Energy-conscious flow shop scheduling under time-of-use electricity tariffs

A time-indexed integer programming formulation is developed and used to identify manufacturing schedules that minimize electricity cost and the carbon footprint under time-of-use tariffs without compromising production throughput. The approach is demonstrated using a flow shop with 8 process steps operating on a typical summer day. Results suggest that shifting electricity usage from on-peak hours to mid-peak hours or off-peak hours, while reducing electricity cost may increase CO₂ emissions in regions where the grid base load is met with electricity from coal-fired power plants. The trade-off between minimizing electricity cost and reducing CO₂ emissions is shown via a Pareto frontier.     

Hall cell energy recovery and anode pre-heating, gauging the opportunity

Anode pre-heating was proposed as an alternative for recycling waste heat from smelting operations, which currently consume substantially more energy than the theoretical minimum required. Aside from direct electricity savings, anode pre-heating can provide extra metal production and reduce carbon dioxide emissions. Public data on energy and aluminum production is analyzed to examine the value of these three potential components and define a research development path. It is concluded that indirect process gains show the most potential value with economies on the order of 3 TWh per year in electricity, an avoidance of about 1.8 million metric tons of CO₂ emissions, and an increase in production capacity of about 200,000 metric tons of aluminum per year without any expansion of installed capacity.     

Thermodynamic Aspects of Iron in Metal Dusting

A theoretical analysis has been conducted into the thermodynamic role of iron and the dependence of possible metal-dusting occurrence on temperature, volumetric ratios of carburizing species CH₄ and CO, and total pressure. The analysis indicates that the equilibrium carbon activity with cementite (a _{C(Fe 3 C/Fe} )) decreases with increasing temperature, and becomes lower than unity in excess of approximately 1000°C. This critical temperature could be the upper-temperature limit for decomposition of cementite in metal dusting. The temperature regime for possible metal dusting in either CH₄–H₂ or CO–CO₂ gas mixtures increases with increasing volumetric ratio of CH₄ or CO. Increasing total pressure leads to a reduction of this temperature regime in CH₄–H₂ but an expansion in CO–CO₂. Comparatively, this temperature regime in CO–CO₂ is shifted more towards lower temperatures, and iron is likely more susceptible to metal-dusting attack in CO–CO₂ due to the higher reverse carbon-activity gradient established.     

Carbon doped α-Al2O3 coatings grown by chemical vapor deposition

Alumina (Al₂O₃) coatings deposited by chemical vapor deposition (CVD) with different modifications and dopants are widely applied as wear resistant coatings on cemented carbide cutting tools. The aim of this work was to investigate the influence of CH₄ addition on the deposition of α-Al₂O₃ by low-pressure chemical vapor deposition (LPCVD). The coatings were deposited at 1005 °C on a TiN–TiCN base layer using a precursor gas mixture of AlCl₃, CH₄, CO₂, HCl, H₂S, and H₂. Coating characterization was conducted by scanning electron microscopy (SEM), glow discharge optical emission spectroscopy (GDOES), X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectroscopy (ToF-SIMS), glancing angle X-ray diffraction (GAXRD), nanoindentation and tribological ball-on-disk tests against Al₂O₃ balls. Additionally, the ball-on-disk wear tracks were investigated by Raman spectroscopy.     

On the oxidation of iron in CO2 + CO mixtures. III: Coupled linear parabolic kinetics

High-purity iron has been oxidized at 1000–1200° C in CO₂ and in CO₂ + CO with different compositions and at different total gas pressures (0.1–1 atm.). The experimental work has comprised thermogravimetric reaction rate measurements and characterization of the wüstite scales by metallography and x-ray diffraction. The overall results have been analyzed in terms of a classical model for coupled linear/parabolic kinetics, where it is assumed that the surface of growing wüstite scales has exactly the same defect structure and defect concentrations as that of bulk wüstite equilibrated in the same gaseous atmospheres. Important discrepancies are found between the predicted and the experimentally observed reaction behavior. Thus, both the linear and parabolic rate constants are found to be dependent on the partial pressure of CO₂ and the total gas pressure of the CO₂ + CO gas mixtures, and furthermore, the reaction in CO₂ + CO is slower than in O₂ and in H₂O + H₂ with the same oxygen activity. In order to explain the experimental results, it is suggested that CO and CO₂ molecules interact with the wüstite surface and thereby affect the defect structure and defect concentrations in a thin surface layer, and that this, in turn, affects both the linear and parabolic reaction rates.     

