Microwave System for Small-Scale Treatment of Rocket Propellant Laboratory Derived Waste
During the normal course of research, development, and testing by the Air Force Research Laboratory (AFRL) at Edwards Air Force Base, small quantities (15 to 30 pounds per month) of environmentally hazardous propellant, explosive, and pyrotechnic (PEP) wastes are produced. CHA Corporation is focused on destroying the PEP waste safely and economically. Phase I and Phase II Small Business Innovation Research (SBIR) projects were awarded to develop a microwave disposal system that economically destroys these small quantities of PEP wastes. This system will meet the customer’s needs to treat their waste materials on-site to simplify regulatory requirements under the Resource Conservation and Recovery Act.
During Phase I (Contract # FA9302-09-M-0003), CHA Corporation experimentally demonstrated that the microwave disposal system will completely eliminate hazardous lab PEP wastes to non-detectable levels. Based on these tests, a system was designed to destroy 15-30 pound per month of lab derived PEP waste. The capital and operating & maintenance costs were estimated based on this design to demonstrate the economic advantages of the CHA microwave technology.
The experimental investigation was performed using various PEP chemical compounds in solution: 5% monomethylhydrazine (MMH), 2% 2-hydroxyethylhydrazine (HEH), 5% 2-nitrotoluene (NT), and 5% ammonium perchlorate. Also, two 6-hour longevity tests were successfully completed using 18 liters of HEH solution sent by the AFRL Propellant Division. To test the destruction of contaminated solids, experiments with contaminated paper towels and rubber gloves were performed in a microwave reactor with silicon carbide at the bottom to initiate the oxidation. The destruction efficiencies for all four compounds were greater than 99.9%. Also, the paper towel and rubber glove wastes were completely destroyed by microwaves without producing any toxic air pollutants. The results of the experimental investigations clearly demonstrate the ability of the technology and the advantages over existing methods
Under the Phase II project (Contract Number: FA9302-12-C-0025), CHA built a Microwave Energetic Lab waste Destruction System (MELDS) prototype that could be used to dispose hazardous PEP waste and PEP-contaminated solid. The MELDS consists of four major process units; (1) Microwave Solid Disposal Unit, (2) Microwave Tar Cracker, (3) Acid-Gas Scrubber and (4) Microwave Oxidation Unit.
The Microwave Solids Disposal Unit is used to destroy Velostat™ bags containing a mixture of solid PEP and PEP-contaminated lab wastes. The PEP-contaminated materials include gloves, wipes, and paper towels. The solid PEP waste is mainly binder mixed with oxidizer. These wastes are contained in 0.5-gallon or 1-gallon Velostat™ bags. The microwave solid disposal reactor is equipped with a 3-kW microwave generator.
The microwave pyrolysis of solids in the Microwave Disposal Unit produces hydrocarbon gases and tar. To avoid the contamination of scrubber water and system plugging, tar in the gas leaving the solid disposal reactor has to be decomposed to hydrocarbon gas or oxidized. The mixture of hydrocarbon gas and tar is introduced into the top of the microwave tar cracker running at 3kW microwave power. Air is also introduced into the top to promote oxidation of the tar and hydrocarbon gas.
The gas leaving tar cracker enters the Acid-Gas Scrubber and is scrubbed with a sodium hydroxide solution to remove neutralize acid components generated from the oxidation of waste containing chlorinated compounds such as ammonium perchlorate. If the waste being treated is known to not contain acid gas forming compounds it can be bypassed.
The gas leaving the scrubber enters into the Microwave Oxidation Unit and the remaining hydrocarbons in the gas are oxidized. The oxidizer is equipped with a 6kW microwave generator and packed with a mixture of oxidation catalyst and silicon carbide. The hydrocarbon vapor and its combustion byproducts are oxidized to CO2, H2O, and N2 in the oxidizer. The gas leaving oxidizer is free from air pollutants and directly vented into the atmosphere. Because microwave energy facilitates low temperature oxidation NOx and dioxins will not be formed.
The prototype operates as a batch system for the disposal of PEP and PEP-contaminated solid waste but can be easily adapted for continuous operation for treating slurry, liquid or vapor wastes.
The microwave prototype has been used to dispose medical waste sent by BioSafe. The medical waste sample was completely oxidized without generating air pollutants. It has also been tested to destroy pharmaceutical wastes. These test demonstrate that the MELDS could be used to dispose medical and pharmaceutical wastes on site without generating air pollutants.