Steam Boiler: Steam Boiler Furnace

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Pressure Fired Boiler

When pressure fired boiler doesn't has an ID Fan (Induced Draft) will may run on positive pressure above several percentage or most of the load variety. In the type of pressure fired boiler, the pressure in furnace ranges when the load is adjusted because of the variance in draft losses together with regard to the load of boiler. Since the amount of firing raises, extra air is delivered by FD Fan (Forced Draft) to raise furnace pressure.

Figure 1 shows the actual design of pressure fired boiler process. An important factor of this type is that the boiler furnace should be flue gastight and also airtight. This is essential to ensure that the quite hot flue gas temperature associated with boiler furnace is unable to leak to environment.

A little leak within these conditions will degrade the material close to it, ultimately doing damage to the boiler furnace walls and also generating a functional risk. Boiler furnace is designed pressure-tight in pressure fired boiler system by using inner casing which is welded or seal somewhere between generating tubes and furnace wall steam. In addition boiler furnace has sealed window to view furnace circumstances and flame.

Figure 1: Pressure Fired Boiler
(Source: Book-Boiler Control System Engineering-G.F Gilman)

Design Furnace Dimension

The firing system can be consists of some components such as burner, combustor, grate, or bed. Design furnace dimension both for shape dimension and area is mainly determined by the dimension of flames and firing devices. Flame impingement should be prevented through provide required clearances burners, that may result in damage and overheating in pressure parts.

The furnace height is decided by firing device type and residence time necessity of specific fuel or volume of furnace. The following table presents a information about furnace residence time for some fuels. Residence time is needed with regard to a particle of fuel to achieve the middle point of furnace exit opening out of the firing device (Centre of heat input, should be accurate), supposing the entire furnace for being at isothermal of Furnace exit gas temperature. The furnace length is determined by flame while the furnace cross section is determined by burners; these considerations are very suitable for design furnace dimension of package boiler which has configuration of gas flow horizontally.

The furnace encounters the maximum temperature of gas inside of boiler. To get the high quantities of heat needed, evaporator surfaces usually are utilized in furnaces due to the excessive rates of heat transfer in boiling process. Tube metal and gas cooling are the best useful solutions.

Carbon steel tubes can be applied for structure in which DNB (departure from nucleate boiling) isn't a probability and also temperature of tube metal seldom go over 50°C above saturation. Considering that no evaporation happens in supercritical boiler, cooling seriously isn't efficient and also temperature of water closes to crucial stage namely 374°C, therefore needing low alloy material for furnace tubes.

Table 1: Variety of Furnace Residence Times pertaining to Several Fuels (in Seconds)
(Source: Book-Boiler for Power and Process-Kumar Rayaprolu)

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

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.

General Thermal Design Steps

Thermal design of steam boiler is the process to design furnace, heat transfer surface, control steam and water temperature, and select suitable burner or without burner method. Design furnace is calculation process to reach the best size and configuration of furnace to perform combustion completely. Furnace can be designed based on some parameters; wall construction, pressure, heat transfer, waste heat recovery, fuel and firing methods.

After design furnace, the next step to design thermal in steam boiler is calculate heat transfer surface in wall tubes, superheater, reheater, economizer, and air heater. The optimum heat transfer is the objective of this step to determine the requirement a number of tubes needed for the listed pressure parts above.

After design furnace and heat transfer surface, the next important step is control water and steam temperature. Control water temperature is used in economizer while steam temperature is performed in steam outlet temperature in superheater and reheater. Control steam temperature is responsible by desuperheater or attemperator.

Then design burner equipment for selected fuel (gas, oil or pulverized coal) and design without burner if steam boiler use stoker firing is the other necessary step to do thermal design process.

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

Classification of Furnace Steam Boiler

Furnace of steam boiler is the space in which combustion of fuel take place. Furnace can be classified in many types. The general classifications of furnace steam boiler are as follow:

§ Based on wall construction:

1. Air cooled refractory setting

2. Solid refractory setting

3. Water wall / water cooled

Fig. 1: Furnace Room of Steam Boiler

§ Based on pressure maintained:

1. Balance draft furnace

2. Pressurized furnace

3. Supercharged furnace

§ Based on fuel:

1. Coal furnace

2. Oil furnace

3. Gas furnace

4. Baggase furnace, etc

§ Based on firing method:

1. Hand fired furnace

2. Stoker fired furnace

3. Pulverized coal fired furnace

§ Based on heat transfer:

1. Radiation furnace

2. Convection furnace

§ Based on waste heat recovery:

1. Recuperative furnace

2. Regenerative furnace

Steam Boiler Furnace

The combustion furnace releases the heat and becomes the heat transfer system. There are three important things required for combustion to take place in the furnace: time, temperature, and turbulence. The control of the furnace draft is required to maintain a negative pressure in the furnace in a balance draft of steam boiler. This pressure is defined by steam boiler manufacturer. Negative 0.5 inches is a common control point. The control set point may be raised during inspection rounds from 0.5 inches to 1.0 inches to minimize the possibility of flame coming out of inspection doors.

The furnace is enclosed by a system completely welded gas tight and water cooled designed to be an integral part. The water cooled furnace envelopes constitutes the principal steam generating surface and proportioned for required cooling of combustion product to predicted furnace exit temperature.

The circulation of furnace envelopes tubes is designed for unrestricted upflow of water and steam, terminating in the steam drum where steam, terminating in the steam drum where steam separation take place. Steam-free water, sub cooled by feedwater, returns to bottom distribution headers through unheated downcomers and large diameter feed pipes.

The furnace is designed with sufficient volume to provide for complete and efficient combustion of all fuels at all loads without flame impingement on superheater elements and on furnace water walls.