Incinerator Air Pollution Control (APC)
Air pollutant emissions have become the focus of public concern and regulatory scrutiny regarding incineration facilities. The fraction of total facility capital cost for system components functionally directed at compliance with air emission limitations is, for many plants, more than 35% of the total investment. In many instances, the award of air emission permits is the largest hurdle in the construction of incineration facilities.
Because of these realities, those wishing to construct and operate incineration systems should explore and understand the relationships between the quantity and characteristics of air pollutant emissions and:
- The specific chemical and physical characteristics of the wastes to be burned.
- The design features of the incinerator.
- The operating conditions in the incinerator.
- The control affected by the air pollution control (APC) devices.
With this knowledge, the incinerator operator can institute controls on the types or relative firing rate of the wastes to be burned and/or configure the plant design and the operating procedures to assure that no adverse air quality impacts detract from the benefits of incineration as a useful tool in waste management.
Air Pollutants from Combustion Processes
In the combustion of fuels and wastes, air contaminants are generated that have significance to the design engineer or system analyst in three areas:
- Obtaining permits from regulatory agencies for installation and/or operation of the system.
- Establishing the specifications for APC systems.
- Establishing the basic design, suggesting modifications to existing designs, or interpreting problems in emission minimization or control.
- Establishing the requirement for a Continuous Emission Monitoring System (CEMS).
There are many air pollutants emitted from combustion processes. The most basic is inorganic particulate matter. The total suspended particulate (TSP) is, predominantly relatively inert “ash”: a mixture of benign compounds primarily composed of silicon, aluminium, calcium, iron, aluminium, and oxygen. However, this portion of combustor emissions also includes the “heavy metals” of lead, mercury, cadmium, arsenic, and other elements that may have significant toxic, carcinogenic, and other health effects.
Finally, the inorganic TSP includes an important portion of the small particle size material denoted as “PM2.5.” PM2.5 is the respirable fraction under 2.5μm in mass mean diameter and has been shown to have an important role in both the onset and aggravation of asthma and other respiratory diseases. Data on fine particulate matter shows sulphates and nitrates to be the most abundant species in atmospheric aerosols with sulphates being the predominant contributor to PM2.5 . A substantial fraction of the total atmospheric PM2.5 is formed outside the stack from SOx and NOx precursors (secondary PM2.5). However, because of the health concerns regarding PM2.5, one might expect more stringent increasing limits to be placed on acid gas and NOx emissions, even if baghouse, ceramic filtration, and wet electrostatic precipitator (WESP) controls effectively capture the primary PM2.5 in the exhaust gases.
A second category of emission includes the combustible solids, liquids, and gases. A portion of these combustibles can be a fraction of the raw waste originally fed to the unit. Beyond this, a complex mixture of products of incomplete combustion (PICs) is usually found. These include carbonaceous soot and char; carbon monoxide; “hydrocarbons”; and representatives of many classes of carbon–hydrogen–oxygen–nitrogen–halogen compounds such as benzene-soluble organic matter (BSO), polycyclic organic matter (POM) (e.g., benzo-(α)-pyrene), and a variety of polyhalogenated hydrocarbons (PHH), including the isomers and congeners comprising the families of polychlorinated and polybrominated dibenzo furans (PCDF, PBDF), dibenzo p-dioxins (PCDD, PBDD), and PCB and polybrominated biphenyls (PBB).
Some of the organic and/or inorganic compounds emitted from incinerators exhibit (or are suspected to exhibit) significant adverse health effects. Some of the compounds react in the atmosphere (especially, under the influence of ultraviolet radiation) to generate ozone and a spectrum of oxygenated reaction products that can irritate the eyes and weaken pulmonary systems. With the great strides in sampling and analysis technology in recent years, the emission of these compounds can be quantified, regulated, and used as a basis for fines and penalties and, of vital importance, continuation of permission to operate.
There are several pollutants where the emission level is directly related to fuel chemistry. These include sulphur oxides, halogens and hydrogen halides, trace elements, and radioactive elements. Others, such as nitrogen oxides, show emission levels that are related to both fuel chemistry and the combustion process. Inert particulate emissions are related to the fraction of “ash” in the feed and the fluid flow and other physical processes that can elutriate and convey the material from the combustion zone. Net pollutant emissions of pollutants such as the PICs arise partly from the waste chemistry (contributing “building blocks”), partly from failures in the combustion process (generating the PICs), and partly from successes in the combustion process (destroying a portion of the PICs).
Equipment Options for Incinerator Air Pollution Control
1. Settling Chambers
A settling chamber is a distinct chamber or zone where the gas velocity is reduced so as to permit gravity settling of the particulate to occur. The controlling physical relationship (Stokes’ law). The capture of particulate by settling is sensitive to re-entrainment, so settling chambers are often designed with a wet bottom wherein a sluice of water is used to both trap and convey solids. Clearly, settling chambers are not highly efficient and their applicability in modern plants is nil as a means to meet emission regulations.
