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Biofuels and Biomass
Fluidized bed torrefaction of commercial wood pellets: process performance and solid product quality Paola Brachi, Riccardo Chirone, Michele MICCIO, and Giovanna Ruoppolo Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01519 • Publication Date (Web): 30 Jul 2018 Downloaded from http://pubs.acs.org on August 12, 2018
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Fluidized bed torrefaction of commercial wood pellets: process performance and
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solid product quality
3 *
Paola Brachi§ , Riccardo Chirone§, Michele Miccio‡, Giovanna Ruoppolo§
4 5 6 7 8 9 10 11 12 13
§
Institute for Research on Combustion, National Research Council, P.le Tecchio 80, 80125 Napoli, Italy ‡
Department of Industrial Engineering, University of Salerno, via Giovanni Paolo II 132, 84084 Fisciano (SA), Italy
ABSTRACT
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This paper reports an experimental study aimed at investigating the influence of fluidized bed
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torrefaction treatment on the quality of commercial wood pellets. In particular, an experimental
16
program was performed, which allowed to investigate, at a laboratory-scale, the impact of the
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torrefaction temperature (200, 230, 250 °C) and the reaction time (7 and 15 min) on: a) the
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distribution and the composition of the main output products of torrefaction process (torrefied
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solids, condensable volatiles and permanent gases); b) the quality of torrefied pellets; and c) process
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performance in terms of mass and energy yields of the solid product. In particular, the quality of
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pellets was characterized in terms of apparent density, bulk density, calorific values, volumetric
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energy density, moisture uptake, swelling behavior, hardness (shore D) and nonstandard durability
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index. Results suggest that light torrefaction (200 °C and 7-15 min) is the most suitable to ensure a
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sustainable production (84-85% mass yield and 94-95 energy yield) of high quality torrefied wood
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pellets (no swelling in water, about 27% decrease in the moisture uptake after 7 days of exposure in
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an environment at a high relative humidity of 80 %, hardness and durability comparable to those of
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untreated wood pellets, only a 2-3% decrease in the volume energy density) in a downstream
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configuration (torrefaction after pelletization). The torrefaction treatment of wood pellets in *
Corresponding author. Tel.: +39 081 5931567; e-mail address:
[email protected] 1 ACS Paragon Plus Environment
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fluidized bed reactor has not been investigated so far, therefore, findings of this work can be useful
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to highlight potential advantages and drawbacks related to use of such a technology in this specific
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application.
32 33 34
Keywords: Torrefaction, Fluidized bed reactor, Wood pellets, Shore D Hardness, Durability, Hydrophobicity.
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1. INTRODUCTION
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Biomass is considered the renewable energy source with the highest potential for replacing
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fossil fuels in the short to medium term. 1,2 However, there are currently a number of challenges
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related to the quality of biomass resources that still prevent them from being used on a large scale
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for heat and power production. Biomass has, in fact, a lower energy content per unit mass compared
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with fossil fuels, which means that a higher load of feedstock is required in the case of biomass-fed
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plant in order get the same amount of energy when compared to fossil fuels. Moreover, biomass is a
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relatively bulky material. Typically, biomass bulk density ranges from 80-100 kg/m3 for agricultural
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straws and grasses to 150-200 kg/m3 for woody resources like wood chips and sawdust3, whereas
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the bulk density of coal is about 700 kg/m3.4 As a consequence of this, the volume of feedstock to
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be handled increases enormously, when biomass is used as a fuel instead of coal, with all the
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consequent issues that this can bring for logistics, e.g., storage and transportation, which are factors
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that greatly affect profit margins and thus the convenience of a biomass-fed plant. This is also the
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reason why it is only economically feasible to transport unprocessed biomass over a distance lower
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than about 200 km.4 Again, biomass cannot be stored outdoor without a careful protection since it is
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prone to natural decomposition over time and breakdown with exposure to moisture and pests (e.g.,
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flies and mosquitos), with consequent loss of quality and off-gas emissions; the high moisture
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content of some kinds of biomass (i.e., agro-industrial residues) also accelerates the decomposition
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process. In this regard, it is worth noting that drying biomass has a little benefit for the 2 ACS Paragon Plus Environment
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improvement of biomass storage behavior. In fact, because of its hydrophilic nature, biomass can
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re-absorb moisture and start to decompose again.
