O Formation in Combustion. the Synthesis and Analysis of HNCO

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The importance of HNCO as a precursor of N20 formation in combustion. The synthesis and analysis of HNCO Margit Ruutelmann

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Titel: The importance of HNCO as a Precursor of NzO Formation in Combustion The Synthesis and Analysis of HNCO

Forfattare: Margit Ruutelmann, CTH

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The Importance of HNCO as a Precursor of NzO Formation in Combustion The Synthesis and Analysis of HNCO

Margit Ruutelmann r ^ DEPARTMENT OF INORGANIC CHEMISTRY CHALMERS UNIVERSITY OF TECHNOLOGY and UNIVERSITY OF GOTEBORG S-412 96 GOTEBORG, SWEDEN Institutionen for Oorganisk Kemi Chalmers Tekniska Hogskola och Goteborgs Universitet Report OOK 93:03

The Importance of HNCO as a Precursor of N2O Formation in Combustion The Synthesis and Analysis of HNCO

Margit Ruutelmann DISCLAIMER

Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. The Importance of HNCO as a Precursor ofN20 Formation in Combustion The Synthesis and Analysis of HNCO

Margit Riiiitelmann

Department of Inorganic Chemistry Chalmers University of Technology and University of Goteborg S-41296 Goteborg Sweden

December, 1993 Abstract

The atmospheric N%0 concentration increases steadily. Nitrous oxide is one of the greenhouse gases thus increasing the global temperature. It also contributes to the stratospheric ozone depletion. Any change in the atmospheric content of N%0 is believed to be anthropogenic. One of the main global source of N%0 emissions is the fossil fuel combustion in the transportation and utility power plant sections. The amount of N%0 emitted by conventional power stations is quite moderate 15-20 ppm but the release of N2O from fluidised bed combustion is relatively high 100-250 ppm, which is serious since one expects the FBC to increase in the future. Due to the demand to reduce the emissions of nitric oxides from combustion systems to the atmosphere, non-catalytic reduction of NOx in the gas phase during urea injection has been proposed, but the increase of N2O and CO has turned out to be a major problem with the urea use. In combustion processes urea decomposes forming HNCO. HNCO is considered to be one of the precursors in the formation of N2O. The work was carried out in two steps. First, HNCO formation and destruction routes in flame combustion, under fluidised bed combustion conditions and during ammonia and urea injection were investigated according to the literature data. Second, HNCO was synthesized from the cyclic trimer of cyanuric acid in laboratory conditions and the analysis of HNCO was carried out by using three different methods:

1. Absorption of HNCO in different aqueous solutions and determination of HNCO in the form of NH4+-ions 2. Determination of HNCO in argon flow by Balzers QMS 311 quadropole mass spectrometer 3. Determination of HNCO by Gas Analyzer Type 1301 which is a Fourier Transform Infra Red (FHR) spectrometer

The aim of the work was to calibrate FTTR for different HNCO concentrations in order to start the measurements of HNCO concentrations in flue gases.

11 Table of Contents

Abstract ii

1. THE ORIGIN OF N%0 AND ITS OCCURRENCE IN THE ATMOSPHERE 1

1.1. Introduction 1 1.2. The Increase of N2O Concentration in the Atmosphere 3 1.3. The Decrease in Stratospheric Ozone 5 1.4. The Greenhouse Effect 6 1.5. Sinks and Sources of Atmospheric N2O 7

2. HNCO AS A PRECURSOR OF N2O FORMATION IN COMBUSTION 9

2.1. Fluidised Bed Combustion 9 2.1.1. The Construction of a Fluidised Bed B oiler 9 2.1.2. The Advantages with FBC 10 2.1.3. N2O Formation in Fluidised Bed Combustion 11 2.2. Nitrous Oxide in Flame Combustion 12 2.2.1. Homogeneous Chemistry - Important Flame Reactions 12 2.2.2. Heterogeneous Chemistry 13 2.3. Pathways of N2O Formation from Fuel-N in Fluidised Beds 15 2.4. The Main Routes Leading to N2O Formation in the Homogeneous Gas Phase in FBC 16 2.4.1. The Reactions of HCN 16 2.4.2. The Reactions of NH3 18 2.4.3. The Decomposition of HNCO 19 2.4.4. The Decomposition of N2O 19 :' 2.5. NOx Abatement by Selective Non-Catalytic NO < Reduction 21 2.5.1. Ammonia Injection 21 2.5.2. Urea or Cyanuric Acid Injection 24

3. THE SYNTHESIS OF HNCO 27

3.1. Physical Constants of Isocyanic Acid 27 3.2. Chemical Properties of Isocyanic Acid 28 3.3. Preparation of Isocyanic Acid 30 3.4. Description of Synthesis 32

m 4. THE ANALYSIS OF HNCO 34

4.1. The Absorption of HNCO in Different Aqueous Solutions 34 4.2. The Determination of NH4+-ions in Different Aqueous Solutions 37 4.3. Balzers QMS 311 Quadropole Mass Spectrometer 41 4.4. Gas Analyser Type 1301 44 4.4.1. The Measurement Principle of FTIR 45 4.4.2. The Basic Principles of Calibration and Measurements 46 4.5. The Calibration of FTIR for HNCO 48 4.6. The Infra Red Spectra of HNCO 50

5. Conclusions 55

6. Acknowledgements 56

7. References 57

8. Appendix 60 1. THE ORIGIN OF N20 AND ITS OCCURRENCE IN THE ATMOSPHERE

1.1. Introduction

Nitrous oxide has recieved much attention recently. The concentration of N2O in the atmosphere has been slightly increasing during the last century [14]. One of the anthropogenic sources of nitrous oxide is fossil fuel combustion. Estimates of the amount of N2O formed during combustion of coal show that this form of power generation is currently not the major global source of N2O. Concern has been expressed about fhture release of N2O from fluidised bed combustion (FBC) when they become a major technology utilised in combustion of coal [14]. Both heterogeneous gas-solid reactions involving fuel char or limestone particles and purely homogeneous gas phase reactions have been suggested to be responsible for the high nitrous oxide emissions in fluidised bed combustion flue gases. HCN, formed by the devolatilization of the fuel, is an important compound in N2O formation reaction route [16]:

+0 +N0 HCN------> NCO------> N20 (1.1)

At high temperatures most of HCN is oxidized to NO, whereas at lower temperatures (1000-1200 K) N2O is the dominating nitrogen oxide. The reaction of formed NCO with H2O and H2 can lead to the formation of isocyanic acid HNCO [16]:

NCO + H20 -> HNCO + OH (1.2)

NCO + H2 -> HNCO + H (1.3)

Oxidation of NH3 can also yield to N2O but only little N2O is formed from ammonia [16]:

+OH,H +NO NH3------>NH------>N20 (1.4)

1 One method in the selective non-catalytic technique (SNR) to reduce nitrogen oxides from combustion systems is the injection of urea. The high level of N2O and CO concentrations is the major problem during urea injection especially in the low end of the temperature range [31]:

The addition of HNCO in the combustion process is found to be the precursor of N2O [16]:

+OH +NO HNCO------> NCO------> N20 (1.6)

At high temperatures (above 1223 K) N2O decomposes rather quickly. Below 1223 K the destruction of N2O can be accelerated by catalysts [1].

2 1.2. The Increase offyO Concentration in the Atmosphere

N%0 is a relatively inert gas at atmosphere concentrations and does not enter the normal nitrogen cycle in the troposphere [14]. Therefore N2O is not an acidifying agent, nor a contributor to the deposition of nitrate, which are the most commonly reported hazards of nitrogen oxides, NO and NO2.

Table 1.1. Atmospheric nitrogen species [21] -

Species Concentration or Range (by volume) ppbv

Molecular Nitrogen (N2) 78.08 % Nitrous Oxide (N2O) 300 ppbv Ammonia (NH3) 0.1-1.0 ppbv Nitric Acid (HNO3) 50-1000 ppbv Hydrogen Cyanide (HCN) 200 ppbv Nitrogen Dioxide (NO2) 10-300 ppbv Nitric Oxide (NO) 5-100 ppbv Nitrogen Trioxide (NO3) 100 ppb** Pan (CH3COO2NO2) 50 ppbv Dinitrogen Pentoxide (N2O5) 1 ppbv ** Pemitric Acid (HO2NO2) 0.5 ppbv * Nitrous Acid (HNO2) 0.1 ppbv Nitrogen Aerosols: Ammonium Nitrate (NH4NO3) 10 ppbv Ammonium Chloride (NH4CI) 0.1 ppbv ppbv - part per billion by volume * - (strong diurnal variation - max cone, during day - time ** - strong diurnal variation - max cone, during night - time

Nitrous oxide has emerged as a pollutant of concern due to its strong absorption of infra red radiation (the greenhouse effect) as well as because of the role it plays in the destruction of stratospheric ozone. It is one of the main greenhouse gases contributing about 6% of the man made greenhouse gases [14].

3 The current concentration of N2O in the atmosphere is around 305 ppb [14]. The tendency has been slightly increasing concentration during the last 25 years. Through analysis of air bubbles trapped in antarctic ice during the past centuries a preindustrial level (year 1800) of about 280 ppb was established [28]. The increase in atmospheric N2O is assumed to be purely anthropogenic. Fig. 1.1 shows the average concentrations in series of samples from two locations over 10 year period [15]. The rate of increase is about 1 ppb/year. The significance of this increase becomes apparent when considering the depletion of stratospheric ozone and the changes in the global energy balance.

