Sorption Hybrid Process for Landfill Gas Cleanup

Sep 15, 1997 - Landfill gas (LFG) consists primarily of methane and carbon dioxide and a few ... sulfur-containing compounds in LFG severely impact th...
0 downloads 0 Views 220KB Size
Download PDF
4100

Ind. Eng. Chem. Res. 1997, 36, 4100-4107

A Catalytic/Sorption Hybrid Process for Landfill Gas Cleanup Chuanteng He, Doug J. Herman,† Ron G. Minet, and Theodore T. Tsotsis* Department of Chemical Engineering, University of Southern California, Los Angeles, California 90089-1211

Landfill gas (LFG) consists primarily of methane and carbon dioxide and a few percent of O2, N2, and H2O. It also contains numerous other organic compounds, many containing halogens and sulfur. Such compounds besides being potentially toxic to human, animal, and plant life, in addition present challenges to the further processing of LFG. For example, halogen- and sulfur-containing compounds in LFG severely impact the life of reforming catalysts used to produce H2, to be utilized in a fuel cell for the production of electric energy. For such processing, it is important to reduce the concentration of such compounds from the level of tens to hundreds of parts per million, typically found in LFG, down to the single-digit parts per million and, most preferably, parts per billion level. Effective processes to accomplish this are not currently available. This paper describes the development of such a process, which is well-suited to fuelcell applications of LFG. The process combines catalytic hydroprocessing, utilizing Co-Mo/ Al2O3 or Ni-Mo/Al2O3 catalysts in an atmospheric pressure reactor, together with sorption technology, utilizing disposable adsorbents, for the removal of HCl and H2S produced during the catalytic treatment. In laboratory, and field investigations the process has been shown to be very effective in removing the S- and Cl-containing organic compounds, for the most part down to their analytical detection limits. In companion field tests, no significant catalyst deactivation was observed during over 1000 h of testing. 1. Introduction Landfills are among the oldest but still the most common means for disposal of municipal and industrial solid waste, both in the U.S. and abroad. Landfilling is considered today the most economical way of solidwaste disposal. In the past, landfills were thought to have a minimal impact on the environment. Accumulated evidence in the last few years, however, has indicated that landfilling still poses a threat to human health and the environment by generating landfill gas (LFG) and liquid leachate (EPA-450/3-90-011a, 1991; Baker et al., 1990). Both contain many undesirable and potentially toxic chemical components, which are produced by the various chemical, physical, and biological waste decomposition processes. Properly dealing with liquid leachate and LFG is the key to the overall successful management of landfills. LFG consists typically of 95-99% methane and carbon dioxide, together with lesser quantities of other gases like N2, O2, H2, H2S, and NH3 and countless organic chemical compounds other than methane (collectively referred to as NMOCs). The latter include a variety of sulfided and halogenated compounds. Among the halogenated compounds, one finds many (if not all) the chloro/fluoro substituted CH4, C2H6, and C2H4 homologues (including carbon tetrachloride, chloroform, methylene, methyl chloride, dichloroethane, PCE, trichloroethane, and tetrachloroethane) mono- and dichlorobenzene, and other polychlorinated aromatics (EPA450/3-90-011a, 1991; Baker et al., 1990). The typical sulfided compounds one encounters, in addition to H2S, include carbonyl sulfide (COS), dimethyl sulfide (DMS), and various mercaptans. The reported concentrations of these contaminants vary widely among the various landfillssfrom less than 100 ppb to more than several hundred parts per million. Even for a given landfill, the composition of LFG not only varies with time, as * Phone: 213-740-2225. Fax: 213-740-8053. E-mail: [email protected]. † Electric Power Research Institute, Palo Alto, CA 94304. S0888-5885(97)00252-2 CCC: $14.00

the landfill ages, but is also different among the various sites of the landfill. This is because of the wide range of solid wastes deposited in a typical landfill during its lifetime. Natural and synthetic products, such as plastics, leather/rubber, household cleaners, pesticides, textiles, and papers, all common constituents of municipal solid waste (MSW), produce through their degradation many different chemicals. Landfill gas presents a number of environmental and safety problems (EPA-450/3-90-011a, 1991). These include the following: explosion hazards, which develop when LFG migrates from the landfill through the surrounding soil into adjacent structures, where it mixes with air in confined spaces (methane, one of the two primary ingredients of LFG, is explosive at concentrations above 5% by volume); effects on surrounding vegetation created, from the anoxic conditions resulting from the LFG replacing the oxygen within the root zone of plants; odorous conditions resulting from H2S and other sulfided compounds; health effects associated with many of the NMOCs found in LFG including benzene, toluene, and other aromatics and the aforementioned halogenated and sulfided compounds; acidity effects associated with carbon dioxide, which when dissolved in water creates acidic conditions which consequently cause soil-erosion problems, and SO2 and HCl produced by LFG combustion, which when released to the atmosphere contribute to acid rain; and global-warming effects attributed to CO2 and CH4, which are both greenhouse gases. CH4, in particular, is a very potent greenhouse gas almost 25 times more effective than CO2. By some accounts, fugitive emissions from landfills are responsible for about 25% of all CH4 released into the atmosphere (EPA Working Draft Report, 1996). There is interest today in the development of economic and efficient technology to deal effectively with the LFG problem. Some of the most promising technologies involve the utilization of LFG as an energy source in power generation (Augenstein and Pacey, 1992; Doorn et al., 1995). There are many incentives in utilizing © 1997 American Chemical Society

