# 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 84101Selenium 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 Fl y Ash in the Combustion System ................. 17
2.2.3. Low-Temperature Reactions of Selenium .............................................................. 19
2.3. Behavior in of Selenium in Ai r 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, Fr ee 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

2Selenium in Coal-Fired Boilers April 4, 2011

5.1. Model Inputs .................................................................................................................. 61
5.2. Modeling Results............................................................................................................ 62
6. Conclusions ................................................................................................................... ........ 65
7. References .................................................................................................................... ......... 65

3Selenium in Coal-Fired Boilers April 4, 2011



## List of Figures
Figure 1. Reaction rate vs. te mperature for reaction of SO 2 or SeO 2 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 SeO 2 . .......................................................................................................................... 14 Figure 4. Equilibrium calculation for starting mixture of CaO in air with gas-phase SeO 2 . ....... 14 Figure 5. Gibbs free energy change for condensation of selenium species. ................................ 15 Figure 6. Predicted low-temperature equilibr ium selenium speciati on as a function of temperature. ............................................................................................................... 16 Figure 7. Se vaporization calcu lated from full-scale data. 15 ........................................................ 17 Figure 8. Relationship between coal iron and Se vaporizatio n, 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 pl ants 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 SeO 2 . ............................................................................................. 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

4Selenium 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 predic ted selenium concentrations in fly ash for Pittsburgh coal. ......................................................................................................................... ... 54 Figure 23. Measured and predicte d selenium concentrations in fly ash for Illinois 6 coal. ........ 55 Figure 24. Measured and predicte d selenium concentrations in fly ash for Kentucky coal. ....... 55 Figure 25. Measured and predicte d selenium concentrations in fly ash for Ohio coal. ............... 56 Figure 26. Measured and predicte d selenium concentrations in fly ash for Wyodak coal. ......... 57 Figure 27. Measured and predic ted 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 partitioni ng to ash for the MPCRF tests (from nonvaporized fraction, reaction with Ca, and reaction with Fe). ..................................... 63 Figure 32. Predicted Se distribut ions for bituminous cases. ........................................................ 64 Figure 33. Predicted Se distribut ions for subbituminous cases. .................................................. 64

5Selenium in Coal-Fired Boilers April 4, 2011



## 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, inpu ts, 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. ...................................................................................................................... 2 5 Table 5. Summary of Se removal data from 2010 ICR sorted by firing system, fuel, and APCDs. ...................................................................................................................... 2 6 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 c oncentration 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 select ed 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 SeO 2 and SO 2 in UofA experiments. ..................... 59

6Selenium in Coal-Fired Boilers April 4, 2011

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 achievab le control technology MPCRF Multipollutant control research facility MW t Megawatt thermal OAQPS Offices of Air Qual ity Planning and Standards ORD Office of Research PM Particulate matter QA Quality assurance QAPP Quality assurance project plan REI Reaction Engineer ing International SCR Selective catalytic reduction

7Selenium in Coal-Fired Boilers April 4, 2011



## Abstract
The behavior of Se during combustion is differe nt from other trace metals because of the high degree of vaporization and high va por pressures of the oxide (SeO 2 ) 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 977 o C and that calcium reacts with sele nium at temperatures less than 800 o C. The model also included competing reactions of SO 2 with calcium and iron in the ash. Predicted selenium distributions in fly ash (c oncentration vs. particle size) were compared against measurements from pilot-scale experiment s for combustion of six coals, four bituminous and two low-rank coals. The mode l predicted the selenium distri bution in the fly ash from the pilot-scale experiments reasonably well for six co als of different compositions. The model was then used to predict selenium partitioning to th e solid phase for comparison with experimental data collected at the EPA Multipollutant Contro l Research Facility (MPCRF). Several model inputs were unknown for these cases; therefore correlations from prev ious data were generated to provide estimates. The model was able to pred ict the average selenium removal across an electrostatic precipitato r (ESP) for the bituminous and PRB coals. However, the model did not correctly predict the removal across a fabric filt er (FF) for the bituminous coal, which suggested that there are additional low-temperature interac tions that occur in the FF, which the model does not presently include. 

## 1. Introduction
Selenium is a chemical element represented by th e chemical symbol Se, with atomic number 34 and having an atomic mass of 78.96. Selenium rarely occurs in its elemen tal state in nature. Selenium is a solid at room temperature (density=4.8 g/cm 3 ) with a melting point of 220°C and a boiling point of 685°C. It is a me mber 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 applic ations, the largest being the manufacture of glass and ceramics. Selenium is used to tint glasse s and glazes red, as well as to “decolorize” ironcontaining glasses. Selenium is used in photoc opying and solar cells, because of its photovoltaic properties. Selenium is also us ed as a toner for photographic prin ts 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 se lenites), in contra st, are very toxic. 2

8Selenium 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 fl ora and fauna. For example, high selenium levels have been found to cause certain congenita l disorders in wetland birds. 3 Human activities can also contribute to selenium cont amination 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; a nd effluents from petroleum refineries. In December 2004, the U.S. Environmental Prot ection Agency (EPA) proposed revised water quality criteria recommending safe levels for s hort-term and long-term selenium exposure in both freshwater and saltwater environments. EPA re leased a draft ambient water quality criterion for selenium in October, 2008. The criteria recomm endations were intended to protect aquatic life under the Clean Water Act. 5 Selenium is also listed as a hazardous air polluta nt under the Clean Air Act. Currently there are no limits on air emissions of selenium from com bustion sources. However, US EPA is in the process of setting maximum achievable contro l technology (MACT) sta ndards for coal-fired power plants, which could include sta ndards for air emissions of selenium.
Selenium is found in coals in trace concentrations . USGS has reported concentrations in coal as high as 150 ug/g 6 , but 0.5 to 5 is a more typical range for US coals. Selenium is often found in coal either associated with metal sulfide mine rals, 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 th at under flame conditions (~1600 K, oxidizing atmosphere) the most thermodynamically stable species of selenium is SeO 2 (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 seleni um in wastewater discharges (ash ponds, scrubber water treatment). In this re port, background information is revi ewed regarding th e speciation of selenium in fly ash from coal combustion syst ems and the conditions for conversion of gaseous SeO 2 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 SeO 2 gas on hydrated lime and other sorbents at temperatures from 400 o C to 1000 o C. They tested hydrat ed lime, kaolinite, bauxite and limestone for sorption capacity with SeO 2 (g) in N 2 and in air. In air, hydrated lime captured more SeO 2 than the other sorbents. The capacity of hydrat ed lime for SeO 2 peaked at 600 o C. Desorption studies showed that CaSeO 3 decomposed at temperatures greater than 825 o C.

