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Ind. Eng. Chem. Res. 2000, 39, 4024-4031
Attainment of the Theoretical Yield of Carbon from Biomass Michael Jerry Antal, Jr.,*,† Stephen G. Allen,† Xiangfeng Dai,† Brent Shimizu,† Man S. Tam,† and Morten Grønli‡ Hawaii Natural Energy Institute, University of Hawaii at Manoa, Honolulu, Hawaii 96822, and SINTEF Energy Research, Thermal Energy and Hydropower, 7034 Trondheim, Norway
Previous work has shown that very high yields of charcoal are obtained when pyrolysis of the biomass feedstock is conducted at elevated pressure in a closed vessel, wherein the pyrolytic vapors are held captive and in contact with the solid products of pyrolysis. In this paper, we show that, for some biomass species, the yield of carbon produced by this process effectively attains the theoretical value predicted to exist when thermochemical equilibrium is realized. Various agricultural wastes (e.g., kukui nut, macadamia nut, and pecan shells) and tropical species (e.g., eucalyptus, leucaena, and bamboo) offer higher yields of carbon than the hardwoods traditionally employed by industry in the U.S. and Europe. Moreover, the yields of carbon from oat and rice hulls and from sunflower seed hulls are nearly as high as the yields of carbon from hardwoods. There is a correlation between the yield of carbon and the acid-insoluble lignin content of the feed. Charcoal briquettes made from agricultural wastes and lump charcoal from tropical species are promising sources of renewable carbon for use in the smelting of metal ores. Introduction The smelting of metal ores is a technological pillar of civilization. Before the dawn of recorded history, charcoal was used to smelt tin needed for the manufacture of bronze tools.1 Modern technology still relies on charcoal to smelt metal ores. For example, wood charcoal (as well as coal and coke) is used to reduce silicon dioxide to silicon in an arc furnace2 according to the idealized reaction SiO2(s) + 2C(s) ) Si(s) + 2CO(g). Very high purity silicon is used to manufacture semiconductors (silicon with impurities in the parts per billion range) and photovoltaic cells (silicon with impurities in the parts per million range). Silicon is also used as an alloy in the production of steel, cast iron, aluminum, and other metals. The Norwegian ferrosilicon industry consumed between 70 000 and 100 000 tonnes (mt) of wood charcoal during 1998. To reduce its consumption of coal, Norway may increase its reliance on charcoal by as much as 300 000 mt/year (depending on the charcoal price). Currently, the Norwegian ferrosilicon industry imports charcoal from Asia and South America (Brazil) at a price (including transportation costs) of about 250 Euro per mt of fixed carbon. Washed coal and coke are much cheaper (i.e., about 180 Euro per mt of fixed carbon). Despite its high price, wood charcoal is able to compete with fossil carbons because of its relative purity (low ash content) and high reactivity. Moreover, the Norwegian ferrosilicon industry anticipates new taxes on fossil carbon that will further improve the competitive position of renewable carbons. In the U.S., charcoal is usually manufactured from hardwoods by pyrolysis in large kilns or retorts.3 Worldwide charcoal production is estimated to lie in the range of 264 to 1005 million mt per year and is growing at an estimated rate of about 3% per year.4 The yield of * Author to whom correspondence should be addressed. Phone: 808/956-7267. Fax: 808/956-2336. E-mail: antal@wiliki. eng.hawaii.edu. † University of Hawaii at Manoa. ‡ SINTEF Energy Research.
