Carbon Capture

Natural gas as found usually contains some percentage of carbon dioxide –the Gorgon gas field contains around 14 percent carbon dioxide while the Jansz gas field has less than 1 percent. In order to liquefy natural gas to enable its efficient transport it is necessary to cool the natural gas to around 162 oC below zero. However at this temperature any retained carbon dioxide will freeze into a solid potentially plugging or damaging the liquefaction equipment. For this reason it is necessary to remove any carbon dioxide before the natural gas is cooled. The technologiesfor removing the reservoir carbon dioxide is well understood and has been used in gas processing plants worldwide for many decades.

In contrast, separation of carbon dioxide from flue gas is a less developed technology. Four techniques are available: absorption on amines, solid physical absorption, low temperature distillation and selective membranes. Of these only amine absorption has been extensively used on flue gases. Finding the optimum separation techniques for the emissions from each plant is a subject of research. In addition, coal gasification as a pre-combustion technology, and oxyfiring technologies are also available for commercial decision making.

A further option is pre-combustion capture where CO2 is removed in a chemical process yielding hydrogen, prior to combustion. Hydrogen rich gas provides a lower emission fuel.

Both proposals incorporate other mitigation methods to reduce greenhouse gas emissions, including increasing the thermal efficiency of generation and refrigeration plants and in the case of Alcoa’s Wagerup and Pinjarra refineries industrial carbonation of bauxite residue (commonly called red mud) which is currently a by product of no value. The carbonation process locks away the CO2 through chemical bonding and improves the pH and handling characteristics of the residue to the point that it can be used for soil amendment, road base or the manufacture of building products.