9Selenium in Coal-Fired Boilers April 4, 2011

10 Using several different measurements, the authors concluded that th ese two reactions are responsible: Ca(OH) 2 => CaO + H 2 O(g) (1)
CaO + SeO 2 (g) => CaSeO 3 (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 S o is the surface area. They determined the activation energy and exponents from their TGA data. They also looked at simultaneous reactions of SO 2 and SeO 2 with hydrated lime. Even when th e gas phase concentrations of SeO 2 and SO 2 were comparable, sulfation (at 600 o 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 se ries of studies on sorption of SeO 2 and SO 2 on CaO. 10 , 11 One study was designed to understand the impact of the product su lfate layer on sorption of SeO 2 and SO 2 . 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 SeO 2 and SO 2 . The sorption rate of SO 2 on fresh CaO was greater than the sorpti on rate on sulfated CaO at 700 o C. The loss of sorption rate at high CaO conversion was only signifi cant when the concentration of SO 2 was high (>100 ppm SO 2 ). This led the authors to conclude that sorption of trace SeO 2 would not be affected by the product layer. Experiments with 26 ppm SeO 2 showed no effect of the degree of CaO sulfation on sorption. Thus, the product layer did not seem to be what reduced SeO 2 sorption in the presence of SO 2 . In a more detailed study of SO 2 and SeO 2 sorption, 11 the authors identified two stages of the SO 2 - CaO reaction: (1) Initial chemical kinetic cont rol at low CaO conversion; (2) Late-stage (high CaO conversion) when control is by di ffusion through the product layer. The authors hypothesized that in stage (1) the nu mber of active sites, which can react with SO 2 or SeO 2 , 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 SO 2 and SeO 2 . Li et al. obtained a reaction order for SeO 2 +CaO of ~1 at 600-700 o C, which increased to 1.52 at 800 o C. The reaction order for SO 2 -CaO was 1. They also gave a graph ( Figure 1 ) with k vs. T for SeO 2 and SO 2 , from which one might deduce the activat ion energy. The rate constant for Se was greater than the rate consta nt for sulfur at T less than 720 o C, but dropped sharply above this temperature, probably because CaSeO 3 decomposed at higher temperatures. m SeO a n
o 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 900k,1/min

Temperature, o C Se

S

Li et al. ES&T 2006, 40, 7919
Figure 1. Reaction rate vs. te mperature for reaction of SO 2 or SeO 2 with CaO. 11

It seems clear that hydrated lime will work best for capture of selenium, if injected between 400 o C and 600 o C (750 o F and 1100 o F), which is to say at the economizer inlet or outlet. The reaction rate of SeO 2 will be higher than that of SO 2 in this temperature range, but the concentrations of SO 2 are much higher. SeO 2 should react with the CaO from hydrated lime in this temperature range, until a pr oduct layer builds up from sulf ation and reduces the number of sites for reaction. Selenium is soluble in silicate glasses, and is us ed in the glass industry to “de-colorize” glass, by reacting with iron to form an Fe-Se compound. Fang and Lynch 12 measured the solubility of selenium in silicate slags in equilibrium with Cu-Se alloy. As the amount of Fe 2+ increased in the slag (relative to Fe 3+ ), the solubility of Se increased in the slag. They reported 0.2 wt% to 3 wt% Se in the slags at temperatures of 1185 o C and 1250 o C. 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, se lenium might dissolve in molten silicate particles in the flame. The presence of iron in the silicates, particul arly under reducing conditi ons, could promote the formation of Fe-Se compounds, which might serve to retain selenium in the ash particles. Calcium hydroxide was compared with sodium carbona te as a sorbent for selenium in the glass industry. As noted above, selenium is often used as an additive in the ma nufacture of flint glass to control color. Some of the selenium vaporizes in the glass furnace. Kircher 13 compared sodium and calcium sorbents by injecting the so rbents in the off-gases of glass furnace. Figure 2 shows the data reported by Kircher for temperatures between 330 o C and 390 o C. At these

11Selenium in Coal-Fired Boilers April 4, 2011

12 temperatures, Ca(OH) 2 was a good sorbent for selenium, but Na 2 CO 3 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 literatu re on reactions of vapor-pha se selenium with iron compounds.



### 0 20 40 60 80 100

2 2.5 3 3.5 4 4.5%SeAdsorbed



### Sorbent loading, g/m 3 

### 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 CaSO 3 , CaSO 4 , CaSeO 4 , FeSO 4 , Fe 2 (SO 4 ) 3 , and Fe 2 (SeO 3 ) 3 as summarized in Table 1 . There are no data on calcium selenite (CaSeO 3 ). However, Buketov 14 reported values of the free energy of formation for CaSeO 3 . The standard entropy was calculated from CaSeO 4 , based on the ratio of entropies of CaSO 3 and CaSO 4 .Selenium in Coal-Fired Boilers April 4, 2011

Table 1. Thermochemical properties of sulf ites, sulfates, selenites, and selenates.

Formula Chemical Name MW Phase H f 0 298 S 0 298 G f 0 298 g/mol kcal/mol cal/(mol*K) kcal/mol Source
Na 2 SeO 4 Sodium selenate 188.9 s -257.88 28.00 -266.22 HSC
Na 2 SeO 3 Sodium selenite 172.91 s -236.50 -35.16 -246.98 HSC CaSO 3 Calcium sulfite 120.138 s -277.10 24.23 -284.32 HSC CaSO 4 Calcium sulfate 136.138 s -343.60 25.50 -351.20 HSC CaSO 4 (A) Calcium sulfate 136.138 s -340.72 26.05 -348.49 HSC CaSO 4 (B) Calcium sulfate 136.138 s -339.66 25.72 -347.33 HSC CaSeO 4 Calcium selenate 183.038 s -265.25 15.10 -269.75 HSC CaSeO 3 Calcium selenite 167.039 s -224.773 14.35 -229.05 Ref. 39 FeSO 4 Iron(II) sulfate 151.905 s -222.00 28.91 -230.62 HSC
Fe 2 (SO4) 3 Iron(III) sulfate 399.867 s -617.35 73.50 -639.26 HSC Fe 2 (SeO 3 ) 3 Iron(III) selenite 492.569 s -407.79 65.11 -427.20 HSC

Equilibrium calculations were carried out for SeO 2 plus Ca(OH) 2 or CaO in air, in order to approximate the TGA experiments of Ghosh-Dastid ar 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 800 o C, but this compound decomposed above 800 o C. The observed decomposition is pred icted by equilibrium. Of course, selenate (Se[VI]) is rarely observed in fly as h samples collected from ESP hoppers. However, there might be low-temperature reactions that co nvert selenate to selenite or conversion of SeO 2 (g) to CaSeO 3 might be the first step in reaction with CaO, but th e second step (oxidation of CaSeO 3 ) might be kinetically limited in flue gas. In any case, CaSeO 4 thermodynamics reproduces the behavior observed in the e xperiments in air, in the range of 400 o C to 1000 o C.

