#optimizingcombustion — Public Fediverse posts

#BoilerManual #OptimizingCombustion #Section9 #Page22

9. Slag viscosity is a term describing how easily slag will flow at a specific temperature. It is important to cyclone furnaces because the slag must be thick enough to hold coal particles, but it must be fluid enough to run out the furnace when an excess builds up. For the cyclone furnace, we need a coal that has ash which melts and flows at 2600 F maximum.

10. The five conditions which affect coal-ash deposits in your furnace are:

........ 1. ___How much ash is in the coal.

........ 2. ___The make-up of the ash itself.

........ 3. ___Firing method.

........ 4. ___Equipment design.

........ 5. ___Operating conditions of the boiler.

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#BoilerManual #OptimizingCombustion #Section9 #Page21

produced runs out the cyclone, down the furnace, and into the lag tank, for final disposal.

6. The five requirements necessary for slag tap furnaces are:

........ 1. The furnace must be able to maintain temperatures above 3000 F and also be able to tolerate these temperatures.

........ 2. The slag tap furnace must be strong enough to support the extremely heavy molten slag, wherever it tends to accumulate.

........ 3. ___The wall of the furnace should be chemically inactive to the hot slag.

........ 4. ___There must be adequate drainage for the slag.

........ 5. ___The slag needs to be cooled for disposal.

7. The three materials in coal are:

........ 1. ___Combustible material, which should be 15% for cyclone furnaces.

........ 2. ___Ash, which should be a minimum

........ 3. ___Moisture.

8. The four steps that can help prevent iron sulfide formation are:

........ 1. ___Make sure the cyclones are getting enough excess air.

........ 2. ___Keep coal sizing as fine as possible.

........ 3. ___Keep iron oxides to a minimum by making sure cyclone firing is balanced at all times.

........ 4. ___Protect tube surfaces from iron sulfides by keeping stud length at least 1/4" or longer, and by using a good refractory coating that will last.

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#BoilerManual #OptimizingCombustion #Section9 #Page20

Answers for optimizing combustion

1. Combustion is the rapid chemical combination of oxygen with the combustible elements in fuel, that produces heat.

2. The Three T's of Combustion are:

........ 1. ____Time__________________________

........ 2. ____Temperature____________________

........ 3. ____Turbulence_____________________

3. Complete combustion occurs when fuel and oxygen are combined and all the fuel is completely burned. However, not all the oxygen supplied was used. Perfect combustion is when all the oxygen supplied is used. No excess oxygen remains. Perfect combustion would be the ideal i the boiler operation, but instead, excess air is supplied to assure complete combustion.

4. The three areas of controllable heat loss in the boiler are:
........ 1. The first area of heat loss is excess air out the stack. The operator should keep a close eye on the fuel/air ratio to help minimize this problem.

........ 2. The second way to prevent heat loss is to make sure there isn't any unburned combustibles, ash or refuse. This includes combustible gases which shouldn't be allowed out the stack.

........ 3. The third area is radiated heat loss through the unit. If the boiler is kept well insulated, you can help reduce this heat loss as well.

5. Fuel is burned in the cyclone and temperatures reach around 3000 F. This temperature melts the ash into a sticky slag which forms a layer on the walls of the cyclone. This sticky slag layer catches the larger coal particles, while rapidly swirling air scrubs the coal particles with oxygen, causing combustion. Excess slag

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#BoilerManual #OptimizingCombustion #Section9 #Page19

7. What three materials in coal are important to its suitability as fuel?

........ 1. __________________________________

........ 2. __________________________________

........ 3. __________________________________

8. What are the four steps that can help prevent iron sulfide formation?

........ 1. __________________________________

........ 2. __________________________________

........ 3. __________________________________

........ 4. __________________________________

9. What is slag viscosity and why is it critical to the cyclone furnace?

10. Name five conditions which affect coal-ash deposits in your furnace.

........ 1. __________________________________

........ 2. __________________________________

........ 3. __________________________________

........ 4. __________________________________

........ 5. __________________________________

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#BoilerManual #OptimizingCombustion #Section9 #Page18

