ABSTRACT

Legislation to control NOX emissions, one cause of acid rain and ozone induced smog, has created an impetus to control NOX emissions. Selective Non Catalytic Reduction (SNCR) using urea chemistry is utilized to control NOX emissions from boilers, municipal waste incinerators, refinery furnaces, recovery boilers, utilities and other stationary combustion sources. Control requires injecting urea-based solutions into the flue gas at specified temperatures. Urea solutions accelerate CaCO8 precipitation in industrial waters used for dilution, and thereby interfere with proper application of the urea solution. The negative effect of urea solutions on hardness stability is discussed as well as how CaCO8 precipitation in urea solution can be controlled by suitable scale inhibitors.

INTRODUCTION

The Clean Air Act Amendment (CAAA) of 1990 under Titles I, III and IV legislate reduction of NOX (N0, N02) emissions by stationary combustion sources. Title I includes imposition of Reasonable Available Control (RACT) Technology for NOX and Volatile Organic Carbon (VOC) emitters, and Title III gives retrofit and new source guidelines for Municipal Waste Combustors (MWC9) . The intent of the NOX emission reduction is to prevent acid rain, induced smog and improve the general air quality. ozone This means industrial operations as diverse as chemical plants, refineries, pulp and paper plants, electric utilities, municipal waste incinerators and metal/metallurgical plants among others must comply in time frames varying between 1995 and the year 2010. Regional and state rules affect approximately one hundred municipalities and counties that are in non-attainment areas, and thereby strongly affect industry within these areas. Among such regions are Southern California, the Northeast and portions of Texas, Florida and Illinois.

NOX can be formed by the combustion of nitrogen compounds in natural gas, fossil fuels, wood, municipal waste or other sludge. NOX is also formed by the oxidation of nitrogen in air under combustion conditions. Equation 1 may occur at temperatures greater than 1100ºC (2000ºF).

(Equation in full paper)

SNCR Operation System Flow. Figure 1 gives a simplified flow diagram of a typical SNCR process. For reasons of economics and volume, a concentrated product - containing typically 50% urea - is shipped to the plant site where it is stored in bulk. As required by plant operating requirements and condition, the concentrate is diluted with plant water to typically 5–15% urea. The diluted urea is then pumped to wall mounted spray nozzles or in-furnace lances where the urea treatment is sprayed into the flue at a point above the burners where the combustion gas is optimally 925ºC-1100ºC. The urea solution is sprayed in such a manner that the entire critical flue volume is covered. Urea will then undergo a series of gas phase reactions that end with the summary reaction shown in Equation 2. These reactions occur within a residence time frame of approximately 0.2 to 4 seconds as a function of temperature and the design of the boiler or incinerator. The location of the injection point(s) is determined by computer modeling based on pyrometer measurements and plant engineering design.

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