Soot and nitrogen chemistry in oxy-propane combustion: interactions and implications for NO reduction

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Dipartimento di Ingegneria

dell’Energia, dei Sistemi, del Territorio e delle Costruzioni

Corso di Laurea Magistrale

in Ingegneria Energetica

Tesi di Laurea

Soot and nitrogen chemistry in

oxy-propane combustion: interactions and

implications for NO reduction

RELATORE

Prof. Ing. Leonardo Tognotti

TUTOR

Prof. Klas Andersson

CANDIDATO

Giovanni Nizzola

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I

Abstract

Oxy-fuel combustion is one of the most promising technologies to capture CO2 from

power plants. The concept is to burn the fuel in a mixture of oxygen and recirculated flue gases instead of air. The resulting flue gases will then mainly be composed by CO2 and

H2O. One interesting feature of oxy-fuel combustion is that the emission of NOx may be

heavily reduced by reburning reactions due to the recycle of flue gases. In addition, ex-periments performed in Chalmers 100 kWth oxy-fuel unit with propane as a fuel showed a

strongly increased reduction of NO when inlet oxygen concentration was increased above 37%. Under these conditions an increased soot formation within the flame was also ob-served. Therefore, could be inferred that soot formation may play some role in the reduc-tion of NO and interacreduc-tions between nitrogen and soot chemistry might take place.

The aim of this work is to test this hypothesis identifying and describing possible soot-NOx interactions, of interest to explain the observed NO reduction trends. The

investiga-tion is based on combusinvestiga-tion modeling, with focus on chemistry. Detailed reacinvestiga-tion mecha-nisms from literature, relevant to nitrogen and soot chemistry were implemented and the sensitivity of results to combustion conditions was investigated.

The modeling results showed that temperature, inlet oxygen concentration and mixing assumptions are important to radical pool and to determine the rates of key-reactions in both soot and nitrogen chemistry. In particular, an increasing soot formation was ob-served when local temperature and inlet oxygen concentration were increased in the mod-el, with major importance of the former. In the same way, when the local stoichiometry was changed to a more oxygen-lean environment, the soot formation increased. On the other hand, nitrogen oxides showed characteristic trends in the reactor in all the simula-tions, with chemistry-based sections clearly detectable. The outlet NO concentration in-creased when the flame and post-flame temperature inin-creased, whereas dein-creased when inlet oxygen concentration was increased. Stoichiometry acted on the local NO concentra-tion and reburning, with low impact on outlet NO. However, the homogeneous NO reduc-tion was not as high as in experiments, confirming the possible intervenreduc-tion of other phe-nomena, such as soot-NO interactions. The mechanisms that were identified in this work consisted essentially in homogeneous and heterogeneous interactions. Homogeneous in-teractions that take place on a soot precursors’ level, mainly seen as competition for radi-cals between reburning and gas-phase soot chemistry. Heterogeneous interactions involv-ing soot particles both as reactants and as catalyzers could be seen in flame and post flame regions. In conclusion, interactions could be a key-process to explain the enhanced NO reduction seen in the experiments at high inlet oxygen concentrations.

Keywords: Oxy-fuel combustion, combustion chemistry, nitrogen oxides, reburning, soot, soot-NOx interactions

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II

To my family, the most important thing

worth fighting for

(Alla mia famiglia, la cosa più importante

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III

Acknowledgements/Ringraziamenti

I’d like to thank my examiner, Prof. Leonardo Tognotti, for being open and giving me the opportunity to go abroad for my thesis work. Also I want to express my gratitude to my tutor at Chalmers, Klas Andersson, for its availability and for receiving me as a Mas-ter’s Thesis student as well as for giving me the opportunity to present my work at an International Conference.

A big, sincere thank you to my two supervisors at Chalmers University, Fredrik Nor-mann and Thomas Ekvall for your sincere enthusiasm and support for my work. Thank you for following me, helping me in the difficult moments and encouraging me to do bet-ter. Remaining in Sweden, a particular acknowledgement to my friends Pedro and Germán Maldonado for your availability, the advices, the discussions and the help as well as for all the dinners together. Also thank you for making me indirectly improve my Spanish a lot. Finally, thank you to all the people of both the oxy-fuel group, for being receptive towards my ideas, and the Department of Energiteknik, for the beautiful atmos-phere at work, that made me enjoy my time at the Department. Thank you also for all the “fika” together. I realized that Swedish fika should definitively be imported in Italy.

Grazie alla mia famiglia, per il sostegno ed i sacrifici fatti durante i miei studi, non so-lo economici, e per avermi educato a lavorare dando sempre il massimo per realizzare i miei sogni. A mio padre, per la fiducia incondizionata e per essere stato un esempio da seguire durante tutta la mia vita. A mia madre, per tutto l’amore dato e per la voglia di imparare che mi ha infuso sin dall’infanzia, con il suo desiderio di conoscenza che, pur-troppo, non ha potuto soddisfare. E grazie ai miei fratelli, Massimiliano e Mariagrazia, per avermi insegnato a combattere e a non arrendersi, anche quando i problemi sono mol-to più grandi di noi. Grazie per essermi stati vicini nei momenti difficili, spero di poter fare anche io lo stesso per voi.

Grazie anche a tutti i miei amici, in Italia e all’estero, per avermi fatto divertire durante tutti questi anni e alimentato in me la voglia di scoprire posti nuovi ed imparare cose nuo-ve.

En fin, gracias a mi nena, María, por creer siempre en mi y por hacerme sentir especial, eres un sol.

Grazie Thank you Gracias Tack,

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IV

List of Tables

Table 1.1 Comparison of some properties of CO2 and N2 at 1 atm and 1000 K (Chen et

al., 2012). ... 5 Table 1.2Measurements ports positions in the Chalmers test furnace in mm from the

burner inlet (Andersson & Johnsson, 2007) . ... 12 Table 2.1 Features of most important detailed particle dynamic models found in

literature. ... 37 Table 2.2 Surface reaction scheme of the ABF model (Appel et al., 2000) . ... 40 Table 2.3 Characteristics of the BINs defined in the MIT model {Richter, 2005 #2311

Soot density is assumed for all the particles to be 1.8 g/cm3 (carbon black). .. 42 Table 2.4 General representation of the various reaction classes in the MIT soot model. ... 43 Table 3.1 Input properties of propane fuel used in the simulations. ... 47 Table 3.2 Oxidizer compositions and calculated properties for the nitrogen chemistry

simulations. ... 47 Table 3.3 Oxidizer compositions and calculated properties for soot chemistry

calculations. ... 48 Table 5.1 Heterogeneous soot-NO model using the MIT model approach (see section

3.2.2). ... 96

List of Figures

Figure 1.1 Typical configuration of an oxy-fuel power plant (Fujimori & Yamada, 2013) highlighting the additional sections compared to a conventional plant, namely ASU, RFG system and CPU. ... 3 Figure 1.2 Historical development of oxy-fuel power plants (Scheffknecht et al., 2011).

