Suppression of methane/coal dust deflagration flame propagation by CO2/fly ash as a flue gas layer

To reveal the suppression characteristics of methane/coal dust deflagration flame propagation by the flue gas loading in local zones. The suppression experiments of flue gas layer (CO₂ and fly ash) with different thicknesses and fly ash concentrations were conducted in semi-open vertical combustion pipe. The flue gas layer was produced by self-designed flue gas layer generator. Flame propagation characteristics which including the flame image, velocity, the ion current and temperature were recorded by the high-speed photography, the ion current probe and the thermocouple. The residues after coal dust deflagration were characterized by FTIR (Fourier transform infrared spectroscopy). The results show that the flue gas layer has a significant suppression on deflagration flame propagation. With the increasing of flue gas layer thickness and fly ash concentration, the flame velocity, height, temperature and the ion current gradually decreases, and the suppression effect was enhanced. The asphyxiation of CO₂, heat absorption and insulation of fly ash were mainly methods for the suppression of coal dust deflagration flame.     

Degradation of heat exchanger materials under biomass co-firing conditions

Abstract

Co-firing biomass in conventional pulverised coal fired power stations offers a means to rapidly introduce renewable and CO₂ neutral biomass fuels into the power generation market. Existing coalfired power stations are both much larger and more efficient than current designs of new biomass combustion systems, so feeding a few percent of biomass feed into an existing large coal fired station will give more biomass derived power than a new dedicated biomass station. Co-firing levels started at ～2% biomass, but this has increased to ～5–10% biomass, with higher levels of biomass co-firing being investigated, although supply of biomass becomes an issue with increasing co-firing levels. The lower levels of biomass co-firing (up to ～5%) can be achieved with relatively minor modifications to existing plants, so avoiding the large capital costs and risks of building new biomass-only fired power systems. However higher levels of co-firing are more difficult to achieve, requiring dedicated biomass supply systems and burners. For existing coal-fired power stations, the co-firing of biomass causes some practical problems, e.g.: the control of co-firing two fuels; changes to bottom/fly ash chemistry; changes to deposition (fouling and slagging) within the boiler; reduced reliability of key high temperature components (e.g. heat exchangers) due to increased corrosion problems relative to those experienced with coal alone.

This paper reports the results of assessments carried out to evaluate the potential operating conditions of heat exchangers in combustion systems with biomass (wood or straw) and coal cofiring, as well as laboratory corrosion tests that have been carried out to give an initial assessment of potential effects of biomass-co-firing.

The corrosion tests have been carried out using the deposit recoat method in controlled atmosphere furnaces. A series of 1000 hour tests have been carried out at typical superheater and evaporator metal temperatures using simulated deposit compositions and gaseous environments (selected on the basis of plant experience and potential fuel compositions). Five materials were exposed in these tests: 1Cr steel, T22 steel, X20CrMoV121, TP347HFG and alloy 625. In order to produce statistically valid data on the actual metal loss from the materials, the performance of the materials in these tests was determined from dimensional metrology before and after exposure. For each material, these data have been used to determine the sensitivity of the corrosion damage to changes in the exposure conditions (e.g. deposit composition, gas composition) thereby producing initial models of the corrosion performance of the materials. The corrosion data and model outputs have been compared with data available from power plants operating on coal, straw or wood fuels.     

Effect of laser surface melted zirconium alloys on microstructure and corrosion resistance

Zirconium alloys were laser surface melted (LSM) using a continuous wave CO₂ laser at energy densities of 4,7 and 10 kJ cm^–2. LSM samples examined using SEM and optical microscopy exhibited resolidified regions with several different microstructures, including ultrafine martensite. Corrosion performance was obtained by steam autoclave tests and immersion tests in 10% FeCl₃ at room temperature. Coarser microstructures performed better than fine microstructures in autoclave tests, while fine microstructures performed better than coarse microstructures in 10% FeCl₃ immersion tests. Accelerated corrosion in the autoclave and immersion tests was observed to occur near the laser beam overlap region. The surface chemistry was examined for alloy segregation using secondary iron mass spectroscopy. Tin and iron alloy elements segregated near the periphery of each melt pool. Segregated regions containing increased iron concentrations associated with each laser pass were responsible for accelerated corrosion.     

