Article pubs.acs.org/EF
Liquid-Quench Sampling System for the Analysis of Gas Streams from Biomass Gasification Processes. Part 2: Sampling Condensable Compounds M. D. Kaufman Rechulski,† J. Schneebeli,† S. Geiger,† T. J. Schildhauer,*,† S. M. A. Biollaz,† and Ch. Ludwig†,‡ †
General Energy Research Department (ENE), Paul Scherrer Institute (PSI), CH-5232 Villigen PSI, Switzerland School of Architecture, Civil and Environmental Engineering (ENAC−IIE), Swiss Federal Institute of Technology at Lausanne (EPFL), Station 6, CH-1015 Lausanne, Switzerland
‡
ABSTRACT: The performance of a continuous liquid-quench-based sampling system is presented, with regard to the sampling of condensable compounds. Toluene is used as a model molecule representing tars commonly found in the gas streams of biomass gasification plants and related research units. It is shown that the operational parameters of the sampling system can be adjusted for a correct sampling of condensable compounds. Moreover, the systematic measurement uncertainty caused by the use of the sampling system is evaluated in the case of the characterization of a raw producer gas from a bubbling fluidized-bed gasifier considering both condensable and noncondensable compounds. It is concluded that the systematic uncertainty is small for H2, CO, CH4, C7H8, and C10H8, comparable to the random measurement uncertainty for C2H4, C2H6, N2, H2O, and C6H6 and larger than the random measurement uncertainty for CO2, C2H2, C3H6, and COS.
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INTRODUCTION In biomass gasification plants and related research units, a quantitative measurement of condensable species, such as water and tars, in their gas streams is of key importance. Sampling of these streams is however not trivial, because at ambient conditions, they tend to condense, leading to erroneous measurements or even practical problems (e.g., blockage of the sampling lines).1,2 For this reason, several initiatives have been undertaken to develop systems that allow for the sampling and quantification of the condensable compounds.1,3−6 From one side, sampling equipments were developed to provide the means for representative measurements. From another side, analytical tools have been adapted or created aiming at the measurement of the different condensable species (especially tars) found in the thermochemical conversion of biomass. The most common sampling techniques successfully employed to sample condensable species can be classified on the basis of their working principle. In one case, the sampling tray is built to avoid the condensation of the compounds of interest, conveying them from the sampling point to the analytical equipment. This is typically performed using heated sampling lines and reducing the dew point of the compounds with a diluting gas and/or reduced pressure.3,7,8 These procedures usually allow for online measurements of the condensables, and the time resolution of the measurements is defined by the analytical tools used. The detection of trace compounds depends upon their transport through the sampling system and the limit of detection of the analytical equipment employed. Another sampling approach is to actively separate the condensable species by capturing them into a solid-phase adsorber bed (SPA) or a liquid solvent, which then have to be analyzed to characterize the sampled gas.9 SPA sampling of tars © 2012 American Chemical Society
is usually not continuous, and the analysis of the samples is typically performed offline, so that no real-time information about the stream is available.4,5 Methods using a liquid solvent to characterize tars are widely used6 and have been developed into a protocol,1,10 which standardizes the sampling tray, sampling method, and sample analysis. In this case, the procedure is performed batchwise: the gas is sampled through a series of impinger bottles, some containing liquid solvents at low temperatures, which promote the condensation of steam and tars. The analysis of the compounds trapped in the solvents provides information about the sampled gas via mass balance. The sampling time might be as long as 1 h, because the compounds of interest have to be concentrated in the liquid solvent to be detected by gas chromatography or gravimetry.11 Analysis of noncondensables in the sampled gas can be performed downstream of the sampling tray by classical detectors, such as a mass spectrometer (MS) or gas chromatograph (GC). Even though it allows for measurements of species at low concentrations, the