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www.lqm.co.uk© Land Quality Management 20131
Chemistry of Gas Works Contaminants
Judith Nathanail, Land Quality Management Ltd
www.lqm.co.uk© Land Quality Management 20132
Gas Works Contaminants
•Ammonia liquors
•Coal tar•Spent oxide•PAHs•Free cyanide,•complex cyanide
•Elemental sulphur, sulphate, thiocyanate,
•Metals•Acidic soils.
www.lqm.co.uk© Land Quality Management 20133
Chemistry of Gas Works
Contaminants
• Follow the molecules and substances from coal via gas works process and into waste stream
www.lqm.co.uk© Land Quality Management 20134
Coal• readily combustible• more than 50% by weight of
carbon.• Comprises carbon, volatiles
and moisture • other constituents include,
nitrogen, ash, and sulphur.• hydrocarbons are fused
aromatic ring compounds, each containing a vast number of rings and aliphatic chains (eg decane) and rings (eg cyclohexane)
• Chemical formula of coal -
C135H96O9NS
CH0.73O0.08N0.01S0.01
www.lqm.co.uk© Land Quality Management 20135
http://www.tutorvista.com/content/chemistry/chemistry-i/coal-petroleum/coal.php#destructive-distillation-of-coal
Purification
Waste/by products
www.lqm.co.uk© Land Quality Management 20137
Retort
• Heat coal in a closed vessel (no O2) “a retort”,
• Drives off volatiles • (including water)
• Thermal decomposition (not combustion)
• Gases go up feed pipes• methane, hydrocarbons,
hydrogen, carbon monoxide, carbon dioxide, nitrogen, HCN, NH3
• Coke in retort• almost pure C
http://www.nationalgasmuseum.org.uk/index.asp?page=history-02
Solid
Gas Solid
Gas
Tar Ammonia
www.lqm.co.uk© Land Quality Management 20138
http://en.wikipedia.org/wiki/History_of_manufactured_gas
Gasworkers withdrawing a completed charge of coke.
Instant flame; quench water became ammoniacal toxic liqour
Coke removed
and quenched
Liquors
produced
Remaining gases within
Coke condense and some dissolve
Tar Ammonia
www.lqm.co.uk© Land Quality Management 20139
Hydraulic main• Hydraulic main.
• gas bubbles through liquid
• Tar and ammonia condenses and mixes with water
Retort
Ascension Pipes
Hydraulic Main
Tar AmmoniaGases condense and some
dissolve
www.lqm.co.uk© Land Quality Management 201310
Condenser
• Further cools down gas and condense water vapour and liquifiable hydrocarbons,
• Volatiles condense into a dark amber liquid called 'liquor', • drained off from the condensers
into a tar pit or 'well'.
• What is coal tar?Gases condense and some
dissolve
Tar Ammonia
www.lqm.co.uk© Land Quality Management 201311
Coal Tar
• Viscous liquid•mixture• Phenols - C6 H5 OH
• alcohol (OH) groups attached to benzene rings:
• PAHs - fused aromatic rings
• heterocyclic compound• a cyclic compound that has atoms of at least two different elements as members of its ring (e.g. C + N)
Pyridine
C5H5N
OH
www.lqm.co.uk© Land Quality Management 201313
Ammonia washer
• Gas is bubbled up through water which absorbs the ammonia in the gas.
• 'ammoniacal liquor' is drained off, often into the same pit as the tar (where it floats on top of the tar).
