Article pubs.acs.org/IECR
Impact of Siloxane Impurities on the Performance of an Engine Operating on Renewable Natural Gas Nitin Nair,† Xianwei Zhang,† Jorge Gutierrez,‡ Jack Chen,‡ Fokion Egolfopoulos,§ and Theodore Tsotsis*,† †
Mork Family Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles, California 90089-1211, United States ‡ Gas Engineering, Engineering Analysis Center, Applied Technologies, Southern California Gas Company, Pico Rivera, California 90660-5100, United States § Department of Aerospace and Mechanical Engineering, University of Southern California, Los Angeles, California 90089-1453, United States S Supporting Information *
ABSTRACT: An internal combustion engine operating on natural gas (NG) spiked with siloxanes has been studied experimentally with the goal of understanding the impact of siloxane impurities on engine performance. These impurities are shown to completely decompose during NG combustion in the engine to form silica microparticulates. These coat the internal metal surfaces in the engine (e.g., the piston heads) as well as the engine’s oxygen sensors and spark-plugs, and they also collect in the engine oil. A certain fraction of them, furthermore, are carried out of the engine in the flue-gas, and they deposit inside a catalyst monolith bed placed downstream of the engine resulting in its severe deactivation. These engine studies are consistent with prior fundamental studies by this team that indicate that siloxane impurities readily decompose in the NG combustion environment to form silica particulates that coat exposed metal surfaces. They also point out the critical importance for engine performance of adequately removing these siloxane impurities from NG prior to its use.
1. INTRODUCTION Biogas from sludge biodegradation in wastewater treatment plants (WWTP) and landfill gas (LFG) generated from the decomposition of solid waste in landfills are both promising renewable fuels, as they contain a large fraction of methane, 40−70% by volume, the rest being CO2, together with smaller amounts of other gases like O2, N2, and Ar.1 There are many facilities worldwide using biogas and LFG for power production (e.g., boilers) and electricity generation,2−4 and concerns about global warming are likely to encourage their further capture and utilization.3 Unfortunately, biogas and LFG contain in addition a number of undesirable trace contaminants (>140 have been identified to date reaching concentrations as high as 2000 mg/ m3).5 They include sulfided (e.g., H2S, COS, CS2) and halogenated compounds (both aromatic and aliphatic)2,6−9 as well as a particularly problematic class of trace constituents known as siloxanes.2 These are organic compounds containing in their backbone alternating silicon, with attached methyl groups and oxygen atoms (see Supporting Information Table S1 for a list of such siloxanes frequently reported together with trimethylsilanol in biogas and LFG).10,11 Siloxanes are found in biogas and in LFG because they are frequently added today into products, such as detergents, shampoos, cosmetics, paper coatings, and textiles,11 and unfortunately end-up in WWTP and in landfills when consumers discard these products. Some of the landfills, for example, have reported siloxane levels as high as 0.03 g/g in their secondary sludge.12,13 During combustion, siloxanes decompose into SiO2 microparticulates5,11 which pose a threat to equipment (see © 2012 American Chemical Society
discussion to follow) but also to human health and the environment, unless adequate measures are taken to remove them from the exhaust. Both are current keen concerns in California, and key drivers for this study, because of the prospect of using biomethane or renewable natural gas (RNG), which results from either biogas or LFG, after their impurities have been removed and their methane content has been upgraded to meet natural gas (NG) pipeline standards. The concern here is with potential malfunction of equipment for siloxane removalsee discussion belowwhich may lead into their accidental release into the NG supply (pipeline) system; this then may result in their combustion in home appliances (e.g., stoves, water heaters, etc.), thus causing harmful environmental emissions and deleterious effects on human health.1,6 Because of the technical challenges siloxanes present to the beneficial use of biogas and LFG, they have attracted the attention of researchers in the renewables area, particularly in recent years. Badjagbo et al.14 and Crest et al.,15 for example, reported on the challenge of sampling and analyzing siloxanes in LFG, this still remaining a major challenge since field instruments are still lacking in sensitivity and reliability. Furthermore, there are numerous reports on different techniques for their removal.11,16 Adsorption is the most Received: Revised: Accepted: Published: 15786
