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Influence of Steam, Hydrogen Chloride, and Hydrogen Sulfide on the Release and Condensation of Zinc in Gasification Maria Benito Abascal,*,† Marc Blas̈ ing,† Yoshihiko Ninomiya,‡ and Michael Müller† †

Institute of Energy and Climate Research (IEK-2), Forschungszentrum Jülich GmbH, Leo-Brandt-Straße, 52425 Jülich, Germany Department of Applied Chemistry, Chubu University, 1200 Matsumoto-Cho, Kasugai, Aichi 487-8501, Japan

‡

ABSTRACT: The aim of this work was to assess the influence of H2O, HCl, and H2S on the condensation and release of zinc. The condensation behavior of the zinc vapors was investigated in a heated flow channel reactor housed in a furnace with a gas cooling zone, where eight glass filters were placed on different cooling stages. The metal species deposited in the filters were determined by means of ion chromatography (IC) and inductively coupled plasma optical emission spectroscopy (ICP-OES). Experiments on the release of the inorganic vapors were carried out in a heated flow channel reactor coupled to a molecular beam mass spectrometer (MBMS). The experiments were carried out under two typical gasification conditions (H2/H2O/He or Ar) and atmospheric pressure with 50 and 500 ppmv of HCl and H2S. Hot gas analysis was done at 900, 800, 700, and 500 °C. The condensation experiments showed that zinc condensed as ZnO, ZnCl2, and ZnS under the conditions considered. The species detected in the gas phase were Zn, ZnO, ZnCl2, and ZnS. The experimental results were compared with Scheil−Gulliver cooling calculations carried out with FactSage 6.3 software. This model was proven to be an excellent tool for the prediction of the behavior of zinc under gasification conditions. Furthermore, the global kinetics of the condensation experiments was clarified with the thermodynamic pseudoequilibrium model recently developed by researchers of Chubu University (Japan). With this work, not only a good understanding of the behavior of the zinc under gasification conditions was obtained, but also the finding and evidence of a powerful tool for predicting fast and easily the behavior of trace metals under gasification conditions. metals.6−9 The majority of these investigations have, however, been carried out under combustion conditions and not under coal gasification conditions.10,11 The behavior of volatile species in the reducing conditions of gasification might be different to that ones that occur during combustion.12 Recent investigations, run under gasification conditions, have demonstrated that water vapor as well as the trace gases HCl and H2S affect the condensation and release of the trace metal cadmium significantly.13 Trace elements can be classified into three groups according to their partitioning during coal combustion and gasification.14 Zinc is an element that belongs to Group 2 and it is considered an element of moderate concern with respect to coal utilization. It is found in concentrations that vary from 5 to 300 ppmv in coal, soil, shale, and crust.14 According to recent investigations based on thermodynamic equilibrium calculations, zinc is an element that volatilizes in the gasifier and can be totally or partially in the gas phase in cleaning conditions.12 Thus, zinc-loaded particles may escape actual particulate control systems and constitute the source of a health hazard.15 To verify the results of the equilibrium calculations, the behavior of zinc under gasification conditions was

1. INTRODUCTION According to recent studies, fossil fuels will continue being the primary source of energy until the year 2035.1 Coal reserves are more abundant than those of other fossil fuel and its supply is relative secure. Global coal demand will increase by 15% to 2040, but the highest increase will occur in the next 10 years.2 However, it is already well-known that this fossil fuel contains trace metals of particular environmental concern that are released during the gasification process. These trace metals are emitted into the atmosphere and also contribute to technical problems in the gasifier unit and downstream processes, decreasing the efficiency and causing technological problems in gasification plants.3 Thus, the development of clean coal technologies is of vital importance for the future use of coal. The integrated gasification combined cycle (IGCC) with gas purification and CO2 capture has been proven to be efficient and commercially viable in Buggenum, Spain, and The Netherlands.4 The development of hot fuel gas cleaning systems can increase the efficiency of power plants and can reduce the release and condensation of trace metals in gasification processes.5 However, efficient clean up techniques to eliminate or reduce trace metals can only be developed once the hot gas chemistry during the gasification process is well understood. Several groups have investigated and affirmed that chlorine, sulfur, and water vapor influence the behavior of trace © 2016 American Chemical Society

Received: Revised: Accepted: Published: 6911

April 28, 2016 June 3, 2016 June 3, 2016 June 4, 2016 DOI: 10.1021/acs.iecr.6b01637 Ind. Eng. Chem. Res. 2016, 55, 6911−6921

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Industrial & Engineering Chemistry Research experimentally studied. Therefore, in this paper, the influence of H2O, HCl, and H2S on the release and condensation of zinc was investigated. A better understanding of the behavior of zinc under gasification conditions is essential for the future development of cleaning systems that reduce its emissions into the environment.

Table 2. Overview of the Experimental Conditions in the Condensation and Gas Phase Experiments experimental run influence of H2O influence of H2O influence of HCl influence of HCl Influence of H2S influence of H2S

2. EXPERIMENTAL SETUP AND THERMODYNAMIC CALCULATIONS 2.1. Experimental Setup. To carry out the experiments two different setups were utilized. An atmospheric flow channel reactor with a cooling zone was used for the condensation experiments and an atmospheric flow channel reactor coupled to an MBMS system was used for the release experiments. The experimental setups have been described comprehensively recently.13 In this paper only a short description of the experimental setups and the experimental conditions will be given. The atmospheric conditions considered were a gas composition of a coal gasifier and a gas composition before the water−gas shift reaction (WGSR) with a steam to CO ratio of 1.5 (see Table 1). However, the search of new atmospheric

H2 (%)

H2O (%)

CO (%)

CO2 (%)

water gas shift (real) gasification (real) water gas shift (lab) gasification (lab)

15 29 14 28

51 2 17 3

34 66

3

H2O (%)

