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O Formation in Combustion. the Synthesis and Analysis of HNCO I The importance of HNCO as a precursor of N20 formation in combustion. The synthesis and analysis of HNCO Margit Ruutelmann aSTRBUHON OF THIS DOCUMENT IS UNLIMITED --------------------------------------------------------------------------------------------- ------ ---------------------- ■ NUTEK, 117 86 Stockholm, Tel 08-744 95 00 (Fran 1 november 1992 nytt nummer: 08-681 91 00) Rapporterna kan besfallas (ran Studsviksbiblioteket, 611 82 Nykoping. Tel 0155-22 10 90, 22 10 00, Fax 0155-26 30 44 Narings- och teknikutvecklingsverket Titel: The importance of HNCO as a Precursor of NzO Formation in Combustion The Synthesis and Analysis of HNCO Forfattare: Margit Ruutelmann, CTH RAPPORT INOM OMRADET FORBRANNING OCH FORGASNING > . * Rapportnummer: FBT-95/9 Projektledare: Oliver Lindqvist Projektnummer: P276 358-4 och P1282-1 Kemiska forlopp FBC CTH Projekthandlaggare pa NUTEK: Rolf Ingman « Pastadress Besoksadress Telefoti Telefax Telex 117 86 Stockholm Liljeholmsvagen 32 08-681 91 00 08-19 68 26 10840 nutek s ISSN 0283-8575- Report OOK 93:03 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.
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