THE FORCED DRAUGHT BURNER2
2.9 OPTIMISING COMBUSTION WITH FORCED DRAUGHT
BURNERS
In this section, we will analyse certain technologies capable of forcibly optimising the combustion process developed using an Forced draught burner. As can be seen, these techniques require the application of plant engineering subsystems, which allow accurate combustion monitoring and regulation, such as:
• Systems for regulating combustion O2;
• Pre-heating combustion supporter air;
• Regulating the number of fan revolutions.
• Burner Management System.
2.9.1 Regulating the O2
As anticipated in section 1, to avoid the presence of unburned combustion particles in the flue gases and therefore obtain total fuel oxidation, a degree of air in excess with respect to the stoichiometric value must be present, which cannot however be too high or the efficiency will suffer.
The excess air is determined and established when the burner is set in relation to the average running parameters for the burner, and measured in relation to the amount of oxygen or carbon dioxide present in the discharge flue gases. The optimum value of the excess air however is variable when the burner is running, in relation to the amount of Blimp for air blown burners
Diagram 75
oxygen required for perfect fuel oxidation.
Therefore the exact amount of air to be supplied to the burner depends on the oxygen content in the air and the characteristics of the fuel; in particular, the parameters that have most influence on the amount of air required are:
• Combustion supporter air temperature: an increase of 10°C in the combustion supporter air temperature corresponds to a decrease in air density of around 3% with the consequent decrease of the oxygen in the air by approximately 0.6%;
• Barometric air pressure: a decrease in the barometric pressure of 10 mbars causes a drop in the air density by approximately 1%
with a consequent decrease in the oxygen present in the air of around 0.2%;
• Calorific value of the fuel: an increase of 5% in the calorific value of the fuel corresponds to an increase in the oxygen requirement of 1%;
• Fuel delivery, temperature and pressure;
• Draught of the flue and back pressure of the furnace;
All the variables mentioned above influence combustion thus determining the amount of oxygen required and, consequently, the excess air. For the best control of the combustion process, the amount of air must be continually modified so that the amount of oxygen in the flue gases emerges as optimum.
This system denominated “regulation of O2 in flue gases” involves a probe for measuring the oxygen in the flue gases, which is installed in the flue gas pipe in the generator, and an
electronic control unit.
The probe is linked to the electronic control unit and reads the oxygen value present in the combustion flue gases. The probes used are generally zircon (ZrO2) in type or electrochemical, as they are reliable, accurate and equipped with a more or less instantaneous response.
The control unit determines the change between the oxygen value measured by the probe and the nominal value set, to determine the exact amount of air to feed to the burner.
By positioning/correction of the burner air regulation damper, controlled by a servomotor, the control unit can guarantee the right contribution of combustion supporter air, and therefore of oxygen, in relation to the output supplied by the burner.
The control of the oxygen in the combustion flue gases makes it possible to set the excess air to the value corresponding to the maximum technical combustion efficiency. In fact, while without the control of the O2 excesses of air must be guaranteed that are greater than the optimum ones for safety reasons, in order to take into account the variable working conditions, with the O2 regulation system, it is possible to set minimum oxygen values in the flue gases, to determine the minimum excess of air to obtain complete fuel oxidation without the need for increasing safety.
In this way, NOx emissions are reduced, due to the smaller amount of oxygen present during combustion.
Minimisation of excess air involves a decrease in the delivery of the burnt gases and, consequently, their temperature. The result is
0
100 150 200 250 300 350 400 450 500
Load (kg/h) O2 in flue gases (%)
Without oxygen regulation With oxygen regulation
Reference values of the oxygen content in flue gases for a gas burner Diagram 76
O2in flue gases (%)
a further increase in technical combustion efficiency.
Diagram 77 clearly shows, for a given temperature of the flue gases, variation in efficiency by varying the percentage of oxygen.
An additional advantage deriving from applying this system is the continual fuel monitoring, making it possible to immediately highlight any malfunctions which can be compensated until an allowed threshold is reached, beyond which it is possible, if necessary, to shut the system down.
2.9.2 Pre-heating the combustion supporter air
This technical solution is adopted to recover the heat contained in the flue gases. The sphere of application is limited to high-temperature heat producing systems, such as diathermic oil systems. In such cases, in fact, the exchange fluid must be heated to a temperature of more than 300°C and, consequently, the flue gases exit the boiler at a high temperature. Generally, the pre-heating temperature of the combustion supporter air that is achieved is around 150°C.
