Boiler Tube Surfaces Research Articles

A Study on Bituminous Coal Base Acid Ratio to the Slagging Factor at Large Scale Boiler

Different types of coal have different characteristics and performances. In thermal coal plant, deposition of coal ash inside the furnace causes slag formation inside the boiler rear path and consequently reduces the heat transfer process and boiler efficiency. Besides, accumulation of ash on the boiler tube surfaces form layers of slags and blocks the flue gas flow out of the boiler. Therefore, the purpose of this study is to investigate the relationship between sub-bituminous coal base acid ratio towards the heat transfer process inside large-scale boilers. The base acid ratio for sub-bituminous coal is measured before firing inside large-scale boiler of studied the thermal plant which has a generation capacity of 700 MW. This study found correlation between high furnace rear path temperature (FRPT) that is observed to be above 800℃, with the build up of ash accumulation inside the boiler, for the studied coal. Thus, a high base acid ratio causes the accumulation of coal ash, thus reducing the heat transfer process which results in high FRPT of the boiler. Therefore, it proves that a base acid ratio is an indicator for coal performance during firing inside the boiler.

Prediction of biomass ash fusion behaviour by the use of detailed characterisation methods coupled with thermodynamic analysis

Ash deposition such as slagging and fouling on boiler tube surfaces is an inevitable, though undesirable consequence of burning solid fuels in boilers. The role of fuel characteristics, in affecting the form and severity of the problem, is significant. In recent years, biomass fuels have gained increasing popularity as an environmentally friendly source of energy in power plants all over the world. This study is based on characterising the fusion behaviour of four biomass fuels (pine wood, peanut shells, sunflower stalk and miscanthus) using ash fusion temperature (AFT) tests, simultaneous thermal analysis (STA) of fuel ashes, calculation of empirical indices and predicting ash melting behaviour with the help of thermodynamic equilibrium calculations. The AFT results failed to show any clear trend between fusion temperature and high alkali content of biomass. STA proved useful in predicting the different changes occurring in the ash. Empirical indices predicted high slagging and fouling hazards for nearly all the biomass samples and this was supported by the possible existence of a melt phase at low temperatures as predicted by thermodynamic calculations.

Some Boiler Tubes Crack Reason Analysis

Aiming at some boiler tubescrack in a power plant, the element composition and the valence of elements in corrosion products on the outer surfaces of the boiler tubes were analysed by Scanning Electron Microanalyzer (SEM) and X-ray Photoelectron Spectroscopy (XPS), and the corrosion reason and mechanism of the outer surfaces of the boiler tubes were studied.The results showed as follows. (1)The content of element S in the corrosion products on the outer surfaces of the fire side boiler tubes was bigger than that in the corrosion products on the outer surfaces of the back side boiler tubes and the corrosion of the outer surfaces of the fire side boiler tubes was more serious than that of the outer surfaces of the back side boiler tubes. (2) the element S in the corrosion products existed in the form of sulfate (SO42-), because the valence of S in the corrosion products on the outer surfaces of the fire side boiler tubes and the back side boiler tubes was +6, not-2. (3) the element Fe in the corrosion products on the outer surfaces of the fire side boiler tubes and the back side boiler tubes existed in the form of Fe3O4, Fe2O3 or Fe2(SO4)3. (4) The corrosion of the boiler tubes was induced by sulphate under high temperature, and aggravated with the temperature of the outer surfaces of the boiler tubes elevated.

Measurement of the thermal resistance of scale on the inner surfaces of boiler tubes

This paper proposes a method of measurement of the thermal resistance of scale on the inner surface of a boiler tube. The principle of the method is to measure the temperature rise due to the scale when the tube is radiantly heated from outside. An experiment was performed with a test piece composed of three tubes 300 mm in length, which had been cut out from a supercritical pressure boiler. A 120 mm × 140 mm area of the test piece was radiantly heated, boiling of water took place inside the tubes, and the temperature at the crest of the tube was measured with welded thermocouples 80 µm in diameter. Similar measurement was performed after cleaning the inside of the tubes with hydrochloric acid, and both measured data were compared. As an example of the results, the difference of the temperature rise was 8.24 K for a heat flux of 100 kW/m2, and the estimated thermal resistance of the scale was 0.082 m2K/kW. © 2002 Scripta Technica, Heat Trans Asian Res, 31(2): 117–127, 2002; DOI 10.1002/htj.10020

Measurement of the thermal resistance of scale on the inner surfaces of boiler tubes

