Transcript Combustion Control for Boilers

Combustion Control for Boilers
1
September 23, 2004
Contents
2
1.
Introduction
2.
Basic type of boilers.
3.
Why need boiler controls?
4.
Combustion control for boilers.
5.
Fuji’s oxygen gas analyzer.
September 23, 2004
1. Introduction
How does a boiler works?
A boiler is a water containing vessel which transfers heat from a fuel
source (oil, gas or coal) into steam which is piped to a point where
it can be used to run production equipment, to sterilize, to provide
heat, to steam-clean, etc.
The energy given up by the steam is sufficient to convert it back
into the form of water. When 100% of the steam produced is
returned to be reused, the system is called a closed system.
Since some processes can contaminate the steam, so it is not
always desirable to feed the condensate back into the boiler. A
system that does not return the condensate is called an open
system.
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1. Introduction
Closed system
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1. Introduction
Open system
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2. Basic type of boilers.
The two main types of boilers are:
1. Firetube
2. Watertube
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2. Basic type of boilers.
1.
Firetube
Fire or hot gases are directed through the inside of tubes
within the boiler shell which are surrounded by water. The
tubes are arranged in banks so that the gases can be passed
through the boiler up to 4 times before passing out the stack.
This system exposes the maximum heat transfer surface to the
water. Firetube boilers are also known as shell boilers and can
produce up to approximately 750 hp or 25,000 lbs of steam per
hour. 80% of boilers in use are of this configuration.
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2. Basic type of boilers.
1.
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Firetube
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2. Basic type of boilers.
1.
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Firetube
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2. Basic type of boilers.
2.
Watertube
Fire or hot gases are directed to and around the outside of
tubes containing water, arranged in a vertical position.
Watertube boilers are usually rectangular in shape and have
two or more drums. The separation of steam and water takes
place in the top drum, while the bottom drum serves as a
collection point for sludge. This system is usually used when
more than 750 hp or several hundred thousand lbs of steam
per hour, are needed.
There are other designs with special configurations, adapting
them to particular applications
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2. Basic type of boilers.
2.
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Watertube
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2. Basic type of boilers.
2.
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Watertube
September 23, 2004
3. Why need boiler controls?
Boiler efficiency relates the boilers energy output to
the boilers energy input and can be expressed as:Boiler efficiency (%) = Heat exported by fluid/Heat provided by fuel
An accurate control of the amount of air is essential to the boiler
efficiency. Too much air will cool the furnace and carried away
useful heat. And too little air and the combustion will be
incomplete. Unburned fuel will be carried over and smoke may be
produced.
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3. Why need boiler controls?
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1.
Increase uptime and availability
2.
Reduce flue gas emissions
3.
Maintain boiler safety
4.
Control operating costs
September 23, 2004
3. Why need boiler controls?
1.
Increase uptime and availability
The primary objective of most boilers operation is
maintaining the uptime and availability. It is essential to
maintain and upgrade the boiler control systems to assure
steam availability.
Modern controls are more reliable and can be readily
adjusts to load swings caused by varying plant operations.
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3. Why need boiler controls?
2.
Reduce flue gas emissions
Failure to comply with the current emissions regulations can
be as costly as loss of utilities. Government mandates are
enforced by fines, threat of closure, or imprisonment will
provide sufficient incentives for plants to comply with the
regulations; thus, modernize controls are necessary.
Improved in combustion efficiency means reduction in
waste disposal problems. And by accurately controlling the
oxygen, fuel flow and stack temperature, you will see
reductions in plant emissions.
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3. Why need boiler controls?
3.
Maintain boiler safety
Modernize control system will have tight integration with
flame safety or burner management system to improve
safety.
Accessing field data, diagnostics functions and alarms can
be achieved by coupling modern electronic controls.
Password security of the configuration software also assures
no unintended changes can be done which can endanger
your personnel and equipment.
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3. Why need boiler controls?
4.
Control operating costs
a)
Reduction in fuel consumption
b)
Reduction in engineering, installation and startup costs
c)
Reduction maintenance costs associated with older
equipment
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a)
Reduction manpower with automatic responds
b)
Provide a flexible control strategy to reduce process upsets
c)
Readily data available for remote monitoring to determine
process optimization, boiler efficiency and load allocations
September 23, 2004
4. Combustion control for boilers.
Burner combustion control generally includes one or a
combination of the following methods:-
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•
Excess air regulation
•
Oxygen trim
•
Burner modulation
•
Air/Fuel cross-limiting
•
Total heat control
September 23, 2004
4. Combustion control for boilers.
1.
