Partitioning of Lead and Lead Compounds under Gasification-Like

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Cite This: Energy Fuels XXXX, XXX, XXX−XXX

Partitioning of Lead and Lead Compounds under Gasification-Like Conditions Marc Bläsing,*,† Maria Benito Abascal,† 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: An investigation of the chemical form, the concentration, and distribution of lead species between the gas and condensed phase was carried out under gasification-like conditions. The influence of hydrogen sulfide, hydrogen chloride, and steam on the speciation of lead was studied. The gaseous species were determined online by molecular-beam-massspectrometry. The condensates were analyzed by standard methods. The dominant chemical forms were metallic Pb, PbCl2, and PbS. The experimental data were compared with the results of thermodynamic Scheil−Gulliver cooling calculations. Further, the experimental data was used as an input for a pseudoequilibrium model aiming at the determination of kinetic information on the underlying transformations.

1. INTRODUCTION A crucial understanding of the chemical speciation (qualitatively and quantitatively) of lead in gasification systems is needed to determine the environmental effect and to install appropriate control mechanisms. Additionally, the speciation of lead is of high importance for the ecotoxicity. Lead is reported as a volatile element under gasification conditions1−6 and combustion conditions7−10 as well. In more detail, Zhou et al.10 published that lead has a high volatilize ratio even at intermediate temperature, e.g. about 37% volatile ratio at 800 °C and about 61% volatile ratio at 1000 °C. However, the experiments of Zhou et al.10 were done under combustion conditions. Further, Font et al.2,3 named lead as an element with high condensation potential. In this meaning lead is an element which is at least partially volatilized and condensed with decreasing temperature in gas cooling steps of the gasification unit. This behavior is of concern because lead is enriched on the fine-grained particles and those particles can escape particulate control systems.11 Sulfide and oxide species are major lead compounds of the condensate. These findings were based on experimental and theoretical studies.12 Further, Diaz-Somoano and MartinezTarazona12 showed that lead sulfide is the major lead compound at low chlorine content and high hydrogen sulfide content. Font et al.2,3 studied samples directly deposited on the cooling system of a working entrained flow gasifier. Both the species and the condensation temperatures of the experimental work were in line with the theoretical work. Progress in this research field lead already to more detailed information on the speciation of lead under gasification conditions. Anyway, the influence of hydrogen sulfide and hydrogen chloride, which are common and notorious trace gases of gasification product gas, is still insufficiently available. Further, experimental data regarding the direct determination of gas phase lead species is not available at all. Therefore, the objective of the present lab-based study is to investigate both © XXXX American Chemical Society

the fate and the distribution of lead and lead species under gasification-like conditions with special emphasis on the trace gases hydrogen chloride and hydrogen sulfide, as well as steam as a major compound in the gasification atmosphere. From a methodic point of view, the study goes one step ahead by combing established condensation experiments with the unique molecular beam mass spectrometry technique which allows for in situ analysis of the hot gas. The experimental investigation of the gas phase as well as the offline analysis of the condensate is supported by thermochemical modeling with FactSage 7.1. This facilitates a comprehensive understanding of the condensation and the release, respectively hot gas chemistry, of lead under gasification-like conditions.

2. EXPERIMENTAL SETUP AND THERMODYNAMIC CALCULATIONS 2.1. Experimental setup. The experimental setup has already been described in detail by Abascal et al.13,15 recently. Therefore, a brief presentation of the setup is given in the following, only. The experiments were undertaken in two different setups. We used a horizontal flow channel tube furnace for the condensation experiments with a continuous temperature gradient provided by 9 independent SiC-heating zones over the length of the corundum tube. For the in situ determination of the hot gas species we used a flow channel reactor with 4 heating zones. This furnace was directly coupled to a molecular beam mass spectrometer system for gas analysis. The composition of the atmosphere during the experiments follows the work of Abascal et al.13,15 Therefore, the atmospheric conditions were based on the producer gas composition of an entrained flow coal gasifier. The temperature during entrained flow gasification is at least 1500 °C, which leads to effective conversion of tars. In our experiments an entrained flow gasifier producer gas composition before the water−gas shift reaction (WGSR) with a steam to CO ratio of 1.5 was used.13,15 However, the atmosphere of the present study does not contain CO, because of the safety regulations in the lab. Received: September 18, 2017 Revised: December 5, 2017

