DAILY NOTES & Key LEARNING POINTS

Deposits in Black Liquor Fired Boiler:

The large quantity and the low melting temperature of the ash make black liquor one of the

most troublesome industrial fuels used for steam and power generation. Check ash melting

Temp of slop.

Deposition of fly-ash on tube surfaces in the upper furnace of recovery boilers is a persistent

problem in many kraft pulp mills. Massive deposit accumulation greatly reduces the boiler

thermal efficiency, may create a corrosive environment at the tube surface, and in severe

cases, may completely plug the flue gas passages, leading to unscheduled shutdown of the

boiler.

Deposits consist of more than 99 wt% water-soluble alkali compounds, mainly sodium sulphate

(Na2SO4) and sodium carbonate (Na2CO3) with a small amount of sodiumchloride (NaCl), and

reduced sulphur compounds, such as Na2S. Potassium (K) is also present as a substitute for

sodium. Check deposits analysis of slop Boiler.

The sticky temperature is an important parameter determining the rate of deposition in the

region upstream of the generating bank. Stickiness is a strong function of deposit temperature,

composition, particle size and velocity. Tube surface conditions also have a great impact on

stickiness. Carryover particles are less sticky when they are covered with a layer of

condensation, and/or when they are mixed with a large amount of unburned black liquor

particles.

In the lower superheater, the flue gas temperature is usually higher than 820 o C (1510 o F).

Carryover particles may still be burning (sparklers) and be at a higher temperature than the

surrounding flue gas. Carryover and ISP are molten droplets, which strike and solidify on the

tubes to form fused and hard deposits. As the deposit grows thicker, the outer surface

temperature increases until it reaches the radical deformation temperature at which point the

surface becomes fluid, slags and thus stops growing. Under such conditions, deposit thickness is

self-limiting, that is, no accumulation occurs after the deposit reaches a certain thickness.

Plugging thus does not occur in this region despite the fused, hard and adherent nature of

underlying deposits.

In the higher superheater and in the region closer to the boiler bank, the flue gas temperature

falls within the deposit sticky temperature zone (Figure 9). Carryover particles and ISP are

always sticky, forming deposits that will continue to grow and will not be self-limiting, since their

surface temperature is lower than the radical deformation temperature. In this region,

therefore, massive deposit accumulation likely occurs if soot blowing is insufficient. The deposits

usually form heavily on the tube leading edge due to the impaction of molten/partially molten

carryover particles. They may eventually bridge the spacing between adjacent superheater

platens.

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Copper Nickle Tubes in Air Heaters:

Copper-nickel tubes from the fan coolers in a nuclear power plant were found to have pitting

corrosion under bacterialdeposits (Fig. 34). Slime-forming bacteria acting in concert with iron-

and manganese-oxiding bacteria were responsible for the deposits.

Pitting corrosion in 90Cu-10Ni tubes from a fan cooler in a nuclear power plant. Pits are located

underthe small deposits associated with the deposition of iron and manganese by bacteria.

Source: Ref 46[corrosion handbook]

It is quite common to have bacterial slime films on the interior of copper alloy heat exchanger

and condenser tubing.

Usually, these films are a problem only with heat transfer as long as the organisms are living.

When they die, however, organic decomposition produces sulfides, which are notoriously

corrosive to copper alloys. Occasionally, NH3-induced stress-corrosion cracking has been

directly attributed to microbial NH3 production.

Pitting. Aqueous solutions of chlorides, particularly oxidizing acid salts such as ferric and cupric

chlorides, will cause pitting of a number of ferrous and nonferrous metals and alloys under a

variety of conditions. The ferritic (400-series) and austenitic stainless steels are very susceptible to

chloride pitting (as well as to crevice corrosion and SCC, which are discussed later in this

section). Molybdenum as an alloying element is beneficial, so molybdenum-containing stainless

steels, such as types 316 and 317, are more resistant than the non molybdenum alloys. However,

most chloride environments require higher alloys containing greater amounts of chromium and

molybdenum, such as Hastelloy alloy G- 3 (UNS N06985), Inconel alloy 625 (UNS N06625), and

Hastelloy alloy C-22 (UNS N06022), for optimum performance. Exceptions are titanium and its

alloys, which show exceptional resistance to aqueous chloride environments (including the

oxidizing acid chlorides), and copper, copper-nickel, and nickel-copper alloys, which are widely

used in marine

applications.

All too often, however, galvanic corrosion caused by contact between dissimilar metals in the

same

environment is harmful. Examples are:

Copper-nickel or stainless steel heat exchanger tubes rolled in plain carbon steel tubesheets

exposed to river water for cooling.

