Influence of Temperature on the Release of Inorganic Species from

Oct 8, 2014 - Victorian Brown Coals and German Lignites under CO2 Gasification ...... (29) Ma, R. P.; Felder, R. M.; Ferrell, J. K. Evolution of hydro...
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Influence of Temperature on the Release of Inorganic Species from Victorian Brown Coals and German Lignites under CO2 Gasification Conditions Joanne Tanner,† Marc Blas̈ ing,*,‡ Michael Müller,‡ and Sankar Bhattacharya† †

Department of Chemical Engineering, Monash University, Wellington Road, Clayton, 3800, Australia Institute for Energy and Climate Research (IEK-2), Leo-Brandt-Strasse 1, 52425 Jülich, Germany

‡

ABSTRACT: The release of volatile inorganic species containing Na, K, S, and Cl during coal gasification is of interest due to the known associated issues related to corrosion and fouling in coal-to-product processes. Therefore, laboratory-scale experiments using two Victorian brown coals and four German lignites were performed at 1100, 1200, and 1400 °C under an atmosphere of 20% CO2 in He to simulate gasification conditions. The reaction products were analyzed online by the wellestablished molecular beam mass spectrometry technique, and qualitative and semiquantitative analysis of the results was performed. The primary release occurred during the initial devolatilization phase, with less volatile and secondary products released during the gasification phase. The main inorganic species detected were 34H2S+, 35Cl+, 36HCl+, 39K+/39NaO+, 58NaCl+, 60 COS+/60NaCl+, and 64SO2+. The release of sodium and chlorine species was highly dependent on temperature and the inherent Al and Si content of the coals. The release of sulfur-containing species was highly dependent on temperature and the inherent Ca content of the coals. Mechanisms for the initial release and subsequent reaction of intermediate inorganic species are offered herein to explain the observed trends with temperature and coal properties.

1. INTRODUCTION Worldwide population growth and the associated elevated demand for power and chemical feedstocks, particularly in developing countries, has prompted an increase in coal-toproducts applications via gasification technologies. In particular, the use of low rank coals in value-adding coal gasification processes is of renewed interest. One of the persistent concerns associated with the gasification of low rank coals is the negative impact of volatile inorganics, inherent in the parent coals, which are released during the gasification process. Alkali, sulfur, and chlorine species, for instance, are well-known to contribute to slagging and fouling in the gasifier unit and downstream processes, leading to decreased plant life and efficiency and increased maintenance costs.1 The mineral and nonmineral forms in which inorganic elements occur in raw coals are reasonably well understood,2−5 and these modes of occurrence directly influence the release mechanisms and species evolved under gasification conditions. Nonmineral inorganic species are present in raw Victorian brown coals as inherent matter in the form of exchangeable cations associated with oxygen-containing functional groups.2 Na, Ca, Mg, Fe, Al, Si, and NaCl may occur, where Al and Si in this form are acid soluble and Fe is nonpyritic.5 These transient inorganic species are weakly bound to the coal structure as complexes with carboxylic and phenolic groups, and as such may be volatilized at relatively low temperatures.6,7 It is these volatile species, in particular, Na, K, S, and Cl, which are of interest in this investigation. The discrete mineral inorganic species in low rank coals are present in very low concentrations and exist predominantly as quartz and clays.2 Their transformations, while also critical to various aspects of the thermal processing of coal, will not be discussed in this paper, except © 2014 American Chemical Society

where they are in direct interaction with the above-mentioned volatile species of interest. Several groups have investigated the release, reactions, and influences of volatile species on low rank coal pyrolysis and gasification processes.8−12 The majority of these investigations have, however, been carried out under conditions which do not adequately represent those used in industrial coal gasification applications and do not record the various species of interest in situ with respect to the location in the gasifier or downstream processes in which they are known to be problematic. Similar investigations, which were performed in situ by the above definition, were run under substoichiometric air atmospheres of up to 7.5% oxygen in helium.13−15 While this atmosphere does in some ways simulate the reducing environment common to gasification processes, it does not provide any information on the effect of the presence of CO2 − one of the primary gasifying agents. Therefore, in this paper, the release of alkali, sulfur, and chlorine species in the presence of CO2 from several low rank coals from Victoria, Australia, and North Rhine-Westphalia, Germany, was investigated. The experiments focused on the influence of temperature on the type and magnitude of inorganic species released from the coal under 20% CO2 in helium. The hot gases were measured directly using the molecular beam mass spectrometry (MBMS) technique, which provided qualitative and semiquantitative data. The temperature range and atmosphere were chosen to mimic some of the pertinent conditions found in industrial gasification processes. Received: July 2, 2014 Revised: September 22, 2014 Published: October 8, 2014 6289

