Study on Intelligent Classification of Aging Heat-Resistant Materials.

High chromium martensitic heat-resistant steel is considered as a candidate material for pressure components of the next generation of incinerators of the subcritical level or above in China due to its excellent high-temperature and corrosion resistance, but in the long-term service, aging will significantly affect the service safety of materials. So, accurate identification of its aging state is important to enhance the safety of a power plant. In this paper, an automatic aging grading model of high chromium martensite heat-resistant steel based on the depth residual network is proposed according to different scales of metallographic data. A multiscale data set is constructed by image reduction to verify the accuracy of the model in identifying microstructure images of high chromium martensitic heat-resistant steel with different scales. The experimental results show that the model using multiscale data sets performs well, and then, through feature pyramid network model training, the accuracy rate is further improved, and a relatively good prediction accuracy model is obtained. The validity of the deep learning method for the classification of damage and aging of P91 steel with different scales is verified.

Corrosion Behavior of High-Cr-Ni Materials in Biomass Incineration Atmospheres.

In this paper, the corrosion test of high-Cr-Ni tubes was carried out in a biomass incinerator by replacing the original heated surface tube with a test tube. The investigated materials are high-Cr-Ni stainless steels (TP347H, SP2215, and Sanicro25) and alloy (HT700T). Long-term services (>4000 h) to investigate the corrosion rates and corrosion characteristics of the materials have been carried out. The appearance, element content, and composition of corrosion products after corrosion of the specimens were analyzed. Analysis indicates that the deposits are mainly composed of alkali metal salts, iron oxides, iron sulfates, and complex salts. Moreover, the corrosion morphology of the materials with different Cr-Ni contents varies greatly. TP347H has a high corrosion rate (0.11 mm/1000 h) with intergranular corrosion cracks and pitting on the windward side. However, the corrosion pattern of HT700T is comprehensive corrosion and the corrosion rate is low (0.015 mm/1000 h). Using corrosion rate as a criterion for corrosion resistance, HT700T has the highest corrosion resistance, while TP347H has the lowest. The corrosion behavior is also related to the corrosion resistance index (CI) value based on the content of critical elements. The order of material corrosion resistance predicted by the CI value is the same as reflected by the corrosion rate.

Effects of Activation Conditions on the Properties of Sludge-Based Activated Coke.

Sewage sludge and waste biomass are unavoidable byproducts of municipal and industrial processes. Both materials have significant carbon contents. Activated coke with a developed pore structure can be obtained after its physical activation. In this study, sewage sludge and waste poplar bark were used as precursor materials to prepare activated coke by steam, carbon dioxide gas, and their mixtures. The effects of different concentrations of activation gas on the activated coke product were investigated. Through nitrogen adsorption analysis, it was found that activated coke has a higher specific surface area and better pore structure when activated in gas containing 20% steam. The activated coke prepared by carbon dioxide gas activation has higher microporosity than that prepared by steam activation. Infrared spectrum analysis shows that steam activation is beneficial to the formation of free hydroxyl functional groups. Through scanning electron microscopy analysis, the pores of activated coke prepared by steam activation appeared to extend deeper in the structure of the coke, while the pores of activated coke prepared by carbon dioxide activation appeared to have fine circular structures. The activated coke prepared by the activation of mixed gas appeared to have ablated particles on the surface due to the ablation of the pore structure. In order to prepare activated coke with excellent adsorption performance, the physical and chemical properties of activated coke under different activation conditions were studied in detail.

Release of Sulfur and Nitrogen during Co-pyrolysis of Coal and Biomass under Inert Atmosphere.

Coal combustion is a major energy provider but also a serious pollutant emitter worldwide. Biomass has attracted increasing research attention due to its carbon neutral and other advantages during co-pyrolysis of coal. Therefore, pyrolysis of corn straw, lignite coal, and their mixed fuel was investigated using a horizontal tube furnace at a temperature range of 300-900 °C in this study by concentrating on emissions of nitrogen- and sulfur-bearing gases. Emissions of HCN, NH3, COS, and SO2 were monitored, and char yield was calculated during pyrolysis. Results showed that char yield decreases with increasing temperature due to the decomposition of heterocyclic aromatic hydrocarbon. Sulfur (42.45 wt %, 900 °C) and nitrogen (73.23 wt %, 600 °C) were released from the mixed fuel during pyrolysis. The synergistic effect between corn straw and coal was evaluated and their experimental and calculated values were compared. The synergistic effect on emissions was remarkable at a temperature range of 600-800 °C due to high biomass content and desynchronization of volatile matter emissions between the coal and biomass. Volatiles in the biomass, which are released before coal, stimulated the volatilization of coal. Several conventional gases, such as CH4 and C2H6, were also analyzed to investigate emission patterns at different temperatures.

Influence of the overfire air ratio on the NO(x) emission and combustion characteristics of a down-fired 300-MW(e) utility boiler.

Down-fired boilers used to burn low-volatile coals have high NO(x) emissions. To find a way of solving this problem, an overfire air (OFA) system was introduced on a 300 MW(e) down-fired boiler. Full-scale experiments were performed on this retrofitted boiler to explore the influence of the OFA ratio (the mass flux ratio of OFA to the total combustion air) on the combustion and NO(x) emission characteristics in the furnace. Measurements were taken of gas temperature distributions along the primary air and coal mixture flows, average gas temperatures along the furnace height, concentrations of gases such as O(2), CO, and NO(x) in the near-wall region and carbon content in the fly ash. Data were compared for five different OFA ratios. The results show that as the OFA ratio increases from 12% to 35%, the NO(x) emission decreases from 1308 to 966 mg/Nm(3) (at 6% O(2) dry) and the carbon content in the fly ash increases from 6.53% to 15.86%. Considering both the environmental and economic effect, 25% was chosen as the optimized OFA ratio.

Improved NOx emissions and combustion characteristics for a retrofitted down-fired 300-MWe utility boiler.

A new technique combining high boiler efficiency and low-NO(x) emissions was employed in a 300MWe down-fired boiler as an economical means to reduce NO(x) emissions in down-fired boilers burning low-volatile coals. Experiments were conducted on this boiler after the retrofit with measurements taken of gas temperature distributions along the primary air and coal mixture flows and in the furnace, furnace temperatures along the main axis and gas concentrations such as O(2), CO and NO(x) in the near-wall region. Data were compared with those obtained before the retrofit and verified that by applying the combined technique, gas temperature distributions in the furnace become more reasonable. Peak temperatures were lowered from the upper furnace to the lower furnace and flame stability was improved. Despite burning low-volatile coals, NO(x) emissions can be lowered by as much as 50% without increasing the levels of unburnt carbon in fly ash and reducing boiler thermal efficiency.