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EPA has recently released its September 24, 1993, draft memorandum on "Implementation of Exposure Assessment Guidance for RCRA Hazardous Waste Combustion Facilities." There are significant issues in this document that require clarification if it is to be used as guidance for cement kiln trial burn plan preparation.
In reading the document, it is obvious that it was written based on the EPA's experience and understanding of the operation of incinerators. There are a few specific references to cement kilns, however the bulk of the document is based on incinerator technology. The design and operation of cement kilns is decidedly different than the operation of an incinerator. Some accommodation to the peculiarities of cement kiln operations should be made.
The following is a list of issues and suggested modifications to the guidance that were generated by a review of the document. The guidance document stresses that this is to be guidance only for the risk assessment phase of the testing. Also, the "Application of Data" section stresses that metals (feed rate) concentrations should be determined from the tests designed to maximize total metals emissions. With this in mind, it should be easier to resolve some of the following issues.
LIST OF EMISSIONS TO BE ANALYZED FOR IN THE STACK
The EPA has targeted six items:
1) The ten BIF metals - this data has already been generated as part of the COC testing. Can this data be used for the assessment? ("Data in lieu of")
2) Carcinogenic compounds - The list includes a full PCB's scan of stack emissions. If the facility does not accept PCBs or PCB contaminated materials as defined by TSCA in their waste, is a scan for PCBs required?
3) Identification and assessment of the 20 largest peaks in a GCMS
of the flue gas. It is not clear if this is to be done on the VOST and
semi-VOST runs together or separately.
4) Full PCDD/PCDF scan (as TEQ). This should not be a problem. Again, can "Data in lieu of" be used?"
5) Any compounds present in high concentration in the waste fuel. A definition of "high concentration" would be helpful.
6) Any PIC's the permit writer feels are important. This list should be presented and discussed as soon as possible by the region. It is notable that the word "feel" was used in this context rather than "determined". What rationale for choosing PICs will be provided? Is the list from Appendix VIII Part 266 appropriate?
The EPA has indicated average emissions are more directly related to risk and might be more appropriate. They then suggest that since they are unaware of a mechanism to prevent an exceedance of average emissions, they will use worst case. If average emissions were not exceeded, they would cease to be averages. The technical justification for the agency's position in this regard seem weak or non-existent.
WASTE FEED CONDITIONS
The facility is asked to use "worst-case wastes" as fuel during the testing. What is a "worst-case" waste, and how is this information to be used in setting permit conditions?
High content of halogens - It may not be possible to secure enough native wastes for a test with a maximum permitted level of chlorine. Can chlorine bearing solvents be added to achieve the necessary chlorine content?
The facility is asked to conduct the test with maximized metals, ash, halogen content and the highest organic levels in the kiln raw feed. This guideline is based on EPA's experience with incinerators where the ash content of the HWF is acting as a catalyzer for reformation of compounds such as PCDD's. Not questioning that reaction, why is there a need to maximize metal feed rates (duplicating the metals emissions testing) knowing that the APC is loaded with CKD consisting of metals known to promote these reactions (copper, iron, zinc, nickel and aluminum)? It is suggested that metals feed rates and raw kiln feed rates not be operating parameters for this testing when conducted in a cement kiln producing salable clinker.
With respect to the HC content of the raw feed, this is a difficult parameter to control. There are certain raw materials that generally increase HC input but not consistently. There is no reliable method of determining in advance, for the tons of material needed for a test, the HC content. Also, it is known that certain operating conditions affect stack HC content. As an example, higher APC inlet temperatures may increase Stack HC content, however high APC inlet temperature and low combustion chamber temperature (to increase PIC formation) would be conflicting conditions. Also, if the HC content in the stack threatened to exceed the current HC limits, the kiln would have to be operated at a lower raw feed input. This is another reason to not require raw kiln feed rate or HC content as an operating limit for HC risk assessment testing. It is suggested that the risk assessment test be conducted at an HC emissions rate that is less than or equal to the HC emission operating limit established during the Alternative HC limit testing (if applicable) and that the HC emissions levels experienced during the Risk Assessment testing not be used as operating limits.
