Article pubs.acs.org/EF
Assessment of the Solid-Phase Adsorption Method for Sampling Biomass-Derived Tar in Industrial Environments Mikael Israelsson,* Martin Seemann, and Henrik Thunman Department of Energy and Environment, Division of Energy Technology, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden ABSTRACT: Low-temperature gasification of biomass is a primary process step for production of biofuels or electricity using gas-fired engines or turbines. In addition to the desired product gas, the raw gas produced through gasification inevitably contains condensable hydrocarbons, known collectively as tar. The amount and composition of the tar have relevance for the efficiencies of the downstream processes. Tar can be measured using both online and off-line methods. However, many of these methods currently lack information regarding their implementation and accuracy levels for large-scale systems. In this work, the SolidPhase Adsorption (SPA) method for measuring tar in industrial applications is evaluated. The individual steps of the method were examined for their effects on the overall performance of the analysis. Sample collection was found to be the most prominent source of error, and this was mainly due to human factors. Omitting to determine the temperature and pressure of the sampled gas contributed to this error, as the sampled volume of gas under normal conditions could not be correctly calculated. Inconsistencies in the treatment and storage of the collected samples were shown to affect the more volatile species with boiling points similar to that of benzene. The gas chromatography (GC) analysis was performed with satisfying accuracy. However, the reliability of the estimations of the average composition and dew-point of the tar mixture were dependent upon the amount of the identified species. The current implementation of the SPA method yields values with a relative standard deviation within 10% for the majority of the compounds in a given sample. However, in line with the result of previous studies, the tar species with boiling points between those of benzene and xylene (i.e., the BTX compounds) were measured with a lower accuracy than those of heavier tars.
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INTRODUCTION The increasing demand for green energy has spurred research into renewable energy sources, such as biomass. Currently, several different applications for the utilization of biomass are being studied intensively. One promising process is gasification, in which solid biomass is converted into a gaseous fuel. Lowtemperature gasification of biomass at 700°−900 °C, which produces a combustible gas mixture, is a first step toward the production of synthetic green fuels. Unfortunately, this process also yields a spectrum of unwanted condensable hydrocarbons, generally referred to as tar. Various definitions of tar have been proposed.1 However, in the present work, tar is defined as all aromatic species that have a boiling point higher than or equal to that of benzene. Tar starts to condense at temperatures around 300−400 °C, which means that extensive gas cleaning is required to prevent damage to downstream equipment. This cleaning may involve wet scrubbing, thermal cracking or catalytic processes to crack or reform the tars.2−5 An advantage of the latter catalytic processes is that the energy bound up in the tar, which represents 5%−15% of the total energy of the fuel,6 is partially transformed to usable energy. Accurate measurements of the amount of tar and tar composition are necessary for improving the ability to optimize the process to obtain a desired product of sufficient quality. Thus, reliable methods with verified accuracies for measuring tars are needed, and these methods should include a tool for determining the error of measurement. Extensive studies have been carried out to develop ways of measuring the tar content of the product gas using both online and off-line methods.7−13 The two main off-line methods are (1) the tar protocol,12 © 2013 American Chemical Society
which is a cold trapping method whereby the tar is collected in a series of cooled impinger bottles that contain a solvent, and (2) Solid-Phase Adsorption (SPA),7 in which tar is adsorbed onto a solid-phase extraction (spe) column containing an amino phase and subsequently desorbed using a solvent. While both methods allow for quantification of individual tar species, they differ with respect to the sampling procedure employed. Previous work14 was performed to compare the two methods which yielded similar results for species heavier than xylene. A secondary adsorbent column was used to improve the measurement of the BTX-species, however, the core amount of heavier species were adsorbed in the first column. The SPA method is widely used, probably due to its less-complicated procedure for sample collection. However, information is sparse regarding the overall accuracy of the method, and practical guidelines for implementing the method in an existing process are lacking. In the present study, raw gas was sampled from the Chalmers 2−4 MWth indirect gasifier,6 which was coupled to a 12 MWth circulating fluidized bed boiler. In this environment, it is not convenient to apply the tar protocol, as safety regulations within the facility mandate that the handling of organic solvents be kept to a minimum. Furthermore, large-scale processes exhibit both rapid and slow variations of unknown frequencies during operation, which affect the amount and composition of Received: September 20, 2013 Revised: November 8, 2013 Published: November 11, 2013 7569
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the tar. The comparatively long sampling times involved in the tar protocol (i.e., 30−60 min)7 render it impractical for the determination of rapid variations, which, instead, are represented as a mean value. If the process variations are large, the above can lead to complications in matching the tar data to the measured gas data, as gas measurements are typically carried out at a significantly higher frequency. The long sampling times also make it difficult to compare different sample results from the same experimental point, so as to determine the error of measurement. A faster version of cold trapping developed at the Paul Scherrer Institute in Switzerland (PSI)15 can be used for resolving variations in the tar output. However, this method still requires the use of a solvent on-site, as well as similar amounts of equipment. The tar spectrum can also be analyzed by online gas chromatography (GC), removing the need for sample collection altogether. While this might reduce the error of sampling, for a GC run time of around 50 min, several GC instruments would be needed to obtain satisfactory resolution. The SPA method can be used to obtain measurements at a relatively high resolution, provided that the measurement error is small. In addition, sample collection for SPA analysis does not require the use of solvents or multiple items of equipment on-site. After consideration of the various strengths and weaknesses of the above-mentioned methods, the SPA method was chosen in the present work based on the practical aspects of its implementation in an industrial system. The principal utilization of and theory underlying the SPA method have been thoroughly described by Brage and others.7,16−18 In the present work, the SPA method for analyzing tar was investigated to determine the overall accuracy and repeatability of the method in large-scale applications. Furthermore, the effects of associated factors, such as sample extraction, gas lines, and sample treatment, were examined.
