The combustion of industrial boilers is not complicated at first glance. The fuel is fed into the furnace, and the combustion-assisted air can start a fire to produce heat, and the heat is transmitted to water or steam.The really tricky thing is to keep the flame stable.The thermal efficiency of the boiler, the exhaust temperature, the service life of the furnace, and the stable steam supply at the back end all depend on the flame to maintain the established form, position and combustion intensity.Once the flame is erratic and shaking, the exothermic heat of the furnace will fluctuate, the heating of the water-cooled wall is unbalanced, and the steam pressure will fluctuate. In severe cases, it will cause flameout, tempering and even furnace deflagration.The incentives behind the phenomena of flame flicker, pulsation, de-ignition and tempering, and incomplete combustion are different, and it is difficult to deal with them with the same set of solutions.
Unstable combustion means that the input conditions have not changed according to the design intention, but the flame position, shape, or exothermic rate continue to deviate or periodically, and the unit will not return to its original working point within tens of seconds.It is not the same thing as normal fluctuations. When the load changes, the air volume and fuel follow the set slope, and the oxygen volume returns to the set zone after a short deviation. Instability means that the deviation cannot converge on its own.
High-frequency jitter is the rapid flicker of the root of the flame, with a frequency of tens of Hz or more, which does not affect the output in the short term, and deform the flame-stabilized parts in the long term.The low-frequency pulsation is the periodic expansion and contraction of the entire flame, and the roar falls from a few hertz to tens of hertz. After matching the acoustic mode of the furnace, the amplitude grows by itself.De-ignition, tempering, and accidental flameout are harder instability.The flame is biased, and the local heat flow density can be twice as high as the design value. The wall temperature rise and the cracks in the furnace wall all start from here.
The reasons are organized into four lines: fuel input, mixing process, detection and control, and air flow.Most faults fall on one line, and a few are superimposed on two.
The gas supply pressure is usually 20 to 50 mbar, and the burner has the lowest and highest allowable pressure.The pressure drops below the lower limit, the flow rate in front of the nozzle decreases according to the square root relationship, the blast does not decrease synchronously, the excess air coefficient rises, and the flame roots rise, become shorter, and turn blue and shiny.When the pressure rises, excess gas flows in, the flame lengthens and turns yellow, and the tail temperature rises.
What is more prone to problems is the dynamic quality of the pressure. The response of the pressure regulating station, the start and stop of upstream equipment, and the insufficient volume of the pipeline all cause the pressure to shake by tens of millibars within a few seconds, and it flashes when it falls on the flame.When the air source is mixed and transported, the Huabai number drifts by five to ten percent, and the same aperture and valve position have different heat dissipation.Looking at the pressure and viscosity of the fuel, the pressure atomization nozzle should be ten to twenty bar, and the temperature of the heavy oil is ten degrees Celsius below the set value. The atomization particle size rises from one hundred microns to more than two hundred microns, and the flame lengthens and deposits carbon.The increase in the filter pressure difference makes the flow rate smaller, and the dead zone appears in the fouling of the control valve spool. The flow rate does not move after ten percent of the command is gone, and it rushes over when it is detached. This kind of step is the easiest to excite oscillation.
The cost of treatment is very clear. The regulator tank suppresses the second-level pulsation at the cost of land area, initial investment and regular inspection; the dual-pass filter is switched online to avoid cleaning and stopping the furnace, at the cost of complex pipelines and additional leakage points; the online calorific value meter incorporates quality into the control, the equipment is expensive, and the calibration is important.These inputs on small-capacity boilers cannot be recovered.
The theoretical air volume corresponds to about 9.5 cubic meters of air per cubic meter of natural gas. In fact, there should be a margin. The excess air coefficient is taken from 1.15 to 1.25, and the oxygen content of the flue gas is roughly three to five percent.When the air is insufficient, the flame lengthens, darkens, and then black smoke rises. Carbon monoxide rises from dozens of parts per million to thousands. The unburned carbon sinks at the rear of the furnace, and the combustion is postponed to the rear section, and the local temperature of the flame is higher.
