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Investigations using a cold flow model of char mixing in the gasification reactor of a dual fluidized bed gasification plant
Investigations using a cold flow model of char mixing in the gasification reactor of a dual fluidized bed gasification plant Stephan Kraft, Friedrich Kirnbauer, Hermann Hofbauer PII: DOI: Reference:
S0032-5910(16)30714-8 doi:10.1016/j.powtec.2016.10.032 PTEC 12031
To appear in:
Powder Technology
Received date: Revised date: Accepted date:
17 May 2016 21 September 2016 13 October 2016
Please cite this article as: Stephan Kraft, Friedrich Kirnbauer, Hermann Hofbauer, Investigations using a cold flow model of char mixing in the gasification reactor of a dual fluidized bed gasification plant, Powder Technology (2016), doi:10.1016/j.powtec.2016.10.032
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Investigations using a cold flow model of char mixing in the gasification reactor of a dual fluidized bed gasification plant Stephan Krafta,∗, Friedrich Kirnbauera , Hermann Hofbauerb
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a Bioenergy 2020+ GmbH, Wiener Strasse 49, A-7540 G¨ ussing, Austria University of Technology, Institute of Chemical Engineering, Getreidemarkt 9/166 Vienna, Austria
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b Vienna
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Abstract
This paper treats the mixing and movement of char in a dual fluidized bed (DFB) biomass gasification plant. In these plants such measurements are trou-
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blesome to perform, and so a cold flow model has been developed to investigate
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this topic. This cold flow model allows simulating the fluidization behaviour of the gasification reactor in the DFB plant in G¨ ussing, Austria. The recirculation
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of the bed material is also possible, and can be easily controlled with a rotary valve. In the cold flow model, bronze is used as the bed material and polyethylene as the char. It is possible to take samples during operation to investigate the char concentration in the bed material recirculation stream. Experiments have shown that the char shows a flotsam behaviour since it is of low density. Furthermore, the investigations have shown that higher fluidization rates and higher bed material recirculation rates enhance the char mixing and increase the char concentration in the recirculation stream. It was found that doubling the overall char concentration in the system does not lead to a doubling of the char concentration in the bed material recirculation stream. Furthermore, the influence of the bed height in the gasification reactor was investigated. It was found that higher bed heights lead to lower char concentrations in the recirculation stream. These initial investigations revealed that much is still unknown ∗ Corresponding
author Email address: [email protected] (Stephan Kraft)
Preprint submitted to Journal of LATEX Templates
October 14, 2016
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about DFB plants, but the knowledge of the behaviour of the different types of particles in the bubbling bed of the gasification reactor helps to further improve and develop the DFB technology.
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Keywords: cold flow modelling, fluidized bed gasification, segregation, scaling
1. Introduction
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Fluidized bed reactors have been in use for a long time and are used for various applications. The advantages of fluidized beds are their good mixing behaviour and gas/solid contact due to the vigorous bubble motion. The dual
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fluidized bed (DFB) steam gasification of biomass is a promising technology for converting biomass into a product gas which consists of CO, CO2 , H2 , CH4 , and steam [1]. The product gas can be further used for various applications, such
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as the generation of electricity and heat, synthetic gases and fuels [2], [3], [4].
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The basic principle of the DFB gasification process is schematically shown in Fig. 1. A schematic drawing of the DFB system is depicted in Figure 2. The
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reactor consists of two connected fluidized beds: a gasification reactor and a combustion reactor, between which a bed material, olivine, is circulating. The gasification reactor is operated in a bubbling bed regime, fluidized with steam. Wood chips are fed into the gasification reactor, where they dry and the volatiles are released. The product gas, which consists mainly of H2 , CO, CO2 , CH4 , and H2 O, leaves the gasification reactor at the top and undergoes further gas cleaning.
The solid residue after drying and the release of volatiles consists mainly of carbon and inorganics and is referred to as ”wood char”. A part of the wood char is transferred with the recirculating bed material through a chute to the combustion reactor. It is operated in a fast fluidized bed regime, and is fluidized with air. In the oxidizing atmosphere the wood char is burnt. Due to the combustion the bed material is heated. At the top of the combustion reactor the particle-laden flue gas enters the cyclone, where bed material particles are separated from the flue gas. Bed material particles flow back to the gasi-
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fication reactor via a loop seal. There, the hot bed material provides heat for endothermic particle drying and devolatilization reactions [5].
In the hot plant not the entire wood char is burnt in the combustion reactor
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but a part of it recirculates back to the gasification reactor. Therefore, in the
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gasification reactor one part of the char is generated by biomass gasification, the other comes from the recirculated bed material stream. Several different types of particles are present in a gasification reactor: un-
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converted biomass, partly converted biomass, wood char, and bed material (olivine). So, the gasification reactor can be considered as a multi particle-
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species bubbling fluidized bed with bed material draining at the bottom. The particle diameter and densities vary over a large range. Table 1 shows the ranges of density and particle diameter for the bed material, biomass, and wood char.
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Regarding the particle diameter and density, the bed material is small and heavy
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whereas the biomass and wood char are large and light. Differences in particle diameter and particle density can lead to segregation. Generally, the larger and lighter particles tend to float on the bed surface, which is called flotsam
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behaviour. Smaller and heavy particles on the other hand tend to move downwards in the bed, called jetsam behaviour [6]. So, the biomass and wood char have a tendency to move to the top of the bubbling bed and accumulate there. So, the potential for segregation in the bubbling bed arises. For the operation of the DFB reactor, it is crucial that a sufficiently large amount of char moves to the combustion reactor to ensure that enough heat is produced by oxidizing the char. This also implies that less product gas has to be recycled and no additional fuels have to be added to the combustion reactor, which increases its overall efficiency. Hence, two opposite tendencies occur: The chute to the combustion reactor is located at the bottom of the gasification reactor but due to its flotsam behavior the char tends to move upwards in the bed. Because of this, the char movement and its dependence on various parameters is of great interest for the operation and further optimization of the DFB process. Up to now, the amount of char that moves to the combustion reactor in a 3
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DFB plant has not been measured in any way. Singh et al. [7] estimate the mass fraction of char for a hot fluidized bed combustor to be around 1–5 wt.-%. Werther and Ogada [8] give carbon concentrations for bubbling beds which are
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in the same range for coal and coke. For the DFB process, only via process
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simulation has it been possible to calculate, via energy balances, the burnt amount of char in the combustion reactor [9]. Since it is not clear if all the char is burnt in the combustion reactor, a char bypass through the system is likely
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and a certain amount is transported back via the loop seal to the gasification reactor. This is likely because the temperature in the cyclone of the hot plant
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is higher than in the combustion reactor. Furthermore, the influence on the movement of the char into the combustion reactor of various parameters, such as the fluidization rate, char concentration, and bed height in the gasification
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reactor, as well as the bed material recirculation rate, are unknown.
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Many cold flow model studies have been conducted and presented in the literature with bubbling beds where the segregation of biomass and sand as bed material has been investigated and where large density and/or particle diameter
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ratios, as shown in Table 1 have been used. Comprehensive studies were carried out by Zhang and co-workers as well as Fotovat and co-workers. Zhang et al. [10] used sand/cotton stalk mixtures in a bubbling bed. They found a decrease of segregation with increasing fluidization velocity. They also measured the time until dynamic equilibrium of the segregation was reached and found that this time decreases with increasing fluidization velocity. Furthermore, they found that first the segregation decreases with increasing gas velocity but then with even higher velocities starts to rise slightly again. Similar results were also found in another publication by Zhang et al. [11]. Zhang et al. [12] investigated the axial segregation in sand/biomass mixtures. Concentration profiles over the height were investigated. They found that for that configuration the concentration of the biomass at the bed surface increases with increasing u0 umf u0 umf
and that the concentration decreases in the bottom layer with increasing .
Fotovat et al. [13] investigated the pressure gradients over a sand/wood 4
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fluidized bubbling bed and found that the pressure gradients are larger at the bottom of the system than at the top. They also investigated the characteristics of the gas distribution and bubble phase, using pressure and voidage signals [14].
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The axial distributions of the biomass particles were also studied, and it was
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found that high biomass concentrations were obtained at the top of the bed [15], [16].
Different biomasses are also used in other studies, e.g. cotton stalk [17], rice
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husk [18], [19], saw dust and groundnut shells [20], and olive pits [21]. Other authors, such as Yu et al. [22] and Wirsum [23], use plastics in their studies.
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Biomass char is used by Park and Choi [24]. They investigate the influence of different shapes of bubbling fluidized bed columns and the fluidization velocity on the segregation behaviour of the biomass char. The used shapes of
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the columns are of a circular, square and rectangular type. They found that
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the char concentration at the top of the bed is higher with higher fluidization velocities. Due to their results, also the column shape has a slight influence on the segregation behaviour.
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In the aforementioned studies, mixtures with flotsam particles like biomass
and plastics and sand as bed material are investigated. Compared to ambient conditions, the gas density decreases approximately by a factor of 5 under operating conditions of biomass gasifiers or combustors (which are typically operated at 700–800 ◦ C. This implies that the ratio of the densities of bed material and gas changes. The ratio ρp /ρg is a scaling parameter, as in, e.g. Reh [25] or Glicksman[26]. It is not matched any more if the biomass/sand mixtures are fluidized with air under ambient conditions. A few studies can be found where the mixing behaviour is investigated in a scaled cold flow model where not only the geometry but also the fluid mechanical conditions are scaled. Bidwe et al. [27] investigated the segregation in a cold flow model of a gasification reactor. The cold flow model is a bubbling fluidized bed with a drain at the bottom and a bed material supply at the top, a simple form of bed material recirculation. The biomass and bed material of the hot plant are scaled according to dimensional laws. Steel powder is used for the scaled 5
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biomass and limestone for the bed material of the hot plant. The investigation of the concentration profile over the height shows that the lighter particles float on the surface of the bed. Since there is no recirculation, but always new bed
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material is supplied, an accumulation was also observed.
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Foscolo et al. [28] use a cold flow model of a gasification reactor consisting of two parallel interconnected fluidized beds. They used copper as the bed material and glass beads to model the biomass. They investigated the movement and
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the circulation times of the glass beads between the two interconnected fluidized beds.
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Both Bidwe et al. and Foscolo et al. investigated the mixing of scaled biomass. However, Bidwe et al. did not use a closed recirculation loop in their experimental device, and Foscolo et al. used an internal circulation principle.