tar protocol is neither able to provide time-resolved nor online characterization of the condensables in the sampled gas. At Paul Scherrer Institute (PSI), a continuous liquid-quenchbased sampling system has been developed for the characterization of condensable and noncondensable species in the gas sampled. The operating principle of the sampling system is based on the separation of these two classes of compounds by promoting condensation. The contacting between sampled gas and liquid solvent is performed in a continuous fashion, which allows for time-resolved measurements, be it online or offline. Received: February 15, 2012 Revised: May 11, 2012 Published: May 30, 2012 6358
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Figure 1. Setup used to investigate the sampling of condensable compounds. was 1-methoxy-2-propanol (CAS number 107-98-2, purchased from Brenntag Schweizerhall AG). This solvent has the advantages of (1) having a relatively high boiling point (121 °C), thus being suited for quenching the sampled gas at high temperatures, avoiding problems such as blocking of the sampling lines by precipitation of tars, (2) having a high autoignition point (246 °C), thus allowing for safe operation, and (3) having a high capacity to solubilize tars even in presence of water, as observed experimentally in the development phase of the sampling system. The sampling system 1 of Figure 1 was the one evaluated in this work. To calculate fgas of toluene in the sampling system 1, it is necessary to know the concentration of toluene in the “clean gas” stream (YiG,out); therefore, a second system was used to sample it. For both sampling systems, the concentration of toluene in the gas sampled can be calculated from measurements of the “fresh solvent” and “used solvent” liquid streams and a mass balance, which is presented below. Fraction of Condensable Compounds in the Sampled Gas. Equations 2−6 were developed to calculate the molar fraction of a condensable compound i in the sampled gas (YiG,in) from its molar fractions in the liquid streams. The general application of sampling a typical producer gas of biomass gasification plants is considered. It is assumed that the known values are flows of the “fresh solvent” and “clean gas” (FL,in and FG,out respectively) streams, as well as the concentration of the main compounds and species of interest present in the “fresh solvent” (XiL,in) and “used solvent” (XiL,out) liquid streams. To calculate the mass balance, given that steam is the condensable found usually in highest concentrations in the gas streams sampled, initially only water is taken into account to have a first guess of the total molar flow of species transferred from the gas into the liquid phase (ΣFiGL; eq 2). The flows of sampled gas (FG,in) and used solvent (FL,out) are then determined (eqs 3 and 4), and consequently, using the measurements of the liquid phase and eq 5, the individual flows FiGL can be estimated. This manner, ΣFiGL is calculated iteratively to correct for changes in the values of FL,out, averaged molecular weights, and densities of the liquid streams. It was assumed that the flow of solvent to the gas phase is negligible because of the low vapor pressure of the solvent under the experimental conditions. The gas dissolved in the liquid stream “used solvent” is not taken into account because of its small contribution to the total molar flows FG,in and FL,out. When the values of FiGL converge, YiG,in is obtained using a known or estimated value of YiG,out and eq 6.
Until now, this sampling equipment has been extensively used;12−16 however, it has not been explained in detail in the scientific literature, and its performance has not been investigated. In the part 1 of this series (10.1021/ef3008147), the equipment has been described and it was analyzed with regard to sampling of noncondensable compounds. In this paper, the performance of the system is evaluated for the sampling of condensable species that are dissolved into the quenching liquid. Additionally, an application of the equipment is presented: sampling of the raw producer gas of a bubbling fluidized-bed gasifier, focusing on both the condensable and noncondensable compounds. An analysis of the measurement uncertainties is conducted. It is shown that, if the sampling system is properly operated, the systematic measurement errors (i.e., bias) caused by the use of the sampling system can be kept small and comparable to the random error (i.e., variance) for most of the compounds measured.
