Coal goes green

To bio-oil and biochar, in the list of materials that can be produced by heating plant biomass in the absence of air, can now also be added bio-coal. This novel material, developed by a team of Chinese chemists, is so termed because it has many of the same properties as normal coal and thus could potentially be used as a fuel in much the same way, but without adding further to global warming.

Normal coal was created as result of ancient plants being exposed to great temperatures and pressures under the ground for millions of years, slowly transforming into a carbon-based, energy-rich material. Mankind’s burning of this material for heat and energy over much shorter timescales, resulting in the release of hundreds of billions of tonnes of carbon dioxide into the atmosphere, has been a major contributor to global warming.

Despite many countries actively reducing their reliance on coal, with more and more power stations in countries such as the UK switching to burning biomass, it is still a major fuel, especially in countries like China, which consumed around 2700 million tonnes of coal in 2017. By potentially providing a direct replacement, bio-coal could reduce the reliance on normal coal and thus help to reduce carbon dioxide emissions over the long term.

Bio-coal actually came out of the challenges involved in upgrading bio-oil. Heating plant biomass – often some form of agricultural residue – in the absence of air, a process called pyrolysis, produces a liquid known as bio-oil and a solid residue known as biochar. The biochar can be used as a soil additive or a solid fuel in its own right, while the bio-oil can potentially be used as a liquid fuel or a feedstock for industrial chemicals, but only after it has been upgraded to remove excess oxygen and moisture.

Scientists are currently developing various ways to cost-effectively upgrade bio-oil, usually based on reacting it with compounds such as hydrogen over a catalyst. Perhaps the simplest upgrade method, however, is distillation, in which the various useful hydrocarbons in the bio-oil are driven off at different temperatures. Simple it may be, but distillation doesn’t actually work very well. This is because it tends to cause many of the compounds in the bio-oil to react together to form a big clump of carbon-rich polymers, reducing the yield of useful of hydrocarbons.

But Chinese chemists led by Han-Qing Yu at the University of Science and Technology of China in Hefei wondered whether the clump of polymers might also prove useful. To find out, they conducted pyrolysis on rice husk at 500°C to produce bio-oil and biochar. Then they performed atmospheric distillation on the bio-oil at temperatures up to 240°C, with the aim of maximizing production of the polymer rather than useful hydrocarbons, although such hydrocarbons were still produced.

As Yu and his team report in a paper in Science Advances, the resulting polymer material looked very much like coal and also had a very similar mass energy density, at 25MJ/kg, compared with between 19MJ/kg and 28MJ/kg for a selection of different coals. As an added advantage, the bio-coal also had very low levels of heavy metals such as cadmium, lead and zinc, meaning it wouldn’t produce much heavy-metal pollution when burnt.

Yu and his team then tried the same distillation process with bio-oil produced from various other types of agricultural residue, comprising sawdust, wheat straw, bagasse and soybean straw. In all cases, it produced polymers with very similar properties to coal, including mass energy densities that varied between 26MJ/kg and 28MJ/kg. Hence, Yu and his colleagues decided to call these polymers bio-coal.

Because this bio-coal is derived from plant biomass, it should be carbon neutral, especially if derived from agricultural residues that would otherwise be burnt. Furthermore, the process would produce biochar and useful hydrocarbons as well. As a consequence, a life cycle assessment by the chemists revealed that the process should produce more energy than it uses, reduce carbon dioxide emissions and be economically viable, as the bio-coal, hydrocarbons and biochar would all generate revenue.

Yu and his colleagues calculated that all the agricultural residues produced by China in a year could produce 402 million tons of bio-coal, sufficient to replace 14% of the country’s current coal consumption.

The views represented here are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd. or of the SCI.