Ind. Eng. Chem. Res., Vol. 36, No. 10, 1997 4101

LFG in “waste-to-energy” conversion systems. In addition to helping conserve valuable alternative energy resources, direct LFG utilization results in reduced NMOCs and CH4 emissions. A number of such facilities are in operation utilizing internal combustion (IC) engines and gas and steam turbines. The choice of the type (and size) of power-generation facility utilizing LFG depends largely on the quantity and rate of landfill gas produced. Steam and gas turbines are appropriate for only very large landfills. IC engines have a smaller capacity but they are, themselves, not free of problems associated with corrosion of engine internal parts and secondary NOx and SOx emissions. Fuel cells offer a number of advantages over turbines and engines. They are spatially compact, easily movable, and adaptable. Fuel cells using H2 produced by reforming of LFG have a much higher energy-conversion efficiency than either gas and steam-turbines or engines. Fuel-cell applications of LFG are not commercial at this point, however. The reason for that can be traced back to the aforementioned halogenated and sulfided VOCs commonly found in LFG. In addition to being corrosive to the fuel-cell structure itself, these compounds have an adverse effect on the reforming catalyst’s life. For example, unpublished experimental deactivation tests by our group designed to investigate the influence of VCM (vinyl chloride monomer), a common constituent of LFG, on the life of a commercial steam-reforming catalyst (NiO/ Al2O3) have indicated that the catalyst life was greatly shortened, if 100 ppm of VCM was present in the feed. Analysis of the deactivated reforming catalyst at the end of the run revealed that its chlorine content was 8 times higher than that of the fresh catalyst. If LFG fuel-cell applications are to become commercial, efficient techniques must be developed for removing the halogenated and sulfided VOCs from landfill gas. The currently available techniques for removal of such compounds involve their sorption on activated carbon. Activated carbon, however, adsorbs most VOCs common in landfill gas with a greater preference for the larger compounds. The consequence of that is the need for frequent regeneration (and disposal of the effluent) or replacement, which drives up the costs. The need, furthermore, to reduce the concentration, preferably down to the parts per billion level, for a variety of compounds, each with different sorptive affinities to the carbon surface, dictates significant overdesign of the carbon-bed column. There is a clear need today for the development of a more efficient technique for the removal of chlorinated and sulfided VOCs from LFG, which is better suited to fuelcell applications. Such a process should allow for uninterrupted long-term operation, should be effective in simultaneously reducing the levels of all the chlorinated and sulfided compounds found in LFG, and should be robust to daily and seasonal variations in their concentration. The technique, furthermore, must not add significantly to either the capital or the operating costs of fuel-cell operation, if it is to be competitive with other waste-to-energy alternative technologies. This paper details the development of a potential such process intended to clean up from landfill gas the commonly found halogenated and sulfided hydrocarbon components, in order to provide a sulfur- and chlorinefree feed for a fuel-cell application at a reasonable cost. The proposed clean-up process consists of two conventional operations: (1) the catalytic hydrotreatment of the landfill gas to convert the halogenated and sulfided

Table 1. Synthetic Simulated Landfill Gas Composition component

concn

CO2 O2 N2 CH4 vinyl chloride dichlorobenzene trichlorofluoromethane DMS COS H2S

38% 1% 9% 52% 50 ppm 50 ppm 50 ppm 25 ppm 25 ppm 25 ppm

compounds into HCl, HF, and H2S and (2) sorption of the HCl/HF and H2S produced on a suitable sorbent. The use of hydrotreating is not novel in the area of chlorinated hydrocarbon destruction (Frimmel and Zdrazil, 1994; Novak and Zdrazil, 1993; Hagh and Allen, 1993; Gioia et al., 1993) and neither is the use of sorption technology for removing H2S from LFG (Samuels, 1990; Sandelli et al., 1994). We are not aware, however, of any published study in which both technologies have been used in tandem to treat LFG. In the paper, we will first describe a “proof of concept” laboratory experimental investigation utilizing simulated LFG. Prior to the initiation of the laboratory experiments, thermochemical calculations were carried out to verify the feasibility of the concept. These will be also described here. The paper will culminate with the description of results from field-scale investigations at the Anoka County landfill in Minnesota. 2. Laboratory Investigations As detailed in the Introduction, actual landfill gas has a complex and varying composition, which shows significant daily and seasonal variations. In the laboratory investigations reported here, a simulated landfill gas (SLFG) was used instead of the real LFG. Ease in operation and simplicity in analysis are the primary motivating factors for the choice in addition to the fact that LFG transport presents serious logistical problems. Using SLFG is appropriate for proof of concept laboratory investigations, involving preliminary screening of catalysts and sorbents and choice of operating conditions. Performing such studies at the field level is prohibitively expensive. Before the commercial application of the process, extensive field investigations must, of course, be carried out utilizing actual LFG. The results of a series of such investigations are reported in subsequent sections of this paper. The simulated landfill gas utilized in this study was purchased from the Sol-Cal Air Gas Company. Its composition, as certified by the provider, is shown in Table 1. 2.1. Thermochemical/Thermodynamic Calculations. Before the initiation of any experiments with the simulated landfill gas, calculations were performed to verify that, based on thermodynamic considerations alone, it is possible to treat the simulated landfill gas under the chosen conditions and attain sub parts per million total levels for both the sulfur- and chlorinecontaining VOCs. The following overall reaction scheme was assumed to describe the hydrotreating process of the SLFG:

