Steam Boiler: Steam Boiler Combustion

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Combustion Flames of Boiler

Combustion flames of boiler are produced as soon as gas fuel or the volatiles in the coal, liquid fuel which form droplets, solid fuel which form small particles are put together with air, after that they are ignited. Combustion flames can remain and pass on via the mixture among certain top and bottom restrictions of air/fuel proportions. These are recognized as the top and bottom flammability restrictions or explosive restrictions. Combustion flames of boiler are different generally between one fuel to other fuel and it is influenced by several conditions, for example pressure and temperature.

In case this kind of a mixture among the restrictions is comprised in a extended tube, the combustion flames will probably go alongside it at a certain velocity. This velocity differs from one fuel to other fuel, and it is influenced by the ratio of fuel/air and also actual conditions such as tube diameter, temperature, pressure, along with the way; down-wards or up-wards of combustion flame journey.

The velocity of combustion flames in the steam boiler are necessary in vaporizing and gas burners, especially in which element of combustion air is pre-combined together with fuel and goes directly into the furnace through a tube. When combustion flames velocity is higher than the mixture velocity within the pipe, the combustion flames could burn up again and also display returning down the supply pipe. Whenever, nevertheless, the combustion flames velocity is significantly lower compared to the mixture out via the burner, the combustion flames can remove through the nozzle and pick up-off. These results, generally known as light-again and raise-off are factors why attention should be obtained in switching burners coming from one particular fuel to a different.

Fuel/air mixes could build up in the boiler furnace or other limited area, and lead to an explosion when it is ignited. Commonly, the extra limited area will make higher impact of explosion. In case gas or oil burners, pulverized fuel drop ignition, serious attention should be obtained to make sure that the total system especially for combustible mixes should be carefully purged prior to an effort is designed to re-turn on the burner. Current programmed systems include flame malfunction recognition with specific purging specifications before any effort at re-ignition. Air fan is normally used to blow and purging through the furnace of steam boiler.

Combustion flames of boiler can be non-luminous or luminous. Coal and oil generate luminous combustion flames with orange color, although LPG and Natural Gas can burn produce non-luminous combustion flames with blue color. Luminous combustion flames are much better during heat transfer through radiation when compared with non-luminous combustion flames. The current steam boiler design enable combustion of any kind of fuel with no essential impact on total output or efficiency. Different designs committed to either oil or coal combustion might be susceptible to overheating on rear tube when fewer heats are shifted in boiler furnace if heating gas.

Burner Management System

Burner Management System (BMS) is intended to make sure a secure, organized performing sequence and procedure of equipment for fuel combustion begin from start-up until shutdown and also to minimize probable faults. The BMS is designed to secure in opposition to failure of combustion equipment as well as related systems. The protection attributes of BMS should be intended to give security in most popular urgent situation; nevertheless, the BMS is unable to substitute a wise operator’s affordable common sense in most conditions. In several stages of function, the system should give allowable interlocks solely to confirm protected startup procedure. When the equipment for firing is on-line, the operator should comply with appropriate protected working procedures.

It's important in which most components of Burner Management System are in excellent being employed and can be operated anytime the burner is operated to give safety. Frequent inspection and servicing in the BMS and also its related equipment is important to get its ongoing protected function. Last and comprehensive design of Burner Management System is needed for each the appropriate design requirements. The BMS consists of procedures needed for complying with Standard Code NFPA 8501 (National Fire Protection Association) for Single Burner Operation.

Burner Management System transmits these kinds of instructions to the firing control procedure:

Quantity of the burners turns on: These types of instructions permit the firing control to choose which usually oxygen set point to apply. Also, they are employed to cut the need to fuel circulation control valves.
Arranged to Light-Off Situation: This order is provided to the firing control procedure, allows the oil/gas stations to light off situations. Whenever the order is taken off, the stations could be launched to automatically.
Arranged to Purge Situation: This order allows the station of firing air to purge situation. On the end of purge, it will certainly head to a lowest if in computerized, or keep when it is operated manually.

The system additionally makes “trip burner” order from the firing control procedure whenever any kind of these disorders occur:

- Burner management instructions are sporadic

- Fuel at a lowest condition.

- The control electric power is dropped.

Boiler Heat Recovery

The greatest heat loss of steam boiler is done by the hot flue gases in the stack. Recovery of several of heat losses can be performed by economizer which utilizes flue gas to heat boiler feedwater or utilize flue gases to heat combustion air via air heater. These methods of boiler heat recovery will certainly increase the overall thermal efficiency and efficiency of steam boiler.

