Reduction and Control Technologies
Flue Gas Treatment Controls
NOx control with flue gas treatment involves the reduction of NOx in the flue gas by injecting a chemical reducing agent into the post-combustion region of a combustion unit. The reducing agents, primarily ammonia and urea, convert the NO in the flue gas to molecular nitrogen at high temperatures, between 1,600 and 2,000°F, without a catalyst. When a catalyst is used, this conversion takes place at a lower temperature range, roughly 575 to 800°F. Flue gas treatment methods without a catalyst are SNCR, while those with a catalyst are termed SCR.
Retrofitting these technologies to boilers typically involves installation of reagent injection nozzles, reagent storage and control equipment, and, in the case of SCR, catalytic reactors. Because flue gas treatment NOx reduction efficiency depends in large part on flue gas temperature, injection nozzle placement is limited to those locations where acceptable process temperatures are present.
Generally, in packaged industrial boilers, available locations for reagent injection and catalyst placement are further limited by space considerations. These units may also operate with wide ranges in boiler steam load that cause flue gas temperature shifts outside the optimum temperature window. Injection of reagents outside the optimum reaction temperature window results in lowered NOx reduction efficiency and emissions of unreacted ammonia.
Two primary types of SNCR control technologies are currently available for retrofit to boilers. The first is based on the use of ammonia (NH3) as the reducing agent, while the second is based on the use of urea (NH2CONH2). The figure below illustrates the basic process:
Generally, similar NOx reduction efficiencies are obtained whether ammonia or urea is used. Under carefully controlled conditions, reduction levels of 70% are possible, but reductions in the range of 40 to 50% are more typical.
The technology can be difficult to apply to industrial boilers that modulate or cycle frequently. This is because the ammonia (or urea) must be injected in the flue gases at a specific flue gas temperature. And, in industrial boilers that modulate or cycle frequently, the location of the exhaust gases at the specified temperature is constantly changing.
Exxon Research and Engineering Company developed and patented an ammonia-based SNCR process known as Thermal DeNOx. The Thermal DeNOx process is based on a gas phase homogeneous reaction between NOx and ammonia which produces molecular nitrogen and water at high temperature. In this process, aqueous or anhydrous ammonia is vaporized and injected into the flue gas through wall-mounted nozzles at a location selected for optimum reaction temperature and residence time. The optimum reaction temperature range for this process is 1,600 to 2,000°F, although this can be lowered to 1,300°F with additional injection of gaseous hydrogen.
Unreacted ammonia is commonly referred to as ammonia slip, breakthrough, or carryover. The amount of ammonia slip also depends in part on the amount of ammonia injected. Achievable NOx reductions for an individual boiler depend on the flue gas temperature, the residence time at that temperature, the initial NOx concentration, the NH3/NOx ratio, the excess oxygen level, and the degree of ammonia/flue gas mixing. Also, stratification of both temperature and NOx in the flue gas can affect the performance of the SNCR control. The optimum placement of SNCR injectors requires a detailed mapping of the temperature profile in the convective passes of the boiler, because of the narrow temperature window.
In one urea-based SNCR process (Nalco Fuel Tech NoxOut), an aqueous solution containing urea and chemical enhancers is injected into the furnace or boiler at one or more locations, depending on the boiler type and size. The urea reacts with NOx in the flue gas to produce nitrogen, carbon dioxide, and water. The main advantage of urea injection over ammonia injection is that urea is a nontoxic liquid that can be safely stored and handled.
Like ammonia injection, SNCR is effective only within a certain temperature range. Without the use of chemical enhancers, urea injection effectively reduces NOx at temperatures between 1,650 and 2,100°F. Residence time at temperature of interest is important. By using proprietary enhancers and adjusting concentrations, greater NOx reduction efficiency can be achieved over a wider temperature window. If the urea is released at too high a temperature, the chemical species can actually be oxidized to form NOx. Below this temperature, urea reacts with NOx to form undesired amounts of ammonia.
Due to residence time and temperature constraints, small packaged watertube and firetube boilers with fluctuating steam loads are difficult applications, and require case-by-case determinations for cost and performance levels.
Selective Catalytic Reduction (SCR)
The SCR process takes advantage of the selectivity of ammonia to reduce NOx to nitrogen and water at lower temperature in the presence of a catalytic surface. Two catalyst formulations are denoted “base metal” – this category including oxides of titanium, molybdenum, tungsten, and vanadium; and zeolites – which are alumina-silicate-based. These formulations may include other components that impart structural stability. Catalysts come in various shapes and sizes, according to the particular application.
Gaseous ammonia is injected with a carrier gas, typically steam or compressed air, into the flue gas upstream of the catalyst. The ammonia/flue gas mixture enters the catalyst, where it is distributed through the catalytic bed. The flue gas then leaves the catalytic reactor and continues to the exit stack or air preheater. Ammonia slip tends to be less with SCR than with SNCR.
SCR operates most efficiently at temperatures between 575 and 800°F and when the flue gas is relatively free of particulate matter, which tends to contaminate or “poison” the catalytic surfaces. Operation above this temperature range can reduce the effectiveness of certain catalysts, and operation at lower temperatures increases the likelihood of ammonium sulfate formation within the bed.
With SCR technology, NOx removal efficiency can approach 90% or higher.
Process control of ammonia flow remains essentially constant during periods of normal operations. The excess ammonia supplied is known as ammonia slip. To account for reaction lag time during startup and shutdown, an ammonia-rich ratio is required for increasing loads, and an ammonia-lean ratio is required for decreasing loads. Ammonia slip must be kept low because ammonia can react with other combustion constituents to form ammonia salts which can result in additional particulate matter (PM) emissions.