Bromine Transformation during Catalytic Pyrolysis of Waste Electronic Circuit Boards (WECBs) in an Auger Reactor over the Dual-Catalyst HZSM-5/CaO

TL;DR

This study introduces a dual-catalyst system to reduce bromine pollution during electronic waste recycling, improving both environmental safety and product quality.

Contribution

A novel dual-catalyst (HZSM-5/CaO) system is introduced for bromine fixation and hydrocarbon upgrading during electronic waste pyrolysis.

Findings

44 wt% of bromine was retained as CaBr2 in solid residue, reducing bromine in pyrolysis oils.

The dual-catalyst system doubled gas yield and suppressed liquid formation during pyrolysis.

Lower WHSV increased lighter aromatics but accelerated coke deposition, emphasizing the need for optimization.

Abstract

Effective bromine
mitigation is a critical challenge in the sustainable
recycling of electronic waste, where the uncontrolled release of brominated
species compromises both environmental safety and product quality.
This study unveils a novel synergistic transformation pathway of bromine
(Br) during ex situ dual-catalyst pyrolysis of waste electronic circuit
boards (WECBs). Experiments were conducted in a continuous auger reactor
integrated with a fixed-bed catalytic unit employing a dual (HZSM-5/CaO)
catalyst system. By tuning the weight hour space velocity WHSV from
0.6 to 1.0 h–1, the catalytic process not only doubled
the gas yield from 2.7 to 6.5 wt % but also selectively suppressed
liquid formation from 18.0 to 12.5 wt %, while driving deeper deoxygenation
and aromatic hydrocarbon enrichment. At lower WHSV, intensified secondary
reactions promoted the generation of lighter aromatics and also accelerated
coke deposition, highlighting the need for WHSV optimization. Mechanistic
insights reveal that brominated phenols and aromatic hydrocarbons
dominate the primary volatile fraction, where Br+ radicals
undergo dual pathways: recombination with H+ and small
fragments forming HBr/CH3Br, or neutralization by CaO to
yield stable CaBr2. Importantly, 44 wt % of total bromine
was retained in the solid residue as CaBr2, drastically
lowering bromine content in pyrolysis oils. The dual-catalyst strategy
thus enables simultaneous Br-fixation, hydrocarbon upgrading, and
catalyst regeneration, drastically reducing bromine in pyrolysis oils.
These findings provide a scalable, mechanistically guided route for
the valorization of cleaner electronic waste, coupling environmental
protection with high-value fuel production.

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