Fast pyrolysis research at Iowa State University centers around several pilot units located at the Biorenewables Research Laboratory (BRL) on the ISU campus and the BioCentury Research Farm (BCRF), which is located approximately seven miles west of campus. These units convert dry biomass by rapidly heating the particles to high temperatures (300 to 600oC) in an environment containing little or no oxygen at atmospheric pressure. Fast pyrolysis results in three main products: pyrolysis liquids or bio-oil, solid residue (biochar), and non-condensable gases.
The reactors located in the BRL include two bubbling fluidized bed reactors, an auger reactor, and a free fall reactor. The bubbling fluidized bed reactors are custom built systems that can pyrolyze up to 1 kg of biomass per hour at temperatures ranging from 400 to 600oC. These reactor utilize a hot fluidized bed of sand with nitrogen or air as the fluidizing agent.
The free fall reactor is a novel system that can pyrolyze 1 to 2 kilograms of biomass per hour at temperatures up to 800oC. Biomass is fed by an auger into the top of a 10’ heated tube. The biomass is heated as it falls, and vapors exit the reactor to be subsequently filtered and condensed while the remaining solids fall in a catch at the reactor bottom. The char is continuously removed from the catch with an auger to allow for continuous operation and prevent further reactions. Heaters along the length of the reactor tube can be independently controlled, allowing the residence time, temperature, and heating rate to be adjusted. The reactor also includes thermocouples and gas chromatography (GC) ports every 6” along the length of the tube. This allows for vapor temperature and composition to be determined as the biomass falls through the reactor and is pyrolyzed.
The auger reactor is another novel unit featuring two co-rotating augers that mechanically mix biomass and a solid heat carrier material. Biomass can be processed at rates of up to 2 kilograms per hour. The heat carrier is typically metered into the system at rates up to 15 kilograms per hour, and can consist of many different materials including steel shot, sand, ceramic beads, and silica carbide particles. Like the fluidized bed reactor, operation conditions include an inert environment at atmospheric pressure and temperatures ranging from 400 to 600o.
Each of the reactor systems in the BRL utilizes cyclone filters to remove solid particulate from the vapor stream. The vapors are then quickly cooled using a combination of condensers and electrostatic precipitators (ESPs). This results in the collection of stage fractions (SF) with distinct chemical and physical properties.
The fast pyrolysis process development unit (PDU) located at BCRF processes up to 8 kilograms of biomass per hour at temperatures ranging from 350 to 600oC. This system utilizes a bed of sand fluidized by compressed nitrogen or recycled non-condensable gas generated during pyrolysis. As the biomass is pyrolyzed, the resulting vapors exit the reactor and enter a pair of cyclone filters, which remove solid particulate. The vapor stream is filtered further through a moving bed granule filter (MBGF). The vapors then pass through a series of condensers and ESPs, where they are condensed to form stage fractions with distinct chemical and physical properties.