| http://www.w3.org/ns/prov#value | - it is also possible to remove some of the carbon dioxide, one of the main greenhouse gases cited as a cause for global warming, but that involves additional steps.U.S. companies working in the field include Oklahoma-based Syntroleum (Charts), Rentech (Charts) in Colorado and Utah-based Headwaters (Charts).According to the Department of Energy, oil needs to remain near $50 a barrel for liquid coal t
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