Figure 1. Steps in the tar measurement process.
ment error should be small enough to allow determination of the variation in tar output. The gas is transferred from the gasifier to the sampling point in heated pipes and it is often cleared of particles using a filter. If soot or entrained bed material is accumulated on the filter it can have a catalytic effect on the amount and composition of the tar. Presumably, this effect would be more prominent if an active bed material was to be used in the gasifier. Furthermore, the amount and composition of the tar could change due to the condensation of heavy tars or the reactions that may occur during prolonged residence times in heated pipes. The degrees of significance of these effects are unit-specific and need to be determined for each unique setup, sampling point, and bed material, so as to obtain an unbiased measurement. The sampling point for SPA measurements typically consists of a heated volume, through which the gas flows, with a septum mounted a short distance from the wall, to avoid melting. The sampling point temperature should be maintained at approximately 350 °C, to avoid condensation of the tar. Currently, there is no standardized design for the SPA sampling point. Inward leakages may occur during sampling in systems that are operated at subatmospheric pressures. This results in inaccurate sample collection, as an unknown fraction of air is extracted with the raw gas. Therefore, the gas sampling line should be designed to enable the detection of leaks, preferably by quenching the gas flow directly after the sample point to minimize reactions with the leaked oxygen. Sample extraction is performed using a sample syringe, pump or other equipment to extract evenly over a period of about 1 min a set volume of raw gas, usually 100 mL, through the spe column. During extraction, to obtain a representative sample from subatmospheric systems, it is important to ensure that there are no leakages in the septum or the extraction device. The column increases in temperature as the gas passes through, as a consequence of the gas temperature and steam condensation. If the increase in temperature is sufficient to decrease tar adsorption, active cooling is needed for reliable measurements. The impact of increased sampling temperatures and, by extension, column temperatures has been thoroughly
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THEORY Gas sampling and analysis systems for gasifiers are available in different sizes and designs, depending on the specific application. Nevertheless, since tar measurements are performed on hot gas, generalizations can be made with respect to the process layout. Figure 1 shows the steps through which the tar passes from creation to quantification by SPA and GC analysis. In this process, tar is produced in the gasifier (Step 1) and is transported with the raw gas through heated pipes and a particle filter (2) before it reaches the sampling point (3), where it is extracted (4). The collected sample (5) is eluted (6), to generate one aromatic and one phenolic sample (7), both of which are analyzed using a GC flame ionization detector (FID) (8). The different steps in this chain are described below, and the challenges faced in each step in order to obtain an acceptable accuracy are highlighted. With the exception of steps 5−7, the above steps are not unique to the SPA method. Therefore, the challenges to be overcome are common to all the methods used for tar measurements. A gasifier that operates with a continuous fuel feed is likely to give variable outputs of gas and tar due to the variations in fuel feed, fuel composition, temperature, and other process-related parameters. Currently, these variations are included in the error calculation for the tar measurement, making it difficult to distinguish between incorrect measurements and the presence of an unstable process. The SPA method might facilitate a sufficiently high resolution of the measurement to allow observations of the process variations. However, the measure7570
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described.19 However, steam was not present in these measurements as the tar components were evaporated in air. On occasion, the needle becomes blocked by pieces of the septum, which complicates both the sample collection and elution of the sample. Therefore, the method for sample collection should allow for the detection of disruptions during sampling, such as the creation of a partial vacuum in the extraction device. SPA columns of different sizes and compositions can be used for tar measurements, depending upon the tar concentration and composition. However, regardless of the application, complete adsorption and desorption of the measured components are, obviously, prerequisites for reliable measurements. The accuracy of the LC-NH2 column16 and the accuracy of the SPA method7 for laboratory measurements have been reported. In the latter publication, it was stated that the laboratory measurements yielded almost complete adsorption of all tar species, with the exceptions of benzene and toluene, which showed significant breakthrough when a column containing 100 mg of amino phase was used. The use of 500 mg of amino phase further decreased the breakthrough of these volatile species to below the level of detection. Compared with online GC, the SPA method yielded roughly 90% of the benzene amount measured by GC, although this was believed to be due to the benzene evaporating during the elution of the column. Similar measurements were performed by ECN and presented during the 20th European Biomass