Torrefaction of herbaceous biomass – A study of
product, process and technology
Torrefaction of herbaceous biomass – A study of
product, process and technology
Proefschrift
ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,
op gezag van de Rector Magnificus prof. ir. K.Ch.A.M. Luyben; voorzitter van het College voor Promoties,
in het openbaar te verdedigen op donderdag 19 november 2015 om 12:30 uur
door Yash Vasant JOSHI
Master of Science in Sustainable Energy Technology geboren te Mumbai, India
This dissertation has been approved by the promotor: Prof. dr. ir. B.J. Boersma
and the copromotor: Dr. ir. W. de Jong
Composition of the doctoral committee:
Rector Magnificus voorzitter
Prof. dr. ir. B.J. Boersma Technische Universiteit Delft, promotor Dr. ir. W. de Jong Technische Universiteit Delft, copromotor
Independent members:
Prof. Dr.-Ing. H. Spliethoff Technische Universität München, Germany Prof. dr. ir. G. Brem Universiteit Twente
Prof. dr. ir. P.M. Herder Technische Universiteit Delft Dr. ir. J.A. van Oijen Technische Universiteit Eindhoven Prof. dr. D.J.E.M. Roekaerts Technische Universiteit Delft, reservelid
Other member:
Prof. dr. ir. J.H.A. Kiel Technische Universiteit Delft
This research was financially supported by E.ON Benelux as a part of the project “Energy from Biomass”.
Cover photo by Stanislav Aristov Design and layout by Despina Sapoutzi
ISBN: 978-94-6186-515-1
S
UMMARY
Co-firing biomass with coal in pulverized fuel boilers is a readily implementable means f0r attaining renewable electricity generation targets. Even as utilities have gained considerable operational experience over the past years with co-firing small quantities (0-3% on energy basis) of assorted biomass and waste residues for power production, they rely on wood pellets when required to co-fire higher fractions (>10% on energy basis). However, wood pellets are 4-6 times as expensive as coal (on an energy basis), resulting in the viability of renewable electricity production from large scale co-firing of biomass being heavily reliant on subsidy. The use of cheaper herbaceous biomass (such as verge grass or bagasse) may be appealing in view of reducing fuel cost, but is hampered in practice on account of technical and logistical challenges emanating from its high moisture content, low bulk density, fibrous structure and seasonality. Torrefaction, alternatively referred to as roasting or mild pyrolysis, is a thermal pre-treatment that can be applied to biomass to improve its physicochemical properties for subsequent thermochemical conversion (including co-firing). The present work is a study of torrefaction as applied to herbaceous biomass in relation to its process, products and technology.Torrefaction involves processing biomass at temperatures between 230 °C and 320 °C under non-oxidizing conditions, leading to production of a fuel with improved heating value, grindability and storability. It can be conceptually placed in between drying and charcoal production with respect to the severity of thermal treatment. Unlike charcoal production, however, the objective of torrefaction is not the removal of volatiles, but the maximization of energy yield while achieving the necessary improvement in fuel properties befitting the end use (including, but not limited to, co-firing).
A complete heat and mass balance of the torrefaction process is critical in assessing its primary viability, and is the main objective of Chapter 3. Considering that torrefaction can benefit significantly from co-siting with another energy intensive industry, the assumed process includes the utilization of a “waste” heat source. The system analysis software Cycle Tempo® is used to simulate a steady state model of
the torrefaction process by linking unit operation blocks of drying and torrefaction along with auxiliary process equipment. Lower torrefaction temperatures were always found to result in a better net system efficiency. Even as the improvement in calorific value (for both the solid bio-coal and torrefaction volatiles) was greater for higher processing temperatures, this was not found to energetically compensate for the higher solid mass loss. The drying unit typically represents the largest fraction of the total heat load, and must be considered as an integral aspect of torrefaction process design.
Considering the temperature of treatment, it follows that the biomass feedstock be completely dry prior to torrefaction. To reduce the energy required in thermally drying wet grass, the raw biomass can be subjected to a preceding mechanical fractionation (pressing) process to remove the free moisture content. Chapter 4 looks into the effects of this pre-treatment on the composition and torrefaction behaviour of grass. It was found that mechanical fractionation applied to verge grass treatment has ancillary benefits of leaching mineral content. This not only reduces the ash content of the feedstock, but also results in a more favourable ash quality. It was also seen that there is a small removal of cellulose and hemicellulose, with greater removal of other extractives (and possibly lignin) from the biomass. There is relatively low mass loss penalty associated with pressing, which is a positive consequence of this inevitable step prior to torrefaction.
Even as small scale experimental studies in torrefaction give important insights into the yield and kinetics of torrefaction, they cannot account for heat and mass transfer limitations, which greatly influence bulk gas-solid reactions. Furthermore, several of these experiments are carried out using apparatus that employ heat transfer modes that are not scalable to an industrial scale of operation. To better simulate conditions that would be encountered in practical scaled up units, a bench scale torrefaction/drying test rig was constructed at the Process & Energy Laboratory in TU Delft, employing a packed bed of biomass with direct contact heating using a convective gas medium (details are contained in Chapter 5). In addition to providing operational experience, the torrefaction test rig enabled the production of larger quantities of torrefied material that could be used for characterisation.
Most torrefaction studies limit the characterisation of the torrefaction products to the elemental and proximate analysis of the solids. However, several improvements in properties due to torrefaction can be attributed to a chemical transformation of specific biomass constituents. Chapter 6, therefore, focusses on investigating the
nature of these transformations and their association with mass and energy yields for varying conditions of torrefaction temperatures and times for verge grass and bagasse. Chemical analysis of the products was carried out using high performance liquid chromatography (HPLC) following a two-step acid hydrolysis that complemented the use of more commonly applied thermal analysis techniques of differential thermogravimetry and bomb calorimetry. It was seen that for a torrefaction temperature of 290°C at a residence time of 15 minutes, approximately 23% glucan and 82% xylan in bagasse is converted as compared to 96% glucan and 97% xylan that is converted in verge grass. In addition to the degradation of structural carbohydrates, the extent of formation of acid insoluble residue (char) was also seen to differ with the choice of feedstock. As a consequence, within the range of torrefaction conditions, a comparable reduction in mass yield is found to lead to a loss of energy yield that is 25% less in the case of verge grass as compared to bagasse. The correlation between the mass yields and energy yields in both cases is remarkably linear and specific to each kind of feedstock.
Biomass torrefaction in gas atmospheres containing oxygen, such as hot flue gases or air, is an interesting concept with potentially positive implications on processing costs and efficiency. Chapter 7 concerns an investigation into the effects of varying concentrations of oxygen in the atmosphere on the torrefaction of sugarcane bagasse in a packed bed at different temperatures. In addition to understanding the development of bed temperature profiles, the study includes an analysis of the torrefied solids (HHV determination using bomb calorimetry) and the torrefaction volatiles (permanent gases: NDIR spectrometry, condensates: HPLC with UV-Vis/RI detection). It was observed that for this specific reactor configuration and heat transfer regime, there are temperature specific limits beyond which an increase in oxygen concentration leads to an oxidative thermal runaway in the packed bed. An increase in torrefaction temperatures from 270°C to 290°C, for e.g. leads to a reduction in the tolerable limits of oxygen concentration from 5% to 1%. It was further determined that away from the ignition zone, moderate addition of oxygen does not drastically reduce mass and energy yields of the solids, and in some cases may also lead to more uniform heating of the biomass bed. Increase in oxygen concentration also leads to a shift in the partitioning of the volatiles in favour of permanent gaseous products, with specifically an increase in carbon dioxide resulting in reduction of the HHV of the volatiles.
Chapter 8 concerns the development of a modelling framework for simulating drying and torrefaction in a packed bed reactor. In addition to the main heat and mass balance model, there are several additional modules that can independently account
for reactor geometry, drying kinetics, fluid/solid properties, transport phenomena and torrefaction kinetics. The model, as a whole, is found to have reasonable capabilities with respect to predicting temperature profiles and torrefaction yields, and can be used as a tool in scale-up. However, a torrefaction kinetics model based on the two-step pseudo-component approach is unable to provide a good dynamic prediction of the weight loss in torrefaction.
