Biomass fuel characterization for NOx emissions in cofiring applications

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Biomass fuel characterization for NO

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emissions in

Co-firing applications

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Biomass fuel characterization for NO

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emissions in

Co-firing applications

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 26 februari 2007 om 15.00 uur

door

Gianluca DI NOLA

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Samenstelling promotiecommissie: Rector Magnificus, voorzitter

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

Prof. dr. L.P.H. de Goey, Technische Universiteit Eindhoven Prof. dr.ir. G. Brem, Universiteit Twente/TNO

Prof. Dr.-Ing. H. Raupenstrauch, Technische Universit¨at Graz Dr.ir. W. de Jong, Technische Universiteit Delft

Dr.ir. W. Willeboer, Essent Energie

Prof. ir. J.P. van Buijtenen, Technische Universiteit Delft, reservelid

Cover designed by Diana M. Guio Torres.

Front: Gas composition/temperature measurements at the Maasvlakte boiler, Rotter-dam.

Back, top to bottom: Novel HWM-FTIR setup, 50 kW electrically heated combustion flow reactor, Flame from the flow reactor during co-firing experiments at 1300◦_{C, NO}

x

This research has been partly funded by the Netherlands Agency for Energy and the Environment (Senter-Novem), Contract 2020-02-12-14-003.

Published by Gianluca Di Nola, Delft ISBN 978-90-9021576-1

Keywords: Biomass co-firing, NOx emission control, Thermogravimetric analysis, Fast

devolatilization, Pyrolysis modeling, NOxprediction.

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

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Summary

In this dissertation the impact of various biomass fuels and combustion conditions on the NOx emissions during biomass co-firing with coal is dealt with.

Fossil fuels dominated the energy scenario since the industrial revolution. However, in the last decades of the 20th_{century, increasing concerns about their availability and}

environmental risks related with their extensive utilisation, like the greenhouse effect caused by CO2, together with rising oil prices are favoring the development of renewable

energies. Increasing the share of renewable energy sources in the power generation sector is strongly promoted in the EU, the European Commission has set its target at 22% of the overall power production by 2010.

Co-firing biomass and waste fuels with fossil fuels appears as a promising route to in-troduce renewable fuels in the power sector and reduce CO2emissions. Biomass co-firing

has been implemented at several coal-fired power stations, generally limited to about 5% of the thermal input. This practice offers several advantages. Besides CO2emission

reduction, it enables flexibility in fuel supply as power operators are not dependent on a constant supply of a particular fuel. Economically, co-firing is encouraged in many countries as fiscal incentives are available for power operators going ”green” and emission trading spreads. In order to increase the fired biomass share, an important requirement that has to be met by power operators concerns NOx emissions, as future EU targets

tighten. It is therefore crucial to characterise the combustion of biomass and waste fuels with pulverised coal, in order to minimise NOxformation.

Experimental and modeling tools have been used to investigate the NOx formation

from fuel-bound nitrogen of biomass fuels and coal/biomass mixtures.

To start with, pyrolysis behaviour of single fuels and their mixtures was investigated at fundamental scale by thermogravimetric analysis. It has been shown that all inves-tigated biomass fuels present similar thermal degradation behaviour, with a maximum weight loss between 300–380℃. Woody and agricultural biomass materials show higher devolatilisation rates than animal waste. When comparing different fuels, the percentage of fuel-bound nitrogen converted to volatile bound-N species (NH3, HCN, HNCO) does

not correlate with the initial fuel-N content. Biomass pyrolysis resulted in higher volatile-N yields than coal, which potentially indicates that volatile-NOx control during co-firing might

be favored. No significant interactions occurred during the pyrolysis of coal/biomass mix-tures at conditions typical of thermogravimetric analysis (slow heating rate). Evolved gas analysis of volatile species confirmed the absence of mutual interactions during woody biomass co-pyrolysis. However, non-additive behaviour of selected gas species was found during slaughter and poultry litter co-pyrolysis. Higher CH4 yields between 450–750℃

and higher ammonia and CO yields between 550–900℃ were measured. Such result is likely attributed to catalytic effects of alkali and alkaline earth metals present in high

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quantity in animal waste ash. The fact that the co-pyrolysis of woody and agricultural biomass is well modeled by simple addition of the individual behavior of its components, permits to predict the mixture’s behavior based on experimental data of single fuels. On the other hand, animal waste co-pyrolysis presented in some cases synergistic effects in gas products although additive behavior occurred for the solid phase.

In order to study the devolatilisation of selected fuels at conditions closer to industrial applications (high temperature, high heating rate) and follow closely the fast evolution of gas compounds, a newly designed heated wire mesh reactor (HWM) has been integrated in an FTIR spectrometer for in-situ analysis of evolved volatile species. Devolatilisation experiments have been carried out with coal and high fuel-N content animal waste. It is shown that peak temperature is the parameter presenting the strongest influence on the devolatilisation process. Devolatilisation of coal/biomass mixtures is characterised by a dual behaviour. On one hand, it is shown that the mixtures follow the parent fuels behavior, showing additivity for the overall weight loss and for the evolution of main C-species. On the other hand, some non-additive behavior is observed in the evolution of N-species. Fuel-bound nitrogen partitioning in volatile-N (NH3, HCN, HNCO) and

char-N was found to be strongly temperature dependent.

As a further step towards the industrial scale, bench scale (co-)combustion experi-ments have been carried out in a newly designed 50 kW pulverised fuel combustion re-actor. Results show that biomass co-firing is an effective technology for controlling NOx

emissions while mitigating CO2emissions from coal-fired plants. The NOxreduction rate

achievable by primary DeNOxmeasures (i.e. air-staging) may increase with biomass

co-firing compared to conventional coal co-firing. Up to a 70% reduction rate was achieved during our experiments. Such reduction rate is closely depending on the combustion conditions, the fuel-N content of the biofuel and its relative partitioning in combustion. With the adoption of air-staging, a minimum in the final NOx emissions is achieved for

primary zone stoichiometries (λ1) between 0.7–0.9, however the optimal λ1 is depending

on the biofuel. Co-firing presents a lower and broader λ1range that minimises final

emis-sions compared with coal firing, due to the fact that enhanced devolatilisation and faster oxygen depletion occur during co-firing. During biomass co-firing, higher fuel-bound ni-trogen release was measured in the reducing zone compared with coal combustion. NOx

precursors were effectively reduced to molecular nitrogen in the burnout zone, resulting in lower conversion of fuel-N to NOx at the exit. The overall fuel conversion was highly

dependent on the particle size of the input fuel. Due to its particle diameter up to mm size, chicken litter co-firing presented lower final fuel conversion than pure coal. The co-firing experiments showed that the final NOxemissions are less than proportional to

the nitrogen content of the input fuel. An empiric correlation between fuel properties and final NOx emissions was proposed.

In order to obtain model input data regarding initial fuel devolatilisation for CFD numerical simulations, the kinetic pools for species release were determined by application of the Kissinger method using a TG-FTIR system with heating rates of 10, 30 and 100 ℃/min. The Functional Group-Depolymerization Vaporisation and Cross-linking (FG-DVC) and FG-BioMass network models were applied to selected fuels. The model was used to extrapolate global kinetic parameters at high temperature and high heating rate conditions.

Finally, global kinetic parameters and fuel nitrogen partitioning (HCN, NH3, tar-N,

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and poultry litter have been modeled and validated against experiments. Results showed that accurate fuel nitrogen partitioning and NOx gas phase chemistry are of primary

importance. Reasonably good results were achieved using a variable char-N conversion into nitrogenous species as function of the local primary stoichiometry.

