Converting Oil Shale to Gasoline via Alberta Taciuk Processor Results in Full Fuel Cycle GHG Emissions 1.5-1.75 Larger Than From Conventionally Produced Gasoline
|GHG emissions for ATP shale (low and high cases) and conventional gasoline in grams of CO2e per MJ of final fuel delivered. Credit: ACS. Click to enlarge.|
Converting oil shale to gasoline via the Alberta Taciuk Processor (ATP)—an above-ground shale retort—results in fuel-cycle greenhouse gas emissions of ~130-150 g CO2 equivalent/MJ of gasoline produced, according to a new analysis by Dr. Adam Brandt at Stanford University. These emissions are 1.5 to 1.75 times larger than emissions from conventionally produced gasoline.
The results depend most sensitively on the grade of shale used, and the rate of carbonate mineral decomposition which causes inorganic CO2 release, reports Brandt in a paper published online 25 August in the ACS journal Energy & Fuels.
Oil shale is a fine-grained sedimentary rock containing kerogen—a solid organic precursor to oil and gas—from which hydrocarbon gases and liquids (HCs) can be obtained through the application of heat. There are two basic approaches to processing oil shale: mining the rock and heating it in a surface retort, and heating the rock in the ground, to then pump up the resulting oil. The largest global oil shale deposits—estimated to be equivalent to 1,500 Gbbl oil equivalent—are found in the Green River formation of Colorado, Utah, and Wyoming.
Recent oil shale efforts have been spurred by US federal support for oil shale research and development, for example, the White River Mine research and development project, proposed by the Oil Shale Exploration Company (OSEC) in response to a Bureau of Land Management call for research proposals. The largest stage of the project was proposed to produce 1.8 Mbbl of shale oil from 2.7 Mton of raw shale, using a 250 ton/h Alberta Taciuk Processor (ATP) retort. More recently, OSEC has proposed using the Petrosix process, another oil shale retorting technology.
Also of importance is a recent project in Queensland, operated by Southern Pacific Petroleum(SPP), which also used the ATP. This project was terminated in 2004 due to cost overruns and opposition on environmental grounds. Current ATP development activities include the construction of an ATP retort for use in the Fushun shale of China.—Brandt, 2009
In the new paper, Brandt models two large-scale deployments of the ATP which have low and high energy and GHG intensities—each of larger scale than existing operations. In each case, the ATP is applied to oil shales of the Green River formation.
The ATP fuel cycle comprises six stages:
- Mining and preprocessing of shale
- Disposal of spent shale
- Onsite upgrading of raw shale oil
- Refining of upgraded shale oil
- Combustion of refined liquid fuels
|Schematic of mass and energy flows in the ATP retort. Credit: ACS. Click to enlarge.|
The ATP retort has lower water requirements than previous surface retort designs, and can also utilize fine particles, thus reducing shale waste. Most or all of the retorting energy is provided by the combustion of char (a carbon-rich shale coke resulting from the heating of the kerogen that remains adhered to shale particles) and produced gas, making the process potentially energy self-sufficient from the point of view of the operator.
Incoming shale is heated to above 500 °C. The temperature of retorting is a design characteristic, Brandt notes, with higher temperatures resulting in shorter retort residence times and somewhat higher oil output. However, at temperatures above 600 °C, there is a tendency to reduce oil yield, likely by cracking of oil. Oil and noncondensable gases (HCs, H2, CO2, CO, and H2S) are removed from the retort as vapors, carrying energy with them. The retorted shale is then moved to the combustion chamber.
The high temperatures can cause carbonate minerals within the shale to decompose. When contained in oil shale, carbonates begin decomposing at ~565 °C (MgCa(CO3)2) or 620-675 °C (CaCO3), depending on the partial pressure of CO2 in the retorting atmosphere. In addition to the release of inorganic CO2, carbonate decomposition is endothermic, increasing the heat demand of retorting.
Brandt performed full life cycle assessments (LCA) for high and low energy cases. Among his findings were:
Producing 1 MJ of reformulated gasoline from shale via the ATP requires the consumption of 0.56 to 0.87 MJ upstream. For comparison, upstream consumption for reformulated gasoline produced from conventional oil is ~0.2-0.25 MJ/MJ fuel.
Much of the energy input comes from the fuel feedstock itself. Nearly all of the energy consumed by the retort is provided by the shale itself and much of the refinery energy input comes from the shale oil refinery feedstock.
Full-fuel-cycle GHG emissions are estimated to be 129 g CO2 equiv/MJ in the low case and 153 g CO2equiv/MJ in the high case. Emissions from carbonate decomposition are important in both cases. These are ~1.5-1.75 times those of gasoline from conventional crude oil on a full-fuel-cycle basis.
By varying key parameters in the model, Brandt found that the results are most sensitive to the richness of the shale: the lower the shale quality, the more inert mineral matter must be heated per megajoule of oil produced. Results are also sensitive to the level of carbonate decomposition.
In his discussion of the results, Brandt notes that the two cases analyzed are conservative and could underestimate the actual operating impacts of oil shale production using the ATP. Upstream energy inputs in the mining of the shale could be higher, and higher rates of carbonate decomposition could also be higher. In addition, the analysis does not account for fugitive methane emissions which could occur during shale and shale oil handling and processing.
It is instructive to consider the implications of a very large oil shale industry that does not practice CO2 mitigation. If we produce, refine, and combust fuel equal to 10% of 2005 US gasoline consumption (3.3 x 108 bbl/y, or 1.8 x 1018 J) from oil shale using the ATP instead of conventional oil, full-fuel cycle emissions could increase from about 42.5 million t of carbon (C as CO2) to 65-74 million t of carbon. This is a rough increase of 20 to 30 million t. To put these figures in perspective, emissions from all sectors in the state of Colorado equaled 24 million t of carbon in 2001. Thus, replacing 10% of US gasoline with shale-derived fuels produced using large-scale ATP projects would result in additional emissions commensurate with the total emissions from the state of Colorado.
Given the uncertainties involved and the potential for large GHG impacts from oil shale production with the ATP, more research attention should be focused on understanding this technology and mitigating its impacts. It is especially crucial that this occur before the development of an oil shale industry in the United States.—Brandt, 2009
Adam R. Brandt (2009) Converting Oil Shale to Liquid Fuels with the Alberta Taciuk Processor: Energy Inputs and Greenhouse Gas Emissions. Energy Fuels, Article ASAP doi: 10.1021/ef900678d