I was involved in biodiesel combustion and emissions research about 20 years ago, prior to my current position.  With respect to biodiesel and other oxygenated fuels, we found that NOx emissions would increase approximately linearly with fuel oxygen content and that ignition delay properties also had a strong impact, particularly in older engines with relatively long ignition delay/low injection pressures.  NOx formation in diesels is dominated by the Zeldovich mechanism, and thus is an exponential function of combustion temperature.  Any efforts to reduce combustion temperature can be effective at reducing NOx formation.  The most common methods that I am aware of for reducing diesel NOx formation are injection timing retard, the use of cooled EGR and reducing air charge temperature.  There are always limits and there are often trade-offs with PM emissions, fuel consumption, and when taken to extremes - even combustion stability and misfire in the case of EGR and injection timing retard.  Sometimes other measures, such as increased injection pressure, can allow for more flexibility in combustion phasing and allow better tradeoffs.  NOx catalysis via urea/SCR or NOx adsorption is revolutionary in that it allows an order of magnitude or greater reduction in NOx while allowing somewhat more favorable combustion phasing (i.e., higher engine-out NOx) and higher efficiency.  While there is a NOx increase with biodiesel fuels, the increase may be a moot point in engines equipped with SCR since NOx reduction is often >>90%.  One potential issue with SCR might be NO2 formation.  Our earlier research showed both increased engine-out NO2 and, perhaps more troubling, increased oxidation of NO to NO2 over PGM catalysts when using biodiesel fuels.  SCR catalysts rely on about 40-60% oxidation of NO to NO2 to promote faster NOx reduction kinetics at low temperatures.  It is possible that biodiesel could promote additional NO2 oxidation that could interfere with SCR low-temperature kinetics. More research is needed in that area, particularly considering how urea/SCR is now nearly ubiquitous in heavy-duty truck engines used in Europe, North America and Asia.

Exhaust Gas Recirculation is one technique to reduce NOx. Several other methods are available like ammonia spray, water spray etc. into the combustion chamber to reduce the temperature. 

Pl refer the attachment.

I agree with the measures mentioned by Anand Pai and Maciej Mikulski, but it has to be stated, that these technologies are for the reduction of NOx in general, not only for biodiesel fuel. The difference in exhaust emission characteristics between crude-oil based diesel and biodiesel stems from the different chemical structure and the varying behavior in mixture preperation and combustion.

Kind regards,

Mario 

I do not understand why you would want to enhance NOx emissions. Shoudn't our objective be to reduce rather than enhance NOx emissions?

I do not know on which sort of engine you are working.

With new diesel engines having SCR there is no problem : the NOx sensors permits to adjust urea to have no more NOx.

But I suppose you have a classical engine. So you know that , with oxygen inside the biodiesel molecules, combustion is locally slightly hotter and that is why there is more NOx but also less soots. But this is only true if biodiesel is like a classical diesel fuel about physical properties (viscosity for exemple). If you were using oil (and not esters) the problem would be more complex.

There is no commercial additive to avoid the phenomena. Many years ago some people made experiments adding compound containing NH2. I am not sure it was working but may be you could try again.

A solution could be a water emulsion but you have to be careful about the stability.

.All the techniques were given by the beloved researcher.

I don't know whether you have all this technique or equipment or experimental set up for that.

As far as i know the simplest way to reduce the NOx formation  is by increasing  the rate of cooling water and reduce the engine temperature.

But in your question you have asked for the techniques to enhance the nox emission.

if that the case reverse the idea.

in case for faster cooling use nano material or PCM

I was involved in biodiesel combustion and emissions research about 20 years ago, prior to my current position.  With respect to biodiesel and other oxygenated fuels, we found that NOx emissions would increase approximately linearly with fuel oxygen content and that ignition delay properties also had a strong impact, particularly in older engines with relatively long ignition delay/low injection pressures.  NOx formation in diesels is dominated by the Zeldovich mechanism, and thus is an exponential function of combustion temperature.  Any efforts to reduce combustion temperature can be effective at reducing NOx formation.  The most common methods that I am aware of for reducing diesel NOx formation are injection timing retard, the use of cooled EGR and reducing air charge temperature.  There are always limits and there are often trade-offs with PM emissions, fuel consumption, and when taken to extremes - even combustion stability and misfire in the case of EGR and injection timing retard.  Sometimes other measures, such as increased injection pressure, can allow for more flexibility in combustion phasing and allow better tradeoffs.  NOx catalysis via urea/SCR or NOx adsorption is revolutionary in that it allows an order of magnitude or greater reduction in NOx while allowing somewhat more favorable combustion phasing (i.e., higher engine-out NOx) and higher efficiency.  While there is a NOx increase with biodiesel fuels, the increase may be a moot point in engines equipped with SCR since NOx reduction is often >>90%.  One potential issue with SCR might be NO2 formation.  Our earlier research showed both increased engine-out NO2 and, perhaps more troubling, increased oxidation of NO to NO2 over PGM catalysts when using biodiesel fuels.  SCR catalysts rely on about 40-60% oxidation of NO to NO2 to promote faster NOx reduction kinetics at low temperatures.  It is possible that biodiesel could promote additional NO2 oxidation that could interfere with SCR low-temperature kinetics. More research is needed in that area, particularly considering how urea/SCR is now nearly ubiquitous in heavy-duty truck engines used in Europe, North America and Asia.

