Diesel engines are reliable, fuel-efficient, durable, easy to repair and inexpensive to operate, but they produce significant levels of particulates (PM) and NOx, mostly when accelerating from a stop. Current estimates indicate that emissions from such engines in the northeast U.S. account for roughly 33 percent of the nitrogen oxides (NOx) and 80 percent of the particulate matter (PM10 ) emitted by all mobile sources.
Diesel engines that power construction equipment are more polluting than equivalent diesel engines for normal highway use because of the lack of any emission controls until 1996. Therefore, reducing these emissions has the potential to improve ambient air quality for the region and has significant air quality benefits for those who live or work in or adjacent to construction areas. A major step was taken in May 2004 with the approval of the new U.S. Environmental Protection Agency Clean Air Non-road Diesel rule, but it will take almost two decades to have the diesel engines that power construction equipment replaced with the new mandated cleaner engines.
PB developed diesel engine retrofit/clean fuels programs during the construction phase of two large projects:
- The Central Artery/Tunnel (CA/T) Project in Boston , Massachusetts , for which PB in joint venture with Bechtel is serving as managing consultant
- The I-95 New Haven Harbor Crossing Corridor Improvement Program (I-95 NHHC) in southern Connecticut on which PB is serving as program manager.
Emission Reduction Technologies
The diesel engine retrofit programs discussed in this article started as ways to reduce emissions until cleaner fuels and cleaner engines become part of the standard manufacturing process five to ten years from now. The most commonly applied technologies fall into three categories:
- Fuel modifications, including synthetic diesel, water-in-diesel emulsions, biodiesel, and ultra low sulfur diesel
- Engine design/fuel modifications, including exhaust gas recirculation, dimethyl ether, and natural gas
- After-treatment/add-on pollution control devices , including oxidation catalysts, diesel particulate filters, lean catalysts, and selective catalytic reduction.
Given the advanced stage of design and construction of the two projects, the programs focused on add-on pollution-control devises with the options of cleaner diesel fuels.
The CA/T Diesel Retrofit Program
The CA/T project spans a 15-year construction period with hundreds of diesel powered construction engines in simultaneous operation. Construction started in 1992. During 1998, in conjunction with Massachusetts Department of Environmental Protection (MDEP) and the Northeast States for Coordinated Air Use Management (NESCAUM), we implemented an emission reduction diesel retrofit program for off-road construction equipment using oxidation catalysts.
Initially started as a pilot program, it was then expanded to include all off-road equipment on more than 20 remaining construction contracts. This program resulted in well over 100 pieces of diesel powered equipment being retrofitted. We also examined the use of an emulsified diesel fuel, which reduces NOx and black smoke compared to regular diesel.
Oxidation catalysts reduce diesel emissions by oxidizing diesel pollutants, such as PM, hydrocarbons (HC) and carbon monoxide (CO), to less harmful emissions, such as H2O and carbon dioxide (CO2 ). Oxidation catalysts were selected over diesel particulate filters for the following reasons:
- Greater reduction in HC that is associated with diesel odors as well as CO and PM10
- Ease of installation and maintenance
- Lower cost (approximately $2,500 versus $10,000 per unit).
Oxidation catalysts are the most broadly applied technology currently certified by the Environmental Protection Agency (EPA). The technology is well proven, since more than one million highway vehicles retrofitted with oxidation catalysts are in circulation to date, but it is not common on construction equipment.
Based on EPA certification data, we anticipated that oxidation catalysts would achieve at least 20 percent reductions for PM, 40 percent reductions for CO, and 50 percent reductions for HC in all heavy-duty engines. The results of the evaluation for 88 pieces of equipment retrofitted during 2000 indicated emission reductions of approximately 90 Kg/day of CO, 30 Kg/day of HC, and 7.4 Kg/day of PM10 .
The CA/T project had also explored the possibility of lowering diesel emissions even further by replacing the diesel fuel with a cleaner alternative. The LUBRIZOL Corp. manufactures a low NOx emission blend of diesel fuel marketed as PuriNOx, TM which consists of a mixture of diesel fuel, water, and an additive to maintain stability of the emulsified mixture. Demonstration projects achieved 10 percent to 30 percent NOx reductions and 10 percent to 50 percent PM reductions. A test performed on one of the CA/T contracts using a Caterpillar excavator for a period of three weeks indicated that PuriNOxTM reduced NOx emissions up to 30 percent and smoke up to 96 percent when compared to No. 2 diesel fuel. Unfortunately, PuriNOxTM was not applied due to budgetary considerations.
The I-95 Nhhc Diesel Emission Control Program
Reconstruction of the 11.5-km (2-mile) -long I-95 NHHC project will take more than ten years to complete and will have more than 200 pieces of diesel powered construction equipment operating during this period. Four emission reduction technologies were considered for this diesel vehicle emission control program:
- Two diesel engine retrofit technologies (oxidation catalysts and four way catalysts)
- Two cleaner fuels (biodiesel B-20 blend and PuriNOxTM ).
Because this was a voluntary pilot program for Connecticut Department of Transportation, it was decided to use the most widely accepted and least expensive emission reduction options. As such, oxidation catalysts and PuriNOxTM were selected to provide the contractors more flexibility in situations where equipment would not remain on site for long periods.
Based on an evaluation of maximum emission benefits (a function of horsepower-hours of operation and fuel consumption) versus practical construction site considerations:
- All diesel powered construction equipment with engine horsepower (HP) ratings of 60 HP and above that were on the project or assigned to a contract for a period in excess of 30 days had to be retrofitted with emission control devices and/or use clean fuels
- Small equipment and specialized equipment to be used for short periods of time was exempted from retrofitting.
By mid 2004 more than 60 pieces of construction equipment were retrofitted with oxidation catalysts (Figure 1).
Figure 1: Construction equipment retrofitted with oxidation catalyst for the I-95 New Haven Harbor Crossing project. |
Conclusions and Observations
This type of program is a cost-effective way to reduce diesel emissions and odor considering that the costs of the oxidation catalysts are roughly one percent of the total cost of the construction equipment to be retrofitted.
Another positive aspect of initiating these programs was the creation of an air quality working group that met regularly and was able to convince all affected parties to buy into the program. It was critical to obtain a clear understanding of what the program benefits and costs were, who was going to pay, and how the concept would be translated into a specification as part of the bid documents early on in the project.
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