Monday, July 27, 2009

Biobutanol-Ultra Low Sulfur Diesel: A perfect blend fuel for the future

A lot has been talked and discussed about biobutanol as a potential fuel for gasoline blend and its advantages, but very less has been researched, discussed and talked about butanol as a DIESEL fuel. I have been working on "butanol as a fuel " for more than two years I have studied, researched and analysed butanol's properties as a gasoline blend fuel as well as diesel blend fuel. More I went in to depth of butanol more I started liking to see butanol as a full blown fuel in either way, the simple reason is blending of butanol improves various shortcomings of gasoline and diesel.

Diesel automobiles are the backbone of the transportation across the globe, high consumption of diesel causing harsh pollutant built up at the atmosphere. To abate pollution caused by diesel engine exhaust US, European and other environment protection agencies are going tougher for diesel emission standards and aiming at Ultra low sulfur diesel (ULSD) or S-15 ( ULSD can contain maximum 15ppm of sulfur, which is 97% less than low sulfur diesel).

Use of ULSD indeed will trim-down the pollution from diesel engine exhaust however there are numerous other issues associated with ULSD as a fuel, biggest of them is lubricity (It is a measure of a fuel's ability to prevent wear in system components). In conventional diesel sulfur present is sufficient for providing desired lubrication, but in a case of ULSD additives for lubrication is required to meet ASTM standard. Another vital difficulty with ULSD is presence of significant higher amount of dissolved water due to hydrotreating (about 10 fold), this water caused freezing in winter and microbial growth in warm weather. Fuel's oxidative stability is its resistance to the formation of gum and insoluble oxidation product which can plug filter and may lead engine to poor perform. Fuels' poor oxidative ability and microbial growth can impact shelf life, and today's clean fuels have very short shelf life compared to fuels of 20 year ago. Shelf life of EPA mandated ULSD is from 3 to 6 months. Increase in amount paraffin (wax) contain in ULSD compared to normal diesel is another glitch of desulfurization process. During cold weather paraffin (wax) present in diesel begins to form crystals, these crystal eventually plug the filter, starve the engine and prevent from starting. Cold start has always been hiccup for diesel engine, low temperature of combustion chamber, piston head and cold air intake makes evaporation of diesel difficult, which leads to engine having difficulty at the time of start and sluggish for a considerable time of starting. Cold start also leads to incomplete combustion and smoke formation.

In today's day and age alcohols are widely accepted as a clean fuel additives , oxygenates and also fuel for the future. There are several limitations to using lower alcohols in compression ignition engine (diesel engine), particularly methanol and ethanol due their lower energy density, low cetane number and limited miscibility with diesel. On other hand n-butanol is completely miscible with diesel and sparingly with water, has higher energy density and higher cetane number compared to ethanol. Though experience with diesel engine running on butanol blended fuel is quite limited, biobutanol has potential to overcome some of drawback of LSD/ULSD and to reduce demand of petroleum fuels without much compromising on energy density and emission properties.

Butanol as Diesel fuel Blend

Fuel consumption and economy is the key factor to consider any renewable fuel for the replacement of fossil fuel. Switching from petro diesel to butanol blend diesel definitely would result in higher consumption of fuel. Heating value of petro ULSD is 19183 btu/lb and that of n-butanol is 14766 btu/lb which is 77% of petro diesel's heating value and almost same as that of biodiesel's(92% of biodiesel). In theory 20% and 40% butanol-diesel blend will result in the fuel consumption penalty of about 5% and 10% respectively. Use of ethanot in gasoline is widely practised as renewable fuel, but the similar use of ethanol in diesel is not widely accepted, one of the barrier to the use of ethanol and lighter alcohols in diesel fuels is limited miscibility especially at lower temperatures and need of the emulsifier. Whereas, butanol can be completely mixed to the diesel fuel in any concentration and it doesn't separate at lower temperature or on addition of water and hence can be mixed with diesel anywhere in the supply chain. Complete miscibilty of butanol-diesel blend and non-corrosive and non volatile nature of butanol makes transportation and handling lot more easier than lower alcohols.

Butanol as diesel fuel additive:

Linear, primary alcohols have better lubrication property than paraffin or iso-paraffin which makes them excellent additive for clean diesel fuel for the improvement of lubricity and butanol can be most preferred due to its similarity with diesel fuel. Atomization characteristic of diesel and even bio-diesel can be improved by blending with butanol. Butanol could be used to reduce the viscosity, density and surface tension of the fuel blend on which the atomization characteristic of any fuel depends.

In theory, a hydrocarbon fuel when burned should produce only water and carbon dioxide as exhausted gases. However, such ideal combustion rarely occurs even in most perfectly adjusted engine. What is actually produced is the large amount of carbon monoxide, oxides of nitrogen, particulates and unburned hydrocarbons. When butanol is blended with fuel (gasoline/diesel) it can serve as an oxygenate and may help to mitigate the emission or air pollution.

Blending of butanol with ULSD can improve engine's cold weather and cold flow performance. Due to low latent heat (less than half of ethanol) of butanol is less likely to cause starting difficulties or problem during warm up. According to the report published by Argonne National Laboratories blending butanol with diesel may also extend cold flow properties and oxidative properties. The problem of plugging of filter due to crystallization of wax at lower temperature can also be curtailed by addition of butanol, as butanol can reduce the solidification temperature of fuel at lower temperature. (Source: Smith and Workman)

Like other fuels, butanol also has limitations like low cetane number, exposure to its vapor may cause eye, nose throat irritation and the biggest challenge is high production cost.



  1. A very interesting concept and it is a fact Diesel cannot be stored for long (a key requiremnt now a days in this globalised world) even if one can come over the other problems with ultra modern cars. The problem as you say is to be able to produce commercially viable quantities. It starts with availabilty of enough bio mass to begin with and then the slow fermentation process - am I right? There is huge hype built on Algae, though not sure if that will lead to something spectacular. Proabably a novice challenge from someone uninitiated - why cant we radically change (something which has not happened for decades?)the refining process of fossils and be able to hydrogenate & oxygenate substantially more, may be by blending with bio at source. The carbon chain becomes longer with these molecules and burns of less toxically?

  2. Ajay you are correct fermetation yield is very poor in case of butanol, reason in toxicity of butanol, in conventional fermentation butanol concentration goes max to 1.6% only, but with genetically modified bugs researchers have been able to achieve up to 4%, so I'm optimistic. I'm closely tracking fermentation and syngas route I have read here and there about algae but really wanna to read in detail. I want to see renewable fuel more in use and keep fossil fuel and water for future generation.

  3. Hello! i found your blog via linkedin, i am also very interested in these topics. Do you know from which concentration, via fermentation process, would butanol then be feasible/promising? from the economical point of view i mean.
    Ethanol is now ¬10% with natural strains, but it has lower value and other separation costs...


  4. Margarida,
    You have a valid point, whether butanol is promising or not? I don't think biobutanol is promising yet, but good news are there various grps have genetically modified bacteria and now they claim their bacteria can tolerate butanol up to 4%. Another approach from syngas route looks promising to me, ofcource far from commercialization.

    AS far as downstream process compared to ethanol less costlier as butanol has limited solubility in water.


Please comment on the content and look-N-Feel of this site and help us improve it.