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Both recirculation systems are different from the DFB principle, where the bed
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material is transported upwards in a separate combustion reactor. For the operation of a DFB reactor, the influence of various operating parameters on the movement of the char are of interest. These are especially the
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fluidization rate of the gasification reactor, the recirculation rate, and the bed height. Since the absolute amount of char in the gasification reactor not yet known, different char concentrations in the system are investigated. This study aims at increasing the mixing in the gasification reactor and therefore increasing the wood char concentration in the recirculating bed material stream. This will ensure that a sufficient amount of wood char is transported to the combustion reactor.
Therefore, a cold flow model was designed and constructed to investigate the char concentration in the recirculated bed material stream of a DFB biomass gasification process. For the investigations, the DFB plant in G¨ ussing, Austria, was chosen as a reference and the cold flow model was constructed to match the following specifications: • fluidization reactor with a bubbling fluidized bed where the same fluidization conditions in the gasification reactor in G¨ ussing can be simulated,
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• possibility of controlling the bed material recirculation stream independently of the fluidization rate of the combustion reactor,
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• possibility of sampling during operation of the cold flow model for investi-
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gations of the char concentration in the bed material recirculation stream. For the first time, the mixing of biomass char in a DFB reactor is investigated in a cold flow model scaled according to scaling relationships. Furthermore, this
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is done with a full bed material recirculation loop, where the recirculation rate can be easily controlled with a rotary valve. Furthermore, sampling is possible
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during operation of the cold flow model. In this paper, the design and first results with this new cold flow model are presented and discussed.
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2. Material and methods
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2.1. Scaling of the cold flow model Glicksman [26] nondimensionalized the governing equations for a two-phase
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flow and obtained a set of nondimensional numbers which he called the full set. Using the full set is often not useful when the cold flow model should be operated with air at ambient conditions. Keeping all the dimensionless numbers of the full set constant only one unique set of values is obtained, including the scaling factor. This often requires unsuitably large dimensions for the cold flow model that should be constructed. To allow more flexibility, Glicksman derived the simplified set with the goal
of reducing the number of dimensionless parameters [29]. Doing so, it is possible to model also large hot plants with smaller cold flow models. The simplified set is as follows: u20 ρp Gs L1 , , , , bed geometry, φp , PSD gL ρg ρp u0 L2
(1)
The first dimensionless number is known as the Froude number. The second number is the density ratio between the gas and the particle phase. The third number is for scaling the bed material recirculation rate, Gs . The fourth is the 7
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scaling factor. For the cold flow model, also the bed geometry, the sphericity φp , and the particle size distribution (PSD) should match between the hot plant and the cold flow model. The simplified set does not contain the Reynolds number
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as does the full set or, e.g. in Reh’s derivation of his regime diagram [25].
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For the cold flow model, a scaling factor of 1:12 is chosen. For the bed material in the cold flow model, a density of about 15,700 kg/m3 is calculated. Therefore, bronze is chosen, which is of high density and is commonly used in
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cold flow model investigations, e.g. [30], [31], [32]. For the current investigations, a mean particle diameter for the bed material of dp,m = 101 µm is calculated
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and bronze powder with a diameter of dp,m = 118 µm is used, which comes close to the calculated value. The calculated and used values for the cold flow model in this study are given in Table 2. The particle size distribution is shown
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in Fig. 3.
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The bed diameter for the DFB plant in G¨ ussing is taken from [33], the density from [34]. The gas properties of the product gas are calculated for a typical composition of product gas according to [33]. The properties of the air
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are taken from the VDI-W¨ armeatlas [35]. The data for the sphericity of the particles are taken from [34]. Db is the diameter of the conical reactor at the bed surface and is estimated from the operating conditions. To calculate the minimal fluidization velocity, umf , the correlation of Grace
[36] is used:
Rep,mf =
p umf dp ρg = 27.22 + 0.0408 Ar − 27.2 µg
(2)
Bosch [34] measured a minimum fluidization velocity for bronze with a particle diameter of dp = 119 µm, obtaining a value of umf = 0.48 m/s, which is very close to the value calculated from the correlation of Grace, which is, for the value dp = 118 µm as used in this investigation, umf = 0.0489 m/s. The calculations give a required superficial velocity of u0 = 0.32 m/s. Table 3 shows a comparison of the relevant dimensionless numbers of the G¨ ussing plant and the cold flow model. With a better match with the dimen-
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sionless numbers, it can be supposed that the cold flow model better simulates the fluid mechanics in the real plant. However, different authors have used different dimensionless numbers for their scaling considerations. F r and the
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density ratio also appear in Reh’s similarity [25], but he also added Rep which
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does not appear in the simplified set. In Glicksman’s full set, Rep is included, though. Glicksman’s simplified set includes the ratio of superficial and minimum fluidization velocity. U ∗ and d∗p are the parameters in the dimensionless
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regime diagram by Grace [37].
For the present study, the relevant dimensionless numbers regarding fluid
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mechanics were calculated for the G¨ ussing plant and for the cold flow model. Also the relative deviation from the hot plant are depicted and calculated as follows:
DF B
xCF M − xDF B , xDF B
stands for the values in the G¨ ussing DFB plant, and
(3) CF M
for the
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where
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rel.dev. =
calculated values of the cold flow model. So, if the values of G¨ ussing are over-
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estimated, then the relative deviation is positive, otherwise negative. In Table 3 all of the the relative deviations are within 20%, except for u0 /umf
and ρp /ρg . The ratio of the velocities cannot be matched exactly since a slightly larger bed material is used. The required high density for the bed material can only be matched with a relative deviation of −0.443. Nevertheless, it is assumed that the fluid mechanics in the cold flow model are similar to the G¨ ussing plant. The scaling of the char particles was conducted as follows. It was assumed
that the ratios of the densities and diameters of the solid particles remain constant: dp,char dp,bed
and
ρp,char ρp,bed
(4)
Polyethylene (PE) is used as char, with the specifications shown in Table 4. Also the ratios according to Eq. (4) are shown. If the values for the char in Table 4 are used and the calculated ratios are applied, a corresponding char density and a coresspondig char diameter for
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G¨ ussing can be calculated. Eq. (4) gives values for the density of ρcorr = 300 kg/m3 and a diameter of dcorr = 12.5 − 13.6 mm.
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2.2. Design and operating principle of the cold flow model
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The flowsheet of the cold flow model is depicted in Fig. 4. The fluidization reactor is operated as a bubbling fluidized bed. The bed material leaves the fluidization reactor through an outlet at the bottom. Furthermore, the outlet
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flow can be closed with a gate valve. Via the upper chute the fluidization reactor is connected with the rotary valve which allows controlling the rate of
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recirculation of the bed material. The upper chute is always filled with bed material to ensure a steady flow through the rotary valve. Via the lower chute the bed material flows further to a riser where the bed material is conveyed
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upwards. In the cyclone the bed is separated from the gas flow and flows back to the fluidization reactor via the loop seal.
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The cold flow model is built with acrylic glass and assembled in an aluminium frame. The air for the cold flow model is supplied by the pressure system of the
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Vienna University of Technology at a pressure of 9 bar. The flow rate for the fluidization reactor, riser and loop seal is measured with one mass flow meter for each. At the gas outlets of the cyclone and fluidization reactor, air filters are installed to separate the fines that are entrained. The fluidization reactor is constructed with a rectangular cross sectional area
and can be divided into three reactors since it is planned to also do experiments with changing bed cross sections. For the current experiments only one reactor is used. Its dimensions are 0.10 × 0.15 m2 , whose hydraulic diameter corresponds to the required one calculated with the scaling relationships. The fluidization reactor is about 0.5 m high and the air is supplied via 12 nozzles from the bottom. The rotary valve used has a throughput of 60–410 dm3 /h. It is driven by a electric engine with 0.25 kW. The rotary valve is operated with a frequency converter and a potentiometer to control its speed. The rotary valve was calibrated and a correlation between the speed and the throughput of the rotary 10
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valve was established. For the bronze used, a bulk density of 5300 kg/m3 was measured, which then leads to a throughput of about 90–600 g/s of bronze. The separation efficiency of the cyclone was about 100% during the experi-
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ments: the separated bronze in one series of measurements was in the range of
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grams and therefore negligible.
The design of the loop seal was chosen according to the work of Peining et al. [38]. They investigated loop seal designs regarding their throughput. Since
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the cold flow model should be as flexible as possible, the design of the loop seal was chosen that enables the highest throughput of bed material.
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The loop seal and the fluidization reactor are connected with a flexible hose. This flexible hose can be disconnected from the fluidization reactor and a sample can be taken. So, sampling can occur without any changes in the operational
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set-up, meaning the rotary valve speed and the fluidization of the cold flow
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model can remain the same.
Compared to the hot plant no char is burnt and the whole amount is fed back to the fluidization reactor. The amount of char that moves to the riser is of
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primary interest since it correlates with the amount of wood char which moves to the combustion reactor in the hot plant. In steady state the amount of wood char in the gasification reactor of the hot plant remains constant. Furthermore, the amounts of char in the different parts in the cold flow model are constant in steady state. Therefore, the char concentration in the bed material recirculation stream does not change moving from the chute to the loop seal, since no accumulation of char occurs. Taking the sample at the aforementioned position enables to measure the char concentration in the bed material recirculation stream after the fluidization reactor. 2.3. Experimental conditions In the present work the parameters varied are as follows: • volume flow rate: 9–24 Nm3 /h, which corresponds to fluidization numbers of
u0 umf
= 3–9; a typical corresponding fluidization number for the DFB
plant in G¨ ussing is about 9; 11
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• throughput of the rotary valve: 173, 258, 344 g/s bronze; a typical corresponding bed material recirculation rate for the DFB plant in G¨ ussing is
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about 300 g/s;
• overall char concentration: 2, 4, 6 mass percent of char in the system; no
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typical corresponding overall char concentration has been estimated yet for the DFB plant in G¨ ussing;
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• bed height in the fluidization reactor: 9 cm, 11 cm, 13 cm for minimum fluidization conditions and only bronze; a typical corresponding bed height
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for the DFB plant in G¨ ussing is between 9–11 cm. The operating points in the fluidization reactor, of the loop seal, and the riser, can be depcited in the diagram by Grace [37] which was extended by
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Schmid [39]. In Fig. 5, all the operating points are depicted. The fluidization
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reactor is operated from the low bubbling fluidization to higher values, which corresponds to the G¨ ussing value. The riser is operated as a pneumatic conveyer,
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which corresponds to the fast fludization condition and to ensure a complete transportation of the recirculating bed material. The loop seal is operated too in a bubbling bed regime at a fluidization number of
u0 umf
= 4 to ensure sufficient
throughput.
