Gas and Bed Axial Composition in a Bubbling Fluidized Bed Gasifier

Subscriber access provided by CORNELL UNIVERSITY LIBRARY

Article

Gas and Bed Axial Composition in a Bubbling Fluidized Bed Gasifier: Results with Miscanthus and Olivine George Lardier, Judit Kaknics, Anthony Dufour, Rudy Michel, Benjamin Cluet, Olivier Authier, Jacques Poirier, and Guillain Mauviel Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01816 • Publication Date (Web): 29 Aug 2016 Downloaded from http://pubs.acs.org on September 5, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Energy & Fuels is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Gas and Bed Axial Composition in a Bubbling Fluidized Bed Gasifier: Results with Miscanthus and Olivine

5

George Lardier a, Judit Kaknics b, Anthony Dufour a, Rudy Michel b, Benjamin Cluet a, Olivier Authier c, Jacques Poirier b, Guillain Mauviel a* a

Reactions & Processes Engineering Laboratory (LRGP), CNRS, Lorraine University, ENSIC, 1 rue Granville, 54001 Nancy, France

b

10

Conditions Extrêmes et Matériaux Haute Température et Irradiation (CEMHTI), CNRS,

Orléans University, 1D Avenue de la Recherche Scientifique, 45071 Orléans Cedex 2, France c

Department of Fluid Dynamics, Power Generation and Environment, EDF R&D, 6 Quai Watier, 78400 Chatou, France

ABSTRACT 15 This article presents a novel air-blown bubbling fluidized bed device that has the ability to sample gas and bed materials at various axial positions during the gasification experiments. The reactor was operated with olivine as bed material, and Miscanthus, a biomass rich in potassium and silica, and thus prone to bed agglomeration. The comparison of gas and char 20

axial profiles along the bed allows a better understanding of the biomass gasification: it shows in particular that O2 consumption and CO2 production at the bottom of the bed are mainly due to char oxidation, even if few pyrolysis gases may also be produced and oxidized near the grid. Regarding bed defluidization, the agglomerate fraction is followed by taking bed samples during the operation: it is shown that the rate of agglomeration is linear while

25

defluidization signs appear when the agglomerate fraction reaches 6% near the grid. Small

ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

agglomerates are observed on the top of the bed, whereas big agglomerates are segregated near the grid. The SEM-EDX analysis shows that the layer that sticks olivine particles together does not strictly correspond to biomass ashes melt: it contains also particles and atoms that come from the erosion of olivine and stainless steel wall. 30


Page 2 of 29

Page 3 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1. INTRODUCTION Among the technologies converting ligno-cellulosic biomass into heat and electricity 35

(cogeneration), gasification is now developing at commercial scale. Bubbling fluidized beds (BFB) are the preferred technology for biomass gasification at the 1 to 20 MW power input scale. BFB provide good heat transfer that allows achieving fast drying and devolatilization in comparison with fixed bed. They also allow long solid residence time that is necessary to gasify the char produced by biomass devolatilization.

40

Despite the several advantages of BFB for biomass gasification, many technical challenges still have to be overcome. The main bottlenecks are related to the tar production [1] and bed agglomeration [2]. Primary tar is mainly composed of sugar compounds, acids, furans, phenols which are generated during the biomass devolatilization [3]. These compounds are converted by several

45

heterogeneous and homogenous reactions into permanent gases (H2, CO, CO2, CH4,…) and aromatic tars : Benzene, Toluene, Xylenes (BTX) and Polycyclic Aromatic Hydrocarbons (PAH) [4]. The final tar content of the producer gas has to be minimized in order to avoid energy losses and plugging of the apparatus (pipes, motors) [3]. Tars can be converted into permanent gases by thermal or catalytic cracking after the gasifier [5, 6]. The cooled syngas

50

may also be washed by water or organic solvents in gas/liquid contactors [7]. These operations of tar reduction increase the cost of gasification process and reduce its energetic efficiency. Thus it is crucial to better understand the tar decomposition inside the gasifier [8]. The impact of bed material (silica sand, olivine, dolomite, supported nickel or iron,…) is important since it may act as a catalyst for tar conversion [9]. Nevertheless these materials

55

should be resistant to attrition, fragmentation, deactivation [10] and agglomeration. Agglomeration is related to the inorganic species present in the biomass [11]. Lignocellulosic biomass contains high levels of alkali metals and silica which form silicates and tend to melt below the process temperature. The presence of liquid phase alters the fluidization behaviour by modifying the inter-particle forces [12]. In bubbling fluidized bed, the two main reverse

60

forces are the capillary and viscous adhesive force between sticky, ash-coated particles and the breaking forces due to bubble activity and particle drag in the bed that result in particle collisions [13]. When the inter-particle forces are greater than the breaking force, the particles will agglomerate resulting in bed segregation and uneven temperature profile along the bed. Agglomeration can lead to the defluidization of the bed [13, 14]. The relative tendency of


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

65

defluidization for some biomass species in descending order is Miscanthus, straw, olive pomace, Reed canary grass, Lucerne, Salix (willow), birch [2]. The defluidization can be noticed by a sharp decrease of pressure below the grid and uneven temperature distribution in the gasifier [15, 16, 17]. Agglomeration highly depends on process temperature [18], the higher the temperature in the reactor the more likely is the

70

agglomeration [15, 16]. The temperature gradient between a fuel particle and bed particle is also substantial as it can promote the transfer of volatized alkali species from fuel to bed particle [2]. Defluidization could be avoided by using lower operational temperature ( 5.5

3

827

0.24

7.4

6.06

56%

76%

n.d.

