United States
Environmental Protection
Agency
National Risk Management
Research Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/SR-96/071
October 1996
EPA Project Summary
Evaluation of Biomass Reactivity in
Hydrogasification for the Hynol
Process
Yuanji Dong and Edward Cole
The reactivity of poplar wood in
hydrogasification under the operating con-
ditions specific for the Hynol process was
evaluated, using a thermobalance reactor.
Parameters affecting gasification be-
havior, e.g., gas velocity, particle size,
system pressure, reaction temperature,
reaction time, and feed gas composi-
tion, were investigated. The experimen-
tal results showed that temperature and
particle size strongly affect biomass
conversion and gasification rates. The
poplar wood conversion is proportional
to the partial pressures of hydrogen
and steam in the feed gas. A conver-
sion of 86-87% was observed when 1/8-in.
(0.32 cm) poplar particles were gasified at
30 atm (2942 kPa) and SOO'C for 60 min
with the feed gas composition simulating
the Hynol recycled gas from the methanol
synthesis reactor. As the reaction time
extended to 2.5 hours, the conversion
increased to 90%. It was found that
gasification involves a rapid reaction
of biomass thermal decomposition and
a slow reaction of residual carbon-
aceous matter with the feed gas. The
activator energies for these reactions
were estimated. A kinetic model was
developed to quantitatively express
gasification rates and biomass conver-
sion as functions of reaction time. The
model was used to correlate the
thermobalance reactor experimental
data. The carbon, hydrogen, and oxy-
gen contents in the charred samples
obtained after different gasification
times were analyzed and compared.
Potassium carbonate was found to cata-
lyze biomass gasification and increase
the carbon conversion of poplar wood.
The gasification reactivity of pressed
switchgrass was briefly evaluated.
This Project Summary was developed
by EPA's National Risk Management
Research Laboratory's Air Pollution
Prevention and Control Division, Re-
search Triangle Park, NC, 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
The Hynol process was proposed to
meet the increasing demand for economi-
cal production of methanol from biomass
and natural gas. The process consists of
three reaction steps: (1) hydrogasification
of biomass with the recycle gas remaining
after methanol synthesis, (2) steam re-
forming of the produced gas with addition
of natural gas feedstock, and (3) metha-
nol synthesis from the hydrogen (H2) and
carbon monoxide (CO) produced. These
three reactions take place in the
hydropyrolysis reactor (HPR), the steam
pyrolysis reactor (SPR), and the methanol
synthesis reactor (MSR), respectively.
After theoretical evaluation, the U.S.
Environmental Protection Agency's Air Pol-
lution Prevention and Control Division
(APPCD) concluded that the Hynol pro-
cess represents a promising technology
for maximizing fuel production inexpen-
sively and with minimum greenhouse gas
emissions. Under contract to APPCD,
Acurex Environmental Corporation has
established laboratory research facilities
to perform supporting kinetic studies of
two principal reactions of the Hynol pro-
cess: biomass hydrogasification and meth-
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ane steam reforming. The studies are
aimed at improving understanding of these
reactions, providing quantitative informa-
tion to support the design and operation
of a bench-scale evaluation, and identify-
ing additional needs for Hynol process
development.
A thermobalance reactor (TBR) has
been installed to evaluate biomass reac-
tivities in hydrogasification under the op-
erating conditions specific for the Hynol
process. The report summarizes the TBR
test results.
Experimental
Figure 1 is a flow diagram of the test
facility. The TBR used in the study is
electrically heated and consists of a 35-
mm stainless steel reaction pipe, a pres-
sure vessel, and a topwork which accom-
modates a weight transducer for measur-
ing sample weight during reaction. To ini-
tiate an experimental run, a basket with
known weight of the biomass sample was
charged into the topwork through the win-
dow gauge. Mass flow controllers were
used to control the flow rates of hydrogen,
methane (CH4), carbon monoxide, and car-
bon dioxide (CO2) to maintain the desired
feed gas composition. The mixed gas was
preheated to 350°C by electric heat trac-
ing. A high performance liquid chromatog-
raphy (HPLC) pump was used to meter
and inject distilled water (H2O) from a res-
ervoir into the gas line where the H2O was
vaporized by the preheated feed gas. The
gas was then preheated to the desired
H2 —»-
CH4 —>•
CO —»-
CO2 —>
Heat Tracing #1
H2o
Thermobalance
Reactor
Trap
Superheater
Condenser
Backpressure
Regulator
Vent
Dry Gas
Meter
Heat Tracing #2
Figure 1. Flow diagram of the thermobalance reactor (TBR) system
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operating temperature in the lower part of
the reactor. The exit gas from the TBR
was cooled in a condenser to remove
moisture, and then passed through a high
pressure filter and a backpressure regula-
tor before it was vented to atmosphere.