2.4. Experimental procedure and data analysis For each operation point point (e.g. u0 /umf = 9, m ˙ rec = 173 g/s, cchar,system =
2%) 10 samples were taken. The samples were taken at the sampling point after the loop seal. Then, the sample was sieved and the masses of the char and bronze were recorded. After that, the sample was put back into the system. To give the system the chance to get into a steady state, 10 times the residence time τ in the fluidization reactor was allowed to pass. After that time the next sample was taken. The char concentration for a sample i was calculated with the following formula:
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mchar,i , mbed,i
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cchar,i =
(5)
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where mchar,i and mbed,i mean the mass of the char and the bed material in the sample, respectively. The mean of all of the samples was calculated. Since the
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bed material leaves the fluidization reactor at the bottom, the measured char concentration corresponds to the concentration at the bottom of the bed in the
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fluidization reactor.
For the diagrams a mixing factor M is calculated: cchar,i
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M=
cchar,overall
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(6)
If M = 0 no char is in the recirculated bed material stream. M = 1 means that the char load in the recirulcation stream is the same as for the system.
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In the following diagrams the fluidization number is shown on the abscissa,
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which is defined as followed:
FN =
u0 . umf
(7)
The fluidization number can also be used as a characterization of the fluidiza-
tion state. A fluidization number of F N = 1 indicates minimum fluidization conditions, fluidization rates of F N > 1 indicate a fluidized bed (while the velocity is less than the terminal velocity).
3. Results
3.1. Visual observations The construction of the cold flow model with acrylic glass allows visually observing the fluidization behaviour. During experiments with char concentrations of 4% and 6% in the bed and fluidization velocities of 3.5 < u0 /umf < 6, a distinct char layer formed at the bed surface. This char layer also damped the bubble eruption at the surface compared to the case where no char in the system is present. Even at the highest superficial velocities (u0 /umf > 8), the
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char showed a tendency to move upwards and form a layer at the bed surface, although a thinner one than with lower fluidization velocities.
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3.2. The influence of the fluidization rate
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In Fig. 6 the influence of the fluidization rate can be seen. The diagram shows the results of experimental runs with a bed height at minimum fluidization conditions of 9 cm, an overall char concentration of 2% in the system, and a
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bed material recirculation rate of 258 g/s bronze.
On the ordinate the mixing factor M according to Eq. (6) is depicted and
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on the abscissa the fluidization number u0 /umf according to Eq. (7). For low fluidziation numbers the mixing factor has a value of about 70%, which indicates segregation in the gasification reactor.
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For higher fluidization numbers the char concentration in the recirculation stream increases and the mixing factor reaches higher values, near 100%, which
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indicates nearly ideal mixing.
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3.3. The influence of the recirculation rate In this section the influence of the recirculation rate is shown. For each of
the three char concentrations (2%, 4%, 6%), a diagram with varying fluidization numbers and bed material recirculation rates is plotted, Figs 7–9. In all three figures the char concentration in the recirculation stream in-
creases with increasing fluidization number, as was already shown in Section 3.2. Furthermore, in all three figures the mixing factor gets higher with an increasing recirculation rate. For m ˙ rec = 344 g/s with a total char concentration of 2% there is a change in the mixing factor for low fluidization numbers, then it decreases and remains nearly constant. The same can be also observed for the 4% case. For the 6% case no such phenomena can be seen. For the other bed material recirculation rates, no such tendencies can be observed. If the char amount is increased from 2% to 6%, the curves in the diagrams come closer and closer, indicating that the influence of the recirculation rate
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decreases with an increasing amount of char in the system. The average ratio between the values of the lowest recirculation rate (173 g/s) and the highest recirculation rate (344 g/s) is for 2% char about 1.14 (Fig. 7), whereas for 6%
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char it decreases to 1.07, indicating a smaller gap between these curves.
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For a char concentration of 6% in the system the absolute amount of char which leaves the fluidization reactor is depicted in Fig. 10. As discussed in the previous paragraph for 6% char, the curves are close together, i.e. the char
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concentration in the recirculation stream does not vary much between the lowest and highest bed material recirculation rates. But since the absolute amount of
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recirculated bed material is also increased, the absolute amount of char that leaves the fluidization reactor increases.
So, the increase of char concentration in the recirculation stream has two
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sources. First, the increasing mixing factor when the recirculation rate is in-
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creased. Second, higher recirculation rates transport more char because more bed material is transported.
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3.4. Influence of char concentration in the system In this section the influence of the char concentration in the system is inves-
tigated at a constant recirculation rate, Figs 11–13. In each figure the mixing factor decreases with increasing char concentra-
tion in the system. For 2% char concentration, only for the lowest fluidization numbers is the mixing factor below 80%, and it approaches 100% (i.e. ideal mixing) for the higher fluidization numbers. In contrast, for the 6% experiments, the mixing factor lowers to 40% for the lowest fluidization number and is only around 80% for the highest fluidization number. Although the mixing factor decreases with increasing char concentration, the absolute amount of char transported to the riser still increases. Fig. 14 shows the absolute amount of char that is transported to the riser for a recirculation rate of 258 g/s. Even though the mixing factor decreases, the absolute amount of char is higher if the char concentration is increased. So, the increase in overall char concentration makes more of a contribution than the decreasing mixing 15
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factor. The dotted line in Fig. 14 is obtained when the values of the 2% curve are doubled. It can be seen that it is higher than the measured values of the 2%
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curve, which indicates that a doubling of the char concentration in the system
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does not lead to a doubling of the char concentration in the recirculation stream. 3.5. Influence of bed height in the fluidization reactor
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In this section the influence of different bed heights in the fluidization reactor is shown, Figs 15 and 16. Therefore, the bed height of bronze alone, where char
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is not considered, at minimum fluidization conditions was increased from 9 to 13 cm in the fluidization reactor.
For the bed height of 9 cm it was already shown that the mixing factor
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increases with increasing fluidization number. For the additional two heights of 11 and 13 cm this trend can also be seen.
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It can be seen that the mixing factor decreases with increasing bed height. Furthermore, in these figures there can also be seen the trend that the mixing
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factor decreases with increasing char concentration, a result that was already found in Section 3.4.
4. Discussion
Generally, the char shows a flotsam behaviour. These results are in accor-
dance with Bidwe et al. [27] and Foscolo et al. [28], who found a flotsam behaviour of the biomass. Since char is even lighter than biomass, a flotsam behaviour is reasonable. From the results it can be seen that the char concentration in the recirculation stream increases with increasing fluidization rate. Since the char acts like flotsam and has therefore a tendency to move upwards in the bed, a higher superficial velocity increases the bubbling motion and therefore the mixing of the char particles in the bed. As a consequence, the char is more evenly distributed over the bed height.
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Higher fluidization rates in the hot plant increase the amount of char that moves with the recirculating bed material stream to the combustion reactor. Therefore, more heat is generated in the combustion reactor and less product
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gas has to be recirculated. As a result, the efficiency of the power plant increases.
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However, high fluidization rates in the G¨ ussing plant are expensive as well since steam has to be generated, and because of the high evaporation enthalpy of water.
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Also more recirculation increases the mixing factor because higher recirculation streams seem to entrain more char from the fluidization reactor. Further-
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more, the effect that a higher recirculation stream entrains more char decreases with increasing char concentration. With higher char concentrations in the system there is more char present in the fluidization reactor and the recirculation
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stream can only entrain a smaller fraction of the total char amount. In the hot
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plant there is no rotary valve: the recirculation rate is set by the fluidization of the combustion reactor, which makes it impossible to investigate the influence of the recirculation stream. Only the use of a rotary valve allows investigat-
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ing the influence of the rate of recirculation of the bed material on the char concentration.
With more char in the system, the bed height increases in the fluidization
reactor since the char has a very low bulk density. Moreover, the flotsam char then moves more to the top of the bed. A decrease of the concentration occurs and consequently the mixing factor decreases. Furthermore, the following question can now be answered: Does the amount of char in the recirculated stream double if the char concentration in the system is doubled? From Fig. 14 this question can be answered with a ‘no’. If the char concentration were to be doubled, e.g. from 2% to 4%, this would correspond to the dotted line in Fig. 14 and this is higher than the measured curve for the 4% overall char concentration. The same for tripling the char concentration from 2% to 6%. In that case the difference is even larger than for the doubling case. So, one can conclude that with more char in the system, the char accumulates at the bed surface. However, as previously stated, the absolute amount of char in the hot 17
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plant is still unknown, which made a series with different char concentrations necessary.
Since the char has a tendency to float on the bed surface, the forming char
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layer moves more and more upwards with increasing bed height and therefore
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the concentration of char at the bottom of the bed decreases. This results in a lower concentration in the recirculated bed material stream, and consequently to a lower mixing factor. For higher beds, the absolute amount of char transferred
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to the riser decreases, a result that is also of relevance for the hot plants. If more char moves to the combustion reactor, less product gas has to be recirculated:
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lower bed heights can enhance char transport. However, lower bed heights also reduce the contact time of the upwards moving biomass particles when fed to the fluidization reactor, and the formation of tar is more likely which then has
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to be removed in the downstream processing.
5. Summary and conclusion
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In this paper, the design of a cold flow model to investigate the char movement in a dual fluidized bed (DFB) system was discussed. The cold flow model is scaled with Glicksman’s simplified set of similarity rules. The basis of the cold flow model is the DFB reactor in G¨ ussing, Austria. Therefore, the cold flow model consists of a fluidization reactor where the fluidization conditions of the gasification reactor of the DFB plant can be simulated. Unlike the G¨ ussing plant, there is a rotary valve to control the bed material recirculation, a riser to pneumatically convey the bed material upwards, a cyclone for gas–solid separation, and a loop seal to separate the air flows of the cyclone/riser and the fluidization reactor. Bronze is used as the bed material and polythylene as the char, to satisfy the scaling relationships. During operation, samples were taken to investigate the amount of char in the recirculation stream. A mixing factor, M , is introduced to quantify the amount of char in the recirulcation stream. If M = 0, no char would be in the recirculation stream and complete segregation occurs. If M = 1, complete and ideal mixing occurs.