Ar (%)

HCl (ppmv)

H2S (ppmv)

water gas shift gasification

14

17

69

0

0

28

3

69

0

0

gasification

28

3

69

50

0

gasification

28

3

69

500

0

gasification

28

3

69

0

50

gasification

28

3

69

0

500

program predicted a temperature of 400 °C in order to evaporate 100 ppmv of zinc under the experimental conditions. This temperature was experimentally proven by evaporating zinc at different temperatures and determining the amount of zinc that went to the gas phase. The duration of the tests was 24 h. Considering the weight loss of zinc during this time, the total amount of zinc evaporated could be calculated. The zinc source for all the experimental runs was metallic zinc. The results showed that at 492 °C the concentration of zinc obtained was 100 ppmv. Although this temperature was higher than the predicted temperature due to kinetics, the sample boat containing the metal was positioned in the experiments at the position of the outer tube where the temperature was 492 °C. The exact concentration of zinc in each experiment was calculated from the weight loss of zinc during the respective experimental runs. 2.1.1. Condensation Experiments. A schematic of the experimental setup used for the condensation experiments is given in Figure 1. The detailed description of this apparatus has been given elsewhere.13 The reaction zone had two parts: the inner tube and the outer tube. Both were made of quartz glass. The inner tube was necessary for introducing the simulated gasification product gas (H2 and Ar) and the zinc source into the experiments. This tube had a 5 cm gap in order to hold the sample boat containing the zinc source. The outer tube, however, was used for introducing the water vapor and the trace gases HCl and H2S into the atmosphere. Two gas atmospheres with two different amounts of water vapor were considered (14% H2/ 17% H2O/69% Ar and 28% H2/3% H2O/69% Ar), as it was been explained before. The total gas flow in the condensation experiments was 510 mL/min and the duration of each experimental run was 72 h. Eight quartz glass filters were placed at different positions along the cooling zone for the deposition of the condensable compounds during the experiments. These filters were filled with quartz rings and had a length of 10 cm. The temperature, position of each filter in the cooling zone as well as the position at which the zinc source was introduced in the experiments is shown in Figure 2. The outer tube as well as the eight filters present in the cooling zone were analyzed after each experiment. The metal species deposited were dissolved and determined by means of ion chromatography (IC) and inductively coupled plasma optical emission spectroscopy (ICP-OES). The relative error of the ICP-OES and IC analysis was ±5−20%. Furthermore, the relative error of the separation methods was approximately

Table 1. Overview of the Experimental Conditions That Most Closely Correspond to the Real Conditions atmosphere

H2 (%)

atmosphere

Ar (%)

69 69

conditions that replace CO by Ar or He, was necessary. The socalled lab conditions would avoid the utilization of CO in the laboratory. For these laboratory conditions the same hydrogen content and oxygen partial pressure as after entrained flow gasification of coal and in the water gas shift reactor was considered. These conditions were obtained by curve fitting the oxygen partial pressure evolution of the real conditions with CO in FactSage 6.3. This simplification was validated by carrying out a set of equilibrium calculations with FactSafe 6.3 under the real laboratory conditions and new laboratory conditions. In these calculations the same species were found and the simplification was considered as valid. Therefore, the so-called laboratory conditions without CO were the experimental conditions for all the experimental runs carried out in this study. The experiments were carried out under two typical gasification conditions (H2/H2O/He or Ar) and atmospheric pressure with different concentrations of HCl and H2S. Concentrations of 50 and 500 ppmv of the trace gases in the experiments were considered. An overview of the atmospheric conditions in the experiments is given in Table 2. Hot gas analysis was done by MBMS at four different temperatures at the end of the reactor. 900, 800, 700, and 500 °C were considered. In these experiments, argon was replaced by helium, since its low atomic mass leads to high signal intensities in the MBMS.16 The desired average zinc concentration in the experiments was 100 ppmv. This is an average concentration of the common content of trace metals in coal (0.1 to 300 ppmv). The necessary temperature for the vaporization of 100 ppmv of Zn in the experiments was calculated with FactSage 6.3. The 6912

DOI: 10.1021/acs.iecr.6b01637 Ind. Eng. Chem. Res. 2016, 55, 6911−6921

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Industrial & Engineering Chemistry Research

Figure 1. Schematic of the experimental setup for the condensation experiments.

Figure 2. Temperature profile in the condensation experiments.

±0−20%. There was a small gap between the filters and the outer tube. That explains that a part of the zinc compounds condensed in the outer tube, and this fraction is represented in the graphs in an extra column. The fraction that condensed in the outer tube predominantly corresponded to fractions that condensed at temperatures where the majority of the zinc species condensed. The systematic procedure to separate the metal compounds in the filters was as follows. ZnCl2, ZnO, and ZnS were dissolved in water, acetic acid (30%), and hydrochloric acid (25%), respectively. The determination of Zn2+ and Cl− by ICP-OES and IC of the solutions allowed the quantification of the zinc compounds in every filter. The filters were weighed before, after the experiments and after dissolving the condensate. Thus, the results of the analyses were compared with the weight gain of the filters during the experiments. The relative variance between the elemental analysis and the gained weight of the filters during the experiments was determined in the range −6.3% to +5.2%. 2.1.2. Release Experiments. The experimental setup for the gas phase experiments was similar to the experimental setup for the condensation experiments. However, as the main goal of these experiments was to determine the species present in the gas phase and their behavior under different experimental conditions, it did not have a cooling zone. The experimental setup only consisted of a reaction zone. Furthermore, the end of the outer tube was coupled to a MBMS in order to analyze the gas in situ. The MBMS instrument has been explained in

