Entrained-flow gasification of coal under slagging conditions: Relevance of fuel–wall interaction...

Oviedo ICCS&T 2011. Extended Abstract

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Entrained-Flow Gasification of Coal under Slagging Conditions: Properties of Solid Wastes and Relevance of Char-Wall Interaction Phenomena

Fabio Montagnaro1, Paola Brachi1 and Piero Salatino2

1 Dipartimento di Chimica, Università degli Studi di Napoli Federico II, Complesso Universitario del Monte di Sant’Angelo, 80126 Napoli (Italy), T: +39 081 674029; F: +39 081 674090; E: [email protected]

2 Dipartimento di Ingegneria Chimica, Università degli Studi di Napoli Federico II; Istituto di Ricerche sulla Combustione, Consiglio Nazionale delle Ricerche, Piazzale Vincenzo Tecchio 80, 80125 Napoli (Italy), T: +39 081 7682258; F: +39 081 5936936; E: [email protected]

Abstract

The aim of this paper is to investigate the properties of solid wastes generated from an

industrial-scale pressurized entrained-flow gasifier, by means of a combination of

experimental techniques: elemental, granulometric and X-ray diffraction analyses,

scanning electron microscopy and energy dispersive X-ray analysis. The results are

critically discussed in the light of the different regimes of char-slag micromechanical

interaction and of the different phases that are established in the gasification chamber.

The discussion allows to give useful insights concerning the properties and partitioning

of carbon among the three main sources (coarse slag, slag fines, fly ash) of solid

residues coming from the gasifier: in particular, differences between coarse slag and

slag fines are highlighted, though these wastes are generated from the same main-

stream. Furthermore, it is observed that residual carbon in slag granules is present in a

segregated embedded form, while slag fines are composed of both porous (high-carbon)

and compact (low-carbon) material. Altogether, the properties of the three residues are

consistent with a mechanistic framework of the bulk-to-wall transfer and partitioning of

solids during entrained-flow gasification of coal developed in a recently published

theoretical paper.


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1. Overview

Entrained-flow coal gasifiers of new generation are characterized by operating conditions

(high operating temperatures and multi-stage feedings of coal and gaseous reactants)

aimed at favoring ash migration/deposition onto the reactor walls, whence the molten ash

(slag) flows and is eventually drained at the bottom of the gasifier [1,2]. Detailed studies

concerning the fate of char particles as they impinge the wall slag layer have been only

recently developed [3–6]. In a recently published paper, Montagnaro and Salatino [7]

addressed the relative importance of the parallel pathways of coal conversion associated

with entrained-flow of carbon particles in a lean-dispersed gas phase vs. segregated flow

of char particles in a dense-dispersed phase in the near-wall region of the gasifier. Char

segregation in the dense-dispersed phase is promoted by bulk-to-wall particle migration

and by inelastic interaction of char particles with the molten slag wall layer. By taking

into account characteristics such as char density, particle diameter and impact velocity,

slag viscosity, interfacial particle-slag tension, theoretical criteria for either char particle

entrapment inside or carbon-coverage of the wall ash layer have been developed. This is

represented in Figure 1, which depicts the following plausible regimes of char-slag

micromechanical interaction: regime E) (entrapment), in which char particles reaching the

slag surface become permanently embedded into the molten layer and further course of

combustion/gasification is hindered; regime S) (segregation), in which char particles

reaching the wall adhere to the slag layer’s surface without being fully engulfed, so that

the progress of combustion/gasification is permitted; regime SC) (segregation and

coverage), in which the coverage of the slag layer with carbon particles is extensive. In

this last regime a dense-dispersed annular phase is established in the close proximity of

the wall ash layer, where the excess impinging char particles which cannot be

accommodated on the slag surface accumulate. This phase is likely to be characterized by

a velocity that is intermediate between that of the fast lean-dispersed phase and that of the

slowly moving wall molten ash layer. This feature is beneficial to C conversion as it gives

rise to a longer mean residence time of carbon particles belonging to this phase.