VCM

C2H3Cl + H2 f C2H4 + HCl

dichlorobenzene

(1)

C6H4Cl2 + 2H2 f C6H6 + 2HCl (2)

4102 Ind. Eng. Chem. Res., Vol. 36, No. 10, 1997

trichlorofluoromethane CCl3F + 4H2 f CH4 + 3HCl + HF (3) DMS

C2H6S + H2 f C2H6 + H2S

(4)

COS

COS + 4H2 f CH4 + H2O + H2S

(5)

The actual reaction scheme is probably more complex. The thermodynamics of the CO2 hydrogenation, water gas shift, and reforming reactions were not taken into consideration in the calculations, however. Though analysis of the exit gas composition showed that it contained 0.5-1.0% CO, indicative of some minimal CO2 hydrogenation and reforming reaction occurring, the set of the overall reactions (reactions 1-5) above was deemed sufficient for the purposes of estimating a rough upper bound in the conversion during the hydrotreating process. The equilibrium constants for the above set of five reactions were calculated based on the literature data on enthalpies, Gibbs free energies of formation, and heat capacities for each component involved (CRC Handbook, 1992). Data that could not be obtained from the literature have been estimated by suitable groupcontribution methods (Reid et al., 1987). The values of the various thermodynamic properties utilized and further details about the calculations can be found elsewhere (He, 1995). Assuming that, at the reactor outlet, equilibrium is reached for all reactions, the exit concentrations for each of the VOC reactant impurities in Table 1 are calculated to be less than 1 ppb (corresponding to a conversion well in excess of 99.99%) at 1 bar and 340 °C. In these calculations, the CH4, H2, and H2O mole fractions were set constant at 52%, 5%, and 2%, correspondingly. Of course, thermodynamic calculations of this kind only provide an idea of what is ideally attainable. Due to kinetic limitations, what is often attained in real practice falls below expectations. The goal is then to select the catalysts, sorbents, and other conditions appropriately so that one closely approaches the expected ideal conversions. This has been the primary purpose of the laboratory investigation reported below. 2.2. Experimental Apparatus. The laboratory experiments were performed in an apparatus specifically designed for this purpose, a schematic diagram of which is shown in Figure 1. Two cylindrical vessels made of stainless steel act as the hydrotreater and the sorbent bed for H2S and HCl/HF removal. A finemetering pump is used to continuously add water into the input gas feed stream, which subsequently passes through an evaporator, whose temperature is maintained constant by a temperature controller. The purpose of the pump/evaporator assembly is to create predetermined amounts of moisture content in the SLFG feed to simulate conditions that are typically found in landfill gas. The flow rates for both SLFG and hydrogen were regulated by mass-flow controllers and flow indicators, which were precisely calibrated before use. Both the hydrotreater reactor and the sorbent bed were heated by multiple-zone heaters controlled by individual controllers to ensure that the temperature profile was uniform along the reactor length. Before entering the hydrotreater reactor, the feed was heated to the desired temperature in a preheater to avoid potential axial temperature gradients. Following the hydrotreater, the water found in the SLFG and the water produced during the hydrogenation of the VOCs are collected in a condenser. Together with the water,

Figure 1. Schematic diagram of the laboratory test apparatus for SLFG cleanup.

