Renewable Energy from Waste: Investigation of Co-pyrolysis between

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Renewable Energy from Waste: Investigation of CoPyrolysis between Sargassum Macroalgae and Polystyrene Ketwalee Kositkanawuth, Arpita Bhatt, Melanie Sattler, and Brian H. Dennis Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b03397 • Publication Date (Web): 18 Apr 2017 Downloaded from http://pubs.acs.org on April 25, 2017

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Renewable Energy from Waste: Investigation of Co-Pyrolysis between Sargassum Macroalgae and Polystyrene Ketwalee Kositkanawuth,§£ Arpita Bhatt,§* Melanie Sattler,§ Brian Dennis¥ §

Department of Civil Engineering, University of Texas at Arlington, 425 Nedderman Hall, 416 Yates St., Box 19308, Arlington, TX 76019, Email: [email protected], [email protected]

£

Present: Department of Environmental Engineering, King Mongkut's University of Technology Thonburi,126 Prachauthit Rd, Bangmod, Tungkru, Bangkok, Thailand 10140, Email: [email protected] ¥

Department of Mechanical and Aerospace Engineering, , University of Texas at Arlington, 425 Nedderman Hall, 416 Yates St., Box 19308, Arlington, TX 76019, Email: [email protected] *

Corresponding Author

ABSTRACT Co-pyrolysis of the seaweed Sargassum and polystyrene was investigated as a potential source of renewable energy. Sargassum is a brown macroalgae, poses a large disposal problem for beaches worldwide; polystyrene is the plastic least recycled in the US. Although macroalgae bio-oil cannot be directly used due to high oxygen content and low heating value, co-pyrolysis of macroalgae with lowoxygen content waste polystyrene can enhance oil quality. Pyrolysis of pure Sargassum was conducted to determine the temperature producing the highest percent oil (600°C). Co-pyrolysis of four different mixture ratios of Sargassum and polystyrene (5%, 15%, 25%, and 33% by weight) was then conducted at 600°C. Feedstocks and pyrolysis products (liquid oil and water, gas phase, and solid phase) were characterized using elemental analysis, thermogravimetric analysis, gas chromatography, surface area and adsorption isotherm analysis, and NMR. Co-pyrolysis with polystyrene improved the quality and quantity of the oil, compared to pyrolysis of Sargassum alone. The oil quantity increased, from 3% for Sargassum alone up to 29% for a mixture of 67% Sargassum and 33% polystyrene. Co-pyrolysis improved oil potential heating value, and decreased its potential for producing air pollution when combusted, by lowering its nitrogen content. Co-pyrolysis

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produced a gas with up to 7% hydrogen and 30% methane, which can be burned as a fuel. Co-pyrolysis of Sargassum and polystyrene therefore shows promise as a method for generating fuel and reducing disposal problems. KEYWORDS Co-pyrolysis, bio-oil, biomass, macroalgae, Sargassum, polystyrene, resin INTRODUCTION Biofuel produced from biomass has many advantages over traditional fossil fuels, including renewability, greater energy security, and foreign exchange savings.1-3 In addition, biofuels from cellulosic feed stocks reduce greenhouse gas (GHG) emissions by about 80% .4 Use of aquatic biomass, especially algae, as a renewable energy resource is particularly promising.5,6 Unlike land sources of biomass, algae can grow in marine water, fresh water, or even in wastewater treatment ponds, and thus does not compete with other land uses, like growing crops for food.7 According to Clarens et al.5, to produce the equivalent amount of biofuel, algae requires 3 times less farming area than corn, canola, or switchgrass . One of the most effective methods for producing fuel from biomass is pyrolysis.8 Pyrolysis takes place in the absence of oxygen and at lower temperature, resulting in higher net calorific values (10-20 MJ/m3) than gasification and combustion (4-15 MJ/m3).9 Pyrolysis yields three final products: bio-oil (3075%), solid residues (10-35%), and non-condensable gases (10-35%) such as carbon monoxide (CO), carbon dioxide (CO2), hydrogen (H2) and light hydrocarbons.10 A large number of studies of pyrolysis of algae have demonstrated its potential as a renewable energy resource.7,11,12 Most previous research has focused on microalgae pyrolysis due to microalgae’s faster growth rate and productivity compared to macroalgae; however, significant barriers in using microalgae for biofuel exist. Microalgae require highly efficient harvesting methods, resulting in high capital and operating costs.13 Consequently, several researchers have begun exploring macroalgae,