Waterwall corrosion in pulverized coal burning boilers: root causes and wastage predictions

Abstract

Waterwall corrosion has become a serious problem in the USA since the introduction of combustion systems, designed to lower NO_x emissions. Previous papers have shown that the main cause of the increased corrosion is the deposition of corrodants, iron sulfides and alkali chlorides, which occurs under reducing conditions. In this paper, the contribution of various variables such as the amount of corrodant in the deposit, the flue gas composition and the metal temperature, is further quantified in laboratory tests, using a test furnace allowing thermal gradients across the deposit and the metal tube samples. Approximate deposit compositions were calculated from the coal composition, its associated ash constituents and corrosive impurities. A commercially available thermochemical equilibrium package was used, after modifications to reflect empirical alkali availability data. Predictions from these calculations agreed reasonably well with the alkali chloride and FeS content found in actual boiler deposits. Thus approximate corrosion rates can be predicted from the chemical composition of the coal using corrosion rates from laboratory tests, adjusted to account for the short duration (100 hours) of the laboratory tests. Reasonable agreement was again obtained between actual and predicted results.     

Unburned carbon measurement in fly ash using laser-induced breakdown spectroscopy with short nanosecond pulse width laser

The unburned carbon in fly ash is one of the important factors for the boiler combustion condition. Controlling the unburned carbon in fly ash is beneficial for fly ash recycle and to improve the combustion efficiency of the coal. Laser-induced breakdown spectroscopy (LIBS) technology has been applied to measure the fly ash contents due to its merits of non-contact, fast response, high sensitivity, and real-time measurement. In this study, experimental measurements have been adopted for fly ash flows with the surrounding gases of N₂ and CO₂, while the CO₂ concentration varified to evaluate the CO₂ effect on the unburned carbon signal from fly ash powder. Two kinds of pulse width lasers, 6?ns and 1?ns, were separately adopted to compare the influence of laser pulse width. Results showed that compared with that using 6?ns pulse width laser, plasma temperature was lower and had less dependence on delay time when using 1?ns pulse width laser, and spectra had more stable background. By using 1?ns pulse width laser, the emission signal from surrounding CO₂ also decreased because of the less surrounding gas breakdown. The solid powder breakdown signals also became more stable when using 1?ns pulse width laser. It is demonstrated that 1?ns pulse width laser has the merits for fly ash flow measurement using LIBS.     

Experimental Study on the Effect of the Water-Cut Conditions on the Performance of L80 Carbon Steel

In the oil production, water and acidic gases, i.e., H₂S and CO₂, are co-produced with the oil. The acidic gases are known to associate with a variety of corrosion damage to the surface facilities leading to costly failures. Also, the acidic gases cause a reduction in the service life of equipment. Corrosion of API L80 tubular carbon steel in sweet media (in the presence of CO₂ gas) was investigated using the linear polarization resistance meter. Experiments using API L80 tubular carbon steel material were carried out in a stagnant flow condition with different ratios of produced water to crude oil at relatively high temperatures (60 °C up to 90 °C). The pressure was about 200 psi (13.8 bar) of CO_2, and the experiments were carried out using a high pressure vessel namely an autoclave cell. Under those experimental conditions, results indicated that at a temperature of 60 °C, the corrosion rate for carbon steel L80 increased as water-cut ratio increased. Also, the results showed that at higher temperature than 60 °C, the formation of iron carbonate scale on the surface of the steel was observed to increase. Consequently, the corrosion rate of the L80 carbon steel was observed to decrease.     

