Study finds well-to-wheels GHG emissions of GTL diesel lower than that of petroleum diesel when co-product displacement is considered
|Gas-to-liquids (GTL) process systems boundary with co-product displacement. Credit: ACS, Forman et al. Click to enlarge.|
Using a co-product displacement method (i.e., substitution) rather than an energy-based allocation method to assess the lifecycle (well-to-wheels, WTW) emissions of gas-to-liquids (GTL) technology results in WTW greenhouse gas (GHG) emissions lower than that of petroleum diesel, according to a paper by a team from Sasol Synfuels International and Baker and O’Brien published in the ACS journal Environmental Science and Technology.
The study also highlights the potential for blending GTL diesel in refineries with heavy crudes that require severe hydrotreating, such as Venezuelan heavy crude oil or bitumen derived from Canadian oil sands and/or in jurisdictions with tight aromatic specifications for diesel, such as California.
Lifecycle assessment (LCA) is an internationally standardized methodology for the systematic and quantitative evaluation of environmental impacts from functionally equivalent products or services, through all stages of the product life cycle. For petroleum-derived transportation fuels, this includes feedstock extraction, fuel production and fuel combustion in the vehicle engine (well-to-wheels).
For a process that results in two or more products, an LCA must deﬁne how environmental impacts will be assigned to those products, Forman et al. note. The framework for handling co-products in an LCA is presented in ISO 14040:2006, and advocates avoiding allocation when possible by including within the boundary of the assessment production processes for materials that are replaced by co-products.
A common approach in LCA and net energy analysis, known as the “system expansion” method (also known as the “substitution” or “displacement” method) credits saved energy and emissions burdens to co-products associated with the products displaced in the market. This method subtracts the energy and materials required to produce the material, along with any downstream benefits or penalties, that are displaced by the co-product. This method has been adopted as the default for dealing with biofuel co-products in transportation LCA models and used in biofuel regulations development, and the substitution method is used wherever needed in the life cycle of a given fuel pathway.
...While the substitution method is generally advocated for conducting LCAs, an unavoidable consequence of its implementation is the outcome can vary greatly depending on the assumptions made in the assessment for processes which produce economically useful coproducts in addition to the fuel. This situation is further exacerbated by the results from LCA being highly sensitive to definitions of system boundaries, life cycle inventories, process efficiencies, and functional units.
...The GTL process can produce GTL diesel, GTL naphtha, and liquefied petroleum gas (LPG) and potentially many other coproducts, such as GTL lubricant base oils, GTL aviation jet kerosene, GTL waxes, and normal paraffins, which all share the same qualities of very low levels of sulfur and aromatics while at the same time being rich in hydrogen relative to conventional analogues.
...These fundamental superior physical properties extend to all GTL products, which can result in a downstream GHG emissions beneﬁt relative to petroleum-derived analogous products, and should require LCA practitioners to thoroughly investigate the GHG emissions benefits of all GTL products using ISO-preferred substitution methodology.
...LCAs of the GTL pathway exist in the literature, but data on the GHG emissions benefits of GTL coproducts is lacking. LCAs of the GTL diesel pathway that have not employed co-product substitution methodology because of lack of available data and substitution complexities have afforded lifecycle GHG emissions slightly higher than current petroleum-derived conventional diesel baselines, while LCAs of GTL diesel that have used the systems boundary expansion method have generally produced lifecycle GHG emissions equivalent to current conventional diesel baselines.
...This study builds on industrial knowledge of the GTL process and GTL products to demonstrate how by using the substitution method, the GHG emissions benefits associated with GTL coproducts aﬀords significant carbon dioxide (CO2) benefits downstream from the production phase, resulting in a different WTW emissions result compared to using the energy allocation method.—Forman et al.
The researchers used two scenarios: Scenario A in which GTL is used as a neat fuel, and Scenario B, in which GTL diesel is blended with petroleum-derived diesel. The GTL pathway model they used is based on global current or imminent GTL production, representing “an industrially relevant average” of the GTL process (the model GTL facility is located in Qatar).
They considered a single product slate from raw gas: LPG, 8.4%; condensate, 17.8%; GTL Naphtha, 23.1%; GTL diesel, 40.7%; GTL normal paraffin, 2.4%; and GTL lubricant base oils, 7.5%. A different mix of GTL products could result in different GHG emissions, they noted.
Condensate and LPG. The associated upstream production process that operates to feed the GTL plant, depending on the characteristics of the gas reservoir and can produce significant amounts of gas condensate, which can add substantially to the overall liquid product output. When assessing the overall WTW GHG emissions impact of GTL (and other natural gas-based fuel pathways), the associated gas condensate produced by the upstream portion of the facilities needs to be carefully considered, Forman et al. suggest.