13Selenium 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, o C 

### Ca(OH) 2 

### + SeO 2 

### in air

SeO 2 (g) CaSeO 4

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

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, o C 

### CaO + SeO 2 

### in air

SeO 2 (g) CaSeO 4

Figure 4. Equilibrium calculation for starting mixture of CaO in air with gas-phase SeO 2 .

At temperatures less than 200 o F (~100 o C), condensation of gaseous SeO 2 might occur via two pathways:
SeO 2 (g) + H 2 O(g) Ù H 2 SeO 3 (4)
SeO 2 (g) Ù SeO 2 (5) The Gibbs free energy change for these two reacti ons as a function of temperature is shown in Figure 5 . The condensation of SeO 2 (g) to SeO 2 solid is more thermodynamically favorable than

14Selenium in Coal-Fired Boilers April 4, 2011

the formation of H 2 SeO 3 . Both reactions are thermodyna mically favorable at scrubber temperatures.

‐ 8.0 ‐ 6.0 ‐ 4.0 ‐ 2.0 0.0 2.0 4.0

200 250 300 350 400deltaG,kcal/mol

Temperature, F SeO 2 (g) + H 2 O(g) = H 2 SeO 3

SeO 2 (g) = SeO 2

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

An equilibrium calculation was carried out for th e temperature and conditions characteristic of a scrubber flue gas. The starting gas composition was assumed to be 10.4% CO 2 , 13.2% H 2 O, 5.3% O 2 , 71.1% N 2 , 39 ppm HCl, 25 ppm SO 3 , and 0.07 ppm SeO 2 . The species and phases used in the calculation are given in Table 2 . Condensed selenium was allowed to be SeO 2 or a mixture of H 2 O(l) and H 2 SeO 3 .

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)

15Selenium in Coal-Fired Boilers April 4, 2011

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 200Moles

Temperature, o F SeO 2 (g) SeO 2 H 2 SeO 3 ‐ H 2 O(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 160 o F (71 o C), first as SeO 2 , then as H 2 SeO 3 . Thus, as the flue gases cool down at the inlet to a scrubber for SO 2 , 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 re action between Se and iron, we believe that there can be limited interaction between Se and iron-containing ash in the coal part icle. A relationship between the iron content of the co al and Se vaporization was derive d from full-scale data and is shown in Figure 8 . This relationship can be used to estimate vaporization of Se.

16Selenium 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 experiment s 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

17Selenium 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 coal s 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 SO 2 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 experime nts, Se was observed in the submicron particles at temper atures greater than 1000 to 1100 o C, suggesting reaction with iron in the submicron ash at high temperatures. A dding iron or calcium to the bituminous coals increased the concentration of Se in fly ash. Iro n 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 ca lcium in the Wyodak coal at temperatures less than 1000 o C, which was consistent with the labora tory observation that the reaction product between calcium and selenium was not stable above 800 o C. Adding Se to the Wyodak coal increased Se concentration in all particle si zes. Adding supermicron iron to the Wyodak combustion experiment resulted in a modest incr ease in Se in the superm icron 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. La rger solubility is indicated for iron-containing silicates under reduci ng conditions. With sufficient calcium in fly ash, selenium might be expected to form a calcium selen ite/selenate at temperatures below about 800 o C. Without sufficient calcium or iron (or in the presence of relatively high concentrations of SO 2 ), some of the selenium might be expected to condense as SeO 2 at temperatures below about 200 o C. Partial condensation would lead to selenium leaving the particulate control device as gaseous SeO 2 , which is observed in bituminous coal-fired plants.

18Selenium in Coal-Fired Boilers April 4, 2011



### 2.2.3. Low

­ Temperature Reactions of Selenium Selenium, and to a lesser extent arsenic, retention by both fly as h and activated carbon has been studied in coal combustion flue ga s 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 120 o C. Two fly ash samples were ev aluated, 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 apparatu s to study retention of As and Se by activated carbons in simulate d coal combustion flue gas a nd gasification syngas. These experiments were also carried out at 120 o 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 capacit y of about 2 mg/g. XPS analysis of the exposed sorbent showed that th e 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 125 o C to 250 o C. SeO 2 was generated by evaporation and carried into a bed of carb on with a nitrogen carrier. Six activated carbons were used, in cluding Norit FGD carbon (which was actually a sulfur-impregnated carbon). The carbons were not extensively described, although surface areas were given. Short-dur ation tests at 125 o C showed that capture of SeO 2 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 m 2 /g) was chosen for more detailed experime nts. Sorption-desorption experiments showed that some of the adsorbed Se was eas ily desorbed, which suggested that there was a mixture of physically and chemically ad sorbed Se on the carbon. Sorption of SeO 2 was noted to be relatively slow, and the authors hypot hesized that the relatively large SeO 2 molecule might have difficulties in diffusing in the carbon micr opores. XPS analysis of the exposed carbon surface showed that selenium was in two fo rms on the surface: elemental Se and SeO 2 . The elemental Se presumably was covalently bound to th e carbon surface. The heat of adsorption of selenium on the carbon was calculate d 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 ga s containing oxidants, selenium on the surface of carbon could be oxidized to Se[VI] by NO 2 . Even without NO 2 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 sili cates under reducing condi tions. With sufficient calcium in fly

19Selenium in Coal-Fired Boilers April 4, 2011

ash, selenium might be expected to form a calcium selenite/selenate at te mperatures below about 500 o C (932 o F). In a coal-fired boiler, this temperat ure corresponds roughly to the inlet of the economizer. For coals with low sulfur, but mode rately high iron, iron selenite might form at temperatures below 300 o C (that is, in the air preheater or downstream). Without sufficient calcium or iron (or in the presence of relatively high concentrations of SO 2 ), selenium might be expected to condense as SeO 2 at temperatures below about 200 o C. Partial condensation would lead to selenium leaving the par ticulate control device as a gas (SeO 2 ), which is observed in bituminous-fired plants. Laboratory work has su ggested the selenium interacts with unburned carbon or activated carbon at low temperatures (120-250 o C).


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

2.3.1. Full Scale Selenium Data
Helble 22 reported an average elemental capture effici ency for Se of 49% for units using an ESP for PM control (16 sites). This average in cluded units firing bituminous, subbituminous, and lignite coals. Se removal efficiency is signi ficantly lower that PM removal due to Se high volatility and tendency to enri ch 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.

