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Technical assessment of fuel cell operation on landfill gas at the Groton, CT, landfill
Energy 28 (2003) 397–409 www.elsevier.com/locate/energy
Technical assessment of fuel cell operation on landfill gas at the Groton, CT, landfill R.J. Spiegel a,∗, J.L. Preston b a
National Risk Management Research Laboratory, US Environmental Protection Agency (EPA), Research Triangle Park, NC 27711, USA b Hydrogen Source Corporation, 60 Bidwell Road, South Windsor, CT 06074, USA Received 6 April 2002
Abstract This paper summarizes the results of a seminal assessment conducted on a fuel cell technology that generates electrical power from landfill waste gas. This assessment at Groton, Connecticut was the second such project conducted by the Environmental Protection Agency (EPA), the first being conducted at the Penrose Power Station near Los Angeles, California. The main objective was to demonstrate the suitability of the landfill gas energy conversion equipment at Groton with different conditions and gas compositions than at Penrose. The operation of the landfill gas cleanup system removed contaminants from the gas stream with essentially the same efficacy as at Penrose, even though the quantity and kinds of contaminants were somewhat different. The fuel cell power plant’s maximum output power improved from 137 kW at Penrose to 165 kW at Groton, due to a 31% increase in the heating value of the Groton landfill gas. Published by Elsevier Science Ltd.
1. Introduction The EPA has previously conducted a demonstration of a landfill gas-to-energy system using fuel cell technology (UTC Fuel Cells PC25 ) in an industrial suburb located 15 miles northwest of downtown Los Angeles, CA. This site, the Penrose Power Station, collects gas from four nearby landfills that contain a significant amount of industrial waste material. One primary reason Penrose was selected as a test site was the stringent environmental regulations and local code requirements that exist for the South Coast Air Quality Management District. Therefore, successful installation and operation at this site would likely be accepted at other landfills throughout the ∗
Corresponding author. Tel.: +1-919-541-7542. E-mail address: [email protected] (R.J. Spiegel).
0360-5442/03/$ - see front matter Published by Elsevier Science Ltd. doi:10.1016/S0360-5442(02)00118-4
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US. Results from verification tests at the Penrose site can be found in prior publications[1,2]. The site also contained internal combustion engines capable of producing approximately 9 MW of electrical power from a pressurized gas stream. Because the 200 kW fuel cell system was fed from the same pressured gas source, the fuel cell gas pretreatment unit (GPU) was designed to accept pressurized gas. The GPU significantly exceeded performance requirements and reduced fuel cell contaminants in the landfill gas consisting of sulfur and halides to total levels below 0.047 ppmv (as H2S) and 0.032 ppmv (as Cl), respectively. The maximum output power from the fuel cell of 137 kW was achieved at approximately 2350 SL/M in flow rate of the landfill gas, which has a heating value of around 17–18 kJ/SL. As the fuel cell is designed to produce 200 kW when operated on natural gas (higher heating value of 36.5–39.1 kJ/SL), the lower output power on landfill gas is a result of its lower heating value. Based on these tests, to achieve maximum power (200 kW) will require higher flow rates of landfill gas without producing unacceptable pressure drops within the fuel cell power plant. The engineering changes required to increase the flow rate and reduce pressure drops are not major modifications to the power plant, and they have been previously elaborated upon [2]. All the electrical power produced by the fuel cell power plant was sold to the local utility, Los Angeles Department of Water and Power. To further assess and test the suitability of the technology that was utilized (fuel cell power plant) and developed (GPU), the equipment was moved to the Groton, CT, landfill, which has been closed since 1983 with 2 million tons of refuse in place. There is a gas collection system in place which feeds an open flare where the gas is incinerated at a rate of approximately 11,300 SL/M (400 SCFM). Based on this flow rate and gas heating value (around 57% methane), it was estimated that the landfill had enough fuel capacity for four 200 kW fuel cells, although only one was installed at the site. The main goals of the new project included: to demonstrate the adaptability of the GPU to operate on somewhat different landfill gas compositions; to obtain additional operational data regarding cost and reliability; and to change the operating regime (gas temperature) of the GPU to determine if a simpler system could be utilized to clean the gas to required levels. Connecticut has a blanket exemption for fuel cells in regard to air emissions; therefore, no air emission permits were required for the Groton site. Water analyses were required from the small overflow of clean condensate from the fuel cell water treatment system. Normally, this small discharge would be dispensed via a sanitary sewer, which was not available at the site. Fig. 1 contains a drawing of the major features of the installation site at Groton. Basically, the same equipment that was installed at Penrose was likewise installed at Groton. As the GPU was originally designed and built to operate in a non-freezing climate in southern California, a building was required to protect against the potential freezing of wet landfill gas and related condensate. The building that housed the GPU and associated equipment is 6.1 × 9.1 m (20 × 30 ft) with the necessary heating, ventilation, and combustible gas sensor to protect the GPU. Another modification for the Groton site was the addition of a gas compressor, which was required because the GPU was originally designed to operate on the 6.9 × 105 Pa (100 PSIG) landfill gas supply at Penrose. A dual-piston, positivedisplacement, lubrication-free, continuous-duty compressor, rated for 2260 SL/M at 2.76 × 105 Pa (40 PSIG), was regulated down to approximately 1.52 × 105 Pa (22 PSIG) at the inlet to the GPU. This paper summarizes the results of a 1-year demonstration test at the Groton landfill, and is a follow up to the original tests at the Penrose site. Emphasis is placed on the contaminant removal performance of the GPU and the reliability/availability of fuel cell operation on landfill gas. More detailed information can be found in an EPA report [3].
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Fig. 1. Groton landfill test system site layout.
2. Groton landfill gas composition Fig. 2 contains plots of major constituents (O2, N2, CH4, and CO2) of landfill gas as a percentage 1 of the total. The measurements were taken over a time interval that stretches approximately 1 2 years, with the first 6 months of data taken before the demonstration started. These data reveal a high quality gas with a higher CH4 content and less diluents than the Penrose gas. For example, the average content of CH4, CO2, N2, and O2 in the gas versus Penrose are 57 vs. 44%, 41.35 vs. 38%, 1.3 vs. 17.6%, and 0.41% vs. 0.4%, respectively. These values yield average gas heating values (kJ/SL) of 21.8 (SD ⫽ 0.08) at Groton versus 16.7 at Penrose. Thus, the Groton landfill represents a “high-end” gas composition, while Penrose is a good representative site with low quality gas.
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Fig. 2. Plots of the groton landfill gas constituents.
