Evaluation of the Behavior and Control of 

Selenium (Se) in Coal-Fired Boilers 

Final Report 

April 4, 2011 

Reference No. CF0524/QT-OH-10-000217 

Prepared for 

Air Pollution Prevention & Control Division 

National Risk Management Research Laboratory 

Office of Research & Development 

U.S. Environmental Protection Agency 

109 T.W. Alexander Drive, E305-1 

Research Triangle Park, NC 27711 

Prepared by 

Reaction Engineering International 

77 West 200 South, Suite 210 

Salt Lake City, UT 84101 
Selenium in Coal-Fired Boilers 

April 4, 2011 

Table of Contents

Table of Contents ............................................................................................................................ 2

List of Figures ................................................................................................................................. 4

List of Tables .................................................................................................................................. 6

Abstract ........................................................................................................................................... 8

1. Introduction ............................................................................................................................. 8

2. Literature Review.................................................................................................................... 9

2.1. Thermochemsitry and Kinetics of Selenium Compounds ................................................ 9

2.2. Behavior of Selenium in Combustion Systems .............................................................. 16

2.2.1. Vaporization of Selenium in Combustion ............................................................... 16

2.2.2. Interaction between Selenium and Fly Ash in the Combustion System ................. 17

2.2.3. Low-Temperature Reactions of Selenium .............................................................. 19

2.3. Behavior in of Selenium in Air Pollution Control Devices ........................................... 20

2.3.1. Full Scale Selenium Data ........................................................................................ 20

2.3.2. ICR Part III Selenium Data ..................................................................................... 24

2.3.3. Effect of ACI on Se Removal ................................................................................. 29

2.3.4. Behavior of Selenium in Scrubbers ........................................................................ 29

2.3.5. Selenium in Fly Ash................................................................................................ 30

3. Model Description ................................................................................................................ 36

3.1. Gas-Solid Interactions .................................................................................................... 38

3.1.1. Heterogeneous Condensation, Continuum Regime ................................................ 39

3.1.2. Heterogeneous Condensation, Free Molecular Regime .......................................... 40

3.1.3. Surface Reaction, Continuum Regime .................................................................... 40

3.1.4. Surface Reaction, Free Molecular Regime ............................................................. 41

3.2. Surface Reaction Kinetics .............................................................................................. 41

3.3. Model inputs ................................................................................................................... 44

4. Model Validation .................................................................................................................. 44

4.1. Modeling Results............................................................................................................ 53

5. Modeling of EPA MPCRF Experiments ............................................................................... 60

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Selenium in Coal-Fired Boilers 

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5.1. Model Inputs .................................................................................................................. 61

5.2. Modeling Results............................................................................................................ 62

6. Conclusions ........................................................................................................................... 65

7. References ............................................................................................................................. 65

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Selenium in Coal-Fired Boilers 

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List of Figures

Figure 1. Reaction rate vs. temperature for reaction of SO2 or SeO2 with CaO.11 ...................... 11

Figure 2. Selenium adsorption as a function of sorbent loading for injection of calcium 

hydroxide or sodium carbonate in the exhaust of glass furnaces.13 ........................... 12

Figure 3. Equilibrium calculation for starting mixture of Ca(OH)2 in air with gas-phase 

SeO2. .......................................................................................................................... 14

Figure 4. Equilibrium calculation for starting mixture of CaO in air with gas-phase SeO2. ....... 14

Figure 5. Gibbs free energy change for condensation of selenium species. ................................ 15

Figure 6. Predicted low-temperature equilibrium selenium speciation as a function of 

temperature. ............................................................................................................... 16

Figure 7. Se vaporization calculated from full-scale data.15 ........................................................ 17

Figure 8. Relationship between coal iron and Se vaporization, from full-scale data. ................. 17

Figure 9. Selenium partitioning in bituminous coal-fired plants.23 ............................................. 21

Figure 10. Comparison of Selenium Removal in Plants A-D. ..................................................... 22

Figure 11. Se Removal from the EPA MPCRF (Ref. 25). ........................................................... 23

Figure 12. Selenium removal in bituminous plants with cold-side ESP only as a function 

of coal sulfur content ................................................................................................. 28

Figure 13. Flow diagram for Selenium model. ............................................................................ 37

Figure 14. Vapor pressure of SeO2. ............................................................................................. 40

Figure 15. UofA furnace time-temperature histories.16 ............................................................... 47

Figure 16. Se concentrations in ash as a function of ash diameter: baseline experimental 

results for bituminous coals. ...................................................................................... 49

Figure 17. Se concentrations in ash as a function of ash diameter: baseline experimental 

results for low-rank coals. .......................................................................................... 49

Figure 18. Se concentrations in ash as a function of ash diameter at Port 14: baseline 

experimental results compared with doping experiments for Pittsburgh coal. .......... 50

Figure 19. Se concentrations in ash as a function of ash diameter at Port 14: baseline 

experimental results compared with doping experiments for Ohio coal. .................. 51

Figure 20. Selenium concentrations in ash as a function of ash diameter at Port 14: 

baseline experimental results compared with doping experiments for 

Kentucky coal. ........................................................................................................... 52

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Selenium in Coal-Fired Boilers 

April 4, 2011 

Figure 21. Selenium concentrations in ash as a function of ash diameter at Port 14: 

baseline experimental results compared with doping experiments for Wyodak 

coal. ............................................................................................................................ 52

Figure 22. Measured and predicted selenium concentrations in fly ash for Pittsburgh 

coal. ............................................................................................................................ 54

Figure 23. Measured and predicted selenium concentrations in fly ash for Illinois 6 coal. ........ 55

Figure 24. Measured and predicted selenium concentrations in fly ash for Kentucky coal. ....... 55

Figure 25. Measured and predicted selenium concentrations in fly ash for Ohio coal. ............... 56

Figure 26. Measured and predicted selenium concentrations in fly ash for Wyodak coal. ......... 57

Figure 27. Measured and predicted selenium concentrations in fly ash for North Dakota 

lignite. ........................................................................................................................ 58

Figure 28. Predicted Se removal on ash for the UofA experiments (from non-vaporized 

fraction, reaction with Ca, and reaction with Fe). ..................................................... 59

Figure 29. Control of PM and Selenium across the PM control device in MPCRF 

tests.46,25 ..................................................................................................................... 61

Figure 30. Time-temperature profiles measured during the MPCRF tests. ................................. 62

Figure 31. Predicted selenium partitioning to ash for the MPCRF tests (from nonvaporized fraction, reaction with Ca, and reaction with Fe). ..................................... 63

Figure 32. Predicted Se distributions for bituminous cases. ........................................................ 64

Figure 33. Predicted Se distributions for subbituminous cases. .................................................. 64

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Selenium in Coal-Fired Boilers 

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List of Tables

Table 1. Thermochemical properties of sulfites, sulfates, selenites, and selenates. .................... 13

Table 2. Species used in equilibrium calculation. ....................................................................... 15

Table 3. Summary of selenium emissions, inputs, and removal in ICR Part III database, 

by fuel type. ............................................................................................................... 24

Table 4. Number of boilers that tested for non-mercury metals in ICR, by fuels and 

APCDs. ...................................................................................................................... 25

Table 5. Summary of Se removal data from 2010 ICR sorted by firing system, fuel, and 

APCDs. ...................................................................................................................... 26

Table 6. Average selenium removal from ICR data set by fuels and APCDs ............................. 27

Table 7. Selenium solubility in water from fly ash samples.32 .................................................... 30

Table 8. Total selenium concentration in ash (μg/g).33 ................................................................ 31

Table 9. Se forms in fly ash.33 ...................................................................................................... 32

Table 10. Selenium speciation from fly ash leaching.33 .............................................................. 32

Table 11. Fly ash composition (Mg- and S-free) of Narukawa et al.34 normalized to 

100%. ......................................................................................................................... 33

Table 12. Concentration of Se, Fe, Ca in ash.34 ........................................................................... 33

Table 13. Selenium speciation by selective leaching.34 ............................................................... 34

Table 14. Solubility of selected fly ash fractions in pH 4.9.35 ..................................................... 34

Table 15. Solubility product constants at 298.15 K for the dissolution of selenates and 

selenites.36 .................................................................................................................. 35

Table 16. Thermal stability temperatures of various iron and calcium compounds with 

selenium or sulfur. ..................................................................................................... 43

Table 17. Parameters used in selenium model. ............................................................................ 44

Table 18. Properties of coals in UofA experiments.16 ................................................................. 46

Table 19. Sampling temperatures from UofA experiments. ........................................................ 48

Table 20. Model predictions for removal of SeO2 and SO2 in UofA experiments. ..................... 59

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Selenium in Coal-Fired Boilers 

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List of Acronyms 

ACI Activated carbon injection 

APCD Air pollution control device 

CEM Continuous emission monitor 

CFD Computational fluid dynamics 

EPA Environmental Protection Agency 

ESP Electrostatic precipitator 

FF Fabric filter 

FGD Flue gas desulfurization 

HAP Hazardous air pollutant 

MACT Maximum achievable control technology 

MPCRF Multipollutant control research facility 

MWt Megawatt thermal 

OAQPS Offices of Air Quality Planning and Standards 

ORD Office of Research 

PM Particulate matter 

QA Quality assurance 

QAPP Quality assurance project plan 

REI Reaction Engineering International 

SCR Selective catalytic reduction 

7 
Selenium in Coal-Fired Boilers 

April 4, 2011 

Abstract

The behavior of Se during combustion is different from other trace metals because of the high 

degree of vaporization and high vapor pressures of the oxide (SeO2) in coal flue gas. 

Furthermore, these gaseous oxides are absorbed on the fly ash surface in the convective section 

by a chemical reaction. The composition of the fly ash (and of the parent coal) as well as the 

time-temperature history in the boiler therefore influences the formation of selenium compounds 

on the surface of the fly ash. A model was created for interactions between selenium and fly ash 

post-combustion. The reaction mechanism assumed that iron reacts with selenium at 

temperatures below 977o

C and that calcium reacts with selenium at temperatures less than 

800o

C. The model also included competing reactions of SO2 with calcium and iron in the ash. 

Predicted selenium distributions in fly ash (concentration vs. particle size) were compared 

against measurements from pilot-scale experiments for combustion of six coals, four bituminous 

and two low-rank coals. The model predicted the selenium distribution in the fly ash from the 

pilot-scale experiments reasonably well for six coals of different compositions. The model was 

then used to predict selenium partitioning to the solid phase for comparison with experimental 

data collected at the EPA Multipollutant Control Research Facility (MPCRF). Several model 

inputs were unknown for these cases; therefore correlations from previous data were generated to 

provide estimates. The model was able to predict the average selenium removal across an 

electrostatic precipitator (ESP) for the bituminous and PRB coals. However, the model did not 

correctly predict the removal across a fabric filter (FF) for the bituminous coal, which suggested 

that there are additional low-temperature interactions that occur in the FF, which the model does 

not presently include. 

1. Introduction

Selenium is a chemical element represented by the chemical symbol Se, with atomic number 34 

and having an atomic mass of 78.96. Selenium rarely occurs in its elemental state in nature. 

Selenium is a solid at room temperature (density=4.8 g/cm3

) with a melting point of 220°C and a 

boiling point of 685°C. It is a member of the sulfur group of nonmetals and is similar to sulfur in 

terms of forms and compounds. Selenium has oxidation states of 6, 4, and -2. 

Selenium is used in a variety of industrial applications, the largest being the manufacture of glass 

and ceramics. Selenium is used to tint glasses and glazes red, as well as to “decolorize” ironcontaining glasses. Selenium is used in photocopying and solar cells, because of its photovoltaic 

properties. Selenium is also used as a toner for photographic prints to extend the tonal range of 

black and white photographs. 

Biologically, selenium is an essential trace element, but is toxic in larger doses. The Tolerable 

Upper Intake Level of selenium for humans is 400 mg/day; exceeding this intake can lead to 

selenosis. 1

 The bioavailability of elemental selenium and most metallic selenides is low. 

Oxyanions of selenium (selenates and selenites), in contrast, are very toxic. 2

8 
Selenium in Coal-Fired Boilers 

April 4, 2011 

Some soils are naturally high in selenium, which can result in high levels of selenium in water 

bodies and a resulting impact on flora and fauna. For example, high selenium levels have been 

found to cause certain congenital disorders in wetland birds.3

 Human activities can also 

contribute to selenium contamination of water bodies, 4

 including: irrigation runoff from highselenium soils; discharges from coal-fired power plant sources such as ash ponds; discharges 

from coal mining or hard rock mining; and effluents from petroleum refineries. 

In December 2004, the U.S. Environmental Protection Agency (EPA) proposed revised water 

quality criteria recommending safe levels for short-term and long-term selenium exposure in 

both freshwater and saltwater environments. EPA released a draft ambient water quality criterion 

for selenium in October, 2008. The criteria recommendations were intended to protect aquatic 

life under the Clean Water Act. 5

Selenium is also listed as a hazardous air pollutant under the Clean Air Act. Currently there are 

no limits on air emissions of selenium from combustion sources. However, US EPA is in the 

process of setting maximum achievable control technology (MACT) standards for coal-fired 

power plants, which could include standards for air emissions of selenium. 

Selenium is found in coals in trace concentrations. USGS has reported concentrations in coal as 

high as 150 ug/g6

, but 0.5 to 5 is a more typical range for US coals. Selenium is often found in 

coal either associated with metal sulfide minerals, predominantly pyrite, or as an organically 

associated element.7

 Selenium has also been observed to be associated with silicates in some 

coals.6

 In low-rank coals, selenium is mostly organically bound, but in bituminous coals the 

element is split between organic and mineral association. 

Selenium is a volatile element and vaporizes to a great extent during coal combustion. Hightemperature equilibrium calculations show that under flame conditions (~1600 K, oxidizing 

atmosphere) the most thermodynamically stable species of selenium is SeO2(g). 

The behavior of selenium in coal-fired power plants is relevant to air emissions of this hazardous 

air pollutant, as well as to emissions of selenium in wastewater discharges (ash ponds, scrubber 

water treatment). In this report, background information is reviewed regarding the speciation of 

selenium in fly ash from coal combustion systems and the conditions for conversion of gaseous 

SeO2 to condensed selenium compounds. 

