Peter J. H. Carnell and Vince Atma Row,
Johnson Matthey Process Technologies,
UK, examine the origins of mercury in
fossil fuels and the means for its removal
and safe disposal.
M
ercury is the only common liquid metal and it rarely occurs free in nature. The chief ore is cinnabar
(HgS), which is found in a limited number of locations. Mercury is found in fossil fuels but the
levels present are variable from a few ppb to 30 000 ppb. Mercury is a potent biocide and hence
it is surprising that it should be found in fossil fuels and also at varying concentrations. This suggests that a
non-biological mechanism may be responsible for its occurrence. Processing fossil fuels releases any mercury
present to the atmosphere and because of its volatility it is now widely distributed. Generally, levels of mercury
in the atmosphere are still extremely low but can be high in locations where fossil fuels are being processed or
used and this can lead to high levels in water and streams. Here, highly toxic organomercury compounds (e.g.
dimethyl mercury) can be formed. Fixed bed absorbents provide a safe and practical method for the removal
of mercury during the processing stages and the spent material can be recycled with the mercury recovered for
auditable storage.
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volcanic activity but other defects such as the Karpinsky
Lineament allow progress towards the surface. This has
resulted in the creation of the so-called ‘mercuriferous belts’
around the globe (see Figure 2).2
Escape to the atmosphere is prevented by meeting an
impervious layer or by chemical reaction to yield non-volatile
compounds. Mercury has a strong affinity for elemental sulfur
and its organic compounds so, unsurprisingly, it is trapped as the
sulfide (cinnabar HgS). This ore was the main source of mercury
from mines in Spain, China, Algeria, USA (California) and
Kyrgyzstan, all of which lie within the mercuriferous belts.
Impervious layers are provided by the formations (salt, etc.)
that trap and retain gaseous and liquid hydrocarbons.
Mercury that has escaped to the atmosphere is
eventually collected and trapped in water. Here it
may be fixed as organomercury or by reaction with
sulfur or sulfur containing compounds.
This chemistry is particularly interesting.
Elemental mercury reacts with elemental sulfur to
form mercuric sulfide and with some transition metal
sulfides to yield mixed sulfides. Ionic mercury reacts
with organic sulfur compounds to yield mercaptides
and sulfides. In fact, ‘mercaptans’ derive their name
from this reaction and the formation of these
compounds was used as a means of characterising
the organic sulfur compounds found in crude oil.
Figure 1. Elements essential for life.1
In the early days of the oil industry, mercuric
chloride was used to ‘sweeten’ kerosene by
precipitating mercury mercaptides and sulfides.3 The
melting points of a few compounds formed between
mercuric chloride and organic sulfur compounds are
given in Table 1.
Unlike oil and gas, the mercury content of coal
has been extensively studied.4,5 The worldwide
chemical element average content (Clarke value) of
mercury in coal is 0.1 ± 0.01 ppm and is the same for
bituminous, sub-bituminous and lignite rank coals.
However, there are variations in the level of mercury
found in any given reserve (both depth and location),
as well as variations in different locations.4,5 The level
of mercury in coal does appear to be linked to the
sulfur content and there is some evidence that
Figure 2. Mercuriferous belts: global distribution of anthropogenic mercury
mercury is deposited by vapour phase absorption or
emissions to air in 2010. (Source: United Nations Environment Programme
by a hydrothermal process rather than as a
(UNEP), Global Mercury Assessment, 2013.)
component of the organic matter.
All fossil fuels contain mercury. The amount and
Table 1. Melting points of mercuro-sulfur compounds
form depends on the nature of the hydrocarbon and
Sulfur compound
Mercury derivative
Melting point (°C)
the location. Thus mercury tends to be present in
coal as pyrites, in oil both as elemental and
176
MeSH
(MeS)2 Hg
organomercury, and in natural gas in the elemental
85
EtSH
(EtS)2 Hg
form as show in Table 2.
Oil and gas occurring in Southeast Asia have
Et2SHgCI2
77
Et2S
particularly high levels of mercury. Figure 3 shows
Pr2S
Pr2SHgCI2
88
the mercury hot spots in the region. Paradoxically,
many gas fields, some even in this area, have low
levels of mercury.
Table 2. Mercury level range in fossil fuels6,7
Most of the mercury found in the earth’s current
Fossil fuel
Mercury compounds present
Amount
atmosphere can be traced back to the combustion of
fossil fuels and much of the blame is laid on
Coal
Pyrites
10 - 100+ ppb
coal-fired power stations. Whilst it is true that a
coal-fired power station will be a very large point
Oil
Elemental and organomercury
10 - 30 000 ppb
source for mercury emissions, more oil and gas is
Natural gas
Elemental
Trace up to 4000 µg/m3
used than coal.
Mercury is not among the 32 naturally occurring elements
deemed necessary for life (see Figure 1). In fact, mercury, or more
precisely its compounds, are among the most powerful biocides
known to man.
Many of the transition metal elements are thought to
have arrived on the surface of the earth as a result of
collisions with meteorites. There is no suggestion that
mercury arrived this way – in fact, its low boiling point
(346°C) makes this unlikely. It is perhaps more likely that its
origin is in the earth’s core.
Like other volatile materials, mercury progresses towards
the surface of the earth. The easiest route is where there is
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2014
Removal of mercury during gas
processing and LNG production
in spite of its high molecular weight and high boiling point
it is surprisingly mobile and reactive. Mercury can easily be
removed from gaseous and liquid hydrocarbons by reaction
with fixed bed absorbents of a variable valency metal sulfide:
Raw natural gas is likely to contain water, carbon dioxide (CO2)
and hydrogen sulfide (H2S). These have to be substantially
removed to avoid corrosion and the formation of hydrocarbon
Hg + MxSy = MxSa + HgS
hydrates. This is achieved in a two stage wash process. The
acid gases are removed with an amine wash and the water with
The reactive metal is incorporated in an inorganic support
a glycol wash. A typical gas processing flowsheet is shown in
and the absorbent is supplied with the reactive sulfide present or
Figure 4.