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As noted above, there are numerous reports of field-scale observations of silica films (often several milimeters thick) forming on internal surfaces of equipment operating on biogas5,11 and proving difficult to remove by either chemical or mechanical treatment. These abrasive silica deposits forming on the inner walls of engines, on gas turbine blades, and on heat exchange surfaces in boilers, have the potential to lead to serious damage, thus necessitating more frequent maintenance and increasing the cost of operating these devices.16,21 In boilers, the silica layer acts as a thermal insulator, interfering with the heat exchange operations.3 In engines, the silica deposits have been reported to clog narrow passages,10 increasing thus the potential for accidental explosions. Martin et al.27 have reported that the combustion of siloxanecontaining biogas in lean-mix, spark-ignition engines results in substantial wear even after short running times. Siloxanes have been reported to interfere also with the operation of catalytic treatment systems of the exhaust gas, often decreasing their efficiency.2,14 The impact of organosilicon compounds on catalytic oxidation catalysts had previously also attracted attention in the context of the abatement of VOC emissions from printing shops. In these studies, organosilicon compounds contained in the printing ink are reported to decompose inside the catalysts and deposit as silica and to mask the active sites.28 Organosilicon compounds such as L2 are also known to poison Pt and Pd supported catalysts by virtue of the coating and blocking of the surface of the precious metals by silicon atoms.29,30 More recent studies of deactivation of Pt/Al2O3 supported catalysts have revealed that the silica deposition on the catalyst surface is promoted by the active catalyst metal.31 As alluded to previously, another potential concern with the fine silica microparticles that form is that they may be carried out in the flue-gas of the NG equipment (e.g., engines, boilers, and furnaces), and unless adequate measures are taken to remove them from the exhaust, they are likely to escape into the atmosphere, where they may pose a risk to both human health and the environment.1,6 The same concern exists, of course, as well with home appliances such as stoves and ovens operating on RNG. In summary, attention has been focused in recent years on the fate of siloxane impurities during RNG combustion in NG equipment and common household appliances and the impact on their performance as well as on human health and the environment. Though there are reports of field observations of inorganic deposits on various internal surfaces of combustion equipment, this team knows of no open literature reports of systematic studies, under well-controlled laboratory conditions, investigating the various phenomena involved. This paper represents the second in a series of publications by this team aiming to remedy the situation and to bridge this knowledge gap. The first paper in the series reports the results of fundamental combustion studies of siloxane decomposition in a simulated RNG/air flat flame using the counterflow experimental technique.32 A key conclusion from that study is that siloxanes readily decompose in the RNG combustion environment to form pure silica particulates that coat exposed metal surfaces. The model combustion experimental configuration allowed, furthermore, the determination of the global reaction kinetics of siloxane decomposition in the RNG flame environment. In this study, the focus is on a well-instrumented IC engine operating on real NG laced with trace amounts of two common siloxanes (L2 and D4). Laboratory engine
common method,10,17−19 but a key problem is that the media utilized are not particularly selective toward siloxanes and they adsorb most other nonmethane organic compounds (NMOC) in LFG; this reduces the bed’s capacity for siloxane adsorption, necessitating frequent regeneration. Siloxanes are difficult to remove effectively from spent adsorbents during regeneration,20 which results in adsorbents of progressively lower capacity, until media replacement becomes necessary, at a great cost. A key challenge with adsorption (and all other physical methods), furthermore, is that it does not change the molecular state of siloxanes, which when released from the beds are still the same as when they entered. Regeneration involves burning the offgas, which releases SiO2 particles into the atmosphere, and consumes methane fuel to operate the incinerators. Absorption at relatively high pressures in solvents (e.g., selexol and methanol) is another approach.5 The problem with absorption is high capital and operating and maintenance costs. Solvent regeneration (and robustness) is also key to success to reduce solvent disposal and operating costs, and to ensure a long-term operation. Refrigeration has been tried also but is not effective on its own.20 Hybrid processes combining refrigeration with adsorption/absorption show more promise.5 However, the high energy consumed for cooling large amounts of wet gas is a major drawback for commercialization. Further, as with adsorption and absorption, siloxanes are not converted and thus require incineration for their destruction. Biological treatment to remove siloxanes has been studied also,2,21,22 but the results, so far, are disappointing with conversions ∼10%. Siloxane removal via membranes has also been proposed;23 however, no experimental data are currently available, and the practical implementation is unclear, as membranes are, in general, not well-suited for removing trace impurities. Reactive approaches have also been utilized, for example, peroxidation to reduce the siloxane content of sludge in a digester producing biogas.24 A reduction of 50−85% was observed, which is not high compared with other competing technologies, and it will require, in most instances, an additional polishing step to comply with siloxane limits by engine manufacturers (Table S2, in the Supporting Information section shows typical such limits proposed by various engine manufacturers). Finocchio et al.25 have studied