2. Cyclones And Inertial Collectors
“Cyclone” denotes the most common member of a family of particulate control devices that depend on inertial forces to remove relatively large, massive particulate materials from a gas stream. The removal efficiency of a cyclone is related to the angular velocity achieved in the vortical flow zone within the cyclone separator. As the rotational velocity within the device increases, progressively smaller particles reach the collection area at the outer wall and are captured. The inlet ducting is often tailored (a tangential, helical, or involute entry) and/or internal vanes are used to facilitate the vortical acceleration and deceleration in order to minimize pressure losses.
Cyclones are fabricated of carbon steel for softer particulate (e.g., wood sawdust or sander dust). Most fly-ash materials are very abrasive and the collection cone of an ordinary carbon steel cyclone would rapidly erode and perforate. Therefore, hardened, abrasion-resistant cast iron or similar materials are used for the cones in fly-ash applications. For high-temperature applications where low
efficiency is acceptable (e.g., removal of the bulk of the coarse particulate from the flue gas leaving an FB sludge incinerator), a refractory lining is cast in a large-diameter steel casing.
3. Wet Scrubbers
Wet scrubbers include a broad class of APC devices with, often, a dual functionality: removal of PM and absorption of one or more gaseous pollutants. The gas cooling and humidification taking place in a wet scrubber may be important (and, sometimes, undesirable) side effects. The focus of design depends on the regulatory and flue gas contamination scenario. Although wet scrubbers can be effective at particulate and gas removal, their operation introduces a process waste (an aqueous discharge) needing either a sewer or a receiving stream. This new effluent may require monitoring and/or treatment and almost certainly requires a permit. These disadvantages often encourage the use of dry collectors as the preferred alternative.
In modern incineration applications with stringent particulate limits, the Venturi and tray/sieve scrubbers are, by far, the most common. In most applications, a “quencher” is installed upstream of the wet scrubber to cool and humidify the combustion gases before entering the scrubber properly. This is particularly important when the scrubbing liquid incorporates dissolved or suspended solids. Without a prequench with relatively clean water, the contact of hot (especially >250°C), dry gas with the liquid leads to flashing of the water and formation of
an aerosol of fine particulate that is hard to collect in the downstream zones of the scrubber.
4. Dry Electrostatic Precipitator Systems
The ESP uses electrical forces to move PM in a flowing gas stream to a collecting surface. The particles are electrically charged by passage through a corona: a region characterized by a luminous blue glow within the ESP containing a high concentration of gaseous ions. Either positive or negative ions or electrons can be generated in a corona but, for industrial gas cleaning operations, the negative
corona is most commonly used. The charging process is most effective with large particles since they sweep a proportionally larger cross-sectional area and accumulate more electrons as they pass through the corona region. The converse can reduce ESP efficiency: inadequate particle charging with high concentrations of submicron particles leading to “space-charge quenching” (high voltages with low currents).
Since the particles form a continuous layer on the collection plates, all of the ion current must flow through the dust layer. This current creates an electrical field in the dust layer that can become large enough to cause local electrical breakdown, injecting new ions (of the wrong polarity) into the space between the discharge electrodes and the collection plates. This reduces the charge on the particles and may cause sparking. This breakdown condition is called “back corona” and leads to a decrease in collection efficiency.
An optimum sparking rate of 90–100 sparks per minute per section of ESP is common. Conventional automatic control devices for ESPs continuously ramp up the voltage until sparking ensues, then back off and start increasing again. If a unit is adjusted to avoid sparking, it generally means that the maximum efficiency potential of the ESP is not being achieved. One must also be careful that misalignment of electrodes, condensation (especially with SO3-containing gases), and/or dust accumulation in and about the high-voltage penetrations of the ESP shell do not have an adverse impact on the sparkover rate.
5. Fabric Filter (Baghouse)
Fabric filtration is an effective means to remove fine particulate from a gas stream. Removal efficiency remains high in comparison with most other control technologies for submicron particulate. This is the particulate fraction that is of special significance in the emissions of respirable particulate (PM2.5 and PM10) and of many carcinogenic and toxic organic aerosols and heavy metals. It is not surprising, therefore, that the use of filtration-based devices in incineration applications has grown significantly as regulations have become more and more stringent.
Filtration devices are of two types: cloth or Fabric Filters and in-depth or bed filters. Fabric Filters are generally found as an array of long, cylindrical bags mounted in a structure (a “baghouse”). Bed filters can be of several types but, in incineration applications, are most often a deep, packed bed (a “gravel bed” filter).
