Release of Inorganic Constituents from Leached Biomass during

860

Energy & Fuels 1999, 13, 860-870

Release of Inorganic Constituents from Leached Biomass during Thermal Conversion D. C. Dayton,*,† B. M. Jenkins,‡ S. Q. Turn,§ R. R. Bakker,‡ R. B. Williams,‡ D. Belle-Oudry,† and L. M. Hill† National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado 80401, Department of Biological and Agricultural Engineering, University of California, Davis, Davis, California 95616, and Hawaii Natural Energy Institute, University of Hawaii at Manoa, 2540 Dole Street, Honolulu, Hawaii 96822 Received November 19, 1998. Revised Manuscript Received March 9, 1999

Leaching of inorganic materials has recently been shown to substantially improve the combustion properties of biomass fuels, especially straw but including other herbaceous and woody fuels. Leaching with water removes large fractions of alkali metals (typically >80% of potassium and sodium) and chlorine (>90%). Smaller fractions of sulfur and phosphorus are also removed. Alkali metals are heavily involved in ash fouling and slagging in combustion and thermal gasification systems. Chlorine is a facilitator of alkali volatilization, and contributes to corrosion and air pollution. The presence of these elements has reduced or eliminated the use of certain biomass fuels in many combustion applications, even where such use might provide significant environmental benefits. Leaching could mitigate the undesirable effects of biomass ash in thermal systems. Reported here for the first time are comparative studies of volatile inorganic species evolving from leached and unleached biomass fuels during thermal conversion. Leached and unleached samples of rice straw, wheat straw, switchgrass, commercial power plant wood fuel, and banagrass (Pennisetum purpureum) were tested in bench-scale combustion studies using an alumina-tube flow reactor housed in a variable temperature furnace and coupled to a molecular beam mass spectrometer (MBMS) system. Sugarcane bagasse, as the leached byproduct of sugar production, was also tested. The MBMS system was used to monitor the combustion products, including inorganic vapors, directly and in real time during each batch combustion event. Total relative amounts of HCl(g), SO2(g), NaCl(g), KCl(g), and other species were compared for leached and unleached samples. The MBMS results were consistent with the levels of alkali metals and chlorine in the samples as determined from the proximate, ultimate, and ash analyses of the samples. The more alkali and chlorine in a given sample, the more gas-phase HCl, KCl, and NaCl detected with the MBMS during combustion of that particular sample. The MBMS results clearly support earlier results, which indicated that leaching biomass effectively reduces or eliminates the release of alkali metal vapors during combustion.

Introduction Biomass fuels offer substantial environmental benefits relevant to managing atmospheric carbon and global climate change. At the local level, proper use of biomass offers benefits in bioremediation and reductions in impacts associated with open burning, wild land fires, landfilling, soil erosion, drainwater management, etc. Although the economics of biomass energy are highly site specific, and in a restructured energy market the current direct costs of biomass power generation are frequently higher than power generated from natural gas and other fossil sources, the important environmental attributes of biomass fuels are likely to lead to increasing use in the future, both for electric power and for liquid fuels. Despite these environmental benefits, technical difficulties in the efficient conversion of bio* Corresponding author. Phone: (303) 384-6216. Fax: (303) 3846103. E-mail: [email protected]. † National Renewable Energy Laboratory. ‡ University of California, Davis. § University of Hawaii at Manoa.

mass remain. In the case of power generation from biomass via thermal conversion (including direct combustion, cofiring, gasification, and pyrolysis), the inorganic constituents of biomass often contribute to adverse impacts on reactors, furnaces, heat exchangers, turbines, emission control devices, and other equipment. Paramount among these are fouling, slagging, and (in fluidized beds) agglomeration of bed media, and the companion problem of corrosion. Processes that can mitigate these effects have been sought not just for biomass systems, but also for coal and other solid fuel facilities that suffer the same undesirable consequences of inorganic transformations at elevated temperatures.1-3 For biomass, key constraints in thermal conversion arise from necessary nutrient elements present natu(1) Raask, E. Mineral Impurities in Coal Combustion; Hemisphere: New York, 1985. (2) Bryers, R. W. Ash Deposits and Corrosion Due to Impurities in Combustion Gases; Hemisphere: Washington, DC, 1977 (and subsequent volumes). (3) Inorganic Transformations and Ash Deposition during Combustion; Benson, S. A., Ed.; Engineering Foundation: New York, 1992.

10.1021/ef980256e CCC: $18.00 © 1999 American Chemical Society Published on Web 04/28/1999

Release of Inorganic Constituents from Leached Biomass

rally in the plant material, including the macronutrients nitrogen and potassium.4 Fuel nitrogen is predominant in the production and emission of NOx; potassium is heavily involved in fireside fouling.5 Other nutrient elements, such as phosphorus, sulfur, calcium, and magnesium, are present in biomass. Inherent inorganic elements exist at relatively low levels in most forest land woods, but fertilization and irrigation of agricultural and herbaceous and short-rotation energy crop plants lead to higher levels of virtually all inorganic species. Chlorine is also generally found in elevated concentrations in annual biomass. Chlorine is a facilitator of alkali volatilization, and like sulfur, is an important contributor to corrosion, metal wastage, and pollution. Compounding the compositional difficulties are the high ash contents of annual species, especially the grasses, which in addition frequently take up and concentrate silica. Although sodium is generally present only in small amounts in most terrestrial biomass (and is toxic to most plants at higher concentrations), halophyte species that may be considered in drainwater management and other bioremediation activities specifically take up and incorporate substantial amounts of this alkali metal. Soil added during timber harvesting and other biomass handling activities adds to the ash loading and contributes to fouling and slagging. At the temperatures typical of thermal conversion, and especially at combustion temperatures, alkali and alkaline earth metals react to form sulfates, chlorides, silicates, hydroxides, and other compounds that participate in the formation of slags and fouling deposits. Fouling of heat exchangers is a critical limitation in the design and operation of biomass power plants. An initially clean heat exchanger tube, such as a superheater tube, will rapidly develop a layer of condensed alkali salt after biomass is first fired in the boiler. Subsequently, via inertial impaction, homogeneous and heterogeneous chemical reaction, and thermophoresis, a deposit layer will grow on the tube, typically forming an aerodynamic wedge on the upstream surface. The gas- and particle-phase compositions and the temperature control the rate at which the deposit grows. Composition and temperature also influence the tenacity and structure of the deposit. Removal or reduction of alkali metal in the fuel decreases the concentrations of both gas-phase and particle-phase alkali species in the furnace, decreases the rate of condensation and chemical reaction, and reduces the amount of liquid formation in particles and hence the rate of deposit accumulation through particle impaction. Although the principal mechanisms of fireside fouling and slag formation are reasonably well understood,6 efficacious means to combat these phenomena once the fuel has been introduced to the furnace or reactor are not generally available. Power plant operators currently rely principally on fuel selection, that is selecting fuels that contain low concentrations of ash, alkali metals, and chlorine, to avoid excess fouling in furnaces and boilers. This practice potentially excludes for power production many fuels that might be available at lower cost, and in some (4) Marschner, H. Mineral Nutrition of Higher Plants; Academic Press: London, 1986. (5) Jenkins, B. M.; Baxter, L. L.; Miles, T. R., Jr.; Miles, T. R. Fuel Process. Technol. 1998, 54, 17-46. (6) Baxter, L. L. Biomass Bioenergy 1993, 4 (2), 85-102.