Gianluca Di Nola

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Karakterisering van Biomassa Brandstoffen voor NOxemissies in Meestook applicaties

Samenvatting

In dit proefschrift wordt onderzoek naar de invloed van verschillende soorten biomassa en verbrandingscondities op de NOxemissies bij het meestoken van biomassa met kolen

beschreven.

Fossiele brandstoffen hebben sinds de industri¨ele revolutie het energiebeeld gedomi-neerd. In de laaste decennia van de 20e eeuw maken toegenomen zorgen betreffende hun beschikbaarheid en milieurisico’s, zoals het broeikaseffect, in relatie tot hun intensieve gebruik, alsmede stijgende olieprijzen, de ontwikkeling van duurzame energiebronnen echter aantrekkelijker. Het verhogen van het aandeel van duurzame energiebronnen in de energievoorziening wordt sterk gestimuleerd in de EU. De Europese commissie heeft als richtpercentage 22% aangegeven voor de totale elektriciteitsproductie vanaf 2010.

Meestoken van biomassa en afvalbrandstoffen in combinatie met fossiele brandstoffen lijkt een veelbelovende optie om duurzame brandstoffen in de energie sector te introduc-eren en om CO2 emissies te verminderen. Meestoken van biomassa is al in verscheidene

kolengestookte elektriciteitscentrales toegepast, waarbij een limiet van ongeveer 5% van de warmteinhoud gehanteerd wordt. Deze methode biedt verscheidene voordelen. Naast de reductie van CO2emissies zorgt het voor flexibiliteit in de brandstoftoevoer aangezien

de energieproducenten niet afhankelijk zijn van een constante toevoer van een specifieke brandstof. Economisch gezien wordt meestoken in veel landen gestimuleerd gezien de aanwezigheid van fiscale impulsen voor energieproducenten die overstappen op ”groene” energie en de verbreiding van emissiehandel. Gezien de toekomstige striktere EU doel-stellingen moeten de energieproducenten aan een belangrijke eis betreffende de NOx

emissies voldoen om het aandeel gestookte biomassa te vergroten. Het is daarom cruci-aal om de verbranding van biomassa en restprodukten met poederkool te karakteriseren om NOx vorming te minimaliseren.

Experimenten en modellen zijn gebruikt om NOx vorming van brandstofgebonden

stikstof in biomassa en kolen/biomassa mengsels te onderzoeken.

Allereerst is het pyrolyse gedrag van pure brandstoffen en hun mengsels onderzocht op fundamentele schaal met behulp van thermogravimetrische analyse. Het bleek dat alle on-derzochte biomassa brandstoffen een vergelijkbaar thermisch afbreekgedrag vertonen, met een maximum gewichtsverlies van 300–380℃. Houtachtige en agrarische biomassa materi-alen vertonen een hogere pyrolysesnelheid dan biomassa van dierlijke oorsprong. Wanneer verschillende brandstoffen worden vergeleken, dan blijkt het percentage brandstofgebon-den stikstof dat is omgezet in vluchtige stikstofverbindingen (NH3, HCN, HNCO) niet te

correleren met het initi¨ele stikstofgehalte in de brandstof. Biomassa pyrolyse resulteert in hogere vluchtige-N component opbrengsten dan steenkool, wat zou kunnen betekenen dat NOxbeheersing tijdens co-firing kan worden bevorderd. Geen noemenswaardige

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ties tijdens houtachtige biomassa co-pyrolyse. Echter, niet-additief gedrag van sommige gasvormige stoffen werd waargenomen tijdens co-pyrolyse van slacht- en pluimveeafval. Hogere CH4 opbrengsten (tussen 450–750℃) en hogere ammonia en CO opbrengsten,

tussen 550–900 ℃, werden gemeten. Deze resultaten zijn waarschijnlijk toe te schri-jven aan catalytische effecten van alkali- en aardalkalizouten, die in grote mate aanwezig zijn in de as van dierlijk afval. Het feit dat de co-pyrolyse van houtachtige en agrarische biomassa goed kan worden gemodelleerd door simpelweg het sommeren van het individu-ele gedrag van alle componenten, maakt het mogelijk om het gedrag van het mengsel te voorspellen gebaseerd op experimentele data van de afzonderlijke brandstoffen. Aan de andere kant vertoonde de co-pyrolyse van dierlijk afval synergistische effecten wat betreft de gasvormige producten, hoewel additioneel gedrag optreedt betreffende het koolresidue. Om de pyrolyse van enkele brandstoffen onder condities dichter bij indutri¨ele ap-plicaties (hoge temperaturen, hoge verwarmingsnelheden) te onderzoeken en de snelle ontwikkeling van de gasvormige componenten nauwkeurig te volgen, is een nieuw ont-worpen ”heated wire mesh reactor” (HWM) ingebouwd in een FTIR spectrometer voor in-situ analyse van de ontstane vluchtige stoffen. Pyrolyse experimenten zijn uitgevoerd met steenkool en dierlijk afval met een hoog stikstofgehalte. De piek temperatuur is de parameter die de grootste invloed op het pyrolyseproces blijkt te hebben. De pyrolyse van steenkool/biomassa mengsels wordt gekarakteriseerd door twee factoren. Aan de ene kant volgen de mengsels het gedrag van de originele brandstof, met additioneel gedrag voor het totale gewichtsverlies en voor de ontwikkeling van de belangrijkste C-componenten. Aan de andere kant wordt er geen additioneel gedrag waargenomen in de ontwikkeling van de N-componenten. De verdeling van de brandstofgebonden stikstof in vluchtige N-componenten (NH3, HCN, HNCO) en char-N bleek zeer temperatuursafhankelijk te

zijn.

Als een verdere stap richting industri¨ele schaal zijn bench-scale meestookexperimenten uitgevoerd in een nieuw ontworpen 50 kW verbrandingsreactor voor poedervormige brand-stof. De resultaten geven aan dat het meestoken van biomassa een effectieve technologie is voor het controleren van NOxemissies en het tegelijkertijd verminderen van de uitstoot

van CO2 in kolengestookte centrales. De reductie van NOx door middel van primaire

DeNOx maatregelen (d.w.z. ”air-staging”) kan, in vergelijking tot conventionele

kolen-verbranding, worden vergroot door het meestoken van biomassa. NOxreducties tot 70%

zijn behaald tijdens onze experimenten. Zulke reducties zijn sterk afhankelijk van de verbrandingscondities, het stikstofgehalte van de biobrandstof en de relatieve verdeling van brandstofgebonden stikstof tijdens verbranding. Door toepassing van ”air-staging” wordt een minimum in de uiteindelijke NOx-emissies bereikt voor primaire-zone- waarden

(λ1) tussen 0.7–0.9. De optimale waarde van λ1 is echter afhankelijk van de

biobrand-stof. Meestoken levert een lager en groter λ1-bereik dat uitlaatemissies minimaliseert in

vergelijking met kolenverbranding, omdat er tijdens meestoken sprake is van verbeterde pyrolyse en snellere zuurstofafname. Tijdens het meestoken van biomassa werd een hogere uitstoot van brandstofgebonden stikstof gemeten in de gereduceerde zone in vergelijking tot kolenverbranding. NOxbronnen werden effectief gereduceerd tot moleculair stikstof

in de uitbrandzone hetgeen resulteerde in een lagere omzetting van brandstofgebonden stikstof tot NOx aan de uitlaat. De totale brandstofconversie was sterk afhankelijk van

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van millimeters voorkwamen in de kippenmest, vertoonde het meestoken hiervan lagere brandstofconversies dan pure kolenverbranding. De meestookexperimenten laten zien dat de uiteindelijke NOxemissies minder dan evenredig vari¨eren met het stikstofgehalte van

de toegevoerde brandstof. Een empirische correlatie tussen brandstofeigenschappen en uiteindelijke NOx emissies is voorgesteld.

