The team at the Chinese Academy of Agricultural Sciences used a high temperature pyrolysis reactor coupled to a tailored catalyst bed to process corn straw. By adjusting reactor temperature, catalyst loading, and catalyst composition, they obtained gas yields above 70 percent by weight, with hydrogen concentrations up to 45.24 percent by volume in a single catalytic stage. When they introduced biochar as a pre catalyst layer, the hydrogen fraction rose further to as much as 48.87 percent by volume.
Lead author Xinyi Zhang notes that catalytic biomass conversion often faces low hydrogen selectivity and heavy carbon deposition that deactivates nickel catalysts. Nickel based materials efficiently break carbon hydrogen and carbon carbon bonds but tend to coke when exposed to tar laden biomass vapors, reducing activity over time. The group therefore configured the reactor so that raw vapors encounter biochar first, which reshapes the reaction environment and product distribution before the stream reaches the nickel catalyst bed.
In this configuration, biochar functions as a first stage reactive filter within the two stage system. The char, which is porous and contains oxygen bearing functional groups, adsorbs large tar molecules and reactive radicals that would otherwise form coke directly on the metal surface. On the char, these macromolecular tars crack and reform into smaller gases that then move to the downstream catalyst as a cleaner, more reactive feed.
"Biochar is not just a byproduct in this system, it becomes an active partner in steering the chemistry toward hydrogen and away from problematic carbon buildup," says corresponding author Lili Huo. "By placing biochar ahead of the nickel catalyst, we extend catalyst life, upgrade the gas quality, and create a more sustainable closed loop for biomass utilization." With biochar pre catalysis, the authors report gas yields rising by up to nearly 9 percentage points while tar yields declined across all tested catalyst formulations.
Downstream of the biochar layer, the researchers tested a series of nickel based catalysts supported on porous alumina and modified with additional metals such as cobalt, iron, molybdenum, and cerium. The cerium containing catalyst NiCeAl2O3 delivered the highest hydrogen content in single stage operation, aided by a redox cycle between Ce3 plus and Ce4 plus that supplies active oxygen, promotes cracking of aromatic tars, and suppresses condensed carbon deposits. Other bimetallic formulations, including NiFe and NiMo, generated strong overall gas yields but tended to favor methane and produced more disordered carbon deposits, indicating that modest changes in metal composition can redirect reaction pathways.
Characterization using X ray diffraction, Raman spectroscopy, electron microscopy, and temperature programmed oxidation showed how the staged configuration alters carbon structures and catalyst surfaces. With biochar pre catalysis, spent catalysts contained less amorphous carbon, more graphitic and ordered deposits, and reduced pore blockage, which signal improved resistance to deactivation. The char itself became more graphitized and lamellar, suggesting that the reactions favor more stable carbon forms rather than tar like residues.
To evaluate potential deployment beyond the laboratory, the team carried out a techno economic assessment for treating one metric ton of corn straw. Incorporating the biochar pre catalytic stage yielded an additional net benefit of 0.31 US dollars per kilogram of biomass compared with conventional non catalytic pyrolysis, due to higher production of hydrogen rich gas and lower costs for tar treatment and carbon related charges. In contrast, a scenario without the biochar layer and with faster catalyst deactivation produced a loss of 2.72 US dollars per kilogram, reflecting higher catalyst expenses and reduced output of valuable gases.
The authors note that nickel catalysts remain a significant cost factor and that the loadings used in their experiments exceed what industry would likely apply. However, they argue that recycling biochar generated within the process back into the reactor as a pre catalyst could reduce fresh catalyst demand, extend nickel catalyst lifetime, and support closed material loops in rural biorefineries. They add that carbon pricing policies, optimization of low cost biochars, and development of robust alternatives to nickel could support scaling this two stage approach into operating renewable hydrogen systems.
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Shenyang Agricultural University
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