Questions for optimizing combustion

1. What is the definition of combustion?

2. What are the Three T's of Combustion?

........ 1. __________________________________

........ 2. __________________________________

........ 3. __________________________________

3. What is the difference between complete combustion and perfect combustion?

4. Name three areas of controllable heat loss in the boiler.

........ 1. __________________________________

........ 2. __________________________________

........ 3. __________________________________

5. Explain how the cyclone furnace works?

6. Name five requirements necessary for slag tap furnaces.

........ 1. __________________________________

........ 2. __________________________________

........ 3. __________________________________

........ 4. __________________________________

........ 5. __________________________________

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#BoilerManual #OptimizingCombustion #Section9 #Page17

decreased boiler efficiency as well as the formation of combustible products that can present a hazardous condition in the convection pass and air heaters, as well as in the furnace.

In view of he great number of factors involved in the combustion of any fuel, it is obvious that the specific requirements for the proper combustion of the fuel must be considered a distinct problem. It is possible, however, from the foregoing to draw certain general requirements of proper combustion.

1. The admission of an air supply that will assure sufficient oxygen for complete combustion, (fuel/air ratio).

2. Since complete combustion is not necessarily efficient combustion, it must be secured without permitting the dilution of the products of combustion with excess air, (fuel/air ratio).

3. The air supply should be admitted at the proper time in such a manner tht the oxygen of the air comes into free and thorough contact with the combustible substances of the fuel, (time and turbulence).

4. The gases must be maintained at a temperature equal to or above their ignition point until combustion is complete, (temperature).

In this section of the operator training manual we have reviewed the principles of the combustion process and have examined the inefficiencies which result in combustion losses. Suitability of fuels in relation to cyclone operation as well as the by-products of combustion such as slag and coal ash, were examined along with the effect of operating variables on these deposits.

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#BoilerManual #OptimizingCombustion #Section9 #Page16

COMBUSTION GUIDES

It is necessary to provide the operating personnel with a device to allow manual or automatic proportioning of the amount of air to the amount of fuel.

The level of excess air is one index that is commonly used to determine the performance of the unit and to guide its everyday operation. Excess air is the amount of air supplied over and above that required for theoretically perfect combustion. It is always necessary to supply some excess air to assure complete combustion of the fuel. Any excess air not actually required constitutes a substantial loss in the form of decreased boiler efficiency and thus, a higher fuel bill. On the other hand, operating boiler efficiency and thus, a higher fuel bull. On the other hand, operating with a deficiency of air flow for the fuel being burned can also result in

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Alt = Labeled Fig. 5 Effect of sintering time. The graph is laid out exactly like Fig. 4 except for how the y axis is incremented (0 to 50, in 10s), and has 3 curves like Fig. 4 but the coal type labels aren't here--each curve is marked in respective order: 168 hours, 15 hours, and 4 hours, and the curves all have steeper curves upward, and are marked with small circles at the points where they cross vertical lines from the x axis. The 168 hours curve terminates slightly beyond where the 1500 F and the 40 mark of psi intersect; the 15 hours curve terminates exactly at the intersection of 1600 F and the 50 mark of psi; the 4 hour curve originates just to the left side of the 1500 F mark just above 0 mark of psi, and terminates just beyond where 1700 F intersects with the 30 mark of psi.

#BoilerManual #OptimizingCombustion #Section9 #Page15

flow is controlled by the velocity damper in each cyclone to maintain the proper fuel-air relationship. This air flow is automatically temperature compensated to provide the correct amount of air under all boiler loads. The total air flow to the cyclone is controlled by the windbox to furnace differential pressure, which is varied as a function of load to increase or decrease the forced draft fan output.

Automatic compensation for the number of cyclones in service has been incorporated along with the additional feature of an oxygen analyzer. This gas analyzer serves as an important aid to the operator in monitoring excess air for optimum firing conditions.

