Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
pubs.acs.org/journal/ascecg
Renewable Biomass-Derived Coke with Texture Suitable for Aluminum Smelting Anodes Yaseen Elkasabi,*,† Hans Darmstadt,‡ and Akwasi A. Boateng† †
ACS Sustainable Chem. Eng. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 08/30/18. For personal use only.
Eastern Regional Research Center, Agricultural Research Service, United States Department of Agriculture, 600 E. Mermaid Lane, Wyndmoor, Pennsylvania 19038, United States ‡ Rio Tinto, Arvida R&D Centre, 1955 Boulevard Mellon, Jonquière, QC G7S 4K8, Canada ABSTRACT: Industrial producers of calcined petroleum coke encounter issues pertaining to product quality and environmental ramifications. While previous attempts to produce biorenewable versions of calcined coke address CO2 footprint, sulfur, and metals poisoning, none have demonstrated a sufficient degree of anisotropy, which is critical for performance in aluminum smelting anodes. Using fast pyrolysis of biomass, we produced bio-oils of sufficient quality whereby coke can be formed through distilling off the volatiles. Tail-gas reactive pyrolysis (TGRP) produced bio-oils with low oxygen content, low viscosity, and high polyaromatic content, all of which are critical for alignment of coke precursors. Distillations at controlled rates and temperatures ensured the alignment of polyaromatic domains leading to coke anisotropy. Bio-oils with oxygen content below 16 wt % successfully produced coke with mixtures of isotropic and anisotropic domains. In coprocessing experiments, mixtures of pyrolysis bio-oil and green petroleum coke coked under similar conditions to produce highly anisotropic textures, although degrees of discontinuity exist within merged particles. When baked with coal tar pitch as an additive, calcined biocokes exhibited resistivity values of 100−200 μΩ·m, which correlated well with the calcination time at 1200 °C. KEYWORDS: Bio-oil, Coke, Biomass, Texture, Optical microscopy, Pyrolysis, Aluminum smelting
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INTRODUCTION Calcined petroleum coke is the only known material capable of serving as anode material for aluminum smelting at industrially relevant scales. Reasons for this singularity revolve around the combination of electrical conductivity, thermal tolerance, low impurity (such as S, Ni, and V) content, high bulk density, and low coefficient of thermal expansion (CTE).1−3 Aluminum smelting uses consumable anodes which produce ∼1.5 tonnes CO2/tonne Al. Industrially, smelters consume more than 25 megatonne/yr of calcined petroleum coke to produce 50 megatonne/yr of aluminum metal.4 Smelters who use hydrogenerated electricity produce 37.5% of their total CO2 footprint from aluminum production.5 One option to reduce the CO2 footprint is using renewable biocoke. In this case, the CO 2 generated during anode consumption would be compensated by the CO2 captured during biomass growth. Manufacture of biomass-derived char for iron production is already performed in Brazil on commercial scale.6 Furthermore, biomass char also has found applications in soil amendment and briquetting.7 Although proposed in the literature,8 use of biomass char in electrodes is not performed commercially. According to laboratory studies,9,10 partial replacement of petroleum coke by biomass char resulted in poorer anode properties. The anode density decreased, whereas anode resistivity and © XXXX American Chemical Society
oxidation increased. This was attributed to low char bulk densities and to the presence of inorganic compounds (such as Na and K) which catalyze anode oxidation. While methods exist for removal of inorganic compounds from biomass char,11 the low bulk density remains an issue. Pressurized pyrolysis, in a manner similar to ablative reactors, can increase the char bulk density,12 but costs have yet to be determined. Furthermore, biomass char usually has an amorphous texture.13 Anodes containing fillers with these textures have a high coefficient of thermal expansion (CTE),14 making them susceptible to thermal shock cracking.15 In anodes, isotropic coke can only be used as a blend component, but not exclusively.16 It can be summarized that the poor performance of biomass char in anodes has several reasons: inorganic compounds present in biomass report to the char and during carbonization, the developing char does not pass through a liquid phase required for the development of the desired graphite-like, anisotropic texture. Rather than using the solid pyrolysis char product, another option is to use pyrolysis oil to produce biocoke. This strategy could address the shortcomings mentioned above for solid Received: June 22, 2018 Revised: August 12, 2018
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DOI: 10.1021/acssuschemeng.8b02963 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
heat profiles would influence the viscosity and hence the development of cokes. As an update to our previous work on the subject,23 we demonstrate how the texture of biocoke can be influenced.