Figs. 3 and 4 contain measurements of one of the major contaminants in landfill gas: H2S. As seen in Fig. 3, data taken on a given sample day (before the demonstration started) with an Industrial Scientific H2S Meter from 28 individual wells show concentrations with a wide variation, from approximately 17 to 2000 ppmv. The corresponding average concentration was 430 ppmv with a SD of 482. Data at the flare inlet taken by Draeger tube as a function of time are shown in Fig. 4. The sample period was approximately 1 year, which corresponded to the demonstration time frame. For whatever reason, the H2S concentrations tended to decline as a function of sampling time, starting at a level of around 500 ppmv and reaching a low of 50 ppmv near the end of the year-long sample period. The average concentration was 281 ppmv with a SD of 137, which was larger than the average concentration (100 ppmv) at Penrose. Analyses for organic sulfur compounds prior to the demonstration start date revealed 0.9 ppmv of dimethyl sulfide. Other organic compounds, such as dimethyl disulfide, methyl mercaptan, carbonyl sulfide, and carbon disulfide, were not detected at a 0.1 ppmv detection level. Table 1 summarizes the results of testing the raw landfill gas for the other category of contaminants that poison fuel cell catalysts, namely halogenated hydrocarbons. As with the sulfur data, 1 the samples were taken over an approximate time span of 1 years that covered the demonstration 2 period. A total of 22 compounds were found in measurable concentrations. Most of the compounds did not change significantly from measurement to measurement, with the most significant percent change occurring in dichlrodifluoromethane which had values ranging between 0.84 and 10.2 ppmv. Total halogenated species varied from a low of 2.27 ppmv to a maximum concentration
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Fig. 3. Hydrogen sulfide levels at individual wells at the Groton landfill.
Fig. 4.
Hydrogen sulfide concentration versus time at the Groton landfill.
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Table 1 Groton landfill gas analysis results for halogenated hydrocarbons Compound
Sample 1b (ppbv)
Sample 2b (ppbv)
Sample 3c (ppbv)
Sample 4d (ppbv)
Sample 5e (ppbv)
Bromobenzene Bromoform Bromomethane Carbon tetrachloride Chlorobenzene Chloroethane Chloroform Chloromethane Dichlorodifluoromethane 1,1-Dichloroethane 1,2-Dichloroethane 1,1-Dichloroethene trans-1,2-Dichloroethylene cis-1,2-Dichloroethylene cis-1,3-Dichloropropylene Methylene chloride Tetrachloroethylene 1,1,1-Trichloroethene Trichloroethylene Trichlorofluoromethane Trichlorotrifluoroethane Vinyl chloride Totals
8 0 165 70 319 464 60 82 3150 128 12 92 48 –a 283 58 18 0 0 12 –a 506 5475
9 0 319 82 339 719 70 93 4170 149 13 113 54 –a 323 66 22 0 0 15 –a 789 7345
31 100 0 0 0 973 100 1070 10,200 22 0 0 0 –a 0 55 12 18 13 212 –a 330 13,136
0 0 0 0 230 640 0 78 –a 110 0 0 0 120 0 58 68 27 30 260 12 640 2273
0 0 0 0 0 810 0 0 840 69 0 0 0 110 0 58 51 20 34 270 23 800 3085
a b c d e
Analysis not performed for this species. Samples 1 and 2 taken on the same day, 10 months before the start of the 1-year demonstration. Sample 3 taken 6 months before the start of the 1-year demonstration. Sample 4 taken 8 months after the start of the 1-year demonstration. Sample 5 taken 11 months after the start of the 1-year demonstration.
of 13.1 ppmv. As Cl (equal to the sum of the individual species times the number of halogen atoms per species), the total halogens varied from 16 to 45 ppmv. Generally, these values were less than those at the Penrose site. 3. GPU The process and instrumentation diagram for the Groton fuel cell landfill gas facility is shown in Fig. 5. A detailed flow diagram of the GPU is contained in Fig. 6. As operation, bed size, and material descriptions of the GPU have been extensively elaborated upon in previous publications (i.e., [1,2]), only a brief overview will be provided here. Basically, the GPU incorporates one non-regenerable absorbent step, plus two stages of refrigeration combined with two regenerable absorbent steps. The use of staged refrigeration provides tolerance to varying landfill gas contami-
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Fig. 5.
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Groton fuel cell facility process and instrumentation diagram.
nants. The first-stage non-regenerable carbon bed converts H2S to elemental sulfur and water via the Claus reaction. The first-stage condenser removes water to a uniform dew point of approximately 1 °C, along with some heavier hydrocarbons. A regenerable dryer bed next reduces the dew point from 1 °C to less than ⫺45 °C to prevent freezing in the second refrigeration step. The very cold gas, as a result of the second refrigeration stage, enhances the effectiveness of the regenerable activated carbon bed in removing halogenated hydrocarbons. After exiting a final filter to remove small particles, the treated gas is fed to the fuel cell. This staged, refrigerated approach is more flexible than utilizing dry bed absorbents alone and has the built-in capability to remove a wide range of contaminant concentrations which can exist from site to site and within a single site, varying with time (i.e., see Fig. 4 and Table 1). Regenerable beds (dryer bed and activated carbon bed) have absorbed contaminants removed by using cleaned, heated gas from the GPU exit, where the regenerable path is shown by the dashed lines in Fig. 6. As each absorbent step contains two beds in parallel, one bed is cleaned while the other is adsorbing. After exiting the final bed, the regeneration gas is fed to a low NOx incinerator (flare) where the mixture is combusted to provide greater than 98% destruction of the contaminants. Because the average concentration level of H2S in the landfill gas at Groton is considerably higher than at Penrose, the H2S removal capacity of the non-regenerable beds in the GPU was increased by adding two external 992 l (35 ft3) capacity tanks that were installed in front of the original beds (see Fig. 5). As with the original beds, they were filled with Westates UOCH-KP
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Fig. 6. Flow diagram of the GPU.
