Characterisation of rig deposits from oxy-coal combustion. Fraser Wigley Imperial College London Ben Goh E.ON UK

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1 Characterisation of rig deposits from oxy-coal combustion Fraser Wigley Imperial College London Ben Goh E.ON UK First Oxyfuel Combustion Conference, 8-11 September 2009, Cottbus, Germany

2 Coal ash deposition In a power station, most coal ash particles leave the boiler with the flue gas and are separated out by electrostatic precipitators. Some ash particles impinge on walls or super heaters and are retained, forming deposits that cause slagging in the radiative zone (boiler) or fouling in the convective pass. Deposits generally densify after deposition. A certain level of deposition is anticipated. Boilers are equipped with soot blowers (steam lances) to remove excess deposit. Operational problems can occur when deposits: Reduce heat removal, raising downstream temperatures Fall and cause damage Obstruct gas flow

3 Impact of oxy-coal combustion on ash behaviour? The oxy coal combustion environment is dramatically different: Chemically (CO 2 + H 2 O instead of N 2 ) Physically y(gas properties p and variable oxygen concentration affect time temperature history) Higher CO 2 partial pressures may affect the transformations of coal carbonate minerals, such as calcite CaCO 3 and siderite FeCO 3. These minerals are important sources of Ca and Fe, which reduce the viscosities of aluminosilicate liquids changing the stickiness of the coal ash particles and rate of sintering of the deposits. Some fly ash is currently sold for use as a partial cement replacement. Although the literature is ambiguous, condensed calcite CaCO 3 on the ash particle surfaces probably accelerates the initial setting slightly but has no effect on final strength.

4 Dissociation of calcite, CaCO 3 CaO + CO 2 DTA (heating at 3 C/min) shows that calcite dissociates at: C in air C in pure CO 2 Calcite reconstitutes at C on cooling in pure CO 2. Siderite FeCO 3 dissociates at ~530 C. Equilibrium thermodynamic calculations suggest that Na, K, Ca, Mg and Fe will tend to form sulphates, rather than carbonates, on cooling in flue gas. However, kinetics and inhomogeneity dominate the transformations of coal minerals during combustion.

5 Combustion Test Facility at E.ON UK

6 CTF deposits for characterisation Oxy coal trials took place on the E.ON UK combustion test facility, a time temperature scaled 1 MW th combustor with a wide range of fuel and combustion condition flexibility. For oxyfuel combustion, the CTF hd had two flue gas recycle streams primary recycle without ih oxygen enrichment for conveying fuel to the burner, and secondary recycle that was enriched with oxygen up to a maximum of 26%. Deposit samples were acquired during the combustion of a single coal under four main conditions: Air firing Oxy firing with 21, 24 or 26% O 2 added to the secondary recycle Two coals were combusted El Cerrejon and Thoresby. T l l f O i th fl t l l f i t i d t Two levels of excess O 2 in the flue gas, two levels of air staging and two deposition locations were also used.

7 Typical El Cerrejon deposits Air-fired Oxy-fired 21% Oxy-fired 24% Oxy-fired 26%

8 Cross-sections sections through typical El Cerrejon deposits Air-fired Oxy-fired 21% Oxy-fired 24% Oxy-fired 26%

9 Typical Thoresby deposits Air-fired Oxy-fired 21% Oxy-fired 24% Oxy-fired 26%

10 Extent of sintering for Thoresby deposits 7 6 Air fired Oxy fired Jones Index of deposit Gas temperature ( C) at deposition point

11 Comparison between Thoresby and El Cerrejon deposits Compared to the El Cerrejon CTF deposits, the Thoresby deposits are slightly: Larger (coal has higher ash content) Darker(ash has higher iron oxide concentration) More sintered (higher iron oxide ash has lower viscosity) Apart from these small differences, the changes in deposit shape, microstructure and phases present with variations in air/oxy firing, % O 2 enriched, excess O 2 and % OFA are the same for both El Cerrejon and Thoresby deposits.

12 Effect of excess O 2 and OFA on deposit microstructure 2% Excess O 2 4% Excess O 2 0% OFA 15% OFA

13 Deposit macrostructures Summary Compared to air fired deposits, oxy fired deposits were: Smaller Different shape (wedge) More densely packed Less well sintered and more friable (weaker) As the level lof O 2 enrichment ih tincreased, oxy fired deposits became larger but showed no significant structural changes. Deposit macrostructure was not significantly affected by the level of excess oxygen or the proportion of over fire air.

