Mercury speciation in pulverized fuel co-combustion and gasification

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Mercury Speciation in

Pulverized Fuel Co-combustion and Gasification

PROEFSCHRIFT

ter verkrijging van de graad van doctor aan de Technische Universiteit Delft

op gezag van de Rector Magnificus prof.dr.ir. J.T. Fokkema, voorzitter van het College van Promoties,

in het openbaar te verdedigen op dinsdag 13 november 2007 om 10.00 uur

door

Shishir Panjabrao SABLE

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Dit proefschrift is goedgekeurd door de promotor: Prof. Dr.-Ing. H. Spliethoff

Samenstelling promotiecommissie: Rector Magnificus, voorzitter

Prof. Dr.-Ing. H. Spliethoff, Technische Universiteit Delft, promotor Prof. dr.ir. A. Stankiewicz, Technische Universiteit Delft

Prof. dr.ir. J. Gross, Technische Universiteit Delft

Prof. dr.ir. L.P.H. de Goey, Technische Universiteit Eindhoven Prof. dr. Kim Dam-Johansen, Denmark Technical University Dr. W. de Jong, Technische Universiteit Delft

Dr. R. Meij, KEMA NL BV

This research has been partly funded by the European Commission under the Fifth Framework with Thematic Programme "Energy, Environment and Sustainable Development (EESD), Contract ENK5-CT-2002-00699.

Published by Shishir P. Sable, Delft ISBN 978-90-9022171-7

Keyword: Mercury speciation, PF (co)-combustion, Gasification

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means without the prior written permission of the copyright owner. An electronic version of this dissertation is available at http://www.library.tudelft.nl

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Mercury Speciation in Pulverized Fuel Co-combustion and Gasification

Summary

This PhD thesis primarily focused on the impact of secondary fuels and the combustion conditions on mercury speciation in pulverized fuel co-combustion of secondary fuels with coal. Additional work is concentrated on understanding the mercury speciation under gasification conditions.

Mercury is well known to human for a variety of reasons and considered to be a poisonous material. Due to the severe environmental and health impacts, it is identified as a very toxic and hazardous pollutant. Coal based power generation is a significant source of mercury emissions to the atmosphere and this has attracted huge attention in the past decade. Mercury emissions from the power plants have been studied from different points of view e.g. study of mercury behaviour in the combustor with different combustion conditions and the capture of mercury in the Pollution Control Devices. Recently, the concerns regarding global warming and need for new energy resources introduced the concept of cofiring of biomass and waste as secondary fuels in the power industry. The addition of a variety of secondary fuels will change the behaviour of mercury in the combustor which will then affect the design of emission control technology.

Investigations regarding mercury speciation/distribution into different forms (Elemental, Ionic and Particulate) under co-combustion conditions are carried out in this thesis with the help of experimental and modelling tools.

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The use of secondary fuels significantly affects NOx emissions due to different ways of nitrogen present in the fuel matrix. Air-staging is a proven in-furnace NOx reduction technology. Reducing air-stoichiometry in the primary zone of the combustor increases unburnt carbon which in turn reduces mercury emissions in the gas phase due to affinity of ionic mercury towards high amount of unburnt carbon in ash. Ash analysis shows the effect of surface area, particle size and unburnt carbon on mercury capture. Calcium variation in the ash was observed due to formation of different slag in reducing and oxidizing conditions and might have affected the mercury capture in combination with above parameters. Low iron concentration of ash does not seem to affect the capture of mercury. A pronounced effect of increase in reactor temperature was found on ash properties. High amount of unburnt carbon at lower temperature capture more mercury. It causes a reduction of total gaseous mercury at lower temperature. However, due to slag formation, which is verified with thermodynamic modelling, mercury is concentrated more into the lower amount of ash and hence shows a rise at 1300ºC for co-firing cases.

A new model indicated in chapter 5 is developed to predict Hg°, Hg+_,Hg2+_{and Hg} p in the post-combustion zone upstream of a Particulate Control Device (PCD). The model incorporates reactions of mercury with chlorinating agents (HCl) and other gaseous species and simultaneous adsorption of oxidized mercury (HgCl2) on fly ash particles in cooling of flue gases. The homogeneous kinetic model from literature has been revised to understand the effect of the OH+NO+M↔HONO+M reaction on mercury oxidation. Being a pressure dependent reaction, the choice of proper reaction rates was very critical. It was found that, the mercury oxidation reduces from 100% to 0% while going from high pressure to low pressure limit rates with 100 ppmv NO. Based on the revised chemistry of SO2, NOx, HCl, Cl, C/H/O, and the comparison with the experimental data in literature, the most suitable mercury reaction mechanism was chosen for overall model development. The heterogeneous model describes selective in-duct Langmuir-Hinshelwood adsorption of mercury chloride on ash particles. The heterogeneous model has been built using Fortran and linked to Chemkin 4.0. This way the simultaneous formation of HgCl2 in the gas phase and capture on ash is ensured. The combined homogeneous-heterogeneous model results were compared with the Information Collection Request (ICR) data of selected power plants and the experimental data from co-combustion tests on the Delft flow reactor. The results are in good agreement qualitatively. However, quantitatively the results differ considerably. The discrepancies are attributed to inaccurate power plant data, adsorption kinetic data and modelling assumptions. In spite of these deviations, the model can serve as a starting point for more rigorous mercury modelling.

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Speciatie van kwik in poederkool co-verbranding en vergassing

Samenvatting

De focus van dit proefschrift is primair gericht op de invloed van secundaire brandstoffen en de verbrandingscondities op kwik speciatie in poederkool co-verbranding van secundaire brandstoffen met kolen. Additioneel werk is gericht op het verkrijgen van inzicht in de speciatie van kwik onder vergassingsomstandigheden. Kwik is om verschillende redenen zeer bekend bij mensen en wordt als giftig materiaal beschouwd. Vanwege de ernstige invloed op mens en milieu is het zelfs gekenschetst als een zeer giftig en schadelijke, vervuilende component. Op kolen gebaseerde elektriciteitsproductie vormt een significante bron van kwik emissies naar de atmosfeer en dit heeft in de afgelopen decade in het bijzonder de aandacht gekregen. Vanuit diverse invalshoeken zijn kwik emissies van elektriciteitscentrales bestudeerd; zo zijn er bijvoorbeeld studies verricht naar het gedrag van kwik tijdens verbranding onder verschillende verbrandingscondities, alsmede de afvangst van kwik in emissie beperkende processen. De afgelopen jaren hebben de bezorgdheid rond de opwarming van de aarde en de noodzaak voor nieuwe energiebronnen geleid tot een introductie van bij- en meestoken van biomassa en afval als secundaire brandstoffen in de elektriciteitsproducerende industrie. Het toevoeren van een wijd spectrum aan secundaire brandstoffen zal het gedrag van kwik in de verbrandingsinstallatie veranderen, hetgeen dan ook het ontwerp van de emissie beperkende technologie zal beïnvloeden.

Studies naar de kwik speciatie/verdeling in verschillende vormen (metallisch, ionisch en deeltjesgebonden) tijdens co-verbranding zijn uitgevoerd, gebruikmakend van experimentele en modelleringstools, en zijn beschreven in dit proefschrift.