O. 306

-5 302

Fig. 1.1. N2O in the atmosphere 1975-1985. +: Sampling over the northern Pacific Ocean. 0: Sampling at Antarctis [15].

4 1.3. The Decrease in Stratospheric Ozone

The concentration of ozone (O3) in the stratosphere is a result of equilibrium between rates of formation and destruction [11].

Formation: 02 + hi) —> 20 (k< 220 nm) (1.7)

0 + O2 + M --> O3 + M (1.8)

Destruction: 0 + O3 —> 202 (1.9)

O3 + hu --> O2 + 0 (X<1100 nm) (1.10)

The most important catalytic cycle for ozone destruction involves NO and NO2 [35]:

NO + O3 —> NO2 + O2 (1.11)

N02 + 0~>NO + 02 (1.12)

Net: O + O3 —> O2 + O2 (1.13)

This sequence is responsible for about 70% of the destruction of O3 in the lower stratosphere. N2O is the only significant source of NO in the stratosphere [21]:

O3 + hu(X< 310 ran) --> O (ID) + O2 (1.14)

O (ID) + N20 ~> 2 NO (1.15)

So it is obvious that increased concentration of N2O in the stratosphere would increase the destruction rate of ozone.

5 1.4. The Greenhouse Effect

The contents of CO2, H2O, CH4, certain CFC-gases and N2O in the atmosphere are known to absorb infra red light emitted from the surface of the Earth. Increasing concentrations of CO2, CH4, N2O etc. will increase the Earth's average temperature, thus causing major climate change. This effect is known as the greenhouse effect. The temperature has shown an increasing tendency during the last century. During 1880-1985 the increase was ca 0.6 °C, but this could have been the result of natural change rather than anthropogenic emissions [14]. Nevertheless, it is a fact that the atmospheric concentrations of CO2, N2O and CH4 have increased during the past years (CO2: 0.5% p.a., N2O: 0.3% p.a., and CH4:1.0%p.a.) [14].

Table 1.2. Current contribution to global warming excluding H2O [22]:

co2 49% CH4 18% CFC 11 and 12 14% N20 6% others (eg O3) 13%

6 2.5. Sinksand Sources of Atmospheric N2O

The backround level of N2O in the atmosphere is the result of equi librium between formation and destruction of N2O. The natural source of N2O is mainly biological activity. The emission of N2O from soil and from the ocean is a result of side reactions in the nitrification/denitri fication cycle. The natural sink of N2O is its destruction in the stratosphere which occurs mainly through reaction forming NO:

0(1D) + N20= 2NO (1.16) and forming N2 [14]:

N20 + O (ID) = N2 + 02 (1.17)

N20 + hu = N2 + O (ID) (1.18)

The natural source of N2O is considered to contribute at a constant rate and thus the change in the atmospheric content is believed to be anthropogenic.

Table 1.3. Direct N2O emissions from different antropogenic sources [22]

Mt(N)/yr

Use of fertilizers 1.0 +- 0.3 Biomass burning 0.5 +- 0.1 Fossil fuel combustion stationary sources 0.2 +- 0.1 mobile sources * 0.4 +- 0.2

Sum of direct anthropogenic emissions 2.1 +- 0.7

* - assuming 50 % of the kilometres are run with gasoline engines equipped withmedium aged 3 - way catalysts.

7 The widespread use of catalysts has shown the increase of the N%0 emission. Especially aged catalysts seem to affect the production of N%0 by a 10-16 fold increase compared to engine exhaust without catalysts [14]. The biogenic source of N2O is sensitive to human activity. The extensive use of fertilizers has been shown to increase the N2O production. The data in the table for stationary combustion sources are based on low emissions (0-5ppm) from pulverized coal combustion and combustion of oil. The stationary combustion source could increase in future if certain DeNOx techniques and fluidised bed combustion are to be used extensively at the present state of these technologies.

8 2. HNCO AS A PRECURSOR OF N20 FORMATION IN COMBUSTION

2.1. Fluidised Bed Combustion 2.1.1. The Construction of a Fluidised Bed Boiler

Fluidisation is a phenomenon where particles behave as a fluid. In fluidised bed combustion, a particle bed is placed on a distributor and a gas flow is introduced below the distributor plate. At a low gas flow rate (fluidisation velocity) the system behaves like a fixed bed with a low pressure drop across the bed. As the fluidisation velocity is increased the pressure drop across the bed increases until a state is reached when the pressure drop across the bed is equal to the weight of the bed. At higher fluidisation velocities the bed expands, the particles become freely suspended in the gas stream, bubbles arise, and the bed attains the fluidised state. The basic components required for an atmospheric stationary fluidised bed boiler is schematically shown in Fig. 2.1 [12].

Air

Fig. 2.1. A schematic representation of an atmospheric stationary fluidised bed boiler [12].

9 The combustion chamber consists of the fluidised bed and the freeboard zone, after which the combustion gases are directed through a convective section. Heat exchangers are normally built into the walls and also as in bed tube bundles. The bed material consists of sand, unbumed fuel (usually coal, 0.2-4%), sorbent (usually limestone), and ashes. '

2.1.2. The Advantages with FBC

The fluidisation provides for excellent mixing and a more complete combustion. The combustion temperature of FBC is relatively low (1073- 1173 K) [33] in comparison with the temperatures of flame combustion (1273-1873). The low temperatures in FBC prevent thermal NO formation and promote NO reducing reactions during the combustion process. The temperature range is also suitable for sulphur capture by limestone addition (limestone reacts with SO2 to form CaS04) [33]. The environmental problem including the FBC is high emissions of N2O compared with the traditional boilers. The fluidised bed technology is being developed and improvements of the atmospheric fluidised combustion method, like the pressurized fluidised bed (PFBC) and the circulating fluidised bed (CFB), are attracting a lot of interest [12]. The PFBC has a more compact design and higher combustion rates. In the CFB the gas velocities are high, resulting in entrained bed material which is separated from flue gas by cyclones, and recycled. The recirculation of bed material improves the combustion efficiency and decreases the emissions of pollutants.

10 2.1.3. N2O Formation in Fluidised Bed Combustion

The low combustion temperatures of 1073-1273 K in FBC decrease the emissions of NO and SO2 but since N%0 does not readily decompose below 950 °C, the amount of N2O released is high. Nitrous oxide can be formed in FBC by diverse gas-solid-phase interactions and homogenously by gas phase reactions[17]. Nitrogen oxides from fluidised bed combustion originate mainly from the oxidation of fuel-nitrogen (fuel-N) present in carboneous fuel. Due to the low temperature, the contribution from the fixation of atmospheric nitrogen in the combustion air is minor. The emission of N2O from fluidised bed combustion depends on the fuel, the temperature, excess oxygen level, bed material and Ca/S ratio [14]. Other variables that can possibly influence the emission are design, pressure, load, temperature profile, air staging pattern and ash recirculation [14]. As shown in Fig. 2.2 [33] during the fuel devolatilization process, some of the fuel-nitrogen is released together with the volatiles (volatiles- N), and the rest remains in the char (char-N). The volatile fuel-N products further pyrolyze to simple amino species, like NH3, and to simple cyano species, like HCN. The distribution between char-N and volatiles-N and the distribution between NH3 and HCN species in the volatiles are poorly known for different fuels. The dependence of both distributions on the combustion conditions is not well known.

Fomuott ot rum Pnauca dMMftattfl 1 tvctuton

' inao/trexm 1 comousoofl H, N,0 NO ■ otCQwadhtr

Fig. 2.2. Pathways of N2O formation from fuel-N in fluidized beds [33].

11 2.2. Nitrous Oxide in Flame Combustion

The temperatures of flame combustion is about 1273-1873 K [33] which are in comparison with the combustion temperatures of FBC quite high. The emissions of N%0 from flame combustion are generally below 20 ppm whereas from combustion of coal in FBC the typical NaO concentrations in the flue gas vary from 50 ppm up to 200 ppm [37].

2.2.2. Homogeneous Chemistry - Important Flame Reactions

During the devolatilization of the fuel, some of the fuel-nitrogen is released together with the volatiles (volatiles-N), and the rest remains in the char (char-N). NH3 and HCN are the main volatile N-containing components during coal combustion.

HCN reactions [161:

HCN + OH —> HNCO + H (2 .1)

HNCO + H->NH2 + CO (2.2)

HNCO + OH -> NCO + H20 (Z3)

HCN + O —> NCO + H (2.4)

NCO + NO --> N2O + CO (2.5)

NCO + OH ~> NO + CO + H (2 .6)

NHi reactions 1191:

NH3 + OH —> NH2 + H20 (2.7)

NH2 + OH ->nh + h 2o (2 .8)

NH + NO —> N2O + H (2.9)

12 NH + OorOH ~> NO + H2O (2 .10)

Reactions 2.9 and 2.10 are in competition for the available NH pool. The high concentrations of O and OH in the flame zone favour reaction 2.10 which reduces the NH concentration, and therefore reduces N20 formation via reaction 2.9.

