fuel bound nitrogen research 2
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RELEASE OF FUEL BOUND NITROGEN IN BIOMASS DURING HIGH
TEMPERATURE PYROLYSIS AND GASIFICATION
Jiachun Zhou
Mechanical Engineering Department
Universityof Hawaii at Manoa
2540 Dole Street
Honolulu, Hawa ii 96822
Phone: (808)956-2342; Fax: (808)956-2335
Stephen M. Mas utanil, Darren M. ishimura,
Scott
Q.
Turn, Charles M. Kinoshita
Haw aii Natural Energy Institute
University of Hawaii at Manoa
2540 Dole Street
Honolulu, Hawaii 96822
ABSTRACT
Pyrolysis and gasification of two biomass feedstocks with
significantly different fuel-bound nitrogen (FBN) content were
investigated to determine the effect of operating conditions on
the partitioning of FB N among gas species. Experiments w ere
performed in a bench-scale, indirectly-heated, fluidized bed
reactor. Data were obtained over a range of temperatures and
equivalence ratios representative of commercial biomass gasi-
fication processes. An assay of all major nitrogenous comp o-
nents of the gasification products was performed for the first
time, providing a clear accounting of the evolution of FBN.
Results indicate that: (1)
NH3
is the dom inant nitrogenous gas
species produced during pyrolysis of biomass; 2) the majority
of FBN is converted to NH3 or N2 during gasification; relative
levels of NH3 and N2 are determined by thermochemical reac-
tions which are affected strongly by temperature; (3) N2
appears to be produced from NH 3 in the gas phase.
INTRODUCTION
During pyrolysis and gasi f ica t ion of b iomass fue ls ,
nitrogenous com pound s, such as ammonia (NH3), hydrogen
cyanid e (HCN), and oxides of nitrogen (NO NO2 or NO,;
N 20 ) may evolve from fuel-bound nitrogen
(FBN).
These gas-
phase pollutants pass through end-use systems with the
synthesis gas, where they can poison catalysts or may undergo
further oxidization and be emitted as NO,, which is the primary
contributor to photo chem ical smog. Although research on
biomass gasification has been pursued for many years, to date
only a few studies have been conducted on the formation,
deposi t ion , and aba tement of n i t rogenous pol lu tants .
Additional effort in this area is warranted given the current
Gasification Combined Cycle) power systems and liquid fuel
syn the s i s .
interest in utilizing biomass gas for
IGCC
(Integrated
'author
to
whom correspondence should be
addressed
Although fuel nitrogen chemistry in coal combustion and
gasification systems has been extensively investigated, it is
unclear whether these results can be applied to biomass, since
nitrogen is bound in different forms in these two solid fuels.
Earlier work suggests that fuel structure significantly
influences FBN evolution.
The format ion of n i t rogen-conta in ing spec ies dur ing
biomass gasification ha:: been investigated by several
researchers ( Ish imura e al. 1994; Furman et
al.
1992;
Leppalahti, 1993; Evans
ef
al. 1988). Thes e studies identified
NH3, HCN, and N2 as the major nitrogenous components of the
synthesis gas and documented effects of varying gasification
conditions on their concenlrations . In all of these studies, N2
levels were inferred from a nitrogen balance rather than
measured d i rec t ly . Hence , som e uncer ta in ty remains
concerning the partitioning of FBN.
Reaction pathways that have been proposed for biomass
FBN evolution are largely based on models developed for coal
combustion and gasification. The extent to which these
models apply is unclear due to the aforementioned differences
in fuel structure.
In order to clarify the mechanisms by which biomass FBN is
released and converted during gasification and pyrolysis, an
investigation was initiated comprising experimental and
mode l ing compone n t s . Th i s pa pe r summ a r i z e s t he
experiments. Results of the modeling study are provided in
Zhou
t
al. (1997).
PROCEDURES
A series of biomass gasification and pyrolysis experiments
were performed. In the p:yrolysis tests, no oxidizing gas was
supplied to the reactor; however, oxidation reactions occurred
due to fuel-bound oxygen and moisture contained in the
biomass.
Experiments were performed in a bench-scale fluidized bed
reactor. Th e reactor consists of an 89 mm i.d. stainless steel
pipe enclosed within a stack of electric heaters that allow
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uniform temperatures to be maintained in the fluidized bed.
0.21-0.42 mm diameter Alum ina beads comprise the bed
which has a static height of about 700 mm. Biomass is fed
into the reactor from a sealed hopper with an auger-type screw
feeder. Gases pass through a high temperatur e char filter
before entering the sampling system . Details of the facility
are given in Zhou (1994) and Ishimura (1994).
The m ajor nitr ogen ous species, NH,, N2, HCN , and NO,,
were quantified either on-line or by off-line analysis of
extracted gas samples. A gas chromatograph (GC), ion-
specific electrodes (ISE), and a chemiluminescence analyzer
(CLA) comprised the principal instrumentation employed in
this study.
