THE UNITED STATES can now officially count two disastrous, expensive failures of carbon capture and storage — a technology key to realizing the dream of “clean coal.” The Kemper County Energy Facility was supposed to burn cheap, dirty lignite coal without emitting climate-altering carbon dioxide. Mississippi ended up with a very expensive natural gas plant that pumps carbon dioxide into the atmosphere like any other.

The idea was to turn coal into a synthetic gas, from which planet-warming carbon dioxide would be removed before it was burned. Backed by federal cash, Kemper was supposed to show that, under the right circumstances, power generators could burn coal without contributing to global warming. The plant was supposed to cost $2 billion. Instead, the technology never operated for long stretches, and the project went nearly $5 billion over budget. Last week, its owners finally relegated the plant to burning natural gas, the most common fuel used in electricity generation in the United States, without any diversion of greenhouse emissions.

This is the second federally funded clean-coal boondoggle. The first, known as FutureGen, was supposed to showcase innovative carbon capture and storage technology at a breakthrough Illinois plant. Despite a strong push from the local congressional delegation, the George W. Bush administration ended the hapless experiment, and the plant never operated, despite an Obama-era effort to revive it. More wasted money.

It is tempting to say this history proves that “clean coal” is a fantasy. To that, there is strong pushback from the coal industry and some environmentalists who believe the technology will be needed to tackle climate change. China has built many coal plants in recent years, facilities that will not be retired for decades; it will be disastrous if they continue belching carbon dioxide into the air for that whole time.

The technology’s advocates point to a federally backed Texas plant that came online in January, which experienced none of the usual budget problems and is supposed to cost-effectively provide captured carbon dioxide to local drillers, which use it to extract hard-to-obtain oil. Other carbon capture technologies in development may work better.

Or maybe not. Scarce dollars supporting clean coal could go to a variety of other on-the-cusp technologies — from innovative batteries that can help scale up renewables to revolutionary nuclear reactor designs. No one, and certainly no one in Congress, knows what a decarbonized energy sector will look like. Lawmakers should resist channeling cash to favored experiments and distribute research money and other incentives on merit.

Much of the government’s complex web of energy programs, tax breaks and other incentives could be eliminated if Congress fostered markets in which the best low-carbon technologies were profitable and would therefore rise into use naturally. The preferences of private companies, entrepreneurs and consumers, spurred by simple price signals, would determine how to wring carbon dioxide out of the economy. This freedom-maximizing, flexibility-preserving, waste-minimizing policy is a carbon tax. It is long past time the country had one.