which results in a decrease of both the
metal surface area and the specific activity.
Modification of the catalyst by some appropriate additives may effect the conversion of CO2
which then methane production. Ni catalysts were modified by alkali metal, alkaline earth
metals, transition metal, noble metal or rare earth metal just to select which promoters could
increase the conversion of CO2 as well as the methane formation. The effect of cerium oxide
as a promoter in supported Ni catalysts was studied by Xavier et al. (1999). They claimed
that the highest activity of CeO2 promoter for Ni/Al2O3 catalysts could be attributed to the
electronic interactions imparted by the dopant on the active sites under reducing conditions.
The testing was evaluated in a high pressure catalytic reactor consists of a stainless steel
reactor of 25 mm diameter and 180 mm length which is mounted vertically inside a furnace.
Methanation activity and metal dispersion was found to decrease with increasing of metal
loading. It is observed that the catalyst doped with 1.5 wt% CeO2 exhibited highest
conversion of CO and CO2 with percentage of conversion increase 3.674 moles/second,
which is 86.34%. The presence of CeO2 in impregnated Ni/γ-Al2O3 catalysts was associated
with easier reduction of chemical interaction between nickel and alumina support hence
increase its reducibility and higher nickel dispersion Zhuang et al. (1991). It showed a

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Natural gas 21

beneficial effect by not only decreasing the carbon deposition rate but also increasing and
maintaining the catalytic activity.
The study of Yoshida et al. (1997) in a bench scale test at ambient temperature and 350oC for
carbon recycling system using Ni ferrite process was carried out in LNG power plant. The
feed gas was passed at a flow rate of 10 mL/min. They found that the amount of methane
formed after CO2 decomposition was 0.22 g (conversion CO2 of to CH4: 77%) in the latter
and 0.49 g (conversion of CO2 to CH4: 35%) in the former. According to their study, the
methanation and carbon recycling system could also be applied to other COz sources such as
IGCC power plant and depleted natural gas plant. Hence, pure CH4 gas can be theoretically
synthesized from CO2 with low concentration in flue gas and H2 gas with the minimum
process energy loss, while conventional catalytic processes need an additional separation
process of CH4 gas formed.
Hashimoto et al. (2002) who revealed that the catalysts obtained by oxidation-reduction
treatment of amorphous Ni-Zr alloys exhibited high catalytic activity with 100% selectivity
formation of CH4 at 1 atm. Around 80% of CO2 was converted at 573 K. They found the
number of surface nickel atom decreases with nickel content of catalyst, because of
coagulation of surface nickel atoms leading to a decrease in dispersion of nickel atoms in the
catalysts. Moreover, Habazaki et al. (1998) reported that over the catalysts prepared from
amorphous Ni-Zr (-Sm) and Co-Zr, nickel-containing catalysts show higher activity than the
Co-Zr catalyst. CO reacted preferentially with H2 and was almost completely converted into
CH4 at or above 473 K in the CO-CO2-H2. The maximum conversion of carbon dioxide under
the present reactant gas composition is about 35% at 575 K.
Most of the previous work used rare earth oxide as a dopant over Ni/Al2O3 catalysts for
hydrogenation reaction. Su and Guo (1999) also reported an improvement in catalytic
activity and resistance to Ni sintering of doped with rare earth oxides. The growth of Ni
particles and the formation of inactive NiO and NiAl2O4 phases were suppressed by
addition of rare earth oxides. The combinations of two oxides lead to creation of new
systems with new physicochemical properties which may exhibit high catalytic performance
as compared to a single component system (Luo et al., 1997). However, the catalytic and
physicochemical properties of different oxide catalysts are dependent mainly on the
chemical composition, method of preparation and calcination temperatures (Selim and El-
Aihsy, 1994).
Ando and Co-workers (1995) had studied on intermetallic compounds synthesized by arc-
melting metal constituent in a copper crucible under 66.7 kPa argon atmosphere. The
hydrogenation of carbon dioxide took place under 5 Mpa at a reaction temperature at 250oC
over LaNi4X. They found that the conversion of CO2 was 93% over LaNi5 and the
selectivities to methane and ethane in the product were 98% and 2%, respectively. The
source of activity can be attributed to the new active sites generated by decomposition of the
intermetallic compounds. However, even under atmospheric pressure, 56% of CO2
converted to CH4 and CO with selectivities of 98% and 2%, respectively.
The promotion of lanthanide to the nickel oxide based catalyst gives positive effects which
are easier reduction of oxide based, smaller particles size and larger surface area of active
nickel (Zhang et al., 2001). Moreover, the highly dispersed nickel crystallites is obtained over
nickel catalyst containing of lanthanide promoter (Rivas et al., 2008). Furthermore, the
methanation of carbon dioxide over Ni-incorporated MCM-41 catalyst was carried out by
Du et al. (2007). At 873 K, 1 wt% of Ni-MCM-41 with space velocity of 115001 kg-1h-1 showed