4

828

0.24

5.5

6.43

61%

81%

n.d.

5

827

0.32

6.6

5.10

58%

82%

> 4.2

6

881

0.33

10.9

4.26

46%

74%

1.5

7

896

0.32

10.6

5.50

64%

88%

0.7

8

915

0.32

5.5

5.55

68%

91%

1.2

250 The tests #1 and #2 are achieved under the same operational conditions (Tbed=826 °C, ER=0.24, U/Umf=10.9) and analyses show that the gas compositions and the corresponding indicators (LHV, cold gas efficiency and carbon conversion) are reproducible.

255

3.2.1. Influence of U/Umf The tests #1 to #4 allow assessing the influence of the ratio U/Umf since the other conditions are identical (Tbed=827±1 °C, ER=0.24). The obtained gas fractions are similar. Considering the performance indicators, the increase of U/Umf from 5.5 to 10.9 causes a significant decrease in cold gas efficiency (from 61 to 46%). This seems to be due to lower carbon conversion (from 81 to 68%) that may be linked to higher char particles elutriation caused by

260

higher gas velocity. Kwapinska et al. [19] observes a similar effect that was linked to higher particle elutriation for torrefied Miscanthus in comparison with raw Miscanthus. 3.2.2. Influence of Equivalence Ratio The influence of Equivalence Ratio was not the main focus of our study since it is well documented [1, 8, 19]. When ER increases from 0.24 to 0.32 (tests #3 and #4 compared to test

265

#5 in Table 4), the produced gas contains less H2 and more N2. Its LHV decreases from 6.2±0.2 to 5.1 MJ m-3(STP) (see Table 5). Nevertheless, the cold gas efficiency is not really affected (around 59±3%) because the higher gas productivity compensates the lower gas


Page 13 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

LHV. The carbon conversion is slightly increased: 82% with ER=0.32 instead of 79±3% with ER=0.24. All these values are very close to the ones obtained in similar conditions [19]. 270

3.2.3. Influence of bed temperature In an adiabatic gasifier, the bed temperature depends directly on the biomass moisture and the ER value through the energy balance. When biomass moisture increases or ER decreases, the temperature of the bed is reduced. In this gasifier, it is however not possible to work on adiabatic mode and the bed temperature is controlled independently of the ER. The tests #5 to

275

#8 allow assessing the influence of the bed temperature at a given ER (0.32±0.01). When the bed temperature rises, the H2 and CO fraction increases. Nevertheless, U/Umf values are not the same in these tests. If we compare the tests #5 and #8 that have similar U/Umf, it is clear that the temperature has a positive effect on cold gas efficiency (58 to 68%). It is essentially due to a better carbon conversion (82 to 91%) mostly related to higher char oxidation kinetic

280

that is in competition with char elutriation. This impact of temperature has already been observed by many authors [8, 19]. The global tar concentration (including benzene) seems to be lowered by higher temperature: from 7.5 g m-3(STP) at 827 °C to 5.8 g m-3(STP) at 915 °C (cf. Figure 3). In similar conditions [19], it was observed 4.9 g m-3(STP) (without benzene and toluene) at 850°C.

285

However, if the comparison is achieved on the basis of tar yield, there is no clear tendency: 12.6 g kg-1daf at 827 °C and 12.8 g kg-1daf at 915 °C. Horvat et al. [29] measured around 15 g kg-1daf (including benzene) at 850 °C with raw miscanthus. The slight decrease of tar concentration with temperature is essentially due to the reduction of the small aromatics (toluene, xylene, styrene) and oxygenated compounds (phenol, cresol, benzofuran). However,

290

these ECN class 3 tars [30] are not harmful for piping and motors and may even be considered as suitable fuel compounds. On the contrary, the increase of ECN class 4 tars, i.e. naphthalene, methylnaphthalene, fluorene and phenanthrene (from 0.5 to 1 g m-3(STP)) is unwanted since these compounds tend to increase the tar dew point. This is confirmed by computing the tar dew point with Thersites [31]. At 827 °C, the computed tar dew point is 35

295

°C, whereas it is 64 °C at 915 °C. We should note that the absolute values of these computed tar dew points are likely to be under-estimated, since the heavier PAH were not quantified by GC analysis.