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de as. De zeer hoge calcium gehaltes in kippenmest vertonen geen enkel effect op deeltjesgebonden of gasvormig kwik. Dit is waarschijnlijk het gevolg van een hogere calciumsulfaat vormingssnelheid in de aanwezigheid van hoge zwavel en chloor hoeveelheden. Echter, bij de co-verbranding van planten residuen, kan calcium hebben gereageerd met chloor, waardoor ionisch kwik naar metallisch kwik is gereduceerd.

Het gebruik van secundaire brandstoffen beïnvloedt de NOx emissies significant, vanwege de verschillende structuren waarin stikstof in de brandstofmatrix gebonden is. Getrapte verbranding (“air staging”) is een bewezen primaire NOx reductie technologie. Door de lucht stoichiometrie in de primaire zone van de verbrandingskamer te reduceren, neemt de hoeveelheid onverbrand koolstof toe, waardoor op haar beurt kwik emissies naar de gasfase worden gereduceerd door de affiniteit die ionisch kwik vertoont ten opzichte van onverbrand koolstof in de as. Analyse van de as toont het effect van deeltjesoppervlak en -grootte, alsmede onverbrand koolstof of kwik-afvangst. Calcium variatie in de as is waargenomen als gevolg van verschillende slakvorming onder reducerende en oxiderende omstandigheden en dit zou de kwik-afvangst kunnen hebben beïnvloed in combinatie met de bovengenoemde parameters. Een laag ijzergehalte van as lijkt kwik-afvangst niet te beïnvloeden. Een uitgesproken effect van een toename van de reactortemperatuur op de as-eigenschappen werd waargenomen. Een grote hoeveelheid onverbrand koolstof bij lagere temperatuur verhoogt de kwik afvangst. Het veroorzaakt een reductie in de totale hoeveelheid gasvormig kwik bij die lagere temperatuur. Echter, als gevolg van slakvorming, welke is geverifieerd door middel van thermodynamische evenwichtsmodellering, wordt kwik meer geconcentreerd in de lagere hoeveelheid as en dus vertoont het een toename bij 1300ºC voor co-verbranding.

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en de experimentele gegevens van co-verbrandingsexperimenten uitgevoerd met de Delftse “Flow Reactor”. Kwalitatief gezien zijn de resultaten in goede overeenkomst met elkaar. Echter, er bestaat een aanzienlijk kwantitatief verschil tussen de resultaten. De verschillen worden toegeschreven aan niet accurate gegevens van de elektriciteitscentrales, adsorptie kinetiek data en modelaannames. Ondanks deze afwijkingen kan het model toch als startpunt dienen voor een meer omvattende kwik modelstudie.

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2 Mercury Emissions from Combustion and Gasification processes: A status review ...11 2.1 Introduction...11 2.2 Theoretical background ...11 2.3 Chemistry of mercury ...13 2.3.1 Homogeneous oxidation ...13 2.3.2 Equilibrium analysis ...14 2.3.3 Kinetics ...15 2.3.4 Heterogeneous oxidation ...19 2.4 Measurement of mercury ...21

2.4.1 Manual methods for Hg measurement...21

2.4.2 Continuous emission monitors for Hg measurement...23

2.5 Mercury control technologies ...25

2.5.1 Low NOx burners ...25

2.5.2 Particulate matter control (ESP or FF)...26

2.5.3 Spray dryer absorption systems ...27

2.5.4 Wet flue gas desulphurization...27

2.5.5 SCR & SNCR ...28

2.5.6 Novel mercury control technologies...29

2.6 Mercury emissions during gasification...34

2.7 Conclusion ...35

2.8 Scope of future work...36

3 Experimental set-ups & Measurement Techniques...39

3.1 Introduction...39

3.2 Flow Reactor...39

3.3 Analysis and sampling techniques...43

3.3.1 Online mercury monitoring...43

3.4 Flue gas analysis ...46

3.5 Ash analysis ...47

3.5.1 Unburnt Carbon (UBC) determination ...47

3.5.2 Surface area measurement ...48

3.5.3 Particle size distribution of ash samples ...48

3.5.4 Mercury analysis of ash samples ...49

3.6 Fuels...49

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4.1 Co-combustion and trace metal emission with pulverized fuels ...53

4.2 Experimental strategy ...55

4.3 Results...56

4.3.1 Effect of secondary fuel ...56

4.3.2 Effect of air-staging ...60

4.3.3 Effect of reactor temperature ...66

4.4 Mercury mass balance...71

5 A Combined Homogeneous & Heterogeneous Model of Mercury Speciation in Pulverized Fuel Combustion Flue Gases ...75

5.1 Introduction and background ...75

5.2 Model development ...75

5.2.1 Gas phase (homogeneous) mercury speciation model...75

5.2.2 Mercury reaction rates selection ...79

5.2.3 Analysis of the homogeneous kinetic mechanism...81

5.3 Homogeneous-heterogeneous model for mercury speciation...82

5.3.1 Relevant mass transfer mechanism...83

5.3.2 Model equations...83

5.3.3 Solution procedure...84

5.3.4 Model parameter estimation ...85

5.4 Results and Discussion ...86

5.4.1 Effect of heterogeneous interaction ...86

5.4.2 Effect of Solids loading ...87

5.4.3 Effect of particle diameter ...88

5.4.4 Effect of unburnt carbon and adsorption kinetics...89

5.4.5 Effect on other gaseous mercury species...90

5.5 Comparison with plant data ...91

5.6 Comparison with PF (co)-combustion experimental data ...94

6 Kinetic Modelling of Mercury Speciation in Gasification Processes ...99

6.1 Introduction...99

6.2 Mercury reaction mechanisms development ...100

6.2.1 Solution procedure...101

6.3 Results and Discussions...101

6.3.1 Simulations of experimental data of Lu et al. (2004) ...101

6.3.2 Numerical simulations ...104

6.3.3 Rate of production (ROP) analysis ...105

6.3.4 Transition from reducing to oxidizing conditions ...107

7 Conclusion & Recommendations...111

7.1.1 Pulverized fuel co-combustion experiments...111

7.1.2 Modelling of mercury speciation in combustion and gasification process ...112

7.2 Recommendations for Future work ...113

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7.2.2 Modelling of mercury speciation in combustion

and gasification process ...113

8 References ...115

9 Nomenclature ...129

APPENDIX A: Details of Flow Reactor ...132

APPENDIX B: Reaction Kinetics of Mercury and Flue Gas Components in Combustion Conditions ...138

APPENDIX C: Solution Procedure of Model Equations ...146

APPENDIX D: Reaction Kinetics of Mercury and Product Gas Components in Gasification Conditions ...154

List of Publications ...163

Acknowledgement ...165

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1 Introduction

Mercury (Hg) is a naturally occurring, highly volatile heavy metal which is known to man for more than 3000 years. It is found in trace quantities throughout the environment in rocks, soils and the oceans. The element constitutes only 0.5 parts per million (ppm) of the earth’s crust, making it scarcer than uranium but more common than gold or silver. Mercury is principally found as the ore cinnabar (mercury sulfide, HgS) but is also found in an uncombined state. Being an element, mercury never breaks down but persists in the environment, cycling through land, air and water. Mercury can exist in the environment in elemental, organic or inorganic forms. According to the International Chemical Safety Council of United Nations, an organic form of mercury – methyl mercury – is one of the six most serious pollution threats to the planet. Elemental mercury is the familiar mercury metal, which is used in thermometers, barometers, some electrical switches and other applications. It can exist both as a vapor and a liquid at room temperature. The two major sources of mercury in the environment are:

1) Natural mobilization of mercury from the earth’s crust. Mercury becomes airborne in large amounts through volcanic eruptions and forest fires.