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including seaweed, as a potential choice for biofuel. Hui et al.14 investigated pyrolysis characteristics and kinetics of seven marine macroalgae. They suggested that the favorable combustion characteristics of algae make it suitable for use as a pyrolysis feedstock. Moreover, Ross et al.15 investigated the thermal behavior of five species of brown seaweed compared to three forms of terrestrial biomass. The authors found a lower proportion of phenolic compounds in seaweeds, which is advantageous since phenolic compounds pose difficulty in the deoxygenation process for upgrading the oil. Bae et al.16 conducted pyrolysis of two brown and one red macroalgae between 300 and 600 °C; the oil yields were comparable to pyrolysis of conventional land biomass. The authors also calculated the heating value of the oil excluding water content, which was comparable to the heating value of oil from wood biomass. Not only would pyrolysis of sea macroalgae provide bio-oil without competing with other land uses, but it would also potentially reduce problems associated with seaweed waste disposal. The tons of macroalgae that deposit onto shorelines release odors as they start to decompose, destroy beach visual aesthetics for visitors, and can block sea turtles from nesting and impact their egg hatching.17 Sargassum is a genus of marine of brown macroalgae (Phaeoplyceae) found in Asia, North America, Australia, and Europe, which poses a particularly large disposal problem. A challenge associated with using macroalgae as a pyrolysis feedstock, however, is high oxygen content of the oil, which results in low heating value, requiring upgrading and refining processes such as deoxygenation and hydro-heating to improve the quality of oil. Low quality oil from pyrolysis is typically inappropriate for direct transportation use because of its low pH and high viscosity, which can lead to corrosion and severe engine deposition, respectively. This problem can be solved by conducting copyrolysis of macroalgae with a waste material low in oxygen content, such as plastic, which enhances oil quality and quantity compared to using biomass alone. Plastic is originally made from polymers which are petroleum products; hence, pyrolysis studies of plastic waste have shown high yields of bio-oil with high heating value.18,19 Several studies have conducted co-pyrolysis between different biomass materials and plastic.20-24 These studies found lower oxygen content over biomass alone. In particular, pyrolysis of 3 ACS Paragon Plus Environment

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polystyrene (PS) has shown promise. PS has a high energy content of about 16,000 BTUs per pound, twice that of coal.27 Pinto et al.28 found that pyrolysis of PS yielded the highest liquid product and the lowest gas yield compared to polypropylene and polyethylene under the same conditions. Siddiqui and Redhwi29 showed that polystyrene had the highest conversion via pyrolysis, compared to polypropylene, low- and high-density polyethylene, and polyethylene terephthalate. Shadangi and Mohanty (2015) found that co-pyrolysis of waste polystyrene enhanced the yield of pyrolytic oil of Karunja and Niger seeds, as well as increased the calorific value.30 Abnisa et al. (2013) conducted co-pyrolysis of palm shell and polystyrene, and Hassan et al. (2016) conducted co-pyrolysis of torrefied wood and polystyrene.31,32 However, no studies of co-pyrolysis between macroalgae and polystyrene have been conducted. Use of polystyrene as a co-pyrolysis feedstock would not only potentially enhance oil quality compared to biomass alone, but also beneficially re-use a problematic waste. 12.8% of the US waste stream is plastic; however, only 9.2% of this plastic is recycled, meaning that most of the rest ends up in landfills, where it is seldom biodegradable.25 Among plastic, polystyrene (PS) resin has the least percent recycled.26 Its lightness makes it difficult to collect from curbside containers: it often blows away, becoming litter. In addition, its bulkiness makes it difficult and expensive to transport. Hence, many municipal recycling programs do not accept it. The overall goal of this study is thus to investigate co-pyrolysis of the macroalgae Sargassum and polystyrene, as a potential source of renewable energy from waste. This is the first study of the copyrolysis of these two materials. Specific study objectives were: (1) To compare the quantity and quality of oil produced from co-pyrolysis of Sargassum and polystyrene at different mixture ratios, (2) To determine whether an interaction between Sargassum and polystyrene increases the amount of oil produced,


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(3) To analyze the quantity and composition of gaseous and solid products from co-pyrolysis, and explore their potential uses. A number of previous studies have revealed an interaction or synergistic effect between biomass and polymers which increases oil production.23, 33-36 The second objective in this study was thus to determine whether a similar effect exists for Sargassum and polystyrene. EXPERIMENTAL SECTION Feedstock Preparation. Recycled polystyrene (Styrofoam) pellets were provided by Dart Container Corporation, which were 3-4 mm. Sargassum was obtained from Whitecap Beach, Corpus Christi, TX. The Sargassum was washed and sun-dried for a day. After that it was placed in an oven at 100 °C to reach moisture of 5% or lower. Algae samples were ground to achieve 3-4 mm size. Later, algae samples were stored in vacuum bag to prevent decomposition. Feedstock ratios of 5%, 15%, 25% and 33% polystyrene by weight mixed with algae were chosen based on thermogravimetric analysis (TGA) and elemental analysis. Our goal was to utilize as much Sargassum as possible, yet still produce oil of a reasonable quantity and quality. Thus, the polystyrene amount was limited to 33%. Future work could explore optimizing the mixture ratio based on various factors. To achieve homogeneous mixtures of feedstocks, polystyrene was soaked in dichloromethane (DCM) solvent to weaken the polymer chain. The mixtures were stirred using a magnetic stirrer until the desired weight of polystyrene dissolved in DCM. The appropriate amount of algae was later soaked in the solution, and the solution was stirred until it was well-mixed. Later, the mixture was completely dried and again ground and sieved using no. 4 and no. 8 mesh size, to achieve particle size ranging between 3-4 mm. Characterization of Feedstocks. Table 1 summarizes the analysis methods used for characterization of feedstocks and pyrolysis products. Algae contains mainly protein (30-70%), carbohydrate (10-30%), and lipid (5-20%), while the main components of woody biomass are cellulose 5 ACS Paragon Plus Environment