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Recent developments in mechanical densification technologies, including pelletization and
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briquetting, have substantially improved the economics of moving biomass around the globe4,5.
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Typically, these technologies increase the biomass energy density (MJ/m3) through the increase of
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its bulk density (kg/m3), but have a very little benefit for the improvement of properties such as the
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low calorific values (MJ/kg) and the remarkable hydrophobic behavior. The lower heating value
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(LHV) of currently marketed wood pellets is, in fact, approximately 15-19 MJ/kg, which still limits
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the mixing ratio that can be used in co-firing with coal typically having a lower heating value
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(LHV) of about 23-28 MJ/kg.6
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Fuel pellets with a very high bulk energy density in the range from 15 to 18 GJ/m3 and a
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lower heating value as high as 20-24 MJ/kg can be obtained when torrefaction, a relatively new
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thermal pretreatment of biomass, is combined with pelletization.6 It is worth mentioning that wood
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pellets, which are known to be a very energy-dense biomass fuel, have a bulk energy density that
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typically range from 7.5 to 10 GJ/m3 whereas that of coal is about 18.4-23.8 GJ/m3.
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Basically, torrefaction is a thermo-chemical treatment where a solid biomass is heated in an
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inert environment up to a temperature ranging between 200 and 300 °C. It is traditionally
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characterized by low particle heating rate (typically less than 50 °C/min) and a relatively long
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reactor residence time that typically ranges from 30 to 120 minutes depending on the specific
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feedstock, technology and temperature. The benefits accomplished by torrefaction include the
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possibility to convert any lignocellulosic raw materials into a hydrophobic solid, which can be
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stored outdoors, can be co-fired efficiently with coal and have improved properties even for other
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applications such as pyrolysis and gasification. The bulk density of the torrefied material, however,
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is generally lower than that of the raw biomass, making transport and storage economically
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challenging. Therefore, combining torrefaction and pelletization has great potential to upgrade raw
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biomass into a standard commodity fuel. 3 ACS Paragon Plus Environment
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So far, two potential pathways, either an upstream (torrefaction before pelletization) or
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downstream (torrefaction after pelletization) configuration, have been assessed for retrofitting
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torrefaction within pellet production facilities. In particular, since a successful upstream integration
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certainly produces highly dense and durable torrefied pellets, whereas a downstream integration
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could potentially results in a deterioration of pellet quality (e.g., a loss in strength and density), most
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of the research efforts performed so far, have been mainly focused on the upstream integration.7,8
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However, many challenges still remain with such configuration, which includes: (a) difficulty in
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densification with a consequent need for binders along with a conditioning step in order to obtain
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reasonable pelletization efficiency;9,10 (b) frequent maintenance requirements due to the abrasive
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nature of torrefied biomass that lower useful life of pellet;11 (c) safety concerns from fine generation
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during grinding and pelletizing torrefied biomass, in particular possibility of dust explosion11.
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Moreover, it is worth noting that, most of the currently used binders are hydrophilic and, hence, in
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addition to impart additional costs and lower the pellet heating value, they may also compromise
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pellet hydrophobicity.11 On the other hand, torrefaction as a downstream operation, where white
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pellets are torrefied to produce black pellets, has the benefit of being a simple bolt-on integration. In
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addition, downstream integration eliminates the need for additional grinding and pelletizing units,
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while minimizing plant contamination due to dust generated from processing torrefied biomass.
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Again, CAPEX can be reduced, since a typical torrefier has significantly higher throughput when
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processing pellets compared to wood chips.11
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In this context, taking into account the operational benefits of downstream integration
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compared to the upstream one, an experimental study on the torrefaction treatment of commercial
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wood pellets, which has been poorly investigated so far12-14, has been focused in this work.