Energy & Fuels

8.0

Tar concentration (g/m3(STP))

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

fluorene, phenanthrene

6.0

napthalenes indenes

4.0

benzene toluene, xylene, styrene

2.0

phenol, cresol, benzofuran

0.0 #3 - 827°C

300

#8 - 915°C

Figure 3. Tar composition at gasifier outlet

3.3. Axial profile of gas composition Local gas sampling is a difficult task and few data are available in the literature on this topic 305

[32; 33]. In this study, the gas has been sampled before the defluidization begins since it has been shown that defluidization affects the gas composition [34]. The local gas composition for test #1 achieved at 826 °C is represented in Figure 4.a for the five main gases. The values at 1200 mm correspond to the values measured on-line after gas cooling. Air gasification consists of incomplete combustion in oxygen deficiency but

310

considerable uncertainties exist with regard to the chemical reactions that consume O2 in emulsion and bubble phases, in particulate phase and/or above the bed. It is very interesting to observe that O2 is consumed rapidly in the olivine bed and that the residual fraction is below 0.5 vol% in the freeboard (that is above z=400 mm). Since CO2 is the main compound produced at the bottom of the bed (around 13 vol%), it indicates that oxygen consumption is

315

due to complete oxidation of some fuel (char and/or pyrolysis gases) near the grid. This point will be discussed hereafter. In the study of Ross et al. [33], there was also a rapid consumption of O2 within the first 50 mm and no O2 was detected beyond 120 mm. CO2 was also produced immediately. It should be pointed out that the gas sampled in the dense bed over-represents the composition of the emulsion phase in comparison with the bubble phase,

320

that may contain more O2 and less CO2 at a given position [35].


Page 14 of 29

Page 15 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

CH4, H2 and CO are produced more progressively in the olivine bed. CO and H2 may be produced by char partial oxidation, biomass pyrolysis and/or primary tar cracking. The production of CH4 under 270 mm indicates that there are some biomass particles that are able to sink under this position, since CH4 cannot be produced by char oxidation. Indeed, CH4 is 325

mainly produced by pyrolysis and secondary conversion of tar [36]. CH4, H2 and CO fractions still increase in the freeboard, whereas the CO2 fraction remains quite stable (around 17 vol%). On Figure 4.b), it can be seen that the nitrogen element (that corresponds only to N2) decreases slightly along the freeboard, whereas C, H and O slightly increases : this indicates that there are some tar molecules that are converted in the freeboard

330

and produce permanent gases. Radmanesh et al. [32] observed exactly the same tendencies with beech powders gasified in silica sand at 800°C and ER= 0.32, even if the absolute values were slightly different (16 vol% CO, 13 vol% CO2, 10 vol% H2, 3 vol% CH4 at the exit). Ross et al. [33] did not get the same tendencies: in the freeboard, they observed a slight reduction of CO and H2 fractions when wood powders were gasified in sand.

335 a)

b)

Figure 4. a) Permanent gas axial composition and coarse char axial mass fraction b) Permanent gases axial elemental composition (test #1, Tbed = 826 °C, ER = 0.24, U/ Umf=10.9) 340 The coarse char fraction profiles (Figure 7.b) are useful to understand the gas axial composition. At the bottom of the bed, there are very few coarse char particles whereas the top of the bed contains a large char fraction. The mean coarse char fraction is only 1.6 wt% at 896 °C, whereas it is 2.7 wt% at 827 °C for the same U/Umf value equal to 10.9. This 345

temperature influence is probably related to char oxidation kinetics. When the gas velocity is lower (U/Umf = 6.6), the mean char fraction for the whole bed is higher (4.3 %). This could be explained by lower char elutriation.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

In the case of the experiment #1 at 826 °C (Figure 4 and Figure 7.b), the coarse char mass fraction is relatively low at the bottom (0.3 wt%). These results could be compared to 350

segregation studies in cold fluidized bed. Cluet et al. [37] did experiments with small balsa chips mixed with the same olivine at room temperature. Balsa has a very low density (159 kg/m3) close to the Miscanthus char density (~110 kg m-3). Since the mean balsa mass fraction in the whole bed was significantly lower (0.5 wt%) than our mean char fraction (2.7 wt%), the comparison should be made with the ratio between bottom and mean char fractions:

355

this ratio is 0.07 in our case, whereas it is 0.46 in the case of the corresponding cold fluidized bed experiment. This comparison allows to assume that char is deeply oxidized at the bottom of the fluidized bed gasifier. But is it possible that pyrolysis gases may also be oxidized at the bottom of the bed? Since the biomass is fed by the top in this fluidized bed, it seems quite unlikely at first glance. The

360

Radioactive Particle Tracking measurements achieved by Fotovat et al. [38] show that some biomass particles sink near the bed wall at a mean axial velocity of ~100 mm s-1 for U/Umf = 4.4. In the case of the experiment #1, the mean axial velocity may be higher since U/Umf = 10.9. This implies that some of the biomass particles that land at the surface of the bed (z ~ 400 mm) may sink at the bottom in less than 4 s. This diving time is correct if the effect of

365

endogenous bubbles characterized by Bruni et al. [39] is not very significant in our high gas velocity conditions. This diving time should be compared to the devolatilization time. The correlation tv = 0.8e1525/Td1.2 developed for spherical particles by Di Blasi and Branca [40] has been used at 826°C and d=8.1 mm (i.e. the equivalent diameter of our cylindrical miscanthus pellets). It yields a duration of 39 s for complete particle devolatilization. As a consequence, it

370

seems possible that a part of the air injected through the grid oxidizes some gases produced by biomass pyrolysis at the bottom of the bed. Obviously there would be more pyrolysis gases mixed with oxygen near the grid if the biomass was injected in this zone. Besides, it is wellknown that it would reduce the tar content of the syngas [5, 41]. 3.4. Characterization of defluidization and agglomeration phenonema

375

3.4.1. Evolution of temperature and pressure Below 830 °C, despite the long experimentation time (5 h), defluidization phenomenon was never observed. On the other hand, defluidization rapidly occurred at operating temperatures above 880 °C (less than 1.5 h in Table 5). The defluidization is characterized by strong

380

variation of temperature and pressure as it can be seen in Figure 5 for the test #7 at 896 °C. A


Page 16 of 29

Page 17 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

decrease of the pressure drop suggests a segregation of agglomerates in the bed. The sudden variation of temperature in the bed is also indicative of poor mixing caused by large agglomerates. This behavior was observed by many authors and recently by GómezHernández et al. [17]. In order to understand this defluidization phenomenon, it is necessary 385

to sample the bed and characterize the agglomerates that are formed along the test.