2) Anthropogenic emissions from mobilization of mercury impurities in fossil fuels, incinerators, chlor-alkali industries, mining, processing/refining of mercury ore, gold mining, as well as the manufacturing of pharmaceuticals, lime, cement and batteries.

Figure 1.1 Fate of mercury in atmosphere

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Chapter 1

disposal and burning of mercury-bearing waste have led to a dramatic presence of this highly toxic metal in air and water.

1.1 Global mercury emissions and regulations

The first quantitative worldwide estimate of the annual industrial or anthropogenic emissions of mercury into the environment was prepared by Nriagu and Pacyna (1988) for the reference year 1979-1980. Pacyna and Pacyna (1996 & 2002) updated the emission inventory from 1979-1980 for the reference year 1990 and later for the year 1995. Minimum and maximum emissions were estimated for more than 150 countries for various industries like coal and oil product combustion, lead, zinc, gold, cement and caustic soda production. The data is shown in Table 1.1.

Table 1.1 Global emissions of mercury (reference year of 1995)

The global anthropogenic mercury emissions in the year 1995 were calculated by (i) collection of emission data from countries where such data were estimated by the national emission experts and (ii) estimates of emissions on the basis of emission factors and statistical data on the production of industrial goods and/or the raw material consumed. Pacyna et al. (2001) emphasize that emission factors have an

Coal combustion Continent/

Country Power plants

t/y Residential t/y Total stationary combustion,t/y Ref. Europe Germany France UK Poland Russia Rest 5.7 1.5 7.8 20.6 16 31 4.4 0.1 1.4 11.3 33 33 185 Pacyna et al., 2001 North America USA Canada South America 65 < 4 230 47 27 US EPA, 1997 Pacyna, 1997 Africa Zaire South Africa 197 90 76 Pacyna & Pacyna, 2001, 2002 Asia India China Japan Korea (Rep.) 60 150 860 117 310 45 44 Pacyna & Pacyna, 2001 CPCB, 2001 Wang et al., 2000 Australia &

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Introduction

associated uncertainty range; e.g. coal combustion has a ± 25% and cement production has a ± 30% error. Therefore the estimates that are calculated from emission factors are meant to give a rough idea of relative emissions and not to be taken too literally.

Coal combustion in European power plants contributed 26% of the total European mercury emissions and coal combustion for residential heat generation also contributed 26% (Sloss, 2002). It is also observed in the studies that the decrease of mercury emissions in western Europe, north America is due to the installation of emission control equipments. However, Pirrone et al. (2001) estimated a rise in mercury emissions from the Mediterranean countries. For 1998, the total emissions from this region amounted to 110 t/y and are predicted to 123 t/y by 2005. It has been estimated that the USA is responsible for around 3% of the global mercury emissions. According to US Departement of Energy (DOE), almost 65 tonnes of total US mercury emissions in 1995 arose from coal fired utility boilers. The Information Collection Request (ICR) data (http://www.epa.gov/ttn/atw/combust/utiltox/ mercury. html) suggest that the annual emission from coal fired power plants was around 48 t/y for the year 2000.

Asia (particularly China and India) is the highest contributor to mercury emissions and these are probably rising continuously at an alarming rate. It contributed about 50% to the total emissions of mercury in 1995. The mercury emissions are clearly due to the increased combustion of coal with rising energy demands. China with an annual 1.4 billion tonnes of coal production was estimated to emit 310 tonnes of mercury through coal consumption processes of which mercury emissions from power plants are 45% (Wang et al., 2000). Data on mercury emissions in India are rarely available. In a short study, by the Central Pollution Control Board (CPCB) of the government of India, it was calculated that, India annually consumes 317 million tonnes of coal in sectors such as coal-fired thermal power plants, iron and steel plants, cement plants, foundries, fertilizer production, paper manufacturing, etc. The power sector, which accounts for over 70 per cent of the total coal consumption, annually consumes around 220 million tonnes. The total mercury pollution potential from coal in India is estimated to be 77.91 tonnes per annum, considering an average concentration of mercury in coal as 0.272 ppm. About 59.29 tonnes per annum mercury is mobilized from coal-fired thermal power plants alone (CPCB, 2001). These data should be interpreted with care as they are calculated based on averaging the mercury concentration and raw material input values.

1.1.1 Legislation

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Chapter 1

will reduce nationwide utility emissions of mercury in two distinct phases. The first phase cap is 38 tons and emissions will be reduced by taking advantage of “co-benefit” reductions – that is, mercury reductions achieved by reducing sulfur dioxide (SO2) and nitrogen oxides (NOx) emissions under CAIR. In the second phase, due in 2018, coal-fired power plants will be subject to a second cap, which will reduce emissions to 15 tons upon full implementation.

• There are a number of international agreements, a few among the european countries like OSPAR (the Oslo & Paris commissions programme), HELCOM (the Helsinki commission programme covering the North Sea), Barcelona Convention (similar to OSPAR & HELCOM for the Mediterranean sea) and Nordic (the Scandinavian cooperation to achieve reduction in emissions of several pollutants including mercury). None of these protocols suggests limits on emissions of mercury from coal fired power plants. Mercury is one of the pollutants listed for consideration under the European commission’s air quality framework directive (Directive 96/62/EC). The commission is carrying out a collective preliminary study on mercury and other trace metals behavior and control from power plants. The future directive will guide all the member countries to consider the actions; they will be required to take under their own air quality strategies (Lee et al., 2000). The current regulation is applied only to municipal waste incineration and limit mercury emissions to 50 µg/m3. • Environment protection agency in New South Wales, Australia, has proposed

a pollution control regulation based on pollutant weightings. Mercury has a high weighting. The proposal will introduce load-based fees which will apply to all industries including coal fired power plants.

• Recently, Japan implemented a new law called the Pollutant Release and Transfer Register (PRTR). The PRTR aims to promote the voluntary improvement of business in the management of specific chemical substances and to prevent any impediment of environmental protection. Under this Law, owners of business that deal with chemical substances in amounts exceeding stipulated values must calculate the quantity of the substances they release into the environment and notify the government of the result. The amounts of the elements treated in a coal-fired power plant were lower than the reference quantities that the PRTR requires to be reported (Ito et al., 2005).

• Scarcity of sufficient mercury emission data and absence of key control equipment in many power plants in China and India might hinder the governments to propose a law on control of mercury emissions. Also major problems of power shortage might bring the mercury emission problems at a secondary level.

1.2 Mercury emissions from coal fired power plants: Technical issues

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Introduction

Table 1.2 Mercury contents of coal

Type of Coal Hg (mg/Kg ) Ref.