Thermal stress relief is generally not one of the better preventive measures, because

ammoniacal SCC occurs at relatively low stress levels. In fairly mild ammoniacal environments,

such as the cooling tower water system mentioned above, the copper-nickel alloys, particularly

90Cu-10Ni, give good service.

Which is better SS 316 Tubes OR Copper - Nickel (90-10) Tubes

Heat Transfer Capability: Admiralty Brass is included in this table because use it is rated at

1.00 for a wall thickness of 18 BWG and is considered the standard to be used when

making comparisons

Material Tube [BWG]

22 20 18

Brass tube 102 101 100

Cu – Ni 98 96 93

SS316 85 81 74

At the wall thicknesses which are commonly used, the 90/ 10 Copper-Nickel is shown to

be superior. Once placed into service, the heat transfer capability of a condenser tube

deteriorates from the “new & clean” condition due to the occurrence of fouling, scaling,

etc. Cleanliness factors are used to adjust the “new & clean” values for in-service

conditions and are based on the propensity for scaling and bio-fouling of the given

material. The potential for growth of bio-fouling organisms in Type 316 SS justifies the use

of a significantly higher cleanliness factor for the 90/10 Copper-Nickel. Thus, in the area of

heat transfer, the 90/10 Copper-Nickel alloy has a definite advantage.

Biological Fouling: is more likely to occur in recirculating cooling water systems than in

once-through systems and is a primary contributing factor to crevice-related attack in

susceptible alloys. Bio-fouling can also have a significant adverse effect on heat transfer

capability. All copper alloys are resistant to bio-fouling. By comparison, all stainless steel

alloys are susceptible to bio-fouling. Manufacturers of stainless steels recommend

increased flow rates, but chlorination is usually still required

General Corrosion Resistance: Both materials would be rated as “Excellent” because

condenser tubes never fail prematurely from general corrosion. Rather, premature failure

occurs because of a selective form of corrosion attack.

Erosion-Corrosion Resistance: The potential for erosion-corrosion is directly related to

cooling water velocity. Both materials are good, but Type 316 SS is better. 8 fps is

recommended as the maximum design water velocity for 90/10 Copper-Nickel in

seawater, and 10 fps max is recommended for fresh water.

Sulfide-Related Pitting Attack: is the “Achilles Heel” of all copper alloys. The reader is

cautioned against the use of 90/10 Copper-Nickel in cooling waters containing greater

than 10 ppm sulfides.

Crevice-Related Pitting Attack:has historically been the “Achilles Heel” for stainless steel

alloys. The increased chromium and molybdenum content of Type 316 SSvs. Type 304 SS

improves the resistance of this alloy to crevice-related pitting attack, but exposure to

temperatures above 70oF in stagnant water for any period of time should be avoided as

the alloy’s resistance to crevice attack decreases above this temperature.

Steam Impingement Resistance: Virtually all copper-nickel and stainless steel alloys

exhibit very good resistance to this type of corrosion.

Resistance to Ammonia Attack: Both 90/10 Copper-Nickel and Type 316 SS are

considered suitable materials for the air removal section of the condenser.

Resistance to Stress Corrosion Cracking: Typical environments in which stress corrosion

can occur include ammonia (for copper alloys) and chlorides (for stainless steels). 90/10

Copper-Nickel is essentially immune to stress corrosion in both of these environments. By

comparison, Type 316 SS has been found to be susceptible to chloride stress corrosion.

Compatibility with other System Materials:Galvanic corrosion is a possibility whenever two

dissimilar materials are in contact with one another as is often the case with condenser

tubes and tubesheets. Traditionally, tubesheet materials have been produced from

copper alloys, and 90/10 Copper-Nickel should be slightly favored when copper alloy

tubesheetsare being used. When cathodic protection systems are utilized, the

manufacturer should be consulted if Type 316 SS is selected as the tube material since

too large of a negative potential can result in hydrogen embrittlement of the tube

material.

Type of Corrosion Attack 90/10 Cu-Ni Type 316 SS

General Corrosion Resistance Excellent Excellent

Erosion-Corrosion Resistance Very High Excellent

Resistance to Sulfide-Related Pitting Poor Excellent

Resistance to Crevice-Related Pitting Excellent Fair

Resistance to Bio-Fouling Very High Poor

Steam Impingement Resistance Good Excellent

Resistance to Condensate (Ammonia) Attack Very High Excellent

Resistance to Stress Corrosion Cracking Excellent Fair

Conclusion: Assuming normal standard operating parameters, economics and heat

transfer capability favor the selection of 90/10 Copper-Nickel. Should it be the alloy of

choice, the reader is cautioned to review cooling water flow rates versus the

recommended design velocities for the alloy and to evaluate the cooling water for the

presence of sulfides. Should Type 316 Stainless Steel be the alloy of choice, the reader is

cautioned to avoid exposure to temperatures above 70oF in stagnant waters (e.g.,

during half condenser operation during tube cleaning) and to fully explore the potential

pitfalls of cathodic protection.