• At larger works - this was followed by Livsey washer
• What is Ammoniacal liquour?• liquid with ammonia dissolved in it
Tar
NH3 and NH4+
www.lqm.co.uk© Land Quality Management 201314
Ammoniacal liquor
• solution of
• ammonia,
• ammonium compounds
• phenols
• sulphur compounds…
• N and S – from coal
• C135H96O9NS
http://marysgasbook.blogspot.co.uk/2009/08/waht-to-do-
with-waste-ammoniacal-liquor.html
www.lqm.co.uk© Land Quality Management 201315
Ammonia
• NH3
• soluble in water (at lower temperatures)• NH4
+
• NH3 and NH4+ both
present in water
• distinctive odor
• Ammonia generated with gas as ammonia vapours, then condensed to ammonia liqour
5 electrons in outer shell
Look at periodic table to show
why 1N bonds with 3H
www.lqm.co.uk© Land Quality Management 201316
The Periodic Table
Li
Na
K
Rb
Cs
Fr
Be
Mg
Ca
Sr
Ba
Ra
Sc
Y
Ti
Zr
Hf
Rf
Mn
Tc
Re
Bh
Co
Rh
Ir
Mt
Cr
Mo
W
Sg
V
Nb
Ta
Db
Fe
Ru
Os
Hs
Ni
Pd
Pt
H
Cu
Ag
Au
B
Zn
Cd
Hg
Al
Ga
In
Tl
C
Si
Ge
Sn
Pb
N
P
As
Sb
Bi
O
S
Se
Te
Po
F
Cl
Br
I
At
He
Ne
Ar
Kr
Xe
Rn
La
Ac
Ce
Th
Pr
Pa
Nd
U
Pm
Np
Sm
Pu
Eu
Am
Gd
Cm
Tb
Bk
Dy
Cf
Ho
Es
Er
Fm
Tm
Md
Yb
No
Lu
Lr
I II III IV VIV VII 0
Transition elements
www.lqm.co.uk© Land Quality Management 201318
Tar Well• Gravity separation
• Lighter ammoniacal liqour floats on top• Decanted liquor removed from upper layer
• Heavier coal tar sinks to base
TarAmmonia
www.lqm.co.uk© Land Quality Management 201319
Gas scrubber• Transfer particulate matter from gas to sorbants
• Remaining ammonia and some H2S and HCN were removed by scrubbing gas with water or weak ammoniacal liquor
• H2S Hydrogen sulphide – stink bombs
• HCN Hydrogen cyanide (aka Prussic Acid) - a systemic chemical asphyxiant
www.lqm.co.uk© Land Quality Management 201321
Purifier
• remove remaining impurities – mainly sulphides and cyanogens
• Early works – SO2 passed over 'slaked lime', (Calcium hydroxide, Ca(OH)2).• Produces foul lime – mixture of slaked lime and calcium
compounds containing sulphur or cyanide• Calcium thiocyanate Ca (SCN)2• Calcium ferrocyanide Ca2 Fe(N)6.12H2O• Calcium thiocarbonate
• Later works – Iron oxides eg bog ore (peat with a lot of iron oxide)• Iron sulphides• Complex cyanides
www.lqm.co.uk© Land Quality Management 201324
Cyanide
• A compound which contains cyano group
• C triple-bonded to N
• cyanide ion (CN-)
• Free cyanide
• Complex cyanide
• Cyanide is toxic because it binds with iron in body – impedes respiration
www.lqm.co.uk© Land Quality Management 201325
Free cyanide
• Cyanide anion itself
• hydrogen cyanide, HCN, • gaseous
• aqueous
• produced by the combustion of certain materials under oxygen-deficient conditions
• Used as chemical weapon in WW1 but replaced by Chlorine
• Very reactive
www.lqm.co.uk© Land Quality Management 201326
Free cyanide - toxicity
• In body CN- binds to haemoglobin (Hb) in red blood cells.
