Prediction of Tar and Char Levels in the Production of Pyrolysis Liquid and Gasification Syngas
The intense heating during fast pyrolysis vaporizes biomass in an oxygen-free environment. The process takes less than half a second and the vapor is cleaned and condensed into a liquid called bio-oil. This project uses a small laboratory scale reactor system, designed and built by Iowa State graduate students, the Center for Sustainable Environmental Technologies, along with Conoco-Phillips. Its purpose is to investigate the fast-pyrolysis kinetics during the formation of char, tar, and gas. Also, a feedstock pretreatment investigation is being done to determine how prewashing can affect product characteristics.
The reactor is designed for a feed rate of 100 to 250 grams-per-hour, using a pneumatic volumetric feeder. The biomass used is flash-heated with a fluidized bed of hot sand. The vapors are first separated from solids with a cyclone filter. Then condensable liquids are collected in an ice bath heat exchanger. The non-condensable gasses are sampled by a micro-gas chromatograph to monitor the total gas production of the system. Figure 1 is a schematic of the system, while Figure 2 is a photo of the system during its initial operation.
The condensed bio-oil yield is 55-65wt% and is composed of hundreds of various chemicals. The bio-oil has a potential to become a very popular source for hydrocarbon derived materials and fuels. The goal of the study is to develop a method for producing clean, energy dense bio-oil that can be easily used to create industrial chemicals and transportation fuels.
Figure 2: Photo of reactor at final stage of construction
Bio-oil production with the bench top reactor also produces char at 5-10wt% and gas at 25-40wt%. The experiments will determine how operation conditions and the type of feedstock affect the overall quality and yield of the bio-oil that is created. The char and product gas are also potential alternative fuel/energy sources for processes such as soil abatement or boiler fuel.
The kinetics of biomass gasification will also be studied in this research project. In gasification, air is used to fluidize the sand bed rather than nitrogen. Oxygen in the air allows some combustion to begin to occur. This raises the steady operation temperature up to 800°C and produces much more gas in comparison to the bio-oil. The gas form gasification is rich with carbon-monoxide and hydrogen. This gas stream could be converted into other forms of more readily usable energy or used directly for combustion fuel.
The specific chemical derivatives that are produced during gasification will be studied with the same apparatus. The formation of bio-oil during gasification, also known as tar, can create problems for downstream processes. Determining the proper conditions to produce less tar and more gas is important for developing a profitable gasification system.
Currently this project is focused on how pretreatments affect the bio-oil properties. Loblolly pine slash is the biomass feedstock being studied. Two separate pretreatments are being explored: water wash to remove mineral matter form the biomass, and torrefaction which involves gently heating the biomass to remove all moisture and create a more coal-like material. The outcome will make a case to whether or not it will be profitable to include prewashing or torrefaction of the feedstock in the design of a large scale pyrolysis plant.
Principal Investigators:
Dr. Robert C. Brown, Iowa State University, rcbrown@iastate.edu
Participating and/or Sponsoring Organizations:
Iowa State University
ConocoPhillips