Om initi¨ele brandstofpyrolyse-gegevens te verkrijgen die benodigd is als invoer voor CFD simulaties, zijn de kinetische pools voor het vrijkomen van specifieke componenten bepaald door toepassing van de Kissinger methode, gebruikmakende van een TG-FTIR systeem met verwarmingssnelheden van 10, 30 en 100 ℃/min. De Functional Group-Depolymerization Vaporisation and Cross-linking (FG-DVC) en FG-BioMass netwerk modellen zijn toegepast op geselecteerde brandstoffen. Het model is gebruikt om globale kinetische parameters te extrapoleren bij hoge temperatuur en hoge verwarmingssnelheid. Tenslotte zijn kinetische parameters en gegevens omtrent de brandstofstikstof verdel-ing (HCN, NH3, teer-N, koolresidue-N) verkregen middels toepassing van de

FG-DVC/FG-Biomass modellen en deze zijn gebruikt als invoergegevens voor CFD voorspellingen van NO-vorming in de verbrandingsreactor. De NOx-uitstoot van een poederkoolvlam en

twee meestookgevallen met slacht- en pluimveeafval zijn gemodelleerd en gevalideerd met experimenten. De resultaten laten zien dat een accurate brandstof-stikstof-verdeling en NOx gasfase-chemie van primair belang zijn. Redelijk goede resultaten zijn bereikt

door middel van een variabele koolresidu-N omzetting tot stikstofhoudende gassen als functie van de lokale primaire stoichiometrie.

Gianluca Di Nola

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”Considerate la vostra semenza fatti non foste a viver come bruti ma per seguir virtute e canoscenza” (Dante Alighieri, Divina Commedia, Inferno canto XXVI, 118-120)

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1

Introduction

In this introductory part, the energy policy in the Netherlands is described and framed within the current world energy scenario, stressing the importance of biomass as a re-newable energy source which can contribute to the abatement of CO2 and NOxemissions.

The different technologies currently available for the implementation of biomass fuels in Power Generation are described, with emphasis on direct Co-firing. Then, the motivation and scope of this research work are illustrated. Finally, the methodology followed and the outline of the dissertation are given.

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1.1 World Energy Outlook

Global energy needs are growing and are likely to continue to grow steadily for at least the next three decades [159]. If governments continue to adhere to current policies, which is the assumption of the World Energy Outlook’s reference scenario, the world primary energy demand is going to expand by almost 60% from 2002 to 2030, an average annual increase of 1.7% per year. The projected rate of growth is, nevertheless, slower than over the past three decades, when demand grew by 2% per year. As it can be seen in figure

1.1, fossil fuels will continue to dominate global energy use for the next three decades. They will account for around 85% of the increase in world primary demand over the period 2002–2030 [159]. Among fossil fuels, natural gas will grow the fastest, however oil will remain the most important energy source. Oil demand will increase from 75 mb/d in 2000 to 120 mb/d in 2030. Coal, which remains important in power generation because of its low cost, will still account for 24% of total primary energy supply. Renewable energy sources will grow in importance, while the share of nuclear power in the world energy supply will drop. More than 2/3 of the growth in world energy use will come from the developing countries, where economic and population growth are the highest [159].

0 1000 2000 3000 4000 5000 6000 7000 1970 1980 1990 2000 2010 2020 2030 M to e

Coal Oil Gas Nuclear Hydro Other

Coal Oil Gas Other renewables Nuclear Hydro

Figure 1.1: World primary energy demand by fuel: Reference Scenario [161]

Furthermore, projections through the year 2030 show a continuous increase in global carbon dioxide emissions, if no new policies and measures are put in place. Under this scenario, CO2 emissions are projected to grow by 69%, slightly more than the growth of

66% in energy supply [159]. The Kyoto Protocol

In order to reduce the global warming, mainly caused by CO2 emissions, several

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1.1. World Energy Outlook 3 of the Parties in Kyoto, Japan, on 11 December 1997, a number of nations have ap-proved an addition to the treaty: the Kyoto Protocol [255]. The Kyoto Protocol calls for those industrialised countries and transition economies listed in its Annex I to reduce their overall greenhouse-gas emissions by at least 5% below their 1990 levels on average over 2008–2012. Annex I includes all the OECD countries except Korea and Mexico. Emission-reduction commitments vary among countries. To achieve their targets, coun-tries can implement domestic emission reduction measures or use international ¸Sflexible mechanisms ˇT. The latter include emissions trading and joint implementation, a scheme in which countries invest in projects to reduce net emissions in other Annex I countries and thereby earn credits towards meeting their own national commitments. To take effect, the Protocol must be ratified by at least 55 nations, and the Annex I countries ratifying the Protocol must represent at least 55% of that groupˇSs total emissions in 1990.

By July 2004, 123 countries, including all EU countries, had ratified the Protocol. Although the ratifying countries included all but eight Annex I countries, their aggregate emissions made up only 44% of total Annex I emissions. Two countries that had not ratified ˝U the United States and Russia ˝U made up almost all the remaining emissions. The Protocol could not come into effect until one of those two countries ratifies it. The United States and Australia had announced that they do not intend to do so. On the 18th _{of November 2004, Russia ratified the Kyoto protocol.}

The Kyoto Protocol, an international and legally binding agreement to reduce green-house gases emissions worldwide, entered into force on 16 February 2005. As of the 7th

of July 2006, 164 states and regional economic integration organizations have deposited instruments of ratifications, accessions, approvals or acceptances. The total percentage of Annex I Parties emissions is currently 61.6%. The updated status of the Protocol can be found at http://unfccc.int.

1.1.1 Energy policy in The Netherlands

In the Netherlands, energy from renewable sources amounted in 2002 to 48 PJ, of which the majority was biomass and renewable municipal solid waste, see table1.1. In absolute terms, the supply of renewables increased threefold from 1990, when only 16.7 PJ of renewables were used. However, owing to the increase in total primary energy sources (TPES), the share of renewables in TPES has increased only from 1% in 1990 to 2% in 2002 [160].

The goals of the Dutch government as stated in the Third White Paper on Energy comprise continuous energy savings, energy efficiency improvements and a further de-velopment of renewable energy from 1% in 1995 to 10% in 2020 [235]. This target is almost fivefold of the 53 PJ in 2003, i.e. 270 PJ in 2020. From this target, 40% (120 PJ) is planned to be realised with energy from waste and biomass [191]. It is thus clear the strategic role given to waste and biomass materials as renewable energy source. The potential of biomass and waste for power/heat generation becomes even more important since waste landfilling is being forbidden in the Netherlands and waste incineration is characterised by very low efficiencies. Following the EU directive to promote electricity production from renewables (2001/77/EC), the Netherlands has agreed to an indicative target of generating 9% of its electricity from renewables by 2010 [160].