carbon, connected in series for maximum protection against breakthrough, and plumbed such that either bed can be isolated from the gas flow, facilitating bed-changeout without shutting the system down. The addition of these extra beds allows a longer time interval before the absorbent carbon material in the tanks needs to be replaced. For typical H2S average concentration levels of 200– 300 ppmv, a conservative changeout interval is 5–6 months. The other major piece of equipment shown in Fig. 5, that was not part of the original GPU, is the gas compressor. As explained earlier, it was added to provide the pressure required by the GPU. At Penrose, the pressurized landfill gas supply required that the GPU be designed to accept this gas. The use of a pressurized gas supply allows the size of the GPU’s absorbent beds, pipes, and valves to be smaller, resulting in a GPU with a smaller “foot print”. 4. GPU performance 4.1. Operation and reliability The GPU operated for 4168 h during the 1-year test period, with the longest run between shutdowns being 827 h. The EPA report [3] contains detailed information regarding start and stop times, run hours, reasons for shutdowns, and corrective actions taken. It suffices to state here that 21 forced outages were due to the GPU. Other shutdowns were caused by six voluntary actions, four grid outages, and two fuel cell related events. Of these shutdowns, about half were one-of-
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a-kind mechanical failures which were corrected and did not recur. They were generally attributed to leaks resulting from the equipment being moved from California to Connecticut. The remainder were due to three system issues which were diagnosed during the demonstration test: (1) a recurring high GPU pressure drop was corrected by adding two new coalescing filters and water traps to prevent landfill gas condensate from entering the H2S removal bed downstream of the compressor (see Fig. 5); (2) periodic freeze-ups of the refrigeration system were eliminated by adding an in-line dryer to the d-limonene refrigerant; and (3) an improved gas compressor exhaust valve was installed to prevent periodic failures caused by excessive wear on the exhaust valves. Most of these periodic failures were caused by the added compressor, and it was estimated that the malfunctioning valves caused outages of about 1050 h during the 1-year demonstration. With these outages in mind, the gross availability of the GPU for the year-long test was 45%. Most of the one-of-a-kind outages occurred during the first 6 months. If the outages that were caused by the added compressor, which is not part of the original GPU design, are removed from the operating hours, the gross availability increases to 56%. After all the system issues were resolved, the GPU availability increased to 70% over the last 6 months of the test. Finally, it should be noted that manpower availability at Groton to promptly diagnose and correct a problem was also an issue, which had an overall negative impact on the overall availability of the GPU. 4.2. Contaminant removal To verify the GPU’s ability to remove contaminants (sulfur and halide compounds) from the raw landfill gas, samples were periodically collected at the GPU exit. Most of the GPU gas exit samples were taken via evacuated stainless steel Summa canisters; however, since it was found that very small amounts of H2S may be absorbed onto the Summa canister walls with time, additional samples were taken with Tedlar bags. These samples were taken prior to the start of the 1-year test period, and then at monthly intervals during the test period. Sampling occurred during the last hour before the regeneration cycle was initiated for the dryer and activated carbon beds. It can be expected that this is a worst case situation because breakthrough of contaminants is most likely to occur during the end of a “make” cycle. The samples were analyzed off-site by Performance Analytical, Inc. in Canoga Park, CA, an independent testing laboratory, using gas chromatography/mass spectrometry for VOCs (including halides) and chromatography/flame photometric detection methods for sulfur compounds. Table 2 summarizes the results for the contaminant removal performance of the GPU. A single data sample was taken at the GPU exit at the beginning of the demonstration period, and then four sets of two samples each were taken at the specified GPU total operating time. Because of a proliferation of outages (described above) during the first 6 months of the test period, it was decided to focus the data collection on the latter part of the test period when the nagging problems were solved and more representative data likely. Consequently, the first (after the initial set) of the four data sets was taken approximately 9 months into the demonstration, and the last three data sets were taken at monthly intervals during months 10–12. In this regard, note that the last data set was collected during a preplanned change in GPU operating conditions. More will be said of this later. As seen in the data in Table 2, the single exit sample at the beginning of the 1-year test measured total sulfur and halides of 0.022 and 0.014 ppmv, respectively. Total sulfur was calcu-
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Table 2 GPU contaminant removal performance GPU operating time (h) Sulfur compounds (ppmv) Hydrogen sulfide Methyl mercaptan Ethyl mercaptan Dimethyl sulfide Dimethyl disulfide Carbonyl sulfide Carbon disulfide Total sulfur (as H2S) Volatile organic compounds Dichlorodifluoromethane 1,1-Dichloroethane Benzene Chlorobenzene Ethyl benzene Styrene Trichloroethene Toluene Tetrachloroethene Vinyl chloride Xylene isomers cis-1,2-Dichloroethene Total halides (as Cl)
0
2325
2331
3194
3198
3483
3485
3966d
3967d
NA ⬍0.004 ⬍0.004 ⬍0.004 ⬍0.002 0.022 ⬍0.002 0.022 (ppmv) NA ⬍0.002 ⬍0.003 ⬍0.002 ⬍0.002 ⬍0.002 ⬍0.002 0.007 ⬍0.002 ⬍0.004 0.001 ⬍0.002 0.014
⬍0.004 ⬍0.004 ⬍0.004 ⬍0.004 ⬍0.002 ⬍0.004 ⬍0.002 nd
⬍0.004 ⬍0.004 ⬍0.004 ⬍0.004 ⬍0.002 ⬍0.004 ⬍0.002 nd
⬍0.004 ⬍0.004 ⬍0.004 ⬍0.004 ⬍0.002 ⬍0.004 ⬍0.002 nd
⬍0.004 ⬍0.004 ⬍0.004 ⬍0.004 ⬍0.002 ⬍0.004 ⬍0.002 nda
⬍0.004 ⬍0.004 ⬍0.004 ⬍0.004 ⬍0.002 ⬍0.004 ⬍0.002 nd
⬍0.004 ⬍0.004 ⬍0.004 ⬍0.004 ⬍0.002 ⬍0.004 ⬍0.002 nda
⬍0.004 ⬍0.004 ⬍0.004 ⬍0.004 ⬍0.002 0.010 ⬍0.002 0.010
⬍0.004 ⬍0.004 ⬍0.004 ⬍0.004 ⬍0.002 0.080 ⬍0.002 0.080
NA ⬍0.001 ⬍0.002 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 nd
NA ⬍0.001 ⬍0.002 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.002 ⬍0.001 ⬍0.001 nd
NA ⬍0.001 ⬍0.002 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.002 ⬍0.001 ⬍0.001 nd
NA ⬍0.001 ⬍0.002 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.002 ⬍0.001 ⬍0.001 nd
NA ⬍0.001 ⬍0.002 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.002 ⬍0.001 ⬍0.001 nd
NA ⬍0.001 ⬍0.002 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.002 ⬍0.001 ⬍0.001 nd
NA ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 0.002 ⬍0.001 0.012b
NA ⬍0.001 0.00042 ⬍0.001 ⬍0.001 0.0001 ⬍0.001 0.00039 ⬍0.001 0.0022 ⬍0.001 0.00015 0.019c
All samples taken with Summa canisters unless noted. nd=non-detected; NA=data not available. a Carbon disulfide was not detected in Summa canister, but was detected in a Tedlar bag sample at 0.017 ppmv at 3483 h and 0.014 ppmv at 3485 h. b Chloromethane detected at 0.012 ppmv and bromomethane at 0.00044 ppmv. c Also detected (ppmv): chloromethane=0.013, bromomethane=0.00046, chloroethane=0.0016, and trichlorofluoromethane=0.00033. d Data taken with regenerable carbon beds operating warmer than normal, as part of testing for a simplfied GPU.