Figure 5. Temperature and pressure over time (test #7, Tbed = 896 °C, ER = 0.32, U/ Umf=10.6) 390

3.4.2. Evolution of local bed composition obtained by bed sampling under hot gasification conditions The solid sampler has been used to determine the evolution of bed composition during gasification tests. Since the delay between two samplings is at least 15 minutes (samples are

395

kept under N2 flow for 10 min), it has been chosen to take the samples always at the same height in the bed. Since the agglomerates tend to sink inside the fluidized bed, the samples are taken just over the grid (the mean position is 20 mm above the grid). At 896 °C (test #7), the agglomerate fraction increases rapidly (Figure 6). When the defluidization signs are detected around 60 min (Figure 5), the agglomerate fraction has

400

reached 6%. This value is slightly higher than the one given by Michel et al. [42]: 3% at 900 °C. They have used 80 g of the same olivine and 1 g of Miscanthus ash that was produced from the same Miscanthus harvest. This slight difference may be due to the fact that their measurement of agglomerate ratio was achieved on the whole bed, whereas in our case, the sample was taken at the bottom of the bed that contains higher fraction of agglomerates.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

405

In the case of test #5 (827 °C), the agglomerate fraction increases slowly. At the end of this test with a longer operation time (240 min), the fraction is only 2.3% and there is no apparent defluidization. Nevertheless, the agglomerate fraction would probably continue to increase until reaching defluidization if the test duration was longer. In the samples at 896 °C (test #7), no coarse char particles (over 630 µm) are recovered at the

410

bottom of the bed whereas at 827 °C (test #5), the coarse char fraction increases and stabilizes around 0.5%. It is due to the competition between char production by pyrolysis, char consumption by oxidation and char elutriation. At 896 °C, char oxidation rate is higher leading to a lower char content in the sampled bed.

415

Figure 6. Evolution of coarse char and agglomerate fraction (20 mm above the grid) during the tests #5 (827 °C) and # 7 (896 °C). The points that are highlighted correspond to agglomerate samples that have been analyzed by SEM-EDX (pictures shown hereafter). 3.4.3. Particles inventory after defluidization and cooling At the end of the tests, the fluidization gas was rapidly shut down to stop fluidization and the

420

bed was cooled under nitrogen flow. The cold bed has been recovered layer by layer and characterized by sieving and flotation. The results are represented in Figure 7. It was thought that the agglomerate fraction in the bottom layer just above the grid (5.7% at 896 °C) would be the same than the one obtained at final time with the solid sampler (7.5% at 896 °C, see Figure 6), but it is not exactly the case. This might be due to experimental uncertainty or to

425

the fact that this bottom layer is 100 mm thick whereas the solid sampler chamber is only 40 mm high. Since coarse agglomerates sink at the bottom of the bed near the grid, it may be logical to observe higher agglomerate concentrations in the solid sampler than in the thick layer. Nevertheless, we should emphasize that the bigger agglomerates (see Figure 11) cannot come into the solid sampler that is 15 mm wide.


Page 18 of 29

Page 19 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

430

Energy & Fuels

In the case of tests #1 and #5 (around 827 °C), the agglomerate fractions are below 0.5% for each layer taken along the bed. In the case of test #7 (at 896 °C), the mean agglomerate fraction is 6% but the local fraction is higher at the top of the bed and lower in the middle. This behavior is explained at least by size differences: it is observed that very big agglomerates (larger than 5 mm) are recovered at the bottom. Small agglomerates (between

435

0.7 and 5 mm) are recovered at the top: they tend to float because they have lower apparent density than olivine. The qualitative difference between the agglomerates will be described below.

440

(a)

(b)

Figure 7. Bed axial final composition for tests #1, #5 and #7: agglomerate (a) and coarse char (b) fractions

445

3.4.4. SEM-EDX surface analysis of agglomerates obtained by bed sampling under hot gasification conditions During the test #7 at 896 °C, agglomerates were collected by the real time sampling device. Figures 8, 9 and 10 show SEM-EDX pictures of the bed particles sampled at three moments during the operation (t0, t0+15 min, t0+75 min). These samples correspond to highlighted points on Figure 6.

