In-bed Tubes Research Articles

From bubbling to circulating fluidized bed combustion—development and comparison

The most significant introduction of fluidized bed combustion technology took place about 50 years ago. Initially the combustion beds were of the bubbling type. Once the designs had reached commercial application, several drawbacks were discovered: Erosion on in-bed heat-exchanger tubes, insufficient combustion and desulphurization efficiencies with coal, unfavourable scale-up to electric utility-size. The problems were solved by applying circulating systems. The present text compares these bubbling and circulating designs. It is concluded that the bubbling bed may not be suitable for coal combustion, but for biomass and organic waste most of the drawbacks disappear, and the bubbling bed, being simpler, may have an economic advantage over CFB that should be considered. In addition, combinations of CFB and BFB are quite favourable in many applications.

Hydrodynamics of gas–solids flow in a bubbling fluidized bed with immersed vertical U-tube banks

We apply a two-fluid model (TFM) from the open-source code Multiphase Flow with Interphase eXchanges (MFIX) to investigate hydrodynamics in a gas–solids fluidized bed with immersed vertical tubes. The cut-cell method implemented in MFIX is used to fully resolve the flow around vertical U-tube banks. Simulations are performed in a bed diameter of 0.145m with square and triangular tube arrangements, for inlet gas velocities of U0/Umf=2.3, 4.5 and 6.8. Simulation results are compared with experimental results from the literature and show very good agreement for the bubble size. The efficiency of vertical tubes in reducing bubble size depends upon inlet gas velocity and tube arrangement. Reduction in bubble size is due to the vertical tubes preventing bubble coalescence and promoting bubble splitting. In-bed vertical tubes result in uniform distribution of bubbles within the bed with increase in bubble frequency. The bubble frequency is higher within the bed for square tube arrangements. For a bed with vertical tubes, the bubble shape is generally elongated, which results in high bubble rise velocity. Axial solid velocity and solids circulation patterns are significantly affected by the vertical tubes, where triangular tube arrangements rarely show any solids circulating zone.

Three-dimensional CFD modelling of a fluidised bed combustor fuelled by biomass and coal

A computational fluid dynamics analysis of a fluidised bed combustor fuelled by coal and biomass is performed. A discrete phase modelling approach is used to track fuel particles. Combustion is modelled using a two-mixture fraction probability density function approach. The standard k–ɛ two-phase turbulence model is used to describe the gas–solids flow. The residence time of coal particles is found to be greater than that of biomass particles. Contours of char burnout and devolatilisation are obtained and combustion is found to be quantitatively complete, and thus no significant change in combustion efficiency is found under full-load conditions. Velocity vectors are obtained and it is found that the velocity increases from the notch region towards the exit of the furnace, owing to the reduction in cross-sectional area. Heat exchange from in-bed superheater tubes to flue gases is greatest at the highest-temperature regions of the tubes.

High temperature degradation by erosion-corrosion in bubbling fluidized bed combustors

Heat-exchanger tubes in fluidized bed combustors (FBCs) often suffer material loss due to combined corrosion and erosion. Most severe damage is believed to be caused by the impact of dense packets of bed material on the lower parts of the tubes. In order to understand this phenomenon, a unique laboratory test rig at Berkeley was designed to simulate the particle hammering interactions between in-bed particles and tubes in bubbling fluidized bed combustors. In this design, a rod shaped specimen is actuated a short distance within a partially fluidized bed. The downward specimen motion is controlled to produce similar frequencies, velocities and impact forces as those experienced by the impacting particle aggregates in practical systems. Room temperature studies have shown that the degradation mechanism is a three-body abrasion process. This paper describes the characteristics of this test rig, reviews results at elevated temperatures and compares them to field experience. At higher temperatures, deposits of the bed material on tube surfaces can act as a protective layer. The deposition depended strongly on the type of bed material, the degree of tube surface oxidation and the tube and bed temperatures. With HCl present in the bed, wastage was increased due to enhanced oxidation and reduced oxide scale adherence.

