Steam Rankine Cycle

This is the classic combustion route where hot gas is used to produce steam which is used to drive a steam turbine which drives a generator to produce power. Process steam can be extracted from the turbine if required and condensed for process heat.

Renewable Energy Fuels that can be used include wood, wood waste, agricultural waste such as corn cobs and stalks, nut shell, chicken litter, etc. Fuels such as shredded tyres and other industrial wastes with calorific value can be burnt in more sophisticated combustors.

Gasification is the partial combustion of fuel to convert a solid or liquid fuel into a gaseous fuel containing hydrogen, CO and CO2. This fuel can be used to power engines or gas turbines to drive generators or can be used to produce liquid fuels via Fischer Tropsch or other similar catalystic processes. Companies such as Coskata (www.coskata.com) and Ineos (www.ineos.com) have produced microbes that are able to convert this gas directly to ethanol. 

Gasification and use of the syngas is typically more energy efficient than combustion.

Fuels that can be used depend on the type of gasification system under consideration. Downdraft gasification is particularly sensitive to fuel shape while updraft has the problem of unburnt tars that need to be removed if the gas is to be used in an engine. Appropriate fuels include wood, wood waste, agricultural waste such as corn cobs and stalks, nut shell, sorted municipal waste, sewerage sludge, etc. Fuels with high concentrations of alkali are more challenging technically though there are systems that can deal with them.

Pyrolysis

Pyrolysis is the conversion of a solid fuel in the absence of oxygen to a mixture of gas, liquid and solid components. Fast pyrolysis maximizes the conversion to liquid oil while slow pyrolysis produces more gas and char.

Pyrolysis should be considered as a concentration step able to convert a low energy density fuel to a high energy density fuel that can easily be transported. Unfortunately, pyrolysis oil is acidic and generally unsuitable for direct use though some specialized equipment has been developed for this role (www.dynamotive.com).

The benefit of pyrolysis is that plants can be positioned to minimize fuel transport costs as the fuel is generally low density eg chicken litter, sawdust or corn cobs. The oil from the distributed plants can be transported to a central facility where the oil can be gasified to produce a syngas suitable for conversion to power, heat or to liquid fuels via FT or other similar technologies. Dynamotive for example have produced high grade fuel from pyrolysis oil.

Fuels that can be used include wood, wood waste, agricultural waste such as corn cobs and stalks, nut shell, chicken litter, etc. Fuels such as shredded tyres and other industrial wastes with calorific value can also be pyrolised.