NoO destruction T331:

N20 + H ~> N2 + OH (2.11)

N20 decomposes easily above 950 °C [24]. The HCN reaction paths lead through either HNCO or NCO. The HNCO path leads to NH2 via reaction 2.2, and into the NH3 chemistry described above, or it can form N20 in reaction 2.5. Any N20 formed in the flame zone is removed by reaction 2.11. The high rate of reaction 2.11 suggest that the flame zone is not the source of N20 in coal combustion. N20 destruction, reaction 2.11, is slowed down by lower temperatures and reduced hydrogen atom concentrations.

2.2.2. Heterogeneous Chemistry

Heterogeneous reactions which can lead to N20 formation are the following [37]:

1. The formation of char nitrogen to N20

2. The reaction of NO with char nitrogen, or the adsorption of an NO molecule onto the char surface where it reacts with a second NO molecule.

3. The devolatilization or gasification of char nitrogen, followed by gas phase reaction.

13 According to the results of several research-work the prevailing reactions contributing to N2O formation in flame combustion are the following [19]:

1. Char acts as a vehicle for carrying bound nitrogen away from the volatile flame zone.

2. A portion of the char-N devolatilizes as HCN within overall fuel-lean regions. Alternatively, HCN may be formed by gasification reactions.

3. A portion of the HCN reacts homogeneously to form N2O via reactions 2.4 and 2.5.

4. As this reaction occurs under postflame conditions, the reduced temperature and H-atom concentrations prevent the removal of N2O by reaction 2.11.

The heterogeneous destruction of N2O occurs over several materials. Fe20s, Fe3C>4, MgO and CaO enhance the degradation of N2O at high temperatures [24]. In conclusion, the fact that N2O emissions from coal flames are low does not mean that little N2O is formed during the flame combustion but in comparison with the fluidised bed combustion relatively small amounts of N2O are released in the flue gas.

14 2.3. Pathways ofN20 Formation from Fuel - N in Fluidised Beds

At present a number of N2O formation paths have been suggested:

1. The oxidationof char-nitrogen [8].

O2 + (C) + (CN).~> (CO) + (CNO) (2.12)

N2O formation: (CN) + (CNO) ~> N20 + 2 (C) (2.13)

2(CNO) --> (CO) + (C) + N20 (2.14)

NO formation: (CNO) -> NO + C (2.15)

(CN) + (CO) ~>NO + 2(C) (2.16)

2. The reduction of NO on char surface [8]. This includes the reaction of NO with char-nitrogen, and the absorption of a NO molecule onto the char surface and its subsequent reaction with a second NO molecule to N2O. .

NO + 2(C) —> (CN) + (CO) (2.17)

NO + (CN) --> N20 + (C) (2.18)

3. The catalytic reactions of nitrogen species on the surface of other particles present in fluidised beds than char [17].

* the role of CaO as a catalyst for N2O formation from NH3 and NO in presence of oxygen. *

* the formation of N2O from NO on surface of reduced CaSOx compounds (the chemical composition of the reduced CaS04, which is named CaSOx was not known)

2NO + CaSOx --> N2O + CaS04 (2.19)

4. The homogeneous gas phase oxidation of fuel - N speciesat low temperaturesis discussed in 2.4.

15 2 A. The Main Reaction Routes Leading to N2O in the HomogeneousGas Phase in FBC 2AJ. The Reactions ofHCN

As shown in Fig. 2.3 [17] most of the HCN reacts first to NCO radical (box 1). This occurs mainly through the following reactions [17]:

a) HCN + 0 —>NCO+ H (2.20)

b) hnc + oh ->cn +h 2o (2.21)

CN + 02->NC0 + 0 (2.22)

For destruction of NCO four main paths were identified [16]:

a) The reaction of NCO with H radical leading to NH (box 4)

NCO + H --> NH + CO (2.23)

b) The reaction of NCO with H2O and H2 leading to isocyanic acid HNCO (box 5) according to equations 1.2 and 1.3.

c) The reactions of NCO with O and OH radicals leading to NO (box 7)

NCO + O > NO + CO (2.24)

NCO + OH --> NO + CO + H (2.25)

d) The reaction of NCO with NO leading to N2O (box 9)

NCO + NO ~> N20 + CO (2.26)

which reactions path dominates is greatly dependant on the local combustion conditions. At high temperatures (1200 K) NCO is rapidly removed by the attack of H, O and OH radicals [17], leading to NH (box 4) and NO (box7) formation. By the subsequent reaction of NH with the radicals, mostly with H radicals, it is converted to an N atom:

NH + H->N + H2 (2.27)

16 and finally

N + OH —> NO + H (2.28)

N + O2 --> NO + O (2.29)

Most of HCN is oxidized to NO at higher temperatures, whereas at lower temperatures (1000-1200 K) as shown in Fig. 2.4, N2O is the dominating nitrogen oxide [17]. At lower temperatures the rates of the reactions 2.23,1.2 and 1.3 are slower, however, due to the lower levels of H, O and OH radicals a significant N2O formation may occur. N2O formation from cyano species increases as the temperature decreases. At high temperatures less N2O is formed because the inter mediate radical NCO is rapidly consumed by reactions that do not lead to N2O.

NO NO| NjO HNC0

Fig. 2.3. Flow diagram for fuel-lean oxidation of HCN at high temperature (T=1200 K). The thickness of flows is proportional to the amount of bound nitrogen from initial HCN converted to the species shown in the boxes. For NHi, i stands for i= 0-2 [17].

17 NO NOj NzO HNCO

Fig. 2.4. How diagram for fuel-lean oxidation of HCN at low temperature (T=1000 K). The thickness of flows is proportional to the amount of bound nitrogen from initial HCN converted to the species shown in the boxes. For NHi, i stands for i= 0-2 [17].

2.4.2. The Reactions ofNH3

By the attack of radicals NHg reacts first to NH2. This occurs mainly via reaction with OH radicals [17]:

NH3 + OH ~> NH2 + H2 (2.30)

By the subsequent conversion of NH2 to NH, mostly via reaction:

NH + NO -> N2O + H (2.31)

But most of NH produced was consumed by faster radical reactions 2.27, 2.28 and 2.29 to NO which explains why only little N2O was formed from ammonia.

18 2.43, The Decomposition ofHNCO

The significant N2O formation from urea addition was found to result from the decomposition ofHNCO to NCO [17]:

HNCO + OH ~> NCO + H20 (2.32)

NCO + NO > N20 + CO (2.33)

2.4.4. The Decomposition ofN20

For the destruction of N20 (box 12) the most important reactions are [17]:

N20 + H ~> N2 + OH (2.34)

N20 + OH -> N2 + H02 (2.35)

Summary: HCN is the dominating nitrogen source for N20 whereas formation from NH3 is found to be smaller. Temperature is the most important single factor affecting the homogeneous N20 formation. N20 is formed within a narrow temperature window about 1000-1100 K (Fig. 2.5) [16]. At high temperatures neither NH3 nor HCN produces N20 when oxidized. N20 formation increases steeply as the temperature decreases. At temperatures below 1000 K most of HCN remains unreacted and hardly any N20 is produced. Higher excess air indicates higher N20 levels at constant temperatures (Fig. 2.6) [16]. The homogeneous gas phase reactions are an important contribu tors for N20 formation in fluidised beds.

19 Fig. 2.5. Distribution of bound nitrogen from oxidation of HCN [16].

Fig. 2.6. Distribution of nitrogen from the oxidation of HCN versus air/fuel stoichiometric ratio (1100 K). Fuel: methane [16].

20 2.5. NOx Abatement by Selective Non-Catalytic NO Reduction

Pollutant species emitted from combustion sources include nitric oxide (NO) and nitrogen dioxide (NO%) that together are called nitrogen oxides (NOx). Nitrogen oxides play an important role in the formation of photochemical smog and acid rain [5]. The major component of NOx emitted from combustion systems is nitric oxide. NO is formed through the oxidation of atmospheric and fuel-bound nitrogen during combustion [6]. The NOx control technologies can be subdivided into two main groups: primary methods (combustion process modifications) and secondary methods (post-combustion gas treatments) [6]. For lower NOx emission level, post-combustion flue gas treatment (FTG) processes are often proposed. FGT processes for reducing nitrogen oxide emissions include a variety of methods. One such class of NO reduction techniques is selective, non-catalytic reduction (SNR). Selective, non-catalytic techniques for removing nitric oxide from the exhaust gases of combustion processes include the addition of ammonia, cyanuric acid or urea to the hot exhausts [6].

2.5.2. Ammonia Injection

An example of the SNR technique is the use of ammonia injection into post-combustion gases containing oxygen at the temperatures about 1200K [6]. NOx is selectively reduced to N2 and H2O [4]. The reduction of NO by NH3 is ruled by the following reaction scheme [20 ]: + O2 [ + 02, OH M; OH; NH2 } NH------> HNO------> NO

+ OH 4- NO

+ NO > NNH

21 The radicals O, H and OH required for reaction

O; OH; H NHg------> NHi (2.37)

are obtained by the relatively slow reaction

NH3 + 02 --> NH2 + H02 (OH + H) (2.38)

The existance of an optimum temperature (850-1050 °C) for NO reduction is due to the following [31]:

* For T > T0pt, large amounts of radicals obtained will result in the production of sufficient quantities of NHi, but, since reaction

NHi —> NO (2.39)

now becomes faster than + NO NHi------>N2, (2.40)

the latter are preferentially consumed for NO formation.

