Boiler Surface Research Articles

Analiza kriterijuma za procenu formiranja naslaga na kotlovskim grejnim površinama pri sagorevanju biomase

Combustion of different fuels in boiler furnaces leads to formation of fireside deposits on irradiated as well as on convective surfaces. This problem is not a new one, but it became one of the main operational problems in boilers using low rank coal and some sort of biomass, especially agricultural. In the paper presents the results of the analysis of the criteria for estimating the formation of deposits on heating surfaces of boilers in the case of biomass combustion. Also, the ash related problems during biomass combustion in the boiler furnace were considered. Key empirical correlations have been tested for slagging and fouling in case of coal or biomass combustion based on testing of different types of fuel in laboratory and real furnace conditions. The analysis has shown that many sorts of agricultural biomass as well as the Kosovo lignite have a high fouling and slagging tendency, as confirmed by indicators based on the chemical composition of fuel ash and ash solubility temperatures.

High-temperature sulfur corrosion behavior of h-BN-based ceramic coating prepared by slurry method

The heating surface of a coal-fired boiler is susceptible to high-temperature sulfur corrosion. To investigate and address this problem, a h-BN-based ceramic coating was prepared on the surface of a 20CrMn steel sheet using the slurry method. After sintering, the coating had a relatively dense surface with only a small number of microcracks. The coating and the substrate exhibited a metallic damascene structure and were well bonded. The high-temperature-corrosion behavior of the uncoated and ceramic-coated steel sheets in sulfur dioxide (SO2), hydrogen sulfide (H2S), and sulfate environments was investigated. The results show that the coating could effectively prevent inward diffusion of sulfur, and it exhibited high-temperature-corrosion resistance. The high-temperature-corrosion resistance of the coated steel sheet was 8, 7, and 1.7 times that of the uncoated steel sheet in the SO2, H2S, and sulfate environments, respectively. After corrosion in the SO2 and H2S atmospheres, potassium sulfate (K2SO4) crystal grains were formed on the surface of the coating. However, because the coating was relatively dense, thus preventing S diffusion, the presence of K2SO4 was not detected inside the coating.

Investigation of thermo-aerodynamic characteristics of banks of tubes with punched spiral finning

An application of tubes with punched spiral-tape finning is promising for heat exchange intensification in convective heating surfaces of boilers and boilers-utilizers. Results of experimental research into thermo-aerodynamic characteristics of specified heating surfaces are presented. As a result of research, heat exchange intensification by 17…32 % due to fin punching was established. Heat transfer increases at an increase in the Reynolds number and a decrease in the degree of finning. Dependence of heat transfer on the accepted parameter for characterizing the bank geometry – relative to longitudinal pitch of tubes – is extreme with peaks in domain of variability relative to longitudinal pitch of 2.7…3.5. Efficiency of punched finning was determined. Results of the study of aerodynamic resistance showed its increase by 18…40 % due to the punching of fins. Resistivity increases at an increase in reduced length of the extended surface, and decreases at an increase in ratio of transverse pitch of tubes of the bank to longitudinal pitch and in Reynolds number. Results of experiment were generalized and formulas for engineering calculations of heat exchange and aerodynamic resistance of in-line tube banks with punched spiral finning were proposed. The formulae hold in domains of variability of defining parameters: finning coefficient y=6.01…9.012, relative longitudinal pitch of tubes in banks s 2 =2…6, Reynolds numbers Re d =6·103…4·104 and Re e =5·10 3 …4·10 4 , ratios of tube pitches S 1 /S 2 =0.4…2.5 and reduced length of extended surface – H/F=4.58…30.45. We established the intervals variability in ratio of pitches of tubes, in which thermo-aerodynamic efficiency of in-line and staggered tube banks is maximal, respectively: 1.0...1.5 and 2.0...3.0. Within these intervals, values of the Kirpichov criterion are, respectively, E=125...150 for in-line and 75...80 for staggered banks. Formulae establish relationship between Nusselt and Euler criterion with geometric characteristics of banks and Reynolds numbers. We determined thermo-aerodynamic efficiency of in-line and staggered banks of tubes with punched spiral finning by results of experimental studies. In-line banks have higher efficiency. As a result of calculation research into thermo-aerodynamic efficiency of four types of heating surfaces of a powerful boiler-utilizer, in-line tube banks with punched spiral finning turned out to be more efficient by this parameter. The Kirpichov criterion for these tube banks, located in one shell, is 319, for staggered tube banks with punched finning – 228.8, for staggered tube banks with continuous finning – 223.8, and for staggered bare-tube banks – 143.0.

