Apr-2008

Conversion of syngas to diesel

An overview of Fischer-Tropsch technologies for the production of diesel from syngas using a variety of feedstocks

Stèphane Fedou, Eric Caprani, Damien Douziech and Sebastien Boucher, Axens

Article Summary

Fischer-Tropsch (FT)-based XtL options (gas, coal, petroleum coke or biomass to liquids) represent viable routes for the production of non-crude oil-based ultra-high-performance motor fuels and speciality products. Improvements in catalyst and reactor technology, coupled with optimised integration and economies of scale, have allowed gas-to-liquids (GtL) complexes to be competitive with LNG and pipeline projects. Most major oil and gas companies are closely following XtL developments. Currently, several large GtL industrial projects have been realised (Oryx in Qatar) or are under construction (Pearl in Qatar, Escravos in Nigeria).

Eni and IFP (French Institute of petroleum) have developed proprietary FT and upgrading technologies in a close collaboration between the two groups. These technologies are based on proprietary catalysts and reactor designs resulting from scale-up criteria developed over 12 years of common research and development. Large-scale hydrodynamic facilities (slurry bubble columns), dedicated bench-scale pilot units and a large-scale FT pilot plant have been developed and operated to minimise reactor and ancillary unit scale-up risks.

A large-scale FT pilot plant has been built and operated since 2001. The plant, located within the Eni refinery in Sannazzaro, Italy, is fully integrated with the refinery utilities and network. The 20 bpd FT synthesis section has been used to optimise slurry handling (loading, make-up and withdrawal), reactor configuration and product separation units. Large-size scale-up has been accomplished through the implementation of several specifically designed scale-up tools to assess the catalyst’s stability under industrial conditions, and study fluid dynamics and liquid-solid separation.

Fuel diversification: from GtL to XtL
The estimated amount of natural gas reserves has continuously increased over the past ten years, moving from the 1997 estimation of 152.2 Tcm to 181.5 Tcm in 2007, with a reserve/production ratio of around 63 years, higher than oil of 38 years.¹ It is thus quite logical to envisage using natural gas (or coal with its R/P ratio above 140 years) as a source of liquid transportation fuels, through natural gas conversion into liquid hydrocarbons, the so-called GtL, which is characterised by an intermediate step of natural gas conversion for producing synthesis gas (hydrogen and carbon monoxide). This production can be divided into two main value chains:
• Production of methanol and methanol-derived products (including acetic acid, DME and so on)
• FT synthesis for production of high-quality middle distillates (ie, kerosene and diesel fuel), pure paraffinic naphtha, lube bases, waxes and, in some cases, olefins and speciality chemicals.

While the final market for LNG, pipeline and wire transportation is the traditional natural gas one, GtL technologies open up new markets (automotive fuel, chemicals) to natural gas producers (Figure 1).

Increasing global demand for ever-cleaner fuels, particularly middle distillates, such as in Europe, should favour the FT GtL route, which answers market needs.²

A relatively recent requirement, but fundamental for decades to come, is that automotive fuels must not only be cleaner, but the impact of their well-to-wheel lifecycle on the greenhouse effect (ie, overall CO2 emissions from production up to consumption of the liquid fuel, including biomass CO2 absorption [in the case of biomass]) must be as low as possible.

This general requirement will be translated into different regional targets. As an example, the European Council stated on 8–9 March 2007: “a 10% binding minimum target to be achieved by all Member States for the share of biofuels in overall EU transport petrol and diesel consumption by 2020.” As it is commonly agreed that the first generation of biofuels (using vegetable oils and sugars) will not be sufficient, the so-called second-generation biofuels will have to come into force. And the most promising technology to achieve this target is BtL-FT (biomass-to-liquid through Fischer-Tropsch: production of FT diesel from a syngas generated by the gasification of biomass), which allows for a decrease in the equivalent CO2 emissions per km down to 90% compared to a conventional diesel.3 Biomass may also be mixed with petroleum residues or coke in order to take advantage of refinery infrastructures and operating experience.

All these XtL options (ie, gas, coal, petroleum coke or residue, biomass-to-liquids, or a mixture of some of them) will allow diversifying fuel sources, producing cleaner fuels and limiting overall GHG emissions. They all require a reliable technology package to transform clean syngas from whatever source into ultra-clean liquid fuels, and this is the scope of the Gasel technology suite (Figure 2).

Fischer-Tropsch technologies: industrial applications
FT technologies can be characterised/differentiated by four parameters:
•   FT catalyst: two main types:
    ν Iron-based catalysts
    ν Cobalt-based catalysts.

•   FT reactor: three main types of reactor:
    ν Fixed bed (the catalyst is located inside tubes)
    ν Fluidised bed (the catalyst is main-tained in suspension by the syngas)
    ν Slurry bubble column (three-phase reactor with synthesis gas, waxes, liquid products and solid catalyst).

•   Operating temperature:
    ν HT-FT: High-temperature Fischer-Tropsch (around 350°C and above)
    ν LT-FT: Low-temperature Fischer-Tropsch (220–240°C).