the decomposition of D3 on the surface of basic (CaO, MgO) and acidic oxides (Al2O3, SiO2), showing that reactive adsorption occurs accompanied by surface silication and release of methane. Adsorbent regeneration is not possible, however, and the basic oxides lose their reactivity when in contact with CO2 due to surface carbonation. UV photodecomposition of L2 has been also investigated by this team in benchtop experiments with promising results,26 but the approach needs still to be fieldtested, and its economics to be further investigated. In summary, conventional methods (adsorption, absorption, refrigeration) face significant technical and economic hurdles for siloxane removal from LFG and biogas but remain in use because there are currently no other commercial processes to replace them. Newer approaches (e.g., biofiltration, membrane separation, reactive approaches, etc.) face challenges of their own and/or have yet to be field-tested. As one starts using RNG, therefore, the potential for these impurities finding their way into NG equipment and common household appliances remains a key challenge and points out the need for systematic studies of the fate of siloxanes during combustion in such devices and their impact on performance and associated emissions. 15787
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0258010078) was installed in both engines. The sensor was installed on the exhaust gas stream before the catalyst bed, and during operation its voltage was monitored via a multimeter. Figure 2 shows the schematic of the overall experimental engine setup. Each engine was connected, during operation, to two GE electric bulbs, 250 and 150 W respectively, corresponding to a total electric load of 400 W (chosen because, according to the manufacturer, the engine runs most efficiently at that load). Performance was monitored through the r.p.m display unit of the engine. The flow of pipeline-quality NG to the engine was controlled by a small valve in the NG regulator, and was monitored via gas-flow meters (American Meter Company, model DTM 200A). The energy efficiency (calculated as η (%) = (W/FEP) × 100, where W is the engine energy output in Watts, F is the gas flow rate in cubic feet per second, E is the energy content of NG equal to 1020 BTU/ft3, and P is a conversion factor equal to 1055.05 J/BTU) was also recorded as an indicator of engine performance. The two engines operated side-by-side for the same period of time and under the same NG flow rate and overall conditions (e.g., air to fuel ratio or A/F), other than the fact that the NG going into one of the engines (hereinafter, named the siloxane engine) was “spiked” with an equimolar mixture of two siloxanes, namely L2 and D4, which are the two most common linear (L2) and cyclic (D4) siloxanes in LFG and biogas (see Table S1 in the Supporting Information section). To generate parts per million level concentrations of siloxanes, a high-precision syringe pump (Harvard Apparatus model PHD 2000) coupled to a quartz Nebulizer with a flush capillary-lapped nozzle (Meinhard model TR-20-A0.5) was utilized to generate a fine siloxane spray into a heated NG stream whose flow rate was controlled by a mass flow controller (Cole Parmer model no. 55932). The siloxane-containing NG stream was then mixed with the main NG stream in order to generate the NG feed-stream into the engine with the desired siloxane concentration. The ability of the setup to generate reliably the required siloxane concentrations was verified by periodic injection of gas samples into a GC/MSD system (Agilent Technologies, 7890A GC System/5975C Inert XL MSD). The GC/MSD system was calibrated using standard liquid samples of siloxanes in ethanol. The GC/MSD measurements were also cross-checked with the estimated concentrations based on the amount of liquid siloxane injected and the gas flow rates. The composition of the exhaust gas coming out of the two engines was analyzed in two locations immediately after exiting the engines (and before entering the catalyst bed) as well as after exiting the catalyst bed. A Testo 350XL Gas Emissions Analyzer was used to measure the concentrations of CO, CO2, CH4, NO, NO2, and NOx. The analyzer was calibrated every 7 days (according to EPA protocol) using standard gas cylinders (450.5 ppm CO in N2; 215 ppm NO and 100 ppm NO2 in N2; and 1394 ppm CH4 in N2). Omega J-type thermocouples were installed at the inlet and outlet of the catalyst bed to measure the bed temperature at these two positions. The catalyst bed was heated by the hot exhaust gas stream and no effort was made to control its temperature (which, however, remained relatively constant during operation). The pressure drop across the bed was also measured by a differential pressure sensor (Dwyer Series 475 digital manometer). Both engines were operated for 500 h. For the first 10 h, the concentration of siloxanes into the siloxane engine was varied. After that initial period, the total concentration of siloxanes (L2
studies, due to their mechanical complexity, cannot provide the fundamental insight counterflow combustion studies provide. They are, however, an important link between fundamental combustion investigations and commercial engines operating under real field conditions. In what follows, the experimental setup and the techniques utilized are first described. The experimental observations are then presented and discussed in terms of the potential implications of the siloxanes’ presence in NG on the operation of real world engines.