When pressure and temperature were sta-
bilized at the desired levels, the sample
basket was lowered into the reaction zone
and the changes in sample weight were
automatically recorded by the transducer
as a function of reaction time. Because
the change in gas composition across the
sample is negligible, the reactions can be
considered to take place at constant con-
ditions. A personal computer, equipped
with LabTech software, was used to con-
trol the TBR facility and log experimental
data.
Poplar wood grown in North Carolina
was used as the representative biomass
sample. It was cut to the desired size and
dried before use. A few tests were also
conducted with pressed switchgrass. The
compositions of poplar wood and switch-
grass used are presented in Table 1.
The reactivities in hydrogasification were
evaluated under the Hynol operating con-
ditions based on the results of the Hynol
process simulation provided by the EPA,
in which the biomass is hydrogasified at
30 atm (2942 kPa) and 800 °C and with a
feed gas composition: H2 = 65.83%, CH4
= 11.63%, CO = 8.95%, CO2 = 2.32% and
H2O = 11.27%. Effects of deviation from
these operating conditions were also in-
vestigated.
Kinetic Model and Data
Treatment
In data treatment, the changes in sample
weight recorded by the weight transducer
were transformed to biomass conversion
as a function of reaction time. Biomass
conversion is defined on an ash-free ba-
sis and can be calculated from the re-
corded variation in sample weight by:
X = (W. - W) / (W. - W0 CA
(1)
where W0 =
W =
Initial sample weight
Recorded sample
weight at time t
CA = Ash content in the
sample analyzed from
the ultimate analysis.
The carbon conversion is equal to the
amount of carbon gasified divided by the
amount of carbon in the initial biomass
sample and can be calculated from the
carbon contents in the sample analyzed
before and after gasification as:
a=1-WCF/(W0Cc
(2)
where WCF =
Weight of the carbon re-
maining in the char aft-
er gasification
Carbon weight fraction
in biomass sample be-
fore gasification
When the analysis of carbon content in
a charred sample is not available, an ap-
proximate carbon conversion can be esti-
mated by assuming that all hydrogen and
oxygen in the biomass sample are con-
verted into a gas product after gasification
and the residual char contains only car-
bon and ash. Thus, the carbon conver-
sion is approximated as a function of the
biomass conversion by:
a=1 -
(3)
To quantitatively evaluate biomass re-
activity and gasification rate, a kinetic
model has been developed. The following
assumptions were made in model devel-
opment:
(a) Two types of reactions involved in
biomass hydrogasification can be classi-
fied in terms of mechanism and reaction
rate: the thermal decomposition reaction
of biomass and the reaction of residual
carbonaceous matter with process gas.
The former reaction is rapid and may be
completed in seconds, while the latter is
very slow and requires hours to finish.
(b) Both reactions are first order with
respect to the remaining solid reactants
and can be expressed by:
dX1/dt = K1(Xc -X,) (4)
and
dX2 / dt = K, (1 - Xc - X2) (5)
Table 1. Analysis Results of Poplar Used in this Study and Comparison with Other Reported Data
Sample Poplar Poplar Poplar Switchgrass
Data Source
Carbon wt.%
Hydrogen
Oxygen
Ash
Sulfur
Nitrogen
This Study
51.52
6.20
41.37
0.47
0.02
0.42
BNL*
51.32
6.16
34.57
6.64
0.13
1.18
Noyes**
51.60
6.30
41.50
0.60
0
0
This Study
47.39
6.15
40.09
5.07
0.06
1.22
Volatile
Fixed Carbon
Higher Heating
Value (Btu/lb)
Moisture Free
Basis
91.38
8.15
8768 7861 8920
76.97
17.96
7836
* Brookhaven National Laboratory
** Noyes Data Corporation
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where X1 and X2 are the conversions by
the rapid and slow reactions, respectively;
Xc is the maximum fraction of the matter
convertible by the rapid reaction; and K,
and Kj are the reaction rate constants for
the rapid and slow reactions. By integrat-
ing Equations (4) and (5), the total biom-
ass conversion can be expressed as:
X=1-XC expf-K, t) -
exp(-K2t) (6)
From Equation (6), the conversion is
zero at t = 0 and would approach 1 as t
approaches infinity. The model has three
parameters: Xc, K,, and K2, which are
functions of sample properties and reac-
tion conditions, and can be determined by
correlating the data of conversion versus
reaction time obtained from the TBR ex-
periments.