Conference and Exhibition (June 21, 2012, in Milan, Italy).18 In this case, a less satisfactory breakthrough behavior was observed, with significant breakthrough of tar species (ranging from benzene to xylene) being observed for a column containing 100 mg of amino phase. Results for 500 mg of adsorbent were not presented. Measurements on 500 mg of aminopropyl-bonded silica adsorbent were performed,19 describing incomplete adsorption of benzene and the effects of altering the temperature of the sampled gas. Other types of columns have also been investigated,19,20 and they have shown promising results for measurements of lighter tar species. However, the present work is limited to determining the performance of the SPA method using the 500-mg LC-NH2 column. The collected columns should be eluted and analyzed directly after sampling, so as to minimize desorption. Unfortunately, this is rarely the case, as laboratory facilities are often located some distance away from the sampling site. Thus, there is a need for information on how well the tar in the samples remains bound to the adsorbent at room temperature or in a freezer. In this respect, clear effects of storage at room temperature, especially for the BTEX compounds (benzene, toluene, ethylbenzene and xylenes), have been reported previously.17 Furthermore, storage at −20 °C resulted in complete desorption of the BTEX compounds after 2 months, as well as slight reductions in the amounts of detected indene and phenol. However, data describing the short-term effects of sample storage are necessary to ensure that the entire sample is analyzed. The practical aspects of the SPA method, with regard to assaying compounds ranging from benzene to coronene, are described elsewhere7 but can be summarized here as follows. Prior to elution, 50−100 μL of one aromatic internal standard (tert-butylcyclohexane) and one phenolic internal standard (4ethoxyphenol), both of which are dissolved in dichloromethane (DCM) to 2−3 mg/mL, are added directly to the column. For elution of the columns, they are first flushed with 1.8 mL of
pure DCM, to separate the aromatic fractions from the column, after which 1.5 mL of a mixture of eight parts DCM, one part isopropanol (IPA), and one part acetonitrile (ACN) is used to elute the phenolic compounds. A weak flow of nitrogen is used to push the solvent through the column for both separated fractions. N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA) is added to the phenolic samples 1 h before the analysis, to generate trimethylsilyl ethers. The main challenges associated with the elution step are related to the separation of the aromatic and phenolic species, as well as evaporation of the sample. Evaporation can occur from the internal standard (before it is added), the column, and the extracted liquid. Thus, the elution step should be performed quickly and excessive heating of the samples should be avoided. It seems likely that the GC analysis will always be the timedetermining step for the SPA method, and eluted samples are typically stored in a freezer until analysis. Samples contained in tightly sealed 2-ml autosampler vials show no visible signs of evaporation even after 1 year of storage in a freezer at −20 °C. Presumably, these vials will also preserve the samples at room temperature, although it is not known for how long sample integrity is maintained under this condition. After the vial septum is pierced, evaporation of the sample is noticeable and at room temperature, the sample can be completely evaporated in a couple of weeks. Thus, good reproducibility and accurate calibration of the gas chromatograph are essential, given that the reliability of the sample will decrease with time. Fortunately, piercing of the septum is usually concomitant with the analysis of the sample, so evaporation is rarely a problem. Nevertheless, the risk of evaporation is a factor that should be considered when additional analysis of a sample is required. The GC is calibrated using external standards for identified species of interest. These identified species can be used for predicting dew-point and average C, O, and H compositions of the tar mixture. Dew-point calculations are based on heavier species, although the majority of the total mass needs to be identified to obtain a reliable value for the average composition. Furthermore, as unknown peaks are typically quantified using the toluene calibration, the error in quantification can be reduced by using standards for all the major components. As individual calibration for the entire spectrum is not feasible, the ratio of the identified mass percentage to the number of calibrated species needs to be optimized. In summary, all the steps depicted in Figure 1 may contribute to the overall error of measurement. Furthermore, with the possible exceptions of the elution and analysis steps, these errors tend to produce an underestimation of the actual amount of tar. In line with many other types of measurement, the steps that present the highest risks for errors are the gas treatment and subsequent sampling steps. However, the individual effects of all stages should be examined to create a clearer picture of the entire method. Furthermore, as the different species may be affected differently at certain stages, e.g., the BTEX compounds during storage, it is difficult to describe the accuracy of the SPA method using only one value for the variation. Therefore, several values for the variation are needed, including those for different species or boiling points, so as to detect errors that only affect a certain part of the tar spectrum.
