Nitrogen compounds in pressurised fluidised bed gasification of biomass and fossil fuels

Nitrogen compounds in pressurised fluidised bed

gasification of biomass and fossil fuels

Nitrogen compounds in pressurised fluidised bed

gasification of biomass and fossil fuels

Proefschrift

ter verkrijging van de graad van doctor

aan de Technische Universiteit Delft,

op gezag van de Rector Magnificus prof. dr. ir. J.T. Fokkema,

voorzitter van het College voor Promoties,

in het openbaar te verdedigen op maandag 7 februari 2005 om 15:30 uur

door

Wiebren DE JONG

scheikundig ingenieur

Dit proefschrift is goedgekeurd door de promotoren:

Prof. Dr. –Ing. K.R.G. Hein

Prof. dr. J.A. Moulijn

Samenstelling promotiecommissie:

Rector

Magnificus,

voorzitter

Prof. Dr. –Ing. K.R.G. Hein,

Technische Universiteit Delft, promotor

Prof. dr. J.A. Moulijn

Technische Universiteit Delft, co-promotor

Prof. Dr. –Ing. H. Spliethoff

Technische Universiteit Delft

Prof. dr. ir. P.J. Jansens

Technische Universiteit Delft

Prof. dr. ir. C. Daey Ouwens

Technische Universiteit Eindhoven

Prof. dr. M. Hupa

Å

bo Akademi University, Finland

Dr. ir. P.D.J. Hoppesteyn

Corus, IJmuiden

Drs. J. Andries heeft als begeleider in belangrijke mate aan de totstandkoming van het

proefschrift bijgedragen.

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Prediker 3:1.

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Ecclesiastes 3:1.

Voor:

Heit en mem

Nitrogen compounds in pressurised fluidised bed gasification of biomass and fossil fuels

Summary

This PhD thesis assesses the experimental and theoretical work which was performed to study the behaviour of nitrogen compounds during airblown pressurised stationary fluidised bed gasification of biomass and brown coal, followed by high temperature ceramic gas filtration.

Fossil fuels have dominated the energy supply in modern societies and will continue to do so in the 21st century. The resources, however, are depleting, especially of oil and natural gas. Therefore, other energy sources are to be exploited further within this century. Biomass is one of the almost CO2 neutral, renewable contributors to the future energy production. Nowadays many modern, high efficiency (combined) power and heat producing systems using biomass are or become commercially available. One promising route to efficient power and heat supply is the Integrated Gasification Combined Cycle. This cycle is particularly of interest for medium to larger scale installations. Pressurised operation of the gasifier offers the advantage of smaller process equipment, including that for the necessary downstream gas cleaning. Also, the work needed to compress the gas for gasturbine use will be much smaller or not needed at all.

High temperature gas filtration offers the benefit of increased overall efficiencies of the power and heat producing cycle. This integrated gasification technology, however, is still in a stage of development and demonstration.

When instead of absorption gas cleaning, high temperature, dry gas filtration is applied, nitrogen compounds, like ammonia (NH3) and hydrogen cyanide (HCN), are not dissolved in the absorption liquid. As a result NOx emissions in gas turbine combustors are produced. NOx is known for its negative effects on the health of humans and animals and acidification of soil and water. Therefore, increasingly stringent emission restrictions are imposed on this component. Both coal and biomass contain nitrogen in their chemical structure and in gasification processes this so-called fuel bound nitrogen is converted to a large extent into NOx precursors.

Although woody biomass contains low amounts of nitrogen (only a few tens of mass percentage on dry fuel basis), there is still a high emission potential based on the fuel's energy content, because of the low calorific value of the fuel, as compared to coal. Thus biomass causes significant NOx emissions when no further measures are taken.

An introduction regarding the use of biomass in energy production, the potential of NOx emissions of a range of young and old fuels and open research questions is given in chapter 1.

A literature overview is presented in chapter 2. A state-of-the-art review of the fluidised bed gasification activities is given. Also, an overview of the modelling of fluidised bed gasification on large and small scale, with emphasis on the emission of NOx precursors is presented. The influence of fuels, additives and process parameters on the release of these compounds is addressed. Both the primary and later stages of the conversion of solid fuels are considered.

In chapter 3 the experimental facilities used to study the thermal conversion behaviour of fuel bound nitrogen are presented. These facilities can be divided into two categories:

1) Pilot scale test rigs; a 1.5 MWthermal Pressurised Fluidised Bed Gasifier situated at the section Energy Technology of Delft University of Technology and a similar 50 kWthermal test rig available at the Institute of Process Engineering and Power Plant Technology of Stuttgart University (Germany).

2) Characterisation equipment to study solid fuel reactivity, especially in the early stages of conversion in gasification processes, namely pyrolysis. Here, two techniques are used representing slow and rapid heating conditions, respectively: TG-FTIR (thermogravimetric analysis coupled to FTIR), situated at AFR inc. (Hartford, CT, USA) and heated grid reactor equipped with in-situ IR diode laser diagnostics, available at the Technical Physics department of Eindhoven University of Technology.

In chapter 4 the experimental results obtained by using the facilities described above are presented and discussed. In the pressurised fluidised bed gasification tests performed at Delft university, miscanthus and wood pellets have been used. Brown coal has been chosen because it is an older, but still comparatively high-volatile, fossil fuel. No significant radial gradients in the concentration of the main and minor gaseous product constituents were observed. The concentrations of the main gasification product gas components were comparable to the limited open literature data, available from other pressurised fluidised bed test rigs at comparable air stoichiometry values.

Axial gradients in the gas concentrations during the pressurised fluidised bed gasification tests could be clearly observed for acetylene, which is related to reactions involving tar and soot precursor formation and destruction.

Under the pressurised fluidised bed gasification conditions studied, the main fuel bound nitrogen component produced is NH3, whereas HCN is formed to a minor extent (only a few percent of the fuel bound nitrogen). This was comparable with results from other bottom-fed FB gasifiers. On the other hand, comparatively low values have been found for a top-fed pressurised FB. HNCO and NO were never detected by means of even a high resolution FTIR spectrophotometer under the pressurised gasification test conditions studied.

Tests with Ca-containing dolomite and a Ca-less additive (MinPhyl, or Pyrophyllite) under otherwise comparable process conditions showed that an increased Ca inventory in the gasifier increases the NH3/HCN ratio significantly.

To obtain basic model input data, flash pyrolysis experiments with miscanthus were conducted using the heated grid reactor set up. This research was focused on measuring the yield of CO, CO2 and NH3 at a heating rate of 280-320 K/s and final temperatures in the range of 1050-1400K.

Unfortunately, NH3 could not be detected, due to condensation or the limited frequency range that could be achieved with the tuneable laser. CO and CO2 yields have been measured in-situ and were compared with the FG-DVC biomass pyrolysis model.

For all fuels used, kinetic parameters for this pyrolysis model have been determined by application of the Tmax method, using a TG-FTIR system with heating rates of 10, 30 and 100 K/min. However, this model does not predict the pyrolysis product yield satisfactorily at high heating rates, based on the kinetics determined by low heating rates. It gave a reasonable quantitative yield prediction for CO but a substantial under-prediction for CO2. The competition between the evolution of primary products like primary tar fragments and carboxylic acids on one side and light gases like CO, CO2 and H2O on the other side is probably the reason. This competition is expected to be heating rate dependent. Apparently the primary pyrolysis products like tars and carboxylic acids, which contain precursor groups for the formation of CO and CO2, are quickly removed from the reaction zone and quenched immediately. Thus, no time is available for further primary tar and carboxylic acid decomposition into CO2 and CO (to a minor extent), which results in low yields. According to this hypothesis the yields of primary tar and carboxylic acids must be significant. This is confirmed by the observation that under pyrolysis conditions the tar yield increases at increasing heating rates.

