Coal is rapidly becoming the world's most popular fuel. Today in the United States, more than half of the electricity produced comes from coal-fired power plants. The demand for coal is expected to triple by the middle of the next century, making it more widely used than petroleum or natural gas.

The unpleasant aspect of this trend is that coal is a relatively dirty fuel. When burned, it releases particulates and pollutants such as carbon monoxide, nitrogen and sulfur oxides into the atmosphere. If the use of coal is to expand continually, something must be done to reduce the hazard its use presents to the environment.

Over the past two decades, therefore, there has been an increasing amount of research on clean coal technologies, methods by which the combustion of coal releases fewer pollutants to the atmosphere. As early as 1970, the United States Congress...