The cost of excess air is that the flame temperature drops. For every zero-one increase in the excess air coefficient, the exhaust heat loss rises by about 1.5 to 2%, and no matter how large the air volume is, the fire will be extinguished.If the oxygen content is pressed below two percent, the best smoke exhaust loss can be obtained, at the cost of only one step away from the inflection point of carbon monoxide; leave three to five percent, and give up a little efficiency for anti-disturbance margin.For units with manual damper ratio, the fuel valve moves the damper first and then the damper. The time difference between ten seconds and one minute is enough for the flame to swing back and forth.
As soon as the geometry of the burner changes, the flame changes.The aperture of the fuel nozzle is worn and enlarged, the flow rate increases by 10 to 20%, the atomization angle widens, the flame becomes shorter and thicker, and the heating surface close to the furnace throat is partially over-temperature.The local spray hole is sealed by the coke block, the atomization cone is notched, the flame is bright on one side and dark on the other, and the air flow in the furnace is biased.
The damage to the cyclone is more concealed, the number of cyclones decreases after the blade is coked and deformed or ablated, the central reflux zone is shortened, and the flame stabilization ability is greatly reduced. In the initial stage, the wind resistance deteriorates, and the load is removed as soon as it rises.The deviation of the nozzle relative to the front and rear of the throat is ten to twenty millimeters, and the degree of wind adherent to the wall changes at a time, and the flame changes from centered to skewed, scouring the furnace wall on one side and burning in the air on the other side.
Poor mixing stems from mismatched momentum ratios.The gas pressure drops below the design value, the jet penetration is not enough, the fuel and air go their own way, the flame is divided into several bright clusters, the noise is loud, and the carbon monoxide is high.The nozzle and cyclone are precision parts. The furnace must be stopped for replacement and reinstalled according to the manufacturer's size. The aperture can be adjusted on site for emergency use. The atomization characteristics have deviated from the design, and subsequent adjustments have been awkward.
Ignition failures are mostly due to energy and clearance.The electrode gap is two to four millimeters. Ceramic cracking or carbon deposition raises the breakdown voltage. The ignition transformer outputs a few thousand volts to 10,000 volts and can't produce a spark. The electrode is biased out of the combustible concentration range of the gas jet, no matter how strong the spark is, it can't be lit.
It is also common for the ignition to be successful but the fire report is not reported.Ionized fire detection relies on flame conductivity. Carbon deposition, poor grounding, and shielding failure of the probe will cause the flame current to fall from a few microamps to below the threshold value. The ultraviolet type is easily disturbed by the background infrared of the furnace and adjacent burners.Both ends of the false signal are dangerous. If there is a fire, the fuel will continue to be fed without fire, and if there is no fire, the normal combustion will be cut off.The last thing to do is to retry continuously after a failure. The remaining gas is not purged cleanly, and the next spark is the source of ignition.
The outlet of the furnace with balanced ventilation is maintained at a negative pressure of ten to fifty pa.The negative pressure is the external restraint of the shape of the flame. When it suddenly becomes larger, cold air penetrates from the observation hole and cracks in the furnace wall, the excess air rises, and the flame is pumped upward; the negative pressure becomes smaller or even positive, and the high-temperature flue gas flows out of the gap, and carbon monoxide accumulates in the boiler room.
The problems of the blower and induced draft fan are most likely to be exposed when the load changes. The impeller accumulates ash, the baffle is stuck, and the frequency conversion hysteresis will all cause instantaneous deviations of several hundred pa.The ash accumulation of the economizer causes the resistance of the flue to slowly rise, and the negative pressure that can be maintained at the same speed is getting smaller and smaller, and the baffle is opened to no margin.The air leakage exceeded five percent, the amount of flue gas and excess air increased significantly, and the flame temperature decreased.These maintenance must be continuously invested, and if you don't do it, you will watch the adjustment margin be eaten up, and finally turn off the fire or breathe fire under positive pressure in a load mutation.
Combustion control is limited by the intersection of the fuel circuit and the air circuit. Air is added first when the load is raised, and fuel is reduced first when the load is lowered. The sequence itself is correct. The problem is that the parameters do not match the object.
The combustion of the boiler has pure hysteresis and inertia. It takes a few seconds for the fuel to go from the valve port to the fire, and the thermal inertia of the furnace is measured in minutes.The proportional band is too small, the integration time is too short, the controller reacts too strongly to each small deviation, the fuel valve and the damper overshoot back and forth, and the oscillation period is 20 to 60 seconds, just falling in the range visible to the naked eye, the damper can not be eliminated, the source of disturbance is the control output itself.
