Chapter 2
Specialised Boiler Designs
Compare different designs of heat recovery steam generators (HRSG): Natural
circulation, controlled circulation and once-through (OTSG).
Describe the purpose and use of specialised steam generator duct arrangement,
including air preheater bypasses, economiser bypasses, and air heater circulation
Explain types and applications of fire tube and water tube boilers/steam generators.
Explain the purpose of boiler fittings, including safety devices, drum internals, and soot
blowers.
Once Through Boilers:
Concept:
The mass flow rate thru’ all heat transfer circuits from Eco. inlet to SH outlet is kept
same except at low loads wherein recirculation is resorted to protect the water wall
system
Drum type boiler:
Steam generation essentially takes place in furnace water walls
Fixed evaporation end point - the drum
Steam -water separation takes place in the drum
Separated water mixed with incoming feed water and fed back to ww lower drum by
means of boiler water circulating pumps (BWCPs).
Natural Circulation Boiler
Circulation through water walls by thermo-siphon effect
Controlled Circulation Boiler
At higher operating pressures (just below critical pressure levels), thermo-siphon
effect supplemented by pumps to ensure safety of furnace wall tubes.
Once Through Boiler
Once -through forced flow through all sections of boiler (economiser, water walls &
superheater),Feed pump provides the driving head Suitable for sub critical & super
critical pressures.
Advantages:
Quick response to load changes
Shorter start up time
Better suited for sliding pressure operation
Steam temperature can be maintained over wider load range under sliding pressure
Higher tolerance to varying coal quality
Suitable for sub critical & super critical pressures
Disadvantages :
Higher feed pump auxiliary power consumption
Cannot operate under conditions of condenser leak
Requirements:
Stringent water quality
Sophisticated control system
Low load circulation system
Special design to support the spiral furnace wall weight
High pressure drop in pressure parts
Higher design pressure for components from feed pump to separator.
Characteristics:
Provides quicker response to TG load changes
Supports achievement of better heat rate at lower loads
Higher furnace wall pressure drop and consequent higher feed pump auxiliary power
consumption
Needs ultra-pure quality feed water - Cannot operate under conditions of condenser
leak.
FLUIDIZED BED COMBUSTION:
Fluidized bed combustion is one method of burning coal. In a fluidized bed boiler, there
is a series of modules each containing a bed of inert granular material, such as ashes or
crushed rocks. Crushed coal from 1.6 mm to 6 mm is injected into the bed using an air
stream. Combustion air is introduced underneath the bed through the plenum chamber
and is blown through the entire bed. The combustion air lifts the bed, as well as the
crushed coal, off the supporting grid and the bed becomes fluidized. Initial ignition of the
bed is accomplished using auxiliary burners. Fig. shows a schematic sketch of the
arrangement.
The advantages of fluidized bed combustion are lower boiler equipment costs because
of efficient heat transfer and elimination of pulverizing equipment. SO2 emissions are
also reduced. If crushed limestone is used in the bed it combines with the sulphur in the
fuel reducing the SO2 in the flue gases.
Fluidized bed boilers can burn low-grade fuels in an environmentally acceptable manner
because of turbulent mixing occurring in the fuel bed. This provides good heat transfer.
Fluidized Bed Combustion
Fig. (a) shows combustion in a shallow bed. Volatile matter (VM) rises above the bed and mixes with the
air during combustion. A limited amount of air is supplied to the shallow bed design to limit combustion
and bed temperature.
The deep bed design in Fig. (b) has in-bed combustion as well as volatiles combusting above the bed.
More air is fed to the deep bed design. The evaporator tubes within the bed control bed temperature.
Fluidized bed combustion can take place at temperatures ranging from 800°C to 900°C instead of
standard combustion temperatures of 1600°C to 1900°C for pulverized coal and oil firing. The ability to
burn material at the lower temperatures is very important. It makes it possible to burn low quality fuels.
The lower combustion temperatures also produce lower emissions of NOx, nitrogen oxides.
Circulating Fluidized Bed combustion has given boiler and power plant operators a greater flexibility in burning a wide
range of coal and other fuels. All this without compromising efficiency and with reduced pollution.
In the olden days blacksmiths used to heat the iron by placing it on a bed of coal. Bellows provide air to the coal from
the bottom of the bed. Fluidized Bed combustion is something similar to this.