In chapter 5 the modelling of pressurised fluidised bed gasification is described and the influence of process parameters is assessed. The model is a steady state plug flow-in-series model with detailed reaction kinetics. Heterogeneous char oxidation and gasification (by H2O, CO2 and H2), heterogeneously catalysed HCN hydrolysis and homogeneous reactions (including nitrogen molecular and radical species and a simplified tar cracking reaction) are taken into account.

The possibility to increase the conversion of NH3 into N2 by varying the process conditions or by adding specific compounds to the gasifier has been studied theoretically. However, NH3 is found to be a very stable compound, which is hardly converted in pressurised fluidised bed gasifiers.

Decomposition into N2 is slightly increased by increasing temperature, but this option is limited due to the risk of bed sintering for alkali containing biomass fuels. The NH3 conversion is only slightly dependent on reactor pressure. A minimum conversion was obtained around 2.5 bar. At higher pressures (10 bar) the NH3 conversion slightly increased. Gas residence time in the reactor did not affect the fuel-N conversion. The destruction of NH3 is only taking place in the presence of O, H and OH radicals, which are consumed very fast in the initial part of the bed.

Larger NH3 conversions can be reached by injecting NO or NO2 into the bed. However, HCN is

formed and NO conversion is limited, which leads to undesired emissions. Addition of O2 favours NH3 conversion. However, the main nitrogen species formed was NO rather than N2. In the most favourable case, at near-stoichiometric conditions, NH3 is converted for 50% into NO and 50% into N2. Moreover, the addition of secondary air decreases the already low LCV gas heating value. The

presence of a high concentration of CH4 in the bed part of the gasifier reduces the NH3 conversion,

probably due to the competition for radicals between CH4, its intermediates (mainly CH3 radicals) and NH3. H2O2 and H2O (steam) addition into the bed did not affect at all NH3 conversion.

A comparison between the model based simulations and the experiments on the 1.5 MWth Delft University pressurised fluidised bed gasification scale and the 50 kWth Stuttgart University rig is presented in chapter 6.

The agreement for N-species prediction and measurements is quite good for the fuels and more in particular for the main fuel bound nitrogen component in the product gas: NH3.

For HCN the concentrations are often underpredicted, probably due to the heterogeneous hydrolysis reaction, taking place at the char surface, which can have slower kinetics than assumed in literature. The model predicts the formation of super-ppmv HNCO and NO concentrations, but they have never been detected by FTIR analysis in our work. Probably, catalytic hydrolysis converts HNCO into NH3 and CO and this reaction has not been considered in the model. For NO, catalytic reduction by ash constituents probably plays a role at the gasification temperatures. It is also possible that neglection of S and Cl chemistry causes deviations between model and experimental results for the minor species. This is less the case for wood. Also, simplification of tar- and char-nitrogen reactions can be significant: the model assumes that the nitrogen which is not available as gas species will be released initially in the form of HCN.

The agreement between model and experimental results for the main product gases is reasonably good. The differences between the calculated and measured values can be attributed to pyrolysis yields of H2. These were obtained from a correlation of literature data and as such used as input in the biomass pyrolysis sub-model. Also, the use of simplified tar cracking kinetics, in terms of possible reactions, product yields and rates probably plays a major role in the deviations observed for CO and CO2 concentrations. The deviations are the highest for wood, and less for miscanthus and brown coal. This is in-line with the hypothesis that the simplified tar cracking kinetics plays a major role, as of the tested fuels wood pyrolysis shows the highest initial tar yields. Differences in the heterogeneous combustion and gasification reactions are not expected to have a major impact, as the carbon conversions are quite well predicted.

Finally, chapter 7 gives an overview of the conclusions and the recommendations specified for pressurised fluidised bed experimental pilot scale research, fuel characterisation and modelling.

Stikstofcomponenten in drukwervelbedvergassing van biomassa en fossiele brandstoffen

Samenvatting

Dit proefschrift behandelt het experimentele en theoretische werk dat is uitgevoerd in het kader van een studie naar het lot van de stikstof componenten tijdens drukwervelbedvergassing in een stationair wervelbed in combinatie met hoge temperatuur filtratie, gebruikmakend van keramische filters. Fossiele brandstoffen hebben de energievoorziening van de moderne maatschappij bepaald en zullen dit blijven doen in de 21e _{eeuw. De voorraden raken echter uitgeput, in het bijzonder die van olie en} aardgas. Daarom moeten andere energiebronnen in deze eeuw worden aangeboord. Biomassa is een van de bijna CO2 neutrale, hernieuwbare brandstoffen voor de toekomstige energievoorziening. Tegenwoordig zijn er al commerciële systemen op de markt voor (gecombineerde) warmte en kracht productie met een hoog rendement op basis van biomassa. Een veelbelovende optie voor efficiënte elektriciteit- en warmtevoorziening is het gecombineerde vergasser – STEG systeem. Dit systeem is in het bijzonder van belang voor middelgrote tot grote installaties. Het onder druk bedrijven van de vergasser biedt het voordeel van kleinere proces apparatuur, inclusief de downstream gasreiniging. Compressie van het geproduceerde gas, hetgeen nodig is voor het bedrijven van moderne gasturbines, zal in dit geval niet of in mindere mate nodig zijn. Hoge temperatuur gasreiniging door middel van bijvoorbeeld keramische filters biedt het voordeel van hogere efficiënties van het warmte en elektriciteit producerende systeem. Deze geïntegreerde vergassingstechnologie, echter, bevindt zich nog steeds in het stadium van ontwikkeling en demonstratie.

Bij toepassing van hoge-temperatuur en dus droge gas filtratie in plaats van natte absorptie technieken zullen stikstofcomponenten, zoals ammoniak (NH3) en waterstofcyanide (HCN), niet oplossen in de absorptievloeistof en zullen NOx vormen in gasturbine verbrandingskamers. NOx heeft negatieve effecten op de gezondheid van mens en dier en veroorzaakt verzuring van de grond en het oppervlaktewater. Daarom worden wereldwijd steeds stringentere emissie eisen opgelegd voor deze emissiecomponent. Zowel kolen als biomassa bevatten chemisch gebonden stikstof en het is deze brandstofgebonden stikstof die in vergassingsprocessen voor een groot deel wordt omgezet in NOx precursors.

Hoewel houtachtige biomassa lage gehaltes aan stikstof vertoont (slechts enkele tiendes massa-procenten op droge basis), is er in vergelijking met kolen toch een hoog emissiepotentieel als de lage stookwaarde van de brandstof in ogenschouw wordt genomen. Daarom draagt biomassa significant bij tot NOx emissies wanneer er geen proces gerelateerde maatregelen worden genomen.

In hoofdstuk 1 van dit proefschrift wordt een inleiding gegeven in de toepassing van biomassa in de energievoorziening, potentiële NOx emissies van een reeks jonge tot oude brandstoffen, alsmede open onderzoeksvragen op dit gebied.

In hoofdstuk 2 wordt een literatuuroverzicht geboden. Hierin wordt een “state-of-the-art” overzicht gegeven van de wervelbedvergassingsactiviteiten op zowel kleine als grote schaal en wordt een overzicht gepresenteerd van de modellering van wervelbedvergassing. De nadruk wordt gelegd op de emissie van NOx precursors, zowel in de primaire als ook de latere stadia van de conversie van vaste brandstoffen. De invloed van verschillende brandstoffen, additieven en proces gerelateerde parameters op het vrijkomen van deze componenten wordt behandeld.