The demonstration with CCS (■), the industrial scale without CCS (▲) and the pilot scale (○) plants for power and year of construnction are represented. ... 4 Figure 1.3 Effect of flue gas recycle ratio on normalized adiabatic combustion (flame)

temperature and volume flow. Adapted from (Scheffknecht et al., 2011). ... 6 Figure 1.4 Flow sheet of the Chalmers 100 kWth test furnace modeled in this thesis, from

(Andersson & Johnsson, 2007; Kühnemuth, Normann, Andersson, Johnsson, & Leckner, 2011). In the figure is shown the NO injection in the recycled flue gas system, which is not used in case of soot measurements and modeling. ... 13 Figure 1.5 Characteristics of oxy-fuel flames with varying inlet oxygen concentration. (a)

Comparison of measured NO reduction (●) and modeled homogeneous reduction (■)(Kühnemuth et al., 2011). Pictures of flame with (b) 25 % and (c)

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40 % inlet oxygen concentration. The yellow collor indicates sooting tendency for increasing inlet oxygen concentration. ... 14 Figure 2.1 Contribution of the different sectors to emissions of nitrogen oxides in 2011

((EEA), 2011). ... 16 Figure 2.2 Schematic representation of the most important processes in the nitrogen

chemistry in combustion (Fredrik Normann, 2010). ... 17 Figure 2.3 Comparison of average NOx emissions in air- and oxy-fuel combustion of coal

in some pilot and lab-scale furnaces with data found in literature (Chen et al., 2012). The dashed line represents equal emissions in both combustion cases, whereas the dotted line is a best fit for NOx emissions in once-through

furnaces (○). Finally, the red line is the trend in the emissions from furnacs with RFG (□). ... 21 Figure 2.4 Left: image of a soot particle aggregate (Stanmore, Brilhac, & Gilot, 2001).

Right: schematic representation of the structure of primary soot particles (H. Wang, 2010). ... 26 Figure 2.5 Schematic representation of the soot formation mechanism. Adapted from

(Saggese et al., 2013). ... 27 Figure 2.6 Representation of the two even-carbon-atom pathways to first ring formation

(M. Frenklach, 2002; Mansurov, 2005). ... 29 Figure 2.7 Propargyl recombination mechanism and intermediate steps to benzene

formation (Miller & Melius, 1992), belonging to the odd-carbon-atom routes. ... 29 Figure 2.8 Representation of the HACA mechanism (R 2.12 and R 2.13) for aromatic

surface growth (M. Frenklach, 2002; Frenklach & Wang, 1994; Mansurov, 2005). ... 30 Figure 2.9 Representation of the coagulation process of spherical soot particles

(coalescence) and of soot clusters (aggregation) (Blanquart & Pitsch, 2009). ... 34 Figure 3.1 Schematic representation of the combustion model in the CHEMKIN PRO®

software (Design, 2012). ... 46 Figure 3.2 Injection profile of the oxidizer representing the mixing assumption

(Kühnemuth et al., 2011; Fredrik Normann, 2010). ... 48 Figure 3.3 Temperature profiles used in the simulations. OF30 profile was taken from

(Kühnemuth et al., 2011) and was used both for NOx and soot simulations, whereas the air, OF21 and OF27 were used only for soot simulations. ... 49 Figure 3.4 Concentration profiles of nitrogen oxides and volatile nitrogen species for the

OF30 case with reference temperature profile obtained by preliminary simulations. The concentration profiles are a result from simulations performed with the DTU model described in Section 4.3.1. It uses the inlet concentrations and reactor set up as described for the OF30 case in section 4.2.1 and 4.2.2. ... 50

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Figure 3.5 Tested temperature profiles for temperature sensitivity analysis. The temperature in a section or in one of the measurement ports was changed, so that the medium temperature in a “target” section would be changed. ... 53 Figure 3.6 Modified oxidizer profiles, compared with the reference one. The curves

actually represent the cumulative amount of injected oxidizer. Profile #1 was taken from (Hjärtstam et al., 2012), whereas the Profile #2 (oxidizer staging) was from(Kühnemuth et al., 2011). ... 54 Figure 4.1 Concentration profiles of the most important species in the combustion

process (OF30). ... 55 Figure 4.2 Simplified reaction path diagram summarizing the steps involved in the

conversion of the products of reburning reactions in NO or N2 (Fredrik

Normann et al., 2009). ... 58 Figure 4.3 Concentration of CO along the first half of reactor for the OF30 case obtained

with the DTU model. The previously defined reactor sections are also indicated. ... 59 Figure 4.4 Local rate-of-production analysis for CO. Some of the most important CO-producing (positive ROP) and CO-consuming (negative ROP) reactions are showed. ... 60 Figure 4.5 Integral ROP of acetylene, [mole/m3], showing the reactions that mostly

contribute to acetylene concentration. The positive ROP values indicates a reaction whose net integral reaction rate gives a production of acetylene, whereas the negative values indicate the reactions that destroy it. ... 61 Figure 4.6 Mole fraction and total rate-of-production of acetylene in the OF30 case. .... 62 Figure 4.7 NO concentration curves (obtained with the tested temperature profiles)

showing the sensitivity of NO concentration to temperature. ... 64 Figure 4.8 CO concentration variation for the first four different temperature profiles

compared to the base case. ... 66 Figure 4.9 Acetylene concentration sensitivity to changes in temperature compared to the

base case. ... 67 Figure 4.10 Sensitivity of NO concentration to oxidizer composition at the end of the

different sections. In these curves, only the oxidizer composition was changed, and the reference temperature profile was used. The Air case sections are also showed in violet symbols (+, ▲, ●, ■), as a comparison. Each symbol corresponds to a section, as for the oxy-fuel cases. ... 68 Figure 4.11 Effect of changing CO2 and O2 concentration on the NO concentration for

two different OF cases. ... 69 Figure 4.12 Concentration of CO for different OF cases with and without CO2. ... 70

Figure 4.13 Effect of changing the OF case on acetylene (top) and hydrogen (bottom) concentration. ... 71 Figure 4.14 Comparison of the NO concentration in the reactor obtained with the

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Figure 4.15 Species concentration in the furnace for the oxidizer profile #1 (Hjärtstam et al., 2012). ... 74 Figure 4.16 Species concentration profiles for the oxidizer staged profile. ... 74 Figure 4.17 Experimental setup of Zaragoza University used for the soot model

validation. ... 76 Figure 4.18 Experimental temperature profiles used in the simulations, adapted from

(Ruiz et al., 2007). ... 76 Figure 4.19 Comparison between the calculated (color) and experimental (gray scale)

outlet gas concentrations obtained in the Zaragoza quartz flow reactor (M. Abián et al., 2012). The different bars for a fixed temperature represent different CO2 concentrations in the inlet stream. The left vertical axis in each

figure is referred to the experimental results, whereas the right one is referred to the calculated concentrations. These axes are the same for ethylene, hydrogen and CO, whereas in acetylene case the modeled axis is double. For temperatures below 1275 K no soot was formed and the results could be regarded as obtained with the sole gas-phase sub-model and used to show its accuracy. ... 77 Figure 4.20 Concentration profiles of the most important species and radicals involved in

aromatic formation calculated with the MIT model. ... 78 Figure 4.21 Integral ROP with respect to benzene. Each bar represents the total

contribution of each reaction to the formation or destruction of benzene [mole/cm3]. ... 79 Figure 4.22 Concentrations of the major PAHs up to coronene (BIN1), including

benzene. The number of aromatic rings is indicated as subscript for Ai PAHs.