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(40-50%), hemicellulose (25-35%), and lignin (10-40%).7,10 Therefore, cellulose, hemicellulose, and lignin analysis was not conducted. Future analysis could include determination of protein, carbohydrate, and lipid content, and how it changes following pyrolysis. Elemental Analysis: To determine the carbon (C), nitrogen (N), and hydrogen (H) content of the feedstocks (polystyrene and algae), a PerkinElmer 2400 Series II CHNS/O elemental analyzer was used. Temperature settings for the instrument followed ASTM D5291-10: combustion temperature of 925 °C, reduction temperature of 640 °C, and detector oven temperature of 84 °C. About 2-5 grams of ground solid sample were loaded into the injector. The CHN mode was used and oxygen content was estimated by percent difference, assuming sulfur and other minerals were negligible. Thermogravimetric Analysis (TGA): TGA has been frequently applied in pyrolysis research to investigate biomass thermal characteristics and kinetics. TGA measures mass changes as a function of temperature and time when a material is decomposed under a controlled atmosphere. In this study, the TGA instrument SDT Q600 was used. About 5-10 mg of prepared sample was heated at a rate of 10 °C/min from room temperature to 1000 °C until no change in mass occurred. In addition, TGA of mixtures of Sargassum and PS were performed to determine whether an interaction effect occurs between Sargassum and polystyrene. Pyrolysis Experiments. Pyrolysis experiments were conducted in a reactor of 1-inch inside diameter and 2-foot length. The top of the reactor was connected to a steel tube and an elbow fitting which allowed vapor to exit. The other end of the reactor was connected to a thermocouple to measure inside temperature. An outside heater wire was wrapped around the reactor, which was then covered with an insulator to prevent heat loss. The reactor was connected to a control system, which consisted of an auto proportional-integral-derivative (PID) controller (CN7523, Omega) and a solid state relay (SSR330DC25, Omega).


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About 70 grams of sample were used for each run. The reactor was initially purged with nitrogen gas for 15 minutes to create an oxygen-free environment. A heating rate of 10 °C/min was used. Based on TGA results of algae, final temperatures of 400-700 °C were applied for pyrolysis. Later, the optimum temperature of 600 °C was used to study pyrolysis of the mixture at different ratios, as summarized in Table 2. At a certain temperature, the samples decomposed and generated water and oil vapors. The vapors were collected in a condenser. The liquid product was collected in a flask, while non-condensable gases exited the condenser through a bubbling system. Gases were collected before, during, and after the reaction using 1-L Zefon Tedlar bags. Characterization of Pyrolysis Products. Oil and Gas: Elemental analysis was applied to the oil samples, as discussed under “Characterization of Feedstocks.” In addition, gas chromatography (GC) was used for the oil and gas analysis (SRI 8610C-for oil, SRI310C- for gas). Oil samples were characterized using a flame ionization detector (FID). The carrier gas was helium (5 psi, 10 ml/min). A capillary column (cat. # 70104, Restek Corporation) was used, and the temperature program was set up following ASTM D2887. The same technique was also used for gas analysis. However, for gas analysis, a thermal conductivity detector (TCD) was used to detect the four main gases from the pyrolysis process, which were carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), and hydrogen (H2). The carrier gas was helium (7 psi, 10 ml/min) and a packed column was used (cat. # 19809, Restek Corporation). An isothermal temperature of70 °C was held for 50 min. Solid Residues: Elemental analysis was applied tp residues, as discussed under “Characterization of Feedstocks.” In addition, tests were conducted to determine whether the residues would be useful as adsorbents for gas-phase or liquid-phase pollutants, as discussed next. In order to evaluate the potential use of the pyrolysis residues as adsorbents for gas-phase pollutants, surface area analysis was conducted and gas-phase adsorption isotherms were determined, and compared with commercially-available adsorbents. Surface area analysis was performed using a Quantachrome Autosorb iQ gas sorption analyzer. Samples were initially cleaned by degassing at 200 °C for 120 minutes to remove moisture and 7 ACS Paragon Plus Environment

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some volatiles. Surface area was determined by passing N2 gas into a sample cell placed inside a liquid nitrogen container. The gas molecules were adsorbed onto the sample surface and into its pores until it was saturated. Surface area was estimated based on Brunauer, Emmett, and Teller (B.E.T) theory. Additionally, isotherms of gas volume adsorbed vs. relative pressure were developed. The N2 isotherm of the residue was compared with commercial carbons, FluePac B (used for gas-phase adsorption) and Filtrasorb 200 (used for gas- and liquid-phase adsorption). In order to evaluate the potential use of the pyrolysis residues as adsorbents for liquid-phase pollutants, liquid adsorption tests were conducted using methylene blue (MeB, C16H18N3SCl)) dye as an adsorbate and residue of pure algae at 600 °C as an adsorbent. MeB is commonly used for liquid-phase effluent studies due to its harmful impacts on receiving waters, humans, and animals.37 The adsorption isotherm of algae residue was compared with a commercial liquid-phase carbon adsorbent, Filtrasorb 200. The equilibrium time for both adsorbents with MeB was identified before adsorption tests were carried out. The concentration of MeB was evaluated using ultraviolet-visible spectroscopy (UV-vis) with 610 nm wavelength. The adsorption experiments were performed following ASTM D3860-98. 3. RESULTS AND DISCUSSION Feedstock Analysis. Sargassum and Polystyrene Composition: Table 3 provides the elemental composition of each feedstock. It is noticeable that Sargassum algae has quite low carbon and high oxygen content, resulting in a high O/C ratio compared to polystyrene. A high oxygen content results in a low heating value because the carbon is already partially oxidized, which is not desirable for a fuel source. It should be noted, however, that oxygen content may have been overestimated, since oxygen was obtained by difference. Mass not counted as carbon, hydrogen, or nitrogen was assumed to be oxygen; however, algae may contain significant sulfur from sulfate polysaccharides. Elemental analyses similar to Sargassum have been reported for a number of macroalgae species including red, green, and brown by