the condenser also collects a large fraction of the HCl/ HF produced, which dissolves in the water collected. The gas out of the water collector is directed either to the adsorber or through the side stream into the gas chromatograph and the rest of the analytical equipment. In the early experiments, before entering the sorbent bed and the analytical equipment, the gas first passed through a guard bed containing drierite and molecular sieve to remove water vapor. It was shown, however, that this guard bed interfered with the measurements by adsorbing H2S. In subsequent experiments, the use of such drying beds was abandoned. 2.3. Analysis of Products. A variety of techniques were utilized in the laboratory for the analysis of the streams exiting the hydrotreater and sorbent bed. An HP 5890 II GC equipped with ECD and FPD detectors was the primary measuring device for the various chlorided and sulfided compounds. The manufacturer reported the sensitivity of the ECD detector is 0.05-1 pg of chlorinated compounds, dependent on the molecular structure. The reported sensitivity of the FPD detector is 20 pg of S-containing VOCs. Our own testing with the VOCs of Table 1 indicate the following limits of detectability: trichlorofluoromethane ) 9 ppb, dichlorobenzene ) 1 ppm, H2S ) 200.0 ppb, COS ) 3 ppb, DMS ) 55 ppb. The ECD detector does a poor job in detecting VCM and is totally insensitive to HCl. For detecting VCM, Drager-tube-type measurements were carried out. The Drager tube’s lower limit of detection for VCM is 500 ppb. For HCl, conventional titration analysis was utilized by collecting the HCl in a bubbler containing

Ind. Eng. Chem. Res., Vol. 36, No. 10, 1997 4103

water and titrating every 2 or 3 days with a dilute NaOH solution using phenolphthalein as the indicator. The calculated detection limit of HCl by this technique is 160 ppb. Dichlorobenzene being the single exception, the minimum levels of detection for all other critical compounds of interest were below the 1 ppm level. The following conditions were chosen for the GC and its ECD and FPD detectors. The column for ECD was GS-Q (30 m × 0.53-mm i.d.), from J &W Scientific. The hydrogen (carrier gas) flow rate was set at 6.7 mL/min. The temperature profile during analysis was 30 °C for 2-min stay and then a ramp from 30 to 200 °C at a rate of 20 °C/min. The column for FPD was Carbograph VOC (30 m × 0.53-mm i.d.), from Alltech. The hydrogen (carrier gas) flow rate was set at 8.7 cm3/min. The temperature profile was 60 °C for 1 min and then a ramp from 60 to 130 °C at 5 °C/min followed by a second ramp from 130 to 240 °C at a rate of 30 °C/min. The temperature for both injectors A and B and the auxiliary gas valve was 140 °C. The injector A (for ECD) septum purge flow rate was set at 8.6 cm3/min, with a split ratio of 10-15; for injector B (for FPD), the septum purge flow rate was set at 14.0 cm3/min, with a split ratio of 10-15. For the ECD detector, we utilized a 5% CH4/ Ar make-up gas with a flow rate of 54 cm3/min, an anode purge flow rate of 6.1 cm3/min, and a temperature of 250 °C. For the FPD detector, we utilized a hydrogen flow rate (not including the flow through the column) of 72 cm3/min, an oxygen flow rate of 26 cm3/min, an auxiliary make-up gas (helium) flow rate of 68 cm3/min, and a temperature of 200 °C. 2.4. Experimental Results. Hydroprocessing is a mature technology used for many years in the petroleum industry for the removal of heteroatom like oxygen, nitrogen, and sulfur. More recently, the technology has been discussed in the context of dehalogenation of various chlorinated VOCs (Frimmel and Zdrazil, 1994; Novak and Zdrazil, 1993; Hagh and Allen, 1993; Gioia et al., 1993). Conventional hydroprocessing catalysts, i.e., Ni-Mo or Co-Mo supported on alumina and/or silica, have been utilized in these applications. Supported noble-metal catalysts have also been investigated and appear to be highly active. Given, however, the fact that LFG typically contains many other compounds, in addition to the chlorinated VOCs, and that noble metals are known to be sensitive to organic impurities, it was decided in this investigation to utilize more conventional catalytic materials. There are many potential sources of commercial hydroprocessing catalysts. A preliminary screening of a number of such catalysts led to the selection of two catalysts for further investigation: a Ni-Mo catalyst from Haldor-Topsøe (TK-525, 3-5% NiO, 15-25% MoO3, 65-75% Al2O3, 5-10% SiO2) and a Co-Mo catalyst from United Catalyst Inc. (C20-6-03, with a manufacturer’s reported composition of 80-90% Al2O3, 1-5% CoO, and 10-15% MoO3). Sulfiding of the catalysts was carried out by purging them with H2S-containing SLFG for 2 days at 360 °C. Within the time limits of the laboratory runs and the requirements imposed on the performance, both catalysts performed equally well. Figure 2 reports the results of a continuous 45-day run with the C20-603 catalyst. The catalyst as received by the manufacturer was packed to occupy 25 cm3 of the hydroprocessing reactor volume and was sulfided in situ following the procedure outlined above. Before the initiation of this run, after the catalyst sulfidation procedure, the catalyst was reduced in flowing hydrogen at 340 °C

Figure 2. Hydrogen sulfide, COS, and DMS concentrations after the hydrotreater. 0 ) hydrogen sulfide. O ) COS. 2 ) DMS.

