Hydrated lime, or sorbent, is injected or sprayed into the center of the duct or pipe that carries the flue gas. This allows the sulfur to be collected in the form of a dry dust or ash . Conversion technologies turn coal into a gas or liquid that can be cleaned and used as fuel. One of the most advanced conversion technologies is called combined-cycle coal gasification. Combined-cycle gasification systems are among the cleanest of the clean coal technologies as the sulfur, nitrogen compounds and particulates are removed before the fuel is burned in the gas turbine.
Coal gasification can also take place underground.
In underground gasification, steam and oxygen are injected into a coal seam through wells drilled from the surface. The coal seam is ignited and partially burned. Heat generated by the combustion gasifies additional coal to produce fuel-grade gases. These are piped to the surface where they are cleaned and used to drive gas turbines. Underground gasification is particularly useful in extracting energy from coal seams that are un-mineable.