This article originally appeared in the 3rd Quarter 2008 issue of The PEI Journal.
Diesel Exhaust Fluid: Using urea to control diesel engine emissions
By Vijay Srinivasan, Ph.D.
Urea is the first significant consumable non-fuel fluid to be added to any car or truck in the history of the automotive industry. Some people prefer to use the term “Diesel Exhaust Fluid” (DEF) instead of “urea” due to its close association to malodorous bodily fluid. Urea is a colorless, odorless substance that is used to meet the Environmental Protection Agency's (EPA) vehicle exhaust regulations that will go into effect in January 2010.
DEF in New Diesel Vehicles
In 2000 and 2001, EPA published the Tier 2 Emission Standards for Light Duty and Heavy Duty Vehicles, mandating that on-highway diesel and gasoline vehicles meet new strict emission control standards beginning January 1, 2010. Effective that date, no new on-road vehicles can be sold without meeting the Tier 2 emission standard: 0.2 grams of nitrogen oxide (NOx) per brake horsepower-hour. Gasoline engines, unlike diesel engines, do not have any problem meeting the new standards. Many end-users, however, prefer diesel engines due to their improved fuel economy on a mile-per-gallon as well as a mile-per-dollar basis.
Vehicle and engine manufacturers have developed Selective Catalytic Reduction (SCR) technology, which uses DEF to convert noxious NOx into harmless nitrogen gas. While some vehicle manufacturers have committed to using the SCR technology, others will use Exhaust Gas Recirculation (EGR) technology. In EGR, a portion of the vehicle exhaust gas is recycled to the engine, minimizing the generation of NOx in the engine itself, allowing the vehicle to meet the Tier 2 standards.
SCR technology has up to a five percent fuel economy advantage over EGR. The use of EGR also requires additional space for the recirculating pipes, potentially decreasing the available payload capacity. Industry projections on SCR acceptance vary, indicating that between 50 percent and 75 percent of all diesel vehicles will use SCR technology.
Below is the current status of committed and uncommitted engine and vehicle manufacturers:
Will Use SCR: Daimler (Mercedes, Freightliner, Sterling, WesternStar), BMW, Ford, Volvo/Mack, MTU/Detroit Diesel, PACCAR (Kenworth, Peterbilt, DAF), Hyundai, Kia, Nissan
Will Not Use SCR: Navistar/International, John Deere, Caterpillar
Undecided: Honda, Toyota/Hino, GM, Chrysler, Cummins, VW, Isuzu.
The EPA's vehicle exhaust regulations will go into effect in January 2010.
SCR vs. EGR
The battle between Selective Catalytic Reduction and Exhaust Gas Recirculation is currently being played out, and there are ardent supporters and detractors on both sides of the issue. In Europe, SCR appears to decisively have won the battle, at least for now. However, the acceptance of SCR is somewhat dampened by unfortunate timing, as SCR-equipped vehicles are hitting the road at the same time as a global slowdown in growth, increased cost of energy and DEF are having a serious economic impact. The timing of this economic hardship has the potential to tilt the battle to one side or the other, but no one is sure which side.
Here is one analysis of the comparative economics: The initial cost of SCR is estimated to be $4,000 per truck, but there are no published estimates of initial EGR cost, so I am going to assume $2,000. Using the following numbers for class 8 heavy duty trucks, SCR saves $2,100 per year:
- Diesel: $5/gallon
- DEF: $3/gallon
- Diesel Savings Using SCR: 5%
- DEF Usage: 2% of diesel usage
- Miles Driven per Year: 70,000
- MPG: 6.3 miles per gallon
This $2,100 represents about a one-year payback for SCR.
Urea Production and Use
Urea is a colorless, odorless crystalline solid with a molecular weight of 60 and molecular formula of (NH2)2CO. DEF solution decomposes to ammonia and carbon dioxide upon extended exposure to elevated temperatures, leading to an odor similar to ammonia-based household cleaning products. Globally, about 130 million tons of urea are produced, primarily for use as a fertilizer, supplying nitrogen, an essential nutrient for plant growth. For fertilizer use, granular urea is generally shipped across great distances to minimize freight cost. Fertilizer distributors receive the granules and dissolve them in water at regional terminals, then ship to end-users by tanker trucks.
At a presentation to NATSO, the National Association of Truckstop Operators, Colonial Chemical Company's Jim Spooner told the audience three important factors to understand related to DEF:
- Purity of DEF is critical for vehicle performance.
- The supply chain determines the purity.
- DEF requires proper storage and handling to assure quality.
DEF used in SCRs is of a much higher purity. Distributors have the option of bringing in granular urea, concentrated urea or the 32.5 percent solution. The selection of the incoming state of urea will be based on the trade-off among storage capacity, throughput and convenience. End-users include truck stops, truck fleet facilities, truck dealers, automotive dealers and automotive service facilities.
