In-Line Localized Monitoring of Catalyst Activity in Selective Catalytic NOx Reduction Systems

Abstract

Localized catalyst activity in an SCR unit for controlling emissions from a boiler, power plant, or any facility that generates NOx-containing flue gases is monitored by one or more modules that operate on-line without disrupting the normal operation of the facility. Each module is positioned over a designated lateral area of one of the catalyst beds in the SCR unit, and supplies ammonia, urea, or other suitable reductant to the catalyst in the designated area at a rate that produces an excess of the reductant over NOx on a molar basis through the designated area. Sampling probes upstream and downstream of the designated area draw samples of the gas stream for NOx analysis, and the catalyst activity is determined from the difference in NOx levels between the two probes.

Description

CROSS-REFERENCES TO RELATED APPLICATIONS[0001]This application is a continuation of application Ser. No. 11 / 112,667, filed Apr. 21, 2005, which is related to U.S. Provisional Patent Application No. 60 / 571,100, filed May 14, 2004, and claims all benefits legally capable of being offered by both such applications. The entire contents of both such applications are incorporated herein by reference.BACKGROUND OF THE INVENTION[0002]1. Field of the Invention[0003]This invention resides in the field of NOx emissions from combustion facilities, and methods and plant equipment using selective catalytic reduction (SCR) for controlling and reducing the emissions.[0004]2. Description of the Prior Art[0005]A widely used process for the reduction of NOx emissions from coal-fired utility boilers and in combustion flue gases in general is the process known as selective catalytic NOx reduction (SCR). In this process, the NOx in the flue gas is reacted with oxygen and ammonia over a solid catalyst whi...

AI Technical Summary

Benefits of technology

[0012]The present invention resides in modules for monitoring catalytic activity on a localized basis in an SCR unit, methods of on-line monitoring of SCR catalyst activity utilizing such modules, and SCR units that contain such modules, all either reducing or eliminating the deficiencies of the prior art methods. Each module is sized to extend over a lateral area of a catalyst layer that is small relative to the full area of the layer as well as the cross section of the gas flow stream through the SCR reactor and includes a pair of sampling probes for drawing samples from the gas stream traversing the area both upstream and downstream of the catalyst plus a reductant supply probe for directing a fresh reductant stream to the area. Suitable reductants are ammonia gas, a mixture of ammonia and air, and aqueous urea. The module is constructed and mounted in such a way that it does not disrupt, block, or interfere with the bulk flow through the SCR unit, and can be activated and deactivated at will without shutting down the unit. The reductant is fed through the reductant supply probe at a rate sufficient to achieve an excess (on a mole basis) of reductant over the NOx entering the catalyst area that the module is positioned over, while the samples from the sampling probes are directed to analyzers and a data processing unit that determine the NOx levels in each probe and calculate the catalyst activity from the difference in NOx levels and the area flow rate according to the following relation, which is derived from Equation (1) above:K=Aνln(1−ΔNOx)  (3)

[0013]In preferred embodiments of the invention, the module further includes a temperature sensor and, in certain embodiments, a gas velocity sensor. Further modules within the scope of the invention also include an air inlet for blowing soot from the catalyst in the module area, and miscellaneous flow control components such as an ammonia injection grid when ammonia is used as the reductant, the grid is sized to distribute the added ammonia over the catalyst in the module area, a static mixer to promote uniform mixing of the added reductant into the gas stream in the module area, and lateral barriers or shrouds sized to surround the catalyst to prevent the divergence of gas flows from the catalyst area to adjacent areas or vice versa.

Problems solved by technology

This in turn results in greater amounts of unreacted ammonia leaving the unit (the “ammonia slip”) and therefore greater cost in operating the unit, a greater risk of pollution, and possible adverse impacts on downstream equipment.

Catalyst degradation is further complicated by the fact that in large catalytic reactors the catalyst is deployed in two or more distinct and separated layers, with different layers tending to degrade at different rates.

Monitoring methods that are in current use introduce inefficiencies of their own, however.

These methods will not differentiate between situations in which all catalyst layers are losing activity at approximately the same rate from those in which upstream layers are losing activity at a greater rate than those downstream.

A further difficulty is that increases in the ammonia slip or in the ammonia content of the fly ash may be the result of factors other than catalyst activity, such as an ammonia injection grid that is not properly adjusted or any other irregularity in the ammonia injection system.

Unless the unit is expressly shut down for the sampling, the time interval between sampling opportunities will be dictated by the outage schedule of the unit rather than concerns over the catalyst activity and can be very long.

A further problem with the withdrawal of catalyst samples is that the analyses of these samples provide no information regarding how any observed decrease in catalyst activity occurred, i.e., whether the decrease occurred slowly and gradually over time or by a step change resulting from a boiler upset, a fuel change, or some other occurrence not related to the SCR unit itself.

Images