Combustion Control for Boilers. Introduction Basic type of boilers. Why need boiler controls? Combustion control for boilers. Fuji’s oxygen gas analyzer. Contents. How does a boiler works? A boiler is a water containing vessel which transfers heat from a fuel
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
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.3. Why need boiler controls?
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.3. Why need boiler controls?
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.3. Why need boiler controls?
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.3. Why need boiler controls?
Reduction in fuel consumption
Reduction in engineering, installation and startup costs
Reduction maintenance costs associated with older
Reduction manpower with automatic responds
Provide a flexible control strategy to reduce process upsets
Readily data available for remote monitoring to determine process optimization, boiler efficiency and load allocations3. Why need boiler controls?
combination of the following methods:-
Excess air regulation
Total heat control4. Combustion control for boilers.
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:-
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 fuel4. Combustion control for boilers.
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
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 override4. Combustion control for boilers.
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.
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 lower4. Combustion control for boilers.
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
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 steam4. Combustion control for boilers.
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.4. Combustion control for boilers.
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.4. Fuji’s oxygen gas analyzer.
Excess Air Coefficient
Exhaust Oxygen, O2 (%)
Energy Loss Ratio,
Exhaust Gas (%)
13.74. Fuji’s oxygen gas analyzer.
(In the case of heavy oil combustion at exhaust gas temperature of 250 °C and atmospheric temperature of 20 °C)
(The data may varies based on the construction and performance of the boilers)
Evaporation rate from boiler
Annual operating hours
Evaporation multiplier factor for boiler
Improved value of excessive air coefficient
1.6 -> 1.2
(O2 gas from 7.9% -> 3.5%)
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 6,069.004. Fuji’s oxygen gas analyzer.
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.
No sampling device is required
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.
Compact and light weight design
The detector and converter weigh about 1.6kg and 3.5kg respectively.
Instrument equipped with indicator and transmitting function
The converter is equipped with an indicator that permits direct readout of the oxygen concentration transmitting output function or RS485 communication.
Alarm and control functions available
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.4. Fuji’s oxygen gas analyzer.
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
Warm up time
Direct insertion type zirconia detector
Applicable gas temperature
-20~+600 °C or -20~+1,590 °C
Sample gas pressure
-20~+60 °C or -5~100 °C
4~20mA or 0~1Vdc
Indication oxygen concentration
03 x LED
Panel or Pipe Mounting
RS-4854. Fuji’s oxygen gas analyzer.
Load Ratio (%)
Standard air ratio
Blast furnace gas
For electrical enterprise
Evaporation rate exceeding 30ton/hr
Evaporation rate from 10~30ton/hr
Evaporation rate not exceeding 10ton/hr
-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.
Standard air Ratio
Metal melting furnace
Continuous steel heating furnace
Metal heating furnace not continuous steel heating type
Continuous thermal treatment furnace
Gas producer and gas heating furnace
Oil heating furnace
Pyrolytic furnace and modification furnace
Alumina kiln and lime kiln
Continuous glass melting kiln
1.34. Fuji’s oxygen gas analyzer.
2. Industrial Furnaces