lated by summing the products of each sulfur compound times the number of sulfur atoms per mole. Likewise, total halides were determined by adding the products of each halide times the number of halide atoms per mole. As these values were well below required levels, the 1-year test commenced. During the remainder of the test period, total halide results showed no detectable concentrations, except for those collected in the last month. These samples had concentrations ranging from 0.012 to 0.019 ppmv, as a result of changing the GPU operating conditions (see below). Using the typical raw landfill gas inlet halide concentrations listed in Table 1, the halide removal efficiency is greater than 99%. With samples collected via Summa canisters, total sulfur detected was also below detection limits, except during the last month when the operating parameters of the GPU were changed. However, duplicate analyses from Tedlar bag samples for one data set (see Table 2) indicated 0.017 ppmv carbon disulfide. As previously discussed, this result indicates that low levels of carbon disulfide may adsorb onto the canister walls. These low levels of organic sulfur are no problem for the commercial fuel cell (PC 25), which contains a fuel
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preprocessor (hot zinc oxide bed) to remove organic sulfur from its more typical fuel of natural gas. As with the halide removal process, the sulfur removal efficiency of the GPU is over 99%. 4.3. GPU operational test change Another goal of the project was to test if the GPU could be made simpler, and hence less costly, by eliminating selected components. The simplification approach was to combine the two dryer beds and the final two activated carbon beds into just two vessels, instead of the four currently used. Additionally, it was desired to eliminate the low temperature refrigeration unit. While it is generally known that the performance of activated carbon is enhanced by cold, dry gas [4], quantitative temperature-based results relative to landfill gas are unknown. To that end, the test objective was to determine if the carbon beds would maintain an acceptable level of performance if the gas inlet temperature to the beds was increased from the normal operating temperature of ⫺18 °C (0 °F) to about 2 °C (35 °F), which is the inlet temperature to the dryer beds. Fig. 7 shows both the resulting GPU total halide and sulfur removal performance versus the carbon bed temperature. Note that, as the bed temperature increases, the removal efficiency decreases. These results indicate that the operation of the GPU at higher temperatures is feasible, consequently allowing the possible reduction in the number of adsorbent vessels and the elimination of the low temperature refrigerator in future renditions of the GPU. The higher levels of sulfur can easily be handled by the existing desulfurizer bed in the fuel preprocessor of the fuel cell. However, the long term detrimental effects of the higher levels of halides on the fuel cell’s fuel preprocessor life will require further review. A commercially available halogen guard material
Fig. 7.
GPU performance versus activated carbon bed temperature.
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(i.e., activated alumina) may be required in the hydrotreater-based preprocessor to remove resulting hydrogenated halide compounds in the polisher bed. 5. Fuel cell performance At the Groton landfill, the fuel cell ran on the cleaned gas for 3313 h during the 1-year demonstration period. Shutdowns of various causes (see above) produced 16 runs, the longest being 827 h on the last run. Of these outages, only one was caused by the fuel cell, resulting in an adjusted availability/reliability for the fuel cell of approximately 96%. To calculate the adjusted availability, the total operating time is divided by the elapsed time minus time adjustments to account for unforced outages not due to the fuel cell (i.e., GPU problems); shutdowns due to operator error; waiting time for replacement parts that probably should have been on hand; and lack of manpower availability to restart the power plant during shutdowns that occurred during weekends or holidays. The actual fuel-cell-related shutdown happened approximately 8 months into the demonstration, and was due to failure of several electrical space heating elements inside the fuel cell. This resulted in freeze-related damage to a feed water pump, solenoid valve, and flow switch. Once these components were replaced, which took about a week, normal operation resumed. The fuel cell produced a maximum power output of 165 kW on the cleaned Groton landfill gas, some 35 kW below its rated output power on natural gas. However, to avoid shutdowns caused by gas quality (lower methane content) fluctuations, the fuel cell was operated at a conservative output level of 140 kW. While operating at a constant level of 140 kW, the fuel cell efficiency was determined over a 9-day period. The efficiency is calculated according to: energy output (kWh) (3602 kJ/kWh)100%/gas consumed (SL)×LHV (kJ/SL). The net energy output was measured using the fuel cell output voltage and amperage sensors, with an adjustment factor based on a comparison of the fuel cell calculations with the utilitycalibrated site meter at the prior demonstration site (Penrose). The fuel cell gas consumption was measured at the GPU exit, using a Yokogawa YFCT Flow Computing Totalizer (Style B). The lower heating values (LHVs) used were the average of two samples taken at the end of the 9day period. The resulting efficiency was 38.1%, which is very close to the nominal value of 40% when the fuel cell is operated on natural gas. 6. Discussion and conclusions The landfill gas-to-energy equipment, which includes the fuel cell and GPU, operated successfully during the 1-year test at the Groton landfill following relocation from the Penrose, CA, site. This test demonstrated the transportability as well as the applicability of the equipment to varying landfill gas compositions (heat content and contaminants). The commercial PC 25 fuel cell operated 3313 h on the landfill gas with a maximum power output of 165 kW, an efficiency of 38.1%, and an adjusted availability of 95.6%. The GPU operated for 4168 h at Groton with an overall contaminant removal efficiency of over 99%. Tests were also conducted to determine the feasibility of simplifying the GPU by the elimination of certain components. Based on the results of the tests, the recommended simplification consists
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of removal of the low temperature refrigeration unit with organic halide removal within the fuel cell power plant. In fact, a system with main features consisting of a non-regenerable KOHimpregnated activated carbon bed for H2S removal, followed by a coalescing filter to remove liquids and a blower to deliver the gas to the fuel cell at the required pressure, has operated successfully at a commercial venture on a landfill in Braintree, MA. In this regard, note that the Braintree landfill gas was relatively contaminant-free, compared to that at Penrose or Groton. Additionally, another version of the GPU has been successfully implemented in an ancillary application: a cleanup system for anaerobic digesters associated with wastewater treatment plants [5]. References [1] Spiegel RJ, Trocciola JC, Preston JL. Test results for fuel-cell operation on landfill gas. Energy 1997;22(8):777–86. [2] Spiegel RJ, Preston JL, Trocciola JC. Fuel cell operation on landfill gas at Penrose power station. Energy 1999;24:723–42. [3] Preston JL, Trocciola JC. Testing of fuel cells to recover energy from landfill gas: groton landfill. EPA-600/R-98126, NTIS PB 99-105199; 1998. [4] Graham JR, Ramaratnam M. Recover VOCs using activated carbon. Chem. Eng. 1993;100(2 Suppl):6–12. [5] Spiegel RJ, Preston JL. Test results for fuel cell operation on anaerobic digester gas. J Power Sources 2000;86:283–8.