more detail in recent publications.17,18 The gas flow in the experiments was 4300 mL/min. Each experiment was carried out four times, each with different temperatures at the end of the reactor. The experiments were conducted at 900, 800, 700, and 500 °C. In the same way as in the condensation experiments, the concentrations of HCl and H2S considered were 0, 50, and 500 ppmv. 2.2. Thermodynamic Calculations. 2.2.1. Scheil−Gulliver Cooling Calculations. Cooling calculations were performed in order to evaluate the condensation and release behavior of zinc under the gasification conditions considered in this study. The aim of using the cooling calculations was to assess the experimentally data obtained in the experiments, in other words, to validate the use of the cooling calculations to predict the behavior of zinc under gasification conditions. The calculations were performed using the Scheil−Gulliver cooling model of FactSage 6.3 Software. The nonequilibrium Scheil− Gulliver Cooling tool was previously proved to be an excellent tool for predicting the condensation and release of cadmium under the same atmospheric conditions.13 This model has been used and explained in detail in some other recent research works.13,19,20 The Equilib module of FactSage enables the performance of cooling calculations and displays the phase transitions and compositions during equilibrium cooling and Scheil−Gulliver cooling. With this module it is possible to calculate all equilibrium (or Scheil−Gulliver nonequilibrium) phase tran6913


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Industrial & Engineering Chemistry Research sitions as a multicomponent mixture is cooled.21,22 This model assumes a nonequilibrium solidification path calculating a sequence of equilibrium states. The calculations consider the composition of the gas phase given at high temperature and then the model decreases the temperature step by step. At every step the equilibrium is calculated and all condensed phases are removed from the calculation. The remaining gas phase is in the next incremental step cooled and the process is repeated.19 The calculations to determine the behavior (temperature and speciation) of zinc were carried out from 1000 °C to room temperature at decreasing temperature steps of 25 °C. The atmospheric conditions of the experiments were the input data for the calculations. Atmospheric pressure was considered. As well as in the experiments the influence of H2O, HCl, and H2S on the release of gaseous species and condensation behavior of the condensable metal species was determined. The results of the calculations are presented as the speciation of the condensed and gaseous phase versus temperature. The results of the calculations will be compared with the experimental results in the Summary and Discussion section. 2.2.2. Thermodynamic Pseudoequilibrium Model. To be able to understand the condensation mechanisms of metal species vapors upon flue gas cooling, a thermodynamic pseudoequilibrium model was developed using ChemApp linked with the FactSage 6.2 database. This model was recently developed by researchers of Chubu University (Japan). The detailed description of this model has been given elsewhere.7,8 With this model is possible to predict the condensation behavior of metal vapors under the influence of cooling rate and chemical reaction control.7−9 In this work this model was applied to the atmospheric conditions of the condensation experiments in order to verify its validity under gasification conditions. This model was proven in recent work to be a good tool for providing important information about the global kinetics in the condensation of cadmium under gasification conditions.13 The schematic of the model is shown in Figure 3.

compounds of the condensation experiments were taken into account. The temperatures considered were an arithmetic mean of the temperature ranges of each filter. A temperature range from 1000 °C to room temperature with an interval of 100 °C and atmospheric pressure was considered in the calculations.

3. RESULTS 3.1. Condensation of Zn Species. 3.1.1. Condensation Experiments. Figure 4 shows the condensation distribution of

Figure 4. Condensation distribution of zinc in the outer tube and in the filters placed along the cooling zone in an atmosphere containing 3% vol water vapor (left bar) and 17% vol water vapor (right bar).

zinc in the filters and in the outer tube when the atmosphere contained H2O(g). The bars on the left for each temperature show the results obtained when the atmosphere contained 3% volume H2O and the bars on the right the results with 17% volume H2O. The results show under both experimental conditions that zinc condensed entirely as ZnO at temperatures below 886 °C. With 3% vol of water ZnO condensed in the second (735−577 °C) and third (577−438 °C) filters. However, with 17% volume of water, ZnO began to condense in the first filter (886−735 °C). The results clearly show that an increase of the water vapor induced the condensation of ZnO at slightly higher temperatures. Figure 5 shows a comparison between the condensation behavior of zinc when the atmosphere did not contain HCl and when it contained 50 and 500 ppmv HCl. In these experiments the amount of water did not vary and was 3% vol. The bars on the left for every temperature show the results obtained when the atmosphere contained 0 ppmv HCl. The bars in the middle represent the results with 50 ppmv HCl, and the bars on the right represent the results with 500 ppmv HCl. The introduction of HCl in the atmosphere promoted the condensation of zinc as a mixture of ZnO and ZnCl2. ZnO condensed generally at higher temperatures (735−163 °C) than ZnCl2 (577−163 °C). In an atmosphere containing 50 ppmv HCl, ZnO condensed in the second and third filter (735−438 °C). ZnCl2 condensed in the third and fourth filter (577−325 °C). When the atmosphere contained 500 ppmv HCl, ZnO condensed predominantly in the third and fourth filter (577−325 °C). ZnCl2 could only be detected in the fifth filter (325−163 °C).

Figure 3. Schematic of the thermodynamic pseudoequilibrium model. Figure reproduced with permission from ref 7. Copyright Elsevier 2011.

In brief, a variable, α, is used to describe the proportion of a metallic vapor that does not condense on a certain cooling stage, which is named supercooling. A rapid cooling of flue gas, “super-cooling”, can cause supersaturation of a vapor causing only a little deposition. An alpha value of zero at a cooling stage shows that all the species have reached the equilibrium. That means that there is not supercooling in that cooling stage and that it does not have a thermodynamic reaction control. Thus, alpha reflects the effect that a rapid cooling has on the condensation of the metal vapor species. The alpha value was determined by curve fitting the results of the condensation experiments. As input for modeling the flue gas compositions and the amount of condensed metal 6914


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Figure 5. Condensation distribution of zinc in the outer tube and in the filters placed along the cooling zone in an atmosphere containing 0 ppmv HCl (left bar), 50 ppmv HCl (middle bar), and 500 ppmv HCl (right bar).

