Modelling of trace element release in oxyfuel PF coal combustion

SupervisorsDr. Marcos Millan-AgorioDr. Graham Reed

Marco A. Jano Itomarco.jano-ito10@imperial.ac.uk

Background: IC involvement in FECUNDUS

Researchers currently involved in FECUNDUS:

• Prof Nigel Brandon (ESE - Professor). • Dr Marcos Millan (CE - Lecturer).• Dr Nigel Paterson (CE - Research Fellow).• Dr Graham Reed (CE - Research Fellow).• Mr Hamish Spender (CE - PhD student).

Groups at IC Involved in Fecundus• Department of Chemical Engineering.

• Pyrolysis and Gasification of Solid Fuels.» Integration of Gasification with Carbon Storage (Flexgas).» Biomass Chars for Steel Production in Electric Arc Furnaces

(Green EAF).• Gas Cleaning.

» Catalytic Tar Cracking.• Fuel Catalytic Upgrading and Characterisation.

» Synthesis and Testing of Hydrocracking Catalysts.

• Department of Earth Sciences and Engineering.• Fuel Cells.

» Fuel Cell Design and Optimisation.» Effect of Gas Contaminants on Fuel Cells.

Background: IC and FECUNDUS

• WP2 - Oxyfuel plus Steam Gasification and Integration with the ICFB Process Scheme.

• WP3 - Selection and Characterisation of Alternative Fuels For Entrained Flow Gasification.

• WP4 - Development and Test of Materials for Advance Gasification Processes.

Up to 1000°C and 30 bar.Resistance-heated reactor shell.Solid feed rate of up to 6 g/min.Gas flow rates of up to 15 ln /min.

Pressurised Fluidised Bed Reactor

V-5

V-4

I-6 I-1 I-4 I-7 I-3 I-5

V-1

V-3

V-2

E-1

D1

M2

E-2

I-2

C2C1 C3

M1

O1

H1

I1

Z1

S1

F1

V1 F2F3

X1

V3

V2

V4

V5

E1

G1

R1

T1

Modelling of trace element release in oxyfuel PF coal combustion

SupervisorsDr. Marcos Millan-AgorioDr. Graham Reed

Marco A. Jano Itomarco.jano-ito10@imperial.ac.uk

Modelling of trace element release in oxyfuel PF coal combustion

Introduction

Objectives

Methodology

Results

Conclusions

Presentation contents

Introduction

- Carbon capture and storage

- Oxy-fuel combustion

- Trace elements in coal and their behaviour in combustion

Conventional PC combustion

Source: Based on Vattenfall (2010) and Centro Mario Molina (2010)

Conventional power generating systems use air to burn coal and produce steam.

Coal is burned in a boiler and flue gases in some cases are cleaned before being emitted into the atmosphere.

Bottom ash

Coal

Boiler

Precipitator

Sulphur removal

Fly ashNOx

Steam turbine

Condenser

Electricity

DeNOX

SOx

Flue gases

Air

Oxy-fuel combustion

Source: Based on Vattenfall (2010) and Centro Mario Molina (2010)

Bottom ash

Coal

Boiler

Steam turbine

Condenser

Electricity

Precipitator

Fly ash

Sulphur removal

SOxWater

Cooler and condenser

Air separation

Air

N2

O2

Recycled flue gases

Oxy-fuel combustion uses oxygen and recycled flue gases instead of air. Recycled flue gases control temperature.

Addition of an Air Separation Unit (ASU), flue gas cleaning and CO2 compression.

The purity of CO2 can be as high as 95%.

Compressor CO2

Trace elements in coal

Source: Clarke, L.B. (1993) The fate of trace elements during coal combustion and gasification: and overview. Fuel, 72 (6), 731 – 736.

These elements are found as traces (less than 1000ppm by weight) in coal.

Their emission during combustion is considered to be a threat to the environment, human health and process operation as well.

These elements can be found as:

Included in coal particles or as minerals and rock fragments.

Associated to organic or inorganic matter.

Trace element behaviour under combustion

Source: Atalla, M., Morgan, S., Riley, K., Bryant, G. and Nelson, P. (2007) Trace element deportment in combustion processes. Cooperative Research Centre for Coal in Sustainable Development (CCSD), Research report 70.

Complex transformations at rapid heating and high temperatures.

Combustion:

Non-volatile trace elements in organic matter transferred to gas phase.

In the mineral matter, volatile elements vaporise.

Non-combustible material is in the bottom-ash, fly-ash and vapour.

Oxy-fuel combustion

Higher heat capacities of CO2 and H2 O (in case flue gas is recycled before being dried).

Higher gas emissivities.

Higher flue gas density.

Hindered diffusion of CO2 from the particle.

Hindered diffusion of O2 to the char surface.

Reaction between CO2 and carbon.

The formation of NOx , SOx and ash particles, as well as the fate of trace elements may be altered

What potential to form carbonate deposits?

Main features of oxy-fuel combustion

Objectives

Development of a procedure to evaluate trace element release in an oxy-fuel combustion system that may be similar to a real plant.

Use MTDATA as a thermodynamic tool to model trace element behaviour at different temperatures.

Analysis of the speciation of trace elements in combustion under O2 /N2 and O2 /CO2 environments, different stages of the processes and different coal compositions.

Analysis of the impact of trace element release on process equipment corrosion.

Air-fired and Oxy-fuel systems

• System based on: “Conceptual design of supercritical O2 -based PC boiler” prepared by Foster Wheeler to the U.S. Department of Energy.

• Provides industry data for air-fired and oxy-fuel processes as well as coal composition (Illinois #6).