A nitrogen species gas sampling system was installed
downstream of the sintered metal char filter. NH3 and HCN
were collected by absorption into liquid solutions and
measured with ISEs. The NH3/HCN samp ling train consisted of
four bubblers arranged in series. Th e first two bubblers were
filled with 15 0 ml of 0.1-M su lfuric acid (H2S0 4) used to trap
NH,. Th e remaining bubblers were filled with a mixture of 145
ml of 0.1-M sodium hydroxide (NaOH) and 5 ml of 0.117-M
lead acetate trihyd rate (Pb Ace03 H20) which reacts with and
absorbs HCN.
Downstream
of
the bubblers, a small slipstream of the
biomass synthesis gas was directed into a CLA to detect and
quantify NO,. The CLA was calibrated before each experiment
with two certified EPA Protocol gas mixtures containing 9 and
90
ppm NO in N2. The CLA was recalibrated after each test run
to assess instrument drift . Since the accuracy of CLA
measurements of NO may be affected by other species present
in the synthesis gas mixture, (e.g., H2 and CO) a correction
recommended by Matthews
et al.
(1977) was applied to the raw
data.
A second sampling train in parallel with the NH,/HCN
bubblers was used to collect synthesis gas samples and
nitrogenous tars. Gases were analyzed with a Perkin-Elm er
Auto System GC equipped with a thermal conductivity detector
(TCD). A stainless steel packed column (12.2 m x 3.2 mm)
from Alltech Associates, Inc. was em ployed to separate the
adjacent Ar, 0 2 , and N2 chromato gram peaks. Analyses were
conducted at low chamber temperatures and carrier gas flow
rates. A PE Nelson Model 1020 Personal Integrator interfaced
with the GC was used to determine gas concentrations from the
chrom atogram s (manual integration w as sometime s used to
infer 0 2 concentrations). The GC was calibrated with two gas
mixture standards that had compositions similar to the
biomass gas.
Difficulty in quantifying N2 to date has prevented a
comprehensive inventory of FBN products. In the present
system, contamination of samp les by ambient air posed the
greatest problem (the tests used an O;?/Ar mixture, ra ther than
air, to gasify the biomass). Since the gasifier was operated at
positive internal pressures and the fuel hopper was sealed, the
possibility
of
tramp air leaking into the gasifier is negligible.
Any contamination therefore arose from air leaks into the
collection bulbs and vials, or the
G
during sampling and
analysis. Fortunately,
air
contamination
of
this type can be
identified via its O2 content.
Since gasification occurs with a deficiency of oxidizer, both
experiments and simulations indicate that residual O2 levels in
the products are negligibly low. 0 detected by the GC
analysis may then be attributed to air contam ination. The
known N2 /0 2 ratio in the air can be applied to quantify the
'contaminant' Nz from the 0 2 measurements. This amount may
then be subtracted from the total N2 detected with the GC to
estimate N2 from FBN.
Two types of biomass, leucaena and sawdust, containing
significantly different amounts of FBN, were gasified and
pyrolized in these experim ents. Leucae na, a fast-grow ing
nitrogen-fixing plant, is being considered as a potential
dedicated energy crop (Hubbard
&
Kinoshita, 1993). Th e low-
FBN sawdust which was used consisted of a mixture of several
hardwood species (e.g., fir, poplar, oak ash). Proxim ate and
ultimate analyses of these feedstocks are provided inTable 1.
TABLE
1
ANALYSES OF FEEDSTOCKS
Leucaena
Proximate A nalysis,
Mo i s tu re 10 .4
Volatile Matter 74 .28
Fixed Carbon 18 .54
A sh 7 .18
Ultimate Analysis, (dry basis)
[C]: Carbon 48. 43
[HI: Hydrogen 5. 64
[O]: Oxygen 36.02
[SI: Sulfur 0.22
[N]: Nitrogen 2.5 1
A sh 7 .18
Sawdust
7 . 6 8
7 7 . 7 0
1 4 . 2 8
0 . 3 4
4 8 . 4 5
5 . 1 1
4 6 . 0 1
0 . 0 3
< 0.1
0 . 3 7
The leucaena, which consisted of leaves and small branches,
was harvested and exposed to air for several day s to reduce its
moisture content. Both sawdust and leucaena feedstocks were
milled to yield particles less than 3 mm in size.
Parametric tests were performed to investigate the effects of
operating parameters on FBN evolution in pyrolysis and gasi-
fication. Parameters that were varied included bed temperature
and equivalence ratio (ER). ER is defined as the actual oxidizer-
to-fuel ratio (mass basis) divided by the stoichiometric
oxidizer-to-fuel ratio.