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Table 1.1: Renewable energy production in the Netherlands, 1990 to 2002 (PJ). (Source: CBS/Novem, 2002. a_{2002 figures are provisional)}

Domestic production 1990 1995 2000 2001 2002a

Hydropower 0.70 0.73 1.18 0.97 1.0

Wind energy 0.46 2.62 6.86 6.81 7.5

Photovoltaics 0.00 0.01 0.07 0.11 0.14

Thermal solar energy 0.07 0.17 0.41 0.48 0.5

Heat pumps – 0.24 0.63 0.80 1.0 Heat/cold storage 0.01 0.07 0.47 0.66 0.8 Sub-total 1.26 3.84 9.62 9.83 10.94 Bioenergy 15.40 17.06 28.07 32.13 37.0 – Waste incineration 6.31 5.58 11.59 12.86 12.8 – Biomass incineration 6.48 6.51 10.67 13.44 18.7 * local - heat 6.48 6.48 7.40 7.40 – * local - energy 0 0 1.49 1.49 – * co-firing 0 0.03 1.78 4.55 – – Biomass gasification 0 0 0 0 0 – Biomass fermentation 2.62 4.97 5.80 5.82 5.4 Total 16.66 20.89 37.68 41.97 48.0

combines security of supply, economic efficiency and high ecological standards. This requires changes in technology, economy and social structures over a long period. The specific objectives for a sustainable energy system are:

– Maintaining the present level of security of supply for electricity and gas;

– Achieving an annual CO2reduction of 9.4 Mt in 2008–2012 (as part of the Kyoto

obligations);

– Improving energy efficiency by 1.3%/year;

– Increasing the share of renewables in electricity supply to 9% of TPES by 2010 and to 10% by 2020.

1.1.2 Biomass as sustainable energy source

The utilisation of biomass as a sustainable energy source to reduce the dependance on fossil fuels and the CO2production by industrial applications will increase steadily in the

first part of this century. According to IEA forecasts [161, 162], biomass used for power generation will triple worldwide over the next three decades, increasing from the actual 1% share to a 2% of the global electricity production. The largest increase will be in OECD Europe (from <2 of today to 4%), driven by strong government support to pro-mote renewables [161, 162]. Biomass will be used mostly for the production of electricity and heat in decentralised applications in industry, or district heating. A percentage of it is expected to be used in co-firing with coal, as a way to mitigate CO2 emissions from

coal-fired power plants [161].

In the Netherlands, the Dutch government and the electricity production companies which operate coal-fired power plants have closed an agreement (so-called coal covenant). In this agreement, the companies are committed to reduce the CO2emissions from coal

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1.2. NOx emissions and their impact 5

co-firing will therefore play a primary role in the Dutch power sector at least for the next decade.

In order to promote its development, a Biomass Action Plan has been developed in co-operation with market parties and has been sent to the parliament in late 2003. At present, the use of biomass is already promoted by providing subsidies for those energy providers that implement biomass co-firing in their units [160].

Table 1.2: Biomass and waste utilisation potential in the Netherlands (PJ) [191]

Technology 1995 2000 2010 2020 Waste incineration 5.6 11.6 15 20 Waste combustion 6.4 7.4 8 8 Co-firing 0.1 1.8 39 42 Stand alone CHP – 1.5 10 40 Waste incineration 5.6 11.6 15 20 Landfill gas/digestion 5.0 5.5 8 10

Table 1.2 reports a prognosis of the biomass and waste utilisation potential in the Netherlands [191], listing different technological choices for the implementation of biofuels in the power sector. Co-firing appears as the dominant technology for the utilisation of biomass in the power generation, with a steep growth between 2000–2010.

1.2 NO

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emissions and their impact

Nitrogen oxides (NOx) comprise various nitrogen compounds like nitrogen dioxide (NO2)

and nitric oxide (NO). These compounds are originated from combustion processes and they are harmful for the environment and for human health. Nitrogen oxides play an im-portant role in the atmospheric reactions that create harmful particulate matter, ground-level ozone (smog) and acid rains. Moreover, NOxemissions contribute to the formation

of fine particles, with consequences on human health (respiratory diseases). NOx

emis-sions also can cause deterioration of water quality (i.e. eutrophication, an alteration of aquatic/terrestrial ecosystem caused by an enrichment in nitrogen content), leading to oxygen depletion that degrades water quality and harms the aquatic flora and fauna. Furthermore, NOx can react with common organic chemical species and ozone to form

toxic products that may cause biological mutations (i.e. nitrates). Last but not least, one of the NOxspecies, nitrous oxide -N2O-, is a strong greenhouse gas. Its concentration in

the atmosphere is about 320 ppbv, increasing by 0.2% annually due to human activity.

N2O contributes to global warming, accumulating in the atmosphere, contributing to the

destruction of the ozone layer and causing a gradual rise of earth’s temperature. In figure

1.2the airborne pollution by NOxin the atmosphere over Europe is shown.

1.3 NO

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emissions: environmental legislation

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Figure 1.2: Air pollution in the atmosphere over Europe by NOx emissions (source:

KNMI, 2001)

recognised pollutants such as SO2and NOx, to reduce the amount of wastes going to

land-fills and address the perceived threat to global warming by mitigating CO2production.

However, the priority assigned by different governments and the policies implementation methods varies greatly.

At European Union level, the EU Large Combustion Plant Directive (LCPD, 2001/80/EC, [261]) sets new limits for the emissions of sulphur dioxide (SO2),

nitro-gen oxides (NOx) and fine dust particles for all new combustion plants (≥ 50 MWth).

The LCP Directive entered into force at the end of 2001, replacing the earlier Direc-tive from 1988 on large combustion plants (DirecDirec-tive 88/609/EEC) and tightening the Community requirements for air pollution control from new combustion plants. The new limits for ¸Sexisting ˇT plants will be binding from 2008. The LCP Directive encourages the combined generation of heat and power and sets specific emission limits for the use of biomass as a fuel in order to regulate NOx emissions. All combustion units with an

installed capacity greater than 20 MW will also have to contend with the EU Emission Trading Scheme as part of its commitment to achieve its Kyoto greenhouse gas emission reduction targets. This CO2 emissions trading scheme is operational since 2005.

The limits set by the EU on the NOx emissions with the Large Combustion Plants

Directive (LCPD) 2001/80/EC [261] and with the Waste Incineration Directive (WID) 2000/76/EG [260] are summarised in table 1.3.

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1.3. NOx emissions: environmental legislation 7

installations comprises:

– The general Netherlands Emission Guidelines (Nederlandse Emissierichtlijn, NER); – A special arrangement under the NER for clean waste wood combustion (NER-BR); – A special arrangement under the NER for biomass pyrolysis (NER-BR voor

pyrol-yse);

– The emission standards for large combustion units (Besluit Luchtemissie Stookin-stallaties, BEES A/B);

– The degree of airborne emissions from waste incineration units (Besluit Verbranden Afvalstoffen , BVA)

Table 1.3: NOx emission limits set by EU Directives. Values are expressed in mg/Nm3

(6% O2)

Large Combustion Plant Directive (2001/80/EC)

Plant type Fuel 50–500 MWth >500 MWth

Existing (until 2016) solid fuel 600 500

Existing (from 2016) solid fuel 600 200

50–100 MWth 100–300 MWth >300 MWth

New solid 400 200 200

New biomass 400 300 200

Waste Incineration Directive (2000/76/EG)

50–100 MWth 100–300 MWth >300 MWth

solid 400 300 200

biomass 350 300 300

The fuel and the size of the installation determine which of the above regulations apply. The limits mentioned in these regulations are guidelines issued by the national government. In practice, the local permit-issuing authority (the municipal and provincial government) may decide which exact limits apply. Table1.4summarises the current NOx

emission limits in the Netherlands.

