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Energy & Fuels 1997, 11, 316-322
Adsorbed Natural Gas Storage with Activated Carbons Made from Illinois Coals and Scrap Tires Jian Sun, Todd A. Brady,† Mark J. Rood,* and Christopher M. Lehmann Department of Civil Engineering, University of Illinois, 205 North Mathews Avenue, Urbana, Illinois 61801
Massoud Rostam-Abadi* and Anthony A. Lizzio Illinois State Geological Survey (ISGS), 615 East Peabody Drive, Champaign, Illinois 61820 Received November 8, 1996. Revised Manuscript Received January 14, 1997X
Activated carbons for natural gas storage were produced from Illinois bituminous coals (IBC102 and IBC-106) and scrap tires by physical activation with steam or CO2 and by chemical activation with KOH, H3PO4, or ZnCl2. The products were characterized for N2-BET area, micropore volume, bulk density, pore size distribution, and volumetric methane storage capacity (Vm/Vs). Vm/Vs values for Illinois coal-derived carbons ranged from 54 to 83 cm3/cm3, which are 35-55% of a target value of 150 cm3/cm3. Both granular and pelletized carbons made with preoxidized Illinois coal gave higher micropore volumes and larger Vm/Vs values than those made without preoxidation. This confirmed that preoxidation is a desirable step in the production of carbons from caking materials. Pelletization of preoxidized IBC-106 coal, followed by steam activation, resulted in the highest Vm/Vs value. With roughly the same micropore volume, pelletization alone increased Vm/Vs of coal carbon by 10%. Tire-derived carbons had Vm/Vs values ranging from 44 to 53 cm3/cm3, lower than those of coal carbons due to their lower bulk densities. Pelletization of the tire carbons increased bulk density up to 160%. However, this increase was offset by a decrease in micropore volume of the pelletized materials, presumably due to the pellet binder. As a result, Vm/Vs values were about the same for granular and pelletized tire carbons. Compared with coal carbons, tire carbons had a higher percentage of mesopores and macropores.
Introduction Despite technical advances to reduce air pollution emissions, motor vehicles still account for 30-70% of all urban air pollutants.1 The Clean Air Act Amendments of 1990 require 100 cities in the United States to reduce their O3 levels within 5-15 years. Hence, auto emissions, the major source of pollution that causes O3, must be reduced 30-60% by 1998.2 Some states like California have set stringent laws to clean up severe air pollution. Beginning in 1997, 25% of all cars sold in California should qualify as low-emissions vehicles (LEVs). By 2005, 75% of the cars sold in California should be LEVs.3,4 This situation has spurred interest in the research and development of alternative fuels for vehicles. Alternative Fuels for Vehicles. (a) Methanol, Liquefied Petroleum Gas, and Hydrogen. Vehicles fueled with M85 (85% methanol/15% gasoline) or M100 (pure methanol) have CO and NOx emissions similar to those of conventional gasoline-fueled vehicles.5 The largest air pollution emission benefit from M100-fueled vehicles is their reduced ozone-producing potential (1.8 † Present address: Intel Corp., W. Chandler Blvd., Chandler, AZ 85226. X Abstract published in Advance ACS Abstracts, February 15, 1997. (1) DeLuchi, M. A.; Ogden, J. M. Transp. Res. 1989, 27, 255. (2) Kreith, F.; Norton, P.; Potestio, D. Transp. Q. 1995, 49, 2. (3) Keller, M. N. Cars 1992, March, 59. (4) Johannson, L. ENVIRO 1992, 13, 11.
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g of O3/mile), compared with gasoline (3.8 g of O3/mile).6 Emissions from liquefied petroleum gas (LPG)-fueled vehicles are comparable to gasoline-fueled vehicles, but their ozone-producing potential is lower (0.7 g of O3/ mile).6 Hydrogen-fueled vehicles theoretically produce no pollutants except NOx, which can be further reduced by lowering combustion temperature.7 However, it is important to consider that hydrogen is generally made by coal gasification or water electrolysis, and these processes generate air pollutants directly or indirectly. (b) Natural Gas. Natural gas as a fuel for vehicles offers many environmental benefits. A natural gas vehicle (NGV) uses a conventional spark ignition engine with only minor modifications. Natural gas burns more completely and produces fewer air pollutants than gasoline. Compared with conventional gasoline-fueled vehicles, NGVs have 76% lower CO, 75% lower NOx, and 88% lower non-methane HC emissions due to better mixing of the gaseous fuel, lean fuel to air ratio, and lack of fuel enrichment to start.5,6 Carcinogenic pollutant (e.g. benzene and 1,3-butadiene) emissions are minimized.6 Because the hydrocarbon constituent in NGV exhaust is predominantly methane, which has (5) Alson, J. A.; Adler, J. M.; Baines, T. M. In Alternative Transportation Fuels; Quorum Books: New York, 1989. (6) Carslaw, D. C.; Fricker, N. Chem. Ind. 1995, Aug 7. (7) Golovoy, A. Proceedings: Compressed Natural Gas; Society of Automotive Engineers: Pittsburgh, PA, 1983.
