N. Boukis

Hydrothermal gasification of biomass and organic wastes

Wet biomass and organic wastes can be efficiently gasified under hydrothermal conditions to produce a hydrogen rich fuel gas. New experiments in two tubular flow reactors and in two batch autoclaves with carbohydrates, with aromatic compounds, with glycine as a model compound for proteins and with real biomass are reported for different residence times, temperatures and pressures. It was found that at 600°C and 250 bar all compounds are completely gasified by addition of KOH or K2CO3, forming a H2 rich gas containing CO2 as the main carbon compound. Concentrations of CO, CH4 and C2–C4 hydrocarbons are low in the product gas (<1, ∼3 and <1 vol%, respectively). Carbon balances for the smaller flow reactor are closed to better than 96%. Ranges of product concentrations are given as estimated from experimental reproducibility. Some reflections for the engineering of a pilot plant are presented.

Factors controlling corrosion in high-temperature aqueous solutions: a contribution to the dissociation and solubility data influencing corrosion processes

Abstract Typical materials for applications in high-temperature, high-pressure aqueous solutions are stainless steels, nickel-base alloys, titanium, tantalum, noble metals, or ceramics. Under these conditions, these materials are often severely attacked, especially in the presence of acids, salts, or gases. Corrosion phenomena in high-temperature aqueous solutions are mainly influenced by the following factors: dissociation of acids, salts, and bases, the solubility of gases, the solubilities of corrosion products, and the stability of the protecting oxide layers. The influences of these factors on the corrosion, together with literature data of dissociation constants and solubilities of oxides, are reviewed in this paper. A recommendation is given for the application of possible reactor materials for oxidizing high-temperature aqueous systems in the presence of different inorganic species.

Review of the Corrosion of Nickel-Based Alloys and Stainless Steels in Strongly Oxidizing Pressurized High-Temperature Solutions at Subcritical and Supercritical Temperatures

Abstract The corrosion phenomena of nickel-based Alloy 625 (UNS N06625), which in the present content is representative of other nickel-based alloys and stainless steels in strongly oxidizing solut...

Corrosion of alumina ceramics in acidic aqueous solutions at high temperatures and pressures

Alumina is resistant against corrosive aqueous solutions and could be used as a reactor material in the Supercritical Water Oxidation (SCWO) process. For this reason, the corrosion resistance of alumina and zirconia toughened alumina (ZTA) ceramics was investigated in aqueous solutions containing 0.1 mol/kg H2SO4, H3PO4 or HCl at T = 240°C–500°C at p = 27 MPa. In sulfuric acid, the solubility of alumina and its corrosion products was high at temperatures of 240°C–290°C. The corrosion rate was still high at higher temperatures (340°C–500°C), but the corrosion products were less soluble and formed a non-protecting scale on the samples. Phosphoric acid was less corrosive due to the formation of berlinite (AlPO4) on the surface of the specimens. In hydrochloric acid, the dissolution of the alumina grains was the predominant corrosion phenomenon at temperatures of 240°C–290°C. At higher temperatures, intergranular corrosion was observed, but a dissolution of the grains did not occur.

Corrosion screening tests of high-performance ceramics in supercritical water containing oxygen and hydrochloric acid

Abstract The corrosion of various ceramic materials in simulated supercritical water oxidation (SCWO) environment was measured. Supercritical water with 0.44 mol kg −1 oxygen and 0.05 mol kg −1 hydrochloric acid was used to simulate typical SCWO conditions after the decomposition of the organic material. The experimental temperature was 465 °C and the pressure 25 MPa. The experiments were performed within a reactor with an inner surface made of alumina. In this very corrosive fluid only a few Al 2 O 3 - and ZrO 2 -based materials did not corrode severely. Homogeneous surface attack and grain boundary diffusion were observed. HIP-BN, B 4 C, TiB 2 , Y 2 O 3 and Y-TZP disintegrated. SiC and Si 3 N 4 -based materials showed a large weight loss, up to above 90%.