Heißkorrosion im Brennergas bei 1200 °C

Hot Corrosion in Burner Gas at 1200°C . The lifetime of a material at high temperatures is controlled by its hot corrosion behaviour against the components (ashes, salts, sulfur) of the burner gas. In this paper Na₂SO₄, Na₂CO₃, NaCl, NaF and V₂O₅ as well as H₂S, SO₂ and HCl were used for the experiments. It could be shown that the hot corrosion behaviour of MoSi₂ coated Nb-and Ta-alloy is excellent against acid additives (NaCl, NaF, V₂O₅) to the burner gas while alcalic melts (Na₂SO₄, Na₂CO₅) corrode the silizide. The solubility of the MoSi₂ is caused by the transport rate of molecular oxygen through the glasses formed by oxidation on the surface of the materials. It can be shown, that the lifetime of sintered MoSi₂ coatings on refractory metals is more than 10 times longer as by NiAl coated superalloys in hot corrosion test.     

Transport properties of high-volume fly ash concrete: Capillary water sorption,water sorption under vacuum and gas permeability

Studying concrete’s resistance to carbonation-induced corrosion usually involves exposing the material to CO₂ for quite some time. To estimate the performance of high-volume fly ash (HVFA) concrete more quickly, two key properties governing this process can be studied, namely water penetrability and gas permeability. With respect to HVFA mixtures optimized for usage in an environment exposed to carbonation with wetting and drying, we adopted the latter approach. This paper presents a full assessment of concrete mixtures with varying fly ash amounts. A 50% fly ash mixture by mass with a binder content of 400 kg/m³ and a water-to-binder ratio of 0.4 had a lower capillary water uptake (?32.6%), water sorption under vacuum (?10.7%) and gas permeability (?78.9%) than a proper reference normally used in this environment. The fly ash applied had an excellent quality regarding loss on ignition (3.5%) and fineness (19% retained on a 45 μm sieve).     

Effect of carbon on corrosion resistance of powder-processed Fe-0·35%P alloys

The corrosion behaviour of phosphoric irons containing 0.35 wt % P, 2% copper, 2% nickel, 1% silicon, 0.5% molybdenum, with/without 0.15% carbon prepared by powder forging route were studied in different environments. The various environments chosen were acidic (0.25 M H₂SO₄ solution of pH 0.6), neutral/marine (3.5% NaCl solution of pH 6.8) and alkaline (0.5 M Na₂CO₃ + 1.0 M NaHCO₃ solution of pH 9.4). The corrosion studies were conducted using Tafel extrapolation and linear polarization methods. The studies also compare Armco iron with phosphoric irons. It was observed that the addition of carbon improved the corrosion resistance of a Fe-0.35%P-2%Ni-2%Cu-1%Si-0.5%Mo alloy in all the environments. Corrosion rates were highest in acid medium, minimal in alkaline medium and low in neutral solution. SEM/EDAX was used to characterize the compositions.     

The corrosion of INCONEL alloy 740 in simulated environments for pulverized coal-fired boiler

The corrosion of a new nickel base superalloy, INCONEL alloy 740, has been studied at 550 and 700 °C on exposure to the synthetic coal ash/flue gas environments by means of XRD, SEM, and EDX. Low temperature hot corrosion of the new alloy occurred at two temperatures. The corrosion started to form the thin Cr₂O₃ scale on the alloy at 550 °C and developed as pitting attack resulted from sulfidation. The frontal attack at 700 °C consisted of two successive stages in which the corrosion mechanism started from the sulfidation and ended up in the fluxing of oxide. The compact and protective Cr₂O₃ scale formed and the internal sulfidation took place during the initial stage. The severe hot corrosion occurred due to the presence of the molten CoSO₄ during the propagation stage. The loose and porous outer layer and the compact inner layer consisted of spinels and oxides, respectively. The sulfides of Cr, Ti, and Nb formed on the front of oxide scale and in Cr-depletion zone. The rapid degradation of corrosion resistance of the alloy can be attributed to the dissolution of both cobalt and cobalt oxide on the surface. The alloy of 25% Cr exhibited better resistance to coal ash/flue gas corrosion as compared to the alloy of 23% Cr in the present case.     

