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Energy & Fuels 2009, 23, 502–506
Mineral Content of Grasses Grown for Seed in Low Rainfall Areas of the Pacific Northwest and Analysis of Ash from Gasification of Bluegrass (Poa pratensis L.) Straw† Gary M. Banowetz,* Stephen M. Griffith, and Hossien M. El-Nashaar United States Department of Agriculture (USDA) Agricultural Research SerVice (ARS), 3450 Southwest Campus Way, CorVallis, Oregon 97331 ReceiVed June 20, 2008. ReVised Manuscript ReceiVed September 30, 2008
Straw residue from grass seed production, along with clippings from turfs derived from that seed, represent a potential supply of feedstock for bioenergy production. Harvesting straw from these production systems removes minerals, including macronutrients and carbon that ultimately must be replaced to sustain soil quality and subsequent crop production. In addition, certain mineral constituents of straw contribute to slag formation at common operating temperatures in thermochemical reactors. The objective of this research was to determine whether genotypic differences in the concentrations of minerals that impact thermochemical conversion and represent macronutrient loss existed within and among Kentucky bluegrass (Poa pratensis L.), perennial ryegrass (Lolium perenne L.), and tall fescue [Schedonorus phoenix (Scop.) Holub]. Grasses were grown at two contrasting locations in the low rainfall region of the Pacific Northwest, and mineral analyses were performed on vegetative tissues from each. Differences in the concentration of most minerals occurred among varieties of Kentucky bluegrass and between the three species. Mineral concentration was dependent upon the location at which the plants were grown but not always correlated with soil content of the respective mineral. On the basis of the average amount of straw available after conservation requirements have been met, we estimate that straw harvest removes 48-96 kg of potassium ha-1, 2-10 kg of phosphorus ha-1, and 662-1029 kg of carbon ha-1. Mineral analyses of ash derived from gasification of Kentucky bluegrass straw showed that nitrogen, phosphorus, potassium, and carbon were recovered in the ash and represented a potential soil amendment. Dioxin and heavy-metal concentrations in the ash were very low.
1. Introduction Approximately 90% of the U.S. production of Kentucky bluegrass (Poa pratensis L.) seed occurs in low rainfall (annual precipitation of 350-600 mm) areas of central and eastern Oregon, eastern Washington, and northern Idaho.1 Smaller acreages of perennial ryegrass (Lolium perenne L.) and tall fescue [Schedonorus phoenix (Scop.) Holub] are also produced under these low rainfall conditions. In many cases, straw produced by these cropping systems was burned because of a lack of local markets and the high cost of transporting straw to distant markets. Regional legislation has curtailed the use of field burning because of citizen concerns about air quality and safety. Straw produced by these farming enterprises, along with clippings from residential and municipal turfs, represent an abundant and sustainable feedstock for bioenergy production.2 Straw from seed production is widely distributed at low density across the landscape, and consequently, small-scale distributed thermochemical technologies have been suggested as a potential † Disclaimer: The use of trade, firm, or corporation names in this publication is for the information and convenience of the reader. Such use does not constitute an official endorsement or approval by the United States Department of Agriculture (USDA) or the Agricultural Research Service (ARS) of any product or service to the exclusion of others that may be suitable. * To whom correspondence should be addressed. Telephone: (541) 7384125. Fax: (541) 738-4127. E-mail:
[email protected]. (1) Holman, J. D.; Thill, D. University of Idaho Extension Bulletin 842, 2005; p 11. (2) Banowetz, G. M.; Boateng, A.; Steiner, J. J.; Sethi, V.; El-Nashaar, H. M. Biomass Bioenergy 2008, 32, 629–634.
