9882
Ind. Eng. Chem. Res. 2008, 47, 9882–9888
Banagrass vs Eucalyptus Wood as Feedstocks for Metallurgical Biocarbon Production† Takuya Yoshida,‡ Scott Q. Turn,‡ Russell S. Yost,§ and Michael Jerry Antal, Jr.*,‡ Hawaii Natural Energy Institute, School of Ocean and Earth Science and Technology, and Department of Tropical Plant and Soil Sciences, UniVersity of Hawaii at Manoa, Hawaii
Excessive emissions of fossil CO2 are known to be a primary cause of global climate change. Emissions from the iron and steel-making industries account for 5-6% of global fossil CO2 emissions. Biocarbon (i.e., charcoal) could be used to replace the coal currently employed to smelt iron ore and thereby reduce fossil CO2 emissions. In Brazil, Eucalyptus wood charcoal is used to smelt iron ore, but there is interest in the use of charcoal produced from other biomass feedstocks. Banagrass, a variety of elephantgrass (Pennisetum purpureum, Schum.), which produces near-record amounts of biomass, is a promising biomass candidate for charcoal production in Brazil and elsewhere. In this paper we describe results of charcoal production from banagrass of different ages and states of demineralization. Mature banagrass provides the highest yields of biocarbon. In addition to its maturity, the structure of the feedstock strongly influences the fixed-carbon yield. Our results indicate that banagrass may be preferred to Eucalyptus wood as a promising feedstock for metallurgical biocarbon production. Introduction Emissions of CO2 are now the biggest issue related to global climate change. Although many countries are making every effort to reduce CO2 emissions, the total amount continues to increase. Global CO2 emissions from fuel combustion in 2005 was 27.1 Gt; of which 4.6 Gt was from automotive transportation and 5.2 Gt was from manufacturing industries and construction.1 The iron and steel industry is one of the largest constituents of the manufacturing sector and plays an important role in global economic growth. CO2 emissions from the iron and steel industry accounted for 1.4 Gt or 5.8% of the global CO2 emissions due to fossil fuel use in 2000.2 World crude steel production increased from 848 Mt in 2000 to 1344 Mt (estimated) in 2007,3 a result of the rapid economic growth in China, India, and other developing countries. Most of the CO2 emissions associated with conventional crude steelmaking results from the reduction process in a blast furnace,4 whereby coke made from hard coal and/or pulverized coal made from steam coal are used to convert iron ore into iron. Prior to the discovery of coal, charcoal (i.e., biocarbon) was used as the reducing agent. CO2 emissions from the iron and steel industry can be diminished by substituting renewable charcoal made from biomass for coal. Some developing countries have made efforts to use charcoal produced from biomass resources in the iron and steel industry. For example, blast furnaces are operated in Brazil using charcoal produced from Eucalyptus wood that is cultivated on a neighboring plantation.5 Because biomass feedstocks possess widely varying properties, a variety of materials is being evaluated for their suitability for biocarbon production. Herbaceous species are promising candidates because of their very high growth rate and relative * To whom correspondence should be addressed. Tel.: 808-956-7267. Fax: 808-956-2336. E-mail:
[email protected]. † We dedicate this paper to Michael Lurvey of Carbon Diversion Corp. and Edward Griffiths of Pacific Carbon and Graphite LLC with thanks for their early support of Flash Carbonization research at UH. § Department of Tropical Plant and Soil Sciences. ‡ Hawaii Natural Energy Institute, School of Ocean and Earth Science and Technology.
