Low-emissions generator uses catalyst system to reduce NOx, meet regulatory standards
By Ted Christiansen, National Oilwell Varco
Standards are a part of our complicated world – ASTM, ANSI, ISO, BS, DIN, SAE, SPE, API, EPA, CARB are just a few acronyms in the alphabet soup of standards.
With most standards, we cannot seem to agree on one standard worldwide, within a country, or even within an industry. Emissions standards are no different, forcing everyone involved in the manufacturing and use of diesel engines to make sense of it all. The intent of this article is to clarify how changing emissions standards will affect drilling and well service operations.
The players, standards
The US Environmental Protection Agency (EPA) is responsible for setting the emissions standards for engines and other environmental matters. Unfortunately, California and several other states believe the EPA is not reducing the emissions limits fast enough, so it has its own regulatory body called CARB – California Air Resources Board. This makes emissions compliance even more complicated and unpleasant. A solution to this problem will be presented here to allow uninterrupted operation of drilling and well service equipment in any state, including California, Wyoming and Colorado – today, 2014 and beyond.
The next EPA-mandated emissions limits for off-road equipment are Tier 4 Interim beginning 1 January 2011 and Tier 4 Final beginning 1 January 2014 (Figure 1). These standards apply to all diesel engines, including generator sets and mobile rigs. They require a significant reduction in NOx (nitrogen oxide) and PM (particulate) emissions compared with Tier 3 levels. In engines above 750 hp, Tier 4 mandates that NOx must be reduced from 6.9 g/bhp-hr to 0.5 g/bhp-hr and PM must be reduced from 0.15 g/bhp-hr to 0.07 g/bhp-hr. This represents 13.8-times and a 2.1-times reductions, respectively.
These limits and reductions are more easily understood when presented graphically, as in Figure 2. The orange-shaded field represents the Tier 3 emissions limits. An engine manufacturer must demonstrate a Tier 3 engine’s emissions falls inside this area. The green-shaded area represents Tier 4 emissions limits. An engine meeting Tier 4 must have emissions that falls inside this area.
In most cases, these new limits will not be achievable through engine design and calibration changes. To comply with these limits, the addition of aftertreatment technologies from automotive and heavy trucks will be necessary.
Diesel engines emit two primary pollutants that are regulated – NOx and PM. Nitrogen oxides are generated during combustion in two forms. Neither is visible but are shown graphically in Figure 3. PM emissions are generated from incomplete combustion, typically in excess fueling conditions such as rapid load changes. PM emissions are the black smoke associated with diesel engines. Note the legend scale is 50 nm (nanometers). To get a sense of scale, the data in compact discs are stored as indentations (known as pits) that are approximately 100 nm deep by 500 nm wide.
PM emissions can be inhaled and become lodged in the lungs, potentially leading to health problems. This is the motivation for regulation and reduction of these emissions, particularly in large, densely populated areas.
If diesel emissions cannot be reduced in the engine (cylinders), how can the 2014 limits be met? The solution is the treatment of the engine exhaust externally by a catalyst system, otherwise known as aftertreatment. The car you drive every day has a catalytic converter installed by the vehicle manufacturer to meet the emissions limits for that particular engine/vehicle combination. Similarly, the National Oilwell Varco Low Emissions Generator also has an aftertreatment system.
Figure 5 shows the emissions reduction achieved by the aftertreatment system on this generator set. The yellow dot represents the emissions as measured by the engine manufacturer. The blue dot represents the emissions with the aftertreatment system. The NOx emissions for this particular engine and aftertreatment system design are reduced 92% from 4.5 g/bhp-hr to 0.375 g/bhp. The particulate emissions are reduced from 0.15 g/bhp-hr to 0.052 g/bhp-hr.
The aftertreatment system on this low-emissions genset makes it EPA 2014 Final-compliant. It can be used in all US states, including California, Wyoming and Colorado, today and in the future.
The NOx aftertreatment system uses urea fluid injected into the exhaust stream to achieve the NOx reduction. The usage rate compared with fuel is in the low single-digit percentage. The storage volumes on the genset skid are proportioned so the urea tank would need a refill at the same time as the diesel fuel tank.
This type of emissions system requires the use of ultra-low sulfur diesel (ULSD) fuel containing a maximum of 15 ppm sulfur. This fuel was mandated nationwide in the US in October 2006. The use of higher sulfur fuel will eventually poison the oxidation catalysts, resulting in lower exhaust stream temperatures. The diesel particulate filter will eventually plug, leading to poor engine performance. The result is a damaged aftertreatment system and emission non-compliance.
Power, logistics, sound
This particular genset designed for National Oilwell Varco’s new 5E AC Mobile Rig for well service uses a 850-hp turbo diesel as its power source and produces 550 kW of electrical power at 600V/60 Hz. The space between the rails is used for a fuel tank and small urea tank. The genset’s house can be used with tailboard trailer for longer-term use on grade, or placed on a trailer next to the mobile rig. The only connections required for rig-up are three electrical connections.
In addition to the low exhaust emissions, this particular genset also has low noise emissions. Figure 6 shows the sound pressure in dBA for everyday situations and generator sets. The NOV Low Emissions Genset is about the same sound intensity at 5 ft as passing trucks at 22 ft (79/80 dBA). Of particular interest is the comparison of this noise level with “open house” gensets at 98.5 dBA and 101.6 dBA. Not only will surrounding communities appreciate (or even demand) the lower noise level, but rig hands will as well.
What about offshore?
This technology can be scaled for use on offshore drilling rigs. Offshore Norway, it may be of particular value and impact the bottom line directly and immediately. The Norwegian government passed a law several years ago that levies a tax on NOx emissions. Note the tax rate per kg of NOx increases 4.6 times from 2007 to 2010.
The tax rate (USD) per rig by year for four selected total installed power is shown in Figure 7. In 2010, a drillship operating in Norwegian waters would be levied a $6 million tax. A 50% reduction in emissions – easily achievable with this technology – could reduce this tax by 50% to $3 million.
The emissions reduction pace will continue for the foreseeable future and will increase both capital and operating expenditures. With the appropriate application of technology, however, these expenditures can be reduced significantly.
Ted Christiansen is director of new technology development at National Oilwell Varco. He has a BSME from GMI Engineering & Management Institute in Flint, Mich., and a MS in computer and systems engineering from Rensselaer Polytechnic Institute in Troy, N.Y., and is a licensed professional engineer in Texas.