MULTIFUNCTIONAL (NOx/CO/O2) SOLID-STATE SENSORS FOR COAL COMBUSTION CONTROL

Abstract

We have made great progress in both developing solid state sensors for coal combustion control and understanding the mechanism by which they operate. We have fabricated and tested numerous sensors and identified the role electrode microstructure plays in sensor response. We have developed both p-type (La 2 CuO 4 ) and n-type (WO 3 ) semiconducting NO x sensing electrodes. We have demonstrated their respective sensing behavior (sensitivities and cross-sensitivities), related this behavior to their gas adsorption/desorption behavior and catalytic activity, and in so doing verified that our proposed Differential Electrode Equilibria is a more comprehensive sensing mechanism.These investigations and their results are summarized below.The composition and microstructure of the sensing electrode is the key parameters that influence the sensing performance. We investigated the effect of electrode microstructure on the NO x sensitivity and response time using a La 2 CuO 4 -based potentiometric sensor.Temperature dependence, cross-sensitivity and selectivities of a La 2 CuO 4 -and WO 3 -based potentiometric NO x sensor were investigated both in N 2 and in a simulated exhaust gas. We performed temperature programmed reaction (TPR) and desorption (TPD) experiments to determine the reaction and adsorption characteristics of O 2 , NO x , CO, CO 2 , and their mixtures on the electrodes, and related the results to sensor performance.In order to optimize the sensor electrode microstructure, powders were prepared using four different powder synthesis routes, resulting in different particle size distributions and BET surface areas. Different sintering conditions were also applied. The microstructure of electrodes, synthesized with the same composition, has a dramatic effect on both sensitivity and response time of potentiometric NO sensors, showing that large surface areas generate a porous morphology with smaller grain size, and that smaller grain size results in a sharper response and faster response time.The relative responses of the La 2 CuO 4 -based sensor under varied concentrations of NO, NO 2 , CO, CO 2 and O 2 were studied. The results showed a very high sensitivity to NO, CO, and NO 2 at 450 ºC in 3% O 2 , whereas the response to O 2 and CO 2 gases was negligible.The NO response at 400 -500 ºC agreed with the NO adsorption behavior. The high NO 2 iv sensitivity at 450 ºC was probably related to heterogeneous catalytic activity of La 2 CuO 4 .The adsorption of NO was not affected by the change of O 2 concentration and thus the sensor showed selective detection of NO over O 2 . However, the NO sensitivity was strongly influenced by the existence of CO, H 2 O, NO 2 , and CO 2 , as the adsorption behavior of NO was influenced by these gases.The WO 3 -based sensor was able to selectively detect NO in the presence of CO 2 in 3% O 2 at 650 ºC. The NO sensitivity, however, was affected by the variation of the NO 2 , CO, and H 2 O concentration. No gas-solid reactions were observed using TPR in the NOcontaining gas mixtur...