100

increases with residence time; increasing the temperature increases the DRE at a constant time. At efficient combustion temperatures, the rate can become mixing limited.

Modern thermal oxidation systems can accomplish +99% DRE for capacities ranging from 1000-500,000 cfm and VOC concentrations of 100-2000 ppmv. Typical residence times are 1 sec or less at temperatures of 1300-1800°F. Inlet VOC concentrations above 25% of the lower explosion limit (LEL) are generally avoided due to the potential explosion hazards. Temperatures near 1800°F and long residence times can lead to elevated nitrogen oxide levels, which may have to be controlled separately (if lowering the combustion temperature is not feasible).

Thermal incinerators are usually coupled to two types of thermal energy recovery systems: regenerative and recuperative. Both methods transfer the heat content of the combustion exhaust gas stream to the incoming gas stream (Ruddy and Carroll 1993). In a regenerative system, an inert material (such as a dense ceramic) removes heat from the gases exiting the furnace. Such a ceramic storage bed eventually approaches the temperature in the combustor, consequently reducing the heat transfer. Therefore, the hot exhaust stream contacts a cooler bed, while the incoming gas stream passes through the hot bed.

As shown in Figure 5.21.1, the VOC-laden gas stream enters bed #1 which warms this gas stream by transferring heat from a previous cycle. Some VOCs are destroyed here, but most of them are oxidized in the combustion chamber. The flue gases from this combustion exit through bed #2 and transfer most of their enthalpy in the process. Within seconds of a heating and cooling cycle, the beds are switched, and the incoming stream now enters bed #2. Consequently, a near steady-state operation is approached, and more heat can be recovered from these systems than from a typical thermal incinerator (about 70%). Using multiple beds can lead to heat recoveries to 95%. The need for any auxiliary fuel depends on the potential thermal energy of the VOC-laden stream.

Recuperative thermal oxidation systems typically use a shell-and-tube design heat exchanger to recover heat from the flue gases for heating the incoming gases. Operating temperatures are reached quickly in these systems. Recuperative heat exchange is also more suited to cyclic operations and variations in VOC feed rates and concentrations. Figure 5.21.2 is a schematic of such a system.

Table 5.21.1 summarizes the emission sources, VOC categories, and typical operating parameters (flow rates and concentrations), as well as the cost aspects of thermal VOC control technologies.

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