Deposit Shedding in Biomass-Fired Boilers: Shear Adhesion Strength Measurements

Ash deposition on boiler surfaces is a major problem encountered in biomass combustion. Timely removal of ash deposits is essential for optimal boiler operation. In order to improve the understanding of deposit shedding in boilers, this study investigates the adhesion strength of biomass ash from full-scale boilers, as well as model fly ash deposits containing KCl, K2SO4, CaO, CaSO4, SiO2, K2CO3, Fe2O3, K2Si4O9, and KOH. Artificial biomass ash deposits were prepared on superheater tubes and sintered in an oven with temperatures ranging from 500 to 1000 °C. Subsequently, the deposits were sheared off by an electrically controlled arm, and the corresponding adhesion strength was measured. The effect of sintering temperature, sintering time, deposit composition, thermal shocks on the deposit, and steel type was investigated. The results reveal that the adhesion strength of ash deposits is dependent on two factors: ash melt fraction, and corrosion occurring at the deposit–tube interface. Adhesion strength inc...

Recent Advances and Current Status of the Characteristic of Deposits on Heating Surfaces from Heating Biomass-fired Boiler

Biomass direct combustion for electricity is the most widely applied and developed biomass conversion and utilization technology, but due to the high content of alkali metals, it will cause deposition and high temperature corrosion of heating surfaces in boiler during biomass combustion, thus reduces the efficiency and influences the long-time safe operation of the boiler. Alkali metals problem especially deposition and high temperature corrosion is urgently need to be solved. A review of literature indicates ash deposition and high temperature corrosion of heating surfaces in biomass-fired boilers, combined engineering practice with experimental study, explored the mechanism of deposition build-up and high temperature.

Combustion performance and slagging characteristics during co-combustion of Zhundong coal and sludge

Zhundong coal (ZDc) with a very large reserve is faced with severe problems of slagging and fouling during combustion in boilers because of the high-Na content. Sludge, the by-product of urban sewage treatment, is also faced with the problem in utilization. In this study, the co-combustion of ZDc and sludge was investigated in a laboratory-scale experimental apparatus before further studies in larger-scale setups. The experimental results confirm an interaction between ZDc and sludge during co-combustion, which was mainly caused by the Na catalytic action and improved the combustion performance of the co-fuels. The catalytic effect was particularly significant at low sludge mixing ratios. The reactions between Na-based compounds in ZDc and Si/Al/P-rich minerals in sludge, forming high-melting-point phosphates and aluminosilicates, not only increased Na retention in residual ash reducing the risk of fouling on tail-heating surfaces in boilers, but also raised the ash fusibility of the co-fuels avoiding low-temperature sintering. Even so, to prevent slagging, the high combustion temperature above 900 °C should be avoided during co-combustion because of the high Na retention in residual ash. Moreover, the high heavy metal retention in residual ash decreased the pollution caused by heavy metal volatilization during sludge combustion.

On the Necessity of Changes in the Strategy of Utilization Ship Boiler Technical Condition Maintenance in the Aspect of LNG Applied as Fuel

Abstract Heavy oils (HFO fuels) used on ships play a part in degradation of technical condition of heat exchange surfaces of utilization boilers especially on the exhaust gas side. Presence of sulphur in these fuels is the main factor favouring degradation. The upper limit for sulphur content in the fuel used outside the SECA areas equal to 3.5% is currently in force, at least until the year 2020 or 2025. The recommended by classification societies overhauls of utilization boilers are, therefore characterized by a specially chosen strategy thanks to which it is possible to maintain their appropriate technical condition. The requirement to use fuels with low sulphur content (LSFO), which are significantly more expensive than MDO fuels, in the areas of controlled sulphur emissions also led to a further introduction of alternative fuels, such as methanol and above all liquefied natural gas (LNG), onto ships. That is especially valid for the ship owners whose vessels e.g. ferries sail mainly within SCECA This article analyses the consequences of the introduced fuel change on utilization boiler maintenance. A change in the technical condition maintenance strategy for utilization boilers has been suggested.

