Combustion – Process of combustion or burner operation – Flame shaping – or distributing components in combustion zone
Reexamination Certificate
2002-07-11
2004-03-02
Basichas, Alfred (Department: 3743)
Combustion
Process of combustion or burner operation
Flame shaping, or distributing components in combustion zone
C431S008000, C110S345000
Reexamination Certificate
active
06699030
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to combustion of hydrocarbonaceous fuel such as coal.
BACKGROUND OF THE INVENTION
Under the dual pressures of economic deregulation and tightening environmental regulation operators of combustion systems face the difficult task of increasing system efficiency to increase productivity, while simultaneously reducing pollutant emissions. In many combustion systems, such as coal or oil-fired utility boilers, enhancing productivity comes at the expense of increased pollutant emissions, or vice versa.
One of the few areas of boiler or furnace operation that can have a negative impact on both pollution control and productivity is the distribution of fuel and air to the individual burners. It is well known that most boilers or furnaces have significant variations in the amount of either air or fuel, or both, fed to individual burners in a multi-burner array. This variation in air and fuel flow leads to significant stoichiometric ratio variations among the burners, which in turn reduces combustion efficiency and increases pollutant formation. Burners are typically designed to operate within a specific range of stoichiometric ratios to provide a reasonable compromise between good combustion efficiency and low pollutant formation. For example, in a boiler fired with coal, if the air to fuel ratio is too high, the burner operates too “lean” (i.e., fuel-lean) and NOx formation is increased. If the stoichiometric ratio is too low the burner operates too “rich” (i.e., fuel-rich) and CO and unburned carbon increases.
A wide range of factors can lead to problems with air and fuel distribution. Typical solid fuel fired burners, such as those fired with pulverized coal, consist of two main flows. One flow is the transport air, or primary air, which is used to transport fuel from a common feed location to the individual burners. The other flow is the combustion air, or secondary air, which is often supplied through a common windbox. The combustion air stream, which may be subdivided into multiple air streams in the burner, does not mix with the transport air until the burner outlet. For liquid or gas fired systems the combustion air stream may be the only air fed to the burners, other than the minor amount of compressed air used for atomization.
In most combustion systems air for both streams is supplied through the use of a blower. Typical supply pressures are relatively low, in the range of tens of inches of water column. Therefore, even subtle variations in system construction or design can lead to some burners being starved of air, or of fuel if the transport air is similarly affected. Many burners have register dampers that can be opened or closed to control how much air is fed from the windbox. These dampers may also serve to split the secondary air stream into separate streams according to the burner design. However, the damper design and the tolerances required to allow long term operation of the burners make precise flow control to the burners problematic if not impossible. In coal fired utility boilers it is not uncommon to find that the flow rates of air to some of the burners are off by more than 30% from the design values.
With entrained solid fuels, such as coal, the problem of fuel distribution to the burners becomes even more serious. In the case of pulverized coal transport air passes through the pulverizer, or mill, entrains coal that has been pulverized to the desired size, and carries it to the individual burners. With this type of system not only are there issues related to transport air flow to the individual burners, similar to those discussed above, but the problem is compounded by the need to transport a two-phase fluid without permitting separation of the phases in the pipe. For example, as the coal-laden air stream passes around a sharp bend the coal tends to concentrate in one part of the air stream. This phenomenon, called roping, can lead to poor distribution of fuel to the individual burners. Reduction of air flow in any given leg of the distribution system can also lead to settling of the coal from the transport air stream as the velocities are not adequate to keep the solids entrained. In coal fired utility boilers it is not uncommon to find that the coal flow rates to some of the burners are off by more than 30% from the design values.
In addition to the problems associated with maintaining a uniform coal and airflow distribution, some systems may actually require biasing of either the coal or air to specific burners in the array. For example, when a burner is situated adjacent to a sidewall comprising water-cooled tubes (i.e. steam tubes) the flame temperature of that burner can be significantly reduced by heat transfer to the water. Although this reduced flame temperature can help reduce the formation of thermal NOx, it can lead to increases in CO and unburned carbon, if the burner is operated under rich conditions. Furthermore, corrosion of the waterwall may become an issue. To overcome both these problems it may be necessary to bias the air or coal flow to that particular burner such that the burner operates slightly more fuel lean, which serves to increase the flame temperature and combustion efficiency. Given the difficulties associated with creating a simple uniform coal-air distribution, biasing the burners in this fashion is well beyond current commercial practice.
A number of solutions have been proposed to better control both fuel and air flow. These solutions have demonstrated that significant improvements in pollutant emissions and combustion efficiency can be achieved. However, as discussed in the next section, currently available control techniques tend to be limited in their ability to maintain burner balance.
Numerous means have been proposed to control the distribution of fuel and air to individual burners. One is the inclusion in most burners of dampers to control the secondary airflow to the individual burner. The damper assembly is used to close down the cross sectional area of the flow openings in order to restrict the flow of air through the duct. The design of the dampers tends to make flow control very imprecise—making optimization of the flow extremely difficult.
A number of systems, such as those disclosed in U.S. Pat. Nos. 5,685,240, 5,593,131, 6,293,105 and 5,879,148, have been proposed to control the distribution of fuel to an array of burners. These systems preferentially increase the pressure drop through a given leg of the fuel distribution system and/or the air distribution system to control the flow of fuel or air to that specific burner. These systems have been reasonably successful for those burners firing liquid or gaseous fuels, but have been less so for solid fuels due to problems inherent in the transport of a two-phase fluid. These problems include separation of the phases in the transport line and, particularly for solid fuels, erosion of the devices used to control the flow.
Other prior disclosures differ from the present invention in one or more significant ways. U.S. Pat. No. 5,697,306 discloses a device wherein a stream of air is supplied through a so-called “hollow plug”. The objective of this device is said to be control of the stoichiometric ratio of the fuel rich portion of a burner. An optimal stoichiometric ratio is disclosed only for this fuel-rich region, based on properties of the fuel. Air is supplied such that it mixes rapidly with the transport air and coal at the exit of the burner to create a mixture with the desired stoichiometric ratio. Even if this invention could be advantageous for controlling the stoichiometric ratio of this fuel rich core, there is no attempt to control the overall stoichiometric ratio of the burner, let alone of an array of burners. Further, by operation of the disclosed device with the addition of a second stream of air based on the coal properties, not on the requirement to balance the burner, operation of this device would quite possibly actually exacerbate the burner to burner unbalance.
U.S. Pat. Nos. 4,903,901 and 5,048
Bool, III Lawrence E.
Kobayashi Hisashi
Basichas Alfred
Black Donald T.
Praxair Technology Inc.
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