Ammonia has been essential to human activities for centuries. It is widely used as feedstock for fertilizers, industrial chemicals, and emission after-treatment systems. Owing to its properties, ammonia has garnered interest as a carrier for hydrogen in energy applications. It can be generated from carbon-free emission sources and pyrolyzed to produce pure hydrogen for various applications. The combustion of ammonia for power generation has been previously reviewed in this journal besides several aspects of ammonia oxidation chemistry, as it relates to emission after-treatment and reburn systems. However, the pyrolysis and oxidation chemistry of ammonia requires further elucidation to improve its use as a hydrogen carrier and as a fuel for combustion systems. This article provides an in-depth review of ammonia pyrolysis and oxidation chemistry in noncatalytic and catalytic systems. The catalytic pyrolysis chemistry of ammonia to produce pure hydrogen is reviewed to understand catalyst and reactor requirements for scaling up this technology. The combustion properties of ammonia as a pure fuel and in mixtures, including ignition, flame propagation, and extinction characteristics; its pyrolysis and oxidation reactions; and its potential to produce pollutant emissions are extensively reviewed. Ammonia combustion reaction mechanisms are reported based on results from pyrolysis and oxidation reactors, shock tubes, rapid compression machines, and research engines. The experimental work is complemented by the development of detailed combustion models via chemical kinetic and quantum chemistry simulations. Herein, recent results on ammonia pyrolysis and oxidation chemistry are introduced and summarized by highlighting the pertinent aspects of this rich and rapidly increasing body of information.
Carbon capture technology is currently considered one of the promising technologies to mitigate atmospheric CO2 concentration. CO2 capture and utilization (CCU) captures anthropogenic waste CO2 and valorizes it into useful products, supporting a circular transition pathway towards carbon neutrality. Unfortunately, the thermodynamic stability of CO2 requires a high-energy input for its conversion, resulting in processes with a net positive carbon footprint. The incorporation of enzymes as biocatalysts in a process is attractive, as it facilitates CO2 conversion under ambient conditions. In Nature, the conversion of CO2 into organic compounds is done through photosynthesis, using an enzyme called ribulose-1,5-biphosphate carboxylase/oxygenase (RuBisCO). RuBisCO plays a central role in the natural assimilation of CO2, making it the enzyme chosen in Nature upon which all life forms depend. However, the slow carboxylation rate of RuBisCO (1–10/s) has caused it to be overlooked by faster enzymes such as carbonic anhydrase (CA), which has a carboxylation rate of 106/s. Despite this, RuBisCO has a rate enhancement of 108 to 1010 times higher than CA. Thus, this review aims to take a closer look at RuBisCO and examine its potential in CCU. Various aspects are considered, such as RuBisCO’s performance in comparison to other enzymes, approaches to overcome its limitations, its applications and implications in CCU, the valuable chemicals that can be derived from it, recent developments in RuBisCO-integrated processes, and its economic and environmental considerations. Through this, RuBisCO’s potential as one of the key enzymes in CCU will be explored.
Fluidized bed reactors (FBRs) are crucial in the chemical industry, serving essential roles in gasoline production, manufacturing materials, and waste treatment. However, traditional up-flow FBRs have limitations in applications where rapid kinetics, catalyst deactivation, sluggish mass/heat transfer processes, particle erosion or agglomeration (clustering) occur. This review investigates multifunctional FBRs that can function in multiple ways and intensify processes. These reactors can reduce reaction steps and costs, enhance heat and mass transfer, make processes more compact, couple different phenomena, improve energy efficiency, operate in extreme fluidized regimes, have augmented throughput, or solve problems inherited by traditional reactor configurations. They address constraints associated with conventional counterparts and contribute to favorable energy, fuels, and environmental footprints. These reactors can be classified as two-zone, vortex, and internal circulating FBRs, with each concept summarized, including their advantages, disadvantages, process applicability, intensification, visualization, and simulation work. This discussion also includes shared considerations for these reactor types, along with perspectives on future advancements and opportunities for enhancing their performance.
Conditional Source-term Estimation (CSE) is a turbulence–chemistry interaction model to simulate reacting flows. This model is similar to the Conditional Moment Closure (CMC) approach in using the conditional scalar field to calculate the conditional reaction rates. However, unlike CMC, where transport equations are solved for the conditional scalars, an integral equation is inverted in CSE to estimate the conditional scalars. The model has been developed and applied to a wide range of combustion regimes, including diffusion, premixed, stratified premixed, mixed-mode combustion in lifted flames, spray combustion and MILD combustion in the past two decades. It has been tested against several Direct Numerical Simulation (DNS) databases in a priori analyses and also coupled with both Large-Eddy Simulation (LES) and Reynolds-Averaged Navier–Stokes (RANS) flow solvers to simulate benchmark burners. The CSE model has also been used in the simulation of practical combustion devices such as internal combustion engines and industrial furnaces. In this paper, the fundamental basis of the CSE model is first presented, and the model’s limitations and strengths are described. The challenges of the application of CSE to different combustion regimes are discussed through a comprehensive review of the past published works. Mathematical and numerical implementation techniques are presented, and future challenges in developing this turbulence–chemistry interaction model are also proposed.