10.1021/ef800490w
approach to reduce transportation costs associated with converting this feedstock into bioenergy.3 One thermochemical approach to use this biomass is gasification, in which controlled heating conditions convert carboncontaining materials into synthesis gas (syngas), a mixture of carbon monoxide, hydrogen, and other gases.4 After the syngas is cleaned, it may be burned to generate electrical power or used in the catalytic production of chemicals or liquid fuels. Syngas quality and yield greatly depends upon the temperature at which the gasifier is operated, and that temperature is dependent upon the mineral composition of the feedstock and the behavior of the minerals within the thermal environment.5 At higher operational temperatures where carbon (C) conversion efficiency is high, silicon (Si), potassium (K), and other alkalis present in crop residue feedstocks can vaporize and react with other mineral components to form a sticky glass-like substance referred to as slag.6 Slag formation and corrosive alkalis formed during the thermal treatment reduce the lifespan and utility of gasifier hardware and diminish the economic feasibility of converting straw to energy. One approach to reduce slag production is to operate the reactor at a lower temperature, but when bluegrass straw was gasified at lower temperatures, C (3) Boateng, A. A.; Banowetz, G. M.; Steiner, J. J.; Barton, T. F.; Taylor, D. G.; Hicks, K. B.; El-Nashaar, H. M.; Sethi, V. K. Biomass Bioenergy 2007, 31, 153–161. (4) McKendry, P. Bioresour. Technol. 2002, 83, 47–54. (5) Jenkins, B. M.; Baxter, L. L.; Miles, T. R., Jr.; Miles, T. R. Fuel Process. Technol. 1998, 54, 17–46. (6) Miles, T. R.; Miles, T. R., Jr.; Baxter, L. L.; Bryers, R. W.; Jenkins, B. M.; Oden, L. L. Biomass Bioenergy 1996, 10, 125–138.
This article not subject to U.S. Copyright. Published 2009 by the American Chemical Society Published on Web 11/14/2008
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Energy & Fuels, Vol. 23, 2009 503
Table 1. Mean Concentration of Selected Minerals in Aboveground Biomass of Kentucky Bluegrass (P. pratensis L.) Varieties Propagated in Commercial Production Fields in Spokane County, WAa Kentucky bluegrass cultivar mineral
Casablanca
Cheetah
Gady
Ginger
Jefferson
Al B Ca Cl Co Fe K Mg Mn P S Si Zn
145 a 64.2 a 3790 b 3700 b 228 a 657 a 36900 b 1890 a 27.1 a 980 d 203 ab 15900 b 1.61 ab
52 d 33.0 b 2210 c 3050 e 214 c 447 b 30500 e 1090 e 19.7 c 1160 b 98 d 8300 de 1.72 a
38 e 26.5 c 1720 d 4300 a 156 d 296 c 43000 a 960 f 15.2 d 940 d 245 a 7600 e 0.80 e
31 fg 33.1 b 1780 d 3270 d 142 de 244 cde 32700 d 1180 d 6.7 g 1290 a 153 bc 12500 c 1.60 ab
(mg kg-1) 24 h 28.4 c 2120 c 3180 de 105 g 124 f 31800 d 1240 c 6.4 g 930 de 103 cd 12600 c 1.20 cd
C N
343 b 13.0 a
463 a 10.8 c
363 b 13.1 a
298 c 13.4 a
(g kg-1) 461 a 10.5 c
Kenblue
Kenblue (no till)
Parkland
Rampart
29 gh 30.2 c 1700 de 2530 f 114 fg 170 ef 25300 g 1250 c 12.5 e 1060 c 109 cd 8700 d 1.41 bc
35 ef 28.3 c 1530 e 2170 g 129 ef 194 def 21700 h 970 f 9.41 f 850 f 109 cd 8300 de 1.08 de
101 c 34.3 b 1860 d 2900 ef 263 ab 569 a 29000 f 1090 e 21.2 b 870 ef 101 d 12000 c 1.73 a
114 b 60.6 a 4290 a 3490 c 257 b 451 b 34900 c 1570 b 12.2 e 1200 ab 103 cd 21600 a 1.60 ab
434 a 12.7 ab
462 a 10.9 c
448 a 11.3 bc
432 a 10.7 c
a Mean (n ) 4) values within a column followed by the same letter are not significantly different at p ) 0.05 using Fisher’s least significant difference (LSD) mean comparison test.