2. EXPERIMENTAL SECTION In these studies two Honda EU2000i gasoline electric generators were utilized. The generators were modified in order to be able to run on NG instead of gasoline. This was accomplished by converting the original gasoline carburetor into a modified NG/gasoline carburetor and then connecting it to a NG regulator. A photograph of one of the engines and of the catalyst monolith attached is shown in Figure 1. Note that
Figure 1. (top) Laboratory engine with the catalyst monolith attached. (bottom) Catalyst monolith connected to the engine muffler.
the catalyst bed attached to the engine (Figure 1, bottom) was purchased separately (DCL Inc., oxidation catalyst model no. RC4x4x1-24) since the engine did not come equipped with it, because its emissions when running on gasoline were below the regulated threshold. The catalyst bed was of monolith shape and the manufacturer reports that it contains V−Zn−Ba on an alumina wash-coat. Though the original engine did not come equipped with an oxygen sensor, in order to investigate the impact of siloxane impurities on this important component of larger size engines, an oxygen sensor (Bosch, model no. 15788
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Figure 2. (top) Schematic of the experimental apparatus for the siloxane engine. (bottom) Schematic of the experimental apparatus for the nonsiloxane engine.
bed of the same composition, which was left connected to the engine for an additional 97 h. Upon termination of the study, the structural characteristics of the second deactivated bed were again analyzed via SEM/EDX, BET (for studying the overall surface area and pore volume), and CO chemisorption analysis to detect any potential decrease in the active sites of the catalyst. After 200 h of operation, the siloxane engine spark plugs were removed and their surface analyzed for the presence of silica deposits via SEM. After 467.5 h of operation, an oxygen sensor was installed and its performance was monitored via the voltage signal coming from the sensor. At the end of the 500 h point of engine operation, the sensors from both engines were removed and their performance was tested (at the Advanced Transportation Technology Center, of Long Beach City College) using a Bosch MTS 5200 Engine Analyzer equipped with a Digital Storage Oscilloscope. Subsequently, the sensors’ surface was analyzed using SEM/EDX. The oil for both engines was replaced every 100 h and analyzed for the presence of metals and other heteroatoms, via a variety of analytical techniques, by Jet Care International, Inc. In order to verify
+ D4) stayed constant at 10 ppmv. This siloxane content (10 ppmv) is very much in line with what is encountered in the LFG from various landfill sites around the world. For example, Wheless and Gary20 report the amount of siloxane found in two landfill sites (C-Modern and C-Kiefer) to be 20 ppmv. According to Dewil et al.,11 the siloxane concentration ranges from 4.8 mg/m3 up to 400 mg/m3, averaging 71.4 mg/m3. The conversion factor between these two common ways to express siloxane concentration depends on the MW of the given siloxane. For example, for L2 1 mg/m3 corresponds to 0.138 ppmv, so the aforementioned range by Dewil et al.11 corresponds to 0.7−55.2 ppmv, averaging 9.9 ppmv, close to the concentration of 10 ppmv used in this study. The catalyst bed connected to the siloxane engine was removed after 403 h of operation (by that point it had completely deactivated, see discussion below). For control purposes, the catalyst in the nonsiloxane engine was also replaced even though it was still active. The silica profiles along the deactivated bed were analyzed using scanning electron microscopy (SEM) and energy dispersive X-ray (EDX). The bed (for both engines) was subsequently replaced with another 15789
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whether silica microparticulates manage to escape the catalyst bed, their presence was analyzed at the end of the exhaust pipe venting into the hood, via the use of a scanning mobility particle sizer (SMPS). Upon completion of the testing, the siloxane engine was completely dismantled, and the internal surfaces and components were examined for the presence of silica deposits.
