Upcycling animal waste into hydrogen and multi-walled carbon nanotubes: effects of feedstock and catalysts

Abstract

This study delved into the technical viability of utilizing thermal treatment on agricultural waste to generate hydrogen as a clean energy source and the creation of carbon nanotubes. This process was integrated with CO2 capture employing a calcined calcium oxide as a sorbent. Representative streams of agricultural waste, specifically chicken and goat manure, were utilized. Each experimental run involved feeding 50 grams of samples on a dry basis into a pyrolysis – CO2 capture – chemical vapor deposition (CVD) system. Subsequently, the gas produced post-CVD reaction was gathered and analyzed, with an emphasis on hydrogen gas content. Multi-walled carbon nanotubes (MWCNTs) were synthesized on various catalysts within the CVD reactor, subsequently collected and analyzed. Three different catalysts were investigated to assess their impact on hydrogen production and catalyst yield. Analysis of the CVD gas revealed a hydrogen yield ranging from 40% to 63% of the total collected gas. Among the three catalysts, the Co-Fe-Mo-MgO catalyst exhibited the highest hydrogen gas percentage at 63% volume. Chicken manure outperformed goat manure as a feedstock, yielding an average of 50% volume of hydrogen compared to 32% volume from goat manure. The Chicken_Co-Fe-Mo-MgO feedstock combination yielded the highest hydrogen percentage at 63% volume. Overall, MWCNT yields ranged from 0.9% to 1.7%. Chicken manure also outperformed goat manure in MWCNT production, yielding 1.67% of the input, which was 47% higher than that produced by goat manure. Characterization of MWCNTs via various analytical methods indicated that Chicken_Co-Ni exhibited a lower ID/IG than Goat-Co-Ni, suggesting a higher degree of graphitization in Chicken-Co-Ni. X-ray diffraction patterns for the Co-Fe-Mo-MgO, Co-Ni-Mo-MgO, and Co-Mo-MgO catalysts showed a significant peak at 43°, deviating from the typical 26° for MWCNTs. An economic evaluation was conducted for a pyrolysis-CVD agricultural waste processing plant by scaling up product ratios to the daily mass of animal manure produced by each farm. A plant with a capacity of 15 tonnes per day for pyrolysis-CVD and 100kg per day for chicken and goat manure respectively was modeled, and the estimated costs, revenues, and profits were calculated. The conclusion drawn was that operating such a processing plant would not be economically viable due to the high operational costs associated with the various catalysts, as well as the capital costs of plant construction and challenges related to storage, drying, and further product treatment. In conclusion, this project affirms the technical feasibility of generating hydrogen and MWCNTs from agricultural waste, while also highlighting the influence of feedstock and catalysts on process efficiency. However, economic analysis underscores the need for further research to identify a more cost-effective catalyst for achieving economic viability in full-scale processing plants. Despite these challenges, the pyrolysis process has demonstrated efficacy as a waste management solution, notably by reducing solid content into valuable solid biochar products.