American Coal • Anthracite • Bituminous • Subbituminous • Lignite 0.18 0.08-0.2 0.08-0.19 0.13-0.22 Kilgroe et al. (2001)

Australian 0.01-0.4 Knott et al. (1983)

Canadian 0.03-1.3 Faurschou et al.(1982)

Columbian (El-cerrejon) 0.04-0.12 This study

German <0.7-1.4 Smith (1987)

Indian Coal • Lignite

• Jammu & Kashmir valley

0.01 – 0.21 0.72 – 0.87 Ghosh et al. (1994) Indonesian 0.02-0.03 Polish 0.07-0.09 Russian 0.07-0.12 South African 0.07-0.14 http://ec.europa.eu/ environment/chemicals/ mercury/pdf/ eurelectric.pdf

Being extremely volatile, mercury releases into the gas phase in its elemental form in high temperature regions of combustion furnaces. Subsequent cooling of combustion gases and the interaction of elemental mercury with the other flue gas constituents and fly ashes results in converting a portion of mercury into other forms. Elemental mercury in flue gas is converted to ionic or oxidized and particulate/solid bound in the cleaning process of flue gas. Some fraction of unconverted mercury, however, remains in elemental form. The global atmospheric level of mercury increases if the mercury is emitted in elemental form as it is water insoluble and hence difficult to remove. In the cleaning process of flue gases, part of the oxidized mercury is captured on fly ash particles which are then collected in Particulate Control Device (PCD), generally an Electrostatic Precipitator (ESP) or a Fabric Filter (FF). This ash bound mercury is termed as particulate mercury. Remaining oxidized mercury is captured in the desulphurization unit due to its solubility in water. A schematic of mercury control in existing power plants is shown in Figure 1.2. As stated earlier, elemental mercury is difficult to remove and therefore raising the rate of mercury oxidation is a key factor in mercury capture. A higher fraction of oxidized mercury will improve the mercury control. The oxidation of mercury is a very complex process which depends upon a number of factors like type of fuel, furnace and flue gas cleaning unit configurations, the combustion operating parameters, Cl, C/H/O, N, S chemistry, the surface and reactive properties of fly ash, time-temperature history of flue gases etc. The complex interactions of these parameters suggest that distribution of mercury in various forms may be different for each power plant.

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Chapter 1

boiler configuration, the furnace operating parameters and the in-furnace technologies like air-staging, low NOx burners, re-burning. The study regarding ‘control’ of mercury involves capture of mercury in the existing air-pollution control devices; SCR/SNCR catalyst, ESP/FF and FGD or development of technologies like activated carbon, FGD solvent development. The output of mercury from the source will form the basis for mercury control strategies. Any changes in fuels or combustion parameters will bring the changes to control operations.

Figure 1.2 Distribution and capture of mercury in power plants (Senior et al., 2005a)

1.3 Biomass in power generation and mercury

Biomass which includes animal or plant waste has been a source of energy for many years. Its usage was restricted to house heating, cooking etc. In last two decades, due to the threat of global warming, greenhouse gas emissions and scarcity of conventional fossil fuels, biomass is considered as a secondary fuel in power production. The co-combustion of biomass or animal waste in coal-fired power plants is a promising technique for the reduction of greenhouse gases and to the solution of the waste disposal problem. It also reduces dependence on fossil fuels to some extent. Extensive experiments in test rigs and power plants have been made to investigate the behavior of blended fuels in pulverized fuel combustion (Bemtgen et al., 1995). Recently, 135 power plants worldwide are co-firing biomass with coal. More than 50% of them are pulverized fuel plants (Koppejan, 2005). The Dutch power plants currently feed around 4-6% of secondary fuels on a thermal basis. The Third White

Paper on Energy by the Dutch government promotes energy saving and energy

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Introduction

Table 1.3 Mercury contents of various biomass and waste

Fuel Hg (mg/Kg) Ref.

Biomass-mix

(A mix of different biomass, Coffee residue, sludge etc.)

0.293

Meat and Bone Meal 0.001

Olive residue 0.004

Palm residue 0.004

Paper plant residue 0.017

B-wood 0.010

Chicken manure/Poultry waste 0.016

This study

Waste paper 0.08

Sewage sludge 0.5-10

RDF 1-10 Zevenhoven (2001)

1.4 Mercury in gasification processes

The gasification of coal and in particular, development of Integrated Gasification & Combined Cycle systems (IGCC) holds the promises of coal conversion processes with both higher efficiencies and lower gaseous emission. Little is known about mercury emissions from gasification processes. The speciation of mercury which is observed in combustion flue gases is believed to differ tremendously in case of gasification product gas, due to the lack of oxygen. Based on the knowledge about mercury speciation in combustion processes and equilibrium modelling, Frandsen et al. (1994) and Sloss et al. (2002), predicted that in a typical O2 blown coal gasifier, only elemental mercury (Hg°) is stable rather than HgO, which is the precursor to form other stable Hg++ species, such as HgCl2 or HgSO4, in the downstream flue gas under standard combustion oxidizing conditions. The measurements carried out on Polk and Wabash River Power plants in the USA in the framework of ICR reveal relatively high amount of elemental mercury in product gas (Kilgroe et al., 2002).

1.5 Motivation and scope

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Chapter 1

As explained earlier, it is believed and also proved with experimental and plant data that mercury exists mainly in its elemental form in gasification processes. However, the fundamental knowledge to understand this behavior is still lacking. The information concerning kinetic mechanisms which can affect mercury speciation in gasification processes is not available.

The main objectives of the research presented in this thesis are

1. To investigate mercury speciation in a pulverized co-combustion environment; the emphasis is to understand the effect of secondary fuels such as biomass, animal and agriculture waste and in-furnace control technologies on mercury speciation.

2. To predict mercury speciation numerically by a new approach of combining elementary reactions of mercury with other flue gases (CO2, H2O, O2, NO, HCl, SO2 etc.) and interaction with fly ashes by means of a surface mechanism.

3. To develop a kinetic model and identify the important elementary reactions and the product gas components which can affect mercury speciation in gasification processes.

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Introduction

Figure 1.3 TOMERED project work package structure

1.6 Thesis outline

This PhD thesis mainly consists of three parts: (i) study of mercury speciation under co-combustion conditions (ii) modeling of homogeneous and heterogeneous interaction of mercury with flue gas and fly ash constituents (iii) kinetic modelling of mercury speciation in gasification processes.

Chapter two focuses on a survey of existing knowledge regarding mercury emissions,

regulations, measurements and existing and modern control technologies. A survey of models for mercury speciation is also included in this chapter.

Chapter three describes the experimental combustor, sampling and analysis facilities

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Chapter 1

The results of bench scale experiments on the Delft FR are presented in chapter four. In co-combustion studies, demolition wood (B-wood), olive residues and chicken manure (poultry waste) are used as secondary fuels with 10 and 20%th share with coal as a primary fuel. Elemental, ionic and particulate mercury are measured. The effect of the in-furnace NOx control technology (air-staging) on gaseous and particulate mercury emissions in co-combustion conditions is studied. It is known that the combustor temperature changes the ash, slagging and flue gas behavior. These parameters are also expected to affect mercury distribution. Therefore the temperature effect is studied and presented in this chapter.

Main focus of chapter five is to extend a homogeneous chemistry model to include a heterogeneous interaction model and try to predict mercury speciation in a more reliable way. A homogeneous reaction model is revised by incorporating correct rate constants for valid pressure and temperature ranges. The heterogeneous model is studied separately and combined with the homogeneous model. The results are compared with the obtained Delft FR data and power plant data selected from the ICR database.

Chapter six is dedicated to a novel kinetic model of mercury speciation in gasification

processes. A kinetic model is developed which explains the interaction of mercury with chlorine along with sub-mechanisms of H/C/O, S, N, Cl and hydrocarbons. The important reactions and the species that play a major role in mercury speciation are identified.