Technical assessment of fuel cell operation on landfill gas at the Groton, CT, landfill R.J. Spiegel a,∗, J.L. Preston b a
National Risk Management Research Laboratory, US Environmental Protection Agency (EPA), Research Triangle Park, NC 27711, USA b Hydrogen Source Corporation, 60 Bidwell Road, South Windsor, CT 06074, USA Received 6 April 2002
Abstract This paper summarizes the results of a seminal assessment conducted on a fuel cell technology that generates electrical power from landfill waste gas. This assessment at Groton, Connecticut was the second such project conducted by the Environmental Protection Agency (EPA), the first being conducted at the Penrose Power Station near Los Angeles, California. The main objective was to demonstrate the suitability of the landfill gas energy conversion equipment at Groton with different conditions and gas compositions than at Penrose. The operation of the landfill gas cleanup system removed contaminants from the gas stream with essentially the same efficacy as at Penrose, even though the quantity and kinds of contaminants were somewhat different. The fuel cell power plant’s maximum output power improved from 137 kW at Penrose to 165 kW at Groton, due to a 31% increase in the heating value of the Groton landfill gas. Published by Elsevier Science Ltd.
1. Introduction The EPA has previously conducted a demonstration of a landfill gas-to-energy system using fuel cell technology (UTC Fuel Cells PC25 ) in an industrial suburb located 15 miles northwest of downtown Los Angeles, CA. This site, the Penrose Power Station, collects gas from four nearby landfills that contain a significant amount of industrial waste material. One primary reason Penrose was selected as a test site was the stringent environmental regulations and local code requirements that exist for the South Coast Air Quality Management District. Therefore, successful installation and operation at this site would likely be accepted at other landfills throughout the ∗
Corresponding author. Tel.: +1-919-541-7542. E-mail address: [email protected] (R.J. Spiegel).
0360-5442/03/$ - see front matter Published by Elsevier Science Ltd. doi:10.1016/S0360-5442(02)00118-4
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US. Results from verification tests at the Penrose site can be found in prior publications[1,2]. The site also contained internal combustion engines capable of producing approximately 9 MW of electrical power from a pressurized gas stream. Because the 200 kW fuel cell system was fed from the same pressured gas source, the fuel cell gas pretreatment unit (GPU) was designed to accept pressurized gas. The GPU significantly exceeded performance requirements and reduced fuel cell contaminants in the landfill gas consisting of sulfur and halides to total levels below 0.047 ppmv (as H2S) and 0.032 ppmv (as Cl), respectively. The maximum output power from the fuel cell of 137 kW was achieved at approximately 2350 SL/M in flow rate of the landfill gas, which has a heating value of around 17–18 kJ/SL. As the fuel cell is designed to produce 200 kW when operated on natural gas (higher heating value of 36.5–39.1 kJ/SL), the lower output power on landfill gas is a result of its lower heating value. Based on these tests, to achieve maximum power (200 kW) will require higher flow rates of landfill gas without producing unacceptable pressure drops within the fuel cell power plant. The engineering changes required to increase the flow rate and reduce pressure drops are not major modifications to the power plant, and they have been previously elaborated upon [2]. All the electrical power produced by the fuel cell power plant was sold to the local utility, Los Angeles Department of Water and Power. To further assess and test the suitability of the technology that was utilized (fuel cell power plant) and developed (GPU), the equipment was moved to the Groton, CT, landfill, which has been closed since 1983 with 2 million tons of refuse in place. There is a gas collection system in place which feeds an open flare where the gas is incinerated at a rate of approximately 11,300 SL/M (400 SCFM). Based on this flow rate and gas heating value (around 57% methane), it was estimated that the landfill had enough fuel capacity for four 200 kW fuel cells, although only one was installed at the site. The main goals of the new project included: to demonstrate the adaptability of the GPU to operate on somewhat different landfill gas compositions; to obtain additional operational data regarding cost and reliability; and to change the operating regime (gas temperature) of the GPU to determine if a simpler system could be utilized to clean the gas to required levels. Connecticut has a blanket exemption for fuel cells in regard to air emissions; therefore, no air emission permits were required for the Groton site. Water analyses were required from the small overflow of clean condensate from the fuel cell water treatment system. Normally, this small discharge would be dispensed via a sanitary sewer, which was not available at the site. Fig. 1 contains a drawing of the major features of the installation site at Groton. Basically, the same equipment that was installed at Penrose was likewise installed at Groton. As the GPU was originally designed and built to operate in a non-freezing climate in southern California, a building was required to protect against the potential freezing of wet landfill gas and related condensate. The building that housed the GPU and associated equipment is 6.1 × 9.1 m (20 × 30 ft) with the necessary heating, ventilation, and combustible gas sensor to protect the GPU. Another modification for the Groton site was the addition of a gas compressor, which was required because the GPU was originally designed to operate on the 6.9 × 105 Pa (100 PSIG) landfill gas supply at Penrose. A dual-piston, positivedisplacement, lubrication-free, continuous-duty compressor, rated for 2260 SL/M at 2.76 × 105 Pa (40 PSIG), was regulated down to approximately 1.52 × 105 Pa (22 PSIG) at the inlet to the GPU. This paper summarizes the results of a 1-year demonstration test at the Groton landfill, and is a follow up to the original tests at the Penrose site. Emphasis is placed on the contaminant removal performance of the GPU and the reliability/availability of fuel cell operation on landfill gas. More detailed information can be found in an EPA report [3].
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Fig. 1. Groton landfill test system site layout.
2. Groton landfill gas composition Fig. 2 contains plots of major constituents (O2, N2, CH4, and CO2) of landfill gas as a percentage 1 of the total. The measurements were taken over a time interval that stretches approximately 1 2 years, with the first 6 months of data taken before the demonstration started. These data reveal a high quality gas with a higher CH4 content and less diluents than the Penrose gas. For example, the average content of CH4, CO2, N2, and O2 in the gas versus Penrose are 57 vs. 44%, 41.35 vs. 38%, 1.3 vs. 17.6%, and 0.41% vs. 0.4%, respectively. These values yield average gas heating values (kJ/SL) of 21.8 (SD ⫽ 0.08) at Groton versus 16.7 at Penrose. Thus, the Groton landfill represents a “high-end” gas composition, while Penrose is a good representative site with low quality gas.
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Fig. 2. Plots of the groton landfill gas constituents.