Hoofdstuk 3 geeft een overzicht van de toegepaste experimentele technieken de gebruikt zijn voor de studie naar het thermische conversiegedrag van brandstofgebonden stikstof. Deze kunnen worden onderverdeeld in twee categorieën:

1) Proefopstellingen op pilot schaal: de 1.5 MWthermisch drukwervelbedvergassings-opstelling in het laboratorium van de sectie Energy Technology van de Technische Universiteit Delft, en de 50 kWthermisch opstelling die beschikbaar is bij het “Institut für Verfahrenstechnik und Dampfkesselwesen (IVD)” van de Universiteit Stuttgart (Duitsland). 2) Fundamentele karakteriseringsapparatuur voor bestudering van de vaste brandstofreaktiviteit, in het bijzonder in de vroege stadia van conversie in het vergassingsproces, namelijk de pyrolyse. Hier worden twee technieken besproken die langzame en snelle verhittingscondities vertegenwoordigen: respectievelijk TG-FTIR (thermogravimetrische analysie gekoppeld met FTIR), bij AFR Inc. (Hartford, CT, USA) en de heated grid reactor voorzien van in-situ IR diode laser diagnostiek, bij de afdeling Technische Natuurkunde van de Technische Universiteit Eindhoven.

In hoofdstuk 4 worden de experimentele resultaten gepresenteerd en besproken. In de Delftse drukwervelbed experimenten zijn miscanthus- en houtpellets als biobrandstoffen gebruikt. Bruinkool is geselecteerd omdat het een oudere, maar toch nog hoog-vluchtige fossiele brandstof is. Er is geen significante gradiënt in het radiaal concentratieprofiel van hoofd- en sporecomponenten waargenomen in de vergasser. De concentraties van de hoofdcomponenten van het productgas waren vergelijkbaar met de beperkte gemeten open literatuurdata van andere drukwervelbed opstellingen bij vergelijkbare stoichiometrie. In de axiale gas concentratie profielen tijdens de Delftse drukwervelbed experimenten werden duidelijke gradiënten waargenomen voor acetyleen, een component welke gerelateerd is aan reakties die teer en roet-precursor vorming en afbraak betreffen.

Bij de toegepaste drukwervelbed vergassingscondities is de belangrijkste brandstofgebonden stikstof component NH3, terwijl HCN in mindere mate wordt gevormd (enkele procenten van de vaste brandstofgebonden stikstof). De conversie naar NH3 en HCN was vergelijkbaar met andere FB vergassers met bodemvoeding, in contrast met de relatief lage waardes die werden waargenomen voor een drukwervelbed met topvoeding. HNCO and NO zijn nooit gedetecteerd, zelfs niet met een hoge-resolutie FTIR spectrofotometer, onder de bestudeerde drukvergassingscondities.

Experimenten uitgevoerd met Ca-houdend dolomiet en een Ca-loos additief (MinPhyl, of Pyrophylliet) onder vergelijkbare procescondities toonden aan dat een verhoogd Ca aanbod in de vergasser leidt tot een significante toename in de NH3/HCN verhouding.

Ter verkrijging van de basis model inputgegevens, zijn heated grid flash pyrolyse experimenten uitgevoerd met miscanthus. Dit deelonderzoek was gericht op het bepalen van opbrengst van CO, CO2 and NH3 als functie van de temperatuur in de range 1050-1400K en opwarmsnelheden van 280-320K/s.

NH3 kon helaas niet worden gedetecteerd, hetgeen te wijten was aan ofwel condensatie of het beperkte frequentie gebied dat kon worden ingesteld met de tuneable laser. De CO en CO2 opbrengsten zijn in-situ bepaald en vergeleken met de FG-DVC biomassa pyrolyse model uitkomsten.

Voor alle toegepaste brandstoffen zijn de kinetische parameters bepaald voor dit pyrolyse model door toepassing van de Tmax methode, uitgaande van TG-FTIR metingen bij verhittingssnelheden van 10, 30 en 100 K/min. De extrapolatie die dit model gebruikt om de pyrolyse productopbrengst bij hoge opwarmsnelheden te voorspellen gebaseerd op de kinetiek parameters die verkregen zijn bij lage opwarmsnelheden, levert een wisselend beeld op. Het resulteert in een redelijk correcte kwantitatieve opbrengstvoorspelling voor CO en een duidelijk te lage voorspelling voor CO2. De competitie tussen het vrijkomen van primaire producten als primaire teerfragmenten en carboxylzuren enerzijds en lichte gassen als CO, CO2 en H2O anderzijds is waarschijnlijk de oorzaak. Deze competitie is waarschijnlijk afhankelijk van de opwarmsnelheid. De primaire pyrolyse producten als teren en carboxylzuren, die precursor groepen bevatten voor CO en CO2 vorming, worden blijkbaar snel uit de reaktiezone verwijderd en direct afgekoeld. Er is dan dus geen tijd beschikbaar voor verdere ontleding in CO2 en (in mindere mate) CO, wat resulteert in lage productopbrengsten.

Volgens deze hypothese moeten de opbrengsten van primaire teren en carboxylzuren significant zijn. Dit wordt bevestigd door de waarneming dat voor pyrolyse bij toenemende opwarmsnelheden de teer opbrengst toeneemt.

In hoofdstuk 5 wordt de modellering van drukwervelbedvergassing behandeld. Hier wordt het model beschreven en de invloed van proces parameters bestudeerd. Het model is een stationair propstroomreactor-in-serie model met gedetaileerde reactiekinetiek. Heterogene kool-residue oxidatie en vergassing (door H2O, CO2 en H2), heterogeen gekatalyseerde HCN hydrolyse en homogene reacties (met inbegrip van stikstofhoudende moleculen en radicalen en een vereenvoudigde teer kraakreactie) worden in het model meegenomen.

De mogelijkheid om NH3 conversie in N2 te vergroten door procescondities te variëren of door toevoeging van specifieke componenten aan de vergasser is theoretisch bestudeerd. Het blijkt dat NH3 een erg stabiele component is, welke nauwelijks kan worden omgezet in drukwervelbedvergassers. Decompositie in N2 neemt iets toe door verhoging van de temperatuur, hoewel deze optie beperkt is door het risico van bed sintering voor alkali-houdende biobrandstoffen. De NH3 conversie is slechts in beperkte mate afhankelijk van de druk. Een minimum in de conversie is gevonden bij 2.5 bar. Bij hogere drukken (10 bar) neemt de NH3 conversie licht toe. De gas verblijftijd in de reaktor heeft praktisch geen invloed op de brandstof-N conversie. Afbraak van NH3 vindt alleen plaats in de aanwezigheid van O, H en OH radicalen, die erg snel reageren in de initiële bedzone.

Hogere NH3 conversies kunnen worden gerealiseerd door injectie van NO of NO2 in het bed.

Anderzijds wordt dan HCN gevormd en wordt ongereageerd NO voorspeld, die beide ongewenste emissiecomponenten zijn. Additie van O2 bevordert de NH3 conversie. De belangrijkste stikstofcomponent wordt dan echter NO en in het meest ideale geval, bij praktisch stoichiometrische condities, wordt de brandstofgebonden N voor 50% in NO omgezet en voor 50% in N2. Toevoegen van secundaire lucht verlaagt de toch al lage LCV gas stookwaarde. Aanwezigheid van een hoge

concentratie CH4 in de bedzone van de vergasser reduceert de NH3 conversie, waarschijnlijk door de

competitie voor radikalen tussen CH4, haar intermediaire omzettingsproducten (voornamelijk CH3 radikalen) en NH3. H2O2 and H2O (stoom) additie in het bed heeft geen invloed op de NH3 conversie. Een vergelijking tussen de met behulp van het in hoofdstuk 5 beschreven model uitgevoerde simulaties en de experimenten uitgevoerd op de 1.5 MWth drukwervelbed schaal en de 50 kWth schaal wordt in hoofdstuk 6 gepresenteerd.

