Steve Besore has been employed by MTU for 25 years and is currently manager of oil and gas markets. He has a degree in business administration from Glendale Community College in Glendale, Ariz. Besore can be reached at firstname.lastname@example.org.
Current EPA Tier 4 Interim emissions standards apply to most new diesel engines used in oil and gas exploration, and even more strict Tier 4 Final regulations are just around the corner. While the onus of compliance is on engine manufacturers, drillers and producers need to be aware of how emissions regulations will affect their operations.
The Tier 4 Interim fracturing engine that meets all current EPA emissions regulations without the need for exhaust aftertreatment. (Photos courtesy of MTU and Stewart & Stevenson)
For decades, diesel engines have found wide application in oil and gas exploration and production, from powering pumps to running mechanical oil rigs to generating onsite electricity. A fairly recent application is driving the powerful pumps used in deep well fracturing operations, where banks of large diesel engines provide the hydraulic forces to fracture petroleum-bearing rock formations to liberate the oil and gas.
In the past, the exhaust gases from these essential workhorses in the oil patch and other nonroad uses were uncontrolled and resulted in emissions of nitrogen oxides (NOx), carbon monoxide (CO), non-methane hydrocarbons (NMHC) and particulate matter (PM or soot). Just as with emissions from on-highway trucks, buses and cars, these uncontrolled nonroad engines contributed their share to damaging air pollution.
Then in 1996, the U.S. Environmental Protection Agency (EPA) began to enforce limits on diesel exhaust emissions from new nonroad diesel engines and then from new stationary diesel-engine generator sets in 2006. Implemented in a series of steps called tier levels, these regulations have introduced successively more stringent limitations on NOx, CO, PM and NMHC.
In response to these regulations, engine manufacturers began introducing innovative design changes and sophisticated engine control systems that have successfully reduced the major pollutants in diesel exhaust to comply with each successive tier level. This article explains the latest and near-term emissions requirements for new nonroad diesel engine applications and discusses the technologies used to comply with the current and pending standards.
How EPA Emissions Regulations Apply to Oil and Gas
While the burden of complying with current and future diesel emissions regulations is on engine manufacturers, it is important to understand how these regulations are applied in the oil patch. Here is a quick summary:
- Current federal EPA emissions regulations are not retroactive to older engines in the field. They apply only to new engines based on their date of manufacture and, in some cases, their application.
- EPA emissions regulations vary by the horsepower rating of the engine (or the metric mechanical kW equivalent).
- Diesel-engine and gas-engine generator sets used strictly for emergency standby power (ESP) are exempt from EPA Tier 4 Interim and Tier 4 Final regulations.
- Nonemergency diesel and gas-engine generator sets used for powering electric drilling rigs and for camp power for crews will be required to comply with the current Tier 4 Interim regulations. Tier 4 Final will be required for small nonemergency generator sets beginning in 2013 and eventually all nonemergency generator sets by 2015.
Figure 1. EPA mobile off-highway emissions reduction achievements
Dramatic Reductions Already Achieved
Since the introduction of regulations in the mid-1990s, nonroad diesel emissions have been dramatically reduced. (See Figure 1.) By the time Tier 4 Final is introduced in 2014 and 2015, NOx and PM emissions from diesel exhaust will have been reduced by 99 percent. The tier requirements have been written and implemented over an 18-year period that will culminate in 2015 with Tier 4 Final. (See Figure 2.)
Figure 2. Nonroad diesel emissions standards vary by the horsepower rating of the engine and have been phasing in since 1996, and 18-year period that will culminate in 2015 with Tier 4 Final.
Emergency Standby Power Exception
As noted above, diesel-engine and gas-engine generator sets used strictly for emergency standby power (ESP) at oil and gas exploration and production sites are exempt from EPA Tier 4 Interim and Tier 4 Final regulations. ESP installations are exempted because Tier 2 and Tier 3 engine generators already exhibit emissions reductions of more than 85 percent that have come about through in-engine design improvements. This means that all current Tier 2 and Tier 3 diesel generator sets in their applicable horsepower categories will be in EPA compliance through 2015 and beyond when used strictly for ESP. (See Figure 3.) ESP installations are also allowed up to 100 hours of running time per year for testing and maintenance.
