Ted Bestor, El Paso Pipeline Group, Houston, Texas;

and Mike Whelan, Pipeline Research Council International, Arlington, Virginia

Compressor station air emissions compliance represents an ongoing and significant long-term financial and operational threat to gas pipelines. To address this concern, the Compressor and Pump Station Technical Planning Committee (CAPSTC) of Pipeline Research Council International, Inc. (PRCI) has been working to develop cost-effective NOx control options to meet the very low NOx levels expected to be imposed on legacy reciprocating engines in the next three to seven years.

Over the past two years, PRCI has established a leadership position in assuring the future operation of the industry’s legacy pipeline compressor units under aggressive emissions control scenarios. A significant R&D program, known as Emissions Reduction for Legacy Engines (ERLE), combines the resources of PRCI, established research contractors, and industry vendors to maximize the likelihood of success.

PRCI is comprised of several technical committees, and the specific mission of the CAPSTC is to minimize the operating costs and capital requirements of compression and pump service while meeting market demands and all applicable environmental regulations. The ERLE project team within the CAPSTC has developed a comprehensive, time and progress-based plan (or “roadmap”) based on industry-wide support and consensus.

The ERLE effort began in December 2004, involving multiple contractors and a proactive industry oversight team. It targets ~2010 emissions compliance, under the premise that future emissions requirements will continue to push NOx requirements lower. The overall ERLE objective is to develop technical options that allow 80% of legacy engines to achieve potential NOx requirements as low as 0.5 gm/hp-hr with no increase in other emissions or fuel consumption, and no decrease in operating range, at costs ranging from 1/6 to 1/3 of engine replacement costs, across the full span of ambient conditions.

A significant, additional impact of the ERLE program is that the engine control strategies and optimized components developed can also be used to operate at maximum fuel efficiency. Thus, significant opportunities for fuel reduction and associated greenhouse gas emissions reduction are inherent to this program.

A joint effort

A perpetual challenge to obtaining value from an R&D program is avoiding disconnects between research and commercialization. The committee has sought to head off this predicament by involving stakeholders in the roadmap process early and often. Currently, the ERLE program is approximately 30% complete, thanks to the efforts of:

  • Sponsor: PRCI (through its member companies)
  • Research contractors: Advanced Engine Technologies, Inc., Colorado State University, Kansas State University, Innovative Environmental Solutions
  • Vendors (providing commercialization channels and cost-share): Hoerbiger, Enginuity, Kistler.

Case for action

Integral reciprocating compressor engines larger than 1,000 horsepower (hp) comprise half of the 16 million hp of installed U.S. gas compression capacity, with two-thirds of these units more than 40 years old. A careful review of multiple, pending federal regulations and the behavior of a set of aggressive (but precedent-setting) state governments leads to the conclusion that in the next three to seven years, a variety of NOx control programs will target these legacy engines.

Future emissions control programs will encompass a much broader geographic area than today. Driven by ozone control, regional haze concerns and New Source Review (NSR) pressure, stringent NOx emission reviews will be conducted by many states that put pipelines at risk for:

  • The loss of grandfathered engine status due to NSR interpretations
  • Requirements for ultra low-NOx emissions levels. that approach electric-equivalent performance. Today, these levels are achieved only by new, high-speed separable engines, electric motors or by rich-burn engines outfitted with NSCR catalysts.

Compliance scenarios that require either widespread engine replacement or engine retrofit to unprecedented emissions levels have severe financial consequences. The engine replacement cost scenario ranges from $2.2 billion to $6 billion, while retrofits could be appreciably less if ultra-low NOx options are developed.

Unlike ten years ago, today such punitive expenditures would be considered “reasonable” measures that fall within the mandates of various federal and state regulations. Therefore, the industry cannot expect to successfully argue against the economic burden created by these regulations, but must respond proactively to this challenge by setting aggressive development targets for cost-effective emissions retrofit equipment that allow legacy engines to meet these outyear targets.

This review further concludes that a nominal NOx level of 0.5 g/bhp-hr is a realistic target endpoint for retrofit technology, because key regulatory initiatives have allowed averaging emissions across units in order to meet state/regional NOx budgets. Thus, higher-NOx units can be offset by ultra-low NOx units, and the ability to achieve ? gram NOx becomes an important option as the overall regulatory targets are squeezed below the previous endpoints of 1.5 to 6.0 g/bhp-hr NOx.

Unfortunately, this quantum NOx reduction appears unavoidable, as consensus seems to be emerging that the public health effects of ground-level ozone exposure require a tighter ozone standard. This will lead to further controls on NOx sources as the new ozone standards are implemented and the regional and state compliance plans are developed. This viewpoint was confirmed by EPA’s recent release of a proposed rulemaking for a more stringent ambient ozone standard that will cause hundreds of counties to enter ozone non-compliance status in a few years.