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It was found that the char acts as a flotsam and shows a tendency to move to the surface of the bed. Putting all the results so far together, the following
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summary can be made:
• Higher fluidization numbers in the fluidization reactor increase the mixing
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factor due to a more vigorous bubble movement.
• Higher recirculation rates also increase the mixing factor, although this
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tendency decreases with increasing char concentration in the system. • Increasing the overall char concentration in the system leads to a decreas-
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ing mixing factor.
• Doubling the overall char concentration in the system does not lead to
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a doubling of the char concentration in the bed material recirculation
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stream.
• If the bed height is increased, the mixing factor also decreases, due to the
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char layer that moves upwards. These results imply that the movement of the char can have a large influ-
ence on the operation of real DFB plants. The insights gained in this research are of great relevance and will help to further improve and develop the DFB technology.
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meaning
SI units
cchar
char concentration
kg/kg
dp
particle diameter
m
Db
bed diameter
m
g
gravitational acceleration
m/s2
Gs
recirculation rate flux
kg/m2 s
L
characteristic length
m
mass
m ˙ rec
bed material recirculation rate
kg/s
M
mixing factor
-
FN
fluidization number
-
u0
superficial velocity
m/s
umf
minimal fluidization velocity
m/s
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NU m
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kg
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Greek letters
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symbol
symbol
meaning
SI units
µg
gas viscosity
kg/m s
φp
sphericity
-
ρp
particle density
kg/m3
ρg
gas density
kg/m3
Dimensionless numbers Ar =
ρg (ρp −ρg )gd3p µ2g
Archimedes number
d u ρ Rep = pµg0 g u2 F r = gD0b Rep U ∗ = Ar1/3
Particle Reynolds number
d∗p = Ar1/3
dimensionless particle diameter
Froude number dimensionless velocity
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Nomenclature
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Acknowledgements
This study was carried out in the framework of Bioenergy2020+. Bioen-
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ergy2020+ GmbH is funded within the Austrian COMET program, with resources from the Austrian government and the regions Burgenland, Niederster-
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reich and Steiermark.
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[30] A. Kreuzeder, C. Pfeifer, H. Hofbauer, Fluid-dynamic investigations in a scaled cold model for a dual fluidized bed biomass steam gasification pro-
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tional Journal of Chemical Reactor Engineering 5. [31] T. Pr¨ oll, K. Rupanovits, P. Kolbitsch, J. Bolhar-Nordenkampf, H. Hofbauer, Cold flow model study on a dual circulating fluidized bed system for chemical looping processes, Chemical Engineering & Technology 32 (2009) 418–424.
[32] J. Schmid, T. Pr¨ oll, H. Kitzler, C. Pfeifer, H. Hofbauer, Cold flow model investigations of the countercurrent flow of a dual circulating fluidized bed gasifier, Biomass Conversion and Biorefinery 2 (2012) 229–244. [33] F. Kirnbauer, V. Wilk, H. Kitzler, S. Kern, H. Hofbauer, The positive effects of bed material coating on tar reduction in a dual fluidized bed gasifier, Fuel 95 (2012) 553–562. [34] K. Bosch, Scale UP der Dampf-Wirbelschicht-Biomassevergasung, Ph.D. thesis, Institute of Chemical Engineering, Vienna University of Technology (2007).
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[35] VDI-Gesellschaft, VDI-W¨armeatlas, Springer-Verlag, 2006. [36] J. Grace, Fluidized Bed Hydrodynamics, Hemisphere, Washington, 1982.
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[37] J. Grace, Contacting modes and behaviour classification of gas-solid and
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other two-phase suspensions, The Canadian Journal of Chemical Engineering 64 (1986) 353–363.
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[38] W. Peining, X. Wenchong, L. Qing, Y. Hairui, L. Junfu, Z. Yonggang, Baiyang, Impact of passage structure on loop seal operating characteristic,
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in: 11th International Conference on Fluidized Bed Technology, 2011. [39] J. Schmid, Development of a novel dual fluidized bed gasification system for increased fuel flexibility, Ph.D. thesis, Institute of Chemical Engineering,
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Vienna University of Technology (2014). flue gas (CO2, N2, H2O, O2)
GASIFICATION
feedstock/fuel (biomass)
bubbling bed
heat
circulation of bed material
800 to 870°C
COMBUSTION fast bed 870 to 940°C
fuel to comb.
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product gas
(H2, CO, CO2, CH4, H2O)
char
steam
air
Figure 1: Basic principle of the dual fluidized bed process (based on Schmid et al. [32]).
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Datenreihen1
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cyclone
5
6
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combustion reactor
4 3 2 1 0
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chute
scale unit: meters
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loop seal
gasification reactor
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Figure 2: Geometry and main parts of the DFB gasification system in G¨ ussing, Austria
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cumulative distribution [-]
100% 80% 60% 40% 20% 0%
0
50 100 150 particle diameter [µm]
200
Figure 3: Particle size distribution (PSD) of the used bronze powder.
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outlet air cyclone
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cyclone
loop seal
riser
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fluidization reactor
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sampling
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outlet air gasifier
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air
upper chute
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air for bottom fluidization
bed material recirculation stream
rotary valve
lower chute
air
Figure 4: Flow sheet of the cold flow model.
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operation point of riser
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operation range of fluidization reactor
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operation point of loop seal
Figure 5: Grace diagram with operating points.
100%
mixing factor [-]
80%
60% 2%, 9cm, 258g/s
40%
20%
0% 2
4
6 u0/umf [-]
8
Figure 6: Influence of fluidization rate.
28
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100%
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60%
40%
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mixing factor [-]
80%
2%, 9cm, 173g/s 20%
2%, 9cm, 258g/s 2%, 9cm, 344g/s
0% 4
6 u0/umf [-]
8
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2
10
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Figure 7: Influence of the recirculation rate with 2% char concentration in the system.
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100%
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mixnig factor [-]
80%
60%
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40%
4%, 9cm, 173g/s
20%
4%, 9cm, 258g/s 4%, 9cm, 344g/s
0% 2
4
6 u0/umf [-]
8
10
Figure 8: Influence of the recirculation rate with 4% char concentration in the system.
Table 1: Diameters and densities of the particles present in the gasification reactor
bed material dp [m] ρp [kg/m3 ]
−4
biomass −3
10
10
2800
–10
−2
600–800
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wood char 10−3 –10−2 150–300
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100%
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60%
40%
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mixnig factor [-]
80%
6%, 9cm, 173g/s
20%
6%, 9cm, 258g/s
2
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6%, 9cm, 344g/s
0%
4
6 u0/umf [-]
8
10
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Figure 9: Influence of the recirculation rate with 6% char concentration in the system.
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mass flow model char [g/s]
18 16 14 12 10 8 6
6%, 9cm, 173g/s
4
6%, 9cm, 258g/s
2
6%, 9cm, 344g/s
0 2
4
6 u0/umf [-]
8
10
Figure 10: Absolute amount of char that is transferred into the riser.
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100%
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60%
40%
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mixing factor [-]
80%
2%, 9cm, 173 g/s
20%
4%, 9cm, 173 g/s
2
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6%, 9cm, 173 g/s
0%
4
6 u0/umf [-]
8
10
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Figure 11: Influence of the char concentration in the system at a recirculation rate of 173 g/s.
100%
mixing factor [-]
80%
60%
40% 2%, 9cm, 258 g/s
20%
4%, 9cm, 258 g/s
6%, 9cm, 258 g/s 0% 2
4
6 u0/umf [-]
8
10
Figure 12: Influence of the char concentration in the system at a recirculation rate of 258 g/s.
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100%
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60%
40%
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mixing factor [-]
80%
2%, 9cm, 344 g/s
20%
4%, 9cm, 344 g/s
2
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6%, 9cm, 344 g/s
0%
4
6 u0/umf [-]
8
10
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Figure 13: Influence of the char concentration in the system at a recirculation rate of 344 g/s.
mass flow model char [g/s]
14 12
10 8 6 4
2%, 9cm, 258 g/s 4%, 9cm, 258 g/s 6%, 9cm, 258 g/s
2 0
2
4
6 u0/umf [-]
8
10
Figure 14: Absolute amount of char tranferred into the riser. The dotted line is obtained if the values of the 2% curve is doubled.
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100%
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60%
40%
4%, 9 cm, 258g/s
4%, 11 cm, 258g/s
20%
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mixing factor [-]
80%
4%, 13 cm, 258g/s
0% 4
6 u0/umf [-]
8
10
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2
Figure 15: Influence of bed height for 4% char concentration in the system and a recirculation
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rate of 258 g/s.
100%
mixing factor [-]
80%
60%
40%
6%, 9 cm, 258g/s 6%, 11 cm, 258g/s
20%
6%, 13 cm, 258g/s
0% 2
4
6 u0/umf [-]
8
10
Figure 16: Influence of bed height for 6% char concentration in the system and a recirculation rate of 258 g/s.
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cold flow model
510
118
2800
φp [–]
0.93
ρg [kg/m3 ]
1
0.208 4.20 · 10−5
u0 [m/s]
1.11
0.32
0.127
0.0489
27
0.30
1.65
0.12
D
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umf [m/s]
1.17
1.82 · 10−5
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µg [kg/m s]
Db [m]
8750
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ρp [kg/m3 ]
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G¨ ussing dp [µm]
m ˙ rec [kg/s]
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Table 2: Used design values for the cold flow model.
G¨ ussing
cold flow model
rel. dev.
Fr
0.077
0.088
0.146
Rep
2.81
2.43
−0.135
13,442
7485
−0.443
u0 /umf
8.8
6.6
−0.253
Ar
430
498
0.158
0.373
0.307
−0.176
7.55
7.92
0.050
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Table 3: Comparison of relevant dimensionless numbers.
ρp /ρg
U
∗
d∗p
Table 4: Data for bronze and for char (polyethylene)
bronze
char
ratio
dp [mm]
0.118
2.5–3.15
21.2–26.7
ρp [kg/m3 ]
8750
940
0.103
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Highlights • We constructed a cold flow model for investigation of char movement in a
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dual fluidized bed biomass gasification plant.