20Selenium in Coal-Fired Boilers April 4, 2011

0% 20% 40% 60% 80% 100% 120% 140%Selenium,%ofInput 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 300 o C. Thus, it seems likely that for bituminous coals, formation of sele nites is not the only path for selenium in the combustion system. The fly ash might contain selenites and SeO 2 , but most of the selenium will remain in the gas phase. It should be noted that when fly ash is sample d from the hot flue gases using a dilution-quench probe (as in Seames’ experiments 16 , 17 ), gaseous SeO 2 might condense as submicron SeO 2 in the sampling system. This submicron SeO 2 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.
Norris 24 studied Se partitioning in f our coal-fired power plants:

21Selenium 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 measurme nt was disgarded. Overall, the Se mass balance on the four plants was slight ly 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 amoun t 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 id entify the fraction of Se that is associated with solids (fly ash, econimizer ash, bo ttom 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 Seleni um 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 furna ce, SCR, FF, ESP, and wFGD. With the ESP in

22Selenium in Coal-Fired Boilers April 4, 2011

use (no FF) PM removal effici ency 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 seleni um will be present in the gas phase at the ESP inlet, while for subbitu minous 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 E SP for bituminous firing and should be almost completely removed across the ESP for subbituminous or lignite firing. Therefore, for bituminous-fired boilers that ha ve a cold-side ESP followed by a wet flue gas desulfurization (FGD) scrubber, a significant frac tion of the selenium input to the boiler should be present at the inlet to the FGD. SeO 2 , which is the form of gase ous selenium thought to be

23Selenium in Coal-Fired Boilers April 4, 2011

present in flue gas at back-end te mperatures, 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 remova l across the scrubber for those three boilers was 73%. Meij 26 published data on removal of selenium acr oss FGDs based on a total of 16 testing campaigns at coal-fired boilers burning bitu minous 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 SO 2 has a very high removal rate across we t scrubbers (greater than 95%), selenium does not. There are a number of possible explanati ons, including formation of H 2 SeO 3 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 Re quest (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 website 27 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 equipmen t 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 remova l 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 s ubbituminous coal units, 99.7% for lignite units, and only 30% for coal refuse units. These rem ovals reflect the composition of the fuel (e.g., sulfur content, calcium in ash) as well as the mi x 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

24Selenium in Coal-Fired Boilers April 4, 2011

The boilers in the ICR data set for non-merc ury metals testing had a variety of APCD configurations, as shown in Table 4 . Half the boilers had no SO 2 control. The predominant SO 2 control device for bituminous boilers was a we t FGD, while a spray dryer absorber (SDA) coupled with a fabric filter was the predominant SO 2 control device for subbituminous boilers. A small number of boilers had circulating dry sc rubbers (CDS) or duct so rbent injection (DSI) for SO 2 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 SO 2 24 30 C

‐ ESP/DSI or CDS 2 1 C

‐ ESP/SDA 2 C

‐ ESP/FGD 33 3
H

‐ ESP/No SO 2 4 1 H

‐ ESP/FGD 4 2 FF/No SO 2 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 boile r firing bituminous coal, the addition of SO 2 control devices increased selenium removal. The type of PM control device also affected Se removal. Without SO 2 control, average Se rem oval ranged from 20% to 76%, depending on the PM control equipment. W ith a wet FGD in place, average Se removal increased to 80%-93%, depending on the PM contro l device. Se removal increased to 99% with a spray dryer for SO 2 capture and a fabric filter for PM control. Average Se removal was higher for conventional boilers firing s ubbituminous coal. Without any SO 2 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 SO 2 control and a FF for PM capture, Se removal averaged 99.5%. Several of the conven tional boilers firing subbituminous coal also injected activated carb on for mercury capture. For cases with no SO 2 control and a FF, ACI slightly increa sed Se removal. For cases with no SO 2 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 dr yers and FF, ACI had a ne gligible effect on Se

25Selenium in Coal-Fired Boilers April 4, 2011

removal, however, removal was very high for thes e cases (>99%). Less data are available for CFB boilers. The addition of a dry scrubber incr ease Se removal from an average of 90% to 99.6% for bituminous-fired units equipped with FF. For CFB units fi ring 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 20 10 ICR sorted by firing system, fuel, and APCDs.

Boiler Fuel SO 2 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%

26Selenium in Coal-Fired Boilers April 4, 2011

When the average selenium removal is broken do wn 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-fir ed 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 SO 2 in the flue gas should have a negative impact on selenium uptake by fly ash (a nd subsequent capture in an ESP). This relationship can be seen in Figure 12

.

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

APCD Configuration Bituminous Sub

‐ bituminous Lignite Coal Refuse
C

‐ ESP/No SO 2 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 SO 2 20.43% 79.08%
H

‐ ESP/FGD 80.56% 99.68%

FF/No SO 2 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%

27Selenium in Coal-Fired Boilers April 4, 2011

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

012345SeRemoval

Wt% S in coal (dry)

Figure 12. Selenium removal in bi tuminous 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 b ituminous 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 co ntrast to bituminous-fired boile rs, 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-f ired boilers and subbituminous-fired boilers, although the latter show much higher removal across hot-sid e ESPs than the former. Selenium removal across boilers firing coal re fuse was noticeably lower than that even on bituminous-fired boilers. In general, wet SO 2 scrubbers remove selenium from the flue gas, although not at the same efficiency as they remove SO 2 . 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 particulat e control devices. It is interesting to note that higher selenium re moval efficiencies are obs erved, 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 contro l device and the scrubber. The higher selenium removals for plants with spray dryers, as compared to those w ith wet scrubbers, suggests that spray dryers are

28Selenium in Coal-Fired Boilers April 4, 2011

29 more efficient at removal of selenium. PM contro l devices also affected Se removal. FF had the largest Se removal efficiency followed by CESP and HESP. The effects of ACI on Se remova l re with ACI while others showed a decrease.

al-

each

control device. There was no co rrelation between Se

can be removed from wastewater

e

,

t

n 4+

.

s
bly due ith ORP. DBA conc entration does correlate with formation of

reduce

n resulted in conversion of selenite to a particul ate form, therefore reducing selenate formation. are unc lear; some units showed improved Se captu


### 2.3.3. Effect of ACI on Se Removal Dombrowski, et al. 28 studied the effect of activated car bon 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 lign ite coals. The plants were e quipped with either an ESP of SD/FF for PM control. Norit ac tivated 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

remova l and ACI rate or coal type.


### 2.3.4. Behavior of Selenium in Scrubbers A significant amount of volatile Se species is cap tured 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 spec iation in the scrubber. Their bench-scale experimentation showed that certai n FGD operating variables can be controlled to minimize selenite oxidation, which is desirable fo r 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-pr ecipitate selenite. Mn 2+ does not promote sele nite 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 possibl e to optimize pH and oxidation-reduction potential (ORP) to maintain high SO 2 removal and high sulfite oxidation, yet minimize selenite oxidation In addition, samples were taken from 8 full scal e 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 show ed a correlation betweenabsorber 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 lo w-sulfur eastern bituminous coal and was equipped with an ESP and wFGD. Initially, Se in the scrubber was 100% selenate. Iron additioSelenium in Coal-Fired Boilers April 4, 2011

Cordoba, et al. 31 investigated trace elements in the wate r streams of two power plants with wet FGDs. Mass balance analysis s howed that the recycling of fi ltered 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 r ecycled back to the FGD. Se enrichment occurred after more recycling steps than mo re soluble species. Th e enrichment may be attributable to the formation of fluoride, sul phate, and/or soluble meta l chloride complexes.