Generally for every 7°C rise in temperature of feedwater, steam boiler can reduce fuel consumption about 1%. For every 20°C rise in temperature of combustion air through air heater, fuel consumption of boiler can be reduced by 1%.

Figure 1: Boundary Layer of Gas Side
(Source: The Boiler Operator Handbook - Graham & Trotmant)

An economizer is an extendable of steam boiler and it is operated at similar pressure, therefore is susceptible to similar protection needs as steam boiler. Moreover circulation of water temperature coming into the economizer should not be very low possibly inducing too much sulfur corrosion, or very high so vapor bubbles form in the tubes. As common guideline temperature of water must be over 55°C and not greater than 20°C below saturation temperature of steam at working pressure of steam boiler that could be attained through steam tables. So as to observe these temps, thermometers are needed at the inlet and outlet of the economizer.

Many heating tools could be developed or designed to utilize preheated combustion air. A tubular air heater in the flue gas flow recuperates heat. Metal air heaters are susceptible to sulfur corrosion if temperatures of flue gas decrease very low. In big installations, stainless steel can be installed to defeat corrosion.

Natural Gas Reforming

Reaction of natural gas reforming is catalytic reaction between natural gas and steam by uses nickel catalyst that is supported by alumina (NI/Al₂O₄). Overall equilibrium reaction of natural gas reforming is endothermic. In industrial world, reaction of natural gas reformation with steam is main process to produce synthesis gas which consists of CO and H₂. If natural gas is represented by CH₄, so main reactions of natural gas reforming can be written in equations as follow:

Equation 1:

CH₄ + H₂O ↔ CO + 3H₂ ΔH_{298 K} = +206.2 kJ/mol
Equation 2:

CO + H₂O ↔ CO₂ + H₂ ΔH_{298 K} = -41.1 kJ/mol
Equation 3:

CH₄ + 2H₂O ↔ CO + 4H₂ ΔH_{298 K} = +165 kJ/mol

At high temperature and low pressure, conversion of CH₄ based on Equation 1 and Equation 3 thermodynamically will increase. Equilibrium reaction of equation 2 is known as water-gas shift reaction which has exothermic properties and does not depend on operating pressure.

The new study of reforming reaction is CO₂ reforming as shown in Equation 4. The reaction is endothermic and conversion of CH₄ thermodynamically will increase at high temperature and low pressure.

Equation 4:
CH₄ + CO ↔ 2CO + 2H₂ ΔH_{298 K} = +247.4 kJ/mol

This last reaction is utilized to get synthesis gas with low ratio of H₂/CO by replace some or all steam with CO₂ in feed gas process. High content of CO₂ can increase carbon forming.

Equilibrium composition in reaction of natural gas reforming, free Gibbs energy thermodynamically can be estimated as shown in Equation 5. For ideal gas, fugacity coefficient (Фi) can be assumed as one, so Equation 5 can be written as Equation 6. Equilibrium composition is reached at minimum free Gibbs energy is mathematically shown in Equation 7.

Where:

a_i : activity of component i

G_i^o : free Gibbs energy in forming component i

n_t : total mol

n_io : initial mol of component i

n_i : mol of component i

y_i : fraction of component i

ɛ : extent of reaction

Фi : fugacity coefficient of component i

Finally, equation of equilibrium reaction can be written as following equations:

K = exp (-ΔG / RT)

K = Π C_i^vi

K = P^ΔvΠ (y_i)^vi

Where:

C_i = equilibrium concentration of component i

K = constant equilibrium reaction

P = total pressure

Δv = difference coefficient of product and reactant

Constant equilibrium reaction (Ki) can be calculated from empirical equation as function of temperature in following equations:

Where: T in Kelvin unit.

Coal Fuel Classification

Coal is fuel that is mostly used as fuel of steam generator in industries and power plant. Based on its hardship, coal fuel can be classified in three types namely anthracite, bituminous and lignite.

Anthracite coal is usually known as hard coal that shiny and black. Semi anthracite coal has ashen color and it is not as hard as anthracite coal. Bituminous coal is usually known as soft coal and it is often used in industrial world because its ignition is easy with long burning. Sub bituminous is also soft coal type but this coal type has lower carbon content than bituminous coal type. Lignite coal has low heating value and high moisture; furthermore this type is seldom used as fuel in steam boiler.