Figs. 3 and 4 contain measurements of one of the major contaminants in landfill gas: H2S. As seen in Fig. 3, data taken on a given sample day (before the demonstration started) with an Industrial Scientific H2S Meter from 28 individual wells show concentrations with a wide variation, from approximately 17 to 2000 ppmv. The corresponding average concentration was 430 ppmv with a SD of 482. Data at the flare inlet taken by Draeger tube as a function of time are shown in Fig. 4. The sample period was approximately 1 year, which corresponded to the demonstration time frame. For whatever reason, the H2S concentrations tended to decline as a function of sampling time, starting at a level of around 500 ppmv and reaching a low of 50 ppmv near the end of the year-long sample period. The average concentration was 281 ppmv with a SD of 137, which was larger than the average concentration (100 ppmv) at Penrose. Analyses for organic sulfur compounds prior to the demonstration start date revealed 0.9 ppmv of dimethyl sulfide. Other organic compounds, such as dimethyl disulfide, methyl mercaptan, carbonyl sulfide, and carbon disulfide, were not detected at a 0.1 ppmv detection level. Table 1 summarizes the results of testing the raw landfill gas for the other category of contaminants that poison fuel cell catalysts, namely halogenated hydrocarbons. As with the sulfur data, 1 the samples were taken over an approximate time span of 1 years that covered the demonstration 2 period. A total of 22 compounds were found in measurable concentrations. Most of the compounds did not change significantly from measurement to measurement, with the most significant percent change occurring in dichlrodifluoromethane which had values ranging between 0.84 and 10.2 ppmv. Total halogenated species varied from a low of 2.27 ppmv to a maximum concentration
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Fig. 3. Hydrogen sulfide levels at individual wells at the Groton landfill.
Fig. 4.
Hydrogen sulfide concentration versus time at the Groton landfill.
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Table 1 Groton landfill gas analysis results for halogenated hydrocarbons Compound
Sample 1b (ppbv)
Sample 2b (ppbv)
Sample 3c (ppbv)
Sample 4d (ppbv)
Sample 5e (ppbv)
Bromobenzene Bromoform Bromomethane Carbon tetrachloride Chlorobenzene Chloroethane Chloroform Chloromethane Dichlorodifluoromethane 1,1-Dichloroethane 1,2-Dichloroethane 1,1-Dichloroethene trans-1,2-Dichloroethylene cis-1,2-Dichloroethylene cis-1,3-Dichloropropylene Methylene chloride Tetrachloroethylene 1,1,1-Trichloroethene Trichloroethylene Trichlorofluoromethane Trichlorotrifluoroethane Vinyl chloride Totals
8 0 165 70 319 464 60 82 3150 128 12 92 48 –a 283 58 18 0 0 12 –a 506 5475
9 0 319 82 339 719 70 93 4170 149 13 113 54 –a 323 66 22 0 0 15 –a 789 7345
31 100 0 0 0 973 100 1070 10,200 22 0 0 0 –a 0 55 12 18 13 212 –a 330 13,136
0 0 0 0 230 640 0 78 –a 110 0 0 0 120 0 58 68 27 30 260 12 640 2273
0 0 0 0 0 810 0 0 840 69 0 0 0 110 0 58 51 20 34 270 23 800 3085
a b c d e
Analysis not performed for this species. Samples 1 and 2 taken on the same day, 10 months before the start of the 1-year demonstration. Sample 3 taken 6 months before the start of the 1-year demonstration. Sample 4 taken 8 months after the start of the 1-year demonstration. Sample 5 taken 11 months after the start of the 1-year demonstration.
of 13.1 ppmv. As Cl (equal to the sum of the individual species times the number of halogen atoms per species), the total halogens varied from 16 to 45 ppmv. Generally, these values were less than those at the Penrose site. 3. GPU The process and instrumentation diagram for the Groton fuel cell landfill gas facility is shown in Fig. 5. A detailed flow diagram of the GPU is contained in Fig. 6. As operation, bed size, and material descriptions of the GPU have been extensively elaborated upon in previous publications (i.e., [1,2]), only a brief overview will be provided here. Basically, the GPU incorporates one non-regenerable absorbent step, plus two stages of refrigeration combined with two regenerable absorbent steps. The use of staged refrigeration provides tolerance to varying landfill gas contami-
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Fig. 5.
403
Groton fuel cell facility process and instrumentation diagram.
nants. The first-stage non-regenerable carbon bed converts H2S to elemental sulfur and water via the Claus reaction. The first-stage condenser removes water to a uniform dew point of approximately 1 °C, along with some heavier hydrocarbons. A regenerable dryer bed next reduces the dew point from 1 °C to less than ⫺45 °C to prevent freezing in the second refrigeration step. The very cold gas, as a result of the second refrigeration stage, enhances the effectiveness of the regenerable activated carbon bed in removing halogenated hydrocarbons. After exiting a final filter to remove small particles, the treated gas is fed to the fuel cell. This staged, refrigerated approach is more flexible than utilizing dry bed absorbents alone and has the built-in capability to remove a wide range of contaminant concentrations which can exist from site to site and within a single site, varying with time (i.e., see Fig. 4 and Table 1). Regenerable beds (dryer bed and activated carbon bed) have absorbed contaminants removed by using cleaned, heated gas from the GPU exit, where the regenerable path is shown by the dashed lines in Fig. 6. As each absorbent step contains two beds in parallel, one bed is cleaned while the other is adsorbing. After exiting the final bed, the regeneration gas is fed to a low NOx incinerator (flare) where the mixture is combusted to provide greater than 98% destruction of the contaminants. Because the average concentration level of H2S in the landfill gas at Groton is considerably higher than at Penrose, the H2S removal capacity of the non-regenerable beds in the GPU was increased by adding two external 992 l (35 ft3) capacity tanks that were installed in front of the original beds (see Fig. 5). As with the original beds, they were filled with Westates UOCH-KP
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Fig. 6. Flow diagram of the GPU.
