Search

In this paper, the effect of solar intensity on the heat pipe tip temperature in a heat pipe type—evacuated-tube solar collector (HP-ETSC) was investigated. A simple relation was proposed, relating the solar intensity to the heat pipe tip temperature generated from the experimental data. This simple empirical relation was applied in a set of heat transfer equations derived to predict the heating medium temperature at the manifold outlet of the evacuated-tube solar collector. The calculated results corresponding to two types of heating medium, i.e., palm oil and water, were compared with experimental results from the literature. The results show that the average error was 6.41% for the case of palm oil and 4.66% for the case of water. Based on the case of water as a heating medium fluid, it was found that the flow rate of the heating medium fluid affected the accuracy of prediction, as the percentage error increased with the heating medium flow rate. The maximum percentage error increased from only 1.83% for a water inlet flowing at a Reynolds number of about 2.4 × 103 to 15.23% for a water flow rate at a Reynolds number of about 2.6 × 104. The correction factor was added into the correlation to predict the heat transfer coefficients of heating medium fluids. With this correction factor, the maximum error could be reduced from 11.78% to 7.29% for the palm oil case and from 15.23% to 5.57% for the water case. The average errors corresponding to palm oil and water cases could be reduced to 0.74% and 1.26%, respectively. [ABSTRACT FROM AUTHOR]

A palm oil-based polygeneration system (POPS), which is a combination of a fixed bed hydrotreating reactor (FBHTR), a three-phase separator, and a series of cryogenic separators, is co-production process of green diesel and liquefied petroleum gas (LPG) named Design 1. The FBHTR model is validated by experiment data and its optimal operating parameters are determined by solving the response surface methodology-based optimization algorithm. Two CO 2 -negative designs for the POPS named Designs 2 and 3 adopt approaches of (i) the evacuated tube solar collector (ETSC) for reducing 35% flue gas from the furnace, (ii) the amine-based CO 2 capture process coupling with pre- or post-separation system for producing the high-purity CO 2 product, and (iii) the heat integration design for reducing the energy duties of hot/cold utilities. Design 2 is validated to achieve the maximum negative net CO 2 emissions. Design 3 not only ensures the negative net CO 2 emissions, but also it produces three high-purity products (98.3% green diesel, 100% LPG, and 99.9% CO 2) simultaneously. [Display omitted] [ABSTRACT FROM AUTHOR]

Biogas is an alternative source of fuel potentially used to run solid oxide fuel cells (SOFCs). It mainly consists of CH4 and CO2 which can degrade the SOFC interconnect, which is typically made of stainless steel. To investigate the effect of each gas constituent, we focussed here on the effect of CO2 on high-temperature oxidation behavior of and scale adhesion on the stainless steel interconnect, Type 430 stainless steel. The samples studied were oxidised in CO2 at contents of 5–100% at 800 °C. The oxidation kinetics were found to be parabolic with the rate constant increasing when the CO2 content increased. The scale adhesion was assessed using a tensile-test method. The scale formed in the atmosphere containing higher CO2 content exhibited poorer scale adhesion, as indicated by a lower strain initiating the first spallation and a larger spallation percentage after the first spallation took place. The worsened scale adhesion relates to pores formed at the scale/steel interface. The adhesion energies were further quantified giving the values of about 40–100 J m–2. Oxidation mechanisms were suggested based on the dependence of the parabolic rate constant on the oxygen partial pressure and the inward diffusion of carbon-bearing species. [ABSTRACT FROM AUTHOR]

Though the classical transport theory does not mathematically express the Arrhenius-like relation between Cr-loss flux due to volatilisation and absolute temperature, such relation was experimentally observed for 409L stainless steel exposed at 600–750 °C. Volatilisation calculation in the present cases can be estimated giving such relation because of the nearly linear relation between mass transfer coefficient and temperature. Cr-loss fluxes in O 2 with 20 and 40 % H 2 O are close to or coincided with the fluxes calculated from MnCr 2 O 4 evaporation. Increasing water vapour could enhance volatilisation contributed from the raised CrO 2 (OH) 2 pressure, reduce oxidation rate, and worsen scale adhesion. • 409L stainless steel was assessed at 600–750 °C for SOFC interconnect application. • Flux of Cr loss by volatility related to temperature in the Arrhenius-like form. • Cr-loss flux equation can be simplified resulting in the Arrhenius-like relation. • Water vapour could help reduce oxidation rate with enhanced scale volatilisation. • Water vapour could worsen the scale adhesion. [ABSTRACT FROM AUTHOR]