Figure 3. Generator sets used exclusively for emergency standby power are exempt from Tier 4 Final. Some smaller generator set engines are able to meet Tier 4 Interim without the need for aftertreatment.
Figure 5. Selective Catalytic Reduction (SCR) will be the principal aftertreatment technology used to further reduce NOx in diesel exhaust.
While in-engine refinements helped engine manufacturers achieve Tier 2, Tier 3 and even Tier 4 Interim standards, Tier 4 Final regulations may require exhaust aftertreatment to further reduce NOx and PM. The most effective and practical aftertreatment technologies include selective catalytic reduction (SCR) to control NOx and diesel particulate filters (DPF) to capture the remaining PM. While most diesel engines require SCR to meet the NOx limits for both Tier 4 Interim and Tier 4 Final, some engine models are able to meet the Tier 4 Interim regulations without either SCR or a DPF.
For example, one engine commonly used in advanced fracturing operations meets all EPA Tier 4 Interim regulations without SCR or a DPF. Instead, it uses exhaust gas recirculation (EGR) combined with advanced combustion controls, dual-stage turbochargers and high-pressure common rail fuel injection to achieve Tier 4 Interim standards. The three major technologies that are being used on diesel engines to meet Tier 4 Interim and Tier 4 Final are discussed in further detail below.
Selective Catalytic Reduction
Selective Catalytic Reduction (SCR) — SCR is being used on some engines to achieve Tier 4 Interim and Tier 4 Final regulations. SCR reduces the remaining NOx in diesel engine exhaust by up to 90 percent. It works by combining the exhaust gases with ammonia in the form of urea (or diesel emissions fluid, DEF) and passing the mixture over a catalyst. The result is exhaust that contains harmless elemental nitrogen, water vapor and carbon dioxide. Roughly one gallon of DEF is required for every 20 gallons of diesel fuel that is burned. (See Figure 5.)
A fracuring rig destined for Russia - the use of emissions compliant engines is growing rapidly in many foreign countries that are committed to reducing pollution.
Diesel Particulate Filter
Diesel Particulate Filter (DPF) is a device that traps and incinerates PM (soot particles) in diesel exhaust. The technology will not be required on most diesel engines to achieve Tier 4 Interim regulations, but it may be required in most power nodes in conjunction with SCR to achieve Tier 4 Final. A DPF uses a mechanical filter to trap soot particles after they have been partially oxidized by a catalyst. Periodically, inert ash will have to be removed from the DPF. (See Figure 6.) In 2014 and 2015, a combined DPF-SCR unit will be used on most engines to meet Tier 4 Final regulations.
A quintaplex fracturing rig
Exhaust Gas Recirculation
Exhaust Gas Recirculation (EGR) is another technology that is used on diesel engines to reduce NOx in the exhaust. EGR works by recirculating a small amount of cooled exhaust gas back into the combustion chamber. This reduces combustion temperatures and effectively reduces the production of NOx. EGR is very effective at reducing NOx, and many EGR-equipped engines may not require SCR aftertreatment. (See Figure 7.)
Beginning in 2014, new diesel engines purchased for oil and gas applications must meet the EPA’s Tier 4 Final regulations. To meet this goal, engine manufacturers have been working for nearly two decades to perfect emissions control technologies on diesel and spark-ignited engines to ensure that compliant systems were in place when the EPA Tier levels took effect. Some new diesel engines used in fracturing applications are able to comply with current Tier 4 Interim regulations without the need for exhaust aftertreatments such as SCR or a DPF.
Figure 6. A diesel particulate filter (DPF) traps the carbon particles in diesel exhaust and converts them to ash, which must be periodically removed.
Current Tier 2 and Tier 3 generator sets are EPA-compliant for most ESP applications through 2015 and beyond.
Tier 4 Final regulations will require aftertreatment technologies such as SCR and DPF beginning in 2014 for applications of 750 horsepower and below, and in 2015 for applications of 750 horsepower and above.
Figure 7. Another emission control technology, exhaust gas recirculation (EGR), cuts NOx by reducing combustion temperatures using recirculated exhaust gases.
To make sure that your new diesel engines for oil and gas applications comply with all the latest EPA emissions regulations, consult your engine manufacturer and local air-quality authorities.
Upstream Pumping Solutions, Summer 2011