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Model-free rate expression for thermal decomposition processes: The case of microcrystalline cellulose pyrolysis
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Technology.ORCID iD: 0000-0002-2068-357X
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Wood Chemistry and Pulp Technology.ORCID iD: 0000-0002-0252-337X
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Energy Processes.ORCID iD: 0000-0001-7995-3151
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Wood Chemistry and Pulp Technology.
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2015 (English)In: Fuel, ISSN 0016-2361, E-ISSN 1873-7153, Vol. 143, 438-447 p.Article in journal (Refereed) Published
Abstract [en]

We explore the possibility to derive a completely model-free rate expression using isoconversional methods. The Friedman differential method (Friedman, 1964) and the incremental integral method by Vyazovkin (2001) were both extended to allow for an estimation of not only the apparent activation energy but also the effective kinetic prefactor, defined as the product of the pre-exponential factor and the conversion function. Analyzing experimental thermogravimetric data for the pyrolytic decomposition of microcrystalline cellulose, measured at six different heating rates and three different initial sample masses (1.5-10 mg), revealed the presence of secondary char forming reactions and thermal lag, both increasing with increased sample mass. Conditioning of the temperature function enables extraction of more reliable prefactors and we found that the derived kinetic parameters show weak dependence on initial sample mass. Finally, by successful modeling of quasi-isothermal experimental curves, we show that the discrete rate expression estimated from linear heating rate experiments enables modeling of the thermal decomposition rate without any assumptions regarding the chemical process present. These findings can facilitate the design and optimization of industrial isothermal biomass fed reactors.

Place, publisher, year, edition, pages
2015. Vol. 143, 438-447 p.
Keyword [en]
Model-free, Isoconversional, Kinetics, Thermogravimetric analysis, Pyrolysis, Cellulose
National Category
Energy Engineering
Identifiers
URN: urn:nbn:se:kth:diva-160361DOI: 10.1016/j.fuel.2014.11.079ISI: 000347688700052Scopus ID: 2-s2.0-84918767293OAI: oai:DiVA.org:kth-160361DiVA: diva2:791340
Funder
Swedish Energy Agency
Note

QC 20150227

Available from: 2015-02-27 Created: 2015-02-19 Last updated: 2017-12-04Bibliographically approved
In thesis
1. Isoconversional analysis for the prediction of mass-loss rates during pyrolysis of biomass
Open this publication in new window or tab >>Isoconversional analysis for the prediction of mass-loss rates during pyrolysis of biomass
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Biomass is the only renewable carbon source that can compete with fossil energy sources in terms of production of materials, chemicals and fuels. Biomass can be transformed into charcoal, liquid and gas through pyrolysis, i.e. pure thermal decomposition. By changing the pyrolysis conditions either solid, liquid or gaseous fractions can become the main product and pyrolysis is thus a very versatile process. Pyrolysis is also the first step in combustion and gasification, two important thermal processes in our society. The importance of biomass pyrolysis has led to extensive research in this area but due to the complexity of the process there is still no general understanding of how to describe biomass pyrolysis, which is essential in order to optimize thermal processes. The research presented in this thesis thus aims at finding a simple yet accurate way to model the decomposition rate of biomass during pyrolysis.

Thermogravimetric analysis, a well known method that is simple to use, was chosen to collect the experimental data used for kinetic evaluation. The reaction kinetics were derived using two different model-free, isoconversional methods, i.e. the non-linear form of the Friedman method and the incremental, integral method ofVyazovkin. By using these two methods and experimental data, complete reactionrate expressions could be derived for commercial cellulose, Norway spruce and seven different samples originating from kraft cooking, the most common process to produce pulp for the paper industry. The derivation of model-free rate expressions have never been performed before for these materials and since the rate expressions are model-free, no assumptions or knowledge about the pyrolysis reactions were required. This is a great advantage compared to the commonly used model-fitting methods that rely on information about these aspects. All therate expressions were successful in predicting mass-loss rates at extrapolated pyrolysis conditions. This is a clear indication of the soundness of the methodologypresented in this thesis.

Abstract [sv]

Biomassa är den enda förnybara kolkällan som kan konkurrera med fossila energikällor när det gäller produktion av material, kemikalier och bränslen. Biomassakan omvandlas till biokol, bioolja och gas med hjälp av pyrolys, dvs termisk nedbrytning. Genom att variera de processförhållanden som råder under pyrolysen kan man få antingen fast, flytande eller gasfasiga ämnen som huvudprodukt, något som gör pyrolys väldigt flexibelt. Utöver detta är pyrolys även betydelsefull vid förbränning och förgasning, två viktiga processer i dagens samhälle. Vikten av biomassapyrolys har resulterat i omfattande forskning inom området men pga biomassas komplexa natur råder det ännu ingen enighet gällande hur biomassapyrolys bör modelleras. Detta försvårar utveckling och optimering av termiska processer matade med biomassa. Forskningen som presenteras i denna avhandling fokuserar således på att finna en enkel men noggrann metod för att beskriva hastigheten med vilken biomassa bryts ned under pyrolys.

Termogravimetrisk analys, en vanligt förekommande metod som är enkel att använda, valdes för att samla in experimentell data som kan användas för att undersöka hastigheten för termisk nedbrytning, dvs kinetiken. Två olika metoder som på engelska går under benämningen “model-free” och “isoconversional” har använts, nämligen den icke-linjära formen av Friedmans metod och den stegvisa, integrala metoden som utvecklats av Vyazovkin. Genom att använda dessa två metoder och experimentell data kunde kompletta reaktionshastighetsuttryck tas fram för kommersiell cellulosa, gran och sju olika material framställda genom sulfatprocessen, den idag vanligast förekommande pappersmassaprocessen. Pyrolyskinetiken för dessa material har aldrig tidigare analyserats med dessa två metoder och fördelarna med metoderna gjorde det möjligt att bestämma hastighetsuttryck utan någon kunskap om de pågående reaktionerna. Detta är en viktig fördel jämfört med andra metoder som är beroende av sådan information. Alla framtagna reaktionshatighetsuttryck kunde användas för att framgångsrikt förutsäga minskningen av massa vid extrapolerade pyrolysförhållanden. Detta är en tydlig indikation på att metoden använd i denna avhandling fungerar väl.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2016. 88 p.
Series
TRITA-CHE-Report, ISSN 1654-1081 ; 2016:23
Keyword
pyrolysis, biomass, kinetics, cellulose, spruce, black-liquor, model-free, isoconversional, prediction, pyrolys, biomassa, kinetik, cellulosa, gran, svartlut, model-free, isoconversional, förutsägelse
National Category
Chemical Engineering
Research subject
Chemical Engineering; Energy Technology
Identifiers
urn:nbn:se:kth:diva-187469 (URN)978-91-7595-997-9 (ISBN)
Public defence
2016-06-10, F3, Lindstedtsvägen 26, Stockholm, 09:57 (English)
Opponent
Supervisors
Note

QC 20160524

Available from: 2016-05-24 Created: 2016-05-24 Last updated: 2016-05-24Bibliographically approved

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