Characterisation of rig deposits from oxy-coal combustion

Fraser Wigley – Imperial College LondonBen Goh – E.ON UK

First Oxyfuel Combustion Conference, 8-11 September 2009, Cottbus, Germany

Coal ash depositionCoal 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, p p g p ,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:Operational problems can occur when deposits:

• Reduce heat removal, raising downstream temperatures

• Fall and cause damage

• Obstruct gas flow

Impact of oxy coal combustion on ash behaviour?Impact of oxy-coal combustion on ash behaviour?

The oxy‐coal combustion environment is dramatically different:

• Chemically (CO2 + H2O instead of N2)

• Physically (gas properties and variable oxygen concentration affect y y (g p p ygtime‐temperature history)

Higher CO2 partial pressures may affect the transformations of coal carbonate minerals, such as calcite CaCO3 and siderite FeCO3. Thesecarbonate minerals, such as calcite CaCO3 and siderite FeCO3. 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.p g p

Some fly ash is currently sold for use as a partial cement replacement. Although the literature is ambiguous, condensed calcite CaCO3 on the ash particle surfaces probably accelerates the initial setting slightly butash particle surfaces probably accelerates the initial setting slightly but has no effect on final strength.

Dissociation of calcite CaCO CaO + CODissociation of calcite, CaCO3 → CaO + CO2

DTA (heating at 3°C/min) shows that calcite dissociates at:

• 600‐700°C in air

• 900‐960°C in pure CO2p 2

Calcite reconstitutes at 1000‐950°C on cooling in pure CO2.

Siderite FeCO3 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.

Combustion Test Facility at E ON UKCombustion Test Facility at E.ON UK

CTF deposits for characterisationCTF deposits for characterisation

Oxy‐coal trials took place on the E.ON UK combustion test facility, a time‐temperature scaled 1 MWth combustor with a wide range of fuel and combustion condition flexibility. For oxyfuel combustion, the CTF h d fl l i l i hhad two flue gas recycle streams – primary recycle without 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% O2 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 tTwo levels of excess O2 in the flue gas, two levels of air staging and two deposition locations were also used. 

Typical El Cerrejon depositsTypical El Cerrejon deposits

Air-fired Oxy-fired21%

Oxy-fired24%

Oxy-fired26%

Cross sections through typical El Cerrejon depositsCross-sections through typical El Cerrejon deposits

Air-fired Oxy-fired21%

Oxy-fired24%

Oxy-fired26%

Typical Thoresby depositsTypical Thoresby deposits

Air-fired Oxy-fired21%

Oxy-fired24%

Oxy-fired26%

Extent of sintering for Thoresby depositsExtent of sintering for Thoresby deposits

7

5

6

t

Air fired

Oxy fired

4

5

x of dep

osit

2

3

Jone

s Ind

ex

0

1

950 1000 1050 1100 1150 1200

Gas temperature (°C) at deposition point

Comparison between Thoresby and El Cerrejon depositsComparison 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 shapeApart from these small differences, the changes in deposit shape, microstructure and phases present with variations in air/oxy‐firing, % O2“enriched”, excess O2 and % OFA are the same for both El Cerrejon and Thoresby deposits.Thoresby deposits.

Effect of excess O and OFA on deposit microstructureEffect of excess O2 and OFA on deposit microstructure

2% Excess O2 4% Excess O2

0% OFA

15% OFA

Deposit macrostructures SummaryDeposit macrostructures – Summary

Compared to air‐fired deposits, oxy‐fired deposits were:

• Smaller

• Different shape (wedge)p ( g )

• More densely packed

• Less well sintered and more friable (weaker)

A th l l f O i h t i d fi d d it bAs the level of O2 enrichment increased, 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.

Typical cross sections through air fired depositsTypical cross-sections through air-fired deposits

15% OFA, 2% Excess O2 15% OFA, 4% Excess O2

Typical cross sections through oxy fired depositsTypical cross-sections through oxy-fired deposits

21% O2 enriched 24% O2 enriched

Deposit microstructures SummaryDeposit 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 asOxy‐fired deposits showed no significant changes in microstructure as the level of O2 enrichment increased.

Deposit microstructure was not significantly affected by the level of th ti f fi iexcess oxygen or the proportion of over‐fire air.

Thoresby deposits were more sintered than El Cerrejon deposits, and became more sintered with increasing temperature.

Crystalline phases in CTF deposits by XRDCrystalline phases in CTF deposits, by XRD

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%)Mullite

Quartz

e intensity

Oxy fired (RR 82.0%, sec O2 21%)

+ Hematite

Relative

0 10 20 30 40 50 60 70

Angle (°2θ)

CTF deposit phasesCTF 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 oxy‐fired deposits than in the air‐fired deposits.

The lower level of mineral transformation was a consequence of lowerThe 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 flamepersisting through the flame.

The two major phases in the XRD spectra (quartz and glass) are not in thermodynamic equilibrium.

Extent of mineral transformationExtent of mineral transformation

Thoresby coal

El Cerrejon coal

Qua

rtz ratio

Mullite/Q

950 1000 1050 1100 1150 1200

Gas temperature (°C) at deposition point

Extent of mineral transformationExtent of mineral transformation

Air fired

Oxy fired

Qua

rtz ratio

Mullite/Q

950 1000 1050 1100 1150 1200

Gas temperature (°C) at deposition point

Conclusions 1Conclusions 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.

Other differences (denser packing and different shape of the oxy‐firedOther differences (denser packing and different shape of the 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 theto 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 can be explained by the higher ash content and ash iron oxidecan be explained by the higher ash content and ash iron oxide concentration of the Thoresby coal.

Conclusions 2Conclusions 2

Coal minerals have undergone the same transformations during oxy‐firing 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.

Once the combustion conditions (and therefore time‐temperatureOnce 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.

AcknowledgementsAcknowledgements

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&DThis 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• Impacts of oxy‐coal firing on plant operation

• Methods of flue gas treatment and CO2 purification

• Generic process implications of oxy‐coal firing on pulverised coal l t d iplant design

OxyCoal UK Phase 1 Project ParticipantsOxyCoal-UK: Phase 1 – Project Participants

Lead companyDoosan Babcock Energy Limited

University

Sponsors / Sponsor ParticipantsScottish and Southern Energy plcScottishPower Energy WholesaleEDF Energy plcUniversity

ParticipantsImperial College LondonUniversity of Nottingham

EDF Energy plcDrax Power LimitedDONG Energy Generation A/S

Industrial ParticipantsAir Products plc

BP Alt ti E I t ti l

Government SupportDepartment of Business, Enterprise and Regulatory ReformT h l St t B dBP Alternative Energy International

LimitedE.ON UK plc

RWE npower plc

Technology Strategy BoardEngineering and Physical Sciences Research Council

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