Figure 6. Condensation distribution of zinc in the outer tube and in the filters placed along the cooling zone in an atmosphere containing 0 ppmv H2S (left bar), 50 ppmv H2S (middle bar), and 500 ppmv H2S (right bar).

As it can be seen in the figure, a larger amount of HCl promoted the condensation of ZnO at lower temperatures. With 500 ppmv HCl a large amount of ZnCl2 condensed in the tube. This amount corresponded to ZnCl2 that condensed in the third and fourth filter. Therefore, this species condensed in both cases at the same temperatures. Thus, it can be concluded that the condensation behavior of ZnCl2 did not seem to be affected by the amount of HCl. The only observed difference was that the zinc amount which condensed as ZnCl2 was a little higher with 500 ppmv than with 50 ppmv HCl. The total amount condensed as ZnCl2 with 50 ppmv and 500 ppmv was 22.8% and 24% of the total zinc species condensed in each experiment. The condensation distribution of zinc when the gas atmosphere contained the trace gas H2S is depicted in Figure 6. A comparison between the condensation behavior of zinc when the atmosphere did not contain H2S and when it contained 50 and 500 ppmv H2S is shown. As well as in the experiments with HCl, the amount of water considered was 3% vol. The introduction of H2S in the atmosphere caused zinc to condense as a mixture of ZnO and ZnS. Both components condensed at 735−438 °C. ZnO condensed in the second (735−577 °C) and third filter (577−438 °C) under both experimental conditions. With 50 ppmv of H2S in the atmosphere ZnS also condensed in the second and third filter while with 500 ppmv of H2S, it predominantly condensed in the second filter. The total amount of zinc that condensed as ZnS did not vary much. It was in both cases approximately 10% of the total zinc condensed. In summary, the increase of the H2S concentration promoted the condensation of ZnS at higher temperatures, causing ZnS to condense at higher temperatures when higher amounts of H2S were present in the atmosphere. On the other hand, the amount of H2S did not seem to affect the condensation temperature of ZnO. This species condensed at 735−438 °C regardless of the amount of H2S.

3.1.2. Scheil−Gulliver Cooling Calculations for the Solid Phase. The condensation calculations performed with the Scheil−Gulliver cooling model of FactSage 6.3 software are presented in Figure 7. The graphs show the molar amount of zinc species that condensed at each temperature. As in the condensation experiments, the influence of H2O, HCl, and H2S in the condensation behavior of zinc was studied. A comparison between the results with 3% vol water and the results with 17% vol water is presented in Figure 7a. The model calculations showed that under both atmospheric conditions zinc condensed as ZnO. When the atmosphere contained 3% vol of H2O, ZnO condensed at temperatures between 640 and 450 °C. However, when the amount of introduced water was 17% vol, ZnO began to condense at 725 °C. In summary it can be said that an increase of the water vapor induced the condensation of ZnO at higher temperatures. Figure 7b shows the results obtained from the Scheil− Gulliver cooling model when 50 and 500 ppmv of HCl were present in the atmosphere. In this case, the results obtained with 50 ppmv of HCl are depicted with a solid line and the results obtained when the atmosphere contained 500 ppmv of HCl are depicted with a dashed line. The cooling calculations show that zinc condensed as a mixture of ZnCl2 and ZnO when HCl was introduced into the experiment. With 50 ppmv of HCl, the simulation predicted the deposition of ZnO at temperatures between 640 and 450 °C. No ZnCl2 condensed under these atmospheric conditions. The calculations showed the deposition of ZnO and ZnCl2 when 500 ppmv of HCl were in the atmosphere. The condensation temperature of this species was 625−275 °C for ZnO and 280− 200 °C for ZnCl2. The amount of zinc condensed as ZnCl2 was very low in comparison to the ZnO deposit. In general, an increase of the HCl amount led to the deposition of ZnO at lower temperatures. In addition, a higher amount of HCl caused zinc to condense as ZnCl2. With 50 ppm of HCl in the atmosphere, the amount of zinc that condensed as ZnCl2 was insignificant. The amount of zinc 6915


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as ZnS at slightly higher temperatures (790−600 °C) than with only 50 ppmv of H2S. The amount of ZnS condensed increased too. Summing up, the increase of H2S shifted the condensation of ZnS to higher temperatures. The condensed amount of ZnS also increased in an atmosphere containing more ppmv of H2S. 3.1.3. Thermodynamic Pseudoequilibrium Model. Figure 8 shows the comparison between the model calculations and the experimental results of the condensation of zinc in an atmosphere containing H2O (3 and 17% vol), HCl (50 and 500 ppmv), and H2S (50 and 500 ppmv). The curve fitting of the experimental results indicate an alpha value of zero for all experimental conditions except for the experiment with 500 ppmv of H2S, where an alpha value of 0.6 fit well the experimental results. As it has been explained before, an alpha value of zero assumes that the species have reached the equilibrium and therefore there is not any reaction or cooling control. In these cases the model calculations predicted very well the experimental results. In the case with 500 ppmv of H2S, as it can be observed in Figure 8 the deposition fraction predicted by the model with an alpha value of zero was far higher than that observed in the experimental results. Thus, it was reasonable to consider that supercooling plays an important role in the partitioning of zinc vapors in this case. Due to it, a portion of the ZnO and ZnS vapors were possible saturated and they barely deposited at high temperatures. They quickly diffused across the high temperature cooling zone in gas form and preferred to condense at lower temperatures. According to the model, the influence of supercooling can be balanced by the presence of H2S and H2O, which causes the condensation of metal vapors at higher temperatures due to the formation of sulfides. Thus, the deposition of heavy metal vapors at high temperature is favored by H2S and H2O. Consequently, formation of new compounds through chemical reactions in flue gas is an important factor that affects the condensation distribution and can greatly compensate the negative effect of supercooling. However, as only the experiment with 500 ppmv of H2S was influenced by supercooling, any firm conclusions cannot be drawn about the influence of the chemical reactions in the condensation behavior of zinc. 3.2. Release of Zn Species. The influence of water vapor, HCl, and H2S on the gas phase speciation of zinc was investigated by MBMS. Experiments were conducted at 900, 800, 700, and 500 °C at the end of the furnace, because the Scheil−Gulliver calculations predicted important variations in the behavior of zinc in this temperature range. As the mass spectrometer only determines the gaseous ionized species based on their mass-to charge ratios (m/z), it was necessary to carry out a calibration before starting the experiments. With this calibration it was possible to correlate the intensities of the isotopes present in the experiments with the trace metal vapor concentration (ppmv) obtained. In the experiments the monitored species were 64Zn+, 64 Zn16O+, 64Zn35Cl2+, and 64Zn32S+. In the experiments, some difficulties were found in measuring the total Zn(g) present in the gas phase. A share of zinc was remaining in the MBMS. For that reason the total amount of zinc in the gas phase could not be measured accurately. Therefore, only the concentration of the minor species present in the gas phase is shown in the results. However, it should be highlighted that Zn(g) was the major species present in the gas phase in all experiments. Furthermore, due to the strong temperature gradient in the