Estimation of Oxidation Kinetics and Oxide Scale Void Position of Ferritic-Martensitic Steels in Supercritical Water

Exfoliation of oxide scales from high-temperature heating surfaces of power boilers threatened the safety of supercritical power generating units. According to available space model, the oxidation kinetics of two ferritic-martensitic steels are developed to predict in supercritical water at 400°C, 500°C, and 600°C. The iron diffusion coefficients in magnetite and Fe-Cr spinel are extrapolated from studies of Backhaus and Töpfer. According to Fe-Cr-O ternary phase diagram, oxygen partial pressure at the steel/Fe-Cr spinel oxide interface is determined. The oxygen partial pressure at the magnetite/supercritical water interface meets the equivalent oxygen partial pressure when system equilibrium has been attained. The relative error between calculated values and experimental values is analyzed and the reasons of error are suggested. The research results show that the results of simulation at 600°C are approximately close to experimental results. The iron diffusion coefficient is discontinuous in the duplex scale of two ferritic-martensitic steels. The simulation results of thicknesses of the oxide scale on tubes (T91) of final superheater of a 600 MW supercritical boiler are compared with field measurement data and calculation results by Adrian’s method. The calculated void positions of oxide scales are in good agreement with a cross-sectional SEM image of the oxide layers.

Heat losses in power boilers caused by thermal bridges

In this article the analysis of heat losses caused by thermal bridges that occur in the steam boiler OP-140 is presented. Identification of these bridges were conducted with use of thermographic camera. Heat losses were evaluated based on methodology of VDI 4610 standard, but instead of its simplified equations, criterial equations based on Nusselt number were used. Obtained values of annual heat losses and heat flux density corresponding to the fully insulated boiler surfaces were compared to heat losses generated by thermal bridges located in the same areas. The emphasis is put on the role of industrial insulation in heat losses reduction.

Modeling and analysis of process configurations for hybrid concentrated solar power and conventional steam power plants

Availability and flexibility of variable renewable energy production can be increased without thermal energy storages by integrating intermittent energy production with load following energy production. This paper focuses on modeling, analysis and comparison of three hybrid configurations for concentrated solar power and conventional steam power plants. The configurations include feedwater preheating, cold reheat line and high pressure turbine concepts, in which linear Fresnel collector solar field with direct steam generation is applied to generate steam parallel with the steam boiler. The modeling is conducted using dynamic simulation software Apros®, which enables investigation of the system operation under varying process conditions. In particular, the focus is on the comparison of thermal load balance and interdependencies of the subsystems. In addition, the focus is on increasing the solar contribution, as the solar share is low in current commercial concepts. As a main conclusion, the more solar steam is fed to the system, the more different turbine sections and heat surfaces of the steam boiler are imbalanced, which leads to challenging system design. In addition, the maximum fuel and emission savings are not only determined by the achievable solar share, but also by the hybrid process design. The design of the hybrid system has to be optimized and modified according to the configuration in order to achieve higher peak solar share and fuel and emission savings than 20% achieved in this study.

Formation of alkali salt deposits in biomass combustion

In the present work, the alkali salts deposit formation on cooled surfaces in biomass-fired boilers has been studied. Different deposition tests with two real biomasses (orujillo and cardoon) as well as with several synthetic materials have been conducted in an entrained flow (pilot scale) reactor for a range of conditions representative of those prevailing at different locations inside industrial boilers. A number of parameters, such as gas temperature around the probe or surface temperature, have been changed in order to analyze their influence on the characteristics of the ash deposits (morphology, thickness, composition…). The deposits' morphologies observed in the experimental samples indicate that the dominant deposition path for alkali salts (in the conditions explored in this work) is thermophoresis of aerosols formed either in the thermal boundary layer next to the deposition surface or already in the external gas stream (in case gas conditions allow for their formation). The contribution of each possible deposition mechanism has also been studied by comparing the obtained experimental results with theoretical predictions from a deposition model developed by Castillo and Rosner. Experimental trends have usually been in good agreement with those calculated according to the mechanisms of arrival of small particles to the deposition surface. Furthermore, the agreement found between calculations and measurements has been good enough to support the applicability of the model due to Castillo and Rosner to the quantitative prediction of deposit formation rates.