A

DOI: 10.1021/acs.energyfuels.7b02803 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 1. Composition of the Atmosphere during the Experimental Runs Experiment Effect Effect Effect Effect Effect Effect

of of of of of of

steam steam hydrogen hydrogen hydrogen hydrogen

chloride chloride sulfide sulfide

Gas composition

Hydrogen (%)

Steam (%)

Water-gas-shift Gasification Gasification Gasification Gasification Gasification

14 28 28 28 28 28

17 3 3 3 3 3

Argon/Helium (%) Hydrogen chloride (ppmv) 69 69 69 69 69 69

Hydrogen sulfide (ppmv)

0 0 50 500 0 0

0 0 0 0 50 500

calculations can be found in Abascal et al.13,15 The following information is very condensed. Cooling calculations are done with the Equilib module of the FactSage 7.1 database (FactPS, FToxid, and FTsalt). Phase transitions and compositions during equilibrium cooling and Scheil−Gulliver cooling of mixture containing multicomponents are calculated by this module.17 For the Scheil−Gulliver cooling calculation, a nonequilibrium condensation routine calculating a sequence of equilibrium states for each cooling step is assumed. The composition of a gas phase at a certain temperature determines the starting point of the calculation. At every cooling step the equilibrium between gas phase and condensed phase is determined. Then the condensed phases are removed from the calculation. The remaining gas phase is in the initial composition of the next cooling step. This procedure goes on until the final temperature has been reached. Froment et al.18 showed recently the application of equilibrium calculations for the prediction of the release and condensation behavior of several inorganics during thermochemical conversions of wood. We used the thermodynamic Scheil−Gulliver cooling calculations in the present study to determine the thermodynamic stable lead species for the cooling range starting at 1000 °C to room temperature in cooling steps of 25 °C under atmospheric pressure and the influence of steam, hydrogen chloride, and hydrogen sulfide. The results of the calculations are presented as the speciation of the condensed and gaseous phase versus temperature. The calculated data sets were used for comparison with the experimentally determined data in order to reach a more detailed knowledge of the underlying mechanisms. 2.2.2. Pseudoequilibrium model. More details regarding the model and the calculations can be found in Jiao et al.19,20 The calculation procedure was developed in-house in the group of Ninomiya at Chubu University using ChemApp linked with the FactSage 6.2 database (FACT, Fact 53, FToxid, and FTsalt). The model can predict the condensation of vapor phase metals taking into account the cooling rate and the chemical reaction control. In sum, it is possible to determine the kinetic effects and rate limiting effects of the condensation of the metals under investigation. Abascal et al.13,15 showed recently for zinc and cadmium that the model is an applicable tool which can provide information about the global kinetics in the condensation process of these metals under gasification conditions. The following very compressed description of the method is based on several recent publications.13,15,19,20 In order to understand the use of

Therefore, the oxygen partial pressure was adjusted by the amount of hydrogen. Thermodynamic calculations by Abascal et al.13,15 showed that this simplification of the atmosphere fits well. As a result, the atmosphere consists of H2, H 2O, He, or Ar and different concentrations of hydrogen chloride and hydrogen sulfide at atmospheric pressure. Concentrations of 50 and 500 ppmv of the trace gases hydrogen chloride and hydrogen sulfide in the experiments were considered. The gas composition is summarized in Table 1. In-situ analysis of the high temperature gas was performed with a molecular beam mass spectrometer (MBMS). The MBMS instrument has been explained in more detail in recent publications.14 The total gas flow in the release experiments was 4300 mL/min. This technique requires helium as a carrier gas to reach high detection sensitivity. The high temperature gas phase experiments were done at different temperatures: 600, 800, and 900 °C. The concentration of lead in the gas was set to 100 ppmv by vaporizing lead from a lead source directly into the argon/hydrogen atmosphere. Further details can be found in refs 13 and 15. The apparatus for the condensation experiments consists mainly of a quartz reactor with an inner and an outer tube. The inner tube included the lead source and was flushed by argon/hydrogen. The outer tube was flushed by argon/steam which included the traces of hydrogen chloride and hydrogen sulfide as well. During the condensation experiments the gas flow was set at 510 mL/min. Each experimental run lasted 72 h in order to sample a significant amount of condensate. The condensate was deposited in 8 quartz glass filter elements filled with quartz rings which were positioned following the temperature gradient in the quartz tube. Each of the filter elements was 10 cm in total length. The filters including the filter rings can be removed after each experiment to analyze the condensate for its chemical composition. Ion chromatography and inductively coupled plasma optical emission spectroscopy were used to determine the chemical composition after dissolving the condensate. PbCl2/PbS and PbOx were dissolved in hydrochloric acid (25%) and nitric acid (20%) according to German standards for sample preparation.16 The amount of Pb2+ and Cl− was determined in each solution. Further, the weight change of the filters over the run time of the experiment was determined and checked with the quantified results provided by the standard analysis as mentioned above. The calculated variance of the condensate sampling process was ±0−20%. 2.2. Thermodynamic calculations. 2.2.1. Scheil−Gulliver cooling calculations. More detailed information regarding the

