If the Secretary determines that the utilization of a clean coal technology is not practicable for a proposed project and that a United States conventional coal technology would constitute a substantial improvement in efficiency, costs, and environmental performance relative to the technology being used in a developing country or country making the transition from nonmarket to market economies, with significant indigenous coal resources, such technology shall, for purposes of sections 13361 and 13362 of this title, be considered a clean coal technology. In the case of combustion technologies, only the retrofit, repowering, or replacement of a conventional technology shall constitute a substantial improvement for purposes of this section. In carrying out this section, the Secretary shall give highest priority to promoting the most environmentally sound and energy efficient technologies.1