Evaluation of near-drum thinning data in recovery boiler generating bank tubes

Near-drum thinning affects the fireside surface of recovery boiler generating bank tubes near the surface of the mud drum. Although sophisticated thickness scanning equipment has been developed to rapidly make tens of thousands of thickness measurements in the portion of a tube that is vulnerable to near-drum thinning, methods for using these data to evaluate fitness-for-service have not shown similar advances. Non-destructive testing companies typically use a technician’s subjective judgment to identify the “thinnest reliable" thickness measurement on each tube. Some mills decide whether tubes can continue in operation or should be plugged or replaced based on this single thickness measurement. However, finite element analysis of the remaining strength of individual tubes thinned in the near-drum area suggests that it is essentially impossible to identify the weakest tubes from simple empirical rules. In the absence of an industry standard for evaluating these data, different mills could reach different conclusions about the fitness-for-service of a tube from the same data set. This paper reviews the technology for scanning the thickness of generating bank tubes and discusses approaches that have been used to identify the tubes most weakened by near-drum thinning and to evaluate the fitness-for-service of individual tubes.

CFD simulations of heat transfer in internally helically ribbed tubes

Abstract Heating surfaces in power boilers are exposed to very high heat flux. For evaporator protection against overheating, internally helically ribbed tubes are used. The intensification of the heat transfer and the maintenance of the thin water layer in the intercostal space, using ribbed tubes, enables better protection of the power boiler evaporator than smooth pipes. Extended inner surface changes flow and thermal conditions by influencing the linear pressure drop and heat transfer coefficient. This paper presents equations that are used to determine the heat transfer coefficient. The results of total heat transfer, obtained from CFD simulations, for two types of internally ribbed and plain tubes are also presented.

Corrosion and life assessment of IntrexTM superheater tubes in a CFB oil shale boiler

Heating surfaces in oil shale boilers are subject to intensive high temperature corrosion (HTC). The protective oxide scale formed on austenitic steels SA213-TP347HFG, X8CrNiMoNb16-16 and X8CrNiMoVNb16-13 has been investigated and analysed (1) under both controlled laboratory conditions and (2) in the IntrexTM superheater of the circulating fluidized bed (CFB) boiler operating at the Narva Thermal Power Plant. Results show that the associated protective oxide scale tends to spall from tube surfaces. A long term investigation of HTC on the oil shale boiler tubes has demonstrated the applicability of these materials for such use and a comparison of HTC tubes manufactured from differing steel grades allows a selection of the most suitable grade. Remaining life assessments for particular tubes have also been conducted on the basis of this long term study of HTC on CFB boiler heating surfaces. This prognosis is vitally important to warranty safe and reliable further operation of thermal equipment.

Role of potassium in deposit formation during wood pellets combustion

Biomass boiler issues regarding slagging, fouling and corrosion are related to alkali species present in fuels. These alkali species are released as gaseous alkali chlorides, hydroxides and/or sulfates during combustion. Alkali chlorides/sulfates later condense on cold boiler surfaces enhancing fouling and corrosion. Subsequent deposition of silica-rich ashes leads to the formation of low temperature melting eutectics, especially alkali/earth alkali-silicates (K/Ca-Si), and creates compact and strong deposits. Condensing chlorides/sulfates and low melting eutectics both reduce boiler performance and its availability.Two different woody biomass fuels and their mixture with two different kinds of aluminosilicate base additives were combusted during this study. Deposits were sampled using two different kinds of probes; cooled deposit probe and un-cooled deposit probe. The macroscopic and microscopic observations and chemical composition of deposit samples are compared using electron microscopic image analysis (EMPA) for both deposit samples. The additive significantly changes the morphology and composition of the deposit samples. The difference in morphology and composition can be explained by the change in potassium chemistry between two cases.