The key characteristics of fabrics for use in gas filtration include the following:
- Temperature: A maximum continuous service temperature higher than the normal working temperature. Peaks in temperature and their duration must be considered if the temperatures do not reliably hold at or below the norm.
- Corrosiveness: Some gas constituents are aggressive and will attack the fabric. The inherent resistance to such an attack is specific: gas to fiber.
- Hydrolysis: Flue gas humidity can affect the strength and dimensional stability of the fabrics.
- Cost: Obvious in importance but evaluated in the light of air-to-cloth (A/C) ratio (affecting the number of bags), unit cost per bag, loss during installation (important with some bags that are sensitive to mishandling), and expected service life (replacement cost).
6. Ceramic Filter
Ceramic filtration is a very effective means to remove fine particulate from a gas stream and allows for easy integration of a Dry Scrubbing system (see below). Ceramic filtration systems operate in a similar method as Fabric Filters, but with the bags replaced with porous ceramic filter elements/candles. The advantages of the Ceramic Filtration over Fabric Filters are:
- Filtration at a higher temperature – filter elements can withstand temperatures of up to 900°C compared to 180-220°C for fabric filters. Besides better endurance to temperature spikes, a higher temperature also allows for more efficient scrubbing of acid gasses.
- Ceramic elements are non-flammable.
- Ceramic elements are resistant to acids and alkalis.
Absorption is a process for the removal of one or more components from a gas mixture. Absorption is often thought of as involving contact of a gas with a liquid (as in a wet scrubber). However, the principles are applicable in completely dry systems (e.g., the absorption of acid gases by injected lime or sodium bicarbonate) or “semidry” systems. For the latter type, the absorption of acid gases occurs on lime slurry droplets where the slurry is injected in quantities and with particle sizes and lime concentrations such that complete evaporation of the slurry water occurs.
The particulate abatement technologies described previously use a variety of physical principles to affect the capture of individual particles. In absorbers, the mechanism by which pollutants are captured involves a mass transfer flux driven by molecular and eddy diffusion from a region of high concentration to one of low concentration. Capture can involve a reversible (equilibrium) association of pollutant (usually present with a large quantity of diluting carrier gas that does not absorb) with an absorbing material.
Wet Packed Towers for Removal of Pollutant Gases
One of the most common absorber devices for the removal of acidic gases is the packed tower. Here, a vertical, cylindrical column is erected. A given depth (Z) of the column is filled with a packing material that is chemically inert and designed to give a minimum of pressure drop and a maximum
of gas–liquid contact area. In accord with good practice, the size of the packing is not more than 1/20th of the column diameter. The column is operated at between 5% and 75% of the flooding rate.
Flooding rate is defined as the gas volumetric flow rate that for the tower diameter and packing would inhibit the flow of liquid down through the tower to the degree that the flow would, substantially, be blocked.
Dry Absorbent Contactors for Acid Gas Pollutant Control
The highest removal efficiency for gaseous pollutants is often achieved using a wet scrubber with appropriate chemical addition such that the equilibrium partial pressure of the target pollutant is very low. An example would be HCl scrubbed with a caustic soda solution.
However, although the wet contactors are efficient, they produce a wastewater requiring treatment and disposal, present not insolvable but problematic corrosion problems, generate a visible plume (unless special and costly countermeasures are taken), and often present numerous operation and maintenance problems with plugging, solids build-up, and the like.
The dry absorbent systems avoid many of these problems and, although they present their own deficiencies, they find use in some incineration applications. This is particularly the case for acid gas abatement.
Dry sorbent injection involves the injection of solid, alkaline material into the flue gases of an incinerator to effect gas–solid reactions in the time available prior to discharge to the stack.
The combination of dry injection with Ceramic and Fabric filters has a special advantage: the alkaline absorbent is deposited with the particulate in the cake on the filtering medium. Thereafter, the filter cake acts as an alkaline absorbent and continues to remove acid gases. This increases the utilization of reagent.
8. Carbon Adsorption
This process technology involves injection of activated carbon into the flue gases followed by physical and chemical adsorption of vapor-phase mercury and its compounds on the carbon. “Doping” of the carbon with chemicals (such as sulphides) that create strong chemical bonds to elemental mercury may enhance capture. In combination with the removal of solid-phase mercury compounds in the particulate control device, high overall control efficiencies are realized.
Of the several technologies for mercury control, there is more experience with injection of dry activated carbon than with alternatives where the carbon is added to the lime slurry tank and injected into the flue gas stream through the SDA system. The captured mercury compounds are ultimately found in association with the fly ash. Capital costs are relatively low and, due to the low dosage rates (about 60 mg/m3 of flue gas), the chemical costs are medium to low (in absolute dollar terms). In concept, activated carbon addition can be made either ahead of or after a spray dryer and