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EXPERIMENTAL SECTION
Experimental Method and Apparatus. The liquid-quench sampling system developed at PSI has been described in more detail in part 1 of this series (10.1021/ef3008147); its working principle is as follows: the sampled gas is withdrawn from the gas pipeline and quenched with a continuously fed solvent. The gas−liquid mixture generated flows concurrently in the tubings of the system and is compressed, cooled, and separated again in a “clean gas” stream and a “used solvent” stream. This way, the condensable compounds present in the sampled gas should be captured into the quenching liquid, which can be analyzed online or offline, providing information about these species in the sampled gas. Figure 1 shows a simplified scheme of two sampling systems connected in series, which is the setup used in this paper, as will be further described. The effectiveness of removal of condensable compounds from the sampled gas into the liquid is determined by the operational parameters of the equipment, such as the amount of quenching solvent and cooling temperature. It can be quantitatively evaluated using the parameter apparent gas-phase partition factor ( fgas; eq 1), for which “clean gas” stands for the gas stream generated after the gas− liquid mixture is separated. The lower the concentration of a condensable in the “clean gas” stream, the better it is captured by the quenching liquid and the lower its fgas. The minimum attainable values of fgas are limited by the vapor−liquid equilibria of the condensable species at the conditions found where the phase separation takes place. i f gas
i YG,out mole fraction of i in clean gas = = i mole fraction of i in sampled gas YG,in
H2O H2O FL,in(XL,out − XL,in )
n i H2O ≅ FGL ≅ ∑ FGL
H2O (1 − XL,out )
i=1
(2)
n
FG,in = FG,out +
i ∑ FGL i=1
(1)
(3)
n
The performance of the sampling system was evaluated using it to sample a gas stream containing on average 0.175%mol (or 7.0 g/m3N) of toluene in nitrogen generated with a saturator. This amount of toluene is comparable to the sum of the amounts of light aromatic and polyaromatic tars typically found in the raw producer gas from fluidized-bed biomass gasifiers.9,17,18 The quenching liquid employed
FL,out = FL,in +
i ∑ FGL i=1
i i i FGL = FL,outXL,out − FL,inXL,in
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(4) (5)
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Table 1. Set Points of the Sampling System 1 for the Evaluation of Its Performance in Sampling Condensable Compoundsa experiment number
gas flow, G (LN/h)
liquid flow, L (mL/min)
G/L
temperature, T (°C)
pressure, P (barg)
number of experimental settings
1 2 3 4
10−60 60 30 57
0.40−1.79 0.87 1.79 2.00
92−2331 1143 279 475
−20 from −20 to +19 from −20 to +19 −18
0 0 0 0−5
8 3 3 4
G is the flow of “clean gas”. L is the flow of “fresh solvent”. G/L stands for the dimensionless ratio of G and L. T and P are the temperature and pressure, respectively, of both the cooling coil and separation column. a
i YG,in =
i i + FGL FG,outYG,out
FG,in
systematic and random measurement errors involved in the use of the sampling system, a gasification experiment was conducted using a bubbling fluidized-bed gasifier fed with wood pellets and air as the gasification agent. The internal diameter of the gasifier is 52 mm, with a height of 1.0 m, and 500 g of Al2O3 was used as the bed material. The composition of the wood pellets is given elsewhere.13 Sampling of the raw producer gas was performed through a metallic hot gas filter (400 °C; 5 μm pore size) at the outlet of the gasifier. The permanent gas composition was monitored at the “clean gas” stream of the sampling system using a micro-gas chromatograph [μGC, Varian CP 4900 with two PLOT columns, MSA5 and PPU, 0.32 mm inner diameter and 10 m long, and thermal conductivity detectors (TCDs)]. The compounds monitored were H2, CO, CO2, CH4, N2, C2H2, C2H4, C2H6, C3H6, H2S, and COS. The operational parameters of the gasifier and sampling system were as follows: wood feed, 0.74 kg/h; air flow, 19.74 LN/min; average bed temperature, 741 °C; and pressure, 109 mbarg. The sampling system was operated with a flow of “clean gas” (FG,out) at 88 LN/h, 0.87 mL/min of “fresh solvent” (FL,in), cooling temperature of −19 °C, and 2 barg of pressure. Samples of the “used liquid” stream were taken every 15 min for offline analysis of condensables. Every time a liquid sample was withdrawn, the gas flow through the sampling system was measured, as well as the temperature and pressure of the cooling coil and separation column. Gas flow measurements were performed using a 100 mL bubble meter column and repeated 3 times. The liquid pump was calibrated prior to the experiment, and the standard error of the calibration curve was calculated to be 0.05 mL/min following the recommendation of Coleman et al.19 Karl Fischer coulometry was used to determine the water content of the samples (Metrohm KF coulometer 737 series 01); each sample was measured 3 times, which provided an average value and a standard deviation. The tar content was measured using the GC/MS previously described. One sample was injected 3 times into the GC/MS, and the relative standard deviation of benzene, toluene, and naphthalene was estimated and considered constant for the other samples analyzed. Evaluation of the Measurement Uncertainties. The real average concentration of a compound in the sampled gas (Y̅real) is unknown, but an approximate value (Y̅ G,in) can be determined using the sampling system and the necessary analytical instruments (μGC, GC/MS, etc.). As shown in Figure 2, the use of the sampling equipment induces a systematic measurement error (or measurement bias, b), which is related for example to the dissolution of gases into the quenching liquid or the incomplete condensation or dissolution of condensables in the quenching liquid, as discussed in parts 1 (10.1021/ef3008147) and 2 (this paper) of this series. fgas is a direct measure of this bias.
