|Fig. 1: An image of a typical pressurized water nuclear reactor. (Source: Wikimedia Commons)|
The demand for energy, population of humans on earth, and carbon emissions will continue to increase over time. In order to mitigate a 2°C increase in temperatures, CO2 and greenhouse gas emissions should be reduced. An efficient method of doing so is by capturing CO2 emissions from hydrocarbon-combustion based power plants. However, the amount of energy required to generate steam for sorbent regeneration renders the carbon capture and sequestration (CCS) process expensive. Due to nuclear energy's promising high plate capacities, low emissions, and high heat generation, this paper will take a look at the feasibility of using nuclear energy as a heat source for CCS coupled with enhanced oil recovery (EOR). In order to understand where nuclear energy can play a role, existing energy inefficiencies and raw material consumption rates will be analyzed for power plants, nuclear energy, EOR, and CCS.
All power plants experience inefficiencies that reduce the amount of energy converted to electricity; in a thermodynamically ideal world, all extractable energy would be converted to electricity. The second law of thermodynamics states that not all energy is transformed into useful energy due to entropic losses (i.e. heat); energy may be lost as waste heat during and prior to electricity generation, and further reduced through ohmic resistance (6% of the electricity being transmitted and distributed annually in the US) in the US power grid.  Aside from producing electricity, all power plants emit greenhouse gases directly as a byproduct or indirectly from the equipment and material used for maintaining the power plant. However, the amount of emissions per power plant vary from each other and per energy source. Electricity can be generated from hydrocarbon-based fuels, nuclear energy, and renewable energy sources; among these sources, hydrocarbon-based fuels emit the most greenhouse gas. 
According to a 2013 EIA annual data on US electricity generation, 67% of electricity was produced by combustible fuels, 19% of US energy was produced from nuclear energy, and 13% was produced from renewable energy sources.  In order to mitigate emissions and be within air standards, hydrocarbon-based power plants employ CCS technology. Although CCS technology helps reduce emissions, applying these technologies also reduces the net output of electricity generated from power plants, and introduces additional constraints and energy inefficiencies.
Nuclear power plants generate electricity through sustaining nuclear chain reactions. These nuclear chain reactions consist of unstable elements undergoing nuclear fission to break into two or more relatively stable products. Aside from these elemental products, a large amount of energy is released in the form of radiation and heat. This energy is absorbed by a coolant, which then heats up water via a heat exchanger, prior to being sent to a steam turbine for electricity generation.  After turning the turbines, the steam then goes through a condenser containing coolant or lake water before being recirculated to the reactor to be used further. For a power plant with a once- through cooling system, steam is continuously cooled by more water that is pumped from an outside source. An example of a nuclear power plant is shown in Fig. 1.
|Fig. 2:An image of steam injection EOR. (Source: Wikimedia Commons)|
For power stations running 24 hours a day for a year, and power stations with once-through systems, nuclear power plants consume 13 ML/MW of energy generated, while fossil/biomass/waste power stations consume 10 ML/MW.  Although nuclear power plants may consume more water than fossil-fired power stations, nuclear power plants generate more electricity with fewer emissions than fossil-fired power stations do. 
Although hydrocarbon-based energy sources have been around for many years, companies involved in extraction of the raw material (i.e. petroleum, coal) can improve the efficiency of their oil recovery projects. In order to numerically determine the efficiency of oil recovery projects, the energy returned on investment (EROI), the ratio between the amount of usable energy obtained from an energy source and energy expended on acquiring that energy source, can be analyzed.  A look at the EROI of different fuel sources (i.e. solar, wind, hydro) and a look at the EROI of combustible fuel sources reveal that natural gas and oil energy sources are favorable. However, when compared to nuclear energy, these combustible hydrocarbon fuel sources posses a lower EROI.  In order to improve the EROI of these processes, new technologies can be applied to improve the efficiency of oil recovery and recover 20-40% more of the anticipated recovered oil. [5,6]
Enhanced oil recovery (EOR) is the implementation of a variety of techniques in order to increase the yield of oil recovered from an oil field. [2,6] EOR can consist of thermal injection (i.e. steam), or gas injection (i.e. CO2, N2, natural gas). In order to produce the steam required for thermal injection, a boiler can burn fuel directly, or concentrated solar mirrors can be used to can be utilized.  Aside from these two methods, steam can also be obtained via cogeneration systems from power plants by use of a once-through heat-recovery steam generator.  If EOR is applied, wells have the potential to improve recovery rates, improve the EROI, but EOR is expensive and favorable as the price of oil increases, and if the compression or generation of steam and CO2 become economically feasible.  An example of steam injection is shown in Fig. 2. In order to reduce the cost of CO2 used for EOR, CCS must improve.
|Fig. 3:An image of a pulverized coal plant applying carbon capture and sequestration technology. (Source: Wikimedia Commons)|
CCS is an energy intensive process where the calculated ideal minimum work varies per source of CO2. The thermodynamic minimum work of separation is inversely related to the concentration of CO2 in the inlet stream, and temperature of the streams as well. However, the real work of separation is higher than the theoretical values due to equipment inefficiencies and real-world entropic losses. Furthermore, the CO2 adsorption and desorption process in CCS are further limited by sorbate properties, CO2 binding kinetics, and regeneration rates; each of which increase the amount of real work needed. An example of a post-combustion carbon capture technology can be seen in Fig. 3.
