Oryx GTL is the first low-temperature Fischer-Tropsch GTL plant outside South Africa dedicated to the production of GTL diesel. The partners intend to increase the capacity of the plant to more than 100,000 bpd and are exploring the construction of a further 130,000 bpd extension of the existing plant.

The plant uses Sasol’s proprietary Slurry Phase Distillate process (SPD) which combines three commercial technologies: autothermal reforming for the production of synthesis gas from natural gas; slurry-phase Fischer-Tropsch synthesis for the conversion of the syngas to a waxy syncrude, and isocracking technology to upgrade the syncrude into liquid fuels. London-based Sasol Chevron will market the GTL diesel worldwide.

The energy and carbon efficiencies of GTL remains a technical challenge. Industry estimates put the overall energy efficiency for the GTL process at about 60% (i.e., for 100 Btu of natural gas in, you get about 60 Btu of hydrocarbon product out). Carbon efficiencies are about 77% (i.e., 23% of the carbon from the feed is lost in the form of CO₂).

A 2004 GTL Life Cycle Assessment Synthesis Report from Five Winds International, commissioned by Sasol Chevron, ConocoPhillips and Shell, concluded after reviewing three different studies that:

Production and use of GTL fuel can contribute less greenhouse gas to the atmosphere than production and use of conventional diesel fuel. The study commissioned by ConocoPhillips indicated the reduction in greenhouse gas emissions is significant if the GTL fuel is produced from associated gas that is otherwise flared in amounts of 10% or greater.

More conservatively, and in cases where the feedstock is from other sources, the greenhouse gas contribution of GTL fuels is comparable to conventional diesel technology. In the expanded GTL technology system, available natural gas is used for space heating and electricity generation, whereas conventional refining technology uses more carbon-intensive light fuel oil and residual fuels respectively, to meet these needs.

While the GHG emissions from production and upstream processes of the GTL system are higher compared to the refinery-based system, the advantages in the use phase, at a minimum, compensate for the disadvantages in those phases.

Total GHG emissions of the GTL system are between 12% less and 11% more than the refinery system, based on varying assumptions and data. The majority of the scenarios suggest an at least neutral if not positive GHG performance (i.e. reduced emissions) for the GTL system...it is safe to conclude that the two technologies are at least equivalent in their GHG contributions.

Disadvantages in the fuel production stages of the technology are offset by the fact that GTL fuels offer slight advantages in the fuel use (combustion) stage, which contribute approximately 75% or more of the total calculated GHG impact of both options.

Refinery technology has a lower total primary energy requirement, requiring between 17% to 29% less energy to meet the same functions as the GTL system.

The discrepancy between GHG and total primary energy results arises from the different resources used. While GTL is based on the hydrogen-rich feedstock natural gas, the conventional system is based on the more carbon-intensive crude oil. The larger energy contribution for GTL is due to less efficient processing of feedstock into products (thermal efficiency of over 60% compared over 90% for the refinery).

Improvements with respect to thermal efficiency in the GTL system can be expected over time since the technology has not gone through the same degree of technological improvement as the conventional system.

Sasol CEO Pat Davies said prior to the opening that Sasol and its partners were aiming to produce 450,000 barrels of GTL fuel worldwide via different projects by 2014.

The Challenge. The Sasol Chevron GTL Challenge Team arrived on schedule in Doha, Qatar yesterday after completing its 11,000-km (6,837-mile) drive from South Africa. (Earlier post.) One of the vehicles—a Toyota Hilux Raider named “African Renaissance”used only neat GTL for the entire trip.

Comparing the oils prior to change. Oil from the GTL vehicle is on the left.

According to the team, the GTL fuel burned significantly cleaner than conventional diesel and provided superior performance. The team estimated that the GTL-fueled vehicle emitted 4,500 cubic meters less of sulfur dioxide and 102,000 cubic meters less of carbon dioxide than its diesel-fueled companions.

The team had a graphic display of the cleaner-burning properties of GTL compared to conventional diesel. At the halfway stage (6,500 kilometers) the oil from the vehicles using traditional diesel had deteriorated and needed to be changed, while the oil the vehicle running on GTL diesel, was barely affected.

We are using a standard commercial motor oil and the implications for overall vehicle economy and reduced running costs are huge. The essential performance difference between GTL and the conventional diesel is related to the cleaner burning of the GTL diesel which leads to less soot formation in the combustion chamber. GTL diesel performs better due to its uniquely clean composition.

—Ian Myburgh

Resources:

• Gas to Liquids Life Cycle Assessment Synthesis Report

• A Life Cycle Assessment Comparing Select Gas-to-Liquid Fuels with Conventional Fuels in the Transportation Sector