Figure 7. Speciation of the condensed phase during cooling from 1000 to 0 °C predicted by the Scheil−Gulliver cooling model in an atmosphere containing (a) 3% vol water vapor (solid line) and 17% vol water vapor (dashed line); (b) 50 ppmv HCl (solid line) and 500 ppmv HCl (dashed line); (c) 50 ppmv H2S (solid line) and 500 ppmv H2S (dashed line).

condensed as ZnCl2 with 500 ppmv HCl was appreciable although the amount of ZnO was much higher than the amount condensed as ZnCl2. The cooling calculations for 50 ppmv H2S and 500 ppmv H2S introduced into the atmosphere containing zinc are shown in Figure 7c. The cooling calculations predicted the condensation of zinc as a mixture of ZnO and ZnS. As it can be observed in the graph, when the atmosphere had 50 ppmv of H2S, zinc condensed as ZnO at 620−450 °C and ZnS at 725−625 °C. The amount deposited of both species was similar. When the atmosphere contained 500 ppmv of H2S, zinc only condensed 6916


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Figure 8. Comparison between the calculated and the experimental results of the condensation of cadmium vapors in the presence of H2O (3−17% vol), HCl (50−500 ppmv), and H2S (50−500 ppmv).

Table 3. Concentration of Minor Gaseous Zinc Species (ppmv) at 900, 800, 700, and 500 °C When the Atmosphere Contained Water Vapor (3−17% vol), HCl (50−500 ppmv), and H2S (50−500 ppmv). Note that Zn(g), the Major Species in the Gas Phase, Is Not Shown Minor gaseous Zn-species (ppmv) 3% H2O experimental run 900 800 700 500

°C °C °C °C

17% H2O

50 ppmv HCl

500 ppmv HCl

50 ppmv H2S

500 ppmv H2S

ZnO

ZnO

ZnO

ZnCl2

ZnO

ZnCl2

ZnO

ZnS

ZnO

ZnS

112.0 103.2 25.4 14.8

138.6 35.2 18.4 18.4

114.0 33.0 25.0 11.0

167.0 167.0 115.0 115.0

54.0 54.0 47.0 24.0

375.0 375.0 375.0 167.0

68.0 68.0 61.0 40.0

122.0 68.0 68.0 41.0

68.0 68.0 11.0 11.0

128.0 48.0 41.0 27.0

3.2.1. Release Experiments. The results of the MBMS measurements are provided in Table 3. The table shows an overview of the relative concentrations (ppmv) of all the minor zinc species detected in the gas phase at 900, 800, 700, and 500 °C. The release experiments showed that Zn and ZnO were the only compounds present in the gas in the experiments with H2O. The amount of Zn present in the gas phase was much larger than the amount of ZnO in every experiment. The amount of ZnO was similar at 900 and 800 °C, but decreased

reaction zone of the furnace, the concentration of zinc evaporated from the sample boat varied from experiment to experiment. Therefore, only a qualitative study of the gas species could be carried out. The main target of these experiments was the determination of the zinc species in the gas phase under different experimental conditions. As in the condensation experiments, the results of the gas phase experiments were compared with the results obtained with the Scheil−Gulliver calculations for the gas phase. 6917


