There are two basic approaches to converting coal to synthetic fuels: indirect liquefaction (ICL) (Fischer-Tropsch) and direct (Bergius). The indirect liquefaction pathway first gasifies the coal to produce a syngas, and then catalytically converts the syngas to linear hydrocarbons or chemicals. Direct liquefaction, by contrast, combines coal with hydrogen over a catalyst for the direct conversion to linear and ring-type hydrocarbons. (There is a hybrid process that combines the two.)

Axens DCL process. Source: Axens. Click to enlarge.

In the US, HRI started DCL development in the early 1970s. The US government, through the US Bureau of Mines and US DOE, invested some $3.6 billion from 1975-2000 to develop DCL technology. Two reactor technologies emerged from the work: a single-stage fluidized bed reactor (H-Coal Process), and a two-stage fluidized bed reactor technology (Catalytic Two-Stage Liquefaction, or CTSL process), the basis for Headwater’s current process. Axens’s H-Coal process is also a two-stage process (two catalytic reactors).

Generally, the ICL process produces high-cetane diesel and low-octane naptha; the DCL process produces on-spec diesel and high-octane naptha. (Headwaters points out the hybrid DCL/ICL process can produce premium gasoline and diesel fuel with minimal refining.)

Building on decades of experiences in DCL and a database on a wide range of coals, both companies have continued to increase liquid yields, improve energy efficiency, lower production costs and reduce the environmental footprint (CO₂ emissions and water consumption).

Headwaters says that compared to ICL, the DCL process can result in:

• 30% lower capital cost for a comparably sized plant
• Up to 50% more liquid product per ton of coal
• Up to 50% less plant generated CO₂ (almost all of which can be captured for enhanced oil recovery)
• Half the water consumption
• 25% higher thermal efficiency
• Balanced power—no need to generate excess power

The DCL process operates on low-ash bituminous coal or sub-bituminous coal. A typical bituminous coal can yield 3 barrels of liquid fuel products (diesel and gasoline) per tonne of coal compared to 2 to 2.4 barrels per tonne of coal for indirect coal liquefaction. The thermal efficiency (energy in products divided by energy in coal x 100%) is approximately 60% compared to 48% for indirect coal liquefaction, according to Headwaters.

In the Headwaters DCL Process, dry pulverized coal is mixed with recycled process liquid and hydrocracked at 435 to 460 ºC in the presence of high-pressure hydrogen (about 170 bars). Most of the coal structure—a solid organic material made up of large, complex molecules containing carbon, oxygen, hydrogen, sulfur and nitrogen—is broken down in the first-stage reactor. Liquefaction is completed in the second-stage reactor.

The proprietary GelCat Catalyst is dispersed in the slurry for both stages. The intermediate coal liquids from the mild hydrogenation step can be further upgraded using conventional refining techniques to produce gasoline, jet and diesel fuels that will meet or exceed existing and future fuel specifications.

(Even the lower CTL plant emissions for DCL—434 kg/barrel, or 10.3 kg/gallon—are substantially higher than conventional diesel refining. According to the California Air Resources Board, crude refining for ultra low sulfur diesel results in CO₂e emissions of 11.41 g/MJ. The comparable DCL plant emissions per MJ diesel, based on the Headwaters data, would be around 75 g/MJ (10.3 kg/gallon, with 1 gallon diesel approximately equal to 137 MJ (LHV)). If 80% of that could be sequestered, that would bring the plant CO₂ down to around 15 g/MJ—still higher than conventional crude refining. Total well-to-tank emissions for either petroleum-based diesel or synthetic would also need to consider recovery of crude or coal; transport; refining or production; and transport of the resulting fuel. In the ULSD pathway for ARB, that results in a total of 19.81 g/MJ.)

Headwaters is licensing its technology internationally, with the first commercial plant having started up 30 December in China—a 20,000 barrel per day plant owned by the Shenhua Group, the largest coal mining company in the world. The installed capital cost of this plant is $1.5 billion (2007 dollars). The estimated break even cost is $35 – 40 per barrel selling price.

Alliance DCL will market the technologies and anticipates offering project-specific services, from feedstock characterization, pilot plant evaluation, feasibility studies and engineering design through plant start-up and ongoing technical support. Axens will also provide the coal liquids upgrading technologies necessary to achieve finished fuel specifications.

Both companies provided technology packages and basic engineering contributing to the successful construction and start-up of the first commercial direct coal liquefaction plant in China in December 2008. Several new DCL projects are currently in development by the alliance.

Resources

• Recent Advances in Direct Coal Liquefaction Technology (Headwaters, 2009)

• Optimizing Refinery Operations & Alternative Fuels Production (Axens, 2009)

• Summary Report of the DOE Direct Liquefaction Process Development Campaign of the Late Twentieth Century: Topical Report (DOE, 2001)