The torrefaction process and its application to the processing of herbaceous biomass (that has been the object of scientific study in this thesis) can be industrially deployed given the appropriate development of technology in a conducive economic and policy landscape (Chapter 9). From a technological perspective, in addition to operating at optimum torrefaction conditions, it is recommended that the equipment design allow for efficient heat integration, be accommodative to variability of feedstock quality and bear features incorporating inherent safety. The policy framework in the Netherlands has returned to favouring electricity production from biomass co-firing and will serve to support the development of torrefaction technology by stimulating demand. An assertive step in the direction of developing appropriate technology will provide opportunities for long term returns that may eventually be sustained, even in the absence of viability gap funding.
Yash Joshi July 2015
S
AMENVATTING
Mee- en bijstook van biomassa met kolen in poederkoolketels is een direct implementeerbare technologie voor het realiseren van hernieuwbare elektriciteitsproductiedoelstellingen. Hoewel de elektriciteitsproductiebedrijven een aanzienlijke mate van ervaring hebben opgedaan de afgelopen jaren met het mee- en bijstoken van relatief kleine hoeveelheden (0-3% op energiebasis) van een reeks biomassa- en restproductsoorten voor elektriciteitsproductie, zijn zij afhankelijk van houtpellets als het gaat om mee- en bijstook van grotere fracties (>10% op energiebasis). Echter, houtpellets zijn 4-6 maal zo duur als kolen (op energiebasis), hetgeen er in resulteert dat de haalbaarheid van duurzame elektriciteitsproductie op basis van grootschalige mee- en bijstook van biomassa sterk afhankelijk is van subsidieverstrekking. Het toepassen van goedkopere, grasachtige biomassa (zoals gras of suikerriet bagasse) zou aantrekkelijk kunnen zijn vanuit de optiek van brandstofkostenreductie, maar in de praktijk wordt dit belemmerd vanwege technische en logistieke uitdagingen die voortvloeien uit de hoge vochtgehaltes, lage bulkdichtheid, vezelachtige structuur en seizoensafhankelijke beschikbaarheid. Torrefactie, ook wel milde pyrolyse of roosteren genoemd, is een thermische voorbehandeling die kan worden aangewend voor biomassa om haar fysisch-chemische eigenschappen te verbeteren voor de erop volgende thermofysisch-chemische conversie (inclusief mee- en bijstook). Dit werk is een studie naar torrefactie, toegepast voor grasachtige biomassa in relatie tot haar proces, producten en technologie.Torrefactie omvat het omzetten van biomassa bij temperaturen tussen 230 °C en 320 °C onder niet-oxiderende condities, leidend tot de productie van een brandstof met verbeterde stookwaarde, maalbaarheid en mogelijkheid tot opslag. Conceptueel kan de technologie worden gepositioneerd tussen het drogen en (houts)kool productie met betrekking tot de intensiteit van de thermische behandeling. Ter onderscheiding van (houts)koolproductie, echter, is het doel van torrefactie niet het verwijderen van vluchtige bestanddelen, maar de maximalisatie van de energieopbrengst terwijl tegelijkertijd de nodige verbetering in de brandstofeigenschappen wordt gerealiseerd ten gunste van het eindgebruik (inclusief, maar niet beperkt tot alleen bij- en meestook).
Een complete energie- en massabalans van het torrefactieproces is essentieel in de evaluatie van de primaire haalbaarheid en dit is de belangrijkste doelstelling van Hoofdstuk 3. Als we in ogenschouw nemen dat torrefactie significant kan profiteren van “co-siting” in samenhang met een andere energie-intensieve industrie, dan houdt het beschouwde proces de toepassing van een “afval” warmtebron in. De systeemanalyse software Cycle Tempo® is gebruikt om een model van de stationaire toestand te simuleren van het torrefactieproces door “unit operation” blokken met betrekking tot drogen en torrefactie te koppelen tezamen met hulp-procesapparatuur. Lagere torrefactietemperaturen resulteerden altijd in een betere netto systeemefficiëntie. Hoewel de verbetering in de stookwaarde (voor zowel de vaste bio-kolen als de van torrefactie afgeleide vluchtige gassen) groter was voor hogere temperaturen, werd dit niet bevonden energetisch te compenseren voor het grotere verlies van massa. De droger vertegenwoordigt typisch de grootste bijdrage aan de totale warmtevraag, en moet worden beschouwd als een integraal aspect van het torrefactie procesontwerp.
Als de voorbehandelingstemperatuur wordt beschouwd, volgt hieruit dat de biomassavoeding geheel droog moet zijn voor torrefactie. Om de energiebehoefte te reduceren in het thermisch drogen van nat gras, kan men de onbehandelde biomassa vooraf een mechanisch fractioneringsproces (persen) laten ondergaan om het vrije vochtgehalte te verwijderen. Hoofdstuk 4 gaat in op de effecten van deze voorbehandeling op de samenstelling en het torrefactiegedrag van gras. Resultaat is dat mechanische fractionering toegepast op bermgras als bijkomende voordeel de uitloging van minerale bestanddelen heeft. Dit reduceert niet alleen het asgehalte van de voeding, maar het resulteert ook in een betere askwaliteit. Ook is waargenomen, dat er een geringe verwijdering was van cellulose en hemicellulose met een grotere verwijdering van andere extracten (en mogelijk lignine) uit de biomassa. Er gaat een relatief gering massaverlies gepaard met het persen, hetgeen een positief gevolg is van deze onvermijdelijke stap voorafgaand aan torrefactie. Hoewel kleinschalige experimentele studies naar torrefactie belangrijke inzichten verstrekken in de opbrengst en kinetiek van torrefactie, kunnen deze niet de effecten van warmte- en stofoverdrachtslimiteringen, die de bulk gas-vaste stof reacties zeer beïnvloeden, verdisconteren. Verder is het zo, dat verscheidene van deze experimenten worden uitgevoerd gebruikmakend van apparatuur die warmteoverdrachtsmethodieken gebruiken die niet opschaalbaar zijn naar een industriële schaal van operatie. Om beter de omstandigheden te simuleren die
voorkomen in praktische, opgeschaalde reactoren, werd een testopstelling op labschaal voor torrefactie/droging geconstrueerd in het laboratorium voor Proces & Energie aan de TU Delft, waarbij gebruik werd gemaakt van een gepakt bed van biomassa met direct contact verwarming middels een convectief gas medium (details staan beschreven in Hoofdstuk 5). Naast het voorzien in operationele ervaring, bood de torrefactie testopstelling de mogelijkheid van het produceren van grotere hoeveelheden aan getorreficeerd materiaal, die konden worden aangewend voor karakterisering.
De meeste torrefactie studies beperken de karakterisering van de torrefactieproducten tot elementanalyse en zogenaamde ‘proximate’ analyse van de vaste stoffen. Echter, verscheidene verbeteringen in de eigenschappen ten gevolge van torrefactie kunnen worden toegeschreven aan een chemische omzetting van specifieke biomassa bestanddelen. Hoofstuk 6 richt zich op het bestuderen van de aard van deze omzettingen en hun relatie tot de massa- en energieopbrengsten voor wisselende condities van torrefactietemperatuur en –tijd voor bermgras en suikerriet bagasse. De chemische analyse van de producten werd uitgevoerd met behulp van “high performance liquid chromatography” (HPLC) gevolgd door een tweestaps zure hydrolyse, welke het gebruik van de meer gangbare thermische analyse technieken van differentiële thermogravimetrie en bomcaloriemetrie complementeerde. Waargenomen werd, dat voor een torrefactietemperatuur van 290°C bij een verblijftijd van 15 minuten, ongeveer 23% van de glucose eenheden en 82% xylaan in het bagasse-torrefactieproduct werd omgezet, in vergelijking met een omzetting van 96% van de glucose eenheden en 97% xylaan in het bermgras-torrefactieproduct. Naast de degradatie van structurele carbohydraten, werd ook waargenomen dat de mate van de vorming van zuur-onoplosbaar residu (‘kool’) varieerde met de keuze van de voeding. Ten gevolge hiervan ging, binnen de reeks aan bestudeerde torrefactiecondities, een vergelijkbare reductie in massaopbrengst gepaard met een verlies aan energieopbrengst van 25% in het geval van bermgras in vergelijking met bagasse. De correlatie tussen de massa- en energieopbrengsten is in beide gevallen opmerkelijk lineair en specifiek voor elk soort voeding.