(6)
The value of YiG,out of system 1, which is necessary for the mass balance, can be determined experimentally when a second sampling system is used to sample its “clean gas” stream, as shown in Figure 1. Again, YiG,out of system 2 has to be estimated or measured, but its influence on the measured fgas values of system 1 is limited to a few percentage points. To illustrate, system 2 was connected in series to a third sampling system to sample its “clean gas”. This way, fgas of both systems 1 and 2 could be measured. Assuming that fgas of system 3 is 0, fgas of system 2 was calculated to be around 4%, which resulted in a fgas of system 1 of only 0.37% higher in absolute values than if fgas of system 2 was 0; the difference of 0.37% is well within the measurement uncertainties. Therefore, when using fgas to evaluate the performance of the sampling system, it is important to be able to determine YiG,out of the sampling system 1, but YiG,out of system 2 can be considered 0.
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EXPERIMENTS CONDUCTED Influence of Operational Parameters on Liquid-Phase Measurements. The performance of the sampling system to sample condensable compounds was investigated to point out which operational parameter is more significant for representative quantification of condensables using the information of the liquid samples. The operational parameters analyzed are the flow of gas through the system, measured at the “clean gas” stream (G), the flow of quenching liquid measured at the “fresh solvent” stream (L), temperature, and pressure of the cooling and separation steps. The sampling system 1 in Figure 1 was the one analyzed, and Table 1 summarizes the set-point values investigated. In total, 18 different experimental settings were tested by varying gas flow, liquid flow, temperature, and pressure within the possible range of the setup using at least three different values for each parameter. The system was kept stable for more than 30 min to ensure high data quality. The cooling temperature and pressure of the sampling system 2 were kept constant at −20 °C and 0 barg, respectively, and its G/L ratio varied from 33 to 600. To determine the concentration of toluene in the “fresh” and “used solvent” streams (XiL,in and XiL,out), samples of them were collected in GC vials for offline analysis with a GC/MS (GC Agilent 6890 with a WCOT column, 60 m in length, 0.25 mm inner diameter, and 0.25 μm film thickness, MS Agilent 5973). Measurements of the residence time of the liquid phase were performed using an ultraviolet−visible (UV−Vis) spectrometer (Ocean Optics single-beam spectrometer USB2000+XR1, with a deuterium tungsten−halogen light source DT-MINI-2-GS) equipped with a flow-through cell (2 mm optical path length and 2 m optical fiber) connected at the “used solvent” stream. Phenanthrene was used as the trace compound in the sampled gas, and the wavelength of 254 nm was monitored with a time resolution of 1 Hz. Comparison of Systematic and Random Measurement Errors. Experiments Conducted. To evaluate the 6360
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RESULTS AND DISCUSSION Influence of Operational Parameters on Liquid-Phase Measurements. In the following results, fgas of toluene in sampling system 1 is reported as a function of the different operational parameters of the sampling system tested. Gas and Liquid Flows. The quenching liquid improves the heat removal from the hot sampled gas and collects the condensing species. The flows of sampled gas and quenching liquid through the system determine their contact time, residence time, and flow regime, from which the comprehensive outcome is evaluated with fgas. Figure 3 indicates that fgas of
Figure 2. Scheme of the average real molar fraction of a compound in the sampled gas (Y̅real), the spread as a result of random variations (sreal), the average concentration measured via the sampling system (Y̅G,in), the measured spread (sYG,in), and the measurement bias (b).