Further analysis is required to find suitable coal as fuel to perform well combustion. Proximate or ultimate analysis is required to identify the best characteristic of coal type. Proximate analysis is used to know humidity, substance that easily evaporate, ash, carbon and sulfur content in coal. Ultimate analysis is used to know elements such as nitrogen, sulfur, oxygen, carbon, ash and hydrogen in coal.

Coal fuel classification is used as consideration to do purchase contract. The method that is used is standard ASTM D 388-38. Coal fuel classification is based on physical properties, carbon value, moisture, mineral matter content in coal.

Coal fuel classification can be categorized in each class and group based on carbon content and volatile matter as follow:

Coal fuel classification if carbon content more than 69% and volatile matter content less than 31%.

Class : Anthracite coal ===> Group : Meta anthracite

Normal anthracite

Semi anthracite

Class : Bituminous coal ===> Group : Low volatile

Mid volatile

Coal fuel classification if carbon content less than 69% and volatile matter content more than 31%.

Class : Bituminous coal ===> Group : High volatile A

High volatile B

High volatile C

Class : Sub bituminous coal ===> Group : Sub bituminous A

Sub bituminous B

Sub bituminous C

Class : Lignite coal ===> Group : Lignite

Brown coal

Description of each group:

- Meta anthracite has carbon content 98-100% and volatile matter content 0-2%.

- Normal anthracite has carbon content 92-98% and volatile matter content 2-8%.

- Semi anthracite has carbon content 86-92% and volatile matter content 8-14%.

- Low volatile has carbon content 78-86% and volatile matter content 14-22%.

- Mid anthracite has carbon content 69-78% and volatile matter content 22-31%.

- High volatile A has heating value more than 14.000 btu/lb.

- High volatile B has heating value 13.000-14.000 btu/lb.

- High volatile C has heating value 11.000-13.000 btu/lb.

- Sub bituminous A has heating value 11.000-13.000 btu/lb.

- Sub bituminous B has heating value 9.500-11.000 btu/lb.

- Sub bituminous C has heating value 8.300-9.500 btu/lb.

- Lignite has heating value less than 8.300 btu/lb.

- Brown coal has heating value less than 8.300 btu/lb.

Forced Convection

Forced convection is a process or form of heat transfer where fluid movement is produced by an additional supply such as suction machine, forced draft fan, induced draft fan, pump, and others. Force convection can occurs in our daily life like air conditioning, heat exchanger, steam boiler and other devices.

Heat transfer process in steam boiler occurs in air heater and economizer. This heat transfer type is forced convection because convection occurs due to flue gases are blown by force draft fan / primary air fan to air heater and economizer.

Forced convection heat transfer can be determined by the following equation (use Newton’s Law of Cooling):

Q = h A (Ts - T∞) or Q = h A ΔT

Where:

Q = forced convection heat transfer rate (W)

h = heat transfer coefficient (W/m² K)

A = exposed area of metal

Ts = temperature of surface heating transfer element (K)

T∞ = free stream temperature (K)

Forced convection is different with natural convection. But sometimes in some cases forced convection is mixed with natural convection. The problem is how to distinguish whether this heat transfer is categorized as forced convection or natural convection. Most engineers use Archimedes number (Ar) parameter to determine dominant convection heat transfer type. The following formula is Archimedes number equation:

Ar = Gr / Re²

Where:

Ar = Archimedes number

Gr = Grashof number

Re = Reynolds number

If the value of Ar more than 1, then natural convection is dominant, if the value of Ar less than 1, then forced convection is dominant.

Residual Fuel Oil

Residual fuel oil is the residual oil of other petroleum products. Residual fuel oil must be free from tendency to corrosion, acid, solid particles and other bond solid materials that may clog or damage the burner. Residual fuel oil also must be free from chemicals that can form flux and crust on the wall boiler furnace.

Some residual fuel oil contains vanadium and sodium in extremely small quantities. This material can result in corrosion when used as fuel for steam boiler or gas turbine. This problem can be solved by using additive magnesium, calcium, zinc, and others. The heating value of residual fuel oil is ranging from 6.260.000 to 6.450.000 Btu/bb.

