Presteaming of the wood raw material takes place in a chip bin which also provides buffer storage of the raw material.
Presteaming is applied to pre-heat the raw material and to remove air from the wood to facilitate impregnation with liquor, e.g., an acid solution in acid-catalyzed hydrolysis.
The bin is of a hopper type design and consists of a cylindrical and conical top section and a conical bottom section. The raw material enters the bin at the top and leaves through the conical bottom section.
In the conical bottom section a vibrating cone is inserted to help prevent bridging of the raw material in the bin. The preheating steam is introduced underneath this cone and distributed both in the middle and around the periphery of the bin where it meets the raw material passing down between the cone and the bin wall. The bin is designed for 15 to 20 minutes retention time.
The wood is then fed by a plug screw feeder into the pressurized reactor.
Second generation biomass material, such as wheat straw, bagasse, corn stover, reed and grasses often do not require impregnation or pre-steaming before the prehydrolysis. It is often sufficient to heat the biomass with steam when it enters the prehydrolysis reactor. At that point also acid can be added to give the desired hydrolysis conditions.
A pin drum feeder ensures smooth and reliable feeding of the biomass to the plug screw feeder which feeds the biomass into the reactor. The pin drum feeder also controls the feed rate of biomass to the plug screw.
The pin drum feeder consists of two speed-controlled revolving drums with sturdy conical pins. The pin drum feeder distributes the raw material in a controlled flow down to a screw conveyor. The screw conveyor then feeds the biomass into the plug screw feeder and a T-pipe which is equipped with an active blow-back damper. The biomass is pre-hydrolyzed in one or more reactor tubes depending on the retention time and the production rate.
Feeding biomass into a pressurized reactor is a crucial step in biorefining. Even and safe feeding of the biomass is important. Whether the biomass is wood, in the form of chips or sawdust, or annual plants such as wheat straw, bagasse, reed, energy cane or grasses it can be fed into a pressurized reactor by a plug screw feeder. In the plug screw feeder, a tight plug is continuously formed of the biomass which seals against the reactor pressure.
Valmet has developed screw feeding technology to ensure safe feeding and minimize the risk of a blow-back. An active counter-pressure cylinder seals off the reactor, which minimizes the amount of steam going in the backward direction, in case of loss of the plug.