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Table 1. Results of the Chemical Analysis of the Coals under Investigation HKN-S− Moisture (air-dried) Ash (dry basis)

20.02 3.58

C H N S Cl Al Fe Ca Mg K Na Si

65.8 4.81 0.78 0.205 0.01 0.024 0.18 0.79 0.30 0.016 0.19 0.009

Fe/S Na/Cl Ca/S

0.50 28.88 3.08

HKN-S+

HKS

Proximate Analysis of the Coals (mass %) 19.76 20.45 4.23 6.59 Chemical Composition (dry basis, mass %) 65.8 62.0 4.98 4.89 0.84 0.69 0.508 0.365 0.03 0.02 0.025 0.099 0.22 0.40 0.90 1.2 0.34 0.43 0.016 0.018 0.19 0.42 0.020 0.60 Elemental Ratios (molar basis) 0.25 0.62 11.33 27.48 1.42 2.58

HKT

LY

MOR

10.27 13.20

11.16 7.99

14.92 3.59

57.3 4.18 0.75 0.478 0.01 1.2 0.20 1.1 0.39 0.069 0.22 2.7

54.4 4.26 1.01 0.84 0.09 0.37 0.1 0.04 0.05 0.027 0.09 2.74

61.6 4.70 1.55 0.88 0.04 0.016 0.39 0.66 0.40 0.011 0.12 0.021

0.24 31.05 1.90

0.05 1.55 0.04

0.25 4.74 0.60

This result, coupled with the high measured aluminum and silicon content in comparison to literature data, indicates that extraneous material is present in the sample. The presence of a high amount of silica in the quartz phase as well as minor amounts of aluminosilicate was confirmed by XRD of the ash which was prepared at 815 °C. The extraneous material, possibly sand or clay from the interseam included during sampling, may influence the release mechanism of sodium as discussed in subsequent sections of this paper. 2.2. Experimental Setup. The release of Na-, K-, S-, and Clcontaining species during gasification under a controlled atmosphere of 20% CO2 in helium was investigated at high temperature in a tubular furnace coupled to a molecular beam mass spectrometer (MBMS). For detailed descriptions of the coupled MBMS system, including schematics of the experimental setup and MBMS apparatus, the reader is referred to Bläsing13,14 and Wolf.23 Only a brief outline with pertinent and differing experimental details is presented here. The reactor consisted of a heated alumina flow channel housed in a furnace with four independent heating zones. Helium was selected as the carrier gas for all experiments, as its low atomic mass ensures high signal intensities in the MBMS.23 All parts of the reactor downstream of the reaction zone were maintained at temperatures above the condensation point of the Na-, K-, Cl, and S-containing species of interest, making it impossible for condensation to occur in this region. A gas flow of 3.2 L/min He and 0.8 L/min CO2 corresponding to 80.0 vol % He and 20.0 vol % CO2 was maintained in the reactor to simulate a gasification-like environment. A platinum sample boat loaded with 100 ± 2 mg of dried coal was inserted into the air cooled end of the heated alumina flow channel and positioned in the reaction zone at the desired temperature by a horizontally displaceable alumina rod. The hot reaction products flowed to the end of the reactor, where they were analyzed by MBMS. MBMS is a reliable method for analyzing gases produced under extreme temperature and pressure conditions and has been applied as such since the 1960s.8,16,17,19,24−28 The main advantage of MBMS is that highly reactive, condensable species are effectively quenched, so that no further condensation and reaction is possible prior to analysis.23 The MBMS consists of three vacuum chambers. Gas from the reactor enters the first chamber through a 0.3 mm diameter front nozzle orifice, where it undergoes supersonic free jet expansion and attains free molecular flow after a distance of approximately 10 orifice diameters. In the second chamber, the core of the free jet expansion is extracted by a conical skimmer of 1 mm diameter and directed into the third chamber, where a hot filament emits electrons at 50 eV and 1 mA. Every 10−4−10−3 molecule is ionized by electron impact and

This enhanced knowledge of the release mechanisms under gasification conditions may therefore be applicable to the improved design and optimization of gasification and downstream operations, such as heat transfer processes, hot gas cleaning, and gas combustion.

