This invention relates to a control system for a fuel fired furnace and more specifically to the control of the stoichiometric ratio within the combustion process occurring within the furnace of a steam generating power plant. Control of the stoichiometric ratio is accomplished by regulating the distribution of air flow to the combustion process in such a manner that the formation of oxides of nitrogen are maintained at consistent levels while simultaneously maintaining carbon in fly ash and carbon monoxide at acceptable levels.
In recent years oxides of nitrogen, also known as NO.sub.x, have been implicated as one of the elements in contributing to the generation of acid rain and smog. Now, due to very strict state and federal environmental regulations demanding that NO.sub.x emissions be maintained at acceptable levels, the control of the formation of NO.sub.x during the combustion process is of critical importance and a major concern in the design and operation of a power plant. As a consequence, combustion control systems must improve to meet these demands.
Oxides of nitrogen are a byproduct of the combustion of hydrocarbon fuels, such as pulverized coal in air, and are found in two main forms. If the nitrogen originates from the air in which the combustion process occurs, the NO.sub.x is referred to as `thermal NO.sub.x.` Thermal NO.sub.x forms when very stable molecular nitrogen, N.sub.2, is subjected to temperatures above about 2800 F. causing it to break down into elemental nitrogen, N, which can then combine with elemental or molecular oxygen to form NO or NO.sub.2. The rate of formation of thermal NO.sub.x downstream of the flame front is extremely sensitive to local flame temperature and somewhat less so to the local mole concentration of oxygen. Thermal NO.sub.x concentration can be reduced by lowering the mole concentrations of N.sub.2 and O.sub.2, reducing the peak flame temperature and reducing the amount of time that N.sub.2 is subjected to these temperatures.
If the nitrogen originates as organically bound nitrogen within the fuel, the NO.sub.x is referred to as `fuel NO.sub.x.` The nitrogen content of coal is comparatively small and, although only a fraction is ultimately converted to NO.sub.x, is the primary source of the total NO.sub.x emissions from a steam generating power plant. The formation rate of fuel NO.sub.x is strongly affected by the rate of mixing of the fuel and air stream in general, and by the local oxygen concentration in particular. The formation of fuel NO.sub.x is a multi-stage process. During initial coal particle heat up the coal is broken down into both volatile matter consisting of reactive cyanogens, oxycyanogens and amine species and char consisting of unburned carbon, hydrocarbons and ash. In an oxygen rich environment the volatile matter will convert largely to NO.sub.x and in a fuel rich environment it can be reduced to N.sub.2. The remaining fuel bound nitrogen is released during char combustion. For char combustion to approach completion, an oxygen rich process is required. As with the volatile released NO.sub.x, the eventual fate of char released nitrogen is dependent upon the specific time, temperature and stoichiometric history.
The stoichiometric ratio, .phi., of a combustion process is defined here as the number of moles of oxygen supplied to combust a given quantity of fuel divided by the number of moles of oxygen theoretically necessary to combust a the same quantity of fuel. Typically, the stoichiometric ratio in a fossil fuel fired steam generating power plant is a quantity greater than or equal to one and can be expressed as a percentage in which case it is referred to as percent theoretical air, .tau.=.phi..times.100. A related term is excess air which is (.phi.-1).times.100 or .tau.-100.
From the preceding it should be apparent that by controlling the distribution and mass flow rate of air to the combustion process the stoichiometric ratio of the process is controlled and thus the formation of NO.sub.x. One method of controlling the mass flow rate of air to the combustion process within a tangentially fired furnace in order to effect a low NO.sub.x condition is through the use of staged combustion. Typically a main burner zone is defined wherein pulverized coal is combusted in a fuel rich environment. This is accomplished by withholding a portion of the total air required for complete combustion. This portion of air, which may appear in multiple segments and is commonly known as overfire air (OFA), is instead introduced above the main burner zone and mixed with the products of incomplete combustion after the O.sub.2 content in the main burner zone is consumed. Staged combustion minimizes NO.sub.x formations via two mechanisms. First, by having a fuel rich atmosphere during the first stage, the initial amount of fuel NO.sub.x formed is reduced because less oxygen is available to combine with the fuel bound nitrogen. Second, lower fuel NO.sub.x results because of the reduced air concentrations during the initial firing stage, thus, primary stage residence time increases. Residence time is the amount of time necessary for a coal particle to combust. The increased residence time provides an environment which is conducive to the reduction of any oxidizable N.sub.2 volatiles that have been formed such as NH.sub.3 or HCN. This is done by entraining and reducing NO.sub.x compounds and the volatiles into their elemental components, oxygen and nitrogen, and combusting the hydrocarbons. Furthermore, staged combustion reduces the peak flame temperatures, resulting in lower thermal NO.sub.x formation.