... 80 Figure 4.23 Integral rate-of-production [mole/cm3] with respect to pyrene, highlighting

the ten most important reactions. ... 81 Figure 4.24 Major soot properties in the furnace compared to the acetylene trend. The

main classes of reactions involved in the soot formation are also highlighted. ... 82 Figure 4.25 Dimensional distribution of soot particles in terms of particle number per

BIN or number of particles of a given diameter calculated at different distances in the reactor. ... 83 Figure 4.26 Comparison of the trends of acetylene, PAHs and soot in the reactor,where

the delays in the formation of the aromatics and soot particles can be seen. . 84 Figure 4.27 Sensitivity of soot volume fraction results to temperature for the OF30 case.

The temperature profiles are showed in Figure 3.5. ... 85 Figure 4.28 Sensitivity of particle number density to temperature for the OF30 case. The

numbered profiles are described in Figure 3.5. ... 85 Figure 4.29 Soot volume fraction for different OF cases and air case using experimental

temperature profiles (Figure 3.3) and comparison with flame pictures from (K. Andersson et al., 2008) ... 86

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Figure 4.30 Separation of the thermal and oxidizer change effects on the soot formation in OF21, OF27 and OF30 cases. ... 87 Figure 4.31 Comparison of peak acetylene concentrations obtained with the two models

and with the peak soot volume fractions for the higher OF cases using the OF30 temperature profile. ... 88 Figure 4.32 Comparison of the peak volume fraction for different combustion cases. The

cases with an * indicate that the soot volume fractions were calculated using the OF30 temperature profile. ... 89 Figure 4.33 Effect of different mixing profiles (Figure 3.6) on the soot volume fraction.

The red line representing the profile #1 is not clear, since the soot amount is almost zero. ... 90 Figure 4.34 Soot number density for different mixing profiles defined in Figure 3.6. ... 90 Figure 5.1 Qualitative estimation of peak soot volume fraction at higher OF cases based

on results obtained in the previous Chapter. A separation of thermal and oxidizer change effects is also attempted. The red line (■) represents the peak fv calculated changing the oxidizer composition and maintaining the reference

temperature profile (OF30). The blue line (●) was obtained changing the temperature profiles, increasing peak temperature according to the increase in adiabatic flame temperature. The difference between blue and red line is an estimation of the thermal effect when changing OF case, whereas the difference between the values of two OF cases on the red line is the effect of changing oxidizer composition. ... 99

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1

Introduction

1.

In the last years, carbon dioxide has been recognized among the main responsible of global climate change with its emissions in the atmosphere become of current concern. Most of CO2 emissions come from combustion of hydrocarbon fuels, in transport,

indus-trial and power generation sectors. In particular, the technologies currently employed for power generation are essentially based on fossil fuels, whose dominant role have been confirmed in the near future (Odenberger & Johnsson, 2010). Therefore, significant ef-forts towards the development of technologies able to reduce the CO2 emissions have

been made in the last few years by industry and academia. One of the possible options towards this objective is the so-called CCS (Carbon Capture and Storage) technologies. It consists in the reduction of CO2 emissions by means of different capturing technologies,

and its successive storage in appropriate sites. In this sense, oxy-fuel combustion has been acknowledged as one of the most promising technologies for the CCS in the last few years. Both industry and academia studied this technology, developing and implementing pilot and demonstration power plants based on this technology.

However, despite oxy-fuel technology could allow to control CO2 emissions, other

pollutants will also have to meet more stringent emission regulations in the next years. In this sense, many studies have been made to investigate the formation behavior of other pollutants, such as NOx, SOx and particulate matter, in oxy-fuel combustion. The results

indicated that the formation of these pollutants follows different paths and their emissions result different from conventional air combustion. These differences have been explained with the different atmospheres and physical conditions in the furnace, deriving from the different combustion concept. Besides, possible interactions between pollutants might be of great interest. In this sense, their study may contribute to the development of new con-trol techniques, which would help to meet the future more stringent emission regulations.

1.1 Oxy-fuel technology

Oxy-fuel combustion has been widely accepted as one of the most promising technol-ogies for the CO2 capture in power plants operated with fossil fuels. Techno-economic

studies (MIT, 2007) demonstrated its competitiveness in the retrofit of existing air-fired plants, in a regulatory scenario characterized by carbon taxes and drivers towards the CCS. Although this technology could be used with a great variety of fuels, the attention of research and industry has been addressed more on the oxy-coal combustion. This is, because coal is believed to continue in his current role in energy supply systems also in the future, due to its high availability and its relative competitive prices (IEA, 2013; Odenberger & Johnsson, 2010).

The oxy-coal combustion concept dates back to investigations of Abraham et al. (Abraham, Asbury, Lynch, & Teotia, 1982), and Horn and Steinberg (Horn & Steinberg, 1982). They proposed this technology essentially to provide a CO2-rich gas for uses in

enhanced oil recovery besides controlling the emissions from coal power plants. From these studies, the Argonne National Laboratory began to investigate better this

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gy, mostly from the system and the combustion point of view. Later on, in the 1990s, the increased worldwide attention to carbon capture determined a higher interest in the tech-nology by both industry and academia, which led to a better understanding of the oxy-fuel process and to the first pilot scale and demonstration plants (Chen, Yong, & Ghoniem, 2012).

The concept of oxy-fuel combustion is to burn the fuel substituting the air with a mix-ture of nearly pure oxygen and recycled flue gases (RFG). The result is that the air-borne nitrogen in the oxidizer is removed and the resulting flue gases are mainly constituted by carbon dioxide and water vapor. Once the water vapor is condensed, the flue gases are composed of about 95% by CO2. This CO2-rich stream is subsequently cleaned, the CO2

is liquefied, compressed and is then sent to a storage. Due to the high CO2 content, the

separation and cleaning process would be less energy intensive compared to air-fuel com-bustion.

A typical oxy-fuel power plant is shown in Figure 2.1 (Fujimori & Yamada, 2013), where can be seen some additional equipment compared to the conventional power plants. For example, the air separation unit (ASU) that generally operates a cryogenic separation of oxygen from the inlet air, resulting in oxygen purity higher than 95%. This component is compulsory in an oxy-fuel power plant (even without CCS) in order to pro-vide oxygen for the combustion unit. However, it is also an energy intensive component, being capable of consume almost 15-20% of the energy produced (Chen et al., 2012) and reducing the efficiency of the plant of almost 5-7 percentage points (Buhre, Elliott, Sheng, Gupta, & Wall, 2005). Furthermore, the RFG system, necessary to regulate the inlet oxygen concentration and the combustion temperature, can assume different config-urations. In fact, it is possible to recycle the flue gases at different points in the flue gas train, depending on whether a dry or a wet recycle is desired. The choice between dry and wet flue gas recirculation depends on many factors, such as fuel, heat transfer and effi-ciency among others. The main difference between these two relies in the different com-position. In fact, the wet recycle is mostly composed by CO2, H2O and recycled pollutant

species, whereas in the dry recycle the gas is mainly composed by CO2 pollutant species,

which are present in higher concentrations, because of the elimination of H2O.

