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1. Advanced Reactor Design for Ammonia Production from Electrochemical Nitrogen and Nitrate Reduction

   Yifu Chen, Wenzhen Li ( November 2021 , AIChE 2021 Annual Conference)

   Motivated by the increasing demand for flexible and sustainable routes of ammonia (NH3) production, the electrochemical nitrogen (N2) and nitrate reduction reaction (NRR and NO3RR) have attracted intense research interest in the past few years1,2. Compared to the centralized Haber-Bosch process that operates at elevated temperature and pressure, the electrochemical pathway features mild operating conditions but high input energy density, allowing for distributed and on-site generation of NH3 with water as the proton source, thereby reducing the transportation and storage costs of NH3 and H23. Besides N2 which is highly abundant in the atmosphere, nitrate-N exists widely in agricultural and industrial wastewaters, and its presence has raised severe concerns due to its known impacts on the environment and human health4,5. In this regard, NO3RR provides a promising strategy of simultaneously removing the harmful nitrate-N and generating NH3 as a useful product from those wastewater streams. While research activities on both NRR and NO3RR are blooming with substantial progress in the field of electrocatalysis, some major challenges remain unnoticed or unresolved so far. Due to the wide existence of reactive N-containing species in laboratory environments, the source of NH3 in NRR measurements is sometimes elusive and requires rigorous examination by control experiments with costly 15N26,7. On the other hand, while the electro-reduction of nitrate is much more facile, additional costs arising from the enrichment and purification of nitrate in contaminated waste resources have challenged the practical feasibility of NO3RR both technically and economically2. In this talk, we will present our latest research progress as part of the solutions to these challenges in state-of-the-art NRR and NO3RR studies, from the perspective of reactor design. By taking advantage of the prior developments in 15N2 control experiments, here we suggest an improved 15N2 circulation system that is effective and affordable for NRR research, allowing for more accurate and economized quantitative assessment of NH3 origins, so that false positives and subtle catalytic activities can be identified more reliably. For NO3RR, we developed a compact reactor system for rapid and efficient electrochemical conversion of nitrate to NH3 from real nitrate-containing waste sources, accompanied by the concurrent separation and enrichment of the produced NH3 in a trapping solution to yield pure ammonium compounds. Our work highlights the importance of advanced reactor design in N-related electrochemistry research, which will facilitate the transformation of the current N-centric chemical industries towards a sustainable future. 

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2. Reports from the Frontier: Electrifying Chemical Transformations and Separations to Valorize Wastewater Nitrogen

   https://doi.org/10.1149/2.F04232IF  

   Liu, Matthew ; Tarpeh, William ( June 2023 , The Electrochemical Society Interface)

   Ammonia is an essential compound to modern society, underpinning fertilizer production and chemical manufacturing. Global ammonia demand currently exceeds 150 million tons a year and is projected to increase over 2% annually. Over 96% of ammonia is currently generated through the Haber-Bosch (HB) process, in which steam-reformed hydrogen reacts with nitrogen under reaction conditions that consume 1–2% of global energy and contribute 1.2–1.4% of anthropogenic CO2 emissions every year. In an environmental context, ammonia is a form of reactive nitrogen. Large amounts of reactive nitrogen, such as HB ammonia, accumulate in the biosphere because 80% of wastewater globally is discharged without treatment. The resulting skew in the global nitrogen cycle leads to imbalanced ecosystems and threatens water quality. Conventional water treatment removes reactive nitrogen by converting it to N2 (biological nitrification–denitrification); at HB facilities, the N2 is then cycled back to produce ammonia. Directly valorizing reactive nitrogen in waste streams would shortcut the use of N2 as an intermediate in water remediation and ammonia production, allowing savings in energy, emissions, and costs. Indeed, treating nitrogen as a resource to recover rather than simply a pollutant to remove aligns with the US National Academy of Engineering’s call to manage the nitrogen cycle, a challenge central to chemical manufacturing and ecosystem protection. 

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3. Unbiased solar H2 production with current density up to 23 mA cm2 by Swiss-cheese black Si coupled with wastewater bioanode

   Lu, L. ; Vakki, W. ; Aguiar, A. J. ; Xiao, C.X. ; Hurst, K. ; Fairchild, M. ; Chen, X. ; Yang, F. ; Gu, J.* ( January 2019 , Energy & environmental science)

   Unbiased photoelectrochemical hydrogen production with high efficiency and durability is highly desired for solar energy storage. Here, we report a microbial photoelectrochemical (MPEC) system that demonstrated superior performance when equipped with bioanodes and black silicon photocathode with a unique ‘‘Swiss-cheese’’ interface. The MPEC utilizes the chemical energy embedded in wastewater organics to boost solar H2 production, which overcomes barriers on anode H2O oxidation. Without any bias, the MPEC generates a record photocurrent (up to 23 mA cm2) and retains prolonged stability for over 90 hours with high Faradaic efficiency (96–99%). The calculated turnover number for MoSx catalyst during a 90 h period is 495 471 with an average frequency of 1.53 s1 . The system replaced pure water on the anode with actual wastewater and achieved waste organic removal up to 16 kg COD m2 photocathode per day. Cost credits from concurrent wastewater treatment and low-cost design make photoelectrochemical H2 production practical for the first time 

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4. Optimal design of combined cycle power plants with fixed‐bed chemical‐looping combustion reactors

   https://doi.org/10.1002/aic.16516  

   Chen, Chen ; Bollas, George M. ( January 2019 , AIChE Journal)

   Process intensification options are explored for near‐carbon‐neutral, natural‐gas‐fueled combined cycle (CC) power plants, wherein the conventional combustor is replaced by a series of chemical‐looping combustion (CLC) reactors. Dynamic modeling and optimization are deployed to design CLC‐CC power plants with optimal configuration and performance. The overall plant efficiency is improved by optimizing the CLC reactor design and operation, and modifying the CC plant configuration and design. The optimal CLC‐CC power plant has a time‐averaged efficiency of 52.52% and CO₂capture efficiency of 96%. The main factor that limits CLC‐CC power plant efficiency is the reactor temperature, which is constrained by the oxygen carrier material. CLC exhaust gas temperature during heat removal and gas compressor to gas turbine pressure ratio are the most important operating variables and if properly tuned, CLC‐CC power plants can reach high thermodynamic efficiencies. © 2018 American Institute of Chemical EngineersAIChE J, 65: e16516 2019

    

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5. Targeting climate‐neutral hydrogen production: Integrating brown and blue pathways with green hydrogen infrastructure via a novel superstructure and simulation‐based life cycle optimization

   https://doi.org/10.1002/aic.17956  

   Lal, Apoorv ; You, Fengqi ( November 2022 , AIChE Journal)

   This article addresses the sustainable design of hydrogen (H₂) production systems that integrate brown and blue pathways with green hydrogen infrastructure. We develop a systematic framework to simultaneously optimize the process superstructure and operating conditions of steam methane reforming (SMR)‐based hydrogen production systems. A comprehensive superstructure that integrates SMR with multiple carbon dioxide capture technologies, electrolyzers, fuel cells, and working fluids in the organic rankine cycle is proposed under varying operating conditions. A life cycle optimization model is then developed by integrating superstructure optimization, life cycle assessment approach, techno‐economic assessment, and process optimization using extensive process simulation models and formulated as a mixed‐integer nonlinear program. We find that the optimal unit‐levelized cost of hydrogen ranges from $1.49 to $3.18 per kg H₂. Moreover, the most environmentally friendly process attains net‐zero life cycle greenhouse gas emissions compared to 10.55 kg CO₂‐eq per kg H₂for the most economically competitive process design.

    

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