Down-time corrosion in boilers

Down-time corrosion can occur on boiler surfaces, e.g. furnace walls, superheaters, or economizers that are covered with hygroscopic deposits, when the temperature drops under 100°C. This scenario takes place when a boiler is shut-down for cleaning, maintenance, or other reasons, such as unplanned shut-downs. Initially, the dry deposits will absorb moisture from the surrounding air, potentially creating a corrosive environment. After this, corrosive acids can form in the deposits. In this study modified online-corrosion probes were used in combination with deposits taken from 6different boilers at various locations (for example, from the furnace, the superheater, and the economizer), where the fuels have been waste, demolition wood or biomass. The deposits were ground and dried in an oven at 160°C for several hours before exposed to a moist environment (RH 65%) during online measuring of the corrosion rate and the pitting activity. Four types of alloys were tested: low-alloy ST45.8-steel, 9% Cr ferritic P91-steel, austenitic stainless steel 304L, and Ni-based super Alloy 625. The results for ST45.8 show that in biomass boilers a corrosion rate from negligible values up to 0.7mm/year can be reached within a week, while waste-fired boilers can have rates as high as 1.8mm/year. Furthermore, for some samples from waste-fired boilers show a high pitting activity already after 24h. The tests with the P91-steel show values up to 0.16mm/year, for samples from different locations in the boiler. For 304L and Alloy 625 the down-time corrosion was negligible even after a two-week exposure. The ability to follow the down-time corrosion online has provided data that show that even though thought to be negligible, the risk of down-time corrosion is of significance, especially if the fuel is waste. Also, even if the initial corrosion rate is low, it increases during the first 24h due to the exposure to moist environment, motivating immediate cleaning of the boilers after shut-down, especially on surfaces of lower steel quality.

화목보일러 화재위험성에 관한 실험적 연구

본 연구에서는 농촌지역을 중심으로 대표적인 겨울철 난방기구인 화목보일러 화재위험성을 평가하였다. 화목보일러 연료투입구 주변 및 연통의 온도를 측정하였으며, 연통 내부에 축적되는 타르의 화재 위험성을 평가하였다. 연료 투입구개방 상태에서 전방의 온도를 측정한 결과 화목 투입구로부터 40 cm 이상 떨어진 영역에서는 온도가 약 <TEX>$40^{\circ}C$</TEX> 이하를 유지하였다. 본 연구에서는 연료 투입구 외부로 비산되는 불티가 60 cm 이내의 범위에 비산되었다. 연통 내부에서는 온도가 <TEX>$600^{\circ}C$</TEX> 이상까지 상승하였으며, 화목보일러 본체로부터 2 m 떨어진 위치의 연통 내부에서 온도가 <TEX>$420^{\circ}C$</TEX> 이상 도달하였다. 연통 내부의 타르를 수거하여 발화온도를 측정한 결과 약 <TEX>$398^{\circ}C$</TEX>를 기록하였다. 화목보일러 본체 및 연통 표면에서 온도가 약 <TEX>$300^{\circ}C$</TEX> 이상 도달하였으며 가연물이 접촉될 경우 착화되었다. In this study, experiments were carried out to evaluate the fire risk of a firewood boiler, which is mainly used as a winter heating apparatus in rural areas. The ambient temperatures of the combustion chamber and the duct were measured. The fire risk of tar inside the duct was also investigated. The temperature decreased less than <TEX>$40^{\circ}C$</TEX> in the region more than 40 cm from the combustion chamber. Fire- flakes were scattered in the range of less than 60 cm from the combustion chamber. The temperature inside the rose to above <TEX>$600^{\circ}C$</TEX>. At 2 m from the boiler body, the temperature inside the duct was increased to about <TEX>$420^{\circ}C$</TEX>. The ignition temperature of tar was about <TEX>$398^{\circ}C$</TEX>. The temperatures of the boiler and duct surface were above <TEX>$300^{\circ}C$</TEX>. Combustible material ignited when it contacted the boiler surface or duct surface.