Energy & Fuels, Vol. 13, No. 4, 1999 861

cases prevents the industry from using crop residues and other fuels that may improve local air quality and reduce land use impacts from open burning and landfilling. Fuel selection excludes undesirable fuel elements from the conversion system, but also denies the system the energy content of the fuel. Another way of excluding undesirable elements is to remove them from the fuel before firing. In some cases, simple screening can remove many undesirable components because they become concentrated in the fine material fraction (e.g., as adventitious soil or leaf fractions that have been pulverized in fuel handling). Where the undesirable constituents are inherent and dispersed throughout the fuel, screening alone offers small benefit. Many of the more important elements, including potassium and chlorine, are water soluble, however, and can largely be removed through leaching that can substantially improve the combustion properties of biomass fuels.7-16 Simple leaching with water removes large fractions of alkali metals (in most cases >80% of potassium and sodium) and chlorine (>90%). Smaller fractions of sulfur and phosphorus are also removed. The leaching of rice straw, for example, increases the ash fusion temperature by 300 °C or more, and by 500 °C for high-alkaline wheat straw.7 Similar improvements have been observed for switchgrass,8 and leaching even removes alkali metal from woody biomass. Leaching is one possible way to mitigate the undesirable effects of biomass ash in thermal systems by the simple technique of eliminating such elements from the fuel entering the furnace or reactor. Technical improvements in fuel properties caused by leaching have been amply demonstrated in laboratory-scale experiments.7,8,10-12 They have also been demonstrated for cereal straws at full scale in commercial boilers.13,16 The direct reduction from leaching in the evolution of volatile species during thermal conversion other than by simple total gravimetric analysis has not been determined, however. This study investigated the comparative release of inorganic materials from a number of biomass fuels both (7) Jenkins, B. M.; Bakker, R. R.; Wei, J. B. Biomass Bioenergy 1996, 10 (4), 177-200. (8) Jenkins, B. M.; Bakker, R. R.; Baxter, L. L.; Gilmer, J. H.; Wei, J. B. Combustion characteristics of leached biomass. In Developments in Thermochemical Biomass Conversion; Bridgwater, A. V., Boocock, D. G. B., Eds.; Blackie Academic and Professional: London, 1997; pp 1316-1330. (9) Bakker, R. R.; Jenkins, B. M. Feasibility of fuel leaching to reduce ash fouling in biomass combustion systems. In Biomass for Energy and the Environment; Chartier, P., Ed.; Pergamon: Oxford, 1997; pp 980985. (10) Turn, S. Q.; Kinoshita, C. M.; Ishimura, D. M. Biomass Bioenergy 1997, 12 (4), 241-252. (11) Jenkins, B. M.; Bakker, R. R.; Williams, R. B.; Baxter, L. L.; Turn, S. Q.; Thy, P.; Sime, M.; Lesher, C.; Sclippa, G.; Kinoshita, C. Measurements of the fouling and slagging characteristics of banagrass (Pennisetum purpureum) following aqueous extraction of inorganic constituents. In Making a Business from Biomass in Energy, Environment, Chemicals, Fibers and Materials; Overend, R. P., Chornet, E., Eds.; Pergamon: Oxford, 1997; pp 705-718. (12) Turn, S. Q.; Kinoshita, C. M.; Ishimura, D. M.; Zhou, J. Fuel 1998, 77 (3), 135-146. (13) Bakker, R. R.; Jenkins, B. M.; Williams, R. B.; Carlson, W.; Duffy, J.; Baxter, L. L.; Tiangco, V. M. Boiler performance and furnace deposition during a full scale test with leached biomass. In Making a Business from Biomass in Energy, Environment, Chemicals, Fibers and Materials; Overend, R. P., Chornet, E., Eds.; Pergamon: Oxford, 1997; pp 497-510. (14) Burvall, J. Biomass Bioenergy 1997, 12 (3), 149-154. (15) Hadders, G.; Olsson, R. Biomass Bioenergy 1997, 12 (3), 171175. (16) Sander, B. Biomass Bioenergy 1997, 12 (3), 177-183.

862 Energy & Fuels, Vol. 13, No. 4, 1999

Dayton et al. Table 1. Fuels and Treatmentsa

no.

fuel type

refa

1a

rice straw

8

1b

rice straw

8

2a 2b 3a 3b 4a 4b 5a 5b 5c

wheat straw wheat straw switchgrass switchgrass wood fuel wood fuel banagrass banagrass banagrass

7 7 8 8 8 8 10, 11 10, 11 10, 11

5d

banagrass

10, 11

6

sugarcane bagasse

10, 11

a

treatment untreated sample of California variety M202 (medium grain Japonica) from field material baled fall, 1994 leached sample of 1a, soaked (40 L kg-1) overnight in deionized water, drained, then soaked (40 L kg-1) and drained again untreated sample of high alkaline Imperial Valley (CA) wheat straw collected summer, 1994 leached sample of 2a, laboratory washed with distilled water at 120 L kg-1 untreated whole stem sample of Nebraska switchgrass collected 1996 leached sample of 3a, laboratory washed with distilled water at 120 L kg-1 untreated commercial California wood fuel collected from power plant boiler feed conveyor, 1996 leached sample of 4a, laboratory washed with distilled water at 120 L kg-1 untreated forage chopped, air-dried banagrass from Hawaii, designated FC-U forage chopped banagrass, fresh pressed to expel juice, then air-dried, designated FC-P forage chopped banagrass, fresh pressed to expel juice, rinsed with tap water at 8 L kg-1, pressed again to dewater, then air-dried, designated FC-PRP cutter milled banagrass, fresh pressed to expel juice, rinsed with tap water at 8 L kg-1, pressed again to dewater, then air-dried, designated JC-PRP composite sample of sugarcane bagasse collected at random intervals from exit conveyors of Hawaiian sugar processing facility, 1995, air-dried

Previous reference detailing treatment method.