Corrosion of alloy 625 in aqueous solutions containing chloride and oxygen

Abstract Alloy 625 (UNS N06625) is used frequently as a reactor material for the oxidation of hazardous organic wastes in supercritical water (supercritical water oxidation [SCWO]). In the presence of chloride (Cl−) and oxygen (O2), all Ni-based alloys corrode fast in high-temperature, subcritical water. High-pressure, high-temperature-resistant tube reactors made of alloy 625 were used as specimens. Coupons were exposed simultaneously inside the test tubes. Experimental conditions included temperatures up to 500°C and pressures up to 38 MPa. Pitting corrosion was observed at temperatures above ≈ 130°C to 215°C. At higher temperatures (up to the critical temperature of water), transpassive dissolution dominated. Under certain conditions, transgranular stress corrosion cracking (TGSCC) appeared in the transition zone between the passive and transpassive regions leading to premature failure of the test reactors. Parts of the corrosion products were insoluble in supercritical water and formed thick layers in...

Methanol reforming in supercritical water for hydrogen production

ABSTRACT The production of hydrogen by reforming of methanol in supercritical water was studied in continuous flow apparatus made of nickel base alloys. Experiments were performed at pressures from 25 to 45 MPa, temperatures in the range of 400 to above 600°C and residence times from few seconds up to few minutes. The feed concentration varied from 5 to 64 wt.% methanol. The main component of the product gas is H2, carbon is converted to CO2, CO, and CH4. Methanol conversion is up to 99.9% without addition of a catalyst. Oxidation of the reactor inner surface before gasification turned out to enhance the reaction rate and to lower the carbon monoxide concentration. Obviously, the heavy metals of the inner surface of the reactors catalyze the reaction. Pilot-plant tests (with a flow rate of 100 kg/h) confirm the laboratory experiments and give data for the energy balance of the process.

Corrosion of Alloy 625 in High-Temperature, High-Pressure Sulfate Solutions

Abstract Corrosion phenomena, of alloy 625 (UNS N06625) were investigated in oxygenated aqueous solutions containing sulfuric acid (H2SO4), sodium hydrogen sulfate (NaHSO4), or sodium sulfate (Na2S...

Transpassive Dissolution of Alloy 625, Chromium, Nickel, and Molybdenum in High-Temperature Solutions Containing Hydrochloric Acid and Oxygen

Abstract Coupons of nickel, molybdenum, chromium, and the nickel-based Alloy 625 (UNS 06625) were corroded in strongly oxidizing hydrochloric acid (HCl) solutions at 350°C and a pressure (p) of 24 MPa, with reaction times between 0.75 h and 50 h. For Alloy 625, the effect of surface roughness also was investigated. Nickel and molybdenum showed strong material loss after only 5 h of reaction as a result of the instability of the solid oxides formed under experimental conditions. The attack on chromium started at the grain boundaries. At longer reaction times, thick, spalling oxide layers formed on the surface. The attack on Alloy 625 also started at the grain boundaries and at inclusions leading to the formation of small pits. On polished surfaces, the growth of these pits occurred faster than on nonpolished surfaces, but fewer pits grew. Corrosion products formed at the surface consisted of oxygen and chromium. On isolated spots, nickel- and chlorine-containing products also were found.

The corrosion of tantalum in oxidizing sub- and supercritical aqueous solutions of HCl, H2SO4 and H3PO4

The corrosion of tantalum was investigated in sub- and supercritical oxidizing solutions of hydrochloric, sulfuric and phosphoric acid at temperatures between 360 and 500 °C. The corrosion rates in HCl and H2SO4 increased strongly above the critical temperature of water, which was attributed to a phase transformation from vitreous to crystalline Ta2O5. Corrosion rates in H3PO4 were low at all temperatures due to the formation of a top phosphate layer.