Chemistry of Gas Works Contaminants13 - ELQF - Chemistry of Gas Works Contaminants 18 July 2013.pdf · Chemistry of Gas Works Contaminants Judith Nathanail, Land Quality Management

www.lqm.co.uk© Land Quality Management 20131

Chemistry of Gas Works Contaminants

Judith Nathanail, Land Quality Management Ltd

[email protected]


Gas Works Contaminants

•Ammonia liquors

•Coal tar•Spent oxide•PAHs•Free cyanide,•complex cyanide

•Elemental sulphur, sulphate, thiocyanate,

•Metals•Acidic soils.


Chemistry of Gas Works

Contaminants

• Follow the molecules and substances from coal via gas works process and into waste stream


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


http://www.tutorvista.com/content/chemistry/chemistry-i/coal-petroleum/coal.php#destructive-distillation-of-coal

Purification

Waste/by products



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


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


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


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


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



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+


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


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


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



Tar Well• Gravity separation

• Lighter ammoniacal liqour floats on top• Decanted liquor removed from upper layer

• Heavier coal tar sinks to base

TarAmmonia


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


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


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


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


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


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


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


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


Transition elements


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



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


Sulphur

• From S in coal

• Sulphur dioxide

• Hydrogen sulphide

• Sulphuric acid


Metals

U

Mg

Mn CoCr

Mo

V Fe Ni Cu Zn

Cd

Hg Pb


Transition elements

U

As


Transition elements

Metals often found on gas works sites


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


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



Land Quality Management Ltd.

The Sir Colin Campbell Building

University of Nottingham Innovation Park

Triumph Road

Nottingham NG7 2TU

Tel: +44 (0)115 7484080

Fax: +44 (0)115 7484083

www.lqm.co.uk

[email protected]

LQM OfficeAspire

From Nottingham

Bus Station (2 mins from

rail station); take i4 or

Indigo; alight at Faraday

Road

Chemistry of Gas Works Contaminants13 - ELQF - Chemistry of Gas Works Contaminants 18 July 2013.pdf · Chemistry of Gas Works Contaminants Judith Nathanail, Land Quality Management

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