Results and Discussion
Model Prediction
The applicability of the developed model
to the biomass hydrogasification under the
Hynol operating conditions was investi-
gated. Figure 2 shows good agreement
between the experimental data and the
model regression curve over a period of
60 min, indicating that the model can be
used to correlate the TBR experimental
data and quantitatively express the varia-
tion in gasification rate as a function of
time. By correlating the 60-min gasifica-
tion data obtained with 1/8-in. poplar par-
ticles at 30 atm and 800°C, the model
parameters determined were Xc = 0.8405,
K, = 18.3 min-1, and K, = 0.0035 min'1.
Substituting these parameters into Equa-
tion (6), the conversions at other gasifica-
tion times were predicted. The calculated
conversions, Xcal, were then compared to
the results, Xexp, obtained from the sepa-
rate experimental tests for various gasifi-
cation times, as shown in Figure 3. The
maximum relative error in the comparison
is 1.7%. The comparison covers a range
of reaction time from 0.2 to 150 min.
Run No. B950302
1/8" poplar particles
30 atm and 800° C
Hynol feed gas
Experimental data
Model prediction
Time (min)
Figure 2. The comparison of experimental conversion data with the model correlation results
0.92
0.90
0.88
0.86
0.84
0.82
0.80
1/8" Poplar 30 atm and 800° C
O 150 min
D 60 min
A 30 min
V 20 min
O 0.5 min
0 0.2 min
O
Model Parameters
Xc= 0.8405
K1 = 18.3
K2 = 0.0035
0.80 0.82 0.84 0.86 0.88 0.90 0.92
Figure 3. Comparison of the experimental data from the separate tests with the model predictions (reaction temperature = 800°C)
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Effects of Particle Size
Four sizes of poplar particles used to inves-
tigate the effects on gasification were 7/16-in.
(1.1 cm) diameter cylinders, 1/4-in. (0.64 cm)
diameter cylinders, 1/8-in. (0.32 cm) cubes
and 20 - 30 mesh sawdust. It was found
that the rate of the rapid reaction increased
significantly as a result of the rate in-
crease in heat transfer and intraparticle
diffusion when particle size was reduced
from 7/16 to 1/8 in. The sawdust showed
the highest conversion. Agglomeration dur-
ing gasification was observed for the pop-
lar particles 1/8 in. or larger, which re-
duced the gas diffusion within the par-
ticles.
The chars obtained by gasifying differ-
ent sizes of poplar particles were ana-
lyzed. Some of the volatile matter remained
in the residual chars after 30 min gasifica-
tion of 7/16-in. poplar particles. However,
nearly all of the hydrogen and oxygen in
1/8 in. poplar and sawdust were converted
in 20 min. The residual cooled, chars after
gasification are fragile and can be easily
ground to fines by attrition.
Effects ofBiomass Residence
Time
The experiments showed that the rapid
reaction stage for 1/8 in. poplar particles
could be essentially completed in less than
0.2 to 0.3 min, converting most of the
biomass into gas product. The conversion
contributed by the slow reaction is rela-
tively small and proceeds very slowly. At
30 atm and 800°C, 84% of 1/8-in. poplar
was converted by the rapid reaction, and
the slow reaction converted an additional
3% in 60 min.
To achieve high biomass conversion, suf-
ficient biomass residence time must be pro-
vided. It was found that an additional 10%
biomass conversion could be obtained when
the reaction time extended from 20 to 150
min. The composition comparison between
the charred samples after 20 and 150 min
gasification indicated that there was virtu-
ally no hydrogen and oxygen in the char
after 20 min. The additional conversion af-
ter 20 min was contributable to the carbon
reactions.
With an estimated residence time of
7.86 h for the bench-scale gasifier, it was
predicted that 94% of the total dry biom-
ass or 88% of the biomass carbon con-
tent can be converted in the Hynol gasifi-
cation.
Effects of Temperature
To investigate the effects of reaction
temperature, the experiments were con-
ducted at five temperature levels: 750,
800, 850, 900, and 950°C. In these tests
1/8-in. poplar particles were exposed to
the simulated Hynol feed gas at 30 atm
pressure. The gasification time was 60
min for these tests to identify the conver-
sion contribution of the slow reaction at
high temperatures. Conversion increased
as temperature was raised from 800 to
900 °C as shown in Figure 4. Reaction
temperature increases biomass conversion
by increasing the rate of the slow reac-
tion.