Excess air regulation
In actual practice, gas , oil, coal burning and other systems
do not do a a perfect job of mixing the fuel and air even
under the best achievable conditions. Additionally,
complete mixing may be a lengthy process. To ensure
complete combustion and reduce heat loss, excess air has
to be kept within suitable range.
The regulation of excess air provides:-
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•
A better boilers hear transfer rate
•
An advance warming of flue gas problems
•
Excess air coming out of the zone of maximum efficiently
•
Substantial savings on fuel
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4. Combustion control for boilers.
1.
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Excess air regulation
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4. Combustion control for boilers.
2.
Oxygen trim
When a measurement of oxygen in the flue gas is available, the
combustion control mechanism can be vastly improved (since the
percentage of oxygen in flue is closely related to the amount of
excess air) by adding an oxygen trim control module, allowing
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•
Tighter control of excess air to oxygen setpoint for better
efficiency
•
Faster return to setpoint following disturbances
•
Tighter control over flue emissions
•
Compliance with emission standards
•
Easy incorporation of carbon monoxide or capacity override
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4. Combustion control for boilers.
2.
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Oxygen trim
September 23, 2004
4. Combustion control for boilers.
3.
Burner modulation
Modulating control is a basic improvement in controlling
combustion. A continuous control signal is generated by a
controller monitoring the steam or hot water line. Reductions in
steam pressure or hot water temperature lead to an increase in
firing rate. The advantages of introducing burner modulation in
combustion control include.
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•
Fuel and air requirements are continuously matched to the
combustion demand
•
Steam pressure or hot water temperature is maintained within
closer tolerances
•
Greater boiler efficiency
•
Weighted average flue gas temperature is lower
September 23, 2004
4. Combustion control for boilers.
4.
Air/Fuel cross-limiting
A cross limiting combustion control strategy ensures that there can
never be a dangerous ration of air and fuel within a combustion
process. This is implemented by always raising the air flow before
allowing the fuel flow to increase or by lowering the fuel flow
before allowing the air flow to drop.
Cross-limiting combustion control is highly effective and can easily
provide the followings
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•
Optimization of fuel consumption
•
Safer operating condition by reducing risk of explosion
•
Fast adaptation to variation in fuel and air supplies
•
Satisfaction of the plant demand fore steam
September 23, 2004
4. Combustion control for boilers.
4.
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Air/Fuel cross-limiting
September 23, 2004
4. Combustion control for boilers.
5.
Total heat control
A cross limiting combustion control strategy ensures that there can
never be a dangerous ration of air and fuel within a combustion
process. This is implemented by always raising the air flow before
allowing the fuel flow to increase or by lowering the fuel flow
before allowing the air flow to drop.
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4. Combustion control for boilers.
5.
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Total heat control
September 23, 2004
4. Fuji’s oxygen gas analyzer.
Top class performance zirconia oxygen analyzer
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4. Fuji’s oxygen gas analyzer.
Accurate O2 Measurement is Essential for Energy Saving!
The oxygen analyzer consists of a compact zirconia detector that
can be inserted directly in wall of the flowing sample gas. The
detector measures the oxygen content in the flowing sample gas
and transmits the signal to the converter. The converter will then
trigger the ON-OFF alarm based on the preset oxygen
concentration and give control signal to other devices.
Fuji Electric’s oxygen analyzer has a unique construction that
eliminates the necessity of aspirating sampling gas or injecting air.
And make it extremely suitable for monitoring and controlling
combustion system like, heater boiler, kiln, melting furnace, low
oxygen warehouse and food packing machine.
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4. Fuji’s oxygen gas analyzer.
Excess air coefficient and energy loss ratio
Excess Air
Coefficient
Exhaust Oxygen,
O2 (%)
Energy Loss Ratio,
Exhaust Gas (%)
1.1
1.9
9.4
1.2
3.5
10.3
1.3
4.8
11.1
1.4
6.0
12.0
1.6
7.9
13.7
(In the case of heavy oil combustion at exhaust gas temperature of 250 °C and atmospheric temperature of 20 °C)
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4. Fuji’s oxygen gas analyzer.
