Processing problems

Heat transfer rates are considered by some researchers to become problematical at large biomass capacities, particularly for ablative pyrolysis which is a surface area controlled process rather than a volumetric process.

Condensation of bio-oil vapours is a known problem. Cooling is not generally difficult but rather the collection of the cooled aerosol. Stable tar aerosols are formed which pass readily through the condensers and require special handling. Direct quenching in hot product oil has been used by Bio-Alternative, and also by Alten in their washing system when cooled separated product water was recycled through a packed tower. NREL and Ensyn also use direct contact with product liquid to aid collection. Electrostatic precipitators are known to work well.

Pretreated feed is claimed to give a "sticky" char product which agglomerates, sometimes in the reactor and sometimes in the downstream equipment causing build up of char-oil cakes resulting in eventual blockage. Another phenomenon is the partially depolymerised lignin which will tend to have a much higher viscosity than cellulose and hemicellulose degradation products. If this is condensed or collected in preference to other constituents due to inadequacies in the design of the liquid cooling and collection system, then blockage may again result. As high temperature char residues encourages product cracking, any char must be removed as it is formed and not allowed to be recycled or remain in contact with the vapours.

Most reactor systems (see Table 4.2) are based on orthodox fluid bed, circulating fluid bed or entrained flow reaction systems and are thus, in principle, able to be designed and operated in conventional ways. It is only the ablative type of reactors that tend to be unique to biomass.