### 2.3.5. Selenium in Fly Ash
Jackson and Miller 32 studied fly ash from twenty-three coal-fired power plants. Selenium was extracted in water or in a TCLP-type solution (ace tic acid). ICP-MS was used to identify the Se species in the extract. Table 7 shows the fraction of water-solubl e 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

30Selenium 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 co mposition. 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, al though 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 ex traction procedure was used to quantify oxidation states of Se species in fly ash. The solutions tested were:

• 70% HClO 4

• 16 M HNO 3

• Aqua regia + 48% HF

• 12 M HCl
The HClO 4 solution and the HF solution gave complete dissolution and recove ry of all selenium compounds. The HCl solution and the water soluti on gave complete recovery of Se[IV] and Se[VI], but were not able to dissolve Se[0]. The authors distinguish ed between Se[IV] and Se[VI] by water extraction w ith 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 decreasi ng 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, on ly 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 th e water-soluble extract exists primarily as Se[IV]. Not all the Se[IV] was so luble in water. In sample SB03 none of the Se[IV] was water-soluble. The au thors 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

31Selenium in Coal-Fired Boilers April 4, 2011

SO 3 , as suggested in the literature. The lack of complete solubili ty of Se[IV] argues against all SeO 2 , 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 speciati on 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 MgCl 2 solution, pH 7 [Exchangeable species]
2. 1 M NaOAcetate [Carbonates]
3. 0.04 M NH 2 OH HCl in HOAcetate [Bound to Fe-Mn oxides]
4. 0.02 M HNO 3 /30% H 2 O 2 [Organically bound]
5. Residue They also did a water extraction; the main diff erence between the water extraction and selective leaching solution #1 is that the pH was not held c onstant for water extraction. Two of the fly ash samples (1 and 6) produced acidic solutions in water extraction. Th e water extracts were analyzed for Se species. The bulk composition of the ash sa mples 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

32Selenium in Coal-Fired Boilers April 4, 2011

in Ca. Ash sample 2 was high in Fe. The concen tration 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 SiO 2 Al 2 O 3 TiO 2 Fe 2 O 3 CaO K 2 O Na 2 O P 2 O 5 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 sign ificant amount of organi cally associated Se. Note that the unburned carbon conten t of the ash was not provided. The water leaching results mirror the selective leach ing 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 seem ed to be little correla tion with Ca/Se ratio or Fe/Se ratio and solubility or speciation of Se.

33Selenium in Coal-Fired Boilers April 4, 2011

Table 13. Selenium speciati on 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-workers 35 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 di fferent values of pH: 2.9 and 4.9. The ash samples were collected near the flame and post co mbustion. The selenium solubility results at pH 4.9 are reproduced below for five coals in Table 14

.

Table 14. Solubility of select ed fly ash fractions in pH 4.9. 35

Combustion Zone Post-Combustion Zone Coal Submicron Supermicr on 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 othe r 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 pa rticles than in the supermicron particles. Furthermore, selenium solubility is higher in the flame zone, sugges ting some post-combustion transformations that render Se less soluble post-combustion. Seames postulated that the Pittsburgh submicr on ash solubility was consistent with iron selenates, but the supermicron as h solubility more likely indica ted calcium-selenium compounds.

34Selenium 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 SeO 2 in the ash. The Wyodak and North Dakota lignite results were attributed to the presence of calcium-selenium compounds. Essington 36 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 seleni tes, for a given cation. For a given oxidation state, the cation associated with the selenium oxya nion also affects the sol ubility. 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 Na 2 SeO 3 = 2Na + + SeO 3 2- 3.51 CaSeO 3 = Ca 2+ + SeO 3 2- 7.65 CaSeO 4 = Ca 2+ + SeO 4 2- 4.77 FeSeO 3 = Fe 2+ + SeO 3 2- 9.99 FeSeO 4 = Fe 2+ + SeO 4 2- 6.52 Fe 2 (SeO 4 ) 3 = 2Fe 3+ + 3SeO 4 2- 23.19

SeO 2 is soluble in water to form selenious acid:
SeO 2 + H 2 O = H 2 SeO 3 (6)

H 2 SeO 3 = H + + HSeO 3 − (pK a = 2.62) (7)
HSeO 3 − = H + + SeO 3 2

− (pK a = 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). SeO 2 is more soluble in water than calcium or iron selenite. It is possibl e that the bituminous fly ash samp les generated by Seames that had the highest solubility contained some SeO 2 , in addition to iron a nd/or calcium selenite. Previous studies measured speciation of Se in water extracts, in which the pH of the solution might affect the selenium speci ation. Huggins and co-workers 37 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.

35Selenium in Coal-Fired Boilers April 4, 2011

Irrespective of the coal ra nk, all the selenium in the ash samples was shown to be selenite, but not predominantly SeO 2 . The ash sample that contained ac tivated 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 bitu minous 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 sele nite in the fly ash samples, although the researchers suggested that a small am ount of selenate might be present. In summary, the solubility of se lenium from fly ash in water had been observed to vary from 0% to as high as 60%. Selenium is enriched in smalle r ash particles, but the so lubility of selenium is not noticeably different between small and large as h 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 appear s to be predominantly Se[IV], either calcium or iron selenite. Bituminous fly ash might also contain SeO 2 . Se[0] has been observed in fly ash, perhaps associated with carbon in fly ash. Se [VI] has been observe d in the water-soluble selenium fraction in fly ash, although it is not clear if th is oxidation state (sel enate) 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 se lenium 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 availabl e 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

.

36Selenium in Coal-Fired Boilers April 4, 2011

Figure 13. Flow diagram for Selenium model.

The model is one-dimensional, and requires the in itial condition of the system: fly ash size and composition, coal composition, time-temperature pr ofile, vapor-phase selenium concentration, and initial particle selenium concentration. The initial sele nium concentration on the ash particles is computed assuming zero concentrat ion for all submicron particles, which are assumed to be formed from ash vaporiza tion 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 io Se 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, C o is the concentration of sele nium in coal (g/g), and C a is the mass fraction of ash in the coal, taken from the ultimate analysis. C Seo,i is the initial concentration of selenium in g/g on size i. This

37Selenium in Coal-Fired Boilers April 4, 2011

equation assumes a uniform distribu tion of selenium on the supermicr on particles. If the initial selenium concentration on a given size is assumed proportional to the concentration of another compound in the ash particle, sa y compound “B”, then the foll owing equation would apply:


## ∑

⋅ ⋅ ⋅−
=

i iiB iB

a o io Se nC C C C C )( )( )1(

, θ , (10)

where (C B ) i is the concentration of compound B in g/g ash, n i is the mass fraction of ash size i, and the summation is taken only over supermic ron particles. Note that for uniform C B , 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 cap tured 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-vapori zed portion of the Se will be distributed among the supermicron ash particles.


