Z. Liu, M. Hughes, Y. Tong et al.
Energy 239 (2022) 121925
biosolids biochar has 11% Fe content (Table 3). Thus, the good cat- alytic performance of certain biochars such as p-sludge and bio- solids biochars was most likely due to the speci fi c metal species. However, if the catalytic metals are fully covered by coke, then the biochar catalytic effect will decrease or stop [47]. Second, pyrolysis vapor constituents can easily adsorb onto biochar and metal surfaces and then dissociate to reactive radicals such as aromatic ring fragments [48 e 51]. Biochar itself also con- tains free radicals via the decomposition of organic matter during pyrolysis [52,53]. Therefore, free-radical reactions are favored un- der high temperatures, converting vapor constituents to smaller molecules and light hydrocarbon gases [48,54 e 57]. For example, though wood biochar has very low metal content with similar microstructure to manure biochar (Table 4), the catalytic effect of wood biochar was always better than manure biochar at high temperatures (Fig. 1). The catalytic performance of wood biochar was even close to that of biosolids biochar (Fig. 1). These results indicate that some speci fi c radicals in wood biochar could have played a role in upgrading pyrolysis vapor. The radical types on biochar and radical reactions over biochar surface require a detailed investigation in future work. Third, the porous carbon structure of biochar acts as the sub- strate support and active sites for highly dispersed metals and radical absorption enhanced reactions. Hence, biochar surface area is another crucial factor to determine the catalytic performance. Poor biochar microstructure reduces catalytic performance. For instance, stover biochar and DDGS biochar have very low surface area compared to other biochars (microstructural analysis is shown in Table 4), so they did not increase py-gas yield (Fig. 1a). The SEM images of studied new biochars are shown in Fig. 10. Corresponding to the surface area, the morphology of new p-sludge was very porous. This microstructure provided enough sites for the catalytic metal of Ca.
Table 3 Metal composition of new biochar catalysts (wt% dry basis).
800 C Biochar Catalyst
Element
stover
DDGS wood manure p-sludge biosolids
Mg
2.13
2.77 6.94
0.6
1.84 0.12 0.08 0.22
0.83
Ca
10.86
39.15
0.41
K
2.3
ND ND ND
0.02 0.15
ND
Na Ni Cu Zn
0.14
0.37 0.25 0.5
0.42 0.02 0.09 0.19 0.36
ND ND ND 0.01 ND ND ND 0.02 ND 0.01 ND 0.04 ND ND 0.02 0.08
ND
0.02 0.06
Mn
ND
Fe Cr Ti Zr
2.73
11.25
0.11
0.02 0.01
0.51
ND ND ND 0.03
ND
0.06 0.68
0.02
ND ND 0.33
0.27
ND ND ND 0.01 0.06 (Metal composition is on an element basis; ND: Not detected; The contents of Mg, Ca and Fe that are over 2% are highlighted in bold). ND
concentration relative to the fi rst cycle (Fig. 4). H 2 has low volu- metric energy content, therefore, lower H 2 concentration led to higher py-gas HHV. In the meantime, the concentrations of high unit energy components such as CH 4 andC 2 H 4 increased, leading to the increase of py-gas HHV. The py-gas energy yields per biosolids pyrolyzed decreased as the catalyst cycle number increased (Fig. 9). Though py-gas HHVs increased after the fi fth cycle for each biochar catalyst, py-gas productions decreased greatly at the same time (Fig. 3), leading to lower py-gas energy outputs compared to cycle 1. Speci fi cally, the py-gas energy was only reduced by 8% (from 8400 kJ to 7700 kJ) from cycle 1 to cycle 5 when using p-sludge biochar, while the py- gas energy reduction percentage using wood and biosolids chars were 17% and 19%, respectively. Meanwhile, the bio-oil energy increased gradually. With the catalyst deactivation during cyclic uses, the py-gas energy yields fi nally approached 0S py-gas energy, implying that only the catalytic temperature and secondary re- actions caused the energy shift to py-gas. In particular, at cycle 1, new p-sludge biochar produced a py-gas with energy over 8400 kJ (almost three times higher than 0B py-gas energy). The signi fi cant shift from bio-oil to py-gas energy may be useful for water resource recovery facilities that already combust anaerobic digester biogas for increased on-site energy recovery by blending biogas and py-gas.
4. Conclusions
This is the fi rst report that p-sludge biochar was a more effective catalyst compared to other carbonaceous waste biochars to in- crease py-gas yield (e.g. p-sludge biochar resulted in the highest py-gas mass fraction of nearly 40% at 800 C). In addition, the production of light non-aqueous phase condensate was eliminated under some catalytic conditions (e.g. p-sludge biochar resulted in the lowest bio-oil mass fraction of approximately 20% at 800 C). P- sludge biochar resulted in the highest py-gas yield and lowest bio- oil yield, lower catalyst loading requirements and longer reuse times, and upgraded py-gas and bio-oil compositions. P-sludge biochar increased the py-gas yield by nearly two-fold compared to the control test. The catalytic activity of p-sludge biochar was stable and produced the greatest shift in energy from bio-oil to py-gas during cyclic uses (i.e. the py-gas energy was only reduced by 8%,
3.5. Factors in fl uencing catalytic effects of biochar
Multiple properties of biochar catalysts could in fl uence pyroly- sis product upgrading. First, some biochars contain metals such as Mg, Ca, and Fe. These metals can play a key role in catalytic destruction of high molecular weight hydrocarbons (e.g. tar) [23,29,45,46]. For example, p-sludge biochar contains 39% Ca and
Table 4 Microstructural analysis of biochar catalysts. 800 C Biochar Catalyst
DFT cumulative surface area (m 2 /g)
DFT cumulative pore volume (cc/g)
DFT pore radius (Å)
Stover DDGS Wood
5.49 0.69 37.7 39.9 52.3 45.7
0.0043 0.00055
15.5
16
0.015 0.058 0.033 0.023
7.26 31.9 9.89 7.26
Manure p-sludge Biosolids
* DFT: Density Functional Theory.
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