Biomassa torrefactie in gas omgevingen die zuurstof bevatten, zoals hete rookgassen of lucht, is een interessant concept met potentieel positieve effecten op verwerkingskosten en efficiëntie. Hoofdstuk 7 betreft een studie naar de effecten van variërende concentraties van zuurstof in de atmosfeer op de torrefactie van suikerrietbagasse in een gepakt bed bij verschillende temperaturen. Naast begripsvorming betreffende de ontwikkeling van bed temperatuurprofielen, omvat de studie een analyse van de getorreficeerde vaste stoffen (HHV-bepaling
gebruikmakend van bomcaloriemetrie) en vluchtig materiaal gevormd bij torrefactie (permanente gassen: Niet-Dispersieve infrarood spectroscopie, condensaten: HPLC uitgerust met UV-Vis/RI detectie). Waargenomen werd, dat voor deze specifieke reactorconfiguratie en bijbehorend warmteoverdrachtsregime, er temperatuur-specifieke grenzen waren waarbuiten een toename in zuurstofconcentratie leidt tot een oxidatieve thermische ‘runaway’ in het gepakte bed. Een toename van de torrefactietemperatuur van 270°C tot 290°C leidt bijvoorbeeld tot een reductie in de toelaatbare grens van zuurstofconcentratie van 5% tot 1% (volumebasis). Verder werd bepaald, dat op een afstand van de ontstekingszone een bescheiden toevoeging van zuurstof niet de massa- en energieopbrengsten van de vaste stoffen drastisch reduceert, en in sommige gevallen kan het ook leiden tot een meer uniforme verhitting van het biomassa bed. Een toename van de zuurstofconcentratie leidt ook tot een verschuiving in de verdeling van het vluchtige materiaal ten gunste van gasvormige producten, met in het bijzonder een toename van kooldioxide, hetgeen resulteert in de reductie van de HHV-waarde van de vluchtige bestanddelen.
Hoofdstuk 8 beschrijft de ontwikkeling van een modelleringsraamwerk voor het simuleren van drogen en torrefactie in een gepakt bed reactor. Naast het centrale massa- en energiebalans model, zijn er verscheidene additionele modules die – onafhankelijk- de reactor geometrie, drogingskinetiek, gas/vaste stof eigenschappen, transportverschijnselen en torrefactiekinetiek verdisconteren. Het model als geheel heeft een redelijke mogelijkheden voor wat betreft het voorspellen van temperatuurprofielen en torrefactieopbrengsten, en kan worden gebruikt als methodiek voor opschaling. Echter, torrefactiekinetiek die gebaseerd is op de tweestaps pseudo-componenten benadering is op zich niet in staat om een goede dynamische voorspelling te leveren van het massaverlies gepaard gaande met torrefactie.
Het torrefactieproces en de toepassing ervan op het verwerken van grasachtige biomassa (hetgeen het doel was van de wetenschappelijke studie in dit proefschrift) kan industrieel worden ingezet, gegeven een geschikte ontwikkeling van de technologie en een stimulerend economisch en beleidslandschap (Hoofdstuk 9). Vanuit een technologisch perspectief, naast het opereren onder optimale torrefactiecondities, wordt aanbevolen, dat het apparaatontwerp rekening houdt met efficiënte warmte-integratie, flexibel is in het licht van variaties van de voedingskwaliteit en gekenschetst wordt door het inbouwen van inherente veiligheid. Het beleidsraamwerk in Nederland is teruggekeerd naar bevordering van elektriciteitsproductie op basis van biomassa bij- en meestook en zal dienen ter ondersteuning van de ontwikkeling van torrefactietechnologie door stimulering van
de vraag. Een ferme stap in de richting van het ontwikkelen van geschikte technologie zal voorzien in mogelijkheden voor winsten op de lange termijn die uiteindelijk duurzaam zouden kunnen zijn, zelfs onder condities van afwezigheid van subsidieverstrekking ter overbrugging van een haalbaarheidsgat.
Yash Joshi Juli 2015
L
IST OFA
BBREVIATIONSAFT Adiabatic Flame Temperature
AIR Acid Insoluble Residue
ATEX Atmosphères Explosibles (European Explosive Atmospheres Code: 94/9/EC)
BOD Biochemical Oxygen Demand
EY Energy Yield
FC Fixed Carbon
FGD Flue Gas Desulfurization
FIT Feed-In Tariff
HGI Hardgrove Grindability Index
HHV Higher Heating Value
HPLC High Performance Liquid Chromatography
HT / LT HEX High Temperature / Low Temperature (Heat Exchangers)
IP Intermediate Pressure (Turbine)
LCV Lower Combustion Value
MBM Meat and bone meal
MEP Milieukwaliteit van de Elektriciteitsproductie (Environmental Quality of Electricity Production) – subsidy programme
MY Mass Yield
NDIR Non-dispersive Infrared (Spectrometry) NOx Mono-nitrogen oxides NO and NO2 pf boiler Pulverised Fuel boiler
RI Refractive Index (Detector)
S(N)CR Selective (Non-)Catalytic Reduction
SDE+ Stimulering Duurzame Energieproductie (Stimulating Sustainable Energy Production) - subsidy programme
SOx Sulfur oxide including many types of sulfur and oxygen containing compounds such as SO, SO2, SO3, etc.
