Distribution and Fate of Trace Elements during Fixed-Bed Pressurized

Sep 15, 2016 - However, trace elements present in coal could be emitted during the ... U, and Be are found to be enriched in the ash in the form of ar...
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Distribution and Fate of Trace Elements during Fixed-Bed Pressurized Gasification of Lignite Shu-Qin Liu,* Cai-Hong Wang, Ming-Yue Liu, and Shang-Jun Zhang School of Chemical and Environmental Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China ABSTRACT: Lurgi fixed-bed pressurized gasifiers have attracted significant commercial interest in China because they are suitable for the production of substitute natural gas. However, trace elements present in coal could be emitted during the gasification process and transferred into gas, water, or solids in downstream processes if not properly removed, and this could then be greatly toxic to human health or lead to environmental pollution. In this study, simulation tests were performed using a laboratory-scale fixed-bed pressurized gasifier, and the distribution and fate of toxic elements including F, Hg, As, Se, Pb, U, and Be were investigated. The results indicate that during pressurized gasification, with increasing temperature from 800 to 1100 °C, the volatility of Pb increases, whereas As, U, and Be exhibit the opposite trend. It is obviously affected by pressure, such that increasing pressure leads to the enrichment of As in the ash. Elements As, U, and Be are found to be enriched in the ash in the form of arsenates, uranium oxide, and beryllium aluminates, respectively. On the other hand, the elements F, Se, and Pb are largely transformed into condensates, with possible forms of hydrogen fluoride, hydrogen selenide, and lead sulfide, which should be controlled in downstream processes in order to avoid adverse environmental issues caused by these toxic materials.

1. INTRODUCTION Fixed-bed pressurized gasification has attracted significant commercial interest in China owing to its suitability for the production of substitute natural gas (SNG). As the core device, the Lurgi IV gasifier has been successfully tested in a 4 billion Nm3 of SNG/year coal-to-SNG plant in Liaoning province in northeastern China.1 The coal-to-gas industry is currently undergoing significant development, and preliminary estimation suggests that the quantity of SNG will be close to 30 billion Nm3 in 2020 and 50 billion Nm3 in 2030.2,3 However, the fixed-bed pressurized gasification of lump coal in large-scale processing also brings enormous challenges in terms of the removal of environmentally hazardous materials in the products, including toxic trace elements. Trace elements in coal could be released during gasification and transformed from the solid phase to the gas or liquid phase, or even become water-soluble, and this could magnify their adverse impact on the environment and give rise to various technical problems during gas utilization.4 Human health problems caused by domestic coal combustion have been reported, and millions of people in the southwest of China have been affected by fluorosis, arsenosis, selenosis, and loss of vision.5−11 It is well-known that the release behavior of trace elements during coal-utilizing processes varies according to content and mode of occurrence of elements, coal type, processing conditions, pollution control devices, etc.12 Reported studies on the behavior of toxic trace elements during coal combustion are quite extensive, in line with the extensive industrial use of coal boilers,13−20 whereas relatively few papers describe their behaviors during the process of gasification.18 The release and partitioning behaviors of trace elements during air-blown fluidized bed gasification21−23 or entrained-flow gasification24−26 have been well conducted, but cases on Lurgi pressurized gasification are quite limited. To compensate for the lack of experimental data in gasification, most of the studies are based on thermodynamic equilibrium calculations. How© XXXX American Chemical Society

ever, the thermodynamic equilibrium conditions may not be satisfied in an industrial gasifier, and the reactions are therefore more kinetically limited.27 Moreover, the thermodynamic equilibrium calculation is generally conducted at medium or higher pressures.28,29 The temperature and pressure in the bed layer of a Lurgi pressurized gasifier are not as high as those of entrained-flow gasifiers. As the coal is fed in, it is first dried and devolatilized by the heat of the rising gas. The devolatilized coal, called “char”, enters the reduction zone followed by the combustion zone, and it is finally burnt to ash. That is, the entire gasifier can be divided into four distinct zones. Thus, the process is more complicated, which would further affect the transformation of trace elements in terms of content and modes of occurrence. Therefore, it is necessary to comprehensively understand the partitioning behavior of trace elements during the fixed-bed pressurized gasification process and then to develop suitable technologies for pollution control. Drying and devolatilizing are the initial stage of fixed-bed pressurized gasification and the release of some toxic elements begins at this stage, so it is essential to understand their migrating behavior during the pyrolysis process. Guo et al.30 have evaluated the transformation behavior of trace elements during fixed-bed pressurized pyrolysis as functions of pressure, temperature, and atmosphere; the results showed that higher pressure inhibited the release of trace elements. The release of As, Pb, Cr, Cd, and Mn was promoted at higher pyrolysis temperatures and in a reducing atmosphere.30 Partitioning of trace elements between ash and gas in the Sasol-Lurgi gasification process using lump coal was investigated by Bunt et al.,27,31−33 and FactSage thermodynamic equilibrium modeling was also used to predict the speciation behavior of Received: July 11, 2016 Revised: September 3, 2016