* For T < Topt, reaction to obtain the radicals is too slow and insufficient O, OH, H radicals are produced. *

* The addition of small amounts of hydrocarbons lower the optiomum temperature, since OH and H production by reaction

RH + O2 -> R + H02 (OH, H) (2.41)

As shown in Fig. 2.7 [14] a slight increase in N20 emissions is noticed over the whole temperature range (1073-1273 K) when ammonia is used as a reductant. The increase does not depend on the extent of NOx reduction.

22 Temperature (K)

AAMANjO emission - 08850 percent deNOx

750 800 850 900 950 1000 1050 1100 Temperature (deg. C)

Fig. 2.7. NOx-reduction and emission of N%0 at molar ratio 1.3. Reductant ammonia. The N0X base level is ca 320 ppm [14].

23 2.5.2. Urea or Cyanuric Acid Injection

The addition of urea to the exhaust of combustion systems has been proposed as a method for reducing nitric oxide emissions. During urea injection instead of ammonia, much higher N%0 emissions are seen. According to the field literature, many different urea decomposition reaction routes are proposed [18]:

NH2CONH2 ~> 2NH2 + CO (2.42)

180 °- 280 °C CH2NCONH2 ------> C3N3(OH)3 + 3NH3 (2.43)

330 °C C3N3(OH)3------> 3HNCO (2.44)

380 °C H2NCONH2 ------_> C3N3(NH2)3 + 6NH 3 + 3C02 (2.45)

Both urea and cyanuric acid can give rise to the formation of HNCO in combustion processes. The reaction routes of urea decomposition forming directly HNCO are the following [4]:

H2NCONH2 --> NH3 + HNCO (2.46)

H2NCONH2 ~> NH2 + HNCO + H (2.47)

H2NCONH2 ~> NH3 + NCO + H (2.48)

Depending on the combustion temperature, urea decomposes mainly in accordance with the reactions 2.43 and 2.44 or form directly NH3 and HNCO in the reaction 2.46. HNCO decomposition can result in the formation of NCO radicals in reaction 2.32. The latter reacts further to N2O or N2:

NCO + NO -0N2O + CO (2.33)

NC0 + N0->N2 + C0 + 0 (2.49)

24 NCO + NO—>N2 + C02 (2.50)

Reaction 2.33 explains the rise of both N2O and CO emissions during urea or cyanuric acid injection. Fig. 2.8 [14] shows that N2O formation is significant at 1100-1300 K for urea or cyanuric acid addition. N2O formation coincides with the formation of CO. At high temperatures, any N2O formed will be rapidly removed by reaction 2.34. which is extremely sensitive to temperature. The urea process is a hybride of the cyanuric acid and NH3 process [6]:

+ OH + NO HNCO

+ H

(2.51)

+ NO

The comparison of the NH3 and the cyanuric acid or urea nitric oxide reduction processes demonstrates that the NO reduction chemistry of each of these processes is very different. Of course, since NH3 is formed when urea is heated, all the reactions for the NH3 case, is also relevant for the SNR process with urea. In addition, HNCO is formed when urea is heated. NO removal path in the NH3 process is based on NH2 path and in the cyanuric acid or urea process is based on NCO chemistry which at relatively low combustion temperatures leads to N2O formation [6], The formation of the latter from NCO + NO is much faster than that from NH + NO [31]. Recently, the exhaust gas treatment process using gaseous isocyanic acid was proposed. The process works by adding gaseous isocyanic acid (HNCO) to an exhaust gas stream [5].

25 Temperature (K)

emission * COQQO percent deNOx

750 800 850 900 950 1000 1050 1100 Temperature (deg. C) Fig. 2.8. NOx-reduction and the emission of N2O at molar ratio 1.3 (N in urea/NO). Reductant urea. The NOx base level is ca 400 ppm. [14].

26 3. THE SYNTHESIS OF HNCO

3.1. Physical Constants oflsocyanic Acid

A HNCO molecule contains one atom each of the four principal elements of organic matter H, C, N and O. The interatomic distances of the HNCO molecule are the following: H-N 0.986, N-C 1.209, C-0 1.166 A and the angle HNC 128 ° [23]. The three heavy atoms N-C-0 are nearly linear.

M (HNCO) = 43.03 g /mol Crystalline form gas Melting point - 81 °C to - 79 °C Boiling point 23.5 °C Density 1.140 g/cm3 (liquid) Solubility HNCO is water, ether, benzene, chloroform and acetic acid soluble.

The vapour pressure of HNCO is approximately 1 mmHg at -80°C, 10 mmHg at -53 °C, 100 mmHg at -19 °C and the normal boiling-point by extrapolation is 23.5 °C [2]. To store liquid HNCO it is necessary to slow down the polymerization process by keeping temperature at -30 °C or below. The other species containing four principal elements H, C, N and O are cyanic acid, fulminic acid and isofulminic acid [23]. Cyanic acid has the structure HOC=N. Contrary to belief isocyanic acid, NH=C=0, is not in equilibrium with cyanic acid, HOC=N [23]. Cyanic acid is kinetically unstable. Fulminic acid, HC=N-0, is stable for a time only in Et%0 solution at low temperatures. It polymerizes rapidly. Interatomic distances of HCNO molecule are H-C 1.027, C-N 1.168 and N-0 1.199 A. Isofulminic acid, HO-N=C, is an intermediate in stability between fulminic acid and isocyanic acid.

27 3.2. Chemical Properties oflsocyanic Acid

Isocyanic acid is reported to be strongly acidic but it is less hazardous to handle than HC1. Liquid and gas form of HNCO has a strongly acid odour.

A. Polymerization Liquid isocyanic acid polymerizes to form a mixture of cyanic acid and cyamelide. These two are easily separeted since cyanuric acid is soluble in hot water whereas cyamelide is not. The proportionof cyanuric acid formed increases with temperature, from 30% at -20 °C to 57% at 20°C [2]. Gaseous isocyanic acid is formed by the depolymerization of both cyanuric acid and cyamelide heated at temperatures above 700 °C [2].

(HNCO)3 (g) ~> 3HNCO (g) (3.1)

Below 350 °C isocyanic acid tends to condensate.

B. Decomposition In the combustion processes during urea injection formed HNCO can react according to the reaction 2.51.

C. Reactions with water In aqueous solutions [2] isocyanic acid undergoes both ionization:

HNCO --> H+ + NCO- (3.2)

and hydrolysis:

HNCO + H20 --> NH3 + C02 (3.3)

Almost no studies have been made to prove the gas phase reaction according to the equation:

HNCO (g) + H20 (g) ~> NH3 (g) + C02 (g) (3.4)

The hydrolysis of HNCO is pH dependant and occurs most rapidly in acid solutions [2]:

HNCO + H20 + H+ --> NH4 ++ C02 (3.5)

28 In more neutral solutions:

HNCO + H20 --> NH3 + C02 (3.6) and in alkaline solutions:

NCO- + 2H20 --> NH3 + HC03 - (3.7)

The reactions shown in equation (3.6) and (3.7) probably occur through the formation of carbamic acid and carbamate as intermediates [2 ]:

+H20 HNCO------> NH2COOH --> NH3 + C02 (3.8)

+H20 +h 2o NCO------> NH2COO------> NH3 + HC03- (3.9)

Carbamic acid is very unstable, whereas carbamate is more stable.

29 3.3. Preparation of Isocyanic Acid

Different methods of preparing isocyanic acid are reported in literature [2]:

A. Depolymerization of cyanuric acid

The isocyanic acid was prepared by the depolymerization of the cyclic trimer of cyanuric acid. Cyanuric acid depolymerizes by heating to yield isocyanic acid according to the equation [9]:

N 'NT (s) ■> (HNCO )3 (g) --> 3HNCO (g) (3.10)

HO OH

Cyanuric acid tends to sublime without reaction. In order to achieve depolymerization the solid cyanuric acid is first vaporized and the gas obtained is heated still further. Cyanuric acid starts to sublime when heated at about 330 °C and a gaseous trimer (HNCO)g is formed. For (HNCO)g depolymerization to 3HNCO the gas stream is heated to 700 °C. The process of depolymerization is never complete and both the monomer and the trimer can be found in the gas phase. The by-products of cyanuric acid decomposition are HCN, CO, CO2, NH and H2O. HCN is turned into HNCO by oxidation of HCN over a silver catalyst Ag20. At -80 °C the gas is turned into liquid which is a mixture of monomer HNCO and trimer (HNCO)g. In order to get rid of trimer distillation is carried out at -32 °C. The solid component (HNCO)g remains behind in the gas wash bottle and liquid HNCO is collected and stored at -80 °C. HNCO should not be stored too long (max 1 week) due to its relatively rapid trimerization.

30 B. Acidification of Alkali Isocyanates

A variety of ways of acidifying alkali isocyanates to produce isocyanic acid has been employed. Oertel and co-workers [2] passed HC1 into suspensions of alkali isocyanates in inert organic liquids and HNCO was yield. In another patent [2] a solution of trichloroacetic acid in hexane was added to a warm solution of NaNCO in hexane.