2. Literature Review

2.1. Thermochemistry and Kinetics of Selenium Compounds

Ghosh-Dastidar et al.8 carried out TGA studies on the sorption of SeO2 gas on hydrated lime and 

other sorbents at temperatures from 400o

C to 1000oC. They tested hydrated lime, kaolinite, 

bauxite and limestone for sorption capacity with SeO2(g) in N2 and in air. In air, hydrated lime 

captured more SeO2 than the other sorbents. The capacity of hydrated lime for SeO2 peaked at 

600oC. Desorption studies showed that CaSeO3 decomposed at temperatures greater than 825oC. 

9 
Selenium in Coal-Fired Boilers 

April 4, 2011 

10 

Using several different measurements, the authors concluded that these two reactions are 

responsible: 

Ca(OH)2 => CaO + H2O(g) (1) 

CaO + SeO2(g) => CaSeO3 (2) 

Agnihotri et al.9

 continued the same type of TGA experiments as in Ref.8 but determined 

reaction kinetics for Eq. 2. They wrote the following rate equation: 

 

 (3) 

where A is the pre-exponential and So is the surface area. They determined the activation energy 

and exponents from their TGA data. They also looked at simultaneous reactions of SO2 and 

SeO2 with hydrated lime. Even when the gas phase concentrations of SeO2 and SO2 were 

comparable, sulfation (at 600o

C) was eight times faster than selenation. This conclusion is 

somewhat contradicted by the experimental results of Li et al., discussed below, at least for the 

chemical kinetic-controlled regime of sorption. 

Li and co-workers carried out a series of studies on sorption of SeO2 and SO2 on CaO.10,11 One 

study was designed to understand the impact of the product sulfate layer on sorption of SeO2 and 

SO2.

10 In some cases, sorbent was pre-sulfated to build up a product layer, to determine if this 

would interfere with the reactions of CaO with SeO2 and SO2. The sorption rate of SO2 on fresh 

CaO was greater than the sorption rate on sulfated CaO at 700oC. The loss of sorption rate at 

high CaO conversion was only significant when the concentration of SO2 was high (>100 ppm 

SO2). This led the authors to conclude that sorption of trace SeO2 would not be affected by the 

product layer. Experiments with 26 ppm SeO2 showed no effect of the degree of CaO sulfation 

on sorption. Thus, the product layer did not seem to be what reduced SeO2 sorption in the 

presence of SO2. 

In a more detailed study of SO2 and SeO2 sorption,11 the authors identified two stages of the SO2-

CaO reaction: (1) Initial chemical kinetic control at low CaO conversion; (2) Late-stage (high 

CaO conversion) when control is by diffusion through the product layer. 

The authors hypothesized that in stage (1) the number of active sites, which can react with SO2

or SeO2, was large enough so that there was no competition. But in stage (2), the number of 

active sites was limited and there was competition between SO2 and SeO2. 

Li et al. obtained a reaction order for SeO2+CaO of ~1 at 600-700o

C, which increased to 1.52 at 

800o

C. The reaction order for SO2-CaO was 1. They also gave a graph (Figure 1) with k vs. T 

for SeO2 and SO2, from which one might deduce the activation energy. The rate constant for Se 

was greater than the rate constant for sulfur at T less than 720o

C, but dropped sharply above this 

temperature, probably because CaSeO3 decomposed at higher temperatures. 

m

SeO a no P RT

E ASr 2 ⎠⎝ exp ⎟

⎞ ⎜⎛− =
Selenium in Coal-Fired Boilers 

April 4, 2011 

0

0.0005

0.001

0.0015

0.002

0.0025

0.003

0.0035

600 700 800 900

k, 1/min

Temperature, oC

Se

S

Li et al. ES&T 2006, 40, 7919

Figure 1. Reaction rate vs. temperature for reaction of SO2 or SeO2 with CaO.11 

It seems clear that hydrated lime will work best for capture of selenium, if injected between 

400oC and 600oC (750oF and 1100oF), which is to say at the economizer inlet or outlet. The 

reaction rate of SeO2 will be higher than that of SO2 in this temperature range, but the 

concentrations of SO2 are much higher. SeO2 should react with the CaO from hydrated lime in 

this temperature range, until a product layer builds up from sulfation and reduces the number of 

sites for reaction. 

Selenium is soluble in silicate glasses, and is used in the glass industry to “de-colorize” glass, by 

reacting with iron to form an Fe-Se compound. Fang and Lynch12 measured the solubility of 

selenium in silicate slags in equilibrium with Cu-Se alloy. As the amount of Fe2+ increased in 

the slag (relative to Fe3+), the solubility of Se increased in the slag. They reported 0.2 wt% to 3 

wt% Se in the slags at temperatures of 1185o

C and 1250oC. Their results were consistent with a 

model that had been proposed, in which selenium was postulated to exist in both a neutral state 

and as an Fe-Se complex in the slag. 

Thus, in a coal-fired boiler, selenium might dissolve in molten silicate particles in the flame. 

The presence of iron in the silicates, particularly under reducing conditions, could promote the 

formation of Fe-Se compounds, which might serve to retain selenium in the ash particles. 

Calcium hydroxide was compared with sodium carbonate as a sorbent for selenium in the glass 

industry. As noted above, selenium is often used as an additive in the manufacture of flint glass 

to control color. Some of the selenium vaporizes in the glass furnace. Kircher13 compared 

sodium and calcium sorbents by injecting the sorbents in the off-gases of glass furnace. Figure 2

shows the data reported by Kircher for temperatures between 330o

C and 390oC. At these 

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Selenium in Coal-Fired Boilers 

April 4, 2011 

12 

temperatures, Ca(OH)2 was a good sorbent for selenium, but Na2CO3 was not as effective. No 

fundamental information on sodium sorbents has been found in the literature to date. 

No data have been found to date in the literature on reactions of vapor-phase selenium with iron 

compounds. 

0

20

40

60

80

100

2 2.5 3 3.5 4 4.5

%Se Adsorbed

Sorbent loading, g/m3

5

Ca(OH)2 ‐ 335 C

Ca(OH)2 ‐ 390 C

Na2CO3 ‐ 330 C

Na2CO3 ‐ 385 C

Figure 2. Selenium adsorption as a function of sorbent loading for injection of calcium 

hydroxide or sodium carbonate in the exhaust of glass furnaces.13 

The HSC database contains thermodynamic data on CaSO3, CaSO4, CaSeO4, FeSO4, Fe2(SO4)3, 

and Fe2(SeO3)3 as summarized in Table 1. There are no data on calcium selenite (CaSeO3). 

However, Buketov14 reported values of the free energy of formation for CaSeO3. The standard 

entropy was calculated from CaSeO4, based on the ratio of entropies of CaSO3 and CaSO4. 
Selenium in Coal-Fired Boilers 

April 4, 2011 

Table 1. Thermochemical properties of sulfites, sulfates, selenites, and selenates. 

Formula Chemical Name MW Phase Hf

0

 298 S0 298 Gf

0

 298 

 g/mol kcal/mol cal/(mol*K) kcal/mol Source 

Na2SeO4 Sodium selenate 188.9 s -257.88 28.00 -266.22 HSC 

Na2SeO3 Sodium selenite 172.91 s -236.50 -35.16 -246.98 HSC 

CaSO3 Calcium sulfite 120.138 s -277.10 24.23 -284.32 HSC 

CaSO4 Calcium sulfate 136.138 s -343.60 25.50 -351.20 HSC 

CaSO4(A) Calcium sulfate 136.138 s -340.72 26.05 -348.49 HSC 

CaSO4(B) Calcium sulfate 136.138 s -339.66 25.72 -347.33 HSC 

CaSeO4 Calcium selenate 183.038 s -265.25 15.10 -269.75 HSC 

CaSeO3 Calcium selenite 167.039 s -224.773 14.35 -229.05 Ref. 39 

FeSO4 Iron(II) sulfate 151.905 s -222.00 28.91 -230.62 HSC 

Fe2(SO4)3 Iron(III) sulfate 399.867 s -617.35 73.50 -639.26 HSC 

Fe2(SeO3)3 Iron(III) selenite 492.569 s -407.79 65.11 -427.20 HSC 

Equilibrium calculations were carried out for SeO2 plus Ca(OH)2 or CaO in air, in order to 

approximate the TGA experiments of Ghosh-Dastidar et al. and Li et al. The equilibrium 

calculations (Figure 3 and Figure 4) show the same behavior as observed in the TGA 

experiments, in that a Ca-Se compound was formed below about 800oC, but this compound 

decomposed above 800oC. The observed decomposition is predicted by equilibrium. Of course, 

selenate (Se[VI]) is rarely observed in fly ash samples collected from ESP hoppers. However, 

there might be low-temperature reactions that convert selenate to selenite or conversion of 

SeO2(g) to CaSeO3 might be the first step in reaction with CaO, but the second step (oxidation of 

CaSeO3) might be kinetically limited in flue gas. In any case, CaSeO4 thermodynamics 

reproduces the behavior observed in the experiments in air, in the range of 400oC to 1000oC. 

13 
Selenium in Coal-Fired Boilers 

April 4, 2011 

0.00E+00

5.00E‐05

1.00E‐04

1.50E‐04

2.00E‐04

2.50E‐04

400 500 600 700 800 900 1,000

Temperature, oC

Ca(OH)2 + SeO2 in air

SeO2 CaSeO4 (g)

Figure 3. Equilibrium calculation for starting mixture of Ca(OH)2 in air with gas-phase 

SeO2. 

0.00E+00

5.00E‐05

1.00E‐04

1.50E‐04

2.00E‐04

2.50E‐04

400 500 600 700 800 900 1,000

Temperature, oC

CaO+ SeO2 in air

SeO2 CaSeO4 (g)

Figure 4. Equilibrium calculation for starting mixture of CaO in air with gas-phase SeO2. 

At temperatures less than 200o

F (~100oC), condensation of gaseous SeO2 might occur via two 

pathways: 

SeO2(g) + H2O(g) Ù H2SeO3 (4) 

SeO2(g) Ù SeO2 (5) 

The Gibbs free energy change for these two reactions as a function of temperature is shown in 

Figure 5. The condensation of SeO2(g) to SeO2 solid is more thermodynamically favorable than 

14 
Selenium in Coal-Fired Boilers 

April 4, 2011 

the formation of H2SeO3. Both reactions are thermodynamically favorable at scrubber 

temperatures. 

‐8.0

‐6.0

‐4.0

‐2.0

0.0

2.0

4.0

200 250 300 350 400

deltaG, kcal/mol

Temperature, F

SeO2(g) + H2O(g) = H2SeO3

SeO2(g) = SeO2

Figure 5. Gibbs free energy change for condensation of selenium species. 

An equilibrium calculation was carried out for the temperature and conditions characteristic of a 

scrubber flue gas. The starting gas composition was assumed to be 10.4% CO2, 13.2% H2O, 

5.3% O2, 71.1% N2, 39 ppm HCl, 25 ppm SO3, and 0.07 ppm SeO2. The species and phases 

used in the calculation are given in Table 2. Condensed selenium was allowed to be SeO2 or a 

mixture of H2O(l) and H2SeO3. 

Table 2. Species used in equilibrium calculation. 

Gas-Phase: Phase 1: Phase 2: Pure Phases: 

Cl(g) HNO3(g) H2O(l) H2O(l) SeO2 

Cl2(g) HOCl(g) H2SO4 H2SeO3 H2O(l) 

ClO(g) N(g) H2SO4*2H2O(l) 

CO(g) N2(g) H2SO4*3H2O(l) 

CO2(g) N2O(g) H2SO4*H2O(l) 

H2(g) NO(g) 

H2O(g) NO2(g) 

H2O2(g) NO3(g) 

H2SO4(g) O2(g) 

HCl(g) OH(g) 

HCO(g) SeO(g) 

HNO(g) SeO2(g) 

HNO2(g) SO3(g) 

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Selenium in Coal-Fired Boilers 

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0.00E+00

1.00E‐06

2.00E‐06

3.00E‐06

4.00E‐06

5.00E‐06

6.00E‐06

7.00E‐06

8.00E‐06

120 130 140 150 160 170 180 190 200

Moles

Temperature, oF

SeO2(g)

SeO2

H2SeO3‐H2O(l)

Figure 6. Predicted low-temperature equilibrium selenium speciation as a function of 

temperature. 

Results of the calculations (Figure 6) show that condensed selenium is predicted to form below 

160o

F (71oC), first as SeO2, then as H2SeO3. Thus, as the flue gases cool down at the inlet to a 

scrubber for SO2, condensation of selenium is thermodynamically favored. 

2.2. Behavior of Selenium in Combustion Systems

2.2.1. Vaporization of Selenium in Combustion

From full-scale data,15 it was noted that Se vaporization ranged from 65% to 100%. Given the 

observation of high-temperature reaction between Se and iron, we believe that there can be 

limited interaction between Se and iron-containing ash in the coal particle. A relationship 

between the iron content of the coal and Se vaporization was derived from full-scale data and is 

shown in Figure 8. This relationship can be used to estimate vaporization of Se. 

16 
Selenium in Coal-Fired Boilers 

April 4, 2011 

0%

20%

40%

60%

80%

100%

Se Vaporized in Combustion

Figure 7. Se vaporization calculated from full-scale data.15 

Figure 8. Relationship between coal iron and Se vaporization, from full-scale data. 

2.2.2. Interaction between Selenium and Fly Ash in the Combustion

System

Pilot-scale data were taken from experiments carried out at the University of Arizona. 16,17 The 

experimental facility was a 2.2 kg/hr refractory-lined, coal-fired furnace. A quench probe was 

17 
Selenium in Coal-Fired Boilers 

April 4, 2011 

used to take size-segregated ash samples near the flame (upper) and at the end of combustion 

(lower). A series of experiments were carried out in which a coal was burned and size-resolved 

ash samples were taken. Six coals were burned in this study: 

• Pittsburgh bituminous coal; 

• Illinois 6 bituminous coal; 

• Ohio 5,6,7 bituminous coal; 

• Elkhorn/Hazard Kentucky bituminous coal; 

• Wyodak PRB coal; 

• North Dakota lignite; 

Doping experiments were performed with several of the coals to further investigate the influence 

of Ca, Fe, Se, or SO2 on selenium capture. 