Table 2. Experimental Results of Condensation Experiments (all data in %) Steam

Hydrogen chloride

Hydrogen sulfide

3%v

17%v

°C

Pb

Pb

Pb

PbCl2

Pb

PbCl2

Pb

PbS

Pb

PbS

45−55 56−77 78−162 163−324 325−437 438−576 577−734 735−885

0.0 0.0 0.0 0.0 0.0 0.0 41.5 58.5

0.0 0.0 0.0 0.0 0.0 0.0 70.5 29.5

0.0 0.0 0.0 0.0 0.0 0.0 65.2 34.8

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0 0.0 0.0 69.3 21.8

0.0 0.0 0.0 0.0 3.0 5.9 0.0 0.0

0.0 0.0 0.0 0.0 0.0 0.0 18.9 54.1

0.0 0.0 0.0 0.0 0.0 0.0 27 0.0

0.0 0.0 0.0 0.0 0.0 0.0 7.5 7.5

0.0 0.0 0.0 0.0 0.0 0.0 37.5 47.5

50 ppmv

500 ppmv

B

50 ppmv

500 ppmv

DOI: 10.1021/acs.energyfuels.7b02803 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels the results of the pseudoequilibrium calculations in the present study, a few important details are given here. The term α is used to describe the proportion of a vapor phase metal that does not undergo condensation at a given cooling temperature. This effect is named supercooling and can be reached in experiments by abrupt cooling of the flue gas causing supersaturation. This means that the gas is overloaded by vapor metallic compounds. The result is a lower amount of condensate as expected from the thermodynamic equilibrium point of view. If all metallic compounds have reached the equilibrium without the influence of supercooling or/and chemical reaction control then the term α is 0. So a value of α between 0 and 1 determines the significance of supercooling or/and reaction control on the condensation behavior of the metal compounds under investigation. In the present study we determined the term α by curve-fitting as described by Jiao et al. recently.19 The input data was given by the amount of the condensed metallic compound and the composition of the atmosphere during the experiments. The temperature steps for the calculation were given by the arithmetic mean of the cooling temperature of each filter element.

hydrogen chloride, which can cause further problems in the gasification system. Influence of hydrogen sulfide. Under the influence of hydrogen sulfide, metallic Pb and PbS were the most significant lead species in the condensate. Pb was the main lead species in an atmosphere containing 50 ppm hydrogen sulfide counting for a fraction of 54.1 mol % at 735−885 °C and 18.9 mol % at 578−734 °C. Increasing the hydrogen sulfide content had a strong influence on the speciation and condensation temperature. The fraction of PbS increased strongly with increasing amount of hydrogen sulfide in the atmosphere, e.g. about 39% increase of PbS (50 ppm hydrogen sulfide) to PbS (500 ppm hydrogen sulfide) at 578− 734 °C. In contrast, the fraction of metallic Pb decreased to a comparably low value of 7.5%vol. 3.1.2. Scheil−Gulliver cooling calculations for the condensate. The results of the Scheil−Gulliver cooling calculations for the condensed phases of lead under the influence of different concentrations of steam, hydrogen chloride, and hydrogen sulfide are summarized in Figure 1.