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EXPERIMENTAL SET-UP AND PROCEDURE
Measurements were performed on the Chalmers 2−4MWth indirect gasifier, using a slipstream from the outlet. This flow passes through a ceramic filter that removes any particles before the gas stream enters 7571
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Figure 2. Left panel: SPA sample point, spe column, and manual part of the extraction device. Right panel: pneumatic robot for consistent sample extraction. the gas cleaning system. The ceramic filter is heated by the extracted gas flow, and electric heaters ensure a minimum temperature of 350 °C. This minimum temperature was calculated using the Thersites dew-point model21 for coronene and was maintained throughout the heated sections, to avoid condensation. The residence time in the heated pipes from the gasifier to the SPA sampling point was 2−3 s. SPA samples were collected before and after the filter simultaneously, to investigate the potential catalytic effects of the filter, condensation, and gas-phase reactions on the amount and composition of the tar. In parallel, the effects on the measurement of different sampling point temperatures were examined. Raw gas was sampled using a 100-mL syringe, B BRAUN Omnifix, and the 500-mg Supelclean LC-NH2 spe tube. The primary sampling point was located just upstream of the gas cleaning system and was constructed as depicted in the left panel of Figure 2. At this sampling point, roughly 2 Nl/min of dry raw gas was transported through a heated gas line (350 °C), followed by a volume that was heated to the same temperature before reaching the quenching point. The heated volume (350 °C) was equipped with a septum mounted a short distance from the wall, to avoid melting, which served as the entry point for the sample syringe. A secondary sampling point of similar design was positioned just upstream of the particle filter. The SPA samples were collected by attaching an spe column to a 100-mL syringe via a universal tube connector, inserting the needle (1.2*50 mm) into the hot gas flow via the septum, and extracting 100 mL of gas through the column using the syringe. The remaining raw gas continued through the quench, after which it was cooled and dried before it reached the online gas analysis equipment. The relatively small flow of gas and the positioning of the quench were selected to ensure a strong response in the N2 and O2 concentrations if leakage occurred during sampling. Initially, the 100-mL syringe was operated manually. Subsequently, it was fitted with a pneumatic robot, which allowed more reproducible sampling (right panel, Figure 2). The robot consists of a pneumatic cylinder, connected to the syringe, which is filled with pressurized air at a flow rate regulated by a needle valve. This flow rate was calibrated to allow sample extraction times of 1 min. After collection of a sample, the pressure in the cylinder was released. If a blockage in the needle resulted in the formation of a vacuum, the syringe piston retracted. When this happened the current column was discarded and replaced. To determine differences in repeatability between the manual and robot-assisted sampling, the maximum and minimum values for all the tar species from naphthalene and upward were compared in two sample series. Naphthalene was chosen as the starting point to exclude the effects of sample evaporation, as the collected quantities of heavier components could be thought of as depending mainly on the sample collection technique. The performed measurements consisted of six spe columns each, collected within a period of about 10 min. The columns, with the needles still attached, were then sealed and stored in a freezer, to minimize desorption of the more volatile components. The columns were eluted, within 24 h of sampling, as described previously,7 with the following modifications: (1) the needle used for sampling was also rinsed with DCM, as heavier components were condensed inside it, and (2) BSTFA was not added to the phenols to form trimethylsilyl
ethers, as the GC column used did not require this step for analysis of phenol, m/p-cresol and o-cresol. To examine the effects of short-term storage at room temperature, measurements were made in which all the collected columns were frozen at the same time. Thus, the first column was left at room temperature for a longer time than the second and so on, allowing more time for the first column to desorb components. The overall sampling time was about 10 min. In a separate investigation, to determine the effects of storage at −20 °C, two sample series were collected simultaneously. One batch was eluted immediately after sampling, while the other batch was stored in a freezer for 24 h. The two gas chromatographs used, the BRUKER GC-430 and GC450, were operated in the split mode using the SGE 4-mm FocusLiner with fused silica wool, autosamplers, FID detectors, and midpolar BR17-ms columns with graphite ferrules. The temperature ramp, which ranged from 50 to 350 °C, was developed to measure components ranging from benzene to coronene. The injector and detector temperatures were set at 350 °C, the split ratio was 20, and the column flow was set to 1 mL/min with helium as the carrier gas. The oven was programed to hold at 50 °C for 5 min, after which the temperature was increased by 8 °C/min until 350 °C was reached, where it was held for an additional 12.5 min to ensure that the entire tar spectrum was retained. The current setup gave a reproducibility level that was within 5% for each