North American urea capacity is about 10 million tons, while consumption is somewhat higher. The balance of the urea is imported from regions with low-cost natural gas. Urea is produced commercially by reacting ammonia and carbon dioxide. Ammonia is produced primarily from natural gas, along with byproduct carbon dioxide.
A key industry benchmark for the price of solid fertilizer-grade urea is US Gulf Granular urea barge load, which was $400 per ton at the end of 2007; it has increased to above $650 per ton, mainly due to the imposition of a 135 percent export duty by the Chinese government. For the 32.5 percent DEF solution used in diesel vehicles, the cost of contained urea at US Gulf is about $1 per gallon. The street price of 32.5 percent DEF suitable for diesel vehicles is expected to be $2 to $5 per gallon. The significantly higher street price is due mainly to logistics and the infrastructure cost of making DEF solution available in small quantities across the country, as well as to the higher purity required for the diesel vehicle application.
The estimated quantity of urea needed to serve the SCR market varies widely, but even the highest estimate is less than one percent of the global urea production. Annual on-highway diesel consumption is about 40 billion gallons. Assume that 50 percent of all new diesel vehicles use SCR. If DEF usage rate is two percent of diesel use, and 50 percent of the fleet in the United States is turned over in five years, then the estimated DEF demand in 2015 would be 200 million gallons (300,000 dry tons). If SCR acceptance increases to 75 percent and DEF usage rate increases to 4 percent, the estimated urea demand in 2015 would be 600 million gallons (900,000 dry tons).
In either case, the amount of DEF solution that needs to move from production facilities to dispensing sites across the nation is substantial. DEF producers are working feverishly to increase the North American production capacity of the high purity DEF solution that is required for SCR and its distribution capabilities to various parts of the country. Chemical distributors are also working to add DEF-handling facilities at existing chemical distribution centers across the country. We have not yet come across any agricultural or petrochemical distributors that plan to stock and distribute DEF.
Filling the Tank
In Europe, where vehicle SCR systems have been in commercial use for about two years, a wide range of dispensing systems are available: hand operated drum pumps, electrical or compressed air-powered pumps, metering systems for drums (typically 55 gallon) and totes (typically 275 gallon), and pumps and metering systems for large stationary tanks.
A potential problem in handling DEF is that it freezes at 11°F. The safest solution is to store and dispense DEF solution indoors. If indoor dispensing is not practical, heated enclosures designed specifically to store and dispense the fluid can be purchased. Excessive heating at elevated temperatures causes the DEF to decompose, releasing ammonia. ISO standards limit the total ammonia to 0.2 percent, and so the shelf life of the fluid is limited if exposed to elevated temperatures for extended periods.
ISO standards have very strict limits on contamination, which limit the materials that the fluid can contact. Per ISO standards, the suitable materials are 304 and 316 stainless steels, titanium, hastelloys and several plastics, so long as they are free of additives, i.e., polyethylene, polypropylene, polyisobutylene, Teflon, PVDF and PFE. Materials specifically not recommended include non-ferrous metals and alloys (copper, aluminum, magnesium, silver, zinc, lead), solders containing non-ferrous metals, and nickel-coated plastics and metals. Bottles, drums and totes are most commonly made of polyethylene. Large storage tanks can be made of stainless steel or coated carbon steel.
Lessons from Europe
SCR-based diesel vehicles were introduced in Europe at the end of 2006. A large number of new diesel-powered vehicles use SCR technology. There have been no major DEF availability problems, with a full range of containers available: 10-liter (2.5 gallon) bottles, 55-gallon drums, 275 gallon IBCs, 250-1,000 gallon plastic tanks and large stainless or coated steel tanks. A variety of materials for constructing the tanks is available. The most popular containers are 275-gallon polyethylene intermediate bulk containers (IBCs) and totes inside a galvanized steel cage with easy-to-install dispensing systems.
While the most favored system varies by country within Europe, a vast majority of systems appear to be of an accuracy that does not meet retail weights and measures standards. Most SCR vehicles are trucks, and most of these trucks return “home” at night where they are usually fueled. Therefore, the dispensing systems do not have to meet onerous weights and measures requirements. In the United States, on the other hand, about three-quarters of the on-highway diesel demand is filled at public filling stations such as truck stops. The cost to implement a weights and measures certified system might well be so high that some of the stations may delay installation of the equipment until there is reasonable certainty of SCR technology adoption. This, in turn, might delay certification of trucks by the EPA, in effect delaying Tier 2 enforcement unless temporary or permanent weights and measures exemption is obtained for low cost alternatives.
PEI will continue to monitor the development of this emerging technology and market.