XPS investigation on AISI 420 stainless steel corrosion in oil and gas well environments

The corrosion behaviour of 13Cr-martensitic stainless steel (AISI 420) was investigated in CO₂-H₂S-Cl^– environments typical of oil and gas wells under different CO₂ and H₂S partial pressures. The corrosion tests indicated that the AISI 420 steel was highly corrosion resistant to CO₂-induced phenomena (general corrosion and carbonate S.C.C.), while in the H₂S environment a high S.S.C.C. (Sulphide Stress Corrosion Cracking) susceptibility and high corrosion rates were found. Moreover, CO₂ in CO₂-H₂S-Cl^– systems inhibited general corrosion and S.S.C.C. phenomena by favouring the formation of a protective film. By means of X-ray photoelectron spectroscopy (XPS) the chemical nature of the films grown on AISI 420 in different environmental conditions was investigated and the following statements were drawn out:

Low-temperature CO2 capture technologies – Applications and potential

CO₂ capture by chemical or physical sorption and membrane separation have been the dominant fields of research within post- and pre-combustion CO₂ capture from power cycles and industrial processes. Except for oxy-combustion capture applications, limited attention has been given to low-temperature capture from flue gas and synthesis gas by phase separation. This paper gives an overview of common CO₂ capture conditions for a broad range of different power cycles and industrial processes. For a selected range of capture conditions, potential applications for low-temperature CO₂ capture have been evaluated with respect to energy consumption and CO₂ capture ratio. For all applications of low-temperature capture, specific power consumption and obtainable CO₂ capture ratio are sensitive to flue-gas or synthesis-gas feed CO₂ concentration. However, for certain applications such as synthesis gas from coal gasification, low-temperature capture shows promising potential and highly competitive energy figures compared to baseline technology.     

Effect of H2S on Fe corrosion in CO2-saturated brine

The effect of H₂S at ppm level concentrations on iron corrosion in 3 wt% NaCl solutions saturated with CO₂ in the temperature range of 25–85 °C is examined using electrochemical and surface science techniques. Small H₂S concentrations (5 ppm) have an inhibiting effect on corrosion in the presence of CO₂ at temperatures from 25 to 55 °C. At 85 °C, however, 50 ppm H₂S is needed to provide significant corrosion inhibition. At higher H₂S concentrations, the corrosion rate increases rapidly, while still remaining below the rate for the H₂S-free solution. Characterization of the iron surfaces after corrosion was carried out using X-ray photoelectron spectroscopy and X-ray diffraction. A sulfur peak (S2p) is observed at a binding energy of 161.8 eV in all cases, attributable to disulfide (\textS₂^2-) ({\text{S}}_{2}^{2-}) formation. Corrosion protection in the temperature range 25–55 °C can be attributed to Fe(II) bonded to S and O. At 85 °C, protection of the iron surface is most likely due to FeS₂ formation. Morphological changes on the iron surface after exposure to H₂S containing solutions were observed by SEM. A thin protective film was seen after exposure to solutions containing 5 ppm H₂S at 25 °C, while at 85 °C, with the addition of 50 ppm H₂S to CO₂-saturated brine solution, a dense protective film was formed on the iron surface.     

Corrosion failures of AISI type 304 stainless steel in a fertiliser plant

A urea plant, operating on ammonia and carbon dioxide (CO₂) gases, had to be shutdown due to corrosion in the intercooler and aftercooler of its CO₂ gas cleaning circuit. Extensive general corrosion of AISI type 304 stainless steel parts, such as sealing strips, fins, demisters and the shell, of these two components which were in contact with the duplex stainless steel tubes, caused the shutdown of the fertiliser plant within 6 months. Investigations of the corrosion products by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) techniques showed the presence of carbon and ammonia based compounds, thus suggesting the role of ammonia and CO₂ gases, or the product of their reactions, in the corrosion of type 304 stainless steel. Electrochemical polarisation studies showed that duplex stainless steel possessed a more positive open circuit potential and a nobler critical pitting potential than type 304 stainless steel thus confirming that the corrosion of type 304 stainless steel was caused by the galvanic action with the duplex stainless steel heat transfer tubes. Hence, it was recommended that (i) the same material (type 304 stainless steel) be used for all parts of the intercooler and aftercooler to avoid galvanic corrosion, (ii) condense water carried over by CO₂ gas by cooling it to low temperatures immediately after it comes out from the scrubber, (iii) slight modification of the process to add up to 0.8% oxygen in the CO₂ gas before entry into the intercooler, which will help in retaining/formation of an effective passive film on type 304 stainless steel.     