3. EXPERIMENTAL RESULTS AND DISCUSSION The flue-gas composition data for the nonsiloxane engine immediately after the engine (and before the catalyst bed) and after the catalyst bed are shown in Figure 3 (note that these
Figure 4. (a) CO and CH4 flue-gas concentrations for the siloxane engine. (b) NO, NO2, and NOx flue-gas concentrations for the siloxane engine.
Figure 3. (a) CO and CH4 flue-gas concentrations for the nonsiloxane engine. (b) NO, NO2, and NOx flue-gas concentrations for the nonsiloxane engine.
data are with the second catalyst bed). Note that the NOx species (defined as the sum of NO + NO2though some of the NO2 converts into NO) and CH4 remain unaffected by this catalyst. However, there is substantial conversion of CO (see also Figure 5, below). Figure 4 shows the corresponding fluegas composition data for the siloxane engine and the behavior is qualitatively similar to that for the nonsiloxane engine. In Figure 4, the data are plotted both in terms of the time of operation but also in terms of the total siloxane amount the engine has been exposed to during the operation of this particular catalyst bed. Figure 5 shows the CO conversion for the nonsiloxane engine, which indicates that the catalytic activity remains constant for the duration of engine operation, with conversion harboring ∼90%. Figure 6 shows the corresponding CO conversion for the siloxane engine, again
Figure 5. CO conversion vs time on stream for the nonsiloxane engine.
plotted both in terms of time on stream as well as with respect to the total volume of siloxanes injected into the engine. Catalytic conversion starts at ∼90% (as is the case with the nonsiloxane engine, see Figure 5), but it declines gradually right from the start reaching values lower than 50% by the time the engine operation was terminated (for the first catalyst bed which stayed much longer on stream, as noted above, the catalyst activity continued to decline throughout the operation of the engine). 15790
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Figure 6. CO conversion vs time on stream and volume of siloxane injected for the siloxane engine.
Figure 7 shows an EDX scan of the catalyst’s surface along the length of the bed which is 8.7 cm long (this EDX profile Figure 8. BET data of the catalyst monolith for both the siloxane and nonsiloxane engines.
iQ (Model no. 373) instrument, was also performed on the same (as with the BET analysis) spent catalyst beds of both engines. The analysis was carried out using a 1.24 g sample (from the inlet section) from each bed at an isothermal temperature of 45 °C, varying the CO pressure from 80 to 720 Torr, measuring the CO uptake at various pressures, and extrapolating the resulting linear plot to 0 Torr pressure to calculate the amount of CO chemisorbed by the catalysts. For the catalyst bed from the engine without siloxanes, the amount was 0.761 μmol/g, while the corresponding amount for the catalyst from the siloxane engine was 0.043 μmol/g, indicating a decrease in the active surface area of ∼94.4% for the silicacoated catalyst. Silica microparticulates also coat the internal surfaces in the engine, as one may expect. (One expectation, prior to the initiation of the study, was that the silica, by coating the spark plug surfaces, would significantly interfere with the engine performance; the expectation of engine performance deterioration did not materialize, however, see further discussion below). After 97 h of operation, for example, the spark plugs from both engines were removed and visually inspected. The presence of silica deposits on the spark plug from the siloxane engine was clearly evident. After 200 h of operation, the spark plugs from both engines were removed again, and their surfaces examined by SEM/EDX. Figure 9 shows a SEM image of the silica film formed on the spark plug metal surface from the siloxane engine. The silica deposit structure consists of a compact layer that appears strongly adhering to the metal surface underneath and a top layer with a more porous and less dense nature that easily flakes from the surface. Figure 10 shows the EDX data of the metal surface of the nonsiloxane engine spark plug as well as that of the siloxane engine spark plug. As expected, no Si was detected on the nonsiloxane engine spark plug surface while only Si and O were observed on the siloxane engine spark plug. However, the spark plug metal surfaces (not shown here) where the engine spark is generated (gap) appear equally clear and free of any deposits in both spark plugs, consistent with the fact that there were no changes in engine operation observed, as noted above. These spark plugs were
Figure 7. Silica profile on the catalyst monolith surface along its length.