Other sources of uncertainty in the measurements are, for example, the inherent random variation of the measured concentration (represented in Figure 2 as a distribution curve), the calibration of the measurement device (e.g., GC/MS), errors in the measurements of flow of sampled gas and quenching liquid, and differences between operators. The importance of the measurement bias as a result of the sampling system was evaluated by comparing it to the random error of the measured molar fractions in the sampled gas. The uncertainty related to the systematic measurement error (b) in the gasification experiment was calculated as the difference between the average measured value (Y̅; eq 7) and a corrected average (Y̅corr), as given in eq 8. Y̅corr is an approximation of the real unknown concentrations in the sampled gas, and it was calculated as the value Y̅ corrected by the corresponding fgas, which was determined on the basis of the independent systematic experiments described in parts 1 (10.1021/ ef3008147) and 2 (this paper) of this series. The uncertainty related to the random variation (i.e., spread around the mean concentration) was given by the standard deviation (s; eq 9) of the concentrations measured N times during a stable operation of the gasifier (Yj, where j = 1, ..., N). Particularly for condensable species, this represents the random uncertainty in the first-order replication level, that is, both uncertainty from sample to sample and the uncertainty of the analysis of each sample (zeroth-order replication level uncertainty).19 Outliers were not taken into account, given that, during the measurement, for a few instances, the μGC was not able to measure the producer gas and measured air instead. For each compound i, si and bi were normalized by the average measured concentration, so that the relative random uncertainty (RRU) and the relative systematic uncertainty (RSU) are calculated (eqs 10 and 11). Y̅ i =
1 N
Figure 3. Influence of gas flow (G) and liquid flow (L) on the apparent gas-phase partition factor ( fgas) of toluene. Results of experiment 1 are shown.
toluene can be kept below 4% (i.e., 96% capture in the quenching liquid) for the range of liquid flow tested (0.4−1.79 mL/min) if the gas flow is below 30 LN/h. Sampling System Concentrates Condensable Compounds. When choosing the liquid flow to be used, one factor to be taken into account is the limit of detection of the analytical tools that will analyze the liquid streams, such as a GC/MS. While high liquid flows are desired to ensure capture of condensable species, the liquid samples generated will be more diluted, which is undesirable if one is interested in trace compounds. When the G/L ratios are correctly manipulated, the sampling system can be used to concentrate a certain compound in the liquid. Figure 4 illustrates the calculated concentration of a trace compound in the “used solvent” stream (Xtrace L,out) when the sampling system is used to characterize a gas containing 20%mol of steam and 0.45 ppmmol of the trace compound (Ytrace G,in ). The mass balance equation (eq 6) was employed, and fgas is assumed as 5%. An analytical tool with a limit of detection (LOD) of 0.5 ppmmol and a limit of quantification (LOQ) of 1.0 ppmmol is considered. Direct analysis of the trace compound using this analytical tool is not possible, because Ytrace G,in is smaller than the LOD; however, when using the sampling system, the G/L ratio can be adjusted to generate a “used solvent” stream sufficiently concentrated with the trace species. In the example given, Xtrace L,out increases asymptotically with higher gas flows (for a fixed liquid flow of 1.38 mL/min) because the condensing water
N
∑ Y ji (7)
j=1
i bi = |Y ̅ i − Ycorr ̅ |
1 (s ) = N−1 i 2
(8) N
∑ (Y ji − Y ̅ i)2 j=1
(9)
RRUi =
si Y̅ i
(10)
RSUi =
bi Y̅ i
(11)