before and after leaching. The fuels studied included rice straw, wheat straw, switchgrass, commercial power plant wood fuel, banagrass (Pennisetum purpureum), and sugarcane bagasse. Bagasse was the only fuel for which an unleached sample was not tested, as bagasse itself represents the leached byproduct of sugar production, and raw cane is not normally considered for direct thermal conversion (although recently interest has developed in the use of sugarcane trash and tops as fuel). Bench-scale combustion studies were conducted in an alumina-tube flow reactor housed in a variable temperature furnace and coupled to a molecular beam mass spectrometer (MBMS) system.17,18 Total relative amounts of HCl(g), SO2(g), NaCl(g), KCl(g), and other species were compared for leached and unleached samples to assess differences in the evolution of volatile species that have been associated with fouling and slagging in biomass power systems. Experimental Section 1. Fuel Preparation. Samples of six biomass fuels were collected and prepared for analysis: rice straw, wheat straw, switchgrass, mixed wood fuel from a commercial power plant, banagrass, and sugarcane bagasse. Each fuel selected has been the subject of a previous investigation into the merits of leaching, and was characterized in terms of fuel properties. A single procedure was not used in treating all fuels; rather the fuels were subjected to varying treatments, each assessing a particular leaching strategy. However, all fuels were fully characterized in terms of leaching effectiveness and resulting fuel composition, and represent a consistent, well-described suite of samples. All samples were air-dried and archived to the time of the present study under dry, airtight conditions. All samples were milled through 20 mesh for the present combustion studies. Except for bagasse and banagrass, two samples of each fuel type were prepared for analysis with the MBMS system: (1) an untreated (unleached) sample, and (2) a leached sample. Four banagrass samples were tested: an untreated sample and three treated samples for which the severity of the treatment was varied. Leaching was accomplished in a number of ways explained briefly here. The (17) Dayton, D. C.; French, R. J.; Milne, T. A. Energy Fuels 1995, 9, 855-865. (18) Dayton, D. C.; Milne, T. A. Laboratory measurements of alkali metal containing vapors released during biomass combustion. In Application of Advanced Technologies to Ash-Related Problems in Boilers; Baxter, L., DeSollar, R., Eds.; Plenum Press: New York, 1995; pp 161-185.

fuels and treatments are summarized in Table 1. Included in Table 1 are references to the original publications giving details of the treatment procedures. Fuel properties are listed in Table 2. Oxygen in Table 2 was computed by difference taking into account the chlorine, carbon, and sulfur in the ash fraction so as not to double count these elements from the ash and ultimate analyses. Relative reductions in inorganic species concentrations in the fuels caused by leaching are listed in Table 3. As noted in Table 1, leaching was accomplished by a number of techniques, depending on the purpose of the original study producing the samples. Leached rice straw was produced as part of an investigation into the membrane concentration of leachate. The rice straw was first soaked for 24 h in deionized water, drained, then soaked and drained again. The rice straw, a medium-grain M202 variety, is currently the most common variety planted in California. The leached wheat straw was produced as part of a study investigating the leaching characteristics of biomass, in which 50 g milled samples (20 mesh) were successively washed with 500-mL increments of distilled water (extraction was nearly complete after an application of 40 L kg-1 as determined by measurements of leachate electrical conductivity, but leaching was continued through a total application of 120 L kg-1). The wheat straw was a sample from the Imperial Valley of California, and contained high levels of alkali and chlorine caused by the saline conditions under which it was grown. The switchgrass was a whole stem sample of standing material from the 1995 season grown in Nebraska and provided by the Nebraska Public Power District (Columbus, Nebraska). Leached switchgrass (Treatment No. 3b) was prepared in the same manner as the leached wheat straw sample. The wood fuel was collected from the feed conveyor of a commercial biomass power plant in California (Wheelabrator-Shasta, Anderson, California). The fuel represents a routine blend of commercial forest species, including Ponderosa pine and Douglas fir chips from unmerchantable material and forest thinnings. The leached wood fuel (Treatment No. 4b) was produced in the same manner as the leached wheat straw and switchgrass samples. Banagrass samples were produced as part of a study investigating the comparative combustion and gasification characteristics of untreated and leached banagrass and sugarcane bagasse.10-12 Fresh Hawaiian banagrass was either forage chopped (as the most likely commercial field harvesting operation), or cut using a rotating knife head to produce a finer particle size and enhance molecular transport in leaching. Several treatments were then employed in investigating the leaching and thermal conversion behavior of the fuel. The banagrass treatment designated FC-P (Treatment No. 5b in



Table 2. Fuel Compositions by Treatmenta fuel

rice straw

wheat straw

switchgrass

wood fuel

sugarcane bagasse

banagrass

treatment no. 1a 1b 2a 2b 3a 3b 4a 4b 5a 5b 5c 5d treatment unleached leached unleached leached unleached leached unleached leached FC-U FC-P FC-PRP JC-PRP dry dry, ash-free

14.75 18.64

15.15 18.45

17.15 19.66

2.05

0.20

2.15

C H N S Cl Ash Ob

37.87 4.61 0.63 0.14 1.01 20.87 35.82

39.53 4.76 0.53 0.08 0.04 17.89 37.21

SiO2 Al2O3 TiO2 Fe2O3 CaO MgO Na2O K2O P 2O 5 SO3 Cl CO2 Undd

15.085 0.015 0.004 0.054 0.434 0.430 0.566 2.463 0.371 0.232 0.849 0.021 0.346

15.944 0.213 0.009 0.077 0.442 0.242 0.057 0.249 0.088 0.073 nd 0.021 0.476

SiO2 Al2O3 TiO2 Fe2O3 CaO MgO Na2O K2O P 2O 5 SO3 Cl CO2 Undd

72.28 0.07 0.02 0.26 2.08 2.06 2.71 11.80 1.78 1.11 4.07 0.10 1.66

89.12 1.19 0.05 0.43 2.47 1.35 0.32 1.39 0.49 0.41 nd 0.12 2.66

42.49 5.12 0.68 0.39 2.02 12.78 38.68 4.580 0.314 0.019 0.124 0.596 0.321 1.342 2.352 0.188 0.698 1.879 0.015 0.353 35.84 2.46 0.15 0.97 4.66 2.51 10.50 18.40 1.47 5.46 14.70 0.12 2.76

6

Higher heating value (MJ/kg dry basis) 18.10 18.74 18.86 18.71 19.92 19.35 19.47 19.49 19.59 20.76

18.20 18.95

18.30 18.88

18.70 19.22

18.60 19.11

17.74 19.38

0.33

Alkali Index (kg alkali oxide/GJ) 0.24 0.02 0.18 0.06

0.71

0.45

0.24

0.11

0.17

45.67 5.71 0.64 0.09 0.21 6.45 41.31

Ultimate Analysis (% dry matter) 45.12 43.18 49.26 45.6 6.48 6.08 5.75 5.97 0.31 0.17 0.92 0.23 0.08 0.03 0.03 0.04 0.04 nd 0.03 nd 3.77 3.25 4.51 4.05 44.23 47.31 39.61 44.16

47.98 5.50 0.60 0.10 0.58 3.94 41.69

48.69 5.61 0.48 0.06 0.29 3.05 41.97

48.84 5.60 0.41 0.05 0.09 2.69 42.37

48.79 5.57 0.31 0.05 0.01 2.66 42.65

45.20 5.48 0.13 0.05 0.06 8.45 40.67

3.985 0.184 0.003 0.104 0.377 0.153 0.224 0.375 0.216 0.108 0.034 0.018 0.669 61.79 2.85 0.04 1.62 5.84 2.37 3.48 5.81 3.35 1.68 0.52 0.28 10.37

