Rotary Cup Burners

Principle of operation :

The fundamental concept behind a rotary cup burner is the mechanical fragmentation of fuel into extremely fine particles. As implied by its name, this burner features a high-speed rotating cup, spinning at 5000 to 6000 rpm, depending on the manufacturer's design. Oil flows over the cup as a thin film. Due to the rapid rotary motion, the fuel particles are propelled away from the cup's center in a specific pattern, breaking into very fine particles. These particles, when dispersed in various directions, form a conical shape that is ignited to combust the fuel.

In a rotary cup burner, separate ducts are designated for primary and secondary air. The primary air primarily aids in the oil atomization process, while the secondary air ensures complete combustion. Typically, the air distribution is 15-20% for primary air and 80-85% for secondary air.

The burner features an oil distribution cup mounted on a rotating shaft, driven by an electric motor via a V-belt. Positioned at the tip of the wind box, the cup is accompanied by primary and secondary air registers. The primary air register receives air at high pressure (7-15 kPa), whereas the secondary air register, located separately, supplies air at a lower pressure (2.5–5 kPa). This is facilitated by the airflow path's design and vane-controlled dampers.

This burner requires pre-heating only to make the oil flow easily, usually heating up to 90°C for furnace oil commonly used in India. Since oil pressure doesn't contribute to fuel atomization, a pressure of 3.5 barg is adequate.

Once inside the burner, the oil is evenly distributed onto the walls of the rotating cup through an oil distributor situated at the cup's inner center. As the oil moves through the cup, it forms a thin sheet, and upon exiting the cup's rim, this sheet is 100 microns thick. Swirling primary air strikes the oil sheet at an axial velocity of approximately 100 m/s, resulting in effective atomization and stable flame. The secondary air supports the rest of the combustion process.

The rotary cup is the burner’s most crucial component. Made from hard chromium-plated low-carbon steel, its center of gravity is near the hub, reducing the load on the structure. The hub is precisely crafted from a light alloy. The cup is powered by an electric motor through a V-belt, with oil supplied via a tube separate from the shaft and belt drive. The shaft is balanced and vibration-free.

Fuel atomization using a high-speed rotating cup is significantly more effective than the pressure jet principle, with primary air distribution further enhancing the process.

This makes the rotary cup burner particularly suitable for handling highly viscous heavy oils, comfortably burning Furnace oil, LSHS, or heavy-grade viscous oils found in European countries.

The most notable features of this technology include a wide turndown range, electrical energy and fuel savings, and the capability to handle challenging fuels. These features are further detailed below.

Effective Combustion, Reduced Emissions due to Rotary Cup Burner:

Micro Carbon Residue, Ramsbottom, and Conradson Carbon Residue are three test methods used to evaluate the same characteristic of diesel and heavy fuel. This residue includes incompletely burned fuel particles and the ash formed during combustion.

Compared to a pressure atomized burner, the rotary cup burner is more effective in reducing unburnt emissions from the stack.

With a pressure atomized burner, the oil droplet size range is larger than with a rotary cup burner (50-300 microns for pressure jet versus 20-150 microns with a rotary cup). Moreover, the pressure atomizing burner relies on oil pressure to maintain the droplet size range. When the burner turns down, the oil pressure decreases, significantly affecting atomizing quality and droplet size. This negatively impacts emissions and contributes to smutting, a common issue with pressure jet burners using heavy fuel oils.

The lower the oil pressure from the pressure jet burner, the worse the situation becomes, resulting in poor atomizing quality, excessive fuel usage, and generally poor combustion figures at low fire.

Another consideration is oil viscosity. Rotary cup burners are highly tolerant of viscosity changes in heavy fuel oils, which are common. In contrast, any viscosity change in pressure jet burners severely affects atomization quality, leading to poor emissions if viscosity varies. The rotary burner manages these viscosity changes better due to its mechanical atomization method. Additionally, with heavy oil and pressure jet burners, the atomizing temperature is a compromise, as the actual temperature needed for proper viscosity through the nozzle would be much higher than used. This results in poor combustion quality and increased smutting.

Since the rotary cup burner does not depend on oil pressure for atomization and allows a very thin oil film to be thrown into the primary air path, the atomizing quality remains consistent as the burner modulates. Coupled with the smaller droplet size range, consistent throughout the modulation range, this enables more complete fuel combustion, reducing smutting and often eliminating it.

The combination of superior atomizing quality, lower oil temperatures, and pressures, along with reduced FD fan ratings, results in better emissions and reduced operating costs for the boiler plant.