CO2分压对20#钢在CO2/H2O气液两相塞状流中腐蚀行为的影响

利用失重法、SEM、EDS、XRD和XPS等分析方法在自主设计的动态腐蚀实验装置上研究了CO_2分压对20#钢在CO_2/H_2O气液两相塞状流中腐蚀行为的影响,对腐蚀试样进行了腐蚀速率分析、腐蚀形貌特征观察以及腐蚀产物成分与膜层结构特征分析。结果表明:随CO_2分压的增加腐蚀速率增加,0.04 MPa、0.28 MPa下分别达到腐蚀速率最小值(1.160 9 mm/a)和最大值(1.898 8mm/a);上管壁腐蚀产物随着CO_2分压的增加最终形成颗粒较大的节瘤状产物,下管壁腐蚀产物由球形颗粒形成初始致密的单层膜逐渐转变为由致密的内层膜和具有网状连通裂纹的片状疏松外层膜构成;经EDS元素分析可知上下壁面的腐蚀产物均由Fe、C、O三种元素构成,XPS分峰图谱显示C 1s、O 1s和Fe 2p均出现了三个拟合峰位,结合XRD分析可知腐蚀产物的主要组成相有Fe_3C、FeCO_3、Fe_2O_3、Fe_3O_4、FeOOH。     

Comparison of ferritic and austenitic plasma nitriding and nitrocarburizing behavior of AISI 4140 low alloy steel

This paper compares the ferritic and austenitic plasma nitriding and nitrocarburizing behavior of AISI 4140 low alloy steel carried out to improve the surface corrosion resistance. The gas composition for plasma nitriding was 85% N₂–15% H₂ and that for plasma nitrocarburizing was 85% N₂–12% H₂–3% CO₂. Both treatments were performed for 5 h, for different process temperatures of 570 and 620 °C for ferritic and austenitic plasma treatment, respectively. Optical microscopy, X-ray diffraction and potentiodynamic polarization technique in 3.5% NaCl solution, were used to study the treated surfaces. The results of X-ray analysis revealed that with increasing the treatment temperature from 570 to 620 °C for both treatments, the amount of ε phase decreased and γ′ phase increased. Nitrocarburizing treatment resulted in formation of a more amount of ε phase with respect to nitriding treatment. However, the highest amount of ε phase was observed in the ferritic nitrocarburized sample at 570 °C. The sample nitrided at 620 °C exhibited the thickest layer. The potentiodynamic polarization results revealed that after plasma nitriding and nitrocarburizing at 570 °C, corrosion potential increased with respect to the untreated sample due to the noble nitride and carbonitride phases formed on the surface. After increasing the treatment temperature from 570 to 620 °C, corrosion potential decreased due to the less ε phase development in the compound layer and more porous compound layer formed at 620 °C with respect to the treated samples at 570 °C.     

