Center for Sustainable Environmental Technologies

This project aims to explore a viable alternative to fluidized bed reactors for bio-oil production via the thermochemical process fast pyrolysis. This conversion method decomposes lignocellulosic biomass in the presence of heat to produce a mixture of organic vapors, bio-char and combustible gas. When quickly cooled, the vapors are condensed into a liquid bio-oil which is a suitable feedstock for further refining into an array of biorenewable products. Fast pyrolysis reactions typically occur in the absence of Oxygen at atmospheric pressure and a temperature of approximately 500°C (932°F).

Fluidized bed reactors have been proven at a commercial scale, and are the most common design used for fast pyrolysis processing. Though they are well understood, provide high biomass throughputs and bio-oil yield, their operation relies on a large volumetric flow of an inert gas such as Nitrogen that must be heated and compressed. Other reactor designs may offer certain advantages such as the reduction or elimination of carrier gas, but they have not been as thoroughly investigated. The auger type of pyrolyzer has been identified as especially appealing for its potential to reduce operating costs associated with bio-oil production. This design may also be well suited for small, portable pyrolysis systems in a highly distributed or decentralized biomass processing scheme. After a thorough research and design process, a bench scale auger reactor system has been developed at Iowa State University. The construction and assembly phase of this alternative reactor system, as seen below, is near completion.

The operating principle of this design is that biomass is continuously pyrolyzed by being brought into direct contact with a bulk solid heat transfer medium referred to as a “heat carrier.” The heat carrier material, such as sand or steel shot, is heated independently before being metered into the reactor. On a gravimetric basis, thermodynamic calculations suggest a heat carrier feed rate 20 times the biomass feed rate. Two intermeshing, co-rotating 1” augers quickly combine biomass and heat carrier in a shallow bed to effectively carry out the pyrolysis reactions. This mechanical mixing process, though not well understood, appears to be the essence of this alternative pyrolyzer design. Volatile vapors and aerosols exit at various ports, while char is transported axially through the 20” long reactor section and stored in a canister with the heat carrier. A detail of the stainless steel augers is depicted below.