The viscosity of residual fuel oil is the most important thing to be considered because of the difficulty in handling and atomizing of thick / viscous oil. The advantages of usage residual fuel oil compared with coal are as follows:

The weight of residual fuel oil 20% lighter and its volume 50% smaller with same heating value.
Destruction does not occur in storage.
Free from sudden fire.
Residual fuel oil can be distributed to combustion furnace so without using man power.
Residual fuel oil has higher combustion efficiency and does not produce smoke.
Residual fuel oil does not generate dust or ash which is harmless to surrounding places.
Reducing the cost of cleaning combustion furnace.
Excess water is needed to perform smaller combustion.

Residual fuel oil with high sulfur levels when used in the steam boiler will tend to be corrosive. Therefore the temperature of flue gas from combustion process that passes through air heater and economizer must be sufficiently high so as not to corrode materials. Residual fuel oil with sulfur content of 6% requires temperature 60^oF higher than residual fuel oil with sulfur content of 2%.

Fuel Heating Value

Fuel heating value is the amount of heat energy that is released by fuel in the oxidation process of chemical elements that exist in fuel. Determination of fuel heating value is important to design combustion furnace of steam boiler. Combustion furnace volume can be determined using following formula:

V_cf = (m_f x LHV x η_sb) / h_rf

Where:

V_cf = combustion furnace volume (m³)

m_f = fuel consumption (kg/s)

LHV = Low Heating Value (kJ/kg)

η_sb = steam boiler efficiency

h_rf = heat release rate (kg/m³.s)

From the above formula can be seen that the volume of combustion furnace can be known if the Lower Heating Value (LHV) is known. Fuel heating value can be classified into two types:

Highest Heating Value

Highest Heating Value (HHV) is the heating value that is obtained from combustion of 1 kg fuel with calculate the heat of steam condensation (water that is produced from combustion in the form of liquid)

Lowest Heating Value

Lowest Heating Value (LHV) is the heating value that is obtained from combustion of 1 kg fuel without calculate the heat of steam condensation (water that produced from combustion in the form of gas / steam).

Fuel heating value (HHV and LHV) can be obtained in the following ways:

Taking heating value from existing literature
Obtaining heating value through testing process in laboratory

Fuel heating value can also be obtained by using equipment in the laboratory; it is oxygen bomb calorimeter as shown in Figure 1. Heating value that is obtained by this equipment can be HHV and LHV. The highest heating value (HHV) can be calculated by following formula:

HHV = (T₂ - T₁ - T_RW) x c_v

While lowest heating value (LHV) can be calculated by following formula:

LHV = HHV – 3240 (kJ/kg)

Where:

T₁ = temperature of cooling water before it is turned on (^oC)

T₂ = temperature of cooling water after it is turned on (^oC)

T_RW = temperature rise of wire igniter = 0.05 (^oC)

c_v = specific heat of equipment= 73529.6 (kJ/kg ^oC)

Figure 1: Oxygen Bomb Calorimeter

Boiler Fuel

The fuel can be defined technically as any material that can burn. While commercially, the fuel can be called as any material that has specific calorific value and is able to react with oxygen in air to produce heat. Generally, fuel can be classified into three main types, namely:

Boiler fuel solid
Boiler fuel liquid
Boiler fuel gas

Based on the occurrence, fuel can be differentiated into natural fuels and artificial fuels. The following below is the differences of boiler fuel based on natural fuel and artificial fuel:

Boiler fuel solid

Natural: Wood, peat, lignite, bituminous, anthracite

Artificial: wood charcoal, coke, briquettes, bagasse, palm oil waste, coconut shell

Boiler fuel liquid

Nature: Crude Oil

Artificial: Gasoline, kerosene, fuel oils

Boiler fuel gas

Nature: The gas methane (CH₄), Ethane gas (C₂H₆), carbon monoxide (CO), LNG, LPG

Artificial: Coal gas, water gas, Raymond gas, high gas furnace, coke oven gas, producer gas.

A steam boiler requires heat source at high enough temperatures to produce steam. Fossil fuels used for generating steam are usually burned directly in the furnace boiler, although the heat for the steam boiler may also be in the form of residual heat from another process.

Combustion can be defined as rapid chemical combination of oxygen with combustible elements of fuel. There are only three important chemical elements which can be burned, namely: carbon (C), hydrogen (H) and sulfur (S). Sulfur usually have little meaning as a source of heat but can be an essential element in terms of problems of corrosion and pollution.

