Design Considerations

The most important design considerations for incineration systems are discussed in the following sections. Manufacturers of incinerators rely on empiri cal data obtained from extensive pilot plant tests. Such information is generally proprietary; therefore, the manufacturers will typically perform the design after receiving the input data, conditions, and functional specifications for the incineration system.

Moisture Content of Feed Sludge Traditionally, combined primary and secondary sludge is usually dewatered to 16 to 25% solids (75 to 84% moisture). Dewatering to this solids concentration means that for every kilogram of solids, 3 to 5.25 kg of water must be evaporated at high temperatures. Such systems require large amounts of auxiliary fuel to evaporate the water. The excess costs of such systems, from the increase in fuel price, have resulted in the shutdown of many incinerators.

Drier sludge cakes are produced at plants that use efficient belt filter presses, high-solids centrifuges, or recessed plate filter presses for dewatering. As a general rule, fluidized-bed furnaces are preferable to multiple-hearth furnaces when the solids content is greater than 30% (WEF, 1998). This is because fluidized-bed furnaces can operate with lower excess air levels while avoiding many of the operating complications of multiple-hearth furnaces. At less than 30% solids content, multiple-hearth furnaces merit consideration, provided that air emission regulations can be met without an afterburner.

Heat Recovery and Reuse Modern sludge incineration processes rely on waste heat recovery and recycle for economic operation. Heat recovery can be for internal reuse or for secondary use. Internal reuse includes direct recovery, in which the sludge is preheated or dried by flue gases, and indirect recovery, in which flue gases are used to preheat combustion air. Preheating the combustion air is the most common and economical approach to heat recovery in sludge incineration. Secondary uses can be in the form of space heating, power generation, or using in an indirect sludge dryer to increase solids concentration and thus to eliminate the need for an auxiliary fuel.

Ash Disposal Although incineration provides the greatest reduction in the volume of sludge, there is still a significant quantity of material to be disposed of. For a specific wastewater treatment plant, the quantity of ash produced will essentially be equal to the inert fraction in the sludge. Incineration of grit, screenings, and scum will also affect the quality and quantity of ash. The quantity of ash will vary seasonally in many facilities as a function of waste characteristics and where there are significant industrial discharges. However, most increases in the quantity of inerts result from infiltration or inflow to the sewer during wet weather periods. Chemicals also affect the quantity of ash and the removal of heavy metals. Thus, where chemicals are employed in the treatment processes, the effect on the quantity of ash and metals on the disposal facilities must be taken into consideration. The quantity of ash generally ranges from 200 to 400 g/kg (400 to 800 lb/ton) of raw dry solids combusted without consideration of grit. Digested sludge will produce pro portionally more ash, about 350 to 500 g/kg (700 to 1000 lb/ton) of dry solids combusted because of the lower volatile content of digested sludge. Ash generally has a specific gravity of 2.4 to 3.0. Dry bulk densities range from 385 to 640 kg/m3 (24 to 40 lb/ft3), and wet bulk densities range from 1440 to 1920 kg/m3 (90 to 120 lb/ft3).

Ash from incinerators can be handled by two principal methods: dry ash handling or wet ash handling. Dry ash handling involves lifting the ash from the furnace with a bucket elevator or by a pneumatic method to a silo for later load-out. The bucket elevator method of lifting ash is generally applied to smaller installations. Lateral movement is usually provided by screw conveyors. This method is noisy, can be dusty, and is severe on the elevators and conveyors due to the abrasive nature of ash. The pneumatic method of lifting ash is generally applied to larger plants and where a lot of lateral movement is required, such as in multiple-unit installations. At the point of discharge, a baghouse or other type of air filter must be provided to capture the aerosol dust released when the ash drops into the silo. For ash load out from the silo, the ash is passed through a wetting device that provides enough water to control the dust that would be emitted during handling and transportation.

In wet ash handling, ash slurried with water is pumped to a lagoon for treatment and disposal. Therefore, this method is appropriate only where there is a feasible holding area (lagoon) on or near the plant site. Its principal merit is that it creates little dust. However, abrasion-resistant heavy-walled pipe and rubber-lined pumps are needed to minimize abrasion and wear.