Ref. http://olinbrass.com/sites/default/files/downloads/Olin-Brass-Fineweld-Tube-

Technical-Letter-C706-vs-S316_0.pdf

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Desuperheater Selection and Optimization: Turndown represents the variability of the

steam flowrate. Certain processes have a constant steam flow, so turndown is not an

important design factor. Other applications, including power generation and food

processing, require large disparities in steam flow. As a general rule, higher turndown

requirements call for more complex and more expensive desuperheaters.

HEAT EXCHANGERS

Another type of heat exchanger, which is specifically designed to realize a large heat

transfer surface area per unit volume, is the compact heat exchanger. The ratio of the

heat transfer surface area of a heat exchanger to its volume is called the area density .A

heat exchanger with 700 m2/m3(or 200 ft2/ft3) is classified as being compact. Examples

of compact heat exchangers are car radiators (1000 m2/m3).

Air vs. Steam for Soot Blowing in Boilers:

Generally speaking, soot blower manufacturer’s lances, with some modifications, will

handle either cleaning media steam or compressed air. Cleaning energy is usually

defined as:

Fluid Horsepower = WV (P x 144)/33,000

Where: W is Flow in lbs. /min

V is Specific Volume in Ft3/lb.

P is Psig at the nozzle

Other Soot Blowing Operational Considerations:

Tube Erosion

When steam is used as a cleaning medium and a soot blower starts is blowing cycle, there

normally is a temperature differential between the soot blower and the steam. When this

happens, steam condenses and slugs of water are ejected from the soot blower nozzle. After

repeated cycles, the slugs may erode the tubes in the boilers requiring plugging of the tubes

and eventually replacement. Tube erosion can often be a more significant problem in a steam

system than in a compressed air system.

Wall Blowers or IR Blowers

Wall blowers are used to clean the furnace walls and will probably use approximately 2,200 to

2,300 scfm. Wall blowers normally operate in one or more pairs depending on the overall cycle.

Their basic job is to reduce slag that has accumulated on the pipes in the upper levels of the

boiler, and superheated region. When the slag builds up on the pipes the rate of heat transfer

decreases, this will lower the temperature of the steam going to the superheated portion of the

system, therefore decreasing overall efficiency of the system.

A normal cycle for a wall blower starts when it energizes and moves out into the furnace to its

outermost position where the control valve opens and air starts flowing. It takes three seconds to

go from zero to full flow. The wall blower rotates one or more times depending on the amount of

cleaning required and then the control valve closes in three seconds and the blower retracts

into the wall.

The time for a wall blower cycle is approximately three to six minutes and the period of zero flow

between wall blower flow to no-flow to flow can be as high as 1 _ minutes in a normal

sequence. This may be reduced if cycle time needs to be reduced by overlapping.

Long Retractable Lances or IR Blowers

Long retractable blowers are used to clean pendant-type radiant surfaces and convection

surfaces in high temperature zones as well as convection passes to reduce slag buildup on the

walls of the boiler and for temperature control. These areas are normally in the super-heater,

reheater, and the economizer section.

The long retractable lance cycle starts when the lance energizes and extends into the boiler

and air starts flowing with the three-second delay from the control valve. The lance travels

outward and rotates at the same time. The linear speed varies from 65 to 150 inches per minute,

depending on the cleaning and temperature zone requirements.

Air Heaters

The long retractable blowers are often also used in the air heater section. Sometimes a “swing

arm” type blower may be used in the air heater section. This will use a lower magnitude of

compressed air, but will generally require a longer cycle.

System Interlocks

Several safety interlocks can be provided on soot blowing systems. One or more of the interlocks

listed below may be used:

These safety interlocks are used to prevent damage due to low air pressure or flow to the lances,

especially the long retractable lances while they are operating in the boiler.

The loss of a lance will reduce boiler efficiency and eventually will lead to a plant shutdown.

Advantages of Sonic Soot Blowers

Sonic soot blowers are a proven alternative to conventional steam soot blowers in power

generation plants which burn a range of fossil fuels and other waste fuels including bio fuels, and

as a result, suffer boiler fouling and slagging problems.

