Ciris Energy today said it raised a series B round to build its first commercial plant for converting underground coal to natural gas.

Khosla Ventures led the funding, which also brought in existing investors Braemer Ventures, Rho Ventures, and GE Energy Financial Services. An SEC document from last week showed that Ciris raised $23.9 million, out of a planned $63.9 million.

Ciris has developed a process to biologically convert underground coal into methane, the main ingredient of natural gas. The company has said that its technology is less expensive than conventional natural gas production and other gasification processes.

As an energy source, natural gas has become one of the favorite options in the U.S. and elsewhere in the world. The U.S. has large reserves, including relatively recent finds in shale rock. Natural gas also emits about half of the carbon dioxide as burning coal and has fewer other air pollutants.

The process of converting underground coal to natural gas--called coal bed methane extraction--is being pursued by other companies, including Luca Technologies. Luca plans to do it by injecting a nutrient-rich solution into coal beds to stimulate growth of bacteria that digest coal to make methane. The methane is then captured and pumped into natural gas pipelines.

In addition to producing a cleaner-burning fuel, methane can be extracted from coal seams that are no longer productive enough for mining.

In a statement, the Centennial, Colo.-based company said the series B round will allow it to deploy its technology in the field.