3. RESULTS 3.1. Condensation behavior of lead in a simulated gasification atmosphere. 3.1.1. Experimental studies. The results of the condensation experiments under the influence of steam and the trace contaminant gases hydrogen chloride and hydrogen sulfide are summarized in Table 2. Significant condensed lead compounds were metallic Pb, PbS, and PbCl2. The amount of these lead species was strongly influenced by the trace compounds hydrogen chloride and hydrogen sulfide, as well as by steam. Details are given in the following. Influence of steam. Metallic Pb was the only significant condensed species under the influence of steam. The main amount of the condensed metallic Pb was found with a fraction of 58.5 mol % in the temperature range 736−885 °C for the atmosphere containing 3 vol % steam. Further 41.5 mol % of metallic Pb condensate was found at 578−734 °C. The condensation behavior of lead changed significantly in the presence of 17 vol % steam. The amount of metallic Pb condensate was 29.5 mol % at 736−885 °C and therefore significantly lower at 17 vol % steam than for 3 vol % steam. Further, the main amount of Pb condensate was found in the temperature range 578−734 °C with a fraction of 70.5 mol %. These results clearly show that the condensation of lead is strongly influenced by the amount of steam in the atmosphere. The major finding is that the increase of the concentration of steam induced the condensation of metallic Pb at lower temperature. Influence of hydrogen chloride. The significant lead species were metallic Pb and PbCl2. In direct comparison with the results of the hydrogen chloride free atmosphere, the condensation temperature of lead strongly decreased. This is shown by the increase of the fraction of metallic Pb at 578− 734 °C from 41.5 mol % (0 ppm hydrogen chloride) to 65.2 mol % (50 ppm of Hydrogen chloride) and to 69.3 mol % (500 ppm of hydrogen chloride). Further, the fraction of metallic Pb decreased at 735−885 °C from 58.5 mol % (0 ppm hydrogen chloride) to 34.8 mol % (50 ppm of hydrogen chloride) and to 21.8 mol % (500 ppm hydrogen chloride). Another important finding is that PbCl2 was not determined at all under the atmosphere of 50 ppm hydrogen chloride, whereas for the atmosphere containing 500 ppm hydrogen chloride, a small, but still significant, amount of PbCl2 was determined. With regard to gasification conditions it seems that the volatility of lead is increased by the presence of

Figure 1. Scheil−Gulliver model calculated composition of the condensates (1000 to 0 °C).

The calculations were carried out at decreasing temperature steps of 25 °C starting from 1000 °C to room temperature. The results show the condensed lead species and their condensation temperature. Influence of steam. The model calculations were done for 3 vol % steam and 17 vol % steam, respectively. The calculated data sets show that lead condensed as metallic Pb(liq) at 825− 550 °C. A negligible amount of Pb(s) was determined at 325− 250 °C. The calculated results for 3 vol % steam and 17 vol % steam fit each other very well. This result indicates that there is no significant effect of the water content on the condensation of lead. The predicted amount of condensed Pb(liq) was alike at the temperatures under investigation. Therefore, according to the cooling calculations, it can be considered that the amount of water vapor introduced in the atmosphere had no significant influence on the condensation behavior of lead in the temperature range under investigation. Influence of hydrogen chloride. The calculations were done for 50 and 500 ppmv of hydrogen chloride, respectively. The results showed that lead condensed mainly as metallic Pb(liq) for both hydrogen chloride concentrations. Further, a C