In CCS, the real work can be calculated by determining the work requirements after breaking down the process into its series of unit operations, and after accounting for known absorption, adsorption, membrane technologies. In CCS, energy is consumed for gas blowing and compression, solvent pumping, and in solvent regeneration. Gas blowing and compression, as well as solvent pumping both require machinery and a direct source of energy to operate on; on the other hand, solvent regeneration also requires energy, but in the form of steam. The amount of steam required for this process is impacted by the solvent properties, temperature, and amount of dissolved CO2 and substrate present.
For this paper, amine sorbates, such as monoethanolamine (MEA), will be inspected due to their prevalent use in industrial processes for CO2 and H2S removal in scrubbing technologies. Next, an analysis of possible configurations of nuclear assisted CCS and EOR will be determined. This analysis will take a look at improved energy efficiencies, existing policies, and significant economic factors to consider if such a configuration were to be implemented.
Nuclear power plants have previously been studied to determine their use as a heat source in the oil industry.  However, heat derived from nuclear energy has not been applied to due its economic feasibility in the existing market, and nuclear wastes.  Aside from the former and latter limitations, the use of nuclear energy is limited by the depth of the well, and properties of the oil being extracted.  A Canadian study of the use of Candu reactors have shown that utilizing light-water reactors (LWR) and heavy-water reactors (HWR) can result in cost savings from 25 to 50% with respect to burning coal for the steam production estimated.  Although these reactors were shown to be advantages, they were limited by temperature and pressure capability. 
In order to overcome these limitations, and be able to recover extra heavy crudes, high temperature gas cooled reactors (HTGRs) can be used in a cogeneration scheme to produce electricity and required process heat. HTGRs can produce steam within the range of 500-900°C at high pressures.  A study on the application of an 1170 MWth HTGR to extract oil from shale, revealed that price of recovered shale oil would be $41 per barrel in the year 2005.  The previous study's calculation, and current oil prices suggest that in order for nuclear energy to become economically feasible, a stable oil market or demand for energy must be present. As a result, oil prices must rise higher than $41 per barrel in order for nuclear-assisted heavy oil exploitation to appear economically feasible. In order to determine the feasibility of a steam source obtained through nuclear a cogen facility. 
For a coal-fired power plant utilizing a 30 wt% MEA solution for removing 90% of the CO2 present in its flue gas, is approximately 2.5 - 3.6 GJ of energy are consumed.  For this plant, the theoretical minimum work for CO2 separation and compression to the required pressure of 150 bar is 0.42 GJ/ton of CO2. From this energy, approximately half of the energy is used to regenerate the CO2 and the remainder is used to compress the CO2.  A look at the available sources of steam reveal that solar-generated steam is the most economic (USD 17/million BTU), while using steam from a cogen facility (USD 20/million BTU) is slightly less economic due to a fraction of thermal energy being used to generate electricity.  Although the cost of steam is higher for cogen facilities, the cost of this steam is 25% less than the cost of steam produced from a boiler system (USD 27/million BTU).  This difference is anticipated since the cost of cogen steam is the result of the oil field's electrical/thermal demand ratio determined by an ideal steam-supply curve on the basis of the marginal fuel cost. 
Running an Integrated Environmental Control Model (IECM) simulation on a typical 650 MW output coal-fired power plant, with 30 wt% MEA CO2, with CO2 compression pressures of 150 bars (15 MPa), yields the following results: Flue Gas Fan Use = 16.60 MW, Sorbent Pump Use = 1.396 MW, CO2 Compression Use = 67.62 MW, and Sorbent Regeneration Equivalent Energy = 180.1 MW. The energy required for solvent regeneration is approximately 20% of the energy of a 800 MW coal-fired power plant. The sum of these values can help us determine that the real work, 266 MW.
From data collected from this simulation, the CO2 capture system used 85.61 MW, and the amount of CO2 removed by that system is 3.8 × 106 tonnes/yr. By using the former and latter values, it was determined that the amount of energy used for steam for regeneration on a per mole of CO2 basis would be 31.52 kJ per mole of CO2 emitted. Overall, the total cooling water consumed is 8.6 & times; 104 tonnes/hr, 3.4 × 104 tonnes/hr from the steam cycle, 782 tonnes/hr for DCC, 4.6 × 104 tonnes/hr for the capture process, and 5783 tonnes/hr for CO2 compressor process.
For this 650 MW output coal-plant, approximately 86 ML of water would be required for cooling (~107,800 liters/MWh) which falls within the ranges of reported fossil-fueled steam with once-through cooling of 75,708 and 189,270 liters of water per MWh. When compared to nuclear power plants' water-withdrawal rates (94,635 to 227,124 L/MWH), coal fired power plants were determined to withdraw 33 percent less than once-through nuclear systems. 
From the IECM values obtained and literature values reported, HTGR nuclear power plants with a cogen scheme are expected to generate enough necessary electricity to power both power plants including drilling operations, as well as enough steam (~60% of water needed for a coal plant with a CCS system) for CCS and EOR steam injection processes.
As expected, the EROI decreases due to existing inefficiencies related to the 2nd Law of Thermodynamics. In order to improve the EROI associated with the extraction of oil, reduce inefficiencies in steam generation for CCS, and overall reduce carbon emissions and water consumption, thermal heat provided from a nuclear power source via a cogen scheme must be supplied. Although grouping these major energy intensive processes may result in a highly efficient power plant, there are many factors that prevent this project from being feasible. Factors such as market stability in oil prices, or environmental and safety concerns hinder current progress in developing this form of integrated infrastructure, and were lightly addressed in this paper. However, one can anticipate that these environmental policies will change in the future as the population size increases, along with the demand for an improved and environmentally friendly energy infrastructure.
© Joel Dominguez. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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