Table 1.4: NOxemission limits at Dutch national level. mg/Nm3(6% O2∗)

NER NER NER BLA BEES A BEES A

(clean waste wood) pyrolysis (<300 MWth) (>300 MWth)

NOx 300 600a 60 100 100 200

∗_{Recalculated from 11%} a_{Only if larger than 250 kW}

thand with more than 80% particleboard in fuel

In order to further reduce NOx emissions, the government has decided to introduce

NOx emissions trading. The national emission target, based on the NetherlandsˇS

inter-national commitments, is 231.000 tonnes of NOx in 2010. The scheme is expected to

increase investments in emissions control technologies because low-NOxtechnologies are

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1.4 Biomass in Power Generation

Biofuels have long been used to generate process steam and electricity. The main tech-nological options available for the implementation of biomass as fuel in power generation (heat and electricity production) are combustion and gasification. Other thermal pro-cesses, such as pyrolysis and/or liquefaction are not yet mature technologies for large scale utilisation, nevertheless they may represent an interesting option for smaller scale CHP and biorefinery (i.e. biofuel production) applications.

The technological options can be briefly summarised as follows [208]: • Direct co-firing: the pre-processed biomass is directly fed to the boiler;

• Indirect co-firing: the biomass is gasified, the produced syngas is fed to the boiler;

• Parallel combustion: the biomass is fired in a separate boiler for steam produc-tion.

All these approaches have already been implemented to a certain extent, either on a demonstration or a commercial basis, in several European countries and in the USA, as a consequence of extensive R&D initiatives sponsored by the European Union and the US Department of Energy (USDOE), within various programmes such the JOULE (Joint Opportunities for Unconventional or Long-Term Energy Supply), APAS (Activit´e de Promotion, D’Accompagnement et de Suivi) and the IEA Bioenergy Task 32, as summarised in several excellent reviews [92, 150, 151, 158, 208, 275, 328–330].

Each of the aforementioned technological options is characterised by its own advan-tages and drawbacks. However, in general, biomass utilisation for power generation in existing coal-fired power plants (centralised generation) presents a number of advan-tages compared with decentralised power generation in small plants firing biomass [150], namely:

– The existing plant capacity represents, even with relatively small biomass fractions of the total fuel input, the base for a high and readily available input potential for biomass;

– The overall efficiency of large power plants is higher compared to that of small, biomass-fired plants;

– In case of seasonal non-availability or shortfalls of bio-fuel supply, the power avail-ability can be guaranteed by coal (i.e. a high security of supply combined with high fuel flexibility);

– The additional investment cost needed for implementing biomass co-firing in ex-isting coal-fired units are relatively low compared to new dedicated combustion systems.

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1.4. Biomass in Power Generation 9

1.4.1 Direct co-firing: Technology and Status

Co-firing, also defined as the firing of two dissimilar fuels -typically coal and biomass-at the same time in the same boiler, has been proposed as a means for using biomass in coal-fired utility boilers in a highly cost-effective manner.

Direct co-firing can be virtually applied to all types of coal-fired boilers (pulverised fuel-fired, bubbling and fluidised bed, grate fired, cyclone and stoker boilers). However, since the vast majority of coal power generation is realised with pulverised fuel (PF) technology, in this dissertation co-firing always refers to this application, unless explicitly mentioned.

Direct co-firing represents the most cost-effective approach to biomass utilisation by the electric utility industry [328]. Compared with other approaches, it is considered to have a short development time, low-risk and high social benefits [31]. This technology has been demonstrated at commercial scales in essentially every (T-fired, wall-fired, cyclone) boiler type, combined with many commercially significant (lignite, sub-bituminous coal, bituminous coal, brown coal) fuel types.

Within the direct co-firing technology, 4 basic options can be outlined [208]:

• Upstream mixing. The biomass fuel is mixed with the coal before the coal feeders. The mixed fuel is sent through the coal mills and distributed across the coal burners. It is the simplest option with the lowest capital cost. It may, however, involve the highest risk of interference with the coal-firing capability. It can be applied for a limited range of biofuels and, due to the density differences, only for biofuel shares <5% on thermal basis. Furthermore, it can be applicable bearing in mind the different milling characteristics between coal and biomass (i.e. not suitable for every biomass fuel);

• Separate handling, joint injection. The biofuel is handled separately from the coal and injected into the pulverised fuel pipeline upstream of the burners or at the burners. It involves higher capital cost as the installation of the biofuel handling/transport system is required. It may complicate the boiler control and operation;

• Separate handling, separate injection. The biofuel is handled, transported and fired completely independent from the coal, via dedicated burners. It involves ever higher capital cost, but it generally lowers the operational risks;

• Biomass reburning. The biofuel is used as a reburn fuel for NOxcontrol, through

an ad hoc-designed reburn system placed above the coal burners. It constitutes a promising option, currently in the testing/demonstration phase.

The selection of the appropriate approach typically depends on a number of fuel- and site-specific factors.

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However, in order to reach the goal of renewable energy produced by co-firing shown in table1.2, it is estimated that 14% on thermal basis of coal should be replaced by biomass and waste fuels in the existing coal-fired units [179]. Therefore, increasing quantities of biofuels will need to be fired in the coming years to comply with such target. Moreover, in order to ensure both capacity and availability of power & heat supply, the operational flexibility of the plant becomes of primary importance. Given the seasonal character of several biomass materials, an important requirement for plant operators is related to the possibility of co-firing different biomass and waste types to ensure fuel supply.

In conclusion, increasing the share of biomass fuels in coal-fired utility boilers cannot be achieved if its effects on at least the NOxemissions and fuel burnout are not known

be-forehand. Fuel characterisation combined with combustion modeling is therefore crucial in order to predict the effects of biomass co-firing on plant operation.

1.4.2 Open challenges

Biomass and waste co-firing can influence the overall performance of a utility boiler, bring-ing operational advantages as well as challenges. Research is ongobring-ing for understandbring-ing and solving a number of issues related to pulverised biomass co-firing, as highlighted in some recent publications [31, 208, 275, 286, 319, 328, 330]. The areas of concern can be divided as follows:

• Upstream. It comprises all issues related to the biomass supply, preparation and handling, upstream the burners. Fuel availability is crucial since biofuels usually need to be sourced locally in order to capitalise the investments. Several biomass feedstocks may only be available seasonally. Given the biomass low bulk density, co-firing implies practical issues related to the biomass transport and storage. Fur-thermore, biomass fuels require typically a dedicated fuel preparation, including milling and drying (to improve combustion efficiency), depending on the biofuel type.

• In-furnace. Biomass co-firing introduces several issues into boiler operation, that may have potential for deleterious effects but can nevertheless be solved. Several biomass types (i.e. straw, fast-growing energy crops, animal waste) may contain high amounts of alkali, alkaline earth metals and chlorine compared to coal. This can cause high temperature corrosion and slagging/fouling on heat exchangers sur-faces, worsening the boiler efficiency and lifespan. NOx emissions are generally

difficult to predict, since biomass fuels may have either lower/higher fuel-bound nitrogen content than coal and final emissions can increase, decrease or remain constant compared to coal firing, heavily depending on boiler operation;

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1.5. Motivation and Scope 11

1.5 Motivation and Scope

Co-firing biomass fuels into coal-fired units impacts the overall plant performance. Biomass fuels present different physical and chemical properties compared to coal. These differ-ences result in physical and chemical changes of the combustion process. Furthermore, a number of upstream and downstream issues arise when co-firing biomass fuel with coal, as earlier discussed.

The practice of biomass co-firing is destined to increase heavily in the Netherlands in the next decades, as foreseen by the current Dutch energy policy. NOx emissions

from power generation is of primary concern, as both national and European legislation tighten on NOxlimits. With the current knowledge of biomass co-firing, it is not possible

to predict the impact of various biomass fuels on the overall NOxemissions.