Depolarised gas anodes for aluminium electrowinning

Consumable carbon anodes are used in the electrowinning of aluminium by the Hall-Heroult process.Emissions of CO2 may be eliminated by introducing an inert oxygen evolving anode,which however will require a higher anode potential.An alternative approach is to use a natural gas or hydrogen gas anode to reduce the CO2 emissions and lower the anode potential.Preliminary laboratory experiments were carried out in an alternative molten salt electrolyte consisting of CaCl2-CaO-NaCl at 680 °C.Porous anodes of platinum and tin oxide were tested during electrolysis at constant current.The behaviour of inert anode candidate materials such as tin oxide and nickel ferrite were also studied.     

Low temperature methane oxidation on differently supported 2 nm Au nanoparticles

Low temperature CH₄ oxidation was studied on 2 nm gold nanoparticles supported on various metaloxides. The differences in reaction rates for the different systems suggest that the support material has an effect on the activity. From TEM analysis, we found that the gold particles were stable in size during the reaction. In addition to full oxidation to CO₂, traces of C₂H₆ were detected when Au/TiO₂ was used, indicating limited partial CH4 oxidation. TiO₂ was found to be the best support for gold nanoparticles both in terms of activity and gold particle stability.     

Synthesis of a NiFe2O4 catalyst for the preferential oxidation of carbon monoxide (PROX)

The aim of this work was to study the NiFe₂O₄ spinel catalyst obtained by combustion reaction in the preferential oxidation of carbon monoxide reaction to conversion reactants H₂, CO and O₂ and to conversion products CH₄, H₂O and CO₂. The powders were prepared according to propellants chemical concept and characterized by XRD, TEM and catalytic tests. The XRD pattern shows the characteristic peaks of the spinel phase. The particle size calculated by TEM was 10.7 nm. The catalyst proved to be more selective to reagents for conversion into O₂ (89.5%) at 350 °C.     

Inert anodes for AI smelters: Energy balances and environmental impact

This paper assesses the energy and environmental consequences of retrofitting aluminum electrolysis cells with inert anodes. The energy consumption and the CO₂ emissions are calculated based on assumptions of what the cell voltage may be. It is crucial not to ignore the cell voltage increase that may be necessary to maintain the heat balance of the cell, and also the sources of electrical energy that would provide the incremental power. Thus, a global environmental analysis of the impact of retrofitting cells with inert anodes is needed. For more information, contact Halvor Kvande Hydro Aluminium Metal Products, N-0240 Oslo, Norway; +47-22-53-9155; fax +47-22-53-7778; e-mail halvor.kvande@hydro.com.     

Thermodynamic Roles of Metallic Elements in Carburization and Metal Dusting

This paper addresses the thermodynamic aspects of metallic elements (typically Fe, Ni, and Cr) in carburization and metal-dusting processes in CH₄–H₂, CO–CO₂, and CO–H₂O–H₂ carburizing gas mixtures, and their possible contributions to understanding of carburization and metal-dusting phenomena. Carburization requires $a_C \left( {gas} \right) > a_C \left( {M_Z C/M} \right) \cdot a_C \left( {M_Z C/M} \right)$ is solely temperature-dependent, while a_C (gas) is dependent not only on temperature but also on gas chemistry and total pressure. In general, metallic elements tend to be carburized at higher temperatures in CH₄–H₂, but at lower temperatures in CO–CO₂ and CO–H₂O–H₂ carburizing gas mixtures. For metal dusting to occur, $a_C \left( {M_Z C/M} \right)$ (gas)> 1 (first-type) and $a_C \left( {gas} \right) > a_C \left( {M_Z C/M} \right)$ ; 1 (second-type) should be satisfied. Possible regimes for first- and second-type metal dusting are discussed for pure Fe and Ni, and the range for first-type metal dusting of Ni is considerably broader than that for Fe.     

Electric Current Effects on the Corrosion Behaviour of High Chromium Ferritic Steels

The high-temperature corrosion behaviour of the Cr containing ferritic alloys Crofer 22 APU and Crofer 22 H were investigated for their potential application as interconnects in planar-type solid oxide electrolysis cells (SOECs) operating at 800 °C for syngas production in steam/CO₂ co-electrolysis mode. To simulate the operating conditions for this application, oxidation tests in relevant atmospheres with and without electric current were conducted. The corrosion behaviour was influenced by the electric current resulting in accelerated oxidation on the negative side and suppressed oxidation on the positive side. The scale structure was influenced by a combination of atmosphere and electric current effects. The modified oxidation of the interconnect steels due to the electric current effect could have detrimental impact for the O₂ side and beneficial effect for the CO₂/H₂O side in an SOEC stack operating in co-electrolysis mode.     