Any fuels which contain hydrogen will produce water (H₂O) as one of product results. The water produced can be a liquid, gas or mixture of two phases. If the water formed during combustion of hydrogen in the boiler fuel can be condensed, the amount of heat that can be obtained will be greater than if the water formed in gaseous form.

Therefore, there are two kinds of combustion value or calorific value:

Higher Heating Value (HHV)

When water vapor of combustion is condensed that must take into account of latent heat of evaporation. The value of the boiler fuel liquid combustion is useful for the calculation of heat loss which can be calculated by using the equation:

HHV = 14500 C + 62000(H₂ – (O/8) + 4000 S

Where:

HHV = Higher Heating Value

C =% carbon in boiler fuel

H =% hydrogen in boiler fuel

O =% oxygen in boiler fuel

S =% of sulfur in boiler fuel

Lower Heating Value (LHV)

When water vapor of combustion is condensed and appears entirely in the form of gas so it does not take into account of latent heat evaporation. Lower Heating Value of boiler fuel liquid is useful for the calculation of heat loss which can be calculated by using the equation:

LHV = HHV - 9720 H₂ - 1110W

Where:

LHV = Lower Heating Value

H₂ = percentage of hydrogen in the boiler fuel

W = content of water steam contained in air

Water Tube Boiler

In water tube boilers, boiler feed water flowing through the tubes into steam drum, water drum and header. Water is heated by gas burners or other fuel combustion to form steam and to be distributed into steam drum. Water tube boiler is selected if its steam and the steam pressure are very high as in the case of boilers for power generation.

Water tube boiler which is very modern designed with steam capacity of 4500-12000 kg / hour, with a very high pressure. Lots of water tube boilers are constructed in a package if used fuel oil and gas. For water tube that uses solid fuel, is not commonly designed package.

Characteristics of water tube boilers are the following below:

- Forced, induced and balanced draft help to improve combustion efficiency

- Less tolerant of water quality resulting from water treatment plant.

- Allows for the higher thermal efficiency.

Figure 1: Water Tube Boiler
(Source: United Nation Environment Program, 2008)

Classification of Flow in Evaporation Process

Pattern of flow in the evaporation process can be classified into five sections as follow:

- Single phase liquid flow

- Bubbly flow

- Slag or plug flow

- Annular flow

- Single phase steam flow

Figure 1: Flow Patterns in Evaporation Process

Although it is difficult to know the type of flow inside water wall tubes and the changes position of flow type from one to another, method to explain position of certain flow type is much needed. A method to describe transitional of flow type inside water wall tubes is in the form of mapping type of flow. This type of flow is illustrated in graphical form. The coordinate of graphic is the function of gas phase superficial velocity (j_g) and liquid phase superficial velocity (j_f). The graphic which explain the fluid flow upward in vertical tube has been made by researchers Hewit & Robert in 1969 as figure below.

Figure 2: Mapping of Flow in Vertical Tube

Annular Wispy Flow in Evaporation Proses

Large bubbles occur in slag or plug flow and will be bigger so as to form a cylinder amid tubes and at the inside surface wall tubes still attach liquid. The condition at this region can be called as annular flow. In annular flow, liquid phase inside tube will be increasingly depleted along with increasing quality of steam formed. Thus reach a point where liquid phase at the wall tubes does not exist anymore. This point is called dryout point. But at dryout point the quality of steam is not reach yet saturation steam point.

Figure 1: Annular & Wispy Annular Flow in Evaporation Process

For large heat flux, the liquid in the surface wall tubes will dry out first, while the liquid in the middle of tubes still has not evaporated. Furthermore cylinder flow will be formed with liquid in the middle of tubes. This flow is called as wispy annular flow. This flow generally occurs at condition Departure Nucleate Boiling (DNB) and the quality of steam in this flow is still low.

The limitation of annular flow according the figure above can be seen the equation below:

ρ_f j_f² < 1000 (kg/ms2)

ρ_g j_g² < 168 (kg/ms2)

The limitation of wispy annular is:

ρ_f j_f² < 1000 (kg/ms2)

ρ_g j_g² < 168 (kg/ms2)

Where:

j_g = superficial velocity of gas phase

j_f = superficial velocity of liquid phase

Slag or Plug Flow in Evaporation Process

There are some kinds of flow in evaporation. One of them is slag flow. Along with the increases quality of steam in the water wall tubes, bubbles steam that arises will be greater, thus forming plug or slag inside tubes. The flow in this region is called as plug or slag flow. However, if in the slag flow there are many small bubbles or bubbly, the flow in this region is often called as random or churn flow.

