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Ind. Eng. Chem. Res. 2000, 39, 626-634
Release of Fuel-Bound Nitrogen during Biomass Gasification Jiachun Zhou,† Stephen M. Masutani,*,‡ Darren M. Ishimura,‡ Scott Q. Turn,†,‡ and Charles M. Kinoshita†,‡ Hawaii Natural Energy Institute, and Biosystems Engineering Department, University of Hawaii at Manoa, Honolulu, Hawaii 96822
Gasification of four biomass feedstocks (leucaena, sawdust, bagasse, and banagrass) with significantly different fuel-bound nitrogen (FBN) content was investigated to determine the effects of operational parameters and nitrogen content of biomass on the partitioning of FBN among nitrogenous gas species. Experiments were performed using a bench-scale, indirectly heated, fluidized-bed gasifier. Data were obtained over a range of temperatures and equivalence ratios representative of commercial biomass gasification processes. An assay of all major nitrogenous components in the gasification products was performed for the first time, providing a clear accounting of the evolution of FBN. Important findings of this research include the following: (1) NH3 and N2 are the dominant species evolved from fuel nitrogen during biomass gasification; >90% of FBN in feedstock is converted to NH3 and N2; (2) relative levels of NH3 and N2 are determined by thermochemical reactions in the gasifier; these reactions are affected strongly by temperature; (3) N2 appears to be primarily produced through the conversion of NH3 in the gas phase; (4) the structural formula and content of fuel nitrogen in biomass feedstock significantly affect the formation and evolution of nitrogen species during biomass gasification. Introduction Biomass gasification is an attractive technology to convert biomass fuels to gaseous products which can be used in power generation and alternative transportation fuel production. During gasification of biomass fuels, nitrogenous compounds, such as ammonia (NH3), hydrogen cyanide (HCN), and oxides of nitrogen (NO + NO2 or NOx; N2O), may be produced from fuel-bound nitrogen (FBN) in biomass feedstock. These gas-phase species pass through end-use systems of the product gas, where they can poison catalysts or may undergo further oxidization and be emitted as NOx, which is the primary contributor to photochemical smog. Although research on biomass gasification has been pursued for many years, to date only a few studies have been conducted on the associated formation, deposition, and abatement of nitrogenous pollutants. Additional effort in this area is warranted given the current interest in utilizing biomass gas for IGCC (integrated gasification combined cycle) power systems and liquid fuel synthesis. It is well recognized that a major source of NOx from coal combustion and gasification is the conversion of fuel nitrogen in the coal. Conversion of volatile nitrogenous species is the primary contributor to fuel NOx emissions.1 These volatile NOx precursors include HCN and NH3.2,3 Other nitrogenous species, such as NOx and N2, are also produced during pyrolysis and combustion of coal.4 Although coal nitrogen emission studies have provided insight into FBN evolution, it is unclear whether those results can directly be applied to biomass, because nitrogen is bound in different forms in the two solid fuels; earlier work suggests that the fuel structure significantly influences FBN evolution.3-5 Nitrogen evolution during thermochemical conversion * To whom correspondence should be addressed. E-mail:
[email protected]. † Biosystems Engineering Department. ‡ Hawaii Natural Energy Institute.
of biomass fuels has not been investigated extensively. Several researchers have studied the formation of nitrogen-containing species during biomass gasification.5-8 These studies identified NH3, HCN, and N2 as the major nitrogenous components of the product gas and documented effects of varying gasification conditions on their concentrations. In these earlier studies, however, N2 concentrations were inferred from a nitrogen balance, rather than being measured directly. A level of uncertainty therefore remains concerning the partitioning of FBN. While previous investigations have provided important information on FBN emission during biomass gasification, results from different studies vary considerably, and the mechanisms for the formation and evolution of nitrogenous species are still unclear. The observed differences may reflect differences in test conditions and feedstocks. The present study attempts to clarify the effects of gasification conditions and fuel on the release and evolution of biomass FBN through parallel experiments utilizing four different biomass feedstocks having significantly different FBN contents. Facility All tests in the present study were performed by employing a bench-scale fluidized-bed gasification system (Figure 1), which includes an indirectly heated fluidized-bed gasifier, a screw biomass feeder, and a hot gas char filter. The reactor consists of an 89 mm i.d. stainless steel pipe enclosed within a stack of electric heaters that allow uniform temperatures to be maintained in the fluidized bed. Alumina beads (0.21-0.42 mm diameter) 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. Oxygen and argon are injected into the gasifier to serve as an oxidizer and a tracer gas, respectively. Product gases exiting the gasifier pass through a high-temperature char filter before entering the sampling system.
10.1021/ie980318o CCC: $19.00 © 2000 American Chemical Society Published on Web 01/29/2000
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Figure 1. Gasification system and sampling train.