Several conclusions related to the mechanism for selenium-ash interactions were drawn from the 

experimental data.16 For most of the bituminous coals, the Se distribution in the ash appeared to 

track the iron distribution. In the bituminous coal experiments, Se was observed in the 

submicron particles at temperatures greater than 1000 to 1100oC, suggesting reaction with iron in 

the submicron ash at high temperatures. Adding iron or calcium to the bituminous coals 

increased the concentration of Se in fly ash. Iron had the larger effect on Se concentration. The 

temperature of the lower furnace sampling location might have been too high to see significant 

Se-Ca reaction. 

There was evidence for reaction between Se and calcium in the Wyodak coal at temperatures less 

than 1000oC, which was consistent with the laboratory observation that the reaction product 

between calcium and selenium was not stable above 800oC. Adding Se to the Wyodak coal 

increased Se concentration in all particle sizes. Adding supermicron iron to the Wyodak 

combustion experiment resulted in a modest increase in Se in the supermicron ash particles. 

Perhaps this reflected mass transfer limitations, because the Se content of the Wyodak coal was 

low. 

At flame temperatures, selenium is soluble in silicate glasses. Larger solubility is indicated for 

iron-containing silicates under reducing conditions. With sufficient calcium in fly ash, selenium 

might be expected to form a calcium selenite/selenate at temperatures below about 800o

C. 

Without sufficient calcium or iron (or in the presence of relatively high concentrations of SO2), 

some of the selenium might be expected to condense as SeO2 at temperatures below about 

200oC. Partial condensation would lead to selenium leaving the particulate control device as 

gaseous SeO2, which is observed in bituminous coal-fired plants. 

18 
Selenium in Coal-Fired Boilers 

April 4, 2011 

2.2.3. Low­Temperature Reactions of Selenium

Selenium, and to a lesser extent arsenic, retention by both fly ash and activated carbon has been 

studied in coal combustion flue gas and gasification syngas. Coal combustion flue gas is of more 

importance, so the discussion will concentrate on those studies. López-Antón et al.18 studied 

capture of As and Se at 120o

C. Two fly ash samples were evaluated, one from a fluidized bed 

combustor and one from a pc-fired boiler. In the ash samples exposed to simulated coal 

combustion flue gas, the equilibrium capacity of the fly ash for Se was 17-18 mg/g. The carbon 

content of the ash samples did not affect the retention of Se in the ash samples. The researchers 

also noted that Se was easily desorbed from the fly ash samples after exposure. 

López-Antón et al.19 used the same experimental apparatus to study retention of As and Se by 

activated carbons in simulated coal combustion flue gas and gasification syngas. These 

experiments were also carried out at 120o

C. Three carbons were used: two low-ash carbons 

from Norit (virgin and sulfur-impregnated) and one high-ash coal-derived carbon prepared by the 

researchers. In simulated combustion gas, the Norit carbons had a capacity for Se of about 3 

mg/g, while the coal-derived carbon had a capacity of about 2 mg/g. XPS analysis of the 

exposed sorbent showed that the selenium adsorbed in the combustion gas was Se[IV]. 

Jadhav et al.20 studied removal of Se by activated carbon in the temperature range of 125o

C to 

250oC. SeO2 was generated by evaporation and carried into a bed of carbon with a nitrogen 

carrier. Six activated carbons were used, including Norit FGD carbon (which was actually a 

sulfur-impregnated carbon). The carbons were not extensively described, although surface areas 

were given. Short-duration tests at 125o

C showed that capture of SeO2 increased as sorbent 

surface area increased, although the relationship was not linear, suggesting that surface 

properties played a role in sorbent, too. The sorbent with the highest surface area (Sornonorit-4, 

1220 m2

/g) was chosen for more detailed experiments. Sorption-desorption experiments 

showed that some of the adsorbed Se was easily desorbed, which suggested that there was a 

mixture of physically and chemically adsorbed Se on the carbon. Sorption of SeO2 was noted to 

be relatively slow, and the authors hypothesized that the relatively large SeO2 molecule might 

have difficulties in diffusing in the carbon micropores. XPS analysis of the exposed carbon 

surface showed that selenium was in two forms on the surface: elemental Se and SeO2. The 

elemental Se presumably was covalently bound to the carbon surface. The heat of adsorption of 

selenium on the carbon was calculated to be in the range of 6 to 8 kcal/mole, which falls between 

physical and chemical adsorption. 

Olson et al.21 noted that in a flue gas containing oxidants, selenium on the surface of carbon 

could be oxidized to Se[VI] by NO2. Even without NO2 present, there are other oxidants in flue 

gas. López-Antón et al.19 observed Se[VI] on the surface of activated carbon exposed to a 

simulated coal combustion flue gas, which did not contain NOx. 

In summary, at flame temperatures, selenium is soluble in silicate glasses. Larger solubility is 

indicated for iron-containing silicates under reducing conditions. With sufficient calcium in fly 

19 
Selenium in Coal-Fired Boilers 

April 4, 2011 

ash, selenium might be expected to form a calcium selenite/selenate at temperatures below about 

500oC (932oF). In a coal-fired boiler, this temperature corresponds roughly to the inlet of the 

economizer. For coals with low sulfur, but moderately high iron, iron selenite might form at 

temperatures below 300o

C (that is, in the air preheater or downstream). Without sufficient 

calcium or iron (or in the presence of relatively high concentrations of SO2), selenium might be 

expected to condense as SeO2 at temperatures below about 200oC. Partial condensation would 

lead to selenium leaving the particulate control device as a gas (SeO2), which is observed in 

bituminous-fired plants. Laboratory work has suggested the selenium interacts with unburned 

carbon or activated carbon at low temperatures (120-250o

C). 

2.3. Behavior in of Selenium in Air Pollution Control Devices

2.3.1. Full Scale Selenium Data

Helble22 reported an average elemental capture efficiency for Se of 49% for units using an ESP 

for PM control (16 sites). This average included units firing bituminous, subbituminous, and 

lignite coals. Se removal efficiency is significantly lower that PM removal due to Se high 

volatility and tendency to enrich in submicron particles. 

A review of full-scale data on selenium partitioning in coal-fired boilers showed that from 27% 

to >90% of the selenium in bituminous-coal fired boilers exited the ESP (presumably in the 

gaseous phase).23 Figure 9 shows the partitioning for some of these boilers. 

20 
Selenium in Coal-Fired Boilers 

April 4, 2011 

0%

20%

40%

60%

80%

100%

120%

140%

Selenium,

% of Input

Stack Gas

Scrubber Out

ESP Ash

Bottom Ash

Bituminous Subbituminous‐

Lignite

Figure 9. Selenium partitioning in bituminous coal-fired plants.23 

For low-sulfur bituminous coals with low calcium in the ash (or in which calcium is unavailable 

for reaction in the ash), iron selenite could form at temperatures below 300o

C. Thus, it seems 

likely that for bituminous coals, formation of selenites is not the only path for selenium in the 

combustion system. The fly ash might contain selenites and SeO2, but most of the selenium will 

remain in the gas phase. 

It should be noted that when fly ash is sampled from the hot flue gases using a dilution-quench 

probe (as in Seames’ experiments16, 17), gaseous SeO2 might condense as submicron SeO2 in the 

sampling system. This submicron SeO2 would be moderately water-soluble, and this could 

explain the higher selenium solubility noted in the submicron ash of the bituminous coals in 

Seames’ experiments. 

Norris24 studied Se partitioning in four coal-fired power plants: 

21 
Selenium in Coal-Fired Boilers 

April 4, 2011 

Plant A: 200 MW, bituminous, SCR, ESP, FGD 

Plant B: 800 MW, bituminous, SCR, ESP, FGD 

Plant C: 290 MW, bituminous, SNCR, ESP, shares FGD with cyclone boiler 

Plant D: 176 MW cyclone, bituminous, SCR, ESP, FGD 

Results from these studies are shown in Figure 10. The flue gas measurement for Plant A was 

35%, which was considered higher than probable so this measurment was disgarded. Overall, 

the Se mass balance on the four plants was slightly low (76%-96%). Fly ash results were in 

agreement for the four plants suggesting 35-55% of the Se was removed with the fly ash, with 

the exception of Plant D which had a large amount or Se in the bottom ash leaving a smaller 

fraction available for fly ash capture. FGD slurry data are in agreement for Plants A and B, 

showing 30-40% selenium removal.Flue gas data for Plants B, C, and D show very little 

selenium escaping with the flue gas (<7%). It is also useful to identify the fraction of Se that is 

associated with solids (fly ash, econimizer ash, bottom ash) and flue gas (Se measured in flue gas 

and absorbed from flue gas by FGD slurry). Se associated with solids was highest for the 

cyclone boiler (75%) and 36-54% for Plants A, B, and C. 

Figure 10. Comparison of Selenium Removal in Plants A-D. 

Hutson, et al.25 studied Se in the EPA Multi-Pollutant Combustion Research Facility (MPCRF), 

which consists of a 4 MMBtu/hr multi-fuel furnace, SCR, FF, ESP, and wFGD. With the ESP in 

22 
Selenium in Coal-Fired Boilers 

April 4, 2011 

use (no FF) PM removal efficiency was 99.33% and with the FF in use (no ESP) PM removal 

was 99.86%, firing either an eastern bituminous, PRB subbituminous, or Texas lignite coal. 

Selenium removal in the PM control device varied widely over the various coals and 

configurations (Figure 11). Selenium removal while firing the PRB coal was very good (99%) 

and didn’t depend on the PM control device. Se removal while firing bituminous coal was 

inconsistent, though the FF appeared to remove more Se. Se removal was also fairly high 

(~85%) while firing TX lignite coal. These data suggest that both coal type and PM control 

device will affect Se removal. 

Figure 11. Se Removal from the EPA MPCRF (Ref. 25). 

The observations from full-scale power plants,15 recent pilot-scale testing,25 and from the 

modeling suggest that for boilers firing bituminous coals most of the selenium will be present in 

the gas phase at the ESP inlet, while for subbituminous and lignite coals, most of the selenium 

will be present in the fly ash at the ESP inlet. Thus, for boilers that have cold-side ESPs, 

selenium is only partially removed by the ESP for bituminous firing and should be almost 

completely removed across the ESP for subbituminous or lignite firing. 

Therefore, for bituminous-fired boilers that have a cold-side ESP followed by a wet flue gas 

desulfurization (FGD) scrubber, a significant fraction of the selenium input to the boiler should 

be present at the inlet to the FGD. SeO2, which is the form of gaseous selenium thought to be 

23 
Selenium in Coal-Fired Boilers 

April 4, 2011 

present in flue gas at back-end temperatures, is soluble in water and therefore might be expected 

to be removed in the scrubber. Figure 9 contained full-scale data for three bituminous-fired 

plants with wet scrubbers. The average removal across the scrubber for those three boilers was 

73%. Meij26 published data on removal of selenium across FGDs based on a total of 16 testing 

campaigns at coal-fired boilers burning bituminous coals in the Netherlands. The average 

removal of Se across the FGDs in the study was 60%, but the range was large, from 0% to 90%. 

The results reported by Meij are consistent with the results complied in Figure 9. While SO2 has 

a very high removal rate across wet scrubbers (greater than 95%), selenium does not. There are 

a number of possible explanations, including formation of H2SeO3 aerosol and absorption 

followed by re-emission into the gas. 

2.3.2. ICR Part III Selenium Data

The US EPA issued an Information Collection Request (ICR) in 2010 for utility boilers in order 

to gain information for developing a utility MACT (Maximum Achievable Control Technology) 

for HAPs (Hazardous Air Pollutant) metals. The 2010 Utility ICR data are available from the 

US EPA website27 in the form of spreadsheets or database files. 

The ICR data collected included Se information. The Se data was categorized based on boiler 

type, fuel type, and air pollution control equipment to analyze how various configurations affect 

Se removal. In total, 214 units were sampled for non-mercury metals. Table 3 gives a 

breakdown of emissions, fuel input, and removal of selenium by primary coal type. 

Bituminous coals had the highest content of selenium on average. Subbituminous coals had an 

average selenium content of about one-tenth that of bituminous coals. Lignites and waste coal 

(coal refuse) had intermediate selenium contents, on average. The average removals of selenium 

were 79% for bituminous coal units, 91% for subbituminous coal units, 99.7% for lignite units, 

and only 30% for coal refuse units. These removals reflect the composition of the fuel (e.g., 

sulfur content, calcium in ash) as well as the mix of air pollution control devices used for a given 

subset, as discussed below. 

Table 3. Summary of selenium emissions, inputs, and removal in ICR Part III database, by 

fuel type. 

 

Average Se 

Emission Std. Dev. 

Average 

Se in Fuel Std. Dev. Average Se Std. N 

 lb/MMBtu lb/MMBtu lb/MMBtu lb/MMBtu Removal Dev. 

Bituminous 3.62E-05 7.72E-05 2.95E-03 2.25E-02 79.07% 35.65% 127 

Subbituminous 7.16E-06 9.68E-06 1.94E-04 4.41E-04 90.69% 12.69% 71 

Lignite 1.27E-05 2.83E-05 8.46E-04 1.59E-04 99.73% 0.32% 7 

Coal Refuse 1.53E-04 3.31E-04 7.51E-04 5.35E-04 30.47% 184.64% 9 

All Plants 3.05E-05 9.21E-05 1.86E-03 1.73E-02 81.19% 49.81% 214 

24 
Selenium in Coal-Fired Boilers 

April 4, 2011 

The boilers in the ICR data set for non-mercury metals testing had a variety of APCD 

configurations, as shown in Table 4. Half the boilers had no SO2 control. The predominant SO2

control device for bituminous boilers was a wet FGD, while a spray dryer absorber (SDA) 

coupled with a fabric filter was the predominant SO2 control device for subbituminous boilers. 

A small number of boilers had circulating dry scrubbers (CDS) or duct sorbent injection (DSI) 

for SO2 control. 

Table 4. Number of boilers that tested for non-mercury metals in ICR, by fuels and 

APCDs. 