overnight. The kinetic run took place at atmospheric pressure conditions also at 340 °C. A hydrogen/SLFG feed mixture at a feed rate of 126 cm3/min containing 12.7% hydrogen was utilized in the first 7 days of the experiments. Subsequently and for the remainder of the run, the hydrogen composition was raised to 21%, with little observable change in VOC removal efficiency. Of the VOCs in Table 1, only H2S, COS, and DMS were detected at the exit of the hydrotreater at levels above their detection limits. Of interest is the behavior of the reactor in terms of the H2S composition. In the first couple of weeks of operation, it harbors around the level of 25 ppm, which is the level of H2S feed composition. This is indicative that any H2S produced by the hydrogenation of the sulfided VOCs is adsorbed either on the catalyst and/or the reactor wall. Later, the level of the exit H2S composition starts increasing gradually until, at the end of the run, it reaches a level of around 75 ppm, corresponding to complete conversion of the sulfided VOCs. Increases in the exit concentration of H2S are not accompanied by significant changes in catalyst behavior and/or catalyst deactivation. A number of other experimental conditions were tested during the proof of concept experimental investigations. The water vapor, which is present naturally in the LFG and might be present in the hydrogenating stream, was of concern for catalyst operation. In several of the experiments, the run started with an inlet stream saturated with water vapor. Subsequently, the water flow was stopped and the run was continued for several more days with dry SLFG. We observed no perceptible changes in the behavior of the hydrotreater, both in terms of the exit level of VOC concentrations and of H2S. Water vapor, of course, is a product of the hydrogenating reactions of SLFG, and it is conceivable that whatever effect it has on catalyst behavior is already being experienced at that level; any added water vapor probably has little additional effect on the catalytic rates. Water vapor does affect the equilibrium exit concentrations in the sorbent beds which, however, under such conditions remain below the level of detection. The presence of water vapor also has significant implications for process design since it has a great solubility for HCl/ HF. A condensation unit like the one indicated in Figure 1 is likely to significantly decrease the load on the HCl/HF sorbent. A number of other issues were of concern for the field studies and were investigated during the laboratory investigations. LFG contains small but finite amounts

4104 Ind. Eng. Chem. Res., Vol. 36, No. 10, 1997

Figure 3. % HCl and HF recovery.

of oxygen and nitrogen. The oxygen-to-nitrogen molar ratio is generally smaller to that found in air. This is because as air is drawn into the landfill, the oxygen participates in various aerobic biodegradation processes. Oxygen (and, to a much lesser extent, nitrogen) is of concern for the operation of both catalysts and sorbents. In the laboratory investigations, however, it was found to have no effect on either the catalyst or sorbent operation. Oxygen, within the accuracy of the water collection and measurement technique, was found to completely convert to H2O in experiments with both wet and dry LFG feeds. Optimum temperature and pressure conditions were also investigated during the study. Typical hydroprocessing catalysts are known to operate effectively under high hydrogen pressures. LFG pressures within the landfills, on the other hand, are slightly above atmospheric. Though LFG can be pumped and delivered at any desired pressure, it is desirable that cleanup takes place at relatively low pressures because of the potential for the condensation of the various chlorinated and sulfided VOCs, which can corrode and plug the pumping equipment. In the laboratory experiments, the pressure was maintained only slightly above atmospheric. The catalysts tested, however, were proven effective for the hydrogenation of all VOCs in the SLFG. The temperatures of operation for both the reactor and sorbent beds were chosen to be 340 °C. Higher temperatures did not provide significant improvement in performance. Finally, the effect of hydrogen concentration was investigated. Though hydrogen availability is not of concern in fuel-cell applications, it is preferable to minimize its level of consumption. One must, of course, provide at the minimum the stoichiometric amount required for oxygen and VOC hydrogenation. In the laboratory investigations, increasing the hydrogen concentration in the feed mixture beyond 5% seemed not to give a noticeable advantage. The behavior in terms of HCl/HF composition at the exit of the hydrotreater parallels that of H2S. In Figure 3, the combined exit concentration of HCl/ HF is shown as a function of run time. The experiments shown in Figure 3 utilize the TK-525 catalyst (25 cm3 of reactor volume), atmospheric pressure conditions, a temperature of 340 °C, and H2/SLFG feed mixture with a feed rate of 121 cm3/min and a H2 concentration of 9%. The overall (HCl + HF) concentration is plotted in terms of the fraction it represents of the overall exit concentration expected based on complete conversion of all the halogenated VOCs in the SLFG. A gradual increase is observed in the exit concentration of the acid. Even at the end of the run in Figure 3, a significant amount was