2. EXPERIMENTAL SECTION 2.1. Fuel Preparation. Samples of four Rhenish lignites from Hambach (HKN-S-, HKN-S+, HKS, and HKT) and two Victorian brown coals (Loy Yang and Morwell) were collected. The Rhenish coals were dried and ground in a mill and then sieved to a particle size 1; ±10% for amounts 0.1−1%, and ±20% for amounts 500 °C)

(2)

S

(3)

2 2(g)

FeS(s) + H 2(g) → Fe(s) + H 2S(g) FeS(s) + H 2O(g) → FeO(s) + H 2S(g)

(4)

(T > 800 °C)

(5)

The absence of inherent sulfates in these coals partly accounts for the delay in release of COS and SO2 from the coals. COS is not evolved until the char gasification phase begins, and the majority of the SO2 is released even later, as shown in Figures 1 and 2. COS is formed by a combination of functional group decomposition and the H2S−COS shift reaction shown in eq 6.29 This reaction does not proceed at a significant or measurable rate until the dilution effect of the initial pyrolysis phase has finished and the initial CO2 concentration is restored; hence, the delayed release of COS. H 2S(g) + CO2(g) ↔ COS(g) + H 2O(g)

(6) 64

+

The more significant delay in the release of SO2 is attributed to the evolution of the stronger organically bound sulfur, e.g. heterocyclics, during conversion of the coal matrix into gaseous products, and to the rerelease of captured sulfur from decomposition of sulfates formed during the devolatilization phase. Additionally, as the gasification reaction rate diminishes due to consumption of the available carbon, the rate of CO production is reduced, further stabilizing the SO2 product. Similarly to the Victorian brown coals, the sulfur in the Rhenish lignites is predominantly present in organic forms with minimal pyrites and sulfates.30 A corresponding similarity in the trends in release of S-containing species between the Rhenish and Victorian coals was also observed. Thus, the MBMS technique and these results appear to distinguish between organically and inorganically bound sulfur. 3.2. Semiquantitative Results of the MBMS Measurements. The majority of the volatile species of interest were evolved entirely during the initial devolatilization phase of the experiments. Therefore, the semiquantitative analysis considers only the time from sample insertion to the end of the devolatilization phase, with the exception of the COS and SO2 signals, for which the averaged area was extended to include the entire peak. Due to the arbitrary nature of the recorded signal intensities, the raw data obtained at each temperature for each sample are not directly comparable. Therefore, the averaged peak area for each m/z ratio was integrated and normalized to the averaged area of the 46CO2+ signal during the 20 s background reading taken prior to sample insertion to facilitate comparison between coals and temperatures. The averaged, 6293

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Figure 3. Averaged, normalized peak areas for 35Cl+.

Figure 7. Averaged, normalized peak areas for 34H2S+.

Figure 4. Averaged, normalized peak areas for 36HCl+.

Figure 8. Averaged, normalized peak areas for 64SO2+.

Figure 5. Averaged, normalized peak areas for 39NaO+. Figure 9. Averaged, normalized peak areas for 60COS+.

fragments for the same m/z ratio are discussed in accordance with the dominant signal, as explained previously. The release and formation of the various chlorine and sodium species is explained as follows. Weakly bound organic chlorine, and chlorine evolved as HCl following surface reaction with coal hydrogen are released at low temperatures. Inherent NaCl and sodium associated with carboxylic functional groups in the coal are also released at 800°C)

CaO(s) + SO3(g) ↔ CaO(s) + SO2(g) +

1

2

O2(g)

(20) (21)

