Staged combustion process

Roy F. Weston, Inc., owns and operates two TISs the TIS-5, capable of treating 7 tons per hour (tph) of waste, and the TIS-20, designed to treat up to 30 tph. Both employ a two-stage combustion process for incineration of hazardous waste. Transportable incineration is generally cost-effective for sites containing more than 1000 tons of contaminated media. 

However, substantial amounts of the unsubstituted aromatics were found, as shown in Figure 13. At 900°C more than 60 percent of each liquid product was unsubstituted aromatics. The amounts of one, two, and three ring molecules (i.e., benzene, naphthalene, and phenenthrene) varied with the molecular weight of the starting material. For example, the liquid product of MD-3 at 900°C was more than 50 percent benzene while naphthalene was more than 30 percent of the liquid product from MD-4 at 900°C. These unsubstituted aromatics are more thermally stable than substituted aromatic molecules and can be considered soot precursors in staged combustion processes. 

Mustard Mustard is immediately destroyed at temperatures above 500°C to S02, HQ, C02 and water vapor. The contractor operated a commercial, transportable 7 MM Btu/hr incinerator which could readily achieve an organic destruction efficiency of 99.9999% and which was used to destroy frozen bulk mustard introduced as solid waste and to thermally treat agent-contaminated metal scrap. The incinerator incorporated a two-stage combustion process, e.g., a rotary kiln (900°C, 1.5 tonnes/hr capacity) and a down-fired secondary combustion chamber (1200°C, 2.5s residence time), backed by a venturi wet flue gas scrubber. System components were erected on a prepared concrete foundation and housed in a prefabricated building to provide shelter for the equipment and operators. Natural gas from a nearby well provided a fuel source. 

Figure 10.16 is a block diagram depicting a two-stage combustion process to dispose of waste products, which, in a single stage combustion process, would have produced a flue gas containing excessive amounts of NO. Such a process comprises ... 

Flame or Partial Combustion Processes. In the combustion or flame processes, the necessary energy is imparted to the feedstock by the partial combustion of the hydrocarbon feed (one-stage process), or by the combustion of residual gas, or any other suitable fuel, and subsequent injection of the cracking stock into the hot combustion gases (two-stage process). A detailed discussion of the kinetics for the pyrolysis of methane for the production of acetylene by partial oxidation, and some conclusions as to reaction mechanism have been given (12). 

The combustion process proceeds in two stages in the primary section the soHd phase bums and volatile gases are driven off in the secondary section, these volatile gases are burned. The combustion of refuse wastes often requires an auxiUary burner to maintain sufficient temperature for complete combustion. Large amounts of excess air, as high as 300%, are frequendy used. 

Staged Burners As was pointed out earher under ToUutant Formation and Control in Flames, the proper staging of fuel or air in the combustion process is one technique for minimizing NO emissions. Gas burners that achieve such staging are available. 

Theoretically, to produce 1 kg of nitric acid requires at least 0.27 kg of ammonia and 4.33 kg of air (or 1.02 kg of oxygen). These weights refer to the content of concentrated acid. Realistically, however, the process is divided into three successive stages combustion, oxidation, and absorption. 

Control of oxides of nitrogen can be accomplished by catalysts or ab-sorbants, but most control systems have concentrated on changing the combustion process to reduce the formation of NOj. Improved burners, change in burner location, staged combustion, and low-temperature combustion utilizing fluidized-bed systems are all currently in use. These combustion improvement systems do not generate waste products, so no disposal problems exist. 

Over the past 15 years, many studies have been conducted to determine the basic physical and chemical mechanisms of combustion and ignition. Although the studies have served to illustrate many of the overall characteristics of these processes, a quantitative characterization of ignition and of combustion has yet to be accomplished. The purpose of this article is to review the studies conducted to date, to indicate the significant accomplishments, and to show the areas which are not understood and therefore need further study. The discussion of ignition is presented first, since it is the initial stage of the combustion process. 

From the discussion in the previous section, it is clear that, above all, NO2, and then NO, are the principal oxidizers produced in the flames of nitrate esters. The reaction of NO2 with aldehydes plays an important role in the combustion of nitrate esters, since these molecules are the major decomposition products of these materials in the first stage of combustion. Pollard and Wyatt studied the combustion process of HCHO/NO2 mixtures at sub-atmospheric pressures.They found that the reaction occurs very rapidly at temperatures above 433 K, the NO2 being reduced almost quantitatively to NO, and the aldehyde being oxidized to CO, COj, and H2O. The order of reaction was found to be one with respect to both reactants. The same result has been reported by McDowell and Thomas.I ] jgg proposed reaction steps are ... 

---