1. Field of the Invention
This invention relates to combustion in general, and specifically to burners and methods of use of same to combust a fuel with an oxidant having an oxygen concentration greater than air.
2. Prior Art
In burner technology employing oxidants having oxygen concentration greater than the oxygen concentration in air, when such burners are used in glass manufacture, one object is to produce a luminous low temperature flame. Often these flames have low gas velocities and very simple (for example, pipe-in-pipe configuration) mixing strategy. Use of burner blocks having cylindrical combustion chambers is known, wherein the major process consists of a fuel-rich core surrounded by oxygen-rich sheath inside the cylindrical cavity. A cylindrical shape burner block (sometimes referred to as a precombustor) with a ratio of length "L" to diameter "D" of the cylindrical cavity between 2 and 6. In this L/D ratio range, the fuel and oxygen velocities (less than 600 ft/s) are chosen for firing ranges up to 20 MM Btu/hr. Here, the objective is to produce a long, lazy and high luminosity oxy-fuel flame using delayed mixing. The production of soot particles due to thermal cracking (in the fuel-rich core) and subsequent combustion provides the flame luminosity. Outside of the above L/D ratio range the flame acquires very high "axial" momentum and it becomes very non-luminous.
While such burners are useful for many purposes, there are disadvantages with such burners. The major disadvantage of such burner blocks is that the flame shape, especially maximum flame diameter and/or flame length, is always dictated by the burner block L/D ratio and fuel and oxidant velocities. The general flame characteristics are long, lazy and high luminosity flame without any significant component for convective heating, larger flame surface area for the increased load coverage, or considerations to reduce the effect of particulate "inspiration effect" near the hot surface of the burner block (recirculation zone). The axial momentum flame of such burners produce a low-pressure region due to the combustion.
The size and strength of above three dimensional recirculation zone is dependent on the momentum of the axial flame combustion products. The higher the flame momentum, the higher would be the inspiration effect of the recirculation zone, and higher would be the magnitude of low pressure region around the hot-face of the burner block.
The low pressure region in the vicinity of the burner block hot-face allows various process particulates (such as glass batch, volatiles, condensates, and the like) to deposit on the burner block hot-face, or sometimes even get pulled inside the burner block cavity (if there is a void between oxidant stream and burner block inner surface). This is very common if the burner block cavity is not designed to fill completely with flame gases. The objective to design a tight burner block without any void becomes very difficult when burner firing rate (amount of fuel and oxidant flows) is varied over a wide range. A slight gap in the burner block around the flame envelope can inspirate combustion products into the burner block cavity due to the presence of the low-pressure region and subsequent pumping action of the recirculation zone. The consequence of plugged burner block can result in increased frequency of maintenance (in terms of cleaning burner and/or block or lower burner/block life) or a catastrophic failure (meltdown of block/burner) due to direct or indirect impingement/deflection of high temperature flame.
Another disadvantage of pipe-in-pipe shape burner block design is the difficulty in producing a flame which is expanded in a radial direction generally perpendicular to the direction of fuel and oxidant gas flows, referred to herein as a "flat" flame. The cylindrical geometry pipe-in-pipe burners have no provision for expanding the flame to develop in the radial direction. The radial, flat flame shape is very common in air-fuel burners for heating a furnace interior with constant heat flux. A simple example is a steel reheat furnace where air-fuel burners are mounted on furnace roof (crown) and they radiate heat to the steel load (billets, plates) below. The advantage of a radially expanding flat flame (usually swirling) is to provide a very little axial heating component and most of the heat is due to radiation from the heated wall. The flat flame is found to hug the wall surface by a coanda effect and produce a heat-source for imparting a uniform radiation. This type of air-fuel burner is known by a tradename "Wall Hugger" in industry. The wall-hugging flame is created by the swirling of air at high velocity. However, the same process is not yet proven using oxy-fuel burners.
Swirling oxidant/fuel burners are known where the oxidant has an oxygen concentration greater than the concentration in air. Typically the burner block has a cylindrical cavity, with the burner recessed within a cylindrical cavity. High velocity fuel injection and swirling, low velocity oxygen injection in the annular space is provided. A swirl stabilized flame is formed within a constant diameter cylindrical block cavity. In this design there is no provision for expansion of diverging oxygen flow stream due to swirling oxygen motion. The resulting flame is a "narrow" cylindrical flame based on exit diameter D. The pipe shape combustion chamber coupled with a narrow exit geometry does not provide sufficient room for the flame to expand in the radial direction or in the extreme situation a formation of flat flame. The constant diameter geometry (pipe shape) negatively affects the maintenance of oxidant swirl due to wall friction. If a swirling fluid stream is not allowed to expand in the radial direction, the intensity of swirl quickly dies due to wall friction effects. On the other hand, the swirling oxidant also quickly reacts with the fuel due to the close proximity in the combustion chamber. This process creates a short intense flame. The cooling of the burner block is also negatively affected due to the quick burnout of swirling oxidant in a relatively narrow diameter burner block.
Further, the entire burner body (usually a metallic pipe) is inserted into the burner block until the burner tip (fuel nozzle tip) is a distance "L" from the hot face of the burner block. The swirling oxidant is introduced just upstream of the nozzle exit and the oxidant is mostly conveyed through a metallic burner body without cooling significant burner block length. It appears that the use of swirling oxidant is to introduce better mixing conditions with the fuel rather than altering flame characteristics in the radial dimension. Due to the fixed burner block geometry (cylindrical shape), flame characteristics such as the flame shape alteration in radial direction, cooling burner tips and block interior using oxidant flow, and cleaning burner block interior with sweeping oxidant stream are severely curtailed.
Therefore, there exists a need in the combustion art for a burner which solves some or all of the above problems with known burners.