Heavy Metals in Colorado and Chinese Oil Shale Semicoke: Disposal Issues, Impediments to Byproduct Conversion † ‡ Beatriz Datangel and Jillian L
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ARTICLE pubs.acs.org/EF Heavy Metals in Colorado and Chinese Oil Shale Semicoke: Disposal Issues, Impediments to Byproduct Conversion † ‡ Beatriz Datangel and Jillian L. Goldfarb*, † Department of Chemistry, Simmons College, 300 The Fenway, Boston, Massachusetts 02115, United States ‡ Department of Chemical Engineering, University of New Hampshire, 33 Academic Way, Durham, New Hampshire 03824, United States ABSTRACT: Oil shale, a fine-grained sedimentary rock, contains a proportionally large amount of kerogen, which can be converted into oil by thermal degradation of the compacted rock. The primary byproduct of current oil shale oil extraction processes is semicoke. Its landfill deposition presents a possible threat to the environment and represents a waste of a potentially useable byproduct. In this work, we explore the heavy metal content of oil shale semicoke pyrolyzed at 500 and 1000 °C to better understand the risks posed by disposal of oil shale processing waste on the nearby environment, as well as impediments to potential byproduct conversion. The greatest potential obstruction to byproduct conversion and the greatest environmental risk posed by open air disposal of oil shale semicoke is likely due to relatively high arsenic concentrations; using X-ray fluorescence spectroscopy, we find arsenic concentrations in semicoke (pyrolyzed at 500 °C) ranging from 25 ppm for Chinese oil shale from the Huadian mine (class C) to 79 ppm for Green River, Colorado, 50 gallons per ton (GPT) of oil shale. Other heavy metal elements analyzed, including barium, copper, lead, manganese, and iron, are well below the United States Environmental Protection Agency (U.S. EPA) regional screening limits. 1. INTRODUCTION are electricity generation by power stations and oil production.5 Over the past several decades, concerns over dwindling fossil Electricity production generates ash as the primary waste, a silica- fuel resources have prompted a surge in research and develop- based powder with little to no volatile organics present, as the ment of alternative energy technologies. Although scientists, compacted rock is burned directly in a combustion chamber. Oil politicians, and ordinary citizens alike agree with the need to extraction from oil shale creates semicoke as a byproduct via develop new energy generation technologies, the choice among pyrolysis of the shale rock to extract the oil. In this scenario, the fi rock is compacted to increase the available surface area (to speci c alternative energy sources available is a hot topic for ° debate. In North America, where tar sand and oil shale deposits increase oil yield) and heated upward of 500 C under an inert are vast, experimentation is ongoing in the areas of alternative gas to prevent oxidation and combustion of the fuel while fossil fuels. Oil shale is a low-grade solid fuel with high ash extracting the oil. The oil yield primarily depends upon the content. A fine-grained sedimentary rock, oil shale contains a amount of kerogen originally present in the shale, as well as the proportionally large amount of kerogen, which can be converted pyrolysis temperature. Whereas ash from electricity generation into oil by thermal degradation of the compacted rock. has low carbon content (less than 0.1% by mass of solid Worldwide deposits of oil in shale are estimated at upward of material), semicoke has a relatively high amount of organic 2.8 trillion barrels of recoverable oil.1,2 Historically, the cost of oil char present, depending upon the aforementioned shale and produced from oil shale is markedly higher than that of conven- extraction conditions. tional oil processes, such as drilling, and this has inhibited Previous work shows that oil shale semicokes have relatively high overall surface areas despite a 2À18% organic char fraction countries, such as Australia and the United States, from devel- fi 2 À1 oping an oil shale industry.3 Interest in oil shale in the United (and thus have high speci c surface areas, upward of 300 m g , unactivated), leading us to posit a potential use of oil shale States was piqued in the 1970s when the price of oil peaked. 