(d)TGA (Differential) Thermogravimetric Analysis
V.G. Verge Grass
VOCs Volatile Organic Compounds XRF X-Ray Fluorescence (Spectrometry)
L
IST OFS
YMBOLS,1 = Drying air flow (kg/s) a
m
,2 = Heating air flow (kg/s) a
m
Total air flow (kg/s)
a
m
A = Solid pseudocomponent 1 (Torrefaction Kinetics) B Solid pseudocomponent 2 (Torrefaction Kinetics) C Solid pseudocomponent 3 (Torrefaction Kinetics) V1 Gaseous pseudocomponent 1 (Torrefaction Kinetics) V2 Gaseous pseudocomponent 2 (Torrefaction Kinetics)
{ { { {{ , , , ,
Higher Heating Value of the Torrefaction Solids Higher Heating Value of the Torrefaction Gas Higher Heating Value of the Raw Biomass Mass flow of the Torrefaction Solids
T s T g R T s T HHV HHV HHV m m { ' {
Mass flow of the Torrefaction Gas Net heat input
g
H
K K
Process Efficiency without torrefaction gas combustion Process Efficiency with torrefaction gas combustion
I II
2 1
Area specific mass flow rate of gas (kg m s )
g g n m M A E 2
= Internal Diameter of the Reactor (m) = Length of Reactor (m)
4 /
Area normal to flow direction (m )
Distance co-ordinate from column bottom (m) Thickness of control volume (m)
n D L D A z dz
D H
2 3
Volume specific heat transfer surface (m m ) Bed porosity (fractional voidage)
Mass of steel pipe per unit length (m) = Thermal Conductivity of the Insulation = Thickness of the in reac w ins ins m k t Insulation U U 3 3 1 1 , 1 1 , ,
bulk density of solids (kg m ) density of gas (kg m )
Specific heat capacity of gas (J kg K ) Specific heat capacity of solid (J kg K )
Specific heat capacity of steel
s s reac g g g p g p s p w c c c (J kg 1K )1 1 1 1 , 1 1 ,
Latent Heat of Water (J kg )
Specific Heat Capacity of Steam (J kg K ) Specific Heat Capacity of (liquid) water (J kg K )
v p steam p water h c c Temperature of gas (°C) Temperature of solid (°C)
Temperature of steel wall (°C)
Temperature of the external environment (°C)
g s w ext T T T T I 2 2 *
Correction Factor to account for difference in mol. wt. of diffusing species Moisture content of the solid phase (kg / kg )
Absolute humidity of the gas phase (kg / kg ) Absolute humidi H O s H O g X Y Y 2
ty of the gas phase at the solid surface (kgH O/ kg )g
2 1 2 1
Heat transfer co-efficient between gas and solid interface (W m K )
Heat transfer coefficient between the gas phase and the steel wall (W m K ) Heat transfer coefficient between wall a
in w l h h h 2 2 1 2 nd environment (W m K ) Mass Transfer Coefficient (kg m )
M H O in
C
ONTENTS
Summary v
Samenvatting ix
List of Abbreviations xiv
List of Symbols xvi
1. Introduction
1.1. Background 2
1.2. Biomass Availability and Cost 4
1.3. Issues in Large Scale Biomass Co-firing 6
1.4. Biomass Pre-Treatment 12
1.5. Torrefaction 17
1.6. Pelletization & Briquetting 18
1.7. Proposed Solution 18 1.8. Thesis Outline 21 2. Torrefaction 2.1. Introduction 26 2.2. Fundamentals of Torrefaction 27 2.3. Advantages of Torrefaction 30 2.4. Torrefaction Technology 31
2.5. Torrefaction: An Enabling Technology 37
2.6. The Future of Torrefaction 38
3. Torrefaction Process Modelling
3.1. Introduction 42
3.2. Modelling of Unit Operations 43
3.3. Process Modelling 46
3.4. Results and Discussion 49
3.5. Conclusions 53
4. Mechanical Fractionation of Verge Grass
4.2. Material and Methods 58
4.3. Results and Discussion 60
4.4. Conclusions 68
5. Torrefaction Test Rig
5.1. Background 72
5.2. Motivation 72
5.3. Design of Torrefaction Test Rig 73 6. Constituent Transformations in Verge Grass and Bagasse
6.1. Introduction 82
6.2. Methodology 84
6.3. Results and Discussion 87
6.4. Conclusions 101
7. Oxidative Torrefaction
7.1. Introduction 106
7.2. Methodology 110
7.3. Results and Discussion 112
7.4. Conclusions 126
8. Reactor Modelling
8.1. Introduction 130
8.2. Model Structure 130
8.3. Energy and Mass Balance Model 131
8.4. Constituent Sub-Models 143
8.5. Example Result and Outlook 153
9. Discussions
9.1. Introduction 156
9.2. Perspectives on Torrefaction Technology 156 9.3. Perspectives on Economics and Policy 165 10. Conclusions and Recommendations
10.1. Conclusions 178
10.2. Recommendations 180
Appendices
A. Structure of Reactor Model Code B. Reactor Model Script
C. MATLAB® Script Code to determine kinetic parameters D. C,H,N,S Analysis of Biomass Samples
E. Ash Composition Analysis of Biomass Samples
Epilogue & Acknowledgements List of Publications
1
INTRODUCTION
“Asoulintensionthat’slearningtofly; Conditiongrounded,butdeterminedtotry” ǦPinkFloyd LearningtoFlyǦAMomentaryLapseofReason(1987)Biomass coǦfiring is an easily implementable means of large scale production of electricityfrombiomass.Severalpowerutilitieshavegainedextensiveexperiencein combusting small quantities of assorted biomass fuels in pulverized coal boilers. However in view of coǦfiring larger quantities of biomass (30+% on a mass basis), thereemergeseveralsignificantissuespertainingtofuelqualityandcost.Seekinga resolutiontotheseissuesbyapplying suitablefuel preǦtreatment techniquesforms thebasisofthisfouryearstudy.
1.1 BACKGROUND
The Energy Sector in the Netherlands has been striving to achieve the optimum balance atop the three pillars of ‘sustainability’: reliability, profitability and
environment.ThelastofthesepillarshasbeenassociatedwiththeEuropeanUnion’s
“20Ǧ20Ǧ20 target” of achieving 20% of all energy consumed to be supplied by renewableresources[1].
Biomassisessentiallyarenewableresourceandmaybeconsideredasastoredform of solar energy. Ever since humanity discovered how to control fire, up until the times of the Industrial Revolution in Europe, biomass combustion served as the principle source of heat. Following the widespread deployment of coal (a fuel that wasmoreenergydensethanwood),biomassfuelsweresoonrelegatedtoonlybeing usedasdomesticfuels.Followingtheadventofpetroleumandnaturalgas,biomass fuels further fell into disuse to the extent that in the ‘developed world’ they were only used for “recreational” value. However, with a newǦfound focus on renewable energy,woody biomasshas returned tobeing afuelsourcefor producingheatand power. Ranging from the rudimentary woodǦfired stoves to modern techniques of highly efficient combustion, gasification and liquefaction systems, biomass energy stillserves as the fourthmost important energy source for mankind (after oil, coal and natural gas) providing more than 10% of the global energy supply [2]. Even as thefoodvs.fuelcontroversysurroundedthesoǦcalledfirstgenerationbiomassfuels, second generation fuels (primarily residueǦbased) are largely considered to be carbonǦneutralandenvironmentfriendly.
One of the fundamental qualms about widespread implementation of renewable energy technologies such as wind or solar power concerns the intermittency of production.Intheabsenceoflargeandefficientenergystoragecapacity,thisleads to issues with grid stabilization during instances of demandǦsupply mismatch, leading to these power sources negatively impacting the reliability of electricity supply. Since existing base load power sources (typically coalǦfired and nuclear power plants) have considerable latency and small turndown ratios, there is a maximumlimittothewindandsolarpowerthatcanbeevacuatedbythegrid.For these reasons, given the present infrastructure landscape, additionally producing electricity from a renewable solid fuel provides an interesting opportunity of increasing the share of renewable electricity production while not affecting the reliabilityofsupply.
For quite some time there has been a consensus between the several stakeholders involved in the Dutch Energy sector that “biomass coǦfiring” is the most readily implementable technology in attaining the EU renewable energy targets. CoǦfiring involvesreplacingapartoftheconventionalfuel(typicallycoalorgas)combustedin thepower plant furnace bybiomass(orbiomassǦderivedfuels).Considering thatit involves almost no modifications to the existing power plant furnace, and also the least investment in additional equipment, direct1coǦfiring has been the technology of choice for most utility companies operating pulverized coal boilers. Apart from enablingproductionofrenewableelectricitywithexistinginfrastructure,coǦfiringof biomass with coal leads to reduced emissions of SOx and NOx.The reduced SOx
emissionsaredue to the lower sulfur contentof biomass whereas thedropin NOx
emissionsisattributedtothechangeintheredoxbalanceinthecombustionzoneas aconsequenceofvolatilesreleasedfromthebiomass[3].Thiseffecthasbeenused byseveralutilitycompaniestorestartoldpowerplantsortoextendtheoperational lifetime of existing aging power plants without installing additional flue gas treatment facilities such as flue gas desulfurization (FGD) units or selective (nonǦ) catalyticreduction(SCR/SNCR)units.