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Industrial & Engineering Chemistry Research significantly at 700 and 500 °C when 3% vol of water vapor was introduced into the outer tube. With 17% vol of water vapor the concentration of ZnO decreased abruptly at 800 °C and then remained constant. As it can be observed in Table 3 under both experimental conditions the concentration of ZnO increased significantly with the temperature. Furthermore, it seems that the concentration of ZnO was slightly higher with 17% vol of H2O than it was with 3% vol of H2O, especially at 900 °C. The release experiments with HCl showed that zinc was present in the gas phase as a mixture of Zn, ZnCl2, and ZnO. The amount of Zn present in the gas phase was much larger than the amount of ZnO and ZnCl2 in every experiment. In both experiments the amount of ZnO increased with the temperature. Besides, the concentration of ZnCl2 was in both cases larger than the one of ZnO. With 50 ppmv of HCl the amount of ZnCl2 increased until 800−900 °C, where the concentration was constant. In the case with 500 ppmv of HCl, the concentration of ZnCl2 was constant at temperatures from 700−900 °C. Thereby, the only difference between both experiments was the concentration of ZnCl2. In general, with 500 ppmv of HCl, the concentration of ZnCl2 was much larger. The release experiments with H2S showed that zinc was present in the gas phase as a mixture of Zn, ZnS, and ZnO. Zn was the major species present in the gas phase in every experiment. In both experiments the concentration of ZnS and ZnO showed a gradual increase with the temperature. In general, the concentrations of ZnS were higher than the concentrations of ZnO. The only difference between the results of both experiments was that at the lowest temperatures (500− 800 °C) the concentrations of ZnO and ZnS with 500 ppmv of H2S were very low in comparison with the concentrations obtained with 50 ppmv of H2S in the atmosphere. This was a result of the different total zinc vaporized (ppmv) in every experiment. 3.2.2. Scheil−Gulliver Cooling Calculations for the Gas Phase. The gas phase composition regarding zinc species predicted by the Scheil−Gulliver cooling model is shown in this section. The calculations show the speciation of the main inorganic gaseous species at temperatures between 1000 and 0 °C under the influence of H2O, HCl, and H2S. The speciation of the gas phase predicted by the Scheil− Gulliver cooling calculations in an atmosphere containing water vapor and zinc is shown in Figure 9. The graph shows a comparison between the results obtained with 3 and 17% vol water vapor in the atmosphere. According to the calculations, Zn(g) was the only gas species that appeared at high concentrations at the temperature range considered. Zn was present between 1000 and 280 °C. The only appreciable difference between both atmospheres was that the concentration of zinc when the atmosphere contained 17% vol of H2O started to decrease at slightly higher temperatures. ZnO appeared in the gas phase at 1000−570 °C. However, its concentration was lower than 1 ppb (log10 (act) ≈ 10−9) and as it was out of the range of the concentrations mainly addressed in this study, it was not considered. The gas phase composition of zinc when 50 and 500 ppmv of HCl were present in the atmosphere predicted by the Scheil− Gulliver cooling model is depicted in Figure 10. The main inorganic species present in the gas phase when the atmosphere had HCl were Zn and ZnCl2. ZnO was present at very low concentrations, as in the case with 3−17% vol of H2O mentioned above. The concentration of ZnCl2 remained

Figure 9. Gaseous zinc-containing species versus temperature in an atmosphere containing 3% vol water vapor (solid line) and 17% vol of water vapor (dashed line) calculated by the Scheil−Gulliver cooling model.

Figure 10. Gaseous zinc-containing species versus temperature in an atmosphere containing 50 ppmv HCl (solid line) and 500 ppmv HCl (dashed line) calculated by the Scheil−Gulliver cooling model.

constant between 650 and 200 °C. At temperatures higher than 650 °C the concentration of ZnCl2 decreased with the temperature. Zn was only present at high concentrations at 1000−650 °C. An increase in the HCl concentration only influenced the behavior of ZnCl2. The concentration of ZnCl2 when 500 ppmv of HCl was in the atmosphere was a little higher than with only 50 ppmv of HCl. Figure 11 shows the release behavior of zinc predicted by FactSage when H2S was introduced into the atmosphere. A comparison between the results with 50 ppmv H2S and 500 ppmv H2S is shown. The calculations showed that Zn, ZnO, and ZnS were present in the gas phase when the atmosphere contained H2S. The concentrations of ZnO and ZnS were less than 1 ppb and therefore, not considered. The concentration of Zn increased with the temperature; 100 ppmv of Zn was present between 1000 and 620 °C when the atmosphere had 50 ppmv of H2S. However, in an atmosphere containing 500 ppmv of H2S, high concentrations of Zn were in the gas phase only until 780 °C. 6918


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0.6 fit the experimental results nicely. ZnO and ZnS were possibly saturated and barely deposited at high temperatures. They diffused across the cooling zone in gas form and condensed at low temperatures. Although it is reasonable to think that supercooling affected the condensation of the experiment with 500 ppmv of H2S, no conclusions could be drawn as this was the only experiment where supercooling affected potentially the behavior of zinc. In the cases with H2O, HCl, and 50 ppmv of H2S, the model was able to predict nicely the total condensation fraction of zinc with the temperature with an alpha value of zero. 4.2. Release Behavior of Zinc. In the release experiments some difficulties were found to measure the total Zn(g) present in the gas phase. Zinc was remaining inside the MBMS and the total amount of zinc could not be measured accurately. Therefore, only the concentrations of the minor species were shown in the results although Zn(g) was the major species present in the gas phase in all experiments. Furthermore, it should be emphasized that due to the strong temperature gradient in the furnace, the vaporization of the exact same amount of zinc in every experiment was not achieved. Therefore, the total concentration of zinc differed from experiment to experiment. That is why only a qualitative analysis could be achieved. The speciation and the behavior of each species present in the gas phase were determined. The release experiments showed that Zn and ZnO were the only compounds present in the gas phase in the experiments with H2O. Furthermore, the amount of Zn present was much larger than the amount of ZnO in every experiment. In the calculations Zn(g) was the only gas species that appeared in high concentrations at the temperature range considered. Although the model predicted a very low concentration of ZnO, this species was found in the gas phase in the experiments at ppm level. Excluding this difference the model was able to predict reasonably well the release behavior of zinc. The experiments with HCl showed that zinc was present in the gas phase as a mixture of Zn, ZnCl2, and ZnO. In both experiments the amount of ZnO increased with the temperature. Besides, the concentration of ZnCl2 was in both cases larger than the one of ZnO. The Scheil−Gulliver calculations showed that the main inorganic species present in the gas phase were Zn and ZnCl2. ZnO was present in the gas phase in concentrations smaller than those addressed in this investigation (1 ppb). This was the only appreciable difference between the experimental results and calculations. Nevertheless, the speciation and the behavior of all species were similar in the experiments and in the calculations. Thus, although some differences were found between the calculations and the experimental results, the Scheil−Gulliver calculations presented a similar tendency as observed in the experiments. The results of the experiments with H2S showed that zinc was present in the gas phase as a mixture of Zn, ZnS, and ZnO. The concentration of ZnS and ZnO showed a gradual increase with the temperature, and the concentration of ZnS was higher than the concentrations of ZnO. The Scheil−Gulliver calculations also showed that Zn, ZnO, and ZnS were present in the gas phase when the atmosphere had H2S. The behavior of ZnO and ZnS in the calculations and in the experiments was found to be similar. However, in the experiments the concentrations of these species were at ppm level and the concentrations of ZnO and ZnS were less than 1 ppb in the calculations. Thereby, although the experimental results were only qualitative and some discrepancies were found between