Fluidized Bed
At the bottom of the boiler furnace there is a bed of inert material. Bed is where the coal or fuel spreads. Air supply is
from under the bed at high pressure. This lifts the bed material and the coal particles and keeps it in suspension. The
coal combustion takes place in this suspended condition. This is the Fluidized bed.
Special design of the air nozzles at the bottom of the bed allows air flow without clogging. Primary air fans provide the
preheated Fluidizing air. Secondary air fans provide pre-heated Combustion air. Nozzles in the furnace walls at
various levels distribute the Combustion air in the furnace.
Circulation
Fine particles of partly burned coal, ash and bed material are carried along with the flue gases to the upper areas of
the furnace and then into a cyclone. In the cyclone the heavier particles separate from the gas and falls to the hopper
of the cyclone. This returns to the furnace for recirculation. Hence the name Circulating Fluidized Bed combustion.
The hot gases from the cyclone pass to the heat transfer surfaces and go out of the boiler.
Bubbling Fluid Bed Boilers (BFB) there are two types of fluidized bed boilers:
• Bubbling fluid bed (BFB)
• Circulating fluid bed CFB)
The bubbling fluid bed boiler (BFB): The particles in the bed are kept in suspension by an upward flow
of air and combustion gases. The bed is in a fluid-like state and is at a distinct level that can be easily
seen. This mixing of air and fuel leads to complete combustion of the fuel. The bed temperature is
between 815°C and 875°C. Depending which type of fuel is used, extra heat transfer surface area may
be added which keeps the bed temperature lower. The heat transfer surface is in-bed tube bundles that
have water flowing through them. The tube bundles can be subject to high erosion rates.
Circulating Fluid Bed Boilers (CFB) A CFB boiler uses more fluidizing air to the bed than a BFB boiler,
and there is no distinct bed level. Fluidizing air causes the fuel and bed material to rise and circulate
through the combustion area. From the combustion area, the fluidizing air goes to the hot cyclone
collector. The solids collected in the cyclone collector are routed back to the bed.
HEAT RECOVERY STEAM GENERATORS: The concept of using waste energy to generate steam is
increasing. This is because of higher fuel costs, and the need to scavenge heat from industrial processes.
Environmental concerns from burning solid fossil fuels like coal have increased the use of cleaner fuels
like natural gas. Gas turbines burn natural gas and turn electrical generators. In power generation, the
waste heat from a gas turbine can supply steam to power a steam turbine. Such combined cycles push
overall power cycle efficiency to nearly 50%.
Differences between HRSGs and Conventional boilers:
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HRSGs use exhaust from a gas turbine as a heat source and do not need a dedicated firing
system (burner, fan, motor etc.)
HRSGs typically do not use fans (draft is from gas turbine exhaust)
HRSGs generate steam at multiple pressure levels to improve heat recovery efficiency
Heat transfer is by convection rather than radiation
HRSGs do not use membrane water walls
HRSGs use finned tubes to maximize heat transfer
HRSG
Conventional Boiler
Type of HRSG :
Three (3) Main Types
•NATURAL CIRCULATION HRSGs
•FORCED CIRCULATION HRSGs
•ONCE THROUGH HRSGs
NATURAL CIRCULATION HRSGs :
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Typically horizontal gas flow and vertical tubes
Tube bundles typically grow thermally down
For gas turbines less than 50 MW, evaporator is shipped to site in single pieces
For larger gas turbines the evaporator is shipped in multiple sections
FORCED CIRCULATION HRSG:
Typically vertical gas flow and horizontal tubes:
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Steam/water mixture circulation through evaporator tubes and to/from drum with a pump
Historically common in Europe due to small footprint
ONCE THROUGH HRSG :
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Typically vertical gas flow and horizontal tubes
OTSGs eliminate the need for steam drums
Phase change from water to steam is free to move throughout the bundle
Compare different designs of heat recovery steam generators (HRSG):
natural circulation, controlled circulation and once-through (OTSG).
CRITICAL PRESSURE:
Natural circulation in a boiler depends upon the difference in the density of a column of water and
the density of a column containing a steam/water mixture. The difference in density produces the
circulation head. At low pressures, the difference in densities is substantial.
The difference in density between water and saturated steam becomes progressively less with
increased pressure and disappears at the critical pressure 22 106 kPa. Critical pressure is
reached when there is no difference in density between water and saturated steam. This is
illustrated graphically in Fig. Above the critical pressure, the density of water and steam is the
same. This means that for a boiler operating at the critical pressure, there can be no natural
circulation, and pumps must be used to provide forced circulation.