Extensive experiments have been performed at the Maasvlakte Power Plant (near Rotterdam) over the past several years in coǦfiring different kinds of fuels. The coǦ fired fuels have included commercially traded wood pellets, meat and bone meal (MBM),coffeebeanresidues,sawdust,bleekaarde(bleachingclay),papersludge,etc. Most of the coǦfiring experiments have been carried out on a small scale, with coǦ firingratiosreachingamaximumofabout6%onanenergybasis.ThebiomassiscoǦ fireddirectly;howeveritmayeitherbemixedwiththecoalontheconveyoraftera processofmixingandloosepelletisationandpassthroughthecoalmills,orelsefed directlyintothefuellinesafterbeingprocessedbyanomnivore,whichisadedicated hermeticallysealedfacilityfortransportation,comminutionandscreening(usedfor hazardouswastestreamssuchasMBM). ForlargescalebiomasscoǦfiring,mostenergyutilitiesinNetherlandsseemtohave favoured (or have returned to favouring) the combustion of commercially traded woodpellets,owingtoconsistencyofquality,necessityoflittle/nomodificationsto theexistingplantandadequacyofsupply.However,thesewoodpelletsaretypically 4Ǧ6timesasexpensiveascoalona€/GJbasisasaconsequenceofwhichlargescale
1
Indirect coǦfiring involves first gasifying the biomass in a separate reactor, followed by combustionofthegasificationproductsintheboilerfurnace
biomass coǦfiring activities in the Netherlands have been primarily driven by subsidy.Thebusinesscasesforalmost allrenewableenergy applications arehighly dependent on subsidies. However, a discord amongst the various ruling Governments regarding the means to achieving a transition to sustainable energy production seems to have created a rather volatile subsidy regime with respect to biofuels. Until 2007, coǦfiring activities were supported by the Dutch MEPǦsubsidy programme.EvenastheMEPwasstoppedin2006,thesubsidytariffsasdetermined bythisprogrammewerepaidouttoexistingcoǦfiringunitsforthestipulatedperiod. Thesubsidytariff(overandabovethewholesaleelectricityprice)forcoǦfiringwood pelletswasfixedat6.5€ct/kWhe,whilethatforfiringagriculturalresidueswasfixed
at 3.8 €ct/kWhe [4]. Translated into an energy based cost, the subsidy amounts to
approximately 7.5 €/GJ for wood pellets, which means that the subsidy would effectivelycompensatetheutilityfortheentirecostofpurchasingwoodpellets. As a part of a “Green Deal” with the Dutch Government, energy companies were supposedtomaintaintheircoǦfiringlevelsinabsenceofsubsidyuntil2015.Beyond this, they were required to ramp up the levels of coǦfiring to 30% on a mass basis (approx. 20% on a thermal basis) until 2020 under future agreements. However, followingtheresignation of theDutchGovernment inApril 2012,thefuture of the “Green Deal” was entrenched in some ambiguity. In the absence of a subsidy mechanism,itwasobservedthatallmajorutilitiesfounditunaffordabletocontinue withcoǦfiringwoodpellets,leadingtoadeclineinrenewableelectricityproduction. Inviewofmeetingrenewableenergytargets,itwasfoundtobenecessaryasof2015 toreintroducethesubsidyforcoǦfiringbiomassundertheSDE+frameworksubject to a maximum limit of 25 PJ. The basic tariff for electricity produced by coǦfiring biomasshasbeensetfor2015at0.108€/kWhforexistingcoǦfiringutilitiesand0.115 €/kWhfornewunits2[5].Itisalsoafurtherrequirementthattheutilitiestowhich the subsidy has been awarded must prove that the biomass fuel is sourced ‘sustainably’.Animportantaspect,however,isthatunliketheMEPprogramme,the SDE+programmedoesnotapplyapreferentialpricingforwoodpellets.Giventhis distinction,thereexistsanopportunityforbusinessestodecreasetheirfuelcostsby utilizingcheaperfuelsincoǦfiring.
1.2 BIOMASSAVAILABILITYANDCOST
2 ThenetsubsidydisbursedisthedifferencebetweenthebasictariffandacorrectionfactorThe fuel standard for coǦcombustion of biomass in western Europe is white wood pelletsimportedprimarilyfromCanada.Theglobalvolumesofannualwoodpellet tradecontinuestoremainquitesmallthoughhavingrisenfrom7Mtonsin2010[2] to around 12 Mtons in 2013. In anticipation of a bioǦbased energy sector, the Netherlands aims at being a dominant player in world biomass trade. One of the goalsofpreferentiallyencouragingwoodpelletscoǦfiringintheMEPsubsidyscheme may have been to stimulate domestic demand for wood pellets to facilitate the creation of a trading and logistics infrastructure that could in the future service larger European or global requirements. A step in this direction was that trading futurescontractsofcommercialbioǦpelletswasinitiatedontheAPXǦENDEXenergy exchangeinearly20123. Woodpelletstypicallyhaveaheatingvalueofaround17MJ/kg,andaretradedata priceofabout120Ǧ150€/tonne,leadingtoanenergybasedpriceofaround7Ǧ9€/GJ; whereashardcoalsaretypicallyreportedtohavepricesintherangeof1.5–2.0€/GJ. Thisis,infact,thebasicreasonwhythecaseforbiomasscoǦfiringwithwoodpellets has been highly dependent on subsidies. Sustainably sourced wood pellets are typicallysourcedfromresiduesofsawmillsandasoutputsof“sustainableforestry”, which refers to utilizing forestry sourced biomass at the same rate at which it is produced. Due to the increasing demand for biomass in coǦcombustion and the limitedforestryindustry,woodpelletpricesmaybeexpectedtorisesignificantly. ItisseeninFigure1.1thatapproximately130PJworthofbiomassisavailablelocally in the Netherlands, the largest fraction of which is available at a minimal cost (sometimesevenbearinganegativeprice).However,directlyutilizingthesebiomass streamsisoftendifficultduetoissuesoffuelqualityrelatedtohigher(andvariable) moistureandashcontents.Furthermore,severalorganicresidueshaveatendencyto undergodecompositionduetobacterialorfungalactionwhenleftinstorageforan extendedperiod.Thisnotonlyleadstoadecreaseintheircalorificvalue(relevantto their potential use as fuels), but also makes their storage biologically hazardous. Hence,theyarepresentlycompostedorlandfilledatanetcosttosociety.Anideal preǦtreatment technique may thus aim at transforming these essentially waste productsintoenergydenseandinertfuelsforelectricityproduction.
3
Figure1.1:AvailabilityofBiomassinNLasaFunctionofCostin€/GJ[6]
Animportantissue,however,concernsthechallengeswithcollectionandtransport of these ‘difficult fuels’. In several cases, the generation of the biomass is far too geographically dispersed to allow for viable collection. However, there do exist specific opportunities in cases where centralized collection occurs or is even necessary.Suchopportunitiescanservetoprovidepotentialsourcesofbiomassfuel forcoǦfiring.
1.3 ISSUESINLARGESCALEBIOMASS
COǦFIRING
Several power plants (mostly in Europe) have been coǦfiring biomass since a long time,withmostlysmallcoǦfiringratios.Anoverviewofsomeoftheseactivitieshas beengivenbyvanLooandKoppejan[7].PulverizedbiomasscoǦfiringcarriedoutat largerscales,however,islimitedtoeitherwoodpelletsorotherdrystrawasfuels[4, 8].Inadditiontoissueswithitseconomicsandlogistics,coǦfiringofbiomassfuels
on larger scales4has been accompanied by several technical challenges with fuel properties,preparationandcombustion[9,10]: 1.3.1 COMBUSTIONBEHAVIOUR Figure1.2:CombustionProperties(LCV&AFT)foratypicalbiomass (50%C,6%H,44%H,ɐ=1.2) Thedifferenceinthecombustionpropertiesofbiomassandcoalareprimarilydueto differencesinthevolatilityandmoisturecontent.Figure1.2showsavariationofthe lowercalorific value(LCV) andadiabatic flametemperature(AFT) as a function of thewetbasismoisturecontentofthefuelascalculatedusingcombustionequations givenbyvanLooandKoppejan[7].Itcanbeseenthatthedrybasiscalorificvalueof biomassfuelsandtheadiabaticflametemperaturesareverystronglydependenton theirmoisturecontent.Onbothaccounts,biomassfuelsarerankedlowerthancoals (which typically have an LCV of around 27 MJ/kgdry and an adiabatic flame
temperature of more than 2100°C). The moisture contained in the biomass evaporatesandleadstothereductionoftheLCVofthefuel.Sincealargepercentage ofthiswatervapourcannotbecondensed,thelatentheatlossesofthepowerplant areincreased.ModernonceǦthroughboilersoperateatveryhigh(oftensupercritical) temperatures and pressures, and are typically designed for heat transfer based on 4 Incentivesforrenewablysourcedelectricityinadditiontoreducedemissionshavealsoin somecasesledtoatotalconversionofanagingcoalfiredunittoabiomass(woodpellet)fired one.EvenastheconversionresultsinapowerdeǦratingofmorethan30%,suchconversions weresuccessfullycarriedoutatTilburyB(RWENpower,UK),DraxPowerPlant(DraxGroup, UK),Ironbridge(E.ON,UK),Provence4(E.ON,France),etal.
