Studying the causes of destruction of steam superheater tubes in boiler equipment

Results of comprehensive studies of samples of prematurely destroyed 57×4 mm steam superheaer tubes of STBA 22 steel used in a boiler unit of Singburi Sugar Co, Ltd factory (Thailand) are presented. The tubes were manufactured at Interpipe Niko Tube Ltd. (Ukraine) according to JIS G 3462 Standard (Japan). They were destroyed in a short (~240 hrs) term of operation. The cause of premature destruction of tubes of the above steel grade and size assortment in the boiler unit has been established. Based on present-day investigation methods (metallography, X-ray diffraction, etc.), it was found that the tubes were operated with violation of fuel combustion conditions and heat-carrying agent circulation. Characteristic features of operation of damaged tubes include high thermal stresses from the side of the fire-chamber and limitation (or absence) of circulation of the heat-carrying agent (blockage in bends, drum heads, etc.). During operation, the tubes were also exposed to significant thermal vibration stresses (unstable combustion conditions). Prolonged overheating occurred at temperatures above 1000 °C because of violation of circulation of heat-carrying agent and unstable combustion mode. High thermal stresses at almost complete absence of a heat-carrying agent, uneven distribution of growing heat flows caused by violation of the combustion mode in the fire-chamber contributed to accelerated degradation of structure and thermal destruction of the tube metal. In a short term of operation (~240 hours), there was a significant change in the tube size (accelerated high-temperature creep) and complete recrystallization of metal structure throughout the entire wall thickness of the damaged tubes. It has been established that the accelerated degradation of metal microstructure in the destroyed tubes was associated with both overheating of the tube wall and the as-delivered metal structure non-recommended for operation at high temperatures and pressures. It was shown that it is necessary to adjust the heat treatment conditions for these tubes at Interpipe Niko Tube Ltd. The study results have made it possible to develop recommendations for eliminating violations of operating conditions and establishing control of actual heat flows in the most thermally loaded sections of the Singburi Sugar Co. Ltd factory’s steam boiler superheater. Taking into account peculiarities of the boiler equipment and its operating conditions, it was also recommended to use a more heat-resistant and refractory steel instead of the currently used material for manufacture of the steam superheater tubes. Keywords: boiler tube, steam superheater, damage, thermal destruction, structure degradation, combustion conditions, heat carrier circulation, overheating.

Field Test of Superheater Pipes Vibration Caused by Sound Energy in the 235 MWe Circulating Fluidized Bed Boiler

This paper presents the results of measurements of vibrations of the pipes of the steam superheater installed in the convection pass of the 235 MWe Circulating Fluidized Bed boiler (CFB) induced using sonic soot blowers of the Nirafon NI-100 type. The measurements were made using two ICP Triaxial 356A16 accelerometers allowing the analysis of accelerations in the maximum range of ±490 m/s2. Simultaneously with vibration measurements, the sound pressure level was recorded using the G.R.A.S. 40BH high-pressure microphone. The measurements and spectral analysis of the recorded signals showed that the acoustic wave of 148 dB is safe for the steam superheater pipes causing vibrations of maximum amplitude not exceeding 1 mm. The field tests confirmed the supposition that the dominant mechanism for cleaning the surfaces of the superheater’s heating pipes are not pipe vibrations, but the breakage of cohesion forces between dust particles.

Thermodynamic Analysis and Optimization of Secondary Overheating Parameters in Turbo-Expander Plants on Low Boiling Working Fluids