Pressure-Density Curve for Water and Steam
Nucleate boiling is a type of boiling that takes place when the surface temp is hotter than the
saturated fluid temp by a certain amount but where heat flux is below the critical heat flux.
Nucleate boiling occurs when the surface temperature is higher than the saturation temperature
by between 40C to 300C.
Steam generators operating above critical pressure are classed as supercritical and steam
generators operating below critical pressure are classed as subcritical. The boiler in Fig. 28 is a
universal pressure boiler built by Babcock & Wilcox. Universal pressure boilers can be designed
for subcritical and supercritical pressures. The single furnace is approximately 15 meters wide
with firing through both front and rear walls. This once-through design steam generator requires
no steam drum or circulating pumps.
Cycle Efficiencies
The advantage of supercritical boilers is the increased thermal efficiency of the overall plant
cycle. A typical plant thermal efficiency of 38%-40 % is obtained with a sub critical cycle. A
supercritical cycle has an overall thermal efficiency of 45%-48%. The overall plant thermal
efficiency increases as the steam generator steam pressure and temperature increase. Other
factors also affect the efficiency such as cooling water temperature and condenser vacuum.
Once-through Circulation
In a once-through boiler, there is no recirculation of boiler water within the unit. The economizer
and superheater in all boilers operate on a once-through principal. In a supercritical boiler, the
boiler or evaporating circuit operates on the once-through principal as well. In a typical unit the
evaporating section surrounds the furnace. Therefore, the major differences in design of the
supercritical boiler surround the design of the furnace tubes.
The water wall tubes of a supercritical unit have a greater temperature rise than standard
subcritical boilers. This is because the steam’s saturation temperature increases as the pressure
increases. There may also a difference in the temperature of adjacent water wall tubes. The
boiler is designed to protect the furnace wall tubes. The flow through the tubes is not adequate to
cool the tube metal at low loads (low steam flows). To accomplish this, manufacturers use
several strategies including:
 Using a boiler recirculation pump for use at low loads and on start-up
 Incorporating a turbine bypass arrangement that recirculates some of the water at low
loads
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Operating the boiler at lower pressures at low loads. This is called a sliding pressure
design. The boiler pressure reaches full pressure only at higher load levels. This design
still uses recirculation using valves or pumps for startup.
Once-through boilers are often called Benson Boilers. Mark Benson patented the concept of the
once-through boiler in 1922. Licenses to build Benson Boilers can be obtained from the Siemens
Company. Some licensees are Babcock-Hitachi, Babcock and Wilcox, Foster Wheeler and
Babcock Borsig. Another once-through design is the spiral-tube Sulzer furnace, technology
owned by ABB of (Switzerland). The Benson design and the Sulzer design look similar. Both use
spiral-wound furnace tubing and separator vessels for start-up. Sulzer license holders include
Korean Heavy Industries (Korea), Mitsubishi Heavy Industries (Japan) and Alstom Power.
Constant Pressure Design A constant pressure type of once-through steam generator is
designed to operate at the same pressure through most of its operating range. For example, the
furnace portion of the boiler is rapidly brought up to operating pressure. It is controlled at this
pressure until it reaches full load. Throttling valves are used to increase the furnace pressure at
low loads. Water and steam are also recycled from the furnace outlet back to the feedwater
pump. The recycle flow is used to keep the flow adequate for furnace cooling. Fig. is a graph of
the flow through a constant pressure design at various loads. It is at full pressure from 20 to
100% load.
Recirculation Flow and Once-through Flow
Full Sliding Pressure Designs:
Sliding pressure designs of supercritical steam generators operate at full pressure for only a
portion of their operating range. They may hit full pressure only for the top 20% of full load
design. There are many variations depending upon the manufacturer of the unit. As is shown in
Fig. 30 the unit can run at partial or sub critical pressures in the lower load ranges. The pressure
increases on a straight line basis as the load increases.
Sliding pressure designs do not require any boiler throttle valves to maintain the pressure at low
rates. A pumped recirculation system is used for low load and start-up operation – usually below
30% of full load. It is needed to supply enough flow to cool the furnace walls and the economizer
section of the steam generator. It also prevents steam from forming in the economizer.
Partial Sliding Pressure Designs Partial sliding pressure designs of steam generators have boiler
outlet throttling valves to keep the operating pressure of the furnace wall system at the design
pressure for most of the load range. The pressure is allowed to slide down for the lower 30% or
so of the load range. Above the 30% range, turbine load control is achieved with the turbine
throttle valves only. This design is well suited for base load operation. Pumped water recirculation
is still required for starting up and for very low loads.