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• We investigated influence of fluidization rate, recirculation rate, char concentration and bed height on char concentration in the bed recirculation
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stream.
• Increasing fluidization rate and bed material recirculation rate increase
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char concentration.
• Increasing bed height decreases char concentration due to its flotsam be-
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haviour.
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S0032-5910(16)30714-8 doi:10.1016/j.powtec.2016.10.032 PTEC 12031
To appear in:
Powder Technology
Received date: Revised date: Accepted date:
17 May 2016 21 September 2016 13 October 2016
Please cite this article as: Stephan Kraft, Friedrich Kirnbauer, Hermann Hofbauer, Investigations using a cold flow model of char mixing in the gasification reactor of a dual fluidized bed gasification plant, Powder Technology (2016), doi:10.1016/j.powtec.2016.10.032
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Investigations using a cold flow model of char mixing in the gasification reactor of a dual fluidized bed gasification plant Stephan Krafta,∗, Friedrich Kirnbauera , Hermann Hofbauerb
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a Bioenergy 2020+ GmbH, Wiener Strasse 49, A-7540 G¨ ussing, Austria University of Technology, Institute of Chemical Engineering, Getreidemarkt 9/166 Vienna, Austria
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b Vienna
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Abstract
This paper treats the mixing and movement of char in a dual fluidized bed (DFB) biomass gasification plant. In these plants such measurements are trou-
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blesome to perform, and so a cold flow model has been developed to investigate
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this topic. This cold flow model allows simulating the fluidization behaviour of the gasification reactor in the DFB plant in G¨ ussing, Austria. The recirculation
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of the bed material is also possible, and can be easily controlled with a rotary valve. In the cold flow model, bronze is used as the bed material and polyethylene as the char. It is possible to take samples during operation to investigate the char concentration in the bed material recirculation stream. Experiments have shown that the char shows a flotsam behaviour since it is of low density. Furthermore, the investigations have shown that higher fluidization rates and higher bed material recirculation rates enhance the char mixing and increase the char concentration in the recirculation stream. It was found that doubling the overall char concentration in the system does not lead to a doubling of the char concentration in the bed material recirculation stream. Furthermore, the influence of the bed height in the gasification reactor was investigated. It was found that higher bed heights lead to lower char concentrations in the recirculation stream. These initial investigations revealed that much is still unknown ∗ Corresponding
author Email address: [email protected] (Stephan Kraft)
Preprint submitted to Journal of LATEX Templates
October 14, 2016
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about DFB plants, but the knowledge of the behaviour of the different types of particles in the bubbling bed of the gasification reactor helps to further improve and develop the DFB technology.
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Keywords: cold flow modelling, fluidized bed gasification, segregation, scaling
1. Introduction
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Fluidized bed reactors have been in use for a long time and are used for various applications. The advantages of fluidized beds are their good mixing behaviour and gas/solid contact due to the vigorous bubble motion. The dual
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fluidized bed (DFB) steam gasification of biomass is a promising technology for converting biomass into a product gas which consists of CO, CO2 , H2 , CH4 , and steam [1]. The product gas can be further used for various applications, such
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as the generation of electricity and heat, synthetic gases and fuels [2], [3], [4].
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The basic principle of the DFB gasification process is schematically shown in Fig. 1. A schematic drawing of the DFB system is depicted in Figure 2. The
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reactor consists of two connected fluidized beds: a gasification reactor and a combustion reactor, between which a bed material, olivine, is circulating. The gasification reactor is operated in a bubbling bed regime, fluidized with steam. Wood chips are fed into the gasification reactor, where they dry and the volatiles are released. The product gas, which consists mainly of H2 , CO, CO2 , CH4 , and H2 O, leaves the gasification reactor at the top and undergoes further gas cleaning.
The solid residue after drying and the release of volatiles consists mainly of carbon and inorganics and is referred to as ”wood char”. A part of the wood char is transferred with the recirculating bed material through a chute to the combustion reactor. It is operated in a fast fluidized bed regime, and is fluidized with air. In the oxidizing atmosphere the wood char is burnt. Due to the combustion the bed material is heated. At the top of the combustion reactor the particle-laden flue gas enters the cyclone, where bed material particles are separated from the flue gas. Bed material particles flow back to the gasi-
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fication reactor via a loop seal. There, the hot bed material provides heat for endothermic particle drying and devolatilization reactions [5].
In the hot plant not the entire wood char is burnt in the combustion reactor
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but a part of it recirculates back to the gasification reactor. Therefore, in the
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gasification reactor one part of the char is generated by biomass gasification, the other comes from the recirculated bed material stream. Several different types of particles are present in a gasification reactor: un-
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converted biomass, partly converted biomass, wood char, and bed material (olivine). So, the gasification reactor can be considered as a multi particle-
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species bubbling fluidized bed with bed material draining at the bottom. The particle diameter and densities vary over a large range. Table 1 shows the ranges of density and particle diameter for the bed material, biomass, and wood char.
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Regarding the particle diameter and density, the bed material is small and heavy
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whereas the biomass and wood char are large and light. Differences in particle diameter and particle density can lead to segregation. Generally, the larger and lighter particles tend to float on the bed surface, which is called flotsam
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behaviour. Smaller and heavy particles on the other hand tend to move downwards in the bed, called jetsam behaviour [6]. So, the biomass and wood char have a tendency to move to the top of the bubbling bed and accumulate there. So, the potential for segregation in the bubbling bed arises. For the operation of the DFB reactor, it is crucial that a sufficiently large amount of char moves to the combustion reactor to ensure that enough heat is produced by oxidizing the char. This also implies that less product gas has to be recycled and no additional fuels have to be added to the combustion reactor, which increases its overall efficiency. Hence, two opposite tendencies occur: The chute to the combustion reactor is located at the bottom of the gasification reactor but due to its flotsam behavior the char tends to move upwards in the bed. Because of this, the char movement and its dependence on various parameters is of great interest for the operation and further optimization of the DFB process. Up to now, the amount of char that moves to the combustion reactor in a 3
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DFB plant has not been measured in any way. Singh et al. [7] estimate the mass fraction of char for a hot fluidized bed combustor to be around 1–5 wt.-%. Werther and Ogada [8] give carbon concentrations for bubbling beds which are
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in the same range for coal and coke. For the DFB process, only via process
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simulation has it been possible to calculate, via energy balances, the burnt amount of char in the combustion reactor [9]. Since it is not clear if all the char is burnt in the combustion reactor, a char bypass through the system is likely
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and a certain amount is transported back via the loop seal to the gasification reactor. This is likely because the temperature in the cyclone of the hot plant
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is higher than in the combustion reactor. Furthermore, the influence on the movement of the char into the combustion reactor of various parameters, such as the fluidization rate, char concentration, and bed height in the gasification
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reactor, as well as the bed material recirculation rate, are unknown.
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Many cold flow model studies have been conducted and presented in the literature with bubbling beds where the segregation of biomass and sand as bed material has been investigated and where large density and/or particle diameter
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ratios, as shown in Table 1 have been used. Comprehensive studies were carried out by Zhang and co-workers as well as Fotovat and co-workers. Zhang et al. [10] used sand/cotton stalk mixtures in a bubbling bed. They found a decrease of segregation with increasing fluidization velocity. They also measured the time until dynamic equilibrium of the segregation was reached and found that this time decreases with increasing fluidization velocity. Furthermore, they found that first the segregation decreases with increasing gas velocity but then with even higher velocities starts to rise slightly again. Similar results were also found in another publication by Zhang et al. [11]. Zhang et al. [12] investigated the axial segregation in sand/biomass mixtures. Concentration profiles over the height were investigated. They found that for that configuration the concentration of the biomass at the bed surface increases with increasing u0 umf u0 umf
and that the concentration decreases in the bottom layer with increasing .
Fotovat et al. [13] investigated the pressure gradients over a sand/wood 4
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fluidized bubbling bed and found that the pressure gradients are larger at the bottom of the system than at the top. They also investigated the characteristics of the gas distribution and bubble phase, using pressure and voidage signals [14].
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The axial distributions of the biomass particles were also studied, and it was
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found that high biomass concentrations were obtained at the top of the bed [15], [16].
Different biomasses are also used in other studies, e.g. cotton stalk [17], rice
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husk [18], [19], saw dust and groundnut shells [20], and olive pits [21]. Other authors, such as Yu et al. [22] and Wirsum [23], use plastics in their studies.
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Biomass char is used by Park and Choi [24]. They investigate the influence of different shapes of bubbling fluidized bed columns and the fluidization velocity on the segregation behaviour of the biomass char. The used shapes of
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the columns are of a circular, square and rectangular type. They found that
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the char concentration at the top of the bed is higher with higher fluidization velocities. Due to their results, also the column shape has a slight influence on the segregation behaviour.
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In the aforementioned studies, mixtures with flotsam particles like biomass
and plastics and sand as bed material are investigated. Compared to ambient conditions, the gas density decreases approximately by a factor of 5 under operating conditions of biomass gasifiers or combustors (which are typically operated at 700–800 ◦ C. This implies that the ratio of the densities of bed material and gas changes. The ratio ρp /ρg is a scaling parameter, as in, e.g. Reh [25] or Glicksman[26]. It is not matched any more if the biomass/sand mixtures are fluidized with air under ambient conditions. A few studies can be found where the mixing behaviour is investigated in a scaled cold flow model where not only the geometry but also the fluid mechanical conditions are scaled. Bidwe et al. [27] investigated the segregation in a cold flow model of a gasification reactor. The cold flow model is a bubbling fluidized bed with a drain at the bottom and a bed material supply at the top, a simple form of bed material recirculation. The biomass and bed material of the hot plant are scaled according to dimensional laws. Steel powder is used for the scaled 5
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biomass and limestone for the bed material of the hot plant. The investigation of the concentration profile over the height shows that the lighter particles float on the surface of the bed. Since there is no recirculation, but always new bed
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material is supplied, an accumulation was also observed.
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Foscolo et al. [28] use a cold flow model of a gasification reactor consisting of two parallel interconnected fluidized beds. They used copper as the bed material and glass beads to model the biomass. They investigated the movement and
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the circulation times of the glass beads between the two interconnected fluidized beds.
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Both Bidwe et al. and Foscolo et al. investigated the mixing of scaled biomass. However, Bidwe et al. did not use a closed recirculation loop in their experimental device, and Foscolo et al. used an internal circulation principle.