Calculation of cost saving with improved combustion efficiency
(The data may varies based on the construction and performance of the boilers)
Item
Case 1
Case 2
Evaporation rate from boiler
5ton/hr
1ton/hr
Annual operating hours
2,000hrs
Evaporation multiplier factor for boiler
12
Improved value of excessive air coefficient
1.6 -> 1.2
(O2 gas from 7.9% -> 3.5%)
Kerosene price
USD 1.05/kg (USD 0.82/ℓ, SG 0.78)
Kerosene consumption rate
5,000kg/hr ÷ 12 = 420kg/hr
2,000kg/hr ÷ 12 = 85kg/hr
Annual saving through the improved
combustion efficiency
420kg/hr x USD 1.05/kg x
(13.7 – 10.3) % x 2,000hrs
= USD 29,988.00
85kg/hr x USD 1.05/kg x
(13.7 – 10.3) % x 2,000hrs
= USD 6,069.00
Note: The data shown in the above table are calculated on an assumption of improvement of energy loss = full reduction ratio,
therefore the combustion efficiency of the boiler must be taken into account for calculating fuel reduction rate accurately. Fuel
reduction ratio will therefore be least several percent higher in actuality.
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4. Fuji’s oxygen gas analyzer.
Advantages:
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1.
No sampling device is required
2.
Compact and light weight design
3.
Instrument equipped with indicator and transmitting function
4.
Alarm and control functions available
5.
Lost cost
6.
Easy maintenance
The instrument requires no gas aspirating pump or ejector for normal measurements; therefore it
can be operated easily. It can be used very conveniently like traditional thermocouple.
The detector and converter weigh about 1.6kg and 3.5kg respectively.
The converter is equipped with an indicator that permits direct readout of the oxygen concentration
transmitting output function or RS485 communication.
Though compact it is compact and lightweight in design, the converter consists of an oxygen
concentration setting mechanism as well as alarm setting and control circuits that can transmit
control signals.
Comparing Zircomat-P with other conventional oxygen analyzers it is much more economical in cost.
Zircomat-P assures easier maintenance comparing to other conventional oxygen analyzers and can
be used under severe site conditions for a long time.
September 23, 2004
4. Fuji’s oxygen gas analyzer.
System Outline
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4. Fuji’s oxygen gas analyzer.
Technical Specifications
Measuring range
Repeatability
Linearity
Response time
Power Supply
Power consumption
Warm up time
Type
Detector
Applicable gas temperature
Converter
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0 to 2 ~ 50%
Within ± 0.5% maximum output signal
± 2% full scale
With 7 seconds
100, 115, 220 or 230 Vac, 50/60Hz
15 + 50VA
15minutes
Direct insertion type zirconia detector
-20~+600 °C or -20~+1,590 °C
Sample gas pressure
-3~+3kPa (-306~+306mmH2O)
Ambient temperature
Output signal
-20~+60 °C or -5~100 °C
4~20mA or 0~1Vdc
Indication oxygen concentration
3-digits LED
Indication operation/settings
16-digits LCD
Mode display
03 x LED
Mounting
Panel or Pipe Mounting
Optional function
RS-485
September 23, 2004
4. Fuji’s oxygen gas analyzer.
Standard air ratio by Energy Economy Law in Japan for conservation of energy
Based on Article 4, Clause 1 of the law regarding rational use of energy (Law No. 49 published in 1979), judging
standard for enterprisers at factories (Notification No. 467 of the Ministry of Commerce and Industry, dated October
1979) has been amended on October 26, 1979 (Notification No. 559 of the Ministry of Commerce and Industry) to
specify standard air ratio.
1. Boilers
Classification
For electrical enterprise
Others
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Evaporation rate exceeding
30ton/hr
Evaporation rate from
10~30ton/hr
Evaporation rate not
exceeding 10ton/hr
Load
Ratio
(%)
Standard air ratio
75~100
Solid
Fuel
1.2~1.3
Liquid
fuel
Gaseous
fuel
1.05~1.1
Blast furnace
gas
1.2
75~100
1.2~1.3
1.05~1.
1
1.1~1.2
1.1~1.2
1.3
75~100
-
1.2~1.3
1.2~1.3
-
75~100
-
1.3
1.3
-
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4. Fuji’s oxygen gas analyzer.
2. Industrial Furnaces
Classification
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Standard air Ratio
Metal melting furnace
1.3
Continuous steel heating furnace
1.25
Metal heating furnace not continuous steel heating type
1.3
Continuous thermal treatment furnace
1.3
Gas producer and gas heating furnace
1.4
Oil heating furnace
1.4
Pyrolytic furnace and modification furnace
1.3
Cement kiln
1.3
Alumina kiln and lime kiln
1.4
Continuous glass melting kiln
1.3
September 23, 2004
The End
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