Currently, almost all oil burners used for residential and commercial buildings applications have pressure atomizing-type burners. For small units, these have one, fixed firing rate. For larger commercial building applications, there may be two or three firing rates achieved through some combination of control of pump pressure and/or multiple nozzles.
On the small end of the spectrum, the minimum firing rate is practically limited to about 20.5 kW (or 0.5 gallons per hour). At firing rates lower than this, industry experience has shown that blockage of the small nozzle slots reduces reliability to an unacceptable level.
Lower firing rate oil burners are needed to address oversizing. Oversizing, particularly in newer construction with lower home heat design loads, leads to excessive cycling, appliances physically larger than necessary and loss of efficiency.
In cases in which boilers are used with tankless coils to provide domestic hot water in addition to space heating, the design burner input is typically dominated by the peak hot water demand; lower burner firing rates are not generally desired unless domestic hot water storage is provided. Modulating or two-stage firing can provide significant benefits, including reduced noise on average, improved efficiency, reduced cycling and capacity to meet periodic high loads.
Several reviews have been done on methods to achieve low or modulating firing rates. For low firing rates, air atomization is one serious option. By achieving high-quality atomization with air, the requirements for high fuel pressure and small oil passages are relaxed.
One residential oil burner with high-pressure air atomization is currently on the market in Europe. This low-NOx burner incorporates an air compressor that provides atomization air in the 70 kilopascals (10 psi) gage range. The burner operates with a nominal firing rate of 8 kW. The burner is considered expensive and incorporates a unique air atomizing nozzle with an integral fuel shutoff valve for “clean” starts and stops.
A second air atomization concept - the Babington system - involves a very unique nozzle involving one or more atomizing spheres. Compressed air is introduced into the spheres that have a small, precision slit in the surface. Fuel covers the outside surface of the spheres and is atomized by compressed air flowing out of the slit. A wide range of burner systems have been developed with firing rates from under 4 kW to about 24 kW. This approach is reported to provide excellent atomization quality and combustion performance.
To avoid the need for an air compressor, Brookhaven has been actively involved with the development of a low-pressure air atomized burner. Here, the atomizing air pressure is on the order of 1,500-2,000 Pa and can be provided by a fan.
To achieve good atomization quality, a very high fraction of the total combustion air flow is introduced through the atomizer. This is in contrast to high-pressure air atomizing systems where only a very small fraction of the total air passes through the nozzle. Good combustion performance with low NOx emissions has been reported over the firing rate range of 4 kW to 20 kW.
Direct VaporizationOther approaches for low firing rates may not involve atomization of the oil, but rather direct vaporization from a surface. One approach, for example, involves a porous surface that has fuel driven through under steady or pulsing pressure. From the opposite surface, fuel is vaporized, mixed with surrounding combustion air and burned. This approach is commonly used now in automotive and marine auxiliary heaters with firing rates as low as 4 kW.
To achieve modulation with pressure atomization, one approach involves “bypass” nozzles. Here, there is a high fuel flow rate through the atomizer swirl slots, but a portion of the oil that passes through these slots is returned back to the fuel pump suction side or storage tank and does not exit the nozzle as part of the spray.
By controlling the amount of bypass flow, the firing rate can be adjusted. One prototype burner of this type, produced in Japan, is currently under test at Brookhaven. This burner uses a pulse width modulated valve in the bypass line to control electronically the burner firing rate between 8 kW to 48 kW. A variable speed fan is used to maintain constant excess air over the firing rate range.
In a second, related approach, an atomizer has been developed that incorporates two sets of swirl slots. One has a fixed rate of fuel flow and the other has a variable fuel flow rate. Electronic modulation of the second flow rate can be used to control firing rates over a 2:1 firing rate range.
With pressure atomization, it is possible to have two-stage firing, or modulation, by simply changing the fuel pressure to the nozzle. The firing rate varies as the square root of the fuel pressure and very high pressures are required to achieve high turndown ratios. Fuel pumps for small burners with two outlet pressures are available and have been integrated into a small number of appliances in Europe.
More recently, a pulsed solenoid fuel pump has been developed that uses a variable frequency driver to vary fuel pressure over the range 700 Pa (100 psi) gage to 5 mPa (700 psi) gage. Incorporated in a low-NOx prototype burner, firing rate modulation over a 3:1 firing rate range has been demonstrated.
Pulsed FlowInstead of having steady flow to a pressure atomizing nozzle, pulsed fuel flow may be used to achieve modulated or two-stage firing. The concept here is to have the fuel arrive at the atomizer pressure in waves that are ideally square in shape with a frequency high enough to prevent significant impact on combustion performance.
Square waves of pressure prevent atomization of fuel at low pressure on the edges of the wave that leads to large drops and poor combustion performance. Frequencies in the range of 50 hz to 200 hz have been targeted. One pump manufacturer has introduced a fuel pump that has two switchable discharges. One provides constant pressure and the other provides pulsed pressure for a turndown ratio in firing rate of 2:1.
Brookhaven has been involved with the development of an alternative concept involving constant fuel pressure and the integration of a rapid cycling microvalve integrated with a conventional atomizer. The valve operates at constant frequency and firing rate can be controlled electronically using a pulse width modulated driver. A key performance parameter with this concept is the impact of modulation on the quality of atomization.
These results show fairly constant atomization quality over the control range. Increasing fuel pressure obviously improves spray quality. In combustion tests, better performance has been obtained with low-NOx (“blue flame”) burners than conventional yellow flame burners. The low-NOx burners have much higher rates of internal recirculation, which “smooth out” the fuel introduction pulses.
With air atomization, firing rate modulation may be somewhat easier to achieve. Brookhaven has developed and field-tested, for example, a two-stage version of the low pressure air atomizing burner concept. At low fire, all of the combustion air passes through the atomizer. For high fire, a secondary air port is opened allowing more air to pass into the combustion zone around the atomizing nozzle. Fuel pressure is fixed at 150 kPa (20 psi) gage and high and low firing rate fuel flow fixed orifices are used to control firing rate. Electronic modulation of fuel flow rate to the nozzle using a pulse-width-modulated valve has also been demonstrated.
A completely different approach toward low firing rate and modulation involves vaporization of the fuel followed by combustion of fully premixed vapors and air.
This has been an area that has received attention in the past and fully functional prototype systems have been developed. System complexity and coke formation have been issues. Recently there have been some important advances in vaporization of the fuel and controlled, low temperature oxidation cool flames to prepare fuel air mixtures for combustion. Here, fuel is sprayed into air that is preheated to roughly 320 degrees C. The fuel is vaporized and then undergoes low temperature oxidation reactions leading to consumption of about 5 percent of the oxygen and a temperature rise to about 430 degrees C.
This process does not lead to ignition but self-extinguishes due to a change in the reaction mechanisms at the higher temperature. The prepared mixture can be burned in a stabilized flow or within a porous matrix. This later approach can be configured for very low NOx emissions.