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of the final signal are shown. In both cases, the flow regime observed was Taylor flow, that is, a sequence of liquid and gas slugs flowing through the tubes with a liquid film wetting the walls. The residence time of the liquid in the system is given by the values of t10% in Table 2. It defines the time delay between the moment an event happens in the sampled gas (e.g., change in the concentration of phenanthrene) and its detection at the “used solvent” stream. A small liquid flow results in a high residence time; consequently, there is a compromise between concentrating the liquid stream to detect trace compounds and having a delay in the measured values. Together with the flow regime, the residence time also defines the degree of internal mixing of the liquid phase. As given by t90% − t10% in Table 2, the signal increase was steep, indicating a limited forward axial mixing of the liquid phase, as expected in a Taylor flow regime. Judex employed an analogous version of PSI’s sampling system but with different tube lengths than the lengths reported here.13 Using a liquid flow of 1.5 mL/ min and gas flow of 60 L/h, resulting in Taylor flow regime, he showed that, when an increasing concentration step is given at the sampled gas, the concentration in the liquid phase takes around 1 min to increase from 10 to 90% of its final value but 10 min in the case of a decreasing step to 10% of the initial signal. Damping of the decrease of sampled gas concentrations by the liquid solvent is therefore important for the measurements of condensable compounds. In the sampling system, the flow regimes observed at the cooling coil within the conditions described in Table 1 ranged from stratified (high G/L) to Taylor flow (low G/L). As explored by Saxena et al.,20 in a twophase stratified gas−liquid flow in coiled tubes, the velocity of the liquid at the interface is faster than the velocity near the walls, resulting in forward mixing of the liquid. Backward axial mixing occurs by diffusion mechanisms. In a Taylor flow, internal mixing of the liquid is rather limited because the contact between the consecutive liquid slugs, which are separated by the gas bubbles, is small. The main mechanism responsible for the back mixing of the liquid is the mass exchange between the liquid film on the wall and the liquid slugs.21 With small slug size and thin film, the axial mass transfer is kept small,21 making Taylor flow regime more
Figure 4. Sampling system can concentrate a compound from the sampled gas into the “used solvent” stream. Liquid flow is assumed constant, at 1.38 mL/min.
from the sampled gas dilutes the “used solvent” stream. The trace compound of interest could be quantified by this analytical tool if the gas flow is set to at least 60 LN/h (G/L of 724). Residence Time of the Liquid and Internal Mixing. The residence time of the liquid is determined by the dimensions of the equipment, the flows of gas and liquid, and the flow regimes. For indicative purposes, the residence time of the liquid phase was measured using two different liquid flows and a constant gas flow. Results are presented in Table 2. Time zero Table 2. Residence Time of Liquid Phase for Different Flows of Quenching Liquid gas flow, G (LN/h)
liquid flow, L (mL/min)
G/L
t10% (min)
t50% (min)
t75% (min)
t90% (min)
t90% − t10% (min)
18.2 20.2
0.99 1.97
307 170
22.0 9.0
26.0 10.0
30.0 11.0
33.0 11.7
11.0 2.7
is the moment the trace compound phenanthrene was injected in the sampled gas, and the times to reach 10, 50, 75, and 90%
Figure 5. Influence of (a) temperature and (b) pressure in the separation column on the apparent gas-phase partition factor ( fgas) of toluene. G/L ratios used are indicated. In panel a, results of experiments 2 and 3 are shown, and in panel b, results of experiment 4 are reported. 6362
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Table 3. Composition of the Producer Gas from the Gasification Experiment: (a) Molar Fractions on a Dry Basis as Measured by the μGC and (b) Determined from the Liquid Samples Assuming s fgas of 0 (a) molar fractions (%dry) H2
CO
CO2
CH4
N2
13.92
13.29
15.25
3.71
52.56
H2O 9.83
C2H2
C2H4
C2H6
C3H6
H2S
COS
0.03 0.78 (b) molar fractions (%wet)
0.28
0.18
0.00
1.90 × 10−4
C6H6
C7H8 −2
6.85 × 10
3.60 × 10
interesting than the stratified flow regime for characterizing condensable compounds in the sampled gas. Temperature and Pressure of the Separation Column. Both the temperature and pressure play a role in determining YiG,out and, therefore, fgas of a given species. High pressures at the cooling coil increase the dew point of the condensable compounds and together with the low temperatures promote their condensation or dissolution in the quenching liquid. Additionally, the temperature and pressure of the separation column determines the conditions for the vapor−liquid equilibrium and, therefore, the partial pressure of toluene in the “clean gas” stream. The effect of both the temperature and pressure on fgas of toluene was investigated and behaved as expected, as