DOI: 10.1021/acs.energyfuels.7b02803 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels negligible amount of metallic Pb(s) and PbCl2(s) was calculated for these conditions. Pb(s) condensed at 265−200 °C and PbCl2(s) at 350−250 °C. The condensed amounts of both metallic Pb(s) and PbCl2(s) were so small that they are barely appreciable in the graph. The calculated amount of condensed metallic Pb(liq) differs significantly for the different concentrations of hydrogen chloride. When the atmosphere contained 50 ppmv of hydrogen chloride, lead condensed as metallic Pb(liq) at 825−550 °C. However, when the atmosphere contained 500 ppmv of hydrogen chloride, metallic Pb(liq) condensed at 800−500 °C. Therefore, it could be concluded that the increase of hydrogen chloride in the atmosphere shifts the condensation point of metallic Pb(liq) to lower temperatures. Influence of hydrogen sulfide. The calculations were done for 50 and 500 ppmv of hydrogen sulfide, respectively. The main condensed phase calculated is metallic Pb(liq) as shown in Figure 1. But also, a negligible amount of PbS(s) was calculated for 525−400 °C. However, the amount of PbS(s) deposited was very small in comparison with the deposits of metallic Pb(liq). The calculated amount of condensed metallic Pb(liq) differs significantly for the different concentrations of hydrogen sulfide. The calculated data showed the condensation of metallic Pb(liq) at temperatures between 800−550 °C when 50 ppm hydrogen sulfide was considered. With 500 ppm of hydrogen sulfide, metallic Pb(liq) condensed at temperatures between 825−550 °C. Hence, the increase in the amount of hydrogen sulfide shifts the condensation point of metallic Pb(liq) to lower temperatures. 3.1.3. Thermodynamic pseudoequilibrium calculations of the condensed phase. The condensation mechanism of lead vapor species during flue gas cooling can be determined through the comparison of the experimental results with the results of thermodynamic pseudoequilibrium calculations. The basics of the calculations have been described recently by Abascal et al.13,15 The results of the calculations are shown in Figure 2. The curve-fitting of the experimental results indicates an alpha value of zero for the experiments with steam and hydrogen chloride. The pseudoequilibrium calculations predicted very well the experimental results. This indicates that the experiments were in the equilibrium state. The experimental results of the experiments with hydrogen sulfide showed a significant variance from the calculated data. The amount of the condensate was smaller than predicted by the calculations. Further, an alpha value of 0.25 for the experiment with 50 ppmv hydrogen sulfide was calculated. For the experiment with 500 ppmv hydrogen sulfide an alpha value of 0.2 was calculated. The results indicate that the lead vapor and its condensation did not reach the equilibrium state during the experiments. This indicates that the lead vapors were most likely oversaturated and condensed at lower than predicted temperature. In sum, a supercooling effect was determined for a hydrogen sulfide containing atmosphere but not for steam and a hydrogen chloride rich atmosphere. Another important finding is that the alpha value decreases from 0.25 at 50 ppmv hydrogen sulfide to 0.2 at 500 ppmv hydrogen sulfide which shows that chemical reaction control slightly compensates for the super cooling effect. The result is a shift of the condensation temperature of lead containing vapors to lower temperature.

Figure 2. Comparison of the experimental results and results of pseudoequilibrium calculation.

3.2. Hot gas chemistry of lead in simulated gasification atmosphere. 3.2.1. Experimental results. The influence of steam, hydrogen chloride, and hydrogen sulfide on the concentration and speciation of gaseous lead was determined by in situ hot gas measurements via a molecular beam mass spectrometer. The monitored species were 208Pb+, 224 PbO+, 240PbO2+, 278PbCl2+, and 242PbS+. The composition of the atmosphere was the same as during the condensation experiments. The total gas flow was 4.3 L/min. The experiments have been conducted at 900 °C, 800 °C, and 600 °C. The concentration of lead in the gas was kept at about 100 ppmv (± 20 ppmv). Only a qualitative analysis could be achieved. An overview of the relative concentrations of all gaseous lead species is given in Table 3. Influence of steam. The experimental results showed that metallic Pb was the predominant gaseous lead compound in the gas phase during the experiments with 3 vol % and 17 vol % steam. Traces of the species PbO and PbO2 were found, but the amount was negligible. Influence of hydrogen chloride. The experiments with hydrogen chloride showed that lead was released as a mixture of Pb and PbCl2. As can be observed in the table, the metallic Pb was the prevalent compound. In general terms, the hydrogen chloride showed a significant influence on the condensation of PbCl2 and shifts the condensation to lower temperature. The concentration of PbCl2 increased with decreasing temperature and increasing concentration of hydrogen chloride. Thus, with 500 ppmv hydrogen chloride, higher concentrations of PbCl2 and PbO2 were present in the gas phase. On the contrary, the concentration of metallic Pb was smaller in the condensation experiment with 500 ppmv hydrogen chloride than in the experiment with only 50 ppmv. Influence of hydrogen sulfide. The species detected in the gas phase were metallic Pb and PbS. Metallic Pb was the D

DOI: 10.1021/acs.energyfuels.7b02803 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