Figure 11. Gaseous zinc-containing species versus temperature in an atmosphere containing 50 ppmv H2S (solid line) and 500 ppmv H2S (dashed line) calculated by the Scheil−Gulliver cooling model.

4. SUMMARY AND DISCUSSION In this section a comparison of the experimental results with the results of the cooling calculations and the thermodynamic pseudoequilibrium model is given. 4.1. Condensation Behavior of Zinc. Zinc condensed entirely as ZnO at temperatures between 735 and 438 °C when the atmosphere only contained water vapor. However, an increase of the water vapor amount induced ZnO to begin condensing at 886 °C. The Scheil−Gulliver calculations predicted the condensation of zinc as ZnO at 640−450 °C when the atmosphere contained 3% vol water vapor. However, the model predicted that the condensation of ZnO started at 725 °C when the water vapor amount was 17% vol. Therefore, it could be concluded that the Scheil−Gulliver calculations matched very well with the experimental results obtained when the atmosphere had water vapor. In the presence of HCl, zinc condensed as mixture of ZnO (735−163 °C) and ZnCl2 (577−163 °C). The condensation behavior of ZnCl2 was not affected by the amount of HCl. However, larger amounts of HCl lead ZnO to condense at lower temperatures (577−163 °C). The Scheil−Gulliver calculations also predicted the condensation of ZnO at lower temperatures when the amount of HCl increased. However, the model did not predict the condensation of ZnCl2 with 50 ppmv HCl in the atmosphere. Despite this difference the model fit very well with the experimental results. Overall, it could be concluded that the cooling calculations predicted the condensation behavior of zinc properly when HCl was present in the atmosphere. In the experiments containing H2S, zinc condensed as a mixture of ZnO and ZnS. Both condensed at 735−438 °C. A high amount of H2S in the atmosphere caused ZnS to condense at slightly higher temperatures. Nevertheless, the condensation of ZnO was not affected by H2S. The cooling calculations predicted quite well the results with 50 ppmv of H2S. However, with 500 ppmv of H2S in the atmosphere the model did not predict the condensation of ZnO as happened in the experiments. Without considering the appearance of ZnO in the deposits with 500 ppmv of H2S, the model predicted the experimental results quite good. According to the results of the thermodynamic pseudoequilibrium model all experiments were in equilibrium except the experiment with 500 ppmv H2S. In this case an alpha value of 6919


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Industrial & Engineering Chemistry Research Notes

the model and the experimental results, it could be concluded that the Scheil−Gulliver model predicted the experimental results obtained quite well. In general, a good agreement between the cooling calculations and the experimental results was found. The speciation as well as the temperatures at which the species were released or condensed under the influence of H2O, HCl, and H2S showed the same trends in the calculations and in the experimental results. However, in some cases small differences were found. These differences could be explained partly by the difficulty of evaporating the same metal concentration in every experiment. Nevertheless, the Scheil−Gulliver calculations can be considered a good tool for the prediction of the condensation and release behavior of zinc. 4.3. Practical Implications. According to this study, Zn species will be present in the gasifier as a mixture of gaseous and solid compounds. Zinc species are moderate volatile and will condense at relatively high temperatures. Special attention should be given to atmospheres where the formation of ZnO and ZnS in high concentrations is possible. ZnO and ZnS will condense at temperatures below 735 °C. However, ZnCl2 condenses at temperatures below 577 °C. Thus, ashes collected from the gasifier could content high amounts of Zn compounds. The deposition of Zn will depend on the concentration of the trace gases HCl and H2S and the water vapor.

The authors declare no competing financial interest.

ACKNOWLEDGMENTS

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REFERENCES

The work described in this paper was supported by Bundesministerium für Wirtschaft und Technologie in the framework of the HotVeGas-EM project (FZK 0327773F) and by Deutsche Forschungsgemeinschaft (DFG) with Grant No. BL 1363/1-1.