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2 Mercury Emissions from Combustion and Gasification processes:

A status review

2.1 Introduction

Although coal is abundant and widely dispersed geographically, with good availability in areas that are not politically sensitive, the case against the fuel is that it is dirty in virtually every respect. Its greenhouse penalty has received enormous media coverage, but its use harbours many pollution problems that are as yet unfamiliar to a suspecting public. One of these is the release of mercury. In the most biologically assimilated form, mercury affects the human reproductive, development, neurological, immunological and endocrine systems. Qualms concerning mercury are normally directed at the disposal of plastics, paints, high intensity lamps, alkaline batteries and sewage sludge. Mercury, however, is also present in trace amount in coal in concentrations varying from 0.02 to 1.5 ppmw, while co-firing of coal and some of the above wastes either in utility boilers or cement kilns may increase this environmental burden.

Mercury is present in trace amounts in coal, waste and other materials and is released as gas phase species when these materials are burned. Whilst a significant fraction of the mercury may be recaptured on ash particles or by downstream control equipment, much of it may also be released into the atmosphere. Once emitted to the atmosphere, mercury deposits on soil, pond and is converted into organic mercury. The hazardous organic mercury consumed by fishes eventually enters into the food cycle.

Mercury emissions from coal-fired power plants are highly dependent upon its distribution in different forms. Mercury is available in gaseous elemental, ionic and particulate forms in the post-combustion zone. The speciation is controlled by the operating parameters and the interaction of mercury species with other flue gas components and fly-ash. In the last decade, several studies (Hall et al., 1991, Laudal et al., 2000, Gasper et al., 1997) have focused on understanding the effects of coal-fired exhaust gas and ash constituents on the behavior and capture of mercury in the post-combustion zone. The implementation of post-combustion controls is not specifically intended to control mercury emissions from coal-fired utility boilers. However, these technologies capture mercury in varying degrees depending on the control equipment and process conditions used as well as the mercury speciation at the inlet to the control devices. The ICR data which assist in speeding up the development of appropriate control technologies are partly explained in this work. A similar initiative has been taken by the European commission however, the data are still unpublished. Apart from analyzing control of mercury in existing Air Pollution Control Devices (APCDs), there are continuous efforts to modify the existing APCDs or developing new technologies to capture mercury. A brief review of these technologies is reported in this study.

2.2 Theoretical background

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Chapter 2

dominant sulphide mineral and mercuric sulphide (HgS) or cinnabar (Swaine, 1984). However it is not unusual that up to 20% of mercury in coal is associated with the organic fraction (Dale, 1993). When the fuel particles of coal reacts with the hot recirculated combustion products in the near burner region, their temperatures rise sharply, resulting in the mercury to be released in the elemental form (Hg0) by evaporation. Figure 2.1 shows a schematic of the partitioning of mercury in a combustor including a summary of the particulate matter emission, which has been thoroughly described by Yousif and Lockwood, (2000). Subsequent cooling of combustion gases and the interaction of elemental gaseous mercury with the other combustion products results in converting a portion of mercury into other forms. There are three basic forms of mercury in the flue gas from coal fired electric utility boilers:

• Elemental Mercury (Hg°) • Oxidized mercury

The main compounds include HgCl2, HgO, HgSO4. Although oxidized mercury doesn’t exist as mercury ions in the boiler flue gas, these compounds are also called ionic mercury. These compounds are measured as ionic mercury by the speciation test methods used to quantify oxidized mercury. • Particulate bound mercury

Figure 2.1 Fate of mercury in solid fuel combustion systems

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Mercury Emissions from Combustion and Gasification processes: A status review

understood. A lot of efforts are invested to understand the transformation phenomena of mercury to greater extent by number of laboratories (EPA, EERC, EPRI). The data obtained show that a combination of some site-specific factors affects the speciation in the flue gas. The factors are:

• Type and properties of coal,

• Combustion conditions in the boiler furnace, • The flue gas temperature profile,

• The flue gas composition, • The fly-ash properties,

• Post-combustion flue gas cleaning technologies.

It is important to understand how mercury speciates in the boiler flue gas because the overall effectiveness of different control strategies for capturing Hg often depends on the concentrations of the different forms of Hg present in the boiler flue gas. The current understanding of the mechanisms by which Hg0 transforms into Hg2+ and Hgp in the flue gas from coal-fired electric utility boilers is discussed in the subsequent section.

2.3 Chemistry of mercury

As mentioned above, the gaseous elemental Hg exiting from the furnace can undergo subsequent oxidation in the flue gas by several mechanisms. The predominant oxidized Hg species in boiler flue gas is believed to be HgCl2. Other possible species may include HgO, HgSO4 and mercury nitrate monohydrate Hg(NO3)2·H2O (EPA report, 2002). The potential mechanisms for oxidation of Hg in the boiler flue gas include:

1. Homogeneous (Gas phase) oxidation. 2. Heterogeneous (Fly ash mediated) oxidation. 3. Oxidation by post-combustion control techniques.

Among these three mechanisms, homogeneous and heterogeneous oxidations are explained in this section in detail, whereas oxidation by post-combustion control technologies is discussed in section 2.5.

2.3.1 Homogeneous oxidation

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Chapter 2

2.3.2 Equilibrium analysis

The thermodynamic equilibrium calculations for mercury using CEA (chemical equilibrium analysis) (Frandsen et al., 1994) utilizes the minimization of the Gibbs free energy for the mercury species. The equilibrium calculations have been performed for mercury in coal by number of researchers and the results are presented in figure 2.2 (Frandsen et al., 1994, Linak & Wendt, 1994, Cenni et al., 1998). Some commercial software tools like Chemkin, Factsage, Mingtsys, Solgasmix (Ericson & Rosen, 1973) can be used to calculate equilibrium.

Hg(0) HgCl2 HgO HgSO4(S) 0 0.2 0.4 0.6 0.8 1 1.2 25 225 425 625 825 1025 1225 1425 Tem perature, °C M o le F ra c ti o n

Figure 2.2 Equilibrium calculations for Hg speciation in standard oxidizing conditions. (Elements considered: C, H, N, O, S, Cl, Hg; concentrations are taken from Frandsen et al., 1994)

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2.3.3 Kinetics

2.3.3.1 Effect of chlorine kinetics in flue gas

Comparison of equilibrium calculations for the mercury system with field measurements strongly implicates kinetic limitations associated with the oxidation of elemental mercury. While little fundamental information on the rates of such oxidation reactions exists for mercury concentrations typical for coal combustion flue gas, some information can be obtained from global reaction rate studies conducted under waste incineration conditions (reactions (1) & (2)). Hall et.al. (1991) for the first time examined the potential homogeneous gas phase reactions of mercury with Cl2, HCl, O2, NH3, NO, NO2, SO2 and H2S at atmospheric pressure and temperature varying from ambient to 900°C in a simulated flue gas. Reactions of elemental mercury were evaluated by means of measurement of total gaseous Hg and Hg° in experiments conducted under both isothermal and decreasing temperature conditions. Reactions with HCl occurred rapidly at temperatures ranging from 500°C to 900°C, with approximately 90% conversion noted in 0.7 s during isothermal measurements at the highest temperature. Reactions with Cl2 were also rapid, with 70% conversion occurring in 1.1s at 500°C. Similarly, Gasper et al. (1997) examined the reaction of Hg in simulated incinerator flue gas at temperatures of 400°C to 900°C and concluded that the process was kinetically limited. Moreover, recent studies on mercury speciation, including ICR boiler tests, indicate that high levels of mercury oxidation are most strongly correlated with high chlorine concentrations in coal indicating that mercury chlorination is the predominant oxidation mechanism. High coal chlorine contents have been shown to correlate with higher levels of mercury oxidation and retention in numerous studies (EPRI, 2000; Sliger et al., 2000).