14 Typical cross-sections sections through air-fired deposits 15% OFA, 2% Excess O 2 15% OFA, 4% Excess O 2

15 Typical cross-sections sections through oxy-fired deposits 21% O 2 enriched 24% O 2 enriched

16 Deposit microstructures Summary Compared to particles in air fired deposits, particles in the oxy fired deposits from both coals were: Similar in size Less well rounded Richer in clay derived particles that were not fully fused Oxy fired deposits showed no significant changes in microstructure as the level of O 2 enrichment increased. Deposit microstructure was not significantly affected by the level of excess oxygen or the proportion of over fire air. Thoresby deposits were more sintered than El Cerrejon deposits, and became more sintered with increasing temperature.

17 Crystalline phases in CTF deposits, by XRD Mullite Quartz Air fired Oxy fired (RR 79.3%, sec O2 26%) Oxy fired (RR 80.0%, sec O2 24%) Oxy fired (RR 82.0%, sec O2 21%) Relative e intensity + Hematite Angle ( 2θ)

18 CTF deposit phases The phases identified in oxy fired deposits were generally the same as those found in air fired deposits; coal minerals have shown the same transformations on combustion. Mullite and glass were less abundant (compared to quartz) in the oxyfired deposits than in the air fired deposits. The lower level of mineral transformation was a consequence of lower flame temperatures. Carbonates might be present in the oxy fired deposits, possibly persisting through the flame. The two major phases in the XRD spectra (quartz and glass) are not in thermodynamic equilibrium.

19 Extent of mineral transformation Thoresby coal El Cerrejon coal Quartz ratio Mullite/Q Gas temperature ( C) at deposition point

20 Extent of mineral transformation Air fired Oxy fired Quartz ratio Mullite/Q Gas temperature ( C) at deposition point

21 Conclusions 1 Consistent differences have been observed between CTF deposits from oxy firing and deposits from air firing. Some of these differences (lower levels of coal mineral transformation and ash particle sintering) are consequences of the lower CTF temperatures during these oxy firing trials. Otherdifferences (denser packing anddifferent different shape oftheoxy fired oxy fired deposits) may be general features of oxy firing. Most of the differences in mineral transformation and deposition appear to result from physical, rather than chemical, differences between the two modes of combustion in these trials. The minor differences between the deposits from the two coals studied canbeexplained explained bythehigherashcontentandashironoxide higher and iron oxide concentration of the Thoresby coal.

22 Conclusions 2 Coal minerals have undergone the same transformations during oxyfiring as observed during air firing, but to a lesser extent because of the lower CTF temperatures in these trials. Current uncertainty about ash behaviour during oxy coal combustion arises mainly from the unknown combustion conditions. Oncethecombustion conditions (and thereforetime temperature Once the combustion conditions (and therefore time temperature history) for oxy firing are specified, ash behaviour in the radiative section of the boiler should be largely predictable from current knowledge.

23 Acknowledgements The characterisation work described here was funded by BCURA and by the EPSRC; sample acquisition was part funded by the UK Technology Strategy Board. The efforts of Mabel Lew in characterising the Thoresby deposits are gratefully acknowledged. This work was undertaken as part of OxyCoal UK, a collaborative R&D project which aimed to address: Fundamental coal characterisation under oxy coal firing conditions Impacts of oxy coal firing on plant operation Methods of flue gas treatment and CO 2 purification Generic process implications of oxy coal firing on pulverised coal plant tdesign

24 OxyCoal-UK: Phase 1 Project Participants Lead company Doosan Babcock Energy Limited University Participants Imperial College London University of Nottingham Industrial Participants Air Products plc BP Alternative ti Energy International ti Limited E.ON UK plc RWE npower plc Sponsors / Sponsor Participants Scottish and Southern Energy plc ScottishPower Energy Wholesale EDF Energy plc Drax Power Limited DONG Energy Generation A/S Government Support Department of Business, Enterprise and Regulatory Reform Technology Strategy t Board Engineering and Physical Sciences Research Council