The rate of decomposition of CaSO4 has also been shown to increase in a CO−CO2−N2 atmosphere in the presence of Fe2O3 at temperatures above 750 °C, and SiO2 and Al2O3 at temperature above 1100 °C. Furthermore, the CaSO4 may react with CO generated by the gasification reactions, releasing additional CO2 and SO2.42 CaSO4(s) + CO(g) ↔ CaSO3(s) + CO2(g)

(22)

CaSO4(s) + CO(g) ↔ CaO(s) + CO2(g) + SO2(g)

(23)

LY is deficient in Ca in comparison to the other five samples, and shows only mild SO2 evolution at 1400 °C. In a complementary trend, H2S evolution decreases significantly at 1400 °C for all samples except LY. Thus, the above mechanism explains the observed trends for 34H2S+, 64SO2+, and 60COS+ for all coals with significant inherent Ca content by the capture of evolved H2S from the pyrolysis phase and the subsequent rerelease of the sulfur as SO2 during the gasification phase. In the case of LY, the Ca content is insufficient to promote this mechanism. Instead, the H2S−COS equilibrium reaction dominates at 1400 °C, resulting in the significant increase in H2S evolution and the minor release of COS and SO2 relative to the other samples. 3.3. Comparison with Release Measurements Performed under O2 Combustion and Gasification Conditions. Previous investigations of the thermal behavior of Rhenish lignites have been undertaken at various temperatures under 20% O2 and 7.5% O2 atmospheres to simulate combustion and gasification conditions, respectively.13,14 A brief, qualitative comparison with the current investigation of gasification under 20% CO2 follows. The devolatilization phase was similar for similar temperatures under all atmospheres tested, with a characteristic sharp reduction in the relevant O2 or CO2 signal. In all cases, the duration of the devolatilization phase decreased with increasing temperature and the total duration was 10−20 s. The initial devolatilization phase, and hence the release of Na and Cl species, is therefore considered to be essentially independent

4. CONCLUSION In this work, the influence of temperature on the release of Na-, K-, S-, and Cl-species on gasification of selected Victorian brown coals and Rhenish lignites was investigated. The coals were gasified under an atmosphere of 20% CO2 in He over the temperature range 1100 to 1400 °C, and the evolved species of interest were measured by molecular beam mass spectrometry. The key chemical species studied were 34H2S+, 35Cl+, 36HCl+, 39 + 39 K / NaO+, 58NaCl+, 60COS+/60NaCl+, 64SO2+, and 74KCl+. 6296

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In addition, 17OH+, 30CO+, and 46CO2+ signals were monitored and recorded. The results were compared with previous investigations conducted under 20% O2 and 7.5% O2. The measured intensity−time release profiles showed two distinct phases, pyrolysis and char gasification, with the majority of the volatile inorganic species released during the initial pyrolysis phase regardless of atmosphere. The presence and partial pressure of O2 or CO2 in the gasification environment were shown to affect not only the heterogeneous reaction with the solid carbonaceous material but also the secondary heterogeneous and homogeneous reactions with inherent and intermediate inorganic constituents, thereby affecting the final inorganic gas phase composition. The amount of HCl, NaOH, and NaCl released is dependent on both temperature and inherent Al and Si content of the coals. The mechanism involves the capture of Na species in the gas phase by silica and aluminosilicates in the solid phase, and the temperaturedependent equilibrium between NaOH and NaCl. The final equilibrium concentrations and time of release of H2S, COS, and SO2 are dependent on temperature, atmosphere, and inherent Ca content of the coals. There is a delay in the release of COS and SO2, which was explained by the proposed release mechanism. Gaseous H2S is released from the coal and captured by CaO in the solid phase as CaS. The CaS is converted to CaSO4 by reaction with CO2 in the atmosphere and finally decomposed to SO2 at high temperatures.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +49-2461-61-1574. Fax: +49-2461-61-3699. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge Brown Coal Innovation Australia (BCIA), Bundesministerium für Wirtschaft und Technologie (FKZ 0327773C), the Go8-DAAD Joint Research Scheme, and the Australian Research Council LIEF scheme (LE120100141) for providing the funding for this collaborative research project.

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