ease of harvest. Banagrass (Pennisetum purpureum), a variety of elephant grass, has demonstrated biomass yields of 50.76 to 67 dry-t/ha/year7 and is one species under consideration. Another advantage of banagrass is that the startup time is far less than with tree crop biomass such as eucalyptus. Banagrass can be seeded or sprigged in and productive within 6 months. Several research studies have been conducted to investigate its fuel properties and its use as a feedstock for energy conversion processes.8-10 In general, fast growing, herbaceous biomass species such as banagrass have higher ash content than woody biomass. The effect of this inorganic fraction on the carbonization process as well as on the metallurgical reduction process is of importance in evaluating the potential of biocarbon substitution for fossil reductants. Mechanical strength and low impurity content are important requirements for carbons used as iron ore reductants. Higher mechanical strength is preferable in order to withstand abrasion, thermal shock, and the weakening reactions with carbon dioxide and alkali that occur in a blast furnace.11 Impurities (moisture, ash, and volatile content, etc.) are generally associated with negative effects (reduced coke energy content and increased slag formation, etc.) on the iron production process; consequently carbons with low ash contents are preferred. If the carbon reductant contains more than 10 wt % ash, the ash chemistrys such as the ratio of basic (Fe2O3, CaO, MgO, Na2O, and K2O) and acidic (SiO2 and Al2O3) components which affect the reactivity of the carbon with CO2 (i.e., the Boudouard reaction C + CO2 f 2CO)smust be acceptable.11 Although many indices of quality have been established for coking coals, there is an absence of similar parameters for metallurgical charcoal, and it is not clear that the coal-based indices are equally applicable to charcoal. The flash carbonization (FC) process12-15 quickly and efficiently converts biomass into charcoal. This process has been demonstrated with various biomass resources and has attained carbon yields approaching the theoretical values defined by thermochemical equilibrium.12-15 Feedstocks used in this work included banagrass samples of different maturities, as well as banagrass that was demineralized using water leaching techniques. Each sample material was carbonized using the FC
10.1021/ie801123a CCC: $40.75 2008 American Chemical Society Published on Web 11/19/2008
Ind. Eng. Chem. Res., Vol. 47, No. 24, 2008 9883 a
Table 1. Elemental Analysis Results of Feedstocks
elemental analysis for given sample IDb Y06
Y12
M05
M12
NL
ash contentc (wt % dry basis) 16.66 12.50 13.30 15.12 7.30 ultimate analysisd (wt %, dry basis) C 38.16 38.94 41.09 41.11 44.70 H 5.37 5.44 5.51 5.45 5.78 O 39.72 39.91 41.38 41.06 39.89 N 1.72 2.13 1.18 0.99 0.86 S 0.14 0.17 0.12 0.10 0.07 Cl na na na na 0.57 ash 15.84 15.00 10.51 9.94 8.15 ash analysis (wt %, ashed sample weight basis) Al2O3 1.15 0.72 na 0.24 0.15 CaO 3.60 4.59 na 3.60 3.28 Fe2O3 1.20 0.79 na 0.28 1.58 MgO 4.66 4.21 na 4.84 5.37 MnO 0.15 0.11 na 0.08 na P2O5 4.05 6.32 na 4.43 6.40 39.74 42.50 na 28.44 21.00 K2 O SiO2 37.88 41.35 na 52.85 50.70 0.46 0.43 na 0.51 0.83 Na2O SO3 0.45 0.68 na 1.35 0.75 TiO2 0.29 0.18 na 0.05 0.07 Cl na na na na 5.79
L 6.38 46.29 5.91 42.17 0.37 0.06 0.05 5.15 0.39 4.79 3.00 2.30 na 1.71 4.65 80.84 0.48 0.37 0.12 0.19
a na ) not analyzed. b Y06, young banagrass harvested on May 2006 at age 4 weeks; Y12, young banagrass harvested on December 2006 at age 4 weeks; M05, mature banagrass harvested on April 2006 at age 10 weeks; M12, mature banagrass harvested on December 2006 at age 10 weeks; NL, shredded nonleached banagrass; L, shredded leached banagrass. The NL and L banagrass were cultivated on the same schedule and harvested at age several years. c Ash content determined by ASTM E 1755-95. d Y06, Y12, M05, and M12 were measured by Huffman Laboratories, Inc. NL and L were measured by Hazen Research Inc.