Depending on the application and boiler plant design, sonic soot blowers usually totally replace

existing high maintenance steam soot blowers whether retractable or rotary. In a few cases,

sonic soot blowers can be used to supplement steam soot blowers.

Mechanisms of Steam Soot Blower Erosion.

written by: Dr V T Sathyanathan • edited by: Lamar Stonecypher • updated: 7/4/2010

There are many mechanisms that can cause steam soot blower erosion of boiler tubes at various

heat transfer sections. Knowing the way these mechanisms contribute to erosion will help to

prevent loss of availability of boiler.

Soot blowers are provided in boilers at various locations like water-walls, superheaters, reheaters,

economizers and air pre-heaters. Steam soot blowers have specific advantage and

disadvantages over other types. The advantages being mainly their low capital cost, operating

cost and the effectiveness of cleaning in areas like furnace, superheaters and reheaters. The

major disadvantages are they need a higher level of maintenance; effectiveness is low in oil

firing mainly in air pre-heater area. They need warm up and condensate draining before startup.

The mechanisms of steam soot blower erosion of heat transfer tubes can be a single factor or

multiple factors acting individually or in unison. There are much more than hundred soot boilers

in boilers generating and supplying steam for a 500 MW and above plants.

Possible mechanisms

o All blowers are set to be set at the right steam pressure recommended by the designer if

this is not done then it leads to poor cleaning or higher rate of tube erosion due to high

steam pressure. This is true for all soot blowers in the boiler starting from furnace to air pre-

heater.

o The alignment of the blower with respect to the furnace walls, superheater tubes,

reheater tubes, economizer tubes and air pre-heater tubes or elements is very critical

and not maintaining this leads to erosion of the tubes and subsequent metal wastage.

The thinning of the tubes finally leads to pinhole failures and many secondary figures due

to this depending upon the orientation of the leak.

o It is required to ensure at least 50 degree centigrade of super heat in the steam being

used for blowing. If the super heat in the steam is lower than required then during

blowing wet steam impinge the tubes at high velocity and the impact force damaging

the heat transfer tubes. This can be identified by the typical spit like metal wastage on

the tubes surrounding the blower’s area of effectiveness.

o The duration of operation of blowers is another main reason for erosion of the heat

transfer tubes. Even if you maintain the correct pressure and temperature the erosion will

take place at a slow phase if duration is more than required.

o In coal fired boiler if alignment is not correct then the ash deposits being cleaned can

get entrained and cause erosion of tubes. However in oil fired boilers it is not a

mechanism that can happen due to the fact that the ash in oil is not significant at all.

o The higher frequency of operation of the soot blowers than needed also leads to tube

erosion.

o Optimizing the soot blower operation is important as operating those blowers where

deposits are not there or very low will lead to metal wastage over a period of time.

o Failure to drain the condensate in the soot blower steam pipes is also contributing

mechanism of tube erosion. The condensate gets entrained in the steam while the

blower operates and has a much higher damaging effect than the lower degree of

superheat in steam.

SLOP [ DISTILLARY SPENT WASH]

On an average 8-15L of effluent is generated for every litre of alcohol produced (Saha et. al,

2005) In India, there are 319 industries producing 3.25 billion litres of alcohol and generating 40.4

billion litres of wastewater annually (Pant et. al, 2007) Molasses spent wash has very high levels of

BOD, COD as well as high potassium, phosphorus and sulphate content contains 2% of a dark

brown pigment called melanoidins that impart colour to the spent wash Melanoidins are toxic to

many microorganisms involved in wastewater treatment High COD, total nitrogen and total

phosphate content of the distillery effluent may result in the eutrophication of the natural water

bodies Spent wash is reported to inhibit seed germination, reduce soil alkalinity, cause soil

manganese deficiency and damage agricultural crops.

FLUIDISED BED BOILER FUNDAMENTALS:

Grate combustion has many more disadvantages than combustion of pulverized coal: lower

combustion efficiency, application limited only to high rank, coarse particle coals, without fine

particles. Bulky and heavy movable parts are exposed to high temperatures. Ash sintering in the

furnace is common. The price of the equipment for flue gas cleaning from SO2, NOx and ash

particles is high compared to the price of the boiler itself and makes the energy production

uncompetitive in the market.

Bulk density of particulate solids is the mass of particles per unit of bed volume. Bulk density is

always smaller than the true density of a solid particle, since the bed volume includes the

volume of voids between the particles. Bulk density depends on the size and shape of the

particles, state of particle surface, density of the solid particle and mode of particle “packing.”