was taken with the first catalyst monolith after 403 h on stream). A silica film covers the whole monolith surface with the heavier coverage observed on the front end of the bed, as expected. The deposition of silica also impacts greatly the pore structure characteristics, as Figure 8 indicates which shows the pore size distribution (PSD) measured using BET for samples taken from both the siloxane and nonsiloxane engine (the data in Figure 8 were generated by taking a 1 g sample from the midsection of the second catalyst bed without crushing it and analyzing it in a Micromeritics ASAP 2010 BET apparatus). When comparing the BET data between the catalyst monolith connected to the siloxane engine with that of the nonsiloxane engine, one observes significant decreases in both the pore volume (from 0.0272 to 0.0242 cm3/g) as well as in the total surface area (from 6.563 to 4.911 m2/g). In addition, the average pore size (as determined using the BJH analysis method of the adsorption branch of the data) of the siloxane engine monolith is shifted toward the smallest pore sizes, indicating a progressive plugging of the support pores in the pore structure. CO chemisorption analysis, using an Autosorb 15791
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was purchased and installed in both engines (these particular sensors were installed at the 460 h point of engine operation). Oxygen sensors are a critical component of large NG internal combustion engines as they determine whether the engine operates either fuel-rich or fuel-lean and help control the operating conditions and performance, including the NOx and the other emissions. To monitor the performance of the sensor, its voltage was recorded. Figure 11 reports the readings for both
Figure 9. SEM image of the spark plug metal surface for the siloxane engine coated by a silica film.
Figure 11. Sensor voltage signals.
the siloxane and the nonsiloxane engines. The voltage signals increased with time on stream for both engines and more so for the siloxane engine. On the other hand, the signals were very noisy and they did not appear to indicate any discernible qualitative differences in their performance. (Since these sensors do not control the Honda engine performance, any damage to them will not manifest itself as a problem for engine operation; that will not be the case, however, for engines whose operation depends on the sensor to control performance and the emissions). Therefore, in order to examine the impact, if any, that siloxanes may have on oxygen sensor performance, both devices were removed at the 500 h point of engine operation for testing using a Bosch MTS 5200 Engine Analyzer, as previously noted. The testing involved monitoring the voltage pulse width and the response time of the sensors, as the engine is first forced to run fuel-rich, and is then allowed to adjust to the proper air-to-fuel mixture, i.e., running with maximum fuel efficiency depending on the load. If an oxygen sensor functions properly, the response time is expected to be less than 100 ms. The testing results indicated that the response time of the nonsiloxane engine sensor was 96 ms, indicating good performance, whereas that of the siloxane-engine oxygen sensor was 124 ms, indicating failed performance. (If this particular sensor was controlling the engine operation, it would trigger the check engine light to come on). After these sensor tests were completed, the sensor surfaces were studied by SEM and EDX. Figure 12 shows the SEM micrograph of the siloxane sensor surface which illustrates the presence of deposits, while EDX indicates the presence of Si and oxygen indicative that these are silica films. The nonsiloxane engine sensor was also analyzed by SEM/EDX as well (data not shown here), and no silica films or deposits were found on it. After the completion of the 500 h run, the siloxane engine was dismantled and its internal surfaces were examined. Extensive silica deposition was observed. Figure 13, for example, shows a photograph of the piston head where a silica
Figure 10. EDX of the metal surface of the nonsiloxane engine spark plug (top) and of the silica coated spark plug surface for the siloxane engine (bottom).
then reinstalled in their respective engines and left on the engines for the duration of the experiment (500 h). In the siloxane engine, the spark plug continued to function well without any problems until the experiment was terminated. After 500 h, the spark plugs for both engines continued to be free of silica deposits on the gap. As noted previously, the Honda engines did not come equipped with an oxygen sensor; so instead, an oxygen sensor 15792
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Figure 12. SEM-EDX results of the siloxane engine oxygen sensor.