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Alt = Labeled Fig. 4 Comparison of sintering characteristics. This graph has its x axis marked in terms of Sintering temperature - F where it increments every 100 degrees from 1400 to 1900. Its y axis is marked Strength of sintered fly ash -- 1000 psi and increments every 4 units from 0 to 20. There are 3 lines on the graph, all originating at the 1500 degree mark just above 0 on the y axis. Topmost line is marked Coal A; middle line is Coal B, and bottom line is Coal C. Small circles mark where their upward curves intersect with each temperature vertical line up to the 1800 F mark, where both Coal B and Coal C end. Coal A ends at the 1700 F mark.

#BoilerManual #OptimizingCombustion #Section9 #Page14

although excess air has no direct effect on deposit strength, the higher gas temperatures caused by increased furnace wall slagging, do affect superheater deposition.

Sintering time, or reaction time, is also a very important factor in determining deposit characteristics. Figure 5 shows that if a deposit is not promptly removed, the strength of the deposit increases many times. Thus, establishing sootblower operating frequency and coverage is also an extremely important facet of the overall problem of ash deposition.

FUEL/AIR MEASUREMENT AND CONTROL

On multi-cyclone installations the feeder drives are calibrated so that uniform fuel flow is delivered for the same master signal. The total air

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Alt = Labeled Fig. 3 Viscosity--temperature relationship. A graph laid out with the x axis in terms of Slag temperature--F incremented every 100 degrees. The y axis is in terms of Viscosity--poise, incremented in logarithmic terms, increment markings as follows: 10, 20, 50, 100, 200, 500, 1000, 2000, 5000 and 10000. There are two curves; the leftmost curve begins on the left just before the intersection of 1000 poise and 2000 degrees, then runs downward to where it stops just after the intersection of 20 poise and 2500 degrees; it's marked Reducing atmosphere.

The rightmost curve starts on the left just past the intersection of 10,000 poise and 2100 degrees, then descends where the curve ends at just under 25 poise where it intersects with roughly 2550 degrees.

#BoilerManual #OptimizingCombustion #Section9 #Page13

nature of ash depositing on boiler surfaces. Deposits are frequently divided into three broad types:

1. Fused slag deposits forming on furnace walls and other surfaces exposed to predominantly radiant heat.

2. High-temperature bonded deposits occurring on convection heating surfaces. especially superheaters and reheaters.

3. Low-temperature deposits occuring on air heaters and economizers.

EFFECT OF OPERATING VARIABLES

Although the predominant factors affecting ash deposition are the amount and composition of the ash, boiler operating conditions have also been demonstrated to affect deposition. Some of the factors that have been studied are excess air, firing method, and deposit-time temperature, which is a function of the gas-tube temperature relationship as well as ash properties.

The effect of excess air variation on viscosity is indicated in Figure 3. It was noted earlier that plastic slag is most difficult to remove from furnace walls, and this figure shows that variations in atmosphere from reducing to oxidizing have a major effect on the nature of the ash. In practical terms this means that care must be exercised in maintaining proper fuel/air ratios at all times. If imbalances are allowed to occur, the slagging may be aggravated. Flame impingement on furnace walls, or operating several cyclones with less than theoretical air required for combustion and others at high excess air levels are typical ways in which this can occur. Increased slagging can also raise temperatures entering the convection pass, which leads to higher gas and deposit temperatures, thereby increasing deposit strength, see Figure 4. Thus,

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#BoilerManual #OptimizingCombustion #Section9 #Page12

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Alt = Labeled at the top as Table 1 Design parameters--coal and coal ash and it's a full page of texts organized into 3 major boxes, then further categorized and subcatagorized to the point that it basically defies alt-text transcription. There is a large box at the top labeled Fuel Characteristics, containing 2 categories labeled Properties of coal substance and Properties of coal ash respectively. Each has a subheader labeled Technological properties and under each of those is an enumerated list of the same. At the bottom of this large box is an arrow pointing to it, which is connected to arrows pointing to the other two boxes laid out below it.

