Hydrothermal Carbonization, Markus Antonietti, Max Planck Institute, of Colloids and Interfaces,
has developed a method of making coal from biomass using an autoclave system. The process is suitable for web biomass and the main byproduct is water, not carbon dioxide. It seems suitable for wet biomass that is difficult to combust. The process is being studied extensively in Germany.

Also see http://stoves.bioenergylists.org/maxphydtherm

More information:
Maria-Magdalena Titirici, Arne Thomas and Markus Antonietti, "Back in the black: hydrothermal carbonization of plant material as an efficient chemical process to treat the CO2 problem?", New J. Chem., 8th March 2007, DOI: 10.1039/b616045j

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