APCD

Configuration Bituminous

Sub‐

bituminous Lignite Coal Refuse

C‐ESP/No SO2 24 30

C‐ESP/DSI or CDS 2 1

C‐ESP/SDA 2

C‐ESP/FGD 33 3

H‐ESP/No SO2 4 1

H‐ESP/FGD 4 2

FF/No SO2 22 16 4 7

FF/DSI or CDS 7 2 2 2

FF/SDA 19 8

FF/FGD 8 7 1

PM Scrubber/FGD 1

IGCC 2

Total 127 71 7 9

A summary of Se removal is shown in Table 5. For a conventional boiler firing bituminous coal, 

the addition of SO2 control devices increased selenium removal. The type of PM control device 

also affected Se removal. Without SO2 control, average Se removal ranged from 20% to 76%, 

depending on the PM control equipment. With a wet FGD in place, average Se removal 

increased to 80%-93%, depending on the PM control device. Se removal increased to 99% with 

a spray dryer for SO2 capture and a fabric filter for PM control. Average Se removal was higher 

for conventional boilers firing subbituminous coal. Without any SO2 control, average selenium 

removal ranged from 79% to 97%, depending on PM control equipment. The addition of a wet 

FGD increased average Se removal to 80%-99%. With a spray dryer for SO2 control and a FF 

for PM capture, Se removal averaged 99.5%. Several of the conventional boilers firing 

subbituminous coal also injected activated carbon for mercury capture. For cases with no SO2

control and a FF, ACI slightly increased Se removal. For cases with no SO2 control and a coldside ESP, ACI slightly decreased Se removal. For units with a wet FGD and FF, ACI slightly 

increased Se removal. For units with spray dryers and FF, ACI had a negligible effect on Se 

25 
Selenium in Coal-Fired Boilers 

April 4, 2011 

removal, however, removal was very high for these cases (>99%). Less data are available for 

CFB boilers. The addition of a dry scrubber increase Se removal from an average of 90% to 

99.6% for bituminous-fired units equipped with FF. For CFB units firing lignite coal and 

equipped with FF, the addition of a dry scrubber had a negligible effect on Se removal, but 

removal levels were very high (>99%). 

Table 5. Summary of Se removal data from 2010 ICR sorted by firing system, fuel, and 

APCDs. 

Boiler Fuel SO2 control 

Primary 

PM control 

# 

Units 

min Se 

removal 

max Se 

removal 

ave Se 

removal st dev 

Conventional bit none FF 11 -18.84% 99.26% 75.61% 44.13% 

Conventional bit none CESP 22 -85.17% 94.82% 47.58% 41.74% 

Conventional bit none HESP 3 -89.06% 93.35% 20.43% 96.55% 

Conventional bit wFGD FF 6 74.01% 100.00% 92.87% 9.61% 

Conventional bit wFGD CESP 28 32.84% 99.98% 87.92% 12.31% 

Conventional bit wFGD HESP 4 34.52% 99.34% 80.56% 30.79% 

Conventional bit Spray Dryer FF 13 97.51% 99.99% 99.50% 0.74% 

Conventional subbit none FF 8 83.63% 99.32% 95.20% 4.85% 

 w/o ACI 5 83.63% 99.32% 94.03% 6.04% 

 w/ ACI 3 96.86% 97.31% 97.16% 0.26% 

Conventional subbit none CESP 28 37.05% 99.97% 85.59% 14.94% 

 w/o ACI 10 41.90% 99.97% 87.99% 12.99% 

 w/ ACI 9 37.05% 96.37% 80.53% 18.19% 

Conventional subbit none HESP 1 79.08% 

Conventional subbit wFGD FF 7 92.73% 99.99% 98.44% 2.66% 

 w/o ACI 3 92.73% 99.01% 96.44% 3.30% 

 w/ ACI 4 99.80% 99.99% 99.93% 0.09% 

Conventional subbit wFGD CESP 2 70.69% 89.45% 80.07% 

Conventional subbit wFGD HESP 2 99.64% 99.72% 99.68% 

Conventional subbit Spray Dryer FF 7 99.09% 99.86% 99.48% 0.26% 

 w/o ACI 4 99.09% 99.86% 99.47% 0.32% 

 w/ ACI 3 99.34% 99.75% 99.49% 0.22% 

CFB bit none FF 8 23.71% 99.98% 90.22% 26.88% 

CFB bit Dry Scrub FF 4 99.26% 99.77% 99.59% 0.23% 

CFB lignite none FF 3 99.71% 99.93% 99.78% 0.13% 

CFB lignite Cir Dry Scrub FF (w/ ACI) 2 99.95% 99.98% 99.96% 0.02% 

CFB 

coal 

refuse none FF 6 99.65% 100.00% 99.83% 0.16% 

 

26 
Selenium in Coal-Fired Boilers 

April 4, 2011 

When the average selenium removal is broken down by fuel and ACPD type, patterns emerge, 

which are consistent with the more detailed site data. Table 6 shows the average selenium 

removal by fuel and APCD type. Previous work showed that bituminous-fired plants with coldside ESPs show low rates of selenium capture, and in Table 6, bituminous plants with a cold-side 

ESP only have an average selenium removal of 48%. Based on observations of the competition 

between selenium and sulfur for capture on fly ash, high SO2 in the flue gas should have a 

negative impact on selenium uptake by fly ash (and subsequent capture in an ESP). This 

relationship can be seen in Figure 12. 

Table 6. Average selenium removal from ICR data set by fuels and APCDs 

APCD

Configuration Bituminous

Sub‐

bituminous Lignite Coal Refuse

C‐ESP/No SO2 47.58% 86.12%

C‐ESP/DSI or CDS 94.24% 99.89%

C‐ESP/SDA 99.78%

C‐ESP/FGD 87.92% 80.07%

H‐ESP/No SO2 20.43% 79.08%

H‐ESP/FGD 80.56% 99.68%

FF/No SO2 82.66% 91.91% 99.78% 20.07%

FF/DSI or CDS 99.69% 98.52% 99.96% 66.87%

FF/SDA 99.45% 99.48%

FF/FGD 92.87% 98.44% 99.13%

PM Scrubber/FGD 71.93%

IGCC 89.20%

27 
Selenium in Coal-Fired Boilers 

April 4, 2011 

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

012345

Se Removal

Wt% S in coal (dry)

Figure 12. Selenium removal in bituminous plants with cold-side ESP only as a function of 

coal sulfur content 

The recent pilot-scale combustion work of Hutson et al.25 showed that when a fabric filter was 

used as the particulate control device for a bituminous coal, selenium removal was greater than 

95%. Table 6 shows an average selenium removal across fabric filters on bituminous-fired units 

of 83%, consistent with the pilot-scale work. 

Subbituminous-fired boilers, in contrast to bituminous-fired boilers, have been shown to have 

high levels of selenium removal across cold-side ESPs. 25 The average selenium removal for 

subbituminous-fired boilers in Table 6 with only a cold-side ESP is almost twice the average 

removal on bituminous-fired boilers. Removal of selenium across hot-side ESPs is lower than 

that across cold-side ESPs for both bituminous-fired boilers and subbituminous-fired boilers, 

although the latter show much higher removal across hot-side ESPs than the former. 

Selenium removal across boilers firing coal refuse was noticeably lower than that even on 

bituminous-fired boilers. 

In general, wet SO2 scrubbers remove selenium from the flue gas, although not at the same 

efficiency as they remove SO2. The average removal in the ICR dataset in Table 6 is higher for 

boilers with wet or dry scrubbers, as compared to boilers with only particulate control devices. It 

is interesting to note that higher selenium removal efficiencies are observed, on average, at 

boilers with spray dryer absorbers (or circulating dry scrubbers) as compared to those with wet 

FGDs. The ICR data for scrubbed units, of course, represent the combined removal of selenium 

across both the particulate control device and the scrubber. The higher selenium removals for 

plants with spray dryers, as compared to those with wet scrubbers, suggests that spray dryers are 

28 
Selenium in Coal-Fired Boilers 

April 4, 2011 

29 

more efficient at removal of selenium. PM control devices also affected Se removal. FF had the 

largest Se removal efficiency followed by CESP and HESP. The effects of ACI on Se removal 

re with ACI while others showed a decrease. 

al-

 each 

 control device. There was no correlation between Se 

can be removed from wastewater 

e 

, 

t 

n

4+

. 

s 

bly due 

ith ORP. DBA concentration does correlate with formation of 

 reduce 

n 

resulted in conversion of selenite to a particulate form, therefore reducing selenate formation. 

are unclear; some units showed improved Se captu

2.3.3. Effect of ACI on Se Removal

Dombrowski, et al.28 studied the effect of activated carbon injection on Se removal from co

fired power plants. Data from nine plants were analyzed, including units firing low-sulfur 

eastern bituminous, PRB, and TX lignite coals. The plants were equipped with either an ESP of 

SD/FF for PM control. Norit activated carbon (Darco Hg or Darco Hg-LH) was injected at

site. The data showed a slight decrease in Se emissions for 4 sites. The 4 sites showed an 

increase in Se removal across the PM

removal and ACI rate or coal type. 

2.3.4. Behavior of Selenium in Scrubbers

A significant amount of volatile Se species is captured in wet FGD scrubbers. The absorbed Se 

partitions between the slurry liquor and solids. Se in FGD liquor may be in many forms, which 

respond differently to wastewater treatment methods. Selenite 

using traditional iron co-precipitation while selenate cannot. 

URS conducted bench-scale experiments and full-scale sampling to better understand Se 

speciation and partitioning in wet FGD systems.29 Samples from bench-scale experiments wer

frozen and shipped to the lab where they were analyzed within 48 hrs of collection, however

there was no certainty that the samples reflected actual Se speciation in the scrubber. Their 

bench-scale experimentation showed that certain FGD operating variables can be controlled to 

minimize selenite oxidation, which is desirable for selenium removal from FGD wastewater. A

FGD conditions, dissolved Fe promotes selenite oxidation to the selenate form and iron oxides 

may absorb and/or co-precipitate selenite. Mn2+ does not promote selenite oxidation while M

does. The promotion of Se oxidation by Mn and Fe is a catalytic effect that is concentration 

dependent. Scrubber additives dibasic acid (DBA) and ethylenediaminetetraacetate (EDTA) can

inhibit selenate formation. It may be possible to optimize pH and oxidation-reduction potential 

(ORP) to maintain high SO2 removal and high sulfite oxidation, yet minimize selenite oxidation

In addition, samples were taken from 8 full scale scrubber systems. The samples were filtered 

on-site and shipped on ice to the lab where Se speciation analyses were conducted within 48 hr

of sample collection. The field data also showed a correlation between absorber ORP and Se 

oxidation. The effect of DBA on selenium oxidation was not apparent in field data, possi

to confounding affects w

unknown Se species. 

Higgins, et al.30 added iron to a full scale FGD absorber to enhance mercury removal and

selenate formation. The 100 MW unit fired low-sulfur eastern bituminous coal and was 

equipped with an ESP and wFGD. Initially, Se in the scrubber was 100% selenate. Iron additio
Selenium in Coal-Fired Boilers 

April 4, 2011 

Cordoba, et al.31 investigated trace elements in the water streams of two power plants with wet 

FGDs. Mass balance analysis showed that the recycling of filtered water from gypsum slurry 

filtration caused an unusually high concentration of Se in streams related to the FGD. Gaseous 

Se dissolves in the gypsum slurry and is continuously recycled back to the FGD. Se enrichment 

occurred after more recycling steps than more soluble species. The enrichment may be 

attributable to the formation of fluoride, sulphate, and/or soluble metal chloride complexes. 

2.3.5. Selenium in Fly Ash

Jackson and Miller32 studied fly ash from twenty-three coal-fired power plants. Selenium was 

extracted in water or in a TCLP-type solution (acetic acid). ICP-MS was used to identify the Se 

species in the extract. Table 7 shows the fraction of water-soluble selenium and the fraction of 

the water-soluble selenium identified as Se[IV]. 

Table 7. Selenium solubility in water from fly ash samples.32

Sample % water-soluble Se

%Se[IV] in watersoluble fraction 

1 11.6 100.0 

2 2.7 83.8 

3 11.4 100.0 

4 1.2 -- 

5 0.0 -- 

6 11.5 100.0 

7 3.6 -- 

8 38.6 100.0 

9 0.0 41.9 

10 51.6 100.0 

11 0.3 100.0 

12 13.8 100.0 

13 13.7 100.0 

14 1.9 37.1 

15 11.4 92.2 

16 13.0 91.3 

17 63.0 93.2 

18 15.1 80.5 

19 0.7 100.0 

20 2.8 100.0 

21 27.4 100.0 

22 63.5 100.0 

23 23.1 100.0 

30 
Selenium in Coal-Fired Boilers 

April 4, 2011 

The fly ash samples were not identified nor were bulk compositions provided, although the pH of 

the water solution provides some clue to the composition. All samples except 9 and 14 gave a 

pH >7 in water solution. Samples 9 and 14 were acidic, with pH < 5. These acidic ashes were 

the only ones with a significant amount of Se[VI] identified, although the fraction of selenium 

that was water-soluble was low for these two ash samples. 

Another study on selenium leaching from fly ash samples was carried out by Mattigod and 

Quinn.33 In this study, a selective extraction procedure was used to quantify oxidation states of 

Se species in fly ash. The solutions tested were: 

• 70% HClO4 

• 16 M HNO3

• Aqua regia + 48% HF 

• 12 M HCl 

The HClO4 solution and the HF solution gave complete dissolution and recovery of all selenium 

compounds. The HCl solution and the water solution gave complete recovery of Se[IV] and 

Se[VI], but were not able to dissolve Se[0]. The authors distinguished between Se[IV] and 

Se[VI] by water extraction with and without reduction of Se[VI] to Se[IV]. 

Both bituminous and subbituminous ashes from the Western U.S. were studied. Se concentration 

generally increased with decreasing particle size. No consistent trend was seen in Se content 

with light and heavy (>2.1 SG) density fractions. 

Table 8. Total selenium concentration in ash (μg/g).33

 < 2.7 um 

Sample Bulk 8-15 um

2.7-9 

um 

< 2.7 

um >2.1 SG <2.1 SG 

SB01 21.6 12.9 24.7 34.6 26.3 62.3 

SB02 5.2 4.3 8.5 12.9 14.7 9.3 

SB03 0.9 0.5 1.2 1.1 1.3 1.1 

B01 9.4 15.1 19.1 19.7 18.1 28.4 

B02 7.9 5.6 17.1 23.6 28.5 15.3 

Based on the selective extraction, only Se[0] and Se[IV] were identified in the fly ash samples 

(Table 9). As shown in Table 10, except for fly ash SB03, Se in the water-soluble extract exists 

primarily as Se[IV]. Not all the Se[IV] was soluble in water. In sample SB03 none of the 

Se[IV] was water-soluble. The authors speculated on what might be the form of Se in the fly ash 

samples. Se[0] could be formed by oxidation of selenides or reduction of Se[IV], for example by 

31 
Selenium in Coal-Fired Boilers 

April 4, 2011 

SO3, as suggested in the literature. The lack of complete solubility of Se[IV] argues against all 

SeO2, which is partially water-soluble, as discussed below. 