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In detail, batch experimental runs were performed at three different temperatures for two
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values of the reaction time in order to investigate the effect of such process variables on both the
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quality of torrefied wood pellets (i.e., elemental composition, bulk and energy densities, sizes and
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shape, hydrophobic behavior, mechanical durability and fine particles content) and process 4 ACS Paragon Plus Environment
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performances (i.e., mass and energy yields). To the best of the Authors’ knowledge, the torrefaction
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treatment of wood pellets in fluidized bed reactor has not been investigated so far. Therefore,
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findings of this work can also useful to highlight potential advantages and drawbacks related to use
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of the fluidization technology in this specific application.
111 112
2. EXPERIMENTAL SECTION
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2.1 Materials sampling and chemical characterization Commercial fir pellets (6 mm diameter and 3.15-4 mm length) were used in this work as biomass
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feedstock. Ticino sand in size range of 150-300 µm (Particle density = 2651 kg/m3; packed bulk
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density = 1537 kg/m3; Sauter mean diameter = 210 µm; minimum fluidization velocity = 4.14·10-2
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m/s at 25 °C) was used for the operation of the fluidized bed reactor.
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The determination of moisture content (M), volatile matter (VM), fixed carbon (FC), and ashes
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(ASH) in raw and torrefied biomass samples was performed by using a TGA 701 LECO
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thermogravimetric analyzer by following the ASTM D5142. The elemental composition (CHN) of
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samples was determined by using a CHN 2000 LECO analyzer according to the ASTM D5373
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standard method. The oxygen content was estimated by subtracting the sum of the percentages (dry
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basis) of C, H, N and ash from 100%. All the analyses were performed in triplicate at least.
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Liquid products were analyzed by gas chromatography coupled to mass spectrometry (GC‐MS
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Agilent HP6890/HP5975, equipped with a capillary column DB-35MS, 30 m ×0.25 mm ID) using
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helium as carrier gas.
128 129
pH measurements of liquid products were measured by using a Thomas Scientific 675 pH/ISE meter at ambient temperature.
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The analysis of permanent gases evolved during the torrefaction treatment of wood pellets was
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performed in real time. In particular, the Testo 350 advanced portable emission analyzer was used
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for the real-time monitoring of evolved carbon monoxide (CO) and carbon dioxide (CO2), which
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are the main components of permanent gases evolved during torrefaction processes11. 5 ACS Paragon Plus Environment
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2.2. Calorific values, mass density and energy density of raw and torrefied pellets The higher heating value (HHV, MJ/kg, dry basis) was measured by using a Parr 6200 Isoperibol
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Calorimeter. The conversion of higher to lower heating values (LHV, MJ/kg, dry basis) was
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performed according to the following Eq. (1):
139 140
LHV (MJ/kg) = HHV (MJ/kg) - 2.442(8.936·H/100)
(1)
141 142
where the constant 2.442 is the enthalpy difference in MJ/kg between the gaseous and liquid
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water at 25 °C, the constant 8.936 is the ratio between the molar masses of water (H2O) and
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hydrogen (H2)15 and H is the weight fraction (%) of hydrogen in the samples on a dry basis, All the
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analyses were performed in duplicate at least.
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The apparent density of wood pellets was calculated as the ratio of the pellet mass to its volume.
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In particular, in this work, the ends of the tested wood pellets were flattened by using sandpaper in
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order to make them perfect cylinders. The length (L) and diameter (D) of cylinders were then
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measured by using a Vernier caliper (Mod. Metrica, 0.05mm and 1/128 inch Accuracy; 0-150 mm
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and 0-6 inch measuring Ranges), which were used to calculate the volume of pellets and hence their
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apparent density. The bulk density was calculated as the ratio of the mass to the volume (including
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the contribution of the interparticle void volume) of a batch of pellets, poured into a 25 ml
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graduated cylinder. The packed bulk density was obtained by mechanically tapping the cylinder
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containing the sample until no further volume change was observed. Density measurements were
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performed after the samples were oven dried at 105 ± 5 °C for 24 h. The density values reported
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later in Fig. 5A are the average of 5 measurements for each sample.
