Fast pyrolysis requires high heating rates. As biomass has a low thermal conductivity, high heating rates throughout the particle either result from small particle sizes or erosion of the char product from around the particle as it pyrolyses as occurs in ablative pyrolysis (see Chapter 2 and 3). The current view of the maximum size feasible for fluid bed or circulating fluid bed or entrained flow reactors for giving high yields of liquids is 6 mm. The exception is ablative pyrolysis which is a surface area controlled process and has no upper size limit as the char layer is constantly eroded or abraded away. This upper particle size limitation for most processes thus carries a cost and energy penalty to grind the feedstock. Some biomass forms exist in a stringy form which cannot be ground in conventional milling equipment. Chopping or slicing mechanisms may work, but extensive testing is needed to evaluate alternative comminution processes. Low bulk density feeds give handling problems and fluidisation problems in bubbling fluid bed reactor systems. Feed that is pretreated such as by acid washing has reduced mechanical strength and gives high fines level which can result in higher solids levels in the product.

As all the feed moisture appears in the product together with all the reaction water, the feed has to be dried to as low a water content as possible. Practically this limit is around 10% moisture. At normal high yields of liquids of 60 to 65 % on a dry liquid basis, the reaction water contributes about 15% moisture to the wet product from a bone dry feed. Each 1% moisture in the feed adds approximately a further 1.25% moisture to the product oil on a wet basis. Drying to much below 10% is generally considered too expensive as well as significantly increasing the fire and safety hazards from handling very dry materials. Forced drying is probably the most expensive pretreatment operation for the feedstock from most primary energy crop sources.

High lignin feeds give higher viscosity products that are also more "sticky" due to the presence of higher levels of partially depolymerised lignin. Optimisation of the time-temperature processing window may improve the product quality and a small sacrifice in yield may be more than compensated for a significant improvement in quality or properties. Resinous feeds give analogous product handling problems from tar products resulting from resin degradation.

All biomass contains some ash, usually appreciable quantities of alkali metals which report to both the char and the liquid. This is compounded by the presence of char in the liquid from incomplete char removal in orthodox cyclones. Although developments are in hand to filter char out in the vapour phase, analogous to hot gas filtration in gasification processes, a proven system is not yet available and preliminary results have suggested that product yields may be adversely affected from the higher vapour residence times.