Academic literature on the topic 'Steam Temperature'

In this research, hydrogen production from conventional slow pyrolysis, flash pyrolysis, steam gasification and catalytic steam gasification of various biomass samples including rice husk, wood pellets, wheat straw and sugarcane bagasse was investigated at ultra-high temperature (~1000 °C). During flash pyrolysis of the waste wood, the gas yield was improved to ~78 wt.% as compared to ~25 wt.% obtained during slow pyrolysis. The addition of steam enhanced the hydrogen concentration from 26.91 vol.% for pyrolysis to 44.13 vol.% for steam gasification. The comparison of pyrolysis, steam gasification and catalytic steam gasification in a down-draft gasification reactor at 950 °C using rice husk, bagasse and wheat straw showed a significant increase in gas yield as well as hydrogen yield. The hydrogen yield was enhanced from ~2 mmoles g-1 for pyrolysis to ~25 mmoles g-1 during steam gasification using a 10 wt.% Ni-dolomite catalyst. The higher hydrogen yield was due to the enhanced steam reforming of hydrocarbons and thermal cracking of tar compounds at higher temperature. When compared with the other catalysts such as 10 wt.% Ni-dolomite, 10 wt.% Ni-MgO, and 10 wt.% Ni-SiO2, the 10 wt.% Ni-Al2O3 catalyst showed the highest hydrogen yield of 29.62 mmoles g-1. The investigation on gasification temperature showed that the hydrogen yield was significantly improved from 21.17 mmoles g-1 at 800 °C to 35.65 mmoles g-1 at 1050 °C. The hydrogen concentration in the product gas mixture was increased from 50.32 vol.% at 800 °C to 67.41 vol.% at 1050 °C. The increase in steam injection rate from 6 to 35 ml hr-1 enhanced the hydrogen yield from 29.93 mmoles g-1 to 44.47 mmoles g-1. The hydrogen concentration increased from 60.73 to 72.92 vol.%. The increase was mainly due to the shift in the equilibrium of the water gas shift reaction as H2:CO ratio increased from 2.97 to 7.78. The other process variables such as catalyst to sample ratio, carrier gas flow rate showed little or no influence on the gas yield and hydrogen yield. The steam gasification of residual biomass char was performed at 950 °C to recover extra hydrogen. The presence of 10 wt.% Ni-Al2O3 in the gasifier improved the hydrogen yield to ~47 mmoles per gram of biomass as compared to the other catalysts such as 10 wt.% Ni-dolomite and 10 wt.% Ni-MgO. The gasification temperature showed a positive influence on hydrogen yield from 750 °C to 950 °C. The increase in steam injection rate from 6 ml hr-1 to 15 ml hr-1 enhanced the hydrogen yield from 46.81 to 52.10 mmoles g-1 of biomass.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2011.<br>Cataloged from PDF version of thesis.<br>Includes bibliographical references (p. 119-123).<br>The commercial nuclear power industry is continually looking for ways to improve reactor productivity and efficiency and to increase reactor safety. A concern that is closely regulated by the Nuclear Regulatory Commission is the exothermic zircaloy-steam oxidation reaction which can potentially occur during a loss of coolant accident (LOCA), and may become autocatalytic beyond 1,200 0C, thus generating a large amount of hydrogen. The concern for the zircaloy oxidation reaction has been heightened since the March 2011 events of Fukushima, Japan. One solution offering promising results is the use of silicon carbide (SiC) cladding in nuclear reactor fuel rod designs. SiC, a robust ceramic which reacts very slowly with water or steam, has many features that meet or exceed that of zircaloy including the ability to withstand higher temperatures due to a higher melting point and the ability to absorb fewer neutrons than zircaloy which would allow for increased safety margins and fuel burnup. An experimental investigation of the oxidation performance of a-SiC during a postulated LOCA event was performed. The test facility was designed and fabricated to test the oxidation rates of zircaloy and SiC in a high temperature, high-purity, flowing steam environment. Studies of zircaloy-4 oxidation were conducted to validate the test facility for this purpose. Thirty six zircaloy-4 tests lasting up to 30 minutes, at temperatures ranging from 800°C to 1,200°C, were completed and compared to existing models and literature data. Additionally, six longer duration a-SiC tests lasting from 8 hours to 48 hours, at temperatures of 1,140°C and 1,200°C, were completed. These tests clearly show that, from an oxidation perspective, SiC significantly outperforms zircaloy in high-flowing, superheated steam. For zircaloy, results from the most intense temperature/duration testing combination of 1,200°C for 30 minutes show 15.6 percent weight gain. For the most intense SiC tests at 1,200°C for eight hours, a weight loss of two orders of magnitude less occurred, a 0.077 percent weight loss. The four 24 hour and 48 hour SiC tests at 1,140°C also correlate well with the expected paralinear oxidation trend and further confirm that SiC is more resistant to oxidation in high temperature steam than zircaloy.<br>by Ramsey Paul Arnold.<br>S.M.