still being adsorbed on the catalyst and/or the reactor wall. The gradual increase in the HCl/ HF concentration is not accompanied by any perceptible other changes in reactor behavior. Before the initiation of the laboratory investigations, a number of sorbents were evaluated for the removal of HCl/HF and H2S. Two sorbents for the removal of HCl/HF and three sorbents for the removal of H2S were selected for further evaluation. The two sorbents for HCl/HF removal were a UCI sorbent (C125-1-02, containing 25-35% Al2O3‚SiO2, 40-50% CaO, 1-5% SiO2, 20-30% ZnO) and a Haldor-Topsøe sorbent (HTG1, containing 15-30% K2CO3 and 70-85% Al2O3). Both sorbents were shown to perform equally well during the investigations. The gas leaving the sorbent bed contained no traces of either acid. The same is not true, however, for the three sorbents chosen for H2S removal. Of the three sorbents studied, only one (Haldor-Topsøe HTZ-5, containing 99-100% ZnO) was shown particularly effective in removing H2S under the experimental conditions tested. It was found, furthermore, that in addition to the chemical composition of a sorbent, other characteristics are also equally important. For example, a sorbent made by the same manufacturer (HaldorTopsøe HTZ-3) and with the exact same composition, but with lower porosity and surface area, was shown to be relatively less effective. 3. Field Studies The catalytic/sorption hybrid process for LFG cleanup (CSHP) tested in the laboratory was scaled-up and tested in a pilot-plant-scale unit using real LFG generated from a landfill in Anoka County, Minnesota. The CSHP system was tested as the second stage (topping stage) to a more conventional LFG treatment system. The reason for doing so was to make sure that, in case of failure of the CSHP system, the landfill gas passing through the system would at least have undergone a certain degree of cleanup. Doing so, of course, also allowed us to exhibit the superiority of the CSHP system over the more conventional technology. The overall system combining the conventional clean-up technology and the CSHP system for polishing duty might, furthermore, have certain economic advantages. The combination of sorbents and methods it affords provides greater flexibility in tailoring the needed level of cleanup to a particular landfill gas contaminant concentration. 3.1. Field Apparatus. A schematic diagram of the test apparatus is shown in Figure 4. In the test apparatus, LFG (the flow rate averaged 27.4 m3/h over the duration of the run, the minimum flow rate being 26.9 m3/h and the maximum 28.3 m3/h) is first compressed to approximately 3 atm. It subsequently passes through a pressurized gas reservoir to dampen the pressure swings from the compressor. After the gas reservoir, the LFG passes through two packed-bed reactors (listed as SulfaTreat reactors in Figure 4) containing conventional commercially available iron oxide sorbent (a proprietary sorbent consisting of Fe2O3 and Fe3O4 on an inert substrate, sold by Gas Sweetener Associates, Metairie, LA) for bulk sulfur removal. The LFG then passes through two packed-bed reactors containing activated carbon for heavy-hydrocarbon removal. Hydrogen is subsequently added (to create an LFG containing 5% hydrogen), and the gas is then heated to approximately 370 °C and subsequently fed into the CSHP unit. In the pilot-plant unit, all three CSHP stages (i.e., the hydrotreater, the HCl/HF, and

Ind. Eng. Chem. Res., Vol. 36, No. 10, 1997 4105

Figure 4. Schematic diagram of the Anoka landfill test system.

Figure 5. Halocarbon removal through the clean-up system (C1’s). Medium-gray bar ) raw gas. Dark-gray bar ) after carbon. Lightgray bar ) after hydrotreater, detection limit.

H2S removal systems) are incorporated into the same reactor. The three units contained the catalytic/sorbent materials which were shown to be optimal during the laboratory investigations. The catalyst was presulfided following the manufacturer’s recommended operating conditions utilizing a flowing nitrogen mixture containing 200 ppm H2S. Samples were withdrawn from four locations (as shown in Figure 4) and analyzed by Interpoll Laboratories, Inc. (Circle Pines, MN). Twenty-nine halogen compounds (together with a number of non-halogenated aromatics) and three sulfur compounds were detected in the raw landfill gas. The halogenated hydrocarbons and the aromatics were collected in virgin 5-L Tedlar bags and analyzed by GC/ MS on a Fison MD800 analytical system equipped with a VRX analytical column which was temperature programmed from 35 to 230 °C using a dual-ramp algorithm over approximately 25 min with a He flow rate of 2 cm3/min. The three sulfur compounds detected were hydrogen sulfide, dimethyl sulfide (DMS), and methylmercaptan. The hydrogen sulfide sampling and analysis were performed using a modification of EPA Method 11 which has been shown to attain lower detection limits than those specified by the method. The DMS and methylmercaptan were analyzed using GC/ FPD. 3.2. Experimental Results. Figures 5-10 summarize the effectiveness of the clean-up system for these different contaminants. The average concentrations of the different halocarbon compounds sampled at three