conversion efficiency was reduced to approximately 35%.3 The high content of these “anti-quality minerals” in straw has limited the implementation of thermochemical approaches to convert this abundant feedstock into energy.6,7 There may be potential to improve the suitability of grass straws as thermochemical feedstock by selective breeding, but little is known about genotypic differences in the accumulation of Si and other minerals among these grasses. Differences among other grasses have been documented.8-11 The amount and seasonal distribution of rainfall is known to impact mineral uptake.12 Rainfall in this region of the Pacific Northwest occurs largely during the period between late fall and early spring.13 Sustainable use of biomass for energy production requires awareness of the quantities of commercial fertilizer required to replace minerals removed from the agroecosystem.14-16 There is potential to sequester C and to replace some of the minerals contained in the biomass by returning co-products of the conversion process to the soil,17 but data are lacking on their concentration in the co-products. Crop management and a variety of biotic and abiotic factors affect the mineral composition of plants,11 and C cycling with grassland agroecosystems
(7) Thompson, D. N.; Shaw, P. G.; Lacey, J. A. Appl. Biochem. Biotechnol. 2003, 105-108, 205–218. (8) Shewmaker, G. E.; Mayland, H. F.; Rosenau, R. C.; Asay, K. H. J. Range Manage. 1989, 42, 122–127. (9) Christian, D. G.; Riche, A. B.; Yates, N. E. Bioresour. Technol. 2002, 83, 115–124. (10) Christian, D. G.; Yates, N. E.; Riche, A. B. J. Sci. Food Agric. 2006, 86, 1181–1188. (11) Lemus, R.; Brummer, E. C.; Moore, K. J.; Molstad, N. E.; Burras, C. L.; Barker, M. F. Biomass Bioenergy 2002, 23, 433–442. (12) Voltas, J.; Romagosa, I.; Munoz, P.; Araus, J. L. Eur. J. Agron. 1998, 9, 147–155. (13) National Climatic Data Center website. 2007 (accessed on Dec 2007). (14) Reynolds, J. H.; Walker, C. L.; Kirchner, M. J. Biomass Bioenergy 2000, 19, 281–286. (15) Edwards, K. A.; Anex, R. P. American Society of Agricultural and Biological Engineers Annual Meeting, Minneapolis, MN, 2007; paper 076078. (16) Wilhelm, W. W.; Johnson, J. M.; Karlen, D. L.; Lightle, D. T. Agron. J. 2007, 99, 1665–1667. (17) Lehmann, J. Nature 2007, 447, 143–144.
is impacted by rainfall, soil-water content, and other local weather conditions.18 The purpose of this research was 2-fold: (1) To determine whether varietal differences affected the concentration of minerals in Kentucky bluegrass, perennial ryegrass, and tall fescue straws produced in contrasting locations within the low rainfall area of the Pacific Northwest and (2) to quantify the quality of char ash produced during gasification as a potential soil amendment. The mineral concentrations in straw from selected varieties of these grasses grown at two locations were quantified, and mineral concentrations, heavy metals, and dioxin content of ash produced from gasification of Kentucky bluegrass straw were analyzed. These data will provide a basis for selection of germplasm appropriate for improving the utility of this biomass as sustainable thermochemical feedstock and in assessing aspects of the sustainability of straw removal. Mineral concentrations of grasses produced in the high rainfall region of the Pacific Northwest will be described in a separate report. 2. Experimental Section 2.1. Plant Biomass. Aboveground plant biomass was collected from Kentucky bluegrass varieties after seed harvest in commercial production fields in Spokane Co., WA (47° 27′ 10.26′′ N, 117° 08′ 4.13′′ W; 714 m elevation). Additional accessions of Kentucky bluegrass, along with perennial ryegrass and tall fescue accessions, were collected at early vegetative stages during the month of April from the USDA Agricultural Research Service Plant Germplasm Introduction farm (46° 44′ 37.50′′ N, 117° 06′ 33.88′′ W; 771 m elevation) in the low rainfall area of eastern Washington. The soil at the Spokane County site was a Freeman silt loam (fine-silty, mixed, superactive, mesic aquandic palexeralfs), while that at Pullman was a Palouse silt loam (silty, mixed, superactive, mesic pachic ultic haploxerolls). Plants were cut 4 cm above the soil surface, transferred to the laboratory, dried at 80 °C for 24 h, ground, and weighed. Three 1 in. diameter soil cores were sampled to a depth of 30 cm from each location. 2.2. Mineral Analysis. Minerals were extracted from plant tissue using microwave-assisted acid digestion [Environmental Protection Agency (EPA) method 3052] with an Ethos D microwave station (18) Knapp, A. K.; Fay, P. A.; Blair, J. M.; Collins, S. L.; Smith, M. D.; Carlisle, J. D.; Harper, C. W.; Danner, B. T.; Lett, M. S.; McCarron, J. K. Science 2002, 298, 2202–2205.