Ash Elementalc (% dry matter) 2.367 2.848 1.693 0.013 0.032 0.634 nd 0.002 0.028 0.032 0.016 0.343 0.233 0.245 0.767 0.063 0.036 0.164 0.014 0.009 0.064 0.441 0.023 0.271 0.159 0.020 0.099 0.057 0.020 0.046 0.005 0.002 nd 0.025 0.040 0.337 0.361 -0.041 0.064 Ash Elementalc (% ash 600 °C) 62.79 87.63 37.53 0.34 0.97 14.06 nd 0.05 0.63 0.85 0.49 7.60 6.19 7.55 17.00 1.67 1.11 3.64 0.36 0.27 1.42 11.70 0.71 6.00 4.22 0.62 2.19 1.50 0.60 1.02 0.13 0.05 nd 0.67 1.22 7.48 9.58 -1.27 1.43

1.756 0.557 0.024 0.301 0.547 0.084 0.053 0.059 0.053 0.026 nd 0.157 0.433 43.37 13.75 0.59 7.42 13.50 2.07 1.32 1.46 1.31 0.64 nd 3.87 10.70

1.339 0.029 0.002 0.031 0.236 0.211 0.039 1.253 0.158 0.100 0.336 0.041 0.163 33.99 0.74 0.05 0.78 6.00 5.36 1.00 31.80 4.00 2.55 8.54 1.05 4.14

1.236 0.031 0.006 0.032 0.227 0.138 0.034 0.785 0.158 0.049 0.119 0.037 0.196 40.53 1.03 0.21 1.05 7.45 4.53 1.11 25.75 5.19 1.60 3.90 1.21 6.44

1.388 0.023 0.005 0.024 0.265 0.107 0.023 0.418 0.153 0.032 0.026 0.053 0.173 51.60 0.86 0.19 0.91 9.84 3.98 0.85 15.55 5.69 1.20 0.95 1.96 6.42

1.676 0.019 0.001 0.025 0.298 0.065 0.015 0.197 0.063 0.015 0.003 0.119 0.164 63.02 0.73 0.05 0.95 11.20 2.44 0.56 7.39 2.37 0.57 0.10 4.47 6.15

3.601 1.957 0.233 1.367 0.249 0.166 0.048 0.251 0.111 0.042 0.006 0.055 0.363 42.62 23.16 2.76 16.18 2.95 1.97 0.57 2.97 1.31 0.50 0.07 0.65 4.29

a nd ) not detected. b Oxygen by difference, including Cl, S, and C in ash. c Ash elemental (% dry matter) computed from ash elemental (% ash) and ash content from ultimate analysis. d und ) undetermined.

Table 1) was forage chopped and then mechanically pressed to expel juice. Treatment No. 5c (FC-PRP) was handled in the same manner, but then rinsed with water and mechanically pressed again to dewater the sample. Treatment No. 5d (JCPRP) was the same as No. 5c, except that the cutting mechanism used produced a finer particle size before pressing and rinsing than the chopper. Sample 5a was the untreated forage chopped banagrass that was simply air-dried following chopping. All other banagrass treatments were also air-dried after processing. The sugarcane bagasse sample was collected from the exit conveyors of a sugar processing plant (Waialua Sugar Co., Hawaii) transporting bagasse to storage. The bagasse was air-dried but no other treatment was employed except milling through 20 mesh for the MBMS analysis. Although the leaching treatments varied, all treated samples, with the exception of sample 5b (banagrass FC-P), are heavily depleted in alkali and chlorine. Some general impacts of leaching are apparent in Table 3. Potassium was depleted by an average of 82% for all samples except the FC-P banagrass that was not fully leached. Chlorine in the fuel was reduced by an average of 95%, and in the ash by an average of 91%. Large depletions (>50%) were associated with sodium and sulfur in ash (total sulfur depleted only 41%

in fuel). Phosphorus was reduced on average by 43%, magnesium by 51%, and nitrogen by 37%. The reductions in potassium, sulfur, and nitrogen are useful from a nutrient recycling perspective. Reductions in sulfur and nitrogen are useful for reducing SOx and NOx emissions from combustion units. Heating value, on a dry basis, was on average increased by 3%, even though the oxygen concentration of the fuels increased by about 6% because total ash was reduced by leaching. The dry, ash free heating values remained essentially unchanged (average increase of 1%). The percentages of carbon and hydrogen also increased in the biomass. Alumina, titania, and to a large extent iron, are present in biomass primarily as a result of soil contamination. The increases or decreases in these species are associated with the extent to which leaching removes or retains soil materials, and are therefore subject to large variation. Alkali index, a measure of fouling potential expressed as the mass of alkali per unit fuel energy, was reduced by an average of 81%. Fouling is thought to be of low severity for fuels of alkali index below 0.17 kg GJ-1 (and of high severity above 0.34 kg GJ-1),19 although there are a number of other considerations in predicting fouling. The values for alkali index in Table 2 show that leaching, by this measure, substantially reduces the fouling potential.


Dayton et al.

Table 3. Reductions (%) in Biomass Properties Due to Leachinga,b

treatment no. dry dry, ash free

C H N S (total) Cl (total) Ash O SiO2 Al2O3 TiO2 Fe2O3 CaO MgO Na2O K2O P2O5 SO3 (ash) Cl (ash) CO2

rice straw

wheat straw

switchgrass

wood fuel

1b

2b

3b

4b

banagrassc FC-P 5b

FC-PRP 5c

averaged JC-PRP 5d

-3 1

-6 2

Heating Value -1 -7 0 -6

-1 0

-3 -1

-2 -1

-3 -1

90

85

Alkali Index 93 68

37

67

84

81

-4 -3 16 43 96 14 -4 -6 -1357 -114 -42 -2 44 90 90 76 68 100 -3

-7 -12 6 77 90 50 -7 13 42 87 16 37 52 83 84 -15 84 98 -18

-1 -2 20 40 50 23 -1 8 -8 -225 -4 4 35 14 37 0 51 65 11

-2 -2 32 50 84 32 -2 -4 21 -159 20 -12 49 42 67 3 68 92 -27

-2 -1 48 50 98 32 -2 -25 33 32 18 -26 69 62 84 60 85 99 -187

-1 -3 37 41 95 25 -6 -8 -233 -28 12 3 51 55 83 43 69 91 -40

4 6 45 63 100 14 -7 -20 -146 nd 50 -5 43 35 95 87 66 67 -57

7 -4 75 -33 100 10 -11 -4 12 16 12 29 49 17 78 46 44 nd 54

a Negative sign denotes increase in value. b nd ) not detected in unleached fuel, therefore no reduction could be determined. c Relative to FC-U. d Excludes FC-P (Treatment 5b) as incompletely leached.