Release of alkali salts and coal volatiles affecting internal components in fluidized bed combustion systems

In spite of the potential advantages of atmospheric fluidized bed systems, experience has proved that, under certain environments and operating conditions, a given material employed for internal components could lead to catastrophic events. In this study, an attempt is made to establish material selection and operational criteria that optimize performance and availability based on theoretical considerations of the bed hydrodynamics, thermodynamics and combustion process. The theoretical results may indicate that, for high-volatile coals with particle diameters (d c ) of 1-3 mm and sand particle size (d s ) of 0.674 mm, a considerable proportion of alkali chlorides may be transferred into the freeboard region of fluidized bed combustors as vapor phase, at bed temperatures (T b ) s ) ≤ 0.2 m and fluidizing velocity (U o ) b > 890 °C and XSA > 30 %, a high oxidation rate of the in-bed tubes may be present. Nevertheless, for these higher T b values and XSA < 10 %, corrosion attack of metallic components, via sulfidation, would occur since the excessive gas-phase combustion within the bed induced a local oxygen depletion.

Modeling of Volatiles Combustion and Alkali Deposition in a Fluidized Bed Coal Combustor

A simplified kinetic model, coupled with the bed hydrodynamics and a volatile evolution region within the bed, was formulated to predict the extent of gas-phase combustion in a laboratory-scale fluidized bed coal combustor (FBC). A close examination has also been made to highlight the relevance of the reducing/oxidizing environment (computed with the present theoretical model) in relation to FBC materials exposed to fireside corrosion at high temperature, under various operating conditions. The model results revealed that, for high-volatile coals with particle diameters (d c ) of 1-3 mm and sand particle size (d s ) of 0.674 mm, over one third of the original coal volatiles may burn in the freeboard region at bed temperature (T b ) ≤ 850 °C and excess air (XSA) ≤ 10 %. These values, together with the computed equilibrium conversion of alkali chlorides to sulfates, may suggest that sodium and potassium salts present in the vapor phase are likely to accelerate hot corrosion of heat exchange tubes above the bed when an FBC operates at T b ≤ 840 °C, XSA ≤ 20 %, d c 890 °C and XSA > 30 %, high oxidation rates may be present for the in-bed tubes. At these higher T b values and XSA < 10 %, a sulfidation mechanism presumably influences the extent of corrosion on the metallic components within the bed.

Strong HCl effect on tube wastage in a simulated bubbling fluidized bed environment

This paper reports initial results from a laboratory fluidized bed test rig where HCl gas was incorporated into the bed. The rig simulates the particle hammering interactions between in-bed particles and tubes within bubbling fluidized bed combustors (BFBCs). A significant increase in material wastage rates in the presence of HCl was found at 450°C as well as at 200°C. The detrimental effect decreased with decreasing HCl concentration.

Effect of HCl on the corrosion and wear of in-bed tubes in a laboratory simulated bubbling fluidized bed

Heat-exchanger tubes in fluidized bed combustors (FBCs) often suffer material loss due to combined corrosion and erosion. The effect of chlorine on in-bed tube wastage and its possible mechanism are being studied using the EPRI/LBNL FBC wastage simulator. This test rig was designed to simulate dense particle impacts on tube bottoms with well-controlled parameters, and has been proven to closely reflect situations found in operating bubbling FBCs. In this study, HCl gas was chosen to be the Cl source and was introduced into the fluidizing air with a 50-ppm concentration. Tests were performed at temperatures ranging from ambient to 400°C using 1018 low carbon steel rods in a bed of commercial SiO 2 sand that had an average size of 800 μm. The wear profile after each test was measured using a profilometer. Results showed an increase in material wastage rates in the presence of HCl, and that the rate was significantly higher with higher test temperatures. Microstructural and chemical analysis of the wear surface and corrosion products are reported. It is concluded that the dominating effect of HCl on the wastage rates is due to an enhanced oxidation and a reduced scale adherence.