A

DOI: 10.1021/acs.energyfuels.6b01692 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 1. Proximate and Ultimate Analysis of Coal Sample (wt %) proximate analysis

a

ultimate analysis

Mad

Aad

Vad

FCad

Cad

Had

Nad

Sad

Oadb

11.50

29.10

28.47

30.93

43.70

3.11

0.57

0.65

11.59

M, moisture; A, ash yield; V, volatile matter yield; FC, fixed carbon; ad, air-dried basis. bOxygen content is by difference.

trace elements in a fixed-bed pressurized gasifier. In addition, the validation of model prediction was further undertaken via turn-out sampling in a Sasol-Lurgi Mark IV gasifier. The results indicated that toxic elements, including Hg, Cd, Pb, and Se, were found to be highly volatile, and most of them were distributed into the gas phase, whereas As and some semivolatile elements, such as Cu, Mo, Ni, and Zn, showed limited devolatilization behavior in the drying and pyrolysis zone of the fixed-bed gasifier. The experimental results were partly contrary to what was predicted within the reduction zone of the fixed-bed gasifier, and this phenomenon implied that the reactions in a fixed-bed gasifier operating on lump coal were kinetically limited because of the mass and heat transfer limitations across coarse coal particles, and thus conditions used for the thermodynamic equilibrium calculation were unsuitable. In this study, the distribution of trace elements through the coal bed in the gasifier has been clarified. However, the partitioning behavior of trace elements in out streams including syngas, condensate, and ash were not considered. In this work, the effects of temperature and pressure on the trace element emissions during fixed-bed pressurized gasification were investigated, and mass fractions were calculated to evaluate the distribution of trace elements in out streams. In addition, thermodynamic equilibrium calculations were conducted to predict the transformation of trace elements in the different phases. The trace elements investigated in this study include F, Hg, As, Se, Pb, U, and Be.

Figure 1. Schematic diagram of the experimental system of lab-scale fixed-bed pressurized gasifier: 1, gas container; 2, thermocouple; 3, thermowell; 4, Al2O3 porcelain ball; 5, sample support; 6, heating furnace; 7, lump coal; 8, reactor; 9, sieve plate; 10, high pressure pump; 11, cyclone separator; 12, condensor; 13, liquid collector; 14, gas outlet. ultrapure water (0.6 g/min) were introduced simultaneously for gasification. During the gasification experiment, the system pressure was kept constant by adjusting the back-pressure valve. The outlet high-temperature syngas (approximately 350 °C) was first cooled by the condenser and then detected by gas chromatography at intervals of 5 min. When the total content of CO and H2 was lower than 1%, the supplies of agents were cut off, followed by nitrogen purging to cool the furnace to room temperature. Experimental conditions were designed to be close to those of the Lurgi pressurized gasification process. Gasification temperatures selected for this study ranged from 800 to 1100 °C in intervals of 100 °C and the pressure varied from atmospheric pressure to 3 MPa. 2.4. Sample Collection. To collect the trace elements retained in the syngas, the gas pressure needs to be decreased to atmospheric pressure first. Solvents of 100 mL of 5% HNO3/10% H2O2 and 100 mL of 4% KMnO4/10% H2SO4 were used to absorb the trace elements from the gas phase according to U.S. Environmental Protection Agency Method 0060, as shown in Figure 2. These solutions were preserved for subsequent trace element analysis and denoted as gas samples. During each test, the total volume of syngas produced was measured using a wet gas flowmeter. All of the bottles used for absorbing and sampling were soaked in a solution of 20%