Because the primary control of NOxemissions from solid fuel combustion relies heavily

upon the management of volatile nitrogen species, it is important to consider the fate of fuel nitrogen during the pyrolysis and combustion of coals and biomass fuels in order to examine the NOx emissions behavior from biomass co-firing.

The objectives of this research work can be summarised as follows:

1. Develop more knowledge on the thermal behavior of various biomass fuels and coal-biomass mixtures, with emphasis on the fuel-bound nitrogen partitioning between volatiles and char.

2. Investigate the impact of biomass co-firing on NOx emissions, in particular with

the adoption of primary DeNOx measures (air staging).

3. Predict NOxformation from bench scale biomass co-firing via numerical modeling,

which represents the first step towards large scale modeling.

1.6 Methodology

This work aims at contributing to a better understanding and prediction of NOxemission

formation/destruction from biomass co-firing. Starting at a fundamental level and mov-ing towards the industrial application of biomass co-firmov-ing, experimental and modelmov-ing activities are carried out in an integrated approach, as shown in figure1.3.

1.6.1 Experimental activities

Experimental activities are carried out on different setups, from lab to bench scale. • Lab scale: two different lab scale setups are utilised, in order to characterise the

thermal behavior of fuels under a wide range of conditions, from slow to fast heating rates.

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Fuel characterisation Thermogravimetric analysis - TG-FTIR Numerical NO predictionsx - CFD tool

Large scale NO modellingx

Funda menta l scale Industrial application Fast devolatilisation - HWM-FTIR

Bench scale co-firing experiments - 50 kW PF flow reactor

Pyrolysis modelling (network models) - FG-DVC, FG-biomass

Figure 1.3: Schematic of the approach followed in this research work, showing the links between its different parts

biomass fuels and to study whether coal-biomass mixtures follow their par-ent fuel behavior during decomposition at slow heating rates, or if synergistic effects take place.

– Fast devolatilisation. Coal and selected biomass fuels are characterised at high heating rates under inert conditions and the fuel-bound nitrogen par-titioning between char and volatiles is quantified. The goal is to study the thermal decomposition of different fuels at fast heating conditions, relevant for industrial applications. Then, the fast devolatilisation of coal-biomass mixtures is studied, to assess whether the mixtures follow their parent fuel behavior or if synergistic effects take place.

• Bench scale: Coal combustion and biomass co-firing experiments are carried out at a 50 kWelPF flow combustion reactor to investigate the combustion behavior

of coal and coal-biomass mixtures. In particular, the influence of air staging on the final NOxemissions, the combustion efficiency and the evolution of main species as

well as NOx precursors in the fuel-rich zone for different primary stoichiometries.

1.6.2 Modeling activities

Related to the experimental activities, modeling work is carried out. The objectives of the modeling activities are:

• to model the pyrolysis behavior of coal and biomass fuels, with emphasis on the evolution of main species and NOx precursors.

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1.7. Outline 13 obtained from the pyrolysis models into a CFD model of the bench scale 50 kWel

PF flow reactor, in order to predict the NOxformation/destruction during biomass

co-firing. This experience will serve as a first step towards the prediction of NOx

emissions from biomass co-firing in large scale PF coal-fired utilities.

1.7 Outline

The dissertation is divided into 9 chapters and organised in the following way.

Chapter 2 presents a theoretical overview on solid fuel combustion and nitrogen oxides formation. Coal and biomass pyrolysis/combustion mechanisms are introduced and a review on NOxformation and destruction is given. A brief introduction on current

advances in solid fuel combustion modeling, with emphasis on network pyrolysis models and NOx models utilised in CFD simulation closes the chapter. Chapter 3 presents

an evaluation of state-of-the-art pyrolysis and combustion equipment. The experimental facilities that were used in this work are described.

Results from lab scale slow heating rate experiments are presented in chapter 4. The pyrolysis behavior of single fuels is characterised by TG-FTIR, the influence of heating rate is analysed, the volatile gas products evolution quantified with FTIR. The partitioning of the fuel bound nitrogen is quantified. Then, coal/biomass mixtures are investigated. The co-pyrolysis behavior of biofuels is studied to evaluate whether the mixture follows the parent fuels behavior or if synergistic effects occur.

Results of fast devolatilisation experiments are shown in chapter 5. First, the de-volatilisation behavior of selected single fuels is investigated and the influence of main pyrolysis parameters (temperature, heating rate and holding time) is evaluated. The partitioning of the fuel bound nitrogen is studied. Then, the influence of temperature on the devolatilisation of coal-biomass mixtures with increasing biofuel share is examined, in order to assess whether additive behavior or synergistic effects are recorded.

Chapter 6 presents the results of the bench scale (co)-firing experiments. The in-fluence of both primary stoichiometry and of various biomass fuels on the final NOx

emission is assessed. Then, the fate of main (CO2, CO, CH4) and nitrogenous species

(HCN, NH3, NO) is analysed during air staged co-firing for selected fuels. Finally,

ex-perimental correlations are drawn, fuel-N conversion to NO and characterisation of the fuel-N release are presented.

Pyrolysis modeling results are reported in chapter 7. The Functional Group-Depoly-merisation Vaporisation and Cross-linking (FG-DVC) and FG-BioMass models are ap-plied to a selection of coal and biomass fuels. The results of the models are compared with experiments.

NOx prediction with CFD simulation are validated with measurements in chapter

8. Formation rates of volatile species and tars, as well as fuel nitrogen partitioning data are obtained from FG-DVC/FG-BioMass and used as input data for the post-processing simulations. A pulverised coal flame and two co-firing cases with slaughter and poultry litter are validated against measurements.

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If we knew what we were doing, it wouldn’t be called research, would it?

Albert Einstein, 1879-1955

2

Overview of NO

x

formation & destruction in

Biomass Co-firing

After the general introduction on renewable energy and biomass co-combustion given in the previous chapter, a more detailed insight into the combustion process of pulverised solid fuels is presented hereby, from devolatilisation to char oxidation, with particular emphasis on the nitrogen reactions. Then, nitrogen oxides formation and destruction mechanisms as well as reduction strategies are briefly illustrated. The background of the modelling activities performed during this research work is explained, from network models for pyrolysis modelling to CFD tools and the NOxsub-models utilised for nitrogen

oxides prediction from biomass co-firing.

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2.1 Coal and Biomass

Coal

Coal is a complex organic polymer consisting of large polycyclic aromatic clusters of several fused rings strung together by assorted hydrocarbon chains of varying lengths and other heteroatom (N,S,O) linkages [12]. With modern analytical techniques (e.g. NMR, FTIR) four major structures have been established: aromatic clusters, aliphatic bridges and loops, side chains and oxygen groups [66, 302], as shown in figure 2.1. As to the heteroatoms linkages, nitrogen is mainly contained in functional groups, oxygen (between 1–30% depending on rank) is present in the organic matrix as carboxylic acid (-COOH), carbonyl (=CO) and hydroxyl (-OH) functional groups and as heterocyclic oxygen [6]. Organic sulphur is present mostly as alkyl sulphides, thiophenes, disulfides and arylsulfides [264, 377].

Figure 2.1: The structure of a hypothetical coal. Adapted from [308, 373]

Coals are usually classified according to their rank ( a measure of the coalification degree), from lignite to anthracite, based on proximate and ultimate analysis data. Table 2.1

summarises such classification.

Table 2.1: Coal properties for increasing rank (on % daf basis, [348]).

Type Moisture Volatiles C H O LHV

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2.1. Coal and Biomass 17 A commonly used parameter based on the proximate volatile matter content is the fuel ratio (Fixed C/Volatile content). The atomic ratios H:C and O:C are also used to describe coal (and biomass) characteristics, as it will be shown later in this section. In figure2.2the differences in the chemical structure for various coal ranks are finally shown.