2. MBMS Experiments. Quartz sample boats containing 20-30 mg of biomass material were placed in a platinum mesh basket attached to the end of a 6-mm (0.24′′) diameter quartz rod. A type-K (chromel/alumel), inconel-sheathed thermocouple was inserted through the tube so the thermocouple junction was located above the sample boat approximately one-fourth of the way over the sample. This thermocouple was used to measure the local gas temperature above the sample boat, not the flame temperature or the sample temperature. The rod was inserted into an 18-mm (0.7′′) inner diameter tubular alumina reactor that was fitted over the sampling orifice of an MBMS system described in the literature.17,18,20 A mixture of 20% O2 in helium flowed through the reactor at a total rate of 3.0 standard L min-1 while the reactor was held inside a variable temperature two-zone furnace at 1100 °C. The residence time of combustion products in the reactor before sampling was approximately 0.18 s. Gaseous combustion products expanded supersonically through a 200-µm diameter orifice into the first stage of the MBMS system. A 1-mm conical skimmer collimated the expanding gases before entering the third stage of the vacuum system where they were ionized by electron impact (25 eV electron energy) and detected by quadrupole mass spectrometry. Mass spectra were recorded in the mass range of 20 to 180 amu at a rate of about once every second throughout the entire combustion process. Six biomass materials were studied, as mentioned earlier. The quartz boat containing the sample was initially held in the cool zone of the reactor. Background spectra were acquired for at least 15 s before the biomass sample was inserted into the heated portion of the reactor. When the sample was inserted, the total ion current (TIC) increased as the combustion process began, indicating the rapid devolatilization of the sample. As the devolatilization was completed, the TIC decreased. A second increase in the TIC corresponded to the consumption of the remaining char in the sample. Acquisition (19) Miles, T. R.; Miles, T. R., Jr.; Baxter, L. L.; Bryers, R. W.; Jenkins, B. M.; Oden, L. L. Biomass Bioenergy 1996, 10 (2-3), 125138. (20) Evans, R. J.; Milne, T. A. Energy Fuels 1987, 1, 123-137.

of spectra continued until the TIC dropped to the initial background level, indicating that the combustion process was complete. The entire combustion process typically lasted less than 40 s. Triplicate samples of each fuel were studied to ensure reproducibility. The ionization energy used in these combustion experiments was set low enough to minimize the fragmentation of sampled species while providing maximum sensitivity. Although fragmentation was not eliminated, the low (25 eV) ionization energy ensured that each ion was only singly charged. Therefore, for most species of interest, the relative intensities of peaks at a particular value of m/z (mass-to-charge ratio) corresponding to the mass of a given species are good approximations of the relative amount of that species released. For example, the amount of NO(g) released at a particular moment during combustion can be found by measuring the ion signal at m/z ) 30 (NO+) in the mass spectrum acquired at that moment. Species such as HCl(g) (m/z ) 36 and 38) and NaCl(g) (m/z ) 58 and 60) have intensity at two masses because chlorine has two isotopes, 35Cl and 37Cl, that have a relative natural abundance of 3:1. Notable cases are peaks at m/z ) 23 and m/z ) 39 and 41. These peaks were consistently assigned to Na+ and the two isotopes of K+, respectively. These are fragment ions produced during the ionization of alkali metal-containing vapors such as KCl and K2SO4, and should not be construed as an indication of the presence of pure alkali metal vapors (such as K(g)).16 Ion intensity observed at m/z ) 28 was assigned to CO+ rather than N2+ for two reasons. First, helium rather than N2 was used as a carrier gas in these experiments; second, all the fuels had a relatively low nitrogen content and high carbon content. Therefore, most of the signal intensity at m/z ) 28 was attributed to CO+ with a small, but unknown, contribution from N2+. Two masses, m/z ) 44 (assigned to CO2+) and m/z ) 32 (assigned to O2+) were not scanned when acquiring data to avoid saturating the detector. However, the relative amount of CO2(g) produced could be detected by the measured intensity at m/z ) 45 assigned to 13CO2+. The 13CO2 isotope has a relative natural abundance of 1.17%.


Figure 1. Average mass spectra recorded during the devolatilization phase (A) and char combustion phase (B) during unleached wheat straw combustion at 1100 °C in 20% O2 in helium. Intensities were normalized to the 34O2+ signal intensity measured before the sample was inserted into the high-temperature reactor.

Results and Discussion The MBMS results were similar to results from earlier studies of biomass combustion17,18 in that two distinct phases were observed for all fuels: a devolatilization phase and a char combustion phase. Background mass spectra were obtained by averaging all mass spectra acquired during the first 15 s of the experiment before the sample was inserted. Representative mass spectra for a particular combustion phase were obtained by averaging spectra recorded throughout the duration of that phase, then subtracting the background spectrum from the averaged spectrum. An example of the average mass spectra recorded during the devolatilization and char combustion phases during unleached wheat straw combustion in 20% O2 in helium at 1100 °C is shown in Figure 1. Signal intensities in Figure 1 were normalized to the 34O2+ signal recorded before the sample was inserted into the reactor. This normalization procedure was performed in an attempt to minimize the instrument response variation from sample to sample and from day to day. Figure 1A displays the average mass spectrum recorded during the devolatilization phase of unleached wheat straw combustion. Clearly, the dominant species detected during this phase of wheat straw combustion were CO(g), NO(g), and CO2(g). The maximum 13CO2+ intensity in Figure 1A is off scale at 43. Several other less intense peaks were assigned to species such as HCl(g), KCl(g), NaCl(g), and SO2(g) that were also


released during the devolatilization phase. These inorganic species are consistent with the fact that the fuel analysis of the unleached wheat straw indicates relatively high concentrations of sulfur, potassium, sodium, and chlorine (Table 2) compared to the other fuels studied. The average mass spectrum recorded during the char combustion phase of wheat straw combustion is shown in Figure 1B. The dominant species observed during the char combustion phase were CO(g) and various alkali metal vapors. The maximum CO+ intensity is off scale in Figure 1B at 124. Approximately 5 times more CO(g) and 6 times less CO2(g) was released during the unleached wheat straw char combustion phase compared to the devolatilization phase. It also appears that more than twice as much alkali metal vapor was released during the unleached wheat straw char combustion phase compared to the devolatilization phase. Much of the K+ intensity was attributed to fragmentation of other parent potassium-containing species (mostly KCl) as described earlier. The K2Cl+ and NaKCl+ peaks are fragments of the dimers (KCl)2(g) and NaClKCl(g). These dimeric species are thermodynamically stable at these temperatures and alkali vapor concentrations; however, their formation is also enhanced during the free jet expansion of combustion gases containing substantial concentrations of very polar molecules such as alkali metal chlorides.21 Having identified the major and minor products released during unleached wheat straw combustion, the evolution of these products can be followed as a function of time during combustion of a small sample of unleached wheat straw. Figure 2 contains three plots that display the time evolution for 10 selected combustion products observed during unleached wheat straw combustion. The time axis was adjusted so that time 0 corresponds to when the sample was inserted into the reactor. Everything before that was considered background and the combustion event was complete after 25 s. Also plotted in the figure is the gas temperature as measured with a thermocouple located above the sample boat. The gas temperature rises very rapidly during the devolatilization phase and finally reaches the furnace temperature in the char combustion phase. The localized flame and sample temperatures were likely higher during the devolatilization phase and reached the furnace temperature faster than measured because of the time it takes for the sampling arm and sample boat to thermally equilibrate. All of the ion traces have been normalized to the relative amount of CO2(g) measured. The ion intensity at m/z ) 45 (13CO2+) was multiplied by 85.4701 to account for the isotopic abundance of 13CO2 compared to the most abundant naturally occurring isotope. The CO2+ signal was then normalized so the maximum intensity was 100. All other ion signals were scaled accordingly. For example, in the top plot in Figure 2, the CO2+ signal peaks at about 2 s after the sample was inserted into the reactor, during the devolatilization phase, with a peak intensity of 100. The CO+ signal does not peak until midway through the char combustion phase, 15 s after the sample was inserted into the reactor, with a peak relative intensity of 7. The 34O2+ (21) Milne, T. A.; Klein, H. M. J. Chem. Phys. 1960, 33, 1628-1637.