The paper presents a thermodynamic analysis of secondary overheating in turbo-expander plants on low-boiling working fluids. The possibility of optimizing the parameters of the working fluid in a secondary stem superheater has been studied. The research was carried out for two typical turbo-expander cycles: with a heat exchanger at the outlet of the turbo-expander, intended for cooling an overheated low-boiling working fluid, and without a heat exchanger. Cycles in T–s coordinates were constructed for the studied schemes. The influence of pressure and temperature in the intermediate superheater on the exergetic efficiency of the turbo-expander unit was studied. Thus, the dependences of the exergetic efficiency and losses on the elements of the turbo-expander cycle are obtained when the temperature of the working fluid changes and pressure of the working fluid not changes in the intermediate superheater, and when the pressure changes and the temperature does not change. As a low-boiling working fluid, the ozone-safe freon R236EA is considered, which has a “dry” saturation line characteristic, zero ozone layer destruction potential, and a global warming potential equal to 1370. It has been determined that increasing the parameters of the low-boiling working fluid in front of the low-pressure turbo expander (regardless of the scheme of the turbo expander cycle) does not always cause an increase in the exergetic efficiency. Thus, overheating of the working fluid at a pressure exceeding the critical pressure causes a positive exergetic effect, but for each temperature there is an optimal pressure at which the efficiency will be maximum. At a pressure below the critical pressure, overheating leads to a decrease in the exergetic efficiency, and the maximum exergetic effect is achieved in the absence of a secondary steam superheater. All other things being equal, a turbo-expander cycle with a heat exchanger is more efficient than without it over the entire temperature range and pressure of the low-boiling working fluid under study.

DEVELOPMENT OF CONCEPTUAL TECHNICAL SOLUTIONS AND METHODS OF THEIR IMPLEMENTATION DURING THE DESIGN OF A DUST COAL STEAM GENERATOR OF SUPER-SUPER CRITICAL PARAMETERS OF STEAM 28 MPa / 600 °С / 600 °C FOR 300 MW ENERGY UNIT. PART 2

In the second part of the work, using the mathematical model described in first unit, a direct-flow pulverized coal boiler with supercritical steam parameters of 28 MPa /600 °C /600 °C was calculated for a 300 MW power unit at loads of 50–70 %. It is shown that: a) the temperature of contaminated screens 1234 °С in the active combustion zone, where 92 % of the initial fuel burns out, is lower than the temperature of slagging beginning (1268 °С) of ДГ-100 coal, which indicates the slag-free operation of the screens of the lower radiation part (LRP) b) over the entire range of the boiler load change of 50–100 %, the temperature at the exit from the active combustion zone will be higher than the minimum permissible temperature of 1250 °C, below which the flame is attenuated; c) upon transition to 50 % load, the temperature of gases at the outlet from the active combustion zone decreases from 1506 °C to 1342 °C, as a result of which the specific thermal stress of the LRP screens decreases by 1.469 times, while the feed water consumption at the inlet of the boiler falls by 2 times, which leads to an increase in the temperature of the LRP steam, the middle and upper radiation parts, screens of the ceiling and rotating chamber; d) to reduce the thermal stratification of the inlet stage of the secondary steam superheater (ППП1), and as a consequence, the pipe wall temperature, it is necessary to divide the ППП1 surface into two packets, installing an intermediate mixing manifold between them with full steam mixing. Bibl. 5, Fig. 13, Tab. 3.

Operational and economic analysis of a Velox-type combined heat & power plant with a throttle valve

For many years, the Department of Power Engineering and Turbomachinery of the Silesian University of Technology has been using a small-capacity (about 500 kWe) steam-gas power plant. Based on many years of experience in operation of this power plant utilizing the Velox-type gas-steam system, an idea arose to modify this type of thermal power cycle to create a combined heat and power (CHP) plant of small capacity, dedicated for distributed heat and power production or production process steam with high temperature. Previous thermodynamic and economic analysis of that type of low-power CHP plant confirmed many advantages of using this type of solution in low-power engineering. The new idea is to add a throttle valve between the economizer and the evaporator for the reference cycle of the gas-steam Velox-type CHP plant. It was further assumed that water would partially be vaporized on the valve. This paper presents a thermodynamic and economic analysis of this type of a system with an added throttle valve. Downstream the valve, the evaporated water fraction is fed directly into the steam superheater. It is determined how such a retrofit affects the system thermodynamic and economic characteristics. The system is modelled in the EBSILON® Professional 14 software.