Nonethe-less, in case of oxy-coal combustion, there are two different streams of RFG entering the boiler: the primary RFG, used for coal transportation, and the secondary RFG in the oxi-dizer stream. Usually more than 2/3 of the flue gases exiting the boiler are recycled, thus the RFG system can also have an impact on the energy balance of the plant (Fredrik Normann, 2010; Scheffknecht, Al-Makhadmeh, Schnell, & Maier, 2011).

Returning to Figure 1.1, other additional components are in the flue gas train, belong-ing to the so-called CO2 purification unit (CPU), which includes a flue gas condenser

(FGC) and other components needed for the successive CO2 processing. In the former,

separation of water from the flue gases is realized to increase the CO2 concentration up to

95%. The latter, includes all the components for CO2 cleaning, condensation and

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The energy penalty associated with the additional systems lowers the efficiency of oxy-fuel plants compared to the corresponding air-fired plants without CCS. For example, considering a coal plant retrofitted to oxy-fuel with CCS, the total reduction in efficiency is higher than 10 percentage points (2007). A lower reduction in efficiency can be at-tained improving the efficiency of the initial plant, e.g. by repowering, or retrofitting to a pressurized oxy-fuel combustion system. This last type of plants, recently addressed by researchers (Hong, 2009), gives the possibility to recover part of the latent heat in the FGC, eliminates the air entrance in the boiler and reduces the consumption of systems like the RFG fan and the CO2 compressor. Therefore, although the energy consumption of

the ASU increases, the energy efficiency of a pressurized oxy-fuel plant is higher than an atmospheric one of almost 3% (Hong, 2009).

Figure 1.1 Typical configuration of an oxy-fuel power plant (Fujimori & Yamada, 2013) highlighting the additional sections compared to a conventional plant, namely ASU, RFG system and CPU.

One important parameter in defining the amount of RFG in oxy-fuel combustion is the recycle ratio of the flue gases (RR). It is defined in the literature as (Chen et al., 2012; Scheffknecht et al., 2011):

̇

̇ ̇ (1.1)

Where ̇ is the mass flow rate of recycled flue gases and ̇ is the mass flow

rate of combustion products and this ratio is always less than unity. In fact, the fraction of recycled flue gases is only a fraction of the total flue gases exiting the boiler. The im-portance of this parameter relies not only in the fact that it controls the quantity of gases in the flue gas train, but, more important, the inlet oxygen concentration is strictly de-pendent on it, since a change in ̇ determines a corresponding change in O2 inlet

con-centration or in the O2/CO2 ratio.

Furthermore, it serves to control the flame temperature, which otherwise would rose to unsustainable values for the furnace materials. Besides, in an oxy-fuel boiler with RFG, the volume of gases passing through the boiler can be reduced by about 80% (Buhre et al., 2005), due to the recycle of flue gases. In fact, varying the recycle ratio (and in turn

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the O2 concentration), the volume of RFG changes and the total volume of gases in the

furnace changes accordingly (see Figure 1.3). This can have effects on the residence times of gases or on furnace dimensions that could be reduced in oxy-fuel cases.

To date, oxy-fuel combustion technology still has not been reached the commercial state, although the number of pilot and demonstration plants is increasing worldwide. Figure 1.2 shows the historical progress in the number and scale of both the existing and planned oxy-fuel plants. In recent years, numerous studies, also supported by experi-mental work, have been carried out to examine the issues related to an oxy-fuel combus-tion environment and investigate their dependence on combuscombus-tion parameters (Buhre et al., 2005; Chen et al., 2012; Fengsham Liu, Guo, & Smallwood, 2003; Wall et al., 2009). Many of these studies pointed to explain some aspects in which oxy-fuel combustion dif-fers from the conventional air combustion, contributing to the current understanding of this process.

Figure 1.2 Historical development of oxy-fuel power plants (Scheffknecht et al., 2011). The demonstration with CCS (■), the industrial scale without CCS (▲) and the pilot scale (○) plants for power and year of construnction are represented.

In the following, some of these aspects will be addressed, based on these studies and on the vast amount of reviews on the argument present in the literature. The idea is to give an overview of the oxy-fuel technology, serving point of reference in the continua-tion of this work.

Oxy-fuel combustion has showed a great potential for retrofitting the existing fossil fuel plants. However, this is considered only a first step in the penetration of this technol-ogy in the power generation sector. As indicated in (Fredrik Normann, 2010), this kind of plant would belong to the first generation of oxy-fuel plants, characterized by design con-straints and combustion conditions given by air-fired boilers. The successive generation of plants is intended to be designed and built when oxy-fuel and CCS would already be

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implemented in the future power and legislative sectors. This type of plants offer addi-tional degrees of freedom in the design of the whole plant and in the optimization of the operating conditions. For example, they could be regulated in inlet oxygen concentration and might also implement new pollutants’ abatement techniques, specifically developed for oxy-fuel conditions.

1.1.1 Combustion atmosphere

The key-point to understand how oxy-fuel atmospheres differ from air combustion at-mospheres is that the N2 of air is substituted with recycled flue gases. Many researchers

stated that the substitution of N2 affects combustion atmosphere from both a physical and

a chemical point of view (Chen et al., 2012; Glarborg & Bentzen, 2008; Wall et al., 2009).

From the physical point of view, oxy-fuel combustion atmosphere is mainly character-ized by the different physical properties between CO2 and N2 (Table 1.1), which are

brief-ly anabrief-lyzed in the following.

Table 1.1 Comparison of some properties of CO2 and N2 at 1 atm and 1000 K (Chen et al., 2012).

Property Unit CO2 N2 CO2/N2

Density kg/m3 0.5362 0.3413 1.57

Specific heat capacity kJ/kg K 1.2343 1.1674 1.06

Volumetric heat capacity kJ/m3 K 0.662 0.398 1.66

Kinematic viscosity m2/s 7.69e-5 1.2e-4 0.631

Thermal conductivity W/m K 7.057e-2 6.599e-2 1.07

Thermal diffusivity m2/s 1.1e-4 1.7e-4 0.644

Absorptivity/emissivity - >0 0 -

Mass diffusivity m2/s 9.8e-5 1.3e-4 0.778

The different density could be also explained considering that the different molecular weights between the two molecules determine, according to a perfect gas assumption, denser flue gases. This fact may have implications on burner aerodynamics and flame shapes resulting different in oxy-fuel conditions, because of the different feed gas veloci-ty of the gases (Chen et al., 2012).