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Banowetz et al.
Table 2. Mean Concentration of Selected Minerals in Aboveground Biomass of Accessions of Tall Fescue [S. phoenix (Scop.) Holub], Perennial Ryegrass (L. perenne L.), and Kentucky Bluegrass (P. pratensis L.) Collected, Propagated at USDA Field Plot in Whitman County, WAa grass species and accession number tall fescue mineral
600689
600898
600899
perennial ryegrass 600970
255175
403887
403891
Al B Ca Cl Co Fe K Mg Mn P S Si Zn
338 a 30.8 ab 5130 b 2580 a 406 ab 470 ab 31600 a 2770 a 116 a 3140 a 545 a 2850 a 27.2 a
245 a 22.2 b 5270 ab 2830 a 304 b 355 b 32700 a 2220 a 98 a 3690 a 528 a 2610 a 39.5 a
419 a 40.1 a 6450 a 3380 a 443 ab 499 ab 30000 a 3000 a 129 a 3100 a 558 a 2610 a 25.1 a
502 a 38.9 ab 5840 ab 3010 a 545 a 626 a 28500 a 2540 a 110 a 3360 a 525 a 2510 a 39.9 a
806 a 64.6 a 6400 a 2150 b 855 a 933 a 31200 a 1840 a 121 a 3380 a 530 a 2000 b 54.7 a
(mg kg-1) 495 b 829 a 46.1 bc 45.1 c 5690 ab 5510 ab 2690 ab 3460 a 534 b 101 c 611 c 900 a 30800 a 28900 a 1820 a 1960 a 108 ab 82 b 3420 a 3120 a 515 ab 538 a 2000 b 2440 ab 66.3 a 69.3 a
C N
396 b 30.5 ab
394 b 27.4 b
411 a 30.8 a
395 b 28.0 ab
401 a 28.2 a
399 ab 24.6 a
(g kg-1) 395 ab 26.0 a
Kentucky bluegrass 403896
25715
25762
34918
371775
578847
646 b 59.4 ab 4980 b 3250 a 666 ab 762 b 28900 a 1850 a 91 ab 3000 a 498 b 2740 a 62.2 a
391 a 38.9 a 3880 c 2690 ab 432 b 500 b 25800 a 1530 c 157 a 4400 a 553 a 2230 b 54.9 b
786 a 53.1 a 6050 a 2170 bc 850 ab 958 a 25100 a 2160 a 116 b 3900 b 555 a 2520 a 67.4 ab
531 a 35.2 b 3700 c 1950 c 635 ab 710 ab 24300 a 1820 b 115 b 3510 b 520 b 2260 b 72.1 ab
564 a 37.1 b 5050 b 2910 a 561 ab 647 ab 24700 a 2090 a 109 b 3740 b 543 a 2160 b 71.2 ab
808 a 38.0 b 5120 b 2580 ab 927 a 104 c 25900 a 1800 b 158 a 2970 c 538 ab 2280 b 89.8 a
393 b 23.0 a
413 a 34.1 ab
406 ab 36.8 a
400 bc 26.1 c
403 bc 31.4 b
399 c 25.2 c
a Mean (n ) 4) values for respective species followed by the same letter are not significantly different at p ) 0.05 using Fisher’s LSD mean comparison test.
