Risk Assessments and Process
To obtain a permit for
a hazardous waste
unit, a Human Health Risk Assessment must be performed and submitted as
part of the permit application. This assessment is usually based on
emissions data obtained during the trial burn and prevailing weather
in the locality.
USEPA Region 6 has
prepared a Human
Health Risk Assessment
Protocol (July, 1998). This protocol provides guidance on conducting
risk assessment. One of the items included in Chapter 2: Facility
is a discussion on why and how process upsets should be considered when
determining emission rates used for performing this assessment. This
Notes addresses this guidance as it applies to cement kilns operating
the BIF regulations.
In Section 2.2.1 it is
that emission rates used to complete the risk assessment will be (1)
average emission rates adjusted for upsets, or (2) reasonable maximum
rates measured during trial burn conditions in order to assure that
assessments are conservative." EPA's concern is that a process upset
result in short periods of greater than normal stack emissions.
discusses the origin of
upsets describing them in general terms and relating specific examples.
The document lists "start-ups", "shut-downs", "malfunctions of the
units or APCDs" or upsets in other units which cascade to the hazardous
waste combustion device resulting in an upset. In more general terms,
process upsets are described as being the result of "when the hazardous
waste combustion unit is not being operated as intended...."
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
and that exceedences of these parameters would result in increased
Cement Kilns Operating Under the BIF
and Process Upsets
In cement kilns
operating under the BIF
a "start-up" process upset is precluded from occurring. The kiln system
must be operating within specified parameters and monitored emission
hazardous waste fuel may be utilized.
Upsets caused by
malfunctions are of
- The most common type is the
of the use of hazardous waste fuel only. This type of shut-down may
due to a variety of reasons from purposeful to the result of an AWFCO.
In these instances, the kiln system continues to operate utilizing the
primary fossil fuel. Critical operating parameters remain well within
specified ranges and the APCDs continue to operate. In these instances,
no increased uncontrolled emissions would occur. An example would be
continuous emissions monitor (CEM) failing its automatic calibration.
- The less common type is a less
discontinuation of HWF utilization, such as the result of a facility
failure. This would immediately result in an AWFCO of the HWF to the
In this instance, the massive residual heat in the tons of processed
(the clinker and raw feed within the system) and the natural draft that
would continue to draw the flue gases through the APCDs would combust
small amount of organics within the combustion zone. Even without
power, the greatly reduced gas velocity and the decaying corona charge
in the ESP would be sufficient to continue to attenuate the emissions
kiln systems inherently have a massive
store of energy and long gas retention times, unlike purpose built
waste incinerators. Consequently, cement kilns have less potential,
or capability to emit hazardous waste constituents due to process upset
conditions which might result from shut-downs and/or equipment
that the cement
be reliably maintained within the selected operating parameters, and
exceedance of these parameters results in increased emissions, is
addressed. Trial burns establish such operating limits as: maximum gas
flow rates, maximum raw feed rates, minimum APCD power, maximum APCD
temperature, maximum HWF feed rates, maximum chlorine feed rates,
trace metals feed rates, and maximum THC and CO emissions
during the testing. This testing, however, must demonstrate that the
are within that allowed by the regulation. Consequently, a conservative
(protective of the environment) operating envelope is established and
since an AWFCO would be initiated by an exceedance of any one
these operating limits even though these limits have already been
to be safe by the trial burn testing. With this understanding, an
of the operational data, as recommended in Section 2.2.5, can only
in a conclusion that there would be no increased emissions due to
upsets in cement kilns operated under the BIF regulations.
operators have an
operate well within this operating envelope to preclude AWFCOs and the
subsequent loss of revenue these interruptions entail. This incentive
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
limits that result in AWFCOs.
conclusion, cement kilns
the BIF regulations would not be expected to experience increased
The operational limits established by the trial burn and enforced by
AWFCOs preclude increased emissions due to start-ups, shut-downs,
and process upsets. Indeed, the routine avoidance of these limits by
operators probably results in decreased emissions when compared to the
trial burn emission rates.
Reduction of Total Annual
Due to Cement Kiln Down Time
facility operators must
routine maintenance on the kiln and ancillary equipment. Much of this
be done without a significant amount of down time. However, one
routine which cannot be avoided and will result in significant down
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
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.
down time period should be considered when estimating the total annual
emissions used to assess risk due to these emissions.Additionally,
are periods when HWF is not consumed. This may be due to the
of suitable wastes or may be associated with the repair of required
During these periods no emissions attributable to the use of HWF can
While more difficult to estimate, this down time should also be
when estimating the total annual emissions used to assess risk.