C. Other methods

a) isocyanic acid is formed when urea is heated

NH2CONH2 --> HNCO + NH3 (3.11)

b) isocyanic acid is formed when a solution of nitrourea in water is heated

NH2CONHNO2 --> HNCO + NH2NO2 (3.12)

c) the removal of hydrogen halide from a carbomoyl halide

H2NCOX --> HNCO + HX (3.13)

d) the pyrolysis of alkyl carbamates (e. g. NH2C02Me)

e) the reaction of molten alkale cyanate with urea, biuret or cyanuric acid

31 3.4. Description of Synthesis

Experimental

Isocyanic acid HNCO was preparedby the thermal decomposition of the cyclic trimer of cyanuric acid CgNgCOKQg. In Fig.3.1 the schematic of the HNCO synthesis is shown. The two furnaces (1,2) were arranged in series. The temperatures of both tube furnaces were controlled by Pt- Pt/ 10% Rd thermocouples connected to the temperature control units which delivered the appropriate voltage output to the furnaces. The reactions were carried out in a quartz tube (3).

Depolvmerization of cvanuric acid

About 15 g of solid cyanuric acid was weighted out in a 50 ml round-bottom flask in the first tube furnace and preliminary heated in vacuo at 200 °C in order to remove water. The temperatures of both tube < furnaces were kept at 200 °C. When water had been removed the temperature of the second tube furnace was increased to 700 °C, while the temperature in the first tube furnace was gradually raised to 450 °C. Then the solid cyanuric acid was volatilized slowly and the resulting vapour was heated still further by passing the gas stream through the tube furnace at 700 °C. The bulk of HNCO formed was collected in a trap (5) cooled to - 80°C with an acetone bath to which dry ice was added [27]. The tube (4) connecting the quartz tube (700 °C) and the trap (-80 °C) was heated electrically to 250 °C - 300 °C and isolated to avoid the transformation of the gas phase into solid phase. On the bottom of the trap (5) AgaO was placed in order to oxidize any HCN formed to HNCO. The whole synthesis was carried out in vacuo and the time needed was 5 hours.

Purification of the product

The liquid phase obtained consists of both HNCO and (HNCO)s. It was transferred to a gas wash bottle (6) and purified by distillation at - 32°C. To obtain the right distillation temperature dry ice was added to acetone and the temperature of the mixture was contineously controlled by ordinary thermometer. The stream of argon (5 ml/ min) was flown through the gas wash bottle containing the liquid HNCO kept at -32 °C and the obtained gas

32 stream was oxidized with Ag%0 and dried over P2O5. Pure HNCO was collected in a trap (7) at -80 °C and stored at the same temperature. HNCO can be also titrated with NaOH (pH=9.85) to check the pu rity of the product [9].

to pump

1 and 2 - tube furnaces 3 - quartz tube 4 - tube connecting the quartz tube and the trap 5 - trap cooled to -80 °C 6 - gas wash bottle (-32 °C) 7 - trap cooled to -80 °C

Fig. 3.1. The schematic drawing of the apparatus of HNCO synthesis.

33 4. THE ANALYSIS OF HNCO

4.1, The Absorption of HNCO in Different Aqueous Solutions

Isocyanic acid undergoes hydrolysis in aqueous solutions according to equation 3.3. The hydrolysis is pH dependant and occurs most rapidly in acid solutions in accordance with equation 3.5. This fact makes it possible to determine the concentration of HNCO in a carrier gas by bubbling the gas through an absorbing solution in a gas wash bottle. The absorbents employed for the absorption of HNCO were mostly acid solutions.

Different absorbing solutions were tested:

100 ml dest. H2O 100 ml 0.1 mM H2SO4 100 ml 0.1 mM HC1 100 ml 1 mM H2SO4 + MgS04 100 ml 0.1 M H2SO4 100 ml 1 M H2SO4

The pH level of the above mentioned solutions were measured by METROHM 654 pH-METER and the results are tabulated in Table 4.1. The carrier gas 20 ml/min argon was flown over the surface of liquid HNCO in the gas wash bottle (1) and then diluted with 1000 ml/min argon in a three-tubed round-bottom flask (2) (Fig. 4.1). The gas was bubbled through 100 ml of absorbent solution placed in a 250 ml gas wash bottle and HNCO was absorbed in the form of NH4+-ions. The bubbling time was 10 min and 20 min. Comparative study of HNCO absorption in different absorbent solutions were carried out when the temperature of HNCO was -32 °C and-36 °C. At -32 °C the concentration of HNCO in the carrier gas flow was higher compared with the results obtained when liquid HNCO was kept at -36°C.

34 iooo ml/mm

1 - gas wash bottle (-32 °C or -36 °C) 2 - three-tubed round-bottom flask 3 - gas wash bottle containing absorbent solution

Fig. 4.1. The apparatus set up for diluting gaseous HNCO in 1000 ml/ min Ar flow.

The amount of HNCO left in argon flow after absorption was determined by FTIR. The concentration of HNCO in argon after absorption was of course lower but the concentration of CO2 much increased due to the reactions 3.3 and 3.5. The results are shown in Fig. 8.1- 8.6. According to the spectrum obtained the increasing acidity of the solutions is followed by better absorption efficiency. The best results were achieved after absorption in 1 M and 0.1 M H2SO4 solutions correspondingly 35.6 ppm (4.9 %) and 48.8 ppm (6.63 %) HNCO was left in the carrier gas. The concentration of CO2 is relatively high when the absorption takes place in 0.1 M and 1 M H2SO4 solutions. Due to the low pH-level of 1 M H2SO4 (almost 0) and high CO2 concentration in the carrier gas 0.1 M H2SO4 was used when FTIR was calibrated and exact HNCO concentrations in the argon flow was determined.

35 Table 4.1. The results of HNCO absorption in different aqueous solutions. The concentration of HNCO in argon flow was C (according to the absorbance 81.66 from spectra obtained) during constant conditions (-32 °C). See also Appendix Fig. 8.1-8.6.

Absorbent solution pH HNCO not absorbed Absorption efficiency

100 ml dest. H%0 5.03 15.94 80.5%

100 ml 0.1 mM H2SO4 2.07 10.66 87.0%

100 ml 0.1 mM HC1 4.00 8.38 89.74%

100 ml 0.1 mM H2SO4 + MgS04 2.08 8.401 89.71%

100 ml 0.1 M H2SO4 1.00 5.396 S>3.4 %

100 ml 1 M H2SO4 0.00 3.975 95.1 %

36 4.2. The Determination ofNH4+- ions in Different Aqueous Solutions

HNCO was absorbed in the form of NH4+-ions in aqueous solutions. Two methods, the direct and indirect method, may be used for the determination of NH4+-ions [33]. The direct method [25]. Ion - selective electrode can be used to determine the concentration of NH4+ in different absorbent solutions. Having quite acidic solutions the correction are made by adding 2.5 M NaOH, 1 M NaOH or 0.5 M NaOH to the probes in order to obtained pH= 6-7. The indirect method. The apparatus shown in Fig. 4.2 was arranged. A is a 250 ml glass flask provided with a two holed rubber stopper which carries a tap funnel B and a glass spray trap C. The purpose of the trap is to prevent any droplets of sodium hydroxide solution being driven over in the subsequent distillation. D is a Liebig's condenser attached to the trap by means of a rubber bung and E is a 100 ml Pyrex conical flask which serves as a receiver.

Fig. 4.2. The apparatus used for indirect determination of ammonia in an ammonium salt.

37 100 ml of the resultant solution of the absorption was transferred into the 250 ml distillation flask. A few fragments of porous porcelain was added to promote regular ebullition in the subsequent disstillation. 35 ml 14.5 g/1 H3BO3 was placed in the receiver and it was controlled that the end of the condenser dipped into the solution. 25 ml 12.5 M NaOH was placed in the funnel. The sodium hydroxide solution was run into the flask by opening the tap F. The tap was closed as soon as all the alkali had entered. The flask was heated so that the contents boiled gently. After about 15 minutes all the ammonia should have passed over into the receiver. The condenser was carefully rinsed with water. Using a calibrated combined glass pH electrode linked to a METROHM automatic titration system, the solution was titrated to the equivalence point (pH=4.9) using 0.05 M H2SO4. The consumed H2SO4 at the equivalence point is a measure of the NH3 in the sample [1].

mjsfH3= c * (v2- vi) * 2 * 10-3 * M ( NH3) (4.1)

H1nh 3 the amount of NH3 in g in the sample c concentration of H2SO4 vi volume of H2SO4 in ml used in zero-probe titration V2 volume of H2SO4 in ml M(NH3) = 17.02 g/mol 2 1 mole H2SO4 is equivalant to 2 mole NH3 10-3 transformation factor from ml to 1

Chemical Reactions:

NH4++ oh - ->NH 3 (g) +H20 (4.2)

NH3+ H3BO3 —> NH4+ + H2BO3- (4.3)

2NH4BO2 + H2SO4 --> (NH4)2 SO4 + 2HB02- (4.4)

38 Table 4.2. The comparison of the results obtained by titrating and measuring the concentration of NH4+-ions by ion-selective electrode in different absorbent solutions. Ar (20 ml/min) was flown through the gas wash bottle containing liquid HNCO (-30 °C) and the bubbling time during absorption was 20 min.