FURNACE DESIGNS :
The major design concern in building supercritical boilers is the once-through operation of the
furnace tubing. The mass flow through the furnace tubes is much lower than in a furnace with
circulation. The mass flow must still be high enough to prevent overheating and departure from
nucleate boiling (DNB) while generating steam at sub critical pressures. At normal loads and at
supercritical pressures the mass flow must be high enough to prevent overheating of tube metal
and variations in steam exit temperatures. The two common methods of overcoming this problem
are spiral furnace wall tubes, and vertical tubes with rifling inside the tubes.
Spiral Furnace Walls:
By using a spiral-wound furnace design, the total number of tubes that encase the furnace is
reduced. The tubes are arranged at an angle and spiraled around the furnace as shown in Fig.
For example, if the tubes are at a 30o angle, the number of tubes is reduced by 50%. The pitch or
spacing between the tubes remains the same as in the vertical design. The path of each tube
includes all four walls of the furnace. This has the advantage of minimizing the temperature
differences of the steam exiting the tubes. As the number of tubes is 50% less than the vertical
design, the mass flow through each tube is higher.
There are several disadvantages to this design. Because the mass flow per tube is higher, the
pressures drop though each tube is also higher. This translates into higher boiler feed pump
power requirements. The furnace is also more difficult to construct, with more tube welds than the
vertical walls. The support system for the walls is also much more complicated to construct than
vertical furnace walls resulting in higher costs.
Vertical Walls
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Less ash deposition on wall
Less mass flow
More number of tubes
More boiler height for same capacity
No uniform heating of tubes and heat transfer
in all tubes of WW
Spiral Walls
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More ash deposition
More fluid mass flow
Less number of tubes
Less boiler height
Uniform heat transfer and uniform heating of
WW tubes
Vertical Furnace Walls:
Vertical furnace walls are also used in supercritical boilers. They have rifled (internally ribbed)
tubing to avoid overheating and DNB at lower pressures. The rifling adds turbulence to the water
and steam mixture in the tube helping to cool the tube metal. Tube rifling is illustrated in Fig. 34
and Fig. 35.
Supercritical Boiler Water Wall Rifle Tube And Smooth Tube
Vertical tube furnace wall designs have a circulation pump for low rates and start up. To ensure
the circulation through each tube is equal, the tubes are fitted with orifices to distribute the flow.
The advantage of vertical furnaces walls is the ease of construction and maintenance. It is also
easier to route tubes around furnace openings.
Describe typical designs, components, and operating strategies for biomass boilers.
Biomass Boilers:
Biomass is anything that is or was alive, such as leaves, grasses, bamboo, vine clippings, sugar
cane, coffee grounds, and rice hulls. Biomass boilers are usually designed to use wood as well.
Wood comes in many forms such as: bark, wood sticks, sawdust, over and under sized wood
chips, and even used wood pallets.
Biomass fuels are most often used in industrial processes where a large supply of energy for
heating and drying is required. The production of pulp and paper, for example, requires large
quantities of mechanical energy for grinding, chipping, and cooking. In order to produce a final
product, the pulp must be dried, usually using steam. These energy requirements, along with the
availability of waste wood products, make firing boilers with wood and biomass products
economical and cost effective.
Steam supplies energy to the food processing industry. Cooking, drying, and canning all require a
source of energy. These processes often leave behind waste products that can be used as fuel.
Some examples are: coffee grounds, sugar cane fibre, coconut hulls, rice hulls, and nutshells.
Often food producers install boilers which burn biomass material. The boilers produce steam,
which can be used as an energy source for the plant. The equipment is similar to that used in the
pulp and paper industry for burning wood by-products.
Some utility plants are fired by biomass fuels. These installations are made economical by either
the high cost of fossil fuels in the area, or by a low cost supply of biomass fuel. Sometimes the
plant is built as a stand-alone unit with a condensing steam turbine, and other times it is
constructed next to a plant that can use exhaust steam.
Biomass is often burned in combination with a fossil fuel such as coal, natural gas or oil. Often
these installations use a traveling grate, with the biomass added to the coal. When biomass is
burned with oil or gas, a traveling grate is used for the biomass, and the gas or oil burners are
located above the biomass fire.
1. HRSG :
https://www.youtube.com/watch?v=hdqrfCN0qbo
2. Once Through Boilers
3.