flame temperatures and burning profiles of coals. CoǦfiring large shares of moist biomasscanleadtotheinabilityofreachingthenecessaryoperatingtemperatures/ pressuresoftheboiler.Thelowerflametemperaturesalsomeanthatthefuelneedsa longer residence time in the combustion zone for complete combustion. Furthermore, biomass particles being less dense than coal fines also experience higher velocities in the furnace, resulting in reduced residence times. Since the originaldesigndoesnotaccountfortheincreasedresidencetimerequirement,the combustionoffuelcanbeincompleteresultinginincreasedparticulatematter(soot) emissions. This also represents a fuel loss, further contributing to a decrease in combustionefficiency[10].
Biomass is characterised by a much higher volatile content (around 70%) as comparedtocoal(around30%).Furthermore,devolatilisationisinitiatedinbiomass fuels at much lower temperatures than that in coal. Biomass combustion can be separated into two distinct steps, firstly deǦvolatilisation and gas phase oxidation, followed by char oxidation. As opposed to the case of coal combustion where the heatismainlyderivedfromcharoxidation,majorityoftheheatreleasedinbiomass combustion comes from the gas phase oxidation of the volatiles. The difference in thenatureofheatreleasefromthetwofuelsleadstotheoperatingconditionsbeing at variance with the design conditions, resulting in a loss in combustion efficiency [10]. The flame stability in pf boilers, however, is generally benefitted due to the higher volatile content of biomass fuels, as long as their moisture content and particlesizesareadequatelysmall.
1.3.2 MILLING
Particle size is one of the most important attributes in solid fuel combustion, and hence the application of appropriate and optimised milling is one of the critical aspectsofoperatingpulverizedfuelboilers.TheeasiestoptionforcoǦfiringistocoǦ mill biomass and coal. However, this is not always possible owing to certain limitationsimposedbythenatureofthebiomass.Sincethemodeofcomminutionin spindle mills is brittle fracture, wood chips and other fibrous biomass cannot be processedeffectively.Woodpellets,however,areeasilymilledtotheoriginalparticle sizedistributionoftheformativesawdust.Biomassparticlesalsotendtoaccumulate in the mill, leading to an increase in the mill differential pressure and power consumption. Due to a lower particle density, biomass particles typically exit the millwithmuchlargertopsizes.Owingtothehigherreactivityofbiomassatlower
temperatures,thetemperatureofthehotprimaryairusedinmillingmustbelimited towellbelow180°Cforsafeoperation.Furthermore,thepresenceofmoistureinthe biomass can affect the heat balance in the mill. For all these reasons, it is recommendedthatcoǦmillingpercentagedoesnotexceed10%(massbasis)[4].
Figure1.3:LayoutofaVerticalSpindleBowlMill
Inlargepowerplants,however,thereareusuallyseveralcoalmillsfeedingmultiple rowsofburners.Inthiscase,amorepreferredalternativeistoreserveone(ormore) of these mills for biomass. This particular mill(s) may then be subjected to all modifications that may be necessary for milling biomass. Some of the important changesthatmaybemadetoaverticalspindlemillforprocessingbiomasspelletsas reportedbyLivingston[8]includethefollowing:
1. Arotaryvalveinstalledinthecoalfeedchute,formingatightsealbetween themillandthebunker
2. Milltooperatewithoutpreheatingprimaryair
3. Deactivation of the rotating classifier, thus leading to lesser material recirculatedthroughthemill
4. Further structural changes (baffles) made to the mill to maintain the primaryairvelocitiesinthemill
Followingthesemodifications,severalpowerplantsreportsatisfactoryperformance ofthemillsforprocessingbiomasspellets.Itisalsosignificanttonotethatbiomass hadalowerHardgroveGrindabilityIndex(HGI)ascomparedtocoal(andwashence more difficult to mill to similar particle sizes with a comparable power consumption).However,sincethereactivityofbiomassishigherthanthatofcoalit may not be necessary to mill the biomass to subǦmillimetre sizes for complete combustion.
It must be reiterated that the modifications made to the existing coal mills only enable them to utilize wood pellets (or essentially compressed sawdust). For grindingwetterandmorefibroustypesofbiomass,othertechnologiessuchashigh energy hammer mills, disc mills and attrition mills have also been suggested. However,severaloftheseproposedtechnologiesareessentiallyscaledupversionsof millsoriginallydesignedforproductionofhighvalueproductswithlowvolumesof production.Evenasthesemillsareabletoproducepulverizedbiomasswithavery welldefined(anddesirable)particlesizedistributions,theyareonlyabletodosoat the cost of a very high energy consumption. However, lower moisture contents in thefuelandtheresultingfriabilityleadtoimprovedgrindabilityofbiomassaswell as a lower energy consumption in milling. Thus, milling of biomass could benefit significantlyfromapreǦtreatmentstepinvolvingdrying.
1.3.3 STORAGEANDHANDLING
Most untreated biomass fuels are characterized by low bulk energy densities as compared to coal. To decrease the logistics costs in the biomass supply chain, it is commonpracticetodensifybiomassatorclosetosourcebeforetransport.Sawdust is usually pelletised, whereas in case of straw / grass baling is favoured. Even as pelletisation allows for reasonable energy densities (~11 GJ/m3) in the case of wood pellets, this value is still around oneǦthird of that in case of coal. With baling, the valueisevenlower(~2GJ/m3),therebyseverelyimpactingtheviabilityofthesefuel beingtransportedoversignificantdistances.
AnotherimportantfactorinthestorageofbiomassisthatofselfǦheatingleadingto spontaneous ignition, a phenomenon which in the past has led to disastrous consequences[11].AsisshowninFigure1.4,selfǦheatinginbiomassmaybearesult of either biological or chemical oxidation. The initial increase in temperature in a biomasspileistypicallyaconsequenceofbiologicalactivity.However,theelevated temperaturescanalsotriggerchemicaloxidationprocessesthatleadtoafurtherrise
in temperatures that may result in ignition. From the energetic perspective, any oxidativeaction(whetherbiologicalorchemical)leadstoanetdrymatterloss[12]. InthecaseofselfǦignition,however,whatisrelevantisnotmerelythenearǦtotaldry matterlossbutalsotherisktolifeandproperty.
Ithas been observedthatwood pellets, stored atroomtemperatureemitCO, with theemissionrateincreasingathighertemperatures[13].Furthermore,othervolatile organic compounds (VOCs) may also be released by wood pellets during storage. TheemissionofCOisaccompaniedbythedepletionofO2whichalone(inabsence
ofignition)hasledtofatalitiesinbiomassstorages.
Figure1.4:TemperaturedevelopmentandprocessesresponsibleforselfǦheatingin biomassstorage[7]
Since the initiation of selfǦheating is a consequence of biological activity, it is also seenthatdrybiomassislesssusceptibletotheseprocessesthanmoistbiomass[7]. Thus, long term storage of biomass thus necessitates that the feedstock be dry in additiontothespecificprecautionsthatneedtobetakentoavoidfirehazards.