Absorbent solution pH Ion-selective Titrating electrode NH3 mol/1 NH4+ mol/1

100 ml dest. H%0 5.03 4.886*10-3 4.365*10-3

100 ml 0.1 mM H2SO4 2.07 5.278*10-3 4.39*10-3

100 ml 0.1 mMHCl 4.00 4.315*10-3 3.465*10-3

100 ml 0.1 mM H2SO4 + MgS04 2.08 4.681*10-3 3.7*10-3

100 ml 0.1 M H2SO4 1.00 1.152*10-3 5.6856*10-3

100 ml 1 M H2SO4 0.00 5.347*10-4 4.658*10-3

The low pH level in the solutions makes it unreliable to determine the concentration of NH4+-ions directly by ion-selective electrode. Due to that the indirect method was chosen to determine the amount of NH4+- ions in different aqueous solutions. The concentration of HNCO in ppm in argon flow was calculated using the following relationship:

C = Ci * 10-3 * 22.4 * 0.1 /t * ( Qi + Q2 ) *10-6 (4.5 )

C HNCO concentration in ppm Ci NH3 concentration in mol/1 t contact time of HNCO absorption in minutes Ql argon flow through HNCO in 1/min Q2 argon flow in which HNCO is diluted 1/min In our case Q2 = 1000 ml/min = 11/min

39 Table 4.3. The results of titrating and the calculated HNCO concentrations in ppm in argon flow. 20 ml/min argon was flown through the gas wash bottle containing liquid HNCO. HNCO was absorbed in 100 ml of 0.1 M H2SO4.

The temperature of liquid -32 °C -36 °C HNCO

1. 2. 3. 4. pH level of the solution 6.815 7.100 6.862 6.710 after absorption ml 0.05 M H2SO4 used to titrate to the eq. point 3.45 6.925 2.6 2.505

The time of bubbling Ar through the gas wash bottle 10 20 10 10 containing liquid HNCO in min

HNCO concentration in Ar flow in ppm 734 748 547 526

The influence of temperature on the HNCO concentration in the outgoing Ar is great, meaning that many efforts have to be made to obtain good thermostating and constant conditions.

40 43, Balzers QMS 311 Quadropole Mass Spectrometer

A brief description of the operating principle of Balzers QMS 311 quadropole mass spectrometer [10] is given below. • Gas samples enter into the ion source section of the analyser via an electrically heated capillary (Fig. 4.3). Electrons formed from a cathode filament subsequently bombard the in-coming gas molecules forming ions which are accelerated and focused towards the quadropole mass filter. In this section, the sample ions are subject to an electrostatic force with the result that only ions with a certain mass filter consists of four cylindrical, highly polished rods symmetrically arranged to the axis. Each pair of opposite rods are interconnected so that between the two pairs an altering voltage exists. The ions entering the mass filter along its axis experience an electric field and consequently oscillate either stably or unstably depending on their mass/charge ratio. The unstably oscillating ions move with an ever-increasing amplitude and thus collide with the rods and become neutralized. Those ions with the selected mass/charge ratio proceed through the quadropole and are able to reach the detector section of the mass spectrometer. The selected ions are registered using an in-line secondary electron multiplier detector. The ions are accelerated up to ca 1.7 kV and strike the first conversion diode releasing secondary electrons which are amplified at each stage giving current gains of 104 - 108. The ion current obtained is then converted into both logarithmic and linear voltages (0-10V) using an electrometer amplifier and the mass signals are monitored on a chart recorder. Two modes of operation are possible with the mass spectrometer; firstly by using a mass scan and secondly, a continual measurement of a single mass/charge ratio.

Fig. 4.3. The schematic of the Balzers QMS 311 quadropole mass spectrometer analyser unit.

41 The analysis of gas composition in the carrier gas flow was followed by using a Balzers QMS 311 quadropole mass spectrometer. As shown in Fig. 4.4a the following species CO+; CQ^+; NCO+; HNCO+; H20+ were performed. The spectrum at the molecular-weight 43 is considered to belong to isocyanic acid HNCO. The spectrum was compared with the spectrum obtained when no HNCO was present in argon flow - no peak was achieved at molecular weight 43 (Fig. 4.4b). As shown in Fig. 4.5 the height of the spectra obtained by mass spectrometer decreases proportionally to the decrease of HNCO concentration in the carrier gas flow due to the reduction of argon flow through the gas wash bottle with liquid HNCO.

Fig. 4.4. Mass spectrogram (a) HNCO present in Ar flow and (b) without HNCO in Ar flow.

42 Fig. Measured pressure in MS

4.5. for m/e 43 (mbar*106 ) The rates QMS Carrier comparison

through 311 e

quadropole

gas the

of gas

the flow Time

wash

mass

height

(min) 43 bottle

(Ar)=1000 spectrometer

the containing

spectra

at ml/min

different

obtained liquid

HNCO.

Ar by

flow Balzer

4.4. Gas Analyzer Type 1301

This analyser is a Fourier Transform Infra Red (FTIR) spectrometer. Its detection principleis based on photoacoustic absorption using Brael & Kjaer's patented photoacoustic measurement chamber. The analyzer can measure almost any gas which is able to absorb infra-red light within the region 4000 cm-1 to 650 cm-1 (2.5 to 15.4 pm) [32]. Detection thresholds are gas dependant, but lie typically in the range of 0.1 to 10 ppm. The analyser has a built-in graphics screen. Absorption spectra, concentration curves or tables of concentrations can be displayed.

FTIR can be used for:

1. qualitative analysis measurements on site 2. unattended monitoring of gas concentrations for up to 7 gases simultaneously 3. industrial processes monitoring 4. industrial hygiene using air-quality measurements 5. detecting accidental gas releases 6. analysis measurements at land-fill sites 7. analysis measurements at hazardous-waste sites

Up to 100 different gases can be defined and stored in a catalogue in the internal memory. Gas Analyser Type 1301 can both analyse and monitor gases. These two types of measurement correspond to the two different measurement modes in the analyser; spectrum-measurement mode and concentration measurement mode.

44 4.4.1. The Measurement Principle ofFTIR

The schematic of the measurement chamber and pump system is shown in Fig. 4.6 [32]. The pump draws air from the sampling point through two air filters to flush out the "old" air in the measurement chamber and fill it with a "new" air sample. The new air sample is hermetically sealed in the analysis cell by closing the inlet and outlet valves. Broad band light from an infra red source is modulated to produce a time-varying signal by passing it through a modified Michelson Interferometer. The infra red light reflected from the interferometer output mirror enters the measurement chamber. There it is selectively absorbed by the gas sample, which causes the temperature of the gas to fluctuate. This produces corresponding pressure fluctuations within the chamber i.e. an acoustic signal. The frequencies of the acoustic signal depend on the wavelengths of the light absorbed by the gases present in the sample. An intensity detector placed in the chamber measures the variations in intensity of the infra red light. This produces the signal which is used for energy normalization of all measured spectra. The photoacoustic signal is measured by the two microphones mounted on the chamber walls to produce an electrical signal proportional to the amount of absorption that has occured. The electrical signal is then filtered and sampled and Fourier transformed to produce an absorption spectra. User-defined filter bands determine individual concentration values. These are obtained from the spectrum by intergrating between the chosen limits for the filter bands and applying a conversion factor. Air Outlet

Optical Window Micropnone 2 Air-snunt

Measurement / Champer

Outlet V Flush Valve \/^lua __

internal Fine Air-filter Inlet Valve t Sampling External Coarse Microphone 1 Tube ■ Fine Air-filter

infra-red rays from Interferometer

Fig. 4.6. Measurement chamber and pump system of FTlR.

45 4.4.2. The Basic Principles of Calibration and Measurements

Calibration

Before the concentration of a gas can be measured, the gas must be calibrated. This means that the numerical relationship between the measured photoacoustic signal from the measurement chamber and the actual concentration of the absorbing gas must be known to the analyser. The measurement used to determine the relationship is known as Span calbration. For the Gas Analyser Type 1301, there are the following two types of calibration [32]:

1. Zero-spectrum calibration 2. Span-calibration

Zero-spectrum calibration

A signal is always measured in the measurement chamber even when no absorbing gas is present. This signal is due to the noise created by the imperfect reflection of infra-red light from the chamber walls. This noise signal is measured during Zero-spectrum calibration . A supply of dry " zero gas " - one that does not absorb infra red light - is connected to the air-inlet of the analyser and measurements are made on samples of this gas. The absorption spectrum derived is termed on the Zero Spectrum. The zero spectrum is always substracted from the measured absorption spectra.

Span-calibration

The relationship between the measured photoacoustic signal and the concentration of the absorbing gas is known as the conversion factor for that gas. It is the factor that is evaluated during Span-calibration. During a Direct Span-calibration, a fixed and known concentration of gas is connected to the air inlet of the analyser and calibration measurements are made on samples of this gas. Before the Span-calibration, a Zero spectrum calibration must have been already made. The Zero-spectrum needs to be substracted from the absorption spectra measured during calibration (4.7).

46 Measured absorption Zero spectrum Absorption spectrum spectrum

Figi 4.7. How the analyzer processes data to produce an absorption spectra.

The most general rule to choose the concentration of a span gas is that it should be at least 100 times its detection limit. If a gas is measured outside the linaer range it is recommended to calibrate twice with the same gas. Once with a low concentration and once with a high concentration. During an Indirect Span-calibration, a Reference spectrum is loaded from disk as a substitute for a spectrum derived from an actual calibration measurement. Reference Spectrum from the Reference Data disk, which is supplied with the analyzer.