Achieving Carbon Neutrality in the Global Aluminum Industry

In the 21st century, sustainability is widely regarded as the new corporate culture, and leading manufacturing companies (Toyota, GE, and Alcoa) and service companies (Google and Federal Express) are striving towards carbon neutrality. The current carbon footprint of the global aluminum industry is estimated at 500 million metric tonnes carbon dioxide equivalent (CO_2eq), representing about 1.7% of global emissions from all sources. For the global aluminum industry, carbon neutrality is defined as a state where the total “in-use” CO_2eq saved from all products in current use, including incremental process efficiency improvements, recycling, and urban mining activities, equals the CO_2eq expended to produce the global output of aluminum. This paper outlines an integrated and quantifiable plan for achieving “carbon neutrality” in the global aluminum industry by advocating five actionable steps: (1) increase use of “green” electrical energy grid by 8%, (2) reduce process energy needs by 16%, (3) deploy 35% of products in “in-use” energy saving applications, (4) divert 6.1 million metric tonnes/year from landfills, and (5) mine 4.5 million metric tonnes/year from aluminum-rich “urban mines.” Since it takes 20 times more energy to make aluminum from bauxite ore than to recycle it from scrap, the global aluminum industry could set a reasonable, self-imposed energy/carbon neutrality goal to incrementally increase the supply of recycled aluminum by at least 1.05 metric tonnes for every tonne of incremental production via primary aluminum smelter capacity. Furthermore, the aluminum industry can and should take a global leadership position by actively developing internationally accepted and approved carbon footprint credit protocols.     

High-Temperature Corrosion of Nimonic-75 and Various Steels in Gases at Low Partial Pressurest

Abstract

Nimonic-75 has been reacted in low partial pressures of CO₂, H₂O, CO, CH₄, N₂, and various mixtures of these gases at temperatures in the range 750–1000°. Ten different steels have been studied in a fivecomponent gas mixture at temperatures between 500–750° and in a number of (CO + H₂+ H₂O) mixtures at 500° and 600°. No gross effects have been found on any of the materials tested under the various conditions of exposure, and the most significant phenomena in environments of practical importance are decarburisation and/or preferential oxidation of certain alloy constituents.     

Effect of Pressure on Metal Dusting Initiation on Alloy 800H and Alloy 600 in CO-rich Syngas

The effect of pressure on metal dusting initiation was studied by exposing conventional alloys 600 and 800H in CO-rich syngas atmosphere (H₂, CO, CO₂, CH₄, H₂O) at ambient and 18 bar total system pressure and 620 °C for 250 h. It was verified that, at constant temperature, increasing the total system pressure increases both oxygen partial pressure (pO₂) and carbon activity (a _C), simultaneously. Both samples exposed at ambient pressure showed very thin oxide scale formation and no sign of metal dusting. By contrast, samples exposed in the high-pressure experiment showed severe mass loss by metal dusting attack. Iron- and chromium-rich oxides and carbides were found as corrosion products. The distinct pressure-dependent behavior was discussed by considering both thermodynamic and kinetic aspects with respect to the protective oxide formation and pit initiation.     

Effect of H2S on metal dusting of iron

A review is given on the effect of H₂S on metal dusting of iron which has been studied by gas carburisation in CO‐H₂‐H₂O‐H₂S and CH₄‐H₂‐H₂S mixtures at 500 and 700°C. The presence of H₂S in carburising gas atmospheres leads to sulphur adsorption on the iron surface, which retards carbon transfer. Segregation experiments and surface analyses have shown that sulphur segregates (and thus adsorbs) on cementite surfaces as well as on iron surfaces. The adsorbed sulphur also suppresses graphite nucleation and thus can stop the reaction sequence of metal dusting. Experiments by thermogravimetric analysis (TGA) have shown that the extent of retardation of metal dusting depends on temperature, carbon activity and H₂S content. The higher the carbon activity, the higher is the H₂S content required for suppression of metal dusting. At carbon activities a_C > a_C(Fe/Fe₃C) the metastable iron carbide, cementite (Fe₃C), occurs as an intermediate phase during metal dusting. Carburisation experiments in CO‐H₂‐H₂O‐H₂S mixtures at 500°C and X‐ray diffraction analysis (XRD) of carburised samples have revealed that at very high carbon activities a second iron carbide, Hägg carbide (Fe₅C₂), forms on the cementite surface. Microstructural investigations have shown that both metastable carbides decompose during metal dusting. Metal dusting experiments on iron at 700°C have been performed in CH₄‐H₂‐H₂S gas mixtures. By adding 15 ppm H₂S to the CH₄‐H₂ atmosphere the onset of metal dusting can be retarded for more than 350 hours. By means of Auger electron spectroscopy (AES), scanning electron microscopy (SEM) and energy dispersive X‐ray analysis (EDX) it was shown that coke contains graphite, cementite and iron particles with adsorbed sulphur.     