The full stroke of the damper actuator is 30 to 90 seconds, and there is still a dead zone of more than ten seconds. According to the dynamic adjustment of the fuel valve, the air circuit has been chasing.The stagnant water in the pressure pipe makes the pressure reading a few hundred pa lower, and the drift of the oxygen probe causes the wind distribution to deviate. These deviations do not alarm, leaving only a slowly climbing line on the trend.The load rises and falls by more than 5 to 10% of the rated speed per minute, and the air volume cannot keep up, so it is necessary to hypoxia before passing the customs.Limiting the variable load rate can suppress the oscillation, at the cost of the deterioration of the ability to adapt to process fluctuations.
Are you struggling to find an industrial boiler that delivers stable combustion and reliable performance? Unstable combustion can lead to operational interruptions, reduced efficiency, and maintenance concerns.
At Bidragon, we supply industrial boilers designed to meet different fuel requirements and operating conditions. Whether you are planning a new boiler installation or looking for a suitable replacement, contact Bidragon to discuss your boiler requirements and explore the right solution for your application.
Flame color, brightness, and shape are first-hand information.The gas is normally light blue with yellow tips, darkening and yellowing is hypoxia, and shortening and whitish is peroxygenation; the fuel is normally orange and yellow evenly, once it is dark red or elongated, atomization or air distribution has gone wrong.The distance, length, and degree of centering of the flame from the nozzle all reflect abnormalities earlier than the instrument.
The low-frequency roar of tens of Hz is mostly related to the acoustic resonance of the furnace, and the high-frequency howling is mostly from the tempering or the jet near the nozzle.Periodic fluctuations in the negative pressure of the furnace indicate that something is driving it.The oxygen content and carbon monoxide should be looked at together. The oxygen content is normal and the carbon monoxide is high, indicating poor mixing or local hypoxia. The two are high at the same time, indicating excessive air distribution but extremely poor mixing.The exhaust temperature and the color of the chimney are slow indicators. The outlet has changed from light gray to dark gray, and the combustion efficiency has fallen for a period of time.
One percent of the volume of carbon monoxide in the dry flue gas corresponds to two to three percent of the heat loss from incomplete chemical combustion. It is common for the thermal efficiency to decrease by several percentage points when the carbon in the soot is stacked.Carbon monoxide, unburned hydrocarbons, and soot have risen together, and the law of nitrogen oxide generation has also been changed. Compliance costs are often noticed earlier than efficiency losses.
When the flame is skewed, the local heat flow density can be twice as high as the design value, the wall temperature rises, the creep life is shortened exponentially, and the nozzle and fire inspection can be burned out within a few minutes of tempering.After turning off the fire, it takes ten minutes to half an hour to complete the purging, ignition, heater, and boost. The process loss of continuous steam supply and product scrap often exceed the maintenance cost.
Troubleshooting ranges from phenomena to data to components.First set the form, high-frequency flicker, low-frequency oscillation, position offset or flameout, the form basically determines the direction; then take the reference at the stable load, fuel pressure, flow rate, damper opening, furnace negative pressure, oxygen content, carbon monoxide.
Looking at the dynamics from the fuel side, the trend curve is pulled for tens of minutes, looking for periodicity and step, the filter pressure difference, the front and rear pressure of the pressure reducing valve, the control valve command and feedback deviation can cover most of the supply failures.Compare the fan current, outlet pressure, actual opening of the baffle and the design curve on the air side, and check the actuator for the difference between the command and the position by 10%.The burner stops to check the nozzle, cyclone, extension depth, electrode and fire inspection.Control the side adjustment record, and manually give a step to see the response time and amplification characteristics.
The fuel depends on the equipment and supply chain. The pressure regulating tank, the dual-pass filter, and the pressure regulating station with switching determine the lower limit of the pressure fluctuation. Long-term air source agreement, fuel batch testing, and solid fuel pretreatment suppress the calorific value fluctuations, and the return is a stable flame and a smaller air distribution margin.
The burner is cleaned and maintained according to the operating hours, the nozzle is replaced according to the cycle, and the cyclone is stopped every time the furnace is cleaned.Starting from the measurement of air volume, measure oxygen and carbon monoxide at several load points, find the inflection point where carbon monoxide rises, and leave a margin of 1.5% to 2% at the operating point.The fan, baffle, flue pressure difference, and furnace wall seal are checked regularly, the sensor is calibrated according to the cycle, and the parameters are re-adjusted after the fuel or burner changes.