Measurements

The Type 1301 analyser has two ways in which it can measure [32]:

1. Concentration mode This mode is used when actual concentrations of individual gases are measured. It is the monitoring mode of the analyzer or quantitative-analysis function. 2. Spectrum mode This mode is used when the absorption spectrum of a gas sample is wanted to obtain in order to analyze its content No concentrations are produced. It is the qualitative-analysis function of the analyzer

The results are displayed in three different forms: 1. Absorption spectra 2. Table of Concentrations 3. "Time History" concentration curves

47 4.5. The Calibration of FTIRfor HNCO

The calibration of FTIR was made directly by connecting a gas supply consisting of HNCO diluted in argon flow to the air inlet of the analyser. At first the analyser was zero-spectrum calibrated. The calibration was made with pure argon. Fixed and known HNCO concentrations are needed for calibration of FTIR. The different HNCO concentrations in the carrier gas flow was obtained by varying argon flow through the gas wash bottle containing liquid HNCO kept at -36 °C. The flow rates of argon were controlled and measured with calibrated mass flow controllers Model 5850 E produced by Brooks Instrument V.B. HNCO concentration in argon flow was determined by absorbing in 0.1 M H2SO4, distillating and titrating the solution containing NH4+- ions with 0.05 M H2SO4. The concentration of NH3 in mol/1 was recalculated to the concentration of HNCO in ppm in argon flow in accordance with the equation 4.5. According to the results FTIR was calibrated in different HNCO concentrations. The results of FTIR calibration are shown in Fig. 8.7 - 8.11. Fig. 4.8 shows the amount of infra red light absorbed by HNCO as a function of HNCO concentration. The concentration of HNCO in argon flow is greatly temperature dependant. The increase of the temperature ca 4 °C (-36 °C —> -32 °C) is followed by higher concentration of HNCO in the carrier gas (see Table 4.4).

500 1000 1500 Concentration of HNCO (ppm)

Fig. 4.8. The calibration curves of HNCO □-^liquid HNCO was kept at -32° C A- liquid HNCO was kept at -36° C

48 Table 4.4. The comparison of HNCO concentrations in argon flow depending on the temperature.

At flow through the gas wash bottle HNCO concentration in ppm containing liquid HNCO (ml/min)

-36 °C -32 °C Absor- Absor- bance(from bance(from spectra) spectra)

5 11.19 131+-3 22.95 183 +-4

10 21.01 265+-6 38.14 365 +-7

20 36.42 537+-10 81.66 730 +-11

30 45.89 804+-17 116.1 1095 +-21

40 56.46 1072+-22 148.3 1460 +-26

49 4.6. The Infra Red Spectra ofHNCO

The spectra of HNCO synthesized according to the above described procedure was displayed as an aid to choose the filter limits in the Gas Set-Up screen. The upper and lower filter limits (Filter Limits A) were defined in the Gas Set-Up and the infra red absorption spectra was predicted within the frequency range 2270 cm-1 and 2245 cm* 1- The spectra of HNCO obtained by using Fi'lR is shown in Fig. 4.9. The main bands of HNCO with the double peaks typical to HNCO were identified. The pseudo-antisymmetric NCO stretch with the peak at 2255 cm-1 and 2285 cm* 1 is by far the strongest band in the spectra of HNCO (Fig. 4.9 A). The spectrum within the region about 3500 cm* 1 belongs to N-H bonding. The highest peaks in this frequency range were identified at 3543 cm* 1 and 3520.5 cm* 1 (Fig. 4.9 B). According to the literature data the pseudo-symmetric NCO stretch at 1320 cm* 1 is really weak and in our case no band was achieved but double peaks of HNCO at 786 cm* 1 and 685 cm* 1 were determined (Fig. 4.9 C)

1913-tl-22 12:48:19

Abt.

ISM

Fig. 4.9. The infra red absorption spectra of HNCO A - see Fig. 4.9 A B - see Fig. 4.9 B C - see Fig. 4.9 C

50 1933-11-22 12:99.53

Abe

Fig. 4.9 A

1993-11-22 i2 41:18

Abe

Lin

Fig. 4.9 B

1993-11-22 17 92:98

Abe

Lin

Fig. 4.9 C

51 The spectra obtained are well corresponding to the infra red spectra achieved by Denmark University of Technology shown in Fig. 4.10 [9].

Fig. 4.10. The infra red absorption spectra of HNCO obtained by Denmark University of Technology [9].

52 s.

The infra red spectrum of HNCO has previously been studied in the gas phase by Reid and Herzberg [13], Morgan and Lawson. Ashby and Werner have investigated the spectrum below 1000 cm-1. These workers' assignments are given in Table 4.5 [25].

Table 4.5. Assignments for HNCO.

n3 %4 %5 n6 (cn>l)

Reid and Herzberg 3531 2274 1327 797 572 670 Morgan and Lawson 3520 2271 1342 572 670 (797) Ashby and Werner 761 660 577 Dixon and Kirkby 660 577 777 HNCO in inert solvents 3473 2255 1321 785 680? 570? HNCO in diethylether complexes 3127 2249 1310 860 - 565?

HNCO / pyridine complex 2849 2244 1306 920 - 565?

The results obtained by using FTIR are also well corresponding to the data summarized in Table 4.5. The infra red absorption spectra of HNCO was compared with the spectra obtained by V.E. Bondybey [3] in order to find out if some other HCNO species can be found in the argon flow. In accordance with the infra red spectra of HCNO and HOCN (Fig. 4.11) no bands at the corresponding wavelengths were identified in the spectra obtained in our case and as follows no HCNO and HOCN exist in the carrier gas flow. R-NCO has a strong absorption of infra red light at ca 2250 cm-1 and 1360 cm-1 [9] but no bands appeared at 1360 cm-1. Accordingly the stretching frequency at about 2250 cm_ l is considered to belong to HNCO.

53 4000 Fig. ABSORBANCE

4.11.

4=± bands The Spectrum are [3].

(b) (C) (a) (a) 3500 v

\ due

strongest

HOCN HNCO Sections HCNO

HOCN

of HNCO. 3000

HNCO J

of — HCNO I ----

are the 1 —

Negative I —

labled. in matrix

absorption

2500 Ne I

11 matrix.

The 54 spectrum y going 2

unlabeled

2000 v bands The

bands x2 [cm

y -1

HNCO are 3

] HCNO

belong 1250

positive labled.

bands

in to =y=J=

Ne (b) HCNO. going 1000

are

matrix. Vibrational -t

labeled —

bands ^A-»A

4 .,

-V y

6 4 — y5 kA- 500 5. Conclusions

Isocyanic acid HNCO was prepared by the thermal decomposition of the cyclic trimer of cyanuric acid CgNgCOH^. The solid cyanuric acid tends to sublime without reaction and in order to obtain almost complete depolymerization to HNCO it has to be volatilized slowly. The concentration of HNCO in the gas phase depends on the temperature of the liquid phase. Liquid HNCO is transferred to the Ar gas flow by passing Ar over the liquid surface. Thus, it is important that the flow of an inert gas as well as the temperature of liquid HNCO is kept at a constant and known level during the calibration. The concentration of HNCO in the gas flow was determined by absorption of HNCO in H2SO4. The absorption effeciency of the solution depends on the concentration of H2SO4, the experimental set-up and must be determined in every occasion. Liquid HNCO can be stored at -80 °C maximum 1 week due to its < rapid trimerization to (HNCO)3.

55 6. Acknowledgements

For financial support I would like to thank the Nordic Council of Ministers, which made it possible for me to perform my project work during 4 month in Sweden.

I am sincerely grateful to Professor Oliver Lindqvist, for introduc ing me to the field of N%0 and HNCO chemistry.

I also wish to thank the following persons:

Ove Nilsson, my supervisor, for patiently listening to my 'little' problems and always having good advices. Miroslawa Abul-Milh, thanks to whose optimism all the days in the laboratory were full of sunshine. Dan Stromberg, Britt-Marie Steenari, Birgitta Olanders, Hakan Kassman, Heije Miettinen, for good advices and help during my studies and all the others at the Department of Inorganic Chemistry who have helped and encouraged me.

At last, I would like to thank my parents, Juta and Ulo Tarno, and my husband, Reimo Riiutelmann, in Tallinn, for their support during my studies at Chalmers University of Technology, far from home.