An emerging impetus is regional haze concerns and the stated intention to protect visibility within the numerous Class I locations (National Parks & Wilderness Areas) found in the Western United States. NOx is regulated as a precursor to particulates and aerosols that contribute to visibility loss. When enacted, each park/wilderness area will have a 250-mile buffer protecting its airshed, in which strict NOx controls can be implemented.

The sequence and relationship of the various EPA regulatory initiatives has been identified in a recurring pattern well into the future. The intersection and overlap of these statutory requirements, continuing growth in population and the economy, and the requirement that ambient air quality standards be periodically revisited leads to the conclusion that over the long-term, NOx emissions will continue to be reduced, without regard to cost, until no technical alternatives remain.

The PRCI Compressor and Pump Station Committee and the PRCI Board of Directors have historically placed a high priority on developing reciprocating engine NOx retrofit options that are widely applicable and cost effective. This previous work has been extremely productive, with hundreds of engines employing technology from the PRCI/GRI program, saving pipelines many millions of dollars of largely non-productive capital expenditures. One single project that reversed a pending EPA rulemaking on formaldehyde control (requiring oxidation catalyst on all engines) saved the industry an estimated $400 million.

The financial exposure from the worst-case outcome of aggressive NOx controls – engine replacement – can be significant. The potential costs extend well into the billions of dollars. It should be noted that the replacement costs of $1,250-$1,500 were established in 2004, and have escalated drastically since then.

Fortunately, this exposure can be mitigated via improved technology options through this R&D. Even if these regional scenarios do not develop, the savings achieved by avoiding related costs would more than pay for this entire multi-year R&D program. The related costs could include large engine NOx retrofit project failures, or requirements to retrofit additional units because very low NOx levels and the corresponding NOx tonnage reduction cannot be achieved from a lesser number of units.

In addition, other drivers for low-NOx retrofit installations include:

  • Horsepower additions at existing compressor stations that require emissions offsets.
  • Facility expansions or increases in engine utilization that trigger Prevention of Significant Deterioration reviews and corresponding implementation of Best Available Control Technologies.
  • “Creative” use of New Source Review regulations to force retirement of fully operational units simply because they are older than permitting authorities feel is appropriate.
  • Stringent permitting and retrofit programs driven by perceived environmental values that select states are employing in their efforts to be eco-friendly and progressive.
  • The periodic need to over-control engines as a display of good faith to expedite permit approvals or to resolve a prior compliance issue.
  • Unit reconstruction or unit removal/re-installation that triggers New Source Review and associated requirement to reach Best Available Control Technology emissions levels.

Objectives

To comprehensively approach this challenge, a thorough review of the state-of-the-art of reciprocating engine NOx controls was conducted to identify a risk-controlled, flexible, multi-year R&D program. This program builds on a foundation of 15 years of productive research efforts and the resulting expansive body of knowledge and commercial experience that exists among operators, aftermarket vendors and R&D providers. The exercise confirmed the following R&D program goals for legacy reciprocating engine emissions NOx control equipment:

  • Achieve 0.5 grams NOx/bhp-hr at full load and speed, and maintain the lb/hr emissions equivalent of this level throughout the full operating range.
  • Result in no fuel consumption penalty.
  • Apply to at least 80% of legacy pipeline engines (> 6 MMhp total).
  • Enable continued compliance with CO, hydrocarbon and formaldehyde requirements.
  • Limit total initial and five-year operating costs to no more than 1/6th to 1/3rd of the unit replacement cost.
  • Maintain the unit’s current operating range and maintain or improve engine reliability.

Approach

A thorough assessment of the factors that impede low NOx operations identified six discrete technical dimensions that govern overall engine performance and emissions:

  • Engine closed-loop controls
  • Emissions aftertreatment
  • In-cylinder fuel and air mixing
  • Air delivery (at the turbocharger and engine frame level)
  • Air management (at the cylinder level)
  • Ignition systems.

Many of the dimensions are interdependent, but it was possible to prioritize them according to their expected marginal impact on NOx reductions, as well as their associated risks and costs. Each of the technology dimensions were evaluated for the following criteria:

  • Technology description
  • State-of-the-art
  • Technology gaps
  • Areas of promising technology development
  • Status of roadmap progress for each technology dimension

For all dimensions, it was recognized that a great deal of investment is ongoing for automotive and stationary power generation reciprocating engines, and that these developments should be closely monitored for their potential applicability to pipeline engines.

Inherent to this was the identification of program “on-ramps” and “off-ramps,” where certain technologies would be down-selected and/or alternative approaches would be initiated. Some technologies evaluated still have significant cost barriers (e.g., homogeneous charge compression ignition, or HCCI) and will not be pursued, but new understandings were gained and documented in project reports. Other technologies, such as oxidation catalysts, have matured in the short time since the roadmap’s inception, and will not be investigated under this program.

As one might expect, a five-year research program comprised of six technology dimensions and multiple projects per dimension quickly becomes a project management challenge. To facilitate this, an oversight team, consisting of representatives from PRCI member companies, was established for each of the six dimensions.