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Both recirculation systems are different from the DFB principle, where the bed
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material is transported upwards in a separate combustion reactor. For the operation of a DFB reactor, the influence of various operating parameters on the movement of the char are of interest. These are especially the
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fluidization rate of the gasification reactor, the recirculation rate, and the bed height. Since the absolute amount of char in the gasification reactor not yet known, different char concentrations in the system are investigated. This study aims at increasing the mixing in the gasification reactor and therefore increasing the wood char concentration in the recirculating bed material stream. This will ensure that a sufficient amount of wood char is transported to the combustion reactor.
Therefore, a cold flow model was designed and constructed to investigate the char concentration in the recirculated bed material stream of a DFB biomass gasification process. For the investigations, the DFB plant in G¨ ussing, Austria, was chosen as a reference and the cold flow model was constructed to match the following specifications: • fluidization reactor with a bubbling fluidized bed where the same fluidization conditions in the gasification reactor in G¨ ussing can be simulated,
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• possibility of controlling the bed material recirculation stream independently of the fluidization rate of the combustion reactor,
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• possibility of sampling during operation of the cold flow model for investi-
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gations of the char concentration in the bed material recirculation stream. For the first time, the mixing of biomass char in a DFB reactor is investigated in a cold flow model scaled according to scaling relationships. Furthermore, this
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is done with a full bed material recirculation loop, where the recirculation rate can be easily controlled with a rotary valve. Furthermore, sampling is possible
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during operation of the cold flow model. In this paper, the design and first results with this new cold flow model are presented and discussed.
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2. Material and methods
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2.1. Scaling of the cold flow model Glicksman [26] nondimensionalized the governing equations for a two-phase
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flow and obtained a set of nondimensional numbers which he called the full set. Using the full set is often not useful when the cold flow model should be operated with air at ambient conditions. Keeping all the dimensionless numbers of the full set constant only one unique set of values is obtained, including the scaling factor. This often requires unsuitably large dimensions for the cold flow model that should be constructed. To allow more flexibility, Glicksman derived the simplified set with the goal
of reducing the number of dimensionless parameters [29]. Doing so, it is possible to model also large hot plants with smaller cold flow models. The simplified set is as follows: u20 ρp Gs L1 , , , , bed geometry, φp , PSD gL ρg ρp u0 L2
(1)
The first dimensionless number is known as the Froude number. The second number is the density ratio between the gas and the particle phase. The third number is for scaling the bed material recirculation rate, Gs . The fourth is the 7
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scaling factor. For the cold flow model, also the bed geometry, the sphericity φp , and the particle size distribution (PSD) should match between the hot plant and the cold flow model. The simplified set does not contain the Reynolds number
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as does the full set or, e.g. in Reh’s derivation of his regime diagram [25].
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For the cold flow model, a scaling factor of 1:12 is chosen. For the bed material in the cold flow model, a density of about 15,700 kg/m3 is calculated. Therefore, bronze is chosen, which is of high density and is commonly used in
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cold flow model investigations, e.g. [30], [31], [32]. For the current investigations, a mean particle diameter for the bed material of dp,m = 101 µm is calculated
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and bronze powder with a diameter of dp,m = 118 µm is used, which comes close to the calculated value. The calculated and used values for the cold flow model in this study are given in Table 2. The particle size distribution is shown
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in Fig. 3.
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The bed diameter for the DFB plant in G¨ ussing is taken from [33], the density from [34]. The gas properties of the product gas are calculated for a typical composition of product gas according to [33]. The properties of the air
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are taken from the VDI-W¨ armeatlas [35]. The data for the sphericity of the particles are taken from [34]. Db is the diameter of the conical reactor at the bed surface and is estimated from the operating conditions. To calculate the minimal fluidization velocity, umf , the correlation of Grace
[36] is used:
Rep,mf =
p umf dp ρg = 27.22 + 0.0408 Ar − 27.2 µg
(2)
Bosch [34] measured a minimum fluidization velocity for bronze with a particle diameter of dp = 119 µm, obtaining a value of umf = 0.48 m/s, which is very close to the value calculated from the correlation of Grace, which is, for the value dp = 118 µm as used in this investigation, umf = 0.0489 m/s. The calculations give a required superficial velocity of u0 = 0.32 m/s. Table 3 shows a comparison of the relevant dimensionless numbers of the G¨ ussing plant and the cold flow model. With a better match with the dimen-
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sionless numbers, it can be supposed that the cold flow model better simulates the fluid mechanics in the real plant. However, different authors have used different dimensionless numbers for their scaling considerations. F r and the
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density ratio also appear in Reh’s similarity [25], but he also added Rep which
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does not appear in the simplified set. In Glicksman’s full set, Rep is included, though. Glicksman’s simplified set includes the ratio of superficial and minimum fluidization velocity. U ∗ and d∗p are the parameters in the dimensionless
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regime diagram by Grace [37].
For the present study, the relevant dimensionless numbers regarding fluid
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mechanics were calculated for the G¨ ussing plant and for the cold flow model. Also the relative deviation from the hot plant are depicted and calculated as follows:
DF B
xCF M − xDF B , xDF B
stands for the values in the G¨ ussing DFB plant, and
(3) CF M
for the
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where
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rel.dev. =
calculated values of the cold flow model. So, if the values of G¨ ussing are over-
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estimated, then the relative deviation is positive, otherwise negative. In Table 3 all of the the relative deviations are within 20%, except for u0 /umf
and ρp /ρg . The ratio of the velocities cannot be matched exactly since a slightly larger bed material is used. The required high density for the bed material can only be matched with a relative deviation of −0.443. Nevertheless, it is assumed that the fluid mechanics in the cold flow model are similar to the G¨ ussing plant. The scaling of the char particles was conducted as follows. It was assumed
that the ratios of the densities and diameters of the solid particles remain constant: dp,char dp,bed
and
ρp,char ρp,bed
(4)
Polyethylene (PE) is used as char, with the specifications shown in Table 4. Also the ratios according to Eq. (4) are shown. If the values for the char in Table 4 are used and the calculated ratios are applied, a corresponding char density and a coresspondig char diameter for
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G¨ ussing can be calculated. Eq. (4) gives values for the density of ρcorr = 300 kg/m3 and a diameter of dcorr = 12.5 − 13.6 mm.
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2.2. Design and operating principle of the cold flow model
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The flowsheet of the cold flow model is depicted in Fig. 4. The fluidization reactor is operated as a bubbling fluidized bed. The bed material leaves the fluidization reactor through an outlet at the bottom. Furthermore, the outlet
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flow can be closed with a gate valve. Via the upper chute the fluidization reactor is connected with the rotary valve which allows controlling the rate of
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recirculation of the bed material. The upper chute is always filled with bed material to ensure a steady flow through the rotary valve. Via the lower chute the bed material flows further to a riser where the bed material is conveyed
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upwards. In the cyclone the bed is separated from the gas flow and flows back to the fluidization reactor via the loop seal.
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The cold flow model is built with acrylic glass and assembled in an aluminium frame. The air for the cold flow model is supplied by the pressure system of the
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Vienna University of Technology at a pressure of 9 bar. The flow rate for the fluidization reactor, riser and loop seal is measured with one mass flow meter for each. At the gas outlets of the cyclone and fluidization reactor, air filters are installed to separate the fines that are entrained. The fluidization reactor is constructed with a rectangular cross sectional area
and can be divided into three reactors since it is planned to also do experiments with changing bed cross sections. For the current experiments only one reactor is used. Its dimensions are 0.10 × 0.15 m2 , whose hydraulic diameter corresponds to the required one calculated with the scaling relationships. The fluidization reactor is about 0.5 m high and the air is supplied via 12 nozzles from the bottom. The rotary valve used has a throughput of 60–410 dm3 /h. It is driven by a electric engine with 0.25 kW. The rotary valve is operated with a frequency converter and a potentiometer to control its speed. The rotary valve was calibrated and a correlation between the speed and the throughput of the rotary 10
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valve was established. For the bronze used, a bulk density of 5300 kg/m3 was measured, which then leads to a throughput of about 90–600 g/s of bronze. The separation efficiency of the cyclone was about 100% during the experi-
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ments: the separated bronze in one series of measurements was in the range of
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grams and therefore negligible.
The design of the loop seal was chosen according to the work of Peining et al. [38]. They investigated loop seal designs regarding their throughput. Since
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the cold flow model should be as flexible as possible, the design of the loop seal was chosen that enables the highest throughput of bed material.
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The loop seal and the fluidization reactor are connected with a flexible hose. This flexible hose can be disconnected from the fluidization reactor and a sample can be taken. So, sampling can occur without any changes in the operational
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set-up, meaning the rotary valve speed and the fluidization of the cold flow
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model can remain the same.
Compared to the hot plant no char is burnt and the whole amount is fed back to the fluidization reactor. The amount of char that moves to the riser is of
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primary interest since it correlates with the amount of wood char which moves to the combustion reactor in the hot plant. In steady state the amount of wood char in the gasification reactor of the hot plant remains constant. Furthermore, the amounts of char in the different parts in the cold flow model are constant in steady state. Therefore, the char concentration in the bed material recirculation stream does not change moving from the chute to the loop seal, since no accumulation of char occurs. Taking the sample at the aforementioned position enables to measure the char concentration in the bed material recirculation stream after the fluidization reactor. 2.3. Experimental conditions In the present work the parameters varied are as follows: • volume flow rate: 9–24 Nm3 /h, which corresponds to fluidization numbers of
u0 umf
= 3–9; a typical corresponding fluidization number for the DFB
plant in G¨ ussing is about 9; 11
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• throughput of the rotary valve: 173, 258, 344 g/s bronze; a typical corresponding bed material recirculation rate for the DFB plant in G¨ ussing is
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about 300 g/s;
• overall char concentration: 2, 4, 6 mass percent of char in the system; no
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typical corresponding overall char concentration has been estimated yet for the DFB plant in G¨ ussing;
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• bed height in the fluidization reactor: 9 cm, 11 cm, 13 cm for minimum fluidization conditions and only bronze; a typical corresponding bed height
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for the DFB plant in G¨ ussing is between 9–11 cm. The operating points in the fluidization reactor, of the loop seal, and the riser, can be depcited in the diagram by Grace [37] which was extended by
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Schmid [39]. In Fig. 5, all the operating points are depicted. The fluidization
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reactor is operated from the low bubbling fluidization to higher values, which corresponds to the G¨ ussing value. The riser is operated as a pneumatic conveyer,
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which corresponds to the fast fludization condition and to ensure a complete transportation of the recirculating bed material. The loop seal is operated too in a bubbling bed regime at a fluidization number of
u0 umf
= 4 to ensure sufficient
throughput.