different locations in the LFG clean-up system (sampling ports 1 before the sulfur removal unit, 3 after the activated carbon treatment, and 4 after the CSHP unitssee Figure 4) are indicated in Figures 5 and 6. The values shown are the time-averaged values over 1000 h of testing. Each of these compounds, after the CHSP unit, was reduced to less than the detection limits of the gas analysis equipment, which in most cases was 20 ppbv, in some cases less than that (e.g., chloromethane), but in the highest case was less than 100 ppbv. Figure 5 shows the removal of C1 halocarbons. The carbon beds removed trichloromethane, tetrachloromethane, bromomethane, and dibromomethane effectively down to the GC/MS detection limits. One is unable, therefore, from these results to infer the effectiveness of CSHP in removing these compounds. The carbon beds were not very effective, on the other hand, in removing some of the other compounds, particularly dichlorodifluoromethane (but also to a lesser extent chloromethane, dichloromethane, and trichlorofluoromethane). The CSHP unit, however, was able to remove them to very low (below detection limits) levels. Figure 5 points out the difficulty in using carbon beds for LFG cleanup. Activated carbon is very specific in its sorption properties for various organic compounds. Its affinity toward such compounds is a strong function of the compound’s chemical structure (see, for example, differences in the absorption capacities for tetrachloromethane and dichlorodifluoromethane in Figure 5) and often also dependent on the composition of the gaseous mixture. This represents a serious disadvantage when it comes to the cleanup of LFG, since it contains so many heteroatom-containing compounds and its composition changes continuously as a function of time and location within a given landfill. The CSHP system’s destruction efficiency, on the other hand, appears to be independent of the type of halogenated compound treated. This represents a distinct advantage in the area of landfill gas cleanup. Figure 6 shows the removal efficiency for the remaining halocarbons, C2 and larger. For nine of the compounds (the tri- and tetrachloroethanes, 1,2,2-trichlorotrifluoroethane, 1,3-dichloropropene, the hexachlorobutadiene, 1,3- and 1,4-dichlorobenzenes, and 1,2,4trichlorobenzene), carbon beds were very effective in removing the compounds down to their GC/MS detection limit levels. For six of the compounds (chloroethene, 1,2-dichlorotetrafluoroethane, cis-1,2-dichloroethene, triand tetrachloroethenes, and chlorobenzene), the carbon beds were very ineffective, but the CSHP system completely removed such compounds down to their detection limits by GC/MS. For the remaining six compounds, the carbon-beds were partially effective, but the CSHP unit was completely effective in reducing them to their detection limits. Again, the unpredictability of activated carbon in removing the various halogenated compounds should be noted (see, for example, the differences in behavior between chlorobenzene, 1,2-dichlorobenzene, and 1,3- and 1,4-dichlorobenzene). Even for the heavier halogenated hydrocarbons, the CSHP method outperforms sorption by activated carbon. Figure 7 shows the removal efficiency of the nonhalogenated aromatics. These compounds are not fuelcell contaminants. They will most likely be completely cracked or reformed in the fuel cell after they exit the clean-up system. As expected, activated carbon has a high affinity for toluene, the xylenes, and ethylbenzene.

4106 Ind. Eng. Chem. Res., Vol. 36, No. 10, 1997

Figure 6. Halocarbon removal through the clean-up system (C2+). Medium-gray bar ) raw gas. Dark-gray bar ) after carbon. Lightgray bar ) after hydrotreater, detection limit.

Figure 7. Aromatic hydrocarbon removal through the clean-up system. Medium-gray bar ) raw gas. Dark-gray bar ) after carbon. Light-gray bar ) after hydrotreater.

It was surprising to see, however, how less effective the carbon beds were in removing benzene, trimethylbenzene, and styrene. With the exception of styrene, the CSHP system for the temperatures and H2 partial pressures utilized had little effect on any of the nonhalogenated compounds. This is an encouraging result because it shows that the hydroprocessing catalyst is very effective in removing the halogens from the compounds but ineffective toward hydrogenating or cracking the aromatic hydrocarbons. The latter was a concern in terms of minimizing hydrogen consumption and catalyst deactivation. It is interesting to note that approximately 5 ppm benzene were created in the hydrotreater, due to the conversion of haloaromatics. The effectiveness of the clean-up system on sulfur compounds including hydrogen sulfide is shown in Figures 8-10. Three different sulfur compounds were detected in the raw landfill gas: hydrogen sulfide, DMS, and methylmercaptan. Hydrogen sulfide is the compo-

Figure 8. Clean-up system effectiveness for hydrogen sulfide removal. 9 ) raw gas. ] ) activated carbon outlet. b ) SulfaTreat outlet. 2 ) polished gas detection limit.

nent with the highest concentration in the raw LFG, varying between 8 and 124 ppm, with an average of about 40 ppm. Figure 8 shows the hydrogen sulfide concentrations sampled in four places: raw gas inlet; after the SulfaTreat reactors (using a commercial iron oxide sorbent) outlet; at the activated carbon-bed outlet, and at the CSHP system outlet. As can be seen from this figure, the CSHP clean-up system was effective in reducing all hydrogen sulfide concentrations to below 100 ppbv. It can also be seen from this figure that the iron oxide was effective in removing the bulk of the hydrogen sulfide but unable for the most part during the testing period to bring the hydrogen sulfide concentration below the 1 ppm level. The activated carbon was generally ineffective in removing hydrogen sulfide beyond the level attained by the SulfaTreat reactors. (It should be noted that the activated carbon used in these