Furthermore, the higher heat capacity of CO2 determines lower flame temperatures

compared to the same quantity of N2. For example, the adiabatic flame temperature in

OF21 (oxy-fuel combustion 21% oxygen) will be lower than in air. This means that, to obtain same temperatures in the two cases, one has to recycle less CO2 by lowering the

RR and thus increase the oxygen concentration (Fujimori & Yamada, 2013). Oxygen concentration (or rather RR) acts thus directly on the adiabatic flame temperature. There-fore, if one has to consider a retrofit or operating an oxy-fuel plant matching the same adiabatic flame temperature as in an air plant, the appropriate oxygen concentration (RR) has to be chosen. In literature, an oxygen concentration of about 30% with RR of 69% has been pointed out to obtain flame temperatures comparable between air and oxy-fuel com-bustion (see Figure 1.3). However, the previous considerations are based on theoretical

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calculations and there are differences between solid and gaseous fuels. As demonstrated in experimental works, the oxygen concentration that allows to match the two conditions is usually lower than the theoretical one. Andersson et al. (Andersson, Johansson, Hjärtstam, Johnsson, & Leckner, 2008) determined that for OF25 oxy-lignite combustion the flame temperature is comparable to that in air-lignite while an O2 concentration of

27% is required to match the air flame temperature in case of propane combustion (Andersson & Johnsson, 2007).

As showed in Figure 1.3, the choice of RR act also on the volume flow through the furnace and, considering a case in which the flame temperature is matched, the corresponding volume flow will be lower. If vice versa the flow is the fixed quantity, the flame temperature will be drastically decreased. Consequently, the choice of a particular value of O2 concentration (or RR) will determine a different volume flow as well as

dif-ferent radiative heat flux in the furnace (see Section 0). However, the possibility to change this parameter is another option to take into account for both the design and the optimization of the oxy-fuel combustion devices (Fredrik Normann, 2010).

Figure 1.3 Effect of flue gas recycle ratio on normalized adiabatic combustion (flame) temperature and volume flow. Adapted from (Scheffknecht et al., 2011).

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7 1.1.2 Chemical effects of CO2

Besides the differences in the physical properties between the two atmospheres, oxy-fuel combustion can be characterized by the fact that carbon dioxide, unlike N2, may

par-ticipate in direct chemical reactions. This aspect, together with the previously described thermal effects, has been identified in several studies to be responsible of changes in the combustion chemistry. All these studies (Glarborg & Bentzen, 2008; Fengshan Liu, Guo, Smallwood, & Gülder, 2001; Watanabe, Arai, & Okazaki, 2013) identified the most im-portant step of the chemical effects of CO2 in the reaction:

R 1.1

This, in conventional air combustion, is the CO oxidation step and is the main reaction responsible of the CO2 production as a combustion product. During oxy-fuel combustion,

however, it is reversed in the sense of CO production, due to the high CO2 concentration

(Fengshan Liu et al., 2001). However, when a certain amount of CO is produced, mainly from fuel decomposition and reaction R 1.1 itself, this reaction proceeds in the conven-tional direction. The enhanced CO2 reactivity has as a main consequence a modification

of the O/H radical pool (Glarborg & Bentzen, 2008). In fact, CO2 competes for the H

rad-ical with the most important chain branching reaction R 1.2 (Fengsham Liu et al., 2003), determining a modification of the amount of H and OH radicals available in the combus-tion zone.

R 1.2

This reaction, together with the reverse reaction R 1.1, is among the key-steps in a combustion process. In particular, it is fundamental because it accelerates the rate of combustion, increasing the amount of radicals (Westbrook & Dryer, 1984). In conven-tional air combustion, reaction R 1.1 contributes to the production of H radicals, thus act-ing to promote the chain branchact-ing reaction and consequently the fuel oxidation. In oxy-fuel combustion the competition between R 1.1 and R 1.2 reactions leads to an overall decrease in the fuel burning velocity (see Section 1.1.4), mainly because of the inhibition of the chain branching reaction (Fengsham Liu et al., 2003). Therefore, in oxy-fuel com-bustion atmospheres the O/H radical pool results modified with changes both in the rela-tive concentrations of the radicals and in their rates of consumption (F. Normann, Andersson, Johnsson, & Leckner, 2010). In general, the H/OH concentration ratio de-creases for increasing CO2 concentration (Watanabe et al., 2013) and the relative amount

of the radicals changes from H>OH>O typical of air combustion to OH>O>H in oxy-fuel combustion, as determined by Normann and coworkers (F. Normann et al., 2010). They explained these changes with a slower consumption of these radicals under oxy-fuel con-ditions due to the buffering effect of the high CO concentration.

Besides, reaction R 1.1 is not the only one where CO2 can participate. Depending on

the combustion parameters, other reactions may play a role in the determination of the CO2/CO equilibrium under oxy-fuel conditions, even though the most important step

re-mains R 1.1. In their investigation on the chemical effects of CO2 in oxy-fuel combustion,

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reactions that convert CO2 to CO. Firstly, the thermal decomposition of carbon dioxide (R

1.3), occurring at high temperatures, that is an endothermic reaction.

( ) ( ) R 1.3

The other class of reactions that the authors highlighted includes reactions of CO2 with

hydrocarbon radicals, relatively abundant in the flame zone. In the investigated case of methane oxy-fuel combustion, they identified reactions of almost all the fuel-generated hydrocarbon radicals with CO2. In order of increasing radical’s reactivity but decreasing

concentration: R 1.4 R 1.5 R 1.6 R 1.7 R 1.8

They report that R 1.4, despite the abundance of radical, was quite slow or pro-ceeded in the opposite direction. In the same way, the last two reactions were of lower importance, because of the usually low concentration of the involved radicals. Therefore, the two most important steps in the CO2-hydrocarbon radicals’ reactions were reactions R

1.5 and R 1.6, with the latter being faster. Furthermore, the authors reported how these steps were responsible, in their modeled conditions, of about 10-20% of the CO2 chemical

reduction (Glarborg & Bentzen, 2008). Furthermore, they conclude the study with some practical implications of the CO2 chemical effects. According to them, the major

implica-tion of the high CO concentraimplica-tion in the near-burner region is in the increase of corrosion and slagging problems in oxy-fuel combustion devices. Nevertheless, due to the slower burning velocity, the CO oxidation would occur in the lower temperature zones slightly downstream as compared to conventional combustion (Glarborg & Bentzen, 2008).

To sum up, differences in the radical pool concentrations can be found in oxy-fuel combustion compared to air. Chemical effects of CO2, expressed by reaction R 1.1or

re-actions with hydrocarbon radicals, mainly explain these differences. The consequences of this different environment determine changes not only in the hydrocarbon chemistry, but also in soot (Beltrame et al., 2001; Fengshan Liu et al., 2001) and nitrogen chemistry (Glarborg & Bentzen, 2008; F. Normann et al., 2010), as will be explained in the next chapters.