De overeenkomst tussen berekende en gemeten waarden is voor de stikstofcomponenten tamelijk goed voor de brandstoffen en in het bijzonder voor de belangrijkste brandstofgebonden stikstofcomponent: NH3. Voor HCN wordt de concentratie vaak te laag voorspeld, mogelijk door de heterogeen gekatalyseerd hydrolyse reaktie, welke plaatsvindt op het kool-residue oppervlak en die een langzamere reaktiekinetiek zou kunnen hebben dan aangenomen is in de literatuur. Het model voorspelt ruim 10 ppmv HNCO en NO, maar deze stoffen zijn nooit door middel van FTIR analyse gedetecteerd. Waarschijnlijk wordt HNCO katalytisch in NH3 en CO omgezet en deze reaktie is niet in het model meegenomen. Voor NO speelt katalytische reductie door as-elementen waarschijnlijk een rol bij de heersende vergassingstemperaturen. Het is ook mogelijk dat verwaarlozing van de S en Cl chemie voor de trace componenten de afwijkingen tussen model en experimenteel resultaten veroorzaakt. Dit is in mindere mate het geval voor hout. De vereenvoudiging van de teer- en kool-residuegebonden stikstof kan ook van betekenis zijn: in het model wordt aangenomen dat de stikstof die niet vrijkomt als gas component bij de pyrolyse, initieel als HCN vrijkomt.

Voor de hoofdcomponenten van het productgas is de overeenkomst tussen de model- en experimentele resultaten tamelijk goed. De verschillen tussen de berekeningen en de metingen kunnen worden toegeschreven aan de op basis van de literatuur aangenomen pyrolyse opbrengst van waterstof in het biomassa pyrolyse sub-model. De onzekerheid aangaande de teerontledingskinetiek, in termen van mogelijke reacties, product opbrengsten en snelheid, speelt waarschijnlijk ook een prominente rol in de waargenomen verschillen voor CO en CO2.

De afwijkingen zijn het hoogst voor hout en minder voor miscanthus en bruinkool. Dit komt overeen met de hypothese dat onnauwkeurigheden in de teerontledingskinetiek hier een belangrijke rol in spelen, omdat hout de hoogste initiële teeropbrengst vertoont in de snelle pyrolyse stap. Verschillen in de heterogene verbrandings- en vergassingsreactiekinetiek hebben naar verwachting een veel minder grote invloed op de voorspelde concentraties, aangezien de koolstofconversie in het algemeen tamelijk goed wordt voorspeld.

Tenslotte wordt in hoofdstuk 7 een overzicht gegeven van de getrokken conclusies en aanbevelingen gedaan voor verder onderzoek, gespecificeerd naar experimentele drukwervelbed pilot schaal onderzoek, brandstofkarakaterisering en modellering.

Table of Contents

Summary

vii

Samenvatting

xi

Notation

xix

Chapter 1: Introduction

1.1 Towards a renewable energy based world 1 1.2 Biomass as part of renewable power generation 3 1.3 Technology for biomass utilisation for heat and power generation 3 1.3.1 Small scale versus large scale processes 4 1.3.2 Technological options for large scale biomass based

heat and power generation 5

1.3.2.1 Combustion 5

1.3.2.2 Gasification 6

1.4 Power production from biomass gasification, open research questions 8

1.5 The fate of fuel bound nitrogen 9

1.6 Outline of this thesis 11

Chapter 2: Fluidised bed solid fuel gasification processes,

overview and analysis of experimental research and modelling

2.1 The fluidised bed reactor applied for solid fuel gasification 13 2.2 Industrial fluidised bed gasification systems 13 2.3 An overview of recent research, development and small scale

demonstration activities 17

2.4 Experimental findings regarding the fate of fuel nitrogen during

fluidised bed gasification 20

2.4.1 Influence of fuel type 20

2.4.2 Influence of air stoichiometry 23

2.4.3 Influence of temperature 24

2.4.4 Influence of pressure 25

2.4.5 Influence of additives 26

2.4.6 Influence of particle diameter 26

2.4.7 Influence of steam 27

2.4.8 Influence of feed location 27

2.5 Fluidised bed gasifier modelling 28

2.5.1 General overview 28

2.5.2 Drying and flash pyrolysis, initial steps in the process 32 2.5.2.1 Experimental techniques and findings 32 2.5.2.1.1 Main components and hydrocarbons 33

2.5.2.1.2 Nitrogen components 53

2.5.2.2 Modelling approaches 60

2.5.3 Heterogeneous char-gas reactions 62

2.5.3.1 Main carbon based reactions 62

2.5.3.2 Heterogeneous and heterogeneously catalysed homogeneous

nitrogen reactions 65

2.5.4 Homogeneous gas phase reaction mechanisms, including nitrogen

chemistry 66

2.6 Potential primary measures for fuel_NOx emission reduction 68

Chapter 3: Experimental set-ups and measurement techniques

3.1 Introduction 71

3.2 The Delft Pressurised Fluidised Bed Gasification (PFBG) test rig 71

3.2.1 Description of the rig 71

3.2.2 Analysis and sampling techniques 75 3.2.2.1 FTIR spectrophotometer 75

3.2.2.2 Gas Chromatography 81

3.2.2.3 On-line Non Dispersive Infrared/UV, colorimetric and

paramagnetism based analysers 83

3.2.2.4 Sampling probes and analysis of tar compounds 86 3.3 The 50 kW(thermal) IVD Pressurised Fluidised Bed Gasifier (DWSA) 89 3.3.1 Description of dimensions and operation 89

3.3.2 Analysis techniques applied 90

3.4 The TG-FTIR set-up at Advanced Fuel Research Company Inc. (USA) 91 3.5 The heated grid reactor at Eindhoven University of Technology 92

Chapter 4: Experimental results

4.1 Choice of fuels, bed materials and additives 99

4.1.1 Fuel choice 99

4.1.2 Fuel composition and related chemical properties 100 4.1.3 Physical property characterisation of fuels and bed materials 101 4.2 Experimental results of PFBG gasification tests 104 4.2.1 Experimental data representation and definitions of relevant parameters 104 4.2.2 Background information on the measurement campaigns 105

4.2.3 Miscanthus gasification 106

4.2.4 Wood gasification 117

4.2.5 Brown coal gasification 124

4.3 Experimental results of DWSA gasification tests 129 4.3.1 Overview of the DWSA measurement programme 129

4.3.2 Wood gasification 130

4.3.3 Brown coal gasification 131

4.4 Experimental results of TG-FTIR pyrolysis tests 133 4.4.1 Overview of the TG-FTIR experimental programme 133

4.4.2 Kinetic analysis approach 133

4.4.3 TG-FTIR analysis results and derived kinetic parameters for miscanthus 135 4.4.4 TG-FTIR analysis results and derived kinetic parameters for wood

(Labee “A quality” energy pellets) 140 4.4.5 TG-FTIR analysis results and derived kinetic parameters for brown coal 144

4.4.6 TG-FTIR analysis discussion 147

4.5 Experimental results of heated grid pyrolysis tests 149 4.5.1 The heated grid experimental programme 149

4.5.2 Miscanthus pyrolysis results 149

4.5.3 Discussion of the results 153

4.6 Conclusions and recommendations 155

4.6.1 Conclusions and recommendations related to PFB gasification 155 4.6.2 Conclusions and recommendations for fuel characterisation 156

Chapter 5: Modelling bubbling fluidised bed gasification,

focussed on nitrogen compounds

5.1 Modelling approach 159

5.2 Description of the model 159

5.2.1 Idealised reactor approach 159

5.3 Simulation results using the idealised reactor modelling approach 169

5.4

Conclusions 182

Chapter 6: Comparison of modelling results and experiments

6.1 Choices made for the comparison between model and experiments 183 6.2 Gasification experiments compared with model results 183 6.2.1 TUD PFBG experimental and simulation results 183 6.2.2 IVD DWSA experimental and simulation results 192