ER
ranged from 0.18 to 0.40 in the
gasification tests. Bed temperatures between 700" to 950"C,
which are representative of commercial gasifiers (Wang,
1991), were investigated. No steam was injected during any of
the tests although the biomass feedstocks contained small
amounts of moisture. Biomass feed rate was about 3 kg/h.
In the pyrolysis tests, the bed was fluidized by injection of
pure argon. Low biomass feed rates (0.73 to 0.87 kg/h) and
high argon flow rates were employed to limit reactions
between char and gas species and minimize their effect on the
gas species concentration data.
EXPERIMENTAL RE SULTS AND DISCUSSION
P v r o l v s i s
Tests
Te s t s w e re pe r fo rme d to de t e rmine the r e l a t i ve
concentrations of gas-phase nitrogen species produced by
pyrolysis of biomass, and to investigate the influence of
temperature on this process. Species concentrations, on an
inert-free basis, are plotted against temperature in Figure 1 .
These results are for leucana feedstock.
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TABLE 2 FUEL NITROGEN PARTITIONING DURING
PYROLYSIS
OF
LEUCAENA
u .
700
750 800 850
9
TP-3
(a) NH3 concentration vs. temperature.
2oOT
7 75 800 850 900
(b) NO and HCN concentration vs. temperature.
T C)
FIGURE
1
NITROGENOUS SPECIES CONCENTRATIONS
IN PYROLYSIS GAS (LEUCAENA)
As pyrolysis temperature increases from 700C to 900"C,
NH3 levels are observed to decline by a factor of six from about
48,000 ppmV to 8,000 ppmV. HCN and NO concentrations
also fall (from around 20 ppmV to less than 10 ppmV for HCN
and from 1 70 ppmV to around 40 ppmV for NO), albeit less
drastically. NH3 was detected at much higher concentrations
than the other two nitrogen species.
These results indicate
that NH3 is the dominant nitrogenous pyrolysis product and
that levels of the three nitrogen pollutants decrease with
pyrolysis temperature.
The measured partitioning of FBN is given in Table 2.
Pyrolysis reactions convert less than 1% of the fuel nitrogen
into HCN and N O at the conditions examined.
As
temperature
increases from 700C to 9OO"C, the fraction of FBN remaining
in the char decreases from about
50
to 41%.
Most of the fuel
n i t rogen appa ren t ly evo lves a s NH3 and N2, with
decomposition of
NH3
being a probable source of the
N2
(Ishimura, 1994).
T C 7 0 0 '750 8 0 0 8 5 0 9 0 0
N(NOx)/Nf% 0.19
0 . 1 9
0 . 1 4 0 . 1 0
0 . 0 8
N(NH3)/Nf,%
5 4 . 1 3 4 8 . 7 4
2 5 . 8 1 1 3 . 4 9
1 0 . 4 8
N(HCN)/Nf% 0.02
0.03 0.01
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35 T
NH3 N2
75
800 850
900
950
TW
FIGURE
3
VARIATION OF NH3 N2 CONCENTRATIONS
WITH TEMPERATURE (LEUCAENA; ER=0.25)
Measured NH3 and N2 concentrations in the gasified leucaena
are plotted against temperature in Figure 3. NH3 decreases
sharply from approximately 31,000 ppmV at 750C to 6,000
ppmV at 900C. A slight increase in NH3 is observed at
950C. Over the same temperature range, molecular nitrogen
(N2 ) generally increases with temperature (9,500 ppmV to
17,000 ppmV); however, the data suggest that a small decrease
in Nz may occur between 900C and 950C. Th e high levels of
NH3 in the product gas are consistent with the results of the
pyrolysis tests. Since NH3 and z exhibit opposite trends in
response to changes in temperature, there may be a basis to
propose that conversion of NH3 to N2 is the critical thermo-
chemical path in the evolution
of
FBN during gasification of
b ioma ss .
Concentrations of NO and HCN are plotted against gasifier
bed temperature in Figure 4. NO decreases from 30 ppmV at
750C to 5 ppmV at 95OOC; HCN concentration falls from
55
ppmV at 750C to 30 ppmV at 950C. The measurements
indicate that these two species exist in the synthesis gas at
levels two to three orders of magnitude smaller than NH3.
A HCN NO
O 40
2
8
3
2
I I
I
750
800
85
900 95
T
( C)
FIGURE
4
VARIATION OF NO HCN CONCENTRATIONS
WITH TEMPERATURE (LEUCAENA; ER=0.25)
The distribution of leucaena FBN among the nitrogenous
species NH,, N2, NO, and HCN for
ER
= 0.25 (7 50 T to 950C)
is presented in Table 3. Mo st of this nitrogen forms NH3 and
N2; less than 1 of the FBN is detected as HCN and NO, The
strong influence of temperature is apparent from the data:
between 750C and 950"C, the fraction of FBN present as NH3
in the synthesis gas decreases from approximately 60% to
10%.