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only 46.5% CO2 conversion and a selectivity of 39.6% towards CH4. Almost no catalytic
activity was detected at 373-473 K and only negligible amounts of products were detected at
573 K. However, this catalyst structure did not change much after CO2 methanation for
several hours, producing the high physical stability of this catalytic system.
In addition, nickel based catalysts that used more than one dopants had been studied by Liu
et al. (2009). Ni-Ru-B/ZrO2 catalyst was prepared by means of chemical reduction and dried
at 80oC for 18 h in air with total gas flow rate of 100 cm3/min. They found that CO2
methanation occurred only when temperature was higher than 210oC. At reaction
temperature of 230oC, the CO conversion reached 99.93% but CO2 conversion only 1.55%.
Meanwhile, Ni-Fe-Al oxide nano-composites catalyst prepared by the solution-spray plasma
technique for the high temperature water-gas shift reaction was investigated by Watanabe et
al. (2009). The CO conversion over 39 atom% Ni-34 atom% Fe-27 atom% Al catalyst achieved
around 58% and yielded about 6% of methane at 673 K.
On the other hand, Kodama et al. (1997) had synthesized ultrafine NixFe3-xO4 with a high
reactivity for CO2 methanation by the hydrolysis of Ni2+, Fe2+ and Fe3+ ions at 60-90oC
followed by heating of the co-precipitates to 300oC. At reaction temperature of 300oC, the
maximum yield (40%) and selectivity (95%) for CH4 were obtained. Moreover, the
conversion of CO2 over NiO-YSZ-CeO2 catalyst prepared by impregnation method was
100% at temperature above 800oC. This catalyst was investigated by Kang et al. (2007). No
NiC phase was detected on the surface of NiO-YSZ-CeO2 catalyst. Yamasaki et al. (1999)
reported that amorphous alloy of Ni-25Zr-5Sm catalyzed the methanation reaction with 90%
conversion of CO2 and 100% selectivity towards CH4 at 300oC.
Furthermore, Ocampo et al. (2009) had investigated the methanation of carbon dioxide over
5 wt% nickel based Ce0.72Zr0.28O2 catalyst which prepared by pseudo sol-gel method. The
catalyst exhibited high catalytic activity with 71.5% CO2 conversion and achieved 98.5%
selectivity towards methane gas at 350oC. However, it never stabilized and slowly
deactivated with a constant slope and ended up with 41.1% CO2 conversion and its CH4
selectivity dropped to 94.7% after 150 h on stream. Catalytic testing was performed under
operating conditions at pressure of 1 atm and a CO2/H2/N2 ratio is 36/9/10 with a total gas
flow of 55 mL/min.
Meanwhile, Kramer et al. (2009) also synthesize Re2Zr10Ni88Ox catalyst by modified sol gel
method based on the molar ratio metal then dried for 5 days at room temperature followed
by 2 days at 40oC and lastly calcined at 350oC for 5 h. The catalytic performance was carried
out by the reactant gas mixture of CO/CO2/N2/H2 = 2/14.9/19.8/63.3 enriched with water
at room temperature under pressure of 1 bar and total flow rate of 125 mL/min. At reaction
temperature of 230oC, almost 95% conversion of CO was occurred and less than 5% for
conversion of CO2 over this catalyst.
The novel catalyst development to achieve both low temperature and high conversion of
sour gasses of H2S and CO2 present in the natural gas was investigated by Wan Abu Bakar et
al. (2008c). It was claimed that conversion of H2S to elemental sulfur achieved 100% and
methanation of CO2 in the presence of H2S yielded 2.9% of CH4 over Fe/Co/Ni-Al2O3
catalyst at maximum studied temperature of 300oC. This exothermic reaction will generate a
significant amount of heat which caused sintering effect towards the catalysts (Hwang and
Smith, 2009). Moreover, exothermic reaction is unfavorable at low temperature due to its
low energy content. Thus, the improvement of catalysts is needed for the in-situ reactions of
methanation and desulfurization to be occurred at lower reaction temperature.

