United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S7-84-009 Mar. 1984
v>EPA Project Summary
A Study of the Steam
Gasification of Organic
Wastes
Michael J. Antal, Jr., William E. Edwards, Henry L Friedman, and Frank Er
Rogers
Chemical kinetic data describing the
pyrolysis/gasification characteristics
of organic waste (biomass) materials is
needed for the design of improved
conversion reactors. Unfortunately, lit-
tle data is available in the literature on
the pyrolysis kinetics of waste materials,
and essentially no data has been pub-
lished on tha rates and products of the
secondary, gas phase reactions. The
objective of this research was to de-
termine the effects of various reactor
conditions (residence time, tempera-
ture, and heating rate) on the rates of
the primary pyrolysis reactions, and the
rates and products of the secondary gas
phase reactions.
This Project Summary was developed
by EPA's Industrial Environmental Re-
search Laboratory. Cincinnati, OH, to
announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
In an age of diminishing fossil fuel
reserves, as reflected in rapidly escalating
prices for fluid fuels, local shortages of
gasoline and fuel oil, and large imbal-
ances in international trade, attention
has recently focused on fluid fuel produc-
tion from renewable biomass* resources.
Pyrolysis is one attractive method for
converting biomass to more useful fuel
^Because their thermochermcal properties are simi-
lar, the words "biomass" and "organic wastes" are
used interchangeably throughout this report.
forms. Unfortunately, the product slate of
most pyrolysis processes includes signif-
icant quantities of relatively valueless
tars, oils, pitches, liquors and some water
soluble organic compounds. These value-
less products of conversion processes
adversely affect process economics be-
cause they reduce conversion efficien-
cies, and require more complicated equip-
ment for their handling and ultimate
disposal.
The primary goal of this research was
to experimentally determine the condi-
tions required for the conversion of these
valueless products into a hydrocarbon-
rich synthesis gas. To accomplish this
goal, the research effort focused primarily
on an elucidation of the chemistry of
waste pyrolysis/gasification in steam.
Since kinetic information is essential for
the development of improved gasification
systems, all experiments were designed
and conducted to provide carefully charac-
terized rate data. When possible, this data
was interpreted using Arrhenius kinetics,
and mathematical models were developed
to fit the experimental data. Although
these models were not intended to be of
fundamental import, they are expected to
be of great use to chemical engineers in
designing advanced gasification systems.
Summary and Conclusions
The gasification characteristics of bio-
mass differ considerably from those of
coal, lignite, peat and other fossil fuels.
Research described in this report has
shown biomass gasification to be a three-
step process:
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1. Pyrolysis. At modest temperatures
(300 ° to 5OO°C) biomass materials
lose between 70% to 90% of their
weight by pyrolysis, forming gase-
ous volatile matter and solid char.
Biomass materials contain much
more volatile matter than does coal,
and biomass loses weight more
rapidly than coal by pyrolysis.
2. Cracking/Reforming of the Volatile
Matter. At somewhat higher temper-
atures (600°C or more), the volatile
matter evolved by the pyrolysis
reactions (step 1 above) reacts in
various ways to form a hydrocarbon
rich synthesis gas. These reactions
occur very rapidly (seconds or less)
and can be manipulated to favor the
formation of various hydrocarbons
(such as ethylene).
3. Char Gasification. At even higher
temperatures (800°C or more) char
gasification occurs by reactions
C + H20 - CO + H2
C + CO2-2CO
C-H/2O2-CO
C + O2 - CO2
These reactions can be used to
describe coal gasification, but pro-
ceed more slowly and under more
demanding conditions than those
required for steps 1 and 2. Since
step 1 produces less than 30% by
weight char, these char gasification
reactions play a minor role in bio-
mass gasification.
