A techno–economic analysis of an integrated GTL, nuclear facility with utilities production
Francis, Michael Craig
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The nuclear industry has undergone a revival in recent years, which has been more commonly termed the nuclear “renaissance”. This renaissance period has brought renewed interest to the commercial nuclear industry as well as to peripheral or related industries, particularly in the areas of research and development. Some of the most common research topics include the integration of nuclear and process technologies, or more specifically the use of nuclear heat energy in process plants. Gas–to–liquids (GTL) technology, although often referred to as an unconventional fossil fuel technology, is a mature technology and successful commercial applications in the state of Qatar are evidence of that. Likewise, thermal desalination processes such as multi stage flash (MSF) and multiple effect distillation (MED) are also very mature technologies that have been in commercial operation for many decades. Both GTL and desalination processes may be regarded as energy intensive processes that demand large amounts of thermal energy, which is typically provided by the combustion of fossil fuels. The use of fossil fuels as a primary energy source, however, has a number of drawbacks: unstable and/or rapidly increasing prices, negative environmental impact as well as concerns over long term sustainability. Nuclear energy is far more attractive from a sustainability perspective and also produces negligible carbon dioxide (CO2) emissions. By utilising nuclear heat energy either directly or through waste heat in a secondary circuit, process plants become more energy efficient whilst also emitting less green house gases. The proposed process design is an integrated nuclear GTL facility: the primary focus is the integration of heat energy in a typical GTL complex. The secondary focus is the use of nuclear energy to drive electricity and potable water production. A typical GTL facility herein refers to the type investigated and proposed in a recent feasibility study conducted by Sasol Technology and Sasol Chevron Holdings Limited in 2006, which is property of Sasol Chevron Holdings Limited and Sasol Chevron Holdings Qatar Limited, as part of the Sasol Chevron Integrated GTL project comprising gas and GTL plants. The proposed integrated facility is a large industrial complex and Qatar was chosen as a suitable geographic location for the study for a number of reasons: * Established GTL industry, which is supported by the government as a means of monetizing their natural gas resources. * Extensive natural gas reserves fed from the world’s largest non–associated gas field * An industrial city, such as Raf Laffan, that contains well established logistical and engineering infrastructure to support a large industrial complex. * Socio–economic considerations that warrant the development of additional utilities generation capacity in Qatar. * Favourable political climate for the introduction of nuclear energy in the region. In the proposed design only a handful of units in the typical GTL complex were identified for heat integration: synthesis gas generation (reforming), hydrogen production unit (reforming) and the process superheaters. The focus area of the GTL complex was then upstream of the Low Temperature Fischer Tropsch (LTFT) reaction units and there were no opportunities for heat integration identified in the downstream product work up (PWU) or refinery units. The process was modelled as a nuclear steam methane reforming (SMR) process, with nuclear heat providing the required endothermic reaction energy for the reforming process. The helium exit temperature from the reforming process was 781.50oC, which meant that the helium could also be used to superheat the complex high pressure (HP) steam. The superheated HP steam was then used as feed to the reformers themselves and to drive a back end Rankine power cycle. A final stage, backpressure turbine then provided low pressure (LP) steam to drive MSF desalination units. Approximately 40 percent of the total available nuclear thermal energy was used in the reforming and superheater units. In the helium Brayton power cycle a significant amount of electricity was generated whilst also providing low temperature waste heat that was utilized for MED desalination units. The proposed integrated design thus combined three technologies that together produced large quantities of their respective products. The integrated nuclear GTL design also required the introduction of a CO2 shift reactor downstream of the reforming units to correct the synthesis gas (Syngas) ratio fed to the LTFT reactors. The CO2 makeup stream was assumed to be imported from offsite. This shift reactor unit was certainly a departure from the conventional GTL process layout and represented a significant CO2 credit opportunity, particularly in the context of a large industrial facility such as that at Ras Laffan. The conventional GTL design also utilizes autothermal reforming technology that requires oxygen feed to the units, while the nuclear SMR process does not require oxygen. Thus another benefit associated with nuclear GTL integration would be the omission of the air seperation units (ASU), which ordinarily require large amounts of energy to drive the unit air compressors. A pressure swing adsorption (PSA) unit and CO2 wash unit were also included upstream of the FT reactors, providing both clean Syngas at the required Syngas ratio as well as a clean, high purity stream of hydrogen to be used in the PWU units. An economic analysis was performed to gauge the realistic viability of the technical proposal. In this analysis simple return on investment (ROI) calculations were performed to provide net present value (NPV) and internal rate of return (IRR) indications. A constant discount rate of 21.25% was used for all economic calculations. The various technologies were also analysed as stand–alone facilities and then together as an integrated facility. The major drivers or levers in each of the respective industries were used as bases for low, high and reference economic analysis. The base case typical GTL complex returned very favourable values with an IRR of 68%. The integrated facility also retuned favourable ROI indictors with an IRR of 42%. In the context of an integrated nuclear GTL facility, the nuclear portion alone was not economically viable as most of the energy was used for process heat rather than power generation. The inclusion of C02 credit revenues only marginally improved the economics of the nuclear portion of the facility, but obviously contributed positively to the overall facility ROI indicators. At a CO2 credit value of 90.62 $/ton the nuclear portion of the integrated facility would become economically justifiable in its own right. However, it may be argued that such a high CO2 credit value is highly unlikely in the short to medium term future. The major technical benefits of a nuclear integrated facility include improved carbon efficiency and measurable CO2 emissions reduction. The typical (base case) GTL facility, however, has an attractive business case without the integration of the nuclear and desalination technologies. A decision to invest in such a large, integrated facility would thus depend heavily on local socio–economic and political factors. The key driver in GTL economics, and hence the proposed integrated design as well, is the product pricing and natural gas/crude oil price differential. This is the main reason for presenting low, high and reference growth cases in the economic analysis. Despite lower NPV and IRR indicators than the GTL base case, the integrated design still represents an attractive investment. The comprehensive facility is also an excellent means to monetize gas resources and provide utilities to a fast growing nation.
- Engineering