carbon, connected in series for maximum protection against breakthrough, and plumbed such that either bed can be isolated from the gas flow, facilitating bed-changeout without shutting the system down. The addition of these extra beds allows a longer time interval before the absorbent carbon material in the tanks needs to be replaced. For typical H2S average concentration levels of 200– 300 ppmv, a conservative changeout interval is 5–6 months. The other major piece of equipment shown in Fig. 5, that was not part of the original GPU, is the gas compressor. As explained earlier, it was added to provide the pressure required by the GPU. At Penrose, the pressurized landfill gas supply required that the GPU be designed to accept this gas. The use of a pressurized gas supply allows the size of the GPU’s absorbent beds, pipes, and valves to be smaller, resulting in a GPU with a smaller “foot print”. 4. GPU performance 4.1. Operation and reliability The GPU operated for 4168 h during the 1-year test period, with the longest run between shutdowns being 827 h. The EPA report [3] contains detailed information regarding start and stop times, run hours, reasons for shutdowns, and corrective actions taken. It suffices to state here that 21 forced outages were due to the GPU. Other shutdowns were caused by six voluntary actions, four grid outages, and two fuel cell related events. Of these shutdowns, about half were one-of-
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a-kind mechanical failures which were corrected and did not recur. They were generally attributed to leaks resulting from the equipment being moved from California to Connecticut. The remainder were due to three system issues which were diagnosed during the demonstration test: (1) a recurring high GPU pressure drop was corrected by adding two new coalescing filters and water traps to prevent landfill gas condensate from entering the H2S removal bed downstream of the compressor (see Fig. 5); (2) periodic freeze-ups of the refrigeration system were eliminated by adding an in-line dryer to the d-limonene refrigerant; and (3) an improved gas compressor exhaust valve was installed to prevent periodic failures caused by excessive wear on the exhaust valves. Most of these periodic failures were caused by the added compressor, and it was estimated that the malfunctioning valves caused outages of about 1050 h during the 1-year demonstration. With these outages in mind, the gross availability of the GPU for the year-long test was 45%. Most of the one-of-a-kind outages occurred during the first 6 months. If the outages that were caused by the added compressor, which is not part of the original GPU design, are removed from the operating hours, the gross availability increases to 56%. After all the system issues were resolved, the GPU availability increased to 70% over the last 6 months of the test. Finally, it should be noted that manpower availability at Groton to promptly diagnose and correct a problem was also an issue, which had an overall negative impact on the overall availability of the GPU. 4.2. Contaminant removal To verify the GPU’s ability to remove contaminants (sulfur and halide compounds) from the raw landfill gas, samples were periodically collected at the GPU exit. Most of the GPU gas exit samples were taken via evacuated stainless steel Summa canisters; however, since it was found that very small amounts of H2S may be absorbed onto the Summa canister walls with time, additional samples were taken with Tedlar bags. These samples were taken prior to the start of the 1-year test period, and then at monthly intervals during the test period. Sampling occurred during the last hour before the regeneration cycle was initiated for the dryer and activated carbon beds. It can be expected that this is a worst case situation because breakthrough of contaminants is most likely to occur during the end of a “make” cycle. The samples were analyzed off-site by Performance Analytical, Inc. in Canoga Park, CA, an independent testing laboratory, using gas chromatography/mass spectrometry for VOCs (including halides) and chromatography/flame photometric detection methods for sulfur compounds. Table 2 summarizes the results for the contaminant removal performance of the GPU. A single data sample was taken at the GPU exit at the beginning of the demonstration period, and then four sets of two samples each were taken at the specified GPU total operating time. Because of a proliferation of outages (described above) during the first 6 months of the test period, it was decided to focus the data collection on the latter part of the test period when the nagging problems were solved and more representative data likely. Consequently, the first (after the initial set) of the four data sets was taken approximately 9 months into the demonstration, and the last three data sets were taken at monthly intervals during months 10–12. In this regard, note that the last data set was collected during a preplanned change in GPU operating conditions. More will be said of this later. As seen in the data in Table 2, the single exit sample at the beginning of the 1-year test measured total sulfur and halides of 0.022 and 0.014 ppmv, respectively. Total sulfur was calcu-
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Table 2 GPU contaminant removal performance GPU operating time (h) Sulfur compounds (ppmv) Hydrogen sulfide Methyl mercaptan Ethyl mercaptan Dimethyl sulfide Dimethyl disulfide Carbonyl sulfide Carbon disulfide Total sulfur (as H2S) Volatile organic compounds Dichlorodifluoromethane 1,1-Dichloroethane Benzene Chlorobenzene Ethyl benzene Styrene Trichloroethene Toluene Tetrachloroethene Vinyl chloride Xylene isomers cis-1,2-Dichloroethene Total halides (as Cl)
0
2325
2331
3194
3198
3483
3485
3966d
3967d
NA ⬍0.004 ⬍0.004 ⬍0.004 ⬍0.002 0.022 ⬍0.002 0.022 (ppmv) NA ⬍0.002 ⬍0.003 ⬍0.002 ⬍0.002 ⬍0.002 ⬍0.002 0.007 ⬍0.002 ⬍0.004 0.001 ⬍0.002 0.014
⬍0.004 ⬍0.004 ⬍0.004 ⬍0.004 ⬍0.002 ⬍0.004 ⬍0.002 nd
⬍0.004 ⬍0.004 ⬍0.004 ⬍0.004 ⬍0.002 ⬍0.004 ⬍0.002 nd
⬍0.004 ⬍0.004 ⬍0.004 ⬍0.004 ⬍0.002 ⬍0.004 ⬍0.002 nd
⬍0.004 ⬍0.004 ⬍0.004 ⬍0.004 ⬍0.002 ⬍0.004 ⬍0.002 nda
⬍0.004 ⬍0.004 ⬍0.004 ⬍0.004 ⬍0.002 ⬍0.004 ⬍0.002 nd
⬍0.004 ⬍0.004 ⬍0.004 ⬍0.004 ⬍0.002 ⬍0.004 ⬍0.002 nda
⬍0.004 ⬍0.004 ⬍0.004 ⬍0.004 ⬍0.002 0.010 ⬍0.002 0.010
⬍0.004 ⬍0.004 ⬍0.004 ⬍0.004 ⬍0.002 0.080 ⬍0.002 0.080
NA ⬍0.001 ⬍0.002 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 nd
NA ⬍0.001 ⬍0.002 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.002 ⬍0.001 ⬍0.001 nd
NA ⬍0.001 ⬍0.002 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.002 ⬍0.001 ⬍0.001 nd
NA ⬍0.001 ⬍0.002 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.002 ⬍0.001 ⬍0.001 nd
NA ⬍0.001 ⬍0.002 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.002 ⬍0.001 ⬍0.001 nd
NA ⬍0.001 ⬍0.002 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.002 ⬍0.001 ⬍0.001 nd
NA ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 0.002 ⬍0.001 0.012b
NA ⬍0.001 0.00042 ⬍0.001 ⬍0.001 0.0001 ⬍0.001 0.00039 ⬍0.001 0.0022 ⬍0.001 0.00015 0.019c
All samples taken with Summa canisters unless noted. nd=non-detected; NA=data not available. a Carbon disulfide was not detected in Summa canister, but was detected in a Tedlar bag sample at 0.017 ppmv at 3483 h and 0.014 ppmv at 3485 h. b Chloromethane detected at 0.012 ppmv and bromomethane at 0.00044 ppmv. c Also detected (ppmv): chloromethane=0.013, bromomethane=0.00046, chloroethane=0.0016, and trichlorofluoromethane=0.00033. d Data taken with regenerable carbon beds operating warmer than normal, as part of testing for a simplfied GPU.