Figure 1: Plug or Slag Flow and Churn Flow in Evaporation Process

Churn flow is formed from the breakage of large bubbles steam in the slag flow. This flow is sometimes referred to as semi-annular flow or slag-annular flow. From the figure above can be known the limitations of slag flow are:

ρ_f j_f² < 5124 (kg/ms2)

ρ_g j_g² < 168 (kg/ms2)

Where:

j_g = superficial velocity of gas phase

j_f = superficial velocity of liquid phase

Thermodynamic Analysis in Furnace

When chemical reactions occur, the bonds between molecules of reagents will be destroyed and atoms, electrons will be rearranged to form product. The energy will be formed when the rapid combustion process is performed by combustible elements. The fuel can be burnt completely if the carbon element contained in the fuel is burnt to be carbon dioxide or all of hydrogen element is burnt to be water and sulfur to be sulfur dioxide.

As an illustration of the theoretical amount of air in the combustion of methane, the combustion product of this reaction contains only carbon dioxide, water and nitrogen.

CH₄ + a(O₂ + 3.76 N₂) → bCO₂ + cH₂O + dN₂

The total mass of each chemical element must be same on both sides of the equation. Although there are different elements of their chemical compounds in the reagent and reaction, amount of moles of reagent with combustion product may differ. Amount of air to supplies sufficient oxygen to perform complete combustion of carbon, hydrogen and sulfur contained in the fuel is called stoichiometric amount of air.

The parameter of equation above are a, b, c, d represents the number of moles of oxygen, water, carbon dioxide and nitrogen. Value 3.76 moles of nitrogen are considered to accompany oxygen. The following value is taken by applying the mass conservative of carbon, hydrogen, oxygen and nitrogen:

C: b = 1

H: 2c = 4

O: 2b + c = 2a

N: d = 3.76 a

So the equation above will be:

CH₄ + 2(O₂ + 3.76 N₂) → CO₂ + 2H₂O + 7.52N₂

Coefficient 2 before (O₂ + 3.76 N₂) is the number of moles of oxygen in the combustion air per mole of fuel not the amount of air. The amount of combustion air is 2 mole oxygen plus 2 x 3.76 moles of nitrogen to give 9.52 mole air per mole of fuel.

Disadvantage Gas Fuel in Steam Boiler

Natural gas as fuel for steam boiler not only has many advantages but also has disadvantages. The following below are disadvantages of steam boiler if using gas as its fuel:

- Location of boiler should near location of natural gas exploration or at least transmission natural gas pipeline can be reached economically.

- If in the economic range distance of transmission natural gas pipeline there is not water which has good quality to be supplied into steam boiler or to be used for cooling system in condenser steam turbine, so additional cost is required to make water treatment or make cooling water treatment such as condenser or cooling water.

- Need additional equipment and instrument to design and install natural gas piping to burner for safety reason because natural gas will be more danger than using oil fuel or solid fuel. So the investment cost for safety requirement is required.

Advantage Gas Fuel for Boiler

There are some fuel classifications; one of them is gas fuel. Almost of gas fuel is from fossil. Using gas as fuel for combustion steam boiler has many advantages. The following below are the advantage using gas fuel in steam boiler:

- Combustion process using gas fuel does not produce ash and soot, so flue gas velocity through tube gaps can be enlarged, thus the tube gaps can be narrowed and diameter of tubes and dimension of boiler to be used can be smaller.

- Because of high velocity through the gaps of tubes, the heat can be transferred higher from gas combustion to water.

- The burner equipment for natural gas can be simpler than use oil or solid fuel because it does not need piping or equipment for heater and atomizer.

- The combustion process using natural gas can perform more perfectly and does not give bad impact to air pollution than use oil or solid fuel.

- The lifetime of boiler using natural gas is longer than using oile fuel or solid fuel because natural gas does not contain sulfur (S), natrium (Na) and vanadium (Va) and does not produce soot and ash.

- Low cost for operation and maintenance.

Bubbly Flow in Evaporation Process

Evaporation is the process to convert water into steam. Evaporation process occurs in steam boiler especially in water wall tubes area. One of evaporation process is bubbly flow. When temperature of water in water wall tubes is equal to saturation temperature of water, the little bubbles will arise especially near metal wall tubes. Saturation temperature of water will be reached faster in area near metal wall tubes because the heat is transferred first from metal wall tubes to water inside. Flow area of little bubbles can be called as Bubbly Flow.

