Risk Assessments and Process Upsets

To obtain a permit for a hazardous waste combustion unit, a Human Health Risk Assessment must be performed and submitted as part of the permit application. This assessment is usually based on stack emissions data obtained during the trial burn and prevailing weather conditions in the locality.

USEPA Region 6 has prepared a Human Health Risk Assessment Protocol (July, 1998). This protocol provides guidance on conducting this risk assessment. One of the items included in Chapter 2: Facility Characterization is a discussion on why and how process upsets should be considered when determining emission rates used for performing this assessment. This Tech Notes addresses this guidance as it applies to cement kilns operating under the BIF regulations.

In Section 2.2.1 it is stated, (EPA) "...expects that emission rates used to complete the risk assessment will be (1) long-term average emission rates adjusted for upsets, or (2) reasonable maximum emission rates measured during trial burn conditions in order to assure that risk assessments are conservative." EPA's concern is that a process upset would result in short periods of greater than normal stack emissions.

Section 2.2.5 discusses the origin of these process upsets describing them in general terms and relating specific examples. The document lists "start-ups", "shut-downs", "malfunctions of the combustion units or APCDs" or upsets in other units which cascade to the hazardous waste combustion device resulting in an upset. In more general terms, these process upsets are described as being the result of "when the hazardous waste combustion unit is not being operated as intended...." "...usually during events and times when the hazardous waste combustion unit is not operating within the limits specified in a permit or regulation." EPA's concern is that the selected operating parameters cannot be reliably maintained and that exceedences of these parameters would result in increased emissions.

In cement kilns operating under the BIF regulations, a "start-up" process upset is precluded from occurring. The kiln system must be operating within specified parameters and monitored emission requirements before hazardous waste fuel may be utilized.

Cement kiln systems inherently have a massive store of energy and long gas retention times, unlike purpose built hazardous waste incinerators. Consequently, cement kilns have less potential, tendency or capability to emit hazardous waste constituents due to process upset conditions which might result from shut-downs and/or equipment malfunctions.

The concern that the cement process cannot be reliably maintained within the selected operating parameters, and that exceedance of these parameters results in increased emissions, is easily addressed. Trial burns establish such operating limits as: maximum gas flow rates, maximum raw feed rates, minimum APCD power, maximum APCD inlet temperature, maximum HWF feed rates, maximum chlorine feed rates, maximum trace metals feed rates, and maximum THC and CO emissions simultaneously during the testing. This testing, however, must demonstrate that the emissions are within that allowed by the regulation. Consequently, a conservative (protective of the environment) operating envelope is established and maintained since an AWFCO would be initiated by an exceedance of any one of these operating limits even though these limits have already been demonstrated to be safe by the trial burn testing. With this understanding, an examination of the operational data, as recommended in Section 2.2.5, can only result in a conclusion that there would be no increased emissions due to process upsets in cement kilns operated under the BIF regulations.

Facility operators have an incentive to operate well within this operating envelope to preclude AWFCOs and the subsequent loss of revenue these interruptions entail. This incentive provides an added margin of safety. An examination of the operational data used to derive an "upset" factor could also be used to derive a "decreased" emissions factor resulting from the operator's avoidance of the operating limits that result in AWFCOs.

In conclusion, cement kilns operating within the BIF regulations would not be expected to experience increased emissions. The operational limits established by the trial burn and enforced by the AWFCOs preclude increased emissions due to start-ups, shut-downs, malfunctions and process upsets. Indeed, the routine avoidance of these limits by the operators probably results in decreased emissions when compared to the trial burn emission rates.

Cement facility operators must perform routine maintenance on the kiln and ancillary equipment. Much of this can be done without a significant amount of down time. However, one maintenance routine which cannot be avoided and will result in significant down time is the inspection and replacement of the fire brick lining of the kiln. Generally, this requires three to five weeks each year from the time the fuel to the kiln is shut off until the use of HWF is restarted. During this period, no emissions attributable to the use of HWF can occur. This down time period should be considered when estimating the total annual emissions used to assess risk due to these emissions.Additionally, there are periods when HWF is not consumed. This may be due to the unavailability of suitable wastes or may be associated with the repair of required equipment. During these periods no emissions attributable to the use of HWF can occur. While more difficult to estimate, this down time should also be considered when estimating the total annual emissions used to assess risk.