Table 3. Relative Concentrations of Minor Gaseous Lead Species (in %) at Different Temperatures with Steam, Hydrogen Chloride, and Hydrogen Sulfide Steam

Hydrogen chloride

Hydrogen sulfide

Atmosphere

3%v

17%v

°C

Pb

Pb

Pb

PbCl2

Pb

PbCl2

Pb

PbS

Pb

PbS

900 °C 800 °C 600 °C

100.0 100.0 100.0

100.0 100.0 100.0

85.8 91.6 97.6

14.2 8.4 2.4

76.3 71.7 67.4

23.7 28.3 32.6

88.1 75.7 100.0

11.9 24.3 0

42.0 92.9 91.0

58.0 7.1 9.0

50 ppmv

500 ppmv

50 ppmv

500 ppmv

concentration of the lead species. In general, the concentration of PbO and PbO2 increases with increasing concentration of steam in the atmosphere. Influence of hydrogen chloride. The speciation of the gas phase predicted by the Scheil−Gulliver cooling calculations in an atmosphere containing 50 ppmv or 500 ppmv hydrogen chloride and lead is shown in Figure 3. The main inorganic species present in the gas phase were metallic Pb, PbCl2, and PbO. PbO2 was in the gas phase in negligible concentration only. At 1000−800 °C a constant concentration of Pb was observed. The concentration of metallic Pb decreases with decreasing temperature from 800−145 °C. The concentration of PbCl2, the second most abundant species in the gas phase in this temperature range, was constant at temperatures between 800−300 °C. At temperatures higher than 800 °C the concentration decreased with the temperature. The concentration of PbO was about the same quantity as the concentration of PbCl2 at temperature above 900 °C. The concentration of PbO decreased strongly with the temperature. Furthermore, an increase of the hydrogen chloride amount of the atmosphere leads to an increased concentration of PbCl2. PbCl2 becomes the most important lead species at temperature lower than 465 °C for an atmosphere containing 500 ppmv hydrogen chloride and 585 °C for an atmosphere containing 50 ppmv hydrogen chloride. In general, the concentration of PbCl2 increases with increasing concentration of hydrogen chloride in the atmosphere. Influence of hydrogen sulfide. The speciation of the gas phase predicted by the Scheil-Gulliver cooling calculations in an atmosphere containing 50 ppmv or 500 ppmv hydrogen sulfide and lead is shown in Figure 3. The calculated gaseous lead species are metallic Pb, PbS, PbO, and PbO2. Metallic lead and PbS are the most abundant lead species in the gas phase. The concentration of PbO was much smaller than the concentrations of Pb and PbS. The amount of PbO2 was negligible. The concentrations of all species increased with the temperature. The concentrations of both metallic Pb and PbS decrease slightly from 825 to 1000 °C. As can be observed in the graph, an increase of the amount of hydrogen sulfide from 50 ppmv to 500 ppmv lead to a significant increase of the amount of PbS in the gas phase.

dominant species in the experiments with 50 ppmv hydrogen sulfide. At 600 °C metallic lead was even the only species detected and no PbS has been determined with 50 ppmv hydrogen sulfide. In general, the concentration of PbS increased strongly with increasing amount of hydrogen sulfide in the atmosphere. Further, the influence of the temperature on the gaseous lead species is significant. The concentration of PbS increased with increasing temperature for both 50 ppmv and 500 ppmv hydrogen sulfide. In the experiments with 500 ppmv hydrogen sulfide, PbS was the major species with 58 vol % at 900 °C. 3.2.2. Scheil−Gulliver cooling calculations. The release behavior of lead under the influence of steam, hydrogen chloride, and hydrogen sulfide was predicted by the Scheil− Gulliver cooling model. The speciation of the main inorganic gaseous species at temperatures between 1000 °C and room temperature is given in the following. Influence of steam. The speciation of the gas phase predicted by the Scheil−Gulliver cooling calculations in an atmosphere containing water vapor and lead is shown in Figure 3. The graph shows the results obtained with 3 and 17 vol %

Figure 3. Scheil−Gulliver model calculated composition of the hot gas (1000 to 0 °C).