2. EXPERIMENTAL SECTION 2.1. Coal Sample. Ulanqab lignite from Inner Mongolia, northern China, was used as the test sample in this study. The proximate and ultimate analyses of coal were performed using ASTM Standards D3173-03 (2005), D3174-04 (2005), D3175-02 (2005), and D31761989 (2005), and the results are given in Table 1, which are on an airdried basis (ad).The coal was crushed, sieved, and dried at 105 °C for 1 h before experiments. Generally, the particle size of coal used for Lurgi gasification lies in the range from 5 to 50 mm. In this study, particles ranging from 10 to 13 mm were used. 2.2. Experimental Apparatus. In the present work, an electrically heated fixed-bed pressurized gasification system was applied, as shown in Figure 1, which consists of a reaction tube, electric heaters, pressure controllers, and a condenser. The reaction tube was made of an 800H nickel−iron−chromium alloy 50 mm in diameter and 600 mm in length. The gasifier was heated via an internal electrical element and three electric heaters to ensure that the reaction zone temperature reached 1200 °C. The bed temperatures were monitored using thermocouples. In addition, a high-pressure pump (Series II, SSI, USA) was equipped to inject ultrapure water into the reactor, which was evaporated at the bottom of the reactor as the gasification agent. There is a back-pressure valve installed to adjust the system pressure, and the value is displayed on a gauge. A condenser for gas−liquid separation was used to remove unreacted steam, and the hightemperature syngas was simultaneously cooled to room temperature. 2.3. Simulation Test. Approximately 40 g of coal particles was put into the furnace and heated at a rate of 50 °C/min to the target temperature after the leak test, and then held at that temperature for 3 h. Nitrogen was chosen as the protective gas in the heating process. When the furnace temperature was stable, oxygen (20 mL/min) and

Figure 2. Absorption method of trace elements in syngas: 1, globe valve; 2, empty bottles; 3, solvents of 5% HNO3 and 10% H2O2; 4, solvents of 4% KMnO4 and 10% H2SO4; 5, wet gas flowmeter. B

DOI: 10.1021/acs.energyfuels.6b01692 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

elements in the fixed-bed pressurized gasification systems modeled, and elements such as Ca, Fe, K, and Na were also considered. The Equilib module of FactSage was used, and the equilibrium state for which calculations were to be performed was defined by specifying the temperature and pressure. Quantities may be specified by fixing the overall amount of each component in the system, i.e., by specifying the amount of reactants. The term ”Slaga” in the context means “liquid oxide solutions” from the FACT FToxide database. For calculations, 100 g of coal was used to calculate the input value, and the amounts of oxygen and steam used in the calculation system were based on the technical parameters obtained from the simulation tests for fixed-bed pressurized gasification. The input for calculations is given in Table 3.

HNO3 for 24 h and then washed and dried before use. Part of the trace elements carried by high-temperature syngas would be condensed or dissolved with the reduction of gas temperature and then transferred into the condensate. When the gasification test was accomplished, the liquid reserved in the condenser was collected, weighed carefully, and preserved as the condensate samples. Residual ash at the bottom of the gasifier was also weighed and preserved as the ash sample. 2.5. Trace Element Analysis. Mercury in the samples was determined using an Hg analyzer (DMA 80, Milestone, Italy), in which samples were heated and the evolved Hg was selectively captured as an amalgam and measured by atomic absorption spectrophotometry. The detection limit of Hg was 0.005 ng, and the linearity of the calibration was in the range 0−1000 ng.34 Fluorine was determined by pyrohydrolysis with an ion-selective electrode, following the methods described in ASTM Standard D5987-96 (2002). Inductively coupled plasma mass spectrometry (ICP-MS; X Series II, ThermoFisher Scientific, USA) was used to determine the other selected elements of As, Se, Pb, U, and Be in the coal and samples. Prior to the ICP-MS analysis, an UltraClave microwave high pressure reactor (Milestone) was used to acid digest the solid samples, and a certain volume of nitric acid and hydrofluoric acid was employed for digestion. For more details about coal or ash digestion and ICP-MS analysis techniques, refer to the study of Dai et al.35 Arsenic and selenium in these samples were determined by ICP-MS using collision-cell technology to avoid disturbance of polyatomic ions.36 Multielement standards (CCS-1, CCS-4, CCS-5, and CCS-6, Inorganic Ventures) were used for calibration of trace element concentrations. The linearity of the calibration curves for ICP-MS determination was considered as satisfying in the range 0−100 μg/L with a determination coefficient r2 > 0.9999. Relative standard deviations for the measurement of trace elements by ICP-MS are less than 5.0%. 2.6. Data Treatment. To understand the volatilization of trace elements studied during the fixed-bed pressurized gasification process, the volatility percentage was calculated based on the following formula:37,38