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9 1.1.3 Heat transfer

Changes in the heat transfer occur in oxy-fuel furnaces as compared to conventional ones. These changes are mainly determined by the different gas composition. The major differences are found in the radiative fluxes, due to the strong variations in the radiative properties in oxy-fuel environments (Table 1.1). In fact, while in air combustion N2 is

regarded to be transparent to the thermal radiation, in oxy-fuel combustion high amounts of tri-molecular compounds are present (e.g. CO2 and H2O). These molecules interact

with the thermal radiation (Siegel & Howell, 1981), absorbing and re-emitting it in spe-cific spectral bands. Therefore, since the CO2 and H2O partial pressures are increased, the

total absorptivity and the emissivity in oxy-fuel flames are strongly increased. In general, the flame radiation was found to be higher for the gaseous fuels due to this effect of CO2

on flame’s emissivity and to the soot particle emission. The two effects, however, are counteracting, since also CO2 has an effect on soot formation. In their study, Andersson et

al. (Andersson, Johansson, Johnsson, & Leckner, 2008) studied the differences in radia-tion intensity in three different propane flames, a reference air-fired flame and two oxy-fuel ones (OF21 and OF27). They found that the radiation in the OF21 case was very close to that of the air case. This was explained by the differences in the soot formed in the two flames as well as by differences in gas emissivity directly correlated to the CO2

concentration. In fact, the air flame radiation was due mostly to the soot particles, given the low emissivity of the gases. On the other hand, the OF21 flame had no soot formed and the radiation emitted was due almost only to the higher CO2 concentration. The OF27

case showed the highest radiation intensity among the examined flames as well as a sig-nificant quantity of soot formed, which explains the higher radiation intensity, despite the lower CO2 concentration compared to the other OF cases. Thus, both CO2 and soot

radia-tion are determining factors in the increase of radiaradia-tion heat transfer in oxy-fuel flames of gaseous fuels. However, for oxy-combustion of solid fuels, the impact of the increased gas radiation may be overlapped by the particle radiation (e.g. soot, char and fly ash), which was found not to be affected to a great extent by oxy-fuel conditions. In this sense, in another work, Andersson et al. (Klas Andersson, Robert Johansson, et al., 2008) inves-tigated the differences in radiation emission between oxy-fuel lignite flames (OF25 to 29) and an air-fired lignite flame. They reported that, the gas radiation increased under OF25 conditions compared to air case due to the marked increase in the CO2 partial pressure,

even though the total radiation intensity was similar for the two cases. This is because particle radiation constituted a significant share in the total radiation intensity, and did not change particularly from air- to oxy-fuel firing. Therefore, although the gas radiation is increased in oxy-lignite combustion, the total radiation intensity was almost the same for the OF25 and air cases as long as they had comparable flame temperature (Klas Andersson, Robert Johansson, et al., 2008). In fact, the change in radiation intensity for the other OF cases was explained as an effect of the changing recycle ratio, which acted on the temperature profile through CO2 thermal effects. However, the main conclusion of

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particle and gas radiation, with resulting similar radiation emission for air and oxy-fuel cases.

In addition, the changes in radiative heat transfer may have indirect effects on the chemistry, since they modify the local temperatures by radiative heat losses both from gases and from particles. Thus, they act on the chemical kinetics altering the reaction rates and activating or deactivating temperature-dependent reaction pathways. This, in general, demonstrates that a strong coupling between radiation and chemistry exists, even in oxy-fuel combustion.

Besides radiation, also convective heat transfer has an important role in furnaces. In general, convective heat flux can be considered proportional to convective heat transfer coefficient and temperature difference between the flue gases and the heat exchangers in the convective Section of a boiler, for example. The first parameter can be expressed in term of dimensionless quantities, whose values depend on the properties of the gas, such as viscosity, thermal conductivity or heat capacity and on the flow velocity (Chen et al., 2012), all functions of the RR. The combination of these parameters determines the heat transfer coefficient in the oxy-fuel case. In general, the higher heat capacity and the lower kinematic viscosity of CO2 and H2O would tend to increase the convective heat transfer.

On the other hand, the increased radiative heat transfer in the radiant Section determines lower temperatures of the flue gases entering the convective Section. These facts, com-bined with the lower gas velocity determined by the different volume flow, determine an overall decrease in the convective heat transfer under oxy-fuel conditions. Furthermore, the resulting convective heat transfer is a function of the recycled CO2, and thus of the

RR, which is an accepted parameter to optimize and balance both the convective and the radiative heat transfer in the boiler. This optimization is needed not only to determine efficient operating conditions, but also in case of retrofit to oxy-fuel conditions of an ex-isting plant. In this case, the matching of the heat transfer is usually preferred to the matching of only the flame temperature, since the two may differ, despite the temperature matching. In this view, an optimal range of recycle ratios can be found, in which approx-imately the same heat transfer coefficients within the boiler can be obtained for air and oxy-fuel combustion, even though this changes the exit temperature of the flue gases from the furnace (Scheffknecht et al., 2011).

1.1.4 Fuel ignition and burnout

Further differences between air- and oxy-fuel combustion are determined by the dif-ferent behavior from the point of view of fuel ignition and burnout. Studies in this sense highlighted how the ignition of the fuel is delayed and its burnout is slower in oxy-fuel combustion (Khare et al., 2008; Molina & Shaddix, 2007).

In general, these changes were attributed to the different properties of CO2 as

com-pared to N2, such as the higher heat capacity and the different diffusivities (Molina &

Shaddix, 2007; Suda, Masuko, Sato, Yamamoto, & Okazaki, 2007). In fact, the ignition of the fuel is dependent primarily on the temperature of the surrounding gas, which de-termines the production of radicals and accelerates the reactions of fuel decomposition.

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Furthermore, the CO2 may have both indirect and direct effects. The indirect effect is on

the temperature and relies on the different heat capacity and the radiative heat transfer modification, as already explained. On the other hand, direct effects of CO2 can be both

chemical and physical. Chemically, it changes the radical pool and thus the amount of available radicals attacking the fuel molecules is changed. Conversely, the physical ef-fects of CO2 on fuel ignition rely on the different thermal and mass diffusivity in the CO2

atmosphere. The first parameter acts on the heating rate of the fuel, whereas the second is responsible of a lower mass diffusion of the reactants (namely O2) towards the fuel

mole-cules (Molina & Shaddix, 2007). All these parameters contribute to the delayed fuel igni-tion when switching from air to oxy-fuel combusigni-tion condiigni-tions.

Nevertheless, the flame burning velocity is strongly influenced by oxy-fuel conditions, resulting in lower flame propagation velocities in oxy-fuel compared to air cases (Suda et al., 2007). This aspect could be explained considering that the laminar flame speed is mainly dependent on the thermal diffusivity and the heat capacity of the bulk gas as well as on chemical kinetics. In fact, flame speed is proportional to thermal diffusivity and inversely proportional to the heat capacity, since a higher heat capacity determines a low-er templow-erature and thus a slowlow-er fuel ignition and consumption rate (Kuo, 2005). In their work, Suda et al. (Suda et al., 2007) showed how the flame propagation velocities in three diluent gases (Ar, N2 and CO2) decrease in the order: SL(Ar)> SL(N2)> SL(CO2). This

re-sult could be explained with the decreasing thermal diffusivities of the three diluent gases and the corresponding increase in heat capacity from Ar to CO2 (Chen et al., 2012).