char. First, the metal concentrations in bio-oil are several orders of magnitude lower than those in biomass char. Furthermore, coke formation passes through a liquid phase, allowing in principle formation of the desired anisotropic coke textures. It is important to note that not all oils yield anisotropic coke. Upon heating of the coke feedstock, its viscosity decreases, and it passes through a fluid stage, allowing alignment and structural ordering. This forms polycrystalline domains still embedded in a deformable fluid, termed mesophase.17 Upon further heating, viscosity increases due to loss of volatile compounds and precipitation of solid carbon (Figure 1). The wider the low-viscosity window is, the more
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EXPERIMENTAL SECTION
Pyrolysis. Fast pyrolysis of various biomass (see Table 1) was conducted using the Eastern Regional Research Center fast pyrolysis
Table 1. Elemental Analyses of Bio-Oils and Their Distillation Residues Based upon Their Final Distillation Temperaturesa oxygen (wt %) biomass
a
bio-oil
distillation residue
distillation temperature (°C)
willow
14
guayule
15
horse litter switchgrass
20 23
hardwood
27
n.d. n.d. 8.1 8.0 7.0 6.8 7.0 14.0 18.3 17.0 16.0 20.4 19.8 18.2 17.1 15.2
250 320 225 250 275 300 325 320 250 275 300 225 250 275 300 325
n.d.: not determined.
tail-gas reactive pyrolysis process.21 Briefly, dried biomass was fed through a fluidized sand bed reactor (temperature: 500 °C, N2 atmosphere) at a rate of 2 kg/h. Any solid char particulates were separated by subsequently passing the vapors through a cyclone, after which compounds with a low boiling point were collected in a series of cold-water condensers. Any remaining oil entrained in the noncondensable gases was precipitated by electrostatic precipitation (ESP). All experiments used the oil obtained from the ESPs. Using a preheater and gas blower, the remaining gases were recycled back into the reactor at a rate of 50−70%. Coking. Prior to coking, light components of pyrolysis oils were removed by batch distillation (final temperature in Table 1), modified from our previous procedure.22 The oils were heated gradually in stages under atmospheric pressure, with the temperature being held for 30 min each at 200, 250, 275, 300, and 325 °C, or until the final temperature noted in the table was reached. Afterward, the residues were removed from heat, cooled, and underwent a simulated calcining procedure in an argon-purged coking oven. Each sample was placed into a ceramic crucible, which was then placed in a graphite crucible. The graphite crucible was closed with a cover, and a hole in the cover allowed volatiles to escape. The closed graphite crucible was placed in a bed of packing coke, which reacted with any O2 that might have infiltrated the oven. A silica plate was placed on the graphite crucible to further minimize any oxygen infiltrations. The coking oven introduced heat at 16 °C/min until a temperature of 900 °C was attained, after which heat was shut off, and the oven was allowed to cool. For co-coking experiments, green petroleum coke as-received (Rio Tinto, Jonquiere, QC, Canada) was crushed using a mortar and pestle prior to use. TGRP bio-oil was mixed with green petroleum coke at a mass ratio of 10:1 oil:coke. Mixtures were placed in a muffle furnace, with the temperature ramped to 150 °C and held for 5 min, then held at 200, 250, 275, and 300 °C for half an hour at each temperature.
Figure 1. Optical microscopy images of coke with (a) amorphous, (b) isotropic, (c) anisotropic sponge, and (d) anisotropic needle texture. (e) Schematic representation of the relationship between viscosity and temperature of reaction mixture during coking, adapted from ref 30.
mesophase alignment and anisotropic coke texture can occur. Due to interactions between the O-groups, the low-viscosity window is much narrower for high-O oil than for low-O oil. Consequently, coke anisotropy increases with decreasing O content of the corresponding feedstock.18 Traditional pyrolysis of biomass produces oils containing 30−35% wt O.5,19 The O content can be reduced by hydrodeoxygenation (HDO), wherein the oil reacts under pressurized hydrogen, in the presence of a heterogeneous catalyst, analogous to petrochemical hydro-treatment.20 The produced low-O oil can be transformed to anisotropic coke. However, the low yield of coke from hydro-treated oil, combined with the relatively high capital and operating costs of HDO, likely make this approach uneconomical. An alternative strategy for long-term biorefinery profitability is to modify the pyrolysis step such that the bio-oil produced is inherently low in O, such as in the tail-gas reactive pyrolysis (TGRP) process.21 An additional advantage of the TGRP process is the high thermal stability of the oils.22 By controlling the rate of distillation, not previously explored before with TGRP oils, the B
DOI: 10.1021/acssuschemeng.8b02963 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering Samples were then removed from the furnace, cooled, and subjected to the simulated coking procedure in the coking oven. Bio-oil distillate residues co-coked with coal tar pitch (CTP; Rio Tinto, Jonquiere, Canada), (softening point = 120 °C; 8% quinolone insolubles) were coked at either 17 or 25 wt % pitch. Temperature ramping of pitch blends was similar to that of distillate residues but with long dwell times at 325 °C. After the appropriate dwell times, samples were held at 1200 °C for 0−2 h, then followed by gradual cooling. Characterization. Elemental analysis (CHNS) was conducted using a Thermo EA1112 CHNS analyzer. The O content was calculated by difference. For microscopic analysis, coke particles were embedded in epoxy resin and successively mechanical polished until a colloidal silica 0.05 μm finishing step. Optical images were recorded with a LEICA DMI500 M reflected light microscope equipped with a 1λ polarizer. The samples were observed in air; no oil immersion method was used. Examination of specimens was done with a magnification between 25× and 500×. Additional details can be found elsewhere.24 Calcined coke samples underwent trace metal analysis using an Axios Max Mineral X-ray Fluorescence spectrometer (PANalytical; Almelo, The Netherlands). Apparent density was measured by weighing the mass of a coke sample that was cut and sanded into a rectangle. Sample dimensions were measured with calipers, and the density was calculated based on the volume. Electrical Measurements. Solidified calcined biocoke was measured either directly or as part of calcined mixtures with coal tar pitch. Solid portions of calcined petroleum coke (received from Rio Tinto) were measured directly as-is. Each sample was cut and sanded into rectangles, with each dimension measured to within 5% accuracy. A layer of colloidal silver paint was applied to each end of the lengthwise dimension where electrical leads were applied. DC current was introduced using a CE Compass 305D variable DC power supply, and associated voltages were measured with a Cen-Tech P37772 multimeter. At increments of 0.5 A, voltage readings were taken, until at least 3 A was attained. This procedure periodically cycled between specific current values and zero current to prevent overheating that can cause artificial reductions in resistance. I−V curves were plotted from the data with the linear slope corresponding to the inverse resistance. Sample dimensions and resistance values were used to calculate resistivity.