⎛ C ⎞ V % = ⎜1 − i Y ⎟ × 100 C0 ⎠ ⎝

3. RESULTS AND DISCUSSION 3.1. Abundance of Trace Elements in Coal. The contents of the trace elements studied in coal are shown in Table 4, together with the average values in Chinese coals for comparison. With the exception of Se, Pb, and U, the contents of F, Hg, As, and Be are higher than the average value for common Chinese coals. Compared to the relevant concentration coefficient (CC), the ratio of element concentration in the investigated coal to that in Chinese coals established by Dai et al.,39 selenium with CC < 0.5 is depleted in the coal. Elements F, Hg, As, Pb, U, and Be have CC values ranging from 0.5 to 2, a normal level compared to common Chinese coals. 3.2. Volatilization of Trace Elements during the FixedBed Pressurized Gasification Process. 3.2.1. Gas Composition under Different Gasification Conditions. The gas compositions from fixed-bed pressurized gasification with variations in temperature and pressure are shown in Table 5. It can be observed that, for an oxygen-steam pressurized gasification process, a medium-heating-value gas is produced, with the total content of H2 and CO above 57.83%. Under a pressure of 3 MPa, the increase of temperature is beneficial to the production of syngas. When the temperature is increased from 800 to 1100 °C, the total content of CO and H2 increases from 57.83 to 73.66%; in contrast, the CO2 content decreases from 28.32 to 11.27%. However, the increase of pressure has the opposite effect on CO: its content decreases from 31.24 to 22.54% as the pressure is increased from atmospheric pressure to 3 MPa. Meanwhile, the content of CH4 increases from 5.77 to 14.32% with increasing pressure. The endothermic reaction, including the reduction of CO2 and steam, is remarkably promoted by the reaction temperature to produce much more H2 and CO. CH4 in syngas is mainly formed from the volatiles emitted from coal pyrolysis and the methanation reaction during the gasification process. Increasing the gasification pressure could enhance the methanation reaction and then restrain the reduction of steam when the pressure is above atmospheric pressure, resulting in an increase of CH4 content. The CH4 content in this study is close to that from an industrial facility, whereas those of CO and H2 are much higher. This is because the industrial gasifier is a selfheating type by successive feeding, whereas the laboratory equipment is an external heating type, which favors the reduction of H2O(g) and CO2. The time−temperature history of the trace elements between the simulation test and industrial operation may be different, and this is a limitation of the study. 3.2.2. Influence of Temperature. Temperature is one of the key factors influencing the gasification reactivity. The volatile behavior of trace elements is significantly affected by temperature, because most reactions and transformations of traceelement-bearing compounds occur at higher temperatures.12

(1)

where V% is the calculated value of the element volatility; Ci and C0 are the contents of trace elements in ash and coal, respectively, μg/g; and Y is the ash yield (%) as shown in Table 2.

Table 2. Ash Yields at Different Gasification Conditions condition temp (°C) (at P = 3.0 MPa) 800 900 1000 1100 press. (MPa) (at T = 1100 °C) 0.0 1.0 2.0 3.0

ash yield (wt %) 15.28 15.95 14.75 14.58 15.39 14.56 14.51 14.58

2.7. FactSage Modeling. The thermodynamic equilibrium model used in this work was FactSage version 6.4, and the FACT Pure Substance (FactPS) and FACT Oxide (FToxide) databases were chosen as the most appropriate data. This provided the Gibbs free energy of each substance to be calculated as a function of temperature, enabling the software to perform calculations to determine the equilibrium species for a defined system. The purpose was to predict the speciation behavior of trace elements occurring in a fixed-bed pressurized gasification system to understand the distribution of trace elements. The calculated system can be defined in terms of the elements or compounds involved. In this work, the major components in coal (e.g., C, H, O, N, S, Cl) were considered with the trace C

DOI: 10.1021/acs.energyfuels.6b01692 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 3. Input for Thermodynamic Equilibrium Calculation for Pressurized Gasification element

mass (g)

C H O N S Cl

43.70 3.11 11.59 0.57 0.65 0.01

components in coal

trace element F Hg As Se Pb U Be H2O(l) O2 H2O(l)

moisture in coal gasification agent

mass (g) 1.50 2.50 5.26 0.90 9.32 2.36 2.37

× × × × × × ×

10−2 10−5 10−4 10−4 10−4 10−4 10−4

oxide SiO2 Al2O3 Fe2O3 CaO MgO K2O Na2O 11.50 8.57 180.00

mass (g) 17.18 5.24 1.70 1.69 0.79 0.80 0.37

Table 4. Contents of Trace Elements in Coal trace element

raw coal (μg/g)

Chinese coalsa (μg/g)

CCb

F Hg As Se Pb U Be

150.37 0.25 5.26 0.90 9.32 2.36 2.37

130 0.16 3.80 2.47 15.10 2.43 2.11

1.16 1.53 1.38 0.36 0.62 0.97 1.12

a

Dai et al.40,41 bCC, ratio of element concentration in investigated coals vs Chinese coals.