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Bulk and apparent energy densities (GJ/m3, dry basis) of raw and torrefied pellets were
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calculated as the ratio of the lower heating value (LHV) to the bulk and apparent density of the
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pellets, respectively. 6 ACS Paragon Plus Environment
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2.3 Hygroscopic behavior of raw and torrefied pellets Two types of tests were used to investigate the hygroscopic behavior of both raw and torrefied
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wood pellets. The first one, coded below as exposure test, involved pellets that were placed in a
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climate-controlled chamber at specified humidity and temperature conditions. The second one,
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coded below as immersion test, involved the immersion of pellets in water. In both cases, pellets
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were analyzed to determine the resulting moisture uptake after a prefixed time. In more details,
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exposure tests were performed by using conventional static desiccator technique.16,17 Specifically, a
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glass desiccator containing a supersaturated solution of either KBr or K2CO3 in water was used as a
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humidity control chamber, which allowed exposing samples to 80 ±2 or 43 ±1 % RH at room
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temperature (i.e., 25 ± 2 °C), respectively. In order to minimize daily temperature changes the
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desiccator was partially submerged in a water bath. Approximately 5 g of oven-dried pellets were
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put into an open weighing bottle and then placed into the desiccator for testing. The humidity and
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temperature in the glass desiccator were checked by using a digital thermo-hygrometer (30.5005
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TFA Dostmann). The moisture content (MC, %wt) of pellets samples after an exposure time of 7
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days was measured by using a Kern DBS Halogen Moisture analyzer.
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Immersion tests were performed putting pellets of similar size into a 50 ml volumetric vials filled
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with about 30 mL of distilled water. At the end of the test, the solid samples were separated from
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water by using a sieve with 1 mm diameter and letting the sample drip water for about 30 minutes.
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After the dripping time, the moisture content (MC, %wt.) of wet pellets was measured by using the
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above mentioned Kern DBS analyzer.
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In both cases, the water uptake (mass gained), Wg (%wt.), with respect the initial mass of dry samples, was calculated by using the following formula:
183 184
= ( ) · 100
(2)
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2.4 Hardness and durability of raw and torrefied pellets
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A non-standard method was purposely set up in this work in order to compare at a small scale
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the quality of raw and torrefied wood pellets in terms of their durability, which represents the ability
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to hand pellets without experiencing unacceptable breakage or generating a significant amount of
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fines. This newly developed method allows for the determination of a pellet durability index
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(PDI)18, which is defined as the weight fraction (%) of whole pellets remaining intact in a sample
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tumbled according the method described below.
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About 5 g of dust-free pellets are placed on a 1 mm mesh sieve. The sieve is tightly covered at
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the top to prevent the loss of the material sample during sifting or mechanical agitation. A pan is
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also used to collect fines passing through the sieve. Four nitrile butadiene rubber (NBR) balls (30
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mm diameters, 70 Shore A hardness) and 7 stainless steel ball bearings (4.5 mm diameter, grade
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100 AISI 316 ) are added to the sieve to make the test more stressing and somehow simulating the
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bulk handling during actual manufacturing and delivery steps. The durability test set (i.e., sieve, pan
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and cover) is placed on an orbital shaker moving at 210 rpm for 30 min. Afterward, fines are
200
removed from the tumbled samples by screening and the remaining pellets are weighed against their
201
initial mass to calculate the PDI as follows:
202 203
!" #$
(%. ) = "
!" #$
∙ 100
(3)
204 205
Analysis of pellets hardness was carried out by using a Shore D durometer (Sauter HBD 100-0)
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in accordance with ASTM D2240. This test method is based on the penetration of a specific type of
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indentor when forced into the material under specified conditions. The indentation hardness is
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inversely related to the penetration and is dependent on the elastic modulus and viscoelastic
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behavior of the material. Shore durometer hardness measurements were repeated at least 10 times 8 ACS Paragon Plus Environment
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for each sample by using both different pellets and different points on each pellet. The average
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values and the standard deviation of measurements were accurately taken into account (see Fig. 8
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below).
213 214
2.5. Apparatus and procedures for pellets torrefaction
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A schematic representation of the laboratory-scale fluidized-bed apparatus is shown in Figure 1.