Biomassdustbeinghighlyreactiveshouldalsobeavoided,sinceitconstitutesafire hazard[11].Caremusttotakentoensurethatbiomassisnotunnecessarilyagitated, andthatthereareadequatesafetymeasuresagainstallpotentialsourcesofignition. Handling biomass flows also poses challenges as compared to handling gaseous or liquidflows.Commonproblemsencounteredduringbiomassflowincludeplugging, segregation, obstructed or limited discharge, erratic flow, sudden uncontrollable flow. Experience has shown that these difficulties routinely cause industrial plants that handle biomass like wood pellets to underperform compared to their counterpartswhichoperatewithliquidsorgases[10].
1.3.4 ASHDEPOSITION
The total ash content of biomass fuels is highly variable, ranging from <1% for certain types of woody biomass to >10% with certain herbaceous biomass. The compositionofbiomassashisalsoquitevaried,withherbaceousbiomasseshaving higherconcentrationsofNaandKandlowerconcentrationofCainashascompared towood.Thehighercontentofalkaliandchlorineinbiomassfuelsandtheresulting low ash fusion temperatures have been linked to increased slagging and fouling tendencyincoǦfiring[7].However90+%ofalkalisinbiomassareinawaterǦsoluble or ionǦexchangeable form and susceptible to vaporization during heating.Typical
ashǦforming components that are leached out by water include alkali sulfates, carbonates, and chlorides. Further elements can be leached out by ammonia, HCl and possibly other organic solvents. However, the feasibility of application of leaching on a very large scale needs to be viewed critically. Additives such as dolomiteandkaolinarecapableofreducingsinteringbyraisingthemeltingpointof ash.Anotheroptionthatisalreadyappliedforcoalblendsisthatofsmartblending, whichinvolveschoosingthemixingratiosofthedifferentfuelsonthebasisoftheir mineral composition, with a view to reducing the slagging / fouling tendency. Similar to the case of coals, this approach may permit the coǦfiring of certain problematicbiomassfuelsaslongastheyareappropriatelyblendedwithotherfuels.
1.4 BIOMASSPRE
Ǧ
TREATMENTMost of the concerns set forth in the Section 1.3 become apparent only at higher biomasscoǦfiringratios(>10%onamassbasis).Furthermore,theseissuesaremore significantwhenburninglowǦgradefuelssuchasagriculturalresiduesascompared to ‘cleaner’ fuels such as wood pellets. Development of viable preǦtreatment
technologies for processing cheap fuels presents an exciting opportunity for power utilitiestoenabletheutilizationofthesefuels.Lowerfuelpricesmayalsohelpthe utility in gaining a competitive edge in the market while pioneering a sustainable energy transition. The essential requirements of such a preǦtreatment technology canbelaidoutasfollows:
x All technologies to be cost effective, considering that the price of biofuels shouldbeaslowaspossible(andhenceascloseaspossibletocoal)tomake themcompetitivewithminimalresorttosubsidy.
x Ability to handle large biomass streams. For a 20% energy basis coǦfiring application with an electricity generation of 1000 MW, the biomass throughput in question is in order of around 90 tonnes per hour of dry matter5.
x Not highly sensitive to particle sizes and capability of handling diverse biomassfeedstock.
ItisalsoaquestionastowherethepreǦtreatmentshouldbelocated.PreǦtreatment at the source of biomass generation can help decrease the logistical overloads, whereascoǦsitingpreǦtreatmentfacilitiesatanindustriallocation(potentiallywhere itisutilized)canbenefitfromutilizationofwasteheatinadditiontopossiblebulk handling infrastructure. In view of the required improvement in properties as laid out in section 1.3 subject to the abovementioned requirements, the following preǦ treatmenttechniquesmaybeconsidered:
1.4.1 BIOMASSDRYING
Dried biomass leads to higher combustion temperature, better and more stable burnǦout of the fuel and a lower requirement for excess combustion air, all contributing to an increase in overall plant efficiency [14]. These advantages in additiontothoseenumeratedinSections1.3.2and1.3.3,leadstotheconclusionthat itwouldbebeneficialtoexternallydrybiomasspriortomillingandcombustion. Thewatercontainedinbiomasscanbeconceptuallydifferentiatedinto[15]:
x Free Moisture characterised by its removal on application of weak mechanicalstrain x BoundWater 5 AssumingaLCVofabout20MJ/kgd.b..Itisnot,however,eithernecessaryorfeasiblethatall thebiomasscombustedispreǦtreatedatonelocation.
o Mechanically bound moisture, contained in pores, removable on applicationofastrongmechanicalstrain
o Physicallyboundwater,eitherabsorbedoradsorbedinbiomass o Chemically bound water, that can be removed by thermal drying
techniquesonly
1.4.1.1 MECHANICALFRACTIONATION(DEǦWATERING)
Ithasbeenshownbythattheenergyconsumptioninmechanicallydewateringwood is much lesser (58%) than what is involved in thermal drying [16]. As a result, it would be beneficial to dry the biomass to the maximum possible extent using mechanicaldewatering.Thesamestudyindicatedthatbiomasscouldbedriedupto 50%(wetbasis)usingrollercompression.
Otheradvancedtechniquesclaimingtoimprovedewateringincludepreheatingthe biomass before compression [17] and using superabsorbent polymers to prevent reabsorption of water [18]. Process equipment using vacuum to enhance liquid recovery from biomass (specifically sugarcane) has also been found [19]. However thesetechnologiesarestillnotwidelyutilizedforlargescaleprocesses.
Someofthedifferentequipmentusedfordewateringinclude:
x Roller presses: Used to achieve high compression, and can be used effectivelyinacontinuousprocess.Achievingagoodmaterialpinchcanbe challenging given the tendency of biomass to slip. However, modifications totherollersurfacesuchastexturingorcorrugationscanassistinoperation. x Screw Presses: Involves a screw with a gradually reducing pitch pressing against a (possibly converging) wire mesh, resulting in an increase in pressure leading to liquid extraction. Screw presses are also referred to masticating juicers, considering its crushing action and resulting size reduction.Theyworkinacontinuousmode.
x Filter Presses: Are used for achieving a high dry moisture content. A filter pressconsistsofaseriesoffilterchambersformedfilterplatessupportedon a metal frame. Once the filter chambers are clamped, the filter press is loadedwithslurry.Theplatesonthefilterpressareclampedtogetherwith hydraulicramsthatgenerateveryhighpressures.However,filterpressesare typicallyoperatedinthebatchmode,thusmeaningthattheyarenotideal forprocessinglargefuelstreams.
Figure1.5:MechanicalDewateringTechniques
(Left:RollerPressSchematic,Right:ScrewPressschematic)
In case of fuels with a high quantity of water soluble minerals, it is suggested that dewateringcaninfactbeusedforitsleachingeffect.Alkalisandchlorideshavebeen showntobesolubleinwater,andmaybecontainedintheliquidextractedfromthe mechanicaldewateringsystem.Theleachingcapacityofmechanicaldewateringcan befurtherenhancedbyaprocesscalledimbibition,whichbasicallyinvolvesadding extra water to the biomass to displace water containing dissolved salts within the biomass. The process is regularly used to improve the yield of sugar in sugar cane juiceextraction[20].
1.4.2 THERMALDRYING
The moisture content remaining after mechanical dewatering can be removed by thermalmeans.ForacoǦfiringsetup,itmaybeadvantageoustouselowgradewaste heat to carry out thermal drying of the biomass. There are several types of dryers with varying sizes, configurations and flows that can be used for this purpose. Mujumdar[21]hasdescribedseveralofthesedryerconfigurations,andenlistedtheir advantagesanddisadvantages.
