LATEST TECHNOLOGY & REMEDIAL MEASURES FOR ASH DEPOSITION PROBLEMS

Ash deposition problem in conventional coal-fired/fuel oil-fired/FBC boilers including mechanisms, latest technologies, and remedial measures.

Ash Deposition – Fouling & Slagging

Introduction

Uneven ash deposition on fireside steam-generating surfaces of boilers may lead to alarming disturbances in circulation. The situation aggravates with higher steam generating parameters i.e. temperature and pressure.

For once-through supercritical pressure boilers, ash deposits on the water walls of the furnace bring about a large amount of temperature maldistribution. A large aggregate of slags detached from the furnace water wall can cause mechanical breakage of water wall tubes at the furnace bottom.

Fly ash clogs convective gas ducts with the effect that greater resistance is imparted to the motion of flue gas. This may result in a loss of steam-generating capacity as high as 50% of the rated value. Fly ash deposits on convective heat transfer surfaces build up additional thermal resistance and lead to a loss of boiler efficiency.

Process

The inorganic impurities in fuel like Sodium, Sulphur, and Vanadium are responsible for the ash deposition problem in boilers. Fuels used for steam generation contain a large variety of impurities in the form of inorganic material apart from the organic material that provides heat energy. During combustion, these impurities change their chemical form by combining with other constituents in the combustion regime. The effect of such combined materials being formed will be different at different sections of the boiler starting from the furnace to the air pre-heaters. 

At the beginning of the deposition process, when the tube is clean, only vapors of KCl, K₂SO₄, NaCl, Na₂SO_4, and fine particles of Al₂O₃ & SiO₂ will accumulate on the surface in a sticky condition. The deposit surface temperature goes up when the thickness of the deposit increases. Consequently, a molten film on the outer part of the deposit can flow down the side and form drops at the bottom of the deposit. Finally, these drops detach the tube when the surface tension is no longer capable of supporting the weight of the drops.

Classification of Ash Slags

Fusibility of Ash

The fusibility of ash is determined by the amount of different metal oxides and silica present in ash. These metal oxides (CaO, MgO, Fe₂O₃) form eutectics with silica (SiO₂). Usually, ash produced by most solid fuels contains metal oxides in the range of 5 to 40 %. The higher percentage of metal oxides lowers the melting point of eutectic giving rise to low melting ash. Al₂O3+ SiO₂ content higher than 80% tends to increase the melting point of ash, which therefore becomes high melting. The mechanical strength of the primary ash deposit affects the slagging of the heat exchange surface of the boiler.

Strength of Deposit

Mechanisms of Ash Deposition

Condensation

Condensation is the mechanism by which vapors are collected on a surface cooler than local gas. It happens either heterogeneously on the tube surface or homogeneously on the surface. 

The inner layer is formed due to the condensation of low melting ash from flue gases on the tube surface as a thick sticky film that arrests the high melting fine ash particles falling on it.

Saturated with fine ash particles, the molten slag on the tube surface solidifies to form a primary dense layer of slag firmly bonded on the tube surface. The temperature of the outer surface of the primary layer then gradually increases and inhibits further condensation of low-melting ash from flue gas.

Therefore, further deposits of high melting ash particles on the rough outer surface of the primary layer become loosely held and they form a secondary layer.

Thermophoresis

Thermophoresis is a force generated by the temperature gradient between the hot gas and the cold wall affecting the particulate movement towards the cold wall. This phenomenon is observed in mixtures of mobile particles where the different particle types exhibit different responses to the force of a temperature gradient. It is caused by the temperature gradient so the movement of fluid molecules in the hot zone and high energy levels in this region displaces the nanoparticles towards the cold region.

This force transmits gas-borne particles of size less than 10 micrometers to 30 micrometers towards the lower temperature regions, such as a radiant section of the boiler. Thermophoresis is also referred to as Thermo migration, Thermo diffusion, or Soret effect. Soret number is the ratio of temperature difference to the concentration. The rate of change of temperature with displacement in a given direction as with an increase in height gives a database for studying the thermophoresis effect. 

Brownian Diffusion

Brownian diffusion is the characteristic random wiggling motion of small particles resulting from constant bombardment by surrounding gas molecules. This pattern of motion typically consists of random fluctuations in a particle position inside a fluid sub-domain, followed by a relocation to another sub-domain.

It is the dominant particle deposition mechanism for small particles over short distances. Brownian coefficient depends on Boltzmann’s constant (1.38×10-23J/K). The larger the value of this coefficient the more rapid the mass transfer from regions of high to low temperatures.