Figure 2. The time evolution of CO2+, CO+, 34O2+, NO+, SO2+, HCl+, K+, KCl+, Na+, and NaCl+ observed during unleached wheat straw combustion. All species intensities are relative to CO2+ with a maximum intensity of 100. The gas temperature as measured with a thermocouple positioned above the sample boat during the combustion event is also shown. The time axis was adjusted so that time 0 corresponds to when the sample was inserted into the high-temperature reactor.

trace is on a scale 100 times smaller than the CO2+ signal. As discussed earlier, m/z ) 32, corresponding to the most naturally abundant isotope of molecular oxygen (32O2+), was skipped in order to avoid saturating the detector. The 34O2 isotope has a relative abundance of 0.4% compared to 32O2. Nevertheless, this trace shows that the O2 concentration dips during the devolatilization phase as oxygen was consumed during the combustion of the volatile products released from the unleached wheat straw. Although the oxygen concentration was diminished during the devolatilization phase, there was still a 30% excess of oxygen in the reactor. The second plot in Figure 2 displays the time evolution of NO(g), SO2(g), and HCl(g) as measured by the corresponding ions. The amounts of NO(g) and HCl(g) peaked during the devolatilization phase. The distribution of the amount of SO2(g) released during unleached wheat straw combustion was bimodal with a peak during the devolatilization phase and a second, more intense peak at the end of the char combustion phase. The SO2(g) released during the devolatilization phase formed from the oxidation of organic sulfur in the plant matrix while the SO2(g) released during the char combustion phase likely occurred because of the decomposition of inorganic alkali metal sulfates at high temperatures and oxygen concentrations.22 The third plot in Figure 2 shows the time evolution of the alkali metal vapors detected during unleached wheat straw combustion. As discussed above, K+ is a

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Figure 3. Average mass spectra recorded during the devolatilization phase (A) and char combustion phase (B) during leached wheat straw combustion at 1100 °C in 20% O2 in helium. Intensities were normalized to the 34O2+ signal intensity measured before the sample was inserted into the high-temperature reactor.

fragment ion formed from the electron impact ionization of potassium-containing species and is not a measure of K(g). The shape of the K+ and KCl+ traces is very similar, indicating that most, if not all, of the K+ occurred because of KCl(g) ionization. These traces also indicate that most of the KCl(g) was released during the char combustion phase. Similar conclusions pertain to the release of NaCl(g). As with the K+ species, most, if not all, of the Na+ occurred because of NaCl(g) ionization. The effects of leaching on products observed during wheat straw combustion are obvious when the results for unleached wheat straw presented in Figures 1 and 2 are compared with the combustion results for leached wheat straw presented in Figures 3 and 4. Figure 3 presents the mass spectra averaged during the devolatilization and char combustion phases observed during leached wheat straw combustion. The intensities have been normalized to the 34O2+ signal measured before the sample was introduced into the reactor in an attempt to minimize the impact of variations in instrument response. Therefore, direct comparisons can be made between the relative intensities in Figures 1 and 2 and the relative intensities in Figures 3 and 4. (22) Hastie, J. W.; Plante, E. R.; Bonnell, D. W. Alkali Vapor Transport in Coal Conversion and Combustion Systems. In Metal Bonding and Interaction in High-Temperature Systems; Gole, J. L., Stwalley, W. C., Eds.; ACS Symposium Series 179; American Chemical Society: Washington, DC, 1982; Chapter 34.


Figure 4. The time evolution of CO2+, CO+, 34O2+, NO+, SO2+, HCl+, K+, KCl+, Na+, and NaCl+ observed during leached wheat straw combustion. All species intensities are relative to CO2+ with a maximum intensity of 100. The gas temperature as measured with a thermocouple positioned above the sample boat during the combustion event is also shown. The time axis was adjusted so that time 0 corresponds to when the sample was inserted into the high-temperature reactor.

The relative amounts of CO(g), NO(g), and CO2(g) released during the devolatilization phase of leached wheat straw combustion (Figure 3A) were consistent with the amounts of these products detected during the devolatilization phase of unleached wheat straw combustion (Figure 1A). Less SO2(g) was released during the devolatilization phase of leached wheat straw combustion compared to unleached wheat straw combustion. The main difference between the combustion results for the unleached and leached wheat straw samples was the lack of HCl(g) and alkali metal vapors detected during leached wheat straw combustion. The lack of alkali metal vapors released during the char combustion phase of leached wheat straw compared to unleached wheat straw combustion is apparent when Figure 3B is compared to Figure 1B. Also evident from Figure 3B is the similarity in relative amounts of NO(g) and CO2(g) released during the char combustion phase of leached and unleached wheat straw combustion. However, about 12 times less CO(g) was released during the char combustion phase of leached wheat straw compared to the amount of CO(g) released during the char combustion phase of unleached wheat straw. The char combustion phase during leached wheat straw was also 4 times shorter than the unleached wheat straw char combustion phase. These observations are consistent with other observations of the burning times of leached and unleached biomass samples.8 Clearly, the leaching process effectively removes alkali metals and chlorine from the fuel, which


correlates to the elimination of HCl(g) and alkali metal vapors released during the combustion process. The removal of alkali metals, however, also appeared to reduce the relative char burnout time during the combustion of the leached versus unleached wheat straw fuels. Although the relative amounts of char produced from the leached and unleached samples was not measured, it seems plausible to assume that less char was formed during leached wheat straw combustion compared to unleached wheat straw combustion based on the relative amount of CO(g) released during char combustion. This is also consistent with the effects of reduced chlorine concentration and the role of halogens as flame inhibitors.8 The time evolution of the same 10 selected ions plotted in Figure 2 is shown in Figure 4 for leached wheat straw combustion, along with the gas temperature as measured with a thermocouple located above the leached wheat straw sample. As in Figure 2, the time axis was set such that time 0 corresponds to when the sample was inserted into the reactor and all ion signals were normalized so the maximum signal for CO2+ was 100. The CO2+ and CO+ signals peak during the devolatilization phase with a corresponding decrease in the 34O2+ signal intensity. Although the oxygen concentration decreased during the devolatilization phase, there was still 30% excess O2 in the reactor atmosphere. The CO2+ signal intensity decays to zero approximately 11 s after the sample was introduced into the hightemperature reactor, indicating that the char combustion phase was much shorter during leached wheat straw combustion compared to unleached wheat straw combustion. The second plot in Figure 4 shows the relative amounts of NO(g), SO2(g), and HCl(g). The amount of NO(g) and SO2(g) released during leached wheat straw combustion peaked during the devolatilization phase. Confirming what was observed in Figure 3, no HCl(g) was detected during leached wheat straw combustion. The third plot in Figure 4 also shows that no alkali metal vapors were detected at any time during leached wheat straw combustion, again confirming what was observed in the average mass spectrum in Figure 3. Similar combustion results were obtained for all of the fuels studied. As expected, CO2(g) was the dominant species detected during the devolatilization phase of all the fuels because of the high carbon contents (38%49%) in all biomass samples (see Table 2). Other products detected during devolatilization of all the fuels were CO(g), NO(g), and SO2(g). The unleached rice straw sample had the second highest chlorine content of all the fuels studied; as a result, HCl(g) was detected during the devolatilization phase of unleached rice straw. HCl(g) release was negligible for the leached samples and for those fuels with very low chlorine contents, such as the wood fuel and switchgrass. The mass spectra averaged over the char combustion phase differed remarkably from one fuel to another. The char phase has been shown to be extremely important in studies of alkali metal release from biomass fuels, as most of the volatile alkali metal-containing vapors are released during this phase.17,18 This was evident from the results presented in Figure 2 for unleached wheat straw combustion. The peaks associated with