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various authors.15,16,38-40 Most species have 30-40% carbon, 35-60% oxygen, 4-6% hydrogen, and 1-3% nitrogen, with the variation of elements likely due to seasonal and environmental factors. The polystyrene pellets were high in carbon content and low in oxygen, which corresponds to a high heating value. Other studies have found similar compositions of PS: 90-91% carbon, 7-8% hydrogen, and 0-1% oxygen.41-43 Table 3 also shows average proximate analysis results obtained from TGA in triplicates, with standard deviations in parenthesis. Higher volatile matter indicates the ability of a material to be converted into fuel. During pyrolysis, a portion of the volatile matter is converted to oil, and some is converted to gases. Fixed carbon (the residue after de-volatilization under an inert atmosphere) can serve as a solid fuel source under oxidizing conditions. The residue that remains unburned after oxidation is ash, which primarily consists of inorganic material, and is not useful as a fuel. Both fixed carbon and ash end up as solid residue following pyrolysis. According to Table 3, the polystyrene consists of all volatile matter and fixed carbon, which can serve as fuel, with no ash. The Sargassum, however, contains a high percent of ash (15.5%). Ross et al.16 suggested that high ash is due to minerals and trace elements found in seaweed. Future work could include determination of the amount of cellulose, hemicelluloses, lignin and extractives present Sargassum. Investigation of Potential Interaction between Sargassum and polystyrene: Figure 1 shows the proximate analysis of the various mixture ratios of Sargassum and polystyrene. It is observed that the volatile matter increases as the ratio of polystyrene in the mixture increases, and vice versa for the fixed carbon, ash content, and moisture. This is due to the fact that polystyrene is a major source of volatiles, while the seaweed is a source of fixed carbon. Additionally, Figure 1 illustrates the comparison between theoretical (predicted – dashed lines) and observed (experimental – solid lines) proximate analysis. The theoretical values were calculated based on weighted averages using information about each pure feedstock. The experimental volatile matter values were 1.5-2% higher than the predicted values. An 9 ACS Paragon Plus Environment

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interaction between algae and polymer likely accounts for the difference between the measured and predicted values. As discussed later, however, this interaction generally results in increased water phase and decreased oil phase, but improves oil quality by removing oxygen. Pyrolysis Product Distributions. Pyrolysis of Pure Feedstock: Pyrolysis of Sargassum was conducted at different temperatures to determine the optimum temperature to be applied for co-pyrolysis. The resulting product distributions are plotted in Figure 2. At different temperatures, significant differences (at α = 0.05 level, a commonly-used value that provides a fairly stringent test of statistical significance) were observed among the percents of all products except for water phase product. As the temperature increases, the gas, water, and oil products increased until 600 °C. Beyond this temperature, the oil yield decreases slightly, but the residue and gas increase. Therefore, 600 °C was chosen as the optimum temperature for subsequent co-pyrolysis experiments to maximize the oil yield. Similar results were obtained by various studies.16,44,45 Park et al.46 explained that incomplete pyrolysis at low temperature leads to high residue, but low gas and liquid product. On the other hand, excessively high temperature results in thermal cracking or secondary decomposition of vapor compounds generating more gases, but lower liquid. The percent solid residue is high for pure Sargassum, as shown in Fig. 2. According to Ross et al. (2008), minerals and trace elements found in seaweed lead to higher ash and higher solid residue compared to other forms of terrestrial biomass.16 Figure 2 also shows that the percent water phase exceeds the oil phase. Pyrolysis of Sargassum by itself would therefore not be a desirable method to produce oil; co-pyrolysis with another waste like polystyrene is needed. For the polystyrene pellet, 90% of the material was converted to oil without any water production. There was a small amount of gas and solid residue from the pellet, approximately 5% for each product. This result concurs with the TGA result, in which the polystyrene was completely degraded and low solid residue remained. Similarly, Liu et al.47 reported 98% liquid yield from PS pyrolysis at 600 °C,


