SULPHUR CAPTURING DURING A FIXED-BED GASIFICATION PROCESS OF COAL

Transcription

1 SULPHUR CAPTURING DURING A FIXED-BED GASIFICATION PROCESS OF COAL M. Pat Skhonde, J. Reginald Bunt, A. Christien Strydom, Harold Schobert 4 th International Conference on Clean Coal Technologies May 2009 copyright, copyright reserved, Sasol Technology, 18 May 2009

2 Background Coal is an abundant resource with a promising future in energy and petrochemicals production Coal is also a potential source of pollution, with increased utilisation leading to even more environmental footprints with sulphur emissions from coal processing plants one of the environmental concerns H 2 S and COS are emitted from coal gasification with SO 2 and SO 3 from coal combustion processes Sasol Fixed Bed Dry Bottom gasification process as deployed at the Sasol plant in Secunda, South Africa consumes more than 30million tons of coal per annum leading to the production of more than barrels/day of fuels and chemicals Coals used in the Secunda operation gasification plant have low sulphur content (1-2%).

3 Gasification process

4 The fate of sulphur in the Secunda plant SO x to FGD Steam plant Sulphur in coals from mines Coal preparation plant gasification recovery unit Gas cooling H 2 S in raw gas Elemental S/H 2 SO 4 product S in ash S in gas liquor tar/oil/naphtha

5 H 2 S emissions reduction options Pre-gasification Selective mining and blending Coal destoning Post gasification Sulpholin process for sulphur production Chemical absorption with Clause Sulphuric acid production In-situ processes??

6 Sulphur during coal gasification (Skhonde et al., 2009) Sulphur species in the crystalline phase in ash (based on XRD) FeS 2 + H 2 Fe (1-x) S + H 2 S Fe (1-x) S + 9O 2 FeO.Fe 2 O 3 Fe 2 O 3 + 4SO 2 + SO 3 pyrite pyrrhotite anhydrite Hematite and magnetite Bottom gasifier height top Skhonde et al., Energy and Fuels,23, 2009

7 In-situ sulphur capturing options Use of calcium-based sorbents for fuel and flue gas desulphurisation Dolomite and limestone used with feed coal for conventional as well as fluidised-bed gasification and combustion processes Applications limited to usage for fine coal processing

8 Objectives The possibility of capturing sulphur in-situ through injection of SO 2 into a packed coal bed under controlled conditions, simulating various zones of a fixed-bed gasification process Utilisation the high-temperature transformation products of limestone and dolomite in the coal for in-situ capturing of the injected sulphur in the form of SO 2

9 METHODOLOGY

10 Properties of the coal used Medium C rank, bituminous coal, from highveld mine in SA. Proximate analysis (%, as received) Ultimate analysis (%, as received) Ash Moisture volatile Fixed C H N S O matter Carbon Coal

11 Typical feed PSD Screen size (mm) Mass (kg) Fractional % retained Total

12 Pipe reactor profile char ash Thermocouple 1 Thermocouple Samples for analysis

13 Pipe reactor vs SFBDB gasifier profiles (Bunt and Waanders; 2009) Bunt and Waanders, Fuel 88, 2009

14 Pipe reactor vs SFBDB gasifier profiles (Bunt and Waanders; 2009) Bunt and Waanders, Fuel 88, 2009

15 Pipe reactor vs SFBDB gasifier profiles (Bunt and Waanders; 2009) Bunt and Waanders, Fuel 88, 2009

16 Analysis done Sulphur forms analysis Total sulphur Mineral sulphur Sulphate sulphur Organic sulphur Mineralogical analysis (XRD)

17 RESULTS & DISCUSSION

18 Total sulphur profile 2 drying zone pyrolysis zone gasificatio n zone combustion and ash zone % total S content (a.r) Base Case Reaction 0 top Sample number bottom

19 Total sulphur captured Sample Reference samples % S Mass S(kg) in coal/mass ash SO 2 treated reaction samples % S Mass S(kg) in coal/ma ss ash Total

20 Total sulphur captured Total mass of sulphur in the reference samples: 1.76 kg Total mass of sulphur in the reaction samples: 2.73 kg Mass of S captured (retained) into the coal mineral structure: Total sulphur in reaction samples - Total sulphur in reference samples = kg = 0.97 kg Mass of S added (from cylinder): 40 ml/min for 2hrs = 2.38 kg % S captured:= Sulphur added (and retained) in coal structure/ sulphur added from cylinder 100 = 0.97 kg/2.38 kg X 100 = 41 %

21 Sulphur forms profile Base Case 1.4 drying zone pyrolysis zone gasificatio n zone combustion and ash zone % content total S Mineral S Organic S Sulphate S Sample number

22 Sulphur forms profile reaction samples 2 drying zone pyrolysis zone gasificatio n zone combustion and ash zone % content Total S Mineral S Organic S Sulphate S Sample number

23 Mineralogical results- reaction samples drying zone pyrolysis zone gasificatio n zone combustion and ash zone 60 CaCO CaMg(CO3)2 CaS CaO MgO Fe2O3 % content % Carbon content CaSO4 Carbon Sample number 0

24 Conclusions Sulphur capturing through SO 2 injection in a packed coal bed, simulating conditions during a fixed bed gasification process achieved with up to 41% efficiency Sulphur capturing reactions taking place at the gasification zone and combustion zone Sulphur captured as mineral sulphur at the gasification zone due to high carbon content XRD results indicates formation of CaS at the gasification zone At lower carbon content anhydrite formation is favoured Testing on a commercial gasifier necessary to prove the concept

25 The end