chromatograph. Each sample vial was analyzed three times sequentially, after which the mean values for all peaks were calculated. A small amount of aromatic species in the phenolic sample was to be expected due to incomplete elution of the aromatic sample. However, for the same reason a similar amount of the aromatic internal standard should also be obtained. Therefore, the aromatic and phenolic samples were analyzed using the same method, to determine the error associated with the elution step. To search for heavy tar components, samples were collected from the gasifier under a wide range of operating conditions, to obtain the widest possible range of tar compositions. These samples were analyzed to determine the behaviors of the heaviest detectable tars, such as coronene. As mentioned above, a temperature of 350 °C was maintained in all the heated pipes and equipment to avoid condensation. Furthermore, the measurements made before and after the filter, together with visual inspection of the transfer lines, served to determine if condensation had occurred. The experimental series for the gasifier are carried out during the winter months, with SPA samples being collected on a daily basis. Several hundreds of columns have been collected and small improvements to the sampling and analysis are being made continuously. To evaluate the overall repeatability of the method when used in industrial applications, a series of eight SPA measurements, each resulting in four to six usable columns, was collected for different operating modes of the gasifier. The tar spectrum was divided into the known compounds and groups of unknown species that exited the chromatographic column between two known compounds, being lumped together as one value. The mean concentrations and relative standard deviations (%RSD) were calculated for each group and known compounds in each SPA measurement and then averaged over the eight measurements, to create an overall picture of the typical concentrations and standard 7572
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deviations. The %RSDs were obtained from the sample standard deviations, calculated according to eq 1, and were also compared to the collected mass fractions of all the groups and species for every SPA measurement. S=
1 N−1
temperature was considerably higher, conservatively estimated at 40 °C. At this temperature, the humidity was 6−7%. Consequently, if one neglects to consider the change in humidity and the 10 °C temperature difference, the resulting error of sampling is approximately 6−7%. Figure 4 depicts the
N
∑ (xi − x ̅)2 i=1
(1)
For the above-mentioned samples, the individual mass percentages of the unknown peaks were calculated using the first column, to determine the possible gain of increasing the number of compounds chosen for calibration. The unknown species of interest were categorized into: group A, consisting of unknown species that comprised more than 1% of the total sample mass each; and group B, which contained between 0.5% and 1% of the total sample mass each. For the studied cases some species will, most likely, be treated as both group A and group B species, depending on the measurement. However, as a first step to increase the identified amount, general information describing the total number of unknown species of interest and their collective amount is needed.
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RESULTS AND DISCUSSION Measurements were performed at different points along the gas line to determine the effects of condensation and filter catalysis on tar composition. Figure 3 shows the measured weight
Figure 4. Dry gas fractions under the normal condition per sample of humid gas.
fractions of dry gas collected in the sample syringe under normal conditions, as a function of the ambient temperature. As the ambient temperature can be assumed to be constant for most sampling points and equipment types, this type of systematic error may not pose a problem for comparisons performed on the local scale. However, for the correct determination of the amount of tar, it is necessary to control or monitor both the temperature and pressure of the sampling system. The maximum and minimum values (in g/Nm3) of all the tar species from naphthalene and upward for a typical, manually sampled point that consisted of six columns are depicted in Figure 5. All the intervals plotted between two known Figure 3. Compositions and total amounts of tar in the raw gas before and after passage through the ceramic filter.
percentages of phenolic compounds, aromatic compounds with one ring, and aromatic compounds that contain two or more rings, before and after the filter. The last bar in the diagram depicts the total concentration (as measured divided by 20) of tar for both measurements in units of g/m3. The comparison clearly illustrates a higher tar content for the raw gas after it has passed through the filter, although the change in composition is within the margin of error for this measurement. It is concluded that the difference in the total tar amount is a function of the difference in ambient temperature between the two sampling points. Furthermore, the effects of filter catalysis and short residence times, at temperatures of around 350 °C, are at this point indistinguishable from the error of the method. The ambient temperature at the primary sampling point was about 30 °C, and it was assumed to remain fairly constant in all the experiments. The sampled raw gas contained approximately 50−60%vol steam, the majority of which condensed in the 100mL syringe, yielding