lated by summing the products of each sulfur compound times the number of sulfur atoms per mole. Likewise, total halides were determined by adding the products of each halide times the number of halide atoms per mole. As these values were well below required levels, the 1-year test commenced. During the remainder of the test period, total halide results showed no detectable concentrations, except for those collected in the last month. These samples had concentrations ranging from 0.012 to 0.019 ppmv, as a result of changing the GPU operating conditions (see below). Using the typical raw landfill gas inlet halide concentrations listed in Table 1, the halide removal efficiency is greater than 99%. With samples collected via Summa canisters, total sulfur detected was also below detection limits, except during the last month when the operating parameters of the GPU were changed. However, duplicate analyses from Tedlar bag samples for one data set (see Table 2) indicated 0.017 ppmv carbon disulfide. As previously discussed, this result indicates that low levels of carbon disulfide may adsorb onto the canister walls. These low levels of organic sulfur are no problem for the commercial fuel cell (PC 25), which contains a fuel
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preprocessor (hot zinc oxide bed) to remove organic sulfur from its more typical fuel of natural gas. As with the halide removal process, the sulfur removal efficiency of the GPU is over 99%. 4.3. GPU operational test change Another goal of the project was to test if the GPU could be made simpler, and hence less costly, by eliminating selected components. The simplification approach was to combine the two dryer beds and the final two activated carbon beds into just two vessels, instead of the four currently used. Additionally, it was desired to eliminate the low temperature refrigeration unit. While it is generally known that the performance of activated carbon is enhanced by cold, dry gas [4], quantitative temperature-based results relative to landfill gas are unknown. To that end, the test objective was to determine if the carbon beds would maintain an acceptable level of performance if the gas inlet temperature to the beds was increased from the normal operating temperature of ⫺18 °C (0 °F) to about 2 °C (35 °F), which is the inlet temperature to the dryer beds. Fig. 7 shows both the resulting GPU total halide and sulfur removal performance versus the carbon bed temperature. Note that, as the bed temperature increases, the removal efficiency decreases. These results indicate that the operation of the GPU at higher temperatures is feasible, consequently allowing the possible reduction in the number of adsorbent vessels and the elimination of the low temperature refrigerator in future renditions of the GPU. The higher levels of sulfur can easily be handled by the existing desulfurizer bed in the fuel preprocessor of the fuel cell. However, the long term detrimental effects of the higher levels of halides on the fuel cell’s fuel preprocessor life will require further review. A commercially available halogen guard material
Fig. 7.
GPU performance versus activated carbon bed temperature.
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(i.e., activated alumina) may be required in the hydrotreater-based preprocessor to remove resulting hydrogenated halide compounds in the polisher bed. 5. Fuel cell performance At the Groton landfill, the fuel cell ran on the cleaned gas for 3313 h during the 1-year demonstration period. Shutdowns of various causes (see above) produced 16 runs, the longest being 827 h on the last run. Of these outages, only one was caused by the fuel cell, resulting in an adjusted availability/reliability for the fuel cell of approximately 96%. To calculate the adjusted availability, the total operating time is divided by the elapsed time minus time adjustments to account for unforced outages not due to the fuel cell (i.e., GPU problems); shutdowns due to operator error; waiting time for replacement parts that probably should have been on hand; and lack of manpower availability to restart the power plant during shutdowns that occurred during weekends or holidays. The actual fuel-cell-related shutdown happened approximately 8 months into the demonstration, and was due to failure of several electrical space heating elements inside the fuel cell. This resulted in freeze-related damage to a feed water pump, solenoid valve, and flow switch. Once these components were replaced, which took about a week, normal operation resumed. The fuel cell produced a maximum power output of 165 kW on the cleaned Groton landfill gas, some 35 kW below its rated output power on natural gas. However, to avoid shutdowns caused by gas quality (lower methane content) fluctuations, the fuel cell was operated at a conservative output level of 140 kW. While operating at a constant level of 140 kW, the fuel cell efficiency was determined over a 9-day period. The efficiency is calculated according to: energy output (kWh) (3602 kJ/kWh)100%/gas consumed (SL)×LHV (kJ/SL). The net energy output was measured using the fuel cell output voltage and amperage sensors, with an adjustment factor based on a comparison of the fuel cell calculations with the utilitycalibrated site meter at the prior demonstration site (Penrose). The fuel cell gas consumption was measured at the GPU exit, using a Yokogawa YFCT Flow Computing Totalizer (Style B). The lower heating values (LHVs) used were the average of two samples taken at the end of the 9day period. The resulting efficiency was 38.1%, which is very close to the nominal value of 40% when the fuel cell is operated on natural gas. 6. Discussion and conclusions The landfill gas-to-energy equipment, which includes the fuel cell and GPU, operated successfully during the 1-year test at the Groton landfill following relocation from the Penrose, CA, site. This test demonstrated the transportability as well as the applicability of the equipment to varying landfill gas compositions (heat content and contaminants). The commercial PC 25 fuel cell operated 3313 h on the landfill gas with a maximum power output of 165 kW, an efficiency of 38.1%, and an adjusted availability of 95.6%. The GPU operated for 4168 h at Groton with an overall contaminant removal efficiency of over 99%. Tests were also conducted to determine the feasibility of simplifying the GPU by the elimination of certain components. Based on the results of the tests, the recommended simplification consists
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of removal of the low temperature refrigeration unit with organic halide removal within the fuel cell power plant. In fact, a system with main features consisting of a non-regenerable KOHimpregnated activated carbon bed for H2S removal, followed by a coalescing filter to remove liquids and a blower to deliver the gas to the fuel cell at the required pressure, has operated successfully at a commercial venture on a landfill in Braintree, MA. In this regard, note that the Braintree landfill gas was relatively contaminant-free, compared to that at Penrose or Groton. Additionally, another version of the GPU has been successfully implemented in an ancillary application: a cleanup system for anaerobic digesters associated with wastewater treatment plants [5]. References [1] Spiegel RJ, Trocciola JC, Preston JL. Test results for fuel-cell operation on landfill gas. Energy 1997;22(8):777–86. [2] Spiegel RJ, Preston JL, Trocciola JC. Fuel cell operation on landfill gas at Penrose power station. Energy 1999;24:723–42. [3] Preston JL, Trocciola JC. Testing of fuel cells to recover energy from landfill gas: groton landfill. EPA-600/R-98126, NTIS PB 99-105199; 1998. [4] Graham JR, Ramaratnam M. Recover VOCs using activated carbon. Chem. Eng. 1993;100(2 Suppl):6–12. [5] Spiegel RJ, Preston JL. Test results for fuel cell operation on anaerobic digester gas. J Power Sources 2000;86:283–8.