DEIFYING AND DEFYING DIESEL
August 12, 2013 § 1 Comment
Diesel rightly deserves a place of pride in the world of transport fuels. The high density fuel combined with a high efficiency engine provides fuel economy which is a full third over that of gasoline. This is why it is the sole fuel of choice for long haul and heavy duty trucks, and off-road equipment that are the backbone of agriculture. 95% of school buses run on diesel. But diesel carries environmental baggage. For that reason, and because it is by and large a product of oil, it invites substitution, especially in urban areas.
Diesel fuel and its use have seen significant advances over the last twenty years. The principal of these is the utilization of low sulfur fuel. While this has been mandated for on-road vehicles, the off-road regulation has been left to the states and is variable. Also, there are nearly a million diesel generators in the US performing a variety of functions. In places like India which are subject to regular electricity shut offs, virtually all the generators use diesel. A high concentration of these is in urban areas.
The other major advances have been in the implementation of particulate filters and urea injectors. Oxides of nitrogen (NOx) are formed at the relatively high temperatures of combustion in a diesel engine. Urea injection removes nearly 90% of the NOx by converting them to nitrogen and water. But despite efforts to date, the preponderance of evidence suggests that the fine particulates from diesel combustion (PM 2.5) are serious contributors to mortality and morbidity.
All of the above has caused diesel displacement, initially by compressed natural gas (CNG) particularly in urban areas. In 1998 the Indian Supreme Court mandated the switch on all public vehicles in Delhi and full implementation took several years. The World Bank reported on significant improvements in mortality and morbidity. Since then Kuala Lampur has done the same, as have many other Indian cities. In the US the pace of adoption has been much slower but is picking up, mostly due to the low cost of natural gas. But, except for the Honda Civic, no passenger vehicle has been designed to run on CNG. The daunting aspect is that the volumetric density is nearly one fourth that of gasoline. But numerous research efforts, many of these funded by the DOE’s ARPA E, are targeting a doubling together with 500 psi storage pressure versus 3500 psi in conventional systems. This last will allow faster and cheaper filling stations and will be an enabler for home filling.
The substitutes are many. They start with diesel produced from natural gas. This is sulfur free and ought to be devoid of any aromatics. Even the appearance is benign: a whitish somewhat translucent fluid. At a meeting I spoke at in Qatar Shell showed off their product by dispensing from a transparent pump. This substitute of course is a simple drop-in and should actually sell for a premium. Biodiesel was much in vogue for a while. It too is a drop-in but the raw material for its production tends to be in pockets of availability. Crop based biodiesel, from Canola, for example is very easy to make; practically a garage operation. But sources such as soy bean have drawn fire for using excessive water in the cultivation. On balance this avenue is likely to remain a boutique.
All the other substitutes come with degrees of difficulty in the engine or the infrastructure and dispensing. The aforementioned CNG is the current leader, and is most applicable to short haul fleets because of the ease of filling logistics. The other candidates are liquefied natural gas (LNG) and dimethyl ether (DME), in that order at this time.
LNG is believed to be more viable for long haul transport than is CNG, simply for reasons of range. Until recently I was bearish on the distribution costs. LNG production plants are large, with even a modest size one having production of about 5 million gallons of diesel equivalent per day (about 9 million gallons of LNG). Given that each truck carries 180 gallons, that plant will need to deliver considerable distances in refrigerated containers at -163 degrees F. But recent reports of a relatively new refrigeration technology, the nitrogen expansion cycle, offer the promise of small footprint production at reasonable cost. These would be in the range of just 30,000 gallon diesel equivalents per day.
The latest entrant is DME, emboldened by the low production cost driven by the current and projected low cost of natural gas. It is clean burning, with reduced NOx emissions due to the lower temperature burn and zero particulates. This last is the big driver. Volvo, and their subsidiary Mack Trucks, have announced the 2015 launch of a standard 13 liter engine running solely on DME. DME is stored and transported much like propane and the viability of economic infrastructure will likely be reliant on small footprint production.
Even methanol is entering the derby, albeit at a research stage for the present. Professor Cohn at MIT has designed and built prototype engines running on a diesel blend with methanol. In his concept methanol is injected at discrete intervals during the piston stroke. The evaporative cooling allows for higher compressions. He claims that his 9 liter engine with these features will do the job of the standard 13 liter diesel engine. A lot of the benefit comes from the reduced weight.
The diesel engine was one of the early internal combustion engines devised and named after the inventor Rudolf Diesel. Gasoline displaced it over the years. Yet, the modern diesel engine is the workhorse of commercial transport, and for good reason. But the health ramifications of fine particulate emissions are driving the desire for substitutes. This is especially the case in non-attainment areas. Cheap shale gas, at least in North America, is a significant enabler. Displacing powerful incumbents is hard. The reasons must be compelling. Here in the US we may have those.