### 3.1. Gas

­ Solid Interactions The vapor-phase selenium is transported to the pa rticle surface where it interacts by one of two mechanisms, heterogeneous condensation, or surf ace reaction. These mechanisms are computed as fluxes, with units of moles SeO 2 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 desc ribe the transport. The concentration of selenium on a gi ven ash particle at a given time t f is given by the following equation, 

## ∫ =

= ⋅⋅

⋅⋅ += tft

t i

ii

Se i Seo i Se dttF d MW CC

0 3 )(

6 ρ π . (11)

Here, ρ i and d i are the ash particle density, and diam eter, respectively, wh ich 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 μ g/g. Eq. 11 is integrated numerically for each particle size, allowing for cha nging gas and transport properties with time. At each time step, the moles of SeO 2 removed from the flue gas is computed as

38Selenium in Coal-Fired Boilers April 4, 2011

, (12) 

## ∑

= Δ⋅⋅=Δ

sizesi ii SeO tnpF n 2

where F i is the flux for the given size, referr ed to above, and defined below, and np i is the number of ash particle of size i for ou r given basis of 1 gram of coal. np i is computed as

3 6 ii i i d nCa np

⋅⋅ ⋅
= ρ π . (13)

The following equations are used to describe the flux of SeO 2 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 1 2

2 ⎥ ⎦ ⎤ ⎢ ⎣ ⎡

⋅+⋅+ +

⋅−⋅

⋅ ⋅⋅
= ∞ Kn Kn Kn PP TR

Dd F
s i i π (14)

where Kn = 2 λ /d i is the Knudsen number, D is the diffusivity of SeO 2 , P ∞ and P s are the bulk and surface pressures of SeO 2 , respectively. The term in brackets in Eq. 14 is the Fuchs-Sutugin correction for the non-continuum behavior of sma ll particles. For larg e particles the term becomes unity. The mean free path is computed as

2/1

2 ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛

⋅⋅ ⋅

⋅= TR MW fg π ρ μ λ . (15)

The diffusivity of SeO 2 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 pl otting the atomic radius versus the diffusion volume of 8 elements and using this result to estimate the diffusion volume for selenium. The surface pressure, P s is computed as the equi librium vapor pressure of the gas in the following reaction: SeO 2 (s) = SeO 2 (g). Figure 14 shows a plot of the vapor pr essure versus temperature. The relatively high vapor pressure at low temp erature implies condensation will only occur at very low temperatures, and relatively high conc entrations of selenium in the vapor-phase.

39Selenium in Coal-Fired Boilers April 4, 2011



### ‐ 6 ‐ 4 ‐ 2 0 2 4 6

0 500 1,000 1,500 2,000log(P,bar)



### Temperature, K

Figure 14. Vapor pressure of SeO 2 .



### 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 1 2 2 π , (17)

where k Se 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 SeO 2 , 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

40Selenium in Coal-Fired Boilers April 4, 2011

41 case of a high-surface area sorbent, the same equa tion 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, ra ther than the surface pressure, so the diffusive resistance is added to the reactiv e 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 T R d F Se

i ⋅⋅⋅

⋅ = ∞ 2 π . (19)

For the free molecular regime, there is no diffusi on 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, dependi ng on the conditions. For temperatures above 800 o C, neither mechanism will occur for calcium. For iron, the surface reaction was assumed to take place at temperatures below 977 o C. As discussed above, if the equilibrium vapor pressure is greater than the bulk partial pr essure, the heterogeneous conde nsation 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 equilibr ium vapor pressure, the reaction fluxes are assume d zero, and only heterogeneous condensation occurs.


### 3.2. Surface Reaction Kinetics A detailed description of experimental ther modynamic and kinetic data is given above. Agnihotri et al. 1 gave the following equation for the SeO 2 -CaO reaction:

( 20)

where x is the conversion of CaO and S o is the initial surface area of the particle. Agnihotri et al. 9 determined that E a = 16.849 kJ/gmol, m =0.25 and n =0.66 from data taken in the range of 400 o C to 600 o C. Li et al. made measuremen ts of the reaction rate from 600 o C to 840 o C. Near 600 o C, the reaction order for SeO 2 , n , was 1.0, increased to 1.5 as the temperature increased to 800 o C. For this modeling effort, we will assume m =1 and n =1 (in Eq. 20) and us e an activation energy of 16.84 kJ/gmol. Li et al. 11 observed that above ~650 o C, the reaction rate fell to zero by 800 o C. 

## ( ) 

## [ ] ⎟ ⎠ ⎞ ⎜ ⎝ ⎛

⋅

−
⋅−⋅= TR E SeO xSkk a n m ofo f exp )1( 2Selenium in Coal-Fired Boilers April 4, 2011

The fall in reaction rate with increasing temperature is indicative of the decomposition of the reaction product, CaSO 3 , as has been noted by Fan and co-workers. Above 800 o C, therefore, the reaction rate will be set to zero. The reaction rate for Se with ir on uses the same form as Eq. 20. The sulfur present in flue gas is able to react with calcium or iron in as h 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 SeO 2 and SO 2 in their TGA experiments. 10 , 11 Yang and Shen 41 measured sulfation rates of crystalline calcium silicates at temperatures up to 900 o C and reported that the rates were comparable to those of the reaction between SO 2 and limestone. Graham also observed sulfation of high-calcium fly ash in laboratory experiments with highcalcium fly ash. 42 Kang and co-workers 43 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 P SO2 as ( ) T S e CaOMPaSO mskgSOk /9210 2 2 2 23.6] /[
− ⋅= ⋅ (21)

The reaction of CaO with SO 2 is reported to be a two-step process, first forming CaSO 3 and then forming CaSO 4 . Simons 44 reported the rate of SO 2 reaction with CaO as 

## [ ] ( ) CTeFasaP kgSOR o T SO 827, 7sec/ /6750 2 2 < ⋅ −= − (22) 

## [ ] ( ) CTC eFasaPx kgSOR o oT SO 1227 827, 106sec/ /000,17 4 2 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 SO 2 with calcium surfaces. All the sulfur in coal is assumed to form SO 2 , and the SO 2 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 SO 2 replace those of selenium oxide. The reaction of iron and SO 2 will be modeled similarly. The same activation energy as for the calcium-SO 2 reaction will be used, in the absence of experimental data.
There are four reactions in the model that oc cur simultaneously. In calcium-containing ash particles, both SeO 2 and SO 2 react with the surface. The Ca-Se reaction is only active at temperatures less than 800 o C. The activation energy for the Ca-Se reaction was taken from Fan and co-workers. The pre-exponential factor for th is reaction was fit to experimental data. The Ca-SO 2 kinetic parameters were taken from the previous arsenic model. 45 In iron-containing ash particles, both SeO 2 and SO 2 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 SeO 2 (g), similarly to the reaction of CaO with SeO 2 (g).