Figure 1: Bubbly Flow in Evaporation Process

In the bubbly flow, bubbles of steam on one side can be small and round and on other side it could be great with a round and flat shape. In this condition the size of the bubbles is not always exactly same, but it is expected have uniform size.

Fuel and Ignition System

Steam boiler needs fuel and ignition system to perform combustion in the furnace. Fuel selection and design ignition system is the most important parameter to do firing perfectly. The following is type of fuel and ignition system which must be considered:

1. Solid fuel such bagasse, coconut shell and wood. It usually uses stoker to carry out the combustion system. The firing process needs perforation and grate to put fuel for combustion. Perforation is used to flow primary air or combustion flow through the slits of grates.

2. Fuel powder for pulverized coal boiler. The fuel is coal powder that be transported by combination of primary air and secondary air and inject it to furnace.

3. Liquid fuels such as fuel oil or diesel fuel. Fuel oil and diesel oil need atomize steam or atomize air to split particle of oil or diesel into very small scale to make firing process easily. Atomize steam or air is injected through nozzle gun in the burner system. Combustion use liquid fuel needs some equipment such as atomizer either for air or steam, pressure system to spray oil, steam and air, and heater system to warm up oil to decrease viscosity of oil.

4. Fuel gas such as natural gas is sprayed into the furnace which is helped by combustion air. Transportation gas fuel need compressor to give pressure in the gas piping.

Pulverized Coal Firing

Steam boiler can be classifiedbase on fuel. One of them is solid fuel such as bagasse, wood and coal. Methods to burning coal can be distinguished by underfeed stoker, overfeed stoker and pulverized coal firing. Pulverized coal firing is generally used for bigger scale of power plant than use stoker firing. The best classification of coal to be used in this method is bituminous type.

The size of coal to be burned should be small and ground so grinding step must be performed by pulverizes coal. Pulverized coal will make combustion occur easily and fast when reach ignition temperature. Pulverized coal should be designed dry by use warm air. Warm Air and pulverized coal are mixed flow though exhauster to furnace and to be burned on temperature about 3000 F. Warm air is taken from heat exchanger process in air heater. Primary air is supplied by force draft fan into pulverizer while secondary fan flow through winbox.

Construction of Water Wall Tubes

Evaporation phase occurs in water wall tubes. Evaporation is the process to convert water into steam. Therefore water wall tubes should be designed and constructed to provide high heat absorption, minimum excess air level and highest boiler efficiency. Construction of water wall tubes should be also constructed to prevent air leakage into steam boiler, eliminate amount of heat losses and permit high heat release and combustion rate in the furnace.

Construction of water wall tubes must provide high quality of the supporting component such as tubes, casing, refractory, lagging, tile, fin, and so on. Best construction will reduce heat loss and maintenance. Construction of water wall tubes can be classified into four types such as:

Figure 1: Tube and Tile Water Wall
Source: See Reference

1. Tube and Tile Water Wall
At the first development of steam boiler, the designer use this type because of limitation of factory or workshop capability to make tubes are rolled into header, water drum or steam drum, so the spacing between tubes is not designed well practically. The effect of this condition is the furnace can not give enough heat transfer surface (see figure 1)

Figure 2: Tangent Tube Water Wall
Source: See Reference

2. Tangent Tube Water Wall
Tangent tube water wall type is similar with tube and tile water wall type but the amount of heat transfer is higher than tube and tile water wall because of higher amount of tubes in this type. But the problem is same with tube and tile water wall type, it is the limitation to give protection in the refractory because it is directly contacted with burner or combustion process (see figure 2).

Figure 3: Studded Tube Water Wall
Source: See Reference

3. Studded Tube Water Wall
This type is continuous development from the above type. The advantage of this type is can give higher efficiency because the construction of studded tube water wall protects refractory effectively. If the corrosion attack the lagging and tubes, so leakage will occurs when flue gas flow through the water wall(see figure 3).

Figure 4: Membrane or Fin Tube Water Wall
Source: See Reference

4. Membrane or Fin Tube Water Wall
Membrane or fin tube water wall is now best design and construction because this type can give more protection in the insulation and give highest efficiency and heat transfer surface (see figure 4).

Reference: Book – Central Boiler Plants – Department of The Army