Impaction

Inertial impaction is the mode of transport for large particles to heat exchange surfaces. The particles subjected to inertial impaction are usually greater than 5-10 micrometers in size.

To impact a force, the large particles must have enough inertial momentum to overcome the drag force of the gas flow on the particles. Smaller particles will tend to succumb to the drag effect and follow the streamlines.

Eddy impaction is another important transport mechanism. It acts on particles in the range of 1-10 micrometers and those susceptible to diffusion and thermophoresis. These particles do not have enough inertia to impact on front side of the tube. However, they may accumulate on the back side of the tube.

Chemical Reaction

Formation of Compounds

Sulfidation

Conditions For Sulfidation

The temperature of the tube surface must be high enough so that mixtures of Na₂S₂O₇ & K₂S₂O₇ exit in the molten state. The exact temperature depends on relative amounts of Na & K in coal. Carbon from unburned coal is unlikely to be sufficiently active to set up a reducing environment at these tube metal deposit interfaces. The decomposition of CO would give a finer, more uniform, and perhaps more active form of carbon within the deposit matrix.

Hydrogen Sulphide/Vanadium

Sulphation

It is a kind of sintering that may occur in the furnace if there is an unfavorable mineral composition in the fuel. If the CaO content in the ash is more than 40% and if SO₂ is present in the flue gas, the process of sintering may start on a loosely deposited layer formed on heating surfaces. This results in the growth of dense/ strongly bonded slag deposits on tube surfaces of water walls, platens, and superheaters.

CaO+SO₂ + 1/2 O₂ = CaSO₄

Ash Deposition in Fuel Oil-Fired Boiler

• Fuel oils burnt in boilers are characterized by low ash content (0.07 – 0.15%), whereas some of them are rich in Sulphur (3 – 3.5%)
• Ash of fuel oil contains an assortment of mineral fractions-mainly the compounds of Ca, Mg, Na, Si, V & Fe
• These specific mineral impurities together with a high content of Sulphur led to ash deposition
• When fuel oil is burned, the combustion products contain all sorts of vanadium oxides i.e., VO, V₂O₃, V₂O_4, and V₂O₅
• Higher valent oxides have lower melting points and hence are more harmful from the point of fouling
• Fouling of waterwall tubes in the form of sticky deposits begins at a temperature above 815K. This sticky film of sodium vanadium (5Na₂O.V₂O₄.11V₂O₅) attracts fly ash particles
• With the increase of surface temperature, the layer becomes more plastic and capable of retaining a higher load of fusible fractions of fly ash
• As a result, the overall process leads to the formation of a strong bonded deposit
• Na/V ratio plays a very important role in determining the characteristics of deposited ash in fuel oil-fired burners. With the increase of this ratio in ash, its sticky property increases. It is found to be maximum when this ratio is in the range of 3.7 – 9.5

Ash Deposition in FBC Boilers

• In fluidized bed combustion solid fuels reduced to 1-6 mm size can be successfully burnt in a fluidized state over a grate at the bottom of which combustion air is blown off
• The velocity of air is so controlled that the fuel particles are lifted off and reciprocated in the vertical flame
• The finer and partially burned coal particles are carried to the upper layer of the fluidized bed, whereupon their flow velocity decreases, and they undergo complete combustion
• The thickness of such bed varies from 0.5 M to 1 M & temp. around 800-1000˚C
• The specific characteristic of a fluidized bed is that it expands in volume by 1.5 – 2 times during operation
• Boiler tubes are placed in the form of inline or staggered tube bundles arranged in and above the bulk of the fluidized bed
• Fluidized bed boilers can be categorized into 2 main groups:

Degree of Fouling

Effect of Gas Flow Velocity on Degree of Fouling

• The degree of fouling is determined by the thickness of loose deposits comprising chiefly of medium fractions (10-30 micrometers)
• Ash particles in this range continuously settle on the tube surface while ash particles in the coarse range (> 30 micrometers) continuously destroy the layer on collision
• Ultimately the final thickness of the deposited loose layer is governed by the dynamic equilibrium between these two processes
• As the flow velocity of flue gas increases, the kinetic energy of ash particles increases & with the rise destructive effect of coarse particles in proportion to the third power of velocity
• Therefore, the net result is that the thickness of loose deposits on the tube surface decreases

