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 ¹ 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.