Table 9. Se forms in fly ash.33

 Total Se[0]+Se[IV] Total Se[IV] Water-soluble Se[IV] 

Sample Bulk < 2.7 um Bulk < 2.7 um Bulk < 2.7 um 

SB01 21.6 34.6 14.7 30.4 12.6 15.5 

SB02 5.2 12.9 4.3 12.4 2.8 9.1 

SB03 0.9 1.1 0.2 0.2 0.2 0.2 

B01 9.4 19.7 8.3 16 3.4 7.8 

B02 7.9 23.6 6.8 19.3 4.8 13.7 

Table 10. Selenium speciation from fly ash leaching.33

 

Fraction as Se[0] 

 

Fraction of Se[IV] water 

soluble 

Sample Bulk < 2.7 um Bulk < 2.7 um 

SB01 31.94% 12.14% 14.29% 49.01% 

SB02 17.31% 3.88% 34.88% 26.61% 

SB03 77.78% 81.82% 0.00% 0.00% 

B01 11.70% 18.78% 59.04% 51.25% 

B02 13.92% 18.22% 29.41% 29.02% 

Narukawa et al.34 used ICP-AES analysis in conjunction with a GC to speciate Se in fly ash 

extracts. Four solutions were used in a sequential extraction procedure: 

1. 1 M MgCl2 solution, pH 7 [Exchangeable species] 

2. 1 M NaOAcetate [Carbonates] 

3. 0.04 M NH2OH HCl in HOAcetate [Bound to Fe-Mn oxides] 

4. 0.02 M HNO3/30% H2O2 [Organically bound] 

5. Residue 

They also did a water extraction; the main difference between the water extraction and selective 

leaching solution #1 is that the pH was not held constant for water extraction. Two of the fly ash 

samples (1 and 6) produced acidic solutions in water extraction. The water extracts were 

analyzed for Se species. 

The bulk composition of the ash samples were reported without any Mg or S. I have converted 

these to oxide composition, but MgO is missing (Table 11). The ash samples were all fairly low 

32 
Selenium in Coal-Fired Boilers 

April 4, 2011 

in Ca. Ash sample 2 was high in Fe. The concentration of Se in the ash samples is shown below 

in Table 12. 

Table 11. Fly ash composition (Mg- and S-free) of Narukawa et al.34 normalized to 100%. 

Sample SiO2 Al2O3 TiO2 Fe2O3 CaO K2O Na2O P2O5 BaO SrO 

1 54.4% 27.7% 1.5% 10.6% 1.5% 0.8% 0.2% 2.2% 0.5% 0.5% 

2 45.5% 29.3% 1.8% 16.1% 4.5% 0.8% 0.4% 1.1% 0.3% 0.1% 

3 69.6% 22.0% 1.0% 3.6% 1.0% 1.7% 0.7% 0.3% 0.1% 0.0% 

4 65.0% 28.4% 1.9% 2.2% 1.4% 0.6% 0.1% 0.1% 0.1% 0.1% 

5 64.9% 20.1% 0.9% 7.6% 3.0% 2.1% 1.0% 0.2% 0.1% 0.1% 

6 66.3% 27.4% 1.2% 1.3% 0.4% 2.8% 0.2% 0.2% 0.1% 0.0% 

Table 12. Concentration of Se, Fe, Ca in ash.34

Sample Se, mg/kg Ca/Se Fe/Se 

1 3.01 2,824 19,136 

2 2.26 11,106 38,673 

3 3.49 1,576 5,874 

4 2.87 2,753 4,321 

5 2.48 7,097 17,258 

6 5.15 447 1,437 

From the selective leaching results, most of the Se was not extractable. Samples 2, 3, and 4 had 

the most exchangeable Se. Sample 6 had a significant amount of organically associated Se. 

Note that the unburned carbon content of the ash was not provided. 

The water leaching results mirror the selective leaching results for exchangeable Se. The results 

are not exactly the same, since pH was not held constant in water leaching. Thus illustrates the 

effect of pH on Se solubility. 

Of the water-soluble Se, ~90% was Se[IV] for the “basic” ash samples (Table 13). The acidic 

ash samples (1 and 6) provided more Se[VI] in the leachate, 20% and 62%, although there was 

not much Se that was water soluble. There seemed to be little correlation with Ca/Se ratio or 

Fe/Se ratio and solubility or speciation of Se. 

33 
Selenium in Coal-Fired Boilers 

April 4, 2011 

Table 13. Selenium speciation by selective leaching.34

 Fraction of Se from selective leaching Se from water leaching 

Sample 

Exchangeable 

(pH 7) 

Organically 

bound 

Nonextractable Water 

leachable 

Fraction 

Se[IV] 

Fraction 

Se[VI] 

1 7.6 0.0 92.4 7.9 20 80 

2 30.5 0.0 69.5 42.5 9 91 

3 42.0 0.0 58.0 37.0 7 93 

4 28.3 0.0 71.7 33.3 8 92 

5 6.7 0.0 93.3 8.4 12 88 

6 15.5 39.0 45.5 16.5 62 38 

Seames and co-workers35 used a pilot-scale coal combustor, a 2.2 kg/hr refractory-lined furnace 

at the University of Arizona, to generate fly ash samples. A quench probe was used to take sizesegregated ash samples at three locations: near the flame (upper), at the end of combustion 

(lower), and at the baghouse inlet. Seames carried out leaching experiments on submicron ash 

samples and supermicron ash samples at two different values of pH: 2.9 and 4.9. The ash 

samples were collected near the flame and post combustion. The selenium solubility results at 

pH 4.9 are reproduced below for five coals in Table 14. 

Table 14. Solubility of selected fly ash fractions in pH 4.9.35

Combustion Zone Post-Combustion Zone 

Coal Submicron Supermicron Submicron Supermicron 

Pittsburgh 42% 29% 50% 0% 

Ohio 14% 0% 0% 0% 

Kentucky 13% 0% 0% 0% 

Wyodak PRB 0% 0% 0% n.d. 

N.Dakota lignite 0% 0% 0% 0% 

The low-rank coals have high calcium in the ash and low sulfur. The Ohio and Pittsburgh coals 

contain high iron and high sulfur. The Kentucky bituminous coal is a low-sulfur coal, which has 

less sulfur than the other two bituminous coals and a moderate iron content (as compared to the 

low-rank coals). 

The solubility data suggest that for the bituminous coals, particularly the Pittsburgh coal, 

selenium is more soluble in the submicron particles than in the supermicron particles. 

Furthermore, selenium solubility is higher in the flame zone, suggesting some post-combustion 

transformations that render Se less soluble post-combustion. 

Seames postulated that the Pittsburgh submicron ash solubility was consistent with iron 

selenates, but the supermicron ash solubility more likely indicated calcium-selenium compounds. 

34 
Selenium in Coal-Fired Boilers 

April 4, 2011 

The Kentucky solubility results were attributed to iron selenates. Seames thought that the Ohio 

ash solubility results reflected the presence of SeO2 in the ash. The Wyodak and North Dakota 

lignite results were attributed to the presence of calcium-selenium compounds. 

Essington36 estimated the solubility product constants for the dissociation of metal selenates and 

selenites, using an estimating method for thermodynamic data. Table 15 gives the values of pK 

at 298.15 K. pK is equal to the negative logarithm of the solubility product constant. The higher 

the solubility product constant, the more soluble the compound is. Thus, values of pK closer to 

zero in the table mean that the reactant is more likely to dissolve. 

Selenates are more soluble in water than selenites, for a given cation. For a given oxidation 

state, the cation associated with the selenium oxyanion also affects the solubility. In terms of 

solubility, Na > Ca > Fe. 

Table 15. Solubility product constants at 298.15 K for the dissolution of selenates and 

selenites.36

Reaction pK 

Na2SeO3 = 2Na+ + SeO3

2- 3.51 

CaSeO3 = Ca2+ + SeO3

2- 7.65 

CaSeO4 = Ca2+ + SeO4

2- 4.77 

FeSeO3 = Fe2+ + SeO3

2- 9.99 

FeSeO4 = Fe2+ + SeO4

2- 6.52 

Fe2(SeO4)3 = 2Fe3+ + 3SeO4

2- 23.19 

SeO2 is soluble in water to form selenious acid: 

SeO2 + H2O = H2SeO3 (6) 

H2SeO3 = H+

 + HSeO3

−

 (pKa = 2.62) (7) 

HSeO3

−

 = H+ + SeO3

2−

 (pKa = 8.32) (8) 

Selenites are most often observed in leachate (and in fly ash, as noted below). Calcium selenite 

is more soluble than iron selenite (or iron selenate). SeO2 is more soluble in water than calcium 

or iron selenite. It is possible that the bituminous fly ash samples generated by Seames that had 

the highest solubility contained some SeO2, in addition to iron and/or calcium selenite. 

Previous studies measured speciation of Se in water extracts, in which the pH of the solution 

might affect the selenium speciation. Huggins and co-workers37 used XAFS to speciate 

selenium in ash samples. This method does not involve leaching selenium and so should provide 

a less ambiguous assessment of Se species in the ash. 

Ten fly ash samples were examined: six bituminous ash samples, three PRB ash samples, and 

one lignite ash sample. One of the PRB ash samples also contained activated carbon. 

35 
Selenium in Coal-Fired Boilers 

April 4, 2011 

Irrespective of the coal rank, all the selenium in the ash samples was shown to be selenite, but 

not predominantly SeO2. The ash sample that contained activated carbon showed evidence of 

some Se[0], perhaps an organoselenium compound. There were differences in the XAFS spectra 

for selenium between the bituminous and low-rank fly ash samples. The authors concluded that 

there might be different selenites formed in bituminous fly ash, as compared to low-rank fly ash. 

XAFS was also used to speciate selenium in fly ash samples from four power plants in 

Australia.38 The analysis identified primarily selenite in the fly ash samples, although the 

researchers suggested that a small amount of selenate might be present. 

In summary, the solubility of selenium from fly ash in water had been observed to vary from 0% 

to as high as 60%. Selenium is enriched in smaller ash particles, but the solubility of selenium is 

not noticeably different between small and large ash particles. Seames observed differences in 

solubilities when the ash was collected from the combustion zone or the post-combustion zone, 

which might suggest transformations of selenium in the flue gas, once it has reacted with or 

condensed on fly ash. Selenium in fly ash appears to be predominantly Se[IV], either calcium or 

iron selenite. Bituminous fly ash might also contain SeO2. Se[0] has been observed in fly ash, 

perhaps associated with carbon in fly ash. Se[VI] has been observed in the water-soluble 

selenium fraction in fly ash, although it is not clear if this oxidation state (selenate) exists in the 

fly ash or is formed in solution during leaching. 

3. Model Description

Previously, a model was developed for the gas-to-solid conversion of selenium in coal-fired 

boiler exhaust gases.39 This was a one-dimensional model that included selenium condensation 

and reaction with calcium and iron on the fly ash as temperatures decrease in the postcombustion section of the boiler. The model took into account the available surface area of the 

fly ash as well as the competition between selenium and sulfur for surface sites. The model was 

intended to predict the concentration of selenium in the vapor phase at the inlet to the air 

preheater. 

A flow diagram describing the Selenium Model is shown in Figure 13. 

36 
Selenium in Coal-Fired Boilers 

April 4, 2011 

Figure 13. Flow diagram for Selenium model. 

The model is one-dimensional, and requires the initial condition of the system: fly ash size and 

composition, coal composition, time-temperature profile, vapor-phase selenium concentration, 

and initial particle selenium concentration. The initial selenium concentration on the ash 

particles is computed assuming zero concentration for all submicron particles, which are 

assumed to be formed from ash vaporization followed by homogeneous condensation and 

agglomeration. Selenium not vaporized is assumed to be uniformly distributed on the 

supermicron ash particles and is computed as 

a

o

Se io C

C C ⋅ −

− = )1(

)1( , φ

θ , (9) 

where θ is the fraction of selenium in coal vaporized, φ is the mass fraction of submicron ash, Co

is the concentration of selenium in coal (g/g), and Ca is the mass fraction of ash in the coal, taken 

from the ultimate analysis. CSeo,i is the initial concentration of selenium in g/g on size i. This 

37 
Selenium in Coal-Fired Boilers 

April 4, 2011 

equation assumes a uniform distribution of selenium on the supermicron particles. If the initial 

selenium concentration on a given size is assumed proportional to the concentration of another 

compound in the ash particle, say compound “B”, then the following equation would apply: 

∑ ⋅ ⋅ ⋅− =

i

iiB

iB

a

o

Se io nC

C

C

C C

)(

)1( )( ,

θ , (10) 

where (CB)i is the concentration of compound B in g/g ash, ni is the mass fraction of ash size i, 

and the summation is taken only over supermicron particles. Note that for uniform CB, Eq. 10 

reduces to Eq. 9. 

The model requires as input the amount of selenium vaporized in the flame. As discussed above, 

selenium is soluble in silicate glasses, where it has been shown to react with iron. The model 

assumes that some portion of selenium in the coal is captured by the large, glassy ash particles at 

flame temperatures. The amount of selenium captured by these residual as particles is assumed 

to be proportional to the bulk iron content of the coal ash, based on a correlation between 

apparent vaporization of selenium in coal-fired boilers and the iron content of the coal. From 

full-scale data,15 it was noted that Se vaporization in this high-temperature combustion zone 

ranged from 65% to 100% (Figure 7) and the relationship between selenium and iron was used to 

estimate Se vaporization (Figure 8). In the model, the non-vaporized portion of the Se will be 

distributed among the supermicron ash particles. 