shown in Figure 5. Important to notice in Figure 5a is the effect of the G/L flow ratio. As discussed before, a higher G/L ratio will result in a more concentrated liquid stream and, therefore, in a higher partial pressure of a condensable compound in the “clean gas” stream. Figure 5b indicates that, relative to the temperature, pressure plays a less important role when the system cools the sampled gas to −20 °C. For these conditions, the capture of condensable compounds into the quenching liquid can be improved by increasing the pressure of the sampling system to 2 barg, after which no significant effect was observed. Pressurizing the sampling system can also be beneficial when aerosols (e.g., potassium chloride) are present in the sampled gas. As discussed by Kowalski et al.,22 the pressurization of the gas−liquid mixture induces supersaturation of the solvent vapor, which condense on the particles, causing them to precipitate into the liquid solvent. Other characteristics of the sampling system that also promote the precipitation of aerosols are the mixing between the gas and liquid solvent at the quench point and the reduction of the temperature. Judex showed that, while impinger bottles at −25 °C present a slip of particles of about 20%, PSI’s sampling pressurized system better captures the particles into the quenching liquid: the slip obtained was smaller than 0.1%.13 Operational Parameters for Simultaneous Gas- and Liquid-Phase Measurements. The experimental results presented indicate that, for liquid-phase measurements, a cooling temperature should be kept as low as possible, preferably below −20 °C, while liquid flows above 0.8 mL/min are desirable for the range of gas flow used (10−60 LN/h). For measurements of gas-phase compounds (e.g., permanent gas), all system parameters investigated affect the measurements in an opposite fashion as the liquid phase [see part 1 of this series (10.1021/ ef3008147)]; therefore, when simultaneously sampling both classes of compounds, an optimized operational point of the sampling system could be suggested as follows: (1) the temperature should be equal to or smaller than −20 °C; (2) the pressure should be equal to or smaller than 2 barg; and (3) the fresh solvent flow should be between 0.8 and 1 mL/min.
C10H8 −2
1.01 × 10−2
Comparing Systematic and Random Measurement Errors. The systematic measurement uncertainties induced by the use of the sampling system were evaluated by comparing it to the random error of the molar fractions measured. The sampling system was used to characterize the raw producer gas of a bubbling fluidized-bed gasifier, which was described above. The composition of the sampled gas, as measured by the μGC and using the liquid samples considering a fgas of 0, is given in parts a and b of Table 3. The RRU and RSU were calculated, and the results are plotted in Figure 6. The values of fgas of steam, benzene, and
Figure 6. RRU and RSU of the species in the raw producer gas of the fluidized-bed gasifier.
naphthalene were estimated on the basis of that of toluene, which is the condensable compound used in the systematic experiments, in a conservative approach. Considering the composition of the gas sampled (Table 3), the dew point of benzene is 2 times lower than that of toluene (12 and 24 °C, respectively); thus, fgas of benzene was assumed to be 2 times higher than that of toluene. The dew points of naphthalene and steam are 81 and 99 °C, respectively (well above the temperature that the sampled gas was cooled); nonetheless, their fgas was assumed the same as that of toluene. The compounds monitored can be divided into three categories, according to how important the bias is that results from the liquid-quench sampling system. In the first category, the bias caused by the sampling system (RSU) is considerably smaller than the spread of the measured concentration (RRU). H2, CO, CH4, C7H8, and C10H8 belong to this class, and their measured values can be considered representative of their real concentration in the sampled gas. The reason here is that the concentration of these species is relatively high, and/or they do not tend to be distributed over both phases because of either very high partial pressure and little solubility (e.g., H2 and CH4) or low vapor pressure (e.g., naphthalene). In the second 6363
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group he applies another or complementary sampling or analysis method.