2D dynamic mesh model for deposit shape prediction in boiler banks of recovery boilers with different tube spacing arrangements

Abstract CFD tools are of great value in the design and operation of boilers. One particular aspect that can be modeled by CFD is the ash deposition and plugging of heat transfer surfaces of boilers. Fouling and slagging are the most typical causes of unscheduled boiler shutdowns. Consequently, appropriate predictions of deposit geometries and rates are of considerable interest. CFD multiphase flow simulations are capable of modeling particle-laden streams and constitute a suitable tool for study of material deposition. Fouling phenomena have a complicated and multidisciplinary nature involving thermo-fluid mechanics, sticking/rebounding of particles, sintering, etc. If the deposit growth rate has been accurately calculated by a multiphase model appropriate for particle-laden flows, CFD dynamic mesh techniques are able to move the interphase fluid-deposit according to the growth rate. This work develops and presents a CFD model for prediction of deposition shapes in a classical boiler bank of a Kraft Recovery Boiler by combining a multiphase and a dynamic mesh model. The effect of tube transversal spacing is also analyzed. The paper highlights the features and effects of the dynamic mesh model.

Effect of accompanying ions and ethylenediaminedisuccinic acid on heavy metals sorption using hybrid materials Lewatit FO 36 and Purolite Arsen Xnp

The results of the new hybrid ion exchangers Lewatit FO 36 and Purolite Arsen Xnp investigations are presented in the article. Their structure, surface chemical properties and sorption ability towards Cu(II), Zn(II), Cd(II), Pb(II) and Cr(VI) ions with and without the presence of S,S-(ethylenediamine N,N′-disuccinic) acid (S,S-EDDS) as a complexing agent as well as the accompanying Cl−, NO3− and SO42− ions are described in detail. S,S-EDDS is suggested as a promising substitute for EDTA in the nuclear energy industry, for chemical cleaning of boiler and heat-transfer surfaces. The sorption process was carried out under batch (static) conditions from single- and multicomponent aqueous solutions to select the ion exchanger that showed the best performance towards the above-mentioned ions. The sorption capacity of Lewatit FO 36 and Purolite Arsen Xnp increased with the increasing metal ions concentration in the solution. The sorption capacity was higher for Purolite Arsen Xnp and equal to: 23.70mg/g for Cu(II), 26.21mg/g for Zn(II), 50mg/g for Cd(II), 139mg/g for Pb(II) and 23.80mg/g for Cr(VI). The detailed characterization of the above mentioned hybrid ion exchangers was made by the electron spin resonance (ESR) measurements using the X-band electron paramagnetic resonance spectrometer. The Lewatit FO 36 shows the superparamagnetic state while Purolite Arsen Xnp does not exhibit such properties.

Correlation between aging grade of T91 steel and spectral characteristics of the laser-induced plasma

T91 steel with favorable mechanical performance has become the representative heat-resistant steel used as heat exchange surfaces in supercritical and ultra-supercritical boilers. The organizational structure and mechanical properties change during the service period, called material aging, which affects the service life and the equipment safety. To develop a fast and easy aging predictive technique of heat exchange metal surfaces, laser-induced breakdown spectroscopy (LIBS) was applied to investigate the plasma characteristics of T91 steel specimens with different aging grades. The metallographic structure, mechanical properties and spectral characteristics of the specimens were analyzed. Then, the correlations between the spectral characteristics and the aging grade were established. The analysis results show that the martensite substructure disappears, and the dimension of the carbide particles among the crystal lattices increases with aging. At the same time, the hardness of the samples gradually decreases. The peak intensities of both the matrix and the alloying element increases then decreases with aging, owing to the change of the metallography structure and mechanical properties. Furthermore, good unique value correlations between the intensity ratio of CrI/FeI, MoI/FeI and the aging grade are found. This demonstrates that LIBS is a possible new way to estimate the aging grade of metal materials.