the oil O content was less than half of typical values found in conventional pyrolysis oil (14 vs >30%19,5). As the distillation temperature increases, the oxygen concentration of the bottoms decreases, suggesting that the majority of oxygen exists as part of light compounds. Coke texture depends on the combined O and S concentration of the corresponding feedstock, where anisotropy increases with decreasing combined O and S concentration.28 The total heteroatom content (O, S) of petroleum coke greatly affects the calcined petcoke anisotropy. While sulfur exists in trace quantities in biocoke, the oxygen levels in biocoke (>6%) exceed what is found for petcoke (2−4%).29 Coke O and S. The O and S content of biocoke was determined for two biomass feedstock materials (hardwood and horse litter). As compared to the coker feedstock (Table 1), the O concentration in the cokes was about 10 times lower (Table 2). Most probably, this reflects the loss of O-containing
RESULTS AND DISCUSSION Pyrolysis Oil. Previous studies on TGRP oil utilized both atmospheric and vacuum distillation to produce coke, along with high ramping rates thereafter. Because coke texture is greatly influenced by the time−temperature profile, we carried out gradual distillation experiments at particular temperatures. We also eliminated vacuum distillation to reduce excessive pores due to sudden volatilization as well as to reduce processing steps. When heating petroleum residues, viscosity decreases, and the polyaromatics pass through a fluid stage at which alignment and structural ordering can occur. This forms polycrystalline domains still embedded in a deformable fluid, termed the mesophase.25 Beyond this optimal point of minimal viscosity (Figure 1e), viscosity increases again due to higher devolatilization and precipitation of fixed carbon. Variation of the bio-oil oxygen content results from a few variables. While the tail-gas recycle rate primarily affects the oil quality, the biomass type can affect the oil oxygen content. When the oxygen content is sufficiently low, TGRP oils tend to exhibit similar product distributions of key compounds, one exception to this trend being algae and guayule-based oils.26 As deduced from previous studies,27 differences in biomass type exhibit negligible elemental differences on the final calcined coke product for TGRP oils: only the metals content may change. Table 1 displays the oxygen contents of both the oils and distillation residues with respect to final distillation temperature before coking. With some pyrolysis feedstock materials,
groups during coking and calcination. The S concentrations in the two cokes were smaller than 0.05 wt %, corresponding to the very low S content of the initial biomass. In blends of green petroleum cokes fed to calciners, the S content ranges from ∼0.7 to nearly 5.7 wt %.30 The corresponding SO2 emissions of the calciner and of the Al smelters are limited by environmental standards. This limits the amount of high-S coke that can be used. Low-S biocoke might serve as a blend component to keep SO2 emissions below these standards. Trace Metal Analysis. Metal impurities present in calcined coke are undesirable for anodes for several reasons, including contamination of the produced Al,31 increase of anode oxidation,32−34 and decrease of the current efficiency.35,36 First, impurities that contaminate the Al and catalyze anode oxidation are discussed (Ni and V). The biocokes had very low concentrations of these impurities (∼10 ppm vs typical coke specifications of 250 and 350 ppm, respectively Table 2). In petroleum coke, these impurities are of considerable concern as their concentrations are already high and continue to increase.30 This makes it difficult to respect impurity specifications for the produced Al. As for S, biocoke might be used to dilute existing petcoke sulfur in blends. Elements that contaminate the produced Al but have no significant impact on anode oxidation or the current efficiency include Mn, Si, Ti, and Zn. For these, Zn was higher in hardwood-derived coke as compared to the coke specifications. However, most Zn present in Al originates from the binder
Table 2. Trace Metals Analysis of Calcined Biocokes from Various Biomass Sources impurity % ppm
a
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C
S O V Ni Mn Ti Zn Si Ca Na K P
typical spec. (max.) 3 350 250 10 20 10 250 200 150 10
hardwood
horse litter