6 However, when oil prices fell, research toward oil production semicoke as a sorbent material. Similar to conventional fossil from shale markedly decreased. However, with oil prices once fuels, oil shale is a mixture of inorganic and organic compounds again setting record highs, oil shale is back at the forefront of formed during prehistoric times. It is these inorganic compounds many discussions as a stopgap fossil fuel source between our that lead us to question whether heavy metals present in the oil reliance on the pipelines of the Middle East and a renewable shale rock remain within the semicoke after processing. This is a potential impediment to using the semicoke as an in situ sorbent. energy future. ffi There are several barriers to commercial use of oil shale as a If there are a su cient amount of metals present, the semicoke domestic source for oil. These include overcoming process water may require extensive pretreatment and nullify the byproduct requirements and ground swelling issues, long-term stability of the manufactured oil, and dealing with the veritable mountains Received: April 25, 2011 of oil shale semicoke produced.4,5 This paper addresses the Revised: June 27, 2011 latter issue. The two primary utilization pathways for oil shale Published: June 29, 2011 r 2011 American Chemical Society 3522 dx.doi.org/10.1021/ef2006386 | Energy Fuels 2011, 25, 3522–3529 Energy & Fuels ARTICLE conversion benefits of disposal cost reduction and environmental and 1000 °C, monitored with an Omega type K thermocouple directly mitigation. The presence of heavy metals represents a potential above the sample accurate to (0.5 K under a helium flow of approxi- À issue to the open-air disposal of the semicoke, because these mately 300 cm3 min 1 in a porcelain crucible. The mass loss from the metals could leach into groundwater and transport through the pyrolysis procedure is attributed mostly to the removal of volatile matter environment. Fathoni et al. note that issues of semicoke storage (kerogen) and then, to a lesser extent, water removal and the decom- and disposal surpass concerns over the stability of extracted oil.4 position of calcite and dolomite in the mineral portion of the shale. The By investigating a variety of oil shale semicoke samples from Colorado 50 GPT and Chinese Maoming mine class A samples were also China and the United States, we probe the feasibility of waste-to- heated in an oxidative environment to yield 100% ash burnoff samples fi byproduct conversion of semicoke, as well as storage and disposal for testing to mimic the waste produced from ring or co-combusting oil shale and semicoke (because of sample size limitations, we could only issues because of entrained heavy metals as a function of the shale ff origin and pyrolysis temperature. perform the 100% burno for these two samples). Organic char contents and BrunauerÀEmmettÀTeller (BET) surface areas of samples pyro- Prior studies document the presence of heavy metals in oil ° 6 shale fly ash. In some parts of China, for example, studies lyzed at 500 and 1000 C are reported in previous work. 2.2. X-ray Fluorescence (XRF) Measurements. demonstrate an accumulation of lead, cadmium, zinc, thallium, A Thermo barium, and arsenic in oil shale flyash.7 These metals are human Fisher Scientific Niton Hand-held XL3t XRF analysis instrument was used to characterize the presence of metals. Because each element has a carcinogens and are categorized by the United States Environ- distinct set of energy levels, it emits X-rays at a unique set of energies. mental Protection Agency (U.S. EPA) as priority heavy metals ff The set of energies allows the instrument to measure the elemental because of their toxic e ects on humans. As a result of open composition of a sample. XRF spectroscopy is rapid and does not disposal, large quantities of heavy metals could enter water damage or alter the sample, making it a preferred method for a variety of reserves, be it groundwater or surface water, and deteriorate applications, such as metal and alloy analysis, mining and geochemical the quality of the environment. Metal compounds change their exploration, art and archeology, and quality control of consumer goods. phases rather than break down and can be transported by air or Environmental applications specifically pertain to soil, dust, air filters, ff 7 11 land to water through seepage and runo from the sediment. As and drywall testing. Nei et al. detail, in Estonia, large quantities of cadmium, copper, We programmed the XRF to follow EPA method 6200 for “field zinc, lead, and other metals are present in oil shale waste; these portable XRF spectrometry for the determination of elemental concen- 8 metals enter soil and water compartments as ions. trations in soil and sediment” for both in situ and ex situ soil samples.12 A secondary pollution exposure pathway may exist when This method was employed because it represents the closest in sample working with or prolonged exposure to products like cement, characteristics to the oil shale semicokes. The analytes measured in this which contain a significant fraction of spent fuel to increase the method include the eight Resource Conservation and Recovery Act long-term stability of the cement. Heavy metals present in the oil (RCRA) metals of arsenic, barium, cadmium, chromium, lead, selenium, shale ash and semicoke must be quantified to model the effects of and silver. The other elements measured using the XRF include exposure on those who use potential byproducts in concrete and molybdenum, strontium, zirconium, rubidium, throium, zinc, copper, other construction materials or spend prolonged time near disposal iron, manganese, vanadium, titanium, scandium, potassium, sulfur, and sites.9,10 The ability to convert the semicoke waste to a useable calcium.