3. Abatement Strategy
In order to achieve these more stringent standards, the diesel industry has had to focus on several strategies to achieve compliance. Initially, this effort focused on better management of fuel flow and the combustion process. Past, present and future regulations have all stimulated a dramatic change in diesel engines, fuel systems and after-treatment technologies. Once viewed as dirtier than their gasoline counterpart, diesel engines are receiving renewed attention because, on a unit-of-work basis, diesel fuel emits fewer greenhouse gases than gasoline. Since NOx and PM have now been lowered dramatically, diesel engines and the fuels that power them can be part of the environmental solution instead of being part of the problem. Given the operating conditions, fuel quality truly determines the attributes of the emission parameters (e.g. improving fuel combustion, increasing cetane level also tends to reduce emissions of nitrogen oxides and particulate matter); reduced sulphur will also decrease the NOx and PM simultaneously (Figure3) and its level of concentration in the diesel also plays a significant role in the performance of the downstream control mechanism. Diesel fuel evaluations carried out in Europe show that reduced sulphur in diesel can lower particulate. For example, lowering the diesel sulphur level from 2000 ppm to 500 ppm reduced the overall particulate from light-duty diesels by 2.4% and from heavy-duty diesels by 13%. The relationship between particulates and sulphur levels was found to be linear; for every 100-ppm reduction in sulphur there is a 0.16% reduction in particulate from light duty vehicles, and a 0.87% reduction from heavy-duty vehicles.
The technology-disabling effect of sulphur in diesel fuel is comparable to lead and sulphur in gasoline. Catalytic converters or NOx absorbers can eliminate much of the NOx emissions from new diesel engines, but sulphur disables them in much the same way that lead poisons the three-way catalyst. Thus, the presence of sulphur in diesel fuel effectively bars the path to low emissions of conventional pollutants. A sulphur content of 10 ppm compared to 50 ppm increases the performance and durability of oxidising catalytic converters, DeNOx catalytic converters and particulate filters, and therefore decreases fuel consumption. Sulphur in diesel fuel significantly affects the reliability, durability and emission performance of catalyst-based DPFs. Sulphur affects the filter performance by inhibiting the performance of catalytic materials upstream of, or on, the filter. Sulphur also competes with chemical reactions intended to reduce pollutant emissions and creates particulate matter through catalytic sulfate formation. There are also lower particulate emissions (due to lower sulphate emissions) with oxidising catalytic converters. In addition to its role as a technology enabler, low-sulphur diesel fuel gives benefits in the form of reduced sulphur-induced corrosion and slower acidification of engine lubricating oil. This leads to longer vehicle maintenance intervals and lower maintenance costs.
Therefore, with regard to diesel fuel, the following policies are recommended: Emerging advanced PM and NOx control technologies capable of achieving very low emissions will require a maximum of 50 ppm sulphur or preferably 10 ppm. While interim improvements in diesel fuel quality will benefit air quality, it is most efficient and cost effective for a refinery to go directly to ULSD rather than via several interim steps.
Engine Controls Mechanism
To meet the increasing stricter demand as per the emission standards, engine manufacturers started as early as the late eighties to develop cleaner diesel engines by employing a number of strategies (Figure 4). These approaches include advanced common rail fuel injection, electronic engine controls, combustion chamber modifications, air boosting, improved air/fuel mixing, and reduced oil consumption. Achieving ultra-low exhaust emission targets requires a systems approach. Engine manufacturers are focusing on ways to control engine operation to reduce engine-out emissions to lowest possible levels and reduce the burden on the exhaust emission control systems.
Post Combustion Control Technologies
Since more than a decade we have witnessed tremendous efforts by various stakeholder organisations (auto manufactures, emission control device manufactures, fuels oil producers and other stakeholders) to bring about changes in the emission control strategy (Fig 5). Today, viable emission control technologies exist to reduce diesel exhaust emissions in a two pronged strategy – reduction in PM emission and NOx separately (Figure 6 & 7).
Technologies designed to control particulate matter (PM) include:
• Diesel oxidation catalysts (DOCs)
• Diesel particulate filters (DPFs)
• Closed crankcase ventilation (CCV)
Technologies designed to control oxides of nitrogen (NOx) include:
• Exhaust gas recirculation (EGR)
• Selective catalytic reduction (SCR)
• Lean NOx catalysts (LNCs)
• Lean NOx traps (LNTs)
All is not lost
• Diesel emissions from mobile sources have raised health and welfare concerns but a number of technologies exist that can greatly reduce emissions from diesel-powered vehicles.
• Diesel oxidation catalysts, diesel particulate filters, exhaust gas recirculation and crankcase emission controls have been successfully rolled out on new 2007 on-road vehicles. These technologies offer opportunities to greatly reduce emissions of particulate matter and other pollutants like toxic HCs.
• Advanced NOx control technologies are being developed to meet EPA’s 2010 on – highway emission standards. Technologies such as lean NOx traps and selective catalytic reduction have been demonstrated to be durable and effective methods of achieving low tailpipe NOx levels from diesel and other lean burn engines.
• The widespread availability of low sulphur diesel (50 ppm max) for on-road vehicles has enabled the application of advanced emission control systems for diesel engines and vehicles. The future availability of ultra low sulphur fuels (10 ppm max) with the introduction of tighter emission regulations equivalent to Euro V/VI will substantially abate the menace of diesel pollution.
• Some of the technologies that reduce particulate matter and NOx are also applicable to the diesel engines already on the road today, offering a cost-effective way to reduce diesel emissions during their remaining life.
• Advanced sensors are being developed to monitor all components of the exhaust control system. These sensors will allow diesel engines to meet the same OBD and emissions requirements already in place for gasoline spark-ignited engines and vehicles.Niranjan Raje Former Director (R&D) Indian Oil Corporation
The views expressed by the author are personal. (Niranjan Raje is a member of the Expert Appraisal Committee of the Ministry of Environment and Forests, and member of the EPCA. He superannuated as Director (Research & Development) of Indian Oil Corporation in 2005. He has over 35 years of experience in the fields of fuels quality, auto emissions and alternative fuels. He has published over 50 research papers in reputed Indian and international journals. He was also awarded the Department of Science and Technology Award by the President of India.)