Sept/Oct 2007

The coal gasification process

By H. E. Robinson

The Canadian Clean Power Coalition, a group of industry suppliers and consumers interested in finding ways to reduce the negative impacts of coal processing, suggested the use of coal gasification techniques. This process has the potential to mitigate environmental effects such as the emission of greenhouse gases, and generate by-products that are useful in other areas of the carbon industry.

Gasification breaks down coal into hydrogen (H2), a synthetic gas called ‘syngas,’ and carbon dioxide (CO2). While the H2 can be used for bitumen upgrading, a high-purity CO2 is released during H2 production that can be captured for enhanced oil recovery or storage. The syngas can be used as a fuel to replace natural gas or go through further refinement to produce more H2 and CO2. All three products in the gasification process have commercial applications.

The initial step involves combining dried and pulverized coal, oxygen, and high-pressure water or steam in a gasifier. The coal is exposed to the steam under high temperatures, while the pressure and oxygen levels are carefully controlled. This produces a mixture of H2, and a combination of CO2 and CO (carbon monoxide) which makes up syngas. The syngas is then cooled using water. The waste water is either treated at a waste management plant, or recycled back into the gasification process. Any particles and trace metals are removed from the syngas before it is ready to be marketed as a substitute for natural gas, or it can be refined again to convert H2 to CO2.

Hydrogen can be used to upgrade a heavy crude oil (bitumen) into petroleum products such as gasoline. Carbon-rich bitumen is extracted from oil sands deposits as a thick and viscous semi-solid fluid. Treating the crude oil with H2 helps remove sulphur and nitrogen, and then upgrades it into a synthetic crude. This, in turn, can be converted into gasoline, jet fuel, and other petroleum products. Natural gas is typically used to produce H2 for this process; however, recent fluctuations in the price and the relatively limited supply of natural gas have made the use of H2 from coal gasification more economic.

The CO2 produced by coal gasification is concentrated, has a high purity, and can be captured so that it is not released into the atmosphere or transported by pipeline for further use. Carbon dioxide is used in enhanced oil recovery (EOR) operations where it is injected into declining oil fields in a process called ‘miscible displacement.’ The gas dissolves the oil, which reduces oil viscosity and maintains reservoir pressure. This improves the flow of oil from the reservoir and results in increased production. One of the criticisms to this technique is that pumping CO2 into the ground often requires the use of more energy, while EOR also frees more fossil fuels for consumption, which only leads to more CO2 being emitted. However, the economic advantages of using CO2 in EOR can compensate for the expense of injecting CO2 into the ground.

Both H2 and CO2 are marketable gases and the coal gasification process produces them in relatively pure forms. While H2 can be used for many applications outside the mining industry, CO2 needs to be captured and stored (see “The search for low-cost CO2 storage” article, page 40).

The final step in coal gasification is converting the remaining ash from the original coal feed into a stable and inert solid that can be used for backfilling, or as asphalt for roads.

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