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4.2 Manganese Based used in Methanation Catalysts

Manganese has been widely used as a catalyst for many types of reactions including solid
state chemistry, biotechnology, organic reactions and environmental management. Due to
its properties, numerous field of research has been investigated whereby manganese is
employed as the reaction catalyst. Although nickel also reported to be applicable in many
process as a good and cheap catalysts, it seems that using nickel as based catalysts will
deactivated the active site by deposition of carbon (Luna et al., 2008). Hence, it is essential to
use other metal to improve the activity and selectivity as well as to reduce formation of
carbon. A proof that manganese improves the stability of catalysts can be shown in
researched done by Seok. H. S et al (2001). He have proven that manganese improve the
stability of the catalyst in CO2 reforming methane. Added Mn to Ni/Al2O3 will promotes
adsorption CO2 by forming carbonate species and it was responsible for suppression of
carbon deposition over Ni/MnO-Ni/Al2O3. Other research done by Li. J et al. (2009) prove
that when manganese doped in appropriate amount, it will cause disorder in the spinal
structure of metal surface and can enhance the catalytic activity of the reactive ion.
According to Ouaguenouni et al, [33], in the development of manganese oxide doped with
nickel catalyst, they found that the spinel NiMn2O4 was active in the reaction of the partial
oxidation of methane. The catalysts show higher methane conversion when calcined at
900°C. This is because the stability of the structure which led to good dispersion of nickel
species. Indeed, the presence of the oxidized nickel limited the growth of the particles
probably by the formation of interaction between metallic nickel out of the structure and the
nickel oxide of the structure.
Ching [34] in his studies found that, 5% of manganese that had been introduced into cobalt
containing nickel oxide supported alumina catalyst will converted only 17.71% of CO2 at
reaction temperature of 300ºC. While when Mn was introduced into iron containing nickel
oxide supported alumina catalyst, the percentage of CO2 conversion does not differ much as
in the Co:Ni catalyst. This may be because manganese is not a good dopant for nickel based
catalyst. This is in agreement by Wachs et al. [35], where some active basic metal oxide
components such as MnO and CeO did not interact strongly with the different oxide
functionalities present on oxide support and consequently, did not disperse very well to
form crystalline phases. Therefore, in research done by Wachs et al. [36] stated that Ru could
be assigned as a good dopant towards MnO based catalyst. They are active basic metal
oxides that usually anchor to the oxide substrate by preferentially titrating the surface Lewis
acid sites, such as surface M-oxide vacancies, of the oxide support.
In addition, the hydrogenation of carbon oxides was also performed over promoted iron-
manganese catalysts. Herranz et al. [37] in their research found that manganese containing
catalyst showed higher activity towards formation of hydrocarbons. When these catalysts
were promoted with copper, sodium and potassium, carbon dioxide conversion was
favoured by alkaline addition, especially by potassium, due to the promotion of the water-
gas shift reaction.
When Najwa Sulaiman (2010) incorporated ruthenium into the manganese oxide based
catalyst system with the ratio of 30:70 that was Ru/Mn (30:70)/Al2O3, it gave a positive
effect on the methanation reaction. The percentage conversion keeps on increasing at 200oC
with a percentage of CO2 conversion of 17.18% until it reaches its maximum point at 400ºC,
whereby the percentage of CO2 conversion is at the highest which is 89.01%. At reaction
temperature of 200oC and 400oC, it showed Mn and Ru enhances the catalytic activity