Anthracite Low-volatile bituminous High-volatile bituminous Sub-bituminous Lignite Coal rank

Figure 2.2: Chemical structures of various coal ranks. Adapted from [343]

Nitrogen in coal

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N H N pyrrole 400,4 eV N pyridine 398,9 eV N OH pyridone 400,6 eV N O + pyridine-N-oxide 403,2 eV N X R X amino 399,4 eV N X + quaternary 401,5 eV N NO2 CN nitro 405,1 eV nitrile 399,5 eV

Figure 2.3: Overview of N functional forms with their binding energies

M ole % N itro ge n T ype

Weight % Carbon (daf) Quaternary

Pyridinic Pyrrolic

Argonne Premium Coal samples

Figure 2.4: N- functional groups in the Argonne coals by XPS. Adapted from [183]

Biomass

Biomass can be defined as an organic, non-fossil material of biological origin which may be used as fuel for heat production or electricity generation. The Directive 2001/77/EG reports: Biomass means the biodegradable fraction of products, waste and residues from agriculture (including vegetal and animal substances), forestry and related industries, as well as the biodegradable fraction of industrial and municipal waste.

Atomic O:C Ratio

Incresing

Heating Value

AtomicH:CRatio

Biomass Peat Lignite Coal Anthracite Wood Lignin Cellulose Anthracite Lignites Coals 0 0.2 0.4 0.6 0.8 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

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2.2. Pulverised solid fuel combustion 19 Figure 2.5 represents the Van Krevelen diagram [32], also known as coalification diagram, showing the transition between coal and biomass based on the H:C and O:C atomic ratios. Biomass is compared with coal and other fuels, by showing approximate boundaries between different types of solid fuels.

Biomass and derived fuels can generally be divided into four different classes: wood and woody materials, herbaceous and other annual growth crops (e.g. straw, grasses), agricultural by-products and residues (e.g. shells, hulls, pits and animal wastes), refuse-derived fuels (RDF) and waste and non-recyclable papers, often mixed with plastics.

Many biomass materials consist mainly of different fractions of cellulose (∼30–50% w/w), hemicellulose (∼10-50%) and lignin (∼20–40%). Cellulose is a linear polymer of D-glucopyranoside units connected by weak β-glycosidic linkages in4_C

1conformation [299]

that forms the main part of plants cell walls. Its structure is shown in figure 2.6(a). Hemicelluloses comprise a wide variety of plant-derived heteropolysaccharides associated with cellulose and lignin. The most common hemicelluloses are: xylan, glucuronoxy-lan, arabinoxyglucuronoxy-lan, glucomannan and xyloglucan. In angiosperms, xylan is the principal hemicellulose component, a polymer of β(1→4)D-xylopyranose (figure2.6(b)). Lignin is a complex polymer which acts as a binder for the cellulose fibres in wood and certain plants and adds strength and stiffness to the cell walls. Its chemical structure consists of 3D spatially arranged aromatic sub-structures. It imparts considerable strength to the wall and also protects it against degradation by microorganisms. Lignin contributes to about 30% of biomass materials [110]. Due to its aromatic structure, lignin has a higher thermal stability than cellulose and hemicelluloses. The molecular structure of lignin can be found in figure2.6(c).

Due to its carbohydrate structure, biomass is highly oxygenated with respect to con-ventional fossil fuels. Typically, 30 to 40% of the dry matter in biomass is oxygen [165]. Fixed Carbon content can vary widely, from 5 up to 60% of dry matter, depending on ash content. Moisture and ash content can be very diverse as well.

Nitrogen in biomass

Nitrogen is a macronutrient for plants and critical to their growth. Nitrogen content in biomass fuels can vary widely, from <<1% as in core wood up to ∼10% content in meat and bone meal, as table2.2shows. Animal manure and waste is typically characterised by high fuel bound nitrogen content. Nitrogen in biomass is present mainly as quaternary, amino-acidic, proteinic and amine groups [137–139]. However, to the author’s knowledge, no systematic study has been carried out yet as for coal with the aim of characterising nitrogen functionalities in biomass fuels. The fuel bound nitrogen plays a fundamental role in the formation of nitrogen oxides pollutant emissions, as it will be shown in the next sections. The nitrogen content of biomass fuels is therefore an important parameter when considering a biofuel for co-firing applications.

2.2 Pulverised solid fuel combustion

The combustion of solid fuels comprises complex phenomena that can be simplified into: 1) drying;

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(a) Cellulose (b) Hemicellulose

(c) Lignin

Figure 2.6: Molecular structures of lignocellulosic constituents

3) char oxidation

Such division is however merely formal, as in most combustion system these phenomena are overlapping each other. Knowledge of these mechanisms is essential for the design and the optimisation of combustion systems. Therefore, in the following part, more insight is given to the devolatilisation and char oxidation mechanism.

2.2.1 Devolatilisation

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2.2. Pulverised solid fuel combustion 21

Table 2.2: Nitrogen content of various biomass and derived fuels

Biomass type N (wt%, daf) Reference

Wood 0.05–0.99 [40, 175, 197] Sawdust 0.02–0.2 [40, 41, 215, 345] Straw 0.04–0.67 [345] Rice husk 0.11 [141] Rice straw 0.95 [218] Eucalyptus 0.15 [90] Bagasse 0.31–0.38 [90, 325]

Olive residue 0.3–1.1 [40, 108], This work

Poplar 0.47 [90]

Miscanthus 0.64 [175]

Brazil nut shell 0.7 [45]

Corn residue 0.6–0.7 [298], This work

Switchgrass 0.77 [4, 218]

Palm kernel 0.45–2.8 [363], This work

Hazelnut shell 1.3 [40]

Safflower seed 3.1 [39]

Rapeseed 3.91 [290]

Chicken manure 1.5–4.9 [101], This work

Sewage sludge 6–8.1 [41, 327]

Meat and bone meal 8.9–11.1 [20, 78], This work

process [50, 157, 306]. When devolatilisation is carried out under inert conditions it is termed pyrolysis. Coal pyrolysis has been studied extensively for more than a century, a handful of excellent reviews is available [12, 111, 157, 287, 302, 306, 311] that presents past experimental and modeling efforts. Figure 2.7illustrates how devolatilisation influences the complete combustion process. For example, volatile species release controls the ig-nition, temperature and flame stability. Soot formation, which plays a primary role in radiative energy transport, is caused by tar decomposition during secondary devolatili-sation. Moreover, the volatile release controls swelling behavior, particle agglomeration, char reactivity and char physical structure [306].

Three main processes can be identified during coal devolatilisation. Upon heating, coal undergoes mild changes, including the disruption of hydrogen bonds, vaporisation and release of non-covalently bonded molecules and low temperature cross-linking (large aromatic fragments) in low rank coals with more than 10% oxygen [311]. These reac-tions occur generally for T < 500 K and are not very important in the whole pyrolysis process. Then, during primary pyrolysis at higher temperatures (500–1000 K), the weak aliphatic bridges connecting large aromatic clusters in the coal matrix are cleaved, pro-ducing fragments. Those fragments will be released as primary tar if their vapor pressure is sufficiently high to escape the coal matrix. The larger fragments will eventually un-dergo moderate temperature ”cross-linking” reactions to attach to the char. At the same time, the release of some of the functional groups attached to the aromatic clusters and some labile bridges leads to the formation of light gases, including CO, CO2 and light

hydrocarbons.

