Shell has concluded a year-long Gas to Liquids (GTL) fuel trial involving six diesel delivery trucks provided by a California-based bottled water distributor, Yosemite Waters.
The results show that using GTL Fuel in a conventional diesel engine reduced all regulated emissions compared to the California Air Resources Board baseline requirements. The chart below comes from a preliminary results presented at the Diesel Engine Emissions Reduction conference at the end of August. The final results may change a bit after further analysis, but this provides a good overview. The researchers measured the emissions using a New York City Bus drive (NYCB) cycle, which is a standard set of simulated driving conditions, and tested with and without a particulate filter.
|GTL Emissions Reductions in NYCB Cycle|
|GTL Fuel, no Filter||1||-0.81%||13.4%||69.0%||23.5%|
|GTL Fuel, with CCRT Filter||1||>99%||17.1%||>99%||97.0%|
GTL Fuel is a synthetic fuel made from natural gas, with virtually no sulfur and aromatics. There are a number of different, but related, technologies used to convert gas to liquids, based on the original Fischer-Tropsch process developed in Germany in the 1920s. Basically, F-T uses catalytic reactions to create complex hydrocarbons from simpler organic chemicals. This process is used to create similar liquids from a variety of feedstocks, including gas, biomass and coal.
Shell uses a low-temperature, cobalt catalyst-based version of the Fischer-Tropsch GTL process that produces an ultra-clean synthetic fraction of gasoil (GTL Fuel) that is virtually free of sulphur and aromatics.
Shell has been pushing hard on GTL, and has operated a medium scale 14,700 bbl/day GTL plant at Bintulu, Malaysia since 1993. The company plans to use its more than a decade’s worth of GTL experience from Bintulu to scale its nascent Qatar GTL operation to 140,000 barrels per day. For feedstock, however, that 140 kbpd output will require 1.6 billion cubic feet per day of natural gas.
GTL clearly has an emissions benefit in the tank. There are other issues to consider, however, one being fuel economy, another the energy and emissions lifecycle cost (well-to-wheel).
In the trial, the GTL-fueled vehicles experienced a decrease in fuel economy that still needs to be quantified and analyzed.
Shell contracted with PriceWaterhouseCooopers to perform a detailed energy and emissions lifecycle analysis (LCA) on its GTL process (Shell Middle Distillate Synthensis—SMDS) compared to a conventional refinery. The results of this are mixed.
The SMDS option has a greenhouse gas (GHG) impact that is at least comparable and possibly incrementally better than the crude oil option. SMDS also generates significantly less waste and particulates emissions compared to the conventional refinery.
But SMDS carries a significantly higher (41%) total primary energy requirement than crude oil refining. The total primary energy requirement can serve as a measure of resource depletion, since it describes the resources consumed by each option in terms of net heat value.
The greater total primary energy requirement of the SMDS option is primarily due to the less efficient GTL process (66% thermal efficiency, which includes upstream gas treatment), compared to refining (92%). In other words, SMDS consumes more resources to deliver the same output as the refinery.
Shell and its partners in this test are not alone in their interest in GTL. DaimlerChrysler recently concluded a GTL test using Sasol GTL fuel, both neat and in blends, with good emissions reductions results. Daimler Chrysler prefers to look to Biomass-to-Liquids (BTL) for the future.
ConocoPhillips also ran some LCA testing, with approximately comparable results in terms of GHG, pollutants and energy. (COP assessed its GTL process as having only a 25% higher energy requirement.)
So where does this leave us?
Pollutants. Using GTL in vehicles reduces pollutants—so from an air quality point of view, its use is a benefit.
Greenhouse Gas Emissions. Producing and using GTL is essentially a wash from a GHG perspective, compared with diesel.
Energy. The GTL production process is less efficient than oil refining, and so carries a significantly higher energy requirement.
The other factor to consider is the market. Given the situation with oil and rising prices for fuel, GTL could serve a short- to medium-term role as a lower-emissions substitute for conventional ultra low-sulfur diesel (ULSD). For oil and gas companies, GTL is very attractive as a solution for getting their natural gas to market—i.e. convert the gas to a liquid product, ship that and sell it.
Its viability, however, depends not only on continued copious supplies of natural gas, but on the construction of very extensive and expensive infrastructure. The Shell Qatar GTL plant is one of the largest—if not the largest—construction projects ever undertaken in Qatar.