The last aspect influencing the flame velocity is the influence of the bulk gas on chem-ical kinetics, which is higher for CO2-rich atmospheres. As explained in Section 0, the

chemical effects of CO2 determine difference in the O/H radical pool, reducing the

con-centrations of H radicals and inhibiting the chain branching reactions. This reflects on the fuel burning rate and thus on the flame propagation velocity. A numerical quantification of this chemical effect of CO2 on the burning velocity of methane and hydrogen in both

air and oxy-fuel conditions was conducted by Liu and coworkers (Fengsham Liu et al., 2003). They developed a numerical methodology to investigate this effect based on the implementation of a fictitious species to substitute CO2 (FCO2) with same thermal and

transport properties as CO2, but chemically inert. In this way, in their results, the

differ-ence between the flame velocity with CO2 and FCO2 represented the chemical effect of

CO2 on the flame velocity. This effect was also found to be higher in methane than in

hydrogen flames.

In conclusion, when switching from air to oxy-fuel combustion, the fuel ignition delay combined with the lower flame speed and the changes in aerodynamics, could lead to problems from the point of view of the flame stability. These are to be solved in a retrofit, for example, by changes in the burner design or adjusting the recycle ratio (Figure 1.3) (Scheffknecht et al., 2011).

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1.2 Experimental setup at Chalmers University

Several investigations were conducted at Chalmers University of Technology on the implications of oxy-fuel atmospheres on pollutant emissions, radiation and combustion chemistry. The studies were based on several experimental campaigns conducted in the test unit built at the University as well as on modeling. In particular, studies on NOx

for-mation under oxy-fuel combustion confirmed the reduction in the amount of such pollu-tants compared to the air case. In this Section the experimental setup of Chalmers test furnace, base for the present work, is briefly reviewed.

The Chalmers 100 kW test furnace flow chart is represented inFigure 1.5 (Andersson & Johnsson, 2007; Kühnemuth et al., 2011),since it is the object of the modeling work in this thesis. The furnace is of down-fired type and can be operated in both air and oxy-fuel conditions and with different fuels. In addition, under oxy-fuel condition it could be oper-ated either with dry or wet flue gas recirculation, but only the first condition was investi-gated here. The fuel used in gas-fired experiments is essentially propane and a fuel input of 81 kW is used in all propane experiments, with a global stoichiometric ratio of 1.15. For experiments on the NO reduction, the test rig could be equipped with a NO injection in the recirculation loop and in the experiments performed in (Andersson & Johnsson, 2007; Kühnemuth et al., 2011; F. Normann et al., 2010; Fredrik Normann, Andersson, Johnsson, & Leckner, 2011) the concentration was maintained at 500 ppm. Measurement ports are present on the furnace sides where the instrumentation can be inserted. The posi-tions of these ports, in terms of distance from the burner inlet, are showed in Table 1.2.

Table 1.2 Measurements ports positions in the Chalmers test furnace in mm from the burner inlet (Andersson & Johnsson, 2007) .

Figure 1.4 The oxy-fuel test furnace at Chalmers.

Port Position [mm] R1 46 R2 215 R3 384 R4 553 R5 800 R6 998

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13 F ig u re 1 .5 F lo w s h ee t o f th e C h a lmers 1 0 0 kWth test fu rn a ce mo d eled in th is th esis , fr o m (A n d ers so n & Jo h n ss o n , 2 0 0 7 ; K ü h n emu th , N o rma n n , A n d ers so n , Jo h n ss o n , & Leck n er, 2 0 1 1 ). I n th e fig u re is s h o w n th e N O in jectio n in th e rec yc led flu e g a s system , w h ich is n o t u sed in c a se o f so o t mea su reme n ts a n d mo d elin g .

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1.3 Motivation

The experiments carried out at Chalmers University of Technology indicated a possi-ble influence of other pollutants, such as soot, on the reduction of nitrogen oxides. In par-ticular, Künnemuth et al. (Kühnemuth et al., 2011) measured the reduction of recycled NO during oxy-fuel combustion of propane with inlet oxygen concentration ranging from 21% to 35% and compared it with the calculated homogeneous reduction, as showed in Figure 1.6a. In this figure is also showed the reduction of NO obtained from additional measures at inlet oxygen concentrations above 40% as well as pictures of the flames at oxygen concentrations of 25% and 40% (Figure 1.6b and c).

These results confirm that for higher inlet oxygen concentration the NO reduction in-creases, showing a clear change in trend compared to the cases with lower inlet oxygen concentration and being much higher compared to the calculated homogeneous reduction. One hypothesis is that the higher reduction of NO at higher inlet oxygen concentration may be explained by increased soot formation, which could be supported by the flame observations in Figure 1.6b and c, where the yellow flame is an indication of soot for-mation. Therefore, these differences could be attributed to interactions between NOx and

soot particles that might take place, which would be of great importance when inlet O2

concentration increases. These interactions have become an issue in the research that is still under discussion. In fact, soot formation mechanisms have not been developed for oxy-fuel combustion and the heterogeneous reaction mechanisms describing the soot-NOx

interactions are still under study.

Figure 1.6 Characteristics of oxy-fuel flames with varying inlet oxygen concentration. (a) Comparison of measured NO reduction (●) and modeled homogeneous reduction (■)(Kühnemuth et al., 2011). Pictures of flame with (b) 25 % and (c) 40 % inlet oxygen concentration. The yellow collor indicates sooting tendency for increasing inlet oxygen concentration.

b)

c) a)

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1.4 Aim of the work

The aim of this work is to investigate and describe possible interactions between soot and nitrogen chemistry under oxy-fuel combustion conditions. In particular, analyzing the chemical formation pathways and the sensitivity to combustion parameters of both pollu-tants, common reaction routes and important aspects of the soot-NOx interactions may be

identified. In this way, considerations about their importance in the NOx reduction at

high-inlet O2 concentrations seen in the experiments might be done.

1.5 Outline of the thesis

The thesis is structured as follows. Chapter 2 gives the present understanding about the mechanisms of formation of nitrogen oxides and soot, referring to the available literature, to set the boundaries of this work. Chapter 3 presents the models used in this work, to-gether with the methodology for the analysis of the results. Chapter 4 shows the results of the two chemical sub-models for soot and NOx in the base case and their sensitivity to

combustion parameters. In chapter 5, a discussion of possible interactions between the two pollutants is carried out, based on some literature and on the previous results. In addi-tion, considerations about the high-inlet-oxygen concentration oxy-fuel cases are made, attempting to explain in a qualitative way the observed behavior. Finally, chapter 6 pre-sents the conclusions of the work, highlighting also some issues that could be further in-vestigated in future works.

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Background

2.

This chapter describes the current understanding of the nitrogen and soot chemistry as well as the influence of oxy-fuel atmospheres on the mechanisms of formation, from the literature. Furthermore, an overview of the most used modeling techniques is given.

2.1 Nitrogen oxides

Nitrogen oxides, also referred as NOx, are produced in almost all the technical

applica-tions that include combustion or high-temperature processes. In Europe, for example, almost 40% of these pollutants are produced by the transport sector and slightly less than one-fourth by the energy production sector, Figure 2.1 ((EEA), 2011).

Figure 2.1 Contribution of the different sectors to emissions of nitrogen oxides in 2011 ((EEA), 2011).