Heat exchangers and steam headers are at the heart of any boiler and are susceptible to a range of failures including tube leaks, ligament cracking, creep and fatigue. These common forms of header failure mechanisms can be exacerbated by local thermal stresses due to temperature and flow maldistribution at full and partial boiler load operations. The purpose of this project is to develop process models of the outlet stubbox header of a final superheater (FSH) heat exchanger in a 620MW coal-fired drum type boiler. The process models were used to assess the impact of steam flow and temperature distribution on the thermal stresses in the header material. The process models were developed using Computational Fluid Dynamics (CFD) and Finite Element Analysis (FEA). Thermocouples were installed at key locations on the stubbox headers to monitor metal temperatures and the measured metal temperatures served as boundary values and for validation of the CFD results. The thermocouple data was analysed for three different steady state boiler loads, namely full load, 80% load and 60% load. It showed that the temperature distribution across these headers was not uniform, with a maximum temperature difference across the outlet stubbox of 40℃ at full load and 43℃ at partial loads. Other relevant power plant data, such as steam pressure, was provided from the power plant's Distributed Control System (DCS) and was used as boundary conditions for the CFD models. The exact mass flow distribution across the inlet stubs of the outlet stubbox header was unknown and was estimated using a CFD model of the inlet stubbox header and steam mass flow values from power plant's DCS system. A CFD model was created for each of the three boiler loads at steady state conditions. The CFD results provided the metal temperature profile, internal steam temperature distribution and pressure distribution across the header. The CFD solid temperatures were validated using the thermocouple readings and found to be in agreement. The CFD results were exported to the FEA models, where specific displacement constraints for thermal expansion were utilised. The FEA models were used to assess the extent of thermal stresses due to thermal expansion only, as well as stresses due to thermal expansion combined with internal pressure. High local stresses were found at the borehole crotch corners of the rear outlet branch and inlet stubs. However, these are below 0.2% proof strength at elevated temperatures. The high local stresses thus did not result in local plastic deformation but contribute to exacerbate steady state failure mechanisms such as creep.

Thermodynamic and transport properties and heat phenomena for steam in a high temperature region were examined. First, a set of computer subroutines was developed -the Fraction code, for generating molar fractions of a dissociating steam and gas mixture for a given pressure and temperature. Second, a set of computer subroutines was developed for the computation of all possible combinations of binary diffusion coefficients and of the diffusion coefficient for the component through a gas mixture. Third, steam properties may be evaluated for an ideal gas mixture which includes no effect of chemical reactions-frozed state, or for a non-ideal solution at equilibrium state which includes maximal effects of chemical reactions-effective properties at equilibrium state. A set of computer subroutines was developed-the Frozen Properties code, for evaluating dissociated steam frozen properties for a given pressure and temperature. A computer code, UODH2O (University of Ottawa Dissociation H$\sb2$O code), was developed using a look-up table and interpolation technique to generate effective properties at pressure from 0.01 to 100.00MPa and temperature from 1000 to 5726.85$\sp\circ$C. Based on the semi-empirical model by Nesterenko et al. (1967 a) for dissociated nitrogen tetroxide (N$\sb2$O$\sb4$#2NO$\sb2$ a two-component gas mixture), a method was developed to determine the Nusselt number of a chemically reacting (dissociated) fluid flow. (Abstract shortened by UMI.)