Over this same temperature range, fu el nitrogen existing
as
N
increases from
40
to about 85%. It also is interesting
to note that the slight increase in NH3 me asured between
900C and 950C exactly compensates for the corresponding
decrease in N2.
TABLE3 FUEL NITROGEN PARTITIONING AS A
FUNCTIONOFTEMPERATURE (LEUCAENA; ER=0.25)
T C 7 5 0 8 0 0 8 5 0 9 0 0 9 5 0
N(NOx)/Nf,% 0.06
0.04
0 . 0 2 0.02
0.01
N(NH3)/Nf,%
6 3 . 5 48.74
2 5 . 8 1 1 3 . 4 9 1 0 . 4 8
N(HCN)/Nf,% 0.11
0.09
0 . 0 8 0 . 0 7 0.07
N(char)/Nf,%
7.7
5 . 2 2 . 0 2 . 0 1.2
N(N2)/Nf,% 38.6
6 9 . 9 8 0 . 3 8 8 . 7
8 5 . 7
Concentration data from the sawdust gasification tests are
plotted in Figure
5.
NH3 is observed to decrease from 950
ppmV at 700C to about
400
ppmV at 900C. NO and HCN are
again detected at much lower levels than NH3. Wh ile the
general trends agree with the leucaena results, differences in
the FBN content of the two feedstocks once again are
manifested in the large difference in absolute values of
concentrations of nitrogenous species. The exception
to
this
is NO, which was detected at higher levels in the gasified
sawdust. This behavior is not presently understood and
warrants additional study.
1000
8 800
8
6
8
400
3
NH3
A HCN
NO
7 750
800
850 900
T PC
FIGURE 5 NITROGENOUS SPECIES CONCENTRATIONS
VERSUS TEMPERATURE (SAWDUST, ER=0.25)
As mentioned previously, this study is the first to report a
comprehensive inventory of FBN for biomass gasification.
The present m easurem ents of N2 confirm that this species
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accounts for the large quantities of fuel nitrogen denoted as
" miss ing" i n p rev ious i nve s t iga t ions . A l though the
possibili ty of contamination of samples with ambient air
introduces a level of uncertainty into the data, relatively good
closure was attained in the nitrogen balance which ranged from
about 98%
to
120% for the leucaena experiments. On the other
hand, the carbon balance for the same tests fell between 94%
and 98%. Th is suggests that further refinement of the nitrogen
spe c i e s me a su re me n t sy s t e ms a nd p ro toc o l s ma y be
worthwhile pursuing.
Theoretical calculations have determined that the optimum
range of equivalence ratios for biomass gasification is 0.2 to
0.4 (Wa ng, 1991). Tests were performed at three values of ER
within this range to examine the effect of this parameter on
Fl3N chemis try. It was discovered that, unlike temperature, ER
does not significantly impact concentrations of nitrogenous
species in the synthesis gas. Figures 6 and 7 summarize the
influence of both ER and temperature on the amounts of NH3
and N2 produced when leucaena is gasified.
30000
a
25000L\^ ^ ^ ^
A ER=0.18
ER=0.25
ERd.32
750 800 850 900 950
TW)
FIGURE6 NH3 CONCENTRATION AS A FUNCTION OF
TEMPERATURE AND ER (LEUCAENA)
20000
15000
e
5
10000
0
ER=O.18
ER=0.25
5000]
~
,
R=O.32
750
800 850 900 950
T
C)
FIGURE7 N2 CONCENTRATION AS A FUNCTION OF
TEMPERATURE AND ER (LEUCAENA)
At temperatures in excesfs of 800C , NH3 conce ntrations
measured at three values of ER are comparable. Th e Nz results,
presented in Figure
7,
appear to be more sensitive to ER . This
may, however, simply reflect the greater uncertainty in these
data.
Figure 8plots the variation of NO and H C N with ER at
800C. Higher ER appears to favor lower concentrations of
both of these species. These changes in NO and HCN levels,
however, do not significantly impact the partitioning of FBN
since NO and HCN account for a very small fraction of the fuel
nitrogen
(4 ).
0.1
0.2
0.3
0.4
ER
FIGURE8 VARIATION OF NO AND HCN WITH ER
AT 8 C (LEUCAENA)
The modest inf lu ence of ER o n concentra tions of
nitrogenous species is supported by the results of the sawdust
gasification experiments. A:< seen in Figure 9 , a change in ER
from 0.25 to
0.37
only induces NH3 concentrations to increase
from 310 ppmV to 350 ppmV. Th e single data point at ER
=
0.18 suggests that NH3 may be affected strongly by this
parameter below ER
=
0.25. No experiments were performed at
ER