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24 Natural Gas

because H2 and CO2 are easily chemisorbed and activated on these surfaces. Murata et al.
(2009) suggested that the high CO2 conversion was probably due to the manganese species
which causes the removal of chlorine atoms from RuCl3 precursor and increases the density
of active ruthenium oxide species on the catalyst which resulted in high catalytic activity.
Furthermore, it is very important to used stable and effective metal oxide catalyst with
improved resistance to deactivation caused by coking and poisoning. Baylet. A et al. (2008)
studies on effect of Pd on the reducibility of Mn based material. They found that in H2-TPR
and XPS test, only Mn3+/Mn2+ is proportional to the total Mn content in the solid support
that leads to the stable catalyst to avoid cooking and poisoning effect. Additionally, Hu. J et
al. (2008) in their research on Mn/Al2O3 calcined at 500°C, shows that manganese oxide
proved to have a good performance for catalytic oxidation reaction and also show better
catalytic performance compare using support SiO2 and TiO2. It is shows that not only doped
material are important in producing good catalyst, based catalyst also play major role in
giving high catalytic activity in catalysts. El-Shobaky et al. (2003) studied, the doping process
did not change the activation energy of the catalyzed reaction but much increased the
concentration of the catalytically reactive constituents without changing their energetic
nature.
Other research made by Chen. H. Y et al (1998) revealed the important of promoting
manganese in catalyst. The studies shows that when Cu/ZnO/Al2O3 promoted Mn as based
catalyst, it shows increasing in catalytic activity, larger surface area of Cu concentration and
elevated Cu reduction temperature compare catalyst without Mn. In XPS studies also
revealed that reaction between Mn and Cu resulting reduction of Mn4+ to Mn3+ as well as
oxidation of Cu0 and Cu+ to higher oxidation state. The most important result is, added Mn
enhanced methanation yield up to 5-10%. This is an agreement with Wojciechowska. M et al.
(2007) where in they found that when using manganese as based in copper catalyst increase
methane yield and activated the catalyst more compare to copper-cooper catalyst.
Wachs et al. (2005) found that some active basic metal oxide components such as MnO and
CeO did not interact strongly with the different oxide functionalities present on oxide
support and consequently, did not disperse very well to form crystalline phases. Therefore,
in research done by Wachs et al. (1996) stated that Ru could be assigned as a good dopant
towards MnO based catalyst. They are active basic metal oxides that usually anchor to the
oxide substrate by preferentially titrating the surface Lewis acid sites, such as surface M-
oxide vacancies, of the oxide support.

4.3 Noble Metals used in Methanation Catalysts

Nickel oxide will lose its catalytic ability after a few hours when it undergoes carbon
formation process. The carbon formation can be avoided by adding dopants towards the Ni
catalyst. Therefore, incorporating of noble metals will overcome this problem. Noble metals
such as rhodium, ruthenium, platinum and iridium exhibit promising CO2/H2 methanation
performance, high stability and less sensitive to coke deposition. However, from a practical
point of view, noble metals are expensive and little available. In this way, the addition of
dopants and support is good alternative to avoid the high cost of this precious metal. For the
same metal loading, activity is mainly governed by the type of metal but also depends on
precursor selection (Yaccato et al., 2005). While, the reaction selectivity depends on support
type and addition of modifier (Kusmierz, 2008).

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Methane production rates for noble metals based catalysts were found to decrease in order
Ru > Rh > Pt > Ir ~ Pd. It may be suggested that the high selectivities to CH4 of Ru and Rh
are attributed to the rapid hydrogenation of the intermediate CO, resulting in higher CO2
methanation activities. Panagiotopoulou et al. (2008) had claimed the selectivity towards
methane which typically higher than 70%, increases with increasing temperature and
approaches 100% when CO2 conversion initiated at above 250oC. A different ranking of
noble metals is observed with respect to their activity for CO2 hydrogenation, where at
350oC decreases by about one order of magnitude in the order of Pt > Ru > Pd ~ Rh. From
the research of Ali et al. (2000), the rate of hydrogenation can be increased by loading noble
metals such as palladium, ruthenium and rhodium. The results showed that all of them
perform excellently in the process of selective oxidation of CO, achieving more than 90%
conversion in most of the temperature region tested between 200oC to 300oC.
Finch and Ripley (1976) claimed that the noble metal promoters may enhance the activity of
the cobalt supported catalysts to increase the conversion to methane. In addition, the noble
metals promoted catalysts maintained greater activity for methane conversion than the non-
promoted catalysts in the presence of sulfur poison. The addition of small contents of noble
metals on cobalt oxides has been proposed in order to increase the reduction degree on the
catalytic activity of Co catalysts (Profeti et al., 2007). Research done by Miyata et al. (2006)
revealed that the addition of Rh, Pd and Pt noble metals drastically improved the behavior
of Ni/Mg(Al)O catalysts. The addition of noble metals on Ni resulted in a decrease in the
reduction temperature of Ni and an increase in the amount of H2 uptake on Ni on the
catalyst.
It well known that ruthenium is the most active methanation catalyst and highly selective
towards methane where the main products of the reaction were CH4 and water. However,
the trace amount of CO was present among the products and methanol was completely
absent (Kusmierz, 2008). Takeishi and Aika (1995) who had studied on Raney Ru catalysts
found a small amount of methanol was produced on supported Ru catalyst but the methane
gas was produced thousands of times more than the amount of methanol from CO2
hydrogenation. The selectivity to methane was 96-97% from CO2. Methane production rate
from CO2 and H2 at 500 K on their Raney Ru was estimated to be 0.25 mol g-1 h-1. The
activity for methane production from CO2 ± H2 at 433 K under 1.1 MPa was much higher
than that under atmospheric pressure. The rate of methane synthesis was 3.0 mmol g-1 h-1
and the selectivity for methane formation was 98% at 353 K, suggesting the practical use of
this catalyst (Takeish et al., 1998).
Particularly suitable for the methanation of carbon dioxide are Ru/TiO2 catalysts. Such
catalysts display their maximum activity at relatively low temperatures which is favorable
with respect to the equilibrium conversion of the strongly exothermic reaction and form
small amount of methane even at room temperature (Traa and Weitkamp, 1999). It can be
prove by VanderWiel et al. (2000) who had studied on the production of methane from CO2
via Sabatier reaction. The conversion reaches nearly 85% over 3 wt% Ru/TiO2 catalyst at
250oC and the selectivity towards methane for this catalyst was 100%.
Meanwhile, a microchannel reactor has been designed and demonstrated by Brooks et al.
(2007) to implement the Sabatier process for CO2 reduction of H2, producing H2O and CH4.
From the catalyst prepared, the powder form of Ru/TiO2 catalyst is found to provide good
performance and stability which is in agreement with Abe et al. (2008). They claimed that the
CO2 methanation reaction on Ru/TiO2 prepared by barrel-sputtering method produced a

