10 on 10 with 10 ppm: A wise step on All Fools’ Day

Starting from today, Delhi leads to sell only clean petrol and diesel fuel with sulphur content as low as 10 ppm

By Anumita Roychowdhury
Published: Saturday 31 March 2018

Today is a big day for the clean air community in India as Delhi celebrates a step forward! The National Capital is joining the august league of cities in the developing world to advance use of the cleanest petrol and diesel to cut emissions and make way for cleaner Bharat Stage VI (BS-VI) vehicles to come in two years. India is the only major vehicle-producing country to skip BS-V emissions standards and move directly to BS-VI in 2020. Today, all fuel retail outlets in the Capital are awash with petrol and diesel with sulphur content as low as 10 ppm or BS-VI fuels—five times less than the BS-IV fuels with 50 ppm sulphur, the current fuel quality in the country.  This proactive move of the Union Ministry of Petroleum and Natural Gas to cut the toxicity of Delhi’s smog has stirred excitement. But people are also asking how will this help when vehicles have not yet moved to BS-VI standards?

Benefits of changing over to clean fuels are several and will only get significantly better when vehicles comply with BS-VI standards in 2020. But as citizens, we need to see through the maze of scientific jargons to understand how this fuel will help us.

Our on-road vehicles will spew less particles: All our vehicles will spew less particles when their tanks are topped up with 10 ppm sulphur fuels. Sulphur in fuels contributes towards the formation of particulate. According to the United States Environmental Protection Agency (USEPA), “the contribution of the sulfur content of diesel fuel to particulate emissions is well-established and there is a general linear relationship between fuel sulfur levels and particulate emission” (see Graph 1: Tons of directly emitted particulate matter from diesel fuels sulfur).  As the sulphur level increases, particulate matter related to sulphur increases linearly. Thus, conversely, if sulphur level reduces, the related particulate emissions will also reduce from the on-road fleet.

Graph 1: Tons of Directly Emitted PM from Diesel Fuels Sulfur

Notes: PPM = parts per million. Only particulate matter (PM) related to sulfur and not the total PM emitted from a diesel engine are reflected in this figure.

(Source: Michael Walsh 2015 based data from United States Environmental Protection Agency (USEPA))

Sulphur dioxide emissions—a deadly gas, is also directly proportional to the amount of sulphur in fuel: Fuel sulphur also increases emissions of sulphur dioxide—a harmful ingredient of smog. Moreover, in the oxygen-rich diesel exhaust, several percent of the sulphur dioxide oxidise to become sulphate particles and contribute to the formation of ultra-fine particles in the exhaust, which are considered especially hazardous because of their ability to penetrate deeply into the lungs. Thus, if sulphur levels are reduced in fuels, the entire on-road fleet will benefit.

10 ppm sulphur fuels allow emissions control systems of on-road diesel cars to perform more efficiently: Older, on-road, duty diesel vehicles that meet standards from BS-II to BS-IV are generally fitted with oxidation catalysts to comply with the standards. With higher sulphur fuel, the conversion of sulpur in the catalyst reduces its (the catalyst’s) effectiveness and leads to catalyst “poisoning” as well as formation of sulphates in the converter which are emitted as additional particles. This problem will reduce substantially and system performance will improve for all on-road vehicles if they run on BS-VI fuel or 10 ppm sulphur fuels.

Less engine wear-and-tear that can reduce emissions: Sulphur content is also known to have effects on engine wear and deposits depending largely on operating conditions. High sulphur content becomes a problem in diesel engines operating at low temperatures or intermittently. Under these conditions, there is more moisture condensation, which combines with sulphur compounds to form acids and results in corrosion and excessive engine wear. Generally, the lower the sulphur levels, the less the engines wear out. Thus, all on-road vehicles benefit from ultra-lowsulphur fuels.

