Pyrolysis and Other Thermal Processing
Solid biomass can be liquefied by pyrolysis, hydrothermal liquefaction, or other thermochemical technologies. Pyrolysis and gasification are related processes of heating with limited oxygen. Conditions for producing pyrolysis oil are more likely to include virtually no oxygen. Pyrolysis oil or other thermochemically-derived biomass liquids can be used directly as fuel, but also hold great promise as platform intermediates for production of high-value chemicals and materials.
Fast pyrolysis is a thermal decomposition process that occurs at moderate temperatures with a high heat transfer rate to the biomass particles and a short hot vapor residence time in the reaction zone. Several reactor configurations have been shown to assure this condition and to achieve yields of liquid product as high as 75% based on the starting dry biomass weight . They include bubbling fluid beds, circulating and transported beds, cyclonic reactors, and ablative reactors.
Fast pyrolysis of biomass produces a liquid product, pyrolysis oil or bio-oil that can be readily stored and transported. Pyrolysis oil is a renewable liquid fuel and can also be used for production of chemicals. Fast pyrolysis has now achieved a commercial success for production of chemicals and is being actively developed for producing liquid fuels. Pyrolysis oil has been successfully tested in engines, turbines and boilers, and been upgraded to high quality hydrocarbon fuels although at a presently unacceptable energetic and financial cost.
In the 1990s several fast pyrolysis technologies reached near-commercial status. Six circulating fluidized bed plants have been constructed by Ensyn Technologies, with the largest having a nominal capacity of 50 t/day operated for Red Arrow Products Co., Inc. in Wisconsin. DynaMotive (Vancouver, Canada) demonstrated the bubbling fluidized bed process at 10 t/day of biomass and is scaling up the plant to 100 t/day. BTG (The Netherlands) operates a rotary cone reactor system at 5 t/day and is proposing to scale the plant up to 50 t/d. Fortum has a 12 t/day pilot plant in Finland. The yields and properties of the generated liquid product, bio-oil, depend on the feedstock, the process type and conditions, and the product collection efficiency.
Biomass Program researchers use both vortex (cyclonic) and fluidized bed reactors for pyrolyzing biomass. The fluidized bed reactor of the Thermochemical Users Facility at the National Renewable Energy Laboratory is a 1.8 m high cylindrical vessel of 20 cm diameter in the lower (fluidization) zone, expanded to 36 cm diameter in the freeboard section. It is equipped in a perforated gas distribution plate and an internal cyclone to retain entrained bed media (typically sand). The reactor is heated electrically and can operate at temperatures up to 700°C at a throughput of 15-20 kg/h of biomass.
Recently, a catalytic steam reformer was coupled to the pyrolysis/gasification system. Like the pyrolyzer, the reformer is an externally heated fluidized bed reactor that will be used to produce hydrogen from pyrolysis gas and vapors generated in the first stage of the process and to clean the gas from tars.
Biomass Program micro-scale pyrolysis systems include externally heated different types reactors coupled to the molecular-beam mass-spectrometer (MBMS). These systems are very efficient tools, especially for studying mechanisms of thermal and catalytic processes and to optimize process conditions for different products from variety of feedstocks. For example, the ongoing research sponsored by Philip Morris resulted in understanding the chemical processes of biopolymer pyrolysis and oxidation leading to aromatic hydrocarbon formation.
Direct hydrothermal liquefaction involves converting biomass to an oily liquid by contacting the biomass with water at elevated temperatures (300-350°C) with sufficient pressure to maintain the water primarily in the liquid phase (12-20 MPa) for residence times up to 30 minutes. Alkali may be added to promote organic conversion. The primary product is an organic liquid with reduced oxygen content (about 10%) and the primary byproduct is water containing soluble organic compounds. Hydrothermal treatment is based on early work performed by the Bureau of Mines Albany Laboratory in the 1970s. Developers include Changing World Technologies (West Hampstead, NY), EnerTech Environmental Inc (Atlanta, GA), and Biofuel B.V. (Heemskerk, Netherlands).