Changes in the volatilities of the studied trace elements as a function of gasification temperature during the conversion of coal to ash are shown in Figure 3. During the pressurized gasification process, coal lumps crack with the release of volatiles and some minerals inside are decomposed or melt with increasing temperature. Trace elements embedded in the coal structure in various forms also migrate to the gas or transform into other phases. From Figure 3, it is clear that F, Hg, and Se are highly volatile and their volatilities increase slightly with increasing temperature. Elements Pb, As, U, and Be show intermediate behavior, and their volatilities are obviously changed with increasing temperature. Elements F, Hg, and Se largely migrate into the gas phase when the temperature reaches 800 °C, with volatilities of 86.7, 95.9, and 79.7%, respectively. This may be due to the lower boiling points of the elements themselves or their compounds. Fluorine in coal is mainly associated with inorganic minerals such as clays and phosphate minerals; some organic F is also occasionally found.42−45 The decomposition features of

Figure 3. Volatilities of trace elements as a function of gasification temperature (pressure 3 MPa).

different F-bearing minerals have been reported,46,47 and it is found that inorganic F-bearing minerals decompose at high temperatures. It is noted that most F is released into the gas phase below 800 °C in the simulation tests with a volatility higher than 85%, which could be inferred that this proportion of F exists in the organic and ions forms that adsorbed on the surfaces of the mineral particles, or in a water-soluble state within the pores. When the temperature increases from 800 to 1100 °C, the volatility of F is increased slightly. This can be attributed to the release of F occurring in the crystal lattice of minerals in the form of isomorphism, which is generally difficult to be released.

Table 5. Gas Compositions under Different Gasification Conditions gas composition (vol %) condition temp (°C) (at P = 3.0 MPa) 800 900 1000 1100 press. (MPa) (at T = 1100 °C) 0.0 1.0 2.0 3.0 industrial Lurgi gasifier (Mark IV)

H2

CO

CO2

CH4

N2

50.86 52.72 53.49 51.12

6.97 7.83 16.38 22.54

28.32 25.80 15.29 11.27

12.89 12.47 13.96 14.32

0.96 1.18 0.88 0.75

51.56 51.32 50.75 51.12 39.36−41.41

31.24 30.09 26.45 22.54 14.90−25.70

10.21 10.43 10.59 11.27 25.29−33.18

5.77 7.14 11.28 14.32 7.58−12.85

1.22 1.02 0.93 0.75 1.20−1.50

D

DOI: 10.1021/acs.energyfuels.6b01692 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels In a previous study,48 the trace elements are divided into three categories depending on the boiling points of trace elements and their oxides. In fact, the volatile behavior of trace elements is tightly related to the operating parameter during gasification.12 Therefore, the volatile categories of the selected trace elements are reclassified in order to clearly describe their release behavior at fixed-bed pressurized gasification conditions: the most volatile group, with volatility higher than 70%; the semivolatile group, with volatility in the range 35−70%; and the least volatile group, with volatility lower than 35%. Thus, the intermediate behaviors of Pb, As, U, and Be with increasing temperature are found. When the temperature is changed from 800 to 1100 °C, the volatility of Pb increases from 44.6 to 87.5%, jumping from “semivolatile” to “most volatile”, whereas elements As, U, and Be exhibit the opposite trend. Arsenic decreases from “most volatile” to “semi volatile”, whereas U and Be become “least volatile.” In the fixed-bed pressurized gasification process, lead in coal combines easily with other elements, such as Cl, Se, and/or S, transforming to compounds such as PbS, PbCl2, PbCl, and PbSe,49 which are easily volatized to the gas phase due to their lower boiling points. The volatility of As decreases from 77.1 to 42.9% with increasing temperature. Arsenic is mainly associated with pyrite in coal, although it is sometimes in the forms of arsenates and silicates.50−54 A proportion of As is released at 800 °C because of pyrite decomposition, as supported by the X-ray diffraction (XRD) analysis of chars from fixed-bed pressurized pyrolysis at pressure of 3 MPa (Figure 4). However, when the temperature

investigated in the present study, and the results are shown in Figure 5. The volatilities of the selected trace elements tend to

Figure 5. Volatilities of trace elements as a function of gasification pressure (temperature 1100 °C).

