Underground coal gasification
Australian company Linc Energy Ltd has announced the first production of diesel from its demonstration coal to liquid (CTL) facility at Chinchilla, Queensland, using underground coal gasification (UCG). The firm claims this is the first time the technology has been combined with CTL, reducing the cost and lifecycle carbon emissions of the fuel.
As the price of oil and gas has increased, interest in CTL fuels, whose plant capital costs were previously considered too expensive, has grown. Such fuels generally prove to be of high quality, offering reduced sulphur content and low raw material costs.
Linc employs indirect liquefaction, in which coal is reacted with steam to produce a
synthetic gas (syngas) consisting of hydrogen and carbon monoxide. The syngas
composition is adjusted using the water shift reaction (CO+H2O+CO2+H2), then cleaned of sulphur compounds before being catalytically converted to liquid fuel. Fischer-Tropsch synthesis produces diesel or (at high temperature and pressure) aviation fuel and petrol. Other catalytic routes are used to generate methanol or dimethyl ether.
Producing syngas underground within coal seams offers reduced energy requirements and eliminates the methane emissions found in conventional mining operations. Total lifecycle carbon emissions of UGC diesel is expected to be 40% less than CTL processes.
The UCG process used at Chinchilla was developed in Russia in the 1960s by the Skochinsky Institute of Mining, which developed into manufacturer Yerostigaz. In 2007, Linc bought Yerostigaz, which operates the world’s only commercial plant for UCG power generation in Uzbekistan.
Underground coal gasification involves two or more wells, typically 30m apart and over 100m deep, joined inside the coal seam. Coal combustion is initiated at the foot of the first well and maintained by compressed air, which is introduced to create an injection well. The conditions are hot enough to produce syngas by reacting with ground water.
The resulting syngas moves under pressure to the second well where it is extracted and cleaned (in practice, a grid of wells is used which enables the location and direction of combustion to be closely controlled to within one metre). Gasification levels are monitored to keep the pressure within the cavity below the groundwater hydrostatic pressure, effectively sealing the ‘reactor’.
The UCG system is said to be able to recover ‘stranded’ coal seams and exploit deposits that are uneconomic to mine. Combustion residues remain underground, eliminating disposal costs, and the reduced plant and energy requirements of UCG lower processing costs.
Linc’s demonstration facility, with a five-to-10 barrel-a-day capacity, will run for three to four years to optimise efficiency and permit joint venture partners to conduct trials.
Construction of a 20,000 barrels/day production plant is set to begin in South Australia in mid-2009, with completion scheduled for 2012. It is expected to produce synthetic diesel at AUS$28/barrel, including capital costs (conventional CTL systems generally cost $55/barrel). The underground facility is scheduled to run for at least 20 years.
Further information: Linc Energy
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