The majority of the combustion-generated NOx is constituted by nitric oxide (NO),

with a smaller percentage of nitrogen dioxide (NO2). Nitrous oxide (N2O) may also occur

in some particular applications, but in general it is not included in the common definition of NOx. This definition usually refers to the composition of NOx at the stack of a

combus-tion system. In fact, in the atmosphere, the NO is rapidly converted to NO2 by reaction

with the oxygen molecules in the air and thus the composition changes. Once in the at-mosphere, nitrogen oxides are responsible, directly or indirectly, of phenomena like pho-tochemical smog, ozone formation in the troposphere and acid rains as well as ozone de-pletion in the stratosphere. Furthermore, they have dangerous effect on human beings since they can irritate and damage for example the respiratory system and might be fatal

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in case of prolonged exposition (Baukal, 2004; Fredrik Normann, 2010; Warnatz, Maas, & Dibble, 2006).

For these reasons, in the last decades the regulatory framework on air pollution has imposed increasingly stringent limitations. These measures have produced a considerable reduction in the emitted nitrogen oxides and the minimization of NOx emissions has

be-come an important constraint in the design of all the combustion devices. To improve a further reduction in the emissions and to meet the future limitations, a fundamental role is played by the development of more efficient control techniques that could be also com-bined with changes in the combustion concepts and with additional control techniques for the flue gas cleaning. In this sense, a deeper understanding of the chemical mechanisms leading to NOx formation and reduction as well as their sensitivity to combustion

parame-ters, could represent the key in the development of these new control techniques in the future generation of power plants.

2.1.1 Formation mechanisms

Four paths for NOx formation are identified in the literature (Bowman, 1992): thermal,

prompt, N2O path and fuel-N path. Each of these paths can be described and modeled

using a chemical kinetic mechanism whose elementary reactions are considered to be reversible. In this way, creating the appropriate conditions, the NOx formation may be

enhanced or inverted, allowing the formed nitrogen oxides to be reduced back to molecu-lar nitrogen. Therefore, as already said, it is important to understand the NOx formation

processes, in order to develop appropriate control techniques, which allow to meet the always more stringent limitations on the emissions of these pollutants. In the following, the four mechanisms of NOx formation are briefly described, as summarized in Figure

2.2, from (Fredrik Normann, 2010).

Figure 2.2 Schematic representation of the most important processes in the nitrogen chemistry in combus-tion (Fredrik Normann, 2010).

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18 Thermal NOx

Thermal NOx are formed in conventional combustion according to the extended

Zeldovich mechanism, which include the following elementary reactions (Zeldovich, 1946):

R 2.1

R 2.2

R 2.3

The overall result of the forward mechanism is the combination of the air-borne N2

with O2 in the combustion chamber, mediated by the O/H radical pool. In fact, it begins

with the N2 decomposition (R 2.1) done by the O radical, forming a first NO molecule

and continues with the recombination of the resulting highly reactive N atom with O2

molecules and OH radicals to form other two NO molecules (R 2.2 - R 2.3).

The first reaction is the controlling step of the whole mechanism. This is because it the mechanism activating step and is characterized by relatively high activation energy, re-quired to break the strong triple bond of the N2 molecule. To demonstrate this statement,

the overall rate of formation of NO given in the previous reaction can be considered, with the hypothesis of a quasi-steady state for the N concentration ( ), determined by the faster reactions R 2.2 - R 2.3 (Warnatz et al., 2006):

(2.1)

(2.2)

Using the previous statement and combining the two above equations, one obtains:

(2.3)

Therefore, the rate of NO production with the Zeldovich mechanism is determined by reaction R 2.1 only, in particular by the N2 and O radical concentration and by its rate

coefficient. The latter parameter is the most important in determining the rate of NO pro-duction through the thermal path. In fact, this reaction rate is strongly dependent on the combustion temperature and generally the thermal-NOx formation takes place only above

1500°C. However, higher temperatures are required for the thermal mechanism to be rel-evant and the NO and N2 concentrations may have a role in the reaction rate of the

ther-mal NOx formation. Furthermore, the Zeldovich mechanism is reversible and, under

par-ticular conditions of temperature, stoichiometry and species concentrations, it could be inverted to reduce the formed NO (Fredrik Normann, 2010).

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Prompt NOx

The prompt NOx formation mechanism, also known as Fenimore mechanism

(Fenimore, 1971), involves reactions of the air-borne nitrogen with hydrocarbon radicals (CHi) in the flame (R 2.4). The result of these interactions is the formation of intermediate

products among which the most important ones are hydrocyanic acid (HCN) and ammo-nia (NH3).

R 2.4

These compounds are likely to be converted to NO in an oxidizing atmosphere or con-verted back to N2 in reducing atmospheres. However, since a reasonable amount of

hy-drocarbon radicals capable to react in the Fenimore mechanism is accumulated under fuel-rich conditions, it is evident that mostly in such conditions it could give a contribu-tion to the total NOx produced. Actually, this contribution is considered of lower

im-portance in air-firing conditions, and it would be lower under oxy-fuel conditions due to the absence of N2.

However, in these conditions a path similar to the prompt mechanism is usually active, acting to reduce the recycled NOx within the flue gases. This mechanism is called

reburn-ing and is used as a control technique in air combustion whereas constitutes an intrinsic reduction mechanism in oxy-fuel combustion.

N2O mechanism

An O radical, like in the Zeldovich mechanism, initiates the nitrous oxide mechanism with an attack to the air-borne nitrogen. The difference here is that in this reaction is in-volved a third body (M) and the product is N2O. The formed N2O may after react with

another O radical to form two molecules of nitrogen oxide.

R 2.1

R 2.2

The first reaction is the most important. It is favored by high pressures, has low activa-tion energy and can thus be active at low temperatures (Warnatz et al., 2006). However, the NO quantity formed via this path is usually insignificant and this is the reason why this mechanism is usually not considered at all. It could give a contribution only in partic-ular conditions, where the other mechanisms are no active. For example, for fuel-lean conditions, low temperatures and absence of fuel-bound nitrogen this is the only mecha-nism that produces NO.

Fuel-NOx

The fuel-NOx formation is important in all the applications in which a

nitrogen-containing fuel is burned. Generally, these fuels are coals or biomasses and this mecha-nism can be to a large extent the dominant one in the NOx production. However, in this

work the fuel-NOx are not considered, but a general description of the mechanism is still

Soot and nitrogen chemistry in oxy-propane combustion: interactions and implications for NO reduction

Dipartimento di Ingegneria

dell’Energia, dei Sistemi, del Territorio e delle Costruzioni

Corso di Laurea Magistrale

in Ingegneria Energetica

Tesi di Laurea

Soot and nitrogen chemistry in

oxy-propane combustion: interactions and

implications for NO reduction

RELATORE

Prof. Ing. Leonardo Tognotti

TUTOR

Prof. Klas Andersson

CANDIDATO

Giovanni Nizzola

Abstract

Acknowledgements/Ringraziamenti

Contents

List of Tables

List of Figures

Introduction

1.

1.1 Oxy-fuel technology

1.2 Experimental setup at Chalmers University

1.3 Motivation

1.4 Aim of the work

1.5 Outline of the thesis

Background

2.

2.1 Nitrogen oxides