AbstractThis paper proposes a method of measurement of the thermal resistance of scale on the inner surface of a boiler tube. The principle of the method is to measure the temperature rise due to the scale when the tube is radiantly heated from outside. An experiment was performed with a test piece composed of three tubes 300 mm in length, which had been cut out from a supercritical pressure boiler. A 120 mm × 140 mm area of the test piece was radiantly heated, boiling of water took place inside the tubes, and the temperature at the crest of the tube was measured with welded thermocouples 80 µm in diameter. Similar measurement was performed after cleaning the inside of the tubes with hydrochloric acid, and both measured data were compared. As an example of the results, the difference of the temperature rise was 8.24 K for a heat flux of 100 kW/m2, and the estimated thermal resistance of the scale was 0.082 m2K/kW. © 2002 Scripta Technica, Heat Trans Asian Res, 31(2): 117–127, 2002; DOI 10.1002/htj.10020

Recovery of EDTA from Power Plant Boiler Chemical Cleaning Wastewater

The deposition of iron oxides, metallic copper, and other impurities on boiler tube surfaces causes loss of heat transfer efficiency at electrical power plants. Hence, the tubes are cleaned every 3–5 years in order to restore the heat transfer efficiency. Chelating agents, mainly ethylenediaminetetraacetate (EDTA) and its salts, have been extensively used for the removal of metal deposits from the boiler tube surfaces. Consequently, the boiler chemical cleaning wastewater (BCCW) contains large amounts of iron, copper, and chelating agents. An electrochemical reduction process that reduces metal ions to their elemental state is tested and evaluated in order to provide the potential option to recover EDTA for reuse from power plant BCCW. Results from controlled direct current electrolysis experiments show that the electroreduction process can successfully remove metal ions from the synthetic solution and field BCCW samples and can deposit elemental metals on the cathodic plate. At a current density of 15.5 ...

Oil Ash Corrosion — A Review of Utility Boiler Experience

Abstract A review of experience with oil ash corrosion is presented along with current design practices used to avoid excessive tube wastage. Factors influencing oil ash corrosion include fuel chemistry, boiler operation, and boiler design. These factors are interdependent and determine the corrosion behavior in utility boilers. Oil ash corrosion occurs when vanadium-containing ash deposits on boiler tube surfaces become molten. These molten ash deposits dissolve protective oxides and scales causing accelerated tube wastage. Vanadium is the major fuel constituent responsible for oil ash corrosion. Vanadium reacts with sodium, sulfur, and chlorine during combustion to produce lower melting temperature ash compositions, which accelerate tube wastage. Limiting tube metal temperatures will prevent ash deposits from becoming molten, thereby avoiding the onset of oil ash corrosion. Tube metal temperatures are limited by the use of parallel steam flow and by limiting steam outlet temperatures. Operating a boiler...

Rational use of caustic magnesite dust in the production of refractories

Additionally conducted investigations showed the necessity of elimination of delivery to firing of fine dust from electrostatic precipitators contaminated with low-melting compounds of sulfur and fluorine and circulating in the kiln ⇆ electrostatic precipitators closed cycle, which makes it possible to reduce the unproductive load on the dust removal equipment and discharge of dust into the atmosphere and to eliminate the crust on waste-heat boiler tube surfaces and corrosion of metal and reinforced concrete structures.

Development of a monitoring system for Ash deposits on boiler tube surfaces

AbstractThe etfect of ash deposits on the output of heat flux meters installed on the water wall tubes in pulverized coal fired utility boilers is studied. The objective is to use commercially available heat flux meters as the sensing devices for detecting and monitoring the growth of ash deposits. Reliable performance is shown and an attractive method is described for processing the heat flux meter output into a signal related to the ash deposit only and independent of variation of boiler load and firing conditions in the combustion chamber. Patents are pending for this Monitoring System.

The Use of Gravity Fractionation Techniques for Assessing Slagging and Fouling Potential of Coal Ash

This paper presents a novel approach toward a fundamental understanding of ash deposit phenomena and describes a laboratory bench-scale technique for assessing the slagging and fouling potential of mineral matter in coal in a boiler firing pulverized fuel. Coal ash deposits collected from boiler tube surfaces have been found to differ from the composition of the coal ash as fired; i.e., selective deposition has occurred. Data are presented to show that analysis of the distribution of mineral matter in the coal matrix can account for this difference. The technique that has been developed consists of a gravity fractionation of pulverized coal so that the ash can be analyzed in groups of particles that are capable of unique behavior in the boiler. The collective behavior of individual ash particles during combustion is not necessarily equivalent to the behavior of a mixture of all ash particles as would be determined by conventional ASTM tests.