42Selenium in Coal-Fired Boilers April 4, 2011

In order to determine the thermal stabili ty of iron-selenium compounds, thermodynamic calculations on reaction equations were carried ou t using HSC. For each reaction equation, the decomposition temperature of the reactant (left-hand side) was calculated from the temperature at which the Δ G 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 ca lcium 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 te mperatures. Selenites are not as thermally stable as selenates. The only thermodynamic data av ailable for an iron com pound (Fe(III) selenite) suggests that this compound is stable at higher temperatur es than calcium selenite. The decomposition of the reaction product of CaO and SeO 2 (g) has been observed experimentally at about 800°C, which is clos e 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 th ermodynamic data on iron-selenium compounds is limited and the exact compound that forms fr om the reaction of iron oxide and SeO 2 (g) is not known. In the model, a decomposition temperat ure of 977°C (1250 K) will be assumed for some iron selenate compound (assumed). The activation energy for the Fe-S e reaction was set to be compar able 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 977 o C (1250 K), because reaction with iron was observed at higher temperatures in the labor atory data, as discusse d above. The activation energy for the Fe-SO 2 reaction was the same as that for the Ca-SO 2 reaction, since there were no data on this reaction. Reaction rate parameters are summarized in Table 17

.

43Selenium 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 < 800 o C
koSO 2 0.000117 moles/s/m 2 /Pa Ca-SO 2 reaction
EaSO 2 56,119.5 J/mol T<827 o C
koSO 2 1 moles/s/m 2 /Pa EaSO 2 141,338 J/mol 827 o C<T<1227 o C koFe 5.0 m/s Fe-Se reaction
EaFe 20,000 J/mol only <977 o C
koFeSO 2 0.1 moles/s/m 2 /Pa Fe-SO 2 reaction EaFeSO 2 141,338 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 seleni um 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-com bustion ash size distribu tion is specified as particle diameter versus mass fraction. In addi tion, 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 combusti on. Because the selenium is present in only trace quantities in the coal th e volatilized selenium as SeO 2 is added to the flue gas directly, without regard to the material balanc e defined by the group 1 inputs. The SeO 2 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.

44Selenium in Coal-Fired Boilers April 4, 2011

• 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, a nd their properties are summarized in Table 18 . The four bituminous coals had a range of sulf ur contents, from 0.8% (Kentuc ky) to 3.6% (Illinois 6). The content of Fe 2 O 3 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 Fe 2 O 3 . 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 SO 2 (SO 2 -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 se t 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 (F e-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.

45Selenium in Coal-Fired Boilers April 4, 2011

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) SiO 2 45.37 49.40 55.00 38.09 43.28 22.00 Al 2 O 3 24.18 19.31 33.76 39.41 17.17 20.38 Fe 2 O 3 20.28 22.04 5.10 13.03 6.32 11.75 TiO 2 1.14 1.01 1.68 2.04 1.38 0.50 P 2 O 5 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 Na 2 O 1.35 0.70 0.32 0.92 1.67 5.12 K 2 O 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 hi gher 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).

46Selenium in Coal-Fired Boilers April 4, 2011

Figure 15. UofA furnace ti me-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 temper ature was approximately equal to 800 o C, while for the Kentucky coal, the sampling temperature was greater than 800 o C. Thus, we expect to see little reaction with calcium in the bituminous coal da ta, but ample time for reaction with calcium in the low-rank coal data.

47Selenium in Coal-Fired Boilers April 4, 2011

Table 19. Sampling temperatures from UofA experiments.
Coal Port 4 Temperature, o C Port 14 Temperature, o C 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 highe r 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 as h, so it is more info rmative 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 su rface area of the submicron particles attracted more Se than the supermicron particles. For the Pittsburgh and Il linois coals, there is evidence that at the high temperature (Port 4) there was vapor-phase SeO 2 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 diff erence between the shape of the Se distributions between Port 4 and Port 14, which might be because of the r eaction of Se with bituminous ash particles at temperatures greater than 1267 o C (2312 o F). There were less data for the low-rank coals; a si gnificant portion of th e North Dakota selenium distribution was not available. N onetheless, it is clear for the Wyodak coal that there was little Se in the submicron particles at Port 4 (1027 o C), but by Port 14 (687 o C), Se concentrations in the submicron particles increased. This is consistent with reaction between calcium and selenium, which takes place at temperatures below 800 o C.

48Selenium in Coal-Fired Boilers April 4, 2011

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 func tion of ash diameter: baseline experimental results for bituminous coals.

Figure 17. Se concentrations in ash as a func tion 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 DakotaSelenium in Coal-Fired Boilers April 4, 2011

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 SO 2 was added to the burner; or (d) additional Se was a dded 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 chan ged the shape of the distribution curve somewhat, in the region for which there are data (only data for diameters great er than 0.5 μm were taken).

Figure 18. Se concentrations in ash as a func tion of ash diameter at Port 14: baseline experimental results compared with doping experiments for Pittsburgh coal.

The Ohio bituminous coal was low in calcium. Th erefore, calcium was adde d 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 be tween Se and calcium.

50Selenium in Coal-Fired Boilers April 4, 2011

Figure 19. Se concentrations in ash as a func tion of ash diameter at Port 14: baseline experimental results compared with doping experiments for Ohio coal.

The Kentucky coal was low in sulf ur (and iron). Therefore, SO 2 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 SO 2 . 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 th e submicron range. Adding iron (as supermicron particles) increased the con centration of Se in the supe rmicron particles modestly.

51Selenium in Coal-Fired Boilers April 4, 2011

Figure 20. Selenium concentrations in ash as a function of ash di ameter at Port 14: baseline experimental results compared with doping experiments for Kentucky coal.

Figure 21. Selenium concentrations in ash as a function of ash di ameter at Port 14: baseline experimental results compared wi th doping experiments for Wyodak coal.

Several conclusions related to the mechanism for selenium-ash interactio ns 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 experime nts, Se was observed in the submicron particles at temp eratures greater than 1000 o C, suggesting reaction with iron in the submicron ash at high temperatur es. Adding iron or calcium to the bituminous coals increased

52Selenium in Coal-Fired Boilers April 4, 2011

the concentration of Se in fly ash. Iron had the larger effect on Se concentration. The temperature of the lower furnace sampling locatio n might have been too high to see significant Se-Ca reaction.
There is evidence for reaction between Se and ca lcium in the Wyodak coal at temperatures less than 1000 o C, which is consistent with the labora tory observation that the reaction product between calcium and selenium is not stable above 800 o C. Adding Se to the Wyodak coal increased Se concentration in all particle si zes. Adding supermicron iron to the Wyodak combustion experiment resulted in a modest incr ease 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 seri es of figures. It is important to note that the experime ntal data (concentration of Se in fly ash) showed higher Se in ash than introduced with the coal so the data ha ve 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 Po rt 14 for combustion experiments with the Pittsburgh coal. The m odel shows that adding calcium or iron to the Pittsburgh coal shifts selenium from submicron to supermicron particles.