process, and the effects of process conditions and the feedstock characteristics on the yield and properties of the product charcoal were evaluated. The aim of the present work was to compare the suitability of banagrass to Eucalyptus wood for use as feedstocks for metallurgical charcoal production. Apparatus and Experimental Procedures. Banagrass samples having different maturities (see Table 1) were collected in Hawaii. Two mature banagrass samples (M05, M12) with an age of 10 weeks were harvested at the Waianae experimental site, cut into pieces less than 25 cm length, and oven-dried or air-dried. Two young banagrasses (Y12, Y06) were also harvested at the Waianae experimental site at age 4 weeks. One of them (Y12) was dried in an oven, and the other (Y06) was air-dried. NL and L banagrass were harvested from wind breaks at the Hawaii Agriculture Research Center’s Kunia station. Although their precise age is not known, the plants are several years old and have been irrigated and fertilized on the same schedule as papaya plantings in an adjacent field. Both the NL and L samples were shredded into small pieces and then airdried (NL) or leached by water and air-dried (L).10 Previous analysis of banagrass leached using these techniques determined that 88 and 98% of the original potassium and chlorine (respectively) were removed, while 91% of the original silicon remained.10 Prior to carbonization the samples were stored in the laboratory; therefore, the moisture contents varied somewhat depending on the conditions in the laboratory on the day of the experiment. Table 1 displays elemental analyses of these samples. Because the ash analyses of M12, Y06, and Y12 reported by Huffman Laboratories were on a dried sample weight basis, we converted them to an ashed-sample weight basis. This study employed procedures similar to those used in our earlier work.12,15 A measured amount of banagrass was placed inside a cylindrical canister. At the same time, additional
samples of the feedstock were taken for analysis of their moisture content. Then the canister was loaded into a pressure vessel. The pressure vessel was subsequently pressurized to 1.14 (150 psig), 1.60 (230 psig), or 2.17 MPa (300 psig) by air. The ignition heater, which was located in the pressure vessel at the bottom of the canister, was turned on to ignite the biomass at the bottom. Ignition occurred within a minute. About 4 min after the heater was energized; the pressure of the reactor was lowered to the desired reaction pressure by releasing gas from the bottom of the vessel. Thereafter pressurized air was delivered from an air accumulator to the top of the vessel. The flame front moved against the air flow from the bottom to the top of the canister, thereby converting the biomass into charcoal. At 6 min after the ignition heater was turned on, it was turned off. When the desired amount of air was delivered, the air flow was halted. The oxygen concentration in the effluent gas from the carbonizer was monitored by an oxygen analyzer (Bacharach, Inc., model OXORII) and recorded. The amount of delivered air was calculated from the pressure levels of the accumulator before and after the experiment. The reactor was depressurized and cooled, and finally, the products of carbonization were removed for analysis. The charcoal product was usually divided into three sections and equilibrated in the open air prior to subsequent analysis. The charcoal in each section was weighed and a representative sample from each section was subjected to proximate analysis according to ASTM D1762-84. In addition, some of the charcoal samples were ashed and subjected to analysis. The majority of the ash analyses were performed by the Huffman Laboratories Inc. We provided feed samples that were ashed in our laboratory at 575 °C (following ASTM E 1755-95) and charcoal samples that were ashed by us at 750 °C (following ASTM D 1762-48). Huffman Laboratories then ashed the samples again at 750 °C to determine their ash content and subsequently subjected the twice-ashed materials to oxidation at 950 °C for quantitative mineral analysis. Because some of the ash constituents volatilized at 950 °C, the ash analyses do not necessarily sum to 100%. Ash analyses of the NL and L feedstocks were performed by Hazen Research Inc. These feeds were ashed at 600 °C, and the resulting ashes were digested and subsequently analyzed using ICP atomic absorption or optical emission. Results and Discussion Table 2 summarizes the reaction conditions and the results of this work. As in our previous work,12 we define the charcoal yield ychar, the fixed-carbon yield yfC, volatile matter (VM) yield yVM, and ash recovery yash as follows: ychar )
mchar mbio
( 100 -%% fCfeed ash ) % VM )y ( 100 - % feed ash ) % char ash y )y ( % feed ash )
(1)
yfC ) ychar
(2)
yVM
(3)
char
ash
char
(4)
where mchar and mbio represent the dry mass charcoal and feedstock respectively, % fC, % VM, and % feed ash denote the percentage of fixed carbon contained in the charcoal, the percentage of volatile matter in the charcoal, and the percentage of ash in the feedstock, respectively.