PULSE-DETONATION STEAM SUPERHEATER

In [1], we patented the innovative pulse-detonation steam superheater (PDSS) for deep oxygen-free and pollutant-free gasification of organic municipal and industrial wastes to produce a gas mixture of H2 and CO. The PDSS is essentially the pulse-detonation engine operating on the ternary fuel-oxygen-steam mixture with fuel represented, e. g., by a gas mixture of H2 and CO (energy gas or syngas). The low-temperature steam (375-380 K) is produced by a standard steam generator. Pulsed detonations in the PDSS are achieved due to intermittent spark ignition of the fuel-oxygen mixture followed by fast deflagration-to-detonation transition and transmission of the detonation wave to the ternary fuel-oxygen-steam mixture. It is implied that the PDSS is directly connected to the waste gasification flow reactor and normally operates on the H2-CO energy gas. However, for starting the plant, the PDSS initially operates on any available hydrocarbon fuel or hydrogen.

ORGANIC WASTE GASIFICATION WITH HIGHLY SUPERHEATED STEAM PRODUCED BY CYCLIC DETONATIONS OF METHANE-STEAM-OXYGEN MIIXTURES

The paper presents the results of testing of the innovative laboratory-scale installation for organic waste anaerobic gasification to syngas (CO and H2) with highly superheated steam (HSS) produced by cyclic detonations of ternary methane-oxygen-steam mixtures in the pulsed detonation steam superheater (PDSSH).

Expediency of using the method of thermochemical regeneration at the reconstruction of a gas thermal power plant

The article is devoted to finding ways for the improvement of technical, economic and environmental characteristics of an existing gas thermal power plant (TPP). One of such ways is the use of thermochemical regeneration (TCR) technology. Thermochemical regeneration is the technology of utilization of the waste-gas heat, which lies in the conversion of fuel due to this heat, as a result of which a new fuel with a significantly higher calorific value is formed. In addition, this fuel contains a significant amount of hydrogen, the combustion of which is accompanied by lower NOx emissions as compared with, for example, natural gas. Thus, TCR enables one to solve simultaneously environmental problems (at least in part). When using this technology, there is a problem of finding a heat source to implement the conversion process. It is shown that the replacement of intermediate steam superheater by thermochemical reactor reduces the efficiency of power plant as a whole. Therefore, we analyze the variant of gas-turbine superstructure over the TPP. Two schemes of the realization of TCR with steam-gas power plant (SGP) are considered: a scheme with the use of air excess for decreasing the temperature of working body before the gas turbine (α > 1) and a scheme with ballast in the form of combustion products. Calculations show that the presence of oxygen in the reagent of conversion significantly reduces its degree, which makes such schemes inefficient, and the use of combustion products as ballast to reduce the temperature of working fluid before the gas turbine gives an increase in efficiency of 3.6% (rel.) as compared with conventional SGP. It is established that the introduction of scheme with ballast in the form of combustion products will save 2790 nm3 / h of natural gas. Keywords: thermal power industry, thermochemical regeneration, steam-gas power plant

The effect of internal pressure and thickness on the creep strain of the superheater pipes

Superheater pipes in turbines commonly are used to produce superheated steam. Internal pressure is critical for steam superheater elements. The pipes in such applications are vulnerable to temperature environments, which can bring the component to enter the creep regime, creep deformation, or even creep fracture. In general, most of the failures in boilers are caused by creep. Creep-resistant materials used in facilities operated at high temperatures must, therefore, be able to withstand the highest possible temperature loads. This study aims to investigate the creep behaviour of a 617 alloys steel steam pipe, which operated within 100,000 hours. The temperature of steam was set at 700?C, and the pressure in the pipe was 35 MPa. Abaqus software based on the finite element method was used in the study. The effect of internal pressure and pipe thickness on the creep strains was observed. The variation of the internal pressure was 35, 37.5, 40, 42.5, and 45 MPa. Whereas, the thickness variations were 30, 35, 40, 45, and 50 mm. The simulation results revealed that an increase in the internal pressure and the decrease of the pipe thickness increase the creep strain. This study can be used to predict the possibility of creep damaged for the superheater pipes operated at high temperatures, which have different thicknesses.