3.1. Gas­Solid Interactions

The vapor-phase selenium is transported to the particle surface where it interacts by one of two 

mechanisms, heterogeneous condensation, or surface reaction. These mechanisms are computed 

as fluxes, with units of moles SeO2 per second per particle. Only one mechanism is assumed to 

occur for a given set of conditions. The equations for each mechanism depend on the gas-phase 

transport regime: free molecular (submicron) or continuum (supermicron). Thus, four flux 

equations are used to describe the transport. 

The concentration of selenium on a given ash particle at a given time tf is given by the following 

equation, 

∫

=

=

⋅⋅

⋅⋅

+=

tft

t

i

ii

Se

Se i Seoi dttF

d

MW CC

0 3

)(

6 ρ

π

. (11) 

Here, ρi and di are the ash particle density, and diameter, respectively, which are both assumed 

constant. The total particle mass is also assumed constant, since the total mass of selenium is 

much less than the mass of the ash particle. Selenium concentrations are measured in μg/g. Eq. 

11 is integrated numerically for each particle size, allowing for changing gas and transport 

properties with time. 

At each time step, the moles of SeO2 removed from the flue gas is computed as 

38 
Selenium in Coal-Fired Boilers 

April 4, 2011 

∑ , (12) =

Δ⋅⋅=Δ

sizesi

SeO ii n tnpF 2

where Fi is the flux for the given size, referred to above, and defined below, and npi is the 

number of ash particle of size i for our given basis of 1 gram of coal. npi is computed as 

 3

6 ii

i

i

d

nCa

np

⋅⋅

⋅ =

ρ

π

. (13) 

The following equations are used to describe the flux of SeO2 to the particle surfaces. 

3.1.1. Heterogeneous Condensation, Continuum Regime

The equation used to describe the condensation flux in the continuum regime is given by: 

( ) , 33.171.11

2 1

2 ⎥

⎦

⎤ ⎢

⎣

⎡

⋅+⋅+

+ ⋅−⋅ ⋅

⋅⋅ = ∞ Kn Kn

Kn PP

TR

Dd

F s

i

i

π (14) 

where Kn = 2λ/di is the Knudsen number, D is the diffusivity of SeO2, P∞ and Ps are the bulk and 

surface pressures of SeO2, respectively. The term in brackets in Eq. 14 is the Fuchs-Sutugin 

correction for the non-continuum behavior of small particles. For large particles the term 

becomes unity. The mean free path is computed as 

2/1

2 ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

⋅⋅

⋅ ⋅= TR

π MWfg

ρ

μ λ . (15) 

The diffusivity of SeO2 is computed using the empirical equation of Fuller, Schettler, and 

Giddings, 40 which utilizes molecular diffusion volumes. The diffusion volume of selenium was 

not available so the value was calculated by plotting the atomic radius versus the diffusion 

volume of 8 elements and using this result to estimate the diffusion volume for selenium. The 

surface pressure, Ps is computed as the equilibrium vapor pressure of the gas in the following 

reaction: SeO2(s) = SeO2(g). Figure 14 shows a plot of the vapor pressure versus temperature. 

The relatively high vapor pressure at low temperature implies condensation will only occur at 

very low temperatures, and relatively high concentrations of selenium in the vapor-phase. 

39 
Selenium in Coal-Fired Boilers 

April 4, 2011 

‐6

‐4

‐2

0

2

4

6

0 500 1,000 1,500 2,000

log(P, bar)

Temperature, K

Figure 14. Vapor pressure of SeO2. 

3.1.2. Heterogeneous Condensation, Free Molecular Regime

The flux in the free molecular regime (submicron particles) is given by 

2/1

2

2( )

)(

TRMW

PPd

F s

i ⋅⋅⋅⋅

−⋅⋅⋅ = ∞

π

πψ , (16) 

where ψ is the accommodation coefficient, or the fraction of successful collisions with the 

particle surface, taken as unity. 

3.1.3. Surface Reaction, Continuum Regime

The flux equation for the surface reactions is given by the following equation: 

⎥

⎥

⎥

⎥

⎦

⎤

⎢

⎢

⎢

⎢

⎣

⎡

⋅ + ⋅⋅

⋅

⋅⋅ = ∞

cfsDFasakd

P

TR

d

F

Sei

i

i 2 1

2 π , (17) 

where kSe is the reaction rate constant in units of (m/s), Fasa is the fraction of the active surface 

area, where reactions are assumed to occur between the SeO2, which is an oxy-anion, and 

calcium cations at the particle surface. Fasa is assumed equal to the mass fraction of CaO in a 

given ash size. The same equation is used for the reaction between selenium and iron. In the 

40 
Selenium in Coal-Fired Boilers 

April 4, 2011 

41 

case of a high-surface area sorbent, the same equation is used, but the surface area of the sorbent 

is used to calculate the active surface area. This effectively neglects intraparticle diffusional 

resistance. 

Eq. 17 is written in terms of the bulk selenium partial pressure, rather than the surface pressure, 

so the diffusive resistance is added to the reactive resistance in the denominator of the term in 

brackets. The corresponding surface pressure for reaction-only is computed by 

FasakdD

PD

P

Se

s ⋅⋅+⋅

⋅⋅ = ∞

2

2 . (18) 

3.1.4. Surface Reaction, Free Molecular Regime

The reaction flux for the free molecular regime is given by 

FasaPk

R T

d Fi Se ⋅⋅⋅ ⋅ = ∞

2 π . (19) 

For the free molecular regime, there is no diffusion resistance, and the surface pressure is equal 

to the bulk pressure. 

The reaction fluxes presented are not expected to occur in parallel, rather, only heterogeneous 

condensation, or surface reaction occurs, depending on the conditions. For temperatures above 

800o

C, neither mechanism will occur for calcium. For iron, the surface reaction was assumed to 

take place at temperatures below 977o

C. As discussed above, if the equilibrium vapor pressure is 

greater than the bulk partial pressure, the heterogeneous condensation flux is assumed zero; 

negative values are not allowed. For the surface reactions, if the surface pressure, Eq. 18 (or the 

bulk pressure for the free molecular regime) is greater than the equilibrium vapor pressure, the 

reaction fluxes are assumed zero, and only heterogeneous condensation occurs. 

3.2. Surface Reaction Kinetics

A detailed description of experimental thermodynamic and kinetic data is given above. 

Agnihotri et al.1

 gave the following equation for the SeO2-CaO reaction: 

 

 ( 20) 

where x is the conversion of CaO and So is the initial surface area of the particle. Agnihotri et 

al.9

 determined that Ea = 16.849 kJ/gmol, m=0.25 and n=0.66 from data taken in the range of 

400o

C to 600oC. Li et al. made measurements of the reaction rate from 600oC to 840oC. Near 

600o

C, the reaction order for SeO2, n, was 1.0, increased to 1.5 as the temperature increased to 

800o

C. 

For this modeling effort, we will assume m=1 and n=1 (in Eq. 20) and use an activation energy 

of 16.84 kJ/gmol. Li et al.11 observed that above ~650o

C, the reaction rate fell to zero by 800oC. 

( ) [ ] ⎟

⎠

⎞ ⎜

⎝

⎛

⋅

− ⋅−⋅= TR

E xSkk SeO m n a

f ofo )1( 2 exp
Selenium in Coal-Fired Boilers 

April 4, 2011 

The fall in reaction rate with increasing temperature is indicative of the decomposition of the 

reaction product, CaSO3, as has been noted by Fan and co-workers. Above 800o

C, therefore, the 

reaction rate will be set to zero. The reaction rate for Se with iron uses the same form as Eq. 20. 

The sulfur present in flue gas is able to react with calcium or iron in ash to reduce the reactivity 

of the ash towards selenium. This important effect is included in the model. Li and co-workers 

noted the simultaneous removal of SeO2 and SO2 in their TGA experiments.10,11 Yang and 

Shen41 measured sulfation rates of crystalline calcium silicates at temperatures up to 900o

C and 

reported that the rates were comparable to those of the reaction between SO2 and limestone. 

Graham also observed sulfation of high-calcium fly ash in laboratory experiments with highcalcium fly ash.42 Kang and co-workers43 measured sulfation rates in a laboratory experiment 

using ash from an Eagle Butte subbituminous coal. They calculated a rate constant that was 

first-order in PSO2 as 

( T )

S mskgSOk CaOMPaSO e

/9210

2

2

2 /[ 23.6] − ⋅ ⋅= (21) 

The reaction of CaO with SO2 is reported to be a two-step process, first forming CaSO3 and then 

forming CaSO4. Simons44 reported the rate of SO2 reaction with CaO as 

[ ] ( ) kgSOR CTeFasaP T o

SO 7sec/ 827, /67502 2 −= ⋅ < − (22) 

[ ] ( ) kgSOR eFasaPx CTCoT o

SO 106sec/ 827, 1227 4 /000,172 2 −= ⋅ << − (23) 

Fly ash has a mixture of CaO (free calcium) and calcium aluminosilicates. The submicron 

particles that contain calcium, which represent the largest fraction of the total ash surface area, 

are more likely to consist of CaO than calcium aluminosilicate, since calcium aluminosilicates 

are formed by the coalescence of supermicron mineral particles during combustion. For the 

model, Eq. 22 and Eq. 23 will be used to calculate the reaction rate of SO2 with calcium surfaces. 

All the sulfur in coal is assumed to form SO2, and the SO2 flux is computed using the flux 

equations for surface reaction, as for selenium, listed in Eqs 17 and 19. In these equations, the 

diffusivity and partial pressure of SO2 replace those of selenium oxide. The reaction of iron and 

SO2 will be modeled similarly. The same activation energy as for the calcium-SO2 reaction will 

be used, in the absence of experimental data. 

There are four reactions in the model that occur simultaneously. In calcium-containing ash 

particles, both SeO2 and SO2 react with the surface. The Ca-Se reaction is only active at 

temperatures less than 800o

C. The activation energy for the Ca-Se reaction was taken from Fan 

and co-workers. The pre-exponential factor for this reaction was fit to experimental data. The 

Ca-SO2 kinetic parameters were taken from the previous arsenic model.45 

In iron-containing ash particles, both SeO2 and SO2 are assumed to react with the surface. There 

is experimental evidence for formation of iron-selenium compounds post-flame.16 If free iron 

exists in the fly ash, it can react with SeO2(g), similarly to the reaction of CaO with SeO2(g). 

42 
Selenium in Coal-Fired Boilers 

April 4, 2011 

In order to determine the thermal stability of iron-selenium compounds, thermodynamic 

calculations on reaction equations were carried out using HSC. For each reaction equation, the 

decomposition temperature of the reactant (left-hand side) was calculated from the temperature 

at which the ΔG of the reaction as written changed sign. The results of these calculations are 

shown in Table 16. 

Table 16. Thermal stability temperatures of various iron and calcium compounds with 

selenium or sulfur. 

 Thermal Stability Temperature 

Reaction C K 

FeSO4 = FeO + SO2(g) + 1/2O2(g) 1035 1308 

Fe2(SO4)3 = Fe2O3 + 3SO2(g) + 

3/2O2(g) 780 1053 

Fe2(SeO3)3 = Fe2O3 + 3SeO2(g) 640 913 

 

CaSO4 = CaO + SO2(g) + 1/2O2(g) 1662 1935 

CaSO3 = CaO + SO2(g) 995 1268 

CaSeO4 = CaO + SO2(g) + 1/2O2 760 1033 

CaSeO3 = CaO + SeO2(g) 540 813 

Selenates are not as stable as sulfates at high temperatures. Selenites are not as thermally stable 

as selenates. The only thermodynamic data available for an iron compound (Fe(III) selenite) 

suggests that this compound is stable at higher temperatures than calcium selenite. 

The decomposition of the reaction product of CaO and SeO2(g) has been observed 

experimentally at about 800°C, which is close to the decomposition temperature of calcium 

selenate. 

Based on the reaction equations, it is expected that iron selenite or selenate will form at a higher 

temperature than calcium selenate. The thermodynamic data on iron-selenium compounds is 

limited and the exact compound that forms from the reaction of iron oxide and SeO2(g) is not 

known. In the model, a decomposition temperature of 977°C (1250 K) will be assumed for some 

iron selenate compound (assumed). 

The activation energy for the Fe-Se reaction was set to be comparable to that of the Ca-Se 

reaction (20 kJ/mol-K). The pre-exponential factor was fit to experimental data. The Fe-Se 

reaction was assumed to occur at temperatures below 977o

C (1250 K), because reaction with iron 

was observed at higher temperatures in the laboratory data, as discussed above. The activation 

energy for the Fe-SO2 reaction was the same as that for the Ca-SO2 reaction, since there were no 

data on this reaction. Reaction rate parameters are summarized in Table 17. 

43 
Selenium in Coal-Fired Boilers 

April 4, 2011 

Table 17. Parameters used in selenium model. 

Parameter Units Comments 

ko 50 m/s Ca-Se reaction 

Ea 16,860 J/mol only < 800oC 

koSO2 0.000117 moles/s/m2/Pa Ca-SO2 reaction 

EaSO2 56,119.5 J/mol T<827oC 

koSO2 1 moles/s/m2/Pa 

EaSO2 141,338 J/mol 827oC<T<1227oC 

koFe 5.0 m/s Fe-Se reaction 

EaFe 20,000 J/mol only <977oC 

koFeSO2 0.1 moles/s/m2/Pa Fe-SO2 reaction 

EaFeSO2 141,338 J/mol T<1227°C 

3.3. Model inputs

The inputs to the model can be split into three groups: (1) coal composition and time-temperature 

profile, (2) ash size distribution and properties, and (3) initial selenium concentration and 

speciation. In order to compute the selenium partitioning, the composition and time versus 

temperature profile of the flue gas are required. The flue gas composition is obtained by 

specifying the coal ultimate analysis (C, H, O, N, S, Cl, Ash, Moisture), the combustion air 

composition, and the stoichiometric ratio, assuming complete combustion. The time-temperature 

profile is input directly. A basis of one gram of as-received coal is taken as a basis for the 

calculation. For input group two, the post-combustion ash size distribution is specified as 

particle diameter versus mass fraction. In addition, ash density, calcium content and iron content 

for each size are specified. The group three inputs require the selenium concentration in the coal, 

and fraction of selenium volatilized during combustion. Because the selenium is present in only 

trace quantities in the coal the volatilized selenium as SeO2 is added to the flue gas directly, 

without regard to the material balance defined by the group 1 inputs. The SeO2 in the flue gas 

will be on the order of 1 ppmv or less. 

