Z. Liu, M. Hughes, Y. Tong et al.
Energy 239 (2022) 121925
were ground and screened to a nominal diameter of 1 e 2 mm. Biomass residue composition data are shown in Table S1 in the supporting information (SI). Proximate analysis was conducted according to ASTM D7582 standard and ultimate analysis was conducted using Vario Micro Cube (Elementar, Hanau, Germany). The oxygen content was calculated by difference. The resulting biochar catalysts were represented by stover, manure, wood, DDGS, biosolids, and p-sludge. The conventional mineral catalysts used in this study were calcined dolomite (CaO $ MgO) and sintered olivine ((Mg,Fe) 2 SiO 4 ), which have been extensively studied because they are widely available and ef fi cient for reducing tar during thermo- chemical conversions [29]. The calcined dolomite and sintered olivine were from Chem Lime Co. (Salinas, CA, USA) and Magnolithe GmbH Co. (Steiermark, Austria), respectively. The dolomite con- tained 17% MgO and 80% CaO (wt %, dry basis) and the olivine contained 48% MgO, 39% SiO 2 , and 11% Fe 2 O 3 (7% Fe) (wt %, dry basis). 2.1.2. Biochar preparation and catalytic pyrolysis tests A lab-scale pyrolytic system shown in SI (Figure S1) was used to produce biochar catalyst and conduct catalytic pyrolysis tests. The system consisted of a stainless-steel pyrolysis reactor vessel (360 cm 3 ), a ceramic radiative heater, a gas purge and release system, a stainless-steel tubular reactor (0.79 cm inner diameter, 27.9 cm long), an ice bath for liquid condensation, and a permanent gas collection valve with a Tedlar ® bag. Biomass residues were pyrolyzed by purging the pyrolytic system with argon gas at 15 mL/ min. The heating rate was controlled at approximately 10 C/min, and the retention time of each test was 30 min after the reactor reached the desired pyrolysis temperature. Argon fl ow was shut off when the desired maximum retention time was achieved. Pyrolysis vapor passed through the tubular reactor to condensers in an ice bath in which bio-oil and py-gas were separated. Py-gas was collected in a Tedlar ® bag and bio-oil was collected in two con- densers. Biochar was obtained in the pyrolysis vessel. The masses of biochar and bio-oil were measured gravimetrically. Coke (i.e. solid carbon formed by carbonization or decomposition of heavy hy- drocarbons) could deposit on the biochar catalyst surface in the tubular reactor. The coke mass yield was calculated from the weight difference (weight gain) of the biochar catalyst before and after the test. The py-gas mass was calculated by difference (i.e. initial bio- solids mass minus the sum of biochar, bio-oil, and coke masses) based on the method used previously [23]. Because coke mass ( < 2% for all the tests) was minor compared to biochar, bio-oil, and py-gas, the coke yield fraction was not presented in the results fi gures. The biochar catalysts were produced at temperatures from 600 C to 800 C in the pyrolysis reactor with an empty down- stream tubular reactor maintained at 500 C. The temperature that was used to produce catalysts was identical to the experimental pyrolysis temperature (e.g., biochar catalyst made at 700 C was used for 700 C catalytic pyrolysis experiments). Biochar and mineral catalysts had a nominal diameter of 0.5 e 1.5mm. In catalytic pyrolysis experiments, wastewater biosolids was used for all of the tests to study the impacts of different catalysts. The tubular reactor was fi lled with each biochar or mineral catalyst ( fi lled length was 26 cm for each catalyst) and preheated to the desired temperature identical to the temperature of the pyrolysis reactor vessel in which biosolids was pyrolyzed (see Table 1). Py- rolysis temperature ranged from 600 C to 800 C. The catalyst/ biosolids mass ratio was 0.33 for all catalysts and additionally mass ratios of 0.2 and 1 for wood, p-sludge, and biosolids biochars were employed. Up to fi ve cycles (i.e., fi ve consecutive batch experiments using the same biochar catalyst sample) for the wood, p-sludge, and biosolids chars were conducted to observe catalyst activity after repeated use. All pyrolysis experiments were conducted in
contaminants of emerging concern such as triclosan, triclocarban and antibiotic resistance genes as well as reduce total estrogenicity of the solid product [14 e 16]. Therefore, biosolids pyrolysis can be favorable in terms of energy recovery, resource recovery, contam- inant removal and release of contaminants to the environment [17]. However, biosolids-derived bio-oil from slow pyrolysis generally accounts for 40% of the total product mass [4]. Also, Fonts et al. [18] stated that full-scale implementation of biosolids pyrolysis is dif fi cult due to the low economic value of raw bio-oil [18]. Thus, reducing bio-oil and increasing cleaner py-gas production could be more practical. To that end, catalytic pyrolysis decomposes bio-oil resulting in higher py-gas production. Previous research focused on pyrolysis vapor upgrading and decomposing the resulting mixture of high molecular weight hy- drocarbons (i.e., tar), which is a predominant bio-oil component [19], using various heterogeneous catalysts including a commercial catalyst (ZSM-5), fl uid catalytic cracking catalyst and biochar [20 e 22]. Often, catalytic cracking leads to deoxygenation by for- mation of light-weight aqueous and gaseous hydrocarbons as well as increased permanent gas production. In particular, El Rub et al. (2008) compared tar decomposition using two biochars (i.e. pine- wood and commercial biochars) to that observed using traditional catalysts (e.g. olivine and dolomite), concluding that the former two biochars were more cost effective [20]. In addition, Liu et al. (2017, 2018) found Ca-loaded biochar (e.g. biosolids biochar) was more effective when converting bio-oil to py-gas [23,24]. In the United States, over 4 million dry tons of wastewater sludge from the pulp and paper sector are produced every year [25]. Paper mill sludge (p-sludge) often has very high Ca content due to the use of lime in the recausticizing cycle. Hence, the resulting p- sludge biochar also has high Ca content, which can be a potential biochar catalyst for upgrading pyrolysis products. However, p- sludge biochar has only been widely studied for pollutant removal [26 e 28]. Research has not been conducted regarding its catalytic performance during pyrolysis process. Therefore, the main objective of this study was to evaluate the catalytic performance of p-sludge biochar during biosolids pyrol- ysis in terms of the yield, composition, and energy of py-gas and bio-oil. The p-sludge biochar was compared to traditional mineral catalysts and other types of biochar that originate from common carbonaceous wastes. The effects of catalyst loading (catalyst to feedstock mass ratio) and catalyst lifetime (the duration for which a catalyst can continue to alter the product yields compared to the non-catalyzed control yields) were studied. Furthermore, possible factors affecting the catalytic effectiveness, such as metal content and surface area, were investigated.
2. Materials and methods
2.1. Material preparation and pyrolysis test
2.1.1. Materials Six biomass residues were chosen as precursors to create bio- char catalysts. P-sludge was provided by a commercial facility producing recycled fi ber for the paper industry and food-grade packaging (Sustana Fiber, De Pere, WI, USA). Other residues were from agriculture, bioethanol industry, forestry, livestock industry, and municipal wastewater industry, which were corn stover, dried distillers grains with solubles (DDGS), pinewood sawdust, cow manure, and wastewater biosolids, respectively. Corn stover, manure, and sawdust were purchased from Drip Trap, Hoffman A H Inc. and Healthy Pet, respectively. DDGS was provided by United Ethanol, LLC (Milton, WI, USA) and wastewater biosolids were the dried biosolids (i.e. Milorganite ® ) from the Milwaukee Metropol- itan Sewerage District (Milwaukee, WI, USA). Biomass residues
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