Ind. Eng. Chem. Res., Vol. 36, No. 10, 1997 4107

Figure 9. Cleanup system effectiveness for dimethyl sulfide (DMS) removal. 0 ) raw gas. O ) SulfaTreat outlet. [ ) hydrotreater outlet, detection limit.

sulfided LFG compounds to sufficiently low levels since it allows them to avoid corrosion and frequent shutdowns. Under investigation in future studies will be the economics of the process. The biggest issues associated with the development of an economic system are the longevity of the catalysts, the ultimate loading of the sorbents, the allowable levels of total halogens remaining in the cleaned gas, and the cost/complexity of generating hydrogen gas on-site. In the field-tests reported here, no significant deactivation of the catalysts was observed during 1000 h of testing. Longer tests will, of course, be needed to validate long-term catalyst stability. In the fuel-cell process of interest here, hydrogen costs are not a concern, since it is envisioned that the hydrogen source for the clean-up process will be the fuel processor subsystem within the fuel cell. For turbine and engine applications, this remains an issue for further study. These issues will be addressed in a future comprehensive EPRI report (FPRI TR-108043: “Design and Testing of a Landfill Gas Cleanup System for Carbonate Fuel Cell Power Plants”). Literature Cited

Figure 10. Cleanup system effectiveness for methylmercaptan removal. O ) raw gas. 0 ) SulfaTreat outlet. ] ) hydrotreater outlet, detection limit.

experiments was selected based on its halocarbon removal efficiency; chemically modified activated carbon is available, which can remove hydrogen sulfide, but it was not used in the tests reported here.) Figure 9 indicates that the SulfaTreat reactors were only partially effective in removing DMS, while the CSHP system reduces the DMS below 1 ppm. On the other hand, the SulfaTreat reactors were very effective in reducing the methylmercaptan concentrations below its detection limit, and therefore, from Figure 10 it is difficult to deduce the removal efficiency of the CSHP system. 4. Conclusions These tests demonstrate the basic feasibility of the CSHP process to clean landfill gas of halogen and sulfur compounds to low levels. The CSHP system could be utilized as a stand-alone system or in combination with more conventional landfill gas clean-up schemes, as demonstrated at the Anoka test site. The system would be applicable to any power-generation device operating on landfill gas, including fuel cells, combustion turbines, and reciprocating engines. Though the emphasis in these tests was on fuel-cell applications, turbines and engines also benefit from cleanup of the halogenated

Augenstein, D.; Pacey, J. Landfill gas energy utilization technology options and case studies. EPA Report-600/R-92-116, June 1992. Baker, L.; Capouya, R.; Cenci, C.; Crooks, R.; Hwang, R. The landfill testing program; data analysis and evaluation guidelines. CAPCOA/CARB Report, Sept 1990. CRC Handbook of Chemistry and Physics, 73rd ed.; CRC: Cleveland, OH, 1992. Doorn, M.; Pacey, J.; Augenstein, D. Landfill gas energy utilization experience: discussion of technical and non-technical issues, solutions and trends. EPA-600/R-95-035, March 1995. EPA Report-450/3-90-011a. Air emissions from municipal solid waste landfillssBackground information for proposed standards and guidelines. March 1991. EPA Working Draft Report. Opportunities for landfill gas energy recovery in California. Draft profiles of candidate landfills and current projects. March 1996. Frimmel, J.; Zdrazil, M. Hydrogenolysis of organochlorinated pollutants: Parallel hydrosulfurization of methylthiophene and hydrodechlorination of dichlorobenzene over carbon-supported Ni, Mo, and Ni-Mo sulfide catalyst. J. Chem. Technol. Biotechnol. 1994, 63, 17. Gioia, F.; Famiglietti, V.; Murena, F. Catalytic dechlorination of 1,2,3-trichlorobenzene. J. Hazardous Mater. 1993, 33, 63. Hagh, B. F.; Allen, D. T. Catalytic hydroprocessing of chlorobenzene and 1,2-dichlorobenzene. AIChE J. 1993, 36, 773. He, C. Catalytic hydrogenation for landfill gas clean-up. M.S. Thesis, USC, Nov 1995. Novak, M.; Zdrazil, M. Effects of sulfidation and synergism in hydrodechlorination of o-dichlorobenzene over Ni-Mo/alumina catalyst. Bull. Soc. Chim. Belg. 1993, 102, 271. Reid, R. C.; Prausnitz, J. M.; Poling, B. E. The Properties of gases and liquids, 4th ed.; McGraw-Hill: New York, 1987. Samuels, A. H2S removal system shows promise over iron sponge. Oil Gas J. 1990, Feb 5. Sandelli, G. J.; Trocciola, J. C.; Spiegel, R. J. Landfill gas pretreatment for fuel-cell applications. J. Power Sources 1994, 49, 143.

Received for review March 24, 1997 Revised manuscript received July 14, 1997 Accepted July 22, 1997X IE970252H

X Abstract published in Advance ACS Abstracts, September 15, 1997.