Most jurisdictions require that BACT be used in all new or modified sources. The regulations seldom specify how to determine BACT beyond defining the term. In Section 4.14.4, the “top‐down” approach developed by EPA for formal determination of BACT is described. The “top‐down” process is thorough, sequential, and complete. However, it can be expensive to conduct and may result in the applicant being required to use a technology not desired. This dilemma can sometimes be avoided.
In most cases, both the applicant and the permitting agency agree on what is the most stringent technology for the source being permitted. If the applicant decides to use that control technology, a “top‐down” BACT process is not needed, and accepting that most stringent technology early in the permitting process can greatly speed up other approval steps.
In other cases, the number of technologies reviewed in the “top‐down” process can be limited by prior agreement with the permitting agency. The number may be limited by citing BACT analyses that have been done for similar processes in similar settings and having the agency accept these analyses as applicable.
In a BACT protocol, the project description can be summarized, assumptions can be stated, and technologies that will be considered as part of the BACT analysis identified. This document can clearly set the boundaries of the BACT analysis and avoid surprises later in the process. Technical feasibility is usually used as the basis for including technologies, and infeasible technologies can be excluded prior to the “official” start of the analysis. After a BACT protocol has been submitted, a telephone call is often adequate to agree on the boundaries of the analysis.
There are many instances when only a few (often as few as two) technologies are considered in the BACT analysis, and in those situations the permitting agency may accept a much less formal analysis than the “top‐down” approach. Brevity usually makes the analysis easier to understand and allows the opportunity to make important points clearly without unnecessary boilerplate.
Top‐Down BACT Analysis
In 1987, EPA issued a memorandum that described a “top‐down” approach to determining what constitutes BACT for a particular source. The memorandum was to be followed up with regulatory language in 1996. The fundamental guide of the “top‐down” approach is that the examination of what control technology constitutes BACT will start with the most stringent technology – that is, the one with the greatest emission reduction. The applicant must demonstrate that this most stringent technology will not work in order to reject it. The basis for it not working can be technical infeasibility, or because of energy, environmental, economic, or other cost impacts that are unacceptable. The assumption starting the analysis is that the most stringent technology will work, and should be used, unless proven otherwise. The process of disproving feasibility of successively less stringent technologies continues until a control technology is found that will work, and that technology becomes BACT for that particular source. In the following subsections, the steps required in a “top‐down” BACT analysis are described.
Identify Technologies
The first step in the “top‐down” process is to identify the control technologies that will be considered in the analysis. Historically, intense arguments have arisen over this question, mostly related to whether a technology proposed by a regulatory agency for inclusion on the “top‐down” list was available. When addressing this issue, the BACT provision to include “any other emission control technique found by the California Air Pollution Control Officers Organization to be technologically feasible ….” can give rise to suggestions to consider many different technologies that may not yet be commercially available (CAPCOA 1993).
The availability criteria used by SCAQMD contains most of the elements used elsewhere. The SCAQMD list includes the following:
- The technology is commercially available.
- The technology is reliable. There has been at least 12 months of demonstrated performance in comparable field conditions.
- The technology is effective as demonstrated by a source test or other performance documentation.
- The technology has been achieved in practice or is technologically feasible and cost effective.
Lists have been compiled of control technologies for different equipment or processes. Descriptions of control technologies are also available. These references, which contain very similar information, are adequate to provide a list of alternatives that must be considered in the “top‐down” analysis for most jurisdictions, and they are updated periodically. Some of the lists include technologies considered LAER as the most stringent technology listed. The logic for including LAER is that this is the most stringent technology, and the applicant must disprove its use in the “top‐down” process.
Process changes that result in lower emissions can also be considered BACT. In some instances, process changes such as changing from a solvent to a water‐based coating can result in much lower emissions and fewer hazardous waste products.
There is opportunity for a great deal of judgment in the selection of technologies for the “top‐down” list, and that judgment is exercised differently by agencies in different states or regions. Including technologies on the initial “top‐down” list that have little or no likelihood of being technically feasible for a particular source increases the amount of work that must be done to exclude that technology from the process.
The term available with respect to control technologies refers to commercial availability. Can you purchase this technology and has it been demonstrated in a commercial setting? The commercial setting means that it must have been part of a commercial process, and not a research effort, pilot test, or demonstration.
There is sometimes disagreement about whether a technology that has been used commercially in a foreign country is considered “available” in the United States. Often there is not good information available about foreign “commercial technologies” to determine whether the use was a commercial setting and not subsidized by the technology vendor or an agreement that could reimburse the user if the technology were adopted as BACT in the United States. Usually, if there are several “apparently commercial” applications in other countries, the assumption of commercial availability is made for the purposes of including the technology in a BACT list. However, of the references cited above for lists of BACT technologies, foreign uses have not been included.
Determine Technical Feasibility
Each of the technologies identified must be technically feasible for the source being permitted. The technology must be used in the particular circumstances of the source being permitted. For example, a technology to reduce NO x from combustion turbines is to use water injection into the combustion chambers. However, for a gas pipeline compressor station in a remote, arid area where water is not available, water injection is not considered technically feasible. Another example would be the use of floating roof organic liquid storage tanks. This technology is considered technically feasible in most areas, but in extremely cold, icy environments such as on the North Slope, the sliding seal on floating roof tanks tends to freeze in place, preventing the roof from moving. Because of this, floating roof tanks are considered technically infeasible in Arctic locations.
In some instances, prior agreement can be negotiated with the agency to exclude technologies that have been shown to be infeasible in recent similar analyses. Such agreements should always be confirmed in writing to assure that a notice of deficiency is not issued for the subsequent application.
There can be some uncertainty about whether a technology is technically infeasible for a particular application, or whether it would just be considered too costly. If the issue were cost, it would need to be addressed in the cost evaluation later in the “top‐down” process.
For most of the arguments of technical infeasibility, an expensive approach can be imagined to make it feasible. For example, for the NO x control by water injection case, drilling for water could be done, and at some depth water could probably be found. The cost of obtaining water would be extremely high but possible! The strategy to use in the BACT analysis should be to propose a limitation as to technical infeasibility. Then the additional effort needed to estimate costs is not necessary – the technology is off the list. If the technical infeasibility argument is rejected, additional analysis will be needed to demonstrate either that the technology is too expensive or that the technology could be defined as BACT. Note that in applications where LAER is required, the cost argument is not usable, so proposing technical infeasibility may be the only choice.
Rank Technically Feasible Alternatives
Once a “short list” of technically feasible control alternatives has been developed, the alternatives are ranked from the most to least effective in reducing emissions from the proposed source. Often it provides clarification to express control level in more than one way, such as percent control and concentration of pollutants in an exhaust stream. The most useful common measure of control effectiveness is the total emissions per year from the source, using the control technology identified. Technologies are listed “top‐down” starting with the most effective technology that is technically feasible.
This short list should be presented to the permitting agency and agreed upon before continuing the BACT analysis. Once the list of technologies has been agreed to between the agency and the applicant, the remaining steps in the “top‐down” process take place one technology at a time, starting with the “top” or most effective technology. The intent of the next steps is to determine whether there are reasons for rejecting the application of this “top” technology to the process or equipment being permitted.
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