4. Model Validation

Pilot-scale data were taken from experiments carried out at the University of Arizona.16,17 The 

experimental facility was a 2.2 kg/hr refractory-lined, coal-fired furnace. A quench probe was 

used to take size-segregated ash samples near the flame (upper) and at the end of combustion 

(lower). A series of experiments were carried out in which a coal was burned and size-resolved 

ash samples were taken. Each data set contained the following data: 

• Coal ultimate and proximate analysis, plus ash chemistry and trace element content. 

44 
Selenium in Coal-Fired Boilers 

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• Time-temperature history of the furnace for individual coals 

• Low-pressure impactor data (0.03 μm to 15 μm) at each of the three sampling locations, 

including mass distribution, major elements and trace elements. 

Six coals were burned in this study, and their properties are summarized in Table 18. The four 

bituminous coals had a range of sulfur contents, from 0.8% (Kentucky) to 3.6% (Illinois 6). The 

content of Fe2O3 in the ash ranged from 5.1% (Kentucky) to 22% (Illinois 6). The Kentucky 

coal was also very low in calcium (1.8% CaO in ash). The low-rank coals, Wyodak 

subbituminous and North Dakota lignite, had high CaO in ash – 22.7% and 30.3%, respectively. 

The North Dakota lignite also had moderate iron in the ash, 11.8% as Fe2O3. 

The datasets are identified as follows: 

1. Pittsburgh bituminous coal; 

2. Illinois 6 bituminous coal; 

3. Ohio 5,6,7 bituminous coal; 

4. Elkhorn/Hazard Kentucky bituminous coal; 

5. Wyodak PRB coal; 

6. North Dakota lignite; 

7. Pittsburgh coal plus injection of Fe-nitrate solution (Fe-doping) in order to increase the 

Fe in the ash equivalent to doubling the moles of calcium in the ash; 

8. Kentucky coal plus added SO2 (SO2-doping) in order to achieve the same SO2 

concentration in the flue gas as in the Ohio coal; 

9. Ohio coal co-injected with Ca(OH)2 (Ca-doping) in order to set the ratio of Ca/As the 

same as in the Wyodak coal; 

10. Wyodak coal plus injection of Se solution into flame (Se-doping) in order to set the ratio 

of Se/Ca to be the same as in the Ohio coal, about an 8 times increase in Se; 

11. Wyodak coal plus injection of Fe-nitrate solution (Fe-doping) in order to set the ratio of 

Fe/Se to be the same as in the Ohio coal, about a 2.5 times increase in Fe. 

45 
Selenium in Coal-Fired Boilers 

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Table 18. Properties of coals in UofA experiments.16 

Coal 

Pittsburgh 

Bitum 

Illinois 6 

Bitum 

Kentucky 

Bitum 

Ohio 

Bitum 

Wyodak 

Subbit 

ND 

Lignite 

Ultimate analysis (wt%) 

C 76.6% 67.7% 74.9% 71.1% 51.2% 38.6% 

H 4.8% 4.7% 4.6% 4.8% 3.6% 2.6% 

N 1.5% 1.2% 1.4% 1.4% 0.7% 0.4% 

S 1.6% 3.6% 0.8% 2.6% 0.3% 0.6% 

Cl 0.1% 0.0% 0.2% 0.1% 0.0% 0.0% 

O 6.9% 9.2% 8.4% 8.1% 12.3% 12.5% 

Ash 7.0% 10.3% 7.4% 9.7% 6.0% 9.4% 

Moisture 1.4% 3.3% 2.3% 2.3% 25.8% 35.9% 

Ash composition (wt% ash, S-free) 

SiO2 45.37 49.40 55.00 38.09 43.28 22.00 

Al2O3 24.18 19.31 33.76 39.41 17.17 20.38 

Fe2O3 20.28 22.04 5.10 13.03 6.32 11.75 

TiO2 1.14 1.01 1.68 2.04 1.38 0.50 

P2O5 0.60 0.13 0.23 0.51 1.78 0.13 

CaO 4.77 4.45 1.81 2.24 22.74 30.25 

MgO 1.03 0.95 0.59 1.02 4.03 8.00 

Na2O 1.35 0.70 0.32 0.92 1.67 5.12 

K2O 1.28 2.00 1.51 2.55 0.45 1.38 

BaO 0.20 0.68 0.50 

SrO 0.49 

Selenium, μg/g coal 0.62 2.2 3.1 1.4 1.08 1.5 

Each coal that was burned in the pilot-scale furnace produced its own temperature profile, as 

shown in Figure 15. The bituminous coals produced higher temperatures than the low-rank 

coals, because of the higher water content of the low-rank coals. The figure shows the 

temperatures from the flame down to Port 14 (lower sample). 

46 
Selenium in Coal-Fired Boilers 

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Figure 15. UofA furnace time-temperature histories.16 

Before examining the modeling results, it is useful to review the experimental data, to observe 

trends and look for underlying mechanisms. The data will be displayed in terms of concentration 

of Se in ppmw as measured in the different stages of the low-pressure impactor. Each stage is 

associated with a mean aerodynamic diameter. 

Low-pressure impactor samples were taken at Port 4 (Upper Samples) and Port 14 (Lower 

Samples). The temperatures at these locations are given in Table 19. For three of the 

bituminous coals, the Port 14 temperature was approximately equal to 800o

C, while for the 

Kentucky coal, the sampling temperature was greater than 800o

C. Thus, we expect to see little 

reaction with calcium in the bituminous coal data, but ample time for reaction with calcium in 

the low-rank coal data. 

47 
Selenium in Coal-Fired Boilers 

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Table 19. Sampling temperatures from UofA experiments. 

Coal Port 4 

Temperature, o

C 

Port 14 

Temperature, oC 

Pittsburgh 1187 792 

Illinois 1187 792 

Kentucky 1267 947 

Ohio 1107 797 

Wyodak 1027 687 

North Dakota 967 598 

In the next series of figures, the experimental data will be examined. All of the experimental 

data (concentration of Se in fly ash) showed higher Se in ash than introduced with the coal so the 

data have been normalized to the total amount of Se in the coal (Table 18). This limits the 

usefulness of the data for investigating total Se removal. Each of the data sets, including the 

doping experiments, reflects 100% Se capture on ash, so it is more informative to compare the 

shapes of the curves. First, Figure 16 and Figure 17 show the distribution of Se as a function of 

particle size for the bituminous coals and low-rank coals, respectively. 

For the bituminous coals, the concentration of Se decreased as particle size increased. This 

pattern suggests that the higher surface area of the submicron particles attracted more Se than the 

supermicron particles. For the Pittsburgh and Illinois coals, there is evidence that at the high 

temperature (Port 4) there was vapor-phase SeO2 in the furnace, which nucleated homogeneously 

in the sampling system to form ultrafine (0.03-0.05 μm) particles. For submicron particles from 

0.1 to 1 μm, there was little difference between the shape of the Se distributions between Port 4 

and Port 14, which might be because of the reaction of Se with bituminous ash particles at 

temperatures greater than 1267o

C (2312oF). 

There were less data for the low-rank coals; a significant portion of the North Dakota selenium 

distribution was not available. Nonetheless, it is clear for the Wyodak coal that there was little 

Se in the submicron particles at Port 4 (1027o

C), but by Port 14 (687oC), Se concentrations in the 

submicron particles increased. This is consistent with reaction between calcium and selenium, 

which takes place at temperatures below 800o

C. 

48 
Selenium in Coal-Fired Boilers 

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49 

1

10

100

1000

0.01 0.1 1 10 100

[Se] (ppmw)

Ash size (um)

Port 4

Port 14

(a) Pittsburgh

1

10

100

1000

0.01 0.1 1 10 100

[Se] (ppmw)

Ash size (um)

Port 4

Port 14

(b) Illinois

1

10

100

1000

0.01 0.1 1 10 100

[Se] (ppmw)

Ash size (um)

Port 4

Port 14

(c) Kentucky

1

10

100

1000

0.01 0.1 1 10 100

[Se] (ppmw)

Ash size (um)

Port 4

Port 14

(d)Ohio

Figure 16. Se concentrations in ash as a function of ash diameter: baseline experimental 

results for bituminous coals. 

Figure 17. Se concentrations in ash as a function of ash diameter: baseline experimental 

results for low-rank coals. 

1

10

100

1000

0.01 0.1 1 10 100

[Se] (ppmw)

Ash size (um)

Port 4

Port 14

(a) Wyodak

1

10

100

1000

0.01 0.1 1 10 100

[Se] (ppmw)

Ash size (um)

Port 4

Port 14

(b) North Dakota
Selenium in Coal-Fired Boilers 

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Doping experiments were performed in which (a) calcium was added to the coal (as Ca(OH)2); 

(b) iron was added to the flame by injecting an iron nitrate solution; (c) additional SO2 was 

added to the burner; or (d) additional Se was added to the flame as a salt solution. Figure 16 

gives the results from adding iron to the Pittsburgh coal compared with the baseline; these are 

results from Port 14. The addition of iron changed the shape of the distribution curve somewhat, 

in the region for which there are data (only data for diameters greater than 0.5 μm were taken). 

Figure 18. Se concentrations in ash as a function of ash diameter at Port 14: baseline 

experimental results compared with doping experiments for Pittsburgh coal. 

The Ohio bituminous coal was low in calcium. Therefore, calcium was added to the Ohio coal. 

The Se distribution curves are fairly similar (0.5 to 9 μm) as shown in Figure 19. Again, the Port 

14 temperature might have been too high for significant reaction between Se and calcium. 

50 
Selenium in Coal-Fired Boilers 

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Figure 19. Se concentrations in ash as a function of ash diameter at Port 14: baseline 

experimental results compared with doping experiments for Ohio coal. 

The Kentucky coal was low in sulfur (and iron). Therefore, SO2 was added to the Kentucky 

coal. As shown in Figure 20, the concentration of Se in the supermicron ash did not change upon 

addition of SO2. 

The Wyodak coal was low in iron and selenium, but high in calcium. Therefore, iron or 

selenium was added to the Wyodak coal, as shown in Figure 21. Adding Se increased the Se 

concentration in the ash, especially in the submicron range. Adding iron (as supermicron 

particles) increased the concentration of Se in the supermicron particles modestly. 

51 
Selenium in Coal-Fired Boilers 

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Figure 20. Selenium concentrations in ash as a function of ash diameter at Port 14: 

baseline experimental results compared with doping experiments for Kentucky coal. 

Figure 21. Selenium concentrations in ash as a function of ash diameter at Port 14: 

baseline experimental results compared with doping experiments for Wyodak coal. 

Several conclusions related to the mechanism for selenium-ash interactions can be drawn from 

the experimental data. For most of the bituminous coals, the Se distribution in the ash appears to 

track the iron distribution. In the bituminous coal experiments, Se was observed in the 

submicron particles at temperatures greater than 1000o

C, suggesting reaction with iron in the 

submicron ash at high temperatures. Adding iron or calcium to the bituminous coals increased 

52 
Selenium in Coal-Fired Boilers 

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the concentration of Se in fly ash. Iron had the larger effect on Se concentration. The 

temperature of the lower furnace sampling location might have been too high to see significant 

Se-Ca reaction. 

There is evidence for reaction between Se and calcium in the Wyodak coal at temperatures less 

than 1000o

C, which is consistent with the laboratory observation that the reaction product 

between calcium and selenium is not stable above 800o

C. Adding Se to the Wyodak coal 

increased Se concentration in all particle sizes. Adding supermicron iron to the Wyodak 

combustion experiment resulted in a modest increase in Se in the supermicron ash particles. 

Perhaps this reflects mass transfer limitations, because the Se content of the Wyodak coal was 

low. 

4.1. Modeling Results

The modeling results are compared to the experimental data in the next series of figures. It is 

important to note that the experimental data (concentration of Se in fly ash) showed higher Se in 

ash than introduced with the coal so the data have been normalized to the total amount of Se in 

the coal (Table 18). This means that the experimental data show 100% Se capture on ash, which 

may not be correct for all cases. Therefore, it is more reasonable to compare the shape of the 

curves and key features rather than the absolute measurements. 

Figure 22 shows the predicted and measured selenium distributions at Port 14 for combustion 

experiments with the Pittsburgh coal. The model shows that adding calcium or iron to the 

Pittsburgh coal shifts selenium from submicron to supermicron particles. 

53 
Selenium in Coal-Fired Boilers 

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(a) Pittsburgh Baseline (b) Fe addition Pittsburgh

(c) Ca addition Pittsburgh 

Figure 22. Measured and predicted selenium concentrations in fly ash for Pittsburgh coal. 

Figure 23 shows the predicted and measured selenium distributions at Port 14 for combustion 

experiments with the Illinois 6 coal. The model predicts the shape of the experimental curve but 

not the absolute concentrations of selenium in the ash. 

54 
Selenium in Coal-Fired Boilers 

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Figure 23. Measured and predicted selenium concentrations in fly ash for Illinois 6 coal. 

Figure 24 shows the predicted and measured selenium distributions at Port 14 for combustion 

experiments with the Kentucky coal. As with the Pittsburgh coal, the shape of the curves is 

similar, but the model does not predict as much Se capture on ash. It is somewhat hard to 

understand the increase in Se concentration with the addition of SO2, particularly without data on 

the submicron particles. 

(a) Kentucky Baseline (b) SO2 addition Kentucky

Figure 24. Measured and predicted selenium concentrations in fly ash for Kentucky coal. 

Figure 25 shows the predicted and measured selenium distributions at Port 14 for combustion 

experiments with the Ohio coal. Again, the Se distribution curves have similar shape but do not 

agree in absolute concentration. However, the model predictions of Se in ash are closer to the 

measurements than with the other bituminous coals. Since the Ohio coal was low in calcium, 

experiments were carried out with the addition of calcium. The model very closely predicts the 

Se on ash for this doping case and also predicts near 100% Se capture. 

55 
Selenium in Coal-Fired Boilers 

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(a) Ohio Baseline (b) Ca addition Ohio 

Figure 25. Measured and predicted selenium concentrations in fly ash for Ohio coal. 