decrease when the pressure increases from atmospheric pressure to 3 MPa. Among these trace elements, the decreasing volatility of As is remarkably distinct, whereas those of the others show slight decreases. This suggests that the volatilities of trace elements are correlated with the gasification pressure, and higher pressure would restrain the migration abilities of the trace elements from coal;25,57,58 thus fewer trace elements would be carried by the high-temperature syngas. With increasing pressure, the volatilities of F, Hg, Se, and Pb decline by less than 10.8 percentage points and Hg exhibited minimum change. These high volatilities are linked to the low boiling points of either the elements themselves or the compounds that contain these elements. Large proportions of F, Hg, and Se evaporate into the gas phase at 800 °C, even under the pressure of 3 MPa, so their volatilities changed slightly with increasing pressure at a higher temperature. Compared to the results in Figure 3, the influence of pressure on the volatility of Pb is less significant than that of temperature. The volatility of Pb is reduced only by 10.8 percentage points when the pressure increases from atmospheric pressure to 3 MPa, and it remains highly volatile. Arsenic is the most concerning trace element because its volatility decreases from 77.5 to 42.9% with increasing pressure. This may be due to the following reasons:57,58 First, the vapor pressure of volatile trace element increases with increasing gasification pressure, which lowers the release rate of As from minerals. Second, based on molecular thermodynamics, the diffusion flux of evaporated trace elements from the mineral surfaces is apparently reduced with increasing pressure, and the elemental collision is accelerated at higher pressures, thus promoting the agglomeration of trace elements back to the volatile matter surface. In addition, the chemical combination of As with calcium-bearing minerals or silicates is intensified and then the element As is encased in the ash again. The effect of elevated pressure on the equilibrium composition of As was evaluated at pressures from atmosphere pressure to 3 MPa, and the results are shown in Figure 6. With increasing pressure, the mole fractions of As2(g), AsS(g), and AsO(g) decrease, whereas that of the reduced species, AsH3(g), remarkably increases. Meanwhile, species in the liquid phase such as As2O3(slaga)

Figure 4. XRD patterns of chars from fixed-bed pressurized pyrolysis at pressure of 3 MPa. K, kaolinite; A, anhydrite; P, pyrite; Q, quartz.

is higher than 800 °C, arsenic tends to react with Ca-bearing minerals or eutectic silicates to form more stable compounds, and finally be fixed or covered by ash layers according to the studies by Quick et al.55 and Clemens et al.,56 resulting in the reduction of volatility. In addition, U and Be show a similar release behavior and possibly a similar transformation mechanism as As in the pressurized fixed-bed gasification process. 3.2.3. Influence of Pressure. Increasing pressure is one of the most important ways to enhance the gasification intensity. With increasing pressure, the evaporation and condensation of trace elements will be affected, even at the same reaction temperature. For better understanding of the influence of pressure on trace element migration, the variation of the volatility of trace elements with pressure at 1100 °C was E

DOI: 10.1021/acs.energyfuels.6b01692 Energy Fuels XXXX, XXX, XXX−XXX

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Table 6. Volatile Levels of Trace Elements under Different Gasification Conditions element condition temp (°C) (at P = 3.0 MPa) 800 900 1000 1100 press. (MPa) (at T = 1100 °C) 0.0 1.0 2.0 3.0 a

F

Hg

As

Se

Pb

U

Be

m m m m

m m m m

m s s s

m m m m

s s m m

s s s l

s s s l

m m m m

m m m m

m s s s

m m m m

m m m m

l l l l

l l l l

m, most volatile; s, semivolatile; l, least volatile.

Figure 7. Mass balance ratios of trace elements after fixed-bed pressurized gasification. Gasification conditions: 1100 °C, 3 MPa, H2O(l) 0.6 g/min, and O2 20 mL/min.

As can be seen from Figure 7, values of the mass balance ratio for all trace elements range from 77 to 123%, within the acceptable scope described in the literature.59 During the simulation tests of fixed-bed pressurized gasification, complete closure of the mass balance is very difficult to achieve because a certain amount of highly volatile elements will condense and accumulate in the pipes with decreasing gas temperature. Furthermore, some errors in sampling and measurement exist. 3.4. Distribution of Trace Elements in Out Streams of the Gasifier. The distribution of trace elements in gasification products could not only provide an indication to explore the transformation mechanism of elements during the gasification process but also help control the emission of trace elements. Those elements carried by high-temperature syngas could be transferred into the liquid phase or still exist in the gas phase after cooling, which would require more attention during gas cleaning. The mass fraction was used to illustrate the distribution behaviors of F, Hg, As, Se, Pb, U, and Be among the gasification products, including ash, condensate, and gas, at the typical operating conditions of fixed-bed pressurized gasification (1100 °C, 3 MPa). The calculated equation can be expressed as

Figure 6. Changes of arsenic species with pressure for fixed-bed pressurized gasification at 1100 °C. (a) gaseous phase species; (b) liquid phase species.

and As2S3(slaga) appear and the mole percentages obviously increase with increasing pressure. The volatilities of U and Be were relatively unaffected by pressure and remained at approximately 30%. A summary of the volatile levels of selected trace elements under designated temperatures and pressures is shown in Table 6. 3.3. Mass Balance of Trace Elements. After gasification is stabilized, the elements introduced into the gasifier will exit the system in three streams: ash, condensate, and gas. The total mass balance ratio (MBR) can be expressed by the following equation: MBR =

Ciashmash + Ci lml + Cigasmgas MR = M0 C0m

(2)

where M0 and MR are the masses of the trace element in coal and products, respectively, μg; C0, Ciash, Cil, and Cigas represent the contents of the trace element in coal, ash, condensate, and gas, respectively, μg/g; m, mash, ml, and mgas represent the weights of coal, ash, condensate, and gas, respectively, g. By calculating the MBR, a picture of the mass balance ratio of each element can be obtained, as shown in Figure 7.

