Lei Zhang

This paper accomplishes a numerical investigation on the effect of two-level over-fire air (OFA) on the combustion and NO_x emission in a supercritical 600 MW wall-fired boiler. Different arrangements of two-level OFA nozzles and different airflow ratios between the two layers are set to examine NO_x emission and the carbon content in fly ash. According to the simulation results, the two-level OFA case releases the NO_x between the single upper layer and the single lower layer of the two-level OFA nozzles in operation. Moreover, the two-level OFA arrangement gives a lower carbon content in fly ash than the single-level OFA cases. In addition, both low NO_x emissions and low carbon content in fly ash can be obtained simultaneously when the two levels of OFAs are injected from Layers 1 and 4 with r_lower at 0.5.

Hydrogen induced stress cracking (HISC) has sometimes been experienced when duplex stainless steels are applied as subsea facilities under cathodic protection (CP). The effects of cathodic potential on HISC of duplex stainless steel were studied by electrochemical methods, slow strain rate tensile test (SSRT) and four-point bending test. At the constant potential, with the hydrogen evaluation reaction proceeding, OH- on the surface of metals increases and the calcium and magnesium deposition layer can decrease the current to a static value. The SSRT results show that the fracture mode changed from ductile to brittle when the duplex stainless steel was polarized to -1.05V (SCE) or more negative potential. The four-point bending test results show the relationship between potential and the sensitivity of HISC in artificial seawater at 3oC when the specimens were loaded up to 98% of yield stress (Rp0.2). The hydrogen concentration of the specimens was also measured and the occurrence of HISC needs a certain hydrogen concentration.

The stress corrosion cracking (SCC) behavior of duplex stainless steel (DSS) in H2S-Cl- environment has been discussed for a long time. However, the high H2S-CO2 pressure and the high Cl- content can coordinate to cause severe pitting or SCC of DSS. There has been limited information on the mechanism of SCC of DSS under high H2S-CO2 pressure with high Cl- content. In this paper, SCC of UNS S31803 DSS was investigated under different H2S-CO2 pressures (300 kPa H2S + 200 kPa CO2, 600 kPa H2S + 400 kPa CO2, 900 kPa H2S + 600 kPa CO2) with 150,000 ppm Cl- at 60 ⁰C. Four-point bend beam test method and scanning electron microscopy analysis were combined to investigate the SCC mechanism of DSS. The effect of high H2S-CO2 pressure on SCC mechanism was discussed. The results indicated that under lower H2S-CO2 pressure, cracks initiated and propagated at pitting areas which were caused by anodic dissolution mechanism. Under 900 kPa H2S + 600 kPa CO2, except the pitting assisted cracking areas, there also existed some hydrogen stress cracking at crack terminals. The higher H2S-CO2 pressure can facilitate both hydrogen embrittlement and anodic dissolution.

This paper developed a heat transfer model of the underground pipe group to study the effect of cooling and heating load imbalance to geothermal field. The results show that, there are the approximate linear relationship between average soil temperature and the total heat flow provided by GSHP system to the soil during the whole running period. The temperature of soil restoration ratio is less than the heat flux ration in the spring and autumn. Simulation results show that when heating/ cooling load equal to 0.5 and the heating flux for the bore hole are 60w/m and -40w/m in the Summer and Winter respectively, after this system run 12 months and 10 hours every day, the average soil temperature around borehole will increase 0.48°C, and the soil close borehole temperature will increase 1.3°C than the initial soil average temperature with the value of 17.6°C, which make GSHP not be efficiently for long term running. The results is hoped to be useful for GSHP design and long term running.