WSRC-MS-99-00944

Deflagration Transient Study of the CIF Incinerator

Thong Hang
Westinghouse Savannah River Company
Savannah River Technology Center
Aiken, SC 29808

This document was prepared in conjunction with work accomplished under Contract No. DE-AC09-96SR18500 with the U.S. Department of Energy.

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Key Words: Environmental Science, Process-Oriented, Risk Analysis.

Abstract

The Consolidated Incineration Facility (CIF) treats solid and liquid RCRA hazardous and mixed wastes generated at the Savannah River Site (SRS). The transient responses of the CIF system to a deflagration, caused by an accidental charge of a modest quantity of solvent (e.g. toluene) into the rotary kiln, were a major safety concern. Using a dynamic computer model, a study was conducted to analyze the transient system responses to the rapid temperature and pressure surge in the kiln. The objective of the study was to determined the maximum pressure, temperature, and gas flow rate in each CIF component (rotary kiln, secondary combustion chamber, quencher, scrubber/cyclone, mist eliminator, reheaters, HEPAs, and ID fans). The resulting data provided a basis for the subsequent structural analysis. This paper will describe the CIF deflagration study in some detail, and present the results of the simulation scenarios.

Process Description

The CIF was designed for treatment of liquid and solid low-level radioactive, mixed and RCRA hazardous wastes. Figure 1 shows a schematic CIF process flow diagram. The incineration system consists of a 13-million Btu rotary kiln (RK) and a 5-million Btu secondary combustion chamber (SCC) to treat both solid and liquid combustible wastes. Normal operating temperatures are 1832° F in the rotary kiln, and 2012° F in the SCC. Maximum solids retention time in the RK is 90 minutes. A 2-second offgas residence time is required in the SCC. No. 2 fuel oil burners are used in the RK/SCC for startup, shutdown, idling and temperature control.

Figure 1. CIF Flow Diagram

A wet offgas system was selected for air emission control. SCC offgas is cooled to saturation in a liquid-recirculating quench. Particulates and acid gases in the quench offgas are removed by a steam-atomized scrubber. The liquid/gas separation of the scrubbing mixture takes place in a cyclone separator. A mist eliminator removes entrained water droplets. Saturated scrubber offgas is reheated, and filtered through HEPAs prior to atmospheric discharge.

The two principal variables to be controlled in the CIF system are temperature and pressure. Temperatures are monitored at the exit of the rotary kiln, the SCC, and the reheater. Pressures are measured at the kiln exit and the mist eliminator inlet. Additionally, the oxygen level in the rotary kiln flue gas is also subject to control in order to maintain the design excess air required for combustion. Figure 2 shows a schematic diagram of the CIF control system.

Temperature control: Control of the rotary kiln and SCC temperature is achieved by modulating fuel oil. The reheater temperature is controlled by regulating the steam flow.

Pressure control: The system pressure is controlled with two independent loops, i.e. the pressure regulating valve loop and the ID fan loop. The valve loop controls the kiln pressure by regulating flow at the scrubber inlet using a 14" butterfly control damper. In the ID fan loop, control of the offgas pressure is achieved by regulating the ID fans vane angle based on input from the pressure sensing device located at the mist eliminator inlet.

Oxygen control: The sensors located at the kiln discharge head continuously measure the level of oxygen at the kiln exit. The measured signal is sent to the oxygen controller and is used to determine a new set point for the solids combustion air flow. The oxygen controller then sends the signal to the flow controller, which modulates the solids combustion air flow to achieve the desired set point.

The CIF began operation in 1995. To support CIF startup and operation, the Savannah River Technology Center (SRTC) designed an extensive test program which included metals partioning (Burns et al., 1994), offgas testing, and RK seal testing.

In complement of the test program, mathematical modeling was utilized to simulate various aspects of the incineration process: flow pattern, heat transfer, dynamic responses to fluctuations in the feed or upsets in the system equipment. This paper focuses on the modeling study of deflagration.

Deflagration Study

During normal CIF operation, a small quantity of flammable solvents (e.g. toluene) may pass X-ray inspection undetected and be accidentally charged to the rotary kiln through the solids ram feeder system. The worst case results from the solvent being contained in a metal container. The kiln high heat release superheats the liquid solvent causing pressurization of the container until it ruptures to rapidly release the content. The dispersed flammable vapors could cause rapid deflagration of all the oxygen present, which would increase the kiln temperature and pressure to a value approaching the adiabatic isochoric complete combustion (AICC) temperature and pressure for a stoichiometric mixture.

Deflagration is a major concern to CIF. An assessment of the system transient responses to deflagration would be essential to address the problem. The objective of this study is to determine the maximum pressure, temperature, and gas flowrate in each component (kiln, SCC, quencher, scrubber/cyclone, mist eliminator, reheaters, HEPAs, and ID fans). The data provide a basis for the structural analysis.