Hg + Cl2 HgCl2 (1) Hg + 2HCl HgCl2 + H2 (2)

In a series of laboratory experiments, Widmer et al., (1998, 2000) examined the oxidation of mercury at two levels of HCl concentration, 3000 and 300 ppmv, and found that mercury becomes oxidized in about one second around 425°C to 525°C. Such a high rate of oxidation may be obtained due to the very high levels of chlorine concentration. Subsequent thermo-chemical analysis was used to deduce the elementary reaction steps involved leading to the overall reaction from HCl and Hg to HgCl2. The mechanism described suggests that the rate-determining step in homogeneous mercury oxidation is the attack on the Hg atom by the Cl atom, which is in accordance with the work of Sliger et al., (1999) and Ghorishi (1999).

The one step global mechanism that is proposed by Widmer et al. (1998, 2000) and Hall et al. (1991) can give plausible qualitative results. But they are generally not suitable for examining the effects of other flue gas components on Hg chlorination. Furthermore, such mechanisms provide little insight into the details of the conversion process.

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Chapter 2

has been used to simulate four sets of experimental data. Their work is based on the following proposed elementary reaction (Sliger et al., 1999) and (Widmer et al., 1998):

Hg + Cl HgCl (3) HgCl + Cl HgCl2 (4)

In the model of Niksa and Helble (2001), a number of sub-mechanisms of chemical reactions are considered including oxidation of Cl, HOCl, moist CO oxidation, H2O and NOx conversion chemistry. The model does not include reactions for hydrocarbon oxidation and SOx conversion chemistry because they were not relevant for the test data considered. In total, 102 elementary reactions are involved in the proposed model to interpret the experimental data from the four laboratory studies. The model quantitatively describes the oxidation of Hg° via HgCl to HgCl2 for a broad range of temperatures and HCl concentrations in the gas.

Edwards et al., (2001) developed a model by extending the reaction steps to encompass 60 elemental reactions involving 21 reactive species in total. The basic finding of them is in the same line as that of Niksa and Helble (2001); the dominant oxidizing species is Cl, which reacts with Hg during quenching of the flue gas. Key parameters are Cl and Cl2 concentrations of the flue gas, the quench rate and the rate by which Cl recombines to Cl2. This model predicts substantial Hg oxidation only during quenching of the flue gas. The performance of the model was assessed by comparing it with results from laboratory scale experiments by Widmer et al. (1998) and Ghorishi (1999). At temperatures below 630°C, the model drastically under-predicted the measured Hg conversion trends. At these lower temperatures, elemental Hg was predicted. One explanation for the discrepancies could be the need for additional chlorination and Hg oxidation pathways, or surface-induced catalytic effects. The model performed better at higher temperatures and correctly predicted the effects of increasing HCl concentrations on Hg conversion.

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Xu et al. (2003) developed a homogeneous mercury speciation model based on the work of Widmer et al. (1998, 2000). This kinetic model includes the oxidation and chlorination of key flue-gas components, as well as six mercury reactions involving HgO with a new reaction rate constant calculated directly from the Transition State Theory (TST). The model mainly focuses on sensitivity of HgO forming/consuming reactions when chlorine concentrations are low and equilibrium conditions are not achieved. Even though oxygen weakly promotes homogeneous Hg oxidation especially for low chlorine concentrations, around 1.5-6% of mercury is predicted to be present as HgO. In total 107 reactions with 30 species are considered in the model and the results are compared with experimental data in literature (Sliger et al. 2000, Mamani paco et al. 2000, Widmer et al., 2000). The discrepancy in matching the data is explained, which is mainly due the absence of accurate rate constants for the existing mechanisms, the addition of reactions involving HgO or surface induced catalytic effects.

Qiu et al., (2003) developed a kinetic mechanism by re-estimating the rate constants of key mercury-chlorine reactions using the TST approach. The overall mechanism was based on work of Widmer et al., (2000) and also the rates of other reactions including C/H/O, N, S, Cl species were reviewed. They predicted the effect of SO2 on Hg oxidation with Cl2 as chlorinating agent reasonably well.

While these model predictions agree reasonably with their own or other experimental data in literature (Sliger et al. 2000, 2001), (Ghorishi et al. 1998), (Mamani-paco et al. 2000), the choice of any single model is difficult. However, the predictions of these models are highly depending on Cl, S, N and C/H/O chemistry. Slight changes in the main reactions of chlorine, sulfur and NO affect mercury oxidation predictions. Several reaction rate constants are available in literature for Cl, S, N, S and C/H/O reaction mechanisms (Senkan et al. 2000), (Mueller et al., 1999, 2000), (Glarborg et al., 1995), (Allen et al., 1997), (Tsan et al. 1991), (Roesler et al. 1995). Choosing proper rates for valid pressure and temperature ranges is a big task. In the next section, effects of other gas components are explained briefly.

2.3.3.2 Effect of other flue gas components on kinetics of mercury oxidation

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Chapter 2

OH. The elimination of OH via reaction number (7) is believed to inhibit Hgo oxidation.

Hg + NO2 HgO + NO (5) 2NO2 2NO + O2 (6) NO + OH + M HONO + M (7)

The presence of SO2 in the flue gas reduces the amount of elemental mercury oxidized by HCl (Schager, 1990, Miller et al., 1997). Qiu et al., (2003) suggested that SO2 reacts with OH radicals similar to NO and forms HOSO. It reduces the source of OH for HOCl formation and therefore reduces mercury oxidation. SO2, like SO3, competes with the reaction of forming chlorine compounds in the post combustion process thereby reducing the chlorine concentration and hence the mercury oxidation (Hocquel et al., 2003). In the same line of discussion, carbon monoxide (CO) appears to reduce mercury oxidation (Lu et al. 2004), which is related by the authors to the probable impact of chlorine free radical scavenging caused by this species. The presence of water vapor also was noted to reduce oxidation of mercury by inhibiting the decomposition of HCl to Cl2 (Niksa et al., 2001; Laudal et al. 2000). However, oxygen is observed to be a weak promoter of oxidation of mercury (Niksa & Helble, 2001; Laudal et al., 2000). 0 20 40 60 80 100 10 100 1000 10000 Quench rate (K/s) % e le m e n ta l m e rc u ry

Figure 2.3 Effect of quench rate on mercury oxidation

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Some of the complex mechanisms of Hg-Cl chemistry are slowly becoming comprehensible to investigators involved in this field. However, quantitative models for predicting homogeneous mercury oxidation in coal combustion systems are still lacking. Advances are needed in experimental methods for sampling and speciating mercury, chlorine and other reactive species to determine the ionic form of mercury.

2.3.4 Heterogeneous oxidation

In reviewing the heterogeneous reactions of mercury in coal combustion gas, it is necessary to consider how both, the properties of solid surfaces and the composition of the flue gas together affect the oxidation and capture of mercury. Numerous studies have been performed to evaluate mercury adsorption properties of actual and simulated fly ashes, activated carbons and noble metals. Generally similar mercury adsorption capacities have been observed for many of these sorbent types when tested in nitrogen. However, these test results can be seriously misleading in the absence of information on the effects of flue gas constituents, which can appear to have a greater effect on surface reactions than the properties of solid materials.