(Milestone, Monroe, CT) and analyzed by inductive coupled plasma optical emission spectroscopy (ICP-OES) (Perkin-Elmer Life and Analytical Sciences, Shelton, CT). A quality control check standard (QCCS), representing the approximate midpoint of the calibration range for each mineral was included as the first sample in each set and was repeated following every 10th sample. All QCCS value determinations were within 10% of the actual concentration. Plant chloride (Cl) content was quantified on tissue samples (25 g) that were extracted with 100 mL of deionized water, shaken for 30 min at 350 rpm, and filtered through Whatman Qualitative Number 42 filters (Florham Park, NJ) that had been washed 3 times with 1% H2SO4 (v/v) and deionized water. The filtrate was analyzed colorimetrically for Cl (QuickChem method 10-117-07-1-C) on a Lachat flow injection autoanalyzer (Hach Co., Loveland, CO). Nitrogen (N), sulfur (S), and C in tissue ground in a Tecator Cyclotec 1093 sample mill were quantified using a Perkin-Elmer 2400 Series II CHNS/O analyzer (Shelton, CT). Soil pH was determined on a slurry consisting of 1:2 ddH2O/soil. Soil organic matter was determined by loss on ignition at 500 °C, after 4 h. 2.3. Quantification of Available Minerals. Available quantities of phosphorus (P), N, K, and S were determined by the Oregon State University Soil Testing Laboratory as described.19 2.4. Analysis of Ash from Gasification of Kentucky Bluegrass Straw. Kentucky bluegrass straw was gasified as described,3 and mineral content of the ash resulting from gasification was quantified using the ICP-OES approach described above. Heavy metals, dioxins, and furans in the ash were quantified by gas chromatography-mass spectrometry (GC-MS) by Columbia Analytical Services (Kelso, WA). Heavy metal analysis was performed using EPA Method 200.8 (arsenic, cadmium, cobalt, lead, molybdenum, nickel, selenium, and zinc) and EPA Method 7471A (mercury). Dioxins and furans were analyzed using EPA method 8290, a high-resolution GC/MS approach. The potential for leaching of heavy metals from the ash was estimated using the standard toxicity characteristic leaching procedure (TCLP; EPA Method 1311). 2.5. Statistical Analyses. Statistical analyses of mean differences between species and plant tissue were calculated by analysis of variance (ANOVA) using (PROC GLM, SAS, Statistical Analysis System Institute, Cary, NC). In the ANOVA, location was considered random and varieties were considered fixed. Means of (19) Gavlak, R.; Horneck, D.: Miller, R.; Kotuby-Amacher, J. Soil, Plant and Water Reference Methods for the Western Region; Utah State University, Logan, UT, 2003.
all minerals were evaluated for main effects, and interaction of the locations and varieties were compared using Tukey’s studentized range (HSD) test (PROC GLM, SAS). Mean differences among tissues within each species were also subject to ANOVA. All differences reported are significant at p e 0.05, unless otherwise stated. Tukey’s test was used for the multiple comparisons of the observed means.
