(489h) On an Integrated Approach to Evaluate Hydrogen As an Energy Product or Feedstock in Biochemical and Thermochemical Processes.

The use of hydrogen either as an energy carrier and/or as feedstock has gained considerable attention in recent years as means to advance renewables and promote biobased production paths. Currently, 98% of the global hydrogen used is produced from fossil fuels, with the predominant production process as steam methane reforming. To mitigate the release of greenhouse gases and achieve carbon-neutral emissions, it seems imperative to switch from natural gas to renewable feedstock, such as biomass, as an alternative for the production of renewable hydrogen. Rather than an energy product, hydrogen is also attractive to use as a valuable feedstock to upgrade oxygen-rich resources, such as those derived from biomass and organic substrates. Given that many waste-based resources are organic in nature, hydrogen offers a promising pathway for replacing fossil-based feedstocks in a wider range of industrial applications. Renewable pathways that produce or consume hydrogen may include electrochemical pathways (especially water electrolysis), biochemical pathways (e.g. dark fermentation, photo fermentation), and/or thermochemical pathways such as gasification and pyrolysis. While technology readiness levels continue to increase (at almost industrial-scale levels), there is no consensus concerning the suitable technology to involve hydrogen. Preferred hydrogen use (as energy product or chemical feedstock) is often unclear and depends on the extent to which complementary technologies can be effectively integrated: technology integration often becomes much more important than the efficiency of the individual technologies.

A systems methodology is employed in the paper that is based on an integrated framework to enable valorization of various organic resources while encompassing low and high-humidity organic waste. The framework provides for different pathways to produce hydrogen either as an energy fuel or as a chemical feedstock to produce higher-density fuels or chemicals. To achieve this, the paper relies on a model-based approach that combines first principle-based and hybrid models to account for the available technologies to integrate. Mathematical optimization is further deployed to optimize interactions and assess the level and the scale of integration required. The proposed framework is outlined in Figure 1. The proposed framework valorizes biochemical and thermochemical paths, it does consume and produce hydrogen, and integrates biomass supplies of different origin and resources.

Thermochemical processes include conventional and gas loop gasification. Biochemical processes are based on dark fermentation. Chemical looping gasification (BCLG) provides for the efficient use of oxygen and involves two reactors: the fuel reactor and the air reactor. The reactors are interconnected by an oxygen carrier that facilitates the transfer of oxygen via reduction-oxidation reactions. BCLG eliminates the need for Air Separation Units since the biomass is not directly gasified with air but with a metal oxide. A simplified version of the individual process is illustrated in Figure 2 as retrieved from Aspen Plus V11. Sharing the basic principles with chemical looping technology, the BCLG process takes place based on many intrinsic components such as the fuel, gasifying agents, reactor configurations, and looping materials. The kinetic approach in gasification process takes into account the tar formation in the reactor. Degrees of freedom include process conditions, looping materials, and reactor configurations. Dark fermentation makes use of Caldicellulosiruptor Saccharolyticus as a promising candidate with a remarkable ability to produce a theoretical maximum yield of 4 mol H2 per mole glucose. The fermentation offers advantages against conventional choices (e.g. utilization of agricultural, industrial, and forest waste as substrates, while eliminating the need for elevated temperature and pressure) and relies on a kinetic model with a substrate conversion efficiency that is low and restricted to 7.5-15% (of the energy contained in glucose that is converted to H2). Monod kinetics include inhibition and the effluent undergoes anaerobic digestion to produce methane as a by-product that is further recycled as a striping gas to lower the partial pressure of hydrogen and promote higher reaction rate, as shown in figure 3.

The optimization addresses degrees of freedom with respect to different biomass feedstocks, reaction pathways and products, as well as different means to involve hydrogen. The framework is optimized against constraints for seasonality, exploring energy integration schemes and product portfolios available. The framework is able to accommodate hydrogen production from any other renewable resources (e.g. wind farms, hydro or solar energy) and can be used to evaluate different business models that include a centralized or a distributed deployment of the process technologies.

References:

[1]: Nguyen, N.M.; Alobaid, F.;Dieringer, P.; Epple, B. Biomass-Based Chemical Looping Gasification: Overview and Recent Developments. Appl. Sci. 2021, 11, 7069

[2]: Ljunggren M, Willquist K, Zacchi G, van Niel EWJ. A kinetic model for quantitative evaluation of the effect of H2 and osmolarity on hydrogen production by Caldicellulosiruptor saccharolyticus. Biotechnol Biofuels 2011;4:31.

[3]: Kokossis, A. C., Tsakalova, M., & Pyrgakis, K. (2015). Design of integrated biorefineries. Computers & Chemical Engineering, 81, 40-56.