Consistently, a schematic diagram of the entrained-flow gasifier is presented in Figure 2

in which, in particular, the presence of three different sources of solid wastes is

underlined, that is: slag phase, yielding coarse slag granules upon interaction with the

quench bath at the bottom of the gasifier; dense-dispersed phase, giving rise to slag fines

upon interaction with the quench bath; lean-dispersed phase, giving rise to fly ash

escaping the gasifier in the gas stream.

https://www.researchgate.net/publication/244067937_A_model_of_char_capture_by_molten_slag_surface_under_high-temperature_gasification_conditions?el=1_x_8&enrichId=rgreq-4050022b-1408-4845-83e8-df4ae5f47588&enrichSource=Y292ZXJQYWdlOzI2MTU0OTgxOTtBUzoxMDI3MjI2NjE1MTkzNjNAMTQwMTUwMjM5OTI4Ng==


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syn‐gasfly ash (WFLY) slag (WSLAG)

lean‐dispersed

dense‐dispersed

char, sootslag

devolatilization/combustion zonecoal (WF)

O2 (WOX)H2O (WS)

syn‐gasslag fines (WSF)

Figure 1 (left). Regimes of C-slag micromechanical interaction (E=entrapment; S=segregation; SC=segregation and coverage). Figure 2 (right). Outline of flow patterns in the entrained-flow gasification chamber (the symbol ‘W’ stands for mass flow rates). 2. Rationale of the present investigation

This investigation was stimulated by the operational experience from an industrial-scale

pressurized (25 bar; cf. [8,9]) entrained-flow gasifier, operated in the slagging regime (at

temperatures around 1700–1900 K; cf. [9,10]). The gasification chamber has an internal

diameter of 3.8 m and a length of 13 m. Mass feeding ratios are: WOX/WF=0.8;

WS/WF=0.1 (see Figure 2). Solid fuel feed rate is WF=30 kg s–1 and, typically, the solid

fuel is a 50:50 coal:pet coke mixture. Practical operation of the gasifier revealed a value

of the mass ratio WFLY/(WSLAG+WSF) smaller than expected at the design stage (circa 0.1

vs. 0.4; cf. [9,11]). This was primarily ascribed to an unexpectedly large value of the mass

flow rate of slag fines (WSF) leaving the quench bath. Moreover, C content in fly ash

resulted rather limited (circa 5%; cf. [11]), while that in slag fines was reported to be quite

high. Finally, a non-negligible organic fraction was detected in the slag waste (cf. [12]),

and this appears consistent with findings reported, for similar systems, by other authors

[13–15]. In this paper, results concerning characterization of solid residues from the

industrial-scale gasifier were interpreted in the light of the different regimes of char-slag

micromechanical interaction (Figure 1) and of the different phases that are established in

the gasification chamber (Figure 2), as presented in the Overview. In this context, it is

useful to remind that the solid waste other than fly ash is quenched in a water bath

generating, besides the slag (sometimes referred to as coarse slag), a black water whence

slag fines are recovered by filtration [13]. The different thermal/conversion history of


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these two solid wastes is likely to strongly influence their properties, in particular as far as

carbon content, morphology and further reactivity is concerned. Only recently did the

question concerning the differences between coarse and fine slag receive consideration:

the reader is referred, for example, to the works [13–15].

3. Materials and experimental techniques

Coarse slag and slag fines samples were generated in the ELCOGAS entrained-flow

gasifier located in Puertollano, Ciudad Real (Spain). This material was provided in the

summer of 2009 and, due to the considerable amount of wastes produced by the plant and

to the variability of the operating conditions of the gasifier, it might not be fully

representative of the ash generated by the industrial gasifier during normal operation.