Methodology

The transient responses of the CIF system to deflagration were analyzed using an existing CIF dynamic computer model. Model development and simulation results of the CIF process dynamics were presented elsewhere (Hang 1992). In this study, the model was modified to add the following features:

Deflagration energy: At the average kiln oxygen level of 13.75% and the kiln operating temperature of 1400° F, the AICC pressure was determined to be 25.6 psig (708" water gauge) for a stoichiometric toluene mixture. To obtain a kiln pressure surge approaching 25.6 psig, an energy source was generated in the kiln at a rate of 683,358 Btu/sec for 0.05 seconds. The energy rate for a different initial kiln operating temperature T_K was approximated by:

rate: Energy rate (Btu/hr)
T_K: Kiln operating temperature (° R)

14" Vent: A 14" vent is located near the top of the ash chute housing to provide kiln overpressure relief. A delay time of 0.1 seconds was assumed. The outflow from this vent was estimated as follows:

flow = 3600 C_d A_vent G_ventG_vent = 7.7 (D P)^0.5

flow: Vent outlet flow rate (lbs/hr)
C_d: Vent discharge coefficient (= 0.5)
A_vent: Vent area (ft²)
G_vent: Mass discharge rate (lbs/ft²-sec)
D P: Pressure difference [Kiln pressure–atmospheric pressure] (psi)

Reverse flow: In general, the design process pressure difference establishes the gas flow in the downstream direction. However, if the pressure of a downstream component becomes larger than the pressure of an upstream component (e.g. rapid pressure decrease due to the pressure relief vent), gas flows in a reverse direction. The reverse flow would affect mass and energy balances in both components. Effects of the reverse flow were accounted for in the rotary kiln, SCC, and the quench.

30" kiln pressure control damper: The 14" damper was replaced with the 30" damper.

Results

Five scenarios were selected for the deflagration simulation to cover a wide range of the CIF operations:

Deflagration at the operating kiln temperature of 1400° F.
Deflagration at the operating kiln temperature of 1832° F.
Deflagration at the operating kiln temperature of 1400° F. The 30" damper valve to control kiln pressure was completely closed.
Deflagration at the operating kiln temperature of 1832° F. The 30" damper valve to control kiln pressure was completely closed.
Deflagration at the operating kiln temperature of 1400° F. The offgas pressure was not controlled. A lower recirculation rate (150,000 lbs/hr) was assumed for the quench.

Scenarios 1 and 2 cover the entire range of the kiln operating temperatures. Scenarios 3 and 4 provide the worst scenarios for upstream components, in which the deflagration causes the 30" damper completely closed. That would significantly reduce the gas flow to the offgas components downstream of the quench and therefore would require a longer time to get the rotary kiln back to the normal operating condition. Scenario 5 conservatively estimates the effect of deflagration on the offgas components downstream of the scrubber/cyclone, when the ID fans failed to control the offgas properly. The quench recirculation rate was assumed at its low flow alarm value of 150,000 lbs/hr.

In each scenario the simulation was carried out in two stages:

Stage 1 – Normal operation prior to deflagration: This stage was to establish the CIF normal operting condition before the liquid solvent was accidentally introduced to the kiln causing deflagration.

Stage 2 – Deflagration: The deflagration was simulated by generation of an energy source within the kiln for 0.05 seconds. The release of this energy source was defined above. All liquid wastes, fuel oil and combustion air injected to the kiln and SCC were assumed to terminate.

The results of the five scenarios are summarized in Table 1, in which the maximum temperature, pressure, and gas flow rate are listed for each process component. For illustration, Figures 3, 4, and 5 below show the transient responses (temperature, pressure, flow rate) of the rotary kiln, and the SCC to deflagration in Scenario 1.

The results showed that the 30" control damper had no significant impact on the rotary kiln and the SCC regarding the peak values of process variables. However, with the 30" control damper completely closed, a longer time was required to reestablish the normal operating condition in the kiln. Scenario 5 also indicated that a failure of the offgas pressure control (e.g. simulated by maintaining a fixed vane angle of the ID fans) would result in a large pressure increase in the HEPAs and the ID fans.

Conclusions

This study shows that deflagration in the rotary kiln causes a tremendous overpressure in the CIF system. The kiln pressure may increase to 525" water and 403" water if deflagration occurs at the kiln operating temperatures of 1400° F and 1832° F, respectively. The simulations indicate that within 5 seconds the kiln pressure returns to negative. If the 30" control damper to control the kiln pressure was completely closed due to deflagration, a longer time (> 10 seconds) would be required to reestablish the normal kiln operating pressure. A failure to properly control the offgas pressure would result in a significant pressure in the HEPA filters and ID fans.

References

Burns, D.B.; H. Holmes Burns; M.G. Looper; and G.R. Hassel. 1994. "Metals Partitioning in a Rotary Kiln Incinerator with a Wet Off-Gas System." In Proceedings of the 1994 International Incineration Conference (Houston, TX, May 9-13). The Regents of the University of California, Irvine, CA, 527-531.

Hang, T. 1992. "Dynamic Simulation Study of the CIF Incinerator." In Proceedings of the 1992 Incineration Conference (Albuquerque, NM, May 11-15). The Regents of the University of California, Irvine, CA, 679-682.

Acknowledgement

The information contained in this paper was developed during the course of work done under Contract No. DE-AC09-96SR18500 with the U.S. Department of Energy.