The key factors for hot, uniform combustion are the constant mixing of air into the material being burned and the use of partially combusted material to heat and ignite new material introduced into the combustion chamber. Three major European grate designs have world-wide application:
Martin process (see Figure 10.9.4). In this design, the grate has a reverse reciprocal action; it moves alternately down and back to provide continuous motion of the refuse. The net motion of the refuse is downward toward the bottom of the furnace, but the agitation caused by oscillation of the grate causes considerable mixing of the burning refuse with the newly introduced material leading to rapid ignition and uniform burning. Van Roll process (see Figure 10.9.5). This design has three sections: the first dries the newly introduced refuse and ignites it, the second serves as a primary combustion grate, and the last reduces the refuse to ash. Grate elements move so that at a given time for any pair of elements, one is moving and one is stationary. This process results in the refuse moving toward the bottom of the furnace, but the shuffling action of the grates agitates the fuel bed enhancing the combustion process. VKW or Dusseldorf process (see Figure 10.9.6). This grate is comprised of several horizontal drums with a diameter of 1.5 m (5 ft). The shafts of the drums are parallel one after the other at a 30° slope. The drums are placed on 1.75-m (about 7-ft) centers. Each drum is built of bars (cast iron) in the form of arched segments
Waste Feed Hopper
Waste Feed Hopper
which are keyed to a central element below. Each drum rests over a separate chamber to control underfire air. The unit rotates in the discharge direction at an adjustable peripheral speed which varies according to the constituents of the waste being burned. The drum shafts lie in the bearings placed in the outside walls of the unit, and each roller is fitted with a driving gear and can be regulated independently of the others. Ignition grates at the front end of the incinerator generally rotate at up to 15 m/hr (50 ft/hr). The burnout grates normally rotate at 5 m/hr (about 16 ft/hr) since they have little waste material to move. The room under the grate is divided into a zone for each roller, to which preheated or cooled flue gas (about 200 to 256°C) can be brought. A special feeding arrangement carries the refuse from the feeding chute to the grate.
Another aspect of grate design is the percentage of air openings provided in the grate. These air openings vary
from 2 to over 30% of the grate area (Velzy 1968). Proponents of the larger openings feel that the siftings (the ash from the fuel bed) should be allowed to fall below the grate as soon as possible and large amounts of air should be permitted to pass through the bed to meet the combustion requirements of varying fuel characteristics. Proponents of the smaller openings cite advantages such as the small volume of siftings, the small amount of un-derfire air that is required, and the resulting shorter combustion flames, all of which reduce particle entrainment in the escaping gas (Velzy 1968).
In a technique employed at some U.S. facilities, waste is pneumatically injected into the furnace system and burned while suspended in the furnace chamber, rather than being burned on a grate (see Figure 10.9.7). With the removal of ferrous metals and other noncombustibles in typical RDF systems, a boiler system has evolved and has been in commercial operation at Biddeford, Maine, since 1987. The controlled combustion zone (CCZ) boiler design is a state-of-the-art boiler design for both wood and RDF boilers (Gibbs and Kreidler 1989).
The hourly burning rate (Fa) varies from 60 to 90 lb of MSW per sq ft of grate area (Velzy and Hechlinger 1987). An hourly rate of 60 lb/sq ft reduces refractory maintenance and provides a safety margin. In coal burning furnaces, the grates are usually covered to a depth of 6 in, which corresponds to an hourly coal load of 30 to 40 lb/sq ft. The heating values and densities of uncompacted MSW are less than half of that. Thus, the same firing densities (on a Btu basis) produced by coal can be produced by MSW when the MSW is supplied at an hourly rate of 60 lb/sq ft and covers the grate to a depth of 3 to 4 ft. The required grate area in square feet is directly proportional to the maximum charging rate F (lb/hr) and inversely proportional to Fa, the grate area A, as follows:
The grate design must also be based on the manufacturer's design criteria. Basically, the only consistent design criteria used by manufacturers is the specified kilogram (pounds) of waste that can be loaded per square meter (square foot) of the grate area. Planners need more empirical data for proper design and must develop a more rational approach to select the proper grate.
The firing furnace capacity is a function of its grate area and volume. The furnace volume is usually determined on the basis of an hourly heat release of 20,000 Btu/cu ft. If the hourly release rate is 20,000 Btu/cu ft and the heating value of the MSW is 5000 Btu/lb, the hourly firing rate is 4 lb/cu ft of furnace volume. A typical design basis is to provide 30 to 35 cu ft of furnace volume for each tpd of incinerator capacity (Velzy and Hechlinger 1987).
The basic requirement of any combustion system is a sufficient supply of air to completely oxidize the feed material. The following chemical and thermodynamic properties must be considered in incinerator design: the elemental composition, the net heating value, and any special properties of the waste that can interfere with incinerator operation. The stoichiometric, or theoretical, air requirement is calculated from the chemical composition of the feed material. Planners must know the percentages of carbon, hydrogen, nitrogen, sulfur, and halogens in the waste as well as its moisture content to calculate the stoichiometric
RDF Feed System
RDF Traveling Grate Stoker With Overbed Feed
CCZ Lower Furnace
RDF Traveling Grate Stoker With Overbed Feed
FIG. 10.9.7 RDF furnace. (Reprinted, with permission, from D.R. Gibbs and L.A. Kreidler, 1989, What RDF has evolved into, Waste Age [April].)
a combustion air requirements and predict combustion air flow and flue gas composition.
Table 10.9.3 shows the stoichiometric oxygen requirements and combustion product yield for each waste component. The stoichiometric air requirement is determined directly from the stoichiometric oxygen requirement with use of the weight fraction of oxygen in air. Given temperature and pressure, the required volume of air can be calculated based on gas laws.
If perfect mixing could be obtained and waste burnout occurred instantaneously, only the stoichiometric requirement of air would be needed. However, neither of these phenomena occurs in real-world applications. Therefore, some excess air is required to ensure adequate waste-air contact. Excess air is usually expressed as a percentage of the stoichiometric air requirement. For example, 50% excess air implies that the total air supply to the incinerator is 50% higher than the stoichiometric requirement.
In general, the minimum excess air requirement for an incinerator depends on the degree of mixing achieved and waste-specific factors. Most incinerators require 80 to 100% excess air to burn all organics in the MSW (Wheless and Selna 1986). Incinerator operation is optimized when sufficient oxygen is provided to achieve complete combustion, but no more. Additional oxygen reduces thermal efficiency and increases nitrogen oxide generation.
The cold air volume required for proper combustion in the incinerator per unit weight of MSW can be calculated as follows (Essenhigh 1974):
Total Cold Air Volume (cu ft/lb) = B (1 - a - M)(S)(1 + e)
B = the dry and inert-free (DIF) heating value, in Btu/lb of MSW
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