Z. Liu, M. Hughes, Y. Tong et al.
Energy 239 (2022) 121925
Table 1 Experimental design.
Reactor Temperature ( C)
Downstream Temperature ( C)
Downstream Loading
Catalyst/Feedstock Mass Ratio
Catalytic
600 700 800 600 700 800 600 700 800
600 700 800 600 700 800 500 500 500
0.5 e 1.5 mm catalyst 0.5 e 1.5 mm catalyst 0.5 e 1.5 mm catalyst 0.5 e 1.5mmsand 0.5 e 1.5mmsand 0.5 e 1.5mmsand
0.33 0.33
0.2 a , 0.33 b , 1 a
Non-catalytic (0S)
0 0 0 0 0 0
Non-catalytic (0B)
Blank Blank Blank
a The catalyst/biosolids mass ratios of 0.2 and 1 were only utilized for wood, p-sludge, and biosolids biochars. b Wood, p-sludge, and biosolids biochars were recycled for up to fi ve times under 800 C at the ratio of 0.33.
cycles, and fi ve cycles) were analyzed by temperature programmed oxidation (TPO) to quantify coke deposition. The TPO of new p- sludge biochar was also conducted for comparison. TPO was per- formed in a thermogravimetric analyzer (TGA 550, TA Instruments, New Castle, DE) by heating 2.5 mg sample at a 10 C/min to 800 C with a continuous 100 cc/min air fl ow and a fi nal dwell time of 10min.
triplicate. Statistical analyses (ANOVA, a level ¼ 5%) were per- formed using OriginLab (Northampton, MA, USA). Two non-catalytic biosolids pyrolysis tests were also conducted as controls (see Table 1). The fi rst non-catalytic test (0S) employed inert sand with nominal diameter of 0.5 e 1.5 mm in place of catalyst in the downstream tubular reactor. The 0S test was conducted to observe any secondary reaction effects among pyrolysis vapor components at high temperature [30]. The second non-catalytic test (0B) employed an empty downstream tubular reactor main- tained at 500 C to prevent pyrolysis vapor components from condensing during transition downstream (secondary reactions were minimized).
3. Results and discussion
3.1. Impact of p-Sludge biochar and other catalysts on mass yields and py-gas composition
2.2. Characterization of py-gas, bio-oil, and biochar
P-sludge biochar catalyst signi fi cantly increased py-gas yield and reduced bio-oil yield at high temperatures (Fig. 1a). At 800 C, p-sludge biochar resulted in the lowest bio-oil mass fraction (approximately 20%) and the highest py-gas mass fraction (nearly 40%). In comparison, bio-oil mass fractions were approximately 36% and 31% in the 0B and 0S controls, respectively. Hence, p- sludge biochar played a signi fi cant role in further decomposing bio- oil in addition to temperature (i.e. downstream temperature). At 700 C, bio-oil yield using p-sludge biochar was still reduced to 30% (wt % of the total product mass) while the 0B control test yielded 36% bio-oil. At the lower temperature of 600 C, product yields under all test conditions were similar, indicating that higher temperatures are required to achieve biochar catalytic activity (Fig. 1a). Bio-oil yield did not signi fi cantly vary as a result of different downstream con- ditions (ANOVA, p ¼ 0.0752) at 600 C, and was approximately 35%. Olivine and dolomite catalysts were also studied at 800 C for comparison. Dolomite reduced the bio-oil yield to 20.5%, but olivine only reduced the bio-oil yield to 27.5%. Devi et al. [32] also found that calcined dolomite was more reactive than olivine with respect to tar decomposition [32]. The catalytic effect of p-sludge biochar on the bio-oil yield (21.5% at 800 C) was comparable to that of the traditional tar reduction catalyst, dolomite. P-sludge biochar catalyst altered py-gas composition at high temperatures (700 C and 800 C) (Fig. 1b). Hydrogen was the predominant gas component in the catalyzed py-gas. P-sludge biochar increased the H 2 fraction to over 40 vol% at 800 C, which was similar to results with wood and manure biochars. At 600 C, though, the product yields barely changed. P-sludge biochar altered the py-gas composition by increasing the H 2 concentration to over 35 vol%. The H 2 increase most likely resulted from tar steam reforming (tar þ H 2 O / CO, CH 4 , and H 2 ), steam methane reforming (H 2 O þ CH 4 # 3H 2 þ CO; 2H 2 O þ CH 4 # 4H 2 þ CO 2 ), and dry reforming (CO 2 þ CH 4 # 2CO þ 2H 2 ). The CO 2 concentration was reduced to less than 20 vol% with biochar catalysts at high temperatures (700 Cand800 C). TheCO 2
2.2.1. Characterization of py-gas and bio-oil Py-gas constituent concentrations (i.e. H 2 , CH 4 , CO, C 2 H 4 , C 2 H 6 , CO 2 , C 3 H 8 ) were determined by gas chromatography with a thermal conductivity detector (GC-TCD) (Agilent Technologies 7890A) described previously [4]. Gas chromatography-mass spectroscopy (GC-MS) (7890B-5977A, Agilent Technologies, USA) and gas chro- matography with a fl ame ionization detector (430 GC-FID, Bruker Daltonics, Inc., USA) were used for the qualitative and quantitative bio-oil analyses. The bio-oil composition analyses were conducted at Iowa State University's Bioeconomy Institute using a published analytical method [31]. The higher heating value (HHV) of py-gas was calculated based on the component concentrations and the corresponding compo- nent HHVs. The biochar and bio-oil HHVs were measured by bomb calorimetry (Parr 1341, Parr Instrument Company, Moline, IL). By multiplying each pyrolysis product yield with its corresponding HHV (i.e. bio-oil yield multiplied by bio-oil HHV; py-gas yield multiplied by py-gas HHV), the product energy per mass of bio- solids pyrolyzed (kJ/kg biosolids) was obtained.
2.3. Characterization of biochar
Surface morphology and elemental composition of both new (i.e. pristine) and used biochar were analyzed using a scanning electron microscope equipped with energy dispersive spectroscopy (SEM-EDS) (JSM-6510LV, JEOL USA, Inc. MA, USA). Biochar micro- structure was measured using a Brunauer-Emmett-Teller (BET) instrument (NOVA 4200e, Quantachrome Instruments, Boynton Beach, FL). The chemical composition of biochar was analyzed by X- ray fl uorescence (XRF-1800, Shimadzu). X-Ray Diffraction (XRD) analyses of new and used biochar as well as raw p-sludge were conducted using Bruker D8 Discover X-ray diffractometer (Cu anode). The used p-sludge biochar catalysts (used for one cycle, three
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