Varying the pressure of the fuel oil immediately before the orifice (nozzle) controls the flowrate of fuel from the burner.
If the fuel flowrate is reduced to 50%, the energy for atomisation is reduced to 25%.
This means that the turndown available is limited to approximately 2:1 for a particular nozzle. To overcome this limitation, pressure jet burners are supplied with a range of interchangeable nozzles to accommodate different boiler loads.
Advantages of pressure jet burners:
• Relatively low cost.
• Simple to maintain.
Disadvantages of pressure jet burners:
• If the plant operating characteristics vary considerably over the course of a day, then the boiler will have to be taken off-line to change the nozzle.
• Easily blocked by debris. This means that well maintained, fine mesh strainers are essential.
Rotary cup burner
Fuel oil is supplied down a central tube, and discharges onto the inside surface of a rapidly rotating cone. As the fuel oil moves along the cup (due to the absence of a centripetal force) the oil film becomes progressively thinner as the circumference of the cap increases. Eventually, the fuel oil is discharged from the lip of the cone as a fine spray.
Because the atomisation is produced by the rotating cup, rather than by some function of the fuel oil (e.g. pressure), the turndown ratio is much greater than the pressure jet burner.
Advantages of rotary cup burners:
• Good turndown ratio.
• Fuel viscosity is less critical.
Disadvantages of rotary cup burners:
• More expensive to buy and maintain.
At present, gas is probably the most common fuel used in the UK.
Being a gas, atomisation is not an issue, and proper mixing of gas with the appropriate amount of air is all that is required for combustion.
Two types of gas burner are in use ‘Low pressure’ and ‘High pressure’.
Low pressure burner
These operate at low pressure, usually between 2.5 and 10 mbar. The burner is a simple venturi device with gas introduced in the throat area, and combustion air being drawn in from around the outside.
Output is limited to approximately 1 MW.
High pressure burner
These operate at higher pressures, usually between 12 and 175 mbar, and may include a number of nozzles to produce a particular flame shape.
Dual fuel burners
The attractive ‘interruptible’ gas tariff means that it is the choice of the vast majority of organisationsin the UK. However, many of these organisations need to continue operation if the gas supply is interrupted.
The usual arrangement is to have a fuel oil supply available on site, and to use this to fire the boiler when gas is not available. This led to the development of ‘dual fuel’ burners.
These burners are designed with gas as the main fuel, but have an additional facility for burning fuel oil.
The notice given by the Gas Company that supply is to be interrupted may be short, so the change over to fuel oil firing is made as rapidly as possible, the usual procedure being:
• Isolate the gas supply line.
• Open the oil supply line and switch on the fuel pump.
• On the burner control panel, select ‘oil firing’. (This will change the air settings for the different fuel).
• Purge and re-fire the boiler.
This operation can be carried out in quite a short period. In some organisations the change over may be carried out as part of a periodic drill to ensure that operators are familiar with the procedure, and any necessary equipment is available.
However, because fuel oil is only ‘stand-by’, and probably only used for short periods, the oil firing facility may be basic.
On more sophisticated plants, with highly rated boiler plant, the gas burner(s) may be withdrawn and oil burners substituted.
Burner control systems
The reader should be aware that the burner control system cannot be viewed in isolation. The burner, the burner control system, and the level control system should be compatible and work in a complementary manner to satisfy the steam demands of the plant in an efficient manner.
The next few paragraphs broadly outline the basic burner control systems.
On/off control system
This is the simplest control system, and it means that either the burner is firing at full rate, or it is off. The major disadvantage to this method of control is that the boiler is subjected to large and often frequent thermal shocks every time the boiler fires. Its use should therefore be limited to small boilers up to 500 kg/h.
Advantages of an on/off control system:
• Least expensive.
Disadvantages of an on/off control system:
• If a large load comes on to the boiler just after the burner has switched off, the amount of steam available is reduced. In the worst cases this may lead to the boiler priming and locking out.
• Thermal cycling.
High/low/off control system
This is a slightly more complex system where the burner has two firing rates. The burner operates first at the lower firing rate and then switches to full firing as needed, thereby overcoming the worst of the thermal shock. The burner can also revert to the low fire position at reduced loads, again limiting thermal stresses within the boiler. This type of system is usually fitted to boilers with an output of up to 5 000 kg/h.
Advantages of a high/low/off control:
• The boiler is better able to respond to large loads as the ‘low fire’ position will ensure that there is more stored energy in the boiler.
• If the large load is applied when the burner is on ‘low fire’, it can immediately respond by increasing the firing rate to ‘high fire’, for example the purge cycle can be omitted.
Disadvantages of a high/low/off control system:
• More complex than on-off control.
• More expensive than on-off control.
Modulating control system
A modulating burner control will alter the firing rate to match the boiler load over the whole turndown ratio. Every time the burner shuts down and re-starts, the system must be purged by blowing cold air through the boiler passages. This wastes energy and reduces efficiency. Full modulation, however, means that the boiler keeps firing over the whole range to maximise thermal efficiencyand minimise thermal stresses. This type of control can be fitted to any size boiler, but should always be fitted to boilers rated at over 10 000 kg/h.
Advantages of a modulating control system:
The boiler is even more able to tolerate large and fluctuating loads. This is because:
• The boiler pressure is maintained at the top of its control band, and the level of stored energy is at its greatest.
• Should more energy be required at short notice, the control system can immediately respond by increasing the firing rate, without pausing for a purge cycle.
Disadvantages of a modulating control system:
• Most expensive.
• Most complex.
• Burners with a high turndown capability are required.
A considerable amount of energy is stored in fuel, and it burns quickly and easily. It is therefore essential that:
• Safety procedures are in place, and rigorously observed.
• Safety interlocks, for example purge timers, are in good working order and never compromised.