Effect of Arrangement of Tubes in Furnace on Degree of Fouling

• The tube fouling depends not only on the type of tube arrangement (staggered/in-line) but also on the longitudinal pitch of bunches
• Under the conditions of identical gas velocity and tube diameter, the fouling coefficient for in-line tubes is about 2 to 3.5 times that for staggered tubes
• It is not advisable to operate the pulverized coal-fired water-tube boiler at gas velocity as low as 3-4 m/s as the degree of fouling of fireside heating multiplies with the fall of flue gas velocity and it increases significantly. Nominal gas velocity at rated load should be 5-6 m/s 

Effect on Heat Transfer Coefficient

Flue Gas Velocity vs Fouling Coefficient

Fouling Coefficient vs Exposure Time

Effect of Coal on Deposit

Technologies Available for Prevention of Deposition Problems in Boiler

• There are some ash behaviour prediction tools like Ash Pro (SM) used to assess the slagging and fouling situations in coal-fired boilers, integrating boiler computational fluid dynamic (CFD) simulations with ash behavior models including ash formation, deposition, deposit growth, and strength development
• These prediction results are very important to assess the overall performance of power plants including fuel quality, ash properties, fouling, slagging, etc.
• The characterization of ash is extremely important to predict the mineral transformation in coal particles during combustion, which controls the characteristics of resulting ash particles
• Computer Controlled Scanning Electron Microscopy (CCSEM) and Scanning Electron Microscopy Point Count (SEMPC) methods are available today to fully characterize ash with respect to the ash deposition process
• As a result, a direct comparison can be made between coal mineralogy and that observed in the ash sample
• This is of fundamental importance as it provides the database from which the relevant ash-deposition models can be constructed
• Thus, it is possible to establish the effect of coal preparation (grinding, pulverizing, and cleaning) and combustion conditions on the size and mineralogy of ash particles

Ash Deposit Characteristics

• Fine, light-colored, reflective, thin layered depositing at W.W. at all types of boilers and dense and molten slag deposit covering patches or entire W.W. area at p.c. fired boilers
• Sticky ash rimming around the burners (called eyebrows) or accumulating on W.W. near Burner
• Highly fused, very porous slag with bubbly appearance (termed vesicular) depositing at 2-3 m above Burner
• Sandstone-like feathered fouling deposits on the front edge of the tube of R.H
• Alternating light grey and brown or red deposits at primary S.H./R.H
• White to reddish brown depositing at L.T.S.H./ECO
• Very fine light brown or grey ash depositing at A.P.H

C.C.S.E.M. AND S.E.M.P.C.

• S.E.M.P.C. involves the systematic microprobe analysis of over 240 points within a suitably prepared sample
• C.C.S.E.M. analysis of ash samples establishes an estimate of the size distribution of major species
• S.E.M.P.C. data is used to establish the amount of liquid phase available in the sample
• Factors which control the probability of deposit initiation are:

1. Size, shape & momentum of ash particle
2. Temperature
3. Amount of liquid phase on the surface of a particle
4. Base/Acid ratio of liquid phase
5. Viscosity & surface tension of liquid phase
6. Structure & nature of metal surface 

Method of Analysis

• The viscosity of various phases is computed by particle basis and data is presented in the form of population Histograms called viscosity distribution
• Viscosity is then calculated at a given temperature
• Usually, the gas temperature in the region of the surface of the target is taken for calculations
• By studying the data one can establish a scale or relative degree of sticking probability of the targeted surfaces
• Ash with a high population of low-viscosity liquid phases will tend to form a deposit on a metal surface than ash with a bulk of amorphous phases with high viscosities
• It is the proportion of the liquid phase and its viscosity in the ash particles that plays the primary role in ash buildup on the tubes
• Coals that form ashes with smaller-sized particles, and lower amounts of surface liquid phases have a low probability of sticking on the targeted surfaces as the initial layer
• From CCSEM and SEMPC analysis, we get the size distribution of the ash particles, the phase distribution, viscosity, and the chemical composition of the liquid phases on a particle-by-particle basis
• Use of this data can be made to gain some insight into the degree or relative degree of sintering the ash particles will undergo within the deposit and that will enable us to assess the degree to which the surface of the deposit will become the dominant factor in the rate of growth
• It is convenient to review the complex process with respect to measurable parameters
• We can use the data from CCSEM and SEMPC analysis to assess the extent to which a captive liquid phase will form and hence the rate of growth

Micro Beam Technology

• Microbeam Technologies Inc (MTI) provides advanced quantitative information on the impurities in fuels, ash behavior, and determining and predicting the effect of ash on power system performance using CCSEM
• MTI processes also identify problematic deposits, slagging, and fouling
• This predicted information is very useful in the reduction of abrasion & erosion, slag flow problems, convective pass fouling, ash resistivity, and deposit strength