The major nitrogenous species, NH3, N2, HCN, and NOx, 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) were the principal instruments employed in this study.
A nitrogen species gas sampling system, shown in Figure 1, was installed downstream of the sinteredmetal char filter. NH3 and HCN were collected by absorption into liquid solutions and measured with ISEs. The NH3/HCN sampling train consisted of four bubblers arranged in series. The first two bubblers were
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Table 1. Feedstock Properties (Dry Basis)
a
process method
leucaena
sawdust
moisture volatile matter fixed carbon ash
10.40 74.28 18.54 7.18
7.68 84.16 15.47 0.37
[C]: [H]: [O]: [S]: [N]: ash
48.43 5.64 36.02 0.22 2.51 7.18
48.45 5.11 46.01 0.03 0.03 0.37
carbon hydrogen oxygenb sulfur nitrogen
Banagrass1 JC-PRPa
Banagrass2 FC-PRPa
Banagrass3 FC-Pa
Proximate Analysis, % 6.40 79.25 14.92 5.83
11.36 80.55 15.7 3.75
7.07 81.52 15.48 3.00
7.07 79.45 16.48 4.07
Ultimate Analysis, % 46.27 5.27 42.41 0.05 0.12 5.83
47.04 5.11 43.81 0.04 0.22 3.75
47.39 5.24 43.76 0.14 0.36 3.00
46.93 5.09 43.01 0.14 0.44 4.07
bagasse
These designators refer to the different feedstock treatments.11
filled with 150 mL of 0.1 M sulfuric acid (H2SO4) used to trap NH3. The 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 trihydrate (PbAce‚ 3H2O), which reacts with and absorbs HCN. Downstream of the bubblers, a small slipstream of the biomass product gas was directed into a CLA to detect and quantify NOx. 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. Because the accuracy of CLA measurements of NO may be affected by other species present in the product gas mixture (e.g., H2 and CO), a correction recommended by Matthews et al.9 was applied to the raw data. A second sampling train in parallel with the NH3/ HCN bubblers was used to collect product gas samples. Gases were analyzed with a Perkin-Elmer Auto System GC equipped with a thermal conductivity detector (TCD). A stainless steel packed column (12.2 m × 3.2 mm) from Alltech Associates, Inc., was employed to separate the adjacent Ar, O2, and N2 chromatogram 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 chromatograms (manual integration was sometimes used to infer O2 concentrations). The GC was calibrated with two gas mixture standards that had compositions similar to the biomass gas. Although the GC was capable of measuring N2 in the collected gas samples to within 2%, contamination of the samples by ambient air posed a problem (note that the present tests used an O2/Ar mixture, rather than air, to gasify the biomass). Because 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 into the GC during sampling and analysis. Fortunately, air contamination of this type can be identified via its O2 content. Because gasification occurs with a deficiency of oxidizer, both experiments and simulations indicate that residual O2 levels in the products are negligibly low. Any O2 detected by the GC may then be attributed to air contamination. The known N2/O2 ratio in the air can be applied to quantify the “contaminant” N2 from the O2 measurements. This amount may then be subtracted from the total N2 detected with the GC to estimate N2 formed from FBN.
b
Oxygen: determined by the difference method.
Overview of Experiments Four types of biomass feedstocks were used in the experimentssleucaena, sawdust, bagasse, and banagrass. Leucaena (Leucaena leucocephala), a fast-growing plant and a potential biomass energy resource,10 has a high nitrogen concentration compared to the other feedstocks. The leucaena feedstock, consisting primarily of leaves and small branches, was harvested from leucaena trees and exposed to air for several days to reduce the moisture content in the raw material. The low-FBN sawdust consisted of a mixture of several hardwood and softwood species (e.g., fir, poplar, oak, ash). Banagrass (Pennisetum Purpureum), a fast-growing tropical grass, is being considered as a possible short-rotation, intensive-culture energy crop.10 In some of the tests reported herein, freshly harvested banagrass was treated using different processing steps to remove or to reduce various inorganic components. These treatments attempt to minimize negative impacts of inorganics on the gasification process and some end-use technologies (e.g., combustion in gas turbines). The FBN content was determined also to depend on the treatment. Details of the treatments and their effects have been documented by Turn et al.11 All feedstocks were milled using the same machine to yield particle sizes generally (>95%) less than 3 mm. Proximate and ultimate analyses of the feedstocks are given in Table 1. A screw feeder was employed to transport the feedstock from a small hopper into the gasifier. The housing of the feeder screw between the feeder and gasifier was water-cooled to minimize thermochemical decomposition of biomass in the screw prior to entering the gasifier. The feeding rate was determined via calibration. Parametric tests were performed to investigate the effects of operating parameters on fuel-bound nitrogen evolution during gasification. The parameters that were varied were bed temperature and equivalence ratio (ER). ER is defined as the actual oxidizer-to-fuel ratio (mass basis) divided by the stoichiometric oxidizer-tofuel ratio. ER ranged from 0.18 to 0.40 in the tests. Bed temperatures between 700 and 950 °C, which are representative of commercial gasifiers,12 were investigated. Each test was conducted at a fixed equivalence ratio with pure oxygen as the oxidizer. Argon was employed as a tracer gas, and no steam was injected although the biomass feedstocks contained small amounts of moisture. All data were obtained after steady-state operation had been attained. The criterion for steady-state operation in this study was stabilization of the temperatures
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Figure 3. NH3 and N2 concentrations vs temperature (ER ) 0.25, leucaena). Figure 2. Major gas species composition vs temperature (ER ) 0.25, leucaena).