water vapor in the atmosphere. The results obtained with 3 vol % water are depicted with a solid line, and the results containing 17 vol % water vapor are depicted with a dashed line. According to the calculations, metallic Pb, PbO, and PbO2 were the only gaseous species present in the gas phase when the atmosphere contained water vapor. A concentration of 100 ppmv of Pb was in the gas phase at temperatures between 1000−850 °C. The concentration of metallic Pb decreased significantly with decreasing temperature. The calculated concentration of PbO was about 5 orders of magnitude lower than the concentration of metallic Pb and decreased with the decreasing temperature. The amount of PbO2 was in principal negligible. Steam shows a significant influence on the

4. DISCUSSION AND PRACTICAL IMPLICATION The experimental results have to be compared with the results of the cooling calculations and the thermodynamic pseudoequilibrium model. The following conclusions of the condensation and release behavior of lead were deduced. Both experimental and Scheil−Gulliver calculations indicate that the most likely and most abundant forms of lead in the gasifier product gas are metallic Pb, PbCl2, and PbS. PbO and especially PbO2 do not play a significant role. These results are in agreement with earlier work.1,21 Anyway, this study aims a E

DOI: 10.1021/acs.energyfuels.7b02803 Energy Fuels XXXX, XXX, XXX−XXX

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little bit further, since it has been shown for the experimental conditions of this study that the proposed and discussed gaseous lead species PbH has not been determined by MBMS at all. The detection limit of the MBMS is about 10 ppbv. Allover, the Scheil−Gulliver model calculation was found to be useful in predicting the lead species. However, it has been shown that a pure-equilibrium model is limited. To pay attention to nonequilibrium factors, a pseudoequilibrium model (kinetically modified equilibrium model) has been successfully applied. The experimental results of the condensation studies were congruent to the results of the pseudoequilibrium calculations for steam and hydrogen chloride conditions, but a significant variance was found for the hydrogen sulfide conditions. The latter is most likely affected by the supercooling effect. With focus on practical implication, this investigation shows that lead forms highly volatile compounds under gasificationlike conditions which means that lead compounds will condense in colder parts of the gasification plant only. Metallic lead, PbCl2, and PbS will most likely be the most abundant, highly volatile lead compounds in the gas phase. Especially, high chlorine fuels have the capability to shift the condensation point of lead compounds to even lower temperature, as shown by the results of this model study. Future work should address the interaction of hydrogen chloride and hydrogen sulfide, and its competing behavior for the formation of lead compounds has to be addressed in the future as well, because both chlorine and sulfur are common fuel components.

Marc Bläsing: 0000-0002-6116-1604 Yoshihiko Ninomiya: 0000-0002-3523-9666 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS

■

REFERENCES

The authors would like to thank Bundesministerium für Wirtschaft und Technologie for the financial support in the framework of the HotVeGas-EM project (FZK 0327773C) and Deutsche Forschungsgemeinschaft (DFG) for the financial support with Grant No. BL 1363/1-1.

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5. CONCLUSION This study shows new findings regarding the chemical form and the concentration of lead under gasification-like conditions at temperatures up to 1000 °C under the influence of sulfur (via hydrogen sulfur), chlorine (via hydrogen chloride), and steam. Most abundant chemical forms of lead were metallic Pb, PbCl2, and PbS. These lead species were determined online in the gaseous phase and put in correlation to gasification conditions for the first time. In general, PbO and PbO2 are minor species as shown by the experimental findings and as predicted by the thermodynamic calculations. The additions of steam, hydrogen chloride, and hydrogen sulfide have a significant influence on the chemical form and the concentration of lead species. For example, the concentration of PbCl2 increases strongly with increasing hydrogen chloride content of the atmosphere. Also the concentration of PbS increases strongly with increasing hydrogen sulfide content. The major lead condensation processes occur at temperatures below 885 °C as shown by the experimental results and by Scheil− Gulliver calculations. In general, the experimental results and the Scheil−Gulliver calculations of the gas phase show good agreement. Increasing the amount of hydrogen sulfide lead to prevalence of PbS in the condensate. The effect of hydrogen chloride is less significant due to the lower amount of PbCl2 in the condensate. Nevertheless, the amount of PbCl2 condensed at lower temperature is increasing with increasing concentration of hydrogen chloride.

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DOI: 10.1021/acs.energyfuels.7b02803 Energy Fuels XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.energyfuels.7b02803 Energy Fuels XXXX, XXX, XXX−XXX