M% =

Mi × 100 MR

(3)

where M% represents the mass fraction; Mi is the mass of trace element in a certain product, μg. The mass fraction values of F

DOI: 10.1021/acs.energyfuels.6b01692 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

The compounds with Ca and As, such as Ca3(AsO4)2, do not occur in the FactSage prediction results. This is because in thermodynamic calculations, with input of the reactants, the reducing atmosphere is dominant, which is not beneficial to the formation of compounds with Ca and As. But for fixed-bed gasification, there exists a thin oxidation layer on the bottom of the gasifier, in which the injected oxygen is used for combustion and in turn favors the formation of the compounds with Ca and As. Ca3(AsO4)2 has been found in coal gasification ash using TOF-SIMS (time-of-flight secondary ion mass spectrometry).60 3.4.4. Selenium. About 17% of selenium is retained in the ash (Figure 8), indicating more than 80% of Se is released from the coal. As with F, approximately 78% of Se in the gaseous phase is transferred into the condensate in the gas-cooling stage. H2Se presented in the gas phase when the temperature is higher than 100 °C according to the thermodynamic calculation results, and it becomes the primary species between 400 and 1100 °C. Small amounts of AsSe(g) and PbSe(g) also form with increasing temperature. The condensed phase of Se exists in the form of PbSe(s) when the temperature decreased below100 °C. 3.4.5. Lead. From the results given in Figure 8, it can be seen that, after pressurized gasification under 3 MPa and 1100 °C, approximately 15% of Pb is enriched in the ash and most Pb in coal is transferred into the gaseous phase. During the cooling stage, approximately 71% of Pb in the gas phase is condensed and transferred into the condensate. Therefore, pollution control of Pb should focus on downstream wastewater treatment. Pb in coal mainly exists in the forms of PbS and PbSe. The thermodynamic calculation indicates that, as the temperature increases, PbS(s) and PbSe(s) gradually decrease and disappear at 400 °C. Then, PbS(slaga) begins to form, accompanied by small amounts of Pb(g), PbS(g), PbSe(g), and PbCl(g). Pb(g) and PbS(g) become the primary gaseous species at temperatures between 800 and 1100 °C. When the temperature is higher than 1000 °C, PbO(slaga) is also observed, and the mole percentage takes up approximately 5%. Therefore, it can be concluded that Pb becomes highly volatile above 800 °C and the redistribution of Pb in the cooling stage is most important to the final removal of Pb. 3.4.6. Beryllium. After fixed-bed pressurized gasification under 3 MPa and 1100 °C, a large proportion of Be in coal is enriched in the ash with the mass fraction higher than 88%. Only 10% of Be-bearing compounds is entrained in the syngas and then transferred into the condensate during the cooling stage. Speciation transformation from thermodynamic modeling shows that, at 200 °C, Be(OH)2 in the solid phase reacts with some aluminum oxides to transform into BeAl2O4(s), which becomes the primary species of Be between 200 and 900 °C. Be(OH)2(g) appears at 500 °C and becomes the main gaseous phase species at temperatures higher than 1000 °C. There is a deviation between thermodynamic calculation and the simulation test since the volatility of Be is lower in the simulated fixed-bed pressurized gasification. This may be because, at high temperatures between 1000 and 1100 °C, the melting of coal ash may occur in the reducing atmosphere, and a proportion of Be-bearing species is easily enclosed in the agglomerates, then reduces the migration of Be into the gas phase.

trace elements are given in Figure 8. Simultaneous thermodynamic equilibrium calculations were carried out to simulate the

Figure 8. Distribution of trace elements among the out streams of the simulation test. Gasification conditions: 1100 °C, 3 MPa, H2O(l) 0.6 g/min, and O2 20 mL/min.

