This invention relates generally to optical monitoring systems and more specifically to optical monitoring systems for enhancing combustion spectroscopy by reducing the interference caused by background radiation.
In various fossil energy processes which require controlled combustion of coal, for example, in special reactor vessels at elevated temperatures (2,000.degree. to 3,000.degree. F.) and pressures (100 to 1,000 psig), optical techniques offer a potential solution to many difficult monitoring problems. Optical methods offer noncontact, noninvasive sensing techniques that avoid many of the usual problems that are associated with high temperatures and pressures.
Various optical methods have been applied to fossil energy processes in recent years. These include techniques ranging from simple pyrometry to sophisticated laser diagnostic systems. However, these attempts to use optical instrumentation on fossil energy processes have been plagued, in varying degrees, by inadequate optical access to the combustion flame. In almost all cases, high temperature dictates the need for some means of cooling windows and/or instrumentation. Quartz windows are most often employed, but the radiative heat loss and cooling requirements can be large for even a moderate-size window. Also, the high pressures dictate the use of thick windows which further reduce optical access.
The field of fiber optics offers several advantages over conventional windows in this type of application. The small size and large light gathering capability of fiber optics along with their ability to allow sensitive detectors to be spaced some distance from the reactor vessel are the most notable features.
In order to apply optical instrumentation successfully to any process, the nature of the optical properties of the process must be well understood. In fossil energy applications, there have been only minimal efforts to characterize critical optical parameters such as spectral emission characteristics and optical depth. The collection of data to define these basic optical properties is an essential step for successful application of optical instrumentation.
Flame spectroscopy is a well-developed science, but the techniques that yield excellent results in a well-controlled laboratory burner are often worthless in a combustion environment. Assuming that the problem of optical access can be solved by the use of fiber optics, one still has to cope with intense thermal background or blackbody radiation from walls and entrained solid particles. The thermal radiation can be orders of magnitude more intense than the emission lines that are sought. Entrained solid particles can absorb some emissions and obscure the field of view as particles swirl in the view of the fiber optics. Rapidly fluctuating flames further complicate the problem. Measurements on a coal-water fuel (CWF) combustor reveal significant frequency components up to a few hundred hertz. Others have reported measuring frequency components at frequencies of 10 kHz or more.
Thus, there is a need for an optical monitoring system in which fiber optics can be employed and which overcomes the problems of flame emission monitoring as outlined above.