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Moving targets: how ever changing air quality regulations are driving process decisions

Historically, the concern of process engineers was the design and operation of plants to do primarily one thing, efficiently meet product specifications. In today’s changing regulatory environment, there are additional concerns design engineers may neglect.

Darin Kennard, EOG Resources, Inc.
Luke Addington, Bryan Research and Engineering
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Article Summary
These concerns include Volatile Organic Compounds (VOCs) and Hazardous Air Pollutants (HAPs) emissions as exceeding thresholds can limit throughput, increase liability to the operator, and add significant lead time to plant construction or modifications. A case study aimed at debottlenecking a large cryogenic gas plant is presented to investigate a legacy VOC and HAP issue as well as maximise overall plant production. The debottlenecking study focused on multiple amine sweeting units and their associated still vent emissions. Interestingly, methanol was found in surprising amounts in the still vent emissions. Control devices were assessed and subsequently installed to alleviate any future VOC and/or HAP issues and allow for increased production.

The regulation of Green House Gases (GHGs) has changed significantly of late, with the likely outcome that treating will be performed in the field at “minor sources”. Producers operating large plants already categorised as “major sources” of air pollutants must pay careful attention to these GHGs. A case study of a cryogenic gas plant is presented where increasing plant throughput was potentially bottlenecked due to emissions of CO2 in excess of Prevention of Significant Deterioration (PSD) permitting thresholds. A thorough analysis was performed of the plant to manage CO2 emissions while still maintaining product specifications. This allowed the operator to avoid a long, costly regulatory review and permitting process while still increasing production.

With current GHG regulations likely to push gas treating into the field, sour gas streams may be the new bottleneck as SO2 emissions may curtail production due to the National Ambient Air Quality Standard (NAAQS) for SO2. Designers will need to begin evaluating the technologies best suited to work around this issue.

Historically, the primary concern of process engineers has been optimising plant design and operation for two purposes: to make product specifications and to do so as economically as possible. However, with ever increasing regulatory rules on the operation of various upstream/midstream facilities, it is no longer sufficient to focus solely on product specifications and operating costs. There is another design parameter-namely air quality permitting and regulatory compliance-that should be considered at the front end engineering design (FEED) stage, if not earlier. Not doing so can create bottlenecks or unanticipated delays in obtaining construction permits in facilities and fields, even though the facilities may otherwise be able to meet product specification economically.

Air quality issues can have a variety of direct impacts on facilities. For starters, bumping up against thresholds for particular contaminants can limit plant expansion if emissions are directly proportional to plant throughput. A plant with emissions approaching a threshold for a given contaminant may not be able to increase throughput without triggering lengthy delays and costly environmental reviews.

Likewise, a design project that does not address emissions thresholds at the FEED stage could find itself greatly delayed by regulatory permitting. Not being aware of the regulatory framework can delay projects unnecessarily by months, if not years, hurting profitability and unduly adding to operating costs.

If air emissions regulations are considered at the pre-FEED stage, operators and designers can take advantage of the current regulatory conditions and “optimise” their facility designs, so to speak, to avoid unnecessary costs and add considerable operational flexibility.

Overview of Current Air Quality Regulations

Air quality regulations are a complex and continuously evolving issue that should be considered in all phases of a project from initial design to final hand-off to operators. The first regulatory consideration is obtaining authorisation to construct a facility. There are multiple permit types and each varies from state to state, with permitting lead time increasing as larger emission thresholds are exceeded. The highest level of pre-construction air permit is known as a Prevention of Significant Deterioration (PSD) permit.1 If a PSD permit is required, construction of new or modifications of an existing facility may not begin until the permit is issued. The elapsed time between applying for a permit and final issuance can be long, taking up to 24 months, potentially creating a prohibitive delay in the execution of a project. If PSD emission thresholds are considered in the initial design of facility, changes could be made to the design to ensure emissions are held below the thresholds, allowing the operator to avoid PSD permitting entirely and apply for a more streamlined permit. Avoiding PSD permitting will reduce the required lead time to obtain a permit and potentially avoid onerous operating restrictions and monitoring requirements that could significantly hinder the economics of a project.

A second regulatory consideration is initial and ongoing compliance with Federal and state air quality rules and the potential requirement to obtain a federal operating permit. For example, if a new gas processing plant has potential to emit (PTE) emissions of Criteria Pollutants (NOx, CO, & VOC) greater than 100 tpy or Hazardous Air Pollutants (HAPs, generally BTEX, n-Hexane and Methanol) greater than 25 tpy, the facility will be considered a major source and therefore subject to significantly more stringent emission control standards, monitoring, recordkeeping and reporting. An initial design optimised to reduce VOC & HAP emissions in conjunction with emission controls may allow the owner/operator to avoid a lengthy permitting process and significantly lower compliance costs in the future. Table 1 contains a list of PSD major thresholds for facilities that are not on the list of 28 named stationary sources.

As new strategies to reduce air pollution have been developed, they have become standard practice, or what the EPA calls “Best Available Control Technology”, or BACT. A project subject to PSD permitting undergoes a lengthy “BACT Review”, a pollutant-by-pollutant analysis of available control technologies to ensure that the most effective method is chosen without being prohibitively expensive. If the project is not using the current BACT, the owner/operator will be required to demonstrate why current BACT is prohibitively expensive and that the use of somewhat less effective emission controls are justified.

The evolution of BACT marches on, year by year, as the regulatory environment continually changes. BACTs, though, are not simply “end of pipe” controls. Optimising facility operations to mitigate emissions is one of many effective strategies of preventing a BACT review by a regulatory agency. So designers and operators should investigate in detail what avenues they have available to reduce the additional costs of air quality issues, especially in terms of operation. This optimisation requires a full understanding not only of the process, but also the regulatory environment in which the plant will operate.

Case Study 1: Large Cryoplant Expansion Bottlenecked by HAPs Emissions
An example of how the regulation of VOC and HAP emissions directly impacts the operation of plants can be demonstrated from a large cryogenic processing facility located in the lower 48-states. This facility receives field gas from both conventional and unconventional sources. As new wells began coming on line, the processor had a strong need to increase throughput in the facility.
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