6.3 Discussion of the results 196

6.4 Conclusions and recommendations 197

Chapter 7: Conclusions and recommendations for further research

7.1 Conclusions 199

7.2 Recommendations 201

Bibliography

203

Appendices

1 Details of analytic measurements

227

1.1 Spectra, spectral windows and calibration curves used for quantitative

species analysis with FT-IR 227

1.2 Calibration curves used for quantitative species analysis with gas

chromatography 241

2

Relevant chemical & physical properties of the gasification product

gas components

249

2.1 Gas phase viscosity 249

2.2 Diffusion coefficients of gas phase components 250

2.3 Gas Phase Thermal Conductivity 251

2.4 Thermodynamic data 252

3 Detailed

homogeneous

reaction scheme “Kilpinen 97”

255

4

Results of TG-FTIR measurements and comparison of FG-DVC

model results with experiments at different heating rates

259

5

Calculation of axial Péclet numbers for PFBG and DWSA tests

simulated

275

6 List

of

publications

277

Dankwoord

281

Notation

Abbreviations

ABFB Atmospheric Bubbling Fluidised Bed ACFB Atmospheric Circulating Fluidised Bed AFR Advanced Fuel Research inc.

ar or a.r. As received

BC Brown Coal

BHF Baghouse Filter

BTX Benzene, Toluene, Xylene CFD Computational Fluid Dynamics daf Dry and ash-free basis

db Dry basis

FBN Fuel Bound Nitrogen

FG-DVC Functional Group-Depolymerisation Vaporisation Condensation FID Flame Ionisation Detector

FTIR Fourier Transform InfraRed

GC Gas Chromatograph

Gtoe Gigaton oil equivalent HCV or HHV Higher Calorific Value

HGR Heated Grid Reactor

HRSG Heat Recovery Steam Generator

IGCC Integrated Gasification Combined Cycle LCV or LHV Low Calorific Value

mf Moisture free

MSW Municipal Solid Waste

Mtoe Megaton oil equivalents NDIR Non Dispersive InfraRed NDUV Non Dispersive UltraViolet ODE Ordinary Differential Equation PFB Pressurised Fluidised Bed PBFB Pressurised Bubbling Fluidised Bed PCFB Pressurised Circulating Fluidised Bed PFBG Pressurised Fluidised Bed Gasifier

RDF Refuse-derived Fuel

RPS Rotating Particle Separator SCADA Supervision Control and Data Acquisition SCO Selective Catalytic Oxidation SCR Selective Catalytic Reduction SPA Solid Phase Adsorption technique SPE Solid Phase Extraction

SRC Short Rotation Coppice

SS Stainless Steel

TCD Thermal Conductivity Detector TGA Thermogravimetric Analysis (or Analyser) TUD Delft University of Technology

WP Wood Pellets

Latin Symbols

A Area of gasifier reactor part [m2_]

A(υ) Absorbance at wave number υ [-]

ai’(υ) Absorption coefficient at wave number υ of species i' [m-1]

Bo Bodenstein number [-]

b Path length through the sample [m]

b Temperature coefficient in the Arrhenius equation [-]

ci’ Concentration of species i' in the sample [-]

cp Specific heat [J.kmol-1.K]

CC Carbon Conversion [%]

d Diameter [m]

Dax Axial dispersion coefficient [m2.s-1]

DT Diameter of gasifier reactor part [m]

Di,m Diffusion coefficient of component i in mixture [m2.s-1]

E Activation energy [J.kmol-1_]

f Radiation frequency [s-1_] g Acceleration of gravity (= 9.81) [m.s-2_] Ga Galilei number [-] H Enthalpy [J.kmol-1_] H Height of bed [m] I Intensity of radiation [*]

ki Rate constant of component i [s-1]

ki0 Frequency factor (or pre-exponential factor) [s-1]

mi Mass fraction of i in fuel [-]

M Heating rate [K.s-1_]

MWi Mole mass of component i [kg.kmol-1]

OF Distance between beamsplitter and fixed mirror [m] OM Distance between beamsplitter and movable mirror [m]

P Pressure [Pa]

Pe Péclet number [-]

R Universal gas constant (=8314.3) [J.kmol-1_.K-1_]

Re Reynolds number [-]

t Time [s]

T Temperature [K]

u Velocity [m.s-1]

v Velocity [m.s-1]

x Fraction of reacted material [-]

X Mass fraction of char remaining [-]

Yi Mass fraction of component or functional group i [-]

Greek Symbols

δ Retardation, or difference in optical path length [m]

ε Volume fraction [-]

η Dynamic viscosity [Pa.s]

η Efficiency [%]

λ Wavelength [m]

υ Wave number [cm-1]

ν Kinematic viscosity [m2_.s-1_]

ρ Density [kg.m-3_]

ρ Parameter in the empirical relation for σ [−]

σ Gaussian distribution of the activation energies [J.kmol-1_]

φ Sphericity factor bed material [-]

Φm Μass flow [kg.s-1]

.

j

Indices

A Absolute

ax Axial direction

b Bubble

bed Bed (section)

C Carbon (char) diff Diffusion e Electrical f Forward reaction g Gas (phase) m Mass max Maximum

mf At minimum fluidisation condition

mix Mixture (gas)

p Particle r Residence

r Backward reaction

s Solid (phase)

SMD Sauter Mean Diameter

Stoich Stoichiometric

th Thermal

Chapter 1

Introduction

1.1 Towards a renewable energy based world

Up till now, fossil fuel utilisation has made a large contribution to the energy systems of our modern age. However, most of the fossil fuel reserves for power and heat supply, transportation and chemicals manufacturing decrease and need to be substituted by alternatives in order to maintain the present way of life. The 20th century was based on these conventional energy carriers, but the 21st century will have to adjust to the decline of the oil and natural gas based economy and the increasing public awareness of the negative effects of environmental pollution from fossil fuel utilisation.

Figures 1.1a through 1.1d give an overview of recent data concerning fossil fuel and uranium reserves, as well as the expected cumulative demand over the period 1990-2050.

Figure 1.1a Global primary Energy Reserves, Commercially and technically exploitable, Global Energy Perspective, [IIASA/WEC, 1998].

Figure 1.1b Global primary Energy Reserves Commercially and technically exploitable, World Economic Outlook, [IEA, 1998].

Oil 343 Gtoe Natural Gas 333 Gtoe Coal 606 Gtoe Uran iu m 57 Gt oe Coal 710 Gtoe Ur ani u m Oil 411 Gtoe Natural Gas 237 Gtoe 51 G toe Oil 261 Gtoe Coal 273 Gtoe Uranium Natural Gas 211 Gtoe 40 Gt oe Oil 3.46 Gtoe Natural Gas 2.06 Gtoe Coal 2.13 Gtoe Uran iu m 0.65 Gto e

Figure 1.1c Cumulative primary energy demand (1990-2050) Energy Reserves,

[IIASA/WEC, 1998].

Figure 1.1d Primary energy utilisation in 1999, [BP Amoco, 2001].

Figures 1.1a-c show that reserves of oil and natural gas are (just) sufficient for the indicated period. Based on previous experience, however, it is expected that more reserves of natural gas will be found in e.g. hydrate fields (clathrates) in deep-sea area’s, but the quantities still have to be investigated.