Heat Flux Meters

• Heat flux meters can be used to study thermal absorption diagnostics in boilers as a part of the boiler-monitoring systems
• These sensors can be installed at power plants by a supplier of soot-blowing systems though it is difficult to quantify the energy saving obtained from the development and application of such a sensor
• Continuous fouling formation on heat transfer surfaces is a great problem in existing conventional coal-fired utility boilers
• A cost-effective way to minimize this difficulty is the continuous monitoring of fouling tendencies in the boiler as a tool of operation

Targeted In Furnace Injection (TIFI) Technology

• Chemicals are injected into the flue gas system after mixing with water and air
• Magnesium hydroxide slurry diluted with water and then atomized with air is the most common application of TIFI technology
• This mixture is sprayed into the furnace at computer-determined ports that allow for complete coverage of problem areas
• These cause a chemical reaction with existing deposits and affect their physical crystal characteristics
• This chemical treatment program is successfully applied for the inhibition and reduction of slag formation in the superheater, reheater, and furnace wall sections
• This technology is used in western USA coal-fired utility boilers to control slagging. fouling and tube cracking
• This technology involves different forms of fluid dynamics modeling

Pulse Detonation Wave Technology

• This technology is based mainly on a supersonic combustion process
• A shock wave is propagated forward due to energy released in a reaction zone behind it, where an ignited fuel/oxidizer mixture burns and releases energy
• The significant characteristics of the pulse detonation technology are high temperature, high velocity and high-pressure waves producing direct impact, thermal stresses, high reflection and wave reverberations and exceptionally high shearing capacity to remove slag and fouling deposits
• P.D.E. (pulse detonation engines) are very effective for producing detonation waves (strong waves) for removing ash deposition without damaging the boiler
• P.D.E. is powered by an electric motor and injection valves feed the combustible fluid into the ignition section. Instruments mounted on different sections of the device are necessary to check the performance of the engine. These instruments include pressure transducers, thermocouples, and thin film gauges
• Researchers have found that softer slags are removed more easily with multiple waves. The Slag on the top of the tube generally breaks up into smaller pieces when exposed to waves, while the slag on the side of the tube and in the webbing shears off

Internal Cameras

• Internal cameras (3.9-micrometer band) can be used to monitor the fouling problem
• Special infrared cameras for this purpose are designed to scan the fouling by filtering the light from the boiler. This allows the camera to see through the flames to the walls by blocking the appropriate wavelengths
• These types of cameras are hand-held and thus must be used through viewpoints
• Internal cameras can be installed in areas that are prone to ash buildup or can be installed in adequate numbers to cover all relevant areas of the boiler
• If rotatable cameras are used, they will also be able to see multiple places of the boiler

Location/Phenomenon/Counter Measures

A. S.H. & R.H.

External	Internal	Phenomenon	Counter Measures
High Temp. Corrosion	Steam Oxidation	Corrosion by low molten coal Alkali Iron Sulphate Oxidation caused by steam	Thermal spray High chromium material Composite tube Additives

B. Furnace Water Wall

Sulfidation corrosion

Phenomenon	Counter Measures
Corrosion loss caused by hydrogen sulphide in  Reducing atmosphere combustion gas and iron sulphide in deposit ash	Improvement in combustion efficiency Thermal Spray Clad Welding

C. Burners

Oxidation, Sulfidation

Phenomenon	Counter Measures
COxidation /Sulfidation at high temperatures (>1800°F)	Heat resistance material

D. Economiser

Low Temperature Corrosion

Phenomenon	Counter Measures
Sulfuric Acid Dew Point Corrosion	High Cold End Metal Temperature Corrosion Resistance Materialrial

E. CFB BOILERS

• It has been possible to battle out ash deposition problem in the most aggressive environment of CFB with the advent of new spray processes, such as Electric – Arc Wire and HVOF (High-Velocity Oxygen Fuel) and improved coating materials like
• Iron – based alloy containing 30 % Cr
• Low carbon steel similar to the material in the existing weld overlay
• The goal of weld overlay is ultimately to improve the properties of the base metal and cladding is a process where different tougher material is applied than the base metal
• Many CFBs have an external heat exchanger which helps to compensate the changes in load conditions and fuel properties for controlling heat absorption rate in the furnace

Author

Pramod P Kate 

Vyankatesha Engineers and Consultants, Nagpur

Boiler Technical Consultant