Heat recovery is achieved using an air/flue gas heat exchanger installed inside the flue gases discharge pipe. The amount of heat recovered is proportionate to the mass-related delivery of
the air for the temperature change caused by the passage through the exchanger. On average, this technique makes it possible to obtain an improvement in efficiency up to 8 %.
It is good practice to install the combustion supporter air thrust fan upstream from the heat exchanger.
When calculating the pressure drops, the real conditions of air use must be taken into account, using the application of the correction factors shown in table 23.
2.9.3 Regulating the fan speed
In section 2.5.1 concerning the fans, we saw how combustion supporter air regulation can be done using a variation in the system characteristic curve or using the fan characteristic curve: the first can be done using the variation in pressure drop introduced by a servo-controlled damper, while the second can be done varying the rotation frequency of the fan motor.
The fan rotation frequency is changed using particular frequency and tension converters called inverters, capable of regulating the fan rotation speed and, consequently, the delivery of the combustion supporter air. The following advantages can be obtained with this:
• reduction in electrical power absorbed by the fan;
Loss of the flue gases for different % of O2
Diagram 77
combustion efficiency, as well as guarantee an efficient supervision of the combustion system, the system can be supplemented by a burner supervision system called Burner Management System, a concept layout of which is shown in Diagram 79.
Using this system, it is possible to unite and supervise all burner regulations and exploit them simultaneously. For example, it is possible to integrate oxygen regulation with the fan rotation speed, thus obtaining a saving in terms of electrical power absorbed as described in the following diagram, or handle the functioning of several burners at the same time.
• reduction in noise levels;
The electrical power absorbed by the fan is directly proportionate to the number of revs, therefore a decrease in the number of revs corresponds to a decrease in the power absorbed.
The reduction in the noise level is obtained both at fan level and with regard to any dampers that are passed by an air flow that has a lower speed.
For these advantages to be effective, the frequency converter (inverter) must guarantee the exact observance of the descent and ascent ramps. This is necessary to maintain the correct fuel/air formula; to obtain the latter the motor must precisely follow the value of the number of nominal revs programmed without any delays.
The saving which can be obtained by introducing the rev converter is equal to approximately 40% of the electrical energy absorbed by the fan. A precise evaluation of the energy saving can be calculated using the graph in Diagram 78.
2.9.4 The Burner Management System
To achieve an improvement in technical
Output absorption saving with inverter employ
0,50 1,51 2,523 3,54 4,55 5,56 6,57 7,58 8,59 9,510 10,511 11,512 12,513 13,514 14,515 15,5
0% 5% 10% 15% 20% 25% 30% 35% 40% 45% 50% 55% 60% 65% 70% 75% 80% 85% 90% 95% 100%
Fan delivery
Output absorption (kW)
Absorption without inverter
Absorption with inverter Output saving
Diagram for the evaluation of the energy saving by means of the inverter Diagram 78
MODEM MODEM
RS 232 (MODBUS PROTOCOL) RS 232 (MODBUS PROTOCOL)
RS 422 (MODBUS PROTOCOL)
RS 422 (MODBUS AND OTHERS PROTOCOL
OR
OR
INFRARED PORT
LAP TOP FOR COMMISSIONING/SERVICE
ANALOG I/O UNIT DIGITAL I/O UNIT
SAMPLING PROBE BOILER ROOM CONTROLS AND ALARMS
LOAD SENSOR
Burner Output (%)
Output absorption (%)
Without O2 regulation
With O2 regulation Electrical power absorption with O2regulation and inverter
Diagram 80
Conceptual representation of a Burner Management System Diagram 79
For pressurised boilers, this efficiency is generally between 90% and 93% and it can be calculated by considerating the fuel efficiency (described in section 1.5.1) and the loss through the shell (which generally are 1÷2%).
Preliminarily, if we only have the effective capacity of the boiler, the capacity at the boiler furnace can be calculated by dividing the effective capacity by 0.9:
If the only data available is the delivery of vapour produced, generally expressed in kg-h or t/h, the furnace thermal output can be calculate using the following equation
Qfurnace=[ Gv CP (Tvapour– Twater) + Gv CLAT VAP] / η
Where:
Gv = mass-related vapour delivery [kg/s]
Cp = specific heat at constant pressure [kW/kg
°C]
Tvapour = vapour temperature [°C]
Twater = water temperature entering the boiler [°C]
CLAT VAP = latent vaporisation heat [kW/kg]
η = efficiency of the vapour generator.
Qfurnace= Quseful 0.90 η100%= Quseful
Qfurnace