Roles of carbon dioxide and steam on the hydrogen embrittlement of 3Cr tube steel in synthetic natural gas environment

The hydrogen embrittlement behaviour of 3Cr has been investigated under mixed H₂ with CO₂ at different strain rates, hydrogen partial pressures, and in the presence/absence of steam. The slow strain rate test results show that the HE susceptibility of 3Cr increased with increasing hydrogen partial pressure, and the plasticity of 3Cr obviously decreased in the presence of steam. The effect of strain rate was negligible in H₂/CO₂ environment but showed a significant difference in H₂/CO₂/steam environment. The fracture was a ductile fracture mode in N₂ environment and a brittle fracture mode in H₂/CO₂/steam environment. The reason for the severe plasticity loss of 3Cr in H₂/CO₂/steam environment was probably that the steam has a preferential adsorption onto the 3Cr surface compared with H₂ and CO₂. Consequences in CO₂ combined with H₂O to form H₂CO₃, which accelerated the anodic dissolution of 3Cr, and the physical adsorption of H₂ on steel was enhanced.     

CARBOX—a computer programme for the numerical evaluation of equilibria involving CO-CO2-H2-H2O-CH4 mixtures and non-stoicheiometric oxides and carbides

Abstract

A computer programme (CARBOX) has been written which makes for easier use of existing methods for representing the oxidising, sooting and carburising tendencies of CO–CO₂–H₂–H₂O–CH₄ mixtures in contact with metals or their oxides and carbides; these have previously found only limited application because of the tedious nature of the calculations. The basic equilibrium relationships have been stated in generalised form, so that problems involving equilibrium with non-stoicheiometric compounds can be dealt. with, provided that the relevant free-energy data are available, and a number of different types of solutIOn have been programmed. As a result it is now possible to examine a variety of cases involving conditions not considered by previous workers. Some typical applications are listed.     

Degradation mechanism of long service life Ti/IrO2-Ta2O5 oxide anodes in sulphuric acid

Ageing performances of long service life Ti/70%IrO₂-30%Ta₂O₅ (at mole fraction) anodes prepared at 450 °C over the whole electrolysis time in H₂SO₄ solution have been investigated. It is showed that, the whole electrolysis processes of these anodes can be divided into three stages consisting of `active', `stable' and `de-active' regions. In the first two stages, the dissolution of coated oxides is dominated (with IrO<sub>2</sub> component preferential loss) which results in decreasing of apparent current for oxygen evolution and voltammetric charge. It is found that, in these two stages, the preferential orientations of (1 1 0) and (1 0 1) planes in IrO<sub>2</sub> rutile decreases with electrolysis time, and that of (0 0 2) plane increases, while the preferential orientations remain to be stabilized in the `de-active' region. Therefore, the loss mechanism of catalyst coatings in this region is changed. Electrochemical impedance spectroscopy measurement shows a slight increase in reaction resistance (R_ct) of oxide catalysts for oxygen evolution in the `de-active' region, while a sharp rise in totally physical impedance of the whole anode. According to the experimental results, a degradation mechanism of the Ti based anodes has been proposed by degradation of Ti/catalyst layer interface resulting from the dissolution and anodic oxidation of metal base alternatively.     

Morphology and activity relationships of macroporous CuO–ZnO–ZrO<Subscript>2</Subscript> catalysts for methanol synthesis from CO<Subscript>2</Subscript> hydrogenation

A series of macroporous CuO–ZnO–ZrO₂ (CZZ) catalysts with different Zn/Zr ratios were successfully prepared by template method and characterized by X-ray diffraction (XRD), N₂ adsorption, reactive N₂O adsorption, scanning electron microscopy (SEM), H₂ temperature-programmed reduction (H₂-TPR), and transmission electron microscopy (TEM). The activity of the catalysts was tested for methanol synthesis from CO₂ hydrogenation. It is found that the increase in the Zn/Zr ratio could lead to the sintering of the catalysts, destroying the macroporous structure integrity. The macroporous CZZ catalysts own lower Zn/Zr ratio, exhibiting a better morphology and activity. For comparison, the conventional nonporous CZZ catalysts were also investigated. The results show that the CZZ catalysts with macroporous structure own smaller particles, higher CO₂ conversion, and CH₃OH yield. It reveals that the macroporous structure could inhibit the growth of the particle size, and the special porous structure is favorable for diffusion and penetration of CO₂, which could improve the catalytic activities.     

Long term corrosion of X70 steel and iron in humid supercritical CO2 with SO2 and O2 impurities

Abstract

The corrosion of X70 steel and iron in supercritical CO₂/SO₂/O₂/H₂O environment were investigated after a 454 h exposure. Optical microscopy was applied to observe the morphology of etch pits and synthesise the three-dimensional morphology. X-ray diffraction and X-ray photoelectron spectroscopy were employed to detect the composition of product scales. Experimental results verified that the localised corrosion occurred on the X70 steel sample under corrosion product deposits. Ferrous sulphate, sulphur and iron sulphide were detected as the corrosion products.