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The torrefaction unit of the apparatus consists of a steel tubular column (38 mm inner diameter and
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350 mm height) surrounded by an electrical heating tape (FGR-100/240V V-ROPE HEATER
218
500W by Omegalux). The thermal insulation of the reactor is ensured by a mineral wool heat-
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insulating cylinder. The temperature of the reactor is regulated by a proportional integral derivative
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(PID) controller (Gefran 600 PID), which reads the bed temperature trough a K-type thermocouple
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inserted in the reactor at 7 cm above the gas distributor. A disk-shaped ceramic fiber felt (6 mm
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thick) set at the bottom of the column and topped by a 15 mm height bed of steel spheres
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(4.5 mm diameter) acts as the gas distributor. A flowmeter (Asameter Model E, by ASA) with a
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100-1000 NL/h flow range supplies nitrogen as a fluidization gas during the torrefaction tests. A
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glass tubular trap followed by a water-cooled Liebig condenser and an air-cooled Liebig condenser
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are used as gas-liquid separators in order to recover the condensable fraction of the torgas evolved
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during the torrefaction treatment of wood pellets.
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Batch torrefaction tests were performed at three different temperatures (i.e., 200, 230 and 250
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°C) and by fixing the reaction time equal to either 7 or 15 min. Torrefaction tests and the resulting
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solid products will be coded below as “TWP-T-t” where TWP stands for torrefied wood pellet
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(TWP), T indicates the torrefaction temperature and t the reaction time: for example,TWP-200-15
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denotes the torrefied pellets obtained from a test performed at 200 °C by fixing the reaction time at
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15 min. In a typical experimental run, about 20 g of raw wood pellets (RWP) having a moisture
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content of approximately 6-7 % wt. was charged into the reactor. The aspect ratio of the granular
235
solid bed, which is defined as the ratio of height to diameter of the bed, was set as high as 4. At 9 ACS Paragon Plus Environment
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startup, the bed of granular solid was heated to the selected torrefaction temperature while using air
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as a fluidizing gas. When the fluidized bed reached its prefixed steady-state temperature, the
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fluidizing gas was switched to nitrogen and the biomass was dropped from the top into the reactor.
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During torrefaction tests, the nitrogen flow rate was set equal to 200 Nl/h corresponding to a
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fluidization velocity in the range from 7.8·10-2 to 9.7·10-2 m/s at the operating temperatures. This
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choice, which was suggested by preliminary trial and error fluidization tests, allowed achieving a
242
good mixing of biomass and inert particles in the bed during torrefaction tests while preventing
243
their elutriation and segregation. The resulting gas−solid contact time, calculated as the ratio of the
244
static bed height to the superficial gas velocity, slightly increased with the rise in the torrefaction
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temperature. Once the prefixed test time was passed, the bed was cooled down to less than 100 °C
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as fast as possible (approximately 2-3 min) by turning the electrical heater off, removing the reactor
247
from the insulating cylinder and blowing cold compressed air onto the reactor surface. Finally, the
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solid and liquid products were recovered and weighted. In more details, the condensable volatiles
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evolved during torrefaction tests were quantitatively recovered from tail end of the tube reactor and
250
the three adopted condensation traps (see Fig. 1) by washing these latter with acetone. Afterward,
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they were collected as trap-1 liquids (T1-L), when recovered from the tail end of the reactor
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column, trap2 liquids (T2-L), when recovered from the glass tubular trap, trap-3 liquids (T3-L),
253
when recovered from the water-cooled Liebig condenser and trap-4 liquids (T4-L), when recovered
254
from the air-cooled Liebig condenser. The solid torrefied product was separated from the granular
255
solid by manual sieving. Mass yields of solid (MYS), liquid (MYL), and gaseous (MYG) products
256
were evaluated on an as-received basis (ar) through the following Eqs. 4−6. It is worth noting that
257
the mass of condensable compounds used in Eq. 2 does not include the mass of the T1-L fraction
258
recovered from the tail end of the reactor column, which was of difficult determination. The energy
259
densification index (IED) and the energy yield (EYS) of torrefied solids were also evaluated on a dry
260
basis by means of Eqs. 7 and 8.
261 10 ACS Paragon Plus Environment
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Energy & Fuels
-.//01203 4.523
262
&'( (%, *+) = ,
263
&': (%, *+) = ,
264
&'? (%, *+) = ,
265
DE (−, GH) =
/67 7..3 80550-4
9 ∙ 100
;.