Although each of these three steps play
a role in commercial reactors designed
for biomass gasification, the ultimate
product slate of the reactor is largely
determined by the second step of the
gasification process. Careful control of
reactor conditions (i.e., gas phase resi-
dence time and temperature) can favor
the production of valuable hydrocarbons
(plefins and paraffins) from biomass over
a less valuable synthesis gas (CO and H2),
or valueless tars, liquors and oils. The
focus of the full report is the determina-
tion of those conditions which favor the
formation of hydrocarbons from biomass
using thermochemical conversion tech-
niques. The conclusion is that surprisingly
mild conditions are required to effect the
conversion of biomass to hydrocarbons.
Those familiar with petrochemical tech-
nologies will recognize similarities be-
tween the results given in the report and
comparable results for steam cracking
naphthas and gas oils to ethylene. In fact,
biomass may be a more desirable source
of olefins than feedstocks traditionally
used by the petrochemical industry. Data
given in the report indicate that from a
thermochemical viewpoint, biomass is a
much more desirable solid fuel feedstock
for the production of fluid fuels and
chemicals than are coal, lignite or peat.
During the past two years, over 60% of
the Princeton research effort on biomass
and waste conversion has focused on
elucidating the rates and products of the
secondary, gas phase reactions of pyro-
lytic volatile matter in steam (step 2
described above). Results of our research
in this area are described in the full
report. This research produced the first
detailed study of secondary, gas phase
reaction rates and products reported in
the literature. The most important con-
clusion resulting from this aspect of the
research is that the secondary, gas phase
reactions are dominated by cracking
chemistry. At temperatures above 700°C,
these cracking reactions reach comple-
tion in less than one second. Thus the
technologies developed for steam crack-
ing naphthas and gas oils are applicable
to biomass conversion, and may prove to
be the favored conversion route as conver-
sion technologies develop and mature.
A significant effort was also made to
study the effects of increased pressure on
the products and rates of the primary and
secondary pyrolysis reactions. Numerous
difficulties were encountered which sub-
stantially impeded our progress; neverthe-
less some early results are reported here.
The remaining part of Princeton's re-
search effort was divided between kinetic
studies of the solid phase pyrolysis reac-
tions (step 1 described above), and some
exploratory model compound studies
aimed at providing an insight into the gas
phase chemistry. Despite the great variety
of biomass materials studied, their rates
and mechanisms of weight loss by pyrol-
ysis were reasonably similar from an
engineering point of view. Kinetic weight
loss models, developed to describe the
solid phase pyrolysis reactions, are pres-
ently being used in a reactor design effort
at Princeton. The model compound stud-
ies focused on the gas phase pyrolysis of
ethanol, used to simulate the cracking
chemistry of cellulose volatiles. Competi-
tive reactions forming ethylene, or Cru,
CO and H2 were studied. Cracking rates
were measured, and found to compare
favorably with literature values. Future
research on model compounds will be
aimed at determining those conditions
which give riseto maximum production of
olefins from biomass by offering an
insight into the gas phase cracking chem-
istry.
Since the research described in this I
report indicates that biomass and wastes
may be more desirable than coal, peat, or
lignite as gasification feedstocks, the
former should be thoroughly investigated
as synfuel sources. A more thorough
investigation of the thermochemical con-
version chemistry of biomass and wastes
to useful fluid fuels would be necessary
before final economic and environmental
judgments can be made. These efforts
should include studies of gasification
mechanisms, kinetics, and catalysis.
More attention- should be given to the
secondary, gas phase reactions of pyro-
lytic volatile matter. Reactors, designed
specifically to study the gas phase chem-
istry, should be developed. Without a
better insight into the gas phase chem-
istry, biomass gasification reactor design
efforts will continue to produce 19th and
early 20th century concepts, rather than
the 21st century processes needed to
compete with today's sophisticated hydro-
carbon processing industry.
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MichaelJ. Antal, Jr., William E. Edwards, Henry L Friedman, and Frank E. Rogers
are with Princeton University. Princeton, NJ O8544.
Walter W. Liberick, Jr., is the EPA Project Officer (see below).
The complete report, entitled "A Study of the Steam Gasification of Organic
Wastes," (Order No. PB 84-143 148; Cost: $13.00, subject to change) will be
available only from:
National Technical Information Service
5285 Port Royal Road
Springfield. VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
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United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
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