The primary focus of this project is to research, develop and evaluate an alternative fast pyrolysis reactor design for the production of bio-oil. The reactor being studied is a radiative free-fall reactor shown in Figure 1.
Fast pyrolysis of biomass is the thermal degradation of lignocellulosic material in the absence of oxygen. When exposed to moderate temperatures between 450 – 550°C the biomass vaporizes forming condensable vapors, char and non-condensable gas. Commercially, fast pyrolysis has been performed in bubbling fluidized bed and circulating fluid bed reactors. These reactors can produce high bio-oil yields (~70 wt %) due to high heat transfer rates and short vapor residence times. Large amounts of carrier gas are used to fluidize the bed material and quickly remove the vapors. Alternative reactor designs may overcome these inefficiencies by minimizing the use of an inert carrier gas, simplifying the design and reducing the number of moving parts.
Figure 2: Process flow diagram
The free fall reactor is essentially a heated, upright pipe through which biomass is fed. The reactor is externally heated to 500°C by ceramic radiative heaters. Biomass is ground down to small particles using a knife mill and fed into the top of the reactor with a volumetric screw feeder (not shown in Figure 1). The particles fall 2m through the length of the pipe before they are completely pyrolyzed. The char is collected in a canister. The pyrolysis vapors and non-condensable gases pass through a heated cyclone before entering a condenser system. Here the vapors are quenched into bio-oil and the non-condensable gases exit to a dry gas meter and a micro GC for further analysis. The gases are then vented. This process is shown in Figure 2.
A recent central composite design of experiments was performed to optimize the free-fall reactor for the production of bio-oil. Red oak biomass was selected as the feedstock for the 30 experiments. The effects of reactor temperature, biomass particle size, carrier gas flow rate and biomass feed rate were investigated. The following levels were chosen:
Heater temperature (°C): 450, 500, 550, 600, 650
Particle size (microns): 200, 300, 400, 500, 600
Nitrogen flow rate (sL/min): 1, 2, 3, 4, 5
Feed rate (kg/hr): 1.00, 1.25, 1.50, 1.75, 2.00
A model of the product yields produced from design of experiments is shown in Figure 3 as a function of the heater temperature. Maximum bio-oil yields around 70 wt % were achieved at 600°C. The bio-oil was analyzed for moisture content, water insoluble content, solids content, Ultimate and Proximate analysis, higher heating value, Total Acid Number and chemical composition by GC/MS. The char and non-condensable gases were also analyzed. Models produced from the design of experiments revealed optimum conditions for maximum bio-oil production and significant factors.
Publications:
Ellens, C.J. Design, optimization and evaluation of a free-fall biomass fast pyrolysis reactor and its products. M.S. thesis, Iowa State University, Ames, 2009.
For further information and inquiries please contact those listed below:
Principal Investigators:
Dr. Robert C. Brown, Iowa State University, rcbrown@iastate.edu
Participating and/or Sponsoring Organizations:
ConocoPhillips Company