Dryingtechnologiescanbeclassifiedinto:
x Flash (pneumatic) dryers: These are typically employed for rapid drying of particles suspended in a pneumatic medium, that also serves to transport these particles. However, these dryers are limited in their application for dryingpowders,cakes,granules,flakes,pastes,gels,andslurries,andcannot
be applied to fibrous solids like biomass that cannot be dispersed (or fluidized)uniformly.
x Convective dryers: Are used with different convective heating media like flue gases, air, superheated steam, and can be applied to drying a wide variety of types of solids. The dryers may either facilitate continuous or batch processing with the modes of gasǦsolid contact including: fixed / movingbeds(conveyors),fluidizedbedsandrotarydryers(tumblingdryers /kilns).
x Conductive dryers: Conduction based drying can use high temperature jacketing,externallywallfiringortheuseofheatedcartridges/inserts.Most commonly used configurations include drum dryers, paddle dryers and jacketed kilns. These types of dryers are often used for applications like drying slurries, when there is a predominance of a liquid phase. For solids drying, these reactors suffer from low solidǦsolid heat transfer coefficients, leading to higher temperatures requirements for the heating surfaces. Scaling up of these reactors is also difficult due to the limited volume to surfaceratio.
x Radiative dryers: These dryers typically the involve the use of microwaves and infrared radiation in several reactor configurations. The advantage of thesereactorsisthattheinputenergycanbeusedoverawellǦdefinedand targeteddryingzone.Microwavesspecificallyhavehighpenetrationandcan effectively target the moisture contained in the bulk of the biomass. However,theprimaryenergysourceforthesedryersiselectricity,leadingto questionsofaffordabilitywithprocessingverylargequantitiesofarelatively lowǦvalueproductinapowerproductionprocess.
Evenasthermaldryingcanleadtoacompleteremovalofmoisture,biomasshasthe potential to reabsorb water from the surrounding air up until it reaches its equilibrium moisture content. Reabsorption can be limited to an extent by pelletisation.However,contactwithliquidwatermuststillbepreventedleadingto therequirementofindoorstorage.
A remarkable (but catastrophic) consequence of liquid water reabsorption was experienced in combating accidental fires in biomass storage silos. Attempts to quench the silo fire with water led to the stored wood pellets reabsorbing a large quantityofwater.Thisresultedinalargeincreaseintheweightofthefuel,resulting
in structural damage to the silo. Hence, the design of firefighting measures employedforwoodpelletsilosmusttakethisfactorintoaccount.
Figure1.6:Waterreabsorptionbypellets
1.5 TORREFACTION
Even as thermal drying can solve several of the issues in combustion, which are a result of the moisture content in the fuel, it does not affect the organic volatile matter content of the biomass. Also, even as dry biomass is more easily stored as compared to wet biomass, it is by no means immune from biological activity, moisture reabsorption and spontaneous ignition. Torrefaction is a preǦtreatment technology aimed at altering the properties of the biomass fuels to bring their combustion properties closer to those of coal. From the perspective of ultimate analysis, biomass fuels preferentially lose oxygen and hydrogen, thus bringing it closer to coal on the van Krevelen diagram (Figure 2.2). On a compositional basis, this is related to the devolatilisation of the hemicellulose fraction (and possibly a partial degradation of cellulosic and lignin fractions). The degree to which the biomasspropertiesarealtereddependsonthesoǦcalled“severity”,whichrelatesto theprocesstimeandtemperatures.
In addition to improving the combustion properties (increased calorific value and adiabatic flame temperatures) and reduced volatility, torrefaction also leads to increased hydrophobicity, better grinding properties and decrease in biological activity[22,23].Torrefactionistypicallyreportedtohaveamassyieldofaround70% corresponding to an energy yield of 90% depending on the severity of the
torrefaction reaction, indicating in an increase in energy density. It has been reported that the torrefaction reaction itself is only mildly endothermic or exothermic [24] and thus the torrefaction gases can be utilized to provide the requiredprocessheat.Anecessaryprerequisitefortorrefactionhoweveristhatthe biomassmustbecompletelydry,andhenceitcaninfactbeviewedasanextension of the drying process. Since torrefaction of herbaceous biomass forms the most importantpartofthisthesis,thetechnologyisdiscussedinmoredetailinChapter2.
1.6 PELLETISATION&BRIQUETTING
Compaction of biomass to make pellets or briquettes is an inevitable step prior to the transportation of low bulk density solid biofuels [7]. The distinction between pelletizing and briquetting concerns not only the difference in the size of the product,butalsothedegreeofdensification.Evenaspelletsaremoreenergydense than briquettes, their production is associated with a higher energy requirement. Thus,thechoiceofthetechnologymustbebasedonoptimisingthecostsassociated with the entire product lifecycle. Prior to pelletizing, it is necessary to mill the biomass to reasonably small particle sizes to increase adhesion and ensure pellet durability. The limits on particle sizes are less stringent with respect to forming briquettes. The cost of milling is further dependent on the nature and preǦ treatments that the biomass has been subjected to. Thus, dried and torrefied biomassismilledatalowercostthanfreshlyharvestedwetbiomass.Animportant concern with respect to pelletizing (or briquetting) biomass is the necessity of binders.Ithasbeenobservedthatbiomasscontainingahighquantityoflignin(for e.g.wood)doesnottypicallyrequireadditivessincethermallysoftenedligninunder acompressiveforceactstobindthepellet.
1.7 PROPOSEDSOLUTION
Owingtotheirlowprices,preǦtreatmentoflowǦgrade,highmoisturebiomassfuels forcoǦfiringpresentsaninterestingchallengeandopportunity.However,asstatedin Section 1.2, there are only a select few cases where such biomass is centrally collected. This present work deals with preǦtreatment of two types of biomass, primarilyselectedduetotheircentralizedcollectionintheexistinglandscape. The first biomass dealt with in this study is verge grass. This refers to the grass growingalongsidewaterways,provincialroadsandhighways.IthasbeenestimatedthatthetotalavailabilityofthisgrassintheNetherlandsisaround0.25Mton(dry) peryear[25],whichtranslatestoathermaloutputofaround4.5PJ.Atpresent,this grass is harvested for purposes of maintenance and subject to composting at a net cost to the public6. In addition to verge grass, a certain fraction of landscape grass that must be disposed (0.51 Mton (dry)/year) can also be utilized for coǦfiring. Appropriate preǦtreatment may allow the utilization of these locally sourced biomassestoproducerenewableelectricity.
Sugarcane is one of the most important cash crops in tropical countries. There already exists an established infrastructure to collect, store and process large volumesofcanecentrallyinlargeindustrialunits.Sugarcanebagasseisthusoneof the most abundant agricultural residues with the total annual global production estimated to be around 270 Mton (dry). A fraction of this bagasse is combusted to provide process heat and electric power for the sugar mills. However, there is typicallyanexcessofbagasseavailableattheplant,thatiseitherbriquettedandsold asanindustrialfuelorisusedtoproduceadditionalelectricpowerthatissoldtothe grid. However, the efficiency of electricity production at bagasse coǦgeneration plantsistypicallynotashighasdedicatedpfboilers.Incountrieswithextendeddry seasonslikeIndia,itiscommonpracticetosundrybagasseinopenfieldstoimprove thecombustion properties of thefuel.However, this is notan option for countries withregularrainfallthroughouttheyear;afactorwhichhasefficiencyimplications for bagasse combustion. Storage of large quantities of moist bagasse is also not possible due to concerns with selfǦignition. PreǦtreatment of bagasse to produce commodity fuels (similar to commercially traded wood pellets) for coǦfiring could thusleadtoamuchmoreefficientutilizationofthebiomassfeedstock.Depending on the price of renewable electricity, it may even make for a better business propositionforsugarmillstoearnfromthesaleofbagassetopfboilersthantoearn fromthesaleofexcesselectricity.
Figure 1.7 represents the conceptual design of the biomass preǦtreatment process. Theprocesschainfortreatingherbaceousbiomassessentiallyconsistsoffourserial functional steps of mechanical fractionation, thermal drying, torrefaction and pelletising/briquetting. The mechanical fractionation in the case of bagasse is essentially the existing process in a sugar mill where juice is extracted from the
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25Ǧ35€/ton(fresh)asreportedin2002[26]H.W.Elbersen,E.R.P.Keijsers,J.vanDoorn, Biorefineryofvergegrasstoproducebiofuel,in:12thEuropeanConferenceonBiomassfor Energy,IndustryandClimateProtection,Amsterdam,TheNetherlands2002.