The exploitation of these fields, though, will be comparatively expensive. Also for oil there are probably unknown reserves, which are by now not yet commercially or technically exploitable. Coal as conventional fuel is expected to be still available for several centuries because of its huge reserves and its broad global reserve distribution. The figures further indicate that the reserves of Uranium, the fuel for power generation based on nuclear fission, are of the same order of magnitude as the expected demand in the indicated period. This situation is not positive for the long term use of this fuel, unless technology is shifted to a more efficient use of fuel sources, like e.g. in fast breeder systems [Gardiner, 1990]. However, radioactive waste disposal and removal problems are still preventing the widespread using of nuclear technology. Also, the danger of proliferation of nuclear energy knowledge and implementation on a global scale might lead to increased nuclear arms proliferation. Furthermore, the world is faced with a growing human population and an increasing level of industrialisation, giving rise to an increasing demand for personal comfort and energy demand per capita. Long term scenario’s, like the one mentioned in figure 1.1 c, differ in the way the energy use per capita increases, but in all scenario’s 50-60% of the world’s energy supply will rely on fossil sources as primary energy supply.

The use of fossil fuels as primary energy source gives rise to an increasing emission of environmentally hazardous species, which is becoming a growing problem. The rising worldwide awareness of environmental constraints leads to legislative actions in most industrialised countries in order to restrict local and regional emissions of acid rain or smog precursor gases (e.g. SOx and NOx) and dust. Furthermore, several industrialised countries are heading for a reduction of the human contribution to the greenhouse effect, by reducing emission of these so-called greenhouse gases, like in particular CO2, CH4 and N2O. Of these gases, CO2 is the most important contributor to the absorption of infrared radiation emitted from the earth’s surface. The increase of the CO2 concentration in the atmosphere is supposed to contribute significantly to the enhanced greenhouse effect (from 55% [Wójtowicz et al., 1993] to 63.5% [Sloss, 2002]). CH4 also absorbs infrared radiation and contributes for approximately 15% -20.5% to the enhanced greenhouse effect. Finally, N2O has a contribution of about 6.5% to the enhanced greenhouse effect. In the Kyoto-conference [UNFCCC, 1997], held in December 1997, it was agreed to reduce worldwide greenhouse gas emissions of the industrialised nations by 2012 to a 5.2% lower level compared to the emissions in 1990.

For the Dutch situation this agreement implies that in the period 2008-2010 compared to the 1990 level 6% less greenhouse gases will be allowed to be emitted according to the “Uitvoeringsnota Klimaatbeleid”[Duurzame energie in uitvoering, 1999]. This will have to be realised under higher economic grow rates and accompanied with a higher consumption level, which potentially causes increasing greenhouse gas emission levels [Energierapport, 1999]. Therefore, a reduction of the use of fossil fuels, which are the major source of CO2 emission, is required.

In order to reach the goal of decreasing CO2 emissions, growing attention is drawn toward two directions. The first is to improve the efficiency of existing fossil fuel conversion processes. The second is the use of sources that are practically CO2 emission free, like wind, solar, geothermal, hydropower, biomass and advanced nuclear sources [Gardiner, 1990]. Biomass is ‘almost CO2 neutral’ and renewable, because CO2 is taken up from the atmosphere during the growth of biomass and released again during combustion in a relatively short cycle time, as compared to fossil fuels.

An increased use of renewable energy sources can support governments to achieve a wide range of policy goals: e.g. improved energy security and diversity, enhanced levels of technology export to countries which are less developed and reduced emissions of greenhouse gases and other pollutants, such as sulphur oxides, nitrogen oxides, particulates and trace metals.

In the world, the USA is the country with the highest energy demand per capita. According to the International Energy Outlook [IEO, 2000], the share of renewables in the US electricity market is projected to be about 20% in 2020. The majority of this renewable power production will be hydroelectrity. Biomass will provide a smaller contribution to the US power supply, which is currently approximately 4% of the primary energy [Costello & Chum, 1998]. The US department of Energy (DOE) has published a figure of 1% use of grid-connected biomass power capacity, which is about 7 GW. A goal set by the US government is to implement an additional 17 GW biomass power the next two decades [DOE, 1996].

The European Union recently published a White Paper on Renewable Energy, stating that Europe could double its use of renewable fuels from 6 % in 1997 to 12% by the year 2010. Currently, in Europe energy from biomass sources accounts for about 45 Mtoe (approximately 3% of the total consumption), while the European Commission proposes that biomass in total will contribute an additional 90 Mtoe per year by 2010 with an increased share in the total energy consumption [EC, 1997].

The Dutch government has agreed to a goal of 10% energy from renewable sources for the year 2020, with biomass as the main contributor [Weterings et al., 1999]. This implies about 120 PJ of bio-energy. Recent expectations are such that in ten years’ time biomass for energy supply could increase from a current level of approximately 13 PJ to a level of 80 PJ [Energierapport, 1999]. For this purpose, the Dutch government uses several instruments in a liberalising electricity market, such as offering fiscal advantages, to stimulate the demand of green electricity as well as the production of energy from renewable sources [Kwant & Leenders, 1999].

1.2 Biomass as part of renewable power generation

Since the dawn of mankind, biomass has been used as food for life and energy for heating and cooking by combustion. It is a form of solar energy stored in organic form. Nowadays it is recognised that the use of biomass for supply of energy offers the advantage of reducing the net CO2 emission. Power generating facilities based on fossil fuels, of which coal is the most important component, cause comparatively high specific emissions of the greenhouse gas CO2. Today around 40% of the world’s electricity generation is based on coal and 20% of the CO2 emissions are caused by coal-fired power plants [Campbell et al., 2000]. Biomass can nowadays be utilised in quite efficient processes for power and heat production, which have been developed to industrial scale in the last decades.

For this purpose, biomass from energy crop cultivation, set-aside land can be used in an economic way in certain countries. This helps to restructure the agricultural situation with the benefit of job creation along with the agro-economic activities for this purpose. A valuable bio-diversity can be created along with erosion protection, depending on the crop used. Drought tolerance and low fertiliser and/or pesticide requirements are important requirements [Hall et al., 1993], [DOE, 1996], [Faaij, 1997].

Also, (industrial) waste streams of biological origin can be used, thereby reducing disposal problems or incineration, which is inefficient compared to electricity (co-) generation.

The current commercial and non-commercial biomass use for energy production is estimated to be between 6 and 17% of world primary energy, most of this is used in developing countries, where biomass accounts for up to one third of energy needs [Bauen & Kaltschmitt, 2001]. By contrast, biomass provides at most 3% of energy in industrialised countries [Gross et al., 2003].

The utilisation of solid fossil together with biomass for heat and power production offers additional advantages compared to the use of biomass as a single fuel [Rüdiger et al., 1996]:

•

variations in the availability of biomass can be met by changing coal-biomass ratio’s;

•

it enables a wider range of system sizes, increasing optimisation possibilities;

•

it can show synergistic effects that reduce operational problems and emissions.

1.3 Technology for biomass utilisation for heat and power generation

An important aspect for the choice of the technology to convert (biomass) fuels is the cycle efficiency for power and heat generation. Efficiency improvements decrease specific fuel consumption and emissions.

Available energy conversion systems for solid fuels have thermodynamic limitations with respect to attainable efficiencies. Power generation in conventional systems is based on either steam turbine or gas turbine technology. The application of advanced efficient systems such as fuel cells is interesting

for the longer term but these are not considered in this study as they are presently comparatively expensive and not technically feasible for large scale biomass/waste based conversion processes. The first systems used at commercial scale were based on a steam turbine cycle with direct combustion of the fuel. In this conversion process heat is transferred from a high (combustion) temperature to a level determined by the maximum allowable steam conditions (pressure and temperature). These conditions are mainly determined by material strength constraints. Due to the relatively large difference in the temperatures mentioned (combustion and steam temperature), the process efficiency is limited thermodynamically by the upper temperature level of the steam cycle. A gas turbine can be used efficiently for generating power using high temperature working fluids, like flue gas from solid fuel or gasification-derived gas combustion. One of the limitations in power generating efficiency in a gas turbine (or Brayton) cycle is in the lower temperature level to which heat can be released. A combination of a gas turbine and a steam turbine, a so-called combined cycle, leads to potentially higher efficiencies as compared to the separate steam and gasturbine cycles. In the following sub paragraph, a comparison is made between small and large-scale systems for power and heat production, as these have their own advantages and disadvantages.