The bottom left box is the next largest of the set, titled Boiler Design and Operation; listed under that are 6 items under which are those items' subdivisions and the best thing to do is to follow the main text regarding these. Off to the right side of this box is a 2-way arrow pointing to itself and the box to the right of it, labeled Sootblower Design and Operation, the smallest of the set and easier to write alt-text for. Like the others, it has an enumerated list of items each with their own subdivision lists.

The items in the smallest box about sootblower design are: 1.Blowing medium; 2.Type of sootblower; 3. Location and spacing of sootblowers {this one has no subdivisions}; Sootblower nozzles; 5. Lance-tube speed; Frequency of blower operation {this item has no subdivisions}. Blowing medium is further subdivided with alphabetic designations, in this order: air or steam; pressure; temperature. Type of sootblower is further divided with alphabetic designations in this order: short retractable; long retractable; fixed position rotating; traveling frame. Sootblower nozzles is further divided with alphabetic designations in this order: type; size; number; angle of attack. Lance-tube speed s further divided with alphabetic designations in this order: rotational; axial.

#BoilerManual #OptimizingCombustion #Section9 #Page11

2,600 F is considered maximum. Somewhat lower temperatures may be desirable for fuels with high moisture content and low heating values.

COAL-ASH DEPOSITION

Ash deposition in various boiler zones is an important factor to be considered by the boiler operator. Initially, ash deposits on furnace walls act as insulation, thereby delaying the cooling of flue gases. This can cause an increase in steam temperature and is one factor that can cause the deposits to advance into normally cooler parts of the boiler. If the deposits are not removed during operation, accumulations forming on the furnace walls may cause excessive gas temperatures downstream, or in some cases, these accumulations may fall and damage pressure components. Accumulations in tube banks may block gas passes and require a boiler outage for cleaning.

The occurrence and severity of ash deposition depend largely on the coal-ash composition and amount of coal-ash, but can be strongly influenced by the method of firing, design of equipment, and operating conditions. Some of the influencing conditions are shown in Table 1.

ASH-DEPOSIT TYPES

A portion of the coal-ash and the combustion by-products is carried by the flue gases through the boiler, regardless of the method of coal firing. Much of the ash passes through the boiler without depositing, or in the case of the slag-tap-furnace, is removed as molten slag. The ash passing through the boiler is subject to various chemical reactions and physical forces which lead to deposition on tube surfaces. Flue-gas particles, metal temperatures, gas velocity, and flow patterns, as well as other factors,

such as particle size and composition, influence the amount and

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#BoilerManual #OptimizingCombustion #Section9 #Page10

2. In order to keep cyclone coal carryover to the furnace to a minimum, keep coal sizing continuously as fine as possible (95% through 1/4" mesh or finer). Also, keep quantities

of primary air to a minimum to keep fires in the cyclone for as long as possible. Operate with as high an air temperature to the cyclone as possible.

3. Keep cyclone firing balanced at all times. Check fuel and air flow calibration for each cyclone frequently to be sure these readings are correct.

4. Other than the above operating variables, additional maintenance should be performed in the cyclone and lower furnace to keep any iron sulfide formed away from the tube surfaces. For this reason it is necessary to protect these surfaces by maintaining stud length (at least 1/4" or longer) and applying a good refractory coating that will remain intact on the tub es for a long period of time.

Item number 4 will be a continuous maintenance item, requiring cleaning and inspecting during outages. The other items are ones which the operator has the capability of controlling. Exercising such control will insure efficient and prolonged operation.

The other important criterion for establishing the suitability f coal for firing in the cyclone is the viscosity of the slag formed from the ash. Since satisfactory combustion of coal depends on the formation of a liquid slag layer in the cyclone, and since ash is removed from the cyclone and primary furnace in fluid form, the viscosity of the slag must permit slag flow at the temperatures experienced within the cyclone and furnace.