© 1997 American Chemical Society
Adsorbed Natural Gas Storage
insignificant tropospheric photochemical reactivity, emissions from NGV are expected to contribute the least to ozone formation (0.2 g of O3/mile).6,8 There are three technologies for on-board natural gas storage: liquefied natural gas (LNG), compressed natural gas (CNG), and adsorbed natural gas (ANG). LNG uses a cryogenic system that is bulky and expensive. CNG has been commercialized worldwide. However, its primary drawback is that the gas must be stored at a high pressure (3000 psig). ANG uses adsorbents and operates at a much lower storage pressure (500 psig) than CNG; thus, ANG has lower capital and maintenance costs. As an automobile fuel, natural gas has emerged as a leading alternative to gasoline. There are about 40 000 NGVs in the United States and about 1 million worldwide.5 In the short term, depot-based commercial fleets (e.g. buses and taxis) will be the first beneficiaries of NGVs because of their present limited range and lack of a national fuel-service infrastructure. Improvement in ANG technologies will offer significant opportunities for reducing capital and operating costs.7,9,10 Adsorbed Natural Gas Storage. A key ingredient for successful commercialization of ANG is the adsorbent. Natural gas storage capacity of an adsorbent is usually assessed in terms of its volumetric methane storage capacity (Vm/Vs, where Vm is the volume of stored methane at standard temperature and pressure and Vs is the volume of the storage container). Commercial development of ANG requires adsorbents with cost e$2/lb and storage capacities g150 Vm/Vs.10 Among all of the adsorbents, activated carbons have the most favorable natural gas storage capacities.7,11 Illinois coal has the potential to be an inexpensive and plentiful parent material for activated carbons in ANG applications. The United States also has large stockpiles of waste tires, growing at a rate of approximately 280 million tires/year.12 Over 80% are landfilled, constituting a loss of significant resources and creating environmental problems. If some of these waste tires were converted into activated carbons, a significant amount of tires could be diverted from landfills. Activated carbons from these sources may meet the cost and adsorption capacity requirements for ANG adsorbents. The overall objective of this study is to develop methods for producing microengineered adsorbents from coal and scrap tires and evaluate the potential application of these materials for use in ANG vehicles. The goal was to produce low-cost adsorbents that meet or exceed the performance and cost targets established for low-pressure natural gas storage materials. This paper describes methods for producing adsorbents from coal and scrap tires. The effects of coal preoxidation, type of activation (chemical or physical activation), and pelletization of the resulting carbon or coal were studied as process variables. The potential of these low-cost adsorbents for application in low-pressure ANG vehicles is also evaluated. (8) Seinfeld, J. H. Air Pollution: Physical and Chemical Fundamentals; McGraw-Hill: New York, 1975. (9) Nelson, C. R. Physical Sciences NGV Gas Storage Research; Gas Research Institute: Chicago, IL, 1993. (10) Wegrzyn, J.; Weismann, H.; Lee, T. Proceedings: Annual Automotive Technical Development; Dearborn, MI, 1992. (11) Quinn, D. F.; MacDonald, J. A.; Sosin, K. Proceedings of the 207th National Meeting of the American Chemical Society, San Diego, CA; American Chemical Society: Washington, DC, 1994. (12) New York Times 1990, May 9, D1.
Energy & Fuels, Vol. 11, No. 2, 1997 317 Table 1. Proximate and Ultimate Analyses of IBC-106 Coal, IBC-102 Coal, and Atlas Scrap Tire proximate analysis (as received) moisture (wt %) volatile matter (wt %) fixed carbon (wt %) ash (wt %) ultimate analysis (dry) ash (wt %) carbon (wt %) hydrogen (wt %) nitrogen (wt %) sulfur (wt %) oxygen (wt %)
IBC-102
IBC-106
tire
14.4 34.1 45.6 5.9
8.3 37.9 45.9 8.0
0.9 69.8 26.2 3.2
6.9 74.1 5.3 1.5 3.3 9.0
8.7 70.3 5.2 1.5 3.7 10.6
3.2 86.2 7.4 0.1 1.7 1.5
Materials and Experimental Procedures Initial Sample Preparation. Adsorbent carbons were produced from two Illinois coals, IBC-106 (free swelling index ) 4.313 ) and IBC-102 (free swelling index ) 3.813). These coals were chosen because of their high carbon and low ash contents in comparison to other Illinois coals. The raw IBC-106 coal, provided by the Illinois Basin Coal Sample Program,13 was ground and sieved from -8 mesh (
