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Experimental and Simulation Studies on Biomass Torrefaction and Gasification

Tapasvi, Dhruv
Doctoral thesis
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http://hdl.handle.net/11250/285982
Date
2015
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  • Institutt for energi og prosessteknikk [3305]
Abstract
The potential for bioenergy in Norway is significant. This potential can be realized by

improving the properties of biomass and making it a convenient and competitive alternative

to other fuels. Torrefaction is the most promising biomass pretreatment technique to date,

improving its effectiveness as a fuel in various thermochemical processes. Torrefaction

considerably reduces moisture content but increases the heating value, hydrophobicity and

grindability of biomass. Torrefaction is influenced by many parameters, including biomass

composition, temperature, holdup time and particle size. To evaluate the feasibility of

torrefaction in a particular region, locally available biomass resources should be investigated.

This approach forms the basis of the present study. To improve the viability of bioenergy in

Norway, I undertook fundamental research on the torrefaction of Norwegian woody biomass

and evaluated the behavior of torrefied biomass in thermochemical processes.

Starting with a detailed literature review on the topic, torrefaction behavior of Norwegian

Birch and Spruce was experimentally investigated. Torrefaction experiments were performed

in a macro-TGA reactor with provisions for continuous measurement of volatiles. Process

temperature (225 and 275 °C), holdup time (30 and 60 minutes) and sample size (10 and 40

mm cubes) were varied. Fuel characterization, derivative thermogravimetric (DTG) curves,

product yields, hydrophobicity tests, grinding energies and particle size distributions are

discussed. Temperature had the strongest effect on the properties of torrefied biomass of all

the studied parameters. Overall, considerable improvements in grindability and

hydrophobicity were obtained in torrefied biomass from both feedstocks.

To obtain information on the intrinsic kinetics of torrefaction, the pyrolysis kinetics of

Norwegian spruce and birch wood was investigated in another study. Micro-TGA was

employed with nine different heating programs, including linear, stepwise, modulated and

constant reaction rate (CRR) experiments. The 18 experiments on the two feedstocks were

evaluated simultaneously using the method of least squares. Part of the kinetic parameters

could be assumed common for both woods without a considerable worsening of the fit

quality. Three pseudocomponents were assumed. Two of them were described using

distributed activation energy models (DAEM), while the decomposition of the cellulose

pseudocomponent was described using self-accelerating kinetics. In another approach, all

three pseudocomponents were described using n-order reactions. A table was calculated to provide guidance about the extent of devolatilization during torrefaction at various

temperatures and residence times.

For understanding torrefied biomass reactivity in oxidative conditions, another micro-

TGA study was conducted with four torrefied wood samples and their original feedstocks

(birch and spruce) at slow heating rate programs. Particularly low sample masses were

employed to avoid self-heating of the samples due to heat of combustion. Linear, modulated

and CRR temperature programs were employed in TGA experiments under gas flows of 5

and 20% O2. The kinetic model consisted of two devolatilization reactions and a subsequent

char burn-off reaction. Cellulose decomposition in the presence of oxygen has selfaccelerating

(autocatalytic) kinetics. Decomposition of the non-cellulosic components of the

biomass was described using a distributed activation model. The char burn-off was

approximated by power-law (n-order) kinetics. Each of these reactions has its own

dependence on oxygen concentration, which was also expressed using power-law kinetics.

The model contained 15 unknown parameters for a given biomass. Certain of these

parameters could be assumed to be identical for the six samples without a substantial

worsening of fit.

Lastly, the behavior of torrefied biomass in a gasification process was evaluated. A twostage

biomass gasification model was selected using Aspen Plus as the simulation and

modeling tool. The model included minimization of the Gibbs free energy of the produced

gas to achieve chemical equilibrium, constrained by mass and energy balances for the system.

Air and steam were used as the oxidizing agents with both untreated and torrefied biomass as

feedstocks. Three process parameters were studied: equivalence ratio (ER), Gibbs reactor

temperature and steam-to-biomass ratio (SBR). A total of 27 cases were included in the

analysis, operating the system below the carbon deposition boundary with all carbon in the

gaseous form in the product gas. Product gas composition [hydrogen (H2), carbon monoxide

(CO), carbon dioxide (CO2) and nitrogen (N2)] was analyzed together with cold gas energy

and exergy efficiencies for all cases. Torrefied biomass gave higher H2 and CO contents in

the product gas, as well as higher energy and exergy efficiencies, than untreated biomass. The

overall efficiency of an integrated torrefaction-gasification process depends on the mass yield

of torrefaction. The results were validated using a C-H-O ternary diagram combined with

results from similar studies.
Has parts
Paper 1; Tapasvi, Dhruv; Tran, Khanh-Quang; Wang, Liang; Skreiberg, Øyvind; Khalil, Roger Antoine. BIOMASS TORREFACTION – A REVIEW. 9TH EUROPEAN CONFERENCE ON INDUSTRIAL FURNACES AND BOILERS

Paper 2: Tapasvi, Dhruv; Khalil, Roger Antoine; Skreiberg, Øyvind; Tran, Khanh-Quang; Grønli, Morten. Torrefaction of Norwegian Birch and Spruce: An Experimental Study Using Macro-TGA. Energy & Fuels 2012 ;Volum 26.(8) s. 5232-5240 http://dx.doi.org/10.1021/ef300993q © 2013 American Chemical Society

Paper 3: Tapasvi, Dhruv; Khalil, Roger Antoine; Várhegyi, Gabor; Tran, Khanh-Quang; Grønli, Morten; Skreiberg, Øyvind. Thermal Decomposition Kinetics of Woods with Emphasis on Torrefaction. Energy & Fuels 2013 ;Volum 27.(10) s. 6134-6145 http://dx.doi.org/10.1021/ef4016075 © 2013 American Chemical Society

Paper 4: Tapasvi, Dhruv; Khalil, Roger Antoine; Várhegyi, Gabor; Skreiberg, Øyvind; Tran, Khanh-Quang; Grønli, Morten. Kinetic Behavior of Torrefied Biomass in an Oxidative Environment. Energy & Fuels 2013 ;Volum 27.(2) s. 1050-1060 http://dx.doi.org/10.1021/ef3019222 Copyright © 2013 American Chemical Society

Paper 5: Tapasvi, Dhruv; Kempegowda, Rajesh Shivanahalli; Tran, Khanh-Quang; Skreiberg, Øyvind; Grønli, Morten. A Simulation study on the torrefied biomass gasification. Energy Conversion and Management 2014 ;Volum 90. s. 446-457 http://dx.doi.org/10.1016/j.enconman.2014.11.027 This article is reprinted with kind permission from Elsevier, sciencedirect.com
Publisher
NTNU
Series
Doctoral thesis at NTNU;2015:175

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