Thermal Incinerators

Katari et al. (1987a and b) summarize incineration techniques for VOC emissions. Particularly useful are a table on the categorization of waste gas streams (see Table 5.21.5) and a flow chart that determines the suitability of a waste gas stream for incineration and the need for auxiliaries. The categorization includes the % oxygen, VOC content vis-a-vis LEL, and heat content. Thus, a mixture of VOC and inert gas with zero or a negligible amount of oxygen (air) and a heat content >100 Btu scf-1 (3.7 MJ m~3) can be used as a fuel mixed with sufficient oxygen for combustion (see category 5 in Table 5.21.5).

Figure 5.21.4 is a schematic of the incineration system. Waste gas from the process, auxiliary fuel (if needed), and combustion air (if needed) are combined in the combustion chamber under conditions (i.e., time, temperature, and turbulence) to achieve minimum conversion (DRE >99%). The temperature inside the combustion chamber should be well above the ignition temperature (1000-1400°F) of most VOCs; residence times of 0.3-1.0 sec may be sufficient. If higher DREs are required (>99.99%), both residence time and temperature may have to be increased depending on the VOC composition. This increase is also necessary the more nonuniform (in terms of VOC components) the waste gas stream is.

The majority of industrial waste gases for thermal destruction fall under category 1 of Table 5.21.5. The following parameters must be quantified (see Figure 5.21.4):

Heat content of the waste gas and Tp from process (Tp 5

Tw, if no heat exchanger is used) The % LEL (VOC content and types) The waste gas volumetric flow rate Tw TE required based on the necessary DRE

The nature of the VOCs in the waste gas stream determine the temperature (TE) at the exit of the combustor (not equal to the adiabatic flame temperature).

The composition of the waste gases determines the combustion air requirements; required temperatures and flow rates determine the auxiliary fuel requirements, furnace chamber size, and heat exchanger capacity. The suggested temperatures are 1800°F for 99% DREs at approximately 1 sec residence time. Applicable incinerator types include liquid injection, rotary kiln, fixed-hearth, and fluidized-beds (of which several variants exist). Dempsey and Oppelt (1993) describe these units in more detail. Although these authors principally address hazardous waste, their article

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