GCI TECH NOTES ©
Destruction and removal efficiency (DRE) is the efficiency of the unit (kiln) in destruction and removal of a particular targeted organic compound. With respect to organic compounds, this is generally referred to as destruction efficiency (DE). The difference is that these hydrocarbons are destroyed or not destroyed, as no attempt is made to remove them from the stack gases if they are not destroyed. DE testing is important as it demonstrates the ability of the facility to destroy targeted hydrocarbons and establishes an important minimum operating parameter that must be met during subsequent hazardous waste fuel use. This efficiency is calculated by determining the mass emission rate of the selected hydrocarbon and dividing this by the mass input rate of this same hydrocarbon. The resultant value is subtracted from one (1) and that result multiplied by 100 to represent the value as a percentage of the selected hydrocarbon destroyed by the process. This DE test is conducted at the lowest sustainable combustion zone temperature the facility is willing to operate at. This temperature then becomes the minimum operating temperature allowed during the use of hazardous waste fuel.
A successful destruction demonstration of 99.99% of the selected POHC(s) would demonstrate a level of risk to the maximum exposed individual (MEI) of less than 1 in 100,000. (Preamble to the Boiler and Industrial Furnace Regulation, Part 3, Section II, A) Additionally, this level of destruction "is protective of risks posed by emissions of organic constituents in the wastes in virtually every scenario of which the Agency is aware." (emphasis added) (Preamble to the Boiler and Industrial Furnace Regulation, Part 3, Section II, A with the added footnote to Engineering - Science NTIS# PB87 173845).
POHCs and POHC Selection
Principal Organic Hydrocarbons (POHCs) are toxic organic compounds listed in Appendix VIII, Part 261, and found, or possibly found, in the hazardous waste fuel. It is logically assumed that the demonstration of the destruction of 99.99% of difficult to destroy compounds would demonstrate a similar destruction ability of any compounds less difficult to destroy. To achieve this demonstration of destruction efficiency a POHC or POHCs are selected based on their difficulty to be incinerated and their presence or potential presence in the waste fuel.
The most widely used incinerability index, and that referenced in EPA trial burn guidance documents, is the index produced by The University of Dayton. The primary researchers of this ranking are Dr. Barry Dellinger and Dr. Phillip Taylor. This index divides several hundred organic compounds into thermal stability classes; very thermally stable, moderately stable, fragile, etc. Demonstrating a 99.99% destruction of a compound of specified class is viewed by Dellinger and Taylor as adequate demonstration of the ability of that device to adequately destroy the other compounds of that class and any lower class. (Appendix D of the "Guidance on Setting Permit Conditions and Reporting Trial Burn Results, Volume II" USEPA.) In addition to the ranking by class, individual compounds are ranked by number, from the most stable with a ranking of 1 to the least stable with a ranking of 320+. Not all of these rankings are based on hard experimental data. Some of these rankings are based on the rankings of similar compounds and on reaction kinetic theory. However, the index produced by The University of Dayton has specified which compounds for which there is hard experimental data. These compounds are indicated by their bold typeface in the listing.