charcoal manufactured from hardwoods in a Missouri kiln operated on a 7-12 day cycle does not exceed 31 wt %.6 Yields ranging from 26 to 38 wt % have been reported by skillful researchers using metal partial combustion kilns.7 Less efficient processes are widely employed in the developing world.8 Such processes are an important cause of deforestation in many tropical countries (including Thailand, Haiti, and Madagascar). Because of pollution associated with the inefficient conversion of biomass to charcoal, the charcoal fuel cycle is among the most greenhouse-gas-intensive energy sources employed by mankind.8 In 1985, the Hawaii Natural Energy Institute (HNEI) initiated research aimed at increasing the yields of charcoal from biomass. Shortly thereafter, we learned that high yields are obtained when pyrolysis is conducted at elevated pressure in a closed vessel, wherein the vapors are held captive and in contact with solid products of pyrolysis. A grant from the State of Hawaii enabled us to build a pilot plant, which demonstrated charcoal yields of 42-62 wt % on a 1-2 h operating cycle.9-11 The charcoal yield (ychar) is a measure of the efficiency of the pyrolysis process. We define the charcoal yield as ychar ) mchar/mbio, where mchar is the dry mass of charcoal produced by pyrolysis of a biomass feedstock with dry mass mbio. Unfortunately, this definition is intrinsically vague because charcoal is not a welldefined chemical compound. Some charcoals are nearly pure carbon, whereas others have undergone only a partial pyrolysis and retain significant amounts of oxygen and hydrogen. To make meaningful comparisons, a better measure of the pyrolysis efficiency is needed. In this paper, we emphasize the efficiency of conversion of the organic matter in biomass to a nearly pure carbon. To measure this efficiency, the charcoal is subject to a proximate analysis according to the standard ASTM test procedure (see below). In essence, the charcoal is carbonized in an inert environment at a high temperature. Thereafter, its volatile matter content is defined as % VM ) 100 × (mchar - mcc)/mchar, where mcc is the dry mass of the carbonized charcoal that remains
10.1021/ie000511u CCC: $19.00 © 2000 American Chemical Society Published on Web 10/11/2000
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after heating. Likewise, its ash content is given by % char ash ) 100 × mash/mchar, where mash is the dry mass of ash that remains following combustion of the carbonized charcoal. The fixed-carbon content of the charcoal is defined as % fC ) 100 - % VM - % char ash. We define the fixed-carbon yield yfC of a carbonization process as yfC ) ychar × [% fC/(100 - % feed ash)], where % feed ash is the percentage ash content of the feed. This yield represents the efficiency realized in the pyrolytic conversion of the ash-free organic matter in the feedstock to a relatively pure, ash-free carbon. In this paper, we report measurements of yfC for several carbonization processes. Values of yfC depend strongly on the process conditions and the composition of the feedstock. When the carbonization process involves pyrolysis of a captive sample of biomass at elevated pressures, values of yfC approach those predicted to exist when thermochemical equilibrium is attained. Apparatus and Experimental Procedures Biomass feedstocks were obtained as grab samples from various sources. Samples of almond shell, bamboo, corn cobs, coconut shell, kukui nut shell, macadamia nut shell, walnut shell, Eucalyptus grandis, and Leucaena leucocephela were collected in Hawaii. Samples of alder (Alnus incana), birch (Betula pubescens), pine (Pinus silvestrus), and spruce (Picea abies) wood were cut from forests outside Trondheim, Norway. Samples of oak (Quercus spec.) wood, garlic processing wastes, pecan shells, oat, and rice hulls were given to us by the ConAgra Corporation. Sunflower seed hulls grown and milled in southern Hungary were obtained from the Hungaro-Sol Co. No pretreatments were employed to improve the yields of charcoal from these samples. At SINTEF, charcoals from pine, birch, spruce, alder, and oak wood were produced at atmospheric pressure in a stainless steel retort (volume ) 3.7 L) placed in a Nabertherm muffle oven. The muffle oven is equipped with a programmable temperature controller, which provides control of the heating rate, temperature, and heating time. The retort has a capacity of approximately 1 kg of dry wood per batch. The typical dimensions of the feedstock pieces were 3 cm × 3 cm × 5 cm. During heating and cooling, the retort was purged with N2 to prevent air leakage into the system. Temperatures inside three wood pieces, as well as the temperature inside the retort, were measured by 0.5-mm type K thermocouples. At the University of Hawaii (UH), high-yield biomass charcoals were produced in two reactors with similar configurations and operating procedures. The process development unit (PDU), which is described in detail elsewhere,11 has an internal volume of 80 L and can produce as much as 10 kg of charcoal per cycle. Because the pyrolytic reactions that transform biomass to charcoal are exothermic,12,13 the pilot plant requires little heat input.11 Recent work has shown that the required heat input (as natural gas, propane, or electric power) is effectively that needed to dry the feed. The laboratory reactor14 is a pressure vessel with an internal volume of 7.2 L. At the bottom of this reactor, there are two internal 4-kW Omegalux cartridge heaters that are controlled by a temperature controller (Omega CN132). Recently, we installed an ARI Industries 700 W rod heater down the central axis of the reactor to improve the uniformity of the charcoal product. To minimize heat losses, the reactor walls are heated by four external