The gas processing facility separates LPG and condensate from the feed gas to the GTL plant at carbon efficiencies (98%) that are significantly higher than those for the GTL process alone (79.1%). The net effect is an increase in the overall efficiency of the entire system relative to the GTL process alone.For the study, the authors assumed that LPG is burned locally in a commercial boiler, displacing natural gas, which is the major energy source in Qatar. They assumed condensate is shipped from the Middle East to China and India (average of 4,000 nautical miles) and displaces petroleum-derived naphtha.
GTL Naphtha. Steam cracking of GTL Naphtha results in a higher olefin yield than cracking conventional naphtha alone, and is substantially more selective to the production of ethylene, propylene and butadiene.
In addition, because GTL Naphtha has an essentially zero aromatics content, cracking of GTL Naphtha results in reduced coking of furnace tubes and catalyst relative to conventional naphtha, allowing for extended run durations. Consequently, the GHG emissions intensity of GTL Naphtha cracking is lower than petroleum-derived naphtha cracking, they conclude.
GTL Normal Paraffin. The GTL C9–C14 normal paraffin stream is an ideal feedstock for linear alkyl benzene (LAB) or linear alkyl benzene sulfonate (LAS) production for detergent manufacture because it is linear and has essentially zero sulfur and aromatics, they note.
Conventional normal paraffin production requires energy intensive pre-fractionation, hydrotreating and separation from petroleum-derived kerosene to produce the LAB or LAS feedstock.
For the study they assumed that processes downstream of normal paraﬃn production are equal in GHG intensity. This assumption is conservative, they added, because an alternative scenario for GTL normal paraffin extraction is separation of an olefin-rich (25% oleﬁn) C9–C14 normal paraffin stream from the Fischer-Tropsch unit, which would significantly reduce the need for the energy intensive dehydrogenation step that is required in LAB production.
GTL Lubricant Base Oils. GTL lubricant base oils are high in saturates, have excellent oxidation stability, are virtually sulfur free and possess a high viscosity index. They can produce 0W-20 and 5W-20 motor oil grades and provide significant quality benefits over petroleum-derived lubricant base oils, matching in many areas those of the more energy-intensive chemically derived synthetic lubricant base oils (polyalphaolefins or PAOs).
These GTL lubricant base oils also have a fuel efficiency advantage over petroleum-derived lubricant base oils gained from the high viscosity index of these oils, enabling the oils to provide good lubrication (i.e., sufficient viscosity) to the engine over a broad range of operating temperatures.
For the study, they assumed that GTL lubricant base oils (50% 0W and 50% 5W) displace existing petroleum Group II lubricant base oils in the US market, and generate an average fuel efficiency saving of 0.85% against petroleum-derived lubricant base oils.
Refinery blending of GTL diesel (Scenario B). GTL diesel is an already refined, hydrogen-rich, essentially zero-aromatic fuel, which when blended in a refinery could potentially reduce the severity of refining processing, thus reducing incremental CO2-intensive hydrogen production. Blending GTL diesel with petroleum diesel typically results in reduced sulfur and aromatics content and density of the blend, with the cetane number increased.
|Comparison of GHG emissions of GTL diesel and Petro diesel pathways using substitution and allocation methodologies, gCO2e/MJ|
|GTL dieselscenario A||GTL dieselscenario B||GTL diesel||ULSD||CARB diesel|
Several issues suggest future work. Using the substitution method, the composition of the product slate from the GTL pathway appears to have an influence on the final lifecycle GHG emissions of GTL diesel, implying that the environmental advantages of GTL could be optimized with changes to the composition of the GTL product slate. Given the relevancy of the refinery blending results in the current study to other paraffinic fuels, such as HVO, a sensitivity analysis of major parameters used when blending GTL diesel and refinery intermediates could be of value.
Technology and system-level improvements suggest further environmental benefits of GTL. For example, recent results suggest that combustion at altitude of a 1:1 blend of synthetic paraffinic kerosene (SPK) aviation fuel, obtainable from the GTL process, and petroleum-derived JetA kerosene results in significantly less ice particles being formed in contrails with particles formed having a larger average diameter (average ice particle diameter = 2 μm) relative to combustion of JetA kerosene alone (average ice particle diameter = 1 μm). This has the effect of reducing the optical depth and radiative forcing effect of linear contrails from 12 mW/m2 in JetA kerosene to 5 mW/m2 in the 1:1 SPK/JetA blend.
The consequence of these effects would be a smaller overall radiative forcing (RF) impact from combustion of the 1:1 SPK/JetA blend relative to combustion of JetA alone. Although the level of scientiﬁc understanding remains low, linear contrails and contrail cirrus are generally thought to be significant radiative-forcing components associated with aviation.—Forman et al.
Grant S. Forman, Tristan E. Hahn and Scott D. Jensen (2011) Greenhouse Gas Emission Evaluation of the GTL Pathway. Environmental Science & Technology DOI: 10.1021/es202101b