• From there it is taken to the body's tissues where it binds to an enzyme called cytochrome oxidase
• Stops cells from being able to use oxygen
Come back to this after discussing ligands
www.lqm.co.uk© Land Quality Management 201327
Complex cyanide
• CN- ion forms ligands with Fe (and other TMs eg Fe, Cu, Ni, Pb, Cd)
• Forms ferrocyanides and ferricyanates
• Complex cyanide less toxic than free cyanide
• Complexes may dissociate to release free cyanide • in acid conditions• in UV to form CN- and so HCN
• May persist for 100s of years
Look at ligand slide
www.lqm.co.uk© Land Quality Management 201328
Jargon buster
What is a Ligand?Ligand = negative ion or
neutral molecule which
can form a complex with
a central metal cation
Complex = ligand plus cation
TM metals can accept extra electrons
Look at
periodic table
to show TMs
www.lqm.co.uk© Land Quality Management 201329
The Periodic Table
Li
Na
K
Rb
Cs
Fr
Be
Mg
Ca
Sr
Ba
Ra
Sc
Y
Ti
Zr
Hf
Rf
Mn
Tc
Re
Bh
Co
Rh
Ir
Mt
Cr
Mo
W
Sg
V
Nb
Ta
Db
Fe
Ru
Os
Hs
Ni
Pd
Pt
H
Cu
Ag
Au
B
Zn
Cd
Hg
Al
Ga
In
Tl
C
Si
Ge
Sn
Pb
N
P
As
Sb
Bi
O
S
Se
Te
Po
F
Cl
Br
I
At
He
Ne
Ar
Kr
Xe
Rn
La
Ac
Ce
Th
Pr
Pa
Nd
U
Pm
Np
Sm
Pu
Eu
Am
Gd
Cm
Tb
Bk
Dy
Cf
Ho
Es
Er
Fm
Tm
Md
Yb
No
Lu
Lr
I II III IV VIV VII 0
Transition elements
www.lqm.co.uk© Land Quality Management 201330
Thiocyanate
(IUPAC = cyanosulfanide)
• Cyanide ion combined with sulfur • anion [SCN]−.
• produced by the reaction of elemental sulfur or thiosulphate with cyanide• 8 CN− + S8 → 8 SCN−
• CN− + S2O32− → SCN− + SO3
2−
• Does dissociate under weak acidic conditions • but has similar complexing properties to cyanide• approximately 7 times less toxic than hydrogen cyanide
www.lqm.co.uk
Jargon buster
The prefix thio-,
• an oxygen atom in the compound has been replaced by a sulfur atom
• Thiosulphate
• Thiocyanate
• Thiocarbonate e.g.
•CSO22-
Sulphate Thiosulphate
Cyanate
ThioCyanate
CS3-xOx2- (x = 0, 1, or 2).
Carbonate
www.lqm.co.uk© Land Quality Management 201336
Distribution/storage
http://en.wikipedia.org/wiki/History_of_manufactured_gas
http://iangrey.org/2008/06/03/lifes-a-gas/
Gas meter – Fakenham Gas Works
Gas continues to drop particles from tar fog/mist into water beneath
Tar
www.lqm.co.uk© Land Quality Management 201338
Sulphur
• From S in coal
• Sulphur dioxide
• Hydrogen sulphide
• Sulphuric acid
www.lqm.co.uk© Land Quality Management 201340
Metals
U
Mg
Mn CoCr
Mo
V Fe Ni Cu Zn
Cd
Hg Pb
I II III IV VIV VII 0
Transition elements
U
As
I II III IV VIV VII 0
Transition elements
Metals often found on gas works sites
www.lqm.co.uk© Land Quality Management 201341
Metals
• Metal oxides = catalysts
• Trace elements in bog ore – hydrated ferric oxide mixed with peat
• Spent oxide
• Foul lime
• Lead in paint and as caulking for gas holders and pipework
www.lqm.co.uk© Land Quality Management 201342
Gas Works Contaminants
• From Coal• Ammonia liquors
• Ash and clinker (metals and unburned coke or coal)
• Coal tar
• spent oxide
• free cyanide, complex cyanide
• elemental sulphur, sulphate, thiocyanate
• Metals
• acidic soils
• PAHs
• BTEX
• Purification processes• spent oxide
• free cyanide, complex cyanide
• elemental sulphur, sulphate, thiocyanate
• metals
• acidic soils
• Coal Tar processing• PAHs
• BTEX
• metals
www.lqm.co.uk© Land Quality Management 201344
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University of Nottingham Innovation Park
Triumph Road
Nottingham NG7 2TU
Tel: +44 (0)115 7484080
Fax: +44 (0)115 7484083
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