Slag will just flow on a horizontal surface at a viscosity of 250 poises. The temperature at which this viscosity occurs (T250) {in the term T250 the number is written as a subscript} is used as the criterion to determine the suitability of a coal from this point of view. The

T250 is calculated from a chemical analysis of the coal ash, and a value of

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#BoilerManual #OptimizingCombustion #Section9 #Page9

Iron sulfide (FeS) can form only in a reducing atmosphere. In other words, an insufficient amount of air exists for complete combustion of the fuel. If excess air is supplied, the iron and sulfur in the fuel will form Fe2O3 and SO2 which are non-corrosive compounds.

The conditions in the furnace change widely and continuously depending upon the extent of carryover from the cyclone and the quantity of air. In order to minimize tube wastage in the cyclone and furnace, the following guidelines have been established:

1. Maintain proper fuel/air ratios at each cyclone. Enough air must be supplied for
complete combustion of the fuel and to avoid reducing conditions. If less than 14
cyclones are in-service, an additional increase in excess air should be made for each out-of-service cyclone.

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Alt = Labeled Fig. 2 Coal suitability for cyclone furnaces based on a tendency to form iron and iron sulfide. It is a graph where the x axis is numbered 0 to 10 incrementally and marked Total sulfide in coal - percentage dry basis. The y axis is in increments of 5s, 0 to 35 and is marked Fe203 divided by Coal ash CaO + MgO ratio. The area below 25 on the y axis where less than the 3 on the x axis is, and below somewhat less than the 7 mark on the x axis but rises no further than somewhat less than 10 on the y axis, is shaded and marked Suitable. The area of the graph outside that area is marked Not suitable.

#BoilerManual #OptimizingCombustion #Section9 #Page8

One of the most important attributes of the slag tap furnace is the coating of sticky ash that covers a portion of the furnace walls near the bottom. The sticky surface of molten ash, deliberately maintained in selected high temperature zones, serves to trap other transient particles. The ash, so collected, drains continuously toward the furnace bottom and is removed through the tap holes. The consequent reduction in the quantity of dust and ash leaving the boiler unit has a definite practical value, since it decreases the amount of dust to be handled by collectors and therefore decreases the size and cost of the dust collecting equipment.

SUITABILITY OF COALS

The suitability of coals is dependent on the moisture, ash and volatile contents of the coal together with the chemical composition of the ash. The volatile matter should be higher than 15% on a dry basis, to obtain the required high combustion rate. The ash content should be a minimum of about 6% to provide a proper slag coating in the cyclone and can be as high as 25% on a dry basis. A wide range of moisture content is permissible depending on coal rank, secondary air temperature, and fuel preparation equipment.

One of the two important criteria for coal suitability is the total amount of sulfur compared to the ratio of iron to calcium and magnesium, Figure 2. This comparison gives an indication of the tendency of the coal to form iron and iron sulfide, both of which are very undesirable in the cyclone furnace. Coals with too high a sulfur content and/or a high iron ratio are not considered suitable.

Observations have been made over many years on the formation of iron sulfide (FeS) and the role it plays during corrosion and wastage of cyclone and furnace tubes in coal-fired boilers. {Additionally, the iron pyrite content in slag is FeS2, FYI}

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#BoilerManual #OptimizingCombustion #Section9 #Page7

SLAG TAP FURNACE

Some of the important requirements for adequate slag tap furnace design are:

1. The slag in the furnace must be kept fluid. The furnace temperature must be high and the slag tap furnace should be designed to withstand the maximum temperature reached during combustion, which is usually in excess of 3,000 F.

2. Since fluid slag is heavy as well as extremely hot, it must be securely contained in those regions of the furnace where it tends to collect.

3. The interior surface of the furnace must be chemically inactive to the constituents of the hot slag.

4. A method must be provided to drain slag from the furnace as fast as it is formed, or at least at frequent intervals.

5. Once the molten slag has left the furnace, it must be cooled to a temperature that renders it suitable for ultimate disposal.

To withstand the high temperatures noted above, all the walls and the floor of the furnace are water-cooled. The molten slag continuously drains from the furnace floor into the slag tank. The molten slag disintegrates as it comes in contact with water in the slag tank, and this final slag product is conveyed to disposal.