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with a negligible amount of gas and char. Work by Williams and Williams’18 and Siddiqui and Redhwi 29 also followed a similar trend. Co-pyrolysis of Sargassum and Polystyrene: The product distribution from co-pyrolysis between the Sargassum and polystyrene is shown in Figure 3. The weight of product for each mixture ratio can be obtained by multiplying the sample weight (70 g) by the percentage for that product. As shown in Fig. 3, as the percent polystyrene increases, the percent residue decreases and percent oil increases. At 33% polystyrene and 67% Sargassum, the solid residue and oil are both about 30% by weight. Increasing the polystyrene content even more would increase the oil percent. To identify a synergistic effect, Figure 3 compares theoretical (dashed lines) and experimental (solid lines) yields. The theoretical values were calculated based on weighted averages using information on each pure feedstock. It should be noted that the moisture content in the original feedstock is excluded from the water phase yield to compare product yield from biomass decomposition, not dehydration. Surprisingly, the oil yield is less than expected, while the water yield is greater, except for 33% polystyrene. Thus, generally, the interaction is not beneficial, since greater oil production is typically desired. The results obtained are similar to the study by Berrueco et al.48, which yielded higher water phase but lower oil than predicted for co-pyrolysis of sawdust and HDPE (1:1) at temperatures varying from 640-850 °C. They noticed a high amount of H2 production. Thus, they claimed that the water elimination process is enhanced during co-pyrolysis. Similarly, in this study, the solid residue from seaweed decomposition could serve as a source of radicals, which could provoke water elimination from the hydroxyl group of the biomass and increase water production. In parallel, the elimination of water reduces the oxygen content of the oil phase, which improves oil quality. The water elimination reaction possibly occurs at a higher rate than the oil-producing reactions; hence, there is lower oil yield. Oil Characterization. Figure 4 a) shows gas chromatograms for pyrolysis oil from pure seaweed and pyrolysis oil from pure polystyrene pellets. Figure 4 b) shows gas chromatograms for co-pyrolysis


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oils. By comparing Figures 4 a) and b) with the ASTM D2887 quantitative calibration mix chromatogram (Figure 4 c)), it was determined that the major components in the pure seaweed oil were between C16C20, with boiling points ranging from 320-390oC. Some C28-C36 compounds with boiling points 400500oC are also present in the pure seaweed oil. For the pellet oil, gas chromatograph peaks corresponded to C16-C18, and C24-C28 with boiling points of 280-320oC, and 390-400oC, respectively. The gas chromatographs for the co-pyrolysis oils indicated that the mixtures are influenced more by plastic rather than biomass. As the plastic weight increases, the two peaks corresponding to the plastic increase. As demonstrated in Table 4, co-pyrolysis with polystyrene enhances oil quality, compared to pyrolysis of Sargassum alone. According to CHON analysis, the polystyrene addition lowered the oxygen content from 9% to 0.3% and increased the carbon content from 74% to 89%. As polystyrene content increases, the carbon content of the oil is significantly higher and oxygen content lower, resulting in a lower O/C ratio. The oil quality of the mixtures becomes more similar to pellet oil, which consists of low oxygen but is rich in carbon, resulting in a low O/C ratio. Increasing the percent polystyrene lowers the nitrogen content, which will mean reduced nitrogen oxide pollutant formation when the oil is burned. Even though the co-pyrolysis oil quality is much better than the original seaweed oil, the H/C ratio is lower than traditional fossil fuels due to the low hydrogen content of the oil. H/C ratios for diesel and gasoline can range from 1.8-2.0, but in this study the H/C ratio for the oil is only 1.05-1.13, likely due to low hydrogen content in both raw materials. A higher H/C ratio gives better oil quality: it reduces sulfur content and increases energy release during combustion. Hydrogenation of the oil could be considered for an upgrading and refining process, or a hydrogen-rich feedstock could be considered for co-pyrolysis. Gas Characterization. The production of CH4 and H2 from co-pyrolysis is advantageous in that methane and hydrogen can be burned for energy. Production of methane gas is preferred over production of CO2, in which the carbon is already completely oxidized, and therefore has no fuel value. CO is a


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criteria pollutant if not oxidized to CO2, and its production should thus ideally be minimized. If the goal is maximizing production of liquid fuel, however, then less transfer of carbon to the gas phase is preferred, even in the form of methane. Table 5 shows gas products as functions of stage of pyrolysis and PS/Sargassum percent. In the initial stage, gas was collected before the first liquid dropped. During the initial stage, the primary gas component was CO2. Bae et al.16 also found that CO2 was initially the major gas produced from samples of two brown and one red macroalgae; CO2 percent then decreases as the temperature increases. The second stage corresponded to the reaction stage, where liquid product was continuously being produced. During the reaction stage, CO2 was still the major gas, followed by CO, and CH4, respectively. The final stage occurred when no observed liquid was found after the desired temperature was reached. H2 appears only in the final stage at high temperature. In the final stage, the mixtures with the highest percents of PS produce higher percent ranges of methane, and lower percent ranges of carbon monoxide. The higher the polystyrene ratio, the less gas product from co-pyrolysis was produced throughout the experimental period. As shown in Figure 3, higher polystyrene ratios correspond not only to lesser gas production but also to greater oil production, which is typically desirable, because oil is the product with the greatest value. For pyrolysis of pure PS, little gas was generated, with more carbon being retained in the liquid phase. Residue Characterization. Elemental Analysis: Table 6 provides elemental analysis of pyrolysis solid residues. There is not much variation in carbon, nitrogen, or oxygen content with mixture ratio. Interestingly, hydrogen generally exhibits a decreasing trend as the mass of polystyrene increases. The residues contain relatively high oxygen, with a considerable amount of nitrogen. This means the residue would not be attractive as a solid fuel: high oxygen content leads to lower heating value of a fuel, while nitrogen can lead to nitrogen oxide emissions when the fuel is burned. On the other hand, the residue could be a good source of fertilizer due to high nitrogen content.


