Combustion systems convert potential chemical energy in fuels (e.g. natural gas, hydrocarbon, or hydrogen) into another form of energy such as heat or kinetic energy. The fuel is mixed with air or oxygen and combusted, whereupon the combustion products and unburned fuel and air are exhausted from the system in a gaseous stream. Well-known combustion systems include a wide variety of devices including boilers, furnaces, and reciprocating engines.
Both the efficiency and performance of a combustion system can be improved by regulation of the combustion process. For example, precise control of the fuel to air ratio can significantly reduce fuel consumption, while simultaneously reducing toxic emissions in the exhaust stream. Regulation is typically accomplished by a control system that monitors the concentration of combustion products such as nitrogen oxides (NO.sub.x), carbon monoxide (CO), and unburned fuel and air in the exhaust stream. The relative concentrations of these exhaust components provides information regarding the combustion system's operation. Important operating parameters such as the fuel to air ratio may then be adjusted to improve operation of the combustion system. Although the requirements of control systems are specific for each different combustion process, the principles governing the control mechanism are similar.
Several problems exist with regard to current sensing technology used in combustion control systems. Most existing sensors are capable of monitoring only a single exhaust gas component at a time in a combustion process. This component, typically O.sub.2, is continuously monitored and the concentrations of other components are estimated based on mass balance and thermodynamic equilibrium. However, combustion is a highly non-equilibrium process, so concentrations determined in this manner may be inaccurate. Potentiometric sensing of other combustion product gases such as NO.sub.x, SO.sub.x, CO.sub.x, and H.sub.2 S, and unspent fuel gases such as CH.sub.4 and C.sub.3 H.sub.8, have been reported but the sensors have demonstrated limited performance. Control systems could be significantly improved if several gaseous components could be accurately monitored simultaneously.
Several ex-situ techniques based on optical phenomena are capable of multi-functionality, but suffer from other disadvantages. For example, Fourier Transform InfraRed (FTIR), and Non-Dispersive InfraRed (NDIR) sensors can accurately monitor multiple gas concentrations, but are very large and expensive. Further, these sensors require the installation of gas sampling lines which makes the sensors cumbersome, and significantly delays control system response. Other ex-situ techniques such as Gas Chromatography (GC), Mass Spectrometry (MS) and most traditional chemical analysis methods are less expensive but are also cumbersome and time consuming. For these reasons, ex-situ sensors based on FTIR, NDIR, GC and MS, and traditional chemical methods are impractical for combustion control systems.
A need exists for a simple, low-cost, multi-functional sensor capable of sensing more than one gas component simultaneously. The sensor should be reliable and accurate, and be able to withstand the harsh environment of the exhaust stream from an operational combustion system.