2.4. Experimental procedure and data analysis For each operation point point (e.g. u0 /umf = 9, m ˙ rec = 173 g/s, cchar,system =
2%) 10 samples were taken. The samples were taken at the sampling point after the loop seal. Then, the sample was sieved and the masses of the char and bronze were recorded. After that, the sample was put back into the system. To give the system the chance to get into a steady state, 10 times the residence time τ in the fluidization reactor was allowed to pass. After that time the next sample was taken. The char concentration for a sample i was calculated with the following formula:
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mchar,i , mbed,i
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cchar,i =
(5)
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where mchar,i and mbed,i mean the mass of the char and the bed material in the sample, respectively. The mean of all of the samples was calculated. Since the
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bed material leaves the fluidization reactor at the bottom, the measured char concentration corresponds to the concentration at the bottom of the bed in the
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fluidization reactor.
For the diagrams a mixing factor M is calculated: cchar,i
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M=
cchar,overall
.
(6)
If M = 0 no char is in the recirculated bed material stream. M = 1 means that the char load in the recirulcation stream is the same as for the system.
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In the following diagrams the fluidization number is shown on the abscissa,
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which is defined as followed:
FN =
u0 . umf
(7)
The fluidization number can also be used as a characterization of the fluidiza-
tion state. A fluidization number of F N = 1 indicates minimum fluidization conditions, fluidization rates of F N > 1 indicate a fluidized bed (while the velocity is less than the terminal velocity).
3. Results
3.1. Visual observations The construction of the cold flow model with acrylic glass allows visually observing the fluidization behaviour. During experiments with char concentrations of 4% and 6% in the bed and fluidization velocities of 3.5 < u0 /umf < 6, a distinct char layer formed at the bed surface. This char layer also damped the bubble eruption at the surface compared to the case where no char in the system is present. Even at the highest superficial velocities (u0 /umf > 8), the
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char showed a tendency to move upwards and form a layer at the bed surface, although a thinner one than with lower fluidization velocities.
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3.2. The influence of the fluidization rate
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In Fig. 6 the influence of the fluidization rate can be seen. The diagram shows the results of experimental runs with a bed height at minimum fluidization conditions of 9 cm, an overall char concentration of 2% in the system, and a
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bed material recirculation rate of 258 g/s bronze.
On the ordinate the mixing factor M according to Eq. (6) is depicted and
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on the abscissa the fluidization number u0 /umf according to Eq. (7). For low fluidziation numbers the mixing factor has a value of about 70%, which indicates segregation in the gasification reactor.
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For higher fluidization numbers the char concentration in the recirculation stream increases and the mixing factor reaches higher values, near 100%, which
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indicates nearly ideal mixing.
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3.3. The influence of the recirculation rate In this section the influence of the recirculation rate is shown. For each of
the three char concentrations (2%, 4%, 6%), a diagram with varying fluidization numbers and bed material recirculation rates is plotted, Figs 7–9. In all three figures the char concentration in the recirculation stream in-
creases with increasing fluidization number, as was already shown in Section 3.2. Furthermore, in all three figures the mixing factor gets higher with an increasing recirculation rate. For m ˙ rec = 344 g/s with a total char concentration of 2% there is a change in the mixing factor for low fluidization numbers, then it decreases and remains nearly constant. The same can be also observed for the 4% case. For the 6% case no such phenomena can be seen. For the other bed material recirculation rates, no such tendencies can be observed. If the char amount is increased from 2% to 6%, the curves in the diagrams come closer and closer, indicating that the influence of the recirculation rate
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decreases with an increasing amount of char in the system. The average ratio between the values of the lowest recirculation rate (173 g/s) and the highest recirculation rate (344 g/s) is for 2% char about 1.14 (Fig. 7), whereas for 6%
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char it decreases to 1.07, indicating a smaller gap between these curves.
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For a char concentration of 6% in the system the absolute amount of char which leaves the fluidization reactor is depicted in Fig. 10. As discussed in the previous paragraph for 6% char, the curves are close together, i.e. the char
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concentration in the recirculation stream does not vary much between the lowest and highest bed material recirculation rates. But since the absolute amount of
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recirculated bed material is also increased, the absolute amount of char that leaves the fluidization reactor increases.
So, the increase of char concentration in the recirculation stream has two
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sources. First, the increasing mixing factor when the recirculation rate is in-
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creased. Second, higher recirculation rates transport more char because more bed material is transported.
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3.4. Influence of char concentration in the system In this section the influence of the char concentration in the system is inves-
tigated at a constant recirculation rate, Figs 11–13. In each figure the mixing factor decreases with increasing char concentra-
tion in the system. For 2% char concentration, only for the lowest fluidization numbers is the mixing factor below 80%, and it approaches 100% (i.e. ideal mixing) for the higher fluidization numbers. In contrast, for the 6% experiments, the mixing factor lowers to 40% for the lowest fluidization number and is only around 80% for the highest fluidization number. Although the mixing factor decreases with increasing char concentration, the absolute amount of char transported to the riser still increases. Fig. 14 shows the absolute amount of char that is transported to the riser for a recirculation rate of 258 g/s. Even though the mixing factor decreases, the absolute amount of char is higher if the char concentration is increased. So, the increase in overall char concentration makes more of a contribution than the decreasing mixing 15
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factor. The dotted line in Fig. 14 is obtained when the values of the 2% curve are doubled. It can be seen that it is higher than the measured values of the 2%
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curve, which indicates that a doubling of the char concentration in the system
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does not lead to a doubling of the char concentration in the recirculation stream. 3.5. Influence of bed height in the fluidization reactor
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In this section the influence of different bed heights in the fluidization reactor is shown, Figs 15 and 16. Therefore, the bed height of bronze alone, where char
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is not considered, at minimum fluidization conditions was increased from 9 to 13 cm in the fluidization reactor.
For the bed height of 9 cm it was already shown that the mixing factor
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increases with increasing fluidization number. For the additional two heights of 11 and 13 cm this trend can also be seen.
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It can be seen that the mixing factor decreases with increasing bed height. Furthermore, in these figures there can also be seen the trend that the mixing
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factor decreases with increasing char concentration, a result that was already found in Section 3.4.
4. Discussion
Generally, the char shows a flotsam behaviour. These results are in accor-
dance with Bidwe et al. [27] and Foscolo et al. [28], who found a flotsam behaviour of the biomass. Since char is even lighter than biomass, a flotsam behaviour is reasonable. From the results it can be seen that the char concentration in the recirculation stream increases with increasing fluidization rate. Since the char acts like flotsam and has therefore a tendency to move upwards in the bed, a higher superficial velocity increases the bubbling motion and therefore the mixing of the char particles in the bed. As a consequence, the char is more evenly distributed over the bed height.
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Higher fluidization rates in the hot plant increase the amount of char that moves with the recirculating bed material stream to the combustion reactor. Therefore, more heat is generated in the combustion reactor and less product
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gas has to be recirculated. As a result, the efficiency of the power plant increases.
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However, high fluidization rates in the G¨ ussing plant are expensive as well since steam has to be generated, and because of the high evaporation enthalpy of water.
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Also more recirculation increases the mixing factor because higher recirculation streams seem to entrain more char from the fluidization reactor. Further-
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more, the effect that a higher recirculation stream entrains more char decreases with increasing char concentration. With higher char concentrations in the system there is more char present in the fluidization reactor and the recirculation
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stream can only entrain a smaller fraction of the total char amount. In the hot
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plant there is no rotary valve: the recirculation rate is set by the fluidization of the combustion reactor, which makes it impossible to investigate the influence of the recirculation stream. Only the use of a rotary valve allows investigat-
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ing the influence of the rate of recirculation of the bed material on the char concentration.
With more char in the system, the bed height increases in the fluidization
reactor since the char has a very low bulk density. Moreover, the flotsam char then moves more to the top of the bed. A decrease of the concentration occurs and consequently the mixing factor decreases. Furthermore, the following question can now be answered: Does the amount of char in the recirculated stream double if the char concentration in the system is doubled? From Fig. 14 this question can be answered with a ‘no’. If the char concentration were to be doubled, e.g. from 2% to 4%, this would correspond to the dotted line in Fig. 14 and this is higher than the measured curve for the 4% overall char concentration. The same for tripling the char concentration from 2% to 6%. In that case the difference is even larger than for the doubling case. So, one can conclude that with more char in the system, the char accumulates at the bed surface. However, as previously stated, the absolute amount of char in the hot 17
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plant is still unknown, which made a series with different char concentrations necessary.
Since the char has a tendency to float on the bed surface, the forming char
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layer moves more and more upwards with increasing bed height and therefore
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the concentration of char at the bottom of the bed decreases. This results in a lower concentration in the recirculated bed material stream, and consequently to a lower mixing factor. For higher beds, the absolute amount of char transferred
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to the riser decreases, a result that is also of relevance for the hot plants. If more char moves to the combustion reactor, less product gas has to be recirculated:
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lower bed heights can enhance char transport. However, lower bed heights also reduce the contact time of the upwards moving biomass particles when fed to the fluidization reactor, and the formation of tar is more likely which then has
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to be removed in the downstream processing.
5. Summary and conclusion
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In this paper, the design of a cold flow model to investigate the char movement in a dual fluidized bed (DFB) system was discussed. The cold flow model is scaled with Glicksman’s simplified set of similarity rules. The basis of the cold flow model is the DFB reactor in G¨ ussing, Austria. Therefore, the cold flow model consists of a fluidization reactor where the fluidization conditions of the gasification reactor of the DFB plant can be simulated. Unlike the G¨ ussing plant, there is a rotary valve to control the bed material recirculation, a riser to pneumatically convey the bed material upwards, a cyclone for gas–solid separation, and a loop seal to separate the air flows of the cyclone/riser and the fluidization reactor. Bronze is used as the bed material and polythylene as the char, to satisfy the scaling relationships. During operation, samples were taken to investigate the amount of char in the recirculation stream. A mixing factor, M , is introduced to quantify the amount of char in the recirulcation stream. If M = 0, no char would be in the recirculation stream and complete segregation occurs. If M = 1, complete and ideal mixing occurs.
