Influence of Salts During Hydrothermal Biomass Gasification: The Role of the Catalysed Water-Gas Shift Reaction

Supercritical water gasification of hyperaccumulators for hydrogen production and heavy metal immobilization with alkali metal catalysts

The high moisture content and heavy metal concentration of hyperaccumulator are the main bottlenecks of resource utilization. Supercritical water gasification technology was used to convert Sedum plumbizincicola (a hyperaccumulator of Zn and Cd) into hydrogen gas and to immobilize HMs into biochar. Homogeneous alkali metal catalysts such as NaOH, Na₂CO₃ and Ca(OH)₂ were added to optimize the experimental conditions. The results showed that NaOH was effective in capturing CO₂in-situ, thereby shifting the water-gas shift reaction equilibrium in the forward direction. And the increase of NaOH concentration had a significant promotion effect on hydrogen production. In the non-catalytic gasification of Sedum plumbizincicola, the highest hydrogen (1.5 mol/kg) and H₂ selectivity (22.9%) with greater carbon gasification efficiency (19.3%) and lower H₂ gasification efficiency (8.7%) of the gas products were obtained at 400 °C with 6 wt% material concentration for 20 min. However, NaOH at 5% mass fraction maximized hydrogen and H₂ selectivity up to 7.5 and 98.2%, respectively. Alkali catalyst not only promoted the generation of hydrogen-rich bio-gas but also enhanced the immobilization efficiency of heavy metals. Compared to non-catalytic, when the addition amount of NaOH was 1 wt%, the Zn、Mn、Cd、Pb、Cr accumulated in biochar increased significantly for 76.8, 42.5, 80.8, 75.6 and 80.0%, respectively. This study highlights the remarkable ability of SCWG with alkali catalyst for hydrogen production and heavy metal stabilization.

Reaction kinetics and pathways of crotonic acid conversion in sub- and supercritical water for renewable fuel production

The degradation of an unsaturated lipid compound in water proceeds via two temperature-driven pathways – ionic and free radical reaction pathways.

Gasification of Biomass in Supercritical Water, Challenges for the Process Design—Lessons Learned from the Operation Experience of the First Dedicated Pilot Plant

Gasification of organic matter under the conditions of supercritical water (T > 374 °C, p > 221 bar) is an allothermal, continuous flow process suitable to convert materials with high moisture content (<20 wt.% dry matter) into a combustible gas. The gasification of organic matter with water as a solvent offers several benefits, particularly the omission of an energy-intensive drying process. The reactions are fast, and mean residence times inside the reactor are consequently low (less than 5 min). However, there are still various challenges to be met. The combination of high temperature and pressure and the low concentration of organic matter require a robust process design. Additionally, the low value of the feed and the product predestinate the process for decentralized applications, which is a challenge for the economics of an application. The present contribution summarizes the experience gained during more than 10 years of operation of the first dedicated pilot plant for supercritical water gasification of biomass. The emphasis lies on highlighting the challenges in process design. In addition to some fundamental results gained from comparable laboratory plants, selected experimental results of the pilot plant “VERENA” (acronym for the German expression “experimental facility for the energetic exploitation of agricultural matter”) are presented.

The use of process simulation in supercritical fluids applications

Modelling and simulation from micro- to macro-scale are needed to attain a broader commercialization of supercritical technologies.

Digested sewage sludge gasification in supercritical water

Digested sewage sludge gasification in supercritical water was studied. Influences of main reaction parameters, including temperature (623-698 K), pressure (25-35 Mpa), residence time (10-15 min) and dry matter content (5-25 wt%), were investigated to optimize the gasification process. The main gas products were methane, carbon monoxide, carbon dioxide and traces of ethene, etc. Results showed that 10 wt% dry matter content digested sewage sludge at a temperature of 698 K and residence time of 50 min, with a pressure of 25 MPa, were the most favorable conditions for the sewage sludge gasification and carbon gasification efficiencies. In addition, potassium carbonate (K2CO3) was also employed as the catalyst to make a comparison between gasification with and without catalyst. When 2.6 g K2CO3 was added, a gasification efficiency of 25.26% and a carbon gasification efficiency of 20.02% were achieved, which were almost four times as much as the efficiencies without catalyst. K2CO3 has been proved to be effective in sewage sludge gasification.

Catalytic hydrothermal gasification of algae for hydrogen production: composition of reaction products and potential for nutrient recycling

Chlorella vulgaris, Spirulina platensis and Saccharina latissima were processed under supercritical water gasification conditions at 500 °C, 36 MPa in an Inconel batch reactor for 30 min in the presence/absence of NaOH and/or Ni-Al(2)O(3). Hydrogen gas yields were more than two times higher in the presence of NaOH than in its absence and tar yields were reduced by up to 71%. Saccharina, a carbohydrate-rich macro-alga, gave the highest hydrogen gas yields of 15.1 mol/kg. The tars from all three algae contained aromatic compounds, including phenols, alkyl benzenes and polycyclic aromatic hydrocarbons as well as heterocyclic nitrogen compounds. Tars from Chlorella and Spirulina contained high yields of pyridines, pyrroles, indoles and pyrimidines. Up to 97% TOC removal were achieved in the process waters from the gasification of the algae. Analyses for specific nutrients in the process waters indicated that the process waters from Saccharina could potentially be used for microalgae cultivation.

Hydrothermal reaction kinetics and pathways of phenylalanine alone and in binary mixtures

We examined the behavior of phenylalanine in high-temperature water (HTW) at 220, 250, 280, and 350 °C. Under these conditions, the major product is phenylethylamine. The minor products include styrene and phenylethanol (1-phenylethanol and 2-phenylethanol), which appear at higher temperatures and longer batch holding times. Phenylethylamine forms via decarboxylation of phenylalanine, styrene forms via deamination of phenylethylamine, and phenylethanol forms via hydration of styrene. We quantified the molar yields of each product at the four temperatures, and the carbon recovery was between 80-100 % for most cases. Phenylalanine disappearance follows first-order kinetics with an activation energy of 144 ± 14 kJ mol⁻¹ and a pre-exponential factor of 10(12.4 ± 1.4)  min⁻¹. A kinetics model based on the proposed pathways was consistent with the experimental data. Effects of five different salts (NaCl, NaNO₃, Na₂ SO₄, KCl, K₂ HPO₄) and boric acid (H₃BO₃) on phenylalanine behavior at 250 °C have also been elucidated. These additives increase phenylalanine conversion, but decrease the yield of phenylethylamine presumably by promoting formation of high molecular weight compounds. Lastly, binary mixtures of phenylalanine and ethyl oleate have been studied at 350 °C and three different molar concentration ratios. The presence of phenylalanine enhances the conversion of ethyl oleate and molar yields of fatty acid. Higher concentration of ethyl oleate leads to increased deamination of phenylethylamine and hydration of styrene. Amides are also formed due to the interaction of oleic acid/ethyl oleate and phenylethylamine/ammonia and lead to a decrease in the fatty acid yields. Taken collectively, these results provide new insights into the reactions of algae during its hydrothermal liquefaction to produce crude bio-oil.