In complete agreement with Joseph, also using diesel-biodiesel mixtures the NOx emission is proportional to oxygen content of the fuel. The oxygen presence in the fuel molecule drives to higher oxygen concentration in high temperature zones than with diesel fuel. For the same reason, in spite of a NOx increase you can generally expertise a soot concentration decrease.

in general and briefly , Nox will increase due to increase of temperature and oxygen and the more oxygen the fuel has the more Nox and engine heat (due to more complete combustion) is seen. reduction of these two can improve the nox but then u will face problems with CO and pm.

We did some studies with soybean methyl ester that suggest that it is the effect of the fuel on the combustion/temperature pressure history that determines NOx levels in a DI engine. By affecting the ignition process, the fuel affects the pressure time history and therefore the NOx levels, it doesn't seem to be a strong chemical process. NOx levels correlated nicely with peak pressure or peak pressure rise rates, irrespective of the fuel.

SAE paper 930934,   "Combustion of Soybean Oil Methyl Ester in a Direct Injection Diesel Engine" found in any of the following SAE references:  New Developments in Alternative Fuels and Gasolines for SI and CI Engines - SP-0958,  Alternative Diesel Fuels - PT-111,  Alternate Fuels - PT-48, or SAE 1993 Transactions: Journal of Engines - V102-3

DME behaves in a similar manner, though we don't have comparable data.

Higher NOx emissions are caused by: (i) Advanced injection timing, (ii)  Presence of unsaturated(double bond) fatty acids, (iii) High temperature combustion (above 1600 oC), (iv) High rate of Combustion, and (v) more premixed combustion, etc. 

In older engines, advanced injection is caused by higher bulk modulus (or high density) of biodiesel fuels. In conventional diesel engines(equipped with in-line fuel injection systems), use of biodiesel causes the pressure waves (generated by fuel pump) to travel faster from pump end to injector end, which causes early/advanced injection and due to advanced injection, the combustion occurs close to TDC. Therefore, pressure and temperature of cylinder gas will rise to above 1600 oc, which generates NOx. By choosing low-density biodiesel, injection can be delayed. Injection timing can be retarded by reducing density of biodiesel, which can be reduced by preheating.

Presence of unsaturated FAMEs, particularly poly-unsaturated esters such as, linoliec acid/ester (C18:2) and linolenic acid/ester (C18:3), causes high temperature combustion, due to presence of double bonds. Increasing unsaturation in biodiesel, leads to more NOx generation. Presence of mono-unsaturation fatty acid/esters (C18:1) gives better results. 

Rate of combustion depends on (CN) cetane number (and also on ignition delay:ID). Biodiesel with low CN, causes more delay in ignition. Delay in ignition causes low rate of combustion and more premixed combustion, which causes more NOx. Therefore, by choosing biodiesel with high CN is the way to reduce NOx.        

In my view, we cannot generalize the linear relationship between NOx and oxygen content (m%) in biodiesel. It may be true for certain feedstocks, but not for all. Example: The coconut biodiesel contains more than 14 % oxygen, but generates very low NOx, due to short and saturated carbon chain.    

http://biodiesel.org/reports/20030201_gen-361.pdf

It has to do with the chemical bonds of feed stock. See attached file. 

Most of the NOx is formed after the fuel is burned in homogeneous combustion, or in diffusion combustion, it is formed in the region where the fuel has burned, and there is an excess of oxygen.  The only way to reach temperatures high enough to form NOx is tor burn the fuel.  So NOx is formed where there is no fuel.

Thus, fuel has an impact on NOx formation only to the extent that it determines conditions after combustion, when the fuel is no longer present.  Adiabatic flame temperatures for most organic fuels are pretty much similar, so the main fuel influence is determining when the fuel burns (ignition delay), which determines the temperature time history for the short period of time after combustion until the NOx concentration freezes. EGR and water injection lower the flame temperature, adding more inert material to be heated up by the same amount of fuel and word the same way regardless of fuel. 

There is a question of how you conduct the experiment to compare the fuels.  If you just change the fuel without adjusting the injection timing, you will most likely see greater differences between fuels.  If you compare the fuels with optimized timing for each specific fuel, the differences should be smaller.  There are some lower order effects, but the NOx formation mechanism is well understood, and not that complicated.  See any engine text from Heywood on.

Generally NOX appear during premix combustion which is characterized by a significant increase in heat. By reducing the temperature peaks during this phase of combustion, the NOx level in post-combustion gases could be considerably reduced.

No that isn't correct, the NOx is formed in the diffusion flame, not in the "premixed flame".  See the work by Dec. For example SAE.Paper 970873, also in SAE Transactions volume 106, page 1319.  There are several other papers by him and co-authors.  He clearly shows the NOx to be formed in a thin zone in the diffusion flame, at the edge of the jet.  The premixed reactions leading up to ignition occur at temperatures too low to form NO, and the "homogeneous" reactions inside the fuel jet are much too rich to form any significant amount of NO.  At the diffusion flame, the temperatures are close to the aidabatic flame temperature, and there is some oxygen present.

There are various strategies that can adopted to reduce the increase in NOx emissions as a result of using bio diesel in diesel engine.

Please refer to the following :

"Optimization strategies to reduce the bio diesel NOx effect in diesel engine with experimental verification "

Energy Conversion and Management 68 (2013) 96–104