Elevated temperature erosion of several thermal-sprayed coatings under the simulated erosion conditions of in-bed tubes in a fluidized bed combustor

A series of elevated temperature erosion tests were carried out on cooled AISI 1018 low-carbon steel and four thermal spray-coated mild steel specimens using a nozzle-type elevated temperature erosion tester. The thermal-sprayed coatings included a high velocity oxy-fuel (HVOF) 75Cr 3C 225NiCr cermet coating, HVOF Cr 3C 2, low velocity flame-sprayed Cr 2O 3 ceramic coatings and an arc-sprayed FeCrSiB coating. Test conditions attempted to simulate the erosion conditions at the in-bed tubes of fluidized bed combustors. The specimens were water-cooled on the backside. Material wastage was determined from thickness loss measurements of the specimens. Erosion test results were compared with those from testing isothermal specimens. The morphologies of specimens were examined by scanning electron microscopy (SEM). It was found that the cooled specimens had greater material wastage than the isothermal specimens. The effect of cooling the specimen on the erosion wastage for 1018 steel was greater than for the thermal-sprayed coatings. The lower material wastage of coated specimens compared to 1018 steel was related to the coatings' composition and morphology. The HVOF Cr 3C 2 coating specimen exhibited the lowest erosion wastage due to its favorable composition and morphology. It had a finer structure and smaller splat size than the other three coatings. The erosion mechanism of the cooled specimens was similar to that of the uncooled specimens, despite the different material wastage.

Summary of results from the Berkely in-bed tube erosion simulator

The Berkeley rig was designed to simulate the particle hammering interactions between in-bed particles and tubes within bubbling fluidized bed combustors. To achieve this in a controlled manner, a rod shaped specimen is actuated a short distance within a partially fluidized bed, consisting quartz particles with an average size of 800 μm. The rig has been used to investigate the characteristics of tube metal loss and its dependence on temperature. Most of the tests were performed isothermally by varying the bed temperature within the studied range, some were carried out by internally cooling the specimen while keeping the bed temperature constant at 700°C. This paper reviews the results that have been obtained from these simulations. Earlier conclusions from cold model studies are summarized, and the effects of temperature, temperature gradient, bed condition and bed material deposits on tube wastage are presented and discussed.

The effect of temperature gradient on steel tube wastage in a bubbling fluidized bed simulator

This paper describes a unique laboratory rig that simulates the interaction between dense particles and in-bed tubes in bubbling fluidized bed combustors. In this test rig, a tubular specimen is actuated periodically in a partially fluidized bed. A coolant can flow continuously through the specimen in order to achieve a surface temperature lower than that of the bed. Preliminary tests performed on 1018 steel both under isothermal and temperature difference conditions are described and compared. During the temperature difference test, the bed was held at 700 °C and the specimen surface at 200 °C. Results showed a wear rate five times higher under these conditions compared to the isothermal test, but neither the wear mechanism nor the circumferential wear distribution was altered. As in previous studies using isothermal test conditions, material loss was mainly caused by abrasion of the metal by the bed particles. The deposition of an adherent layer composed of bed material onto the specimen surface acted as a protection against much of the damaging impact. The degree of such deposition was significantly less on specimens tested under the temperature difference condition, and this is believed to be the main cause for the observed increase in wear.

Heat transfer characteristics in a small-scale fluidized bed boiler

Heat transfer characteristics in a small-scale fluidized bed boiler (2MWth) were studied using lignite and corn cob as fuels. Depending on air velocity, the heat transfer rates from bed to water membrane wall and from hot flue gas to convective tube bank were in the ranges 75–55% and 25–45% of the total heat absorbed by the boiler, respectively. At designed capacity, the heat transfer flux based on bed cross sectional area and on water membrane wall area were about 0·45 and 0·15 MWm−2, respectively. Under the conditions studied, it was found that the overall heat transfer coefficient between bed and water membrane wall was 100–300 W m−2 K−1, whereas that between flue gas and convective tube bank was 10–30 Wm−2 K−1. The study of heat transfer to a horizontal tube immersed in the bed as well as placed in the freeboard region were also studied. The effective heat transfer coefficients were found to be 300–800 W m−2 K−1 for in-bed tube and 30–150 W m−2 K−1 for the freeboard region, depending on air velocity. Comparison of these data with those predicted by both modelling and correlation reported in the literature was also made. For the immersed tube, good agreement was observed for low air velocity, while at high air velocity the experiment produced results twice those estimated from modelling and correlation. For the freeboard region, the model gave a fair prediction.