9884 Ind. Eng. Chem. Res., Vol. 47, No. 24, 2008 Table 2. Experimental Conditions and Resultsa moisture content feedstock run date in feed (%), no. yymmdd g, wet wbb 1 2 3 4 5 6 7 8 9 10 11h 12 13 14i 15j 16j 17j 18 19 20 21 22 23 24 25
061017 061130 061206 070111 070126 061220 061227 070522 060512 070124 061026 070209 070216 070517 070524 070529 070531 070726 070828 071025 070822 070720 070820 071010 071017
Y06 Y06 Y06 Y06 Y06 Y12 Y12 Y12 M05 M05 M05 M12 M12 M12 M12 M12 M12 NL NL NL NL NL L L L
514.17 182.11 182.11 182.11 182.11 182.11 182.11 236.81 514.17 182.11 514.17 182.11 182.11 200 273.75 273.75 273.75 426.13 426.13 426.13 426.13 426.13 426.13 426.13 426.13
13.3 11.4 15.6 11.1 11.8 7.3 7.8 8.2 8.8 7.9 8.4 5.5 5.6 10.1 10.6 10.2 10.2 10.3 10.6 9.9 10.5 11.0 9.7 9.6 9.3
reactor pressure (MPa)
ABRc of delivered air (kg/kg)
total ABRd (kg/kg)
superficial velocitye (mm/s)
ychar (%)
yfC (%)
yVM (%)
yash (%)
2.17 2.17 1.14 1.14 1.14 1.14 1.14 1.14 2.17 1.14 1.14 1.14 1.14 1.14 1.69 1.48 1.14 1.14 1.14 1.14 1.14 1.14 1.14 1.14
0.88 1.29 1.35 1.14 1.50 1.11 0.59 1.23 1.04 1.46 1.18 1.20 1.30 1.32 1.12 1.15 2.48 2.80 2.81 1.72 1.35 1.72 1.83 1.76
0.96 1.53 1.48 1.26 1.63 1.23 0.71 1.32 1.12 1.58 0.09 1.29 1.32 1.37 1.40 1.24 1.23 2.53 2.85 2.86 1.78 1.40 1.77 1.88 1.81
5.3 5.4 10.5 7.2 4.0 7.0 7.4 5.8 5.7 4.3 7.0 11.5 7.0 4.8 3.8 4.5 10.4 11.1 18.1 10.3 8.4 10.2 13.9 17.9
42.9 33.8 36.2 38.0 na 30.7 38.2g na 35.2 35.1 na 40.0 36.0 40.9 41.5 38.1 39.3 30.0 27.6 25.6 na na 28.9 26.7 25.7
18.5 13.3 18.8 17.9 na 15.2 19.1g na 19.2 18.3 na 22.9 22.0 23.8 23.2 22.9 22.5 20.5 20.4 18.4 na na 20.9 20.8 20.3
12.1 7.9 7.2 9.5 na 4.9 9.8g na 7.6 9.1 na 8.9 6.3 10.8 11.7 8.3 8.7 3.3 1.8 1.6 na na 4.1 2.0 1.2
104.8 97.4 87.3 91.3 na 104.8 103.7g na 89.5 73.4 na 86.0 79.2 77.2 77.8 76.7 84.8 103.6 90.8 92.4 na na 85.4 84.2 86.6
remarksf O U-t, high O2 PU-t PU-t, high O2 U-t
U-t U-t, m, high O2
a na ) not analyzed. b wb ) wet basis. c ABR ) air-biomass ratio. d ABR of initial loaded air in reactor + delivered air. e Superficial velocity of the air flow at 298 K and reactor pressure. f U-t:, uncarbonized top section; O, overcarbonized; PU-t, partly carbonized top section; U-t,m, uncarbonized top and middle sections; high O2, oxygen concentration in the effluent gas exceeded 5%. g Charcoal was analyzed mixed with uncarbonized banagrass. h Conducted without air delivery. i With 144.74 g of corncob at bottom. j 91.25 g of chopped banagrass (
