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Radiation is the principal mode of heat transfer in furnaces. Models for gaseous radiative properties have been well-established for air combustion. However, there is uncertainty regarding their applicability to oxy−fuel conditions. In this paper, a new and complete set of weighted sum of gray gases model (WSGGM) is derived, which is applicable to computational fluid dynamics (CFD) modeling of both air−fuel and oxy−fuel combustion. First, a computer code is developed to evaluate the emissivity of any gas mixture at any condition using the exponential wide band model (EWBM), and the calculated results are validated in detail against data in the literature. Then, the validated code is used to generate emissivity databases for representative air- and oxy-firing conditions, for each of which a refined WSGGM with new parameters is derived. The practical way to implement the model to CFD simulations of combustion systems is given. Finally, as a demonstration, the new model is implemented to CFD modeling of two furnaces of very different beam lengths. The CFD results are compared to those based on the widely used WSGGM in the literature, from which some useful guidelines on oxy−fuel modeling are recommended.

Hydrothermal liquefaction (HTL) of biomass is emerging as an effective technology to efficiently valorize different types of (wet) biomass feedstocks, ranging from lignocellulosics to algae and organic wastes. Significant research into HTL has been conducted in batch systems, which has provided a fundamental understanding of the different process conditions and the behavior of different biomass. The next step towards continuous plants, which are prerequisites for an industrial implementation of the process, has been significantly less explored. In order to facilitate a more focused future development, this review—based on the sources available in the open literature—intends to present the state of the art in the field of continuous HTL as well as to suggest means of interpretation of data from such plants. This contributes to a more holistic understanding of causes and effects, aiding next generation designs as well as pinpointing research focus. Additionally, the documented experiences in upgrading by catalytic hydrotreating are reported. The study reveals some interesting features in terms of energy densification versus the yield of different classes of feedstocks, indicating that some global limitations exist irrespective of processing implementations. Finally, techno-economic considerations, observations and remarks for future studies are presented.

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