Scientists have dramatically increased the efficiency of producing clean hydrogen fuel from plant waste in a breakthrough that could one day lead to petrol stations being replaced by a network of roadside “bioreactors” for refuelling cars.
A study funded by Shell Oil has shown that it is possible to convert all 100 per cent of the sugar stored in corn stover – the stalks, cobs and husks leftover in a harvested maize field – into hydrogen gas with no overall increase in carbon dioxide emissions to the atmosphere.
The researchers perfected the process by mixing the raw biomass with a watery solution containing a cocktail of ten enzymes that turned the plant sugars xylose and glucose into hydrogen and carbon dioxide, said Professor Percival Zhang of Virginia Tech in Blacksburg, Virginia.
Previously it has only been possible to convert between 30 per cent and 60 per cent of the plant’s sugars into hydrogen using either fermenting microbes or industrial catalysts. However, the latest technique converts 100 per cent of the plant sugars into hydrogen, Professor Zhang said.
Producing pure hydrogen gas from crop waste and biomass is seen as one of the most important goals of the green economy because of the need to produce clean alternatives to petrol. However, existing methods are inefficient, costly and are dogged by the problem of how to distribute the hydrogen once it is made.
“All the products produced by the process are gases so they can be separated and collected easily from the biomass substrate. Over its lifecycle, the process is carbon neutral and we have achieved a 17-fold increase in the rate of the reaction which makes it economically viable,” Professor Zhang said. “This means we have demonstrated the most important step toward a hydrogen economy – producing distributed and affordable green hydrogen from local biomass resources,” he said.
One of the critical developments in the process is being able to directly use “dirty” biomass as the fuel rather than relying on highly processed sugars as the source of hydrogen. In addition to being more efficient, this means it should also be possible to build large bioreactors the size of petrol stations near to sources of biomass, so leading to a network of green re-fuelling stations distributed around the country, Professor Zhang explained.
“The next problem is to work on how to scale it up. But if we receive further funding I think in three to five years we should be able to build a bioreactor that is something like a gas station which can produce 200 kilos of hydrogen fuel a day. This would be enough to re-fuel about 40 or 50 cars,” he told The Independent.
The key step in the study was to identify the precise combination of enzymes that would work together on the plant waste to convert all of its xylose and glucose – which together account for 90 per cent of the sugars in plant waste – into hydrogen and carbon dioxide, which can be collected separately.
These 10 enzymes were initially made in microbial fermenters using genetically engineered bacteria. The separated enzymes were then added to the solution of plant waste where they continued to work for several weeks. However, the aim eventually is for these enzymes to continue working for months or years without being replaced, Professor Zhang said.