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The
natural gas market world-wide is
huge. Although there is a need to
provide a standardized gas such that all
the end users’
gas burners will function as intended,
there are regional differences in
specifications. The US market has this
challenge that makes the
interchangeability of gases difficult,
and the cost and feasibility of
standardizing has been considered but
discarded.
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In the UK, however, a similar
conversion was done area by area
in the 1960s and 1970s as the
market was converted from
‘town
gas’
to
‘North
Sea gas’.
(Town gas was synthesized by
gasification of coal.) Town gas
was common in Europe until the
advent of gas finds in the North
Sea. Pipelines from these and
Russian fields serves this
market today. North America has
had a change of fortune in
recent years by technology
enabling the production of
so-called shale gas. There have
also been LNG projects
developed, with more coming on
stream in the next few years.
Gas is challenged by other forms
of energy.
Although existing users are to
an extent
‘sitting
ducks’
due to investments made,
provision costs of gas must be
kept in check to keep its market
share. Electricity is the
immediate competitor in the
retail market, and that in turn
could be provided through the
combustion of gas, coal or oil,
and other sources are nuclear
power plants and
hydroelectricity. The more
alternatives that are available
in any one market, the more the
focus on provision cost of
energy in the market. Deeper
discussions of these issues may
be found elsewhere (BP, 2011;
IGU, 2013a,b; Natural Gas Supply
Association, 2005). |
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Specifications of natural gas as a
product is a very interesting topic in
many ways and the specifications really
determine what treatment a gas
eventually needs. There are two
dimensions to this. One is the transport
system that supplies a market and what
treatment the gas needs to uphold flow
assurance in the supply chain. The other
is the end market with its appliances
where gas burners have been fitted with
certain gas properties in mind.
Interchangeability of gas cannot be
taken for granted. There are many
stumbling blocks to this (IGU, 2011).
Methane, or natural gas, is less
reactive than their heavier analogues
like ethane, propane and so on. As
feedstock for making hydrogen as in the
ammonia process it is the preferred
starting point as the ratio of hydrogen
to carbon is highest in methane. For
this reason, and because of the pricing,
natural gas is the feedstock of choice
for this purpose.
The C2+
fraction of the natural gas has in the
main a higher market value as feedstock
than as fuel. Hence the opportunity to
separate these components from the gas
is often taken. The economics of this
has varied over time though.
Sizes
For various assessments it is valuable
to have a feel for sizes of plants and
associated variables. The question
being, what is big, what is small, what
is a challenge and what is trivial.
Plant sizes and complexities will vary
widely. Perhaps the simplest gas
treating plant to be encountered in this
context will the end-of-pipe solution
scrubber where some contaminant is to be
removed from an effluent gas stream
before being released. Maybe this
scrubber has a packing height of 3m and
a diameter of 2 m, and furthermore when
the absorbent has done its job, it may
be returned to the process without
further ado. A 400MW CCGT (Combined
Cycle Gas Turbine) power plant that
needs CO2 abatement will have a gas
stream in the order of 1.8millionm3/h,
and the absorber would have a diameter
around 17m if there is one train only.
A large synthesis gas train may have a
gas flow in the order of 10 000 kmol/h.
This would be 224 000Nm3/h. However, the
pressure could be around 25 bar if this
was an ammonia plant, and this would
imply a real gas stream in the order of
10 000m3/h. In natural gas treating
there is a wide range of plants.
A fairly small one might be 10 MMSCFD.
This is a typical way of specifying
plant size in North America. MMstands
for
‘mille-mille’,
which is Latin inspired, meaning 1000
×
1000 (or a million). SCF is Standard
Cubic Feet, and D implies per 24 hours
(a Day). In North America
‘Standard’
means the gas volume is at 60ºF
and an absolute pressure of 14.696 psi (psi
=pounds per square inch).
the
‘standard’
pressure may also be 14.73 psi, which is
based on a pressure of 30 in. of a
mercury column. Beware; if you are
buying gas the difference in what you
get is 0.23%, which is not to be given
away easily in negotiations. A large gas
plant could be in the region of 2million
Sm3/day. This is typical of a gas field
in the North Sea. This is in metric
units, and the
‘standard’
now implies 15ºC and 1.013 bar. If this
was indeed the gas’s
temperature and pressure it would be at
its
‘standard
conditions.’
Note that 15ºC and 60ºF are not
identical. European and American
standard conditions are not equal:
something to be kept in mind when
selling and buying.
An often used specification for H2S
allowed in natural gas is 0.25 grain per
100 SCF. This is a US term. One
‘grain’
is 1/7000th of a pound (lb). LNG plants
are usually referred to in million
tonnes of LNG per year. A plant of
3million tonnes per year was considered
big less than 10 years ago, but
one-train capacities have been stretched
to 5–7
and there is a new generation of plants
with a third refrigeration loop that
could take the capacity to 10 million or
more.
A large ammonia plant today would
typically be 2000 tonnes per day. This
is almost the double of what was usual
around 1970. Cryogenic air separation
units (ASU) could be as big as 3500
tonne of oxygen per day, but this size
of plant is rare. Traditionally they
have been built to provide oxygen for
steel works. However, they figure in
present day studies on oxy-fuel plants.
That is, power plants where
hydrocarbons, or coal more likely, is
combusted with oxygen to make the CO2
resulting more easily accessible for
capture and storage.
It is good to develop an intuitive sense
for plant sizes and put them into
perspective. The ability to distinguish
between the various
‘standard’
units of gas quantity is a must. To help
in this direction and to summarize the
earlier discussion of plant sizes.
Ambient Conditions
Plants have been built in all sorts of
places. Some are hot, some are cold and
some are to be found at a high altitude
where the air is thin. When comparing
plant costs and efficiencies, this must
be kept in mind. An LNG plant will of
course have a better efficiency if the
heat sink is at 5ºC compared 35ºC. On
the other hand winterization may be
costly. Special precautions must be made
if it is to be operated for weeks on end
at
−40ºC.
References:
1. Gas Treating - Absorption Theory
and Practice - DAG A. EIMER
2. Fundamentals of Natural Gas, Arthur
J. Kidnay & William R. Parrish |
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