Omegalux band heaters totaling 4.4 kW. In some experiments, the temperature of the reaction is monitored by two type K thermocouples inserted in an annulus at the top of the reactor. The pressure within the reactor is controlled by a back-pressure regulator. Combustible gases released by the back-pressure regulator are burned in a flare. Between 0.5 and 1 kg of biomass is placed inside a stainless steel canister (i.d. ) 11.4 cm, length ) 71.1 cm), which is subsequently loaded into the laboratory reactor. Typical dimensions of a wood feedstock are 3 cm × 3 cm × 5 cm. The moisture content of the feedstock is determined by oven drying grab samples of the material at 105 °C until no further decrease in weight is observed (typically after 24 h). The reactor is closed, and its wall is heated to 170 °C by the external band heaters. The bottom heaters and the rod heater are then turned on. Steam and volatile matter quickly lift the system pressure to a set-point value of 1 MPa, which is controlled by the back-pressure regulator. Pyrolysis is complete when the reactor pressure remains steady for at least 20 min. The heaters are turned off, and the reactor is depressurized and cooled overnight. Subsequently, the charcoal is removed from the canister and divided into three sections (i.e., top, middle, and bottom). The charcoal in each section is weighed, and small samples of each section are subjected to proximate analysis according to ASTM D 1762-84. Essentially, in this analysis, the charcoal is heated (“carbonized”) in a covered crucible to 950 °C and held at this temperature for 6 min. The measured weight loss is defined to be volatile matter (VM), and the residual solid is carbonized charcoal. Subsequently, the carbonized charcoal is heated in an open crucible to 750 °C and held at this temperature for 6 h. The material that remains in the crucible is defined to be ash. The reported charcoal yield is the dry mass of charcoal taken from the canister divided by the dry mass of the feedstock loaded into the canister. The reported volatile matter and ash contents of the charcoal are weighted averages of the values of each section. The reported fixed-carbon content is 100% less the sum of the volatile matter and ash contents. SINTEF employs the same procedures in its proximate analyses, except that the sample is held at 950 °C for 7 min during carbonization. Elemental analyses of the various charcoals are obtained from a commercial laboratory (Huffman). To determine the cellulose, hemicellulose, and lignin content of the biomass feedstocks, the methodologies of Moore and Johnson15 were used for quantitative saccharification with some modification. The subsequent analysis by high-performance liquid chromatography (HPLC) is based on procedures developed by the National Renewable Energy Laboratory (NREL, Golden, CO)16 and was approved for use in the NREL Bioethanol Program. Subsamples of the feed were air-dried for at least 1 day and then dried at 105 °C for 3 h, comminuted to -40 mesh, and dried for 1 h. The moisture content of each sample was determined from the weight loss upon drying (before comminution). A representative portion (0.3 g) was then subjected to primary acid hydrolysis by adding 3 mL of 72 wt % sulfuric acid and heating the slurry to 30 °C for 2 h with periodic mixing. Dilution to 87 mL produced a 4 wt % acid solution, which was then autoclaved at 121 ( 1 °C for 1 h, the secondary hydrolysis. A portion of these hydrolysates was neutralized to pH 5-6 with calcium carbonate,
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Table 1. Summative Analyses of the Feedstocks feed
glucose (wt %)
xylose (wt %)
galactose (wt %)
arabinose (wt %)
mannose (wt %)
cellulose (wt %)a
hemicellulose (wt %)a
lignin (wt %)b
alder wood almond shell bamboo birch wood coconut shell corn cob eucalyptus wood garlic waste kukui nut shell leucaena wood macadamia nut shell oak wood oat hull pecan shell pine wood rice hull spruce wood sunflower seed hull walnut shell
35.9 27.5 43.9 39.7 26.9 29.2 47.8 26.9 16.2 45.3 29.9 38.3 53.8 6.2 46.8 34.3 45.7 29.7 23.3
25.2 28.7 20.0 27.7 28.0 23.6 10.5 4.6 14.0 15.2 20.2 19.2 17.0 3.1 6.6 15.8 5.9 19.4 18.9
1.5 2.0 0 0 0 1.7 4.4 3.2 0 1.7 0 1.9 0 1.2 2.5 1.2 3.0 1.6 2.4
0 0 0 0.9 0 3.2 0 0 0 0 0 0 1.3 0 0 2.1 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 10.8 0 14.4 0 0
32.3 24.7 39.5 35.7 24.2 26.3 43.0 24.2 14.6 40.8 26.9 34.5 48.4 5.6 42.1 30.9 41.1 26.7 21.0
23.5 27.0 17.6 25.1 24.7 25.2 13.2 6.9 12.3 15.0 17.8 18.6 16.1 3.8 17.7 16.8 20.9 18.4 18.8
24.8 27.2 25.2 19.3 34.9 16.3 25.3 8.5 60.1 26.9 40.1 28.0 16.2 70.0 25.0 35.9 28.0 27.0 32.7
a Calculated using the following formulas: % cellulose ) % glucose × 0.9 and % hemicellulose ) (% galactose + % mannose) × 0.9 + (% xylose + % arabinose) × 0.88. b Measured acid-insoluble lignin.