speciation transformation of trace elements in the gasification process, and the results are shown in Figure 9. 3.4.1. Fluorine. From the measured results given in Figure 8, it can be seen that fluorine shows a high volatility during fixedbed pressurized gasification and less than 15% is retained in the ash. A large proportion of F is finally present in the condensate after gasification, with the mass fraction taking up to 82%. F in coal mainly exists in the form of CaF2. The thermodynamic calculation indicates that when the temperature is higher than 200 °C, it transforms to gaseous HF, which becomes the dominant species between 400 and 1100 °C. In addition, trace amounts of F may combine with the elements H, O, C, Cl, Si, K, Mg, Na, U, etc., which could not be shown in Figure 9 due to the scale. Because HF(g) is highly water soluble, it can be dissolved into the condensate as the gas cools. 3.4.2. Mercury. Mercury is the most volatile trace element and gaseous mercury is dominant in the temperature range of gasification, which has been reported in previous studies.31,37 Based on the results, 88% of Hg is present in the gas and less than 1% is retained in the ash after gasification, which may be caused by the absorption of carbon/char. Speciation transformation from thermodynamic modeling shows that Hg(g) is the only gas species above 100 °C. When the gas temperature decreases below 100 °C, Hg is still retained in the gas phase. Thus, more attention needs to be paid during the gas-cleaning stage. 3.4.3. Arsenic. More than 68% of arsenic is found to be enriched in the ash after fixed-bed pressurized gasification at 3 MPa and 1100 °C, and thus it requires more attention during ash storage. Arsenic in the gaseous phase is redistributed as the high-temperature syngas flows through the condenser, and approximately 30% of As is trapped in the condensate. When the temperature is lower than 300 °C, As2S3 and As2S2 are the main forms of As existing in coal. At 400 °C, As4(g) appears and takes up 90%, together with trace quantities of gaseous AsSe, AsS, AsO, AsN, and AsH3. With increasing temperature, the AsH3(g) content significantly increases and becomes the main species of As between 600 and 1100 °C. Species of As2O3(slaga) and As2S3(slaga) appear at the gasification temperature of 1000 °C, and their contents gradually increase with increasing temperature. G

DOI: 10.1021/acs.energyfuels.6b01692 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

Figure 9. Equilibrium compositions of trace elements under oxygen−steam gasification at pressure of 3 MPa. (a) F, Hg; (b) As; (c) Se; (d) Pb; (e) Be; (f) U.

3.4.7. Uranium. The partitioning behavior of U during fixedbed pressurized gasification is similar to that of Be. Approximately 87% of U is retained in the ash, and the rest is transferred into the condensate. According to the thermodynamic calculation data of U, it is clear that U is very stable and exists in the form of UO2(s), even at gasification conditions of 3 MPa and 1100 °C. Only

when the temperature is above 1100 °C are some UO3, UO2F, and UO3(H2O) found in the gaseous phase. Based on the above study, As, U, and Be tend to be retained in the solid phase and require more attention for their further migration from ash. Mercury is retained even in the cooled gas, and its migration in the gas phase is concerning. A large proportion of F, Se, and Pb are transferred into the liquid H

DOI: 10.1021/acs.energyfuels.6b01692 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

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phase, together with some As, U, and Be, which should be a matter of concern and controlled in the downstream processes.

4. CONCLUSIONS The distribution behavior of F, Hg, As, Se, Pb, U, and Be was investigated using a laboratory-scale fixed-bed pressurized gasifier, and the influence of temperature and pressure was explored. The results are as follows: 1. The volatilities of As, Pb, U, and Be exhibit intermediate behavior under a pressure of 3 MPa. With increasing temperature from 800 to 1100 °C, the volatility of Pb increases from 44.6 to 87.5%. That is, Pb, a semivolatile element, becomes the most volatile. However, As, U, and Be exhibit the opposite trend, and their volatilities decrease from 77.1, 53.6, and 61.7% to 42.9, 30.9, and 32.6%, respectively. This means that the volatility of As decreases from “most volatile” to “semivolatile”, whereas U and Be become “least volatile”. F, Hg, and Se always have high volatility above 80%. 2. The volatilities of trace elements tend to decrease when the pressure increases from atmospheric pressure to 3 MPa. Arsenic is obviously affected by the pressure, and its volatility remarkably declines with increasing pressure, leading to the enrichment of As in the gasification ash. 3. For the out-stream distribution, As, U, and Be tend to be enriched in the ash in the form of arsenates, uranium oxide, and beryllium aluminates, respectively, after fixed-bed pressurized gasification under 3 MPa and 1100 °C, whereas large proportions of F, Se, and Pb in coal are transferred into the condensate in the form of hydrogen fluoride, hydrogen selenide, and lead sulfide, respectively, together with some As, U, and Be, which should be a matter of concern and should be controlled in the downstream processes. Mercury is mostly emitted with the gas, even when the gas temperature is below 100 °C, and thus its migration in the gas phase needs to be managed.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +861062339156. Fax: +861062339156. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support provided by the National Basic Research Program of China (973 Program) (No. 2014CB238905), the National Natural Science Foundation of China (No. 51476185), and the Fundamental Research Funds for the Central Universities (No. 2009QH13). The authors would like to express their gratitude to Prof. Shifeng Dai and his research group for the assistance during analysis of the samples.

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DOI: 10.1021/acs.energyfuels.6b01692 Energy Fuels XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.energyfuels.6b01692 Energy Fuels XXXX, XXX, XXX−XXX