• Cases: Air-fired case, Oxy-fuel with 72.1% recycled flue gases and Oxy-fuel with 65.5% recycled flue gases.

Source: Seltzer, A., Fan, Z. and Robertson, A. (2006) Conceptual design of supercritical O2-based PC boiler. Foster Wheeler Power Group, Inc., DE-FC26-04NT42207.

Oxy-fuel cases

Coal analysis

Illinois No 6 bituminous coal (US DOE 2008), % db

Ash 10.9

S 2.82

Cl 0.33

ash minerals, % on ash db

Silica 45

Alumina 18

Iron Oxide 20

Calcium Oxide 7

Trace element composition

Average for Illinois mines shipped coal (USDOE 2008), ppm (db)

As 7.5 Mn 38

B 90 Mo 8.4

Be 0.7 Ni 14

Cd 0.9 Pb 24

Co 1.3 Sb 0.8

Cr 14 Se 1.9

Cu 9.2 V 31

Hg 0.09 Zn 84.4

Activities performed

Air-fired and oxy-fuel modelling

Trace element release

• Simulation of the behaviour of 15 trace elements using MTDATA.

• Preliminary analysis of the behaviour of trace element speciation for the air-case and oxy-fuel cases.

• Mass balance of the processes.

• Adiabatic flame temperature.

Modelling of trace element release

• Chemical equilibrium calculations provide an initial understanding of speciation.

• MTDATA minimizes the Gibbs energy function in complicated multiphase multicomponent systems.

• Scientific Group Thermodata Europe (SGTE) for element properties.

Main species formed

Vapour Condensed

140°C-380°C 380°C-800°C 800°C- 1200°C

140°C-380°C 380°C-800°C 800°C-1200°C

Mercury HgCl2HgCl2 , Hg,

HgO Hg, HgO ------ ------ ------

Arsenic As4 O10As4 O10 ,

As4 O7, As4 O8,AsO, AsO2 As2 O5 ------ ------

Selenium SeO2 SeO2 , SeO SeO2 , SeO, Se,SeH ------ ------ ------

Lead ------ PbCl2PbO, PbCl, PbCl2 , Pb PbSO4 PbSO4 ------

Antimony SbCl3 SbCl3 , Sb4 O6SbCl, SbO,

Sb4 O6Sb2 O5 , SbO2 SbO2 ------

Chromium ------ CrH2 O4 , CrO2 Cl, CrO2

CrH2 O4 , CrO2 Cl, CrO2

Cr2 S3 O12Cr2 S3 O12 ,

Cr2 O3Cr2 O3

Cadmium ------ CdH2 O2 , CdCl2

Cd, CdH2 O2 , CdCl2 , CdOH CdSO4 ------ ------

Nickel ------ NiCl2 ,NiO2 H2 , NiCl2 ,

NiOH, NiCl NiSO4NiSO4 , Ni2 SiO4

Ni2 SiO4

Main species formed

Vapour Condensed

140°C-380°C 380°C-800°C 800°C- 1200°C

140°C-380°C 380°C-800°C 800°C-1200°C

Manganese ------ MnCl2 MnCl2 , MnCl MnSO4MnSO4 ,

Mn2 O3 , Mn3 O4Mn2 SiO4

Cobalt ------ CoCl2 , CoH2 O2

CoCl2 , CoH2 O2

CoSO4CoSO4 , Co2 SiO4

Co2 SiO4

Beryllium ------ BeH2 O2BeH2 O2 , BeSO4

BeSO4BeSO4 , Be2 SiO4

------

Boron BH3 O3 , BClH2 O2

BH3 O3 , BHO2 , BClH2 O2

BHO2 , BKO2 , BH3 O3 , BO2

BHO2 ------ ------

Copper ------ CuCl, CuCl2 , Cu3 Cl3

CuCl, CuCl2 , Cu, CuOH CuSO4 CuSO4 ------

Vanadium VOCl3 V4 O10 , VOCl3 ------ V2 O5 V2 O5Ca2 O7 V2 ,

V4 O10 , VO2

Zinc------ ZnCl2

Zn, ZnCl2 , ZnH2 O2 , ZnO, ZnCl, ZnOH

H2 O5 SZn, ZnSO4

ZnSO4 ,Zn2 SiO4

Zn2 SiO4

Interactions between minor and trace elements

Considerable interaction

No interactionInteraction

Condensed species formed at convection zone tube wall temperatures (800 to 380oC)

Element Condensed form

Element Condensed form

Sb SbO2 Cu CuSO4

As As2 O5 Pb PbSO4

Be Be2 SO4 , Be2 SiO4

Mn MnSO4

Cd CdSO4 Ni NiSO4

Cr Cr2 O3 , Cr2 O12 S3

V V2 O5

Co CoSO4 Zn ZnSO4

Conclusions (1)

• Air-fired and oxy-fuel cases present similar trace element speciation trends and small differences were found.

• At high temperatures, trace elements were primarily found in the chlorinated and oxidised forms.

• At low temperatures, trace elements condensed as sulphates and silicates.

• Trace elements interacted primarily with chloride, sulphur and silicon.

Conclusions (2)

For the coal studied, trace element speciation is unchanged between conventional PC and oxyfuel PC boilers, but:

The recycle configuration (wet or dry) can affect S concentration

Coal Cl, S and Si concentration sensitivity studies show:

• decreased Cl lowers Pb, Cd and Cu volatility

• decreased S allows carbonate to form if:

– Ca:S>1

and

– temperatures are low ( ~ 140oC)

• decreased Si (i.e. low ash) increases Co, Ni and Zn volatility at high temperatures