3. Results and Discussion Apparent genotypic variation in mineral concentrations occurred between the eight Kentucky bluegrass varieties produced in Spokane County, WA (Table 1). Over 2-fold differences in the accumulation of calcium (Ca), Cl, K, S, and Si, minerals frequently associated with slag formation during thermochemical processing, occurred among these varieties. Differences in the concentration of P, a mineral with fertilizer value and one that occurs in boiler deposits,6 were significant but smaller in magnitude. The range of Si accumulation varied almost 3-fold among these varieties, but concentrations of C fell within a relatively narrow range. Straw from “Gady” contained the greatest quantities of Cl, K, and S but contained the least amount of Si. In contrast, “Casablanca” straw contained the greatest quantities of aluminum (Al), boron (B), iron (Fe), magnesium (Mg), and manganese (Mn). “Ginger” and ‘Rampart’ contained the most P. “Kenblue” contained relatively small quantities of Ca, Cl, K, and P. The Ca, Mg, P, K, and Si concentrations measured in these varieties were similar to those found in reed canarygrass20 and Cave-in-Rock switchgrass,21 although the Cl concentration was greater in these Kentucky bluegrass varieties. In contrast, the concentrations of K in three varieties of switchgrass propagated in Pennsylvania22 were less than those measured in these Kentucky bluegrass varieties. Their switchgrass plants were harvested 70-80 days later in the season than the Kentucky bluegrass varieties in our study. Delayed harvest significantly reduces the mineral content, including that of K, in grasses.10,20,21,23 The contrasting mineral concentrations among Kentucky bluegrass varieties propagated under similar environmental and soil conditions in Spokane County suggests a potential genetic basis for mineral accumulation among these genotypes. Genetic mechanisms regulating Si uptake in rice have been described,
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Energy & Fuels, Vol. 23, 2009 505
Table 3. Total and Available Mineral Content of Soils Collected in Spokane and Whitman Counties, WA, and Ash Derived from Gasification of Kentucky Bluegrass (P. pratensis L.) Strawa Spokane County mineral
total
Whitman County available
total
ash
available
total
available
(mg kg-1) Al B Ca Cl Co Cu Fe K Mg Mn P S Si Zn NO3-N NH4-N
63311 (1479) 675 (14) 10954 (245) NA nd 22.3 (0.6) 23747 (671) 88377 (1004) 6073 (242) 839 (30) 687 (16) 248 (3.4) 154354 (2279) 84 (2) NA NA
ndb nd nd nd nd nd nd 135 (17) nd nd 43.(6) 78 (6) nd nd nd nd
25548 (17) 254 (2) 6680 (50) NA 24255 (21) NA 25980 (21) 4783 (290) 5538 (468) 683 (83) 843 (89) 535 (6.8) 205400 (7700) 177 (15) NA NA
C N pH
17.4 (1.17) 3.37 (0.19) 5.01
nd 1.00 (0.03)c
21.8 (1.4) 5.38 (0.22) 6.20
nd nd nd nd nd nd nd 182 (16) nd nd 120 (13) 83 (3) nd nd nd nd
32260 (5807) nd nd 814 (190) nd 63 (2.07) 53690 (6820) 94855 (9874) 7850 (826) 450 (5.27) 1493 (179) 4650 (0.095) 33255 (2904) 63.3 (3.26) 1.37 (0.018) 17.2 (2.36)
nd nd nd nd nd nd nd 7795 (2270) nd nd 189 (11) 2200 (10) nd nd nd nd
nd 1.15 (0.03)
463 (16.7) 49.1 (2.6) 8.35
nd 18.6 (2.4)
(g kg-1)
a
Values represent mean (n ) 4) with standard error in parentheses. b Not determined. c Available N ) nitrate-N + ammonia-N.
Table 4. Comparison of Aboveground Biomass Mineral Concentration by Three Species of Grass Grown at a USDA Field Plot in Whitman County, WAa grass species mineral
tall fescue
Al B Ca Cl Co Fe K Mg Mn P S Si Zn
377 b 33.0 b 5670 a 2890 a 424 b 487 b 30690 a 2630 a 113 b 3320 b 539 a 2640 a 32.9 b
perennial ryegrass (mg kg-1) 694 a 53.8 a 5640 a 2950 a 767 a 801 a 29910 a 1870 b 100 b 3230 b 520 b 2300 b 63.1 a
398 b 29.2 b
397 b 25.5 c
a
arsenic cadmium cobalt lead mercury molybdenum nickel selenium zinc a
EPA Method 200.8 EPA Method 200.8 EPA Method 200.8 EPA Method 200.8 EPA Method 7471A EPA Method 200.8 EPA Method 200.8 EPA Method 200.8 EPA Method 200.8
ash content (mg L-1)
5 100 1 5 5 0.2 1 5
nd nd nd 1.57 nd nd nd nd
nd ) not detected.