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It was found that the char acts as a flotsam and shows a tendency to move to the surface of the bed. Putting all the results so far together, the following
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summary can be made:
• Higher fluidization numbers in the fluidization reactor increase the mixing
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factor due to a more vigorous bubble movement.
• Higher recirculation rates also increase the mixing factor, although this
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tendency decreases with increasing char concentration in the system. • Increasing the overall char concentration in the system leads to a decreas-
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ing mixing factor.
• Doubling the overall char concentration in the system does not lead to
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a doubling of the char concentration in the bed material recirculation
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stream.
• If the bed height is increased, the mixing factor also decreases, due to the
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char layer that moves upwards. These results imply that the movement of the char can have a large influ-
ence on the operation of real DFB plants. The insights gained in this research are of great relevance and will help to further improve and develop the DFB technology.
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meaning
SI units
cchar
char concentration
kg/kg
dp
particle diameter
m
Db
bed diameter
m
g
gravitational acceleration
m/s2
Gs
recirculation rate flux
kg/m2 s
L
characteristic length
m
mass
m ˙ rec
bed material recirculation rate
kg/s
M
mixing factor
-
FN
fluidization number
-
u0
superficial velocity
m/s
umf
minimal fluidization velocity
m/s
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NU m
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D
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kg
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Greek letters
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symbol
symbol
meaning
SI units
µg
gas viscosity
kg/m s
φp
sphericity
-
ρp
particle density
kg/m3
ρg
gas density
kg/m3
Dimensionless numbers Ar =
ρg (ρp −ρg )gd3p µ2g
Archimedes number
d u ρ Rep = pµg0 g u2 F r = gD0b Rep U ∗ = Ar1/3
Particle Reynolds number
d∗p = Ar1/3
dimensionless particle diameter
Froude number dimensionless velocity
20
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Nomenclature
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Acknowledgements
This study was carried out in the framework of Bioenergy2020+. Bioen-
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ergy2020+ GmbH is funded within the Austrian COMET program, with resources from the Austrian government and the regions Burgenland, Niederster-
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reich and Steiermark.
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tional Journal of Chemical Reactor Engineering 5. [31] T. Pr¨ oll, K. Rupanovits, P. Kolbitsch, J. Bolhar-Nordenkampf, H. Hofbauer, Cold flow model study on a dual circulating fluidized bed system for chemical looping processes, Chemical Engineering & Technology 32 (2009) 418–424.
[32] J. Schmid, T. Pr¨ oll, H. Kitzler, C. Pfeifer, H. Hofbauer, Cold flow model investigations of the countercurrent flow of a dual circulating fluidized bed gasifier, Biomass Conversion and Biorefinery 2 (2012) 229–244. [33] F. Kirnbauer, V. Wilk, H. Kitzler, S. Kern, H. Hofbauer, The positive effects of bed material coating on tar reduction in a dual fluidized bed gasifier, Fuel 95 (2012) 553–562. [34] K. Bosch, Scale UP der Dampf-Wirbelschicht-Biomassevergasung, Ph.D. thesis, Institute of Chemical Engineering, Vienna University of Technology (2007).
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[35] VDI-Gesellschaft, VDI-W¨armeatlas, Springer-Verlag, 2006. [36] J. Grace, Fluidized Bed Hydrodynamics, Hemisphere, Washington, 1982.
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[37] J. Grace, Contacting modes and behaviour classification of gas-solid and
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other two-phase suspensions, The Canadian Journal of Chemical Engineering 64 (1986) 353–363.
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[38] W. Peining, X. Wenchong, L. Qing, Y. Hairui, L. Junfu, Z. Yonggang, Baiyang, Impact of passage structure on loop seal operating characteristic,
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in: 11th International Conference on Fluidized Bed Technology, 2011. [39] J. Schmid, Development of a novel dual fluidized bed gasification system for increased fuel flexibility, Ph.D. thesis, Institute of Chemical Engineering,
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Vienna University of Technology (2014). flue gas (CO2, N2, H2O, O2)
GASIFICATION
feedstock/fuel (biomass)
bubbling bed
heat
circulation of bed material
800 to 870°C
COMBUSTION fast bed 870 to 940°C
fuel to comb.
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product gas
(H2, CO, CO2, CH4, H2O)
char
steam
air
Figure 1: Basic principle of the dual fluidized bed process (based on Schmid et al. [32]).
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Datenreihen1
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cyclone
5
6
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combustion reactor
4 3 2 1 0
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chute
scale unit: meters
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loop seal
gasification reactor
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Figure 2: Geometry and main parts of the DFB gasification system in G¨ ussing, Austria
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cumulative distribution [-]
100% 80% 60% 40% 20% 0%
0
50 100 150 particle diameter [µm]
200
Figure 3: Particle size distribution (PSD) of the used bronze powder.
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outlet air cyclone
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cyclone
loop seal
riser
MA
fluidization reactor
NU
sampling
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outlet air gasifier
TE
D
air
upper chute
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air for bottom fluidization
bed material recirculation stream
rotary valve
lower chute
air
Figure 4: Flow sheet of the cold flow model.
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operation point of riser
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operation range of fluidization reactor
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operation point of loop seal
Figure 5: Grace diagram with operating points.
100%
mixing factor [-]
80%
60% 2%, 9cm, 258g/s
40%
20%
0% 2
4
6 u0/umf [-]
8
Figure 6: Influence of fluidization rate.
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100%
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60%
40%
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mixing factor [-]
80%
2%, 9cm, 173g/s 20%
2%, 9cm, 258g/s 2%, 9cm, 344g/s
0% 4
6 u0/umf [-]
8
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2
10
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Figure 7: Influence of the recirculation rate with 2% char concentration in the system.
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100%
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mixnig factor [-]
80%
60%
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40%
4%, 9cm, 173g/s
20%
4%, 9cm, 258g/s 4%, 9cm, 344g/s
0% 2
4
6 u0/umf [-]
8
10
Figure 8: Influence of the recirculation rate with 4% char concentration in the system.
Table 1: Diameters and densities of the particles present in the gasification reactor
bed material dp [m] ρp [kg/m3 ]
−4
biomass −3
10
10
2800
–10
−2
600–800
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wood char 10−3 –10−2 150–300
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100%
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60%
40%
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mixnig factor [-]
80%
6%, 9cm, 173g/s
20%
6%, 9cm, 258g/s
2
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6%, 9cm, 344g/s
0%
4
6 u0/umf [-]
8
10
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Figure 9: Influence of the recirculation rate with 6% char concentration in the system.
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mass flow model char [g/s]
18 16 14 12 10 8 6
6%, 9cm, 173g/s
4
6%, 9cm, 258g/s
2
6%, 9cm, 344g/s
0 2
4
6 u0/umf [-]
8
10
Figure 10: Absolute amount of char that is transferred into the riser.
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100%
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60%
40%
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mixing factor [-]
80%
2%, 9cm, 173 g/s
20%
4%, 9cm, 173 g/s
2
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6%, 9cm, 173 g/s
0%
4
6 u0/umf [-]
8
10
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Figure 11: Influence of the char concentration in the system at a recirculation rate of 173 g/s.
100%
mixing factor [-]
80%
60%
40% 2%, 9cm, 258 g/s
20%
4%, 9cm, 258 g/s
6%, 9cm, 258 g/s 0% 2
4
6 u0/umf [-]
8
10
Figure 12: Influence of the char concentration in the system at a recirculation rate of 258 g/s.
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100%
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60%
40%
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mixing factor [-]
80%
2%, 9cm, 344 g/s
20%
4%, 9cm, 344 g/s
2
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6%, 9cm, 344 g/s
0%
4
6 u0/umf [-]
8
10
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Figure 13: Influence of the char concentration in the system at a recirculation rate of 344 g/s.
mass flow model char [g/s]
14 12
10 8 6 4
2%, 9cm, 258 g/s 4%, 9cm, 258 g/s 6%, 9cm, 258 g/s
2 0
2
4
6 u0/umf [-]
8
10
Figure 14: Absolute amount of char tranferred into the riser. The dotted line is obtained if the values of the 2% curve is doubled.
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100%
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60%
40%
4%, 9 cm, 258g/s
4%, 11 cm, 258g/s
20%
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mixing factor [-]
80%
4%, 13 cm, 258g/s
0% 4
6 u0/umf [-]
8
10
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2
Figure 15: Influence of bed height for 4% char concentration in the system and a recirculation
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rate of 258 g/s.
100%
mixing factor [-]
80%
60%
40%
6%, 9 cm, 258g/s 6%, 11 cm, 258g/s
20%
6%, 13 cm, 258g/s
0% 2
4
6 u0/umf [-]
8
10
Figure 16: Influence of bed height for 6% char concentration in the system and a recirculation rate of 258 g/s.
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cold flow model
510
118
2800
φp [–]
0.93
ρg [kg/m3 ]
1
0.208 4.20 · 10−5
u0 [m/s]
1.11
0.32
0.127
0.0489
27
0.30
1.65
0.12
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umf [m/s]
1.17
1.82 · 10−5
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µg [kg/m s]
Db [m]
8750
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ρp [kg/m3 ]
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G¨ ussing dp [µm]
m ˙ rec [kg/s]
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Table 2: Used design values for the cold flow model.
G¨ ussing
cold flow model
rel. dev.
Fr
0.077
0.088
0.146
Rep
2.81
2.43
−0.135
13,442
7485
−0.443
u0 /umf
8.8
6.6
−0.253
Ar
430
498
0.158
0.373
0.307
−0.176
7.55
7.92
0.050
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Table 3: Comparison of relevant dimensionless numbers.
ρp /ρg
U
∗
d∗p
Table 4: Data for bronze and for char (polyethylene)
bronze
char
ratio
dp [mm]
0.118
2.5–3.15
21.2–26.7
ρp [kg/m3 ]
8750
940
0.103
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Highlights • We constructed a cold flow model for investigation of char movement in a
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dual fluidized bed biomass gasification plant.
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• We investigated influence of fluidization rate, recirculation rate, char concentration and bed height on char concentration in the bed recirculation
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stream.
• Increasing fluidization rate and bed material recirculation rate increase
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char concentration.
• Increasing bed height decreases char concentration due to its flotsam be-
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haviour.
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