(1) BP Energy Outlook 2035. http://www.bp.com/content/dam/ bp/pdf/energy-economics/energy-outlook-2016/bp-energy-outlook2014.pdf (accessed January 2014). (2) OECD/IEA. World Energy Outlook; International Energy Agency: Paris, 2014. (3) Bakker, W. High Temperature Corrosion in Gasifiers. Mater. Res. 2004, 7, 53. (4) Higman, C.; Van Der Burgt, M. Gasification, 2nd ed.; Gulf Professional Publishing: 2008. (5) Müller, M. Integration of Hot Gas Cleaning at Temperatures Above the Ash Melting Point in IGCC. Fuel 2013, 108, 37. (6) Miller, B.; Dugwell, D. R.; Kandiyoti, R. The Influence of Injected HCl and SO2 on the Behavior of Trace Elements during Wood-Bark Combustion. Energy Fuels 2003, 17, 1382. (7) Jiao, F.; Cheng, Y.; Zhang, L.; Yamada, N.; Sato, A.; Ninomiya, Y. Effects of HCl, SO2 and H2O in Flue Gas on the Condensation Behavior of Pb and Cd Vapors in the Cooling Section of Municipal Solid Waste Incineration. Proc. Combust. Inst. 2011, 33, 2787. (8) Jiao, F.; Zhang, L.; Yamada, N.; Sato, A.; Ninomiya, Y. Effect of HCl, SO2 and H2O on the Condensation of Heavy Metal Vapors in Flue Gas Cooling Section. Fuel Process. Technol. 2013, 105, 181. (9) Jiao, F.; Zhang, L.; Song, W.; Meng, Y.; Yamada, N.; Sato, A.; Ninomiya, Y. Effect of Inorganic Particulates on the Condensation Behavior of Lead and Zinc Vapors upon Flue Gas Cooling. Proc. Combust. Inst. 2013, 34, 2821. (10) Querol, X.; Fernández-Turiel, J.; López-Soler, A. Trace Elements in Coal and their Behaviour during Combustion in a Large Power Station. Fuel 1995, 74, 331. (11) Reddy, M. S.; Basha, S.; Joshi, H. V.; Jha, B. Evaluation of the Emission Characteristics of Trace Metals from Coal and Fuel Oil Fired Power Plants and their Fate during Combustion. J. Hazard. Mater. 2005, 123, 242. (12) Díaz-Somoano, M.; Martínez-Tarazona, M. R. Trace Element Evaporation during Coal Gasification based on a Thermodynamic Equilibrium Calculation Approach. Fuel 2003, 82, 137. (13) Benito Abascal, M.; Bläsing, M.; Ninomiya, Y.; Müller, M. Influence of Steam, Hydrogen Chloride, and Hydrogen Sulfide on the Release and Condensation of Cadmium in Gasification. Energy Fuels 2016, 30, 943. (14) Clarke, L. B. The fate of Trace Elements during Coal Combustion and Gasification: an Overview. Fuel 1993, 72, 731. (15) Shi, G. L.; Lou, L. Q.; Zhang, S.; Xia, X. W.; Cai, Q. S. Arsenic, Copper, and Zinc Contamination in Soil and Wheat during Coal Mining, with Assessment of Health Risks for the Inhabitants of Huaibei, China. Environ. Sci. Pollut. Res. 2013, 20, 8435. (16) Bläsing, M.; Müller, M. Mass Spectrometric Investigations on the Release of Inorganic Species during Gasification and Combustion of German Hard Coals. Combust. Flame 2010, 157, 1374. (17) Wolf, K. J.; Müller, M.; Hilpert, K.; Singheiser, L. Alkali Sorption in Second-Generation Pressurized Fluidized-Bed Combustion. Energy Fuels 2004, 18, 1841. (18) Bläsing, M.; Melchior, T.; Müller, M. Influence of Temperature on the Release of Inorganic Species during High Temperature Gasification of Rhenish Lignite. Fuel Process. Technol. 2011, 92, 511. (19) Froment, K.; Defoort, F.; Bertrand, C.; Seiler, J. M.; Berjonneau, J.; Poirier, J. Thermodynamic Equilibrium Calculations of the

5. CONCLUSIONS In this work, the influence of water vapor, HCl, and H2S on the condensation and release of zinc was investigated. Two experimental setups were utilized in order to carry out the experiments. The experiments were carried out under two gasification conditions, each with different amounts of water vapor. Different amounts of HCl and H2S (50 and 500 ppmv) were added into the experiments in order to assess their influence on the behavior of zinc. Hot gas analysis was done by MBMS at 900, 800, 700, and 500 °C. In the condensation experiments ZnO, ZnCl2, and ZnS were found in the deposits. The species found in the gas phase were Zn, ZnO, ZnCl2, and ZnS. The release and condensation of zinc compounds were strongly influenced by steam, HCl, and H2S. The comparison of the experimental results with the results of the cooling calculations performed by the Scheil−Gulliver model from FactSage 6.3 showed the great capacity of the model to predict the behavior of zinc under the gasification conditions taken into account. Thus, the speciation as well as the temperatures at which the species condensed and were in the gas phase had the same trend in the calculations as in the experiments. Furthermore, the pseudothermodynamic model was of great help to clarify and understand the global kinetics of the experiments. With this work a better understanding of the behavior (condensation and release) of zinc under gasification conditions has been obtained. Furthermore, the Scheil− Gulliver tool has been discovered to be a potent tool for obtaining reliable predictions of the behavior of this trace metal under gasification conditions.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +49 246161 5710, Fax: +49 246161 3699. E-mail: m. [email protected]. 6920


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Industrial & Engineering Chemistry Research Volatilization and Condensation of Inorganics during Wood Gasification. Fuel 2013, 107, 269. (20) Gheribi, A. E.; Audet, C.; Le Digabel, S.; Bélisle, E.; Bale, C. W.; Pelton, A. D. Calculating Optimal Conditions for Alloy and Process Design using Thermodynamic and Property Databases, the FactSage Software and the Mesh Adaptive Direct Search Algorithm. CALPHAD: Comput. Coupling Phase Diagrams Thermochem. 2012, 36, 135. (21) Bale, C. W.; Chartrand, P.; Degterov, S. A.; Eriksson, G.; Hack, K.; Ben Mahfoud, R.; Melancon, J.; Pelton, A. D.; Petersen, S. FactSage Thermochemical Software and Databases. CALPHAD: Comput. Coupling Phase Diagrams Thermochem. 2002, 26, 189. (22) Bale, C. W.; Bélisle, E.; Chartrand, P.; Decterov, S. A.; Eriksson, G.; Hack, K.; Jung, I.-H.; Kang, Y.-B.; Melancon, J.; Pelton, A. D.; Robelin, C.; Petersen, S. FactSage Thermochemical Software and Databases  Recent Developments. CALPHAD: Comput. Coupling Phase Diagrams Thermochem. 2009, 33, 295.

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