1.3.1 Small scale versus large scale processes

Electric power from biomass can be generated in a decentral or central way. Large scale centralised power production offers the following main advantages (with specific biomass related points indicated in italic):

+

the availability of electricity is well secured;

+

the economics of scale cause comparatively low power production costs;

+

emissions can be controlled adequately, due to the gas cleaning equipment being relatively well known on large scale and operated by skilled personnel.

Disadvantages are:

−

users are dependent on a grid, which is sometimes not reliable in developing countries;

−

significant losses in the distribution and transportation of electricity, although in a densely populated country, like e.g. the Netherlands, this loss effect can be ignored;

−

transportation distances of fuel are relatively long, with accompanied increased costs and CO2

emission;

−

the availability of especially biofuels can be insecure.

In the decentralised option, which was also characteristic for traditional biomass use, electricity is produced by small scale units in local communities, or farms located far away from densely populated areas. This has the following advantages:

+

users are not dependent on the electric grid, which especially in countries in the developing world is not always secured, a critical issue for e.g. hospitals;

+

transportation and distribution losses of electricity are relatively small;

+

the average distance of transportation of biomass fuels is short.

Disadvantages are:

−

emissions of small local power producing equipment cannot be controlled as adequate as those of large scale power plants;

−

maintenance of the units requires special skills and is possibly less secured as compared to large-scale power production;

−

fuel conversion, gas cleaning (both for protection of downstream equipment and

emission reduction) and prime mover equipment is relatively expensive due to the negative economics of scale;

−

availability is not always secured, as skilled personnel is not always there and also because there can be problems in biomass supply.

In the Dutch situation the advantages of large-scale systems outweigh the disadvantages and therefore the focus of this study will be on large-scale systems.

1.3.2 Technological options for large scale biomass based heat and power generation

For large scale heat and electricity generation using biomass, the main technological options are combustion or gasification. Pyrolysis, or liquefaction in combination with a combined cycle e.g. is still in a conceptual stage [Siemons, 2002]. These techniques are more interesting for smaller scale decentralized heat and power production (scale-up is still under development) or for producing biofuels for transportation.

1.3.2.1 Combustion

Co-combustion of biomass in large-scale utility boilers with, as additional option, co-production of electricity and heat is already a commercial common practice [Spliethoff, 2000].

The energy conversion systems based on combustion basically consist of a primary combustion section, a boiler section and a steam turbine. The combustion section ensures conversion of the chemical energy bound in the solid fuel into heat in the presence of an overall over-stoichiometric amount of air. This heat is transferred by radiation and convection to the water and the steam in the boiler and sensible heat of the flue gases; with conduction playing a minor role as heat transfer mechanism. The steam is expanded in a steam turbine to drive the generator for electricity production. Part of the energy content in the steam that cannot be converted into power, generates heat on a relatively low temperature level and can be used for e.g. district heating provided that a consumer net is commercially possible. Often the heat produced cannot be used to its full extent.

The amount of power generated divided by the amount of heat produced of most biomass-fired power stations amounts to values less than 0.5 [Van den Heuvel & Stassen, 1994]. Depending on the local requirements variable amounts of heat can be utilized, but comparatively high electric power generation efficiency is the driving force in most situations.

The electric efficiency of common steam cycles is determined by:

•

the temperature of steam at the turbine inlet; the higher the temperature (accompanied with a higher pressure) the higher the efficiency; the maximum temperature is determined by the material properties of the steam generator;

•

the steam pressure at the turbine outlet (condenser pressure), which is determined by the temperature to which the outlet steam can be cooled down; the lower this temperature, the higher the overall efficiency;

•

thermal losses of the boiler system;

•

combustion efficiency;

•

generator losses.

There are several commercial combustion system configurations available for both biomass and solid fossil fuels, with the following reactors as primary combustor types:

•

fixed bed combustors (practically not applied anymore for solid fossil fuels), like e.g.: fixed flat or inclined grate firing units;

moving flat or sloped grate ovens; combustors with bottom screw feeding;

•

fluidised bed combustion reactors, such as: bubbling beds;

circulating beds;

pulverised solid fuel burners (entrained flow reactors); cyclone burners.

When compared to other combustion technologies fluidised beds offer the advantages of:

•

very high flexibility with respect to fuel properties like size distribution, density, moisture and ash content;

•

good heat transfer leading to installations with relatively small specific reactor volumes;

•

in-situ primary gas cleaning by addition of sulphur binding compounds, like e.g. limestone, so that downstream cleaning is less extensive;

•

relatively low reactor temperatures, leading to less corrosion, deposition problems and potentially lower NOx and SOx emissions than high temperature systems.

Problems of these fluidised bed combustion systems are fouling and slagging of heat exchanging equipment in the system (boiler) and sintering tendencies of ash and bed material especially with fuels containing alkali metals and chlorine. Also, for fluidised bed combustion of coal comparatively high N2O emissions have been observed.

1.3.2.2 Gasification

An alternative to direct combustion of biomass and/or coal for conversion of fuel into power and heat is conversion by thermal gasification. In principle there are three basic gasification reactor configurations:

the fixed bed gasifier (dry ash or slagging), which can be distinguished into three subtypes:

•

the co-current down flow reactor;

•

the counter-current reactor, and

•

the cross-current gasifier;

the fluidised bed gasifier (non slagging operation), which can be divided into two sub configurations:

•

the bubbling fluidised bed and

•

the circulating fluidised bed; the entrained flow gasifier (slagging).

The process can be configured as autothermal or allothermal. In the first, most commonly found configuration, the heat necessary to drive the endothermic gasification reactions is provided in-situ by partial combustion of the fuel. For the latter configuration, the heat needed for the endothermic reactions is provided externally.

Application of oxygen/steam as gasification medium is common for coal gasification, because smaller equipment can be applied when compared to air blown gasifiers.

Gas cleaning is a critical issue for the successful implementation of gasification technology for electricity generation. The combination of the gasifier, down stream followed by high temperature dry gas cleanup processes offers the advantage of higher overall efficiencies than systems using wet, low temperature, raw gas cleaning techniques. This holds especially for downstream applications that are using hot product gas.

Relatively high power efficiencies can be attained by application of Integrated Gasification & Combined Cycle (IGCC) technology. High efficiency, natural gas fired combined cycles are applied on a large scale throughout the world with natural gas as fuel. These units require comparatively low investments. The IGCC process, characterised by a relatively high power to heat production, has been demonstrated already for coal as solid fuel on large scale and seems to be also especially attractive in the medium-size power production range of 20-150 MWe [Kurkela et al., 1993b].

Examples of technically successful coal-based IGCC demonstration projects in Europe are "Buggenum" (253 MWe, see figure 1.2 a and b) [Ploeg, 2000] and "Puertollano" (300 MWe) [Schellberg, 2000]. In the USA, coal-based IGCC projects are: "Polk Country" (250 MWe, 4.5 year demonstration project ended in 2001, now continuing to operate commercially), "Wabash River" (262 MWe, 5 year demonstration project ended in 1999, now continuing to operate commercially) and

"Piñon Pine" (100 MWe), see e.g. [Campbell et al., 2000]. The first two mentioned can be characterised as technically successful, with availability data as high as 70-80%; the last one has ended less successfully [Henderson, 2003]. Furthermore, also in Japan and Australia IGCC projects based on coal have been launched [Henderson, 2003].

Figure 1.2 a. Picture of the Buggenum IGCC b. Process scheme of the plant. power station; [Scheibner&Wolters, 2002].

