A variant of slow pyrolysis includes a steam gasification step, a technology developed by
Eprida in collaboration with the University of Georgia ().
Adding steam to the pyrolysis reaction liberates additional syngas from the biochar product,
mainly in the form of hydrogen. The biochar that remains after this ‘secondary’ pyrolysis
displays rather different properties from the primary product, differing in pore size and carbon
to oxygen ratio (Demirbas, 2004).
Syngas can be purified through a sequence of operations to yield pure streams of the
constituent gases: hydrogen (50% of gas yield), carbon dioxide (30%), nitrogen (15%),
methane (5%), and lower molecular weight hydrocarbons, as well as some carbon monoxide
(Day et al., 2005). There is a small energy penalty associated with these steps.
Slow pyrolysis research plants currently process feedstock at a rate of 28–300 kg hr-1 on a
dry mass basis, and commercial plants operate at 48–96 t d-1. Comparing the efficiency of
pyrolysis plants is complex since the mix and use of products vary, and the composition and
heat value of syngas differs. Feedstock quality and moisture content is also variable, and
there is a conversion loss in the generation of electrical power through steam or gas turbines.