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Effects of pyrolysis conditions and ash formation on gasification rates of biomass char Anna Strandberg, Per Holmgren, David R. Wagner, Roger Molinder, Henrik Wiinikka, Kentaro Umeki, and Markus Broström Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 15 May 2017 Downloaded from http://pubs.acs.org on May 15, 2017
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Effects of pyrolysis conditions and ash formation on
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gasification rates of biomass char
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Anna Strandberg †*, Per Holmgren †, David R. Wagner †,§, Roger Molinder ‡, Henrik Wiinikka ‡,
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Kentaro Umeki #, and Markus Broström †
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†
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Conversion Laboratory, SE-901 87 Umeå, Sweden
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‡
RISE Energy Technology Center, Box 726, SE-941 28 Piteå, Sweden
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#
Luleå University of Technology, Division of Energy Science, Energy Engineering, SE-971 87
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Luleå, Sweden
Umeå University, Department of Applied Physics and Electronics, Thermochemical Energy
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ABSTRACT: Pyrolysis conditions and the presence of ash forming elements significantly
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influence char properties and its oxidation or gasification reactivity. In this study, intrinsic
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gasification rates of char from high heating rate pyrolysis were analyzed with isothermal
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thermogravimetry. The char particles were prepared from two biomasses at three size ranges, and
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at two temperatures. Reactivity dependence on original particle size was found only for small
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wood particles that had higher intrinsic char gasification rates. Pyrolysis temperature had no
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significant effect on char reactivity within the range tested. Observations of ash formation
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highlighted that reactivity was influenced by the presence of ash forming elements, not only at
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the active char sites, but also through prohibition of contact between char and gasification agent
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by ash layer formation with properties highly depending on ash composition.
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1. INTRODUCTION
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For the design of different thermochemical conversion reactors for solid biomass fuels the rate
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of char conversion is an important design criterion. Devolatilization is relatively rapid and thus
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the slower char gasification is considered to be the rate limiting step of the process,1,2 implying
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that high char reactivity is important to achieve high specific capacity. The conditions during
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devolatilization affect the chemical and physical properties of the char and therefore also the char
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reactivity; high pyrolysis temperature and prolonged retention time generally reduce the
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reactivity of char particles.2-4 Furthermore, high heating rates during pyrolysis tend to increase
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the char reactivity for both oxidation and gasification.2,5 This can be explained by development
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of higher surface area of chars generated at higher heating rates.5 Similar results have been
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reported from studies on coal char.6
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Milling of biomass to an acceptable size distribution is a bottleneck in the development of
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systems for combustion or gasification of biomass bulk solids. It is energy demanding and also
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many biomass bulk solids are often associated with feeding problems connected to their
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structure.7 By using larger particles the milling costs could be reduced.8 However, previous
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studies on the effect of particle size shows that larger particles can be incompletely converted in
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the short residence time of an entrained flow gasifier and thereby reduce the efficiency of the
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process.9-11 Larger particles have larger temperature gradients due to thermal inertia, and the
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particle size influences the mass and heat transfer, and to some extent, secondary reactions inside
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the particles.11-13 In addition, the relative importance of gravity becomes higher with an increase
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in particle size, resulting in shorter particle residence time (i.e. lower char conversion).
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Several studies have addressed the correlations between particle size and apparent reactivity
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from different perspectives. Moilanen et al.14 evaluated the effect of the particle size on char
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quality for wood particle sizes between 0.5 and 10 mm. Less reactive char was produced from
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larger sized particles, probably connected to intra-particle transport and secondary
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transformation of pyrolysis products. Similar findings were published by Gómez-Barea et al.15
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who studied the gasification reactivity of different biomass char particle sizes at different CO2
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partial pressures and temperatures in a thermogravimetric analyzer (TGA). The measured
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gasification reactivity decreased as the particle size increased. Zanzi et al.16 also concluded that
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the reactivity of char was increased when small fuel particles were subjected to rapid pyrolysis.
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Several other publications also investigated how initial particle size can affect the char reactivity
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by mass diffusion.10,17 A less elaborated effect is how the intrinsic char properties are affected by
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the different temperature history experienced by particles of different size in the ranges relevant
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for entrained flow reactors.
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Besides pyrolysis conditions, the ash forming elements present in the fuel are also known to
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play important roles in reactivity and char burnout. Elements from the alkali metal group,
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primarily K, are known to “catalyze” conversion reactions and thereby increase reactivity.18-23
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The change in reaction rates between different biomasses have been studied by others and the
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suggested mechanism of catalytic ash elements has been complemented with descriptions of a
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deactivating effect of counter ions such as Si, P and Al.24-28 Supported by reactivity
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measurements, the hypothesis is based on K being captured in ash forming reactions, forming
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e.g. silicates or phosphates instead of being available on active char sites. Simple and useful, but
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empirical and fuel specific indexes have been used to correlate contents of ash forming elements
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to the inhibiting effects observed;25,26,28 in short, ratios describing the presence of catalytic
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potential. To date, detailed ash formation reactions have not been mechanistically coupled to the
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“deactivating” observations.
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The first objective of this study was to evaluate intrinsic gasification rates of chars generated
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from rapid devolatilization of biomass particles with different size, to study if and how char
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reactivity changes depending on the parameters varied. Two different fuels were included to
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represent both woody and agricultural biomass: wood from Scots pine and straw from wheat.
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Isothermal TGA analyses of ground char produced in a drop tube reactor was performed in a
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CO2 atmosphere to determine the effect of initial particle size and pyrolysis conditions on the
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intrinsic rate of char gasification.
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The second objective of this study was to compare gasification profiles of chars from the two
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fuels, and to further develop the theories of fuel-ash interactions using compositional information
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together with observations on reactivity profiles and ash formation during gasification. This part
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was accomplished using the same samples and methods mentioned above, but complemented
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with interrupted gasification experiments to enable chemical and morphological analysis of
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gasification residues at different degrees of conversion.
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2. EXPERIMENTAL SECTION
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2.1. Biomass feedstock and char preparation
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Wood from Scots Pine (Pinus sylvestris L.) and straw from wheat (Triticum aestivum) was
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dried at 105 °C and milled to