Table 7. Dioxin and Furan Content of Ash Derived from the Gasification of Kentucky Bluegrass (P. pratensis L.) Strawa TEF-adjusted (ng kg -1) concentrationb
dioxin
404 a 30.7 a
Table 5. Heavy-Metal Content of Ash Derived from the Gasification of Kentucky Bluegrass (P. pratensis L.) Strawa analysis method
regulatory limit (mg L-1)
arsenic barium cadmium chromium lead mercury selenium silver
616 a 40.5 b 4760 b 2460 b 681 a 771 a 25160 b 1880 b 131 a 3700 a 541 a 2290 b 71.1 a
a Mean (n ) 4) values for respective species represent pooled data from multiple accessions of each species. Values within rows followed by the same letter are not significantly different at p ) 0.05 using Fisher’s LSD mean comparison test.
analyte
analyte
Kentucky bluegrass
(g kg-1) C N
Table 6. Heavy-Metal Leachability of Ash Derived from the Gasification of Kentucky Bluegrass (P. pratensis L.) Straw as Determined by the Standard TCLP Method (EPA Method 1311)a
(mg kg-1) 10.7 0.19 18.3 9.04 nd 47.0 105 nd 95.5
nd ) not detected.
suggesting the potential for genetic improvement in reducing the Si concentration in other grasses.24 In our study, “Kenblue” contained smaller quantities of Ca, Cl, K, P, and Si relative to those measured in “Rampart”. Tillage methods used for crop
2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) 1,2,3,7,8-pentachlorodibenzo-p-dioxin 1,2,3,4,7,8-hexachlorodibenzo-p-dioxin 1,2,3,6,7,8-hexachlorodibenzo-p-dioxin 1,2,3,7,8,9-hexachlorodibenzo-p-dioxin 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin 1,2,3,4,6,7,8,9-octachlorodibenzo-p-dioxin 2,3,7,8-tetrachlorodibenzofuran 1,2,3,7,8-pentachlorodibenzofuran 2,3,4,7,8-pentachlorodibenzofuran 1,2,3,4,7,8-hexachlorodibenzofuran 1,2,3,6,7,8-hexachlorodibenzofuran 1,2,3,7,8,9-hexachlorodibenzofuran 2,3,4,6,7,8-hexachlorodibenzofuran 1,2,3,4,6,7,8-heptachlorodibenzofuran 1,2,3,4,7,8,9-heptachlorodibenzofuran 1,2,3,4,6,7,8,9-octachlorodibenzofuran a nd ) not detected. TCDD.