53Selenium in Coal-Fired Boilers April 4, 2011

( a ) Pittsbur g h Baseline ( b ) Fe addition Pittsbur g h

( c ) Ca addition Pittsbur g h

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

Figure 23 shows the predicted and measured selenium distributions at Po rt 14 for combustion experiments with the Illinois 6 coal. The model pr edicts the shape of the experimental curve but not the absolute concentrations of selenium in the ash.

54Selenium in Coal-Fired Boilers April 4, 2011

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

Figure 24 shows the predicted and measured selenium distributions at Po rt 14 for combustion experiments with the Kentucky coal. As with th e 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 concen tration with the addition of SO 2 , particularly without data on the submicron particles.

( a ) Kentuck y Baseline ( b ) SO2 addition Kentuck y Figure 24. Measured and predicte d selenium concentrations in fly ash for Kentucky coal.

Figure 25 shows the predicted and measured selenium distributions at Po rt 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 m odel predictions of Se in ash are closer to the measurements than with the other bituminous coal s. 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 al so predicts near 100% Se capture.

55Selenium in Coal-Fired Boilers April 4, 2011

( a ) Ohio Baseline ( b ) Ca addition Ohio Figure 25. Measured and predicte d selenium concentrations in fly ash for Ohio coal.

Figure 26 shows the predicted and measured selenium distributions at Po rt 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 ma tched the experimental data in the supermicron particles.

56Selenium in Coal-Fired Boilers April 4, 2011

( a ) W y odak Baseline ( b ) Se addition W y oda k

( c ) Fe addition W y odak

Figure 26. Measured and predicte d 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. Ag ain, the model predicts 100% Se capture on ash and the distribution matches the experimental data well.

57Selenium in Coal-Fired Boilers April 4, 2011

Figure 27. Measured and predicte d selenium concentrations in fly ash for North Dakota lignite.

Table 20 and Figure 28 summarize the predicted levels of removal of SO 2 and SeO 2 under the UofA experimental conditions (that is, between Port 4 and Port 14). There are no data on the removal of SO 2 or SeO 2 , but the model predictions give some insight into how the model behaves. The unvaporized fracti on (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 reac tion 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 pr edicted as the lower te mperature required for condensation were never reached. Adding Fe to the Pittsburgh coal decreased SO 2 removal and SeO 2 removal stayed the same. The measured Fe in ash was not drastically in creased for the Fe-doping case, hence the model did not show an increase in Fe-S e reaction (labeled as “Fe-Se” in Figure 28 ). Adding Ca to the Pittsburgh coal decreased the SeO 2 removal. However, the meas urement 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 s ubmicron data were available, this greatly reduced the available surface area for reaction. The Se removal predicted for the Kentucky cas es was very small due to the high Port 14 temperature and limited time spent in th e Fe-Se reaction temperature region. Adding Ca to the Ohio coal increased SO 2 removal by more than a factor of 10 and doubled SeO 2 removal. Adding Se to the Wyodak coal (8 times increase) did not affect the percentage removal of SeO 2 (but the absolute amount of SeO 2 removal increased), and SO 2 removal

58Selenium in Coal-Fired Boilers April 4, 2011

decreased (competition with SeO 2 ). Adding Fe to the Wyodak coal decreased SO 2 removal and did not affect SeO 2 removal. Table 20. Model predictions for removal of SeO 2 and SO 2 in UofA experiments.
SeO 2 removal SO 2 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).

59Selenium in Coal-Fired Boilers April 4, 2011



## 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 MW t ). It is capable of firing all types of pulverized coal, natural gas and No. 2 fuel oil. The combustor is equipped with lo w 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) - posit ioned 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 desulfurizati on (FGD) scrubber for control of SO 2 emissions. The system is equipped with continuous emissions monito rs (CEMs) for on-line measurement of O 2 , CO 2 , NOx (NO and NO 2 ), SO 2 , CO and Hg. There are multiple sampling ports throughout the gas flow path. The facility is designed for ease of modifi cation so that various co ntrol 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 se en 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: bitu minous, 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 a nd 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 wa s near 100% with either the ESP or FF. Removal was 88% while firing the lignite coal with the ESP for PM control.

60Selenium in Coal-Fired Boilers April 4, 2011

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 co mposition, 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 avai lable 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 av ailable data. Sufficient data were not available for lignite coal to form these correlations; ther efore 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 durin g the tests and input di rectly 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 da ta 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 th e modeling results.

61Selenium in Coal-Fired Boilers April 4, 2011

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



### 5.2. Modeling Results Predicted selenium partitioning to ash from the unvaporized fr action, reaction with Ca, and reaction with Fe is shown in Figure 31 . The unvaporized fraction (l abeled as “initial” in Figure 31 ) was distributed among the supermicron particle s 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 pr edicted 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, SeO 2 (g) has been shown to react w ith 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 reacte d 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

62Selenium in Coal-Fired Boilers April 4, 2011

increased Ca reaction (~98% of the reacted Se). The PM cont rol device did not affect the experimental results for the subbituminous cases b ecause 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 di stributions in the ash. The Ca and Fe distributions were not measured for these tests and were therefore estimat ed 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 incr eased 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).

63Selenium in Coal-Fired Boilers April 4, 2011

Figure 32. Predicted Se distribut ions for bituminous cases.

Figure 33. Predicted Se distribut ions for subbituminous cases.

64Selenium in Coal-Fired Boilers April 4, 2011



## 6. Conclusions
At flame temperatures, selenium is soluble in silicate glasses. La rger solubility is indicated for iron-containing silicates under reduci ng conditions. With sufficient calcium in fly ash, selenium might be expected to form a calcium selen ite/selenate at temperatures below about 800 o C (1470 o F). Without sufficient calcium or iron (or in the presence of relatively high concentrations of SO 2 ), some of the selenium might be expected to condense as SeO 2 at temperatures below about 200 o C. However, no SeO 2 was predicted to condense in th e model. Partial condensation would lead to selenium leaving the particulate control device as SeO 2 (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 hi gh temperatures (below 977 o C) – perhaps this reaction is with FeSi-Al glasses and only happens for sufficien tly high temperatures to result in low viscosity of the ash;

• Ca reacts with Se at temperatures less than 800 o C;

• SO 2 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. Th e 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 betw een 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). 

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