450


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 29

Figure 8. SEM picture of the bed material (test #7) sampled during the fluidization at the beginning of the test (after heating up to 896°C) Figure 8 shows a global view of olivine particles before agglomeration. Particles exhibit angular shapes with a relative good homogeneity in shape and size.

a)

b)

c)

d)

455

e)

Mg

Ca

K

Figure 9. SEM-EDX analysis of the bed material sampled during the fluidization after 15 min of biomass gasification (test #7 at 896°C) 460 Figure 9.a shows some agglomerates already formed after 15 min of gasification at 896 °C. Figure 9.b and Figure 9.c display a zoom on a zone with a fused structure (formed from ashes as analysed by EDX, not shown). This structure seems to be formed by ash melting over the former macroporous structure of biomass (formed by vascular bundles and cell walls). 465

Miscanthus has a high silicon uptake from the soil to enhance strength and rigidity. Silicon is integrated into the plant tissues and when the biomass particles are converted into char upon gasification, it maintains the form of the original structure up to high temperature [43]. Char is further oxidized and porous matrix is enriched in ashes which undergo melting. Figure 9.d


Page 21 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

shows a zoom on a typical agglomerate formed from olivine particles (similar particles as in 470

Figure 8, at the beginning of test) with a melted zone of ashes between them. EDX analysis (Figure 9.e) demonstrates that this binder is rich in Ca and K (the markers of ashes) and it is poor in Mg (a marker of olivine). These observations are in agreement with agglomeration data obtained by [18] with olivine and Giant Reed or Sweet Sorghum bagasse. A similar mechanism of agglomeration was observed on the agglomerates produced at 827 °C (not

475

shown).

In the case of experiment #7 at 896 °C, the same type of analysis has been achieved for agglomerates sampled at 75 min, and after the complete defluidization-cooling of the bed (see supplementary material). The agglomerates exhibit similar morphology than the ones sampled 480

at 15 min. There are no visible effects of the method used to collect the agglomerates (in-situ sampling vs agglomerates recovered after cooling). This explains why the analyses presented hereafter are only achieved with particles recovered after defluidization and cooling of the bed. 3.4.5. Analysis of agglomerates obtained after defluidization

485 Figure 10 presents the agglomerates collected from the gasification test #8 at 915 °C. Figure 10.A shows an agglomerate collected from the grid. The channels of gas flow can be observed along its left side. Figure 10.B shows shining black agglomerates rich in carbon. They were collected from the upper part of the bed (200-300 mm from the grid). In Figure 10.C, 490

different types of agglomerates can be observed. The gas flow formed them into a drop-like shape while the ashes were still molten.

Figure 10. Pictures of agglomerates collected after gasification and defluidization (test #8 at 915°C) 495


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 29

Figure 11 presents three agglomerates collected from the grid. The morphology of the bottom and upper parts of the agglomerates differed. The bottom side is rounded and molten, while the upper side is rough.

B1

A1

B2

A2

500 Figure 11. Pictures of three agglomerates (noted I, II, III) collected from the grid (test #8 at 915°C): side A is the bottom side in contact with the grid, whereas side B is the upper side. SEM-EDX analysis of the agglomerate I collected from the grid: upper side (B1,B2) and bottom side (A1,A2) 505 The SEM-EDX analysis of agglomerate cross-sections reveals the difference in the composition of the two sides. Figure 11.B1 and Figure 11.B2 show the upper, rough side of an agglomerate. The molten ash is mainly composed of K, Ca and Si but also contains Fe, Al and Cr. In the bottom side (Figure 11.A1-A2), the inclusion of Fe-Cr particles can be 510

observed and the molten ash is extremely rich in Fe. The olivine particles have an outer layer containing Al, Ca and Cr. The point by point SEM-EDX analysis in Figure 12 shows that the concentration of Fe is higher in the molten ash than in the olivine particles. Therefore we assume that the Fe derives from an outer source, in particular from the erosion of the grid and the gasifier wall. Indeed, the gasifier is made of Inox 310 S containing 0.12 wt% C, 23-26

515

wt% Cr, 18-21 wt% Ni, 1 wt% Si, 1 wt% Mn, 50-53 wt% Fe. The quantity of Fe and Cr are in the same order of magnitude as the inclusion in Figure 11.A1, which strengthens our hypothesis. On the other hand, olivine is a natural crystal with Cr and Al impurities, therefore the elevated Cr might come from the accumulation of the impurities during the gasification.


Page 23 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

520

Energy & Fuels

Figure 12. Point-by-point SEM-EDX analysis of one olivine particle covered by molten ash (test #8 at 915°C)

Figure 13 presents the SEM-EDX analysis of agglomerates from the test #8. In Figure 13.A, the fragmented surface of the olivine particles and small olivine lamellas can be observed in 525

the molten ash. Figure 13.B show that in some areas the molten ash consists of two phases. Both phases contain oxides of Mg, Ca, K, Fe and Si, but the darker one is richer in K and Fe oxides, while the lighter one is richer in Ca and Mg oxides. These phenomena were not observed during laboratory interaction tests performed in air [44]. They may be linked to the fragmentation of olivine particles during bed movement and the erosion of the gasifier wall.

530

This latter introduces a high level of Fe into the molten ash phase and it can increase the adhesion between the molten ash and bed particles.

Figure 13. Agglomerates from test #8 at 915°C: (A) olivine lamellas in the molten ash, (B) two different phases in the solidified molten ash 535


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

4. CONCLUSION A novel bubbling fluidized bed device for studying solid fuels gasification was presented. A 540

specific system was designed for bed material collection under high temperature conditions: it allows real-time sampling and quenching of bed materials at desired bed height. It provides accurate information on the temporal evolution of agglomerate and char fractions at a given position, whereas conventional bed sampling layer by layer after gasification test allows obtaining a global view of the bed at the final time.

