Despite the growing adoption of solar power and other renewables, fossil fuels still rule our energy world. That makes steps to make them cleaner all the more vital. Chemists report today in Science the discovery of an additive that sharply cuts carbon emissions from an industrial process that can convert coal, natural gas, or agricultural biomass to liquid fuels such as diesel or gasoline.
“It’s remarkable,” says Mark Saeys, a chemist at Ghent University who was not involved in the study.
The ability to convert coal and other carbon-containing compounds into liquid fuels was invented a century ago by a pair of German chemists, Franz Fischer and Hans Tropsch, and is now known as the Fischer-Tropsch process. The process requires a feedstock called syngas, a mixture of two gases, carbon monoxide (CO) and hydrogen (H2), that’s created by cooking the coal under high temperatures and pressures. Then, catalysts cause this syngas to react to form hydrocarbons that can be further altered to make fuels and other valuable chemicals.
The process was used by Germany in the 1930s to fuel the Nazi war machine and by South Africa during apartheid to produce fuels from the nation’s abundant coal reserves. Although relatively expensive, the approach is still used today to satisfy strategic fuel security needs or in places with abundant feedstocks such as coal or natural gas.
The chemistry is highly polluting, however. The energy needed to create the syngas is usually supplied by burning fossil fuels, and the Fischer-Tropsch process generates secondary reactions that convert much of the carbon in syngas to carbon dioxide (CO2). In one typical industrial setup, one-third of all the carbon contained in syngas ends up as CO2 vented into the atmosphere.
The standard Fischer-Tropsch process uses iron-based catalysts to clip carbon atoms from CO molecules and link them to H2 molecules. Additional carbons and hydrogens can then link up to form a range of different olefins, or short hydrocarbon chains. The trouble is that the leftover oxygen atoms that were separated from the CO molecules can also pair up with hydrogen to form water. And aided by the same catalyst, that water can undergo a separate reaction with CO to form a mixture of CO2 and H2. The H2 can be captured and reused, but the CO2 winds up as pollution.
For years, chemists have tried to shield their iron catalysts with water-repelling coatings to prevent the unwanted CO2-generation. But the improvements have been modest, notes Ding Ma, a chemist at Peking University who led the new effort. In hopes of getting a better result, Ma and his colleagues added trace amounts of compounds called halomethanes to their syngas mixture. When they added about 20 parts per million of a halomethane called methyl bromide, the bromine atoms glommed onto the surface of the iron catalyst particles, blocking water molecules from binding and breaking apart, the first step in the CO2-generating reaction, Ma says.
As a result, the amount of carbon in the syngas that ends up as CO2 dropped from roughly one-third to less than 1%, Ma and his colleagues report. The additive not only makes the process cleaner, but more efficient. By ensuring that nearly all of the syngas carbon is converted to hydrocarbons, “it reduces the cost of production of liquid fuels,” Saeys says.
Ma says his team’s additive is already being commercialized by Synfuels China, a major fuel producer. For large plants that emit millions of tons of CO2 annually, the new process could reduce emissions by hundreds of thousands of tons of CO2 per year, he says. “The economic and environmental benefits are both substantial.”