1.00'
0.95-
o
CD
CD
*±
CO
o
O
0.90-
0.85'
0.80
Sample: 1/8" Poplar cubes
Pressure: 30 atm
700 750 800 850
Temperature (°C )
900
950
Figure 4. Effect of reaction temperature on poplar conversion after 60 min
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The activation energies for the rapid
and slow reactions were estimated from
the temperature dependencies of their ini-
tial reaction rates. The activation energies
determined from the Arrhenius plots are
2.8 kcal/mol (11.7 kJ/mol) for the rapid
reaction and 33.4 kcal/mol (140 kJ/mol)
for the slow reaction. Very low activation
energy of the rapid reaction implies that
the overall reaction rates observed were
restricted by either the heat transfer or the
intraparticle diffusion. Since an initial tem-
perature drop was always observed in the
TBR testing, it is suggested that the heat
transfer rate is slower than diffusion rate
during the rapid reaction stage. There-
fore, in the TBR the thermal decomposi-
tion rate is dominated by heat transfer.
Effects of Feed Gas
Composition
The effect of feed gas composition was
investigated by varying the flow rates of
individual gas components under constant
system pressure. Helium was used as an
inert "makeup" gas for this purpose. The
60-min gasification tests of 1/8-in poplar
particles at 30 atm and 800°C showed
that conversion under pure helium is about
6% lower than that under pure hydrogen.
The gasification conversion was pro-
portional to the partial pressures of hydro-
gen and steam in the feed gas as shown
in Figures 5 and 6. The hydrogen in the
feed gas increases biomass conversion
by promoting both the rapid and slow re-
£ 0.86
't> 0.84
0.82
1/8" Poplar cubes
800 °C
O
O 100% He and 30 atm
D Hynol feed gas and 30 atm
A 100% H 2 and 30 atm
V Hynol feed gas and 50 atm
O 65.83% H2 and 34.17% He
10
15
20
25
30
H2 pressure (atm)
Figure 5. Effect of hydrogen partial pressure on conversion after 60 min
0.90
0.89
0.88
0.87
0.86
O 0.85
0.84
0.83
1/8" Poplar cubes
30 atm and 800°C
H2 = 65.8%, CO = 9%, C02 = 2.3%
I
23456
Steam partial pressure (atm)
actions, while the steam in the feed gas
mostly affects the rapid reaction.
Within the range of experimental condi-
tions used, compositional changes in CH4,
CO, and CO2 in the feed gas showed
negligible effects on biomass conversion.
Effects of Catalysts
The catalytic effects of potassium car-
bonate (K2CO3) on poplar wood gasifica-
tion were investigated. Catalyst was de-
posited on the poplar samples by evapo-
ration from solution at 105°C. The gasifi-
cation experiments were conducted with
Figure 6. Effect of steam concentration on poplar wood conversion after 60 min
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both 1/8-in. poplar particles and sawdust
at 30 atm and 800°C for 60 min. The
experimental results were interpreted in
Figure 7 where the carbon conversion data
were calculated based on the actual car-
bon loss in the sample analyzed after
gasification. Statistical calculations were
conducted using student's t to determine
the catalytic effects on carbon conversion.
It was found that, with a 95% confidence
level, the minimum increases in carbon con-
version by KjCOj were 4.5% for 1/8-in.
poplar particles, and 4.2% for sawdust. The
charred samples after gasification with
K2CO3 catalyst were soft, and no agglom-
eration was observed during gasification.
Reactivity of Pressed
Switchgrass
The gasification at 30 atm and 800°C
for 60 min showed that 81-82% of pressed
switchgrass could be converted into a gas
product approximately equivalent to a car-
bon conversion of 62%. Under the same
gasification conditions, the biomass con-
version for 1/8-in. poplar particles is 87%.
The reasons for lower conversion obtained
with pressed switchgrass are not clear
and need further study.
100
o
CD
CD
*±
CO
c
g
'
o
o
o
_a
CO
O
95-
90 -
85-
O
80 -
75-
30 atm and 800°C
Hynol feed gas
O Saw dust
D 1/8" particles
O
O
O
D
D
O
70
34
K2CO3 (
Figure 7. Effect of K2CO3 catalyst on carbon conversion of poplar hydrogasification
-------
Yuanji Dong and Edward Cole are with Acurex Environmental Corp., Research
Triangle Park, NC 27709.
Robert H. Borgwardt is the EPA Project Officer (see below).
The complete report, entitled "Evaluation ofBiomass Reactivity in Hydrogasification
for the Hynol Process," (Order No. PB96-187638; Cost: $44.00, 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:
Air Pollution Prevention and Control Division
National Risk Management Research Laboratory
U. S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection Agency
Center for Environmental Research Information (G-72)
Cincinnati, OH 45268
Official Business
Penalty for Private Use $300
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT NO. G-35
EPA/600/SR-96/071
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