Spannungsrißkorrosion in hochreinem Wasser von 3½ % NiCrMoV-Vergütungsstählen für Dampfturbinen-Scheiben und -Wellen

Stress Corrosion Cracking in High Purity Water of 3½ % NiCrMoV – Quenched and Tempered Steel for Steam Turbine Discs and Shafts In recent years intergranular stress corrosion cracking has occured world-wide in the shrink-fitted discs of low pressure turbine rotors made of low alloy steels. Only in a few cases steam impurities such as NaOH, Na₂CO₃, Na₂SO₄, H₂S, or NaCl, which initiate SCC, could be found. The stress corrosion cracking behaviour of the turbine disc steel 26 NiCrMoV 14 5 with a yield strength of approx. 850 N/mm² was examined under special corrosion conditions. Gaseous and other impurities of the water, which lead to higher conductivity can initiate stress corosion cracks and increase the stress corrosion crack velocity insignificant. Stress corrosion crack initiation can be prevented by shifting the pH-value and the free corrosion potential in the region of passivity. Unfavourable crevice conditions must be avoided. Solutions are shown, how to prevent stress corrosion cracking of steam turbine discs.     

Mössbauer spectroscopy study on the corrosion resistance of plasma nitrided ASTM F138 stainless steel in chloride solution

Plasma nitriding of ASTM F138 stainless steel samples has been carried out using dc glow discharge under 80% H₂–20% N₂ gas mixture, at 673 K, and 2, 4, and 7 h time intervals, in order to investigate the influence of treatment time on the microstructure and the corrosion resistance properties. The samples were characterized by scanning electron microscopy, glancing angle X-ray diffraction and conversion electron Mössbauer spectroscopy, besides electrochemical tests in NaCl aerated solution. A modified layer of about 6 μm was observed for all the nitrided samples, independent of nitriding time. The X-ray diffraction analysis shows broad γ_N phase peaks, signifying a great degree of nitrogen supersaturation. Besides γ_N, the Mössbauer spectroscopy results indicated the occurrence of γ′ and ε phases, as well as some other less important phases. Corrosion measurements demonstrate that the plasma nitriding time affects the corrosion resistance and the best performance is reached at 4 h treatment. It seems that the ε/γ′ fraction ratio plays an important role on the resistance corrosion. Additionally, the Mössbauer spectroscopy was decisive in this study, since it was able to identify and quantify the iron phases that influence the corrosion resistance of plasma nitrided ASTM F138 samples.     

Effect of treating atmosphere in plasma post-oxidation of nitrocarburized AISI 5115 steel

In this study quenched and tempered AISI 5115 steel was plasma-nitrided and nitrocarburized at 550 °C for 5 h in atmospheres of 80% N₂ balanced with various amounts of CO₂ and H₂ gases. The amount of CO₂ varied from 0 to 10 vol%. The highest amount of ε phases was formed in the compound layer after treating in atmosphere containing 7 vol% CO₂. Optimized compound layer was post-oxidized for 1 h at 450 °C under O₂/H₂ volume ratios of 1/1 and 3/1 as well as 100% oxygen. The treated samples were characterized using metallographic techniques, XRD, SEM, roughness measurement and potentiodynamic methods. The results showed that the growth rate of the oxide layer increased with increasing O₂ in the oxidizing gas mixture. X-ray diffraction analysis of oxidized layers confirmed the formation of highest amount of magnetite at post-oxidation in an atmosphere with the O₂/H₂ volume ratio of 1/1. Electrochemical polarization tests proved the enhancement of corrosion resistance by plasma post-oxidation and the highest corrosion resistance obtained after oxidizing under an O₂/H₂ volume ratio of 1/1.     

Exergy recuperative CO2 gas separation in pre-combustion capture

The integrated coal gasification combined cycle (IGCC) can achieve higher power generation efficiency than conventional pulverized coal combustion power plants. However, a CO₂ capture process prevents improving power generation efficiency of IGCC, because CO₂ separation from gas mixtures requires huge amounts of energy. Therefore, in this study, we analyzed the CO₂ separation process in the pre-combustion capture process using a process simulator (PRO/II) in the steady state, and proposed a new process using a modularity based on self-heat recuperation (SHR) technology to decrease energy consumption. Pre-combustion capture was applied in the IGCC plant, which involved coal gasification and CO-shift conversion with CO₂ capture. The results show that the energy consumption for the CO₂ separation process using SHR was decreased by two-thirds. This means that the power generation efficiency can be improved by SHR compared with conventional IGCC with a CO₂ capture process.