545

In the case of Miscanthus gasification in olivine bed, different observations were achieved. First, the composition of the gas at the exit of the gasifier is typical for this type of gasifier: CO and H2 fractions increase with bed temperature, whereas the gas LHV decreases with Equivalence Ratio. A relatively novel observation is the impact of gas velocity: the increase of U/Umf causes a significant decrease of carbon conversion that may be linked to higher char

550

particles elutriation. Tar concentrations are also measured at the exit of the gasifier: the bed temperature rise reduces slightly the global tar concentration, but it increases the concentration of class 4 tars (naphthalene, fluorene, phenanthrene,…) that are troublesome for gas engine applications. The comparison of gas and char axial profiles along the bed allows a better understanding of

555

the dense bed behavior: it shows that O2 consumption and CO2 production at the bottom of the bed are mainly due to char oxidation, even if few pyrolysis gases may also be produced and oxidized near the grid. This type of data will be useful for further model development and assessment. Thanks to the real-time sampling system, it is possible to show that the char fraction at the

560

bottom of the bed is very stable, whereas the agglomerate fraction increases slowly at 827 °C and rapidly at 896 °C. Defluidization signs appear clearly when the mass fraction of agglomerates at the bottom reaches 6%. The corresponding agglomerates are quite big (> 5 mm), whereas there is a large fraction of small and light agglomerates at the top of the dense bed.

565

The SEM-EDX analysis of agglomerates revealed that the liquid phase that stick olivine particles together is mainly composed of the oxides of K, Ca, Si, Mg and Fe. The amount of Fe is greater than in the olivine, also more Cr and Al were found in the agglomerates. It is possible that the elevated level of Fe, Cr and Al come from the impurities of natural olivine but the high metal content can also derive from the erosion of the reactor wall. The intense


Page 24 of 29

Page 25 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

570

Energy & Fuels

erosion of olivine is also observed by SEM-EDX analysis. To conclude, it is shown that the layer that sticks olivine particles together does not strictly correspond to biomass ashes melt: it contains also particles and atoms that come from the erosion of olivine and stainless steel wall. Other fluidization tests with olivine and Miscanthus will be carried out to validate our

575

findings in an autothermal fluidized bed pilot plant (50 kg h-1) built at LERMAB (Epinal, France). Other type of biomasses and bed materials (silica sand, dolomite, olivine with additives…) will be tested in this small BFB to reduce agglomeration and increase tar cracking.

580

ASSOCIATED CONTENT Additional SEM-EDX pictures are provided as supporting information. This material is available free of charge via the Internet at http://pubs.acs.org.

585

ACKNOWLEDGEMENT The authors acknowledge the financial support of ANR (Agence Nationale de la Recherche) to the GAMECO project (BioE-2010). Pascal Beaurain, Mathieu Weber, Christian Blanchard, Franck Giovanella, David Brunello and Richard Lainé are acknowledged for their help to conceive and build the fluidized bed

590

device.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. 595 NOMENCLATURE AND UNITS

600

BFB Bubbling fluidized bed BTX Benzene, Toluene, Xylenes GC/MS-FID Gas Chromatography / Mass Spectrometer – Flame Ionisation Detector


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

HX ER d

605

610

SEM-EDX T U Umf PAH

Xc

Heat exchanger Equivalence Ratio diameter Lower heating value of the syngas (MJ m-3) Lower heating value of the biomass (MJ kg-1) Mass of biomass gasified (kg) Mass of syngas produced (kg) Scanning Electron Microscope - Energy Dispersive Spectroscopy Temperature (°C) Gas superficial velocity (m s-1) Minimum fluidization velocity (m s-1) Polycyclic Aromatic Hydrocarbons Volume of syngas produced (m3) Carbon conversion (%)

615 Greek letters

Cold gas efficiency Mass fraction of gas species i in the syngas Mass fraction of carbon in the species j

620


Page 26 of 29

Page 27 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

REFERENCES 625

(1) Campoy, M.; Gómez-Barea, A.; Fuentes-Cano, D.; Ollero, P. Tar Reduction by Primary Measures in an Autothermal Air-Blown Fluidized Bed Biomass Gasifier. Ind. Eng. Chem. Res. 2010, 49, 11294-11301. (2) Liliedahl, T.; Sjöström, K.; Engvall, K.; Rosén, C. Defluidisation of Fluidised Beds during Gasification of Biomass. Biomass Bioenergy 2011, 35 (1), S63-S70.

630

(3) Milne, T. A.; Evans, R. J.; Abatzoglou, N. Biomass Gasifier “Tars”: Their Nature, Formation, and Conversion; NREL/TP-570-25357; National Renewable Energy Laboratory, 1998. (4) Devi, L.; Ptasinski, K. J.; Janssen, F. J. J. G. A Review of the Primary Measures for Tar Elimination in Biomass Gasification Processes. Biomass Bioenergy 2003, 24 (2), 125-140.

635

(5) Corella, J.; Herguido, J.; Alday, F. J. Pyrolysis and Steam Gasification of Biomass in Fluidized Beds. Influence of the Type and Location of the Biomass Feeding Point on the Product Distribution. In Research in Thermochemical Biomass Conversion; Bridgwater, A. V., Kuester, J. L., Eds.; Springer Netherlands, 1988; pp 384–398. (6) Pfeifer, C.; Hofbauer, H. Development of Catalytic Tar Decomposition Downstream from a Dual Fluidized Bed Biomass Steam Gasifier. Powder Tech. 2008, 180 (1–2), 9-16.