The circumferential distribution of wastage on in-bed tubes in fluidized bed combustors

The circumferential distribution of metal loss about heat exchanger tubes within fluidized bed combustors usually shows one of two distinct patterns. In one case, flats are formed on either side of the tube bottom and there is little or no loss at the bottom (Type A); in the other case, the wastage is concentrated at the bottom and falls off to either side (Type B). Previous explanations for this difference in behaviour have mostly relied on an assumption of a difference in the actual damage process or material response. In this paper, a simple computer simulation shows that Type A behaviour can result from a high degree of constraint on the flow conditions around tubes, and that the introduction of a small degree of flow variability into such a system will lead to Type B behaviour.

Effect of particle-tube collision frequency on material wastage of in-bed tubes in the bubbling fluidized bed combustor

The metal wastage mechanisms and rate that occur in 1018 carbon steel used in in-bed tube of the bubbling fluidized bed combustor (BFBC) are discussed with particle-surface collision frequency around the tube. Electrostatic impact probes based on the triboelectric effect of moving particles were developed and used to measure the particle-surface collision frequency, which is one of the major parameters characterizing the material wastage of in-bed tubes in the BFBC. The distribution of the particle-surface collision frequency around the tube was affected by the location of the tube and bed height. A series of material wastage tests was carried out using 742 μm, quartz silica (SiO 2) on 1018 steel at elevated temperature (300°C) which occurs in-bed locations of BFBC. The morphologies of the specimens were examined and analyzed using scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). The eroded surface morphology under high particle velocity (30 m s −1) exhibited typical ductile behaviour. The maximum collision frequency around the periphery of the tube was found at 45°, 315° from the bottom centre, 0°, which was correlated with maximum material wastage rate of the specimens.

Corrosion and Coating of Advanced Materials

Most metals are thermodynamically unstable and tend to change to more stable phases by reaction with their service environment. These oxidation or corrosion processes are usually controlled by kinetic factors, and the rate often increases with increasing temperature. Degradation by these processes is an important, often life-limiting, factor for critical components in thermal-power systems and other high-temperature process plants, and it can have considerable economic and technical consequences. Corrosion can be particularly severe, for example, inGas-turbine blades and vanes, Gas coolers in coal-gasification systems, Superheaters and reheaters in coaland oil-fired boilers, In-bed tubes in fluidized-bed coal combustion systems, Components in high-temperature fuel cells, Components in steam-reforming plants, Components in incinerator plants, and Exhaust systems (e.g., automobiles).Although there is considerable awareness of such problems and much attention is given to the materials requirements for high-temperature systems, failures still occur and can be catastrophic (fire, explosion, burst tube, etc). Plant conditions and control are particularly important since damage can occur due to overheating and overstressing when a material is taken beyond its physical, mechanical, and chemical limits or due to so-called dew-point corrosion when the system is cooled and acidic vapors condense to electrolytically corrode unprotected surfaces.The oxidation of metals and alloys in oxygen at high temperatures is now reasonably well-understood. Indeed, alloys have been successfully developed to resist environmental attack in such atmospheres. The oxidation resistance of these materials is due to the presence of alloying elements—such as chromium, aluminum, and silicon—which can react with oxygen to form stable, protective, external oxide scales on the component surface, thereby inhibiting further high-temperature degradation.

Temperature Dependence of Steel Wastage in a Bubbling Fluidized Bed Simulator

Abstract Tubes within bubbling fluidized bed combustors have in many instances suffered wastage. The wastage can be quite high at temperatures near 300°C, but it typically shows an abrupt decrease at approximately 400°C. Superheater tubes that operate at higher temperatures generally do not experience wastage. It is widely believed that this decrease in wastage with temperature is due to the development of a continuous oxide layer that protects the metal substrate by virtue of its hardness and resistance to spalling. In this study, the temperature effect is examined using a wear rig specially designed to simulate the impact conditions relevant to in-bed tubes. It was discovered that wastage for mild steel can decrease from a relatively high value to essential zero within the temperature range of 400 to 430°C. This decrease was attributable not to the presence of an oxide scale but to the development of a protective deposit layer. The deposit consisted of an agglomeration of submicron bed material particle...