Air Pollution Control Incineration of sludge results in the production of a sterile nonodorous ash, an approximately 90 to 96% volumetric reduction, and a large volume of combustion gases. Accompanying this process is the potential for significant degradation to air quality unless effective control technologies are employed to reduce emissions in the combustion gases.

If properly designed and operated, incineration can provide complete combustion of the organics in wastewater sludge to produce principally carbon dioxide, water, and sulfur dioxide. However, incomplete combustion can produce unacceptable intermediate products such as hydrocarbons, other volatile organics, and carbon monoxide. These products are often referred to as products of incomplete combustion. Some of these products can produce offensive odors; therefore, special attention is needed to minimize nuisance odor emissions.

Incineration of sludge has the potential for discharge of excessive particu-lates. These are the predominant air contaminants from thermal destruction, and they include both solid particles and liquid droplets (excluding uncom-bined water) that are swept along by the gas stream or formed through condensation of the flue gases. Particulates from incineration of sludge are enriched with volatile trace metals such as cadmium, lead, and zinc. Particles sizes are mostly smaller than 2 |im, and volatile elements are primarily in the submicrometer sizes (WEF, 1998). Technologies for controlling particulates from gas streams include (1) mechanical collectors, (2) wet scrubbers, (3) fabric filters, and (4) electrostatic precipitators. Selection of a particular collection system depends on the nature of the particulate matter, conditions of the gas streams, and emission limits.

Mechanical collectors exert inertia forces for particle separation. They have relatively low collection efficiency and are generally used as precollec-tors upstream of main particulate control devices. The three types of mechanical collectors in use include: (1) a settling chamber, which uses the low gas velocity through the chamber [less than 3 m/s (10 ft/s)] to settle the heavy particles; (2) an impingement separator, which directs the gas stream against collecting bodies, where particles lose momentum and drop out of the gas; and (3) a cyclone separator. In a cyclone separator, gas enters tangentially at the top of the cylindrical shell and is forced down in a spiral of decreasing diameter in a conical section. This action lets the particles spiral downward to the bottom through an airlock and lets the gas return back up at the center of the vortex and discharges from the top.

Wet scrubbers employ water to separate dusts or mists from gas streams. They are the most widely used emission control equipment for sludge incineration. They have the added advantage of removing water-soluble contaminants such as hydrogen chloride, sulfur dioxide, and ammonia. The four types of wet scrubbers are (1) spray towers, in which particles are captured by the droplets from a liquid spray at the top of the column against the rising gas stream; (2) cyclone scrubbers, in which centrifugal forces increase the momentum of the collision between the particles and the liquid droplets; (3) ejector-Venturi scrubbers, in which a high-pressure liquid jet scrubs the gas and provides the draft for moving the gas; and (4) Venturi scrubbers, in which the gas stream accelerates across the Venturi or orifice, and a scrubbing liquid is sprayed and mixed with the gas at the throat. The high turbulence causes collisions between liquid droplets and particulates; consequently, particulates are captured.

Fabric filters (baghouses) collect particulates in the gas stream by passing the gas through a filter medium or fabric. When the pressure drop across the filter increases as the dust accumulates, the filter is cleaned by mechanical shaking, pulse jet, or reverse airflow. Fabric filters are highly efficient (greater than 99% particulate removal efficiency); however, they can be used in an incineration system only if the gas temperature is reduced to 150 to 177°C (300 to 350°F).

There are two types of electrostatic precipitators: dry electrostatic precipi-tators and wet electrostatic precipitators. In a dry electrostatic precipitator, a negative charge is imparted to the particulates in the exhaust gases passing through a large chamber. The negatively charged particulates are then attracted to the positively charged collector plates in the chamber. The collected particles are removed by periodic vibration or rinsing. A wet electrostatic precipitator is similar to a dry electrostatic precipitator except that it contains a washing mechanism to counteract the buildup of volatile or par-

ticulate matter on the plates. Electrostatic precipitators can achieve removal efficiencies of 99% or greater with negligible pressure drop.

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