Figure 5. Relative amounts of HCl(g) released during the combustion of raw banagrass and of banagrass subjected to a variety of treatments, at 1100 °C in 20% O2 in helium. The banagrass samples listed in the legend refer to the sample processing procedure as described in Table 1.

alkali metal-containing vapors were not observed during combustion of the leached samples. The rice straw spectra showed characteristics similar to the wheat straw spectra; the untreated rice straw spectrum contained peaks of significant intensity attributed to alkali metal- and chlorine-containing species, most of which were absent from the average char combustion phase spectrum obtained during combustion of the leached sample. The decreased amounts of HCl(g), KCl(g), and NaCl(g) released during combustion of the leached straws compared to the unleached straws was consistent with the reductions in alkali metal and chlorine content listed in Table 3. Only CO2(g), CO(g), and NO(g) were detected in the mass spectra averaged during the char combustion phases of bagasse and of unleached and leached wood fuel and switchgrass combustion. Little or no alkali metal- or chlorine-containing species were detected, which is consistent with their fuel analyses. The banagrass samples represent a set of fuels that have been processed with varying degrees of severity, and it was possible to explore the combustion products detected as a function of sample processing severity. The banagrass processing appeared to have the largest impact on the potassium and chlorine contents of the fuel. Table 3 indicates that the amount of chlorine in the banagrass sample can be reduced significantly as the sample process severity was increased. The unprocessed banagrass sample (FC-U) contained 0.58% Cl. By simply pressing the banagrass to expel the juice, it was possible to remove 50% of the chlorine in the sample (FC-P). If the pressed sample were rinsed and re-pressed it was possible to remove >80% of the chlorine in the banagrass sample. This same press/rinse/press procedure applied to the more finely cut banagrass (JC-PRP) was successful in removing >90% of the chlorine in the sample. Figure 5 shows the relative time evolution of HCl(g) during combustion of each of these banagrass samples. Again, time 0 corresponds to when the samples were inserted into the high-temperature reactor. Clearly, the amount of HCl(g) detected during banagrass combustion was a function of the amount of chlorine in the sample, which was influenced by the severity of the sample processing.

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Figure 6. Relative amounts of KCl(g) released during the combustion of raw banagrass and of banagrass subjected to a variety of treatments, at 1100 °C in 20% O2 in helium. The banagrass samples listed in the legend refer to the sample processing procedure as described in Table 1.

Figure 7. Relative amounts of SO2(g), HCl(g), KCl(g), and NaCl(g) detected during the combustion of unleached and leached biomass at 1100 °C in 20% O2 in helium.

Similar results were obtained for the relative amount of KCl(g) released during combustion of the banagrass samples as shown in Figure 6. The unprocessed banagrass sample contains 1% K by weight. Pressing the banagrass sample removes 37% of the potassium from the sample. If the sample were rinsed and re-pressed, 67% of the potassium in the banagrass was removed. More than 80% of the potassium was removed from the banagrass sample by applying the press/rinse/press procedure to the finely cut banagrass sample. These reductions in fuel potassium content correlate well with similar reductions in the relative amount of gas-phase KCl detected during banagrass combustion as shown in Figure 6. The mass spectral combustion results obtained for all the biomass samples studied are summarized in Figures 7 and 8 in order to show the effects of leaching on sulfur, chlorine, and alkali metal vapor release. The intensity versus time profiles (examples are shown in Figures 2 and 4) for SO2(g) (m/z ) 64), HCl(g) (m/z ) 36), KCl(g) (m/z ) 74), and NaCl(g) (m/z ) 58) recorded during combustion of all the samples investigated were inte-


Figure 8. Relative amounts of SO2(g), HCl(g), KCl(g), and NaCl(g) detected during the combustion of raw banagrass and of banagrass subjected to a variety of treatments, at 1100 °C in 20% O2 in helium.

grated to determine the total relative amount of the given combustion product. These integrated intensities were normalized to the sample mass and the 34O2+ intensity measured before each sample was inserted into the high-temperature reactor. As described earlier, this normalization procedure was an attempt to minimize variations in instrument response over time. The results from the triplicate samples were averaged and standard deviations were computed. Figure 7 shows the relative (normalized) amounts of SO2 (g), HCl(g), KCl(g), and NaCl(g) measured during combustion of all the biomass samples except banagrass. The results are the average of a set of triplicate samples, and error bars represent one standard deviation. As shown in Table 2, the untreated wood fuel and switchgrass samples contained little alkali metal and chlorine, although leaching still achieves a substantial reduction in potassium. As a result of the low concentrations of alkali metals and chlorine in the fuel, the amounts of alkali metal- and chlorine-containing species detected during combustion of these fuels were nearly the same for leached and unleached versions and were near or below the detection limit of the instrument. Similarly, little HCl(g), NaCl(g), and KCl(g) were released during bagasse combustion, consistent with the fuel analysis shown in Table 2. On the other hand, substantial amounts of alkali metal- and chlorine-containing species were detected during combustion of the untreated wheat straw and rice straw samples. As depicted in Figure 7, the relatively large amounts of HCl(g), NaCl(g), and KCl(g) detected during the combustion of these samples reflect the fuel compositions listed in Table 2. Upon leaching, the relative amounts of alkali metal- and chlorine-containing species detected were considerably lower. In the case of wheat straw, KCl(g), HCl(g), and NaCl(g) were reduced by 98%, 90%, and 91%, respectively; for rice straw the reductions were 95%, 84%, and 80%, respectively. These results compare well with the reductions in potassium and chlorine for wheat and rice straw listed in Table 3. For both fuels, the decrease in the integrated K+ intensity was identical to that in the KCl+ intensity. This means that nearly all the potassium contained in the unleached wheat and rice straws