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100% yield of CH4 at 160oC which was significantly higher than that required in the case of
Ru/TiO2 synthesize by wetness impregnation method and Gratzel method. Barrel-
sputtering method gives highly dispersed Ru nano particles deposited on the TiO2 support
which then strongly increase its methanation activity.
Another research regarding CO-selective methanation over Ru-based catalyst was done by
Galletti et al. (2009). The γ-Al2O3 to be used as Ru carrier was on purpose prepared through
the solution combustion synthesis (SCS) method. The active element Ru was added via the
incipient wetness impregnation (IWI) technique by using RuCl3 as precursor. Three Ru
loads were prepared: 3%, 4% and 5% by weight. All of the catalysts reached complete CO
conversion in different temperature ranges where simultaneously both the CO2 methanation
was kept at a low level and the reverse water gas shift reaction was negligible. The best
results were obtained with 4% Ru/γ-Al2O3 in the range of 300–340oC, which is 97.40% of CO
conversion.
For further understanding about methanation over Ru-based catalysts, Dangle et al. (2007)
conducted a research of selective CO methanation catalysts prepared by a conventional
impregnation method for fuel processing applications. It well known that metal loading and
crystallite size have an affect towards the catalyst activity and selectivity. Therefore, they
was studied the crystallite size by altering metal loading, catalyst preparation method, and
catalyst pretreatment conditions to suppress CO2 methanation. These carefully controlled
conditions result in a highly active and selective CO methanation catalyst that can achieve
very low CO concentrations while keeping hydrogen consumption relatively low. Even
operating at a gas hourly space velocity as high as 13500 h-1, a 3% Ru/Al2O3 catalyst with a
34.2 nm crystallite was shown to be capable of converting 25–78% of CO2 to CH4 over a wide
temperature range from 240 to 280oC, while keeping hydrogen consumption below 10%.
In addition, Gorke and Co-workers (2005) had carried out research on the microchannel
reactor which coated with a Ru/SiO2 and a Ru/Al2O3 catalyst. They found that the Ru/SiO2
catalyst exhibits its highest CH4 selectivity of only 82% with 90% CO2 conversion at a
temperature of 305oC, whereas a selectivity of 99% is obtained by the Ru/Al2O3 catalyst at
340oC with CO2 conversion of 78%. However, Weatherbee and Bartholomew (1984)
achieved a CH4 selectivity of 99.8% with CO2 conversion of only 5.7% at reaction
temperature of 230oC using Ru/SiO2 catalyst.
Mori et al. (1996) investigated the effect of reaction temperature on CH4 yield using Ru-MgO
under mixing and miling conditions at initial pressures of 100 Torr CO2 and 500 Torr H2. No
CH4 formation was observed at the temperatures below 80oC under mixing conditions over
Ru-MgO catalyst. It reached 31% at 130oC but leveled off at 180oC. CH4 formation over this
catalyst under milling condition increased from 11% at 80oC to 96% at 180oC. They found
that incorporating of MgO, a basic oxide to the Ru, promotes the catalytic activity by
strongly adsorbing an acidic gas of CO2. According to Chen et al. (2007), Ru impregnated on
alumina and modified with metal oxide (K2O and La2O3) showed that the activity
temperature was lowered approximately 30ºC compared with pure Ru supported on
alumina. The conversion of CO on Ru-K2O/Al2O3 and Ru-La2O3/Al2O3 was above 99% at
140–160°C, suitable to remove CO in a hydrogen-rich gas and the selectivity of Ru-
La2O3/Al2O3 was higher than that of Ru-K2O/Al2O3 in the active temperature range. While
methanation reaction was observed at temperature above 200ºC.
Other than that, Szailer et al. (2007) had studied the methanation of CO2 on noble metal
supported on TiO2 and CeO2 catalysts in the presence of H2S at temperature 548 K. It was