b
nd nd nd nd nd 4.348 25.196 nd nd nd nd nd nd nd nd nd nd
4.35 × 10-2 2.52 × 10-3
TEF ) toxic equivalency factor relative to
establishment had relatively small impact on mineral accumulation by “Kenblue” straw. Significant genotypic variation in mineral concentration also occurred in grasses collected from the USDA Plant Germplasm Introduction and Testing research farm in Pullman, WA (Table 2), but the variation among varieties was less than that observed in straws collected from Spokane County. Variation in P and
506 Energy & Fuels, Vol. 23, 2009
Si concentrations was apparent, but Si concentrations in tissues collected from the Pullman site were 3-6-fold lower than the levels measured in Kentucky bluegrass from Spokane County. The plants collected in Pullman consisted solely of vegetative tissues representing growth stages of early spring, in contrast to the postharvest tissues collected in Spokane County. The data collected from plants grown in Pullman are likely more representative of mineral concentrations that would occur in clippings from turfs developed from the respective varieties. As a consequence, a comparison of mineral concentrations between plants from the two locations is not representative of what might be expected with straws collected after seed harvest. Mineral concentrations and tissue partitioning change markedly during plant growth as minerals are translocated from vegetative to reproductive tissues.25 Calcium levels in vegetative tissues of grasses from Pullman were generally greater than those measured in whole plant tissue collected in Spokane County, even though the soil content of this mineral was similar in both locations (Table 3). When pooled data from each accession of tall fescue, perennial ryegrass, and Kentucky bluegrass grown at Pullman were compared, significant variation was found among the species (Table 4), although differences in Ca, Cl, K, and Si were relatively small. On average, the concentration of minerals in Kentucky bluegrass that impact thermochemical conversion was less than that of the other two species. A previous study demonstrated that the amount of available grass seed or cereal grain straw remaining after conservation residue requirements are met is approximately 2.2 Mg ha-1 in the Pacific Northwest.2 On the basis of the range of minerals presented in Tables 1 and 2, straw harvest would remove 48-96 kg of K ha-1, 2-10 kg of P ha-1, and 662-1029 kg of C ha-1. Much of the K and P can be recovered in ash derived from the gasification of this straw (Table 3). The practical use of ash to recover macronutrients and sequester C is, however, dependent upon whether undesirable minerals or compounds are concentrated in the ash or formed within the thermal environment. Heavy-metal analyses showed that the ash contained relatively small quantities of arsenic, cadmium, cobalt, lead, molybdenum, nickel, and zinc (Table 5). All heavy metals, except nickel, that were analyzed were present at levels below 100 ppm, and six of the nine metals were present at less than 20 ppm. Mercury and selenium were not detected. Leachability analyses indicated that very small quantities of chromium were leached from the ash, but no leaching was detected for seven of the eight analytes (Table 6). Dioxin and furan formation can occur at elevated temperatures, where carbon and chlorine are present.26 Analyses of ash derived from the gasification of Kentucky bluegrass straw (20) Burvall, J. Biomass Bioenergy 1997, 12, 149–154. (21) Dien, B. S.; Jung, H.-J. G.; Vogel, K. P.; Casler, M. D.; Lamb, J. F. S.; Iten, L.; Mitchell, R. B.; Sarath, G. Biomass Bioenergy 2006, 30, 880–891. (22) Adler, P. R.; Sanderson, M. A.; Boateng, A. A.; Weimer, P. J.; Jung, H.-J. Agron. J. 2006, 98, 1518–1525.
Banowetz et al.
showed that dioxins and furans were present at minimal levels with hepta- and octachloro-dioxin derivatives detected at the parts per trillion level (Table 7). The remaining 13 analytes were not detected. 4. Conclusions Straw produced from perennial grass seed production in the low rainfall areas of the Pacific Northwest has potential for use as a nonfood feedstock for bioenergy production. Genotypic differences in the concentrations of minerals that impact thermoconversion of straw exist, and the 2-fold or greater range of these differences suggests that germplasm with improved combustion characteristics could be selected. The range of variability was greater in straw collected after seed harvest than that observed in vegetative material collected earlier in the plant life cycle. Even at current energy costs, straw conversion to bioenergy represents a value-added enterprise secondary to seed production; therefore, genetic improvement will need to be accomplished without impacting seed yield and quality. Straw from mature plants contained 4-8 times the concentration of Si, a mineral associated with slag production, suggesting a greater immediate potential for use of turf clippings in thermochemical processes. The Kentucky bluegrass cultivar “Gady” contained the least amount of Si, at 7600 mg kg-1, while “Rampart” contained the greatest quantities, at 21 600 mg kg-1. In contrast, “Gady” contained the greatest quantities of K and Cl, minerals also associated with slagging in thermal reactors. The relatively low quantities of heavy metals (