640

645

(7) Zwart, R. W. R.; Van der Drift, A.; Bos, A.; Visser, H. J. M.; Cieplik, M. K.; Könemann, H. W. J. Oil-Based Gas Washing-Flexible Tar Removal for High-Efficient Production of Clean Heat and Power as Well as Sustainable Fuels and Chemicals. Env. Prog. Sust. Energy 2009, 28 (3), 324-335. (8) Narvaez, I.; Orio, A.; Aznar, M.P.; Corella, J., Biomass gasification with air in an atmospheric bubbling fluidized bed. Effect of six operational variables on the quality of the produced raw gas, Ind. Eng.Chem. Res. 1996, 35, 2110-2120 (9) Virginie, M.; Adánez, J.; Courson, C.; de Diego, L. F.; García-Labiano, F.; Niznansky, D.; Kiennemann, A.; Gayán, P.; Abad, A. Effect of Fe–olivine on the Tar Content during Biomass Gasification in a Dual Fluidized Bed. Appl. Cat. B 2012, 121–122, 214-222.

650

(10) Miccio, F.; Piriou, B.; Ruoppolo, G.; Chirone, R. Biomass Gasification in a Catalytic Fluidized Reactor with Beds of Different Materials. Chem. Eng. J. 2009, 154 (1–3), 369-374. (11) Bartels, M.; Lin, W.; Nijenhuis, J.; Kapteijn, F.; van Ommen, J.R. Agglomeration in fluidized beds at high temperatures: Mechanisms, detection and prevention. Prog. Energy Combust. Sci. 2008, 34, 633–666.

655

(12) Li, S.; Shang, L.; Teng, H.; Lu, Q. A model for agglomeration in bio-fuel fired fluidized bed. J. Therm. Sci. 2010, 19, 451–458. (13) Khadilkar, A.; Rozelle, P. L.; Pisupati, S. V. Models of Agglomerate Growth in Fluidized Bed Reactors: Critical Review, Status and Applications. Powder Technol. 2014, 264, 216-228.

660

(14) Zhu, Y.; Piotrowska, P.; van Eyk, P. J.; Boström, D.; Wu, X.; Boman, C.; Broström, M.; Zhang, J.; Kwong, C. W.; Wang, D; Cole, A. J. ; de Nys, R.; Gentili, F. G.; Ashman, P. J. Fluidized


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Bed Co-Gasification of Algae and Wood Pellets: Gas Yields and Bed Agglomeration Analysis. Energy Fuels 2016, 30 (3), 1800-1809. (15) Olofsson, G.; Ye, Z.; Bjerle, I.; Andersson, A. Bed Agglomeration Problems in Fluidized-Bed Biomass Combustion. Ind. Eng. Chem. Res. 2002, 41, 2888–2894.

665

(16) Lin, W. Agglomeration in bio-fuel fired fluidized bed combustors. Chem. Eng. J. 2003, 96, 171– 185. (17) Gómez-Hernández, J.; Serrano, D.; Soria-Verdugo, A.; Sánchez-Delgado, S. Agglomeration Detection by Pressure Fluctuation Analysis during Cynara Cardunculus L. Gasification in a Fluidized Bed. Chem. Eng. J. 2016, 284, 640-649.

670

675

(18) Fryda, L. E.; Panopoulos, K. D.; Kakaras, E. Agglomeration in Fluidised Bed Gasification of Biomass. Powder Technol. 2008, 181 (3), 307-320. (19) Kwapinska, M.; Xue, G.; Horvat, A.; Rabou, L. P. L. M.; Dooley, S.; Kwapinski, W.; Leahy, J. J. Fluidized Bed Gasification of Torrefied and Raw Grassy Biomass (Miscanthus × Gigantenus). The Effect of Operating Conditions on Process Performance. Energy Fuels 2015, 29 (11), 7290-7300. (20) Lewandowski, I.; Clifton-Brown, J.C.; Scurlock, J.M.O.; Huisman, W. Miscanthus: European experience with a novel energy crop, Biomass Bioenergy 2000, 19, 209-227 (21) Kaknics, J.; Defoort, F.; Poirier, J. Inorganic Phase Transformation in Miscanthus Ash. Energy Fuels 2015, 29(10), 6433–6442.

680

685

(22) Michel, R.; Rapagnà, S.; Di Marcello, M.; Burg, P.; Matt, M.; Courson, C.; Gruber, R. Catalytic Steam Gasification of Miscanthus X Giganteus in Fluidised Bed Reactor on Olivine Based Catalysts. Fuel Proc. Technol. 2011, 92 (6), 1169-1177. (23) Xue, G.; Kwapinska, M.; Horvat, A.; Li, Z.; Dooley, S.; Kwapinski, W.; Leahy, J. J. Gasification of Miscanthus X Giganteus in an Air-Blown Bubbling Fluidized Bed: A Preliminary Study of Performance and Agglomeration. Energy Fuels 2014, 28, 1121-1131. (24) Abdelouahed, L.; Authier, O.; Mauviel, G.; Corriou, J. P.; Verdier, G.; Dufour, A. Detailed Modeling of Biomass Gasification in Dual Fluidized Bed Reactors under Aspen Plus. Energy Fuels 2012, 26, 3840-3855.