in the gasifier. Because pressure in the gasifier typically exceeds atmospheric pressure and varies slightly over time as the outlet filter resistance increases, parameters such as the residence time were adjusted to account for pressure variations. Mass balance calculations for major elements, O, H, C, and N, were performed for all parametric tests. Temperature strongly affects chemical reactions in the gasifier. Because a uniform temperature distribution greatly facilitates interpretation of the data, the heaters and the heater control were configured to obtain a uniform temperature distribution within the fluidizedbed section. Within the bed, temperature varied by less than 15 °C during steady-state operation.13 Maximum departures from the set-point temperature were less than 4%; the largest departures occur at a location near the biomass inlet. At the inlet, rapid pyrolysis and exothermic oxidation reactions which occur when the fresh biomass contacts oxygen-rich gas entering from the distributor elevate temperature about the set point. Results and Discussion Effect of Temperature. Tests were conducted employing leucaena and sawdust feedstocks. Gasifier temperature was varied between 750 and 950 °C (leucaena) or between 700 and 900 °C (sawdust) at a constant equivalence ratio. Because trends were similar for all ERs, a single case at ER ) 0.25 will be discussed. Gas composition results for major species from leucaena, shown in Figure 2, indicate that both H2 and CO increase with increasing temperature. Between 750 and 950 °C, hydrogen increases from 26% to 33% (dry, inert free; DIF) and CO increases from 29% to 41% (DIF); CO2 decreases from 34% to 18% (DIF), and C2H2 decreases from 2% to 0.6% (DIF). The temperature does not significantly influence the concentration of CH4. The gas yield increases from about 1 to 1.37 m3/kg between 750 and 950 °C. Carbon and hydrogen balances performed for this test range from 94% to 98% and from 88% to 99%, respectively. The observed trends in gas composition and gas yield as functions of temperature agree
with the results of previous experimental studies14 and the theoretical analysis conducted by Wang.12 Measured NH3 and N2 concentrations in the leucaena product gas are plotted as functions of temperature in Figure 3. NH3 decreases sharply from 31 240 ppmV at 750 °C to 6060 ppmV at 900 °C. A slight increase in the NH3 concentration was observed at 950 °C. Over the same temperature range, molecular nitrogen (N2) generally increased with increasing temperature (from 9480 to 16 750 ppmV); however, a slight decrease in the N2 concentration was detected at 950 °C. The high levels of NH3 in the product gas suggest that it may be the major nitrogenous species formed when biomass is rapidly pyrolyzed after entering the gasifier. Because NH3 and N2 exhibit opposite trends as the temperature varies, there is a basis to propose that the conversion of NH3 to N2 is the dominant thermochemical process which determines the final fate of FBN in the gas phase under gasification conditions examined in the present study. To assess the relevance of equilibrium on the evolution of FBN, an equilibrium analysis of the biomass gasification process was performed in an earlier study using the STANJAN program.14 The results of the analysis indicated that the majority of FBN is converted to N2 at equilibrium under typical biomass gasification conditions (700-900 °C; ER 0.2-0.4). The absolute concentrations of nitrogenous species (including N2) that were predicted at equilibrium were observed to differ greatly from experimental data. For example, at 800 °C, the predicted and measured NH3 concentrations are about 10 and 18 000 ppmV, respectively. Equilibrium HCN levels are less than 4 ppmV versus measurement data indicating about 40 ppmV. These differences emphasize the importance of kinetics in the distribution of FBN among nitrogenous components of the gasified biomass. The present measurements of N2 support the contention by Ishimura14 and others that biomass FBN not detected as amines, cyanogens, NOx, or tars exists as N2. Although several sources of measurement uncertainty discussed above affect the nitrogen balance, relatively good closure of the N-atom inventory was attained in the present investigation, ranging from 105% to 116% after correcting for air leakage.
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Figure 4. NO and HCN concentrations vs temperature (ER ) 0.25, leucaena).