Coarse slag and slag fines were characterized by: carbon elemental analysis, performed

by a LECO CHN-2000 instrument; granulometric analysis, performed by either a Malvern

Instruments Master Sizer 2000 laser granulometer (operated down to a minimum particle

size of 0.02 μm) or mechanical sieving (in 10 size ranges between 0 and 9.5 mm); X-ray

diffraction (XRD) analysis, performed by a Bruker D2 Phaser diffractometer (operated at

diffraction angles ranging between 10 and 60°2θ with a scan velocity equal to 0.02°2θ s–1);

scanning electron microscopy (SEM), performed by a FEI Inspect microscope equipped

with an energy dispersive X-ray (EDX) probe (operated up to magnifications of 3000×).

4. Experimental results

A preliminary elemental analysis on coarse slag and slag fines revealed no appreciable

carbon content (for the former) and a carbon content as high as 57.4% (for the latter).

Figures 3 and 4 show absolute and cumulative particle size distributions for slag and slag

fines, respectively. Due to the much coarser size of the slag material, the particle size

analysis for this waste was carried out by mechanical sieving instead of laser

granulometry. From Figure 3, it can be observed that slag particles size extended over a

broad range, with maximum size equal to about 9 mm (cf. [12]). Nonetheless, a very

distinct peak for the absolute distribution could be appreciated at 1.7 mm; the mean

Sauter diameter for this distribution was equal to 1.18 mm. Finally, a median value (d50)

of 1.27 mm was obtained from the cumulative distribution. Slag fines were characterized

by much smaller values of particle size (Figure 4): particles coarser than 700 μm were not

observed, the peak and the Sauter diameter for this distribution were equal to 100 and 20


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μm, respectively, and the d50-value for the cumulative distribution was 72 μm.

Particle diameter, mm

0 1 2 3 4 5 6 7 8 9

Abs

olut

e si

ze d

istri

butio

n, -

0.0

0.1

0.2

0.3

0.4

0.5

0.6

peak=1.7 mmSauter=1.18 mm

Particle diameter, mm

0 1 2 3 4 5 6 7 8 9

Cum

ulat

ive

size

dis

tribu

tion,

-

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

d50=1.27 mm

Particle diameter, μm

0 100 200 300 400 500 600 700

Abs

olut

e si

ze d

istri

butio

n, -

0.00

0.01

0.02

0.03

0.04

0.05

0.06

peak=100 μmSauter=20 μm

Particle diameter, μm

0 100 200 300 400 500 600 700

Cum

ulat

ive

size

dis

tribu

tion,

-

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

d50=72 μm

Figure 3 (upper row) and Figure 4 (lower row). Absolute (left) and cumulative (right) particle size distributions for coarse slag (Figure 3) and slag fines (Figure 4).

Diffraction angle, °2θ

10 15 20 25 30 35 40 45 50 55 60

Peak

inte

nsity

, a.u

.

A

P

coarse slag

slag fines

Figure 5. XRD analysis for coarse slag and slag fines (A=anhydrite, CaSO4, CPDS#228; P=periclase, MgO, CPDS#2693). Figure 5 reports XRD patterns for the wastes under investigation. The results for coarse

slag revealed no distinct peaks, due to the amorphous and vitreous nature of this material

related to its very peculiar thermal history in the gasifier [10,12,16]. Though deriving

from the same main-stream, slag fines separated from the slag after the quench bath

showed a different crystalline microstructure, the peaks of anhydrite and periclase being


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clearly recognizable. While elemental analysis on coarse slag did not indicate the

presence of carbon in this material, interestingly, different results were obtained when

cross-sections of coarse slag granules were analyzed by SEM-EDX (Figure 6a and Table

1).

Figure 6. SEM micrographs of cross-sections of: a) a whole coarse slag particle (magnification=50×); b–c) selected zones of slag particles (magnification=1600×), displaying carbon-rich patches. Table 1. EDX elemental analysis results (%wt) referring to SEM micrographs reported in Figures 6 and 7.