The IGCC process comprises a gasifier with appropriate product gas cleanup and both a gas turbine and a steam turbine for electricity generation. The competitiveness of the process alternatives has been based on the overall price of electricity, discarding the environmental benefits related to IGCC technology. In the future, the environmental factors and sustainability aspects may probably be of higher importance and competitiveness of IGCC systems based on a variety of solid fuels will then be significantly improved. However, the competitiveness of IGCC cannot be based only on environmental superiority. The overall feasibility of IGCC technology also has to be improved. This can be attained by developing and improving equipment components of IGCC units. In addition, the fuel flexibility has to be further improved – one can consider all kinds of waste material in this respect - accompanied by reduction of operational costs.

Fluidised bed technology is an attractive process for gasification in comparison to the fixed bed or entrained flow alternatives, because of:

+

the well-established principles of operation;

+

the fact that the process can be scaled successfully to the medium-size power range ;

+

its flexibility with respect to feedstock characteristics (type, particle size);

+

the favourable heat and mass transfer properties;

+

the energy-efficiency of the process?.

A main disadvantage is that the fluidised bed process is limited in the temperature range to be applied, due to possibility of bed material sintering, which causes fuel conversion to be several percent points less than 100%. This disadvantage holds most for relatively unreactive fuels, such as bituminous coals, but not so for reactive biomass and brown coal.

The most attractive gasification medium in the fluidised bed gasification process is air since no expensive air separation unit is necessary.

A pressurised gasification process has as a major advantage that compression of the product gas, necessary for combustion in (advanced) gas turbines, is not required to such an extent as for atmospheric gasification. Compression of air for the gasifier is in this sense cheaper and easier to accomplish than compression of the fuel gas. Finally, reduced size and, hence, less costly equipment is sufficient for the gasifier and gas cleanup section as compared to atmospheric gasification processes.

As pressure resistant materials are more costly this argument holds for scales in a range from 20-30 MW electric and higher.

As an example the process scheme of the 18 MWth pressurised fluidised bed IGCC demonstration plant at Värnamo is given in figure 1.3. The commercialisation of combined cycle power generation based on the gasification of solid biomass fuels has been slower than expected about one decade ago. An overview of these processes is given in chapter 2.

Figure 1.3 Process scheme of the Värnamo pressurised fluidised bed IGCC demonstration unit [Ståhl,

2001]

1.4 Power production from biomass gasification, open research questions

There are still partly unanswered questions, which have to be answered by means of measurement and process modelling. These questions are related to the fate of species harmful to equipment and environment based on biomass gasification with fluidised beds as primary solid fuel converters. They are summarised below:

•

What is the mechanism of the formation and destruction of higher hydrocarbon species (tars), which can clog process equipment parts like fuel control valves and gas analysis equipment and can cause a decreased downstream gas turbine combustion efficiency and soot emission?

•

What is the fate of trace elements; will they be emitted to the environment in solid form as ashes (from bed and fly ash) or in gaseous form (air toxics) in the exhaust gas?

•

How is fuel bound sulphur partitioned in the gasifier into mainly H2S and COS and subsequently SOx during gas turbine combustion?

•

What is the fate of nitrogen species (like NH3 and HCN) in NOx formation in processes including gas turbine combustors downstream of the gasifier, when dry, high temperature gas cleaning processes are applied [Hoppesteyn, 1999]?

The first open research question is not dealt with in this thesis. The second is the subject of another thesis from our institute [Ünal, 2005]. Regarding the remaining questions a further consideration is given below.

Table 1.1 gives an overview of typical elementary compositions of a selection of coal and biomass species. The biomass examples are from agricultural waste origin (straw types), energy crop cultivation (miscanthus) and forestry.

Coals presented in the table below are brown coal (from the German Hambach open mining) and black bituminous coals from the UK (Daw Mill coal) and the USA (Pittsburgh and Utah coal). Data for peat (from eastern Finland), a fuel intermediate between biomass and coal, is also given.

Table 1.1 Fuel composition of several solid fuel sources (on dry basis) related to their nitrogen and sulphur species emission potential.

Biomass C (mass%, dry) H (mass%, dry) O (mass%, dry) N (mass%, dry) S (mass%, dry) HHV (MJ/kg) (kg/GJ) N (kg/GJ) S Rice straw 1) _{39.2 5.1 35.8 0.6 0.1 15.4 0.4 0.07} Miscanthus 2) _{48.2 5.4 42.8 0.58 0.16 19.3 0.3 0.08} Sawdust 3) _{48.5 5.1 46.0 0.03 0.03 19.2 0.02 0.02}

Wood pellets (‘clean’)2) 51.4 6.0 42.2 0.17 0.11 20.3 0.08 0.05

Peat and Coal

Finnish peat 4) _{54.5 5.6 33.6 1.80 0.25 21.8 0.8 0.1}

Hambach Brown coal 2) _{64.5 4.4 26.1 0.64 0.39 25.2 0.3 0.2} Daw Mill coal 69.4 4.4 10.0 1.2 1.6 30.0 0.6 0.8

Pittsburgh coal 1) 75.5 5.0 4.9 1.2 3.1 31.8 0.4 1.0

Utah coal 1) _{77.9 6.0 9.9 1.5 0.6 33.0 0.5 0.2}

1) [Reed,1981] 2) this study 3) [Zhou,1998] 4) [Kurkela et al., 1992]

Coal is a sedimentary rock composed of both organic and inorganic constituents and formed through partial decomposition of plant debris under the action of heat, pressure and time, [Bend, 1992], [Van Krevelen, 1993]. The main difference between coal and biomass is the coal’s higher heating value, which is closely related to the oxygen content in the fuels. During carbonisation, oxygen and hydrogen are reduced and coals of increasing rank from lignite to anthracite are formed, see [Speight, 1983]. Here, coal rank is defined as an indicator for the stage of alteration, or degree of coalification, attained by a particular coal. The greater the alteration, the higher the rank of the coal. Related to this difference in elementary composition is the much higher reactivity of biomass as compared to coal in thermochemical conversion processes, because less stable oxygen containing structures are present in the fuel.

Absolute sulphur content is higher in the coals presented than in biomass types shown. This is also the case, although somewhat less pronounced, when the sulphur contents are compared on an energy basis. As sulphur species emission for biomass gasification is not a major problem for a broad range of these fuels, this issue is not further studied in this context.

Although the absolute values of the nitrogen content on mass basis in biomass materials in most cases is significantly lower than coal, the nitrogen quantity on an energy basis is comparable for certain biomass species, like miscanthus and straw. This indicates the relatively high emission potential of significant nitrogen containing species, like NH3 and HCN as NOx precursors. The chemical structures in which the nitrogen is bound, however, are of importance too for the still not completely understood partitioning behaviour. This is the reason why the fate of nitrogen species will be the focus of this thesis.

1.5 The fate of fuel bound nitrogen

The emission of NOx, a collective noun for NO and NO2, is a worldwide occurring regional problem as its effects are both on the environment (plants, buildings) as well as human and animal health. One of its effects is contribution to photochemical smog formation, in which NOx, CH3COO2NO2 (peroxylacetyl nitrate, PAN) and O3 play a role [Leighton, 1961]. Smog affects health negatively and reduces visibility. NOx also participates in the formation of O3 in the troposphere causing problems for vegetation. It is also a contributing agent in the greenhouse effect. Nitrous oxide, N2O, is known as “laughing gas”. When it is inhaled, it can give uncontrollable laughter, further inhalations lead to enjoyable hallucinations and longer exposure leads to anaesthesia [Hayhurst & Lawrence, 1992]. N2O has a relatively long lifetime in the troposphere of approximately 150 - 170 years [Badr & Probert,