Treating flue gas minimizes or eliminates both the environmentally unacceptable nitrogen oxide (NOx) and sulfur dioxide (SO2) emissions as well as the collecting of unburned solid particles before they escape into the atmosphere. Both techniques involve a number of steps
NOx Reduction Techniques
Control of NOx pollutants begins with the proper fuel selection. A coal with a sufficiently low fuel nitrogen (less than 1.5%) as shown in a routine fuel analysis may eliminate the need for any NOx reduction techniques. Natural gas has no nitrogen in the fuel; fuel oil typically has a lower nitrogen content than coal. Coal reactivity may also be decreased to slow down combustion and decrease temperatures to minimize NOx production. Lower flame temperatures will result in a lower level of NOx production in oil and gas-fired systems. Boiler systems that have highly turbulent flames and high temperature furnaces usually need lower fuel nitrogen than is normally available in the required quantities. Reduction techniques would then be needed.
This step is only practical if a new facility is planned. By increasing the furnace cooling surface, the high temperature and time aspects of NOx production can be reduced. Another benefit is the increased flexibility in coal purchasing especially in specifications and price.
Selecting a burner relative to furnace size limits the oxygen availability to form NOx while simultaneously shaping the flame to minimize the 2800 degree residency time. This permits the use of moderate and low-nitrogen coal and meeting NOx emission regulations. This method generally produces good results.
Low Excess Air Combustion
This technique also limits the availability of oxygen and increases the efficiency of a high-turbulence burner. There is little change in the 2800-degree residency time and only fair results can be expected. Also, when this technique is used, a fairly complex series of controls must be installed to maintain the best combustion. Coal ash problems may also arise due to the reduced oxygen levels.
This technique also limits oxygen availability by adding excess oxygen, needed for complete combustion, through overfire air ports. The rate of combustion is lessened and the 2800 degree residency time is decreased. It produces good results with moderate-to-high fuel nitrogen coals. There is the possibility of decreased carbon burnout and furnace heat absorption and an increase in fireside deposits and potential corrosion.
This method also limits oxygen availability and flame temperature but for different levels of burners on large units. It is fairly successful and easily applied to existing units. As in the two-stage firing technique, there can be carbon loss and increased slagging.
Flue Gas Recirculation
This method takes advantage of the reaction that tends to drive fuel nitrogen towards N2 in the presence of NO. With NO present, there is a tendency to minimize the formation of thermal NOx by driving the reaction toward the more stable N2. This is a “last resort” technique when regulations must be met with high-nitrogen coal. It is the most effective method but it is the most expensive and difficult to install. Combustion control equipment and operating requirements with fans, ductwork and air balancing increase the complexity and can create problems.
Selective Non-Catalytic Reduction (SNCR) and Selective Catalytic Reduction (SCR)
There are two types of SNCR control technologies for retrofit to industrial boilers; one uses ammonia as the reducing agent; the other urea.
They reduce NOx in the flue gas to molecular hydrogen at high temperatures between 1600 and 2000 degrees Fahrenheit without a catalyst. With a catalyst the conversion takes place at a much lower temperature range, roughly 575-800 degrees Fahrenheit. This is called SCR. Typically these agents are injected in the post-combustion region.
Because of the significant load variations in industrial boilers which cause the optimum temperature zone to shift location in the boiler, the application and effectiveness of this type of flue gas treatment is limited.
Solid Particle Removal
The removal of solid particles from the flue gas (also called particulate emissions) is an important part of the combustion process, as proper system selection and the maintenance of that system can significantly affect plant operating costs, as well as legislative compliance. These solid particles are basically the nonburnable elements in coal that leave the furnace and boiler after combustion.
There are a number of control techniques that can be applied, varying with the type of coal and combustion equipment installed (stoker, fluidized-bed or pulverized-coal firing):
This is the oldest form of particulate collection. It extracts ash particles from the flue gas circular air current, which forces the particles to the outer portion of the current and downward into a storage hopper. It is typically found in stoker-fired boilers. Some spreader stoker fired boilers use mechanical collectors ahead of precipitators or baghouses for reinjection of the flycarbon and for an increase in overall collection efficiency.
This is an additional technique applied to mechanical collection to improve collection efficiency. In operation, some 10-20% of the flue gas is removed from the bottom hopper of the collector and cleaned in a small baghouse. This can in-crease ash collection efficiency by up to 35-50%.
These devises operate on the principle that the ash particles can accept an electrical charge. Particles pass through an electrical field and are attracted to a vertical metal plate, where, periodically, they are shaken loose and collected in the collection hopper.
These systems, quite simply, work on the same principle as a household or industrial bag-type vacuum cleaner. The ash is removed in one of two ways; a reverse stream of air is blown through the bag during collection shutdown, which re-moves the ash coating and channels it into a collection hopper. The other method involves collection of the ash on the outside of the bag. A high-pressure pulse of air is periodically forced down through the bag, shaking the ash from the bag and into the ash hopper.
These devises cause the ash to be mixed with water droplets in a high-velocity air stream. The ash-laden droplets are then collected in a down-stream scrubber demister section. Care must be taken in the disposal of the contaminated water, which will contain sulfuric and hydrochloric acid from the chemical combination of water and fly-ash. Also, additional care must be taken to assure the water is properly and completely removed from the flue gas. A wet scrubber has an advantage since additional heat is removed from the flue gas and can be recovered by exchangers for heating makeup water For comparison purposes, here are the cost differentiation factors between the various solid particulate collection systems, starting with the assumption that the mechanical collector is a factor of one:
Sulfur Dioxide Removal and Control
All coal and oil contain some sulfur. As a result, there is bound to be some amount of sulfur dioxide generated in the combustion process. Just how its emission is minimized depends on a number of available techniques.
Using washed coal is considered the best alternative for meeting sulfur regulations. Factors such as transportation, availability and price need to be considered. This practice is not as common as it used to be, given the availability of lower-sulfur coal.
Wet Nonregenerative Scrubbers
These systems can operate in a “throwaway” mode, where the sulfur dioxide gas reacts with a chemical, such as limestone, and the combined compound is disposed of or sold for gypsum. With additional processing, the elemental sulfur can be separated and made available for sale. Solids and pH levels are continuously monitored from a slipstream takeoff.
Wet Regenerative System
These scrubbers substantially speed up the collection process. However, their effectiveness requires the use of expensive sodium hydroxide or sodium carbonate, which require recovery systems. A major benefit, however, is the lack of sol-ids buildup, scaling, or critical pH control.
Here the flue gas is combined with chemicals in a water-based spray. The heat in the flue gas dries up the moisture, leaving a solid product, collectable in the baghouse. Critical elements in these systems include residence time in the chamber, flue gas temperature, which must be high enough to assure 100% moisture evaporation and adequate mixing of the chemical with the flue gas.