was released in the form of KCl(g). This is consistent with the fact that no other potassium-containing species such as K2SO4 or KOH were observed during combustion of these two fuels. Leaching also affected the amount of SO2(g) released during combustion of fuels containing significant amounts of sulfur, such as rice straw and wheat straw. This again was consistent with the fact that the fuel analyses showed a lower sulfur content for the leached straws compared to the unleached straws. The fuel analyses indicated a decline in sulfur content of 43% for rice straw and 77% for wheat straw. By comparison, the amount of SO2(g) detected during combustion of these fuels was reduced by 43% for rice straw and 67% for wheat straw. The MBMS results were in good agreement with the fuel analyses, showing nearly the same amount of reduction in the volatile sulfur content of these fuels. On the basis of the MBMS results, leaching appeared to have no effect on the release of NO(g) during combustion of these fuels, even though there was an average 37% reduction in fuel nitrogen (Table 3). The relative amounts of SO2(g), HCl(g), KCl(g), and NaCl(g) measured during the combustion of the four banagrass samples are plotted in Figure 8. In comparison to the rather large amounts of KCl(g) and NaCl(g) measured during the combustion of unprocessed banagrass (FC-U), the gas-phase alkali-chloride levels were much lower for the processed banagrass samples. In fact, the percent reduction was related to the degree of treatment to which the banagrass was subjected. When the forage-chopped banagrass was pressed (sample FCP), there were reductions of approximately 46% in the amount of KCl(g) and 31% in the amount of HCl(g) detected during combustion. By comparison, the potassium content of the fuel decreased by 37%, and chlorine content decreased by 50%, as shown in Table 3. The levels of NaCl(g) and SO2(g) released during combustion of the untreated sample were near or below the detection limits of the instrument, so the decline in the release of these species was difficult to quantify. A significant amount of NO(g) was detected during combustion of raw banagrass, but this value did not appear to change for the treated fuels, again even though there is a 20%-48% reduction in fuel N caused by leaching (Table 3). When the banagrass was subjected to water rinsing and a second pressing process (sample FC-PRP), the amounts of HCl(g) and KCl(g) released during combustion were reduced by 68% and 85%, respectively, compared to the release of these products during untreated banagrass combustion. Cutting the banagrass with a mill to a finer particle size and performing the press, rinse, press treatment (sample JC-PRP) reduced the amounts of HCl(g) and KCl(g) detected during combustion by 82% and 92%, respectively, compared to the relative amounts of these products detected during combustion of the unprocessed sample. The JC-PRP treatment was the most effective for removing alkali metal and chlorine from banagrass and, as a result, at reducing the amount of HCl(g) and alkali metal vapors released during banagrass combustion. Conclusions Alkali metal vapors released during biomass combustion have been the focus of attention because of their


role in fouling and slagging in biomass power plants. The presence of chlorine in a biomass fuel tends to enhance the vaporization of alkali metals as chlorides (because of the thermodynamic stability of alkali chlorides at combustion temperatures), inhibits flaming combustion, increases corrosion rates, and potentially leads to the formation of toxic compounds. Most woody biomass fuels tend to have low ash contents and correspondingly low potassium and chlorine contents. Therefore, woody biomass has more desirable fuel properties for electricity production in furnaces and boilers because of the generally lower ash concentrations. Agricultural residues, such as straws, and fastgrowing biomass crops, such as grasses, tend to have high potassium and chlorine contents that make them less desirable biomass fuels. Leaching provides a technical solution to their use in extant facilities without the need to incorporate new technology, such as gasifiers (which also benefit from the removal of alkali metals and halogens). Leaching has proven to be a very effective method for removing alkali metals and chlorine from biomass. Previous studies with the biomass fuels used in the present work have documented the effectiveness of leaching major and minor inorganic ash constituents from biomass. The current experiments confirm that reducing the amount of chlorine and potassium in a fuel by leaching reduces the amount of chlorine and alkali metal vapors released during combustion of biomass fuels. The alkali metal vapors released during biomass combustion were reduced largely in proportion to the reduction in potassium and sodium fuel concentrations. In the cases of fuels such as rice straw and wheat straw, which contain high percentages of alkali metal and chlorine, the reduction in the measured amounts of sodium- and chlorine-containing species released during combustion of these fuels was >80%; for potassiumcontaining species it was >95%. For banagrass, these species were removed most effectively with a fine particle size and an aggressive pressing, rinsing, and second pressing treatment. This treatment decreased the amounts of potassium-containing species released to the vapor phase during combustion by 88% or more and HCl(g) by more than 81%. Reducing the alkali metal content of a biomass fuel also appears to result in less char production during the combustion process. This was inferred from the char combustion phases for the leached wheat and rice straws being 4 times shorter than the char combustion phases observed during combustion of the unleached samples. Gas-phase NO levels were unchanged even though fuel N contents were reduced 20%-50% by leaching. This suggests that while leaching reduces fuel nitrogen, it may also affect the nitrogen combustion chemistry in that a larger fraction of the fuel-bound nitrogen was converted to NO(g) during combustion of the leached samples compared to the unleached samples.

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This is consistent with other information concerning the conversion fractions of fuel nitrogen as a function of nitrogen concentration. Leaching is clearly an effective means of improving the fuel properties of a given biomass feedstock that might not otherwise be considered for electricity production. Removing volatile alkali metals and chlorine from biomass fuels translates into a reduction in alkali metal vapors in biomass combustion systems. Sugarcane bagasse is unique in that it represents a leached fuel that is a byproduct of sugar production; however, for agricultural residues such as straws, an extra fuel processing step would have to be added to a biomass power plant. Another option for biomass leaching would be to change agricultural harvesting practices to take advantage of natural leaching. For example, studies have shown that potassium and chlorine in rice and other straws can be reduced by leaving straw in the fields after harvest through the wet season.8,9,23,24 Another way to avoid the accumulation of potassium and chlorine in certain biomass energy crops is to alter fertilization practices. The chlorine content of certain grasses and straws can be reduced by applying chlorine-free fertilizers.16 However, restricting potassium fertilization may have undesirable consequences for crop yield and farm revenue. Field leaching (e.g., by rain washing) provides a means to recycle potassium to subsequent crops. Ultimately, the decision to leach a fuel is an economic one and the feasibility depends on both the fuel and the application. From a technical perspective, however, leaching clearly reduces the alkali and halogen volatilization during thermal conversion, and provides a means to handle otherwise high fouling biomass fuels. Acknowledgment. We are grateful for the support of the Solar Thermal, Biomass Power, and Hydrogen Technologies Division of the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy. Special thanks go to Richard Bain and Thomas Milne for technical and programmatic support and guidance. The authors would also like to thank the following organizations for their support of various portions of this work: California Energy Commission, Electric Power Research Institute, National Renewable Energy Laboratory, University of California, Hawaii Natural Energy Institute, Sandia National Laboratories, Waialua Sugar Co., Wheelabrator-Shasta Energy Co., Inc., Nebraska Public Power District, and Hydra-Cooperations, Inc. The contributions of these organizations are greatly appreciated. EF980256E (23) Knudsen, N. O.; Jensen, P. A.; Sander, B.; Dam-Johansen, K. Possibilities and evaluation of straw pretreatment. Proceedings Biomass for Energy and Industry, Tenth European Conference and Technology Exhibition, Wurzburg, Germany, 1998; pp 224-228. (24) Jørgensen, U.; Sander, B. Biomass Bioenergy 1997, 12 (3), 145147.

Release of Inorganic Constituents from Leached Biomass during

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