The difficulty in tapping slag of high fluid temperature is most evident during low load operation. Under these conditions, even a coal with a slag of meedium fluid temperature may not be suitable for slag tapping, since the furnace temperature may not be sufficiently high to attain the degree of fluidity necessary for tapping.

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#BoilerManual #OptimizingCombustion #Section9 #Page6

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Alt = This image isn't labeled but it has appeared in the manual twice before: as Fig. 14 in Section 2 on page 15, and as Fig. 1 in Section 4 on page 2. It's the poor quality phototype image of a cyclone's innards, with all the same labels as appeared in the other two cases.

#BoilerManual #OptimizingCombustion #Section9 #Page5

450,000 to 800,000 Btu/cu.ft. per hour and gas temperatures exceeding 3,000 F are developed. These temperatures are sufficiently high to melt the ash into a liquid slag, which form as a layer on the walls of the cyclone. The incoming coal particles (except for a few fines that are burned in suspension) are thrown to the walls by centrifugal force, held in the slag, and scrubbed by the high-velocity tangential secondary air. Thus, the air required to burn the coal is quickly supplied, and the products of combustion are rapidly removed.

The release of heat per cubic foot in the cyclone furnace is very high. However, there is only a small amount of surface in the cyclone and this surface is partially insulated by the covering slag layer. Heat absorption rates range from 40,000 to 80,000 Btu/sq. ft. hour. This combination of high heat release and low heat absorption assures the high temperature necessary to complete combustion and to provide the desired liquid slag coating.

The gaseous products of combustion are discharged through the water-cooled re-entrant throat of the cyclone, Figure 1, into the gas-cooling furnace.Molten slag in excess of the thin layer retained on the walls continually drains away from the burner end and discharges through the slag tap opening, to the boiler furnace, from which it is tapped into a slag tank, solidified, and disintegrated for disposal.

By this method of combustion the fuel is burned quickly and completely in the small cyclone chamber, and the boiler furnace is used mainly for cooling the flue gases. Most of the ash is retained as a liquid slag and tapped into the slag tank under the boiler furnace. Thus the quantity of flyash is low.

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#BoilerManual #OptimizingCombustion #Section9 #Page4

combustion. Excess air would not be required if it were possible to have perfect combustion of air and fuel. It is necessary, however, to keep the excess at a minimum in order to hold down the stack loss. The excess air that is not used in the combustion of the fuel leaves the unit at stack temperature. The heat required to heat this air from room temperature to stack temperature serves no purpose and is lost heat.

In summary, there are certain heat loses over which there is no control, and certain others which are subject to some control. The inherent losses are the result of (1) the discharge of the products of combustion at a temperature higher than ambient, and (2) the moisture content of the fuel plus the combination of some of the hydrogen with the oxygen in the fuel.

The heat losses which are controllable by careful operation, can be minimized by:

1. Careful control of fuel and air ratios on a per cyclone basis.

2. Tolerating virtually no unburned solid combustible matter in ash or refuse.

3. Permitting no unburned gaseous combustibles in the exit gases.

4. A well-insulated settling for the steam generating unit to reduce radiation loss.

The efficiency of combustion in a heat exchanger or boiler is 100 minus the sum of the heat losses expressed in percent.

CYCLONE COMBUSTION

The combustibles are burned from the fuel at heat release rates of

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#BoilerManual #OptimizingCombustion #Section9 #Page3

COMBUSTION LOSSES

It may be well to distinguish here between perfect and complete combustion. Perfect combustion is the result of supplying the exact amount of oxygen for burning all of the combustible elements of the fuel and utilizing in combustion all of the oxygen supplied. Complete combustion, on the other hand, results in the combustion of all the combustible elements of the fuel but does not utilize all of the oxygen supplied. If perfect combustion could be accomplished in the boiler furnace, there would be no unavoidable combustion losses. When combustion is complete, but not perfect, there are losses due to the supplying of too great an amount of oxygen, and hence air (4.32 lbs. of air yields 1 lb. of oxygen). The more nearly complete combustion can be made to approach perfect combustion, the less the loss that will occur in burning the fuel.