from the siloxane engine when compared to the clean oil and the used oil from the nonsiloxane engine oil. The presence of a large concentration of silica particulates in the engine oil indicates the potential for accelerated engine wear of moving parts, such as pistons, likely to lead to the need for frequent replacement. Despite the coating of the internal engine surfaces with silica deposits including the spark-plugs and the piston head, there was little discernible evidence of any negative impact on engine performance, however. For example, the rpm data indicated no changes in the power output of the engine. As noted previously, one of the concerns with the siloxanes’ presence in RNG is the potential for the silica microparticulates escaping into the environment. This prospect is more of concern, of course, with home appliances (e.g., stoves, ovens, indoor furnaces, etc.) rather than with NG equipment which generally operate outdoors. As previously noted, for the measurement of the particulates in the engine flue-gas, a scanning mobility particle sizer was utilized, manufactured by TSI (model 3936). The flue-gas was sampled at the exit of the catalyst bed. Figure 15 shows the particle size distribution for both the siloxane and the nonsiloxane engines. The SMPS is unable to detect any particulates in the nonsiloxane engine, while the same is not true for the siloxane engine for which the SMPS detects particles ranging in size from 10−180 nm with a mean particle size of ∼73 nm, and a mass concentration of 13 μg/m3. One should be reminded, furthermore, that these silica particulates are those that have managed to escape the catalyst monolith.
Figure 13. Photograph of the piston head of the siloxane engine showing the silica film deposition.
film is clearly visible. EDX analysis of these films (not shown here) confirmed that they consisted of Si and O. Visual examination of the engine valve surfaces indicated substantial coatings on the valves from the siloxane engine as compared to the valves from the nonsiloxane engine. The EDX results of one of these valve surfaces, shown in Figure 14, confirm that the deposits are silica. The presence of silica deposits on the internal engine surfaces suggests the presence of these particulates in the engine oil as well. The engine manufacturer recommends oil change after every 100 h of operation. After two of these oil changes, the used oil from both engines was analyzed for the presence of metals. The results are shown in Table S3 in the Supporting Information section. There was a significant concentration of the silicon element in the used oils
4. CONCLUSIONS Biogas and landfill gas are both promising renewable fuels; however, they contain trace amounts of siloxanes which during 15793
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Figure 14. EDX results of the siloxane engine valve.
In this study, an internal combustion engine operating on natural gas spiked with two common siloxanes has been studied experimentally, with the goal of understanding the impact of siloxane impurities on engine performance. These impurities were shown to completely decompose during NG combustion in the engine to form silica microparticulates. These coat the internal metal surfaces in the engine, such as the piston heads, as well as the engine’s oxygen sensors and spark plug, and they also collect in the engine oil. A certain fraction of them, furthermore, are carried out of the engine in the flue-gas and they deposit inside a catalyst monolith bed placed downstream of the engine resulting in its severe deactivation. In addition, a fraction of these particles of submicrometer size escape through the catalyst bed. These engine studies are consistent with prior fundamental studies by this team that indicate that siloxane impurities readily decompose in the RNG combustion environment to form silica particulates that coat exposed metal surfaces. They also point out the critical importance for engine performance of adequately removing these siloxane impurities from RNG prior to its use.
Figure 15. Particle size distribution in the flue-gas.
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combustion decompose into SiO2 particles which pose a threat to equipment and the environment, unless adequate measures are taken to remove them from the exhaust. This is an issue of current concern because of the imminent prospect of using either biogas or LFG in the form of renewable natural gas. Current techniques for siloxane removal from LFG and biogas face significant technical and economic hurdles, so the potential for these impurities finding their way into NG equipment and common household appliances remains a key challenge and has motivated this systematic study by this group on the fate of siloxanes during combustion in such devices.
ASSOCIATED CONTENT
S Supporting Information *
Table S1 listing some of the most common siloxane compounds in LFG and some of their properties, Table S2 describing the siloxane limits in LFG and digester gas recommended by various engine manufacturers, and Table S3 including the oil analysis data for two used oil samples from the siloxane and nonsiloxane engines. This material is available free of charge via the Internet at http://pubs.acs.org/. 15794
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AUTHOR INFORMATION
Corresponding Author
*Tel.: 213 740 2069. Fax: 213 740 8053. E-mail address:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors would like to acknowledge the Southern California Gas Company for their support and Dr. Constantinos Sioutas and his team from the Civil and Environmental Engineering Department, at USC, for their help with the SMPS measurements. The support of the National Science Foundation is also gratefully acknowledged.
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dx.doi.org/10.1021/ie302751n | Ind. Eng. Chem. Res. 2012, 51, 15786−15795