Current status

A summary of where we are along the roadmap is provided below.

  1. Closed-loop controls
    1. Cylinder-level sensors
      1. Where we are. Ongoing field evaluation of prototype ION-Sense technology to be commercialized by Altronic, Inc.; and characterization of advanced Kistler pressure sensors.
      2. Where we are going next. Complete field demonstration of Altronic pre-production ION-Sense technology, and evaluate potential for Kistler sensors as closed-loop control elements.
    2. Predictive controller
      1. Where we are. Development and demonstration of a fast controls prototyping system
      2. Where we are going next. Prototype hardware and software development leading to a field test in 2008.
    3. Cylinder and cycle level controls
      1. Where we are. Evaluated root causes of instability; reviewed new and previous data on cycle-level combustion variation and its relationship to key engine control parameters and set points.
      2. Where we are going next. Start development of control algorithm using fast prototyping system.
  2. After treatment controls
    1. Selective catalytic reduction
      1. Where we are. Completed on-engine (slip-stream) evaluation of five catalyst formulations (Figure 1) to characterize the load-following performance of SCR catalyst using anhydrous ammonia reagent.
      2. Where we are going next. Full-scale laboratory, on-engine evaluation of catalyst down-selected from slip-stream tests, evaluation of aqueous ammonia performance.
    2. NSCR state-of-art
      1. Where we are. Initial stages of remote monitoring of NSCR systems installed on multiple engines in field gathering service to characterize their emissions performance.
      2. Where we are going next. Identify and evaluate durable sensors (lambda or other) to control NSCR systems. Quantify unwanted side effects of NSCR, such as ammonia formation, and identify the conditions that cause NSCR systems to drift outside their desired operating range.
  3. Air management
    1. Active air control
      1. Where we are. Work is underway to evaluate the effects of balancing the scavenging air delivered to each power cylinder. Fabrication of a two-cylinder, static flow bench is also underway (Figure 2).
      2. Where we are going next. Completion of modeling and bench testing. Design and evaluate multi-cylinder engine prototype air balancing system, pending participation by a commercialization partner.
  4. Combustion performance (mixing)
    1. HCCI
      1. Where we are. Re-evaluated HCCI based on recent advances in European research. Costs and timeframe to implement on legacy pipeline engines are prohibitive.
      2. Where we are going next. No research projects identified at this time.
  5. Air delivery
    1. Turbocharger peaking system
      1. Where we are. Evaluated options for turbo peaking and selected a burner/booster system (Turbocharger Booster System, aka TuBS). Modeled, designed and tested a laboratory bench-scale system (Figure 3).
      2. Where we are going next. Pending participation by a commercialization partner, we intend to design, fabricate and evaluate a field prototype.
    2. Turbocharger Component Matching System (TuCMs)
      1. Where we are. Development and programming for the background system associated with a turbocharger component selection and optimization tool is complete. This is intended to reduce the performance variance of upgraded/re-aeroed turbochargers for NOx retrofit projects.
      2. Where we are going next. Provided there is sufficient support by turbocharger suppliers, we intend to develop a graphical users’ interface for TuCMs. A future enhancement would be to populate the application with vendor-specific component data.
    3. Turbocharger Monitoring System (TuMS)
      1. Where we are. Alpha and Beta prototypes are currently installed on five to six mainline legacy engines for two pipeline companies. This technology will enable the user to determine when turbocharger maintenance or overhauls are necessary, before a failure or sustained combustion instabilities occur.
      2. Where we are going next. We are developing a low-cost version of the current hardware and are seeking commercialization partners.
  6. Ignition
    1. Advanced pre-combustion chamber concepts
      1. Where we are. Identified candidate pre-chamber technologies based on recent literature and prior industry research. Completed on-engine, single-cylinder testing for six of these concepts.
      2. Where we are going next. Design and test a multi-cylinder system based on the PCC technology down-selected from the single-cylinder evaluations. Potential commercialization partners are currently involved in the technical review of this project and as co-funders.

Conclusion

So far, so good. In general, research is a risky and challenging endeavor, fraught with setbacks and unreasonable expectations. Multi-year programs are also usually beset with problems, such as uncertain funding or ever-changing influences and priorities. Although the Emissions Reduction for Legacy Engines program combines both of these elements, close cooperation between PRCI, research contractors, and industry vendors is proving successful at meeting the program goals. We are within budget, and at the 40% mark along the time-line, we are only slightly behind schedule, as measured by mile markers along the roadmap. But, we are not going to exceed the speed limit to arrive at .5 gm/hp-hr NOx to meet up with ERLE by 2010 – we’ll get there safely and comfortably – and the industry should be able to utilize a robust set of options to confront the multiplicity of NOx control challenges it will face in the near future.

Acknowledgment

Based on a paper presented at the Gas machinery Conference, held in Dallas, Texas, October 1-3, 2007.