The real measure of the efficiency of combustion is found in the relationship existing between the amount of air theoretically required for the burning of the fuel and the amount of air actually supplied. In other words, the individual cyclone fuel/air ratios dictate the efficiency of combustion taking place inside that cyclone.

Not all of the Btu's available in the fuel are converted to heat and absorbed by the steam generation equipment. Some of the fuel may be unburned, leaving carbon in the ash, or carbon may be burned incompletely to form some CO instead of CO2. Usually all of the H2 in the fuel is burned. By far the greatest heat loss is that lost up the stack. Since the heat in the fuel is determined from a base of ambient temperature, all the products of combustion must be cooled to the same ambient temperature if all the heat is to be utilized. Higher temperature than ambient exiting the stack then represents a loss.

From a practical point of view, it is necessary to use more than the theoretical air requirements to assure sufficient oxygen for complete

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#BoilerManual #OptimizingCombustion #Section9 #Page2

turbulence, and sufficient time for complete combustion. These factors are referred to as the three T's of combustion.

In the boiler furnace, the heat energy evolved from the combination of combustible elements with oxygen depends on the ultimate products of combustion and not on any intermediate combinations that may occur in reaching the final result.

A simple demonstration of this law is the combustion of 1 pound of carbon with oxygen to produce a specific amount of heat (about 14,100 Btu/lb). The combustion process may be in two steps, first to form CO (carbon monoxide), producing a much smaller amount of heat (3,960 Btu/lb carbon) and second, the combustion of the CO is obtained to form CO2 (carbon dioxide), , releasing 10,400 Btu/lb carbon. However the sum of the heat released in the two steps equals the 14,100 Btu/lb evolved when carbon is burned in one step to form CO2 as the final product.

The combustion of hydrogen is accomplished without difficulty, but the successful combustion of carbon to CO2 requires special measures to assure a continued supply of oxygen in contact with the carbon particles as long as they remain unburned. Not only must there be thorough mixing of the coal particles and air, there must also be sufficient turbulence to remove the combustion products as they form at the surface of the fuel and provide fresh air at the fuel surface for continued combustion. The greater the turbulence the more rapid the process; hence, less time is required for combustion.

Failure to successfully burn carbon completely to CO2 will result in appreciable losses in combustion efficiency and in the amount of heat released by the fuel. Only about 28% of the available heat in the carbon is released if CO is formed instead of CO2.

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#BoilerManual #OptimizingCombustion #Section9 #Page1

Optimizing combustion

The function of the boiler is to obtain the maximum amount of heat from a given quantity of fuel. If this objective is to be realized, it is important you understand the broader principles involved in the combustion process. In this program we will review these principles and also explore the inefficiencies which result in combustion losses. The combustion process beginning in the cyclone will be examined, and we will discuss the suitability of fuels as related to cyclone operation. We will also cover the by-products of combustion such as slag and coal ash, along with the effect of operating variables on these deposits.

PRINCIPLES OF COMBUSTION

Combustion may be defined as the rapid chemical combination of oxygen with the combustible elements of the fuel. There are only three combustible chemical elements of significance - carbon, hydrogen, and sulfur. Sulfur is usually of minor significance as a source of heat, but it can be of major significance in corrosion and pollution problems.Carbon and hydrogen, when burned to completion, combine with oxygen according to the following:

C + O2 = CO2 + 14,100 Btu/lb of C
2H2 + O2 = 2H2O + 61,100 Btu/lb of H2

Air is the source of oxygen in a boiler furnace. he heat released in these combustion reactions is about 14,100 Btu/lb of carbon burned and 61,100 Btu/lb of hydrogen burned.

The objective of good combustion is to release all of this heat while minimizing losses from combustion inefficiencies and excess air. The combination of the combustible components of a fuel with all the oxygen requires temperature high enough to ignite the constituents, mixing or

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