690

(25) Michel, R.; Ammar, M. R.; Poirier, J.; Simon, P. Phase Transformation Characterization of Olivine Subjected to High Temperature in Air. Ceramics Int. 2013, 39 (5), 5287-5294. (26) Ergudenler, A.; Ghaly, A. E. Agglomeration of alumina sand in a fluidized bed straw gasifier at elevated temperatures. Bioresource Technol. 1993, 43, 259–268.

695

(27) Öhman, M.; Nordin, A.; Skrifvars, B.-J.; Backman, R.; Hupa, M. Bed Agglomeration Characteristics during Fluidized Bed Combustion of Biomass Fuels. Energy Fuels 2000, 14(1), 169–178. (28) Chirone, R.; Miccio, F.; Scala, F. Mechanism and prediction of bed agglomeration during fluidized bed combustion of a biomass fuel: Effect of the reactor scale. Chem. Eng. J. 2006, 123(3), 71–80.


Page 28 of 29

Page 29 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

700

Energy & Fuels

(29) Horvat, A.; Kwapinska, M.; Xue, G.; Rabou, L. P. L. M.; Pandey, D. S.; Kwapinski, W.; Leahy, J. J. Tars from Fluidized Bed Gasification of Raw and Torrefied Miscanthus X Giganteus. Energy Fuels 2016. DOI: 10.1021/acs.energyfuels.6b00532 (30) Van Paasen, S. V. B.; Kiel, J. H. A. Tar Formation in a Fluidised Bed Gasifier: Impact of Fuel Properties and Operating Conditions; ECN-C--04-013; 2004. (31) Thersites : http://www.thersites.nl/

705

(32) Radmanesh, R.; Chaouki, J.; Guy, C. Biomass Gasification in a Bubbling Fluidized Bed Reactor: Experiments and Modeling. AIChE J. 2006, 52 (12), 4258-4272. (33) Ross, D.; Noda, R.; Horio, M.; Kosminski, A.; Ashman, P.; Mullinger, P. Axial Gas Profiles in a Bubbling Fluidised Bed Biomass Gasifier. Fuel 2007, 86 (10–11), 1417-1429.

710

(34) Kuo, J.-H.; Lin, C.-L.; Wey, M.-Y. Effect of Agglomeration/defluidization on Hydrogen Generation during Fluidized Bed Air Gasification of Modified Biomass. Int. J. Hydrogen Energy 2012, 37 (2), 1409-1417. (35) Grace, J.; Bi, H.; Zhang, Y. Pitfalls in Gas Sampling from Fluidized Beds. Chem. Eng. Sc. 2009, 64 (10), 2522-2524.

715

(36) Dufour, A.; Masson, E.; Girods, P.; Rogaume, Y.; Zoulalian, A. Evolution of Aromatic Tar Composition in Relation to Methane and Ethylene from Biomass Pyrolysis-Gasification. Energy Fuels 2011, 25 (9), 4182-4189. (37) Cluet, B.; Mauviel, G.; Rogaume, Y.; Authier, O.; Delebarre, A. Segregation of Wood Particles in a Bubbling Fluidized Bed. Fuel Proc. Technol. 2015, 133, 80-88.

720

(38) Fotovat, F.; Ansart, R.; Hemati, M.; Simonin, O.; Chaouki, J. Sand-Assisted Fluidization of Large Cylindrical and Spherical Biomass Particles: Experiments and Simulation. Chem. Eng. Sc. 2015, 126, 543-559. (39) Bruni, G.; Solimene, R.; Marzocchella, A.; Salatino, P.; Yates, J. G.; Lettieri, P.; Fiorentino, M. Self-Segregation of High-Volatile Fuel Particles during Devolatilization in a Fluidized Bed Reactor. Powder Technology 2002, 128 (1), 11-21.

725

(40) Di Blasi, C.; Branca, C. Temperatures of Wood Particles in a Hot Sand Bed Fluidized by Nitrogen. Energy Fuels 2003, 17 (1), 247-254. (41) Kern, S.; Pfeifer, C.; Hofbauer, H. Gasification of Wood in a Dual Fluidized Bed Gasifier: Influence of Fuel Feeding on Process Performance. Chem. Eng. Sc. 2013, 90, 284-298.

730

(42) Michel, R.; Kaknics, J.; de Bilbao, E.; Poirier, J. The Mechanism of Agglomeration of the Refractory Materials in a Fluidized-Bed Reactor. Ceramics Int. 2016, 42 (2, Part A), 25702581. (43) Kaknics, J. Gazéification de La Biomasse En Lit Fluidisé Bouillonnant: Interactions à Haute Température Entre Les Composés Inorganiques et Les Matériaux Granulaires; Orléans, 2014.

735

(44) Kaknics, J.; Michel, R.; Richard, A.; Poirier, J. High-Temperature Interactions between Molten Miscanthus Ashes Materials in a Fluidized-Bed Gasifier. Energy Fuels 2015, 29 (3), 1785– 1792.


Gas and Bed Axial Composition in a Bubbling Fluidized Bed Gasifier

Recommend Documents