Chromium vaporisation from chromia-forming alloys when exposed to high temperatures can deteriorate the protective property of oxide scale. This problem affects the lifetime of solid oxide fuel cells and solid oxide electrolysers using chromia-forming alloys as interconnectors. In this work, AISI 441, a commercial grade stainless steel which chromium vaporisation investigation has not been reported so far, was chosen for study. Chromium vaporisation rates of bare AISI 441 and AISI 430 and AISI 441 and AISI 430 coated with Mn-Co spinel by electroplating method were measured in an atmosphere of 5 % HO in O over the temperature range of 650-900 °C with the linear velocity of the humidified oxygen stream varied in the range of 0.8-3.0 cm s. For uncoated samples, AISI 441 showed a lower chromium vaporisation rate in comparison with that of AISI 430. Mn-Co spinel coating could reduce the chromium vaporisation rate from AISI 430 surface. However, on the contrary, the Mn-Co spinel coating could not only fail to suppress the chromium vaporisation, but it also promoted the chromium vaporisation from AISI 441 surface by 2 times. [ABSTRACT FROM AUTHOR]

• A type 430 stainless steel is coated by Mn-Co spinel by a slurry method. • Continuous Cr 2 O 3 formed by pre-oxidation before coating reduces a scaling rate. • Pre-oxidation helps reduce Cr loss by volatilisation. • Coating structure e.g. pore size affects the flux of Cr loss. • Dependence of gas diffusivity on the coating structure is graphically proposed. An AISI 430 stainless steel was coated by Mn-Co spinel using a slurry method. Pre-oxidation before the coating helped reduce the oxidation rate of the coated steel at 800 °C in O 2 -5%H 2 O, relating to the formation of the continuous chromia layer which inhibited the outward diffusion of iron to the coating layer. It also helped reduce the mass flux of Cr loss due to the volatilisation. The combined molecular and Knudsen diffusion of the volatile species through the coating layer was suggested to explain the reduced volatilisation rate of the coated sample with pre-oxidation. [ABSTRACT FROM AUTHOR]

AISI 441 sheets were oxidized from 4 to 24 h at 800 °C in wet atmosphere. Micrometric oxide nodules containing Ti, Nb, Cr and Mn were observed to form on the passive scale. The internal microstructure of these objects was investigated using STEM–EDX and FIB-SEM tomography. Experimental results reveal a complex microstructure linked with the presence of Si and Nb and their competition for interfacial oxidation. Eventually, the chemical compositions of the nodule and of the oxidation affected zone in the nodule vicinity are discussed in relation with thermodynamic calculations investigating the stability of the different oxides. [ABSTRACT FROM AUTHOR]

Less mass gain and poorer scale adhesion assessed by the tensile test of the AISI 430 stainless steel exposed to O 2 -40%H 2 O at 800 °C were observed in comparison with the exposure to O 2. The reduced mass gain was partly due to Cr-species volatilisation while the worse scale adhesion related to pores formed at scale/steel interface. The Brouwer diagrams were proposed to help discuss the water vapour effect on altering defect concentrations that could affect the oxidation rate and scale adhesion of the studied steel. • AISI 430 stainless steel was exposed to O 2 -40%H 2 O and O 2 at 800 °C. • The steel oxidised in O 2 -40%H 2 O had less mass gain and poorer scale adhesion. • Cr volatilisation in O 2 -40%H 2 O had significant effect on reducing mass gain. • Pores were observed at the interface between steel and scale formed in O 2 -40%H 2 O. • Brouwer diagrams of Cr, Mn and H interstitials in O 2 -H 2 O were proposed. [ABSTRACT FROM AUTHOR]

A Co-27Cr-6Mo alloy was oxidised in pure O 2 between 800 and 1000 °C for durations up to 96 h. The flow rate was varied between 2 and 5 cm.s–1. In these conditions, volatilisations of chromium, cobalt and molybdenum were observed. The chromium volatilisation values were in good agreement with calculations assuming (CrO 3)g volatilisation limited by diffusion in the gas boundary layer. On the contrary, the measured flux of evaporated Co was higher than the calculated Co volatilisation flux from the metallic Co. The theoretical relation between the solid Co particle size and its vapour pressure was suggested to help explaining such difference. • Volatilisations of Cr, Co and Mo from Co-27Cr-6Mo alloy were examined in dry O 2. • A strong influence of O 2 flow rate on Cr volatilisation was observed. • Cr volatilisation from Co-27Cr-6Mo alloy is not neglectable in dry atmosphere at temperatures lower than 1000 °C. • Co volatilisation was observed, higher than predicted by classical thermodynamic calculation. [ABSTRACT FROM AUTHOR]