In a typical configuration for a staged firing system utilizing OFA, combustion air is supplied by a forced draft fan to a common vertical plenum, known as the windbox, and then distributed to the furnace through a number of parallel ducts. Flow rates of combustion air are modulated by individual dampers. For control purposes the dampers are grouped into three categories: fuel/air dampers, adjacent to the fuel elevations, auxiliary air dampers, located between fuel elevations, and overfire air dampers, located above the fuel elevations. The OFA dampers can be further divided into two groups: close-coupled overfire air dampers which feed directly off of the top of the windbox and separated overfire air dampers which supply air to the upper levels of the furnace. Total flow of secondary air to the furnace is controlled by the forced draft fans. The auxiliary air dampers are used to control the windbox-to-furnace pressure differential, dp, as a function of total unit air flow. The fuel/air damper positions are set as a function of the coal feeder speed and the overfire air damper positions are set as a function of unit load or in some cases unit air flow.
Typical prior art combustion control systems consist of a means by which to measure total air flow to the furnace coupled with a means by which, in a preprogrammed manner, overfire air dampers are sequentially opened as unit air flow and unit load are increased; and in a reverse manner are sequentially closed as unit air flow and unit load decrease. This sequencing is based upon the designer's experience and must be field adjusted for a particular unit, at a given load, burning a given fuel. Thus, current combustion control technology makes no attempt to monitor or control main burner zone stoichiometry.
Achieving low NO.sub.x emissions comes at a cost, i.e. as NO.sub.x emissions diminish there is a concomitant increase in carbon monoxide and the presence of carbon in fly ash. The carbon monoxide and carbon in fly ash parallel one another as air is apportioned amongst the various overfire air levels. Theoretically, the ability to achieve low NO.sub.x emissions while simultaneously maintaining acceptable levels of carbon monoxide and carbon conversion efficiency depends heavily upon maintaining the proper main burner zone stoichiometric ratio. This has been substantiated by both field and laboratory testing. However, there are inherent difficulties in controlling main burner zone stoichiometry using the existing control methodology outlined above. The deficiencies include:
1. The main burner zone stoichiometry and the unit stoichiometry can not be adjusted independently. With the existing control method, if the flow of air through the forced draft fan is increased, so as to increase the excess air, the control action of the windbox-to-furnace pressure differential control loop will tend to redistribute a portion or all of the additional air to the main burner zone. Thus, the main burner zone stoichiometry will also increase as the excess air is increased. Corresponding decreases in main burner zone stoichiometry will result when excess air is decreased. PA1 2. There is no means for directly setting a prescribed value of main burner zone stoichiometry. In fact, the nonlinear relationship between overfire air damper positions and main burner zone stoichiometry makes it difficult to even anticipate the amount of position adjustment required to produce a desired amount of change in stoichiometry. These factors increase the difficulty in field tuning the unit to obtain a desired system performance. PA1 3. Changes in the windbox-to-furnace pressure differential setpoint schedule will change the main burner zone stoichiometry unless the overfire air damper positions are also adjusted. Thus, adjustments to the windbox-to-furnace pressure differential which may be made to vary firing conditions can become coupled to optimum overfire air damper settings, increasing the difficulty in field tuning the unit. PA1 1. the same fuel is used PA1 2. the fuel flow is constant for a given load PA1 3. the same windbox to pressure differential exists PA1 4. the same total air is present for a given load PA1 5. the same boiler cleanliness exists
The new control method addresses these problems by providing a means for directly setting and maintaining main burner zone stoichiometry. By current methods, maintaining main burner zone stoichiometry is problematic in that fuel flow is not accurately measured, air flow to the main burner zone is typically not measured. Furthermore, fuel analysis, and therefore theoretical air requirements, is not accurately known. The conventional approach is to use trial and error to find a damper position versus boiler load curve that gives an "optimal" main burner zone stoichiometry. But this approach gives a main burner zone stoichiometry under fixed operating conditions, i.e.:
The proposed method solves the problem of maintaining main burner zone stoichiometry by first calculating the unit stoichiometry from the measured % O.sub.2 in the flue gas. From the unit stoichiometry it is determined how much overfire air is needed for a desired main burner zone stoichiometry. Finally, the air requirements for the main burner zone are determined by subtraction and thus the main burner zone stoichiometry can be calculated.