Experience at Edmonton (GLC) of generating electricity using refuse as fuel

The paper describes a continuous power-generation process incinerating untreated domestic/industrial refuse mix as a sole fuel for electricity production based on conventional power-station practise. Details of the background, design and expected performance of the plant, as commissioned, are given. Problems of initial operations included difficulties with specialised refuse and residuals, handling equipment, as well as catastrophic failures of boiler tubes from flue side corrosion/erosion within an average 4000 operating hours for individual incinerator/boiler heat exchangers. Explanations are provided, and details given, of remedial measures involving considerable modifications to equipment. The experience of final sustained successful operations until January 1975 is discussed, including the qualities and performance of refuse as sole fuel for steam generation. Operating and thermal efficiencies, relevant to energy conversion and stage processes, including ferrous metal recovery from residuals, are noted. Conclusions arrived at relate to successful commercial operations being achieved by attention to obtaining really efficient combustion and balancing the requirement for dealing with the heavy flue-fouling characteristics of refuse burning against the effects of renewed corrosion/erosion on exposed or re-exposed boiler-tube surfaces. The economics of overall operations and achievement of viable large-scale electricity production with refuse disposal are reported.

History and Causes of On-Load Waterside Corrosion in Power Boilers

On-load corrosion affects the mild or low-alloy steel surfaces of boiler tubes where steam is being generated at temperatures up to 360°c, and pressures up to 180 bar. The corrosion rate under these conditions is low in pure water or dilute solutions, but is much higher in more concentrated solutions af either high or low pH. On-load corrosion results from the local formaiion of either concentrated alkaline or acidic solutions. These are generated by the concentration of dilute solutions by boiling when the heated surface is obstructed by a steam blanket, a crevice or a porous deposit.Corrosion due to concentrated alkaline solutions was recognised first in the thirties. The alkalinity of boiler water consequently tended to be reduced and corrosion due to acidic solutions increased during the forties and fifties. The effect of acid-forming contaminants was demonstrated in the sixties and acidic corrosion due to oxygenated chloride solutions was established even more recently. Understanding the mechanism of on-load corrosion has led to the present satisfactorily low incidence of corrosion to boilers.Prospects of changes in water treatment, design or alloy composition are briefly discussed.

Corrosion and Deposits From Combustion of Solid Waste—Part IV: Combined Firing of Refuse and Coal

Combined firing of processed municipal refuse with coal has been investigated in a stoker-fired boiler. The refuse preparation consisted of shredding, magnetic separation of metals, and air classification. The nature of the deposits and the extent of corrosion of simulated boiler tube surfaces was determined by inserting probes into the boiler. For amounts of refuse up to 35 percent on a heating value basis, the corrosion rate of carbon steels was reduced substantially below that found from previous measurements for refuse alone, and below that calculated on the basis of dilution by the coal. The temperature of the boiler flue gases also was found to be an important factor in corrosion. The emissions of sulfur oxides from high sulfur coal were reduced by the action of the alkaline components of the refuse.

Corrosion and Deposits From Combustion of Solid Waste—Part III: Effects of Sulfur on Boiler Tube Metals

Sulfur plays a significant part in the corrosion of heat transfer surfaces in municipal incinerators. The nature of the deposits and the extent of corrosion on simulated boiler tube surfaces has been investigated in a combined field and laboratory study. The corrosion resulting from the accumulation of deposits and the action of flue gases on boiler tube metals has been measured by probes exposed in large municipal incinerators. In addition, laboratory experiments were conducted under controlled environments to provide additional data for determination of reaction mechanisms. The amount of sulfur in the solid waste was found to be a controlling factor in the corrosion, and increasing the amount of sulfur resulted in reduced corrosion rates on boiler tube metals. This effect has important implications for the combined firing of refuse and fossil fuels.

A Corrosion Problem in Large Steam Generating Stations

Abstract Internal corrosion of boiler tube surfaces is a serious problem in the electric utility industry. Such corrosion usually takes the form of a pinhole leak which at first does not affect the boiler operation or attract the attention of the operator in the control room. The pinhole increases in size until it is large enough to cause the tube to be torn apart. Heavy deposits of a black oxide frequently are found in a tube at or near the point of failure. Possible explanations for the presence of this oxide are: (1) Iron removed from the piping and equipment in the pre-boiler circuit may be carried back into boiler when the oxygen-free alkaline environment favors its precipitation as black iron oxide (Fe3O4), and (2) Some factors may prevent the formation of the protective film which is needed to prevent bare metal from coming in contact with water. Protective measures based on both premises were put into use. One method of combatting this type of corrosion involves the maintenance of an optimum conce...