Figure 6a Figure 6b Figure 6c Figure 7a Figure 7b Figure 7c Figure 7d Figure 7e C 9.27 48.77 54.23 86.47 82.27 84.89 18.36 13.90 O 26.95 11.50 8.67 6.71 8.18 6.98 39.21 40.94

Na 0.14 0.14 0.13 n.d. n.d. n.d. 0.33 0.25 Mg 0.79 0.46 0.44 n.d. 0.18 n.d. 0.64 0.64 Al 14.91 9.20 8.31 0.31 0.35 0.26 11.00 10.92 Si 32.62 19.10 17.82 0.51 0.38 0.29 22.08 24.13 S n.d. 1.24 1.70 3.03 3.39 2.91 0.69 n.d. K 2.25 1.41 1.44 0.20 0.17 n.d. 2.86 1.51

Ca 7.38 4.75 3.96 n.d. n.d. n.d. 2.52 4.80 Ti 0.59 0.38 0.40 n.d. n.d. n.d. 0.70 0.44 V 0.38 0.19 0.23 0.34 0.28 0.31 0.46 0.30

Fe 4.72 2.86 2.67 2.15 4.32 3.96 1.14 2.17 Ni n.d. n.d. n.d. 0.28 0.47 0.41 n.d. n.d. The whole slag particle shown in Figure 6a appeared to be mostly vitreous and dense, in

agreement with XRD results. While the inorganic fraction is primarily constituted by

Si+Al (47.5%) together with Ca (7.4%, related to the feeding of limestone to the gasifier

as fluxing agent), Fe (4.7%) and K (2.2%), a remarkable carbon content (9.3%) was also

detected. Carbon could not be mainly ascribed to the possible presence of CaCO3: even if

all the calcium were present as carbonate, the ‘free’ carbon would anyway be as large as

7.1%. Analyses performed on the cross-sections of other whole particles led to

comparable observations. Figures 6b–c (and Table 1) report SEM-EDX results obtained

carrying out the analysis at a greater magnification on the cross-section of two selected

zones of slag particles. In both cases the occurrence of darker patches was observed.

a) b) c)


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Pointwise quantitative EDX results refer to these regions: C-contents as high as 48.8–

54.2% were obtained. This finding contributes to the assessment of the relevance of

carbon entrapment (regime E), Figure 1) in slag particles. It is worth noting that elemental

analysis did not show the presence of any organic fraction and this was essentially so

because C was permanently entrapped into the slag matrix in a way that could not have

been disclosed by thermal analysis. Only the cutting procedure associated with the SEM-

EDX analysis of granules cross-sections was able to disclose the unreacted carbon, which

appeared to be somewhat segregated (in the patches) with respect to the inorganic slag

matrix. By taking into account that quantitative results relevant to Figure 6a referred to

the cross-section of a slag particle, an average carbon content around 3–4% was estimated

for this waste. Figure 7 and Table 1 report the results of the SEM-EDX analysis

performed on whole slag fines particles. In particular, particles having prevailing either

porous (Figures 7a–c) or compact (Figures 7d–e) structures were observed. In any case,

the carbon content was larger than the value obtained from the inspection of coarse slag

particles, in line with results of elemental analysis. This is particularly evident for porous

particles (Figures 7a–c): C-content ranged between 82.3 and 86.5%, and Fe (2.2–4.3%)

and S (2.9–3.4%) could also be appreciated. Thus, these particles should be mainly

associated with unreacted char present in the dense-dispersed phase (Figure 2) giving rise

to slag fines upon impingement on the quench bath. On the other hand, dense particles

(Figures 7d–e) display morphological and chemical features that are closer to those of

coarse slag particles, at least as far as SEM-EDX results are concerned: C-content ranged

between 13.9 and 18.4%, the Si+Al-fraction was as high as 33.1–35.0% and Ca (2.5–

4.8%), K (1.5–2.9%) and Fe (1.1–2.2%) could also be detected. It is also highlighted that

elements such as Na, Mg, Al, Si, K, Ca and Ti, while revealed in smaller amounts (or not

revealed at all) in high-C porous slag fines, were present in larger percentages in both

coarse slag and low-C dense slag fines particles. The opposite is true for S and Ni. The

results are consistent with the previously-reported C-content of slag fines (about 57%),

obtained by standard elemental analysis carried out on waste containing both high-C

porous and low-C dense materials. Moreover, the more compact slag fines should be

regarded as having intermediate properties between porous slag fines and coarse slag: this

observation, jointly with XRD results, highlights again the establishment of a dense-

dispersed phase that, together with the slag phase, generates both streams: slag and slag

fines (Figure 2).


