Environmental Engineering Reference
In-Depth Information
oligomers and polymers. The addition of methanol or ethanol to fast-pyrolysis bio-oil at the 10 wt%
level was found to be effective in largely retarding these reactions (Diebold and Czernik 1997).
Furthermore, elimination of contact with air, addition of antioxidants, and mild hydrogenation are
effective measures to increase storage stability of pyrolysis bio-oils.
8.3.2.5 catalytic upgrading of Pyrolysis Bio-oil
Pyrolysis bio-oil has limited applications as a fuel other than direct combustion in furnaces
to provide heat or power. As a result, bio-oil must be upgraded to serve as fuel in vehicular
transportation. The goal of upgrading is to increase volatility through molecular-weight reduction,
enhance storage stability, and eliminate oxygen to raise product fuel heating value. Catalytic
upgrading using hydrogen is accomplished using two main methods: hydroprocessing and catalytic
cracking. These upgrading reactions are similar in nature to hydrotreating reactions that occur in
conventional petroleum refinery processes. The advantage of producing hydrocarbon fuels from
pyrolysis bio-oils, similar to FT reactions of biomass gasification synthesis gas, is the compatibility
of the final fuel product with distribution infrastructure in pipelines and with conventional and
high-efficiency engines.
Recent reviews have appeared on upgrading pyrolysis bio-oils to liquid hydrocarbon fuels
through hydrotreatment and hydrocracking (Furimsky 2000; Huber et al. 2006; Elliott 2007).
In hydroprocessing, oxygen is removed from compounds in pyrolysis bio-oils through reactions
with hydrogen to produce water plus hydrocarbons, which can be isolated as immiscible separate
phases. Various heterogeneous catalyst materials to carry out hydroprocessing reactions on
bio-oils include sulfide catalysts found in the petroleum refining industry and precious metal
catalysts.
The use of acidic cracking catalysts is well known in petroleum refining to reduce molecular
weight and convert fuel components to more aromatic structures. When these catalysts are applied
to pyrolysis bio-oils, similar results are observed, but with high levels of coke formation. Zeolitic
acid catalysts, such as HZSM-5, yield high ratios of aromatic to aliphatic compounds and have been
used to crack the product of pyrolysis bio-oil hydrotreatment. This yields a 5:1 ratio of aromatic to
aliphatic product with 30-50% conversion to coke. Coke is burned to recover catalyst and generate
process heat. However, a continuing challenge is achieving a good heat balance (NSF 2008).
Long-term catalyst stability and life has not been demonstrated in pyrolysis bio-oils that have
been catalytically upgraded. Currently the longest test lasted 8 days before significant hydrotreating
catalyst deactivation was observed. Deactivation of cracking catalysts has limited in-process life to
only a few cycles in recent experiments. Catalyst deactivation has been attributed to the presence
of water on oxide structures of catalysts and to the presence of trace contaminants such as minerals
in biomass feedstocks, but the actual mechanisms are not currently well understood (NSF 2008).
8.3.2.6 Power uses of Pyrolysis Bio-oil
Pyrolysis bio-oil can be combusted similarly to fossil fuels in boilers, gas turbines, and diesel engine
power applications. Co-firing with coal, natural gas, and fuel oil are also options for utilization of
bio-oil as a renewable feedstock. Bio-oil has an advantage compared with wood chips and pellets
because of its wider application in co-firing applications, which allows easier adaptation to higher-
efficiency power generation (conventional natural gas) and advanced power (combined cycle power).
A review of pyrolysis bio-oil power generation can be found in a recent publication by Czernik and
Bridgwater (2004), showing that bio-oil can be successfully used as boiler fuel and in diesel engine
and gas turbine applications.
8.4 BIochemIcal conversIons Plant Woody BIomass
Biochemical conversion of lignocellulosic biomass to fuels and other chemicals generally
focuses primarily on the isolation of sugars in their monomeric form from hemicellulose and