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Natural gas 27

observed that in the reaction gas mixture containing 22 ppm H2S, the reaction rate increased
on TiO2 and on CeO2 supported metals (Ru, Rh, Pd) but when the H2S content up to 116
ppm, the all supported catalysts was poisoned. In the absence of H2S, the result showed that
27% conversion of CO2 and 39% conversion of CO2 to methane with the presence of 22 ppm
H2S after 4 hours of the reaction.
Moreover, the addition of Rh strongly improves the activity and stability of the catalysts
(Wu and Chou, 2009), resistance to deactivation and carbon formation can be significantly
reduced (Jozwiak et al., 2005). Erdohelyi et al. (2004) studied the hydrogenation of CO2 on
Rh/TiO2. The rate of methane formation was unexpectedly higher in the CO2 + H2 reaction
on Rh/TiO2 in the presence of H2S. At higher temperature of 673 K, around 75% of
selectivity for CH4 formation and CO was also formed from the reaction. Choudhury et al.
(2006) presented the result of an Rh-modified Ni-La2O3-Ru catalyst for the selective
methanation of CO. However, the performance of the prepared catalysts was reported to be
that CO2 conversion appeared to be less than 30% when CO converted completely.
It had been reported that the addition of Pd had a positive effect for hydrogenation of CO or
CO2 because of its higher electronegativity with greater stability of Pd0 species compared to
those of Ni0 under on stream conditions (Castaño et al., 2007). In contrast, Pd/SiO2 and
Pt/SiO2 catalysts showed poor activities at temperature lower than 700 K with the CO
conversion was not greater than 22% at temperature 823 K over these catalysts (Takenaka et
al., 2004). A Pd–Mg/SiO2 catalyst synthesized from a reverse microemulsion has been found
to be active and selective for CO2 methanation (Park et al., 2009). At 450oC, the Pd-Mg/SiO2
catalyst had greater than 95% selectivity to CH4 at a carbon dioxide conversion of 59%. They
claimed that the similar catalyst without Mg has an activity only for CO2 reduction to CO.
these results support a synergistic effect between the Pd and Mg/Si oxide.
Furthermore, platinum-based catalysts present an activity and a selectivity that are almost
satisfactory. Finch and Ripley (1976) claimed that the tungsten-nickel-platinum catalyst was
substantially more active as well as sulfur resistant than the catalyst in the absence of
platinum. It was capable to show a conversion of 84% of CO after on stream for 30 minutes
in the presence of less than 0.03% CS2. No catalytic activity was observed under the poison
of 0.03% CS2 without the addition of Pt. The platinum group promoters enabled the
catalysts to maintain good activity until the critical concentration of poison was reached. Pt
catalysts were most well known as effective desulfurizing catalysts. Panagiotopolou and
Kodarides (2007) found that the platinum catalyst is inactive in the temperature range of
200oC-400oC, since temperatures higher than 450oC are required in order to achieve
conversion above 20%.
Moreover, Nishida et al. (2008) found that the addition of 0.5 wt% Pt towards cp-Cu/Zn/Al
(45/45/10) catalyst was the most effective for improving both activity and sustainability of
the catalyst. At 250oC, the conversion of CO was achieved around 77.1% under gas mixture
of CO/H2O/H2/CO2/N2 = 0.77/2.2/4.46/0.57/30 mL/min. Pierre et al (2007) found that the
conversion of CO over 5.3% PtCeOx catalyst prepared by urea gelation co-precipitation
(UGC) which was calcined at 400oC reached about 92%. This catalyst is more active and
shows excellent activity and stability with time on stream at 300oC under water gas shift
reaction.
Recently, Bi et al. (2009) found that Pt/Ce0.6Zr0.4O2 catalyst exhibited a markedly higher
activity with 90.4% CO conversion at 623 K for the water gas shift (WGS) reaction. The
methane selectivity was only 0.9% over this catalyst which has been prepared by wetness

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impregnation method. Meanwhile, Utaka et al. (2003) examined the reaction of a simulated
reforming gas over Pt-catalysts. At temperatures from 100oC to 250oC, high CO conversions
of more than 90% were obtained but most of the conversion was caused by water gas shift
reaction. The use of platinum catalyst in conversion of cyclohexane was conducted by
Songrui et al. (2006). They found that the cyclohexane conversion over Pt/Ni catalyst
prepared by impregnation method (55%-53%) was obviously higher than that over Pt/Al2O3
catalyst (30%-20%).