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Figure 7. SEM micrographs of different whole slag fines particles: a)–c) at magnification=1600×; d)–e) at magnification=3000×. 5. Critical discussion

It has been observed that the slag fines, generated in high amount in the gasifier (cf.

Section 2), are very rich in carbon (cf. elemental and SEM-EDX analysis results). This is

believed to be a clue in favor of the existence of a dense-dispersed phase (regime SC),

Figures 1 and 2), which would be the source of high-C slag fines upon interaction with

the quench bath. On the other hand, the slag waste presented a non-negligible content of

carbon, mostly entrapped (in a segregated fashion) into the slag matrix. This would be

consistent with the occasional establishment of regime E). It is interesting to analyze these

findings in the light of results recently published by Li et al. [5,6]. These authors

investigated the char-slag transition during entrained-flow oxidation of coal particles,

observing a distinct transition from porous/non-sticky char to fluid/sticky slag occurring

at temperatures above the ash flow temperature only when carbon conversion exceeded a

certain threshold (around 90%). This means that the coexistence of regimes SC) –as the

prevailing regime– and E) might be possible. If one assumes, based on Montagnaro and

Salatino’s findings [7], that regime SC) is the dominant regime under typical operating

conditions of entrained-flow gasifiers, then some char particles belonging to the dense-

dispersed phase and in the late burn-off stage could well be permanently embodied into

the slag layer, according to Li et al.’s findings [5,6]. Although the carbon content of these

a) b) c)

d) e)


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particles is likely to be modest, it is anyway not negligible and could possibly justify the

estimated C-value of 3–4%. As far as fly ash is concerned, its carbon content (5%) was

unexpectedly small. This finding can be interpreted by considering that bulk-to-wall

transfer of char/ash particles is dominated by the inertial mechanism associated with

turbophoresis and centrifugal forces due to swirl/tangential flow. This mechanism would

make wall transfer of coarser particles more effective than transfer of fines [7].

Considering, as reported by Wu et al. [13], that the carbon content of coarser particles is

generally higher, this mechanism would imply a more effective transfer of carbon to the

wall, as compared with ash transfer. These two aspects (coexistence of regimes and

preferential mass transfer) could explain why fly ash resulted selectively C-depleted and

why slag fines did show such a large mass flow rate and C-content, contrary to the

expectations. Moreover, it is recalled that part of the fly ash might derive by nucleation

and growth of fine (carbon-free) inorganic particles from the gas phase under the

extremely high temperature conditions experienced by the fuel in the flame region of the

oxidizer. This would further justify the comparatively small carbon content of fly ash.

6. Conclusions

The properties and partitioning of carbon among the three main sources (coarse slag, slag

fines, fly ash) of solid residues coming from an industrial-scale entrained-flow coal

gasifier have been characterized by a combination of experimental techniques. The carbon

content of slag fines is very large, in the order of 60%. The carbon content of fly ash is

around 5%. The carbon content of slag granules as assessed by standard elemental

analysis techniques is negligible, but combined microscopy and EDX analysis of the

granules’ cross-sections indicates that residual carbon is present in slag granules as

segregated embedded carbon-rich patches, amounting to about 3% by mass of the sample.

The properties of these three types of residues are consistent with a mechanistic

framework of entrained-flow gasification of coal developed by the authors, which

considers the bulk-to-wall transfer of solids and the establishment of segregated phases in

the near-wall region of the gasifier.

Acknowledgement

The authors wish to express their gratitude to Mr. F. García Peña, Dr. A. M. Mozos and Dr. P. Coca

(ELCOGAS, Spain) for having supplied raw materials and for useful discussion. Dr. M. Urciuolo and

Mr. S. Russo (IRC-CNR, Italy) are gratefully acknowledged for their support in solid characterization.


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