The section called "Difficult to burn wastes such as viscous liquid wastes or high solids wastes" is not applicable to cement kiln technology. Cement kilns, unlike incinerators, have long gas residence times and very long solids residence time. Also, cement kilns operate at much higher temperatures than incinerators. Cement kilns are specifically designed to remove water and carbon (predominately carbonates) from mineral substrates then, through intense heat, produce a set of inorganic crystals that make up cement clinker. The entrainment of CKD into the APC is a part of cement kiln operation, the APC has been designed for this purpose. Any unburned wastes that might accumulate in the CKD would be subject to the control of 266.112. To date, the presence of organics in waste CKD in excess of the health based limits has not been reported. It is suggested that fluid viscosity and solids content limits are unnecessary as operating parameters at cement kilns for liquid fuel streams. Any separate containerized waste feed should be maximized for this test.
It is interesting to note that the section, Mixed Wastes With Easily Volatilized Components, calls for "normal" operating conditions in contrast with the rest of the document. Cement kilns that burn solid wastes in containers can and have fed a variety of container types and sizes as well as a wide variety of contents. Previous testing has centered on input rate as the limiting condition. This is realistic for a cement kiln because of the BIF requirements for metals and chlorine input limits. However, the concern with soot formation, and hence PCDD/PCDF reformation and similar reactions is similar to the concern with difficult to burn materials noted above. There is no evidence that organic materials in excess of the health based limits set out in 266.112 (which the EPA readily admits are excessively restrictive) have accumulated in the waste CKD.
Similarly, the type of container (steel, plastic, bag, etc.) would have no impact on the effect of the waste entering the kiln as long as the kiln exit oxygen concentration is above a specified minimum. It is suggested that the control parameter for solids and liquids (in containers) entering the kiln continue to be mass input per unit time.
1) Combustion temperature and residence time
Currently, cement kilns must conduct DE tests with selected and approved POHCs to determine the minimum combustor temperature operating limits. This test however may not achieve the high APC inlet temperature needed to release HCs from the raw feed in long wet kilns. It is suggested that the minimum combustor temperature continue to be determined during the DE testing and not become an operating limit parameter during the risk assessment test.
2) Amount and Distribution Combustion Air
Incinerators often have air injection nozzles distributed throughout the combustion chamber to impart turbulence and preclude low oxygen pockets. Cement kilns operate with two or possibly three sources of air, all near the same location, the kiln hood. Turbulence is generated not only from the configuration of the air inlet devices but also high gas velocity and interaction with the load in the kiln.
The Guidance suggests that adequate combustion air, and its distribution, can be monitored by observing CO and/or THC in the stack gasses. This is not applicable to most cement kilns. It has been demonstrated that for cement kilns CO and THC concentration in the stack can move inversely to one another when O2 is changed.
It is suggested that a minimum O2 concentration at the kiln exit would be established for a safe operation of the kiln, instead of a maximum CO and THC concentration. Operating above this minimum should be sufficient to demonstrate that the device has sufficient air.
3) Mixing (Fuel and Air)
Mixing should not be of concern in cement kilns because of the exceedingly high Reynolds' numbers.
4) Rate of Waste Combustion
Operation at maximum waste fuel impact rates is not a problem. However, maximum expected excess air rates is opposite the conditions established in Item 2 of the guidance regarding minimum excess O2. Minimum O2 is clearly the worst case of those scenarios.
5) Temperatures at inlet to the APC
A high inlet temperature to the APC and low combustion chamber temperature are conflicting parameters. It is suggested that the PIC risk assessment test be conducted at as low a combustion chamber temperature as possible, that still produces salable clinker, and also has an APC inlet temperature at the maximum. The APC inlet temperature would then become a limit, the combustion chamber temperature would not, having been established during the DE test.
6) Other Conditions
At the present time there are no other factors that require the establishment of limits.