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Exploration of Potential Use of Solid Residues as Adsorbents for Gas-Phase Pollutants: The potential of the pyrolysis residue as an adsorbent was determined based on surface area and pore volume, as shown in Table 7. The residue exhibits extremely small surface area and total pore volume compared to the commercial adsorbents. Bird et al.49 observed BET surface areas for biochars from pyrolysis eight different species of green macroalgae from 1.15 m2/g to 4.26 m2/g, which is similar to the surface area of 4.60 m2/g in this study. The adsorption isotherms under N2 atmosphere for Sargassum and commercial adsorbents are illustrated in Figure 5. The seaweed residue adsorbs less than 10 cc/g of the gas volume at STP, whereas the adsorption capacity of Filtrasorb 200 and Fluepac-B are as much as 250 and 180 cc/g of gas at STP, respectively. This result corresponds to the higher surface area and pore volume of the commercial adsorbents. Exploration of Potential Use of Solid Residues as Adsorbents for Liquid-Phase Pollutants: Initially, the equilibrium times of the seaweed residue and Filtrasorb 200 were determined. Sargassum char reached equilibrium in about 4-5 hours, while the commercial activated carbon took 5 days to reach the equilibrium, since it has higher surface area and is capable of adsorbing more adsorbate. The seaweed residue adsorption capacity was about ten times lower than Filtrasorb 200 (275 mg adsorbate/g adsorbent). Hence, its use as a liquid-phase adsorbent is not promising. An additional metal analysis on the residue of pure seaweed was conducted using inductively coupled plasma mass spectrometry (ICP-MS) to determine the concentration of various metals, which could contribute to ecological problems if the residue is deposited in the environment. The metals analysis results are listed in Table 8. The main metals in the seaweed are alkali and alkali earth metals including Na, K, Ca, and Mg, since the seaweed comes from the marine environment. Besides, high levels of Si and Al are observed, as well as some trace amounts of Sr, Ni, Cu, and Zn. To determine whether these


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concentrations pose a concern, a toxicity characteristic leaching procedure (TCLP) test would need to be conducted. Water Phase Characterization. Although the hydrocarbons and aromatics would be anticipated to exist primarily in the oil phase, some degree of partitioning into the water phase would be expected. The water phase product was thus analyzed by nuclear magnetic resonance (NMR) to determine structures of any organic compounds. According to NMR results as shown in Figure 6, the peaks primarily come at early resonance, which possibly indicates an aliphatic or ether compound. The main spectra are exhibited between 1.5−2.5 ppm, which possibly indicates CH3−, CH2−, or CHα−bonded to aromatic ring. Spectra between 3−4 ppm are also observed, which may represent a ring methylene group joined with two aromatic rings. A small amount of compounds at 0.5−1.5 ppm is also detected, which probably corresponds to CH3−, CH2−, or CHα− that was removed from aromatic rings. Sensoz and Kaynar (2006) also observed organics in the water phase.50 CONCLUSIONS Co-pyrolysis of Sargassum and polystyrene therefore shows promise as a method for generating fuel and reducing disposal problems associated with both wastes. Co-pyrolysis with polystyrene improved the quality and quantity of oil produced, compared to pyrolysis of Sargassum alone. As the percent polystyrene increased, the oil quantity increased, from 3% for Sargassum alone up to 29% for a mixture of 33% polystyrene/67% seaweed. Polystyrene addition lowered the oxygen content from 9% to 0.3%, while raising the carbon content from 74% to 89%, which would raise the oil’s heating value. It also lowered the nitrogen content of the oil, which would mean that the oil would produce less nitrogen oxide emissions when burned. The H/C ratio found in this study was only 1.05-1.13, a bit lower than diesel and gasoline (1.8-2.0). Future work should explore hydrogenation for upgrading the oil. An interaction between the two feedstocks was observed during co-pyrolysis. The interaction decreased the oil phase by up to 5% and increased the water phase by up to 3%, depending on the percent 15 ACS Paragon Plus Environment

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of polystyrene in the pyrolysis feedstock. However, the interaction also improved the quality of the oil, by removing oxygen. The gas produced from co-pyrolysis of Sargassum and polystyrene contains mostly carbondioxide, with some hydrogen (up to 7%), methane (up to 30%), and carbon monoxide (up to 40%). The hydrogen content and methane mean the gas can be used as a fuel. Combusting the gas would oxidize the criteria pollutant carbon monoxide to carbon dioxide. The best potential use of the solid residues obtained from co-pyrolysis of Sargassum and polystyrene is applied to soil as a nutrient source for plants, due to their high nitrogen content. For this application, potential leaching of aluminum and silicon should be examined using the toxicity characteristic leaching procedure. The residues contain a large amount of oxygen, and a substantial amount of nitrogen, which would lead to low heating value and nitrogen oxide emissions, respectively, if the residue were to be burned as a fuel. The gas and liquid adsorption capacities of the solid residue were much lower than commercially available adsorbents. ACKNOWLEDGEMENTS The lead author (Ketwalee Kositkanawuth) would like to thank Mr. Norman Hall and Mr. Michael Westerfield from Dart Container Corporation. Sincere thanks to Mr. Kevin Koster, Mr. Jim, Mr. Derek Herzog, and Ms. Stacie Talbert from the City of Corpus Christi, Texas, for assisting and providing raw materials for the research.