4.4 Supports for the Methanation Catalysts

The presence of the support was recognized to play an important role since it may influence
both the activity and selectivity of the reaction as well as control the particle morphology.
Insulating oxides such as SiO2, y-Al2O3, V2O5, TiO2 and various zeolites usually used as
material supports. These supports are used to support the fine dispersion of metal
crystallites, therefore preparing them to be available for the reactions. These oxides will
possess large surface area, numerous acidic/basic sites and metal-support interaction that
offer particular catalytic activity for many reactions (Wu and Chou, 2009).
Alumina is often used as support for nickel catalyst due to its high resistance to attrition in
the continuously stirred tank reactor or slurry bubble column reactor and its favorable
ability to stabilize a small cluster size (Xu et al., 2005). Alumina does not exhibit methanation
activity, it was found to be active for CO2 adsorption and the reverse spillover from alumina
to nickel increases the methane production especially for co-precipitated catalyst with low
nickel loading (Chen and Ren, 1997). Happel and Hnatow (1981) also said that alumina
could increase the methanation activity although there was presence of low concentration of
H2S. Additionally, Chang et al. (2003) believed that Al2O3 is a good support to promote the
nickel catalyst activity for CO2 methanation by modifying the surface properties. Supported
Ni based catalyst in the powder form usually used by many researchers and only few
researchers used a solid support for their study.
Mori et al. (1998) had studied the effect of nickel oxide catalyst on the various supports
material prepared using impregnation technique. They revealed that the reactivity of the
catalysts depended on the type of supports used which follow the order of Al2O3 > SiO2 >
TiO2 > SiO2·Al2O3. The reason for the higher activity of Ni/Al2O3 catalyst with 70%
methanation of CO2 at 500oC was attributed to the basic properties of the Al2O3 support on
which CO2 could be strongly adsorbed and kept on the catalyst even at higher temperatures.
However, Takenaka et al. (2004) found that the conversion of CO at 523 K were higher in the
order of Ni/MgO (0%) < Ni/Al2O3 (7.9%) < Ni/SiO2 (30.0%) < Ni/TiO2 (42.0%) < Ni/ZrO2
(71.0%). These results implied that Al2O3 support was not appropriate for the CO conversion
but suitable support for CO2 conversion.
In addition, Seok et al. (2002) also prepared various Ni-based catalysts for the carbon dioxide
reforming of methane in order to examine the effects of supports (Al2O3, ZrO2, CeO2, La2O3
and MnO) and preparation methods (co-precipitated and impregnated) on the catalytic
activity and stability. Catalytic activity and stability were tested at 923 K with a feed gas
ratio CH4/CO2 of 1 without a diluent gas. Co-precipitated Ni/Al2O3, Ni/ZrO2, and
Ni/CeO2 showed high initial activities but reactor plugging occurred due to the formation
of large amounts of coke. The gradual decrease in the activity was observed for Ni/La2O3
and Ni/MnO, in which smaller amounts of coke were formed than in Ni/Al2O3, Ni/ZrO2,
and Ni/CeO2. The catalyst deactivation due to coke formation also occurred for