This study has focused on solid oxide electrolyser cells for high temperature steam electrolysis. Solid oxide electrolysis is the reverse operation of solid oxide fuel cells (SOFC), so many of the same component materials may be used. However, other electrode materials are of interest to improve performance and efficiency. In this work anode materials were investigated for use in solid oxide electrolysers. Perovskite materials of the form L₁₋ｘSrｘMO₃ , where M is Mn, Co, or Fe. LSM is a well understood electrode material for the SOFC. Under electrolysis operation LSM performed well and no interface reactions were observed between the anode and YSZ electrolyte. LSM has a relatively low conductivity and the electrode reaction is limited to the triple phase boundary regions. Mixed ionic-electronic conductors of LSCo and LSF were investigated, with these materials the anode reaction is not limited to triple phase boundaries. The LSCo anode had adherence problems in the electrolysis cells due to the thermal expansion coefficient mismatch with the YSZ electrolyte. The LSCo reacted with the YSZ at the anode/electrolyte interface forming insulating zirconate phases. Due to these issues the LSCo anode cells performed the poorest of the three. The performance of electrolysis cells with LSF anode exceeded both LSM and LSCo, particularly under steam operation, although an interface reaction between the LSF anode and YSZ electrolyte was observed. In addition to the anode material studies this work included the development of solid oxide electrolyser tubes from tape cast precursor materials. Tape casting is a cheap processing method, which allows for co-firing of all ceramic components. The design development resulted in a solid design, which can be fabricated reliably, and balances strength with performance. The design used LSM anode, YSZ electrolyte, and Ni-YSZ cathode materials but could easily be adapted for the use of other component materials. Proper sintering rates, cathode tape formulation, tube length, tape thickness, and electrolyte thickness were factors explored in this work to improve the electrolyser tubes.

Internal leakages of high pressure and temperature steam valves have been identified as a potential contributor to a loss in power generating plant efficiency. These losses are often neglected due to it being difficult to detect problematic valves and quantify the internal leakages through them. A non-intrusive NOT technique that detects and quantifies internal leakages through valves will be a very favourable tool to any power generating plant as it will allow for the early detection of internal leakages and could possibly provide considerable financial savings. This research evaluates different monitoring techniques suitable for detecting and quantifying internal leakages through valves and selects a technique that is most suitable for application in a power generating plant environment. The proposed technique utilises infra-red thermography to calculate pipe surface temperatures on a length of un-insulated pipe located downstream of a valve that is leaking internally. As the leakage steam flows through the length of un-insulated pipe, it will lose a portion of its heat energy through the pipe wall to the surrounding environment. This will result in a drop in temperature of the steam from the upstream to downstream points of the un-insulated length of pipe. By calculating the heat loss and the drop in temperature of the leakage steam, a mass flow rate of the leakage steam can be determined. A mathematical model was derived which with inputs of upstream and downstream pipe surface temperatures of the un-insulated pipe, pipe properties and ambient air conditions, calculates the heat loss, the temperature drop and the resulting mass flow rate of the leakage flow through the valve. A detailed experimental study was conducted to validate the proposed technique in determining internal leakages thought steam valves. Steam generated from a mini steam generating plant was allowed to flow through an experimental test rig, which contained a length of un-insulated pipe, at different flow rates. Pipe surface temperature measurements of the un-insulated pipe were made using an infrared thermal camera and a mass flow rate of the steam was calculated using the derived mathematical model. In all experiments, the mass flow rate calculated using the mathematical model was compared to a mass flow rate acquired from a flow measuring device installed in-line with the experimental test rig. The results indicate that an increase in mass flow rate causes an increase in pipe surface temperatures of the un-insulated pipe which translates to an increase in heat loss of the leakage steam through the length of un-insulated pipe. The mass flow rate calculated using the proposed technique closely approximates the mass flow rate acquired from the flow measuring device. This indicates that the proposed technique, using infrared thermography, is capable of detecting and quantifying possible internal valve leakages encountered in online operation. Onsite tests were performed using the proposed technique on two different boiler drain valves at Majuba Power Station. It was found that one of the valves was internally leaking steam to the atmosphere at a rate of 0.039 kg/s whilst the other valve was sealing correctly. A comprehensive financial impact study was conducted, and it was found that this leakage steam will result in a total loss of R 730 108 per annum if the leak is left unattended. This is the loss for a single valve that has a relatively small leak. The financial loss for a combination of all valves that are internally leaking in a power plant could be substantial and can clearly justify plant personnel in utilising the proposed technique to identify problematic valves. With its portability, non-intrusiveness and ease of use the proposed technique provides a cost-effective means to determine internal leakages through power plant valves.