56 7. References

1. Abul-Milh, M„ Miettinen, H., Nilsson, 0., Lindquist, 0., "Heterogena N2O reaktioner, FBC", Institution for Oorganisk Kemi, Goteborgs Universitet och Chalmers Tekniska Hogskola, Goteborg, 1989. 2. Belson,D.J., "Preparation and Properties of Isocyanic Asid", Chem. Soc. Rev., 1982,11,41. 3. Bondybey, V.E., "Infrared Spectra and Isomerization of CHNO Species in Rare Gas Matrices", J. Mol. Spectrosc., 1982,92,431. 4. Carlsson, M., "Lustgasbildning vid termisk reduktion av NOx med urea -Etapp 2A och en litteraturstudie-Etapp 1", Stiftelsen for Varmeteknisk Forskning, Box 6405, 11382 Stockholm, Juli 1992. 5 . Caton, J.A., Siebers, D.L.," Removal of Nitric Oxide from Exhaust Gas with Cyanuric Acid", Combustion and Flame 79: 31-46,1990. 6. Caton, J.A., Siebers, D.L.," Comparison of Nitric Oxide Removal by Cyanuric Acid and by Ammonia", Combust. Sci. and Tech., 1989, 65, p 277-293. 7. Chapman, S., "A theory of upper-atmosphere ozone", Mem. Roy Meteorol. Soc, 1930,3,103. 8. Cooper, DA., "Some Aspects of NOx Control in Fluidized Bed Combustion", Thesis, Chalmers University of Technology and University of Goteborg, 1989. 9. Dam -Johansen, K., Christensen, P.G., Jensen,S. H., "Resultat af litteratutsogning og forsog, Fremstilling og m&ling av isocyansyre (HNCO)", Institut for Kemiteknik, Byg. 229 DTH, Lyngby, Denmark. 10. Dawson, P.H., "Quadropole mass spectrometry and its applications", Elsevier, Amsterdam, 1976. 11. Finlayson-Pitts, B.I. and Pitts, J. N. Jr, "Atmospheric Chemistry", John Wiley & Sons, New York, 1986. 12. Ghardashkhani, S., "The Chemistry Behind the Sulphur Dioxide Removal by Calcium Oxide in Fluidised Bed Combustion", Department of Inorganic Chemistry, Chalmers University of Technology and University of Goteborg, 1991. 13. Herzberg, G. and Reid, C., "Infra-red Spectrum and Structure of the HNCO Molecule", Discuss. Faraday Soc. 1950,9, 92. 14. Hulgaard, T., "Nitrous Oxide from Combustion", Ph. D. thesis, Department of Chemical Engineering, Technical University of Denmark, Lyngby, Denmark, 1991. 15. Khalil,M.A.K. and Rasmussen, RA., "Increase and seasonal cycles of nitrous oxide in the earth's atmosphere",Tellus,1983, 35B, p. 161-169.

57 16. Kilpinen, P., Hupa, M„ "Gas Phase Formation and Destruction of N2O - A short review", 5th International Workshop on Nitrous Oxide Emissions, Tsukuba, Japan, July 1-3,1992. 17. Kilpinen, P., Hupa, M., "Homogeneous N2O Chemistry at Huidised Bed Combustion Conditions: A Kinetic Modelling Study", Combustion and Flame 85:94-104,1991. 18. Knol, K. E. Bramer, E. A.and Valk, M„ "Reduction of nitrogen oxides by injection of urea in the freeboard of a pilot scale fluidised bed combustor", Department of Thermal Engineering, Twente University of Technology, PO Box 217,7500 AE Enschede, the Netherlands, 1989. 19. Kramlich, J.C., Cole, JA., McCarthy, J.M., and Lanier, Wm. S., "Mechanisms of Nitons Oxide Formation in Coal Flames", Combustion and Flame 77: 375-384,1989. 20. Kulish, O.N., Zaslonko I. S., "Noncatalytic Technology of NOx Reduction in Flue Gases", The Coal Research Forum, Imperial College, London, 20-22nd April,1993. 21. Levin, J. in EPA/ IFP European Workshop on the Emissions of Nitrous Oxide from Fossil Fuel Combustion, EPA-600/ 9-89-089, October 1989, 22. Levin, J. in LNETI/EPA/IFP European Workshop on the Emissions of Nitrous Oxide (1990), Lisbon, Portugal, June 1990, p. 1-256. 23. Macintyre, J.E., Dictionary of Inorganic Compounds, Chapman & Hall, 1992,1, Ac - CIO, p. 416-417. 24. Miettinen, H., Stromberg, D., and Lindquist, O., "The Influence of Some Oxide and Sulphate Surfaces on N2O Decomposition", 11th International Conference on Fluidised Bed Combustion, Montreal, Canada, April 21-24,1991. 25. Nelson, /., "Hydrogen-bonded complexes of iso-cyanic acid: Infrared spectra and thermodynamic measurements", Spectrochim. Acta, 1970,26, p. 109-120. 26. Nielsen, C., Jodal, M., "A Comparative Study of Ammonia and Urea as Reductants in Selective Non-Catalytic Reduction of Nitric Oxide", ACHEMASIA'89.11-17 October, Beijing. 27. Nordin, R., "Elementar Kemisk Laboratorieteknik", OKP - litteratur, Vreta Kloster 1989, p. 145. 28. Pearman, G. Etheridge, D., de Silva, F., and Fraser, PI., "Evidence of changing concentrations of atmospheric air bubbles in Antarctic ice", Nature, 1986,320, p. 248-250. 29. Puromaki, K., Analysrapport Nr. 155, hnstitutionen for Oorganisk Kemi, Chalmers Tekniska Hdgskola och Gotegorgs Universitet, Juni 1992. 30. Smart, J.P., Maalman, T., "Analytical procedure for the quantitative determination of NH3 and HCN in combustion systems", Jmuiden, IFRF Doc. No. F 72/a/16, October 1987.

58 31. de Soete, G.G., "The role of NO and N%0 formation/destruction chemistry in coal combustion control techniques", The Coal Research Forum, Imperial College, London, 20-22nd April,1993. 32. Technical Documentation of Gas Analyser Type 1301, Bruel & Kjaer, November 1992. 33. Amand, L-E., Leckner, B., and Andersson, S., "Formation of N2O in Circulating Fluidised Bed Boilers", Department of Energy Conversion, Chalmers University of Technology, S-41296 Goteborg, Sweden, 1991. 34. Vogel, A., "Quantitative Inorganic Analysis", 1962, p. 254-256. 35. Wayne, R.P., Chemistry of atmospheres", Clarendon, Press, Oxford, 1986, p. 126. 36. Weast, R.C., "CRC Handbook of Chemistry and Physics", 60 th Edition (1979-1980), CRC Press, Inc. Boca Raton, Florida 33431, p. C-252. 37. Wojtowicz, M.A., Pels, J.R., Moulijn, JA., "The Trade-off Between NOx and N2O Formation in Fluidised-Bed Combustion of Coal", The Coal Research Forum, Imperial College, London, 20-22nd April,1993.

59 8. Appendix 1993-11-26 10:56:14

X : 2255.0 Y : 15.94E+00 Abs , 20. 1E+00 zxX : ^Y :

Lin

Fig. 8.1. The infra red absorption spectra of HNCO after absorption in dest.HzO.

f- 1993-11-26 10:50:38

X : 2255.0 V : 10. BGE+00 Rbs, 18.7E+00 : zxY :

Lin to

/cm

Fig. 8.2. The infra red absorption spectra ofHNCO after absorption in 0.1 mM H2SO4. 1993-11-26 16:02:41

X : 2255.0 V : 8.380E+00 fibs. 20.2E+00 /\X : vs.Y :

20 ml/minflr+HNCO efter0. ImMHCl

Lin 10

8 5

-966E-03 I 'I I II 4000.0 3500 3000 2500 2000 1500 1000 /cm

Fig. 8.3. The infra red absorption spectra of HNCO after absorption in O.lmMHCl. 1993-11-26 11:03:53

X 2255.0 Y : 8.401E+00

fibs 20.0E+00 > X :

20 ml/mlnAr+HNCO oftbrlmMH2SO4+MgS04

15-

Lin 10- 2

-961E-03 i i i i i i i {' 11 mil ii| 1111 in i ii| i nil i ii i| i ii iTTrrrri i 4000.0 3500 3000 • 2500 2000 1500 1000 /cm

Fig. 8.4. The infra red absorption spectra of HNCO after absorption in 1 mM H2SO4 + MgSC>4. 1993-11-26 11:09:67

X : 2255.0 Y : S.396E+00 fibs 35.0E+00 A.X : a Y :

Lin Si

/cm

Fig. 8.5. The infra red absorption spectra of HNCO after absorption in O.IMH2SO4. 1993-11—2G 16:14:00

X 2255.0 Y 3.975E+00 fibs, 37.1E+00 zxX zxY

Lin

/cm

Fig. 8.6. The infra red absorption spectra of HNCO after absorption in 1 M H2SO4. 1993-11-22 12:55:39

X : 2255.0 y : 11.19E+00 fibs 13.5E+00 : z\Y :

Lin

/cm

Fig. 8.7. The infra red absorption spectra of HNCO - 131ppm. 1993-11-22 13:01:33

21. 01E+00 fibs 25.5E+00

HNCO 26p ppm

Lin

-1 .27E+00 rnTTTTTTrTTTTTrrrrpr 3500 3000 2500 2000 1500 1000 /cm

Fig. 8.8. The infra red absorption spectra of HNCO - 265 ppm. 1993-11-22 11:03:20

X : 2255.0 Y : 3B.42E+00 fibs 44.GE+00 : aY : HNCO 53? ppm 40-

30-

Lin 20-

10-

-2.19E+00 I I I I" I I in 11 itt 1111111 ri 1111 i"H i "i 1 111 n | i'i 11 rn 1111111 1 m 11 1 ' n 11 1 " 4000.0 3500 3000 2500 2000 1500 1000 /cm

Fig. 8.9. The infra red absorption spectra of HNCO - 537 ppm. 1993-11-22 13:11:37

X : 2255.0 Y : 45.89E+00 Abs. 56.1E+00 zxX : /xY :

Lin

/cm

Fig. 8.10. The infra red absorption spectra of HNCO - 804 ppm 1993-11-22 11:09:25 X

2255.0 V : 56.46E+00 X fibs. 70.0E+00 < z^Y : HNCO 1072 ppm

60-

40- Lin

20-

------AAJ -3.42E+00 tVTT i i I i i rr ii lini TTTHTTTTT II I I'll I I I'TTT H'l I I IT ITITI I I I I 4000.0 3500 3000 2500 2000 1500 1000 /cm

Fig. 8.11. The infra red absorption spectra of HNCO -1072 ppm.