Gasification

Gasification is a thermal process that converts any carbon-based material into energy without burning it.

The carbon-containing feedstock is reacted with either air or oxygen in a gasifier, which breaks down the mixture into “synthesis gas” or “syngas”. Emissions from gasification can be easily captured because of the high pressure and, often, concentrations.

Coal is the primary feedstock for gasification and will continue to be the dominant feedstock for the foreseeable future. The current growth of coal as a gasification feedstock is largely as a result of new Chinese coal-to-chemicals plants. Modern gasification technologies incorporate significant improvements compared to earlier versions – including increased flexibility, vastly increased scale and new applications are driving gasification technologies to gain greater prominence.

Gasification may also be one of the best ways to produce clean-burning hydrogen for tomorrow’s cars and power-generating fuel cells. Hydrogen and other coal gases can be used to fuel power-generating turbines, or as the chemical building blocks for a wide range of commercial products, including diesel and other transport fuels.

Applications

Chemical production is the most common application of gasification worldwide. Synthetic fuels – both liquid and gaseous – are also growing in importance. The second most common application is liquid fuels.

IN SUMMARY:
  • Gasification
  • Integrated gasification combined cycle (IGCC)
  • Underground coal gasification

Photo: Duke Energy Edwardsport IGCC

Integrated gasification combined cycle (IGCC)

Gasification can also be used for power generation. IGCC plants use a gasifier to convert coal (or other carbon-based materials) to syngas, which drives a combined cycle turbine. Coal is combined with oxygen and steam in the gasifier to produce the syngas. The gas is then cleaned to remove impurities, such as sulphur, and the syngas is used in a gas turbine to produce electricity. Waste heat from the gas turbine is recovered to create steam which drives a steam turbine, producing more electricity – hence a combined cycle system.

By adding a ‘shift’ reaction, additional hydrogen can be produced and the CO can be converted to CO2, which can then be captured and stored. It is hoped to reach efficiencies of over 40%, and possibly as high as 45% with IGCC.

Underground coal gasification

Gasification-Howitworks

Gasification typically takes place in an above-ground gasification plant – however, the reaction can also take place below ground in coal seams. 
Underground coal gasification (UCG) is a method of converting unworked coal - coal still in the ground - into a combustible gas. UCG technology allows countries that are endowed with coal to fully utilise their resource from otherwise unrecoverable coal deposits.

UCG uses a similar process to surface gasification. The main difference between both gasification processes is that in UCG the cavity itself becomes the reactor so that the gasification of coal takes place underground instead of at the surface.

The basic UCG process involves drilling two wells into the coal, one for injection of the oxidants (water/air or water/oxygen mixtures) and another well some distance away to bring the product gas to the surface.

The coal at the base of the first well is then heated to temperatures that would normally cause the coal to burn. However, through careful regulation of the oxidant flow, the coal does not burn but rather separates into the syngas. The syngas is then drawn out of the second well.

The advantages in the use of this technology are the low plant costs (as no surface gasifiers are required) and the absence of coal transport costs. UCG also presents the opportunity to reduce emissions as there are fewer surface emissions. UCG technology could also have synergies with CCS as the CO2 could be stored in the coal cavity after gasification. However, a number of issues remain to be resolved before wider deployment can be achieved.