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(15) Ross, A.B., Jones, J.M., Kubachi, M.L., and Bridgeman, T. Bioresour. Technol. 2008, 99, 64946504. (16) Bae, Y.J., Ryu, C., Jeon, J., Park, J., Suh, D.J., Suh, Y., Chang, D., and Park, Y. Bioresour. Technol. 2011, 102, 3512-3520. (17) Fox JM. Proceedings from Sargassum Symposium, 2008, Accessed at May 25, 2011, available at: http://www.sargassum.org/ (18) Williams, E.A., and Williams, P.T. J. Chem. Technol. Biotechnol. 1997, 70, 9-20. (19) Lopez, A., Marco, I., Caballero, B.M., Laresgoiti, M.F., and Adrados, A. J. Waste Manage. 2010, 30, 620-627. (20) Brebu, M., Ucar, S., Vasile, C., and Yanik, J. Fuel, 2010, 89, 1911-1918. (21) Caglar, A., and Aydinli, B. J. Anal. Appl. Pyrolysis. 2009, 86, 304-309. (22) Marin, N., Collura, S., Sharypov, V.I., Beregovtsova, N.G., Baryshnikov, S.V., Kuznetsov, B.N., Cebolla, V.L., and Weber, J.V. J. Anal. Appl. Pyrolysis. 2002, 65, 41-55. (23) Sharypov, V.I., Marin, N., Beregovtsova, N.G., Baryshnikov, S.V., Kuznetsov, B.N., Cebolla, V.L., and Weber, J.V. J. Anal. Appl. Pyrolysis. 2002, 64, 15-28. (24) Zhou, L., Wang, Y., Huang, Q., and Cai, J. Fuel Process. Technol. 2006, 87, 963-969. (25) US Environmental Protection Agency, 2015. Advancing Sustainable Materials Management: 2013 Fact Sheet. Washington D.C. Accessed 12 May 2011, available at: https://www.epa.gov/smm/advancing-sustainable-materials-management-facts-and-figures-report (26) US Environmental Protection Agency, 2009. Municipal solid waste generation, recycling and disposal in the United States detailed tables and figures for 2008. Technical report, Office of Resource Conservation and Recovery. U.S. EPA, Washington. (27) American Chemistry Council, 2009. Take a closer look at today’s polystyrene packaging. Accessed January 15, 2011, available at: http://www.americanchemistry.com (28) Pinto, F., Costa, P., Gulyurtlu, I., and Cabrita, I. J. Anal. Appl. Pyrolysis. 1999, 51, 39-55. 18 ACS Paragon Plus Environment

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(47) Liu, Y., Qian, J., and Wang, J. Fuel Process. Technol. 2000, 63, 45-55. (48) Berrueco, C., Ceamanos, J., Esperanza, E., and Mastral, J.F. J Therm Sci. 2004, 8(2), 65-80. (49) Bird, M.I., Wurster, C.M., Silva, P.H., Bass, A.M., and Nys, R. Bioresour. Technol. 2011, 102, 1886-1891. (50) Şensöz S, Kaynar Đ. Bio-oil production from soybean (Glycine max L.); fuel properties of Bio-oil. Industrial Crops and Products. 2006, 23(1), 99-105


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Table 1. Summary of Analysis Methods for Feedstocks and Pyrolysis Products Samples Feedstock Gas products Oil products

Solid residues

Water Phase

Characteristics Element: C, H, O, N Moisture, volatile, and fixed carbon Gas Composition Boiling Point Range Element: C, H, O, N Element: C, H, O, N Moisture, volatile, and char

Method Elemental Analysis

Instrument PerkinElmer 2400 Series II

TGA analysis

SDT Q600

GC GC Elemental Analysis Elemental Analysis TGA analysis

SRI 310C SRI 8610C PerkinElmer 2400 Series II PerkinElmer 2400 Series II SDT Q600 Quantachrome Autosorb iQ Agilent 8453 UV-visible spectrophotometer JEOL Eclipse plus 500 MHz Spectrometer

Surface area

Surface Area Analysis

Adsorption Isotherm

Gas and Liquid Isotherm

Structure of Compounds

Nuclear Magnetic Resonance (NMR)


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Table 2. Feedstock Ratios and Temperatures of Pyrolysis Experiments Feedstock

Ratio (by %weight) 100:0

Sargassum:Polystyrene 95:5 85:15 75:25 67:33 0:100

Temperature (°C) 400 500 600 700

600


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Table 3. Elemental and Proximate Analysis of Feedstocks Elemental Analysis Feedstock

%C

%H

%N

% Oa

H/C

O/C

Sargassum 34.1(+1.60) 3.90(+0.57) 1.30(+0.26) 45.2(+1.85) 1.37 1.34 Recycled polystyrene 90.0(+0.23) 3.81(+0.21) 0.00(+0.00) 6.19(+0.44) 0.51 0.05 pellet a percent oxygen obtained by difference, after the ash content is subtracted

Proximate Analysis % % Volatile % Fixed % Ash Moisture matter Carbon 4.00(+0.37) 61.3(+1.52) 19.2(+0.28) 15.5(+1.57) 0.00(+0.00) 98.5(+0.95) 1.50(+0.11) 0.00(+0.00)


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Table 4. Elemental analysis of pyrolysis oil for different mixture ratios of Sargassum and polystyrene Feedstock %C 100% Sargassum 74.4(+0.95) 95% Sargassum/5% PS 80.8(+0.31) 85% Sargassum/15% PS 83.2(+0.03) 75% Sargassum/25% PS 87.9(+0.41) 66% Sargassum/33% PS 89.3(+0.72) 100% Polystyrene 91.2(+0.60) Optimum temperature = 600 oC