category, the spread of the values measured is at most about as large as the bias. This is the case for C2H4, C2H6, N2, H2O, and C6H6. Here, the relative concentrations are lower and/or the solubility in the quenching liquid is higher than for the previous group. At last, the measurement bias is considerably larger than the spread of the measured values for CO2, C2H2, C3H6, and COS. Besides CO2, these species have in common a high solubility in the quenching liquid and a relatively low concentration. CO2 has not only a high solubility but also a high concentration in the sampled gas; it seems that its random error is damped by the sampling system, because CO2 presented a small standard deviation, whereas it would be expected to be comparable to the standard deviation of CO. The measured concentration of H2S was 0; therefore, it was not included in Figure 6. This can be explained with the combination of two phenomena: (1) the low sulfur content of the gasified wood pellets and, as a result, the low H2S concentration in the sampled gas (from a mass balance, the maximum amount of sulfur in the producer gas is around 36 ppmmol, even if the sulfur contained in the ashes is assumed to be 0) and (2) the adsorption of H2S on the filter cake and on metallic parts of the system (e.g., the filter candles).
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from the Swiss Federal Office of Energy (BfE, Project SI/500479) and Competence Center Energy and Mobility (CCEM, Project Syngas Diagnosis) is gratefully acknowledged. The authors also acknowledge Philip Edinger for the assistance with the UV−Vis measurements.
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CONCLUSION The performance of a continuous liquid-quench-based sampling system was evaluated considering the sampling of condensable compounds. It was shown that the flows of sampled gas and quenching liquid affect fgas of toluene (as a model compound for tars) over the equipment and that a liquid flow of at least 0.8 mL/min is recommended to keep fgas small. The ratio between gas and liquid flows can be adjusted if one is interested in capturing trace compounds into the liquid phase, but the residence time of the liquid will also be affected. It was also discussed that the Taylor flow regime in the cooling coil is desired, because it minimizes the internal mixing of the liquid phase and, therefore, can provide a sampling technique sensitive to the small variations of the molar fraction of the condensable compounds. The cooling temperature also considerably affects fgas of condensable compounds; if combined with high G/L ratios, the cooling temperature should be as low as possible to ensure condensation or dissolution in the quenching liquid. At −20 °C, fgas is not strongly affected by the pressure of the separation column; however, the benefits of compressing the gas−liquid mixture cannot be neglected, such as a high efficiency in capturing particles from the sampled gas into the liquid. Generally, the optimized configuration of the sampling system to characterize noncondensable compounds is the opposite of the system desired for the condensable compounds. In case simultaneous characterization of both classes of compounds is desired, a compromise has been suggested on the basis of the experimental evidence. Finally, an analysis of the measurement uncertainties has been conducted by presenting a sampling application (characterization of the raw producer gas from a bubbling fluidized-bed gasifier). It was shown that the systematic uncertainty caused by the use of the sampling system is small for H2, CO, CH4, C7H8, and C10H8, comparable to the random measurement uncertainty for C2H4, C2H6, N2, H2O, and C6H6 and larger than the random measurement uncertainty for CO2, C2H2,C3H6, and COS. With the presented results, the user of the sampling system can decide which accuracy he wants to achieve with respect to which group of species and for which
NOMENCLATURE b = systematic measurement uncertainty F = molar flow fgas = apparent gas-phase partition factor RRU = relative random uncertainty RSU = relative systematic uncertainty s = random measurement uncertainty X = molar fraction in the liquid phase Y = molar fraction in the gas phase Y̅ = averaged molar fraction in the gas phase
Subscripts
corr = corrected measured value G or gas = gas phase GL = mass transfer between gas and liquid phases in = entering the sampling system j = measured data L = liquid phase out = leaving the sampling system real = real (unknown) concentration Superscripts
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i = generic compound
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