A number of the high ranking compounds are excluded from selection due to their toxicity or difficulty in management. As an example, hydrogen cyanide is a class 1 compound and is ranked number one as the most difficult compound to destroy. In addition to being extremely toxic, the compound is a gas and would be difficult to add to the hazardous waste fuel. Another example is benzene. Benzene (Class 1, Rank 3) is a liquid and, until a few years ago, was a commonly used solvent. Now, because it is classed as a carcinogen, the management requirements mandated by a variety of regulations virtually preclude using benzene as a POHC. Additionally, neither of these compounds are likely to be major constituents in hazardous waste fuel. Other high ranking compounds are excluded because they are solids at ambient temperature thus making them difficult to blend with liquid waste fuels. Or, the compounds may be the products of incomplete combustion (PICs) of the primary fuel, coal, in which case their presence as PICs from the combustion of coal would interfere with the DRE determination. Consequently, there are a limited number of high ranking compounds that can be selected as POHCs. Of these, tetrachloroethene (also called tetrachloroethylene and perchloroethylene, CAS 127-18-4) and 1,2,4-trichlorobenzene (CAS 120-82-1) are excellent choices. Tetrachloroethene is a class 2 compound and ranked at about 36 in it's difficulty to be destroyed. 1,2,4-trichlorobenzene is a class 1 compound and is ranked at about 26 in it's difficulty to be destroyed. Tetrachloroethene is a very common non-flammable solvent most commonly known as a "dry cleaning" solvent for clothing. 1,2,4-trichlorobenzene is much less common, but is easily differentiated from other hydrocarbons that may be present in the flue gas. Both of these are listed in Appendix VIII of Part 261. Both of these compounds can be detected using the SVOST sampling and analytical method with detection limits in the 3 to 4.5 µg/M3 range. Tetrachloroethene can also be sampled in the flue gas using the VOST sampling and analytical method.
Input and Output Determination
The amount of the targeted POHCs in the hazardous waste fuel is determined by subjecting a representative sample of the fuel to gas chromatographic analysis. The concentration of the POHC in the fuel is multiplied by the fuel feed rate to produce the POHC mass feed rate. However, since there most probably is an insufficient amount of POHC ranked as "difficult to destroy" in the hazardous waste fuel, additional POHC must be purchased and added to the batch of fuel intended to be consumed during the DE testing or the POHC must be metered into the fuel as it is delivered to the burner assembly. If the latter method is chosen, a careful record of the injection rate during each of the stack gas sampling periods (runs) must be kept. These injection rates times the chemical purity of the injected material (supplied by the vendor) will produce a mass input rate for the metered POHC added to the fuel. The sum of the two mass input rates, the mass input of the POHC present in the hazardous waste fuel and the mass input rate of the injected POHC, constitute the total input rate of the POHC into the kiln.
The mass emission rate of the POHC is determined by analysis of the stack flue gas to determine the concentration of the particular compound. This concentration is multiplied by the stack gas flow rate to produce the mass emission output rate. The concentration of the POHC in the flue gas is determined by sampling and analyzing the flue gas utilizing EPA approved methods. There are two methods for sampling and analysis; the VOST method (EPA Method 0030) and the Semi-VOST method (EPA Method 0010). Depending on the POHC chosen the method which properly samples and analyzes that POHC must be selected. For some compounds either method may be suitable. If two POHCs are chosen for testing it is not unusual for the DE testing to require both sampling/analytical methods to be performed. The stack gas flow rate is determined for each sample collection period (run). There are usually three runs to a DE test sequence.
These measured values are than inserted into the formula DE = (1 - (Wout / Win ) ) x 100; where Win = mass feed rate of one POHC in the hazardous waste fed to the kiln and Wout = mass emission rate of the same POHC present in the stack gas. If the DE value exceeds 99.99%, the DE requirement has been met.
The Effect of Analytical Detection Limits
The DE of the facility may be so high that the small amount of POHC present in the flue gas is below the detection limit of the analysis. If this is the case, this detection limit will be used as the concentration of the POHC in the flue gas. In that event, the DE value will be proceeded by the "greater than" sign (>). It is this analytical limitation that frequently requires the intentional injection, "spiking", of POHC into the hazardous waste fuel. A typical lower detection limit is 3.5 micrograms per dry standard cubic meter. However, the actual concentration of the POHC in the flue gas may be one-half, one-third or one-tenth of this value. Consequently, it is necessary to ensure that there is sufficient POHC input into the kiln so that the use of the lower detection limit value when used in the DE formula will demonstrate a value greater than the required 99.99%. In the early days of DE testing, this was the cause of a number of combustion devices "failing" the DE standard particularly where the lower detection limits were one or two orders of magnitude higher than they are today.