Environmental Engineering Reference
In-Depth Information
Silo
(storage)
Mechanical size
reduction
Pyrolysis
Pre-heater
Truck unload wood chips
Pyrolysis oil
Hydrogen
Pump
Natural gas
Steam
reforming
PSA
Off gas
Vapor
Hydrotreater
(2 stages)
Gasoline
Light st
a
ble oil
Distilation
Distilation
Storage
Oil
H.C.
Diesel
Heavy stable oil
to hydrocracker
FIGure 8.5
Pyrolysis-based process for production of liquid transportation fuel from woody biomass.
approximately 1000°C, and ultrashort residence times (< 0.5 s). Vacuum pyrolysis, similar to fast
pyrolysis, produces predominantly bio-oil at 400°C, with a 2- to 30-s residence time, and medium
heating rate. Hydropyrolysis uses water as a reducing reaction medium to convert woody biomass
to bio-oil (mostly at < 500°C), using a high heating rate and a 10-s residence time. Because water
is the reaction medium, hydropyrolysis has the advantage that it can accept biomass with field
moisture without a drying step. Methanol is used as the reaction medium in methano pyrolysis, but
the main products are chemicals rather than fuels, and reactor conditions include a high heating
rate, a residence time of less than 10 s, and temperatures of more than 700°C.
8.3.2.2 Fast Pyrolysis
Because the main focus of this volume is energy products, fast pyrolysis and utilization of bio-oil
in energy and fuels applications will be covered in more detail, whereas additional information
on other aspects of pyrolysis can be found in several excellent reviews (Bridgewater and Peacocke
2000; Mohan et al. 2006) and the references listed therein. Fast pyrolysis achieves high yields of bio-
oil from biomass and lower recoveries of synthesis gas and char by carefully controlling the reaction
environment. Typical yields for fast pyrolysis are bio-oil (60-75%), synthesis gas (10-20%), and
char (15-25%). A further analysis of bio-oil reveals that water comprises approximately 20-30% by
weight, the remaining being oxygenated organic compounds of varying molecular weight. Organic
compounds in bio-oil from the carbohydrate fraction of wood are more water soluble, whereas
those from lignin are considered more hydrophobic and are higher in molecular weight (NSF 2008).
Requirements for fast pyrolysis are rapid heat transfer rates; a finely ground dry (5%) moisture
biomass feedstock to achieve high heat transfer and reaction rates; and rapid cooling of pyrolysis
vapors and aerosols to avoid degradation, dehydration, and fragmentation.
8.3.2.3 Properties of Pyrolysis Bio-oil
Before discussing conversion of pyrolysis bio-oil into liquid transportation fuels or the use of bio-oil
in heat or power applications, it is necessary to describe the chemical nature and properties of bio-oil