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Fly Ash

Fragmentation

Char Oxidation or Gasification (900 - 1800 C)

Ignition and Soot Formation (900 - 1800 C)

Devolatilization and swelling (400 - 700 C)

Softening (350 - 600 C)

Heating

Figure 2.7: The different steps in coal combustion. Adapted from [306]

to participate in secondary pyrolysis at high temperatures. Between 1000 and 1300 K, functional groups and side chains attached to the aromatic rings in the tar will thermally decompose releasing additional gases, usually comprising CO, CO2, light hydrocarbons,

H2and heteroatom species (HCN, NH3, SO2, etc.) [27,291]. At T>1200 K and prolonged

residence time, the aromatic rings in the tar will combine to form larger clusters in a process similar to the cross-linking reactions in the char. The size of the clusters will continue to grow until soot nuclei form in the flame. It is the generation of such nuclei from the initial gas-phase reactants that initiate the soot particle inception process. On these nuclei the coagulation, agglomeration and aggregation processes will start and develop further [209].

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Nitrogen partitioning during primary/secondary pyrolysis

Primary pyrolysis is the first step during solid fuel combustion (after moisture evapo-ration), therefore the nitrogen release during this step has an important impact on the subsequent secondary reactions. Coal releases a negligible amount of nitrogen as light gases at temperatures < 400 ℃ [104, 182], probably due to the fact that, as already seen, nearly all nitrogen in coal is contained into stable aromatic structures. Tar release represents the major transport mechanism during primary pyrolysis [66, 305]. Nitrogen appears to be initially released mainly with the tar and the tar release is found to be proportional to the coal rank [68]. N-5 and N-6 functionalities, incorporated as already shown in the previous section into a stable aromatic structure, are released intact as components of tar molecules if secondary reactions are minimised [68].

During secondary coal pyrolysis carried out on their radiant reactor, Chen et al. [67] found that HCN is the main N-species increasing from 6 to 40–48% as tar is converted into soot. A minor amount of HCN might also be released during secondary pyrolysis by the char through ring rupture. When soot formation begins at 1300 K, a portion of the coal nitrogen in the tar is incorporated into the soot matrix, as much as 10% for a sub-bituminous coal and 25% for a high volatile bituminous coal [67], the latter being consistent with the findings of Haussmann [142].

Secondary reactions of tar and char decomposition at high temperatures result in the release of nitrogen species into the gas phase as HCN, NH3, HNCO and N2 [27, 67, 104,

195, 201]. Ammonia and HCN are by far the most important nitrogen volatile species, although N2has been found as the dominant specie from slow heating rate experiments

on fixed bed [19,197]. Minor amounts of HNCO were also detected during rapid pyrolysis experiments [233]. There is ongoing dispute in the scientific world regarding the origin of HCN and ammonia during pyrolysis. Some believe that they are originated from a similar source since the onset temperature of their formation is very close [231], others assume that ammonia is converted to HCN under severe conditions [66]. More recently, it has been proposed that HCN may be the primary N-specie during pyrolysis and that ammonia is partly formed through HCN hydrogenation [27,107,131,185,195,197,281,324,325,362]. Hydrogenation of HCN to NH3is further complicated by the fact that more ammonia has

been found when higher concentrations of oxygen containing species were present, and pyrolysis of low rank coals [192]. Moreover, the measurement method used to quantify N-species is particularly important, since wet techniques do not allow for instance the detection of HNCO, that is measured as ammonia [195, 197, 325]. Several factors can influence the nitrogen partitioning during pyrolysis, such as heating rate, temperature, local stoichiometry and experimental apparatus.

Finally, table 2.3 summarises the findings of previous studies on the nitrogen par-titioning during coal pyrolysis, illustrating the experimental conditions and the main N-products formed. The large variation reported in the measured HCN and NH3yields

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Table 2.3: Summary of previous studies on the nitrogen partitioning in the gas phase during coal pyrolysis

1st _Author _{Exp. apparatus} _{Temp/Heating rate} _Ref _Comments

Haussmann Entrained flow reactor 1050–1400℃/105

K/s [142] HCN is the

Freihaut Heated wire mesh, efr n.i.a_/104

[104, 105] main specie

Chen Radiant flow reactor 1200–1580/104

[67] formed

Williams Coal fired boiler n.i./105

[358]

Blair Pyroprobe max 1400/2-20·103

[42] HCN and

Rees Lab-scale combustor n.i. [276] NH3are

Ledesma Fluidised bed 600–1000/104

[195] formed.

Kambara Pyroprobe, efr max 1400/103

[176–178] HCN is Phong-Anant TGA, Drop-tube reactor 200–1200/0.1–103

[269] the primary

Niksa Radiant flow reactor 1450/104

[238] product

Nelson Fluidised bed 500–1050/104

[231]

Kidena Curie-point pyrolyser 700–1100/10–103

[185]

Chen Entrained flow reactor max 1600/105

[70] HCN for

Friebel Fixed bed max 1000, 5 K/min [107] high rank

Tan Fixed bed 500–1000/0.1–2000 [324, 325] NH3for

Li Fluidised bed 400–1000/104

[203] low rank

Leppalahti Fixed bed 600-950℃/10K/min [197] NH3is

Bassilakis TG-FTIR max 900/30K/min [27] the main

Aho Entrained flow reactor 800/ n.i. [5] specie

a_{not indicated}

2.2.2 Char oxidation

The heterogeneous carbon oxidation and char gasification step is the second process to occur in solid fuel combustion systems, proceeding simultaneously or after devolatilisa-tion, depending on reaction conditions. The time required for the complete combustion of a char particle can be several orders of magnitude larger than that for devolatilisation, typically in the seconds range, and it is often the rate-determining step in the overall combustion of pulverised fuels. The chemical structure of the fuel does not control the re-action processes to the same extent as devolatilisation, but, due to the high temperatures generally associated with char oxidation, pore diffusion and external diffusion often play a primary role [302]. Thus the physical fuel structure, including pore structure, surface area, particle size and inorganic content are important for understanding and modelling char oxidation processes.

During char oxidation the reactant, usually oxygen, diffuses from the bulk phase through the boundary layer to the surface of the particle and into the particle’s pore system. The oxygen reacts with carbon at the pore walls, producing CO and CO2. CO

can react in the gas phase in the vicinity of the particle in some combustion regimes to form CO2, which further reacts with the carbon in the char. Because of the local

depletion of carbon in the solid phase, the pore structure of the solid evolves as a function of time and temperature, affecting available surface area, active-site concentrations and pore-diffusion characteristics.

Biomass fuel characterization for NOx emissions in cofiring applications

Biomass fuel characterization for NO

emissions in

Co-firing applications

Biomass fuel characterization for NO

emissions in

Co-firing applications

Proefschrift

Gianluca DI NOLA

Summary

Samenvatting

Table of Contents

1

Introduction

1.1

World Energy Outlook

1.1.1

Energy policy in The Netherlands

1.1.2

Biomass as sustainable energy source

1.2

NO

emissions and their impact

1.3

NO

emissions: environmental legislation

1.4

Biomass in Power Generation

1.4.1

Direct co-firing: Technology and Status

1.4.2

Open challenges

1.5

Motivation and Scope

1.6

Methodology

1.6.1

Experimental activities

1.6.2

Modeling activities

1.7

Outline

2

Overview of NO

x

formation & destruction in

Biomass Co-firing

2.1

Coal and Biomass

Atomic O:C Ratio

Incresing

Heating Value

AtomicH:CRatio

2.2

Pulverised solid fuel combustion

2.2.1

Devolatilisation

2.2.2

Char oxidation