Phase behavior and stabilization of phosphorus in sub- and supercritical water gasification of cyanobacteria

Evolution of phosphorus with the promotion of KOH in supercritical water gasification of dewatered cyanobacteria from ion perspective

Phosphorus is a very important resource, and dewatered cyanobacteria contains a large amount of it. Basic additives, such as KOH, are often used to promote hydrogen production during supercritical water gasification (SCWG) of biomass, but their effects phosphorus transformation have rarely been investigated. In this study, SCWG of dewatered cyanobacteria with potassium salt and KOH was conducted in autoclave at 400 °C for 10 min, to investigate the effect of K⁺ on the transformation of phosphorus under neutral and alkaline conditions. Results showed that K⁺ increased the proportion of phosphorus in the solid phase from 88.4% to 90.8-98.3%. Furthermore, K⁺ could promote the transformation of iron-combined phosphorus to calcium-combined phosphorus and occluded phosphate. Only when the reaction environment was alkaline, the proportion of phosphorus in the solid phase was significantly reduced to a minimum of 26.1%. When the amount of OH^- was sufficient, can this part of phosphorus and organic phosphorus, which was decomposed and transformed by the promotion of OH^-, be transferred to the liquid products. Results from this study laid a foundation simultaneously for hydrogen production and phosphorus recovery more environmentally and high-effectively.

Formation and inhibition of polycyclic aromatic hydrocarbons from the gasification of cyanobacterial biomass in supercritical water

Polycyclic aromatic hydrocarbons (PAHs) formation and inhibition from supercritical water gasification (SCWG) of cyanobacterial biomass were investigated. High reaction temperature, long residence time, and low feedstock concentration favoured higher molecular weight (HMW) PAH formation. The total PAH yield reached 34.80 μg g^-1 at 500 °C, 22.5 MPa, and 10 min. The main PAHs formed in the liquid phase and the solid residue were 3-ring and 4-ring PAHs, which were generated from the cycloaddition reaction of lower molecular weight (LMW) PAHs. In addition, 2-ring PAHs were produced from the Diels-Alder reaction of phenols and unsaturated hydrocarbons. The possible control methods for PAH formation during the SCWG of cyanobacterial biomass were proposed. H₂O₂ addition effectively inhibited the reaction pathways underlying PAH formation, and the addition at more than 1.0% concentration suppressed H₂ production. The work revealed that the inhibition of PAHs was achieved in terms of improving the oxidisation condition during the SCWG process for converting wet biomass or organic wastes to energy sources.

Catalytic Gasification of Sewage Sludge in Supercritical Water: Influence of K2CO3 and H2O2 on Hydrogen Production and Phosphorus Yield

In this work, the catalytic gasification of sewage sludge in supercritical water was investigated in a batch reactor (460 °C, 27 MPa, 6 min), and the separate and combined effects of the catalyst on the H2 production and phosphorus yield were investigated. The experimental results indicated that K2CO3 alone improved the H2 yield, gasification efficiency (GE), and carbon gasification efficiency (CE). The largest H2 yield of 54.28 mol/kg was achieved, which was approximately three times that without a catalyst. Furthermore, the inorganic phosphorus (IP) yield increased with the addition of K2CO3. However, when H2O2 was added, the H2 yield quickly decreased with increasing H2O2 coefficient, and more than 97.8% of organic phosphorus (OP) was converted into IP. The H2 yield increased with the addition of various K2CO3/H2O2 ratios, whereas the IP yield decreased.

A review of research trends in the enhancement of biomass-to-hydrogen conversion

Different types of biomass are being examined for their optimum hydrogen production potentials and actual hydrogen yields in different experimental set-ups and through different chemical synthetic routes. In this review, the observations emanating from research findings on the assessment of hydrogen synthesis kinetics during fermentation and gasification of different types of biomass substrates have been concisely surveyed from selected publications. This review revisits the recent progress reported in biomass-based hydrogen synthesis in the associated disciplines of microbial cell immobilization, bioreactor design and analysis, ultrasound-assisted, microwave-assisted and ionic liquid-assisted biomass pretreatments, development of new microbial strains, integrated production schemes, applications of nanocatalysis, subcritical and supercritical water processing, use of algae-based substrates and lastly inhibitor detoxification. The main observations from this review are that cell immobilization assists in optimizing the biomass fermentation performance by enhancing bead size, providing for adequate cell loading and improving mass transfer; there are novel and more potent bacterial and fungal strains which improve the fermentation process and impact on hydrogen yields positively; application of microwave irradiation and sonication and the use of ionic liquids in biomass pretreatment bring about enhanced delignification, and that supercritical water biomass processing and dosing with metal-based nanoparticles also assist in enhancing the kinetics of hydrogen synthesis. The research areas discussed in this work and their respective impacts on hydrogen synthesis from biomass are arguably standalone. Thence, further work is still required to explore the possibilities and techno-economic implications of combining these areas for developing robust and integrated biomass-to-hydrogen synthetic schemes.