2.3.4.1 Effect of surface properties on mercury sorption

In full-scale boilers, mercury capturs on fly ash alone without sorbent injection has been reported to range from 0% to 90% for some western coals (Butz et al., 2000; Butz & Albiston, 2000) and up to about 60% for certain US and British bituminous coals (Butz et al., 2000; Gibb et al., 2000) depending on temperature. The level of unburned carbon in fly ash and the catalytic effects of inorganic fly ash constituents still have not been fully identified. The mercury adsorption capacity of the inorganic fraction is typically low (Hower et al., 2000), although certain fly ashes with low carbon content may still exhibit significant mercury capture (Butz & Albiston, 2000). Results of mercury analysis performed on different fractions of fly ash show that mercury enrichment often correlated directly with carbon content of fly ash (Hower et al., 2000; Gibb et al., 2000). However, high amounts of unburnt carbon do not necessarily mean high mercury capture on ash. Surface area of fly ash particles plays a major role in mercury capture. Mercury tends to be concentrated in the finer fly ash fraction for both low and high rank coals (Butz & Albiston, 2000; Hassett & Eylands, 1999). Norton et al. (2002) investigated the effect of magnetic (Fe-rich) and non-magnetic properties of coal in mercury adsorption on fly ash. The expected results were a high adsorption capacity of mercury for magnetic fly ash due to high iron content. On the contrary, non-magnetic fly ash was found to have high mercury capture due a higher surface area than that of magnetic fly ash. From their study it appears that surface area is an important factor in determining Hg adsorption and reactivity with ash. The capture of mercury on fly ash increased as the temperature of flue gas reduced below approximately 400oC (Gibb et al., 2000; Dunham et al., 1998) and increased with extending the contact time of flue gas and fly ash (Butz et al., 2000; Butz & Albiston, 2000).

2.3.4.2 Effect of acid gases and fly ash components

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Chapter 2

magnesium. Iron oxide (Fe2O3) is also found in ash as a complex with calcium oxide (CaO) and calcium sulphate (CaSO4). In the presence of sufficiently high flue gas concentrations of HCl or Cl2, metallic oxides in fly ash may be converted to metal chlorides such as cuprous chloride (CuCl). Three-components and four –components artificial fly ashes as well as five actual fly ashes were investigated by Ghorishi et al. (1999) and Lee et al. (2000). The three-component fly ashes were prepared by adding Fe2O3 or CuO at various weights to a base mixture of Al2O3 and SiO2. An additional three-component fly ash was prepared by adding CuCl2 to a base mixture of Al2O3 and SiO2. The four-component fly ashes contained CaO as well. The simulated combustion gas was passed over a fixed bed with the model fly ash. The results showed that Fe2O3 and CuO had a large effect on the oxidation of mercury when HCl was present in the simulated combustion gas. The catalytic effect of CuO was far greater than that of Fe2O3. The effect of added CaO in the fly ash was to slow down the oxidation when HCl was present in the combustion gas. When CaO is present, it will remove the HCl partly and thereby slow down the oxidation of mercury (Ghorishi, 1999). A test with a cement kiln dust containing 29% CaO showed results similar to a test with model fly ashes containing CaO. However, some oxidation was observed with SO2. The combination of SO2 and water vapor inhibited the oxidation process. In tests with the four-component model fly ashes with NOx in the gas, Al2O3 and SiO2 became active in the oxidation process. It was speculated that the Deacon process in which chlorine gas from HCl is formed over the oxides of iron and copper is the cause of the increased oxidation of mercury. In experiments with actual fly ashes in the presence of NO, one of the fly ash (bituminous coal ash) oxidized Hg° completely but showed little oxidation in the presence of HCl. This was explained by its high Fe2O3 contents. The ash derived from sub-bituminous coal did not oxidize mercury at all in presence of NO or HCl whereas other sub-bituminous coal derived fly ash showed 30 to 35% oxidation of Hg in the presence of NO but no oxidation in the presence of HCl. In summary these experiments showed that the mercury oxidation in the presence of HCl and NOx is related to the Fe2O3 and/or CuO content. In a different study on three coal chars from two bituminous and one sub-bituminous coal, the adsorption of mercury was found to depend on coal rank (Senior et al., 1998, Wu et al., 2000). Removal of HgCl2 was higher for bituminous coal chars and removal of Hg° higher for the sub-bituminous coal char. The removal of Hg° was found to correlate with higher levels of organic sulphur content in the char, but the capture of HgCl2 was not related to the sulphur content. Other studies performed to evaluate carbon sorbents have shown a link between mercury adsorption capacity and sulphur content (Krishan et al., 1994, Vidic & McLaughlin, 1996).

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combustion flue gases is not surprising. Moreover, a recent study by EPA (EPA, 2002) shows that fly ashes derived from the same coal but under different plant operating conditions have different capacity for mercury oxidation. Thus process conditions may play an important role in contributing to the reactivity of the ashes.

2.4 Measurement of mercury

Accurate measurements of the various forms of Hg present in the flue gas from a coal- fired electric utility boiler are important for:

1. Understanding the behaviour of Hg in different combustion processes and configurations.

2. To evaluate the Hg removal efficiency of emission control technologies. A variety of measurement techniques, both manual and continuous monitoring, are available for measuring total Hg and speciated forms.

Mercury speciation measurements from coal-fired electric utility boilers have only recently been investigated, with the majority of research on the subject occurring within the last 6-7 years (EPA, 2002). Much of this work begins with examining and understanding the measurement performance under very controlled and simplified conditions, primarily through the use of blended gases in a laboratory setting. Ultimately applying these investigations to pilot-scale coal combustion and finally to full scale, field applications. At each stage, the measurement complexity increases. The complexities associated with the combustion of different coal types, relative amount of coal combustion emissions (e.g. SOx, NOx, HCl, Cl2, particulate matter) and pollution control device availability and configuration all have an impact on the ability to perform high quality Hg measurements.

2.4.1 Manual methods for Hg measurement

Manual methods are well established for measuring total Hg emissions from a variety of combustion sources (Table 2.1). The Method 101 was developed to measure total Hg (ionic, elemental and particulate) emissions from combustion sources such as sewage sludge incinerators and municipal waste combustors. These reference methods were developed and used to support total Hg regulatory needs. The Ontario-Hydro method (www.EPA.gov/ttn/emc/prelim/pre-003.pdf) also called the OH method presently is the method of choice for measuring Hg species in the flue gas from coal fired electric utility plants. This method has been submitted to the American Society of Testing and Materials (ASTM) for acceptance as a standard reference method (EPRI, 1997).

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Chapter 2

insoluble (EPRI, 1997). When the aqueous solutions are positioned immediately after the filter, the Hg2+ is captured and Hgo passes through them to the oxidizing solution where it is then captured. These solutions are analysed separately to determine the distribution of oxidized and Hgo within the sampling train.