Figure 26 shows the predicted and measured selenium distributions at Port 14 for combustion 

experiments with the Wyodak subbituminous coal. The model reproduces the trends of the 

Wyodak Se distribution from the experiments well. The model predicts 100% Se capture on ash 

for all Wyodak cases. When additional Se was added to the coal, the model correctly reproduced 

the peak apparent in the smallest particles (0.05 μm). For addition of iron to the Wyodak flame, 

the model predicted concentrations of Se that matched the experimental data in the supermicron 

particles. 

56 
Selenium in Coal-Fired Boilers 

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(a) Wyodak Baseline (b) Se addition Wyodak

(c) Fe addition Wyodak 

Figure 26. Measured and predicted selenium concentrations in fly ash for Wyodak coal. 

Figure 27 compares the predicted North Dakota lignite Se distribution with the experimental. 

Data are missing for several particle sizes. Again, the model predicts 100% Se capture on ash 

and the distribution matches the experimental data well. 

57 
Selenium in Coal-Fired Boilers 

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Figure 27. Measured and predicted selenium concentrations in fly ash for North Dakota 

lignite. 

Table 20 and Figure 28 summarize the predicted levels of removal of SO2 and SeO2 under the 

UofA experimental conditions (that is, between Port 4 and Port 14). There are no data on the 

removal of SO2 or SeO2, but the model predictions give some insight into how the model 

behaves. The unvaporized fraction (labeled as “initial” in Figure 28) was distributed among the 

supermicron particles before time integration. No condensation was predicted because the 

temperatures were not low enough. The low rank coals show increased reaction with Ca (labeled 

as “Ca-Se” in Figure 28) compared to the bituminous cases; this is because the time-temperature 

profiles of the bituminous cases allow for very limited time in the Ca-Se reaction temperature 

region (high Port 14 temperatures). The longer residence times of the low rank coals result in 

100% Se capture on ash. No condensation was predicted as the lower temperature required for 

condensation were never reached. 

Adding Fe to the Pittsburgh coal decreased SO2 removal and SeO2 removal stayed the same. 

The measured Fe in ash was not drastically increased for the Fe-doping case, hence the model 

did not show an increase in Fe-Se reaction (labeled as “Fe-Se” in Figure 28). Adding Ca to the 

Pittsburgh coal decreased the SeO2 removal. However, the measurement of Ca in ash did not 

show an increase for the Ca-doping case, therefore the model did not predict and increase in CaSe reaction. Also, because no submicron data were available, this greatly reduced the available 

surface area for reaction. 

The Se removal predicted for the Kentucky cases was very small due to the high Port 14 

temperature and limited time spent in the Fe-Se reaction temperature region. 

Adding Ca to the Ohio coal increased SO2 removal by more than a factor of 10 and doubled 

SeO2 removal. Adding Se to the Wyodak coal (8 times increase) did not affect the percentage 

removal of SeO2 (but the absolute amount of SeO2 removal increased), and SO2 removal 

58 
Selenium in Coal-Fired Boilers 

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decreased (competition with SeO2). Adding Fe to the Wyodak coal decreased SO2 removal and 

did not affect SeO2 removal. 

Table 20. Model predictions for removal of SeO2 and SO2 in UofA experiments. 

 SeO2 removal SO2 removal 

Pitts 28.13% 2.18% 

Pitts_Ca 19.70% 0.61% 

Pitts_Fe 28.23% 1.10% 

Illin 29.01% 1.58% 

Kentucky 12.65% 0.25% 

Kentucky_SO2 12.31% 0.21% 

Ohio 41.94% 0.34% 

Ohio_Ca 86.65% 4.78% 

Wyodak 100.00% 13.21% 

Wyodak_Se 100.00% 9.68% 

Wyodak_Fe 99.94% 4.69% 

ND 100.00% 11.74% 

Figure 28. Predicted Se removal on ash for the UofA experiments (from non-vaporized 

fraction, reaction with Ca, and reaction with Fe). 

59 
Selenium in Coal-Fired Boilers 

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5. Modeling of EPA MPCRF Experiments

The MPCRF is a pilot-scale combustion research facility located in the High Bay Building at 

EPA's Research Triangle Park, NC campus. The combustor is rated at 4 MMBtu/hr 

(approximately 1.2 MWt). It is capable of firing all types of pulverized coal, natural gas and No. 

2 fuel oil. The combustor is equipped with low NOx burners and a selective catalytic reduction 

(SCR) unit (with Cormetech catalyst) for NOx control. The system has an electrostatic 

precipitator (ESP) and a fabric filter (FF) - positioned in parallel - for Particular Matter (PM) 

control. The flue gas can be directed to either the ESP or the FF. The facility also uses a wet 

lime-based flue gas desulfurization (FGD) scrubber for control of SO2 emissions. The system is 

equipped with continuous emissions monitors (CEMs) for on-line measurement of O2, CO2, NOx 

(NO and NO2), SO2, CO and Hg. There are multiple sampling ports throughout the gas flow 

path. The facility is designed for ease of modification so that various control technologies and 

configurations can be tested. The facility has a series of heat exchangers to remove heat such that 

the flow path of the flue gas has a similar temperature profile to that seen in a full-scale power 

plant. 

HAP emissions data were collected at the MPCRF from February to July 2010.46,25 Three coals 

were burned during this test period: bituminous, sub-bituminous, and lignite. Method 5 

particulate determination was performed along with Method 29 metals analysis at the inlet and 

outlet of the PM control device. The removal of total PM and metals across the PM control 

device was then calculated. Selenium and PM removal results are summarized in Figure 29. Se 

removal was much less consistent than PM control, as has been seen at full scale plants. While 

firing a bituminous coal, Se removal ranged from 45-74% with the ESP and up to 99.9% with the 

FF. While firing a subbituminous coal, removal was near 100% with either the ESP or FF. 

Removal was 88% while firing the lignite coal with the ESP for PM control. 

60 
Selenium in Coal-Fired Boilers 

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Figure 29. Control of PM and Selenium across the PM control device in MPCRF tests.46,25

5.1. Model Inputs   

The inputs required by the model are the coal composition, air composition, Se in coal, Se 

vaporization, time-temperature profile, stoichiometric ratio, and fly ash distributions (size, 

composition, density). Several key modeling inputs were not available from the MPCRF tests, 

namely the ash distributions. Correlations for size, Ca in ash, and Fe in ash (as a function of 

particle size, coal rank, and Ca or Fe in coal) were developed based on available data. Sufficient 

data were not available for lignite coal to form these correlations; therefore the lignite test 

(7/21/2010) was not modeled. Se vaporization is estimated based on coal Fe content as 

discussed previously. This resulted in 82% Se vaporization for the bituminous coal and 85% for 

the subbituminous coal. Se measurements at the PM control device inlet were used as the initial 

Se concentration in coal. 

Time-temperature profiles were measured during the tests and input directly to the model (Figure 

30). The first temperature was recorded at around 0.7 seconds for most tests, so it was assumed 

that the temperature was constant before this measurement. The model requires a fairly small 

time step in the profile for integration, so data points were added between measurements by 

linearly interpolating. The model is extremely sensitive to the temperature profile (due to the 

kinetics and temperature limits for reaction) so slight uncertainties associated with the profile 

may greatly affect the modeling results. 

61 
Selenium in Coal-Fired Boilers 

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Figure 30. Time-temperature profiles measured during the MPCRF tests. 

5.2. Modeling Results

Predicted selenium partitioning to ash from the unvaporized fraction, reaction with Ca, and 

reaction with Fe is shown in Figure 31. The unvaporized fraction (labeled as “initial” in Figure 

31) was distributed among the supermicron particles before time integration. Vaporized Se 

captured in the ash through reaction with Ca or Fe (labeled “Ca-Se” and “Fe-Se” in Figure 31) 

represents a significant fraction of the Se in ash. No condensation was predicted because the 

temperatures were not low enough. The model predicted 60%-76% of the selenium in the ash 

phase for the Bit-ESP cases and 61% for the Bit-FF case. On average, the model predicts the 

Bit-ESP Se removal fairly well, but under -predicts the Bit-FF case. There may be low 

temperature reactions happening in the filter cake which the model does not account for. 

Unburned carbon in fly ash might adsorb selenium in the FF filter cake. As discussed above, 

SeO2(g) has been shown to react with activated carbon in laboratory, fixed-bed tests. There is 

not enough information to speculate further on the low temperature reactions of selenium and fly 

ash. 

For the bituminous cases, roughly half of the reacted selenium reacts with Ca and half with Fe. 

The model predicted >95% Se to the ash phase for the subbituminous cases. This is due to 

62 
Selenium in Coal-Fired Boilers 

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increased Ca reaction (~98% of the reacted Se). The PM control device did not affect the 

experimental results for the subbituminous cases because nearly all the Se is captured by the ash. 

The predicted Se distributions are shown in Figure 32 and Figure 33. These distributions are 

most affected by the Ca and Fe distributions in the ash. The Ca and Fe distributions were not 

measured for these tests and were therefore estimated based on literature data. This limits the 

significance of the predicted Se distributions. The Se content of the ash increases with 

decreasing particle diameter highlighting the increased reaction with submicron particles due to 

their high surface area. The discontinuity of the subbituminous Se distributions reflects 

discontinuities in the estimated Ca and Fe distributions. 

Figure 31. Predicted selenium partitioning to ash for the MPCRF tests (from nonvaporized fraction, reaction with Ca, and reaction with Fe). 

63 
Selenium in Coal-Fired Boilers 

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Figure 32. Predicted Se distributions for bituminous cases. 

Figure 33. Predicted Se distributions for subbituminous cases. 

64 
Selenium in Coal-Fired Boilers 

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6. Conclusions

At flame temperatures, selenium is soluble in silicate glasses. Larger solubility is indicated for 

iron-containing silicates under reducing conditions. With sufficient calcium in fly ash, selenium 

might be expected to form a calcium selenite/selenate at temperatures below about 800o

C 

(1470o

F). Without sufficient calcium or iron (or in the presence of relatively high concentrations 

of SO2), some of the selenium might be expected to condense as SeO2 at temperatures below 

about 200o

C. However, no SeO2 was predicted to condense in the model. Partial condensation 

would lead to selenium leaving the particulate control device as SeO2(g), which is observed in 

bituminous-fired plants. 

A model was created for interactions between Se and fly ash post-combustion. The reaction 

mechanism assumed the following: 

• Iron reacts with Se at high temperatures (below 977o

C) – perhaps this reaction is with FeSi-Al glasses and only happens for sufficiently high temperatures to result in low 

viscosity of the ash; 

• Ca reacts with Se at temperatures less than 800o

C; 

• SO2 reacts with calcium and iron, but more strongly with calcium. 

The Se distributions in fly ash (concentration vs. particle size) were reasonably well predicted in 

UofA pilot-scale experiments for six coals. The average Se partitioning to fly ash in the MPCRF 

tests was predicted well for bituminous and subbituminous fired cases with ESPs. For the 

bituminous fired case with a FF, the model under predicted Se removal by ash. There may be 

low-temperature reactions happening in the filter cake which the model does not account for. 

The experimental data showed no difference between the subbituminous case with either an ESP 

or FF due to the high Se removal upstream of the particulate control device (because of the high 

Ca content of the fly ash). 

7. References

1. National Institutes of Health; Office of Dietary Supplements. "Dietary Supplement Fact 

Sheet: Selenium". http://ods.od.nih.gov/factsheets/selenium.asp#h7. November 12, 2009. 

(Accessed 2010-01-02). 

2. Wilber, C.G. Toxicology of selenium. Clinical Toxicology 1980, 17 (2), 171–230. 

3. Ohlendorf, H. M. "Ecotoxicology of selenium". Handbook of Ecotoxicology. Boca Raton: 

Lewis Publishers. 2003, 466–491. 

4. EPRI. Site-Specific Water Quality Criteria for Selenium: Development of 

Implementation Analysis Protocol. March 2008 

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http://www.mydocs.epri.com/Fdocs/public/000000000001016481.pdf (Accessed 2010-

01-02) 

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Related to Developing a Recommended Aquatic Life Criterion. Federal Register: 

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6. Akers, D.J., et al. HAPs-R : Precombustion Removal of Hazardous Air Pollutant 

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of sequential leaching to quantify the modes of occurrence of elements in coal. 

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1998. 

8. Ghosh-Dastidar, A.; Mahuli, S.; Agnihotri, R; Fan, L.S. Selenium Capture Using Sorbent 

Powders: Mechanism of Sorption by Hydrated Lime. Environ. Sci. Technol. 1996, 30, 

447-452. 

9. Agnihotri, R; Chauk, S.; Mahuli, S.; Fan, L.S. Selenium Removal Using Ca-Based 

Sorbents: Reaction Kinetics. Environ. Sci. Technol. 1998, 32, 1841-1846. 

10. Li, Y.; Tong, H.; Zhuo, Y.; Chen, C.; Xu, X. Simultaneous Removal of SO2 and Trace 

SeO2 from Flue Gas: Effect of Product Layer on Mass Transfer. Environ. Sci. Technol. 

2006, 40, 4306-4311. 

11. Li, Y.; Tong, H.; Zhuo, Y.; Wang, S.; Xu, X. Simultaneous Removal of SO2 and Trace 

SeO2 from Flue Gas: Effect of SO2 on Selenium Capture and Kinetics Study. Environ. 

Sci. Technol. 2006, 40, 7919-7924. 

12. Fang, L.; Lynch, D.C. "Evaluation of the Behavior of Selenium in Silicate Slag." Metall. 

Trans. B 1987, 18B, 181-187. 

13. Kircher, U. “Waste Gas Treatment of Soda Lime Silica Glass Furnaces – Investigations 

with Different Absorption Agents.” Ceramic Trans. 1998, 82, 75-80.National Institutes 

of Health; Office of Dietary Supplements. "Dietary Supplement Fact Sheet: Selenium". 

http://ods.od.nih.gov/factsheets/selenium.asp#h7. November 12, 2009. (Accessed 2010-

01-02). 

14. Buketov, E. A. “Standard free energy potentials of the formation of selenites.” Vestnik 

Akademii Nauk Kazakhskoi SSR 1965, 21(8), 30-5. 

15. Senior, C.L. “The Fate of Arsenic and Selenium in Air Pollution Control Devices in 

Coal-Fired Power Plants.” Presented at American Chemical Society Spring National 

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16. Seames, W.S. The partitioning of trace elements during pulverized coal combustion, 

Ph.D. dissertation, University of Arizona, Tucson, AZ, 2000. 

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