%H 8.60(+0.71) 7.10(+0.54) 7.40(+0.30) 8.30(+0.16) 8.40(+0.08) 7.90(+0.27)

%N 8.00(+0.12) 6.60(+0.29) 5.50(+0.91) 2.20(+0.05) 2.00(+0.21) 0.00(+0.00)

%O 9.00(+0.47) 5.50(+0.10) 3.90(+0.74) 1.60(+0.12) 0.30(+0.03) 0.90(+0.02)

H/C 1.39 1.05 1.07 1.13 1.13 1.04

O/C 1.21E-03 0.65E-03 0.45E-03 0.17E-03 0.00 8.22E-05


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Table 5. Pyrolysis gas product distribution as functions of pyrolysis stage and Sargassum/PS mixture ratio Feedstock Initial stage 100% Sargassum 95% Sargassum/5% PS 85% Sargassum/15% PS 75% Sargassum/25% PS 66% Sargassum/33% PS Reaction stage 100% Sargassum 95% Sargassum/5% PS 85% Sargassum/15% PS 75% Sargassum/25% PS 66% Sargassum/33% PS Final stage 100% Sargassum 95% Sargassum/5% PS 85% Sargassum/15% PS/ 75% Sargassum/25% PS/ 66% Sargassum/33% PS/

Gas production, % wt. H2 CO CH4

CO2

0.0 0.0 0.0 0.0 0.0

1-2.5 0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0 0.0

97-98 100 100 100 100

0.0 0.0 0.0 0.0 0.0

5-10 5-10 6-12 5-12 5-12

0.5-3 2-10 4-12 3-7 2-9

86-93 80-93 75-93 80-90 80-90

4.5-6.5 5-7 5.5-6.5 4.5-7.5 5-6.5

25-35 30-40 30-35 25-30 20-30

15-25 15-25 20-25 25-30 25-30

40-45 40-45 35-40 40-45 40-45


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Table 6. Elemental analysis of pyrolysis solid residue as a function of mixture ratio of Sargassum and PS Feedstock 100% Sargassum 95% Sargassum/5% PS/ 85% Sargassum/15% PS/ 75% Sargassum/25% PS/ 66% Sargassum/33% PS/

%C 54.1(+1.41) 53.9(+0.42) 54.7(+0.53) 53.1(+0.62) 55.2(+0.98)

%H 1.40(+0.19) 0.80(+0.07) 0.70(+0.03) 0.00(+0.00) 0.20(+0.04)

%N 1.20(+0.04) 1.40(+0.11) 1.50(+0.71) 1.60(+0.04) 1.40(+0.06)

%O 16.7(+1.14) 17.1(+0.34) 16.2(+0.49) 17.7(+0.33) 16.1(+1.04)


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Table 7. BET surface area and pore volume of pyrolysis residue vs. commercial adsorbents

Materials Sargassum residue Filtrasorb 200 Fluepac-B

BET Surface Area (m2/g) 4.60 845 522

Total Pore Volume (cc/g) 0.013 0.387 0.239


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Table 8. Metals analysis of pyrolysis residue Metals Na Mg Al Si K Ca Cr Fe Cd

Concentration (ppm) 12,700 1030 2650 2360 3160 4080 Not detected 82 Not detected

Metals Mn Co Ni Cu Zn As Sr Pb

Concentration (ppm) Not detected Not detected 3.80 3.50 6.75 Not detected 181 Not detected


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Figure Captions Figure 1. Comparison of predicted (*) and observed volatile matter, fixed carbon, and ash content at different Sargassum/polystyrene mixture ratios Figure 2. Percent pyrolysis product distribution of pure Sargassum versus temperature Figure 3. Comparison of predicted (*) and observed product distribution at different Sargassum/polystyrene mixture ratios Figure 4. Gas chromatograph spectra for (a) pyrolysis oil from pure Sargassum and polystyrene, (b) co-pyrolysis oil from various Sargassum/polystyrene mixtures, and (c) ASTM D2887 quantitative calibration mix Figure 5. Adsorption isotherm of (a) Sargassum under N2 and (b) commercial activated carbon under N2 Figure 6. NMR spectra of water phase product


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Figure 1. Comparison of predicted (*) and observed volatile matter, fixed carbon, and ash content at different polystyrene/Sargassum mixture ratios 119x90mm (300 x 300 DPI)

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Figure 2. Percent pyrolysis product distribution of pure Sargassum versus temperature 138x100mm (300 x 300 DPI)


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Figure 3. Comparison of predicted (*) and observed product distribution at different Sargassum/polystyrene mixture ratios 151x105mm (300 x 300 DPI)


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Figure 4. Gas chromatograph spectra for (a) pyrolysis oil from pure Sargassum and polystyrene, (b) copyrolysis oil from various Sargassum/polystyrene mixtures, and (c) ASTM D2887 quantitative calibration mix 215x179mm (300 x 300 DPI)


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Figure 5. Adsorption isotherm of (a) Sargassum under N2 and (b) commercial activated carbon under N2 215x93mm (300 x 300 DPI)


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Figure 6. NMR spectra of water phase product 159x191mm (300 x 300 DPI)


Renewable Energy from Waste: Investigation of Co-pyrolysis between

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