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


4/6

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

fuel bound nitrogen research 2

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