filtered, and analyzed by HPLC. Deionized water was delivered at a flowrate of 0.6 mL/min by a Waters 510 HPLC pump to a Bio-Rad Aminex HPX-87P column (300 × 7.8 mm) heated to 85 °C. A deashing guard column, maintained at room temperature, preceded the analytical column. Monosaccharides and carbohydrate degradation products present in a 50 µL sample were detected by a Waters 410 differential refractometer. Calibration curves for each of these components were verified daily using standard solutions at the beginning and end of each batch of samples. These standard solutions were also analyzed in the middle of large batches of samples. In addition, known amounts of standard sugars were subjected to the same primary and secondary hydrolysis steps as the samples. The recoveries obtained were used to correct for losses during the hydrolysis steps. Finally, this methodology was routinely verified using National Institute of Standards and Technology (NIST, Gaithersburg, MD) standard reference materials 8491 (sugar-cane bagasse) and 8492 (populus deltoides). The amount of acid-insoluble lignin present in the feed was determined gravimetrically by filtering (M grade Gooch) the raw hydrolysis liquors (the product of the secondary hydrolysis step) prepared for the carbohydrate analysis. The residue collected was washed with deionized water until the filtrate had a neutral pH. The amount of acid-insoluble lignin collected was determined after this solid was dried to constant mass at 105 °C (4-5 h). We did not quantify the extractive content of the feedstocks. Results and Discussion Polymers of glucose (i.e., cellulose) and xylose (i.e., xylan) are the main structural elements of most biomass. Other sugars (i.e., galactose, mannose, and sometimes arabinose) are also incorporated into the polymeric structure of the hemicellulose fraction of biomass. Lignin is the non-carbohydrate, organic fraction of the biomass that is not easily extracted by ordinary solvents. Table 1 displays measured values of the cellulose, hemicellulose, and acid-insoluble lignin contents (i.e., the “summative analyses”) of the feedstocks employed in this work. The pecan and kukui nut shells are
noteworthy for their high lignin content, whereas the oat hulls contain 72% carbohydrates and only 16% lignin. Table 2 presents elemental (ultimate) analyses of the feedstocks and their ash contents. Unlike coal and peat, most biomass contains little ash, sulfur, and nitrogen. On the other hand, the carbohydrate content of the biomass manifests itself in the high values of %O and %H listed in Table 2. Table 3 displays the results of rapid and slow pyrolyses of five different wood species at a pressure of 0.1 MPa. The charcoal yields for rapid heating range from 28.9% (pine with % fC ) 69.6%) to 33% (spruce with % fC ) 69.1%); hence, the fixed-carbon yields range from 20.2% to 22.9%. Less than half of the carbon contained in these feedstocks is converted to fixed carbon by the rapid pyrolysis at 0.1 MPa. The bulk of the carbon leaves the reactor as tars and gases (i.e., volatile matter). An efficient carbonization process must minimize the formation of these volatile byproducts by converting them to solid carbon, water, and carbon dioxide. The scientific literature17 offers guidance concerning conditions that improve the yields of carbon from biomass. Larger particle sizes and slow heating favor the formation of carbon by enhancing the contact time of the volatiles with the solid carbon product. It is wellknown12,13,18 that the volatiles are not stable at elevated temperatures in the presence of charcoal or decomposing solid biomass. The volatiles adsorb onto the surface of the solid and quickly carbonize, releasing water, carbon dioxide, and methane as byproducts. We remark that slow heating increases the yield of char from pure cellulose, because low temperatures favor one of the competing, solid-phase pyrolysis reactions that preferentially forms char.19-21 But studies of char formation from small samples of biomass by thermogravimetry, where the vapors are quickly removed from the vicinity of the sample, indicate that slow heating has little effect on charcoal yields.17 Evidently, the presence of hemicellulose, lignin, and ash in the biomass obscures the solid-phase pyrolysis behavior of the cellulose component. Because of the findings reported in these earlier studies, we believe the improved yields of charcoal that result from slow heating of large samples to be an
Ind. Eng. Chem. Res., Vol. 39, No. 11, 2000 4027 Table 2. Elemental Analyses of Feedstocks
a
feed
C (wt %)
H (wt %)
O (wt %)
N (wt %)
average standard deviation (()a alder wood almond shell bamboo birch wood coconut shell corn cob eucalyptus wood garlic waste kukui nut shell leucaena wood macadamia nut shell oak wood oat hull pecan shell pine wood rice hull spruce wood sunflower seed hull walnut shell
0.61
0.07
0.26
0.03
48.27 49.94 47.65 48.07 52.37 48.22 52.87 37.85 55.76 45.95 58.30 50.13 46.00 55.27 49.41 38.86 48.91 50.37 49.95
6.02 5.79 5.77 6.00 5.91 6.20 6.14 4.97 5.60 6.06 8.12 5.98 5.91 4.56 6.11 4.86 6.02 5.62 5.87
45.11 45.01 44.23 45.56 42.34 42.94 39.79 43.12 37.99 41.23 32.77 44.76 43.49 34.75 44.07 37.15 44.65 42.64 42.52
0.30 0.17 0.27 0.17 0.23 1.57 0.16 0.49 0.34 2.42 0.36 0.08 1.13 0.84 0.11 0.42 0.12 0.33 0.62
S (wt %)