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Natural gas 29

impregnated 5 wt% Ni/γ-Al2O3 catalysts. Addition of MnO onto this Ni/γ -Al2O3 catalyst
decreased the amount of deposited coke drastically and 90% of initial CO2 conversion was
maintained after 25 h.
Zhou et al. (2005) had synthesized the Co-Ni catalyst support with activated carbon for CO
selective catalytic oxidation. The conversion of Co-Ni/AC catalyst always keeps at above
97% in a wide reaction temperature of 120-160oC. However, the CO conversion dramatically
decreases with the increasing temperature when the reaction temperature is beyond 160oC.
Activated carbons are used efficiently in many environmental remediation processes due to
their high adsorption capacity, which makes their use possible in the removal of great
variety of pollutants present in air aqueous medium. This is because, besides their high
surface area, they possess several functional surface groups with an affinity for several
adsorbates, justifying the extreme relevance of this adsorbent for the treatment of the
pollutant (Avelar et al., 2010).
Kowalczky et al. (2008) had revealed that the reactions of trace CO2 amount with hydrogen
(low COx /H2 ratios) are dependent on the Ru dispersion and the kind of support for the
metal. Among the supports used in the present study (low and high surface area
graphitized carbons, magnesia, alumina and a magnesium–aluminum spinel), alumina was
found to be the most advantageous material. For similar Ru dispersions, CO methanation
over Ru/Al2O3 at 220oC was about 25 times and CO2 methanation was about 8 times as high
as ruthenium deposited on carbon B (Ru3/CB). For high metal dispersion, the following of
sequence was obtained: Ru/Al2O3 > Ru/MgAl2O3 > Ru/MgO > Ru/C, both for CO and CO2
methanation. It is suggested that the catalytic properties of very small ruthenium particles
are strongly affected by metal–support interactions. In the case of Ru/C systems, the carbon
support partly covers the metal surface, thus lowering the number of active sites (site
blocking).
Takenaka et al. (2004) also found that at 473 K, Ru/TiO2 catalyst showed the highest activity
among all the catalysts but when the reaction temperature increased to 523 K, the CO
conversion was follows the order of Ru/MgO (0%) < Ru/Al2O3 (62.0%) < Ru/SiO2 (85.0%) <
Ru/ZrO2 (100.0%) = Ru/TiO2 (100.0%). Similarly to Görke et al. (2005) who found that
Ru/SiO2 catalysts exhibits higher CO conversion and selectivity, compared to Ru/Al2O3.
Meanwhile, Panagiotopolou and Kodarides (2007) demonstrated that Pt/TiO2 is the most
active catalyst at low temperatures exhibiting measurable CO conversion at temperature as
low as 150oC. Conversion of CO over this catalyst increases with increasing temperature and
reach 100% at temperature of 380oC. While, platinum catalyst supported on Nd2O3, La2O3
and CeO2 become active at temperature higher than 200oC and reach 100% above 400oC.
MgO and SiO2 supported platinum catalysts are practically inactive in the temperature
range of interest.
The detailed studied on the SiO2 and Al2O3 support was investigated by Nurunnabi et al.
(2008) had investigated the performance of γ-Al2O3, α-Al2O3 and SiO2 supported Ru
catalysts prepared using conventional impregnation method. They found that γ-Al2O3
support is more effective than catalysts α-Al2O3 and SiO2 supports for Fischer-Tropsch
synthesis under reaction condition of P=20 bar, H2/CO=2 and GHSV=1800/h. γ-Al2O3
showed a moderate pore and particle size around 8 nm which achieved higher catalytic
activity about 82.6% with 3% methane selectivity than those of α-Al2O3 and SiO2 catalysts.
Pentasil-type zeolite also could be used as a support for the catalysts which exhibited high

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30 Natural Gas

activity and stability due to not only the basicity of alkaline promoters but also the
incorporation with zeolite support (Park et al., 1995).
Furthermore, Solymosi et al. (1981) reported a sequence of activity of supported rhodium
catalysts of Rh/TiO2 > Rh/Al2O3 > Rh/SiO2. This order of CO2 methanation activity and
selectivity was the same as observed for Ni on the same support by Vance and
Bartheolomew et al. (1983). These phenomena can be attributed to the different metal-
support electronic interactions which affects the bonding and the reactivity of the
chemisorbed species.

5. Conclusion
Natural gas fuel is a green fuel and becoming very demanding because it is environmental
safe and clean. Furthermore, this fuel emits lower levels of potentially harmful by-products
into the atmosphere. Most of the explored crude natural gas is of sour gas and yet, very
viable and cost effective technology is still need to be developed. Above all, methanation
technology is considered a future potential treatment method for converting the sour
natural gas to sweet natural gas.

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38 Natural Gas

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 Natural Gas
Edited by Primož PotoÄÂ​nik

ISBN 978-953-307-112-1
Hard cover, 606 pages
Publisher Sciyo
Published online 18, August, 2010
Published in print edition August, 2010

The contributions in this book present an overview of cutting edge research on natural gas which is a vital
component of world's supply of energy. Natural gas is a combustible mixture of hydrocarbon gases, primarily
methane but also heavier gaseous hydrocarbons such as ethane, propane and butane. Unlike other fossil
fuels, natural gas is clean burning and emits lower levels of potentially harmful by-products into the air.
Therefore, it is considered as one of the cleanest, safest, and most useful of all energy sources applied in
variety of residential, commercial and industrial fields. The book is organized in 25 chapters that cover various
aspects of natural gas research: technology, applications, forecasting, numerical simulations, transport and
risk assessment.

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