The typical problem of gas-fired boilers is pressure fluctuation and poor mixing. The pressure drops by tens of millibars within a few seconds, and the flame immediately shortens and brightens, and then lengthens and turns yellow; the momentum ratio of the nozzle and the swirling air does not match, the flame is divided into several bright clusters, the carbon monoxide is high and the oxygen content is normal.Fire detection faults account for a large proportion of such boilers, and grounding and carbon deposition of ionization probes are the most common.Fuel-fired boilers look at atomization and oil pressure. Nozzle wear causes the flow rate to change together with the atomization angle, and blockage causes the atomization cone to exit the gap. If the oil temperature is low, the particle size is large and the flame is elongated. Black smoke.
Coal-fired boilers are mostly problems with the transportation and distribution of pulverized coal. The deviation of the powder volume of the primary air duct of the same coal mill can be more than 10%, and the flame is strong on one side and weak on the other; the fineness becomes thicker, the fire is delayed, and the center of the flame moves down; the proportion of one or two winds is out of balance, and the flame sticks to the wall.The variable of biomass boiler is fuel, the moisture content rises from 15% to 30%, and the heat of fire increases significantly; the particles are uneven, the small particles burn in advance, and the large particles cannot be burned through; the pulsation of the feed makes the flame cycle fluctuate.
The circulating fluidized bed depends on the gentle fluidization state of the bed, and the bed temperature is maintained at 850 to 950 degrees Celsius, which reduces the efficiency and improves the focus.Changes in particle size, deviation of coal feed point, insufficient fluidized air, or clogging of the air distribution plate will cause the bed temperature to deviate laterally by tens of degrees Celsius. When the fluidization is abnormal, a local dead zone of the bed is formed, and the combustion is transferred from the dense phase zone to the dilute phase zone.
Maintenance is planned according to the parts, not according to the overhaul of the entire boiler.The burner is calculated by operating hours, the fan and duct are calculated by quarter or half a year, and the fuel system pressure difference and leakage are calculated.
The cleaning return is the highest, the filter press is poor and the core is changed, the nozzle and air passage are stopped every time the furnace is cleaned, and the fan impeller is cleaned regularly.The ignition device and the fire inspection do functional tests every time the furnace is stopped to measure the electrode gap, insulation and signal strength.The parameters should be kept in the trend. The fuel pressure, negative pressure, oxygen content, and carbon monoxide are stored by the hour. Look at the slope of a few days and weeks. The probe drift and resistance rise are hidden there.The participation of professionals cannot be saved. Experience and qualifications are required for combustion adjustment and control adjustment. The risk of on-site self-handling is greater than the cost saved.
The burner management system is responsible for the ignition sequence, purging, flame monitoring, fuel cut-off and interlocking. The integrity cannot be weakened, and the bypass, shorting, and shielding interlocking cannot be once.After turning off the fire, cut the fuel before purging, and confirm that there is no remaining combustible gas in the furnace before considering ignition.Cut the fuel immediately after tempering, and don't use wind to push the flame back.If the pressure is abnormal, check the induced draft fan and flue first, and immediately stop the furnace, cut off the fuel, and evacuate the ventilation if the leakage is suspected.
The manufacturer's instructions and local regulations are the lower operating limits. The minimum gas pressure, maximum ignition times, purge air volume, and interlock lists are all in the equipment documents. Copying other equipment habits is a common mistake.Repeated ignition and bypassing the interlocking are the most dangerous. The accumulated gas in the furnace enters the combustible range, and a single spark is enough to deflate. Both the furnace door and the flue are pressure relief outlets.Inspection, troubleshooting and restarting are handed over to qualified people.
Combustion instability often arises from overlapping issues like erratic fuel supply, poor air fuel matching, burner wear, faulty flame detection and unstable furnace airflow. Troubleshooting relies on baseline readings and cross checking pressure, negative pressure, oxygen and CO data. As your boiler supplier, Bidragon advises regular burner maintenance, load aligned parameter tuning and periodic sensor calibration. Though these require planned downtime, they keep combustion consistent and cut unplanned stops.