In solid fuel fluidized bed combustion boilers 30–50% of the total generated heat is transferred to

the exchanger surfaces which are in contact with the bed of inert material. Heat transfer

surfaces may be tube bundles immersed into the fluidized bed, or water-tube furnace walls in

contact with the bed

During start-up from such low bed temperatures there are two critical moments. Up to the

temperature of 500–600 °C, there is no combustion of volatiles, so that a larger amount of char

accumulates in the bed than needed for normal boiler operation. When the ignition

temperature of volatiles is achieved, a sudden increase of bed temperature may occur, as well

as bed overheating and even ash sintering and agglomeration of inert material. In order to

prevent such sudden temperature rises, the start-up of the boiler is begun with a smaller coal

flow rate than needed for steady state operation, and coal feeding may be periodically

interrupted, and then restarted, when bed temperature drops are noticed.

The following construction and design solution can reduce the amount of heat and power

needed by the start-up chamber for reasonably fast start-up of FBC boilers:

division of the fluidized bed (furnace) into several sections. Only one, the smaller, section is

heated by combustion products and the fuel is injected in it first. The remaining, larger section of

the furnace does not operate during start-up. Heating of the remaining part of the furnace is

performed by periodical fluidization and mixing of bed material with heated material from the

start-up section. Fuel feeding into this section of the furnace begins when adequate

temperature is achieved,

heat exchanger surfaces are typically not built into the start-up section of the furnace, and even

any furnace water-tube walls are coated with firebricks in this section, and

start-up of the boiler begins using a decreased amount of inert material in the bed, so that the

heat transfer surface remains above or outside this section of the bed.

Fuels which have low char combustion rate (low reactive coals), such as anthracite, coke and

high rank coals burn in FBC boilers with very low efficiency independent of their volatile content.

The basic reason for this is that the residence time of char particles smaller than 1 mm is

insufficient to allow for complete combustion. Losses due to incomplete combustion of particles,

which are elutriated from the furnace, can be very high. When burning these coals it is essential

to recirculate the fly ash particles caught in cyclones or bag filters in order to increase efficiency.

Combustion temperature is the dominant parameter influencing the combustion rate of low

reactive coals, and therefore combustion efficiency, so in these cases a higher bed

temperature must be employed, although this may adversely affect sulphur capture or NOx

emissions.

The maximum temperature cannot exceed 900–950 °C because of problems with ash softening

and possible bed sintering and increased NOx emissions.

In the case of limestone derived beds, if they become flooded with water during the cool down

process, then the bed materials may require major efforts to remove the mass of cement like

material that will form once liquid water is present. For this reason, special attention should be

paid to the processes of erosion and design of any heat exchangers in the bed.

ASME Boiler and Pressure Vessel Codes (BPVC):

Section I – Rules for construction of Power Boilers

Section IV – Rules for construction of Heating Boilers

Section VI – Recommended Rules for Care and Operation of Heating Boilers

Section VII – Recommended Rules for Care and Operation of Power Boilers

Section VIII – Pressure Vessels, Divisions 1 and 2c (rules for construction of pressure vessels

including deaerators, blowoff separators, softeners, etc.)

Section IX – Welding and Brazing Qualifications (the section of the Code that defines the requirements

for certified welders and welding.)

B-31.1 – Power Piping Code

CSD-1 – Controls and Safety Devices for Automatically Fired Boilers (applies to boilers with fuel

input in the range of 400 thousand and less than 12.5 million Btuh input)

National Fire Protection Association (NFPA) Codes

NFPA - 30 – Flammable and Combustible Liquids Code

NFPA - 54 – National Fuel Gas Code

NFPA - 58 – Liquefied Petroleum Gas Code

NFPA - 70 – National Electrical Code

NFPA - 85 – Boiler and Combustion Systems Hazards

Code (applies to boilers over 12.5 million Btuh input)

Suggested Matter and Data to Record in Boiler Air heater outlet air temperature: Monitoring the heated air temperature along with flue gas inlet and outlet temperatures provide an indication of fouling of the heat transfer surfaces, Feedwater temperature: The amount of steam a boiler can generate is dependent on feedwater temperature. Lower temperature feedwater will reduce the capacity of the boiler to generate steam. It has an effect on evaporation rate and overall plant performance. The temperature is also indicative of deaerator performance. Feedwater temperature: The amount of steam a boiler can generate is dependent on feedwater temperature. Lower temperature feedwater will reduce the capacity of the boiler to generate steam. It has an effect on evaporation rate and overall plant performance. The temperature is also indicative of deaerator performance.