Petrol vehicles will also benefit: Sulphur in petrol reduces the efficiency of catalysts and adversely affects heated exhaust gas oxygen sensors. If sulphur levels are lowered, existing vehicles on road equipped with catalysts will generally have improved emissions. According to the USEPA, laboratory testing of catalysts has demonstrated reductions in efficiency due to higher sulphur levels. Studies have also shown that sulphur adversely affects exhaust gas oxygen sensors and may affect the durability of advanced on-board diagnostic (OBD) systems. There is one estimate available from the USEPA that shows that by reducing sulphur level in gasoline (petrol) from 30 ppm to 10 ppm, overall emissions of NOx from on-road vehicles in 2017 reduced by 8 per cent, carbon monoxide by 8 per cent, and volatile organic compounds by 3 per cent.

(Source: Michael Walsh 2015 based data from United States Environmental Protection Agency (USEPA))

Opens up opportunity for retro-fitment of advanced emissions control systems:  Advanced and sophisticated emissions control systems that are needed to control particulate emissions in BSVI diesel vehicles can also be retrofitted in the current on road fleet if 10 ppm sulphur fuel is available. These advanced systems are extremely sensitive to sulphur. Available evidence shows that if BS-VI vehicles that are fitted with advanced and highly sensitive emissions control system do not operate with compatible fuel quality (10 ppm sulphur) the efficiency of the system will reduce significantly.

Cleaner fuel like 10 ppm sulphur fuels opens up the opportunity to retro-fit advanced emissions control systems in the existing models—especially buses and trucks—along with engine re-powering to reduce emissions from the on-road fleet substantially. But there is no retro-fitment policy in India to certify retro-fitment of existing makes and models of on-road vehicles with advanced emissions control systems. This is essential as retrofitment is not an off-the-shelf solution.

Will the auto industry make a proactive move?

As Delhi moves to 10 ppm sulphur fuels and the National Capital Region (NCR) is slated to move by April 2019, can the vehicle industry bring BS-VI vehicles in advance in this region? Already, auto companies are announcing that they are ready with BS-VI models. For instance, the S 350d model of Mercedes Benz released in India is a petrol car that claims to have met BS-IV emissions standards already. Several petrol car models will be able to meet BS-VI standards quickly and at affordable costs.

Getting the 10 ppm sulphur fuel all across therefore becomes critical. Otherwise, fuel sulphur can damage advanced particulate traps, and exhaust gas recirculation (EGR) needed to control NOx. Diesel particulate filters in BS-VI vehicles can have over 95 per cent efficiency in controlling particlate matter with 10 ppm sulphur fuel use. But if BS-VI vehicles run on sulphur rich fuels, efficiency can drop significantly. In fact, studies show that emissions efficiency can drop to zero with 150 ppm sulphur fuel and particulate emissions can more than double over the baseline with 350 ppm sulphur fuel.  Exact impact with 50 ppm sulphur on on-road fleet is not available. The systems can recover their original efficiency to some extent if the use of near-zero sulphur fuels is restored, but recovery takes time due to sulphate storage on the catalyst.

Fuel sulphur also hinders durability and reliability of EGR system due to sulphuric acid formation. Acid formation raises system costs, due to the need for premium components and increased maintenance costs.

Similarly, effect of sulphur on Selective Catalytic Reducing System fitted for NO control in diesel vehicles, can be harmful. Sulphur does not reduce conversion efficiency in SCR systems as directly as in other advanced control technologies, but emissions are impacted in different ways. Fuel sulphur will increase the PM emissions from the downstream oxidation catalyst. Sulphur reactions in urea-based SCR systems can also form ammonium bi-sulfate, a severe respiratory irritant.

Overall, higher sulphur levels in fuels cause more serious degradation over time. The percentage benefits of reducing sulphur levels in fuels increases as vehicles are designed to meet stricter standards. The price of BS-VI fuels in Delhi must remain lower than the price of BS-IV fuels in NCR.  On March 31, when I checked out at the retail outlets, it was encouraging to see lower fuel prices in Delhi compared to NCR—the price of diesel in Delhi was Rs 64.40 per litre as opposed to Rs 65.57 across the border in Haryana; price of petrol in Delhi was Rs 73.55 as opposed to Rs 74.40 per litre in Haryana. The price differential in favour of cleaner fuel must remain.

Air pollution is a big fight. Several pieces have to come together to clean up the air. Clean transport fuels is one big step towards reducing toxicity of our breath.  

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