Chemistry of Gas Works Contaminants

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1 Chemistry of Gas Works Contaminants Judith Nathanail, Land Quality Management Ltd 1

2 Gas Works Contaminants Ammonia liquors Coal tar Spent oxide PAHs Free cyanide, complex cyanide Elemental sulphur, sulphate, thiocyanate, Metals Acidic soils. 2

3 Chemistry of Gas Works Contaminants Follow the molecules and substances from coal via gas works process and into waste stream 3

Coal readily combustible more than 50% by weight of carbon.

a vast number of rings and aliphatic chains (eg decane) and

4 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 - C 135 H 96 O 9 NS CH 0.73 O 0.08 N 0.01 S

5 Purification Waste/by products 5

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methane, hydrocarbons, hydrogen, carbon monoxide, carbon dioxide, nitrogen, HCN, NH 3 Coke

7 Solid Retort Solid Gas Heat coal in a closed vessel (no O 2 ) 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, NH 3 Coke in retort almost pure C Tar Gas Ammonia 7

Coke removed and quenched Liquors produced Remaining gases within Coke condense and some dissolve http://en.wikipedia.

8 Coke removed and quenched Liquors produced Remaining gases within Coke condense and some dissolve Tar Ammonia Gasworkers withdrawing a completed charge of coke. Instant flame; quench water became ammoniacal toxic liqour 8

9 Hydraulic main Hydraulic main. gas bubbles through liquid Tar and ammonia condenses and mixes with water Hydraulic Main Ascension Pipes Retort Gases condense and some dissolve Tar Ammonia 9

10 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 10

11 Coal Tar Viscous liquid mixture Phenols - C 6 H 5 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) OH Pyridine C 5 H 5 N 11

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13 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 NH 3 and NH 4 + Tar 13

14 Ammoniacal liquor solution of ammonia, ammonium compounds phenols sulphur compounds N and S from coal C 135 H 96 O 9 NS 14

15 Ammonia NH 3 soluble in water (at lower temperatures) NH 4 + NH 3 and NH 4+ 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 15

16 The Periodic Table I II III IV V VI VII 0 H He Li Na Be Mg Transition elements B Al C Si N P O S F Cl Ne Ar K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe Cs Ba Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn Fr Ra Rf Db Sg Bh Hs Mt La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr 16

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18 Ammonia Tar Tar Well Gravity separation Lighter ammoniacal liqour floats on top Decanted liquor removed from upper layer Heavier coal tar sinks to base 18

19 Gas scrubber Transfer particulate matter from gas to sorbants Remaining ammonia and some H 2 S and HCN were removed by scrubbing gas with water or weak ammoniacal liquor H 2 S Hydrogen sulphide stink bombs HCN Hydrogen cyanide (aka Prussic Acid) - a systemic chemical asphyxiant 19

20 Purifier remove remaining impurities mainly sulphides and cyanogens Early works SO 2 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) 6.12H 2 O Calcium thiocarbonate Later works Iron oxides eg bog ore (peat with a lot of iron oxide) Iron sulphides Complex cyanides 21

21 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 24

22 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 25

23 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 26

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

24 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 27

25 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 Look at periodic table to show TMs TM metals can accept extra electrons 28

26 The Periodic Table I II III IV V VI VII 0 H He Li Na Be Mg Transition elements B Al C Si N P O S F Cl Ne Ar K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe Cs Ba Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn Fr Ra Rf Db Sg Bh Hs Mt La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr 29

27 Thiocyanate (IUPAC = cyanosulfanide) Cyanide ion combined with sulfur anion [SCN]. produced by the reaction of elemental sulfur or thiosulphate with cyanide 8 CN + S 8 8 SCN CN + S 2 O 3 2 SCN + SO 3 2 Does dissociate under weak acidic conditions but has similar complexing properties to cyanide approximately 7 times less toxic than hydrogen cyanide 30

28 Jargon buster The prefix thio-, an oxygen atom in the compound has been replaced by a sulfur atom Thiosulphate Thiocyanate Sulphate Cyanate Thiosulphate Thiocarbonate e.g. CSO 2 2- ThioCyanate Carbonate CS 3-x O x 2- (x = 0, 1, or 2).

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30 Distribution/storage Gas meter Fakenham Gas Works Tar Gas continues to drop particles from tar fog/mist into water beneath 36

31 Sulphur From S in coal Sulphur dioxide Hydrogen sulphide Sulphuric acid 38

32 Metals Metals often found on gas works sites I II III IV V VI VII 0 Mg Transition elements V Cr Mn Fe Co Ni Cu Zn As Mo Cd Hg Pb U 40

33 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 41

34 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 42

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36 LQM Office Aspire Land Quality Management Ltd. The Sir Colin Campbell Building University of Nottingham Innovation Park Triumph Road Nottingham NG7 2TU Tel: +44 (0) Fax: +44 (0) From Nottingham Bus Station (2 mins from rail station); take i4 or Indigo; alight at Faraday Road 44

Families on the Periodic Table

Families on the Periodic Table Elements on the periodic table can be grouped into families based on their chemical properties. Each family has a specific name to differentiate it from the other families