Table 2.1 Manual methods for mercury measurement. Sampling train configuration Impinger configuration Methods PM & Hgp Gaseous Hg First set Second set Third set Analytical method EPA 29 Glass fibre wool Impinger solution HNO3 -H2O2 dry H2SO4 -KMnO4 CVAAS EPA 101 Glass fibre wool Impinger solution H2SO4 -KMnO4 None used None used CVAAS OH method Glass fibre wool Impinger solution KCl HNO3 -H2O2 H2SO4 -KMnO4 CVAAS Tris-Buffer Glass fibre wool Impinger solution Tris solution HNO3 -H2O2 None used CVAAS MESA Glass wool Sorbent bed KCl soda lime Iodated carbon None used CVAFS

Tris solution: tris(hydroxymethyl) aminomethane in a solution of EDTA in water CVAAS: Cold Vapor Atomic Absorption Spectroscopy

CVAFS: Cold Vapor Atomic Fluorescence Spectroscopy

The OH method along with the other methods are listed in Table 2.1 and they were thoroughly evaluated for their appropriateness for performing speciated Hg measurement from coal fired combustion sources (Laudal et al., 1997). Laudal et al. (1997) studied manual mercury measurements with firing of Blacksville coal. These bench-scale results show that EPA Method 29 is affected by the SO2 concentration. Also the MESA method did not speciate mercury correctly in the presence of NOx and high concentrations of SO2. In both the methods, these flue gas constituents resulted in an overestimation of the oxidized fraction of mercury. However, it is suggested that EPA method 29 could improve the results by using KCl or Tris-buffer solutions. The OH and Tris-buffer methods were the most promising.

As explained in the last section, coal fly ash constituents have affinity towards oxidizing and/or adsorbing mercury. When sampling takes place upstream of a particulate matter control device, the sampling train filter has the potential to collect a high loading of fly ash. The fly ash on the filter can adsorb gaseous Hg from the flue gas or reactive fly ashes can also oxidize gaseous Hgo_{entering the filter. These} processes can alter the total Hg measurement significantly and also its distribution in different forms.

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2.4.2 Continuous emission monitors for Hg measurement

Continuous Emission Monitors (CEMs) are preferable for multiple reasons to manual methods for Hg measurement. A CEM is capable of providing a real time continuous Hg measurement for a long period in time, whereas manual methods are capable of only infrequent ‘snapshot’ Hg measurement in time. As a result, CEMs are able to distinguish the magnitude and duration of short-term emission characteristics as well as perform long term emission measurements to truly characterize the process’ temporal emissions.

Typically, Hg CEMs measure only Hgo. These CEMs measure total Hg through the use of a conversion system that converts (reduces) the gaseous non-elemental mercury (Hg2+) to elemental for detection. As a result all the available Hg CEMs measure only total gaseous mercury (TGM). The conversion of gaseous non elemental mercury is commonly accomplished by liquid reducing agents (e.g. tin chloride (SnCl2), sodium terahydroborate (NaBH4), ascorbic acid (C6H8O6)). This technique is least preferable, though more established. The use of wet chemical reagents is considered to be a limitation to Hg CEM use. The wet chemicals typically possess corrosive properties and require frequent replenishment. The spent reagents are believed to have hazardous properties giving waste disposal problems. In addition, the reducing ability of reagents such as SnCl2 can be affected by high levels of SO2. In addition to the more established wet chemistry conversion methods, dry conversion methods are also used. These techniques use high temperature catalysts or thermal reduction units to not only convert non-elemental mercury, but also condition the sample for analysis by removing selective interferants (EPA, 2002). This approach does much to minimize the size of the conversion system as well as maintenance requirements.

Because the particulate form is difficult to transfer and is also often a measurement interferent, the particulate matter is typically filtered out and Hgp remains unmeasured. This could impart negative bias to the total Hg quantification. This bias could be further amplified as certain types of particulate matter may actually capture gas-phase mercury species. This may not be a significant issue for low particulate concentrations but can produce a major impact for high particulate emission sources. Therefore, measurement of Hgp is also important and should not be ignored.

Speciated Hg measurements are important to characterize combustion process emissions and evaluate control strategies. While there are no commercially available CEMs that directly measure the various speciated forms of Hg, several total gaseous Hg CEMs are enhanced to indirectly measure speciated Hg (ionic and elemental) forms by determining the difference between Hg° and total gaseous Hg. This difference is recognized as the oxidized mercury. Separate Hg measurements are made before and after conversion step in order to calculate the oxidized form. This indirect speciation method is referred to as ‘speciation by difference’.

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Chapter 2

techniques are susceptible to flue gas constituents like SO2, NOx, HCl and Cl2. These gases can act as measurement interferant as well as degrade the performance of concentrating devices. SO2 is a major spectral interferant with most CVAAS detection systems. The effect of SO2 is commonly negated through the use of a gold trap. The sample gas is directed through the gold trap where Hg forms an amalgam with gold. The loaded trap is then flushed with carrier gas and sent to the detector. Conditioning of sample gas before gold trapping is needed as HCl or NOx can poison the gold surface. Potassium hydroxide solution is also used to scrub SO2 before detection. Recently, the Zeeman Effect background is used to reduce the effect of interference on Hg measurement (www.lumex.ru). The UND/EERC has evaluated the performance of Hg CEMs during field tests at different coal fired electric utility power plants (Laudal et al., 1999, Laudal & French, 2000). The tests demonstrated a distinct advantage of the CVAFS over CVAAS. Below a concentration of 5 µg/m3 _of mercury, the AAS systems exhibited lower signals to noise ratios. At these concentrations, the AFS based systems are better choice. An alternative to Hg measurement approach is AES. With this technique Hg is ionized by a high energy source (plasma) and the emission energy is detected. The advantage is that all forms of mercury, including particulate bound Hg are capable of being ionized and detected. Although this technology is not quite as developed, another major advantage is that the ionization source and detector can be located directly at the source, avoiding sample delivery issues. In addition, AES is not susceptible to spectral interferences from common flue gas constituents.

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Table 2.2 CEMs for measurement of speciating mercury.

CVAAS: Cold Vapor Atomic Absorption Spectroscopy CVAFS: Cold Vapor Atomic Fluorescence Spectroscopy AES: Atomic Emission Spectroscopy

ZVAAS: Zeeman modified Vapor Atomic Absorption Spectroscopy UVDOS: Ultra Violet Differential Optical Spectroscopy

2.5 Mercury control technologies

The Information Collection Request (ICR) data (http://www.epa.gov/ttn/atw/combust/ utiltox/utoxpg.html) provide insight into the effectiveness of existing technologies and their ability to reduce mercury emissions. While coal type certainly plays a major role in the chemical transformations of mercury that occur in boilers and pollution control devices, each technology, or combination thereof, can also impact the form and quantity of mercury that is captured or emitted. Due to the vast nature of this subject, a brief introduction on capturing of mercury with the existing control technologies is given in this section.

2.5.1 Low NOx burners

Low NOx burners are one of the NOx reduction technologies which reduce NOx emissions by controlling fuel and thermal nitrogen conversion. Low NOx burners cause an increase in unburnt carbon in ash which can increase the mercury capture efficiency. Gibb et al., (2000) reported that the retention of mercury in solids

Analyzer Hg+2 Reduction method Analytical method Reference Aldora/EcoCem Technologies CVAAS

Argus Hg 1000 Thermocatalytic AES Durag HM 1400 TR Thermocatalytic CVAAS Genesis Laboratory Systems Quick-Silver Hg monitor CVAAS Lumex mercury

CEM Wet chemistry ZVAAS

Nippon MS-1/

DM-5 Wet chemistry CVAAS

Opsis AB Hg 200 Thermocatalytic UVDOAS

Opsis AB CVAAS

PSA,

Sir Galahad II Wet chemistry CVAFS

ST2 Technologies CVAAS

Sick UPA GmbH CVAAS

Mercury speciation in pulverized fuel co-combustion and gasification