Effects of tube orientation and gas velocity on the erosion of a tube inserted into a fluidized bed

Erosion of in-bed tubes, water wall and downstream heat transfer surfaces by bed materials and elutriated particles has persistently plagued most fluidized bed combustion (FBC) systems. This paper presents a systematic experimental study of the effects of tube arrangement and flow condition on embedded tube erosion by using erosion-prone wax cylinders in a 21 cm × 21 cm bench-scale cold fluidized bed. The erosion tests were conducted with 550 μm microbeads glass shot at various combinations of superficial fluidizing velocity, tube orientation and location, tube-to-distributor clearance, and tube circumferential angle. It was found that tube erosion occurs at a threshold fluidizing velocity (26 cm s −1) close to the minimum fluidizing velocity (19 cm s −1), and increases almost linearly with increasing superficial velocity. The specific erosion rate of embedded wax cylinders is high and in the order of 1 mg cm −2 h −1. Under the same condition, the weight loss of in-bed horizontal tubes (0°) is the maximum, followed by 30° inclined tubes, 60° inclined tubes, and vertical tubes (90°). The erosion around a horizontal tube showed that the very bottom part of the tube experienced the heaviest erosion, which amounted to 5 times that of the tube top.

Some theoretical aspects of mixing in the British coal twin-bed combustor/pyrolyser

British Coal, in collaboration with Senior Green Ltd., has incorporated a novel twin-bed coal-fired combustor into a 2.8 MWt water tube boiler. The firing system comprises two interconnected shallow fluidized beds operated as a pyrolyser and a char combustor, respectively. Gas temperatures well in excess of bed temperatures are achieved when the hot gas streams from the two beds are mixed in a common chamber. Particular attributes of the system are high combustion intensity (without in-bed tubes), inherently low NOx emission levels and wide turndown range (up to 10: 1). As part of the ongoing R&D programme, mathematical modelling techniques are being applied to provide a sound theoretical understanding of the physical and chemical processes occurring within the system. In order to assess the factors affecting scale-up of the twin-bed unit, Gloucestershire College of Arts and Technology (GLOSCAT), in collaboration with the Coal Research Establishment of British Coal, is currently developing a comprehensive mathematical model of the system. The paper describes one aspect of the system model, namely solids mixing. The application of simple analytic methods has been found useful to describe the process. Assuming uniform conditions in the vertical plane, mixing of coal and sand within a bed has been regarded as a diffusion process, and a system of ordinary differential equations have been established to predict carbon concentration and temperature profiles for each bed. Although a number of simplifying assumptions have been made, the models provide useful sensitivity analyses, allowing an assessment of the factors influencing scale-up, e.g. the effect of mixing rate and char particle size on temperature profiles. Analytic solutions are presented, and the sensitivity analyses are illustrated using standard computer graphics.

MEASUREMENTS OF IN-BED TUBE BUNDLE EROSION AND PARTICLE-TUBE COLLISION FREQUENCY IN A GAS FLUIDIZED BED

ABSTRACT Erosion of an in-bed tube bundle by bed materials (ash, coal, and sorbents) impingement has caused serious problems to many fluidized bed combustion (FBC) boilers. Experimental investigation of the tube erosion processes was made by placing the erosion-prone wax cylinders in a bench-scale, cold fluidized bed to simulate the long term erosion effect. This paper discusses the results of systematic measurements of weight losses of an in-bed tube bundle under various test conditions. The specific erosion rate of immersed wax specimens ranges from 25 to 175 µg/ (cm2 hr) when impacted by 0.55 mm glass beads at a superficial velocity of 75 cm/s. The tube bundle height and configuration were found to have a prominent influence on the mean solid flow pattern and hence the tube erosion. A comparison of the erosion rates of narrow-pitch and wide-pitch staggered bundles, as well as the in-line bundle are presented. Electrostatic impact probes based on the triboelectric effect of moving particles were develop...

Wastage of In-Bed Heat Transfer Surfaces in the Pressurized Fluidized Bed Combustor at Grimethorpe

The wastage of in-bed heat transfer surfaces has recently emerged as a potentially serious problem for both pressurized and atmospheric fluidized bed combustors. The experimental pressurized fluidized bed combustion facility at Grimethorpe has accumulated a considerable quantity of data covering the wastage of in-bed tubes obtained during a total of just over 3600 h of operation. The combustor operated with different tube banks, with some degree of wastage being experienced with each one. The data are described, evidence for possible solutions is presented, and plans for future tests at Grimethorpe to prove these solutions are outlined.