Figure 5. Nitrogen species concentration vs temperature (ER ) 0.25, sawdust).
Table 2. Distribution of Fuel Nitrogen as a Function of Temperature (Leucaena, ER ) 0.25) T, °C N(NOx)/Nfuel, % N(NH3)/Nfuel, % N(HCN)/Nfuel, % N(char)/Nfuel, % N(N2)/Nfuel, %
750
800
850
900
950
0.06 63.5 0.11 7.7 38.6
0.04 48.74 0.09 5.2 69.9
0.02 25.81 0.08 2.0 80.3
0.02 13.49 0.07 2.0 88.7
0.01 10.48 0.07 1.2 85.7
NO concentrations measured with the CLA and HCN concentrations determined by the ISE method are plotted versus the nominal gasification temperature in Figure 4. The nitric oxide concentration decreases from 30 ppmV at 750 °C to 5 ppmV at 950 °C, and HCN concentrations fall from 55 ppmV at 750 °C to 30 ppmV at 950 °C. The NO and HCN concentrations are 2-3 orders of magnitude lower than those of NH3 and N2. The distribution of FBN among the nitrogenous species NH3, N2, NO, and HCN for gasification of leucaena at ER ) 0.25 (750-950 °C) is given in Table 2. Most of the fuel nitrogen resides in NH3 and N2; less than 1% of the FBN is detected as HCN and NO. The strong influence of temperature is apparent from the datasbetween 750 and 950 °C, the fraction of FBN as NH3 in the product gas decreases from approximately 64% to 18%. Over this same temperature range, fuel nitrogen bound in N2 increases from 40% to about 85%. Nitrogen species concentrations for sawdust gasification are plotted in Figure 5. NH3 in the synthesis gas was observed to decrease from 950 ppmV at 700 °C to about 400 ppmV at 900 °C. NO and HCN are once again detected at much lower levels than NH3. The general trends in nitrogen species concentrations exhibited in the sawdust tests are similar to those recorded in leucaena tests; however, concentrations are lower because of the reduced FBN (2 orders of magnitude) in sawdust. The exception is NO, which was detected in greater amounts in the gasified sawdust. A possible explanation for this will be proposed when the effects of feedstock are discussed. Effect of Equivalence Ratio. Theoretical computations have determined that the optimum range of equivalence ratios for gasification of biomass is 0.20.4.12 Tests were performed at three values of equiva-
Figure 6. Major gas species composition vs ER (800 °C, leucaena).
lence ratio within this range to determine how ER impacts FBN chemistry. Inert-free product gas composition at 800 °C for leucaena gasification is plotted in Figure 6 as a function of the equivalence ratio. The data indicate that CO and H2 decrease, and CO2 increases, as the equivalence ratio increases. Concentrations of higher hydrocarbon compounds, which include methane, remain virtually constant. These trends are consistent with earlier predictions and experimental results.12,14 Unlike temperature, the equivalence ratio does not significantly impact the concentrations of nitrogenous species in gasified biomass. Figures 7 and 8 summarize the influences of both the equivalence ratio and temperature on the amounts of NH3 and N2 produced when leucaena is gasified. At temperatures in excess of 800 °C, NH3 concentrations measured for the three values of ER are comparable and do not show significant variation for values of ER between 0.18 and 0.32. The N2 concentration (Figure 8) appears to be more sensitive to ER. This may, however, simply reflect the greater uncertainty in the N2 data. The mass balance calculation results, presented in Table 3, indicate that the percentages of FBN existing as NH3 or N2 do not vary significantly with ER.
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Figure 9. NO and HCN concentrations vs ER (800 °C, leucaena). Figure 7. Influences of temperature and ER on the NH3 concentration (leucaena).
Figure 10. Nitrogenous species concentrations vs ER (800 °C, sawdust). Figure 8. Influences of temperature and ER on N2 concentration (leucaena). Table 3. Fuel Nitrogen Distribution in NH3 and N2 as Functions of ER and T (°C) (Leucaena) T, °C 750
800
850
ER ) 0.18 ER ) 0.25 ER ) 0.32
63.1 63.5 47.2
N(NH3)/Nfuel, % 50.5 27.6 40.5 24.0 44.7 27.6
ER ) 0.18 ER ) 0.25 ER ) 0.32
37.8 38.6 42.3
N(N2)/Nfuel, % 67.8 97.6 69.9 80.3 51.0 67.8
900
950
12.8 16.0 11.0
10.6 18.2 17.4
108.2 88.7 81.0
114.9 85.7 90.6
The dependence of NO and HCN concentrations on ER at 800 °C is shown in Figure 9. Higher ER promotes lower concentrations of these species in the product gas. This effect, however, does not significantly impact the partitioning of FBN because NO and HCN amount to only a very small fraction of the fuel nitrogen (
