Hydrogen from biomass for power

Biomass has become an important raw material for many process sectors, especially power generation. Many types of biological materials, from agricultural waste products to chicken droppings, are used to fire power stations in various parts of the world.

According to Eckhard Dinjus of the Institute for Technical Chemistry in Karlsruhe, Germany, biomass could also be a source of hydrogen for the next generation of fuel cell-based power plants.

Dinjus and his colleagues have worked out two methods of generating hydrogen from waste wood — one for dry material, and one for wet, where the moisture content is above 70%.

For dry wood, Dinjus uses a two-step process. First, the wood undergoes a fast pyrolysis at around 500°C, in small, compact plants to produce a stable slurry of oil and char which can be stored or transported.

The researchers have developed a twin-screw reactor, a type of mechanised fluidised reactor that does not require a fluidising gas, which allows very fast and effective pyrolysis. Dinjus believes that several of these plants — maybe several dozen — located at sources of biomass, would produce the feed for a central gasifier.

The slurry is fed into a central entrained-flow gasifier, where it is treated with heat and oxygen to produce synthesis gas (syngas), a mixture of carbon monoxide and hydrogen.

For wet biomass, conversion can be done in a single step, using a supercritical water gasification process. Slurried biomass is treated with supercritical steam at 300bar and 600°C, which converts it directly into hydrogen and carbon dioxide; a carbon dioxide scrubber can be incorporated into this system, so that it produces fairly pure, high-pressure hydrogen, Dinjus says.

The team has built a 150kg/hr pilot plant to demonstrate the feasibility of the system, he says, and results are so far extremely promising.