A substantial amount of the mercury present
in the raw gas will be removed in these wash
stages. Regeneration of the liquids involves
flashing off dissolved gases and then heating the
wash liquor near to its boiling point, around 100°C
for the amine and 200°C for the glycol.
Regeneration results in the mercury being
discharged to the atmosphere in the vent gas or
fuel gas. Thus mercury levels as high as
28 000 ng/m3 have been found in stripper gas.
Around half of any mercury present in the raw gas
will be released to the atmosphere at these
stages.9
LNG production requires gas drying to a lower
dew point than can be achieved with glycol driers,
and these plants use molecular sieves in the final
drying stage. These will adsorb mercury and
Figure 3. Mercury hot spots for oil and gas found in Southeast Asia.8
Distribution of mercury presence in hydrocarbon production streams in the
release it during regeneration at levels as high as
region. (Source: BPMIGAS – NORAD – PETRAD – INTSOK – CCOP: seminar
60 000 ng/m3 during the initial regeneration stage.
on stranded gas including low permeability reservoirs and mercury issues.)
This gas is usually fed into the plant fuel gas
system.
Mercury distribution on
gas plants
Little data has been published on the distribution of
mercury on gas processing plants and its monitoring
has tended to be only from specific feed and product
streams. Over the last few years a number of surveys
have been carried out by Johnson Matthey on gas
processing plants located in the UK, North Africa, the
Far East and South America. Measurements were
made using the Sir Gallahad II mercury analyser.
The nature of the plants and the difficulty of
carrying out the measurements meant that it was not
possible to carry out a mass balance on the
distribution of mercury through the various process
streams. Instead a number of readings were obtained
showing the steady state concentrations of mercury at
the various process stages. Not all of the plants had
all of these processing stages and only two of the
seven plants quoted had mercury removal beds.
Figure 5 clearly shows that mercury is distributed
right through the plant and is adsorbed on all the
metal surfaces, which makes maintenance and
decommissioning of redundant equipment highly
hazardous.
Figure 4. Acid gas removal and drying stages of a gas processing plant.
Removal of mercury
from gaseous and liquid
hydrocarbons
Mercury is most commonly present in gaseous
and liquid hydrocarbons in the elemental form.
Furthermore, mercury vapour is monatomic so that
Figure 5. Composite of mercury distribution on gas processing plants.10
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this is formed by reaction with H2S in the hydrocarbon to be
treated.10
The mercury laden spent absorbent is then recycled through
metal smelters and the mercury is recovered for safe storage.
It is possible to use fixed bed reactors to remove mercury
from the vent gases. However, these tend to be at modest
pressures and the reaction kinetics would require the use of
relatively large beds so it is preferable to treat the raw gas where
it is at a high pressure.
Fixed bed technology
The presence of mercury in crude oil, natural gas and natural
gas liquids is largely ignored unless there is the risk of corrosion
of metal equipment or damage to catalysts on the processing
plants. This approach can result in the release of mercury to the
atmosphere from refineries and gas processing plants during
the production of pipeline gas and transportation fuels. The
high mobility of mercury also means that elemental mercury is
present in the product hydrocarbon gas and liquid streams and
this is released to the atmosphere when used.
Johnson Matthey has developed PURASPECJMTM* fixed bed
technology that is well suited for the removal of mercury from
raw natural gas and liquid hydrocarbon product streams. The
spent absorbent is shipped to accredited smelters where, after
processing, a certificate of safe disposal is obtained. The
handling and transportation procedures are now well
established.
There are more than 250 proprietary PURASPECJM mercury
removal reactors in service on a range of hydrocarbons, with
more than 100 units in commercial operation on liquid
hydrocarbons.
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More widespread adoption of this technology would help
reduce the level of mercury from refineries and gas processing
plants.
Note: *PURASPEC is a trademark of the Johnson Matthey group
of companies.
References
1. Williams, R. J. P, and Rickaby, R. E .M. ‘Evolution’s
Destiny’, RSC Publishing, August 2012, ISBN
9781849735582.
2. Bailey, et. al., ‘Mercurifous Belts’, 1973.
3. Bruson, H. A., Resinous Products & Chem. Co., US patents
2,237, 584 2, 258, 130, 1941, 2,277,267, 1942.
4. Emmet Reid, E., ‘Organic Chemistry of Bivalent Sulfur’,
Volume 1, 1958.
5. Emmet Reid, E., ‘Organic Chemistry of Bivalent Sulfur’,
Volume 2, 1960.
6. Yudovich, Y. E., and Ketris, M. P., ‘Mercury in coal: a
review Part 1. Geochemistry’, International Journal of Coal
Geology, 62, 2005, pp. 107 - 134.
7. Carnell, P. J. H. and Row, V. A., ‘A Re-think of mercury
removal from LNG Plants’, Barcelona, Spain, 23 - 26 April
2007.
8. BPMIGAS – NORAD – PETRAD – INTSOK – CCOP Seminar
on Stranded Gas Including Low Permeability Reservoirs
and Mercury issues.
9. Catchpole, S., ‘Mercury Removal in Hydrocarbon Streams’,
PTQ Catalysis, Volume 14, 2009, pp. 39 - 45.
10.Carnell, P. J. H., and Opensahw, P. J., ‘Mercury distribution
on gas processing plants’, GPA Europe, Spring Mtg.,
Dublin, Ireland, 19 - 21 May 2004.